Deltaic and Shallow Marine Deposits (Developments in Sedimentology, Volume 1)

DEVELOPMENTS I N SEDIMENTOLOCY VOLUME I

DELTAIC A N D SHALLOW M A R I N E DEPOSITS

F U R T H E R TITLES I N T H I S S E R I E S

G . C. A M S T U T Z , Editor

SEDIMENTOLOGY A N D ORE GENESIS A . H . B O U M A and A . B R O U W E R , Editors

TURBIDITES R . F. D I L L SUBMARINE EROSION

DEVELOPMENTS IN SEDIMENTOLOGY VOLUME I

DELTAIC AND SHALLOW MARINE DEPOSITS PROCEEDINGS OF THE SIXTH INTER NATIONAL SEDIMENTOLOGICAL CONGRESS THE NETHERLANDS AND BELGIUM

-

I963

E D I T E D BY

L. M. J. U.

VAN

STRAATEN

Geology Department State University, Groningen The Netherlands

ELSEVlE R PUBLISHING COMPANY AMSTERDAM

LONDON

1964

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W l T H 162 ILLUSTRATIONS AND 24 TABLES

ALL RIGHTS RESERVED THIS BOOK OR ANY PART THEREOF MAY NOT BE REPRODUCED IN ANY FORM, INCLUDING PHOTOSTATIC OR MICROFILM FORM, WITHOUT WRITTEN PERMISSION FROM THE PUBLISHERS

THE SIXTH INTERNATIONAL SEDIMENTOLOGICAL CONGRESS THE NETHERLANDS AND BELGIUM, 1963

COMMITTEES

HONORARY COMMITTEE

Prof. Dr. D. J. Doeglas, Wageningen, Utrecht Prof. Dr. C. H. Edelman, Wageningen Prof. Dr. A. Hacquaert, Ghent Prof. Dr. Ph. H. Kuenen, Groningen

Ing. W. P. van Leckwijck, Brussels

ORGANIZING COMMITTEE

Prof. Dr. R. Tavernier, Ghent, President Prof. Dr. A. J. Wiggers, Amsterdam, Vice-president Ing. M. K. Gulinck, Brussels, Secretary Dr. C. Kruit, Rijswijk, Secretary Dr. J. D. de Jong, Haarlem, Treasurer

Prof. Dr. M. Lecompte, Louvain, Member Prof. Dr. J. P. G. Michot, Litge, Member Prof. Dr. L. M. J. U. van Straaten, Groningen, Member

PREVIOUS INTERNATIONAL SEDIMENTOLOGICAL CONGRESSES AND THEIR PROCEEDINGS

1. Belgium, 1946.

Ghent and Brussels. La GPologie des Terrains Rkents dans l’Ouest de I’Europe. Session extraordinaire des SOC. belges de CPologie. Hayez, Bruxelles, 495 pp.

2. France, 1949.

La Rochelle and Angouleme. S6dimentation et Quaternaire. Led Sam, Bordeaux, 321 pp.

3. The Netherlands, 1951. Groningen and Wageningen. Proceedings of the Third International Congress of Sedi-

mentofogy. Nijhoff, Den Haag, 332 pp. 4. Germany, 1954.

Gottingen. Geol. Rundschau, 1955, 43 (2): 307-599.

5. Switzerland, 1958.

Genkve and Lausanne. Eclogae Geol. Helv., 51 (3) : 485-1 177.

THE SIXTH INTERNATIONAL SEDIMENTOLOGICAL CONGRESS THE NETHERLANDS AND BELGIUM, 1963 REPORT

MAY

2428.

FIELD TRIPS, HELD BEFORE SESSIONS (THE NETHERLANDS)

Fieldtrip A: Estuarine sedimentation in the southwestern Netherlands Leader: L. M. J. U. van Straaten. Number of participants: 77. Localities visited: ( I ) Shoal with megaripples in Westerschelde near Kruiningen. (2) Mud flat with ebb gullies near Bath. (3) Sand flat near Bergen op Zoom. (4) Salt marsh near Woensdrecht. (5) Excavation in estuarine series near Willemstad. FieldfripB: Coastal-plain sediments in North-Holland and former Zuiderzee area Leaders: J . D. de Jong and A. J. Wiggers. Number of participants: 114. Localities visited: ( I ) Excavation at Amsterdam, showing 10 m thick section through Holocene deposits, including Lower Peat, lagoon clay, tidal-channel, tidal-flat and salt-marsh deposits, Upper Peat and top clay layer. (2) North Sea beach at Zandvoort. (3) Excavation in dune deposits south of Zandvoort. (4) Several points in Oostelijk Flevoland polder (Zuiderzee area), showing deposits of lacustrine and lagoonal origin, with great variety of sedimentary structures.

MAY

29-30.

MEETING AT AMSTERDAM

The congress at Amsterdam was held in the “Instituut voor de Tropen” and was attended by 283 participants. It was opened by R. Tavernier, who, among other things, expressed the appreciation of the organizing committee for the help, received from several institutions, especially the ‘‘Koninklijke/Shell Exploratie en Productie Laboratorium” at Rijswijk, the Geological Survey at Haarlem, the “Rijkswaterstaat” at The Hague, and the Geology Departments of the Free University at Amsterdam and the State University at Groningen.

VIIT

THE SIXTH INTERNATIONAL SEDIMENTOLOGICAL CONGRESS

F. P. Shepard (La Jolla), president of the International Association of Sedimentology, read an address on “Criteria in modern sediments, useful in recognizing ancient sedimentary environments”. Under chairmanship of J. Bourcart (France), W. P. van Leckwijck (Belgium), L. Trevisan (Italy) and E. Seibold [Germany), 17 speakers presented papers dealing with processes of recent sedimentation and erosion, and some other general topics, viz. Ph. H. Kuenen, P. McL. D. Duff, J. Ph. Mangin, H. Fuchtbauer, G. V. Chilingar, A. J. de Groot, E. T. Degens, A. Bersier, R. Goldring, D. J. Shearman, W. D. Briickner, J. R. Curray, J. H. Hoyt, D. G. Moore, A. Guilcher, D. J. J. Kinsman and A. J. Wells. I n addition, R. Goldring and F. E. J. Arbey read the papers by J. R. L. Allen and A. Rivitre resp., and J. E. Sanders presented a motion picture on sand transport and ripple formation in the laboratory and in natural rivers. On the evening of May 29, a reception was given to the congress members in the “Stedelijk Museum” by the municipality of Amsterdam, who also offered them a boat trip through the Amsterdam canals at the end of the following day. On the evening of May 30, the International Association of Sedimentology held a general assembly in the Physics Department of the Free University, where among other things, a British proposal was accepted to hold the next congress in Great Britain. A new council of the 1. A. S. was elected, composed as follows: President: J. H. Taylor (London); Vice-president: L. Trevisan [Pisa); General Secretary: D. J. Doeglas (Wageningen); Treasurer: A. H. Bouma (Utrecht); Members: K. Birkenmayer (Krakow), P. V. Dehadrai (Dehradun), 0. Riba ISaragosse), F. P. Shepard (La Jolla, Calif.); T. Sudo (Tokyo), J. J. Veevers (Canberra). During the meetings at Amsterdam, the participants had the opportunity to study a n exhibition of sedimentological work done in The Netherlands and Belgium. This exhibition was organized under direction of D . J. Doeglas.

MAY

31.

EXCURSION TO HARINGVLIET WORKS, SYMPOSIUM AT D E L ~AND EVENING SESSION AT ANTWERP

On their way from Amsterdam to Antwerp, most of those attending the congress visited a large excavation in the Haringvliet estuary under the direction of J. H. J. Terwindt. This excavation permits three-dimensional studies of the most recent estuarine deposits, the formation of which could be accurately dated by comparison of a long series of hydrographic maps. The other participants met in the Geology Department of the Technological University at Delft for a symposium on “Sedimentology and ore genesis” (total number of participants: 80), organized by Prof. Dr. G. C. Amstutz (Missouri, U.S.A.) and Prof. Dr. F. J. Faber (Delft). Fifteen papers were presented, which will be published in a separate volume, edited by G. C. Amstutz. On the evening of this day, a session was held in the “Nationaal Bouwcentrum” at Antwerp. Here, under chairmanship of D. S. Gorsline (U.S.A.), A. H. Bouma pre-

MEETINGS AND FIELDTRIPS

Y(

sented and explained a motion picture by R. F. Dill on sediment transport in the heads of submarine canyons.

JUNE

1.

MEETING AT ANTWERP

The last day of the congress was devoted to papers dealing especially with pre-Quaternary formations. Chairmen were: J. H. Taylor (England) and D. J. Doeglas [The Netherlands). The papers were presented by H. G. Reading, W. Zimmerle, P. Wurster, C. Pomerol, G. Kelling, T. R. Owen, R. H. Dott, A. Hallam and B. Mamet. In the closing session the organizers of the congress were thanked by J. H. Taylor (England), A. Bersier (Switzerland), H. Harder [Germany), L. Trevisan (Italy) and F. Macau Vilar (Spain). In the evening the congress had the opportunity to visit the Town Hall at Antwerp. June 2. During the morning of this day a number of participants visited the Hydraulics Laboratory at Borgerhout, near Antwerp.

JUNE

3-8.

FIELDTRIPS HELD AFTER SESSIONS (BELGIUM)

Fieldtrip C: Terrigenous sediments of the Lower Devonian. Rhythmic organogenic

Fieldtrip D: Fieldtrip E:

Fieldtrip F: Fieldtrip G:

and terrigenous sediments during the period of reef formation covering the Middle Devonian and Frasnian (Upper Devonian). Leader: M. Lecompte. Number of participants: 27. Idem. Number of participants: 16. The Namurian in the region of Andenne and Vist. Cyclic sedimentation. Arenaceous sediments of the Upper Devonian (Famennian) in the valley of the Hoyou. Arenaceous sediments with pseudonodules of the Lower Devonian of the Ardennes. Leaders: P. Macar, P. Michot and W. P. van Leckwijck. Number of participants: 20. Idem. Number of participants: 17. The Carboniferous limestone of the Meuse and Vesdre valleys. Rhythmic sedimentation, breccias, biostromes. Leaders: P. Michot, R. Conil, J. M. Graulich. Number of participants: 22.

X Fieldtrip I:

Fieldtrip J: Fieldtrip K:

Fieldirip M:

Fieldtrip N: Fieldtrip 0:

Fieldtrip P:

THE SIXTH JNTERNATIONAL SEDIMENTOLOGICAL CONGRESS

The Carboniferous limestone of Hainaut. Transition into the Namurian. Karst relief and infilling. Lacustrine and deltaic sediments of the Wealden. Shales and siltstones of the Cambro-Silurian of Brabant. Leaders: A. Delmer, R. Legrand, R. Marlikre and G. Mortelmans. Number of participants: 20. Idem. Number of participants: 12. Tertiary marine clayey deposits (Rupelian and Ypresian). Marine Eocene and Neogene sands. Continental sediments of the Pleistocene [loess, cover sands) in west BAgium. Leaders: R. Martchal and J. de Heinzelin. Number of participants: 18. Littoral arenaceous sediments of the Oligocene and the Eocene. Transition facies to continental deposits with slump phenomena. Fresh water deposits of the Oligocene. Leader: M. Gulinck. Number of participants: 14. Idem. Number of participants: 15. Pleistocene estuarine and river deposits. Cover sands, dunes and loess in the east of Belgium. Miocene sands and lignites. Lagoonal deposits of the Oligocene. Leader: F. Gullentops. Number of participants: 12. Idem. Number of uarticiuants: 8. 1 1

EDITORIAL

For various reasons, the committee organizing the Sixth International Sedimentological Congress had decided to focus attention on one central theme, viz. : Deltaic and shallow-marine deposits. The present Proceedings volume contains all papers (of participants) dealing with this topic, and submitted before the deadline, January 1, 1963. The papers had to be written in English, French or German. In several of the contributions in which authors, had written in a language not their own, the text was modified to a smaller or larger extent at the suggestion of the Editor. Though he believes that in some cases this has led to an improvement in the linguistic quality of these papers, he is aware that the result of his activities is still far from faultless. Two papers written in English were corrected by Prof. Dr. P. Allen (Reading), while Ing. W. P. van Leckwijck (Brussels) took care of a few contributions in the French language. The editor hereby expresses his sincere gratitude to these colleagues for their kind help. L. M. J. u. VAN STRAATEN

This Page Intentionally Left Blank

CONTENTS The Sixth International Sedimentological Congress. Committees

..............

Previous international sedimentological congresses and their proceedings

..........

V VI

The Sixth International Sedimentological Congress, The Netherlands and Belgium, 1963. Meetings and fieldtrips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .XI11

Editorial.. Contents

. ,

Presidential Address. Criteria in modem sediments useful in recognizing ancient sedimentary environments F. P. SHEPARD (La Jolla, Calif., U.S.A.) . . . . . . . . . . . . . . . . . . . . . . . Sedimentation in the modem delta of the river Niger, West Africa J. R. L. ALLEN (Reading, Great Britain). . . . . . . . . . .

.............

Structure en creneaux et brkhe cyclopknne en milieu detritique-paralique A. BERSIER (Lausanne, Suisse) . . . . . . . . . . . . . . . . . . .

.........

Specific features of deltaic deposits in coal-bearing and cupriferous formations L. N. BOTWNKINA and V. S.YABLOKOV (Moscow, U.S.S.R.) . . . . . . .

........

1 26 35 39

Devonian biostromes and bioherms of the southem Cantabrian Mountains, northwestern Spain A. BROUWER (Leyden, The Netherlands) . . . . . . . . . . . . . . . . . . . . . . .

48

Heavy mineral distribution on the continental shelf off Accra, Ghana, West Africa W. D. BRUCKNER and H. J. MORGAN (St. John's, Newfoundland, Canada) . . .

54

......

On shallow-water origin of phosphorite sediments G. I. BUSHINSKI (Moscow, U.S.S.R.)

.........................

62

Relationship between porosity, permeability and grain-size distribution of sands and sandstones G. V. C H I L I N G A R ( L O S Calif., A ~ ~ ~U.S.A.) ~~~, . . . . . . . . . . . . . . . . . . . . .

71

Holocene regressive littoral sand, Costa de Nayarit, Mexico J. R. CURRAY (La Jolla, Calif., U.S.A.) and D. G. MOORE (San Diego, Calif., U.S.A.)

76

....

u ber biogeochemische Umsetzungen im Friihstadium der Diagenese E. T. DECENS (Pasadena, Calif., U.S.A.).

.......................

83

Origin and transport of mud (fraction < 16 microns) in coastal waters from the Western Scheldt to the Danish frontier 93 A. J. DE GROOT(Groningen, The Netherlands) . . . . . . . . . . . . . . . . . . . . . Features in the heads of submarine canyons; narrative of underwater film R.F.DILL(SanDiego,Calif.,U.S.A.) . . . . . . . . . . . . . . . Ancient deltaic sedimentation in eugeosynclinal belts R. H. Don JR. (Madison, Wisc., U.S.A.) . . . . .

.........

..................

101 105

Trend surface analysis of sedimentary features of the Modioluris Zone, east Pennine coalfield, England P. McL. D. DUFFand E. K. WALTON (Edinburgh, Great Britain) . . . . . . . . . . . . . 114 Thickness variations of the sandy Almere deposits (Holocene) in the former Zuiderzee area (The Netherlands) P. J. ENTE(Kampen, The Netherlands) . . . . . . . . . . . . . . . . . . . . . . . . 123

,'

Alreconnaissance survey of the environment of Recent carbonate sedimentation along the Trucial Coast, Persian Gulf G. EVANS, D. J. J. KINSMAN and D. J. SHEARMAN (London, Great Britain) . . . . . . . . . 129

XIV

CONTENTS

Trace-fossils and the sedimentary surface in shallow-water marine sediments R. GOLDRING (Reading, Great Britain) . . . . . . . . . . . . . . . . . . . . . . . .

136

Beach studies in west Florida, U.S.A. D. S. GORSLINE (Los Angeles, Calif., U.S.A.)

. . . . . . . . . . . . . . . . . . . . .

144

La sidimentation sous-marine dam la partie orientale de la Rade de Brest, Bretagne A. GUILCHER (Pans, France) . . . . . . . . . . . . . . . . . . . . . . . . . . . .

148

Liassic sedimentary cycles in western Europe and their relationship to changes in sea level A. HALLAM (Edinburgh, Great Britain) . . . . . . . . . . . . . . . . . . . . . . . .

157

Lagoon sediments in Greenland K. HANSEN (Copenhagen, Denmark) . . . . . . . . . . . . . . . . . . . . . . . . .

165

Late Pleistocene and Recent sedimentation, central Georgia coast, U.S.A. (Golden, Colo., U.S.A.) and V. J. J. H. HOYT(Sapelo Island, Ga., U.S.A.), R. J. WEIMER HENRY JR.(Sapelo Island, Ga., U.S.A.) . . . . . . . . . . . . . . . . . . . . . . . . 170 Sediment transport in part of the Lower Pennant Measures of South Wales G. KELLING (Swansea, Great Britain). . . . . . . . . . . . . . . . . . . . . . . . .

177

The Recent carbonate sediments near Halat el Bahrani, Trucial Coast, Persian Gulf D. J. J. KINSMAN (London, Great Britain) . . . . . . . . . . . . . . . . . . . . . . .

185

Sedimentary facies in Bay of Fundy intertidal zone,Nova Scotia, Canada G. DE V. KLEIN[Philadelphia, Pa., U.S.A.) . . . . . . . . . . . . . . . . . . . . . .

193

Aspects gdneraux de la ddimentation argileuse dans les facits littoraux du Paltoghe Nord Aquitain A. KLINGEBIEL et C. LATOUCHE (Talence, France). . . . . . . . . . . . . . . . . . . . 200 Pivotability studies of sand by a shape-sorter PH. H. KUENEN (Groningen, The Netherlands) . . . . . . . . . . . . . . . . . . . . .

207

Typical features of a fluviomarine offlap sequence R.LAGAAY and F. P. H. W. KOPSTEIN (Rijswijk, The Netherlands) . . . . . . . . . . . . 216 Rhaetic-Jurassic-Lower Cretaceous sediments from deep wells in North Jylland, Denmark G . LARSEN (Hellerup, Denmark) . . . . . . . . . . . . . . . . . . . . . . . . . .

227

Reflexions sur la systtmatique et la g e n k des bassins de ddimentation N. LLOPIS LLADO (Madrid, Espagne) . . . . . . . . . . . . . . . . . . . . . . . . .

236

Sedimentology of beaches on the north coast of the Sea of Azov and I. N. REMIZOV (Moscow, U.S.S.R.) . . . . . . . . . . . . . . . 245 N. V. LOGVINENKO Zur Gerollmorphornetrie von Transgressionskonglomeraten G. LUTTIC(Hannover, Deutschland) . . . . . . . . . . . . . . . . . . . . . . . .

253

Sedimentary environments of the Weald Clay of southeastern England J . D. S. MACWUGALL and J. E. PRENTICE (London, Great Britain) . . . . . . . . . . . . 257 Sdlmentation des facies “marbres noirs” de la Belgique et du nord de la France B. MAMET (Bruxelles, Belgique) . . . . . . . . . . . . . . . . . . . . . . . . . . .

264

La dquence-unite et les dries kdirnentaires J. PH. MANGIN (Dijon, France) . . . . . . . . . . . . . . . . . . . . . . . . . . .

269

Sedimentary framework of the drowned Pleistocene delta of Rio Grande de Santiago, Nayarit. Mexico (La Jolla, Calif,. U.S.A.) . . . . 275 D. G. MOORE (SanDiego, Calif., U.S.A.) and J. R. CURRAY Die Korngrossenverteilung in den rezenten Sedimenten des Golfes von Neapel G MULLER (Tiibingen, Deutschland). . . . . . . . . . . . . . . . . . . . . . . . .

282

xv

C0NTENTS

Unterschiede in den chemischen und physkalischen Eigenschaften von fluviatilen, brackischen und marinen Sedimenten W. MULLER (Hannover, Deutschland) . . . . . . . . . . . . . . . . . . . . . . . . 293 The tectonic framework of Carboniferous sedimentation in South Wales T. R.OWEN(Swansea, Great Britain). . . . . . . . . . . . . . . .

The Frio sedimentation on the Rayne field of southwest Louisiana W. R.PAINE (Lafayette, La., U.S.A.) . . . . . . . . . . . . . Shallow-water origin of Early Paleozoic oolitic iron ores J. P E T R ~ N(Praha, EK Czechoslavakia). . . . . . . . .

. . . . . . . . .

301

. . . . . . . . . . . .

308

................

3 19

Origine et conditions de sidimentation des depots sableux dans le Golfe bartonien du Bassin de Paris C. POMEROL (Paris, France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 Couches intraformationnelles a galets primitivement mous dans I’Ordovicien moyen de la region de Caen J . P o ~ c ~ ~ ( C a e n , F r a n c.e ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 Mesozoic and Cenozoic deltas of Tian-Shan and Pamir A. A. PHILLIPOV, A. A. BOGOIAVLENSKY and R. Y.MUZAV.I.POPOV,S. D. MACAROVA, PHAROVA (MOSCOW, U.S.S.R.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 A review of the factors affecting the sedimentation of the Millstone Grit (Namurian) in the Central Pennines H. G. READING (Oxford, Great Britain) . . . . . . . . . . . . . . . . . . . . . . . . 340

Le Flysch: definition; depdt de faible profondeur? M. REcH-FROLLO (Toulouse, France). . . . . . . . . . . . . . . . . . . . . . . . .

347

Contribution a l’etude de la skdimentologie des sediments carbonates A. RIVI~RE et S. VERNHET (Paris, France) . . . . . . . . . . . . . . . . .

356

The penecontemporaneous deformation of heavy mineral bands in the Torridonian sandstone of northwest Scotland R. C. SELLEY (London, Great Britain) . . . . . . . . . . . . . . . . . . . . . . . . 362

On the penecontemporaneous disturbance of bedding by “quicksand” movement in the Devonian rocks of North Devon D. J. SHEARMAN (London, Great Britain) . . . . . . . . . . . . . . . . . . . . . . . 368 Near-shore and shallow-water deposits of Aptian and Albian age in Moscow region U.S.S.R.) . . . . . . . . . . . . . . . . . . . . . . . . . M.S.SHVETZOV(MOSCOW,

371

Primary structures in a part of the Nile delta sand beach S. M. SOLMAN (Cairo, Egypt) . . . . . . . . . . .

379

. . . .

Distribution and lateral variability of heavy minerals in the Annot Sandstones (Vicksburg, Miss., U.S.A.) . . . . . . . . . . . . . . . . . . . . . . . D. J. STANLEY

388

Traits particuliers mineralogiques-geochimiques de I’assise terrigtne carboniftre inferieur dans la region Ouralienne-Volgienne G. I. THEODOROVITCH, N. N. SOKOLOVA, E. D. ROSONOVA et M. V. BAGDASSAROVA (Moscou, U.R.S.S.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399 Crittres de sensibilitt appliqds aux indices de forme des grains de sable v. TONNARD (Gernbloux, Belgique) . . . . . . . . . . . . . . . . . . . . . . . . .

410

Effect of the origin of the Lower Carboniferous clays in the western part of the Moscow Basin

on the alterations of their clay minerals M.F. VIKULOVA(MOSCOW, U.S.S.R.). . . . . . . . . . . . . . . . . . . . . . . . .

417

XVI

CONTENTS

Present day precipitation of calcium carbonate in the Persian Gulf A . J . WELLSand L . V. ILLINC(Rijswijk, The Netherlands) . . . Delta sedimentation in the German Keuper Basin P. WURSTER (Hamburg. Germany) . . . . . . .

. . . . . . . . . . . . . 429

. . . . . . . . . . . . . . . . . . .

Sedimentology of a Tertiary beach sand in the Subalpine Molasse Trough w. Z~MMERLE (Hamburg. Germany) . . . . . . . . . . . . . . . .

436

. . . . . . . . . Index I. according to main topics of papers . . . . . . . . . . . . . . . . . . . . . . .

447

. . . . . . . . . . . . . . . . . . . . . . .

462

Index 11. according to main areas concerned

459

Presidential Address

CRITERIA I N MODERN SEDIMENTS USEFUL I N RECOGNIZING ANCIENT SEDIMENTARY ENVIRONMENTS' F R A N C I S P. S H E P A R D Scripps Institution

of OceanoLgraphy,University of California, La Jolla, CalF (U.S.A.)

INTRODUCTION

The great contributions of CAYEUX (1941) have been a major factor in recognizing the environments in which sediments were deposited. However, I can certainly agree with BROUWER(1962) that Cayeux was too pessimistic in saying that it is uniquely by the analysis of ancient sediments that we can get to understand the conditions in marine basins of the past. It is quite possible that some of these ancient deposits were formed under conditions that no longer exist, but it seems likely that our failure to find analogies between past and present is due largely to the sparcity of studies in modern sedimentary basins. Compared to the thousands of geologists who are investigating ancient sediments there are only a handful of us who have been studying the Recent although we now have many new recruits. It is said by some that the present is a poor time to learn about past sedimentary conditions because of the large amount of tectonic activity that has taken place in the Cenozoic and the broad emergence of the land masses in contrast to the extensive inland seas of the past. However, we still have extensive inland or at least partly landlocked seas, the study of which has only just been started. For example, there are the Gulf of Thailand, the South China Sea, the Yellow Sea, the Timor-Sahui shelf, the Java Sea, the vast embayments in the Arctic, the Caspian, the Black Sea, the Mediterranean, the North Sea, the Baltic, and the Persian Gulf. Need we name more? As a possible objection to the significance of this enumeration it could be pointed out that much of the area consists of relatively deep seas, whereas the seas of the past have been considered by most geologists as largely shallow. Yet that answer cannot be given with as much appeal since my old friend professor Kuenen and others have done so much to demonstrate that turbidity currents in deep water can produce most of the criteria that were used to diagnose shallow water conditions. Perhaps we have had many more deep water basins in the past than we now recognize. Furthermore, turbidity currents can carry shallow water fossils, including wood, out into the greatest of depths and are capable of transporting organisms with little or no signs of wear. Much of the work reported here was supported by grants from the American Petroleum Institute (A.P.I. 51) and from the Office of Naval Research under contract nonr 2216(01).

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Fig.1. Position of sea level during the past 20,OOO years indicated by "C dates of near sea level organisms and plants plotted against their depth below or above present sea level. All samples come from relatively stable areas. The samples from Australia that are above sea level are believed by recent investigators to represent kitchen middens and are therefore suspect.

Another objection to comparing ancient and present day environments is that the present is a time that is abnormal in that the sea level has been fluctuating rapidly over the past several hundred thousand years due to the alternate growth and dispersal of great ice sheets. As to how important this rapid fluctuation is in making the present an abnormal period depends on whether or not there have been as many up and down movements of the sea during the past few thousand years as claimed by a few geologists. For some years I have been accumulating 14Cdates that bear on this problem and as shown in the accompanying graph (Fig.1)I have come to the conclusion, along with most of the investigators of Holland and most Gulf Coast geologists, that the sea has been virtually at a standstill for the past 3,000 years and rose so slowly from about 6,000 to 3,000 years B.P. that it would not have differed very much from the slow sinking of sedimentary basins in the past. The hypothesis of a high sea level during the

RECOGNIZING SEDIMENTARY ENVIRONMENTS

3

climatic optimum, a few thousand years before the present, seems to be disappearing along with many other geological misinterpretations related to the sea. Recent investigations of what formerly seemed to be good evidence of uniformly elevated terraces

Fig.2. Box corer nioditied by A. H. Bouma from design by REINECK (1963). Noteconipass attachment for orienting sample. Structures are often well preserved in the box cores.

4

F. P. SHEPARD

have proved that these terraces are too old for a postglacial high stand and the terraces are far from uniform in height. Therefore, it now seems that there has been time for deposition to occur under the relatively stable conditions that existed during most of the past. Another difficulty in comparing recent and past sedimentation has been that modern sediments obtained by cores are too much disturbed to allow sedimentary structures to be found. This trouble has now been partially eliminated by the use of a large box sampler based on the system of a device of REINECK (1963) which has been redesigned by A. H. Bouma for use in all depths of water (Fig.2). With sections of these rectangular cores, X-ray photographs, and sedimentary peels, we have found cross-bedding, lamination, ripple marks, graded bedding, small folds, and reworking by organisms (Fig.3). Progress in understanding the conditions of sedimentation in the past has come from photographs of the ocean bottom. The finding of ripple marks on seamounts, in submarine canyons (Fig.4), and even in troughs at great ocean depths has helped us understand how completely wrong was the old idea that ripple marks are confined to shallow water deposits‘. Bottom photographs have also emphasized the importance of organisms in working over near surface sediments (Fig.5). The operations of SCUBA divers using Cousteau’s aqua-lung have revolutionized our ideas on submarine erosion and transportation. In the past the main emphasis in the recognition of environments has been on paleontology, the comparison of fossil habitats with those of the present day. This has, of course, been enormously helpful, but it can be deceptive. We have only to think of a few examples to illustrate this point. At present we have bears living from the Arctic to the equatorial regions. If only the polar bears had survived, is it not possible that bear skeletons in ancient deposits would be used as evidence for a cold climate? Coral reefs are generally considered as indicators of shallow and warm water, but, as TEICHERT (1958) and others have emphasized, ahermatypic coral reefs grow as far north as Norway and in water several hundred fathoms deep. Coal has generally been considered as developing in the tropics but the work on Arctic tundras has shown that certain types of coal could have developed equally well in cold climates. The very helpful criteria coming from depth zonation of foraminifera1 assemblages are used extensively in the interpretation of Tertiary sediments. However, it is not known to what extent the zonation is related to depth as a factor of pressure or to what extent to temperature that is not necessarily controlled by depth. BRADSHAW’S ( 196 1) controlled experiments in culturing Foraminifera at Scripps Institution may give some of the answers but work of this type is still in an early stage. Clearly we need to use evidence from the physical and chemical nature of sediments to supplement the faunal assemblages in diagnosing the ancient environments.

Actually, according to D. L. Inman (personal communication, 1963), certain types of ripples are definitely related to shallow water waves.

RECOGNIZING SEDIMENTARY ENVIRONMENTS

5

Fig.3. X-ray photograph of slice from box sample showing various structures. Sample is from Santa Maria Canyon, Baja California, in 335 m.

6

F. P. SHEPARD

Fig.4. Current ripple marks obtained by the writer from the floor of a Japanese submarinecanyon at a depth of 510 m. Photograph by an Edgerton underwater camera.

RECOGNIZING SEDIMENTARY ENVIRONMENTS

7

Fig.5. Showing the trails, burrows, and other disturbances of the bottom by organisms. Photo by an Edgerton underwater camera at a depth of 1,350 m. The disturbance of the sediment is caused by a smsll flat fish visible in picture (left center).

8

F. P. SHEPARD USEFUL METHODS FOR INTERPRETING THE PAST

During the course of some 40 years of sampling and studying recent sediments I have investigated a great many environments that have ranged all the way from beaches and coastal dunes out to the deep sea floor. Climatically these have ranged from glacial marginal to tropical. I n the course of this work I have been looking for criteria and methods for comparing ancient and modern sediments. During my direction of American Petroleum Institute Project 51 I had the cooperation of a considerable group of scientists with a variety of skills attacking this problem from samples collected in the northwest Gulf of Mexico (SHEPARD et al., 1960). The study of the faunas provedvery useful; Foraminifera, ostracods, and molluscan assemblages were all of help. Clay minerals gave some indications but were rather inconsistent so far as our work went. Size analysis along with sorting and skewness of sediments has given some idea of environmental control. Roundness of sand grains served to distinguish adjacent beaches and dunes and orientation of grains proved helpful in some environments. Sedimentary strilctures provided a helpful criterion, especially around river mouths. I have been particularly impressed with a method which I have called “coarse fraction analysis.” This consists of making counts of the constituents of the various sieve sizes of the sand and gravel fraction of a sediment. The types of constituents to be used should depend both on training of the operator and on the particular environment being studied. Such constituents as terrigenous including nonmicaceous light minerals, dark minerals, and micas can be separated easily and no great skill is required to count the benthonic and planktonic Foraminifera, the molluscan fragments, the echinoid fragments, the sponge spicuIes, etc. The ratio of abundance of these different groups has proved to be very helpful in interpreting environments. In fact, when we tested various methods on a series of samples with information on the environments withheld during the test, we found that we got the best scores by determinations based on the coarse fraction constituents. This test may apply only to Gulf Coast shallow water sediments but at least it shows that similar analyses of ancient sediments should be useful along with various other tests.

EXAMPLES OF CRITERIA COMING FROM MODERN SEDIMENT STUDIES

In order to illustrate the type of criteria which may come from sediment studies I should like to point to some results that my colleagues and I have had from our work. I do not mean to imply that these are by any means the only criteria that can be used but just wish to show how the studies of one small group of investigators have indicated the importance of modern sediments. Distinguishing dunes, beaches, and nearshore sediments During the course of the Gulf Coast studies we had the opportunity to compare the

9

RECOGNIZING SEDIMENTARY ENVIRONMENTS

sands from dunes, beaches, and nearshore environments. Later these comparisons were extended to coastal areas from various other parts of the world, with especial emphasis on the Hawaiian Islands. We found that where there were predominantly onshore winds we could distinguish about 95 % of the dune and beach sands because the former had greater roundness, higher silt content, and greater abundance of heavy minerals, especially of the easily recognized zircon crystals; whereas the latter had greater abundance of shells and mica (SHEPARDand YOUNG,1961). We could not find, however, any reason to believe that the skewness or kurtosis of the sands could be used to distinguish these environments as has been claimed by some investigators. As a criterion to separate beach from nearshore sands we found that outside fine sand beaches there was a fairly consistent increase of the very fine sand in the offshore samples along with the appearance of appreciable silt that is virtually missing in the beach sands (SHEPARD and MOORE,1955). Along the Gulf Coast sorting did not appear to differ much between beaches and nearshore sand but in the Hawaiian Islands where most sands are the remains of calcareous organisms, we found that sorting is rather consistently worse in the nearshore zone (Fig.6) (SHEPARDet al., 1962), and that the sediments are more negatively skewed nearshore because of the abundance of relatively large shell fragments and less erosion of the large Foraminifera. The well-known sedimentary structures of dunes and beaches can, of course, supplement these other criteria, but in our cores from drillings in the Gulf Coast area we

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10 F. P. SHEPARD

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RECOGNIZING SEDIMENTARY ENVIRONMENTS

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found very little bedding of any kind to help this interpretation. Whether or not these structures are found it seems likely that most continuous outcrops that traverse these three environments would have characteristics which should make it possible to find the boundaries between dune, beach, and nearshore. Distinguishing arid and humid lagoonal deposits from shallocc, shelf seditncnts The work along the coast of Texas provided some helpful criteria for the recognition of ancient lagoonal deposits and means of identifying the climatic conditions prevailing in the areas ( P A R K E R , 1960; PHLEGER, 1960; RUSNAK, 1960; SHEPARD and M O O R E , 1960). In addition to the distinguishing characteristics of the faunal assemblages the constituents of the coarse fraction and the physical characteristics of the sediments are important. Thus the bay muds differ from muds on the continental shelves in lacking the glauconite that is so common on the shelf. Almost all of the oyster reefs, especially those with Crassostrea virginica, are confined to the bays. Echinoid fragments are common in the shelf deposits and rare in the bays. The bay muds have at least as many Foraminifera as the shelf muds but consist largely of a few species in contrast to the numerous species of the shelf. At the lower end of the bays, especially near inlets, the sediments are commonly sandy clays or clayey sands with only small quantities of silt. Such sediments are very rare on the shelf. In the bays of humid areas the benthic faunas churn up the bottom in such a way as to eliminate most stratification except near stream mouths where the combination of fast deposition and low salinity with little bottom life results in the preservation of laminae. Most bay cores in central Texas lack any indication of stratification even in borings in which we have traced the bay sediments down to as much as 70 ft. below the bay floor (Fig.7a). On the other hand the cores in semiarid Laguna Madre of south Texas are mostly stratified (Fig.7b) because of the scarcity of the bottom sediment churning organisms. This in turn is due to a considerable extent to the covering of the bottom by algal mats that prevent oxygen from entering. In semiarid regions the sediments contain a considerable quantity of chemically precipitated lime. This may occur as thin lime coatings on sand grains or as oolites (RUSNAK, 1960). Locally, gypsum and anhydrite develop in the deposists of Laguna Madre. These chemical precipitates are missing in the more humid bays farther north. Relatively clean sand deposits are found over the part of the Laguna Madre adjacent to the sandy barrier island. These are derived from wind blown sand. This source is not nearly as important in the lagoons inside the barrier islands to the north because there the dunes are more anchored by vegetation. In the bay deposits marginal to the barrier islands in the humid areas sediments are much muddier and less well sorted. Distinguishing marine delta facies

In our studies of the shallow water sediments in the northwest Gulf of Mexico we were impressed by the fact that quantitatively the deposits forming around margins of

F. P. SHEPARD

Fig.8. Showing the growth of the Mississippi delta between 1859 and 1940. Much of the growth took place in a much shorter period. Water depths are given in feet. (Taken from maps and soundings by the U. S. Coast and Geodetic Survey.)

deltas far exceed those of all other environments. This must also be true along most other coasts at the present time. Aside from catastrophic events such as large landslides it is only near the mouths of rivers that deposition is so fast that marked changes are indicated between successive surveys. Where human records go back for thousands of years, as in the Persian Gulf, we find that deltas have built forward for hundreds of miles. At the mouth of the Mississippi the far shorter records show an even more im-

13

RECOGNIZING SEDIMENTARY ENVIRONMENTS

pressive rate of sedimentation (Fig.8). Why, then, do we have so few descriptions of ancient deltaic deposits in geological literature? One answer could be that during the past the land areas were smaller in general and, therefore, delta formation was less important than it is today. Although this is probably true, there seem always to have been great shield areas with the rivers necessary to build deltas and according to paleogeographic maps the lands were sufficiently large to make delta building an active process. Furthermore, most of the seas were more protected by land barriers so that delta building should have been less hampered by the wave and current action that tend to disperse the sediment at present day stream mouths. An alternative explanation for the missing deltas may be that geologists have misinterpreted many ancient delta deposits. One reason that this may have been the case is that the typical textbook diagram of a delta (Fig.9) gives the impression that there are steeply dipping foreset beds lying between essentially horizontal bottomset and topset beds. The soundings outside the great deltas of the present day, however, show that the advancing slopes to which the name foreset bedding should be applied are almost universally very gentle. It is rare that they have inclinations of as much as 1" (SHEPARD, 1956, pp.2615-2620). If this were true also of most of the larger ancient deltas, it is easy to see how misinterpretations could have developed. The discordance in the dip of the foreset beds would be so slight that it would take a precise study before it could be recognized. Needed, therefore, are other criteria. The extensive work on the Mississippi delta (RUSSELL,1936; FISK,1944; FISKand MCFARLAN,1955; SHEPARD,1956, 1960; SCRUTON, 1960; LANKFORD and SHEPARD, 1960), on the Fraser delta (MATHEWS and SHEPARD. 1962), on the Rhone delta (VANANDEL,1955; KRUIT,1955; VANSTRAATEN, 1959), on the Orinoco delta (VANANDELand POSTMA,1954; NOTA,1958), and on the Volga delta (KLENOVA, 1956), to mention some recent studies, certainly have yielded sufficient information to provide the needed criteria. Much has been made of the socalled sand fingers of the Mississippi Passes lying between clay wedges of the interdistributary troughs (FISKand MCFARLAN, 1955). Theoretically such an arrangement should exist in deltas since the currents are, of course, stronger in the channels than in the overflow passageways and, hence, the sediments should be coarser. Unfortunately the very numerous cores and the shallow borings which we took in the Mississippi delta area during the American Petroleum Institute project failed to substantiate any marked differences in the sediments of these facies (SHEPARD and LANKFORD, 1959; SCRUTON, 1960). We found on the other hand that thick sand deposits coming from

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14

F. P. SHEPARD

the wells of the U. S. Army Engineers (KOLBand VAN LOPIK,1958; LANKFORD and SHEPARD, 1960) had characteristics that were indicative of stream channel point bars which are in a sense allied to the sand finger concept, although in the present stream channels they are discontinuous and bordered by mud deposits. Useful criteria for the marine facies of many deltas are the following: (I) Laminations (Fig.7~)consisting of very fine sand and silt alternating with silty clays. These are well developed on the delta front platforms around the Mississippi, Fraser, and Orinoco. (2) A coarse fraction in which Foraminifera and molluscs are scarce, but wood fibers are abundant and ostracods may be common. (3) Unusually large amounts of mica in the sediments because of the excessive mica transport by streams. (4) A very minor discordance between the well-laminated topset beds and generally unlaminated foreset beds. (5) Another minor discordance between the foreset beds and the equally unlaminated bottomset beds, the latter having a considerably greater quantity of Foraminifera and other organisms than found in the foresets. Transgressive deposits of the continental shelves One of the great problems confronting geologists has been to explain the wide extent of certain marine sand formations. At the present time sands are being deposited only rather near shorelines in the sea, and yet in the ancient sediments sand bodies of marine origin can be traced for hundreds of miles away from what is generally considered to be the direction of the shoreline. The study of continental shelf sediments has shown that sands are quite extensive on many shelves but these sands are ordinarily not contiguous to the shore and are likely to occur o n the outer shelf. An examination of the offshore sands (NOTA,1958; CURRAY,1960; NIINO and EMERY,1961) has shown that they are rather different in composition from sands found along the present shores, often being coarser and not infrequently more calcareous. They are clearly not being carried across the shelf from the lands at the present time, but are relict of a period of lower sea level. The finding of shallow water continental and lagoonal fossils in the sands confirms this assumption. In CURRAY’S (1960) study of the Texas shelf a considerable number of shells from the coarse sediments were collected and analyzed for age by 14C. The dates show that these deposits represent a transgressive series encroaching onto the lands as the sea rose after the last glacial stage. If a transgressing sea could produce widespread sand deposits off the Texas coast, it seems equally likely that many transgressions had the same effect in the past. When seas came in slowly across broad flat lands with gentle slopes, sand-carrying rivers should have provided sand deposits for deposition over the broad shallow water zones along the receding shore. Some of the faunas of the ancient marine sandstones suggest that the sands were

15

RECOGNIZING SEDIMENTARY ENVIRONMENTS

O-IOV.

