The Application of Ichnology to Palaeoenvironmental And Stratigraphic Analysis (Geological Society Special Publication No. 228)

The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis

Geological Society Special Publications Society Book Editors R. J. PANKHURST (CHIEF EDITOR) P. DOYLE F. J. GREGORY J. S. GRIFFITHS A. J. HARTLEY R. E. HOLDSWORTH

J. A. HOWE P. T. LEAT A. C. MORTON N. S. ROBINS J. P. TURNER

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It is recommended that reference to all or part of this book should be made in one of the following ways: MclLROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228. MANGANO, M. G. & BUATOIS, L. A. 2004. Ichnology of Carboniferous tide-influenced environments and tidal flat variability in the North American Midcontinent. In: MC!LROY, D. (ed.) The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 157-178.

GEOLOGICAL SOCIETY SPECIAL PUBLICATION NO. 228

The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis

EDITED BY

D. McILROY Sedimentology & Internet Solutions Ltd, UK

2004

Published by The Geological Society London

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Contents MclLROY, D. The application of ichnology to palaeoenvironmental and stratigraphic analysis: introduction

1

MclLROY, D. Some ichnological concepts, methodologies, applications and frontiers

3

PEMBERTON, S. G., MACEACHERN, J. A. & SAUNDERS, T. Stratigraphic applications of substrate-specific ichnofacies: delineating discontinuities in the rock record

29

GLAUB, I. Recent and sub-recent microborings from the up welling area off Mauritania (West Africa) and their implications for palaeoecology

63

GOLDRING, R., CADEE, G. C, D'ALESSANDRO, A., DE GIBERT, J. M., JENKINS, R. & POLLARD, J. E. Climatic control of trace fossil distribution in the marine realm

77

MANNING, P. L. A new approach to the analysis and interpretation of tracks: examples from the Dinosauria

93

UCHMAN, A. Phanerozoic history of deep-sea trace fossils

125

MARTIN, K. D. A re-evaluation of the relationship between trace fossils and dysoxia

141

MANGANO, M. G. & BUATOIS, L. A. Ichnology of Carboniferous tide-influenced environments and tidal flat variability in the North American Midcontinent

157

BANN, K. L., FIELDING, C. R., MACEACHERN, J. A. & TYE, S. C. Differentiation of estuarine and offshore marine deposits using integrated ichnology and sedimentology: Permian Pebbley Beach Formation, Sydney Basin, Australia

179

BALDWIN, C. T., STROTHER, P. K., BECK, J. H. & ROSE, E. Palaeoecology of the Bright Angel Shale in the eastern Grand Canyon, Arizona, USA, incorporating sedimentological, ichnological and palynological data

213

MclLROY, D. Ichnofabrics and sedimentary facies of a tide-dominated delta: Jurassic He Formation of Kristin Field, Haltenbanken, offshore Mid-Norway

237

BANN, K. L. & FIELDING, C. R. An integrated ichnological and sedimentological comparison of non-deltaic shoreface and subaqueous delta deposits in Permian reservoir units of Australia

273

BUATOIS, L. A. & MANGANO, M. G. Animal-substrate interactions in freshwater environments: applications of ichnology in facies and sequence stratigraphic analysis of fluvio-lacustrine successions

311

MELCHOR, R. N. Trace fossil distribution in lacustrine deltas: examples from the Triassic rift lakes of the Ischigualasto-Villa Union Basin, Argentina

335

GENISE, J. F., BELLOSI, E. S. & GONZALEZ, M. G. An approach to the description and interpretation of ichnofabrics in palaeosols

355

DROSER, M. L., JENSEN, S. & GEHLING, J. G. Development of early Palaeozoic ichnofabrics: evidence from shallow marine siliciclastics

383

TWITCHETT, R. J. & BARRAS, C. G. Trace fossils in the aftermath of mass extinction events

397

GENISE, J. F. Ichnotaxonomy and ichnostratigraphy of chambered trace fossils in palaeosols attributed to coleopterans, ants and termites

419

BROMLEY, R. G. A stratigraphy of marine bioerosion

455

Index

481

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The application of ichnology to palaeoenvironmental and stratigraphic analysis: introduction DUNCAN McILROY Sedimentology & Internet Solutions Ltd, 29 Proctor Road, Hoylake, Wirral CH47 4BE, UK (e-mail: [email protected])

Ichnology is the study of trace fossils, which preserve the activity of animals as recorded by their tracks, trails, burrows and borings. Rather than giving information about the taxonomic affinities of a given type of organism, trace fossils yield information about an animal's behaviour in response to its environment. Trace fossils are almost always in situ, are commonly specific to a particular suite of environmental conditions, can be readily studied in core and may be common in strata devoid of body fossils. They are invaluable in thorough sedimentological analysis and are thus of great utility to petroleum geologists, sedimentologists and palaeontologists alike. Over the last 30 years or so, ichnology has been a rapidly developing branch of palaeontology that not only has important applications in classical palaeobiology (e.g. Donovan 1994; Bromley 1996), but is also of great value in the more applied disciplines of palaeoenvironmental and stratigraphical analysis. Much progress has been made in the development of this discipline, but there remain many fascinating and challenging issues, particularly in combining ichnology and sedimentology. This book aims to provide a summary of recent progress, with an up-todate summary of most themes in modern ichnology. The volume stems from the 2003 Lyell Meeting sponsored by The Geological Society, The Palaeontological Association, BP, Shell, Exxon Mobil, Statoil, Total and Amerada Hess. The introductory paper by Mcllroy (a) provides a condensed summary of some ichnological themes and frontiers, and outlines a practical approach for the description of trace fossils and identification of key stratigraphic surfaces. The sequence stratigraphic theme is taken up by Pemberton et «/., illustrated by their work on the Mesozoic of Canada, both in outcrop and in core. The recognition of key (sequence) stratigraphical surfaces is addressed in part by the detailed studies of Bann et a/., Mcllroy (b) and Bann & Fielding in shallow to marginal marine depositional systems, and by Buatois & Mangano in their review of non-marine systems. One of the earliest applications of ichnology with widespread use was that of the ichnofacies

concept (Seilacher 1964, 1967), which was widely used to determine palaeobathymetry. Recent work detailed in the paper by Glaub extends the use of trace fossils to determine bathymetry to include microborings in shells. The use of ichnology to determine ancient nonmarine environments has been taken on in recent years largely through the work of the Argentine ichnology groups, as reviewed by Buatois & Mangano and furthered by the detailed study of lacustrine deltas by Melchor. Complementary to the ichnofacies approach to the study of trace fossils is the study of ichnofabrics as developed in sedimentary rocks. Among these, the most complex fabrics probably arise from the combination of pedogenic and biogenie processes found in soils. A new approach to the description of such complex fabrics is proposed by Genise et «/., with illustrated examples from their recent work. In addition, an ichnofabric approach to detailed facies characterization of tidal deltaic facies is adopted by Mcllroy (b). Ancient depositional environments are reviewed from the 'bottom up' from deep water environments - both oxygen rich (Uchman) and oxygen poor (Martin) - through shallow marine (Bann & Fielding) and marginal marine (Baldwin & Strother; Bann et aL\ Mangano & Buatois; Mcllroy (b)) to non-marine settings (Buatois & Mangano). Insights into the formation and preservation of fossil tracks by Manning stand to revolutionize the way that ichnologists interpret vertebrate tracks, and demonstrate how trace fossils may be used to determine the saturation of ancient non-marine deposits. The last theme addressed in the book is how trace fossil assemblages have changed through time. Evolutionary ichnofaunas are reviewed from the Cambrian by Droser et al. with respect to ichnofabrics and substrate changes, and in the aftermath of extinction events by Twitchett & Barras. Comprehensive Phanerozoic-long reviews of borers and traces of the 'denizens of the deep' are provided by Bromley and Uchman respectively. The stratigraphic record of the radiation of bees, termites, ants and other progenitors of chambered burrows is reviewed by Genise, who also organizes them into ichnofamilies.

From: MC!LROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 1-2. 0305-8719/04/S15.00 © The Geological Society of London.

2 References

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SEILACHER, A. 1964. Biogenic sedimentary structures. In: IMBRIE, J. & NEWELL, N. (eds) Approaches to BROMLEY, R. G. 1996.Trace Fossils: Biology, Taphon-Paleoecology. Wiley, New York, 296-316. omy and Applications. Chapman & Hall, London. SEILACHER, A. 1967. Bathymetry of trace fossils. DONOVAN, S. K. (ed.) 1994. The Palaeobiology of Trace Marine Geology, 5, 413^28. Fossils. Wiley, Chichester.

Some ichnological concepts, methodologies, applications and frontiers DUNCAN McILROY Sedimentology & Internet Solutions Ltd, 29 Proctor Road, Hoylake, Wirral CH47 4BB, UK (e-mail: [email protected]) Abstract: Ichnology straddles the boundary between palaeontology and sedimentology, and is becoming an increasingly important tool in both fields. For the palaeontologist, trace fossils allow insight into behaviour and biomechanics of animals that would otherwise be the subject of conjecture. For the sedimentologist, trace fossils have a marked impact on the interpretation of sedimentary rocks in that they destroy primary sedimentary structures, but can also reveal subtle palaeoenvironmental information beyond the resolution attainable by analysis of primary physical sedimentary structures. This contribution aims to review the major developments in the field of ichnology, and to highlight some of the tools and approaches currently used by ichnologists. A personal ethos for the study of trace fossils in core is outlined as a model ichnological protocol, and some of the frontiers of the science as a whole are briefly discussed.

Some landmarks in the history of ichnological research From nomenclatural chaos to stability (ICZN) Structures that we now recognize as trace fossils have been recorded and named in the literature for centuries. In the early history of palaeontology, a profusion of names was created for shapes preserved in rocks. In many subdisciplines of palaeontology, resolving these early names was a comparatively simple process. For ichnologists, however, sifting through the plethora of names of taxa commonly misidentified as sponges, seaweeds and plants has been a monumental task. The individual to whom we owe the greatest debt is Hantzschel (1962, 1965, 1975) for his monographic works on the numerous synonymies of many forms that were originally poorly documented and illustrated. The task of ichnotaxonomy was made even more difficult by the International Zoological Congress (to whom all taxonomists look for guidance in taxonomic procedure), who initially insisted that, to be valid, all trace fossil names erected after 1930 were to be accompanied by a statement identifying the trace-making animal (ICZN 1964). The net effect of this was to render most post-1930 trace fossil names invalid. This is because ichnologists can seldom identify trace-making organisms; indeed a single trace may be made by many different taxa and - in the case of compound trace fossils - a single trace may be the work of several organisms (e.g. Pickerill & Narbonne 1995; Rindsberg & Martin 2003).

The approach taken by most ichnologists in response to the 1964 decision of the ICZN was to continue to apply the rules of the ICZN without the blessing of officialdom. However, in anticipation of a revised version of the ICZN, Sarjeant & Kennedy (1973) published a draft proposal for a separate ichnological code, adapted from the ICZN and its sister publication, the ICBN (International Code for Botanical Nomenclature). A less radical approach was taken by Hantzschel & Kraus (1972) and Sarjeant (1979), who proposed specific amendments to the pre-existing ICZN, which were eventually integrated into the subsequent version (Melville 1979; ICZN 1985) despite some fierce opposition to the inclusion of non-reproducing forms (e.g. Lemeche 1973). The ICZN (1985) therefore overturned the original ruling regarding the identification of trace-making organisms, rendering post-1930 ichnotaxa valid under the code regardless of whether a trace-maker could be identified, and vilifying the personal decision of most ichnotaxonomists to persist with applying the rules of the ICZN despite not being bound by them. In the most recent edition of the ICZN (1999) ichnology seems to have been largely embraced by the zoological community. Trace fossil genera (ichnogenera) established after 1999 must have a designated type species (ichnospecies) (ICZN 1999, Article 66.1); for earlier established ichnotaxa no type species need be designated but may be assigned at a later date according to the rules (ICZN 1999, Article 69). The status of ichnotaxonomy is thus now firmly established as a subdiscipline of taxonomy and - thanks to the new provisions within the

From: MclLROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 3-27. 0305-8719/04/S15.00 © The Geological Society of London.

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Table 1. The archetypal Seilacherian ichnofacies Ichnofacies

Predominant trace fossil types

Inferred control (Seilacher)

Skolithos

Vertical traces of suspension feeders

Cruziana

Horizontal and vertical deposit feeders

Zoophycos Nereites

Pervasive deposit feeders Shallow burrows with complex morphologies showing highly programmed behaviours Traces characteristically preserving scratches, mostly of suspension feeders Non-marine traces

Bathymetry (above fair-weather wavebase, FWWB) Bathymetry (between FWWB and storm wavebase, SWB) Bathymetry (shelf and slope below SWB) Bathymetry (basin-floor with turbidites)

Glossifungites Scoyenia

ICZN (1999) and the sensible taxonomic practices of ichnologists over the last 3(MK) years ichnology can only grow as a rigorous science. Among the problems that do remain is the exclusion of modern traces by the International Code of Zoological Nomenclature (ICZN 1999, article 1.2.1), which restricts the use of ichnotaxa to fossil and not to modern traces. This regulation complicates the description of modern borings and burrows, but, as such incipient trace fossils may be incomplete, some taxonomic problems may thereby be avoided. The simplest means of making comparisons between modern traces and their ancient counterparts is to use the prefix 'aff.' to denote their affinity to a trace fossil while acknowledging that modern traces have no validity under the current ICZN. A grey area also exists in defining when a modern trace becomes a trace fossil: at abandonment of the burrow, at burial, or at lithification? Other issues, particularly the exact definition of what constitutes a trace fossil, are currently under discussion (see Bertling et al. 2003). Most modern ichnologists consider rootlets and other plant traces as trace fossils, though the ICZN is reluctant to include non-animal taxa for obvious reasons. It may therefore be that the simplest solution to these issues is for a separate ichnological code to be created and given some form of approval by the ICZN and ICBN. Such issues are as yet unresolved and remain a challenge.

Ichnofacies approach The observations of Seilacher (1964, 1967) that recurrent associations of trace fossils could be recognized in the rock record represents the first widely applicable use of ichnology. Initially, six recurrent sets of trace fossils (named ichnofacies) were recognized by Seilacher (1967) and named for a characteristic trace fossil. The eponymous trace fossil need not be present for

Firm surfaces associated with incipient submarine lithification Freshwater conditions (red-bed deposition)

identification of an ichnofacies, with the types of trace fossils/feeding strategy being diagnostic. These ichnofacies are widely used in palaeoenvironmental interpretation (Table 1). The archetypal Seilacherian ichnofacies (Table 1) were based largely on assemblages of traces in a particular lithofacies and related to a bathymetric gradient from shallow Skolithos to deep Nereites ichnofacies. The Scoyenia ichnofacies was however created differently, being an environment-led definition in contrast to the other behaviourally defined ichnofacies. The recognition of these basic ichnofacies groupings was of great utility to sedimentologists as an aid to palaeoenvironmental interpretation. This work was immediately grasped by both the palaeontological and sedimentological communities, and was seminal in inspiring refined sedimentary facies models and in stimulating further classification of associations of trace fossils into additional ichnofacies. The subsequent proliferation of ichnofacies has been reviewed recently (Bromley 1996; Pemberton et al. 2001), and the most important are listed in Table 2. The controls on the distribution of ichnofacies have been conclusively demonstrated to be more than simply bathymetric (Ftirsich 1975; Ekdale et al 1984; Frey et al 1990; Bromley & Asgaard 1991; Gierlowski-Kordesch 1991; Wetzel 1991; Fig. 1), but the ichnofacies themselves retain their usefulness albeit in modified form. As can be seen from Table 2 there is no consistent ethos behind the creation of ichnofacies. The most anomalous of these are the vertebrate footprint ichnofacies and coprofacies, which are more likely to be trace fossil assemblages related to local palaeoecology and palaeobiology of producers than ichnofacies of inter-regional applicability. In addition, Bromley's proposed Fuersichnus ichnofacies has been demonstrated from a variety of non-marine environments (Buatois & Mangano 2004).

ICHNOLOGY & PALAEOENVIRONMENTAL ANALYSIS

5

Table 2. History of the development of the ichnofacies concept, highlighting the inferred palaeoenvironments of the later ichnofacies Ichnofacies

Palaeoenvironment

Author(s)

Paleodictyon and Nereites ichnosubfacies

The deep marine ichnofacies was subdivided into Paleodictyon for sand-rich proximal turbidites and Nereites for mud-rich distal turbidites

Rusophycus

Fluvial/shallow lacustrine

Fuersiehnus

Originally inferred to be representative of shallow lacustrine settings, below FWWB. Has subsequently been shown to extent to a variety of freshwater settings (Buatois & Mangano 2004) Lithic/hardground substrates Woody (xylic) substrates Freshwater shallow lacustrine and fluviatile settings

Seilacher (1974) recognizes the first subdivision of one of the archetypal ichnofacies. Building on the work of Ksiazkiewicz (1970); Crimes (1973) Bromley & Asgaard (1979) as an ichnocoenosis, and as an ichnofacies by Bromley (1996) Bromley & Asgaard (1979) as an ichnocoenosis and as an ichnofacies by Bromley (1996)

Trypanites Teredolites Redefined Scoyenia Curvolithus Psilonichnus Arenicolites Mermia Entobia Gnathichnus Termitichnus Laoporus Brasilichnium Brontopodus Caririchnium and 'Shorebird ichnofacies' Coprofacies Coprinisphaera Ophiomorpha rudis ichnosubfacies

A subset of the Cruziana ichnofacies, found in settings with high sedimentation rates. Particularly delta and fan delta deposits Coastal dunes A subset of Cruziana ichnofacies (opportunistic colonization of event beds) Lacustrine turbidites Subdivision of Trypanites (boring traces) Subdivision of Trypanites (rasping traces of organisms feeding on the surface of lithic substrates) Palaeosol ichnofacies including coprolites, rhizoliths and traces in xylic matter e.g. leaves The variety of footprint assemblages represent a diverse array of palaeoenvironments, though their facies specificity is in doubt, and the separation of Laoporus and Brasilichnium on stratigraphic grounds is not well founded Based on the distribution of various coprolite types. The facies-specificity of coprolites is in doubt and has not been widely used to date Palaeosols with insect nests Subdivision of Nereites ichnofacies proposed for channel and lobe to lobe fringe environments but is only recognized from Eocene and younger strata

The similarities between some non-marine and marine ichnofacies, as highlighted by Bromley (1996), demonstrates parallel behavioural evolution in the non-marine and marine realms, presumably due to comparable environmental controls. The notable exception is the nearabsence of a deepwater mudstone ichnofacies in non-marine settings, which is probably due - at

Frey & Seilacher (1980) Bromley et al (1984) See emendation of original definition by Frey et al. (1984); Buatois & Mangano (1995) Lockley et al. (1987) based on the earlier Curvolithus ichnocoenose of Heinberg & Birkelund (1984) Frey & Pemberton (1987) Bromley & Asgaard (1991) Buatois & Mangano (1993) Bromley & Asgaard (1993) Bromley & Asgaard (1993) Smith et al (1993), replaced by Coprinisphaera of Genise et al. (2000) Lockley et al. 1994

Hunt et al. (1994) Genise et al. (2000) Uchman (2001)

least in part - to the predominance of anoxia resulting from thermal stratification in lakes (Buatois & Mangano 2004). This increased reliance upon interpretation of sedimentary environment before ichnological characterization (subjective ichnofacies sensu Reading 1978) suggests that there is little utility in continuing to create archetypal ichnofacies - a stance

Fig. 1. Summary diagram showing current thinking on the likely distribution of the main soft/loose and firmground ichnofacies (based on Bromley 1996) and based on condition of still-stand of sea-level. Ar, Arenicolites ichnofacies; Cu, Curvolithos ichnofacies; Co, Coprinisphaera ichnofacies; Cr, Cruziana ichnofacies; Fu, Fuersichnus ichnofacies; Gl, Glossifungites ichnofacies; Ne, Nereites ichnofacies; Ps, Psilonichnus ichnofacies; Ru, Rusophycus ichnofacies; Sc, Scoyenia ichnofacies; Sk, Skolithos ichnofacies; Zo, Zoophycos ichnofacies. During marine flooding events and sea-level fall some ichnofacies become more widespread (e.g. Glossifungites ichnofacies).

ICHNOLOGY & PALAEOENVIRONMENTAL ANALYSIS

7

Table 3. Characteristics that make fossils good zone fossils; the ideal zone fossil would fulfil all criteria Characteristic

Rationale

Rapidly evolving, i.e. narrow stratigraphic range Widespread distribution Good preservation potential Abundance Facies independence Easy identification

Improves resolution of biozone Eases interregional correlation Improves chances of occurrence in a given rock unit Improves chances of occurrence in a given unit Allows correlation independent of palaeoenvironment Allows use by non-experts

supported by Goldring (1993, 1995) and discussed further below in terms of ichnofabric analysis and ichnocoenoses.

robust of these have been the schemes of Crimes (1975, 1987, 1992), which included a vast dataset of Neoproterozoic to Cambrian occurrences but rely on unpublished stratigraphic inferences. The most widely used ichnozones are those based on the Neoproterozoic-Cambrian type section in southeastern Newfoundland erected by Narbonne et al. (1987). Indeed the boundary itself was defined at the junction between the Harlaniella podolica and Phycodes pedum ichnozones (Brasier et al. 1994). Other radiation events. The early terrestrialization event has the potential to yield biostratigraphic data, but such sections do not yield abundant ichnological data and there is a potential problem with likely endemicity and diachroneity of early non-marine faunas/ichnofaunas. Rapid radiation is also recorded after major extinction events (see Twitchett & Barras 2004), such as the Permo-Triassic extinction event, which is estimated to have eradicated 96% of all family-level diversity (Jablonski 1991). Importantly, however, no phylum is known to have become extinct at that particular stratigraphic level so - although there was rapid radiation - no fundamentally new body plans evolved or died out, which limits the potential usefulness of ichnostratigraphy, but the stepwise reappearance of ichnotaxa can be of stratigraphic utility (Twitchett & Barras 2004).

Ichnostratigraphy Biostratigraphy is the methodology by which stratigraphers can subdivide the rock record, and correlate from region to region using the record of evolution and extinction of fossil taxa. The basic subdivision of stratigraphic time is the zone, and the fossils that define those zones are known as zone fossils. In accordance with the criteria in Table 3, the best zone fossils are likely to be rapidly evolving organisms with a pelagic portion to their lifecycle (improves distribution), and should comprise distinctive hard body parts. Thus trace fossils generally make poor zone fossils - due largely to the benthic lifestyle of trace-making organisms - except during intervals where benthic organisms that produce distinctive burrows evolve rapidly. Convergent behavioural evolution is the norm in ichnology, which accounts for the longevity of most ichnotaxa; convergent evolution of burrowing organisms has also been demonstrated (cf. Seilacher 1994). Bio-events that have the potential to include trace fossils of biostratigraphic significance include radiation events, and the evolution of distinct tracemaking groups. Radiation events Neoproterozoic-Cambrian. The Cambrian Radiation is perhaps the most dramatic of all the radiation events in the stratigraphic record, being the period of time in which most anatomical design becomes established. Many of the taxa represented by this diversification of body form were benthic in nature, and 'experimentation' in body form and behaviour are to be expected. It is thus unsurprising that this period has been identified by a number of authors as having excellent potential for ichnostratigraphy (Crimes 1975, 1987, 1992; Alpert 1977; Fedonkin et al 1983; Narbonne et al. 1987). The most

Evolution of distinct trace-making groups Palaeozoic arthropods. One of the first direct applications of ichnology to petroleum geology was the development of an ichnostratigraphic scheme for the correlation of peri-Gondwanan shallow marine 'unfossiliferous' quartzites (Seilacher 1970, 1985, 1992, 1993; Fig. 2), which are important reservoir intervals throughout the Middle East and North Africa. The main trace fossils involved in this ichnostratigraphic scheme are ichnospecies of Cruziana and related arthropod trace fossils. Such traces are abundant both in core and in outcrop, but body fossils are notoriously rare. The concept of basing an ichnostratigraphic scheme on Lower Palaeozoic arthropod traces is thus well founded in that

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Fig. 2. The Cruziana ichnostratigraphy of Gondwana (redrawn from Seilacher 1992).

they are abundant and widely distributed, and the trace-makers (trilobites and other arthropods) were rapidly evolving during this phase in their history. The drawback with the use of the scheme is that the trace-making arthropods are benthic and thus prone to provincialism (cf. Magwood & Pemberton 1988). In addition, many of the characters used to define the ichnospecies upon which the scheme relies are only rarely seen in material other than the exquisitely preserved type material. The majority of material examined in the field can thus be difficult to identify by the non-expert. Trias sic—Jurassic. During the Permian through to the Jurassic footprints of the archosaurs are comparatively common, particularly in Europe (Haubold 1984), North America (e.g. Olsen 1980) and South Africa (Ellenberger et al. 1970). The rapid evolution of the archosaur faunas is reflected in their changing footprint morphologies from early (Triassic) Cheirotherium-type footprints to later (Jurassic) tridactyl footprints such as the ichnogenus Grallator. Such schemes are reliant upon an abundance of well-preserved surface tracks, and have been well calibrated by accessory biostratigraphic data (e.g. Cornet & Traverse 1975). The difficulties of recognizing surface tracks make an awareness of possible under-track artefacts an invaluable skill (Manning 2004). In particular, features such as detached heel-like structures and 'spurs' in some ichnospecies of Brachycheirotherium may be related to transmitted heel structures (see Manning 2004, figs 17c, 21b,

22c). Vertebrate footprint ichnotaxonomy is a particularly difficult field, and much work needs to be done to fully appreciate which characters are useful for ichnotaxonomy. Tertiary. The radiation of terrestrial insects is well reflected in the fossil record of their burrow chambers, which is a taxonomic character widely used to identify modern insect taxa. The radiation of insects in the Tertiary was extremely rapid, and their distribution is highly sensitive to regional climatic shifts (Genise 2004). The Insecta are widely dispersed owing to their commonly airborne adult phase, presence in a range of non-marine environments, and their easily characterized egg chambers that have a high preservation potential. The Insecta with their staggered first occurrence datums thus fit the optimal characteristics of a zone fossil (see Table 3).

Seafloor and sediment oxygenation One of the most fundamental controls on the distribution of benthic animals and their trace fossils in aqueous environments is the availability of dissolved oxygen. This may be present either in bottom waters or in porewaters, but is essential for all metazoan life. The links between ichnological/benthic macrofossil distributions and bottom water oxygenation are well established (Bromley & Ekdale 1984; Savrda & Bottjer 1987; Ekdale & Mason 1988), though recent work (Schieber 2003) has demonstrated the need for

ICHNOLOGY & PALAEOENVIRONMENTAL ANALYSIS

careful assessment of apparently unbioturbated sediments through work with image analysis. Appreciation of the role of sediment anoxia in ancient successions is immature, though the impact of sediment anoxia on modern shallow marine taxa is well known (e.g. Pike et al. 2001 and references therein). Likewise, Wignall (1993) has correctly highlighted the fact that changing substrate conditions can favour depauperate ichnofaunas similar to those that typify anoxic bottom-water conditions. Sediments deposited in low-oxygen settings results are generally rich in organic carbon and thus have a high source rock potential to petroleum systems (e.g. Oschmann 199la, b; Wignall 1994). Determination of the palaeo-oxygenation of such sediments can be approached by geochemical means as well as through ichnology and palaeoecology (Wignall & Myers 1988; Wilkins et al. 1996; Wignall & Newton 2001). The organically rich nature of these facies also means that they are a potential treasure-trove of nutrients for deposit-feeding organisms (Diego & Douglas 1999). Colonization during amelioration of low-oxygen conditions by an opportunistic fauna is a common phenomenon that is documented in both modern and ancient sediments (e.g. Sagemann et al. 1991; Savrda & Bottjer 1991; Wignall & Pickering 1993; Bromley et al. 1995; Smith et al. 2000; Martin 2004; Fig. 9). Indeed, the ecology of deep-sea sites in general is now becoming much better known (e.g. Kaufmann & Smith 1997). Recent work has also highlighted the role of chemosymbiosis as a feeding strategy in such settings (Paull et al. 1984; Hovland & Thomsen 1989) and has led to the reinterpretation of some trace fossils as being the result of such behaviour (Seilacher 1990; Fu 1991).

Sequence stratigraphy Perhaps the most significant predictive stratigraphic tool developed in recent years is that of sequence (seismic) stratigraphy, which was developed by Exxon Production Research Co. in the late 1970s (Vail et al. 1977), and has since been further refined and debated (see the excellent review of Nystuen 1998). The basic model involves the recognition of unconformity-bound packages of sediment (sequences), which can be related to cycles of relative sealevel change. Within these sequences higher frequency increases in relative sea-level can be recognized (flooding surfaces), which (envelope) progradational sedimentary packages known as parasequences. The study and correlation of

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these parasequences is known as high-resolution sequence stratigraphy (Howell & Aitken 1996). The majority of integrated ichnological/ sequence stratigraphic approaches have employed the use of trace fossils, either in the recognition of key stratigraphic surfaces (e.g. Bromley & Goldring 1992; Taylor & Gawthorpe 1993; Ghibaudo et al. 1996; Oloriz & RodriguezTovar 1999; MacEachern et al. 1999; Malpas 2000; Pemberton et al. 2000; Uchman et al. 2000) or for improved broad-scale facies interpretations based on a refined ichnofacies-based approach (e.g. Vossler & Pemberton 1988; Frey & Howard 1990; Savrda 1991; Brett 1998; Siggerud & Steel 1999; Pemberton et al. 2001). Despite the firm establishment of the ichnofabric/ichnocoenosis approach to improved facies and stratigraphic analysis (Bockelie 1991; Taylor & Gawthorpe 1993; Taylor & Goldring 1993; Taylor et al. 2003; Schlirf 2003), this methodology has been underused (but see Bockelie 1991; Martin & Pollard 1996; Schlirf 2003; Mcllroy 2004). The advantage of the ichnofabric/ichnocoenosis approach is that its focus is improved characterization of facies - the fundamental building block of sequence stratigraphy - through improved understanding of trace fossil fabrics and seafloor ecology with respect to the host sediments. Once established, an ichnofabric scheme can be used to assess stacking patterns, while simultaneously enhancing ichnological characterization of key stratigraphic surfaces (Mcllroy 2004). It is regrettable that most sedimentology textbooks focus on Seilacherian ichnofacies (e.g. Frey & Pemberton 1984; Pemberton et al. 1992, 2001), rather than also encompassing the more flexible ichnofabric/ichnocoenosis approach outlined below (the notable exception being Goldring 1999). An ichnological ethos What follows is a personal approach to the study of trace fossils and ichnofabric, which is designed largely for the study of marine clastic depositional systems (the author's own main focus of research). The basic methodology is of wider application, but should be adapted to the needs of the particular ichnological/sedimentological problem addressed, e.g. non-marine terrestrial systems (see Genise et al. 2004) and carbonate systems (e.g. Curran 1994). The reader should be aware that there are many ways to approach ichnological studies, and that no single approach is correct. All have their strengths.

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Fig. 3. Comparison of two cross-bedded sandstones with the bivalve escape burrow aff. Lockeia (arrowed): (a) represents a non-marine crevasse splay sandstone from the Carboniferous of Northumberland, England (scale bar in mm); and (b) a tidally deposited sandstone Jurassic, Neuquen Basin, Argentina. Ichnotaxonomically, the two specimens are ichnologically similar but sedimentological observations allow recognition of a tidal depositional environment through tidal bundling.

Scale of observation As with most sedimentological studies, the ultimate aim of an ichnological study commonly determines the resolution at which data are recorded. For example, when looking for long timescale changes in bioturbation, in a thick, sedimentologically homogeneous Neoproterozoic-Cambrian succession, Mcllroy & Logan (1999) used decimetre-scale observations of ichnofabric index (sensu Droser & Bottjer 1986, 1989, 1991). In contrast, when studying the highly heterogeneous tidal deposits of a tidedominated deltaic system, the same author made ichnological and sedimentological observations on a centimetre scale (Mcllroy 2004). The key to producing scientifically valid, usable, data is thus to choose an appropriate scale at which to collect ichnological and sedimentological data. The constraints are commonly time available (often a problem when working to industry deadlines), volume of data required/desired, and the inherent variability of the sedimentary succession. Observations should of course always be made in close detail if possible, but if nothing is found in a thick package of homogeneous sand there is little advantage in making numerous statements describing the lack of sedimentological/ichnological features. A conspicuous lack of trace fossils is in itself revealing though, and an important observation in need of explanation.

Sedimentological context In getting the maximum amount of information from a sedimentary rock it is imperative that

no information is disregarded. In the same way that many sedimentologists record trace fossils as 'bioturbation', so many palaeontologists/ ichnologists record the host sediment as sandstone/shale etc. without due regard to the physical sedimentary structures contained therein (Fig. 3). The sedimentologist should look to ichnology to help understand sedimentologically homogeneous rocks (e.g. Gowland 1996; Martin & Pollard 1996; Mcllroy 2004). Likewise, the ichnologist may learn about the likely spatiotemporal distribution of ichnofabrics by full consideration of likely sandstone body geometries and stacking based on sedimentological information. Although ideally we should all be transdisciplinary geoscientists, the reality is that for most geologists collaboration and open discussion is the way forward; this is especially so between sedimentologists and ichnologists.

Quantification of bioturbation The need to quantify the extent to which animals modify sedimentary fabrics has been recognized since the early days of modern ichnology. Many attempts to produce an easy-to-use scheme for the documentation of this bioturbation have been proposed (e.g. Moore & Scrutton 1957; Reineck 1963; Howard & Frey 1975; Frey & Pemberton 1984; Droser & Bottjer 1986, 1989, 1991; Taylor & Goldring 1993). Most of these rely upon estimating the proportion of sedimentary fabric/laminations destroyed by the burrowing activity of animals (i.e. bioturbation). These need to quantify the fabric as seen in vertical cross-section is a bias introduced by the predominance of studies of shallow box

ICHNOLOGY & PALAEOENVIRONMENTAL ANALYSIS

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Fig. 4. Ichnofabric indices exemplified by flashcards according to the scheme of Droser & Bottjer (1989); redrawn with permission. Shows the proportions of sediment reworked by bioturbation as seen in vertical cross-section.

cores (modern sediments) and sediment/rock cores (e.g. in oil field studies). The most usable of these schemes is the semiquantitative flashcards of Droser & Bottjer (1986, 1989, 1991; Fig. 4), which have been used with success by a number of authors (e.g. Droser & O'Connell 1992; Mcllroy & Logan 1999). This approach has recently been extended to include a flashcard methodology for quantification of the extent of bioturbation on bedding planes (Miller & Smail 1997; Fig. 5). A similar, but more sophisticated, means of graphically representing quantitative and semi-quantitative aspects of trace fossil fabrics in vertical section has been proposed by Taylor & Goldring (1993), and is discussed in detail below (Fig. 6).

Ichnofabric analysis The component of a sediments texture created by the action of animals is known as its ichnofabric. Ichnofabric may be created either by bioturbation (in loose sediment) or by bioerosion (in lithified sediment) by a diverse array of organisms from microbes (e.g. Glaub 2004; Fig. 7a) to dinosaurs (e.g. Manning 2004; Fig. 7b). One of the features of trace-making organisms is that they are commonly highly sensitive to their environment and can thus provide a record of

Fig. 5. Flashcards showing the proportion of bedding planes covered by trace fossils from Miller & Smail (1997), as classified into 'bedding plane bioturbation indices'. Column A represents example bedding planes covered by trace fossils of even size and shape with even distribution; Column B represents example bedding planes covered by trace fossils of different sizes and shapes and with uneven distributions (redrawn with permission).

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Fig. 6. Example of a modified ichnofabric constituent diagram adapted from Taylor & Goldring (1993) expressing the ichnofabric of an outcrop from the Lajas Formation, Neuquen Basin, Argentina. Ast, Asterosoma; Pa, cf. Parahaentzchelinia; Th, Thalassinoides. Note that the horizontal scale is used at the base of the diagram, which is the author's personal preference.

palaeoenvironmental conditions before, during and after deposition of a bed (Fig. 8). When considering physical sedimentary structures alone, information can be gleaned only about conditions at the time of deposition, which in many cases (e.g. hurricane-deposited sandstone beds on the normally quiescent proximal shelf) can be anomalous. The modern sedimentologist should therefore not only be able to record the presence of bioturbation but also be able to combine information from sedimentary structures and other macro/micropalaeontological data in

order to fully characterize their facies and understand their palaeoenvironment of deposition. As discussed above, ichnofabrics are best investigated on a bed-by-bed scale, which normally requires sedimentary logging at a scale of at least 1:50. The features of ichnofabric that should be recorded during routine investigation of sedimentary rocks include: intensity of bioturbation; diversity; relative abundance;

Fig. 7. Different scales of ichnofabric development: (a) artificial casts of microborings similar to Fasciculus in a shell clast (Recent of Mauritania courtesy of I. Glaub); (b) vertical cross-section through a dinosaur track showing bioturbation by a large bipedal dinosaur from the Jurassic Scalby Formation of Yorkshire, UK ('dinoturbation' of some authors) (courtesy of P. Manning).

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Fig. 8. Cored section with Diplocraterion parallelum seen in (a) longitudinal and (b) transverse cross-sections. The greatly different proportions of the cut surface covered by traces is dependent on the section taken. The percentage by volume of core bioturbated is in both cases c. 40%.

ichnometry; infaunal tiering; succession of bioturbation; colonization styles. Intensity of bioturbation Sedimentologists should approach this using one of the many 'bioturbation index' schemes as

described in the section above. The present author recommends either the ichnofabric index schemes of Droser & Bottjer (1986, 1989, 1991), or bed-by-bed estimation of bioturbation as a percentage. It is emphasized, however, that the parameters outlined below should also be investigated in order to make the most of the available ichnological data. It must also be remembered

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Fig. 9. Example ichnofabrics as developed under a range of palaeoenvironmental and sedimentological conditions.

that ichnofabric is a 3D phenomenon, and that examination of a 2D surface (e.g. a cut surface of a core) can be misleading; if possible, an impression of ichnofabric in the opposite plane should be sought in order to estimate the percentage bioturbation as a volume (see Fig. 8). Diversity Ichnological diversity is comparatively simple to measure from both core and outcrop sections, with experience. The number of trace fossils

present within a rock unit can be simply quantified, though care must be taken (especially in core) to ensure that a single trace in different orientations (e.g. different cross-sections of the same trace) is not counted twice. Although trace fossil diversity cannot be directly related to biological diversity it is usually considered as a reasonable proxy. For example, a diverse fauna of shallow infaunal bivalves may only produce two trace fossils (Lockeia as the resting trace and Protovirgularia as the locomotory

ICHNOLOGY & PALAEOENVIRONMENTAL ANALYSIS

trace). In most cases, however, high ichnological diversity corresponds to amenable palaeoenvironmental conditions, and low diversity or lack of bioturbation to harsh palaeoenvironments. The exact causes of environmental stress are diverse and must be assessed on a case-by case basis using ichnological, palaeontological, sedimentological and sometimes geochemical/ palynological means. It is also observed that ichnological diversity is more difficult to assess in core materials where identification below the level of ichnogenus is seldom possible and the morphology of complex branching forms is difficult if not impossible to recognize (except on the limited number of bedding surfaces). Relative abundance Merely presenting the number of different trace fossils present within a rock unit can be a misleading piece of information where the assemblage is dominated by a single ichnotaxon with minor accessory components. Documentation of the relative volumetric proportions of all traces within an ichnofabric and their relative chronology is the fundamental procedure behind good ichnofabric analysis. This can be particularly instructive if combined with simple

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(but tentative) interpretation of the likely trophic niche of the trace-maker. Ichnometry In addition to documenting the diversity and abundance of traces, the dimensions of the burrows themselves can be of great importance. The parameter that is most easily recorded is burrow diameter. Studies have shown that trace fossils become narrower with increased salinity stress (Hakes 1976; Gingras et al 1999) and decreasing dissolved oxygen in porewaters/ bottom waters (e.g. Bromley & Ekdale 1984). This reflects well-known biological trends in such settings (e.g. Milne 1940; Remane & Schlieper 1971). In addition, during some periods of rapid evolution there is a stratigraphic component, in which burrow size increases with time, e.g. the Cambrian explosion (Mcllroy & Logan 1999). Infaunal tiering The distribution of organisms (and their traces) below the sediment is known as tiering. The preservation of a tiering profile is reliant upon rapid killing off of the community (e.g. burial under an event bed or death of the community

Fig. 10. Ichnofabrics in outcrops of the Pacoota Sandstone, Amadeus Basin, central Australia showing: (a) environmental deterioration represented by a decrease in burrow size of Skolithos', (b) development of an early Arenicolites-dommated ichnocoenosis (Arenicolites ichnofacies) cross-cut by a later, Skolithos-domm&ted, ichnocoenosis (Skolithos ichnofacies). The upper bed probably represents a single spatfall as all burrows are the same (small) size. The small size of traces may represent burrows of juveniles buried during a phase of gradually increasing bioturbation.

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due to an anoxic event), because - with continuing deposition - deeper burrows tend to overprint shallower ones (Fig. lOb). This tiering of the infaunal community is typically considered to be a response to partitioning of the infaunal realm into different niches occupied by organisms with different feeding strategies (Bottjer & Ausich 1982; Bromley 1990, 1996; Wetzel & Uchman 1998). The occupants of these different niches, which share similar feeding strategies, have been grouped into 'ichnoguilds' by Bromley (1990), though these rely to a large degree on interpretation of behaviour, which is notoriously difficult to determine with accuracy for most trace fossils. Diagrammatic representation of tiering may be done using either the tiering diagrams of Bromley (1996, p. 295, fig. 12.11) and Wetzel & Uchman (2001) or the more integrated ichnofabric constituent diagrams (ICD) of Taylor & Goldring (1993; see the modified ICD in Fig. 6). More importantly however, several authors appear to use 'complex tiering' for all visually complex ichnofabrics (Taylor & Goldring 1993; Taylor et al. 2003). Complex ichnological tiering is defined herein as being formed

in a sediment in which the trace-making organisms are vertically partitioned into tiers containing more than one trace fossil (see Fig. 9). Multiple occupancy of a given tier should be demonstrable by mutual cross-cutting of the tier's occupant traces (see below). Succession of bioturbation Understanding the order of emplacement of burrows and tracks is a fundamental skill that all modern ichnologists should incorporate into studies aimed at palaeoenvironmental analysis. The recognition of successive cross-cutting palaeocommunities (ichnocoenoses, see below) associated with the same sedimentary unit can lead to an improved understanding of the colonization history and/or changing palaeoenvironmental conditions subsequent to the deposition of a given bed (cf. Wetzel & Uchman 2001). One of the features of such cross-cutting relationships is that later burrows or tracks tend to obscure earlier burrows, and deeper burrows tend to overprint shallower ones with continuing sedimentation (see Figs 9, lOb). In many cases one or a few trace fossils (termed elite trace

Fig. 11. Styles of colonization of sedimentary surfaces: (a) transport of adult and colonization from above; (b) spatfall; (c) equilibration and colonization from below.

ICHNOLOGY & PALAEOENVIRONMENTAL ANALYSIS

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fossils by Bromley 1996) visually dominate an ichnofabric. The elite trace fossil is often produced by a late-stage bioturbator(s), or may be visually striking due to a prominent fill/burrow lining. Colonization styles The colonization of sedimentary surfaces may be achieved in a variety of ways, which can be basically summarized as being from: (a) a depositional surface; (b) an eroded surface; (c) beneath the sediment surface (usually of a migrating bedform); (d) a higher stratigraphic level (Fig. 11). Options (a) and (b) can be difficult to distinguish if the eroded sediment is unlithified and thus not a classical 'Glossifungites surface' (cf. MacEachern et al. 1992; Fig. 6). Option (c) represents the behaviour of a fauna well adapted to conditions with high sedimentation rates (cf. the mouth-bar facies of Mcllroy 2004). Colonization of sediments from a higher stratigraphic level is a common feature of turbidites (e.g. Kern 1980; Buatois & Mangano 2004), though colonization can be from below (e.g. Uchman 1995). The ecology of colonization of disturbed/ defaunated sediment has been a focus of marine ecological research for many years, and has many applications in understanding the palaeoecology of marine event beds. The colonization of marine hardgrounds is generally considered by ecologists to be largely by larval settling (Roughgarden et al. 1985). In soft marine sediments, however, colonization by larval stages is also supplemented by passive (currents) or active (locomotion) relocation of adults (Hall 1994; Snelgrove & Butman 1994; Cummings et al. 1995; Shull 1997). The potential contribution to the sediments ichnofabric of relocated adults is much greater than larval settling (spatfall; see Fig. 1 Ib), and is best developed in regions with high flow strengths where hydraulic transport of adults is common. In addition, spatfall may be seasonal in nature, in which case the colonization window (sensu Pollard et al. 1993) may be controlled by biological rather than physical phenomena. A sedimentological/ichnological description that encompasses all the points above should correspond to an excellent basis for further interpretation. The process of interpreting ichnofabrics and bioturbated sediments is an important step, which should involve careful appraisal of the succession studied. Caution should be exercised when assessing such detailed datasets, so that over-interpretation is avoided.

Fig. 12. Coarse-grained multi-storey tidal channel sandstones with rip-up clasts (upper arrow) with anomalously intense bioturbation (from arrowed surface) by Diplocraterion with a well-developed teardrop shape reflecting ontogenetic growth. From the Tilje Formation, Norwegian Shelf. Core is 10cm wide.

For example, it is well known that modern marine communities are highly patchy in distribution and not predictable relative to prevailing currents etc. This is demonstrated in the few studies of spatial distributions of ichnofaunas

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and ichnofabrics that have been performed thus far (Palmer & Palmer 1977; Goldring et al. 1998; Uchman 2001)

Ichnocoenoses The term 'ichnocoenose' was originally introduced by Davitashvili (1945) to mean the traces of a biological community or biocoenose (fide Radwanski & Roniewicz 1970); ichnocoenose in its original sense thus means the ichnological equivalent of standing crop. Terms for the buried and fossilized ichnocoenose - taphocoenose and orictocoenose respectively - were also introduced by Davitashvili (1945), but have fallen into disuse. The term 'ichnocoenosis' was subsequently independently introduced by Lessertisseur (1955) to encompass fossil assemblages of ichnotaxa equivalent to biocoenose, and was adopted in the latter sense by several authors (Hantzschel 1962; Radwanski & Roniewicz 1970). As noted by Pickerill (1992), communities can rarely, if ever, be recognized in the fossil record, owing to the effects of time averaging. Trace fossils - though study of their cross-cutting relationships (ichnofabric analysis) - can give a detailed impression of the work of several successive communities within a bed. It is useful, therefore, to retain ichnocoenosis as an approximation of its original meaning of the traces of a biological community. Ichnocoenoses should therefore ideally be considered as comprising a group of trace fossils that can be demonstrated - by ichnofabric analysis - to have been formed by the action of what approximates to a single benthic community or a succession of similar communities (based on Ekdale et al. 1984). The normal condition in the stratigraphic record is that beds are colonized by a succession of different communities, and that several superimposed ichnocoenoses can be recognized. Such time-averaged trace fossil fabrics are probably best known as assemblages. An ichnological assemblage is thus made up of all the trace fossils found within a given rock unit (usually a bed), regardless of their relative chronology, and may be composed of one or more ichnocoenoses. Ichnocoenoses and Seilacherian ichnofacies As highlighted by Bromley (1990, 1996), the use of ichnocoenosis as being synonymous with Seilacherian ichnofacies (Dorjes & Hertweck 1975; Frey & Pemberton 1987) is erroneous and problematic. Seilacherian ichnofacies were founded on the recognition of ichnological

assemblages, not ichnocoenoses, and it is important that the two are not confused. An element of ichnofabric analysis is needed to recognize the presence of the Arenicolites ichnofacies (Bromley & Asgaard 1991), which is commonly overprinted by the Cruziana or Skolithos ichnofacies (Fig. 11 a). A number of marine ichnofacies rely upon ichnofabric analysis to distinguish palimpsest fabrics representing different ichnofacies. Some authors, however, do not generally use these more subtle ichnofacies (e.g. Pemberton et al. 1992, 2001, who do not discuss the Arenicolites and Curvolithus ichnofacies). In addition, the use of the prefixes proximal/distal/depauperate is increasingly common (e.g. Pemberton et al. 2001; Bann & Fielding 2004; Buatois & Mangano 2004). It would seem therefore that although ichnofacies are useful as a starting point for ichnological studies - ichnologists working on shallow marine systems have outgrown ichnofacies, and indeed some ichnofacies actually hide important palaeoenvironmental information (cf. Frey & Goldring 1992). The future direction of ichnological work in the shallow marine and ultimately the non-marine is probably through the creation of bespoke ichnological models on a basin-by-basin scale, incorporating description of assemblages (cf. Fursich 1976 for discussion of assemblages) and ichnocoenoses (as resolved by ichnofabric analysis) rather than a reliance on the creation and application of ever more Seilacherian ichnofacies. The study of the non-marine is, however, still in its comparative infancy, and there is still much merit in using an ichnofacies-type approach (cf. Buatois & Mangano 2004; Genise 2004).

Ichnofabric stacking patterns as a correlative tool Having used some variety of the protocol outlined above to describe and understand the ichnology of the stratigraphic succession in question, the next challenge is to use these data in a meaningful manner. As described above, ichnocoenoses are the building block of applied ichnological studies, in both outcrop and core. In many cases beds may contain more than one ichnocoenosis - i.e. comprise an assemblage or association that makes up the sediment's ichnofabric. In core studies, ichnofabric is the most usable stratigraphic unit. In outcrop, however, where good vertical sections are not always available, associations/assemblages of interface traces and the more prominent of the pervasive traces should suffice; quantification of the ichnology is nonetheless still important.

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By integrating ichnological and sedimentological data, a sedimentologist should be able to produce a refined fades model, which may then form the basis for stacking pattern analysis at a variety of scales. Through application of Walther's law (Walther 1894), trends in relative sea-level within a given sedimentary package can be established, progradational trends being recorded by deeper water facies/ichnofabrics/ associations being overlain by shallower water facies/ichnofabrics/associations (as resolved in the integrated conceptual facies/ichnofabric model). Analysis of ichnofabric stacking patterns in such a way has been used to great effect to understand complex or difficult bioturbated successions (Bockelie 1991; Taylor & Gawthorpe 1993; Martin & Pollard 1996; Mcllroy 2004) at either the sequence or parasequence scale. The methodology is identical to that used by sedimentologists in routine stratigraphic studies, but incorporates both palaeontological and sedimentological data for improved facies characterization. The above approach may also be used at a coarser scale with ichnofacies as the building blocks. Departures from the expected succession of facies predicted by applying Walther's law (Walther 1894) to the conceptual facies model need to be explained by either autocyclic or allocyclic means. The surfaces thus identified are known as key stratigraphic surfaces and are discussed below.

Recognition of key stratigraphic surfaces The recognition of key stratigraphic surfaces lies at the heart of the sequence stratigraphic approach to understanding and predicting sedimentological phenomena. The classification of such surfaces has been gradually refined, partly through the recognition of the Glossifungites ichnofacies (e.g. MacEachern et al. 1992; Pemberton et al. 2004), but also through other styles of facies dislocation (Taylor & Gawthorpe 1993; Taylor & Goldring 1993; Goldring 1999; Schlirf 2003; Taylor et al. 2003). Noteworthy phenomena are generally changes to the normal ichnological patterns of a given sedimentological succession, such as: horizons with anomalously intense bioturbation (Fig. 13); horizons with anomalously low or high ichnodiversity (Fig. 13); horizons with anomalous ichnofauna, e.g. a horizon with marine trace fossils in an otherwise non-marine succession (Fig. 13);

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horizons that, by ichnofabric/ichnofacies analysis, can be demonstrated to be host to a succession of ichnofaunas recording gradual deepening or shallowing events (Fig. 13); horizons showing anomalously large burrows that evince growth to adult size while living at a single horizon, e.g. loop-shaped Diplocraterion (Fig. 12); horizons across which there is a dislocation of facies as evinced by ichnological and/or sedimentological analysis (Fig. 6). Hypothetical model examples of the ichnological expression of key stratigraphic surfaces are by no means intended to be exhaustive. Each depositional system has its own unique character, and models such as that of Taylor et al. (2003) should be used for guidance, but departures from such idealized cases are to be expected.

Ichnological frontiers Progress As outlined above, the field of ichnology has progressed apace, especially since the early 1970s when the seminal compilations of Crimes & Harper (1970, 1977) exemplify the state of rapid advancement of the subject and the burgeoning interest in its applicability. Since the early 1970s ichnology has become increasingly relevant to a variety of related disciplines, including zoology, ecology, archaeology, geochemistry, diagenesis, sedimentology, sequence stratigraphy, petroleum reservoir characterization and petroleum exploration. In recent years the ichnological understanding of non-marine depositional systems has improved enormously, largely through the work of South American ichnological research groups (e.g. Buatois & Mangano 1993, 2004; Genise et al. 2000, 2004). There is, however, still much to do by way of integrating this improved understanding with sedimentological and sequence stratigraphic work, as outlined by Buatois & Mangano (2004).

Ichnology and the petroleum industry The most applied aspect of ichnological work involves studies that are of relevance to the petroleum industry. The utility of trace fossils stems from the comparatively simple study of ichnofabrics in core, and the excellent palaeoenvironmental information that they hold (cf.

Fig. 13. Intercalation of marine and non-marine ichnofabrics from the Cloughton Formation, Jurassic, Yorkshire Coast, UK: (a) field photograph of the interval with marine flooding and bioturbation; (b) ichnofabrics associated with the marine flooding surface; (c) ichnofabric constituent diagram of the ichnofabric in (b) above.

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Chamberlain 1978 and many authors since) is thus invaluable to industry. The demands of the modern petroleum industry on accurate characterization of facies are highly exacting. With the increased use of reservoir modelling of sedimentary facies and highly detailed petrophysical studies of reservoir units, the need for high-quality interpretation of sedimentary facies is greater than ever. If facies are misidentified, the foundation stone of most other elements of the reservoir characterization process falls down, and nasty surprises may lie in store during field development. By using all information available to interpret sedimentary facies, such risks are minimised.

the sediment, which can be involved in diagenetic reactions that may reduce porosity and permeability after diagenesis (Worden & Morad 2003). Trace fossils may also connect sand layers separated by impermeable mudstones, thereby improving reservoir characteristics (Gingras et al. 1999; Pemberton et al. 2001, 2004) and systematically clean sandstones of clay (Frey & Wheatcroft 1989; Prosser et al 1993). Recent experimental ichnology has also demonstrated that clay mineral authigenesis can occur in the guts of organisms, making the clay mineral assemblage of sediments metastable, and thereby influencing the of creation of clay mineral cements upon burial (Mcllroy et al. 2003).

Facies characterization The use of ichnology to characterize sedimentary facies is well developed in shallow and marginal marine depositional systems (e.g. Bockelie 1991; Mcllroy 2004) but is much less so in the nonmarine (see the recent inroads made by Genise 2004; Buatois & Mangano 2004), where depositional systems are much more variable and prone to the variable effects of climate and the preservation potential of many trace fossils is comparatively low. Future work on non-marine ichnology should work towards incorporating animal and plant trace fossils (Bockelie 1994) in sedimentological and sequence stratigraphic models. In recent years much exploration effort has been directed toward deep water turbidite plays, e.g. Gulf of Mexico, west of Shetland, west of Africa. This trend has been reflected in the increased research into characterization of turbidite architectural elements. These data have not, however, been well integrated with ichnological studies despite there being many ichnologists specializing in the ichnology of deep marine facies. The potential for combination studies involving sedimentology and ichnology alongside data from provenance techniques, palynological techniques (biostratigraphic and palynofacies; see MacEachern et al. 1999) and geochemical techniques (especially in carbonates) is massively underused at present, and provides yet another rich source of potential information for petroleum geologists to use.

New technologies Of the burgeoning technologies being developed by the petroleum industry perhaps the most exciting for the ichnologist is that downhole imaging (FMI) is now just bordering on a resolution whereby ichnology may become useful (e.g. Bourke 1992; Salimullah & Stow 1995), and can only get better. Advantages include the potential for recovery of image data down the full length of the well, which means that the sedimentologist/ ichnologist need not work exclusively on cored intervals. The image data are challenging to interpret, but ichnologists have been working with difficult sections of trace fossils in core for many years, and should be adaptable enough to exploit this potentially rich data source.

Reservoir quality Parameters of interest to the petroleum geologist are the porosity and permeability of potential reservoir intervals. These two parameters are to a large extent controlled by sedimentological heterogeneity but also by diagenesis. One feature of bioturbated sandstones is that clay-grade material is commonly mixed into the matrix of

Experimental and neoichnological studies Experimental and neoichnological studies have a rich history, including the classical studies of modern sediments of both intertidal and subtidal settings using box-coring and serial sectioning or X-ray analysis (e.g. Reineck 1958; Howard & Reineck 1972). Such techniques need to be improved and more closely related to depositional events in modern settings to facilitate more informed interpretation of ancient environments. Facilities for visualizing such data, including tomography of serial sections (e.g. Fu et al. 1994; Sutton et al. 2001), X-ray and NMR imaging techniques, have improved. In addition, time-series X-rays of ichnofabrics in laboratory experiments has never been fully exploited and should be very revealing. In addition, ecological information concerning infauna of many modern environments is well established and ideal for incorporation into models of ancient depositional environments (e.g. Reed 2002 on deep marine traces; Mcllroy 2004 on tidal depositional environments).

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D. McILROY

A. Martinius, P. Manning and M. Carton are thanked for their reading of an early version of this manuscript. The critical reviews of A. Uchman and M. Schlirf are acknowledged with thanks. A. Taylor and S. Gowland are thanked for discussion prior to writing of this manuscript. Statoil asa and its employees past and present are also thanked for nurturing my involvement in the challenges of the modern petroleum geologist and for presenting me with interesting and pertinent challenges over the last six years or so.

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nology and Sedimentology of Shallow to Marginal Marine Systems. Geological Association of Canada Short Course Volume 15. PEMBERTON, G. S., MACEACHERN, J. A. & SAUNDERS, T. 2004. Stratigraphic applications of substratespecific ichnofacies: delineating discontinuities in the rock record. In: MC!LROY, D. (ed.) The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 29-62. PICKERILL, R. K. 1992. Carboniferous nonmarine invertebrate ichnocoenoses from southern New Brunswick, eastern Canada. Ichnos, 2, 21-35. PICKERILL, R. K. & NARBONNE, G. M. 1995. Composite and compound ichnotaxa: a case example from the Ordovician of Quebec, eastern Canada. Ichnos, 4, 53-71. PIKE, J., BERNARD, J. M., MORETON, S. G. & BUTLER, I. B. 2001. Microbioirrigation of marine sediments in dysoxic environments: implications for early sediment fabric formation and diagenetic processes. Geology, 29, 923-926. POLLARD, J. E., GOLDRING, R. & BUCK, S. G. 1993. Ichnofabrics containing Ophiomorpha: significance in shallow-water facies interpretations. Journal of the Geological Society, London, 150, 149-164. PROSSER, D. J., DAWS, J. A., FALLICK, A. E. & WILLIAMS, B. P. J. 1993. Geochemistry and diagenesis of stratabound calcite cement layers within the Rannoch Formation of the Brent Group, Murchinson Field, North Viking Graben (Northern North Sea). Sedimentary Geology, 87, 139-164. RADWANSKI, A. & RONIEWICZ, P. 1970. General remarks on the ichnocoenose concept. Bulletin de I'Academic Polonaise des Sciences, Serie des Sciences Geologiques et Geographiques, 18, 51-56. READING, H. G. (ed.) 1978. Sedimentary Environments and Facies. Blackwell, Oxford. REED, C. 2002. Lighting the mysteries of the abyss. Geotimes, 47, 24-25, REINECK, H. E. 1958. Kastengreifer und Lotrohre 'Schnepfe' Gerate zur Entnahme ungrestorter, orientierter Meeresgrundproben. Senckenbergiana Lethaea, 39, 42^8, 54-56. REINECK, H. E. 1963. Sedimentgefiige im Bereich der siidlichen Nordsee. Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft, 505, 1-107. REMANE, A. & SCHLIEPER, C. 1971. Biology of Brackish Water. Wiley, New York. RINDSBERG, A. K. & MARTIN, A. J. 2003. Arthrophycus in the Silurian of Alabama (USA) and the problem of compound trace fossils. Palaeogeography, Palaeoclimatology, Palaeoecology, 192, 187-219. ROUGHGARDEN, J., IWASA, Y. & BAXTER, C,

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SAGEMANN, B. B., WIGNALL, P. B. & KAUFFMANN, E. G. 1991. Biofacies models for oxygen deficient facies in epicontinental seas: a tool for palaeoenvironmental analysis. In: EINSELE, G., RICKEN, W. & SEILACHER, A. (eds) Cycles and Events in Stratigraphy. Springer, Berlin, 542-564. SALIMULLAH, A. R. M. & STOW, D. A. V. 1995. Ichnofacies recognition in turbidites/hemipelagites using enhanced FMS images: examples from ODP Leg 129. The Log Analyst, 36, 38-49. SARJEANT, W. A. S. 1979. Code for trace fossil nomenclature Palaeogeography, Palaeoclimatology, Palaeoecology, 28, 147-166. SARJEANT, W. A. S. & KENNEDY, W. J. 1973. Proposal for a code for the nomenclature of trace fossils. Canadian Journal of Earth Science, 10, 460-475. SAVRDA, C. E. 1991. Ichnology in sequence stratigraphic studies: an example from the Lower Paleocene of Alabama. Palaios, 6, 39-53. SAVRDA, C. E. & BOTTJER, D. J. 1987. The exaerobic zone, a new oxygen deficient marine biofacies. Nature, 327, 54-56. SAVRDA, C. E. & BOTTJER, D. J. 1991.Oxygen-related biofacies in maritime strata: an overview and update. In: TYSON, R. & PEARSON, T. H. (eds) Modern and Ancient Continental Shelf Anoxia. Geological Society, London, Special Publications, 58, 201-219. SCHIEBER, J. 2003. Simple gifts and buried treasures: implications of finding bioturbation and erosion surfaces in black shales. The Sedimentary Record, 1,4^8. SCHLIRF, M. 2003. Palaeoecologic significance of Late Jurassic trace fossils from the Boulonnais, N. France. Ada Geologica Polonica, 53, 123-142. SEILACHER, A. 1964. Biogenic sedimentary structures. In: IMBRIE, J. & NEWELL, N. (eds) Approaches to Paleoecology. Wiley, New York, 296-316. SEILACHER, A. 1967. Bathymetry of trace fossils. Marine Geology, 5, 413-428. SEILACHER, A. 1970. Cruziana stratigraphy of 'non fossiliferous' Palaeozoic sandstones. In: CRIMES, T. P. & HARPER, J. C. (eds) Trace Fossils. Geological Journal Special Issue, 3, 447-476. SEILACHER, A. 1974. Flysch trace fossils: evaluation of behavioural diversity in the deep-sea. Neues Jarbruchfur Geologic und Paldontologie Monatshefte, 4, 233-245 SEILACHER, A. 1985. Trilobite palaeobiology and substrate relationships. Transactions of the Royal Society of Edinburgh, 76, 231-237. SEILACHER, A. 1990. Aberrations in bivalve evolution related to photo- and chemosymbiosis. Historical Biology, 3, 289-311. SEILACHER, A. 1992. An updated Cruziana stratigraphy of Gondwanian Palaeozoic sandstones. In: SALEM, M. J. (ed.) The Geology of Libya. Elsevier, Amsterdam, 1565-1580. SEILACHER, A. 1993. Problems of correlation in the Nubian Sandstone facies. In: THORWEIHE, U. & SCHANDELMEIR, H. (eds) Geoscientific Research in Northwest Africa. Balkema, Rotterdam, 329-333. SEILACHER, A. 1994. How valid is Cruziana stratigraphy? Geologische Rundschau, 83, 752-758.

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VAIL, P. R., MITCHUM, R. M. et al. 1977. Seismic stratigraphy and global changes of sealevel. In: Pay ton, C. (ed.) Seismic stratigraphy: Applications to Hydrocarbon Exploration. American Association of Petroleum Geologists, Memoirs, 26, 49-212. VOSSLER, S. M. & PEMBERTON, S. G. 1988. Skolithos in the Upper Cretaceous Cardium Formation: an ichnological example of opportunistic ecology. Lethaia, 21, 351-362. WALTHER, J. 1894. Einleitung in die Geologic als Historische Wissenschaft, Bd. 3. Lithogenesis der Gegenwart. G. Fischer, Jena, 535-1055. WETZEL, A. 1991. Ecologic interpretation of deep-sea trace fossil communities. Palaeogeography, Palaeoclimatology, Palaeoecology, 85, 47-69. WETZEL, A. & UCHMAN, A. 1998. Deep-sea benthic food content recorded by ichnofabrics: a conceptual model based on observations from Paleogene flysch, Carpathians, Poland. Palaios, 13, 533-546. WETZEL, A. & UCHMAN, A. 2001. Sequential colonization of muddy turbidites in the Eocene Beloveza Formation, Carpathians, Poland. Palaeogeography, Palaeoclimatology, Palaeoecology, 168, 171186. WIGNALL, P. B. 1993. Distinguishing between oxygen and substrate control in fossil benthic assemblages. Journal of the Geological Society, London, 150, 193-196. WIGNALL, P. B. 1994. Black Shales. Clarendon Press, Oxford. WIGNALL, P. B. & MYERS, K. 1998. Interpreting benthic oxygenation levels in mudrocks: a new approach. Geology, 16, 452-455. WIGNALL, P. B. & NEWTON, R. 2001. Black shales on the basin margin: a model based on examples from the Upper Jurassic of the Boulonnais, northern France. Sedimentary Geology, 144, 335356. WIGNALL, P. B. & PICKERING, K. T. 1993. Palaeoecology and sedimentology across a Jurassic fault scarp, NE Scotland. Journal of the Geological Society, London, 150, 323-340. WILKINS, R., BARNES, H. & BRANTLEY, S. 1996. The size distribution of framboidal pyrite in modern sediments: an indicator of redox conditions. Geochimica et Cosmochimica Acta, 60, 38973912. WORDEN, R. H. & MORAD, S. 2003. Clay minerals in sandstones: controls on formation distribution and evolution. In: WORDEN, R. H. & MORAD, S. (eds) Clay Mineral Cements in Sandstones. International Association of Sedimentologists, Special Publications, 34, 3-41.

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Stratigraphic applications of substrate-specific ichnofacies: delineating discontinuities in the rock record S. GEORGE PEMBERTON1, JAMES A. MAcEACHERN2 & TOM SAUNDERS1 1

Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada, T6G 2E3 2 Department of Earth Sciences, Simon Eraser University, Burnaby, British Columbia, Canada, V5A 1S6 Abstract: Trace fossils represent both sedimentological and palaeontological entities, providing a unique blending of potential environmental indicators in the rock record. Trace fossils and trace fossil suites can be employed effectively to aid in the recognition of various discontinuity types and to assist in their genetic interpretation. Ichnology may be employed to resolve surfaces of Stratigraphic significance in two main ways: (1) through the identification of discontinuities using substrate-controlled ichnofacies (the firmground Glossifungites ichnofacies, the hardground Trypanites ichnofacies and the woodground Teredolites ichnofacies); and (2) through careful analysis of trace fossils in vertical (softground) successions (analogous to facies successions). Integrating the data derived from substrate-controlled ichnofacies (so-called omission suites) with palaeoecological data from vertically and laterally juxtaposed softground ichnological successions greatly enhances the recognition and interpretation of a wide variety of stratigraphically significant surfaces. When this is coupled with conventional sedimentary facies analysis and sequence stratigraphy, a powerful approach to the interpretation of the rock record is generated.

Trace fossil assemblages can be employed effectively to aid in the recognition of various discontinuity types, as well as to assist in their genetic interpretations. Ichnology can be utilized to resolve surfaces that may have Stratigraphic significance in two main ways: (1) through the identification of discontinuities using substratecontrolled ichnofacies; and (2) through careful analysis of trace fossils in vertical (softground) successions (accomplished by using either ichnofacies or ichnofabric analysis). Though ichnological analysis is a valuable tool, it continues to remain highly under-utilized in most sedimentologically driven genetic stratigraphic studies. Integrating the data derived from substrate-controlled, omission-related ichnofacies with palaeoecological data from vertical ichnological successions greatly enhances the ability to recognize and interpret a wide variety of Stratigraphic surface types. When this is coupled with conventional facies analysis and sequence stratigraphy, a powerful approach to the interpretation of the rock record is generated. Trace fossils have proven to be one of the most important groups of fossils in assisting in the delineation of stratigraphically important boundaries related to genetic stratigraphy (e.g. MacEachern et al 1991, 1992, 1998, 1999b; Savrda 199la, 1991b, 1995; Pemberton et al 1992a, 2001; Taylor & Gawthorpe 1993; Pemberton & MacEachern 1995; Ghibaudo et al 1996;

MacEachern & Burton 2000; Savrda et al 2001; Taylor et al 2003) and event stratigraphy (Seilacher 1962, 1982; Vossler & Pemberton 1988; Frey & Goldring 1992; Pemberton & MacEachern 1997). Genetic stratigraphy lies at the core of three main Stratigraphic paradigms: genetic Stratigraphic sequences (Galloway 1989a, 1989b), allostratigraphy (Walker & James 1992), and sequence stratigraphy (Van Wagoner et al 1990). The recognition of Stratigraphic breaks is essential in any genetic Stratigraphic paradigm, but also is commonly a difficult task, particularly in subsurface analysis. Discontinuities may reflect processes that are external to the depositional system (allocyclic), which may initiate or terminate deposition of sedimentologically related facies successions (Walker 1990). Interpreting the origin of the discontinuity, essential to sequence stratigraphy and to genetic Stratigraphic sequences, is vital in resolving the depositional environments of associated deposits and in determining the allocyclic controls on the depositional systems. To accomplish this requires the integration of ichnofacies relationships (Pemberton et al 2001) or ichnofabrics analysis (Taylor et al 2003; Mcllroy 2004), physical sedimentology and sequence Stratigraphic techniques. Ichnofacies and reconstructed ichnocoenoses are part of the total aspect of the rock, imparted by the depositional environment, and therefore - like lithofacies -

From: MclLROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 29-62. 0305-8719/04/$ 15.00 © The Geological Society of London.

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are subject to Walther's law. For example, isolated bored shells or clasts do not, in themselves, constitute the Trypanites ichnofacies. Rather, there must be some semblance of stratification, lateral continuity and/or vertical succession before an ichnofacies can be applied. This paper expands upon and updates the work already published in Pemberton et al. (2001).

Substrate-controlled ichnofacies and the recognition of stratigraphic discontinuities: the use of omission suites One of the most important factors in the distribution of organisms in modern environments is substrate type. In their recent review Taylor et al. (2003) summarized the different substrate types (Fig. 1) as: soupground (water-saturated mudrocks and micrites); softgrounds (muddy and micritic sediment with some dewatering); loosegrounds (sandy sediments where permanent burrows require stabilized margins); stiffground (stabilized sediment where burrows are unlined); firmground (firm dewatered, often compacted sediment); hardgrounds (lithified substrate surface; variations can be shellground, a cemented shell bed; and rockground, with tectonic omission). To this list could also be added woodgrounds (xylic substrates) and the closely related loggrounds (substrate composed of distinct logs) (Bromley et al 1984; Savrda et al 1993). Three substrate-controlled ichnofacies have been

established (Ekdale et al 1984): Glossifungites (firmground), Trypanites (hardground) and Teredolites (woodground). In clastic settings, most of these trace assemblages are associated with erosionally exhumed (dewatered and compacted or cemented) substrates, and hence correspond to erosional discontinuities. Depositional breaks, in particular condensed sections, may also be semi-lithified or lithified, presumably at their upper contacts (or downlap surfaces), and may be colonized without associated erosion. In general, however, the recognition of substrate-controlled ichnofacies may be regarded as being equivalent to the recognition of discontinuities in the stratigraphic record. Determining whether these discontinuities are autocyclically generated or allocyclically generated, and hence stratigraphically important, is considerably more problematic. Although certain insect and vertebrate burrows in the terrestrial realm may be properly regarded as firmground in character (e.g. Voorhies 1975; Fursich & Mayr 1981; Smith 1987; Groenewald et al 2001) or, more rarely, hardground suites, they have a low preservation potential and are relatively minor in the geologic record. The overwhelming majority of these assemblages originate in marine or marginal marine settings. A discontinuity may be generated in either subaerial or submarine settings, but colonization of the discontinuity is most likely to occur in association with marine influence, particularly in pre-Tertiary intervals. This circumstance has important implications for the interpretation of the discontinuity's genesis. Substrate-controlled ichnocoenoses typically cross-cut the pre-existing softground suites, and hence reflect conditions that post-date both the initial deposition of the unit and the erosion of

Fig. 1. Relationship of substrate type and the distribution of the named ichnofacies.

KEY STRATIGRAPHIC SURFACES

that unit. These omission suites actually correspond to the period of time between the erosional event (which exhumes the substrate) and final burial of the discontinuity beyond reach of the benthic community of the overlying unit. During such a hiatus infaunal organisms are free to colonize the substrates: the specific

31

characteristics of the ichnofauna are thus determined by the nature of the substrate and the palaeoenvironmental conditions prevailing during the hiatus. By observing (a) the underlying and cross-cut softground trace fossil suite (contemporaneous with deposition of the underlying unit), (b) the omission suite and ichnofacies

Fig. 2. Schematic development of a Glossifungites demarcated erosional discontinuity based on the Jurassic Arab D interval in Saudi Arabia. 1: The muddy carbonate substrate is deposited and buried. 2: A transgressive surface of erosion is generated by a sea-level rise. This exposes previously deposited dewatered sediment to the sediment-water interface, where it is burrowed by firmground organisms. The burrows are filled with grainstone that is deposited on the surface as a ravinement deposit. 3: After several rises in sea-level a complex framework results that is characterized by mappable surfaces that are characterized by a Glossifungites assemblage.

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associated with the exhumed substrate and (c) the ichnocoenosis of the overlying unit, it is possible to make some interpretation regarding the origin of the discontinuity and the allocyclic or autocyclic mechanisms responsible (Fig. 2).

The Trypanites ichnofacies The Trypanites ichnofacies (Fig. 3) develops in fully lithified substrates such as hardgrounds, reefs, rocky coasts, 'beach rock' and other omission surfaces (Pemberton et al. 1980; Gruszczyhski 1986, 1998). Development of this ichnofacies therefore also corresponds to discontinuities that have major sequence stratigraphic significance. Bromley & Asgaard (1993) subdivided the Trypanites ichnofacies into two ichnofacies: the Entobia ichnofacies for rocky shorelines (see also De Gibert et al. 1998), and the Gnathichnus ichnofacies for bored shells and boulders found further offshore. Bored shells and boulders, however, are not substrates that can be correlated, because it is difficult to ascertain when the boring activity was initiated. The traces in the Trypanites ichnofacies are characterized by: cylindrical to vase, tear- or U-shaped to irregular domiciles of suspension feeders or passive carnivores; raspings and gnawings of algal grazers and similar organisms (mainly chitons, limpets and echinoids);

moderately low diversity, although the borings and scrapings of individual ichnogenera may be abundant; borings oriented perpendicular to the substrate that may include large numbers of overhangs. In contrast to the Glossifungites ichnofacies, the walls of the borings cut through hard portions of the substrate rather than skirting around them.

The Teredolites ichnofacies The Teredolites ichnofacies (Fig. 4) consists of a characteristic assemblage of borings or burrows in woody or highly carbonaceous substrates. Woodgrounds differ from lithic substrates in three main ways: They may be flexible instead of rigid; They are composed of carbonaceous material instead of mineral matter; They are readily biodegradable (Bromley et al. 1984). Such differences dictate that the means by which, as well as the reasons for which, these two types of substrate are penetrated are also different. As currents can raft woody substrates, it is important to determine whether the bored substrates are autochthonous or allochthonous. Only the autochthonous forms are true members of the Teredolites ichnofacies. These assemblages may also be important in defining sequence and

Fig. 3. Trace fossil association characteristic of the Trypanites ichnofacies.

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Fig. 4. Trace fossil association characteristic of the Teredolites ichnofacies.

parasequence boundaries (Savrda 199la). The use of bored logs to define bounding surfaces, as in the concept of log-grounds (Savrda et al. 1993) should be avoided, because it is very difficult to ascertain when the logs were bored. Such logs are clasts and should not be treated in the same manner as the Teredolites ichnofacies sensu strieto. The Teredolites ichnofacies is characterized by: sparse to profuse, club-shaped borings; boring walls that are generally ornamented with the texture of the host substrate (i.e. tree ring impressions and other xyloglyphs); stumpy to elongate subcylindrical excavations in marine or marginal marine settings; shallower, sparse to profuse non-clavate excavations (isopod borings) in freshwater settings.

The Glossifungites ichnofacies The Glossifungites ichnofacies is environmentally wide ranging, but develops only in firm, unlithified substrates such as dewatered muds or highly compacted sands (Fig. 5). Dewatering results from burial, and the substrates are made available to trace-makers if exhumed by later erosion (e.g. Pemberton & Frey 1985).

Exhumation can occur in terrestrial environments, as a result of channel meandering or valley incision; and in shallow-water environments, as a result of meandering tidal channels, tidal scour erosion, erosive shoreface retreat associated with wave ravinement, or as a result of submarine channels cutting through previously deposited sediments. Such exhumed surfaces commonly correspond to stratigraphic discontinuities, and the specific characteristics of their colonization are critical to their sequence stratigraphic interpretation (Saunders & Pemberton 1986; MacEachern et al. 1992, 1998, 1999a, 1999b; Pemberton et al. 1992b, 2001; Pemberton & MacEachern 1995; MacEachern & Burton 2000; Gingras and Pemberton 2000; Taylor et al. 2003; Mcllroy 2004). The Glossifungites ichnofacies (Fig. 6) is characterized by: vertical, cylindrical, U- or tear-shaped, commonly scratch-marked burrows and sparsely to densely branching dwelling burrows; protrusive spreite in some burrows that develop mostly through animal growth (funnel-shaped Rhizocorallium and Diplocraterion [formerlyGlossifungites})', animals that leave the burrow to feed (e.g. crabs) as well as suspension feeders; low diversity, but commonly high abundance.

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Fig. 5. The Glossifungites ichnofacies is environmentally wide ranging, but develops only in firm, unlithified substrates. (A) Develops in dewatered exhumed muds, Albian Viking Formation, Willesden Green Field, Alberta, 10-35-40-7W5, 2327m or (B) highly compacted sands. (B) Develops in compacted sandstone where an inclined Thalassinoides (T) crosscuts a lined Ophiomorpha (O) in the Price River A core. (C) Gallup Sandstone, San Juan Basin, New Mexico with multiple incision surfaces seen in the top metre of the unit.

Firmground traces are dominated by vertical to subvertical dwelling structures of suspensionfeeding organisms (Fig. 7). The most common structures correspond to the ichnogenera Diplocraterion, Skolithos, Psilonichnus, Arenicolites and unnamed flask-shaped domichnia comparable to Gastrochaenolites (Fig. 8). Dwelling structures of deposit-feeding organisms are also constituents of the ichnofacies, particularly where exhumed substrates occupied more sheltered or distal settings during colonization, and include firmground Thalassinoides, Spongeliomorpha and Rhizocorallium. More recently,

MacEachern & Burton (2000) and Savrda et al (2001) have shown omission-related firmground Zoophycos associated with discontinuities colonized in very distal settings of the shelf and slope. Caution should be exercised in distinguishing firmground assemblages from stiffground assemblages (Wetzel & Uchman 1998) that can be localized and may be related to deep tiers. In proximal, high-energy settings, the assemblages attributable to the Glossifungites ichnofacies are dominated by vertical domichnia. The presence of vertical shafts within shaly intervals is anomalous, as these structures are not capable of being maintained in soft muddy substrates. Glossifungites ichnofacies elements are typically robust, commonly penetrating 20-100 cm below the bed junction. Many shafts tend to be large in diameter (e.g. 0.5-1.Ocm), in particular Diplocraterion habichi and Arenicolites. This scale of burrowing contrasts markedly with the predominantly horizontal and diminutive ichnogenera that typify the exhumed shaly intervals. The firmground traces are generally sharp-walled and unlined, reflecting the stable, cohesive nature of the substrate at the time of colonization and burrow excavation. Further evidence of substrate resilience, atypical of soft muddy beds, is the passive nature of most burrow fills. This demonstrates that the structure remained open after the trace-maker vacated the domicile, thus allowing material from subsequent depositional events to infiltrate the open tube. The post-depositional origin of the Glossifungites ichnofacies is clearly demonstrated by the ubiquitous cross-cutting relationships with the previous softground assemblage. The final characteristic of the suite is the tendency to reflect colonization in large numbers (Fig. 7). In numerous examples, 7-15 firmground traces, most commonly Diplocraterion habichi, have been observed on the bedding plane of 9 cm (3.5 in) diameter cores, corresponding to a density of between 1100 and 2300 shafts per m2. Similar populations have been observed from the modern coast of Germany (Schafer 1972), the modern Georgia coast (Basan & Frey 1977; Morris & Rollins 1977; Pemberton & Frey 1985), and Willapa Bay (Gingras et al. 1999). Selected case studies of ichnological applications to sequence stratigraphy

Regressive surfaces of erosion (RSE) and sequence boundaries (SB) Although subaerial exposure and/or erosion during relative sea-level lowstand may produce

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Fig. 6. Trace fossil association characteristic of the Glossifungites ichnofacies.

Fig. 7. Characteristics of trace fossils that are associated with the Glossifungites ichnofacies: (A) the burrows tend to be robust, unlined domiciles; (B) are found in very high densities; (C) commonly display scratch marks; and (D) cross-cut the original softground trace fossil assemblage.

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Fig. 8. Glossifungites assemblages: (A) Skolithos from the sequence boundary at the base of an interpreted forced regression, Kaybob Field, Viking Formation, 11-35-61-20W5, depth 1759.1m; (B) Glossifungites suite of conglomerate-filled Thalassinoides, Cardium Formation, Pembina Field, 12-9-51-10W5, depth 1596.2m; (C) firmground Arenicolites marking a transgressive surface of erosion (TSE), Cretaceous Viking Formation, 07-19-62-19W5, 1652m; (D) Thalassinoides at surface in the Lower Cretaceous Dun vegan Formation, Jayar Field, 6-11-62-3W6, 2523.6m; (E) Diplocraterion at transgressive surface in the Upper Cretaceous Horeseshoe Canyon Formation in outcrops near East Coulee, Alberta; (F) Glossifungites ichnofacies consisting of Rhizocorallium excavated into offshore shales and cross-cutting a resident softground suite of Helminthopsis, Planolites, Schaubcylindrichnus, Chondrites and Zoophycos. Albian Viking Formation, Willesden Green Field, Alberta, 10-35-40-7W5, 2327m.

KEY STRATIGRAPHIC SURFACES

widespread development of dewatered, firm or cemented substrates (corresponding to regressive surfaces of erosion and sequence boundaries), most are unlikely to become colonized by substrate-controlled trace fossil suites unless the surfaces are subsequently exposed to marine or marginal marine conditions prior to burial. In most cases, deposition of significant thicknesses of non-marine strata generally precludes development of these omission suites on the RSE or SB themselves. There are a number of scenarios, however, where such discontinuities may be preferentially colonized by trace-makers of substrate-controlled assemblages. Such settings include RSE developed beneath forced regressive shorefaces, SB underlying lowstand shorefaces, SB comprising submarine canyon margins, and SB lying at the estuarine mouths of incised valleys, prior to transgressive infill. All of these settings are conducive to colonization of the discontinuity because the surfaces were excavated subaqueously in a marine or marginal marine environment. The marginal marine component of the sequence boundary within an incised valley is aerially restricted and difficult to discern from the transgressively modified sequence boundary during initial transgression. For that reason, this latter scenario is discussed in the context of amalgamated sequence boundaries and flooding surfaces (FS/SB) of incised valleys. Incised submarine canyons There are few published ichnological assessments of an ancient submarine canyon margin. Outcrops of the lower Miocene Nihotupu and Tirikohua formations in Northland, New Zealand, contain a noteworthy firmground trace fossil assemblage of the Glossifungites ichnofacies related to submarine canyon incision (Hayward 1976). The underlying Nihotupu Formation consists of volcanogenically derived siltstones, sandstones and subaqueous mass flow conglomerates, together with submarine andesite pillow-pile complexes. The underlying softground assemblage is sparse and sporadically distributed, characterized by localized individual occurrences of Thalassinoides, Planolites and Scalarituba. These deposits are interpreted as turbidites that were emplaced at bathyal water depths (based on body fossil content) within an inter-arc basin on the lower eastern flanks of the west Northland volcanic arc. The contact with the overlying Tirikohua Formation is sharp and erosional and exhibits visible relief. The exhumed substrate is demarcated by a firmground omission assemblage consisting of Skolithos, Rhizocorallium and ?Thalassinoides attributable to the Glossifungites ichnofacies.

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Hayward (1976) interpreted the erosional discontinuity as a submarine canyon wall, excavated into bathyal to neritic inter-arc sediment gravity flow deposits as a result of basin margin tectonic uplift. Colonization of the canyon walls by the firmground trace-makers preceded the gradual burial of the canyon margins by neritic turbidite deposits of the Tirikohua Formation. The infill of the submarine canyon probably corresponds to late stage relative sea-level lowstand and early transgression. In the subsurface, examples of submarine canyon incision with the development of trace fossil assemblages of the Glossifungites ichnofacies have been recognized in the Miocene of the Nile Delta (Fig. 9). The canyon walls were excavated during lowstand incision and colonized by shrimps that constructed robust Thalassinoides. The interpretation of the surface is critical to correct correlation of the canyon fill and to the recognition of point source turbidites. Fine-grained facies outside the canyons are totally bioturbated by Phycosiphon, Planolites and Helminthoida. Similar facies within the canyon system reflect episodic mud turbidites and remain virtually unburrowed. Forced regressive and lowstand incised shorefaces Forced regressive and lowstand shorefaces constitute two sequence stratigraphic scenarios by which sharp-based shoreline sandstones may form, and both are associated with falling limbs of relative sea-level. Sharp-based shoreface sand bodies, however, have been variably assigned to the progradation of late highstand successions (e.g. Van Wagoner 1995), forced regressive (falling stage) systems (e.g. Hunt & Tucker 1992; Walker & Bergman 1993; Bergman 1994; Davies & Walker 1993), lowstand systems (e.g. Flint et al 1988; Posamentier et al 1992; Posamentier & Chamberlain 1993; Mellere & Steel 1995; Walker & Wiseman 1995), and transgressively incised complexes (e.g. Downing & Walker 1988; Raychaudhuri et al 1992; MacEachern et al 1998, 1999b). Despite the wide range of sequence stratigraphic contexts that facilitate such deposits, many workers continue to regard sharp-based shoreface sandstone bodies to be exclusively of falling stage or lowstand origin. From a facies perspective, however, the sharpbased shoreface successions generated in all three systems tracts are virtually identical. Their principal difference lies in the character of the basal contact with the underlying facies. One distinction, however, is that highstand examples overlie autocyclic basal surfaces that lack evidence of incision into and concomitant truncation of

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Fig. 9. Glossifungites assemblage associated with submarine canyon incision, West Ahken Field, Nile Delta: (A) Skolithos filled with anomalous sediment, West Ahken-1 core, 4379.8ft; (B) sequence boundary at base of submarine canyon fill characterized by a Glossifungites assemblage with Thalassinoides, West Ahken-1 core, 4375.3ft. regional markers in the underlying succession. Such sharp-based but non-incised highstand shoreface deposits also display a genetic affinity of facies across the autocyclic basal surface that commonly corresponds to the erosional bases of individual storm beds. In contrast, forced regressive, lowstand and transgressive shorefaces overlie allocyclic discontinuities that truncate regional markers, reflecting incision into underlying units. Incised-shoreface deposits may correspond to forced regressive, lowstand or transgressively incised systems (Fig. 10). The forced-regressive shoreface overlies a regressive surface of erosion (RSE) cut by wave action during the falling stage

of relative sea-level. Likewise, the lowstand shoreline overlies the marine part of the sequence boundary (SB) and is also cut by wave erosion. The transgressively incised shoreface, however, overlies a wave ravinement surface, cut during relative sea-level rise. The wave ravinement surface commonly amalgamates with or truncates the earlier sequence boundary (FS/SB). This succession reflects a period of shoreline progradation during overall transgression, when sediment supply outpaced relative rise of sea-level. All three scenarios favour the development of substrate-controlled omission suites on the basal discontinuities, as each is excavated within a marine setting, and permit early

Fig. 10. Differentiation of forced-regressive, lowstand, and transgressively incised shoreface complexes. Sharpbased, discontinuity-bound (incised) shoreface successions can be ascribed to one of three main sequence stratigraphic settings. Model 1 reflects forced regression (falling stage), showing the initial fall of relative sea-level and the development of successive shorefaces sitting on regressive surfaces of erosion (RSE). Note that although a correlative conformity (CC) may be produced seaward of each RSE, successive sea-level fall makes these susceptible to erosional removal, and therefore they have a low preservation potential. Model 2 shows the development of the lowstand shoreface, which reflects the most seaward position of the shoreline associated with the lowest position of sea-level. Note that the erosional component of the sequence boundary extends only as far seaward as fair-weather wavebase (FWWB), where it passes into a correlative conformity (CC). In model 3, rise of relative sea-level generates a low-energy flooding surface in basinal positions that passes landward into a transgressive ravinement surface, as it floods across the lowstand and forced-regressive shorefaces. Where the surface cuts across or incises through the old sequence boundary, it produces an FS/SB. Note that the rise of sea-level drowns and preserves the CC of the lowstand shoreface. During a pause in transgression, shoreface progradation occurs, producing a transgressively incised shoreface. In basinward positions, offshore mudstones deposited below fair-weather wavebase may directly overlie the erosional component of the FS/SB because the surface was cut while sea-level occupied a lower position but deposition did not occur until after significant deepening. (Modified from MacEachern et al. 1998)

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colonization of the exhumed substrate. Differentiation between these incised complexes is difficult, but can be achieved through careful documentation of the erosional extent of the basal discontinuity (MacEachern et al. 1999b). Considerable discussion surrounds the validity of differentiating lowstand from forced regressive deposits (e.g. Hunt & Tucker 1992, 1995; Kolla et al. 1995). The work of Helland-Hansen & Gjelberg (1994) and Mellere & Steel (1995), however, illustrates the utility of discriminating falling-stage systems tracts associated with forced regression from the final lowstand shoreline corresponding to maximum fall of sea-level, but prior to transgression. A forced-regressive or falling-stage origin has been proposed for sharp-based sandstone bodies of the Viking Formation in the Garrington Field (Davies & Walker 1993) and Kaybob Field (Pemberton & MacEachern 1995). Posamentier et al. (1992) and Posamentier & Chamberlain (1993) interpreted sharp-based sandstone deposits at Joarcam to reflect a lowstand shoreface deposit. Lowstand shoreface deposits have also been interpreted in the Lindbrook and Beaverhill Lake fields (Walker & Wiseman 1995), although their figure 6 suggests that the 'Lindbrook a' deposit probably reflects a falling-stage shoreface, given that the 'Lindbrook b' shoreface lies farther basinward and likewise overlies a regressive surface of erosion. The Judy Creek Field, which lies along strike to these deposits, contains a lowstand incised shoreface deposit as well. The differentiation between sharp-based, incised shorefaces and deltas of forced regressive versus lowstand origin, however, is problematic. Both forced regressive and lowstand shoreface deposits tend to be fairly thin, in response to the diminished accommodation space associated with relative lowstand of sea-level (Flint 1988; Posamentier et al. 1992; Van Wagoner 1995). Walker & Wiseman (1995) have further suggested that the damping of wave energy across broad shallow platforms lying outboard of these shorefaces contributes to this, because it inhibits incision into the underlying firmly compacted mud. As a result, facies tracts within these shoreface types may be attenuated or even absent. Lowstand shorefaces may be slightly thicker because they may be developed during late lowstand, where a slow rise in relative sea-level may be initiated with associated increased accommodation space. Sandbody widths and width/thickness ratios have also received some consideration as a means of discriminating between these systems, but because of the effects of such controls as variations in

sediment supply, rate of change of accommodation space, basin gradient and duration of shoreline progradation, caution must clearly be exercised. The perceived regional stratigraphic context of the sharp-based shoreface and/or delta deposits has principally been used as a basis for their interpretation. Ainsworth & Pattison (1994) have discussed the problem of attached versus detached lowstand complexes, highlighting some of the difficulties in differentiating between the two scenarios. Falling-stage (forced regressive) interpretations are most commonly based on the presence of additional incised shoreface and/or delta deposits lying basinward of them within the same sequence. Lowstand deposits, on the other hand, tend to be identified mainly on the basis of the absence of additional basinward shorefaces. Given that such deposits may be detached and lying considerably basinward, the sequence stratigraphic interpretation of these intervals may, in some cases, be highly suspect. Walker & Wiseman (1995) concede this uncertainty with respect to the Viking Formation Lindbrook shorefaces of Alberta. MacEachern et al. (1999b) have argued, however, that there are distinctive facies relationships that can also be employed in order to differentiate the various sequence stratigraphic scenarios. Forced-regressive shoreface and/or deltaic deposits overlie regressive surfaces of erosion (RSE). These surfaces are cut in submarine conditions and pass basinward into conformable surfaces analogous to correlative conformities (Fig. 10). The RSE are cut by wave erosion as relative sea-level falls, bringing more basinal facies into the zone of wave attack. Continued sea-level fall results in the subaerial exposure of the falling-stage shorefaces, and their subsequent cannibalization by later regressive surfaces of erosion and, ultimately, the sequence boundary. The preservation potential of these deposits is considerably less than that of the lowstand shoreface, and the correlative conformities of the RSE are, in particular, unlikely to be preserved in the rock record. Kolla et al. (1995) regard the RSE that bound falling-stage deposits merely as higher-order sequence boundaries reflecting incremental rather than continuous fall of relative sea-level. Lowstand shorefaces directly overlie sequence boundaries and, basinward, their correlative conformities (Plint et al. 1988; Posamentier et al. 1992). Landward of the shoreface, the sequence boundaries are cut by subaerial erosion. At the base of the shoreface, however, sequence boundaries are cut within a marine setting and therefore favour colonization by

KEY STRATIGRAPHIC SURFACES

substrate-controlled assemblages, particularly where they incise into dewatered offshore muds. As the lowstand shoreface lies in the most seaward position prior to ensuing sea-level rise, the marine expression of the sequence boundary and the correlative conformity have a high preservation potential, facilitating their differentiation from forced regressive and transgressively incised counterparts (MacEachern et al. 1999b). In weakly storm-influenced settings, the erosional component of the RSE and sequence boundaries is unlikely to persist basinward of fair-weather wavebase and therefore defines a sharp base to the lower shoreface. Basinward of this position, finer-grained offshore deposits overlie the correlative conformity and during progradation grade upwards into lower shoreface muddy sandstones. As a result, the Glossifungites ichnofacies and other omission suites are unlikely to occur in positions below fairweather wavebase. In these basinal positions, coarse-grained lag deposits are likely to be absent as well. The correlative conformity may, however, represent a sharp but depositional facies contact, marked by an abrupt change in proximality of facies, grain size and trace-fossil assemblage. In storm-dominated shorefaces, however, the extent of the allocyclically generated marine expression of the RSE or sequence boundary may be masked by autocyclic storm erosion surfaces and appear to extend to stormweather wavebase. This scenario results in the development of a series of vertically stacked and offlapping, aerially restricted autocyclic surfaces, rather than a single allocyclic surface, but recognition of this condition may be problematic unless outcrop exposure is exceptional. In the subsurface, recognition of this situation would be extremely difficult. However, these autocyclic surfaces are rapidly buried by tempestites and are therefore not readily colonized by trace-makers of substrate-controlled ichnofacies. Both forced regressive and lowstand complexes are typically sharp based in proximal positions and gradationally based in basinal positions. Consequently, only the lower shoreface, middle shoreface and upper shoreface deposits directly overlie the erosional expression of the sequence boundary or the RSE and may be demarcated by the Glossifungites ichnofacies. Landward, a ravinement surface may become amalgamated with the sequence boundary and RSE during the ensuing transgression (e.g. Flint et al. 1986; Flint 1988; Pemberton & MacEachern 1995). It appears reasonable that the correlative conformity of the RSE has an exceedingly low preservation potential in falling-stage deposits, in contrast to that of lowstand-

41

shoreface deposits. The forced-regressive deposits are subjected to subsequent erosion and subaerial exposure during continued fall of relative sea-level, as well as the potential of transgressive ravinement during ensuing rise of relative sealevel (Fig. 10). The lowstand deposits, on the other hand, are produced at the lowest position of relative sea-level fall and are unlikely to be subsequently ravined, because water depths are deepened and fair-weather wavebase shifted landward during later transgression. The presence of a correlative conformity might be taken as a significant support for a lowstand interpretation of a deposit. The subsurface Viking Formation of the Kaybob and Judy Creek fields highlights the similarities between a forced regressive shoreface succession and a lowstand one. The Kaybob Field of central Alberta contains a sharp-based, incised shoreface excavated into underlying open marine distal parasequences (Pemberton & MacEachern 1995; MacEachern & Pemberton 1997; Fig. 11). The underlying distal parasequences consist of moderately to abundantly bioturbated (BI4-5) mudstones, silty mudstones and sandy siltstones, displaying diverse trace fossil assemblages attributable to the Zoophycos ichnofacies and distal and archetypal expressions of the Cruziana ichnofacies, respectively. The parasequences reflect progradation of shelf to upper offshore cycles of an underlying highstand systems tract. The erosional discontinuity is principally demarcated by firmground omission suites of Skolithos, Diplocraterion and Thalassinoides of the Glossifungites ichnofacies (Fig. 11C, D). Where the discontinuity is excavated into sandier expressions of the underlying facies, the surface is demarcated by a palimpsest softground suite of Diplocraterion habichi and Skolithos (Fig. 11 A, B). In all cored intervals where the discontinuity is preserved, it is erosional and is overlain by silty to muddy sandstones, interpreted to reflect lower shoreface and middle shoreface deposits (Fig. 11B, D). The sandstones are moderately and sporadically bioturbated (BI 2-3), characterized by abundant, remnant hummocky cross-stratified beds separated by burrowed beds. The succession shows an upward decrease in the number and thickness of burrowed beds and a concomitant increase in the number and thickness of hummocky crossstratified beds, consistent with upward shallowing (Fig. 11 A). A fully marine, diverse, proximal expression of the Cruziana ichnofacies dominates the more thoroughly burrowed successions and passes upwards into suites consistent with the Skolithos ichnofacies. Locally, trough crossbedded sandstones containing sporadically

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Fig. 11. Forced regressive shoreface. (A) Box shot of core from the Kaybob incised shoreface; base of the interval is to the lower left, and top to the upper right (T). The lower unit consists of bioturbated (BI5—6) silty and sandy mudstones, and (in column 4) muddy sandstones of the underlying regional Viking Formation, reflecting progradation of lower offshore, upper offshore and lower shoreface environments respectively. These are truncated by a regressive surface of erosion (RSE), and overlain by coarser-grained lower and middle shoreface laminated to bioturbated sandstones of a moderately storm-influenced shoreface. Well 11-35-61-20W5; 1757.6-1762.1 m. (B) Close-up photo of the RSE in photo A, showing a palimpsest softground suite consisting of Skolithos (S) demarcating the stratigraphic discontinuity. The incised shoreface in this locality has cut into bioturbated (BI 5) muddy sandstones with softground Ophiomorpha (O), and Palaeophycus (Pa), reflecting a proximal expression of the Cruziana ichnofacies. The overlying lower shoreface sandstones of the forced regressive shoreface contain sideritized mudstone rip-up clasts and Palaeophycus (Pa). Well 11-35-61-20W5, 1759.2m. (C) Box shot of core from the Kaybob incised shoreface in a position more distal than that of photo A. Base of the interval is to the lower left, and top to the upper right (T). The lower unit consists of bioturbated (BI 5-6) silty and sandy mudstones of the underlying regional Viking Formation, reflecting progradation of lower to upper offshore environments. These mudstones are truncated by an RSE, and overlain by lower shoreface muddy sandstones, passing into middle shoreface sandstones. Well 10-15-6219W5; 1667.3-1673.0m. (D) Close-up photo of the RSE in photo C, showing nrmground Thalassinoides (Th) of the Glossifungites ichnofacies demarcating the discontinuity. The discontinuity in this position is incised into bioturbated (BI 6) sandy mudstones containing the archetypal Cruziana ichnofacies with Phycosiphon (Ph) and Chondrites (Ch). The overlying lower shoreface muddy sandstone contains a proximal expression of the Cruziana ichnofacies with Diplocraterion (D) and Palaeophycus (Pa). Well 10-15-62-19W5, 1671.8m.

KEY STRATIGRAPHIC SURFACES

distributed assemblages of the Skolithos ichnofacies may directly overlie the sequence boundary or grade upward from the middle shoreface sandstones, and are interpreted as upper shoreface deposits. The upper contact of the succession is truncated and, locally, cemented with haematite-stained siderite, interpreted to reflect subaerial exposure. In basinward positions, where one might expect to find the offshore sandy mudstones and siltstones equivalent to the Kaybob lower shoreface sandstones, the interval has been removed by later cycles of incision. The basal discontinuity is not preserved seaward of fairweather wavebase, making sequence stratigraphic interpretation of the surface and therefore of the overlying deposit problematic. The succession is consistent with an incised shoreface, but whether forced regressive, lowstand or transgressively incised cannot be unequivocally demonstrated on the basis of the deposits themselves. The presence of additional shorefaces lying basinward of the Kaybob deposit strongly suggests that the deposit cannot reflect the lowstand shoreface of the sequence (Fig. 10). Furthermore, preferential removal of the discontinuity in a seaward position would be difficult to accomplish during transgression, and appears inconsistent with observed relationships in transgressively incised examples (e.g. Downing & Walker 1988; Raychaudhuri et al. 1992; MacEachern et al 1998, 1999b). Additionally, the iron-stained, siderite cemented sandstone at the upper truncated margin of the interval is consistent with continued relative sea-level fall, exposure and subaerial erosion of a forced regressive shoreface. As such, the basal discontinuity is interpreted to reflect a regressive surface of erosion (RSE). By comparison, further basinward of the Kaybob forced regressive shoreface lies an incised shoreface sandstone in the Judy Creek Field. In contrast to the more storm-influenced successions of the Kaybob deposit (MacEachern & Pemberton 1992), the Judy Creek deposit is weakly storm affected and is characterized by thoroughly bioturbated (BI5), pebble- and granule-bearing sandy mudstones, muddy sandstones and silty sandstones of the upper offshore, lower shoreface and middle shoreface respectively (Fig. 12). Bioturbation within these facies is generally uniformly distributed. All facies contain highly diverse trace fossil assemblages. The upper offshore sandy mudstones display suites consistent with the archetypal Cruziana ichnofacies. The lower shoreface muddy sandstones contain trace fossil assemblages corresponding to proximal expressions of the Cruziana ichnofacies.

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The middle shoreface silty sandstones, however, contain trace fossils recording a distal expression of the Skolithos ichnofacies. The Judy Creek incised shoreface deposits clean and coarsen upward, and erosionally overlie the distal counterparts of the same shelf to offshore parasequences truncated by the landward-lying Kaybob forced regressive shoreface. The erosional discontinuity is locally demarcated by well-developed firmground Thalassinoides and less commonly by Spongeliomorpha, attributable to the Glossifungites ichnofacies. In all cored intervals containing an erosional expression of the discontinuity, the overlying facies comprise bioturbated muddy sandstones or silty sandstones, interpreted to reflect deposition above fair-weather wavebase (Fig. 12A, B). In the few instances, however, where the Judy Creek deposit is initiated by upper offshore sandy mudstones, the bounding surface shows no evidence of erosional truncation of the underlying parasequences (Fig. 12C, D). This is significant, as it records the character of the bounding surface in positions below fair-weather wavebase. This non-erosional surface shows an increase in grain size associated with progradation of the Judy Creek incised shoreface, but lying seaward of erosional modification by waves. The surface in this position is interpreted as the preserved correlative conformity of the discontinuity lying landward beneath the incised shoreface sensu stricto (Fig. 12D). In even more distal positions the correlative conformity is difficult to identify, and the succession appears to reflect simple progradation. The correlative conformity survived in this position because, after this cycle of progradation, relative sealevel rose again, thereby flooding the area and causing erosional back-stepping of the shoreline. This is consistent with a lowstand shoreface interpretation, and indicates that the underlying erosional discontinuity corresponds to the sequence boundary, rather than the RSE of a forced regressive shoreface (Fig. 10).

Transgressive surfaces Transgressive surfaces are manifest by (1) mainly non-erosional marine flooding surfaces (MFS) and bay margin flooding surfaces, and (2) lowrelief, erosional (ravinement) surfaces. The ravinement surfaces may be produced by either wave or tidal scour processes, and are referred to as transgressive surfaces of erosion (TSE). Analysis of transgressive surfaces of erosion has had a relatively long history since Stamp (1921) originally defined the term 'ravinement'. A

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Fig. 12. Lowstand shoreface. (A) Box shot of core from the Viking Formation incised shoreface at Judy Creek. Base of the interval is to the lower left, and top to the upper right (T). In this proximal position, bioturbated silty mudstones of the regional Viking parasequences, interpreted as lower offshore deposits, are incised into by moderately to intensely bioturbated (BI4—5) silty sandstones of the Judy Creek lowstand shoreface. The discontinuity is interpreted as a sequence boundary (SB). Well 02/10-19-63-11W5; 1423.4-1427.1 m. (B) Box shot of core from the Viking Formation incised shoreface at Judy Creek, slightly distal of that in photo A. Base of the interval is to the lower left, and top to the upper right (T). The interval shows storm-influenced lower offshore silty mudstones with rare tempestites (lower offshore) incised into by bioturbated (BI 5) muddy sandstones of the Judy Creek lowstand shoreface. The discontinuity (SB) corresponds to the same sequence boundary in photo A. The Judy Creek shoreface sandstones in this position are muddier than those of photo A, and reflect lower shoreface deposition. Well 12-26-63-11W5; 1453.3-1457.6m. (C) Box shot of core from the Viking Formation incised shoreface at Judy Creek, distal of the position indicated in photo B. Base of the interval is to the lower right (B), and top to the upper left (T). The succession shows shelf through upper offshore mudstones at the base, sharply overlain by coarser-grained upper offshore sandy mudstones and muddy sandstones of the Judy Creek lowstand shoreface. The sharp contact corresponds to the correlative conformity (CC) of the sequence boundary present in photos A and B. Well 10-35-64-13W5; 1482.3-1486.7 m. (D) Close-up of the correlative conformity (CC) in photo C, separating a finer-grained sandy mudstone facies below from a pebble bearing (Pe), coarser-grained sandy mudstone above. Both facies contain an open-marine, archetypal Cruziana assemblage with Helminthopsis (H), Terebellina (T), Planolites (P), Phycosiphon (Ph), and Asterosoma (As), interpreted to reflect upper-offshore environments. Well 10-35-64-13W5; 1483.0m. number of landmark papers have discussed the characteristics and implications of ravinement surfaces, particularly with respect to their processes of formation, their depths of incision, the interplay of rate of relative sea-level rise with

pre-existing topography and shoreface depth on the preservation potential of coastal-plain deposits, surface diachroneity and associated facies (e.g. Fischer 1961; Swift 1968; Belknap & Kraft 1981; Pilkey et al 1981; Nummendal & Swift

KEY STRATIGRAPHIC SURFACES

1987). MacEachern et al. (1992) discussed the ichnological suites associated with ravinement surfaces and their associated facies. Marine flooding surfaces Marine flooding surfaces (MFS) are typically abrupt contacts across which there is evidence of an increase in water depth. These surfaces are mantled with dispersed sand, granules or intraformationally derived rip-up clasts, indicating some erosion. The preservation of underlying markers indicates, however, that the degree of erosion is minimal. MFS are typically characterized by the abrupt juxtaposition of offshore, shelf or prodelta shales onto shallow marine sandstones, and are easily identified on geophysical well logs. Such surfaces may demarcate parasequence boundaries, parasequence sets or even systems tracts, depending upon their regional extent (Bhattacharya 1993). The Lower Cretaceous Viking Formation in western Canada contains numerous MFS separating coarsening-upward, regionally extensive parasequences. These parasequences are interpreted to reflect shelf through distal lower shoreface progradation under fully marine conditions. Three facies comprise a complete coarsening cycle, although the minor cycles rarely comprise a complete cycle. The basal facies consists of intensely bioturbated (BI5) silty mudstone. Trace fossils are uniformly distributed and diverse (eight ichnogenera), constituting the archetypal Zoophycos ichnofacies to a distal expression of the Cruziana ichnofacies. Bioturbated sandy mudstone facies grade upward from the silty mudstones and are intensely burrowed (BI5) with a uniformly distributed and highly diverse suite (18-21 ichnogenera) of the archetypal Cruziana ichnofacies. Muddy sandstone facies grade upward from the sandy shale facies and are intensely bioturbated (BI5) with a diverse (18 ichnogenera) and uniformly distributed, proximal expression of the Cruziana ichnofacies. The cycles reflect coarsening upward of facies associated with shoaling, under fully marine conditions, developed during a highstand systems tract. The marine flooding surfaces (MFS) in the major cycles are commonly marked by the return to lower offshore or shelf deposition, and are typically abrupt (Fig. 13A). These flooding surfaces are rarely significantly disrupted by the diminutive trace-makers that characterize the lower offshore and shelf settings. In other cases, cycles may show considerable biogenic modification of the MFS or transgressive surface of erosion (TSE), particularly where lower shoreface deposits are overlain by upper offshore

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sandy mudstones (Fig. 13B). Such contacts may appear gradational, owing to the biogenic homogenization of the surface by the more robust and penetrative trace-makers common in these settings. Elsewhere, the upward transition from shallow to deeper water deposits may occur over intervals of several decimetres or more, reflecting very gradual relative sea-level rise. Similar stacking of such coarsening upward, but thoroughly bioturbated, parasequences separated by pronounced marine flooding surfaces occurs in the Early Permian of the Sydney Basin (Bann 1998; Bann et al. 2004), the Jurassic Heather Formation of the Norwegian North Sea (MacEachern & L0seth 2003), and the Turonian Cardium Formation of Alberta (Vossler & Pemberton 1988, 1989). Transgressive surfaces of erosion Transgressive surfaces of erosion (i.e. ravinement surfaces) afford the most elegant manner of generating widespread substrate-controlled trace fossil assemblages and palimpsest softground suites, because the exhumed surfaces are both widespread and produced within a marine or marginal marine environment. This favours colonization by organisms as the ravinement surface is excavated prior to accumulation of significant thicknesses of overlying sediment (MacEachern et al. 1992a,b; Pemberton & MacEachern 1995). The upper portion of the Albian Viking Formation in the subsurface of central Alberta contains numerous transgressive surfaces of erosion (TSE), recording a complex history of transgression, which culminated in maximum flooding of the Western Interior Seaway. Bann (1998) and Bann et al. (2004) have assessed the ichnological characteristics of TSE from the Early Permian Pebbley Beach Formation of the Sydney Basin and found comparable characteristics to those of the Mesozoic successions of the Western Interior Seaway. The recognition of discrete TSE is difficult on the basis of sedimentology alone, particularly when dealing with the upper Viking Formation, where there exist abundant, sharp-based pebble stringers and thin, trough cross-stratified, coarse-grained sandstones, intercalated with interbedded sandstones, siltstones and shales. A few of these coarse stringers could reflect the veneer on transgressive ravinement surfaces, but owing to their abundance it is difficult to pick those that have regional stratigraphic significance. Similar complexities have been encountered in the Early Permian Pebbly Beach Formation of the Sydney Basin, Australia (Bann 1998), the Middle Jurassic Oseberg Formation (Soegaard & MacEachern 2003),

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Fig. 13. Marine flooding surface/transgressive surface of erosion. (A) Non-erosional marine flooding surface (MFS) separating upper offshore sandy mudstones below from lower offshore/shelf silty mudstones above. The sandy mudstones contain the archetypal Cruziana ichnofacies, with Palaeophycus (Pa), Diplocraterion (D), Asterosoma (As), Planolites (P), Phycosiphon (Ph), Teichichnus (Te) and Rosselia (Ro). The overlying silty mudstones contain a distal expression of Cruziana ichnofacies and show Phycosiphon (Ph) and Teichichnus (Te) Well 12-17-39-27W4; 1604.6m. (B) Bioturbated transgressive contact with palimpsest softground Thalassinoides (Th), subtending into muddy sandstones of the lower shoreface, abruptly overlain by gritty, pebble (Pe) bearing sandy mudstones and silty mudstones with thin tempestites. The contact probably reflects a TSE with only minor erosion. The underlying muddy sandstones contain a proximal suite of the Cruziana ichnofacies with Skolithos (S), Asterosoma (As), Planolites (P), and Phycosiphon (Ph). The overlying mudstones display the

KEY STRATIGRAPHIC SURFACES

and the Tarbert and Heather formations (MacEachern & L0seth 2003) of the Norwegian North Sea. However, in each of these units, virtually every TSE incised into, or ravined across, shaly sediments exhibits an omission suite attributable to the Glossifungites ichnofacies (Fig. 13D, E F) or palimpsest softground suites of the Skolithos ichnofacies (Fig.ISC). Many firmgrounds also appear to have been developed on sideritecemented intervals within the shales (Fig. 13D). Whether the siderite is formed during ravinement, as a chemical response related to deep substrate penetration by the firmground tracemakers, or whether pre-existing, siderite-cemented bands formed resistant layers through which the TSE could not incise, is uncertain. In the latter scenario, however, soft-bodied fauna would have to have been capable of penetrating a highly compacted or cemented layer. Elsewhere, the TSE have been developed on sandy substrates and are marked by palimpsest softground omission suites (Fig. 13C), typically dominated by Diplocraterion habichi and Skolithos (e.g. Middle Jurassic Heather Formation, Norwegian North Sea, MacEachern & L0seth 2003; Early Permian Pebbley Beach Formation, Australia, Bann et al 2004). In a few exceptional cases, TSE excavated across coal layers (Fig. 14) are demarcated by Teredolites longissimus, Diplocraterion parallelum and, more rarely, Diplocraterion habichi, attributable to the Teredolites ichnofacies (e.g. Campanian Horseshoe Canyon-Bearpaw transition, Drumheller Alberta, Saunders & Pemberton 1986; Lower Jurassic Neil Klinter Formation, East Greenland, Dam 1990; and Lower Palaeocene Clayton Formation, Alabama, Savrda 1991b). The firmground omission suites are predominantly manifest by Diplocraterion (typically D.

47

habichi), Skolithos, Arenicolites, Rhizocorallium and Thalassinoides (Fig. 13D, E), attributable to the Glossifungites ichnofacies. In some Viking Formation TSE, firmground Zoophycos with associated Thalassinoides and Rhizocorallium have also been identified (Fig. 13F), where initial colonization of the discontinuity occurred in more distal or sheltered settings during continued sea-level rise (MacEachern & Burton 2000). Savrda (2001) has also identified Zoophycos as part of an omission suite in the shelf and slope deposits of New Jersey. The proximal omission assemblages record predominantly suspensionfeeding behaviours associated with the period of higher energy prevalent during active ravinement and/or the energy conditions at the substrate during substrate colonization (Fig. 13C, D, E). Colonization of these exhumed surfaces post-dates erosive shoreface retreat but presumably occurs prior to significant deepening. These higher-energy (proximal) TSE are commonly overlain by conglomeratic lags. Transgressive surfaces of erosion that are not colonized until after deepening record higher proportions of domichnia of deposit-feeding organisms and are typically overlain by marine pebbly and sandy shales or muddy sandstones. In the most distal settings, the omission suite may consist entirely of firmground domichnia and feeding structures of deposit-feeding organisms (Fig. 13F; MacEachern & Burton 2000). Transgressively incised shorefaces Several Viking Formation oil and gas fields in central Alberta produce hydrocarbons from NW-SE trending, sharp-based sandstones, interpreted to rest upon transgressive surfaces of erosion incised into underlying facies. These include Chigwell (Raychaudhuri et al. 1992),

archetypal Cruziana ichnofacies with Chondrites (Ch), Helminthopsis (H), Phycosiphon (Ph), and Diplocraterion (D). Well 11-24-65-18W5; 1358.0m. (C) A palimpsest softground of Diplocraterion (D), subtending from a regionally extensive TSE excavated landward of the Joffre Embayment Complex. The palimpsest suite cross-cuts remnant lower shoreface sandstones of the regional Viking Fm parasequences. The underlying sandstones contain a proximal expression of the Cruziana ichnofacies, with Helminthopsis (H), Siphonichnus (Si), and Zoophycos (Z). Well 16-34-38-25W4; 1433.9m. (D) A proximal expression of a regionally extensive TSE in the Viking Formation with a pebble lag passively infilling firmground Diplocraterion (D) and Skolithos of the Glossifungites ichnofacies. The omission suite penetrates siderite cemented silty mudstones with visible Palaeophycus (Pa) and Chondrites (Ch). Well 12-31-40-02w5; 1860.8m. (E) A proximal TSE overlain by a pebble lag that passively infills firmground Diplocraterion (D). The omission suite cross-cuts lower offshore silty mudstones with stacked tempestites, containing abundant Phycosiphon (Ph), Helminthopsis (H), and Chondrites (Ch), comprising a distal expression of the Cruziana ichnofacies. Well 09-15-39-27W4; 1549.4m. (F) A distal TSE from the Viking Formation of the Hamilton Field. The underlying mudstones correspond to shelf deposits and contain the Zoophycos ichnofacies. The TSE is demarcated by a firmground omission suite, consisting of Thalassinoides and Zoophycos of the Glossifungites ichnofacies. The overlying sandy mudstones are pebble bearing (Pe) and show the archetypal Cruziana ichnofacies containing Zoophycos (Z), Thalassinoides (Th), Planolites (P), Helminthopsis (H), and Palaeophycus (Pa). Well 06-13-35-09W4; 904.6m.

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afkldjfhjsdkfhjksdfhdkajsfhksdjfhakdjlfhlaskd asdfasdfasdfadsfadfadfadfadfadfadsfadfadsfadsf

Fig. 14. Summary diagram showing multiple sites of compacted mud/peat exhumation in a transgressive barrier-island setting. These include: back-barrier tidal creeks (1) and channels (2), tidal inlets (3), and the open shoreface (4). In all of these settings, developments of the Glossifungites and Teredolites ichnofacies can intermingle in response to localized change's in xylic properties. As shown, the erosional resistance of compacted peat can exert an overriding control on the depths of both channel erosion and shoreface ravinement. Trace fossil assemblages may therefore reflect complex histories repeated burial and re-exhumation.

Joffre (Downing & Walker 1988; MacEachern et al 1998, 1999b), Gilby (Raddysh 1988) and Giroux Lake (Stelck et al 2000). These successions can be regarded as high-energy parasequences bounded by ravinement surfaces. Although transgressively incised shorefaces tend to display thicker successions than do falling-stage systems, reflecting the increased accommodation space available, the 'transgressive' interpretation has rested mainly with the perceived position of the deposits in the regional stratigrapnic framework rather than with any intrinsic characteristics of the succession itself. For example, Posamentier et al. (1992) and Posamentier & Chamberlain (1993) interpreted the Joarcam deposit as a lowstand shoreface. In

contrast, Walker & Wiseman (1995) reinterpreted it as a transgressive shoreface, primarily on the basis of the observation of underlying and basinward shoreface deposits at Lindbrook that they regarded as lowstand in origin. Furthermore, despite the Lindbrook deposits being given a lowstand interpretation, Walker & Wiseman (1995) indicated that should an additional shoreface deposit within their sequence 1 be discovered farther to the northeast, the 'incision at Lindbrook a would then represent a transgressive incision formed during movement of the shoreline to the southwest' (Walker & Wiseman 1995, p. 136). It has, however, been suggested by MacEachern et al. (1999b) that this uncertainty

KEY STRATIGRAPHIC SURFACES

could be alleviated through evaluation of the erosional extent of the underlying discontinuity. Transgressive ravinement causes an erosional discontinuity that ultimately lies seaward of fair-weather wavebase during subsequent periodic progradation. This is because the modified surface was cut prior to shoreface progradation, while sea-level lay at a stratigraphically lower position (Fig. 10). Consequently, in the transgressive scenario, lower offshore and upper offshore deposits, reflecting deposition below fair-weather wavebase, can overlie the erosional component of the basal discontinuity. This is a situation that cannot be accommodated by either a forced regressive or a lowstand scenario (MacEachern et al. 1999a), and is diagnostic of transgressively incised systems. In fact, Walker & Wiseman (1995) noted that an erosional surface always underlies the offshore transition mudstone in such settings, though the implications of that observation were not explored further. As transgressive surfaces of erosion are commonly colonized by firmground omission suites, widespread firmground assemblages attributable to the Glossifungites ichnofacies can be generated, directly overlain by thin gravel lags and basinal facies reflecting offshore and shelf deposition. This facies relationship stands in marked contrast to either forced-regression or lowstand systems where basinal facies overlie the nonerosional correlative conformity and lack demarcation by the Glossifungites ichnofacies. Ultimately, the ravinement surfaces of transgressively incised shorefaces pass seaward into nonerosional marine flooding surfaces (Fig. 10). The Viking Joffre Shoreface Complex (Sequence 2) of the Gilby-JofTre trend (MacEachern et al 1998, 1999b) contains a sharpbased transgressively incised shoreface, excavated into underlying stacked marine parasequences. The incision surface (interpreted as an FS/SB) slopes steeply seaward along its landward edge and flattens out basinward, forming an asymmetric, one-sided erosional scarp. Granules and small pebbles of chert locally mantle the erosional discontinuity. More commonly, the surface is demarcated by firmground Thalassinoides, Diplocraterion and Skolithos of the Glossifungites ichnofacies, in both proximal and distal positions (Fig. 15). The FS/SB is overlain by a coarsening-upward (shallowingupward) succession of gritty sandy shales and muddy sandstones containing a fully marine, diverse and uniformly distributed trace fossil assemblage that corresponds to an archetypal to proximal expression of the Cruziana ichnofacies, respectively. These facies reflect the

49

short-lived progradation of upper offshore and lower shoreface environments of a transgressively incised, weakly storm-influenced shoreface complex. In proximal positions (Fig. 15A, B), the transgressively incised shoreface is virtually indistinguishable from either the forced regressive (Fig. 10) or lowstand incised shorefaces (Fig. 10); the basal discontinuity shows firmground suites directly overlain by lower shoreface sandstones. The transgressive origin of an incised shoreface's basal discontinuity is demonstrated, instead, in distal positions. Distally, firmground omission suites of the .Glossifungites ichnofacies demarcate the discontinuity, indicating that it continues to be erosional, even where it is overlain by deposits that accumulated below fair-weather wavebase (Fig. 15 C, D). This indicates that the surface was cut while sea-level was lower and the area within the zone of wave attack and was colonized during transgressive deepening. Ultimate burial of the discontinuity occurred either during a pause in the rate of transgression, or when sediment supply to the shoreline outpaced deepening and a period of shoreline progradation ensued. In distal positions, the transgressively ravined discontinuity was buried beneath offshore sandy mudstone prior to shoreface deposition (Fig. 10), as is diagnostic of a transgressively incised origin. A comparable succession was described from the Viking Formation of the Chigwell Field by Raychaudhuri et al (1992). The Turonian Cardium Formation of the Pembina field, central Alberta, also contains a series of conglomeratic bodies associated with underlying transgressive surfaces of erosion, interpreted as transgressively incised shorefaces (cf. Vossler & Pemberton 1989; Walker & Eyles 1991). Below this erosion surface, lower offshore silty shales are abundantly bioturbated (BI5) and contain a diverse ichnological assemblage corresponding to a distal expression of the Cruziana ichnofacies. The erosional discontinuity is incised into these silty shales and is marked by robust, pebble-filled firmground Thalassinoides (Fig. 16) and rare Skolithos of the Glossifungites ichnofacies. The conglomerates are largely devoid of bioturbation but pass upward into overlying marine shelf shales that contain trace fossil suites attributable to the Zoophycos ichnofacies. The erosional discontinuity corresponds to the E5 surface of Plint et al. (1986), interpreted as a surface of initial transgression (Plint 1988; Plint et al. 1988), upon which the conglomerates rest. Firmground colonization of the E5 surface corresponds to a hiatus in deposition between the initial transgressive generation of E5 and shoreface progradation of the conglomerates

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S. G. PEMBERTON ET AL.

Fig. 15. Transgressively incised shoreface. (A) Box shot of core from the Joffre Shoreface Complex. Base of the interval is to the lower left, and top to the upper right (T). The FS/SB here lies in a proximal position. Silty and sandy mudstones of the regional Viking Formation, reflecting lower offshore and upper offshore conditions respectively, are erosionally truncated by the basal discontinuity, Overlying the FS/SB are conglomeratic sandstones of the transgressive lag, passing into bioturbated muddy sandstones of the lower shoreface. Well 09-16-39-27W4; 1560.8-1565.1 m. (B) Close-up photo of the FS/SB from photo A, demarcated by firmground Thalassinoides (Th) of the Glossifungites ichnofacies. The discontinuity is overlain by a pebble (Pe) lag and muddy sandstones of the lower shoreface containing a proximal expression of the Cruziana ichnofacies. Skolithos (S), Planolites (P), Asterosoma (As), Palaeophycus (Pa), and Siphonichnus (Si) are visible. Well 09-1639-27W4; 1562.5m. (C) Box shot of core from the Joffre Shoreface Complex, in a position distal to that of photos A and B. Base of the interval is to the lower left, and top to the upper right (T). Here, lower and upper offshore mudstones of the regional Viking Formation are erosionally truncated by the FS/SB, but overlain by upper offshore sandy mudstones of the Joffre Shoreface Complex. Well 08-14-38-25W4, 1431.6-1434.5 m. (D) Close-up photo of the FS/SB of photo C, showing the distal expression of the FS/SB. Here, the omission suite demarcating the discontinuity also consists of firmground Thalassinoides (Th) of the Glossifungites ichnofacies. However, it is overlain by bioturbated (BI5) gritty, pebble (Pe) bearing sandy mudstones of the upper offshore, with Phycosiphon (Ph), Chondrites (Ch), Asterosoma (As), Cylindrichnus (Cy), Palaeophycus (Pa), and Planolites (P) corresponding to the archetypal Cruziana ichnofacies. Well 08-14-38-25W4, 1434.0m.

KEY STRATIGRAPHIC SURFACES

51

such surfaces may also include the discontinuities at the bases of some transgressively incised shorefaces (e.g. E-T surfaces of Flint 1988; Flint et al 1986, 1988; FS/SB of MacEachern et al. 1992, 1998, 1999a, 1999b), the majority are associated with incised valley systems. Incised valley discontinuities may correspond to subaerially exposed areas, such as delta plains, fluvial floodplains, interfluves, or transgressively modified sequence boundaries within the estuarine valley fills themselves. Some reserve the term TS/SB' for discontinuities developed on valley interfluves, where deposition did not occur until the valley was filled and the area transgressively ravined during ensuing relative sea-level rise (e.g. Van Wagoner et al. 1990).

Fig. 16. Transgressively incised Cardium shoreface. (A) The FS/SB marking the base of the Cardium Formation conglomeratic shoreface of the Pembina field. Moderately bioturbated (BI4-5) sandy mudstones of the upper offshore, containing Chondrites (Ch) and Helminthopsis (H), are erosionally truncated by overlying pebble conglomerates. The discontinuity is demarcated by gravel-filled firmground Thalassinoides (Th) of the Glossifungites ichnofacies. Well 04-13-51-11W5, 1636.1 m. (B) The same discontinuity in a nearby core, showing lower offshore silty mudstones truncated by pebble conglomerates of the Cardium Formation incised shoreface. The firmground omission suite also consists of gravel-filled Thalassinoides (Th) of the Glossifungites ichnofacies. Well 12-09-51-10W5, 1596.8m.

during a pause in the rate of transgression. The conglomeratic shoreface was ultimately drowned (MFS), and locally removed (TSE), during resumed transgression. Amalgamated sequence boundaries and marine flooding surfaces Amalgamated sequence boundaries and transgressive surfaces are commonly colonized by substrate-controlled trace-makers. The lowstand erosion event typically produces widespread firmground, hardground and woodground surfaces, corresponding to RSE or SB. The following transgressive event, commonly accompanied by erosion, generates a TSE that tends to remove most or all of the lowstand deposits and exposes the original discontinuity to marine or marginal marine conditions. During this phase of transgression, organisms are able to colonize the re-exhumed substrate. Although

TSE across subaerially exposed surfaces or interfluves Numerous units in coastal margin (delta plain and coastal plain) settings display initially subaerial surfaces subsequently flooded and eroded during transgressive influx of brackish to marine waters. Such scenarios are conducive to the development of substrate-controlled ichnofacies demarcating the discontinuities. The Cenomanian Dunvegan Formation consists of a stacked succession of prograding delta lobes that varied during its history of deposition from river-dominated to wave-dominated in character (Bhattacharya & Walker 1991). The stacked delta lobes and individual shingles within the lobes are separated by marine flooding surfaces of varying scales and which are locally erosional (Bhattacharya 1993). In the subsurface of the Jayar Field, central Alberta, a TSE, overlain by a transgressive sandstone, cuts across rooted and subaerially exposed delta plain deposits of the underlying delta lobe (Bhattacharya & Walker 1991). The erosional discontinuity is demarcated by a firmground Thalassinoides of the Glossifungites ichnofacies, passively filled with coarse-grained sand derived from the overlying transgressive sand sheet. Similarly, the Lower Albian Mannville GroupJoli Fou Formation contact in the Kaybob Field of central Alberta is manifest by a regionally developed FS/SB. In this case, rooted incipient palaeosols developed on floodplain mudstones of the Mannville Group are crosscut by robust, firmground Thalassinoides, reflecting the Glossifungites ichnofacies (Fig. 17A), passively filled with muddy sand and large siderite-cemented clasts. The overlying silty shales contain a distal expression of the Cruziana ichnofacies and, more rarely, the Zoophycos

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Fig. 17. FS/SB Interfluve. (A) Glossifungites ichnofacies-demarcated FS/SB at the Mannville Group-Joli Fou Formation contact, reflecting an interfluve area. Rooted incipient palaeosols are colonized with firmground Thalassinoides (Th), and capped by a transgressive lag. Overlying facies reflect offshore to shelf deposition. Well 11-03-60-19W5; 1894.3m. (B) Rooted (r), incipient palaeosols of the Upper Boulder Creek Formation, corresponding to floodplain conditions, are transgressively eroded and overlain by a thin transgressive lag and brackishwater bay mudstones of the Paddy Formation. The FS/SB of the interfluve is demarcated by firmground Thalassinoides (Th) of the Glossifungites ichnofacies. The overlying brackish-water mudstones contain an impoverished Cruziana ichnofacies, with visible Planolites (P) and exceedingly rare Helminthopsis (H). Well 08-13-69-11W6; 1907.2m.

ichnofacies, recording deposition in lower offshore to outer shelf environments. The amalgamated surface corresponds to an interfluve (Van Wagoner et al. 1990) that was transgressively overrun during basin-wide flooding of the Joli Fou Seaway. This transgression marks a major period of marine inundation of the Western Interior Seaway of North America. The Upper Albian Paddy Member is separated from the underlying Cadotte Member in the Peace River area of Alberta by a regionally extensive disconformity that was excavated during a relative sea-level fall and subsequently transgressively modified during flooding of the basin (Leckie & Singh 1991). This continued flooding eventually led to the return of offshore to shelf conditions reflected by the overlying Shaftesbury Formation (Leckie et al. 1990). In

the subsurface, coastal plain to floodplain mudstones and siltstones, alternating with palaeosols, correspond to the Upper Boulder Creek Formation (post-Cadotte Member but pre-Paddy Member). These represent the preserved terrestrial deposits that accumulated during late Cadotte Member highstand conditions and initial sea-level fall, which survived incision during generation of the Paddy Member disconformity. The Upper Boulder Creek palaeosols locally consist of rooted silty light grey mudstones with hematite-stained spherulitic siderite (Leckie et al. 1989). These palaeosols are truncated by a transgressive surface of erosion that locally displays an omission suite of thin, sharp-walled Skolithos and more rarely Rhizocorallium of the Glossifungites ichnofacies (Fig. 17B). The overlying granule-bearing, sandy mudstones of the Paddy Member are of low diversity, archetypal to distal expressions of the Cruziana ichnofacies, and reflect restricted bay conditions during regional flooding of the coastal margin. Regionally, the Paddy Member occupies a major estuarine valley excavated during initial lowstand conditions (Leckie & Singh 1991). The flooding of the estuary margins during late Paddy time resulted in transgressive modification of the interfluve area. Continued transgression resulted in the deposition of open marine mudstones of the Shaftesbury Formation and the return to shelf al conditions (Leckie et al. 1991). Incised valley complexes: demarcation of valley surfaces For the most part, lowstand deposits rarely dominate incised valley complexes, as the system is largely a zone of sediment bypass during incision (Van Wagoner et al. 1990). Much of the sediment accumulation in these systems occurs during late lowstand and transgression, and is therefore characterized by estuarine infill. The juxtaposition of facies into which the valley is incised, the presence of remnant fluvial deposits within the valley, accumulations of estuarine intervals during ensuing transgression, and the excavation of numerous internal discontinuities within the valley fill, result in highly complex successions that ichnology is ideally suited to help resolve. Ichnological suites are also effective at differentiating salinity changes and, more specifically, salinity reductions, assisting in the differentiation of fully marine, brackish and freshwater deposits (e.g. Pemberton et al. 1982; Beynon et al. 1988; Ranger & Pemberton 1992, 1997; MacEachern & Pemberton 1994; MacEachern et al. 1999a).

KEY STRATIGRAPHIC SURFACES

53

Fig. 18. Schematic model of incised valley surface types commonly demarcated by the Glossifungites ichnofacies (modified after MacEachern & Pemberton 1994).

This, coupled with the presence of substratecontrolled assemblages associated with erosional discontinuities within the valley fill, allows detailed mapping of valley components and assists in the resolution of the sequence stratigraphic history of valley excavation and infill. The Viking Formation produces hydrocarbons from estuarine incised valley fills in at least five fields of central Alberta. The facies successions and their distributions indicate that they accumulated in a barrier estuary or wave-dominated embayed estuary setting, in the sense of Roy et al (1980) and Dalrymple et al. (1992). In most of the incised valley systems of the Viking Formation the valley margins are demarcated trace fossil suites of the Glossifungites ichnofacies, indicating that the valley probably did not fill until active transgression (i.e. no lowstand deposits are preserved). Either the valley served as a zone of sediment bypass and possessed no fluvial deposits, or any lowstand deposits were subsequently eroded and reworked during the subsequent transgression, producing an amalgamated (co-planar) sequence boundary and initial transgressive surface. This initial transgressive surface of erosion most likely reflects tidalscour ravinement. The base of the estuarine valley fill therefore serves both as the sequence boundary and as the base of the transgressive systems tract. The valley fill, as well, contains a number of internal stratigraphic discontinuities, locally reflecting re-incision of the valley, erosive back-stepping of the barred mouth, lateral and landward shift of the tidal inlets at the barrier mouth, and lateral shift of tidal creeks and tidal

channels within the valley (Fig. 18). The valley margins are excavated into coarsening upward, regional Viking highstand marine parasequences. These parasequences contain fully marine, high diversity and abundantly burrowed (BI5) distal to proximal expressions of the Cruziana ichnofacies. This contrasts markedly with the ichnological suites developed within the valley fills. In the Crystal Field the valley margins are demarcated by firmground Diplocraterion, Thalassinoides and unnamed clavate burrows similar in morphology to Gastrochaenolites, assigned to the Glossifungites ichnofacies (Fig. 19B, C). At Willesden Green, the valley base contains firmground Arenicolites, Skolithos, Diplocraterion, Rhizocorallium and Thalassinoides (Fig. 19F).

Incised valley complexes: ichnology of estuarine valley fills The wave-dominated barred estuary systems can be separated into the bay-head delta, including the active channels and distributary channels, the central basin or lagoon, and the estuary mouth. The bay-head delta complex (Fig. 19A) is generally characterized by weakly and sporadically burrowed, parallel-laminated sandstones. Trace fossil diversities can be high (up to 16 ichnogenera), although burrow concentrations are irregular and the numbers of individual forms are low. Bioturbation intensities are also generally low (BI 1-2). The trace fossils comprise an impoverished expression of the Skolithos

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S. G. PEMBERTON ET AL.

Fig. 19. Incised Valley Complex. (A) Parallel-laminated sandstone of the bayhead delta front, Crystal Field. The sandstone is sporadically burrowed with a low-diversity expression of the Skolithos ichnofacies. Unit shows Bergaueria (Be) and Diplocraterion (D). Well 16-24-45-04W5; 1801.3m. (B) Box shot of core from the incised valley of the Crystal Field. Base of the core is to the lower left, and top to the upper right (T). Underlying lower offshore silty mudstones of the regional Viking Formation parasequences have been truncated by an amalgamated flooding surface and sequence boundary (FS/SB) along the margins of the Crystal incised valley. The valley fill at this locality consists of sandstone-dominated central basin deposits that have onlapped the valley margins. Well 04-01-46-04W5; 1802.1-1806.2 m. (C) Close-up of the contact visible in photo B. Lower offshore silty mudstones of the regional Viking contain a distal expression of the Cruziana ichnofacies with Helminthopsis (H), Asterosoma (As), and Palaeophycus (Pa). The FS/SB is demarcated by a firmground omission suite of Skolithos (S), and unnamed flask-shaped domichnia similar to Gastrochaenolites (G). The overlying bay deposits show dispersed pebbles (pe) at the base with a low-diversity suite attributable to the mixed Skolithos-Cruziana ichnofacies. Well 04-01-46-04W5; 1804.5m. (D) Sandy central basin deposits from the Willesden Green incised valley. Current ripple laminated sandstone is intercalated with strongly burrowed (BI4-5) muddy sandstone showing a low-diversity suite of the mixed Skolithos-Cruziana ichnofacies. The facies displays Thalassinoides (Th), Macaronichnus (Ma), Palaeophycus (Pa), Siphonichnus (Si), Bergaueria (Be), Rosselia (Ro) and Planolites (P). Well 06-36-40-07W5; 2322.7m. (E) Bioturbated (BI4) muddy sandstone from the estuary mouth complex of the Crystal incised valley. The suite corresponds to the Skolithos ichnofacies, comprising Thalassinoides (Th), Ophiomorpha (O), Skolithos (S), Diplocraterion (D), Planolites (P), Helminthopsis (H), Palaeophycus (Pa), Siphonichnus (Si), and Teichichnus (Te). Well 08-16-48-03W5; 1529.1 m. (F) Tidal inlet channel fill deposit from the Willesden Green incised valley. Lower offshore, silty mudstones of the regional Viking Formation parasequences with visible Phycosiphon (Ph), and Chondrites (Ch). The discontinuity corresponds to a transgressive scour ravinement (TSR) surface, demarcated by a firmground omission suite of Rhizocorallium (Rh), and Thalassinoides (Th). The channel fill sandstone displays dispersed pebbles (pe), and isolated, robust Ophiomorpha (O). Well 11-31-40-06W5; 2285.6m.

KEY STRATIGRAPHIC SURFACES

ichnofacies. The central basin complex (Fig. 19B, C, D) consists of delicately interstratified sandy mudstones, dark mudstones and thin sandstones. Synaeresis cracks are present throughout and are locally common. The trace fossil assemblages show moderate to low diversities of ichnogenera, moderate though variable bioturbation intensities (BI2-5; typically BI4), and reflect the mixed Skolithos-Cruziana ichnofacies. Trace fossil suites in the central basins are dominated by Teichichnus, Planolites, diminutive Rosselia, Cylindrichnus and Palaeophycus (Fig. 19D). Salinity fluctuations, episodic deposition and variable substrate consistency appear to be the dominant stresses imparted on the infauna. The estuary mouth complex (Fig. 19E) consists of moderately to abundantly burrowed (BI3-5) current and oscillation ripple-laminated sandstones, with minor intercalated mud beds. The trace fossil suites show high diversities of ichnogenera, corresponding to the Skolithos ichnofacies, but the distribution of individual elements reflects the presence of environmental stresses, in particular, episodic deposition. Channel-fill fades associations (Fig. 19F) predominantly consist of the deposits of relatively small, migrating subaqueous dunes. The amalgamation of the trough cross-beds into thick intervals supports a high sediment aggradation rate. The trace fossil suite is sporadically distributed, low in diversity (seven ichnogenera) and corresponds to the Skolithos ichnofacies. The most common elements include Ophiomorpha, Cylindrichnus, Palaeophycus and Diplocraterion. The burrowing demonstrates that most of the channel complexes accumulated in marine or marginal marine conditions, although the degree of salinity stress is difficult to determine. The main stress imposed on the trace fossil suite appears to be related to migration of bedforms and avalanching of sand into the dwelling structures. In the more tidally influenced estuaries of the Alberta Basin [e.g. the McMurray Formation (Pemberton et al. 1982; Ranger & Pemberton 1992, 1997; Bechtel et al. 1994); the Grand Rapids Formation (Wightman et al. 1987; Beynon et al. 1988); and the Glauconite Formation (Leroux et al. 1999)], the valley fills consist predominantly of channel sandstones and lateral accretion deposits manifest by inclined heterolithic stratification (IHS). The channel sandstones are dominated by stacked trough cross-stratified beds, lesser low-angle planar cross-stratified units, and thin current ripple laminated beds. Bioturbation is generally very low (BI1-2) and sporadically distributed, and typically is associated with thin mudstone

55

interbeds. Suites are characterized by sparse Planolites, Palaeophycus and Skolithos, with rare Cylindrichnus, Teichichnus and Arenicolites comprising the secondary elements. Very uncommon ichnogenera include Conichnus, Gyrolithes and fugichnia. IHS intervals are characterized by stacked, trough cross-stratified beds and current-rippled beds alternating with thin, depositionally inclined (3-7°) mudstone beds, interpreted to reflect tidal modification of river flow through the channel system. Bioturbation intensities are variable but moderate to low (BI2-4), and ichnogenera are sporadically distributed. Trace fossil suites are dominated by moderate to common Planolites, with moderately common to rare Teichichnus, Cylindrichnus and Skolithos. Secondary elements comprise rare Gyrolithes, Palaeophycus, fugichnia and roots. Accessory elements include very rare Arenicolites, Rosselia, Thalassinoides, Chondrites, Bergaueria, Rhizocorallium, Lockeia and Ophiomorpha. Some intervals consist of monogeneric assemblages of Planolites, Cylindrichnus or Gyrolithes, particularly within the betterstudied McMurray Formation palaeo-valleys. More saline elements of the suite comprise Chondrites, Rosselia, Rhizocorallium, Bergaueria and Ophiomorpha, all of which have only been described from McMurray Formation intervals (Bechtel et al. 1994; Ranger & Pemberton 1997). The trace fossil assemblages within the incised valley facies associations correspond to simple structures produced by trophic generalists. These are referred to as r-selected (opportunistic) behaviours, and are characteristic of stressed environmental settings (Pianka 1970), particularly those subject to salinity fluctuations. The episodic nature of deposition and the variability in substrate consistency lead to the development of trace fossil assemblages that constitute an impoverished expression of the mixed Skolithos-Cruziana ichnofacies (Pemberton et al. 1992a).

Bay-head delta, channel and embayment deposits In the Viking Formation of the JofTre Field, an amalgamated sequence boundary and flooding surface with a scarp-like geometry truncates underlying regional Viking marine parasequences and, locally, the transgressively incised Joffre Shoreface Complex (MacEachern et al. 1998, 1999a). The Glossifungites ichnofacies, characterized by firmground Skolithos, Diplocraterion and Thalassinoides, locally helps to demarcate this erosional discontinuity (Fig. 20A).

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Fig. 20. Joffre Embayment. (A) Joffre Embayment Complex showing regional Viking Formation lower offshore silty mudstones with Phycosiphon (Ph) and Planolites (P), truncated by a regionally extensive FS/SB, and overlain by embayment sandstones. The discontinuity is demarcated by firmground Thalassinoides (Th) of the Glossifungites ichnofacies. Well 14-11-39-27W4; 1572.7m. (B) Glauconitic pebbly (pe) muddy sandstones of the Joffre Embayment Complex corresponding to a transgressive sand sheet at the base of the regional FS/SB. The sandstone is moderately well bioturbated (BI4) containing a proximal expression of the Cruziana ichnofacies. The sandstone contains Planolites (P), Rosselia (Ro), Helminthopsis (H), Palaeophycus (Pa), and Chondrites (Ch). Well 14-05-38-24W4; 1372.3m. (C) Trough cross-stratified sandstone of the Joffre Embayment

KEY STRATIGRAPHIC SURFACES

The deposits overlying the discontinuity constitute the Viking reservoir facies at Joffre and reflect three stacked, conglomeratic embayment parasequences that prograded toward the northeast. The reservoir facies are dominated by trough cross-stratified and low-angle planar stratified sandstones, pebbly sandstones and conglomerates, concentrated along the southern margin of the amalgamated sequence boundary and flooding surface. The coarse elastics progressively inter-finger with, and ultimately pass into, interbedded mudstones and fine-grained sandstones in a northward and eastward direction. Near the base, the coarse elastics contain glauconite, siderite-cemented mudstone interbeds, mud inter-laminae and resistant mudstone rip-up clasts, and display moderate to low degrees of burrowing, diminishing in intensity upward (Fig. 20B). The trace fossil suite corresponds to the Skolithos ichnofacies. Overlying facies are dominated by well-sorted, unidirectional trough cross-bedded and low-angle planar stratified coarse elastics, locally in fining upward cycles with scoured bases (Fig. 20C, D). The elastics contain mudstone rip-up clasts and thin mudstone interbeds. Burrowing is of low abundance (BI1-2), and reduced diversity, with Diplocraterion, Skolithos, Palaeophycus and Ophiomorpha of the Skolithos ichnofacies. The interbedded mudstone and sandstone deposits contain oscillation ripples, wavy lamination, combined flow ripples and rare current ripples (Fig. 20E, F). These heterolithic intervals are weakly burrowed (BI 1-3) with a sporadically distributed, lowdiversity (salinity stressed?) trace fossil suite of the mixed Skolithos-Cruziana ichnofacies (MacEachern et al 1998, 1999a). Dominant elements comprise Teichichnus, Cylindrichnus, Planolites and Palaeophycus. Detailed ichnological, sedimentological and stratigraphic analyses demonstrate that the coarse elastics overlying the discontinuity comprise at least three parasequences. These parasequences onlap the discontinuity in a southwest direction and inter-finger with mudstones to the northeast. Toward the north end of the field, erosional amalgamation of the coarse elastics is

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more pronounced, and parasequence boundaries cannot be delineated easily. Near the southern end of the field, these parasequences partition the reservoir along the seaward (and structurally up-dip) edge of the deposit. Amalgamation of the reservoir facies at the north end limits the degree of partitioning. The final parasequence of the embayment complex is truncated by a regionally extensive flooding surface, typically manifest as a wave ravinement surface. The wave ravinement surface is commonly demarcated by the Glossifungites ichnofacies, or where excavated across sandy underlying facies, a palimpsest softground suite of Diplocraterion (Fig. 13C). Facies overlying the marine flooding surface reflect fully marine conditions. Conclusions The main applications of ichnology to genetic stratigraphy are twofold. The most obvious use lies in the demarcation of erosional discontinuities. To date, substrate-controlled ichnofacies have been underutilized but are gaining recognition as a viable means of recognizing and mapping these stratigraphically important surfaces, both in outcrop and subsurface. Locally, many surfaces are obvious on the basis of sedimentology alone; however, their appearance can change markedly across the study area, making correlation difficult. Substrate-controlled ichnofacies, such as the Glossifungites ichnofacies, are also important to the genetic interpretation of erosional discontinuities in marine-influenced siliciclastic intervals, as the many examples cited in the paper demonstrate. Hence, even where discontinuities can be recognized purely sedimentologically or stratigraphically, the associated trace fossils enhance their sequence stratigraphic interpretation. In many cases, the genetic interpretation of the discontinuity has come principally from the trace fossil assemblages that are associated with the discontinuity and the overlying units. The continued integration of substrate-controlled ichnofacies with detailed

Complex with sporadically distributed lined Diplocraterion (D). Well 02-05-39-26W4; 1573.3m. (D) Trough cross-stratified sandstone of the Joffre Embayment Complex with sporadically distributed lined Diplocraterion (D). Well 11-07-39-26W4; 1548.1 m. (E) Heterolithic succession of oscillation rippled, combined flow rippled and low-angle parallel-laminated sandstone and dark, weakly burrowed (BI 2-3) mudstone corresponding to open bay deposits. Trace fossils reflect a stressed, low-diversity expression of the mixed Skolithos-Cruziana ichnofacies, characterized by Teichichnus (Te), Cylindrichnus (Cy), and Planolites (P). Well 02-18-38-24W4; 1459.2m. F) Heterolithic succession of oscillation rippled (osc), current-rippled (cr), and low-angle parallellaminated sandstone, with dark, weakly burrowed (BI 2) mudstone corresponding to open bay deposition. Trace fossils reflect a stressed, low-diversity expression of the mixed Skolithos-Cruziana ichnofacies, characterized by Teichichnus (Te), and Planolites (P). Well 03-24-38-25W4; 1437.7m.

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stratigraphic and sedimentological analysis will undoubtedly enhance and refine the developing genetic stratigraphic paradigms. The second use is more subtle, and is concerned with trace fossil behaviours and their palaeoenvironmental implications. Trace fossils, when used in conjunction with primary sedimentary structures, are useful in the delineation and interpretation of facies and facies associations. When these behavioural and substrate-controlled aspects of ichnology are integrated fully with other sedimentological and stratigraphic analyses, the result is a powerful approach to the recognition and genetic interpretation of discontinuities in the rock record.

USA) reinterpreted as lowstand shoreface deposits. American Association of Petroleum Geologists Bulletin, 64, 184-201. BEYNON, B. M., PEMBERTON, S. G., BELL, D. A. & LOGAN, C. A. 1988. Environmental implications of ichnofossils from the Lower Cretaceous Grand Rapids Formation, Cold Lake Oil Sands Deposit. In: JAMES, D. R. & LECKIE, D. A. (eds) Sequences, Stratigraphy, Sedimentology: Surface and Subsurface. Canadian Society of Petroleum Geologists, Memoirs, Calgary, Alberta, 15, 275290. BHATTACHARYA, J. P. 1993. The expression and interpretation of marine flooding surfaces and erosional surfaces in core: examples from the Upper Cretaceous Dunvegan Formation, Alberta foreland basin, Canada. In: POSAMENTIER, H. W.,

The authors would like to thank the Natural Science and Engineering Research Council of Canada (NSERC) for research funding. The senior author would like to acknowledge the Canada Research Chairs programme for their support of his research. D. Robbins assisted with some of the drafting, and we are grateful for his contribution.

(eds) Stratigraphy and Facies Associations in a Sequence Stratigraphic Framework. International Association of Sedimentologists, Special Publications, Oxford, 18, 125-160. BHATTACHARYA, J. & WALKER, R. G. 1991. Allostratigraphic subdivision of the Upper Cretaceous Dunvegan, Shaftesbury and Kaskapau formations, northwestern Alberta subsurface. Bulletin of Canadian Petroleum Geology, 39, 145-164. BROMLEY, R. G. & ASGAARD, U. 1993. Two bioerosion ichnofacies produced by early and late burial associated with sea level change. Geologische Rundschau, 82, 276-280. BROMLEY, R. G., PEMBERTON, S. G. & RAHMANI, R. A. 1984. A Cretaceous woodground: the Teredolites ichnofacies. Journal of Paleontology, 58, 488-498. DALRYMPLE, R. W., ZAITLIN, B. A. & BOYD, R. 1992. Estuarine facies models: conceptual basis and stratigraphic implications. Journal of Sedimentary Petrology, 62, 1130-1146. DAM, G. 1990. Paleoenvironmental significance of trace fossils from the shallow marine Lower Jurassic Neill Klinter Formation, East Greenland. Palaeogeography, Palaeoclimatology, Palaeoecology, 79, 221-248. DAVIES, S. D. & WALKER, R. G. 1993. Reservoir geometry influenced by high-frequency forced regressions within an overall transgression: Caroline and Garrington fields, Viking Formation (Lower Cretaceous), Alberta. Bulletin of Canadian Petroleum Geology, 41, 407-421. DE GIBERT, J. M., MARTINELL, J. & DOMENECH, R. 1998. Entobia ichnofacies in fossil rocky shores, Lower Pliocene, northwestern Mediterranean. Palaios, 13, 476-487. DOWNING, K. P. & WALKER, R. G. 1988. Viking Formation, Joffre Field, Alberta: shoreface origin of long, narrow sand body encased in marine mudstones. Bulletin American Association Petroleum Geologists, 72, 1212-1228. EKDALE, A. A., BROMLEY, R. G. & PEMBERTON, S. G. 1984. Ichnology: Trace Fossils in Sedimentology and Stratigraphy. Society of Economic Paleontologists and Mineralogists, Short Course Notes, Tulsa, Oklahoma, 15.

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KEY STRATIGRAPHIC SURFACES Ross, C. A. & VAN WAGONER, J. C. (eds) Sea-Level Changes: An Integrated Approach. Society of Paleontologists and Mineralogists, Special Publications, Tulsa, Oklahoma, 42, 357-370. PLINT, A. G., WALKER, R. G. & BERGMAN, K. M. 1986. Cardium Formation 6. Stratigraphic framework of the Cardium in subsurface. Bulletin of Canadian Petroleum Geology, 34, 213-225. FLINT, A. G., WALKER, R. G. & DUKE, W. L. 1988. An outcrop of subsurface correlation of the Cardium Formation in Alberta. In: JAMES, D. P. & LECKIE, D. A. (eds) Sequences, Stratigraphy, Sedimentology: Surface e. Canadian Society of Petroleum Geologists, Memoirs, Calgary, Alberta, 15, 167-184.