OF TOTAL FORAMINIFERA

~ ' ' 10-~ 30% ~ ~ ~ ~ ~ ~ ~ ~ ~ ' ~ ~ ~ ~ ~ ~ ~

73 0 - M %

PER CENT PLANKTONIC IN TOTAL

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FORAMINIFERA

i

Fig.10. Map showing the general increase in planktonic Foraminifera in the total percentage of Foraminifera in crossing the continental shelf off the coast of Texas and southwestern Louisiana.

deposited far from the shore. This may mean, however, that the sands were reworked by waves and currents after the land had sunk, and the sands, although deposited near shore, were brought into an offshore zone by transgression without any appreciable covering because currents had inhibited deposition or because of scarcity of fine sediments in the overlying water. This is exactly what has happened along the Texas coast. A core we obtained in 80 m of water contained about 1 m of sand. All along the length of this sand there were Foraminifera characteristic of present day depths, but in addition there were macro-organisms of a shallow water facies (PARKER,1960). This mixture can be best interpreted as a shallow water sand low in Foraminifera that has been reworked under relatively deep water conditions, introducing the benthic Foraminifera of the present depth range. Distinguishing facies of the continental shelf

The study of the organisms of the continental shelf off the Gulf Coast of the United States has shown that it is possible to recognize assemblages of nearshore, inner shelf, and outer shelf (F. L. PARKER,1954; CURTIS,1960; R. H. PARKER, 1960; PHLEGER, 1960; LUDWICK and WALTON,1957). It is not only possible for an expert to distinguish these facies for the Gulf Coast, but rather easy for a geologist with a rudimentary knowledge of the difference between benthonic and planktonic Foraminifera to determine whether a sample is from the inner, intermediate, or outer shelf by the ratio of benthonic to planktonic that decreases outwardly (Fig.10). Also by finding the total per cent of Foraminifera in the coarse fraction similar results can be obtained since the Foraminifera also increase outwardly (CURRAY, 1960, fig. 12d). In some areas

16

F. P. SHEPARD

Fig.1 I . Showing the variation in sand content of a set of cores extending seaward from St. Joseph Island along the Texas Coast.

it was found that cores taken at successively greater distance from shore showed a rather constant decrease in sand lenses with increasing distances (Fig. 11). On the other hand little success has come from attempts to show a continuous decrease in the median diameters of the sediment with increasing distance from shore. The same is true of most other shelf areas of the world, although as EMERY ( I 952) has pointed out one of the reasons for the lack of outward decreasing grain-size is the presence of relict sediments on the outer and intermediate shelves. Distinguishing glacial marine from normal marine

The sediments in basins off many of the glaciated coasts of the world appear to have

RECOGNIZING SEDIMENTARY ENVIRONMENTS

17

characteristics which should make it possible to recognize the former existence of glaciers along the margins of ancient basins. The vast quantities of melting ice that existed along the coasts a few thousand years before the present resulted in the forming of icebergs that transported gravel, stones, and even boulders out to the basins. The forming of icebergs around the margin of Antarctica and Greenland at present is comparable to what existed more widely during the ice age. The present day bergs from Antarctica and Greenland are carrying away such quantities of coarse material that the sediments for hundreds of miles are influenced. For example, off the Grand Banks the slope sediments contain large quantities of rock fragments that may be partly residual from the Pleistocene but could be explained as well by the present day icebergs. Gravel and stones are found mixed with fine sediments in almost all samples from the Gulf of Maine (SHEPARD et al., 1934). Similar deposits arz reported from the Barents Sea (KLENOVA, 1960) and from the shelf off Norway (HOLTEDAHL, 1955). These anomalous sediments indicate that deposition has been rather slow since the last ice retreated from the coast. Otherwise the stones should be buried i n modern sediments. Probably the rivers in these areas still bring in a supply of rocks in ice flows during the spring, but it does not seem likely that these would be carried widely out over the continental shelves. In ancient sediments the finding of a considerable quantity of angular unweatherzd rock fragments scattered through fine-grained sediments is fairly good evidence of glacial marine origin. Other sources such as kelp, marine mammals, and mats of vegetation do not ordinarily transport morz than an occasional erratic to a basin, whereas icebergs bring great quantities of coarse dzbris. Iceberg stones are less likely to be rounded than stones introduced by marine mammals and are more likely to be of heterogeneous character than stones from the other sources since the continental glaciers have a larger source area. The presence of glacial striations, as reported by Holtedahl for the Norwegian shelf, would be confirmatory evidence. Recognition of deep water sands

The fine contributions made by Kuenen and his school of geology have led to the interpretation of turbidites, see list of references by KUENEN and HUMBERT (1964). This has explained the finding of deep water Foraminifera and other deep water organisms in sedimentary rocks associated with sand and gravel layers. The numerous findings of sand layers in cores from present-day deep water basins, troughs, and valleys are similarly intwpreted. Some authors, however, have begun explaining almost all marine sands as turbidites. For a complete understanding of the turbidite problem we need more study of the modern sediments as well as observations on turbidity currents in operation. To date they have been observed only in artificial lakes (GOULD,1951) where they are very slow moving and in Lake Geneva where slightly higher velocities are reported by DILL( 1964b). In recent years there have been rather extensive sampling operations

18

F. P. SHEPARD

along the California and lower California coast that have led to a considerable amount of information on the sand layers that are found in the slope valleys and on the fans as well as in the basins and troughs beyond (EMERY, 1960, pp.21&240; SHEPARD and EINSELE, 1962). Most recently with the help of the box sampler, A. H. Bouma, working with me off these same coasts, has obtained samples that are particularly helpful in interpreting the nature of deep sea sands. One thing which is becoming constantly more evident is that the sands that are carried to the deep basins and troughs move down the slopes along the axes of submarine valleys. Coring along slopes between valleys has almost universally failed to produce the alternating sand and mud layers that are typical of turbidites. On the other hand, most cores from submarine canyons have sand layers, particularly if the canyons extend in near to the shore. The studies of the stability of sediments carried on by MOORE(1961) and his associates at the San Diego Navy Electronics Laboratory, have shown that the sediments in the canyons are metastable and hence subject to sliding, whereas the slope sediments outside of canyons are in general stable. This discovery along with the finding that sand does move out along canyons implies that the basins of the past, into which turbidites were transported, also had submarine canyons of some sort to allow the transport. This in turn indicates that canyons of the sea floor are by no means a special feature of Pleistocene glacial stage origin. There is some disagreement on the definition that should be applied to graywackes, bLt I think that most geologists agree that graywackes are coarse sediments with a muddy matrix. There has been an attempt by some authors to explain graywackes as a product of turbidity currents and incidentally of deep water origin. We have analyzed inany deep water sands, both those obtained on Scripps Institution expeditions in the Pacific and many of the cores obtained by Lamont Geological Laboratory from the Atlantic (SHEPARD,1961). The result of these analyses has been that virtually all are fine to very fine sands, although a few have gravel and shells as wel! as coarse sand. Few samples of coarse sediment with a muddy matrix have been found. A core with gravel taken by Lamont from Hudson Canyon (shown to me by D. B. Ericson), proved to have the gravel embedded in a muddy matrix (Fig.12). Dredging along a channel in outer La Jolla Canyon showed that there were rounded claystone fragments, and R. F. Dill photographed some rounded boulders from the window of the bathyscaph “Trieste” while moving along the muddy floor in the same area (Fig. 13). Therefore, it may well be possible for coarse debris to be transported outward by some form of creep or mud flow along the canyons. This has long been recognized by Kuenen. Such an origin is suggested by the boulders found in a muddy matrix in the Plixene of the Ventura area of California (WINTERER and DURHAM, 1962, fig.63). It s e c n s likely, however, that at present most sands are being transported in a form that prevents the incorporation of coarse sand in a muddy matrix. This suggests that it is unwise to attribute graywackes to a turbidity current origin unless there is confirmatory evidence of another nature as Kuenen has found in some localities. The structures found by BOUMA(1962, fig.8) in his study of the French Alps have a characteristic alternation of graded sand covered by a laminated material of somewhat

RECOGNIZING SEDIMENTARY ENVIRONMENTS

19

finer grain-size and then by cross-bedded and ripple-marked fine sands in turn covered by mudstone. In one of the cores taken by the box corer in the outer La Jolla sub1 marine channel, Bouma found almost exactly this sequence (Fig. 14). Another: example, found by L. M. J. U. van Straaten (personal communication, 1963) is that

Fig.12. Pebbles embedded in muddy matrix found in core from the deep axis of Hudson Canyon. (Courtesy of D. B. Ericson, Lamont Geological Observatory.)

20

F. P. SHEPARD

Fig.13. Boulders photographed hy R. F. Dill from the port of the bathyscaph “Trieste” at 550 rn along the floor of La Jolla submarine channel outside the rock canyon. (Courtesy of Navy Electronics Laboratory.)

of a 17 cm thick graded turbidite layer from the small abyssal plain in the southeastern Adriatic. The structures in this bed are, from the base to the top: (a) horizontal laminations, (6) current ripple laminations, (c) convolute laminations, ( d )very fine horizontal laminations. This serves to show that the currents which emplaced the Alpine turbidites were probably similar in nature to the ones which have led to the deposits just mentioned. These certainly are remarkable examples of how the sediments of the past were laid down by processes still in operation at present. The recognition of ancient turbidites as noted by DOTT(1963) cannot be assured because of the presence of graywackes, graded bedding, or of sole marks and other similar criteria. All of them are developed as well or even better under other condi-

RECOGNIZING SEDIMENTARY ENVIRONMENTS

21

Fig.14. X-ray of box core taken in the f m channel off La Jolla Canyon at 1,028 m. The lower portion is a graded sand bed. This is covered by laminated sands including clay pebbles and the upper portion shows cross-bedding and ripple marks. The f o r e s t slopes, according to compass orientation, are down canyon. Cracks are due to disturbance in handling sediment slice. (Courtesy of A. H. Bouma 1

22

F. P. SHEPARD

tions. The repetition of relatively coarse graded beds with considerable fine intervening sediment and the presence of displaced and mixed faunas in the coarser sediments are apparently the most satisfactory indicators of turbidity current activity. I should like to emphasiz6the point that I have not tried to cover the entire field of comparison between ancient and modern sediments. Many important studies such as faunal assemblages, facies models, and mechanics and dynamics of sedimentation have not becn included, partly bxause of time limitations and partly because of my greater familiarity with the observational type of criteria. In conclusion I believe that one can now predict that acontinuation of studies along all of these fields should make it possible in the not distant future to determine the environments of deposition of virtually all sedimentary rocks that are not greatly altered by diagenesis.

SUMMARY

Evidence is presented favoring the hypothesis that “the present is the key to the past”. It is felt that the difficulties experienced by geologists in the use of modern sediments as a means of interpreting ancient sediments are due to the common lack of knowledge of sedimentary basins of the present day. Recent studies are rapidly filling in the gaps in this information. It is felt that there are plenty of present day basins in which this information is available and the sea level has been essentially constant for a long enough period to allow sediments of the various types to accumulate. Criteria that should be helpful in recognizing ancient environments have been gathered during the extensive operations conducted by the Scripps Institution of Oceanography along with many other organizations. In addition to faunal assemblages, criteria which have proved useful include the following: ( I ) Dune sands can usually be distinguished from adjacent beach sands by greater roundness, more heavy minerals, higher silt content, and smaller content of shells; along with the special aeolian type of cross-bedding. (2) Beach sands can be distinguished from adjacent shallow water formations by lower silt content and, as far as calcareous sands are concerned, by better sorting and less well-preserved fossils. ( 3 ) Lagoonal and estuarine deposits can often be distinguished from shallow shelf deposits by presence of abundant oysters, by scarcity or absence of glauconite and echinoids that are common on the shallow shelf, and by the finding of sandy clays low in silt that are rare on the shelf. (4) Marine delta facies can be recognized by the abundance of lamination, the scarcity of marine organisms along with an abundance of wood fibers, and in some cases by a very small discordance in dip between topset, foreset, and bottomset beds. (5) Extensive marine sand formations are often attributable to a transgressing sea with little deposition except along the advancing shoreline. (6) Glacial marginal deposits can be differentiated from normal marine deposits of

RECOGNIZING SEDIMENTARY ENVIRONMENTS

23

the same depth range by the presence of numerous scattered stones, mostly angular and often striated. (7) Deep water sands can be distinguished from those of shallow water by the repetition of numerous alternations between sand and mud, the sharp lower boundary of the sands, and the frequently graded nature of the sand beds. Here the help of paleontologists is particularly important since the deep sands often have mixed shallow and deep water Foraminifera. Differences in the ripple marks of shallow and deep water sands are also a promising approach to the problem.

REFERENCES

BOUMA, A. H., 1962. Sedimentology of some Flysch Deposits. A Graphic Approach to Furies Interpretation. Elsevier, Amsterdam, 168 pp. BRADSHAW, J. S . , 1961. Laboratory experiments on the ecology of Foraminifera. Contrib. Cushmon Found. Forum. Res., 12 (3) : 87-106. BROUWER, A,, 1962. Past and present in sedimentology. Sedimentology, 1 ( I ) : 2-6. CAYEUX, L., 1941. Causes ancienncs et Causes actuelles en Gkologie. Masson, Paris, 81 pp. CURRAY, J. R., 1960. Sediments and history of Holocene transgression, continental shelf, northwest F. B. PHLEGER and TJ. H. VAN ANDEL(Editors), Recent SediGulf of Mexico. In: F. P. SHEPARD, ments, Northwest Gulfof Mexico. Am. Assoc. Petrol. Geologists, Tulsa, pp. 221-266. CURTIS, D. M., 1960. Relation of environmental energy levels and ostracod biofacies in east Mississippi delta area, Bull. Am. Assoc. Petrol. Geolqyists, 44 (4) : 471494. DILL,R. F., 1964a. Features in the heads of submarine canyons; narrative of underwater film. In: L. M. J. U. VAN STRAATEN (Editor), Deltaic and Shallow Marine Deposits. Elsevier, Amsterdam, pp. 101-104. DILL,R. F., 1964b. Submarine Erosion. Elsevier, Amsterdam. In preparation. DOIT JR., R. H., 1963. Dynamics of subaqueous gravity depositional processes. Bull. Am. Assor. Petrol. Geologists, 47 (1) : 104-128. EMERY, K. O., 1952. Continental shelf sediments of southern California. Bull. Geol. SOC.Am.. 63 : 1105-1 108. EMERY, K. O . , 1960. The Sea ofSouthern California.Wiley, New York, 366 pp. FISK,H. N., 1944. Geolqyical Invest4yation of the Alluvial Valley of the Lower Mississippi River. Miss. River Conunission, Vicksburg, 78 pp. FISK,H. N. and MCFARLAN JR., E., 1955. Late Quaternary deltaic deposits of the Mississippi River. In: A. POLDERVAART (Editor), Crust of the Earth - Geol. SOC.Am., Spec. Papers, 62 : 279-302. COULD,H. R., 1951. Some quantitative aspects of Lake Mead turbidity currents. In: J. L. HOUGH Editor), Turbidity Currents and the Transportation of Coarse Sediments to Deep Water -Soc. Eron. Paleontologists Mineralogists, Spec. Publ., 2 : 34-52. HOLTEDAHL.H., 1955. On the Norwegian continental terrace, primarily outside More-Romsdal: its geomorphology and sediments. With contributions on the Quaternary geology of the adjacent land and on the bottom deposits of the Norwegian Sea. Univ. Bergen, Arbok, Natunitenskap. Rekke, 14 : 209 pp. M. V. (Editor), 1956. Contemporary Sediments of the Caspian Sea. Acad. Sci., U.S.S.R. KLENOVA, Moscow, 303 pp. (Translated into English by Am. Geol. Inst., Washington, D. C.). KLENOVA, M. V., 1960. The Geolqyy of the Barents Sea (Russ.). Acad. of Sci., U.S.S.R., Moscow, 367 pp. English abstract in Marine Geology - Intern. Geol. Congr., 21 st., Copenhagen 1960, Repts. qf Soviet Geologists, 130 pp. KOLB,C. R. and VANLOPIK,J. R., 1958. Geology of the Mississippi River deltaic plain, southeastern Louisiana. U. S. Corps Engrs., Waterways Expt. Sta., Tech. Repts., 2 vols. : 3 4 8 3 and 3484. KRUIT, C., 1955. Sediments of the Rhdne Delta. I . Grain-size and Microfauna. Thesis, Rijksuniv. Groningen. Mouton, The Hague, 14 pp. - Verhandel. Ned. Geol. Miinbouwk. Gcnoot., 15 (3) : 357499. KUENEN, PH. H. and HUMBERT, F. L., 1964. Bibliography of turbidity currents and turbidites. In: A. H. BOUMA and A. BROUWER (Editors), Turbidites. Elsevier, Amsterdam, in press.

24

F. P. SHEPARD

LANKFORD, R. R. and SHEPARD, F. P., 1960. Facies interpretation in Mississippi delta borings. J. Geol., 68 (4) : 408426. LUDWICK, J. C. and WALTON, W. R., 1957. Shelf-edge, calcareous prominences in northeastern Gulf of Mexico. Bull. Am. Assoc. Petrol. GeoloLyisrs,41 (9) : 2054-2101. W. H. and SHEPARD, F. P., 1962. Sedimentation of Fraser River delta, British Columbia. MATHEWS, Bull. Am. Assoc. Petrol. Geologists,46 (8) : 1416-1438. MOORE, D. G., 1961. Submarine slumps. J. Sediment. Petrol., 31 (3) : 343-357. N I I ~ OH. , and EMERY, K. 0..1961. Sediments of shallow portions of East China Sea and South China Sea. Bull. Ceol. Soc. Am., 72 : 731-762. NOTA,D. J. G., 1958. Sediments of the Western Giiiana Shelf- Reportsqfthc Orinoco Shelf Expedinon, 2 - Mededel. Landbouwhogeschool Wageniqpen, 58 (2) : 1-98. PARKER, F. L., 1954. Distribution of the Foraminifera in the northeastern Gulf of Mexico. Harvard Coll. Moseum C o n p . Zaol. Bull., I I 1 (10) : 453-588. PARKER, R. H., 1960. Ecolcgy and distributional patterns of marine macroinvertebrates, northern Gulf of Mexico. In: F. P. SHEPARD, F. B. PHLEGER and TJ. H. VAN ANDEL (Editors), Recent Sedinlents, Norrhuest Gu!fq/’Mexico. h i . Assoc. Petrol. Geologists, Tulsa, pp. 302-337. PHLEGER, F. B., 1960. Sedimentary patterns of microfaunas in northern Gulf of Mexico. In: F. P. SHEPARD, F. B. PHLEGER and TI. H. VAN ANDEL(Editors), Recent Sediments, Northwest Cu!f of Mexico. Am. Assoc. Petrol. Geologists. Tulsa, pp. 267-301. REINECK, H. E., 1963. Der Kastengreifer. Natur Museum, 93 : 65-68. RUSNAK, G. A,, 1960. Sediments of Leguna Madre. In: F. P.SHEPARD, F. B. PHLEGER and TJ. H. V A N ANDEL(Editors), Recent Sediments, Northwesr Gulfof Mexico. Am. Assoc. Petrol. Geologists, Tulsa, pp. 153-196. RUSSELL, R. J . , 1936. Physiography of the Lower Mississippi River delta. Louisiana Geol. Surv., Geol. Bull., 8 : 3-199. SCRUTON, P. C., 1960. Delta building and the deltaic sequence. In: F. P. SHEPARD, F. B. PHLEGER and TJ. H. VAN ANDEL (Editors), Recent Sediments, Northwest Gulf of Mexico. Am. Assoc. Petrol. Geologists, Tulsa, pp. 82-102. SHEPARD, F. P., 1956. Marginal sediments of Mississippi delta. Bull. Am. Assoc. Petrol. Geologists, 40 ( I 1 ) : 2537-2623. SHEPARD, F. P., 1960. Mississippi delta: marginal environments, sediments, and growth. In: F. P. SHEPARD, F. B. PHLEGER and TI. H. VAN ANDEL(Editors), Recent Sediments, Northwest Gulf of Mexico. Am. Assoc. Petrol. Geologists, Tulsa, pp. 5 6 8 1 . SHEPARD, F. P., 1961. Deep sea sands. Intern. Geol. Coqgr., 21 st., Copenhagen 1960, Rept. Session, Norden, pp, 26-42. SHEPARD, F. P. and EINSELE, G., 1962. Sedimentation in San Diego Trough and contributing submarine canyons. Setfinientolqg, 1 (2) : 8 I- 133. SHEPARD, F. P. and LANKFORD, R. R., 1959. Scdinientary facies from shallow borings in Lower Mississippi delta. Bull. Am. Assoc. Petrol. Ceolqyists. 43 (9) : 2051-2067. SHEPARD, F. P. and MOORE, D. G., 1955. Central Texas coast sedimentation: characteristics of sedimentary environment, recent history, ar.d diagenesis. Biill. Am. Assoc. Petrol. Geologists, 39 (8) : 1463- I 593. SHEPARD, F. P. and MOORE,D. G., 1960. Bays of central Texas coast. In: F. P. SHEPARD, F. B. PHLEGER and TJ. H. VAN ANDEL(Editors), Recent Sediments, Northwest Gulf of Mexico. Am. Assoc. Petrol. Geologists, Tulsa. pp. 117-152. SHEPARD, F. P. and YOUNG, R., 1961. Distinguishing between beach and dune sands. J . Sediment. Petrol., 31 (2) : 19&214. SHEPARD, F. P., PHLEGER, F. B. and VAN ANDEL, TJ. H. (Editors), 1960. Recent Sediments, Northwest Gu!f qf Mexico. Am. Assoc. Petrol. Geologists, Tulsa, 394 pp. SHEPARD, F. P., TREFETHEN, J. M. and COHEE, G. V., 1934. Origin of Georges Bank. Bull. Geol. Soc. Am., 45 : 281-302. SHEPARD, F. P., MOBERLY JR., R., OOSTDAM, B. L. and VEEH,H. H., 1962. Beaches of ffawwii Abstract in Prqgr. Ann. Meeting Geol. Soc. Am., 1962 : 140-141A. TEICHERT, C., 1958. Cold- and deep-water coral banks. Bull. Am. Assoc. Petrol. Geolocgists, 42 ( 5 ) : 1064- 1082. VANANDEL, TJ. H. and POSTMA, H., 1954. Recent Sediments of the Gulf of Paria - Reports of Orinoco ShelfExpedition 1. North-Holland, Amsterdam, 245 pp.

RECOGNIZING SEDIMENTARY ENVIRONMENTS

25

VAN STRAATEN, L. M. J. U., 1959. Littoral and submarine morphology of the RhBne delta. ln: R . J. RUSSELL (Editor), 2nd Coasrd Geography C o n ? ,Natl. Acad. Sci. Natl. Res. Council, Washington, D.C., pp. 233-264. WINTERER, E. L. and DURHAM, D. L., 1962. Geology of southeastern Ventura Basin, Los Angeles County, California. U.S. Geol. Surv., Profess. Papers, 334-H : 275-366.

SEDIMENTATION IN THE MODERN DELTA O F THE RIVER NIGER, WEST AFRICA I. R. L . A L L E N Sedinientology Research Laboratory, Geology Department, University of Reading, Reading (Great Britain)

INTRODUCTION

The Niger delta on the West African coast is the focus of drainage from a vast hinterland. It is a sandy delta of “classical” form merging westward with a beach barrierlagoon complex extending from the Lekki area to the River Volta (Fig. lc). The deposits top a thick pile of sediment, much of which is probably deltaic, accumulated from Cretaceous times onward in a downwarp athwart the continental margin (Fig. la). The delta has not previously been described sedimentologically, and this paper is written to outline for the first time salient characters of its depositional environments and lithofacies. Aspects of the delta are discussed by PUGH(1954), NEDECO(1954, 1959, 1961), and ALLENand WELLS(1962). The present study is based on observations made at more than 1,000 sample stations during a survey jointly organised and financed by Shell Internationale Research Maatschappij N.V. and The British Petroleum Company Limited; these companies are thanked for opportunities to study the Niger delta.

SEDIMENT SUPPLY AND DISPERSAL

Sediment entering the Niger delta area comes mostly from the coarsely crystalline basement complex and basement-derived Mesozoic-Tertiary sediments cropping o u t in the Niger and Benue drainage basins. The medium sand grade and finer material from the Niger Basin is characterised by hornblende, epidote, sillimanite, and zircon. Hornblende, epidote, zircon, and garnet typify the very coarse sand and finer debris from the Benue catchment, which for all its small size yields about 60% of the delta’s sand. Minor sediment contributions come from the Volta Basin via the coast, and from the basins of the Ogun, Oshun, and Cross Rivers. Epidote, hornblende, kyanite, and sillimanite mark the Volta’s contribution. NEDECO (1959, 1961) established that the yearly discharge of freshwater into the delta is about 200.109 nl3, of sand (< 0.1 mm) approximately l.106 m3, and of silt and clay ( < 0.1 mm) about 17-106m3. Patterns of dispersal in the delta area are shown in Fig.lb. Swift distributaries

b- DISPERSAL

RIVERS

/ 7

TIDAL CHANNELS LITTORAL CURRENTS

2

WAVE8 AND T I D A L

1

GUINEA

*/

CURRENTS

CURRENT

DEEP CURRENTS ( - 2 5 t m i l

-

0

SEA MILES 5 0

0

C-ENVIRONMENTS

n

KM

I W

d-GROSS LITHOLOGY

n

GRAIN SIZE

..." LIMIT MANGROVE SWAMP

/LIMIT

BEACH AND BEACH RIDGES

OF CLAYEY OF CLEAN

LIMIT OF VERY

SILT SILT FINE SAND

FINE GRNNED AND COARSER SAND

UNIFORM LAYERED UNIFORM

FINE oEw DEPOSITS COARSE

DEPOSITS

MIXED UNIFORM FINE AND LAYERED DEPOSITS MIXED UNIFORM COARSE NONDEPOSITIONAL

AREAS

1_] AN0

LAYERED DEPOSITS

Fig.1. Environmental and sedimentological features of the Niger delta area.

. . I

I

,

.

..

N

00

Fig. 2. Lithofacies based on core samples and grain size spectra of depositional environments in the Niger delta. Environments referred to in numbered columns: (1) floodplain; (2a) mangrove swamp; (2b) channel in mangrove swamp area; (3a) beach; (3b) channel in beach-ridge zone; (4) estuary; (5) river mouth bar; (6) delta platform; (7) prodelta slope; (8) open shelf; (9) continental slope; (10) non-depositional area. Sediment grades: vcs = very coarse sand; cs = coarse sand; rns = medium sand;,fs = fine sand; vfs = very fine sand; csi = clean silt; CIS = clayey silt; scl = silty clay;pd = plant debris. The cores shown in the lower part of the figure are about 0.075 natural size.

SEDIMENTATION IN THE MODERN NIGER DELTA

29

spread discharges radially but unequally through the delta proper from a centre near Aboh. The Nun exit passes to the sea about 26 % of the Niger’s freshwater discharge, and the Ramos and Forcados exits 29 and 15 % respectively; sediment discharges are correspondingly great. The remaining 30 % of the freshwater discharge divides between afurther 19 major exits. Once at sea sediment is dispersed by currents with directions parallel or at steep angles to the delta coast. Long swell waves from the south-southwest drive sand eastward along the beaches in the Lagos-Lekki area (barrier complex), northwestward in the western delta, and eastward in the eastern delta. Yearly drifts vary from 77,000 m3 in the central delta to 380,000 m3in the eastern and western sectors. Tidal currents disperse sediment at steep angles to the coast. They are strongest on the river mouth-bars, but still powerful enough to shift very fine sand or coarse silt into depths as great as about 20 fathoms. Doubtless rip currents also facilitate the transport of coarse scdiment from the littoral into deeper water. Parallel to the coast, silt and clay are dispersed inshore chiefly by strong, wave-generated, littoral currents and farther out by the slow west-east Guinea Current. According to BERRIT(1959). currents setting to the north or northwest operate on the continental shelf below 25 fathoms.

DEPOSITIONAL ENVIRONMENTS A N D LlTHOFAClES

Classification of the environments. Depositional environments in the Niger delta divide between terrestrial, transitional, and marine realms (Fig. Ic). The terrestrial realm is represented by the floodplain environment. The transitional realm includes the mangrove swamp, beach with beach-ridge, and estuarine environments. The marine realm divides into the river mouth-bar, delta platform, prodelta slope, opm shelf, continental slope, and non-depositional environments. Most of these are further divisible. Gross lithology of the delta. The delta sediments fine away from the dispersal centre near Aboh. Fig. Id shows the maximum distances from this centrz attained by sediments of different grades, using WENTWORTH’S (1922) classes for sand, and for finer material an arbitrary division into clean silt, clayey silt, and silty clay. Adapting MOOREand SCRUTON’S (1957) classification of bedding, the surface delta deposit (,< I m) is classified (Fig. Id) according to whether it is an essentially uniform one of fine sediment [silt, clayey silt, silty clay), an essentially uniform one of coarse sediment (sand, gravel), or a layered one of more or less closely interbedded fine and coarse sediments (e.g., silty clay and fine sand). This classification distinguishes between material deposited from suspended load, that deposited from bed load, and that deposited alternately from suspended load and bed load. Bed load deposition occurs chiefly in the terrestrial and transitional realms, while deposition from suspended load is most important in the marine realm. Variations in grain size and bedding over the delta are summarised in Fig.2 showing grain size spectra and lithofacies. Thus, away from the dispersal centre, the delta sediments become en mane better sorted as well as finer grained.

30

I. R. L. ALLEN

Reference should be made to Fig. 1 and 2 in following the descriptions of environments and lithofacies below. Floodplain environment. In this environment (8,400 kmz) sediment is erratically but persistently on the move. The meandering distributaries flow swiftly (50-1 20 cmlsec) over hummocky beds mostly of fine to medium sand. Typically, the channel-bed sands are cross-stratified and incorporate leaf or wood fragments, alternating very fine sand and clayey silt appearing locally. The point-bars are mostly of interbedded crossstratified and evenly stratified sand with some bands of clayey silt. During the March to November rains the distributaries top their banks to flood extensive backswamps. Levees of very fine sand with some clayey silt are built up, while in the backswamps accumulate close interbeds of plant debris, clayey silt, and very fine sand. Mangrove swamp environment. This is essentially a vegetated intertidal flat (9,000 km2) grading from the toe of the floodplain in the central delta but otherwise lapping onto pre-delta sediments. The swamps are crossed and connected to the sea by an intricate system of meandering creeks which feed with salt water and drain the inter-creek flats. Since the tidal capacity of the swamp is about 480.106 m3 (tidal range 1-2 m), the discharge from it of salt water vastly exceeds that of freshwater during each tidal cycle. The dominant plant is the red mangrove, a tree or shrub according to situation, its prop-root systems checking currents and trapping sediment. Quantities of the riverborne silt and clay flocculated in the brackish swamp water are deposited on the flats and creek point-bars as a black, organic-rich clayey silt. Sand remains largely on the creek beds, where it becomes unevenly interbedded with dark clayey silts and plant debris. Estuarine environment. The Calabar estuary (460 km2) is flanked by mud flats and mangrove swamps. Nowhere deeper than 5 fathoms and mostly shallower than 2.5 fathoms, the estuary bottom comprises large shoals elongated parallel to the tidal flows. Despite strong currents (up to 100 cmlsec) sand is in short supply, that drifting along the coast from the west toward the estuary becoming trapped on shoals at its western point. The estuary deposits are mostly interbedded dark gray clayey silts with plant debris, clean silts, and very fine to very coarse sands. Thick sands are restricted to the channels. Beach with beachridge environment. At the river exits sediment is sorted into coarse and fine fractions. Intense wave action drifts the coarse fraction away along the coast, sorting and rounding it further and building up continuous sand beach barriers between the river mouths. NEDECO (1954, 1961) report that the beach surfaces are gentle (gradients 1/50-1/100) and tolerably wide (50-200 m). Typically, the beaches are of clean, fine grained sand with gently inclined even laminae, but medium sand is present locally. Plant debris drifting from the river mouths often becomes buried in hollows on the beach surfaces under fresh sand influxes. Pavements of clay pebbles patchily cover beaches on eroding stretches of the coast. Back of the beach is a ridge from 1.5-3 m high followed inland by more ridges forming a zone a few hundred metres to I6 km wide (1,800 kmz). The densely forested ridges show an upper layer of leached sand with destroyed lamination, their freshwater trees contrasting sharply

SEDIMENTATION IN THE MODERN NIGER DELTA

31

on aerial photographs with the brackish-loving mangroves farther inland. Below root level even lamination is preserved. The ridges record earlier strand lines. River mouth bar environmenf. The river mouth-bars (1,600 km2) (PUCH, 1954; NEDECO,1961) are curved mounds of sediment lying out to sea just off the river exits. Each bar provides a shallow submerged connection between the beaches flanking the exit. The shape and character of each bar is a resolution of the local conflict between the drifting of sand by waves along the coast and the several discharges from the delta interior. Bar crests, swept by powerful waves and tidal currents, lie in depths generally less than 2 fathoms. They consist of clean, fine or medium sand with even lamination, cross-lamination, or cut-and-fill. Conditions are less extreme on the gently inclined bar flanks, where evenly laminated or cross-laminated, clean, very fine sand is found. Deeper still, near the toes of the bars (2.5-7 fathoms), are close interbeds of very fine sand, clean silt, and clayey silt with plant debris. Delta platform evironment. The delta platform (4,500 km2) is a gently inclined, terrace-like feature from 8-25 km across which extends from the beach to a break of slope between 4.5 and 10 fathoms. It is persistently but erratically supplied with suspended silt and clay by flows of turbid water emanating from the river exits and thereafter hugging the coast. Wave-generated and tidal currents are strong enough to move very fine sand and coarse silt across the platform from the vicinity of the beach. Inshore occurs evenly laminated or cross-laminated, fairly clean, very fine sand and coarse silt. Farther out on the platform is closely interbedded coarse silt and dark gray clayey silt with finely divided plant debris, thin bands of very fine sand appearing locally. Bioturbation structures and shell concentrations are common on the outer platform. Prodelta slope environment. This environment (7,800 km2) reaches from the delta platform to a depth of about 22 fathoms, where a thermocline separating two major water bodies intersects the sea bed (ALLENand WELLS,1962). The Guinea Current, the uppermost water body, drifts suspended silt and clay over the prodelta slope. Tidal and wave-generated currents, their strength diminishing with depth, are just competent to shift on the bottom whatever coarse silt or very fine sand attains the slope from nearer shore. Thus the shallower slope consists of thick layers of gray clayey silt and silty clay with plant debris interbedded with thin layers of evenly laminated or crosslaminated, clean, coarse silt and occasionally very fine sand. Bioturbation structures and shell pockets are plentiful. Farther out on the slope is bioturbated gray to greenish gray clayey silt. Open shelf environment. The open shelf (10,000 km2) is a smooth area extending from the 22 fathom line to the shelf break (42-108 fathoms). Tidal and wave-generated flows, unable to drag coarse bottom materials over the long haul from the coast, are competent to rework sand for short distances from non-depositional areas encompassed by the environment. Gentle currents steadily supply the open shelf with clay and silt. Grayish green silty clays devoid of lamination dominate the open shelf sediments, clayey silts being restricted to the central delta, and sand layers to the vicinity of non-depositional areas. Intense bioturbation points to the comparative slowness of deposition.

32

J. R. L. ALLEN

Continental slope environment. Conceivably deriving sediment from numerous sources, the continental slope environment (16,000 km2) is only known between the shelf break and 500 fathoms. The slope deposits are almost exclusively grayish green, intensely bioturbated, silty clays. At one station (253 fathoms) is a mass of shelly quartz sand, possibly slid from a non-depositional area at the shelf edge. Non-depositional environment. This term covers “outcrops” of quartz sand (5,100 km2)of Late Glacial to earlier Holocene age (JELCERSMA, 196 1) on which no significant deposition has occurred subsequently. The outcrops divide between a near-shore zone and a deeper one around the shelf edge. The shallower sands are typically clean and well sorted, being fine to very coarse grained, with plentiful abraded shell and rare glauconite. Minimum radiocarbon ages range from 3,380 150 to 1,190 k I50 years BP. Usually the deeper sands are muddy and rich in glauconite, shell, and foraminifera. They vary from coarse silt to coarse sand grade, showing abundant bioturbation structures. Minimum ages range from 12,250 240 to 10,750 f 250 years BP. Echograms show the exposed sands to be part of an otherwise deeply buried sheet which encompasses the entire continental shelf and probably reaches into the hinterland. Locally no thicker than 19 cm, the sheet rests disconformably on sediments of an earlier (?) delta.

COASTAL BARRIER COMPLEX

WEBB(1958) and HILLand WEBB(1958) briefly described beach barrier-lagoon complexes of the West African Coast. That complex lying immediately west of the Niger delta, and within the influence of the delta, embraces two large lagoons, Lekki Lagoon and Lagos Lagoon, both mostly shallower than 1.5 fathoms and divided from the sea by sandy beach-ridges. Direct connection with the sea is achieved at Lagos. Brackish water fills the lagoons in the dry season but during the rains is largely flushed out. Rivers building small deltas bring varied sediments to the lagoons. Organic-rich mud is accumulating in the deeper parts and clean, medium grained sand near the shores. Mangroves fringe the lagoons locally.

SEDIMENTARY FRAMEWORK A N D EVOLUTION

The stratigraphy of the Niger delta is as follows: younger suite - later Holocene older sands - Late Glacial-earlier Holocene pre-older sands - pre-Late Glacial The younger suite covers lithofacies in the floodplain, mangrove swamp, beach with beach-ridges, estuarine, delta platform, river mouth-bar, prodelta slope, open shelf, and continental slope environments. Older sands are exposed in the non-depositional areas, the pre-older sands lying immediately beneath them. Fig.3 amplifies this SUC-

33

SEDIMENTATION I N THE MODERN NIGER DELTA ONITSHA

NUN R

NEAR

BRASS R

5 T NICHOLAS R

BRASS

LEGEND

I__j

Den0 plotform foc,es

Prodelto slope f o c m

rj

B e a c h focles

BOoen

shelf focles

Mouth bar focies

Fig.3. Schematic facies relations in the Niger delta. a. Section along axis of delta. b. Section parallel to coast along beach-ridge zone.

cession by summarising facies distributions in the Niger delta as inferred from surface and subsurface (very scanty) data. Offshore and probably in coastal areas the delta mass commences with the sheet-like older sands analogous to the strand-plain deposits of the Mississippi and Rhbne deltas (FISKand MCFARLAN, 1955; SCRUTON,1960; KRUIT, 1955; VANSTRAATEN,1959, 1960). The floodplain facies grades seaward into sheet-like bodies of offshore silty clay and clayey silt, nearshore mixed clayey silt, clean silt and sand, clean littoral sand, and inland mixed swamp and channel-fill deposits. Thus the delta differs from the Mississippi bird-foot in framework in not having finger-like river sands (FISK,1961), but agrees essentially with other “classical” deltas like that of the RhBne (KRUIT,1955; VANSTRAATEN, 1959, 1960). The Niger delta evolved simply. After a low-stand of sea-level which brought the river mouths and strand line close to the edge of the continental shelf, the transgressing Late Glacial to earlier Holocene seas advanced inland to deposit the littoral older sands. Once sea level became stable the strand retreated seaward, carrying with it the several lithofacies of the younger suite. Thus in the Niger delta is recorded a transgression and subsequent regression of the sea in a subsiding region gathering sediment from a vast source area. Its deposits constitute a cyclothem: coarse grained littoral deposits at the base are succeeded by fine grained offshore sediments which grade upward into littoral sands and delta-top deposits.

SUMMARY

The cycle of deposits present in the modern Niger delta has been built up during a transgression of the sea (Late Glacial to earlier Holocene) and a subsequent regression (later Holocene). Quartz sands enriched with glauconite or shell debris were deposited at an advancing strand line during the transgression. The regression of the sea has been accompanied by the distribution, by wave and current action, of terrigenous sand, silt,

34

J . R. L. ALLEN

clay, and plant debris between the following principal environments: floodplain, mangrove swamp, estuary, beach with beach-ridges, river mouth bar, delta platform, prodelta slope, open shelf, and continental slope. The lithofacies deposited in these environments overlie the earlier transgressive sands.

REFERENCES

ALLEN,J. R. L. and WELLS,J. W., 1962. Holocene coral banks and subsidence in the Niger delta. J . Geol., 7 0 : 381-397. BERRIT,G. R., 1959. Rbultats scientifiques des campagnes de la “Calypso”: Golfe de Guinee. 11. Octanographie physique. Ann. Inst. OcPanop., Paris, 37 : 37-73. FISK. H. N., 1961. Bar-finger sands of Mississippi delta. In: J. A. PETERSON and J. C. OSMOND (Editors), Geometry of Sandstone Bodies.Am. Assoc. Petrol. Geologists, Tulsa, pp. 29-52. FISK,H. N. and MCFARLAN, E., 1955. Late Quaternary deltaic deposits of the Mississippi River. In: A. POLDERVAART (Editor), The Crust of the Earth. - Geol. SOC.Am. Spec. Papers, 62 : 279-302. HILL, M. B. and WEBB,J. E., 1958. The ecology of Lagos Lagoon. 11. The topography and physical features of Lagos Harbour and Lagos Lagoon. Phil. Trans. Roy. SOC.London, Ser. B, 241 : 319-333. JELGERSMA, S., 1961. Holocene sea level changes in The Netherlands. Mededel. Ceol. Sficht., Ser. c-VI, 7 : 1-101. KRUIT,C . , 1955. Sediments of the RhBne delta, I. Grain size and microfauna. VerhandeL Koninkl. Ned. Geol. Mijnbouwk. Genoot., Geol. Ser., 15 : 357-514. MOORE,D. G. and SCRUTON, P. C., 1957. Minor internal structures of some Recent unconsolidated sediments. Bull. Am. Assoc. Petrol. Geologists, 41 : 2723-2751. NEDECO(Netherlands Engineering Consultants), 1954. Western Niger Delta, Report on Investigation. Nedeco, The Hague, 57 pp. NEDECO,1959. River Studies and Recommendations on Improvement of Niger and Benue. North-Holland, Amsterdam, 1,000 pp. 1961. The Waters ofthe Niger Delta. Nedeco, The Hague, 317 pp. NEDECO, PUCH,J. C., 1954. Sand movement in relation to wind direction as exemplified on the Nigerian coastline. Res. Notes Univ. Ibadan, Nigeria, 5 : 14 pp. SCRUTON, P. C., 1960. Delta building and the deltaic sequence. In: F. P. SHEPARD,F. B. PHLECER and TJ. VAN ANDEL(Editors), Recent Sediments, Nortliwest Guyof Mexico, 1951-58. Am. Assoc. Petrol. Geologists, Tulsa, pp. 82-102. VANSTRAATEN. L. M. J. U.. 1959. Littoral and submarine momholoev of the RhBne delta. In: R. J. L u, RUSSELL(Editor), Proc. 2nd. Coastal Geogruph. C o n ? , Louisiana State Univ. Baton Rouge, pp. 233-264.

VANSTRAATEN, L. M. J. U., 1960. Some recent advances in the study of deltaic sedimentation. Liverpool Manchester Geol. J . , 11 : 41 1 4 2 . WEBB,J. E., 1958. The ecology of Lagos Lagoon. I. The Lagoons of the Guinea coast. Phil. Trans. Roy. SOC.London, Ser. B, 241 : 307-318. WENTWORTH, C . K., 1922. A scale of grade and class terms for clastic sediments. J . Geol., 30 : 377-392.

STRUCTURE E N CRENEAUX ET BRECHE CYCLOPGENNE EN MILIEU DfiTRITIQUE PARALIQUE ARNOLD BERSIER MusPe gPologique, Lausanne [Suisse)

Une paroi rocheuse d’une exceptionnelle grandeur a t t t rckemment dtgagte 8 Vennes, au nord de Lausanne (Suisse) pour le passage d’une autoroute dans une colline de molasse de 1’Aquitanien suptrieur. Des affleurements si vastes n’existent pas dans la nature, en raison de l’alttration rapide de cette roche. Celui-ci a mis en tvidence un exemple de b r k h e cyclopkenne de marne dans le grts, due ti une crtnelure d’trosion, qui depasse de beaucoup en &endue et signification les structures de ce genre, difficilement observables dans les petits affleurements ordinaires de ce facits. La paroi est taillte dans une masse de g r b q u i a I’apparence d’une couche unique. Prts du sommet apparait un alignement discontinu et crtnelt de plaques d’une marne gris-jaune, finement stratifite (Fig. I). Les fragments sont kpais de 0 , 3 4 6 m, longs de 1-3 m, laissant entre eux des intervalles de mCme ordre. Dans l’ensemble ce chapelet est horizontal, paralltle 8 la stratification gtntrale, mais les fragments sont plus ou moins inclines et certains sont tordus. Ces morceaux de marnes sont complttement enrobts par le grts. Au mur, au toit et de chaque cbtt, le mCme sable les entoure. C’est 18 le caracttre le plus tnigmatique de a

50 m

b

I----------

1

. . . _. . ._-. .- -. -.-.- .. . . ....... .. .. .. .. . . ..................... ......................

....i...... -A.

.... I

b

I 1

1

.:.c:. . . ..:. .:: .

10 m

2.5rn

. . . . . . . .1 . . . . . . . . . . . .

I

’ ..:+.:. ................................................. ........................

Fig.1. a. Ensemble de l’affleurement. b. Partie ouest de l’affleurement avec les crkneaux de mame, reliques de couche dtniveltes. c. Detail de I’affleurementA structure de brkhe marneuse cyclopkenne dam le gres, et cicatrice d’erosion dissimulk sable sur sable, mais jalonnk par des galets m o m

36

A. BERSIER

cette structure. A premiire vue ces gros blocs ont une apparence allochtone; leur transport n’ttant absolument pas de la compttence du courant transporteur du sable ambiant, quel pourrait Ctre l’agent de mise en place et de dissemination de cet alignement de blocs, intervenant brusquement en cours de stdimentation sableuse? Le mystkre est d’autant plus grand que ces blocs de marne ne forrnent pas seulement une ligne. Les travaux d’excavation de la paroi ont rtvtlt leur ttendue dans tous les sens. 11s sont disposts cornme des pavts largement espacts. Dtnivelts et souvent bascuIts, ils ne se superposent jamais. La gentse d’une telle structure sdimentaire, examinte en elle seule et dans son seul cadre immtdiat, resterait inexplicable, ou inciterait i irnaginer l’intervention de courants transporteurs d’une violence extraordinaire. Replacte au contraire dans le cadre gtiitral de la stdimentation molassique paralique, cette structure en crtneaux devient explicable par analogie avec d’autres formes stdimentaires du meme faciis. Elle n’en garde pas rnoins une originalitt et une signification qui mtritent inttrtt. I1 se peut que d’autres brtches cycloptennes - i tltments gigantesques - des pseudo-nodules de grande taille, des pseudo-concrttions, reltvent d’une origine semblable. Rappelons que cette format‘on molassique est constitute de couches de grbs (molasses a ciment calcaire, macignos h grain plus fin et A ciment argilo-calcaire), de marnes et argilites, tous ces ternies formant une gamme granulomttrique complkte. Ces couches sont assocites en groupes ou stquences cycliques: les cyclothkrnes, qui montrent une granodtcroissance progressive de bas en haut. Ces particularitts ont t t t dtcrites anttrieurement de faGon plus complkte (BERSIER,1958). Les sommets ou toits des cyclothtmes ont t t t frtquemnient dtcapts par une erosion prtctdant le dCpBt du sable de base du cyclothtme suivant. L‘trosion est bien apparente si le grts de base se superpose 2 une marne ou argilite, en moulant par ses indentations les ichancrures de cette dernikre. Si l’trosion a t t t plus profonde, entaillant jusqu’au corps sableux encore non consolid6 du cyclothtme prtctdent, la cicatrice est beaucoup rnoins visible: le sable dtpost aprts l’erosion se confond avec le sable attaqut par I’trosion. D a m la phase finale de l’trosion, quand le courant dirninuait et que le depBt s’amorcait, le m&me sable ttait arracht ou dtpost, selon d’infimes variations de compttence dtpendant de la rugositt du fond du chenal. Sable trodt et sable nouvellenient apportt se sont donc mClts alors en une m h e phase granulomttrique. Ce mimttisme a dissimult toute cicatrice, les deux bancs de grks sont coalescents et n’en sirnulent qu’un seul. Pourtant des cicatrices dissiniulees peuvent &tremises en evidence dans de semblables cas, soit par continuitt avec des traces lattrales d’trosion mieux marqutes sur des ttnioins marneux, soit par des difftrences de straticulation, soit surtout par des “galets mous” arrachts aux couches marneuses voisines. L’exanien minutieux de l’affleurement de Vennes rtvble la prtsmce de tels galets mous, souvent tcrasts, en chapelet entre les blocs de marne (Fig.lc). Ils soulignent les sillons d’trosion dans les intervalles. Les crtneaux apparaissent ainsi cornme des reliqiies d’une couche marneuse con-

STRUCTURE EN CRBNAUX ET

BRBCHE

CYCLOP~ENNE

37

Fig.2. Formation de la brkhe par erosion en cours de stdiniei Lation. a. Wash-out et attaque de la couche marneuse par ]’erosion. b. Morcellement de la couche par les chenaux. c. Affouillement du sable, basculernents et slumping. d. Enfouissement des blocs par reprise de la sedimentation sableuse.

tinue, entamke et dtcoupte par l’trosion. Aprks avoir rompu la carapace marneuse non encore lapidifiie, ? l’ttat i de vase compacte et probablement thixotropique, 1’61-0sion a affouillt plus facilement le sable sous-jacent encore meuble. Le sapement lateral sous les tables marneuses a provoqut leur tassement sur place, leur basculement avec ou sans translation. Des diplacements et basculements complexes ont pu ainsi se produire par ttapes successives. La mise en porte en faux des tronqons marneux plastiques a dtterniint des ecoulements ou slumpings de leurs tranches. No11 seulement les rnorceaux ont t t t denivelts, mais ils ont pris de ce fait des formes bizarres et contourntes. Une telle difference de rtsistance B I’trosion est conforme aux exptriences de HIULSTROM [ l939), dont les diagrammes devenus classiques montrent qu’un courant de I’ordre de 30 cm/sec peut eroder des grains de 0,l-1 mm de dianiktre (dimension de nos sables) sans Ctre capable d‘arracher des grains de 0,001-0,l mni (correspondant ii nos marnes) dont la cohesion acquise par compaction est plus tlevte que celle des sables. Des graphiques de cet auteur, on peut conclure que le vitesse des ourants trosifs des sables Ctait comprise entre 0,3 et 1 mjsec et qu’elle a dQ&tresuptrieure a ce dernier chiffre pour attaquer les marnes, cette dernikre donnte itant incertaine car i’action abrasive du sable sur les sediments fins a t t t vive. Le dipat des sables seuls a pu s’effectuer eiisuite avec des vitesses de courant s’abaissant B enviroii 5 cmlsec. L‘intervention de courants extraordinaires, B m&mede dtplacer des blocs, est superflue dans la genkse de cette structure. L’alternance des phases de skdimentation et

38

A. BERSIER

d’trosion, maintes fois manifestte dans ce milieu molassique paralique ii faibles variations du niveau de base, et command& par les divagations d’embouchures, est une cause suffisante. Elle suggtre de larges wash-outs par trosion en nappe au devant des embouchures mouvantes, distributrices des stdiments.