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SAUNDERS, T. & PEMBERTON, S. G 1986. Trace Fossils and Sedimentology of the Appaloosa Sandstone: Bearpaw-Horseshoe Canyon Formation Transition, Dorothy, Alberta. Canadian Society of Petroleum Geologists, Field Trip Guide Book, Calgary, Alberta. SAVRDA, C. E. 199la. Teredolites, wood substrates, and sea-level dynamics. Geology, 19, 905-908. SAVRDA, C. E. 1991b. Ichnology in sequence stratigraphic studies: an example from the lower Paleocene of Alabama. Palaios, 6, 39-53. SAVRDA, C. E. 1995. Ichnologic applications in paleoceanographic, paleoclimatic, and sea-level studies. Palaios, 10, 565-577. SAVRDA, C. E., OZALAS, K., DEMKO, T. H., HUTCHINPOSAMENTIER, H. W. & CHAMBERLAIN, C. J. 1993. SON, R. A. & SCHEIWE, T. D 1993. Log grounds Sequence Stratigraphic analysis of Viking Formaand the ichnofossil Teredolites in transgressive tion lowstand beach deposits at Joarcam field, deposits of the Clayton Formation (Lower PaleoAlberta, Canada. In: POSAMENTIER, H. W., cene), western Alabama. Palaios, 8, 311-324. SUMMERHAYES, C. P., HAQ, B. U. & ALLEN, G. P. SAVRDA, C. E., BROWNING, J. V., KRAWINKLE, H. & (eds) Stratigraphy and Fades Associations in a HESSELBO, S. P. 2001. Firmground ichnofabrics in deepwater sequence stratigraphy, Tertiary Sequence Stratigraphic Framework. International Association of Sedimentologists, Special Publicaclinoform-toe deposits, New Jersey slope. Palaios, tions, Oxford, 18, 469-485. 16, 294-305. POSAMENTIER, H. W., ALLEN, G. P., JAMES, D. P. & SCHAFER, W. 1972. Ecology and Palaeoecology of Marine Environments. Oliver & Boyd, EdinTESSON, M. 1992. Forced regressions in a sequence Stratigraphic framework: concepts, examples, and burgh/University of Chicago Press, Chicago. exploration significance. American Association of SEILACHER, A. 1962. Paleontological studies in turbidite Petroleum Geologists Bulletin, 76, 1687-1709. sedimentation and erosion. Journal of Geology, 70, RADDYSH, H. K. 1988. Sedimentology and 'geometry' 227-234. of the Lower Cretaceous Viking Formation, SEILACHER, A. 1982. Distinctive features of sandy tempesGilby A and B Fields, Albert. In: JAMES, D. P. tites. In: EINSELE G. & SEILACHER, A. (eds) Cyclic and Event Stratification. Springer, Berlin, 333-349. & LECKIE, D. A. (eds) Sequences, Stratigraphy, Sedimentology: Surface and Subsurface. Canadian SMITH, R. M. H. 1987. Helical burrow casts of therapSociety of Petroleum Geologists, Memoirs, sid origin from the Beaufort Group (Permian) of Calgary, Alberta, 15, 417^30. South Africa. Palaeogeography, Palaeoclimatol. RANGER, M. J. & PEMBERTON, S. G. 1992. The sedimentology and ichnology of estuarine point bars in the SOEGAARD, K. & MACEACHERN, J. A. 2003. Integrated McMurray Formation of the Athabasca Oil Sands sedimentological, ichnological and sequence Deposit, northeastern Alberta, Canada. In: PEMStratigraphic model of a coarse clastic fan delta reservoir: Middle Jurassic Oseberg Formation, BERTON, S. G. (ed.) Applications of Ichnology to Petroleum Exploration: A Core Workshop. Society North Sea, Norway. Abstract Volume, AAPG Annual Convention, Salt Lake City, Utah, May of Economic Paleontologists and Mineralogists, Core Workshops, Tulsa, Oklahoma, 17, 401-421. 2003, p. A160. RANGER, M. J. & PEMBERTON, S. G. 1997. Elements of a STAMP, L. D. 1921. On cycles of sedimentation in the Stratigraphic framework for the McMurray Eocene strata of the Anglo-Franco-Belgian basin. Geological Magazine, 58, 108-114, 146-157, 194Formation in south Athabasca. In: PEMBERTON, 200. S. G. & JAMES, D. P. (eds) Petroleum Geology of the Cretaceous Mannville Group, Western STELCK, C. R., MACEACHERN, J. A. & PEMBERTON, S. G. Canada. Canadian Society of Petroleum Geolo2000. Foraminiferal biostratigraphic analysis of gists, Memoirs, Calgary, Alberta, 18, 263-291. the Viking Formation, Kaybob North and RAYCHAUDHURI, I., BREKKE, H. G., PEMBERTON, S. G. Giroux Lake fields, central Alberta: a comparison & MACEACHERN, J. A. 1992. Depositional facies with the Hasler Formation biostratigraphy of and trace fossils of a low wave energy shoreface northeastern British Columbia. Canadian Journal succession, Albian Viking Formation, Chigwell of Earth Science, 37, 1389-1410. Field, Alberta, Canada. In: PEMBERTON, S. G. SWIFT, D. J. P. 1968. Coastal erosion and transgressive (ed.) Applications of Ichnology to Petroleum stratigraphy. Journal of Geology, 76, 444-456. Exploration: A Core Workshop. Society of TAYLOR, A. M. & GAWTHORPE, R. L. 1993. Application Economic Paleontologists and Mineralogists, of sequence stratigraphy and trace fossil analysis Core Workshops, Tulsa, Oklahoma, 17, 319-337. to reservoir description: examples from the ROY, P. S., THOM, B. G. & WRIGHT, L. D. 1980. HoloJurassic of the North Sea. In: PARKER, J. R. (ed.) cene sequences on an embayed high-energy coast: Petroleum Geology of Northwest Europe, Proceedan evolutionary model. Sedimentary Geology, 26, ings of the 4th Conference. Geological Society of 1-19. London, 317-335.

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WALKER, R. G. & BERGMAN, K. M. 1993. Shannon sandstone in Wyoming: a shelf ridge complex reinterpreted as lowstand shoreface deposits. Journal of Sedimentary Petrology, 63, 839-851. WALKER, R. G. & EYLES, C. H. 1991. Topography and significance of a basin wide sequence-bounding erosion surface in the Cretaceous Cardium Formation, Alberta, Canada. Journal of Sedimentary Petrology, 61, 473-496. WALKER, R. G. & JAMES, N. P. (eds) 1992. Facies Models: Response to Sea Level Change. Geological Association of Canada, St John's, Newfoundland. WALKER, R. G. & WISEMAN, T. R. 1995. Lowstand shorefaces, transgressive incised shorefaces, and forced regressions: examples from the Viking Formation, Joarcam area, Alberta. Journal of Sedimentary Research, 65, 132-141. WETZEL, A. & UCHMAN, A. 1998. Biogenic sedimentary structures in mudstones: an overview. In: SCHIEBER, J., ZlMMERLE, W.

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Shales and Mudstones 1. E. Schweizerbart'sche Verlagsbuchhandling, Stuttgart, 351-369. WIGHTMAN, D. M., PEMBERTON, S. G. & SINGH, C. 1987. Depositional modeling of the Upper Mannville (Lower Cretaceous), central Alberta. Implications for the recognition of brackish water deposits. In: TILLMAN, R. W. & WEBER, K. J. (eds) Reservoir Sedimentology. Society of Economic Paleontologists and Mineralogists, Special Publications, Tulsa, Oklahoma, 40, 189-220.

Recent and sub-recent microborings from the upwelling area off Mauritania (West Africa) and their implications for palaeoecology INGRID GLAUB Geologisch-Palaontologisches Institut, Senckenberganlage 32-34, D-60325 Frankfurt am Main, Germany (e-mail: [email protected]) Abstract: Late Quaternary dead molluscan shells off Mauritania (West Africa) from the intertidal zone to 220-300 m water depth were studied for microborings. The study gives preliminary data on microborings in upwelling areas and their implications for the fossil record. In total 18 ichnotaxa are described. They are considered to be produced by cyanobacteria, green algae, red algae, fungi and foraminifera. The ichnotaxonomic composition shows minor differences relative to tropical/subtropical areas of investigation. No ichnotaxa are believed to be specific to upwelling areas. Bathymetrical distribution patterns revealed different depth ranges for individual ichnotaxa. Relative to areas with similar latitude but not influenced by upwelling, the absolute depth of the photic zone is shallower. The majority of ichnotaxa observed are already known from the fossil record (tropical and subtropical study areas) and should also be expected from ancient upwelling areas.

The term 'microborings' is used for boring systems in hard substrates with individual tunnel diameters of less than 100 urn. They are commonly found in calcareous substrates, such as shells and ooids (e.g. Golubic et al 1975; Budd & Perkins 1980; Glaub 1994; Bundschuh 2000; Vogel et al. 2000), but are also rarely observed in phosphatic substrates, such as teeth and bones (e.g. Konigshof & Glaub in press). Research on modern and fossil microborings has intensified since the development of the casting embedding technique (Golubic et al. 1970). This preparation method is based on the filling of boring systems by polymer resin and subsequent dissolution of the infested substrate. The resulting artificial casts allow a three-dimensional visualization of the various borings by SEM. The fully detailed morphology of microborings yields information on the producing microbial endoliths (belonging to cyanobacteria, green algae, red algae, fungi etc.) and provides the basis for comparison with other fossil and recent microborings. Studies of tropical to subtropical modern and fossil microborings are numerous (e.g. Radtke et al. 1997 and references therein; Gektidis 1997; Vogel et al. 1999). In contrast, studies on modern and fossil microborings from high latitudes are still rare (Bromley & Hanken 1981; Akpan & Farrow 1984; Akpan 1986; Young & Nelson 1988; Schmidt & Freiwald 1993; Glaub et al. 2002; Vogel & Marincovich in press). In this context, microborings from upwelling areas in low latitudes are of great interest. They are considered to display the influence of temperature on microboring distribution patterns under a similar angle of light incidence as in tropical to subtropical study areas.

The sampling activity of the Meteor Cruise 25/ 1971 off Mauritania (West Africa) provided an excellent database from which to obtain an initial impression of the microboring inventory in an upwelling area (initial documentation in Glaub et al. 2002), A great amount of Quaternary shell material (mainly molluscan shells) was collected, ranging from the intertidal down to 300 m water depth. The present study addresses the following questions: (1) Does the ichnotaxonomic composition differ from non-upwelling localities? (2) What information do the samples give on bathymetric distribution patterns of microborings in upwelling areas? (3) What are the palaeoecological implications? Material and methods The cruise 25/1971 of R.V. Meteor collected molluscan shells at 24 stations (Fig. 1). The activity of the scientific crew members included additional coastal field trips. Sampling depths range from the intertidal to 300m water depth. Samples were taken by dredges, grab samplers, box samplers and vibrocorers (Einsele et al. 1977). The present investigation is based on approximately 150 Holocene molluscan shells, examined by SEM after application of the casting embedding technique (Golubic et al. 1970). The first intention was to focus on Recent material, because of the good quality hydrographic data (light, temperature, currents) available. However, study of the upper layers of profiles (box sampler, vibrocorers) produced

From\ MclLROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 63-76. 0305-8719/04/S 15.00 © The Geological Society of London.

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Fig. 1. Left: area of investigation. Right: ship track of sampling stations and bathymetry (in metres). Below: shelf zonation, stations and sampling equipment (GS, grab sampler; BS, box sampler; Dr, dredge; VC, vibrocorer). Reproduced with permission from Einsele et al (1977).

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no results, even from fresh-looking samples. As a result the studied substrates derive from sampling with a dredge and grab sampler and thus represent a time interval of some thousands of years (Einsele et al. 1977). This revised sampling method is, however, a more realistic analogue for fossil assemblages, which tend to be time-averaged to some degree. The area of investigation is characterized by upwelling water masses and by a cold, mainly south-directed surface current belonging to the Canary current system. Water temperatures of about 15°C to 17°C are reported from 50m water depth and salinity values are about 35.7%o (Mittelstaedt 1972). The area currently belongs to the cold-temperate region. Near the coast the suspension load is high. Plankton blooms may also cause local, temporary reduction of light transparency conditions in the water column. For the naming of the borings observed, established ichnotaxa were used where available. As for the remaining taxa, informal names were

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given (e.g. 'Tripartitum Form'). It was decided to apply the names of existing ichnotaxa to modern traces for the following reasons: (1) if a modern trace and a fossil trace are identical, it is consistent to give both the same name; and (2) ichnological studies on modern microborings have a high potential to aid palaeoecological reconstructions, even if the producer is unknown. As long as the trace-maker is not identified, one has to find a name for the trace observed and cannot use biotaxonomy. In this case the application of established ichnotaxa is more precise and less wordy than informal names (e.g. Tripartitum Form', 'Fasciculus dactylus-lis should not be used in establishing new ichnotaxa, because there is still the opportunity to find the trace-maker in future and to use a biological binomen. This approach serves to avoid the creation of ichnotaxa based on parallel taxonomy that could otherwise increase in an irresponsible way.

Fig. 2. Artificial casts of microborings in molluscan shells off Mauritania; SEM pictures. Simplified drawings are added where needed, (a) Caverna pediculata. Station 69 (41 m water depth), (b) Saccomorpha clava. Station 66 (88-89 m water depth), (c) Fasciculus acinosus. Intertidal Baie de St Jean, (d) Fasciculus dactylus together with Tripartitum Form (thinner tunnels). Station 70 (20m water depth), (e) Fasciculus isp. 1. Station 69 (41 m water depth), (f) Fasciculus isp. 2. Station 58 (74m water depth), (g) Polyactina araneola. Station 69 (41 m water depth), (h) Orthogonum fusiferum. Station 70 (20m water depth), (i) Orthogonum lineare together with Saccomorpha clava. Station 77 (151m water depth).

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Description In total, 18 ichnotaxa are characterized by a brief morphological description, complemented by taxonomic comments and data on their geographical distribution and stratigraphic range. There are several other borings that are not described herein because their rarity precludes confident characterization of their morphology. In addition, borings similar to those of endolithic bryozoa and endolithic sponges are observed. Cavernula pediculata Radtke 1991 (Fig. 2a) Description. Cavernula pediculata is characterized by bag-shaped cavities orientated perpendicular to the substrate surface. The borings

measure 20-30 um in diameter at their greatest width and are 30-60 um in length. They are connected to the substrate surface by thin, short, occasionally ramified tunnels. One of these tunnels is usually 6-7 um wide and 6-7 um long, whereas the others display 1-2 um in diameter and are of similar length. Taxonomic comment. Similar modern borings are affiliated to Codiolum-stages of the green alga Gomontia polyrhiza (Lagerheim) Bornet & Flahault 1889. Distribution in modern and ancient environments. Records of its modern geographic distribution concentrate on the northern hemisphere, where it inhabits tropical to non-tropical environments (e.g. Nielsen 1972; Radtke 1993; Guiry & Nic Dhonncha 2002). The oldest fossil record

Fig. 3. Distribution of microborings at different stations, sorted by water depth.

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dates back to the Triassic (Schmidt 1992). The occurrence of Cavernula pediculata off Mauritania is rare (Fig. 3). Saccomorpha clava Radtke 1991 (Fig. 2b) Description. The boring system consists of clubshaped borings (10-30 jam in diameter at their greatest width, interconnected by small tunnels 1 um in diameter). Several varieties of clubshaped borings are developed, in some cases occurring within a single branching system: some clubs show gradually increasing width from the proximal area near the substrate surface to the distal tips, whereas others look like a ball or an ellipsoid on a stem, and still other clubs are heart-shaped, caused by a small distal notch. A branching system mainly connects the club-shaped borings in their proximal portions, but some tunnels branch off at the distal part of the club. No collar was observed. Taxonomic comment. The ichnotaxon Saccomorpha clava Radtke 1991 is used for borings morphologically similar to those of the modern fungus Dodgella priscus Zebrowski 1936. Distribution in modern and ancient environments. Dodgella priscus is known from tropical and non-tropical modern environments down to 2350m water depth (Hohnk 1969; Zeff & Perkins 1979; Budd & Perkins 1980; Golubic et al. 1984). Saccomorpha clava is recorded from the Triassic (Schmidt 1992), Jurassic (Glaub 1994), Cretaceous (Hofmann 1996) and Tertiary (Radtke 1991). Saccomorpha clava borings are very abundant in the sampling area off Mauritania. These findings confirm earlier data, which indicate that Saccomorpha clava borings usually become more common with increasing water depth. Saccomorpha clava is a key ichnotaxon of the index microboring ichnocoenosis for the aphotic zone (Glaub 1994; see below). Fasciculus acinosus Glaub 1994 (Fig. 2c) Description. The most characteristic feature of Fasciculus acinosus is that small sphaeroid cavities (4-8 um in diameter) are arranged close to each other, so as to resemble a bunch of grapes. At the distal portion a prominent tunnel is developed (6-8 jam in diameter, 2040 jim in length). Taxonomic comment. Fasciculus acinosus Glaub 1994 is an ichnotaxon displaying a similar boring pattern to the modern cyanobacterial species Hyella balani Lehmann 1903. Distribution in modern and ancient environments. Hyella balani is widely reported from the Indian Ocean (Guiry & Nic Dhonncha 2002), NE Atlantic (Nielsen 1972), Caribbean (Gektidis

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1997) and Mediterranean (Le CampionAlsumard 1978). The fossil record of Fasciculus acinosus dates back to the Permian (Glaub et al. 1999). Fasciculus acinosus is mainly restricted to the shallow euphotic zone II, which corresponds to the intertidal (sensu Glaub 1994). Observations in Mauritanian sample material confirm this environmental restriction and extend the geographic distribution of the ichnotaxon to up welling areas. Fasciculus dactylus Radtke 1991 (Fig. 2d) Description. The Fasciculus dactylus borings are bunches of tunnels radiating from an area of entry into the substrate. Individual tunnels measure 5-9 jam in diameter. Tunnels usually display rounded tips and can only rarely be demonstrated to branch. Fasciculus dactylus may also be developed spreading parallel to the substrate surface with smaller (5-6 jam), commonly branching tunnels. Taxonomic comment. The boring pattern is known to be produced by Hyella caespitosa Bornet & Flahault 1889 in the Recent, but may also be produced by other Hyella or Solentia species (cyanobacteria). Fasciculus dactylus was introduced by Radtke (1991). Distribution in modern and ancient environments. Hyella caespitosa is known from many tropical and non-tropical study areas: NE Atlantic, SE Pacific, Indian Ocean (Guiry & Nic Dhonncha 2002). Fasciculus dactylus is quite abundant off Mauritania (Fig. 3). It is index ichnotaxon for two subzones of the euphotic zone: Fasciculus acinosus j Fasciculus dactylus ichnocoenosis of the upper euphotic zone II for the intertidal and Fasciculus dactylus jPalaeoconchocelis starmachii - ichnocoenosis for the well-illuminated subtidal (Glaub 1994). It is known since the Permian (Balog 1996; Glaub et al. 1999). Fasciculus isp. 1 (Fig. 2e) Description. Fasciculus isp. 1 is a unique clusterforming boring system characterized by tunnels radiating from a central area of penetration. The tunnels of some clusters display perpendicular to parallel orientation to the substrate surface, whereas other clusters are typified by only substrate-parallel tunnels. In both cases, three tunnels are typically visible at the entrance area. The tunnels measure 7-9 jim in diameter. Tunnel constrictions are present in some cases. The commonly developed regular dichotomous branching is distinctive. Taxonomic comment. Fasciculus isp. 1 is probably produced by Hyella Stella, a modern cyanobacterium classified in the order Pleurocapsales

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and first described by Al-Thukair & Golubic (1991). Distribution in modern and ancient environments. The first description of Hyella Stella was based on specimens in modern ooids from the Arabian Gulf, Indian Ocean (Al-Thukair & Golubic 1991). Gektidis (1997) registered Hyella Stella rarely while evaluating study experiments off the Bahamas. A further report from modern environments is given for similar borings (Radtke 1993, Fasciculus sp). For records from ancient environments, refer to Green et al. (1988). They reported findings of microborings with preserved organic remains similar to Hyella Stella, named Eohyella dichotoma Green et al. 1988. Their studies are based on Proterozoic (700-800 Ma) silicified pisoliths in Greenland. Fasciculus isp. 2 (Fig. 2f) Description. Tunnels 1—4 um in diameter are developed parallel to the substrate surface (rarely observed in perpendicular position). Ramification abundantly occurs, following angles of up to 90°. The boring system forms densely infested patches. Poorly developed tunnel constrictions are visible every 2-8 um. Taxonomic comment. Fasciculus isp. 2 displays the characteristics described by Radtke (1991) for the ichnogenus Fasciculus. The producer of this boring is unknown, but the size and ramification pattern suggest a bacterium. Distribution in modern and ancient environments. The record in modern and ancient environments is confused. In some respects, borings of the Tertiary Fasciculus parvus (Radtke 1991, taf. 10, fig. 6), orientated parallel to the substrate surface, look similar. In addition, Radtke (1993) observed borings called Fasciculus parvus showing parallel development in modern substrates off Lee Stocking Island (Bahamas). Fasciculus isp. 2 is very abundant off Mauritania (Fig. 3) Polyactina araneola Radtke 1991 (Fig. 2g) Description. The Polyactina araneola boring is composed of three morphological elements: entry tunnel; distal globular enlargement; and thinner tunnels with tapering ends. The entry tunnel (8-10 jam in diameter) is orientated perpendicular to the substrate surface. Deeper in the substrate the tunnel extends its diameter to a nearly globular enlargement 30-40 jam in diameter. All around this widening, thinner tunnels (8-10 jam in diameter) radiate in different directions. These tunnels are characterized by gradually decreasing diameters and tapering ends 1-2 um in diameter. Many tunnels turn back towards the substrate surface, where they

run parallel to the surface for several lOOum, to be connected with another Polyactina araneola boring. Together with boring systems clearly identifiable as being Polyactina araneola are those that might represent initial stages. These possible initial stages are stemmed globular borings that widen distally, where one to four thinner tunnels are developed. Taxonomic comment. Conchyliastrum enderi Zebrowski 1936, a lower fungus (order Chytridiales), has been proposed as the producer of the boring Polyactina araneola by Radtke (1991). Distribution in modern and ancient environments. Conchyliastrum is known from tropical and non-tropical environments (Hohnk 1969; Zeff & Perkins 1979; Budd & Perkins 1980). The corresponding ichnotaxon Polyactina araneola has a long geological record back to the Silurian (Bundschuh 2000). Boring activity of marine endolithic Conchyliastrum species seems to increase in the deep euphotic zone and deeper parts of the water column. Off Mauritania, its distribution is exclusive to samples from station 58, 59, 68, and 69 (Fig. 3) Orthogonum fusiferum Radtke 1991 (Fig. 2h) Description. Orthogonum fusiferum borings are characterized by thin tunnels (l-2um in diameter) with typical spindle-shaped enlargements (5-7um in diameter, lOum long). Besides this type of development there is a smaller one. It displays tunnel diameters of approximately 0.5 jam and enlargements 4um in diameter and 6um long. The smaller version shows up to four tunnels branching off at the spindle-shaped broadening, whereas the bigger variety is unbranched at widenings, or a single tunnel may originate. Taxonomic comment. Orthogonum fusiferum Radtke 1991 is the ichnotaxon for borings similar to those of the modern lower fungus Ostracoblabe implexa Bornet & Flahault 1889. Distribution in modern and ancient environments. Ostracoblabe implexa is known from studies in the southern part of the NW Atlantic (e.g. Radtke 1993) and the Mediterranean (Le Campion-Alsumard 1978). According to studies of Le Campion-Alsumard, the boring activity of Ostracoblabe implexa extends down to 200 m water depth. Orthogonum fusiferum is only rarely documented off Mauritania (Fig. 3). As for ancient environments, the oldest record of Orthogonum fusiferum is that of Bundschuh (2000) from the Silurian. Orthogonum lineare Glaub 1994 (Fig. 2i) Description. Tubular borings, 5-15 um in diameter, display a highly organized boring

MICROBORINGS IN UPWELLING AREAS

system, orientated parallel to the substrate surface, dominated by ramification angles of 90°. Tunnels running parallel to each other are common. Taxonomic comment. The ichnotaxon Orthogonum lineare was erected by Glaub (1994) for fossil borings. Its producer is unknown (see below). Distribution in modern and ancient environments. Boring patterns similar to that of Orthogonum lineare are reported from several places under numerous informal names - modern and fossil, topical and non-tropical - mainly from the aphotic zone. Reports on modern borings are given by: Zeff& Perkins (1979) from the Caribbean at 435-775 m as Tubular branching borings'; Budd & Perkins (1980) from the Caribbean at 119-530 m as Tubular-Form F; Schmidt & Freiwald (1993) from Norway as Type C at 20-90 m; and Krutschinna (1997) from Norway at 230-300 m as Tubulare Spur 1'. Fossil forms have been described by Glaub (1994) from the Jurassic of Europe (inferred to be from the deep euphotic zone to aphotic zone) and by

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Hofmann (1996, taf. X, fig. 5-6, taf. XI, fig. 1) from the Cretaceous of northern Europe. Orthogonum lineare is, together with Saccomorpha clava, a key ichnotaxon for recognition of the aphotic zone ichnocoenosis (of Glaub 1994). Off Mauritania, Orthogonum lineare is common in samples from station 77 (151m) (Fig. 3). Orthogonum spinosum Radtke 1991 (Fig. 4a) Description. Tubular tunnels, characterized by diameters of 10-3 8 um and mainly rectangular ramifications. The tunnels are in some cases developed in some distance to the substrate surface and are in that case abundantly connected with it by perpendicular tunnel junctions. They display a conspicuous boring surface, which is surrounded by short hair-like extensions (1-2 jam in diameter, 10-15 um maximum length) all around the tunnels, but in some cases restricted to distinct portions of the tunnel (Fig.4a). Taxonomic comment. The first description of Orthogonum spinosum was given by Radtke

Fig. 4. Artificial casts of microborings in molluscan shells off Mauritania; SEM pictures. Simplified drawings are added where needed, (a) Orthogonum spinosum. Station 66 (88-89 m water depth), (b) Orthogonum tubulare. Station 77 (151 m water depth), (c) Reticulina elegans. Station 69 (41 m water depth), (d) Scolecia filosa. Station 69 (41 m water depth), (e) Scolecia serrata. Station 69 (41 m water depth), (f) Globodendrina monile; thinner tunnels belong to other taxa. Station 70 (20m water depth), (g) Tripartite Form. Station 69 (41 m water depth).

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(1991) based on Tertiary samples. Similar modern borings are known, but yet not attributed to a distinct producer. Distribution in modern and ancient environments. Spinose borings, usually characterized by longer hairs than described above, are very abundant in modern waters, mainly in deep water (e.g. Zeff& Perkins 1979; Budd & Perkins 1980; Glaub 1994; Krutschinna 1997). In contrast, records of fossil spinose borings are rare: Radtke (1991, Tertiary), Schmidt (1992, Triassic). Orthogonum tubulare Radtke 1991 (Fig. 4b) Description. The boring systems usually spread parallel to the substrate surface, branching rectangular. The main characteristic feature is the extremely varying tunnel diameter. At the point of ramification irregularly-shaped enlargements are developed. The diameters of the interconnecting tunnels measure 10-30|im, whereas the enlargements reach more than 40 jim in diameter. The tunnel surface of both elements, which is more clearly visible at the enlargements, is verrucose. Taxonomic comment. The ichnotaxon Orthogonum tubulare Radtke 1991 was established from Tertiary study material. The biological identity of its producer is as yet unknown. Distribution in modern and ancient environments. Borings with similar morphology but smaller dimensions have been documented in foraminiferan tests from the Atlantic Ocean at 21952323m water depth (Golubic et al. 1984). As for the fossil record, besides the description of Orthogonum tubulare from the Tertiary (Radtke 1991), it is known from the Cretaceous (Hofmann 1996) and the Jurassic (Glaub 1994). Off Mauritania, Orthogonum tubulare borings were observed in samples from five stations (Fig. 3). Reticulina elegans (Radtke) Bundschuh 2000 (Fig. 4c) Description. Reticulina elegans borings occur as densely ramified tunnel networks. They are characterized by dichotomous branching, which gives the boring system a zigzag pattern. The tunnel diameter ranges from 2um to 5|im. In the Mauritanian samples studied, Reticulina elegans was generally developed in the substrate around sponge macroborings. There is no clear explanation for this, but it seems as though Reticulina elegans borings were developed after the sponge tissue decomposition. Taxonomic comment. The characteristics of the ichnotaxon Reticulina elegans (Radtke) Bundschuh 2000 correspond to those of the boring pattern known from the modern siphon-

ally organized green alga Ostreobium quekettii Bornet & Flahault 1889. Distribution in modern and ancient environments. A long list of distributional data exists for Ostreobium quekettii, which indicates nearly global distribution. This endolithic chlorophyte is eurybathic and can survive in euphotic intertidal habitats (shaded positions) as well as under dysphotic conditions (deepest record in 220 m water depth near the Bahamas at approximately 0.01 % of surface light; Fredj & Falconetti 1977). The maximum density of occurrence is recorded from the deep euphotic zone. The corresponding ichnotaxon Reticulina elegans is known from Silurian times (Bundschuh 2000). Reticulina elegans is key ichnotaxon of the index ichnocoenosis for the deep euphotic zone (Glaub 1994). Scolecia filosa Radtke 1991 (Fig. 4d) Description. Scolecia filosa is characterized by curved running tunnels of 1-3 jam in diameter. The borings may form dense networks commonly displaying loops. Ramification rarely occurs but, if observed, it shows the typical Xor Y-ramification patterns. Taxonomic comment. The ichnotaxon Scolecia filosa was named by Radtke (1991). Its boring pattern is comparable with that of the modern cyanobacterium Plectonema terebrans Bornet & Flahault 1889. Distribution in modern and ancient environments. In modern environments Plectonema terebrans is abundantly observed. Its bathymetrical distribution ranges from the intertidal to 370m (Lukas 1978). It has been suggested that Plectonema terebrans may live as a facultative heterotroph (Glaub 1994; Glaub et al. 2001). Fossil borings classified as Scolecia filosa are known from deposits as old as the Silurian (Glaub et al. 1999; Bundschuh 2000). The general observation that the boring pattern described is widespread in modern as well as in ancient environments is confirmed by the findings off Mauritania. Also in good accordance with earlier observations (see discussion in Glaub et al. 2001) is the fact that these borings represent bathymetrically the deepest cyanobacterium off Mauritania. Scolecia serrata Radtke 1991 (Fig. 4e) Description. The boring system is identified by thin tunnels 0.5-2um in diameter, running in a slightly zigzag pattern. The borings may form mat-like patches, caused by meandering tunnels developed close to each other. In general, the boring systems are arranged parallel to the substrate surface.

MICROBORINGS IN UPWELLING AREAS

Taxonomic comment. Boring systems in fossil substrates developed similarly to those described above were called S cole da serrata by Radtke (1991). According to her study the producer is unknown, but most probably a bacterium. Distribution in modern and ancient environments. Scolecia serrata is known from different modern environments. Radtke (1991) gives citations for observations in the Atlantic and the Pacific Ocean. According to Radtke (1993), Scolecia serrata is found as deep as 1600m water depth. Although modern borings like Scolecia serrata are recorded from many places (tropical and non-tropical), its taxonomic affinity remains unknown. Findings in ancient deposits are so far restricted to the Tertiary Period (Radtke 1991; Glaub et al 2002). Globodendrina monile Plewes et al. 1993 (Fig. 4f) Description. The boring consists of a spheroid, verrucose swelling (60-100 um in diameter) from which a single tunnel (20-30 um in diameter) branches off, running parallel to the substrate surface. This tunnel continues branching at low angles, forming anastomosing tunnel fusions in some cases. The branching system developed on one side of the globular swelling may be 400 um wide and 500 um long. Hair-like extensions are observed all over the boring system and are also developed as connections to the substrate surface. Taxonomic comment. The taxon Globodendrina monile was erected by Plewes et al. (1993) for borings of a foraminifer. The modern analogue is so far not identified at genus or species level (Cherchi & Schroeder 1991). Distribution in modern and ancient environments. There are a few reports on modern boring foraminifera (e.g. Smyth 1988; Cherchi & Schroeder 1991; Freiwald & Schonfeld 1996). The X-ray documentation of Cherchi & Schroeder (1991) shows both the globular boring system and the producing foraminifera within, although fine details such as hair-like extensions are not visible. Their sample material derives from 180m water depth off Scotland. Hitherto, the casting embedding technique has rarely been used for documentation of modern foraminifera and their borings, which makes comparison of modern and fossil boreholes hard to access. The data on the fossil record of Globodendrina monile date back to the Jurassic (Plewes et al. 1993; Glaub 1994, see 'Semidendrina Form') or even the Carboniferous (personal communication K. P. Vogel, Frankfurt). Future studies should focus on determining its geographic distribution more precisely as it has considerable potential to aid palaeoecological reconstructions.

71

Tripartitum Form (Fig. 4g) Description. The boring system is composed of three parts: (a) interwoven tunnels 1-8 um in diameter; with (b) episodically integrated ellipsoidal to globular enlargements 3-12um in diameter; and (c) in some cases connected to perpendicularly orientated clusters of three or more tunnels (8-20 um in diameter) with slightly pointed tips. Taxonomic comment. The informal name Tripartitum Form' was chosen to describe boring patterns known from the endolithic conchocelis stage of the modern rhodophyte taxa Porphyra and Bangia. Variations of tunnel diameters suggest that different species may have caused the borings observed. Chain-like arrangements of ellipsoidal to globular widenings, which are typical for the fossil biotaxon Palaeoconchocelis starmachii Campbell et al. 1979 and its modern counterpart Porphyra nereocystis Anderson 1892 (in Blankinship & Keeler 1892), were observed only in one sample of the Mauritanian study material. Distribution in modern and ancient environments. As for endolithic red algae, more than 100 modern Porphyra and some Bangia species are known (Guiry & Nic Dhonncha 2002), but their endolithic conchocelis stages are usually not documented with the help of the casting embedding technique. Therefore the comparison of modern and ancient red algal borings is usually performed only for Porphyra nereocystis and Palaeoconchocelis starmachii. Borings, so far summarized under the term Tripartitum Form, occur abundantly off Mauritania (Fig. 3). Pygmy Form (Fig. 4h) Description. Thin tunnels approximately 0.3— 0.5um in diameter running parallel to the substrate surface. They display ramification at different angles, mainly between 60° and 90°, and are slightly curved. Taxonomic comment. The name 'Pygmy Form' was chosen according to the term for those borings observed near the Bahamas (Radtke 1993). The boring organism is yet not identified, but a bacterium is suggested, owing to the extremely small tunnel diameter. Distribution in modern and ancient environments. The Pygmy Form is commonly observed in the Mauritanian samples. It holds the deepest boring record in the Mauritanian samples (station 62, 220-300 m). Together with Radtke's observation near the Bahamas this seems to be only the second record. No fossil record of the Pygmy Form is known, and it is most probable that the extreme small tunnel diameter reduces its preservation potential.

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Echinoid Form (Fig. 4i) Description. The boring system consists of an entry tunnel connected with a fan-like widening. The entry tunnel measures 15-30 um in diameter and is up to 60-70 um long. At the point of entry it is usually orientated at a high angle to the substrate surface, and is recurved back to the surface. Near the latter contact to the surface the tunnel broadens to a flat, lobed, fan-like widening (60-120 um wide and 45-80 um long). The Echinoid Form is characterized by a slightly verrucose surface texture to the boring and has hair-like extensions, which may occur all over the boring, including the contact between the fan-like area and the substrate surface. Taxonomic comment. The informal name 'Echinoid Form' was used by Radtke (1993) for similar modern borings. So far nothing is known about either the taxonomic affinity of the borer or the fossil record of the trace. Distribution in modern and ancient environments. Besides its occurrence off Mauritania, the Echinoid Form is so far only recorded from modern environments near Mexico (Giinther 1990) and near the Bahamas (Radtke 1993). Interpretation

Ichnotaxonomic composition The ichnotaxonomic composition is in good accordance with data from tropical study areas (e.g. Puerto Rico, Budd & Perkins 1980; Bahamas, Radtke 1993). All ichnotaxa described were also found by Radtke in the Bahamas except Orthogonum spinosum. In contrast to her studies, borings similar to the following ichnotaxa are lacking off Mauritania: Scolecia maeandria, Fasciculus parvus, Fasciculus grandis, Eurygonum nodosum and Saccomorpha sphaerula. Comparison with high-latitude assemblages is not possible, because of the small database available. As a result, none of the ichnotaxa observed can be considered to be restricted to the up welling area. The taxonomic inventory of inferred tracemakers is dominated by heterotrophs. Three ichnotaxa are considered to be produced by fungi, whereas one is believed to originate from foraminiferal boring and seven are of unknown origin (most probably bacterial or fungal heterotrophs). In addition, the endolith producing Scolecia filosa borings is suspected to live as a facultative heterotroph, among other reasons because of its deep occurrence (see discussion in Glaub 1994, Glaub et al. 2001). Six ichnotaxa are attributed to photoautotrophs (three to

cyanobacteria, two to green algae, one to a red alga).

Borings and bathymetric interpretation Microborings have been demonstrated to be important tools for palaeobathymetric reconstruction (Glaub 1994, 1999; Vogel et al. 1995; Gektidis 1997). This scheme is based on depthrelated assemblages of traces made by microendoliths. Such assemblages have enabled the identification of subzones of the euphotic zone and the aphotic zone. Subsequent studies have demonstrated the applicability of the scheme to sedimentary basins from the Palaeozoic onwards (e.g. Vogel et al. 1995, 1999; Glaub & Bundschuh 1997; Bundschuh 2000). The euphotic zone covers the supratidal, the intertidal and the well-illuminated sub tidal. The lower limit of the euphotic zone is defined as the depth where the surface light (between 350 and 700 nm wavelength) is reduced to approximately 1% (for definition and further literature see Glaub 1994). This depth is almost identical with the depth at which the photosynthesis rate equals the respiration rate of photoautotrophic organisms. Photoautotrophic endoliths dominate the euphotic zone (e.g. Golubic et al. 1975), many of them being obligate photoautotrophs. However, one living cyanobacterial species (Plectonema terebrans, which produces a boring similar to the ichnotaxon Scolecia filosa) and one living chlorophycean species (Ostreobium quekettii, with its boring being comparable to the ichnotaxon Reticulina elegans) can cope with less than 1% of surface light (between 350 and 700 nm wavelength). Consequently, they are not restricted to the euphotic zone (Glaub 1994, Gektidis 1997, Vogel et al. 1999, Glaub et al. 2001). It demands further investigations to understand how Plectonema terebrans and Ostreobium quekettii can exist under these low light conditions (see discussion in Glaub 1994). On the basis of microboring assemblages, a subdivision of the euphotic zone is given. The shallow euphotic zone I is equivalent to the supratidal zone. In modern environments this is dominated by cyanobacteria capable of protecting themselves from sunburn damage by sheath pigmentation. No index ichnocoenosis for microborings was defined for this zone, because sampling has not been extended to this euphotic subzone so far. The shallow euphotic zone II corresponds to the intertidal zone; its microboring index ichnocoenosis is called the Fasciculus acinosus I Fasciculus dactylus-ichnocoenosis. It is

MICROBORINGS IN UPWELLING AREAS

characterized mainly by cyanobacterial borings orientated perpendicular to the substrate surface, which are able to deal with the changing hydrographic conditions in the intertidal. For shallow euphotic zones I and II the endolithic communities are influenced by both photic and hydrodynamic factors. For this reason, the photic and hydrodynamic zonation units are identical. Shallow euphotic zone III encompasses wellilluminated sub tidal settings. In contrast to the aforementioned subzones, its microboring community is characterized by cyanobacteria along with borings of red and green algae. The predominant boring pattern is perpendicular to the substrate surface. The Fasciculus dactylus/ Palaeoconchocelis starmachii-ichnocoQnosis is the corresponding index assemblage. The deep euphotic zone is defined as the lessilluminated part of the euphotic zone down to approximately 1% of surface light. The endolithic community is dominated by red and green algae. Boring parallel to the substrate surface typifies this association. The index assemblage of this euphotic subzone is called Palaeoconchocelis starmachii/Reticulina elegansichnocoenosis. The dysphotic zone extends from the 1 % level to about 0.01% or 0.001% of surface light. The most characteristic feature of the dysphotic zone is the dominance of chemoheterotrophic endoliths (mainly fungi), which are accompanied by Scolecia filosa and Reticulina elegans. The endolithic assemblage of the aphotic zone consists of heterotrophs only. The index assemblage for this zone is called the Saccomorpha clava/ Orthogonum lineare-ichnocoenosis.

Application of the bathymetric model Several samples collected in the intertidal zone by the land excursion related to the Meteor cruise were considered to be recent. Their microboring assemblage shows the typical elements of the Fasciculus acinosus/Fasciculus dactylus-ichnocoenosis (Glaub 1994). In total, 11 microboring ichnotaxa are observed (three affiliated to cyanobacteria, three to algae, four to heterotrophs and the probably facultative heterotroph producer of Scolecia filosa) (Fig. 3). Thus the identification of the upper euphotic zone II by means of microborings is clearly applicable. Demonstration of upper euphotic zone III is, however, more problematic. Only three samples from station 70 (20 m) clearly show the typical composition of the Fasciculus dactylus/ Palaeoconchocelis starmachii-ichnocoQnosis. For further confirmation, Fasciculus dactylus is

73

developed perpendicular to the substrate surface, and a decrease in cyanobacterial borings is observed compared with the intertidal zone. In contrast to data from tropical study areas (summarized in Glaub 1994), off Mauritania heterotrophs dominate over photoautotrophs (one cyanobacterium, two algae, six heterotrophs and the probably facultative heterotroph producer of Scolecia filosa). The typical change in microendolithic association from the shallow euphotic zone to the deep euphotic zone is also hard to follow off Mauritania. Only one sample from station 55 (25m) clearly shows the Palaeoconchocelis starmachii/ Reticulina elegans-ichnocoQnosis of the deep euphotic zone with parallel development of Fasciculus dactylus. Samples from 41 m water depth down to 74m water depth display a rather confused distribution pattern of microborings. These stations are characterized by Einsele et al. (1977) as mixed assemblages. Older shell material indicating lower water depth is mixed with remains of modern shell-bearing organisms. This fact is confirmed by the findings of the present study. Several samples show a mixture of ichnocoenoses from the shallow euphotic zone, the deep euphotic zone and probably the dysphotic zone, which reflect exposure to different light conditions. Despite this diffuse picture, the occurrence of some microboring taxa is restricted to this bathymetric interval (the borings affiliated to heterotrophs: Polyactina araneola and the Echinoid Form), whereas others have their shallowest occurrence in this mixing zone (the borings affiliated to heterotrophs: Orthogonum spinosum and Orthogonum tubulare) or disappear here (the borings affiliated to photoautotrophs: Cavernula pediculata, Fasciculus dactylus. Fasciculus isp. 1, Reticulina elegans and Tripartitum or lineare, which are considered to have heterotrophic trace-makers, are observed in samples from shallower stations as well as in those from deeper stations, but not in the mixing zone. The zone between 41 m and 74 m shows the usually observed displacement of photoautotrophs by heterotrophs, but there is no clear gradient visible within this zone. The lower boundary of the euphotic zone is expected most probably at the top of the mixing zone or between the 25m and the 41m station. The parallel development of Fasciculus dactylus is considered as an indicator of this boundary. Further support derives from light measurements (Morel 1982) (see below). Samples deriving from 88-89 m (station 66) down to sampling depth 220-300 m (station 62)

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clearly display the characteristics of the Saccomorpha clava/Orthogonum linear e-ichnocoenosis. A variety of borings known from heterotrophs (mainly fungi) were identified, associated by Scolecia filosa (down to 148-149 m). Two samples from 74m (station 58) displaying no features of reworking, in contrast to the other samples from this depth, indicate aphotic conditions. Thus the boundary between the dysphotic zone and the aphotic zone is inferred between 74m and 88-89 m water depth. The absolute depths estimated for the boundaries between the individual zones and subzones are in good accordance with measurements of the optical properties off Mauritania (Morel 1982). Remarkably, they are shallower than those found in other areas of similar latitudinal position. One explanation for this observation is a high load of planktic content and resuspended sediment. Morel's long-term light measurements off Mauritania record 10-60 m for the lower limit of the euphotic zone. According to Jerlov's water type III (Jerlov 1976) the euphotic zone extends to approximately 30 m water depth, whereas the aphotic zone onset is in approximately 70m water depth. Geological significance Most of the ichnotaxa observed in this study are known from the fossil record (14 taxa). Only for the following ichnotaxa are there no clear data or no data at all from ancient deposits: Fasciculus isp. 2, Tripartitum Form, Pygmy Form and Echinoid Form. The high number of Mauritanian ichnotaxa with fossil equivalents makes the results meaningful for palaeoenvironmental studies. As for palaeo-depth reconstructions by microborings, the main requirement is the autochthony of the samples studied, because only in this case do borings of photoautotrophic endoliths reflect light conditions at the depth at which bioerosion occurred. The studies of Mauritanian samples demonstrate that investigations on microboring itself can aid in identification of allochthonous samples, if mixing of ichnocoenoses is observed. I am very indebted to D. Herm, K. Vogel, M. Gektidis and G. Radtke for information, discussion and for making available important samples. My gratitude is extended to A. Mutter, O. Sagert, R. Schade and J. Tochtenhagen for their technical assistance. The investigations were kindly funded by the German Research Foundation (DFG-Projekt Vo 90/21). Sincere thanks go to D. Mcllroy and R. Bromley. The manuscript was very much improved by an intensive, friendly discussion on

ichnotaxonomy with them and by their competent suggestions. An anonymous reviewer also deserves my appreciation.

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GUIRY, M. D. & NIC DHONNCHA, E. 2002. AlgaeBase. www. algaebase. org GUNTHER, A. 1990. Distribution and bathymetric zonation of shell-boring endoliths in recent reefs and shelf environments. Cozumel, Yucatan (Mexico). Fades, 22, 233-262. HOFMANN, K. 1996. Die mikro-endolithischen Spurenfossilien der borealen Oberkreide NordwestEuropas. Geologisches Jahrbuch, A 136, 1-151. HOHNK, W. 1969. Uber den pilzlichen Befall kalkiger Hartteile von Meerestieren. Bericht Deutsche Wissenschaftliche Kommission fur Meeresforschung, 20, 129-140. JERLOV, N. G. 1976. Marine Optics. Elsevier, Amsterdam, 1-231. KONIGSHOF, P. & GLAUB, I. (in press). Traces of microboring organisms in Palaeozoic conodont elements. Geobios. KRUTSCHINNA, J. 1997. Untersuchungen an der Tiefwasserkoralle Lophelia pertusa in verschiedenen pra- und postmortalen Stadien unter besonderer Beriicksichtigung des mikroendolithischen Befalls. Diploma thesis, FB Biologic, Johann Wolfgang Goethe-Universitat, Frankfurt am Main. LE CAMPION-ALSUMARD, T. 1978. Les eyanophycees endolithes marines. Systematique, ultrastructure, ecologie et biodestruction. PhD thesis, Universite d'Aix-Marseille II, Marseille. LEHMANN, E. 1903. Uber Hyella balani nov. spec. Nyt Magazinfor Naturvidenskap, 41, 77-87. LUKAS, K. J. 1978. Depth distribution and form among common microboring algae from the Florida continental shelf. Geological Society of America, Abstracts, 10, 448. MITTELSTAEDT, E. 1972. Der hydrographische Aufbau und die zeitliche Variabilitat der Schichtung und Stromung im nordwestafrikanischen Auftriebsgebiet im Fruhjahr 1968. 'Meteor'-Forschungsergebnisse, A, 11, 1—57. MOREL, A. 1982. Optical properties and radiant energy in the waters of the Guinea Dome and the Mauritanian upwelling area in relation to primary production. Rapports et Proces-Verbaux des Reunions, 180, 94-107. NIELSEN, R. 1972. A study of shell-boring marine algae around the Danish island Laeso. Botanisk Tidsskrift, 67, 245-269. PLEWES, C. R., PALMER, T. J. & HAYNES, J. R. 1993. A boring foraminiferan from the Upper Jurassic of England and Northern France. Journal of Micropalaeontology, 12, 83-89. RADTKE, G. 1991. Die mikroendolithischen Spurenfossilien im Alt-Tertiar West-Europas und ihre palokologische Bedeutung. Courier Forschungsinstitut Senckenberg, 138, 1-185. RADTKE, G. 1993. The distribution of microborings in molluscan shells from recent reef environments at Lee Stocking Island, Bahamas. Fades, 29, 8192. RADTKE, G., HOFMANN, K. & GOLUBIC, S. 1997. A bibliographic overview of micro- and macroscopic bioerosion. Courier Forschungsinstitut Senckenberg, 201, 307-340.

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SCHMIDT, H. 1992. Mikrobohrspuren ausgewahlter Faziesbereiche der tethyalen und germanischen Trias (Beschreibung, Vergleich und bathymetrischen Interpretation). Frankfurter geowis12, 1-228. SCHMIDT, H. & FREIWALD, A. 1993. Rezente gesteinsbohrende Kleinorganismen des norwegischen Schelfs. Natur und Museum, 123, 149-155. SMYTH, M. J. 1988. The foraminifer Cymbaloporella tabellaef shells. Journal of Foraminiferal Research, 18, 277-285. VOGEL, K. & MARINCOVICH, L. in press. Paleobathymetric implications of microborings in Tertiary strata of Alaska, USA. Palaeogeography, Palaeoclimatology, Palaeoecology. VOGEL, K., BUNDSCHUH, M., GLAUB, L, HOFMANN, K., RADTKE, G. & SCHMIDT, H. 1995. Hard substrate ichnocoenoses and their relations to light intensity and marine bathymetry. Neues Jahrbuch fur Geologie und Palaontologie, Abh., 195, 49-61.

VOGEL, K., BALOG, S.-J., BUNDSCHUH, M., GEKTIDIS, M., GLAUB, I., KRUTSCHINNA, J. & RADTKE, G. 1999. Bathymetrical studies in fossil reefs with microendoliths as paleoecological indicators. Profil, 16, 181-191. VOGEL, K., GEKTIDIS, M., GOLUBIC, S., KIENE, W. E. & RADTKE, G. 2000. Experimental studies on microbial bioerosion at Lee Stocking Island, Bahamas and One Tree Island, Great Barrier Reef, Australia: implications for paleoecological reconstructions. Lethaia, 33, 190-204. YOUNG, H. & NELSON, C. 1988. Endolithic biodegradation of cool-water skeletal carbonates on Scott shelf, northwestern Vancouver Island, Canada. Sedimentary Geology, 60, 251-267. ZEBROWSKI, G. 1936. New genera of Cladochytriaceae. Annals of the Missouri Botanical Garden, 23, 553564. ZEFF, M. L. & PERKINS, R. D. 1979. Microbial alteration of Bahamian deepsea carbonates. Sedimentology,26, 175-201.

Climatic control of trace fossil distribution in the marine realm ROLAND GOLDRING1, GERHARD C. CADEE2, ASSUNTA D'ALESSANDRO3, JORDI M. DE GIBERT4, RICHARD JENKINS5 & JOHN E. POLLARD6 l

Geoscience Building, School of Human and Environmental Sciences, University of Reading, Whiteknights, PO Box 227, Reading RG6 6AB, UK (e-mail: [email protected]) 2 Netherlands Institute for Sea Research, NIOZ, Paleobiology Department, PO Box 59, 1790 AB Den Burg, Texel, The Netherlands (e-mail: [email protected]) 3 Dipartimento di Geologia e Geofisica, Universita di Bari, Campus universitario, Via E. Orabona 4, 70125 Bari, Italy (e-mail: [email protected]) 4Departament d'Estratigrafia, Paleontologia i Geociencies Marines, Universitat de Barcelona, Marti Franques s/n, 08028, Barcelona, Spain (e-mail: [email protected]) 5 The South Australian Museum, North Terrace, Adelaide, South Australia 5000 (e-mail: [email protected]) 6Department of Earth Sciences, University of Manchester, Manchester Ml 3 9 PL, UK (e-mail: John .pollard@ man.ac. uk) Abstract: Modern coastal and shoreface faunas exhibit strong latitude (climate) controlled distributions. In contrast, most ichnotaxa are long-ranging, and ichnofacies are widely distributed geographically. This is readily explained by the dominantly warmer and more equable climates of much of the past, as well as the diversity of the producers of most ichnotaxa. Nevertheless, in the Pleistocene, and in the Eocene, cool-water ichnofabrics can be recognized. The latitudinal distributions of thalassinidean crustaceans and infaunal spatangoid echinoids are examined because of their propensity to form distinctive and often abundant trace fossils. Three climatic zones are tentatively recognized from modern shore and shoreface sediments, and which are considered to extend back to the Mesozoic: tropical and subtropical with pellet-lined burrows (Ophiomorphd), echinoid burrows and other traces; temperate with echinoid burrows and mud-lined or non-lined thalassinidean burrows (Thalassinoides), but without Ophiomorpha; and arctic (cold waters) with only a molluscan and annelid trace fossil association. Examples demonstrating this climatic trend are drawn from the Cenozoic and Pleistocene.

Trace fossils, with the exception of those utilized in Palaeozoic ichnostratigraphy and footprint stratigraphy, are well known for their long stratigraphic ranges, and sedimentological research has focused on the ichnofacies concept and the relative constancy of the Seilacherian ichnofacies through the Phanerozoic. Ichnotaxa are generally regarded as having wide geographical extent as well as long time ranges. This is undoubtedly, in part, attributable to the more equable climates that prevailed over much of the Phanerozoic, as well as the recognition that individual longranging ichnotaxa were formed by a variety of different animals. However, there are rare to uncommon ichnotaxa, and others that are more common, that have more restricted distributions, For example, Diplocraterion of decimetre depth and width, are known from the Cambrian to the Miocene. Taylor et al. (2003) pointed out that specimens with these dimensions are unknown from modern environments, and hinted that the originator may be known off tropical deltas, but

not its burrow. Diplocraterion is, of course, generally recognized as the work of a range of animals, and at present it is impossible to recognize any general climatic control in its distribution. Differences in trace fossil suites of climatically distinct continental sediments are well established for the Permian (e.g. desert sandstones, Brady 1947; Braddy 1999), lacustrine red-beds (Walter 1980, 1982, 1983), proglacial lakes (Savage 1971; Anderson 1981), and Triassic lacustrine red-beds (Clemmensen 1979). The evolutionary trends in environmental expansion and ecospace utilization seen in continental ichnofaunas (Buatois & Mangano 1993; Buatois et al. 1998), especially insect trace fossils in palaeosols (Genise et al. 2000), in part reflect their response to climate. Pemberton et al. (1992) referred to the distinctions between early Cretaceous, late Cretaceous and Cenozoic suites of trace fossils in non-marine environments, which may be readily attributable to the evolution of angiosperms, insects and crustaceans rather than climate per se. Savrda (1995) discussed the

From: MclLROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 77-92. 0305-8719/04/S15.00 © The Geological Society of London.

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application of ichnology to understanding climatically controlled decimetre-scale bedding rhythms, scour and redox cycles. Stratigraphic facies shifts of Ophiomorpha and Zoophycos have been documented (Bottjer et al 1987, 1988), but these do not refer to climatic control; rather Glaub (1994, 1999, 2004), Perry (1998) and Vogel et al (1995) showed how the distribution of microborings in skeletal carbonates is determined by photic control (depth), and Glaub et al (2002) and Vogel & Marincovich (in press) were able to extend this to latitudinal trends. Climatic effects on trace fossil distributions will obviously be most apparent in continental, marginal marine and neritic facies, because of the greater diversity and disparity present in the equivalent environments than in pelagic facies. To establish such effects in the fossil record it is necessary to show that one is dealing with ichnological differences between similar depositional environments that are of similar age. It is also important to attempt to demonstrate that absence of an ichnotaxon is due to climatic rather than to other palaeoecological factors. Absence of an ichnotaxon from a facies in which it would be expected to occur may be due to real absence (impossible to prove positively), or apparent absence due to hydraulic factors (i.e. it was once there, but was removed by penecontemporaneous erosion), or apparent absence due to loss of identity by reworking. It is in the last situation that the role of tiering in aggradational settings is significant, because the traces of deeper-tier burrows tend to overprint and eliminate the activities of shallow-tier bioturbators from the rock record, leading to a distinct bias in the ichnofabric and preferential preservation as elite traces (Bromley 1996; Goldring et al. 2002). Cadee (2001), in a section of his review paper on the sediment dynamics of bioturbating organisms in the coastal zone, drew attention to the modern latitudinal variation in bioturbation, and certain changes in the diversity of groups of organisms with latitude. The purpose of our contribution is to demonstrate that such changes may with confidence be related to climatically induced latitudinal change, and can be recognized in the fossil record. We extend this discussion to include the shoreface, which is of greater geological significance, but lack of information prevents extension to arctic shoreface settings. Cadee (2001) considered two aspects that might be applied to modern bioturbated sediments: first, that the overall degree of sediment reworking between arctic coasts and warm coasts increased more than fivefold. But such measurements cannot be applied to fossil examples, where sedimentation rate and event

stratigraphy greatly influence the degree of bioturbation, as discussed by Taylor et al. (2003) and Mcllroy (2004). Ausich & Bottjer (1982) and Bottjer & Ausich (1982) showed that the beginning of the Mesozoic witnessed a massive increase in the depth of bioturbation, though this may be somewhat modified if the 1 m depth of Thalassinoides reported by Droser & Bottjer (1989) in Ordovician limestones is included. Droser & Bottjer (1988, 1993) and Bottjer et al (2000) considered that they could detect a general increase in the degree of bioturbation seen between late Neoproterozoic-Cambrian and Ordovician sediments of the shoreface, which they attributed to the increase in diversity and abundance of the infauna. Secondly, Cadee (2001) referred to, and expanded on, the increase in diversity of coastal life from high to low latitudes. Diversity is not readily or reliably applied to ancient bioturbated sediments, because the effects of salinity and the colonization window have to be eliminated (Taylor et al. 2003), and because ichnodiversity is quite distinct from the diversity of body fossils. But changes in diversity through a succession are most useful, especially when applied to the recognition of marginal marine facies. To apply diversity change to ancient sediments requires extensive examples of more or less contemporaneity. Cadee (2001) drew attention to the lack of a diverse fauna of callianassids and crabs and their activities from arctic and temperate coasts in contrast to their richness in warm waters. This is of much geological interest because the trace fossils formed by these crustaceans are so conspicuous in the fossil record. But body fossils are very uncommon, and there are only a few distinctive types of burrow. Independent criteria that may be used to assess palaeoclimate of an ichnological section are: overall palaeogeography and regional climate; facies, especially those that are climatically sensitive - glaciogenic, desert; information from associated body fossils, particularly diversity, and information from modern and ancient associations in carbonates (foramol, chlorozoan, Lees & Buller 1972; Lees 1975); morphological factors such as shell thickness, growth banding in plants, corals and shells (Insalaco 1996), and geochemistry/isotope geochemistry of shells; abundance (frequency) - a complex ecological factor, but one that may be of value when the overall picture is taken. We first discuss aspects of the distributions and ichnology of typical crustaceans and spatangoid

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echinoids, before considering some case studies and drawing conclusions. Ichnology of certain crustaceans Dworschak (2000) reviewed the present-day latitudinal distribution of thalassinidean species, which show the highest diversity between 3040°N and 20-30°S (Fig. 1). Thalassinideans are unknown from coastal and shoreface sediments in latitudes higher than about 70°N and 50°S. Many species construct substantial burrow systems to appreciable depths (partially reviewed by Bromley 1996). Griffis and Suchanek (1991) recognized four types of thalassinidean burrow, to which we add additional types (of unproven thalassinidean origin) from the fossil record (below). The burrows are commonly, but not invariably, lined (when excavating a relatively loose substrate). The lining can be of several types: mucus, which is unlikely to be preserved fossil; a mud lining (see Bromley 1996), which may be of different thickness in different taxa; a mud lining pressed into the burrow margin (tamping, Stamhuis et al 1996); or a pelleted lining (the hard lining of Griffis & Suchanek 1991), associated with type 4 burrows (Griffis & Suchanek 1991). The pellets are inserted by the animal into the

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burrow margin, and then smoothed off on the inside (Frey et al. 1978). Type 4 burrows are distinguished by the absence of sediment mounds, but with restricted apertures, and a deep reticulate system of galleries. It is the pelletal lining that is so readily observed in the fossil record. Such traces are often referred to Ophiomorpha, even though the diagnostic burrow morphology of Ophiomorpha may not be apparent. The pellets of Ophiomorpha vary considerably in composition between occurrences, though without apparent facies change. Most commonly they are of muddy sand (as described by Frey et al. 1978), in which case they do not compact. In other cases they may be more muddy, or rich in plant debris, and may become strongly compacted on burial (Pollard et al. 1993). Frey et al. (1987) also described a combination of a pelletal lining, and a further mud inner lining. Other crustaceans make lined burrows, and in the stomatopod Squilla, the mud lining may be up to 5 mm thick for a burrow 10-20 mm diameter (Hertweck 1972). Unattributed crustacean burrows constructed in Cretaceous Chalk, representing a lithified coccolith ooze, may also be lined by fish bones and scales (Bromley 1996), gymnospermous leaves or echinoderm elements (Bather 1911), or remain unlined. It is the lining, or its absence, as well as the overall burrow morphology, that is of

Fig. 1. Global map with indications of latitudinal distributions of Callichurus major, thalassinideans and infaunal echinoids. (1) 70°N to 50°S, northern and southern limits of thalassinidean, and northern limit of infaunal echinoids. (2) 34°N to 27°S, northern and southern limits of Callichurus major. (3) 42°S probable southern limit of infaunal echinoids.

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main concern to palaeontology, and it is unfortunate that these features have been less considered by biologists, for obvious practical considerations. Three ichnogenera are widely recognized, and attributed to the work of thalassinidean, or thalassinidean-like crustaceans: Ophiomorpha, Thalassinoides and Spongeliomorpha (but see Fursich 1973; Schlirf 2000). The overall burrow morphology generally relates to type 4 of Griffis & Suchanek (1991). In Thalassinoides the margin of the burrow is smooth, whereas in Spongeliomorpha the margin displays distinct bioglyphs. Other features used to distinguish crustacean ichnotaxa are discussed by Schlirf (2000, and references therein). Five other crustacean ichnotaxa may also be considered. The upright spiral Gyrolithes may lead into one or other of the three ichnotaxa. It generally does not have a pelletal lining. The sinuous Sinusichnus (Gibert 1996a, Gibert et al 1999) from the Pliocene of the northwestern Mediterranean, which has an unlined burrow, was probably constructed by a decapod crustacean. Pliocene examples of Psilonichnus (Fursich 1981), with a Y-, J- or Uform burrow, were linked to Pholeus (Nesbitt & Campbell 2002), and to thalassinideans (Gingras et al 2000), though Frey et al. (1984) interpreted

similar burrows as the work of the ghost crab Ocypode (ocypodid). The upright bow-form burrow Glyphichnus (Bromley & Goldring 1992), with crustacean-type bioglyphs, from lower Cenozoic sediments, may be linked with certain Cylindrichnus (Goldring et al. 2002). Associated Meyeria sp. (Glypheoidea) were probably responsible for Lower Cretaceous examples (Goldring 1996). Thalassinoides, Ophiomorpha and Spongeliomorpha may all be present in one specimen to form a compound trace fossil, reflecting the need of the constructor to deal with the burrow margin in different ways. Clearly, with a firm substrate the organism needs only to render the margin attractive/unattractive to intruders or microorganisms. The common observation in sandy sediments is for a pellet-lined upper section of the burrow system to pass down to unlined or top-lined galleries, reflecting passage into a firmer level. Some species of callianassid thalassinideans appear to colonize a range of substrates: sand, sandy mud, silty mud. Those that pellet-line their burrows and live in coastal sands, or in sandy shoreface settings (Fig. 2 right), appear to have a restricted latitudinal range. Thus, on western Atlantic coasts, Callichurus major,

Fig. 2. (a) Diagram to show the formation of meniscate backfill by the modern, globular echinoid (Echinocardium cordatum) in longitudinal and transverse (centre) section (modified from Bromley & Asgaard 1975), and a wedge-shaped echinoid (Schizaster). Spines are shown diagrammatically. An active respiratory shaft (funnel) and an abandoned shaft indicate advance of Echinocardium. (b) Composite diagram of an Ophiomorpha-Planolites-mottlQd ichnofabric, with restricted shaft, constricted aperture, and lined, top-lined and unlined portions of shafts and galleries.

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forming Ophiomorpha-typeburrows, does not today extend north of the North CarolinaSouth Carolina state boundary (about 34°N) (personal communication Curran 2002) or south of southern Brazil (about 27° S) (Dworschak 2000). Thalassinideans in muddier sediment are present to higher latitudes: e.g. Callianassa subterranea in the North Sea (Reineck 1963; Stamhuis et al 1997) and their burrows, which are not pellet-lined, may be referred to Thalassinoides or Spongeliomorpha. For fossil distributions the question to be raised is whether modern distributions can be applied to ancient occurrences, because different taxa will have been responsible (at least at lower orders). The question that must also be posed is whether the recognized ichnospecies of Ophiomorpha, distinguished on the form and arrangement of the pellets lining the burrow margin, had the same or different ecologies, e.g. O. nodosa, O. annulata and O. irregulaire. Tentatively, we suggest that occurrences of pelletlined burrows in shoreface, lower beach and estuarine sandy sediments represent complex constructions in warm tropical to subtropical environments by decapod crustaceans. Although there is little likelihood for confusing Ophiomorpha and Thalassinoides, it is useful to keep in mind the nature of the substrate associated with each occurrence. The Upogebiidae appear to have a similar latitudinal range to the Callianassidae (Dworschak 2000). The burrows are of type 5 (Griffis & Suchanek 1991), and of the form generally referred to the ichnotaxon Psilonichnus. The type ichnospecies, P. tubiformis, is unlined, as are other ichnospecies, though P. upsila (Frey et al. 1984) and P. lutimuratus (Nesbitt & Campbell 2002) have a distinctive mud lining. Spatangoid echinoid ichnology Burrowing echinoids evolved rapidly in the Cenozoic, though many Mesozoic echinoids were able to plough through the sediment. Tchoumatchenco & Uchman (2001) record the oldest deepwater Scolicia that is reasonably attributable to an echinoid, from the Tithonian (latest Jurassic), though the age of the oldest shallow-water three-dimensional backfilled burrow that can be related to an echinoid is much less certain, probably because of the low preservation potential (below). Spatangoid echinoids burrow to depths no greater than 20cm, and mostly shallower (reviewed by Bromley & Asgaard 1975) (Fig. 2 left). Their burrows can be very abundant as well as prominent, especially in

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relatively poorly sorted sandy sediment (Kanazawa 1992, 1995). Sand dollar activity has not yet been recorded from ancient sediments. Echinoid burrows are found in lower beach, upper and middle shoreface settings, and, off the Georgia coast (Dorjes 1972), in offshore relict sands, as well as deeper-water environments (below). The latitudinal distribution of shallow marine infaunal spatangoids extends from the tropics to 70°N (North Cape) for African and European species (Hayward & Ryland 1990), and to at least 42°S (Fell 1948) (Fig. 1), thus considerably greater than that of crustaceans forming pellet-lined burrows. The ichnotaxonomy of spatangoid locomotion-feeding trace fossils was reviewed by Uchman (1995). Bichordites refers to traces made by Echinocardium-typQ spatangoids, with a single drain, and Scolicia (and associated preservational variants) to Spatangus-typQ echinoids, with a double drain (Fig. 2 left). The resting trace Cardioichnus may be found in close association with Scolicia or Bichordites (Smith & Crimes 1983). Discussion on modern distributions of infaunal echinoids and Ophiomorphaforming crustaceans The present latitudinal distributions of infaunal echinoids and burrowing crustaceans would seem to offer opportunities for application to fossil occurrences, especially because of their distinctive traces. But there are several problems, not only with respect to the well-known difficulty of extending modern ecologies to those of fossil taxa. It is important to understand the preservation potential and regional facies distribution of the traces. We thus address ecological aspects, and the sedimentologically related aspects of tiering and tier preservation. Perhaps the first consideration is in respect of the salinity tolerance of each group: echinoids, being stenohaline, are normally excluded from estuarine environments, whereas crustaceans enjoy a wider salinity tolerance. Where infaunal echinoids and burrowing crustaceans are present in the same general area, such as the southern North Sea (Reineck 1963) and Georgia coast (Dorjes 1972), they tend to occupy different specific areas, with different sediment characteristics, and penetrate to different tier levels. Two distinct infaunal activities are represented: locomotion/feeding traces by echinoids, and more or less permanent to semi-permanent dwelling structures by crustaceans. The latter require wall stabilization and thus the availability,

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amensalism by other bioturbators. Although forming relatively deep burrows in the lower beach and shoreface they must maintain a connection with the substrate surface by constructing a ventilation shaft, which must be regularly replaced as the animal progresses forward (Fig. 2 left). Any obstruction to this upwardly built shaft is disadvantageous to the animal below. Burrowing echinoids would be inhibited from colonizing areas with pellet (hard)-lined shafts of Ophiomorpha, or lined tubes of Skolithos (cf. sand mason Lanice). On the other hand, population disturbance by the activity of the burrowing echinoid Brissopsis lyrifera (Widdicombe & Austen 1998) had a marked negative effect on overall diversity. We also note the effect on deepwater spatangoids on the preservation of Zoophycos, with truncation of the initial stages of construction (Kotake 1989). We restrict our discussion to shallow marine environments and facies, because arthropod and echinoid distributions in modern deepwater, basinal environments have not yet been fully analysed in respect of ecological parameters. Mesozoic and younger turbidites yield many examples of Ophiomorpha and Scolicia, but the environmental factors that determine their distriBelow a distal turbiditic event bed, or storm butions are largely unknown. Furthermore it is event bed, where little (minimal) penecontemnot clear whether constructor populations were poraneous erosion had taken place prior to necessarily endemic. Occurrences of Ophiomorthe event sedimentation. This is the classic pha in basin-floor settings have been frequently situation for the preservation and casting of regarded as the work of relocated individuals pre-event, shallow graphoglyptids on the (e.g. Follmi & Grimm 1990), though Uchman soles of distal turbidites (Orr 1994; Uchman (1995) showed that Ophiomorpha in Miocene 1995). flysch must be regarded as a normal component Close to the upper surface of event beds, where of the deep-sea trace fossil assemblage. The the event bed was colonized temporarily prior same problem applies to deep-sea echinoids, to the renewal of 'normal' sedimentation. but it may be considered that, following initial In inclined heterolithic sedimentation, with relocation from the shelf, populations became thin amalgamated or closely spaced event adapted to the deep seafloor (Wetzel & beds, where little erosion and little sedimentaUchman 2001). Trace fossils are notoriously tion took place between successive events. poorly preserved in hemipelagic mudrocks Gibert (1996b) described this situation from between turbidite beds. But photographic studies the Middle Jurassic of Central England, in and modern deep-sea cores (Fu & Werner 2000), oolitic and bioclastic grainstones, where each from coarse silts and fine sands from the north'event' is capped by a thin micrite. Deposition east Atlantic, show that infaunal echinoids, and took place in a marginal marine environment, their activity, are of common occurrence. Overpossibly representing lateral deposition on a printing of shallower tiers is to be expected, 'point bar'. though Baldwin & McCave (1999) recorded In association with large-scale cross-stratificafrom their sampled area off Nova Scotia the tion, with impure packstone event units (0.1ephemeral nature of the shallower tiers due to 1.0m thick), as described from Rhodes and penecontemporaneous erosion associated with Italy (below), where the 'events' represent frequent benthic storm events. Fu (personal comgrain-flow avalanching off the edge of a promunication 2000) suggested that the tiering in grading platform. Similar preservation is to be deep-sea sediments is often a temporal matter, expected under avalanching in other situations. and deeper-tier burrowing organisms are inhibA further problem for the preservation of ited by an abundance of burrowing echinoids. echinoid burrows is that they are subject to Temporal aspects, amensalism and population

either in suspension, within the sediment, or at or at the sediment-water interface, of fine-grained mud or organic matter. Both groups construct burrows to an appreciable depth, but whereas many fossil and recent crustaceans' burrows extend to a metre or more, and may be regarded as deep-tier structures, echinoid burrows are relatively shallow-tier. This is an aspect that relates to the relative preservation of the tiers, especially, under conditions of more or less continuous sediment aggradation. It is to be expected that spatangoid traces would be readily overprinted by other deeper traces, and possibly eliminated from the fossil record. Indeed, observations by JMdeG and RG on the Miocene shelfal sandy biosparites of Alicante (Spain) (below) show that this is indeed the case. But only in the Miocene of the Maltese Islands have echinoid burrows been strongly overprinted to apparent obscurity by deeper-tier crustacean activity (below). Preservation of shallow tiers may be favoured by several situations (Taylor et al. 2003; Mcllroy 2004), in which event beds are particularly involved (we do not refer to tier preservation associated with geochemical changes, such as oxygenation, e.g. Wignall 1994):

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density are not readily assessed for ancient body or trace fossils, but are relevant factors in respect of shallow as well as deep marine environments. It follows from the above discussion that turbidites with Ophiomorpha, owing to their being relocated animals, might have originated in tropical or subtropical waters. However, we reserve judgement on endemic populations, except that, following Dworschak (2000), it is unlikely that water depth would be greater than 2000m. Further, Tchoumatchenco & Uchman (2001) pointed out that the environmental range of trace fossils is considered mainly at the ichnogenus level, but the ichnospecies of Ophiomorpha in deepwater facies differ from the common O. nodosa of shallow water, in being mostly hypichnia with long, smooth segments. Uchman (1999) attributed this to substrate conditions, but barrier prospecting (Jensen & Atkinson 2001) may also be an explanation. Off the Georgia coast (Dorjes 1972), where Callichirus major is common in the beach environments, the echinoid Moira bioturbates medium- to coarse-grained relict sand, which may be too clean for thalassinidean burrowing. In contrast, in the southern North Sea Callianassa subterranea occupies silty mud, whereas Echinocardium cordatum occupies fine- to medium-grained sand. In the Gulf of Gaeta (Mediterranean) (Dorjes 1971) recorded Echinocardium cordatum but no extensive activity by thalassinideans. The spatial separation of modern echinoids and thalassinideans seems to be related to substrate preference, but it might be expected that pellet-lined burrows would be formed in sandy substrates in the North Sea if higher temperatures were present! The thalassinidean present in the North Sea forms Thalassinoides-type burrows, but not Ophiomorpha. We have few data from modern coastal waters, and there are probably many more records available from the Pacific and Indian oceans. We can tentatively recognize three latitudinal zones in modern coasts and shoreface settings: in the tropics and sub tropics, where pelletforming thalassinideans and burrowing echinoids are present, though in different facies; in temperate latitudes, where spatangoid echinoids are present with Thalassinoides producers (in different facies), but not Ophiomorpha producers; in arctic latitudes, where neither group is present, and the principal bioturbators are annelids and bivalves (Aitken et al. 1988). We can apply this model to Pleistocene and Cenozoic occurrences with a certain degree of confidence, depending on the data available,

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but with caution to Cretaceous and older settings in respect of the distribution of Ophiomorpha producers. Ophiomorpha and spatangoid trace fossils With infaunal echinoid trace fossils not being recorded prior to the Tithonian (Tchoumatchenco & Uchman 2001) (and mostly later), their usefulness as climatic indicators is stratigraphically limited. Echinoid burrows have been extensively described from the Pleistocene of the Mediterranean, though less frequently from Cenozoic strata. Ophiomorpha is commonly recorded from older Mesozoic sediments and onwards. Tropical and subtropical associations (Table 1) Eocene of southern England Pollard et al. (1993) described several ichnofabrics containing Ophiomorpha from units of the Eocene in southern England where the ichnotaxon is prominent (Fig. 2 right), and made comparisons with other occurrences, especially from the southeast coast of the USA (Frey et al. 1978). Muddier units contain abundant spatangoid echinoids (Lewis 1989). Echinoid burrows were tentatively recognized in the highly bioturbated mudstones (Pollard et al. 1993). The climate of the Eocene is generally recognized as subtropical (Purton & Brasier 1997), with a flora that is compared to that of southeast Asia today (e.g. Collinson 1983). Oligocene of New Zealand Ward & Lewis (1975) recognized well-preserved spatangoid burrows in the Arno Limestone, a bioturbated calcarenite with associated largescale cross-beds, and scour channels. Ophiomorpha nodosa is uncommon, and Ward & Lewis (1975) noted the somewhat disparate occurrence of echinoid burrows (Scolicid) and Ophiomorpha, though with extensive overlap. Scolicia is also recorded in association with Cardioichnus from the Upper Miocene to Lower Pliocene (Gregory 1985). Miocene of Austria, Denmark and Poland Radwanski et al. (1975) and Radwanski (1977) described echinoid burrows, which can be assigned to Bichordites, in shallow marine heterolithic sands and muds from Denmark, in association with a diverse ichnofauna, including Ophiomorpha nodosa. O. nodosa is not, however, present in the same lithology as Bichordites.

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Table 1. Occurrences of Bichordites/Scolicia and Ophiomorpha in tropical/ subtropical, temperate and arctic climatic zones in the Cenozoic. For most, the palaeoclimate can be corroborated from associated body fossils Arctic

? Pliocene, Alaska

Temperate

Eocene/Oligocene, South Australia Pliocene, Washington State, USA ?Pleistocene, Washington State, USA Pleistocene, Mediterranean ?Pleistocene, Korea

Tropical/subtropical

Eocene, southern England Oligocene, New Zealand Miocene, Amazonia, Brazil Miocene, Austria, Denmark and Poland Miocene, Mediterranean Miocene, Patagonia, Argentina Pliocene, northwestern Mediterranean Pleistocene (Tyrrhenian), Tunisia Pleistocene, Jamaica

Radwanski et al. (1975) noted that the depositional environment was similar to that described from the modern southeastern North Sea coast, though recognizing the warmer climatic conditions from the presence of warm water molluscs, and burrows that they attributed to warm water holothurians and amphipods. Uchman & Krenmayr (1995) described occurrences of Ophiomorpha and Bichordites from shoreface sands and muds (molasse facies) of the Lower Miocene of Austria. This represents the oldest record of Bichordites. IScolicia and Ophiomorpha were also recorded from transgressive siliciclastics in SE Poland (Rajchel & Uchman 1999). Miocene and Pliocene of the Mediterranean area The molluscs and corals show that the climate of the Mediterranean area during the Middle to late Miocene was warm to subtropical. Ophiomorpha nodosa has been described from a number of sites (Table 1). In Alicante (Spain) Bichordites dominates the sandy biosparites of the Middle to late Miocene in the Bateig Hill area of Novelda, where the Miocene sediments occupy one of the small foreland Eastern Prebetic Basins (Sanz de Galdeano & Vera 1992). Bateig Hill is extensively quarried for architectural stone (Bland et al. 2001), and the variety Bateig Fantasia displays Bichordites ichnofabric (Fig. 3) in 5-20 cm thick, planar event beds, occasionally associated with Maretia sp. O. nodosa is occasionally present, and is more prominent in stratigraphically adjacent facies, where it may be the dominant ichnotaxon. In the Maltese Islands, in contrast to Alicante, shallow, pelagic wackestones of the Globigerina

Limestone Formation of the Maltese Islands, neither echinoid burrows nor O. nodosa are common. This is in spite of the high frequency and diversity of infaunal echinoids (especially Schizaster), which are in or close to life position (Rose 1974; Rose et al. 1992). Echinoid burrows have been observed (Goldring et al. 2002) at a few specific levels associated with event beds, where the shallow tiers have not been overprinted by deep-tier, bow-form burrows in Cylindrichnus-mode preservation (Goldring et al. 2002). The Pliocene marginal marine basins of the northwestern Mediterranean were described by Gibert & Martinell (1998, 1999). Echinoid burrows are present in the Roussillon Basin in

Fig. 3. Bateig Fantasia, Miocene, Alicante. Slab cut parallel to stratification with Bichordites isp. cut by other traces.

CLIMATIC CONTROLS ON MARINE ICHNOLOGY

sandy blue clays in the Planolites—TeichichnusThalassinoides association. In the contemporaneous Baix Llobreget Basin, Scolicia is present in the more distal deposits whereas Ophiomorpha is present in more proximal and coarser sandy channel sediments, associated with Macaronichnus isp. and Skolithos linearis. The bioturbation index does not exceed BI2. The Lower Pliocene sediments are regarded as having been deposited under warm water conditions (Martinell et al. 1984). Miocene of Patagonia In the Lower Miocene of Patagonia (Argentina), Ophiomorpha nodosa is prominent in shoreface silty sandstones of Chubut Province (Buatois et al. 2003; Carmona & Buatois 2003), and a diverse suite of trace fossils is present in associated lower shoreface sediments. Scolicia is recorded. The authors suggested that the climatic signature was cold-temperate due to polar currents, based on independent analysis of microfossil elements, and cetacean and penguin skeletal remains. This would appear to question our model. However, the palaeoclimatic model for the Middle Miocene of the area (Valdes et al. 2000) predicts warmer conditions, not dissimilar from those of the Miocene of the Mediterranean area (above). Also, evidence from insect trace fossils in palaeosols, fossil plants and mammals in the contemporary Punturas Formation in inland Patagonia at the same latitude indicates a tropical to subtropical climate (Bown & Laza 1990). Miocene of Brazil Gingras et al. (2002) described inclined heterolithic stratification (tidally influenced point bar sediments) from the Miocene of Amazonia, deposited in an environment of fluctuating salinity, with the co-occurrence of Ophiomorpha and Scolicia, the latter indicating colonization during saline incursions. Pleistocene of Jamaica and Tunisia Bichordites is also recorded from the Pleistocene of Jamaica (Pickerill et al. 1993), in stormdominated, medium- to coarse-grained, commonly amalgamated sandstones. Most examples are seen only on upper surfaces, but in association with Ophiomorpha, Thalassinoides, Skolithos and rare Chondrites. The occurrence probably represents warm conditions and thus a mixed association. In the late Pleistocene (Tyrrhenian) of Khunis (Tunisia), Plaziat & Mahmoudi (1988) and Mahmoudi (1988) drew their examples of Bichordites from a Skolithos ichnofacies association with

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Ophiomorpha, Monocraterion, Skolithos and small, curved burrows, with the only mollusc being Loripes lacteus (a lucinoid bivalve). The Tyrrhenian of the Mediterranean represents warm-water conditions (Amore et al. 2000).

Temperate associations The Lower Tertiary of South Australia Ophiomorpha is not observed in the well-known and extensive Tertiary deposits of southeastern Australia, probably a reflection of the high to mid-latitudinal placement of this region, lending support to James's (1997) and James et al.'s (1997, 2001) recognition of many marine calcarenites of this province as 'cool-water carbonates'. Most of these carbonates are principally made of current transported bryozoan remains, and distinctive trace fossils are rare in the more offshore shelf settings. However, distinctive and exquisitely preserved crustacean remains, generally of characteristic Indo-West-Pacific aspect, are common (Jenkins 1977, 1985). Despite the abundance of echinoids in Australian basinal calcarenites, examples of their traces are rare. A spectacular bed of Scolicia with straight and closely meandering backfilled burrows 8-10 cm wide forms the upper threequarters of a 1 m thick cross-bedded shoreface, sand-rich calcarenite above the major flooding surface, marking the base of the transgressive Early Oligocene Aldinga Member of the Port Willunga Formation at Port Willunga (Fig. 4). A likely echinoid producer is occasionally found in association with the traces, the bigger examples of which can be attributed to Meoma

Fig. 4. Scolicia at the base of the transgressive Early Oligocene Aldinga Formation at Port Willunga. Hammer graduated in 5 cm intervals.

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tuberculata (McNamara et al. 1986), but fragments of another unidentified smaller spatangoid are also present. Thalassinoides appears with the transgression at the beginning of the Late Eocene, and at Port Noarlunga, south of Adelaide, metre-deep examples penetrate into cracks in the pre-lithified Tortachilla Limestone before ramifying the soft, decalcified underlying sands. Subpolygonal developments of the ichnotaxon are silicified in the overlying lower parts of the Gull Rock Member of the Blanch Point Formation. However, it is in the Late Oligocene to older Miocene of the Port Phillip basin, Victoria, where common phosphatized examples of Thalassinoides include associated remains of producers such as Ctenocheles (Callianassidae). Paired claws of Callianassa that died in their burrows are seen in the shoreface Pliocene at Port Willunga, south of Adelaide. The fully articulated, stalk-eyed crab Ommatocarcinus, clearly indicative of a warm-water influence during the late Early and Middle Miocene, apparently died in sloping burrows which show a stacked septation (Jenkins 1975). The abundant foraminiferal, faunal and floral indications of warm climatic episodes in southern Australia during the Cenozoic are well documented (Wright et al. 2000).

Swinbanks & Murray (1981) noted that the former lines its burrow with mud and mucus, whereas the latter does not line its burrow. Only the Pleistocene deposits contain Ophiomorpha associated with bay sediments. Although the authors do not explain whether the Pleistocene deposits represent glacial or interglacial sedimentation, we suggest that they may represent interglacial sedimentation, warmer than at present.

Pleistocene of Mediterranean The Pleistocene of the Mediterranean offers a number of examples of shallow water, shoreface facies, several of which show abundant traces due to burrowing echinoids. D'Alessandro & Massari (1997), in a comprehensive and integrated study of the Pliocene and Pleistocene deposits in southern Italy, showed that the Middle Pleistocene (Calcarenite della Casarana) contains a cool-water (foramol) fauna including Arctica islandica (Raffi 1986). Facies 1 includes metre-scale cross-stratification with laminatedto-bioturbated calcarenitic event beds. The bioturbated intervals are 5-10 cm thick, with Bichordites isp., whereas the laminated part can be much thicker. In some intervals the bioturbated beds are complex and up to a metre thick. The laminated-to-bioturbated units represent grain flow avalanche deposits, subsequently burrowed. Pliocene of Washington State, USA Ophiomorpha is recorded as very rare. The clinoCampbell & Nesbitt (2000) and Nesbitt & Camp- form stratification is truncated by a unit (up to bell (2002) have provided a convincing account 2 m thick) of fine-grained calcarenites, intensely of an active margin, storm-flood influenced bioturbated by Thalassinoides boxworks. The Pliocene estuary fill, grading to shelfal (basinal) Calcarenite della Casarana as a whole represents muddy sediments. Whereas the shelfal sediments coarse sediments deposited at relatively lower contain echinoid burrows, more proximal sandy, sea-level during forced regression. estuarine sediments contain Psilonichnus latimurThe succeeding Middle Pleistocene (preatus. Campbell & Nesbitt (2000) recorded only Tyrrhenian) Sabbie della Serrazza is a more rare Ophiomorpha. Psilonichnus appears to have heterogeneous unit that coarsens upwards from a wider latitudinal distribution than Ophiomor- micaceous sandy muds to coarse-grained pha, and, as Campbell & Nesbitt (2000) note, calcarenites. It is associated with a relatively its producers appear to have a tolerance to cool-water (D'Alessandro & Massari 1997) lower salinity. (cooler than present-day) shelly fauna with numerous Pseudamussium septemradiatum and Pleistocene of Washington State, USA A. islandica. Thalassinoides networks and Gingras et al. (1999, 2000) compared the modern boxworks are present, which in some instances trace-forming biota from Willapa Bay with the are of Spongeliomorpha type when piped into trace fossils in closely similar Pleistocene Early Pleistocene marls. Similar large-scale cross-stratification in grainestuarinent nized as of common occurrence in several stones to packstones was described by Bromley Pleistocene facies. This appears to refute our & Asgaard (1975), and Hanken et al. (1996) model, as it is unlikely that subtropical condi- from the Pleistocene Cape Arkhangelos calcartions reached to 47°N, even during warmer inter- enite facies of the Rhodes Formation of glacials. Thalassinoides and 'Ophiomorpha-\ike' Rhodes. The unit represents a spectacular, and burrows were recognized from the modern unusual, example of such stratification, which sediments at Willapa Bay and linked to Upogebia forms large asymptotic clinoforms in beds with pugettensis and Callianassa californiensis, though dips up to 30°. The facies is composed of event

CLIMATIC CONTROLS ON MARINE ICHNOLOGY

beds, from less than 10 cm to over 1 m thick, each fining upwards. As in Italy, the beds appear to represent grain-flow avalanches on clinoforms constructed at the outer margin of a shallowwater, carbonate sand body that was prograding into deeper water. The beds are intensely bioturbated by Echinocardium, which forms winding traces of Bichordites. Ophiomorpha nodosa is absent, but is recorded from lower units of the Rhodes Formation, as are large corals, which are of late Pliocene age, representing warmwater sedimentation. Ophiomorpha is well known from ancient cross-stratified siliciclastic and carbonate sands, though often relatively sparse (Pollard et al. 1993). There are several possible explanations for the absence of crustacean activity crosscutting echinoid burrows in the clinoform stratification of Rhodes: (a) temporal exclusion as in the deep sea (above); (b) the short duration of the colonization window between successive avalanches, though this does not seem to have prevented the formation of Ophiomorpha in Cretaceous and Cenozoic cross-stratified strata (Pollard et al. 1993); (c) climatic control. The association with foramol facies in Italy suggests that a climatic control is the most likely explanation. In these examples from Italy and Rhodes Ophiomorpha appears to be virtually absent, but Thalassinoides is present in the same stratigraphic units associated with echinoid activity, thus corresponding to present-day occurrences. Hanken et al. (1996, figs 8, 14) interpreted the Cape Arkhangelos as representing highstand sedimentation, but this is somewhat inimical for a cool-water (glacial) interval, and may be better attributed to local tectonic activity. Confirmation of the climatic control is seen in the Novoli Graben (Salento Peninsula, Puglia, southern Italy), where 9m-scale sequences have been described by D'Alessandro et al. (in press). The sediments predominantly comprise bioclastic packstones and grainstones to rudstones, and represent foramol-type calcarenites. Biogenic mottling is ubiquitous, and discrete trace fossils include Thalassinoides, Bichordites, and (restricted to certain horizons) Ophiomorpha, Cylindrichnus, Gyrolithes and Tasselia. The vertical changes in the body and trace fossils within the sequences indicate water depth changes corresponding to known sixthorder glacio-eustatic climate fluctuations, with Ophiomorpha entering the stratigraphy with

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rising sea-levels. Most of the sequences are bounded by subaerial, karstic unconformities. A different association is recorded in another sequence with O. nodosa associated with Arctica islandica, a bivalve that today is found only in waters where the February temperature are equal or lower than 5-6 °C, i.e. cool to cooltemperate (Raffi 1986). However, O. nodosa is atypical, being less than 1 cm in diameter with a lining of numerous small pellets, and a different overall morphology, with scarcely branched long shafts. Pleistocene of Korea Kim & Heo (1997) described marginal marine and shelfal siliciclastics from the Early Pleistocene Seogwipo Formation of Jeju Island (Cheju-do) (latitude 33°N), Korea. Probable Bichordites or Scolicia (recorded as Laminites) are present in association with a Cruziana ichnofacies assemblage. Thalassinoides is recorded, but not Ophiomorpha. The palaeoenvironment and molluscan assemblage were described by Kang (1995), who recognized the influence of coldwater currents, and a southward migration of boreal species.

Arctic associations Eyles et al. (1992) described Pliocene? glacially influenced continental shelf and slope trace fossils from the Yakataga Formation of Alaska. We are not convinced by their identification of Ophiomorpha (Eyles et al. 1992, fig. 10), or that the backfilled burrow (op. cit. fig. 11) is attributable to echinoid activity. Thus their trace fossil assemblages may be better interpreted as attributable to annelid and molluscan activity in an arctic setting. Conclusions Our analysis of a number of Cenozoic and Pleistocene occurrences of echinoid and crustacean burrows, together with modern examples, suggests a general model of climate control for this geological interval. We tentatively recognize three latitudinal zones in modern coastal and shoreface settings: Tropical and Subtropical Zone, where pellet-forming thalassinideans and burrowing echinoids are present, though in discrete facies; Temperate Zone, where spatangoid echinoids are present with facially separated Thalassinoides producers, but not Ophiomorpha producers;

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R. GOLDRING ET AL. Arctic Zone, where neither group is present, and the principal bioturbators are annelids and molluscs.

Although we have referred to relatively few examples, and with differing degrees of confidence, our model for climatic distribution of shallow marine trace fossils appears to be robust. There are two anomalous examples from the Pliocene and Pleistocene. The Pliocene example we regard as probable misidentification of both the crustacean and echinoids traces, though we must reserve judgement on the Pleistocene example from Washington State. The occurrence in Italy of O. nodosa in association with Arctica islandica is also anomalous, but the size and overall morphology of the trace fossil are quite atypical. The association of Ophiomorpha in apparently cold-temperate sediments in the Miocene of Patagonia is anomalous, but the palaeoclimate model suggests much warmer conditions than those suggested by the micropalaeontological data. We also tentatively suggest that these assessments may be applied to deep water (turbiditic) occurrences. We recognize the problem of extending this zonation further back in the geological record to Mesozoic occurrences of Ophiomorpha because of the problems in knowing what the constructor(s) were, and whether they were capable of forming a pellet lining to the burrow. We also raise some of the questions that need to be addressed by ichnology: the types and methods of construction of burrow linings, turbiditic distributions of Ophiomorpha and related crustacean traces, and ecological tolerances of spatangoid echinoids. The distribution of other ichnotaxa should be investigated for possible latitudinal distributions. We also emphasize the necessity for careful identification of ichnotaxa, and the identification of the sedimentary surface actually colonized. We are grateful to the management and quarrymen of Bateig Laboral SA (Alicante) for access to the quarries of Bateig Hill, Novelda. E. Stamhuis (Groningen) generously provided a copy of his thesis, and L. Buatois (Tucuman), P. Dworschak (Vienna) and J.-Y. Kim (Chungbuk, Korea) kindly provided relevant literature. B. Sellwood (Reading) discussed and provided a CDROM for Miocene climates. A. Curran (Smith College, Massachusetts), K. Campbell and M. Gregory (Auckland), Shaoping Fu (Bochum) and L. Nesbitt (Washington) are acknowledged for generous discussion. L. Buatois and J. Genise constructively reviewed the paper. Participation of JMdG forms part of the activities of the consolidated research group 2001SGR-00077 of the University of Barcelona, and research project BTE 2000-0584 of the Ministerio de Ciencia y Technologia of Spain.

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HAYWARD, P. J. & RYLAND, J. S. (eds) 1990. The Marine Fauna of the British Isles and North- West Europe. Vol. 1: Introduction and Protozoans to Arthropods. Clarendon Press, Oxford HERTWECK, G. 1972. Georgia coastal region, Sapelo Island, USA: sedimentology and biology. V, Distribution and environmental significance of lebensspuren and in-situ skeletal remains. Senckenbergiana Maritima, 4, 125-166. INSALACO, E. 1996. The use of Late Jurassic coral growth bands as palaeoenvironmental indicators. Palaeontology, 39, 413^431. JAMES, N. P. 1997. The cool-water carbonate depositional realm. In: JAMES, N. P. & CLARKE, J. A. D. (eds) Cool-Water Carbonates. Society for Sedimentary Geology, Special Publications, Tulsa, Oklahoma, 56, 185-203. JAMES, N. P., BONE, Y., HAGEMAN, S., GOSTIN, V. A. & FEARY, D. A. 1997. Cool-water carbonate sedimentation during the terminal Quaternary, high-amplitude sea-level cycle: Lincoln Shelf, southern Australia. In: JAMES, N. P. & CLARKE, J. A. D. (eds) Cool-Water Carbonates. Society for Sedimentary Geology, Special Publications, Tulsa, Oklahoma, 56, 53-76. JAMES, N. P., BONE, Y., COLLINS, L. B. & KYSER, K. 2001. Surficial sediments of the Great Australian Bight: facies dynamics and oceanography on a vast cool-water carbonate shelf. Journal of Sedimentary Research, 71, 549-567. JENKINS, R. J. F. 1975. The fossil crab Ommatocarcinus corioensis (Cresswell) and a review of the related Australasian species. Memoirs of the National Museum of Victoria, 36, 33—62. JENKINS, R. J. F. 1977 A new fossil homolid crab (Decapoda, Brachura), middle Tertiary, southeastern Australia. Transactions of the Royal Society of South Australia, 101, 1-10. JENKINS, R. J. F. 1985. Fossil spider crabs from Australia. In: MURRAY LINDSAY (ed.) Stratigraphy, Palaeontology, Malacology Papers in Honour of Dr Nell Ludbrook. Department of Mines and Energy South Australia, Special Publications, 5, 145-165. JENSEN, S. & ATKINSON, R. J. A. 2001. Experimental production of animal trace fossils, with a discussion of allochthonous trace fossil producers. Neues Jahrbuch fur Geologic und Palaontologie, Monatshefte, 2001, 594-606. KANAZAWA, K. 1992. Adaptation of test shape for burrowing and locomotion in spatangoid echinoids. Palaeontology, 35, 733-750. KANAZAWA, K. 1995. How spatangoids produce their traces: relationship between burrowing mechanism and trace structure. Lethaia, 28, 211—219. KANG, S. S. 1995. Reconstruction of the paleoenvironment and molluscan assemblage of the Lower Pleistocene Sogwipo Formation, Cheju Island, Korea. PhD thesis, Niigata University, Japan. KIM, J.-Y. & HEO, W.-H. 1997. Shell beds and trace fossils of the Seogwipo Formation (Early Pleistocene), Jeju Island, Korea. Ichnos, 5, 89-99. KOTAKE, N. 1989. Paleoecology of the Zoophycos producers. Lethaia, 22, 327-341.

CLIMATIC CONTROLS ON MARINE ICHNOLOGY LEES, A. 1975. Possible influence of salinity and temperature on modern shelf carbonate sedimentation. Marine Geology, 19, 159-198. LEES, A. & DULLER, A. T. 1972. Modern temperatewater and warm-water shelf carbonate sediments contrasted. Marine Geology, 13, M67-M73. LEWIS, D. N. 1989. Fossil Echinoidea from the Barton Beds (Eocene, Bartonian) of the type locality at Barton-on-Sea in the Hampshire Basin, England. Tertiary Research, 11, 1-47. MclLROY, D. 2004. A review of some ichnological concepts, methodologies, applications and frontiers. In: MclLROY, D. (ed.) The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 3-27. MCNAMARA, K. J., PHILLIP, G. M. & KRUSE, P. O. 1986. Tertiary brissid echinoids of southern Australia. Alcheringa, 10, 55-86. MAHMOUDI, M. 1988. Nouvelle proposition de subdivisions stratigraphiques des depots attribues au Tyrrhenien en Tunisie (region de Monastir). Bulletin Societe geologique de France, 1988 (8), IV, 431-435. MARTINELL, J., DOMENECH, R. & MARQUINA, M. J. 1984. Molluscan assemblages in the north-east Spanish Pliocene. Annales Geologiques des Pays Helleniques, 32, 35-56. NESBITT, E. A. & CAMPBELL, K. A. 2002. A new Psilonichnus ichnospecies attributed to mud-shrimp Upogebia in estuarine settings. Journal of Paleontology, 76, 892-961. ORR, P. J. 1994. Trace fossil tiering within event beds and preservation of frozen profiles: an example from the Lower Carboniferous of Menorca. Palaios, 9, 202-210. PEMBERTON, S. G., MACEACHERN, J. A. & FREY, R. W. 1992. Trace fossil facies models: environmental and allostratigraphic significance. In: WALKER, R. G. & JAMES, N. P. (eds) Facies Models. Geological Association of Canada, 47-72. PERRY, C. T. 1998. Grain susceptibility to the effects of microboring: implications for the preservation of skeletal carbonates. Sedimentology, 45, 39-51. PICKERILL, R. K., DONOVAN, S. K., DIXON, H. L. & DOYLE, E. N. 1993. Bichordites monastirensis from the Pleistocene of southeast Jamaica. Ichnos, 2, 225-230. PLAZIAT, J.-C. & MAHMOUDI, M. 1988. Trace fossils attributed to burrowing echinoids: a revision including new ichnogenus and ichnospecies. Geobios, 21, 209-233. POLLARD, J. E., COLORING, R. & BUCK, S. G. 1993. Ichnofabrics containing Ophiomorpha: significance in shallow-water facies interpretation. Journal of the Geological Society, London, 150, 149-164. PURTON, L. & BRASIER, M. 1997. Gastropod carbonate 618O and 813C values record strong seasonal productivity and stratification shifts during the late Eocene in England. Geology, 25, 871-874. RADWANSKI, A. 1977. Present-day types of trace in the Neogene sequence; their problems of nomenclature and preservation. In: CRIMES, T. P. & HARPER, J. C. (eds) Trace Fossils 2, Geological Journal, Special Issue, 9, 227-264.

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A new approach to the analysis and interpretation of tracks: examples from the dinosauria PHILLIP L. MANNING

The Manchester Museum, University of Manchester, Oxford Road, Manchester, M13 9PL, UK Abstract: Tracks can potentially offer unique sources of information, providing insight into the environments, gait and posture, locomotion and behaviour. Track preservation can yield important information on substrate consistency and enable the recognition of transmitted subsurface tracks. The ability to recognize transmitted tracks has broad implications for the understanding of palaeoenvironments and interpretation of ichnological assemblages. In order to gain an understanding of how tracks are formed in three dimensions, and of their variability of expression in different substrates, controlled laboratory simulations were undertaken. Experiments were designed to recover subsurface track layers, yielding for the first time detailed information on subsurface morphology that could be related to 'true' surface track features. It was found that subsurface track relief can be correlated with the magnitude and distribution (across a foot) of load acting on the surface sediment. This pressure is transmitted through the sediment, and deforms successive layers at depth, producing an undertrack. The most significant factor controlling track morphology, whether surface or subsurface, was found to be the moisture/density relationship within the substrate at the time of track formation. Variability in the dimensions of simulated tracks, relative to the 'true' surface track, indicates that caution should be exercised when using fossil tracks to calculate hip height, speed, age, and population dynamics. In addition, comparison of experimental tracks with dinosaur tracks from the Yorkshire coast suggests that many morphological differences between vertebrate ichnotaxa reflect sediment rheology and taphonomy rather than taxonomy of the track-maker.

Fossil tracks have the potential to reveal information on the size, gait and speed of individuals, as well as clues to their behaviour and the environments in which the animals lived. However, the interpretation of tracks is often difficult, in that what is available for study is, in many cases, not an original track surface. If fossil tracks are to be a useful tool for interpreting behaviour and environments, it is essential that preservational types are recognized and can be related to a realistic surface trace. Improved understanding of track formation and preservation could also assist in diagnosing the properties and behaviour of sediment at the time of track formation. This study provides some insight into, and interpretation and understanding of, the complex, threedimensional processes occurring beneath the surface of a track (subsurface deformation) at the time of track formation. Guidelines are also provided for the interpretation of fossil tracks and their use. Experimental equipment was designed and built to optimize repeatability of experiments, to control rheological conditions, and to enable recovery of subsurface track layers. Foot templates used were designed to be comparable to dinosaur feet, though a similar methodology could be used for any foot morphology or gait.

Field study of Middle Jurassic dinosaur tracks was undertaken on the Yorkshire Coast and compared with experimental tracks. Recent work has highlighted the importance of understanding and interpreting the formation and preservation of dinosaur tracks in the field (Romano & Whyte 2003 and references therein). The diverse dinosaur track morphologies found within the Middle Jurassic track assemblages of the Yorkshire coast were studied with a view to improving their interpretation through comparison with laboratory track simulations. History of experimental vertebrate palaeoichnology The study of vertebrate tracks and traces - vertebrate palaeoichnology - has concentrated on describing the trace, with little or no interpretation of track formation/preservation. The first laboratory investigations into vertebrate track formation and preservation were carried out in December 1827 at Oxford University by William Buckland (Sarjeant 1974). Buckland persuaded a crocodile and then a tortoise to walk across a soft pie-crust (presumably of dough) and also over wet sand and soft clay

From'. MclLROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 93-123. 0305-8719/04/S 15.00 © The Geological Society of London.

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Fig. 1. Stacked tracks as recognized by Hitchcock (redrawn from Hitchcock 1858).

(Buckland 1828). Later work by Hitchcock (1858) on fossil tracks from the Lower Jurassic of Connecticut, USA, astutely recognized transmitted tracks (Fig. 1). It took over 100 years for ichnologists to rediscover this phenomenon through experimental work by Lingen & Andrews (1969) on horse tracks. Subsequent work has refined our understanding of tracks and track formation through a variety of approaches.

Preservation of skin/scale impressions (Lockley 1991). Presence of 'messy tracks' in which clay adhering to the foot distorts the foot outline (Bird 1944). Detached mud clasts (presumed to have been once adherent to the foot) within the track (Whyte & Romano 1993a, 1994a; Allen 1997). Collapse/flow features in which the track margin overlies the sole of the track (e.g. Lockley et al 1989; Allen 1997). The presence of a raised displacement rim or bourrelet, which is a direct reflection of the sediment's consistency at the time of track formation. Cohesive sediments tend to bulge into an unbroken, smooth rim, but more friable sands tend to bulge into a radially cracked displacement rim, and spill onto the track surface (Thulborn 1990; Allen 1997). A scenario that clouds this issue of surface track recognition is created when a limb punctures into underlying laminated strata, causing the presence of an underprint in which only the deeper portion of the limb is impressed into the sediment (e.g. Thulborn 1990). These underprints are distinct from transmitted tracks (Fig. 1), which are the primary focus of this paper.

Experimental neoichnology Detailed study of vertebrate trace fossils A number of authors have studied deposits with abundant well-preserved tracks from which a variety of broad-scale observations about sediment conditions and track preservation have been made. It is generally considered that firmground conditions produce the best preservational conditions for surface tracks (e.g. Tucker & Burchette 1977) and that soft/soupground conditions resulted in poor surface track preservation (e.g. Whyte & Romano 1981) (terminology for sediment consistency from Dodd & Stanton 1990). Detailed study of Neogene avian and mammalian ichnofaunas by Scrivner & Bottjer (1986) documented wide morphological diversity in artiodactyl tracks. They attributed the difference in morphology to variations in sediment water content at the time of formation. It was also noted by Scrivner & Bottjer (1986) that surface track preservation was improved by rapid casting by sand.

A second approach to understanding trace fossil preservation has come through observations of known organisms in identifiable sedimentological/rheological conditions. As mentioned above, experimentation has been a fundamental tool in vertebrate ichnology since the early experiments of Buckland (1828). Recent advances in reptilian, avian and mammalian neoichnology have come through careful consideration of foot anatomy and the kinematics of locomotion. Of particular importance to this study is the work on a variety of dinosaur-like modern taxa, from komodo dragons to ostrich (e.g. Padian & Olsen 1984; Demathieu 1987; Farlow 1989; Gatesy et al, 1999). Preservation of surface tracks is improved by microbial mat growth on the track surface (Thulborn 1990) and, in desert environments, by moistening by dew before casting by sand (McKee 1947).

Application ofindenter theory Determination of surf ace tracks The recognition of surface tracks is facilitated by:

Experimentation using artificial indenters in order to understand track formation and morphology was first undertaken by Allen (1989).

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He suggested that mechanical theory offered a number of insights into the likely character of animal tracks in the field. In particular, it suggests that: the track shaft is surrounded by a substantial deformed zone; the deformed zone is likely to include faulted as well as folded layers; and the character of the track is likely to vary with the stratigraphy of the affected sediment. Allen (1989) found that the use of an indented plastic material in laboratory tests qualitatively reproduced all the essential features of real tracks. In the laboratory experiments the limb and foot cut a shaft into the sediment, creating an extensive zone of deformed sediment around and below the shaft; the degree of deformation increased with increased depth of penetration. The deformed zone comprised an axial downfold, in which downward-decaying undertraces were preserved, and a marginal upfold, with associated shear and fracture zones. Allen (1989) found that, where the sole of the foot was complex in shape, anatomical details - in the form of cross-folds (sensu Allen 1989) were preserved in the undertrack.

Extrapolation of biomechanics from tracks An initial contribution to the understanding of tracks in relation to biomechanics was the suggestion that the greater the force borne by a limb, the deeper will be its underprint impression (Demathieu 1987). Hence 'relative foot [track] depth' (relative to the surface on which the animal walked) was proposed as a crucial parameter for understanding the mechanics of track formation, and for estimating the centre of gravity of track-makers. This statement predicts that deeper underprint impressions will represent the dominant weight-bearing limbs, such as the manus or even particular parts of the limb (e.g. distal portion of digits). Through study of a rich dinosaur track deposit in Australia, Thulborn & Wade (1989) distinguished three distinct phases of track creation during footfall, termed: touch down (T-phase); weight bearing (W-phase); and kick-off (K-phase). Some of the morphological variability was attributed to sediment consistency, but Thulborn & Wade (1989) also related some differences between tracks to variability in the footfall cycle. Variation in the track shape with depth

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was noted as a common phenomenon, with underprints often differing greatly from the surface tracks. They noted that digit III was probably the thickest in the track-maker's foot, and normally produced a correspondingly broad impression. However, sometimes this principal load-bearing digit generated a thixotropic reaction in the underlying sediment, where on withdrawal of the foot the walls of digit III were sucked inwards, generating a narrow middle digit (cf. Figs 6c, 19). In addition the authors found that the foot did not sink into the substrate during their T-phase or the Wphase, leading to gaps in the trackways. The first attempt at interpreting complex three-dimensional surface failure in association with dinosaur tracks was undertaken by Gatesy et al (1999). Theropod tracks from the Fleming Fjord Formation (Norian-Rhaetian), East Greenland, were compared with tracks produced by a modern avian theropod, a helmeted guineafowl (Numida meleagris). The Triassic tracks indicated that the animals had walked over substrates in quite variable conditions (dry to saturated), producing a diverse assemblage. The serial sectioning of tracks confirmed that Triassic theropod feet sank, moved forward and were extracted with convergent toes, in a similar fashion to guinea-fowl (Gatesy et al. 1999). Although great inroads have been made into understanding vertebrate tracks, a thorough study of variability of tracks in three dimensions in relation to transmitted features is lacking. Presented herein is a first attempt to link surface features described from dinosaur tracks Thulborn & Wade (1989), modern tracks (Allen 1997; Gatesy et al. 1999) and the experimental indenter approach (Allen 1989) to the subsurface. A continuous trackway from dry to saturated sediment is easy to follow on a modern beach; however, the interpretation of fossil tracks in various stages of water cover is not always obvious, especially with less complete track material. The depth of the water may have affected a dinosaur's method of locomotion and the resultant track morphology. The tracks of wading dinosaurs would be quite different from those of 'swimming' dinosaurs. The 'swimming' (actually punting of a partially buoyant individual off the bed of a water body) animals may take longer strides, because they tended to be buoyed up between one footfall and the next, and their tracks may show only the tips of the toes (Coombs 1980; Thulborn 1990; Whyte & Romano 1994b; Romano & Whyte 1996). Current strength would also affect stride length.

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Experimental tracks The methodology used herein is designed to recover data from transmitted tracks generated by experimentation. The transmitted tracks can then be studied and compared with respect to the sediment conditions under which they were formed. The analysis of the distribution of ground reaction forces on indenter templates was undertaken to assist interpretation of load and pressure distribution during track formation and its bearing on final track morphology. The equipment used, an optical pedobaragraph, also allowed determination of the centre of pressure during footfall. This provided a dynamic insight into the distribution of pressure across the indenter foot for comparison with track morphology. Laboratory equipment included indenter templates, indenter assembly, Newton compression meter, and various sediment-testing boxes in which sediments could be constructed and moisture content could be varied. Indenting a substrate to record the formational and preservational history of a track appeared a simple task at the outset. However, the surface features were analogous to icebergs, with most of the information locked beneath the surface of the track as a complex, three-dimensional structure. The indenter templates varied a great deal in form and functional ability, allowing a study of the relationship between form, function and substrate. The grain size and moisture content were varied between individual track simulations, as Scrivner & Bottjer (1986) recognized these as important variables affecting track preservation. Experimental equipment

Construction of indenter templates Both simple and complex shape templates were cut from 50mm thick, rigid plastic. The shapes were chosen to represent a variety of dinosaur pes morphologies in plan view. The surface area of the seven (templates T1-T7) rigid, plastic templates (Fig. 2) was calculated using a planimeter. This study primarily concerns a semi-rigid T8 that consists of three converging steel bars threaded into an aluminium heel with a moulded silicone rubber outer. Silicone rubber was applied to the frame when held within a plaster mould. The two-part mould had been formed around a sculpted foot, of generic tridactyl type

Fig. 2. Templates used in creation of surface tracks. Surface areas for the templates were: Tl, 750.6mm2; T2, 764mm2; T3, 624.2mm2; T4, 1294mm2; T5, 1257mm2; T6, 757mm2; T7, 1005mm2.

similar to the tridactyl tracks of the Yorkshire Coast. Porous plate box Previous studies have only allowed the introduction of fluids to a sediment from above, often disrupting or destroying any constructed laminae. A better approach is to introduce water in a controlled fashion from underneath via a porous plate. A large plastic box (with reinforced struts to prevent deformation of the box under load) had a porous plate inserted 40mm above the floor of the box. The porous plate was constructed from a piece of 5mm thick plastic, drilled with a close, regular array of 3 mm diameter holes and covered in a fine (105 um) polyester monofilament mesh before its insertion into the box. Separate inlet and outlet valves were inserted into the box, beneath the level of the porous plate. The outlet valve allowed the water level to be monitored, and hence the saturation of the sediment could be controlled. When the sediment had reached the desired saturation point, flooding of the sample could be stopped immediately. When it was required to expel water from the sample, the hydrostatic head tube was removed and the water could drain away freely (Fig. 3). The porous-plate testing box had to withstand temperatures of at least 100°C during oven drying. The temperature was incrementally

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allowing the template to be indented by hand to a given force. The freedom of the NCM allowed more natural movements of the templates to be mimicked, at the price of some experimental control. The NCM was invaluable with the porous plate generated tracks, as the indenter assembly would have proved difficult to apply within the confines of the testing box. This method was employed for most of the experiments discussed below. Experimental methods Fig. 3. The porous-plate testing box as used for subsurface track simulations.

increased from 30 °C to 100 °C over 8-10 days in an oven. Indenter assembly An indenter assembly was constructed, and initially used in track simulations, so that the templates were driven into the sediments in the same way and at a controlled rate (Fig. 4). The indenter assembly allowed a template to be attached and pulled against a counterbalance by means of a spring balance, so that the template indented the sediment below. The disadvantage of this assembly was that it gave the template a forward shunt on initial contact with a sediment surface, and very unnatural reverse motion on retraction of the template. It was also difficult to generate large forces through the template without the apparatus moving or lifting with the template as the pivot. Owing to the natural rolling motion observed in almost all bipedal vertebrates when walking, a second indenting device was adapted to mimic this motion. A Newton force compression meter (NCM) was adapted so that a template could be threaded onto the end of the device,

Fig. 4. Indenter assembly used for surface track simulations.

The optical pedobaragraph (OPB) The OPB experiments were either dynamic or static loaded tests. In some experiments additional load was exerted on a specific digit or digits to record possible effects of an uneven gait. The OPB recorded the dynamically loaded tests at a frame capture rate of either 25 frames per second (25 Hz) or 16.7 frames per second (16.7 Hz). The apparatus consists of a glass plate illuminated at two opposing edges by strip lights and covered by a thin sheet of white deformable film, which is the surface on which a force can be applied (Fig. 5). Light rays are totally internally reflected within the glass plate, except at points where the white film touches the glass. At these points the light rays are refracted out of the glass plate and scatter back from the white film. The film surface is undulating in appearance on the microscopic scale, and an increase in pressure on the film results in these undulations being deformed into intimate contact with the glass plate. The greater the pressure applied to the film, the greater the area of contact with the film and glass, and the greater the quantity of light that escapes from the glass.

Fig. 5. The optical pedobaragraph.

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The result is a continuous grey-scale image of the indenter, in which intensity is proportional to the applied pressure (Betts et al. 1991). A colour image is generated from the digitized data, with the colour of the image being related to the intensity and distribution of pressure at a given time. From such images it is possible to see where high pressures occur, and to assess the general pressure-distribution changes from

the initial strike of an indenter to its toe-off. Pressure is expressed in kgcm~ 2 , which can be converted to the official SI unit of pressure, the pascal (Pa). The conversion factor is: lkgcnT 2 = 98.1kPa. It is also possible to calculate the progression of the centre of pressure throughout the sequence of frames, enabling assessment of the loadbearing properties of any indenter, as well as to

Fig. 6. (a) Tridactyl dinosaur track (F00813) sectioned parallel to digit III. Scalby Formation, Scalby Bay, Yorkshire, (b) Foot template 8 (T8) used in laboratory track simulations (scale 10cm). (c) Track simulation T8/F13 layer 3, top surface of plaster track layer 3T, from a depth of 1.3cm below the surface track layer (scale bar increments 1 cm), (d) Track simulation T8/D1.7b using T8, indented into sediment D with a force of TON when dry (moisture content 0.3%). (e) Track simulation T8/D2.7 using T8 indented into sediment D with a force of 49 N when moist (moisture content 7.3%).

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produce a combined frames image. The combined frames image is a useful indicator of pressure distribution over an indenter throughout a complete dynamic cycle, which can then be compared with static loading of the same indenter. Template 8 was tested in both static and dynamic runs on the OPB. Sediment was also placed on top of the transducer material to measure the transmission of forces. Templates 1-7 were found to be too rigid for the OPB transducer material, and gave no usable data. The results generated by the OPB provided useful data on the relationship between the distribution of load over the sole of a foot in both static and dynamic load situations. Such information is applied to the interpretation of preservation features observed in both fossil and laboratory-simulated tracks below.

Recovery of surface track features The initial indenter tests were undertaken to record surface features relating to the formation and preservation of tracks, using characterized, homogeneous sediments. The sediment was placed in a large tray 1 m long by 500 mm wide and 150mm deep. The sediment was at first indented dry, and then moisture content was increased incrementally. Moisture content was calculated at the beginning and end of each indenter test. For the surface tests the static indenter assembly was used. Templates 1-8 were indented into the substrate, with the sediment mixed and levelled before each experiment. A 300mm x 300mm wire grid of 10mm squares was lightly impressed on the surface of the sediment before indentation, to emphasize the surface displacement features (e.g. Fig. 6d, e below). The indenter force applied and the depth, width and length of each resultant track were recorded, as was the slope angle of the track wall and other distinct characteristics. Any transient features observed at the time of track formation were also noted, such as dewatering of sediment around a track, or suction of the floor of a track above true track surface. The main surface features relating directly to the foot are as shown in Figure 7.

Recovery of subsurface track features A prerequisite for retrieving subsurface deformed track layers is that separable laminae be present. Homogeneous sediments were not suitable for recording subsurface deformation, and so laminated sediments had to be

Fig. 7. Generic tridactyl track showing position of surface features recorded that relate to track morphology: digit length and interdigital angle (IDA).

constructed for each track simulation. Kaffir D™ plaster of Paris (British Gypsum™) was the material used to inter-laminate with the characterized sand and clay samples. Kaffir D™ plaster has suitable properties, including a setting time of 8 min, a high compressive strength (once dry), minimal effect on the behaviour of surrounding sediments, and a very low linear expansion on drying (around 0.25%). Other materials were tried, including cement, various muds and even flour, but none of them fulfilled all the above criteria. Laminated sediments were constructed when dry, as this enabled up to 16 layers of sand and plaster to be constructed (sometimes including laminae of damp clay sandwiched between dry sand), without affecting the dry plaster. The alternating layers of sand and plaster were individually sieved into the porous-plate testing box, with the uppermost and lowermost layers always consisting of sand. Equal volumes of sediment were used in all layers (approximately 900 ml per layer) and equal volumes of plaster (approximately 200ml per layer). Plaster was not sieved right up to the edges of the porousplate testing box, so that water would pass freely through the sand, around and between the plaster layers. Once the laminated sediment had been constructed, it could be indented either dry or moist (by introducing water via the porous plate). There was a time interval of approximately

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4min (with Kaffir D™ Plaster of Paris) before the plaster began to harden. In this time water had to be introduced via the porous plate, and the sediment had to be indented. As soon as a sample had been indented (using template 8 for this set of experiments), four 3 mm thick copper rods were inserted vertically through the sediment, puncturing each plaster layer immediately around the track to leave reference points. A further thin layer of sand was added to the surface track feature to act as a release agent, enabling an additional layer of wet plaster to be poured into the surface track to record its morphology (relative to the copper reference rods). When the top layer of plaster was dry, the copper rods were removed from the sample. The porous-plate testing box, sediment and track were placed in an oven at 30 °C for 24 h. The temperature was then increased every daily by 10 °C until the oven reached 100 °C. After 8—10 days the porous-plate testing box was removed from the oven, and the sample was removed. The sample then had to be returned to the oven for a period of 5 days to expel any remaining moisture. The sediment had to be completely dry because of the delicate plaster layers, which are easily destroyed when damp. In addition, cohesive sands hamper recovery of plaster layers. When removed from the oven, the sides of the sample were brushed clear of sand until the layers of plaster appeared as relief ledges. The depth of each layer, relative to the surface-track layer, was measured. The lowermost sand layer was first brushed away, revealing a cast of the upper surface of the lowermost horizon. Any feature that was associated with transmitted or underprint features was mapped onto an acetate sheet and photographed. The reference points made by the copper rods were also mapped onto the acetate sheet, and a track layer reference number was assigned. A spatula was then used to gently lift the plaster layer, so that the lower surface could be brushed clean, mapped onto an acetate sheet and photographed. The process of acetate sheet mapping and photographing was repeated for each successive track layer.

defined as those that have not been transmitted from overlying sediments. In the laboratorysimulated tracks this category encompasses tracks from the top sand layer, infilled by the uppermost plaster layer and best seen as a cast. The nomenclature used here is for laboratorygenerated tracks, where the unique situation exists of knowing the exact dimensions and morphology of the track-maker's pes. Nomenclature is used in this study to include the track dimensions and angles recorded from track morphology (Figs 7, 8a). Many of these terms have been used to describe fossil tracks (Haubold 1984; Leonardi 1987; Gillette & Lockley 1989; Thulborn 1990). It is difficult, if not impossible, to associate some features of fossil tracks with the morphology of a track-maker's pes or manus, owing to the vagaries of preservation. Descriptive terminology for subsurface track morphology This terminology relates to transmitted track features from the recovered subsurface layers and puncture features associated with undertracks (Fig. 8b). It is important that, if a term is used, the layer from which the feature is described is made clear, i.e. Aw, Bw, C" etc., where n is the layer number and surface type (upper or lower). Subsurface tracks were all produced using template 8 (T8). As discussed earlier, the template was very basic in form, representing the simplified morphology of a theropod or very gracile ornithopod dinosaur pes. The proximal convergence of the digits produced an unnatural intersection of the median lines for each digit. The median lines of dinosaur's digits do not usually, if ever, converge in any example of a dinosaur's skeletal pes. However, it was convenient in the current study that the position of the heel and digits of the template made measurements of track dimensions much easier, as well as being able to relate track morphologies between layers more easily. Data recording

Track description Descriptive terminology for surface track features Surface track features are taken to include the upper surface of the base of shafts of undertracks (sensu stricto Thulborn 1990). Surface tracks are

Experiments were recorded using the abbreviation for the template followed by the experiment number: for example, T8/F2 was the second experiment carried out using template 8 (T8). Six types of data were recorded from the recovered plaster track layers. The maximum zone of deformation (MZD) (Fig. 8b) was recorded for the top (T) and base

Fig. 8. (a) Track dimensions recorded for surface tracks; (b) features recorded from recovered subsurface layers. Data were used for subsequent ichnometric analysis and measured relative to known points described by puncture holes made by copper rods.

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(B) of each plaster track layer (except for T8/F2, where only one of the basal track surfaces could be recorded). The coordinates were plotted for each track simulation on a standard grid (300 mm x 300 mm), to allow comparison between layers within an individual track and their relationship with other track simulation experiments. Individual track surface tracings were generated and placed in successive order, for ease of interpretation. The direction of travel was marked on the first plot of each track data series. The position of the 'heel' was recorded in all successive layers, as was the most anterior point attributable to the distal end of each digit (Fig. 8b). The coordinates were recorded and plotted in the same way as for the MZD. Track length was recorded for the top and base surfaces of each plaster track layer. The track length was taken as the length of the MZD (Fig. 8b), as this was the most persistent track length parameter to be recorded at any depth. Track width was recorded for the top and base surfaces of each plaster track layer. The width of the track was taken as MZD track width, as shown on Figure 8b. The width of the MZD was taken as the maximum width of the track as measured perpendicular to the track axis (axis of digit III) (as shown on Fig. 8b). Digit length was also studied, taking the length measured from the most anterior point attributable to the distal end of each digit to the posterior margin of the 'heel' from each track layer (Fig. 8b). The IDA was measured as the angle of divarication at the point of intersection of the median axis of the digits about the 'heel' (Fig. 8b). Interpretation of results

Indenter theory The force exerted on T8 (Fig. 6b) during each of the subsurface track experiments was considerably smaller than those expected for a dinosaur with a similar-sized foot. A load of 5 kg (49 N) was applied over the surface area of template 8 (470mm2), producing a force of 1.06kN/m2 (1.06kPa). When different-sized structures retain the same shape, they are considered to scale with isometry or to be geometrically similar (Swartz & Biewener 1992). The surface area (S) increases in proportion to the square of its linear dimensions (/):

However, the volume (V) increases even faster, in proportion to the cube of its linear dimensions (/): Each time the dimensions of T8 are doubled, the surface area of the foot increases by a factor of 4 (FL2) FL (Footlength) and the volume increases by a factor of 8 (FL ). This means that, if the dimensions of the template 8 are doubled, the surface area increases from 470mm (125mm long foot) to 1880mm2 (250mm long foot) and the weight of the animal (load) increases from 5kg to 40kg. This means that eight times the load is exerted over four times the area, so that the pressure under the bigger foot is twice that of the smaller foot. The application of a 40 kg load was not possible in the existing experimental testing frame, so the scaled load of 5kg was applied. The larger foot would be dynamically similar to the smaller one, but twice the load would be transmitted through the sediment, affecting the resulting scale but not the morphology of a resultant track. It is impossible to achieve a quantitative test for all fossil dinosaur tracks as there are too many variables to account for. These variables include the moisture content at the time of track formation, the weight of the dinosaur, the true morphology of the dinosaur's foot, and the exact gait of the dinosaur at the time of track formation. Although these variables are controlled in the current laboratory track simulations, it is impossible to know them from a fossil track. This means that the quantitative data produced in this study can be used only as a qualitative guide to the conditions prevailing, and the foot morphology of the track-maker, at the time of track formation. The magnitude of the maximum zone of deformation (MZD) (e.g. Fig. 6d) and related features was in proportion to the load applied to the isotropic sediment. The MZD marked the distribution of vertical pressure at the point of failure at the surface and within the sediment. In transverse cross-section the deformation has an onion-shaped force bulb, as reflected in the width of the MZD with depth (Fig. 9). In longitudinal section the force bulb is found to be distorted, owing to the dynamic nature of track formation (Fig. 10). These results are in line with the predictions of Boussinesq (1883), who solved the problem of stresses reproduced at any point in a homogeneous, elastic and isotropic medium as the result of a point load applied on the surface of an infinitely large half-space. Boussinesq's elastic analysis is represented by the following

CONTROLS ON PRESERVED TRACK MORPHOLOGY

Fig. 9. MZD length/depth plot for track simulation T8/F8 showing a characteristic onion-shape force bulb.

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at a horizontal distance r from the line of action. Application of Boussinesq's equation to a hypothetical homogeneous sediment with a bearing capacity of approximately 0.4 N m"2 loaded with a 49 N force produces a failure zone (force bulb) (Fig. 11) that is remarkably similar to that seen during experimentation (Fig. 9). Boussinesq's theory relates the distribution of a static load at depth, but the current study applied dynamic loading, and therefore the application of Boussinesq's theory was qualitative rather than quantitative. Variations from the expected onion-shaped bulb were produced by the experiments, owing to the dynamic (forward-directed) loading (Fig. 10). One of the longitudinally cross-sectioned fossil tracks studied (F00813) also displayed a distorted force bulb (Fig. 6a). Experiments T8/F6 and T8/F7 were the only interlaminated clays and sands (non-homogeneous sediments) tested, and it was observed that they also generated a distorted force bulb consistent with possible failure predictions using Boussinesq's theory.

Fig. 10. MZD length/depth plot for simulation T8/F8 displaying an anteriorly displaced force bulb.

Kinematics

where P is the point load (P), and
The standardized method for indenting template T8 into the sediment was based on kinematic analysis of modern bipedal vertebrates (Clark & Alexander 1975; McMahon 1984; Gatesy & Biewener 1991). As the laboratory simulations were to be compared with tracks of bipedal dinosaurs from the Middle Jurassic of the Yorkshire coast, the force needed to be applied to the experiment's sediment surface in a realistic

Fig. 11. Pressure bulb for a hypothetical 49 N impact on a homogeneous sediment, as predicted by Boussinesq's equation. Values for av are in Nm~ 2 . Dotted line delineates failure zone (force bulb) for sediment with a hypothetical bearing capacity of approximately 0.4 Nm~ 2 .

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manner. This was facilitated by use of the Newton compression meter, as described above. All known Middle Jurassic bipedal dinosaurs locomoted with a primitive hip-based retraction mechanism for the movement of the hind-limb. The support phase of the hip-based step cycle (sensu Gatesy & Biewener 1991) has three distinct phases:

(1) the heel-down phase; (2) the rotation phase, where the foot rolls forward and the centre of mass of the animal passes over the foot; and (3) the toe-off phase, where the animal places its weight on the distal ends of its digits before pushing off the ground (Fig. 12). The angle of action of the force acting on the foot at the heel-down phase of a limb cycle appears remarkably similar among living bipedal animals, and ranges from 56° to 73° (measured from the horizontal), depending on the type of animal and the speed at which it is travelling. The angle to which the limb rotates forward before the toe-off phase of the step cycle also shows little variation between living bipedal animals, and ranges between 106° and 138° from the horizontal in the direction of travel (Gatesy & Biewener 1991; Fig. 12). For the purpose of experimentation, the limb angle at the heel-down phase of the step cycle for this study was emplaced at approximately 70° (Fig. 12a). The limb was then loaded and rotated to the toe-off phase and withdrawn at an angle of 120° (Fig. 12c). It is noted that substrate type and condition can cause organisms to alter the angle of limb position at the heeldown and toe-off phase of a step cycle. For example, a slippery surface might necessitate a deliberate flat placement of the foot (at a high angle, approximately 90°) to prevent instability.

Subsurface failure Indenting sediment causes a variety of failure types. There are four distinct types of trackrelated failure recognized in the present experimental study:

Fig. 12. The phases of track formation, showing the limb angle at: (a) heel down; (b) forward rotation; (c) toe-off; (d) withdrawal of foot.

In general shear failure, continuous failure surfaces develop between the edge of the indenter and the sediment surface, causing distinct surface anterior displacement rim. The distribution of pressure (load) through the sediment is spread downwards and outwards, delineated by the position of subsurface shear surfaces (Fig. 13a); In local shear failure there is a significant compression of the sediment under the template/ foot. Associated failure surfaces (ISZ and displacement rims) do not reach the soil surface (Fig. 13b); Puncture shear failure comprises vertical shearing (failure) that occurs immediately around the template, creating a shaft. There is no surface development of failure surfaces such as ISZ or displacement rims (Fig. 13c);

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The word 'soil* has different meanings to workers of various disciplines. The definition for soil used herein is an engineering one, where a soil is defined as any loose sedimentary deposit, such as gravel, sand, clay or a mixture of these materials (Smith 1981). The size and distribution of soil particles and the air or water occupying voids between the solid particles affect the mechanical properties of a soil, as do its porosity and permeability. The sediments tested were silty-fine to mediumgrained sands that had little resistance to shearing when dry, but when the moisture content was increased so too did shear strength, up to the critical hydraulic gradient. Increasing moisture content effectively increases the bulk density of sediment, as water replaces air contained in the voids between soil particles. The denser a soil becomes, the greater its shear strength (Karafiath & Nowatzki 1978). However, if moisture content increases beyond the soil's critical saturation point (critical hydraulic gradient), where the soil particles no longer come into contact owing to porewater pressure, the soil fails and becomes a soupground (sensu Dodd & Stanton 1990).

Fig. 13. Modes of failure: (a) general shear; (b) local shear; (c) puncture shear (after Craig 1992); (d) liquefaction failure (Atkinson & Bransby 1978). Shear/failure surfaces are shown in dotted lines, and displaced sediment is shaded dark grey.

Liquefaction failure occurs where areas of intense pressure are created quickly (e.g. end of push-off phase in a step cycle), causing the sediment to reach its liquid limit, whereupon the sediment flows into the track, destroying all evidence of foot morphology of the trackmaker (Fig. 13d).

Soil mechanics Track preservation is strongly related to the mechanical and physical properties of a substrate and their effect on a resultant track. A track is formed when the yield strength of the substrate is exceeded owing to the locomotion of the track-maker. The mechanical properties of a substrate directly influence the resultant features associated with footfall. To fully understand the formation and preservation of tracks, an understanding of the mechanics of soils is required.

Track preservation and moisture/density relationships Footprints have the effect of mechanically compressing (densifying) a soil. Although the mechanisms that control compaction are not fully understood, it has been demonstrated that soil moisture content at the time of compaction is critical, especially for fine-grained soils such as silts (Karafiath & Nowatzki 1978). The simplest and perhaps most widely accepted hypothesis (Lambe 1961) for the relationship between moisture and density at a given compressional event is that, as water is added to a soil, air is expelled. As soil particles adsorb water a surface film is formed that permits particles to slide over each other more easily. Failure occurs when an external load is applied if the critical saturation point of the soil is exceeded. As the thickness of this water film is negligible compared with the diameter of particles in coarse-grained soils, e.g. sand-grade sediments, the effect is not as pronounced as for finer-grained silty soils (Karafiath & Nowatzki 1978). In fine-grained soils the lubricating effect of the water on the soil exists up to a certain threshold. When additional water no longer replaces air in the soil voids, and the amount of entrapped air remains essentially constant, the water has the opposite effect: that is, it occupies pore space that

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could be filled with soil particles upon the application of load (Karafiath & Nowatzki 1978). When a load is applied to such a sediment the water escapes between the soil particles. The degree to which this occurs is controlled by the permeability of the sediment. The load is then transferred from the virtually incompressible water to the compressible skeleton of soil particles, which takes up the load. There is an optimum moisture content for a given soil under a given compaction energy that will allow the wetted soil particles to come as close together as possible. As the subsequent drying of the soil does not affect this spacing, the compaction of the soil at this optimum moisture content results in a maximum dry density. The variation of dry density over a range of mixing moisture contents constitutes the moisture/density relationship (moisture condition value) for a given soil compacted under a given type of load (related to foot morphology and gait) and the amount of energy exerted on that load (a function of size and speed of organism). As moisture is removed (or added) from a silt to silty fine to medium-grained sand, as in this study, the sediment passes through a series of states: liquid, plastic, semi-solid and solid. The transition from one state to another occurs at points known as consistency limits (Smith 1981). The important role that moisture content has in affecting (along with grain size) track morphology has been recognized before (Tucker & Burchette 1977; Scrivner & Bottjer 1986; Allen 1997). This study has, however, generated and recovered well-defined subsurface tracks in both dry and saturated sediments, previously considered poor preservational media (Tucker & Burchette 1977; Scrivner & Bottjer 1986; Gatesy et al. 1999). The poor surface development of a track should, in many cases, be treated as an indicator that a definable subsurface track feature has been transmitted or punctured at depth. Permeability and residual strength The sediments chosen for the laboratory track simulations were within the range of silty fine to medium-grain sands, but clays and silts were additionally used in some simulations (T8/F6 & T8/F7). The values of permeability for the sediments used in this study were typical for silty fine to medium-grade sands (from 1.3xlO~ 0 5 to 2.4 x lO^^ms" 1 ), displaying moderate to high permeability (Atkinson & Bransby 1978). The similarity in permeability of the sediments

enabled comparison of the moisture/density relationship of each simulation, as permeability could be treated as a constant. The internal angle of shearing resistance of the dry sands was dependent on the grading and relative density of each sample: therefore the loose-packed character of each test run resulted in a relatively low density. However, the introduction of moisture to the sand had the effect of initially reducing the volume of the sand owing to consolidation, by increasing the relative density of the sample. The position of the water table relative to the indent of the template had an important effect on the ultimate bearing capacity of the sands. The track simulations indicated that the bearing capacity of fine-grained sands increased with water content, but the medium-grained sand decreased in strength with higher moisture content. The saturated fine and medium-grained sediments displayed comparable shear strength, as opposed to their dry state, where the finegrained sediments were weaker. Non-cohesive soils, such as sand, have a low bearing capacity under static loading (British Standard 8004: 1986. Given that the load (force) delivered by a footfall has direction, the resulting failure has few comparisons in the field of soil mechanics, owing to the speed and dynamic nature of the compaction event. However, the types of failure for static loading situations bear a resemblance to the failure observed in the track simulations in this study, where the template indented the sediment beyond its ultimate bearing capacity. The ultimate bearing capacity of a sediment is defined as the minimum pressure required to cause shear failure along a surface (shear surface) of the supporting soil immediately below or adjacent to a foundation (Craig 1992) or, in the current study, under a foot. Prandtl (1920, 1921) recognized that the failure surface was not linear throughout the soil, but consisted of active and passive Rankine zones separated by a radial zone (Fig. 14). Craig (1992) identified three distinct modes of failure (Fig. 13a-c): general shear, local shear and puncture shear. The track morphologies and surrounding maximum zone of deformation (MZD) in the current study can be explained as the result of one or a combination of the three failure modes identified by Craig (1992). The MZD of each of the track simulations and the MZD length/depth and MZD width/depth plots help differentiate between modes of failure. In the case of general shear failure, continuous failure surfaces develop between the edges of the template and the ground surface (Fig. 13a). As

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Fig. 14. Shear surfaces (failure surface) beneath a loading point, showing the relative position of the Rankine zones: I, active Rankine zone; II, radial Rankine zone, arrows indicate direction of force dissipation; III, passive Rankine zone, arrows depict direction of sediment displacement; IV, displacement rim (after Karafiath & Nowatzki 1978).

Fig. 15. Shear surfaces as developed in a soil with an upper strong sand underlain by a weaker sand, showing the preferential development of local shear zones in the weaker sediment (after Braja 1994).

pressure increases toward the bearing capacity of the sediment, a state of plastic equilibrium is reached, initially in the sediment around the edges of the template and gradually spreading downwards and outwards. When the condition of plastic equilibrium has been reached in a sediment, shear failure is imminent through the whole sediment mass (Craig 1992). Ultimately the state of plastic equilibrium is fully developed through the sediment above the failure surfaces. Heaving (bulging) of the ground surface occurs on all sides of the template. In the present experiments the effect is most prominent anterior to and between digits II and III, and III and IV. Local shear failure is typified by a significant compression of the soil under the template, accompanied by only a partial development of the state of plastic equilibrium. The failure surfaces do not reach the ground surface, and only slight heave occurs. Local shear failure (Fig. 13b) is associated with soils of high compressibility, and is characterized by the occurrence of relatively large settlements. Puncture shear failure (Fig. 13c) occurs when there is relatively high compression of the soil under the template, accompanied by shearing in the vertical direction around the edges of the template. There is no heaving of the ground surface away from the edges. This study additionally included a fourth mode of track failure, liquefaction failure (Atkinson & Bransby 1978). This occurs when

the liquid limit is reached and sediment is saturated, causing liquefaction and flow (Fig. 13d), resulting in contorted laminae (when present) and squelch features (Tucker & Burchette 1977). Sediment strength affects its bearing capacity and the scale of shear failure (Braja 1994). When a strong layer under a point load is relatively thin, failure takes place by the force being transmitted through the strong sand layer, followed by general shear failure in the underlying, weaker sand layer (Fig. 15). When an upper strong sand layer is relatively thick, failure may be fully located in the strong sand (Fig. 14). Static loading of sediment causes predictable failure at an ultimate bearing capacity, but a dynamic load causes variation in the distribution of pressure through sediment. Tracks are formed by the action of such dynamic loads, distorting the distribution of forces transmitted through sediment, in the direction of travel of the animal. This is further complicated by the movement of the foot in relation to the body and the kinematics of the step cycle (Fig. 12). Experimental results The tracks generated in the laboratorycontrolled conditions yielded a variety of track

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morphologies (surface and subsurface) indicative of the prevailing rheological conditions. Several of the laboratory-simulated tracks (T8/F4, T8/F9, T8/F11, T8/F13) were formed while the sediment was saturated. The rapid loading associated with track formation caused excess pore pressure to develop within the sediment, as the fluid in the pore spaces had limited travel at its predetermined rate of permeability (&), and was therefore unable to keep up with the rate of loading. Thus, in some cases, track formation generated excess pore pressure in the sediments, which caused liquefaction failure (Fig. 13d), whereupon the sediment could potentially flow under its own weight (Smith 1981), as observed in track simulation T8/F13 at a depth of 15mm (Fig. 6c). Dinosaur tracks cannot be described in terms of simple soil mechanics, although the mode of failure (Craig 1992; Atkinson & Bransby 1978) for the sediment can be recognized.

Laboratory-simulated surface tracks The surface track simulations indicated that the shape and rigidity of a template affected the behaviour of sediment into which it was indented. The initial eight templates used had flat, rigid surfaces, except for T8. The flexible digits of template 8 meant that, on impact with sediment, penetration was to a greater depth, resulting in deeper tracks than for any of the other templates. The shape of template 8 promoted the development of radial shear zones (Fig. 14), indicative of general and local shear failure. However, templates 1-7 compacted sediment beneath them (in the active Rankine zone: Fig. 14) resulting in puncture shear failure (Fig. 13c). It was decided at an early stage of the study that the behaviour of the sediment and resultant tracks created by T8 was most natural, so the template was used for all subsequent simulations. A feature commonly recorded in dry and occasionally for some moist sediments (<2% moisture content) was the development of interdigital shear zone (ISZ) features (Fig. 6d). The surface ISZ features tended to be more developed in dry sediments (<1% moisture content), sometimes resulting in multiple ISZ features (Fig. 6d): these were usually observed as a subsurface features. The surface development of multiple ISZ features was usually due to an increase in the load applied through the template, allowing the surface development of subsurface shear fronts, as a result of a more developed general shear failure than that observed with smaller loads.

A displacement rim was commonly generated along the posterior margin of digit IV (Fig. 6d). This was a result of an unnatural posterior shunt caused by the indenter assembly used for the surface track simulations. It was because of this unnatural type of locomotion and the static step cycle that the indenter assembly was not used for subsurface track simulations, and the NCM assembly was used instead. The surface track simulations also generated radiating surface tension features within a moisture content range of 3-10%. These radiating features were associated with the posterior margin of the heel, distal end of digits, and hypices of the track. A radial tension feature (radial crack) anterior to digit III was observed on several tracks (Fig. 6e), and is interpreted as a surface development of an ISZ. The radial tension features occurred at a sediment moisture content of between 4% and 8% and was poorly developed, although a larger load would increase the development of the feature, as seen in the ISZ feature in Figure 6d. The failure surface for this feature was considered brittle, as the surface feature did not plastically deform, more commonly observed with higher moisture contents. Recognizable surface tracks were formed at a sediment moisture content range of 2—24%, beyond which surface track features collapsed. The coarser-grained sands retained track definition at the highest moisture contents (up to 25% moisture content), whereas fine-grained sands collapsed at a moisture content of 25%. The relationship between moisture content and grading of the sediment suggested that, as grain size increased, the liquid limit of the sediment also increased; the opposite applied to finergrained sediments. Fine-grained sediments retained an outline of the track, even if some of the surface track digits had partially collapsed. However, coarsergrained sediments quickly collapsed (<10min), leaving a low-relief surface track feature. The collapse of the tracks was related to the permeability of the indented sediment. Medium-grained sediments had a higher permeability than the fine-grained sediments. The coefficient of permeability of the fine-grained sediment (sediment A) used in surface track simulations T8/A1.7-T8/A4.7b (Fig. 16a-d), was in the order of 1.36 x KT^ms"1. The coefficient of permeability of the mediumgrained sediment (sediment D) used in surface track simulations T8/D1.7-T8/D4.7 (Fig. 6d), was in the order of SxKT^ms" 1 . Track features in medium-grained sediments flooded more rapidly than those in fine-grained sediments. This caused surface track features to

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Fig. 16. (a) Track simulation T8/A1.7 using T8 indented into sediment A with a force of 49 N when dry (moisture content 0.2%). (b) Track simulation T8/A2.7 using T8 indented into sediment A with a force of 49 N when moist (moisture content 4.1%). (c) Track simulation T8/A3.7 using T8 indented into sediment A with a force of 49 N when moderately moist (moisture content 26%). (d) Track simulation T8/A4.7b using T8 indented into sediment A with a force of 49N when sediment was saturated (moisture content 29.3%).

collapse and infill with water, resulting in the leaching of a fine intergranular component of the sediment into the track (matrix infiltration). The fine-grained sediments used in surface track simulations allowed the template to indent the sediment to a depth of 110mm when saturated (Fig. 16d). However, similar moisture contents (30%) for coarser-grained sediments allowed the same template, under the same load, to indent to a depth of only 50 mm. The strength of the sediment decreased with increasing moisture content, although a larger grain size increased the threshold at which the optimum moisture content of the sediment is reached. This effectively increases the strength of coarsergrained sediments, as the water film around the sediment particles is negligible compared with the diameter of the sand grains. However, the surface film of water on fine-grained sands

permits the particles to slide over each other more easily when an external load is applied.

Laboratory-simulated subsurface tracks Template 8 was applied using the NCM apparatus for all the subsurface simulations, with a load of 5 kg applied during each experimental run. Dry fine-grained sands (sediments A, B and E) had lower shear strength values than dry medium-grained sands (sediments C, D and F), which allowed template 8 to affect layers up to a maximum of 92mm below the surface track layer. The minimum affected sediment depth for the template in dry fine-grained sediments was 75mm below the surface track layer. The dry, fine-grained sediments of track simulations T8/F2, T8/F10 and T8/F12 all displayed features

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typical of general shear failure (similar to Fig. 6d). Well-developed interdigital shear (ISZ) surfaces spread downwards and in front of the tracks, and an anterior displacement rim was displayed on the surface track layer of track simulations T8/F10 and T8/F12. However, track simulation T8/F2 showed surface puncture of the top track layer caused by the template, apparent by the presence of separate entrance and exit points for digits II and IV. The depth of puncture was not more than 40 mm, or the template would have punctured layer 5 (Fig. 17b). A distinct subsurface feature was observed on the anterior track margins of all the dry, fine-grained sands tested (Fig. 17a). The feature

was the transmitted position of overlaying posterior margins of digits II and IV, and was named a multiple digital transmission (MDT) feature (Fig. 8b). The MDT feature was observed only as a transmitted subsurface feature, and with dry, fine-grained sands occurred on layers at depths of 30-60 mm. The deeper MDT feature (92mm) observed with track simulation T8/F2 resulted from template 8 puncturing layer 9 by up to 40 mm, generating a deeper MDT feature. Key features of track simulations in dry, finegrained sediments: Lower shear strength than dry mediumgrained sediment.

Fig. 17. (a) Track simulation T8/F2, layer 2, top surface of plaster track layer 3 at a depth of 8 cm below the surface track layer, showing multiple digital transmission (MDT) features and well-developed interdigital shear zones, (b) Track simulation T8/F3, layer 5, basal surface of plaster track layer, at a depth of 2.9 cm below the surface track layer with interdigital shear zones, (c) Track simulation T8/F7, layer 9, basal surface of plaster track layer at a depth of 8 cm below the surface track layer. Shows collapse of digits and posteriorly directed projections (cf. Fig. 21b). (d) Track simulation T8/F6, layer C, basal surface of plaster track layer from a depth of 2.3 cm below the surface track layer.

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Features deeper and better developed (MDT, ISZ) in fine-grained than medium-grained sediment. Simulations typically displayed general shear failure. Well-developed ISZ and displacement rim features. Digital positions transmitted to 69-92 mm depth. Track relief increased anteriorly from 30— 40 mm depth. Distal end of digit III compacted track layers to a depth of 60-90 mm. MDT feature occurred at 30-60 mm depth. Clear anterior track displacement with increasing depth. Increasing interdigital angle with depth. Decrease in digit length with depth. For dry, medium-grained sands (sediments C, D and F), track simulations influenced layers up to a maximum depth of only 65 mm below the surface track layer. The minimum depth of influence in dry medium-grained sediments was 46 mm below the surface track layer. Simulations T8/F3 and T8/ F8 displayed features typical of general shear failure. Well-developed interdigital shear (ISZ) surfaces spreading downwards and forwards were also evident in the tracks, combined with an anterior displacement rim displayed at the surface of track layer 7 of simulation T8/F3. The ISZ features of simulation T8/F3 (Fig. 17b) were well developed in layer 5B, giving the misleading appearance of interdigital webbing. Key features of track simulations in dry, medium-grained sediments: Simulations typically displayed general shear failure. Features shallower and less developed than in fine-grained sediment. Well-developed ISZ feature and sometimes an anterior displacement rim. Digital positions transmitted to a depth of 39-45 mm. Track relief increased anteriorly from 2932 mm depth. Distal end of digit III compacted track layers to depth of 39-54 mm. MDT feature occurred at 32-56 mm depth. Tracks displayed clear anterior displacement with increasing depth. Interdigital angle increased with depth. Digit length decreased in length with depth. Saturated fine-grained sands (sediments A, B and E) had shear strengths comparable to those of the saturated medium-grained sands (sediments C, D and F), in contrast to the dry state

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of these sediments. The saturated strength of the fine-grained sands was greater than that of the dry sediments. However, the saturated strength of the medium-grained sand decreased, compared with its dry state. Track simulation T8/F7 (Fig. 17c) was an exception to this trend, inasmuch as it exhibited a decrease in strength, with a maximum depth track feature visible at 99mm below the surface. However, this was due to puncture failure to a depth of 58mm, owing to the influence of interlaminated clays in this track simulation, given that the failure terminated at a clay horizon. Key features of track simulations in finegrained saturated sediment: Increased shear strength compared with dry state. Liquefaction failure displayed at depths up to 15mm. Local shear failure occurred from 15mm downwards. Poorly developed (low-relief) ISZ and displacement rim features. Digital positions transmitted to a depth of 49-57 mm. Track relief increased anteriorly from a depth of 30 mm. Distal end of digit III was compacted to a depth of 60 mm. MDT features occurred at 33-52 mm depth. Anterior track displacement occurred at depths below liquefaction or puncture failure. Interdigital angles and digit length varied with depth, displaying no discernible trend. Saturated medium-grained sands (sediments C, D and F) exhibited shear strengths comparable to those of saturated fine-grained sands (sediments A, B and E). However, the shear strength of the medium-grained sands decreased when saturated (track simulations T8/F4 and T8/F9), compared with the dry strength of the sediments. Saturated medium-grained sediments had lower shear strength than dry medium-grained sands (sediments C, D and F), allowing template 8 to affect layers up to a maximum of 94 mm below the surface track layer (layer 9 of simulation T8/F4). The minimum affected sediment depth for the template in saturated medium-grained sediments was 40mm below the surface track layer of simulation T8/F6. The saturated sediment (F) used for track simulation T8/F6 was medium-grained; however, an underlying layer (10 mm thick) of fine clay prevented the transmission of track features beyond a depth of 40 mm (Fig. 17d). Key features of track simulations in mediumgrained, saturated sediments:

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Shear strength decreased compared with dry state of sediment. Surface layers (up to 33-34 mm depth) display puncture failure. General shear failure occurs below 34mm depth. Well to poorly developed ISZ features and displacement rims. Digital positions transmitted to a depth of 40-66 mm. Track relief increased anteriorly from a depth of 3 5-46 mm. Distal end of digit III compacted to a depth of 60mm. Development of MDT features was restricted to 10-15 mm depth. Anterior track displacement occurred at depths below puncture failure. Interdigital angle and digit length displayed little variation with depth.

Discussion This study recognizes two main groups of influences on the morphology of dinosaur tracks, whether fossil or laboratory-simulated: (1) kinematic and morphological influences of the animal on the track, and (2) preservational influences of rheological conditions etc. on the track. Track-maker morphology influences the size, gait, step cycle (kinematics) and distribution of weight over a foot, affecting final track formation. The preservational influences account for the mechanical and physical properties of a substrate and their effect on resultant tracks. The laboratory-simulated tracks provided information not only on the preservation of tracks, but also on the influence of track morphology. How an animal interacts with a substrate is integral to understanding any resultant preservational features. How an organism's foot interacts is dependent upon the anatomy of its limb, the morphology of the foot, the movement of a limb during the step cycle, and the speed and behaviour of the animal at the time of track formation. The substrate in which tracks are made also affects their formation, inhibiting or enhancing the animal's ability to traverse the substrate, and the gait adopted in some cases. The composition, properties and condition of a substrate also have implications for the preservation potential of a track.

Dinosaur anatomy and gait The form of the vertebrate skeleton has a direct influence on the function of a limb, as it provides the anchor points for the musculature that drives locomotion, and delineates the degree of possible movements of a limb. The body fossil record preserves information on the skeletal anatomy, size, and hence the inferred gait of some dinosaurs. This has enabled workers to generate functional models and to reconstruct the locomotion of many dinosaurs (Romer 1923; Alexander 1985; Norman 1986; Johnson & Ostrom 1995).

Comparison of experimental tracks and OPB data The results from the optical pedobaragraph (OPB) confirmed that the distribution of pressure over the sole of T8 corresponded with the location of many track features. High pressure values corresponded with high-relief features, and conversely low pressures with shallow or missing features were associated with the original template morphology. The dynamically loaded OPB track simulations all recorded the highest point of pressure exerted over the sole of the template beneath the distal end of digit III. A consistent feature observed in almost all the subsurface track simulations was the compaction of track layers at depth under the area of the distal end of digit III. The OPB traces also indicate that the main load-bearing digits in both static and dynamic loading situations were digits II and III (III being the dominant load-bearing digit), with digit IV often leaving little or no pressure trace. The most common digits to be transmitted to lower subsurface layers in the laboratory track simulation were likewise digits II and III.

Comparison of laboratory-simulated tracks with fossil tracks The current study interpreted the morphological features generated in laboratory-simulated tracks by a tridactyl template, while varying sediment grain size and moisture content. The Yorkshire Coast Middle Jurassic track assemblage is dominated by tridactyl tracks (see Romano & Whyte 2003 for review), and many of the preservation features observed in the tridactyl tracks were comparable to those from the laboratory simulations.

CONTROLS ON PRESERVED TRACK MORPHOLOGY Burniston dinosaur tracks Tridactyl dinosaur tracks from the Middle Jurassic Scalby Formation, Burniston Bay, North Yorkshire, were observed in the field,

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The tracks displayed a variety of features eomparable to some of the laboratory-simulated tracks, generating seven categories of tracks studied at this locality:

Fig. 18. (a) Tridactyl dinosaur track (A) and additional digit (B) from an adjoining track. Track preserved as a cast feature (5 m south of steps, towards Cromer Point), Burniston Footprint Bed, Scalby Formation, Long Nab Member, Burniston Bay, Yorkshire (scale bar 10cm). (b) Track simulation T8/F4, layer 4, basal surface of plaster track layer (4B) from a depth of 4.6cm below the surface track layer, showing distinct anterior displacement (scale bar 10cm). (c) Tridactyl dinosaur track A displays pronounced digits II and III with digit II leaving almost no mark. Tridactyl dinosaur track B displays collapsed digits, leaving only a pronounced 'heeF cast. Tracks preserved as cast features on the sole of a sandstone bed. Burniston Bay (15m south of steps, towards Cromer point), Burniston Footprint Bed, Scalby Formation, Long Nab Member, Burniston Bay, Yorkshire, (d) Detail of a single track preserved on the underside of a sandstone block as cast in relief. Burniston Bay, Scalby Formation, Long Nab Member, Burniston Bay, Yorkshire. Radiating from the track and related to its formation are desiccation track-like features.

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1. High moisture content. The single tridactyl track (A) from the Scalby Formation (Fig. 18a) - with an additional digit (B) cutting into the complete track - shows a faint interdigital shear zone feature (ISZ). Similar surface collapse features were observed with simulation T8/D4.7, when medium-grained sand was saturated at the time of indentation. The surface track indicates that the foot of the dinosaur penetrated at least as far as the upper surface of the track block, which subsequently collapsed on withdrawal of the foot. The basal tridactyl track (A) increases in relief anteriorly, indicating that it was transmitted from a depth within the thickness of the track block (70-80 mm). Laboratory-simulated tracks in similar saturated medium-grained sediments in simulation T8/F4 (Fig. 18b) displayed puncture failure, with collapsed digit features similar to track A. As measured from the most posterior point of the heel to the distal end of digit III on the top surface, the track length of the specimen in Figure 18a is 150mm. This increases to 175mm on the underside of the block, and shows an anterior displacement of at least 25 mm relative to the faint tridactyl track on the upper surface. The increase in track length on the underside of the block could have been caused by an underlying layer of clay. Similar increase in track length was observed in simulation T8/F6, which diverted the downward-directed transmission forces, causing anterior displacement of the weaker sediment above the lowermost clay. The fossil track (Fig. 18a) shows evidence of having been transmitted to the top of a thick clay layer, which has subsequently been weathered, leaving traces of the clay adhering to the margins of the track digits. Such a track is likely to have been formed as shown diagrammatically in Figure 19 through comparison with laboratory simulation T8/F6.