Une structure en crtneaux formte de fragments de marne dans un grts molassique aquitanien a CtC dtcouverte par la construction d’une autoroute au nord de Lausanne (Suisse). Les creneaux sont dtsalignts et simulent une br2che cycloptenne (brkhe A Cltments gtants). La genkse de cette structure est explicable par un tpisode d’trosion en cours de stdimentation, analogue a ceux que montre ordinairement cette formation dttritique cyclique en milieu paralique, constquences du dtplacement periodique des embouchures sur le delta. Le wash-out a segment6 la couche de marne, affouillt le sable inftrieur, et provoqut le basculement et le slumping des fragments de marne encore non consolidts. La skdimentation sableuse a repris ensuite. La cicatrice d’trosion invisible, sable sur sable, est soulignte par des galets mous.

SUMMARY

A structure of large, tilted blocks of marl, embedded in Aquitanian molasse sandstone was recently exposed during the construction of a through-way north of Lausanne, Switzerland. Together, the blocks form a breccia on a giant scale. Its origin may be explained by assuming an interlude of erosion, interrupting a depxitional period, such as have been found elsewhere in this cyclical, detrital formation of paralic facies, and which can be ascribed to periodic shifts of the river mouths in a delta. Wash-outs have cut through the marl layer into the underlying sand, after which the blocks of unconsolidated marl were undermined by lateral erosion of sand, became tilted and slid down sideways. Subsequently, deposition of new sand took place. The erosion surface, which is at most places invisible owing to its position between two sands of the same composition, is locally marked by small pebbles of marl.

BIBLIOGRAPHIE

BERSIER, A.. 1958. Sequences detritiques et divagations fluviales. E c l q p e Geol. Helv., 5 1 (3) : 842-853. HIULSTROM, F., 1939. Transportation of detritus by running water. In: P. D. TRASK (Editor), Recent Marine Sediments. Am. Assoc. Petrol. Geologists, Tulsa, pp. 5-3 1 .

SPECIFIC FEATURES O F DELTAIC DEPOSITS I N COAL-BEARING AND CUPRIFEROUS FORMATIONS L. N . B O T V I N K I N A

and v . s.

YABLOKOV

Geological Institute, Academy of Sciences, Moscow (U.S.S.R. )

INTRODUCTION

Deltaic deposits, as well as coastal-marine sediments, have been distinguished in fossil formations for quite some time. However, this has been done mostly by the place they occupy between typically marine and typically continental deposits. In this distinction the entire complex of deposits formed in deltaic environments has usually been considered as a whole. Yet, it mostly comprises strongly different facies. A major division can be made in continental deposits, formed on the landward side of the delta shoreline [river-, peat bog-, lake sediments etc.) and of marine sediments of the subaqueous parts, among which one may mention bands of various coastal-marine deposits. Detailed lithological researches carried out in recent years made it possible to establish textural, structural and other features characteristic for the (usually arenaceous) deposits of the submerged parts of the deltas and their differences as compared with other coastal-marine deposits. The established features permitted us to determine the deltaic origin of a number of sandstones in the coal measures of the Donetz Basin as well as in the series of cupriferous sandstones in the southern Urals and other areas (BOTVINKINA et al., 1956; ZHEMCHUZHNIKOV et al., 1959, 1960; BOTVINKINA et al., 1963; BOTVINKINA, 1963). With regard to the brief character of this communication these features are presented as a general result of all data obtained. They can be subdivided into two groups: (I) Features discernible directly in rock specimens or exposures, textural, structural (especially lamination), inclusions, etc. Features of this group are mostly similar in deltaic deposits of different sedimentary formations. (2) Specific features of the major structures of the deltas and their relations to the adjoining deposits. Here we see both similarities and differences, depending on the conditions under which the deltaic lenses have been formed in different environments.

LITHOLOGY AND STRUCTURES

Deposits of the (originally) submerged parts of the deltas are at most places repre-

40

L. N. BOTVINKINA AND V. S. YABLOKOV

sented by sandstones, mainly of medium grain. In some cases they are coarser and may pass into conglomerates, while in other cases they are of finer grain. The sorting of the grains varies; usually it is medium. They frequently contain pebbles and slightly rounded fragments of more fine-grained rocks (argillites, aleurolites), derived from underlying beds. It is interesting that the admixture of such fragments and pebbles often took place without a concomitant general coarsening of the enclosing arenaceous material. In this respect they differ from river deposits, where the appearance of pebble material is usually accompanied by a general coarsening of the sandstone. Sometimes a rhythmical sorting of grains is recorded in each oblique lamina, but it is expressed rather poorly and not so characteristic as in river deposits. Typical for deltaic formations is the presence of coarse-grained deposits in their middle part. Herein they differ from river sediments, where the lower part is generally the coarsest and usually marked by wash-out structures. A very characteristic feature of the delta deposits is their lamination (BOTVINKINA, 1962). In some cases these were formed under the influence of complicated hydrodynamic conditions in the basin near the estuarine parts of the rivers. Just in front of its mouth the flow of a river, in general, continues by inertia, but it spreads out, the velocity thereby becoming lower and lower the further it gets from the coastline. The influence of the normal marine currents and of the waves (swell) in the basin thereby becomes more pronounced. The flow of the river shows periodic increases (for instance during river floods) and decreases. It often happens that the main direction of outflow into the sea varies from one flood stage to the next one. As a result, the oblique sets of laminae in the arenaceous deposits of the submerged part of the delta shows a wide variation in orientation. In this manner a cross-laniination of large dimensions is produced. It is likely that the conditions in the investigated fossil deltas were of the above described type. The azimuths of the dip of the laminae in these sediments may vary within a range of approximately 180”. The angles of inclination of the laminae and the boundaries of sets of laminae show a strong variation. They fluctuate probably from 3 0 4 ” . A very characteristic feature is that the dip angle of the laminae increases within single sets. Along with large cross-laminated sets several tens of centimeters thick, there are also smaller sets (less than 10 cm in thickness). Cross-lamination can be manifest by minor changes of the granulometric coniposition of the laminae, as well as by the occurrence of various admixtures on or close to the planes of separation of the laminae. In coal measures these admixtures often consist of plant debris. In cupriferous formations the “lamination planes” are often marked by copper-containing minerals. In a number of cases one finds inclusions of relatively large, charred or mineralized vegetable remains. In the cupriferous formations of Priuralie they are associated with copper carbonates (malachite, azurite), which form incrustations and coatings on the surface of organic remains. No fauna has been found in the deposits of the submerged part of the deltas, neither in the coal-bearing, nor in the cupriferous ones. These deposits are characterized by the presence of various pebbles, which by their preferred orientation render the cross-

SPECIFIC FEATURES OF DELTAIC DEPOSlTS

41

lamination still more conspicuous. In the investigated cupriferous sediments, formed under conditions of arid climate, the place of such pebbles is often taken by carbonate and ferruginous concretions washed out from the underlying beds. Deltaic deposits of the cupriferous sandstone formations are also characterized by a more varied composition of pebbles than is usually found in marine sandstones of the same formation. Along with rounded fragments of adjacent and underlying beds of aleurolites and argillites, one here encounters also pebbles of many other rock types which cropped out in the source areas eroded by the rivers.

FACIES CHANGES AND RELATIONS TO UNDERLYING AND BORDERING FORMATIONS

The lower contact of the sandstones formed on the submerged parts of the deltas is usually sharp and often produced by erosion of the underlying sediments but it is more even than at the base of river channel deposits. The wash-outs become less distinct as one advances in the (original) seaward directions. A very typical feature of the deposits of the submerged part of the deltas is the combination of cross-laminations and beds with ripple-mark laminations and with coastal-marine laminations. These beds were, apparently, formed where the river flow became weaker or where its direction changed and waves and swell of the sea basin became a predominant hydrodynamic factor. Also characteristic are the ways in which the submerged delta deposits pass into adjacent sediments, both in horizontal and in vertical sense. Both the coal-bearing and cupriferous delta sediments are usually developed in the regressive series of facies in which a gradual change is found from deeper-basin types to more shallow-basin sediments and finally to continental deposits. The relations with other sediments depend to a large extent on the general environmental conditions during which these rocks were formed, but also on the position, which each point of deposition occupied in respect to the coast line. Obviously, the influence of littoral processes increases with decrease of the distance to the coast. In this direction the deposits of a submerged delta gradually acquire more features in common with those of typical alluvium (in other words continental river deposits). The further from the coast, the stronger the features of the newly formed sediment reflect the influence of the sea. The enclosing facies and, hence, the entire succession will, correspondingly, be of either more continental and littoral or of more marine character. As to the general environments, their influence is much more complicated. Delta formations originating in a humid climate, such as the coal measures in the Carboniferous depression of the Donetz Basin, are characterized by their superposition, often even with gradual transitions, on typically marine deposits. After deposition of a sufficiently thick (up to 15-20 m) sandy series, corresponding to the submerged part of the delta environment, the sediments gradually acquire morc and more features pointing to shallowing of the water. The final stage of complex deltaic deposition is usually represented by littoral (marine or lagoon) sediments of the swell-ripple zone, which

42

L. N. BOTVINKINA A N D

V. S.

YABLOKOV

originate under conditions of a very shallow sea. At higher levels these deposits change gradually into deposits of lagoons, lakes and land bogs, alternating sometimes with typical alluvium, the open sea environment having been replaced by the continental deltaic environment. Deltaic deposits described by us from the Upper Permian formations of the Suburalian depression also overlie basin deposits but usually with a sharp wash-out contact, which in the coal measures of the Donetz Basin has been recorded only in profiles near the original coast line. In Priuralie, on the other hand, the sediment composed of river borne material, deposited in the forelying sea, are of much smaller thickness (3-5 m). The sandstones of the submerged delta become more and more homogeneous and fine-grained as one moves upwards along the profile and change at the top again into basin deposits (though the depths of these basins apparently were very small). Thus, the succession here does not terminate with deposits of continental facies (Fig.4). On the contrary, these features indicate formation at a greater distance from the old coastline. An explanation of all these facts should, probably, be sought in the character of the general environment, and in the regime of the streams which created these deltas. In the Donetz Basin these latter were rather large rivers, emptying into a wide sea gulf. In Priuralie the streams werc much smaller, but more torrential. They poured periodically into shallow water areas, that were sometimes, perhaps, of the lacustrine type. Owing to their temporary high current velocities, they badly eroded previously deposited bottom sediments. The activity of these streams died down much faster than that of the rivers of the Donetz Basin and the original more or less quiet regime of sedimentation was re-established in the basins, their floor not having emerged above the water level.

EXAMPLE OF COAL MEASURE DELTA IN THE DONETZ BASIN

As a result of our regional and sectional researches (ZHEMCHUZNIKOV et al., 1959-1960;

BOTVINKINAet al., 1956) the relations between the deposits of the submerged part of the delta and the various other facies can be presented in much detail (Fig.], 2). On Fig. 1 we see how the area occupied by arenaceous sediments of the submerged part of the delta (having a rather complicated configuration) is bordered on two sides by arenaceous-aleuritic deposits of littoral shallow water and lacustrine facies and on the side of the open sea by aleuritic marine deposits. In a profile (Fig.2) the complicated relations are seen of the deltaic lens with the adjacent facies as traced in detail by a number of closely spaced bore holes. Laterally, delta deposits change quite distinctly into sandy deposits of spits, bay-bars and bars, or into stretches of littoral-marine sandstones extending along the coast line of the sea and barring the lagoon areas. Landwards they change into linearly extended sandstone bodies typically formed in rivers and orientated perpendicularly to the old coast line (Fig.3).

43

SPECIFIC FEATURES OF DELTAIC DEPOSLTS

2

A

z

4km

2

0

Fig.1. Areal relations of deposits of the submerged part of the delta with other deposits. Cycle Ii-L, of C! suite in Dolzhansk region of the Donetz Basin. Symbols: 1 = sandstones of the submerged part of the delta; 2 = arenaceous-aleuritic deposits of the swell-ripples zone of the tidal-lagoon coast; 3 = argillaceous-aleuritic and arenaceous deposits of coastal lakes; 4 = id., but more finegrained and with well preserved plant remzins; 5 = thin intercalations of aleurolites in the zone of swells of the littoral part of the sea. A-A = section line (seeFig.2). Circles = bore holes.

0

loo0

2000

3

4

3000

5

6

4000

8

5ow

9

10

7000

6000

11

12

13

8000

14

15

9000

10000

l l W m

16

Fig.2. Relations between the deposits of the submerged part of the delta with other deposits in a profile. Facies profile along the line A-A (see Fig.1) compiled by Botvinkina. Vertical lines = sections of bore holes. Brackets on the right give the boundaries of a cycle with coal seam 1: and limestone L,. T = transgressive part of the cycle. P = regressive part of the cycle. In this diagrdm the base of limestone L, is taken as a horizontal line. Symbols: 1 4 = marine deposits (I = limestone; 2 = calcareous argillite with marine fauna; 3 = argillite with msrine fauna; 4 = argillite and fine-grained aleurolite without fauns). 5 = argillite or fine-grained aleurolite with a brackish lagoon fauna. &8 = intercalation of aleurolites in the swell zone of the littoral part of the sea (6 = thin intercalation with a thickness of lamina of 1-2 mm; 7 = intercalation with a thickness of lamina up to 1-2 cm; 8 = thick intercalation with a thickness of lamina up to several tens of centimeters). 9 = arenaceous-aleuriticdeposits of thezone of swell ripples of the tidal-lagoon coast. 10 = arenaceous deposits of the submerged part of the delta (river miterial carried into the sea). 11-12 = lacustrine deposits (11 = aleurolites and sandstones with vegetable detritus; 12 = aleurolites and argillites with a well preserved flora). 13-16 = continental peat bog deposits (13 = mostly aleuritic deposits of swampy plains: “subsoil” of a coal seam; 14 = argillites and aleurolites with root remains: floor of a coal seam; 15 = coal argillites of silted peat bogs; 16 = coal seams, deposits of peat bogs).

44

L.

N.

BOTVINKlNA AND V. S . YABLOKOV

Fig.3. Transitions of arenaceous deposits of different facies. Paleogeographic map compiled for the H

end of the regressive part of the cycle with coal seam IS, suite Ci of the Donetz Basin. Compiled by Botvinkina. Symbols: 1 = bogged land; 2 = river sandstones; 3 = coast line; 4 = smdstones of the submerged part of the delta; 5 = sandstones of spits, bay-bars and bars; 6 = sandstones of bottom nisrine currents; 7 = arenaceous-aleuritic deposits in the zone of swell ripples of the lagoon-tidal coast; 8 = aleuritic deposits of the swell zone of the littoral part of the sea.

GENERAL CONCLUSIONS

Thus, on account of their position, the sandstones may be regarded with sufficient confidence as the deposits of the submerged part of the delta. For this reason we are justified in using the features of these deposits as indications of sediments of the submerged parts of deltas also in other cases. The generaUy recurrent complex of these features both structural-textural and generally geological, permitted to distinguish deltaic deposits in many sedimentary coal-bearing and variegated rock masses of the coal basins: Donetzk, Moscow, on Sakhalin Island, Udokansk, Dzhezkazgansk, in Priuralie, etc. (BOTVINKINA et al., 1956; BOTVINKINA, 1962; BOTVINKINA, 1963; BOTVINKINA et al., 1963; ZHEMCHUZKNIKOV et al., 1959-1960; BAKUNet al., 1958; SALNIKOV, 1960; SHULGA,1960; etc.). Both in coal-bearing and cupriferous paralic formations fossil deltaic deposits are characteristic for the regressive series of facies and develop mainly during the regressive stage of sedimentation cycles. This fully agrees with the concept formulated by SAMOILOV (1952) for recent deltas, that the sinking of the coast hinders, and that uplift favours the formation of deltas in general and of its submerged beds in particular. The possibility is not excluded that in a number of cases the lower horizons of deltaic deposits begin to get formed at the end of the transgressive stage. This happens

SPECIFIC FEATURES OF DELTAIC DEPOSITS I Scale (rn)

Lithological section

I1

4s

I11

Type of Genetic lamination type

12.

34.

567I..

.I

B9.

10 r 11 -

Fig.4. Section of littoral-marine deposits including the submergzd part of the delta in the Upper Permian rock rnxs of Priuralie. Symbols: I = Lithological designitions: 1 = pebbles; 2 = mediumgained sandstone; 3 = fine-grained sandstone; 4 = fine-grained smdstone with bands of aleurolites; 5 = coarse-grained aleurolite; 6 = thin intercalation of coarse-grained aleurolite with fine-grained aleurolite and argillite; 7 = argillite. I1 = Structures: 8-11 = lamination (8 = large criss-cross; 9 = cross wave-like; 10 = fine wave-like; 11 = horizontal); 12 = fracture cracks. 111 = Genetic types of deposits: 13 = deposits of the submerged part of the delta; 14 = deposits of the marginal part of the delta; 15 = marine deposits ofthe zone of swells; 16 = dzposits of shallow water parts, sometimes drying out; 17 = littoral-marine deposits; 18 = mzrine deposits.

when uplifts are alrzady taking place on the continent and abundant quantities of detrital material begin to be supplied to the basin by rivers. Thus, fossil deltaic deposits formed under differznt physico-geographical conditions have many specific features in common, which pxmit recognizing them everywhere, just as deposits of the submerged parts of deltas. However, despite their general similarity, there are also differznces, determined by climate, tectonic regime and general paleogeographic environments, peculiarities of the source arza and the character of the basin, in which the deposits of the submerged delta are being formed. The recognition of truly deltaic deposits in fossil sedimPnt&ryrock masses and their distinction from other littoral-marine deposits is of a very substantial practical importance, inasmuch as many deltaic arcas are the sites of concentration of useful minerals, which often decrease in abundance where the deltaic facies pass into facies of other types.

46

L. N. BOTVINKINA AND V. S. YABLOKOV SUMMARY

Detailed lithological researches have made it possible to establish typical features of arenaceous deposits, formed in the submerged parts of deltas and to determine their differences from other littoral-marine sediments. Deltaic sandstones are recognized in the coal measures of the Donetz Basin and in the rock mass of cupriferous sandstones of Priuralie, Dzhezkazgan and other areas. The paper discusses two groups of features of deltaic deposits. The first group consists of features observable directly in rock specimens or exposures: texture, structure (especially bedding), inclusions, organic remains, etc. Features of this group are generally alike in different deltaic deposits. More especially, it is possible to distinguish, on account of such features, the submerged parts of fossil deltas. The second group of features includes specific characteristics in the structure of the entirz formation of the submerged part of the delta and their relations with adjacent facies. Here we find both differences and similarities, depending on the formation conditions of the deltaic rock masses in various environments. The differences are determined by climate, tectonic regime, general paleogeographical environments, specific properties of the source areas and the character of the basin, in which the deposits of the submerged part of the delta are formed. Both in coal-bearing and in cupriferous paralic formations fossil deltaic deposits are mostly characteristic of the regressive series of facies. The establishment of truly deltaic deposits in fossil formations and their delineation against other littoral-marine deposits is of a substantial practical value inasmuch as deltaic deposits are often the sites of concentration of many useful minerals, which may decrease in abundance where the deltaic facies pass into other types of facies.

REFERENCES

BAKUN,N. N., V o L o D i N , R. N. and KRENDELEV, F. P., 1958. Main specific features in the geological structure of Udokansk deposits of cupriferous sandstones and the direction of its further prospecting. Izv. Vysshikh Uchebn. Zavedenii, Geol. i Razvedka, 5 : 61-83 (Russ.). BOTVINKINA, L. N., 1962. Bedding of sedimentary rocks. Tr. Geol. Inst., Akad. Nauk S.S.S.R., 59 : 541 pp. (Russ.). BOTVINKINA, L. N., 1963. Some specific features in the genetic types of deposits and laws governing their stratification in paralic formations of various climatic areas. Tr. Geol. Inst., Akad. Nauk S.S.S.R., 81 : 332-373 (Russ.). BOTVINKINA, L. N . , ZHEMCHUZHNIKOV, J. A., TIMOFUEV, P. P., FEOFILOVA, A. P. and YABLOKOV. V.S., 1956. Atlas of Lithogenetic Types of Middle Carboniferous Coal Measures of the Donetz Basin. Akad. Nauk S.S.S.R., Moscow, 368 pp. (Russ.). BOTVINKINA, L. N., SELIVERSTOV, V. A., SOKOLOVA, T. N. and YABLOKOV, V. S., 1963. Some genetic types of Tatarsk red beds of the Orenburg Priuralic. Izv. Akad. Nauk S.S.S.R., Ser. Geol., 5 : 47-66

(Russ.). SALNIKOV, B . A,, 1960. Strircture and Main Features of Coal Content of the Verkhnednisk Suite on the Western Coast of Sakhalin. Dissertation, 25 pp. (Russ.). SAMOILOV, I. V., 1952. River Mouths. Geografiz, 526 pp. (Russ.).

SPECIFIC FEATURES OF DELTAIC DEPOSITS

47

SHULGA,V. F., 1960. Conditions of Coal Measures Formation in Aleksinsk Regiotl of Moscow Coal Basin. Dissertation- Tr. Lab. Geol. qyliu, 6 : 398410 (Russ.). ZHEMCHUZHNIKOV, J. A,, YABLOKOV, V. S . , BOGOLJUBOVA, L. I., BOTVINKINA, L. N., FEOFILOVA, A. P., RITENBERG, M. I., TIMOFEJEV, P. P. and TIMOFWEVA, Z. V., 1959-1960. Structure and Accumulation Conditions of the main Coal Measures and Coal Seams of Middle Carboniferous A,pe in the Done/z Basin. 1-2. Akad. Nauk S.S.S.R.,Moscow, 679 pp. (Russ.).

DEVONIAN BIOSTROMES AND BIOHERMS OF THE SOUTHERN CANTABRIAN MOUNTAINS, NORTHWESTERN SPAIN AART BROUWER

Department of Strat(@apliy and Paleontology, University of Leyden (The Netherlands)

INTRODUCTION

Along the southern slope of the Paleozoic part of the Cantabrian Mountains, the Devonian succession shows, over large areas, a remarkable uniform alternation of sandstones and shales on the one hand, and of limestones with dolomites on the other. Although local variations as to thickness and details of lithology do occur, the general succession remains essentially the same between the Valsurvio Dome in the east and the Luna River in the west, a distance of nearly 100 km. From a lithological point of view the sandstones are well sorted and consist of well rounded grains. Cross-stratifications, mud cracks, furrows and trails are common features of the sandstones or shales. The limestones are partly well bedded, finegrained, sometimes argillaceous limestones, partly typical biostromal limestones. From a paleontological point of view the whole succession is characterized by abundant remains of brachiopods, bryozoans, crinoids, stromatoporoids and corals. All features referred to are indicative of deposition in shallow water. In the northern part of the province of Palencia a different development of the Devonian succession has been discovered (BROUWER, 1963a, b), which differs lithologically as well as paleontologically from the succession referred to above. Its main elements are shales and fine-grained argillaceous limestones, whereas sandstones occur only in the lowermost and in the uppermost part of the succession. From a palaeontological point of view the differences are even more marked, ammonoids, tentaculites, pelecypods and conodonts being the most characteristic faunal groups. Generally this succession is much less fossiliferous. Tentatively it is suggested that this succession developed further away from the shore and possibly in somewhat deeper water.

FEATURES OF BIOSTROMES

Biostronies are of widespread occurrence in the Santa Lucia and the Portilla Limestone Formation (Fig.1). In the lower part of the Devonian biostronies are relatively rare. Only one biostrome is known so far in the La Vid Formation and a few others of small

W

EISW

ME

Fig. 1. Generalized lithostratigraphic section of the Devonian in the southern Cantabrian Mountains, east of Esla River. Bioherms of transition zone between Asturo-leonesian facies (left) and Palencian facies (right) indicated by oblique hatching. Data from KANIS(1956) (San Martin-Ventanilla area), KOOPMANS (1962) (Valsurvio dome), F. A. Sprenger van Eijk (unpubl.) (Esla area), J. F. Noorthoom van der Kruijff(unpub1.) (San Martin-Ventanilla area), P. A. C. de Ruiter (unpubl.) (Polentinos area) and the author's personal observations.

50

A. BROUWER

dimensions occur in its continuation towards the north in the Palencian facies. The most characteristic groups involved in biostrome formation are rugose and tabulate corals and stromatoporoids. Algal remains are of minor importance, but locally bryozoans and crinoids occur abundantly. The only one known biostrome of the La Vid Formation is near Colle. Its main constituents are branching Rugosa, of which several large specimens between 1 and 2 m in height are still in position of growth. Broken fragments of the same corals are the main constituent of the sediment, particularly in the upper part. The uppermost part of the biostrome consists of a bed of very large favositid colonies, up to 50 cm in diameter. Apparently this sudden faunal change has been caused by increased turbulence of the water. The biostrome seems a local feature as none of the other La Vid exposures, many of which have been studied in detail, showing any biostromal development. In the area of the Palencian facies the Lebanza Limestone Formation, which is chronologically the equivalent of the La Vid Formation, shows a few small biostromes built by massive stromatoporoids, by small favositids or by small branching Rugosa. It is, however, only in the Santa Lucia Formation that biostromal development starts on a much larger scale. Massive stromatoporoids and favositids are by far the most important biostromal elements. Sometimes they occur next to each other, both in position of growth, indicating that they may have lived together in the same habitat. In other places, however, large massive stromatoporoids predominate, or are even the only group present. Bituminous black shale intercalations between the biostromal limestones sometimes show many small colonies of favositids in living position, which is usually not their most stable position. At some levels branching forms, mainly corals, occur, but in general these are of much less importance. Crinoids and bryozoans are sometimes present abundantly, but rarely together with stromatoporoids or corals. Biostromes made upof the several groups referred to, repeatedly alternate in a vertical sense. Many of such biostromes can be traced laterally over several hundreds of meters, or even more, but the same sequence is never repeated in exactly the same way in the next outcrop. The vertical and the horizo~italdifferentiations suggest that the habitats preferred by the various groups of organisms were slightly, but distinctly, different. The deposition of the sands and shales of the Huergas Formation apparently brought the development of biostromes temporarily to an end. It was resumed, however, shortly after the deposition of the Portilla Limestone Formation had started. Usually this formation begins with thinly bedded limestones in which brachiopods and single corals abound. Thereupon biostromal development started anew, usually with a thin layer, 2&30 cm thick, of lamellar stromatoporoids. Contrary to the picture presented by the Santa Lucia biostromes, compound rugose corals are the dominant faunal element in nearly all Portilla biostromes. Branching corals and massive stromatoporoids are restricted to a few beds. Only in the higher parts of the Portilla Formation do tuberose or ramose Thamnopora become the dominant element.

DEVONIAN REEFS IN NORTHWESTERN SPAIN

51

As in the Santa Lucia Formation, the repeatedly alternating biostromal beds dominated by various groups suggest that the distribution of the organisms involved was controlled by minor, but nevertheless distinct changes in environmental conditions.

FEATURES OF BIOHERMS

I n view of the tremendous development of biostronies, the scarcity of bioherms is a noteworthy feature of the Devonian succession in the Cantabrian Mountains. A very few and small isolated bioherms occur in the Portilla Formation throughout the area. Less than ten of them are known so far. Their horizontal dimension is 25 m at most, their vertical dimensions never surpass 4 ni. They are dome-shaped structures, made up of massive, unbedded limestones in which few organic remains are visible to the naked eye. The massive biohermal limestone interfingers with the surrounding, normal, well bedded limestone. The interfingering shows dips which differ markedly from those of the surrounding sediments, suggesting that the structures once rose above the sea-bottom. Further evidence of this is brought forward by the slightly increased coinpaction of the underlying beds, caused by the extra weight of the bioherms. Typical screes of biohermal detritus are nowhere to be seen, and it is therefore improbable that the bioherms were reefs in the sense of wave-resistant structures. Microscopical investigation of the biohermal limestone suggests that massive stromatoporoids were of prime importance in building these structures. There is only one area which seems to have offered more favourable conditions for biohermal growth. It is situated in the northern part of the province of Palencia, east of Cervera de Pisuerga. Reef building started already in Lower Devonian time and continued until well into the Upper Devonian. Lamellar and massive stromatoporoids and massive and branching corals, both Tabulata and Rugosa, were important bioherm building organisms. Sedimentary features show that at least some of the bioherms were wave-resistant structures. Horizontally the biohermal limestone passes into biostromal limestones and fine-grained clastic sediments [shales and mudstones).

REGIONAL SETTING OF BIOSTROMAL AND BIOHERMAL DEVELOPMENT

In the southern part of the Cantabrian Mountains the impressive biostromal development of the Asturo-leonesean facies is now known to occur over 100 kin in an east-west direction. No doubt it continues even further through the arch of outcropping Devonian sediments. It re-occurs in the same facies along the Asturian coast ( R A D I G , 1961). Tn the eastern part of the southern limb the Asturo-leonesean facies passes rather abruptly into the distinctly different Palencian facies. The former represents a sublittoral evironment on a relatively stable shelf, characterized by an abundant benthonic fauna. The latter should rather be regarded as a less stable, or at least less regularly subsiding basin, derived during the greater part of its history from coarse detrital

52

A. BROUWER

material, and characterized by a fauna in which pelagic elements take a predominant position. The area of the Asturo-leonesian facies offered excellent conditions for a potentially reef-building fauna. The stable conditions prevailing throughout the greater part of the Devonian prevented the development of reef structures or even bioherms. This, however, does not imply that the biostromes developed exclusively in quiet water below the zone of wave turbulence. On the contrary, several features suggest that many biostromes developed in very shallow agitated water. In the area of the Palencian facies the water was probably too deep to allow the permanent establishment of potentially reef-building organisms. Only in Lower Devonian times did they temporarily get some foothold, but by that time the facies difference had not yet markedly developed, as is well evidenced, by lithological as well as palaeontological similarities between the La Vid and the Lebanza Formations. It now is of interest to note that the most prolific growth of bioherms took place exactly in the area where the Asturo-leonesean facies passes into the Palencian facies. This change in facies coincides with the northern border of the Leonides (DESITTER; 1962). The details of this transition are partly obscured by Carboniferous rocks, but it is clear enough that towards the Palencian facies the area of the Asturo-leonesean facies ends with a belt in which even more stable conditions prevailed than in the area of the Asturo-leonesean facies proper. That this belt stood as a submarine ridge, as has been suggested by KOOPMANS (l962), seems very doubtful, as neither the lithology nor the paleontological contents of the sediments in this belt favour such conditions. It seems rather a belt of intermittent, but on the whole smaller subsidence. On the outward side of this belt, towards the Palencian facies basin, water depth and subsidence offered the most favourable conditions for the development of bioherms.

SUMMARY

Extensive biostronial growths occur in the Santa Lucia and the Portilla Limestone Formations, resp. of Lower/Middle and Middle/Upper Devonian age, of the southern slope of the Cantabrian Mountains. Development of biostromes took place on a relatively stable shelf in a shallow sea. Uniform conditions prevailed over a large area. In northern Palencia this stable shelf facies passes rather abruptly into a less stable basin facies characterized by fine-grained sediments and a pelagic fauna. Bioherms occur at several stratigraphic levels in the transitional zone between the shelf and the basin facies.

REFERENCES

BROUWER. A., 1963a. Recherches stratigraphiques dans le Paltozoique des Montagnes cantabriques (Espagne nordouest). Leidse G o t . Mededef., in press.

DEVONIAN REEFS IN NORTHWESTERN SPAIN

53

BROUWER,A., 1963b. Deux types faciels dans le Dtvonien de la partie mtridionale des Montagnes cantabriques. Compt. Rend. la R6union Natl. Geol., Oviedo, 1962, in press. DESITTER, L. U.: 1962. The Hercynian orogenesis in northern Spain. In: K. CoE(Editor), Some Aspects of the VariscianFold Belt. Manchester Univ. Press, Manchester, pp. 1-18. KANIS, J., 1956. Geology of the Eastern Zone of the Sierra del Brezo (Palencia-Spain). Thesis, Univ. Leiden. Also published in Leidse Geol. Mededel., 21 (2) : 377-445. KOOPMANS,B. M., 1962. The Sedimentary and Structural History of the Valsurvio Dome, Cantabrian Mountains, Spain. Thesis, Univ. Leiden. Also published in Leidse Geol. Mededel., 26 : 121-232. RADIG,F., 1961. Zur Stratigraphie des Devons in Asturien (Nordspanien). Geol. Rundschau, 51: 249-267.

HEAVY MINERAL DISTRIBUTION ON THE CONTINENTAL SHELF O F F ACCRA, GHANA, WEST AFRICA

w.

D . BRUCKNER

and

H. J . MORGAN

Memorial University, St. John's, Newfoundland (Canada)

INTRODUCTION

The purpose of this paper is to give a condensed account of results obtained by a quantitative study of the heavy minerals in samples from the recent sediment cover of the continental shelf off Accra, Ghana, on the northern side of the Gulf of Guinea. A more detailed report including further investigations may be published later.

ORIGIN OF SAMPLES, CHIEF STEPS OF THEIR STUDY, AND DATA OBTAINED

The samples used for this study were taken along a line extending from the beach a the lighthouse of Accra across the shelf to slightly beyond its outer edge (see Fig. I). Sample numbers (l-21), distances of the sampling locations from the shoreline (here defined as the foot of the beach slope barely exposed at low tide), and their elevations (above and below sea level) are listed in Fig.2. These 21 samples were first divided into grain size fractions by sieve and sedirnentation analysis, and the calcareous particles, chiefy of modern organic origin, were removed from each fraction by hand-picking and by means of dilute HC 1. Fig.2 shows the total size ranges thus determined, the quantitatively larger fractions indicating the frequency peaks, the maximum diameters of mineral grains or (non-calcareous) rock fragments, and the median diameters. The fractions (except the coarse ones and the finest) were then divided with bromoform into their light and heavy mineral components. The fractions so treated, and those of them quantitatively richest in heavies, are also marked in Fig.2. Representative portions of the heavy mineral concentrates were then mounted on slides and the relative abundances of the minerals determined under the microscop:. Of these mineral counts only those made of the fractions between the 45 and 100 p limits appeared to bs generally reliable for further evaluation. In order to reveal the chief trends of arzal mineral distribution as free as possible from vanations of subordinate or incidental nature, average mineral and mineral group percentages of the 45-100 p fractions were calculated for each sample and further combined to obtain average values for four sample groups which represent successive stretches of the sampling line across the shelf. Only these combined values are dealt with in this

HEAVY MINERALS ON THE CONTINENTAL SHELF OFF ACCRA

55

Fig.1. Sketch map of coastal zone at Accra, Ghana, showing sampling line across continental shelf and chief bedrock groups (the bedrock groups after BATES,1955). IA = Acidic gneisses and schists of Dahomeyan Group 1B = Basic gneisses (older( ?) Precambrian), 2 = Cape Coast granite complex (mid( ?)-Precambrian), 3 = Quartzites and phyllites of Togo Series (young( ?)-Precambrian), 4 = Sandstones and shales of Accraian Series (mid-Devonian).

1

paper. It was found useful for the interpretation of the laboratory results to prepare diagrams showing the distribution of certain mineral groups and of individual minerals within these groups, each on an appropriate scale, rather than only one table or diagram comprising all minerals on the same scale. Fig.3-7 with their legends should be self-explanatory in this respect. A few heavy minerals observed in many slides, but rare (apatite, clinozoisite, monazite, topaz, zoisite), have been omitted as well as biotite, muscovite, and chlorite whose ranges of specific gravity overlap that of bromoform. Early during the investigations it was found that the samples from the outer shelf (especially numbers 19 and 21) contained fragments of fairly well consolidated rock: fossiliferous sandy limestone and calcareous sandstone, which apparently outcrop locally on the shelf floor. These fragments are similar to certain types of beach rock associated with remnants of a slightly raised, subrecent shoreline along the West African coast (ANDERSON and BRUCKNER,1957). It seems therefore that they were, or are being, reworked from a fossil beach rock that was formed while sea level was much lower than today (Pleistocene?) and that has since been only partially covered by the sediments of the outer shelf. In order to test this assumption and to determine, if possible, the significance of reworking processes for the formation of the recent outer shelf sediments, two representative specimens of the rock fragments were selected, disintegrated by dissolving their calcareous matter in HCI, and then subjected to the

56

w. D. BRUCKNERAND H . J. MORGAN

-

! E

-

i E

rlancc? urnshahop

1

12

2

4

3

0.

4 5

6

i;

LOG

:.I m

' Z

iC-

rppror

+

-

125

9

250

10

1850

11

2750

12

4150

13

5500

14

7400

437

+ 0.3

-

100

107

1.5

40

I

490

3.0

0.1 0.0 ow Lids iea l e n l 20 - 0.5

8

j

/E: ;.Cj ! -.

-

912 1.3

210

1.5

227

3.5

1.1)

153

4.0

!,:

130

1 .o

- 4.5 - 9.5 - 12.0 - 16.5

- 22

2.5

70

0.7

56

1.1

52

0.11

52

0.7

41)

0.9

67

15

9200

- 31 - 35

0.6

LO

16

13000

-42

1 -4

62

17

15000

- 45

1.3

70

18

17000

2 .o

120

- 53 - 53

19

lS000

20

22000

- 60

21 -

27500

-90

2.1

2.5

150

1

135

-

220

~

Reworked

rock

fraaments

Fig.2. Diagram showing sample locations, grain size distributions (after removal of constituents soluble in dilute HCI), and sample fractions used for heavy mineral study. Solid lines include all size fractions present; stippled areas mark fractions compiising each more than 10 weight % of total sample. Broken lines include all fractions in which heavy mineral analysis was attempted; hachured, areus mark fractions containing more than 10 weight 76 of heavy minerals per fraction. (Line 14 max. mineral grain diameter, read 0.6.)

Sample groups comprising samples: 1-9 010

30

,.

10-13

16-17

22.23

18-21 010

70

Fig.3. Average weight percentages of light and heavy minerals respectively, calculated for groups fo samples from their 45-100 p fractions (all minerals = 100%).

HEAVY MINERALS ON THE CONTINENTAL SHELF OFF ACCRA

57

same procedure as the other samples. The results are shown in Fig.2-7 under the sample numbers 22 and 23.

INTERPRETATION SUGGESTED BY THE DATA

Fig.2 shows that the overall grain size distribution of the beach and inner shelf sediments changes fairly regularly: the beach sands become coarser downslope reflecting the increasing power of the breaking waves, and the inner shelf sands and silts become gradually finer with increasing distance from the coast, increasing water depth, and decreasing power of the undertow. On the outer shelf from sample 14 on, however, irregularities exist, double frequency peaks occur, and a general but somewhat irregular grain-size increase outwards is evident. The fact that the reworked rock fragments from the outer shelf are also rather coarse-grained (especially sample 23) suggests that most, if not all, of the coarser grains in the recent sediments of this area are derived from the underlying rock (the irregularities being probably due to its local variations), while only the finer and finest-grained material can have made the journey this far from the shore. The heavy mineral rich fractions marked in Fig.2 show a similar trend: progressively decreasing grain sizes from the shoreline outwards and additional frequency peaks in the coarser grades on the outer shelf. The relative enrichment of the heavies in the finest fractions of each sample can be ascribed to the lighter minerals of these grain size ranges having been winnowed away and carried farther out in considerable quantity. Among the diagrams giving the average mineral percentages of the 45-100 p fractions, the Fig.3, 5, 6, and 7 show again that the mineral distribution is fundamentally

S a m p l e groups comprising samples: 1-9

I I

10-13

14-17

22.23

(8-21

Opaque minerals

Fig.4. Average grain number percentages of opaque and non-opaque heavy minerals respectively, calculated for groups of samples from their 45-100 p fractions (all heavy minerals = 100%).

w. D.

58

BRUCKNERAND H. J . MORGAN

different on the inner and on the outer shelf. From the beach across the inner shelf the specifically heavier minerals generally give way progressively to the specifically lighter ones (Fig.3,5; zircon in Fig.6; staurolite, epidote and kyanite in Fig.7). On the outer shelf, however, the mineral distribution in the recent sediments is obviously governed, or at least strongly influenced, by supply from the underlying rock (shown particularly well in Fig.5 and 6, and for epidote, kyanite, and tourmaline in Fig.7). If the agreement of values is less good for the remaining minerals in Fig.7, and also for all

S a m p l e groups comprising sainples 1-9 010

.

-20 m L

\

0 1

*.

mw-10 G?

ZY %-:

14-17 2 2 , 2 3

10-13

\.

18-21 010

Non-opaque minerals o f specific gravities 3 - 4

"

80- 1

o n

"\ ---

-

/

.Li'rr=-.

90-2; 0

.

loo-= ,"

Non-opaque minerals o f soecific oravities L - 5

0

m

5:

\,-

Fig.5. Average grain number percentages of non-opaque minerals of specific gravities 4 5 and 3 4 respectively, calculated for groups of samples from their 45-100 p fractions (all non-opaque heavy minerals : 100%).

Sample groups comprising samples : 1-9

10-13

14-17

22.23

18-21

'lo 60

50

(0

LO

30

20

10 0

0

Fig.6. Average grain number percentages of individual non-opaque minerals of specificgravities 4-5, calculated for groups of samples from their 45-100 p fractions (all non-opaque minerals of specific gravities45 = 100%).

HEAVY MINERALS ON THE CONTINENTAL SHELF OFF ACCRA

59

heavies together in Fig.3, it has to be realised that the comparison of source rock and recent sediment is based on no more than two of the reworked rock fragments, whose average composition can only give an approximate picture of the gross averages of the reworked beds.

Sample groups comprising samples : 1-9

010

-40

(:I

-30

10-13

’

14-17

22.23

18-21

010

40.

A= E= K = P=

S=

Amphiboles Epidote Kyanite Pyroxenes Staurolite

30.

-

.2 0

20

-10

10 -

lo

0

1

Fig.7. Average grain number percentages of predominant individual non-opaque minerals of specific gravities 3 - 4 , calculated for groups of samples from their 45-100 p fractions (all non-opaque minerals of specific gravities 3 4 = 100%).

The largest discrepancy found concerns the opaque minerals (Fig.4). A search for its reason revealed that it was, most likely, introduced during the treatment of the samples with acid. A large proportion of the opaque minerals consists of more or less porous grains of limonite, which is moderately soluble in HCI. The reworked rock fragments had to be subjected to prolonged treatment with fairly concentrated acid in order to achieve their complete disintegration and therefore a fairly large amount of their limonitic grains was probably lost by solution. As the heavy mineral distribution discussed here includes the grains down to sizes of 45 p, it seems justified to assume that not only the coarser grains but also most of the finer-grained material on the outer shelf has its source in the underlying rock.

60

w. D. BRUCKNER AND H. J.

MORGAN

HEAVY MINERAL SOURCES ON THE COAST AND TRANSPORT DIRECTION OF THE COASTDERIVED GRAINS

As there are only minor streams in the coastal zone around Accra, which usually deposit their load in lagoons, the clastic material deposited on the beach and inner shelf is almost exclusively provided by wave erosion of the coastal cliffs, in which bedrock outcrops alternate with stretches exposing only the Quaternary mantle. Of the main bedrock groups (Fig.l), the clastic rocks of the Accraian series at Accra itself, and also those of the Togo series west of Accra, can probably be ruled out, such rocks being notoriously poor heavy mineral sources. The Cape Coast granite, with its nearest major outcrops as far as 40 km west of Accra, seems too distant; the same applies to other formations reaching the coast still farther west. The Dahomeyan group east of Accra, however, is known to be rich in the heavy minerals that predominate in the shelf sediments: In its acidic division bands of epidote-bearing hornblende schist and of tremolite-kyanite marble occur (west of Labadi); there are sequences of staurolite, kyanite, and garnet-bearing mica schists (in many outcrops between Accra and Tema), as well as granite gneisses from which the zircon could be derived. Around Kpone the basic division of the Dahomeyan, rich in hornblende, garnet, and pyroxenes reaches the shore. Rutile and tourmaline may be derived from quartz veins of the Dahomeyan belt. The most likely source of the haematite and limonite grains which predominate among the opaque minerals is the Quaternary mantle where these two minerals are common as ferruginous coatings, impregnations, concretions, and hardpans. In order to reach the beach and inner shelf at the sampling line off Accra, the Dahomeyan-derived heavy minerals must have travelled chiefly in a west to southwest direction. This is rather surprising as the prevalent current in the surface waters off Accra (the Guinea current) flows from west to east. However, the Benguela stream, opposite in direction, is known to replace the Guinea current at the coast of Ghana from time to time for periods of a few days (BUCHANAN,1958), and as it is colder than the latter it can perhaps be assumed that it penetrates under it onto the inner shelf more regularly than has so far been realised. If this were correct the distribution of the coast-derived heavies could be attributed chiefly to the joint action of the Benguela stream and the undertow.

ACKNOWLEDGEMENTS

Sampling on the shelf off Accra was done in 1955, 1957, and 1958, first by J. B. Buchanan, later on by W. D. Briickner together with Dr. D. T. Gauld, which all were members of the University College of Ghana at that time. The detailed study of the samples was begun in Ghana by J. B . Buchanan and W. D. Briickner. After 1958, it was carried on at the Memorial University of Newfoundland, where the major share of the laboratory work - including especially the mineralogical part - was taken

HEAVY MMERALS ON THE CONTINENTAL SHELF OFF ACCRA

61

over by H. J. Morgan, with D. Carmichael and J. M. Dawson assisting for some time. The figures of this paper were drawn by J. Drover. The authors wish to thank their colleagues for their help; the financial aid given by the University College of Ghana and by the National Research Council of Canada is also gratefully acknowledged.

SUMMARY

A quantitative study of the heavy minerals in sea bottom samples taken along a line across the continental shelf off Accra shows that their distribution on the inner shelf is governed by their specific gravities, the heavier minerals decreasing in quantity more rapidly outwards than the lighter ones, whereas their abundances on the outer shelf areclosely related to those of the underlying (Pleistocene?) rock, which has been amply reworked. The non-opaque heavies derived from the coast appear to indicate transportation in a west to southwest direction, opposite to the flow of the Guinea current which predominates at the surface of the sea.

REFERENCES

ANUERSON, M . M . and BRUCKNER, W. D., 1957. Note on raised shore-lines of the Gold Coast. In: J. D. CLARK(Editor), Third Pnn-Afi-[can Colyess on Prehistorv, Liviqpfone. 1955. Chatto and Windus, London, pp. 86-92. BATES,D. A , , 1955. Ceolqeicalniap ofthe Gold Coast, 1 : 1,OOO,OOO.Gold Coast Geol. Surv., Accra. BUCHANAN, J. B., 1958. The bottom fauna cornmunities across the continental shelf off Accra, Ghana (Gold Coast). Proc. Zool. Sor. London, 130 : 1-56.

ON SHALLOW WATER ORIGIN O F PHOSPHORITE SEDIMENTS G. I . BUSHINSKI

National Conitiiittee of Geologists in the U.S.S. R . , Moscow ( U . S . S .R . )

INTRODUCTION

Shallow marine formations containing phosphorites are of widespread distribution They are found for example in the Ordovician of Estonia, the Leningrad region and the southern part of the Siberian platform (phosphorites rich in shells), the Jurassic and Cretaceous of the Russian platform (nodular phosphorites with quartz and glauconite sand), Florida (conglomeratic phosphorites). the Eocene and Cretaceous of North Africa (rich zones of bedded phosphorites), the Permian of the western states of the U.S.A.,the Cambrian of Caratau (U.S.S.R.) and Cunian (China) and the Sinian of Tzin-Sian and Cayan (China). Almost the entire world production of marine phosphorite comes from shallow water deposits, the deep-sea ones being of no commercial value.

INDlCATIONS OF SHALLOW WATER ORIGIN

Indications of the shallow water origin of mcst phosphorite deposits are among others: the smooth, more or less worn condition, apparently due to rolling of phosphorite nodules and associated pebbles, the presence of sand grains of various origin and of arenaceous phosphorite material, the occurrence of angular and worn fragments of the shells and skeletoix of molluscs, brachiopods, sponges, Foraminifera and other marine organisms, the inclination of the bedding, the presence of thick-walled benthonic Foraminifera (in the phosphorite sands of Morocco), the alternation of phosphorite layers with shallow marine sands or pebbly sands, or their direct transition into one another, both in vertical and lateral directions. These characteristics, pointing to shallow water formation are typical not only for secondary, but also for primary, unreworked (rich) phosphorites. The primary origin of apparently shallow marine phosphorites is proved by the presence of phosphorite casts of mollusc shells of the same age as the phosphorite layer and also by the growth of oyster shells and worm tubes on phosphorite concretions. Furthermore, these shells and tubes often contain phosphorite, filling pseudomorphs testifying to the slow formation of the phosphate accumulation. Cross bedding in phosphorites has been observed in the Sinian deposit of Tzin-

SHALLOW WATER ORIGIN OF PHOSPHORITE SEDIMENTS

63

Fig.1.Cross lamination in phosphorites. At left: Deposit of Tzin-Sian, Sinian, Hubei, China. At right: Deposit of Janitas, Cambrian, Caratau range, Kazakhstan, U.S.S.R.

Sian in China and in the Cambrian deposit of the Caratau range in Kazakhstan (Fig. I). The layers may be inclined in one direction only, testifying to a constant current direction, or in different directions, proving the inconstancy of water movements. Inclined bedding is often found in shallow water, but it may appear at any depth where water currents exist. Definite signs pointing to the small depth of the sea are calcareous Algae or reef corals. Normally, however, neither the former nor the latter

Fig.2. Benthonic calcareous Algae in phosphorites. At left: Colleniu, deposit of Tzin-Sian. Sinian, Hubei, China. At right: Oncolithes, deposit of Janitas, Cambrian, Caratau range, Kazakhstan, U.S.S.R.

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G . I. BUSHINSKI

are found in phosphorites. Yet, recently remnants of the calcareous reef building Algae Collenia and Oncolithes have been found in the phosphorite deposits of TzinSian and Caratau (Fig.2). Comparable Algae live at the present tim: in depths of 5-50 m, rarely up to 100 m. Hence, it may be concluded that the inclined bedding in the phosphorites discussed above was formed in shallow water. A no less important indication for the relatively small depths at which phosphate accumulation took place, is provided by the direct transition of many phosphorites into sand or pebbly sand deposits of doubtless shallow water origin. Thus the Sinian and Cambrian phosphorites of Asia are often interbedded with or covered by coarse detrital sediments or ore deposits and show traces of interruptions in the depositional process. The same has been observed in the Jurassic and Cretaceous phosphorites of the Russian platform, as well as in the Permian phosphorites of the U.S.A. Pebbles of various rocks may also form a strong indication for shallow water origin, but in bedded phosphorites they are comparatively rare. Rolled, worn grains of sand size, composed of phosphorite, which also point to small depths are found much more often. But it is not always possible to distinguish them from phosphate ovules or grains of other origin, either by their form or by their inner structure. A characteristic feature of many phosphate sand grains is that they are composed of an aggregate of minute oolites, which at the surface of the grains have been ground away by abrasion (Fig.3). Such sand grains are of common occurrence in all phosphorite layers of the Cunian deposit in China, and at Jani-tas and Cockjon in Caratau (U.S.S.R.). The shallow water facies, characteristic for most (rich) phosphorite deposits is also shown by the facies profile through the Caratau basin (Fig.4). I n the zone of very sha1low:water sediments as well as in the adjoining zones of somewhat greater depth

Fig.3. Phosphorite grains in bedded phosphorite of Caratau range: x 60.

65

SHALLOW WATER ORIGIN OF PHOSPHORITE SEDIMENTS

sw

NE Janytas

Akjar

Koksool

Fig.4. Facies profile of the Cambrian phosphate series ofthe Little Caratau range in the direction from shallow water zones (Cockjon, Jani-tas) to deeper-water zones (Ackjar, Cocksu). 1 = Dolomites; 2 = Chert, occasional spongolites; 3 = Phosphorite with oolites and phosphorite pebbles; 4 = Oolite phosphatic; 5 = Pellet phosphorite, sometimes with underdeveloped coating and rare oolites; 6 = Siliceous clay shales. Composed after the data of MENSIANHUA(1959).

(the Cockjon and Jani-tas deposits) oolites and pellets of phosphorites with phosphorite sand grains and granules are present throughout the layers. Phosphorite gravel is seldom met with. The phosphate grains usually show a thin coating of phosphate of a different nature. In the direction of originally deeper water the number of phosphorite sand grains, granules and oolites decreases, phosphate pellets with or without coating beginning to prevail instead. The dimensions of the phosphate pellets decrease from 0.3-1 mm to 0.1-0.2 mm. At the same time the phosphorite layer starts wedging out and lenses of dolomite and layers of siliceous-argillaceous shales appear. The thickness of the phosphorite bearing layers range from 20 m in Cockjon to 50 m in Akjar. Further to the northeast the abundance of phosphate pellets with coating and of phosphate oolites increases again. The siliceous rocks are here and there extremely rich in sponge spicules.

PHOSPHORUS SOURCE

When the rock sequences, not only of poor but also of rich phosphorite deposits are analyzed, the presence of quartz sandstone layers or of quartz sand admixtures form a conspicuous feature. This is the case, for example, in the Sinian deposits of Junhe. Kaian and the western Baikal territory, in the Cambrian deposits of Cunian and Karatau, the Permian deposits of the western states of the U.S.A., and i n the Jurassic, Cretaceous and Tertiary deposits of the Russian platform. This association can hardly be accidental. If we take into consideration that the seas in which the accumulation of phosphates took place were apparently bordered by lowland coasts, then involuntarily the question arises whether the land or the rivers flowing from it

66

G. 1. BUSHINSKI

were the direct source of the phosphates deposited in the sea. In order to check this idea we shall make an approximate calculation. The concentration of dissolved phosphate in river waters in the lower courses is on the average 5&80 mg P,O, per ms of water; upstream waters contain 150-200 mg P,O, per m3 of water, i.e., 2-3 times more. Rivers, however, also carry away a considerable quantity of phosphorus. Thus, for example, the Volga river brings into the Caspian Sea 6,000 tons of dissolved phosphate phosphorus yearly. If all that phosphorus should be precipitated during one million years, the result would be a layer of 40 billion tons, as calculated by 30% phosphorite. This would correspond already to a phosphate deposit comparable in size to that of the fossil basins. It is true that not all dissolved phosphate phosphorus necessarily falls out in phosphorite form, but if we take into consideration that the Volga brings twice as much dissolved and undissolved organic phosphorus and that the accumulation of phosphate may last several million years, then it comes close to the mentioned figure of 40 billion tons. Almost all dissolved phosphorus brought by the rivers into the sea is immediately assimilated by the plankton and deposited on the bottom, very little being carried away into the deeper parts of the basin. This means that a biological filtering of the phosphorus takes place. But in small depths where the motion of bottom waters is intensive enough, the phosphorus deposited under the influence of organisms returns again into circulation and may thereby be carried away far from the river mouths. Together with the phosphorus the rivers also bring much dissolved silica which is likewise assimilated by organisms, deposited on the sea floor and accumulated with the phosphorites in the form of siliceous sediments. The circumstance that quartz sandstones are often associated with phosphorites does not mean that the more there is of the former, the more there should also be of the latter. In fact, just the opposite is the case: where much quartz sandstone is found there are usually no phosphorites at all or they are few in number and thin. Obviously little sandy material reached the place of maximum phosphate accumulation. In order to provide in the mud on the sea floor a sufficiently high concentration of dissolved phosphates required for their precipitation as phosphorite, a rather plentiful rain of organic matter and a quick decomposition of it should take place. This is possible in the case of heightened phosphate concentration in the phytoplankton zone (the upper water layer 100-150 m thick, rarely up to 200 m) and of shallowness, so that there is no time for the sinking, dead organisms to become decomposed on their way to the bottom. The rate of decomposition of the organic matter is increased by high temperatures. Rich concentrations of dissolved phosphates, necessary for an intensive development of the organic life, may be caused by submarine hydrothermal sources, volcanic exhalations, upwelling of the sea water and by fluvial supply from the land. The production of organic life in areas of upwelling waters and i n estuaries may be of enormous importance. Hydrothermal and exhalational sources of phosphorus are of far smaller significance. Accordingly, their influence on the abundance of the plankton bio-mass is negligible and usually remains unnoticed, both in modern seas and in

SHALLOW WATER ORIGIN OF PHOSPHORITE SEDIMENTS

67

reservoirs of the geological past. The paleogeographical properties of basins containing phosphorite deposits renders it likely that rivers formed their most important phosphorus source.

PRECIPITATION FROM BOTTOM MUD SOLUTIONS

Almost all phosphorite layers consist of phosphate grains or pellets (mainly coprolites) and phosphate oolites. Their formation, as well as that of phosphorite nodules, took place in the mud on the sea bottom by processes of diagenesis. New studies of marine hydrochemistry lend much support to this view. According to data of BRUEVITCH and SAITZEVA ( I 958) the phosphate P content in near-bottom waters of shallow- and deep-water stations of the Bering Sea vary within the range of 0.080.13 mg/l, i.e., the extremes lie close t o each other. In shallow-water stations the mud solutions contain 2.50-7.48 nig P per 1, or 30-90 times more than in near-bottom waters. For the northern part of the Caspian Sea the corresponding excess equals 150 times. Together with the phosphorus the alcali reserve (alcalinity), ammoniac nitrogen and silica in bottom mud solutions from shallow-water stations are also much higher. In deep-water stations the phosphate content in mud solutions is only 2-3 times higher than in near-bottom waters. This happens because only stable organic matter reaches great depths. The latter decomposes very slowly, and therefore it changes the composition of mud solutions very little, causing only weak diagenetic processes. The strongest phosphate concentrations in marine environments are found in mud solutions of shallow-water zones. They are scores of times greater than the phosphate concentration in bottom waters. Therefore it is logical to suppose that the phosphate is not precipitated out of the basin water as KASAKOV(1937) considered, but out of the mud or ground water solution, though this is usually characterized by lower pH values (BUSHINSKI, 1954). The depths where the highest concentrations of dissolved phosphates in mud are found are within the range of about 30-200 m, which agrees with the depths of phosphorite formation deduced from the geological data of ARKHANGELSKI ( I 927), KASAKOV (1 937) and other geologists. Two circumstances determine these depth limits. The first is that the shallow water zones are connected with the greatest plankton bio-mass. The second being that in shallow-water the greatest quantity of dead plankton reaches the bottom. Mud solutions in hollows between shoals are especially rich in dissolved phosphates. The structural forms of phosphorite pellets, oolites and concretions are the result of a concentration of phosphate dispersed in deposits, dissolved or hard, but via the solution stage. Most pellets or ovules are phosphatised coprolites. Further phosphate concentration took place owing to repeated washing of the deposits by turbulent water. As a result fine clay, carbonate and silica particles were washed out and carried away into deeper parts of the basin, the phosphate pellets. oolites and concretions remaining in their place and forming phosphorite deposits.

68

G. I. BUSHINSKI

The accumulation of phosphorites in this way is possible only with alternating stages of quiet and rough water. During a stage of quiet water the remains of dead plankton sink to the bottom even on shoals, where as a result of their decomposition phosphorite pelIets, oolites and concretions are formed. In a stage of rough water or storms these structural elements are washed and fine admixtures are carried away. Traces of washing in the form of sand grains and pebbles are observed throughout all the rich and thick phosphorite deposits of Cunian and Caratau. Thus according to the biochemical hypothesis the concentration of phosphorus leading to the formation of phosphorites passed through the following five stages: (1) The influx of waters with a high phosphate content from rivers and possibly also from deeper parts of the sea, by currznts flowing upward into shallow areas with depths of about 30-200 m. (2) The concentration of dissolved phosphorus by organisms and its sinking to the sea floor after their death. (3) The concentration of phosphorus in mud solutions as a result of the decomposition of dead organisms on the sea bottom. On shoals, the deposition on the bottom of dead organisms and their decomposition took place only during conditions of calm weather. ( 4 ) The concentration of phosphates dispersed in mud with the formation of phosphate pellets, oolites, concretions, cement and various pseudomorphs. (5) The concentration of relatively coarse structural elements of the phosphorite, thus formed, owing to the washing out of fine admixtures by waves during stages of stormy weather. The interchange of periods with much and with little water turbulence could also be the result of tectonic elevations and lowerings of the bottom of the basin causing alternative shallowing and deepening of the water. Later concentrations or impoverishments of the phosphorites may take place by late-diagenetic, epigenetic and hypergenetic processes. The first, second and third of these stages are hypothetical, no direct indications having been found in the rocks. Many poor deposits, such as at Junhe, Tzin-Sian, Laibo, Emay, Belka and elsewhere, did not pass the fifth stage or it is only slightly expressed in them. The expounded biochemical hypothesis is still far from perfect. It does not go into the question of what peculiarities of the bacterial or biochemical processes in shallow water areas are responsible for the accumulation of phosphates at some places and the absence of accumulation at others. It may be supposed that for the genesis of phosphorites these processes are almost decisive, but their influence on the accumulation of phosphates in modern seas is still unstudied.

INFLUENCE OF ARID CLIMATE ON ACCUMULATION OF PHOSPHORITES

In the paleoclimatological respect it is interesting that large and rich phosphorite

SHALLOW WATER ORIGIN OF PHOSPHORITE SEDIMENTS

69

basins are situated in former arid zones. STRAKHOV (1962), who has discovered this relationship, explains it by the fact that the arid zone is characterized by strong and very constant trade-winds which cause upward flows or upwelling of waters bringing phosphates onto the continental shelves, and also by the fact that the seas of the arid zone are characterized by relatively high calcium concentrations, which stimulate the precipitation of phosphates. As we understand it, the influence of aridity on the accumulation of phosphates must lie primarily in the fact that few rivers flow here to the sea and, hence, little terrigenous material is supplied which could dilute the phosphorites. As far as appreciable quantities of phosphates were supplied by rivers, this material came from far upstream, from the humid zone. This is in agreement with the well-rounded, smooth and worn condition of the quartz sand grains and by the almost complete absence of admixture of unstable terrigenous minerals. A good example of this is supplied by the map of the Phosphoria formation deposits (MCKELVEY et al., 1959). It shows clearly the wedge of quartz sandstone, the material of which was brought from the north or northeast, i.e., from the humid to the arid zone. The phosphorites, together with silica rocks with sponge spicules are found in a zone extending from this wedge in seaward direction. Further to the south and southeast the phosphate-silica precipitations are replaced, first by dolomites, and then by red beds with gypsum and anhydrite, i.e., precipitations of a hypersaline basin.

SUMMARY

This paper presents evidence showing that among fossil marine phosphate deposits those formed in shallow water environment predominate. This predominance is i n agreement with analyses of the content of dissolved phosphate in the interstitial water of muds from the recent sea floor. In shallow water muds these contents are far higher than those in the seawater circulating directly over the bottom. The difference between the two is maximum at depths between 30 and 200 m. The phosphate is apparently precipitated from the bottom mud solutions. Further concentration may take place after deposition, both mechanically, by washing out of fine-grained clastic material, and chemically, by diagenetic processes.

REFERENCES

AKKHANGELSKI, A. D., 1927. Stratigraphy and g:ological conditions of formation of Russian phosphorites. In: M. M. TETIAEV (Editor), Phosphorites of the U.S.S.R.,pp. 13-21 (Russ.). S. V. and SAITZEVA, E. D., 1958. On the chemistry of the Bering Sea sediments. Papers BRUEVITCH, Inst. Oceanol., 26 : 8-108 (Russ.). BUSHINSKI, G. I., 1954. Lithology of’Cretaceous Deposits of the Dnieper-Donets Trough. Acad. Sci. U.S.S.R., 290 pp. (Russ.). KASAKOV,A. V., 1937. Phosphorite facies and phosphorite genesis. In: B. M. GIMELFARB (Editor), Geologic Investigations of Agronomic Ores of the U.S.S.R.O.N.T.I.,pp. 100-1 19 (Russ.).

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MCKELVEY, V. E. et al., 1959. The Phosphoria, Park City and Shedhorn formations in the western phosphate field. US.Geol. Surv., Profess. Papers, 313A : 1 4 . 1959. On the petrography of the Karatau Basin’s phosphorites. Trans. Acad. Sci. MENSIANHUA, U.S.S.R., 126 (6) : 13261330. (Russ.). STRAKHOV, N. M., 1962. Fundamentals of the Theory of Lithogenesis. 3 . Acad. Sci. U.S.S.R., 95 pp. (Russ.).

RELATIONSHIP BETWEEN POROSITY, PERMEABILITY, AND GRAINSIZE DISTRIBUTION OF SANDS AND SANDSTONES GEORGE V. CHILINGAR

University of Southern California, Los Angeles, Car$ ( U . S . A . )

INTRODUCTLON A N D REVIEW OF LITERATURE

As pointed out by GRIFFITHS (1952a, 1952b, 1958), the definition of a specimen of sandstone in terms of petrographic properties may be presented as:

P = f (m,s, sh, o , p ) where P is some defining index expressed as a function (f) of the mineral composition (m),grain size (s), grain shape (sh), grain orientation (o), and grain packing @). In many instances, it is also very important to characterize a sandstone or a sand in terms of permeability and porosity (mass properties of rocks). Porosity can be easily related to petrography and in many reservoir sands porosity is related to permeability (exponential function). According to GRIFFITHS (1958, p. 1 9 , when permeability is plotted on a logarithmic scale and porosity on an arithmetic scale, a linear relationship emerges. This relationship, however, is not exact as can be seen from the scatter around the trend lines (GRIFFITHS, 1958, p. 16, fig. 1). According to KOTYAKHOV (1949, p.31) the average radius of pores, rc, can be expressed in terms of porosity and permeability by the following formula:

where m is the coefficient of porosity and k is the coefficient of permeability in c d . KOTYAKHOV (1949, p.32) also calculated that the effective diameter of grains, d,, is related to permeability and porosity as follows:

Both of these equations, however, seem to be oversimplified, because Kotyakhov apparently assumed that all grains are of spherical shape.

12

G . V. CHILINGAR

Some data on the relationship between porosity and permeability had been assembled by MUSKAT (1949 ,pp. 168-176). KHANIN(1956; see also CHILINGAR, 1957)conducted a comprehensive statistical study on relationship between the effective porosity and permeability, and found that the granulometric composition of sandstones has to be considered in order to establish correlation between these two variables. KHANIN (1958) also started to study relationship between the porosity and permeability of unconsolidated and weakly cemented sands (Fig. 1). Khanin prepared artificial sand packs with sands of known granulometric composition (at 0, 110, 190, 370 and 560 kg/cni2 compaction pressures) and determined their porosities and permeabilities. Although unconsolidated sands defy any accurate permeability measurement, the

$ 35

>;

e

8

30

25 200

5’

10

15

20

25

30

35

40

45

50

Permeability, D

Fig.1. Relationship between porosity and permeability (to gas) of dry sand packs. I = < 0.1 mni in diameter; I1 = 0 . 1 4 . 2 5 mm in diameter; I11 = > 0.25 mm in diameter; IV = 50y0of sand is < 0.1 mrn, and 50% is 0 . 1 4 . 2 5 nim in diameter; V = 50% of sand is > 0.25 mrn, and 50% is < 0.1 mrn in diameter; VI = 50% of sand is 0.1-0.25 mm, and 50% is > 0.25 mni in diameter. (After KHANIN,1958,

fig. 1.)

graphs such as those presented in Fig. 1 will enable sedimentologists at least to estimate the permeability of sands of known porosity and grain size distribution.

EXPERIMENTAL TECHNIQUES

Permeabilities were measured by air in a permeameter having a capillary-tube flowmeter. The measured values were corrected to infinite mean pressure from the data of HEIDet al. (1950). Their average correction for the appropriate permeability was applied to each sample. Bulk volumes were obtained with a pycnometer using mercury, whereas solid volumes were measured in an Oilwell research porosimeter (BEESON, 1950). It is a Boyle’s law instrument operated with helium. The samples used by the writer were ordinary bottom-hole, consolidated sandstone cores, which were thoroughly cleaned by solvent extraction and distillation and were dried in a vacuum oven for 24 hours at a temperature of 100°C. The porosity and permeability measurements were made on dry cores and on cores containing an irreducible minimum of interstitial formation water.

INTERRELATIONS BETWEEN SOME SANDSTONE PROPERTIES

73

Porosity. %

Fig.2. Relationship between porosity and permeability of very coarse-grained, coarse- and mediumgrained, silty and clayey sandstones.

The interstitial water content (irreducible minimum) was determined with an apparatus using principles of capillarity (BEESON,1953, p.21). Inasmuch as only the interstitial water content in the pendular region is desired, a single measurement may be made at a pressure sufficiently high to be on the nearly vertical portion of the water saturation ( S , )versus the capillary pressure (P,) curve (BEESON,1953, p.19). A generalized dispersion routine (KRUMBEIN and PETTIJOHN, 1938, pp.70-75) was used in the preparation of samples before determining the grain size distribution. The sandstones containing more than 5 0 x of 1-2 mm fraction were called very coarsegrained; 0.5-1 mm, coarse-grained; 0.254.5 mrn, medium-grained; and 0.1-0.25 mrn, fine-grained. Sandstones containing more than 10% of silt (< 0.1 mm) were called “silty”, whereas if clay content (< 0.004 mm) was in excess of 7 % the sandstones were called “clayey”. The latter value, however, is not final and is subject to changes with further research.

EXPERIMENTAL RESULTS

In Fig.2, the permeability was plotted on a logarithmic scale, whereas porosity was

74

G . V. CHILlNGAR

plotted on an arithmetic scale. As shown in this figure, when the grain size distribution is taken into consideration there is a correlation between the porosity and the permeability of cores containing an irreducible minimum of interstitial water. Such a correlation, however, does not seem to exist between the permeability and porosity of dried samples of sandstones, although in many instances these “dry” points do fall on proper curves in Fig.2. Apparently, on measuring the porosity of many dry cores, in addition to measuring the volume of the larger pores (through which the major part of the flow occurs) one also measures the pore volume of minute interstices and cracks which do not affect permeability. The scattering of points in Fig.2 shows, however, that variables other than porosity and grain size distribution influence permeability; and future experiments will be designed to study the effect of other petrographic properties. In the case of silty sandstones, there is a break in the porosity versus logarithm of permeability curve at about 100 md, whereas for coarse- and medium-grained sandstones the break occurs at about 1,000 md.

CONCLUSIONS

On considering grain size distribution, there is a correlation between the porosity and permeability of sandstones, containing an irreducible minimum of interstitial formation water, and unconsolidated sands. Considerable amount of research work, however, still remains to be done in this field.

ACKNOWLEDGEMENTS

The writer is greatly indebted to the following ex-petroleum engineering students (now petroleum engineers working for various oil companies) for their contribution to the present study: R. C. Main, Ali Sinnokrot, Myron Smith Jr., Bill Graham, Frank Lortscher and Edward Zinzer. The help extended by Dr. C. M. Beeson is also greatly appreciated.

SUMMARY

The porosity, permeability, and grain size distribution of 610 sandstone cores (dry and containing an irreducible minimum of interstitial water) from various parts of U. S. were determined by the writer and his students. Porosity (in %) was then plotted versus the logarithm of permeability (in md) separately for (a) very coarse-grained, (b) coarse- and medium-grained, (c) fine-grained, ( d ) silty, and (e) clayey sandstones. Correlation has been found to exist between the porosity and permeability of sandstone cores containing interstitial formation water.

INTERRELATIONS BETWEEN SOME SANDSTONE PROPERTIES

75

REFERENCES

BEESON,C. M., 1950. The Kobe porosimeter and the Oilwell porosimeter. Trans. A.I.M.E., 189 : 313-31 8. BEESON,C. M., 1953. Core Analysis Principles and Experiments. Univ. Southern Calif. Publ., Los Angeles, 55 pp. CHILINGAR, G. V., 1957. Khanin's classification of reservoir rocks. Compass, 34 (4) : 335-339. GRIFFITHS, J. C., 1952a. Grain size distribution and reservoir rock characteristics. Bid/. An?. Assoc. Petrol. Geologists, 36 : 205-229. GRIFFITHS, J. C., 1952b. Measurement of the properties of sediments (abstract). Bull. Ceol. SOC. Am., 63 : 1256. GRIFFITHS. J. C., 1958. Petrography and porosity of the Cow Run sand, St. Marys, West Virginia. J. Sediment. Petrol., 28 : 15-30. HEID,J. G., MCMAHON,J. J., NLELSEN, R. F. and YUSTER, S . T., 1950. Study of the permeability of rocks to homogeneous fluids. A.P.I. Drill. Prod. Pract., 1950 : 23&246. KHANIN,A. A., 1956. About classification of petroleum and natural gas reservoir rocks. Razvedka i Okhrana Nedr., 1 : 7-16. KHANIN,A. A., 1958. Toward question of determining reservoir properties of non-cemented sandstones. Tr. Vses. Nauchn. Issled. Inst. Khim. Pereraborki Gazov, 4 (12) : 8 pp. KOTYAKHOV, F. I., 1949. Interrelationship between major physical parameters of sandstones. Nefr. Khoz., 12 : 29-32. KRUMBEIN, W. C . and PETITJOHN,F. J., 1938. Manual of Sedimentary Petrography. AppletonCentury-Crofts, New York, 549 pp. MUSKAT,M., 1949. Physical Principles of Oil Production. McGraw-Hill, New York,922 pp.

HOLOCENE REGRESSIVE LITTORAL SAND, COSTA D E NAYARIT, MEXICO' JOSEPH R . CURRAY

and

D A V I D G. MOORE

Scripps Institiition of Oceanography, University of California, La Jolla, Calif.; U.S . Navy Electronics Laboratory, San Diego, CaliK ( U . S . A . )

INTRODUCTlON

The coastal plain and continental shelf of the west coast of mainland Mexico have been investigated as a part of the Scripps Institution of Oceanography project on the geology and oceanography of the Gulf of California. One of the areas of greatest geological interest is the area south of Mazatlan, Sinaloa, and north of San Blas, Nayarit, on the mainland side of the Gulf of California (Fig. 1). Both the coastal plain and the continental shelf in this region are dominated by the influence of the Rio Crande de Santiago, one of the major rivers of the west coast of Mexico, with lesser influence by the smaller adjacent rivers to the north and south. During Pleistocene periods of lower sea level, the Rio Crande built a complex delta system on the continental shelf, and in places prograded the edge of the shelf seaward into deeper water. The Holocene transgression moved the shoreline across the exposed upper surface of this delta complex to a position several kilometers inland of its present position. During the last few thousand years, the shoreline has regressed by deposition of a sheet-like body of littoral sand. This paper represents a progress report dealing with the general distribution of the sediments of the regressive sand body, its relationship to the underlying and overlying sediments, and the mechanism of deposition and regression at work along this coastline. The submerged Pleistocene delta complex of the adjacent continental shelf is the subject of another progress report (MOOREand CURRAY, 1963).

REGIONAL DESCRIPTION

The coastal plain (Fig. I ) consists of a low-lying marsh essentially at sea level, overlapping on the seaward-dipping flood plain surface of the Rio Grande de Santiago and the smaller adjacent rivers to the north and south. The coastal marsh exists mainly Contribution from the Scripps Institution of Oceanography, University of California, La Jolla, Calif.

I \ , I '

< \ '/ /

2

\

7 /

\

/

/.

Fig.1. Bathymetric and physiographic chart of the Costa de Nayarit continental shelf - coastal plain complex. Coastal plain consists of system of abandoned Holocene beach-dune ridges, shown diagrammatically, overlying pre-Holocene-transgressive alluvium, and in turn overlain in part by younger Holocene alluvium. Protuberance in continental shelf edge is Pleistocene delta system of Rio Crande de Santiago. (After MWREand CURRAY, 1963.)

78

J. R. CURRAY A N D D. G . MOORE

Fig.2. Oblique air views of abandoned Holocene beach-dune ridges during flood season. Average width of ridges shown is about 80 m. (Photos courtesy of F. B Phleger.) a. Landward half of strand plain of abandoned beach ridges covered with mangroves. b. Closer to ocean than upper photograph, showing discontinuity between adjacent sets of ridges. Younger ridges closer to ocean are built higher and closer together than older ridges, and coastline was I eoriented before formation of younger ridees.

REGRESSIVE LITTORAL S A N D OF COSTA DE N A Y A R I T

79

in the depressions between scores of parallel abandoned beach-dune ridges (Fig.2). This strand plain of abandoned beach ridges averages about 5 km in width for the entire 225 km distance from Mazatlan to San Blas, and about 10 km in width for the better developed cer,tral 130 km length. Maximum width from the present beach to the oldest ridge is 17 km. The upper surface of the strand plain is uniformly furrowed by the parallel ridges, which are typically spaced 30-200 ni apart. About 250 ridges can be counted in air photographs from one of the better developed parts of the plain 12 km wide, giving an average width of about 50 ni per ridge. Relief varies from less than I n i to a maximum of about 5 m above the surrounding level. Detailed leveling shows that the entire plain lies at about sea level. The sand body is continuous between the ridges and all the way across the strand plain (Fig.3). Some depressions between the ridges contain elongate surface lenses of modern alluvium, but sand is always encountered at depth under the alluvium. Some of the smaller rivers of the area discharge directly into the ocean, but others discharge into the tidal marsh between the sand ridges. Seasonal flooding accompanied by influx of alluvial muds is gradually prograding the river flood plains over the landward edge of the strand plain. Only the depressions between the ridges have been filled with modern alluvium in most of the area, but locally some of the older ridges near the landward edge of the plain have been completely buried. The continuous sand sheet overlies the pre-transgressive or Pleistocene flood plain deposits of the coalescing river system (Fig.3). This alluvial surface has been traced by drilling under the coastal strand plain, and it has been delineated in detail on the shallow continental shelf by acoustic reflection niethods (MOORE and CURRAY. 1963). The surface is irregular as a result of subaerial erosion, and sand-filled river channels

mPRE-TRANSGRE5SlVE (PL EIST OC EN E) A L L U V I U M

Fig.3. Diagrammatic cross section through coastal plain and inner continental shelf. Formation of ridges is by successive accretion of submerged longshore bars. During period of low waves, bar is built to surface and becomes new beach, thereby isolating former beach and creating a narrow lagoon. Lagoon is partially filled by washover and later covered with modern alluvium.

80

J. R. CURRAY A N D D. G. MOORE

have been detected both under the strand plain and at sea. Average slope of the irregular surface under the strand plain is about 1.3 m/km (7 ft./mile). Sands composing the strand plain are of fine grain and well sorted, with a high proportion of feldspar and volcanic rock fragments. The older alluvial deposits consist of alternating sandy muds and muddy sands, while the younger alluvium consists mainly of fine mud.

MECHANISM OF FORMATION OF SAND RIDGES

Each ridge was formed individually as a shoreline deposit; the oldest lie farthest from the ocean, and the youngest lie closest to the ocean. The present shoreline is analogous in most respects to each of the ridges at its time of formation. The mechanism of formation proposed for these features is that each ridge started as a submerged longshore bar in front of the existing beach. Laboratory wave tank experiments (MCKEEand STERRETT, 1961) and beach and wave studies (KEULEGAN, 1948; SHEPARD, 1950; D.L. Inman, personal communication, 1961) have shown that longshore bars form initially at the plunge point of breakers. With sufficiently high rate of supply of sand, and with conditions of low wave action, a bar can build to the surface. If this occurs during a high tide and if the low wave conditions persist through the low tide and several tidal cycles thereafter, the longshore bar in effect becomes the new beach, and the former beach is isolated. This process has apparently repeated itself cyclically since the time sea level came approximately to its present position. Cause for the periodicity might be fluctuation in the amount of sand supplied to the beach shoreface, fluctuation in wave and tidal conditions, or the process may occur under conditions of more or less uniform rate of supply and oceanographic conditions. The writers favor the latter explanation: that rate of supply and wave and tidal conditions were approximately uniform throughout the period of formation of the entire strand plain. After formation of each successive new beach ridge, the process started over again after a sufficient supply of sand was brought into the area either by longshore transport from the rivers, or bv reworking of older sediments on the shallow continental shelf. The shoreface was then built seaward to provide a base for the formation of a new longshore bar, and the new bar was built to above sea level. This in effect creates a self-regulating cyclic process, independent of external periodic influence. Age of the oldest abandoned beaches and development of the strand plain is not yet known. The only radiocarbon date available suggests that sea level came approximately to its present position 3,000-5,000 years ago. Assuming a uniform and regular rate of formation, an average of 12-20 years were required for the formation of each ridge. It is known, however, that long term periods of change of conditions and interruption of the regular sequence did occur, and the entire coastline was reoriented to produce discontinuities. (Fig.2b). Another major reorientation of the coastline may be occurring at the present day, because the coastline is locally eroding or trans-

REGRESSIVE LITTORAL SAND OF COSTA DE NAYARIT

81

gressing landward with consequent better development of the present beach ridge. The problems of ages of the ridges, rate of formation, and position of relative Late Holocene sea level in this area cannot be given final answers until further information has been acquired by radiocarbon dating.

SIGNIFICANCE OF REGRESSIVE SANDS

Factors controlling the processes of transgression and regression (CURKAY, 1963) are change of relative sea level and net rate of deposition or erosion along the shoreline. Inasmuch as a geological record is commonly preserved only under conditions of rising relative sea level (usually subsidence of the basin of deposition), the common alternation between transgression and regression is due to periodic pulsation of the rate of subsidence or periodic fluctuation of the rate of sediment supply. Holocene sediments of the Costa de Nayarit display a transgressive-regressive record. The transgression across the continental shelf occurred as a result of the very rapid rise of sea level following the Late Wisconsin (Wiirm 11) glaciation, interrupted by periods of delta advance or regression. With final slowing or cessation of the rise of sea level in the last few thousand years, deposition along the shoreline became dominant, and regression started by the process described. Regressive sands are well known from all geological ages and from all continents. The mechanism of the process of regression should be understood in order to interpret the deposits properly. The process described here is one possible mechanism. but perhaps not the only one. With long continued stillstand of sea level and continued high rate of supply of sand, this deposit could develop to dimensions comparable to those of ancient regressive sands. Alluvial deposition and marsh growth over the top of the sand body will complete the regressive aspect with peat or thin coal measures and an overlying continental shale body.

SUMMARY

The coastal plain of the mainland coast of west central Mexico consists of a strand plain marsh of about 250 parallel abandoned beach-dune ridges. The sand of the individual ridges coalesces to form a sheet sand with a furrowed upper surface. The sand overlies the pre-Holocene-traiisgression flood plain surface of the Rio Grande de Santiago and other small adjacent rivers. The sand is in turn partially covered, especially in the depressions between the ridges, with younger or modern Holocene alluvium. The mechanism of formation of the sand body has been regression after approximate stabilization of Holocene sea level by successive accretion of longshore bars in front of the existing beach. Each successive bar was built to sea level by the rapid supply of sand during periods of low wave intensity, and thereby became the new

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J. R. CURRAY AND D. G . MOORE

beach. The former beach was isolated, and the newly-created lagoon behind the new beach was partially filled by sand. Regressive littoral sand bodies are common in ancient sediments. This is one possible mechanism for their formation. With long continued stillstand of sea level and abundant supply of sand, this deposit could grow to dimensions comparable to those of ancient deposits.

REFERENCES

CURRAY, J. R., 1963. Transgressions and regressions. In: Papers in Marine Geology, Shepard Cotiimemorative Volume. Macmillan, New York, in press. G. H., 1948. An experimental study of submarine sand bars. U.S . Beach Erosion Board KEULEGAN, Tech. Rept. 3: 40 pp. MCKEE,E. D. and STERREIT,T. S., 1961. Laboratory experiments on form and structure of longshore and J. C. OSMOND(Editor), Geometry qf Sandstone Bodies. bars and beaches. In: J. A. PETERSON Am. Assoc. Petrol. Geologists,Tulsa, pp. 13-28. J. R., 1963. Sedimentary framework of the drowned Pleistocene delta of MOORE,D. G. and CURRAY, Rio Grande de Santiago, Nayarit, Mexico. In: L. M. J. U. VAN STRAATEN (Editor), Delfoic and Shallow Marine Deposits. Elsevier, Amsterdam, pp. 275-28 1. SHEPARD, F. P., 1950 Longshore-bars and longshdrktroughs. U. S . Beach Erosion Board Tech. Mem. 15: 32 pp.

UBER BIOGEOCHLMISCHE UMSETZUNGEN IM FRUHSTADIUM DER DIAGENESE’ EGON

T.

DEGENS

Cali‘jomia Institute of Technology, Pasadena, CalF (U.S.A.)

EINLEITUNG

Nur ein geringer Prozentsatz der im Verlauf geologischer Zeiten erzeugten organischen Substanzen hat sich abgelagert und ist in den Sedimenten erhalten geblieben. Der uberwiegende Teil ist als Nahrung biologisch verwertet worden oder aber wurde im Verlauf der Diagenese biogeochemisch umgesetzt. Bereits im Friihstadium der Diagenese vollziehen sich grundlegende Veranderungen in der biochemischen Struktur organischer Substanzen. Dieser erste Abschnitt der Diagenese, welcher den Zeitraum zwischen Absterben des Organismus und dem Ausklang rnikro-biologischer Tatigkeit im unverfestigten Sediment umfasst, modifiziert das organische Spektrum in grundlegender Weise. Unter dern Begriff “Friihdiagenese” fallen dabei alle biogeochemischen Vorgange, die sich wahrend Transport, Ablagerung und irn Bereich der ersten Sedimentmeter vollziehen. Die physikalisch-chemischen Reaktionen, die das organische Material irn weiteren Verlauf der Diagenese verandern, sind, gernessen an den friihdiagenetischen Prozessen, in ihrer Auswirkung n i!r minimal. Im wesentlichen handelt es sich dabei nur noch um eine langsame “Geochemische Reife” der primar oder friihdiagenetisch fixierten Anlagen. Kondensationen bzw. Polymerisationen, Inkohlungsprozesse, Anreicherung durch Migrationsvorgange und andere Phenomene mehr zahlen zu derartigen diagenetischen Maturationen. Was die Verteilung organischer SubstanZen in Oberflachenwassern und Sedimenten angeht, so sind wir relativ gut inforrniert hinsichtlich der absoluten Mengen (total organic matter), die in diesem oder jenem Wasser oder Gestein zu erwarten sind. Eine iibersichtliche Darstellung der in der Literatur vorliegenden Ergebnisse vermitteln die Arbeiten von GOLDBERG (1961), EMERY (1960) und HUNT(1962). Dagegen liegen nur wenige Angaben uber die Art und Verteilung biochemischer Molekiile in Sedimenten und Wassern vor. Folglich fehlen auch viele Voraussetzungen fur die genaue Auswertung und Deutung biogeochernischer Umsetzungen, die sich zweifelsohne im Verlauf der Diagenese am organischen Material vollzogen haben. Die wesentlichen Daten auf diesem Sektor der organischen Geochernie sind in den Arbeiten von VAL-

’ Contrib. no. 1116, Division of Geological Sciences.

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E. T. DEGENS

LENTYNE (1957), ABELSON (1959), EMERY (1960), RITTENBERG et al. (1963) und DECENS et al. (1963a, b) zusammengefasst.