Fig. 20. Sketch of the upper surface of a track block displaying collapsed digits. The faint interdigital shear zone (ISZ) is highlighted.

A second specimen shows collapsed digits and development of a faint ISZ feature (Fig. 20) on the upper surface of a track block, indicating saturated conditions at the time of track formation (cf. simulation T8/F4: Fig. 18b). The transmitted track on the basal surface of the block displays increasing relief anteriorly and no ISZ or MDT features; however, the median line of digit III indicates that it has collapsed. 2. Moisture content 20% (saturated). Several tridactyl dinosaur tracks from the Burniston Footprint Bed had only partial digits preserved (Fig. 18c), displaying collapsed distal ends to digits, often leaving pronounced heel casts. The Burniston Footprint Bed sandstones (fine- to medium-grained) are underlain by silty shale, which played an important role in the excellent preservation displayed by many of the dinosaur tracks from this locality. The marked phalangeal nodes (pads) of track digits and presence of heel impressions indicate that the tracks were formed

Fig. 19. Hypothetical cross-section of a track along the median line of digit III, showing variation in track length (Li surface track and L2 basal track) and displacement (D) with depth. The diagram is based on the fossil specimen in Figure 6a and the experimental track in Figure 18b.

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near, but not at, the surface. The partial collapse of the most distal end of digits and the complete collapse of digits indicate that tracks were formed at different moisture conditions (approximately 20% and 30% respectively). None of the tracks shows any transmitted features (such as relief increasing anteriorly with depth or MDT features), and they display a consistent depth of relief for the length of track features. The dinosaurs that made the Burniston tracks walked over a fine- to medium-grained sand, into which their feet sunk by approximately 1040mm, until encountering the stronger subsurface silty horizon, which plastically deformed, rather than being punctured. The tracks with a distinct heel but collapsed digits were the shallower features (10-20 mm), where sand collapsed around the digits (Fig. 18c) on extraction of the foot, leaving only a distinct heel impression (cast). The well-defined transmitted tracks show distal or no collapse features on digits. These were deeper (20-40 mm) track features, where the foot of the dinosaur compacted the sand under its feet into the silty horizon, along the whole length of its foot. There were no shear features (ISZ or MDT) associated with these particular tracks from Burniston. In terms of soil mechanics, the pressure of footfall equivalent to the active Rankine zone (Fig. 20) - compacted an image of the foot onto the upper surface of the silt. However, the greater shear strength of the silts did not permit the development of the radial shear zone, resulting in a well-defined track image. The absence of a toe-off phase feature in any of the tracks (e.g. Fig. 18c) suggests that the substrate may have affected the way in which animals traversed the area. It was noted that, during experimentation, when T8 was applied in a threephase step cycle the template would slide on moist or saturated clays underlying a 20-40 mm thick sand. Only when the template was exerted at right angles to the sediment (simulation T8/ F6) did the template leave a stable track, which in turn allowed the recovery of subsurface track layers. Track simulation T8/F6, in saturated mediumgrained sands underlain by a clay layer (at depth 40 mm), exhibited many track features observed in the Burniston Footprint Bed (Fig. 17d). The pronounced heel, with collapsed digits formed as a near-surface feature, in which the heel compacted the sands beneath it, but did not puncture the underlying clays, the overlying saturated sand caused the digits to collapse on withdrawal of the template. The resulting track was almost identical to some Burniston Footprint Bed tracks (e.g. Fig. 18c). During

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experimentation, the compacted area of sand was pushed into the clay layer, producing welldefined tracks (Fig. 17d); however, the phalangeal nodes (pads) are better defined at a lower moisture content (Fig. 6e), which suggests that tracks with pronounced heels and collapsed digits formed while the sediment had a high moisture content (saturated), and that the better-delineated phalangeal tracks formed at lower moisture contents (c. 20%). 3. Transmitted tracks with decreasing moisture content. Detail of one of the tracks studied (Fig. 18d) shows that the desiccation cracks often originate from the distal end of track digits, indicating that the tracks formed before the desiccation cracks. The tridactyl tracks (Fig. 18d) increase in relief anteriorly and are poorly defined, indicating that they were transmitted features. The distal ends of the digits were joined by what might be the transmitted remnant of an interdigital shear zone (ISZ) feature. The distal end of digit III (Fig. 18d) undercut the siltstone into which the basal cast feature was transmitted, from within the 600700 mm depth of the sandstone block. The siltstone probably acted as a termination point for transmitted forces from the footfall, owing to its relatively high shear strength compared with the overlying sands. A similar transmitted track was observed on a basal clay layer of simulation T8/F6. The track was formed while an overlying, medium-grained sand layer was saturated (as was the underlying clay layer). The clay layer had the effect of diverting the force bulb laterally, expanding the track's dimensions, and causing the distal end of the digits to almost join. On drying, desiccation cracks formed in the clay 40mm below the sand. This indicates that desiccation cracks are not necessarily a good indicator for recognition of a true surface track horizon. 4. Dry sands punctured to lower horizons with higher moisture content. An excellent tridactyl pes track (Fig. 2la) was observed on the underside of a fine-grained sandstone block preserved in hyporelief. Associated with the transmitted pes imprint, an anterior displacement rim comprising multiple interdigital shear zones (ISZ) - resulting in an extensive maximum zone of deformation (MZD). The distal end of digit III clearly displays a broad hoof impression (Fig. 2la), typical of those expected of an ornithopod dinosaur, such as Camptosaurus from the Middle Jurassic (Weishampel et al. 1990). The sediment between and anterior to digits II and III displays a faint ISZ feature, but this is swamped by the anteriorly displaced, concentric failure feature. The

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Fig. 21. (a) Tridactyl pes (right foot) preserved as a cast in hyporelief, associated with an anterior displacement rim consisting of several concentric failure surfaces, yielding an extensive maximum zone of deformation (MZD) around the pes. A well-defined 'hoof is also present on digit III, which has punctured to the base of this sandstone. From the Scalby Formation, Long Nab Member, Burniston Bay, Yorkshire (scale bar 10cm). (b) Transmitted tridactyl dinosaur track, preserved on the underside of a sandstone block as a cast in hyporelief, with digit IV only partially recovered. Arrow indicates a shear surface in the interdigital shear zone (ISZ). Compare the distinct posterior projection with that in Figure 17c. From the Scalby Formation, Long Nab Member, Burniston Bay, Yorkshire, (c) Large tridactyl dinosaur track with associated smaller tridactyl dinosaur track (top left) preserved as a cast in hyporelief. A faint interdigital shear zone (ISZ) feature is present, and is cut by the isolated digit. From the Scalby Formation, Long Nab Member, Scalby Bay, Yorkshire (scale bar 10cm). anterior displacement rim is a development of the combined ISZ features of digits IV-III and III-II, resulting in a combined multiple interdigital shear zone feature. The type of failure seen in the fossil track was observed in track simulation T8/F2, which was

performed upon a dry, fine-grained sand, suggesting similar soil conditions for the fossil track. The presence of such well-developed ISZ features was a typical feature of dry, fine-grained track simulations. The broad hoof (digit III) marks the maximum penetration of the foot

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(punctured to the base of the sand) at the toe-off phase of the step cycle. The rest of the track is a transmitted feature. The preservation of the 'hoof suggests perforation to a horizon with higher moisture content than the overlying dry sands, accounting for the preservation of this deep track feature. Similar anterior concentric failure rings have been observed in previous studies, attributing the sand crescents to upslope and downslope features generated by locomotion over sand dunes (Reynolds 1989). However, several horizontally bedded tracks observed in the current study displayed anterior sand crescents, suggesting that their formation was influenced by the dynamics of locomotion combined with the foot morphology of the track-maker and the mechanical properties of the substrate, rather than by local topography. Similar anterior displacement rims (sand crescents) were generated in laboratory simulations (Fig. 17a). The laboratory-simulated tracks also indicate that the displacement rims were not affected by the surface topography of the sediments in the porous-plate testing box, as they were horizontally bedded. 5. Deep transmitted in saturated conditions. A tridactyl track (Fig. 21b) from Burniston Bay displays features typical of a transmitted track. The track increases in relief anteriorly and the heel is poorly transmitted, represented only by a faint, posteriorly projecting feature. Digit III is the dominant digit, indicating that it was the main load-bearing digit of the track-maker. The mixed nature of the sediment within digits II and III suggests that the foot punctured or compacted underlying layers to penetrate the exposed horizon. A faint multiple interdigital shear zone (ISZ) feature between digits II and III is partially preserved at the hypex of the digits (Fig. 21b, arrowed). The track simulation T8/F7 produced a deep (80mm) transmitted track feature (Fig. 17c), very similar to the transmitted track described above (Fig. 21b). The simulated track displayed all the features observed in the fossil track, including the faint posteriorly projecting feature at the heel. The simulated track indicates that the posteriorly projecting feature may relate to a multiple digital transmission feature (MDT), unique to transmitted tracks. Digit III of the simulated track also displays puncture features, where overlying layers were compacted into lower layers. The fossil track indicated that the exposed basal layer was at least 60mm below the true surface on which the organism had walked. The saturated, fine-grained sands used in the laboratory simulation reproduced (at depth) a track feature very similar to the fossil

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track, also formed in similarly fine-grained sands. The fossil track (Fig. 21b) was most probably formed when the sediment had a high moisture content (if not saturated), and is a deep (at least 60 mm) transmitted feature. 6. Liquefaction failure close to true surface track. A large (300mm long) tridactyl track (Fig. 21c) collected from the Scalby Formation was preserved as a low-relief cast on the underside of a fallen block. Digits II and III were clearly defined, with digit IV being relatively faint. The track increases in relief anteriorly. The track infill was paler than the surrounding sediments. A smaller tridactyl track was located anterior of digit II of the large track. The smaller track post-dated the large track, as it deforms the interdigital shear zone. The serial-sectioned track (F00813), also from the Scalby Formation (Fig. 6a), displayed a similar track prior to sectioning. The cross-section along the median line of digit III (Fig. 6a) shows a region of down-warped sediments in the heel area (A) that represents deformation caused at the heel-down phase of the step cycle (Fig. 12a). The mid-region (B) of the section of digit III represents the second forward rotation phase of the step cycle (Fig. 12b). The third and most distinct point of the step cycle, the toe-off phase, is clearly represented by a severely down-warped area (C) of sediment (Fig. 12c, d). The track displays all three phases of a step cycle that would be expected for walking using a hipbased retractor mechanism. The basal features for the tracks in Figure 6a and Figure 21c show some evidence that they were transmitted: the increase in relief anteriorly, the faint heel feature, and the dominant relief of digits II and III (load-bearing digits). However, it is the cross-section along the median line of digit III of track F00813 (Fig. 6a) that indicates that track F00813 and possibly that in Figure 21c are deep transmitted features, with MZD (track length) reducing incrementally with depth. Track F00813 was formed in fine-grained sand, with very fine interlaminated silt and organicrich horizons. Based on the data generated from the laboratory track simulations, the incremental reduction in MZD length in a finegrained sand indicates that the sand was saturated (or had a high moisture content) at the time of track formation. The presence of what appears to be liquefaction failure at the anterior margin of the track F00813 (Fig. 6a (C)) also supports the notion that the sediment had a high water content at the time of track formation. The presence of a very faint ISZ feature on the larger track (Fig. 21c) suggests that it was also formed while the sediment had a high

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moisture content, but closer to the surface than track F00813. 7. Transmitted with liquefaction failure. Three tridactyl tracks (Fig. 22) preserved as surface tracks were defined only by the highly deformed/contorted sediment within the tracks,

suggesting that the sediment underwent liquefaction failure at the compactive event of track formation. All digits display medial collapse structures, indicating that the track collapsed on withdrawal of the dinosaur's foot. Figure 22b also shows a natural, end-on cross-section

Fig. 22. (a) Tridactyl dinosaur track (dorsal surface of track) defined only by the highly deformed/contorted sediment within the track. Scalby Formation, Long Nab Member, Scalby Bay, Yorkshire (scale bar 10cm). The outline is marked with chalk for clarity, (b) Tridactyl dinosaur track (dorsal surface of track) defined only by the highly deformed/contorted sediment within the track. Scalby Formation, Long Nab Member, Scalby Bay, Yorkshire (scale bar 10cm). (c) Tridactyl dinosaur track (lower surface of track) defined by the deformed sediment within the track. Anterior transmitted track (A) with transmitted posterior displacement feature (B). Scalby Formation, Long Nab Member, Scalby Bay, Yorkshire (scale bar 10cm). The outline is marked with chalk for clarity.

CONTROLS ON PRESERVED TRACK MORPHOLOGY

of the heel area of the track, defined by the highly deformed/contorted down-warped sediment, more pronounced near the surface of the track. Fragments of the paler overlying fine-grained sandstone have been pushed down into the heel area, floating in the darker, grey silty sand infilling this feature. The grey, silty sand infilling delineates a possible feature relating to an incremental decrease in the maximum zone of deformation (MZD) with increased depth. Owing to the fine grade of the sediment, this suggests that the sediment had a very high moisture content, and was possibly saturated, at the time of track formation. The track in Figure 22c displays two distinct areas relating to its formation. Area B could be interpreted as a fleshy pad at the back of a foot, or as a transmitted heel feature. Area A represents the distorted area of sediment created on extraction of the track-maker's foot. Area A is interpreted as the dynamic result of a foot puncturing the saturated sediment, with the resultant track feature migrating anteriorly with depth. Area B, if interpreted as a transmitted feature, represents the entrance point of the track-maker's heel at the true track surface, transmitted to depth. The track simulation T8/F11 and simulation T8/F13 (Fig. 6c) produced features comparable to the three fossil tracks, in saturated, finegrained sediments. The simulated track showed liquefaction failure of the surface track. The natural section of the heel of the fossil track also indicates that the intensely deformed sediments were restricted to the top 30mm of the track. Track simulation T8/F13 clearly shows the overturned and contorted elements of the layers that underwent liquefaction failure (Fig. 6c). The three fossil tracks all display similar failure; however, those in Figure 22a and 22b are surface tracks, whereas that in Figure 22c is a transmitted track. The incremental decrease in MZD with depth, combined with the fine grade of the sediment, suggests that the sediment had a very high moisture content, and was possibly saturated, at the time of track formation. The liquefaction failure of the surface sediments and digit collapse features also support the inferred interpretation of high moisture content at the time of track formation. Utilization of fossil tracks An understanding of laboratory-simulated tracks can assist in unravelling how fossil tracks are formed and the processes that subsequently altered them. The use to which many workers have applied fossil tracks must be reviewed critically. Studies using size of fossil

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tracks and morphology to determine the taxonomic affinities require supporting evidence generated by laboratory track simulations, and an appreciation of morphological differences between surface and transmitted track features. The interpretation of features observed in laboratory-simulated tracks from this study can be usefully applied to fossil tracks. Vertebrate ichnofacies Lockley et al. (1994) suggested that vertebrate tracks could be compared to invertebrate traces, claiming that their distributions were both controlled by the same sedimentological and stratigraphic principles. They reasoned that repeatedly occurring track assemblages (ichnocoenoses) in various terrestrial deposits might help to define distinctive vertebrate ichnofacies. The term 'ichnofacies' was considered to equate to recurrent ichnocoenoses associated with particular ancient environments, preserved as a distinctive lithofacies (Lockley et al. 1994). The term 'ichnofacies' was originally introduced by Seilacher (1964,1967) to describe recurring associations of trace fossil assemblages associated with specific lithofacies and depositional environments. The ichnofacies reflected the environmental conditions (salinity, substrate character and bathymetry) and the organism's often intimate relationship with a specific substrate (see review in Mcllroy 2004). The current study suggests that the composition, properties and condition of a substrate directly control the 'type' of track preservation and resultant morphology. The use of a single or recurring vertebrate ichnocoenosis to diagnose specific facies relationships (ichnofacies) is rejected, on the grounds that vertebrate tracks are not substrate-specific and are a function of the soil conditions (controlled by the prevailing moisture/density relationship at a given time). Laboratory simulation generated a diverse range of track morphologies that could assist in the interpretation of substrate conditions prevailing in a specific environment at the time of track formation. However, the transient and often ephemeral relationship between vertebrates and specific environments makes it difficult, if not impossible, to define environments based on ichnocoenoses, given that many terrestrial vertebrates are not restricted to a particular palaeoenvironment - particularly the Dinosauria. A tangled tale of vertebrate track taxonomy Fossil vertebrate track nomenclature and that of other trace fossils is hotly debated (see Sarjeant 1990 for review). The diverse track morphologies generated using a single template in the course of

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this study have shown how little, if any, of the track-maker's morphology is preserved in transmitted tracks. When naming tracks, are ichnologists identifying a specific animal's track or the trace of an action? A trace fossil is a sedimentary structure that is produced by biological activity, and has a form that is determined in part by that activity, often reflecting the morphology of the track-maker to only a limited extent (Sarjeant 1990). However, one of the aims of a vertebrate ichnologist is still to identify the track-maker, a goal that is seldom possible. The morphological variability on which ichnotaxa are often erected relates to the often complex formation and preservation of a track, not the foot morphology of the progenitor. Invertebrate and vertebrate palaeoichnologists have for many years followed the guidelines set down in the International Code of Zoological Nomenclature (ICZN) when naming new ichnotaxa. Modifications to the ICZN guidelines (Basan 1979) have provided a defined status for trace fossils within zoological taxonomy (see Mcllroy 2004). However, some authors have suggested that a completely separate code of nomenclature is still required for trace fossils, in which ichnotaxa are differentiated from other natural animal taxa, and that the aim of the ichnologist should be restricted to identifying either the track-maker or its activity (Sarjeant 1990; Mcllroy 2004). Some ichnologists (Farlow 1992; Farlow & Lockley 1993) disagree with Sarjeant (1990), and have placed ichnotaxa within osteologically based orders and classes. This approach has potential problems, such as when the tracks of a tyrannosaur were described (Tyrannosauropus Haubold 1971) that have since been attributed to a hadrosaur (Lockley 1991). Under the rules laid down in the ICZN code, the first name given to a taxon takes priority over subsequent reinterpretation, however unsuitable or misleading the name. Sarjeant (1990) argues clearly that such an approach has an element of absurdity, because it gives equal systematic status to the animal itself and to the effect of its actions. The current study has produced many overprint, true-surface and subsurface track features, all of which assist with the understanding of the interaction of a foot with the substrates tested in this study. The information gained on the preservational circumstances and effects, from the laboratory track simulations, far outweighed the information gained on the track-maker's foot (template) morphology. An ichnotaxon, whether an ichnospecies, ichnogenus or ichnofamily, should reflect the morphology of the

track, its formational history, and preservation features (Peabody 1948). The results of this study, however, indicate that an ichnological approach has merit over an osteologically based approach, and suggests that many track features have little use as ichnotaxobases. Sarjeant (1990) concluded similarly that: we should remember at all times that we are naming, not the animal itself, but the trace of its act as reflected in the sediment and preserved in the fossil record. The need for a standard approach to the description of tracks is vital if the trace of an action or preservational features of an animal's activity are to be the criterion by which it is named. Leonardi (1987) and Thulborn (1990) provide the most consistent approach to track description and terminology, but terms were not available to describe some of the features observed in the current study. The terminology adopted in this study has utilized some existing terms and has adapted and developed terms specifically relating to the surface and subsurface morphology of a track (see Figs 7, 8). Terms such as the maximum zone of deformation (MZD), interdigital shear zone (ISZ), and multiple digital transmission (MDT) describe morphological features that are the result of sediment behaviour caused by a limb's interaction with a substrate during track formation. The MZD, ISZ and MDT features should not be used as characters to define an ichnotaxon, only to help interpret them, as they relate to sediment behaviour in relation to the dynamic formation of a track, and not to animal behaviour. The variations in MZD, ISZ and MDT are useful tools for assisting in the interpretation of a track (vertebrate or invertebrate), as are the digital lengths and interdigital angles (IDA). The use of digit length and IDA as descriptive terminology has proved useful in the current study, but the instability of these values within a single track also makes them unsuitable ichnotaxobases. In summary it is considered that vertebrate ichnotaxa should reflect the morphological differences resulting from track formation and preservation and not the affinity of an alleged maker. Conclusions A controlling factor in the formation of track features is the moisture/density relationship prevailing in the sediment at the time of indentation.

CONTROLS ON PRESERVED TRACK MORPHOLOGY

Subsurface track (simulated) morphology demonstrated the dynamic nature of track formation by the anterior displacement of track features with depth, producing 'stacked' tracks. The direction of the displacement of the subsurface track features corresponded with the direction and magnitude of the force applied and the physical properties of the sediment through which the force travelled. Multiple digital transmission (MDT) features were unique to subsurface deformation, indicating a transmitted track layer. The MDT features were developed over a greater depth of track within dry sediments, with a vertical reduction of MDT features with increasing moisture content. Alteration of grain size and moisture content in the laboratory-simulated tracks affected the type and extent of the development of the maximum zone of deformation track length (MZD track length). Dry medium-grained sands displayed a trend towards large incremental reduction in MZD track length with increasing depth. Dry fine-gained sands displayed trends towards little or no reduction in MZD track length with depth. Saturated (moisture content >29%) mediumgrained sands displayed trends towards little or no reduction in MZD track length with depth. Saturated (moisture content >29%) finegrained sands displayed trends towards a large incremental reduction in MZD track length with depth. Optical pedobaragraph (OPB) studies confirmed that high pressure values corresponded to high-relief features in tracks, and conversely low pressures corresponded to shallow or absent track features. The distribution of pressure across a foot relates to the kinematics of the step cycle, and implies the limb retraction mechanism adopted by that animal. The correlated track relief of simulated and fossil tracks can possibly be used to define the limb retraction mechanism adopted by dinosaurs from transmitted track features that display the distribution of pressure over the sole of the foot. Track length within different 'layers' of a single transmitted track were dependent on sediment consistency and properties. Only the 'true' surface track 'layer' should be used to calculate the hip height, based only on the foot length, as transmitted track lengths can vary greatly from the 'true' surface track length. The presence of cohesive sediment (clay) below non-cohesive sand increases the preservation potential of tracks (in the non-cohesive sediment) at the junction between the two.

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Desiccation cracks post-dating a track do not always indicate aerial exposure of the track surface. Cohesive soils overlain by moist to saturated non-cohesive soils can be indented by a foot generating a transmitted track at depth. On drying, the transmitted track in the cohesive soil cracks, and is subsequently infilled by the overlying non-cohesive soils, giving the false impression of a surface track feature (albeit with transmitted track morphology). The use of dinosaur tracks in comparative multivariate studies should be restricted to surface track features, for comparison with other surface track features. The inclusion of transmitted tracks in such studies invalidates any inferred taxonomic or osteological relationships, owing to the disparity between surface and subsurface track morphology and size. Any multivariate study based on morphological variability in tracks and trackways will be viable only if the three-dimensional variability of track morphology is understood. Future multivariate studies must approach the task of understanding the three-dimensional components of a track before valid comparison can be made with other tracks within a three-dimensional framework. Vertebrate ichnotaxa should reflect the morphological differences resulting from behaviour, not the affinity of an alleged track-maker or artefacts of formation and preservation. The methods developed for the recovery and interpretation of laboratory subsurface track layers may go some way to assisting with the interpretation of fossil tracks. However, many more track simulations are required in varying sediment types, and with realistic indenters, to allow a more complete understanding of processes that form and subsequently alter surface and subsurface track morphology and composition. This research would have not been possible were it not for a studentship awarded by the University of Sheffield and supervised by M. Whyte and M. Romano. K. Padian, P. Ensom, M. Benton, J. Pollard are thanked for critical comments and J., A. & K. Manning for their support. Thanks also to D. Mcllroy for editorial help, for his patience, and for redrafting the figures.

References Alexander, R. M. 1985. Mechanics of posture and gait in some large dinosaurs. Zoological Journal of the aasd

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ALLEN, J. R. L. 1989. Short paper: Fossil vertebrate tracks and indenter mechanics. Journal of the Geological Society, London, 146, 600-602. ALLEN, J. R. L. 1997. Subfossil mammalian tracks (Flandrian) in the Severn Estuary, SW Britain: mechanics of formation, preservation and distribution. Philosophical Transactions of the Royal Society, London, Series B, 352, 381-518. ATKINSON, J. H. & BRANSBY, P.L. 1978. The Mechanics of Soils: An Introduction Critical State Soil Mechanics. McGraw-Hill, London. BASAN, P. B. 1979. Trace fossil nomenclature: the developing picture. Palaeogeography, Palaeoclimatology, Palaeoecology, 28, 143-167. BETTS, R. P., FRANKS, C. I. & DUCKWORTH, T. 1991. Foot pressure studies: normal and pathologic gait analyses. Part II: Biomechanics of the foot and ankle. In: JAHSS, M. H. (ed.) Disorders of the Foot and Ankle-Medical and Surgical Management, 2nd edition, W B Saunders, Philadelphia, Volume 1, Chapter 18, 484-519. BIRD, R. T. 1944. Did brontosaurs ever walk on land? Natural History, 53, 60-67. BOUSSINESQ, J. 1883. Application des potentials a I 'etude de I'equilibre et du mouvement des solides elastiques. Gauthier-Villars, Paris. BRAJA, M. D. 1994. Principles of Geotechnical Engineering. PWS, Boston, MA. BRITISH STANDARDS INSTITUTION, 1986. BS8004, Code of Practice for Foundations, BSI, London. BUCKLAND, W. 1828. Note sur les traces de tortues observees dans les gres rouge. Annals de Sciences Natural, Paris, 13, 85-86. CLARK, J. & ALEXANDER, R. M. 1975. Mechanics of running by quail (Coturnix). Journal of Zoology London, 176, 87-113. COOMBS, W. P. 1980. Swimming ability of carnivorous dinosaurs. Science, 207, 1198-1200. CRAIG, R. F. 1992. Soil Mechanics. Chapman & Hall, London. DEMATHIEU, G. R. 1987. Thickness of the footprint reliefs and its significance: research on the distribution of weights upon the autopodia. In: LEONARDI, G. (ed.) Glossary and Manual of Tetrapod Footprint Palaeoichnology, Department Nacional du Producao Mineral, Brasil, 60-61. DODD, J. R. & STANTON, R. J. 1990. Paleoecology: Concepts and Applications. Wiley, New York. FARLOW, J. O. 1989. Ostrich footprints and trackways: implications for dinosaur ichnology. In: GILLETTE, D. D. & LOCKLEY, M. G. (ed.) Dinosaur Tracks and Traces. Cambridge University Press, Cambridge, 243-248. FARLOW, J. O. 1992. Sauropod tracks and trackmakers: integrating the ichnological and skeletal records. Zubia, 10, 89-138. FARLOW, J. O. & LOCKLEY, M. G. 1993. An osteometric approach to the identification of the makers of Early Mesozoic tridactyl dinosaur footprints. In: LUCAS, S. G. & MORALES, M. (eds) The Nonmarine Triassic. New Mexico Museum of Natural History and Science Bulletins, 3, 123-131. GATESY, S. M. & BIEWENER, A. A. 1991. Bipedal locomotion: effects of speed, size and limb posture

in birds and humans. Journal of the Zoological Society, London, 224, 127-147. GATESY, S. M., MIDDLETON, K. M., JENKINS, F. A. & SHUBIN, S. H. 1999. Three-dimensional preservation of foot movements in Triassic theropod dinosaurs. Nature, 399, 141-144. GILLETTE, D. D. & LOCKLEY, M. G. (eds) 1989. Dinosaur Tracks and Traces. Cambridge University Press, Cambridge. HAUBOLD, H. H. 1971. Ichnia amphibiorum et reptiliorum fossilium. In: KUHN, O. (ed.) Handbuch der Palaoherpetologie, Part 18. Gustav Fisher, Stuttgart, 1-7. HAUBOLD, H. H. 1984. Saurierfahrten. Wittenberg Lutherstadt, Germany: Ziemsen 231pp. HITCHCOCK, E. 1858. Ichnology of New England. A Report on the Sandstone of the Connecticut Valley, Especially its Fossil Footmarks. W. White, Boston, MA [reprinted 1974 by Arno Press, New York]. JOHNSON, R. E. & OSTROM, J. H. 1995. The forelimb of Torosaurus and an analysis of the posture and gait of ceratopsian dinosaurs. In: THOMASON, J. J. (ed.) Functional Vertebrate Morphology in Vertebrate Paleontology. Cambridge University Press, Cambridge, 205-218. KARAFIATH, L. L. & NOWATZKI, E. A. 1978. Soil asdfasdfsadfasdfad Transglobal Technical Publications, Aedermannsdorf. LAMBE, T. W. 1961. Soil Mechanics. Wiley, New York. LEONARDI, G. 1987. Glossary and Manual of Tetrapod Footprint Ichnology. Ministerio das minas e energia, Departamento National da Produ9ao Mineral, Brasil. LINGEN, G. J. & ANDREWS, P. B. 1969. Hoof-print structures in beach sand. Journal of Sedimentary Petrology, 39, 350-357. LOCKLEY, M. G. 1991. Tracking Dinosaurs. Cambridge University Press, Cambridge. LOCKLEY, M. G., MATSUKAWA, M. & OBATA, I. 1989. Dinosaur tracks and radial cracks: unusual footprint features. Bulletin of the National Science Museum, Tokyo, 15, 151-160. LOCKLEY, M. G., HUNT, A. P. & MEYER, C. A. 1994. Vertebrate tracks and the ichnofacies concept: implications for palaeoecology and palichnostratigraphy. In: Donovan, S. (ed.) Palaeobiology of Trace Fossils. Belhaven Press, New York, 241-268. MclLROY, D. 2004. In: MC!LROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 3-27. McKEE, E. D. 1947. Experiments on the development of tracks in fine cross-bedded sand. Journal of Sedimentary Petrology, 17, 23-28. McMAHON, T. A. 1984. Muscles, Reflexes, and Locomotion. Princeton University Press, Princeton. NORMAN, D. B. 1986. On the anatomy of Iguanodon asdasdfsadfasdfsdafasdfasdfasdfasdfasdfasdfasdfdasfadf Bulletin de I'lnstitut Royale des Sciences Naturelles de Belgique: Sciences de la Terre, 56, 281-372.

CONTROLS ON PRESERVED TRACK MORPHOLOGY PADIAN, K. & OLSEN, P. E. 1984. Footprints of the Komodo Monitor and the trackways of fossil reptiles. Copeia, 3, 662-671. PEABODY, F. E. 1948. Reptile and amphibian trackways from the Lower Triassic Moenkopi Formation of Arizona and Utah. University of California Publications, Bulletin of the Department of Geological Sciences, 27, 295-468. PRANDTL, L. 1920. Uber die Harte plastischer Korper. Nachricten von der Koniglichen Gesellschaft der Wissenschaften zu Gottingen (Mathematisch physikalische Klasse aus dem jahre 1920), Berlin. PRANDTL, L. 1921. Uber die Eindringungsfestigkeit (Harte) plastischer Baustoffe und die Festigkeit von schneiden. Zeitscrift fur angerwandte Mathematik und Mechanik, 1, 15-20. REYNOLDS, R. E. 1989. Dinosaur trackways in the Lower Jurassic Aztec Sandstone of California. In GILLETTE, D. D. & LOCKLEY, M. G. (eds) Dinosaur Tracks and Traces. Cambridge University Press, Cambridge, 285-292. ROMANO, M. & WHYTE, M. A. 1996. Fossils explained 16: Trace fossils 3 - dinosaur tracks. Geology Today, 12, 75-79. ROMANO, M & WHYTE, M. A. 2003. Jurassic dinosaur tracks and trackways of the Cleveland Basin, Yorkshire: preservation, diversity and distribution. Proceedings of the Yorkshire Geological Society, 54, 185-215. ROMER, A. S. 1923. The pelvic musculature of saurischian dinosaurs. Bulletin of the American Museum of Natural History, 48, 605-617. SARJEANT, W. A. S. 1974. A history and bibliography of the study of fossil vertebrate footprints in the British Isles. Palaeogeography, Palaeoclimatology, Palaeoecology, 16, 265-378. SARJEANT, W. A. S. 1990. A name for the trace of an act: approaches to the nomenclature and classification of fossil vertebrate footprints. In: CARPENTER, K. & CURRIE, J. C. (edsj Dinosaur Systematics: Perspectives and approaches. Cambridge University Press, Cambridge, 299-307. SCRIVNER, P. J. & BOTTJER, D. J. 1986. Neogcne avian and mammalian tracks from Death Valley

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National Monument, California: their context, classification and preservation. Palaeogeography, Palaeoclimatology, Palaeoecology, 57, 285-331. SEILACHER, A. 1964. Sedimentological classification and nomenclature of trace fossils. Sedimentology, 3, 240-252. SEILACHER, A. 1967. Bathymetry of trace fossils. Marine Geologist, 5, 413-428. SWARTZ, S. M. & BIEWENER, A. A. 1992. Shape and scaling. In: BIEWENER, A. A. (ed.) Biomechanics Structures and Systems: A Practical Approach. Oriel Press, Oxford, 21-43. SMITH, M. J. 1981. Soil Mechanics. Longman Scientific & Technical, London. THULBORN, R. A. 1990. Dinosaur Tracks. Chapman & Hall, London. THULBORN, R. A. & WADE, M. 1989. A Footprint as a history of movement. In: GILLETTE, D. D. & LOCKLEY, M. G. (eds) Dinosaur Tracks and Traces. Cambridge University Press, Cambridge, 51-56. TUCKER, M. E. & BURCHETTE, T. P. 1977. Triassic dinosaur footprints from South Wales: Their context and preservation. Palaeogeography, Palaeoclimatology, Palaeoecology, 22, 195-208. WEISHAMPEL, D. D., DODSON, P. & OSMOLSKA, H. (eds) 1990. The Dinosauria. University of California Press, California. WHYTE, M. A. & ROMANO, M. 1981. A footprint in the sand of time. Journal of the University of Sheffield Geological Society, 7.6, 323-330. WHYTE, M. A. & ROMANO, M. 1993. Footprints of a sauropod dinosaur from the middle Jurassic of Yorkshire. Proceedings of the Geologists' Association, 104, 195-199. WHYTE, M. A. & ROMANO, M. 1994a. Probable sauropod footprints from the Middle Jurassic of Yorkshire, England. Gaia, 10, 15-26. WHYTE, M. A. & ROMANO, M. 1994b. Lower Middle Jurassic sequences between Whitby and Saltwick. In: SCRUTTON, C. (ed.) Yorkshire Rocks and Landscape: A Field Guide. Ellenbank Press, Mayport, 158-164.

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Phanerozoic history of deep-sea trace fossils ALFRED UCHMAN

Institute of Geological Sciences, Jagiellonian University, Oleandry 2a, 30-063 Krakow, Poland (e-mail: [email protected]) Abstract: The Phanerozoic diversity of deep-sea trace fossils, based on 151 flysch formations, displays distinct, non-linear changes through the Phanerozoic, with peaks in the OrdovicianEarly Silurian and Early Carboniferous, lowered in the Permian-older Late Jurassic, a peak in the Tithonian-Aptian, lowered in the Albian, and the maximum in the Eocene. The contribution of graphoglyptids in trace fossil assemblages rises gradually up to the end of the Cretaceous, shows a peak in the Palaeocene-Eocene but a depression in the Oligocene. All the changes were probably influenced by competition for food and food supply, bottom water temperatures and oxygenation, and frequency of flysch habitats. There is no clear influence of major biotic crises (Ordovician/Silurian, Cretaceous/Tertiary, Palaeocene/Eocene) on the diversity of deep trace fossils, except the Eocene/Oligocene crisis.

It is obvious that environmental conditions can affect evolutionary processes. Vice versa, in some cases organisms can influence environments. These environment-organism relationships are reconstructed from the fossil record, which in the case of deep-sea invertebrate benthic animals is full of gaps. These gaps can be partially filled by studying trace fossils. In this paper, the Phanerozoic diversity of deep-sea trace fossils, stratigraphic ranges of selected ichnotaxa, general composition of trace fossil assemblages (contributions of some behavioural groups and some ichnogenera) and contribution of graphoglyptids in trace fossil assemblages are considered. Attempts at this sort of study have been undertaken since the 1970s, but reconsideration is necessary because of the huge amount of new ichnological data becoming available during the last decade. Seilacher (1974, 1977b, 1978) based his work on 16 selected formations. McCann (1990) used 34 formations, but lumped some units together inappropriately - for instance the Jurassic-Tertiary flysch of the Polish Carpathians, which includes several formations deposited in different basins (Ksia_zkiewicz 1977). Orr (2001) considered 50 formations within the Ordovician-Carboniferous interval. In this paper, 151 flysch formations are considered for the entire Phanerozoic. Flysch is understood as a facies, of which nine jfeatures were defined by Dzulynski & Walton (1965). The illustrated specimens are housed in the Institute of Geological Sciences, Jagiellonian University, Krakow, Poland (acronym 154P), and in the Institute of Palaeontology, Wurzburg University, Wurzburg, Germany (acronym PIW 1998IV).

Trace fossil diversity: previous studies and methods There is no consensus on changes of diversity of deep-sea trace fossils through geological time. Crimes (1974) documented a sharp increase in the number of ichnogenera in CretaceousTertiary flysch deposits, but suggested that this could be an artefact partly caused by 'more detailed studies' on formations of this age. According to Seilacher (1974, 1976), the diversity of deep-sea trace fossils increased through the Phanerozoic with a rapid acceleration in the Cretaceous. Subsequently, Seilacher (1977b) evoked a gradual increase of diversity of 'flysch ichnocoenoses' through the Phanerozoic, but later (Seilacher 1978) proposed a 'mid-Cretaceous diversity burst' (see also Frey & Seilacher 1980). These views were tested by McCann (1990), who concluded that the increase of diversity was neither gradual in most of the Phanerozoic nor rapid in the Cretaceous. According to Orr (2001), the diversity of deepsea trace fossil assemblages displayed differences in particular periods of the Palaeozoic, with a distinct increase from the Cambrian to Ordovician, a low peak in the Ordovician, and further increase in the Carboniferous. The diversity of graphoglyptid trace fossils, which are the most characteristic component of the deep-sea Nereites ichnofacies (Seilacher 1967; Frey & Seilacher 1980), was analysed by Uchman (2003). It was low from the Cambrian to the Middle Jurassic, with a low peak in the Ordovician followed by a drop in the Silurian. In the Late Jurassic, after the late Palaeozoic low, the graphoglyptid diversity started to increase gradually and markedly in the Late Cretaceous, probably during the Turonian. It dropped

From: MC!LROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 125-139. 0305-8719/04/S15.00 © The Geological Society of London.

Fig. 1. The diversity diagram and diversity curve of deep-sea trace fossils through the Phanerozoic. The dashed line shows the questionable depressions in the curve. The numbers from 1 to 151 refer to formations according to a list in the author's database (housed in the Geological Society of London). The timescale which is based on Gradstein & Ogg (1999) can be obtained from the Society Library or the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, UK as Supplementary Publication No. SUP 18209 (60 pages). It is also available online at http://www.geolsoc.org.uk/SUP18209.

Fig. 2. Diagram of the proportion of graphoglyptids in trace fossil assemblages that contain 10 or more ichnotaxa. The curve shows average values. The numbers refer to formations according to a list in the author's database. The timescale is based on Gradstein & Ogg (1999).

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again in the Palaeocene, and subsequently increased to a maximum in the Eocene. The Oligocene was marked by a sharp decrease in graphoglyptid diversity and frequency, followed by an increase in the Miocene (Uchman 2003). There is also no uniform method for estimating trends in diversity. Seilacher (1974, 1977b, 1978) selected 16 well-understood deep-sea formations and considered their number of ichnospecies. McCann (1990) analysed the numbers of both ichnospecies and of ichnogenera. Orr (2001) considered only ichnogenera in Cambrian-Carboniferous formations. Apart from the number of ichnogenera in a particular formation, he used the 'average diversity calculated by period' as a parameter. This parameter is rather questionable, because many formations cross period boundaries, and it is difficult to tell which ichnogenera should be considered in which period. The diversity can change very much within periods, and the average values can mask the changes. Moreover, there is always under-representation of trace fossil associations of low diversity, which do not attract the attention of ichnologists, and it is unclear how such data might skew the results. In this paper, the total number of ichnogenera in each of the 151 formations (database housed in the Geological Society of London) is plotted against geological time (Fig. 1), as in Orr (2001). Consideration at the ichnogeneric level is more convenient and objective than at the ichnospecies level, because of many unresolved ichnotaxonomic problems at ichnospecific level. The data are taken from the literature, own observations, and unpublished data from flysch deposits of Turkey, Greece, the Crimea, the Balkans and the Polish Carpathians. Deep-sea deposits are not limited to flysch facies, but the other facies display much less diversity, mostly because of their lower preservational potential (e.g. Wetzel 1984). The data were revised with respect to ichnotaxonomy in a consistent way, through application of concepts previously developed by the author (Uchman 1995, 1998, 1999, 2001). The considered formations differ not only in thickness, stratigraphic range, facies, and exposure, but also in the amount of attention by researchers. This problem is, however, difficult to avoid. Abundance can be measured by the number of formations in which a given trace fossil occurs (Uchman 2003). A curve of changes in diversity was constructed on the basis of the most diverse trace fossil associations (one association for one formation) in any given period. The selected formations are not uniformly distributed in time. The Permian-Jurassic gap, which Seilacher

(1974) recognized, has been partially filled by subsequent research (e.g. Yang 1986, 1988; Kozur et al 1996; Tchoumatchenco & Uchman 1999), but the ichnology of deep-sea formations of this age is still poorly known, as is also true for the Upper Cambrian and the Middle-Upper Devonian. The largest number of considered formations (46%) derive from the Upper Cretaceous-Eocene, representing a mere 12% of Phanerozoic time. Only 8% of the formations cover the Permian-Jurassic, representing 27% of the Phanerozoic. Post-Miocene deep-sea formations are only marginally considered because they are underrepresented in the fossil record: most of them are still covered by the sea, and trace fossil (mostly graphoglyptid) diversity tends to be grossly underestimated in cores. Obviously, the higher the number of formations in a given period, the more objective the assessment of diversity, as can be judged from statistics rules. The proportion of graphoglyptid ichnogenera to total number of ichnogenera was considered for formations that contain 10 or more ichnogenera. This limitation was applied because in smaller trace fossil assemblages each graphoglyptid taxon changes this parameter excessively. The percentage values were plotted against the Phanerozoic timescale of Gradstein & Ogg (1999) (Fig. 2). Potential construction of a curve of the contribution of graphoglyptids on the basis of the highest values in a given period was rejected as too subjective, because some authors seem to omit 'less attractive', commonly simple, non-graphoglyptid ichnotaxa. Instead, a curve of the average values of the percentage of graphoglyptids was constructed (Fig. 2). Diversity through time During the Phanerozoic the diversity fluctuated, and displays characteristic anomalies (Fig. 1). There was a distinct increase of diversity from the Cambrian to Ordovician. It continued to rise during the Early Silurian, when it reached an early Palaeozoic peak in the Aberystwyth Grits Formation of Wales, UK (no 17 in Figs 1, 2) (Crimes & Crossley 1991). Diversity dropped slightly in the Early Devonian. There is only one Middle-Upper Devonian formation among the available data, which shows much less diverse trace fossil assemblages than the older formations. The data by Orr (2001) for this period do not point to a diversity higher than eight ichnogenera, but these data are not available for the author of this paper. It is

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unclear whether this is a virtual or apparent drop of diversity resulting from environmental changes or from under-representation of data from flysch formations of this age, but the latter possibility seems very likely. The next peak of diversity occurred in the Early Carboniferous Culm deposits of the Czech Republic (no 31 in Figs 1, 2) (Lang et al 1979; Pek & Zapletal 1990 and references therein). Diversity dropped again in the Late Carboniferous and reached a depression in the Permian and most of the Triassic. It rose during the Jurassic to the levels of the Palaeozoic peaks in the TithonianAptian. There is a questionable depression in the younger Middle Jurassic and older Late Jurassic that probably results from too low a number of formations of this age. The Albian shows a distinct depression in the diversity curve. Diversity rose from the Cenomanian to

the Coniacian up to the Palaeozoic and the Tithonian-Aptian levels and rose further to a Phanerozoic maximum during the Eocene [Beloveza Formation, Polish Carpathians (no 122 in Figs 1, 2; Ksia_zkiewicz 1977; Uchman 1992) and the Zumaya flysch (no 111 in Figs 1, 2; Leszczynski & Seilacher 1991 and references therein)]. Diversity decreased after the Eocene. Stratigraphic range of selected ichnotaxa and composition of trace fossil assemblages Stratigraphic ranges of ichnotaxa can be used as a Stratigraphic tool. The Cruziana ichnostratigraphy of Gondwana (Seilacher 1970, 1994) is a good example that is very useful in Lower Palaeozoic shallow-marine siliciclastics. In deep-sea trace fossil assemblages there is no group of

Table 1. Occurrences and Stratigraphic ranges of Dictyodora ispp Ichnotaxa

Occurrence

References

'Dictyodora' simplex Seilacher

Lower Cambrian, Salt Range, Pakistan Ordovician, Stara Planina, Bulgaria Middle Ordovician-Lower Silurian, New Brunswick, Canada Lower Llandovery, Gala and Penkill groups, Hawick Rocks, UK Ordovician, Barrancos, Portugal Lower-Middle Ordovician, Upper Hovin Group, Norway, Lower Llandovery, Gala and Penkill groups, Hawick Rocks, UK Middle Ordovician-Lower Silurian, Metapedia Group, New Brunswick, Canada Llandovery, Devil's Bridge Formation, Neuadd Fawr, Welsh Basin, UK Lower Silurian, Waterville Formation, Maine, USA ?Llandeilo-Caradoc, Hauptquarzit, Thuringia, Germany Lower Ordovician, Skiddaw Group, UK Lower Ordovician, Manx Group, UK Lower-Middle Ordovician, Upper Hovin Group, Norway Silurian, Qinling, China Lower Carboniferous, Culm, Thuringia, Germany Lower Carboniferous, Culm, Moravia and Silesia, Czech Republic

Seilacher 1955 Acenolaza & Yanev 2001 Pickerill 1980; Pickerill et al 1987; Benton & Trewin 1980

Dictyodora tennis (M'Coy)

Dictyodora cf. tennis (M'Coy) Dictyodora scotica (M'Coy)

Dictyodora zimmermanni Hundt

Dictyodora ?zimmermanni Hundt Dictyodora silurica Yang Dictyodora liebeana (Geinitz)

Lower Carboniferous, Culm, Frankenwalds, Germany Lower Carboniferous, East Menorca, Spain Lower Carboniferous, Culm, Minorca, Spain

Neto de Carvalho 2001 Hanken et al. unpublished Benton & Trewin 1980 Pickerill 1980; Pickerill et al. 1987 Orr 1995

Orr & Pickerill 1995 Benton 1982 Orr 1996 Orr & Howe 1999 Hanken et al. unpublished

Yang & Hu 1992 Benton 1982 and references therein Lang et al. 1979; Pek & Zapletal 1990 and references therein Stepanek & Geyer 1989

Orr l994;Onetal. 1996 Llompart & Wieczorek 1997

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trace fossils having similar value, but certainly some attempts can be made. Most ichnotaxa show wide stratigraphic ranges, but some of them show characteristic evolutionary trends (e.g. Seilacher 1977a, Uchman 2003). In some instances, abundance can also be used for this purpose. Thus, for example, in deep-sea facies the ichnogenus Dictyodora ranges from the Ordovician to the Carboniferous, though its ichnospecies display narrower ranges and could, with caution, be used in stratigraphy (Table 1, Fig. 3). Nereites irregularis (Schafhautl) (Fig. 4a) (formerly Helminthoida labyrinthica Heer) ranges from the Turonian, questionably from the Aptian. Phycosiphon incertum Fischer-Ooster (Fig. 4c) occurs in deep-sea facies from the Late Devonian. Scolicia (Fig. 4b) and Ophiomorpha (Fig. 5b) occur from the Tithonian, but became more common only since the Turonian (Tchoumatchenco & Uchman 2001). Cladichnus fischeri (Heer) (Fig. 6) ranges from the Coniacian to the Eocene, and Rotundusichnium zumayense (Gomez de Llarena) (Fig. 5a) from the Maastrichtian to the Eocene. Most graphoglyptids have their first occurrences in the Upper Cretaceous or Palaeogene (Uchman 2003). For instance, Glockerichnus alata (Seilacher) (Fig. 7a) occurs only in the Eocene. These facts can be used as an accessory tool for stratigraphy. However, some of graphoglyptids have long breaks in their stratigraphic record, e.g. Helminthorhaphe flexuosa Uchman (Fig. 4d) from the Cambrian to the Late Cretaceous. It is very probable that this trace fossil was made by animals having different stratigraphic ranges (Elvis ichnotaxon, cf. Erwin & Droser 1993). The composition of flysch trace fossil assemblages changes with time. Lower Cambrian flysch assemblages are poorly diversified, and dominated by simple forms (Planolites, Palaeophycus) with significant contribution of arthropod traces and the so-called 'shallow-marine' ichnotaxa (Skolithos, Monocraterion, Phycodes). They display a very low proportion of pascichnia and agrichnia (Orr 2001). However, the Middle Cambrian (age according to Pickerill, personal communication) Meguma Group (Nova Scotia, Canada) deep-sea deposits already contain 17 ichnogenera (Pickerill & Williams 1989). A significant break took place after the Middle Cambrian, when the agrichnial makers of graphoglyptids migrated from shallow-marine environments to the deep sea (Crimes et al. 1992; Crimes & Fedonkin 1994), probably at or near the Cambrian-Ordovician boundary. The colonization of the deep sea by graphoglyptid producers initiated the development of the

Fig. 3. Stratigraphic ranges and abundance of selected ichnotaxa. For Dictyodora see Table 1. Range of Cladichnus fischeri (Heer) based on Uchman (1998, 1999). Ranges of the remaining ichnotaxa are based on the database housed in the Geological Society of London. The question mark indicates that the determination is questionable.

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Fig. 4. Selected deep-sea trace fossils, (a) Nereites irregularis (Schafhautl) and Chondrites intricatus (Brongniart) (in the left corner), Ofterschwanger Beds (upper Albian-upper Cenomanian), Obsermaiselstein, Rhenodanubian Flysch, Germany, PIW 1998IV3. (b) Scolicia isp., Zarzecze Beds (lower Eocene), Konina, Magura Nappe, Polish Carpathians, 144P10. (c) Phycosiphon incertum Fischer-Ooster, Bleichernhorn Series (Maastrichtian), road-cut between Schnifis and Thuringerberg, Rhenodanubian Flysch, Austria, PIW 1998IV11A. (d) Helminthorhaphe flexuosa Uchman, Beloveza Beds (Eocene), Slopnice, Magura Nappe, Polish Carpathians, 144P11. Scale bars - 1 cm.

Fig. 5. Other selected deep-sea trace fossils, (a) Rotundusichnium zumayensis (Gomez de Llarena), Sievering Beds (Maastrichtian-Palaeocene), Miihlberg, Vienna Woods, Rhenodanubian Flysch, Austria, 145P9, Scale bar jkasdhkjlasdhkjalksdjfhjkasdfhalskdjfhlaskjdfhljkdasfhadsjklfhadjksfhladjksfhsdjklfhsdjkfhsdjkfhhf photograph, scale - 10cm.

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Fig. 6. Cladichnus fischeri (Heer), Zementmergel Series (Maastrichtian), Kalkgraben quarry, Rhenodanubian Flysch, Germany, PIW 1998IV120, scale in mm.

Fig. 7. Other stratigraphically useful deep-sea trace fossils, (a) Glockerichnus alata (Seilacher), Eocene flysch of the Carman Prealps, Italy, based on a field photograph, (b) Desmograpton pamiricus (Vialov), Tavrida Flysch (Upper Triassic-Lower Jurassic), Crimea, Ukraine, based on a field photograph.

archetypal Nereites ichnofacies of Seilacher (1967) (Orr 2001). Ordovician-Carboniferous trace fossil assemblages are relatively similar in that they contain characteristic trace fossils such as Dictyodora, Nereites (but not Nereites irregularis (Schafhautl)), Neonereites (included in Nereites by many authors; Rindsberg 1994; Uchman 1995), Protovirgularia, Megagrapton and Squamodictyon. Oldhamia ispp. are present in some muddy facies from the Neoproterozoic to the Early Carboniferous (Seilacher 1997). In younger Palaeozoic strata, Phycosiphon, Lophoctenium and Spirodesmos can be added to the list. Permian-Middle Jurassic trace fossil assemblages are less distinctive. They commonly contain Chondrites, Lophoctenium, Phycosiphon, Nereites, Megagrapton and Agrichnium, Desmograpton pamiricus (Yialov) (Fig. 7b) is a rare but characteristic graphoglyptid in TriassicMiddle Jurassic flysch. In Lower Cretaceous trace fossil assemblages it is difficult to select characteristic ichnotaxa; however, Helminthopsis is quite common, and Belorhaphe zickzack (Heer) and Glockerichnus glockeri (Ksi^zkiewicz) are relatively common at that time. Upper Cretaceous-Neogene trace fossil assemblages commonly contain a different suite of graphoglyptids co-occurring with Scolicia, Nereites irregularis (Schafhautl), Phycosiphon and Zoophycos. In sandstone-rich facies, Ophio-

DEEP SEA ENVIRONMENTS AND EVOLUTION

morpha rudis (Ksi^zkiewicz) and O. annulata (Ksi^zkiewicz) are common. Contribution of graphoglyptids Graphoglyptids form probably the most important component of deep-sea trace fossil assemblages from the Ordovician. Their producers are well adapted to relatively stable oligotrophic conditions (Seilacher 1977b; Miller 1991; Uchman 2003), and they are typical indicators of a K-selected strategy, indicating stable environments (Ekdale 1985). The contribution of graphoglyptids in their palaeoecological context has been considered in Alpine flysch (e.g. Tunis & Uchman 1996). Orr (2001) considered agrichnia (mostly graphoglyptids) and pascichnia together for the Cambrian-Carboniferous deepsea formations, and concluded that their contribution to trace fossil assemblages in the Cambrian was distinctly lower than in younger periods. The curve constructed herein of the proportion of graphoglyptid ichnogenera to the total number of ichnogenera (Fig. 2) shows an increase of the value since the Early Cambrian to Ordovician from about 10% to about 20%. The value then grows slowly and gradually through the rest of the Palaeozoic and the Mesozoic up to about 25%, reaching a maximum in the Palaeocene and Eocene of about 40% and then dropping in the Oligocene below 35%, only to rise again in the Miocene. Discussion The curves of diversity and contribution of graphoglyptids, stratigraphic ranges of some ichnotaxa and main changes in composition of trace fossil assemblages imply evolutionary changes of trace fossils. The changes were related to environmental and palaeoecological events and changes. Their potential controls are discussed below. Palaeozoic There is a distinct rise of diversity and contribution of graphoglyptids from Late Cambrian to Early Ordovician flysch (Crimes 1974). At that time, graphoglyptids, which originated in shallow-marine environments in the Early Cambrian, migrated to the deep sea (Crimes & Fedonkin 1994), probably at or near the CambrianOrdovician boundary (Orr 2001). According to

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previous views (Crimes 1974; Crimes & Droser 1992), the low diversity of the Cambrian deepsea trace fossil assemblages was a reflection of poor oxygenation of bottom waters and insufficient supply of organic detritus. Orr (2001) challenged this view, suggesting that increased competition for ecospace and/or resources in shallow-water environments pressed tracemakers to migrate to the deep sea. Diversity from the Ordovician to the Early Carboniferous remains at a relative high, with two peaks in the Ordovician-Early Silurian and Early Carboniferous (Fig. 1). However, the possible Middle-Late Devonian decrease of diversity remains uncertain because sufficient data are lacking. The distinct decrease of diversity after the Early Carboniferous and especially during the Permian (by about 50%) can be related to the formation of Pangea and lowering of temperature of deep-sea waters as a result of the Gondwanan glaciations. Formation of a single supercontinent, Pangea, and one superocean, Panthalassa, shortened active margins and the abundance of related flysch deposits. It is probable that the decreased habitat area lowered the diversity of its inhabitants. This crisis also influenced the composition of trace fossil assemblages. Dictyodora, Spirodesmos and Oldhamia disappeared from the fossil record. Seilacher (1974) supported a suggestion proposed by Wolf (1960), who regarded the evolution of the abyssal benthos to be strongly influenced by temperature changes of bottom waters. Glacial intervals in the Quaternary show reduced benthic species diversity, including the deep sea (Cronin & Raymo, 1997). Seilacher (1974), however, having a much smaller dataset than presented herein, concluded that the Upper Palaeozoic glaciations had no influence on the diversity of trace fossils. Most probably reduction of bottom temperatures was an important stress factor, which favoured opportunistic trace-makers, and a corresponding lowering of ichnofaunal diversity. Moreover, drying of the climate in the Permian reduced vegetation and probably decreased phytodetrital input - a possible but indirect food source - to the deep sea via turbiditic currents. There is no clear evidence of influence in the ichnological dataset presented herein of the Ordovician/Silurian boundary crisis, which was caused by the Late Ordovician glaciation (Barnes et al 1996), on the diversity of deep-sea trace fossils in general (Fig. 1). However, McCann (1990) noted that a drop of diversity occurred in the Welsh Basin at that time. Crimes et al. (1992, fig. 6) postulated a gradual increase of all deep-sea trace fossils from the

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Late Precambrian (Vendian) to the Early Silurian, but according to Orr (2001) the average diversity dropped slightly after the Ordovician. Also, the number of graphoglyptid ichnotaxa and the number of their first occurrences dropped after the Ordovician (Uchman 2003). The Late Ordovician glaciation was much shorter and less extensive than that of the Late Carboniferous and Permian. This could partially explain the differences in their influence on the deep-sea temperatures and resulting trace fossil diversity. Triassic-Early Cretaceous There is no evidence of influence of the Permian/ Triassic boundary crisis on deep-sea trace fossils. It is very probable that diversity increased gradually during the Triassic to the older Late Jurassic apart from the steps on the curve in the Middle/ Late Triassic (Fig. 1), which may be an artefact. For this period a correspondingly small number of flysch deposits are known, which moreover occur mostly in central Asia and in some areas of the circum-Pacific tectonic belt. Many of these await more detailed investigation, which, potentially, could modify the curve. Still, this was a period with rather limited areas of flysch habitats. The distinct increase in diversity within the Tithonian-Aptian is recorded in the Carpathian Flysch of the Silesian Unit (Ksia_zkiewicz 1977). The Tithonian deep-seafloor was colonized by irregular echinoids producing Scolicia and larger crustaceans producing Ophiomorpha (Tchoumatchenco & Uchman 2001). These efficient bioturbators strongly influenced the seafloor, intensively ploughing it to ingest particles for food and perhaps improving its oxygenation by irrigation of oxygenated water. As a result, they increased the competition for food, which favoured diversity of behavioural adaptations and consequently increased the diversity of trace-makers (e.g. Wetzel 1991). Better oxygenation improved the habitats and enabled the expansion of other organisms into deeper tiers. These processes caused a minor revolution in the substrates like that of the major late Precambrian revolution, i.e. the transition from the mat grounds to mix grounds (Seilacher & Pfltiger 1994; Seilacher 1999). These processes were stopped to some extent by the Lower Cretaceous anoxic events (e.g. Jenkyns 1980). In the Albian, the diversity curve shows a depression. Many flysch facies of this age, at least within the Tethyan and Atlantic realm, were influenced by reduced oxygenation, which possibly reduced

benthic activity (e.g. Jacobs & Lindberg 1998 and references therein). Late Cretaceous-Neogene Following the Albian, the diversity of deep-sea trace fossils increased quickly up to the Phanerozoic optimum in the Eocene. Most probably this rapid growth was caused by further increase in the competition for food and further improvement of the deep-sea habitats, similarly to the diversity of graphoglyptids (Uchman 2003). Flysch deposition related to the active margins of the Tethys and circum-Pacific systems became more common than in the Mesozoic. Enlargement of habitats may have once again enhanced the possibility for new adaptations. Frey & Seilacher (1980) suggested that the divergence of continents resulted in the enlargement of ocean basins, which in turn may have resulted in increased speciation. Echinoid burrows (Scolicia) and large crustacean burrows (Ophiomorpha) show a distinct increase in frequency at that time, up to an optimum in the Eocene. Also, small but efficient deposit-feeding structures (Nereites irregularis} show similar trends (Fig. 3). Most graphoglyptids, whose trace-makers are well adapted to competition for food, exhibited their first occurrences or became more frequent after the Late Cretaceous, and their frequency reached a maximum in the Eocene (Uchman 2003). In sum, new types of trace fossil assemblages were established with common contributions by Scolicia, Ophiomorpha, Nereites irregularis and diverse graphoglyptids. Older trace fossil assemblages, especially those in the Triassic-Jurassic, are more similar to their Palaeozoic counterparts, but do not contain Dictyodora, as was previously noted by Seilacher (1978). The Late Cretaceous changes may also be related to increased input of phytodetritus to the deep sea owing to the coeval evolution of terrestrial angiosperms. Radiation of the graphoglyptid trace-makers may be a response to the increased flux of non-refractory organic matter, mainly cellulose, into the oligotrophic deep-sea environment, where it could be used by burrowers for microbial farming (Seilacher 1974, 1977a, 1977b). More probably, however, they were influenced by the coeval increase of organic matter from plankton (Miller 1991; Uchman 2003), because they were produced in pelagic-hemipelagic mud deposited between turbiditic flows. Phytodetritus is typically utilized by the trace-makers of Scolicia and Ophiomorpha, which burrow in order to exploit organic

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Fig. 8. The diversity curve of deep-sea trace fossils through the Phanerozoic (based on Fig. 1) in relation the sea-level curve (after Hallam 1989), the mean global temperature curve (after Frakes 1979) and the global diversity curve for marine genera (after Sepkoski 1995). G indicates major glaciations. Arrows indicate major biotic crises. The timescale is based on Gradstein & Ogg (1999).

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matter deeply buried by turbiditic sediments and maturated by microbes. The Eocene optimum in diversity (Fig. 1) and frequency of deep-sea trace fossils is very distinct. Graphoglyptids at that time display the largest contribution in trace fossil assemblages (Fig. 2). In the Julian Prealps (Tunis & Uchman 1996), Carpathians (Ksiajzkiewicz 1977; Uchman 1998) and the Rhenodanubian Flysch (Uchman 1999) the increase of graphoglyptid diversity had already begun in the late Palaeocene and started to decrease in the latest middle Eocene. The high diversity is probably related to the advent of oligotrophic conditions in the late Palaeocene-early Eocene (Tunis & Uchman 1996), which is a widespread phenomenon (Hallock et al. 1991) related to global warming (Savin et al. 1975; Boersma & Premoli Silva 1991). Global warming produced dense saline waters that flowed to the deep sea and caused a significant increase in deepwater temperatures (e.g. Brass et al. 1982; Shackleton 1986). These phenomena are rooted in global tectonic and palaeoceanographic changes (Rea et al. 1990; Oberhansli 1996; Pickering 2000). Calcareous nannoplankton diversity displays a second diversity peak in the middle Eocene (Bown et al. 1992). The generic richness of benthic foraminifera also shows a maximum in the middle Eocene (Lutetian) (MacLeod et al. 2000). The Oligocene decrease in diversity and contribution of graphoglyptids (Figs 1, 2) is very distinct. After the Eocene no new graphoglyptid ichnotaxa appear (Uchman 2003). This phenomenon can be related to the Eocene/Oligocene boundary crisis, which also influenced plankton (Corliss 1979), including foraminifera, dinoflagellates and nannoplankton, whose diversity dropped drastically after the Eocene (Cavelier et al. 1981). World temperatures dropped as well (Buchard 1978). The longitudinal Atlantic circulation and connection to the Arctic Ocean was established (Prothero 1994). Cold polar water flowed in several deep-sea basins, such as in the Carpathians and in the Alps, with locally anoxic conditions (Leszczynski 1997 and references therein). Further improvement of the climatic conditions in the Miocene (Savin et al. 1975) did not result in an increase of diversity of deep-sea trace fossils (Fig. 1). Only the frequency of graphoglyptids (Uchman 2003) and their contribution to trace fossil assemblages increased (Fig. 2). Benthic niches have remained filled with graphoglyptids since the Oligocene. Their increased contribution in trace fossil assemblages indicates increased oligotrophy and competition

for food. Diversity of planktonic and benthic foraminifera increased at the beginning of the Miocene (Thunell 1981; MacLeod et al. 2000), as did the diversity of the calcareous nannoplankton (Bown et al. 1992). In general, the diversity curve of deep-sea ichnogenera is similar to the curve of diversity of marine animal genera (Fig. 8) (Sepkoski 1995). However, most of the major biotic crises such as the Ordovician/Silurian, Frasnian/Famenian, Triassic/Jurassic, and Cretaceous/Tertiary, did not influence the diversity of deep trace fossils except for the lower rank Eocene/Oligocene crisis. There is no correlation with the mean global temperature curve (Frakes 1979) except for the discussed decrease of temperature in the Late Carboniferous-Early Permian and after the Eocene. There is also no direct correlation with sea-level (Hallam 1989). Conclusions The diversity of deep-sea trace fossils displays distinct changes through the Phanerozoic, with peaks in the Ordovician-Early Silurian and Early Carboniferous, a depression in the Permian-older Late Jurassic, a peak in the Tithonian-Aptian, a depression in the Albian, and the maximum peak in the Eocene. The contribution of graphoglyptids rose gradually up to the end of the Mesozoic, with a peak in the Palaeocene-Eocene and a depression in the Oligocene. The diversity of deep-sea trace fossils and their evolutionary trends were influenced mostly by competition for food, bottom water temperatures, sediment oxygenation, and also, indirectly, by changes of the frequency of flysch deposits (i.e. suitable deep-sea habitats). There is no clear influence of the major biotic crises, such as the Ordovician/Silurian, Frasnian/Famenian, Triassic/Jurassic and Cretaceous/Tertiary, on the diversity of deep trace fossils except for the lower rank Eocene/ Oligocene crisis. However, the Ordovician/ Silurian, Cretaceous/Tertiary and Palaeocene/Eocene crises influenced graphoglyptid ichnodiversity and their relative abundance (Uchman 2003). Changes in the diversity of deep-sea trace fossils show unpredictable dynamics, not simple linear changes through time. This observation confirms that the time-stability hypothesis of Sanders (1968) is not valid. This publication was supported by the Jagiellonian University (funds DS/V/ING). Sponsors of the Lyell

DEEP SEA ENVIRONMENTS AND EVOLUTION Meeting in 2003 supported the participation of AU in the Lyell Meeting, London, 2003, where this paper was presented. A. K. Rindsberg (Alabama) improved the English and provided helpful comments. Some of the illustrated specimens were collected during tenure of a grant from the Alexander von Humboldt Foundation. R. K. Pickerill (Fredericton), A. Wetzel (Basel) and D. Mcllroy (Wirral) critically reviewed the manuscript and proposed helpful suggestions and further linguistic improvements.

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A re-evaluation of the relationship between trace fossils and dysoxia KATE D. MARTIN Millbank Lodge, Kirkton of Mary culler, Aberdeen AB12 5FS Abstract: Geochemical and palaeontological methods are used to determine the oxygenation histories of Jurassic sequences at Ravenscar, North Yorkshire, and Lyme Regis, Dorset. The ichnology of these sequences is compared with interpreted oxygen levels, allowing current models of oxygen-related trace fossil occurrence to be tested. These case studies support pre-existing models of trace fossil occurrence in demonstrating that burrow diversity, diameters and depth of infaunal tiering increase with increasing oxygen levels. The case studies suggest that trace fossil ethologies may not always be a reliable indicator of palaeo-oxygenation: in some cases, substrate consistency may have a greater influence over ethology than oxygen levels. Chondrites is confirmed as a common constituent of dysoxic settings; however, other trace types may also be indicative of such settings.

Palaeontological studies in dysoxic environments have been motivated by the importance of such environments in the formation of petroleum source rocks (e.g. Savrda et al. 1984), their possible use as global transgression indicators (e.g. DeMaison & Moore 1980), as analogues for the Proterozoic evolution of life (e.g. Rhoads & Morse 1971) and, more recently, as indicators of likely anthropogenic effects on the marine biota (e.g. Oschmann 1991; Tyson & Pearson 1991). Studies of trace fossils in dysoxic environments can be considered as particularly important owing to the reduced diversity of calcareous forms in such settings (e.g. Rhoads & Morse 1971), resulting in a relative paucity of body fossils. A number of models have been developed that attempt to use trace fossils to assess oxygen gradients within dysoxic strata, incorporating information on the specific ichnogenera present in addition to more general observations on depth and extent of bioturbation, burrow size and burrow morphology (e.g. Bromley & Ekdale 1984; Savrda & Bottjer 1986; Ekdale & Mason 1988). Although these models are supported by numerous examples of dysaerobic ichnocoenoses, other studies have suggested that they do not always accurately predict the characteristics of such ichnocoenoses (e.g. Hudson & Martill 1991; Wignall 1991). This suggests that there is a need to evaluate dysaerobic ichnocoenoses during different periods of the Phanerozoic. The oxygen-related trace fossil models currently in use have been based primarily on modern oxygen-restricted environments, determining the characteristics of the incipient traces and using these to enable recognition of ancient examples of dysaerobic trace fossils. The majority of studies of ancient dysaerobic ichnocoenoses

have not been linked to independent evidence for oxygenation. It is thus possible that ichnocoenoses interpreted as dysaerobic may have been controlled by factors other than oxygenation. This work therefore adopts a slightly different approach from many previous studies of dysaerobic trace fossils. Two Jurassic case studies are presented. In each of these, oxygenation histories have been determined using a combination of geochemical and palaeontological methods. The ichnological succession was then compared with the palaeo-oxygenation histories. This method allows some of the assumptions behind the current oxygen-related trace fossil models to be tested. Current models of oxygen-related trace fossil occurrence The case studies presented here will be used to demonstrate that some of the assumptions made in current oxygen-related trace fossil models may not always apply. A brief review of the current models is therefore appropriate.

Savrda and Bottjer model: tiering and oxygen-related ichnocoenoses Bromley & Ekdale (1984,1986) demonstrated how the cross-cutting relationships of trace fossils could be used to determine tiering profiles in the rock record, the deepest traces cross-cutting all shallower ones, and the shallowest traces being cross-cut by all other trace types. The determination of tiering profiles in various Mesozoic trace fossil associations allowed Bromley & Ekdale (1984) to infer that the vertical partitioning of

From'. MclLROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 141-156. 0305-8719/04/S15.00 © The Geological Society of London.

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Fig. 1. Cartoon of the anatomy of an idealized indistinctly burrowed bed.

the infauna was controlled primarily by porewater oxygenation. This recognition led to the development of a trace fossil model for identifying changes in oxygen levels within dysoxic strata (Savrda & Bottjer 1986, 1987, 1989a, 1989b), derived from late Mesozoic and younger examples. Trace fossil preservation can be considered as a continuum between two end members: distinctly burrowed and indistinctly burrowed (burrow mottled) beds (Savrda & Bottjer 1991). Both end members have well developed 'piped zones' (Fig. 1) - areas where burrows were emplaced into underlying laminated strata, resulting in the preservation of discrete trace fossils. Distinctly burrowed beds also have wellpreserved traces within the primary strata (very bioturbated sediments that have passed through the surface-mixed layer). Savrda & Bottjer (1986, 1987) developed and applied tiering models to distinctly burrowed beds to determine their oxygenation histories. Savrda & Bottjer's model was based on three fundamental factors that have been shown in some modern environments to change with

changing degrees of bottom water oxygenation: burrow diversity, diameters, and penetration depth. With declining oxygen levels, biodiversity is generally considered to decrease (e.g. Diaz & Rosenberg 1995). Maximum burrow diameters are also seen to decrease under lower oxygen conditions (e.g. Savrda et al. 1984). The loss of larger infaunal forms under such conditions tends to result in reduced irrigation and aeration of the sediment. A consequent rise in the position of the redox potential discontinuity (RPD) causes the remaining burrows to be emplaced at higher levels within the sediment than they would be under better-oxygenated conditions, thus reducing the vertical extent of the burrows (e.g. Bromley & Ekdale 1984). These three criteria were used by Savrda & Bottjer to characterize units deposited under similar oxygen levels, and were referred to as oxygen related ichnocoenosis (ORI) units. Once determined, the authors used the changing sequence of ORI units to recognize oxygenation and deoxygenation events, also determining in each case whether the change in oxygen levels had been gradual or rapid. Gradual deoxygenation results in a progressive loss of ichnogenera, with a consequent simplification of the ichnofabric, whereas rapid deoxygenation can be recognized by the presence of 'frozen' profiles (Fig. 2). Determination of the nature and extent of changes in oxygen allows the reconstruction of a relative oxygenation curve for the strata under investigation. Savrda & Bottjer (1989a) considered that the tiering model can accurately be applied only to

Fig. 2. Tiering patterns predicted for gradual and rapid deoxygenation events in distinctly burrowed beds. From Wignall (1994).

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distinctly burrowed beds, and proposed a method for determining relative durations and magnitudes of oxygenation episodes within indistinctly burrowed beds of dysoxic strata. Short-term oxygenation events (from several days to hundreds of years) are inferred to produce relatively thin primary strata, with ichnogenera showing simple vertical segregation without extensive cross-cutting relationships. In contrast, protracted oxygenation events (from hundreds to millions of years) are recognized by thicker primary strata and extensive crosscutting relationships due to the upward migration of the infaunal associations through the oxygenation event (see Mcllroy 2004). Magnitudes of palaeo-oxygenation are recognized using the diversity, size and depth relationships of piped zone burrows: increases in these factors are taken to reflect increases in levels of palaeooxygenation. Consideration of Savrda & Bottjer's model indicates that it may be complicated by the difficulties of recognizing ancient tiering relationships in the fossil record, particularly where the earliest emplaced traces have been completely obscured by later ones (Fig. 3). A similar problem may be identified in situations where frequent oxygenation events of relatively low magnitude allow bioturbation sufficient to disturb any laminated strata deposited within

Fig. 4. Cartoon showing the potential for frequent low-magnitude oxygenation events to allow bioturbation sufficient to obliterate evidence of intervening anoxic periods, (a) Oxygenation event produces an indistinctly burrowed bed. (b) Return to anoxic conditions; deposition of laminated strata, (c) Oxygenation event allows bioturbation sufficient to disturb laminated strata, (d, e) as (b) and (c).

Fig. 3. Cartoon showing the potential for early traces to be obliterated by later ones: (a) initial emplacement of piped zone traces; (b, c) production of further traces obscures those produced by the earliest colonists.

intervening anoxic periods (Fig. 4). This could feasibly result in a totally bioturbated sediment fabric, which would be interpreted as reflecting continuous oxygenation. Ichnocoenoses in which tiering has been controlled by factors

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other than oxygen levels may also be difficult to identify.

Ekdale and Mason model Ekdale & Mason (1988) documented their observations on characteristic late Palaeozoic and modern deepwater trace fossils, suggesting that certain morphological (and therefore behavioural) types are typical of low-oxygen environments. Domichnia-dominated aerobic biofacies are seen to be succeeded by pascichnial associations (typically including Scalarituba, Spirophyton and Phycosiphori) where porewaters become dysoxic and bottom waters either dysoxic or anoxic. Endostratal pascichnia are found (according to Ekdale & Mason 1988) in dysoxic but not anoxic sediments and are rare in oxic sediments. This is attributed to the producing organisms requiring some interstitial oxygen, as no surface connection is maintained, but being limited in oxic sediments by the paucity of appropriate food. Lower dysoxic environments (settings where anoxic interstitial waters are overlain by dysoxic waters at the SWI) are typified by fodinichnia-dominated associations. Ekdale & Mason (1988) state that such traces can potentially penetrate to depths of several decimetres or even metres, and this may well be true in better-oxygenated environments; however, in dysoxic settings they are normally emplaced at shallower (centimetre to decimetre) levels (e.g. Smith et al 2000). The maintenance of open burrows by the deposit-feeding organisms that produce fodinichnial traces is inferred as enabling them to exploit the rich organic matter content of the sediment while circulating dysoxic water through the burrow, for respiration. Ekdale & Mason (1988) consider Chondrites and Zoophycos to be characteristic of such environments.

Zoophycos ichnofacies and ZoophycosChondrites ichnoguild Nine recurring associations of contemporaneous environmentally related traces (ichnofacies) were recognized by Seilacher (1967). These ichnofacies were originally interpreted as reflecting bathymetry, but they are now recognized as reflecting a wide range of environmental parameters, related only indirectly to water depth (Frey et al. 1990). Seilacher's Zoophycos ichnofacies was first thought to be typical of 'sublittoral to bathyal; quiet-water, offshore type conditions; impure

silts and sands; below storm wavebase to upper continental slope or equivalent areas free from turbidity flows' (Frey 1975, p. 17). More recently, the Zoophycos ichnofacies has been interpreted as characteristic of stressed, quiet-water environments, especially those experiencing anoxia (Pemberton et al. 1992), over a relatively wide range of water depths. Ichnofossils considered characteristic of the Zoophycos ichnofacies are typically present at low diversities, though individual traces can be abundant. The traces are typically horizontal or slightly inclined spreiten structures (Pemberton et al. 1992), including Zoophycos, Phycosiphon and Spirophyton (Frey & Pemberton 1984) and Chondrites (Bromley & Asgaard 1991). Bromley (1990) also identified a series of recurring trace fossil associations, based on ethological characteristics of the trace fossils rather than their environmental setting. These trace fossil associations are described as ichnoguilds and defined as 'a group of ichnospecies that express a similar sort of behaviour, belong to the same trophic group and occupy a similar tier or location within the substrate' (Bromley 1996, p. 345). Bromley's ichnoguilds were defined using data from Cretaceous Chalk ichnofacies, yet one of the ichnoguilds can be seen as applicable to dysaerobic biofacies in general. The Chondrites-Zoophycos ichnoguild is considered to comprise mainly deep-tier, deposit-feeding (or possibly chemosymbiotic - Bromley 1996) structures produced by non-vagile organisms that are the dominant traces in ichnofabrics from oxygen-restricted environments, and are also characteristic of the deepest tiers in settings with better oxygenated bottom-water (Bromley 1990). The dominant ichnogenera belonging to this ichnoguild are several size classes of Zoophycos and Chondrites. Savrda (1992) also includes Teichichnus and Trichichnus. Bromley & Ekdale's (1984) paper has perhaps been the most widely followed of all work concerning dysaerobic trace fossils. Bromley & Ekdale identified Chondrites as 'a trace fossil indicator of anoxia in sediments', citing examples of Chondrites occurring as the sole trace fossil in strata interpreted as having been deposited under dysoxic bottom-water conditions. The presence of Chondrites (often in association with a range of other trace fossils) has subsequently been used erroneously by many authors as evidence for dysoxic bottom-water conditions (e.g. Jarvis et al. 1988). However, Bromley & Ekdale (1984) and Bromley & Asgaard (1991) emphasize the point that, where Chondrites is present in completely bioturbated sediments, it does not indicate oxygen-deficient seafloors, but

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exploitation of organic matter in deeper tiers within the sediment where porewaters may be reducing. Methods used to determine oxygen levels Both palaeontological and geochemical parameters can be used as a proxy for palaeo-oxygenation. Geochemical techniques are not subject to the same biases of preservation or sampling as palaeontological methods, though they can be influenced by the effects of time-averaging. Use of both geochemical and palaeontological datasets in evaluating levels of palaeo-oxygenation therefore provides a more robust framework against which to compare trace fossil data. Gamma-ray spectrometry, pyrite framboid analyses and the assignment of the benthic macrofauna to oxygen-restricted biofacies are used herein to interpret oxygenation.

Gamma-ray spectrometry Gamma-ray spectrometry has been employed in the evaluation of sedimentary rocks since the 1950s, significant works including those of Adams & Weaver (Adams & Weaver 1958; Adams et al 1959). Adams & Weaver (1958) recognized that the U and Th contents of finegrained elastics varied according to the depositional redox conditions. More recently this concept has been developed by Myers & Wignall (1987) and Wignall & Myers (1988), who showed that Th/U ratios can be used to distinguish between anoxic, dysoxic and oxic depositional settings. Th/U ratios of 3, 2 and 0.5 are taken as the boundary between dysoxic and oxic strata in shales, marls and limestones respectively (Wignall & Myers 1988; Martin 2001). Data were collected according to the criteria of Myers & Wignall (1987).

Pyrite framboid size—frequency analyses Pyrite is an extremely common authigenic mineral in mudrocks. It is typically present as a number of different morphologies, including framboids, clusters, aggregates of bladed crystals and equant crystals. Recent work (Wilkins et al. 1996; Wignall & Newton 1998) has shown that the size distributions of pyrite framboids in modern environments reflect the depositional oxygen conditions, and that this can be used to accurately reconstruct the palaeoredox conditions of ancient clastic environments. Analyses

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of pyrite framboid size-frequency distributions were performed on samples from a number of the case studies investigated. The analyses were carried out on carbon-coated polished blocks and polished thin sections using scanning electron microscopy.

Oxygen-restricted biofacies The principal of using species richness to determine oxygen gradients within dysoxic strata was recognized by Rhoads & Morse (1971) and Byers (1977). This method was developed by Wignall & Hallam (1991) in their oxygenrestricted biofacies (ORB) scheme. Wignall & Hallam used shelly macrofaunal and lithological data from British Jurassic black shales as a basis for six biofacies that distinguish between anoxic, lower- and upper-dysoxic environments. The macrofauna of the case studies presented here have been used to classify the sections according to the ORB scheme, thus providing a measure of palaeo-oxygenation against which to compare the trace fossil data. Toarcian of Ravenscar, North Yorkshire The early Toarcian in Britain is characterized by a number of small-scale extinction events (Little & Benton 1995; Hallam 1996) that may have been caused by an oceanic anoxic event (OAE) (Jenkyns 1988). The exaratum Subzone (Fig. 5) is thought to represent the maximum development of this anoxic event (Saelen et al. 1996); subsequent subzones record the recovery from the event. Coastal sections in North Yorkshire provide the best record of the British Toarcian and have consequently been intensively studied (e.g. Dean 1954; Howarth 1962; Knox 1984). However, the ichnology of this interval has received little attention and is therefore investigated herein. Fieldwork was carried out on wave-cut platform and cliff exposures on the foreshore between Old Peak [NZ 980 025] and Blea Wyke [NZ991 016] near the village of Ravenscar, North Yorkshire. Approximately 90 m of strata, from the falciferum to the levesquei ammonite zone (Fig. 5), were logged (Figs 6a, b), beginning at bed x of Howarth (1962). Parts of the section were inaccessible, however, preventing the collection of data. Gamma-ray spectrometry was carried out at ~1 m intervals, and the occurrence of hard-shelled fauna was noted throughout the sequence. These data provide Th/U ratios and

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Fig. 5. North Yorkshire Toarcian lithostratigraphy, informal nomenclature and ammonite zonation. Shaded box shows the position of the sequence studied. Compiled from data in Knox (1984) and Howarth (1992).

ORB with which to interpret the depositional oxygen levels.

Evidence for oxygenation Evidence for oxygenation is provided using Th/U ratios and ORB. Within the lower part of the Jet Rock Member, Th/U values under 3 indicate the presence of authigenic U, and therefore oxygen-restricted depositional conditions. Above the middle of bed xii Th/U values range between 3 and 8, indicating oxic depositional conditions. The Jet Rock Member contains a common nektonic fauna of ammonites, fish debris and belemnites. Isolated crinoid ossicles occur from bed xix upwards. Epibenthic bivalves (Pseudomytiloides dubius, Meleagrinella substriata and

Bositra buchii} occur as autochthonous shell pavements on some bedding planes. The abundance of this epibenthic fauna varies within the Jet Rock Member, decreasing upwards from the middle of bed xii to the top of bed xiv, where the fauna is sparse. The lower part of the member is therefore assigned to ORB 4 and the upper part to ORB 3, indicating that episodically dysoxic conditions prevailed throughout the deposition of the member, and that periods of improved oxygenation were of longer duration in the lower part. The lower 3 m of the Alum Shale Member contain the same benthic faunal assemblage as the Jet Rock Member, though with greater abundance and therefore interpreted as belonging to ORB 4. A gap in the section of approximately 4m occurs at this point; during this interval Pseudomytiloides dubius disappears and the

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hrace fossils within the Jet Rock Member and Alhale Member of the Toarcian of Ravenscar, North Yorkshire. Bed numbers are those of Howarth (1962); logs for the inaccessible parts of the sequence are compiled from observations of cliff exposures and data in Knox (1984).

infaunal bivalve Dacryomya ovum appears. The benthic fauna remains the same up to bed xxvii, after which point there is another gap in exposure within which the infaunal bivalve Pleuromya costata appears. The last occurrence of Meleagrinella substriata is recorded in bed xxxii; the trigonid Vaugonia pulchella appears 4.5m above bed xliii. The benthic assemblage of B. buchii, D. ovum, P. costata and V. pulchella continues at least up to bed 37, after which there is a ~23 m gap in exposure. The fauna of the Alum Shale Member is assigned to ORB 4 and indicates that the member was deposited under lower dysoxic conditions. The faunas are

scattered throughout the sediment rather than restricted to particular bedding planes: thus conditions are interpreted as having been persistently lower dysoxic. The lack of accessible exposure of the Peak Mudstone Member and most of the Fox Cliff Siltstone Member has meant that ORB cannot be determined for these units. Where the exposure resumes, towards the top of the Fox Cliff Siltstone Member, the fauna is sparse and poorly preserved. Shelly fragments are abundant, but the recognizable fauna consists only of abraded specimens of belemnites, the trigonid Vaugonia pulchella and serpulids.

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Fig. 6. (b) Distribution of trace fossils within the Alum Shale Member, Peak Mudstone Member, Fox Cliff Siltstone Member and Grey Sandstone Member of the Toarcian of Ravenscar, North Yorkshire. Roman numerals are bed numbers of Howarth (1962), Arabic numerals are bed numbers of Dean (1954).

The combined data from Th/U ratios and ORB indicate that the Jet Rock Member was deposited under episodically dysoxic conditions, with periods of improved oxygenation being of longer duration in the lower than the upper part of the member. The Alum Shale Member was deposited under persistent lower dysoxic conditions. Determination of palaeooxygenation in the Peak Mudstone Member and lower Fox Cliff Siltstone Member has not been possible owing to the lack of accessible exposure. Poor preservation of the hard-shelled fauna within the top of the Fox Cliff Siltstone Member and the Grey Shale Member precludes

the determination of ORB; however, Th/U ratios give evidence for deposition under oxic conditions. The entire sequence is interpreted as recording a gradual increase in palaeo-oxygenation following the early Toarcian OAE. This broadly agrees with the interpretation of Saelen et al. (1996), based on their analyses of stable O and C isotopes from belemnite rostra. However, the evidence from this study indicates that the Alum Shale Member was deposited under lower dysoxic conditions, whereas Saelen et al. (1996) consider that the water column was oxygenated by the top of the Jet Rock Member.

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The distribution of trace fossils is shown in Figures 6a and b. Eight trace types are recognized; however, only four of these are assignable to recognized ichnogenera. The four remaining trace types are described briefly below.

Burrow widths range between 2mm and 10 mm; preserved lengths can exceed 28cm. The burrows are partially pyritized. Where pyrite is present the burrow margin is irregular; where pyrite is absent the margin is smooth. These traces were probably formed by pascichnial activity.

Trace type 1 are elongate, straight to sinuous, non-branching, approximately horizontal traces with a diffuse pyrite fill. Widths range from 4mm to 6mm; preserved lengths from 2cm to 3cm. Their morphology suggests a pascichnial origin. Trace type 2 are horizontally oriented, nonbranching, straight to slightly sinuous burrows with a subcircular cross-section and a dense pyrite fill. Burrow diameters are constant within individual traces; between traces they vary from 1 mm to 3 mm. Preserved burrow lengths reach a maximum of 5 cm. Their morphology suggests that they were the product of pascichnial activity. Trace type 3 are simple, vertically oriented burrows with a subcircular cross-section of 1-3 mm diameter and a dense pyrite fill. Their morphology indicates a domichnial origin. Trace type 4 comprises horizontal to obliquely oriented, straight to curved, elongate traces without wall or internal structures (Fig. 7).

Distinct trace fossils, of type 4, first appear within bed xx (Fig. 6a). The 3m of overlying strata contain no distinct traces, but from bed xxiv trace types 2 and 3 occur and increase in abundance over ~2m, after which point there is a ~6m gap in exposure. On the resumption of exposure, in bed xxxii, trace types 2 and 3 are present. Type 3 traces were not recorded between beds xxxiv and xlii; however, type 4 traces were recorded in bed xxxv, and type 1 traces in beds xxxvi and xxxvii. Possible examples of Chondrites are present within bed xxxviii: these traces are predominantly horizontal and display sparse low-angle branching. From bed xliv 5 m of strata contain only trace types 2 and 3, which decrease in abundance upwards until they disappear (Fig. 6b). Small Chondrites (~lmm diameter) are present within bed 42; above this point there is a gap in exposure of 20m. The exposure resumes towards the top of the Fox Cliff Siltstone Member, revealing traces of types 2 and 3 co-occurring with Rhizocorallium in a background of burrow-mottled

Trace fossils

Fig. 7. Trace type 4. Bed xx, Alum Shale Member, Toarcian, Ravenscar.

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Fig. 8. Cartoon of trace fossil occurrence with relation to oxygenation in the Toarcian of Ravenscar.

sediments. Type 2 and 3 traces disappear before the top of the member, just beneath the level at which Chondrites reappears. The Grey Sandstone Member is characterized by completely bioturbated sediments, with Chondrites and Rhizocorallium, from bed 71 Thalassinoides, and, at the very top of the studied section, Taenidium. The first traces to appear (type 4) have maximum diameters of 6mm. Following a gap in trace fossil occurrence, maximum diameters increase from 1 mm in bed xxiv to 2 mm in bed xxvi. Maximum diameters of 3mm occur over 13.5m of strata from bed xxxii, with the exception of a single occurrence of trace type 4 in bed xxxv, which has a diameter of 10mm. Isolated examples of Chondrites at the top of the Alum Shale Member record maximum diameters of 1 mm. The maximum trace diameter recorded in the upper part of the Fox Cliff Siltstone Member is 10 mm; this increases to 25mm within the Grey Sandstone Member. Pascichnia (type 4 traces) are the first traces to appear in the Alum Shale Member. Pascichnia dominate throughout the member, though domichnia and, occasionally, fodinichnia (Chondrites) are also present. The Fox Cliff Siltstone and Grey Sandstone Members are dominated by domichnia, though pascichnia and fodinichnia also occur.

Comparison with current oxygen-related trace fossil models The trace fossils of the Whitby Mudstone Formation and Blea Wyke Sandstone Formation at Ravenscar were formed during a transition

from lower dysoxic to oxic depositional environments (Fig. 8). The diversity of distinct traces increases in parallel with the increase in oxygenation, and maximum burrow diameters show a similar general increasing trend. This supports the model of Savrda and Bottjer (1986, 1987). Ekdale and Mason's (1988) model predicts that an increase in oxygenation will produce a sequence of fodinichnia-, pascichnia- and then domichnia-dominated ichnological associations. All three behavioural categories are represented in the Ravenscar section, but, in contrast to Ekdale and Mason's model, a sequence of pascichnia-dominated assemblages followed by domichnia-dominated assemblages parallels the increase in oxygenation. The observations from this study do not therefore support Ekdale & Mason's model. Wignall (1991) reported ichnocoenoses from the Upper Jurassic Kimmeridge Clay in which a pascichnia-dominated association of trace fossils occurred under lower oxygen levels than other ethological types. Wignall suggested that this assemblage was controlled by substrate consistency, inferring that soupy substrates favour the production of traces that did not require the maintenance of a connection to the sediment-water interface (i.e. pascichnia). The Alum Shale Member has some evidence for soft substrate conditions - ammonites within bed xx of the member were found in vertical and oblique orientations in addition to the more usual horizontal orientation. The occurrences of pascichnia-dominated ichnocoenoses in this study may therefore support WignalFs (1991) conclusions. The identification of Chondrites as the sole occurring trace under dysoxic conditions (Bromley & Ekdale 1984) is not supported by

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the Ravenscar case study. Although Chondrites occurs within the strata deposited under lower dysoxic conditions, it is preceded by four different trace types. Blue Lias of Lyme Regis, Dorset The Blue Lias is a widely used descriptive term that refers to successions of alternating shales and limestones deposited over much of northwest Europe (Callomon & Cope 1995) during the latest Triassic and early Jurassic (Hettangian-Sinemurian). The sediments of the Blue Lias are typically developed in repeating depositional cycles (Fig. 9). These cycles are not always complete; some cycles pass from shale to marl and back to shale, whereas others lack the pale marls and limestones. Milankovitch cyclicity, particularly obliquity (House 1985), has been suggested as the controlling mechanism behind these rhythms. The origin of the limestones (primary versus diagenetic) has previously been the subject of debate; they are now widely accepted as being diagenetic (e.g. Hallam 1986; Bottrell & Raiswell 1990). The section at Lyme Regis was measured approximately 1 km southwest of The Cobb, at SY 3285 9110. The studied section lies within the conybeari Subzone of the bucklandi Zone (Sinemurian). The base of the section measured here corresponds to bed 21 of Lang (1924).

Evidence for oxygenation The Blue Lias at Lyme Regis contains a relatively diverse fauna, including well-preserved skeletons of marine reptiles (e.g. Martill 1995).

Fig. 9. Idealized sedimentary cycle within the Blue Lias. Based on field logging within the conybeari Subzone, Lyme Regis, Dorset.

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The shales generally contain a low-diversity allochthonous faunal assemblage made up of fish scales, echinoid spines, bivalve fragments and, occasionally, ammonites. Large scours, up to 60cm in length and 6cm deep, occur at the base of the shales of bed 6; these are infilled with calcareous material containing abundant fragmentary fauna, including Ostrea, other bivalves, rhynchonellids and crinoids. The marls contain a body fossil assemblage that normally includes Gryphaea, Ostrea, a small unidentified bivalve, fish scales and fragments of bivalves. The marls of bed 5 additionally contain abundant Calcirhynchia calcaria. The limestones contain a more diverse fauna than the marls; this is probably the result of early diagenetic cementation of the limestones allowing better preservation of the fauna rather than the original faunal communities being more diverse. The limestones normally preserve many of the following: C. calcaria, Gryphaea, Ostrea, small unidentified bivalves, the gastropod ?Pleurotemaria, ammonites, crinoids and echinoid spines. Bed 7 is characterized by particularly abundant crinoid debris. Bed 13 is highly distinctive, containing abundant very large ammonites and nautiloids throughout. Additionally, a layer of very abundant mixed crinoid and bivalve debris is present towards the top of the bed; these shell fragments are occasionally concentrated within the chambers of ammonoids. At the top of bed 13, Gryphaea is very abundant, and there are large pieces of wood with encrusting oysters. Changes in ORB through the sequence are predominantly coincident with changes in substrate from shales to marls. The shale of bed 3 is devoid of all fauna and is thus assigned to ORB 1, indicating that the water column was anoxic. The shales of beds 9 and 15 contain nektonic forms and small fragments of bivalves, which are considered to be allochthonous: thus they are assigned to ORB 2, reflecting deposition in conditions of anoxia with a habitable water column. Beds 6, 11 and 12 contain shales with rare occurrences of the benthic form C. calcaria; these are assigned to ORB 3 and reflect episodically dysoxic conditions. The marl-limestone units generally contain common benthic forms, including thick-shelled bivalves (e.g. Gryphaea, Ostrea) and crinoids. These forms are indicative of oxic depositional conditions. Analysis of the size-frequency distribution of pyrite framboids was carried out on a shale sample from bed 3. The sample contained abundant small framboids, giving a size-frequency distribution characteristic of deposition under

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euxinic conditions. The marls and limestones did not contain sufficient pyrite to be analysed using this method. Pyrite framboid and ORB data thus indicate that conditions were persistently anoxic during the deposition of the shales of bed 3. The rest of the shale beds are interpreted as having been deposited under conditions of episodic dysoxia. Oxic conditions are considered to have occurred during the deposition of the marl-limestone units. The improvement in oxygenation is considered to have occurred relatively rapidly, as a diverse hard-shelled fauna is usually present close to the base of the marls. There is typically

an abrupt break in the presence of hard-shelled fauna at the top of the limestone-marl units. This implies that the decrease in oxygen levels was rapid. However, the erosive bases of the shales mean that an undetermined amount of sediments and their associated fauna have been removed, complicating the interpretation.

Trace fossils The ranges of the ichnogenera recognized within the sequence are shown in Figure 10. The traces can be divided into two groups, based on their

Fig. 10. Ichnological data from the Lyme Regis section. Behavioural types: Dom., domichnia; Fod., fodinichnia; Pasc., Pascichina. Oxygen levels: 1, anoxic/episodically dysoxic; 2, lower dysoxic; 3, upper dysoxic; 4, oxic.

TRACE FOSSILS AND DYSOXIA

occurrence relative to the interpreted oxygen levels. The first includes those traces that are present only within the marl-limestone units; the second comprises traces that are present at the transition from shales to marls. All of the trace fossils recognized occur within the marl-limestone units (Fig. 10). Large horizontal expanses of the beds on the wave-cut platforms provide good exposures of the associations. This allows recognition of the cross-cutting relationships between the ichnogenera, and shows that complex, tiered ichnocoenoses were developed (Fig. 11) (see discussion of ichnocoenoses in Mcllroy 2004). The upper tier was occupied by organisms that thoroughly bioturbated the substrate but did not leave distinct traces. The middle tier included the producers of Arenicolites, Thalassinoides, Diplo crater ion, Palaeophycus and Taenidium, whereas the deepest tier contained the organisms responsible for Chondrites and Planolites. Trace fossil assemblages within the marllimestone units are moderately diverse, containing between two and eight distinct ichnogenera. The beds with the lowest diversity tend to be those in which the recognizable traces are poorly preserved members of the deepest tier: thus the apparent low diversities may be a function of preservation. Maximum burrow diameters within the units are relatively large (Fig. 10), varying between 8mm and 40mm. Penetration depths are difficult to measure owing to the lack of vertical exposure; an example of Diplo crater ion from bed 4 had a minimum penetration depth of 7cm. Most of the units contain fodinichnial, domichnial and pascichnial traces (Fig. 10); a few lack domichnial traces, whereas others lack pascichnia.

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The transition from laminated shales up into bioturbated marls represents the change from anoxic/episodically dysoxic conditions to oxic conditions that prevailed during the deposition of the marl-limestone units. The shale/marl boundaries can be categorised as one of two types. In the first type (bed 3) the shale passes directly into homogenised marl lacking distinct traces; this marl probably represents sediment that was originally laminated but was disturbed by organisms within the surface-mixed layer following the increase in oxygenation. In the second type, the bed boundary region contains a piped zone of distinct traces emplaced within the tops of the laminated strata. Piped zones are present within beds 9, 11, 12 and 15. The piped zone of bed 9 contains Chondrites, that of bed 11 Planolites, that of bed 12 Chondrites and Thalassinoides, and that of bed 15 Chondrites and Diplocraterion. Although the Thalassinoides of bed 12 and the Diplo crater ion of bed 15 occur within the piped zones, cross-cutting relationships indicate that they were emplaced later than the piped-zone Chondrites, and also later than other traces, which were emplaced within the (oxic) marl-limestone units. The Sinemurian sequence at Lyme Regis section reflects an environment that was subject to long-term (thousands of years) fluctuations in oxygen levels, from anoxic or episodically dysoxic during the deposition of the laminated shales to oxic during the deposition of the marl-limestone units. Trace fossils were not emplaced during the deposition of either the anoxic or episodically dysoxic shales. The initial colonists under reoxygenation gradients were small organisms that disturbed the shale laminae but did not produce distinct traces. Chondrites

Fig. 11. Cartoon of ideal tiered community developed within marl-limestone units. Based on field logging within the conybeari Subzone, Lyme Regis, Dorset.

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and Planolites were the first distinct traces to be emplaced and penetrated between 4cm and 10cm into the underlying shales - deeper than the initial colonists. These observations indicate that penetration depths, burrow diameters and trace diversity increase with increasing oxygenation, conforming to the predictions of Savrda & Bottjer (1986, 1987). The occurrence of Chondrites as one of the first distinct traces to be emplaced within the reoxygenation gradient supports the observations of Bromley & Ekdale (1984); however the co-occurrence of Planolites shows that other trace types may also be characteristic of restricted oxygen conditions. Both fodinichnia and pascichnia occurred under the lowest oxygen conditions; this does not conform to the model of Ekdale & Mason (1988). Conclusions The two Jurassic case studies presented here use geochemical and palaeontological data to interpret palaeo-oxygen levels, allowing the occurrence of trace fossils relative to oxygenation to be investigated. Savrda & Bottjer's (1986, 1987) models of oxygen-related trace occurrences, based on changes in trace diversity, diameters and penetration depths, applied to both of the case studies. This supports their assertions that oxygen levels have a strong control over these parameters, and that they can be used to distinguish between strata deposited under different oxygen conditions at different temporal scales. Ekdale & Mason's (1998) behaviourally based model was found not to apply in either case study, implying that factors other than oxygenation - possibly substrate stability - may control the occurrence of the trace-producing organisms. Chondrites occurred under conditions of bottom-water dysoxia in the Toarcian at Ravenscar, and was present both in piped zones in reoxygenation gradients and as a deep-tier trace in oxic sediments from the Sinemurian of Lyme Regis. These observations support those of Bromley & Ekdale (1984). However, other trace fossils were found to co-occur with Chondrites in dysoxic settings, or to be present at lower oxygen levels: thus other ichnogenera may also be characteristic of sediment dysoxia/ anoxia. This work was funded by NERC as part of a PhD thesis and carried out at the University of Leeds under the supervision of P. Wignall to whom thanks are due. P. Doyle, D. Mcllroy and C. R. C. Paul provided helpful reviews

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allostratigraphic significance. In: WALKER, R. & JAMES, N. (eds) Facies Models: Response to Sea Level Change. Geological Association of Canada, Ottawa, 42-72. RHOADS, D. & MORSE, J. 1971. Evolutionary and ecologic significance of oxygen deficient marine basins. Lethaia, 4, 413^28. SAELEN, G., DOYLE, P. & TALBOT, M. 1996. Stableisotope analyses of belemnite rostra from the Whitby Mudstone Formation, England: surface water conditions during the deposition of a marine black shale. Palaios, 11, 97-117. SAVRDA, C. 1992. Trace fossils and benthic oxygenation. In: MAPLES, C. & WEST, R. (eds) Trace Fossils. Palaeontological Society, Knoxville, Short courses in Palaeontology, 5, 172-196. SAVRDA, C. & BOTTJER, D. 1986. Trace fossil model for reconstruction of palaeo-oxygenation in bottom waters. Geology, 14, 3-6. SAVRDA, C. & BOTTJER, D. 1987. Trace fossils as indicators of bottom water redox conditions in ancient marine environments. In: BOTTJER, D. (ed.) New Concepts in the Use ofBiogenic Sedimentary Structures for Palaeoenvironmental Interpretation. Society of Economic Palaeontologists and Mineralogists, Pacific Section, Los Angeles, 52, 3-26. SAVRDA, C. & BOTTJER, D. 1989a. Anatomy and implications of bioturbated beds in 'black shale' sequences: examples from the Jurassic Posidonienscheifer (southern Germany). Palaios, 4, 330-342. SAVRDA, C. & BOTTJER, D. 1989b. Development of a trace fossil model for the reconstruction of palaeo-bottom-water redox conditions: evaluation and application to Upper Cretaceous Niobrara Formation, Colorado. Palaeogeography, Palaeoclimatology, Palaeoecology, 74, 49-74. SAVRDA, C. & BOTTJER, D. 1991. Oxygen related biofacies in marine strata: an overview and update. In: TYSON, R. & PEARSON, T. (eds) Modern and Ancient Continental Shelf Anoxia. Geological Society, London, Special Publications, 58, 201-220. SAVRDA, C., BOTTJER, D. & GORSLINE, D. 1984. Development of a comprehensive oxygen-deficient marine biofacies model: evidence from Santa Monica, San Pedro and Santa Barbara Basins, California Continental Borderland. Bulletin of the American Association of Petroleum Geologists, 68, 1179-1192. SEILACHER, A. 1967. Bathymetry of trace fossils. Marine Geology, 4, 413^28. SMITH, C. R., LEVIN, L. A., HOOVER, D. J., McMuRTRY, G. & GAGE, J. D. 2000. Variations in bioturbation across the oxygen minimum zone in the northwest Arabian Sea. Deep Sea Research II, 47, 227-257. TYSON, R. & PEARSON, T. 1991. Modern and ancient continental shelf anoxia: an overview. In: TYSON, R. & PEARSON, T. (eds) Modern and Ancient Continental Shelf Anoxia. Geological Society, London, Special Publications, 58, 1-24. WIGNALL, P. 1991. Dysaerobic trace fossils and ichnofabrics in the Upper Jurassic Kimmeridge Clay of southern England. Palaios, 6, 264-270. WIGNALL, P. 1994. Black Shales. Oxford University Press, Oxford.

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WIGNALL, P. & HALLAM, A. 1991. Biofacies, stratigraphic distribution and depositional models of British onshore Jurassic black shales. In: TYSON, R. & PEARSON, T. (eds) Modern and Ancient Continental Shelf Anoxia. Geological Society, London, Special Publications, 58, 291-309. WIGNALL, P. & MYERS, K. 1988, Interpreting benthic oxygen levels in mudrocks: a new approach. Geology, 16, 452-455.

WIGNALL, P. & NEWTON, R. 1998. Pyrite framboid diameter as a measure of oxygen deficiency in ancient mudrocks. American Journal of Science, 298, 537552. WILKINS, R., BARNES, H. & BRANTLEY, S. 1996. The size distribution of framboidal pyrite in modern sediments: an indicator of redox conditions. Geochimica et Cosmochimica Acta, 60, 38973912.

Ichnology of Carboniferous tide-influenced environments and tidal flat variability in the North American Midcontinent M. GABRIELA MANGANO & LUIS A. BUATOIS Conicel-Insugeo, Casilla de correo 1, correo central, 4000 San Miguel de Tucumdn, Argentina (e-mail: ichnolog@ infovia.com.ar) Abstract: Trace fossils are sensitive indicators of environmental fluctuations and, accordingly, ichnological studies have the potential to improve facies characterization of marginal-marine systems. Carboniferous intertidal deposits in eastern Kansas and western Missouri accumulated under contrasting palaeoenvironmental conditions, ranging from the open shoreline to fluvio-estuarine transitions. Comparative analysis of these exposures illustrates lateral variations in trace-fossil content and allows characterization of the intertidal ichnofaunas developed in three sub-environments: open marine, restricted bays and fluvio-estuarine transitions. Openmarine tidal flat ichnofaunas are characterized by (1) high ichnodiversity, (2) marine ichnotaxa produced by both euryhaline and stenohaline forms, (3) the presence of both infaunal and epifaunal traces, (4) the presence of simple and complex structures produced by presumed trophic generalists and specialists respectively, (5) dominance of horizontal trace fossils of the Cruziana ichnofacies, (6) presence of multispecific associations, (7) high density, and (8) wide size range. This ichnofauna is present in heterolithic deposits and reflects the activity of a biota that inhabited tidal flats dominated by normal-marine salinities and connected directly to the open sea. Restricted-bay ichnofaunas display (1) low ichnodiversity, (2) ichnotaxa commonly found in marine environments, but produced by euryhaline organisms, (3) dominance of infaunal traces rather than epifaunal trails, (4) simple structures produced by opportunistic trophic generalists, (5) a combination of vertical and horizontal trace fossils from the Skolithos and Cruziana ichnofacies, (6) the presence of monospecific associations, (7) variable density, and (8) small size. This assemblage occurs in heterolithic facies and records the activity of a brackish-water benthic fauna inhabiting intertidal areas of estuarine basins and embayments. Fluvio-estuarine ichnofaunas are characterized by (1) moderate to relatively high diversity, (2) forms typically present in continental environments, (3) the dominance of surface trails and absence of burrows, (4) temporary structures produced by a mobile deposit-feeding fauna, (5) a mixture of trace fossils belonging to the Scoyenia and Mermia ichnofacies, (6) moderate density of individual ichnotaxa, (7) absence of monospecific suites, and (8) small size. This assemblage occurs in tidal rhythmites and records the activity of a typical freshwater/terrestrial benthos inhabiting tidal flats that were developed in the most proximal zone of the inner estuary under freshwater conditions. Through integration of ichnological and sedimentological data, conventional sedimentological interpretations of marginal-marine depositional systems can be refined and enhanced. Tidal flats are complex depositional environments that are extremely variable, depending on coastal physiography, climate, dominant sedimentary processes and tidal regimes, among other parameters. They develop in a wide spectrum of coastal environments, such as open coasts, bays, lagoons and estuaries. In this study, tidal flats are considered as that region extending between spring highand spring low-tide levels, grading landward into the supratidal marshes and seaward into the subtidal region (after van Straaten 1954). In contrast, some authors also include flat to gently sloping upper-subtidal areas and lower-supratidal areas in the definition of the 'tidal flat' (Reineck 1967; Weimer et al. 1981; Mcllroy 2004). Tidal flats are dissected by genetically related meandering tidal creeks. Intertidal areas represent rigorous environments, where marine organisms experience extreme changes in temperature, duration of subaerial exposure, salinity, hydrodynamic

energy, and substrate types and consistency (e.g. Reise 1985; Mangano et al 2002a). Although numerous studies have focused on the animalsediment interactions in modern tidal flats (see review in Mangano et al. 2002a), relatively few authors have attempted to apply observations on modern animal-sediment interactions to the study of ancient tidal flats (e.g. Goodwin & Anderson 1974; Miller & Knox 1985; Wescott & Utgaard 1987; Simpson 1991). Late Palaeozoic eustatic sea-level rises caused the development of extensive epicontinental seas over the cratonic USA Midcontinent (Moore 1964; Heckel 1977; Ross & Ross 1987; Watney et al. 1989). The development of extensive Carboniferous-Permiantide-influenced coastlines resulted in the formation of tidal-flat areas that are preserved within equatorial carbonate/siliciclastic cyclothems (Mangano et al. 2002a). Alternating sandstone and mudstone

From: MC!LROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 157-178. 0305-8719/04/S15.00 © The Geological Society of London.

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Fig. 1. Distribution of outcrops of the Marmaton, Kansas City, Pleasanton, Lansing, Douglas, and Shawnee Groups and the localities studied. Localities are numbered as follows: 1, Bandera Quarry (Bourbon County); 2, quarry 1 mi southeast of Eudora, near Coleman Creek (Douglas County); 3, road-cut along the Kansas Turnpike US Highway 70, 1 km west of the intersection with Kansas Highway 7 (Wyandotte County); 4, road-cut along Kansas Highway 24, 1.5 km west of the intersection with Kansas Highway 7 (Leavenworth County); 5, site south of the Kansas City International Airport (Platte County, Missouri); 6, road-cuts on Kansas Highway 166, 3km west of Peru (Chautauqua County); 7, Toronto Lake (Woodson County); 8, Buildex Quarry (Franklin County); 9, Lonestar spillway (Douglas County); 10, Waverly trace fossil locality, 7 km west of the town of Waverly (Coffey County); 11, road-cut along Country Road 6, 4 km south of Stull (Douglas County); 12, road-cut 1 km west of Kanwaka (Douglas County); 13, outcrop east of Perry Lake, 2.5km northeast of Lakewood Hills (Jefferson County).

layers in these intertidal deposits preserve a variety of biogenic structures that provide valuable insights into the palaeoecological and depositional dynamics of these ancient shorelines. Carboniferous intertidal deposits in eastern Kansas and western Missouri (Fig. 1) accumulated under palaeoenvironmental conditions that ranged from open shoreline to the fluvioestuarine transition. The intertidal deposits analysed in this paper occur in several Carboniferous units of eastern Kansas and western Missouri (Fig. 2). Comparative analysis of these exposures illustrates lateral variations in trace-fossil content, and allows characterization of intertidal ichnofaunas that are developed in three subenvironments: open marine, restricted bay, and the fluvio-estuarine transition. The aims of this paper are: (1) to propose an integrated ichnological and sedimentological model of intertidal flat and subtidal sandbar complexes and (2) to characterize and compare the ichnofaunas of

Carboniferous tidal flats formed in openmarine shorelines with ichnofaunas of restricted bays and the fluvio-estuarine transition. An integrated sedimentological and ichnological model of tide-influenced tidal flat and subtidal sandbar complexes Our present knowledge of the ichnology of tideinfluenced shallow-marine environments lags behind that of wave-dominated settings. Integration of ichnological and sedimentological observations has led to the establishment of the 'shoreface model' (MacEachern & Pemberton 1992; MacEachern et al 1999; Pemberton et al. 2001). Although a comparable model for tideinfluenced shorelines is still lacking, we have outlined in previous contributions some of the most relevant characteristics of ichnofaunas from tidal flat and subtidal sandbar complexes (Mangano

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Fig. 2. Pennsylvanian stratigraphy of Kansas and western Missouri, outlining the lithostratigraphic units analysed. Asterisks indicate the stratigraphic position of the different ichnofaunas. Stratigraphy based on Moore et al. (1951).

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Well sorted, fine-grained sandstones with large-scale cross stratification (eolian dunes). Dominance of dwelling and locomotion traces of invertebrates (crabs and insects) and vertebrates. Low ichnodiversity.

Intensely rooted mudstones and dwelling traces of invertebrates (crabs). Massive or parallel laminated mudstones and rare sandstone interbeds. Lenticular bedding. Inclined heterolithic stratification in small to medium-sized tidal creeks. Dominance of horizontal, poorly defined, feeding traces of deposit feeders. Low ichnodiversity. Mudstones and very fine-grained sandstones with wavy and flaser bedding. Inclined heterolithic stratification in medium-sized tidal creeks. Dominance of horizontal feeding traces of deposit feeders, detritus feeders and grazers; great variety of ethological categories and moderate ichnodiversity. High variability depending on tidal regime. Current ripple crosslaminated fine- to very fine-grained sandstones with mudstone partings in micro- to mesotidal settings. Medium to fine-grained sandstones with planar, trough and herringbone cross-bedding and upper-flow parallel lamination in meso- to macrotidal regimes. Inclined heterolithic stratification in large-sized tidal creeks. Dominance of horizontal feeding traces of deposit feeders, detritus feeders and grazers, great variety of ethological categories and high to moderate ichnodiversity in micro- to mesotidal sand flats. Lowichnodiversity assemblages of vertical dwelling and escape traces of passive predators and suspension feeders in macrotidal sandflats. Medium to fine-grained sandstones with planar, trough and herringbone cross-bedding. Dominance of vertical dwelling and escape traces of passive predators and suspension feeders along reactivation surfaces. Low ichnodiversity.

Fig. 3. Integrated sedimentological and ichnological model of intertidal flats and subtidal sandbar complexes (modified from Buatois & Mangano 2000).

& Buatois 1999; Buatois & Mangano 2000; Buatois et al. 2002a; Mangano et al 2002a). In this section we shall expand these observations to propose an integrated ichnological and sedimentological model for some tide-influenced environments (Fig. 3). This model is based not only on observations in the Late Palaeozoic of the North American Midcontinent, but also in a wide spectrum of tidal deposits in Argentina, ranging in age from the Cambrian to the Miocene. Based on the integration of observations from modern and ancient deposits, Klein (1971, 1977) proposed a facies model for tide-dominated shorelines that includes a supratidal region, the upper-, middle- and lower-intertidal zones, and the subtidal area (see summary in Dalrymple 1992). Because tidal energy increases seaward, tidal flats generally coarsen seaward, in contrast to wave-dominated shorelines, which coarsen landward. Therefore a typical tidal-flat profile in a landward direction consists of a lowerintertidal sandflat, a middle-intertidal mixed

(sand and mud) flat, and an upper-intertidal mudflat. Landward of the mudflat, supratidal salt marshes may be present, and the subtidal zone occurs seawrard of the sandflat. Tidedominated parasequences are, consequently, upward fining (Van Wagoner et al. 1990). The supratidal area may be vegetated, forming salt marshes where sedimentary fabric is obliterated by root traces. Animal traces include representatives of the Psilonichnus ichnofacies (Frey & Pemberton 1987). Supratidal marshes grade landwards into a wide variety of terrestrial environments characterized by different tracefossil assemblages that are mostly included in the Scoyenia and Coprinisphaera ichnofacies (Buatois & Mangano 1995; Genise et al. 2000). The upper zones of the tidal flat, referred to as the mudflat, are dominated by deposition of finegrained suspended sediment. Mudflats consist of laminated mudstone with rare siltstone and sandstone interbeds and interlaminae. Lenticular bedding is the dominant bedding style. Scarcity of

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sandstone layers limits trace fossil preservation in mudflat deposits, and an indistinct, mottled texture is common. Interface trace fossils of the Cruziana ichnofacies can, however, be preserved in the rare sandstone intercalations (e.g. Mangano et al. 2002a). Middle-intertidal areas (mixed flat) are typified by sedimentation from traction alternating with fallout from suspension. Heterolithic bedding is typical, mostly represented by flaser and wavy bedding. Elements of the Cruziana ichnofacies are characteristic of the mixed flat. Alternation of sandstone and mudstone layers enhances preservation of horizontal interface traces, such as those that typify the Cruziana ichnofacies. The lower zones of the tidal flat, referred to as the sandflat, are characterized by bedload traction of sand-sized sediment. As is the case for the lower shoreface in wave-dominated shorelines (MacEachern & Pemberton 1992), the sandflat is the most variable intertidal zone in terms of both sedimentary facies and trace fossil content. Whereas the character of facies in the lower shoreface varies, depending on the intensity and frequency of storms (see MacEachern & Pemberton 1992; Pemberton et al. 2001), that of the lower tidal flat is controlled mainly by tidal range (Buatois et al. 2002a). In a macro tidal regime, characterized by high current speeds, migration of largescale bedforms, such as two-dimensional and three-dimensional dunes (Dalrymple 1992), produces thick cross-bedsets. Macrotidal estuaries may display upper-flow regime horizontal planar parallel lamination and rare current ripples (Dalrymple et al 1990; Dalrymple 1992). As a consequence, the lower intertidal zone of macrotidal shorelines is very difficult to distinguish from sub tidal areas. High-energy, rapidly migrating bedforms generally preclude the establishment of a mobile epifaunal and/or shallow infaunal biota, inhibiting development of the Cruziana ichnofacies. Bioturbation is restricted to vertical elements of the Skolithos ichnofacies, typically reflecting colonization windows along reactivation surfaces (Pollard et al. 1993; MacEachern & Pemberton 1994; Mangano et al. 1996). Conversely, in microtidal to mesotidal regimes, migrating bedforms are commonly represented by small current ripples - producing cross-lamination - under low-energy conditions. High-diversity assemblages of the Cruziana ichnofacies are characteristic of this type of sandflat. Tidal flat deposits are typically dissected by a network of meandering tidal channels and creeks that migrate across the intertidal zone, producing lateral accretion of bedsets in point bars (Reineck 1958; Bridges & Leeder 1976;

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Weimer et al. 1981; de Mowbray 1983; Thomas et al. 1987; Dalrymple 1992). Inclined heterolithic stratification is the typical structure formed by this tidally influenced lateral accretion (Thomas et al. 1987). In the upper muddy zones of the tidal flats channels are small to medium size, but in the lower sandy areas they tend to coalesce, forming wider, deeper channels (Dalrymple 1992). The degree of bioturbation is lower in the point bars than in tidal flats, most likely reflecting higher rates of sedimentation along unstable channel margins (cf. Gingras et al. 1999; Mangano et al. 2002a). The subtidal zone is characterized by maximum energy with high current velocities (Dalrymple 1992). As a result, large-scale bedforms, such as dunes, migrate across the subtidal areas, forming sandbars (compound dunes) and ridges. Wilson (1982, 1986) noted that, in modern subtidal regions, few benthic species are able to survive in zones of actively migrating bedforms. Accordingly, faunal diversity increases toward areas with smaller bedforms and in the outer regions where dunes are replaced by small ripples and interbeds and interlaminae of mud. Studies of marine benthic ecology also demonstrate that suspension feeders are the dominant trophic type in high-energy subtidal environments (Wilson 1982). As in the case of macrotidal sandflats, vertical trace fossils of the Skolithos ichnofacies are the dominant elements, commonly extending down into the sediment from reactivation surfaces (e.g. Pollard et al. 1993). Subtidal sandbar sands grade seaward into outer shelf muds, commonly characterized by the Zoophycos ichnofacies. To summarize, the ichnofacies gradient in tideinfluenced shorelines is opposite to that in wavedominated shoreface environments. As overall tidal energy increases from supratidal to subtidal settings, the Skolithos ichnofacies tends to occur seaward of the Cruziana ichnofacies (Mangano et al 2002a) (Fig. 3). This shoreward decrease of energy parallels a decrease in oxygenation, sand content, amount of organic particles in suspension, and mobility of the substrate. This gradient is consistent with information from modern tide-influenced environments, where the highest faunal diversity is present around mid-tide level (Beukema 1976). Modern analogues of the Cruziana ichnofacies are common in tidal flat deposits (e.g. Bajard 1966; Howard & Dorjes 1972; Swinbanks & Murray 1981; Ghare & Badve 1984; Frey et al 1987; Gingras et al 1999). The preservational potential of biogenic structures in intertidal settings is, however, highly variable, with a clear bias in the fossil record towards deeper-tier structures,

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particularly in post-Palaeozoic examples (see Mangano et al. 2002a for discussion).

the slip face of duneforms. These sandstone deposits are interpreted as a subtidal sandbar complex that probably was formed in the lower zone of a tide-dominated estuary. Evidence of estuarine incision at the base of Stratigraphic and depositional setting the Rock Lake Shale Member was presented by Intertidal ichnofaunas analysed in this paper were Feldman et al. (1993), who analysed the Garnett recorded from exposures of the Desmoinesian palaeovalley from outcrops in east-central Bandera Shale Formation (Marmaton Group), Kansas. The valley is incised into the underlying the Missourian Rock Lake Shale Member (Stan- Stanton Limestone and Vilas Shale, and is ton Formation, Lansing Group), the Virgilian oriented northwest to southeast. They estimated Stranger Formation (Douglas Group), the Virgi- the estuarine valley to be 9.8m deep and 300m lian Lawrence Shale Formation (Douglas wide. Outcrops in northeast Kansas and northGroup), and the Virgilian Stull Shale Member west Missouri also include tidal flat deposits (Kanwaka Shale Formation, Shawnee Group). that were formed along a brackish-water, These units outcrop in narrow belts across east restricted embayment (Russell 1972, 1974; Kansas and west Missouri (Fig. 1). The analysed Hakes 1976, 1977, 1985; Mangano et al. 1999). The Douglas Group represents deposition siliciclastic intervals are separated by laterally continuous transgressive limestone units (e.g. Haskell within two estuarine valleys, namely the TongaLimestone, Toronto Limestone, Spring Branch noxie and Ireland palaeovalleys (Archer et al. Limestone) that serve as marker beds. This situa- 1994a). The Tonganoxie sequence records the tion allows spatio-temporal comparisons to be infill of a northeast to southwest oriented estuareffected with a degree of accuracy that typically ine valley, incised during the latest Missourian is otherwise possible only in modern environ- sea-level fall and filled during the subsequent ments. These deposits, however, contain biogenic transgression in the earliest Virgilian (Lanier structures that have already passed through the 1993; Lanier et al. 1993; Gibling et al. 1993; taphonomic filter - that is, the fossilization barrier Archer et al. 1994a; Feldman et al. 1995; Buatois - and are thus better analogues to other ancient et al. 1998a). The Tonganoxie palaeovalley is tide-influenced depositional systems. Integration incised into the underlying Weston Shale as of sedimentological, sequence-stratigraphic, well as units of the Lansing Group and is about palaeoecological, and ichnological evidence with 41 m deep, 11 km wide, and 240km long (Feldpalaeogeographic reconstructions indicates that man et al. 1995). Palaeocurrent studies indicate these units record, for the most part, deposition the existence of southwesterly flowing rivers in embayments and estuaries, opening to the sea and ebb-dominated tidal dunes (Gibling et al. towards the south (Archer et al. 1994a; Mangano 1993). Northwesterly palaeocurrents are evident et al. 2002a). Tidal flats were formed in different higher in the sequence and reveal a reversal in geomorphological positions, such as fluvio-estuar- sediment transport, reflecting the establishment ine transitions, restricted bays and open-marine of a flood-dominated tidal system during the subsequent transgression (Gibling et al. 1993). The environments. Outcrop and subsurface data indicate that the Ireland palaeovalley is incised into the Stranger Bandera Shale Formation was deposited in tide- Formation; although its morphology is not well dominated embayments (Brownfield et al. 1998). known, the overall succession appears similar These authors suggested that sand was trans- to that of the Tonganoxie palaeovalley (Archer ported to the basin by fluvial systems incised et al. 1994a). Tidal flat deposits recorded from during a sea-level fall and reworked by marine these units accumulated in different geomorphoprocesses during the subsequent transgression. logical settings. The succession of the TongaThe Bandera Shale ichnofauna is recorded from noxie Sandstone Member at Buildex Quarry Bandera Quarry (Bourbon County, Southeast (Franklin County, eastern Kansas) consists, for Kansas). The succession at this locality consists the most part, of planar-laminated coarseof interbedded sandstone and shale and large- grained siltstone deposited on an upper tidal scale cross-stratified sandstone (Brownfield et al. flat, close to or at the fluvial-estuarine transition 1998). The sandstone and shale are rhythmically of a macrotidal estuarine palaeovalley (Lanier interbedded and display flaser and wavy bedding, et al. 1993; Buatois et al. 1997). Outcrops of the mudstone rip-up clasts and flat-topped ripples, Lawrence Formation at Lonestar spillway suggesting tidal processes in an intertidal envir- (Douglas County, Northeast Kansas), on the onment. The large-scale cross-stratified sand- other hand, consist of mudstone and very finestone has sinuous crested dunes preserved at grained sandstone displaying flaser, wavy and the top. Current ripples are locally preserved on lenticular bedding, and record deposition in the

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intertidal to subtidal zones of the middle estuary (Archer et al, 1994a). Outcrops of the Ireland Sandstone exposed in Toronto Lake (Woodson County, East Kansas) consist of laterally persistent, parallel-laminated sandstone that probably formed in upper-flow regime sandflats of the lower estuary (Archer et al. 1994a). Finally, a succession of the 'Stalnaker' Sandstone exposed in road-cuts on Kansas Highway 166, 3km west of Peru (Chautauqua County, Southeast Kansas), consists of fine-grained sandstone and mudstone with abundant trace fossils and body fossils. These deposits are thought to have been formed in tidal flats outside the estuarine embayment, directly connected with the open sea (Archer et al. 1994a; Mangano & Buatois 1997). The Stull Shale Member exhibits lateral changes of facies from restricted deposits formed in an embayment in northeastern Kansas to open-marine intervals to the southwest (Mangano & Buatois 1997; Mangano et al. 2002a). Ichnological and sedimentological studies of outcrops of the Stull Shale in Douglas and Jefferson Counties indicate marginalmarine, brackish-water conditions (Hakes 1976, 1985; Mangano & Buatois 1997; Mangano et al.

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2002a). Further to the southwest in Coffey County, tidal flats were formed outside the embayment in direct connection with the open sea (Mangano et al. 2002a). Ichnofaunas and tidal flat variability Comparative analysis of these Carboniferous exposures illustrates lateral variations in tracefossil contents and allows characterization of three different types of intertidal ichnofauna, which correspond to three different sub-environments: open marine, restricted bays, and fluvioestuarine transitions. Although our study is based on outcrops in Kansas and Missouri, we provide comparisons to show that similar ichnofaunas are recognized in other late Palaeozoic areas of the American Midcontinent and the Appalachians.

Open-marine intertidal ichnofaunas Tidal flats may form on open coasts outside embayments. Examples of ichnofaunas from

Fig. 4. Physical sedimentary structures of tidal-flat deposits as seen in bedding-plane views, (a) Flat-topped ripples. Stull Shale. Waverly trace-fossil site. Lens cap is 5.5cm in diameter, (b) Relict troughs and wrinkle marks. Stull Shale. Waverly trace-fossil site. Lens cap is 5.5cm in diameter, (c) Ladderback ripples and flat-topped ripples. Note associated Gyrochorte isp. Stalnaker Sandstone. Road-cut on Kansas Highway 166. Coin is 2.4cm across, (d) Dendritic runnel marks. Tonganoxie Sandstone. Buildex Quarry. Bar= 1 cm. (e) Foam marks. Tonganoxie Sandstone. Buildex Quarry. Bar= 1 cm. (f) Raindrop impact structures on an otherwise rippled surface. Tonganoxie Sandstone. Buildex Quarry. Bar — 1 cm.

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such complexes have been analysed from two main outcrops in Kansas: the Waverly tracefossil site in Coffey County (Stull Shale Member), and road-cuts on Kansas Highway in Chautauqua County ('Stalnaker' Sandstone). The former was studied in detail by Mangano et al. (2002a), and the latter was briefly referred to in a number of publications (e.g. Archer 1993; Archer et al. 1994b; Mangano & Buatois 1997; Mangano et al. 2002b). These interbedded fine- to very fine-grained sandstones and siltstones contain a wealth of physical and biogenic structures. Physical sedimentary structures include asymmetric to symmetric ripples, flat-topped ripples (Fig. 4a), wrinkle marks (Fig. 4b), relict troughs (Fig. 4b), ladder-back ripples (Fig. 4c), interference ripples, gutter casts, flute marks, load casts, and sand volcanoes. Flaser, wavy and lenticular bedding are the dominant bedding styles. The association clearly indicates deposition in the intertidal zone. There is significant evidence for wave influence in these open-marine tidal facies. Facies analysis suggests sedimentation in sandflats, mixed flats, and mudflats, dissected by intertidal channels (Mangano et al. 2002a). Trace fossils are particularly abundant in the sandflat deposits (Figs 5, 6 and 7). Marine

body fossils (e.g. bivalves, brachiopods, crinoids) are also common. Ichnofaunas present in these tidal flat deposits are extremely rich, including resting traces (Asteriacites lumbricalis, Lockeia ornata, Lockeia siliquaria, Rusophycus isp.), locomotion traces (Cruziana problematica, Cruziana isp., Curvolithus simplex, Curvolithus multiplex, Gyrochorte isp., Protovirgularia rugosa), grazing traces (Nereites cambriensis, Nereites imbricata, Nereites jacksoni, Nereites missouriensis, Planolites beverleyensis, Psammichnites grumula, Psammichnites implexus, Psammichnites plummeri), feeding traces (Chondrites isp., Halopoa isp., Parahaentzschelinia ardelia, Phycodes palmatus, Phycodes isp., Phycosiphon incertum, Rhizocorallium irregulare, Teichichnus rectus, Trichophycus isp.) and dwelling traces (Arenicolites isp., Conichnus conicus, Diplo crater ion isp., Palaeophycus tubularis, Pentichnus pratti, Rosselia socialis, Skolithos isp., Solemyatuba isp.). Associated orthomyalinid bivalve shells have been extensively affected by bioerosion, displaying borings by acrothoracican barnacles, polydorid worms (ichnogenus Caulostrepsis), and ctenostomatid bryozoans (Baker 1995). Open-marine tidal flat ichnofaunas are characterized by:

Fig. 5. Idealized reconstruction of open-marine tidal-flat ichnofaunas showing the most typical ichnotaxa.

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Fig. 6. Base of a sandstone bed showing typical occurrence of intertidal ichnofaunas in the open-marine setting. Surfaces are totally covered by trace fossils, and ichnodiversity is very high. Cs, Curvolithus simplex; Cm, Curvolithus multiplex; Ls, Lockeia siliquaria; D, Diplo crater ion isp.; Al, Asteriacites lumbricalis; Cp, Cruziana problematical Pb, Protovirgularia bidirectionalis. Note subparallel orientation of specimens of Protovirgularia bidirectionalis. Stull Shale. Waverly trace-fossil site.

high ichnodiversity; marine ichnotaxa produced by both euryhaline and stenohaline organisms; the presence of both infaunal and epifaunal traces; the presence of simple and complex structures produced by presumed trophic generalists and specialists respectively; dominance of horizontal trace fossils of the Cruziana ichnofacies; the presence of multispecific associations; high density; wide size ranges. Most trace fossils are interface traces, and thus the degree of bioturbation is relatively low. Bedding planes are, however, generally covered by trace fossils, forming multispecific assemblages (Fig. 6). Dominant trace-makers are attributed to bivalves, ophiuroids and annelids.

Heterogeneous distribution of biogenic structures is remarkable at both small and large scales (Mangano et al. 2002a). At the larger scale, zonational distribution is expressed along the entire tidal range; on a smaller scale (i.e. within each sub-environment), spatial segregation of species may reflect distinct microhabitats and partitioning of energy resources. Detailed analysis of bivalve trace fossils at the Waverly site by Mangano et al. (1998) reveals the existence of palimpsest surfaces. These surfaces were formed by a succession of colonization events, including an initial colonization event, erosive scouring and subsequent sedimentation and eventual re-colonization. This succession of events produced time-averaged surfaces that record the work of several communities (ichnocoenoses) of burrowing bivalves. The presence of time-averaged associations may have artificially exaggerated ichnodiversity; the work of

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Fig. 7. Selected trace fossils from open-marine intertidal deposits, (a) Asteriacites lumbricalis, showing horizontal repetition. Base of sandstone bed. Stull Shale. Waverly trace-fossil site. Coin is 1.4cm across, (b) Protovirgularia bidirectionalis, displaying V-shaped markings with opposite directions meeting at a central point. Note that the direction of movement is from the centre to the ends. Base of sandstone bed. Stull Shale. Waverly trace-fossil site. Bar = 1 cm. (c) Nereites missouriensis with well-developed lateral lobes. Top of sandstone bed. Stull Shale. Waverly trace-fossil site. Bar= 1 cm. (d) Rusophycus isp. (R) and Cruziana isp. (C) preserved on the base of a sandstone bed. Stalnaker Sandstone. Roadcut along Kansas Highway 166. Bar = 1 cm. (e) Specimen of Lockeia ornata connected with chevron locomotion traces (Protovirgularia rugosa). Chevron orientation indicates that the animal exited the resting structure. Base of sandstone bed. Stull Shale. Waverly trace-fossil site, (f) Curvolithus simplex preserved on the top of a sandstone bed. Stull Shale. Waverly trace-fossil site. Lens cap is 5.5cm in diameter, (g) Solemyatuba isp. Cross-section view. Stalnaker Sandstone. Roadcut along Kansas Highway 166. Bar = 1 cm. (h) Psammichnites implexus preserved on the top of a rippled sandstone and showing guided meanders on ripple troughs. Stull Shale. Waverly trace-fossil site. Bar= 1 cm. (i) Psammichnites grumula with well-developed holes along a median line and prominent levees on both sides of the trace. Base of sandstone bed. Stull Shale. Waverly trace-fossil site. Bar = 1 cm. successive communities may be expressed at a single bedding plane. These ichnofaunas reflect the activity of biotas that inhabited tidal flats dominated by normal or near-normal marine salinities. Benthic faunas

inhabiting open-marine intertidal areas experience less stress than those developed in estuaries and brackish bays, characterized by steep salinity gradients combined with fluctuating temperature, oxygen and turbidity, among

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other variables (Wightman et al. 1987; Pemberton & Wightman 1992; MacEachern & Pemberton 1994; Pemberton et al. 2001). This fact is reflected by the high diversity of the openmarine tidal flat ichnofaunas and by the presence of a Cruziana ichnofacies.

Restricted-bay intertidal ichnofaunas Restricted-bay intertidal deposits accumulate in incised valleys, lagoons and embayments. Examples of late Palaeozoic ichnofaunas from restricted-bay environments are widespread in Kansas and Missouri, and include those recorded in the Bandera Quarry (Bourbon County, Bandera Shale), several outcrops of the Rock Lake Shale Member in northwest Missouri and northeast Kansas (Leavenworth and Wyandotte Counties; see Mangano et al. 1999), successions of the Douglas Group in Lonestar spillway (Douglas County) and Toronto Lake (Woodson County), and several outcrops of the Stull Shale Member in Douglas and Jefferson Counties (see Mangano et al. 2002a). Some of these ichnofaunas have been discussed by Bandel (1967b), Hakes (1976, 1977, 1985), Mangano & Buatois (1997) and Mangano et al. (1999, 2002a). Other examples of Carboniferous restricted-bay ichnofaunas in the United States include those reported from the Upper Mississippian Hartselle Sandstone of Alabama (Rindsberg 1994), the

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Lower Pennnsylvanian Fentress Formation in eastern Tennessee (Miller & Knox 1985), the Lower Pennsylvanian Caseyville and Tradewater formations of southern Illinois (Devera 1989), the Middle Pennsylvanian Kanawha Formation in West Virginia (Martino 1989, 1996), and the Upper Pennsylvanian Tecumseh Shale Member in Kansas (Hakes 1976). Similar ichnofaunas have been documented in the United Kingdom, but have been placed within a deltaic context (Eagar et al. 1985; Pollard 1988; Buckman 1992). The assemblages occur in heterolithic facies consisting of very fine-grained sandstone and mudstone displaying flaser, wavy and lenticular bedding. Flat-topped ripples and wrinkle marks are relatively common. Beds are commonly stacked, forming fining-upward parasequences that reflect tidal-flat progradation. Sand- and mud-filled intertidal channels typically cut the tidal-flat deposits. Coal beds are present in most outcrops. In contrast to tidal flats from open shorelines, there is less evidence for wave influence in these restricted tidal facies. Marine body fossils are notably absent. In fact, some of these deposits were originally thought to have formed in continental environments, but ichnological evidence (i.e. the presence of marine ichnotaxa such as Asteriacites and Psammichnites) indicates marine influence (Hakes 1976, 1977, 1985). Ichnofaunas from restricted-bay intertidal deposits are not diverse (Figs 8, 9). With the

Fig. 8. Idealized reconstruction of restricted-bay, tidal-flat ichnofaunas showing the most typical ichnotaxa.

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Fig. 9. Selected trace fossils from restricted-bay intertidal deposits. All bars = 1 cm. (a) Psammichnites plummeri. Base of sandstone bed. Ireland Sandstone. Toronto Lake, (b) Lingulichnus isp. Base of sandstone bed. Rock Lake Shale. Quarry near Coleman Creek, SE of Eudora. (c) Gyrochorte isp. (arrow). Top of sandstone bed. Bandera Shale. Bandera Quarry, (d) Teichichnus rectus. Stull Shale. Roadcut along Country Road 6, south of Stull (locality 8 of Hakes 1976). (e) Asteriacites lumbricalis with V-shaped striations ornamenting the arms. Note the presence of two partially preserved, shallower impressions toward the upper left documenting vertical repetition. The escape strategy is also recorded by an elite bivalve trace (upper left). Base of sandstone bed. Rock Lake Shale. Site south of the Kansas City International Airport, (f) Dense assemblage of small specimens of Lockeia siliquaria. Base of sandstone bed. Rock Lake Shale. Quarry near Coleman Creek, SE of Eudora. (g) Palaeophycus tubularis (Pt) and small Protovirgularia (P). Top of sandstone bed. Stull Shale Member. Roadcut west of Kanwaka (locality 5 of Hakes 1976).

exception of Lingulichnus, no ichnotaxa were found to occur exclusively in these deposits; several components of the open-marine assemblage, including resting traces (Asteriacites

lumbricalis, Lockeia siliquarid), locomotion traces (Gyrochorte isp., Protovirgularia rugosa), grazing traces (Nereites isp., Psammichnites implexus, Psammichnites plummeri), feeding

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traces (Teichichnus rectus) and dwelling traces (Diplo crater ion isp., Lingulichnus isp., Palaeophycus tubular-is), were present. These ichnofaunas display: low ichnodiversity; ichnotaxa commonly found in marine environments, but produced by presumed euryhaline organisms; a dominance of infaunal traces rather than epifaunal trails; simple structures attributed to opportunistic trophic generalists; a combination of vertical and horizontal trace fossils from the Skolithos and Cruziana ichnofacies; monospecific associations; variable abundances; small sizes. The abundance of trace fossils in the restricted bay deposits is highly variable. The presence of thick packages of restricted bay deposits with only a few trace fossils is not unusual. However, high-density assemblages may be common locally as well. Monospecific suites of Psammichnites (Fig. 9a) or Lingulichnus (Fig. 9b) are very common. Association of bivalve (Lockeia-Protovirgularid) and ophiuroid (Asteriacites) trace fossils are very common as well (Fig. 9e). In contrast to the conventional view of Asteriacites as a normal-marine salinity indicator, Mangano et al. (1999) demonstrated that this ichnotaxon is widespread in brackish-water facies of the American midcontinent. This is consistent with evidence from modern environments, which shows that some echinoderms, and particularly asterozoans, are able to inhabit brackish-water environments (e.g. Binyon 1972; Stancyk 1973; Turner 1980; Pagett 1980, 1981). It has been noted that reduced size is one of the most notable features of brackish-water assemblages (Hakes 1976, 1985). This is in agreement with studies of marine benthic ecology, which documented reduced size in brackish-water faunas, particularly ophiuroids, bivalves and worms (Remane & Schlieper 1971; Spaargaren 1979, 1995; Gingras et al. 1999). In contrast, crustaceans usually produce large burrows in brackishwater environments (Gingras et al. 1999). It has further been postulated that size reduction in response to salinity occurs either as a morphologic adaptation or as a result of population dynamics (Gingras et al. 1999). In the first case, decreasing size allows the organism to increase its surface area to mass ratio to control osmotic transfer. In the second case, large populations of small forms that attain full growth result in the same biomass. As a consequence of stressful

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conditions, brackish-water faunas are less diverse than their marine and freshwater equivalents (e.g. Croghan 1983; Hudson 1990; Pickerill & sdgfsdfgsdfgsdfgsdfgafgsdfgsdfg should not be equated with biodiversity (see sdfgsdfgsdfgsdfgsdfgsdfgsdfgfdgfsdfgdfgsdfddddd may provide some rough information on general trends in species richness if used with caution. Notably, the reduction in faunal diversity experienced by brackish-water faunas is reflected by a parallel decrease in ichnodiversity. Restricted-bay ichnofaunas record the activity of a brackish-water benthos inhabiting intertidal areas of estuarine basins and embayments. As such, they essentially display the diagnostic features of brackish-water trace-fossil assemblages, as documented by Wightman et al. (1987), Pemberton & Wightman (1992), MacEachern & Pemberton (1994) and Pemberton et al. (2001). Minor differences were discussed by Buatois et al. (2002b), who suggested that Pennsylvanian estuarine ichnofaunas differ from their postPalaeozoic counterparts in having lower ichnodiversity, a lower degree of bioturbation, scarce crustacean burrows, apparent lack of firmground suites, and an absence of specialized morphologic adaptations designed to protect organisms from salinity fluctuations. Interestingly, freshwater organisms tend to disappear rapidly, even with slight increases in salinity, whereas marine organisms experience a more gradual decrease in number under dilution of normal-marine salinity (Pemberton & Wightman 1992). Accordingly, brackish-water ichnofaunas typically resemble impoverished marine ichnofaunas rather than a mixture of freshwater and marine ichnotaxa (Pemberton & Wightman 1992).

Fluvio-estuarine intertidal ichnofaunas In estuaries, tidal influence commonly extends further landward than the saltwater intrusion, particularly in macrotidal systems (Fairbridge 1980; Allen 1991; Dalrymple et al. 1992; Buatois et al. 1997). Accordingly, tidal flats may develop in the uppermost zone of an estuary, under freshwater conditions. Ecological conditions in these fluvio-estuarine transitional settings are remarkably different from those in tidal flats formed along embayment margins or in direct connection with the open sea (Buatois et al. 1997). Our characterization of fluvio-estuarine intertidal ichnofaunas is based on outcrops and cores of the Tonganoxie Sandstone Member at the Buildex Quarry (Franklin County, Kansas). The Buildex ichnofauna was analysed in detail in a series of papers (Bandel 1967a; Buatois

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et al 1997, 1998a, 1998b, 1998c; Mangano et al. 1997). Other examples of virtually identical ichnofaunas in similar depositional settings are present in the Lower Pennsylvanian Whetstone Beds, Mansfield Formation, in Indiana (Archer & Maples 1984, Archer 1993; Archer et al 1994b), the Middle Pennsylvanian Pottsville Formation of Alabama (Cincosaurus and Haplotichnus assemblages of Rindsberg 1990), and the Middle Pennsylvanian Crane succession of the Mansfield Formation in Indiana (Kvale & Barnhill 1994; Mangano et al. 2001). At the Buildex Quarry, the Tonganoxie Sandstone Member overlies the Ottawa Coal and consists of very fine-grained sandstone, siltstone, and mudstone. Outcrop information was integrated with subsurface data derived from 10 drilled cores recovered from the quarry. The deposits consist of laterally persistent, normally graded and parallel-laminated siltstone beds that are replaced upward by a channelized siltstone body and a thicker-bedded, planar-stratified siltstone that is capped by a thin, pervasively rooted, silt-rich coal (Lanier 1993; Lanier et al.

1993). Facies are stacked in symmetrical cycles that each display a gradual increase and subsequent decrease in bed thickness. Climbing ripples, mudstone drapes and soft-sediment deformation structures, such as convolute lamination and micro-faults, are common. Beddingsurface structures are abundant, including tool marks, drainage or seepage rill marks, runnel marks (Fig. 4d), runoff washouts, foam marks (Fig. 4e), raindrop impressions (Fig. 4f), fallingwater marks, and wrinkle marks. Preservation of autochthonous upright plants is common. Recurrent thickness fluctuations indicate that these deposits must be regarded as tidal rhythmites, with thicker strata representing deposition near spring tide and thinner ones recording conditions during neap tide periods (Lanier et al. 1993). Some of the bedding-surface structures (e.g. raindrop impressions, rill marks, runnel marks) indicate brief periods of subaerial exposure. The Buildex Quarry succession is interpreted to have been deposited in tidal flats close to or at the fluvio-estuarine transition of a macrotidal estuarine valley (Lanier et al. 1993;

Fig. 10. Idealized reconstruction of fluvio-estuarine tidal-flat ichnofaunas showing the most typical ichnotaxa.

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Buatois et al. 1997). Animal trace fossils are exclusively preserved on bedding planes (Figs 10, 11); root traces are the only biogenic structures seen in cross-section. Accordingly, the core expression of such an assemblage is one of parallel-laminated deposits with minimal or no bioturbation (Buatois et al. 1998a). Ichnofaunas from fluvio-estuarine intertidal environments are relatively diverse. They are

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dominated by arthropod trackways (e.g. Dendroidichnites irregulare, Diplichnites gouldi, Diplopodichnus bifurcus, Kouphichnium isp., Mirandaichnium famatinense, Stiallia pilosa, Stiaria intermedia) and grazing traces (e.g. Gordia indianaensis, Helminthoidichnites tenuis, Helminthopsis hieroglyphica). Subsurface-feeding traces (Circulichnus montanus, Treptichnus bifurcus, T. pollardi), apterygote insect resting and

Fig. 11. Selected trace fossils from fluvio-estuarine transition intertidal deposits. All photos are bases of siltstone beds from the Tonganoxie Sandstone Member at the Buildex Quarry. All bars = 1 cm. (a) Tonganoxichnus buildexensis. (b) Stiallia pilosa (Sp) and Treptichnus bifurcus (Tb). Note that the trackway is cross-cutting Treptichnus bifurcus on the left, (c) Stiaria intermedia (Si) and Tonganoxichnus ottawensis (To), (d) Helminthopsis hieroglyphica. (e) Gordia indianaensis cross-cut by a series of thin, long tool marks, (f) Undichna britannica. (g) Diplichnites gouldi.

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Fig. 12. Relationship of species diversity, trace fossil assemblages, and salinity levels (modified from Remane & Schlieper 1971; Wightman et al. 1987; Buatois et al. 1997). Open-marine intertidal areas display the maximum ichnodiversity and are characterized by a Cruziana ichnofacies. The reduction in faunal diversity experienced by brackish-water faunas is reflected by a parallel decrease in ichnodiversity, illustrated by the presence of a depauperate Cruziana and Skolithos ichnofacies. The relatively high ichnodiversity of the fluvio-estuarine transition may be understood as recording the secondary peak in diversity typically associated with the activity of freshwater/terrestrial biotas along a salinity gradient (mixed Scoyenia and Mermia ichnofacies in tidal rhythmites).

feeding traces (Tonganoxichnus buildexensis, T. ottawensis), fish traces (Undichna britannica, U. simplicitas) and tetrapod trackways are also common. These ichnofaunas are characterized by: moderate to relatively high diversity; forms typically present in continental environments; the dominance of surface trails and absence of burrows; temporary structures produced by mobile deposit-feeding fauna; a mixture of trace fossils belonging to the Scoyenia and Mermia ichnofacies; moderate density of individual ichnotaxa; the absence of monospecific suites; small size. Interestingly, these ichnofaunas are in marked contrast to those recorded from brackish-water settings. The relatively high ichnodiversity may be understood as recording the secondary peak in diversity typically associated with the activity of freshwater/terrestrial biotas along a salinity

gradient (Fig. 12). This is consistent with the taxonomic composition of these ichnofaunas, which consist entirely of ichnotaxa commonly, though not exclusively, recorded from continental settings. No ichnotaxa indicative of marine influence have been detected. Arthropods are the dominant trace-makers. In fact, the fluvioestuarine ichnofaunas seem to display a mixture of elements of the continental Scoyenia and Mermia ichnofacies (Buatois & Mangano 1995, 1998). Undoubtedly, taphonomy played a role in increasing the ichnodiversity. Suppressed erosion during rising tides allowed excellent preservation of delicate surficial structures (Archer et al. 1994b). Additionally, the absence of pervasive burrowers in such palaeoenvironments improves the preservation potential of such surface traces because such activity by infaunal organisms commonly leads to the destruction of the uppermost tiers (Bromley 1996; Buatois et al 1997). The occurrence of assemblages dominated by trackways and trails in tidal rhythmites has puzzled researchers for some time (e.g. Archer

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& Maples 1984; Rindsberg 1990; Archer 1993; Archer et al 1994b) and, accordingly, a number of clastic successions have been regarded as either marine or non-marine. Although the presence of these continental ichnofaunas in tidal rhythmites makes it difficult to reconcile the sedimentological and ichnological evidence, understanding the ecological conditions existing in fluvio-estuarine transitions of macrotidal estuaries helps to solve this apparent paradox (see discussion in Buatois et al. 1997). The assemblages documented herein occur in tidal rhythmites, and record the activity of typical freshwater/terrestrial biotas inhabiting tidal flats developed in the most proximal zone of the inner estuary under freshwater conditions. The presence of these mixed freshwater/ terrestrial ichnofaunas in tidal rhythmites corresponds to the zone situated between the maximum landward limit of tidal action and the seaward salinity limit. Therefore recognition of these ichnofaunas helps to delineate fluvioestuarine transitions with great precision. The

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freshwater benthos inhabiting this zone do not have the special adaptations necessary to survive in the brackish environment. Whereas fully marine ichnofaunas gradually decrease in diversity into brackish-water settings, ichnofaunas from fluvio-estuarine transitions do not intergrade with those from brackish water. This ichnological signal reflects the fact that freshwater organisms disappear rapidly with even slight increases in salinity, whereas marine organisms experience a more gradual decrease under dilution of normal-marine salinity. Stratigraphic implications The ichnofaunas characterized in this study indicate not only certain depositional zones within coastal tide-influenced environments, but also particular depositional stages. Incised valleys may begin to fill during a lowstand, but sediments typically accumulate during the subsequent sealevel rise (Zaitlin et al. 1994). If lowstand,

Fig. 13. Palaeoenvironmental distribution of the intertidal trace-fossil assemblages analysed.

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coarse-grained sediments are deposited and preserved, they are replaced vertically by finergrained facies of the transgressive systems tract. Commonly, during a lowstand the valley acts as a bypass zone, or lowstand deposits are eroded and reworked during the subsequent transgression (MacEachern & Pemberton 1994). In such cases, transgressive deposits directly overlie the sequence boundary, resulting in the formation of co-planar surfaces (MacEachern et al. 1992). Scoyenia-Mermia intertidal ichnofaunas characterize not only deposition in the proximal portion of the inner estuary, but also the basal transgressive deposits immediately overlying the co-planar surface (Buatois et al. 1998a). In this specific depositional setting and at this particular stage of estuarine valley evolution, freshwater conditions coexist with tidal influence. As transgression proceeds, backstepping brackish-water deposits accumulate. The ichnologic signature of such a change in depositional conditions is reflected in the upward replacement of a mixed Scoyenia and Mermia ichnofacies by a mixed depauperate Skolithos and Cruziana ichnofacies. In cores, this change is evidenced by the transition from parallel-laminated deposits with minimal or no bioturbation to deposits displaying bioturbation due to the activity of infaunal organisms. If transgression proceeds, a diverse Cruziana ichnofacies eventually becomes established, recording the passage to normal-marine salinity conditions. Concluding remarks Our comparative study of a number of Carboniferous intertidal ichnofaunas in the North American Midcontinent emphasizes the importance of ichnology in the delineation of marginal-marine environments and allows characterization of three different types of intertidal trace fossil assemblages. These three ichnofaunas clearly correlate with three different sub-environments within coastal, tide-influenced depositional systems: open marine, restricted bays, and fluvioestuarine transitions (Fig. 13). Most of these tidal-flat deposits cannot be distinguished on the basis of physical sedimentology alone, because they have virtually the same set of physical sedimentary structures. We stress that although lithofacies distribution is, for the most part, salinity-independent, the distribution of the benthos and therefore biogenic structures is not. Therefore ichnological studies of marginal-marine depositional systems can provide the resolution necessary to refine facies and sequence-stratigraphic models previously

constructed only on the basis of sedimentological evidence. We thank Sigma Delta Epsilon, the Paleontological Society, the Mid-America Paleontological Society (MAP Award) and the Antorchas Foundation for providing financial support. Research for this paper was undertaken during tenure at the Kansas Geological Survey and funded by an external grant from the Argentinean Research Council (CONICET) awarded to both authors. We are very grateful to several colleagues who supplied valuable information on Pennsylvanian facies and trace fossils, including S. Beaty, P. Daniel, B. Lanier, R. Leisler, C. Maples, T. Stanley and R. West. R. West is also thanked for his valuable comments on an early draft. J. MacEachern and M. Gingras provided careful reviews that considerably improved the paper. We thank D. Mcllroy for the invitation to participate in the Lyell Meeting, 2003. We are also grateful to D. Ruiz Holgado and M. P. Jimenez for the drawings.