UBERBLICK

Nachvorsichtigen Schatzungen (HUNT,1962;WEEKS,1958) sind insgesamt 3,8 . IOI5 t organischer Substanz in Sedimenten fixiert; davon entfallen auf tonige Absatze 3,5 * 1515t ; der Rest ist an Sandsteine und Karbonate gebunden (0,3 1015 t). Die iiberwiegende Menge der im Sediment konservierten Biomasse ist dabeimit feinklastischen Flachmeerabsatzen assoziiert. Hier liegen die organischen Substanzen durchweg in fein verteilter Form vor und erreichen Konzentrationen, die sich im Mittel zwischen zwei und drei Prozent bewegen. Organische Konzentrate wie Kohle und Erdol sind mengenmassig demgegeniiber nur unbedeutend. So werden die Kohlenvorrate der Welt von verschiedenen Autoren mit 6 * 10l2 t angegeben, wahrend die des Erdols sogar nur 0,2 10I2 t betragen (WEEKS,1958; HUNT,1962). Mit andern Worten, das Verhaltnis KohleErdol zu feinverteilten organischen Substanzen liegt in der Grossenordnung von 1j500. Wegen ihres hohen Anteils an organischer Substanz in Verbindung mit der weiten geologischen Verbreitung kommt damit den feinklastischen Flachmeerabsatzen fur Untersuchungen auf dem Gebiete der organischen Geochemie besondere Bedeutung zu. Vide noch offenstehende Fragen von akademischen und praktischem Interesse konnen von hier aus rnoglicherweise ihre Antwort finden. Zwei der wichtigsten Fragen betreffen dabei die Evolution der organischen Materie und die Entstehung des Erdols. Um die eine oder andere der auftauchenden Fragen beantworten zu konnen, muss man zunachst einmal die wichtigsten diagenetischen Prozesse und ihre Auswirkung auf das biochemische Spektrum der Sedimente kennen, bevor sich weitreichende geologische Schliisse in dieser oder jenen Richtung ziehen lassen. So hat sich immer mehr herausgestellt, dass ohne eine zufriedenstellende Klarung der Vorgange in den ersten Sedimentmetern, Aussagen iiber fossile organische Spektren im Spekulativen stehen bleiben. Der in der klassischen Geologie allgemeingultige Satz, dass die Gegenwart der Schiissel zur Vergangenheit ist, hat seine tiefere Bedeutung auch fur das Gebiet der organischen Geochemie. Vom Verfasser und seinen Mitarbeitern wurden im Verlauf der letzten Jahre biogeochemische Untersuchungen an marinen Sedimenten und Meerwassern ausgefiihrt, welche die vorliegende Fragenstellung zum Thema hatten. Als Arbeitsgebiet wurden Sedimentationsbecken ausgesucht, die der Kuste Siidkaliforniens vorgelagert sind. Von allem, was wir iiber den geologisch-strukturellenAufbau dieser Becken wissen '(EMERY,1960), ahnelt er weitgehend dem fossiler Geosynklinalraume. Systematisch fentnommene Sedimentprofile aus reduzierendem und oxydierendem Milieu, sowie Wasserproben aus verschiedener Meerestiefe (0-4.000 m) wurden auf eine Reihe biochemischer Substanzen analysiert. Besonders eingehend untersucht wurden Eiweisse und Kohlenhydrate, wahrend die Untersuchungen auf Phenole, Indole, Fette,

-

-

BIOGEOCHEMISCHEUMSETZUNGEN IM FRUHSTADIUM DER DIAGENESE

85

Huminsauren und Nukleinsauren noch in ihren Anfangen stehen. Die ersten Ergebnisse aus diesen Studien sind im nachfolgenden Abschnitt kurt dargestellt. Die Untersuchungen wurden weitgehend ermoglicht durch die Unterstiitzung der National Science Foundation, der American Chemical Society (Petroleum Research Fund) und der American Association of Petroleum Geologists (Research Fund). Den genannten Organisationen sei hiermit gedankt.

DISKUSSION

Der Gehalt des Meerwassers an organischen Substanzen wird durch eine Reihe von Faktoren bestimmt, unter denen die geographische Lage (landfern-landnah), Meerestiefe, Wasserbewegung, Temperatur, und die Menge und Art geloster Nahrsubstanzen eine bedeutende Rolle spielen. Die Durchschnittswerte liegen zwischen 1 und 10 mg/l, doch sind wesentlich hohere Gehalte im kustennahen Flachmeerbereich zu finden (KREY,1961). Zu den wichtigsten organischen Substanzen des Ozeans zahlen neben Phytoplankton, Zooplankton und Tripton (particulate matter) auch die im Wasser gelosten organischen Stoffe. Die drei erstgenannten organischen Bestandteile gelten allgemein als die wichtigsten Lieferanten fur die organischen Substanzen, die in das marine Sediment eingehen, wahrend den gelosten Stoffen nur wenig Bedeutung in dieser Richtung beigemessen wird. Die organische Trockensubstanz des marin erzeugten Planktons besteht im wesentlichen aus Eiweiss, Kohlenhydraten und Fetten in einem durchschnittlichen Verhaltnis von etwa 50/30/20. Pigmente sind demgegeniiber quantitativ unbedeutend; die irn kontinentalen Pflanzenreich so dominierende biochemische Gruppe der Lignine fehlt im allgemeinen ganzlich. Das in Suspension befindliche organische Material (Detritus) wird in seiner Hauptmasse als abgestorbenes Plankton angesehen, welches sich in verschiedenen Stadien der Zersetzungbefindet. Die bei Stoffwechselumsetzungen und organischem Zerfall freiwerdenden Molekiile werden vom Meerwasser aufgenornmen und bilden einen wesentlichen Bestandteil der sich in Losung befindlichen organischen Stoffe. Der organische Detritus sinkt allmahlich zum Meeresboden und liefert das Ausgangsmaterial fur die sich im Sediment langsam konsolidierenden organischen Substanzen. Das ist in grossen Ziigen das Konzept iiber Herkunft, Chemismus und Diagenese organischer Substanzen wie wir es aus der Literatur kennen. Diese Vorstellung ist in vielen Punkten sicherlich richtig, in einigen jedoch bedarf sie entschieden einer Neuformulierung. Korrekt sind die Angaben iiber die biochemische Zusammensetzung des Planktons; die Ansichten iiber organische Sedimentation und Diagenese sind hingegen zum Teil revisionsbediirftig. Anhand der Verteilung von Aminosauren, Zucker, Phenolen und anderer biochernischer Substanzen im Meenvasser und rezenten marinen Sedimenten wird nachfolgend ein Konzept entwickelt, das Details iiber den Ablauf biogeochemischer Umset-

86

E. T. DEGENS

zungen im Friihstadium der Diagenxe vermittelt und ergaiizend Aussagen iiber Art und Ursprung organischer Substanzen in marinen Flachmeersedimenten zulasst. In Abb. 1 ist das Aminosaurespektrum von Plankton, Tripton, gelosten Substanzen und marinen Sedimenten dargestellt. Aus Vergleichsgrunden sind die Materialien dem gleichen Sedimentationsraum entnommen (offshore California). Zur besseren Ubersicht wurden die einzeliien Aminosauren gruppenweise zusammengefasst (basisch, sauer, neutral, aromatisch); wegen zu geringzr Konzentration blieben schwefelhaltige Aminosauren (Cystin, Methionin) unberucksichtigt.

lob

A ROM AT I SC H

SAUER

Phenylolonin

Fl Volln __

Plankton

NEUTRAL

Leucin und lsoleucin

0

Ozeon - qebunden

Threonin

m

Ozeonfrei

Prolin

0

Sediment

~~

Abb.1. Art und Verteilung von Aminosiuren in ( I ) Plankton, (2) Ozean, gebunden, (3) Ozean, frei und ( 4 ) rezentern Sediment (Mol. yo).

Aus der Art und Verteilung der Aminosauren lassen sich verschiedene Schliisse ziehen. Zunachst ergibt sich die interessante Tatsache, dass Substanzen, die dem Meerwasser nahezu oder vollig fehlen, im Sediment erheblich angereichert sind, doch auch eine Verarmung an bestimmten Aminosauren im Sediment und eine entsprechende Anreicherung im Ozean ist zu beobachten. Die biochemischen Wechselbeziehungen, welche sich daraus ergeben, lassen sich am besten anhand ausgesuchter Beispiele veranschaulichen:

BLOGEOCHEMISCHE UMSETZUNGEN IM FRUHSTADIUM DER DIAGENESE

87

Arginin, Ornithin und Asparaginsaure stehen durch den sogenannten “Harnstoff Zyklus” miteinander in Verbindung. In Anwesenheit von Arginase spaltet sich Arginin enzymatisch in Ornithin und Harnstoff auf. Die Armut des Ozeans an Arginin und sein Reichtum an Ornithin und Harnstoff (DEGENS et al., 1963a), mag so seine Erklarung finden. Andererseits lasst sich aus Ornithin und Asparaginsaure iiber verschedene Zwischenprodukte wie Citrullin, das Arginin biologisch erzeugen. Bakterien und andere im Sediment aktive Organismen scheinen demnach das im Meerwasser in freier Form vorliegende Ornithin zusammen mit der Asparaginsaure zu verwerten, wobei Arginin als Endprodukt auftritt. Wasserproben, die dem unmittelbaren Kontakt Ozean-Sediment entstammen (Wassertiefe 3.800 m), zeigen gegeniiber hohergelegenen Wassern [Wassertiefe 0-3.000 m) eine auffallige Verarmung an gelosten biochemischen Substanzen. Diese Beobachtung spricht dafiir, dass neben Ornithin rnoglicherweise auch andere freie Substanzen einigen Sedimentbewohnern als Nahrung dient. Besonders eindrucksvoll ist der Nachweis des im Sediment neugebildeten P-Alanins, einer Aminosaure, die sowohl dem Plankton als auch den iibrigen organischen Substanzen des Meerwassers fehlt. Das Auftreten von P-Alanin l a s t sich einmal durch eine metabolische Decarboxylierung von Asparaginsaure erklaren, zum andern sind Dipeptide wie Anserin und Carnosin bekannt, die P-Alanin als Baustein enthalten. Es lassen sich noch viele biochemische Mechanismen dieser Art aus dem Aminoslurespektrum ableiten (DEGENS et al., 1963a, b). Einzeln auf sie einzugehen ist aber im Rahrnen dieser Arbeit nicht moglich. Im Ganzen jedoch sprechen sie dafiir, dass das abgestorbene marine Plankton, welches den Meeresboden letztlich erreicht, von den Sedimentbewohnern nahezu vollkommen verspeist wird, wobei de novo Eiweisse anderartiger Arninosaure-Zusammensetzung erzeugt werden. Ein ahnliches Bild ergibt sich auch fur die Kohlenhydrate. Die im Meerwasser so reichlich vorhandene Glukose, Mannose und Galaktose wird im Sediment gewissermassen “biometasornatisch” nach Rhamnose, Arabinose, Xylose und Ribose verdrangt. Die im Sediment vorliegenden Zucker sind damit weitgehend in situ gebildet worden. Stellt man abschliessend eine Bilanz der Proteine und Kohlenhydrate auf, so ergibt sich die interessante Feststellung, dass die von Meeresorganismen im Ozean gebildeten Substanzen von den Sedimentbewohnern biologisch verwertet werden. Diese Umsetzungen erfolgen schneller und vollstandiger unter oxydierenden als unter reduzierenden Sedirnentationsbedingungen. Die Sedimentationsrate ist hierbei ebenfalls ein entscheidener Faktor: Die im Sediment neugebildeten Eiweisse und Kohlenhydrate haben wie das Plankton im Ozean nur eine kurze Existenzdauer. Als Beweis dafur sollen die biogeochemischen Analysen von RITTENBERG et al. (1963) dienen, die an Sedimenten der “Experimental Mohole” ausgefiihrt wurden. Das Mohole-Profil [ 170 m) urnfasst den stratigraphischen Zeitabschnitt vom Miozan bis zur Gegenwart. Bereits nach wenigen Metern Tiefe ist das Aminosaure-und Zuckerspektrum, wie es von den Sedimentbewohnern erzeugt wurde, ganzlich verandert. Auch mengenmassig gehen die Werte auf

88

E. T. DEGENS

das zehn- bis zwanzigfache zuriick. Hochst verwunderlich wirkt zunachst die Alinlichkeit mit dem biochemischen Spektrum des Triptons (particulate matter). Bevor eine Deutung dieses Phenomens iiberhaupt moglich ist, muss der biostrukturelle Aufbau der Materie im Wasser und Sediment bekannt sein. Es geht dabei ausschliesslich um die Frag:, ob die Aminosauren und Zucker in freier (molecular dispersed) oder gebundener Form in den verschiedenen Medien vorliegen. Im letzteren Fall ist auch die Bindungsart von Interesse. In rezenten Sedimenten liegt das Verhaltnis von freien zu gebundenen Substanzen in der Grossenordnung von 11250. Bei den gebundenen Substanzen der obersten Sedimentmeter handelt es sich in1 wesentlichen um Proteine, Peptide und Polysaccharide. Mit zunehmenden geologischen Alter werden diese weitgehend eliminiert. Die dem Sedinimt verbleibenden Zucker- und Aminosaurekomponenten sind entweder adsorptiv gebunden (z.B. an Tone oder Kerogen) oder aber liegen als nicht-eiweiss bzw. nicht-kohlenhydratartiger organischer Komplex vor (DEGENS et al., 1963a, b). Besonders b-fahigt zu einer derartigen Komplexbildung sind Humussubstanzen. Die Synthese von HLimin-Aminosaure Polymeren, die chemisch den Humusextrakten von Boden ahneln, ist SWABY (1956-1958) gelungen. Die organisshe Substanz der untersuchten Flachmeersedimente besteht aus 30-50 % sogenannter Huminsauren, wobei darunter die Substanzen verstanden werden, die sich mit 0.3 N NaOH aus dem Sediment extrahieren lassen (DEGENSund REUTER, 1962). Die isolierten Humusextrakte ergeben nach Hydrolyse mit 6 N HCI grossere Mengen u.a. an Arninosauren und Phenolen. Nach allem, was wir iiber Natur und Herkunft von Huminsauren wissen, kommen Ligninsubstanzen als ihr g:eigneteste Ausgangsmaterial in Betracht. Gleiches gilt auch fur viele Phenole, so wie sie in Boden, Sedimenten und Oberflachenwassern auftreten. Es s:i in diesem Zusammenhang erwahnt, dass das Phenolspektrum der untersuchten Sedim-nte und Wasser dem kontinentalen Boden vollig entspricht. Auch die aus terrestrischen und marinen Humusextrakten gewonnenen Phenolsauren unterscheiden sich biochemisch und damit faziell durch nichts voneinander (MORRISON,1958; DEGENS et al., 1963a, b). Es wurde anfangs erwahnt, dass Lignin am Aufbau mariner Organismen so gut wie nicht beteiligt ist. Die im Sediment nachgewiesenen Humusextrakte entstarnmen daher aller Voraussicht nach iiberwiegend dem Kontinent. Beriicksichtigt man zudem die Aminosaure- und Zuckerspektren in den Sedimentabschnitten unterhalb der Zone rnikrobiologischer Tatigkeit, so sprechen alle Anzeichen dafiir, dass die erhaltungsFahigen organischen Substanzen grosstenteils kontinentalen Ursprungs sind. Das terrestrische organische Material gelangt in die marinen Sedimentationsbecken entweder in geloster Form oder via Detritus, wobei besonders feinklastisches Material wie Tone als Tragersubstanzen fungieren konnen. Tone sind ausserdem befahigt, das sich in Losung befindliche organische Material adsorptiv aufzunehmen. Das im Meerwasser als Tripton vorliegende organische Material l h s t sich als eine komplexe Mischung zwischen marinen und terrestrischen biochemischen Substanzen auffassen. Eine mechanische Trennung beider Arten ist bisher nicht gelungen. Die

BIOGEOCHEMISCHEUMSETZUNGEN IM FRUHSTADIUM DER DIAGENESE

89

quantitativen Anteile lassen sich in etwa “biochemisch” abschatzen, kennt man die durchschnittliche Verteilung von Aminosauren in Boden und im Plankton. Tripton weist biochemisch starkere kontinentale als marine Ziige auf (70-90 %). Diese Ergebnisse werden weitgehend unterstiitzt durch Kohlenstoff-lsotopenanalysen. Bekanntlich liegt das 613C mariner Organismen zwischen -80 und -17, relativ zum PDB I Standard (CRAIG,1953, 1957)’. Fur terrestrisches Material liegen die Werte generell zwischen -20 und -30. Auf die biologischen und physikalischchemischen Fraktionierungsvorgange, welche diese Isotopenunterschiede zwischen marinen und terrestrischen Organismen hervorrufen, sol1 hier nicht naher eingegangen werden; Einzelheiten sind den Arbeiten von CRAIG(1953, 1957) und VOGEL(1959) zu entnehmen. Die aus dem Flachmeerbereich (San Diego Becken) isolierten Huminextrakte haben ein 613Cvon -22. Dieses Ergebnis stimmt mit dem ECKELMAN’S et al. (1962) uberein, die auf Grund umfangreicher Kohlenstoff-Tsotopenanalysenan rezentem und fossikrn organischen Material zu dem Schluss gelangten, dass die seit paliiozoischen Zeiten in marinen Geosynklinalraumen abgesetzten bzw. erhaltenen organischen Substanzen wahrscheinlich dem Kontinent entstammen. Damit ergeben sich vollig neue Aspekte f i r viele wissenschaftliche und angewandte Fragen in der Geologie. Beispielsweise finden - nimmt man fur Kohlenwasserstoffe ein terrestrisches Ausgangsmaterial an - die leichten Kohlenstoffisotopenwerte fur Petroleum ihre Erklarung (SILVERMAN und EPSTEIN,1958; ECKELMANN et al., 1962). Ob dabei die aus rezenten Sedimenten bekannten Kohlenwasserstoffe im C,,-,, Bereich (MEINSCHEIN, 1961) bereits in kontinentalen Boden entstanden sind oder aber in marinen Sedimenten aus terrestrischen organischen Substanzen im Verlauf der Diagenese hervorgegangen sind, ist dabei nur von sekundarem Interesse. Auch das oftmals sehr hohe 14C-Alter rezenter mariner Sedimente (EMERY,1960) ist moglicherweise durch eine Zumischung toter oder wenig I4C-aktiver terrigener Substanzen bedingt. Der hohe Gehalt an “Humus” in feinklastischen Flachmeerabsatzen in Verbindung mit seiner Kondensationsfahigkeit mit andern Substanzen wie Aminosauren, Zuckern, Nukleinsauren, Fettsauren, Phenolen u.a., dem relativ geringen Nahrwert und der oftmals sogar bakteriziden Wirkung, ein Ergebnis seines phenolischen Charakters, machen ihn zu dem gegebenen Ausgangsmaterial fur Kerogen. In diesem Zusammenhang werden auch die Beobachtungen von HUNT(1 962) und GEHMAN (1962) iiber die unterschiedliche Verteilung von organischen Substanzen in Schiefern und Karbonaten besser verstandlich. Rezente Tone und Kalke enthalten im Durchschnitt etwa ‘2-3 % organische Substanz. Wahrend in Schiefertonen dieser Wert beibehalten wird, betragt er in fossilen Kalksteinen und Dolomiten nur noch 0,1-O,3 %. Untersucht man daraufhin die Art der biochemischen Substanzen im rezenten Kalkmaterial, so stellt man fest, dass sie zu iiber 90 % aus Eiweiss und zu 5 % aus Kohlenhydraten bestehen; Pigmente und Huminsauren sind demgegenuber nur in Spuren vertreten. Protein- und Kohlenhydratsubstanzen sind dagegen in rezenten Tonen mit 613C ist die Abweichung des 13C/12CVerhaltnis in

o/Iw

relativ zu einern Standard.

90

E. T. DEGENS

weniger als 10 % a m Aufbau der organischen Materie beteiligt. Diese Tatsache, dass Humusstoffe den grossten Prozentsatz der organischen Substanzen in feinklastischen Sedimenten ausmachen, darf als der Crund angesehen werden, dass Tone ihren organischen Bestand wahrend der Diagenese zwar verandern aber doch weitgehend als solchen verhalten, wahrend Karbonatgesteine ihn in grosserem Umfang verlieren.

ZUSAMMENFASSUNG

Die wichtigsten Ergebnisse, die sich im Verlauf der biogeochemischen Untersuchungen an Wassern und Sedimenten siidkalifornischer Meeresbecken abgezeichnet haben, lassen sich wie folgt festhalten: (I) Die Verteilung van Aminosauren und Zuckern im marinen Plankton ist grundlegend verschieden von dem im ozeanischen Detritus und Flachmeersediment. (2) Die im Ozean gebildeten organischen Substanzen werden weitgehend von den Sedimentbewohnern konsumiert bzw. biochemisch umgesetzt. (3) Die im Sediment de novo erzeugte organische Substanz (Aminosauren und Kohlenhydrate) wird innerhalb der ersten Sedimentmeter biologisch verwertet. (4) Die nach Ausklang mikro-biologischer Tatigkeit demim Sediment verbleibenden Aminosauren und Zucker ahneln in ihrer Zusammensetzung weitgehend denen des ozeanischen Detritus, der seinerseits kontinentale Charakterziige aufweist. (5) Extrahierbare Humussubstanzen sind mit 30-50 % am Aufbau der organischen Fraktion mariner Flachmeersedimente beteiligt. Lignine kontinentaler Herkunft werden als ihr eigentliches Ausgangsmaterial angesehen. Melaninartige Produkte mariner Herkunft mogen zwar zur Huminsare-Fraktion der Sedimente beitragen, jedoch wird dieser Beitrag aus biogeochemischen Uberlegungen als relativ gering bet rac h tet . (6) Aminosauren, Phenole, Kohlenhydrate und andere biochemische Produkte sind chemosorptiv an die Humussubstanzen gebunden. Sie gehen miteinander moglicherweise spezifische Polymere ein von der Art, wie sie z.B. in Melanin oder Melanoidin Strukturen vorliegen. (7) Derartige Substanzen bilden ein potentielles Ausgangsmaterial fur Kerogen. (8) Das leichte Kohlenstoff-Isotopenverhaltnis der Humusfraktion in Verbindung mit seinem biochemischen Spektrum spricht fur seine iiberwiegend kontinentale Herkunft. (9) Das im marinen Milieu erzeugte organische Material (Plankton) ist reich an Nahrstoffen. Dies fiihrt dazu, dass viele Meeresorganismen und Sedimentbewohner weitgehend oder ausschliesslich von dem planktonischen Substrat leben. Terrestrischer Detritus ist demgegeniiber nahrstoffarm. Diese beiden Faktoren fiihren letzlich dazu, dass die marin erzeugten organischen Substanzen im Verlauf der friihen Diagenese bevorzugt abgebaut werden, wahrend ihre kontinentalen Vertreter dieses Stadium relativ unbeschadet iiberdauern. (10) Unterschiede im Totalgehalt organischer Substanzen zwischen fossilen Kar-

BIOCEOCHEMISCHE UMSETZUNGEN I M FRUHSTADIUM DER DIAGENESE

91

bonaten (niedrig) und Schiefertonen (hoch) sind biochemisch bedingt. Rezente Karbonate enthalten uberwiegend nahrstoffreiche und biologisch leicht zugangliche marin-gebildete Eiweisse, wahrend Tone vornehmlich nahrstoffarme terrigene Humussubstanzen fiihren.

SUMMARY

Organic substances in sea waters and recent marine sediments offshore California are significantly altered during the early stages of diagenesis. Kind and distribution pattern of biochemical materials such as amino acids, sugars, phenols, indoles, and humic acids are indicative of certain reaction mechanisms going on during transportation, deposition, and within recent sediments. They furtheron permit some statements as to the principal source of the organic matter in terms of marine versus continental.

LITERATUR

ABELSON, P. H., 1959. Geochemistry of organic substances. In: P. H. ABELSON (Editor), Researrhes in Geochemisfry.Wiley, New York, pp. 79-103. CRAIG,H., 1953. The geochemistry of the stable carbon isotopes. Ceochinr. Cosmochini. Acta, 3 : 53-92. CRAIG, H., 1957. Isotopic standards for carbon and oxygen and correction factors for inass-spectrometric analysis of carbon dioxide. Ceochini. Cosmochim. Acta, 12 : 133-149. DEGENS, E. T., REUTER, J. H. and SHAW,K. N. F., 1963a. Biochemical compounds in offshore California sediments and sea waters. Geochim. Cosmochirn. Acta, im Druck. DEGENS, E. T., EMERY, K. 0. and REUTER, J. H . , 1963b. Organic materials in Recent and ancient sediments. 111. Biochemical compounds in San Diego Trough, California. N e w s Jahrb. C ~ o l . Palaontol., Monatsh., 1963, im Druck. DEGENS, E. T. and REUTER, J. H., 1963. Analytical techniques in the field of organic geochemistry. Proc. Intern. C o t y r . Ocyanic Geochem., Milan, 1962, im Druck. ECKELMANN, W. R., BROEKER, W. S., WHITLOCK, D. V. and ALLSUP,J. R., 1962. Implications of carbon isotopic composition of total organic carbon of some recent sediments and ancient oils. Bull. Am. Assoc. Petrol. Geolqyists., 46 : 699-704. EMERY, K. O., 1960. The Seaofsouthern California: A modern Habitat ofPetroleuni. Wiley, New York, 366 pp. GEHMAN JR., H. N., 1962. Organic matter in limestones. Ceochim. Cosmochini. Acfa,26 : 885-897. GOLDBERG, E. D., 1961. Marine geochemistry. Ann. Rev. Phys. Chem., 12 : 2 9 4 8 . HUNT,J. M., 1962. Geochemical data on organic matter in sediments. Proc. Intern. Sci. Oil Conf., Budapest, 1962, im Druck. KREY,J., 1961. The balance between living and dead matter in the oceans. Pitbl. An?. Assoc. Advari. Sci., 61 : 539-548. MEINSCHEIN, W. G., 1961. Significance of hydrocarbons in sediments and petroleum. Geochim. Cosniochim. Acta, 22 : 58-64. MORRISON, R. I., 1958. The alkaline nitrobenzene oxidation of soil organic matter. J . Soil Sci.. 2 : 13&140. RKITENBERC, S. C., EMERY, K. 0..HULSEMANN, J., DEGENS, E. T., FAY,R. C., REUTER. J . H.. GRADY, J. R., RICHARDSON, S. H. and BRAY.E. E., 1963. Biogeochemistry of sediments in Experimental Mohole. J. Sediment. Petrol., 33 (1). SILVERMAN, S. R. and EPSTEIN,S., 1958. Carbon isotopic compositions of petroleums and other sedimentary organic materials. Bull. Am. Assoc. Petrol. Geologists, 42 : 998-1012.

92

E. T. DEGENS

SWABY, R., 1956-1958. Soil organic Matter. 8th, 9th, lOrh C.S.I.R.O., Annual Reports. Government Printers, Sydney, Australia. VALLENTYNE, J. R., 1957. The molecular nature of organic matter in lakes and oceans, with lesser reference to sewage and terrestrial soils. J . Fisheries Res. Board, Can., 14 : 33-82. VOGEL,J. C., 1959. Uber den Isotopengehalt des Kohlenstoffs in Siisswasserkalkablagerungen. Geochini. Cosmochini. Acra, 16 : 236-242. WEEKS, L. G.. 1958. Habitat of oil and some factors that control it. In: L. G. WEEKS(Editor), Habitat of Oil. Am. Assoc. Petrol. Geologists, Tulsa, pp. 1-61.

ORIGIN AND TRANSPORT OF MUD (FRACTION < 16 MICRONS) IN COASTAL WATERS FROM THE WESTERN SCHELDT T O THE DANISH FRONTIER A . J. D E G R O O T

Institute for Soil Fertility, Groningen (The Netherlands)

INTRODUCTION

The conclusions dealt with in this paper are partly based on analyses of bottom samples of newly deposited niuddy or clayey material (fractions < 16 p). The samples were taken from fresh, brackish and salt parts of estuaries, at different levels with regard to the water level and hence in different environments as to vegetation, groundwater conditions etc., and from similarly varied places on tidal flats and young salt marshes in and along the Dutch and German Wadden Sea. The analyses are compared with others from older marsh deposits, outside the dikes and in polders, that have been embanked within the last 100 years. In total, several thousands of analyses were carried out. No samples of sufficient size could be obtained from material suspended in the coastal waters. A detailed account of the investigations mentioned in the present paper can be found in the author’s thesis (DE GROOT, 1962).

THE MANGANESE SITUATION IN THE EXAMINED DEPOSlTS

A main division of the Mn compounds in the investigated sediments can be made into exchangeable Mn and higher oxides. The exchangeable Mn is bound as manganous ions to the soil colloids and is the most mobile form. The higher oxides form a sequence of compounds with varying reactivity. They can be distinguished by their oxidizing power with respect to reducing agents. The conversions between these Mn forms may be schematized as follows:

c

~- -

Mn2+ exchangeable

~

-

-

~-

~

~

~

~

MnO, --+ easiiy reducible

__

_

_

__

_

_

_

_

I ~

MnO, rather inert

The redox equilibrium between the exchangeable and easily reducible Mn is determined by the pH and the oxygen concentration in the sediment. The oxidation of bivalent Mn, as indicated above, can take place by purely chemical

_

-

~

94

A. J. DE CROOT

processes, at pH values above 4.9, whereas microbiological oxidation may occur at pH's ranging from 4.8 to 8.9. During transport in aerated river or sea water the mud cannot lose Mn, because the latter is almost exclusively present as insoluble higher oxides. Most of it is present in M n contents e x t r a p o l a t e d t o l 0 O 0 / o o f t h e f r a c t i o n <16microns

3 , 4 0 0 - p . p . m . of t o t a l 3,200

Mn

-

3,0002,8 00 2.600-

2.463

-

2,200.

2,0001.800 -

1.6 00 -

1.4001.2 00 1.000-

800600-

Ohof f r a c t l o n < 1 6 m i c r o n s

0

Sediments o f t h e Ems Sediments of t h e Flemish Banks

Fig.1. Linear relationship between Mn content and percentage of fractions

< 16 p.

ORIGIN AND TRANSPORT OF M U D IN COASTAL WATERS

95

the grain size fractions of 0-35 p, as coatings around clastic particles or attached to clay flakes or as individual particles. In connection with this preferred occurrence in the fine fractions, a linear relationship is found to exist between the contents of Mn (both total and reducible Mn) and the fraction < 16 p (in % of the CaC0,-free mineral constituents), at least as long as the origin of the sediments and the environment of deposition are the same. This relationship makes it possible to characterize a whole group of co-genetic sediments with different granulometric compositions by a single value. The value, chosen in the present study, is the Mn content (in p.p.m.) obtained by extrapolation of the relationship to 100 % of the fraction < 16 p (see Fig.1). After deposition, reduction can take place to a smaller or larger extent, depending on the redox potential of the environment, followed by loss of Mn by way of the exchangeable form. I n deposits of marshes and polders lying above the ground water table, this removal probably takes mainly place by drainage of percolating water. It is less obvious in what manner the Mn is carried off from sediments with very low redox potentials on mud flats. Possibly slow movements of the pore water, and diffusion of Mn ions are at least partly responsible. The degree and the velocity of this removal, in conditions of low redox potential, depends on the salt content, the salt ions dissolved i n the pore water tending to replace the Mn attached to the sediment particles. In fresh water environments the Mn content decreases only very slowly. Notwithstanding this loss of Mn, the relationship between Mn contents and percentages of the fraction < 16 p remains approximately linear, so that the characteristic values (for 100 % of the fraction < 16 p) can still be determined in the same way as for freshly deposited material. In areas with sediments coming from the same source, thesevalues are therefore more or less indicative of the degree of ageing. In general, salt marshes yield lower values than the forelying tidal flats, while the values for embanked polders are distinctly lower again than those for the marshes (see Fig.3-5).

CHOICE OF REPRESENTATIVE SAMPLES

For drawing conclusions about the origin and the direction of transport of mud on account of the Mn contents, only a part of the investigated samples could be used. For the tidal flats, the choice of these samples had to be limited to the most recently deposited material (not older than a few days), which was scraped from the surface with a knife. Even so it was found that some values were abnormally low, e.g., those obtained from muds that were deposited after heavy storms, during which much older material in the areas had been resuspended. In other cases the variation of the Mn contents was too large to allow calculation of a representative linear relationship from which a characteristic value could be extrapolated. This variation may probably be attributed t o the mixing of freshly supplied mud from the rivers or the sea, with various locally reworked older materials. In general, only the highest values in each area could be considered as significant in this respect.

96

A. J. DE GROOT ORIGIN AND TRANSPORT OF MUD

The origin of the mud deposits in Zeeland, investigated along the Western and Eastern Scheldt estuaries appears to be twofold. A minor part is supplied by the river Scheldt, the major part coming from the North Sea (Fig.3). The Scheldt river mud has the higher Mn content of the two. Its sedimentation is mainly confined to the eastern part of the Western Scheldt. The river influence decreases towards the west. Its relatively small importance is especially evident when the Mn contents in the muds of the Western and Eastern Scheldt are compared (Zuid Sloe > Noord Sloe; Kreekrakpolder > Hogerwaardpolder). The precise origin of the mud carried from the North Sea into these estuaries is not y:t certain. One possible source is the waters of the British Channel, entering the North Sea through the Straits of Dover. Another source, which has been suggested in the literature on this subject, could be the mud layers in, and in front of the mouth of the Western Scheldt (Flemish Banks and Wielingen), which have Mn contents lying not very far below those of the Zwarte Polder area, the Zuid Sloe and the Noord Sloe. However, the circumstance that their Mn contents (1092) are still rather high, notwithstanding the relatively high salinities of the water, may indicate that these mud

Fig.2. Water movements in the North Sea, according to KALLE(1937).

ORIGIN A N D TRANSPORT OF M U D IN COASTAL WATERS

97

T h e sedimentation a r e a of the southwestern Netherlands

p/ I.

"5 \

-

~~

o

5

k

m

layers are not old at all, but that they were deposited recently. Consequently, the material of the Flemish Banks and the Wielingen looks rather like another deposit of the same mud supplied via the Straits of Dover. By investigating the sediments of the fresh water tidal delta of the Biesbosch (Fig.2) it was found that little difference exists between the Mn contents of the mud supplied by the Rhine and those of the Meuse. Approximately the same values were obtained for the Haringvliet and the Brielse Gat, whereas in the Grevelingen the value is lower. The latter is probably the result of mixture with mud supplied from the south. The mud transported into the western Wadden Sea, and deposited along the shore of Friesland (Fig.4) has the same value. This inay point to a direct transport of material from the Rhine and the Meuse, along the coast of South and North Holland in northeasterly direction (2 on Fig.2), an interpretation which seems to be corroborated by analyses of mud in the harbour of IJmuiden. The lower Mn contents off the shore of Groningen inay be partly due to temporary deposition in the Wadden Sea, followed by further transport in eastward direction over the Wadden Sea flats. A similar decrease is found when the analyses of the young

98

A. J . DE CROOT .

!

I

The Dutch Wadden coast. t h e Dollard and t h e

'

.

. . .................. 2.

Leybocht

I

I

I

. . . .

/

. . . . . . . . .e;' . . . ... . . .. .. . .. .. . ..... .. . . . . . . . .

-d

. .

. ..:.

Friesland

5

0

I0

I5

20

25Lm

~

Fig.4. For explanation. see Fig.3.

polder deposits of Friesland and Groningen are compared with each other. The Mn contents of the mud carried seaward by the river Ems are considerably higher than those of the Meuse and Rhine sediments (Fig.4). Passing along the shores of the Dollard from the Enis mouth via the southern part to the Punt van Reide ( R ) , this influence is seen to diniinisli gradually. Except for the Punt van Reide itself. the Mn contents i n the Em-Dollard area are everywhere higher than those of comparable deposits in Friesland and northern Croningen, thus clearly showing the significance of the Ems for the sedimentation in the Dollard. The deposits of the reclaimed area of the Leybocht correspond, in Mn content, to the sediments (of comparable degrees of ageing) along the shore of Friesland, which may point to a common origin, viz. out of the North Sea. The mud deposited along the western shores of Schleswig Holstein (Fig.5) has a complex origin. Part of it comes from the Elbe and another part from the older Holocene marine clay beds. eroded on the shallow sea floor in front of Dithmarschen and North Friesland. Other sources may contribute to a smaller degree. The Elbe mud contains more Mn than any other mud investigated. The old clay layers on the sea floor i n front of the coast, on the other hand, have very low Mi1 contents (e.g., Nordstranddanim). The Mn values of the mud deposited at present along this coast therefore depend on the ratio of mixture of the two. The contribution of the Elbe component carried northwards with the sea currents (3 in Fig.2) decreases from the mouth of the Elbe via the Meldorfer Bay to the mouth of the Eider. However, further northward, along the Hindenburgdamm the Mn contents are again very high. The same variations are found in the young polders along this coast. In this connection it is interesting to observe that thc variations in Mn content in the sampled areasoutside the dikes are accompanied by similar changes i n the contents of organic matter. The highest values for organic material in this region were obtained in the deposits along the Elbe. Along the shores of the Meldorfer Bay and the mouth

99

ORIGIN AND TRANSPORT OF MUD IN COASTAL WATERS

of the Eider the contents are much lower (admixture of old marine material, poor in organic matter). The sediments at the Hindenbut-gdamm again show relatively high values. In conclusion it may be remarked that at most only a very subordinate part of the mud deposited along the continental coast can be supplied by the river Thames. This

7 Western coast

of S c h l e s w i q - H o l s t e i n

I I

DENMARK

FRIESLAND

dstranddornrn

1

Weser 0

5

10

15

20

25Lm

Fig.5. For explnnation, see Fig.3.

4'2 8 4

100

A. J. DE GROOT

appears from the low Mn contents of the Thames muds sampled along the shore of Essex. A significant transport of Thames mud across the sea is also unlikely on account of the water circulation in the North Sea (Fig.2). Owing to the flow of water through the Straits of Dover. the waters along the English coast in the vicinity of the mouth of the Thames are carried northeastward, towards the centre of the southern North Sea, rather than across this sea to the shores of Belgium and Holland.

SUMMARY

To determine the contents and character of manganese compounds in Holocene deposits [fractions < 16 p) along the North Sea coast of Holland and Germany, several thousands of analyses have been made. It was found, among other things, that the manganese content is very variable, owing to postdepositional migrations. When this influence is allowed for, regional differences appear to exist, which can be interpreted as the result of primary differences in Mn content of the source material. Using Mn as a tracer, the following conclusions can be drawn about the origin of the mud and the directions of transport along the coast: ( I ) The mud in the Eastern and Western Scheldt is supplied only for a small part by the river Scheldt. The river influence is strongest in the eastern part of the Western Scheldt. Most of the mud deposited in these two estuaries is brought in from the North Sea. Originally it probably comes from the British Channel. (2) The mud of the rivers Rhine and Meuse, which is not deposited in the estuaries, is carried in northeasterly direction. along the coast of South and North Holland and is partly deposited in the Wadden Sea. ( 3 ) Supply of mud by the river Ems is a n important factor for the sedimentation in the Dollard. The contribution of river material is seen to diminish when going along the Dollard shores from the mouth of the Ems via the southern part to the Punt van Reide. (4) Deposition along the western shores of Schleswig Holstein takes place under influence of the supply of mud by the river Elbe and by erosion of older Holocene deposits on the flats in front of the shores of Dithmarschen and North Friesland. ( 5 ) The supply of mud by the river Thames has no visible significance on the composition of the fine grained sediments along the coasts of the continent.

REFERENCES

DE GROOT, A. J., 1962. Mangaantoestand van Nederlandse en Duitse Holocene Seditiaienten it1 Vcrband met Slibtratisport en Bodenyenese. Thesis, Univ. of Utrecht, 205 pp. Also Verslag. Lundboitwk. Onderzoek., 69 (7) : 164pp. KALLE, K., 1937. Nahrstoffenuntersuchungenals hydrographischesHilfsmittel zur Untersuchungvon Wasserkorpern. Ann. Hydrograph. Maritini. Meteorol., 65 : 1-1 8.

FEATURES IN THE HEADS OF SUBMARINE CANYONS NARRATIVE OF UNDERWATER FILM ROBERT

F.

DILL

U S . Navy Electronics Laboratory, San Diqgo, CaliJ ( U . S . A . )

1NTRODUCTION

Geological observations to study the features present in the heads of submarine canyons have been made during the last five years by geologists wearing SCUBA (Self Contained Underwater Breathing Apparatus) equipment. Under-water shots have been made from which two motion pictures were put together. These films, shown at the sixth International Sedimentological Congress, deal with sedimentation and erosion phenomena in the heads of Scripps canyon near La Jolla (Calif., U.S.A.) and of Los Frailes and San Lucas Canyons on the east side of the southern tip of Baja California (Mexico). The Scripps Canyon is cut in Eocene sedimentary rocks, consisting of fine grained sandstones and shales, while the Mexican canyons are cut in hard granite. Notwithstanding the differences in material in which the canyons are cut, the sedimentation and erosion features show nearly complete similarities. In general, it can be said that these submarine canyons start close to the shore line with many small tributaries, with bowl-shaped heads, leading to the branches of the canyons. Sediment that is trapped in the bowl-shaped heads of the tributaries slumps down intermittently, causingerosion on the bedrock bottom and the sides of the tributaries and branches. Often an hourglass shape is the result of the erosion. The upper part of the tributary or branch has a rough surface with many forms of sessile life, while the wider bottom part has smoothed and polished surfaces with such features like truncated burrows, gouge marks, plucking scars, etc., which give evidence of submarine erosion.

SCRIPPS CANYON. CALIFORNIA, U.S.A.

The first sequence shows a diver inspecting one of the most active tributaries leading into the head of the Sumner Branch of Scripps Submarine Canyon. The depth of the water is 8Oft. (24 m). At the time of the photographs the sediment thickness was 21 ft. (6.4 m). The surface sediments in this area continually slump and creep down-canyon

102

R. F. DILL

at an average rate of 2 ft.jmonth at the surface, and approximately I ft./month at the base of the sediment-mat fill. I n areas of slowly moving sediment, erosional features are observed that can only be caused by submarine processes. Smooth polished rock surfaces, truncated burrows of marine organisms, overhanging walls, gougz marks. and plucking scars are some of the primary features giving evidence to this erosion. The second sequence i n Suniner Branch shows the nature o f the canyon at a depth of 100 ft. (30.5 m). The steep-to-overhanging rock walls arz unique to the submarine canyon and have no counterpart in the land canyons of the region. Beneath the haystack-like algal and sediment f i l l of this portion of the canyon the rock walls expand and have an hourglass profile. The surface of the contact zone between the fill and the rock walls is extremely smooth and polished. Material is continually being deposited in this area throughout the year. However, small slumps and slides arc often observed during storm periods which prevent the sediment slopes from exceeding 33-37', the angle of repose of the sediment f i l l in this area. This fill point is also reflected in the eroded rock walls and marks the boundary between active biological erosion and corrasion by the slowly moving sediment-mat. The third sequence shows the same area one month later. The marker stakes that were put in the sediment-mat a month before have moved and tipped down-slope. The surface J f the sediment-mat has a much higher algal content. Gas is bubbling from the fill and is most active between the rock wall-sediment fill contact and in areas of rzcent slump scars. The gas consists of methane and hydrogen sulfide derived from the breakdown of previously buried Algae and sea grasses. The high mica content in the water is shown by a sequence of photographs taken with an underwater light. The almost complete coverage of exposed rock surface by organisms is shown i n their natural color. The pictures were taken at a depth of 95 ft. (29 m) using high speed Ektachrome film and a 2,500 watt color corrected light. A well developed hanging valley enters Sumner Branch of Scripps Canyon at a depth of 55-73 ft. f17-22 m). Exceptionally well developed corrasional erosion features are found in this tributary that grade upward into an area where erosion is dominantly controlled by marine organisms. Observation over a five-year period has shown that the sediment fill of this area is confined to the areas where corrasion is evident. Marker stakes placed in the f i l l are slowly displaced down-slope by slow gravity creep and intermittent slumps. Sediments which slump rarely travel more than 25 ft. (7.6 m) before they reach a stable slope and compensate the stresses that lead to their instability. The inorganic portion of the sediment in this area is a fine sand with a high mica content. I n sttu porosities are between 50 and 55 7:. Relative densities are quite variable and range betwen 1.6-1.9 g/cm3.

BAJA CALIFORNIA. MEXICO

In this region of Baja California a number of large submarine canyons extend u p to

HEADS OF SUBMARINE CANYONS

103

within a short distance of the shoreline. They are cut in hard granite and, like Scripps

Canyon. have features that can only have been caused by submarine erosion. Three expeditions have been made to this region. The deeper portions of the canyons have ( 1963), and SHEPARD (1964). Their results have been studied by BWMAand SHEPARD been correlated with those of the present writer in the shallow nearshore heads of the canyons where diving techniques could be used to photograph and measure the processes active in depths down to 250 ft. (76 m). Two major areas have been studied in detail: (I) Los Frailes Canyon which lies approximately 45 miles northeast of the tip of Baja California, and (2) Sail Lucas Canyon which cuts into the very tip of Cape San Lucas, Mexico. In both areas, stakes were placed in the sediment. It has been observed, by following expeditions, that they move down-slope, and erosion features form by the corrasion of flowing sediment. A transition zone between organically caused surface roughness to an area of relatively smooth rocks has been found only in areas where flowing sand causes corrasion of the older rough surfaces. Niches or eroded “U-shaped” indentures i n the upper lips of the steep canyon walls are often observed where flowing sand falls into deeper portions of the canyon. Many of these niches develop bzlow hanging valleys which are common in most rock-walled submarine canyons.

RIVERS OF SAND, S A N V FALLS

One of the most spectacular submarine phenomena are the “rivers of sand” and “sand falls” that have bzen observed i n San Lucas Canyon. This process was first observed by the late Conrad Limbaugh. Janies Stuart, and Wheeler North i n 1959 durinp an et al., expedition to this area under the direction of Dr. F. P. Shepard (LIMBAUGH 1961). Similar sand flows. but on a much smaller scale, have been observed in other canyons but none can compare with those found at Cape San Lucas. The upper portion of the tributary, shown i n the motion picture, has a broad bowlshaped head. Sand flows slowly down this area during periods of storm when swell inducd currents cut back the beaches and shallow water fill that accumulates slowly during periods of quiescence. The average slope for the sediment fill when at rest is 30”. However, when flowing at a velocity of between 0.1-0.16 knots the slope will increase to as high as 37”. Sediments at rest can be artificially set in motion by diggin? away a part of the fill. Movement continues until a stable slope (approximately 30”) is again established. Velocity is accelerated around objects interfering with normal flow. ln areas of confinement flow has been observed to meander from one side ofthe channel to another. Streaks of dark sand develop where reduced sediment is brought to the surface by turbulence. Objects such as rocks up t G 6 inches (15 cm) in diameter have been observed to be carried by flowing sand. Divers have followed this type of flowing sand to depths of 250 ft. (76 m) and could at this depth see it continuing 011 into deeper water to a depth of at least 350 ft. (1 10 ni).

104

K. F. DILL

Spectacular sand falls develop where there are sharp drop-offs in the canyon’s axial profile. In these areas, the pulsation of the sand movement is readily apparent. In one of the fault controlled tributaries, fresh granite surfaces are exposed where the force of the falling sand has plucked and gouged weak and broken granite pieces from the face of the rock wall. Such surfaces are truly examples of contemporary submarine erosion.

SUMMARY

This paper deals with creep of sediment and rapid flow of sand over the steeply sloping floor of some submarine canyons off California and Baja California. The sediment motion apparently causes scour of the canyon floor and walls. The phenomena were directly observed and filmed by geologists diving in the upper parts of the canyons.

REFERENCES

BOUMA, A. H. and SHEPARD, F. P., 1963. Large rectangular cores from submarine canyons and sand valleys. Bull. Am. Assoc. Petrol. Geologists, in press. LIMBAUGH, C., NORTH,W. and STEWARD, J., 1961. Rivers of Sand (Underwater motion-picture report of submarine sand movement). Dept. of Oceanography, Scripps Institution of Oceanography, Univ. of Calif., San Diego, La Jolla, Calif. SHEPARD, F. P., 1964. Sea floor valleys of the Gulf of California. In: Marine Geology of the Gulfof California- Am. Assoc. Petrol. Geologists, Spec. Publ., in press.

ANCIENT DELTAIC SEDIMENTATION I N EUGEOSYNCLINAL BELTS’ ROBERT H . DOTT

JR.

University of Wisconsin, Madison, Wisc. ( U . S . A . )

INTRODUCTION

Ancient eugeosynclinal belts containing important volumes of volcanic materials are typically regarded as sites of dominantly graded “graywacke” and mudstone deposition. Furthermore, many (if not most) of the sandy sequences are commonly interpreted as having been deposited in relatively deep, quiet marine water by turbidity currents -and doubtless much of it has been. Exceptions to this exist, however, and in significant volumes in most eugeosynclines. The general problem of characterization of geosynclinal sediments has been discussed by KAY(195 1) and VANANDEL(1958). The writer has discussed elsewhere (DOTT, 1961) a few examples of widely varied eugeosynclinal rock types, and several examples are summarized below for illustration. Many others exist, of course. Extensive marine carbonate rocks, including organic reefs, typify the far western North American Permo-Triassic as well as the famous east Alpine Triassic. Even relatively well sorted and pure (i.e., very mature) quartz arenites2 are very important in some areas, for example in the Ordovician of the central Nevada-Idaho eugeosynclinal belt described by KAY(1960), in the Middle Jurassic of central Oregon, and in scattered Mesozoic localities of the Antarctic (Palmer) Peninsula (HALPERN, 1963). Sedimentary environments in eugeosynclines have varied as much as petrographic sediment types. Thick non-marine volcanic and sedimentary sequences occur and are in striking contrast to the more familiar marine ones. A striking example is that of the central Chilean Andes where volcanism was extreme throughout Mesozoic and Cenozoic time. In the earlier Mesozoic, volcanic islands were surrounded by areas of marine deposition (MuRoz CHRISTI,1956). From medial Cretaceous to the present, this region has received enormously thick accumulations of entirely non-marine volcanic and interstratified sedimentary deposits. These possess Late Cretaceous and Tertiary floras and are typified by lavas with colummar jointing, agglomerates, flow breccias, slightly re-worked volcanic conglomerates, tuffs, and tuffaceous sandstones Investigations in Oregon and California are supported in part by funds supplied by the Wisconsin Alumni Research Foundation. Investigations in Chile and Antarctica are supported by the National Science Foundation, Universidad de Chile and Empresa Nacional de Petroleo. Classification of sandstones in this paper employs the terminology of WILLIAM^ et al. (,1954) together with the maturity concepts of FOLK(1951).

106

R. H . DOTT J R .

and shales. This linear volcanic pile lies along the core of the Andean orogenic belt and appears to change along the same axis both north into Peru and south into southern Chile into much less volcanic, marine Cretaceous and Early Tertiary assemblages. Volcanism so exceeded the rate of subsidence in Chile that an unusually large volcanic land was built and persisted above sea level. Yet essential qualities of a eugeosyncline were retained, namely very thick accumulation of volcanic and sedimentary materials in a linear zone. The above examples are cited to indicate that sediment types, depositional mechanisms and environments have all been extremely variable in eugeosynclines. Dorninantly deep marine, shallow marine and even non-marine situations have existed simultaneously in different parts of the same belt and have also succeeded one another within many areas. And successions from deeply submerged to strongly emerged seem to be reversible. This may be influenced by eustatic sea level changes as well as local behavior. In the present paper special attention is devoted to examples of deltaic and littoral sediments in eugeosynclines. Examples discussed are from the Cordillera of the Americas.

OREGON AND CALIFORNIA, U.S.A.

L a t e Cretaceous, Cape Srbastiun urea. Oregon

The Klamath Province of southwestern Oregon and northwestern California contains a familiar type of eugeosynclinal assemblage, namely very immature “graywackes”, mudstone, bedded chert and pillow lavas of Jurassic age. Several intermittent orogenic pulses are recorded therein accompanied by severe faulting, regional metamorphism and emplacement of intermediate, mafic and ultramafic plutons. Very coarse, in part remarkably graded. conglomerates reflect vigorous elevation of irregular land masses i n Late Jurassic and Early Cretaceous. Temporary relative stability was achieved sonietime in the medial Cretaceous, with most of the Cordilleran region above sea level and with a general cessation of volcanism. During Late Cretaceous, local transgression occurred in response to eustatic rise of sea level or epeirogcnic crustal subsidence (or both). In marginal marine regions such as the Cape Sebastian area of the extreme southwestern coast of Oregon, moderately mature littoral sands (feldspathic quartz arenites) and gravels accumulated initially. These possess some medium-scale channeling. cross stratification, ripple niarks and scattered molluscs (Fig. I). They grade u p into a rhythmically alternating sequence of micaceous, less sorted fine sandstone (feldspathic quartz arenite and quartz wacke) and inudstone units. This sequence superficially resembles “turbidites”. but is essentially barren of graded bedding (DOTTand HOWARD. 1962). On the other hand, the thin, fine sandstone units have fine cross laminae throughout their total thickness. Current and load-formed sole markings are common. Mega- and microfossils are rare, but evidence of burrowing activity is abundant.

ANCIENT DELTAIC SEDlMENTATlON I N EUCEOSYNCLINAL BELTS

I07

Fig.1. Relationships of Late Cretaceous strata, Cape Sebastian area, southwestern Oregon, U.S.A. Lower littoral sandstones grade up to top-set deltaic mudstone and sandstone with probable local bar finger sand bodies.

Interstratified within the sandstone-mudstone sequence are many thicker, fine to medium, well sorted sand bodies with conspicuous current structures, chiefly fine cross-laminae, ripple bedding and prominent convolute laminae (Fig. I). However, parallel lamination is also very conspicuous. Grading is totally lacking. Coarse sandstone with larger cross strata and boulder conglomerate lenses characterize some of these masses. Abundant plant debris occurs in scattered laminae that locally are rich enough to be classed as thin, discontinuous coal seams. With exception of a few oyster shell fragments, fauna is totally lacking in these sandstone bodies. The basal sandstone sequence is interpreted as a littoral and near-littoral, very shallow, agitated marine assemblage. The mudstoiie~sandstonesequence is interpreted as representing a shallow (perhaps in part brackish) marine, top-set deltaic environment. Even lamination. abundant tractional current features, impoverished fauna but 1960). The thicker sandstone abundant plant detritus and mica suggest this (SHEPARD, masses within this sequence appear to represent bar finger sands (FISK,1961). They show little scouring at their bases, they are generally concordant, and at the top (and presumably laterally) are gradational with the dominant mudstone and thin, "dirtier" sandstone lithology. Good sorting, abundant fine current lamination. parallel laniination, plant debris, fine mica and lack of fauna also are consistent with this suggestion. Late Eocene, Coos Bql: area, Orqyon

Late Cretaceous stability was short-lived in southwest Oregon, for renewed volcanism, subsidence and transgression produced accumulation of very extensive submarine pillow lavas and Paleocene? to Early and Middle Eocene graded and non-graded

108

R. H. DOTT JR.

“graywackes” (lithic-volcanic wackes) and mudstones. North of the Klamath region, much of the sand was deposited by turbidity currents in relatively quiet, probably deep water (SNAVELY and WAGNER, 1962). General regression began in Late Eocene time, and littoral and deltaic facies shifted progressively northwestward from the Klamath upland. Great delta complexes developed at Coos Bay are characterized by large, prominent lenticular sandstone bodies with complexly channeled basal surfaces and containing angular chunks of contemporaneous, inter-channel laminated mudstones. Contorted stratification developed through gravity mass failures as the channels were cut. Contortion also is present within the channel-fill sands due to both current and gravity deformation of medium and large-scale cross-strata. Convolute lamination is present locally ( D o n and HOWARD, 1962). Current features abound in both topset and channel sediments, but parallel lamination is also very prominent in the very carbonaceous, micaceous and well sorted channel sands. Ripples, grooves, cross-stratification on all scales, contortions, and rolled-up sand balls are characteristic of the entire sequence. Compactional load structures, including sandstone dikes, are well developed as well. Important coal is associated with the higher sandstone bodies. These deposits are interpreted as delta distributary channels with top-set, laminated finer marine interdistributary sequences and, farther landward (south), non-marine inter-channel coal swamp deposits. Bar fingers have not been recognized. A complex delta plain was built of imbricated distributary sands, littoral bars, natural levees and coal swamp deposits that intertongued northward with marine top-set and fore-set deposits. Animal burrow features occur throughout much of the sequence. Foraminifera and some molluscs typify the interdistributary sediments, and sand-beach-dwelTABLE I SUMMARY OF MLNERALOGIC AND TEXTURAL DATA FOR TYPICAL EOCENE SANDSTONES, COOS BAY, OREGON, U.S.A. -

~~

Sample no. ~

~~~

55-1 3 55-14 59-16 59-2 1 59-24 59-25

~~

Qaartz

~~~

Feldspar

~

~

~

Volcanic rock fragments

~~~

Other

MI

r70

-

-

~~~

32“/0 55% 20 Yo 25 % 21 74

28 % 17% 27 % 27% 23

34% 17% 45 % 42 Yo 52%

6% 11% 8%

-

-

-

-

6% 4%

-

0.33m m 0.31 mm 0.12m m 0.36m m

-

1.12 I .03 1.24 1 .oo

_ _

ling molluscs and echinoderms (Dendraster) together with rill structures are found locally in littoral sandstone bodies. Two constructional phases followed by marine destructional phases can be tentatively recognized in the delta complex. Graded bedding is sparingly present in the initial fore-set deposits associated with the lowest channels. Apparently some turbidity current deposition occurred on the fore face and beyond the delta generated by mass failures of sediment as postulated by SHEPARD

ANCIENT DELTAIC SEDIMENTATION IN EUGEOSYNCLINAL BELTS

SUGGESTED

LATE

EOCENE

109

PALEOGEOGRAPHY

Fig.2. Hypothetical paleogeographic map of Oregon and Washington, U.S.A., during Late Eocene Littoral and deltaic environments are shown in relation to land areas and offshore volcanic islands Coos Bay, Oregon is located at the lower left (marked “Coaledo”); Cape Sebastian is farther south near the lower left corner.

(1955) on the Mississippi delta margin. Paleogeographic setting of the Eocene delta complex was rather unique. The Klamath upland with varied rocks lay to the south. Pillow lavas immediately underlie the deltaic sandstones (and may even be partially interstratified with them). Contem-

110

R. H. DOTT J R ,

poraneous volcanic islands existed offshore only 50 miles to the north (Fig.2). The sands of the complex, though mature texturally as to sorting and rounding i n keeping with a deltaic interpretation, are exceptionally immature inineralogically. Most are classed as lithic arenites with a high percentage of volcanic rock fragments (Table I). Associated mudstones contain overwhelming abundance of montmorillonoid minerals, apparently also reflecting an unusually volcanic-rich provenance.

Cretaceous to Holocene, Sacratncnto Vallej7,Culifnniia

During Cretaceous and Early Tertiary time, a long marine embayment occupied much of California. The Klamath highland separated it from the areas of marine deposition i n western Oregon discussed above. According to ANDERSON(1938), a Cretaceous delta-like complex was built at the northwestern end of this embayment and concentrations of sand i n the subsurface suggest at least one other possible deltaic mass on the northeast side. The northwestern example contains important conglomerate 1956) with, in tongues as well as very carbonaceous sandstone and shale (MURPHY, part, shallow marine faunas (ANDERSON, 1938). Transport of much if not most of the clastic sediments away from the postulated northern, coarsely clastic deltaic areas was southward along the Sacramento enibayment chiefly by mass flow and turbidity currents into presumably deeper water (CROWELL, 1957). Though there were marked oscillations of the strand, a gross southward (longitudinal) sedimentary filling and regression has characterized Late Cretaceous to Holocene time. Complex alluvial and deltaic deposits form the valley floor today and modern deltaic sedimentation continues i n northern San Francisco Bay.

SOUTHERN CHILE

At the southern end of the east flank of the Andean orogenic belt a thick sequence of well exposed. rhythmic flysch-like Cretaceous strata pass up into younger molasse-like sandstone and conglomerate with coal seams much like the Coos Bay sequence. Both suites have been likened to the pre-Alpine region (CECIONI, 1957; ZEIL,1958). Graded bedding is common in many repeated units i n the former. Grooves and flutes are common and indicate currents from north to south. Within the sequence are large, discontinuous masses of coarse conglomerate. some remarkably well graded, and a few masses of unsorted pebbly mudstone. Geologists of the Enipresa Nacional de Petroleo have shown (personal comniunication) that regional offlap occurred i n Late Cretaceous and Early Tertiary time along the geosynclinal axis from north to south as well as eastward. This carried the deltaic and littoral sedimentation gradually southward through this time interval, so that it arrived in the present Straits of Magellan area in the Oligocene.

ANCIENT DELTAlC SEDIMENTATION IN EUGEOSYNCLlNAL BELTS

Ill

CONCLUSIONS

Deltaic sediments, far from being oddities in eugeosynclinal belts, are very important i n many areas. Thoiigh they tend to be “young”, that is more or less syn-orogenic or post-orogenic, they can not be so interpreted rigidly i n any supposed “geosynclinal cycle”. Commonly flysch- and molasse-like deposits accumulated siniultaneo~islyi n adjacent areas. Whenever and wherever sufficient stability developed (even if only temporary) and rapid sedimentation prevailed, littoral and deltaic deposits soon formed and at any time in the history of a given region! Chief factors were local relations of rapidity of subsidence and of sediment in-put. Tt is important to note that the net upparent rate of subsidence may be complicated by external eustatic changes quite independent of the prevalent tectonic behaviour within the geosynclinal belt itself. Sediment in-put is in turn complexly related to topography, climate and source rock types, and modifications could be introduced by sediment basin morphology as this might influence current and wavc patterns. But, as deltas reflect abnornially rapid rates of sediment influx, they will apparently develop almost inevitably i n spite of all other circumstances if suficient detritus and run-off are available. It seems axiomatic that there must be many more ancient deltas preserved than have been recognized. Deltaic sediments of eugeosynclinal belts differ from more familiar ones chiefly i n possessing typically immature mineral suites, although those of the modern Mississippi delta are themselves surprisingly immature (VANANDEL.1960). This probably i n part reflects extensive Pleistocene glaciation in the upper Mississippi drainage basin, for sands of the sub-tropical Orinoco delta are slightly more mature mineralogically. The sediments of the ancient examples discussed above all average coarser than i n most large. moderii deltas. This is more a function of local geographic factors; wide variation i n median diameter of materials should be expected from delta to delta. Textural maturity, however, was attained and to essentially the same degree as i n modern ones. The high energy, equilibrium depositional systems characteristic of deltas have operated quite as effectively in eugeosynclinal belts as elsewhere i n spite of overall tectonic instability of such belts. KUENEN (1958) has suggested that geosynclinal filling requires more than just local (internal) island sources. In-put from large river systems draining relatively large parts of continental areas are also required, he feels. Patterns such as those discussed suggest that, at least in younger geologic time, large river systems such as envisioned by Kuenen have contributed a large share of sediments. The southwesterii Oregon Cretaceous and Eocene depositional patterns represent largely lateral “filling” seaward from the rising Cordillera. Patterns like that of the Sacramento Valley, California and southern Chile are virtually identical to that postulated long ago by STAMP (1922) and modified by TAINSH (1950) for the Late Tertiary of Burma. These involved strong longitudinal filling components along the orogenic axes. This pattern of growth or “accretion” along a mobile belt may be a far more important one than has been realized, as Kuenen suggested. On the other hand, such patterns are difficult indeed to discern - and test - in the older geosynclinal strata.

112

R. H. DOTT JR. SUMMARY

Deltaic sediments are more common in eugeosynclinal belts than literature would indicate. They differ from deltaic deposits elsewhere only in a tendency for less mineralogic maturity and slightly coarser detritus. Eocene deltaic sands at Coos Bay, Oregon possess high percentages of volcanic rock fragments, yet they are moderately well sorted and rounded. Associated shales contain chiefly montmorillonite. Deltas have formed, even in these tectonically unstable areas, when and wherever sufficient detritus and fluvial discharge were available so that rate of sedimentation could equal or exceed subsidence. When these conditions are met, deltas appear to form almost inevitably. The deltaic sediments discussed apparently were supplied by major rivers draining rather large land areas. In Oregon the net “filling” pattern was essentially lateral, i.e., seaward (westward) out from the flank of the Cordillera. But in the Cretaceous-Tertiary of northern California and southern Chile, like the Cenozoic of Burma, much of the deltaic “filling” was longitudinal or parallel to the orogenic axis. The latter may be a common pattern of geosynclinal “growth”, but its quantitative importance through geologic time is difficult to assesss.

REFERENCES

ANDERSON, F. M., 1938. Lower Cretaceous deposits in California and Oregon. Geol. SOC.Am., Spec. Papers, 16 : 339 pp. CECIONI, G., 1957. Cretaceous flysch and molasse in Departamento Ultima Esperanza, Magallanes Province, Chile. Bull. Am. Assoc. Petrol. Geologists, 41 : 538-564. CROWELL, J. C., 1957. Origin of pebbly mudstone. &ill. Geol. SOC.Am., 68 : 993-1010. DOTTJR., R. H., 1961. Permo-Triassic diastrophism in the western Cordilleran region. Am. J. Sci., 259 : 561-582. D o n JR., R. H. and HOWARD,J. K., 1962. Convolute lamination in non-gradedsequences. J. Geol., 70 : 114-121. FISK,H. N., 1961. Bar-finger sands of Mississippi delta. In: J. A. PETERSON and J. C. OSMOND (Editors), Geometry of Sandstone Bodies. Am. Assoc. Petrol. Geologists, Tulsa, pp. 29-52. FOLK, R., 1951. Stages of textural maturity in sedimentary rocks. J. Sed. Petrol., 21 : 127-130. HALPERN, M., 1963. Cretaceous Sedimentation in Base O’Higgins Area of the Northwest Antarctic Peninsula. Ph. D. thesis, Univ. of Wisconsin, 97pp, (unpubl.). KAY,M., 1951. North American geosynclines. Geol. SOC.Am. Mem., 48 : 143 pp. KAY,M., 1960. Paleozoic continental margin in central Nevada, western United States. Intern. Geol. Cony-.,21st Copenhqen, 1960, Rept. Session, Norden, 12 : 94-103. KUENEN, PH. H., 1958. Problems concerning source and transportation of flysch sediments. Geol. Mijnhouw, 20 : 324-339. MuRoz CHRISTI,J., 1956. Chile. In: W. F. JENKS(Editor), HandOook of South American Geology - Geol. SOC.Am., Metn., 65 : 187-214. MURPHY, M. A,, 1956. Lower Cretaceous stratigraphic units of northern California. Bull. A m . Assoc. Petrol. Geologists, 40 : 2098-21 19. SHEPARD. F. P.,1955. Delta-front valleys bordering Mississippi distributaries. Bull. Geol. SOC.Atn., 66 1489-1498. SHEPARD,F. P., 1960. Mississippi delta: marginal environments, sediments and growth. In: F. P. SHEPARD,F. B PHLEGER and TJ. H. VAN ANDEL(Editors), Recent Sediments, Northwest Gulf of Mexico. Am. Assoc. Petrol. Geologists, Tulsa, pp. 5 6 8 1 .

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SNAVELY JR., P. D. and WAGNER, H. C., 1962. Tertiary geologic history of western Oregon and Washington (abstract). Progr. Ann. Meeting Ani. Assoc. Petrol. Geolqists, Sun Francisco, 1962 : 55. STAMP,L. D., 1922. An outline of the Tertiary geology of Burma. Geol. Mag., 59 : 481-501. TAINSH,H. R., 1950. Tertiary geology and principal oil fields of Buima. Bull. An]. Assoc. Petrol. Ceologisrs, 34 : 823-855. VANANDEL,TJ. H., 1958. Origin and classification of Cretaceous, Paleocene and Eocene sandstones of western Venezuela. Bull. Am. Assoc. Petrol. Geologists, 42 : 7 3 6 7 6 3 . VANANDEL,TJ. H., 1960. Sources and dispersion of Holocene sediments, northern Gulf of Mexico. In: F. P. SHEPARD, F. B. PHLEGER and TJ. H. VAN ANDEL(Editors), Recent Sediments, Northwest Giiyof Mexico. Am. Assoc. Petroleum Geologists, Tulsa, pp. 34-55. WILLIAMS, H., TURNER,F. J. and GILBERT,C. M., 1954. Petrography. Freeman, San Francisco, 406 pp. ZEIL,W., 1958. Sedimentation in der Magallanes-Geosynklinale mit besonderer Berucksichtigung des Flysches. Geol. Rimdscharr, 4 1 :4 2 5 4 3 .

TREND SURFACE ANALYSIS OF SEDIMENTARY FEATURES OF THE MODIOLARIS ZONE, EAST PENNINE COALFIELD, ENGLAND P.

M C L .D .

DUFF

and E . K .

WALTON

Grant Institiite of Geology, University of Edinburgh, Edinbuyh (Great Britain)

INTRODUCTION

In a previous paper we showed the results of a statistical study of the Coal Measure sediments of the East Pennine Coalfield (DUFFand WALTON,1962). The work was limited to the development of cycles of sedimentation, to their definition and significance in the area as a whole. In this paper we continue with the quantitative study of the sediments and are concerned with the areal variation and inter-relationships of czrtain characters or‘ the sediments. The variation is expressed in terms of partial trend surfaces (KRUMBEIN, 1959), and some attempt is made to assess the amount of data required to provide reliable estimates of the variation within the area. The investigation is restricted to the Modiolaris Zone (Lower Westphalian) because of our present lack of detailed information from the remaining part of the succession and because this is in the nature of a pilot study in the use of these techniques.

TREND SURFACE ANALYSIS

The areal variation of a sedinientary feature can be expressed in terms of a polynomial function based on the geographic co-ordinates ( U , V ) of the observation stations (GRANT,1957; KRUMBEIN, 1959; see also DAWSON and WHITTEN, 1962, for a useful summary of the basic principles). The complete trend of GRANT(1957) is the surface of “best fit” to a set of map data, and usually involves polynomial terms of high order. KRUMBEIN (1 959) and WHITTEN (1959) showed that partial trend surfaces containing linear, or linear plus quadratic‘ or linear plus quadratic plus cubic’ terms often demonstrated significant regional geological changes while the deviations (differences between the computed and the observed values) although including some local “noise” (due to faulty observation etc.) could also show important small-scale local geological effects. ALLENand KRUMBEIN [ 1962) have discussed this last point with regard to the mineralogical content of the Top Ashdown Pebble Bed. Subsequently referred to as quadratic and cubic surfaces.

TREND SURFACE ANALYSIS

:.-

- EAST

115

PENNINE COALRELD

U

.

Fig.]. The East Pennine Coalfield (exposed portion outlined) showing the main anticlines (based on EDWARDS, 1951, fig.39). Dashed lines are isopachs (in feet) of Modioluris Zone based on 129 localities (dots). Inset: Location of East Pennine Coalfield. (Structural features based on WILCOCKSEN, 1947 and EDWARDS, 1951.)

116

P. McL. D. DUFF AND E. K. WALTON

.............

. ..

,_.,

mL

..., .I.._

'.: s,

'..'._

a

U I

I

I

I

Fig.2. Partial trend surfaces and deviations. a. Pr. I: Thickness in feet: linear. b. Pr. I : Thickness in feet: linear plus quadratic. c. Pr. 11: Thickness in feet: linear. d. Pr. 11: Thickness in feet: linear plus quadratic. Zero deviation lines indicated with hatching open to negative areas. Localities for Pr. I1 indicated by dots in c. L = Leeds; D = Doncaster; S = Sheffield;W = Worksop; N = Nottingham. National Grid (10 km squares) indicated on margins. Full Grid Reference for the southwest comer of all maps is SK 400 300.

TRENDSURFACE ANALYSIS - EAST PENNINE COALFIELD

117

In this study the thickness of the Modiolaris Zone was obtained from 124 localities. Further information on number of cycles and sandstone thickness was available only from 29 of these localities. Two projects (Pr.) were therefore devised and the trend surfaces computed using the programme set up by Drs. W. C. Krumbein and C. Faulkner for the IBM 650 (London IBM Library Number 60704). In the first (Pr. I), partial trend surfaces for the thickness of the zone were obtained from the 124 localities. In the second jPr. 11) partial trend surfaces for the thickness were re-computed for comparative pusposes using only those 29 points for which additional information was available. The surfaces for other variables (sandstone thickness and number of cycles) were also computed on the basis of the 29 control points. Throughout the study the geographic coordinates were based on the National Grid.

THICKNESS

An isopacli map showing the thickness of the Modiolaris Zone represents the raw, untreated, data for this variable (Fig. I). The niap shows a simple decrease in thickness to the east i n conformity with the well known general thinning of the Coal Measures in that direction, together with some local variations. Our isopach map differs from that of ~ D W A R D S(1951, fig. 10) because of new borehole information and consequent improved correlation. As might be expected the partial trend surfaces reflect the easterly thinning. The linear surface (Fig.2a) strikes just east of north and accounts tor a large part of the variation in thickness (63.8 % of the sums of squares). The quadratic surface represents a better fit to the data (78.0% sums of squares) but the cubic surface (78.0% s.s.) is little better than the quadratic. Both quadratic and cubic surfaces have contours ruaning generally north-south but bulging convexly towards the east. These general trends are very reminiscent of those suggestcd by WILLS(1956. fig.3) i n his g?neral reconstruction of conditions i n Cornniunis and Modiolaris times (Wills’ Palstage 1 b). 1 he deviation maps (based on the differences between observed and computed values of thickness) for both the linear and the quadratic surfaces are broadly similar (Fig.2a. b). Areas of negative deviations (representing a deficiency of thickness conlpared with the general variation as indicated by the surfaces) occur to the north of Nottingham, east of Worksop and in a belt north of Doncaster. The partial trend surfaces (Pr. TI) deriked from the 29 localities (Fig.2c, d) are strikingly similar to those in Pr. I. The proportions of the sunis of squares accounted for by the surfaces are again significantly high (linear, 60.6%; quadratic, 78.5 %; cubic 79.4%) though again the cubic contribution is small. Compared with the surfaces in Pr. I the general decrease in thickness to the east is again evident and the quadratic and cubic surfaces are convex towards the east; the linear surface dips slightly more south of east. In deducing the general variation, if the 29 points rather than the 124 points had been used the error involved would be very small. The maximum error would occur in the southeast where there is a difference in the computed values of

118

P. M c L . D. DUFF AND E.

................... ............. .. ...'.L 'j

K. WALTON

I

'..

..... . .. . . . .... ...

.......

... .. ..__

I ...

;240L*/

2 0 ; 0

20

140

26 0

Sandstone thickness

'..

.:.. . .... .... .. ..._. ............

f

.1

i

40

p

400

900

Total

30

....'..

thlckners

....

w

u U 7-

U

C

d

9 00

400

Totol

thickness

Fig.3. Partial trend surfaces and deviations (ornament as in Fig.2). a. Pr. 11: No. of cycles: linear plus quadratic. b. Pr. 11: Sandstone thickness in feet: linear. c. Pr. 11: Sandstone thickness in feet: linear plus quadratic. d. Co-variation of sedimentary characters.

TREND SURFACE ANALYSIS - EAST PENNINE COALFIELD

119

50 ft. in 600 ft., an error of about 8 %. In the quadratic and cubic surfaces the discrep ancies between the two sets of results are never more than 4 %. It is clear that for most purposes the variation in thickness could have been determined using the smaller number of localities. A maximum error of 8 % would be involved in the use of the linear surface within the area but this would be confined to a small southerly portion. Any extrapolation eastwards on the basis of the 29 points would involve rather greater errors. The deviation maps (Fig.2c, d) differ rather markedly from those in Pr. I, but it is noteworthy that while the general form of the maps may not coincide, the negative areas north of Nottingham, in the Worksop area and north of Doncaster still persist.

NUMBER OF CYCLES

The figures used in this section are based on the number of seat-earths present, as described in our earlier work (DUFFand WALTON, 1962). The linear surface (not illustrated) is the same as that calculated for thickness in so far as the “18 cycles” contour coincides with the 600 ft. contour, the 21 with the 700 ft., etc. This is partly a reflection of the high degree of positive correlation ( r = 0.74) between the two variables as indicated in the scatter diagram (Fig.3a) though the precise co-incidence of the two surfaces is a chance feature. In any event the linear surface is a poor representation of the variability, accounting for only 29.9% of the sums of squares. The quadratic surface (Fig.3a) which accounts for 56.9% of the sums of squares is also similar to the surfaces of thickness but in the deviation maps the negative areas are somewhat displaced.

+

SANDSTONE THICKNESS

The partial trend surfaces for the thickness of sandstone show a decrease towards the southeast, the linear contours striking northeasterly (Fig.3b). The quadratic contours (Fig.3~)show a convexity towards the east only in the west of the area; in the southeast the direction of convexity is reversed. In both cases the proportion of the sums of squares accounted for by the surfaces is low [linear, 37.7%; quadratic, 42.7%), pointing to the greater variability of this parameter compared with the total thickness. This may be due in part to the fact that the proportion of sandstone in beds where it is interbanded with siltstone and/or mudstone was difficult to estimate from generalised written or graphic borehole logs. Local anomalies could also be caused by the development of thick masses of sandstone in restricted channels. Nevertheless, analysis of variance shows that while the cubic and quadratic components are weak the linear partial trend surface has a confidence level above 99 %. There is a positive correlation (r = 0.5) between sandstone thickness and total thickness of the zone (Fig.3d). Negative deviations occur near Nottingham, Worksop and Doncaster, as before,

+

120

P. McL. D. DUFF AND E. K . WALTON

but their shape and size are slightly different from those associated with the other surface (Fig.3b, c).

DlSCUSSlON

The thickness of a sedimentary succession is a function of regional subsidence and compactional effects. In so far as the compactional effects are dependent on the nature of the previously deposited sediments, in the Coal Measures they presumably reflect the distributary pattern of deltas or sub-deltas. The subsidence due to epeirogenic movements could be regional and the partial trend surfaces may, in the first instance, be taken to rcpreseiit these regional effects. On the other hand local diastrophic riiovements cannot be overlooked (e.g.. in the Scottish Carboniferous, GOODLET, 1957) and it is instructive to compare the deviation maps with the tectonic structures of an area in an attempt to determine how far local contemporary movements may have been important. When this is done (Fig.1, 2, 3) there seems to be little coincidence and this suggests that as far as pre-Permian structures are concerned local structural control during Modioluris times seems to have been very small. This is in contrast to WILCOCKSON’S (1947) conclusions rzgarding the sediments of the Sirnilis-PulcAru Zones, higher in the succession. But it is highly significant that Wilcockson himself suggested that the close relation between tectonic structures and thickness became less i n the lower part of his succession and that “ . . .the tectonic features were gradually coming into existenc: during Coal Measure times; early i n the period differential movements had hardly appeared”. Tf this is the case then the deviation maps may simply indicate local variations i n the sedimentary pattern. Over a period of time the positive deviations could indicate the more persistent paths of distributaries. Elongate maxima wo~ildindicate superposition of channels; point maxima would indicate the locus of intersection of “changing” distributary courses. Considering the generally assumed deltaic nature of the sedimentation it is likely that chance point maxima may be more usual than elongate maxima. The areas of positive deviation south of Nottingham, north of Worksop may be areas of heavier sand accumulation which was lacking in the areas of negative deviations. It is perhaps significant that though not coinciding precisely the deviation map of the sandstone thickness is similar to that of total thickness. More accurate data than presently available would be needed to establish these conjectures. The similarity between the partial trend surfaces of the number of cycles and the thickness and the high positive correlation of the two variables is an interesting 1950: WELLER, feature which has been commented on in similar successions (WANLESS, 1956; READ,1961). It is possible for the relative rates of sedimentation and subsidence to be such that the top of the sediments never reaches closely enough to water level to allow the establishment of a swamp. Under other conditions, and these must have obtained during Coal Measure times in the East Pennine area, sedimentation may be rapid enough to overtake subsidence and allow the establishment of a swamp.

TREND SURFACE ANALYSIS

- EAST PENNINE COALFIELD

121

Vegetation persists until regional subsidence overtakes peat accumulation and ininiersion occurs. The return of sediments brought about by distributary changes (MOORE, 1959) causes the basin to fill up again to sea level. The intercalation of cycles in regions of thicker sedimentation reflects then a critical balance between rates of subsidence and sedimentation. The general variation in sandstone thickness again illustrates the regional control of sand distribution. The degree of correlation between the sandstone thickness and the number of cycles is low (r = 0.3; Fig.3d) and not significant at conventional levels. This can be seen as the interaction of two opposing tendencies, as follows. Distribution of sandstone, like number of cycles, is controlled by regional subsidence and a high degree of positive correlation might be expected between the two variables. But this is offset by the fact that the sand distribution is controlled very closely by the distributary pattern and in the vicinity of the channel itself, so long as that channel persists the development of a number of cycles (which may be forming elsewhere on the delta) is prevented. This, together with the occurrelice of sandstone masses in washouts which may have destroyed one or more cycles, would lead to a negative correlation between the two characters. A preliminary study of this type can only hint at the ultimate usefulness of the technique of trend surface analysis. Although much remains t o be learned about the method, and especially about the geological meaning of the deviations, the results of the two computations of the partial trend surfaces of thickness give a striking illustration that the general variation in sedimentary characters may be obtained from very restricted data.

+

ACKNOWLEDGEMENTS

W e are indebted to the Chief Geologist, National Coal Board for access to the information on which this paper is based, and are particularly grateful to Messrs. R. E. Elliot and R. F. Goossens for providing that information and for their invaluable assistance i n problems of correlation. One of us (E.K.W.) was fortunate to have the opportunity of visiting Northwestern University and of being introduced to the technique of trend-surface analysis by Drs. W. C. Krunibein and E. H. T. Whitten. To them he would like to express his grateful thanks for their generous help. Dr. W. C. Krumbein also kindly read and criticised the manuscript. The cost of using an TBM Computer was met by a grant from the Carnegie Trust forAhe Universities of Scotland, which we gratefully acknowledge.

SUMMARY

The areal variation of the thickness of the sediments of the Motliolaris Zone of the

122

P. McL. D. DUFF AND E. K. WALTON

East Pennine Coalfield can be shown by trend surface analysis using relatively few control points. Partial trend surfaces of the number of cycles and the sandstone thickness are similar to those of the total thickness of the sediments and reflect the overall control of regional subsidence. Deviation m a p of thickness suggest no control, at the time of sedimentation, by local tectonic features. A high degree of positive correlation between total thickness and number of cycles is taken to indicate critical relations between rates of subsidence and sedimentation.

REFERENCES

ALLEN,P. and KRUMBEIN, W. C., 1962. Secondary trend components in the top Ashdown Pebble Bed: A case history. J. Geol., 70 : 507-538. K. R. and WHITTEN,E. H. T., 1962. The quantitative mineralogical composition and variaDAWSON, tion of the Lacorne, La Motte, and Preissac Granitic Complex, Quebec, Canada. J . Petrol., 3 : 1-37. DUFF,P. McL. D. and WALTON,E. K., 1962. Statistical basis for cyclothems: a quantitative study of the sedimentary succession in the East Pennine Coalfield. Sedimentology, 1 (4) 235-256. EDWARDS,W., 1951. The Concealed Coalfield of Yorkshire and Nottinghamshire. Geol. Surv. G I . Brit. Mem., 3rd Edn., 285 pp. GOODLET, G. A., 1957. Lithological variation in the Lower Limestone Group of the Midland Valley of Scotland. Bull. Geol. Sirrv. G I .Brit., 12 : 52-65. GRANT,F., 1957. A problem in the analysis of geo-physical data. Geophysics, 22 : 309-344. KRUMBEIN, W. C., 1959. Trend surface analysis of contour-type maps with irregular controlpoint spacing. J. Geophys. Res., 64 : 823-824. MOORE,D., 1959. Role of deltas in the formation of some British Lower Carboniferous cyclothems. J . Geol., 67 : 522-539. READ,W. A., 1961. Aberrant cyclic sedimentation in the Limestone Coal Group of the Stirling Coalfield. Trans. Eclinbirrgh Geol. SOC.,18 : 271-292. WANLESS, H. R., 1950. Late Palaeozoic cycles of sedimentation in the United States. Repts. 18th Intern. Geol. Congr., London, 4 : 17-28. WELLER,J. M., 1956. Argument for diastrophic control of late Palaeozoic cyclothems. Bull. Am. Assoc. Petrol. Geologists, 40 : 17-50. WHI-ITEN,E. H. T., 1959. Composition trends in a granite: modal variation and ghost stratigraphy in part of the Donegal granite, Eire. J. Geophys. Res., 64 : 835-848. WILCOCKSON, W. H., 1947. Some variations in the Coal Measures of Yorks. Proc. Yorkshire Geol. SOC.,27 : 58-81. WILLS,L. J., 1956. Concealed Coalfields. Blackie, London, 208 pp.

THICKNESS VARIATIONS O F THE SANDY ALMERE DEPOSITS (HOLOCENE) I N THE FORMER ZUIDERZEE AREA (THE NETHERLANDS) P . J . ENTE

AcqriculturalResearch Department, Zuiderzee Polders Developnient Authority, Kampen (The Netherlands)

GEOLOGICAL OUTLINE

An extensive description of the Holocene development of the former Zuiderzee area has been given by PONSand WIGGERS(1959, 1960); see also WIGGERS(1955) and, (1947). MULLER and VANRAADSHOVEN Factors that have greatly influenced this development are: the surface relief of the Pleistocene, the relative rise of sea level and the alternation of periods of transgression and regression. The top of the Pleistocene forms a plain which slopes towards the west. In the centre of the area it is found at a depth of roughly 10 m below mean sea level. The plain is mainly formed by eolian “cover sands”. It has a relief caused by a braided-river system. At the transition from the Pleistocene to the Holocene dunes were formed along the channels of this system, the sand being derived from the streambeds. During the Atlantic substage of the Holocene era, the sea level rose from 17 m to 5 or 6 m below its present position. This hampered drainage and promoted the formation of peat (“Lower Peat”) along the eastern border of the sea. The marine (littoral) sediments of this age are known as “Deposits of Calais”. In the Atlantic the area of their deposition extended into the peat zone as far as the centre of the former Zuiderzee. Here the “Deposits of Calais” are called “Unio Clay”. Deposition continued into the Subboreal. The Unio Clay was deposited in a fresh to brackish environment with some tidal influence, the latter being responsible for a differentiation into levees and backswamps. After the deposition of the Unio Clay had ceased the levees and backswamps became covered with peat, which did not prevent some of the gullies remaining open. In these gullies and in streams in the peat landscape the “old detritus-gyttja” was formed, a sediment, partly composed of the remains of water flora and fauna and partly of detritus from the adjacent peat. Around 1650 B.C. the sea invaded the area again, causing considerable erosion. Deposition was mainly limited to the former drainage channels. Here, the “Cardium Clay” was formed. The latter represents the “Deposits of Duinkerken” of the western part of The Netherlands.

124

P. J. ENTE

Since the end of this transgression (before 1250 B.C.) a large part of the area remained open water, having a small connection in northerly direction with the sea. l n the lakes “young detritus-gyttja” was formed. After the beginning of the Christian era, the forementioned connection was enlarged and the Alinere beds were deposited. Much of the remaining peat was eroded and the older Almere deposits consist for a great part of the resulting detritus (“peat detritus” facies). Lateron more silty and sandy material was supplied from the north (sandy facies). The end of the Almere sedimentation may be put at about 1550 A.D. To complete the given outline it can be mentioned that since 1550 A.D. the Zuiderzee deposits were formed, in an environment with increased salinity. This ended in 1932 when the dam, separating the Zuiderzee from the North Sea, was closed. In the closed-off basin, now called IJsselmeer, local erosion and re-deposition caused the formation of the IJsselmeer deposits.

SEDIMENTATION AS RELATED TO DIFFERENTIAL COMPACTION OF THE SUBSTRATUM

The sandy facies of the Almere deposits shows, in many places, strong thickness variations over short distances. The examples of this phenomenon discussed in the present paper are situated in the northwestern part of the Eastern Flevoland polder. The upper side of these sandy Almere beds is very even and almost level. It slopes very gently (1 m in 5 km) towards the north, and is found at a depth of about 4.50 m below mean sea level. It is covered by about 40 cm of IJsselnieer and Zuiderzee deposits. In striking contrast to the evenness of the upper side, the lower side is strongly undulated. Consequently the thickness of the sandy facies varies greatly, that is, from a few decimetres to a few metres. The primary causes of this strong relief of the base of the Alinere deposits are the following two factors. The first one consists in the presence of numerous river bank dunes, rising to appreciable heights above the normal Pleistocene surface (Fig. I). The tops of many of these dunes almost reach to the upper side of the sandy facies.

-

q -

/

=

river bonkdunes

U”l0 cloy

Fig 1 . Distribution of Pleistocene liver bank dunes and Holocene Unio Clay (“Deposits of Calais”) within 1.5 rn below the surface of the Eastern Flevoland polder.

125

THICKNESS VARIATIONS OF DEPOSITS I N THE ZUIDERZEE AREA

Pleistocene

-

erOIl0”

rn below mean

wrfoce

Fi g2 Schematic geological profile from the northwesternpart of Eastern Flevoland.

The second factor is the relief of the top of the Unio Clay, due to its former gullies, levees and backswamps (Fig.2). At present in many places, the levee deposits attain almost the same level as the upper side of the Almere deposits, whereas the surface of the backswamp deposits lies 2 4 m lower. The former bottom of small gullies lies 1 or 2 m and the former bottom of large gullies several metres below the top of the levee deposits. Where they are well developed, the levees consist to a considerable depth of rather compact clay, whereas the clay of the backswamps (and of the small gullies) is soft. This can be demonstrated by the figures of the apparent specific gravity of both (Table Ia, b). It should be noted that the compactness of the levee deposits dates from the time of their formation. It can be explained by the higher position and consequently the better drainage during sedimentation, together with the evaporation of moisture from the soil, due to the climate and the vegetation at that time. As a result of this inherited compactness of the levee deposits the possible compaction of later date is negligible. However, the backswamp clays, the peat layer overlying the former backswamps and the filling material of the former gullies may have been compacted after their deposition. For, although for the backswamps the figures of the apparent specific gravity are already rather low, they were probably even lower at the time of deposition (see Table Ib, estimated original values). The “estimated original values” are based upon the relation A = 20 n ( L b H ) (cf. SMITS,1953; ZONNEVELD, 1960) and on the variation with depth of the factor n in this formula, according to R. J. de Glopper (personal communication, 1963). In this formula A stands for the water content of 100 g

+

+

126

P. J . ENTE

TABLE I SOIL ANALYSES D A T A ~~

~

~~

-~~

Esrini. w ( y . app. spec.

%

Orp. matter ”/,

Present app. spec. gravity

100-200 200-300 30&400 400-500 500-600

45 50 46 50 49

4 3 4 7 18

0.86 0.83 0.76 0.65 0.54

-

b. Backswamp Unio Clay

3O(k400 400-500 5 w 6 0 0

44 47 49

20 8 12

0.42 0.65 0.58

0.36 0.46 0.42

c. Old detritus-

45G500 5m600 6W700 700-800 800-850

29 24 27 22 21

33 33 36 38 42

0.31 0.36 0.32 0.32 0.31

0.24 0.25 0.23 0.24 0.23

Depth’

a. Levee Unio Clay

~

I

~~

~

~

~~~~

Clay

~~~

~~

graviiy

-

~

In centirnetres below the surface of the polder.

of dry sediment, L for the percentage of clay (particles < 2 p), H for the percentage of organic matter and h for the ratio between water-absorbing capacity of organic matter and of clay. The differences between the figures for the present apparent specific gravity and those inferred for the original situation correspond in this case to a compaction of more than one metre since the time the backswamp deposits were formed. The peat likewise shows low figures for the apparent specific gravity. Also here compaction is plausible, when these figures (0.214.27) are compared with those of non-compressed peat of other areas (0.09-0.13). The compaction may amount to the same value as the thickness of the layer. The “peat detritus” facies of the Almere deposits shows no difference in apparent specific gravity between samples from above the crzsts of the former levees and from above the former backswamps. Furthermore, the figures of samples from an area, where no sandy facies is present, are of the same magnitude. This means that the great differences in depth, of 2 m or more, at which the top of this layer is found, cannot be explained by compaction of the layer itself, but at best by compaction of the substratum. In the former gullies the figures for the apparent specific gravity of the old detritusgyttja are low (Table Ic). By comparing them with the estimated original values, it follows that the compaction, since the time of the deposition, may have been about 1.7 m. The “peat detritus” facies of the Almere deposits above the former gullies shows somewhat higher figures for the apparent specific gravity (0.67) than elsewhere in this area (0.57-0.60).