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Differentiation of estuarine and offshore marine deposits using integrated ichnology and sedimentology: Permian Pebbley Beach Formation, Sydney Basin, Australia KERRIE L. BANN1, CHRISTOPHER R. FIELDING2, JAMES A. MAcEACHERN3 & STUART C. TYE4 1 1Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton,

Alberta, CanadaT6G 2E3 (e-mail: [email protected]) 2 2DepartmentofGeosciences, 214 Bessey Hall, University of Nebraska-Lincoln,

NE 68588-0340, USA (e-mail:[email protected]) 3Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 (e-mail: [email protected]) 4 4Husky Energy, 707 8th Ave Calgary, SW Alberta, Canada T2P 3G7

(e-mail: [email protected]) Abstract: This study integrates ichnology and sedimentology to refine the palaeoenvironmental and sequence stratigraphic interpretations of the Early Permian Pebbley Beach Formation, in the southern Sydney Basin, Australia. This succession has been interpreted previously to reflect entirely inner to outer shelf and slope environments of deposition. Detailed analysis of the formation reveals ichnological and sedimentological characteristics that contradict a fully marine interpretation. Instead, the interval reflects the vertical superposition and lateral juxtaposition of brackish-water and fully marine units. Marine facies comprise: (1) thoroughly bioturbated muddy siltstone (lower offshore); (2) thoroughly bioturbated sandy siltstone (upper offshore); (3) interbedded bioturbated sandy siltstone and laminated sandstone (delta-influenced offshore transition); (4) thoroughly bioturbated muddy sandstone (distal lower shoreface); (5) interbedded laminated sandstone, bioturbated muddy sandstone and dark claystone (delta-influenced lower shoreface); and (6) bioturbated, laterally variable sandstones (transgressive sand sheets). Estuarine facies comprise: (1) channelized heterolithic sandstone-mudstone (active estuarine channels); (2) sheet-like heterolithic sandstone-mudstone (active estuarine basins); and (3) laminated mudstone (abandoned estuarine channels and basins). The interpreted fully marine deposits contain ichnological suites that exhibit moderate to intense bioturbation, high diversities (31 ichnospecies belonging to 20 ichnogenera), uniform distributions of ichnogenera, and significant numbers of structures reflecting specialized feeding/grazing behaviours. In marked contrast, interpreted estuarine (brackish-water) deposits contain impoverished ichnological suites (9 ichnogenera), show variable but significantly reduced degrees of bioturbation intensity, pronounced variability in ichnogenera distributions and the predominance of a few, simple forms representing simple feeding strategies of resilient trophic generalists. The new analysis allows the recognition of a series of highly top-truncated and condensed sequences (cycles of relative sea-level fall and physical rise), which can be physically correlated over several kilometres. Sequence boundaries typically cut down through shoreface sandstones to directly overlie offshore facies, leading to an interface with little apparent lithological contrast. In the absence of laterally continuous exposure, these surfaces may be recognized by careful ichnofacies evaluation. Thus the re-evaluation presented herein has facilitated a more realistic sequence stratigraphic analysis of the Pebbley Beach Formation. Although facies models for indented coastal systems are well-established (e.g. Dalrymple et al. 1992), there is nonetheless a shortage of reliable diagnostic criteria to allow discrimination between fine-grained facies of coastal origin and those of marine shelf origin. The possibility therefore exists for confusion between facies that may be superficially similar but which record widely separated environments of deposition. The integration of ichnology into

stratigraphic analysis provides a useful way of discriminating between lithologically similar facies. Literature on the ichnology of brackishwater environments is limited, particularly for successions of late Palaeozoic age (but see Buatois et al. 2002). Here, we describe a succession of Early Permian age that contains both offshore marine and estuarine facies that are lithologically similar, but which can be separated successfully using a combination of ichnology

From: MC!LROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 179-211. 0305-8719/04/S15.00 © The Geological Society of London.

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and sedimentology. From this fades analysis we present a sequence stratigraphic model for the succession, highlighting the usefulness of ichnology and providing new data on the composition

of brackish-water trace fossil assemblages of Permian age. The Pebbley Beach Formation crops out along the coast of southern New South Wales,

Fig. 1. Location maps: (a) position of Sydney-Bowen Basin; (b) southern Sydney Basin; (c) Pebbley Beach Formation study area. Measured section localities are marked by bullet points.

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Australia, between Point Upright in the south and Clear Point in the north (Fig. 1). This paper focuses on the middle and upper portions of the formation, which are well exposed in sea cliffs at Point Upright, Mill Point and Clear Point. A previous facies scheme for the Pebbley Beach Formation (Gostin & Herbert 1973) proposed a shallow marine to broadly coastal environment of deposition; in contrast, Eyles et al. (1998) concluded an entirely inner to outer shelf and slope environment of deposition for the interval. In that study, Eyles et al. (1998) regarded a series of channel features prominent in the middle and upper parts of the unit as the product of shoreface erosion and deposition in a shallow marine setting. The ichnological characteristics of the various facies compel us, however, to dispute this interpretation, as well as the assertion that the succession is entirely of fully marine origin. The brackish-water and fully marine units of the Pebbley Beach Formation are lithologically and (to a lesser extent) sedimentologically similar, contributing to the confusion regarding the depositional setting. Although there are primary sedimentological features that occur persistently within the various facies of both depositional systems, the ichnological differences are profound. The differences in bioturbation intensity, uniform versus sporadic distribution of bioturbation and, most importantly, the trace fossil assemblages themselves permit the reliable differentiation of brackish-water (estuarine) deposits from fully marine offshore and lower shoreface intervals. The recognition of these disparate depositional environments has a profound impact upon the high-resolution sequence stratigraphic interpretation of the succession. The integration of ichnology with the sedimentological facies model has led to the identification of sequence boundaries, marine flooding surfaces and transgressive surfaces of erosion, demonstrating that the Pebbley Beach Formation has a complex sequence-stratigraphic architecture. The recognition of brackish-water deposits based on ichnological characteristics is principally derived from studies of: (1) the Holocene from the North Sea (e.g. Schafer 1962; Reineck et al. 1967, 1968; Hertweck 1970) and the Georgia coast of the US (e.g. Frey & Howard 1972; Howard & Frey 1973, 1975; Howard et al. 1975); and (2) Cretaceous units of the Western Interior Seaway of North America (e.g. Pemberton et al. 1982; Ekdale et al. 1984; Howard & Frey 1984; Wightman et al. 1987; Beynon et al. 1988; Ranger & Pemberton 1992; MacEachern & Pemberton 1994). The trace fossil assemblages of the Pebbley Beach Formation serve as

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excellent late Palaeozoic comparators to the Cretaceous and Holocene estuarine trace fossil models. Geologic setting The Sydney Basin comprises the southernmost portion of the larger, north-south elongate Sydney-Gunnedah-Bowen Basin (Fig. la), of Late Carboniferous to Triassic age, one of a series of contiguous basins that formed along the Panthalassan margin of Gondwana. The basin has a complex, multiphase history. Late Carboniferous (Pennsylvanian) to Early Permian rifting in a back-arc environment was followed by middle Permian thermal subsidence (Schiebner 1974; Battersby 1981; Murray 1990). The basin then evolved into a retro-arc foreland basin during the Late Permian to Middle Triassic, adjacent to the New England Fold Belt (Holcombe et al. 1997). The Pebbley Beach Formation forms a part of the Early Permian (Sakmarian to Artinskian: Briggs 1998) succession in the southernmost portion of the exposed, onshore Sydney Basin (Fig. 2). It overlies the shallow marine Wasp Head Formation and is in turn overlain by the shallow marine Snapper Point Formation (Gostin & Herbert 1973; Tye et al. 1996). The Wasp Head Formation forms a part of the Talaterang Group, which includes the Clyde Coal Measures and represents deposition during the initial extensional phase of basin formation (Tye et al. 1996). The Pebbley Beach Formation comprises the basal unit of the Shoalhaven Group (Gostin & Herbert 1973), which was deposited late in the extensional phase and during the ensuing phase of passive thermal subsidence. The fully marine and nearshore coastal deposits of the Pebbley Beach Formation can be correlated lithostratigraphically with the fluvial Yadboro Conglomerate to the immediate west (Tye et al. 1996). The interpreted palaeogeography for this period involves a northsouth-elongate shoreline that was at most times somewhere to the immediate west of the exposures described herein, with sediment dispersal from continental drainage systems from west to east (Tye et al. 1996). During Early Permian time, eastern Australia was located along the Panthallassan margin of the Gondwana landmass, connected to Antarctica and adjacent to the palaeo-Pacific Ocean (Veevers & Powell 1987). The South Pole was near the eastern edge of Antarctica (Crowell & Frakes 1975), close to eastern Australia, suggesting that the latitude of the southern Sydney

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Fig. 2. Stratigraphy of the southern Sydney Basin (modified from Tye et al. 1996). YCM, Yurrunga Coal Measures; N-m, non-marine; Art., Artinskian; Kun., Kungurian.

Basin was similar to that of the present-day Ross Ice Shelf. The presence of (1) glendonites (a pseudomorph after the mineral ikaite which iforms at temperatures less than 5 °C, and from which cold climate conditions have been invoked extensively in the Permian of eastern Australia: Kaplan 1979; Carr et al. 1989), (2) large, exotic outsized clasts, and (3) the distinctive coldwater Eurydesma fauna (Runnegar 1979; Dickens 1984) provides convincing evidence for persistent cold climate throughout the deposition of the Pebbley Beach Formation. The cold period was associated with a widespread Gondwana glaciation that began in Late Carboniferous time and persisted until the Early Permian (Dickens 1984; Veevers & Powell 1987).

Facies analysis Nine facies are defined here, based on lithology, primary sedimentary structures and ichnology. Measurement of bioturbation intensity (BI) in the field follows the scheme illustrated in Figs 3 and 4 (adapted after Reineck 1963; Taylor & Goldring 1993), and trace fossils are listed in decreasing order of abundance. These facies record a depositional gradient from lower offshore environments in an open marine setting, through shoreface environments (locally affected

by proximity to a contemporaneous delta system), to coastal environments that are broadly estuarine (brackish water) in character. The use of the terms 'upper offshore' and 'lower offshore' follows the usage of MacEachern & Pemberton (1992) and Pemberton & MacEachern (1995, 1997), modified from Howard (1971, 1972), Howard & Reineck (1981), Howard & Frey (1984), Vossler & Pemberton (1989) and Frey (1990). The upper offshore lies below fair-weather wavebase but adjacent to (and grading into) the lower shoreface. The proximity to the lower shoreface results in the upper offshore receiving significant amounts of sand and silt with clay, producing characteristic sandy mudstone and sandy siltstone facies. The lower offshore grades basinward into the shelf. The lower offshore receives more silt than sand, with clay deposition predominant during fair-weather conditions, resulting in the production of silty mudstone.

Marine facies Facies 1: Thoroughly bioturbated siltstone Sedimentology This facies comprises siltstone and silty mudstone with few if any preserved sedimentary

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Fig. 3. Legend of symbols used in the graphic logs.

Fig. 4. Bioturbation intensity (BI). Measurement of the intensity of bioturbation in the Pebbley Beach Formation, modified from Bann (1998), originally adapted and modified after Reineck (1963) and Taylor & Goldring (1993).

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Fig. 5. Fades 1 and 2. (a) Heavily bioturbated (BI5) Fades 2, capped by a marine flooding surface (FS), overlain by bioturbated (BI 4-5) Fades 1. The marine flooding surface is overlain by a thin, gritty sandstone layer. Arrows mark Diplocraterion habichi (D), Phycosiphon (Ph) and Chondrites (Ch). Fig. 15, 15.5m. (b) Large, tabular glendonite (Gl) in Facies 1 (lower offshore), Fig. 15, 16m. (c) Thoroughly bioturbated (BI4-5) Facies 2. A thin, remnant tempestite showing wavy parallel lamination is present in the centre of the photo. The trace fossil assemblage reflects the archetypal Cruziana ichnofacies, and comprises Chondrites (Ch), Teichichnus (T), Zoophycos (Z), Phycosiphon (Ph), Planolites (P), Asterosoma (As) and Palaeophycus (Pa). Fig. 14, 17-18.3m. (d) Thoroughly bioturbated (BI4-5) Facies 2 containing a remnant parallel-laminated tempestite. The trace fossil suite reflects the archetypal Cruziana ichnofacies, manifest as Planolites (P), Chondrites (Ch), Rosselia (Ro), Palaeophycus (Pa), Asterosoma (As), Zoophycos (Z), Helminthopsis (H), Phycosiphon (Ph) and fugichnia (fu). Fig. 14, 17-18.3m. (e) Intensely bioturbated (BI5) Facies 1 with large, outsized, exotic clast. Fig. 1, Depot Beach.

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structures (Fig. 5a). Interstitial sand (very fine- to MacEachern 1997). The presence of abundant, fine-grained) is rare. Also rare are thin (<1 cm), large, exotic clasts within these fine-grained sharp-based, very fine- to fine-grained sandstone deposits (Fig. 5e) has been used by previous beds that contain low-angle undulatory parallel workers (Gostin & Herbert 1973; Eyles et al lamination, possible small-scale hummocky 1998) as evidence for persistent delivery of icecross-stratification (HCS), combined flow-ripple rafted debris. Glendonites are also strong and current-ripple cross-lamination. Glendonites evidence for very cold climatic conditions. This facies reflects deposition in a lower off(Fig. 5b), outsized clasts ranging from 5mm to greater than 1 m in diameter, carbonaceous detri- shore, open marine environment, affected by tus, pyrite staining and wood fragments are very cold climatic conditions, but only rarely by severe storms. locally common. Ichnology Facies 1 is characterized by more or less pervasive bioturbation, varying from BI4 to BI6, but typically BI5. Bioturbation is locally sporadic, decreasing in intensity in the thin sandstone beds. Most ichnogenera are uniformly distributed throughout the facies, with less common elements sporadically distributed. The trace fossil assemblage is dominated by Phycosiphon, Planolites, Rosselia socialis, lesser Rosselia rotatus and small tightly curved Taenidium (type A). Common but subordinate elements comprise Zoophycos (Fig. 5c), Chondrites (Fig. 5d), Teichichnus, Palaeophycus tubularis and Palaeophycus heberti. Uncommon elements consist of Helminthopsis and Asterosoma. The thin sandstone beds also locally contain Diplocraterion habichi, rare Skolithos and fugichnia. Phycosiphon occurs as both robust and diminutive elements, whereas Rosselia, Zoophycos, Diplocraterion, Skolithos and Asterosoma are diminutive. The assemblage is interpreted to reflect a diverse, distal expression of the Cruziana ichnofacies. The introduction of some Skolithos ichnofacies elements is attributed to the sporadic emplacement of sand beds during storms. Interpretation The fine-grained, bioturbated lithology of Facies 1 indicates a standing-water depositional environment beyond the reach of most currents or waves. The presence of marine invertebrate fossils points to a quiet-water, open marine setting. The thin sandstone beds record exceptional storms and are characteristic of distal tempestites that have been partially biogenically reworked by small, simple burrow types during fair-weather periods. Facies 1 contains moderately diverse trace assemblages produced by deposit-feeding and grazing/foraging behaviours, typical of open marine environments lying well below fair-weather wavebase. The preservation potential of distal tempestites is high, owing to emplacement well below fair-weather wavebase where physical processes are not competent to modify them (Wheatcroft 1990; Pemberton &

Facies 2: Thoroughly bioturbated sandy siltstone Sedimentology This facies comprises thoroughly bioturbated siltstone and sandy siltstone units with rare but upwardly increasing numbers of discrete, thin (0.5-2 cm), very fine- to fine-grained sandstone beds. Sandstone units display remnant lowangle, undulatory, parallel lamination and lesser wave-ripple lamination (Fig. 5c, d). Interstitial sand is pervasively distributed throughout the siltstone units, Synaeresis cracks are rare and are associated with decreased levels of bioturbation. Dispersed outsized clasts, clast clusters, allochthonous logs, soft-sediment deformation structures and glendonites are locally abundant. Ichnology Facies 2 is characterized by more or less pervasive bioturbation, with intensities that vary from BI 5 to BI 6, and typically BI 6. Most ichnogenera are uniformly distributed throughout the facies, though elements associated with intercalated sandstone beds are more sporadically distributed. Uncommon elements are also sporadically distributed. The trace fossil suite is dominated by Rosselia socialis, Zoophycos, Phycosiphon, Planolites, Palaeophycus tubularis, Palaeophycus heberti, Teichichnus and Rhizocorallium irregulare. Subordinate elements comprise Diplocraterion habichi, small Taenidium (type A), Rosselia rotatus, Skolithos, Bergaueria and fugichnia. Rosselia and Zoophycos occur as diminutive elements. The assemblage reflects two phases of infaunal colonization. The main assemblage is interpreted to reflect a diverse expression of the archetypal Cruziana ichnofacies. The presence of Skolithos ichnofacies elements (e.g. Diplocraterion, Skolithos and Bergaueria) reflects infaunal tempestite colonization. Interpretation Facies 2 is considerably more variable than Facies 1, owing to the greater numbers of

Fig. 6. Fades 3. (a) Weakly to moderately bioturbated (BI2), interbedded sandstone, sandy siltstone, and mudstone with tempestites with low-angle, undulatory, parallel lamination. Some sandstone beds show wave ripple lamination and normal grading. The trace fossil suite comprises a diverse expression of the archetypal Cruziana ichnofacies, manifest by Phycosiphon (Ph), Chondrites (Ch), Planolites (P), Diplocraterion (D) and fugichnia (fu). Fig. 1, Mill Point, (b) Bedding plane view of silty sandstone tempestite showing a BI of 3. The trace fossil suite comprises the archetypal Cruziana ichnofacies, and consists of Psammichnites (Ps), Phycosiphon (Ph), Taenidium (type A, Ts) and Planolites (P). Fig. 1, Mill point, (c) Weakly to moderately bioturbated (BI 1-3) sandstone, sandy siltstone, and mudstone with wave ripples and lesser low-angle, undulatory, parallel lamination. Mudstone interbeds locally contain very rare, diminutive synaeresis cracks (sy). The trace fossil suite comprises the archetypal Cruziana ichnofacies, and contains Palaeophycus (Pa), Planolites (P), Chondrites (Ch), Phycosiphon (Ph), Taenidium (type A, Ts), Diplocraterion (D) and fugichnia. Fig. 1, Mill Point, (d) Close-up, slightly to the right of the lower part of photo (c), showing more intensely bioturbated (BI3) sandstone and clayey siltstones. The sandstones display remnant low-angle, undulatory, parallel lamination and lesser wave ripple lamination. The trace fossil suite consists of Chondrites (Ch), Phycosiphon (Ph), Psammichnites (Ps), Palaeophycus (Pa), Planolites (P), Taenidium (type A, Ts) and Diplocraterion (D). Some diminutive fugichnia are visible locally. Fig. 1, Mill Point.

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tempestites. Remnant, low-angle, undulatory, parallel lamination is interpreted as HCS. The mudstone and silty sandstone beds, coupled with the high ichnological diversities and bioturbation intensities, suggest deposition in an environment that experienced lengthy periods of fair-weather near, but below fair-weather wavebase. The diverse trace fossil assemblage, dominated by traces produced by depositfeeding and, to a lesser extent, grazing/foraging behaviours, is characteristic of the archetypical Cruziana ichnofacies and reflects quiescent conditions in environments lying between storm and fair-weather wavebase. Vertical burrows that represent the dwellings of suspensionfeeding organisms (e.g. elements of the Skolithos ichnofacies) correspond to the opportunistic colonization of the storm beds. More intense reworking of tempestites is the product of: greater vertical penetration by more robust deposit feeders; increased sizes and abundances of vertical burrows; and increased time between storm events, facilitating complete colonization and development of deeply penetrating structures. This facies is interpreted to reflect deposition in an upper offshore environment. Outsized clasts are interpreted to have been introduced by ice rafting. Decreased levels of bioturbation associated with rare, locally occurring soft-sediment deformation and synaeresis cracks (Plummer & Gostin 1981; but see Pratt 1998) may indicate a combination of increased sedimentation rates, sporadic deposition, fluctuating salinity levels, periods of reduced oxygenation and soupy substrates (Coates & MacEachern 1999, 2000).

Facies 3: Interbedded bioturbated sandy siltstone and laminated sandstone Sedimentology This facies is dominated by sparsely to moderately bioturbated sandy siltstone beds intercalated with thin (generally <10cm), low-angle undulatory, parallel-laminated, very fine- to fine-grained sandstone beds, and thin (<5cm), dark mudstone beds (Fig. 6a, b). Sandy siltstone beds are commonly truncated from above, and their bases are indistinct owing to biogenic mixing across the interface with underlying sandstone units. Laminated sandstone beds are generally erosionally based and display laminated to burrowed bedding, colloquially referred to as 'Lam-Scram'. Relict HCS, low-angle planar

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lamination, wave and combined flow-ripples are commonly preserved (Fig. 6c, d). Sandstone beds are locally draped with unbioturbated, dark claystone containing organic detritus and locally abundant and generally shallow synaeresis cracks. Dispersed pebbles, clast clusters, allochthonous logs and shelly lags are locally common. Ichnology Facies 3 is characterized by persistent though largely sporadically distributed bioturbation. Bioturbation intensities vary from BIO to BI4 and typically BI3-4. Most ichnogenera are uniformly distributed throughout the facies, though elements associated with intercalated laminated sandstone beds are more sporadically distributed. Rosselia and Zoophycos constitute exceptions, typically increasing in abundance and size within some intervals. Uncommon elements are also sporadically distributed. The trace fossil assemblage can be subdivided into one associated with the bioturbated sandy siltstone, and one reflecting infaunal colonization of the laminated sandstone units. The suite associated with the sandy siltstone is dominated by Rosselia socialis, Teichichnus, Palaeophycus tubularis, Palaeophycus heberti, Phycosiphon and Planolites. Common but subordinate elements comprise Chondrites, Zoophycos, Rhizocorallium irregulare, Diplocraterion habichi, Taenidium (type A) and Conichnus. Helminthopsis, Skolithos and Cylindrichnus are uncommon. The laminated sandstone beds contain an assemblage dominated by Diplocraterion habichi, Diplocraterion parallelum, very large Rhizocorallium irregulare (type A), Phycosiphon and Taenidium (type A). Common but subordinate elements include Palaeophycus tubularis, Macaronichnus isp., Planolites, Rosselia socialis, Rosselia rotatus, Teichichnus, fugichnia, Skolithos and Palaeophycus heberti. Uncommon elements include Chondrites, Psammichnites, Rhizocorallium irregulare, Zoophycos, Cylindrichnus and Asterosoma. Diplocraterion parallelum and Rhizocorallium irregulare (type A) are commonly large. Interpretation Facies 3 records a marine depositional environment that was periodically affected by storms, leading to the superimposition of sharp-based sand lenses and tabular beds as distal tempestites over muds. Interbedded muds settled from suspension or were distributed by gentle currents, and must therefore record periods of more subdued (fair) weather. Facies 3 records a fairweather resident trace fossil suite occupying

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sandy siltstones, erosionally truncated and overlain by sandstones with HCS, locally with a basal lag, and commonly containing escape structures. The fair-weather suite, dominated by the structures of deposit-feeding and grazing/foraging organisms, represents a diverse expression of the archetypal Cruziana ichnofacies. In contrast, the trace fossil suite in the sandstone beds is dominated by vertical burrows of opportunistic suspension feeders, resilient surface detritus feeders and passive carnivores and is indicative of the Skolithos ichnofacies. This recurring juxtaposition of the Cruziana and Skolithos ichnofacies is the basis for recognizing the mixed Skolithos-Cruziana ichnofacies (Howard & Frey 1984; Pemberton & Frey 1984; MacEachern & Pemberton 1992; Pemberton & MacEachern 1997). The overall mixed, diverse SkolithosCruziana ichnofacies suite is complex and is characteristic of interbedded fair-weather units and tempestites deposited within the upper offshore, lying close to but below fair-weather wavebase in a moderately storm-influenced setting. This zone reflects the transition from the lower shoreface to the offshore/shelf (e.g. offshore transition; cf. Howard & Reineck 1981). The storm assemblage represents the activities of opportunistic organisms re-colonizing the substrate following storm disruption (Pemberton & Frey 1984; Vossler & Pemberton 1988; Pemberton & MacEachern 1997). The tops of the sandstone units contain trace fossils that represent initial opportunistic colonization. The sandstones grade upward into sandy siltstones containing a fair-weather resident trace fossil suite that indicates a return to quiescent conditions following storm abatement (Pemberton et al. 1992a, 1992b; Pemberton & MacEachern 1997). Storm activity presumably disrupted bottom fauna and probably resulted in mass stranding and the transportation of organisms to other environments (Rees et al. 1977; Dobbs & Vozarik 1983; Butman 1987). The largediameter living tubes of Diplo crater ion parallelum and large Rhizocorallium irregulare (type A) are commonly filled with cleaner and/ or coarser sediment within some assemblages of the mixed Skolithos-Cruziana ichnofacies. The coarse infills of these structures are interpreted as 'tubular tempestites' (Wanless et al. 1988; Tedesco & Wanless 1991), and provide good evidence for the prior existence of storm beds. The diversities of the trace fossil assemblages generally decrease in response to increases in storm intensity and/or frequency, and with increasing deltaic influence. Thin, unburrowed claystone drapes with synaeresis cracks occurring at the tops of tempestites at some localities may

reflect minor deltaic influence on the depositional environment. Deltaic settings are associated with increased levels of fluvial discharge and large amounts of associated organic detritus and clay material, transported basinward during and immediately following storm events. During post-storm conditions, this mud mantles the tops of storm beds, shielding them from opportunistic colonization, particularly by suspension feeders. High organic contents in the mud result in its rapid oxidation, and resulting oxygen depletion at the seafloor: as a consequence, many organisms are unable to colonize these substrates (e.g. Leithold 1989; Raychaudhuri & Pemberton 1992; Saunders et al. 1994; Coates & MacEachern 1999, 2000; Bann & Fielding 2004). The common association of synaeresis cracks with the dark mudstone drapes suggests a close linkage between the storm events and concomitant heightened precipitation, increased surface runoff and rapid fluvial discharge into the basin following storm abatement. The heterolithic character of this facies and, in particular, the presence of the dark claystones with synaeresis cracks suggests that the setting lies along depositional strike, but downlongshore drift, of a contemporaneous delta complex (e.g. Coates & MacEachern 1999, 2000; Bhattacharya & Willis 2001; Bann & Fielding 2004). Facies 4: Thoroughly bioturbated muddy sandstone Sedimentology This facies comprises intensely bioturbated, 90180cm thick muddy sandstone units, interbedded with minor, thin (<15cm), erosionally based, fine- to medium-grained sandstone beds. Some sandstone beds display low-angle, undulatory, parallel lamination, interpreted as HCS and wave ripple cross-lamination. Shell fragments, dispersed pebbles and granules, carbonaceous detritus and allochthonous logs are locally common. Ichnology Facies 4 is characterized by pervasive bioturbation, with intensities that vary from BIS to BI6, typically BIS (Fig. 7a, b). Bioturbation intensity is more or less uniform. Most ichnogenera are uniformly distributed. The trace fossil assemblage is dominated by Rosselia socialis, Phycosiphon, Planolites, Diplocraterion habichi, Rhizocorallium irregulare, Teichichnus, Rhizocorallium irregulare (type A),

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Fig. 7. (a) Thoroughly bioturbated (BI5) silty sandstone with clay laminae (Fades 4). Proximal expression of the Cruziana ichnofacies including Phycosiphon (Ph), Palaeophycus (Pa), Chondrites (Ch), Teichichnus, Diplocraterion (D), Helminthopsis (H) and Planolites. Fig. 15, 11-13m. (b) Thoroughly bioturbated (BI5) silty sandstone with clay laminae (Facies 4). Proximal expression of the Cruziana ichnofacies including Palaeophycus (Pa), Phycosiphon, Planolites (P), Teichichnus (T), Chondrites (Ch), Rhizocorallium (Rh) and Psammichnites (Ps). Shell debris (sh) is locally present. Fig. 15, 11-13m. (c) Moderately to intensely bioturbated (BI3-5) sandstone (Facies 5) with Rosselia rotatus (Ro) containing Teichichnus-\ike expressions of the living tube (Te). Fig. 1, south Pebbley Beach, (d) Sandstone (Facies 5) with abundant Teichichnus (Te) that represent the lower tubular portion of Rosselia socialis and R. rotatus. Fig. 1, Mill Point, (e) Plan view of the same bed figured in (d) showing a cross-section through the associated Rosselia mud balls (Ro).

Macaronichnus isp., Palaeophycus tubular is and Palaeophycus heberti. Common but subordinate elements are Taenidium (type A), larger, less sinuous Taenidium (type B), sandy, robust IZoophycos, Skolithos, fugichnia, Psammichnites and Chondrites,

Interpretation The dominance of sharp-based sandstone beds in Facies 4 together with the sedimentary structure and presence of marine fossils indicates a shallow marine environment of deposition frequently affected by storms. Rare, thin, sharp-based

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Fig. 8. Fades 5. (a) Weakly bioturbated (BI1-2) HCS and wave rippled sandstone, siltstone and thin, dark claystone. The trace fossil suite comprises Diplocraterion habichi (D), Planolites (P), Rosselia (R) and fugichnia (fu), corresponding to a proximal expression of the Cruziana ichnofacies. Fig. 14, 14.5-16.2m. (b) Plan view of Diplocraterion habichi (D) in a tempestite. Fig. 1, Mill Point, (c) Moderately well bioturbated (BI4) sandstone showing remnant low-angle, undulatory, parallel lamination and clay interlaminae. The trace fossil suite reflects a proximal expression of the Cruziana ichnofacies, and comprises Planolites, Rosselia (R), Phycosiphon, Chondrites, Helminthopsis, Teichichnus (Te), Diplocraterion and Palaeophycus. Fig. 1, Mill Point, (d) Very large Rhizocorallium irregulare (Rh) and Diplocraterion habichi (D) in a tempestite bed. Fig. 1, Mill Point, (e) Non-burrowed to weakly bioturbated (BIO-1), HCS and wave-rippled sandstone and dark claystone.

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HCS sandstone beds represent the deposits of particularly severe storms. The trace fossil suite, comprising a diverse mixture of robust, complex deposit- and detritus-feeding structures, is a proximal expression of the Cruziana ichnofacies. Facies 4 is interpreted to reflect deposition in a well-oxygenated, open marine setting at or just above fair-weather wavebase, consistent with the distal lower shoreface. The intensity and uniformity of burrowing, and the scarcity of preserved primary sedimentary structures, suggest considerable time breaks between storms, during which the fair-weather fauna reworked the substrate. The presence of considerable numbers of vertical burrows, such as Diplocraterion habichi in some localities (Fig. 7a), suggests that storm beds were rapidly colonized by opportunistic, suspension-feeding organisms, prior to their re-colonization and thorough reworking by the resident fair-weather community.

Facies 5: Interbedded laminated sandstone, bioturbated muddy sandstone and dark clay stone Sedimentology This facies is composed principally of tabular to lensoidal sandstone beds, typically 0.2-0.5m thick. These beds can be erosionally amalgamated, or interbedded with bioturbated muddy sandstone, or in some cases separated by thin (0.2-6.0 cm), dark claystone partings. A characteristic feature of this facies is the presence of interbedded lenses or thin beds of very coarsegrained to granular/pebbly sandstone, the tops of which form sets of large (height 2-5 cm, wavelength 15-20 cm), symmetrical, gravelly ripples. Some of these beds are internally composed of a single set of unidirectional cross-stratification. Most other sandstone beds show one or more of the following: well-preserved HCS, low-angle planar cross-stratification, small symmetrical wave ripples and combined flow ripple crosslamination. The muddy sandstone beds resemble those of Facies 4. Shelly debris along with dispersed pebbles and granules and carbonaceous detritus are locally common. Claystone interbeds

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are dark, organic-rich (carbonaceous detritus), generally silt-poor, and contain common to abundant, short synaeresis cracks. Ichnology This facies is characterized by highly variable bioturbation intensities, ranging from BIO to BI4. Laminated sandstone beds, interpreted as tempestites, range from BIO to BIS, and typically BI2. Bioturbated muddy sandstone beds range from BI3 to BI4, typically BI4. Intervening thin claystone beds are generally nonburrowed but rarely have a BI of 1. Bioturbation is sporadically distributed in the facies and 'LamScram' bedding is common. The trace fossil assemblage is dominated by Rosselia socialis, R. rotatus and Rosselia (type A) that exhibits a lateral shift of the mud-ball and contains lateral spreiten (Figs 7c-e, 8a, c, h; Bann 1998). All forms of Rosselia may occur with or without Teichichnus-\ikQ expressions of the dwelling tube. Phycosiphon (Fig. 9a), Diplocraterion habichi (Fig. 8b), Rhizocorallium irregulare (type A, Fig. 8d), Diplocraterion parallelum (Fig. 9c, d), Macaronichnus isp. and fugichnia are also abundant. Common but subordinate elements comprise Palaeophycus tubularis, Taenidium (type A), Skolithos, Conichnus (Fig. 9e), Psammichnites (Fig. 9f) and sandy, robust IZoophycos (Fig. 8f, g). Uncommon elements include Lingulichnus (Fig. 9h), Chondrites, Macaronichnus segregatis, Taenidium (type B) and Cylindrichnus. Fragmented Rosselia mud balls are present within the tempestites, recording erosional truncation, reworking and deposition as clasts. The claystone interbeds contain small, rare Planolites (Fig. 8e). The assemblage, overall, is interpreted to reflect a diverse expression of the proximal Cruziana ichnofacies (Fig. 9b). Interpretation The strongly heterolithic character of Facies 5 reflects marked variations in physical energy and sedimentation rates. Most of the physical structures reflect storm deposition, particularly HCS, low-angle planar cross-stratification, combined-flow and wave ripples. The laminated sandstone units record episodic colonization

Carbonaceous detritus occurs within the sandstone beds. The trace fossil suite consists of diminutive Planolites (P), Chondrites (Ch) and Phycosiphon (Ph). Note that, despite the presence of Phycosiphon, small synaeresis cracks (sy) are present in the claystone. Fig. 14, 18.4-19 m. (f) Bedding plane view of a tentatively identified Zoophycos (Z) comparable to that shown in cross-section in photo (g). The structure occurs in a silty HCS sandstone bed interpreted as a tempestite. Fig. 15, 12.9-14.9 m. (g) Weakly bioturbated (BI2), HCS sandstone, siltstone and dark claystone with a bedding plane expression of the structures displayed in photo (f). The structure (a cone-like, sand-filled depression) is tentatively identified as Zoophycos (Z). Fig. 14, 14.5-16.2m. (h) Example of a complex Rosselia rotatus (R). Fig. 1, south Pebbly Beach.

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Fig. 9. Trace fossils in Fades 4 and 5. (a) Phycosiphon (Ph) in a tempestite. (b). Idealized graphic representation of characteristic trace fossils in lower shoreface deposits in the Pebbley Beach Formation. The distal lower shoreface is characterized by a diverse suite of structures produced by complex deposit- and detritus-feeding behaviours and specialized grazing behaviours. The vertical burrows of opportunistic suspension feeders are associated with rare, thin tempestites. The overall trace fossil assemblage represents a proximal expression of the Cruziana ichnofacies. The delta-influenced proximal lower shoreface is characterized by interbedded tempestites, bioturbated fair-weather deposits and thin, unburrowed claystone beds with synaeresis cracks. The trace fossil assemblage comprises a diverse mixture of robust, complex detritus- and deposit-feeding structures, abundant fugichnia and the burrows of opportunistic suspension feeders, and

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Fig. 10. (a) Fades 4, bioturbated (BI5) silty sandstone erosionally truncated by a transgressive surface of erosion (TSE) and overlain by trough cross-bedded, medium-grained sandstones of Facies 6. The cross-bedded sandstone shows a BI of 2, and contains moderate numbers of Diplocraterion habichi (Dh), reflecting the Skolithos ichnofacies. Fig. 1, Mill Point, (b) Moderately bioturbated (BI 3-4) sandstone (Facies 6). The unit contains abundant, elongate Diplocraterion habichi (Dh) and reflects the Skolithos ichnofacies. Fig. 15, 18.4-19m.

following rapid sand emplacement, with higher proportions of fugichnia, lesser numbers of trace fossils, but otherwise ichnological suites comparable to the muddy sandstone beds. In contrast, the claystone interbeds show highly reduced bioturbation intensities and restricted suites of the Cruziana ichnofacies, probably reflecting stressful environmental conditions. Facies 5, overall containing a diverse Cruziana ichnofacies, is consistent with sedimentation within the lower shoreface. The unburrowed claystone interbeds are interpreted as poststorm mud drapes, and are believed to be associated with heightened precipitation, increased surface runoff at the coast, and rapid fluvial discharge through distributaries from nearby delta lobes (as described in Facies 3). Such delta lobes presumably lie along depositional strike and updrift of the facies. Gravelly symmetrical-rippled beds are believed to record periods of minimal sediment supply, allowing winnowing of sediment by waves and concentration of the coarse fraction. Facies 5 is interpreted as the product of sediment accumulation in an open marine proximal lower shoreface

environment downdrift of a contemporaneous delta complex.

Facies 6: Bioturbated, laterally variable sandstone facies Sedimentology This facies is composed of sandstone beds, 40100cm thick, with minor granule-, pebble- and shell-rich horizons. Most beds have erosional basal contacts (Fig. lOa). Facies 6 also shows considerable lateral variability at the outcrop level. Individual sandstone beds are, in some cases, intensely bioturbated, whereas others show HCS and trough cross-bedding (Fig. lOa). Cross-sets show sigmoidal foresets with mudstone partings on foresets. Both small-scale and large, gravelly, symmetrical wave ripples (as for Facies 5) are also common. Some coarsergrained sandstone beds comprise a single crossset, whereas others preserve co-sets of sigmoidal cross-beds. This facies differs from Facies 4 and 5 in that it displays a greater range of grain sizes, a broader range of physical sedimentary

represents a slightly more diverse expression of the proximal Cruziana ichnofacies. The deltaic influences on the depositional environment force the trace fossil assemblage to remain Cruziana in expression rather than shifting to a distal expression of the Skolithos ichnofacies, as would be expected in a non-delta-influenced proximal lower shoreface environment. Rosselia socialis (Rs), R. rotatus (Rr), Rosselia (type A, Rm), Phycosiphon (P), Planolites (PI), Diplocraterion habichi (Dh), D. parallelum (Dp), Rhizocorallium irregulare (Rh), Teichichnus (T), Macaronichnus isp. (M), M. segregatis (Ms), Palaeophycus tubularis (Pt), P. heberti (Ph), Taenidium (type A, Ts), sandy, robust ?Zoophycos (Z), Skolithos (S), Psammichnites (Ps), Chondrites (Ch), Conichnus (C), Lingulichnus (L), Taenidium (type B, Ta), Cylindrichnus (Cy), fugichnia (fu), Lam-Scram (LS), synaeresis cracks (sy), truncated Rosselia mud balls (tR). (c) Plan view of large Diplocraterion parallelum (D). (d) Vertical section through a large Diplocraterion parallelum (D). (e) Vertical section through a sand-filled Conichnus (C). (f) Plan view of Psammichnites (Ps). (g) Vertical section through Diplocraterion habichi (D). (h) Plan view of Lingulichnus (Li). Examples c-h are from Mill Point.

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structures, and greater variety in the intensity and character of bioturbation. Palaeocurrent data from cross-beds indicate a predominantly west to northwest sediment dispersal direction. Shell material occurs as dispersed fragments and disarticulated valves, or concentrated pebbly lags largely composed of robust articulated Eurydesma hobartense. Other taxa present include shallowly burrowing bivalves such as Megadesmus, Pyramus, Schizodus and Stutchburia, vagrant epifaunal forms including Aviculopecten and Peruvispira, the bellerophont Warthia, the spiriferid brachiopod Ingelarella and a biplicate species of the terebratuloid Gilledia (Runnegar 1979). Carbonaceous detritus, including allochthonous logs, and large (up to 2m diameter), ovoid concretions are locally common. Ichnology This facies is characterized by highly variable bioturbation intensities, ranging from BI2-5. Cross-bedded sandstone units tend to be less thoroughly bioturbated (BI2-3), with sporadically distributed burrows. The trace fossil suite consists of Diplocraterion parallelum, D. habichi (Fig. lOa, b), Rosselia socialis, Macaronichnus isp. and Skolithos. The pebbly, shelly lags and pebbly sandstone beds range from BI3—5, and typically BI5. The trace fossil assemblage is dominated by Diplocraterion habichi. Common but subordinate forms include Diplocraterion parallelum, Phycosiphon, Rosselia socialis, Planolites, Teichichnus and sandy, robust IZoophycos. Some sandstone beds are intensely bioturbated (BI4-5), with a diverse trace fossil assemblage dominated by Diplocraterion habichi, D. parallelum and Rhizocorallium irregulare (type A). Common but subordinate elements include fugichnia, Phycosiphon, Rosselia socialis, R. rotatus (with or without Teichichnus-like expressions of the dwelling tube), Teichichnus, Planolites, Palaeophycus tubularis, Macaronichnus isp. and Skolithos. Most occurrences of this facies directly overlie erosional discontinuities, associated with firmground trace fossil assemblages. These assemblages comprise vertical, unlined, and passively infilled domichnia that penetrate into the underlying facies and cross-cut the underlying trace fossil assemblage. Interpretation The presence of marine invertebrates again suggests an open marine environment of deposition for Facies 6, but the range of sedimentary structures and lithologies points towards deposition in

a variety of water depths. The well-preserved cross-bedding and other structures suggest shallow, regularly agitated water above fair-weather wavebase for the most part. The overall trace fossil assemblage is dominated by burrows of opportunistic suspension-feeding organisms, with subordinate detritus- and deposit feeders, and is interpreted to reflect a distal expression of the Skolithos ichnofacies. The firmground assemblages subtending from underlying erosion surfaces are characteristic of the Glossifungites ichnofacies, and indicate infaunal colonization of firm but unlithified substrates during periods of depositional hiatus. Eurydesma hobar tense is interpreted as an opportunistic species that flourished on silt-free, current-swept, sublittoral substrates (Runnegar 1979). Units that directly overlie Facies 6 (typically the fine-grained marine Facies 1, 2 and 3) tend to reflect deposition in significantly deeper (more distal) depositional environments than those that underlie it. This suggests that the erosion surfaces beneath Facies 6 units represent transgressive surfaces of erosion (TSE). This facies is therefore interpreted to represent transgressive sand sheets that underwent winnowing and concomitant concentration of coarse-grained detritus and shelly debris during periods of rising relative sea-level. The composite nature and lateral variability of this facies suggests that it reflects deposition in various well-oxygenated, sediment-starved, shallow marine settings. Predominantly westward sediment dispersal direction is also consistent with deposition under transgressive conditions. Estuarine facies

Facies 7: Channelized, heterolithic sandstone-mudstone Sedimentology This facies comprises interlaminated and thinly interbedded siltstone and fine-grained sandstone beds, confined to channelized bodies up to 6m thick and typically 100-250 m wide, though composite bodies (Facies 7/9: see below) may reach several hundred metres in apparent width. These bodies incise through all other facies, including other Facies 7 units. Successions pinch out laterally into thin (a few cm), laterally persistent beds of siltstone with fine-grained sandstone laminae (which are representatives of Facies 8). Channel bases are typically marked by thin, laterally discontinuous but persistent lags of well-rounded granules and pebbles, with common coalified log casts (some petrified) and

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other carbonaceous plant debris. Lags pass upward abruptly into the heterolithic facies noted above, which typically show little systematic upward change in lithology. The basal one metre or so of the channel bodies are locally composed of trough cross-bedded, medium- to very coarse-grained sandstone. Palaeocurrent measurements display a bipolar distribution of bedform migration directions. The heterolithic facies typically display 50:50 sand:silt ratios, with individual beds ranging up to 5cm thick. The facies shows linsen, lenticular and wavy bedding (Fig. 11 a, c). Sandstone beds contain a spectrum of interlamination structures ranging from linsen (pinstripe), through lenticular and wavy bedding to flaser bedding, Ripple crosslamination is predominantly unidirectional with some evidence of wave modification (symmetrical sand drapes over unidirectional crosslamination sets. Furthermore, the migration directions of current ripples define a markedly bipolar distribution in most outcrops. Some ripple sets are overlain by a continuous mud drape (e.g. Fig. 11 a) and rhythmically interlaminated sand-mud couplets are also common (Fig. lib). Some intervals show convolute bedding and other soft-sediment deformation structures, including synaeresis cracks (Fig. 11 a). Another characteristic feature of this facies is inclined heterolithic stratification (IHS). At the southern end of Point Upright, a channel with IHS also shows flat lying, discordant stratification (intrasets) between throughgoing inclined bedding surfaces. Palaeocurrent data indicate broadly westward and eastward sediment dispersal.

least moderately sinuous. The predominance of linsen, lenticular and wavy bedding indicates that subequal proportions of sand and mud were transported through the channels by unidirectional flows of modest strength, and the resulting bedforms were later modified by lowenergy waves in some cases. The bipolar distribution of ripple migration directions, and the presence of mud drapes and rhythmic sandmud couplets, confirm that tidal currents were active in the channels. The low-diversity, lowabundance trace fossil assemblage is extremely impoverished and represents a very restricted expression of the mixed Skolithos-Cruziana ichnofacies that is characteristic of inshore coastal environments of deposition. These features contrast strongly with the characteristics of the other heterolithic facies interpreted to have formed in offshore and offshore transition environments. Bipolar current flow, suggesting reversing tidal flows in otherwise low-energy settings, is characteristic of inshore settings, and uncommon in the offshore; tidal flow in distal settings is typically rotary rather than reversing. The impoverished ichnological suites are typical of such estuarine settings, which are subjected to highly variable and generally reduced salinities. The abundant and persistent synaeresis cracks also support a brackish-water interpretation. Facies 7 is therefore interpreted to reflect active fill of laterally migrating, estuarine channels.

Ichnology This facies is characterized by very low bioturbation intensities, ranging from BIO-1. Trace fossils are sporadically distributed and occur in very low numbers. Diversity of ichnogenera is extremely low. Sandier expressions of the heterolithic facies are largely devoid of bioturbation. There are no ichnological differences between intervals that clearly form inclined heterolithic stratification (IHS) and those that do not. The assemblage consists of Planolites (Fig. lib, c), rare fugichnia and rare Skolithos and is extremely impoverished.

Sedimentology This facies resembles Facies 7 except that it occurs as tabular units, rather than being confined to channels, it lacks IHS, it contains less plant debris, and it lacks both the large log casts and the coarse lags seen in Facies 7. Many of the ripple-scale structures show evidence of wave modification in the form of symmetrical drapes and bidirectional crosslamination superimposed on unidirectional sets. Micro-HCS is common, suggesting the increased incidence of combined flow processes. Palaeocurrents are generally bimodal and broadly bipolar, but this reflects wave-modified current ripple cross-lamination as well as a bipolar distribution of current ripple directions. Mud drapes and rhythmic sand-mud couplets are also common (Fig. lid). Synaeresis cracks and softsediment deformation structures are generally less common than in Facies 7 but are locally common (Fig. lie).

Interpretation Facies 7 is interpreted to reflect fine-grained deposition within estuarine channels. IHS is interpreted as lateral accretion surfaces associated with tidally modified channel flow (Howard et al 1975; Thomas et al 1987; Shanley et al. 1992), indicating that the channels were at

Facies 8: Sheet-like, heterolithic sandstone— mudstone

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Fig. 11. (a) Fades 7. Non-burrowed (BIO) wavy bedding with oppositely oriented current ripples, and thick mudstone drapes. Synaeresis cracks (sy) are locally developed within some mudstone interbeds. Fig. 15, 20.2-21 m. (b) Facies 7. Very weakly bioturbated (BI1) wavy bedded sandstone and mudstone. Unit shows undulatory parallel drapes of silt and clay as well as oppositely oriented ripples. Diminutive Planolites (PI) is the only visible trace fossil. Synaeresis cracks occur locally. Fig. 15, 20.2-21 m. (c) Facies 7. Sandstonedominated wavy bedding, showing current ripple lamination, some combined flow ripple lamination and parallel-laminated drapes. Bioturbation intensities reach BI 1-2, with some evidence of soft-sediment deformation in the lower part of the photo. The assemblage consists of Planolites (PI) and exceedingly rare, diminutive Skolithos. Synaeresis cracks are common. Fig. 15. 22.2-25m. (d) Facies 8. A flaser-bedded interval of reactivated current ripples and combined flow ripples, with some evidence of aggradation. The facies shows low bioturbation intensities (BI 1), manifest by isolated, diminutive Planolites (PI). Small synaeresis cracks (sy) are locally developed. Fig. 14, 5.5-6.6m. (e) Facies 8. Lenticular- and linsen-bedded sandstone and mudstone. The unit shows very low bioturbation intensities (BI 1), consisting of diminutive Planolites (PI). Synaeresis cracks (sy) are common. Fig.l, north Mill Point, (f) Facies 8. Mudstone-dominated interval with siltstone and sandstone ripples and rare synaeresis cracks (sy). The unit shows moderate bioturbation intensities (BI 3), consisting of Planolites (PI), Teichichnus (T) and Cylindrichnus (Cy), and reflects a low-diversity expression of the Cruziana ichnofacies. Fig. 1, south Mill Point.

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Fig. 12. Fades 8. (a) Sandstone dominated, wavy to flaser bedding abruptly overlying Fades 1 mudstones, marking the position of a sequence boundary (SB). The overlying heterolithic unit shows remnant wave, combined flow and rarer current ripple lamination. This interval is moderately to abundantly bioturbated (BI3-4), and displays a suite comprising Palaeophycus (Pa), Planolites (P), Rosselia (Ro), Teichichnus (Te) and unnamed equilibrium-adjustment burrows (e-a) reflecting a low-diversity expression of the Cruziana ichnofacies. Note that some of the equilibrium-adjustment burrows subtend across the sequence boundary, reflecting a palimpsest softground omission suite. Fig. 1, north Mill Point, (b) Bedding plane view showing well-developed equilibrium-adjustment burrows (e-a), Planolites (P) and Taenidium (type B, Ta). Bioturbation intensity is BI3. Fig. 15, 1-4m.

Ichnology Fades 8 displays highly variable bioturbation intensities ranging from BI 0 to BI 4. Trace fossils are sporadically distributed, and some ichnogenera are confined to local outcrop areas, though most are persistent within the facies. Suites are locally moderately diverse, containing Planolites (Fig. llf), Skolithos, fugichnia, Psammichnites, Teichichnus, Conichnus, unnamed equilibriumadjustment structures (Fig. 12a, b), 1 Rosselia socialis, possible Diplo crater ion parallelum, Taenidium (type B, Fig. 12 b), Siphonichnus, Cylindrichnus, and large, sandy, cone-shaped IZoophycos. Interpretation This facies is dominated by wave ripples, wavemodified current ripples and combined flow ripples (micro-HCS). There is no evidence to support periods of subaerial exposure, suggesting

that deposition occurred in a permanently subaqueous setting (i.e. not an intertidal flat). Reversing, tidal currents are indicated by less common, opposed current ripples, mud drapes and rhythmic sand-mud couplets. The trace fossil assemblage, although moderately diverse, is sporadically distributed and is characterized by structures that reflect simple deposit-feeding behaviours, with less intercalated suspension-feeding behaviours that are typical of opportunistic suites. The sporadic presence of IZoophycos at specific horizons within the facies suggests short-lived, episodic development of fully or near-fully marine conditions in an otherwise brackish-water setting. The suite corresponds to a highly stressed expression of the Cruziana ichnofacies, with moderately diverse expressions of the mixed SkolithosCruziana ichnofacies sporadically distributed throughout. The local abundance of synaeresis

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Fig. 13. (a) Soft-sediment deformed inclined heterolithic stratification (IHS) of Facies 7 (lower third of photo), capped by dark mudstone of Facies 9. The mudstones are truncated by a transgressive surface of erosion (TSE) and overlain by bioturbated (BI4) silty sandstone of Facies 5. The TSE is ichnologically demarcated by a firmground trace fossil assemblage that consists of unlined, robust, passively filled domichnia. The assemblage contains Diplocraterion habichi (Dh), Skolithos and Planolites and represents the Glossifungites ichnofacies. The photographed interval is approximately 50cm high. Fig. 1, north Point Upright, equivalent to Fig. 14, 14-14.5m. (b). Finely laminated dark silty mudstone (Facies 9). The facies is non-burrowed (BIO). Fig. 14, 11.5-14.4m. (c). Dark, non-burrowed (BIO) mudstone erosionally truncated by a transgressive surface of erosion (TSE) and overlain by bioturbated (BI4) Facies 6. The TSE is ichnologically demarcated by a Glossifungites ichnofacies. Diplocraterion habichi (Dh). Fig. 15, 4.3m. (d) Plan view of robust, unlined, sharply outlined Diplocraterion habichi (Dh) that represent the Glossifungites ichnofacies. The burrow infill is composed of coarse-grained sand from the overlying unit, and the structures stand out clearly from the mudstone host facies. Fig. 1, south Pebbly Beach. (E) Robust, sharply outlined, passively filled Arenicolites (Ar) subtending from a TSE into an underlying sandstone bed. This burrow represents colonization of a palimpsest softground during transgression. Fig. 1, north of Clear Point.

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cracks associated with the facies is also consistent with generally brackish-water conditions. The facies is interpreted to reflect deposition in protected, wave-influenced and periodically tidally influenced brackish-water basins that formed laterally adjacent to the estuarine channels of Facies 7.

Facies 9: Laminated mudstone Sedimentology This facies consists of laminated to blocky, medium to dark grey claystone and clayey siltstone, occurring either as channel fill or as tabular units (Fig. 13a, b). The mud-filled channel deposits are lateral equivalents to Facies 7 and form part of the channel complexes described above, with a fill geometry that is broadly form-concordant with the basal erosion surface. Tabular mudstone units typically overlie Facies 8. Thin (<3cm thick) sandstone and siltstone layers with linsen (pinstripe) lamination are persistent, and locally normally graded. Small pyrite concretions and coaly plant debris are locally present, but rare. At Point Upright the mudstone facies occurs within a channel with a well-developed basal clast layer. Ichnology Bioturbation is absent (BIO) except where the mudstone is truncated by a discontinuity surface (Fig. 13c). Interpretation The complete absence of bioturbation suggests anaerobic and/or reduced salinity conditions. Robust, passively filled domichnia that crosscut the mudstone are characteristic of the Glossifungites ichnofacies and indicate depositional hiatus and colonization of a firm but unlithified substrate (see Fig. 13d, e for other examples of omission suites and Glossifungites ichnofacies in the Pebbley Beach Formation). Facies 9 is interpreted to reflect deposition in abandoned estuarine channels and basins. Summary of depositional environment The middle and upper parts of the Pebbley Beach Formation are interpreted to record a range of coastal and nearshore marine depositional environments (Figs 14, 15). Open marine facies (1-5) record lower offshore to lower shoreface water depths, with shallower shoreface and shoreline facies largely absent (see Sequence stratigraphy below). Coastal facies (7-9) record sediment

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accumulation in estuarine channel and basin environments, in which tidal currents played an important role and waves were generally subdued. This implies a highly indented coastal planform, with perhaps large funnel-shaped estuaries crossed by sinuous channels that were flanked by (or drained into) shallow-water basins. The lack of high-energy wave structures in the estuarine facies suggests that estuaries may have been barred, and this notion is also favoured by the presence of Facies 6, recording transgressive complexes that may have originated as estuary-mouth barriers that migrated inboard during transgressions (cf. Dalrymple et al. 1992). A summary of the ichnological differences between the estuarine and offshore facies is illustrated in Figure 16a-c. Similar arrays of facies to those recorded here, and comparable interpreted depositional settings, are presented by Shanmugam et al. (2000), Beets et al. (2003) and Takano & Waseda (2003). Sequence stratigraphy of the Pebbley Beach Formation The vertical stacking patterns of facies in the Pebbley Beach Formation pose some considerable challenges to sequence stratigraphic interpretation (Fig. 16d, e, Fig. 17). Nevertheless, a series of unconformity-bounded, cyclical stratal packages can be recognized from the vertical arrangement of facies. In the part of the Pebbley Beach Formation under consideration, these cycles are < 10 m thick, and thus could represent high-frequency (fourth- to sixth-order) parasequences (Van Wagoner et al. 1988; see Naish & Kamp 1997), intermediate-scale packages (e.g. Swift et al. 2003), or true sequences formed in a low-accommodation setting (e.g. Kidwell 1997; Fielding et al 2000). Given the long period apparently recorded by the Pebbley Beach Formation (c. 14 Ma, according to Briggs 1998), it seems likely that the cycles under consideration are true sequences (thirdorder or lower). Regardless of the differing timeframes over which these cycles may have formed, many of the examples cited above show a condensed character, with little preservation of lowstand systems tracts and considerable erosional truncation of the highstand systems tract. This pattern is also evident in the Pebbley Beach Formation, where most sequences show evidence of significant erosion during the falling limb of relative sealevel cycles. The greatest degree of erosion is associated with the incision of the estuarine

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Fig. 14. Graphic log of the Pebbley Beach Formation at Point Upright.

channels. The amount of section removed varies locally, in some cases an entire sequence apparently removed and the estuarine channel fill juxtaposed above facies from an earlier depositional cycle. This juxtaposition of fine-grained coastal deposits onto lithologically similar

facies from previous cycles adds to the challenge of developing a realistic and useful sequence stratigraphic model. The integration of ichnofacies analysis and sedimentology has provided a powerful tool for the reliable discrimination between lithologically

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Fig. 15. Graphic log of the Pebbley Beach Formation at Clear Point.

similar but nonetheless disparate environments, Ichnology in particular has proven to be invaluable in the discrimination of fully marine and brackish estuarine deposits. In addition, the recognition of substrate-controlled trace fossil

suites, and in particular the Glossifungites ichnofacies, has facilitated the delineation and genetic interpretation of key surfaces, notably sequence boundaries and transgressive surfaces (Fig. 18a-e). Surfaces recognized in this study have

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Fig. 16. Summary diagram of the ichnological signature of brackish-water versus open marine deposits in the Pebbley Beach Formation showing the differences in bioturbation intensity, uniformity versus sporadic distribution of bioturbation and trace fossil assemblages. Block diagrams show idealized representation of the characteristic trace fossil assemblage in (a) fine-grained fully marine and (b) estuarine deposits, (c) Trace fossil assemblage key and indication of typical bioturbation intensity and uniform versus sporadic distribution. See Figure 4 for an explanation of bioturbation intensity (BI). (d) Vertical view through heterolithic interval illustrating the minimal differences in lithology between the offshore facies and the estuarine channel fill, (e) Close-up view of the juxtaposition of heterolithic estuarine deposits onto heterolithic offshore units. This surface is very subtle and in some instances marked by an omission suite of Diplocraterion habichi (D). Fig. 15, 18-23 m.

Fig. 17. Photomosaic of IHS estuarine channel fill of Fades 7 (IHS) truncating underlying delta-influenced lower shoreface sandstone (LSF) and offshore transition siltstones and sandstones of Facies 3 (OST). The base of the channel fill is interpreted as a transgressively modified sequence boundary (FS/SB). The estuarine channel is capped by dark mudstone of Facies 9. The estuarine complex is truncated by a transgressive surface of erosion (TSE), overlain by heterolithic lower shoreface sandstones and siltstones (LSF), which is truncated by another TSE and overlain by lower offshore deposits (LOS). Approximately 150 m long and 22 m high.

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Fig. 18. Discontinuity surfaces, (a) Thoroughly bioturbated (BI5) muddy siltstone (Facies 1, lo), overlain by non-bioturbated, coarse-grained, channel base of Facies 7 (ec). The boundary between the two units is ichnologically demarcated by Skolithos (Sk) of the Glossifungites ichnofacies, and represents an amalgamated sequence boundary/flooding surface (i.e. FS/SB). The offshore siltstone contains a distal expression of the Cruziana ichnofacies, dominated by diminutive Rosselia (Ro), Phycosiphon, diminutive Zoophycos (Z), Helminthopsis, Teichichnus (Te) and Planolites (P). Fig. 15, 21.1-21.85m. (b) Thoroughly bioturbated (BI5), muddy siltstone (Facies 1, lo), overlain by moderately bioturbated, wavy to lenticular bedded sandstone and mudstone (eb, Facies 8). The boundary between the units contains grit-filled Conichnus (C) interpreted to constitute part of the Glossifungites ichnofacies, and represents a FS/SB. Fig. 1, north Mill Point, (c) Thoroughly bioturbated (BI 5), muddy siltstone (Facies 1, lo), overlain by moderately bioturbated, wavy to lenticular bedded sandstone and mudstone (eb, Facies 8) with equilibrium adjustment structures (ea) protruding across the sequence boundary. Fig. 1, north Mill Point, (d) Lenticular to wavy bedded sandstone and silty mudstone (Facies 7, ec), overlain by thoroughly bioturbated (BI 5) muddy siltstone (Facies 1, the unit seen in the basal half of photo A, lo). The boundary between the two units is marked by a thoroughly bioturbated (BI 5) pebbly sandstone horizon that hosts a palimpsest suite of Diplocraterion habichi (D) and is

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been physically traced over the extent of cliff outcrops and can be shown to be continuous over the distance from Point Upright to Clear Point (c. 5km straight line distance, considerably further around the coastline; Fig. 1). Over the vertical interval considered, eight sequences can be recognized. Of these, two (Sequences 2 and 6) are not readily divisible into systems tracts, but rather seem to record a stack of coarseningupward parasequences. The other sequences, however, display the highly condensed and truncated architecture described above. Sequences 4 and 5 are highly complex, recording multiple generations of channel incision at Mill Point (Fig. 18f), and may indeed each represent more than one sequence. Furthermore, these two sequences become amalgamated as they are traced southward towards and along Point Upright (Fig. 19).

Sequence boundaries Erosion surfaces at the base of the estuarine facies are interpreted to be sequence boundaries. These discontinuities represent a significant basinward shift of facies, and they are generated during lowstand in relative sea-level. In the Pebbley Beach Formation, heterolithic estuarine channel and basin deposits commonly directly overlie fine-grained or heterolithic offshore and offshore transition deposits. The boundary between the two facies is locally subtle, involving no significant change in lithology and only modest changes in physical sedimentary structures (Fig. 16d). Nevertheless, profound changes in bioturbation intensity, sporadic versus uniform of bioturbation, and details of the trace fossil assemblages themselves, all serve to indicate significant changes in depositional environment. Estuarine channel bases locally contain coarse-grained, trough cross-bedded sandstone fill. Glossifungites ichnofacies assemblages and palimpsest softground suites consisting of Diplocraterion habichi, Conichnus and subordinate

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Skolithos also occur locally (Fig. 18a, b). The sandstone is overlain by heterolithic sandstone and siltstone. At other localities the sandstone is absent, and the erosive channel bases are directly overlain by either heterolithic fill or dark grey siltstone (Facies 9). The heterolithic deposits probably represent backfilling of the estuarine channels during ensuing sea-level rise. The presence of firmground suites of trace fossils along the sequence boundaries adds strength to the argument for channel filling during rising sea-level (transgressive systems tract) as, although subaerial exposure and/or erosion during lowstand may generate widespread dewatered or firm substrates, such surfaces are unlikely to have become colonized unless they were subsequently exposed to marine or marginal marine conditions (Pemberton & MacEachern 1995). Sequence boundaries in the Pebbley Beach Formation are therefore amalgamated with marine flooding surfaces (i.e. FS/SB).

Transgressive surfaces Discontinuity surfaces across which a significant deepening in depositional environment can be demonstrated are abundant in the Pebbley Beach Formation, and are interpreted as transgressive surfaces. Transgressive surfaces occur either as largely non-erosive, low-energy marine flooding surfaces (FS) or as low-relief, high-energy transgressive surfaces of erosion (TSE). Flooding surfaces in the Pebbley Beach Formation occur as sharp contacts across which there is evidence of an increase in water depth. These surfaces are mantled locally with dispersed granules and shelly material. The surfaces also commonly host palimpsest softground suites of Diplocraterion habichi, which protrude down into underlying facies and cross-cut the original resident trace fossil assemblage. The degree of biogenic reworking of flooding surfaces varies locally. Where muddy siltstone

interpreted as a transgressive surface of erosion (TSE). Fig. 15, 20.8-21.3m. Lens cap is 5cm across, (e) Transgressive surface of erosion veneered with shelly pebbly lag. This surface has abundant Diplocraterion habichi (D) of the Glossifungites ichnofacies descending into the underlying, thoroughly bioturbated (BI5) Facies 3. Fig. 15, l l . l m . (f). Outcrop view of the complex nature of the stacking patterns in the Pebbley Beach Formation (sequences 3-6). Lenticular to wavy bedded Facies 8 (EB) are overlain (to the left of the photo) by HCS Facies 5 (LSF). The boundary between these two basal units is interpreted as a transgressive surface of erosion (TSE). These facies are truncated by lenticular to wavy bedded Facies 7 showing IHS and occupying a channel. The base of this channel is interpreted as an FS/SB that amalgamates to the right with the underlying TSE. The channel fill is truncated by a TSE that is overlain by burrowed silty sandstone (Facies 4). This is truncated by another channel to the left that wedges out along strike to the north (right) and is truncated by burrowed silty sandstone (Facies 4). See Figure 19, Mill Point south end.

Fig. 19. Proposed sequence stratigraphic framework of the Pebbley Beach Formation showing lateral continuity of facies.

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fades (lower offshore deposits) overlie intensely bioturbated sandy siltstone fades (upper offshore deposits), the FS is generally not visibly disturbed by the diminutive surface-grazing trace-makers that are characteristic of the lower offshore environment. However, where intensely burrowed upper offshore sandy siltstones overlie sandy lower shoreface deposits, the FS is generally more gradational in expression, largely as a result of the complete reworking of the contact by the comparatively more robust deposit feeders that are abundant in the upper offshore. Transgressive surfaces of erosion in the Pebbley Beach Formation occur as low-relief, extensive discontinuity surfaces that show evidence of excavation by wave and current processes, associated with erosional shoreface retreat during transgression ('ravinement': Swift 1975; Arnott 1995). These surfaces are generally veneered by a pebbly lag locally built into large (height 3-5 cm, wavelength 15-20 cm), symmetrical ripples. The lags can also contain abundant shell material (Fig. 18e). Carbonaceous detritus including allochthonous logs is also locally common. Most TSE in the Pebbley Beach Formation host passively filled, palimpsest suites of trace fossils that subtend from the discontinuity surface and cross-cut the underlying trace fossil suite. Where the underlying units consist of finer-grained mudstone and siltstone, the palimpsest suites reflect the Glossifungites ichnofacies. In contrast, where the underlying stratum is sandstone, a softground palimpsest ichnological suite is more typically developed. Suites of the Glossifungites ichnofacies in the Pebbley Beach Formation consist of vertical to subvertical domichnia, produced by opportunistic, predominantly suspension-feeding organisms during hiatus (erosional and/or nondepositional). The burrows are generally sharpwalled, robust and unlined, reflecting the firm but unlithified nature of the substrate at the time of colonization and burrow excavation. The passive nature of the burrow fills indicate that the structures remained open after the inhabitants had vacated them, and they were subsequently filled with sediment from the successive depositional event. The most abundant element of the Glossifungites ichnofacies is densely spaced Diplocraterion habichi, with subordinate Skolithos and Conichnus. Palimpsest softground suites in the Pebbley Beach Formation are also dominated by Diplocraterion habichi. Diplocraterion parallelum and the lower, Teichichnus-like tubes of Rosselia are subordinate elements.

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Implications of the sequence stratigraphic interpretation As suggested above, the thin and condensed sequences recognized in the upper Pebbley Beach Formation indicate a continental margin environment where sediment accumulation in the nearshore realm was limited by accommodation. Sequence boundaries may be recognized with care from a combination of lithological and ichnological criteria, but apart from local coarse-grained ?alluvial sandstones preserved at the base of some channels, no lowstand systems tract deposits are evident. Transgressive systems tract facies are well preserved though typically thin, and highstand systems tracts are typically erosionally truncated by the overlying sequence boundary. This has led to a highly complex stratigraphic architecture with a dominance of heterolithic sandstone-mudstone facies, and no thick sandstone bodies preserved (Fig. 19). One corollary of this architecture that would not have been evident from previous analyses of the unit is that significant volumes of shoreface sand are missing from these sequences. We suggest that, during falling sea-level, much of the record of the previous coastal progradation (perhaps 10-20 m vertical interval of dominantly sand) must have been exported into greater offshore, to the east of the exposures. Thus, although the exposed succession is of little direct interest to hydrocarbon exploration owing to a lack of viable reservoir, it may provide vectors towards more substantial reservoir development down-palaeoslope to the east. Conclusions The Pebbley Beach Formation in the southern Sydney Basin contains a mixture of: fully marine, fine-grained, moderately to intensely bioturbated offshore deposits; moderately to intensely bioturbated sandy shoreface successions; weakly to moderately bioturbated, interlaminated sandy and silty delta-influenced shoreface successions; and weakly to moderately bioturbated, generally mostly fine-grained estuarine deposits. Marine deposits contain abundant and diverse trace fossil suites. Offshore deposits in the Pebbley Beach Formation contain highly diverse trace fossil assemblages that comprise a complex mixture of structures produced by depositfeeding and grazing/foraging behaviours and reflect the archetypal Cruziana ichnofacies.

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Vertical burrows that represent the dwellings of predominantly suspension-feeding organisms (e.g. elements of the Skolithos ichnofacies) correspond to the opportunistic colonization of distal tempestites. Intensely burrowed sandy shoreface intervals in the Pebbley Beach Formation contain trace fossil assemblages that contain diverse mixtures of robust, complex deposit- and detritus-feeding structures. The assemblages reflect proximal expressions of the Cruziana ichnofacies and are characteristic of welloxygenated, open marine settings at or immediately above fair-weather wavebase. Delta-influenced shoreface successions are strongly heterolithic and contain sporadically distributed, diverse trace fossil suites that record proximal expressions of the Cruziana ichnofacies. Unburrowed clay stone interbeds reflect heightened precipitation, increased surface runoff at the coast, and enhanced fluvial discharge through distributaries of nearby delta lobes that lie updrift and along depositional strike. In general, the fully marine successions in the Pebbley Beach Formation contain trace fossil suites that display moderate to intense bioturbation, are characterized by a high diversity of forms, contain significant numbers of structures reflecting specialized feeding/grazing behaviours, and display uniform ichnogenera distributions, all characteristics of equilibrium communities within fully marine environments. Estuarine deposits in the Pebbley Beach Formation contain extremely impoverished ichnological suites. In general, the facies show variable but significantly reduced degrees of bioturbation intensity, pronounced variability in the distribution of individual ichnogenera, and the dominance of a few, simple forms. The dominant elements represent simple feeding strategies of resilient trophic generalists. Estuarine active channel deposits are sparsely burrowed by trace fossil suites comprising less than three ichnogenera, and generally only one. The low-diversity, low-abundance mixed Skolithos-Cruziana ichnofacies assemblages and the abundance of synaeresis cracks in these units reflect fluctuating salinity levels, episodic deposition, and variability in substrate consistency. Estuarine basin deposits contain slightly more diverse trace fossil assemblages, and some suites are moderately diverse locally. The suites can be regarded as impoverished marine assemblages, and reflect a low-diversity expression of the mixed Skolithos-Cruziana ichnofacies. Estuarine abandonment deposits reflect hostile bottom-water conditions (e.g. anaerobic and/or reduced salinity) that precluded colonization.

The middle and upper parts of the Pebbley Beach Formation have been divided into a series of sequences, each reflecting a cycle of fall and rise in relative sea-level. The sequences are thin, condensed and top-truncated, but are nonetheless similar to some other published examples of continental margin successions accumulated under low-accommodation conditions. The basal sequence boundary for each sequence is demarcated by a channelized erosion surface, in some cases with a coarse sandstone recording the lowstand systems tract. The overlying estuarine channel and/or basin deposits, transgressive surface and fining-upward marine facies record the transgressive systems tract, and fine-grained to coarsening-upward nearshore marine facies are erosionally truncated at the top by the next sequence boundary. Key discontinuity surfaces in the Pebbley Beach Formation are generally ichnologically demarcated by palimpsest omission trace fossil suites. Firmground assemblages reflecting the Glossifungites ichnofacies occur at the base of estuarine channel and basin facies, suggesting that sequence boundaries in the Pebbley Beach Formation are amalgamated with marine flooding surfaces (i.e. FS/SB). Transgressive surfaces of erosion in the Pebbley Beach Formation are also demarcated by firmground suites consisting of vertical to sub vertical domichnia, produced by opportunistic, predominantly suspension-feeding organisms during periods of depositional hiatus (erosional and/or non-depositional). The re-evaluation of the middle and upper Pebbley Beach Formation presented herein provides a vivid example of the value of ichnology to stratigraphic analysis of nearshore marine to coastal facies successions. The resulting sequence stratigraphic model is consistent with all field data, and has predictive capabilities that may be useful in the search for hydrocarbons in this under-explored basin. Funding was supplied by a University of Queensland Research Fellowship to KLB and by a research grant awarded to KLB and CRF by Oil Company of Australia Ltd and SANTOS Ltd. JAM was funded by NSERC Operating Grant #184293. Professors Brian Jones and Tony Wright are gratefully acknowledged for insights into the Pebbley Beach Formation. R. Higgs and R. McNaughton are thanked for their reviews.

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Palaeoecology of the Bright Angel Shale in the eastern Grand Canyon, Arizona, USA, incorporating sedimentological, ichnological and palynological data CHRISTOPHER T. BALDWIN1, P. K. STROTHER2, J. H. BECK2 & EBEN ROSE3 1

Department of Geography & Geology, Sam Houston State University, Huntsville, Texas 77341, USA 2 Palaeobotanical Laboratory, Weston Observatory of Boston College, Department of Geology & Geophysics, 381 Concord Road, Weston, Massachusetts 02493, USA 3 Department of Geology & Geophysics, Yale University, New Haven, Connecticut 06520, USA Abstract: The Middle Cambrian Bright Angel Shale in the eastern Grand Canyon contains a depauperate normal marine fauna, but trace fossils and palynomorphs are abundant throughout the formation. Conventional interpretations place the depositional setting of this shale below wavebase as the distal component of a shelfal transgression, but the palynological signature in the mudstones of the Bright Angel Shale indicates a freshwater source to these muds. Examination of several sections in the vicinity of Proterozoic monadnocks and the integration of sedimentological, ichnological and palynological observations yield a more robust model for the palaeoecology of the Bright Angel Shale. Initial correspondence between organic matter content in mudstones and feeding type and intensity (as indicated by traces) is consistent with an estuarine setting for this deposit. The level of organic activity preserved in these sediments indicates that the carbon flux into shallow marine settings due to terrestrial runoff was substantial by middle Cambrian (Glossopleura biozone) time.

Although the Grand Canyon of Arizona is known as a geological wonder, modern research on the palaeoecology and depositional environments of some of its significant strata is lacking. The middle Cambrian Bright Angel Shale (BAS), situated between the underlying Tapeats Sandstone and the overlying marly Muav Limestone (these three together form the Tonto Group), has long been considered to be the distal 'fining-up/cleaning-up' clastic facies of the basal Cambrian Sauk transgression. The classic work of McKee & Resser (1945) certainly left the impression that the BAS contains abundant fossils of marine invertebrates, particularly trilobites, that lend support to this interpretation. But in the eastern Grand Canyon body fossils are generally quite rare, and with few exceptions are relegated to specific coarser sandy beds. Within the olive-green mudstones that dominate the BAS, body fossils are limited to rare Lingulella and rare impressions of small trilobites. Given this paucity of palaeontological evidence, contemporary palaeoenvironmental interpretation of the BAS, and hence of the Sauk cratonic sequence of this part of North America, has relied more on sedimentological and lithofacies criteria than on biological data or on combinations of these. It is this restricted interpretation based on physical sedimentological criteria that

continues to pervade most introductory texts that deal with the classical Cambrian transgression of Laurentia, resulting in overly simplistic transgressive models. The abundance of trace fossils in some sections of the BAS in the eastern Grand Canyon is impressive (Schuchert 1918). These sediments have preserved a snapshot of an active ecosystem that persisted throughout the entire depositional sequence of the BAS and on upwards into the Muav Limestone. In addition to its rich ichnofauna, we show below that the BAS also contains a rich, sporadically distributed palynoflora (see also Strother & Beck 2000). These organicwalled microfossils include spores of probable land plants and terrestrial algae in addition to cuticle fragments and probable egg cases of metazoans. Significantly, the lack of acanthomorphic acritarchs in the microflora points to minimal marine influence during the deposition of the primary muds and silts. The recovered microflora indicates a distinctive freshwater provenance to portions of the BAS sequence. The non-marine microfloral signature characteristic of pelitic lithologies appears in stark contrast to that of the 'marine' lithological and ichnological signatures derived from the intercalated coarser beds. This is contrary to the standard interpretation of clastic deposition

From: MC!LROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 213-236. 0305-8719/04/S15.00 © The Geological Society of London.

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applied to the section by McKee, wherein finer grains were supposed to be the more marine distal representation of proximal sands. It should be noted also that many of the trace fossils, although preserved in convex hyporelief at the base of sandstones, were in fact produced on or in muds such that the apparently paradoxical juxtaposition of marine and non-marine indicators is very intimately intertwined. It is this intimacy that allows us to refine the palaeoecological setting for the BAS by combining the preserved elements of trace fossils with their potential food. Thus we are herein concerned with some of the gastronomic drivers that stimulated trace-makers to behave in certain different and distinctive ways - and to leave preservable traces of their behaviour in a complex sequence of rocks. It is now possible to integrate ichnological and palynological data within McKee's original palaeogeographic setting to create a composite picture of deposition and ecology of the BAS in the eastern Grand Canyon. Trophic structure ascertained from ichnofabric analysis is made more robust when integrated with organic maceral analysis. In essence, these sediments have preserved the traces of feeding behaviour (and abundance of metazoan consumers) while simultaneously preserving an index of food type and abundance in the non-bioturbated portions of beds and the presumably indigestible residues (cf. Mcllroy & Logan 1999) in the bioturbated portions. This integrated approach to facies interpretation, employing lithofacies, ichnofacies and palynofacies criteria - giving as much weight as possible to the cryptic palaeoenvironmental signature of the mud rock components - produces a more convincing palaeoenvironmental interpretation. The result is an environmental model that intrinsically implies greater temporal and spatial complexity than that derived from any single facies criterion or from macroscopic and sanddominated ichno/litho-facies combinations. Prior studies on the palaeoecology of the Tonto Group The Tonto Group (Fig. 1) of the Grand Canyon region is composed of three intergradational formations - in ascending order, Tapeats Sandstone, Bright Angel Shale, Muav Limestone - that form a prominent, largely planar benchlike feature directly above the relatively narrow inner canyon that is cut into various Proterozoic rocks. Schuchert (1918) spent only five days studying the Tonto Group, but he made some critical

observations supporting the shallow marine nature of Tonto deposition. He emphasized the local character of basal Tapeats deposition, observing that much of the Tapeats sediment was derived locally from immediate underlying Proterozoic basement rocks. He concluded that the Tapeats was a deltaic deposit, 'on the land side rather than under the influence of the invading sea' (Schuchert 1918, p. 366). He argued that the greenish shales that begin in the upper Tapeats and persist through the BAS into the Muav were derived from lengthy rivers draining a long eroded 'Precambrian' peneplain. He interpreted the Bright Angel Shale to be 'very shallow, covering wide flats near to shore'. The overlying Muav was likewise interpreted to be a 'shallowwater, near-shore, marine deposit'. Schuchert considered the abundant glauconite as evidence of organic activity, and he commented that the extent of burrowing in the Muav was the greatest he had ever seen in any Palaeozoic section. Schuchert's observations overlapped with the work of Noble (1910, 1922), who did not speculate about the primary depositional settings, but did mention that boulders up to 30 ft in diameter were to be found in Tapeats sediments adjacent to Proterozoic basement highs that poke through the basal Cambrian sediments. Noble (1910) likened the palaeotopography to the contemporary Laurentian shield, citing the isolated monadnocks of resistant Baraboo Quartzite in Wisconsin as a specific analogue. In the Grand Canyon, resistant monadnocks of Neoproterozoic Shinumo Quartzite were gradually onlapped by the muds of the transgressing Sauk Sea. McKee & Resser (1945) expanded on these ideas, focusing on the Cheops section, which they referred to as 'Tapeats Bay'. They measured current directions in this embayment and determined that the predominant flow direction was nearly perpendicular to regional flow (continental runoff) (Fig. 2). Sedimentation immediately adjacent to the Proterozoic 'islands' can be quite spectacular; it is possible today to stand on debris slumps of Tapeats Sandstone age and view the cliffs from which the 500 Ma old breccias were derived. All previous authors have remarked on the fact that the sediments in the basal units adjacent to the monadnocks were formed in situ and were not significantly transported. But even though McKee & Resser (1945) recognized both the embayed and proximal nature of these sediments, they still envisioned only a marine origin for them. Reinterpretation by Wanless (1973) of shallow-water deposition for the overlying Muav Limestone was extended down-section into the

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Fig. 1. Stratigraphic context of the Bright Angel Shale (BAS) in the early Palaeozoic sequence of the Grand Canyon of Arizona, USA. The Tonto Group onlaps, oversteps and drapes various components of the Proterozoic substrate, which locally forms a distinctive monadnock unconformity surface.

'ironstone' beds of the BAS. He concluded that the BAS 'ironstones' are preserved laterites developed subaerially on deflation surfaces. Wanless identified dololaminite sequences in the upper Muav Limestone of the extreme western Grand Canyon that he argued were analogous to modern tidal flat deposits of Andros Island. This reinterpretation undermined a basic tenet of the McKee & Resser (1945) model, i.e. that the Muav Limestone, especially of the western Grand Canyon, represents the deepest and most distal offshore deposits of a relatively uncomplicated eastward transgression. To some extent it was a return to Schuchert's original interpretation of the Muav. Perhaps all of the Tonto Group was deposited in a much shallower-water setting than McKee & Resser (1945) had originally suggested. Hereford (1977) postulated fluvial processes within the underlying Tapeats Sandstone of central Arizona. Similarly, Fedo & Cooper (2001) described fluvial influences in distal braidplain settings for

deposition in the Lower Cambrian Zabriskie Quartzite and Wood Canyon Formation, which are lithological (though not allostratigraphic) equivalents of the Tapeats Sandstone to the west in the southern Great Basin. Depositional environments of Lower Palaeozoic sheet sandstones are notoriously difficult to interpret (Haddox & Dott 1990; Myrow et al 2003). However, the prevalence of well-preserved channels, overbank deposits and extensive lateral accretion surfaces within the 'unfossiliferous' Tapeats Sandstone brings fluvial braidplain to the forefront among possible palaeoenvironments, an interpretation that compares favourably with the high bedload fluvial 'combing' model proposed by Todd & Went (1991) for the Cambrian Aldernay Sandstone Formation of the Channel Islands and the Devonian Glashabeg Formation of Ireland. Middleton & Elliott (2003) have suggested that the Tapeats Sandstone channels represent tidal deposition in a subtidal setting, but the architectural style of the Tapeats

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Fig. 2. Location map showing interpreted palaeogeography for the eastern Grand Canyon. Shaded regions are outcropping Precambrian monadnocks; underlying diagram depicts the reconstructed topography plus the generalised sediment transport directions. Sample localities of measured sections shown (see text for location names).

Sandstone contrasts sharply with models of multi-storey submarine channels as described by Peakall et al (2000). Previous workers (Schuchert 1918; McKee & Resser 1945; Seilacher 1970; Elliott & Martin 1987) noted the abundance of trace fossils in the Tonto Group. All considered the sections to be fully marine. Elliott & Martin (1987) discussed exclusively shallow shelf, storm-influenced ichnocoenoses in the BAS. Such interpretations were heavily influenced by the prevailing notion that the Cruziana ichnofacies as originally proposed by Seilacher (1967) is essentially shelf al in character. To some extent there has been a positive feedback loop reinforcing deeper-water interpretations of the Cruziana ichnofacies, as many traces are found interbedded with mudstones (barren of body fossils) that themselves are cited as evidence of distal (mid-to-outer) shelf deposition. But, in the BAS, there is truly a mixture of trace fossil types. As described elsewhere (e.g. Droser 1991; Pemberton et al. 1992), vertical burrows typical of Skolithos ichnofacies occur in the same beds as horizontal traces related to the Cruziana ichnofacies, and in the BAS these mixtures of ichnofacies components are juxtaposed with sedimentological and palynological indicators that suggest water depths much less than middle to outer shelf.

Methods In the eastern Grand Canyon there is an approximately 60 km stretch of outcrop associated with clastic deposition adjacent to and abutting directly onto Proterozoic monadnocks that were subaerially exposed during mid Cambrian time (Fig. 2). We have examined sections in four drainage regions of this area: Thunder River (TC), Demaray Point, Cheops Pyramid (CP) and Red Canyon (RC). Two sections were examined in detail at Sumner Butte (SB), which are halfway between the topographic highs at Demaray Point and Cheops Pyramid, a total distance of 7 km. The first of these is a complete section through the BAS, a thickness of close to 100m (Figs 3, 5). In a lateral gully, a second partial section was measured near the base of the BAS. This subsection was informally referred to as Teichichnus Gully' (Fig. 4). Samples for palynology were taken from shale intervals in measured sections at Sumner Butte with an average sampling frequency of one every 7m. Elsewhere, spot samples were taken during logging. Organic residues were extracted by treatment in hydrofluoric acid and oxidized either in Schultze's reagent or in nitric acid. Further separation was achieved by flotation in zinc chloride (s.g, = 2), by sieving, or both. Trace fossil distributions and ichnofabric indices

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Fig. 3. A graphic log of the Sumner Bute section through the Bright Angel Shale, 'm' indicates various forms of red and iron-rich sandstone units including those previously referred to by McKee as 'Magenta Beds'. Note the increase in thickness of these 'm' beds up the section. Details of representative 'm' sequences in (A) are shown in: (B) 18-23m; (C) 50.5-53 m. Facies interpretations summarized in Table 1 are included on the detailed sequence logs.

(ii) (Droser & Bottjer 1986) were noted in the field as sections were logged. At Sumner Butte 34 trace fossil assemblages were recorded from outcrop and from localized float. Results

Lithofacies Lithofacies associations in the BAS were originally described by Noble (1922) and well characterized by McKee & Resser (1945). A summary (Table 1) is presented here, based on recent field descriptions from the eastern Grand

Canyon in combination with McKee's more general work. This classification draws attention to the conversion and overprinting of various heterolithic lithofacies (Facies 1) into variously configured facies (Facies 2) that represent different forms and amounts of biogenic overprinting of the original lithofacies. An additional pair of glauconitic and other iron-rich lithofacies (Facies 3 and 4) are distinct from the ambient, heterolithic lithofacies. Brief descriptions and labels for the four facies are summarized in Table 1. The sediments of the BAS are dominated by light olive grey (5Y 5/2) to dark greenish grey (5Gy 4/1) mudstones, siltstones and shales (Figs

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Fig. 4. A section from Teichichnus Gully' (Fig. 3) showing the complex intermixing of feeding strategies within the framework of the lithostratigraphy: (a) lithostratigraphy; (b) guild type (ichnofacies); (c) composite facies type; (d) qualitative trace fossil designations. Palynological samples are designated by the boxed T1-T4.

5b, 6a, b). These mudrocks in field exposure display varying degrees of platyness and lamination (Figs 6a, b, 7a), and typically include scattered glauconite grains throughout. Earlier authors noted the abundance of micaceous bedding

planes, especially within the lower transition zone beds. The mudstone successions are punctuated by sandstone beds of varying thickness, as noted in the facies and section descriptions (Figs 3,

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Fig. 5. Distinctive iron-rich sandstone beds ('Magenta Beds') forming regionally mappable horizons with the Bright Angel Shale, (a) Two cliff-forming indurated iron cemented red sandstones (Facies 4) separated by a poorly exposed interval of heterolithic Facies 1 and 2. The more prominent cliff in centre view is composed of a glauconitic sandstone that becomes progressively more indurated and red. It culminates in a vertically burrowed, wave worked surface, (b) The upper portion of the Sumner Butte section shows the repeated stacks of glauconitic and red iron-rich horizons. Elevations within the section (see Fig. 3) are shown on the outcrop.

4, 5). These sandstone beds are typically apparently well laminated (ii 1-2) (Figs 6b, 7a) and contain what appears to be muscovite, but upon closer field and petrographic inspection are found to be weathered glauconites. In these finer-grained sandstones, peloidal glauconites do not dominate the texture, but they do appear as a pervasive component. Peloidal glauconites can make up a substantial fraction of the coarser ironstones of Facies 3 and 4, and glauconite-rich beds in the middle of the section

at Sumner Butte impart a dark green colour to the section (Figs 5, 6). The distinctive liver-coloured ironstone horizons ('Magenta Beds' of McKee & Resser 1945) split and coalesce over several kilometres in the eastern Grand Canyon in regionally traceable composite beds (Fig. 5). Phosphatized and variously eroded inarticulate brachiopod, trilobite, echinoderm and hyolithid fragments are locally concentrated in irregular patches within and particularly on the tops of these poorly

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Table 1. Summary of fades designations Fades label

Observations/notes

Fades 1 (ii < 1) Fla Parallel-laminated shales/siltstones Fib Interlaminated planar shales and fine sandstones Flc Flaser and lenticular shales and sandstones Fid Discrete, thin sandstone stringers in heterolithic shales/siltstones/sandstones Fie Flat-bedded sandstones Flf Cross-bedded/ripple bedded sandstones

Primary depositional lithofacies

Biogenic overprint of lithofacies Facies 2 (ii > 1) F2a Tunnelled siltstone/sandstone [reworked Fla or Fib] F2b Discrete tunnelled sandstone stringers in tunnelled heterolithics [reworked Fid] F2c Tunnelled heterolithics [reworked Fib or Flc] F2d Tunnelled sandstones [reworked Fie or Flf] Facies 3 (ii 1-5) F3a Glauconitic heterolithics F3b Glauconitic cross-bedded sandstones F3c Glauconitic wave rippled sandstones F3d Glauconitic sandstones Facies 4 (ii 1-5) F4a Red sandstones F4b Red ironstones

Glauconitic May represent precursors to Facies 4 or incomplete development of estuarine bars

Red/iron = Magenta Beds Bar/estuarine sand build-ups

Further combinations to facies listed above combined as subscript descriptor (e.g. F3bBF): Iron-rich mud drape from suspension DT Red mudstone draped top Colonized omission surface BT Burrowed top Switch from current to oscillatory flows RT Wave-rippled top Burrows 'hanging' below foreset surfaces BF Burrowed foresets Hematite/glaucony Fe Iron-rich pellets ii = Ichnofabric Index (Droser & Bottjer 1986)

sorted arkosic ironstones. Magenta beds are typically medium to coarse quartz sandstones, cemented by haematite, but with varying amounts of peloidal glauconite of similar size and lesser amounts of lithic fragments, feldspar and shell fragments. They range in thickness from a centimetre or so up to perhaps 2m, but typically form beds 2-5 dm thick (Figs 5a, 6a). The distribution of Facies 4 (Red Ironstone facies) is shown for a relatively thick, perfectly exposed section of the BAS at Sumner Butte (Fig. 3a). The sequential stacking of the different combinations of all facies is shown in examples of magenta bed sequences from a number of sample sites in the eastern Grand Canyon (Figs 3, 4). The BAS is characterized by repeated coarsening-upward sequences, typically 1.5-5.Om thick but ranging up to l l m thick, which are typically marked in the field by ferruginous sandstone caps. Several of these cycles are visible in Fig. 6a. They begin with non- to low-bioturbated (ii 1-3, 0-2) green shales and mudstones inter-

laminated at a centimetre scale with fine-grained parallel-laminated or ripple-laminated mottled (ii 0-1) quartz sandstones (Facies 1). These grade upward into more heavily bioturbated (ii 2-5) siltstones (ii 2-4) (Facies 2) that are capped with coarse, dark-green or magenta-coloured, crossbedded ironstones (Facies 3 and 4) typically with scoured bases (Figs 5a, 6a) and with very variable ichnofabrics (ii 1-4 locally). At Sumner Butte, haematite staining tends to increase as one moves up-section into the Muav Limestone above. These units higher in the section (approximately 30-70 m) are ultimately erosively capped by decimetre-scale amalgamated and trough and tabular cross-bedded and herringbone crossstratified coarse-grained ironstones, or variously haematitic coarse-grained arkosic sandstones of Facies 4 (Figs 5, 6a). Underlying the distinctive magenta beds are dark green glauconitic interbedded sandstones, siltstones and shales (Figs 5, 6). Many glauconitic sandstones are bidirectionally cross-bedded with 180° bipolar set. At a number of different

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Fig. 6. Outcrop at 30m, Sumner Butte section: (a) sharp lower contact of red sandstone facies (Facies 4) with heterolithic Facies 1 and 2; (b) close-up of the heterolithic facies, demonstrating the intertwined nature of the Palaeophycus burrowing within what was once a muddy substrate, and displaying tunnelled sandstone stringers within sharply laminated heterolithic units of Facies la and Ib.

locations vertical U-shaped and straight burrows extend downwards from sharply defined foreset surfaces for a few centimetres, implying a distinctly episodic sedimentation. Rusty redbrown, iron-rich silt and clay 'biscuits' are present along with clear quartz granules, and

these frequently define diffuse foresets. At the top of these units that underlie the distinct reddened beds the cross-bedded sands give way to burrowed and destratified (in part ii 5-6) units of sand/shale lenticles (Fig. 6b), plus biogenie pads composed of various Cruziana-formmg

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Fig. 7. Intrastratal burrows (possibly 'burrowed Cruziantf or Palaeophycus) at 32 m in the Sumner Butte section, sample GC98-13: (a) cross-section showing the unburrowed centre of the bed; (b) lower bedding plane view; (c) lower bedding plane view in outcrop, showing the almost complete coverage of the active surface near its active margin.

subunits ('dig marks' of Baldwin 1976, p. 137) and various superimposed Arenicolites, Phycodes, Teichichnus and indeterminate burrow fills. Also present in this fades are rare but recurrent Teichichnus and lArenicolites that, in comparison with specimens from this and coeval sections, seem particularly large and cut down through multiple laminae and/or multiple sandstone beds. It should be noted that these larger forms are larger only in the sense of the extended tier depth, and that in all other morphological respects they are identical to the shallower forms. It therefore seems unlikely that the larger

forms represent a different set of burrowers or burrowing taking place under different physical conditions or at significantly different times. The large traces, which contrast so markedly with the uniform tier depth of smaller ('normal') forms (Fig. 8), appear to represent exploratory probings, whereas the normal forms exploited a 'discovered' persistent food source. Numerous examples of the tops of the distinctive 'Magenta Beds' are further distinguished by: dense packing of Skolithos and Arenicolites (ii locally 4) (Fig. 9a) that cross-cut multiple

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Fig. 8. Dense packing of Teichichnus and Phycodes typical of the underside of sandstone stringers within Facies 2b in the lower 2m of ''Teichichnus Gully' exposures (see Fig. 4a).

foresets and sometimes penetrate units of climbing ripple cross-lamination; individual species and superimposed multiple species of epichnial grooves; interference and wave ripples; thin sheet-like or isopachous dark red mud drapes that cover these ripples. Other distinctive green glauconitic heterolithic units occur above and below the arkoses, where they exhibit almost no penetrative bioturbation or contain small Phy codes/Teichichnus-like spreiten bearing walls that 'hang' from the base of some sharply defined sandstone beds (cf. Droser et al 2002a, 2002b) and are confined to the next adjacent underlying mudstone layer.

Ichnoguilds At a very coarse scale, trace fossils were grouped into three associations or ichnoguilds (sensu Bromley 1990, p. 211) based on deduced general feeding strategies: Sediment feeding, as composed of mostly horizontal furrowing and burrowing ichnogenera such as Cruziana, Palaeophycus, Phycodes, Rusophycus (including 'dig marks' and other components of obvious arthropodan origin) and Teichichnus (Figs 7, 8, lOd). Filter I suspension feeding, composed of vertical and 'IT burrows assignable to Arenicolites, Diplocraterion, Monocraterion and Skolithos (Figs 9, lib, d). Surface feeders are a mix of trail-formers, some of which produce positive relief casts on the upper surfaces of sandstone beds and

includes Palaeophycus and Planolites. This group is often found in association with suspension feeders (Figs lOc, 11 a, b, d). This guild contains evidence of shallow-tier burrowing and possibly surficial furrowing or sediment feeding right at the sediment water interface (Figs lib, d). The beds and layers demonstrating sedimentfeeding strategies are ubiquitous. They vary from individual lamina-like simple horizontal burrows (Fig. 6a, 7a, b) to galleries that may be either discrete and separate, or overlapping (Fig. 7c). Beds subjected to sediment feeding can be completely disrupted (ii 4-5) (F3 b/c in Fig. 5a). Sediment feeding is common in heterolithic units and also in sandstone beds in the range of 5-25 cm thick. Hypichnial traces are visually dominant in these sandstone field exposures, with sediment-feeding galleries marking the lower interface of most discrete sandstones (of any size) where a variety of tierlike layers extend down into the mudstone below (Figs 6, 9a). However, it should be noted that a significant number of burrows extend through the overlying sandstone beds, and that sediment browsing of the tops of the same beds may be equally prevalent. An example of this is illustrated in Figs 9a, b, which reveal a similar trace density at the top and bottom of the same slab. This situation somewhat contrasts with the Droser et al. (2002a) observations of earlier NeoproterozoicCambrian ichnofabrics, where 'unattached' forms predominate. Suspension feeders are recognized by the presence of vertical burrows. Skolithos communities

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Fig. 9. Burrow forms within iron-rich sandstone horizons (including fully developed Magenta Beds) in transition beds near the base of Cheops Pyramid: (a) completely tunnelled sand beds passing upwards into horizons of discrete Skolithos and various 'U' burrows on and within foreset surfaces; (b, c) various densities of packing of sprieten-bearing U burrows on the upper surfaces of iron cemented sandstones.

are common, particularly in the basal transition zone (Fig. 9). On exposed upper surfaces in thinner sandstone beds, it is not uncommon to see both Monocraterion and paired holes corresponding to Diplocraterion (Fig. lib, d). The basal section and parts of the underlying Tapeats Sandstone contain many different versions of the 'dumbbell-shaped' impressions that mark the various levels of exhumation of U-shaped burrows (Fig. 9b, c). It seems likely that much of the Tonto habitat prior to the

influx of Bright Angel muds was quite favourable to filter/suspension feeding. Filter feeding is exclusively associated with sand beds, and mostly with thick, frequently multi-storey units of sandstone that may or may not include varieties of cross-bedding (Fig. 9). Some thin units in which the primary depositional texture has been obliterated by vertical burrows (ii 5-6) occur infrequently throughout the section, indicating minor hiatuses (see Mcllroy 2004; Pemberton et al 2004).

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Fig. 10. Sharply defined biogenic marks (mostly trilobite scratches and dig marks) from both the upper and lower interfaces of sandstones from Facies 2: (a) hypichnial Dimorphichnus and Monomorphichnus wipes on the base of a thin fine sandstone; (b) epichnial grooves and paired raised welts on the upper surface of a fine sandstone; (c) hypichnial casts and fills on a wave-rippled sandstone; (d) hypichnial casts of Cruziana and other arthropod digging marks.

Surface traces are numerous throughout the BAS. They may be true furrows or the imprints of the lower surfaces of exhumed burrow fills (Figs lOb, 11 a, c), and they are often preserved as positive casts (e.g. Fig. lib, c). Beds such as those in Figure lib and d appear to retain evidence of two guild types, as they contain a spatial distribution of both burrow pairs in

addition to the surficial cover of Palaeophycus, and therefore may represent pre-depositional and post-depositional suites. Surface furrow traces (i.e. grooves excavated into the top surfaces of detail-retaining cohesive muds and sands) such as Monomorphichnus (Fig. lOa), Cruziana and Rusophycus (Baldwin 1977) show exceptional detail of preservation, with no sign of sand-mud

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Fig. 11. Upper Surface traces, (a) Wrinkle marks ('interference "tadpole" ripples' of McKee & Resser 1945), which include spatulate, tablespoon-sized depressions and a few trails. These sedimentary structures have been proposed as shallow water indicators and also to have been formed by adhesion from the binding properties of microbial cover (Hagadorn 1997). (b) Detail of the bedding surface in (d), showing the tops of paired vertical tubes and the casts of trails formed on rippled beds, Sumner Butte section 11 m. (c) Upper surface traces on thin sandstone bed, again demonstrating the ability of formerly wet sand to retain its shape after the subsequent influx of mud. (d) Upper bedding surface of a rippled sandstone bed at Sumner Butte section 11 m; outline demarcates enlargement in (b).

substrate intermixing (cf. Droser et al. 2002a, 2002b). Burrowing forms (e.g. Arenicolites, Planolites and Teichichnus) typically increase up section over several metres (Fig. 4b, c) with commensurate destruction of laminae and increasing presence of glauconitic peloids. The resultant texture is silty and highly bioturbated (ii 4—5), comparable to the 'green crumbly siltstone' facies of McKee & Resser (1945) (Fig. 5).

As with most field (versus core) descriptions or utilizations of trace fossils there is a strong tendency to consider the sandier facies; it is after all sands that tend to cast and ultimately preserve and expose trace fossils as physical, discrete entities, whereas the muds are poorly preserved. In the BAS, cast and filled structures predominate but - given the variety and intermixing of original lithotopes, particularly as

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represented by the heterolithic fades (Fades 2c, but also 2a and 2b, Table 1) - we are reasonably confident that we have observed a broad distribution of traces and feeding strategies in both sandstones and other finer-grained lithologies.

Palynofacies Palynofacies studies are typically limited to a determination of the 'marine' or 'non-marine' character of an assemblage. This is achieved by assessing the non-marine fraction in a typically mixed assemblage. In Devonian and younger strata the provenance of palynological elements can be quite obvious, particularly for land plants where spores and pollen grains fit into well-known plant classifications. This makes the characterization of palynofacies relatively clear. Prior to the Devonian, we are dependent upon the cryptospore record to provide the 'terrestrial' index that is used in palynofacies construction. In this way, terrestrial palynofacies have been recognized as far back as the lower Ordovician (Wellman et al 2003). With the BAS assemblages, the primary hurdle to a basic palynofacies assessment is that no recognizable marine elements are present in the sequences. Acanthomorphic acritarchs, which are characteristic of marine palynomorphs from this time (Servais et al. 1996), are entirely missing from the BAS, as are zonal taxa of any sort. This leads to an assertion that the assemblage is of non-marine or estuarine origin (Strother & Beck 2000). Such an interpretation is in consort with older conclusions about tidal features and the 'embayments' of the eastern Grand Canyon proposed by McKee & Resser (1945). The Middle Cambrian palynomorphs recovered from the BAS do not match any younger assemblages exactly, but in overall character they are similar to assemblages from known paralic sequences (e.g. the Tucsarora Formation in Pennsylvania: Strother & Traverse 1979; Johnson 1985). Although there is little doubt from the palynological perspective that the BAS assemblages are heavily influenced by estuarine and/or nonmarine elements, without a taxonomic basis from which to proceed it is not yet possible to generate a quantitative assessment of assemblage heterogeneity and hence palynofacies distribution. It is possible to broadly classify palynomorphs and palynodebris from the BAS, and such an attempt leads to sorting into 'fades' based both on their elemental composition and on their quality of preservation (degradation).

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Type A represents primary-sourced organic matter that is dominated by cryptospores and non-marine algal cell clusters. Metazoan fragments are rare or missing, but overall preservation is excellent. Type B contains a more mixed assemblage that includes thinner-walled cells and cell clusters with relatively fewer thicker-walled (cryptospore) forms. This assemblage contains noticeable fragments of metazoan origin (cuticles and structural elements), and may contain larger leiospheres that could be protoctist or metazoan derived. Preservation is mixed. Type C represents a degraded assemblage that contains significant metazoan remains dominating primary photosynthetic biota and algal debris. An additional distinctive assemblage was recovered immediately adjacent to Proterozoic highgrounds in Red Canyon drainage. These samples are characterized by excellent preservation of large tissue fragments and 'leiospheres' in addition to pervasive non-marine cryptospores and terrestrially derived algal cell clusters. In addition, mats of filaments, similar to the tissues that comprise the Silurian terrestrial plant Nematothallus, are found in the Red Canyon samples. However, as this assemblage is not characteristic of palynomorphs from Sumner Butte, where trace fossil data were collected, it will not be discussed further. Figure 12 illustrates the primary components of the most degraded palynofacies type C. Photosynthetic debris includes degraded clusters of cells (Fig. 12a-c, f, 1, m) and a few cryptosporelike specimens such as tetrads (Fig. 12k) and dyads (Fig. 12d, h). The preservation of this material is generally poor, and the recovery from this sample, T-l, was very low. A cuticular fragment (Fig. 12q) is the best preserved of any of the organic pieces recovered. Just higher in the section at Teichichnus Gully' is a slightly better preserved assemblage (sample T-2, Fig. 13), which can still be considered a type C palynofacies. Smooth well-preserved cuticle (Fig. 13t) is similar to that seen in the prior sample. However, this assemblage is more diverse in its cuticular remains, including some pseudo-cellular cuticles reminiscent of nonmarine debris found in younger rocks (Fig. 13g, q). Most of the spore-like cells and clusters in this group are physically damaged, presumably as a result of physical reworking of the sediment, although it is still possible to recognize both thinwalled, 'leiosphere-like' cells (Fig. 13a, b, p) and thicker-walled components (Fig. 13c, d). Again, as in sample T-l, the recovery in terms of absolute abundance of organic matter was poor.

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Fig. 12. Palynofacies Type C assemblage, sample T-1: (a) degraded cell cluster; (b) polyad cluster, genus V; (c) degraded cell cluster; (d) cryptospore dyad; (e) envelope-enclosed cluster; (f) broken cryptospore tetrads and polyad; (g) degraded dyad pair; (h) well-preserved minute dyad; (i) poorly preserved thin-walled dyad; (j) degraded tetrad; (k) degraded tetrad; (1) polyad; (m) small cluster of minute cells; (n) degraded and broken large 'leiosphere'; (o) degraded fragment; (p) degraded thick cuticle; (q) large thin cuticle fragment.

Fig. 13. Palynofacies type C assemblage, sample T-2: (a) thin-walled 'leiosphere'; (b) thin-walled folded 'leiosphere'; (c) large broken dyad; (d) large broken tetrad; (e) distorted dyad; (f) moderately well-preserved cell cluster; (g) degraded reticulate tissue fragment; (h) smaller distorted dyad; (i) envelope enclosed spore-like body; (j) broken enclosed spore-like body; (k) small enclosed monad; (1) damaged large enclosed monad; (m) cluster of polyads, genus V; (n) degraded cluster of polyads, genus V; (o) degraded cluster of thin-walled cells; (p) large 'leiosphere' with broken wall; (q) thick reticulate cuticle forming a pseudocellular pattern; (r) cuticle? fragment; (s) small tissue fragment with embedded tubular structures; (t) large folded fragment of smooth cuticle.

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Fig. 14. Palynofacies type B assemblage, sample GC97-23: (a) polyad cluster; (b) poly ad cluster; (c) irregular tetrad; (d) polyad cluster; (e) thin-walled dyads; (f) attached, thin-walled cluster; (g) planar cryptospore tetrad with contact thickenings; (h) irregular to planar tetrad with envelope; (i) loose clump of small dyads and monads; (j) loosely associated dyad and monad; (k) small cryptospores loosely associated; (1) attached dyad pair - note thickened contact regions and envelope; (m) pair of attached cryptospore dyads; (n) large irregular cluster; (o) amorphous cuticle fragment; (p) amorphous cuticle fragment; (q) amorphous cuticle fragment; (r) structural metazoan fragment; spine?

An assemblage of palynomorphs from heterolithic fades F2b was recovered at 26m in the Sumner Butte section (roughly equivalent to the shale seen in Fig. 6a). This assemblage is intermediate between the degraded type C and the pristine type A samples, and is designated a type B assemblage (Fig. 14). It contains the ubiquitous photosynthetically derived elements of spore-like tetrads and dyads (Fig. 14g-m), in addition to a particularly distinctive form, 'genus V (Fig. 14a-f). Unlike the more pristine type A palynofacies assemblage, however, bits of possible metazoan tissue are preserved (Fig.l4o-r). Therefore, significantly, in spite of the moderate burrowing present in this heterolithic sample, preservation is quite good, and the shales seem to contain remains of both

primary (i.e. photosynthetically derived) and secondary (metazoan) organisms. An assemblage from shales resting immediately on a sandstone surface colonized by trace fossils of presumed filter feeders in Teichichnus Gully' (T-4) is the model for a type A palynofacies. This sample (Fig. 15) is well preserved, without signs of physical damage, and contains no metazoan debris. The cryptospores include symmetric tetrads (Fig. 15a, b, j) and dyads (Fig. 15d, f, o, q). Some of these components can be seen in their more degraded forms in assemblage type A. For example, small dyads (Fig. 15o), clusters of small spherical cells (Fig. 15n) and irregular clusters (Fig. 15m) appear to have degraded equivalents in the type C assemblages. This seems reasonable given the

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Fig. 15. Palynofacies type A assemblage, sample T-4: (a) tetrahedral cryptospore tetrad; (b) tetrahedral cryptospore tetrad showing clearly the nature of the thickened contact rings that characterize these forms; (c) a more irregular form; (d) thin-walled cryptospore dyad; (e) large dyad; (f) small envelope-enclosed dyad; (g) compact cryptospore tetrad with thin envelope; (h) more or less planar tetrad; (i) large thin-walled attached cells; (j) well-formed spore/cryptospore tetrad; (k) linear spore cluster; (1) monad cluster; (m) poly ad; (n) cluster of smaller cells; (o) small dyad; (p) loosely attached cell pair; (q) envelope-enclosed dyad; (r) planar tetrad of envelope-enclosed cells; (s) thin-walled cells forming an open planar tetrad.

assumption that assemblage type A represents the signature of the primary photosynthetic input into this ecosystem. Palynomorph abundance is high in this assemblage - probably 103 times that seen in the type C assemblage.

Discussion Distal storm versus estuarine models of deposition Elliott & Martin (1987) proposed that the sandshale interbeds of the BAS represent shelf storm

deposits, and that open furrow traces (e.g. Rusophycus) were preserved by rapid suspension settling from waning post-event flows. But sandshale laminae may also occur in any number of settings, with a subtle variety of distinctive features. Stochastic or episodic sheet sands deposited during storm events should result in grading, extensive scouring, and possibly diffuse sand-to-mud transitions (B-C beds of the Bouma sequence) (Seilacher 1991; Seilacher & Aigner 1991; Walker 1992; Walker & Flint 1992). However, typical BAS sandstone interbeds have sharp upper and lower contacts (Fig. 6) with well-preserved traces on both surfaces.

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For example, detailed sole markings such as Monomorphichnus (Fig. lOa) may occur on the same thin sandstone interbeds whose tops are covered with pre- and post-depositional suites including Planolites and others (Fig. lOa, b). Some exposures of sand-shale interbeds show cyclic thickening and thinning of sandstone layers strongly suggestive of tidal bundles (Davis et al. 1998). Tidal signatures abound in the BAS, including herringbone cross-stratification showing time/tidal asymmetry (Klein 1970) within the highly scoured and sometimes amalgamated glauconitic sandstones and ironstones that are most likely the product of very high hydrodynamic energy in very shallow water. The coarsening-up to ironstone-cap facies successions of the Bright Angel Shale are comparable to those described by Kim & Lee (2000) from the Ordovician Dongjeom Formation of Korea, and may represent repeated submergence and emergence of metre-scale intertidal to sub tidal cycles. These, sometimes amalgamated, ironstones exhibit certain characteristics typical of condensed sections (Loutit et aL 1988), including a concentration of coarse grains, extensive haematite cement, and what appear to be fossil steinkerns and boxstones. The tops of these beds were reworked to some extent by both currents and waves. They are capped by what appears to be a phase of wave reworking, subsequent abandonment or still-stand linked to colonization by filter feeders and some sediment grazers, and subsequent draping with suspension fallout iron-rich muds. Top surfaces of magenta beds may therefore represent periods of maximum flooding or abandonment. In this context the parasequences that form the BAS may represent repeated sand build-ups that punctuated a muddy, tidally influenced, estuary - but an estuary that was not incised in the often used modern sense (cf. Dalrymple et al. 1994). Even though these sequences comprise a transgressive package that required some 100m of accommodation space, this was not a simple proximal-to-distal fill sequence. The shales of the Bright Angel are not substantially bathymetrically different from either the Tapeats Sandstone clastic 'carapace' below or the ccleaned-up' Muav Limestone above. This low-relief estuarine system was subjected to the migration of bar-like sand bodies, whose movement was driven by normal lateral sedimentation and migration of estuarine sand bars or from episodes during which pulses of coarse sediment were brought into or released into the estuary from more offshore, marine environments. The concentration of nearly every potentially marine invertebrate element into allochthonous

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sandy beds attests to this source. Release of the pulses of sand was perhaps storm controlled or storm influenced, but there is little evidence that storm mechanisms were the principal sedimentological drivers actually within the estuarine domain. Capping sandstones may have been initiated by preferential erosion of softground substrates (the green crumbly siltstone of McKee & Resser 1945; Facies 3 herein), but tides and currents rather than waves acted as the primary sediment-moving agent. Bar sands on occasion built up sufficiently that their tops perhaps became emergent for finite periods of time, but were subsequently abandoned, drowned and became moribund in the terminology of North Sea sand ridges (Stride et al. 1982). In combination with palynological and ichnological data, the basic sedimentology of the BAS does not lend strong support to a storminfluenced subtidal depositional setting as promoted by Middleton & Elliott (2003 and references therein). The non-marine organic signature of the mud rocks simply does not support the predicted uniformity or homogeneity of such an exclusively marine setting. The mud rocks in the BAS tell a very different input story, and an estuarine model better explains the whole package of partitioned and nonuniform facies and sequences. Deposition of the BAS in the eastern Grand Canyon occurred within the context of an estuarine to paralic system for which modern North Atlantic storm-influenced shelf and inner shelf analogues are inappropriate. Certainly the classical Sauk transgression of Sloss (1963) is very real, but it is also very complex, not only in terms of its often acknowledged diachronous properties but also in terms of preserved environments of deposition. Combined biological and sedimentological criteria indicate that in the eastern Grand Canyon the transgression manifests itself as a heterolithic set of beds representative not of a shelf but of coastal plain compartments situated between subdued structural highs. Sediment type and supply, not distance from shore, was the most important factor in determining the resultant rock sequence. The most convenient label to apply to these compartments is estuaries.

Trophic reconstruction based on integrated palynofacies and ichnofacies Palynomorph assemblages represent residual organic remains trapped in muds after burial. Given the overwhelming abundance and sheer volume of burrowing activity in the BAS, it

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seems reasonable to assume that the organic content of the Bright Angel mudstones could represent a subsample of the primary food sources driving this middle Cambrian ecosystem. The preservation of refractory organic matter in a depositional system is affected by a myriad of factors, including: primary abundance of organic material from autochthonous and allochthonous sources; in situ physical chemistry of the depositional environment - in this case, the Eh as it affects on oxidative degradation of refractory organic matter; microbial degradation of organic matter, which may be syndepositional or early postdepositional in character; maceration by metazoan feeding. The recovery of preserved refractory organic matter in shales indicates that decay and degradation was incomplete or arrested at some point in time during the burial process. When organic matter is completely missing, we cannot reconstruct its prior history, except to propose that the sources were originally absent, the organic matter was completely recycled within the primary ecosystem, or post-depositional physical chemical processes were driven to completion. In the case of the BAS, however, the immediate presence of traces indicates that food was abundant in the primary system. Thus at least we can rule out that primary organic matter was lacking. Palynomorphs and associated microdebris also record indicators of both microbial and chemical degradation. Microbial degradation is indicated by patterned wall destruction, essentially holes that form in spore walls or permeate other tissues. The formation of pyrite, as small crystals, also occurs within spore walls, leaving an unambiguous indicator of reducing chemical conditions prior to cessation of active chemistry after burial. The quality of spore wall preservation may vary from perfectly smooth to granular and imperfect. Such variation is difficult to assess with respect to its timing in the overall sequence of organic burial and degradation; however, when well-preserved and imperfectly preserved spores co-occur in a sample, it does indicate varying degrees of early stages of degradation. All samples contain a background component of spherical to subspherical clusters of cells, most of which appear to be the resting cysts of terrestrial (freshwater) algae. These remains represent allochthonous non-marine material brought into the system from fluvial sources. They also could be the autochthonous remains

of phytoplankton and other fallout from the overlying column, but this seems to us to be highly unlikely given the complete absence of other recognizable marine palynomorphs. Additionally, there is no evidence to suggest that this inferred estuarine-like environment was characterized by anything approaching an unmixed water wedge with buoyant fresh water sitting above a saltwater unit below. Such an arrangement would surely have again left at least some mud rocks with a marine palynomorph signature. This is not the case anywhere that we have sampled the BAS. Thus organic-rich muds, including some of terrestrial derivation, form the background and perhaps ambient motif to sedimentation in the BAS environment. The depositional environment was not likely anaerobic because accumulations of amorphous organic matter are rare: there are no black shales here. Instead, currents and surface mixing were supplying plenty of oxygen to the respiring animals that produced the trace fossils. Shallow surface waters above both muds and sands must have been well oxygenated, either by atmospheric diffusion or by well-mixed terrestrial runoff. Preserved organic matter in the Bright Angel shales consists of particulate refractory materials - principally spores, cell clusters and cuticular fragments. This is very unlike the microgranulate amorphous organic (kerogen) matter typically extracted from anaerobic shales. These sediments have preserved traces that have developed at an ecological timescale that can be interpreted as successional transitions from grazed surfacial microbial mats developed on low-water-content muds to fully bioturbated and more vertically burrowed, high-watercontent muds. The tops of BAS sequences are capped with both vertical Skolithos-facies burrows immediately underlain by traces of Cruziana, Palaeophycus, Teichichnus and Rusophycus burrowing/furrowing traces. The sands that overlie the heavily burrowed muddy tops became bioturbated by vertical Arenicolites, Diplo crater ion and Skolithos, some of which may have housed suspension feeders, and also by a variety of grazers. The preservation detail seen in the BAS sandstones of various thickness indicates that the primary sands were bound and stabilized enough to retain fine structure. Presumably, there was an abundance of microbial exudate and/or binding meiofauna that provided this stabilization of the substrate. The formation of micro-reticulate 'wrinkle marks' ('interference "tadpole" ripple marks' of McKee & Resser 1945, plate 12c), which occur high in the section at Sumner Butte (Fig. 1 la), has been ascribed to

PALAEOECOLOGY OF THE BRIGHT ANGEL SHALE

the binding strength of near-surface microbial mats (Hagadorn 1997). Modern analogues of this form occur in muddy supratidal environments on Padre Island in Texas, although they can range into subtidal habitats as well (Hagadorn 1997). The presence of grazing trails on numerous similar surfaces (Fig. 9b) is a clear indication that nutrient was available at the sand surface, reinforcing the notion that food sources were readily abundant. It seems reasonable to speculate that some of that food was in the form of either microbial mats or a meiofauna (cf. Mcllroy & Logan 1999) whose polysaccharide-rich extracellular exudates aided in binding sediment. Shales immediately overlying these grazed and burrowed upper surfaces contain a type A palynoflora. These are assemblages dominated by well-preserved spore and spore clusters with only minimal metazoan and/or proctoctist remains. These represent primary (phototrophic) sources of organic matter that were brought into place with source muds, although it is possible that estuarine aquatic algae may have contributed to the preserved organic content. Regardless of exact source, suspension feeders capable of living in muddy waters would have had ample food in this environment. Sample T-4 in 'Teichichnus Gully' is of this type, occurring low in the section near the transition zone in a part of the section dominated by dense Skolithos and U-shaped burrow colonization.

Ecosystem reconstruction incorporating multiple sources of data When viewed together, the descriptive sequences of lithofacies, ichnoguilds and palynofacies can be placed into a partially constrained palaeogeographic setting to create a robust depositional model for the Bright Angel system. At the outset, the sheer volume of traces indicates an exceptionally rich ecosystem, possibly one that was fed by heterogeneous sources, and inhabited by a variety of different kinds of organism. This ecosystem appears to have been fed by a rich terrestrial organic runoff derived from freshwater algal and probably subaerial plant sources. Therefore it remains unlikely that the physical setting for the BAS deposition included waters of normal marine salinity and certainly not of the type that we would expect to be present in any domain to which the epithet 'shelf might be applied. The preservation of both trace fossils and palynomorphs supports this interpretation. Excessively bioturbated beds (ii 5-6) contain

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the most degraded palynomorphs, and contain metazoan fragments in high relative abundances. The mixed thin sandstone/shale beds (ii 2-4) are of intermediate quality of spore preservation. And shales basal to sequence-topping horizons that contain traces produced by active filter feeders appear to contain the highest percentages of cryptospore tetrads and dyads, forms that have the highest degree of affinity with subaerial land plants. It appears that the distinctive iron-rich facies of the BAS can be considered as somewhat exotic but regionally extensive aerobic facies that repeatedly punctuated a muddy estuarine shoreline. The muds were characterized by high levels of organic input, but the high degree of trace-making activity indicates that oxygen supply was not generally depleted. Both glauconitic and haematitic sands appear to represent the ingress of tidal and other bar sand buildups into an otherwise quiet and moderately low-relief, muddy estuarine setting. Conclusions In the absence of palaeontological data from which to infer depositional settings through uniformitarian methodology, it is extremely difficult for geologists to infer or reconstruct palaeoenvironments with any degree of certainty. Although that seems to have been the case for the Bright Angel Shale, in fact there is a wealth of palaeontological information preserved both in the trace fossils and in the microscopic organic remains recovered from the ubiquitous shales of the formation. When viewed together, the overwhelming weight of evidence indicates that the depositional setting was estuarine and not fully marine. Marine invertebrates are almost exclusively restricted to allochthonous ('Magenta Beds'), but the in situ Lingulella species, which is the only invertebrate of note preserved in the shale matrix, has long been considered a very shallow-water indicator. Rudwick (1970, p. 158) indicates that Lingulides may well have tolerated brackish conditions, so that an estuarine setting for Lingulella is by no means out of the question. This fits the proposed model. Trace fossils associated with the Cruziana ichnofacies of Seilacher are generally considered to be shelfal. But this interpretation can be modified or overturned in the light of other evidence. Other researchers have discovered this ichnofacies in demonstrably deltaic environments (Legg 1985) and Cruziana/ Skolithos associations in estuarine settings. The BAS preserves thin sandstone beds with

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Mono crater ion I Skolithos on the tops and Cruziana on the bottoms of the same beds. These ichnotaxa may impart different environmental meaning on paper, but in this case they were formed in the same bathymetric setting. Given the complete lack of a marine signal in the palynological assemblages extracted from the enclosing muds, that ecosystem was probably estuarine in nature. The intimate admixture of burrowing activity and organic deposition has the potential to tightly characterize and partially reconstruct the trophic structure of this ancient ecosystem. Even though the analysis presented here is a work in progress, crude correlations among palynomorph assemblage composition, sample preservation quality, and trace fossil activity have been noted. In the most actively bioturbated sediments, preserved structural organic matter is most degraded, and metazoan remains (e.g. cuticle fragments) form a significant component of the overall recovery. The opposite is seen in muds deposited near the base of sedimentary packages that rest directly on beds with grazed surfaces and vertical burrows. These surface-grazing and filter-feeding trace fossil communities were inundated by muds that retain a primary signature of fallout of photosynthetic food sources. Ichnofacies characterized by mixed sandstone/shale interbeds with a moderate degree of burrowing contain an intermediate palynoflora, reflecting both primary input (photosynthetic organism) and secondary remains (autochthonous fossils). The great abundance and diversity of traces in the BAS have yet to be quantified; yet they are truly vast. The influx of carbon needed to fuel these animal communities was substantial, and would have been comparable to that seen in modern estuaries and other shallow marine settings that support up to 100% mixing of burrowed substrate. Recovery of palynomorphs reveals the source of at least part of the primary, photosynthetic organisms that drove carbon production in this system. Ironically, these are not marine algae, but what appears to be a mix of non-marine algae and ancestral land plants that produced resistant-walled spores (cryptospores sensu Strother & Beck 2000). Thus the ecosystem under study in the BAS represents an important first step in characterizing and quantifying the earliest known example of carbon dynamics at the terrestrial/marine interface. For fieldwork assistance we thank C. Lenk, B. Haley, L. Marschall, J. Strother and T. McNulty; for Geochemistry: K. Harrison. Funding support for the project includes the following: National Geographic

Society (Strother & Baldwin); American Chemical Society (Strother); Sam Houston State University Office of Research (Baldwin); AAPG Grant-in-Aid, Grand Canyon Association, SEPM, Friday Lunch Club (Rose). We also appreciate the valuable guidance provided by our reviewers: Mary L. Droser, M. Moczydlowska Vidal and Duncan Mcllroy.

References BALDWIN, C. T. 1976. The trace fossil stratigraphy of some shallow marine Cambro-Ordovician rocks from Brittany, NW Spain and the UK. PhD thesis, Liverpool University. BALDWIN, C. T. 1977. Internal structures of trilobite trace fossils indicative of an open surface furrow origin. Palaeogeography, Palaeoclimatology, Palaeoecology, 21, 273-284. BROMLEY, R. G. 1990. Trace Fossils: Biology and Taphonomy. Special Topics in Palaeontology, 3, Unwin, London. DALRYMPLE, R. W., BOYD, R. & ZAITLIN, B. A. (eds) 1994. Incised-Valley Systems: Origin and Sedimentary Sequences. Society of Economic Paleontologists and Mineralogists, Tulsa, Oklahoma, Special Publications, 51. DAVIS, R. A. JR, ALEXANDER, C. R. & HENRY, V. J. 1998. Tidal sedimentology: historical background and current contributions. In: ALEXANDER, C. R., DAVIS, R. A. & HENRY, V. J. (eds) Tidalites: Processes & Products, Society of Economic Paleontologists and Mineralogists, Tulsa, Oklahoma, Special Publications, 61, 1-4. DROSER, M. L. 1991. Ichnofabric of the Paleozoic Skolithos ichnofacies and the nature and distribution of Skolithos piperock. Palaios, 6, 316-325. DROSER, M. L. & BOTTJER, D. J. 1986. A semiquantitative field classification of ichnofabrics. Journal of Sedimentary Petrology, 56, 316—325. DROSER, M. L, JENSEN, S. & GEHLING, J. G. 2002a. Trace fossils and substrates of the terminal Proterozoic-Cambrian transition: implications for the record of early bilaterians and sediment mixing. Proceedings of the National Academy of Sciences, 99, 12572-12576. DROSER, M. L, JENSEN, S., GEHLING, J. G., MYROW, P. M. & NARBONNE, G. M. 2002b. Lowermost Cambrian Ichnofabrics from Chapel Island Formation, Newfoundland: implications for Cambrian Substrates. Palaios, 17, 3-15. ELLIOTT, D. K. & MARTIN, D. L. 1987. A new trace fossil from the Cambrian Bright Angel Shale, Grand Canyon, Arizona. Journal of Paleontology, 61, 641-648. FEDO, C. M. & COOPER, J. D. 2001. Sedimentology and sequence stratigraphy of Neoproterozoic and Cambrian units across a craton-margin hinge zone, southeastern California, and implications for the early evolution of the Cordilleran margin. Sedimentary Geology, 141-142, 501-522. FEDO, C. M. & PRAVE, A. R. 1991. Extensive Cambrian braidplain sedimentation: insights from the southwestern USA Cordillera. In: COOPER, J. D.

PALAEOECOLOGY OF THE BRIGHT ANGEL SHALE & STEVENS, C. H. (eds) Paleozoic Paleogeography of the Western United States-H, Vol 1. The Pacific Section Society of Economics Paleontologists and Mineralogists, Los Angeles, Book 67, 227-236. HADDOX, C. A. & DOTT, R. H. JR. 1990. Cambrian deposits in northern Michigan. Journal of Sedimentary Petrology, 60, 697-716. HAGADORN, J. W. 1997. Wrinkle structures: microbially mediated sedimentary structures common in subtidal siliciclastic settings at the ProterozoicPhanerozoic transition. Geology, 25, 1047-1050. HEREFORD, R. 1977. Deposition of the Tapeats Sandstone (Cambrian) in central Arizona. Geological Society of America Bulletin, 88, 199-211. JOHNSON, N. G. 1985. Early Silurian palynomorphs from the Tuscarora Formation in central Pennsylvania and their paleobotanical and geological significance. Review of Palaeobotany and Palynology, 45, 307-360. KIM, Y. & LEE, Y. I. 2000. Ironstones and green marine clays in the Dongjeom Formation (Early Ordovician) of Korea. Sedimentary Geology, 130, 65-80. KLEIN, G. De V. 1970. Depositional and dispersal dynamics of intertidal sand bars. Journal of Sedimentary Petrology, 40, 1095-1127. LEGG, I. C. 1985. Trace fossils from a Middle Cambrian deltaic sequence, north Spain. In: CURRAN, H. A. (ed.) Biogenic structures: their use in interpreting depositional environments. SEPM Special Publication, 35, 151-165. LOUTIT, T. S., HARDBENOL, J., VAIL, P. R. & Ross, G. R. 1988. Condensed sections: the key to age dating and correlation of continental margin sequences. In: WILGUS, C. K., HASTINGS, B. S., KENDALL, C. G. ST C., POSAMENTIER, H. W., Ross, C. A. & VAN WAGONER, J. C. (eds) Sea Level Changes: An Integrated Approach. Society of Economic Paleontologists and Mineralogists, Tulsa, Oklahoma, Special Publications, 42, 183-213. MclLROY, D. 2004. Some ichnological concepts, methodologies, applications and frontiers. In: MclLROY, D. (ed.) The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 3-27. MclLROY, D. & LOGAN, G. A. 1999. The impact of bioturbation on infaunal ecology and evolution during the Proterozoic-Cambrian transition. Palaios, 14, 58-72. McKEE, E. D. & RESSER, C. E. 1945. Cambrian History of the Grand Canyon Region. Carnegie Institute Publication, Washington DC, 563. MIDDLETON, L. T. & ELLIOTT, D. K. 2003. Tonto Group. In: BELTS, S. S. & MORALES, M. (eds) Grand Canyon Geology (2nd edn). Oxford University Press, Oxford, 90-106. MYROW, P. M., TAYLOR, J. F., MILLER, J. F., ETHINGTON, R. L., RlPPERDAN, R. L & ALLEN, J. 2003.

Fallen arches: dispelling myths concerning Cambrian and Ordovician paleogeography of the Rocky Mountain region. Geological Society of America Bulletin, 115, 695-713.

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NOBLE, L. F. 1910. Contributions to the geology of the Grand Canyon, Arizona; the geology of the Shinumo area. American Journal of Science, ser 4, 29, 369-528. NOBLE, L. F. 1922. A section of the Paleozoic formations of the Grand Canyon, at the Bass trail. USGS Professional Papers, 131-B, 23-73. PEAKALL, J., MCCAFFREY, B. & KNELLER, B. 2000. A process model for the evolution, morphology, and architecture of sinuous submarine channels. Journal of Sedimentary Research, 70, 434^448. PEMBERTON, G. S., MACEACHERN, J. A. & FREY, R. W. 1992. Trace fossil facies models: environmental and allostratigraphic significance. In: WALKER, R. G. & JAMES, N. (eds) Facies Models and Sea Level Change (3rd edn). Geological Association of Canada, St. Johns, Newfoundland, 47-72. PEMBERTON, G. S., MACEACHERN, J. A. & SAUNDERS, T. 2004. Stratigraphic applications of substratespecific ichnofacies: delineating discontinuities in the rock record. In: MclLROY, D. (ed.) The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 29-62. RUDWICK, M. J. S. 1970. Living and Fossil Brachiopods. Hutchinson, London. SCHUCHERT, C. 1918. The Cambrian of the Grand Canyon of Arizona. American Journal of Science, ser 4, 45, 362-369. SEILACHER, A. 1967. Bathymetry of trace fossils. Marine Geology, 5, 413-428. SEILACHER, A. 1970. Cruziana stratigraphy of nonfossiliferous Palaeozoic sandstones. In: CRIMES, T. P. & HARPER, J. C. (eds) Trace Fossils. Geological Journal, Special Issue 3, 447-476. SEILACHER, A. 1991. Events and their signatures. In: ElNSELE, G., RlCKEN, W. & SEILACHER, A. (eds)

Cycles and Events in Stratigraphy. Springer, Berlin, 222-226. SEILACHER, A. & AIGNER, T. 1991. Storm deposition at the bed, facies, and basin scale: the geological perspective. In: EINSELE, G., RICKEN, W. & SEILACHER, A. (eds) Cycles and Events in Stratigraphy. Springer, Berlin, 249-267. SERVAIS, T., MOLYNEUX, S. G., BROCKE, R., FATKA, O. & LE HERISSE, A. 1996 Value and meaning of the term acritarch. Ada Universitatis Carolinae Geologica, 40, 631-643. SLOSS, L. L. 1963. Sequence in the cratonic interior of North America. Geological Society of America Bulletin, 74, 93-114. STRIDE, A. H., BELDERSON, R. H., KENYON, N. H. & JOHNSON, M. A. 1982. Offshore tidal deposits: sand sheets and sand bank facies. In: STRIDE, A. H. (ed.) Offshore Tidal Sands, Process and Deposits. Chapman & Hall, Edinburgh, 95-125. STROTHER, P. K. & BECK, J. H. 2000. Spore-like microfossils from Middle Cambrian strata: expanding the meaning of the term cryptospore. In: HARLEY, M. M., MORTON, C. M. & BLACKMORE, S. (eds) Pollen and Spores: Morphology and Biology. Royal Botanic Gardens, Kew, 413424.

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STROTHER, P. K. & TRAVERSE, A. 1979. Plant microfossils from Llandoverian and Wenlockian rocks of Pennsylvania. Palynology, 3, 1-21. TAYLOR, K. G. & CURTIS, C. D. 1995. Stability and facies associations of early diagenetic mineral assemblages: an example from a Jurassic ironstone-mudstone succession, UK. Journal of Sedimentary Research, A65, 358-368. TODD, S. P. & WENT, D. J. 1991. Lateral migration of sand-bed rivers: examples from the Devonian Glashabeg Formation, SW Ireland and the Cambrian Alderney Sandstone Formation, Channel Islands. Sedimentology, 38, 997-1020. WALKER, R. G. 1992. Turbidites and submarine fans. In: WALKER, R. G. & JAMES, N. P. (eds) Facies

Models: Response to Sea Level Change. Geological Society of Canada, St. Johns, Newfoundland, 239-263. WALKER, R. G. & PLINT, A. G. 1992. Wave- and stormdominated shallow marine systems. In: WALKER, R. G. & JAMES, N. P. (eds) Facies Models: Response to Sea Level Change. Geological Society of Canada, St. Johns, Newfoundland, 219-238. WANLESS, H. R. JR. 1973. Cambrian of the Grand Canyon: a re-evaluation of the depositional environments. PhD dissertation, Johns Hopkins University. WELLMAN, C. H., OSTERLOSS, P. L. & MOHIUDDIN, U. 2003. Fragments of the earliest land plants. Nature, 425, 282-285.

Ichnofabrics and sedimentary fades of a tide-dominated delta: Jurassic He Formation of Kristin Field, Haltenbanken, Offshore Mid-Norway DUNCAN McILROY Sedimentology and Internet Solutions Ltd, 29 Proctor Road, Hoylake, Wirral, CH47 4BB, UK (e-mail: [email protected]) Abstract: Tide-dominated deltas are poorly known from the stratigraphic record and are notoriously complex, owing to the wide spectrum of facies encountered and their spatial/ temporal variability. The tide-dominated deltaic palaeoenvironment combines the ecological harshness of brackish-water settings with complex tidal channel/tidal-flat type facies architecture on the delta top, in association with more classic deltaic facies-stacking patterns. The He Formation is interpreted herein as a tide-dominated delta deposited in a microtidal setting. Its palaeoenvironments are interpreted based on a combination of ichnology, ichnofabric analysis and sedimentology. Ichnofabric stacking patterns are used to elucidate the internal architecture of the notoriously problematic aggradational multi-storey tidal channel units. The tide-dominated deltas of the He Formation have a distinctive ichnological signature that may be used to characterize tide-dominated deltas. In comparison to typical riverdominated deltas the Skolithos ichnofacies is less well developed and ichnodiversity is lower than expected in wave-dominated deltas. The ichnofabric model presented has potential to be used, with modification, in other tide-dominated deltaic settings.

Introduction The ichnology of tide-dominated brackish-water settings is complex, owing to the wide range of environmental conditions experienced by the infauna. In most case studies investigating the ichnology of ancient tidal deposits, the palaeoenvironment is that of a drowned river valley, i.e. an estuary (e.g. Goldring et al. 1978; MacEachern 1989; Wrightman et al 1987; Mattison et al 1989; Pattison 1992; Pemberton & Wrightman 1992; Pemberton et al 1992; Buatois et al 1998; Buatois & Mangano 2004). All tidal systems comprise a complex mosaic of environments in which a range of physical and chemical conditions affect infaunal (trace-making) organisms. Understanding of detailed trace fossil distributions within such complex tidal settings is commonly restricted to the observation that tidal (particularly estuarine) facies are characterized by a mixture of the Cruziana and Skolithos ichnofacies (e.g. Pemberton et al 2001). It is noted, however, that a similar admixture of the Cruziana and Skolithos ichnofacies can characterize lower shoreface event beds (Bromley & Asgaard 1991; Frey & Goldring 1992). The relative importance of various hydrodynamic regimes (e.g. waves versus tides) allows definition of a system as being 'influenced' or 'dominated' by a particular depositional process. The recognition of the character of the predominant hydrodynamic regime is integral to the construction of realistic facies models, and is ideally based on a combination of diagnostic sedimentary structures along with palaeoenvironmental data

from geochemical, palaeontological and ichnological studies (as appropriate). The major challenge with understanding petroleum reservoirs in tide-dominated depositional settings is confidence in distinguishing between different facies at a suitable resolution to reduce uncertainty in reservoir geology. Tide-dominated deltas contain both deltaic and brackish-water facies, similar to estuaries, and therefore pose special problems. The resolution of facies analysis can be significantly improved in highly heterogeneous reservoir facies, such as characterize tidal deposits, through detailed ichnological study using either a bespoke assemblage or an ichnofabric approach such as presented herein. It is noted that in some (mostly Palaeozoic) cases the ichnology of tidal environments can constitute almost entirely bedding parallel traces (e.g. Buatois et al 1998; Buatois & Mangano 2004), which leave very little ichnofabric when seen in vertical cross-section and restrict the use of core analysis to the limited bedding surfaces between core segments. Objectives The primary objectives of this work were thus to assess the sedimentology and ichnology of the He Formation on Kristin Field through sedimentary logging at a 1:50 scale, and to provide ichnologically constrained facies models and a framework for integration with other in-house datasets (e.g. M0rk et al 1997; Stilling 2000; Greger et al

From: MC!LROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 237-272. 0305-8719/04/S15.00 © The Geological Society of London.

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Fig. 1. Location of the Kristin Field in relation to other fields in the Haltenbanken area, offshore MidNorway.

2001) and as input to stochastic reservoir models of this important reservoir interval.

Studied core This study focused on the He Formation in the four cored wells from Kristin Field (6406/2-5; 6506/11-6; 6406/2-3-T3; 6406/2-5A-T2) (Fig. 1). Previous in-house studies on the field (M0rk et al 1997; Stilling 2000; Greger et al 2001) were not available to the author, and this work is independent of their content. The scale of the current study was such that bed-by-bed sedimentological and ichnological observations could be made. Study of the He Formation on Sm0rbukk Field (see Fig. 1) confirms that the ichnofabric scheme proposed herein could be applied with slight modification to adjacent fields.

Sedimentology and stratigraphy of the He Formation The He Formation is a shallow-marginal marine succession of Aalenian age that forms part of the Fangst Group (Fig. 2). It is widespread in distribution on Haltenbanken and is generally between 80m and 90m in thickness. The formation is strongly progradational at its base, comprising an upward-coarsening delta front succession that is in gradational contact with the underlying Ror Formation. This is overlain by a broadly aggradational delta top succession with tidal channels that cut into and rework the adjacent tidal flats. The upper portion of the lie Formation records a gradual flooding of the system and is dominated by subtidal flat and prodelta deposits (see Fig. 3). Good-quality sandstone reservoirs in the He Formation are largely reliant upon the presence of grain-coating

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communities of organisms. Although description of ichnological assemblages is a worthwhile exercise in itself, it becomes a much more powerful tool when the ichnological assemblage is deconstructed into its component parts using ichnofabric analysis (see Taylor & Goldring 1993; Taylor et al. 2003; Mcllroy 2004). This can lead to a greater understanding of conditions before, during and after deposition of a bed, and is especially useful in the recognition of surfaces of Stratigraphic importance (cf. Taylor & Gawthorpe 1993; Taylor & Goldring 1993; Taylor et al. 2003; Mcllroy 2004). Once characterized, ichnofabrics are conventionally named, e.g. PlanoliteslAsterosoma ichnofabric, with the eponymous trace(s) being either highly visible component(s) of the ichnofabric or its most characteristic component(s).

Fig. 2. Stratigraphic units of the Triassic and Jurassic on Haltenbanken; major reservoir intervals shaded grey.

chlorite, which inhibits quartz cementation (see summary in Ehrenberg 1993; Ehrenberg et al 1998). In order to assess the importance of palaeoenvironment in controlling the distribution of chlorite-coated grains a refined palaeoenvironmental/facies model was constructed for comparison with other datasets.

Ichnofabric description This study has focused upon the ichnology of the He Formation within the context of broaderscale sedimentological studies on Haltenbanken. This integrated approach to reservoir description is fundamental to rigorous facies analysis, in which palaeontological/palaeoenvironmental information is considered alongside evidence from physical sedimentary structures. Most descriptions of sediments are taken with reference to beds, within which observations of physical and biogenic sedimentary structures are made. This necessitates sedimentary logging at at least 1:50 scale to capture most individual beds. Indeed, the most easily described unit in ichnological studies is an ichnological assemblage, which comprises all the trace fossils found within a bed (see Mcllroy 2004). The term 'ichnological assemblage' does not discriminate between the relative ages of the component trace fossils, and may represent time-averaged burrowing resulting from several successive

Combining ichnological and sedimentological approaches Trace fossils are best considered as biogenic (animal/plant-made) sedimentary structures that contribute to the sediment texture as seen in core/outcrop. Their study is therefore as much a branch of sedimentology as it is a subdiscipline of palaeontology. Like physical sedimentary structures, trace fossils can yield important information about the environment at the time of sedimentation. Facies are defined using physical sedimentary structures (to understand the depositing current) along with trace and body fossils (Reading 1996). Further refinement of this approach can be achieved by studying the trace fossil assemblage in more detail. For example, a trough crossbedded tidally influenced channel facies will contain similar sedimentary structures (at core scale) along its entire length (may be hundreds of kilometres), but the distribution of trace fossil assemblages/ichnofabrics, as well as being affected by current strength, will also be influenced by the salinity gradient. In this way a single sedimentary facies can be subdivided, on a combination of ichnological and sedimentological grounds, into its component parts. This is important for the recognition of subtle stacking patterns in apparently aggradational successions [e.g. stacked tidal channels (herein) and shorefaces (Martin & Pollard 1996)]. Such variations in stacking patterns are the fundamental data that sedimentologists/sequence stratigraphers use to predict inter-well facies architectures by application of Walther's law (Walther 1894), and in this respect sedimentary facies, ichnofabrics, ichnological assemblages and archetypal

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Fig. 3. Summary stratigraphic section through the He Formation as expressed on Kristin Field, combined from features of all four cored wells.

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ichnofacies can be treated in exactly the same manner. Ichnology of the He Formation

Trace fossils of the He Formation The He Formation contains a diverse assemblage of trace fossils that show a strong palaeoenvironmental control on their distribution. These palaeoenvironmental controls on trace fossils allow highly refined facies analysis of their host sediments when combined with sedimentological interpretations. As the most fundamental unit of stratigraphy is that of facies, improved facies characterization is integral to the construction of robust conceptual facies models, sequence stratigraphic models and the petroleum geological applications thereof. The diverse assemblage of ichnofossils documented from the He Formation described during this study (Table 1) indicates a shallow marine depositional setting with no diagnostically non-marine ichnotaxa represented.

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Ichnofabrics of the He Formation What follows is an account of the ichnofabric types documented from the He Formation. It results from many individual observations of trace fossils from the four wells studied. The ichnofabric scheme is by necessity fairly extensive owing to the large total ichnodiversity and sedimentological/palaeoenvironmental disparity that characterizes the He Formation (see below). The 25 ichnofabrics are ordered alphabetically with respect to their abbreviated ichnofabric title to ease comparison with the reservoir descriptions and the sedimentological descriptions below, and are integrated into the conceptual model (see sedimentology section below). In all cases, ichnofabric description is supplemented with sedimentological implications and distribution in sedimentary facies. Ichnofabric constituent diagrams for all ichnofabrics are presented in Appendix Figs 1 and 2 using a modified version of the ICD method proposed by Taylor & Goldring (1993) (see Mcllroy 2004).

Table 1. Trace fossils found in the uppermost Ror and He Formations and their facies distributions (see Table 2) Trace fossil

Palaeoenvironmental range in the He

Arenicolites isp. Asterosoma isp. Bergaueria ?perata Chondrites isp. Diplocraterion parallelum Diplocraterion polyupsilon Gyrochorte isp. Gyrolithes polonicus Lockeia amygdaloides Monocraterion isp. Ophiomorpha nodosa Ophiomorpha irregularis Palaeophycus tubularis Palaeophycus heberti Phoebichnus trochoides Phycosiphon incertinum Planolites beverleyensis Rhizocorallium irregularis Rosselia rotatus Siphonichnus isp. Skolithos verticalis Taenidium serpentium Taenidium isp. Teichichnus rectus Teichichnus zigzag Schaubcylindrichnus isp. Thalassinoides suevicus

PDMB, TC, IDMBTC PDMB, STF, OTDC, DMB STF TC, IDMBTC, PDMB, PPD, DMB DMB, DDMB DMB, DDMB DMB TC ITF, STF, TC, PDMB, DMB, DDMB, ATC TC, IDMBTC PDMB, IDMBTC, TC PDMB, TC, IDMBTC, DMB STF, ATC, PDMB, DMB, PPD, DPD PPD, DDMB, DMB PPD, DPD DPD, PPD, DDMB, DMB, TC, STF, OTDC DPD, PPD, DDMC, DMB, PDMB, OTDC, STF, ITF, ITMF, IDMBTC, TC, ATC DPD, PPD, CB STF, DMB, ATC DMB, ATC DDMB, DMB, PMB, IDMBTC, TC PPD PPD, PPD, PMB PPD, PPD, PMB, STF, TC PPD PPD PPD, PPD, STF, ATC, TC

Trace fossils are identified from cross-sections in core, and identifications are tentative to a variable degree.

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Fig. 4. (a) Chond/Skol ichnofabric showing colonization from the slightly erosive base of a sandstone. The traces figured are thinly lined small Skolithos (sk) and cross-sections of the horizontal portion of Chondrites (ch), which is typically low-density. Note the shallow-tier burrow mottling, (b) Diplo ichnofabric showing colonization of the upper surface of a parallel-laminated sandstone (DMB facies) by abundant small Diplocraterion (di) and overlain by a heavily bioturbated mudstone with Planolites (pi) and Thalassinoides (th). (c) Gyro/esc ichnofabric showing a sparsely bioturbated sand-rich succession with abundant clay-draped ripple cross-laminations and Gyrochorte (gy). (d) Detail of P. herbi ichnofabric with abundant Palaeophycus heberti (he) showing the diagnostic thick-walled burrows in longitudinal and horizontal cross-section. The burrows are dark brown coloration due to sideritization during diagenesis. Width of core 10cm; all cores displayed with oldest strata bottom left.

Chondrites I Skolithos ichnofabric (Chond/Skol) (Fig. 21a) The Chondrites /Skolithos ichnofabric found in association with coarse tidal channel sandstones is - like the Trichichnus ichnofabric described below - primarily a post-depositional colonization ichnofabric. However, unlike the Trichichnus ichnofabric, the fluid mud deposits that directly overlie the Chond/Skol horizons are commonly heavily bioturbated (see Phyco, PI/ Tae, Pl/Phyco, Pl/Thal below). The ichnofabric is a secondary ichnofabric that normally overprints a burrow-mottled ichnofabric, which is

typically composed entirely of shallow-tier traces, sometimes with discernible Planolites isp. (Fig. 4a) Associated ichnofabrics include the Thalassi-

noides /Chondrites (Thal/Chond)

ichnofabrics,

which may even be interbedded within the same channel-fill succession. Diplocraterion ichnofabric (Diplo) (Fig. 2la) The Diplocraterion ichnofabric is a post-depositional ichnofabric composed of low to moderate intensities of bioturbation and small burrow

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width (3—5mm). The ichnological assemblage is generally monotypic (Fig. 4b), but may be overprinted by trace fossils in the overlying mudstone unit. The Diplocraterion-bearing sandstones are commonly overlain by moderately to highly bioturbated sandy heterolithic facies, especially PljTeich, PI I Thai and PI/ Phyco ichnofabrics. The Diplocraterion ichnofabric is rather restricted in sedimentological range, being confined to the central to distal portion of distributary mouth-bar facies. The Diplocraterion show particular affinity for the parallel-laminated sandstones that are inferred to be deposited from a buoyant sediment plume by suspension fallout (see Brettle et al 2002). The Diplocraterion are exclusively protrusive in nature (i.e. the latest formed U-tube is the deepest and sandinfilled). The trace fossils are also uniformly small, which is taken to support the interpretation of a single phase of opportunistic colonization with the traces of immature organisms being preserved (cf. Frey & Goldring 1992). Dwarfism in suboptimal conditions is also possible, though such an interpretation is considered unlikely given the inferred sedimentological setting (delta front). Gyrochorte/escape burrow ichnofabric (Gyro/esc) (Fig. 2la) Ichnofabrics with abundant Gyrochorte in association with v-shaped escape burrows (Fig. 4c) are common only in distributary mouth-bar facies of the He Formation. The Gyrochorte/ escape burrow ichnofabric is characteristic of ripple cross-laminated sandstones with moderate to high intensities of bioturbation. Sedimentary horizons with the equilibrium-style Gyro/esc ichnofabric tend to have low ichnodiversity. A post-depositional suite may be present, which typically constitutes either the Pl/smAst or Diplo ichnofabrics in depositionally active mouth-bar settings. Alternatively, the Gyro/esc ichnofabric may be interbedded with the diverse ichnological assemblages of subtidal flats (Ross/ Asf) during longer hiatal periods in which subtidal flats are generated by reworking during delta lobe abandonment. The burrows themselves record the life-action of organisms living in areas of high sedimentation rates and well adapted to escape from burial under sandbeds. The v-shaped escape burrows in this ichnofabric are probably created by bivalve molluscs (in bedding plane expression the trace fossil is Lockeia amygdaloides). Gyrochorte burrows are more difficult to interpret, with their distinctive 'm'-shaped morphology in transverse cross-section. Commonly associated

243

with the eponymous trace fossils are Phycosiphon and more rarely Asterosoma isp. Palaeophycus heberti ichnofabric (P. herbi) (Fig. 21a) Trace fossil assemblages composed exclusively of Palaeophycus heberti are uncommon in the lie Formation, but may be of great stratigraphic significance. Ichnofabrics containing the trace fossil P. heberti are normally found in prodelta and distal mouth-bar heteroliths (P. herbi/ Schau ichnofabric), but the trace fossil itself is generally completely absent from the more proximal heterolithic facies such as tidal flats and tidal channels. A single horizon of mudstone completely bioturbated by P. heberti within a channel fill succession (Fig. 4d) could therefore be interpreted as reflecting a marked (albeit temporary) shift in palaeoenvironmental conditions. However, given that the facies above and below this horizon are similar, the change in conditions that allowed development of the intense P. heberti ichnofabrics are likely to be of local rather than regional importance. Either a minor flooding event or a phase of abandonment could be responsible for such a horizon. This is an excellent example of a case where ichnological study can supplement sedimentology to produce improved palaeoenvironmental interpretations. Palaeophycus hebertijSchaubcylindrichnus ichnofabric (P. herbi/Schau) (Fig. 2la) Ichnofabrics containing the thickly lined tubular trace fossils Palaeophycus heberti and Schaubcylindrichnus isp. are common in the upper Ror to lower He Formation, and are commonly associated with intense bioturbation of heterolithic sands and muds (from 60% to 100%; Fig. 5ai,ii). The traces are typically predominantly horizontal with some vertical pipes represented (cf. Frey et al. 1990) and are associated with current ripples, minor wave ripples and synaeresis cracks. In some cases, the current ripples can be demonstrated to show mud couplets deposited at the ebb and flood slacks. The two eponymous traces are typically the earliest component of the ichnofabric, and comprise the primary ichnocoenosis. As cross-cutting relationships between the two are relatively common - neither is systematically preceded by the other - they are thus considered contemporaneous. The primary ichnocoenosis is typically reworked by numerous small Phycosiphon isp. that pervade the whole ichnofabric and vary in proportion from 10% to 50% as seen in vertical section.

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Fig. 5. (ai) Complex ichnofabrics of the lower He Formation with early Palaeophycus heberti (he), Schaubcylindrichnus (sc) and Thalassinoides (th) that are re-burrowed by a later Phycosiphon (pc) ichnocoenosis. (aii) Detail of a vertically emplaced specimen of Schaubcylindrichnus (sc). Burrow diameter is 6mm. (b) The Ophio ichnofabric containing Planolites (pi) and Ophiomorpha (op) colonizes the top of a flat-laminated medium-grained sandstone with an earlier ichnofabric comprising escape burrows (lo) and Skolithos (sk) burrows, (c) Ophio/Tae ichnofabrics in sand-rich heterolithic facies with intense bioturbation by Ophiomorpha irregularis (op), Planolites (pi) Palaeophycus (ph) and Taenidium (ta). (d) OphiojChond ichnofabrics with large Ophiomorpha nodosa (on) and Chondrites (ch) overprinting an earlier ichnofabric with escape burrows (lo). Width of core 10cm; all cores displayed with oldest strata bottom left.

Ophiomorpha ichnofabric (Ophio) (Fig. 2la) Within the He Formation there are numerous examples of upward-fining successions from parallel-laminated sandstones with mud couplets into ripple cross-laminated sandstones, approximately 10-30 cm thick, with draped ripple cross-laminations also showing mud couplets (PDMB and DMB). Trace fossils within the parallel-laminated sandstones are restricted to small escape burrows, rare vertical Skolithos burrows and horizontal unlined Planolites burrows. A syndepositional PljSkol or P//esc suite may be cut by this post-depositional Ophiomorpha (Ophio) suite, which consists of shallow-tier Planolites and mid- to deep-tier thick-walled Ophiomorpha irregularis burrows (Fig. 5b). This ichnolfabric can therefore be separated into bioturbation that occurred at the same time as deposition of sand from a decelerating sediment plume, and by postdepositional colonization by large Ophiomorpha

and Planolites. Cessation of sedimentation is attributed to autocyclic lobe switching or seasonal changes in sediment supply/sediment discharge rather than regional flooding by allocyclic means. Ophiomorpha/Taenidium ichnofabric (OphiojTae) (Fig. 21a) Ichnofabrics with the thinly mud/organic-lined Ophiomorpha irregularis and the meniscate backfilled Taenidium are common in proximal distributary mouth-bar settings in the He Formation (Fig. 5c) along with Planolites and Palaeophycus. The bioturbation in this facies is typically intense (60-80%). The diversity of the assemblage is comparatively low, but may be supplemented by Planolites and occasionally Diplocraterion; indeed it would seem that the Diplo and Ophio I Tae ichnofabrics are closely related. The palaeoenvironmental conditions experienced by this ichnofauna appear to be

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good for both suspension- and deposit-feeding organisms. The depositional environment is probably a high-energy setting with some nutrition provided within the sediment, associated with thin mud laminations. Ophiomorpha/Chondrites ichnofabric (OphiojChond) (Fig. 2la) Coarse-grained upward-fining sandstone successions with thick micaceous drapes are typical of the inter-distributary mouth-bar tidal channels (IDMBTC) and some of the tidal channels (TC) in the He Formation. In some of these a distinctive and easily recognizable ichnofabric is developed, which is identified by the presence of large deep-tier Ophiomorpha nodosa burrows that have thick pellet-lined walls to both their horizontal and vertical shafts. The Ophiomorpha nodosa are associated with, but commonly overprinted by, similarly deep-tier Chondrites. The OphiojChond ichnofabric may overprint an earlier Ophio ichnofabric with Ophiomorpha irregularis owing to changing palaeoenvironmental conditions (see Fig. 5d). This ichnofabric is also associated with the ChondjSkol ichnofabrics typical of some tidal channels (TC) in the He Formation, possibly as a result of minor hiatuses in intra-channel deposition. There are similarities to the estuarine Ophiomorpha ichnofabrics of Pollard et al. (1993), though the authors did not specifically discuss deltaic occurrences of Ophiomorpha. Phoebichnus ichnofabric (Phoe) (Fig. 2la) Ichnofabrics containing Phoebichnus are' uncommon in the He Formation on Kristin Field but are abundant in the upper portion of the upper Ror Formation, where they are typically found at the top of high-order shoaling cycles. Phoebichnus ichnofabrics are typically intensely bioturbated and comprise a diverse ichnofauna composed entirely of fully marine ichnotaxa (Fig. 6a). Dominant among these are Thalassinoides, Phycosiphon, Planolites and Chondrites. The depositional environment in which the ichnofabric is developed is inferred to be a normal marine setting with moderate to low rates of sedimentation in the proximal to distal prodelta (PPD-DPD). The significance of documenting offshore ichnofacies genetically related to reservoir units is that it allows us to recognize the full scope of biogenic fabrics, such that when they are interbedded with reservoir facies we can estimate the amount of flooding represented. This improves understanding of the flooding events and their likely impact on facies architecture.

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Phycosiphon ichnofabric (Phyco) (Fig. 2la) This heavily bioturbated mudstone facies consists of an ichnofabric composed entirely of Phycosiphon burrows. It is found inter bedded with thick cross-bedded upward-fining tidal channel sandstones (TC) in thick mudstones interpreted as fluid-mud deposits. Phycosiphon burrows have a thick halo of siltstone surrounding the mud-filled burrow core and do not crosscut themselves (Fig. 6b). The monotypic nature of this ichnofabric suggests opportunistic colonization of fluid mud deposits. From studying the facies distribution of Phycosiphon in the He Formation it is clear that the ichnogenus is not tolerant of low salinity. Phycosiphon is generally absent in tidal channel facies, which would have had brackish waters. The exception is this ichnofabric (Fig. 6b), where the Phycosiphon colonize thick fluid mud deposits. The fluid muds are inferred to have been deposited in the turbidity maximum and are intimately associated with the mixing zone (see discussion in the sedimentology section above). It may therefore be the case that this ichnofabric is developed opportunistically when the turbidity maximum retreats landward (for example during low discharge periods such as summer). Tidal channels deposits with Phycosiphon-bioturbated fluid muds are thus interpreted to be formed at the distal (i.e. marine) end of the tidal channel system or in channels subject to anomalous conditions such as those that prevail during a marine flooding event or periods of anomalously low discharge. Planolites ichnofabric (Planol) (Fig. 2la) Planolites ichnofabrics (Planol} are composed exclusively of large sand-filled Planolites, are common in the channelized facies of the lie Formation (TC), and are intimately associated with fluid mud deposits as described in the sedimentology section above. The intensity of bioturbation is highly variable (typically between 60% and 80%), and is made up of a large sand-filled species of Planolites, which is probably P. beverlyensis (Fig. 6c). The mudstones, where preserved between the burrows, are characteristically well laminated with either parallel laminations or small ripple cross-laminations defined by silt or sandstone stringers. Colonization occurred subsequent to deposition and followed early lithification of the fluid mud, as the burrows commonly have very sharp margins and can be almost uncompressed. This suggests that the mudstones were deposited in a hostile environment without bioturbators, before conditions (perhaps

D. McILROY

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Fig. 6. (a) Phoebichnus ichnofabric showing intense bioturbation and high ichnodiversity typical of proximal to distal prodelta settings. Traces figured include Phoebichnus (po), Thalassinoides (th), Phycosiphon (pc) and Chondrites (ch). (b) Phycosiphon ichnofabric from a 3 cm thick fluid mud deposit within a tidal channel succession. The trace has very little contrast between the fill and halo and is difficult to photograph, (c) Succession including two fluid mud deposits showing bioturbation of firm fluid mud deposits. The lower is a monotypic Planol ichnofabric with large idiomorphic Planolites (pi), whereas the upper mudstone contains Planolites (pi) and Thalassinoides (th), and is an example of the PIj Thai ichnofabric. The upper sand is an example of the OphiojChond ichnofabric. The upper sandstone bed contains Ophiomorpha (on) and Chondrites (ch) of the (OphiojChond) ichnofabric. (d) Ichnofabric composed of v-shaped escape traces (lo; probably produced by bivalve molluscs) and unlined horizontal tubular burrows of Planolites (pi). The primary P//esc ichnofabric is overprinted by a Pl/smAst ichnofabric. Width of core 10cm; all cores displayed with oldest strata bottom left. salinity or turbidity) ameliorated colonization.

to

allow

Planolites/escape burrow ichnofabric (P//esc)

(Fig. 21a)

Primary equilibrium-style ichnofabrics composed of ripple cross-laminated sandstone with

a low-diversity trace-fossil assemblage composed entirely of escape burrows and Planolites (Fig. 6d) are common throughout the He Formation. The low ichnodiversity may be a function of slightly reduced salinity but is mostly the result of high rates of sedimentation. The trace fossils are the result of the equilibrium fauna of the

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fades that they are found in, i.e. they are syndepositional and may be overprinted by post-depositional ichnofabrics (Fig. 6d).

Planolites/Phycosiphon ichnofabric (PljPhyco) (Fig. 21b)

This composite ichnofabric is found in association with heterolithic fluid mud deposits within upward-fining channel sandstones (TC), in which the fluid muds are initially colonized by a Planolites ichnocoenosis (Planol ichnofabric) that is subsequently re-burrowed by a low-density Phycosiphon ichnocoenosis (low intensity Phyco ichnofabric) (Fig. 7 a). The latter ichnocoenosis is similar to the Phycosiphon ichnofabric

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that colonizes laminated unbioturbated muds as described above, but is also present in medium-grained sandstones. In the present case, initial colonization of the fluid mud by a Planolites (Planol) ichnofabric occurs soon after deposition by subhorizontal Planolites-making organisms that created open burrows that subsequently became sand-filled. The early Planolites ichnofabric was then reburrowed by Phycosiphon that reworked the Planolites burrows and the intervening (semilithified) fluid mud deposits. This ichnofabric is comparatively rare and may represent a transitional environment in which the fluid muds at the proximal end of the turbidity maximum

Fig. 7. (a) Intra-channel heterolith comprising small Planolites (pi) and a low-density Phycosiphon (pc) trace fossils, interbedded with thick, coarse-grained, trough cross-bedded sandstones, (b) Pl/Pph ichnofabric in a clean, well-sorted sandstone overlying a fluid mud deposit with a Sipho ichnofabric with Siphonites (si), and cut by an erosive-based sandstone with climbing ripples. The Pl/Pph ichnofabric is dominated by Planolites (pi) and Palaeophycus (ph), which in this example has destroyed all primary laminations and is thus purely a biogenic fabric, (ci) Cross-bedded sandstone showing neap-spring cyclicity and a v-shaped escape burrow centred upon a vertical burrow and attributed to Skolithos (sk) (Monocraterion morphology), (cii) Bedding plane view of a vertical Skolithos burrow showing the concentric cone-in-cone laminations diagnostic of Skolithos (sk) (Monocraterion). (d) Detail of Pl/Schau ichnofabric developed in a heterolithic ripple crosslaminated sandstone. Trace fossils present are Planolites (pi) and Schaubcylindrichnus (sc). Width of core 10cm; all cores displayed with oldest strata bottom left.

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zone are initially colonized by the normal channel fauna (producing the Planolites ichnofabric described above) during landward penetration of the salt wedge - perhaps during periods of decreased flow (Planolites ichnofabric described above) - and then, with further decrease in discharge, the euryhaline Phycosiphon tracemaker was able to colonize the sediment. Planolites I Palaeophycus ichnofabric (Pl/Pph) The predominant component of this ichnofabric is a burrow-mottled background bioturbation with poorly defined horizontal burrows without linings - referred to Planolites isp. - along with a lower proportion of Palaeophycus burrows that are defined by a thin clay lining to the outer margin of the burrow (Fig. 7b). The low ichnological diversity but high intensity of bioturbation (80-100%) is typical of a stressed sedimentary environment with low rates of sedimentation. The burrows that make up the ichnofabric are both likely to have been made by deposit-feeding organisms, which suggests a high primary organic content. As this ichnofacies is interbedded with more classic mouth-bar facies it is interpreted along with them as a delta front deposit. The apparently low rates of sedimentation in coarse-grained sandstones may suggest colonization of temporarily abandoned parts of the delta front, with diversity perhaps being restricted by lowered salinity or simply very high faunal densities - i.e. amensalism. Planolites I Skolithos ichnofabric (Pl/Skol) (Fig. 21b) Coarse- to medium-grained sandstones with thick micaceous mud-drapes are common in the transitional zone between tidal channel (TC) and inter distributary mouth-bar tidal channel facies (IDMBTC). The sandstones in these settings are commonly rapidly deposited and show a primary P//esc ichnofabric (as described above) that is overprinted by, or contemporaneous with, a Planolites/Skolithos ichnofabric. The Pl/Skol ichnofabric is a syndepositional fabric in which vertical Skolithos burrows and Planolites burrows were contemporaneous with deposition. This is evidenced in some cases by the presence of equilibration behaviour of Skolithos trace-makers producing Monocraterion (Fig. 7ci, ii). The interpretation of this facies as occurring in the distal (seaward) part of the tidal channel is supported by the observation that it is commonly overprinted by the Ophiomorpha ichnofabric (see also Fig. 5b). The palaeoenvironmental conditions inferred for this setting are slightly reduced salinity and high rates of sedimentation, which is reflected

in low ichnodiversity and low intensity of bioturbation. Planolites I Schaubcylindrichnus ichnofabric (Pl/Schau) (Fig. 21b) Ichnofabrics in which the major contributors to the fabric are Planolites and Schaubcylindrichnus are comparatively rare in the He Formation. They may be found in heterolithic facies composed of interbedded ripple cross-laminated sandstone and burrow-mottled mudstone. Bioturbation intensity is comparatively intense, constituting 50-70% of the sedimentary fabric. Ichnodiversity is moderate and includes Planolites, Phycosiphon, Schaubcylindrichnus and rarely Palaeophycus heberti (Fig. 7d). The ripples in this facies are asymmetrical and claydraped with a sparse, syndepositional escape burrow ichnofabric. The moderate ichnodiversity and moderate intensity of bioturbation suggest an equable palaeoenvironment. The inferred palaeoenvironmental setting for this ichnofabric is thus one of a distal distributary mouth-bar (DDMB) in which the effluent currents are fairly weak, and near-normal marine conditions prevail. Planolites/Thalassinoides ichnofabric (PI/Thai) (Fig. 21b) The PII Thai ichnofabric is found in association with tidal channel complexes in the He Formation, and is formed in firmground mudstones. The fabric is composed mainly of Planolites ispp. including large (P. beverleyensis) and small (P. montanus), along with large, slightly oval, sand-filled burrows with a laminated draft-infill interpreted as shallow-tier firmground Thalassinoides ?suevicus (Fig. 8a). In some cases a late Phycosiphon-dominated (Pl/Phyco) ichnofabric can overprint this primary ichnofabric. This ichnofabric is characteristic of the outer reaches of the tidal channels (Fig. 12), and is often associated with Chond/Skol- and Trichbioturbated sandstones (Fig. 4a & 9d). Planolites ITaenidium ichnofabric (Pl/Tae) (Fig. 21b) Interbeds of 2-3 cm thick silty mudstone may rarely be found in association with proximal distributary mouth-bar facies (PDMB). These mudstones are interpreted as fluid mud deposits, which have been formed right out at the distributary mouth through mixing of seawater and clay-rich freshwater. The fact that this is a comparatively rare ichnofacies suggests that the turbidity maximum - in which these mudstone deposits are formed - is rarely forced out as far as the delta front by high fluvial discharge.

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249

Fig. 8. (a) Thick mud with abundant sand-filled Planolites (pi) and ?Thalassinoides (th), constituting the PljThal ichnofabric. (b) PljTae ichnofabric in a thin fluid mud deposit showing moderately intense bioturbation by Planolites (pi), Palaeophycus (ph) and Teichichnus (te). (c) Pl/smAst ichnofabric with small Asterosoma isp. (sm) in a ripple cross-laminated sandstone with escape traces, (d) Rosselia/Asterosoma ichnofabric dominated by Rosselia rotatus (ro) with the diagnostic meniscate fill of the subvertical pipe. Width of core 10cm; all cores displayed with oldest strata bottom left.

These mudstones, once deposited, are typically nutrient-rich and thus, after deposition, are heavily bioturbated by Planolites and meniscate burrows in the equable conditions of the delta front by an ichnofauna comparable to that of the PljPph ichnofabric in adjacent sandstones (Fig. 8b). The ichnofauna itself comprises an opportunistic colonization ichnofauna of Planolites, Palaeophycus, Teichichnus and Taenidium. Although this is not a diverse or complex ichnofauna, its intensity does reflect equable conditions. The range of traces demonstrating a deposit-feeding mode of life of the majority of the infauna distinguishes it from other associated PDMB ichnofabrics. PlanolitesI small Asterosoma ichnofabric (Pl/smAst) (Fig. 21b) The primary ichnocoenosis comprises an intense monotypic fabric generated by Planolites cf. montanus and escape burrows, which is of variable intensity (ranging between 5% and 90%). The Planolites are small in comparison with the P. beverlyensis that make up the idiomorphic Planol ichnofabric. The shallow-tier Planolites can commonly be demonstrated to be cut by small, poorly formed Asterosoma isp. that have

finely laminated thin concentric walls and a comparatively wide central sand-filled burrow (3-5 mm in diameter) (Fig. 8c) and are morphologically distinct from the more classic morphologies of the Asterosoma found in the Rosselia/Asterosoma ichnofabric described below (Fig. 8d). Accessory traces also described from this ichnofacies include rare Teichichnus, which is otherwise recorded only from the PI/ Tae ichnofabric (described above) in fluid mud deposits deposited near the distributary mouth. The absence of Rosselia distinguishes this from the Ross/Ast ichnofabric, which is an important component of subtidal flat facies. Rosselia/Asterosoma ichnofabric (Ross/Ast) (Fig. 21b) Variably sand-rich current-rippled heterolithic sandstones are common in the He Formation and can be found as part of upward-coarsening packages that do not pass into mouth-bar facies with tidal channels but rather into intertidal flat deposits in complete uninterrupted successions. The ichnofauna of these subtidal flats is more diverse than that of similarly heterolithic distributary mouth-bar facies (DMB). The ichnofabric is ubiquitously composed of a primary equilibrium escape-burrowed fabric in

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the ripple cross-laminated sandstones, which is overprinted by a colonization fabric of mid-tier Asterosoma and shallow-tier Rosselia rotatus (Fig. 8d), sometimes with accessory Arenicolites. Both Asterosoma and Rosselia are interpreted as mud-lined burrows of detritus-feeding organisms and dominate the ichnofabric. Interestingly, the only occurrences of Rosselia in the He Formation are in demonstrably subtidal settings, supporting the empirical observations of the author based on the ichnology of the similarly tide-dominated Lajas and Tilje Formations that Rosselia is a good indicator of subtidal settings, particularly subtidal flats.

Siphonichnus ichnofabric (Sipho) (Fig. 21b) Sipho ichnofabrics are characterized by intensely bioturbated horizons (80-100%) with lowdiversity associations of small Siphonites of around 5-8 mm diameter, commonly with Planolites. The eponymous trace fossil is a vertical trace with a meniscate-lined tube wall in which the meniscae are orientated convex down (Figs 7b, 9a). The original description of this trace attributed these meniscae to protrusive burrowing into the sediment through the life and growth of a bivalve (Stanistreet et al. 1980), though the present material differs somewhat from the type material in being thinner, deeper

Fig. 9. (a) Vertical Siphonichnus (si) originating from a mud-rich abandonment horizon (Sipho ichnofabric) and cutting through it. Notice also the large 'crab burrow' disturbing the horizon at the bottom of the photograph, (b) ThaljChond ichnofabric with small Thalassinoides (th) developed from a burrow-mottled horizon in a tidally bundled trough cross-bedded tidal channel sandstone with rare Planolites (pi) burrows, (c) Intensely bioturbated DDMB/OTDC facies comprising heterolithic ripple cross-laminated sandstones and mudstones with large Thalassinoides (th) along with Phycosiphon (pc), overprinting an earlier ichnofabric with escape burrows (lo), Planolites (pi) and Asterosoma (as), (d) Sparsely bioturbated cross-bedded sandstones with isolated mud-filled Trichichnus (tr). Width of core 10cm; all cores displayed with oldest strata bottom left.

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and more curved. The ichnofabric itself is uncommon in the He Formation and, where present, is commonly found to be associated with facies shifts attributable to sudden reduction in sediment supply. Two examples of such a scenario are tidal channel abandonment and delta lobe abandonment: cases of both are suggested to have occurred in the He Formation, and are both probably autocylic in nature. Owing to the hiatal nature of the surfaces associated with Siphonichnus, bioturbation intensity tends to be great and ichnodiversity typically increases markedly from the same surface upward into the abandonment facies. Thalassinoides I Chondrites ichnofabric (ThaljChond) (Fig. 21b) Ichnofabrics composed of small, deep-tier Thalassinoides and penecontemporaneous Chondrites are unusual components of tidal channel facies in the He Formation. They are normally postdepositional ichnofabrics associated with the upper surface of coarse-grained cross-bedded sandstones with tidal bundling (see Fig. 9b). The presence of Thalassinoides in tidal channel sandstones is unusual. Its close relative Ophiomorpha is more common in this setting as its pelletal wall lining is an adaptation for life in mobile substrates. The presence of Thalassinoides could therefore be taken to indicate that the sandstones containing this ichnofabric may have undergone early diagenesis or were otherwise particularly firm at the time of burrowing. The well-developed shallow-tier burrow-mottling, found as part of the same ichnofabric, would appear to support the inference of low sedimentation rates and a hiatus prior to development of the ThaljChond ichnofabric. This ichnofabric is typical of tidal channel facies in the He and is commonly associated with the ChondjSkol ichnofabric. Thalassinoides I Phycosiphon ichnofabric (ThaljPhyco) (Fig. 21b) Thalassinoides I Phycosiphon ichnofabrics are typically intensely bioturbated with a diverse ichnofauna dominated by the Thalassinoides and Phycosiphon along with Palaeophycus heberti, Schaubcylindrichnus and Palaeophycus isp. (Fig. 9c). The ichnofabric is found in heterolithic ripple cross-laminated sands interpreted to have been deposited in the distal distributary mouth-bar to proximal prodelta region. The intense bioturbation and high ichnodiversity identify this ichnofabric as characterizing a fully marine palaeoenvironment with good living conditions for the infauna.

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Trichichnus ichnofabric (Trich) (Fig. 21b) This ichnofabric is found exclusively in association with beds of coarse to very coarse sandstone with granule/pebble lags in upward-fining successions 4-6 m in thickness. These sands may also contain 1-10 cm thick beds of homogeneous, unbioturbated mudstone layers. The ichnofabric itself is composed entirely of mudfilled Trichichnus burrows with colonization surfaces at sand-sand contacts. Levels of bioturbation are typically low, seldom exceeding 10% of the sedimentary fabric (Fig. 9d). The paucity of bioturbation and the monotypic nature of the assemblage suggest that environmental conditions during deposition of this facies were particularly harsh, and allowed colonization only during restricted periods. The broad-scale upward-fining packages in which this facies is found suggest that it was deposited in a tidal channel that - given the evidence for mixing of fresh- and saltwater provided by the presence of fluid muds - was strongly influenced by freshwater and would have been variably brackish during the tidal cycle (see below). Thus the major controls on the infauna are likely to have been current strength and either periodically or permanently reduced salinity. These factors served to exclude other trace-making organisms from this facies, with the Trichichnus utilizing rare colonization windows afforded by amelioration of environmental conditions. The absence of bioturbation in the nutrient-rich fluid mud deposits highlights this as a particularly hostile palaeoenvironment, even harsher than that of the PIj Thai ichnofabric described above. Problematic ichnofabric (Probl) The top of the He Formation is characterized by a distinctive ichnofabric, containing abundant Thalassinoides burrows, that is reworked by a small, unnamed, mud-filled trace of variable morphology that commonly shows numerous upward-directed projections from a curved subvertical stem. Other traces include resting traces Rhizocorallium irregulare and Teichichnus isp. (Fig. 10). The mud-rich nature of this facies invites comparison with the central basin of a wave-dominated estuary (Fig. 11), but this remains somewhat tentative, as is discussed in more detail in the sedimentology section below. Sedimentology of the lie Formation on Kristin Field Sedimentary facies of the He Formation can be grouped according to broad-scale sedimentary

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Fig. 10. Possible estuarine facies with abundant bioturbation by Thalassinoides (th), Rhizocorallium (rh) and an unidentified small feathery mud-filled burrow (fy). The core is 10cm wide.

environment and subdivided with respect to sedimentary facies, which in turn can be split into subfacies based on a combination of their sedimentology and ichnology. Tidal range can be estimated where continuous successions from subtidal to supratidal facies can be determined (i.e. the stratigraphic thickness from subto supratidal approximates tidal range) (Klein 1971). In the case of the He Formation on Kristin Field intertidal facies are scarce but, where they are present, the thickness of intertidal facies is seldom more than 2-3 m, suggesting a microtidal regime. The He Formation comprises a diverse array of sedimentary palaeoenvironments similar to those described from the Jurassic Lajas Formation of the Neuquen Basin, Argentina (Mcllroy et al. 1999). The range of sedimentary facies is

almost identical, but their relative volumetric importance differs between the two systems. What is remarkable is that, despite the low tidal range, sedimentation of the He Formation on Kristin Field is still dominated by tidally modulated flow. This is taken to indicate that the basin was sheltered from wave action and probably had a comparatively small width in the direction of the prevailing winds. Through sedimentological assessment of the He Formation carried out during this study, a conceptual model for the spatial distribution of the observed sedimentary facies has been constructed (Fig. 12). It must be noted, however, that the diagram represents a freeze-frame and does not truly represent the extent of sedimentary facies in ancient deposits (e.g. meander belts extend below the tidal flats). The diagram does not account for the relative preservation potential of these facies. For example, in a tidal deltaic setting the tidal channels will - with progradation - cannibalize their own distributary mouth-bars (Fig. 13), resulting in a delta top sand sheet of amalgamated tidal channels (cf. Monahan et al. 1993). The major sedimentary facies associations documented from the He Formation are tidally influenced fluvial/tidal channels (tidal currents being suppressed by riverine outflow), tide-dominated intertidal to subtidal flats, and distributary mouth-bar through delta front/prodelta to offshore (Fig. 12). These facies associations can be broken down further through careful sedimentary analysis and further subdivided by a combined sedimentological and ichnological approach (see Fig. 14). The full range of sedimentary facies distinguished during this study is presented below (see Table 2).

1. Delta front palaeoenvironments The delta front includes the most distal facies represented in the He Formation on Kristin Field, with the highest ichnodiversity and some of the highest intensities of bioturbation recorded. Sedimentation of these deltaic facies occurs though rapid deposition of sediment at the interface between the fluvial and marine realm. The most distal facies represented are strongly heterolithic and variable in sandstone content. Superficially, this facies association resembles the offshore transition zone of classic wave-dominated systems, but the sediments are devoid of the diagnostic hummocky cross-stratification (HCS) (cf. Dott & Bourgeois 1982), and contain indicators of tidal processes such as mud

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Fig. 11. Idealized model for distribution of fades in a wave-dominated estuary (redrawn from Dalrymple et al. 1992). This depositional setting is only one of several protected back-barrier scenarios; determining the most likely will require a regional perspective beyond the scope of this work.

couplets and thinly laminated beds showing tidal bundling. Distinguishing between shoreface and deltaic environmental settings is critical for realistic reservoir modelling of the He Formation (see also Bann & Fielding 2004). Shoreface settings should contain extensive coast-parallel reservoir sandstones, whereas a tide-dominated delta will contain numerous smaller sandstone bodies oriented perpendicular to the coastline, with the concomitant implications for production planning. la. Proximal distributary mouth-bar fades (PDMB) This sedimentary facies is characterized by parallel-laminated to trough cross-bedded medium- to coarse-grained sandstones, commonly associated with micaceous mudstone horizons up to 10cm thick (Fig. 15a). Bioturbation is irregular in distribution and is commonly either intense or absent. The most common trace fossils are Skolithos, Ophiomorpha, Chondrites, Trichichnus, Piano lit es, Palaeophycus and Taenidium. In distributary mouth-bar settings, currents are ebb-dominated and salinities may be extremely variable, dependent on effluent discharge. Sedimentation rates are commonly high owing to the rapid deceleration of ebb currents upon meeting the effectively stationary marine waters, and flocculation of clays is common (e.g. Gibbs 1977; Martinius et al. 2001). The net effect of these palaeoenvironmental conditions is that facies deposited in such environments in the He

Formation contain a low-diversity ichnocoenoses indicative of stressed conditions. The variable salinity and high sedimentation rates encourage opportunistic colonization by organisms with a generalist feeding strategy and trace-making organisms with a tolerance for salinity stress. This environmental diversity also accounts for the disparity of ichnofabrics found in this facies. Ichnofabrics associated with this sedimentary facies are Chond/Skol, Ophio, OphiojTae, Ophioj Chond, Pl/Skol, Pl/Tae, Pl/Pph and Planol, as described in the ichnofabrics section above. Ib. Central distributary mouth-bar facies (DMB) Medium- to fine-grained sandstones with clay drapes, ripple cross-laminations and abundant escape burrows are characteristic of this facies (Fig. 15b). Climbing ripples and occasionally herringbone cross-lamination are recorded from this facies, which - like PDMB - is largely sand-dominated with occasional thin (and sometimes highly micaceous) mudstone horizons. The ichnofauna is typically more diverse than in proximal mouth-bar settings (PMB), and bioturbation is more constant in its intensity at 30-50%. In the central part of distributary mouth-bars, deceleration of the current may allow deposition of both the tractional and suspended load (e.g. Wright 1977). The sediment plume emitted from the feeder channel can be either buoyant or dense depending on suspended load and

Fig. 12. Conceptual fades model for the relative positions of fades in the tide-dominated deltas of the He Formation on Kristin Field. Acronyms are facies abbreviations as used in the text.

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Fig. 13. Model to explain the sharp juxtaposition of tidal channel fades (lie Formation) upon mouth-bar fades (Ile-Ror Formation) in relation to the interplay between available accommodation space and channel incision. Diagrams a-d and e-h are in time-stratigraphic order with the oldest at the base, (a-d) Show autocyclic development of a delta front in a low accommodation space setting with a sharp grain-size change expected between delta front and feeder channel as seen in the tide-dominated deltas at the base of the He Formation, (e-h) Represents a similar, but high accommodation, setting in which more of the delta front is preserved as seen in the Lajas Formation, Argentina (Mcllroy unpublished).

salinity contrast between the outflow and marine waters, and is invariably tidally modulated to some degree (see discussion in Brettle et al. 2002). The more distal position of this fades relative to the PDMB is inferred based on the smaller size of bedforms and higher ichnodiversity. The increased ichnodiversity in this facies probably reflects the more normal marine salinities found further out from the distributary channel mouth. In addition, rates of sedimentation are likely to be lower and more episodic (than in DMB), which encourages colonization (Levington 1970; Grassle & Grassle 1974; Frey & Goldring 1992). Ichnofabrics associated with this sedimentary facies are P//esc, Gyro/Qsc, Pl/smAst and Sipho (following lobe switching) as described above. Ic. Distal distributary mouth-bar facies (DDMB) This is a heterolithic facies composed of thinly interbedded fine-grained ripple cross-laminated and parallel-laminated sandstones interbedded with bioturbated siltstones and mudstones (Fig. 15c). The proportion of sandstone in this facies is defined as being less than 50%. Bioturbation is moderate to intense and ichnodiversity is typically comparatively great (60-90%). The parallel and ripple cross-laminated sandstones both contain mud-couplets and are much less

bioturbated than the intervening mudstones and siltstones. These heteroliths have Gyro/esc, P//esc, Diplo, P. herbijSchau and Pl/Schau ichnofabrics. The parallel-laminated sandstone horizons are interpreted as resulting from deposition by suspension fallout from a decelerating buoyant effluent plume, which from the presence of mud-couplets can be see to be tidally modulated and deposited in a subtidal setting (cf. Brettle et al. 2002). In contrast, the ripple crosslaminated sandstones indicate deposition from traction, showing that the depositing flow was highly variable in character (Fig. 15b). It must be emphasized that thinly interlaminated sandstones with clay layers can look similar to hummocky cross-stratification (HCS) in core because bedding surfaces in both are commonly very low amplitude and/or long wavelength (cf. Dott & Bourgeois 1982; Brettle et al 2002). The physical sedimentological processes responsible for these two bedform styles are quite distinct, however, and allow discrimination between wave- and tide-dominated deltaic settings. Hummocky cross-stratification is not exclusive to wave-dominated depositional systems; indeed HCS can occur in estuarine and lagoonal settings during exceptional events. However, its general absence in offshore heterolithic facies of the He

Fig. 14. Conceptual fades model for the relative positions of fades and ichnofabrics in the tide-dominated deltas of the He Formation on Kristin Field. Acronyms are ichnofabric abbreviations as used in the text.

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Table 2. Palaeoenvironments and fades of the He Formation along with fades abbreviations as used in the text Inferred environment of deposition

Fades

Abbreviation

Deltaic

Central distributary mouth-bar Distal distributary mouth-bar Proximal prodelta Proximal distributary mouth-bar Anomalous massive bedded sandstone Intertidal mud flats Intertidal sand flats Subtidal flats Offshore tide-dominated coastline Strongly fluvially influence channels Tidal channels Inter-distributary mouth-bar channels Abandoned tidal channel ?Central basin ?Bayhead delta

DMB DDMB PPD PDMB

Tidal flats

Channels

? Estuarine

Formation on Kristin Field precludes the presence of a normal wave-dominated shoreface. In the present case, the strong tidal signature in the rest of the overlying lie Formation, and the lack of typical mid-lower shoreface facies with HCS, are taken as evidence that sedimentation occurred in the distal parts of a tide-dominated delta similar to the Fly River Delta (Alongi 1991; Harris et al. 1993) and the lower Lajas Formation (Mcllroy unpublished). Characteristic ichnofabrics of this distal distributary mouth-bar sedimentological setting in the He Formation on Kristin Field include Diplo, P. herbi, PljSchau and P. herbijSchau. Id. Proximal prodelta (PPD) The proximal prodelta is defined herein as the region in which biogenic reworking of sediment through bioturbation destroys most of the sedimentary laminations. Grain size is typically silt-very fine sand, and the sediment tends to be mud-rich, with mud being mixed into the sediment by bioturbation. Intensity of bioturbation is typically great (90-100%) and ichnodiversity is high (Fig. 6a). Ichnofabrics in this setting are complex and include Phoe and Thal/Phyco as described above. The occasional thin, finegrained sandstone horizons that are preserved comprise ripple cross-laminated sandstones with clay-draped foresets.

2. Tidal flat palaeoenvironments Tidal flats are normally defined as that part of the marine depositional system that is influenced

ITMF ITF STF OTDC FIC TC IDMBTC ATC CB BHD

by tidal sedimentation and may be subject to subaerial exposure during part of the tidal cycle. Herein, in line with some earlier authors (Reineck 1967; Klein 1971; Weimer et al. 1982), this is extended to cover the subtidal portion of a sedimentological continuum that extends below the reach of even the spring tides. In most normal marine systems storm-dominated sedimentation would be expected in this subtidal regime; however, in the He - and in similar tidedominated systems such as the Tilje Formation (Martinius et al. 2001) and the Lajas Formation (Mcllroy et al. 1999) - wave energy is subordinate to tidal energy even in the shallow subtidal realm (Fig. 16). The volumetric predominance of subtidal facies over intertidal/supratidal facies in the He Formation on Kristin Field is taken to indicate a small tidal range as discussed (Fig. 16). This of course assumes incremental increases in accommodation space rather than gradual accommodation space generation, which could then be filled by aggrading intertidal flats. For example, in a microtidal setting, a 10m flooding event would result in 10 m of accommodation space, of which approximately the upper 2m would be filled with intertidal facies and the remainder would be composed of subtidal sediments (Fig. 16). In addition, the upper portion of a succession is most likely to be subsequently eroded during flooding ravinement or channel erosion. 2a. Intertidal mudflats (ITMF) This uncommon He facies is composed of interbedded mudstones and fine- to medium-grained ripple cross-laminated sandstones with neither

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Fig. 15. Sedimentary facies of the deltaic facies association: (a) proximal distributary mouth-bar facies (PDMB) with rip-up clasts and fluid muds at the base and fining upward to parallel-laminated micaceous sands and silts; (b) central distributary mouth-bar facies (DMB) showing typical transitions from low-angle cross-laminated sands and muds showing tidal bundling into wave and current ripple cross-laminated sandstones; (c) distal distributary mouth-bar facies (DDMB) showing intense bioturbation of fine sands with remnants of ripple cross-laminations and thin fluid mud deposits. Width of core 10cm; all cores displayed with oldest strata bottom left.

bioturbation nor rootlets (Fig. 17a). The ripples are primarily oscillation ripples that are inferred to have been formed by wave action in shallow water at states of high tide. This facies is known from only two decimetre-thick horizons in the He on Kristin field; in both cases it is found at the top of a blocky or upward-fining succession of intertidal sandflat facies in which bioturbation, grain size and bed thickness grade into this facies.

The rarity of this facies in the He Formation is probably a function of a variety of parameters discussed below, including the inferred microtidal range. The propensity for mudflats to be eroded by both auto- and allocyclic processes exacerbates their infrequency. Erosion is due to either their association with tidal channels or their position at the top of parasequences where they stand to be eroded either during or after relative sea-level rise (Mcllroy et al 1999).

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Fig. 16. Idealized prograding tidal flat successions in tide-dominated macrotidal settings, tide-dominated microtidal settings and wave- and tide-influenced macrotidal settings. The variable grain-size profiles and facies associations can be used to help determine depositional setting.

2b. Intertidal sand flats (ITF) This facies comprises thin successions of fine- or medium-grained sandstone with trough crossbedding, ripple cross-lamination (both wave and current ripples), and rarer planar-laminated beds. Grain size within a given intertidal sandflat succession is generally uniform but may show upward-fining trends up to 2m thick, passing either into subtidal flats or into intertidal mudflats depending on stratigraphic setting. The foresets of trough cross-beds are commonly, but not always, clay draped, and bioturbation

is typically of low intensity and low diversity (Fig. 17b). Occasional thick clay beds, up to 5 cm in thickness, are also documented. The range of sedimentary structures found in these sandstones is indicative of variable, but generally high, flow regimes, and they grade into channel-margin deposits. The abundance of escape traces related to the bivalve trace fossil Lockeia is also taken as supportive evidence. The typical intertidal sandflat ichnofacies of the He Formation is P//esc with Trich, Sipho and Chond/Skol sometimes present (described

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Fig. 17. Sedimentary facies of the tidal flat fades association: (a) intertidal mudflat facies (ITMF) with parallel and ripple cross-laminated interbedded mudstone and sandstone; (b) intertidal sandflat facies (ITF) showing a mineralogically immature sandstone with parallel (upper flow regime planar beds) and trough cross-bedding interpreted to form adjacent to tidal channels; (c) subtidal flat facies (STF) showing the typical heterolithic ripple cross-laminated sandstones with clay-draped ripple cross-laminae and abundant diverse bioturbation. Width of core 10cm; all cores displayed with oldest strata bottom left.

above). However, this facies is also commonly completely unbioturbated, when it is characterized solely on physical sedimentary structures and the rather aggradational stacking patterns

of beds typical of tidal sandflats. The restricted ichnodiversity is probably due mainly to temperature stress on the intertidal flats during low tides, as discussed (cf. Johnson 1965).

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2c. Subtidalflats (STF) Sub tidal flat fades are typified by thinly bedded, very fine to medium-grained sandstones, interbedded with siltstones and mudstones. The sandstones are 2—5cm thick and are wave and current ripple cross-laminated, commonly with mud-couplets on their foresets and abundant escape burrows. The upper surfaces of sandstone beds are characterized by opportunistic styles of colonization with ichnofabrics rich in mud-lined burrows, especially Rosselia and Asterosoma, that colonize the upper surface following sand deposition (Fig. 17c). The presence of mud couplets and the comparatively high ichnodiversity attest to the subtidal setting of this ichnofacies, and the absence of HCS precludes deposition in a storm-dominated offshore transition zone (Dott & Bourgeois 1982). The ichnodiversity is greater than is typical of He Formation sandy intertidal flat settings on Kristin Field. The sand-mud proportion is much lower, and ichnodiversity higher, than the lithologically similar distal distributary mouth-bar facies described above. The ichnofabrics associated with He Formation subtidal sandflat facies are RossjAst, PI/ esc, Pl/Tae and Planol 2d. Offshore tide-dominated coastline (OTDC) This facies is characterized by intense bioturbation with rare thin ripple cross-laminated horizons of fine-grained sandstone. Ichnodiversity is high and ichnofabrics are typically rich in Phycosiphon (Fig. 17d). Although it is clear that, away from the major axes of progradation (deltas), intertidal flats and subtidal flats with their distinctive sedimentology form the normal prograding coastline (Fig. 12), no pre-established model is in existence for how such facies change offshore in a tide-dominated setting (see Fig. 16) and grade laterally into prodelta environments. This facies represents the equivalent of the offshore transition zone of storm-dominated settings. Distinguishing this facies from the proximal prodelta is difficult on both ichnological and sedimentological grounds. The complete absence of parallel/low angle laminated (suspension fallout) distal distributary mouth-bar sandstones is one possible criterion (cf. Fig. 15b) but is unlikely to be practical owing to the high intensity of bioturbation in normal marine settings (see also Bann & Fielding 2004). Ichnofabrics associated with offshore tidal coastlines are likely to be similar to those of prodelta settings within the same formation. Salinity stress, sedimentation rates and palaeooxygenation are likely to have been the

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controlling (though weak) environmental stresses in both geomorphological settings. Ichnofabrics documented from the base of prograding subtidal flat successions include Phoe, P. herbij Schau and Thal/Phyco.

3. Tidal channel palaeoenvironments The lower-middle portion of the He Formation on Kristin Field comprises numerous stacked upward-fining, coarse- to very coarse-grained, trough cross-bedded sandstones (Fig. 3) with anomalous mudstones/siltstones. These upward-fining successions have erosional bases with coarse pebbly lags, sometimes with woodclasts, and are interpreted as channel sandstones deposited in meander belts on the delta top. In some cases, tidal signatures can be demonstrated in the form of tidal bundling and mud-couplets (cf. Allen 1981), but more commonly in the He Formation their tidal origin is betrayed by the presence of thick mudstones interpreted as fluid mud deposits (Allen et al 1980; Wolanski & Gibbs 1995) and the presence of some marine trace fossils. Fluid mud deposits are interpreted as having formed within the turbidity maximum of a tidal channel in brackish conditions by rapid flocculation (cf. Allen et al 1980; Wolanksi & Gibbs 1995). Tidal signatures are not, however, ubiquitous within such channel sandstones, and some intervals of sandstone show no physical sedimentological evidence that sedimentation was tidally influenced. This is particularly true of the most coarse-grained facies within the He Formation. One of the challenges, therefore, is the detailed characterization of these sandstones in an attempt to be able to resolve facies-stacking patterns within multi-storey aggradational fluvial/tidal channel sandstones. Tidal channels can show a range of sedimentary structures characteristic of flow within a channel, but few of these as seen in core are diagnostic of high-resolution proximal-distal trends (Howard & Frey 1973; Frey & Howard 1986). According to idealized facies models of marginal marine tidal channels (e.g. Dalrymple et al. 1992), channels are typically meandering at their proximal end, becoming straighter towards the sea - from which some interpretations about likely facies architecture could be extrapolated. However, the rheology of the background delta top sediment can strongly dictate the geomorphology of tidal channel deposits. By analogy with fluvial systems, channels would be expected to have narrower meander belts in mud-rich background facies (e.g. Fenies & Faugeres

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1998). It should also be noted that the influence of tidal currents can often be felt large distances into the delta plain through the slowing of riverine flow during spring tides in a macrotidal regime. The landward limit of such tidal effects is known as the tidal reach and influences the sedimentology of tidal channels far beyond penetration of the salt wedge (e.g. Harris et al. 1993). The penetration of the salt wedge is a function of discharge volume versus angle of the delta plain and tidal range. Ichnological characterization of the channel sandstones and their contained mudstones/ heteroliths enables identification of a diverse array of tidal channel ichnofacies that occur along the gradient of hydrodynamic and chemical conditions found in the transition from fluvial through brackish to marine conditions within such channels. This is due to the sensitivity of burrowing infaunal organisms to salinity changes (Milne 1940; Dorjes & Howard 1975). With these factors in mind, sharp-based upward-fining successions typically described

simply as tidal channels are described and classified based on combined study of their physical and biogenic sedimentary structures (Fig. 12). 3a. Fluvially influenced channels (FIC) FIC are composed of very coarse- to coarsegrained sandstone facies with thick trough cross-bedded sandstone beds, gravel lags with woody debris, which may be rounded into wood clasts. Neither the foresets nor toesets of bedforms are mud draped. The sandstones are entirely unbioturbated and show only unidirectional palaeocurrents. Sandstones of this facies are most easily interpreted as proximal tidal channel facies with a strong fluvial influence. The absence of fluid mud deposits and diagnostically marine trace fossils in these coarse sandstones suggests highly reduced salinity or even freshwater conditions at the channel floor and deposition landward of the turbidity maximum (see Schubel 1968). The fact that the coarsest sediments in

Fig. 18. Sedimentary facies of the tidal channel facies association, (a) Tidal channel facies (TC) showing typical trough cross-bedding and thick fluid mud deposits in between bedsets and low intensities of bioturbation. (b) Modern channel-margin tidal flats of the River Dee Estuary, Wirral, UK, showing partially reworked fluid-mud deposits formed in the summer of 2001 and both semi-lithified by October 2001. (c) The same deposits were highly reworked by November 2001 and were common as clasts associated with ebb-tidal dunes, (d) Succession of cross-bedded tidal channel facies with bioturbated fluid mud deposits overlain by highly bioturbated abandoned tidal channel facies (ATC). Width of core 10cm; all cores displayed with oldest strata bottom left.

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the He Formation on Kristin Field are to be found in this fades also supports its inferred proximality. 3b. Tidal channels (TC) This fades comprises medium- to coarse-grained sandstones organized in upward-fining sedimentary packages with erosional bases and lags of extra-basinal pebbles, rounded mud clasts and plant debris. Sandstone beds are generally trough cross-bedded with clay- or organic debris-draped foresets. The clay drapes may be organized into demonstrable tidal bundles, sometimes with superimposed ripple crosslaminations. Sandstones may also be interbedded with 1-10 cm thick mudstone or rippled muddy siltstone horizons (Fig. 18a). Bioturbation is typically of low intensity and low diversity, with a predominance of vertical burrows and escape burrows in sandstone beds. The ichnofaunal assemblage of the associated mudstones is highly variable, showing colonization while soupground or softground conditions prevailed (e.g. Phyco, P. herbi and Pl/Phyco ichnofabrics), or representing firmground assemblages of open sand-filled burrows (e.g. Planol, PI/Thai ichnofabrics) as discussed above (Figs 19, 20). The presence of such thick mudstones in association with coarse-grained sandstones is apparently anomalous. However, sedimentation of faecal pellets, and rapid flocculation in the turbidity maximum of brackish environments, coincident with decreased current velocities associated with states of high and low tide, can allow thick layers of mud to accumulate (e.g. Kranck 1981; Edelvang & Austen 1997). These water-rich muds, known as fluid muds, can be up to 1m thick in the Fly River depositional system (Wolanski & Gibbs 1995) and are common in most estuaries to some degree. These fluid mud deposits become rapidly dewatered and compacted in intertidal settings and undergo early lithification over a period of weeks (unpublished observations from the Dee Estuary Cheshire, UK). Erosion within tidal channels either at the base or on channel margins can rework such semi-lithified muds into mud clasts that are commonly seen in the toesets of duneforms at the base of tidal channels but also occasionally on channel-margin tidal flats (Fig. 18b, c). The ichnology of the tidal channel sandstones as described above is invaluable in distinguishing between tidal channel facies and aids recognition of tidal channel stacking patterns, which are notoriously difficult in multi-storey channel deposits. Ichnofabrics recognized in the He Formation tidal channels are the Trich, Chond,

Fig. 19. Conceptual model showing the distribution of tidal channel ichnofabrics from both the fluid mud deposits and channel sandstones. Symbols as in Fig. 3.

Fig. 20. Vertical cross-sections showing multi-storey tidal channel successions and their ichnofabric stacking patterns with interpretation. Based on a composite database from Kristin Field. Facies hatchings as in Fig. 12.

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P//esc, Thal/Chond, Chond/Skol, Planol, PI/ Phyco, PI I Thai and Phyco ichnofabrics described in the previous section (see also Figs 19, 20). 3c. Abandoned tidal channels (ATC) In the He Formation some upward-fining successions with erosional bases overlain by trough cross-bedded sandstones, commonly with coarse gravel lags and thick mudstone laminae/ drapes 1-5 cm in thickness, are capped by more clay-rich facies (Fig. 18d). The sand: mud ratio is generally less than 60% sand, but in some cases much higher (approximately 80% sand): in such cases the sand-rich channel fill is ichnologically rather similar to subtidal flat facies but is distinguished by the more variable grain size and slightly lower ichnodiversity. The sedimentological character of the lower portion of an abandoned channel typically concords with one of the types of active tidal channel described above. The marked increase in intensity of bioturbation associated with the abandonment event is thought to reflect avulsion at the proximal end of the tidal channel. Following abandonment, channels are influenced primarily by marine waters that are much less hostile to animal life, and, as a result, a diverse ichnofauna can develop. This is in marked contrast to the salinity stress and high sedimentation rates that are typical of more active fluvially connected tidal channels. A similar case is currently present in the Fly River Delta, in which the abandoned portion of the delta contains relict channels that support a dominantly marine ichnofauna (Alongi 1991). Ichnofabrics commonly found in association with the post-abandonment phase of channel fill are Sipho, Phoe, ThaljPhyco, Ross/Ast, Planol and Pl/Skol, as described above. 3d. Inter-distributary mouth-bar tidal channels (IDMBTC) Medium- to fine-grained upward-fining sandstone successions with trough cross-bedding and ripple cross-laminations or planar crosslaminations characterize IDMBTC facies. Foresets of most bedforms are draped in highly micaceous clay (Fig. 18e). These thick micaceous horizons may be up to 10cm thick in extreme cases and are usually unbioturbated or show only a Planol ichnofabric (described below) with large sand-filled Planolites cf. beverleyensis. The foresets of bedforms are largely unbioturbated but may show opportunistic colonization ichnofabrics including Chond/Skol, Ophioj Chond, Pl/smAst and equilibrium colonization as PI/esc ichnofabrics (see discussion of colonization in Mcllroy 2004).

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The ichnofabrics documented from this facies are transitional between those from proximal delta front mouth-bars and those from more proximal tidal channels. It is clear that there is significant palaeoenvironmental stress on the trace-making infauna of this facies, as evidenced by the low ichnodiversity and low intensity of bioturbation (0-30%). The upward-fining nature of the sedimentary packages suggests deposition in a channel setting in distributary channel thalwegs between laterally and downstream accreting longitudinal bars. This is also borne out by the high flow regime, as indicated by trough cross-bedding and planar lamination. Lithological and ichnological similarities with proximal mouth-bar facies and tidal channel facies suggest a transitional location at the distributary mouth, where channel forms are present between the emergent bars at the delta front.

4. ?Estuarine palaeoenvironments The uppermost beds of the He Formation on Kristin field comprise an unusual association of facies that is not seen at lower stratigraphic levels. It is also seen on adjacent fields (e.g. Smorbukk). The facies present are inferred to be broadly estuarine in character in that they show evidence of harsh palaeoenvironmental conditions - as evidenced by the ichnology as described below. In addition the facies typically overlie a coarse-grained lag that may be interpreted as either a ravinement lag or an erosional lag caused by incision during relative sea-level fall. No evidence was available to reliably determine which is the case. The facies association as a whole is dominated by wave-generated bedforms, and suggests that the whole depositional system may have changed to one in which wave action was no longer suppressed. The broad-scale depositional setting envisaged for the facies described below is one of a backbarrier environment with significant salinity stress, predominantly low current strength and perhaps hypoxia, as is common in, but not exclusive to, protected microtidal estuarine settings (e.g. Diaz & Rosenburg 1995). A candidate depositional environment is represented in Fig. 11. 4a. ?Central basin (CB) The uppermost few metres of the He Formation are characterized by an unusual sedimentary facies (Fig. 10), as developed on Kristin, Mikkel, Smorbukk and Sm0rbukk S0r fields (see Fig. 1) at the transition to the overlying Not Formation (Fig. 2). The succession is

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Fig. 21. (a) Example ichnofabric constituent diagrams of idealised ichnofabrics from the He Formation on Kristin Field.

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Fig. 21. (b) Example ichnofabric constituent diagrams of idealised ichnofabrics from the He Formation on Kristin Field.

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broadly upward coarsening and overlies a horizon with evidence for erosion in another Kristin well, but is marked by coal deposition in other parts of Haltenbanken (Midgard Field; Fig. 1). The surface itself is typically overlain by a coarse lag of granule- to pebble-sized extrabasinal clasts, commonly in a muddy matrix. Alternatively, this surface may be a palimpsest colonization surface with Pl/smAst ichnofabrics being overprinted by Phyco and P. herbi ichnofabrics. The sediments above the coarse basal lag are typically dark, mud-rich and heavily bioturbated by Thalassinoides and a problematic feather-like mud-lined vertical burrow (Probl ichnofabric; Fig. 10). There are some thick unbioturbated mudstone horizons, a high proportion of detrital organic matter and thin stringers of very coarse sandstone. The lower bounding surface of this facies is erosional on a regional scale and shows a succession of ichnofabrics that appear to demonstrate a flooding event. The coarse sedimentary layer itself could be interpreted as either a channel lag formed during sea-level fall or a ravinement lag formed by winnowing during sea-level rise. At present this is not possible to determine through lack of a more regional perspective. The overlying sediments are highly bioturbated (70-90%), but ichnodiversity is low, suggesting hostile conditions ideally suited to a few specialist species (Levington 1970) and probably due to low salinity and/or oxygenation in conjunction with low sedimentation rates and palimpsesting of identical ichnocoenoses due to aggradation (see Mcllroy 2004). The trace fossils recorded are Thalassinoides and Rhizocorallium, which are normally considered to be marine in nature, and the feather-like trace (Fig. 10). Few sedimentary structures are present, but the marked contrast in grain size is suggestive of a normally quiescent environment that is subject to periodic events capable of transporting large grain sizes. The sedimentological setting that best fits the range of sedimentary and chemical conditions is that of a protected central basin environment (cf. Zaitlin & Schultz 1984, 1990). The central basin of a wave-dominated estuary is influenced by marine waters but protected from strong currents by the presence of what is effectively a standing body of water constrained on the seaward side by a barrier bar. Occasional storm events may account for the thin but very coarse sand stringers present in this facies, either through increased transport from the fluvial system (through the bayhead

delta) or as storm wash-over deposits (cf. Andrews 1970; Hubbard & Barwis 1976; Carter 1978). 4b. ?Bayhead delta (BHD) Upward-coarsening mud-rich sandstones may overlie the mudstones of facies 4a (CB) at the top of the He Formation. The sandstones are largely unbioturbated and show symmetrical oscillation-rippled surfaces. Core recovery is poor in this interval, however, and interpretation is based on well-log signatures and comparison with adjacent fields. The dearth of bioturbation and coarse grain size of this facies may be attributed to deposition in a proximal estuarine setting, in which a strong freshwater influence may have discouraged the infauna (see review in Mangano & Buatois 2004). Given the preferred interpretation of the underlying sediments as having been formed in a central basin setting, such an upward-coarsening fluvially influenced succession may be interpreted as a bayhead delta.

Anomalous massive bedded sandstone facies The last of the facies described from the He Formation is an anomalous facies comprising stacked massive sandstones. The unbioturbated sandstones are medium to coarse grained and of remarkably even grain size. They occasionally contain faint clay drapes or thick muddy ripple cross-laminated heteroliths. This facies is commonly associated with the IDMBTC facies and as such is probably deposited at the distributary mouth by rapid deceleration of currents with high suspended concentrations of well-sorted sediment. Given that the sands are completely unbioturbated and without sedimentary structures, objective interpretation cannot extend beyond extrapolation from common association with other facies, to which the anomalous facies is presumed to be genetically related.

Application of ichnofabric stacking patterns In attempting to resolve the interrelationships of facies the known environmental tolerances of trace fossils and community structure (described as ichnofabric) have been combined with information from sedimentary structures and common association of facies (ichnofabrics) using Walther's law. Once a model for the distribution of both sedimentary facies and ichnofabrics has been constructed (Figs 12, 14), the

A JURASSIC TIDE-DOMINATED DELTA, NORWAY

vertical stacking of facies and their ichnofabrics in core can be used to assess changes in relative sea-level. During the course of this study special focus was centred on the ichnology of centimetrethick mudstone horizons within tidal channel successions. This enabled production of a detailed model for the spatial distribution of tidal channel ichnofabrics (Fig. 19). This conceptual model for the distribution of tidal channel ichnofabrics can be used to interpret stacking patterns (Fig. 20). This technique is particularly pertinent to the He Formation, owing to the homogeneity of the lower (petroleum-bearing) tidal channel-rich interval (Fig. 3). Throughout this study, the main depositional environments have been determined using a combined sedimentological/stratigraphical approach, and further refined using ichnology. As discussed above, tidal sediments may be deposited in a diverse suite of environments with unique physiochemical conditions that affect the distribution of the infauna. The range of palaeoenvironments in which a given trace fossil is found is an invaluable tool for the study of problematic facies, and greatly aids facies characterization. Conclusion The He Formation is a widespread reservoir interval in the Haltenbanken area offshore Mid-Norway with excellent, though complex, sandstone reservoir intervals that are predominantly in tidal channel and tidal flat facies. Challenges vary from field to field but are in many cases improved by high-quality facies characterization. This study has aimed to incorporate all available sedimentological and ichnological evidence to produce a refined understanding of facies distributions. The facies models and ichnofabric distribution models presented herein allow assessment of facies-stacking patterns that are the fundamental basis of sequence stratigraphic analysis and prediction of facies architecture in inter-well areas. This bespoke sedimentological/ ichnofabric approach has been demonstrated to be highly effective in characterizing the complex tidal-deltaic facies of the He Formation on Kristin Field. Preliminary work with the adjacent Sm0rbukk field has demonstrated that, with little modification, the same scheme can be applied on a semi-regional basis. However, in different basins of different stratigraphic ages the palaeoenvironmental tolerance of tracemaking organisms is likely to differ and require a

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similar scheme to be built from the available dataset. The ichnology of tide-dominated deltas in general remain poorly known; however, comparison of these data with data from wave- and river-dominated deltas (Gingras et al. 1998) suggests that: (1) the Skolithos ichnofacies is better developed in river-dominated deltas; and (2) ichnodiversity is lower than in typical wavedominated deltas. Tide-dominated deltas would seem therefore to represent an intermediate condition between the wave- and river-dominated end members that probably results from an increased marine influence in delta front to delta top settings. This study was supported by Kristin License and Statoil. Logistical support and technical discussions with K. Dale Grov, C. Elfenbein (Statoil), C. Brostr0m (Kristin License, Statoil) & T. Laerdal (University of Bergen) are recognized with thanks. Critical appraisal of the manuscript by A. Martinius and R. Twitchett is gratefully acknowledged. In addition, Kristin License are thanked for permission to publish these findings.

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An integrated ichnological and sedimentological comparison of non-deltaic shoreface and subaqueous delta deposits in Permian reservoir units of Australia KERRIE L. BANN1 & CHRISTOPHER R. FIELDING2 1

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 (e-mail: [email protected]) Department of Geosciences, 214 Bessey Hall, University of Nebraska-Lincoln, NE 68588-0340, USA (e-mail: [email protected]) Abstract: Permian sequences in the Denison Trough of Queensland, eastern Australia, are productive conventional gas reservoirs. Previous attempts to interpret the reservoir bodies in terms of depositional environments have relied largely on sedimentary facies analysis and palynology. Sequences have been re-evaluated from a detailed, integrated ichnological and sedimentological perspective, resulting in a significant increase in the precision and resolution of the palaeoenvironmental interpretation. Various marine, coastal and upper delta plain deposits are recognized. Ichnological signatures have facilitated the differentiation of subaqueous delta deposits from those deposited in non-deltaic offshore and shoreface environments. Delta front facies are further subdivided by integrating ichnological and sedimentological data. Permian offshore and shoreface successions in the Denison Trough contain ichnological signatures that exhibit high diversities (25 ichnogenera comprising 32 ichnospecies), moderate to intense levels of bioturbation, uniformity of burrowing, and a wide variety of structures representing specialized feeding strategies. Examples from an additional reservoir dataset, the Tern Formation from the offshore Bonaparte Basin in north Western Australia, also clearly demonstrate the ichnological complexity of Permian shoreface successions. In contrast, Permian deltaic deposits contain ichnological signatures that reflect stressed environmental conditions. Assemblage diversity is reduced (16 ichnospecies), bioturbation intensity is significantly reduced, and uniformity of burrowing is sporadic.

A delta is generally understood to be a discrete shoreline protuberance formed where a river enters an ocean or other large body of water and supplies sediment more rapidly than it can be redistributed by basinal processes (Elliott 1986). In tide-dominated settings the shoreline may be strongly embayed rather than protuberant, but sediment is nonetheless supplied more rapidly than can be redistributed, and the system still progrades, irrespective of shoreline morphology (Dalrymple 1999). Ancient deposits are difficult or even impossible to characterize as 'deltaic' on the basis of a single core or outcrop section; a reconstructed plan view of the system or good three-dimensional control of facies patterns is necessary (Bhattacharya & Walker 1992). Ancient deltas are economically important owing to their association with coal, oil and gas reserves. Deltaic reservoirs generally consist of an amalgamation of several different sandstone facies, e.g. distributary channels and mouth bars, and therefore form multilateral and multistorey systems comprising a wide variety of facies types and reservoir characteristics (Coleman & Wright 1975; Allen & Chambers 1998).

An accurate genetic model of deltaic (and shoreface) successions is required to refine exploration techniques and optimize production and field development for these types of reservoir. Coastal and deltaic deposits, in general, form similar upward-coarsening and shallowing successions, so that the specific type of depositional environment and relative importance of fluvial, wave and tidal processes cannot be analysed on the basis of well logs alone (Allen & Chambers 1998). In many cases, diagnosis of deltaic deposits has relied exclusively on physical sedimentary structures such as current-generated structures, unimodal or dispersive palaeocurrent distributions, and the broad cross-sectional and plan geometry of a unit. Numerous researchers have recognized that trace fossils record animal responses to subtle changes in environmental parameters such as sedimentological, hydraulic, chemical and bathymetric regimes (Seilacher 1964; Frey & Pemberton 1985; Pemberton et al 1992). The interpretation of sedimentary rocks and palaeoenvironments is therefore enhanced considerably by incorporating ichnology (Seilacher 1955). This is particularly true in the evaluation of

From: MclLROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 273-310. 0305-8719/04/S 15.00 © The Geological Society of London.

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shallow marine successions, where distinction between deltaic and non-deltaic deposits is challenging and potentially contentious. The ultimate objective of this study is to portray the facies implications of various trace fossil assemblages and then to establish the differences between deltaic and non-deltaic shoreface successions. Marine ichnofacies represent archetypal facies models based upon recurrent ichnological assemblages (Seilacher 1967, 1978; Frey & Pemberton 1984, 1985). An integrated approach combining several potential environmental indicators, ichnology, sedimentology and palaeontology has significantly enhanced the palaeoenvironmental interpretations in this study of Permian successions from Australia. The Permian succession of the Denison Trough of central Queensland, Australia, contains a number of shallow marine and deltaic units. The depositional context of several stratigraphic units is uncertain and insufficient information exists on the plan and cross-sectional geometry of these units to firmly establish a deltaic origin. In this paper we interpret the ichnology of formations that have been independently established as deltaic and of shallow marine in origin. An additional dataset from a basin 3000km to the west (Tern Formation, Bonaparte Basin, Western Australia) is also presented as it further demonstrates the diverse ichnological characteristics of Permian shoreface successions. We then apply the ichnological and sedimentological criteria to the diagnosis of a formation, the context of which is complex and cryptic, in order to demonstrate the value of ichnology in this process. Geological setting The Denison Trough forms the southwestern part of the Bowen Basin of east-central Queensland, Australia (Fig. 1). The Trough exists as a concentration of extensional grabens and half-grabens formed during a period of limited though widespread crustal extension across eastern Australia in the Late Carboniferous to Early Permian (Holcombe et al 1997; Fielding et al 2000, 2001). Subsequent extension-related and thermal sag-related subsidence formed the broader feature known as the Bowen Basin, which was then further modified by foreland loading associated with the long-lived Hunter-Bowen contractional event (Fielding et al 2000, 2001). The Denison Trough therefore forms part of the cratonic (western) margin of the Bowen Basin. During late Early to Late Permian times, a series of coarse clastic formations were deposited

along the western margin of the Bowen Basin (Fig. 1). These units are extensive along depositional strike, in a north-south direction, but fine, thin and pinch out eastward into the basin. Palaeocurrent data from these units indicate persistently eastward sediment dispersal into the basin, from source areas located in earlier Palaeozoic fold belt strata to the west (Fig. 1). The coarse-grained formations are separated from each other by intervals of fine-grained shale, rich in marine invertebrate fossils, interpreted to record predominantly offshore marine and shelf conditions. The succession can therefore be regarded as a series of progradational cycles separated by marine shale intervals (Fig. 2a). Many of the sandstone-dominated units of the Denison Trough succession are prospective for hydrocarbons, and several producing gas fields in the area deliver from reservoirs within these formations. The oldest stratigraphic unit recognized in the Denison Trough - the Reids Dome Beds - comprises a locally thick (up to > 1 km) succession of siliciclastics and some thick coal seams, and is interpreted as the mainly fluvial and lacustrine deposits of technically active rift basins. The contact with the overlying Cattle Creek Formation is known to be strongly diachronous. In the northern Denison Trough, where the Cattle Creek Formation is thickest, five alternating mudrock and sandstone-dominated members have been recognized, and are interpreted as offshore marine and broadly coastal respectively (Fielding & Lang 1988, Fig. 2a). Overlying the uppermost mudrock unit of the Cattle Creek Formation, the Sirius Mudstone Member, is the Lower Aldebaran Sandstone. This unit is largely confined to extensional sub-basins, but isopachs show the gradually diminishing influence of the extensional topography. The Lower Aldebaran Sandstone was interpreted by Baker (1991) as a deltaic deposit. The Upper Aldebaran Sandstone unconformably overlies the Lower Aldebaran Sandstone, the angularity of the relationship being apparent only from seismic reflection data. Unlike the underlying unit, the Upper Aldebaran Sandstone is not confined to the early extensional lows, but rather overlaps basement highs and extends considerably further eastward into the basin than older units. The Upper Aldebaran Sandstone shows a prominent arcuate bulge into the basin in plan geometry, and was interpreted by Baker (1991) and subsequent workers as the deposit of a large, eastwardprograding, mixed-influence delta. The top of the Upper Aldebaran Sandstone is marked by a marine flooding surface, and overlain by the Freitag Formation. The lower,

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Fig. 1. Location maps of the Denison Trough.

mainly fine-grained part of this unit was deposited in an offshore and shelf environment, whereas the upper half consists of a number of progradational parasequences that (in the north) include coastal facies. The unit thickens significantly to the north, into an area of limited subsurface data, and the possibility exists that the Freitag Formation in the Denison Trough

could represent the southern margin of a large delta complex. The top of the Freitag Formation is represented by a marine flooding surface of inter-regional extent, traceable throughout the length of the Bowen-Gunnedah-Sydney Basin System (Fielding et al. 2001). In the Denison Trough, this surface marks the base of the Ingelara Formation, a laterally extensive

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Fig. 2. (a) Stratigraphic column of the Denison Trough succession, (b) Legend of symbols to Figures 11-14.

marine mudrock interval. The Ingelara Formation passes upward into the sandstonedominated Catherine Sandstone and its lateral equivalent the German Creek Formation to the north. The Catherine Sandstone was interpreted

by John & Fielding (1993) to be of mostly nondeltaic shallow marine to coastal plain origin, with a small number of modest deltaic depocentres; however, integrated sedimentological and ichnological analysis suggests that a

ICHNOLOGY OF SHOREFACE VS DELTAIC FACIES

number of the shallow marine intervals of the Catherine Sandstone are of deltaic origin. The German Creek Formation to the north, which hosts economically important coal deposits, has been interpreted as the deposits of a large, eastward-dispersing, mixed-influence delta complex (Falkner & Fielding 1993), with some parts of the succession recording deposition in shallow shoreface and foreshore environments (Bann & Fielding 2001). The overlying Peawaddy Formation is another broadly coarsening-upward unit that is interpreted as a further prograding delta system. However, in contrast to underlying units, the volcanic lithic Peawaddy Formation shows evidence of axial dispersal southward into the basin, and thus the section in the Denison Trough represents only a distal margin of this system. Furthermore, the plan shape of the Peawaddy Formation sandstone facies indicates an elongate (birdsfoot) morphology, and together with a general lack of tidal indicators suggests a fluvially dominated delta style. The succeeding Bandanna Formation is of a similar sedimentary character, and ranges up to the latest Permian coal measures, in the Denison Trough. The formations considered herein represent the late extensional to passive thermal subsidence phases of the basin's development. All coarsegrained formations are derived from the craton to the west of the Bowen Basin, and all were accumulated under similar environmental conditions. The climate is interpreted to have been cool temperate and then humid in the aftermath of the late Palaeozoic Gondwanan glaciation (Crowell & Frakes 1971, 1975; Veevers & Powell 1987; McLoughlin 1993), with some floating ice still rafting outsized debris into the marine basin. Previous work (Fielding & Lang 1988; Baker 1991; Falkner & Fielding 1993) has interpreted deltaic depositional environments for some of the coarse-grained units, largely on the basis of gross plan shape of formations, and the interpretation of thick sandstones as distributary channel and mouth bar deposits. These criteria are in some cases difficult to apply, or may be misleading. Ichnofacies analysis provides a powerful additional criterion with which to identify deltaic and deltaically influenced marine environments. Facies analysis A re-evaluation of depositional environments of Permian formations in the Denison Trough and adjacent areas, incorporating the extensive and

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previously underutilized ichnological database, has led to a substantial revision of the facies scheme originally outlined and illustrated by Fielding et al. (1996). A summary of the new facies scheme is given in Table 1. Eight facies associations (A to H) are recognized, each containing a number of component facies, and representing depositional environments ranging from shelf to coastal plain. Association H facies (upper delta plain interdistributary lakes and mires) are outside the area of focus of this paper. A complete spectrum of shallow marine deposits is preserved, ranging from shelfal through offshore and offshore transition to shoreface and foreshore environments. Offshore terminology used in this paper follows the work of Howard (1971, 1972), Howard & Reineck (1981), Howard & Frey (1984), Vossler & Pemberton (1989) and Frey (1990), and subsequently modified by MacEachern & Pemberton (1992) and Pemberton & MacEachern (1995, 1997). A brief summary is given in Bann et al. (2004). Most of the shallow marine facies in the Denison Trough have all the hallmarks of wave-dominated successions (Johnson & Baldwin 1996). Additionally, a separate suite of facies has been defined to accommodate shallow marine facies with somewhat different character, believed to record deltaic environments (Association E: Table 1). Association E includes facies interpreted to record prodelta, through distal delta front, proximal delta front into mouth bar and river mouth settings (Table 1). The interpretation of these facies follows an integrated approach based on their context within vertical sequences, their lithology, cross-sectional geometry and contained physical and biogenic sedimentary structures. Deltas occur in a wide variety of shapes and forms, depending on the types and energy of the coastal processes and the volume and grain size of the fluvial sediment influx (Coleman 1981; Reading 1986; Bhattacharya & Walker 1992). Protuberant deltas generally comprise three major depositional environments: the delta plain, the delta front, and the prodelta. Each of these environments forms a depositional system that consists of a distinct suite of lithologies, facies and reservoir potential (Coleman & Wright 1975; Allen & Chambers 1998). The delta plain comprises the subaerial part of the delta and will not be discussed in any detail herein. The subaqueous portion of the delta exists in the shallow marine zone (or lacustrine zone not discussed here), below the low tide mark (subtidal), and fringes the lower part of

Table 1. Facies

Lithology

Al: Laminated moderately to Claystones and siltstones, laminated, thoroughly bioturbated moderately to intensely bioturbated, dispersed coarser-grained sediment, locally claystone and siltstone fossiliferous A2: Moderately to thoroughly Siltstone or claystone with minor bioturbated muddy siltstone interlaminated and admixed sandstone, moderately to thoroughly bioturbated, locally fossiliferous Bl: Bioturbated sandy siltstone B2: Interlaminated, bioturbated sandy siltstone and laminated sandstone

Siltstone with interlaminated and admixed sandstone, intensely to thoroughly bioturbated, minor discrete sandstone beds <3 cm thick, locally fossiliferous Sandy siltstone and interlaminated, commonly discrete and sharp-bounded sandstone beds < 10 cm in thickness, locally fossiliferous

CIA: Thoroughly bioturbated Intensely bioturbated muddy sandstone units with rare, discrete, sharp-bounded muddy sandstone sandstone layers < 15 cm in thickness, minor dispersed gravel, locally fossiliferous C1B: Interbedded bioturbated Interbedded moderately to thoroughly bioturbated muddy to clean sandstone and muddy sandstone and sharp-bounded, tabular, sparsely to laminated sandstone moderately bioturbated clean sandstone beds < 1 m thick, dispersed gravel and thin conglomerate beds C2: Interbedded massive and Thickly interbedded sandstone in sharpbounded, sparsely to moderately laminated sandstone bioturbated tabular beds and composite units < 1.5 m thick, minor thin conglomerate beds and dispersed gravel C3: Amalgamated laminated Clean, well-sorted sandstone in and cross-bedded sandstone amalgamated tabular beds and composite units, un- to moderately bioturbated, minor thin conglomerate beds and dispersed gravel

Primary Structures

Ichnofacies

Interpretation

Lamination, local glendonites, local outsized clasts

Zoophycos ichnofacies - distal Cruziana ichnofacies

Shelf

Local lamination, local linsen and Distal Cruziana ichnofacies lenticular bedding, rare micro HCS and small-scale ripple crosslamination, local glendonites

Lower offshore

Linsen, lenticular and wavy bedding, current ripple crosslamination, micro hummocky-cross stratification (HCS) As for fades Bl, plus small-scale HCS

Diverse archetypal Cruziana ichnofacies with mixed SkolithosI Cruziana ichnofacies in thin discrete sandstone beds Diverse archetypal Cruziana ichnofacies with Skolithos ichnofacies in the sandstone beds

Upper offshore

Low-angle, undulatory parallel lamination (HCS), oscillation ripple lamination

Diverse proximal Cruziana ichnofacies with Skolithos ichnofacies in the clean sandstone beds Distal Skolithos ichnofacies

Distal lower shoreface

Archetypal Skolithos ichnofacies with uncommon elements from the proximal Cruziana ichnofacies

Middle shoreface

Skolithos ichnofacies, typically impoverished. Pervasive Macaronichnus segregatis assemblage common

Upper shoreface

Thicker sandstones show local basal and top surface gravel lags, flat and low-angle lamination, abundant HCS, symmetrical ripples, interference ripples, large gravelly symmetrical ripples Similar structures to facies C1B, HCS is large-scale

Basal and top surface gravel lags, multi-directional trough crossbedding, low-angle bidirectional cross-bedding, HCS (typically swale dominated), internal erosion surfaces

Offshore transition

Proximal lower shoreface

Dl: Flat-laminated sandstone Well-sorted, sparsely to unbioturbated quartzose sandstone, tabular beds and composite units, minor gravel D2: Cross-bedded, coarsegrained sandstone

Coarse- to very coarse-grained sandstone and conglomerate in erosionally-based (channelized) beds <3m thick, some fining upward, variable levels of bioturbation, local shell debris

El: Unbioturbated gravelly sandstone

Sandstone, sharply-based but typically coarsening upward, some dispersed gravel, intraformational clasts, plant debris, coaly traces Medium-grained, well-sorted, largely unbioturbated amalgamated sandstone units with tabular internal and external geometry and some coarsening upward, common plant debris and coaly traces Thickly interbedded tabular intervals of sporadically bioturbated sandstone in sharpbounded beds and composite units <1.5m thick, common plant debris and carbonaceous detritus, minor dispersed gravel and conglomerate beds, rythmic bedding style

E2: Amalgamated tabular sandstone

E3: Interbedded tabular sandstone and moderately bioturbated sandstone

E4: Interbedded sandstone, bioturbated silty sandstone and claystone

Flat and low-angle or vague Impoverished Skolithos lamination typical, some low-angle ichnofacies seaward-dipping tabular crossbedding Cross-bedding in sets <0.3 m thick, Skolithos ichnofacies often bipolar (heringbone), sigmoidal foresets, local mud drapes (some bundled), minor flat lamination, internal erosion surfaces Abundant cross-bedding, internal erosion surfaces, less common flatlamination and ripple crosslamination, very rare HCS Dominated by HCS (typically swale-dominated), flat and lowangle lamination, common internal erosion surfaces

Thicker sandstone beds show basal and top surface gravel lags, flat and low-angle lamination, abundant HCS, local onshore-directed crossbedding, internal erosion surfaces, some bed tops covered by symmetrical ripples, interference ripples or gravelly symmetrical ripples Dominated by small-scale HCS, Thinly interbedded and interlaminated lam-scram common, also planar sandstone, siltstone and claystone with common discrete sharp-bounded sandstone parallel to low-angle cross-bedding, soft-sediment deformation beds, common plant material and structures, pin-stripe (linsen) to carbonaceous detritus, dispersed gravel, lenticular, wavy and flaser bedding, locally fossiliferous ripple cross-lamination, carbonaceous detritus and wood material

Foreshore

Tidal inlet

No bioturbation observed

River mouth (expansion of outflows)

Sporadically distributed, stressed proximal Cruziana ichnofacies

Mouth bar

Sporadically distributed, stressed expression of the proximal Cruziana ichnofacies

Proximal delta front

Sporadically distributed, stressed Cruziana ichnofacies, very rare Skolithos ichnofacies

Distal delta front

Table 1. Continued Facies

Lithology

Primary Structures

Ichnofacies

E5: Interlaminated siltstone, claystone and silty sandstone

Siltstone with interlaminated and admixed sandstone, sporadically sparsely to intensely bioturbated, locally fossiliferous, abundant carbonaceous material

Linsen, lenticular and wavy bedding, current ripple crosslamination, micro HCS, common soft-sediment deformation and synaeresis cracks

Sporadically distributed, Prodelta stressed expression of the distal Cruziana ichnofacies

Fl: Biopolar cross-bedded sandstone

Erosionally-based, thick (typically 8-30 m), channelized, fining upwards sandstone bodies, some muddy sandstone beds, minor siltstone partings (some show bundling), some gravel, locally moderately bioturbated

Locally common Skolithos ichnofacies

Lower delta plain distributary channel

F2: Trough cross-bedded sandstone

As for facies Fl but no bioturbation, no bundling of fine-grained partings

Trough cross-bedding dominates, sets <2 m thick, large-scale/lowangle (epsilon) cross-bedding (herringbone), flat-lamination, ripple cross-lamination, local softsediment deformation structures As for facies Fl but no bipolar cross-bedding

None

Upper delta plain distributary channel

Gl: Sparsely to moderately bioturbated, interbedded sandstone and siltstone

Interlaminated and thinnly interbedded sandstone and siltstone (beds <10cm), dispersed gravel, bioturbation is variable, common plant debris and coaly traces, sand content 10-80%

Mixed Skolithos I Cruziana ichnofacies, size and diversity restricted

Sandy bays, lagoons and flats

Mixed Skolithos I Cruziana ichnofacies, size and diversity restricted

Muddy bays, lagoons and flats

Parallel lamination, rare softsediment deformation structures

Very rare, size and diversity restricted Cruziana ichnofacies

Isolated lagoon

Parallel lamination

No ichnofacies, sparse rootlets

Coastal plain mire

Flat and low-angle lamination, minor cross-bedding

Skolithos ichnofacies

Storm washover

Cross-bedding, less common flatlamination and ripple crosslamination, rare HCS, common symmetrical ripples

Skolithos ichnofacies

Flood tidal delta

G2: Sparsely bioturbated mudstone

Siltstone and or sandy siltstone, <10% interlaminated sandstone in beds <10cm thick, bioturbation variable but generally sparse, common plant debris and coaly traces, rare dispersed gravel Laminated siltstone and claystone, very rare G3: Laminated mudstone bioturbation, common plant debris and coaly traces G4: Carbonaceous shale/coal Carbonaceous siltstone, very rare sandy detritus, coal Sandstone, sharp-bounded bed <50cm G5: Sparsely bioturbated sandstone thick, local gravel, variable bioturbation, tabular or channelized cross-section G6: Sparsely to moderately Sandstone, composite units with minor bioturbated sandstone siltstone partings, some show overall coarsening or thickening upward, bioturbation variable

Linsen, lenticular and wavy bedding, micro HCS, current and wave-generated ripples, symmetrical, ladder and interference ripples, local softsediment deformation and synaeresis cracks As for facies Gl

Interpretation

ICHNOLOGY OF SHOREFACE VS DELTAIC FACIES

the delta plain. The proximal portion of the subaqueous delta (termed the delta front) can in turn be subdivided into proximal and distal components. The more seaward or distal portion, the prodelta, is where suspended fluvial silt and clay settles out. Sand is transported to the subaqueous delta through distributaries and accumulates at the distributary mouth, as a direct result of the decrease in current velocity, to form mouth bar deposits. Mouth bar deposits form good reservoirs, although their size, geometry and internal facies patterns vary according to the degree of fluvial/wave/tide influence and sediment input (Coleman & Wright 1975; Allen & Chambers 1998; Brettle et al 2002). Mouth bar deposits have been previously identified in the Denison Trough from mapping of both vertical successions and lateral facies relationships (Falkner & Fielding 1993; Fielding et al 1996). Opencut coalmine highwalls containing mouth bar deposits at Gregory and Oaky Creek mines were shown by Falkner & Fielding (1993) to pass laterally into distributary channel facies. Delta front and prodelta deposits in the Denison Trough show a greater abundance of current-generated sedimentary structures relative to their non-deltaic offshore and shoreface counterparts. These, however, are not sufficiently distinctive on a purely physical sedimentological basis to confidently differentiate them from nondeltaic offshore and shoreface deposits. This paper focuses on ichnofacies analysis of the offshore/shoreface and subaqueous delta deposits, in order to facilitate a more robust distinction between these two palaeoenvironments. Measurement of bioturbation intensity follows the scheme outlined in Bann et al. (2004), adapted after Reineck (1963) and Taylor & Goldring (1993). Non-deltaic offshore and shoreface deposits A summary of the characteristics of the 23 component facies is given in Table 1.

Fades Al: Laminated, moderately to thoroughly bioturbated claystone and siltstone Sedimentology This facies comprises flat/parallel-laminated, light to dark grey, fine siltstone or claystone with rare scattered sand and/or gravel clasts (Fig. 3a, also see Fig. lib). Laterally extensive horizons containing outsized clasts, marine

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invertebrate fossils and glendonites (pseudomorphs after the mineral ikaite CaCO3.6H2O: Kaplan 1979) are common. Ankeritic and sideritic carbonate-cements occur locally. Ichnology Bioturbation, where discernible, is uniformly distributed and intense (BI4—6). Characteristic ichnogenera include pervasive, small Phycosiphon, abundant Helminthopsis, common Chondrites, Helminthoida, and Zoophycos. Facies interpretation The very fine-grained nature and intensity of biogenic reworking of this facies indicates slow, continuous deposition under quiescent, fully marine conditions. The trace fossil suite represents grazing and foraging behaviours, with the traces of deposit feeders less common. This assemblage is indicative of the Zoophycos ichnofacies or, in some cases, a distal expression of the Cruziana ichnofacies, and is characteristic of quiescent marine shelf environments, well below maximum storm wavebase. Although grazers and foragers may have been abundant, upon compaction and dewatering their traces are not easily preserved (MacEachern & Pemberton 1992). Inhabitants of soupy sediments and watery softgrounds tend to cause diffusive turbulence during passage through the substrate and produce a structureless fabric (Bromley 1996). Laterally extensive horizons of outsized clasts within shelf deposits are convincing evidence for ice rafting. Glendonites also provide evidence for very cold climatic conditions as ikaite is unstable at temperatures above 5°C, at which point it decomposes to calcium carbonate (Shearman & Smith 1985; Jansen et al. 1987). Ikaite has been recorded in several modern, cold climate localities including Antarctica (Suess et al. 1982), Zaire deep-sea fan (Jansen et al. 1987) and Alaska (Kennedy et al. 1987). Glendonites are relatively common in the Permian of Eastern Australia and have been recorded from several localities (Carr et al. 1989; Bann 1998; Bann et al. 2004).

Facies A2: Moderately to thoroughly bioturbated silty muds tone Sedimentology The siltstone and silty mudstone of this facies contain minor proportions of interlaminated or (more commonly) admixed very fine- to finegrained sandstone with few preserved sedimentary structures. Flat/parallel lamination imparted by grain-size variations, pinstripe

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Fig. 3. (a) Fades Al. GSQ Springsure-18, 220m. (b) Fades A2. Black arrows indicate Phycosiphon; white arrows indicate scattered outsized clasts. Five cent piece is 10mm in diameter, Catherine Sandstone, Arcturus Core -6:2. (c-g) Facies B. (c) Pervasive Phycosiphon (Ph), Facies B2, Rolleston Core -12:1, Freitag Formation. (d) A section through the basal portion of a Rosselia mud-ball (R), Phycosiphon (Ph), Facies B2, Rolleston

ICHNOLOGY OF SHOREFACE VS DELTAIC FACIES

(linsen) and lenticular bedding occur locally. Rare (
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indicates deposition in a lower offshore, open marine environment, affected by very cold climatic conditions, but only rarely by severe storms. Facies Bl: Bioturbated sandy silts tone Sedimentology This facies is characterized by heavily bioturbated siltstone with minor to moderate amounts of interlaminated and interstitial very fine- to fine-grained sandstone (Fig. 3e). Minor discrete, sharp-bounded sandstone beds <5cm thick occur locally and display remnant low-angle undulatory parallel lamination, laminated to scrambled bedding ('lam-scram', Fig. 3e) and rarer oscillation ripple lamination. Dispersed outsized clasts, fossiliferous layers, carbonatecements (typically ankerite/siderite) and glendonites occur locally. Ichnology Bioturbation in this facies is characteristically uniformly distributed and intense, varying from BI5 to BI6, and typically BI6. The trace fossil suite (Fig. 3e) is dominated by Rosselia socialis, Zoophycos, Phycosiphon, Planolites, Palaeophycus heberti, Teichichnus and Rhizocorallium irregulare. Subordinate elements are Asterosoma, Helminthoida, Chondrites, Palaeophycus tubularis, Macaronichnus simplicatus, Diplocraterion habichi and fugichnia. The dominant ichnogenera are generally uniformly distributed throughout the facies, but subordinate elements occur sporadically. Facies interpretation This facies differs from Facies A in that it contains a greater number of sandstone beds with remnant, low-angle undulatory parallel lamination (interpreted as HCS and associated with evidence of tempestites). The intensely bioturbated, muddy and silty sandstone beds suggest deposition in an environment that experienced lengthy periods near, but below, fair-weather wavebase.

Core -12:1, Freitag Formation, (e) Interbedded intensely bioturbated sandy siltstone and very fine- to finegrained, thin, sharp bounded laminated sandstone beds that display remnant low-angle undulatory parallel lamination and 'lam-scram' bedding (LS). Note: above this interval of 'lam-scram' is another interval, and a Rosselia (Rl) truncated by a tempestite displays a readjustment of the burrow in an upward direction (R2). The fair-weather assemblage represents a diverse expression of the archetypal Cruziana ichnofacies. Visible structures here include Palaeophycus heberti (Pa), Phycosiphon (Ph) and Chondrites (Ch). Elements of the Skolithos ichnofacies include Diplocraterion habichi (Dh). Facies Bl, Tern Formation, (f) Large truncated Rosselia (R) overprinting a pervasively bioturbated mottled background texture (BI 5) comprising abundant Planolites (P), Chondrites (Ch) and Phycosiphon (Ph). Facies B2, Tern Formation, (g) Planolites (P), Phycosiphon (Ph) and Asterosoma (A) representing components of the diverse Cruziana ichnofacies. Facies B2, Tern Formation.

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The highly diverse trace fossil suite, dominated by the structures of deposit-feeding and, to a lesser extent, grazing/foraging behaviours is characteristic of the archetypical Cruziana ichnofacies and reflects quiescent conditions in upper offshore environments lying between storm and fair-weather wavebase. Vertical burrows (i.e. elements of the Skolithos ichnofacies) such as Diplocraterion habichi suggest that opportunistic suspension-feeding organisms colonized storm beds. Intensely reworked tempestites are the product of greater vertical penetration by more robust deposit feeders, increased size and abundance of vertical burrows and increased time between storm events that facilitates complete colonization and development of deeply penetrating structures (cf. Frey & Goldring 1992).

Fades B2: Inter laminated, bioturbated sandy siltstone and laminated sandstone Sedimentology This heavily bioturbated facies consists of interlaminated and thinly interbedded siltstone and very fine- to fine-grained sandstone. Sandy siltstone beds are commonly truncated, and bases are indistinct owing to biogenic mixing across the interface with underlying sandstones. Laminated sandstone beds are generally discrete, erosionally based, 5 cm), and contain 'laminated to scrambled' bedding profiles. Outsized clasts and shelly fossil debris are locally common. Pinstripe (linsen) to lenticular, wavy and flaser bedding, internally comprising ripple cross-lamination, ripple form sets, flat lamination and micro HCS (ripple scale) are common. Small-scale HCS and soft-sediment deformation structures occur locally. Ichnology This facies is characterized by persistent though locally sporadically distributed bioturbation with intensities varying from BIO to BI5 and typically 3-5. Most ichnogenera are uniformly distributed throughout the facies, though elements associated with intercalated laminated sandstone beds are more sporadically distributed. The trace fossil suite can be subdivided into one associated with the sandy siltstone, and one reflecting infaunal colonization of the laminated sandstone units. The sandy siltstone beds are intensely bioturbated (BI4-5) and dominated by Rosselia socialis, Phycosiphon (Fig. 3c, d), Teichichnus, Helminthopsis, Zoophycos, very large and small varieties of Rhizocorallium irregulare (Rhizocorallium type A is very large, type B is small) and

Planolites (Fig. 3f). Common but subordinate forms include Rosselia rotatus, Palaeophycus tubularis, Palaeophycus heberti, Asterosoma (Fig. 3g), Chondrites, Diplocraterion habichi, small Taenidium, Skolithos, Cylindrichnus and ?Thalassinoides. The cleaner, well-laminated sandstone beds are less intensely bioturbated and contain a 'lam-scram' appearance, with the BI ranging from 0 at the base to 4 upwards. The trace fossil assemblage is dominated by Diplocraterion habichi, Diplocraterion parallelum, large Rhizocorallium irregulare (Type A), Phycosiphon, fugichnia and Taenidium 'synyphes\ Common but subordinate elements comprise Palaeophycus tubularis, Macaronichnus isp., Planolites, Rosselia socialis, Rosselia rotatus, Teichichnus, Skolithos and Palaeophycus heberti. Uncommon elements include Psammichnites, Conichnus, Rhizocorallium irregulare Type B, Cylindrichnus and Arenicolites. Facies interpretation The palaeoenvironment represented by this facies records fair-weather sandy siltstones, erosionally truncated and overlain by HCS tempestites, locally with a basal lag, and commonly containing escape structures. The diverse trace fossil assemblage, dominated by the structures of deposit-feeding and grazing/foraging organisms, represents a diverse expression of the archetypal Cruziana ichnofacies and reflects infaunal reworking of the substrate during fair-weather. The trace fossil assemblage associated with HCS sandstone units is dominated by vertical burrows of opportunistic suspension-feeders, resilient surface detritus-feeders and passive carnivores, and is indicative of the Skolithos ichnofacies. This recurring juxtaposition of Cruziana and Skolithos ichnofacies (mixed Skolithos-Cruziana ichnofacies: Howard & Frey 1984; Pemberton & Frey 1984; MacEachern & Pemberton 1992; Pemberton & MacEachern 1997) is characteristic of interbedded fair-weather units and tempestites deposited within the upper offshore, lying close to but below fair-weather wavebase in a moderately storm-influenced setting. This zone reflects the transition from the lower shoreface to the offshore/shelf (e.g. offshore transition: cf. Howard & Reineck 1981).

Facies CIA: Thoroughly bioturbated muddy sandstone Sedimentology

This facies consists of intensely bioturbated muddy sandstone units with rare, thin

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(< 15 cm), discrete, sharply bounded (erosionally based), fine- to medium-grained sandstone beds. Sandstone beds locally contain low-angle, undulatory parallel lamination (HCS), oscillation ripple lamination, shelly debris, dispersed pebbles and granules and carbonaceous detritus.

moderately to weakly storm-influenced distal lower shoreface.

Ichnology Bioturbation in this facies is pervasive (BI5-6, Fig. 4), with hazy, indiscrete bioturbate textures dominating (Fig. 4c-e). Sandstone interbeds, though present, are largely biogenically reworked. Most ichnogenera are uniformly distributed throughout the facies. The trace fossil suite is dominated by Rosselia socialis (Figs lid, 12d), Rosselia rotatus, Phycosiphon (Fig. 4a, c), Diplocraterion habichi, Teichichnus (Fig. 4h), Rhizocorallium irregulare Type B (Fig. 4b, g-h), Rhizocorallium irregulare Type A, Zoophycos (Fig. 4a), Diplocraterion parallelum (Fig. 4f), Macaronichnus isp. (Fig. 4c), Planolites (Fig. 4j), Palaeophycus tubularis (Fig. 4c) and Palaeophycus heberti (Fig. 4b, d). Common but subordinate elements are Conichnus, Taenidium isp., Taenidium, Skolithos, Cylindrichnus (Fig. 4e), Siphonichnus, Schaubcylindrichnus, Asterosoma, fugichnia, Psammichnites, Scolicia, Helminthopsis (Fig. 4a) and Chondrites (Fig. 4i).

Sedimentology This facies is composed of thinly to thickly interbedded, tabular intervals of bioturbated muddy sandstone and fine- to medium-grained sandstone, in discrete, tabular to lensoidal beds, typically 0.2-0.5 m in thickness (Fig. 5a). The muddy sandstone beds are similar to units within Facies CIA. Thicker sandstone beds generally display erosional bases, and units are commonly amalgamated. Minor scattered gravel and discrete thin conglomerate beds and single clast layers are locally common. Siltstone and thinly interbedded siltstone/sandstone intervals occur locally and contain pinstripe (linsen) and lenticular bedding. Thicker sandstone beds show gravel lags (basal and/or top surface), flat and low-angle lamination, and HCS are generally laminated to burrowed or sparsely to nonbio turbated. Bed tops contain symmetrical ripples, interference ripples or coarse-grained symmetrical ripples. Shelly debris, dispersed pebbles and granules, soft-sediment deformation structures and carbonaceous detritus are locally common.

Facies interpretation Intensity and uniformity of bioturbation in the muddy sandstones and the lack of primary sedimentary structures indicate either limited storm influence on the environment or considerable time between storms, during which the fairweather assemblage reworked the substrate. The fair-weather trace fossil suite, dominated by a very diverse mixture of robust, complex structures produced by deposit- and detritusfeeding organisms, is characteristic of a diverse proximal expression of the Cruziana ichnofacies. Considerable numbers of vertical structures, such as Diplocraterion parallelum, reflect a component of the Skolithos ichnofacies and suggest that storm beds were rapidly colonized by opportunistic, suspension-feeding organisms, prior to their recolonization and thorough reworking by the resident fair-weather community (cf. Frey & Goldring 1992). Locally occurring massive sandstone beds with indistinct bioturbate textures suggest that an infaunal community occupied a soupy substrate with high porewater content. Rare, thin, sharp-based HCS sandstone beds represent tempestites. These features indicate deposition in a welloxygenated, open marine setting at or just above fair-weather wavebase consistent with a

Facies C1B: Interbedded bioturbated muddy sandstone and laminated sandstone

Ichnology The interbedded, bioturbated muddy sandstone and laminated sandstone units are characterized by highly variable bioturbation intensities, ranging from BI0 to BI5. Bioturbated sandstone beds range from BI 3 to BI 5, whereas the laminated sandstone units have bioturbation intensities between BIO and BI4. Bioturbation and trace fossil distribution is more or less persistent within the muddy sandstone beds but sporadically distributed in the laminated sandstone layers, where 'lam-scram' bedding predominates. The trace fossil assemblage in the muddy sandstone beds is dominated by robust Rosselia socialis (Figs 5a, b, 12e), Rosselia rotatus, Diplocraterion habichi, Diplocraterion parallelum (Figs 5b, lie), Teichichnus, Phycosiphon (Fig. 5a-c), Palaeophycus tubularis (Fig. 5b-c), Planolites (Fig. 5a-c) and Rhizocorallium irregulare types A (Fig. 5b) and B. Common but subordinate elements include Macaronichnus isp., Arenicolites, Cylindrichnus and Skolithos. The trace fossil suite in the laminated sandstone beds is dominated by Diplocraterion habichi and fugichnia (Fig. 5c). Diplocraterion parallelum (Fig. 5d), Skolithos (Fig. 5d), Rhizocorallium jenense,

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Fig. 4. Characteristic ichnofossils in Facies CIA. (a-d) Intensely reworked muddy sandstone with a mottled bioturbate texture (BI5-6), Tern Formation. Bioturbation has occurred in several phases, with mottled background textures overprinted by larger more discrete structures such as IZoophycos (Z?) and Rhizocorallium irregulare type B (Rh). These structures have been reworked by small gazing and foraging organisms

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Arenicolites, Cylindrichnus and Conichnus represent subordinate elements. Fades interpretation The heterogeneity of this fades demonstrates variations in physical energy and sedimentation rates. Most of the physical structures reflect storm deposition, particularly HCS, low-angle planar cross-stratification, and small- to largescale waning-stage combined flow ripples. Nonburrowed siltstone interbeds are interpreted to reflect post-storm draping of tempestites during waning flow conditions, rather than fair-weather deposition. Alternations between episodic tempestite emplacement and thorough bioturbation of the interbedded muddy sandstone record changes in sedimentation rates and storm influence. The abundance of fugichnia in the tempestites supports episodically high sedimentation rates and erosional amalgamation of the beds. The bioturbated sandstone beds contain a relatively diverse trace fossil suite that records a distal expression of the Skolithos ichnofacies, consistent with sedimentation within the more proximal portion of the lower shoreface.

Fades C2: Interbedded massive and laminated sandstone Sedimentology This facies consists of thickly interbedded, tabular intervals of both clean, massive sandstone and laminated fine- to medium-grained sandstone. Gravel, clast layers and discrete thin conglomerate beds are common locally. Thick sandstone beds have erosional bases, gravel lags (basal and/or top surface), flat and low-angle planar cross-stratification, large-scale HCS and swaley cross-stratification (SCS). Bed tops are covered by symmetrical ripples, interference ripples or symmetrical gravel ripples locally.

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Ichnology Bioturbation is variable, ranging from BI0 to a very rare maximum of 5. The average BI is 1-3, many beds have little or no bioturbation preserved, and 'lam-scram' profiles are common. The trace fossil assemblage is less diverse than in Facies C1B. The suite is dominated by robust vertical structures such as Diplo crater ion parallelum, long deeply penetrating Diplocration habichi (Fig. 6c), Skolithos and Arenicolites. Common but subordinate elements comprise Macaronichnus segregatis, robust vertical mudfilled structures such as Rosselia socialis, Cylindrichnus, and Parahaentzschelinia surlyki (Fig. 6a, b). Fine conglomerate beds are generally unbioturbated but locally contain Skolithos, Arenicolites and Diplocraterion habichi. Facies interpretation This facies is characterized by sedimentary structures that reflect deposition in a highenergy, wave-dominated environment. The thick amalgamated units of SCS, HCS and lowangle planar cross-stratified sandstones are characteristic of storm deposits. The thickest amalgamated units indicate deposition in a storm-dominated setting above storm wavebase. More heavily bioturbated units represent the biogenic reworking of storm-deposited sand during fair-weather periods in a well-oxygenated open marine environment above fair-weather wavebase. The absence of muddy interbeds also reflects deposition above fair-weather wavebase. The trace fossil suite is dominated by burrows of suspension-feeding organisms, and represents the Skolithos ichnofacies. The general lack of complex, horizontal burrows formed by deposit-feeding organisms indicates high storm intensity and/or frequency, and reflects a paucity of interstitial food material for infaunal deposit feeders (MacEachern & Pemberton

1992). Vertical, robust, mud ball (Rosselia-type)

interpreted as Helminthopsis (H) and Phycosiphon (Ph). The mottled background texture represents the repeated reworking of successive storm and fair-weather units during fair-weather periods in a distal lower shoreface environment. The trace fossil suite comprises a mixture of subvertical/vertical structures, representing elements of the Skolithos ichnofacies, such as Palaeophycus tubularis (Pt), Diplocraterion parallelum (Dp) and Skolithos (S), and the structures of a diverse range of grazing/foraging, deposit- and detritus-feeding organisms and passive carnivores representing a diverse proximal expression of the Cruziana ichnofacies. Structures visible here include Phycosiphon (Ph), Planolites (P), Palaeophycus heberti (Pa), Chondrites (Ch), Rosselia (R), Schaubcylindrichnus (Sc) and Macaronichnus sp (M). (e) Thoroughly bioturbated silty sandstone with a mottled bioturbate texture overprinted by long thin Cylindrichnus (Cy), Catherine Sandstone, Arcturus Core -6:3. (f) Heavily bioturbated sandstone with Diplocraterion parallelum (Dp), GSQ Springsure Core -17. (g) An example of the top portion of a Rhizocorallium irregulare type B (Rh), a structure that is very common in Permian shoreface facies. Complex structures such as this are often difficult to identify in core, GSQ Springsure Core -17. (h) Teichichnus (Te), GSQ Springsure Core -17. (i) Pervasively bioturbated sandstone, the bioturbate texture comprising tubular structures such as Phycosiphon (Ph), Planolites (P) and Palaeophycus tubularis (Pt), Tern Formation, (j) Pervasively bioturbated mudstone bed, Planolites (P), Tern Formation.

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structures reflect the domiciles and associated waste disposal activities of resilient detritusfeeding organisms. These structures are common components of the Skolithos ichnofacies in Permian deposits from Australia.

Pemberton et al (2001) also considered Rosselia to be typical of the Skolithos ichnofacies. The physical characteristics and dominance of the Skolithos ichnofacies suggest a middle shoreface environment.

Fig. 5. Characteristic bedding styles and ichnofossils in Facies Clb. (a-c) Interbedded laminated clean sandstone and heavily bioturbated muddy sandstone, Tern Formation. The trace fossil assemblage is dominated by a relatively diverse mixture of vertical and horizontal structures such as Diplocraterion parallelum (Dp) and Skolithos (S), robust heavily lined Rosselia socialis (R), Phycosiphon (Ph), large Planolites (P), large Palaeophycus tubularis (Pt), Chondrites (Ch), and possible large Rhizocorallium irregulare type A (Rh). Small pervasive Helminthopsis (H) and Phycosiphon (Ph) have reworked the muddy linings of other larger structures. Fugichnia are common in the laminated sandstone beds (f). (d) Clean sandstone with examples of Skolithos ichnofacies elements, Diplocraterion parallelum (Dp) and Skolithos (S), Lower Aldebaran Sandstone, Arcturus Core -6:4. (e) Bioturbated sandstone with Skolithos, Staircase sandstone, GSQ Core Springsure -16.

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Fig. 6. (a-c) Outcrop examples of Fades C2, German Creek Formation, Bundoora Dam, Queensland, (a, b) Large, vertical, mud-filled burrows interpreted as Parahaentzschelinia surlyki. (c) Long curved Diplocraterion habichi. (d) Tiering profile for Facies C3. Shallower tiers are occupied by the subvertical to vertical dwelling structures of passive carnivores and opportunistic suspension-feeding organisms. Ichnotaxa include Palaeophycus tubularis (Pt), Diplocraterion habichi (Dh), Conichnus (C), Rhizocoralium jenense (Rj), Arenicolites (A), Diplocraterion parallelum (Dp), Skolithos (S) and very robust, vertical Cylindrichnus (Cy). The deepest tier is occupied by Macaronichnus segregatis (Ms). This trace fossil suite represents the Skolithos ichnofacies. (e) Outcrop example of pervasive Macaronichnus segregatis in Facies C3, Freitag Formation, Fairbairn Dam, Queensland.

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Fades C3: Amalgamated laminated and cross-bedded sandstone Sedimentology Thick amalgamated tabular beds and composite units of clean, moderately to well-sorted, quartzose, generally medium-grained sandstone are typical of this facies along with scattered gravel and discrete beds and single clast layers of conglomerate. Beds show basal and internal erosion surfaces, gravel lags (basal and/or top surface), trough cross-stratification and low-angle bidirectional planar cross-bedded sets. Low-amplitude/ long-wavelength and swale-dominated HCS are locally preserved. Ichnology Trace fossils are locally common but rarely abundant, and assemblage diversity is generally low. Most units have little or no bioturbation, with the BI ranging from 0 to a very rare maximum of 4. The average BI is 0-2. Where the degree of bioturbation is high, the trace fossil assemblage is usually dominated by one ichnospecies. Lam-scram profiles are preserved locally but are uncommon. The trace fossil suite is characterized by vertical structures and pervasive Macaronichnus segregatis (Fig. 6d-e). Different ichnotaxa are locally more prevalent, but the most common forms overall include Macaronichnus segregatis, Skolithos, Conichnus, Arenicolites and Diplocraterion parallelum. Diplo crater ion habichi, Rhizocorallium jenense, Palaeophycus tubularis and vertical, robust Cylindrichnus are locally common but subordinate overall. A general lack of silt in burrow fills and linings produces a pervasive bioturbation that is not always obvious, but can be seen clearly in some examples (Fig. 6e). Facies interpretation Sedimentary structures in this facies reflect deposition by shore-parallel wave-driven currents interacting with currents that are generated by translatory flow and plunging waves (MacEachern & Pemberton 1992). Such structures are typical of upper shoreface deposits (Reinson 1984). Storm deposits in the upper shoreface occur as ridge and runnel systems rather than major depositional events, as they are in the lower and middle shoreface, and indicate erosion of the beach face and transport of sediments to the more distal parts of the shoreface (MacEachern & Pemberton 1992). The trace fossil suite represents an impoverished expression of the Skolithos ichnofacies. Most of the trace fossils are heavily lined, vertical to subvertical, and represent the domiciles of

deeply burrowing suspension-feeding organisms that were able to withstand the continually migrating bed-forms. The Macaronichnus tracemaker is most common in deposits from very high-energy environments, typically from around and above the upper shoreface-foreshore contact (Saunders & Pemberton 1986; Saunders et al. 1994). The preservation potential of Macaronichnus segregatis was high, despite the fact that it represents the activities of a deposit feeder, owing to the deep-tier position that it occupied (Saunders & Pemberton 1986; Pemberton et al. 2001). In the intertidal and innermost surf zone of wave-exposed beaches, oxygenated surface waters can circulate several metres into the sand, well below the reach of surface wave disturbance (Pemberton et al. 2001). In addition, large volumes of dissolved and fine particulate organic matter, most of which is mineralized at depth, is filtered through the porous sandy system of the beach (McLachlan et al. 1984). This phenomenon creates a second habitat at depth, separate from the physically controlled habitat at the surface. The deeper habitat is stable, predictable, and is constantly replenished with food material (Pemberton et al. 2001). Saunders & Pemberton (1986) suggested that the Macaronichnus organism fed on microorganisms several metres below the sediment/water interface. Deltaic deposits Facies El: Unbioturbatedgravelly sandstone Sedimentology This facies is characterized by sharp-based, coarsening-upward intervals of unbioturbated, fine- to coarse-grained sandstone. Scattered gravel, intraformational clasts, plant debris and associated coaly fragments are common. The dominant visible structure is high-angle trough cross-bedding in sets typically <0.5m thick. Internal erosion surfaces, flat parallel-bedded cross-lamination and ripple cross-lamination are preserved locally. Hummocky cross-stratification is rare. Ichnology No trace fossils have been recognized in this facies. Facies interpretation Lack of bioturbation in this facies suggests an inhospitable depositional environment. Migrating large-scale bedforms present a problem for infaunal colonization, so that only deeply

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penetrating trace fossils might be expected (MacEachern 2001). The lack of vertical burrows, which reflect the domiciles of opportunistic suspension feeders, may reflect rapid and/or alternating sedimentation rates, salinity fluctuations, increased turbidity levels and rapidly shifting bedforms in a river mouth environment. Biogenie structures produced in such high-energy environments stand little chance of preservation (Bromley 1996).

Fades E2 Amalgamated, tabular sandstone Sedimentology This facies comprises amalgamated intervals of generally medium-grained, well-sorted, quartzose sandstone with rare gravel. Sandstone bodies are tabular (Fig. 7a), with generally tabular internal architecture also, and locally display overall coarsening-upward trends and abundant internal erosion surfaces. Low amplitude but long-wavelength, swale-dominated forms of HCS, flat parallel and low-angle lamination are common. Ichnology Trace fossils are mostly absent but, where they are present, bioturbation is of low intensity and is very sporadically distributed. The trace fossil suite is of very low-diversity and is usually dominated by Macaronichnus isp. (Figs 8a, b, 14d) that contains a diverse and complex range of burrow infill (passive, active and meniscate forms, Fig. 9c). Teichichnus, Palaeophycus tubularis (Fig. 8b) and Planolites (Fig. 8a) are very rare. At first glance the burrows interpreted herein as Macaronichnus isp. appear to have the characteristics of more than one ichnogenus (Palaeophycus, Planolites and Macaronichnus isp.). Closer inspection, however, indicates that the tubular burrows actually represent different components of the same biogenic structure. The cross-sectional shape and size of the burrows is consistent, but the linings and infill of the structures are quite variable (Fig. 9b-c). Whereas some parts of the burrow system appear to have been lined and then remained open (thus being passively infilled), there is evidence that other parts are actively filled and are thought to represent backfilled or waste-stuffed chambers. The composite morphology of this structure suggests a time lag across the mantle/infill discontinuity (conformant reprobing/branching) in the form of meniscate infill, local collapse etc. (T. Saunders personal communication 2003). Many examples do not contain an obvious

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lining; rather the burrow mantle reflects selective particle ingestion with dark micaceous material discarded to the outer edge of the structure. This type of specialized, grain-selective deposit feeding is also associated with other Macaronichnus species (Saunders & Pemberton 1986; Saunders et al. 1994; Pemberton et al. 2001). Facies interpretation The well-sorted, medium-grained nature of this facies and the abundance of large-scale, swaledominated HCS and flat parallel and low-angle cross-lamination suggest deposition in a wavedominated marine environment above fairweather wavebase. An abundance of internal erosion surfaces supports this interpretation. The well-sorted tabular sand bodies suggest that the depositional environment was exposed to continuous reworking (i.e. by river outflow currents and sea waves). The facies is characteristic of wave-reworked mouth bar deposits, the finer fraction continuously removed and swept out to sea where it is then deposited in more distal delta front and prodeltaic settings. The paucity of vertical burrows and the low-diversity trace fossil suite (dominated by pervasive Macaronichnus isp.) represents an ichnologicalassemblage composed of the sporadically distributed burrows of passive carnivores and deposit-feeding organisms. The assemblage reflects a stressed but proximal expression of the Cruziana ichnofacies, suggesting that for the most part the sandy substrate was inhospitable to infaunal organisms and especially so to suspension feeders. The physical sedimentary structures, vertical facies succession and ichnological characteristics indicate deposition in a mixed river- and wave-influenced mouth bar environment.

Facies E3: Interbedded tabular sandstone and moderately bioturbated sandstone Sedimentology This facies consists of thickly interbedded, tabular intervals of fine- and medium-grained sandstone in sharp-bounded (erosionally based) beds and composite units <1.5m thick interbedded with bioturbated sandstone. Interlaminated siltstone/fine-grained sandstone beds, scattered gravel, discrete thin conglomerate beds and single clasts occur locally. The thick sandstone beds contain basal and/or top surface gravel lags, flat parallel and lowangle lamination and HCS. Onshore-directed cross-bedding is rare. Locally, bed tops are

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Fig. 7. (a) Outcrop view of Fades E2, Crocker Sandstone, Capricorn Highway Blackwater to Emerald, Queensland, (b-d) Facies E3, Lower Aldebaran Sandstone, (b) outcrop view, (c, d) equivalent core from GSQ Springsure -16 (see sedimentological log, Fig. 13). (b) Macaronichnus isp. with burrow crossover marked by arrow. Cattle Creek Nianda, Queensland, (c) Sandstone units with low-angle lamination and Rosselia mud-ball rip-up clast (R), Phycosiphon isp. (Ph) and Macaronichnus isp. (M). (d) Amalgamated low-angle laminated sandstone with Macaronichnus isp. (M) and truncated fugichnia (f).

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Fig. 8. (a, b) Facies E2. Medium-grained sandstone with Macaronichnus isp. (M), Palaeophycus tubularis (Pt) and Planolites (P). (c-e) Facies E3. (c) Outcrop view of pervasive Macaronichnus isp. individual burrows are clearly discernible and have a micaceous mantle (M). Lower Aldebaran Sandstone, Cattle Creek Nianda, Queensland, (d, e) Soft-sediment deformation structures in core. Also visible is the sporadic nature of bioturbated intervals characteristic of this and the other deltaic facies. An interval of moderately bioturbated silty sandstone containing Macaronichnus isp. (M) and Phycosiphon (Ph) is visible in (e). Catherine Sandstone, GSQ Springsure core -18. covered by symmetrical ripples, interference ripples or coarse-grained symmetrical ripples. Soft-sediment deformation structures (Fig. 8d, e) and carbonaceous detritus are common.

Ichnology This facies is characterised by a sporadically distributed, moderate to low-diversity trace fossil assemblage. Bioturbation intensities are highly

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Fig. 9. Fades E4. (a) Discrete laminated clean sandstone beds and interbedded bioturbated siltstone and sandstone visible in outcrop. Lower Aldebaran Sandstone, Cattle Creek Nianda, Queensland, (b) Macaronichnus isp. (M) forming a pervasive bioturbate texture. Note the overprinting of individual burrow cross-sections. Aldebaran Sandstone, Arcturus Core -6:4. (c) Macaronichnus isp. (M) in core, showing variation in burrow fill (one of the unusual characteristics of this ichnospecies). Freitag Formation, GSQ Springsure -17. (d) Carbonaceous detritus (ca) and wood fragments (w) in core, Freitag Formation, GSQ Springsure -17. (e) Distal delta front deposits in core showing interbedded low-angle cross-bedded sandstone (LA) and bioturbated interlaminated siltstone and sandstone with a very low-diversity trace fossil suite comprising Macaronichnus isp. (M), Planolites (P) and Phycosiphon (Ph). Lower Catherine Sandstone, GSQ Springsure -18.

variable throughout the fades, and range from 0 to 4. Bioturbated intervals are thin (<20 cm), and are interbedded with non-bioturbated laminated sandstone intervals. Overall the trace fossil suite is dominated by Macaronichnus isp. (Figs 7b-d, 8c). In this facies these burrows display an even wider variety of preservational forms than in

facies E2. Corkscrew-shaped varieties are locally common (Fig. 14c). This type of morphology would ordinarily be classified as Gyrolithes. Phycosiphon (Fig. 7c), Planolites, Rosselia socialis and Cylindrichnus are rare subordinate elements that occur locally. Rosselia mud balls, preserved as rip-up clasts (Fig. 7c), and fugichnia are

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common in some laminated sandstone beds (Fig. 7d). There is a noticeable paucity of vertical burrows, but Diplocraterion habichi occurs locally. Fades interpretation This facies is broadly similar to Facies C2 and C3 in that it comprises mainly amalgamated laminated sandstone bedsets interbedded with bioturbated sandstone beds. These features indicate that the depositional environment was characterized by breaking waves, surf zone conditions and ultimately wave swash, typical of facies deposited above fair-weather wavebase in conditions similar to that of the middle to upper shoreface. There is significant evidence for storm wave reworking of the substrate such as large-scale HCS (amplitude 0.3-0.5m, wavelength up to 20m), internal erosion surfaces and the excavation and re-deposition of Rosselia mud balls as rip-up clasts. Ichnologically, this facies differs from shoreface deposits in subtle ways. The trace fossil suite is similar to that from mouth-bar deposits, but bioturbated intervals here are thicker and more abundant, and intensities are higher. The trace fossil assemblage represents a stressed but proximal expression of the Cruziana ichnofacies. Integrated with the sedimentological characteristics and vertical facies succession, this ichnofacies suggests deposition in a wave-influenced proximal delta front environment. The delta front, in general, usually forms a relatively steep slope, and high sedimentation rates and slope instability result in penecontemporaneous deformation, slumping and gravity movements, producing soft-sediment deformation structures such as those seen in Figure 8d-e.

Facies E4: Interbedded sandstone, bioturbated silly sandstone and clay stone Sedimentology This facies comprises discrete, sharp-bounded, laminated clean sandstone beds, bioturbated sandstones, interlaminated and thinly interbedded siltstone and very fine- to fine-grained sandstones and thin claystone beds (Fig. 9a). Outsized clasts and shelly fossil debris are common locally. Small-scale HCS is the dominant primary structure in the laminated sandstones, and many beds display 'lam-scram' bedding and common fugichnia. The coarsest grained sandstone beds contain planar parallel cross-bedding or lowangle cross-bedding. The interlaminated and interbedded sandstone and siltstone intervals contain common soft-sediment deformation structures, pinstripe (linsen) to lenticular, wavy and flaser bedding with internal ripple cross-

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lamination. Ripple form sets, flat parallel lamination and micro-hummocky cross-stratification (ripple scale) are common in these intervals. Claystone beds generally occur as thin (<3cm) drapes on the tops of sandstone beds, are rarely bioturbated (Fig. 13b), and locally contain synaeresis cracks. Carbonaceous detritus and woody material are abundant throughout this facies (Fig. 9d). The facies passes conformably upwards into proximal delta front and mouth bar deposits (Figs 13, 14). Many intervals show rhythmic interbedding of sand and silt/clay. Ichnology Bioturbation in this facies is sporadically distributed, and many intervals are not burrowed (Fig. lOa, c). Bioturbation intensities are variable, with the overall BI ranging from 0 to 5. The trace fossil suite is dominated by large Macaronichnus isp. (Figs 9b, c, lOa, c, d, e, 13b, 14b) and Phycosiphon (Figs lOa, d, 14b), which occurs as both very robust and diminutive forms. These two trace fossil types occur in sporadically distributed but pervasively bioturbated horizons (Fig. 13b). Fugichnia comprise a common but subordinate element of the assemblage. Teichichnus, Rosselia socialis, Rosselia rotatus (Fig. lOb), Chondrites (Fig. 13b), Rhizocorallium irregular e, Asterosoma, Psammichnites, Planolites (Fig. 9e) and Zoophycos occur locally but are rare. Teichichnus, Rosselia, Asterosoma and Zoophycos occur as diminutive forms. Vertical burrows such as small Diplocraterion habichi occur in sparse quantities. Facies interpretation The interbedded laminated sandstones and bioturbated intervals clearly display characteristics similar to those of a storm-dominated lower shoreface deposit. Examples include HCS, planar parallel to low-angle cross-bedding, lamscram bedding and heavily bioturbated intervals that reflect periods of quiescence associated with fair-weather. Ichnologically, however, this facies differs subtly from lower shoreface deposits. The trace fossil suite is reduced in diversity, individual ichnogenera differ in size as compared with their non-deltaic shoreface counterparts, the uniformity of burrowing is sporadic, degree of bioturbation is more variable and generally less intense, and there is a noticeable paucity of vertical burrows. The trace fossil suite represents a depauperate expression of the Cruziana ichnofacies, which indicates that the depositional environment was physically or chemically hostile to infaunal organisms. The non-bioturbated, carbonaceous claystone layers represent post-storm drapes on top of

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Fig. 10. Facies E4. (a) Sporadic distribution of bioturbated intervals and intervals that lack any evidence of burrowing activity. The trace fossil suite is of very low diversity and comprises Macaronichnus isp. (M) and Phycosiphon (Ph). Lower Catherine Sandstone, GSQ Core Springsure -18. (b) Slender Rosselia rotatus (R) in low-angle cross-bedded sandstone that shows little evidence of other burrow types. The base of this bed sharply truncates the underlying non-bioturbated, laminated silty sandstone unit. Freitag Formation, GSQ Core Springsure -17. (c) Sharp-based laminated sandstone that truncates a bioturbated interval with Macaronichnus isp. (M) and Phycosiphon (Ph). Freitag Formation, GSQ Core Springsure -17. (d) Characteristic trace fossil suite in Permian distal delta front deposits, sporadically distributed large Macaronichnus isp. (M) and robust Phycosiphon (Ph). Freitag Formation, Rolleston Core -11:2. (e) Pervasively burrowed silty sandstone interval within distal delta front facies. The trace fossil suite is of very low diversity and comprises Macaronichnus isp. (M) that displays burrow overlap and a variety of burrow infill, Teichichnus (T) and Phycosiphon (Ph). Aldebaran Sandstone, Arcturus Core -6:4.

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tempest! tes. Similar clay stone drapes have been described from delta front deposits by Saunders et al (1994), Gingras et al (1998) and Bann et al. (2004). Physical structures and ichnological characteristics similar to those in this facies are known from wave-dominated deltas in the Upper Cretaceous Dunvegan and Basal Belly River Formations in North America (Gingras et al. 1998; Coates & MacEachern 1999). This facies reflects deposition in the distal delta front of a wave-influenced delta.

Facies E5: Interlaminated siltstone, clay stone and silty sandstone Sedimentology This facies underlies Facies E3 and E4 (Figs 13a, 14a) and consists of mainly siltstone and claystone with minor to moderate proportions of interlaminated and admixed very fine- to finegrained sandstone. Discrete, sharp-bounded sandstone beds <5cm thick are rare. Interlamination structures range from pinstripe (linsen) to lenticular and wavy bedding, internally comprising ripple cross-lamination, ripple form sets, flat lamination and micro-hummocky cross-stratification (ripple scale). Scattered outsized clasts, marine body fossils and shell debris are rare. Abundant carbonaceous detritus, softsediment deformation structures and carbonate cements (typically ankerite/siderite) occur locally. The claystone beds also contain sparse, small synaeresis cracks. Ichnology As for the sandier deltaic deposits described above, this facies is characterized by variable levels of bioturbation. The degree of reworking varies from BIO to BI5. The trace fossil suite (Fig. 15a) is dominated by small Macaronichnus isp. and Phycosiphon. Diminutive Asterosoma, Planolites, Chondrites, fugichnia and Helminthopsis are relatively common but subordinate. Teichichnus and Zoophycos comprise uncommon elements of the assemblage, both occurring as diminutive forms. Vertical structures are generally absent. Facies interpretation The trace fossil suite in this facies is dominated by a relatively diverse but diminutive and sporadically distributed mixture of structures, produced by grazing/foraging and deposit-feeding behaviours. This assemblage represents a stressed distal expression of the Cruziana ichnofacies, and indicates fully marine conditions

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with persistent environmental fluctuations. The fine-grained, interlaminated nature of the facies records fair-weather deposition in a relatively quiescent environment, below fair-weather wavebase. The sparse, thin sandstone beds represent distal tempestites. The association of synaeresis cracks with the claystone layers suggests fluctuating salinity levels associated with heightened precipitation and increased fluvial discharge during storm events. The facies has similar physical characteristics to offshore deposits, but the vertical facies succession and stressed ichnological signature suggest deposition in a prodeltaic environment.

Case study 1: The Lower Aldebaran Sandstone The Lower Aldebaran Sandstone is recognized as a mixed-influence (wave-tide-fluvial-interactive) delta deposit (Baker 1991). This diagnosis is based on the presence of erosively-based, crossbedded sandstone deposits (Facies Fl, distributary channel), the radiating pattern of palaeocurrent directions, and the arcuate plan shape of the body (Baker 1991; Fielding et al. 1995, 2000). This study identified a significant proportion of Facies E components, further supporting a deltaic interpretation for the Lower Aldebaran Sandstone. The unit overlies the uppermost mudstone member of the Cattle Creek Formation, the Sirius Mudstone Member (Figs 2, 9a, 13). In outcrop, the contact is defined on the basis of exposure character: the Sirius Mudstone Member forms deeply gullied, but flat terrain, whereas the overlying Lower Aldebaran Sandstone forms hilly country with small sandstone cliff exposures. The contact is easily traced from aerial photographs. In many subsurface cores and surface exposures, such as in Cattle Creek, the contact marks a significant and abrupt change in grain size and depositional setting, and is interpreted as a sequence boundary. In other outcrops and cores however, the contact is often cryptic, with a continuous coarsening upward trend from one unit into the other (e.g. Fig. 13a). The characteristic sedimentary succession of the Lower Aldebaran Sandstone is seen in Figure 13a. The interval consists of a coarsening-upward facies succession, showing a transition from the muddier prodelta deposits of the upper Sirius Mudstone Member into the sandier delta front and mouth-bar facies of the Lower Aldebaran Sandstone. The ichnological signature of this succession is one of a fully marine trace fossil assemblage that experienced significant

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Fig. 11. (a) Sedimentological section of the contact between the Moorooloo Mudstone Member and the overlying Staircase Sandstone Member (both components of the Cattle Creek Formation), from GSQ Core Springsure -16. The section comprises a coarsening and shallowing upwards succession. The bioturbation is intense (BI5-6) and uniformly distributed throughout the interval. Diminishing BI readings in the upper half of the succession represent 'lam-scram' bedding in tempestites. The overall trace fossil suite is diverse and contains a mixture of structures produced by the activities of grazing/foraging behaviours, simple and complex deposit-feeding strategies and detritus-feeding organisms. Vertical structures become common in the distal

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environmental and possibly chemical stresses (Fig. 13b). In comparison with the ichnological signature of non-deltaic shoreface successions the trace fossil suites are reduced in diversity, individual ichnogenera differ in size, uniformity of burrowing is sporadic, the degree of bioturbation is more variable and significantly less intense, and there is a distinct paucity of vertical burrows. A section of core (Fig. 13b) from around the 340m mark on the core in Figure 13a represents distal delta front deposits. The trace fossil suite is dominated by Macaronichnus isp. and Phycosiphon. The Macaronichnus isp. displays a variety of morphology and burrow fills. The large examples in the basal half of the core distinctly suggest a Gyrolithes-type spiral morphology, whereas the smaller examples in the upper part of the core appear to maintain a more consistently horizontal nature, possibly suggesting that burrow construction (in the larger example) took place during increased sedimentation rates. The deltaic succession passes up into interbedded muddy and sandy interdistributary bay fill deposits with highly reduced trace fossil suites characteristic of Permian opportunistic, brackish-water trace fossil assemblages (see Bann et al. 2004). The lower delta plain interval is overlain by sandy, cross-bedded distributary channel deposits. This succession (Figs 11 a, 12a) displays many of the characteristics of deposits formed in prograding wave-dominated shoreface settings (Curray et al. 1969; Fielding 1989; Boyd et al. 1992; MacEachern & Pemberton 1992; Walker & Plint 1992 and references therein; Bann et al. 2003). Prograding successions are also characteristically deposited during progradation of delta lobes (Elliott 1986; Coleman & Wright 1975; Bhattacharya & Walker 1992; Gingras et al. 1998; Coates & MacEachern 1999). Until recently, successions such as this could be interpreted only as a prograding wave- or storm-dominated shoreface

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and could not be positively associated with a delta unless good three-dimensional control was available (Bhattacharya & Walker 1992). However, in the Permian successions in this study, ichnological signatures have provided a powerful additional means in the recognition and identification of deltaic depositional environments.

Case study 2: The Freitag Formation The enigmatic Freitag Formation overlies the coarse-grained Upper Aldebaran Sandstone in the northern Denison Trough (Fig. 2a). An accurate plan shape is indicated by isopachs, but the northern extent of the unit is difficult to define owing to sparse data. The arcuate, eastwardbulging plan shape, though incomplete, is suggestive of a delta. Previous work by Fielding & McLoughlin (1992) on a large surface exposure at Fairbairn Dam in the northernmost Denison Trough suggested a non-deltaic coastal platform, although the exposed section included a thick erosionally based cross-bedded sandstone body (Facies Fl) interpreted as a major distributary channel deposit. This study, and other recent work by Bann & Fielding (2001) and Trueman (2002), suggested that the Freitag Formation contains a complex array of progradational cycles, some representing deltas and others coastal platform. The base of the unit is a well-defined flooding surface, with fine-grained facies passing upward into sandstone-dominated facies. A series of broadly coarsening-upward parasequences 540m thick are recognized here (Fig. 14a) from the cored stratigraphic drill-hole GSQ Springsure-18. A complete coarsening-upward parasequence, bounded by interpreted transgressive surfaces of erosion, is visible in Figure 14a. The succession is physically and sedimentologically very similar to the prograding shoreface

lower shoreface and increase upwards to be abundant in the proximal lower shoreface deposits, (b-e) Changing upwards through the prograding shoreface succession, (b) Facies Al, shelf al mudstone, in core. Some vague primary lamination is visible; trace fossil types are difficult to ascertain. Ingelara Formation, GSQ Core Springsure -17. (c) Characteristic upper offshore deposits. Bioturbation is intense and persistent throughout the facies, and the trace fossil assemblage is dominated by structures produced by grazing/foraging organisms such as Zoophycos (Z) and Phycosiphon (Ph), and complex deposit-feeding behaviours such as Asterosoma (A). Ingelara Formation, GSQ Core Springsure -17. (d) Intensely bioturbated distal lower shoreface sandstone dominated by the basal tubes of Rosselia (R). Rosselia is one of the most abundant trace fossil types in Permian shoreface deposits, and represents an element of both the Cruziana and Skolithos ichnofacies. In this example Rosselia represents a component of a proximal expression of the Cruziana ichnofacies. Freitag Formation, GSQ Core Springsure -17. (e) Characteristic proximal lower shoreface deposits. A passively filled Diplocraterion parallelum (Dp) represents an element of the Skolithos ichnofacies and overprints a mottled background texture that reflects biogenic reworking during fair-weather, Tern Formation.

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Fig. 12. (a) Sedimentological section of a characteristic prograding shoreface succession, from the Permian Tern Formation, Bonaparte Basin, Australia. The bioturbation intensity (BI) decreases upwards in relation to coarsening and shallowing. Diminishing BI readings in the proximal lower shoreface deposits represent 'lamscram' bedding associated with the biogenic reworking of the upper portions of tempestites. The succession is truncated by an erosion surface that hosts a firmground assemblage of trace fossils and represents the Glossifungites ichnofacies. The trace fossil assemblages in the offshore to offshore transition consist of a very diverse mixture of structures produced by grazing/foraging, detritus and deposit feeders. The suite reflects the

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successions illustrated in Figures 11 and 12, suggesting a similarity in depositional processes and bathymetry. However, when the ichnological characteristics are compared there are significant differences. Bioturbation intensity (BI, see Fig. 2b) in the Freitag Formation section (Fig. 14a) is reduced, and uniformity of burrowing is sporadic. In addition, the trace fossil assemblage diversity is significantly reduced in comparison with the trace fossil suites in the bathymetrically equivalent (offshore and offshore transition) deposits in Figures 11 and 12. Figure 14b-d illustrates the characteristics of the deltaic facies. Figure 14b shows an example of a distal delta front interval from the Freitag Formation that is pervasively bioturbated by Macaronichnus isp. in the basal portion of the photo and by Phycosiphon in the upper part. An example of proximal delta front deposits from the upper Freitag Formation in GSQ Core Springsure-17 is illustrated in Figure 14c. Once again bioturbation is sporadic, and the trace fossil assemblage is dominated by Macaronichnus isp. and Phycosiphon. The unusual spiral-like morphology seen in this (Macaronichnus isp.) example is typical of the morphology of Gyrolithes. This morphology may reflect an escape mechanism to avoid burial and suffocation from emplacement of the overlying sandy tempestite. Clean sandstones interpreted as mouth-bar deposits are dominated by Macaronichnus, and display a paucity of vertical structures (Fig. 14d). Discussion and conclusions Permian offshore and shoreface deposits from reservoirs in Australia contain ichnological signatures characterized by moderate to intense bioturbation, high assemblage diversities (25 ichnogenera comprising 32 ichnospecies), uniform lateral (and to a lesser extant vertical) distribution of bioturbation, and significant

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numbers of structures reflecting specialized feeding/grazing behaviours. Shelf and offshore deposits contain diverse expressions of the Zoophycos, distal Cruziana and archetypal Cruziana ichnofacies. In the more proximal shoreface successions, diverse assemblages composed of the robust burrows of deposit- and detritus-feeders are mixed with a diverse array of sub vertical to vertical structures that represent the burrows and activities of carnivores, scavengers and opportunistic suspension-feeders. This complex assemblage represents a diverse proximal expression of the Cruziana ichnofacies mixed with a significant proportion of elements from the Skolithos ichnofacies. This suite reflects a well-oxygenated fully marine environment that evidently provided a plentiful supply of food material and stable conditions under which mature infaunal communities were able to thrive, even in the more proximal environments that show evidence of strong and repetitive storm influence. In contrast, the ichnological signature of the Permian subaqueous delta deposits displays low-diversity trace fossil assemblages (16 ichnospecies overall) that are generally dominated by one or two ichnotaxa. Bioturbation is sporadically distributed and significantly reduced in overall intensity. The assemblage represents a stressed expression of the Cruziana ichnofacies. Elements of the Skolithos ichnofacies are largely absent except for rare Diplocraterion habichi in the proximal to distal delta front. The ichnological differences between Permian prodelta and offshore deposits are illustrated in Figure 15. In prodeltaic intervals (Fig. 15a) primary lamination generally dominates over biogenic reworking. The trace fossil suite is relatively diverse, but individual ichnospecies are reduced in size and are sporadically distributed throughout the interval. Bioturbated horizons are usually dominated by Phycosiphon and Macaronichnus isp. The thin sandstone layers

activities of a mature fair-weather community of organisms with specialized feeding strategies, and represents a diverse expression of the distal to archetypal Cruziana ichnofacies. Vertical structures that reflect the influence of suspension-feeding organisms represent elements of the Skolithos ichnofacies and indicate opportunistic colonization associated with storm deposition. These structures increase in abundance upwards through the succession and dominate the trace fossil suite from the proximal lower shoreface through the middle shoreface and up into the upper shoreface deposits, (b-f) Succession illustrated in (a) showing the characteristics of facies upwards through the core from (b) upper offshore to (f) upper shoreface, and in particular the change in morphology of Rosselia with increasing wave energy, (b) Permian Rosselia is generally diminutive, slender and vertical to inclined in offshore deposits, (c, d) In offshore transition to distal lower shoreface deposits it is generally truncated but very robust, and displays a variety of complex transitional forms, (e) In the proximal lower shoreface to upper shoreface (and even into the foreshore in some localities) Permian Rosselia is long, vertical, heavily lined but slender in nature as compared with specimens from more distal shoreface deposits, (f) Clean, well-sorted upper shoreface deposits with rare Skolithos.

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Fig. 13. Sedimentological section of an upward-coarsening and shallowing deltaic succession from the top of the Sirius Mudstone Member (Cattle Creek Formation) and overlying Lower Aldebaran Sandstone, GSQ Core Springsure -16. The contact between the two formations is interpreted as a sequence boundary. Bioturbation is sporadic throughout this succession. The overall trace fossil assemblage is reduced in diversity, especially when compared with the trace fossil suites in the sections illustrated in Figures 11 and 12, and represents a stressed expression of the Cruziana ichnofacies. Elements of the Skolithos ichnofacies are conspicuous in their absence from the sandier facies, where the trace fossil suite is of very low diversity and is dominated by the unusual tubular burrows interpreted as Macaronichnus isp. (b) This portion of core (from 340m depth on the sedimentological section) shows the characteristics of Permian distal delta front deposits. The facies contains interlaminated and thinly interbedded siltstone and very fine- to fine-grained sandstones with low-angle cross-bedding, and thin claystone beds. The trace fossil suite is dominated by Macaronichnus isp. (M) and Phycosiphon (Ph). The thin claystone drapes are rarely bioturbated but here contain Chondrites (Ch).

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Fig. 14. (a) Sedimentological section of a prograding deltaic succession from the lower Freitag Formation, GSQ Core, Springsure -18. Bioturbation in this succession is sporadically distributed and reduced in diversity. The suite is dominated by Phycosiphon and Macaronichnus isp. and contains rare Planolites, Chondrites, Teichichnus and Zoophycos. The trace fossil suite in the proximal delta front is also dominated by Macaronichnus isp. and lacks the vertical burrows of suspension-feeding organisms. The mouth-bar deposits are unbioturbated. This succession is interpreted as a mixed river/wave-influenced delta package. (b-d) Characteristics of coarsening-upwards deltaic facies from distal delta front to mouth bar deposits, (b) An intense bioturbate texture from a distal delta front interval dominated by Macaronichnus isp. and Phycosiphon (Ph). Freitag Formation, GSQ Core, Springsure -17. (c) A sparsely bioturbated proximal delta front interval with Macaronichnus isp. and a structure that displays the characteristics of Gyrolithes. Upper Freitag Formation, GSQ Core, Springsure -17. (d) Macaronichnus isp. in clean sandy mouth-bar deposits, Freitag Formation, GSQ Core, Springsure -17.

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Fig. 15. Graphic representation of the differences between prodeltaic and offshore deposits in core, (a) Prodeltaic deposits contain interbedded siltstone and claystone with minor, interlaminated very fine- to finegrained sandstone layers with synaeresis cracks (sy) and soft-sediment deformation (ss). Bioturbation is sporadically distributed, the assemblage diversity is reduced, and individual ichnospecies are reduced in size. The trace fossil suit is characteristically dominated by Phycosiphon (Ph) and Macaronichnus isp. (M). Asterosoma (A), Chondrites (Ch), Helminthopsis (H), Teichichnus (T) and Zoophycos (Z) occur as diminutive, subordinate elements. Fugichia (f) are common. The trace fossil suite represents a stressed distal expression of the Cruziana ichnofacies. In contrast, offshore deposits (b) are characterized by uniformly distributed intense bioturbation comprising a very diverse mixture of structures produced by grazing/foraging, detritus- and deposit-feeding activities. Ichnospecies include Phycosiphon (Ph), Rosselia (R), Rhizocorallium irregulare (Rh), Planolites (P), Palaeophycus heberti (Pa), Asterosoma (A), Chondrites (Ch), Helminthopsis (H), Teichichnus (T), Helminthoida (He), Macaronichnus isp. (M) and Zoophycos (Z). Thin sandstone beds contain remnant parallel and low-angle lamination, and are associated with elements of the Skolithos ichnofacies such as Diplocraterion habichi (Dh) and Palaeophycus tubularis (Pt). This assemblage represents fair-weather deposits containing a very diverse expression of the archetypal Cruziana ichnofacies and minor influence from the Skolithos ichnofacies associated with tempestites.

ICHNOLOGY OF SHOREFACE VS DELTAIC FACIES

commonly contain soft-sediment deformation structures (ss), synaeresis cracks (sy) and fugichnia (f). In contrast, Figure 15b is a diagrammatic representation of the characteristics of Permian non-deltaic offshore deposits. Bioturbation intensity is high (BI5-6), it is uniformly distributed, and the trace fossil suite is very diverse. Thin sandstone layers (tempestites) have been largely reworked by biogenic activity. A high proportion of the facies has been completely homogenised by the repeated reworking during fair-weather by a diverse infaunal assemblage of grazing/ foraging and specialized deposit-feeding organisms. Ichnologically, shoreface deposits and delta front deposits differ in significant ways (Fig. 16). Lower shoreface deposits contain a diverse fair-weather assemblage (Fig. 16a, b) that represents a proximal expression of the Cruziana ichnofacies, and clearly reflects an environment with plentiful food supplies and long periods of stable fully marine conditions. Tempestites in the lower shoreface (Fig. 16c) contain a relatively diverse assemblage of vertical traces that represents a distal expression of the Skolithos ichnofacies, suggesting that a variety of organisms were able to colonize the substrate even during periods of repeated storm influence. In contrast, trace fossil assemblages in distal delta front deposits (Fig. 16d-e) reflect the activities of a stressed infaunal community only rarely inhabited by diminutive grazing/foraging organisms and deposit-feeders and dominated by opportunistic deposit-feeding organisms. The vertical burrows of opportunistic suspension-feeders are very rarely present. The overall paucity of evidence of infaunal suspension-feeders in the deltaic deposits may reflect high levels of water turbidity, caused by river discharge into the basin. High levels of suspended sediment within the water column produce unfavourable conditions for suspension-feeding organisms by interfering with their filter-feeding apparatus and reducing the percentage of food per unit of material ingested (Moslow & Pemberton 1988; Buatois & Angriman 1992; Gingras et al. 1998; Coates & MacEachern 1999). Large quantities of suspended fine sediment and organic detritus are associated with heightened precipitation, increased surface runoff at the coast, and elevated fluvial discharge in deltaic settings. Freshwater input and thermal variations during increased fluvial discharge would be additional environmental stresses in the deltaic environment (cf. also Mcllroy 2004). The thin claystone drapes on tempestites suggest that the storm beds were rapidly covered with organic-rich mud, shielding them from

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opportunistic colonization, particularly by suspension feeders (MacEachern 1994; Saunders et al. 1994; Gingras et al. 1998). In addition, high levels of organic detritus in the muds may have produced rapid oxidization and oxygen depletion at the seafloor, which would potentially inhibit most organisms from colonizing the substrate (Coates & MacEachern 1999, 2000). The apparent absence of the vertical burrows that would generally characterize an opportunistic storm suite may also reflect high sedimentation rates, and increased levels of sediment reworking in the delta front environment (Gingras et al. 1998). Internal erosion surfaces, lam-scram bedding and the truncation and redeposition of Rosselia mud balls as rip-up clasts provide evidence that storm induced exhumation produced the amalgamation of units and the possible removal of previously established infaunal communities. Stressed, low-diversity trace fossil assemblages such as recorded from the Permian deltaic deposits reflect environments that do not provide optimum living conditions for infaunal organisms. Hence most of the inhabitants are opportunistic species that flourish periodically in environments that are, at other times, too inhospitable for even the most resilient of trophic generalists. In general these Permian deltaic facies are characterized by physical structures and ichnological signatures that indicate rapid sedimentation rates, high degrees of salinity variation, a mixture of current and wave processes, and large quantities of organic detritus. This evidence includes: the low-diversity, sporadically distributed trace fossil assemblage that represents a stressed expression of the Cruziana ichnofacies; common fugichnia but a noticeable paucity of vertical burrows, even in the more proximal facies where opportunistic suspension-feeders would ordinarily be expected; thick coarsening-upward facies successions; thick organic-rich prodelta siltstone intervals; common soft-sediment deformation structures; abundant organic detritus and siderite cements; organic claystone drapes on tempestites with common synaeresis cracks; and abundant wave-generated sedimentary structures. Such evidence strongly supports the interpretation of this deltaic facies association as that of prograding, mixed river- and wave-influenced delta lobes, and clearly differentiates this facies from non-deltaic offshore and shoreface deposits.

ICHNOLOGY OF SHOREFACE VS DELTAIC FACIES Funding for this project was provided by the University of Queensland in the form of a Post-Doctoral Research Fellowship to KLB. Oil Company of Australia Ltd and Santos Ltd are also gratefully acknowledged for their generous funding of this research. Staff from the Department of Mines, Zillmere, are thanked for laying out of core. M. Wilkinson, J. Trueman and T. Grimison are thanked for field assistance. Santos Ltd is also thanked for granting permission to publish material from the Tern Formation. J. Lerette is thanked for useful editorial comments. M. Gregory and D. Mcllroy are thanked for their helpful reviews.

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BANN, K. L., FIELDING, C. R., MAC£ACHERN, J. A. & TYE, S. C. 2004. Differentiation of estuarine and offshore marine deposits using integrated and sedimentology: Permian Pebbley Beach Formation, Sydney Basin, Australia. In: MC!LROY, D. (ed.) The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 179-211. BHATTACHARYA, J. & WALKER, R. G. 1992. Deltas. In: WALKER, R. G. & JAMES, N. P. (eds) Fades Models, Response to Sea Level Change. Geological Association of Canada, St. John's, Newfoundland, 157-158. BOYD, R., DALRYMPLE, R. & ZAITLIN, B. A. 1992. Classification of clastic coastal depositional environments. Sedimentary Geology, 80, 139-150. BRETTLE, M. J., MC!LROY, D., DAVIES, S. J., ELLIOTT, T. & WATERS, C. N. 2002. Identifying cryptic tidal influences within deltaic successions: an example from the Marsdenian (Namurian) interval of the Pennine Basin, UK. Journal of the Geological Society, London, 159, 379-391. BROMLEY, R. G. 1996. Trace Fossils: Biology, Taphonomy and Applications (2nd edn). Chapman & Hall, London. BUATOIS, L. A. & ANGRIMAN, O. L. 1992. The ichnology of a submarine braided channel complex: the Whiskey Bay Formation of James Ross Island, Antarctica. Palaeogeography, Palaeoclimatology, Palaeoecology, 94, 119-140. CARR, P. F, JONES, B. G. & MIDDLETON, R. G. 1989. Precursor and formation of glendonites in the Sydney Basin. Australian Mineralogist, 4, 3-12. COATES, L. & MACEACHERN, J. A. 1999. The ichnological signature of wave- and river-dominated deltas: Dun vegan and basal Belly River Formations, West-Central Alberta. In: WRATHALL, B., JOHNSTON, G., ARTS, A., Rozsw, L, ZONNEVELD, J-P., ARCURI, D. & MCLELLAN, S. (eds) Digging Deeper, Finding a Better Bottom Line. CSPG and Petroleum Society 1999 Core Conference Paper, Calgary, Alberta, p. 99-114.

Fig. 16. Comparison of the ichnological signatures of Permian lower and middle shoreface deposits with delta front deposits, (a) Graphic representation of ichnospecies abundance in lower shoreface, distal delta front, middle shoreface and proximal delta front deposits, (b) Tiering profile of the fair-weather assemblage for Permian lower shoreface deposits. The suite reflects a very diverse mixture of grazing/foraging structures, detritus-feeding and complex deposit-feeding structures, and represents a diverse proximal expression of the Cruziana ichnofacies. (c) Tiering profile for event beds in Permian lower shoreface deposits, reflecting a relatively diverse mixture of structures produced by opportunistic suspension-feeders and robust detritusfeeders, which represents a distal expression of the Skolithos ichnofacies. (d) Tiering profile representing the trace fossil signature from the most environmentally stable portions of distal delta front deposits. The trace fossil suite contains a relatively diverse, but diminutive and sporadically distributed, assemblage of structures produced by grazing/foraging and deposit-feeding behaviours. All of the ichnospecies illustrated are rare but tend to occur in association with each other, and are interpreted as representing the least stressed component of the deltaic infaunal assemblage. The assemblage represents a stressed expression of the Cruziana ichnofacies and contrasts markedly with the fair-weather assemblage from the non-deltaic lower shoreface deposits, (e) Representation of the tiering profile of distal delta front deposits that were colonized only by trophic generalists. The assemblage is dominated by Macaronichnus isp. and Phycosiphon. All other vertical structures are absent except for rare examples of small Diplocraterion habichi. This trace fossil suite comprises a lowdiversity mixture of opportunistic ichnospecies, and represents a stressed proximal expression of the Cruziana ichnofacies.

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FIELDING, C. R., KASSAN, J. & DRAPER, J. J. 1996. Geology of the Bowen and Surat Basins, eastern Queensland. Australasian Sedimentologists Group Field Guide Series, Geological Society of Australia, Sydney, 8, 1-126. FIELDING, C. R., SLIWA, R., HOLCOMBE, R. J. & KASSAN, J. 2000. A new palaeogeographic synthesis of the Bowen Basin of central Queensland. In: BEESTON, J. (ed.) Bowen Basin Symposium 2000, 22-24 October 2000, Geological Society of Australia Coal Geology Group & Bowen Basin Geologists Group, Brisbane, 287-302. FIELDING, C. R., SLIWA, R., HOLCOMBE, R. J. & JONES, A. T. 2001. A new palaeogeographic synthesis for the Bowen, Gunnedah and Sydney Basins of eastern Australia. In: HILL, K. C. & BERNECKER, T. (eds) Eastern Australasian Basins Symposium, Petroleum Exploration Society of Australia, Special Publications, Australasian Institute of Mining & Metallurgy, Melbourne, 269-278. FREY, R. W. 1990. Trace fossils and hummocky crossstratification, Upper Cretaceous of Utah. Palaios, 5, 203-218. FREY, R. W. & COLORING, R. 1992. Marine event beds and recolonization surfaces as revealed by trace fossil analysis. Geological Magazine, 129, 325-335. FREY, R. W. & PEMBERTON, S. G. 1984. Trace fossils facies models. In: WALKER, R. G. (ed.) Facies Models, 2nd edition. Geoscience Canada (Reprint Series, 1), Toronto, Ontario, 189-287. FREY, R. W. & PEMBERTON, S. G. 1985. Biogenic structures in outcrops and cores: 1. Approaches to ichnology. Bulletin of Canadian Petroleum Geology, 33,72-115. GINGRAS, M. K., MACEACHERN, J. A. & PEMBERTON, S. G. 1998. A comparative analysis of the ichnology of wave- and river-dominated allomembers of the Upper Cretaceous Dun vegan Formation. Bulletin of Canadian Petroleum Geology, 46, 5173. HOLCOMBE, R. J., STEPHENS, C. J. et al. 1997. Tectonic evolution of the Northern New England Fold Belt: the Permian-Triassic Hunter-Bowen event. In: ASHLEY, P. A. & FLOOD, P. G. (eds) Tectonics and Metallogenesis of the New England Orogen, Geological Society of Australia, Special Publication, Sydney, 19, 52-65. HOWARD, J. D. 1971. Comparison of the beach-to-offshore sequence in modern and ancient sediments. In: HOWARD, J. D., VALENTINE, J. W. & WARME, J. E. (eds) Recent Advances in Paleoecology and Ichnology. American Geological Institute, Short Course Lecture Notes, Alexandria, Virginia, 148-183. HOWARD, J. D. 1972. Trace fossils as a criteria for recognizing shorelines in the stratigraphic record. Society of Economic Paleontologists and Mineralogists, Special Publications, Tulsa, Oklahoma, 16, 215-225. HOWARD, J. D. & FREY, R. W. 1984. Characteristic trace fossils in nearshore to offshore sequences, Upper Cretaceous of east-central Utah. Canadian Journal of Earth Sciences, 21, 200-219.

ICHNOLOGY OF SHOREFACE VS DELTAIC FACIES HOWARD, J. D. & REINECK, H.-E. 1981. Depositional fades of a high energy beach-to-offshore sequence: comparison with low-energy sequence. American Association of Petroleum Geologists Bulletin, 65, 807-830. JANSEN, J. H. F., WOENSDREGT, C. F., KOOIESTRA, M. J. & VAN DER GAAST, S. J. 1987. Ikaite pseudomorphs in Zaire Deep Sea Fan: intermediate between calcite and porous calcite. Geology, 15, 245-248. JOHN, B. H. & FIELDING, C. R. 1993. Reservoir potential of the Catherine Sandstone, Denison Trough, east-central Queensland. Australian. Petroleum Exploration Association Journal, 33, 176-187. JOHNSON, H. D. & BALDWIN, C. T. 1996. Shallow clastic seas. In: READING, H. G. (ed.) Sedimentary Environments: Processes, Fades and Stratigraphy. Blackwell Scientific Publications, Oxford, 232-280. KAPLAN, M. E. 1979. Calcite pseudomorphs (pseudogaylussite, jarowite, thinolite, glendonite, gennoishi, White Sea Hornlets) in sedimentary rocks: origins of the pseudomorphs. Lithology and Mineral Resources, 14, 623-636. KENNEDY, G. L., HOPKINS, D. M. & PICKTHORN, W. J. 1987. Ikaite, the glendonite precursor, in estuarine sediments at Barrow, Artie, Alaska. Geological Society of America, Abstracts with Programmes, 19, 725. MACEACHERN, J. A. 1994. Integrated ichnological-sedimentological models: applications to the sequence stratigraphic and palaeoenvironmental interpretation of the Viking and Peace River Formations, West-Central Alberta. PhD thesis. University of Alberta. MACEACHERN, J. A. 2001. Ichnology and sedimentology in a sequence stratigraphic framework: integrated facies models for subsurface analysis. Rocky Mountain Association of Geologists, Short Courses, Denver, Colorado, 1. MACEACHERN, J. A. & PEMBERTON, S. G. 1992. Ichnological aspects of Cretaceous shoreface successions and shoreface variability in the Western Interior Seaway of North America. In: PEMBERTON, S. G. (ed.) Applications of Ichnology to Petroleum Exploration: A Core Workshop. Society of Economic Paleontologists and Mineralogists, Core Workshops, Tulsa, Oklahoma, 17, 57-84. MclLROY, D. 2004. Ichnology and facies model of a tide-dominated delta: Jurassic upper Ror and He Formations of Kristin Field, Halten Terrace, Offshore Mid-Norway. In: MclLROY, D. (ed.) The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 237-272. McLACHLAN, A., COCKCROFT, A. C. & MALAN, D. E.

1984. Benthic faunal response to a high energy gradient. Marine Ecology Progress Series, 16, 51-63. MCLOUGHLIN, S. 1993. Plant fossil distributions in some Australian Permian non-marine sediments. Sedimentary Geology, 85, 601-619. MOSLOW, T. F. & PEMBERTON, S. G. 1988. An integrated approach to the sedimentological analysis of some Lower Cretaceous shoreface and delta

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front sandstone sequences. Canadian Society of Petroleum Geologists Memoir, 15, 373-386. PEMBERTON, S. G. & FREY, R. W. 1984. Ichnology of storm-influenced shallow marine sequence: Cardium Formation (Upper Cretaceous) at Seebe, Alberta. In: STOTT, D. F. & GLASS, D. J. (eds) The Mesozoic of Middle North America. Canadian Society of Petroleum Geologists Memoir, Calgary, Alberta, 9, 281-304. PEMBERTON, S. G. & MACEACHERN, J. A. 1995. The sequence stratigraphic significance of trace fossils: examples from the Cretaceous foreland basin of Alberta, Canada. In: VAN WAGONER, J. C. & BERTRAM, G. (eds) Sequence Stratigraphy of Foreland Basin Deposits: Outcrop and Subsurface Examples from the Cretaceous of North America. American Association of Petroleum Geologists, Memoirs, Tulsa, Oklahoma, 64, 429^75. PEMBERTON, S. G. & MACEACHERN, J. A. 1997. The ichnological signature of storm deposits: the use of trace fossils in event stratigraphy. In: BRETT, C. E. (ed.) Paleontological Event Horizons: Ecological and Evolutionary Implications. Columbia University Press, New York, 73-109. PEMBERTON, S. G. MACEACHERN, J. A. & FREY, R. W. 1992. Trace fossil facies models environmental and allostratigraphic significance. In: WALKER, R. G. & JAMES, N. P. (eds) Facies Models: Response to Sea Level Change. Geological Association of Canada, St John's, Newfoundland, 47-72. PEMBERTON, S. G., SPILA, M., PULHAM, A. J., SAUNDERS, T., MACEACHERN, J. A., ROBBINS, D. & SINCLAIR, I. 2001. Ichnology and Sedimentology of Shallow to Marginal Marine Systems. Ben Nevis and Avalon Reservoir, Jeanne d'Arc Basin. Geological Association of Canada, Short Course Notes, St John's, Newfoundland, 15. READING, H. G. (ed.) 1986. Sedimentary Environments and Facies. Blackwell Scientific Publications, Oxford, 113-154. REINECK, H.-E. 1963. Sedimentgefuge im Bereich der sudlichen Nordsee. Abhandlungen der Senckenbergische Naturforschende Gesellschaft, 505, 138p. REINSON, G. E. 1984. Barrier-island and associated strand plain systems. In: WALKER, R. G. (ed.), Facies Models (2nd edn). Geological Association of Canada, Geoscience Canada Reprint Series, Kitchener, Ontario, 1, 119-140. SAUNDERS, T. & PEMBERTON, S. G. 1986. Trace Fossils and Sedimentology of the Appaloosa Sandstone: Bearpaw-Horseshoe Canyon Formation Transition, Dorothy, Alberta. Canadian Society of Petroleum Geologists Fieldtrip Guide Book, Calgary, Alberta. SAUNDERS, T. D. A., MACEACHERN, J. A. & PEMBERTON, S. G. 1994. Cadotte Member Sandstone: progradation in a boreal basin prone to winter storms. In: PEMBERTON, S. G., JAMES, D. P. & WIGHTMAN, M. (eds) Mannville Core Conference, Canadian Society of Petroleum Geologists, Calgary, Alberta, 331-349. SEILACHER, A. 1955. Ecological significance of fossil tracks and trails. Geological Society of America Bulletin, 66, 1663.

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Animal-substrate interactions in freshwater environments: applications of ichnology in facies and sequence stratigraphic analysis of fluvio-lacustrine successions LUIS A. BUATOIS & M. GABRIELA MANGANO Conicet-Insugeo, Casilla de correo 1, correo central, 4000 San Miguel de Tucumdn, Argentina (e-mail: [email protected]) Abstract: At present, three continental archetypal ichnofacies are widely accepted: the Scoyenia, Mermia and Coprinisphaera ichnofacies. The last is present in palaeosols, and the first two occur in fluvio-lacustrine environments. Additionally, the Skolithos ichnofacies may be present in relatively high-energy fluvio-lacustrine deposits. The ichnofauna from active fluvial channels is characterised by low-diversity assemblages of simple vertical burrows and escape traces, referred to the Skolithos ichnofacies. Abandoned or inactive channel deposits characteristically contain low-diversity assemblages dominated by meniscate traces. Floodplain water bodies that experienced progressive drying (desiccated overbank deposits) may contain abundant arthropod and vertebrate trackways, backfilled meniscate traces, ornamented burrows and bilobate traces with scratch marks, which allow recognition of the Scoyenia ichnofacies. Floodplain water bodies that are filled by overbank vertical accretion without experiencing desiccation (overfilled overbank deposits) include simple grazing trails, locomotion trails and horizontal dwelling burrows, representing impoverished occurrences of the Mermia ichnofacies. Hydrologically closed lakes are very stressful environments in which subaqueous ichnofaunas are rare. The richest ichnofaunas in closed lakes are present at the lake margins, and record the activity of terrestrial rather than aquatic faunas (Scoyenia ichnofacies). Hydrologically open lakes host relatively diverse and abundant ichnofaunas, comprising the Scoyenia ichnofacies in low-energy, lake-margin areas, and the Mermia ichnofacies in permanent subaqueous lacustrine zones. Sediments deposited in relatively high-energy lacustrine environments, such as wave-dominated shorelines and delta mouth-bars, commonly are represented by the Skolithos ichnofacies. Although continental trace fossils have not been extensively used in sequence stratigraphy, they have potential for future integrated study. Softground trace fossils are commonly well developed in overfilled lake basins and are useful to delineate parasequences and parasequence sets. In balanced-fill and underfilled lake basins, softground ichnofaunas are poorly developed because of stressful conditions. In contrast, firmground suites are rare in overfilled lake basins, but widespread in lowstand deposits of balanced-fill and underfilled lake basins. Early lowstand amalgamated incised fluvial channels are usually unbioturbated, but palaeosol ichnofaunas (e.g. the Coprinisphaera ichnofacies) may delineate sequence boundaries in interfluve areas. Increasingly isolated fluvial channels encased in overbank deposits develop during late lowstand, and lacustrine deposits accumulate during transgressions, favouring preservation of biogenic structures.