THICKNESS VARIATIONS OF DEPOSITS IN THE ZUIDERZEE AREA

127

Fig.3. Profile showing the different inclinations of the laminae of the sandy facies of the Almere deposits.

From the figures given for the estimated compaction of the beds underlying this “peat detritus” facies it appears possible that originally this formation had an almost horizontal position.

128

P. J. ENTE

In this connection it may be remarked that the lower and upper boundaries of the “peat detritus” facies are undoubtedly time boundaries. In the case of the lower side of the beds this is proven by the presence of a thin layer of sand, eroded from the top of a Pleistocene dune (Fig.2, right side). As for the upper boundary, it follows from the lithology of the Almere deposits themselves. If the “peat detritus” facies would be at least partly synchronous with the sandy facies, this would imply the very improbable situation of sedimentation of sand on the lower spots and of peat detritus and clayey material on the higher spots of the lake bottom of that time. In the foregoing it is made clear that the sedimentation of the sandy facies began on an almost level lake bottom, consisting of the “peat detritus” facies. Where sufficient sand was laid down, its load could cause an increased compaction of the underlying beds, thus leading to subsidence of the lake bottom. A depression was formed in which more sand was deposited, and so on. It follows that the horizontal variations in the amount of sand deposited in the lake, depended on the different degrees to which the substratum could be compacted. A last argument to prove this is found in the sandy facies itself. It contains a fine lamination with bands of peat detritus. At those places where the sandy facies becomes thicker, that is in the direction of a former backswamp or a former gully of the Unio Clay, these bands are seen to diverge at intermediate angles between the upper and lower boundaries of the sand (Fig.3). Accepting the idea of the originally horizontal position of each lamina at the time of its formation, the present inclinations of the deeper laminae imply compaction of the substratum during sedimentation of the sand.

SUMMARY

The thickness variations of the sandy facies of the Almere deposits are attributed to differential compaction (during their sedimentation) of the older, underlying beds. The amount of cornpaction is computed on the base of apparent densities, as measured today and inferred for the past. It is probable that the bases of both the lower and the upper division of the Almere deposits are time boundaries, which originally had subhorizontal positions.

REFERENCES

MULLER, J. en VANRAADSHOVEN, B., 1941. Het Holoceen in de Noordoostpolder. Tijdschr. Koninkl. Ned. Aardrijkskundig Cenoot., 64 : 153-185. PONS,L. J. en WIGGERS, A. J., 1959-1960. De Holocene wordingsgeschiedenis van Noord-Holland en het Zuiderzeegebied. Tijdschr. Koninkl. Ned. Aardrijkskirndig Genoot., 16 : 104-152, I1 : 3-51. SMITS,H., 1953. Over de Inklinkiq van Oostelijk Flevoland. Directie Wieringermeer, Kampen, 56 pp. WIGGERS, A. J., 1955. De Wording van het Noordoostpoldergebied. Thesis, Univ. of Amsterdam. Tjeenk Willink, Zwolle, 216 pp. I. S., 1960. De Brabantse Biesbosch. A study of Soil and Vegetation of a Freshwater Tidal ZONNEVELD, Delta. Thesis, Landbouw Hogeschool, Wageningen, 210 pp.

A RECONNAISSANCE SURVEY OF THE ENVIRONMENT OF RECENT CARBONATE SEDIMENTATION ALONG THE TRUCIAL COAST, PERSIAN GULF G . E V A N S , D. J. J. K I N S M A N ~ ~ ~ J . DS .H E A R M A N

Imperial College, London (Creut Brituin)

INTRODUCTION

The Persian Gulf is a relatively shallow sea being rarely deeper than 50 fathoms' (90 m). Communication with the Indian Ocean is through the Strait of Hormuz, which although narrow is not restricted by a submarine sill, as occurs for example at the southern end of the Red Sea. The deepest water lies, in fact, at its entrance. The Gulf is asymmetric in cross section with the deeper water axis lying close against the Persian shore, inland of which rise the Zagros Mountains. Along the southern or Arabian half of the Gulf the water is nowhere deeper than 20 fathoms (Fig. 1). EMERY ( I 956) made a description of the sediments and waters of the area, based on records and samples obtained by Royal Naval and United States Survey vessels. He showed that the area is one of carbonate sedimentation with appreciable amounts of terrigenous detritus occurring only around the Tigris-Euphrates delta at the head of the Gulf and along the Persian shore. HOUBOLT (1958) made a detailed study of the offshore sediments to the north and east of the Qatar Peninsula. The shallow water sediments were found to be essentially skeletal calcarenites and these pass laterally into calcilutites and marls towards the deeper water axis. The coastal lagoons of Qatar were investigated by BRAMKAMP and POWERS(1955) where they reported a series of carbonate and evaporitic sediments to be accumulating. SUCDEN(1963) studied the Recent sediments of the shallow Gulf of Salwa (to the west of the Qatar Peninsula) where he found a widespread development of aragonite muds, ooliths and pseudo-ooliths. He also made a regional survey of the variation in salinity, with particular reference to the remarkably high values occurring in the Gulf of Salwa and along the Trucial Coast, where salinities as much as 60 % above that of normal ocean water occur. He concluded that these high salinities had little effect on the diversity and abundance of the marine fauna. Recently, WELLSand his co-workers (1962) made the interesting discovery of the formation of Recent dolomite in the sediments of the higher parts of the tidal flats around the Qatar Peninsula and CURTISet al. (1963) All depths are expressed in fathoms below Admiralty Chart Datum (approximately Low Water Spring Tides).

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PERSIAN

GULF

/

TRUCIAL

COAST

Fig.1. Inset map of Persian Gulf showing broad bathymetry and asymmetric character in cross section. Map of Trucial Coast showing nearshore bathymetryand distribution of islands and ernbayments.

showed that both dolomite and anhydrite occur in the sediments of the Sabkha near Abu Dhabi. WELLSand ILLING(1963) have reported most interesting observations on the instantaneous precipitation ofcalcium carbonate in the waters of the southern Gulf. It was against the background of the work by Emery, Houbolt and Bramkamp and Powers, that the Departments of Geology, and Geophysics of the Imperial College of Science and Technology, London, initiated in 1961 a programme of research on the Recent sediments along the Trucial Coast. The nearshore zone above 5 fathoms (9 m) was found to be an area of carbonate sedimentation in which skeletal calcarenites, oolite sands and aragonite muds are being deposited, together with biohermal accumulations and accessory contributions from windblown materials. An evaporitic facies occurs immediately inland. The purpose of this paper is to describe the physiographic setting in which these various sediments occur, in the region between Ras Ghanadha in the northeast and Jabal Dhanna in the west, with more detailed reference to the area around Halat el Bahrani. The sediments of the latter area are described in the accompanying paper by KINSMAN (1963).

PHYSIOGRAPHY

The region consists of a broad coastal strip, up to 15 miles wide, of islands and penin-

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sulas which enclose a series of large, shallow tidal embayments. In the west, the islands are joined by sandy shoals and are separated from the mainland by a long lagoon, Khor a1 Bazm, which is not completely closed at its eastern end (Fig.1). Apart from a few small channels which cut across this offshore barrier the main ebb tide drainage is westwards. In the east, however, the offshore islands are separated from each other by deep ebb channels which drain the large shallow embayments behind them (Fig. 1, 2, 3). Initially the small ebb channels have a dendritic pattern but these coalesce to form one or more major channels which then drain between the islands and across broad sub-aqueous deltas before reaching the open sea. The long profiles of the ebb channels are of particular interest. The channels deepen as they approach the gap between the islands, to depths of 5-7 fathoms, but shallow again, commonly to less than 1 fathom as they flow out across the seaward edge of the sub-aqueous deltas. In some places the channels lose their identity completely. The deltas are bounded seawards by the 1 fathom depth contour and shelve steeply at their outer edge to 3 fathoms. They are simple in form at the mouth of a single ebb channel, e.g., that northeast of Abu Dhabi island, or complex where more than one channel discharges between the islands, as for example the deltas northeast and southwest of Halat el Bahrani. They are being deflected partly southwestwards by the dominant wave approach which is largely controlled by the prevalent northwest or “Shamal” winds. The deltas probably have a complex origin, perhaps representing several stages of deposition separated by periods of elevation which may correlate with Pleistocene and post-Pleistocene sea-level changes. They are also in part erosional platforms cut into earlier Quaternary limestones on which the Recent sediments are accumulating. However, their general arcuate form and their close relationship to the present channel systems suggests that they are dominantly of tidal origin. The seaward islands are composed largely of unconsolidated Recent sediments. Limestones, probably Quaternary in age, underlie these desiments in some parts, and on the mainland often overlie older Tertiary deposits. This complex of lagoons, embayments and islands is bordered in some places on its landward margin by broad, low beach ridges and isolated hills of Tertiary and Quaternary sediments; in other places, extensive areas of intertidal flats, with wide stretches of Algae in some localities, occur along the inner coastline. Inland of the mainland coast is a wide flat sandy plain. often with a saline crust, the Sabkha. The Sabkha lies just above the level of high spring tides and may be up to 15 miles wide. It is bounded on its inland margins by low Tertiary hills capped occasionally by Quaternary limestone and often covered locally with a thin veneer of blown sand. In places a few, flat-topped, steep-sided, isolated hillocks, buttes, stand up from the Sabkha surface, surrounded by a fringe of blown desert sand. To the west the hills reach the coast and the Sabkha here occurs as isolated patches occupying the low ground. A detailed description of the area between Halat el Bahrani and Abu Dhabi will now be given; this area is fairly typical of the eastern part of the coastal region. The seaward facing coasts of these islands are aligned southwest to northeast and parallel the general coast trend. The beaches are fairly steep and are backed by large frontal

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Fig.2. Aerial mosaic of Abu Dhabi region. (Royal Air Force Photograph, Crown Copyright Reserved.)

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Fig.3. Map showing salient features of area covered by aerial mosaic. Inset area is discussed by KINSMAN (1963).

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dune ridges. The maximum tidal range is about 6 ft. but this may be increased during periods of strong northwesterly winds. The orientation of the beaches and the frontal dune ridges is a response to this onshore wind. The steep beach faces are characterised by the development of cusps and are fronted by a narrow low tide terrace. At the top of the beaches one or more berms are usually developed, often extensively colonised by the burrowing crab Ocypoda aegyptica. The berms are succeeded landwards by a wind stripped terrace. Low, hummocky, partly vegetated dunes lead inland from this upper terrace and merge into the frontal dune ridge which on Halat el Bahrani reaches 30 ft. in height and is the highest part of the island. Behind this ridge is an extensive flat, wind stripped area with a surface lag deposit of coarser debris; smaller dunes are present, especially near the inner southeastern corner of the island. In the southwest of Halat el Bahrani, however, behind the frontal dune ridge is a wide swampy area with a typical dendritic drainage pattern of creeks. This area is densely vegetated by Arthrocnemum glaucum. The sheltered inner coasts of Halat el Bahrani are characterised by a succession of spits. These are arranged en khelon and are all directed inwards toward the mainland. The tongues between the spits are colonised by large numbers of gastropods and crabs. Faecal pellets and trails cover much of the gastropod zone, which is inundated by every tide. The small burrowing crab Scopimera lives in the higher zone which is inundated only during high spring tides; the surface of this zone is covered by the burrowing pellets produced by Scopimrra, although these are destroyed whenever the zone is flooded. Wide cjab flats extend along most of these coasts, often landwards of the spit area. The gastropod flats become very extensive in the extreme southeast of the island. Further again southeast a n extensive area of tidal swamps is developed extending almost to the mainland coast. Between the deltas, directly fronting the islands are coral reefs, predominantly of Acropora with subordinate Platygyra and other massive corals. The main ebb channel flowing across the delta between Halet el Bahrani and Abu Dhabi bifurcates 3 miles from its mouth around the island of Jazirat el Ftaisi. This latter island is thus separated from the open sea by the wide shoal area of the delta and does not have the coastline typical of the more seaward islands. The shoal area enclosed by the main branches of the ebb channel is another area of coral growth. The coral here is diffuse and occurs in patches, whereas on the steep walls of the ebb channels the growth of the dominant coral, Acropora, is extremely prolific.

ACKNOWLEDGEMENTS

Grateful acknowledgement is made to D.S.I.R. for financing this project; to the Hydrographer of the Royal Navy, Captain and crew of H.M.S. Dalrymple for much assistance and co-operation; to the British Museum of Natural History for identification of fauna and flora; to Abu Dhabi Petroleum Co., and to Mrs. F. Kelk for assistance in the preparation of the figures.

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SUMMARY

The physiographic setting of an area of Recent carbonate sedimentation along the Trucial Coast, Persian Gulf is described. Previous work on the sediments and waters of the Persian Gulf is briefly reviewed. The paper forms the background for the description of the sediments of part of the area discussed (see KINSMAN, 1963).

REFERENCES

BRAMKAMP, R. A. and POWERS, R. W., 1955. Two Persian Gulf lagoons. (Abstract). J. Sedinimt. Petrol., 25 : 139-140. CURTIS, R., EVANS, G., KINSMAN, D. J. J., SHEARMAN, D. J., 1963. Association of dolomite and anhydrite in the Recent sediments of the Persian Gulf. Nature, 197 (4868) : 679480. EMERY, K. O., 1956. Sediments and water of Persian Gulf. Bull. A m . Assoc. Petrol. Ceogists, 40 : 2354-2383.

HOUBOLT, J. J. H. C., 1957. Surface Sediments of the Persian Gulf near the Qatar Peninsula. Thesis, Univ. of Utrecht, Mouton, The Hague, 113 pp. KINSMAN, D. J. J., 1963. The Recent carbonate sediments near Halat el Bahrani, Trucial Coast, Persian Gulf. In: L. M. 5. U. VAN STRAATEN (Editor), Deltaic andShallow Marine Deposits. Elsevier, Amsterdam, pp. 185-192 SUGDEN, W., 1963. 1. Some aspects of sedimentation in the Persian Gulf. 2. The hydrology of the Persian Gulf and its significance in respect to evaporite deposition. In press. WELLS,A. J., 1962. Recent dolomite in the Persian Gulf. Nature. 194 (4825) : 276275. WELLS,A. J. and ILLING,L. V., 1963. Present day precipitation of calcium carbonate in the Persian Gulf. In: L. M. J. U. VAN STRAATEN (Editor), Deltaic and Shallow Marine Deposits. Elsevier, Amsterdam, pp. 429-435.

TRACE-FOSSILS AND THE SEDIMENTARY SURFACE IN SHALLOW-WATER MARINE SEDIMENTS R. GOLDRING

Department of Geology, University of Reading, Reading (Great Britain)

INTRODUCTION

In three papers, SEILACHER (1954, 1955, 1958) has distinguished two broad assemblages of trace-fossils (ichnofossils) in marine deposits. One may be associated with what are considered to be shallow-water sediments, and the other with what are considered to be relatively deeper-water sediments, in particular turbidites. In the broadest synonymy, one is associated with a flysch facies; the other with shallow marine facies. Few ichnogenera are common to both facies. The turbidite facies is characterised by a preponderance of grazing traces (Pascichnia), whilst resting traces (Cubichnia) and dwelling burrows (Domichnia) are rare or absent. The two latter are well represented in epicontinental sediments. Besides these two distinct trace-fossil assemblages, it is considered that the trace assemblages associated with shallow-water and shelf sediments may be usefully divided into two groups. Although the sedimentological characters of each are substantially distinct, a consideration of the trace assemblages may add useful information towards an appreciation of the sedimentary environment. In analysing epicontinental assemblages, Seilacher has drawn on such examples as the Mesozoic sandstones of Wiirttemberg and the Cambrian of the Salt Range, which include resting traces, often aligned, and trails made on, from, or very close to the sea floor. Migrating burrows constructed below the surface, such as Teichichnus, Ptiycodcs and Rhizocorallium, together with other feeding burrows, are also present. The sea floor must have been quite rapidly covered with accumulating sediment, for those traces, made close to it, to have been preserved. Seldom, if at all, was it subjected to any considerable penecontemporaneous erosion. Vertical repetitions of Asteriacites lumbricalis (SEILACHER 1953, p.100) in sandstone showing primary current lineation, indicate a fairly continuous accumulation of about 2 cm, and associated upward migrating (retrusive) Diplocraterion sp. of at least 15 cm. I n contrast, there are other sediments which, from their sedimentological characteristics and associated body-fossils, must also be considered as epicontinental and neritic. These rarely, if ever, yield any trace-fossil, in position of formation, that was made on or immediately below the sedimentary surface. This does not necessarily mean that such markings were not made in these sediments, but that if they were, then they were never fully preserved.

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CH 0NDR ITES

The trace-fossil Chondrites has been redescribed by SIMPSON(1957). It is known from the Ordovician to the Tertiary, and occurs in several types of preservation (Fig.1). With bed-junction preservation, a change in the nature of the sediment accumulating takes place, and the new sediment fills in the tunnel system excavated by the organism. The lithological change may be gradual, and without penecontemporaneous erosion. If there is no sedimentary change, only subsequent diagenetic differentiation between the sediment filling the tunnels and the matrix, will render them observable. Sharp changes in lithology, as, for instance, between a sandstone and a shale, may be associated with slight penecontemporaneous erosion removing portions of the proximal shafts, and give rise to concealed bed-junction preservation. Burial preservation is where the sediment surrounding the mucus-lined tubes has been winnowed away leaving a pile of Fucus-like strands, indicating definite penecontemporaneous erosion, and the removal of the sedimentary surface from which the tunnel system was initiated. In the latter two types, it is not however possible to measure the amount of erosion that has taken place. Bed-junction preservation and diagenetic preservation were noted by Simpson to occur abundantly i n the Lower Lias of the south coast of England, but burial preservation is absent. Burrowing pelecypods and Pinna in their attitude of growth, and Lima also in the growth position, with encrusting oysters on the posterior part of the shell (HALLAM,I960), also show the infrequency of penecontemporaneous erosion. In the Carboniferous limestone of the Bristol district, only a few examples of burial preservation have been found. Bed-junction preservation greatly predominates, such as cylinders of limestone piping downwards from a limestone into an argillaceous band, and vice versa, but diagenetic preservation is also present in the calcite-mudstone bands. In contrast, in the Upper Emsian Lynton Beds and the Givetian Ilfracombe Beds of southwest England, concealed bed-junction preservation predominates in the slate bands, and burial preservation is not infrequent. Penecontemporaneous erosion was of frequent occurrence. The Lynton Beds and the Ilfracombe Beds are neritic sediments, and both are conformably underlain by continental deposits. Other trace-fossils have not yet been described from the beds but, so far as the writer is aware, no exogenic forms occur. The type of preservation of Chondrites in these sediments contrasts sharply with that in the Dinantian, Lias, and elsewhere in the Mesozoic (SIMPSON, 1957, p.490).

THE UPPER DEVONIAN BAGGY BEDS

The Baggy Beds of southwest England stratigraphically lie in a conformable sequence between continental and neritic sediments. They almost completely lack surface-made traces and shallow burrowings are only locally abundant. DiplocrationJoyo,

138 R. GOLDRING

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a migratory U-burrow, vertical to the bedding, is the most common trace-fossil. It occurs in a banded lithology of fine-grained sandstone and mudstone; the laminae are from 0.1-1.0 cm thick. This lithology accounts for approximately 15-20% of the Baggy Beds (400 m). GOLDRING (1962) has interpreted the behaviour pattern of this species as a response to repeated phases of sedimentation and erosion. Both retrusive and protrusive forms are present. Only a few examples of what are considered to be the actual aperture to the burrow are known, and in the majority of specimens, which are retrusive forms, only a remnant of the original structure is preserved beneath a sandstone capped erosion surface. In spite of the overall accumulation of sediment, there must have been repeated phases of degradation of the sedimentary surface. The overall amount of erosion is not easy to determine, but was generally in the order of 5-10 cm for most specimens, whilst individual phases were probably less than 1 .O cm. I n nearly every case where the species occurs, erosion of sediment can be deduced, with at least twice the amount of sediment preserved having been originally deposited. Only organisms which were able to penetrate to a depth of 5-10 cni or more, were likely to leave any trace of their existeiice in the fossil record. The open U-tube Arenicolites curvatus is present in more massive sandstone. The tubes must have been frequently subjected to erosion, as fragments commonly occur in interbedded congloFig.1. The amount of sedimentation or erosion as indicated by the adjustment to depth and mode of preservation of various species. Heights of solid arrows show amount of sedimentation or erosion. ( I ) Movement pattern of burrowing pelecypod with single siphonal opening e.g., Mya (after REINECK. 1958). With stationary sedimentary surface (a) growing organism burrows deeper. With rapid sedimentation (b) organism migrates towards surface, leaving infilled burrow of same breadth as shell. With degradation of surface (c) organism migrates downwards, leaving burrow of same breadth as shell. (2) Movement pattein of polycheate worm, e.g., Nereis (after REINECK, 1958). Older colonised surface ( a ) is rapidly covered by sediment ( b and c) and during deposition of sediment, paths of escape are directly upwards. New colonisation surface (c) with irregular burrows. Burrows in ( a ) and ( c ) generally mucus lined; in (b) unlined. Similar pattern observed by author in sands of Bagshot Beds (Eocene) at Bramshill (SU 756610). (3) Movement pattern of organism dwelling in single tube, e.g.. Cerianthus (after SCHAFER, 1956). With sedimentation, the coral moves upwards leaving an infilled burrow. A similar pattern might be expected in traces attributed to Skolithos and Monocraterion. ( 4 ) Movement of Asteriocites lunibricolis (after SEILACHER, 1953): (a) with sedimentation the ophiuroid migrates upwards in stages (i-iii), and ( b ) combined plan of all impressions. ( 5 ) Preservation pattern of Chondrites (after SIMPSON, 1957). The tunnel system (a) is infilled (b) following a change in the type of sediment being deposited (bed-junction preservation). Slight degradation of the surface (c) removes the proximal shafts before further sediment, of a different type, accumulates (concealed bed-junction preservation). Renewed degradation of the surface winnows away the sediment, leaving the mucus-lined infilled tunnels as burial preservation (d). (6) Preservation pattern of Arenicolites curvatus (after GOLDRING, 1962). The sedimentary surface with the open U-tubes (a) has been degraded (b), the mucuscemented tube fragments accumulating in an intraformational conglomerate; sediment has filled the tubes in (b). (7) Movement pattern of Diplocration yoyo (after GOLDRING, 1962). In the Upper Devonian Baggy Beds of north Devon, this trace occurs in the various types shown in (F),where all have been truncated to a common erosion surface. It is considered that repeated phases of erosion and sedimentation led to the development of the various types. Stage (A), development of burrow ( I ) . With degradation of the surface, this tube migrates downwards, and at intervals, new tubes (2 and 3) are constructed ( E and C ) . Sedimentation follows (Dand E) but some of the tubes are abandoned. Stage ( F ) , all tubes are abandoned and erosion reduces them to a common base.

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merates. It is not possible to determine how much of the tubes were eroded, though there are fragments up to 5 cm in length. None of the body fossils (thick-shelled pelecypods and gastropods) occur in their growth position. It is interesting to attempt a determination of what the sedimentary surface must have often been like in Baggy times. From a freak preservation at one locality, it can be deduced that the sedimentary surface locally carried a prolific number of DFlocraterion. Judging from the large number of specimens of Lingula, frequently wellpreserved, lying on bedding surfaces elsewhere in the Baggy Beds, other sedimentary surfaces must often have been extensively burrowed by this brachiopod, though not to the depth to which Diplocraterion penetrated. There were probably also banks of the thick-shelled pelecypod Dolabra (Arcacea), none of which have been preserved, the specimens occurring only in cross-bedded sandstone units. If local areas were at times, quite highly populated with surface-dwelling and burrowing organisms, this would indicate (REINECK, 1958), so far as these areas are concerned, periods of only slight sedimentation and erosion. Such areas if they existed at the present day would likely be quite readily observable (e.g., tidal flats) in contrast with areas, such as channels where rapid changes may occur. Scarcely any surface that could represent such an area has been preserved.

AN INTERMEDIATE ASSEMBLAGE

The Baggy Beds and the basin facies of the Blue Lias, or the Lower Jurassic of Wiirttemberg, may be considered as the two extremes for the preservation of sedimentary surfaces. What might be considered as intermediate members, are sections of the Carboniferous of northern England and southern Scotland. There, although there is a good deal of evidence of penecontemporaneous erosion, nevertheless sedimentary surfaces with fossils in their growth positions, and surface-made traces, are not infrequently preserved. The upper part of the Carboniferous limestone and the succeeding Yoredales (Visean to Namurian), together with their Scottish equivalents, comprise repetitive successions of limestone, marine and non-marine shale, sandstone and occasional coals. In palaeoecological studies on the marine shales, CRAIG (1952, 1 9 5 4 , 1956) and FERGUSON (1962) have shown that, occasionally, surface-living organisms such as Productus concinnus and pectiniform pelecypods, and more frequently burrowers such as Lingula and Edrnondia, occur in their position of growth. However, sedimentary scour structures and the occurrence of concentrations of disarticulated and fragmentary shells, show that the sedimentary surface was always likely to be swept by currents. The trace-fossils associated with the series are not well documented, and have often been described merely as “fucoids”. DONALDSON and SIMPSON (1962) have described an association from the Visean (D) limestones of north Lancashire. Besides numerous surface-made, exogenic trails, one bed is covered by conical mounds of castings, Cliornatichnus, above vertical burrows, pushed to the surface by an unknown organism. Judging by the density of the castings (about

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30/m2), they must have been formed on a sedimentary surface of some duration. They were then rapidly covered, and no reworking took place. Spirophyton, Cock’s tail, markings may be connected with the castings, and there are also endogenic burrows, some attributable to Teichichnus and Chonrirites. Cauda-galli markings are common elsewhere in the series, and probably represent the central laminated trace of a sediment-eating organism, migrating through the sediment close to the sea-floor. A Corophium-like burrow with thick walls, near to Tisoa siphonalis MARCEL DE SERRES (BATHER1924), which extends about 5 cm into shale, is common at Earlsferry, Fife, in the Carboniferous Limestone Series. Footprints have been recorded from the Calciferous sandstone (SMITH,1891). In general, the faunal, algal and lithological association indicate a fairly shallow-water sedimentary environment for those parts of the cyclothems considered as marine.

CONCLUSIONS

These analyses, albeit brief, of trace-fossil assemblages formed in no great depth of water, lead to three main conclusions. Firstly, two distinct types of assemblages are represented, possibly the end members of a continuous series, (1) with surface or near-surface made traces, as well as traces made at greater depths, and (2) with only traces made some depth below the sedimen tary surface. Secondly, on the one hand, the frequent occurrence of abundant surface-made traces, or those made very close to the sedimentary surface, indicate fairly continuous sedimentation, and the preservation of nearly every sedimentary surface. On the other hand, the complete absence of these trace types, associated with frequent evidence of erosion, is testimony of discontinuous sedimentation. and the eventual preservation of only a fraction of the actual sediment deposited. REINECK (1960) has shown that only 1/lO,OOO-I/lOO,OOO part of the sediment deposited i n recent shallow water sediments in tidal seas, is actually fossilised. Similar conditions must have existed during the deposition of much of the Baggy Beds. Thirdly, the adjustments of certain organisms attempting to maintain a constant depth below the sedimentary surface, can be used to estimate the amount of sedimentation or erosion that has taken place. Also, the mode of preservation of others is an indication of this. REINECK (1 958) has shown the effect of slow and fast sedimentation and erosion on Mya, Nereis and other organisms in recent sediments. Vertical repetition of Asteriacites and retrusive Diylocraterion give an indicatioii of the amount of sedimentation. These and other examples are shown diagrammatically in the accompanying text-figure. The sedimentological causes for the division are probably complex. To permit continuous sedimentation with only limited penecontemporaneous erosion, the environment must have been under conditions leading t o continuous aggradation. If sediment accumulated beyond a certain rate, colonisation by burrowing organisms would be

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impossible, and most existing organisms would attempt t o migrate elsewhere (REINECK, 1958; MIDDLEMISS, 1962). Discontinuous sedimentation at the present day, is particularly well developed in the intertidal zone of continental shelves, and is a typical feature of prodelta sediments.

SUMMARY

Two distinct trace-fossil assemblages from shallow-water marine sediments are recognised: (a) with traces made on, or close to the sedimentary surface as well as those penetrating deeper, and (h) with only traces made well below the sedimentary surface. Whereas there is evidence for only limited penecontemporaneous erosion in sediments associated with the former type, there is evidence of considerable and frequent penecontemporaneous erosion associated with the latter. The amount of sedimentation and erosion can be estimated from the behaviour pattern and mode of preservation of certain trace-fossils.

REFERENCES

BATHER, F. A,, 1924. Tisoa siphonalis MARCEL DE SERRES, a supposed Liassic annelid. Naturalist, 1925 : 7-10. CRAIG,G. Y., 1952. A comparative study of the ecology and palaeoecology of Lingula. Trans. Edinbrrrgh Geol. SOC.,15 : 110-120. CRAIG,G . Y , 1954. The palaeoecology of the Top Hosie Shale (Lower Carboniferous) at a locality near Kilsyth Quart. J . Geol. SOC.London, 110 : 103-1 19. CRAIG,G. Y.,1956. The mode of life of certain Carboniferous animals from the west Kirkton Quarry, near Bathgate. Trans. Eilinbirrgh Gcol. Soc., 16 : 272-219. D. and SIMPSON, S., 1962. Choniatichtius. a new ichnogenus, and other trace-fossils of DONALDSON, Wegber Quariy. Liverpool Manrhester Geol. J., 3 : 73-81. FEROUSON, L., 1962. The palaeoecology of a Lower Carboniferous marine transgression. J. Paleontol., 36 : 1090-1101. GOLDRING, R., 1962. The trace fossils of the Baggy Beds (Upper Devonian) of North Devon, England. Paliiontol. Z., 36 : 232-257. HALLAM, A., 1960. A sedimentary and faunal study of the Blue Lias of Dorset and Glamorgan. Phil. Trans. Roy. SOC.London, Ser. B, 243 : 1 4 . MIDDLEMISS, F. A,, 1962. Vermiform burrows and rate of sedimentation in the Lower Greensand. Geol. Mqy.. 99 : 3 3 4 0 . REINECK,H. E., 1958. Wuhlbau-Gefiige in Abhangigkeit von Sediment-Umlagerungen. Senckcviber,niana Lethaca, 39 : 1-23, 54-56. REINECK, H. E.. 1960 Uber Zeitliicken in rezenten Flachsee-Sedimenten. Geol. Rundschair,49 : 149-161. SCHAFER, W., 1956. Wirkungen der Benthos-Organismen auf den jungen Schichtverband. Srtickenbet-yiatia Lethaea, 37 : 183-263. SEILACHER, A., 1953. Studien zur Palichnologie. 11. Die fossilen Ruhespuren (Cubichnia). Neites Jahrb. Geol. Palaontol. Abhandl., 98 : 87-124. SEILACHER, A., 1954. Die geologische Bedeutung fossiler Lebensspuren. Z. Derrt. Grol. Ges., 105 : 2 14-227. SEILACHER, A,, 1955. Spuren und Fazies irn Unterkambrium. In: 0. SCHINDEWOLF und A. SEILACHER, BeitrLige zur Kenntnis des Kambriirrns in der Salt Range (Pakistan) - A k a d Wiss. Lit. Mainz Abhandl. Math. Nut. KI., 1955 : 313-399.

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SEILACHER, A., 1958. Zur okologischen Charakteristik von Flysch und Molasse. Eclogae Ceol. Helv., 51 : 1062-1078. SIMPSON, S., 1957. On the trace-fossil Clionrlrites. Quart. J . Geol. SOC.London, 112 : 475-500. SMITH,J., 1891. Note on the occurrence of footprints in the calciferous sandstone between West Kilbride and Fairlie. Trans. Geol. SOC.GlaTyow, 9 : 201-203.

BEACH STUDIES IN WEST FLORIDA, U.S.A.l D. S. CORSLINE

Department of Geolo,q, University of Southern Calfornia, Los Angeles, C a l f . ( U.S.A.)

INTRODUCTION

During the period from January to December, 1962, a series of fifteen beach stations distributed along the Florida Panhandle Coast were occupied monthly and profiles and other characteristics measured and recorded. The primary objective of the survey was to determine the magnitude of the seasonal variations of the beach dimensions and also to investigate the associated changes in the sediments and Foraminifera populations of the beach zone. This short report will note some of the initial conclusions based on the topographic and sedimentologic data. A more detailed paper is in preparation. Fig. 1 illustrates the station locations. This study was part of a larger study of bays of west Florida. I

ALA.

FLA

bJ\J1,

d

Fig.1. Locations of the fifteen beach profile stations. All stations were located on the open Gulf shore except S.J.P.C., Royal Bluff and East Point.

DISCUSSION

The data collected during the survey included detailed topographic profiles taken from fixed bench marks located well above the high water level and extended out beyond the surf zone. Profiles were determined using modified Jacob's staffs after a method desContribution no. 194 of the Florida State University, Oceanograc:,.c Institute.

145

BEACH STUDIES IN WEST FLORIDA

cribed by EMEKY(1961). Sediment samples were collected at intervals of one and onehalf meters on the first survey and then only a t the berm. swash line. mid breakerswash zone and beyond the surf on all subsequent profiles since these were found to be sufTicient to describe the character of the beach material. Water samples were collected for later analysis, temperatures of air and water, wind direction and velocity, biological samples of the swash line. wave period and height and wave direction were all measured and recorded. Sediment samples were analysed texturally using an Emery Settling Tube (EMERY,1938). Biological samples were stained with a protein specific dye (Rose Bengal) so as to mark living organisms. I n order to ascertain the representativeness of nlonthly profiles a series of profiles were measured at several test beaches at intervals of one hour. one to two days. one week and one month. These preliminary studies established that the monthly changes a r e a good sample of the important beach variations effected by changes i n tide and wave trains. TABLE I C F N F R A L SHORE CHARACTERISTICS

A rcrc!ycJ

Slclliulr

IllPUll

110.

iilttl

~-

o

Keaton Beach

I 2

Mashe Island Alligator Berch S.J.P.C. Royal Bluff East Point West Pass San Blas Beach Mexico Beach Panama City Grayron Beach Destin Beach Pensacola Reach Gulf Beach Gulf Shores

3

i 5

5 8 I2 14 I6

17 19 20 21

311.5 5!3.0 17!3.0 712.0 10:3.0 812.0 2514.0 231'4.0 2715.0 2716.0

3015.0 3015.0 3015.0 2415.0 35l5.0

5 10 30 10

2 2 8

20 15 50 35 40 45

3

50 60 75 45 65

~

30

-7

3 10 10 12 10 8 8 17

I5 17

-

100 80 60 35 85 40 60 40 50 35 50

0.28 0.36 0.33 0.27 0.27 0.44 0.31 0.32 0.42 0.30 0.36 0.29 0.43 0.44 P.75

See Fig.1. '?

Average of six months observations.

:' Mean sediment diameters of all samples collected at the swash line at each station Preliniinary examination of the data (Table I) indicates an expectable increase i n wave energy (in terms of wave heights) as the observer progresses from east to west. I n addition, the bay stations are all of the low energy type due to the protection from wave attack by the off<'- barrier islands. Maximum monthly profile changes occur region. Negligible changes are observed at Keaton Beach, at the western port;,

146

D. S. GORSLlNE

Mashe’s Island and the Apalachicola Bay stations (Fig. 1). PRICE’S (1954) original classification of the Big Bend region as a zero energy coast is confirmed. Net vertical changes in the zone of maximum variation range from no change to as much as two meters on the moderate energy beaches of the western sector. The size of the profile “envelopes” at each station are proportional to the much larger changes observed in a similarly executed study of the high energy beaches of the Pacific Coast Of the U.S. ([NGLE, 1962). Drift direction tabulations are surprising in that they are not mono-directional except at two stations (Royal Bluff and Mexico Beach) where offshore features screen out all but a small sector of possible wave approaches. The observations contrast with the evidence of general shoreline trends on aerial photographs and charts which seem to indicate dominant westerly construction in the vicinity of the station locations. There may be a roughly defined association of westerly drift directions and the highest waves and thus, the major long term shore trends may represent the effect of relatively short periods of strong wave action. The greatest fetch for wave generation is from the south and southwest. Sediments are essentially uniform i n character on the Florida beaches. Therefore, no marked changes in regional averages of mean diameter are observed (Table I). However, there appear to be real differences in individual sample profiles. These often show best sorting and markedly different mean diameters (as compared to berm and offshore) in the swash zone. At many beaches shells, shell fragments, drift wood, kelp and other debris accumulate in this zone. Maps of the wave front bearings at all stations for each month produce an interesting pattern. The area can be readily divided into a western and eastern sector west and east of Cape San Blas. Wave directions in each sector are always opposed either divergently or convergently and cross seas are always observed at Cape San Blas. This implies that either a topographic or meteorologic boundary is present. Since the Cape San Blas shoal is not sufficiently large to influence the patterns over such an extended region, it is suggested that the wave evidence is indicative of a meteorologic zonation. Cape San Blas Shoal may itself be a product of the converging seas. This boundary may also delineate sediment provinces but the gross similarity in the Florida shore sands prevents a clear conclusion on this point. A preliminary study of the heavy mineral assemblages of a few samples suggests that the western sands have appreciably smaller heavy mineral contents and also that the opaques (principally magnetite and 1960). ilmenite) are in much greater abundance in the heavy fraction (CARPENTER, More detailed studies are in progress. ACKNOWLEDGEMENTS

The writer wishes to acknowledge the aid and financial support of the study by the Geography Branch of the Office of Naval Research. Much of the hard work of surveying was made possible by the devoted assistance of Messers Noel Plutchak, Robert Kunzler, Richard Evans, Ronald Kolpack, John Schnable, Richard Mitterer, Charles

BEACH STUDIES IN WEST FLORIDA

147

Early, Don Milligan, Vernon Finney and Charles Pflum. Mr. Plutchak and Mr. Kunzler did much of the sedimentologic analysis and Mr. Evans and Mr. Kunzle: also drafted most of the profiles. Salinities were measured by Miss Gloria Brown. Laboratory and field facilities were provided by the Oceanographic Institute and the Department of Geology of the Florida State University. Mr. Ronald Kolpack read the report manuscript and offered many useful criticisms. This study was supported by funds under contract nonr. 988 (07) with the Geography Branch, Office of Naval Research.

SUMMARY

Topographic profiles, sediment and water samples, and climatic data have been collected in a monthly schedule at fifteen stations distributed along the western Florida Panhandle coast during 1962. These observations and associated wave measurements have been studied to determine the magnitude of the beach variations associated with different energy ranges of wave attack. Preliminary conclusions are that the patterns show a direct relation between change i n beach profile versus increasing wave energy and further, that a regional climatic-oceanographic boundary can be defined on the basis of beach and wave data.

REFERENCES

CARPENTER, J. W.. 1960. Heavy Mineral Assemblavs in Florida Beach Sands. Unpubl. res. rept. Dept. of Geol., Florida State Univ.. Tallahassee, Fla., 12 pp. EMERY.K . 0.. 1938. A rapid method of mechanical analysis of sands. J . Sediiiient. Peirul., 8 : 105-1 I I . EMERY,K . 0.. 1961. A simple method of measuring beach profiles. Linxol. O c e a n q . , 6 : 90-93. INCLE JR., L. C., 1962. Tracing movement of beach sands by means of fluorescent dyed sand. Shore Beach, 30 (2) : 3 1-37. PRICE,W. A., 1954. Dynamic environments - reconnaissance mapping, geologic and geomorphic of the continental shelf of Gulf of Mexico. Trans. Gfibr. Coast Assoc. Geol. SOC.,4 : 75-107.

L A SEDIMENTAI'ION SOUS-MARINE DANS LA PARTIE ORlENI'ALE DE LA RADE DE BKEST, BRETAGNE

C O N D I T I O N SD E L A S ~ D I M E N T A T I O N

La Rade de Brest (Fig. I ) est une assez vaste ttendue d'eaux marines, qui ne communique avec I'Ocean que par u n goulet de 2 k n i de large et 50 ni de profondeur. Dans sa partie orientale. seule ttudiee ici, on distingue deux tltnieiits topographiques essentiels: ( I ) Des baiics, dont la profondeur n'atteint iiulle part 10 m sous les plus basses mers, et dont quelques parties tmergent aux grandes basses rners.

Fig.1. Topographic sous-marine de I'est d e la Rade de Brest. 6quidistance: 5 m (.sauf courbe de 3 m par endroits). Profondeurs en metres sous les plus basses mers.

(2) Deux valltes sous-marines encaisstes dans la partie axiale, q u i sont les prolongeinents des Rivikres de Chiteaulin et de Daoulas, ennoytes par la transgression flandrienne, et dont le goulet d'entrte de la rade est la partie aval; la profondeur dtpasse u n peu 30 rn dans I'ouest de la region ttudite. Dans la Baie de Daoulas, la vallte sousmarine nord a ktt cornblte par la stdimentation. La vallte de Chlteaulin est inter-

S~DIMENTATION SOUS-MARINE DANS LA R A D E IIE RREST

149

roinpue par u n seuil. la Traverse de I’H6pita1, q u i doit Ctre un ancien mtandre en majeure partie stdimentt: il en rtsulte i cet endroit u n decrochement dans la direction de la vallte. En outre, d a m I’Anse du Poulniic existent quatre valltes beaucoup rnoins profondes: l’une d’entre elks peut Etre u n autre mtandre de la vallte de Chiteaulin. recoupe par des processus Huviatiles avant la transgression; les trois autres sont vraisemblablement de petites valltes affluentes, beaucoup plus courtes, et moins profondes avant to u te sed i men t a t i on m ari nr: . Comine la houle du large ne penktre pas en Rade de Brest, ce milieu n’est atrecti que par des vngues nCes localenient, sur des fetches atteignant au maximum 24 kin. et gtneralement inferieurs: les vagiies ont donc une trts courte longueur d’onde et s’aniortissent tres rapidement en profondeur. Par contre, les courant.~de mur& sont notables, mais d’iniportance variable selon les lieux. D’apres les cartes de courants de surface d’heure en heure du Service Hydrographique francais. en vive eau rnoyenne (coefficient 100) ils atteignent jusqu’i I ,7 noeud dans I’axe de la vallte de Chiteaulin: I ,3 noeud sur le Banc de Skivieg; I ,O noeud dans le sudest de la Baie de Daoulas; 0.9 noeud dans I’Anse du Poulrnic; 0.8 noeud sur le Banc du Loc’h; 0,7 noeud sur le Banc du Poulmic. La Rade de Brest est u n milieu hybride, different d u milieu estuarien et du milieu bien ouvert sur le large, et participant des deux. C’est de Ih que dtcoulent les caracteres de sa sedimentation.

C A L C I M ~ T K IET E P ~ O G K A P H I DES E S~DIMENTS

Deux cent quinze tchantillons ont Ite draguts d a m I’aire ttudite, bord du petit bateau oclanographique “Kornog“, au moyen d’une drague Rallier du Raty. Les Fig. 2 et 3 permettent de voir quelle est la densitt des prtlkvements. Ceux-ci ont It6 faits dans toutes les positions topographiques, reptrtes au sondeur, sauf sur les hauts et al., 1957). d’estrans qui avaient d6jh ttt ttudits avant (GUILCHER La teneur en CaCO, de ces stdimelits est un premier caracttre trts important. On voit sur la Fig.2 que cette teneur est, en rnoyenne, trks tlevCe: dans I’irnmense ma,joritt des echantillons. elk dtpasse 20%. et il l u i arrive parfois d’exctder 80%. Pourtant, les roches de cette region, qui sont des schistes devoniens avec u n certain nonibre de filons basiques, sont trks gtntralement siliceuses: des gisements calcaires existent sur les bords de la rade, mais ils sont rares et trks peu ttendus. La forte teneur en calcaire provient donc presque uniquement de la fraction organogkne, et elk permet de mesurer la trks grande importance de cette fraction dans la skidimentation sous-marine. I1 y a cependant des difftrences dans les teneurs: les pourcentages depassant 50% se rencontrent dans la partienordouest et ouest, dans l’axe NE-SW de la Baie de Daoulas, et d a m la partie nordest (Traverse de I’H6pital et Banc de Gwaskellou). Au contrairte, les teneurs sont faibles d a m le sudouest (Anse du Poulmic), le sud (Banc du Loc’h), I’est (Landevennec), et d a m le nordouest et le sudest de la Baie de Daoulas.

150

A. GUILCHER

On a aussi determint, pour chaque echantillon, la teneur en Ca CO, de la fraction suplrieure a I mm, et de la fraction infkrieure a 60 p. Ces opkrations ont montre que, cornme en bien d'autres sediments marins, la teneur en calcaire varie en raison directe de la grosseur des particules. Au-dessus de 1 mm, les teneurs supirieures a 90% sont

Fig.2. Calcinittrie. Les chiffres donnent les pourcentages globaux de CaCO,,. Lirnite en trait plein: 50%; et de part et d'autre.