The study of continental ichnofaunas has shown Archer 1989; Bromley & Asgaard 1991; Buatois an explosive development during the last decade. & Mangano 1995a; Labandeira 1997, 1998, This is revealed not only through careful docu- 2002; Buatois et al 1998; Genise et al 2000; mentation of individual cases integrating ichno- Miller & Labandeira 2003). logical, sedimentological and palaeobiological This paper focuses on the ichnology of fluviodata (e.g. Gierlowski-Kordesch 1991; Pickerill lacustrine depositional systems. Our emphasis 1992; Buatois & Mangano 1993; Genise & will be on the aquatic trace fossil assemblages Bown 1994a, 1994b; MacNaughton & Pickerill of continental environments; fully terrestrial 1995; Metz 1995; Buatois et al. 1996a, 1997; ichnofaunas preserved in palaeosols are reviewed Mangano et al. 1997, 2001), but also through by Genise et al. (2004). The objectives of this renewed attempts to provide a conceptual back- paper are: ground to the analysis of organism-substrate interactions in non-marine successions b y t o review present knowledge of freshwater means of definition of archetypal i c h n o f a c i e s i c h n o f a c i e s and the evaluation of temporal and spatial to evaluate the utility of biogenic structures in trends in trace fossil distribution (e.g. Maples & facies analysis by providing a summary of the From: MclLROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 311-333. 0305-8719/04/$ 15.00 © The Geological Society of London.

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ichnology of fluvio-lacustrine environments; and to explore the potential uses of trace fossils in sequence stratigraphic analysis of continental successions. The ichnofacies model: its expansion into continental environments Archetypal or Seilacherian ichnofacies are trace fossil assemblages that recur through long intervals of geologic time and are characteristic of a given set of environmental conditions (Frey & Pemberton 1984, 1985). The ichnofacies model, originally put forward by Seilacher (1967), has been expanded and refined in a series of papers. Modifications include: definition of additional marine ichnofacies, such as the Psilonichnus and Trypanites ichnofacies (Frey & Seilacher 1980; Frey & Pemberton 1987); refinements in the characterisation of previously defined ichnofacies, such as the Glossifungites ichnofacies (Pemberton & Frey 1985; MacEachern et al. 1992); further analysis of the hardground ichnofacies (Bromley & Asgaard 1993; de Gibert et al, 1998); and expansion of the model to cover continental environments (Smith et al. 1993; Buatois & Mangano 1995a; Genise et al. 2000). In his original model, Seilacher (1967) recognised only one ichnofacies for continental environments, the Scoyenia ichnofacies. He proposed the Scoyenia ichnofacies for 'non-marine sands and shales, often red beds, with a distinctive association of trace fossils' and referred to a previous schematic illustration of this ichnofauna (Seilacher 1963, fig. 7), which included

Fig. 1. Ichnofacies model of continental environments.

meniscate burrows, arthropod trackways and bilobed traces, as well as several physical sedimentary structures (e.g. desiccation cracks). Frey et al. (1984) noted that the Scoyenia ichnofacies subsequently was used as a catchall for all assemblages of continental trace fossils. As noted by these authors, the Scoyenia ichnofacies has precise environmental implications because it characterizes low-energy continental deposits periodically exposed to air or inundated, intermediate between aquatic and non-aquatic (see also Frey & Pemberton 1984, 1987). The fact that the Scoyenia ichnofacies was only one of the recurrent trace fossil assemblages of continental environments, and that continental environments are as diverse as marine settings, has long been acknowledged by ichnologists. However, it was not until recently that studies addressing the problem of recognizing additional continental ichnofacies were published (e.g. Smith et al. 1993; Buatois & Mangano 1995a; Bromley 1996; Genise et al. 2000). At present, three continental archetypal ichnofacies are widely accepted: the Scoyenia, Mermia and Coprinisphaera ichnofacies (Fig. 1; see review in Mcllroy et al. 2004). The last is present in palaeosols and is analysed in detail by Genise et al. (2004); only the first two occur in fluviolacustrine environments, and are therefore discussed herein. Bromley (1996) also tentatively proposed the Rusophycus ichnofacies for fluvial to shallow-lacustrine environments and the Fuersichnus ichnofacies for lacustrine settings below the fair-weather wavebase. The Rusophycus ichnofacies is dominated by locomotion and resting traces of arthropods, and cannot be distinguished at present from the Scoyenia

NON-MARINE PALAEOENVIRONMENTS & ICHNOLOGY

ichnofacies. The Fuersichnus ichnofacies is based on examples in which the eponymous ichnogenus occurs in non-marine settings. However, the 'type' examples suggested are from fluvial deposits (MacNaughton & Pickerill 1995) and ephemeral alluvial plain/sandflat facies (Gierlowski-Kordesch 1991) rather than relatively deep lacustrine settings. As currently defined, the Fuersichnus ichnofacies cannot be distinguished from the Scoyenia ichnofacies. Highenergy continental environments, such as lacustrine wave-dominated shorelines, fluvial channels and delta mouth-bars, commonly contain simple vertical burrows (Skolithos), Ushaped vertical burrows (Arenicolites) and escape structures (e.g. Bromley & Asgaard 1979; Mangano et al. 1994). Such assemblages, included by Bromley & Asgaard (1991) in the Arenicolites ichnofacies, are hardly distinctive from the marine Skolithos ichnofacies, so it is best to consider them as non-marine occurrences of the Skolithos ichnofacies (Buatois & Mangano 1995a, 1998). The Scoyenia ichnofacies (Fig. 2), as redefined by Frey et al. (1984) and Buatois & Mangano (1995a), is characterised by: horizontal meniscate backfilled traces produced by mobile deposit feeders; locomotion traces, including both trackways and trails; vertical domiciles; a mixture of invertebrate (mostly arthropod), vertebrate and plant traces; low to moderate ichnodiversity; and

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localized high abundance. Among the meniscate traces, Scoyenia, Beaconites and Taenidium are typical components. Arthropod trackways are represented by a wide variety of ichnotaxa, including Umfolozia, Merostomichnites, Diplichnites, Hexapodichnus, Permichnium and Acripes. Other horizontal traces include bilobate locomotion (Cruziana) and resting structures (Rusophycus), simple forms (Pianolites and Palaeophycus), sinuous crawling traces (Cochlichnus) and banana-shaped traces (Fuersichnus). Vertical burrows are represented by Skolithos and Macanopsis. Short vertical burrows are commonly referred to as Cylindricum (e.g. Pollard 1981). Crayfish burrows, included in the ichnogenus Camborygma, are common components of post-Palaeozoic examples (Wells 1977; Hasiotis et al. 1993). Vertebrate tracks may be abundant, including those produced by amphibians (e.g. Limnopus, Anthichniuni), reptiles (e.g. Chirotherium, Rhynchosauroides) including Dinosaur (e.g. Grallator, Eubrontes), mammals (e.g. Mylodontidichnum, Neomagatherichnum) and birds (e.g. Jindongornipes, Koreanornis) (Gand & Haubold 1984; Aramayo & Bianco 1987a, 1987b, 1996; Fuglewicz et al. 1996; Lockley 1991). Although the Scoyenia ichnofacies is commonly represented by lowdiversity ichnocoenoses of meniscate trace fossils (Frey et al. 1984), Palaeozoic examples usually consist of moderate diverse assemblages of arthropod trackways (Buatois & Mangano 1995a). A narrower definition was recently proposed by Bromley (1996), who considered the

Fig. 2. Schematic reconstruction of the Scoyenia ichnofacies. Based on Buatois et al. (2002).

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Fig. 3. Schematic reconstruction of the Mermia ichnofacies. Based on Buatois et al. (2002).

Scoyenia ichnofacies as a continental equivalent of the firmground Glossifungites ichnofacies of the marine realm. The Scoyenia ichnofacies may be subdivided into two distinct suites: one characterised by meniscate, backfilled structures without ornamentation (e.g. Taenidium, Beaconites) developed in a soft substrate, and the other typified by striated traces (e.g. Scoyenia, Spongeliomorpha), cross-cutting the former and developed in a firm substrate (Buatois et al. 1996b; Buatois & Mangano 2002). The resulting palimpsest surfaces or composite ichnofabrics reflect progressive desiccation of sediment. This view was recently adopted by Savrda et al. (2000), who documented an assemblage of backfilled trace fossils without ornamentation (Taenidium) in soft substrates of a fluvial environment. The Scoyenia ichnofacies occurs in low-energy deposits periodically exposed to air or periodically inundated, and intermediate between aquatic and non-aquatic environments (see also Frey & Pemberton 1984, 1987). Associated physical structures are indicative of periodic subaerial exposure, such as desiccation cracks and raindrop imprints. In fluvial systems, this ichnofacies is present in floodplain deposits, covering a wide variety of sub-environments, such as ponds, levees and crevasse splays (Frey & Pemberton 1984, 1987; Frey et al. 1984; Buatois & Mangano 1995a, 2002). In lacustrine complexes, the Scoyenia ichnofacies typically characterizes lakemargin deposits, being present in both open and closed lake basins, and in both ephemeral and perennial systems (Buatois & Mangano 1998). The Scoyenia ichnofacies also occurs in wet interdunes (Buatois & Mangano 1996).

The Mermia ichnofacies (Fig. 3), proposed by Buatois & Mangano (1995a), is characterised by: dominance of horizontal to subhorizontal grazing and feeding traces produced by mobile deposit feeders; subordinate occurrence of locomotion traces; relatively high to moderate ichnodiversity and abundance; and low degree of specialisation of grazing patterns. Typical components of this ichnofacies include a variety of unspecialised grazing traces (e.g. Mermia, Gordia, Helminthopsis, Helminthoidichnites and Cochlichnus), simple feeding structures (e.g. Treptichnus and Circulichnis\ locomotion traces (e.g. Maculichna) and fish trails (e.g. Undichna). In a subsequent paper, Buatois & Mangano (1998) explicitly stated that high ichnodiversity does not necessarily equate with species richness. These authors noted that different ichnogenera recorded in this ichnofacies might result from minor variations of behaviour of a very simple, unspecialised grazing pattern developed by a single trace-maker (e.g. Helminthopsis, Helminthoidichnites, Gordia, Mermia). The Mermia ichnofacies characterizes finegrained sediments that occur in well-oxygenated, low-energy, permanently subaqueous zones of lacustrine systems (Buatois & Mangano 1995a). Under anoxic conditions the ichnofacies is suppressed. This ichnofacies is typical of open perennial siliciclastic lacustrine systems, but has been recognised in carbonate lakes also (Buatois et al. 2000a; de Gibert et al. 2000). The Mermia

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ichnofacies comprises sediments deposited in fully lacustrine environments, extending from shallow to deep bathymetric zones. There are no archetypal trace fossil associations that clearly distinguish shallow from deep-lacustrine settings, probably because of the wide variability of lacustrine systems. Because of this, Buatois & Mangano (1998) regarded the Mermia ichnofacies as a continental equivalent of the Cruziana, Zoophycos and Nereites ichnofacies in the classical Seilacherian scheme. It is not uncommon for trace fossil assemblages typically recorded from relatively deep lacustrine areas to be found in shallower zones. For example, Pickerill (1992) documented the Mermia ichnofacies in lacustrine shoreface deposits, being deeper zones of the lake characterised by anoxia. Buatois & Mangano (2002) expanded the environmental extent of the Mermia ichnofacies to cover ichnofaunas produced in floodplain water bodies under subaqueous conditions. This is supported by recent studies on the ichnology of modern floodplains (Mikulas 2003). The lower ichnodiversity of these floodplain ichnocoenoses in comparison with lacustrine assemblages may reflect less stable conditions and the temporary nature of floodplain ponds. As stated in its original definition (Buatois & Mangano 1995a, table 2), the Mermia ichnofacies may occur in fjord settings under freshwater conditions owing to glacial melting (see discussion in Buatois & Mangano 2003). This is a common situation in late Palaeozoic postglacial deposits of Gondwana, where the Mermia ichnofacies developed in fjord environments affected by a strong discharge of freshwater due to melting of the ice masses during deglaciation (Buatois et al 2001; Pazos 2002; Buatois & Mangano 2003). It should be stressed that ichnofacies are not indicators of particular sedimentary environments but reflect sets of environmental factors instead. This is not exclusive to continental ichnofacies; it is a lesson learnt from the history of marine ichnology (Buatois & Mangano 2002). As stated by Frey et al (1990), ichnofacies are not intended to be simply indicative of palaeobathymetry. For example, although typical of foreshore to upper-foreshore settings, the Skolithos ichnofacies may occur in more distal deposits, from lower-shoreface and offshore tempestites to deep-marine turbidites (e.g. Crimes 1977; Pemberton & Frey 1984). Regardless of the depositional environment involved, ichnofacies development reflects a set of environmental conditions. The same certainly applies for continental ichnofacies that are essentially controlled by a set of local environmental factors. Water

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availability is of prime importance in controlling trace fossil distribution in continental environments, particularly in alluvial settings (Gierlowski-Kordesch 1991). In turn, sediment water content strongly influences substrate degree of consolidation. The role of substrate consolidation as controlling trace fossil preservation is emphasised in this paper and, accordingly, the Scoyenia and Mermia ichnofacies can be seen, at least in some sense, as taphofacies sensu Bromley & Asgaard (1991). Ichnology of fluvial systems Trace fossils in fluvial successions are usually restricted to certain beds and depositional surfaces. Fluvial ichnofaunas have been reported from two main sub-environments: channel and overbank settings (Buatois & Mangano 1996). Fluvial channel ichnofaunas Fluvial channels are characterised by high to relatively high-energy, rapid fluctuations in rates of sedimentation and erosion, and coarser grain sizes than those typically deposited in adjacent environments. Channels therefore represent stressful environments for benthic organisms, and production and/or preservation of biogenic structures is usually precluded. Two main trace fossil associations can be distinguished in fluvial channels, broadly corresponding to active and abandoned channels. The ichnofauna from active channels is characterised by low-diversity, typically monospecific, suites of simple vertical burrows and escape traces (Fig. 4). Up to 30cm deep Skolithos, forming dense assemblages, were documented in Permian channel-pebble conglomerate and trough cross-bedded fine- to coarse-grained sandstone of Southern Victoria Land, Antarctica (Fitzgerald & Barrett 1986). Similar vertical burrows in fluvial channel facies have been recorded in other Devonian and Permian units of Antarctica (Bradshaw 1981; Zawiskie et al. 1983; Woolfe 1990). Escape traces were recorded in accreting parallel-laminated sandstones formed in channel bars (Sarkar & Chaudhuri 1992). The identity of the trace-maker and the functional significance of the vertical burrows in fluvial channel facies are poorly understood. Fitzgerald & Barrett (1986) suggested that 'worms' [sic] responded to a fluctuating water table in presumably ephemeral fluvial systems in the same way that polychaete annelid worms adjust their burrows to changes in water level

Fig. 4. Taphonomic pathways of fluvial ichnofaunas, showing transitions between different channel and overbank trace-fossil assemblages. Substrate consolidation plays a major role in controlling ichnofacies occurrence.

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in intertidal marginal marine environments. Burrowing crustaceans and other invertebrates are able to adapt to sandy channels as far as 3000km upstream from the fluvial mouth (Zawiskie et al. 1983). The presence of vertical burrows in relatively high-energy deposits invites comparison with the Skolithos ichnofacies, and suggests their functional interpretation as domiciles of suspension feeders. Abandoned or inactive channel deposits are characterised by low-diversity assemblages dominated by meniscate traces that have been assigned to Be aconites or Taenidium (Fig. 4) (Keighley & Pickerill 1994; Goldring & Pollard 1995). Accessory components include vertical to inclined burrows (Skolithos) and simple horizontal burrows (Palaeophycus). Examples of this type of ichnofauna have been documented repeatedly in the literature (e.g. Allen & Williams 1981; Graham & Pollard 1982; Bamford et al. 1986; Sarkar & Chaudhuri 1992; Miller & Collinson, 1994; Miller 2000). This ichnofauna reflects colonization of sandstone deposits after channel diversion ('abandonment') or, more rarely, during periods of low discharge virtually characterised by non-deposition ('inactive'). Meniscate burrows most likely reflect the activity of vagile organisms moving into the substrate in search for food, revealing a combination of bypassing and ingestion. Vertical to inclined burrows have several functions, including permanent domiciles, semi-permanent shelters, nests and passageways. Sarkar & Chauchuri (1992) noted that, in contrast to typical vertical burrows in marine settings, linings are absent. This suggests that construction of non-marine vertical burrows was probably by insects, because insects do not usually produce discrete reinforcements to burrow walls. These unlined insect burrows have commonly been referred to Cylindricum (Pollard & Lovell 1976). Some of these traces are probably not strictly dwelling structures, but temporary shelters in which insects live for relatively short periods of time (Stanley & Fagerstrom 1974). Some vertical burrows have basal bulbous terminations, representing nests excavated and/or constructed by insects for breeding purposes, which are included in the ethological category calichnia (Genise & Bown 1994a). The presence of nesting structures in abandoned channel deposits reflects the ability of insects to colonize different substrate types (Genise et al. 2000). Additionally, vertical burrows may serve as passageways connecting adjacent bioturbated layers. Congruence of size distributions and common intergradations between the different morphotypes present in abandoned channel deposits point to a common producer in most

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cases (Stanley & Fagerstrom 1974; Miller & Collinson 1994). Trace-producers are inferred to be behavioural generalists recording an opportunistic strategy (Miller & Collinson 1994). Overall, this kind of trace fossil association is similar to those from overbank deposits. This is hardly surprising, because abandoned channels lead to the formation of ponded areas, representing a process of floodplain construction (abandonedchannel accretion of Miall 1996). A somewhat anomalous occurrence of fluvial channel ichnofaunas has been reported by MacNaughton & Pickerill (1995) from the Triassic of Eastern Canada. In contrast to the usual situation, these channel deposits are more intensely bioturbated and display higher ichnodiversity than adjacent overbank deposits. These authors suggested preferential colonization of stream channels within a fluvial system developed under arid conditions with limited water supply.

Overbank ichnofaunas Overbank deposits usually host the most diverse and abundant trace fossil assemblages in fluvial depositional systems (e.g. Fordyce 1980; D'Alessandro et al. 1987; Buatois et al. 1997; Buatois & Mangano 2002). It is not unusual for the only trace fossils in a fluvial succession to be preserved in fine-grained overbank deposits interbedded within unbioturbated, stacked channel deposits. The presence of distinct trace fossil-bearing pond deposits within an otherwise unfossiliferous fluvial succession may thus be regarded as taphonomic and colonization windows (Buatois et al. 1997). The presence of these trace fossil suites reflects minor breaks in sedimentation, representing local autocyclically induced hiatal surfaces, but which are of limited lateral extent. Conversely, during periods of rapid continuous deposition invertebrate colonization is precluded. Trace fossil distribution in fluvial settings is largely a function of variability in stream discharge and the amount of time between depositional episodes (D'Alessandro et al. 1987). Taphonomic constraints are key to the final shaping of fluvial ichnofaunas. Ratcliffe & Fagerstrom (1980) have shown that Holocene overbank deposits are very rich in invertebrate traces, but their ancient counterparts are considerably poorer, indicating the low preservation potential of these biogenic structures. Maples & Archer (1989) analysed this problem further, suggesting that preservation of most of the very shallow insect traces in floodplain deposits requires exceptional conditions, including deposition of fine-grained heterogeneous sediment,

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little or no reworking, and enough time between depositional events to allow colonization, but not so much time that plant colonization obliterates animal traces (Fig. 4). Overbank settings include a wide variety of deposits, such as floodplains, crevasse splays and levees. The ichnology of these deposits has recently been reviewed by Buatois & Mangano (2002), who recognised two main recurrent trace fossil associations, which are herein referred to as the 'desiccated overbank' and the 'overfilled overbank' associations. Desiccated overbank deposits correspond to floodplain water bodies that experienced progressive drying (Fig. 4). Ichnofaunas from these deposits include abundant arthropod trackways (e.g. Diplichnites, Trachomatichnus), vertebrate trackways (e.g. Limnopus, Hyloidichnus), backfilled meniscate traces (e.g. Scoyenia, Taenidium), ornamented burrows (e.g. Spongeliomorpha, Tambid), and bilobate traces with scratch marks (Cruziana, Rusophycus). Vertical burrows, referred to Skolithos and Cylindricum, are accessory components. Insect and arachnid nesting structures may also be present. Examples of this type of association have been documented by numerous authors (e.g. Bromley & Asgaard 1979, their Scoyenia and Rusophycus assemblages; Biron & Dutuit 1981; Bracken & Picard 1984; Squires & Advocate 1984; D'Alessandro et al 1987; Debriette & Gand 1990; Sarkar & Chaudhuri 1992; Smith 1993; Gand et al 1997; Kim & Paik 1997; Eberth et al 2000; Savrda et al 2000; Aramayo & Bocanegra 2003). Invertebrate ichnodiversity is low and only rarely

moderate, but vertebrate traces may be relatively diverse. Associated physical structures are indicative of periodic subaerial exposure (e.g. desiccation cracks and raindrop imprints). Desiccated floodplain ichnofaunas represent an example of the Scoyenia ichnofacies (Buatois & Mangano 2002). Some of the trace fossils from this assemblage display features indicative of firm substrates, such as striated walls in Scoyenia and Spongeliomorpha, sharp scratch marks in Tambia, Cruziana and Rusophycus, and welldefined appendage imprints in arthropod trackways. In fact, desiccated floodplain ichnofaunas may be subdivided into two distinct suites: one 'pre-desiccation suite' characterised by meniscate, backfilled structures without ornamentation (e.g. Taenidium, Beaconites) developed in a soft substrate, and the second 'desiccation suite' typified by striated traces (e.g. Scoyenia, Spongeliomorphd), cross-cutting the former and developed in a firm substrate (Fig. 5a, b) (Buatois et al. 1996b). The resulting palimpsest ichnocoenoses/assemblages reflect changing substrate conditions during progressive desiccation of floodplain sediments (Fig. 4). Desiccated overbank ichnofaunas tend to be dominant in distal overbank settings and/or arid to semiarid settings. Overfilled overbank deposits correspond to floodplain water bodies that are filled by overbank vertical accretion without experiencing desiccation (Fig. 4). Ichnofaunas from overfilled overbank deposits are dominated by simple grazing trails (e.g. Helminthopsis, Helminthoidichnites), locomotion trails (e.g. Cochlichnus)

Fig. 5. Two suites of the Scoyenia ichnofacies in desiccated overbank deposits. Permian, La Colina Formation, La Rioja province, western Argentina. All bars are 1 cm. (a) Beaconites barretti. Meniscate backfilled trace fossils lacking striated walls, suggesting emplacement in a softground. (b) Firmground meniscate striated trace fossils cross-cutting the softground suite.

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Fig. 6. Depauperate Mermia ichnofacies in overfilled overbank deposits. Permian, La Golondrina Formation, Santa Cruz Province, southern Argentina. All bars are 1 cm. (a) Poorly defined sinusoidal trails of Cochlichnus anguineus, showing intense deformation and presence of irregular levees. Sinusoidal trails cross-cut by shafts of Ctenopholeus kutscheri. Morphologic features indicate emplacement in water-saturated substrates, (b) Overlapping of well-defined specimens of Cochlichnus anguineus emplaced in a more cohesive substrate.

and horizontal dwelling burrows (e.g. Ctenopholeus, Palaeophycus). Accessory components include resting traces of bivalves or conchostracans (e.g. Lockeia). Tetrapod traces, arthropod trackways and bilobate traces with scratch marks are either absent or represent very minor components of the assemblage, in contrast to the desiccated overbank assemblage. Backfilled traces are typically absent. Examples of these ichnofaunas are also well documented (Turner 1978; Fordyce 1980; Miller 1986; Pollard & Hardy 1991; Gluszek 1995; Buatois el al. 1997; Buatois & Mangano 2002). Ichnodiversity is thus generally low but may be moderate in some cases. The assemblage is characterised by superficial to very shallow structures (Fig. 6a, b). Physical structures indicative of subaerial exposure are absent. Additionally, morphological details are very poorly preserved (Fig. 6a), suggesting that the traces were formed in a water-saturated substrate (e.g. Buatois et al. 1997). Features of these ichnofaunas reflect the subaqueous nature of the associated environment. Poorly preserved traces may be cross-cut by better-defined softground reflecting improving taphonomic conditions due to increasing compaction (Figs 4, 6b). Overbank vertical accretion with little associated erosion commonly allows good preservation of the ichnofauna and precludes desiccation of the water bodies. Buatois & Mangano (2002) noted that, although formed in floodplains, these ichnofaunas lack most of the diagnostic features of the Scoyenia ichnofacies, and suggested that they represent examples of an impoverished Mermia ichnofacies. Although overfilled overbank settings are of shorter lifespan than permanent lakes, freshwater bodies formed in

floodplain basins support the establishment of an aquatic biota. Floodplain ponds represent suitable environments for the development of low- to moderate-diversity suites of invertebrates, such as insects, crustaceans, nematodes, nematomorphs, oligochaetes, molluscs, ostracodes and conchostracans. The lower ichnodiversity of these water bodies in comparison with their equivalents from lacustrine basins is probably an expression of the less stable conditions and temporary nature of overbank environments. However, differences in stability and temporal persistence between these two broad environmental settings is irrelevant for opportunistic organisms able to rapidly colonize stagnant waters in floodplain environments. Overfilled overbank ichnofaunas tend to dominate in proximal overbank settings and/or temperate and humid settings. Ichnology of lacustrine systems Recent interest in lacustrine ichnology has paralleled renewed research into the sedimentology and stratigraphy of lake successions (e.g. Anadon et al. 1991; Gierlowski-Kordesch & Kelts 1994, 2000). Lacustrine biogenic structures probably have the highest preservation potential of all continental trace fossils. Preservation of biogenic structures is particularly favoured in low-energy settings. For example, alternation of very fine sand and mud deposited from underflow currents is ideal for preservation of tiny surface to very shallow traces (Buatois & Mangano 1995a, 1998). Underflow currents deposit sediments well below wavebase, where physical reworking is rare. Low-density, fine-grained

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turbidity currents can usually create a similar taphonomic scenario. In low-energy shoreline areas, trace fossil preservation may commonly be linked to rapid influx of sand via non-erosive sheet floods entering the lake (e.g. Zhang et al. 1998). Lacustrine systems can be divided into hydrologically open (i.e. with an outlet) and hydrologically closed (i.e. without an outlet) (Gore 1989).

Closed lake ichnofaunas Closed lakes are characterised by high salinity and rapidly fluctuating shorelines (Gore 1989). This type of lake represents stressful environmental conditions for the components of such ecosystems. Accordingly, faunal diversity is typically extremely low, and biogenic structures are few, if not completely absent. In some cases,

TAPHONOMIC PATHWAYS LACUSTRINE ICHNOFAUNAS

Fig. 7. Taphonomic pathways of lacustrine ichnofaunas. Hydrologically open lakes contain more varied softground ichnofacies. Vertical ichnofacies changes reflect lake regression. Hydrologically closed lakes display limited softground ichnofacies, but abundant firmground suites in lake-margin deposits.

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Fig. 8. Densely superimposed trackways in marginal deposits of playa lakes. Permian? La Rioja Province, western Argentina, (a) General view of a sandstone top exhibiting high density of arthropod trackways. Coin is 1.6cm. (b) Close-up of the tracked surface. Bar is 1 cm.

trace fossils are restricted to certain beds and record short periods of reduced stress, such as periods of increased productivity or reduced salinity (Price & McCann 1990). The trace fossil Beaconitesfiliformis, attributed to chironomids, occurs in hypersaline lacustrine deposits (Uchman & Alvaro 2000). Notably, an ichnofauna of moderate diversity has been documented in Neogene lacustrine evaporites of Spain (Rodriguez-Aranda & Calvo 1998). The ichnofauna consists of plant, invertebrate traces of chironomids, coleopterans and annelids, and vertebrate traces of mammals and birds. Undoubtedly, the richest ichnofaunas in closed lake systems are present at the lake margins and record the activity of terrestrial rather than aquatic faunas (Fig. 7). These lake-margin trace fossil assemblages are typical examples of the Scoyenia ichnofacies, dominated by meniscate backfilled traces, striated burrows and arthropod trackways. Under appropriate taphonomic conditions, omission surfaces totally covered by trackways are preserved (Figs 7, 8a, b). Associated physical structures indicate subaerial exposure (e.g. desiccation cracks, raindrop imprints). Examples of these ichnofaunas in playa lake settings have been reported in a number of studies (Bromley & Asgaard 1979; Gierlowski-Kordesch 1991; Dam & Stemmerik 1994; Kozur & Lemone 1995; Clemmensen et al. 1998; Zhang et al. 1998; Schlirf et al. 2001).

ichnofaunas. The Scoyenia ichnofacies is typically present in low-energy, lake-margin areas, whereas the Mermia ichnofacies characterizes permanent subaqueous lacustrine zones. Sediments deposited in moderate to high-energy lacustrine environments, such as wavedominated shorelines and delta mouth-bars, are commonly characterised by the Skolithos ichnofacies (Fig. 9). These deposits contain simple

Open lake ichnofaunas Open lakes are characterised by low salinity and relatively stable shorelines (Gore 1989). Such lakes may host relatively diverse and abundant

Fig. 9. Elements of the Skolithos ichnofacies in highenergy continental deposits. Arenicolites isp. deltaic mouth-bar deposits. Triassic, Tanzhuang Formation, central China.

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vertical burrows (Skolithos), U-shaped vertical burrows (Arenicolites) and escape structures (e.g. Bromley & Asgaard 1979; Mangano et al. 1994). The distinction between lake-margin and fully subaqueous deposits can be established through careful ichnological analysis. Lake-margin deposits are characterised by meniscate, backfilled trace fossils and arthropod trackways of the Scoyenia ichnofacies. Meniscate trace fossils are well documented from marginal lacustrine facies (e.g. Daley 1968; Metz 1995, 1996; Hester & Lucas 2001), whereas assemblages dominated by arthropod trackways have been recorded in the same setting (e.g. Pollard et al. 1982; Pollard & Walker 1984; Walker 1985; Bandel & QuinzioSinn 1999; Cook & Bann 2000). Desiccation of marginal lacustrine deposits allows construction and preservation of striated meniscate traces and burrow galleries, such as Spongeliomorpha (Metz 1993) in firmground substrates. More rarely, bioerosion in stromatolites has been documented (Ekdale et al. 1989). Metz (1996) noted that, in Triassic lacustrine deposits of the Newark Basin, elements of the Mermia ichnofacies are replaced by typical representatives of the Scoyenia ichnofacies during shoreline regression. Vertebrate trace fossils are extremely common in marginal lake facies (e.g. Olsen et al. 1978; Lockley et al. 1986; Lim et al. 1989; Prince & Lockley 1989). Avian trackways are also useful to delineate palaeoshorelines in lacustrine successions (Alonso 1985; Yang et al. 1995). A similar situation is recorded by dinosaur trackways that occur at the top of prograding, shallowingupward lacustrine successions of the Jurassic Morrison Formation of Colorado, recording the position of the ancient shoreline (Lockley et al. 1986; Prince & Lockley 1989). Dinosaur and fitosaur trackways are present in Triassic lacustrine mudflat facies of the northwest United States, associated with successive maximum regressive phases of the lacustrine system (Olsen et al. 1978). Therefore, vertebrate trackways help in delineation of cycles of expansion and contraction of water bodies (Lockley 1986, 1989). Additionally, vertebrate trackways provide valuable information on palaeoclimatic conditions. For example, trackways of theropod dinosaur dominate over those of herbivorous dinosaur in semiarid lacustrine systems (Leonardi 1989). In the permanent subaqueous zone of lakes, under conditions of low energy and a high degree of environmental stability, the Mermia ichnofacies develops. Feeding and grazing traces of detritus and deposit feeders are the

dominant component of the ichnofacies, with arthropod trackways subordinate. Vertebrate traces are represented by the fish trail Undichna (Anderson 1976; Higgs 1988; Turek 1989; Buatois & Mangano 1994; de Gibert et al. 1999), and the amphibian trackways include Lunichnium and Gracilichnium (Turek 1989). Oxygen content, energy, food supply and substrate are important controls on trace fossil distribution in lacustrine systems. Oxygenation is commonly a limiting factor; in lakes with thermal stratification the hypolimnion becomes anoxic/dysoxic, and bioturbation is absent. Turbidity and underflow currents may provide oxygen to lake bottoms, allowing the establishment of benthic communities. Examples of permanently subaqueous freshwater ichnofaunas have been reported in various studies (Gibbard & Stuart 1974; Gibbard 1977; Gibbard & Dreimanis 1978; Walter 1985; Miller et al. 1991; Pickerill 1992; Buatois & Mangano 1993; Buatois et al. 1996a, 2000a; Walter & Suhr 1998; de Gibert et al. 2000; Melchor et al. 2003; Melchor 2004). Ichnological analysis helps to distinguish between density underflows and turbidity currents. Both processes commonly operate in open lacustrine systems and are difficult to differentiate on sedimentological criteria alone. Turbidites are deposited by episodic currents that involves re-deposition of sediment initially emplaced under unstable conditions. Once mobilised, a dense turbid fluid flows downslope to the basin-floor to deposit the turbidite. Underflow currents are a relatively continuous phenomenon that represents the uninterrupted transport of river-borne sediment into the lake basin, and are influenced by geostrophic effects (Pharo & Carmack 1979). Turbidites contain trace fossils at bedding tops, recording colonization of opportunistic organisms after episodic emplacement of the event bed (Buatois & Mangano 1998). Underflow current deposits display distinctive suites of trace fossils in each lamina or lamina-set (e.g. Buatois & Mangano 1995b; Melchor 2001), reflecting animal activity contemporaneous with sedimentation instead of re-colonization after deposition of the turbidite bed (Buatois & Mangano 1998). The contrasting colonization styles allow differentiation between near-continuous deposition from river-fed density underflows and episodic sedimentation from turbidity currents. Biogenic structures are also useful to distinguish between marine and lacustrine turbidites, which are virtually indistinguishable in terms of physical sedimentary structures. Deep-sea turbidites are characterised by a high diversity of ornate grazing traces and graphoglyptids that

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reflect highly specialised feeding strategies recorded by the Nereites ichnofacies (Seilacher 1967; Miller 1991; Uchman 1995, 2004). Deep marine environments are typically more stable than lacustrine systems and host a more diverse benthic fauna. In contrast, deep-lacustrine settings are characterised by non-specialised grazing and feeding traces (Buatois & Mangano 1998). Non-specialised feeding patterns are exemplified by the ichnogenus Mermia, which displays looping and common self-crossing, recording the repeated passage of the tracemaker across the same portion of sediment. Such non-specialised trophic strategies probably relate to the abundance and accessibility of food in lacustrine systems (Buatois & Mangano 1995a). The comparatively lower ichnodiversity of lakes in comparison with deep marine settings results from the more ephemeral nature of the former (Buatois & Mangano 1998). Structures referred to Paleodictyon in freshwater assemblages (Archer & Maples 1984; Pickerill 1992) are remarkably simpler than those from marine turbidites. A feeding trace referred to Nereites in lacustrine turbidites (Hu et al. 1998) lacks the internal structure of this ichnogenus and only superficially resembles this marine form. Although most ichnological studies have focused on siliciclastic systems, a few papers dealing with the ichnology of ancient carbonate lakes have been published in recent years. Invertebrate ichnofaunas in Cretaceous lacustrine lithographic limestones of Spain are restricted and dominated by small, horizontal, shallowtier trace fossils of detritus and deposit feeders (de Gibert et al. 2000; Buatois et al 2000a). Associated vertebrate ichnofaunas comprise crocodilian trackways (Moratalla et al. 1995), pterosaurian tracks (Lockley et al. 1995) and fish trails (de Gibert et al. 1999). Additionally, it has been noted that these lacustrine ichnofaunas differ in taxonomic composition, proportion of infauna and ethologic significance from trace fossil assemblages described from marine lithographic limestones (Buatois et al. 2000a). In large, deep lakes, depth-related trace fossil zones can be established. For example, Walter & Suhr (1998) documented a bathymetric zonation in Pleistocene glacial lakes of Germany. In these lakes, shallow-lacustrine trace fossil assemblages are dominated by arthropod trackways (e.g. Warvichnium, Glaciichnium, Lusatichnium), whereas grazing trails are abundant in deeper zones (e.g. Cochlichnus, Gordia, Helminthoidichnites). Although there are no archetypal, recurrent ichnofacies that clearly distinguish shallow- from deep-lacustrine settings,

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ichnological zonations may prove to be useful at the basin scale. Applications of ichnology in continental sequence stratigraphy In comparison with their marine counterparts, continental trace fossils have not been extensively used in sequence stratigraphy. In marine siliciclastic successions, biogenic structures aid in sequence stratigraphy in two main ways: (1) identification of stratigraphic discontinuities using substrate-controlled ichnofacies, and (2) recognition of palaeoenvironmental changes through detailed documentation of vertical changes in softground trace fossils (see recent reviews in Pemberton et al. 2001; Mcllroy 2004; Pemberton 2004). In particular, the firmground Glossifungites ichnofacies develops in stable and cohesive substrates, reflecting erosive exhumation of the substrate (MacEachern et al. 1992). The hardground Trypanites ichnofacies and the woodground Teredolites ichnofacies are used to a lesser extent (Pemberton et al. 2001). Currently recognised allostratigraphic surfaces are incised valley surfaces, incised submarine canyon surfaces, regressive surfaces of marine erosion, ravinement surfaces (transgressive) and co-planar surfaces of lowstand and transgressive erosion (MacEachern et al. 1992; Pemberton et al. 2001; Pemberton 2004). Softground ichnofacies and ichnofabrics are mostly used to detect sharp palaeoenvironmental changes across discontinuity surfaces or gradual palaeoenvironmental changes across parasequences. In the former case, they aid in the classification of stratigraphic discontinuities; in the latter, they allow palaeoenvironmental zonation of parasequences and help in identifying trends in parasequence stacking patterns. The use of trace fossils in continental sequence stratigraphy cannot be simply based on the extrapolation of concepts established from the analysis of the marine stratigraphic record. Discrepancies result from both differences in the nature of continental versus marine ichnofaunas and peculiarities of continental depositional systems with respect to the marine system. For example, substrate-controlled trace fossils in continental successions only rarely indicate erosional exhumation; they are commonly related to desiccation of water bodies (e.g. Buatois et al. 1996b). Moreover, continental firmgrounds form rapidly under conditions of subaerial exposure, without implying a significant hiatus (Flirsich & Mayr 1981). Additionally, some sequence stratigraphic concepts (e.g. parasequence) are difficult

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Fig. 10. Trace fossil assemblages and lacustrine sequence stratigraphy: (a) overfilled lakes; (b) balanced-fill lakes; (c) underfilled lakes. Stratal patterns illustrated after Bohacs et al. (2000).

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to apply in certain continental settings, such as fluvial depositional systems (Van Wagoner et al. 1990). Lacustrine systems also defy direct application of the sequence stratigraphic model established for marine siliciclastic systems. As noted by Bohacs et al. (2000), lakes differ from oceans in several ways, including the smaller volumes of sediment and water included in lacustrine systems, the direct link between lake level and sediment supply, and the fact that lake shoreline migration may be due not only to progradation but also to withdrawal of water. Finally, in continental systems other allocyclic controls, such as tectonism and climate, play major roles (Shanley & McCabe 1998). Bohacs et al. (2000) provided a workable sequence stratigraphic framework for the analysis of lacustrine successions. These authors recognised three different types of lake basin: overfilled, balanced-fill and underfilled. Overfilled lake basins are characterised by rate of sediment/water exceeding potential accommodation. Fluvio-lacustrine siliciclastic deposits that accumulate in hydrologically open lakes are the most common, and parasequence development is driven mainly by shoreline progradation and delta-channel avulsion. Balanced-fill lake basins occur when the rates of sediment/water supply are in balance with potential accommodation. Carbonate and siliciclastic facies can accumulate in lakes that are alternatively hydrologically open and closed. Successions record not only progradational parasequences, but also aggradation of chemical sediments due to desiccation. Underfilled lake basins are characterised by rates of accommodation exceeding rate of supply of sediment/water. In hydrologically closed lakes deposition of evaporites dominates, and parasequences record vertical aggradation due to desiccation. Softground trace fossils are commonly well developed in overfilled lake basins (Fig. lOa), and are useful to delineate parasequences and parasequence sets. Fluvial discharge into overfilled lakes usually generates density currents that oxygenate lake bottoms, allowing the establishment of epifaunal and infaunal communities. Additionally, hydrologically open lakes promote freshwater conditions and no stress due to hypersalinity occurs, leading to the development of a relatively diverse benthos. Upward-shallowing successions due to delta progradation are common. Distal facies commonly consist of underflow current deposits that display elements of the Mermia ichnofacies in each lamina or lamina-set (e.g. Buatois & Mangano 1995b; Melchor 2001). Intermediate facies may contain wave-dominated delta-front deposits, including

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hummocky cross-stratified sandstones deposited by storm events, as well as fair-weather waveand combined-flow ripple cross-laminated sandstone. Grazing trails of the Mermia ichnofacies may occur forming colonization suites at the top of tempestites in such settings (e.g. Buatois & Mangano 1995b; Melchor et al. 2003). However, assemblages are usually impoverished with respect to those of the more distal facies (Buatois & Mangano 1998). Under conditions of continuous wave agitation, elements of the Skolithos ichnofacies may occur. Proximal facies include distributary channel, trough and tabular crossbedded sandstones that are commonly unbioturbated. Locally, escape traces and vertical - or more rarely horizontal - domiciles of suspension feeders may be present, recording an occurrence of the Skolithos ichnofacies (e.g. Melchor et al. 2003). In the case of deep overfilled lake basins, lake floor turbidite systems can develop. Turbidite lobe successions are either progradational or aggradational. Ichnofaunas are relatively diverse; pre-depositional horizontal trace fossils of the Mermia ichnofacies are common on the soles of thin-bedded turbidite sandstones (e.g. Buatois et al. 1996a, 2000b). Post-depositional suites occur either on the base or at the top of turbidites and are commonly less diverse (Buatois & Mangano 1998; Buatois et al. 1998). Land-plant-derived organic matter is the prime source of nutrients, favouring the development of a deposit-feeding benthic fauna in permanently subaqueous, low-energy zones. Firmground suites are rare because such large lakes usually do not experience desiccation. Balanced-fill lake basins commonly contain abundant firmground trace fossils, but softground assemblages are usually depauperate (Fig. lOb). During lowstands, shallow balanced-fill lakes are characterised by relatively thin aggradational parasequences due to desiccation (Bohacs et al. 2000). Lowstand deposits contain abundant and widespread ichnofaunas of the Scoyenia ichnofacies. Striated trace fossils, such as Scoyenia and Spongeliomorpha, recording the firmground suite of the Scoyenia ichnofacies, are extensively developed during lake desiccation (e.g. Bromley & Asgaard 1979; Metz 1995; Clemmensen et al. 1998). Subsequent flooding is associated with rapid influx of sand, allowing preservation of biogenic structures. In contrast, relatively thick aggradational parasequence sets form in lake-floor turbidite systems in deep balanced-fill lakes during lowstands (Bohacs et al. 2000). Under these conditions, firmground trace fossils are absent. Lake hydrology is closed during lowstands and salinity usually increases (Bohacs et al. 2000), therefore

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imposing a stress factor on the lake biota and impoverished softground ichnofaunas. Ichnofaunas from turbidite systems in balanced-fill lakes are less abundant and diverse than those from overfilled lake turbidites. During transgression, parasequences are relatively thick and display retrogradational stacking patterns, while highstand parasequences are variable in thickness and are either aggradational or progradational (Bohacs et al. 2000). Freshwater conditions are common during transgression, but dysaerobic conditions prevail, therefore producing a stress factor on the benthic biota. Trace fossils may occur locally in transgressive and highstand carbonates. However, ichnodiversity is low, and any trace fossils are almost exclusively those produced by epifaunal organisms. This dominance of surficial trace fossils and the paucity of infaunal traces result from brief periods of bottom-water oxygenation, but permanently anoxic interstitial waters that discourage infauna (e.g. Buatois et al. 2000a). A depauperate Mermia ichnofacies is present in these deposits. Scarcity of biogenic structures due to oxygen depletion was also noted in transgressive and highstand siliciclastic deposits of balanced-fill lakes (e.g. Olsen 1989; Mangano et al. 1994, 2000; Metz 1995). Bioturbation is occasionally present at the top of turbidite sandstones, indicating increasing oxygenation in the aftermath of turbidity currents (Mangano et al. 1994, 2000). Additionally, elements of the Skolithos ichnofacies may occur in delta mouth-bars during highstand progradation of deltaic systems (Bromley & Asgaard 1979; Mangano et al. 1994, 2000). The Scoyenia ichnofacies is widespread in underfilled lake basins, but the Mermia ichnofacies is commonly suppressed (Fig. lOc). Deposition during lowstands is restricted to evaporite accumulation in remnant pools developed in the zones of maximum subsidence (Bohacs et al. 2000). Evaporite pools are among the most stressful environments and, with few exceptions, lack biogenic structures. In the remaining zones, sediments that accumulated during the previous highstand experience extreme desiccation during lowstand (Bohacs et al. 2000). The Scoyenia ichnofacies is associated with desiccated substrates formed during lowstand conditions in underfilled lakes (e.g. GierlowskiKordesch 1991; Metz 1996, 2000). Density of arthropod trackways may be high, forming tracked omission surfaces (e.g. Zhang et al. 1998). These omission surfaces are commonly sequence boundaries expressed by co-planar surfaces of lowstand and subsequent flooding. During pluvial periods, underfilled lakes

experience rapid expansion and flash floods reach the basin, leading to deposition of sandy inundites. Trace fossil preservation is mostly linked to rapid influx of sand via sheet floods entering into the lake (Zhang et al. 1998). Hypersalinity usually prevents the establishment of a subaqueous Mermia ichnofacies during transgression and highstand. However, elements of the Mermia ichnofacies may occur, albeit in reduced numbers, in very shallow water thin deposits immediately above flooding surfaces at the base of parasequences. This assemblage is abruptly replaced upward by the Scoyenia ichnofacies reflecting lake regression (e.g. Metz 1996, 2000). Additionally, dwelling traces possibly produced by aquatic chironomid larvae may be present (Rodriguez-Aranda & Calvo 1998; Uchman & Alvaro 2000). Transgressive systems tracts recorded by thin retrogradational parasequence sets usually reflect drastic ichnofaunal changes, from terrestrial assemblages (Coprinisphaera ichnofacies) to transitional terrestrialsubaqueous assemblages (Scoyenia ichnofacies) and salinity-tolerant subaqueous monospecific assemblages of Beaconites filiformis attributed to chironomids (Uchman & Alvaro 2000). In alluvial settings, the sparse distribution of trace fossils primarily reflects changes in depositional systems that may, in turn, be linked to systems tracts. Amalgamated and interconnected, incised fluvial channels are commonly developed in response to low rates of accommodation during early lowstands (Legarreta et al. 1993; Legarreta & Uliana 1998; Shanley & McCabe 1998; Posamentier & Allen 1999). High-energy conditions associated with widespread, intense erosion typically lead to extensive reworking of fluvial deposits, and high sedimentation rates prevent formation and/or preservation of biogenic structures in fluvial channels. However, palaeosols are commonly developed in interfluve areas, and terrestrial trace fossils representing the work of social insects are very common, particularly in Cretaceous and younger strata (Genise et al. 2000; Genise et al. 2004). In particular, the Coprinisphaera ichnofacies (and other palaeosol ichnofacies still unnamed) may delineate sequence boundaries. Increasingly isolated fluvial channels encased in overbank deposits develop as a result of increasing fluvial accommodation during late lowstand. Eventually transgressive lacustrine and marsh deposits accumulate when rate of accommodation exceeds sediment supply (Legarreta et al. 1993; Posamentier & Allen 1999). Increased accommodation favours the preservation of biogenic structures. A trend is apparent towards the progressive replacement of vertical

NON-MARINE PALAEOENVIRONMENTS & ICHNOLOGY

dwelling burrows and escape traces of the Skolithos ichnofacies in active channels by lowdiversity assemblages of meniscate traces in abandoned channels and both the softground and firmground suites of the Scoyenia ichnofacies and even the subaqueous Mermia ichnofacies in overbank deposits. This trend is reversed under increased sediment supply and decreased fluvial accommodation, leading to deltaic progradation and increased channelization during highstand. Conclusions Fluvial ichnofaunas have been reported from two main sub-environments: channel and overbank settings. The ichnofauna from active channels is characterised by low-diversity suites of simple vertical burrows and escape traces, referred to the Skolithos ichnofacies. Abandoned or inactive channel deposits contain lowdiversity assemblages dominated by meniscate traces. Accessory components include vertical to inclined burrows and simple horizontal burrows. Overbank deposits usually host the most diverse and abundant trace fossil assemblages in fluvial environments. Floodplain water bodies that experienced progressive drying (desiccated overbank deposits) host the Scoyenia ichnofacies, which includes abundant arthropod and vertebrate trackways, backfilled meniscate traces, ornamented burrows and bilobate traces with scratch marks. Floodplain water bodies that are filled by overbank vertical accretion without experiencing desiccation (overfilled overbank deposits) host depauperate examples of the Mermia ichnofacies, represented by simple grazing trails, locomotion trails and horizontal dwelling burrows. Lacustrine biogenic structures probably have the highest preservation potential of all continental trace fossils. Hydrologically closed lakes are very stressful, and subaqueous ichnofaunas are rare. The richest ichnofaunas in closed lakes are present at the lake margins and record the activity of terrestrial rather than aquatic faunas (e.g. arthropod trackways, striated traces), recording the Scoyenia ichnofacies. Hydrologically open lakes host relatively diverse and abundant ichnofaunas. The Scoyenia ichnofacies develops in low-energy, lake-margin areas, whereas the Mermia ichnofacies is present in permanent subaqueous lacustrine zones. Sediments deposited in relatively high-energy lacustrine environments, such as wave-dominated shorelines and delta mouth-bars, commonly are characterised by the Skolithos ichnofacies.

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Although continental trace fossils have not been extensively used in sequence stratigraphy, they have potential for further studies. Softground trace fossils are commonly well developed in overfilled lake basins and are useful for delineation of parasequences and parasequence sets. Firmground suites are rare because these lakes usually do not experience desiccation. In contrast, balanced-fill lake basins commonly contain abundant firmground trace fossils in lowstand deposits, but softground assemblages are usually depauperate owing to oxygendepleted conditions. The Scoyenia ichnofacies is also widespread in desiccated lowstand deposits of underfilled lake basins, but the Mermia ichnofacies is commonly suppressed owing to hypersalinity. Early lowstand amalgamated incised fluvial channels are usually unbioturbated owing to high-energy conditions, widespread and intense erosion and high sedimentation rates. Palaeosol ichnofaunas (e.g. the Coprinisphaera ichnofacies) delineate sequence boundaries in interfluve areas. Increasingly isolated fluvial channels encased in overbank deposits develop during late lowstands, and lacustrine deposits accumulate during transgressions. This is paralleled by the progressive replacement of vertical dwelling burrows and escape traces of the Skolithos ichnofacies in active channels by low-diversity assemblages of meniscate traces in abandoned channels and both the softground and firmground suites of the Scoyenia ichnofacies and even the subaqueous Mermia ichnofacies in overbank deposits. This trend is reversed owing to deltaic progradation and increased channelization during highstand. We thank the Antorchas Foundation and the National Agency of Science and Technology for providing financial support. We are very grateful to D. Mcllroy for the invitation to participate in the Lyell Meeting 2003. F. Fursich and S. Lucas provided useful reviews. We also thank M. Jimenez and D. Ruiz Holgado for the drawings.

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Trace fossil distribution in lacustrine deltas: examples from the Triassic rift lakes of the Ischigualasto-Villa Union basin, Argentina RICARDO N. MELCHOR Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET) & Universidad Nacional de La Pampa, Av. Uruguay 151, L6300CLB Santa Rosa, La Pampa, Argentina (e-mail: [email protected]) Abstract: This paper reports six trace fossil assemblages from lacustrine deltas of the Triassic Ischigualasto-Villa Union rift basin, northwest Argentina. They were recognized in three correlated stratigraphic sections separated by about 100km, and come from river- and wave-influenced deltas developed in low- and high-accommodation lacustrine basins. Trace fossil assemblages correspond to delta front (six), delta plain (two) and marginal lacustrine (one) facies associations. Each trace fossil assemblage is described, together with a detailed lithofacies characterization of the trace fossil-bearing interval. They are analysed in conjunction with previously described ichnological assemblages from partially correlative sections of the same basin. Delta front facies contains a high-diversity assemblage (22 ichnotaxa), including Cochlichnus (a ubiquitous form), Gordia, Helminthoidichnites, Helminthopsis, Didymaulichnus, Diplichnites, Stiaria, Cruziana, Bifurculapes, Protichnites, Diplopodichnus, Archaeonassa, Palaeophycus, Treptichnus, Rusophycus, Avolatichnium, 'rhomboidal traces', 'fusiform structures' and 'millimetre burrows'. Trace fossil assemblages from delta plain facies are much less diverse (seven ichnotaxa), but display representatives of a greater variety of ethological categories, including Rhynchosauroides, Skolithos, Palaeophycus, 'horseshoeshaped structures', escape trace and drab-haloed root traces. Marginal lacustrine deposits of a river-dominated delta yielded a monospecific assemblage consisting of Cochlichnus anguineus. Comparison of trace fossil assemblages in wave- and river-dominated lacustrine deltas from the basin (mainly those of the delta front facies) revealed important differences in ichnodiversity that can be useful in the discrimination between these lacustrine delta types. Lacustrine delta deposits contain trace fossils that can be ascribed to three different ichnofacies: a high-diversity occurrence of the Mermia ichnofacies in subaqueous delta front sediments, a low-diversity occurrence of Mermia ichnofacies in subaqueous marginal lacustrine facies, the Skolithos ichnofacies in high-energy upper delta front/lower delta plain facies, and the Scoyenia ichnofacies in intermittently exposed upper delta plain facies. The analysed trace fossil assemblages from delta front and marginal lacustrine settings suggest environmental gradients within the Mermia ichnofacies.

The understanding of trace-fossil distribution in lacustrine basins has witnessed important advances recently, through the identification of the Mermia ichnofacies for fully subaqueous freshwater environments (Buatois & Mangano 1995) and the improved documentation of trace fossil distributions in different lacustrine environments, including evaporitic lacustrine basins (Rodriguez-Aranda & Calvo 1998), shorelines (e.g. Lockley et al. 1992, 1994; Metz 1996; Doyle et al. 2000; Kim et al. 2002) and floodplain lakes (e.g. Buatois et al. 1997; Buatois & Mangano 2002). The Mermia ichnofacies typifies fine-grained sediments from well-oxygenated, low-energy, permanently subaqueous zones of lacustrine systems including floodplain lakes and the landward, freshwater part of fjords (Buatois & Mangano 1995, 1998, 2002, 2003). Trace fossils in high-energy settings of lacustrine basins (e.g. wave-dominated shorelines, delta mouth-bars) have been attributed to continental

occurrences of the Skolithos ichnofacies (Mangano et al. 1994; Buatois & Mangano 1995, 1998; Melchor et al. 2003). Intermittently emergent shallow-lacustrine settings include ichnofossils on softground and firmground (desiccated) substrates that are best ascribed to the emended Scoyenia ichnofacies (Buatois & Mangano 1995; Metz 1996; Melchor et al. 2003). At present, there is scarce documentation of the ichnofossil distribution in particular sedimentary facies of freshwater deltas. Table 1 contains a summary of the published ichnologic information and trace fossil distribution in this environment. The examples of Permian postglacial sequences of South Africa, the Falkland Islands and probably Antarctica, where no agreement about the salinity of the lake basin is available (e.g. Kingsley 1981; Miller et al. 1991; Miller & Smail 1996; Seegers-Szablewski & Isbell 1997; Trewinef al. 2002), must be regarded only as possible case studies. Some authors

From\ MclLROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 335-354. 0305-8719/04/S15.00 © The Geological Society of London.

Table 1. Trace fossil assemblages of lacustrine deltas and related environments Country /region Depositional environment

Age

Unit/basin

Recent

Atchafalaya Basin Mississippi delta Mouth-bar plain Delta front

Recent

Lake Manyara

Tanzania

Pleistocene

Saxony

Germany

Miocene

Lan Krabu Fm

Thailand

Lower Jurassic

Anyao Fm

China

Lower Jurassic

Portland Fm

USA

Permian-Triassic

Beaufort Gr

South Africa

Permian

Ripon Fm

South Africa/ Eastern Cape Province

Ethology-diversity Ichnofossils

Source

Unknown Unknown

Roots traces and burrows Significantly less bioturbation than prodelta. Bioturbation decreases upward Highly bioturbated (>75% volume) Seven kinds of mammalian track and one bird track type. Insect burrows

Tye & Coleman (1989)

Fine trails and burrows, including Cochlichnus Warvichnium, Glaciichnium, Lusatichnium, fine trails and burrows

Walter & Suhr (1998)

Unknown Moderate diversity, locomotion tracks Locomotion and Proximal glaciolacustrine delta front grazing traces Arthropod Distal glaciolacustrine delta front locomotion traces, grazing traces Locomotion and Open lacustrine grazing traces unknown Mouth-bar Lower mouth-bar to Unknown prodelta Open lacustrine Moderate Sublacustrine fan lobes diversity, feeding & grazing traces of deposit-feeding organisms. Burrows dominate Sandstone delta lobes Prodelta Upper delta plain

Cohen et al. (1991 , 1993)

Cochlichnus (dominant), fine trails Bioturbation decreases toward the top Isolated vertical burrows Bioturbation s.L Cochlichnus, Helminthoidichnites, Helminthopsis, Monomorphichnus, Paracantoraphe, Vagorichnus, Tuber culichnus, 'tiny grazing trails', 'irregularly branching burrows'

Flint et al. (1989)

Buatois et al. (1996)

McDonald & LeTorneau (1988) Arthropod trackways, burrows and trails Van Dijk et al. (1978) Lacustrine shelf of deposit-feeding organisms, coprolites 'Scolicitf (= Archaeonassal}, Undichna, Upper prodelta/lower arthropod trackways, horizontal spreiten delta front structures, coprolites Rootlets and rare vertical burrows Mouth-bar Rootlets, arthropod trackways, 'minute Interdistributary bays burrows', leaf mines Delta-fed turbidites Low diversity, low Umfolozia, Undichna, 'small meandering Kingsley (1981) grazing trails' density, locomotion and grazing traces

(s.L)

Bivalve escape traces, fish coprolites

Upper Permian

Fort Brown Fm

South Africa/ Eastern Cape Province

Brenton Loch (Cantera Mb)

Falkland Islands Distal turbidite lobes

Prodelta/delta front

Proximal turbidite lobes

Lower Permian

Brenton Loch (Saladero Mb) Mackellar Fm

Delta slope Transantarctic Mountains

Turbidite fan lobes

Upper Carboniferous Pagoda Fm - Lower Permian

Transantarctic Mountains

Shallow glaciolacustrine delta

Upper Carboniferous

Argentina

Delta-related turbidite lobes. Episodic deposition

Agua Colorada Fm

Delta front

Delta-related turbidite lobes. Underflow currents Deep lacustrine. Muddy turbidity currents Middle Devonian

Hornelen Basin

Norway

Fan-delta (highsinuosity distributary channels)

Low diversity, moderate density, locomotion, grazing and ?dwelling traces

Undichna, 'gastropod trails', 'unidentified Kingsley (1981) burrows'

Undichna ispp., Planolites, Diplocraterion, vertical and inclined burrows with spreite High diversity, Umfolozia, Koupichnium, Cochlichnus, high density, Haplotichnus , Helminthoidichnites, locomotion traces Spirodesmos, Planolites, arthropod dominant resting traces Undichna ispp., Planolites, Spirodesmos, Diplocraterion Surface grazing Mermia, Cochlichnus, Isopodichnus (= Cruziana) Low diversity Isopodichnus (— Cruziana) Very low diversity, low to high density feeding and grazing traces Low diversity, moderate density surface grazing trails High diversity and density, surface grazing and locomotion traces High diversity and density, dominant surface grazing

Planolites, Palaeophycus, Isopodichnus (= Cruziana) Helminthoidichnites, Mermia

Gordia, Haplotichnus, Helminthoidichnites, Helminthopsis, Mermia, Or Chester opus, Undichna ispp., Treptichnus Circulichnus, Cochlichnus, Gordia, Helminthoidichnites, Helminthopsis, Mermia, Or Chester opus, Treptichnus, 'string pits', 'rhomboidal traces' Siskemia, Merostomichnites, Diplopodichnus, 'bilobed trails', 'ribbon trails', 'fine sinuous trails'

Trewin et al (2002)

Miller et al (1991); Miller & Smail (1996) Seegers-Szablewski & Isbell (1997) Isbell et al (2001)

Buatois & Mangano (1993a)

Pollard et al (1982); Keighley & Pickerill (1996)

338

R. N. MELCHOR

consider that these sequences were related to a large lake (e.g. Kingsley 1981; Trewin et al. 2002), and other researchers envisage the basin as a brackish sea (e.g. Visser 1993; Pazos 2002) or a sea with normal marine salinity (e.g. Stanistreet et al. 1980; Johnson et al. 2001). In addition, the comparison of ichnofossil assemblages of lacustrine deltas of different ages listed in Table 1 should consider the secular variations in the extent and depth of bioturbation and behavioural complexity recorded in the continental ichnofossil record (Buatois et al. 1998). The most diverse and best-documented trace fossil assemblages in lacustrine deltas are delta-fed turbiditic lobes, followed by shallower-water upper delta plain and shoreline assemblages (Table 1). On the other hand, there are few records of trace fossil associations from lacustrine delta front settings. This scarcity of ichnological studies in lacustrine deltas contrasts with the extensive documentation of trace fossil distributions in marine deltas (e.g. Eagar et al. 1985; Moslow & Pemberton 1988; Pollard 1988; Coates & MacEachern 1999; Bann & Fielding 2004; Mcllroy 2004). Pollard (1988) found some recurrent ichnocoenoses in interdistributary bay and mouth-bar or crevasse splay sediments of deltaic coal-bearing sequences. Ichnological and sedimentological features that distinguish Cretaceous river- and wave-dominated marine delta sequences from shorefaces have been proposed (Gingras et al. 1998; Coates & MacEachern 1999). Moslow & Pemberton (1988) and Coates & MacEachern (1999) noted that, in river-dominated delta successions, prodelta deposits are devoid of bioturbation and delta front deposits display a low-density, moderate-diversity Cruziana assemblage. Coates & MacEachern (1999) typified wave-dominated delta successions as having a diverse, low-density, stressed Cruziana assemblage in prodelta sediments, and a moderately diverse, locally high-density mixed SkolithosCruziana assemblage in delta front deposits. In the latter case, a marked decrease in the abundance and diversity of trace fossils is attributed to higher-energy conditions (Coates & MacEachern 1999). Whether these relationships can be extrapolated to the freshwater realm is unknown. The purposes of this paper are: to document the detailed stratigraphic distribution of trace fossils in lacustrine deltas of the Triassic Ischigualasto-Villa Union rift basin of Argentina; to compare these examples with the trace fossil assemblages from shallow-shelf deltas from

the same basin (Melchor et al. 2003) and with case studies from the literature; to ascribe the described trace fossil assemblages to archetypical ichnofacies; and to assess possible ichnological signatures of different environments within lacustrine deltas. The described examples come from highstand deltas of flexural-margin and accommodation zone margin of the half-graben, including wavedominated successions and river-dominated deltas that prograded into either high-accommodation anoxic or low-accommodation welloxygenated lacustrine basins. Geological setting The Ischigualasto-Villa Union Basin from northwest Argentina is one of the NW-SE-trending rifts developed on the west margin of southwestern Gondwana during the Early Triassic (Uliana & Biddle 1988; Uliana et al. 1989; Tankard et al. 1995; Fig. la). The basin fill is entirely continental and reaches a maximum thickness of approximately 4000 m (e.g. Milana & Alcober 1994; Kokogian et al. 1999). The oldest deposits are the red-beds of the Talampaya and Tarjados Formations, which are succeeded by thin volcaniclastic deposits of the Chanares Formation and widespread lacustrine strata of the Ischichuca, Los Rastros and Lomas Blancas Formations (Fig. 2). The reader is referred to Stipanicic & Bonaparte (1979), Lopez Gamundi et al. (1989) and Kokogian et al. (1999) for further details on the stratigraphy of the basin. Except for the lower part of the Ischichuca Formation, which contains shallow lacustrine non-deltaic deposits, the lacustrine succession is typically arranged in coarseningand shallowing-upward cycles (parasequences) that record delta progradation (Lopez Gamundi et al. 1989; Milana 1998; Bellosi et al. 2001; Melchor et al. 2003) (Fig. 3). The lacustrine succession of the basin contains sediments of different freshwater to saline palaeolakes that varied from shallow and well oxygenated (i.e. less than 10m deep) to moderately deep (up to 80m deep) and thermally stratified, the latter with anoxic bottom waters (Milana 1998; Melchor unpublished data). The deltaic lacustrine succession of the basin is envisaged to reflect humid climatic conditions (e.g. Bonaparte 1969) and was developed at tropical latitudes (about 35-36°S after Prezzi et al. 2001). The freshwater nature of the lakes where the deltas prograded is well documented by various

ICHNOLOGY OF LACUSTRINE DELTAS

339

Fig. 1. Location map. (a) Position of the study area in South America (left) and extension of Triassic rift basins in northwest Argentina (right). The rectangle shows position of Fig. Ib. (b) Geologic map of the Ischigualasto-Villa Union Basin showing the localities of studies 1-3. Modified from Stipanicic & Bonaparte (1979).

independent lines of evidence: low carbon/sulphur ratios in the black prodelta shales, abundance of branchiopods (mainly conchostracans and notostracans), presence of the freshwater algae Plaesiodictyon mosellanum Brenner & Foster 1994 (Zavattieri & Melchor 1999), and the rarity or absence of evaporites. The recognized facies associations and the abundance of plant, insect and palaeoniscid fish remains are

also in agreement with a freshwater lacustrine setting. The studied localities are located in the northwestern (quebrada or canyon de Ischichuca, locality 1), eastern (Rio Gualo, Talampaya Park, locality 2) and southeastern (La Torre, locality 3) area of the basin (Fig. Ib). All the examples described in this paper come from the lacustrine deltaic interval of the basin. The first

340

R. N. MELCHOR

Fig. 2. Stratigraphy of the Ischigualasto-Villa Union basin showing the relationships of the lacustrine units and different formational names used in the three studied localities.

two localities are located in the flexural margin, and the third locality corresponds to an accommodation zone margin at the southern end of the half-graben. Sedimentological and stratigraphical attributes suggest that these deltas are best compared with the tropical lacustrine deltas of the East African rift lakes, and in particular with those of low depositional slope (cf. Johnson et al, 1995). Detailed logging and stratigraphic analysis (including tracing of lacustrine flooding and ravinement surfaces) permitted basinward correlation of the parasequences (Fig. 3) for over 100km. This correlation suggests that the stacking pattern of the lacustrine deltas reflects lake-level changes. The three sections are roughly stratigraphically equivalent, but have markedly different thickness: the thickest succession is located at the northernmost locality, and the thinnest succession is found at the southern-most locality (cf. Bossi 1971). These differences are explained by contrasting tectosedimentary regimes in a half-graben setting (Melchor 2002), and are strongly linked with the wedge-shaped stratal geometry that is typical of half-grabens (e.g. Leeder & Gawthorpe 1987). Invertebrate and plant trace fossils occur with different density and diversity in some parasequences, which are described below (Fig. 3).

Facies associations This section contains a brief and general description of the main facies associations recognized in the deltaic lacustrine successions of the basin, including representative aspects of the described deltaic lithofacies (Figs 4, 5). A more detailed lithofacies characterization of each trace fossil assemblage is given below, together with the description of the ichnofossils in Table 2. Parasequences range in thickness from 12 to 80m, and the thickest ones are found at quebrada de Ischichuca. Both river-dominated and wavedominated deltas were recognized. A further distinction is made on the basis of the nature of the body of water where these deltas prograded: low-accommodation, shallow-water deltas were small and emplaced on a broad lacustrine shelf, and high-accommodation deepwater deltas were composed by large and low sloping delta lobes (Table 2).

Offshore lacustrine (OL) This facies association displays contrasting features in the three analysed sections and appears to lack evidence of bioturbation. At quebrada de Ischichuca it is represented by thick (up to

ICHNOLOGY OF LACUSTRINE DELTAS

Fig. 3. Schematic stratigraphic sections at quebrada (canyon) de Ischichuca, Rio Gualo and La Torre areas showing facies associations, correlation surfaces and trace fossil assemblages.

341

342

R. N. MELCHOR

Fig. 4. Field photos of delta front fades association, (a) Succession of offshore lacustrine (OL), delta front (DF) and delta plain (DP) facies associations from wave-dominated delta at Rio Gualo; person for scale (circled), (b) Siltstone-dominated, graded underflow deposits, (c) Hummocky cross-stratification from delta front deposits at Rio Gualo (arrowed); circled hammer is 0.35m long, (d) Underflow plus overflow deposits from delta front deposits of a river-dominated delta at quebrada de Ischichuca (staff divisions are 0.10m). (e) Wave-rippled siltstones and sandstones from upper delta front deposits at La Torre.

42m) and monotonous successions of papery locality although fossils are less abundant. black shales with as much as 3% total organic These successions are interpreted as reflecting carbon content and no evidence of disruption deposition from suspension in a moderately of the lamination (Fig. 5a, d). The shales have shallow lacustrine setting, with well-oxygenated yielded varied fossil remains including fish, waters or temporary water stratification. conchostracans, insects, plants and palynomorphs. These shales are interpreted to have been deposited from suspension in a deep fresh- Delta front (DF) water lake with anoxic bottom waters favoured This facies association is composed of coarseningby thermal stratification. At Rio Gualo and La Torre (localities 2 and 3 and thickening-upward, thinly bedded, siltstonerespectively) this facies association is represented dominated successions with minor sandstone by olive grey or brown shales showing fine intercalations. These successions show their base parallel lamination that compose intervals up transitional to offshore lacustrine shales, and to 11 m thick (Fig. 4a). Massive or poorly lami- their top commonly is defined by a sharp contact nated intervals and fine siltstone interbeds can with overlying distributary channel sandstones occur. Fossil content is similar to the previous of the delta plain facies association (Fig. 4a).

ICHNOLOGY OF LACUSTRINE DELTAS

343

Fig. 5. Field photos of delta plain and marginal lacustrine fades associations from quebrada de Ischichuca (locality 1, Fig. Ib). (a) Upper part of the second parasequence (Fig. 3) showing partially covered offshore lacustrine and delta front deposits, capped by delta plain sediments. Arrow indicates the stratigraphic position of Fig. 5b, c, (b) Detail of lateral accretion surfaces of interdistributary bay deposits sharply overlain by coarser-grained channel sediments, (c) Detail of the previous figure showing wave-rippled siltstones interbedded with carbonaceous mudstones. Lens cap is 5.8 cm wide, (d) Field view of the succession covered by Fig. 7a that includes offshore lacustrine, delta front and marginal lacustrine facies associations. Note banded delta front sediments in the lower left corner. OL, offshore lacustrine facies association; DF, delta front facies association; DP, bay + channel = delta plain facies association; ML, marginal lacustrine facies association. TF-A to TF-C, trace fossil assemblages.

Parallel-laminated, graded siltstones with laminae 0.5-3 cm thick compose rhythmic intervals that can reach several metres thick (Fig. 4b, d). Some laminated siltstone intervals display wave-ripple structures (Fig. 4e). Interbedded sandstones can show parallel lamination, ripple cross-lamination and hummocky cross-stratification (Fig. 4c). Soft-sediment deformation structures are rare. This facies association displays the highest diversity and density of trace fossils. Delta front facies associations of river- and wave-dominated deltas are distinguished by the common occurrence of wave and combined flow structures in the latter.

Delta plain (DP) Delta plain deposits are essentially composed of cross-bedded distributary channel deposits and minor interdistributary bay sediments assigned to crevasse channel, crevasse delta and levee sub-environments. The delta plain is characterized by fining-upward successions defined by the presence of fine- to coarse-grained sandstones with trough cross-stratification arranged in

upward-fining cosets up to 9m thick (major channels, Fig. 5a), overlain by fine-grained sandstones, siltstones, tuffs, and occasional palaeosols (interdistributary deposits). The cross-bedded sandstones have erosive bases and commonly contain soft-sediment deformation structures. Although uncommon, lateral accretion surfaces can be associated with these deposits (Fig. 5b, c). Trace fossils are scarce and commonly restricted to the uppermost finer-grained lithologies. This facies association represents active bedload deposition by moderate-sinuosity fluvial channels in a low-gradient setting, in which channels shifted position frequently. Both major and secondary distributary channels can be recognized. The fine-grained interval is interpreted as abandonment facies, which could be exposed subaerially and modified by pedogenic processes.

Marginal lacustrine (ML) These littoral deposits are part of shallow shelf deltas at the top of the Ischichuca Formation

Table 2. Summary of trace fossil assemblages from lacustrine deltas of Ischigualasto-Villa Union basin organized by fades association Fades associations

Delta front

Name

TF-A

TF-D

f

R-D

W-D

Undichna Diplichnites Stiaria Cruziana Deep Bifurculapes. Diplichnites Protichnites Undichna ispp. Cruziana Diplopodichnus Didymaulichnus Shelf Archaeonassa Undichna Deep

R-D

Upper delta front Upper delta front

R-D

Shelf

R-D

shelf

Minor channel Channel

R-D

Deep

R-D

Shelf

Mouth-bar

R-D

Shelf

iv*

Channel

R-D

Shelf

TF-C

Nearshore bars

W-D

Shelf

II

f

TF-B TF-F III

Marginal lacustrine

Lower delta front Underflow deposits Lower delta front Underflow deposits

Delta Lake Ethological categories type* basin Repichnia Pasichnia

Middle delta front

TF-E

Delta plain

Facies

1

Archaeonassa

Diversi ty* Densitv (BPBIJ3 Domichnia

Cochlichnus

Cochlichnus Gordia Helminthoidichnites

Palaeophycus

Cochlichnus Gordia Helminthoidichnites Helminthopsis Cochlichnus

Palaeophycus

Cochlichnus

Palaeophycus 'fusiform structures'

Palaeophycus Skolithos

Rhynchosauroides Cochlichnus

Fodinichnia

Cubichnia

Fugichnia

Plant traces

Fine burrows

Rusophycus

1

2

Treptichnus

Rusophycus Avolatichnium 'rhomboidal traces'

18

3

7

2

2

2

3

2

1

2

1

2

2

2

4

2

1

2

Escape trace Root traces Palaeophycus ispp.

'Horseshoe-shaped structures'

*R-D, river-dominated; W-D, wave-dominated. ^Melchor et al. (2003). * Expressed as the number of ichnotaxa. ^Maximum bedding plane bioturbation index of Miller & Smail (1997).

ICHNOLOGY OF LACUSTRINE DELTAS

at quebrada de Ischichuca (Fig 5d). They are characterized by thick siltstone-dominated successions (up to 50 m thick) with secondary intercalations of wave-rippled or parallel-laminated, fine-grained sandstone beds. They commonly display sole marks and are associated with occasional isolated hummocky lenses (anisotropic hummocky cross-stratification of Midtgaard 1996) and rhythmic graded heterolithic beds. These deposits are interpreted as products of sedimentation in a fully subaqueous, nearshore lacustrine setting. Deposition is attributed to river-fed underflows and to oscillatory and combined flows with occasional modification under storm wave-generated oscillatory flows.

Trace fossil assemblages A total of six trace fossil assemblages were identified, which correspond to the following facies associations: delta front (three assemblages), delta plain (two assemblages) and marginal lacustrine (one assemblage). They are described in detail below and summarized in Table 2. This table also contains the trace fossil assemblages of the correlative Los Rastros Formation at Ischigualasto Park described by Melchor et al. (2003). They are included for consideration in the discussion because they are partially correlative and represent similar environmental settings. Some of the typical trace fossils of the delta front facies association are also illustrated (Fig. 5).

Delta front assemblages These assemblages were recorded in the three analysed localities and are TF-A, TF-D and TF-E, which correspond to the Ischichuca, Los Rastros and Lomas Blancas Formations, respectively (Figs 3, 7). They belong both to river-dominated deltas, including deltas that prograded in high-accommodation (TF-A) and low- accommodation basins (TF-E), and to wave-dominated deltas that prograded in a high-accommodation basin (TF-D). Trace fossil assemblage A (river-dominated delta, high-accommodation basin) TF-A from quebrada de Ischichuca (Figs 3, 7a) corresponds to lower delta plain facies and occurs in silty underflow deposits. It was recorded from a 4m thick interval showing interbedded siltstone, mudstone and shale with occasional convolute lamination that is replaced towards the top by a 10m thick rhythmite

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interval (Figs 4d, 7a). It is represented by a moderately diverse ichnofauna dominated by locomotion traces (Cruziana problematica, Undichna britannica, Diplichnites isp., Stiaria isp.), although resting (Rusophycus stromnessi), grazing or locomotion (Cochlichnus anguineus) and feeding structures (very thin, less than 1 mm in diameter, oblique burrows) were also recorded. The most abundant traces are Cruziana and Rusophycus, which are found in the lower part of the interval along with most of the remaining ichnofossils. Arthropod locomotion traces and Cochlichnus are restricted to the upper part of the trace-bearing interval. Trace fossil assemblage E (rived-dominated delta, low-accommodation basin) This assemblage was recorded at La Torre (locality 3, Figs Ib, 3) within the Lomas Blancas Formation (Figs 3, 7c) and corresponds to wave-rippled upper delta front facies. Delta front sediments at this locality are characterized by decimetre-thick beds that display a normal grading from grey tuffaceous siltstone to mudstone with parallel lamination. The laminated muds grade upward to trace-bearing red siltstones with parallel lamination, wavy bedding and symmetrical ripples (Fig. 4e). Rare finegrained sandstone beds display trough crossbedding, convolute lamination and occasional recumbent folding (probable slump). This succession records deposition from underflow currents and from settling associated with exceptional floods in a moderately shallow lake basin (probably about 20-25 m deep as inferred from the thickness of the parasequences and the characteristics of associated offshore lacustrine deposits). The dominant ichnofossil of this assemblage is the sinusoidal burrow Cochlichnus anguineus, which occur together with the trail Archaeonassa fossulata. Trace fossil assemblage D (wave-dominated delta, high-accommodation basin) This assemblage occurs in two stacked parasequences that are 32 and 50m thick, and crop out at Rio Gualo (locality 2, Figs Ib, 3) and belong to the Los Rastros Formation. Most of the thickness of each parasequence represents siltstone-sandstone-dominated lower delta front facies. Trace-bearing facies at this locality are essentially composed of laminated light-grey siltstones and minor sandstone interbeds (Figs 4a, 7b). The sandstones display parallel lamination, hummocky cross-stratification or wave ripple cross-lamination. Thin (5-1 Ocm thick) fine-grained sandstone/siltstone laminae with

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Fig. 6. Representative trace fossils from lacustrine delta front facies association, (a) Fish trail Undichna britannica. (b) Grazing or locomotion trails Cochlichnus anguineus (C), Gordia marina (G), Helminthoidichnites tenius (H). (c) Long specimens of Cruziana problematica (locomotion trace of arthropods), (d) Arthropod locomotion and resting traces: Protichnites isp. (P), Cruziana isp. (Cr) and 'rhomboidal traces' (r). (e) Feeding burrows: Treptichnus pollardi. (f) Bedding plane with moderately high bioturbation (BPBI — 3; Miller & Smail 1997). Most burrows can be assigned to Cochlichnus. Scale bar = 5cm.

normal grading and parallel lamination are common (Fig. 4b). Thick bedsets of hummocky cross-stratification are also recorded, but they are devoid of ichnofossils (Fig. 4c). Trace fossils were recorded mostly on the tops but also on the bases of finely laminated graded beds, which are interpreted as underflow deposits (Fig. 4b). They represent semi-permanent lake floor sedimentation in a well-oxygenated setting punctuated by deposition of finer-grained sediments from settling (overflow deposits) and by sedimentation of coarse-grained sediments during high-energy events (storms). Trace fossils of both parasequences compose a high-diversity (18 ichnotaxa) and moderate- to high-density assemblage (Figs 3, 7b) (Melchor 2001). It contains grazing/feeding burrows or trails, locomotion and resting/feeding traces of arthropods, and fish trails (Figs 6a, b, 7b). The first group includes Cochlichnus anguineus

(Fig. 6b, f), Gordia marina (Fig. 6b), Helminthoidichnites tenius (Fig. 6b), Palaeophycus tubularis and Treptichnus pollardi (Fig. 6e). Arthropod ichnofossils are diverse and reveal locomotion (Cruziana problematica, Fig. 6c, Diplopodichnus biformis, Didymaulichnus lyelli, Bifurculapes isp., Diplichnites isp., Protichnites isp.) and resting/feeding activities (Rusophycus stromnessi, Avolatichnium isp. and 'rhomboidal traces', Fig. 6d, comparable to those described by Buatois & Mangano 1993a). Fish trails are assigned to Undichna britannica (Fig. 6c), U. bina and U. cf. insolentia. The most common ichnotaxa are Cochlichnus anguineus (burrows that appear with high densities at some stratigraphic levels), Cruziana problematica (and its taphonomic variants Diplopodichnus and Didymaulichnus, see Keighley & Pickerill 1996), and Undichna britannica. Cochlichnus appears frequently with deformed burrow walls. Besides

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Fig. 7. Detailed sedimentologic logs of selected examples of the trace fossil assemblages. See Fig. 3 for stratigraphic position, (a) River-dominated deltaic and marginal lacustrine deposits from the Ischichuca Formation, (b) Second parasequence of a wave-dominated delta from the Los Rastros Formation at Rio Gualo. (c) Second parasequence of a river dominated delta from the Lomas Blancas Formation at La Torre.

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this compositional characterization, the upper parasequence displays a preferred distribution of some ichnotaxa in delta front fades: the lower part (below the first medium-grained sandstone bed) contains a larger number of ichnotaxa than the upper part (Fig. 7b). TF-D contains the higher density of burrowing (BPBI = 3 in the scheme of Miller & Smail 1997) of all analysed assemblages.

Delta plain assemblages Two low-diversity and low-density ichnological assemblages were recorded from delta plain settings of river-dominated deltas: TF-B from a high-accommodation lake basin (Ischichuca Formation), and TF-F from a low-accommodation lake basin (Lomas Blancas Formation). Trace fossil assemblage B (river-dominated delta, high-accommodation basin) This assemblage is represented by escape traces in minor channels of the delta plain from quebrada de Ischichuca (locality 1, Fig. 3), within the Ischichuca Formation. Traces occur in a 7m thick fining-upward cycle bounded by a sharp and erosive lower surface that includes (from bottom to top) planar cross-stratified sets with reactivation surfaces and a single through cross-bedded set (both showing soft-sediment deformation structures) in medium-grained sandstones, which are covered by fine-grained heterolithic deposits. The latter comprises lateral accretion surfaces and wave-rippled sandstones, and siltstones with climbing ripples and lenticular bedding, which are interbedded with dark plant-bearing mudstones (Fig. 5b, c). This cycle is laterally correlative with crevasse deltas and levee deposits. Escape traces have been recorded from the top of the trough cross-bedded set. This cycle represents deposition in a minor distributary channel or a crevasse channel of the delta plain, as suggested by the sedimentological attributes, the presence of common reactivation surfaces, and lateral correlation with crevasse deltas and levee deposits. These channels were dominated by lateral accretion onto point bars (lateral accretion surfaces), and suffered common stage changes (reactivation surfaces) and probably frequent avulsions (thin finingupward cycles). It is possible that sedimentation was rapid, thus promoting sediment instability (soft-sediment deformation features) and transient high sedimentation rates (escape structure). Escape traces are composed of a central disrupted zone (1cm wide and 8cm high) surrounded by downward-deflected laminae.

Trace fossil assemblage F (river-dominated delta, low-accommodation basin) This assemblage occurs in distributary deposits of the top of the third parasequence from La Torre (locality 3, Figs Ib, 3) within the Lomas Blancas Formation (Fig. 7c) and is composed of root traces. Distributary channel deposits are 5-6.5m thick, characterized by medium-grained sandstones with trough cross-bedding associated with parallel-laminated or massive fine-grained sandstones. The parasequence is capped by fine-grained, slightly reddened sandstone with moderately abundant root traces. The delta plain interval of the overlying parasequence is similar, although it also contains heterolithic deposits. They consist of fine-grained sandstone, siltstone and mudstones with abundant carbonaceous material, which display trough cross-bedding, parallel lamination and synsedimentary microfaulting, and are succeeded by lateral accretion deposits with abundant wavy and lenticular bedding. The coarse-grained sediments of the described parasequences reflect the progradation of a delta lobe in lake waters with common wave reworking. These sediments are sharply covered by channel deposits that were exposed subaerially, thus favouring the development of incipient soils. Heterolithic lateral accretion deposits are related to sedimentation in a moderately sinuous distributary channel. Root traces are typically 0.01-0.8 cm in diameter, up to 10 cm long, display occasional bifurcations and have a conspicuous yellow-grey halo with a maximum thickness of about 0.7cm. Former root cavities are filled with fibrous silica, carbonate and remains of probable silicified plant tissue. These root traces are similar to the drab-haloed root traces of Retallack (1983). Of the common origins postulated for this type of root traces, both incipient waterlogging and anaerobic decay of the organic matter of the root during early burial are possible in this case (cf. Retallack 1983, 1990).

Marginal lacustrine assemblage A single assemblage has been recorded from a subaqueous marginal lacustrine setting of a river-dominated delta that prograded in a shallow shelf (Ischichuca Formation). Trace fossil assemblage C (river-dominated delta, low-accommodation basin) This assemblage was identified at quebrada de Ischichuca (locality 1, Figs Ib, 3) from the homonymous formation. It is a monospecific assemblage (Figs 5d, 7a), restricted to a 3m

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thick interval interpreted as subaqueous nearshore lacustrine deposits laterally associated with shallow shelf deltas. The deposits commonly are arranged in coarsening-upward cycles composed of graded heterolithic beds and laminated siltstones with sandstone interbeds, which are capped by parallel-laminated and wave-rippled sandstones. Rare isolated sandstone hummocks (anisotropic hummocky cross-stratification) can occur in the upper part of the cycles (Fig. 7a). The description of the ML facies association contains further details. Trace fossils are represented by a low- to moderate-density, monospecific assemblage composed of Cochlichnus anguineus burrows. They are restricted to the tops of fine-grained sandstone beds showing symmetrical or interference ripples. Discussion

Environmental and stratigraphic repartition of trace fossils The most diverse trace fossil assemblages are found in the delta front facies association, which shows a fairly high ichnodiversity (22 recorded ichnotaxa). Among these, the trace fossil assemblages corresponding to distal or intermediate settings of the subaqueous delta lobe contain the greater number of ichnotaxa. This relationship holds for both river- and wave-dominated deltas, although the examples from wave-dominated deltas of the Los Rastros Formation at Rio Gualo are by far the most diverse assemblages (TF-D, Table 2). The upper part of the delta front contains an impoverished assemblage, with ubiquitous sinusoidal grazing or locomotion trails assigned to Cochlichnus. This trace fossil is found in six of the eight subaqueous assemblages, but comprises monospecific assemblages only in marginal lacustrine deposits (Table 2). Cochlichnus has been recognized in a large variety of marine, transitional and continental environments, which range in age from Precambrian to Holocene (Buatois et al. 1997). The apparent vertical zonation found in TF-A & D could reflect partitioning of assemblages into proximal and distal delta front deposits (cf. Buatois & Mangano 1993b). Ethologically, the assemblages are dominated by locomotion and grazing traces with subordinate resting and feeding traces and rare dwelling structures. Locomotion traces are almost exclusively assigned to arthropods, whereas the remaining repichnial ichnotaxa are ascribed to fishes or gastropods.

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Trace fossil assemblages from the delta plain facies association are considerably less diverse than those from delta front facies, showing a total of seven ichnotaxa that preferentially occur on delta tops associated with lowaccommodation basin states. In contrast, delta plain facies related to high-accommodation lake basin states are almost devoid of ichnofossils, limited to a single record of an escape trace in TF-B. The low ichnodiversity contrasts with the large number of behavioural categories represented, including lacertoid vertebrate tracks (Rhynchosauroides), dwelling burrows (Skolithos, Palaeophycus), probable resting traces of arthropods ('horseshoe-shaped structures'), escape trace and root traces (Table 2). Within these assemblages there are also indicators of desiccated substrates, evidenced by the presence of fine striations in Palaeophycus striatus from trace fossil assemblage IV of the Los Rastros Formation, Ischigualasto Park (Melchor et al. 2003). Ichnofossils from the studied Triassic lacustrine deltas are almost exclusively restricted to shallow penetrating traces on some bedding planes. Most of the bedding planes with ichnofossils display less than 10% bioturbation (BPBI = 2 of Miller & Smail 1997), with scarce examples reaching as much as 25% bioturbation (BPBI = 3) (Table 2). The restriction of burrowing by benthic organisms to bedding planes and common low density of bioturbation has been documented in other Late Palaeozoic and Triassic lacustrine and fluvial successions (Buatois et al. 1998; Miller et al. 2002). The single example of a shallow subaqueous marginal lacustrine trace fossil assemblage (TF-C) resembles those found in upper delta front settings, especially because of the exclusive occurrence of simple grazing trails and the association with waveripple structures. In addition to tracing the differences in trace fossil content along proximal to distal gradients within individual lacustrine deltas, sequence stratigraphic correlation allows comparison of laterally equivalent, but different, deltas. In particular, trace fossil assemblages B (locality 1), D (second parasequence at locality 2) and E + F (locality 3) occur in parasequences that are bounded by correlative flooding surfaces (Figs 3, 7). There is a striking difference between the almost lack of bioturbation in the highaccommodation rived-dominated delta of TFB, the high diversity and high density of the high-accommodation wave-dominated deltas of TF-D, and the depleted ichnocoenoses of the low-accommodation, river-dominated delta of TF-E and F.

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Controls on trace fossil distribution In the permanently subaqueous delta front settings, oxygenation, food supply, water turbidity, erosion and sedimentation rates influence trace fossil distribution. It is well documented that density currents, both discrete turbidity currents and semi-permanent underflow currents, supply oxygen and food to deep, subaqueous settings, thereby favouring the establishment of a diverse biota (e.g. Buatois et al. 1996; Buatois & Mangano 1998). This is especially true for oxygen-deficient bottom waters, as envisaged for TF-A. Higher erosion and turbidity in river-dominated delta front environments may explain the low diversity and density of trace fossils in these settings, in contrast to wavedominated delta front settings. The later were emplaced in bottom waters with higher oxygenation than the deep anoxic and non-bioturbated intervals of the Ischichuca Formation lakes, thus allowing the establishment of a resident fauna with less dependence on the supply of oxygen and food from turbidity currents. Erosion produced by storms may have destroyed the trace fossils produced on previous underflow deposits. Thick amalgamated hummocky crossstratified sandstones lack evidence of bioturbation, which is interpreted as reflecting low potential of preservation of shallow-penetrating traces (cf. Frey & Goldring 1992). In the upper part of delta front successions, where there are indications for higher erosion and sedimentation rates, trace fossil assemblages are reduced in diversity and density, or are absent. Sedimentologic evidence suggests that the sinusoidal burrow Cochlichnus is restricted to subaqueous settings, and the frequent burrow-wall deformation observed in some specimens suggests that they were produced in highly water-saturated substrates (cf. Buatois et al. 1997). Energy and substrate water content are envisaged as the most important factors that controlled the formation and preservation of ichnofossils in delta plain settings of the studied examples. Actively filled fluvial channel deposits are devoid of traces, but trace fossils do occur in intermittently exposed channel margin deposits in the form of striated burrows, footprints and root traces.

Comparison with other trace fossil assemblages The composition of trace fossil assemblages from delta front settings is comparable with typical occurrences of the Mermia ichnofacies (e.g.

Buatois & Mangano 1993a, 1995). An important difference is the dominance of locomotion and resting traces attributed to arthropods in these assemblages, instead of shallow surface-grazing trails that characterize the Mermia ichnofacies. As suggested by Buatois & Mangano (1998) and Melchor et al. (2003), trace fossil assemblages from shallow-lacustrine high-energy settings are best ascribed to the Skolithos ichnofacies, and intermittently exposed lacustrine shoreline deposits contain ichnofossils that suggest assignation to the Scoyenia ichnofacies (Buatois and Mangano 1995, 2004). These three ichnofacies are present in a single parasequence only in river-dominated low-accommodation deltas at Ischigualasto Park locality (Fig. Ib). The deeper or permanently subaqueous deltaic successions contain assemblages that are less variable. Nevertheless, as exemplified by the apparent vertical zonation in TF-A & D (Fig. 7a, b), it is possible that future studies will recognize characteristic and repetitive assemblages within the Mermia ichnofacies. Buatois & Mangano (1996) have also noted an increase in the number of arthropod locomotion traces in littoral lacustrine facies with relation to deepoffshore facies, which are dominated by surface-grazing trails. The monospecific assemblage from a subaqueous marginal lacustrine setting is regarded as an impoverished occurrence of the Mermia ichnofacies. The compositional comparison with marine deltaic sequences reveals significant differences. However, there are similar trends in freshwater and marine deltaic successions, with an upward decrease of ichnodiversity and abundance in shallower and more energetic settings. In addition, a greater diversity and density of trace fossils is recorded from wave-dominated settings (Table 2). Conclusion This study documents the composition and facies repartition of trace fossil assemblages from different locations of a Triassic lacustrine deltaic succession. Trace fossils from all subaqueous non-marine settings are assigned to the Mermia ichnofacies. However, it is envisaged that additional detailed studies may allow further discrimination within this ichnofacies. This inference is supported by documentation of atypical Mermia-type assemblages herein with moderate to very high ichnodiversity and facies-dependent vertical zonation of trace fossils (e.g. TF A & D). Trace fossil assemblages from the Triassic lacustrine succession of Ischigualasto-Villa

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BELLOSI, E., JALFIN, G., Bossi, G., MURUAGA, C., BOGGETTI, D. & CHEBLI, P. 2001. Ambientes sedimentarios en cuencas triasicas de Argentina. Boletin de Informaciones Petroleras, 68, 54—83. BONAPARTE, J. F. 1969. Datos sobre la evolution paleoecologica en las formaciones Triasicas de Ischigualasto-Villa Union (San Juan-La Rioja). Acta Geologica Lilloana, 10, 189-206. Bossi, G. E. 1971. Analisis de la Cuenca IschigualastoIschichuca. Primer Congreso Hispano-LusoAmericano de Geologia Economica, Madrid, 2, 611-626. BRENNER, W. & FOSTER, C. B. 1994. Chlorophycean algae from the Triassic of Australia. Reviews of Palaeobotany & Palynology, 80, 209-234. BUATOIS, L. A. & MANGANO, M. G. 1993a. Trace fossils from a Carboniferous turbiditic lake: implications for the recognition of additional nonmarine the absence of traces in oxygen-deficient offichnofacies. Ichnos, 2, 237-258. shore deposits; BUATOIS, L. A. & MANGANO, M. G. 1993b. The the high diversity of simple grazing trails in paleoenvironmental and paleoecological signifidelta-fed turbidites (Buatois & Mangano cance of turbiditic lake ichnocoenoses from the 1993a; Buatois et al 1996; Trewin et al 2002); Late Carboniferous of the Paganzo Basin, Argenthe high diversity and apparent dominance of tina. Comptes Rendus XII International Congress arthropod traces in lower delta front deposits, Carboniferous-Permian, 2, 409^20. which decrease in diversity toward the top of BUATOIS, L. A. & MANGANO, M. G. 1995. The paleoenvironmental and paleoecologic significance of progradational successions; and the lacustrine Mermia ichnofacies: an archetypical the low diversity of ichnofossils representing subaqueous nonmarine trace fossil assemblage. different ethologic categories in fine-grained, Ichnos, 4, 151-161. occasionally exposed delta plain deposits. BUATOIS, L. A. & MANGANO, M. G. 1996. Icnologia de ambientes continentales: problemas y perspectivas. No diagnostic ichnological signature for identifiAsociacion Paleontologica Argentina, Publication cation of key stratigraphic surfaces in lacustrine Especial, Buenos Aires, 4, 5—30. deltas was found, though improved ichnofacies BUATOIS, L. A. & MANGANO, M. G. 1998. Trace fossil characterization may enable ichnofacies-stacking analysis of lacustrine facies and basins. Palaeopatterns to be used in stratigraphic analysis, as is geography, Palaeoclimatology, Palaeoecology, conventionally performed in marine successions. 140, 367-382. In addition, identification of a diverse trace fossil BUATOIS, L. A. & MANGANO, M. G. 2002. Trace fossils assemblage in deltaic lacustrine successions may from Carboniferous floodplain deposits in western Argentina: implications for ichnofacies models of aid in recognition of distal delta lobe deposits continental environments. Palaeogeography, and thus help to locate potential reservoir facies. Palaeoecology, Palaeoecology, 183, 71-83. Funding for this research was obtained from research BUATOIS, L. A. & MANGANO, M. G. 2003. Caracterizacion icnologica y paleoambiental de la localidad grants PICT 6156 (ANPCyT) and PEI 157/98 (CONItipo de Orchesteropus atavus Frenguelli, Huerta CET), both from Argentina. The Universidad Nacional de Huachi, provincia de San Juan, Argentina. de La Pampa provided logistic support and partial Ameghiniana, 40, 53-70. funding for fieldwork (project no. 136 of the Facultad de Ciencias Exactas y Naturales). L. Buatois, A. BUATOIS, L. A & MANGANO, M. G. 2004. Animalsubstrate interactions in freshwater environments: Martin, D. Mcllroy and P. Pazos reviewed the manuapplications of ichnology in facies and sequence script and made pertinent suggestions that improved stratigraphic analysis of fluvio-lacustrine succesthe paper. sions. In: MclLROY, D. (ed.) The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, References Special Publications, 228, 311-333. BANN, K. L. & FIELDING, C. R. 2004. An integrated BUATOIS, L. A., MANGANO, M. G., Wu, X. & ZHANG, G. 1996. Trace fossils from Jurassic lacustrine ichnological and sedimentological comparison of turbidites of the Anyao Formation (central non-deltaic shoreface and subaqueous delta China) and their environmental and evolutionary deposits in Permian reservoir units of Australia. significance. Ichnos, 4, 287-303. In: MclLROY, D. (ed.) The Application oflchnology to Palaeoenvironmental and Stratigraphic Analysis. BUATOIS, L. A., JALFIN, G. & ACENOLAZA, F. G. 1997. Geological Society, London, Special Publications, Permian nonmarine invertebrate trace fossils from southern Patagonia, Argentina: ichnologic 228, 273-310.

Union basin attain maximum diversity and density in wave-dominated delta front fades, are absent in anoxic offshore lacustrine fades, and are scarce in delta plain facies. These differences may aid in the stratigraphic analysis of lacustrine successions and help to distinguish between wave- and river-dominated lacustrine deltas. Trace fossil assemblages in the analysed lacustrine deltas mimic density/diversity patterns in marine deltas, although they are dominated by non-marine ichnotaxa. This study contributes to the documentation of the general trace fossil distribution in lacustrine deltas, from distal to proximal areas (Tables 1, 2). Identified trends include:

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of Sedimentologists, Special Publications, Oxford, 2, 225-239. VISSER, J. N. J. 1993. Sea-level changes in a back-arcforeland transition: the late Carboniferous-Permian Karoo Basin of South Africa. Sedimentary Geology, 83, 115-131. WALTER, H. & SUHR, P. 1998. Lebensspuren aus kaltzeitlichen Bandersedimenten des Quartars. Abhandlungen des Staatlichen Museums fur Mineralogie und Geologie zu Dresden, 43/44, 311-328. ZAVATTIERI, A. M. & MELCHOR, R. N. 1999. Estudio palinologico preliminar de la Fm. Ischichuca (Triasico), en su localidad tipo (Quebrada de Ischichuca Chica), provincia de La Rioja, Argentina. Asociacion Paleontologica Argentina Publicacion Especial, 6, 33-38.

An approach to the description and interpretation of ichnofabrics in palaeosols JORGE F. GENISE1, E. S. BELLOSI2 & M. G. GONZALEZ2 1

CONICET, Museo Paleontologico Egidio Feruglio, Av. Fontana 140, 9100 (Trelew), Chubut, Argentina (e-mail: [email protected]) CONICET, Laboratorio de Icnologia, Museo Argentina de Ciencias Naturales, Av. Angel Gallardo 470, (1405) Buenos Aires, Argentina Abstract: Studies on ichnofabrics have focused mainly on marine environments. Attempts to apply the ichnofabric methodology and theoretical framework to continental deposits bearing palaeosols are few and poorly developed. Methodologies analysed in this contribution include the applicability of current ichnofabric indexes and diagrams, the assessment of the destruction of the original bedding by ichnofabrics and by other soil characters separately, and the relationships between different stages of palaeosol and ichnofabric development. Many soil features may be formed without the intervention of bioturbation, or may be the result of interactions of physical, chemical and biological processes, in which traces of organisms may have only a subsidiary role. Ichnofabrics can be well developed in palaeosols devoid of other soil characters and, conversely, palaeosols showing a well-developed soil structure can bear almost no trace fossils. This fact adds a third component to classical methods that normally consider only original bedding and ichnofabrics. Theoretical analysis includes the possibility of recording composite ichnofabrics in palaeosols, and the value of individual ichnotaxa as possible indicators of subaerial conditions and environmental changes. The ecological preferences and requirements of trace-makers provide the key to understanding composite ichnofabrics; however, only complex traces can be certainly attributed to particular modern taxa. Insect nests, pupal chambers and earthworm burrows are the most reliable indicators of subaerial exposure and, in many cases, very particular environmental conditions.

Ichnofabric analysis has become an increasingly important tool in ichnological analysis, providing a methodology for documenting and comparing the extent of bioturbation and relative chronology of infaunal tiering (Ausich & Bottjer 1982; Ekdale & Bromley 1983, 1991; Bromley & Ekdale 1986; Droser & Bottjer 1986; Bottjer & Droser 1991, 1994; Taylor & Goldring 1993; Taylor et al. 2003). Most studies of ichnofabrics have been focused on marine environments (Taylor & Goldring 1993; Bottjer & Droser 1994). Few, poorly developed, attempts have been made to apply ichnofabric methodology and its concepts to continental deposits bearing palaeosols. The few references involve the utilization of the ichnofabric index (Droser & Bottjer 1986) as a comparative scale to evaluate the degree of development of soil structure (Retallack 1990), tiering in palaeosols (Hasiotis et al. 1993; Hasiotis & Dubiel 1994; Gonzalez et al. 1998), the proposal of informal names for ichnofabrics produced by particular groups of arthropods (Hasiotis 1997), and the use of the term terrestrial ichnofabrics to describe trace fossil assemblages in palaeosols (Miller & Mason 2000). The aim of this contribution is to present examples of palaeosol ichnofabrics in order to

analyse the utility of different aspects of ichnofabric in continental deposits bearing palaeosols. In order to apply the ichnofabric method to palaeosols the effects of bioturbation must first be distinguished from physical and chemical pedogenic processes. Other aspects required for complete palaeosol description are the interrelationships between different stages of palaeosol and ichnofabric development. Another particular point to be considered herein in detail is the value of the individual components of palaeosol ichnofabrics as possible indicators of subaerial conditions and changes in environment, and the possibility of recording composite ichnofabrics (sensu Bromley & Ekdale 1986) in palaeosols. Methodology Review of existing methodology in ichnofabric analysis There are different methods for measuring the extent of bioturbation and describing ichnofabrics, which are reviewed elsewhere (Taylor & Goldring 1993; Bottjer & Droser 1994; Miller & Smail 1997; Netto 2000). Two basic philosophies are recognized: those that use a semi-quantitative

From: MclLROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 355-382. 0305-8719/04/S 15.00 © The Geological Society of London.

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approach (e.g. Reineck 1963; Droser & Bottjer 1986; Miller & Small 1997) and those that are mostly descriptive (e.g. Ekdale & Bromley 1991; Wetzel & Uchman 1998). Semi-quantitative methods recognize different categories of ichnofabric (ichnofabric indices or bioturbation indices) based on the degree of disruption of the original bedding by bioturbation, which are represented in schematic diagrams developed for different sedimentary settings (Droser & Bottjer 1986; Bottjer & Droser 1994; Miller & Smail 1997). Most of them have been developed for studying ichnofabrics in vertical sections, whereas that of Miller & Smail (1997) was elaborated for bedding planes. In turn, descriptive diagrams (tiering diagrams, TD) were proposed to illustrate relationships of ichnofabrics with palaeoenvironmental factors, rate of sedimentation, and biological activity (e.g. Ekdale & Bromley 1991; Pollard et al 1993; Wetzel & Uchman 1998 and references therein) at particular environments or localities. A comprehensive approach to combine in a single analysis the evaluation of the extent of bioturbation (bioturbation index, BI), as in semi-quantitative methods, with a visual representation of ichnofabrics (ichnofabric constituent diagram, ICD) was proposed by Taylor & Goldring (1993). The ICD considers the ichnotaxa present, ichnodiversity, density and order of emplacement. However, no single method has been widely accepted. Critical analysis of the applicability of different methods has been made elsewhere (e.g. Frey & Wheatcroft 1989; Ekdale & Bromley 1991; Pemberton et al 1992; Miller & Smail 1997). Some authors stated that ichnofabric indices may be useful for describing simple ichnofabrics, whereas descriptive methods are more suitable for complex ones (e.g. Ekdale & Bromley 1991). Others concluded that semi-quantitative methods are more appropriate for large-scale studies, involving hundreds of box cores or thousands of metres of stratigraphic section, whereas the descriptive method has been used with core material or outcrops of limited size (e.g. Miller & Smail 1997). In addition, Taylor & Goldring (1993) claimed that descriptive methods provide the possibility of assessing and comparing a more complete set of data, including also the degree of bioturbation contemplated in the semi-quantitative method. In practice, however, the methodology is so time consuming that its use is restricted, in most cases, to describing key stratigraphic surfaces or summarizing recurring ichnofabrics within a stratigraphic unit (e.g. Mcllroy 2004b).

Methodology for palaeosol ichnofabric analysis The case studies presented herein involve complex ichnofabrics, small-scale, high-resolution observations and detailed palaeoenvironmental analysis. The inherent complexity of soil fabrics is such that conventional methodologies were deemed inadequate, and some modifications to previous descriptive methods are introduced for their description and as an aid to their analysis. Additionally, the bioturbation indices proposed by Taylor & Goldring (1993) have been used to complete the descriptions, as well as a semiquantitative approach to quantification of bioturbation of each tier used for composite ichnofabrics (as suggested by Ekdale & Bromley 1991). Seven grades of BI, based on Reineck (1963), were recognized by Taylor & Goldring (1993), which were adapted herein for description of palaeosols according to burrow density and the amount of burrow overlap. These BI are: 0, no bioturbation; 1, sparse bioturbation, few discrete traces; 2, low bioturbation, low trace density; 3, moderate bioturbation, traces discrete, overlap rare; 4, high bioturbation, high trace density with overlap common; 5, intense bioturbation, later burrows discrete; and 6, complete bioturbation, repeated overprinting. The previous authors also utilized the sharpness of the primary sedimentary fabric to define the grades (Taylor & Goldring 1993). However, in palaeosols the original bedding can also be heavily disrupted by other soil processes that are non-biological, or only partially related to bioturbation: thus this disruption of the sedimentary fabric is not exclusively related to bioturbation as in the classical marine examples. In both the semi-quantitative and descriptive methods only two variables are considered: the original bedding and the ichnofabric. In palaeosols, soil structures other than ichnofabrics should be also considered as destructive agents of the original bedding. Different soil features that can be recognized in palaeosols, such as horizons, peds, cutans, glaebules, crystals, and root and animal traces, modify the original sedimentary fabric of the deposits in which soils develop (Teruggi 1971; Yaalon 1971; Andreis 1981; Bown & Kraus 1987; Retallack 1990; Nettleton et al, 2000). Root and animal traces are the direct products of the activity of organisms, and as such they are considered trace fossils and deserve an ichnotaxonomical treatment, whereas other soil characters are the result of interactions of purely physical-chemical processes, or mixed physical-chemical-biological

ICHNOFABRIC ANALYSIS OF PALAEOSOLS

processes, in which traces of organisms may only be involved to different extent (e.g. Brewer 1976; Fitzpatrick 1984; Retallack 1990). Regarding Ekdale & Bromley's (1983) definition of ichnofabrics, it would be possible to consider the whole soil structure as an ichnofabric because it may be a direct or indirect result of bioturbation. However, soil features such as peds, cutans, glaebules and crystals may be formed as well, without the necessary intervention of bioturbation (e.g. Buol et al 1990; Retallack 1990). For instance, Blokhuis et al. (1990) stated that Vertisols are typical cases of ped formation by physical processes of shear failure, along inclined surfaces, and cracking. Repeated wetting and drying of a dispersed cracking clay soil results in the fragments parting into fine wedge-shaped aggregates, because of the stress-strain regime of the swelling soils (Blokhuis et al. 1990). Even when the traces of organisms would have been involved in the origin of soil characters (e.g. granular and crumb peds), they might be unpreserved or so distorted as to become recognizable, thereby precluding an ichnotaxonomical approach. Apart from its formal definition, the common usage of the ichnofabric concept involves fabrics completely produced by traces of organisms, more commonly than fabrics in which traces are only partially involved. Finally, the development of an ichnofabric may be independent of other soil characters. Ichnofabrics can be well developed in immature palaeosols, obliterating the original bedding almost completely (e.g. Genise & Bown 1994b). Conversely, palaeosols showing a well-developed ped structure may preserve almost no trace fossils (Melchor et al. 2001). In conclusion, the fabric directly and completely produced by root and animal traces is considered herein to be the ichnofabric, whereas the fabric produced by other soil characters, resulting from the interactions of physical, chemical and biological processes, is termed herein the pedofabric. It should be noted that pedofabric is distinct from the terms 'soil fabric' and 'soil microfabric' (e.g. Brewer 1976; Fitzpatrick 1984; Bullock et al. 1985; Retallack 1990; Buol et al. 1990), which are applied mostly to the microstructure of the fine-grained part of the soils. The stages of palaeosol development may be used to evaluate the degree of pedofabric development. There are two scales for estimating stages of palaeosol development, which were proposed by Bown & Kraus (1987) and Retallack (1988) respectively. Both scales are broadly based on similar criteria. The first three stages are characterized by (1) no horizon formation; (2) A horizon and incipient B horizon, and (3) A and B horizons well defined,

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whereas the last two stages (4, 5) are defined mostly by the increasing thickness of A and B horizons. In terms of BI, a highly developed palaeosol will comprise volumetrically high percentages of pedofabric and, accordingly, low to moderate percentages of primary sedimentary fabric and ichnofabrics. Hence the single BI value does not necessarily indicate the percentage of disruption of the original bedding (by animals and plants), and conversely the single percentage of disruption is not a true reflection of the degree of bioturbation. In palaeosols, the proportions of all three processes must be properly described in order to analyse meaningfully the fabric of the deposits in which they occur. A comparison of five study cases is presented herein utilizing a pedofabric/ichnofabric ternary diagram (PITD). The PITD (Fig. 1) includes the bioturbation index (BI), and percentages of original bedding (OB), ichnofabrics (IF) and pedofabrics (PF). BI and percentage of IF were measured following Taylor & Goldring's (1993) adapted table based on burrow density and overlap. The percentage of the original bedding (OB) was measured according to the preservation of primary sedimentary structures. The pedofabric (PF) was calculated considering the development of soil features other than the recognizable trace fossils. Two types of diagram have been proposed for representing descriptive methodologies: tiering diagrams (TD) (e.g. Ekdale & Bromley 1991) and ichnofabric constituent diagrams (ICD) proposed by Taylor and Goldring (1993). The ICD represents events consecutively, starting with the primary fabric and then ichnotaxa in order of emplacement. This order reflects the infaunal succession produced by changes in the environmental conditions (e.g. Bromley & Ekdale 1986; Buatois et al. 1997). In turn, the original tiering - the vertical partitioning of the habitat in response to environmental gradients (e.g. Ausich & Bottjer 1982; Bromley & Ekdale 1986) - may be not represented if it is not the result of the order of emplacement. In marine environments, where colonization of substrates may progress from the surface downwards, the deepest tier is also the last emplaced (Bromley & Ekdale 1986; Ekdale & Bromley 1991; Bromley 1994). In these cases ICDs may represent tiering and order of emplacement simultaneously. However, in palaeosols the first colonizers of the deposit may belong to the deepest tier, as shown herein. Both original tiering (Hasiotis & Dubiel 1994; Gonzalez et al. 1998) and infaunal succession (Hasiotis et al. 1993) have been recorded from palaeosols, and so descriptive diagrams such as TDs, which can illustrate both features of ichnofabrics simultaneously,

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Fig. 1. Proposed diagram for assessing palaeosol ichnofabric, pedofabric, original bedding and grades of bioturbation index, BI. 1, Ischigualasto Formation, well-developed Vertisols; 2, Ischigualasto Formation, poorly developed Vertisols; 3, Laguna Palacios Formation, Entisols; 4, Laguna Palacios Formation, Alfisols; 5, Asencio Formation, Ultisols; 6, Jebel Qatrani Formation, Inceptisols; 7, Sarmiento Formation, Andisols; 8, Sarmiento Formation, Alfisols.

are of fundamental importance. In addition, percentages of bioturbation, which can sometimes be time consuming to calculate, are instrumental for the construction of the ICDs (Taylor & Goldring 1993). In TDs, the thickness of the study section, ichnotaxa, tiering and crosscutting relationships are readily understandable (e.g. Bromley & Ekdale 1986). In conclusion, the study cases presented herein are represented in TDs in which the BI of different tiers is indicated, the pedofabric is depicted along with and independently from the ichnofabric, and are completed with the compositional diagram (Fig. 1) showing total percentages of bioturbation, pedofabric and original bedding. Theoretical background One of the most important results of the study of ichnofabrics is the recognition of the tiered pattern of organisms in substrates and subsequent modifications of it through time in relation to environmental change (Ausich & Bottjer 1982; Bromley & Ekdale 1986). The vertical stacking of ichnofossils, or tiering, is the response of trace-makers to biological, physical and chemical gradients within a substrate (Bromley & Ekdale 1986). In the marine realm, the vertical accretion

of the seafloor or major changes in palaeoenvironmental conditions produce either the upward migration of the communities or a shift from one to another. This is reflected in the juxtaposition of tiers producing composite ichnofabrics (Bromley & Ekdale 1986; Mcllroy 2004a). In modern soils the tiering of organisms is also a well-known phenomenon (e.g. Hasiotis & Bown 1992), but scarcely described from palaeosols. A few examples of infaunal changes and tiering in palaeosols have been documented until now, which show the importance of soil moisture and palaeo-water table as controls on organism distribution (Hasiotis et al. 1993; Hasiotis & Dubiel 1994). There are different sedimentological, palaeopedological, geochemical and ichnological indicators that can contribute to the understanding of fluctuations of ancient water tables (e.g. Bown & Kraus 1983; Retallack 1990; Hasiotis et al. 1993). The development of independent ichnological criteria, which allow comparisons with data gathered from other sources, may be based on two different approaches. One methodology, proposed by Bown & Kraus (1983), is based on the detailed record and comparison of the occurrence of trace fossils in particular palaeosol types and horizons. This approach is particularly important when dealing with trace fossils whose producers are unknown or

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uncertain. Regrettably, papers that analyse trace fossils and palaeosols with such a degree of detail are too few to provide a broad reference database (e.g. Down & Kraus 1983; Hasiotis et al 1993; Hasiotis & Dubiel 1994; Retallack 200 Ib; Genise et al. 2002). A second evaluation of palaeoenvironmental changes involves the recognition of the producers of ichnotaxa recorded from palaeosols. If the trace-maker can be identified, the ecological preferences and requirements of that taxon can then provide the key to understanding composite ichnofabrics. A word of caution on the attribution of trace fossils to modern groups should be introduced before an analysis. Trace fossils range from very simple to very complex morphologies. Along this spectrum, the more complex the trace, the more reliable will be its attribution to a particular modern taxon. The principle of special quality of behavioural homology (Wenzel 1992) states that the more complicated the behaviour, the stronger will be the postulate of homology. Behavioural homologies are defined as those behaviours that are similar because of their origin in a common ancestor, whereas analogies are those similarities that are shown by groups that are not phylogenetically related (Wenzel 1992). To extrapolate the ecological preferences and requirements of modern organisms to the producers of trace fossils, it is necessary to postulate a close phylogenetic relationship for both. Only behavioural homologies, confirmed by macro-and micromorphological studies and/or the high complexity of traces, are useful for postulating this phylogenetic relationship between a modern and a fossil trace-maker. In subaerial settings, insect nests and pupation chambers, and earthworm, millipede and crayfish burrows are complex enough and/or have been well studied macro- and micromorphologically to satisfy this special quality criterion (e.g. Bown & Kraus 1983; Hasiotis & Mitchell 1993; Genise et al 2000; Retallack 200Ib). Conversely, other trace fossils in palaeosols remain to be studied in more detail because their morphologies are very simple and can be attributed indistinctly to different groups of organisms (e.g. Ratcliffe & Fagerstrom 1980). In modern groups, other criteria that cannot be used with trace fossils are also employed to establish behavioural homologies (Wenzel 1992). In turn, a certain degree of congruence with the body fossil record can be proposed as an additional ichnological criterion to postulate homologies. For instance, the proposal that Triassic bee cells (Hasiotis et al. 1995) largely predate the appearance of angiosperms and the oldest fossil bees, known from the Cretaceous,

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has received criticism and general lack of acceptance (e.g. Engel 2001; Genise 2004). The compiled information on recorded trace fossil assemblages from palaeosols shows that they are composed of insect nests and pupation chambers, earthworm, millipede and crayfish burrows, root traces and other ichnofossils whose affinities are obscure (Genise et al. 2000, table 2; Retallack 2001b). Insect nests as a whole are the most common trace fossils in palaeosols, comprising a distinct ethological category termed calichnia (Genise & Bown 1994a). These authors explained their abundance, arguing that nests are constructed structures and not merely excavated ones, and consequently have a high preservational potential. The nesting activities involve the provision of nests with different kinds of organic matter to feed larvae by adults. Both provisions and larvae are very sensitive to soil temperature and moisture, requiring very specific environmental conditions (e.g. Michener 1979; Grasse 1984; Cane 1991; Hanski & Cambefort 1991; Genise 1999). Adults achieve these conditions basically in two ways: (1) by the morphology and constructional materials of nests and breeding chambers, and (2) by selecting specific nesting sites and soil depths to locate them. Constructed walls and linings, along with the general morphology of nests, provide the necessary isolation from soil to maintain a suitable temperature and humidity inside breeding chambers (e.g. Halffter & Edmonds 1982; Grasse 1984). Accordingly, these morphological devices increase the complexity of the whole structure, providing the diagnostic characters for recognition of behavioural homologies that permit the attribution to particular insect taxa (e.g. Genise 1999). In contrast, other structures excavated by different groups of insects for shelter, feeding or dwelling are in most cases more simple, and these analogous structures cannot be attributed to any particular invertebrate taxa (e.g. Ratcliffe & Fagerstrom 1980). Hence, for the purpose of palaeoenvironmental analysis, trace fossils that can be certainly attributed to particular invertebrate taxa and those whose attribution is doubtful should be analysed separately. Some broad-scale relationships between emplacement of trace fossils and position of an ancient water table can be easily addressed knowing the physiological requirements of producers. For instance, insects that are air breathers will inhabit the soil above the water table, whereas crayfishes and other crustaceans, which need free water to breathe, live in burrows under the water table or connected to watercourses (Retallack 1990; Hasiotis

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& Mitchell 1993). However, a more accurate picture of soil moisture preferences and tiering can be achieved by analysing particular groups of trace-makers involved within the context of the resultant ichnofabric. Selected study cases The few examples presented herein were selected particularly (1) to test the application of present methodology, (2) to show the independent development of ichnofabrics and pedofabrics, (3) to demonstrate tiering in ancient soils, and (4) to illustrate the presence of composite ichnofabrics in palaeosols. It is not the purpose of this contribution to review the palaeoenvironmental conditions or any other aspects of the sedimentary sequences included in the study cases. In the case of the Jebel Qatrani Formation from Egypt, only one of us (JFG) conducted ichnological research at that locality: hence the sedimentological and palaeoenvironmental setting is taken exclusively from the literature (Bown 1982; Bown & Kraus 1988). However, the importance of this case study for the previously mentioned purposes of this contribution necessitates its inclusion herein.

Palaeosols from the Ischigualasto Formation (Argentina) The Ischigualasto Formation (lower Late Triassic) is a continental unit of the IschigualastoVilla Union Basin, which crops out in San Juan Province, western Argentina (Stipanicic 2001). It is composed mainly of grey claystones, greenish tuffaceous siltstones and bentonites, carbonaceous mudstones and few thin coal beds intercalated with fine- to coarse-grained, crossbedded sandstones (Stipanicic & Bonaparte 1979; Bellosi et al 200la). At the surveyed localities in the Ischigualasto Provincial Park, the fluvial facies are organized in fining-upward cycles formed by channelized sandstone bodies with trough cross-bedding and downstream and lateral accretion surfaces, grading to thick floodplain deposits (Fig. 2). General alluvial architecture and the anatomy of sandstone bodies indicate an avulsion-controlled fluvial system, having perennial, moderate to high sinuosity, channels, mixed load and frequent splays (Bellosi et al. 2001a; Spalletti 2001). Floodplains were predominantly poorly drained, favouring the development of palustrine conditions locally (Milana & Alcober 1994). Landscape and basin configuration varied slightly

during Ischigualasto times. Trunk and more steady braided rivers occupied the central area of the Ischigualasto Park, whereas in northern and southern areas floodplains were better developed (Milana et al 1998). Previous workers (Milana & Alcober 1994) recognized the presence of 'root traces, bioturbation and burrows' at many horizons. In addition, Melchor et al (2001) stated that Ischigualasto palaeosols exhibit vertic features, tabular root systems and drab colours. Palaeosols of the lower section are better drained, and present calcic and mottled horizons, whereas those of the upper section are more clayey and rooted (Martinez et al 1998). In general, these alluvial palaeosols are simple and show moderate to poor maturity, in accordance with rapid sedimentation deduced from the presence of fresh feldspars (Milana & Alcober 1994). Most mudrocks exhibit greyish to greenish hues, whereas some beds are reddish, indicating better-drained conditions. Fossil amphibians, primitive dinosaurs and therapsids are abundant, and occur together with diverse plant remains belonging to the Dicroidium Flora (Kokogian et al. 1999; Zamuner et al. 2001). Vegetation along the margins of fluvial channels was dominated by subtropical (seasonal) evergreen and sclerophyllous forests, with low diversity, whereas on floodplains a herbaceous-arbustive community developed (Artabe et al. 2001). The two studied palaeosols are intercalated in a small-scale (10m thick in average) sequence in the upper section of the formation (Fig 2). Several of these sequences are intercalated between main channel and typical floodplain deposits, and are associated with carbonaceous mudstones deposited in lacustrine/backswamp environments and fine- to medium-grained sandstones that accumulated in levees and as splay lobes, or in subordinated channels. Such sequences are comparable to avulsion belt deposits (Kraus & Gwinn 1997; Farrell 2001), which generally support high sedimentation rates (Kraus 1996; Kraus & Asian 1998). One example is a 60 cm thick, olive grey (5Y5/ 1), smectitic clay-rich palaeosol (Figs 2, 4a) in which no relict of the original bedding is recognized. Pedofabric is uniform and characterized by small, angular blocky peds (wedge-shaped) and closely spaced slickensides. The palaeosol shows no horizonation, and it is eroded at top by a medium-grained sandstone with convolute bedding accumulated in a minor avulsion channel. Micromorphologically it has a welldefined blocky microstructure (sensu Bullock et al. 1985) and abundant connected streaks of bright clay, which are aligned around ped

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Fig. 2. (a) Location of stacked vertisols (asterisks) in a coarsening-upward avulsion sequence (CU Seq) of the upper section of the Ischigualasto Formation. Arrows show position of Vertisols with in situ Equisetales stems. The CU sequence is formed by suspension clay deposits (SC), splay sand lobes (SL) with ripple lamination and minor sandstone channels (Ch) with deformed trough cross-bedding. Tiering diagrams from well-developed Vertisols (b) and poorly developed ones (c) in this Formation.

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surfaces. Truncated stems of equisetales in lifeposition filled by green sandstone represent the only bioturbation (BI1) (Genise et al 2001; Fig. 4a). This palaeosol probably underwent advanced pedogenic homogenization. Thus it is interpreted as a very well-developed Vertisol, as the original bedding is totally obliterated by the pedofabric. Another example is a less-developed Vertisol, where the original bedding is clearly recognized (Figs 2, 4b). The upper 20cm are composed of a very coarse siltstone, light olive grey (5Y 6/1), having relict ripple lamination and lessdeveloped short slickensides and wedge-shaped peds. In the lower 20cm wedge-shaped clay peds and slickensides are also present, but a fine lamination is still preserved. Bioturbation is higher than in the case described previously (BI2). Ichnofabric is composed of equisetales stems and Skolithos linearis in the upper siltstone (Genise et al. 2001; Fig. 4b). The preservation of parallel and ripple cross-lamination suggests a poorly developed Vertisol.

Palaeosols from the Laguna Palacios Formation (Argentina) The Laguna Palacios Formation is an orange to reddish brown Upper Cretaceous (SantonianMaastrichtian) tuffaceous succession, which constitutes the upper part of the Chubut Group (Feruglio 1949; Sciutto 1981). It records terrestrial sedimentation at the northwest periphery of the San Jorge Basin and north Deseado Massif, in central Patagonia, Argentina (Bellosi & Sciutto 2002). Palaeogeographic reconstruction and thickness variation indicate that it accumulated on basin margin topographic highs (Sciutto 1981). The Laguna Palacios Formation is divided into lower, middle and upper members (Sciutto 1981). The only recorded body fossils from this formation are a few stems and trunks (Sciutto 1981). Two main facies type are recognized where the unit reaches its maximum thickness (Bellosi & Sciutto 2002). Pyroclastic facies predominate, including massive sheet strata of vitrie-crystal tuffs, very fine tuffs with accretionary lapilli, and bioturbated tuffaceous mudstones and claystones (Sciutto 1981; Genise et al 2002). The remaining facies are fining-upward cross-bedded pyroclastic sandstones and scarce intra-formational conglomerates, which are subordinate, albeit significant, because they are mostly restricted to incised channels. Several of these channel bodies, in the middle and upper members, comprise a thin basal siliciclastic sandstone with (fluvial) current

structures abruptly covered by a primary white tuff, suggesting non-permanent streams. Most of the pyroclastic deposits (60% in thickness) are pedogenized, commonly occurring as stacked non-calcic palaeosols (Sciutto 1981; Bellosi et al. 2002a). Detailed sedimentary, ichnologic and pedogenic features were recently published by Genise (2000) and Genise et al. (2002). Welldeveloped and more frequent palaeosols occur in the middle and upper members (Sciutto 1981). The Laguna Palacios Formation is considered to be a volcaniclastic-influenced, loess-palaeosol sequence, formed in a temperate subhumid/-seasonal climate and relatively dry conditions (Bellosi & Sciutto 2002; Bellosi et al. 2002a; Genise et al. 2002). Fluctuations of the water table, suggested by scarce hydromorphic features such as Fe-Mn nodules (Genise et al. 2002), are also consistent with a fossil stump in growth position of the Middle Member, which would have required rapid burial in a waterlogged habitat (Driese et al. 1997; Retallack 200 la). The depositional setting of the Laguna Palacios Formation can be compared to the Pleistocene Pampean loess plains, with few rivers and ponds, and savannah or prairie soils with herbaceous or low vegetation (Teruggi 1957; Iriondo 1999; Sayago et al. 2001). Most sediments were supplied as distal volcanic ash falls and subordinately by fluvial streams (Genise et al. 2002). The development of this loess-palaeosol sequence was regulated by the balance supply versus pedogenesis, which ultimately could have been governed by fluctuations in the climate regime (Bellosi et al. 2002a). The first case of palaeosol ichnofabrics shown herein occurs in a vitric Entisol from the Upper Member (Genise et al. 2002, Fig. 3B, 4B; Figs 3, 4c). The parent material is very fine (clay size grade) vitric volcanic ash. Glass shards are entire and fresh or slightly weathered. This isolated bed, 60cm thick, moderately bioturbated, and very pale orange (10YR8/2), is a distinct light-coloured pyroclastic deposit interbedded in a suite of brownish palaeosols. The original bedding of this air-fall ash is massive or crudely stratified, and pedofabric is wholly absent. Micromorphologically no soil feature is present, and no crystalline mineral was identified in XRD analysis. The ichnofabric shows two tiers (Fig. 3). The upper tier is dominated by Taenidium barretti and a few root traces, resulting in a moderate bioturbation (BI3). The lower tier includes fossil bee nests, Cellicalichnus chubutensis, and also a few root traces, resulting in low intensities of bioturbation (BI 2) (Genise et al.

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Fig. 3. (a) Section of the upper member of the Laguna Palacios Formation containing two different palaeosol types. These facies are included in a 300m thick pyroclastic loess-palaeosol succession, with intercalated fluvial channel deposits, (b) Entisol (star) developed in very fine vitric tuff (volcanic dust), (c) Intensely bioturbated Alfisol (asterisk) developed in tuffaceous sandstones.

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Fig. 4. (a) Truncated stem of equisetales in growth position (left of hammer), in a well-developed Vertisol. Ischigualasto Formation, (b) Abundant Skolithos in a poorly developed Vertisol, which preserves ripple lamination. Ischigualasto Formation. Scale bar: 1 cm. (c) Eroded surface of a vitric Entisol showing fossil bee nests (Cellicalichnus chubutensis) tunnels in relief. Laguna Palacios Formation. Scale bar: 10cm. (d) Detail of Cellicalichnus in plan view in the same Entisol. Laguna Palacios Formation. Coin 2 cm. (e) Structured and moderately bioturbated Alfisol; the pedofabric is composed mainly of blocky peds. Laguna Palacios Formation, (f) Coleopteran pupal chambers (Rebuffoichnus casamiquelai) from the palaeosol horizon shown in (e). Laguna Palacios Formation. Lens cap: 5cm. 2002). The nests, which originally would have reached the surface of the soil, could not be traced upwards into the upper tier, suggesting that the later activity of the shallow Taenidium

trace-makers and roots of plants destroyed this part of the nests. The second case is also included in the Upper Member, separated by 3m from the previous

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one (Fig. 3). The ichnofabric is present in a welldeveloped Alfisol, included in a sequence of composite tuffaceous palaeosols (Genise et al. 2002, figs 3C, F, 4B). It is the most common palaeosol type in the Laguna Palacios Formation. The original bedding is only very scarcely preserved in the upper horizon. The palaeosol shows a yellowish to light brown colour (5 YR 6/4), a well-developed, upper, elluvial, horizon, having platy peds, and a lower illuvial one, having angular to subangular blocky peds (Fig. 4e). The original deposit is a light-coloured, horizontal-bedded turfite (vitric medium tuffaceous sandstone), preserved as a C horizon in some cases. Bioturbation is high in the upper horizons (BI4) and low to absent in the C horizon. Recognizable trace fossils are coleopteran pupation chambers, attributable to Rebuffoichnus casamiquelai (Fig. 4f), Taenidium barretti, Skolithos linearis, Beaconites coronus (sensu Keighley & Pickerill 1994) and thin root traces. Rebuffoichnus casamiquelai is not cross-cut by the other traces, which suggests that it was the last emplaced trace. Its occurrence is patchy, in contrast to the other ichnogenera that display a more homogeneous pattern of lateral distribution in the deposit.

Palaeosols from the Asencio Formation (Uruguay) The Asencio Formation is a late CretaceousPalaeogene red-bed unit cropping out in western Uruguay (Bossi 1966) and partly in eastern Argentina (Genise & Zelich 2001). It is thin, 515m in average thickness, and composed of three to six stacked palaeosols, developed on sandstones of probable fluvial origin (Genise & Bown 1996; Figs 5a, 7a). Natural outcrops are small, but the formation is well exposed in many stone quarries. No body fossils were recorded from this formation despite its containing one of the most diverse, well-preserved and abundant assemblages of bee and coleopteran nests in palaeosols (Genise & Bown 1996). Original bedding is nearly absent with the exception of a few channelized bodies at the base of the unit (Pazos et al. 1998; Goso Aquilar & Guerequiz 2001) and poorly preserved in trough crossbeds in Dumestre quarry. The outstanding pedogenized character of this unit was first recognized by Ford (1988a, 1988 b), who also proposed that fossil insect nests were restricted to 'conglomerates' [sic] sourced from coeval Oxisols developed in humid tropical conditions. Regarding the ichnological content and sedimentological characteristics of the Asencio Formation, Genise &

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Bown (1996) interpreted the clasts of Ford's conglomerates as palaeosol peds. Finally, Gonzalez et al. (1998) proposed the existence of two different horizons characterized by different types of ped (prismatic and nodular) and ichnological content. The abundant ichnological assemblage, composed of 12 ichnogenera, was described by Roselli (1938, 1987) and redescribed by several subsequent authors (Genise & Bown 1996; Genise & Hazeldine 1998a, 1998b; Genise & Laza 1998; Genise & Verde 2000). This kind of red ferruginous palaeosol, as exemplified by the Asencio Formation, is usually produced by a complex of iterative destructionreconstruction stages related to hydromorphological processes (Tardy 1992). The original Bt horizons of mature Ultisols (Gonzalez 1999) subsequently supported lateritization and induration (crusts) (Caorsi & Gorli 1958; Ford 1988c), becoming ferricretes or cuirasses (sensu Nahon 1976; Fig. 7a, b). Lateral changes in the structure from nodular to vacuolar and massive is normal in cuirasses because of iron transfer, leaching or dissolution of kaolinite and quartz grains, formation of voids, and ferruginization (Nahon 1986). The beds with nodular structure are interpreted herein as being produced by dismantling of such iron duricusts (Tardy 1992; Tardy & Roquin 1992; Temgoua 2001; Figs 7a, c). Most of them are residual deposits showing no evidence of transport. Intense and widespread bioturbation (e.g. termite activity) is considered to be responsible for surficial hematite duricrust dismantling (Eschenbrenner 1986; Grasse 1986). However, no undoubtedly termite-made nest structure was identified. Irregular geometries and transitional boundaries between the ferricrete-ferricrete lag are the response of gradual dismantling processes, which may occur at the surface, inside, or at the bottom of ferricretes (Temgoua 2001). Most ferricretes develop under seasonally tropical climates: mean annual temperature 30 °C and rainfall 1300-1700 mm per year, with several dry months (Tardy & Roquin 1992). As they usually develop along large periods of time (105 to 106 years) it is probable that the thin Asencio sequence represents several million years. Changes in pedoenvironment produce smallscale variations in mineralogy and structure. For instance, red ferralitic soils (with haematite and kaolinite-haematite micronodules) are generally located in well-drained upland areas (Schwertmann 1988). In the selected example, two interfingered horizons are identified, one nodular and poorly consolidated (dismantling horizon) and the other well indurated, showing columnar

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Fig. 5. (a) Asencio Formation. The originally structured Ultisols contain abundant insect nests, supported laterization, ferricrete development and later dismantling (intraformational conglomerates), (b) Tiering diagram shows the different fabrics in the dismantling horizon and ferruginized crust, (c) Stratigraphic interval in the middle section of Jebel Qatrani Formation showing high-sinuosity fluvial facies and the position of the studied Inceptisol (star) (taken from Bown 1982). (d) The Inceptisol developed in fine-grained sandstones that preserve the original bedding originated in the upper part of a channel point-bar, with abundant rhizoliths and fossil termite nests (Fleaglellius pagodus).

structure (duricrust) in which the original bedding is completely disrupted (Figs 5a, 7a). The dismantling horizons are massive, having variable quantities of clay matrix (Ford 1988a). Their individual thickness is l-3m, and their geometry is lenticular. The base is, with rare exceptions, clearly non-erosive. Lateral boundaries of the columnar horizons are always transitional and irregular (Gonzalez 1999). In some exposures, dismantling horizons surround remains of duricrusts. Nodules are subrounded

and reddish, and their mean size varies from 3 to 10cm. The mineral composition and micromorphology of the nodules are similar to those of the duricrust, where hematite-kaolinite aggregates prevail. The smallest nodules are the more rounded ones. Clay matrix proportion also varies from 10% to 60%. Matrix percentage and roundness increase as nodule size decreases. The predominance of Monesichnus, Uruguay and Coprinisphaera in the nodular horizons, either in their original positions or rotated, was pointed

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out by Gonzalez et al (1998; Fig. 7c). The bioturbation is moderate (BI3). The duricrusts are 0.5-2.0 m thick, 200-300 m long, undulatory, and wedge shaped (Ford 1988b). Internal structure is defined by coarse columnar to angular blocky peds (Gonzalez et al. 1998), with reddish to orange clay cutans, pedotubules and glaebules (Ford 1988a). Columns are often inclined at 10-30°. Laterally and subordinately the structure becomes massive or vacuolar, with empty channels (3—6 mm wide) and macro voids (alveoles). Some horizons in the lower section of the unit include abundant iron nodules or brecciated carbonate patches, and others show mottled and bleached (yellowish) levels (Ford 1988b). Micromorphologically these horizons are characterized by a clayeyhaematitic ground mass and thick laminated ferroargillans coating voids and grains (Gonzalez 1999). Mineral composition is dominated by weathered and fractured quartz and hematite cement. Kaolinite is abundant and decreases upwards in the profile, whereas montmorillonite increases upwards (Ford 1988a). These horizons are dominated by Palmiraichnus and Teisseirei located in situ and rizoliths up to 30cm long (Gonzalez et al. 1998; Fig. 7b). Bioturbation is also moderate (BI 3).

Palaeosolsfrom the Jebel Qatrani Formation (Egypt) The Eocene-Oligocene Jebel Qatrani Formation of northern Egypt is a sequence of fine-grained to gravelly sandstones, mudstones, scarce carbonaceous shales and freshwater limestones, most of them showing evidence of damp pedogenesis (Bown 1982; Bown & Kraus 1988). This unit records the sedimentation of high-sinuosity rivers and overbank floods on a Palaeogene lowland coastal plain (Bown & Kraus 1988). Fluvial sediment accumulation was controlled by sea-level fluctuations in a stable tectonic setting. Variegation of the rocks in tabular bands is the most striking feature of palaeosol formation. Root and insect traces, cutans, clay-lined vugs and red duricrusts are additional evidence of widespread pedogenesis. The rhizolith assemblage contains roots and stems of fossil bushy and reedy plants and trees. Curiously, vegetal and animal bioturbation are not present in associated argillic horizons (Bown & Kraus 1988). The recognized alluvial palaeosol types include wet Entisols, Inceptisols, mature gley Spodosols and very mature Ultisols, some of them with fragipan horizons (Bown 1982; Bown & Kraus

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1988). Fossil leaves, fruits and woods indicate a wet tropical to subtropical (probably monsoonal) climate. Mangrove swamps prevailed in coastline areas, whereas in the forested interior a large and varied vertebrate fauna proliferated, especially diverse mammals including Old World primates (Bown 1982; Bown et al. 1982). Apart from body fossils, ichnofossils of mammals (rodents), invertebrates (insects, annelids, crabs and crayfishes) and plants (rhizoliths) are also very common in the Jebel Qatrani Formation. Most traces are exceptionally preserved in pedogenized fluvial-channel sandstones, because particular geochemical conditions prevailed during early diagenesis (Bown 1982). In some localities these trace fossils formed intricate, dense masses of chambers and tubes (Bown & Kraus 1988). The selected example comes from the middle part of the Jebel Qatrani Formation, where a yellowish brown, poorly developed Inceptisol occurs in a fine sandstone deposit, corresponding to the upper part of a point bar of a meandering river (Genise & Bown 1994b; Fig. 5b). Relict trough cross-bedding is the only remaining primary physical sedimentary structure. Inceptisols are the most common palaeosol group in the Jebel Qatrani Formation, especially in the coarser channel deposits (Bown & Kraus 1988). Their poor development reflects iterative depositional-erosional conditions in the active meander belt and minor pauses in the sedimentation (Bown & Kraus 1988). No horizons or soil structure were recognized. The ichnofabric is uniform along the palaeosol. Bioturbation intensity is high (BI4), with frequent trace fossil overlap. Rhizoliths and termite nests are particularly abundant. Oblate chambers and narrow galleries compose the fossil termitaria (Fleaglellius pagodus), which occur in the upper part of densely rooted palaeosols, where it is difficult to distinguish between galleries and small rhizoliths (Genise & Bown 1994b; Fig. 7d). The nest system, comprising polychambered structures forming high towers interconnected by galleries, occupies a large volume of the palaeosol.

Palaeosols from the Sarmiento Formation (Argentina) The Sarmiento Formation is an Eocene-lower Miocene pyroclastic succession known mostly because of its abundant and varied fossil vertebrates. The mammal assemblages (marsupials, xenarthrans, astrapotherians, notoungulates, primates and rodents) are considered the stratigraphic standards for the South America land

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mammal ages (Ameghino 1906; Simpson 1940; Cifelli 1985). It is broadly exposed in central and north Patagonia (Argentina), covering more than 200 000 km2 and showing a relatively uniform lithology, characterized by chonites (mud and clay-size tuffs), fine tuffs, bentonites, and intraformational conglomerates (Mazzoni 1979, 1985). Likewise, the presence of palaeosols bearing trace fossils is another significant and well-known feature (Frenguelli 1938; Andreis 1972; Andreis et al 1975; Spalleti & Mazzoni 1977, 1979). At the type locality, Gran Barranca, south of Chubut province, where the unit is divided in three members, there are numerous well-exposed palaeosols in the middle and upper members. The dominant parent material for palaeosols is fine volcanic ash composed of rhyolitic-dacitic glass shards and subordinate plagioclase (andesine). Opal phytoliths are also frequent or abundant in some levels, reaching up to 7% of the sample (Mazzoni 1979). Pyroclastic deposits resulted from distal ash falls, sourced from plinian-type eruptions more than 500km to the NNW (Mazzoni 1985). Most of the Sarmiento Formation tuffaceous beds are interpreted as an aeolian loessite (Spalletti & Mazzoni 1979). At Gran Barranca, the middle section (Puesto Almendra Member) is composed of primary vitric tuffs, bentonites, cross-bedded volcanic sandstones, intraformational conglomerates (generally palaeosol fragments) and a few lenticular basaltic flows (Spalletti & Mazzoni 1979; Fig. 6). A late Eocene-early Miocene age has been established for this member (Kay et al. 1999). Pyroclastic deposits were partly recycled by fluvial and aeolian processes, and supported pedogenesis. Most of the tuffaceous palaeosols are bioturbated and possess a significant argillic horizon, Fe and Mn nodules, a moderate to well-developed b-fabric and significant argilans and ferromangans (Bellosi et al. 200Ib). Dominant trace fossils are dung-beetle nests (Coprinisphaerd) (Spalletti & Mazzoni 1979; Laza 1986; Escribano & Delgado 1996; Genise et al. 2000; Bellosi et al. 200Ib), but there are also frequent root traces, other coleopteran traces (Teisseirei), bee cells (Celliforma), Beaconites cor onus, and large horizontal burrows (Bellosi et al. 200Ib). Scarce calcareous palaeosols (calcretes) contain almost exclusively Celliforma in association with charopid land gastropods (Bellosi et al. 2002c). Two palaeosols of Puesto Almendra Member were selected to show tiering and pedofabric/ichnofabric relationships (Figs 6, 7e, f). The first example is in the middle section of the west end profile of the Gran Barranca (stake MZ-10

from Kay et al. 1999; fig. 1). The palaeosol occurs in a fine (coarse silt grain size) massive tuff, 2.4m thick, highly porous, and light grey (N 8). Bed lithology and thickness are laterally very uniform on a kilometre scale (mantle bedding). The bases of beds are sharp, horizontal erosional surfaces, overlain by a thin upwardfining intra-formational breccia. Two horizons may be recognized in this palaeosol (Figs 6, 7f). The upper one is 0.40m thick, indurated, very light grey (5YR7/1), and shows intense bioturbation and scattered specimens of Coprinisphaera (BI5). The intense bioturbation is composed of a boxwork of sinuous interconnected burrows, 1-2 mm wide and 20mm long, having smooth walls and irregular chambers. Micromorphologically this horizon is characterized by abundant glass shards and a vesicular structure, with an undifferentiated b-fabric, abundant voids and channels, some of them coated by thin argilans, organic nodules and a few braided strands. Some features of this boxwork resemble modern termite nests. Coprinisphaera balls are densely perforated inside this boxwork, but in the remaining palaeosol they are undamaged. The lighter-coloured lower horizon is 2.0m thick with a transitional upper contact, showing coarse columnar structure. Preservation of the original bedding is difficult to assess because of the massive character (ash fall) of the primary sediment. Bioturbation is sparse (BI1), and composed of very thin, long root traces. Owing to a lack of clay (which precluded the formation of an argillic horizon), the abundance and good preservation of vitric shards (volcanic ash) and the thick profile, this palaeosol is classified as a weak to moderately developed Andisol. This example is comparable to the Luquem palaeosol of Retallack et al. (2000), a type of inmature Andisol, which preserves opportunistic vegetation of low specific diversity that grew on volcanic ash substrates. The second case is included in a series of stacked, reddish-orange indurated palaeosols (stake MZ-17 in Kay et al. 1999; fig. 1) associated to the upper erosive unconformity of this member (Kay et al. 2001; Bellosi et al. 2002b; Figs 6, 7e). Palaeosols differ in horizon thickness and boundary, textural features, ped type and abundance of iron nodules (pedofabric), and especially in the trace fossil content (ichnofabric). In the selected palaeosol the original deposit preserved in the lower 80 cm, is massive, greyish orange (10YR7/4), with an intraformational conglomerate. A rough stratification describes the poor preservation of the original bedding. The surface horizon is 30-40 cm thick and argillic. The pedofabric is composed of

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Fig. 6. (a) Stratigraphic position of the selected palaeosols from the Puesto Almendra Member of Sarmiento Formation. Fades association records a discontinuous sedimentation (erosional surfaces and abundant palaeosols), with distal volcanic ash falls, local basalt flows and fluvial reworking by a low-sinuosity system, (b) The lower example (asterisk) is an Andisol preserving the C horizon, (c) The upper example (star) is an Alfisol located near the top of a series of stacked palaeosols related to an unconformity.

subangular blocky peds (subordinate crumb peds), with ferruginized crusts at the top (Fig. 7e). Fed size varies from coarse blocky (2-5 cm) to medium crumb (2-5 mm) (Soil Survey Staff 1975), and the shape is slightly equidimensional. Peds are commonly not interlocked, being separated by clay materials. The crusts are dark orange (10 YR 5/6), 2-3 cm thick and folded.

Micromorphology is characterized by abundant fresh glass shards, a well-developed spongy structure, laminated clay, and Fe-cutans, along with Fe nodules. Some interconnected pores generate a blocky microstructure. The contact between both horizons is undulatory and relatively sharp. This palaeosol is interpreted as a moderately developed Alfisol because of the

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Fig. 7. (a) Laterized profile of Ultisols from the Asencio Formation. Massive to coarse columnar duricrust (arrow) covered by a nodular dismantling horizon, (b) Duricrust showing blocky peds, rhizoliths and Palmiraichnus castellanosi (arrow). Asencio Formation. Scale in cm. (c) Dismantling horizon showing nodular structure and the fossil bee nest Uruguay auroranormae (arrow). Scale in cm. (d) Inceptisol of the Jebel Qatrani Formation showing tiered chambers of the fossil termite nests (Fleaglellius pagodus) and abundant vertical rhizoliths. Lens cap: 5 cm. (e) Alfisol showing subangular blocky peds and folded ferruginized crusts showing a dung-beetle brood mass (Coprinisphaera) (arrow). Sarmiento Formation, (f) Andisol showing an upper horizon with granular peds and intense bioturbation possibly produced by termites and Coprinisphaera (arrows). The lower horizon show a coarse columnar structure and low bioturbation. Sarmiento Formation.

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coarse well-defined peds, thick argillans and undifferentiated b-fabric in the ground mass. The ichnofabric is arranged in two tiers (Fig. 6). In the upper one, Celliforma is the only discrete trace, resulting in a low bioturbation (BI2). The lower tier includes Coprinisphaera, Teisseirei barattinia, pan-shaped traces, and large horizontal burrows 1.8cm in diameter (Fig. 7e). Bioturbation is moderate (BIS), discrete traces are frequent, and cross-cutting is rare. Discussion

Ichnofabrics and pedofabrics The different cases described herein show how pedofabrics and ichnofabrics may independently exhibit variable degrees of development, how bioturbation and other soil processes may disrupt the original bedding by themselves or in combination, and how soil processes (e.g. homogenization, lateritization) may also disrupt ichnofabrics. In palaeosols of the Ischigualasto Formation ichnofabric is almost absent - just a few stems of equisetales and Skolithos linearis in the less-developed palaeosols. Even so, the degree of ped development in these Vertisols is such that the original bedding is visible only in one of the study cases. The absence of a welldeveloped ichnofabric in those palaeosols may be attributed to different causes: • the absence of constructed, more preservable, structures, such as those made by bees, dungbeetles and termites before the Cretaceous (Genise & Bown 1994a); • the frequent waterlogging and reducing conditions found in these soils (Retallack 1990); • ichnofabric destruction by intense pedogenic homogeneization in Vertisols (Melchor et al. 2001). The last hypothesis suggests not only that nonbiological soil processes may disrupt the original bedding by itself, but also that they may destroy the ichnofabric of the deposits, thus emphasizing the importance of studying both pedofabric and ichnofabric in palaeosols. According to independent sedimentological data, these alluvial Vertisols are immature, as maturity was defined as a function of time by Bown & Kraus (1987). In addition, the scales of palaeosol development are based mostly on particular types of horizon (Bown & Kraus 1987; Retallack 1988) that are not present in palaeovertisols (Nettleton et al. 2000). Consequently, and in the absence of important bioturbation, the stage of development of these palaeosols is evaluated herein in

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terms of destruction of the original bedding by the ped-forming processes. A still clearer case of ichnofabric destruction by other soil processes occurs in the Ultisols of the Asencio Formation. These palaeosols show well-developed pedofabrics and ichnofabrics, both responsible for the obliteration of the original bedding, which is absent in almost all outcrops. In addition, the lateritization process dismantled the duricrusts, partially destroying the original ichnofabric. The only preserved trace fossils in these dismantling horizons are insect nests with thick constructed walls. Many nests are disturbed relative to their original positions, as evidenced by random orientation of their nest entrances, as most bees and dungbeetles are very constant in the orientation of their cells and brood masses (e.g. Halffter & Matthews 1966; Stephen et al. 1969). The ferricretization process also modified the original pedofabric of Ultisols in which insects nested and bioturbation developed. At the other extreme, the study case from the Jebel Qatrani Formation shows Inceptisols almost deprived of pedofabric, where intense bioturbation results in a complex intense ichnofabric of BI4. Entisols from the Laguna Palacios Formation and palaeosols from the Sarmiento Formation show intermediate cases in which pedofabric is poorly or undeveloped, whereas ichnofabric shows different degrees of development. Alfisols of the Laguna Palacios Formation show welldeveloped ichnofabrics and pedofabrics, whose relative degree of development varies laterally along the exposed palaeosol on a 100m scale. Most of the Laguna Palacios and Sarmiento palaeosols differ from the remaining alluvial examples in that they developed in pyroclastic aeolian systems. The contrasting mechanisms of sediment supply and hiatuses controlled the soil-forming processes and soil stratigraphy in a variety of different ways. The independent evaluation of ichnofabrics and pedofabrics in these cases favours a more complete analysis of the evolution of these deposits. Different characters of the pedofabric can be used to estimate the relative duration of formation of soils (Retallack 1994). For instance, the clayey Bt horizon of the Alfisol from Laguna Palacios Formation suggests 104 to 103 years of formation because of the well-developed clay skins, ground mass, and homogeneous sepic microfabric (Retallack 1994). During this period of subaerial exposure water tables fluctuated seasonally, as revealed by hydromorphic features and illuviation of clays (Genise et al. 2002). The evidence of subaerial exposure provided by coleopteran pupation chambers in

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these palaeosols falls in a very different timescale. Insect activity involved in the construction of Rebuffoichnus probably indicates the subaerial exposure of the deposits during only one or two seasons. Pedofabrics of the Ischigualasto Formation appear to have been developed in less time than that of Laguna Palacios because of the rapid sedimentation rate in the first formation, as discussed previously. The nesting activity of termites that produced the ichnofabric dominated by Fleaglellius pagodus in the Jebel Qatrani Formation suggests several years of colony growth by apposition of chambers and simultaneous foundation of new colonies (Genise & Bown 1994b). This fact, along with the abundance of rhizoliths, indicates a longer period of subaerial exposure and lower water tables than that suggested by the nesting of solitary insects in the Laguna Palacios, Asencio and Sarmiento Formations. Nests of solitary insects may be constructed, provisioned and closed in only one day, whereas those of social insects may be maintained for years, and their micromorphology can even reveal their age (e.g. Jonkman 1980). Regardless of this, high concentrations of nests of solitary insects, along with well-developed pedofabrics, such as from the Asencio Formation, may indicate a very long period of subaerial exposure and low water tables, interrupted only by heavy seasonal rains, attested to by the presence of abundant dismantled horizons. The Egyptian example demonstrates the independence of ichnofabric and pedofabric development, as the intense subaerial bioturbation produced by termites and roots is not accompanied by the development of other soil characters. Bee and coleopteran trace fossils The ichnofabrics of Laguna Palacios, Asencio and Sarmiento show fossil bee cells and/or dung-beetle brood masses as their dominant components. Remains of nests or cells attributable to bees are included in the ichnofamily Celliformidae, comprising the ichnogenera Palmiraichnus, Celliforma, Corimbatichnus, Uruguay, Ellipsoideichnus, Rosellichnus and Cellicalichnus (Genise 2000). In soil bee nests, excessive moisture causes liquefaction or decay of provisions by fungal attack, which may also destroy the larvae (Michener 1979; Rozen in. litt. in Michener 1979). In addition, in waterlogged soils, larvae are exposed to poor oxygen diffusion (Visscher et al 1994). There are a few exceptional records of bees nesting in periodically or sporadically submerged sites

(e.g. Michener 1979; Roubik & Michener 1980; Cane 1991; Visscher et al. 1994). The waterproofed breeding cells of an aggregation of Epicharis zonata (Anthophorinae) remain beneath the water table during the wet season (Roubik & Michener 1980). An aggregation of Calliopsis pugionis (Andreninae) that nested in a site that was flooded after a heavy, unusual winter rain was recorded by Visscher et al. (1994). Flooding was not fatal, although bees emerged later in the season, affecting the pollen collection and reproductive success. Accordingly, most bee nests are located in well-drained soils (Linsley 1958; Batra 1984), and the cell wall is lined with water-repellent lipids to maintain the moisture conditions (Cane 1991). The ichnogenera Coprinisphaera, Fontanai and Monesichnus, present in the Asencio and Sarmiento ichnofabrics, are considered to be brood-masses of dung-beetles (Sauer 1955; Roselli 1987; Genise 1993; Genise & Laza 1998). Poorly drained and occasionally flooded soils are unfavourable for many digging species of dung-beetle, whereas the period and duration of flooding determine the success or failure of reproduction (Lumaret 1983). One species nesting in soils that were flooded during winter showed a high mortality of eggs and larvae (Kirk 1983). In addition, Hanski and Cambefort (1991) stated that waterlogged soils are generally poor for all dung-beetles owing to alteration of the droppings that these insects utilize to provision their nests. In conclusion, in nests of bees and dung-beetles, excessive water content in the soil produces problems related to larval mortality, decay of provisions, oxygen diffusion and alteration of droppings. Thus the ichnotaxa mentioned previously may be regarded as indicators of well drained to sporadically flooded palaeoenvironments and low water tables at the time of their emplacement in the soils. Apart from calichnia, other recognizable coleopteran structures, fossil pupation cells are common in palaeosols (Roselli 1938, 1987; Retallack 1984; Johnston et al. 1996; Genise 1999; Genise et al. 2002). They are represented in the present study by the ichnogenera Teisseirei in the Asencio and Sarmiento ichnofabrics and by Rebuffoichnus in the Laguna Palacios locality. Such structures are chambers constructed by larvae of different groups of coleopterans to contain and protect pupae before emergence as adult (Retallack 1984; Johnston et al. 1996). It is difficult to attribute these chambers to particular coleopteran taxa with known ecological preferences (Genise et al. 2002), but at least as the trace-makers are air breathers it is possible to ascertain that they were constructed under subaerial conditions above the water table.

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Ant and termite trace fossils The ichnogenus Fleaglellius, which is dominant in the ichnofabric of the study case from the Jebel Qatrani Formation, is considered to be a termite nest (Genise & Bown 1994b). Another possible termite nest presented herein is that recorded from the second case study of the Sarmiento Formation. Fossil termite nests are one of the most common trace fossils in palaeosols (Bown & Laza 1990; Genise & Bown 1994b; Genise 1997), sometimes comprising complex termitic ichnofabrics (Genise & Bown 1994b). Termites as a whole prefer high atmospheric and soil moisture (e.g. Collins 1969; Grasse 1986), and consequently their nests may be found in periodically waterlogged soils (e.g. Lee & Wood 1971; Grasse 1984). However, as with bee or dung-beetle nests, waterlogging produces similar problems of gas exchange (e.g. Schmidt 1960; Roy-Noel 1972; San Jose et al 1989) or fungal or microbial infections (e.g. Grasse 1984). Thus species that inhabit soils that are seasonally waterlogged show particular behaviours or nest features, such as those that retreat into epigeous mounds during the wet season (Lee & Wood 1971; Matthews 1977). Other species provide their nests with special chimneys (Roy-Noel 1972), perforation systems, and air or sand envelopes (Schmidt 1960) to maintain the microenvironmental conditions inside nests, which are very specific in terms of moisture and concentrations of C>2 and CC>2 (Grasse 1984). A similar case is that of fossil ant nests, also common in ichnofabrics of Tertiary palaeosols in North and South America (Laza 1982; Bown et al 1997). In contrast to termites, which are restricted mostly to the stable microenvironment of their nests, most ants construct less elaborate structures but have the ability to move their eggs and larvae from place to place in response to environmental changes (e.g. Wheeler 1910; Holldobler & Wilson 1990). Colonies move frequently - flooding is one of the factors that can trigger these movements - although there are records of ants surviving several hours or even days submerged by floodwater (Holldobler & Wilson 1990). There are few records of ants that nest in low lands and also construct mounds to which they retreat during the wet season to avoid high water tables (e.g. Bruch 1916; Bonetto et al. 1961), like termites. In summary, ants and particularly - termites nest in well-drained to seasonally flooded soils. In the latter case nests are provided with particular devices, such as epigeous mounds, chimneys, or special walls. In terms of soil moisture, ichnofabrics dominated

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by fossil termite nests such as those of the Jebel Qatrani Formation would indicate higher moisture conditions and more frequently flooded soils than ichnofabrics dominated by bee and dungbeetle traces. Fossil plants and mammals of the Jebel Qatrani Formation also indicate a wet climate (Bown 1982; Bown et al 1982; Bown & Kraus 1988). In conclusion, ichnofabrics dominated by calichnia and pupation chambers indicate subaerial conditions. The construction of nests and other underground activities take place during periods of subaerial exposure of deposits. It is important to note that insect nests can be transported, and that they have been recorded as clasts in conglomerates (e.g. Andreis 1981; Bown & Ratcliffe 1988). Therefore not only the identification of nests, but also their position in situ, are needed before the subaerial exposure of deposits can be determined. Earthworm trace fossils Fossil earthworm burrows indicate a moist soil environment. Edaphichnium lumbricatum Bown and Kraus 1983 is considered to be an earthworm burrow because of its morphology, the presence of faecal pellets, and the concentration of calcium carbonate in the burrows and pellets. One of the most important requirements of earthworms is adequate soil moisture, because respiration depends on the diffusion of gases through the moistened body wall (Lee 1985). Consequently, earthworms inhabit soils in which water is confined to films on the surface of soil aggregates or is held in pore spaces by capillary forces, and the relative humidity of air-filled spaces is 100% or slightly less (Lee 1985). In waterlogged soils or those where free water is no longer present, earthworms cannot survive (Lee 1985). In seasonal tropical climates earthworms construct spherical aestivation chambers to spend the dry season at deeper layers in the soil profile (Jimenez et al. 2000). This type of trace, of great environmental value, has recently been found in palaeosols (Verde et al. 2002). In accordance with the moist environments preferred by earthworms, E. lumbricatum was found in gleyed palaeosols of the Willwood Formation (Bown & Kraus 1983). It should be noted that a complete ichnotaxonomical review of pellet-filled burrows from different environments (e.g. Richter & Richter 1939; Pickerill 1989 and references therein) is still lacking, and would be critical to correctly distinguish earthworm from other similar burrows.

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Trace fossils of uncertain affinities In contrast, another group of palaeosol ichnofossils is that composed of those ichnotaxa that (1) cannot be attributed unequivocally to a particular group of producers, and (2) are recorded from different continental and marine deposits. As Retallack (1990) pointed out, these tubular trace fossils, which are common in modern soils, are not diagnostic of them because they are also found in lacustrine and marine environments (e.g. Ratcliffe & Fagerstrom 1980). Ichnofabrics of Ischigualasto, Laguna Palacios, and Sarmiento show this type of trace fossil. In contrast, they are absent from those showing the most developed subaerial assemblages, Asencio and Jebel Qatrani. It is possible that, in the future, micromorphological studies may aid in the attribution of these trace fossils to modern taxa, but currently they are of uncertain affinities. The identification of ichnotaxa belonging to air breathers is critical for assessing the subaerial exposure of the deposits, particularly for those showing poorly developed palaeosols. Palaeosols and/or subaerial trace fossils may occur in fluvial (e.g. Bown & Kraus 1983), lacustrine (e.g. Edwards et al. 1998) and marine (e.g. Curran 1994) deposits during periods of subaerial exposure. In theory, if palaeosol development is very weak, a trace fossil suite related to the original subaqueous conditions of these deposits could be preserved along with trace fossils produced under subaerial conditions (e.g. Curran 1994; Buatois et al. 1998; Retallack 200 Ib). It has been stressed that the presence of root traces is a post-depositional process, independent from the origin of deposits, that indicates subaerial exposure and change of environment (Bockelie 1994; Curran 1994). Similarly, a subaqueous assemblage could be developed if a subaerially exposed deposit were later to be submerged (e.g. Driese & Foreman 1991; Curran 1994), and subaerial and subaqueous trace fossils may occur together in deposits subsequently submerged and exposed to air (Frey et al. 1984 and references therein). Recently, Retallack (200Ib) analysed the possibility of recording fossil burrows representing aquatic environments predating soil formation or resulting from later inundations by lake or lagoonal waters. Different criteria, such as density of burrows in relation to other soil characters, cross-cutting relationships with carbonate nodules, burrow collapse and fillings and associated ichnofauna, were used to demonstrate that Scoyenia beerboweri was produced during the period of soil formation (Retallack 1985, 2001b).

Ichnotaxa such as Skolithos, Scoyenia, Taenidium, Beaconites, Macanopsis, Cylindricum and other named and unnamed trace fossils have been cited from palaeosols (e.g. Genise et al. 2000; Hasiotis 2000; Retallack 200Ib) as well as from marine and lacustrine environments (e.g. Alpert 1974; Hantzschel 1975; Bown & Kraus 1983; Frey et al. 1984; Keighley & Pickerill 1994; Buatois & Mangano 1995). With the exception of Scoyenia beerboweri (Retallack 200Ib), it is impossible to distinguish between specimens in subaerial environments from those occurring in subaqueous ones. The attribution of most of these trace fossils to particular groups of insects, or even invertebrates, is thus, in such cases, at best speculative. Furthermore, to extract palaeoecological and other inferences from them (e.g. Hasiotis & Dubiel 1994; Hasiotis 2000) without a previous accurate analysis of their affinities is of dubious utility. The Triassic meniscate burrows attributed to soil bugs by Hasiotis & Dubiel (1994) are similar to those recorded from subaqueous environments (Keighley & Pickerill 1994). In addition, in the original contribution by Willis & Roth (1962) cited as source by Hasiotis & Dubiel (1994), it is stated that no tunnels or channels are present, a fact already mentioned by RatclifTe and Fagerstrom (1980). Soil bugs have their mouthparts adapted for sucking fluids from roots or other plant materials (e.g. Carver et al. 1991). Observations of the feeding habits of these insects are particularly important in the interpretation of Triassic meniscate burrows. Their menisci are ungraded, and stained with alternating zones of oxidized and unoxidized iron compounds (Hasiotis & Dubiel 1994). This pattern is commonly interpreted as the result of the original alternation of organic-rich (faecal) material with organic-poor (sediment) material (D'Alessandro & Bromley 1987; Hasiotis et al. 1993; Keighley & Pickerill 1994). The presence of such alternating menisci containing faecal material in burrows made by insects, which do not ingest sediment, is unlikely (e.g. Frey et al. 1984), and even more in soil bugs that feed on root fluids. This type of meniscate burrow in Willwood palaeosols was attributed to earthworms by Bown & Kraus (1983). The only possibility of gathering some environmental data from this group of trace fossils of uncertain affinities is when its presence is analysed in combination with other palaeosol and sedimentological data and compared with those from other sequences (Bown & Kraus 1983; Retallack 1985, 2001b). The 'adhesive meniscate burrows' from palaeosols of the Willwood Formation described by Bown &

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Kraus (1983) were later interpreted as indicators of increased soil moisture and fluctuating water table, owing to their presence in the lower half of the palaeosol profile (Hasiotis et al. 1993). In addition, Hasiotis et al. (1993) recorded the presence of Scaphichnium hamatum, a dung-beetle nest, in the upper half of the same profile in a better-drained and drier environment. Similarly, in the Gleysols from the Triassic Chinle Formation the adhesive meniscate burrows are abundant in the deepest tier just above the water table, characterized as having high, stable moisture content (Hasiotis & Dubiel 1994). In these two cases Taenidium is recorded from the deepest tiers of the vadose zone of gleyed palaeosols. The possible producers of meniscate burrows were analysed in detail by Frey et al. (1984), who pointed out the difficulties in establishing their affinities and palaeoenvironmental significance. In conclusion, these authors stated that some meniscate burrows (e.g. Scoyenia gracilis and Beaconites coronus) occur preferentially in moist to wet substrates in shallow aquatic deposits periodically exposed to air or in subaerial deposits periodically submerged (Frey et al. 1984). These scarce data suggest a clear distinction between the drier and welldrained conditions preferred by insect to nest or pupate and the moister and less-drained conditions selected by the producers of these meniscate trace fossils. In contrast, Scoyenia beerboweri was described from well-drained palaeosols in a tropical, seasonally dry, semi-arid palaeoclimate (Retallack 200 Ib). Apart from vertical tiering, soil organisms show patchy lateral distributions controlled by soil texture, soil carbon content, vegetation, and population dynamics (Ettema & Wardle 2002). Accordingly, insect fossil nests and pupation chambers commonly show a laterally heterogeneous distribution in palaeosols, as in the case of Rebuffoichnus described herein from one example of the Laguna Palacios Formation. In contrast, meniscate trace fossils in the same ichnofabric, such as Taenidium and Beaconites, have an extended and homogeneous lateral distribution, suggesting that these trace fossils were not controlled by the lateral heterogeneity of soils. Horizontal patterns of variability of ichnofabrics have been less documented than vertical ones, but more homogeneous lateral distributions are known from subaqueous trace fossils, so much that concentration of burrows (e.g. Skolithos, Ophiomorpha, Zoophycos) along bedding planes has been used as marker beds in stratigraphy (e.g. Ekdale et al. 1984). In conclusion, this group of ichnogenera of simple morphology is by now not clearly indicative of a particular set of palaeoenviron-

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mental conditions. The few possible clues are that: they are mostly recorded from subaqueous environments; in some cases their lateral distribution is homogeneous, suggesting that it is not controlled by soil environmental factors; and several studies suggest that some of them occur in moister conditions than insect nests. Considering that soils may be sporadically, seasonally or permanently waterlogged (e.g. Retallack 1990), a possibility is that these ichnogenera, in contrast to those attributed to insect nests and earthworms, may be recording periods of high moisture or even waterlogging of soils.

Composite ichnofabrics in the study cases In the Ischigualasto Formation ichnofabrics, Skolithos are found at the top of poorly developed palaeosols in association with remains of the original bedding (e.g. ripple cross-lamination), suggesting that these trace fossils were probably produced by subaquatic organisms. In contrast, in Entisols of the Laguna Palacios Formation, the upper tier of Taenidium can be demonstrated to obliterate the entrance of Cellicalichnus, suggesting that bees opportunistically colonized the deepest layers of the soil first, and that Taenidium is a later component of this ichnofabric. This composite ichnofabric would thus reflect a shift of palaeoenvironmental conditions from drier conditions (early deepesttier Cellicalichnus ichnocoenosis) to moister conditions (later shallow-tier Taenidium ichnocoenosis), probably because of a raised water table. Curiously, Savrda et al. (2000) found a similar, but extreme, form of composite ichnofabric in the Cretaceous Tuscaloosa Formation, having recent Cellicalichnus-like nests crosscutting Taenidium-dommatQd fabrics. In Alfisols of the Laguna Palacios Formation, ichnofabrics are dominated by Taenidium, Beaconites and Skolithos, which show a homogeneous horizontal and vertical distribution in the palaeosol, whereas, Rebuffoichnus - a clear indicator of subaerial conditions - is laterally restricted in the same palaeosol, in which they cross-cut the previously mentioned group of traces. This composite ichnofabric thus indicates changes in the soil conditions from moist or even waterlogged palaeonvironments (Taenidium, Beaconites, Skolithos) to subaerial exposure (Rebuffoichnus). Accordingly, illuviation of clays and hygromorphic features of this palaeosol

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indicate seasonality and a mobile water table (Genise et al 2002). On the other hand, tiers composed of different subaerial ichnotaxa (1) sharing common environmental preferences, such as bee cells and dungbeetle brood masses, and (2) showing little, at random, or no cross-cutting are interpreted herein as the original tiering of soils. In the case study of the Asencio Formation, ichnofabric reveals a tiered structure, albeit somewhat blurred by the laterization processes. In the dismantling horizon the upper tier is composed mostly of Uruguay, Monesichnus and Coprinisphaera, whereas in the crusts Teisseirei and Palmiraichnus prevail. Similarly, in the Sarmiento Formation Celliforma constitutes the upper tier, whereas in the lower one Coprinisphaera, Teisseirei and two other undetermined trace fossils are dominant. Conclusions Many soil features that disrupt the original bedding of the deposits may be formed without the intervention of bioturbation, or may be the result of its interactions with physical and chemical processes. For ichnofabric analysis these soil features are considered to constitute the pedofabric of the deposit, which is distinguished from the ichnofabric (the fabric directly and completely produced by plant and animal traces). The usefulness of this distinction is supported by the analysis of study cases that show that ichnofabrics can be intense in palaeosols devoid of other soil characters and, conversely, palaeosols showing a well-developed pedofabric can bear almost no trace fossils. In addition, bioturbation and other soil processes may disrupt the original bedding by themselves or in combination, and soil processes (e.g. homogenization, lateritization) may also disrupt ichnofabrics. The pedofabric is an additional component to classical ichnofabric analysis, which normally considers only original bedding and ichnofabrics. This complexity of palaeosol fabrics requires some modifications to previous methodologies to better describe and interpret palaeosol ichnofabrics. The analysis comprises: • tiering diagrams, in which the bioturbation index of different tiers is indicated; • the pedofabric, depicted along with and independently from the ichnofabric; and • a pedofabric/ichnofabric ternary diagram showing percentages of bioturbation, pedofabric and original bedding. The record of palaeoenvironmental changes in composite ichnofabrics of palaeosols may be

based on two different approaches. One approach comprises the detailed record and comparison of the occurrence of trace fossils in particular palaeosol types and horizons, which is particularly important when dealing with the more simple trace fossils, whose producers are unknown or uncertain. A second approach, particularly important when dealing with complex trace fossils such as insect nests, is based on the identification of trace-makers and the evaluation of ecological preferences and requirements of their recent representatives. Different groups of trace fossils in the study cases show a spectrum from the drier conditions preferred by bees and dung-beetles to the moister ones of ants, termites and trace fossils of uncertain affinities. The subaerial environment is clearly indicated only by trace fossils certainly attributable to air breathers (insect nests and pupation cells, earthworm and millipede burrows). In contrast, trace fossils of uncertain affinities may be subaerial or subaqueous in origin. A possibility is that they may be recording periods of high moisture, waterlogging of soils, or even a subaqueous environment before or after the soilforming process. The duration of subaerial exposure will be directly related to ichnofabric development by the subaerial suite and, as such, it will be a key factor, along with the development of the pedofabric, in understanding the dynamics of post-depositional soil processes. The authors thank D. Mcllroy, G. Retallack and J. de Gibert for the critical review of the manuscript. This research was partially supported by grants from the National Scientific Research Council of Argentina (CONICET-PIP 717/98), the National Agency of Scientific and Technical Promotion of Argentina (FONCYT-PICT 6156/99), and the National Science Foundation of the USA (EAR 00-87636).

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Development of early Palaeozoic ichnofabrics: evidence from shallow marine siliciclastics MARY L. DROSER 1 , S0REN JENSEN1 & JAMES G. GEHLING2 1

Department of Earth Sciences University of California, Riverside, CA 92521, USA (e-mail: [email protected]) 2 South Australian Museum, Division of Natural Sciences, North Terrace, Adelaide 5000, South Australia, Australia Abstract: Earliest Cambrian fine-grained sediments appear to have been firm close to the sediment-water interface. As a result there was a high preservational potential of shallow tiers. There is limited evidence for a mixed layer at this time; rather, most of the preserved trace fossils were open burrows. Later in the Cambrian, depth of sediment mixing increased but firmground conditions are still found relatively close to the sediment-water interface. The development of the mixed layer and properties of Cambrian muddy sediments have numerous stratigraphic and ichnological consequences. These include secular trends in the preservation of event beds and shallow-tier trace fossils including Rusophycus and Cruziana.

Trace fossils and ichnofabric have long played a role in stratigraphic and sedimentological analyses (e.g. Bromley 1996). The advent of sediment mixing has important implications for a number of sediment properties. Sediment mixing by animals greatly influences oxygenation of the sediments, the geochemistry of the sediments and the distribution of organic material, and is important for nutrient cycling (Aller 1983; Mcllroy & Logan 1999). The onset of bioturbation might have played a part in shifts in the carbon and oxygen isotope record, nutrient cycling, and the distribution of organic material (Brasier 1990; Brasier & Mcllroy 1998; Mcllroy & Logan 1999). Another aspect of sediment mixing is the destruction of primary bedding and changes in the rheological properties of the sediment. The preservation potential of physical sedimentary structures was highest before the advent of infaunal activity in the latest Proterozoic (e.g. Eriksson et al. 1998). An additional aspect of the rise of bioturbation is the increased destruction of thin storm event beds. Sepkoski et al. (1991) proposed a secular trend in event bed preservation as a result of increased infaunal activity: that is, as depth and extent of bioturbation increase, event beds are less well preserved. The upper portion of the sediment that is actively being burrowed and thoroughly mixed is known as the sediment mixed layer. Applied originally to deep-sea sediments (Berger et al. 1979), this term is now commonly used to refer to the actively burrowed zone from a variety of marine environments (e.g. Bromley 1996; Savrda & Bottjer 1989). Related, but certainly not synonymous, is the concept of tiering. Tiering is the depth below the sediment-water

interface at which an animal(s) lives (Ausich & Bottjer 1982). Thus, in modern marine environments, invertebrate animals live at a variety of depths within the sediment, and thus occupy a number of different tiers. Of course, animals may live in, but not mix, the sediment. Suspension-feeding animals that build a domicile open vertical burrow, such as a Skolithos-type burrow, do not actively mix the sediment. Many researchers have assumed that the development of the sediment mixing and thus the mixed layer occurred at or near the Neoproterozoic-Cambrian transition, but recent work has indicated a more complicated history. Earliest Cambrian strata show very little evidence of a mixed layer. Indeed, data suggest that the lack of a well-developed mixed layer resulted in finegrained sediments that were firm close to or at the sediment-water interface at this time because of a lack of bioturbation or mixing and a resultant well-developed mixed layer (Droser et al. 2002a, 2002b). We have previously suggested that firm muddy sediments that form at a shallow depth (e.g. are not exposed at the surface because of erosion of overlying soft or soupy sediments) can be directly related to the shallow sediment mixing. In this paper we review the evidence for firm substrates in Lower Cambrian strata and present relevant data from Lower Cambrian through Cambro-Ordovician strata to evaluate the nature and timing of the development of the mixed layer. We have examined several aspects of the Cambrian ichnological record including: trace fossil preservation; preserved depth of bioturbation;

From: MC!LROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 383-396. 0305-8719/04/S15.00 © The Geological Society of London.

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nature of ichnofabric (all aspects of the trace fossil record include features such as mottled bedding resulting from sediment mixing where discrete trace fossils cannot be identified); and nature of the substrate, which has been recognized as a factor in trace fossil preservation. This paper is based on field and laboratory examination of a large number of Cambrian siliciclastic units, trace fossils and sedimentary structures. The bulk of the record through this interval is siliciclastic, and trace fossils are best observed in relatively thin-bedded siliciclastic sediments. These include units of the CambroOrdovician Bell Island Group (~120m) and Lower Ordovician Wabanna Group (20m), Newfoundland; the Lower Cambrian Mickwitzia sandstone (10m), Sweden; the Cambro-Ordovician Bynguano Formation (30m), New South Wales, Australia; the Lower Cambrian Lontova (10m) and Liikati (15m) formations, Estonia; the Lower Cambrian Wood Canyon (100m), Pioche (20m) and Harkless (5m) formations, western USA; the Lower Cambrian Arumbera Sandstone (200m) and the Cambro-Ordovician Pacoota Sandstone (200m), Northern Territories, Australia. We examined parts of these units that represent deposition on the shelf below fairweather wavebase and above maximum storm wavebase, as determined by independent sedimentological criteria such as hummocky cross-stratification, presence of event beds and stratigraphic context. The measured portions of these units (indicated in parentheses above) are characterized by heterolithic bedding ranging from the centimetre to decimetre scale. Detailed logs were made of all sections, and selected intervals were described at the centimetre scale. The sedimentology, ichnofabrics and trace fossil taxa were described in relation to the sedimentary context (e.g. preservation). Rock samples were taken at decimetre intervals where possible, cut into slabs and polished. Selected slabs were X-rayed. Characteristics of the ichnological record of lowermost Cambrian strata Below we summarize results from previous work. The greatest diversity of Cambrian trace fossils have been reported from settings representing deposition below fair-weather wavebase and above storm wavebase (e.g. shelf) that have a heterolithic bedding characterized by moderately thin, generally centimetre-scale, interbedded

sandstone, siltstone and mudstone. In this setting, even in earliest Cambrian strata, trace fossils are conspicuous. Indeed trace fossils are an important part of lowermost Cambrian stratigraphy and define the base of the Cambrian system (Crimes 1987; Narbonne et al 1987; Brasier et aL 1994). Of the units that we examined, the Lower Cambrian formations exhibit a number of shared ichnological and sedimentological characteristics. These characteristics are ubiquitous in Cambrian strata of the earliest Cambrian (Nemakit-Daldynian) (Fig. 1). Preserved depth of bioturbation In modern settings, burrows, tracks and trails produced near the surface have a very low chance of preservation because of physical processes resulting in erosion, and because the potential trace fossils are destroyed by those animals that subsequently burrow deeply into the sediment. Lowermost Cambrian sediments preserve a range of trace fossils that are interpreted as representing shallow tiers: that is, the burrows did not extend more than a few centimetres at most below the sediment-water interface (Droser et aL 2002a, 2002b). These shallow-tiered burrows include various 'treptichnids' (including Treptichnus pedum). They consisted of additions of curved elements; the burrows themselves were open and the tops extended to the sediment-water surface (Fig. 1). The geometry and style of preservation of these trace fossils suggest that they formed less than a few centimetres below the sediment-water surface (Droser et al. 2002b). Other burrows include Gyrolithes, a corkscrew-type trace fossil that has a preserved depth of 1-2 cm. The diameter of this burrow is of the order of millimetres, and it is interpreted to have been an open-burrow system. Gyrolithes is abundant in the Chapel Island Formation of Newfoundland and in the Lower Cambrian units of Baltica (Fedonkin 1983; Jensen & Mens 1999) Quality of preservation Even though 'treptichnid' burrows were constructed close to the sediment-water interface they have sharp walls without actively reinforced margins, and in certain cases delicate surface ornamentation is preserved. Compaction of the burrows is also relatively minor. Several other shallow-tier trace fossils show excellent preservation of detail, including Gyrolithes and Rusophycus. This quality of preservation is

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Fig. 1. Schematic representation of the ichnofabric and sediment response to storm-related erosion positioned at approximate stratigraphic level. Sand is indicated by stippled pattern, mud by white. The mixed layer is depicted by dense patterning. Increased depth of mixing leads to increased erodability and partial removal of the shallower tier.

ubiquitous in the Lower Cambrian units examined.

Styles of preservation In most shallow marine settings, particularly in the Palaeozoic, well-preserved burrows on the base of sandstone beds are created by animals that burrow through the sand to the interface with the underlying finer-grained sediment (Seilacher 1970, 1985). In Lower Cambrian strata a fundamentally different style of preservation appears to be common, for shallow tiers in particular. The burrow may be cast by a source bed to which it remains attached, or it may be cast by sand that bypassed the seafloor, and thus the cast is subsequently attached to the base of a different bed (adhered preservation), or may even be preserved as a sand-filled burrow completely in silt (floating preservation) (Droser et al. 2002b). This is a common type of preservation of treptichnids, Gyrolithes and Palaeophycus/Planolites-type burrows, and is the most common style of trace fossil preservation in all the Lower Cambrian units examined. This type of preservation does occur in the Phanerozoic, and reflects penecontemporaneous erosion (e.g. Hallam 1975) This type of preservation requires that burrows are open and possibly empty. The preservation of shallow-tier trace fossils in this manner suggests that the muddy sediment was

rather resistant to erosion allowing the trapping of sand in burrows rather than the destruction of the burrows (Fig. 1).

The nature of ichnofabric Some animals that burrow do not leave welldefined discrete trace fossils. Instead, the record produced is one of some degree of homogenization, where primary sedimentary structures are not preserved. The final texture has a mottled appearance. This is direct evidence of a mixed layer. For example, in a modern setting under normal marine conditions in environments not characterized by rapid deposition such as prodelta settings, where there is alternating deposition of sand and mud, these two lithologies could be completely mixed, depending on the rates of deposition. In sedimentary rocks of the earliest Cambrian, rare isolated homogenized beds occur less than 1 cm in thickness. Otherwise, there is no evidence of sediment mixing. Sand and mud beds remain remarkably discrete, with sharp boundaries.

Properties of earliest Cambrian muddy sediments The features described above, such as the preservation of sharp burrow margins with delicate scratch marks and the low degree of compaction,

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characterize firmground conditions (Goldring 1995). A firmground indicates stiff but uncemented sediment. In modern settings, firmgrounds are exposed at the surface after erosion of upper layers; firm conditions at depth generally result from advanced dewatering and compaction (Bromley 1996). In the Early Cambrian, compaction would not be a requisite process, but, rather, silty mud would be deposited and in the absence of bioturbators would tend to dewater more rapidly (Droser et al. 2002a). These firmgrounds are not comparable to firm carbonate sediments that exhibit early lithification. That is, when eroded, these sediments presumably could have quickly disaggregated into individual grains rather than erode as mud chips, though flat pebble conglomerates and chips resulting from organic binding can occur. In recent years there has been a growing body of evidence that terminal Proterozoic sediment surfaces were bound by microbial mats to a far greater extent than would be typical of most of the Phanerozoic (e.g. Seilacher & Pfluger 1994; Gehling 1999). The siltstone-sandstone succession of the earliest Cambrian units that we examined shows no typical mat-related structures such as peetee structures, elephant skin texture or pyrite-rich horizons. Wrinkle marks that have been interpreted as mat-related structures (cf. Hagadorn & Bottjer 1997) do occur on a few surfaces in several, but not all of the units. Thus there is no compelling evidence that mats were an important component of these finegrained sediments (but see Goldring & Jensen 1996). That Cambrian and Lower Ordovician sediments were firm near the surface is also suggested by the presence of particular sedimentary structures that must have been formed close to the sediment-water interface. This includes Kullingia-type scratch circles, which form when a tethered organism is rotated by currents (Jensen et al. 2002). The circles are formed in silts or fine sands, and are cast by overlying coarser material. Their preservation requires that sediment just beneath the sediment-water interface is firm enough to imprint delicate concentric structures and also to withstand the erosion of currents in subtidal shallow-marine settings. Scratch circles are most common in lowermost Cambrian terrigenous clastic strata including the Chapel Island, the Uratanna, and the Tornetrask formations, as well as the Mickwitzia sandstone of the units that we have examined, and they are reported from the Khmelnitsk Formation of the Ukraine (see Jensen et al. 2002) and Arumbera Sandstone (D. Mcllroy, personal communication, 2003).

Ichnological record of the late Early Cambrian to earliest Ordovician There was an increase in the diversity of trace fossils throughout the Early Cambrian, and the majority of marine ichnogenera had appeared by the Atdabanian (Crimes 1992). Of particular note is the increase in range of size, which is significant as, other factors being equal, the rate of sediment mixing is proportionally linked to animal cross-section (Piper & Marshall 1969; Thayer 1983; Mcllroy & Logan 1999). Trilobites appear in the Atdabanian (though predated by trilobite trace fossils), and were among the larger animals that disturbed sediment. All of the units examined share a number of significant ichnological characteristics in spite of the fact that they represent a variety of shallow subtidal terrigenous clastic environments. In comparison with the earliest Cambrian strata: Shallow-tier burrows (less than 5 cm in depth) such as Treptichnus pedum and Gyrolithes are not generally common in Upper CambrianLower Ordovician strata. However, both of these trace fossils can be common in some of the Atdabanian units, such as the Mickwitzia Sandstone and the Pacoota Sandstone. Other common Cambrian burrows, such as Phycodes, Teichichnus, Rusophycus and Cruziana, are interpreted to be relatively shallow in depth by most workers, but would have been emplaced deeper than T. pedum and Gyrolithes, perhaps shallower than between 5 and 10 cm (Figs 2-5). With the exception of vertical burrows such as Skolithos and Arenicolites (and perhaps some Teichichnus) emplaced in sand, this second, deeper tier represents the bulk of the common Cambrian trace fossil record. This tier is unlikely to be preserved in Recent normal marine shelfal settings (e.g. Bromley 1996). Indeed, the presence of this tier is commonly used to suggest that conditions were anoxic or otherwise, not normal marine (Gibert & Ekdale 1999). Preservation asfloatingand adhering burrows (Droser et al. 2002b) remains common through Upper Cambrian-Lower Ordovician strata (Fig. 4b), and also occurs in younger strata (Simpson 1957; R. Goldring, personal communication 2003). Open mud burrows cast by sand are also common in these strata. Quality of preservation (e.g. scratch marks and minor compaction) is very high in all of these units. A conspicuous element of this interval is the trace fossils Rusophycus and Cruziana, which are common in all the formations examined in spite of the fact that they represent a

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Fig. 2. Ichnology and ichnofabric of the Lower Ordovician of Grebs Nest Point Formation, Grebes Nest Point, Bell Island, (a) Lower surface of bed with diverse trace fossils preserved by sand in mud. Prominent trace fossils include Cruziana and several Trichophycus. Diameter of coin is about 2.5cm. (b) Abundant sand-filled Trichophycus in mud and sharp erosively based event bed. Scale bar is 20 mm.

range of depositional environments. Scratch marks are routinely exquisitely preserved. Nearly all these units have at least thin mud beds that are thoroughly mixed as viewed in cut slab and/or X-ray. This represents direct evidence of a mixed layer. However, at least at the scale of centimetres, mud and sand beds remain discrete, although burrows

commonly pipe sediment from one to the other (Figs 3, 4). This is in contrast to shelfal sediments of the Mesozoic that, when viewed in core, are thoroughly mixed (e.g. Bockelie 1991, figs 2, 3). Compared with the earliest Cambrian it is more common to find burrows that penetrate sands.

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Fig. 3. Polished vertical section of heterolithic bedding in the Grebes Nest Point Formation (Lower Ordovician), Grebes Nest Point, Bell Island. Note prominent vertical spreite to the left of centre and narrow vertical burrows (Phycodes wabanensis) in lower portions cutting through sandy beds. Top event bed shows minor bioturbation. Scale bar is 20mm.

Tops of sands are nevertheless generally intact, and storm-generated subtidal heterolithic bedding generally is well preserved (Droser & Li 2001, and see below). Where sedimentation rate was low, the tops of sands may be reworked. For example, the Mickwitzia Sandstone and Liikati Formation have beds that are disturbed by Diplo crater ion and Rhizocorallium (e.g. Opik 1929; Jensen 1997). All these units can and do have several tiers, which result in intervals of complex ichnofabric, with trace fossils exhibiting cross-cutting relationships.

Properties of late Early Cambrian-earliest Ordovician sediments Evidence for a near-surface firmground, such as the occurrence of the shallow-tiered trace fossils Treptichnus pedum and Gyrolithes and Kullingia-type scratch circles, is rare above the Rusophycus avalonensis zone. All the characteristics discussed above are consistent with the presence

of a firm substrate below a shallow mixed layer, in contrast to the Lowermost Cambrian, where evidence for a mixed layer is scant (Fig. 1). Our hypothesis is illustrated in Fig. 1. A shallow mixed layer would be present in mud exposed at the surface. During storm events, we envisage some erosion of the mixed layer followed by deposition of sand. Below we discuss specific examples of ichnofabric. An example of heterolithic ichnofabric: the Lower Ordovician Grebes Nest Point Formation. The Grebes Nest Point Formation of the Wabana Group (Newfoundland) provides a particularly instructive example of CambroOrdovician ichnofabric. In a detailed study of the ichnology of the Cambro-Ordovician Bell Island and Wabana groups, Fillion & Pickerill (1990) recognized that the muds were cohesive in a variety of intertidal and shallow subtidal facies, based on sharp burrow outlines and preservation of fine surface sculpture. Our data support these conclusions. Bases of beds often show a high diversity of traces fossils (Fig. 2a). In addition to Rusophycus

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Fig. 4. Polished vertical sections from the Grebes Nest Point Formation (Lower Ordovician), Grebes Nest Point, Newfoundland. Scale bars are 10mm. (a) Sand cut by Phycodes wabanensis. (b) Rusophycus polonicus in adhering preservation, (c) Sharply defined Cruziana isp. Note darker sediment in fill of burrow compared with adjacent sand, suggesting an intrastratal formation for this particular specimen, (d) Lower portion of slab has thin sands partly reworked by Planolites montanus; truncated and overlain by sandstone with small-scale cross-lamination.

and Cruziana (discussed further below), Trichophycus, Teichichnus and Phycodes are common. The different ichnospecies of Phycodes and Teichichnus are probably shallow tiers (Pillion & Pickerill 1990). Phycodes wabanensis is generally preserved with relatively distinctly defined

burrow margins (see also Pillion & Pickerill 1990). Phycodes burrows cross-cut thin mud and sand beds (Figs 3, 4a). Trichophycus occurs commonly in some intervals. Preservation ranges from floating to adhered to cast. Burrows may also be 'washed

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Fig. 5. Field photographs of the Lower Cambrian Arumbera sandstone, Northern Territories. Scale bar is 20 mm. (a) Abundant sand-filled burrows in mud. Member 3 at Daily Gorge, (b) Diverse preservation of sandfilled burrows in mud. Member 3 at Shannon Bore.

out' where the type of preservation is dependent upon the depth of erosion of overlying material. Burrow margins are sharply defined, as are scratch marks. Trichophycus burrows preserved within the mud (attached to the overlying sandstone or not) act as tubular tempestites (Fig. 2b) (cf. Wanless et al 1988): that is, open burrows that are infilled with sediment that would otherwise bypass this palaeoenvironment. These

Trichophycus can be cross-cut within the mudstone by Teichichnus, providing further evidence that Trichophycus was not deeply emplaced. Teichichnus is a common and conspicuous Cambrian trace fossil (see Bland & Goldring 1995; Droser & Li 2001), and can also occur as a tubular tempestite (Jensen 1997). Relatively sharply defined margins with bioglyphs suggest that the sediment was moderately cohesive at

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the time of burrow construction (Pillion & Pickerill 1990). In the Grebes Nest Point as well as the Lower Cambrian Mickwitzia sandstone and Liikati Formation, mudstones may be bioturbated. However, bedding down to the centimetre scale is generally preserved (Fig. 3). Thus thin tempestites are recognizable. Interestingly, in spite of the fact that these mudstones apparently were bioturbated, they still become cohesive at a relatively shallow depth, as indicated by the prevalence of preserved shallow-tier burrows.

The relevance of Skolithos piperock: the Cambro-Ordovician Pacoota and Bynguano formations By the latest Tommotian-Atdabanian an ichnofabric dominated by vertical trace fossils such as Skolithos and Diplocraterion (e.g. Westergard 1931; Droser 1991; Mcllroy 2004) first appeared in the stratigraphic record. These burrows extended to depths of decimetres and commonly create a dense fabric often referred to as 'pipe rock'. These are typical of high-energy sand-dominated shallow marine environments. There has been some debate as to the role of this fabric in the context of the evolution of the early infauna (e.g. Bottjer & Ausich 1985; Miller & Byers 1984). As pointed out by Thayer (1983) among others, these burrows most likely represent dwelling burrows and do not generate intense bioturba-

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tion. Although individual burrows do not result in significant particle mixing, they would have allowed the oxygenation of sediment to depths of several tens of centimetres. Especially under conditions of slow sedimentation the fabric may be dominated by these burrows (cf. Mcllroy 2004, fig 12). Also within this high-energy setting may be found preservation of shallow tiers formed in mud. Examples include Rusophycus latus in the Pacoota Sandstone, Northern Territories, Australia, and the Bynguano Formation of New South Wales (Droser et al 1994). The preserved details indicate that the mud must have been cohesive at the time of trace fossil emplacement. Both the Pacoota Sandstone and the Bynguano Formation include successions of sandstones with Skolithos burrows piping down through abundant, well-preserved Rusophycus occurring on the base of beds (Fig. 6). In some cases, where the Skolithos do not penetrate the base of the bed, it is possible to see original primary bedding such as cross-stratification in the beds casting the Rusophycus. Amalgamation and erosion surfaces are common. Mudstone is rarely preserved. Despite ubiquitous amalgamation, there are numerous levels of Rusophycus (Fig. 7), which indicate firmground conditions within the depth of trilobite burrowing. Primary sedimentary structures appear to continue to the base of the Rusophycus, and this is most consistent with a casting scenario of preservation (Fig. 8, mode k; see also Baldwin 1977).

Fig. 6. Field photograph (oblique lower view) of beds in the Pacoota sandstone, Ellery Creek, Northern Territories. Note several basal surfaces covered with Rusophycus latus, and sides of sandstone beds with Skolithos. Scale bar is 50 mm.

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Fig. 7. Three types of preservation of sandstone and ichnocoenoses of the Bynguano Formation, New South Wales (modified from Droser et al. 1994). (a) Amalgamated sandstones. No predepositional (e.g. Rusophycus) preserved, (b) Amalgamated sandstones with some trace fossils preserved as casts on the base of sandstone beds, (c) Sandstones interbedded with mudstones. Burrows are commonly cast on the base of sandstone beds and appear to reflect open firmground burrows.

Fig. 8. Possible pathways for the formation of Rusophyus. (a) Muddy sediment with a shallow mixed layer, beneath which is firmer sediment, (b, c) Intrastratal burrowing along the interface of sand and mud creates a situation where the burrow is immediately cast. Subsequent erosion may lead to removal of sand except for the burrow cast (f), followed by deposition of sand (i) or mud (h), resulting in styles of preservation that appear to be particularly common in the Lower Palaeozoic, (d) Erosion of mud without deposition of sand leads to exposed firm muds. Preservation of open mud burrows generally is not considered likely. The relatively stiff nature of Lower Palaeozoic muds close to the sediment-water interface may, however, have made this rather common, (k) A fill showing features of physical deposition is to be expected, (e) It can also be envisaged that burrowing took place within mud and that firmer sediment was subsequently exposed and cast (g, k).

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Preservation of Rusophycus and Cruziana Trilobite traces provide useful indicators of sediment properties in part because they are relatively large and three-dimensional. It is thus relatively easy to determine the relationship of the burrows to the encasing sediments in the field. The quality of preservation of the scratch marks can also be instructive. As discussed below, these also may provide insights into the properties of sediments at this time. It is generally assumed that the arthropod-type trace fossils Rusophycus, and in particular Cruziana, did not form at great depths below the sediment-water interface and, in the case of traces such as Cruziana semiplicata, probably no more than a few centimetres below the sedimentwater interface. Delicate preservation of leg imprints suggests that the sediment encountered was relatively cohesive. Whether such features could be preserved as casts of originally open surface furrows or must have originally formed along a sand-mud interface has been a matter of some contention (see Seilacher 1970; Crimes 1975; Baldwin 1977; Goldring 1985). Regardless of the precise mechanism of preservation it is likely that a cover of less cohesive mud was washed out (cf. Miller & Rehmer 1982). The conditions for the preservation of this type of trace fossil may therefore have been particularly favourable. The processes leading to the preservation of Rusophycus and Cruziana are illustrated in Fig. 8. All of these scenarios require that sediments be firm at relatively shallow depths, either so that trilobites burrow down to the firmground through a mixed layer (Fig. 8e) or so that it is shallow enough that erosion of the overlying mixed layer is commonplace (Fig. 8b, c, d). This suggests that the mixed layer was relatively shallow, following the argument of Droser et al. (2002a, 2002b). Discussion The characteristics described above are based on examination of the 11 units listed in addition to the Neoproterozoic-earliest Cambrian units studied for previous work (Droser et al. 2002b). We present these characteristics as a model for understanding early Palaeozoic substrate conditions. We view these as generalizations to which there will probably be exceptions. In particular, trace fossil preservation and substrate conditions will depend on the depositional environment and rates of sedimentation. Indeed, many of these features may be present in any strata of Phanerozoic age, and Phanerozoic-style preservation

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may occur in the Cambrian, all depending on the physical conditions. There are a number of implications of these findings. Below we briefly list potential implications of these data, ranging in order from the obvious to the conjectural.

Increase in the depth and extent of the mixed layer There is an increase in trace fossil size (Mcllroy & Logan 1999) and diversity (Crimes 1992) from the earliest Cambrian to the Early Ordovician. In the earliest Cambrian the very shallowest tiers were commonly preserved. Shallow-tier burrows emplaced a few centimetres or even millimetres were preserved through the Early Ordovician. Data from Neoproterozoic-Ordovician strata indicate that there was virtually no mixed layer in the Neoproterozoic and a minimal mixed layer in the earliest Cambrian. Atdabanianearliest Ordovician strata appear to have had a shallow mixed layer of the order of 5-1 Ocm. Throughout this latter interval, bioturbation remained of moderate depth in shelfal settings. This supports suggestions of a late origination of extensive mixing (e.g. Larson & Rhoads 1983).

Decreased preservation potential of event beds The results presented here further substantiate the suggestion of Sepkoski et al. (1991) that there is a secular trend in the preservation of event beds. Results from this study suggest that, in particular, thin event beds will be best preserved in the Neoproterozoic and earliest Cambrian, but that Cambrian and earliest Ordovician event beds also have high preservation potential. This will, again, depend on the depositional environment and rates of sedimentation.

Increased depth of firmground conditions from near surface in the Early Cambrian As a consequence of the shallow mixed layer, muddy sediments were firm relatively close to the sediment-water interface. Evidence from lowermost Cambrian strata indicates the presence of firmground conditions at or near the sediment surface. All of the formations examined for this paper have extensive evidence of firmground conditions, of the order of 5-1 Ocm below the sediment-water interface. Future work will

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determine the subsequent nature of the development of the mixed layer and thus the deepening of firmground conditions within the sediment. Related to this, Bottjer et al. (2000) have suggested that many Early Cambrian metazoans were well suited for matground conditions. Presumably, they would have also been ideally suited for firmground conditions.

1970). Trilobites do decline in diversity through this interval (Adrain et al. 1998). However, a deepening of the mixed layer through the Palaeozoic would result in the deterioration in the preservation potential of this type of trace fossil. Clearly, a number of other factors must also be considered and tested, but changes in the preservation potential of these trace fossils may be important.

Decreased trace fossil preservation potential of shallow-tier burrows

We acknowledge partial funding for the fieldwork from National Science Foundation (grant EAR-9219731 to MLD) and the National Geographic Society. This paper benefited from conversations with N. Hughes, P. Myrow, M. Kennedy and G. Narbonne. V. Droser, R. Droser, A. Dzaugis and M. Dzaugis provided field assistance. D. A. Droser facilitated fieldwork. This paper also benefited greatly from reviews by R. Goldring and J. Hagadorn. We are also grateful to D. Mcllroy for organizing the symposium and providing a very helpful review.

Cohesive sediment close to the sediment-water interface may explain the rich Lower Palaeozoic record of well-preserved marine trace fossils. The common occurrence of trace fossils preserved as floating or adhering trace fossils (Fig. 8) is probably a result of firm muddy sediments. Casting of open or washed-out burrows may have been particularly likely at this time. The greatest potential for the preservation of marine trace fossils is in Precambrian and earliest Cambrian sediments. In earliest Cambrian strata, very shallow-tiered millimetre-scale Gyrolithes and Treptichnus pedum occur in abundance. Although they are present in younger strata and sometimes in some abundance, they are never as ubiquitous. The presence of graphoglyptid trace fossils such as Palaeodictyon in Cambrian shallow-water sediments could be a further indication of preservation of shallow-tiered trace fossils in this setting (Crimes & Fedonkin 1994).

Nature of Lower Palaeozoic trace fossil record Most of the discrete trace fossils preserved in these heterolithic settings, with the exception of those piping into the sand, are firmground trace fossils. These consist largely of open burrows in mud. The evidence for very earliest Cambrian mobile deposit feeders in muds is low (Droser et al. 2002b). In the earliest Cambrian virtually all of the trace fossils were emplaced in firmgrounds. As the mixed layer further developed, and thus firmground conditions receded into the sediment, softground trace fossils would have become more common and ultimately would have dominated the ichnofauna.

Decreased preservation of Rusophycus and Cruziana Rusophycus and Cruziana decline in occurrence and abundance through the Palaeozoic (Seilacher

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SAVRDA, C. E. & BOTTJER, D. J. 1989. Anatomy and implications of bioturbated beds in Black Shale sequences: examples from the Jurassic Posidonienschiefer (Southern Germany). Palaios, 4, 330-342. SEILACHER, A. 1970. Cruziana stratigraphy of 'nonfossiliferous' Palaeozoic sandstone. In: CRIMES, T. P. & HARPER, J. C. (eds) Trace Fossils. Seel House Press, Liverpool, 447-476. SEILACHER, A. 1985. Trilobite palaeobiology and substrate relationship. Transactions of the Royal Society of Edinburgh, 76, 231-237. SEILACHER, A. & PFLUGER, F. 1994, From biomats to agricultural revolution: In: KRUMBEIN, W. E., PATERSON, D. M. & STAL, L. J. (eds) Biostabilization of Sediments. Bibliotheks und Informationssystem der Carl von Ossietzky Universitat, Oldenburg, 97-105. SEPKOSKI, J. J., BAMBACH, R. K. & DROSER, M. L. 1991. Secular changes in Phanerozoic event bedding and

the biological overprint. In: EINSELE, G., RICKEN, W. & SEILACHER, A. (eds) Cycles and Events in Stratigraphy. Springer, Berlin, 298-312. SIMPSON, S. 1957. On the trace-fossil Chondrites. Quarterly Journal of the Geological Society of London, 112, 475-500. THAYER, C. W. 1983. Sediment-mediated biological disturbance and the evolution of marine benthos. In: TEVESZ, M. J. S. & McCALL, P. L. (eds) Biotic Interactions in Recent and Fossil Benthic Communities. Plenum, New York, 479-625. WANLESS, H. R., TEDESCO, L. R. & TYRRELL, K. M. 1988. Production of subtidal tubular and surficial tabular tempestites by Hurricane Kate, Caicos Platform. Journal of Sedimentary Petrology, 58, 73-75. WESTERGARD, A. H. 1931. Diplocraterion, Monocraterion and Scolithus from the Lower Cambrian of Sweden. Sveriges Geologiska Under sokning, C372, 1-25.

Trace fossils in the aftermath of mass extinction events RICHARD J. TWITCHETT1 & COLIN G. BARRAS2 3 1

School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK (e-mail: [email protected]) 2Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK ^Department of Earth Sciences, School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK Abstract: Ichnology has great potential to advance our understanding of mass extinction events and yet is currently an underutilized resource in such studies. Here we review published ichnological studies for the Ordovician-Silurian, Permian-Triassic and Cretaceous-Tertiary extinction-recovery intervals. In addition, new information regarding the Triassic-Jurassic ichnological record from England, Austria and the western USA is presented. Trace fossils provide important information on the ecological response of the benthic community at such times. In the immediate post-extinction aftermath, the ichnodiversity, burrow size, depth of bioturbation, and ichnofabric index of the sediments are all much reduced. There is an increase in all these parameters through the post-extinction recovery period. In some cases, the stepwise reappearance of certain distinctive ichnotaxa (e.g. Diplocraterion, Rhizocorallium and Thalassinoides) may be of some stratigraphic use. Evidence from Permian-Triassic studies indicates that recovery took longer at low (tropical) palaeolatitudes than mid-high palaeolatitudes. Trace fossils also provide important information on palaeoenvironmental change through the extinction-recovery interval. The application of ichnology to mass extinction studies is in its infancy, but should prove a valuable tool in future research.

Understanding mass extinction events is crucial to understanding the evolutionary history of life on Earth. One tool that is currently underutilized in such studies, but which has much to offer, is ichnology. Our objective is to demonstrate that studying trace fossils can enhance our understanding of mass extinction episodes and, in particular, of the post-extinction recovery period. We provide a review of previous studies in this area, including new data from the Permian-Triassic and Triassic-Jurassic events, and hope to stimulate others to use the trace fossil record in future analyses of extinctionrecovery episodes. The focus of attention is directed towards the marine trace fossil record. However, terrestrial ichnofaunas also have much unexplored potential. One recent example is Olsen et al.'s (2002) study of the effects of possible extraterrestrial impact events on the evolution of the Triassic-Jurassic terrestrial vertebrate faunas of North America, which relied entirely on trackway evidence left by dinosaurs and other tetrapods. To date, this represents the only extinction-related investigation that has primarily involved terrestrial trace fossil evidence.

Application of ichnology to mass extinction studies Palaeoenvironmental analysis The use of trace fossils for palaeoenvironmental analysis is commonplace, and can be applied to all parts of the geological column, including extinction-recovery intervals. Ichnofacies analysis, based on Seilacher's (1967) concepts, allows environmental parameters such as substrate consistency, bathymetry and hydrodynamic energy to be inferred from the occurrence of particular suites of trace fossils (see Bromley 1990 for a recent discussion). The amount of bioturbation present in a particular sedimentary unit also has palaeoenvironmental significance. Sediments that are thoroughly bioturbated were obviously deposited under conditions that were amenable to both benthic colonization (adequate oxygen, food supply etc.) and trace fossil preservation. Sediments lacking bioturbation were deposited under conditions that prevented colonization (e.g. anoxia, high sedimentation rates) and/or were unsuitable for preserving trace fossils. The observed ichnofabric is related to the

From: MC!LROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 397^18. 0305-8719/04/S15.00 © The Geological Society of London.

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interplay between the levels of colonization and preservation. The amount of bioturbation in vertical section can be estimated semi-quantitatively by using Droser & Bottjer's (1986) ichnofabric index (ii); for bedding plane exposures, Miller & Smail's (1997) scheme can be applied. One area of palaeoenvironmental analysis that is of particular importance to extinction studies is the reconstruction of benthic oxygen levels. In their recent review of Phanerozoic extinction events, Hallam & Wignall (1997) concluded that most were associated with episodes of oceanic anoxia. Ichnology is particularly useful in the assessment of ancient benthic oxygen levels. As oxygen levels decline, the amount and depth of bioturbation decreases, as do ichnodiversity and burrow diameter (e.g. Rhoads & Morse 1971; Savrda & Bottjer 1986). Dysaerobic sediments tend to be poorly bioturbated by a single ichnotaxon, such as Chondrites, which typically occupies the deepest tier in normal, aerobic environments (Bromley & Ekdale 1984; Savrda & Bottjer 1987). Sediments deposited under continuously anoxic conditions lack bioturbation.

Palaeoecological studies Trace fossils are records of the activities of organisms, and therefore also provide biological and ecological information. In the marine realm, most ichnotaxa are the products of the actions of soft-bodied, or lightly mineralized, invertebrates, which are rarely (if ever) fossilized. Yet, in modern marine ecosystems, and presumably in ancient ones too, soft-bodied organisms comprise the vast bulk of the benthic biota (Allison & Briggs 1991, p. 26). Usually our only glimpse of these taxa in the fossil record is through the record of Konservat-Lagerstatten (e.g. the Burgess Shale). However, no known KonservatLagerstatte spans an extinction episode, and no extinction episode has Konservat-Lagerstatten in the immediate pre-extinction and postextinction intervals. For example, in order to attempt an assessment of how the Permian extinction events affected soft-bodied marginal marine taxa, Briggs and Gall (1990) were forced to compare the Anisian Ores a Voltzia lagerstatte with several Carboniferous ones. Thus trace fossils are the only record of the direct response of the soft-bodied benthic invertebrate community to mass extinction events. Do changes in ichnodiversity reflect real changes in benthic diversity? Trying to match a particular trace fossil to a particular trace-

maker is not usually possible, and ichnotaxa are only of limited use in assessing benthic diversity. However, certain trace fossils can be attributed to certain classes of organism. For example, Thalassinoides and Ophiomorpha are attributed to anomuran crustaceans, based on comparisons with modern burrows (e.g. Bromley 1990). Resting traces (cubichnia) are often particularly easy to assign as the shape of the trace fossil reflects the morphology of the tracemaker (e.g. Lockeia traces are attributed to bivalves, Asteriacites lumbricalis to ophiuroids etc.). Such assignments allow the presence or absence of certain groups to be documented, which may enable comparison with modern benthic communities. For example, crustaceans are usually the last group to reappear after modern, low-latitude hypoxic events (e.g. Harper et al. 1991). Similarly, in the low palaeolatitude sediments of northern Italy, Thalassinoides is one of the last ichnotaxa to reappear after the Permian-Triassic anoxic event (it is absent until the Anisian). Another important ecological aspect of the trace-making organisms that can be assessed through study of their trace fossils is that of size. Body size is a key element in animal evolution (Jablonski 1996). It is a fundamental character of living organisms, with implications for many aspects of an animal's biology, behaviour and ecology. A growing number of studies have shown that the aftermaths of mass extinction events are characterized by fossil animals of unusually small size. Termed the 'Lilliput effect' by Urbanek (1993), this phenomenon affects a range of animal groups and all extinction events studied to date (e.g. Girard & Renaud 1996; Twitchett 2001). Burrow diameter is a good proxy for size of the trace-making organisms, and certainly a burrowing animal cannot have a diameter larger than its burrow, so the Lilliput effect may be expected in post-extinction ichnogenera too. Indeed, results from northern Italy show that there is an order of magnitude decrease in burrow diameter across the Permian-Triassic extinction event (Twitchett 1999). In shelly taxa, the Lilliput effect is a temporary phenomenon, affecting the immediate postextinction aftermath, and may be related to a decrease in food supply (Twitchett 2001; PriceLloyd & Twitchett 2002) or some other temporary environmental disturbance such as low oxygen levels, temperature change or salinity fluctuation (Hallam 1965). It is followed by a subsequent increase in body size in the later stages of post-extinction recovery, presumably as conditions improve. Likewise, there is an increase in burrow size through the later recovery

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intervals of the Early Triassic (Twitchett 1999) and Early Jurassic (discussed more fully below). No burrow size data have been published for any other extinction episode. Trace fossils are primarily records of behaviour, and hence provide information on the ecological response of the benthic community to extinction events. For example, they can be used to identify opportunistic behaviour (e.g. Ekdale 1985; Vossler & Pemberton 1988), which may be one of the life strategies likely to characterize the surviving communities of the immediate post-extinction interval (Harries et al. 1996). Changes in feeding strategy are also likely to occur during extinction intervals, for example in response to food chain collapse. During the end-Cretaceous event, plankton productivity collapse appears to have led to the preferential extinction of filter feeders and the preferential survival of detritivores (Sheehan et al. 1996). Under such conditions, it might be predicted that trace fossil assemblages in the immediate aftermath of such an event would be dominated by the traces of deposit feeders, and that the dwelling burrows of infaunal suspension feeders would not return until suitable levels of primary production were re-established. Finally, trace fossils are critical in studies of the tiering of infaunal communities. Different taxa are adapted to live at different depths (i.e. within different tiers) below the seafloor, and the term 'tiering' describes the vertical distribution of the infaunal community that results. In their major study of Phanerozoic tiering of suspension-feeding communities in shelf sea settings (below wavebase), Bottjer & Ausich (1986) showed that during the early Palaeozoic only the shallowest tiers (down to 12cm below the seafloor) were occupied. During the Carboniferous, organisms began to exploit the deeper tiers (down to 1 m below the seafloor), and infaunal tiering remained at these depths through to the present. The results of this initial study suggested that the level of infaunal tiering was unaffected by mass extinction intervals, implying greater stability of infaunal communities, compared with epifaunal ones, in the face of environmental change (Bottjer & Ausich 1986). However, recent research (e.g. Twitchett 1999) has shown that the depth of infaunal tiering can be severely reduced during extinction episodes. This has been demonstrated for the Permian-Triassic extinction event, and is reflected in Ausich & Bottjer's (2001) Phanerozoic tiering diagram (Fig. 1). However, with the exception of the TriassicJurassic event described below, the infaunal tiering histories of the remaining major Phanero-

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Fig. 1. Maximum levels of tiering of suspensionfeeding communities in marine environments during the Phanerozoic. Vertical axis shows distance from sediment-water interface in centimetres. PC, Precambrian; Mz, Mesozoic; Cz, Cenozoic. Arrows mark positions of the five major mass extinction events of the Phanerozoic. Redrawn from Ausich & Bottjer (2001).

zoic extinction events have yet to be investigated, and doubtless the picture will change further as our knowledge improves.

Ichnos tra tigraphy In certain situations trace fossils have a stratigraphic use, and have proved to be a valuable tool in correlation, especially during the Precambrian-Cambrian transition (e.g. Crimes 1992) and the lower Palaeozoic (e.g. Seilacher et al. 2002). Crimes (1987) gave a comprehensive account of the distribution of ichnotaxa through a number of Precambrian-Cambrian boundary sections, showing that there is a stepwise appearance of ichnogenera through the earliest Palaeozoic, and that the order of appearance is similar worldwide. The stepwise appearance is to be expected as different trace-making groups originate and/or novel behavioural and burrowing strategies evolve. The first appearance datum of the ichnotaxon Treptichnus (formerly Phycodes) pedum actually defines the base of the Cambrian (Landing 1994). The value of these earliest Cambrian traces is that they are present in rocks lacking abundant shelly remains, and hence are the only potentially useful stratigraphic tools available. Similar conditions may also apply in the aftermath of mass extinction events. There is certainly a stepwise reappearance of ichnotaxa after some of the major extinction episodes, which is unrelated to facies change and is comparable in different localities. One example is in the aftermath of the end-Permian event (Twitchett 1997), which is discussed further below. In addition, post-extinction strata often contain a depauperate and poorly preserved shelly fauna,

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which can make traditional biostratigraphy difficult. In the immediate extinction aftermath there may be a 'dead zone' (Harries & Kauffman 1990), where body fossil taxa are completely absent, resulting in significant gaps in the ranges of biostratigraphically useful groups such as ammonoids or conodonts (e.g. Looy et al 2001; Twitchett et al 2001). In such cases ichnostratigraphy may be one solution to the problems of correlation. Ordovician-Silurian (O-S) event The Late Ordovician (Ashgill) interval witnessed extinction of some 24% of marine families (Benton 1995) and is unique among Phanerozoic extinction events in being intimately linked with a short (less than 1 Ma) episode of glaciation (e.g. Brenchley et al 1994). The patterns of extinction and recovery are diachronous in a number of marine groups, and were clearly driven by changes in climate, oceanographic state and sea-level (e.g. Armstrong 1996; Finney et al 1999). Thus far, MeCann's (1990) study of the deep-sea ichnofaunal record of the Llandeilo to Upper Llandovery formations of the Welsh Basin is the only detailed study of trace fossils through the Ordovician-Silurian mass extinction event and recovery (Fig. 2). The data of McCann (1990) indicate an overall increase in ichnodiversity from the Llandeilo through into the Llandovery. This is probably, in part, a local expression of the global increase in the number of taxa colonizing deep-sea environments through the lower Palaeozoic (Orr 2001) and in part reflects changes in trace fossil preservation through the different formations (McCann 1990). However, superimposed on this overall increase in ichnodiversity is a shortterm decrease associated with the O-S boundary. The decrease in ichnodiversity from the uppermost Ordovician Llangranog Formation to the basal Silurian Gaerglwyd Formation can be simply explained by the facies change from sandstone-dominated to mudstone-dominated facies (Fig. 2). Indeed, this facies control on ichnodiversity is recorded throughout the succession (McCann 1990). However, when only the sandstone-dominated formations are compared (in order to reduce the problems of facies control) a decrease in ichnodiversity is still recorded across the O-S boundary, with the earliest Silurian Allt Goch and Grogal Formations 'having lower abundances than might be predicted' (McCann 1990). The uppermost Ordovician Llangranog Formation contains eight ichnotaxa - Chondrites, Circulichnus, Cochlichnus, Gordia, Helminthopsis,

Palaeophycus, Planolites and Protopaleodictyon — whereas the sediments of the lowest Silurian Allt Goch Formation preserve just four: Chondrites, Helminthopsis, Palaeophycus and Planolites. Post-extinction ichnodiversity is 50% of the preextinction levels, and, according to McCann (1990), the ichnotaxa that remain are the most common members of the pre-event ichnofauna. Time is required for ichnodiversity to recover: the missing ichnotaxa do not reappear until the Aberystwyth Grits Formation (Upper Llandovery). Are these ichnodiversity changes the result of real diversity changes in the benthic infauna, or are they simply due to changing sea-level? Although the drop in ichnodiversity across the O-S boundary also corresponds with a transgression, it is difficult to determine actual depths from the sediments (McCann 1990). The trace fossil record is interpreted as being, at least partly, the result of real biotic change in tracemaker communities. Ecological changes, such as organism size and depth of burrowing, may also have occurred during the O-S interval, but have yet to be studied. McCann (1990) also showed that the ichnodiversity of deep-sea environments has changed over the Phanerozoic. There are broad similarities with curves of marine family diversity based on shelly fossils (e.g. Sepkoski 1984; Benton 1995): an initial Cambrian-Ordovician increase, low diversity through the latest Palaeozoic and early Mesozoic, and a dramatic increase through the later Mesozoic and Tertiary. Decreases in ichnogeneric diversity of deep-sea environments occur across the O-S, P-Tr and K-T mass extinction intervals, as well as through the Carboniferous-Permian interval (McCann 1990). Late Devonian event The Late Devonian event is not considered further in this present work because there are no detailed studies of the trace fossil record of this event, although the presence of bioturbation is sometimes mentioned in sedimentological studies (e.g. Chen & Tucker 2003). Permian-Triassic (P-Tr) event The P-Tr mass extinction event was the most severe of the Phanerozoic, with some 48% of marine families, and possibly in excess of 95% of species, becoming extinct (e.g. Erwin 1993; Hallam & Wignall 1997). Study of the P-Tr

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401

Fig. 2. Stratigraphy, ichnotaxa ranges and sea-level curve through the Ordovician-Silurian succession of the Welsh Basin. Arrow indicates direction of sea-level rise. Solid line indicates presence in formation; dashed line indicates absence. From data in McCann (1990).

event has undergone something of a renaissance since the late 1980s and through the 1990s (Benton 2003). A significant part of this recent effort has included detailed ichnological analyses. One of the triggers that led to this renewal of interest was Hallam's (1989) suggestion that the P-Tr event was caused by an episode of oceanic anoxia. A number of field studies soon followed (e.g. Wignall & Hallam 1992) and, because of their utility in assessing benthic oxygen levels, trace fossils often featured in these investigations.

Pre-event ichnofauna Typically, uppermost Permian (Changhsingian) sediments are well-bioturbated, with a diverse trace fossil assemblage that may include Diplocraterion, Palaeophycus, Planolites, Rhizocorallium, Skolithos, Thalassinoides and Zoophycos (Table 1). This diverse assemblage disappears in the very latest Changhsingian or earliest Triassic (Griesbachian) with the onset of benthic oxygen restriction, which is diachronous on a global scale (Wignall £ Twitchett 2002a).

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Table 1. Latest Permian (Changhsingian) ichnofaunas documented to date Locality

Formation

Ichnotaxa

Reference

Northern Italy

Bellerophon Fm.

Twitchett (1999)

Spitsbergen

Kapp Starostin Fm.

East Greenland Sichuan, China

Schuchert Dal Fm. Dalong Fm.

Diplo crater ion, Planolites, Rhizocorallium, Skolithos, Zoophycos Chondrites, Diplo crater ion, Planolites, 'Zoophycos Palaeophycus, Planolites, Rhizocorallium Thalassinoides , Zoophycos

Northern Oman

Maqam Fm.

Chondrites, Palaeophycus, Rhizocorallium, Skolithos, Thalassinoides

pers. observ.

Wignall et al. (1998) Twitchett et al. (2001) Wignall etal (1995)

the only ichnotaxon present in the immediate post-extinction environments of the earliest Griesbachian is Planolites, which represents the fodinichnia of deposit-feeding vermiform organisms (Twitchett & Wignall 1996; Twitchett 1999). In contrast, the pre-extinction trace fossil assemblage was a diverse mixture of domichnia and fodinichnia (Fig. 3; Table 1), indicating the presence of both suspension-feeding and deposit-feeding animals, including crustaceans. The temporary disappearance of suspensionfeeding organisms could indicate a collapse in primary production and inadequate food supply (cf. Sheehan et al. 1996). However, the disappearance of suspensionfeeding domichnia could also be related to low oxygen conditions. In a study of the infauna of the Cretaceous Greenhorn Formation, Sageman & Bina (1997) showed that domichnia were confined to well-oxygenated intervals, and as oxygen levels decreased, domichnia disImmediate post-extinction aftermath appeared until only the fodinichnia of deposit Earliest Griesbachian ichnofaunas are typically of feeders were left. Small size could also be the low diversity and small size (1-5 mm), with shal- result of either oxygen stress or inadequate low penetrating (< 1 cm) bioturbation confined food supply (or both). Modern low-energy, olito a few discrete horizons separated by metres of gotrophic environments are characterized by a laminated, unbioturbated sediments. This pattern small, sparse, deposit-feeding infauna (Jumars indicates that environmental stress (of some sort) & Wheatcroft 1989). A size decrease is also prevented benthic colonization; only when condi- observed in the shelly macrofauna through tions improve slightly can a pioneering benthic the Permian-Triassic interval, which has been community colonize the substrate. This is typical related to food shortage (Price-Lloyd & Twitchof dysoxic environments, although Erwin (1993, ett 2002). Reduction in the depth of burrowing p. 246) has argued that the absence of bioturba- is probably more likely to be caused by low tion at this time is the result of the extinction of oxygen conditions, although food supply may the benthos, rather than low oxygen conditions, also have an effect (Jumars & Wheatcroft and that benthic conditions were otherwise 1989). Other evidence of low productivity normal. However, this argument does not explain levels can be found, such as a negative shift the observed changes in burrow size and depth of in <S13C and the low organic carbon content of burrowing, and, in addition, there is plenty of the sediments (e.g. Twitchett 2001). Thus the independent geochemical evidence for low immediate post-extinction trace fossil record oxygen conditions at this time (Wignall & Twitch- may be the result of more than one type of environmental stress: inadequate food ett 1996, 2002a). Are there any other possible causes of environ- supply and low oxygen conditions are both mental stress at this time? In northern Italy, contenders.

Disappearance of the pre-event ichnofauna usually occurs over several decimetres or metres of strata. In southern Austria, ichnofabric index declines from ii5-6 to iil-2 over some 15m of strata (Twitchett & Wignall 1996). In East Greenland similar changes occur over just 50cm of strata, which probably represent only a few tens of thousands of years (Twitchett et al. 2001). Disappearance of the pre-event ichnofauna is interpreted as reflecting the collapse of the latest Permian benthic ecosystem, which, in East Greenland at least, occurs at the same time as collapse of the terrestrial plant communities (Looy et al. 2001; Twitchett et al. 2001). As well as a reduction in the amount of bioturbation and ichnodiversity, an accompanying decrease in burrow diameter is also recorded (e.g. Wignall et al. 1995; Wignall & Hallam 1996; Twitchett 1999).

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Early Trlassie recovery Despite the increasing number of studies that have documented ichnofaunal change through the P-Tr boundary, there are still relatively few studies of trace fossil response and recovery through the later Early to Middle Triassic. Only the shallow water, low palaeolatitude, mixed carbonate-siliciclastic Werfen Formation (of northern Italy) has been investigated in any detail (Twitchett & Wignall 1996; Twitchett 1999). The domichnia of suspension feeders (e.g. Skolithos, Arenicolites) reappear in the lower Siusi Member (late Griesbachian) of the Werfen Formation (Fig. 3). These sediments were still deposited under some degree of benthic oxygen restriction (Wignall & Twitchett 1996), but the P-Tr anoxic event was already beginning to weaken in this region (Wignall & Twitchett 2002a). Arenicolites and Skolithos penetrate only some 1-2 cm into the substrate, and burrow diameters are small (1-2 mm). They represent the activities of a post-event pioneering community, penetrating the upper surfaces of thin, distal storm beds. There is an improvement in benthic oxygenation in the lower-middle Siusi Member (Wignall & Twitchett 1996) and a subsequent increase in ichnodiversity, burrow size and the amount and depth of bioturbation. Increase in burrow size at this time is mirrored by a similar increase in the size of the shelly macrofauna (Price-Lloyd & Twitchett 2002). Ichnotaxa present include Cochlichnus, Catenichnus, Lockeia and Palaeophycus. In the shallower stormdeposited, micaceous fine sandstones of the upper Siusi Member (lower Dienerian in age) depth of bioturbation increases again with the reappearance of Diplo crater ion (Twitchett 1999). The Dipio crater ion burrows at this level are usually 3-4 mm in diameter, and may reach 10cm in depth. The overlying Gastropod Oolite Member, which is Dienerian in age, records a further increase in burrow size (Twitchett 1999), ichnodiversity (Twitchett & Wignall 1996), and the amount of bioturbation. Ichnofabric indices (sensu Droser & Bottjer 1986) may reach ii5 for some beds, but are typically ii3. The sediments record deposition in a mixed carbonatesiliciclastic environment above storm wavebase and below fair-weather wavebase. Both preevent and post-event ichnofaunas are present, and benthic conditions were clearly improving. The overlying Smithian age Campil Member, however, records a temporary return to poorly bioturbated sediments (iil-2) containing an

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ichnofauna of small size (Twitchett 1999). Ichnodiversity remains fairly high, however, and the reddish micaceous fine sandstones and siltstones were clearly deposited in a shallow, welloxygenated environment. Monotaxic assemblages of Asteriacites lumbricalis (produced by ophiuroids) are common within this unit. The Campil Member was rapidly deposited, and may have been affected by low salinity levels (Twitchett 1999), which could have affected both the size of the benthos and the amount of burrowing activity that occurred. The Spathian sediments of the Dolomites region were deposited in a range of environments, from peritidal settings to offshore shelf. Bioturbation is usually high (ii3-6), and burrow diameters are larger than at any other time in the Early Triassic. Spathian sediments record the first reappearance of Rhizocorallium, which commonly exceeds 10mm in diameter. Of the common late Permian ichnotaxa, only Thalassinoides fails to appear by the Spathian. However, the overlying Middle Triassic units (separated by an unconformity from the underlying Werfen Formation) are often well bioturbated by large Thalassinoides.

Early Triassic ichnostratigraphy Can the stepwise reappearance of ichnotaxa through the Early Triassic recovery be used to correlate strata? Certainly, the pattern of reappearance of different ichnotaxa, as well as the increase in burrow size and amount of bioturbation through the Werfen Formation of the Dolomites, can be used to correlate with less fossiliferous deposits in adjacent regions (Twitchett 1997). The Servino Formation of Lombardy is one example. The Servino Formation is a relatively thin, lateral equivalent of the Werfen Formation, which contains very few macrofossil remains (Cassinis 1968). It is lithologically variable across Lombardy, and correlation with the Werfen Formation is often problematic (e.g. Assereto et al. 1973; Posenato et al. 1996). In addition, outcrop of the Servino Formation is often patchy on the grass-covered hillsides, which makes detailed mapping difficult. The discovery of conodonts (Twitchett 1997, 2000) may aid correlation in the future, but a systematic study of this microfauna has yet to be undertaken. Meanwhile, trace fossils remain the best means of correlation in the field. Two key ichnostratigraphic levels can be identified. The first is the appearance of Diplocraterion, which occurs in the thin storm beds

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Fig. 3. Generalized stratigraphy of the Permian-Triassic strata of the Dolomites, northern Italy, showing distribution of ichnogenera and sea-level curve. Ichnogeneric ranges from Twitchett & Wignall (1996) and Twitchett (1997, 1999). Sea-level curve and stratigraphy modified from Broglio Loriga et al (1986). AH, Andraz Horizon; GOM, gastropod oolite member; sp, supratidal; in, intertidal; sb, subtidal; 1, oolites; 2, mud cracks; 3, wave ripples.

of micaceous fine sandstone, interbedded with laminated siltstones, in the lower pelitic unit (Twitchett 1997; Fig. 4). Despite the common occurrence of Diplocraterion, burrow density is never very great, and so there is minimal disturbance to the primary sedimentary structures (ii2). These Diplocraterion assemblages can be

correlated with those of the upper Siusi Member of the Werfen Formation, suggesting a lower Dienerian age for this part of the Servino Formation. This supports previous lithological correlation, as the overlying carbonate unit is usually correlated with the Gastropod Oolite Member (Assereto etal. 1973; Fig. 4).

ICHNOSTRATIGRAPHY & MASS EXTINCTION

405

Fig. 4. Generalized stratigraphy of the Servino Formation of eastern Lombardy (after Assereto & Rizzini 1975) showing distribution of trace fossils (data from Twitchett 1997, 2000, and recent field observations) and correlation with the Werfen Formation.

The second important marker is the appearance of Rhizocorallium. The upper silt stonedominated units of the Servino Formation contain common Rhizocorallium (Fig. 4). Ichnofabric index is high (ii3-5), and burrow diameters are larger than in the underlying parts of the Servino Formation. In the Werfen Formation, Rhizocorallium first appears in the Spathian (Val Badia through to San Lucano Members). This age designation agrees well with other evidence, such as the rare occurrences of the foraminiferan Meandrospira pusilla within the upper parts of the Servino Formation, and the fact that the underlying Middle Pelitic Unit is lithologically similar to the Campil Member (Fig. 4; Twitchett 1997). Globally, the first appearances of Diplocraterion and Rhizocorallium in the Early Triassic may

also have stratigraphic value (Twitchett 1999). Certainly, there is some correspondence between the trace fossil record of the Lower Triassic deposits of the western USA and that of northern Italy. Both regions record deposition in similar environments at similar (tropical) palaeolatitudes, and record similar post-extinction patterns of ecological recovery in the shelly fauna (cf. Schubert & Bottjer 1995; Twitchett 1999). Both regions record increasing burrow size, depth of bioturbation, amount of bioturbation and ichnodiversity through the Early Triassic. The highest levels of bioturbation occur in the Spathian deposits of the western USA (Schubert & Bottjer 1995), although even here burrow depths do not exceed 10cm. Preliminary assessment by one of us (RJT) indicates that the Griesbachian Dinwoody

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Formation contains just simple horizontal fodinichnia such as Planolites. U-shaped burrows with spreiten first occur in the Sinbad Limestone Member of the San Rafael Swell (southern Utah), which is upper Nammalian (i.e. probably Smithian) in age (Schubert & Bottjer 1995). These small-diameter burrows are steeply inclined to the bedding, but are never vertical, and penetrate just a few centimetres into the substrate. They could be classified as either Diplocraterion or Rhizocorallium. The first undoubted Rhizocorallium occur within the Spathian Virgin Limestone Member of the Moenkopi Formation. In southern Nevada, the Virgin Limestone contains a diverse ichnofauna, which also includes Arenicolites, Asteriacites, Cochlichnus, Planolites and Skolithos. Finally, it appears that Thalassinoides, which is common in similar facies in the Permian (Schubert & Bottjer 1995), is completely absent from the Lower Triassic sediments of the western USA, but does occur in the Middle Triassic (D. Bottjer personal communication). This is identical to the post-extinction distribution of Thalassinoides in northern Italy (Twitchett 1999). Faster recovery at higher latitudes? At low palaeolatitudes, Thalassinoides, attributed to the activities of crustaceans, is one of the last ichnotaxa to reappear (in the Middle Triassic, some 10 million years after the P-Tr extinction event). After recent, small-scale anoxic events, crustaceans are typically last to become re-established (e.g. Harper et al. 1991). This suggests that patterns of benthic ecological response may be broadly similar at vastly different temporal scales, and may indicate that the reappearance of Thalassinoides can be used to mark the return to 'normal' marine conditions. However, the record at higher northern palaeolatitudes is somewhat different. In East Greenland, small, shallow-tier Thalassinoides are present in the upper part of the Wordie Creek Formation (personal observation), which is dated as latest Griesbachian-early Dienerian in age (Wignall & Twitchett 2002b). In Spitsbergen, Thalassinoides occurs in the Dienerian-age, shallow shoreface deposits of the Vardebukta Formation, soon after disappearance of oceanic anoxia (Wignall et al. 1998). These data indicate that, following the end-Permian event, crustaceans reappeared sooner in mid-high palaeolatitudes than in the tropics. One common element of the pre-event ichnofauna that apparently fails to reappear, even by the Middle Triassic, is Zoophycos. No Early or

Middle Triassic records are known, although Zoophycos is common in many later Mesozoic and Tertiary shelf sea sediments, and was extremely abundant in Upper Permian sediments (e.g. Wignall et al. 1998). One possible explanation is that the Permian Zoophycos tracemaker(s) became extinct during the P-Tr crisis, and it then took many millions of years for the behaviour to be 'reinvented' by other organisms (an Elvis ichnotaxon?). Alternatively, the habitat preferences of the Zoophycos trace-maker(s) may have changed, and they may have become restricted to habitats that are (so far) unrecorded in the earliest Mesozoic sedimentary record. Support is provided by Bottjer et al. (1988), who demonstrated that, since the Palaeozoic, Zoophycos has been restricted to deeper-water environments.

Summary The P-Tr trace fossil record has been well studied at a number of localities worldwide, representing a range of depositional environments. Ichnodiversity, burrow size, depth and amount of bioturbation are all severely reduced across the P-Tr boundary. Only the small (millimetresized), simple fodinichnia of deposit-feeding animals are present, sporadically, in the immediate post-extinction aftermath, and penetrate just a few centimetres below the sediment surface. Patterns of infaunal recovery are less well understood than details of the biotic crisis, but several interesting ecological patterns are beginning to emerge. There is an increase in burrow size, ichnodiversity and depth of bioturbation through the Early Triassic, but the timing of recovery varies between regions. Lower-latitude regions apparently took longer to recover than higher latitudes (at least in the northern hemisphere). The stepwise reappearance of certain ichnotaxa (Diplo crater ion, Rhizocorallium and Thalassinoides) may have some stratigraphic use for correlation between regions at similar palaeolatitude. Triassic-Jurassic (Tr-J) event The Tr-J extinction event was identified by Newell (1967) as one of the five major Phanerozoic extinction episodes, with an estimated 80% of species becoming extinct (e.g. Palfy et al. 2000), although our understanding of this event remains poor, as few complete marine boundary sections are known. Some authors have argued that levels of extinction may have

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407

Fig. 5. Sedimentary log of the Triassic-Jurassic of Pinhay Bay, southern England, showing the distribution of trace fossils and ichnofabric indices (ii). Horizontal ichnofabric calculated from methods of Miller & Smail (1997). Vertical ichnofabric follows Droser & Bottjer's (1986) scheme.

been overestimated, as disappearances usually coincide with major facies change (Cuny 1995; Hallam 2002). The tracks of Late Triassic and Early Jurassic tetrapods have been used to study the faunal response of terrestrial vertebrates through the Tr-J interval (Olsen et al. 2002). This represents the only dedicated study of the terrestrial trace fossil record through any of the major extinction events. Here we present the first analysis of the marine trace fossil record through the Tr-J boundary, from our investigations in southern England, Austria, and Nevada. England Sections through the Uppermost Triassic Lilstock Formation and the Lower Jurassic Blue Lias Formation have been examined at St Audries

Bay (Somerset) and Pinhay Bay (Dorset). Despite differences in the thickness of the Blue Lias at these two sites, the trace fossil records of both show remarkable similarity, and so only Pinhay Bay is discussed in detail here (Fig. 5). To the west of Pinhay Bay (SY 295895), the top of the Rhaetian Lilstock Formation is locally heavily bioturbated by 6-1 Omm diameter Diplocraterion, which penetrate up to 13 cm. The lack of a more diverse ichnofauna in the Lilstock Formation is probably due to deposition under unusual salinity conditions (Hallam & El Shaarawy 1982). Diplocraterion disappears at the Formation boundary, and does not reappear until the angulata Zone of the Hettangian, where burrow diameter is significantly smaller than in the Rhaetian (Fig. 5). The lowest pre-planorbis beds of the Blue Lias Formation are laminated and unbioturbated,

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Fig. 6. Changes in the depth of bioturbation through the Triassic-Jurassic of Pinhay Bay. Open circles represent mean burrow depth; closed circles, maximum burrow depth. P values indicate the statistical significance (Kolmogorov-Smirnov test) between Lilstock Formation and planorbis Zone datasets, and between planorbis Zone and bucklandi Zone datasets.

which may indicate an episode of anoxia. Arenicolites and small Thalassinoides appear in the upper prQ-planorbis beds at Pinhay Bay, and there is a coincident increase in vertical ichnofabric index (sensu Droser & Bottjer 1986). Burrow depth, measured from Arenicolites, never exceeds 5 cm. Relatively small burrow size, shallow depth of bioturbation and low ichnofabric index are consistent with an oxygen-restricted (dysaerobic) environment (e.g. Rhoads & Morse 1971; Savrda & Bottjer 1986). A recent study (Wignall 2001) revealed the presence of abundant pyrite framboids in the lowest pre-planorbis beds, while spectral gamma ray analysis showed an exceptionally low Th/U ratio. These observations, too, are indicative of anoxic deposition. However, Arenicolites and Thalassinoides are not typical dysaerobic ichnotaxa, and the small size of the infauna may also be due to other factors, such as a decrease in food supply, and is typical of immediate post-extinction intervals (cf. Urbanek 1993). Ichnofabric indices are variable, and may reflect changes in sedimentation rate or population density of the infauna. Palaeophycus, Planolites, Rhizocorallium and Thalassinoides are recorded from the overlying planorbis Zone. There is an associated increase in ichnofabric index (Fig. 5). However, there appears to be little change in depth of burrowing

between the prQ-planorbis and planorbis beds (Fig. 6). Both the ichnodiversity and the degree of bioturbation continue to increase through the overlying liasicus Zone with the (re)appearance of Chondrites (Fig. 5). Maximum ichnodiversity is reached in the angulata Zone, which contains Arenicolites, Chondrites, Diplocraterion, Palaeophycus, Planolites, Rhizocorallium and Thalassinoides. Ichnodiversity remains at this level through the Sinemurian (bucklandi and semicostatum Zones). Maximum depth of burrowing also increases in the angulata Zone to 17cm (Fig. 6), as a consequence of the reappearance of Diplocraterion. However, the diameters of these Diplocraterion burrows are still significantly smaller than those of the Rhaetian examples, and pre-Jurassic size is not reached in Diplocraterion until the Sinemurian (bucklandi Zone). Most ichnotaxa, except Rhizocorallium and Chondrites, which show no size increase, are small when they first (re)appear, and increase in size through the Hettangian and into the basal zones of the Sinemurian (Fig. 7). This increase in tracemaker body size may be a response to improving environmental conditions such as food supply or oxygen levels, and mirrors the size increase of the marine shelly macrofauna during the same interval (e.g. Hallam 1975).

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Fig. 7. Size changes of selected ichnogenera through the Triassic-Jurassic of Pinhay Bay. Open circles represent mean burrow diameter; closed circles, maximum burrow diameter. P values indicate the statistical significance between the datasets shown by the arrows at the end of each line (Kolmogorov-Smirnov test)

Central Austria Two sections were studied by one of us (CGB), in the Salzkammergut region of Central Austria. The Rhaetian sediments of Austria contain an

ammonite fauna and, unlike the Rhaetian of England, were deposited under normal marine conditions similar to those of the overlying Hettangian. Ichnofaunal changes across the Tr-J boundary of Austria are thus more likely

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Fig. 8. Composite sedimentary log of the Triassic-Jurassic sections of Central Austria exposed at GaiBau and Kendelbach Gorge (see text for details). Key as in Fig. 6. Vertical ichnofabric index (ii) follows Droser and Bottjer's (1986) scheme.

to reflect the effects of the end-Triassic mass extinction, rather than facies change. The Rhaetian Kossen Formation, exposed in a road cutting near GaiBau, is well bioturbated (ii3—4) and contains Diplocraterion, Planolites, Rhizocorallium, Skolithos and Zoophycos (Fig. 8). The Rhaetian ichnodiversity of Austria is therefore higher than in England, where the sediments were deposited under unusual salinity conditions (Hallam & El Shaarawy 1982). The Kossen Formation was examined again approximately 60m up-section (Zankl 1971), in nearby Kendelbach Gorge. Deposition of the Kossen Formation continues to the Tr-J boundary, and at Kendelbach Gorge there is exposure of the boundary itself. However, although the upper Kossen Formation limestones are similar in appearance and lithology to those at GaiBau, they contain no readily observed trace fossils (Fig. 8). At Kendelbach Gorge, the Hettangian Kendelbach Formation contains shallow marine limestones and is overlain by the deeper-water Sinemurian Adnet Formation. No

identifiable trace fossils, and only limited evidence of bioturbation, could be found in the Hettangian sediments. Limestone samples from the Kossen Formation and overlying Kendelbach Formation at Kendelbach Gorge are completely homogeneous in vertical section. It is, however, unclear whether this homogeneity reflects the uniform nature of original deposition, or is a result of total bioturbation, or diagenesis. The apparent absence of trace fossils from the upper Kossen Formation may suggest a deterioration of environmental conditions approaching the Tr-J boundary in Central Austria. The failure of the ichnofauna to reappear at any point within the overlying Hettangian sediments at Kendelbach Gorge implies that any disturbance to the marine ecosystem continued to affect the tracemaking taxa well into the Early Jurassic. In contrast, the shelly fossil record suggests that recovery at Kendelbach Gorge was relatively rapid, and occurred even in the latest Rhaetian (Hallam 1990). Seven metres below

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Fig. 9. Sedimentary log of the Triassic-Jurassic section in New York Canyon, Nevada. Key as in Fig. 5. Asterisked letters show stratigraphic positions of photographs shown in Fig. 10. Vertical ichnofabric index (ii) follows Droser & Bottjer's (1986) scheme.

the Tr-J boundary, within the Kossen Formation, there are shale horizons containing an impoverished bivalve fauna (Hallam 1990). This correlates with the decrease in bioturbation and ichnodiversity. However, prior to the first appearance of the ammonite Psiloceras planorbis in the Hettangian, Hallam documents an increase in faunal diversity, and the appearance of forams, ostracods, crinoids and brachiopods.

Nevada In the New York Canyon area, Gabbs Valley Ranges, Nevada, marine limestones and siltstones from the Upper Triassic (Rhaetian) Gabbs Formation and the Lower Jurassic (Hettangian) Sunrise Formation were examined. An ammonite fauna is present throughout the section, and so, as in Austria, salinity changes across the Tr-J bound-

ary of Nevada appear to have been minimal. Trace fossil changes are thus not complicated by salinity considerations. There is, however, a facies change in the upper Gabbs Formation (Muller Canyon Member), with a reduction in the number of resistant limestone horizons with bedding plane exposure (Fig. 9). A relatively diverse ichnofauna is present in the Mount Hyatt Member of the Gabbs Formation (Fig. 9), consisting of Arenicolites, Planolites, Rhizocorallium, Skolithos and Thalassinoides (Fig. lOe). Ichnofabric index is low, however (maximum ii2; Fig. lOf). The majority of these ichnotaxa are absent from the overlying Muller Canyon Member, which straddles the Tr-J boundary (Taylor et al. 1983). Indistinct burrows, here referred to Planolites, occur rarely (Fig. lOd). The Muller Canyon Member contains relatively few limestone beds and little horizontal bedding plane exposure. In the underlying and overlying units, trace fossils appear to be confined to the

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Fig. 10. Polished blocks and field photographs indicating evidence for the presence or absence of ichnofauna: (a) Rhizocorallium burrows from the Ferguson Hill Member; (b) polished block showing mottled fabric cut by horizontal burrows from the Ferguson Hill Member; (c) polished block showing traces of original bedding from the Ferguson Hill Member; (d) polished block showing mottled fabric cut by large burrows from the Muller Canyon Member; (e) Thalassinoides burrow from Mount Hyatt Member (see Fig. 9 for stratigraphic positions); (f) polished block showing well-preserved bedding cut by rare burrows in the Mount Hyatt Member.

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limestones and are most easily observed on horizontal surfaces. Thus facies change is the most likely explanation for the low ichnodiversity of the Muller Canyon Member. An earlier study (Hallam & Wignall 2000) reported intense bioturbation from the central part of the Muller Canyon Member, with a thoroughly mottled fabric of indistinct traces cut by abundant Helminthoida. However, other than rare Planolites burrows, no other ichnotaxa were found from the Muller Canyon Member in the current study, and we were unable to confirm this observation. Furthermore, although cut and polished rock samples from this interval often showed a burrow-mottled fabric, the rocks themselves were relatively fissile, suggesting that some of the original bedding remains. Arenicolites, Planolites, Rhizocorallium, Skolithos and Thalassinoides all reappear in the Ferguson Hill Member of the Sunrise Formation (Fig. lOa), which is dated as upper Hettangian to lower Sinemurian (Taylor et al. 1983). Chondrites and Diplo crater ion are also present in this unit. The amount of bioturbation recorded in individual beds varies. Some beds have a mottled fabric, cut by prominent burrows (e.g. Fig. lOb), whereas others show the remains of original bedding (e.g. Fig. lOc). There is little difference in burrow diameter between the ichnotaxa of the Gabbs and those of the Sunrise Formations. This suggests that palaeoenvironments of the Ferguson Hill Member and Mount Hyatt Member were similar.

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Summary These data have shown that the nature of the trace fossil record changes through the Tr-J boundary of each of these three regions (Fig. 11). Some of these changes, such as those at New York Canyon, can be attributed to facies change, but others, such as those through the Blue Lias Formation of England, appear to occur with no significant change in sedimentary facies. These results show that normal marine environments of Rhaetian age contain a diverse ichnofauna, which may include Arenicolites, Diplo crater ion, Planolites, Rhizocorallium, Skolithos, Thalassinoides and Zoophycos. There is some evidence that ichnodiversity, burrow size, depth of bioturbation and ichnofabric index are much reduced in the uppermost Rhaetian (e.g. pre-planorbis beds of England), presumably as a result of environmental deterioration, which may be attributed to dysoxia. There is an increase in ichnodiversity, ichnofabric index, burrow size and depth of burrowing through the Hettangian as benthic oxygen levels improve, although rates of recovery vary. Ichnodiversity returned to Rhaetian levels by the angulata Zone of the Hettangian (at least in England and Nevada; Fig. 11). Ichnotaxa recorded at this time include Arenicolites, Chondrites, Diplocraterion, Palaeophycus, Planolites, Rhizocorallium and Thalassinoides. Burrow size of some ichnotaxa (e.g. Diplocraterion, Fig. 7) continues to increase through into the Sinemurian, which

Fig. 11. Trends in ichnodiversity through the Triassic-Jurassic intervals of England, Central Austria and Nevada, USA. Gap in the Austrian record is the result of diagenesis (see text for details).

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parallels the Early Jurassic rise in body size of macroinvertebrates such as ammonites and bivalves (Hallam 1975; Dommergues et al. 2002). Changes in the depth of burrowing (Fig. 6) indicate that in the latest Rhaetian and earliest Hettangian (planorbis and Hastens Zones) only the shallowest tiers are occupied (cf. Fig. 1). The stepwise reappearance of some ichnotaxa, such as Chondrites and Diplocraterion, may have ichnostratigraphic value. Certainly, in the UK, Diplo crater ion appears to be absent from Jurassic strata until the angulata Zone beds of the Blue Lias (Fig. 5). If the angulata Zone is considered to be the level at which 'normal' infaunal communities have reappeared, then post-extinction recovery is clearly far more rapid than, for example, after the P-Tr event. Cretaceous-Tertiary (K-T) event Trace fossils have, thus far, contributed relatively little to our understanding of the benthic response to the K-T event. The only study of the marine trace fossil record through the extinction-recovery interval is Ekdale and Bromley's (1984) account of the ichnology of the K-T boundary chalks of Denmark. They found that in deeper basinal areas a diverse ichnofauna containing Zoophycos, Chondrites and Thalassinoides continues to the top of the Maastrichtian, where there is a sudden change to the Thalassinoides-dominatQd ichnofauna of the lower Danian limestones. The disappearance of Zoophycos, and its absence from the lower Danian sediments, is attributed to shallowing across the boundary and a change to more soupy substrates (Ekdale & Bromley 1984). Certainly, there is no evidence for a change in oxygen levels, as the sediments are well bioturbated throughout. However, there may be a preservational issue here as the lower Danian chalks are highly compacted, making burrow identification problematic (Ekdale & Bromley 1984). In shallower parts of the basin, Zoophycos declines in abundance in the Upper Maastrichtian chalks, which is again attributed to sea-level fall (Ekdale & Bromley 1984), so that the uppermost, grey Maastrichtian chalks are dominated by Thalassinoides. These are overlain by the Lower Danian Fish Clay, which records an episode of oxygen restriction evinced by laminated, black, carbonaceous sediments. Ichnofabric index, ichnodiversity and burrow size all apparently decline in the lower part of the Fish Clay, although quantitative data are lacking, and increase again in the transition to the Danian chalks above. These changes in the trace fossil record are clearly simi-

lar to the patterns observed through the P-Tr and Tr-J intervals. Trace fossils have also been used in novel palaeoenvironmental analyses of the so-called 'tsunami beds' of the southern USA (Savrda 1993) and Mexico (Ekdale & Stinnesbeck 1998). Following the Alvarez et al (1980) impact hypothesis, and especially with the identification of the crater itself beneath Chicxulub (Hildebrand et al. 1991), evidence was sought for the giant, impact-generated, tsunamis that were hypothesized to have devastated the coastal plains surrounding the present-day Gulf of Mexico. Several spherule-bearing sandstones, up to a few metres thick and of K-T age, were soon identified as tsunami deposits, based on limited sedimentological evidence such as their coarse grain size (in otherwise fine-grained sediments) and apparently rapid deposition (e.g. Bourgeois et al 1988). Although the interpretation of these units as tsunami deposits fitted perfectly with the expectations of supporters of the impact model, such 'hypothesis-driven interpretations' (Stinnesbeck et al 1994) were not universally accepted. Bohor (1996) suggested that they were turbidite deposits, which may have been triggered by tsunamis or seismic events related to the impact. Others argued against any seismic or tsunami involvement and instead interpreted them as the normal transgressive infilling of incised valleys cut during an episode of sea-level fall (e.g. Savrda 1993; Stinnesbeck et al 1993). One key prediction of this latter hypothesis is that deposition was not catastrophic, but took place over an extended period of time. The trace fossil studies of Savrda (1993), who investigated the Clayton Sands of Alabama, and Ekdale & Stinnesbeck (1998), who analysed the sandstones of the Mendez Formation, northeastern Mexico, have proven to be crucial in the correct interpretation of these deposits. In both cases, the 'tsunami beds' could be divided into three subunits, each with a different suite of trace fossils. For example, the lower unit of the Clayton Sands contains rare Ophiomorpha, emplaced prior to deposition of the overlying middle unit, which contains Thalassinoides. The upper unit contains Thalassinoides and a number of horizons with small diameter (35 mm), shallow penetrating (less than 1 cm) Planolites. The multiple horizons of trace fossil emplacement indicate that deposition of the Clayton Sands took place over an extended period of time, with breaks in deposition allowing the burrowing infauna to colonize the substrate (Savrda 1993). The results of Ekdale & Stinnesbeck (1998) were similar, and so the

ICHNOSTRATIGRAPHY & MASS EXTINCTION tsunami origin for these units could be rejected. The studies of Savrda (1993) and Ekdale & Stinnesbeck (1998) show the immense value of ichnology in palaeoenvironmental analysis, and how it is crucial to correct interpretation of sedimentary units, especially when testing hypotheses of catastrophic deposition. Despite the conclusions of these two studies, so-called tsunami deposits continue to be identified in the Gulf of Mexico region (e.g. Tada et al. 2002) without any reference to the trace fossil assemblages they contain. Summary The trace fossil records of four of the five major Phanerozoic mass extinction events have been analysed, but coverage is variable. To date, the Permian-Triassic trace fossil record has been the most intensively studied of these and has been important in palaeoenvironmental analysis, and crucial in providing insights into the ecological response of the marine benthos. Trace fossils are the only records we have of the response of the soft-bodied burrowing infauna to major biotic events of the past. The post-extinction stepwise reappearance of ichnotaxa may have some ichnostratigraphic use, but only within similar environmental settings at similar palaeolatitudes. Trace fossils have great potential for increasing our understanding of these important events in the evolution of life on Earth: we have only just begun to scratch the surface. CGB's fieldwork in Austria was funded by a Sylvester Bradley Award (Palaeontological Association). Fieldwork in England was funded by the John Ray Trust, and fieldwork in Nevada was funded by the Geological Society of London. R. Scott and W. Barras are thanked for assistance in the field. C. Paul is thanked for taking us through the coastal sections west of Pinhay Bay. RJT thanks D. Sciunnach for sharing his knowledge of the Lower Triassic sediments of eastern Lombardy, and D. Bottjer, M. Fraiser and S. Pruss for similar discussions on the trace fossil record of the western USA. Thanks to staff in the Department of Earth Sciences (University of Bristol, UK) and Department of Earth and Planetary Science (UC Berkeley) for use of facilities. Thorough reviews by P. Wignall and M. Garton greatly improved the quality of this publication. References ALLISON, P. A. & BRIGGS, D. E. G. 1991. Taphonomy of nonmineralised tissues. In: Allison, P. A. & Briggs, D. E. G. (eds) Taphonomy: Releasing the Data Locked in the Fossil Record. Plenum Press, New York, 25-70.

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Ichnotaxonomy and ichnostratigraphy of chambered trace fossils in palaeosols attributed to coleopterans, ants and termites JORGE F. GENISE CONICET, Museo Paleontologico Egidio Feruglio, Av. Fontana 140, 9100 (Trelew), Chubut, Argentina (e-mail: [email protected]) Abstract: Most recorded trace fossils in palaeosols are burrows and chambers attributed to bees, ants, termites and coleopterans. Ichnogenera attributed to bees are grouped in the ichnofamily Celliformidae, whereas those attributed to ants, termites and coleopterans are included herein in the new ichnofamilies Pallichnidae, Krausichnidae and Coprinisphaeridae respectively. Shape, type of wall, fillings and associated burrows of chambers are the main morphological ichnotaxobases used for this classification; they are weighed with regard to the behaviour and architecture of the supposed trace-makers. Coprinisphaeridae are spherical, pear-shaped or ovoid structures, having active or passive fillings and constructed walls. The ichnogenera included are: Fontanai, Coprinisphaera, Eatonichnus, Monesichnus, Teisseirei and Rebuffoichnus, attributed to coleopterans. The similar Pallichnidae show lined or structureless walls, and include Pallichnus, Fictovichnus and Scaphichnium, also attributed to coleopterans. The Krausichnidae constitutes trace fossils composed of chambers of different shapes interconnected by burrow systems of inconsistent diameter or isolated chambers associated with burrow systems of different diameters. The Krausichnidae include Attaichnus, Parowanichnus, Krausichnus, Archeoentomichnus, Tacuruichnus, Vondrichnus, Fleaglellius, Termitichnus and Syntermesichnus, attributed to ants and termites. The stratigraphic ranges of insect ichnotaxa in palaeosols are reviewed and compared with the body fossil record of potential tracemakers, revealing that in most cases insect trace and body fossils show similar ranges. As stated by earlier authors, the Cretaceous was a critical period during which the oldest body fossils of dung-beetles, bees, termites and ants are recorded, whereas the trace fossils of these groups are recorded from this period or shortly after, during Cenozoic times.

The grouping of ichnogenera in ichnofamilies is an uncommon but commendable trend in some branches of ichnology, where highly significant ichnotaxobases can be used to make coherent groupings of ichnotaxa (e.g. Genise 2000; Bertling et al. 2003; Rindsberg & Martin 2003). Roselli (1939), a pioneer in insect palaeoichnology, first suggested a higher taxonomy for the hymenopteran and coleopteran trace fossils that he described. In his contribution he grouped the supposed hymenopteran nests in the family 'Nidus Himenopterogenosidae* and those of coleopterans in 'Nidus coleopterogenosidae' (Roselli 1939). Recently, Genise (2000) created the first ichnofamily for insect trace fossils in palaeosols, Celliformidae, to include Celliforma and allied ichnogenera. This contribution represents an attempt to classify many of the remaining chambered trace fossils in palaeosols, attributed to Coleoptera, Hymenoptera and Isoptera (Genise 1999), and to provide a general picture of the stratigraphic ranges of insect ichnotaxa in palaeosols. It also represents an attempt to identify the major behavioural features of these groups as reflected in the morphology of their trace fossils. Few trace fossils constructed by organisms other than insects can be compared morphologically with insect traces. The more complex the

architecture, the more difficult it is to find analogous structures outside the insect realm, Probably the most similar trace fossils are those produced by another group of arthropods: crustaceans. Tunnels associated with chambers are known from crayfishes, whose trace fossils are included in the ichnogenus Camborygma Hasiotis & Mitchell 1993. However, Camborygma shows a distinctive bioglyph composed of scrape and scratch marks, knobby and hummocky surfaces, pleopod and body impressions, all of which distinguish it from similar ichnogenera attributed to social insect nests. In addition, interconnected tunnels of very different diameters are absent, as in other known modern crustacean constructions (Bromley 1990). The ichnogenera Ophiomorpha and Thalassinoides, also attributed to crustaceans, commonly show burrow systems devoid of chambers (Bromley 1990), but Verde & Martinez (2004) described chambers having tiny tunnels radiating vertically from the upper part of the chambers, in connection with both ichnogenera. Spongeliomorpha shows burrow systems in association with chambers in one ichnospecies: S. sicula D'Alessandro & Bromley 1995. The presence of large vertical shafts, small chambers below the maze of tunnels and the typical criss-cross pattern of grooves in

From: MC!LROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 419^53. 0305-8719/04/$15.00 © The Geological Society of London.

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Spongeliomorpha (D'Alessandro & Bromley 1995) and the upper radiating tunnels in the chambers associated with Ophiomorpha and Thalassinoides separate these trace fossils from those of social insects. The morphology of insect ichnogenera devoid of tunnel systems has few similarities with other known ichnological structures. The specimen illustrated by Hantzschel (1975) of Amphorichnus papillatus Myannil 1966 superficially resembles Fictovichnus (Johnston et al. 1996); however, the former is a filling of an amphora-like hollow ending in a distinct apical protuberance (Pemberton et al. 1988; Edwards et al. 1998). Another case is that of the cocoons of earthworms and leeches described by Manum et al. (1991) attributed to the genera Burejospermum, Dictyothylakos and Pilothylakos. The former two were previously considered as a seed and a palynomorph respectively. These cocoons are more likely secretions of organisms (Manum et al. 1991) than structures resulting from their activity, and as such they are ruled out as trace fossils (Bertling et al. 2003). Also, the structure of these cocoons differs from that of insects (Manum et al. 1991), whose cocoons are true trace fossils. The ichnofossil Lithoplaision ocalae Diblin et al. 1991 may superficially resemble an insect chamber, but its conical shape and marine invertebrate remains in the wall are important differences from insect chambers. Continental ichnology, and particularly insect palaeoichnology, is an exciting topic that is developing quickly in a changing scene, in which new discoveries occur daily. Thus this contribution is written with the conviction that the classification and stratigraphy of trace fossils proposed herein will probably need to be updated in the near future. However, a first impression is presented herein, with the understanding that it will help to order the somewhat chaotic ichnotaxonomy of insect trace fossils, providing a new standpoint from which to observe and analyse insect behaviour as reflected by trace fossils. Theoretical background Even the most complex insect trace fossils in palaeosols can be morphologically divided into two components: burrows (tunnels, shafts and galleries) and chambers. The latter term has no specific definition in the ichnological literature and glossaries (e.g. Ekdale et al. 1984; Bromley 1990). However, it is commonly used to name distinct enlargements of burrows in the entomological literature (e.g. Stephen et al.

1969; Halffter & Edmonds 1982; Grasse 1984; Holldobler & Wilson 1990). These excavated chambers are used without further modifications for nesting or pupation, and in other cases they are used to house more complex constructions for nesting or pupation. The knowledge of insect nest architecture was developed mostly through entomological studies, with each group of insects having its own nomenclature for excavated chambers and constructed structures of different functions. Some terms used in the entomological literature are pupation cell (e.g. Scholtz 1988; Skelley 1991), brood cell (Stephen et al. 1969; Batra 1984), brood ball chamber (Halffter & Matthews 1966; Halffter & Edmonds 1982), fungus garden chamber (Holldobler & Wilson 1990) and royal cell (Grasse 1984), among others. The functions of these chambers are diverse, but usually they are related to nesting activities or pupation - in sum, to the successful development of larvae. Fossil nests and pupation cells of Coleoptera, Hymenoptera and Isoptera respectively are the most common insect trace fossils in palaeosols (Genise et al. 2000), a fact that was related to the high potential of preservation of these constructed or lined structures (Genise & Bown 1994a). Females of most solitary bees and wasps nesting in soils, as well as dung-beetles, prepare chambers or structures constructed inside them, in which they provision food (pollen, nectar, prey, carrion or vertebrate excrement), lay an egg, and close the entrance immediately after. A single larva feeds on these provisions and completes its development without the assistance of adults in most cases (Evans 1963; Halffter & Matthews 1966; Batra 1984). In other groups of solitary insects having recorded trace fossils in palaeosols, such as weevils (e.g. Lea 1925; Johnston et al. 1996; Genise et al. 2002b), the larvae are not restricted to chambers prepared by adults. Their development takes place in the soil, in which they move freely, feeding on vegetable matter until their pupation, whereupon they prepare a cell that protects them during this critical period before emergence as adults (e.g. Loiacono & Marvaldi 1994). In turn, social insects such as ants, termites and some bees provision and inhabit the underground nests in which they lay eggs, and rear larvae that are confined to the interior of chambers (Michener 1974; Grasse 1984; Holldobler & Wilson 1990). These main behavioural differences, and a large number of more specific ones, gave rise to the great morphological diversity of insect nesting and pupation structures in soils and palaeosols. Thus the available ichnotaxobases by

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which to classify ichnogenera in ichnofamilies depend on the features of chambers and associated structures, especially shape, wall, fillings and burrows. Accordingly, the former ichnofamily proposed earlier for insect trace fossils in palaeosols, Celliformidae Genise 2000, was based on various characters of cells, the usual name that hymenopterists give to the brood chambers made by wasps and bees (Evans 1963; Stephen et al. 1969; Michener 1974). Ichnotaxobases Four ichnotaxobases, the most common characters used as the basis of ichnotaxa, were listed and analysed by Bromley (1990): general form, type of wall structure, type of branching, and nature of the fill. Of these ichnotaxobases, branching is a less important character for classifying insect fossil nests, than the cross morphology of the entire burrow system is considered herein as an effective ichnotaxobase for insect traces. Shape Excavated and constructed chambers show a morphological continuum that ranges from flat and tabular shapes to spherical ones. Fortunately, discontinuities exist within this spectrum and also some complementary characters that favour the separation of the range as a whole into discrete units such as ichnogenera. The shape of the chambers is an important character for termite and ant nests, but it is even more critical for separating trace fossils attributed to bees and beetles, because in the latter cases the associated burrows are rare. Celliformidae (attributed to bees) can be recognized by the presence of cells having rounded backs and flat or conical tops showing a spiral design that was constructed from the outside by the adult bee (Fig. 1). In contrast, closed pupation cells attributed to beetles have both extremes rounded and smoothed because the larvae themselves construct them from the inside. Among trace fossils attributed to beetles, there is a clear morphological discontinuity between spherical or pear-shaped traces, namely Coprinisphaera Sauer 1955, Fontanai Roselli 1939 and Pallichnus Retallack 1984, and ovoid ones: Monesichnus Roselli 1987, Fictovichnus Johnston et al. 1996, Teisseirei Roselli 1939, Rebuffoichnus Roselli 1987 and Eatonichnus Bown et al. 1997. Among the ovoid forms, it is impossible to distinguish different shapes, with the single exception of Teisseirei, which shows a

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depressed outline in cross-section (Fig. 3c). Scaphichnium Bown & Kraus 1983 is unique because it has a peculiar hamate or lunate shape. Fossil termite and ant nests show a more or less continuous spectrum, from flat chambers in Krausichnus Genise & Bown 1994b, Fleaglellius Genise & Bown 1994b and Archeoentomichnus Hasiotis & Dubiel 1995a, to spherical chambers in Attaichnus Laza 1982, Termitichnus Bown 1982 and Vondrichnus Genise & Bown 1994b. In most cases, other complementary characters are needed to separate these ichnotaxa. However, some morphological discontinuities can be recognized in the spectrum of shapes. Krausichnus and the unnamed termite nests from the Pliocene and Pleistocene of Africa (Coaton 1981; Schuster et al. 2000) display low, flat, tabular chambers, whose floor and roof are parallel. Vertical pillars commonly accompany these chambers, probably to reinforce the whole structure (Fig. 5b). These tabular chambers are clearly distinguishable from other flattened, but more oval, high chambers (e.g. Fleaglellius) in which the roof is more arched with respect to the floor (Fig. 6b). Tabular chambers are apparently also present in Archeoentomichnus (Hasiotis & Dubiel 1995a) and in Termitichnus namibiensis Miller & Mason 2000. However, in the latter the tabular chambers seem to be more likely the result of a tiered arrangement of meniscate burrows, probably made by another organism. The complex architecture of social insects may result in secondary chamber systems within a primary chamber system. This is true, for instance, in Krausichnus trompitus, where the tabular chambers are grouped in spindle-shaped structures that are, in turn, connected with other spindles (Genise & Bown 1994b). Similarly, in Tacuruichnus, a single chambered trace fossil supports a boxwork with chambers in the thick peripheral wall (Genise 1997) (Fig. 5c). Types of wall This ichnotaxobase is the most difficult to analyse because of the different common usages of the term 'wall' (e.g. Retallack 200Ib) and the complexity that walls can reach in insect traces. Commonly speaking, the term 'wall' is applied indistinctly to two different structures: twodimensional surfaces (e.g. 'burrow boundary' of Bromley 1990, 1996) and discrete three-dimensional constructions. This fact introduces some confusion, and is discussed below. In an excavated chamber the wall is the boundary between the cavity and the soil - for instance the brood ball chamber wall in dung-beetle

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Fig. 1. Schematic drawings to demonstrate common features in nesting structures, showing different types of wall built by some dung-beetles, bees and termites. In all cases the insects first excavate a chamber in soil and then construct within it one or more brood balls (dung-beetles), a cell (bees) or a complete nest (termites), respectively. Constructed walls may show an inner lining (bees) or may bear a system of galleries and chambers with lined walls (termites). In dung-beetles and termites, constructed structures are separated from the excavated chamber by a space, whereas in bees the constructed wall is built against the chamber. Drawings are based on data from Coprini (Scarabaeidae) (Halffter & Edmonds 1982), Emphorini (Apidae) (Hazeldine 1997; Genise & Poire 2000) and Nasutitermitinae (Termitidae) (Grasse 1958; Grasse 1984). nests (e.g. Halffter & Edmonds 1982) (Fig. 1). In these nests one or more brood masses, each one having its own constructed wall, may be located inside this excavated brood ball chamber (Halffter & Edmonds 1982) (Fig. 1). In a

constructed bee cell, the wall is commonly a three-dimensional structure removable from the soil. This constructed wall is in turn contained within an excavated chamber with its own twodimensional wall (e.g. Hazeldine 1997) (Fig. 1).

ICHNOSTRATIGRAPHY OF TRACE FOSSILS IN SOILS

Furthermore, in some termite nests the external wall of the central part of the nest is a construction that contains a boxwork that has lined walls separated by a space (paraecie) from an excavated cavity (e.g. Grasse 1958) (Fig. 1). In sum, each constructed wall in a soil produces at least three surfaces: the inner and outer surfaces of the constructed structure and the surface of the bearing cavity, each surface of which may in turn be considered to be a wall. Different types of wall may be present and related in various ways within a single nest and potentially the same trace fossil. Confusingly, in nests of different insect taxa the wall may bear different names, particularly in termite nests, in which the specific nomenclature is more developed (e.g. Grasse 1984). The complexity of chamber walls in insect traces is related to the necessity of maintaining very specific environmental conditions inside nests to protect larvae and provisions. To achieve these conditions insects commonly line or construct walls by adding different types of organic matter to the soil material (Michener 1979; Grasse 1984; Cane 1991; Hanski & Cambefort 1991; Genise 1999). Genise & Bown (1994a) proposed that insect fossil nests are the most common trace fossils in palaeosols because of the original incorporation of organic matter in the wall structure, which enhances the probability of being consolidated by diagenetic processes. Bromley (1990, 1996), when describing the most common taxobases for invertebrate ichnotaxonomy, recognized seven categories of wall structure: no structure (no lining), dust film, constructional walling (constructional lining), zoned fill, wall compaction, diagenetic haloes, and wall ornament. At least five of these are represented in insect architecture and have been recorded in different insect trace fossils in palaeosols. In two ichnogenera, both attributed to ants namely Attaichnus Laza 1982 and Parowanichnus Bown et al. 1997 - the chamber wall shows no particular structure, a fact that probably reflects the absence of linings in the walls of ant nests (Bown et al 1997). However, most recorded insect trace fossils in palaeosols have lined or constructed walls. To line a wall is to cover a surface (e.g. excavated chamber wall) with a 'plaster' or coating (e.g. secretions, clay, faecal material), whereas to construct a wall is to add 'bricks and mortar' (e.g. soil pellets or coarse grains and fine material) to produce a discrete, three-dimensional structure (Fig. 1). A further complication in insect architecture is that insects may also actively line the inner surface of constructed walls with fine material (Noirot 1970; Hazeldine 1997). Bromley

423

(1990, 1996) considered those lined walls covered by 'dust films' to be collected passively in mucuslined burrows. Insects actively produce similar linings with fine material by two main methods: the addition of transported material, or fluidization. Among those that transport material from elsewhere, many species of termite usually line the internal surfaces of constructed or excavated chambers with faecal and/or regurgitated material (e.g. Noirot 1970; Grasse 1984). This lining is well preserved in the ichnofossil Krausichnus trompitus Genise & Bown 1994b (Fig. 5b). Some halictine bees are known to line cells with clay material transported from the tunnel (Sakagami & Michener 1962). The clay lining of Rosellichnus arabicus is probably derived in this manner (Genise & Bown 1996). Bees commonly line cells with water-repellent lipids (Cane 1991) after smoothing the chamber wall (Batra 1984). This smoothing behaviour triggers a fluidization process in the soil adjacent to the chamber that results in a distinct lining of fine material sourced from the same surrounding soil (Genise & Poire 2000). The bee's movements against the water-saturated soil pellets of the wall during its construction increase the pore pressure, which in turn produces the escape of water, drawing the fine material towards the inner surface of the wall (Genise & Poire 2000). Fossil bee cells included in Celliforma Brown 1934, Uruguay Roselli 1939, Ellipsoideichnus Roselli 1987; Palmiraichnus Roselli 1987 and Cellicalichnus Genise 2000 show this type of lining. Similar layers of clay material are found in the walls of the coleopteran pupation chambers Pallichnus Retallack 1984 and Fictovichnus Johnston et al. 1996, who proposed that they were built by coleopteran larvae. Lined walls are easily recognizable because they show a clear discontinuity and a smooth internal surface, whereas externally they intergrade gradually with the host sediment (Retallack 1984; Johnston et al. 1996). Accordingly, trace fossils having lined walls are usually preserved as detached casts of smooth surfaces, or they are preserved in situ in rocks, from where they can be distinguished because of the different texture and colour of the lining. Of all the categories of wall distinguished by Bromley (1990, 1996), the constructed ones are the most common in insect fossil nests. Usually, their producers first excavate a chamber and then build a wall through successive addition of soil pellets or sand grains (e.g. Halffter & Matthews 1966; Stuart 1969; Noirot 1970; Lee & Wood 1971; Hazeldine 1997). There are several important differences between lined and constructed walls that arise from the particular

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behaviours involved. Usually, linings are thin in comparison with constructed walls; however, intermediate thickness may produce some confusion that would preclude a clear distinction. Linings are usually made from fine soil or plant material, secretions, or excretions that are applied as a coating on an excavated or constructed surface (e.g. Sakagami & Michener 1962; Stephen et al 1969; Stuart 1969; Noirot 1970; Lee & Wood 1971; Cane 1991). In contrast, constructed walls are made from unsorted soil material, within which coarse grains or dry faecal pellets are added one at a time and bound by fine soil or faecal material (e.g. Halffter & Matthews 1966; Stuart 1969; Noirot 1970; Hazeldine 1997; Cosarinsky 2001). In lined walls the insect interacts with only one (inner) surface of the chambers, whereas in constructed walls both inner and outer surfaces result from the behaviour of the insect. Thus the outer as well as the inner surface may show a bioglyph, as in Eatonichnus (Fig. 2d). Constructed walls are discrete and resistant structures that can be removed entirely from soils and, when preserved, from the host rock. Accordingly, many trace fossils bearing them are preserved not only as casts or in situ in palaeosols, but also, and more frequently, as complete structures, removed from the rock matrix, showing internal and external bioglyphs. In addition, insect trace fossils having constructed walls can be transported and re-deposited (e.g. Andreis 1972, 1981) in contrast to most trace fossils, which are generally in situ (e.g. Ekdale et al. 1984). These walls are characteristic of Palmiraichnus Roselli 1987, Rosellichnus Genise & Bown 1996, Uruguay Roselli 1939, Coprinisphaera Sauer 1955, Fontanai Roselli 1939, Monesichnus Roselli 1987, Teisseirei Roselli 1939, Rebuffoichnus Roselli 1987, Eatonichnus Bown et al. 1997, Termitichnus Bown 1982, Fleaglellius Genise & Bown 1994b, Vondrichnus Genise & Bown 1994b, Tacuruichnus Genise 1997 and Archeoentomichnus Hasiotis & Dubiel 1995a. Bromley (1990, 1996) mentioned diagenetic haloes as a particular category of wall lacking ichnotaxonomical value. Hasiotis et al. (1993) interpreted the external coating of Scaphichnium hamatum as diagenetic cement, but no particular wall was described for this ichnofossil (Bown & Kraus 1983; Hasiotis et al. 1993). The last category mentioned by Bromley (1990, 1996) - wall ornamentation - seems to be more probably a wall feature than a type of wall in itself. Ornamentation may, potentially, be present in several different types of wall. Edwards et al. (1998) described unnamed trace fossils from the Bernbridge Limestone Formation (England), which

are generally preserved as ovoid internal casts, whose complex surface sculpture can be interpreted as scratch marks (cast in positive relief) from depressions in the chamber wall. Similar casts preserving the original sculpture of the chamber wall are known in Teisseirei barattinia (Melchor et al. 2002, fig. 12 I). The constructed wall of both ichnospecies of Eatonichnus shows a helical pattern that is present on both wall surfaces, although more clearly in the outer surface. E. utahensis also exhibits a distinct superimposed bioglyph (Bown et al. 1997) (Fig. 2d). External bioglyphs are more pronounced when a space is present between the constructed wall and the excavated chamber. The external ornamentation of bee (Apidae, Emphorini) cells shows flattened pellets because the constructed wall is built against the cavity boundary (e.g. Hazeldine 1997; Genise & Poire 2000) (Fig. 1). In contrast, the wall of Apicotermitinae (Termitidae) nests which are separated by a space from the bearing excavated chambers - show a very pronounced, complex sculpture (e.g. Grasse 1984). Fillings Filling material and structure is another important taxobase in invertebrate ichnotaxonomy. Passive fill enters a burrow gravitationally, whereas active fillings are emplaced by the trace-maker (Bromley 1990, 1996). Both kinds of fill occur in insect trace fossils. As described previously, adult insects provision their nests with different kinds of organic matter to rear their larvae. Some dung-beetles are known to arrange the provision of their brood masses in a meniscate pattern (Halffter & Matthews 1966; Halffter & Edmonds 1982) (Fig. 3a). Such meniscate fills are seen in Coprinisphaera, Monesichnus, Eatonichnus and Scaphichnium (Bown & Kraus 1983; Hasiotis et al. 1993; Bown et al. 1997; Genise & Laza, 1998; Duringer et al. 2000a, 2000b) (Figs 2d, 3a). These authors attributed their material to dung-beetles based on the meniscate structure of the fills. The absence of an active fill in Rebujfoichnus, Pallichnus, Fictovichnus and Teisseirei is concomitantly taken to suggest that these ichnotaxa represent coleopteran pupation cells (Retallack 1984; Johnston et al. 1996; Genise et al. 2002a; Melchor et al. 2002). Despite the fact that bees, ants and termites also provision their nests actively (e.g. Stephen et al. 1969; Grasse 1984; Holldobler & Wilson 1990), their trace fossils show no recognizable active fill (e.g. Laza 1982; Genise & Bown 1994b; Hasiotis & Dubiel 1995a; Bown et al. 1997; Genise 2000).

ICHNOSTRATIGRAPHY OF TRACE FOSSILS IN SOILS

425

Brown 1934; Uruguay Roselli 1939; Palmiraichnus Roselli 1987; Ellipsoideichnus Roselli 1987; Burrows associated with fossil bee cells are Rosellichnus Genise & Bown 1996; Corimbatichuncommon (e.g. Cellicalichnus and Ellipsoideich- nus Genise & Verde 2000; and Cellicalichnus nus) (Genise 2000), and the same is true for Genise 2000. These ichnogenera can be recogcoleopteran trace fossils. The coleopteran trace nized by the presence of cells having rounded fossil Pallichnus is the single exception: it shows bases and flat tops, which commonly bear a an associated burrow system, albeit only obser- spiral closure. Cells may be isolated or clustered, vable microscopically (Retallack 1984). The and may be connected by tunnels of similar diapoor preservation of burrows in fossil nests of meters. solitary insects may be a result of the absence The second group comprises the new ichnoof constructed or lined burrow walls, in contrast family Coprinisphaeridae introduced herein, to those of brood and pupation cells (Genise & and includes unnamed trace fossils and spherical, Bown 1994a). pear-shaped or ovoid ichnogenera with active or The burrows within nests of solitary insects passive fill and constructed walls. Ichnogenera have a constant diameter that corresponds to included are: Fontanai Roselli 1939; Teisseirei the body diameter of their constructor, as in Roselli 1939; Coprinisphaera Sauer 1955; Moneother invertebrate trace fossils (Ekdale et al sichnus Roselli 1987; Rebuffoichnus Roselli 1984). The situation is different in ant and ter- 1987; and Eatonichnus Bown et al. 1997. These mite nests, whose burrow systems result from ichnogenera are interpreted as the brood the cooperative work of many. Burrows are com- masses of dung-beetles or pupation chambers monly very complex, and burrow diameter is of various coleopteran taxa. variable. In many cases, larger burrows (first The third group, also representing trace fossils order) are connected with medium-sized ones attributed to pupation chambers and brood (second order) and smaller ones (third order), masses of beetles, consists of ichnofossils lacking the latter representing individual passages that a constructed wall. Such ichnotaxa are included match the size of the workers (Grasse 1984; in the new ichnofamily Pallichnidae, and include: Sands 1987; Genise & Bown 1994b) (Fig. 6c, Scaphichnium Bown & Kraus 1983; Pallichnus d). Even when individual chambers are similar Retallack 1984; and Fictovichnus Johnston et al. to those constructed by coleopterans, the asso- 1996. Pallichnus shows two features that distinciated burrow systems clearly distinguish the guish it from somewhat similar coprinisphaerid constructions of social from solitary insects. ichnogenera. The Pallichnidae lack a discrete The burrow systems may: constructed wall. This fact suggested to Retalarise from the base of isolated chambers (e.g. lack (1984) that these trace fossils would probably represent pupation cells of Geotrupinae or Tacuruichnus)\ be present in the wall or in the interior of Scarabaeinae rather than brood masses. Also, chambers (e.g. Tacuruichnus, Termitichnus the chambers are connected to lateral tunnels that radiate from a vertical shaft, another characqatranii); or connect different chambers (e.g. Krausichnus, ter that is absent in Coprinisphaeridae. FictovichTermitichnus, Vondrichnus, Fleaglellius, Arche- nus resembles Pallichnus in having a lined wall oentomichnus) (Genise & Bown 1994b; Hasio- defined on its inner surface by a clay layer (Retallack 1984; Johnston et al. 1996). Fictovichnus has tis & Dubiel 1995a; Genise 1997). also been attributed to pupation chambers of This ichnotaxobase distinguishes ant and termite beetles (Johnston et al. 1996). Scaphichnium nests from others and enables them to be classi- lacks a lined or constructed wall, and its characfied together in a suprageneric group. teristic hamate or lunate shape with a bulbous termination is very different from both Pallichnus and Fictovichnus. Its inclusion in Pallichnidae is Ichnofamilies of palaeosol trace fossils tentative. The meniscate filling of Scaphichnium probably reflects provisioning organic matter attributed to insects inside the excavated chambers for rearing Insect trace fossils in palaeosols can be grouped larvae, as seen in modern examples of the Geointo four morphological groups or ichnofamilies trupinae (Hasiotis et al. 1993). based on the ichnotaxobases described above. Some internal casts of Coprinisphaeridae may The first ichnogeneric group comprises the ich- be indistinguishable from Pallichnidae. Such is nofamily Celliformidae, erected by Genise the case for internal casts of Rebuffoichnus (2000), and includes the following ichnogenera, resembling Fictovichnus (Fig. 3e), as well as interall of which are attributed to bees: Celliforma nal casts of Coprinisphaera, which may resemble

Burrow system

426

J. F. GENISE

Key to separate ichnofamilies of chambered trace fossils in palaeosols 1 2

3

Trace fossils composed of cells having rounded bases and flat tops that commonly bear a spiral closure Celliformidae Genise Trace fossils showing another combination of characters 2 Trace fossils composed of: (a) chambers of different shapes interconnected by a boxwork showing burrows of different diameters; or (b) isolated chambers bearing an internal boxwork; and/or (c) chambers from which a burrow system radiates. Intersecting grooves, scratch and scrape markings absent Krausichnidae ifam. nov. Trace fossils showing another combination of characters 3 Trace fossils composed of isolated or clustered, spherical, pear-shaped or ovoid chambers, surrounded by a discrete, constructed wall Coprinisphaeridae ifam. nov. Trace fossils composed of spherical, ovoid, hamate, or lunate chambers lacking a constructed wall Pallichnidae ifam. nov.

Pallichnus. However, internal casts are commonly associated with complete specimens in outcrop, leaving no doubt as to the identity of the trace fossils (e.g. Genise et al. 2002a). Ovoid trace fossils included in the Coprinisphaeridae and Pallichnidae may commonly show a broken end, or may be perforated or flattened following the emergence of the adult insect. In these cases, specimens may take a celliformid-like shape (e.g. Edwards et al. 1998). However, the flattened end lacks the spiral design as of well-preserved Celliformidae. The fourth group, the new ichnofamily Krausichnidae, consists of trace fossils composed either of chambers of different shapes interconnected by burrow systems of different diameters, or of isolated chambers associated with burrow systems of different diameters. Ichnogenera include: Attaichnus Laza 1982; Termitichnus Bown 1982; Syntermesichnus Bown & Laza 1990; Krausichnus Genise & Bown 1994b; Fleaglellius Genise & Bown 1994b; Vondrichnus Genise & Bown 1994b; Archeoentomichnus Hasiotis & Dubiel 1995a; Parowanichnus Bown et al. 1997; and Tacuruichnus Genise 1997. These ichnogenera are attributed to the work of social insects, namely ants and termites. The presence of tunnels of different diameters in the same structure is the distinctive feature for recognition of traces constructed by the cooperative work of specialized types of individual (of differing sizes) within a society (Genise 1997). A recently described but unnamed trace fossil from the Pleistocene of Chad (Duringer et al. 2000a, 2000b) deserves further comment. The trace fossil represents several specimens of Coprinisphaera connected by a tunnel system. As such, it could be seen as a link between Coprinisphaeridae and Krausichnidae that would almost preclude their separation. Such structures are unknown among dung-beetle traces, a fact that originally suggested to Duringer et al. (2000a, 2000b) that specimens of Coprinisphaera and

the interconnecting tunnels were constructed by different trace-makers (Duringer et al. 2000b). Later, the same authors recognized termites as possible constructors of these tunnel systems (Duringer et al. in press). The traces are thus a composite trace fossil (sensu Pickerill 1994) in which the termites exploit food reserves intended for larvae of Coleoptera. Systematic ichnology

Ichnofamily Coprinisphaeridae ifam. nov. Type ichnogenus. Coprinisphaera Sauer 1955. Diagnosis. Trace fossils consist of spherical, subspherical, pear-shaped, ovoid, or sub-ovoid chambers, generally isolated, rarely clustered. Chambers are surrounded by a discrete constructed wall, which may show a circular or ovoid hole. Some ichnogenera show empty or passively filled chambers, whereas in others active infill is the norm. Ichnogenus Fontanai Roselli 1939 (Fig. 2c) v*1939 Fontanai Roselli p. 79, figs 23, 24, 31(8). p. 1975 'Nidos fosiles petreos de Coleopteros' Francis p. 553. 1976 Fontanaichnus Roselli p. 167 (junior synonym), p. 1981 'Nidos fosiles de Coleopteros' Sprechmann, Bossi and Da Silva p. 266. 1982 Fontanaichnus Roselli; Martinez p. 58. v!987 Fontanaichnus Roselli; Roselli p. 34, pi. II, fig. 6; pi. Ill, fig. 5. p. 1988 'Nidos de insectos fosiles (coleopteros)' Ford p. 47. 1990a Fontanaichnus Roselli; Retallack, p. 219, figs 201 A,B. 1993 Fontanai Roselli; Genise p. 53. 1994 Fontanaichnus Roselli; Donovan p. 209, figs 8.5 A, B. 1994a Fontanai Roselli; Genise & Bown p. 112.

ICHNOSTRATIGRAPHY OF TRACE FOSSILS IN SOILS

427

Key to ichnogenera within Coprinisphaeridae 1 2 3 4 5

Spherical, subspherical or pear-shaped chambers 2 Ovoid or sub-ovoid chambers 3 Spherical chambers showing a raised rim surrounding the emergence hole Fontanai Roselli Spherical, subspherical or pear-shaped chambers lacking a raised rim to the aperture Coprinisphaera Sauer Ovoid structures, external wall showing a helical design Eatonichnus Bown et al. Ovoid structures showing smooth or rugose outer wall 4 Chambers showing meniscateMonesichnus Roselli Empty or passively filled chambers 5 Chambers having a depressed outline in cross-section; inner surface may show scratch marks; some specimens show a short tunnel at the entrance or antechamber Teisseirei Roselli Chambers having rounded outline in cross-section Rebuffoichnus Roselli

1996 Fontanai Roselli; Johnston, Eberth & Anderson p. 522. 1998 Fontanai Roselli; Genise & Laza p. 213. 1998 Fontanaichnus Roselli; Buatois, Mangano, Genise & Taylor p. 226. 2000 Fontanai Roselli; Genise, Mangano, Buatois, Laza & Verde p. 54. 2000 Fontanai Roselli; Krell p. 891. 2002 Fontanaichnus Roselli; Buatois, Mangano & Acenolaza p. 189. Type and only known ichnospecies. Fontanai kraglievichi Roselli 1939. Diagnosis. Spherical chambers having a thick constructed wall and an emergence hole surrounded by a raised rim or neck. Remarks. This ichnogenus is pending the ichnotaxonomic revision along with the closely related Coprinisphaera (Laza, personal communication). Possible trace-makers are dungbeetles (Scarabaeinae). The presence of a neck is the only feature that allows the separation of Fontanai from Coprinisphaera. The neck may be interpreted as the remains of a separate egg chamber that some dung-beetles construct over the provision chamber in their brood masses (Halffter & Matthews 1966). However, if the neck is interpreted as the remains of a former egg chamber, it would be possible to trace a continuous morphological series between both ichnogenera. Ichnogenus Coprinisphaera Sauer 1955 (Fig. 2a, b) v!938a 'Bolas de escarabeidos' Frenguelli p. 348. v!938b 'Pallottole di Scarabeidi' Frenguelli p. 77, fig. 5; pi. VII, figs 1-8. v!939 Devincenzia Roselli p. 81, figs 26, 27, 28 and 31 (5-6) (non Kraglievich 1932). v!939a 'Nidos fosiles de Escarabeidos' Frenguelli p. 270. v!939b 'Nidos fosiles de Escarabeidos' Frenguelli p. 379, figs 4^-9, pis I-II. v!940 'Nidos fosiles de Escarabeidos' Frenguelli p. 70.

v!941 'Nidos de escarabeidos' Frenguelli p. 87. 1950 'Bolas de Cangagua' Bruet p. 280, pi. I, figs 2,3. *1955 Coprinisphaera Sauer p. 123, figs 1-5. 1955 Cangabola Lengerken p. 937, figs 6-8. 1956 Coprinisphaera Sauer; Sauer p. 550, figs 1—4. 1959 'Nidos de Scarabaeidae' Halffter p. 174. 1959 Coprinisphaera Sauer; Sauer p. 119. 1962 Coprinisphaera Sauer; Hantzschel W189. 1966 'Fossil scarab balls' Halffter & Matthews p. 154. 1966 'Nidos fosiles de escarabeidos' Camacho p. 490, pi. XVI figs n, o. 1972 'Nidos de escarabajos' Andreis p. 91. p. 1975 'Nidos fosiles petreos de Coleopteros' Francis p. 553. 1975 Coprinisphaera Sauer; Hantzschel p. W52. 1976 Devincenzichnus Roselli p. 167 (junior synonym). 1977 'Paleonidos de escarabeidos' Spalletti & Mazzoni p. 267. 1981 'Nidos de escarabeidos' Pascual & Bondesio p. 125. 1981 'Nidos de escarabajos' Andreis p. 34. p. 1981 'Nidos fosiles de Coleopteros' Sprechmann, Bossi & Da Silva p. 266. 1982 'Nidos de escarabeidos' Alonso, Gonzalez & Pelayes p. 2. 1982 Coprinisphaera Sauer; Martinez p. 48. 1982 Devincenzichnus Roselli; Martinez p. 48. 1986a 'Icnofosiles de Scarabaeinae' Laza p. 13. 1986b 'Nidos de Scarabaeinae' Laza p. 19. 1987 'Heliocopris dung ball' Sands p. 423. v!987 Devizenzichnus Roselli, Roselli p. 21, pi. I, fig. 1. *v!987 Martinezichnus Roselli p. 22, pi. I, figs 2, 2a (new synonym). *v!987 Madinaichnus Roselli p. 23, pi. I, fig. 3 (new synonym). *v!987 Microicoichnus Roselli p. 49, pi. I, fig. 8 (new synonym). p. 1988 'Nidos de coleopteros' Ford p. 47. v!990 'Nidos de escarabeidos' Laza & Reguero p. 245.

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Fig. 2. Coprinisphaeridae. (a) Coprinisphaera ecuadoriensis Sauer 1955: thick constructed wall, empty chamber, and emergence hole. Eocene-Miocene Sarmiento Formation, Argentina; MACN-LI1802. Bar: 1 cm. (MACNLI, Museo Argentine de Ciencias Naturales, Laboratorio de Icnologia.) (b) Holotypes of Devincenzichnus murguiai Roselli 1939 (left) (MFLR 479), Martinezichnus francisi Roselli 1987 (centre) (MFLR607), and Madinaichnus larranagai Roselli 1987 (right) (MFLR 641). Note the almost continuous range of ball and hole sizes. Late Cretaceous-Early Tertiary Asencio Formation, Uruguay. Bar: 1 cm. (MFLR: Museo Francisco Lucas Roselli, Nueva Palmira, Uruguay.) (c) Fontanai kraglievichi Roselli 1939: raised rim around the emergence hole. Late Cretaceous-early Tertiary Asencio Formation, Uruguay; MACN-LI1786. Bar: 1 cm. (d) Eatonichnus utahensis (Guilliland and La Rocque, 1952). Holotype: external bioglyph and internal chamber with meniscate filling. Palaeocene Colter Formation, USA; OM20201. Bar: 1 cm. (Photograph courtesy of T. Bown.) (OM: Ohio State University Orton Geological Museum.)

1990a Coprinisphaera Sauer; Retallack p. 219, fig. 201 G-I. 1990a Devincenzichnus Roselli; Retallack p. 219, fig. 201 C, D.

1990b 'Dung beetle trace fossils' Retallack p. 436. 1991 Coprinisphaera Sauer; Retallack p. 182. 1993 Coprinisphaera Sauer; Genise p. 50.

ICHNOSTRATIGRAPHY OF TRACE FOSSILS IN SOILS

1993 Devincenzichnus Roselli; Genise p. 50. 1994 Coprinisphaera Sauer; Donovan p. 209, fig. 8.5 G-I. 1994 Devincenzichnus Roselli; Donovan p. 209, fig. 8.5 C, D. 1994a Coprinisphaera Sauer; Genise & Bown, p. 109, figs 3, 4. 1995 Coprinisphaera Sauer; Genise & Cladera p. 78, figs IE, Fig. IF, left and right. 1995 Martinezichnus Roselli; Genise & Cladera p. 78, figs IE, Fig. IF, centre. 1995 'Nidos de escarabajos' Fontaine; Ballesteros & Powell p. 12. 1996 'Nidos de escarabajos' Iriondo & Krohling p. 43. 1996 Devincenzichnus Roselli; Veroslavsky & Martinez p. 41, pi. I, fig. 5. 1996 Devincenzichnus Roselli; Johnston, Eberth & Anderson p. 522. 1996 Coprinisphaera Sauer; Johnston, Eberth & Anderson p. 522. 1998 Devincenzichnus Roselli; Buatois, Mangano, Genise & Taylor p. 226. 1998 Martinezichnus Roselli; Buatois, Mangano, Genise & Taylor p. 226. 1998 Madinaichnus Roselli; Buatois, Mangano, Genise & Taylor p. 226. 1998 Microicoichnus Roselli; Buatois, Mangano, Genise & Taylor p. 226. 1998 Coprinisphaera Sauer; Buatois, Mangano, Genise & Taylor p. 227. 1998 Microicoichnus Roselli; Genise & Laza p. 213. 1998 Madinaichnus Roselli; Genise & Laza p. 213. 1998 Martinezichnus Roselli; Genise & Laza p. 213. 1998 Devincenzichnus Roselli; Genise & Laza p. 213. 1998 Coprinisphaera Sauer; Genise & Laza p. 220. 1999 Coprinisphaera Sauer; Genise p. 110. 1999 Martinezichnus Roselli; Genise p. 110. 1999 Madinaichnus Roselli; Genise p. 110. 1999 Microicoichnus Roselli; Genise p. 110. 2000 Coprinisphaera Sauer; Genise, Mangano, Buatois, Laza & Verde p. 54. 2000 Martinezichnus Roselli; Genise, Mangano, Buatois, Laza & Verde p. 54. 2000 Madinaichnus Roselli; Genise, Mangano, Buatois, Laza & Verde p. 54. 2000 Microicoichnus Roselli; Genise, Mangano, Buatois, Laza & Verde p. 54. 2000 Coprinisphaera Sauer; Krell p. 890. 2000 Microicoichnus Roselli; Krell p. 891. 2000 Madinaichnus Roselli; Krell p. 891. 2000 Martinezichnus Roselli; Krell p. 891. 2000a 'Fossil dung-beetle brood ball' Duringer, Brunet, Cambefort, Likius, Mackaye, Schuster & Vignaud p. 277, figs 3-6.

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2000b 'Boules de bousiers fossiles' Duringer, Brunet, Cambefort, Beauvilain, Mackaye, Vignaud & Schuster p. 259, figs 1-10. 2000 'Boules-nids fossiles de bousiers' Schuster, Duringer, Nel, Brunet, Vignaud & Mackaye p. 17. 200la Coprinisphaera Sauer; Retallack p. 142. 200la Devincenzichnus Roselli; Retallack p. 142. 200la Martinezichnus Roselli; Retallack p. 142. 200la Madinaichnus Roselli; Retallack p. 142. 2002 Martinezichnus Roselli; Buatois, Mangano & Acenolaza p. 22. 2002 Microicoichnus Roselli; Buatois, Mangano & Acenolaza p. 189. 2002 Madinaichnus Roselli; Buatois, Mangano & Acenolaza p. 189. 2002 Devincenzichnus Roselli; Buatois, Mangano & Acenolaza p. 189. 2002 Coprinisphaera Sauer. Buatois, Mangano & Acenolaza p. 189. Type ichnospecies. Coprinisphaera ecuadoriensis Sauer 1955. Diagnosis. Spherical, subspherical and pearshaped chambers having a constructed wall. Some specimens may show a hole (possibly an emergence hole). Internal cavities empty or containing a meniscate or passive fill. Included ichnospecies. Coprinisphaera murgiai Roselli 1939; C. francisi Roselli 1987; C. larranagai Roselli 1987; C. lafurcadai Roselli 1987; C. ecuadoriensis Sauer 1955; C. frenguellii Genise & Bown 1994a. Remarks. Coprinisphaera is one of the most common trace fossils in the Tertiary palaeosols of South America, and it was appropriately one of the first recorded insect trace fossils (Frenguelli 1938a; Roselli 1939). It has subsequently been described from different localities and ages of South and Central America, Asia, Antarctica and Africa (Genise et al 2000; Krell 2000 and references therein). The mention of 'nidos de escarabajos o escarabeidos' [nests of dung-beetles or scarabs] is common in the Argentinean sedimentological and palaeontological literature (e.g. Andreis 1972, 1981; Spalletti & Mazzoni 1977; Pascual & Bondesio 1981; Alonso et al. 1982; Laza 1986a, 1986b; Laza & Reguero 1990; Fontaine et al. 1995; Iriondo & Krohling 1996). In these cases, it is clear that the authors recognized the typical spherical or pear-shaped forms described by Frenguelli (1938b, 1939b), and as such these occurrences are attributed herein to Coprinisphaera ispp. The trace fossils described by Frenguelli (1938b, 1939b) are the same as those recorded by HahTter (1959), Camacho (1966) and Halffter & Matthews (1966). In turn, Uruguayan sedimentologists (e.g. Francis 1975; Sprechmann et al. 1981; Ford 1988)

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mentioned coleopteran nests described by Roselli (1939) which may be assigned indeterminately to Coprinisphaera or Fontanai. Roselli (1987) created Martinezichnus, Madinaichnus and Microicoichnus based on the different sizes of the chambers and the sizes of emergence holes, but these ichnotaxobases show significant overlap, precluding any clearcut distinction between the different ichnogenera (Fig. 2b). They are formally considered herein as synonyms following Genise (1999). The probable trace-makers are dung-beetles (Scarabaeinae). Ichnogenus Eatonichnus Bown et al. 1997 (Fig. 2d) v*1997 Eatonichnus Bown, Hasiotis, Genise, Maldonado & Browers p. 52, figs 8C-F, 9. 1998 Eatonichnus Bown, Hasiotis, Genise, Maldonado and Browers; Genise & Laza p. 214. 1999 Eatonichnus Bown, Hasiotis, Genise, Maldonado & Browers; Genise p. 111. 2000 Eatonichnus Bown, Hasiotis, Genise, Maldonado & Browers; Genise, Mangano, Buatois, Laza & Verde p. 54. 2000 Eatonichnus Bown, Hasiotis, Genise, Maldonado & Browers; Krell p. 891. v2001 Eatonichnus Bown, Hasiotis, Genise, Maldonado & Browers; Genise, Cladera & Tancoff p. 45. Type ichnospecies. Eatonichnus utahensis (Gilliland & La Rocque 1952). Diagnosis. Trace fossils composed of closely appressed whorls, tightly spiralled around a central cylindrical cavity and converging terminally, thereby forming a closed, spindle-shaped helix. Helices may be either sinistral or dextral, with whorls inclined away from the transverse axis. Whorl diameter constant along the helix and invariably greater than the diameter of the central cavity. Central cavity fill closely packed, with or without meniscae (Bown et al. 1997). Included ichnospecies. E. utahensis (Gilliland & La Rocque 1952); E. claronensis Bown et al. 1997. Remarks. The helical design of the outer wall is unique in Coprinisphaeridae, but its ovoid shape and meniscate fillings relate it to the unpatterned Monesichnus. The distinction between the two named ichnospecies of Eatonichnus is based on their very dissimilar sizes, whorl inclination, cavity fill and external ornamentation. A third, unnamed, ichnospecies has also been described based on its larger size and external bioglyph (Bown et al. 1997). Later, Genise et al. (2001) recorded Argentinean specimens of E. claronensis that show intermediate sizes, suggesting that size may be a misleading ichnotaxobase for this

group. Possible trace-makers are dung-beetles (Scarabaeinae) (Bown et al. 1997). The affinity with dung-beetle brood masses is suggested by its shape and meniscate fillings, similar to the ichnogenus Monesichnus. In addition, some modern dung-beetles are known to excavate helical tunnels (Bown et al. 1997). However, the producer of this trace fossil is unknown because there are no modern analogues for this structure. Ichnogenus Monesichnus Roselli 1987 (Fig. 3a) v*1987 Monesichnus Roselli p. 39, pi. I, fig. 7. v!994 Monesichnus Roselli; Laza, Genise & Bown p. 397. 1997 Monesichnus Roselli; Bown, Hasiotis, Genise, Maldonado & Browers p. 52. 1998 Monesichnus Roselli; Buatois, Mangano, Genise & Taylor p. 226. v!998 Monesichnus Roselli; Genise and Laza p. 213, figs 3-5. 1999 Monesichnus Roselli; Genise p. 110. 2000 Monesichnus Roselli; Genise, Mangano, Buatois, Laza & Verde p. 54. 2000 Monesichnus Roselli; Krell p. 891. 2001 Monesichnus Roselli; Retallack p. 142. 2002 Monesichnus Roselli; Buatois, Mangano & Acenolaza p. 189. Type and only known ichnospecies. Monesichnus ameghinoi Roselli 1987. Diagnosis. Discrete structures, fusiform to ovate, composed of a constructed, unpatterned wall (sometimes showing a longitudinal groove), and an internal cavity that in some cases is empty and in others exhibits a meniscate fill (Genise & Laza 1998). Remarks. Monesichnus is morphologically similar to Eatonichnus, but its unpatterned wall clearly distinguishes it from the latter. Its meniscate fill, interpreted as the provisions of a dung-beetle brood mass (Fig. 3a), distinguishes it from other ichnogenera such as Teisseirei andebuffoichnus, which are interpreted as pupation chambers. Some broken specimens of Monesichnus may also be empty, their fills probably lost by weathering. Even in these cases, the rounded cross-section of the chamber distinguishes it from Teisseirei, and the absence of a rounded emergence hole precludes its assignment to Rebuffoichnus.These structures are similar to brood masses of the representatives of the modern dung-beetle genera Dichotomius, Gromphas and Oruscatus (Scarabaeinae) (Genise & Laza 1998). Ichnogenus Teisseirei Roselli 1939 (Fig. 3b, c) v*1939 Teisseirei Roselli p. 82, figs. 29, 30, 31(7). 1976 Tesseirichnus Roselli p. 167 (junior synonym). 1982 Tesseirichnus Roselli; Martinez p. 61.

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Fig. 3. Coprinisphaeridae. (a) Monesichnus ameghinoi Roselli 1987 (left): wall and meniscate filling. Late Cretaceous-Early Tertiary Asencio Formation, Uruguay; MACN-LI233. A sectioned sample of a brood mass of the dung-beetle Oruscatus davus (Scarabaeinae) (right) showing the same type of wall and fillings. Bar: 1 cm. (b) Teisseirei barattinia Roselli 1939: antechamber and bioglyph in the chamber wall; included in a piece of rock matrix. Late Cretaceous-Early Tertiary Asencio Formation, Uruguay; MACN-LI876. Bar: 1 cm. (c) T. barattinia Roselli 1939: cross-section showing the constructed wall and the depressed outline of the chamber. Eocene-Miocene Sarmiento Formation, Argentina; MPEF-IC 253. Bar: 1 cm. (MPEF-IC: Museo Paleontologico Egidio Feruglio-Icnologia). (d) Rebuffoichnus casamiquelai Roselli 1987. Late Cretaceous Laguna Palacios Formation, Argentina; MACN-LI1202. Bar: 1 cm. (e) R. casamiquelai: internal cast with remains of the outer wall. Late Cretaceous Laguna Palacios Formation, Argentina; MACN-LI1181. Bar: 1 cm. v!987 TeisserichnusRoselli p. 24, pi. I, fig. 5 (lapsus). v!987 Isociesichnus Roselli p. 38, pi. II, fig. 5 (new synonym). 1990a Teisseirichnus Roselli; Retallack p. 219. 1993 Teisseirei Roselli; Genise p. 53.

1996 Teisseirei Roselli; Veroslavsky and Martinez p. 41, pi. I, fig. 4. 1998 Teisseirei Roselli; Buatois, Mangano, Genise & Taylor p. 226. 1998 Isociesichnus Roselli; Buatois, Mangano, Genise & Taylor p. 226.

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1998 Teisseirei Roselli; Genise & Laza p. 213. 1998 Isociesichnus Roselli; Genise & Laza p. 213. 2000 Teisseirei Roselli; Genise, Mangano, Buatois, Laza & Verde p. 54. v2001 Teisseirei Roselli; Genise & Zelich p. 44. 2001 Teisseirei Roselli; Retallack p. 142. v2002 Teisseirei Roselli; Melchor, Genise & Miquel p. 35, fig. 12 A-E, I. Type and only known ichnospecies. Teisseirei barattinia Roselli 1939. Diagnosis. Depressed chambers slightly arched downwards and having constructed walls. Some specimens may show a small, rounded antechamber. Inner surface of the wall displays, in well-preserved specimens, a distinct lining bearing small elliptical scratches oriented mostly longitudinally (modified from Melchor et al 2002). Remarks. The internal bioglyph (Fig. 3b) and the depressed cross-section (Fig. 3c) distinguish this ichnogenus from other Coprinisphaeridae. These trace fossils commonly show three preservational morphologies: (1) empty or passively filled chambers found in situ in palaeosols; (2) isolated, detached clasts composed of the chamber fillings; or (3) empty or passively filled chambers surrounded by a thick wall. Specimens found in situ in palaeosols of the Asencio Formation suggested that these chambers were merely excavated structures (Genise 1999). However, the recent finding of hundreds of specimens bearing constructed walls in the Tertiary of Patagonia and re-examination of previous discoveries suggests that Teisseirei barattinia has a constructed wall. The holotype (and only documented specimen) of Isociesichnus diplocamara Roselli 1987 is actually a structure composed of two attached specimens of Teisseirei barattinia. Thus it is regarded herein as a junior synonym of Teisseirei. Despite the excellent preservation of many specimens that display an internal bioglyph showing the scratches of the constructors, the origin of Teisseirei is unknown. Its shape and the absence of active fill indicate the lack of original provisions, and suggest that it is probably a coleopteran pupation chamber. Ichnogenus Rebuffoichnus Roselli 1987 (Fig. 3d, e) 1925 'Calcareous insect puparia' Lea p. 35, pi. I, figs1-20. v* 1987 Rebuffoichnus Roselli p. 24, pi. I, fig. 4; pi. Ill, fig. 4. p. 1996 Fictovichnus Johnston, Eberth and Anderson p. 516, figs 2, 3C-D. 1998 RebuffoichnusRoselli; Genise and Laza p. 213. 1998 Rebuffoichnus Roselli; Buatois, Mangano, Genise & Taylor p. 226.

v!999 Rebuffoichnus Roselli; Genise, Sciutto, Laza, Gonzalez & Bellosi p. 29. 2000 Rebuffoichnus Roselli; Krell p. 892. 2000 Rebuffoichnus Roselli; Genise, Mangano, Buatois, Laza & Verde p. 54. 200la Rebuffoichnus Roselli; Retallack p. 142. v2002 Rebuffoichnus Roselli; Genise, Sciutto, Laza, Gonzalez & Bellosi p. 230, figs 5D, 7. 2002 Rebuffoichnus Roselli; Buatois, Mangano & Acenolaza p. 189. v2002 RebuffoichnuRoselli: Genise, Laza, Fernandez & Frogoni p. 160. Type and only known ichnospecies. Rebuffoichnus casamiquelai Roselli 1987. Diagnosis. Sub-ovoid to subcylindrical structures composed of a wall, whose exterior aspect is rugose and lumpy, whereas the interior is smooth or showing a faint bioglyph. The internal chamber is ovoid and has a circular cross-section. The wall may be perforated by a rounded hole (Genise et al. 2002b). Remarks. Rebuffoichnusdiffers from other more or less ovoid insect trace fossils, such as Monesichnus, in lacking any active fill and in the presence of a rounded hole. Trace fossils from the Quaternary of Australia described by Lea (1925) and Read (1974) were assigned to an unnamed ichnospecies of Fictovichnus by Johnston et al. (1996). However, the type ichnospecies of Fictovichnus lacks a constructed wall as seen in the material of Lea (1925) and Read (1974). These specimens have a general aspect and similar features to those of Rebuffoichnus casamiquelai (Genise et al. 2002b). R. casamiquelai is attributed to pupation chambers of Curculionidae, Scarabaeidae, or Tenebrionidae (Johnston et al. 1996). However, species of the Curculionidae are the most likely constructors because of the circular body outline, which would accord with the circular holes in the trace fossils (Genise et al. 2002b). This assumption is also confirmed by the discovery of the constructor within a Rebuffoichnufrom Australia (Lea 1925).

Ichnofamily Pallichnidae ifam. nov. Type ichnogenus. Pallichnus Retallack 1984. Diagnosis. Group of ichnogenera comprising subspherical, ovoid, hamate or lunate chambers lacking a constructed wall. Chambers are surrounded by linings or diagenetic haloes. Fillings may be passive or meniscate. Remarks. Scaphichnium, the first named ichnogenus of the ichnofamily (Bown & Kraus 1983), shows peculiar characters that make its inclusion in the Pallichnidae tentative. Pallichnus Retallack

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433

Key to identify the ichnogenera of Pallichnidae 1 2

Hamate to lunate with bulbous termination, unlined walls, meniscate filling Scaphichnium Bown and Kraus Spherical or ovoid shape, lined walls, empty chambers 2 Spherical shape Pallichnus Retallack Ovoid shape Fictovichnus Johnston et al.

1984 is herein designated as the type ichnogenus. The presence of associated tunnels as found in Pallichnus is an unusual trait for this ichnofamily and is therefore not considered as diagnostic. Apart from the known ichnogenera, many unnamed ovoid-shaped trace fossils that are devoid of constructed walls may be included in this ichnofamily, including cocoons (e.g. Bown et al. 1997), pseudoeggs (e.g. Hirsch 1994a), misidentified eggs (e.g. Johnston et al. 1996) and pupation chambers (e.g. Edwards et al. 1998). Ichnogenus Scaphichnium Bown & Kraus 1983 (Fig. 4a) *1983 Scaphichnium Bown & Kraus p. 106, figs 4C, 5E-G, 6C, E, 9B, D. 1993 Scaphichnium Bown & Kraus; Hasiotis, Asian & Bown p. 2, figs 2, 4. 1998 Scaphichnium Bown & Kraus; Genise & Lazap. 214. 1998 Scaphichnium Bown & Kraus; Buatois, Mangano, Genise & Taylor p. 227. 1999 Scaphichnium Bown & Kraus; Genise p. 110. 2000 Scaphichnium Bown & Kraus; Genise, Mangano, Buatois, Laza & Verde p. 54. 2000 Scaphichnium Bown & Kraus; Krell p. 892. Type and only known ichnospecies. Scaphichnium hamatum Bown & Kraus 1983. Diagnosis. Discrete, hook-shaped to lunate, meniscate endostratal burrow fills, oriented with long axis vertical to concave upward and with rounded, bulbous, lower (distal) terminations (Bown & Kraus 1983). Remarks. Scaphichnium hamatum is an unusual insect trace fossil in many aspects. Its particular shape, meniscate fill and lack of a lined or constructed wall confer to this structure a very distinctive architecture unknown in other trace fossils. Consequently, its inclusion in Pallichnidae is tentative. These trace fossils are attributed to brood masses of Scarabaeinae beetles, possibly Geotrupinae (Hasiotis et al. 1993). Ichnogenus Fictovichnus Johnston et al. 1996 (Fig. 4c) 71982 'Ovoid vesicles' Freytet & Plaziat p. 65, pi. 49, figs G, H. 71987 'Cocoons' Ritchie p. 435, pi. 11.14, figs 9, 10. 71994 'Cocoons' Thackray p. 796.

71994 'Egglike concretions' '? lizard eggs' Mikhailov, Sabath & Kurzanov p. 106, figs 7.16D-F, 7.20. ?1994a 'Pseudoeggs' Hirsch p. 281, fig. 11.3A. ?1994b 'Pseudoeggs' Hirsch p. 145, fig. 10-6F. 71996 Celliforma sp. Veroslavsky & Martinez p. 41, pi. I, fig. 2. v!996 'Nodulos ovoidales' Sciutto & Martinez p. 74. * 1996 Fictovichnus Johnston, Eberth & Anderson p. 521, figs l,3A-Band4. v!997 'Wasp traces' Bown, Hasiotis, Genise, Maldonado & Browers p. 48, figs 6A, C-E, 8A-B. v!998 'Cocoon-like trace fossils' Edwards, Jarzembowski, Pain & Daley p. 25, figs 3-6. 1999 Fictovichnus Johnston, Eberth & Anderson; Genise, Sciutto, Laza, Gonzalez & Bellosi p. 29. 2000 Fictovichnus Johnston, Eberth & Anderson; Genise, Mangano, Buatois, Laza & Verde p. 54. 2002 Fictovichnus Johnston, Eberth & Anderson; Genise, Sciutto, Laza, Gonzalez & Bellosi p. 230. v2002 'Cocoons' Melchor, Genise & Miquel p. 24, fig. 12G-H. Type ichnospecies. Fictovichnus gobiensis Johnston et al. 1996. Diagnosis. Ellipsoid chambers enveloped by thin clay-rich zone, the inner surface of which is clearly defined and smooth. The outer surface of the clay-rich zone is gradational with the surrounding matrix. The long axis of the chamber is parallel to bedding, with a terminal exit hole that may be subterminal or medial (on the upper surface of the chamber relative to bedding). Where the ichnogenus occurs in calcretized soils, the chamber is commonly surrounded by variably developed calcite halo. Trace fossils occur as hollow structures with a wall formed of a clay-rich zone and calcite halo, or as eggshaped internal moulds, depending on local calcretization and induration of host sediment (Johnston et al. 1996). Included ichnospecies. F. gobiensis Johnston et al. 1996 (=parvus Johnston et al. 1996 syn. nov.). Remarks. Both named ichnospecies of Fictovichnus - F. gobiensis and F. parvus - were diagnosed on the basis of differing size, probably reflecting the fact that they were made by different species of insect. However, the potential

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Fig. 4. Schematic drawings of Pallichnidae and Krausichnidae described by Bown & Kraus (1983), Retallack (1984), Johnston et al (1996), Bown et al. (1997), and Hasiotis & Dubiel (1995a). (a) Scaphichnium hamatum Bown & Kraus 1983: Lower Eocene Willwood Formation, USA. (b) Pallichnus dakotensis Retallack 1984: Oligocene Brule Formation, USA. (c) Fictovichnus gobiensis Johnston et al. 1996: Late Cretaceous Djadokhta Formation, Mongolia, (d) Parowanichnus formicoides Bown et al. 1997: Palaeocene-Eocene Claron Formation, USA. (e) Idealized reconstruction of Archeoentomichnus metapolypholeos Hasiotis & Dubiel 1995a: Late Triassic Chinle Formation, USA.

existence of trace fossils of intermediate size would confuse the ichnospecific taxonomy: thus the two ichnospecies are considered herein to be synonymous. A third, unnamed ichnospecies from the Quaternary of Australia, included in Fictovichnus by Johnston et al. (1996), is transferred to Rebuffoichnus owing to the presence of a constructed wall in the Australian material (Genise et al. 2002a). Many ovoid casts were described or mentioned as 'cocoons', 'ovoid structures' and/or illustrated

showing an ovoid shape (e.g. Freytet & Plaziat 1982; Ritchie 1987; Thackray 1994; Veroslavsky & Martinez 1996; Sciutto & Martinez 1996; Bown et al. 1997; Melchor et al. 2002). They are attributed herein tentatively to Fictovichnus ispp. In addition, Johnston et al. (1996 and references therein) mentioned a long list of doubtful vertebrate fossil eggs or pseudoeggs that are also probably attributable to Fictovichnus (e.g. Hirsch 1994a, 1994b; Mikhailov et al. 1994). However, most of these cocoons or pseudoeggs

ICHNOSTRATIGRAPHY OF TRACE FOSSILS IN SOILS

lack any evidence of the emergence hole, an important diagnostic character to definitively associate these dubiofossils with this ichnogenus. In contrast, the cocoon-like trace fossils described by Edwards et al. (1998) show evidence of an emergence hole, a complex bioglyph and, in some cases, an associated burrow, features important enough to suggest a new ichnospecies of Fictovichnus. This ichnogenus is attributed to pupation chambers of Coleoptera, probably Curculionidae, Tenebrionidae or Scarabaeidae (Johnston et al. 1996). Ichnogenus Pallichnus Retallack 1984 (Fig. 4b) *1984 Pallichnus Retallack p. 580, figs 7, 9, 10. 1990b Pallichnus Retallack; Retallack p. 221, figs 204, 206. 1993 Pallichnus Retallack; Genise p. 50. 1994b Pallichnus Retallack; Genise & Bown 109. 1996 Pallichnus Retallack; Johnston, Eberth & Anderson p. 522. 1998 Pallichnus Retallack; Buatois, Mangano, Genise & Taylor p. 227. 1999 Pallichnus Retallack; Genise p. 110. 2000 Pallichnus Retallack; Retallack, Bestland & Fremd p. 178. 2000 Pallichnus Retallack; Genise, Mangano, Buatois, Laza & Verde p. 54. 2000 Pallichnus Retallack; Krell p. 891. 2000 Pallichnus Retallack; Genise & Poire p. 7. 2002 Pallichnus Retallack; Melchor, Genise & Miquel p. 29. Type and only known ichnospecies. Pallichnus dakotensis Retallack 1984. Diagnosis. Nearly spherical chambers, defined by thin wall of dark clay and organic matter; inner boundary of wall sharp and smooth, such that the internal cast is easily separated from rock matrix. The outer boundary of the wall grades outward into the surrounding matrix. One side of the chamber is disrupted to form large, irregularly circular, exit cavity, usually about half the diameter of the main chamber. Each nearly spherical chamber is arranged at end of short branches from the vertical burrow, so that the exit cavity faces into the branch burrow; both vertical and branch burrows of slightly lesser diameter than the nearly spherical chamber (Retallack 1984). Remarks. The presence of a lined wall clearly distinguishes this trace fossil from those included in Coprinisphaeridae. The preservation of tunnels is unusual among those ichnogenera attributed to the work of solitary insects, grouped in Celliformidae, Coprinisphaeridae and Pallichnidae. Accordingly, tunnel remains in Pallichnus are detected only in thin sections

435

(Retallack 1984). These trace fossils are interpreted as pupation cells of scarabaeid beetles, particularly Geotrupinae and Scarabaeinae (Retallack 1984).

Ichnofamily Krausichnidae ifam. nov. Type ichnogenus. Krausichnus Genise & Bown 1994b. Diagnosis. Group of ichnogenera showing chambers associated with burrow systems, composed in most cases of burrows of very different diameters. Burrows are devoid of scratch marks and/or intersecting grooves. Chambers have no radiating tunnels from their upper parts and are commonly linked to a burrow system that normally interconnects them with other chambers. Chambers may be empty, passively or actively filled, and/or they may contain secondary systems of burrows of different diameters and smaller chambers on or within their inner walls. Remarks. The first described and most representative ichnogenera of this ichnofamily, Attaichnus Laza 1982 and Termitichnus Bown 1982, bear names relating to ants and termites respectively. However, names such as Termitichnidae or Attaichnidae should be avoided because Krausichnidae comprises trace fossils that can be attributed indistinctly to ants or termites, and also because trace fossil names should not make reference to possible producers (e.g. Bromley 1990). In contrast, Krausichnus shows one of the most complex morphologies of the group, and its name does not refer to the presumed constructor. Among the ichnotaxa included within the Krausichnidae, Termitichnus, Vondrichnus and Fleaglellius constitute a well-defined morphological group that may deserve the creation of an ichnofamily distinct from the Krausichnidae once they are better known. In addition to named ichnogenera, there is an extensive pedologic literature in relating to the origin of laterites, dealing with unnamed trace fossils attributed to termites, mostly from the Pliocene and Pleistocene of tropical areas (e.g. Machado 1983; Grasse 1986; Schaefer 2001). It is almost impossible to deal with the ichnotaxonomy of this material because in most cases the supposed nests were only studied micromorphologically and show no clear boundaries. Meaning that structures were only studied micromorphologically (not macromorphologically) macromorphological studies are lacking. Conversely, this kind of study is needed in already named trace fossils, and could usefully be conducted in order to evaluate the degree of reliability of the attribution of these ichnogenera to termites

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Key to the ichnogenera of Krausichnidae 1 2

3 4

5

6 7

8

Chamber walls neither constructed nor lined 2 Chamber walls constructed or lined 3 Spherical chambers having only one main vertical burrow connected to the lower part or, rarely, another one at the top of the chamber. Secondary burrows connecting the main ones and chambers at different points. The distribution of chambers is similar throughout the structure Attaichnus Laza. Oblate to subspherical chambers interconnected by burrows of similar diameter arising from top, bottom and sides of chambers. Chambers and burrows more numerous in the central part of the nest Parowanichnus Bown et al. Structures composed of tiered, tabular and flat chambers supported by pillars and ramps 4 Structures composed of subspherical, ovoid or oblate chambers 5 Tiered, tabular and flat chambers composing spindle-shaped to columnar compound chambers lacking an external, discrete, wall. Compound chambers interconnected by a burrow system and/or isolated burrows Krausichnus Genise & Bown Tiered, tabular and flat chambers composing a columnar, central structure surrounded by a discrete wall. A burrow system connects this structure to peripheral oblate chambers and possibly other columns Archeoentomichnus Hasiotis & Dubiel Structures composed of a single, large, cup-shaped chamber surrounded by a thick constructed wall bearing a boxwork of secondary burrows and chambers and a peripheral system of radiating burrows with differing diameters Tacuruichnus Genise Structures composed of a system of chambers having similar sizes 6 Chambers connected by primary burrows of similar sizes 7 Chambers connected by primary and secondary burrows 8 At most three apposed, obovate chambers. Commonly having a rind of anastomosing burrows at top and sides of chambers. Diffuse arrangement of chambers Vondrichnus Genise & Bown Two or more apposed chambers commonly forming towers. Bases and tops of chambers convex upwards without a rind of anastomosing burrows Fleaglellius Genise & Bown Spherical to subspherical chambers. Burrows, simple or compound and well differentiated from chambers, having much smaller diameters Termitichnus Bown Elongate to oblate chambers. Burrows consistently simple, in some cases comparable in size to chambers Syntermesichnus Bown & Laza

or ants. For instance, whereas Krausichnus, Termitichnus and Attaichnus leave few doubts about their termite and ant origin respectively, Tacuruichnus is known from a single specimen and requires further confirmation of its structure using more material. Vondrichnus, Fleaglellius and Parowanichnus are comparatively simple structures lacking the distinctive boxwork (sensu Ekdale et al. 1984) composed of tunnels of different diameters. Finally, Syntermesichnus and Archeoentomichnus were described from fragmentary material, from which the whole structure and ichnogeneric diagnoses were inferred. Micromorphological confirmation of the biological affinities of these trace fossils is critical, moreover, as the Triassic Archeoentomichnus predates the oldest (Cretaceous) termite body fossils by about 150 million years (Hasiotis & Dubiel 1995a) (Table 1). The taxonomy of this ichnofamily is complex because the trace fossils were described using different criteria to select taxobases and particularly because the diagnoses were strongly influenced by the architecture of modern nests of the sup-

posed constructors (e.g. Bown & Laza 1990; Laza 1995, 1997; Hasiotis & Dubiel 1995a). Ichnogenus Attaichnus Laza 1982 (Fig. 5a) v*1982 Attaichnus Laza p. 112, pis II, III. 1993 Attaichnus Laza; Genise p. 53. 1997 Attaichnus Laza; Bown, Hasiotis, Genise, Maldonado & Browers p. 45. 1998 Attaichnus Laza; Buatois, Mangano, Genise & Taylor p. 227. 1999 Attaichnus Laza; Genise p. 111. 2000 Attaichnus Laza; Genise, Buatois, Mangano, Laza 7 Verde p. 55. 2001a Attaichnus Laza; Retallack p. 143. Type and only known ichnospecies. Attaichnus kuenzelii Laza 1982. Diagnosis. System of spherical chambers interconnected by primary and secondary burrows. Primary burrows connected to the chambers vertically to the lower part, forming inside a folded rim in some cases. A second primary burrow can be connected at the opposite side of the chamber. The secondary burrows interconnect chambers with primary ones. The

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Fig. 5. Krausichnidae. (a) Attaichnus kuenzelii Laza 1982: chamber and burrow casts. Late Miocene Cerro Azul Formation, Argentina (MACN-LI1787, 1788, 1791, 1792). Bar: 1 cm. (b) Krausichnus trompitus Genise & Bown 1994b, close-up of the holotype: flat chambers with parallel roofs and floors, vertical pillars (centre), and dark linings and fillings. Late Eocene-Oligocene Jebel Qatrani Formation, Egypt. Bar: 1 cm. (c) Tacuruichnus farinai Genise 1997: holotype. Close to the knife it is possible to see the external wall bearing a system of burrows and chambers; on the left, large burrows radiating from the cup-shaped nest. Late Pliocene Barranca de Los Lobos Formation, Argentina. Bar: 10cm.

chamber system occupies a conical area up to 7 m in diameter and 3 m in height, in which the chambers are regularly distributed (modified from Laza 1982). Remarks. This trace fossil is known only from the type locality. However, its morphological features are clearly distinguishable from those

of the other representatives of Krausichnidae. The large (140-170 mm) spherical chambers connected by one or at most two primary burrows at the base and at the top are unmistakable traits. This trace fossil is preserved as detached casts of chambers and burrows having no indication of the presence of a lined or constructed wall

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(Fig. 5a). Laza (1982) attributed this trace fossil to ants of the genus Atta because of the gross morphology of the structure, the size and shape of chambers and burrows, the folded rim of primary burrows entering to the chambers, and the secondary burrows. Ichnogenus Parowanichnus Bown et al., 1997 (Fig. 4d) *1997 Parowanichnus Bown, Hasiotis, Genise, Maldonado & Brouwers p. 45, figs 4, 5. 1999 Parowanichnus Bown, Hasiotis, Genise, Maldonado & Brouwers; Genise p. 111. 2000 Parowanichnus Bown, Hasiotis, Genise, Maldonado & Brouwers; Genise, Buatois, Mangano, Laza & Verde p. 55. Type and only known ichnospecies. Parowanichnus formicoides Bown et al. 1997. Diagnosis. System composed of oblate to subspherical chambers interconnected by burrows of similar diameter. Burrows connected at tops, sides and bottoms of chambers. The gallery network is crudely trellate in plan, forming a grid-like lattice with descending shaft galleries and lateral tunnel galleries set more or less perpendicular to one another. Lined or constructed walls are lacking. Burrows, and the chambers they connect, radiate away from the centre of the structure and gradually decline in number. The chamber system occupies an area up to approximately 1 m in height and 3.3m in width (modified from Bown et al. 1997). Remarks. Parowanichnus differs from Attaichnus in having: (1) a much smaller total nest volume; (2) much smaller and less densely packed chambers; (3) chambers oblate to hemispherical rather than globular; (4) galleries of one basic size (smaller than in Attaichnus); (5) galleries providing access equally to top, sides and bottom of chambers; and (6) chambers and burrows more densely grouped in the central part of the structure (Bown et al. 1997). As with Attaichnus, chambers and burrows lack linings or constructed walls. This trace fossil is attributed to ants because its structure is composed of burrows connected with chambers, and it lacks lined or constructed walls (Bown et al. 1997). Ichnogenus Krausichnus Genise & Bown, 1994 (Fig. 5b) 71981 'Fossilized nests of Hodotermitidae' Coatonp. 79, fig. 1. 1993 'Chevron-shaped chambers' Bown & Genise p. A58. v*1994b Krausichnus Genise & Bown p. 169, figs6H-I, 7, 8D, 9-11, 12A. 1998 Krausichnus Genise & Bown; Buatois, Mangano, Genise & Taylor p. 227.

V71998 Krausichnus Genise & Bown; Genise, Pazos, Gonzalez, Tofalo & Verde p. 12. 1999 Krausichnus Genise & Bown; Genise p. 112. 2000 Krausichnus Genise & Bown; Genise, Buatois, Mangano, Laza & Verde p. 55. 2000 Krausichnus Genise & Bown; Miller & Mason p. 210. 72000 'Termitieresfossiles' Schuster, Duringer, Nel, Brunet, Vignaud & Mackaye p. 15, figs 3-5. 2002 Krausichnus Genise & Bown; Pazos, Tofalo & Sanchez-Bettuci p. 34. 2002 Krausichnus Genise & Bown; Buatois, Mangano & Acenolaza p. 189. Type ichnospecies. Krausichnus trompitus Genise & Bown 1994b. Diagnosis. Tiered arrangement of tabular, flat chambers with roofs and floors flat and parallel. Chambers exhibit vertical pillars, partition walls and conspicuous linings. Successive chambers are connected by tiny passages. Arrangement of tiered chambers may take various forms, such as spindles or columns, resulting in compound chambers without surrounding walls. These compound chambers may be interconnected by isolated or interconnecting burrows (modified from Genise & Bown 1994b). Included ichnospecies. K. trompitus Genise & Bown 1994b; K. altus Genise & Bown 1994b. Remarks. The original ichnogeneric diagnosis included details of the shape and arrangement of compound chambers, but it is now clear that such taxobases are more useful at ichnospecific level. A third, unnamed ichnospecies (Coaton 1981; Schuster et al. 2000) demonstrates that the morphological features that compose a recurrent architecture of ichnogeneric rank include tabular and tiered chambers showing pillars and forming compound chambers without external walls. The shape of the compound chambers and their interconnections are useful to distinguish ichnospecies. The whole structure may consist of a single compound chamber or a system of related compound chambers. Another ichnogenus that shows tiered tabular chambers is Archeoentomichnus, in which these structures are connected with simple elongate oblate chambers. In K. trompitus and K. altus, plus the Chadian material (Schuster et al. 2000), the combination of chambers and burrows indicates a social insect. The conspicuous linings and the constructed pillars and vertical walls specifically suggest termites (Genise & Bown 1994b; Schuster et al. 2000). Accordingly, Coaton (1981) attributed his unnamed fossil nests to the Hodotermitidae.

ICHNOSTRATIGRAPHY OF TRACE FOSSILS IN SOILS

Ichnogenus Archeoentomichnus Hasiotis & Dubiel 1995a (Fig. 4e) 1993 Archeoentomoichnos Hasiotis & Dubiel p. 177 (nomen nudum). 1994 Archeoentomichnus Hasiotis & Dubiel p. 8 (nomen nudum). *1995a Archeoentomichnus Hasiotis & Dubiel; Hasiotis & Dubiel p. 121, figs 3-5. 1995b Archeoentomoichnus Hasiotis & Dubiel; Hasiotis & Dubiel p. 86, fig. 1.6 (lapsus). 1998 Archeoentomichnus Hasiotis & Dubiel; Buatois, Mangano, Genise & Taylor p. 225. 1999 Archeoentomichnus Hasiotis & Dubiel; Genise p. 112. 2000 Archeoentomichnus Hasiotis & Dubiel; Genise, Buatois, Mangano, Laza & Verde 2000 p. 54. 2000 Archeoentomichnus Hasiotis & Dubiel; Hasiotis p. 154. 200la Archaeoentomonichnus Retallack p. 143 (lapsus). Type and only known ichnospecies. Archeoentomichnus metapolypholeos Hasiotis & Dubiel 1995a. Diagnosis. Peripheral part of the structure composed of large and small anostomosing galleries and elongate and oblate chambers. Central part columnar and partially subterranean, internally with stacked floor levels and slender, steeply inclined, connecting ramps. Galleries connect the central part to elongate oblate chambers and possibly to other columnar parts (modified from Hasiotis & Dubiel 1995a). Remarks. The original diagnosis is slightly modified to avoid interpretative terms such as periecie or endoecie that correspond specifically to termite architecture. This ichnogenus is described from fragmentary material. The diagnosis is based on an idealized reconstruction, rather than the holotype's morphology as illustrated by Hasiotis & Dubiel (1995a). The reconstructed morphology is similar to that of Krausichnus, but in Archeoentomichnus the tiered chambers are connected with elongate, oblate, simple chambers, which are lacking in Krausichnus. Moreover, the tabular chambers are thicker, and the central part of the nest less organized in general aspect than in Krausichnus. Ichnogenus Tacuruichnus Genise 1997 (Fig. 5c) v*1997 Tacuruichnus Genise p. 140, figs 2, 3. 1999 Tacuruichnus Genise; Genise p. 112. 2000 Tacuruichnus Genise; Genise, Buatois, Mangano, Laza & Verde p. 55. Type and only known ichnospecies. Tacuruichnus farinai Genise 1997.

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Diagnosis. Cup-shaped structure composed of a wall bearing a net of anostomosing burrows surrounding a chamber. Exteriorly the wall is connected to a gallery system composed of burrows of different diameters (Genise 1997). Remarks. Tacuruichnus looks like a large specimen of Termitichnus qatranii; however, in the latter ichnogenus the chambers are spherical and closed, because they would have been located completely below ground (Genise & Bown 1994b; Genise 1997). Tacuruichnus is the only krausichnid represented by a structure composed of a single large chamber. However, the characteristic burrow and chamber systems of the ichnofamily are present in the chamber's external wall and periphery (Fig. 5c). Its architecture closely resembles the hypogeous part of the nest of Cornitermes cumulans (Nasutitermitinae) (Genise 1997). The presence of a single chamber, interpreted as a nest, is also compatible with nests of C. cumulans. It would be important to find more specimens to determine the complete morphology and the taxonomic affinity of this trace fossil. Ichnogenus Vondrichnus Genise & Bown, 1994 (Fig. 6a) *1994b Vondrichnus Genise & Bown p. 165, figs 5J; 6A, B. 1998 Vondrichnus Genise & Bown; Buatois, Mangano, Genise & Taylor p. 227. 1999 Vondrichnus Genise & Bown; Genise p. 112. 2000 Vondrichnus Genise & Bown; Genise, Mangano, Buatois, Laza & Verde p. 54. 2002 Vondrichnus Genise and Bown; Buatois, Mangano & Acenolaza p. 189. Type and only known ichnospecies. Vondrichnus obovatus Genise & Bown 1994b. Diagnosis. Diffuse, polychambered, excavated subterranean systems. Obovate chambers occur in dense swarms of near 300 in cross-section. Burrows simple, branched or unbranched, exiting from one or more points on periphery of chamber and comprising a dense mass of anastomosing burrows that may connect chambers. Sediment in the centre of the chambers is alveolar and commonly arranged in concentric bands. Chambers expanded by apposition of 1-3 chambers against one another (Genise and Bown 1994b). Remarks. Vondrichnus differs from Termitichnus in: (1) consistently smaller mean chamber size; (2) obovate form of chamber; (3) tighter packing of associated chambers; (4) absence of isolated chambers; (5) larger number of galleries in each cluster; (6) lack of gallery ornamentation; (7) greater density of galleries between chambers;

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Fig. 6. Krausichnidae. (a) Vondrichnus obovatus Genise & Bown 1994b. Late Eocene-Oligocene Jebel Qatrani Formation, Egypt. Bar: 1 cm. (b) Fleaglellius pagodus Genise & Bown 1994b: three-chambered tower. Late Eocene-Oligocene Jebel Qatrani Formation, Egypt. Bar: 1 cm. (c) Syntermesichnus fontanae Bown & Laza 1990: interconnected chambers lined with a lighter fine material; cross-section of an individual thin passage (on the left). Miocene Pinturas Formation, Argentina; MACN-LI81. Bar: 1 cm. (d) Termitichnus simplicidens Genise & Bown 1994b: main burrows (particularly visible on the right) radiate from the base of the chamber. Late Eocene-Oligocene Jebel Qatrani Formation, Egypt. Bar: 10cm.

and (8) absence of compound galleries. Vondrichnus differs from Fleaglellius in: (1) never having more than two apposed chambers; (2) rarely having only vertically apposed chambers; (3) not having convex-upward chambers; (4) commonly possessing a rind of anastomosed peripheral galleries at the tops and sides of chambers; and (5) occurring as diffuse but interconnected groups of chambers (Genise & Bown 1994b). Some of the characters stated by the

authors, such as size, packing of chambers and number of burrows, are undoubtedly very useful for dividing the morphological complex of Termitichnus-Fleaglellius-Vondrichnus; however, they are not very satisfactory as ichnotaxobases in a more general context. Vondrichnus and Fleaglellius show a primary burrow system devoid of secondary tunnels that distinguish them from Termitichnus. Vondrichnus can be clearly separated from Fleaglellius because in

ICHNOSTRATIGRAPHY OF TRACE FOSSILS IN SOILS

the latter the chambers are convex upwards and form 'towers' by vertical apposition (Genise & Bown 1994b) (Fig. 6a, b). Possible trace-makers are termites, possibly Macrotermitinae (Genise & Bown 1994b). The very simple architecture of this trace fossil necessitates further examination of micromorphological characters to confirm its affinities. Ichnogenus Fleaglellius Genise & Bown, 1994 (Fig. 6b) 1994b Fleaglellius Genise & Bown p. 167, figs 6C-G, 8E-F. 1998 Fleaglellius Genise & Bown; Buatois, Mangano, Genise & Taylor p. 227. 1999 Fleaglellius Genise & Bown; Genise p. 112. 2000 Fleaglellius Genise & Bown; Genise, Mangano, Buatois, Laza & Verde p. 54. 2002 Fleaglellius Genise & Bown; Buatois, Mangano & Acenolaza p. 189. Type and only known ichnospecies. Fleaglellius pagodus Genise & Bown 1994b. Diagnosis. Diffuse, polychambered, excavated subterranean system. Chambers oblate, with bases and tops of chambers almost invariably convex-upward and generally apposed. Successive chambers are added vertically or near vertically, with the top of the lower completely overlapped by the base of the upper chamber, such that completed structures make up towers of 2-35 chambers. Towers are widely dispersed in groupings of up to 40 specimens and are connected by dense masses of simple galleries. Galleries, both branched and unbranched, connect different towers at all levels (Genise & Bown 1994b). Remarks. This ichnogenus differs from its closest morphological counterpart, Vondrichnus, in that Fleaglellius'. (1) always has more than one and up to 35 apposed chambers, always added vertically; (2) invariably has individual chambers that are noticeably convex-upwards; (3) lacks a peripheral rind of anastomosed galleries at tops and/or sides of chambers; and (4) lacks a diffuse distribution of individual chambers (Genise & Bown 1994b). Possible trace-makers are termites of unknown affinities (Genise & Bown 1994b). The particular kind of enlargement of nest by vertical apposition of chambers is unknown in subterranean termites, but resembles that of certain subaerial nests. This ichnogenus requires micromorphological analysis of its affinities. Ichnogenus Termitichnus Bown, 1982 (Fig. 6d) v*1982 Termitichnus Bown p. 259, figs 2-7. 71984 Termitichnus Bown; Tandon & Naug p. 285, figs 7B, 9B-E. 1993 Termitichnus Bown; Genise p. 54.

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71993 Termitichnus Bown; Smith, Mason & Ward p. 592, fig. 16. V1994b Termitichnus Bown; Genise & Bown p. 160, figs 4, 5, 8A-C. 1997 Termitichnus Bown; Genise p. 140. 1998 Termitichnus Bown; Buatois, Mangano, Genise & Taylor p. 227. 1998 Termitichnus Bown; Smith & Mason p. 555, figs 12, 13, 14A. 1999 Termitichnus Bown; Genise p. 112. 2000 Termitichnus Bown; Genise, Mangano, Buatois, Laza & Verde p. 54. 2000 Termitichnus Bown; Miller & Mason p. 208, figs 12-16. 2001a Termitichnus Bown; Retallack p. 143. 2002 Termitichnus Bown; Buatois, Mangano & Acenolaza p. 189. Type ichnospecies. Termitichnus qatranii Bown, 1982. Diagnosis. Diffuse, polychambered excavated subterranean systems with spherical to subspherical chambers connected to one another by a network of simple and/or compound galleries of different diameters. Primary burrows arise from the base of chambers. Chambers may be isolated, as expanded globular clusters of chambers or in associated constellate aggregations of tens to hundreds of chambers (modified from Genise & Bown 1994b). Included ichnospecies. T. qatranii Bown 1982; T. simplicidens Genise & Bown 1994b; T. namibiensis Miller & Mason 2000. Remarks. The diagnosis has been modified to exclude characters such as size and fill of chambers, which are more useful for ichnospecific diagnosis. Also excluded are those characters referred to 'nest expansion', which is a concept linked to the supposed constructors rather than to documented morphology. The basal position of the primary burrows is an important character added to the diagnosis, because it is constant and very representative in both ichnospecies. Differences from Fleaglellius and Vondrichnus were commented on in previous sections. Attaichnus has consistently spherical chambers always, and a single primary burrow arises from the base of chambers, and in some cases other from the top. The architecture of Syntermesichnus is clearly different from that of Termitichnus', however, in terms of ichnotaxobases it is difficult to translate these differences avoiding measurements and other taxobases of dubious merit. In addition the commonest type of Termitichnus comprises spherical or subspherical chambers from which one to five primary burrows arise, some of which may connect another distant chamber (Fig. 6d). In contrast, Syntermesichnus takes on the aspect of a boxwork of burrows

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and chambers of rather similar diameters (Fig. 6c). Not all of the trace fossils attributed to Termitichnus after its original description (Bown 1982) and redescription (Genise & Bown 1994b) belong to that ichnogenus. Smith et al. (1993) reported supposed Termitichnus from the late Pleistocene of Namibia, and proposed a Termitichnus ichnofacies, but their figured specimen does not closely resemble Termitichnus. Probable Termitichnus have subsequently been illustrated from the Tertiary of Namibia by Smith & Mason (1998): these are discussed below with unnamed Krausichnidae. An additional ichnospecies of Termitichnus from early Pleistocene deposits of South Africa, T. namibiensis, is composed of isolated or interconnected chambers, surrounded by a thick wall and a network of simple tunnels (Miller & Mason 2000). Chambers of T. namibiensis are filled with meniscate, tiered, galleries. A thick wall and peripheral tunnel system surround isolated or interconnected subspherical chambers typical of Termitichnus. The attribution to this ichnogenus to fossil termite nests is also supported by the tiered arrangement of shelves, as in other fossil and modern termite nests (e.g. Sands 1987). However, some characters raise doubts about the ichnotaxonomical placement of this trace fossil. Chambers apparently lack the characteristic primary tunnels of Termitichnus, and the interpretation of the internal structure is confusing. It is not clear how to interpret a gallery that extends horizontally and also shows a meniscate filling, which commonly results from the backfilling of burrows of similar diameter from that of the trace-maker (e.g. D'Alessandro & Bromley 1987; Keighley & Pickerill 1994). Also, the turning points at the end of galleries are unusual, in that the thin layers between two successive shelves commonly become thicker towards the sides before joining the external wall (e.g. Sands 1987). These thickenings help to reinforce the whole structure. In T. namibiensis the thickenings are disconnected from the outer wall at the turning points. In addition, T. namibiensis also lacks the ramps, pillars and openings that are common in Krausichnus, as well as the thin-layered ovoids and the hive of modern termite nests (e.g. Sands 1987; Genise & Bown 1994b). In sum, the internal structure of the chambers of T. namibiensis (Miller & Mason 2000, fig. 12) plus the abovementioned characters leave some doubts about the trace-makers of this internal structure, which may be different from those of the chambers. These characters, plus the absence of primary burrows arising from the base of

the chambers, suggest that the inclusion of this trace fossil in Termitichnus would require further work. On the other hand, although the absence of external wall makes some specimens comparable to Krausichnus, these specimens are incomplete and eroded (Miller & Mason 2000). Possible trace-makers include termites belonging to Macrotermitinae (Genise & Bown 1994b). Ichnogenus Syntermesichnus Bown & Laza 1990 (Fig. 6c) v*1990 Syntermesichnus Bown & Laza p. 74, figs 2-4. 1993 Syntermesichnus Bown & Laza; Genise p. 54. 1995 Syntermesichnus Bown & Laza; Genise and Cladera p. 80, fig. 2C-D. 1995 Syntermesichnus Bown & Laza; Constantino p. 460. 71997 Syntermesichnus Bown & Laza; Smith & Kitchingp. 41, figs 16, 17. 1997 Syntermesichnus Bown & Laza; Bown, Hasiotis, Genise, Maldonado & Browers p. 46. 1998 Syntermesichnus Bown & Laza; Buatois, Mangano, Genise & Taylor p. 227. 1999 Syntermesichnus Bown & Laza; Genise p. 112. 2000 Syntermesichnus Bown & Laza; Genise, Mangano, Buatois, Laza & Verde p. 54. 2002 Syntermesichnus Bown & Laza; Buatois, Mangano & Acenolaza p. 189. Type and only known ichnospecies. Syntermesichnus fontanae Bown & Laza 1990. Diagnosis. Peripheral part of the structure, tabular, with large and small anastomosing burrows and elongate, oblate chambers. Large burrows arising from chambers, but small burrows branch from larger ones. Systems of small passages at one level extend the structure in the horizontal plane and permit communication to other levels. The same style of branching is present at all levels. Walls of burrows and chambers lined with compacted sediment (finer than host matrix) (modified from Bown & Laza 1990). Remarks. Modifications of the original diagnosis were made to avoid any reference to modern termite nests. Syntermesichnus shows one of the simplest morphologies of Krausichnidae and, consequently, one of the most difficult to define and separate from the other Krausichnidae. The whole structure lacks definite limits and, in the field, resembles a diffuse boxwork of tunnels and chambers of more or less similar diameter occupying entire beds (Genise & Bown, unpublished data). The abundance of similar Syntermesichnus-\ikG traces in pyroclastic

ICHNOSTRATIGRAPHY OF TRACE FOSSILS IN SOILS

deposits from the Cretaceous and Tertiary of southern South America (Genise, unpublished data) gives this trace fossil a particular importance. The chamber and burrow systems, lined with fine material, suggested to Bown & Laza (1990) that Syntermesichnus was a fossil termite nest. However, the lack of a definite architecture (e.g. nest limits) and its pervasive presence in beds preclude a definite attribution without micromorphological analysis of the lined walls. Trace fossils comparable to Syntermesichnus are described from the Jurassic Elliot Formation of South Africa (Smith & Kitching 1997), although this attribution needs further analysis. Possible trace-makers are termites of the genus Syntermes (Bown & Laza 1990). However, as discussed above, this attribution requires micromorphological study and the examination of further material to look for well-delimited nests. In addition, as Constantino (1995) noted, the descriptions of Syntermes nests in the literature are inadequate, so that the affinities of Syntermesichnus should be considered doubtful. Unnamed trace fossils attributable to Krausichnidae Various trace fossils from the Pliocene Laetoli Formation of Tanzania, all attributable to Krausichnidae, were described by Sands (1987). He recognized seven basic types. The first type is composed of systems of anastomosing burrows of different diameters, which may show vertical shafts associated with flattened chambers. These systems are comparable to the structures attributed by Smith et al. (1993) to Termitichnus. However, these structures are not clearly compatible with the ichnogeneric diagnosis. The second type with thick-layered ovoids (= chambers) shows particular features, such as the presence of a surrounding cavity, that are unknown from other fossil termite nests. These ovoids lack an external wall and associated burrow systems. They somewhat resemble very roughly ichnospecies of Krausichnus in having a layered structure, but the layers are unusually thick. A third type of trace fossil with shafted chambers is not comparable to any other Krausichnidae. This type is composed of a bell-shaped chamber from which vertical shafts arise, which are capped by smaller shafts, chambers and pores. The fourth type, composed of a single ovoid, is similar to the second type, but has an external wall and ramps and passages from one floor to the next. The fourth type can be attributed to neither Krausichnus nor Termitichnus because Krausichnus does not have an outer wall and Termitichnus does not have a comparable layered

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structure. The fifth type is represented by three small ovoids showing pits, protrusions, ramps and openings respectively. These resemble similar structures from the Tertiary of Namibia attributed to Termitichnus by Smith and Mason (1998), and also unnamed trace fossils from the early Miocene of Ethiopia (Bown & Genise 1993). They superficially resemble Termitichnus, but lack the associated burrow system, which is diagnostic. The sixth type is similar to the first type but also has chambers and pores arranged in anastomosing columnar and alveolar structures. No other trace fossils are comparable to the sixth type. The last type, from the upper part of the Laetoli beds, consists of thin-layered ovoids that show a layered internal structure having ramps and connecting openings surrounded by a sculptured wall. This type is very similar to unnamed trace fossils described by Coaton (1981) and Schuster et al. (2000) from the Pleistocene of South Africa and the Pliocene of Chad respectively, and is attributable to a distinct ichnospecies of Krausichnus. However, the ovoids from Laetoli have a constructed wall, an unusual trait for Krausichnus that is lacking in the other African material. Miller & Mason (2000) considered these structures attributable to their ichnospecies Termitichnus namibiensis. Sands (1987) related most of these types of trace fossil to different parts of Macrotermitinae nests at different stages of development and/or constructed in different types of soil, with two exceptions: the vertical shaft with flattened chambers, which more probably relates to ant nests of Camponotus, and the thin-layered ovoids, which are attributable to neither the Macrotermitinae nor the Hodotermitidae (Sands 1987). In summary, the Laetoli material (Sands 1987) represents a spectrum of samples of krausichnids that share common features with other representatives of the ichnofamily from Africa (e.g. Coaton 1981; Bown & Genise 1993; Smith et al. 1993; Genise & Bown 1994b; Smith & Mason 1998; Schuster et al. 2000). This similarity reflects the origin of these trace fossils in common lineages of African termites. Ichnotaxonomically, the Laetoli material presents some problems. Considering the architecture of Macrotermitinae nests, each type described is more likely a part of a more complex structure than a complete structure in itself. This fact reiterates a common trend in evolution of behaviour in insects, in which complex architectures and behaviours are the result of the addition of simple behaviours and architectures respectively. One possibility is to include the types one and six

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(tunnel systems) in Syntermesichnus, interpreting this ichnogenus as diffuse boxworks of tunnels and chambers. It would also be possible to include the thick- and thin-layered ovoids in Krausichnus or Termitichnus, but important features distinguish these ovoids from the known ichnospecies. Another standpoint might be to include each Laetolian type in a new ichnotaxon, even if each is part of a more complex structure; this procedure would be supported by the fact that some of these types are recurrent as individual trace fossils in other localities and ages in Africa (e.g. Coaton 1981; Bown & Genise 1993; Smith & Mason 1998; Schuster et al. 2000). Other unnamed Krausichnidae were described by Laza (1995, 1997) from Pliocene and Pleistocene deposits of Argentina. They include seven types of trace fossil: two attributed to termites, and five to ants. One type of termite nest is currently being redescribed by Laza (personal communication) and two ant nests described in 1995 were redescribed by Laza (1997). Hasiotis & Demko (1996) also described a supposed ant nest from the Jurassic Morrison Formation. The importance of all this material for our knowledge of the diversity of Krausichnidae is unquestionable. However, in all cases the diagnoses and descriptions are influenced by the supposed modern analogue, which, in combination

with the fragmentary nature of the material, makes any ichnotaxonomical consideration very difficult. There are still other unnamed trace fossils, cited as termite or ant nests (e.g. Tandon & Naug 1984; Iriondo & Krohling 1996; Tauber 1996; Andreis & Cladera 1998) that are only mentioned and will require analysis and proper description. A sound description of this unnamed material, in the light new understanding of krausichnid morphology, will aid incorporation of this material into the emerging ichnotaxonomic framework. Ichnostratigraphy There are 25 described ichnogenera attributed to insect trace fossils in palaeosols, 19 of which are reviewed herein, and the remaining six in a previous contribution (Genise 2000) (Table 1). Almost all of them show comparable stratigraphic ranges that, in turn, accord with our present knowledge of the evolutionary history of their trace-makers: bees, beetles, ants and termites (e.g. Crowson 1981; Kuschel 1983; Krishna 1990; Jarzembowski & Ross 1996; Grimaldi et al. 1997; Grimaldi 1999; Engel 2000; Grimaldi & Agosti 2000; Krell 2000; Schaefer 2001; Nel et al in press).

Table 1. Stratigraphic ranges and abundance of insect ichnotaxa in palaeosols 1

Pleistocene Pliocene Miocene Oligocene Eocene 1 Palaeocene Upper Cretaceous Middle Cretaceous Lower Cretaceous Jurassic Triassic Permian Carboniferous Devonian Silurian Ordovician Cambrian

2

3 4

4 4 3 2 5 2 1 1

5

6

7

1

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

1 1

1 3 1

1

1

4 1 1 3

&

1

1

1 1 1 1 1 1

1

3 1 1 1 1 2 1 1 1 1 1 1 1

2 4 3 3 1 1 2

i 2 1

1 1

i

Coprinisphaeridae (1-6), Pallichnidae (7-9), Krausichnidae (10-18) and Celliformidae (19-25). 1, Fontanai; 2, Coprinisphaera; 3, Eatonichnus; 4, Monesichnus; 5, Teisseirei; 6, Rebuffoichnus; 7, Pallichnus; 8, Fictovichnus; 9, Scaphichnium; 10, Attaichnus; 11, Parowanichnus; 12, Krausichnus; 13, Archeoentomichnus; 14, Tacuruichnus; 15, Vondrichnus; 16, Fleaglellius; 17, Termitichnus; 18, Syntermesichnus; 19, Palmiraichnus; 20, Celliforma; 21, Corimbatichnus; 22, Uruguay; 23, Rosellichnus; 24, Ellipsoideichnus; 25, Cellicalichnus. Numbers in cells represent formations in which the ichnotaxon is recorded. Circles indicate the oldest body fossil of the potential trace-maker.

ICHNOSTRATIGRAPHY OF TRACE FOSSILS IN SOILS

The Coprinisphaeridae Among the Coprinisphaeridae, Fontanai has been recorded only from the Late CretaceousEarly Tertiary Asencio Formation of Uruguay (Roselli 1939). These redbed deposits could not be dated precisely until now because of the complete absence of datable rocks or fossil remains other than the fossil nests (Genise et al. 2004). Genise et al. (2002a) suggested a possible Eocene age for this unit, owing to the presence of an important unconformity with the underlying Cretaceous Yapeyu Formation (Pazos et al. 2002), comparison of the ichnofauna with other Palaeogene deposits in South America, and the abundance and diversity of dung-beetle nests. This abundance and diversity could reflect the increased availability of herbivore faeces corresponding to the Eocene diversification and abundance of South American herbivores. In Table 1 all the records from this formation are considered to be Eocene. Because this formation is one of the richest in fossil insect nests - it includes 10 of the 25 described ichnogenera - it produces a peak of diversity for this age that may be an artefact, not simply because the age of this formation is unknown, but also because intensive research on other formations and continental deposits has just begun. Nevertheless, it cannot be overlooked that, during the early Eocene, a climatic optimum occurred (e.g. Zachos et al. 2001), which would have favoured the abundance and diversification of some groups of insects (e.g. Wilf & Labandeira 1999). Coprinisphaera and Celliformaare the most widely recorded ichnogenera in palaeosols. Coprinisphaera has been recorded from the aforementioned Asencio Formation in Uruguay (Roselli 1939, 1987; Genise et al. 2004), the early Eocene Casamayor Formation, Argentina (Frenguelli 1938b; Laza 1986a), the late Eocene Musters Formation, Argentina (Andreis 1972; Laza 1986a), the late Eocene La Meseta Formation, Antarctica (Laza & Reguero 1990), the Eocene-Miocene Sarmiento Formation (Laza 1986a; Bellosi et al. 2001), the Oligocene Deseado Formation, Argentina (Frenguelli 1938b; Laza 1986a), the early Miocene Pinturas Formation, Argentina (Genise & Bown 1994a), the late Miocene Collon-Cura Formation, Argentina (Frenguelli 1939; Laza 1986b), the late Miocene Paso de las Carretas Formation, Argentina (Pascual & Bondesio 1981), the Pliocene Monte Hermoso Formation, Argentina (Laza 1986b), the Pliocene Piquete Formation, Argentina (Alonso et al. 1982), the Pliocene Laetoli Formation, Kenya (Sands 1987), the

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Pliocene Chad Formation, Chad (Duringer et al. 2000a, 2000b), the Pleistocene Ensenada Formation, Argentina (Frenguelli 1938a), the Pleistocene Tezanos Pinto Formation, Argentina (Iriondo & Krohling 1996), the Pleistocene Tafi del Valle Formation, Argentina (Fontaine et al. 1995), and from an unnamed Pleistocene formation, from Ecuador (Sauer 1955). Eatonichnus has been recorded from the Palaeocene Colter Formation, USA (Gilliland & La Rocque 1952), the late Palaeocene-Eocene Claron Formation, USA (Bown et al. 1997), and the early Palaeocene Penas Coloradas Formation, Argentina (Genise et al. 2001). Monesichnus is another ichnogenus that has been recorded exclusively from the Late Cretaceous-Early Tertiary Asencio Formation of Uruguay (Roselli 1987). The previously mentioned ichnogenera of Coprinisphaeridae are attributed to Scarabaeinae (Frenguelli 1938; Laza 1986b; Bown et al. 1997; Genise & Laza 1998; Genise 1999; Krell 2000), whose oldest body fossils come from the Late Cretaceous strata (Krell 2000). In this case the body fossils shortly predate the trace fossils of the group, as there is no ichnological record for the Cretaceous. Moreover, the presence of Coprinisphaera is taken as indicative of Cenozoic deposits in South America (Genise et al. 2000). HalfTter & Edmonds (1982) and Cambefort (1991) suggested that dung-beetles might have diversified by the end of the Cretaceous, helped by the radiation of the herbivorous dinosaurs and/or the increase of mammal excrement. The only Cretaceous trace fossils attributable to dung-beetles (non-Scarabaeinae) are those described by Chin & Gill (1996) from dinosaur coprolites. The other two ichnogenera of Coprinisphaeridae deserve particular attention. Teisseirei has been recorded from the Late Cretaceous-Early Tertiary Asencio Formation, Uruguay (Roselli 1939), the Eocene Gran Salitral Formation, Argentina (Melchor et al. 2002), and the EoceneMiocene Sarmiento Formation, Argentina (Bellosi et al. 2001). Despite the well-preserved macro- and micromorphological characters of this trace fossil, as well as its abundance in Tertiary deposits of South America, the trace-maker cannot yet be identified more precisely than as probably being a coleopteran. Hence it is impossible to compare the stratigraphic range of Teisseirei with that of the body fossil record of any particular coleopteran. Rebuffoichnus has been recorded from the Late Cretaceous Laguna Palacios Formation, Argentina (Genise et al. 2002b), the Late Cretaceous-Early Tertiary Asencio Formation, Uruguay (Roselli 1987), and the

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J. F. GENISE

Pleistocene of Australia (Lea 1925; Johnston et aL 1996). This ichnogenus, probably attributable to the work of weevils (Genise et al. 2002b), is one of only two that crosses the K-T boundary, being recorded from the Late Cretaceous to the Pleistocene. As such, it is one of the oldest trace fossils that can be definitely attributed to insects (Genise et al 2002b). The body fossil record of their probable constructors, the curculionids, extends back to Upper Jurassic-Lower Cretaceous deposits (Crowson 1981; Kuschel 1983; Jarzembowski & Ross 1996). As a whole, the body fossil record of possible trace-makers predates the Coprinisphaeridae, even though the thick constructed walls confer to these trace fossils a high preservation potential.

The Pallichnidae The Pallichnidae pose a different problem from that of the Coprinisphaeridae. Scaphichnium has been recorded exclusively from the Eocene Willwood Formation, USA (Bown & Kraus 1983), whereas its possible producers, the Geotrupinae (Hasiotis et aL 1993), are recorded since the late Oligocene (Krell 2000). Pallichnus has been recorded only from the Oligocene Brule Formation in the USA (Retallack 1984), whereas its possible producers, the Geotrupinae or Scarabaeinae (Retallack 1984), are recorded from the late Oligocene and Late Cretaceous respectively (Krell 2000). Fictovichnus, in a broad sense, may include very simple trace fossils showing a morphology that has been recorded from the Late Jurassic Morrison Formation, USA (Hirsch 1994b), the Late Cretaceous Barun Goyot Formation, Mongolia (Mikhailov et al. 1994), the Late Cretaceous Djadokhta Formation, Mongolia (Johnston et al. 1996), the Late Cretaceous Bajo Barreal Formation, Argentina (Sciutto & Martinez 1996), the Palaeocene of Uruguay (Veroslavsky & Martinez 1996), the Eocene Claron Formation, USA (Bown et al. 1997), the Eocene of France (Freytet & Plaziat 1982; Hirsch 1994a), the Eocene Gran Salitral Formation, Argentina (Melchor et al. 2002), the late Eocene Bembridge Limestone Formation, England (Edwards et al. 1998), the Miocene Higewi Formation, Kenya (Thackray 1994), and the Pliocene of Tanzania (Ritchie 1987). It is impossible to determine a unique trace-maker for Fictovichnus, whose makers may have included the Tenebrionidae, Curculionidae, Scarabaeidae (Johnston et al. 1996), or even other families of Coleoptera. This precludes any precise comparison with the body fossil record of the coleopteran families.

The Krausichnidae Representatives of Krausichnidae show, in most cases, similar characteristics: body fossils predating trace fossils and scarcity of data, with some exceptions. Attaichnus and Parowanichnus, the two ichnogenera attributed to ants, have been recorded exclusively from the Miocene Epecuen Formation, Argentina (Laza 1982) and from the Eocene Claron Formation, USA (Bown et al. 1997) respectively. The oldest body fossils of ants come from the Cretaceous of the USA and France (Grimaldi et al. 1997; Grimaldi & Agosti 2000; Nel et al. in press). Krausichnus has been recorded from the Late Cretaceous-Early Tertiary Asencio Formation, Uruguay (Genise et al. 1998), the late Eocene Qasr el Sagha Formation and late Eocene-Oligocene Jebel Qatrani Formation, Egypt (Genise & Bown 1994b), the late Miocene Baynunah Formation, Abu Dhabi Emirate (Bown & Genise 1993), the Pliocene of Chad (Schuster et al. 2000), and the Pleistocene of South Africa (Coaton 1981). Tacuruichnus has been recorded exclusively from the late Pliocene Barranca de los Lobos Formation, Argentina (Genise 1997), whereas Vondrichnus and Fleaglellius from the late Eocene-Oligocene Jebel Qatrani Formation, Egypt (Genise & Bown 1994b). Termitichnus shows a broader distribution, being recorded from the late Eocene-Oligocene Jebel Qatrani Formation, and the Late Eocene Qasr el Sagha Formation, Egypt (Bown 1982), the early Miocene Kashab Formation, Egypt (Genise & Bown 1994b), the Plio-Pleistocene Boulder Formation, India (Tandon & Naug 1984), the late Pleistocene Homeb Silts, Namibia (Smith et al. 1993), the late Pleistocene Sossus Sand, Namibia (Smith & Mason 1998), and the Pleistocene of South Africa (Miller & Mason 2000). Finally, Syntermesichnus has been recorded from the Miocene Pinturas Formation, Argentina (Bown & Laza 1990), and there is a doubtful record from the Early Jurassic Elliot Formation, South Africa (Smith & Kitching 1997). With the exception of this last record, all other occurrences are predated by the oldest fossil termites from the Lower Cretaceous (Krishna 1990). The case of Archeoentomichnus from the Triassic Chinle Formation, USA (Hasiotis & Dubiel 1995a) deserves particular attention because it is the only datum that does not fit the general picture shown in Table 1. These authors attributed this trace fossil to termites despite the fact that it would predate the oldest body fossils by 150 million years (Hasiotis & Dubiel 1995a). Many invertebrate trace fossils are more preservable than their producers:

ICHNOSTRATIGRAPHY OF TRACE FOSSILS IN SOILS

accordingly, fossil bee nests predate the oldest bees (Elliott & Nations 1998; Genise 2000). However, the few million years involved in this difference between the oldest bee trace fossils and the oldest fossils of the probable constructors is an expected one. It is consistent with our previous knowledge of the evolutionary history of bees and their relationship with the coevolving plant groups (Grimaldi 1999; Engel 2001). In the case of Archeoentomichnus, the palaeoentomological significance that would result from the discover of a Triassic termite nest predating the oldest termites by 150 million years would require sound proof, such as micromorphological studies, to be accepted as such. At present, pending further work, this occurrence is not accepted. The whole interpretation of Mesozoic evolution of palaeosol ichnofaunas attributed to insects by Hasiotis (2000) is mostly unsupported, in that the attribution of these early purported trace fossils to insects is not well documented or well founded. The conclusions are based largely on poorly supported interpretations of Triassic and Jurassic trace fossils from the Chinle and Morrison Formations. These interpretations are at odds with the strong evidence that indicate that ants, termites, bees and dungbeetles arose and diversified during the Cretaceous (Krishna 1990; Labandeira 1998; Grimaldi 1999; Krell 2000; Engel 2001; Schaefer 2001). This conclusion is also supported by the ichnological record of insect nests in palaeosols (Table 1). Accordingly, Labandeira (1998), Grimaldi (1999), Genise (2000) and Engel (2001) objected to such interpretations of Chinle and Morrison trace fossils because of inadequate documentation. Triassic and Jurassic trace fossils are unknown from other studied deposits of similar age, which adds significance to the Chinle and Morrison trace fossils, as increasing the necessity for sound descriptions and interpretations of the fossils. Other Triassic and Jurassic palaeosols studied are of low ichnodiversity, composed mainly of simple trace fossils such as Skolithos, Macanopsis, Taenidium and Edaphichnium, none of which can be certainly attributed to insects (Retallack 1976, 1980; Smith & Kitching 1997; Melchor et al 2001; Genise et al 2004). The morphology of trace fossils in palaeosols ranges from very simple to very complex. The attribution of traces of very simple morphology to modern taxa is a misleading procedure because they can commonly be attributed to several different groups of organisms (Ratcliffe & Fagerstrom 1980; Retallack 1990b; Genise et al 2004). Trace fossils in Mesozoic palaeosols that

447

can unequivocally be attributed to insects are few, and are recorded from only three Late Cretaceous formations in the USA, Mongolia and Argentina (Johnston et al. 1996; Elliott & Nations 1998; Genise et al. 2002b). The record of insect fossil nests from rocks of this age accords with the body fossil record of their probable constructors and/or reflects their supposed evolutionary history. A scenario proposed by Genise & Bown (1994a) stated that the diversity of insect fossil nests in palaeosols increased significantly after the Cretaceous and not earlier. This hypothesis is based on the fact that most common trace fossils in palaeosols are constructions belonging to termites, bees and dungbeetles, groups that arose during that period based on body fossil evidence. This hypothesis was later corroborated by the discovery of Late Cretaceous bee nests and coleopteran pupal chambers (Johnston et al. 1996; Elliot & Nations 1998; Genise 2000; Genise et al. 2002b). The Genise and Bown hypothesis is also supported by the increase in abundance of records of insect fossil nests in Tertiary palaeosols, in contrast to the general absence of records from pre-Cretaceous deposits. Although Labandeira & Sepkoski (1993) stated that the appearance and expansion of angiosperms had no influence on insect familial diversification, their quantitative analysis did not evaluate the ecological importance of the families involved. The origin of termites, ants, bees and dung-beetles during the Cretaceous was probably related to the origin and diversification of angiosperms and to the broad-scale ecological changes that resulted in the K-T mass extinction event. These insect tracemakers would have played a major role in this event as the new soil colonizers of the emergent ecosystems. The record of latosols, from the Cretaceous onwards in the stratigraphic column, has been strongly related to the radiation of termites and angiosperms (Schaefer 2001). In contrast to other groups that became extinct or less dominant following the K-T event, insect ichnofossils in palaeosols show that their producers were part of the new flourishing ecosystems. This evolutionary scenario is reflected in the stratigraphic range and abundance of insect trace fossils in palaeosols reflected in Table 1, which shows the absence of Celliformidae, Coprinisphaeridae, Pallichnidae and Krausichnidae in pre-Cretaceous rocks. It also shows that the oldest record of two of these ichnofamilies came from Cretaceous rocks, which contain a lower diversity and abundance of nests than Tertiary ones. Even so, Cretaceous nests are very important ones, in relation to the origin of

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important groups of insects and their building behaviour. Tertiary rocks have the most diverse, abundant and well-preserved assemblages of insect fossil nests, in accordance with the diversification of insects and their building behaviour. The initial manuscript benefited from comments by A. Uchman and A. Rindsberg. I thank also D. Mcllroy, M. Verde and A. Rindsberg for improving the final version. This research was partially supported by a grant from the National Agency of Scientific and Technical Promotion of Argentina (FONCYT-PICT 6156/99) to the author.

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A stratigraphy of marine bioerosion RICHARD G. BROMLEY Geological Institute, University of Copenhagen, 0ster Voldgade 10, DK-1350 Copenhagen K, Denmark (e-mail: [email protected]) Abstract: About 65 ichnogenera and a number of bioerosional trace fossils that are unnamed are catalogued with respect to their stratigraphic ranges. In most cases, corresponding stratigraphic studies of the trace-makers are not possible because (1) the rank of taxonomic ascription is too high to be meaningful and (2) not all members of a high taxon are bioeroders. For example, radulation traces of chitons are known from Jurassic to Recent, whereas chitons have a body fossil record back to the Early Palaeozoic. Similarly, whereas the round drill-hole Oichnus paraboloides is known from Cambrian to Recent, the only identified makers of this trace fossil, naticid gastropods, range from Cretaceous to Recent. The stratigraphic ranges of bioerosion ichnotaxa emphasize the two marine revolutions of the Phanerozoic: there is marked increase in diversification during the Ordovician-Devonian interval and since the Triassic.

Most groups of trace fossils are generally considered of little use as stratigraphic indicators, because they provide more information on the behaviour of the trace-maker than on its biological identity, and because most ichnotaxa are stratigraphically long-ranging. Among the exceptions to this are the hard substrate trace fossils and their bioeroding progenitors; but even within this group, many trace fossils are long-ranging and remain biologically anonymous. Some attempts at treating bioerosional ichnotaxa stratigraphically have been made before. Kobluk et al (1978) covered the Early Palaeozoic; Wilson & Palmer (1990, 1992) and Palmer (1982, fig. 5) the Cambrian to Cretaceous; Ekdale et al (1984, fig. 10.12) had a single figure; and Bromley (1994) included a generalized statement. Taylor & Wilson (2003) cited the stratigraphic range of the more important macrobioerosion ichnotaxa in their table 2. Palmer (1982) expressed pessimistic feelings as he embarked on a synthesis of hardground communities through time. Nevertheless, despite a widespread lack of consensus on the nomenclature of hard-substrate trace fossils, and a highly variable degree of accuracy in the identification of the trace-makers, an updating of the stratigraphy of bioerosional trace fossils is attempted here. In the present study, all modes of bioerosion are treated, i.e. internal bioerosion (boring, durophagy) and external bioerosion (scratching, etching) at all scales (macro, meso and micro). On the other hand, no attempt is made at completeness of treatment. Revision of the bioerosional ichnotaxa continues, but still leaves much to be desired. Kobluk et al (1978, p.163) said 'the taxonomy of macroborings is sadly confused and crude,' and much the same might be said

today. Many ichnotaxa are badly in need of revision; many forms in the 'grey area' that surrounds ichnology have been described but not named (e.g. damage to prey skeletons during attempts at predation); some apparent trace fossils, such as embedment structures (e.g. Tremichnus), may not strictly be trace fossils at all; and some trace fossils are very poorly known or understood and therefore provide minimal stratigraphic or biological information. In her unpublished thesis, Plewes (1996) made good progress in the revision of several ichnotaxa, having had singular success in borrowing type material from museums. Some of her taxonomic discoveries are followed in the nomenclature used in this paper. Attribution to a trace-maker Attempts at attribution of hard-substrate trace fossils to the organism that produced them involve all degrees of precision. A few can be ascribed to individual biological species (e.g. several endolithic algae and Cyanobacteria). Indeed, considering the far greater potential for preservation as fossils that these microborings have over their trace-makers, it is surprising these trace fossils were not included in The Fossil Record 2 (Benton 1993). In the case of most endolithic Bryozoa, the boring moulds the external shape of the animal precisely. This has led bryozoologists to treat the borings as body fossils and, therefore, the names as biological taxa (Pohowsky 1974). Although Taylor (1993) considered this an 'unfortunate dual nomenclature', as long as the same name is used by bryozoologists as ichnologists there need be little misunderstanding. This

From: MC!LROY, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 455-479. 0305-8719/04/S15.00 © The Geological Society of London.

Fig. 1. Stratigraphic ranges of bioerosive ichnotaxa and some other bioerosive structures. The chart has been coloured so as to emphasize the possible four 'ages of bioerosive activity'.

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perfection of body moulding by the boring is also seen in the acrothoracican barnacles, microbial borers and phoronids (Plewes 1994). Tavernier et al. (1992) have commented in depth on this situation in respect of microbial borings. At the other end of the scale are the borings of sponges and worms, where the accuracy of trace-maker identification in some cases leaves even the phylum in doubt. Nevertheless, some attributions are wildly exaggerated. Rigby et al. (1993), apparently considering 'Clionidae' and 'boring sponge' to be synonymous (as did de Laubenfels 1955 in the Treatise on Invertebrate Paleontology), extended the range of Clionidae from Early Cambrian to Recent on the basis of trace fossils. It seems unlikely, however, that the family arose before Jurassic and possibly much later. Hard substrate trace fossils are classically divided into macroborings versus microborings, for the study of which a hand lens or a scanning electron microscope is used, respectively. However, as Taylor et al. (1999) pointed out, there is an intermediate category (mesoborings) for which the optical microscope is a necessary tool. There are no firm boundaries between these size classes, and they are not used here. In the following, ichnogenus is the basic rank for treatment. Ichnogenera are grouped either under major trace-maker group (bivalve, sponge, etc.) and trace type (boring, surficial etching trace, etc.), or, in trace fossil types where the trace-makers are unknown or variable, in morphological groups (e.g. 'small round holes in shells'). Ichnofamilies have been characterized by some workers for a few groups but not all, so these are not used here. Stratigraphic ranges of the ichnogenera and various other trace fossils are arranged in Figure 1. Algal, cyanobacterial and fungal borings The microborings produced by endolithic photoautotrophic microorganisms (algae and Cyanobacteria) are ubiquitous in carbonate substrates of the illuminated seafloor today, and the chemoheterotrophs (mainly fungi) occur in virtually all marine environments. Fungal borings are common over a broad bathymetric spectrum. Algal, cyanobacterial and fungal borings have therefore received much attention as indicators of palaeobathymetrical conditions (e.g. Glaub et al. 2001,2002). The fossil record of these microorganisms has received close study over the last few decades and their trace fossils are known to be the oldest,

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microborings of endolithic Cyanobacteria having been found in the Proterozoic (Campbell 1982; Golubic et al. 1985, 1999; Knoll et al. 1986, 1989). The earliest is dated at 1700 Ma (Zhang & Golubic 1987, Fig. 1) or more probably 1500 Ma (Golubic et al. 1999). These Proterozoic borings (some containing bodily preserved borers) are identical to living cyanobacterial borings today. Although earlier studies were mostly made using thin sections, for example by Hessland (1949) in Ordovician limestones, real advances in biotaxonomy and ichnotaxonomy were made only when three-dimensional casts of the microborings could be made in resin (Golubic et al. 1975; Golubic 1990). The morphology of the borings of microorganisms in many cases is species-specific and allows precise identifications with living tracemaking species. However, some forms are extinct, having no living counterparts. Ichnotaxa based on boring morphology are now used, so that all forms are treated alike as trace fossils. Indeed, it is good to see that a description of a new species of endolithic alga, Hyella vacans, includes the morphology of its boring (Gektidis & Golubic 1996); it is Recent and so, as unfossilized material may not form the basis of new trace fossil taxa, the boring itself is not named. Cavernula Radtke 1991 The ichnogenus Cavernula is represented so far by one species. Its morphology is similar to the codiolum stage of the green alga Gomontia polyrhiza. Cavernula pediculata has been described from the Palaeogene (Radtke 1991), the Cretaceous (Hofmann 1996), the Jurassic (Glaub 1994) and the Triassic (Schmidt 1992). Eurygonum Schmidt 1992 The only species of this ichnogenus represents a boring similar to that of the living cyanobacterium Mastigocoleus testarum. It has been reported from the Upper Jurassic (Glaub 1994), the Triassic (Schmidt 1992; Balog 1996) and the Permian (Balog 1996). Fasciculus Radtke 1991 This ichnogenus represents the borings of several species of the living cyanobacterium Hyella. However, one ichnospecies, F. grandis Radtke, has been compared with the endolithic rhizoid of the green dasycladacean macro-alga Acetabularia crenulata (Radtke et al. 1997). Fasciculus is known from the Palaeogene (Radtke 1991), the Cretaceous (Hofmann 1996), the Upper Jurassic (Glaub 1994), the Triassic (Schmidt 1992; Balog 1996), the Permian (Balog 1996) and the Silurian (Bundschuh 2000).

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Hyellomorpha Vogel, Golubic & Brett 1987 The shape of this trace fossil is also considered to closely resemble that of the boring of the young thallus of the living cyanobacterium Hyella. This form is described from the Late Cretaceous (Maastrichtian) by Schnick (1992) and the Devonian by Vogel et al (1987). Orthogonum Radtke 1991 Named for its somewhat right-angular branching pattern, this ichnogenus comprises three rather contrasting ichnospecies. O. tubulare Radtke is considered to be the work of an unknown heterotrophic endolith. O. spinosum Radtke also appears to be the boring of a heterotrophic microorganism, and O. fusiferum Radtke is compared with the heterotrophic fungus Ostracoblabe implexa. Schmidt (1992) added an ichnospecies considered comparable to a red alga, and Glaub (1994) has added three further ichnospecies, one compared with a modern heterotroph of unknown taxonomic affiliation, whereas the other two ichnospecies have no modern counterparts so far. Considering this mixture of different tracemakers, it is unlikely that the stratigraphic range of the ichnogenus will be meaningful; but this is a general problem in ichnology. Orthogonum has been reported from the Palaeogene (Radtke 1991), the Cretaceous (Glaub 1994; Hofmann 1996), the Upper Jurassic (Glaub 1994), the Triassic (Schmidt 1992) and the Silurian (Bundschuh 2000). Schmidt (1992) considered a microboring in Ordovician ostracods described by Olempska (1986) to be conspecific with his O. tripartitum. Palaeoconchocelis Campbell, Kazmierczak & Golubic 1979 This name covers the bodily remains of the endolithic phase of the bangiacean red alga Porphyra nereocystis. Borings of this type are common today and have been described from the Palaeogene (Radtke 1991), the Upper Jurassic to Lower Cretaceous (Glaub 1994), the Triassic (Schmidt 1992) and the Silurian (Bundschuh 2000). The type material was Silurian in age (Campbell et al. 1979). It is so well preserved (deriving from a well core) that organic remains of the boring rhodophyte are preserved. The name applies to the alga, and in fact the boring remains unnamed. However, in the other references here, the name has been applied to the boring. Planobola Schmidt 1992 The four ichnospecies are compared with modern borings of fungi and cyanobacteria. This ichnogenus is known from the Cretaceous

(Glaub 1994; Hofmann 1996), Jurassic (Glaub 1994), Triassic (Schmidt 1992; Balog 1996), Permian (Balog 1996) and Silurian (Bundschuh 2000). Polyactina Radtke 1991 This ichnogenus represents the borings of fungi of, among others, the genus Conchyliastrum. It is known from the Middle Eocene to Upper Oligocene (Radtke 1991), the Cretaceous (Hofmann 1996), the Middle Jurassic (Glaub 1994), the Triassic (Schmidt 1992; Balog 1996), the Permian (Balog 1996) and the Silurian (Bundschuh 2000). Reticulina Radtke 1991 This microboring has been compared with that of the living green alga, Ostreobium quekettii. It occurs in the Lower Eocene and Oligocene (Radtke 1991), the Cretaceous (Hofmann 1996), the Jurassic (Glaub 1994), the Triassic (Schmidt 1992; Balog 1996) and the Silurian (Bundschuh 2000). Rhopalia Radtke 1991 This ichnogenus equates with borings of the green-algal genera Eugomontia and Phaeophila, and occurs in the Eocene and Oligocene (Radtke 1991), the Cretaceous (Hofmann 1996), the Upper Jurassic (Glaub 1994) and the Triassic (Schmidt 1992). Saccomorpha Radtke 1991 Representing the borings of the living fungal genera Dodgella, Lithopythium and Phythophthora, this ichnogenus occurs in the Eocene and Oligocene (Radtke 1991), the Cretaceous (Glaub 1994; Hofmann 1996), Middle and Upper Jurassic (Glaub 1994) and the Triassic (Schmidt 1992). Scolecia Radtke 1991 This ichnogenus contains ichnospecies that compare with both green algae and Cyanobacteria. It has been described from the Eocene and Oligocene (Radtke 1991), the Cretaceous (Hofmann 1996), the Jurassic (Glaub 1994), the Triassic (Schmidt 1992; Balog 1996), the Permian (Balog 1996) and the Silurian (Bundschuh 2000). Small resetted borings This group includes the Dendrina-\ikQ rosette trace fossils, particularly common in the Cretaceous and known otherwise from the Devonian. The group has no modern counterparts (Hofmann 1996). There is probably room for some synonymy among the following forms, and the

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group needs revising and rationalizing. T. J. Palmer (personal communication 2003) considers it likely that all these forms are borings of Foraminifera. Dendrina Quenstedt 1848 The type ichnospecies, D. belemniticola Magdefrau 1937, was compared to Hyellomorpha by Schnick (1992), who considered it likely to be the work of an alga. Hofmann (1996), who introduced several new ichnospecies, found these never to occur together with light-dependent borings (with one exception), suggesting that the organism responsible for Dendrina ispp. was not a photoautotroph. Bertling & Insalaco (1998) attributed an ichnospecies of Dendrina to boring foraminifera. This species-rich ichnogenus has been reported from the Cretaceous and Jurassic. Dendrorete Tavernier, Campbell & Golubic 1992 The only ichnospecies, D. balani, resembles Clionolithes Clarke, but lacks certain details of the branching structure of that ichnogenus (Plewes 1996). D. balani occurs in Pliocene barnacle skeleton. Nododendrina Vogel, Golubic & Brett 1987 This and the following two ichnogenera were considered junior synonyms of Clionolithes by Plewes (1996), which she considered valid. Certainly, Nododendrina nodosa Vogel, Golubic & Brett 1987 looks very like C. radicans Clarke 1908, but this decision cannot be taken in this paper. Both Clionolithes, which was considered a sponge boring by Clarke (1908) and Plewes (1996), and Nododendrina are based on Devonian material. Platydendrina Vogel, Golubic & Brett 1987 Hofmann (1996) described a new ichnospecies from the Upper Cretaceous. The original description was of Devonian material (Vogel et al 1987). Ramodendrina Vogel, Golubic & Brett 1987 Vogel et al. (1987) described Devonian material. Globodendrina Plewes, Palmer & Haynes 1993 The authors considered G. monile Plewes, Palmer & Haynes 1993 to have been made by a foraminiferan. The monotypic ichnogenus is of Late Jurassic age. Sponge borings The borings of sponges possess several characteristics that distinguish them from the work of

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other trace-makers. The structural plan is generally an anastomosing network of canals that in most cases swell to form rounded chambers. Commonly the chambers dominate the boring and obscure the design of the network. Numerous apertures connect the chambers and canals to the substrate surface. The number of chambers ranges from a single cavity to hundreds. The wall of the boring has a special sculpture created by the removal of chips of substrate by the borer. The preservation potential of the boring in carbonate substrates is far superior to that of the opaline spicules of the skeleton of the sponge. Details of spiculation are necessary, both of megascleres and microscleres, however, for the identification of the sponge, and so the taxonomy of the borers is usually unknown. Nevertheless, recent studies by Calcinai et al. (2003) indicate that the chip pattern on the walls may show itself to be diagnostic to genus level. The characteristic wall sculpture allows even the smallest sponge borings, possibly representing borings by the earliest post-larval growth stage, little larger than a millimetre, to be recognized (Bundschuh et al. 1989; Glaub 1994). Sponge borings become abundant in the Mesozoic and remain so to today. Most of these borings are referred to ichnogenus Entobia. However, a number of papers describing sponge borings of Palaeozoic age usually do not refer these to an ichnotaxon. Mikulas (1994b), nevertheless, has described a Devonian boring that closely resembles Entobia. These Palaeozoic sponge borings constitute an intriguing, but little-known group of trace fossils. Entobia Bronn 1837 Ichnogenus Uniglobites Pleydell & Jones 1988 is considered a junior synonym of Entobia, having but a single chamber. Several sponges today fuse their expanding chambers together with growth, some ending with only a single chamber (Bromley & D'Alessandro 1989). Sponges of different families produce similar borings. Sponges of the family Clionidae are the dominant endolithic sponges today. However, species of other living groups of sponges, e.g. the genus Aka [Siphonodictyon] of the family Adociidae, also produce borings that may be included in Entobia (Riitzler 1971; Bromley & D'Alessandro 1989; Reitner & Keupp 1991). The stratigraphic range of Entobia is undoubted from Jurassic to today. Jurassic entobians have been reported by many authors (e.g. Fursich et al. 1994; Bertling 1999; Perry & Bertling 2000).

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Reports of Triassic sponge borings are usually associated with question marks (Szulc 1990). Resin casting, however, has revealed tiny borings showing sponge-chip wall ornament in the Triassic (Schmidt 1992). The Middle to Late Devonian entobian documented by Mikulas (1994b) is very convincing. Further convincing Ordovician sponge borings have been reported by Kobluk (198la), less convincingly by Pickerill & Harland (1984) and Lindstrom (1979). In Lower Cambrian archaeocyath reefs, however, Kobluk (1981b) described cavities showing scalloped walls, and associated sponge chips and spicules. It thus seems clear that boring sponges were active, but rarely documented, throughout the Palaeozoic. Topsentopsis de Laubenfels 1955 De Laubenfels erected this (ichno)-genus to replace the homonym Topsentia Clarke 1921 (non Berg 1899, fide de Laubenfels 1955). He himself regarded its sponge affinities as doubtful. The specimen consists of a cavity from which a few straight canals radiate. Plewes (1996) reexamined Clarke's holotype and concluded that the central structure was a random hole and not a trace fossil. She concluded that Topsentopsis is a nomen dubium. It is interesting that Mikulas (1992) has documented Early Cretaceous structures having a rather similar appearance to Topsentopsis. They are, however, much more regularly developed and more convincing as trace fossils; he named them Entobia Solaris.

Worm borings Caulostrepsis Clarke 1908 U-shaped borings that have a vane connecting the limbs of the U-boring. Polychaete annelids produce Caulostrepsis today. Spionid polychaetes of the genus Polydora are the bestdocumented producers, and this has led to the belief that all Caulostrepsis are produced by Spionidae (e.g. Hantzschel 1975). However, other polychaetes also produce these borings today, such as the small eunicid Lysidice ninetta (Bromley 1978). Boring polychaetes are almost never preserved as body fossils, but see Cameron (1969). The ichnogenus Caulostrepsis ranges from Devonian (Clarke 1908) to Recent, and is abundant in the Jurassic (e.g. Fiirsich et al. 1994) and Cretaceous (e.g. Voigt 1971). Bodily preserved spionid worms have been described from the Cambrian (Glaessner 1976), but very few taxa of that large family are borers. Conchotrema Teichert 1945 This name is in common use. However, the realization that Conchotrema is identical to and thus a junior synonym of Talpina (Voigt 1972; Plewes 1996) renders it of uncertain value, and it will not be considered in this paper. Cunctichnus Fiirsich, Palmer & Goodyear 1994 This is a millimetre-wide boring having very short and stubby branches. The terminations of the branches have a stunted and slightly broadened appearance. It has only been reported from the Jurassic (Fiirsich et al. 1994; Bertling & Insalaco 1998).

Clionolithes Clarke 1908 The validity and coverage of this ichnogenus has been discussed by many authors, and opinions are very varied (Clarke 1921; Solle 1938; Teichert 1945; Vogel et al. 1987). Plewes (1996) examined Clarke's type material and concluded that the ichnogenus is valid; surface sculpture of similar, better preserved material indicated that Clionolithes is likely to be the boring of a sponge. The trace fossil seems to occur chiefly in the Devonian.

Helicotaphrichnus Kern, Grimmer & Lister 1974 The spionid worm Polydora commensalis lives commensally together with hermit crabs, boring a Trypanites-likG tube up the columella of the shell. The trace fossil has been found in Recent, Pleistocene and Pliocene material of California and Baja California (Kern et al. 1974; Walker 1989; Feige & Fiirsich 1991), in the Miocene of Europe (Kern 1979; Baluk & Radwanski 1979a, 1979b), and in the Eocene of Mississippi (Walker 1992).

Cicatricula Palmer & Palmer 1977 Found in a Middle Ordovician hardground, Cicatricula retiformis Palmer & Palmer rather resembles Clionolithes and may also be placed among the probable sponge borings. It differs in covering a larger area and apparently lacking a roof, being an open, etched network on the substrate surface.

Lapispecus Voigt 1970 Ichnogenus Lapispecus is a winding, cylindrical boring. It differs from Trypanites in having a short, lateral vane extending along it locally, especially on the inside of curves (Voigt 1970, 1971; Bromley 1972). A single Early Pleistocene occurrence has been reported (Bromley & D'Alessandro 1987).

A STRATIGRAPHY OF MARINE BIOEROSION

Maeandropolydora Voigt 1965 Voigt (1965) attributed Maeandropolydora, based on two ichnospecies, to spionid polychaetes because the morphology includes pouch-like developments closely similar to Caulostrepsis individuals. These pouches are connected by winding cylindrical tubes. The ichnogenus has been identified in Recent material (Feige & Ftirsich 1991), and three further ichnospecies have been defined on the basis of Pleistocene material (Bromley & D'Alessandro 1987). The trace fossil is common in the Cretaceous (Voigt 1965; Wilson 1986) and Jurassic (Hoffmann & Krobicki 1989; Bertling & Insalaco 1998), and possibly occurs in the Triassic (Schmidt 1992). Palaeozoic reports are less convincing; Mikulas (1994a) identified Maeandropolydora of Devonian age and Bundschuh (2000) used the name for small fragments in the Silurian. Palaeosabella Clarke 1921 Examination of type material led Plewes (1996) to conclude that Palaeosabella Clarke is a valid and useful name. This provides a name for long, tubular borings that, in contrast to Trypanites, expand distally as an acute cone. Palaeosabella is thus a senior synonym for Specus Stephenson, a name that has been little used. This causes a problem in the present case because it means that some trace fossils named Trypanites may in fact be Palaeosabella. Only in very careful studies (e.g. Pemberton et al. 1980) can this be detected, using three-dimensionally exposed material. Palmer et al. (1997) suggest that Trypanites is dominantly a post-Palaeozoic trace fossil, whereas Palaeosabella is chiefly distributed in the Palaeozoic. For the moment, the following evidence is available. Some sipunculans make Palaeosabella-likQ borings today (Rice 1969). The boring of a commensal worm within the bivalve Corbula is clearly Palaeosabella, having a Miocene to Recent range (J. K. Nielsen 1999). The partly bioclaustrated tubes in oysters that Voigt (1965) called Ostreoblabe may be referred to Palaeosabella', they are of Late Cretaceous age. Stephenson (1952) based Specus on Late Cretaceous material. On the other hand, Cameron (1969) and Pemberton et al. (1980) described Palaeosabella from Devonian material (using the names Vermiforichnus and Trypanites, respectively). The record would appear to be incomplete. Spirichnus Fursich, Palmer & Goodyear 1994 These branched and axially spiralled tubes appear to be known only from the Late Jurassic (Fursich et al. 1994; Bertling & Insalaco 1998).

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The small spiral bioclaustration structures Helicosalpinx Oekentorp 1969 (Devonian) and Torquaysalpinx Plusquellec 1968 (Devonian), considered commensals with tabulate corals and stromatoporoids (Stel 1976), somewhat resemble Spirichnus. However, they are not branched and are probably not in any way related to that ichnogenus. Talpina von Hagenov 1840 The work of Voigt (1972, 1975, 1978) demonstrated that Talpina networks are the borings of phoronid worm pseudocolonies. He also demonstrated (Voigt 1972) that Conchotrema Teichert 1945 is a junior synonym of Talpina, as was confirmed by Plewes (1996). Talpina is abundant today and is well known from the Cretaceous (Voigt 1972) and Jurassic (Fursich et al. 1994). The ichnogenus ranges back to the Late Devonian (Thomas 1911; Rodriguez & Gutschick 1970). Trypanites Magdefrau 1932 The revalidation of the ichnogenus Palaeosabella has rendered many records of Trypanites untrustworthy (Plewes 1996; Palmer et al. 1997). These two forms are similar, Trypanites being cylindrical whereas Palaeosabella slightly expands distally. Thus only where the trace fossil has been described fully or illustrated in full profile can a record be accepted. The statement that Trypanites is the oldest macroboring and is abundant in the Early Cambrian may still be true; James et al. (1977, fig. 3B) illustrated what does appear to be true Trypanites, but this needs to be confirmed. Relatively fat and short Trypanites, about 1— 5 mm in diameter, are produced today by sipunculan worms (Rice 1969; Bromley 1978), whereas thin and slender ones are bored by polychaetes (Bromley 1978). Trypanites is common throughout the Cenozoic and Mesozoic (e.g. Bromley & D'Alessandro 1987; Cole & Palmer 1999). Pickerill (1976) described (under the name Vermiforichnus Cameron 1969) Late Ordovician borings in brachiopod shells that are probably Trypanites. The oldest undoubted Trypanites may be middle Ordovician (Kobluk & Nemcsok 1982). These authors recorded scolecodont remains in the borings, which was considered to indicate a polychaete trace-maker. Bivalve mollusc and barnacle etchings Centrichnus Bromley & Martinell 1991 Very shallow etching scars on carbonate substrates are produced by the basal surface of

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balanid cirripeds (Boekschoten 1967; Miller & Brown 1979). The rather deeper scars produced by verrucid cirripeds and the byssal plug of anomiid bivalves have been named C. concentricus and C. eccentricus respectively. Bromley (1999) published a Pleistocene C. eccentricus that lay in situ directly beneath the byssal notch of its anomiid trace-maker. C. eccentricus occurs from Campanian to Recent, and C. concentricus from Miocene to Recent (Radwanski 1977; Bromley & Martinell 1991). The earliest anomiids from Middle and Late Jurassic do not appear to produce Centrichnus (Fursich & Palmer 1982; Todd & Palmer 2002a). Bivalve borings Gastrochaenolites Leymerie 1842 This ichnogenus includes clavate borings in lithic substrates. These are mostly borings of endolithic bivalves, but other trace-makers may be involved. It should be noted that Recent coralliophilid (=magilid) gastropods bore chambers in coral that might be included in Gastrochaenolites, although fossil examples of these do not appear to have been described (Wenz 1939; Gohar & Soliman 1963; Soliman 1969; Bromley 1970a, fig. 4d; Massin 1982, 1983, 1988, 1990, 1992; Zibrowius & Arnaud 1995). Nevertheless, some of these gastropod borings differ from Gastrochaenolites in having an extra canal diverging from near the aperture (Massin 1987). Some sipunculan worms may also produce Gastrochaenolites (Rice 1969; Ekdale et al. 1984). Gastrochaenolites becomes abundant and diverse in the Jurassic (e.g. Kelly & Bromley 1984; Oschmann 1989). The oldest Gastrochaenolites containing its trace-maker (Lithophaga sp.) is Triassic (Kleemann 1994). Another Gastrochaenolites producer, Gastrochaena, has its bodily first appearance also in the Triassic (J. G. Carter in Wilson & Palmer 1990). Early Pennsylvanian Gastrochaenolites have been described from rocky shore environments (Webb 1993, 1994; Wilson & Palmer 1998). Although in this case the borer is not preserved, the morphology of these (G. anauchen) corresponds exactly to that of bivalve borings. The oldest Gastrochaenolites, also lacking its tracemaker, is G. oelandicus of Early Ordovician age (Ekdale & Bromley 2001; Ekdale et al. 2002). Petroxestes Wilson & Palmer 1988 The mytilid bivalve Coralliodomus was found fitted well within short, deep grooves on the under side of Late Ordovician stromatoporoids

(Pojeta & Palmer 1976). This is the earliest direct evidence of mytilid bioerosion. The grooves, locally abundant, have since been found empty, i.e. lacking the bivalve, and have received the name Petroxestes pera Wilson & Palmer 1988. Tapanila & Copper (2002) reported Petroxestes from the Silurian. Recently, a single specimen of a groove in a Miocene oyster shell has been identified as Petroxestes pera (Pickerill et al. 2001). Until further convincing material is described, however, such drastic extension of the range of this ichnospecies must remain uncertain. Teredolites Leymerie 1842 Clavate borings in wood (lignic) substrates have apparently all been attributed to bivalve tracemakers (Kelly & Bromley 1984). These belong to two families: the Teredinidae (shipworms) from Early Cretaceous to today, and the Pholadidae from Middle Jurassic to today (Kelly 1988; Evans 1999). At present, two ichnospecies are recognized. T. clavatus Leymerie is produced by species ofMartesia and Xylophaga today, and fossil trace-makers include Martesia, Xylophaga and Opertochasma, whereas T. longissimus Kelly & Bromley is produced by the teredine shipworms. Both ichnospecies become abundant by the end of the Cretaceous. For example, Mikulas et al. (1995) documented crowded individuals of T. clavatus of Cenomanian age, and Huggett et al. (2000) recorded a diversity of six teredinid species producing T. longissimus in an Eocene community. Gastropod borings and surficial etchings It was noted above that some gastropods bore cavities in coral substrates today that might be assigned to ichnogenus Gastrochaenolites. One group of boring gastropods, however, produces borings that are characteristic enough to be assigned an ichnogeric name: the vermetid gastropods (Lamy 1930; Bromley 1970a; Savazzi 1996). Renichnus Mayoral 1987b Mayoral (1987b) assigned the name R. arcuatus to etchings produced by vermetids that spiralled at an angle to the substrate surface. As a result, the substrate was etched only where the spiral came into contact with the surface. This produced a series of kidney-shaped depressions hence the name. However, many vermetids spiral in the plane of the interface, in which case the whole spiral is etched into the substrate.

A STRATIGRAPHY OF MARINE BIOEROSION

As all intermediates are found, the one name is probably adequate for all vermetid etchings (Radwanski 1977). The depth to which the gastropod etches itself into the substrate varies, and these trace fossils are intermediate between borings and surface scars. Renichnus is known from Pliocene to today (Mayoral 1987b; Taddei Ruggiero 1999). Gastropod etched and rasped pits and scars Many different groups of gastropods etch or abrade (or both) home depressions in their substrate. Most celebrated are the homing scars of intertidal patelliform gastropods (limpets) (e.g. Orton 1914; Bromley & Hanken 1981; Lindberg & Dwyer 1982). Some modern limpets make pits on the surface of other living molluscs (Vermeij 1998). The great holes supposedly punched in giant ammonites by Late Cretaceous playful or hungry mosasaurs (KaufTman & Kesling 1960; Kauffman 1990), or due simply to implosion damage (Keupp 1991), have been reinterpreted as the homing pits of limpets (Kase et al. 1994, 1998; Seilacher 1998). Scars produced on host shells beneath stationary parasitic capulid gastropods are also well known today (Sharman 1956; Boss 1965; Bromley 1981), and are reported from the Pleistocene (Matsukuma 1978). Less well known are the scars produced by hipponicid gastropods on the shells of other living gastropods. Vermeij (1998) described and illustrated Recent material and found corresponding pits in Early Pliocene and Late Miocene material. The presence of hipponicid body fossils in the same beds increased confidence in attributing the pits, but Noda (1991) actually found a Pliocene hipponicid in place over its etched scar. Pit etching has been very rarely mentioned in association with the genus Crepidula (slipper limpets). However, Walker (1992) has found that Crepidula adunca, which attaches to the external surface of gastropod shells while the host snail is living and after hermit crabs have taken over, etches a characteristic scar in the shell. Comparable scars have been found on Eocene gastropod shells. None of these scars have received ichnotaxonomic treatment. Gastropod and chiton radulation The radula of most patellid and acmaeid gastropods contains opal and goethite (Runham 1961;

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Lowenstam 1971) and is therefore considerably harder than calcite. Similarly, chiton radulae contain denticles hardened with magnetite (Lowenstam 1962). The radulae of these molluscs are therefore adapted to surficially eroding carbonate substrate and allowing the animals to exploit the endolithic microbes as a food source. The fine sculpture that this activity imparts to the substrate has a relief in the region of 100 jam and therefore has a fair fossilization potential. The sculpture has been named Radulichnus inopinatus Voigt. This name covers both gastropod and chiton radulation, which can be distinguished on the basis of morphology. The holotype is a gastropod radulation sculpture. Radulichnus Voigt 1977 The trace fossil is abundant and widespread today, both of chiton origin (e.g. Jiick & Boekschoten 1980) and gastropod (e.g. Bromley & Hanken 1981). Reports of fossil examples are not common, undoubtedly because of their small scale, but the study of acmaeid Radulichnus on Cretaceous ammonites by Akpan et al. (1982) is outstanding. Radulichnus was also found in association with the limpets living on ammonites described by Kase et al (1998). It is not known when Radulichnus was first produced. Chitons (Runnegar et al. 1979) or related forms (Stinchcomb & Darrough 1995) occur in the Late Cambrian. Radulation trace fossils are known from the Cambrian, but only in matgrounds, not in hard substrates (Seilacher 1995; Bottjer 2000; Dornbos & Bottjer 2000; Dornbos et al. in press). Bryozoans Owing to the 'dual nomenclature' for bryozoans, as mentioned above, the degree of 'splitting' as biotaxa is excessive in an ichnological context. Therefore the biofamily level has been chosen, where this is known, and the following is based largely on the work of Pohowsky (1978) and Taylor (1993). Our present knowledge of stratigraphical ranges suggests that first appearances of endolithic bryozoan taxa occur rather smoothly through time. Most of these appear to belong to the Ctenostomata. In the following, the taxa are discussed in stratigraphical order of first occurrence, youngest first. Mayoral (1987a) designated the ichnogenus Stellichnus for the boring produced by the Early Pliocene ctenostome bryozoan Paravinella (Mayoral & Reguant 1995).

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The Immergentiidae and Terebriporidae both appear in the Late Cretaceous and continue to Recent. Immergentia cruciata (Magdefrau 1937) is the oldest known immergentiid, Santonian, Late Cretaceous (Taylor 1993). Vogel et al (1987) transferred /. devonica Richards (1974) to Orbignyopora Pohowsky 1978. Taylor (1993) gave the range for the Terebriporidae as Late Cretaceous (Maastrichtian) to Recent. Mayoral (1988), considering Terebripora a biogenus, has published the ichnogenus Pinaceocladichnus as an ichnological equivalent. Mayoral et al. (1994) have claimed Ordovician orthids to contain Pinaceocladichnus, but this form may be more closely related to the Ropalonariidae (Todd 2000). Foraripora Voigt & Soule 1973 comprises a single ichnospecies, F. pesavis (somewhat resembling a minute bird trackway). Its age is Late Cretaceous (Voigt 1979). The Spathiporidae range from Jurassic to Recent (Taylor 1993). Mayoral (1988), considering Spathipora to be a biogenus, has published the ichnogenus Pennatichnus as an ichnological equivalent. The Penetrantiidae range from Triassic (Rhaetic) to Recent (Taylor 1993). Pohowsky (1978) declared Iramena danica Boekschoten 1970 (Danian) a junior synonym of Penetrantia. Fiirsich et al. (1994) recorded Iramena isp., of Late Jurassic age. The Cookobryozoonidae are known from two localities only, of Triassic age, and the Voigtellidae range from the Permian to Maastrichtian (Taylor 1993). Fischerella is restricted to the Late Mississippian and Casterella to the Late and Middle Devonian (Pohowsky 1978). The family Orbygnyoporidae ranges from Silurian to Triassic (Taylor 1993), although Cuffey et al. (1981) claimed Orbignyopora isp. indet. from the Maastrichtian. The Ropalonariidae comprise one certain (ichno)species, Ropalonaria venosa Ulrich 1879 from the Late Ordovician, the oldest known bryozoan boring. Two other (ichno)species, of Late Jurassic and Early Cretaceous age respectively, possibly belong to Ropalonaria (Pohowsky 1978). The latter two (ichno)species were accepted by Taylor (1993), who thereby gave a range for the Ropalonariidae as Ashgillian to Aptian. Leptichnus Taylor, Wilson & Bromley 1999 is a surficial etching trace of cheilostome bryozoans. At least nine Recent species of cheilostome produce it today. There are two ichnospecies. Palmer & Plewes (1993) published a fine illustration under the nomen nudum Bothrioichnus. The

earliest occurrence of Leptichnus is Maastrichtian, Late Cretaceous. Brachiopod etching While the morphology of some attachment scars of pediculate brachiopods may vary (Bromley & Surlyk 1973), the large majority conform to a similar plan and have been covered by a single ichnospecies, Podichnus centrifugalis Bromley & Surlyk, despite the wide taxonomic variety of the trace-making brachiopods. Where the etching trace was emplaced in a living brachiopod, the shell can exhibit considerable reaction in the form of wound tissue (Taddei Ruggiero 1999). Podichnus Bromley & Surlyk 1973 The trace fossil is usually a clearly visible mesoboring etched shallowly into the substrate. However, several occurrences have been recorded in epoxy casting of substrates, e.g. in the Triassic (Schmidt 1992; Glaub & Schmidt 1994), the Devonian (Vogel et al. 1987) and the Silurian (Bundschuh 2000). Alexander (1994) has also recorded Podichnus from the Carboniferous. The range is Silurian to Recent. Crustacea: acrothoracican borings The borings of acrothoracican cirripeds are small cavities shaped somewhat like a sock, which in many ichnospecies has a partial calcitic lining (Seilacher 1969; Tomlinson 1969). Biologists treat the fossil borings as body fossil moulds, in the same way as the description of Bryozoa. Ichnologists, however, use the first 'genus' available, Rogerella, as the ichnogenus (Bromley & D'Alessandro 1987; Plewes 1996). Rogerella Saint-Seine 1951 Acrothoracican borings are abundant today and back through the Mesozoic. Webb (1994) found them in rocky shore environments of the Early Mississippian. Rodriguez & Guttschick (1970) described them from Late Devonian material. The oldest examples may be those commensal with the gastropod Platyceras, itself parasitic on crinoids of Middle Devonian age (Baird et al. 1990). The unabraded aperture of this form is much wider than is usual for Rogerella. Bundschuh (2000) reported Silurian acrothoracican borings, but her illustrations are not very convincing. The range, then, is Devonian, maybe Silurian, to Recent.

A STRATIGRAPHY OF MARINE BIOEROSION

Crustacea: ascothoracican borings Small, round holes, usually millimetric in size, are produced by endoparasitic ascothoracican cirripeds in irregular echinoids. They tend to be placed near the genital pores of the apical system, but are larger than these (Brattstrom 1936, 1937, 1947). In some cases the holes lie at other locations on the aboral surface of the echinoid (Madsen & Wolff 1965). The published fossil record of ascothoracican borings is negligible. This must be due in part to the great similarity of the borings to drillholes of predatory gastropods. Madsen & Wolff (1965) attributed two holes in the test of a Late Cretaceous holasteroid echinoid to ascothoracican parasites. Two round holes that Kier (1981) described in an Early Cretaceous spatangoid echinoid may be of ascothoracican origin, although that author interpreted them as the work of parasitic gastropods. Voigt (1959, 1967) attributed cyst-like cavities, having a slit-like entrance, in Danian octocorals to the parasitism of ascothoracicans, naming the (unpreserved) animals Endosacculus moltkiae and Endosacculus? najdini. These may be bioclaustration structures. Shell chipping and peeling Chipping and peeling of shells is a common source of trace fossils. It is a form of durophagy, the damage of hard skeletal material by predation. Durophagy can be extremely difficult to distinguish from physical or diagenetic damage (Elliott & Bounds 1987; Lescinsky & Benninger 1994). Today, shell chipping of bivalves and shell peeling of prey gastropods is dominantly the work of crustaceans (Cadee 1968; Shoup 1968; Bishop 1975; Vermeij 1982, 1983b; Shigemiya 2003), but fish and sharks (see below) and even some asteroids (Mauzey et al. 1968) are known to be durophagous. Among brachyuran crabs the habit is especially abundant and, in one study, peeling frequency in a given area of seafloor was correlated loosely with the number of resident species of the shameface crab, Calappa (Vermeij et al. 1980; Vermeij 1982). However, shell peeling is also practised by many other genera of crabs. The stratigraphy of shell chipping and peeling is fairly straightforward. Durophagous shell breakage is abundant in the Neogene (Radwanski 1977; Martinell 1989). In fact this abundance begins in the Jurassic, and PreJurassic shell breakers have been considered

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comparatively weak and rare (Vermeij et al. 1982). Sublethal injury and repair was reported in Carboniferous ammonoids by Bond & Saunders (1989). Devonian brachiopods (Leighton 2001) and gastropods (Lindstrom & Peel 2003) were predated by unidentified durophages, at which time another increase in predation level has been detected (Signor & Brett 1984). Elliot & Brew (1988) found the beak of a nautiloid lodged within the valve of a Devonian brachiopod, and Peel (1984) described damage to Silurian gastropods that compared to that inflicted by Sepia today, suggesting cephalopod predation. There are also reports of Early Palaeozoic durophagy. Indeed, according to Peel et al. (1996) and Ebbestad & Peel (1997), Silurian and Ordovician gastropods show 'heavy damage' by predators. Alexander (1986) considered nautiloids as likely producers of predator damage on Ordovician brachiopods. Cambrian evidence of durophagy is dominated by bitten trilobites (Babcock 1993; Pratt 1998). Crustacean smashing traces Crustaceans produce still other forms of trace fossils in hard substrates that are particularly difficult to classify. One of these, the durophagous activities of Stomatopoda or mantis shrimps, has received an ichnogenus, Belichnus Pether 1995. These are holes produced in gastropod and bivalve prey skeletons by blows struck by the club-shaped chelipods of smasher stomatopods. Such damage is difficult to distinguish from other damage unless there is evidence of impact. The inwardly bent manner of chipping of the shell provides this evidence. Belichnus Pether 1995 Stomatopods are active durophagous carnivores today (Caldwell & Dingle 1975, 1976). Pether (1995) based the ichnotaxonomy on Holocene material, but Geary et al. (1991) interpreted Plio-Pleistocene shell damage as stomatopod activity, and Bamk & Radwanski (1996) have extended this to the Miocene. Borings in coral and rock by Decapoda Many crustaceans bore various forms of chambers in coral (e.g. Andre & Lamy 1933; Zibrowius 1984) and rock. The thalassinideans are perhaps best known: for example, Upogebia operculata bores branched passages in coral (Kleemann 1984; Scott et al. 1988; Williams &

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Ngoc-Ho 1990; Fonseca & Cortes 1998) and Alpheus saxidomus produces sizable chambers within basalt (Holthuis 1980; Fischer & Meyer 1985). Such structures do not seem to have been described from the fossil record. Echinoid borings Several species of regular echinoid bore bowlshaped, hemispherical or deeper pits in rock surfaces in shallow water (Markel & Maier 1966, 1967; Warme 1975). The boring process is mechanical, so the activity is not restricted to carbonate substrates: for example, Allouc et aL (1996) described Recent ones in dolerite, and Abel (1935) in granite. Circolites Mikulas 1992 Fossil structures interpreted as echinoid borings have been reported many times from the Pliocene (Martinell & Domenech 1986; Agusti et al 1990; Watkins 1990; Gibert et aL 1998), and Miocene examples were published by Radwanski (1965, 1969, 1970). Cenomanian material was described by Gtazek et al. (1971), Lewy & Avni (1988, in association with body fossils of the echinoid Goniopygus menardi) and Mikulas (1992). Jurassic echinoid pits were reported by Fursich (1979). The range is from Jurassic to Recent. Echinoid bite traces Bite traces produced by plucking and grazing regular echinoids are abundant in the present marine environment (e.g. Krumbach 1914; Krumbein & Van der Pers 1974; Bak 1994). They are less common in the fossil record, undoubtedly because of their shallow topography and their poor preservation potential. Gnathichnus Bromley 1975 There are surprisingly few reports of Gnathichnus of Neogene and Palaeogene age (e.g. Martinell 1982; Barrier & D'Alessandro 1985). In contrast, descriptions of Cretaceous examples are numerous (e.g. Bromley 1970b, 1975,1994; Voigt 1972,1975, 1996; Breton et al. 1992). Jurassic occurrences have been documented by Palmer (1982) and Riegraf (1973), and from the Triassic by Michalik (1977, 1980) and Fursich & Wendt (1977). Bite traces attributed to fish and sharks There are many reports of bite traces caused by fish predation and grazing in the Recent (e.g.

Monteillet et al. 1982; Nebelsick 1999). Correspondingly, many cases of bite traces in the fossil record have been ascribed to fish and sharks. Fish grazing on coral reefs was considered by Hubbard et al. (1990) to be chiefly important from the base of the Palaeogene onward. Ichnological nomenclature has not been attempted for this variable group of trace fossils. Reports of trace fossils produced by such activity, however, are particularly numerous in Cretaceous prey skeletons (e.g. KaufTman 1972; Thies 1985; Giessler 1991; Dortangs 1998; Neumann 2000). Older occurrences were reported by Holder (1973) on Early Jurassic belemnites and Devonian ammonites, and by Hansen & Mapes (1990) on Carboniferous and Devonian nautiloids. Small round holes in shells A large variety of trace-makers make and have made small round holes in shells and tests for a variety of reasons. Some holes penetrate right through the shell, others end blind because the drilling process was interrupted, and still others end blindly through intent. The purpose of drilling the holes includes predation (penetrative when complete), parasitism (either penetrative or non-penetrative), or mere attachment to the substrate (chiefly non-penetrative). This variability has led to the designation of three ichnogenera: Oichnus Bromley 1981 for penetrative holes, and Sedilichnus Miiller 1977 and Tremichnus Brett 1985 for non-penetrative pits. However, because ichnotaxa are based on morphology, and because the two morphological forms are not clearly different, there has recently been discussion as to whether Tremichnus should be considered a junior synonym of Oichnus (Pickerill & Donovan 1998; Nielsen & Nielsen 2001, 2002; Todd & Palmer 2002b; Donovan & Jagt 2002; Donovan & Pickerill 2002.) This suggested synonymy is followed here, even though Sedilichnus may be the senior synonym. Oichnus is produced by a wide range of tracemaking organisms. As the trace fossils contain few fingerprints to indicate the taxonomic position of the trace-maker, confident identification of the borer is not common. Small (millimetric) round drill-holes are dominantly produced today by predatory gastropods of the Naticidae and Muricacea, both arising in the Cretaceous (Taylor et al. 1983; Tracey et al. 1993). The holes produced by these two groups are considered to be morphologically distinguishable (Carriker 1981), as Oichnus paraboloides and O. simplex respectively (although Herbert & Dietl

A STRATIGRAPHY OF MARINE BIOEROSION

2002 found some Recent muricids produce borings indistinguishable from those of naticids). Thus a good guess at the trace-maker may be made for material of Cretaceous or younger age (e.g. Carriker 1998). Nevertheless, it cannot be ignored that octopods can make drill-holes closely similar to those of muricaceans, and that both ichnospecies occur throughout the Palaeozoic. Recently, the identity of some presumed early naticid gastropods has been revised, transferring them to the non-boring family Ampullospiridae (Kase & Ishikawa 2003). These authors claim that the Naticidae first appeared in the Late Cretaceous Campanian, and that the four species of gastropod that accompany the supposed earliest naticid borings in the English Middle Albian (Mid-Cretaceous) Blackdown Greensand are not naticids but ampullospirids. Concerning the nature of the trace-maker, the Albian (and other O. paraboloides dated earlier than the Campanian) 'remain mysterious' (Kase & Ishikawa 2003, p. 406). The life-long borings made by ectoparasitic capulid gastropods usually have diagnostic features that allow their attribution (Orr 1962; Matsukuma 1978; Bromley 1981). In the Palaeozoic, the parasitic gastropods of the family Platyceratidae have been found in place over their drill-hole in Mississippian crinoids (Baumiller 1990) and Devonian brachiopods (Baumiller et al. 1999) and blastoids (Baumiller 1996). The boreholes are close in shape to O. simplex. Some of the borings generated by predatory octopuses are morphologically characteristic as O. ovalis (Bromley 1993), but not all. Neither do all living species of octopus bore holes. The earliest identified octopus boring is Pliocene in age (Robba & Ostinelli 1975), or possibly Cretaceous (Harper 2000), whereas rare, octopus-like body fossils date back to the Carboniferous (Kluessendorf & Doyle 2000). Boreholes have a much better preservation potential than octopus bodies (Engeser 1990), and this may indicate that the hole-drilling habit is relatively new in the octopods. Small round holes in echinoids, produced by endoparasitic ascothoracican barnacles, are treated under crustacean borings. Oichnus Bromley 1981 Small round holes in shells are abundant today and since the Late Cretaceous, when the muricacean and naticid gastropods began boring (e.g. Vermeij et al. 1980; Kowalewski et al. 1998; Kase & Ishikawa 2003). They are also common in Jurassic bivalves and brachiopods (Harper

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et al. 1998; Harper & Wharton 2000), and have been found in parasitized crinoid columns (Feldman & Brett 1998). Oichnus appears to be unimportant from the Carboniferous to Triassic. However, Fiirsich & Jablonski (1984) found borings resembling O. paraboloides of Triassic age, and Ausich & Gurrola (1979) found both O. simplex and O. paraboloides in Carboniferous brachiopods. Baumiller (1990), Baumiller et al. (1999) and Hoffmeister et al. (2003) reported drill-holes in Carboniferous crinoids and brachiopods. The Devonian period has provided evidence of much shell-drilling (Kowalewski et al. 1998). Devonian blastoids contain holes attributed to platyceratid gastropods (Baumiller 1993, 1996; Baumiller & Macurda 1995), and Devonian brachiopods display drill-holes resembling both O. paraboloides (Smith et al. 1985) and O. simplex (Leighton 2001). Much-parasitized Silurian crinoids were described by Brett (1978). The oldest trace fossils referable to Oichnus occur in the Cambrian (Matthews & Missarzhevsky 1975; Conway Morris & Bengtson 1994) and Late Precambrian (Bengtson & Zhao 1992), the latter in the tube of the earliest known animal to produce a mineralized skeleton! Minute round holes in shells: borings by and in Foraminifera Increasing interest is being shown in the minute borings in microfossils, especially foraminifera. Some of these have been produced by foraminifera. This group of trace fossils represents a size-class of its own (K. S. S. Nielsen 1999; Nielsen & Nielsen 2001). Several new ichnospecies of Oichnus have been based on these minute trace fossils, ranging from about 10 to 60 um in diameter, and some new ichnogenera are being established, e.g. Dipatulichnus Nielsen & Nielsen 2001 and Stellatichnus Nielsen & Nielsen 2001 (see also Nielsen et al. 2003). The minute, globular pit Planobola macrogota Schmidt 1992 is comparable with Oichnus in Foraminifera, being 20-100 um in diameter (e.g. Bundschuh 2000). It occurs in trilobite, brachiopod and coral skeleton, and has not been reported in foraminiferans. The parasitic foraminiferan Planorbulinopsis parasita creates rounded, flat-floored pits about 0.5 mm wide in its host, the foraminiferan Alveolinella quoii (Banner 1971). The pits were not named, the material being Recent. Late Cretaceous orbitoid Foraminifera have been reported that contain relatively large (nearly 1 mm wide) borings (Baumfalk et al.

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1982). These borings have a looped or horseshoe shape to fit the limitations of the substrate, but have side branches and crossovers. They were not named, and were interpreted as the work of an anomalinid foraminiferan, Talpinella cunicularia, specimens of which were usually present within the borings. In fact, Foraminifera themselves have been shown to produce a variety of borings (Matteucci 1980; Cedhagen 1994; Freiwald & Schonfeld 1996; Venec-Peyre 1996). Hallock & Talge (1994) and Hallock et al (1998) described the predatory borings of Floresina amphiphaga in other foraminifera. Several reports of fossil borings attributed to foraminifera include some resetted borings such as Globodendrina monile, Late Jurassic (Plewes et al. 1993; Bertling & Insalaco 1998). The Jurassic foraminiferan Troglotella incrustans bores in the juvenile growth stage (Schmid & Leinfelder 1996). Venec-Peyre (1996) identified 20 species known to bore from Jurassic to Recent. Fossil borings and bioclaustration of uncertain origin Grahn (1981) described minute round holes made in Ordovician chitinozoans, supposedly by bacteria and fungi. Phrixichnus Bromley & Asgaard 1993 This strange boring somewhat resembles Gastrochaenolites, but has a growth-line ornament and a somewhat quadrate cross-section. P. phrix is the only ichnospecies. It occurs today (Bromley & Asgaard unpublished observations), yet the trace-maker remains unknown. It was first published in Pliocene material (Bromley & Asgaard 1993), and has been reported from the Miocene rocky shore environment (Domenech et al. 2001). Planovolites Mikulas 1992 This trace fossil comprises depressions about 50 cm wide and 1-1.5 cm deep, and is considered to be an algal-grazing area, perhaps by echinoids or molluscs. P. homolensis (Early Cretaceous) is the only ichnospecies (Mikulas 1992). Diorygma Biernat 1961 This is a bioclaustration structure caused by an endoparasitic or commensal worm in Devonian atrypid brachiopods. The worm was considered to be a phoronid by MacKinnon & Biernat (1970). There is no sign of bioerosion, and this may not be a true trace fossil.

The small bioclaustration structures, Ordovician to Devonian, including Helicosalpinx Oekentorp 1969, Torquaysalpinx Plusquellec 1968, Chaetosalpinx Sokolov 1948, Catellocauda Palmer & Wilson 1988, Hicetes Clarke 1908 and others, are considered commensal with bryozoans, tabulate corals and stromatoporoids (Stel 1976; Tapanila 2002, in press). As yet, little is known about them. Discussion The present survey of ranges of bioerosional ichnogenera is more comprehensive than previous ones (e.g. Kobluk et al. 1978; Palmer 1982), but it does not reveal many surprises. As seen in Fig. 1, there are two periods of diversification, both of which have been recognized before. The 'Mesozoic marine revolution' is well marked by the abundance and diversity of bioerosional trace fossils (Bertling 1999; Perry & Bertling 2000). Vermeij (1977, 1983a) saw a substantial increase in durophagous and drilling predation in the middle Mesozoic, and corresponding changes in gastropod shell architecture. Brett (1988) drew attention to the much greater abundance of deeply boring bivalves and sponges in the Cretaceous than earlier. Palmer (1986) suggested that an increase in predation might have triggered this trend. According to Reitner & Keupp (1991), the sponge genus Aka appeared in the Triassic and became endolithic in the Jurassic, producing giant borings. By the Cretaceous, Aka tended to occupy deeper, tranquil waters, whereas the newly appearing endolithic clionid sponges generally colonized the shallow, energetic waters. It is hoped that new work on ichnogenus Entobia will support these trends. The apparent dearth of Oichnus in the Jurassic reported by Kowalewski et al. (1998) does not seem to be real (Harper et al. 1998; Harper & Wharton 2000). A Mid-Palaeozoic precursor of the Mesozoic revolution was identified by Signor & Brett (1984) and Brett (1988) as the diversity of hardground communities increased markedly within the Ordovician and Devonian, and this is also clearly represented by the record of bioerosive trace fossils. Gastrochaenolites and Petroxestes make their first appearance at this time, as do Talpina, acrothoracican barnacles and Caulostrepsis (e.g. Wilson & Palmer 1998). There is also a rapid appearance of durophagous predators in the Devonian: placoderms, other fish and some arthropods (Signor & Brett 1984). Before the first, mid-Palaeozoic revolution, the bioerosion process was not impressive. There

A STRATIGRAPHY OF MARINE BIOEROSION

were boring worms producing Trypanites and Palaeosabella; endolithic sponges were present but uncommon; drilling durophagy existed, apparently on a small scale. All these forms were present in the Early Cambrian. In the Late Proterozoic we have an isolated but remarkable occurrence of Oichnus as well as several records of Cyanobacteria. The earliest bioerosion demonstrated is an endolithic cyanobacterium from the Early Proterozoic dated at 1700 Ma. A further slight increase in bioerosional ichnodiversity is suggested for the Neogene. It is not clear whether this is a genuine mini-revolution, or merely a function of taphonomy. Generally speaking, these latest sediments have suffered less diagenesis than the older, which may have led to an increase in the preservation potential of trace fossils. Conclusions The approximately 65 ichnogenera catalogued in this study plus several types of unnamed trace fossil have very variable quality of information to impart. However, those that appear to have a meaningful stratigraphic representation combine to emphasize the diversity of trends of the Phanerozoic (Fig. 1). The Cambrian radiation did not express itself among the bioerosional trace fossils. However, the Mid-Palaeozoic and Mesozoic marine revolutions played a strong role. Most of the behavioural-biological groups of bioerosion that have been outlined in this paper have survived to the present day. The present bioerosion community may therefore be classified as comprising three chronological categories. The Early Palaeozoic forms include Trypanites, possibly Palaeosabella, Oichnus, sponge and cyanobacterial borings, and damage by predatory attack. The Late and Mid-Palaeozoic forms include the endolithic algae, acrothoracican barnacles and mytilid boring bivalves, ctenostome bryozoans, much durophagy, and Podichnus. The Mesozoic-Cenozoic bioerosion community is very diverse, both biologically and ichnologically, and is dominated by endolithic sponges, a variety of boring and etching bivalves, rasping gastropods and chitons, biting echinoids and a host of minor groups. There appears to be yet another increase in bioerosive biodiversity in the Neogene. This effect may have been assisted somewhat by taphonomy: relative to the older sediments, the younger sediments have generally been affected only mildly by diagenetic processes that might decrease the preservation potential of trace fossils.

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I. Glaub is thanked for having provided considerable assistance with the literature on microbioerosion during the preparation of this paper.

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Index Note: Page numbers in bold refer to tables; those in italics refer to figures (while some are included within page ranges). Alberta, Canada, Viking Formation 40, 41, 42, 44, 45, 49, 50, 53, 55-7, 56 algal borings 455-6 Alum Shale Member, North Yorkshire 146, 147, 148 amalgamated sequence boundaries 51-7 animal behaviour 399 ant trace fossils 373, 421, 423 see also Krausichnidae Arab-D interval, Saudi Arabia 31 Archeoentomichnus 434, 439, 446-7 archosaurs see dinosaur tracks Arenicolites 153, 164 Denison Trough 285, 286, 287 He Formation 241 Argentina palaeosol ichnofabrics 360-5, 367-71, 375 Triassic lacustrine deltas 335-54 Arizona, Bright Angel Shale 213-36 arthropod trackways 7-8, 313, 318, 321, 322, 326, 346 Asencio Formation, Uruguay 365-7, 366, 370 Asteriacites 164, 165, 166, 167, 168 Asterosoma 241, 249-50, 249, 250, 267, 303 Attaichnus 436, 437 Australia Cambro-Ordovician formations 390, 391, 392 Pebbley Beach Formation 179-212 shoreface/deltaic facies 273-310 Austria, Triassic-Jurassic extinction 409-11, 410 avulsion belt deposits 360, 361 Bandera Shale Formation, Kansas 162 barnacle etchings 461-62 bathymetric reconstruction 4, 72-4 bayhead delta 55-7, 268 Beaconites 317, 321, 365 bee trace fossils 362, 363, 364, 371, 372, 422, 423 see also Celliformidae beetles see coleopteran trace fossils behavioural homologies 359 Belichnus 465 benthic fauna 146-7 benthic oxygen levels 398 Bergaueria 164, 241 BI see bioturbation indices Bichordites 83^, 84, 85, 87 bioerosional trace fossils 455-79 see also borings; etchings biogenic marks 224 biomechanics of dinosaurs 95 bioturbation see also ichnofabrics claystone and siltstone 281-83 depth and extent 10-11, 393, 397-8, 408 intensity 13-14, 183, 296

laminated sandstone 283-5, 287-8 measurement 355-8 muddy sandstone 188-91, 283, 285-7, 286 mudstone 281 preserved depth 383 sandstones 193-4, 193, 283-* siltstone 182-8, 279-83 succession 16-17 timing 383 bioturbation indices (BI) 356, 357 bite traces 466 bivalve trace fossils 10, 461-62 Blue Lias, Lyme Regis, Dorset 151^ body size 398-99 borings see also microborings acrothoracicans 464 algal, cyanobacterial and fungal 457—8 ascothoracicans 465 attribution 455-7 bivalves 461 Decapoda 465-6 echinoids 466 foraminifera 467-8 gastropods 462-3 in shells 466-7 small rosetted 458-9 sponges 459-60 worms 460-1 brachiopod etchings 464 brackish-water deposits 179-81, 202, 237 see also tidal flats Bright Angel Shale, Arizona 213-36 ecosystem reconstruction 233—4 facies 220 ichnoguilds 223-7 lithofacies 217-23 palynofacies 227-30, 228-30 stratigraphy 215 bryozoans 455, 463—4 Burniston dinosaur tracks 113-19, 113, 116, 118 burrows chambered trace fossils 425, 435, 436-8 dimensions 15 echinoids and crustaceans 80, 82 escape 10, 242, 243, 245-6, 246 Glossifungites ichnofacies 35 homogenization 11 horizontal 319 insects 317,420 interstratal 222 iron-rich sandstones 225 meniscate 375 Phycosiphon 245

482

burrows (cont.) Savrda and Bottjer's model 142-3 shallow-tier 386, 394 thalassinideans 79 U-shaped 225 vertical 313, 314,315, 317 Bynguano Formation, Australia 391, 392 Callichurus major 79, 83 Calliopsis 372 Cambrian base 399 Bright Angel Shale, Arizona 213-36 earliest muddy sediments 385-6 ichnological record 386—93 radiation event 7 canyons, submarine 37, 38 Carboniferous tidal environments 157-78 Cardium Formation, Alberta 49, 51 Caulostrep sis 460 Cavernula 65, 66-7, 66, 457 cavities see microborings Cellicalichnus 362, 363, 364, 423 Celliforma 423, 445 Celliformidae 372, 419, 421, 425 Centrichnus 461-62 chambered trace fossils 419-53 see also insect nests burrows 425 construction 420-1 fillings 424 ichnofamilies 425-44 ichnotaxobases 421-25 identification 426, 427, 433, 436 shape 421 wall types 421-24 channel environments 261-8, 315-17, 316 channelized sandstone—mudstone 194—5, 196 chiton radulation 463 Chondrites deep-sea facies 130 Denison Trough 282, 288, 304 ichnofabric 245, 251, 266, 267 ichnoguild 144-5 He Formation 241 oxygen levels 144, 149-50, 150, 153, 154 Chondrites-Skolithos ichnofabric 242, 242, 266 Cicatricula 460 Circulichnis 170 Cladichnus 131 claystones 281,295-7, 360 climatic control of distribution 77-92 Clionolithes 460 closed lake ichnofaunas 320-1 coarse-grained facies 274 coarsening upward sequence 297, 298, 302 coastal facies 51-2, 194-9, 208, 299 Cochlichnus 345, 346, 349

INDEX cocoons 420, 435 coleopteran trace fossils 364, 365, 371, 372, 421, 422 see also Coprinisphaeridae; Pallichnidae colonization styles 16, 17-18 communities see ichnocoenoses; trace fossil assemblages composite ichnofabrics 375-6 Conchotrema 460 Conichnus 289 continental environments see also deltaic facies; fluvial facies fluvial ichnofaunas 215-16, 262, 315-19 ichnofacies model 312-15, 572 lacustrine deltas 335-54 lacustrine ichnofaunas 319-23 river-dominated facies 255, 344, 345, 347 sequence stratigraphy 323-7 Coprinisphaera 312, 368, 369, 425, 427-30, 428, 445 Coprinisphaeridae 425, 426-32, 427, 428, 445-6 Cornichnus 197 correlation tool 18-19 Cretaceous graphoglyptids 134 mass extinction event 414-15 palaeosols 362-7, 445-6, 447 cross-bedded sandstones 287, 295 see also hummocky cross-stratification crustaceans distribution 81-3 ichnology 79-81 trace fossils 419, 464-5 Cruziana 222, 345, 346, 386, 387 preservation 393, 394 tidal environments 164, 165, 166 Cruziana ichnofacies 7, 41, 42, 45, 49, 284 Bright Angel Shale 233 diverse proximal expression 301 Pebbley Beach Formation 184, 186, 188, 189, 190, 191-3, 197, 208 tide-influenced shorelines 161—2 Cunctichnus 460 Curvolithus 164, 165, 166 cyanobacterial borings 457-8 cyclic sediments 199-200 Cylindricum 317 Decapoda, borings 465—6 deep-sea trace fossils 125-39, 130 diversity 125-33, 135 time range 726, 127-8 deltaic facies 237 analysis 277-81 coarsening-upward sequence 297, 298, 302 delta front 252-7, 342-3, 342, 345-6, 346 delta plain 343, 343, 348 Denison Trough, Australia 273-81, 290-1, 303 distributary mouth-bar 253-5, 255, 257, 258 prodeltaic deposits 304, 301

INDEX tide-dominated 237-72, 258 Tonto Group 214 Triassic rift lakes, Argentina 335-54 Dendrina 459 Dendroidichnites 170 Dendrorete 459 Denison Trough Permian sequences, Australia 273-310,275,276 deltaic facies 273-81, 290-301, 303 facies analysis 277-81, 282 offshore and shoreface deposits 281-90, 306 deoxygenation events 142 Desmograpton 131 Devonian mass extinction event 400 Dietyodora 128, 129 Dimorphichnus 224 dinosaur tracks 93-123, 322 anatomy and gait 112 experimental work 93-5, 96-100, 107-12 laboratory simulations 108-12 Diorygma 468 Diplichnites 170, 171, 345 Diplocraterion 13, 17, 41 Denison Trough 284, 286, 285-9 ichnofabric 242-3, 242, 266 He Formation 241 oxygen levels 153 Pebbley Beach Formation 188, 191, 193, 204, 205 tidal environments 164, 165, 167 Triassic 403-4, 407 wide time range 77 Diplopodichnus 170 discontinuities 29-62, 204 see also omission suites marine flooding surfaces 43, 45, 46, 51-7, 184 regressive surfaces of erosion 34-7, 38^4 sequence boundaries 34-8, 51-7, 202, 205, 207 transgressive surfaces of erosion 39, 43-52, 203, 205-7 distributary mouth-bar facies 253-5, 257, 258 diversity 14-15, 125-33, 135 see also mass extinction events aftermath bioerosional fossils 468-9 changes 78 salinity levels and assemblages 772 Triassic-Jurassic extinction 408, 413 Dolomites, Italy, ichnostratigraphy 403-6 Douglas Group, Kansas 162 downhole imaging 21 drowned river valley 237 dung-beetle nests 371, 372, 422 duricrusts 365, 367 dyads 227, 225, 229 dysoxia-trace fossils relationship 141-56 earthworm trace fossils 373 Eatonichnus 424, 428, 430 Echinocardium 80, 83

483

Echinoid Form 66, 69, 72, 74 echinoids 81-3, 466 ecology see palaeoecology ecosystem reconstruction 233 Ekdale and Mason model of oxygenation 144, 150 Ellipsoideichnus 423 England Blue Lias, Dorset 151-4 Pinhay Bay, Dorset 407-8, 407-8 Scalby Formation, Yorkshire 98, 113-19 Toarcian, North Yorkshire 145-51 Triassic-Jurassic extinction 407-9 Entisol 362, 363 Entobia 459-60 Eocene 136, 367-71 epichnial grooves 224 Equisitales 361, 362 erosion regressive surfaces 34-7, 38-41, 39, 42, 43 transgressive surfaces 39, 43-52, 203, 205-7 escape burrow ichnofabric 242, 243, 245-6, 266 escape traces 315, 316 estuarine facies basin fill 202 Bright Angel Shale 213-36 channel fill 202, 203 differentiation from offshore marine 179-212 He Formation 252-7, 252, 253 palaeoenvironments 265 Pebbley Beach Formation 194-9, 208 valley fills 53-5 etchings 461-3, 464 euphotic zone 72 Eurygonum 457 experiments on dinosaur tracks 93-5, 96-100, 107-12 facies 21 see also deltaic facies; estuarine facies; ichnofacies; lithofacies Bright Angel Shale, Arizona 220 characterization 21 Denison Trough, Australia 277-81, 252 lacustrine deltas 340-5, 341, 344 palaeocurrents, He Formation, Norway 257 southern Sydney Basin 182-99 tidal channels 262 tide-dominated deltas, He Formation 254, 256 failure of substrate 104-5, 106-7, 707, 116-19 fair-weather conditions 187 Fasciculus 65, 66, 67-8, 72-3, 74, 457 feeding traces 314 ferricretes 365 Fictovichnus 423, 425, 433-5, 434, 446 fillings chambered trace fossils 424 estuarine facies 53-5, 202, 203 fine-grained sands 109-11

484

firmgrounds early Cambrian 386, 393-4 Glossifungites ichnofacies 33-4 ichnofacies distribution 6 lacustrine environments 324, 325, 327 omission suites 46, 47, 49 fish trace fossils 314, 322, 346, 466 flaser bedding 195, 797 flashcards 11 Fleaglellius 366, 372, 373, 441-2, 440, 446 flooding see marine flooding surfaces floodplain ichnocoenoses 315, 319 fluvial ichnofaunas channels 215-16, 262, 315-17, 316 intertidal 169-73, 170, 171 overbank374, 315-11,318 flysch trace fossil assemblages 129 Fontanai 426-7, 428, 445 footprints see also track morphology; trackways dinosaurs 8, 113-19 indenter theory equipment 96-7, 98 foraminifera borings 465-6 forced-regressive shorefaces 37^3, 39 Freitag Formation, Australia 299, 300 freshwater ichnofaunas Bright Angel Shale 213-36 fluvial 215-16, 262, 315-19 lacustrine 319-23 lacustrine deltas 38-47 sequence stratigraphy 326-7 frontiers 19-22 fugichnia 191, 193, 197 fungal borings 457-8 gamma-ray spectrometry 145 Gastrochaenolites 462 gastropod trace fossils 462-3, 467 genetic stratigraphy 29 Globodendrina 66, 69,11, 459 Glockerichnus 131 Glossifungites ichnofacies 4, 31, 33-4, 35, 49, 56 assemblages 36, 38 omission suites 41, 42, 46, 47 Pebbley Beach Formation 205, 207 trace fossil associations 35, 41, 42 Gordia 170, 171, 346 Grand Canyon, Bright Angel Shale 213-36 graphoglyptids 129-31 grazing trails 314, 318 Grebs Nest Point Formation 387, 388-91 Gyrochorte 168, 241, 242, 243, 266 Gyrolithes 241, 386, 388 hardgrounds 30, 32 HCS see hummocky cross-stratification Helicotaphrichnus 460 Helminthoidichnites 170, 346

INDEX Helminthopsis 170, 171, 283, 286, 304 Helminthorhaphe 130 heterotrophs 72, 73 hiatus see omission suites horizontal burrows 319 hummocky cross-stratification (HCS) 187, 190, 191, 193, 197, 255, 287 Hyellomorpha 457 hypichnial casts 224 ichnocoenoses 18, 30-1 floodplain 315, 319 oxygen-related 141-4 ichnofabrics see also bioturbation; palaeosol ichnofabrics composite 375-6 constituent diagram 12 He Formation, Norway 239, 241-51 marine/non-marine intercalation 20 methods of analysis 11-18, 355-8 palaeoenvironments 14 scales of development 12 stacking patterns 18-19, 264, 268-9 tidal channels 263 ichnofacies see also Cruziana ichnofacies; Glossifungites ichnofacies; Skolithos ichnofacies concept 4-7, 18 continental environments model 312-15, 312 Corinisphaera 312 distribution 30 fluvio-lacustrine succession 311-33 incised valley complexes 54 Mermia 312, 314-15, 316, 319, 320, 322, 324, 335, 348 Nereites 4 palaeoenvironments 5, 6 Pebbley Beach Formation 200, 201 Scoyenia 4, 312-14, 318, 320, 322, 324, 326, 327 substrate types 30-4 Teredolites 32-3 Trypanites 32 vertebrates 119 Zoophycos 4, 41, 42, 45, 144-5 ichnofamilies see insect ichnofamilies ichnofaunas fluvial 215-16, 262, 315-19 fluvio-estuarine intertidal 169-73, 170, 171 intertidal 162-3 lacustrine 319-23 latest Permian 401-2, 402 open-marine intertidal 164-7, 185, 202 restricted-bay intertidal 167-9, 167 Rhaetian 405, 409-13 salinity levels and diversity 772 trace fossil assemblages 336-7 ichnogenera estuarine facies 194-9

INDEX Lyme Regis, Dorset 152-4, 752 marine fades 182-94 range and diversity 126, 127-8 ichnoguilds Bright Angel Shale, Arizona 223-7 Zoophycos-Chondrites 144-5 ichnometry 15 ichnostratigraphy 7-8 Cambrian 399-400 Coprinisphaeridae 445-6 Krausichnidae 446-8 Pallichnidae 446 Triassic 403-6 ichnotaxa attribution 359 bioerosional 455-79 climatic control 78 stratigraphic range 77, 128-33, 128, 729 ichnotaxobases 421-25 ichnotaxonomy 3^ chambered trace fossils 419-53 composition 72 microborings 65-6 vertebrate tracks 119-20 ICZN see International Code of Zoological Nomenclature IHS see inclined heterolithic stratification He Formation, Norway 237-72 conceptual facies model 254, 256 facies 255, 257, 261-8 ichnofabrics241-51 palaeoenvironments 252-61, 257 sedimentology 238-41, 240, 251-69 trace fossil assemblages 241 incised shoreface deposits 37-43, 44 incised submarine canyons 37, 38 incised valley complexes 52-5 inclined heterolithic stratification (IHS) 195, 198, 203 indenter theory 94-5, 96-7, 98, 102-3 infaunal communities 15-16, 81-3 insect ichnofamilies 419-20, 425-44 Celliformidae 419, 421, 425 Coprinisphaeridae 425, 426-32, 427, 428 Krausichnidae 426, 432, 433-42, 436, 437, 440 Pallichnidae 425, 432-5, 433, 434 stratigraphic range 444 insect nests 8, 315, 419, 420 ants 421 bees 362, 363, 364, 370, 371, 372 beetles 364, 365, 371, 372, 420 termites 366, 370, 372, 373, 421 wall types 421-24, 422 International Code of Zoological Nomenclature (ICZN) 3-4, 120 intertidal flats see also tidal flats fluvial ichnofaunas 169-73, 770, 777 model 158-62,160

mud 257-8, 260 open-marine 164—7, 164, 166 restricted-bay 167-9, 167 sand 258-9, 260 trace fossil distribution 775 iron-rich sandstones 219-20, 279, 222-3, 231 Ischigualasto Formation 360-2, 364, 375 Ischigualasto-Villa Union basin 338-10, 339, 340 Jebel Qatrani Formation, Egypt 367, 370 Jet Rock Member, North Yorkshire 146, 147 Joffre Shoreface Complex 49, 50, 55, 56 Judy Creek Field, Alberta 41, 43, 44 Jurassic see also Rhaetian archosaur footprints 8 Blue Lias, Dorset 151-4 He Formation, Norway 237-72 mass extinction event 406-14 palaeosol ichnofaunas 447 Toarcian, North Yorkshire 145-51 Kansas, USA, outcrops and stratigraphy 158, 759 Kaybob Field, Alberta 41-3, 42 key stratigraphic surfaces 19, 29-62 kinematics of track morphology 103^4 Konservat-Lagerstatten 398 Kossen Formation, Austria 410-11 Kouphichnium 170 Krausichnidae 426, 435-43, 437, 438, 441, 446-7 Krausichnus 420, 423, 435-6, 438-9, 438, 443^ Kristin Field, offshore Norway 238-41, 238, 240 laboratory-simulated tracks 108-12 lacustrine deltas facies associations 340—45, 341, 344 marginal environments 343-5, 343, 347-9 trace fossil assemblages 336-7, 341, 343, 345-9 lacustrine ichnofaunas 319-23 Laguna Palacios Formation 362-5, 363, 364, 375 lake basin types 324, 325 laminated mudstone 199 laminated sandstone 187-93, 284, 285-90 Lapispecus 460 latitudinal distribution 78, 79, 81-3 Lias see Blue Lias; Toarcian Lingulella2l3,233 Lingulichnus 168 liquefaction failure 117-19 lithofacies, Bright Angel Shale 217-23, 220 Lockeia 164, 165, 166, 167, 168, 241 locomotion traces 313, 314, 318, 345, 349 loess sequence 362, 363 loosegrounds 30 Lower Aldebaran Sandstone 292, 297-9, 302 lower shoreface deposits 792 lowstand incised shorefaces 37^43, 39, 44 Lyme Regis, Dorset, Blue Lias 151-4

485

486

INDEX

Macaronichnus Denison Trough 289-96, 297-9, 304, 306 Pebbley Beach Formation 189, 191, 194 Maeandropolydora 461 Magenta Beds, Arizona 219-20, 219, 222-3, 231 Mannville Group-Joli Fou Formation contact 51, 52 marginal lacustrine environment 343-5, 343, 348-9 marine bioerosion 455-79 attribution to trace-maker 455-7 borings 457-61, 462, 464-6, 468 etchings 461-63, 464 radulation 463 round holes 466-8 stratigraphic range 456 marine environments climatic control 77-92 ichnofabric 20 ichnofacies 5-6 siliclastic sediments 383-97 marine facies 253 estuarine deposit differentiation 179-212 Pebbley Beach Formation 182-94, 202, 208 marine flooding surfaces (MFS) 43, 45, 46, 51-7, 184 mass extinction events aftermath 397^16 Cretaceous-Tertiary event 414-15 late Devonian event 400 Ordovian—Silurian event 401 Permian-Triassic event 400-406 Triassic-Jurassic event 406-14 Mauretania, microborings 63-76 maximum zone of deformation (MZD) 100-2 medium-grained sands 111-12 meniscate trace fossils 313, 314, 317, 322, 375, 424 Mermia 323 Mermia ichnofacies 312, 314-15, 316, 319, 320, 322, 324, 335, 350 Mesozoic bioerosion diversity 468 graphoglyptids 134 trace fossil diversity 125, 126, 128, 131 methodology of ichnology 9-19 MFS see marine flooding surfaces microborings 63-76, 455-7 bathymetric interpretation and model 72^ morphology 66-72 minute round holes 467-8 Miocene 83-4, 136 Mirandaichnium 170 Missouri, USA, outcrops and stratigraphy 158, 159 mixed layers see bioturbation models conceptual facies 254, 256 continental ichnofacies 310-13, 312 oxygenation 141^, 150 tidal flats 158-62 moisture/density relationships 105-6, 114-19 mollusc shell microborings 63-76, 65

Monesichnus 430, 431 Monocraterion 241 Monomorphichnus 213, 224 mouth-bar facies 253-5, 253, 255, 265 mud flats 161 muddy sandstone 188-91, 281-5 mudstones 281, 385-6 MZD see maximum zone of deformation nektonic fauna 146 Neogene graphoglyptids 133-6, 134-6 neoichnology 21, 94 Nereites 130, 164, 166, 167 Nereites ichnofacies 4 nests see insect nests Nevada, USA, Triassic-Jurassic extinction 411-13 Nododendrina 459 nomenclature 3—4, 65-6 see also ichnotaxonomy non-deltaic shoreface deposits 281-90 non-marine ichnofabric 20 non-marine ichnofacies 5-6 Norway, Jurassic tide-dominated delta 237-72 oceanic anoxic event 145 offshore deposits 281-90 lacustrine facies 340-42 marine 179-212 tide-dominated 261 trace fossil assemblages 303, 304 Oichnus 466-7 Oligocene diversity 136 omission suites 30^ firmgrounds 46, 47, 49 Glossifungites ichnofacies 41, 42, 46, 47 OPB see optical pedobarograph open lake ichnofaunas 321-23 open marine ichnofaunas 164-7, 164, 166, 185, 202 Ophiomorpha 79, 80-1, 80, 130 Cenozoic occurrences 84 ichnofabric 244-5, 244, 266 He Formation 241 organism distribution 81-3 spatangoid trace fossils 83-7 optical pedobarograph (OPB) 97-9, 112 ORB see oxygen-restricted biofacies Ordovician Grebs Nest Point Formation 387, 388-91 ichnological record 386-93 mass extinction event 400, 401 Orthogonum 65, 66, 68-70, 69, 74, 458 Ostrebium 72 overbank ichnofaunas 317-19, 318 oxygen-restricted biofacies (ORB) 145, 146-8, 151-2 oxygenation 8-9 evidence in Toarcian deposits 146-8 lacustrine environments 314, 322 Lias, Dorset 151-2

INDEX oxygenation (cont.) measurement 145 models 141^ Pacoota Sandstone Formation, Australia 75, 391 palaeobathymetry 4, 72-4 palaeoclimate criteria 78 Palaeoconchelis 73, 458 palaeoecology analysis 398-9 Bright Angel Shale, Arizona 213-36 microborings 63-76 Tonto Group 214-16 palaeoenvironments analysis 1, 12, 395-6, 414-15 delta front 252-7 estuarine 265 ichnofabrics 14 ichnofacies 5, 6 He Formation, Norway 257 tidal channel 261-8 tidal flats 173, 257-61,2(50 wave- and tide-influenced 259, 290—2 Palaeogene diversity 126, 127 palaeogeography, eastern Grand Canyon 216 Palaeophycus 227, 222, 346 Denison Trough 282, 284, 288-90, 293, 304 ichnofabric 242, 243, 248, 266, 267 He Formation 241 oxygen levels 153 Pebbley Beach Formation 185, 187, 189 tidal environments 164, 167, 168 Palaeosabella 461 palaeosol ichnofabrics 355—82 analysis 355-8, 371-2 Argentina 360-5, 367-71, 375 Asencio Formation, Uruguay 365-7, 366, 370 bedding and structures 356-7 composite ichnofabrics 375—6 Jebel Qatrani Formation, Egypt 367, 370 palaeosols chambered trace fossils 419-53 ichnotaxa range 436 palaeotopography 162, 214, 216, 276 Palaeozoic arthropod trace fossils 7-8 bioerosion 468 diversity 125, 126, 127-31 graphoglyptids 133-4 ichnofabrics 383-96 Pallichnidae 425, 432-5, 433, 446 Pallichnus 423, 425, 434, 435, 446 Palmiraichus 422, 423 palynofacies 213-14, 227-30, 228-30 palynomorphs 231-3 Parowanichnus 434, 437 pascichnia traces 150 Pebbley Beach Formation, Australia 179-212

487

discontinuity surfaces 204 estuarine facies 194-9 ichnofacies 200, 201 marine facies 182-94 sequence stratigraphy 199-207, 200, 201, 206 trace fossil summary 202 transgressive surfaces of erosion 203, 205-7 pedofabric/ichnofabric ternary diagram (PITD) 357, 358 pedofabrics 357, 360, 368, 371-2 pellets 79 Pennsylvanian stratigraphy, USA 759 permeability of substrate 106-7 Permian Denison Trough, Australia 273-309 ichnofaunas 401-2, 402 mass extinction event 400-6 Pebbley Beach Formation, Australia 179-212 petroleum industry 19-21 Petroxestes 462 Phoebichnus 241, 245, 246, 266 photoautotrophic endoliths 72 Phrixichnus 468 Phycodes 223, 388 Phycosiphon 130 burrows 245 Denison Trough 282, 281-3, 288, 287, 292-6, 297-301, 306 ichnofabric 245, 246, 247, 251, 266 He Formation 241 Pebbley Beach Formation 185, 187, 188, 191, 194 Pinhay Bay, Dorset, England 407-8, 407-8 piped zones 142, 143 PITD see pedofabric/ichnofabric ternary diagram Planobola 457, 467 Planolites 80, 153 Denison Trough 283, 287, 293 ichnofabric 245-9, 266, 267 He Formation 241 Pebbley Beach Formation 185, 187, 188, 195, 197 Planovolites 468 plant fossils 367 Platydendrina 459 Plectonema 72 Pleistocene assemblages 85, 86-7 Pliocene assemblages 84-5, 86 Podichnus 464 Polyactina 65, 66, 68, 458 porous plate box 96-7 preservation controls, track morphology 93-123 Cruziana and Rusophycus 393, 394 potential of lacustrine ichnofaunas 327 quality 384-5 styles 385 prodeltaic deposits 304, 305 progradational cycles 274-7 prograding shoreface succession 300

488

INDEX

Protovirgularia 164, 165, 166, 167, 168 Psammichnites 164, 166, 167, 168, 197 Pygmy Form 65, 66, 71, 74 pyrite framboids 145, 151-2 quantification of bioturbation 10-11, 355-7 radiation events 7 radulation 463 Radulichnus 463 Ramodendrina 459 Ravenscar, Yorkshire, Toarcian deposits 145-51 ravinement 43-5, 49 Rebuffoichnus 364, 365, 372, 432, 434, 445 Recent microborings 63-76 red-bed succession 365-7 regressive surfaces of erosion (RSE) 34-7, 38-41, 39, 42,43 Renichnus 462-3 repartition 349 research landmarks 3-9 reservoirs 21, 273 residual strength of substrate 106-7 restricted-bay intertidal ichnofaunas 167-9, 167 Reticulina 66, 69, 70, 458 Rhaetian ichnofaunas 405, 409-14 Rhizocorallium 149, 153, 164 Denison Trough 284, 286, 288, 287, 289 He Formation 241 Pebbley Beach Formation 185, 188, 191 Servino Formation 405, 406, 412 Rhopalia 458 rift basins 274 rift lakes 335-54 ripple structures 195, 196, 243, 253 river-dominated facies 253, 344, 345, 347 see also fluvial ichnofaunas Rogerella 464 Rosellichnus 423, 424 Rosselia Denison Trough 278, 284, 288, 287, 292, 296, 300 ichnofabric 249-50, 249, 267 He Formation 241 Pebbley Beach Formation 185, 187, 188, 191, 194 Rotundusichnium 130 RSE see regressive surfaces of erosion Rusophycus 166, 386, 388, 391, 392, 393, 394 Saccomorpha 65, 66, 67, 74, 456 salinity levels 81-2, 772 sandbars 160, 231 sands 109-12, 258-9, 260 sandstones bioturbated 193-4, 193 cross-bedded 290, 295 deltaic 290-301 laminated 187-93, 284, 285-9 muddy 188-91, 286, 284-7, 288

sheet-like 195-9, 197 silty 281^,295-7 tabular 291-5 tidal channels 17 sandy siltstones 185-7, 186 Sarmiento Formation, Argentina 358, 367-71 Savrda and Bottjer model of oxygenation 141^ SB see sequence boundaries Scalby Formation, Yorkshire 98, 113-19 scale of observation 10 Scaphichnium 425, 433, 434, 446 scarabaeid beetles 435 scars 463 Schaubcylindrichnus 241, 243, 244, 247, 248, 266, 267 Scolecia66, 69, 70-1,72,458 Scolicia 81, 82, 83, 84, 85, 130 Scoyenia ichnofacies 4, 312-14, 313, 318, 320, 322, 324, 326, 327 sediment feeders 223 sedimentary structures 163, 164, 276 see also cross-bedded sandstones; soft sediment deformation sedimentology context 10 ichnofabrics 4 He Formation, Norway 239-41, 240, 251-69 intertidal flats and subtidal sandbars 160 lacustrine deltas 347 Pebbley Beach Formation, Australia 179-212 Seilacherian ichnofacies 4-7, 18 semi-quantitative methods 355-6 sequence boundaries (SB) 34-7, 38, 51-7, 202, 205, 207 sequence stratigraphy 9 case studies 34-51 fluvial ichnofacies 326-7 lacustrine ichnofacies 324, 325-6 Pebbley Beach Formation 199-207, 200, 201, 206 Servino Formation, Italy 403-6, 405 shallow marine siliclastics 383-96 shallow-tier burrows 386, 394 shear surfaces 107 sheet-like sandstone-mudstone 195-9, 797 shelf storm deposits 230-1 shell borings 465, 466-7 shorefaces comparison with delta front 306 facies 278, 281-90 forced regressive and lowstand incised 37^3, 44 prograding 300 transgressively incised 39, 47-51, 50 siltstones 182-8, 184, 281-4, 297 silty sandstones 281-3, 295-7 Silurian mass extinction event 400, 401 simulated dinosaur tracks 98, 108-12, 709, 770 Siphonichnus 241, 250-1, 250, 267 Skolithos3l5 Denison Trough 287, 288, 289

INDEX ichnofabric 247, 248, 267, 285, 289 He Formation 241 palaeosol 365 Pebbley Beach Formation 194, 195, 197 piperock 391 Skolithos ichnofacies 4, 42, 53, 161, 188, 312, 316, 320, 321, 324, 327 Skolithos-Cruziana ichnofacies 188, 195, 208, 284 small borings 458-9, 467-8 soft sediment deformation structures 293, 293, 304 softgrounds 6, 30, 41, 42, 324, 325, 327 soil mechanics 105 soil moisture content 105-6, 114-19 soils see palaeosols Solemya 166 soupgrounds 30 spatangoid trace fossils 80, 81, 83-7 Spirichnus 461 sponge borings 459-60 Spongeliomorpha 80-1 stacking patterns 18-19, 264, 268-9 Stiallia 170, 171 Stiaria 170, 171, 345 stiffgrounds 30 storm deposits 187-8, 189-91, 230-1, 287-8, 385 stratigraphic range 77 bioerosive ichnotaxa 456 ichnotaxa 77, 128-33, 128, 729 insect ichnotaxa 436 lacustrine delta assemblages 336-7 stratigraphic surfaces 19, 29-62 stratigraphy see ichnostratigraphy; sequence stratigraphy striated traces 314 sub-recent microborings 63-76 subaerially exposed surfaces 51—2 submarine canyon incision 37, 38 substrate-controlled ichnofacies 30-4 subsurface tracks 99, 104-5, 109-12 subtidal environments 158-62, 160, 259-61, 260 subtropical associations 83-5 supratidal zone 160-1 surface traces 225, 226 surface track simulations 99, 108—9 surfaces see also discontinuities key stratigraphic surfaces 19, 29-62 shear surfaces 107 subaerially exposed 51-2 suspension feeders 223-5 Sydney Basin, Australia 179-212 Syntermesichnus 440, 442, 446 tabular sandstone 288-92 Tacuruichnus 437, 439, 446 Taenidium 153, 185, 315, 362, 363, 365, 375 ichnofabric 244-5, 244, 248-9, 249, 266, 267 He Formation 241

Talpina 461 taphonomic pathways 316, 320 taxonomy see ichnotaxonomy Teichichnus 167, 168, 389, 390 Bright Angel Shale 223 Denison Trough 281, 285, 303 He Formation 241 Pebbley Beach Formation 185, 187, 189-91 Teisseirei 366, 372, 421, 424, 430-2, 431, 445 temperate association 85-7 Teredolites 462 Teredolites ichnofacies 32-3 termite nests 366, 370, 372, 373, 421, 422-3 see also Krausichnidae Termitichnus 441-2, 440, 443^, 446 Tertiary diversity 136 graphoglyptids 133-6, 134-6 insects 8 mass extinction event 414—15 palaeosol ichnofaunas 444-6 tetrads 227, 228, 229 Tetrapod tracks 170 textures see ichnofabrics thalassinidean crustaceans 79, 81 Thalassinoides 41, 80-1, 86, 87, 153 ichnofabric 167, 248, 249, 250, 251 He Formation 241 Triassic 406, 412 tidal channel environments 17, 253-6, 257, 261—8 tidal flats 157-78 see also subtidal environments; supratidal zone facies associations 260 ichnofaunas variability 163-73 model 158-62, 160 palaeoenvironments 257-61, 260 sedimentary structures 163, 164 stratigraphic implications 173-4 successions 259 variability 175-8 tide-dominated deltas 237-72, 259 tiering 15-16, 78, 82 Blue Lias 153 diagrams 357-8, 397 oxygenation events 141^, 142 palaeosols 357, 358, 362, 363, 376 time range and diversity 126, 127-8 Toarcian deposits, North Yorkshire 145-51 Tonganoxichnus 170, 171 Tonto Group, palaeoecology 214-16 Topsentopsis 460 trace fossil assemblages see also ichnofacies Blue Lias, Dorset 152-4 continental 311-33 deep-sea ichnotaxa 128-33 delta front 345-6, 346 delta plain 348

489

490 trace fossil assemblages (cont.} distribution controls 349-50 diversity and salinity levels 772 dysoxia relationship 141-56 graphoglyptid proportions 132, 133 He Formation, Norway 241 intertidal 173 lacustrine deltas 336-7, 341, 345-9, 346, 347 lacustrine sequence stratigraphy 324 mass extinction events aftermath 397-418 Mesozoic 131 Neogene 131 palaeosols 359-60 Palaeozoic 129-31 Pebbley Beach Formation 202 prodeltaic v. offshore deposits 303, 304 proximal Cruziana ichnofacies 191, 192 repartition 349 Toarcian, North Yorkshire 147, 148, 149-50 track morphology 93-123 Burniston dinosaur tracks 113-19 description and terminology 100—2 experimental work 93-5, 96-100, 107-12 indenter theory 94-5, 96-7, 102-3 moisture/density relationships 105-6, 114—19 surface and subsurface features 99-100, 101 trackways arthropods 7-8, 313, 318, 321, 322, 326, 346 vertebrates 313, 318, 322 transgressive surfaces of erosion (TSE) 39, 43-52, 203, 205-7 transgressively incised shorefaces 39, 47-51, 50 Treptichnus 170, 777, 346, 386, 388, 399 Triassic archosaur footprints 8 ichnostratigraphy 403-4 lacustrine deltas, Argentina 335-54 mass extinction events 399^14 palaeosols 360-2, 446-7 Triassic-Jurassic mass extinction event 406—16 Central Austria 409-11, 410 England 407-8, 407-9 Nevada, USA 411-13 Trichichnus ichnofabric 251, 267 Trichophycus 387, 389-90 tridactyl dinosaur tracks 98, 113, 116, 118 Tripartum Form 66, 69,11, 74 trophic reconstruction 231-3

INDEX tropical associations 83-5 Trypanites 461 Trypanites ichnofacies 32 TSE see transgressive surfaces of erosion tsunami beds 414—15 tubular trace fossils 374-5 tuffaceous palaeosols 362-5, 363, 368 tunnels 67-8 see also burrows; graphoglyptids turbidites 322, 325-6 U-shaped burrows 225 UK see England Ultisols 366, 370 Undichna 170, 171, 345, 346 upwelling areas 63-76 Uruguay, Asencio Formation 365-7, 365, 370 valleys 52-5, 162, 237 vertebrates body fossils 367 dinosaur tracks 93—123 ichnofacies 119 track taxonomy 119-20 trackways 313, 318, 322 vertical burrows 313, 314, 315, 317 Vertisols 360-2, 361, 364 Viking Formation, western Canada 40, 41, 42, 44, 45, 49, 50, 53, 55-7, 56 Vondrichnus 440, 441, 446 walls in chambered trace fossils 421-24 wave-dominated environments 259, 291, 344, 345-6, 347 wavy bedding 195, 196, 197 woodgrounds 30, 32-3 worm borings 460-1 wrinkle marks 226 Yorkshire, UK Burniston dinosaur tracks 113-19, 773, 116, 118 Toarcian 145-51, 146 zone fossil characteristics 7 Zoophycos 284, 286, 287, 306, 406 Zoophycos ichnofacies 4, 41, 42, 45, 144-5 Zoophycos-Chondrites ichnoguild 144—5