+

~

frequentes, et beaucoup plus gtneralisees dans I'espace. Au contraire, en-dessous de 60 p. les teneurs sont gentralement 10-30 %. Cette difitrence est essentielle; cependant, on remarquera que la fraction fine est loin, en general, d'Ctre complktenient dtnuie de calcaire: celui-ci y est seulement beaucoup moins abondant. Pour prtciser la nature des elements grossiers, I'exarnen macroscopique des echantillons a suggtrt la niithode suivante: on a procide, sur les tchantillons contenant une fraction sufisante entre I et 2 mni (c'est i dire, en fait, presque tous les echantillons). a un comptage du pourcentage des particules classtes en trois categories: Lithothamnium culcareum, appelt "maerl" en Bretagne; dtbris de coquilles; dtrnents minlraux. La Fig.3 donne le pourcentage des particules de Lithothamnium. Cette algue calcaire apparait comme ttant trks souvent prtdominante: c'est le cas le plus repandu sur les bancs, q u i sont un milieu extremement favorable, oh habituellement on recueille c6te B c6te du maerl vivant et du maerl mort. Cependant, dans la vallte sous-marine de Chiteaulin, la teneur en maerl devient le plus souvent faible ou trks faible. Nous attribuons ce fait ce que le maerl demande un bon kclairage pour vivre, et que la vallte est trop profonde pour qu'il y prospkre. Ceci implique que la sedimentation se fait sur les lieux de vie, donc que les transports de sediments sont faibles. Au contraire, les dtbris de coquilles sont extrCmement prtdominants dans la vallte sous-rnarine de Chiteaulin. Quant ri la fraction minerale a 1-2 mm, e l k est presque partout insigni-

S ~ D I M E N T A T I O NSOUS-MARINE DANS LA RADE DE BREST

151

-

Fig.3. Pourcentages de grains de rnaerl (Lithothamniuni ralcarrun~)dans la fraction 1-2 rnrn. Les tirets delimitent les vallees sous-marines de Chlteaulin et de Daoulas.

fiante: consistant en fragments de schistes ou plus rarement de mica et de quartz, ses pourcentages les plus frlquents vont de 0-3%, et ceux q u i dtpassent 10% sont trts rares. L’abondance du calcaire organogtne est a her au caracttre mixte de la Rade de Brest: dans les estuaires proprement dits, en effet, la fraction calcaire est en gentral trts inftrieure aux chiffres atteints ici. Comnie le calcaire organogkne est trts fragile, il peut paraitre ttonnant qu’il soit plus abondant aux grandes dimensions qu’aux petites: on peut prtsumer que le sediment subit une dtcalcification, qui soustrairait h mesure i la fraction fine ce qui devrait normalement lui revenir d’apports incessants de tests d’organismes.

A de rares exceptions prts, les stdiments de cette aire marine sont htttromttriques, contenant des particules de toutes tailles depuis celles inftrieures i 50 p jusqu‘aux graviers, en passarit par les sables. Le stdiment a gtntralenient une texture vaseuse comnie ceux des estuaires, mais il contient une fraction grossitre plus abondante que la moyenne des vases estuariennes. La encore apparait le caracttre mixte de la rade. On n’a pas fait la micro-granulomttrie de la fraction inftrieure a 50 p, mais on a tamist sous l’eau chaque tchantillon sur un tamis de 50 p, determint ainsi le pourcentage pondtral global de la fraction fine, et passt ensuite a sec la fraction de plus de 50 p dans une colonne de 9 tamis. On a ainsi pu dtterminer les principaux caracttres granulomitriques, et notamment les contrastes en fonction des situations topogra-

152

A . GUILCHER

phiques. Ces contrastes sont representis ici sous fornie de diagrammes triangulaires. la poudre et I'argile (silt and clay) etant grouptes ensemble, et les autres pointes des diagraninies etant attribuees au sable et au gravier. La Fig.4 donne la cle de la nomenclature, inspiree de celle de SHEPARD (1954).

sah!e

25

I

sable

50

75

Fig.4. Classification des categories diniensionnelles dans les diagramnies triangulaires. (d'Apres SHEPARD, 1954).

( / ) Sur la Fig.5 sont report& les points figuraiit les sediments des bancs du sud (Anse et Banc du Poiilniic, Banc du Loc'h, Banc Rouge), et de la Baie de Daoulas. Malgre une assez forte dispersion, on voit que ces points tendent ri se situer surtout dans les categories poudre sableuse et poudre graveleuse. et que mCme un certain argile. Ces sediments sont donc, comme on nombre sont d a m la categorie poudre

+

Fig.5. Granulometrie des sediments des bancs du sud et de la Baie dc Daoulas

S ~ D I M E N T A T L OSOUS-MARINE N D A N S LA RADE DE BREST

153

I'a dit, htttronittriques, A part trois ou quatre, mais, en gtneral, la fraction fine y est dominante ou tres dominante. (2) Sur la Fig. 6 sont figurts les points correspondant aux stdinients des bancs du nord (bancs de Skivieg, du Bendi, de Gwaskellou). Ici, la fraction fine n'est pas pre-

Fig.6. Granulometrie des sediiiients des bancs du nord, sauf la Baie de Daoulas.

dominante comme prtctdemment: les points sont groupts en grande niajorite dans la partiecentraledu diagramme, oh le gravier, le sable et la fraction poudre-argile sont tous prtsents en quantitts importantes et relativement tgales. Ce sont donc ceux des stdiments oh la dispersion granulomttrique est la plus grande, et ils sont nettement moins fins que ceux des bancs du sud et de la Baie de Daotilas. ( 3 ) Sur la Fig.7 sont indiquts, en cercles noirs, les points correspondant aux

Fig.7. Granulometrie des sediments des vallees sousmarines de Chiteaulin et de Daoulas. et de la Traverse de 1'88pital (cercles noirs), et des valltes sous-marines de I'Anse du Poulmic (croix).

154

A. GULLCHER

tchantillons des deux valltes sous-marines de Chlteaulin et de Daoulas, et ceux de la Traverse de I’Hbpital. Par rapport aux cattgories prtctdentes, ces points sont decalts vers les fractions sables et graviers: ils tendent ri se grouper dans la droite du cadre central. et dans le cadre du sable graveleux et du gravier sableux. Autrement dit, la fraction grossitre est encore plus importante que sur les bancs du nord. ( 4 ) Enfin, sur la Fig.7 tgalement, les croix indiquent les tchantillons des quatre petites vallees sous-marines peu profondes de I’.4nse du Poulmic. Ces stdiments sont complttement difftrents deceux des grandes valltes axiales: ce sont les plus fins de tous. Ces difftrences entre les cattgories de stdiments sont explicables par les courants de mar& (voir les vitesses de courants cittes plus haut). Les courants sont canalists surtout par la vallie sous-marine de Chlteaulin, et de Iri ils franchissent le barrage de la Traverse de I’H6pital. II est comprthensible que dans cette rtgion oh le courant est le plus fort, la fraction fine soit la moins abondante (Fig.7, cercles noirs). Sur les bancs du nord (Fig.6), le courant circule aussi assez vite. Au contraire, les bancs du sud sont abritts par la pointe de Peiin ar Vir qui dtvie le courant de flot vers le nord: I’Anse et le Banc du Poulmic et le Banc du Loc’h sont une aire i courants faibles, d’ou I’abondance des particules fines sur les baiics et dans les petites valltes peu profondes de cette rtgion (Figs. 7, croix). De m h e . la Baie de Daoulas est dans u n rentrant assez abritC (Fig.5). On remarquera d’autre part la correspondance entre les teneurs en calcaire relativement fortes et la stdimentation assez grossitre des bancs du nord, et la correspondance entre les teneurs en calcaire faibles et la skdinientation fine des bancs du sud et de la Baie de Daoulas (cf. Fig.2, 5 et 6). Ceci est une autre manitre d’exprimer le fait que plus les sediments sont grossiers, et plus ils sont calcaires. Enfin, un fait remarquable est la prtsence. en de trts nombreux tchantillons, de deux maxima granulomttriques distincts dans le gravier et dans le sable. Cette particularitt nous semble due a ce que le maerl est, par sa forme branchue et sa faible densitt, un mattrial trts particulier. q u i a ses rtgles propres de stdimentation (BERTHOIS et GUILCHER, 1959): il se melange aux particules d’autres origines en donnant un ensemble hybride. Et ce fait s’ajoute encore aux particularitis du milieu marin que nous ttudions ici.

REMERCIEMENTS

Ce travail rlsulte d’une recherche effectute ri bord du “Kornog”, puis au laboratoire de I’lnstitut de Gtographie de Paris, et i laquelle ont participt Mme Ters et M. Elhai, Maitres-assistants; Melle Pruleau, qui a rtcoltt les stdiments et a fait la plus grande partie des manipulations; Mesdames et Melles Kermarec, Malglaive, Paloque, Schiano, Sztokman, et MM. Ciblat, Leroi et Sztokman, ttudiants; M. Guez, Collaborateur technique; ainsi que l’tquipage du “Kornog”, MM. Le Lann, Kerros et H u h .

S~DIMENTATION SOUS-MARINE DANS LA RADE DE BREST

155

La partie orientale de la Rade de Brest comprend des bancs peu profonds (0-10 m). et des valltes sous-marines atteignant jusqu'h 30 m sous les plus basses mers. Les vagues y sont faibles, mais les courants de m a d e sont sensibles dans la partie axiale. Les stdiments sont trks calcaires, surtout dans leur fraction grossikre, beaucoup rnoins d a m leur fraction fine; le carbonate de calcium est fourni ii peu pres exclusivement par du Lithotiiamniwn cdrareum et pas des dtbris coquilliers; mais le premier est peu reprisente dans les valltes profondes, oh les conditions de vie ne lui sont pas favorables. Les elements noii organogenes sont t r b peu nombreux dans la fraction grossiere. Les stdiments sont htttrometriques, comprenant ii la fois des tltnients grossiers, moyens et fins. 11s presentent des differences locales, qui s'expliquent par la plus ou moins grande force des courants de maree: plus grossiers d a m les vallies sousmarines et dans le nord, ils sont plus fins d a m le sud. Ces sediments sont caracteristiques d'un niilieu d'eaux peu profondes intermidiaire entre les estuaires et les eaux ouvertes sur le large.

SUMMARY

The eastern part of the Rade de Brest comprises shoals (0-10 m depth) and submarine valleys, with maximum depths of 30 m below the level of the lowest tide. Wave action is of small intensity, but tidal currents reach high velocities, notably in the axial parts. The sediments are highly calcareous. The calcium carbonate, which is predominantly present in the coarse fraction, the fine fraction containing much less of it, is almost exclusively supplied by Lithothamnium calcareum and by shell debris. Little Litliothamnium material is found in the deep valleys where conditions are unfavourable for its growth. Inorganic elements are very scarce in the coarse fraction. The sediments show poor sorting of grain sizes, and contain elements of coarse, medium and fine size grades. They show local variations, which can be explained by differences in strength of the tidal currents: coarser material in the submarine valleys and in the north, and finer material in the south. The deposits are typical for a shallow water environment, intermediate between estuaries and the open sea.

BTBLlOGRAPHlE

BERTHOIS, L. et GUILCHER. A., 1959. Les Bancs de Saint-Marc et du Moulin-Blanc (Rade de Brest), et remarques sur la stdimentation du maerl (Lithnthunminrn culcurerm7). Cuhiers Oriunqy., 1 1 : 13-23, COLLIN. L., 1921. Geologie probable du fond de la Rade de Brest. Bull. Sor. Ciol. MinPrul. Bretape, 2 : 354-372. P., ANCRAND, J . P. et GALLOY, P., 1957. Les cordons littoraux de la Rade GUILCHER, A., VALLANTIN, de Brest. Bull. ComitP Ociunog. I?tudeles C&s, 9 : 21-54. GuiLcHm, A. et PRULEAU,M., 1962. Morphologie et skdimentologie sousmarines de la partie orientale de la Rade de Brest. Coni. Truv. Hisr. Sci., Bull. Sect. GPopruph., 75, sous presse.

156

A. GUILCHER

SHEPARD, F. P., 1954. Nomenclature based on sand-silt-clay ratios. J. Sediment. Petrol., 24 : 151-158. VACHW, A., 1919. La Rade de Brest et ses abords, essai d'interpretation morphogenique. Ann. Gdograph., 28 ; 177-207.

LIASSlC SEDIMENTARY CYCLES I N WESTERN EUROPE AND THEIR RELATIONSHIP TO CHANGES IN SEA LEVEL A. HALLAM

INTRODUCTION

One does not tiorinally associate the west European Lias with absorbing problems of cyclic sedimentation. Admittedly the development of cycles is neither so spectacular nor common as in, say, the Upper Carboniferous coal measures, but the Lias possesses the inconiparable advantage of a rich ammonite sequence, which allows for extremely refined correlation. This means that one can often compare sediiiientary environments over vast distances for very restricted intervals of time. and has led to results of considerable interest which could well have general application.

ERACHIOPODS. ROBUST P E L E C Y P O D S , ECHINODERM DEBRIS. REEF CORALS

INTERMEDIATE FAUNA

AMMONITES FRAGILE PELLCYPCDS

(1

C13y

Laminated shale

Fig.1. Diagrammatic illustration of a complete limestone-clay cycle of Kliipfelian type, and its equivalent in a sandstone-clay succession. The laminated shale and oolite are additions to Kliipfel's scheme.

158

A. HALLAM

The germinal idea i n the present work has been provided by the German geologist KLUPFEL ( I 9 17, 1920), whose study of the Lias of Lorraine shows an admirable combination of insight and meticulous observation. Kliipfel perceived a regular cyclic alternation in the limestone and clay succession of the Lias and Dogger. His complete cycle b-gitis with a clay containing a comparatively sparse fauna of ammonites and thin-shelled p-lecypods. This passes up gradually into more calcareous beds and eventually into a compact limestone with robust pelecypods, corals etc. At the top is a distinctive “Dachbank” or roofbed, which is capped by an erosion surface (Emersionsfliiche) marked by organic borings and incrustations. Slow deposition of the roofbed is signified by an abundance of phosphate and bone fragments, driftwood and shells. Directly above the roofbed there may be a layer of phosphatic nodules and incrusted pebbles, seemingly derived from the underlying limestone. The transition to clay of the overlying cycle is generally sudden (Fig.1). A sandy variant of the Kliipfelian cycle is also illustrated in Fig. 1. Kliipfel attributed the formation of his cyclic sequence to progressive shallowing of the sea followed by comparatively sudden deepening consequent upon regional epeirogenic movements of the sea floor, a seemingly plausible interpretation supported 1925; H E t D o R N , 1928; SOLL, 1957). In my own by other German geologists (FREBOLD, investigations on the Lias I have been intrigued to find cyclic sequences in Britain closely comparable with those described by K L U P F E L (1917) and have been led to pursue them laterally in the hope of learning more about the underlying mechanism.

LATERAL CHANGES OF FACIES

Any attempt to elucidate the problems of cyclic sedimentation by comparative study i n different regions obviously depends upon a thorough understanding of the variolrs environments of deposition. In the Lias of western Europe marine sedimcntation can be related to a series of basins and swells (Fig.2). Basins are regions of comparative downwarping characterised generally by thick, more or less uninterrupted clay sequences. Swells are topographic highs (either isolated marine shoals or land margins) over which sedimentation was slow. They are usually signified by thin calcareous or sandy deposits with frequent erosional breaks. The different environments of deposition may be treated in more detail with reference to the model illustrated in Fig.2a. Steady subsidence in the land-locked basin A ensures regular sedimentation in quiet water which, because of lack of renewal, is anaerobic below what is termed here the limit of oxygen availability. In consequence the sediments in the deepest parts of the basin lack a normal bottom fauna and are rich in bituminous matter, which marks out a very fine lamination probably annual in origin. Passing upslope towards the shallower reaches of the basin, but still below wave base, inorganic precipitation of calcium carbonate becomes increasingly important as terrigenous sedimentation declines and the waters become warmer, so that deposits become increasingly calcareous though remaining fine-grained. Benthonic organisms flourish above the limit of oxygen avail-

RELATION OF LIASSIC SEDIMENTARY CYCLES TO SEA LEVEL CHANGES

159

a

A

b

F i g 2 The effect of sea level changes on Liassic environments. Full explanation in text. Ornamentasea level: W.B. = wave base; L.O. V. = limit of oxygen availability. No tion as in Fig.1. S.L. quantitative significance is implied in the spacing of these three levels. 1

ability and destroy any lamination originally present. On the swell B, comparatively resistant to downwarping and isolated from substantial terrigenous influx by a zone of deeper water C,sedimentation is comparatively slight. Above wave base finer material is winnowed away, leaving little but ooliths and skeletal carbonate. The shallow near-shore basin C suffers less downwarping than A and never reaches the limit of oxygen availability. It is constantly fed by a rich supply of sediment from the land D, towards which the bottom deposits become increasingly silty and, above wave base, sandy. The sandy deposits are characteristically well sorted medium grained and cross-bedded, having been reworked from deltaic distributaries by tidal and longshore currents and waves. Though the basin is never deep the intense sedimentation from the

160

A. H A L L A M

neighbouring land ensures a high degree of dilution of precipitated calcium carbonate and calcareous muds rarely form. At certain times and places, for reasons still poorly understood, the seas may become enriched in iron, with the consequent formation of oolitic ironstones above and sideritic and chamositic niudstones below wave base instead of oolitic limestones and calcilutites respectively. I n off-shore waters the transition from shale to liniestone is nornially accomplished via an intermediate zone of alternating niarls and argillaceous calcilutites. These minor cycles are thought to be the combined result of minor oscillations of some primary factor (climate or depth of sea) and rhythmic segregation during early diagenesis. Most regional facies variations within the Lias can readily be accommodated within the framework of this model, which also provides the key to the understanding of cyclic variations in the vertical succession. It has been possible, by using these environmental criteria, to demonstrate that Kliipfelian cycles in Britain, Germany and northern France are the local expressions of extremely widespread and essentially synchronous variations i n depth of sea which seen1 to have been independent of a complex pattern of basinsand swells (HALLAM,1961). This fact suggests that the underlying control may have been the eustatic rise and fall of sea level rather than local epeirogenic movements.

EUSTATIC CONTROL

Let us return to Fig.2. Fig.2b illustrates the consequence of a eustatic rise while Fig. 3c shows a lowering of sea level. In the first case a transgression over the land will be accompanied bya shiftingof sedimentary zones so that deeper water facies will come to lie on shallower. Conversely a lowering of sea level will result i n a regression and a replacement of deeper by shallower water facies. accompanied perhaps by erosion over the emergent swells. This is all simple and obvious but it should be borne i n mind that the sedimentary changes will not be equally pronounced in all environments. As clays and niarls may be deposited over a considerable range i n depth a change i n sea level may have little to no effect on the sedimentation in a given locality. If a mud zone is brought above wave base for a brief period without the opportunity of further sedimentation the only record of the event may be a thin band of winnowed and disarticulated shells. High on a swell the only significant disturbance may be an erosional break within an otherwise uniform sequence of limestone. To prove beyond reasonable doubt the operation of eustatic control, it is necessary not merely to correlate likely changes of sea level in the same sense at particular!iorizoiis but also to relate them to major transgressions and regressions which affected different continents, so ruling out the possibility of local tectonic control. This is not an easy undertaking at present since we lack fine stratigraphic detail in most area< outside Europe. Furthermore, local epeirogenic movements can complicate the picture. and the evidence for transgressions over swells may often have been all but destroyed by peneconternporaneous erosion. There is an illuminating instance of this

RELATION OF LIASSIC SEDIMENTARY CYCLES TO SEA LEVEL CHANGES

161

near Caen in Normandy, where a Domerian succession rests upon the Palaeozoic massif, but pockets of shells within fissures on the old land surface are of Carixian age (RIOULT,1958), evidence that could easily have been missed in any but a meticulous survey . Nevertheless there is already good evidence in at least two cases of extensive marine transgressions coinciding with a change to deeper water facies within areas of marine deposition. Domerian-Toarcian The change from the Middle- to Upper Lias from Britain to Germany is perhaps the most striking in the whole of the Lower Jurassic, almost everywhere a series of shallow water sandstones, oolitic limestones and ironstones and shell limestones being directly succeeded by deeper water shales. Especially striking, the Falcifer Zone (Lower Toarcian) is represented over a vast area by finely laminated bituminous shales suggestive of deep water within an extensive land-locked sea. A change of this character is not confined to the northwestern and central part of the continent but extends southwards at least as far as the Jura Mountains (FRANK,1930) and the Aquitaine Basin (DUBAR,1925). Evidence of Lower Toarcian transgressions over old land masses and swells is present i n southwest England (KELLAWAY and WELCH,1948), the flanks of the Armorican Massif (BIGOT,1930; GABILLY, 1961) and possibly the old Vindelician Massif in Switzerland where, however, the oldest proved transgressive deposits (at Vattis) are of Bifrons age (TRUMPY,1949). At any rate the deposition of the Middle Lias of this region was followed by a period of widespread emergence (TRUMPY,1952). More significantly, there is transgressive Lower Toarcian in the Donetz Basin in Russia and over a huge region extending from Arabia to East Africa and Madagascar (ARKELL, 19%). In both areas the age of the oldest transgressive deposits corresponds with the probable time of deepest water (Falcifer Zone) in Europe. There was also an extensive Lower Toarcian transgression over the western and northern parts of the Canadian Shield, with the earliest deposits of Fulcifer or Bifrons age (FREBOLD, 1957, 1960). Sinmurian-Carisian Though the change is usually less striking than in the previous example there is evidence of the commencement of a new sedimentary cycle in many parts of northwestern Europe at the beginning of the Carixian or Lower Pliensbachian (HALLAM, 1961), as also in the Helvetic Alps (TRUMPY,1949). This coincides with the time (Jamesorzi Zone) of an extremely widespread transgression, over the old land mass that extended from southeastern England to the Ardennes (MELVILLE, 1961; HALLAM, 1961), northern Europe (TROEDSSON, 1951) and East Greenland (DONOVAN, 1957), apparently also in Basse Provence, Aquitaine and the Pyrenees (DUBAR, 1925; LECALVEZ and LEFAVRAISRAYMOND, 1961), where the early Carixian sea returned after extensive end-Sine-

162

A . HALLAM

muri..i:i emergence. The oldest dated rocks of the important transgression over the Arniorican Massif are dated as Zbes Zone (BIGOT, 1930) but the sea could well have started to advance in Jarnesoni Zone times. Altogether a total of eleven stratigraphic units corresponding locally with more or less complete Kliipfelian cycles has been recognised in northwestern Europe (HALLAM, 1961) but so far there is inadequate evidence to prove that these all signify the operation of eustatic movements, though the balance of probability favours this alternative. There are already quite reasonable grounds, however, for believing that widespread transgressions took place i n the Hettangian, Lower Sinemurian and Lower Domerian.

THE INFLUENCE OF REGIONAL EPEIROGENY

The eustatic changes themselves are doubtless the consequence of epeirogenic movements in both positive and negative senses in the ocean basins, but we have so far neglected the influence of local movements within Europe. It is generally accepted that while the basins have been subjected to more or less continued downwarping the intervening land masses have undergone periodic uplift. I n the first place this may help to explain the comparative constancy of palaeogeography throughout the Lias in Europe, with only limited transgressions for the most part because uplift of the land would act in opposition to rise of sea level. Secondly, it may help to account for the characteristic asymmetry of the sedimentary cycles. At first glance the abrupt transition at the top of a cycle from limestones and sandstones to clay suggests a comparatively sudden deepening of the sea following a gradual sliallowing. However, subsidence of the basins and their margins, where most of the deposition took place, would accentuate the change due to eustatic rise. Moreover, the concomitant retreat of shorelines would probably serve to reduce the supply of sediment from the land so that the early, transgressive phase of a cycle sho~ildnormally be signified by condensed beds (ALLEN, 1959). This seems frequently to be the case, with the base of a given cycle being marked by a layer of phosphatic nodules, glauconite and/or worn and broken shells (Fig.1). Hence the positive and negative movements of sea level may well have taken place at similar speeds. Intracontinental uplifts should have strictly local effects on the sedimentation in contrast to the more extensive though possibly milder results of eustatic movements. Thus regional uplift became important in Britain only towards the end of the Lias, as evidenced by the diachronous sand formations in the Upper Lias of southwestern England and the irregular and local pre-Bajocian warping and erosion in the Midlands and Yorkshire where several ammonite zones disappear below the Dogger within the distance of a few miles.

RELATION OF LIASSIC SEDIMENTARY CYCLES TO SEA LEVEL CHANGES

163

RELATIONSHIP WITH LIASSIC COAL MEASURE CYCLES

As the familiar Upper Carboniferous cyclothems have been related by several people

to eustatic movements (WELLS, 1961) considerable interest attaches to the presence of Rhaeto-Liassic coal measures in southern Sweden. TROEDSSON (1 95 1) has described a cyclic sequence of non-marine sandstones, clays and coals and partly marine or brackish water pelecypod beds embracing the Rhaetic (three cycles) and Hettangian (Helsingborg Stage - nine cycles). This pattern of sedimentation was brought to an end by a major marine transgression in the Lower Sinemurian, when the first ammonites entered the area. These ammonites include species of Arniorerus, Agussireras, Euagassiceras together with an arietitid probably related to Corotiiceras reynesi (SPATH), which indicate a lower Sernicostcitirrn Zone age. This is highly significant, since it corresponds precisely with a marine transgression across the North Sea, in western Scotland, and the beginning of a new marine sedinientary cycle elsewhere. This is unfortunately the only exact correlation that can be made with the marine successions in western Europe, but it is clear that Troedsson’s twelve cycles must iiiore or less correspond with only four of the cyclic units I have recognised. It seems to follow either that the coal measure cycles have been controlled by additional, minor Buctuatioiis of sea level or that they are at least partly the result of some local factor.

CORRELATION WITH FAUNAL CHANGE

That the Liassic cyclic boundaries correlate with important changes in the ammonite succession has long been recogiiised (KLUPFEL, I9 17; FREBOLD, 1924). Remarkably similar cycles in the Upper Jurassic and Cretaceous of Switzerland (FICHTER, 1934) are also related to faunal breaks. I have been able to extend these conclusions by demonstrating a detailed correlation overwide areas between sedimentary boundaries reflecting changes in depth of sea and evolutionary and migrational changes in ammonite genera through the whole of the Lias. The important eustatically controlled sedimentary changes discussed in this paper both correlate with significant changes in the fauna as a whole. It is especially interesting that the phase of widespread bathyal conditions i n the Lower Toarciaii corresponds with the disappearance (probably by extinction) of almost the whole invertebrate fauna, which was followed in the later Toarcian by a repopulation of species with Middle Jurassic affinities.

SUMMARY

Taking as a starting point a characteristic asymmetrical clay-limestone cycle widely developed in northwestern Europe the sedimentary evidence for periodic and extensive changes in depth of sea is outlined. A model of marine environments is developed and criteria for the recognition of eustatic changes proposed. Evidence is put forward to

164

A . HALLAM

suggest that on at least two occasions a widespread and essentially synchronous deepening of the sea in Europe coincided with an extensive marine transgression over several continents, indicating the operation of eustatic control. Discussion follows on the complicating effect of regional epeirogeny and an explanation of the asymmetry of the cycles is given in terms of the effect of this factor combined with variation in sediment supply controlled by changing sea level. The possible relationship with coal measure cycles in Sweden is briefly outlined and a correlation of the marine cycles with the invertebrate fauna noted. REFERENCES

ALLEN,P., 1959. The Wealden environment: Anglo-Paris Basin. Phil. Trans. Roy. SOC.London, Ser. B, 242 : 283-346. ARKELL, W. J., 1956. Jurassic Geolo,ry ofthe World. Oliver and Boyd, Edinburgh, London, 757 pp. BIGOT,A., 1930. Sketch of the geology of Lower Normandy. Proc. Geologists’ Assoc. Engl., 41 : 363-395. DONOVAN, D. T., 1957. The Jurassic and Cretaceous systems in eastern Greenland. Medd. Groenlnnd, 155 (4) : 214 pp. DUBAR, G., 1925. etudes sur le Lias des Pyrtntes franCaises. Mgm. Soc. GPol. Nord, 9 : 332 pp. FICHTER, H. J., 1934. Geologie der Bauen-Brisen-Kette an1 Vierwaldstattersee und die zyklische Gliederung der Kreideund des Malm der helvetischen Decken. Beitr. Geol. Karte Schweiz.69 : 128 pp. FRANK. M., 1930. Beitrage zur vergleichenden Stratigraphie und Bildungsgeschichte der Trias-LianSedimente im alpin-germanischen Grenzgebiet der Schweiz. NfwesJahrb. Mineral. Ceol. Paliiontol., Abt. B, 64 32-25, FREBOLD, H., 1924. Ammonitenzonen und Sediiiientationszyklen und ihre Beziehung zueinander. Zentr. Mineral. Grol. Puliiontol., 1924: 3 13-320. FREBOLD, H., 1925. Ueber cyklische Meerf,ssedirrrentotion.Leipzig. FREBOLD, H., 1957. The Jurassic Fernie Group in the Canadian Rocky Mountains and Foothills. Mem. Geol. Surv. Con., 287 : 197 pp. FREBOLD,H., 1960. The Jurassic faunas of the Canadian Arctic: Lower Jurassic and lowermost Middle Jurassic ammonites. Bull. Geol. Surv. Can.,59 : 33 pp. GABILLY, J., 1961. Stratigraphie et paltogtographie du Lias dans le detroit Poitevin. Dans: J. ROGER (,Rtdacterrr),Colloqire sur le Lias en France, ChambPry, 1960 - Cornpt. Rend. Soc. Savantes, 85 : 475486. HALLAM, A., 1961. Cyclothenis. transgressions and faunal change in the Lias of northwest Europe. Trans. Edinbiryh Geol. Soc., 18 : 125-174. HEIDORN,F., 1928. Paliogeographisch-tektonische Untersuchungen im Lias zeta von Nordwestdeutschland. N e w s Juhrb. Mineral. Geol. Palaontol., Abt. B, 59 : 117-244. KELLAWAY, G . A. and WELCH,F. B. A.. 1948. British Regional Geology: Bristol and Gloucester District., 2 ed. H. M. Stat. Office, London, 91 pp. KLUPFEL, W., 1917. Uber die Sedimente der Flachsee in1 Lothringer Jura. Ceol. Rundschau, 7 : 97-109. KLUPFEL, W., 1920. Der Lothringer Jura. I. Lias. Jahrb. Preuss. Ceol. Lanrlesanst., 39 : 165-372. LE CALVEZ,Y . et LEFAVRAIS-RAYMOND, A,, 1961. Le Charmouthien du Loth. Dans: J. ROGER Savantes, 85 :793-802. (Rtdacteur), ColloqiresrrrLiasc.nFrance, ChanrbPry,.l960- Compt. Rend. SOC. MELVILLE, R. V., 1961. In:Surnm. Pro$r. Geol. Surv. G t . Brit. 1960, p. 52. RIOULT,M., 1958. Le Lias de Feugerolles-sur-Orne (Calvados). Bull. Soc. Linnt;enne Normaiidie, 9 : 3540. SOLL,H.. 1957. Stratigraphie und Ammonitenfauna des mittleren und oberen Lias (Lotharingien) in Mittel-Wiirttemberg. G a l . Jahrb., 72 : 367426. G., 1951. On the Hoganas Series of Sweden. Lunds Univ. Arsskr., Avd. 2,47 : 268 pp. TROEDSSON, TRUMPY, R., 1949. Der Lias der Glarner Alpen. Denkschr. Schweiz. Naturforsch. Ges., 79 : 1-192. TRUMPY, R., 1952. Der Nordrand der Liasischen Tethys in den schweizer Alpen. Grol. Rundschau, 40 : 239-242. WELLS,A. J., 1961. Cyclic sedimentation: a review. Geol. Mag., 97 : 389403.

LAGOON SEDIMENTS IN GREENLAND K. HANSEN Gatnineltofiqyade 16,Copenhagen (Deriniark)

East of the village Godhavn on the south coast of the Island of Disco in North Creenland is a lagoon separated from the sea by a barrier beach of dark grey sand (Fig.]). The lagoon is 270 m long, 1 10 m wide, 1.5 m deep and has an area of 2.24 hectares. It is connected with the sea by a 100 m long channel. Two brooks enter the lagoon on the north coast. The eastern brook dries up during the summer, the other one has a very large discharge throughout the whole year (PORSILD,1925). The tide ruiis westward along the south coast of Disco, and shows a pronounced difference in amplitude of the two oscillations of each day. The amplitude of the large tide is 2.3 m at spring tide and 0.4 m at neap tide. The small tide has the more constant amplitude of 0.4 m (RIIS-CARSTENSEN, 1948). During winter the Disco Bay is covered with ice as far out as a line from Godhavn to Egedesminde. When the winter ice disappears (in the last part of May) a current system develops, running eastward along the south coast of the bay and westward along the south coast of Disco. The country north and west of the lagoon consists of gneissic rocks. East of the lagoon is a low plain, bounded on the north by a cliff of basalt-breccia. The barrier, a sand ridge 140 m wide and 3 m high and with a steep cliff against the sea, has a sparse vegetation of Psamma areriaria. Fig.2 and Table I show the granulometric composition of the sand from this barrier. TABLE I GRANULOMETRIC COMPOSITION OF T H E S A N D FROM THE BARRIER BEACH EAST OF GODHAVN ~

~

~~

~

~

~

Md

Analysis no.

IL ~

45 1

5 20

452 453

850 640

~

P

-

520 1300 lo00

~~

~~

~

~

Q,

Q3

%

so

IL ~~

~

~~

210 720 430

~-

1.29 1.30 1.54 -

0.94 0.39 1.05 ~~-

-

Sample 451 is from the outer part of the barrier, 452 from the tidal zone on the beach and 453 from the northern part of the barrier. The sediments are all well sorted, medium-coarse sand.

166

K. HANSEN

Fig.]. The lagoon east of Godhavn, Island of Disco. The contour lines for each 2 rn show, that the surrounding land rises steeply to the north and west. The black rectangle is the arctic station of the University of Copenhagen. The numbers 448464 correspond with the granulometric analysis.

Sample 451 has 847; of the grains in the fractions 25&-1,000 p, 453 is appreciably coarser, with 63 % of the material in the fractions 5W2,OOO p and all the sand from the beach is coarser than 500 p. The sand is black or dark grey and mainly composed of basaltic material, the remainder consisting mostly of grains of gneiss and of clear, colourless quartz. The grains > 0.5 mm are strongly rounded. I n the sand from the barrier only the grains < 0.125 mm have sharp edges, but in sample 452 sharp-edged grains up to a grain size of 0.4 mm are present. The barrier also contains fist sized pebbles of basaltic rocks. The western and the northern coasts are rocky and pebbly. In the southernmost part of the lagoon two banks are present, consisting of gravel and coarse sand. The

167

LAGOON SEDIMENTS IN GREENLAND

inner side of the barrier is fringed by a broad sand flat (Watt), which at some places is covered by a thin layer of mud. Farther out, the bottom consists of compact sand. In the deeper parts of the lagoon, however, the bottom is silty and muddy. The vegetation on the flat does not seem to play any part in the sedimentation processes. Fig.3 and Table I1 show the granulometric composition of the sediments from the flat and the bottom of the lagoon. Sample 448 is sand from the flat covered by 20 cin of water at low tide. It is of nearly the same type as the sand from the barrier, having 43.4 % in the fractions 500-1,000 p and 36 % in the fractions 3W500 p. The grains consist of gneiss and basalt and only those < 0.15 mni are sharp-edged. Samples 447,449 and 450 were taken from different parts of the muddy flat. They are all strongly sandy and may be characterised as silty, fine to medium sand. The sorting is not good. In fresh condition these samples are black and soft. After drying

Fig.2. Granulometric composition of the samples from the barrier beach.

Zoo0

1000

500

100

so

20

to

4//

Fig.3. Granulometric composition of the samples from the lagoon.

I68

K. HANSEN

they form small coherent lumps of a markedly fine sandy appearance and of a dirty grey colour. The fine fractions, < 63 p, are rich in diatom frustules. Sample 455 is from a special kind of sediment. In fresh state it is bluish-black, soft and soapy; dried, it forms dirty yellow, very light lumps which remind one of diatom earth. Further examination shows that the fine fractions, < 62 p, predominantly consist of diatom frustules and that there is no clay mineral material at all. The content of TABLE I1 GRANULOMETRIC COMPOSITION OF THE SEDlMENTS FROM THE LAGOON EAST OF GODHAVN

_ _-~

~

Analysis

Md

Q3

Q,

no.

!*

I*

EL

~~

447 448 449 450 455 464

140 510 230 440 11.5 12.5

160 725 500 725 36 34

so

62

4

o/ /I1

Yo

63.3 98.5 60.0 74.0 18.5 9.7

5.9

~

30 390 18 60 6 9

3.47 1.37 5.75 3.49 2.25 1.94

0.74 1.04 0.41 0.47 1.28 1.96

0.1 4.8 12.5 16.7

organic matter (loss of ignition) is 17.3 %, with 5.65 % C and 0.70 % N , which means that nearly the whole part of the fine fractions is organogenic. Sample 464 was taken in the middle of the lagoon at a depth of 1.30 m. In fresh condition it is soft and black, with a brown surface. After drying it forms blackish-grey lumps. The granulometric composition is nearly the same as that of sample 455, the fine fractions also containing a lot of diatoms. However, the whole appearance is clearly different from that of no. 455. It is very astonishing that the mud from the flats is mainly of organic origin, because only about 1 km east of the entrance to the lagoon, the river Rode Elv brings suspended red mud out in the sea in such quantities, that not only the river water itself, but also the sea water far from the coast is pink coloured; so it could be expected that the red mud would enter the lagoon with the tide and be deposited on the flats. This not being the case, the explanation must be, that the discharge of fresh water from the brook entering the lagoon on the northern coast is so large, that there is a continuous outgoing surface current in the channel from the lagoon to the sea. The fresh water from the Rode Elv is spread out as a very thin layer upon the sea water. HOLMQUIST (1959) states, that the salinity in the sea outside the lagoon on July 3, 1956, amounted to 33.4"/,, at a depth of 2 m, but only to 1.7°/00at the surface. Probably there is an inward going density current along the bottom of the channel, which brings some mud into the deeper part of it. Unfortunately no determinations have been made of the salinity in the lagoon, but the larger discharge of freshwater from the brook indicates, that the water must be brackish. It is known that diatoms are common on the tidal flats along the southern and

LAGOON SEDIMENTS IN GREENLAND

169

western coast of the North Sea, where they help to bind the mud particles after deposition (WOHLENBERG, 1954). However, in all the North Sea estuaries mass development of special marine diatoms is limited to the brackish zones (BROCKMANN, 1950).Probably it is the brackish character of the lagoon of Disco that is responsible for the luxuriant growth of diatoms on the flats. The diatoms in the samples 455 and 464 are nearly all (benthonic) freshwater species.

SUMMARY

In a lagoon on the Island of Disco, north Greenland, there is no real salt marsh on the jurrounding flats, but a heavy upgrowth of diatoms.

REFERENCES

BROCKMANN, C., 1950. Die Watt-Diatorneen der schleswig-holsteinischen Westkuste. Abhand. Senekenberg. Naturforsch. Ges., 478. HOLMQUIST, C., 1959. Problems on Marine-Glacial Relicts on Account of Investi,oations on the Genus Mysis. Thesis, Univ. of Lund. PORSILD, A. E., 1925. Iagttagelser over den grsnlandske kildeis og dens virkninger p i vegetationen og jordoverfladen. Geograf. Tidsskr., 28. RIIS-CARSTENSEN, E., 1948. Dengrmlanhke Lods. Kebenhavn. WOHLENBERG, E., 1954. Sinkstoff, Sedin?ent und Anwachs am Hindenburg-Danirn. Die Kiiste, 2.

LATE PLEISTOCENE AND RECENT SEDIMENTATION, CENTRAL GEORGIA COAST, U.S.A.l JOHN H.

HOYT, ROBERT J.

WEIMER

and VERNON J.

HENRY

JR.

Marirre Institrrte, University of Georgia, SapeIo Island, Ga. (U.S.A.) I. Colorado School ofhfines, Golden, Colo. ( U . S . A . ) ; Marine Institute, University qf Georgia, Sapelo Island, Ga. (U.S.A.)

INTRODUCTION

The islands bordering the Georgia coast are the result of a complex history of development from Late Pleistocene to the present. These islands and related features on the Georgia mainland have been studied to determine the sequence of the events and the associated environments. The area of study is shown in Fig. 1. Contributions to the geology of coastal Georgia have been made by VEATCHand STEVENSON (191 l), FLINT(1940), COOKE (1944). MACNEIL(1950), ZEICLER(1959), DOERING (1960), and others. The tidal range in this area averages 7 ft. and is greater here than for coastal areas several hundred miles to the north and south. Breaker heights average 9 inches (HELLE, 1958). Offshore water depths are shallow, averaging 3 5 4 5 ft. ten miles from shore. The nearshore and estuarine waters are turbid, generally limiting below-surface visibility to about 20 inches and less. The barrier islands of the study area are associated with several environments which include: (I) Littoral and shallow neritic, composed mainly of marine sands commonly with even, parallel stratification at low angles; (2) The barrier islands, composed primarily of marine sands as above, overlain by dune ridge sands with divergent, truncated, and commonly steeply dipping stratification; (3) The salt marsh and tidal channels landward from the barrier islands made up of silts, clays, sands, and organic debris; . ( 4 ) The migrating channel inlets, which separate the barrier islands, characterized by marine sands near the inlet interfingering with silts and clays near the salt marsh. The marine sands of the migrating inlet commonly have a steeply dipping depositional interface. This research was supported by National Science Foundation Grant G16426. SubContribution no. 46 from the University of Georgia Marine Institute, Sapelo Island, Ga. (U.S.A.).

LATE PLEISTOCENE AND RECENT SEDIMENTATION ON GEORGIA COAST

171

Fig. 1 . Map showing area of study, distribution of Pleistocene and Recent sediments, and location of cross section.

surface information was obtained with a portable drilling rig loaned by the Jersey Production Research Corporation, Tulsa, Oklahoma.

GEOLOGIC HISTORY

Late Pleistocene and Recent sedimentation is intimately related to sea level fluctuations. Methods for dating the events of the Late Pleistocene are not precise, so the assignment of sea level positions to certain time intervals is tentative and subject to revision. However, the sediments of the study area provide interesting information on the sequence of events which produced the coastal features.

Pamlico shoreline During Late Pleistocene time, when sea level was 20-25 ft. higher than at present, barrier islands and lagoons succeeded by salt marshes were extensively developed. These former islands now occupy a position 6-10 miles inland from the present shoreline of the Atlantic Ocean. The estimated sea level elevation is based on the height of

172

J . H. HOYT, R . J . WElMER A N D V. J. HENRY JR.

GEORGIA OMLDPMENT

a

MID-SANGAMON

OF

PAMLICO

LATE

C

MID-SANGAMON

SANGAMON

DEVELOPMENT RECENT

OF

ANNE

BLUFF

INTERGLACIAL

RECENT

SAPELO

ISLAND

I

A'

ISLANOS

STAGE

BARRIER

INTERGLACIAL

DEVELOPMENT OF SILVER LATE

BARRIER

INTERGLACIAL

DEVELOPMENT OF PRINCESS

I

I

MUNANJ

ISLANDS

STAGE

BARRIER

ISLANDS

STAGE

BARRIER

ISLANDS

EPOCH .O 10

~II

SCALE

-MILES

SILT AND CLAY

Fig.2. Cross sections showing evolution of the barrier islands.

marsh sediments developed behind the Pamlico islands. The Pamlico barrier islands now rise as high as 50 ft. above mean sea level and were therefore 20-30 ft. above the Pamlico sea level. Based on sea level curves compiled by FAIRBRIDGE (1961), the development of these islands was synchronized with the Mid Sangamon interglacial stage. Fig.2a is a cross section showing the distribution of sediments during the formation of the Pamlico islands. Princess Anne shoreline

After development of the Pamlico islands, sea level stabilized at the 10-15-ft. lever (Fig.2b). A series of barrier islands formed along the front of the Pamlico Islands and

LATE PLEISTOCENE AND RECENT SEDlMENTATlON ON GEORGIA COAST

173

were separated from the Pamlico islands by a narrow lagoon-salt marsh. The stabilization at the 10-15-ft. level is tentatively placed in late mid-Sangamon time. Evidence for the stillstand elevation comes from the burrows of the lower beach dwelling arthropod Calliarzassa major SAY(WEIMER and HOYT,1962) and from beach structures developed along the island front. The burrows of C. major are helpful in establishing former sea level positions and depositional environments. These decapods live in tube shaped burrows in the lower foreshore of the beach (low littoral) and shallow neritic environments, and are found almost exclusively along the open ocean. The burrows occur in abundance to about mean sea level and less commonly a foot or two higher. Thus, the upper limit of the fossil burrows may be used to mark the approximate position of mean sea level. Silver Bluff shordine

In late-Sangamon time, sea level stabilized at the plus 2-5-foot level (Fig.2~).It was during this period that the main portion of Sapelo Island was formed. Evidence for the sea level position comes from Callianassa major burrows and salt marsh silts and clays that were deposited behind the islands. Recent sliorelim

The accretion of beach and dune material on the front of the Silver Bluff barrier island was terminated by the Wisconsin glaciation and the resultant drop in sea level to around the minus 330-foot elevation (FAIRBRIDGE, 1961). The following rise of sea ievel has been the subject of considerable investigation (ROYALNETHERLANDS GEOLOG~CAL and M ~ N I NSOCIETY, G 1954; BROECKER et al., 1960; FAIRBRIDGE, 1961; MCFARLAN, 1961; and others). In the area of study, there appears to be little evidence that sea level during the Recent has been much higher than the present level. In the Sapelo Island area, the Rzcent sediments form only a narrow barrier along the front of the main part of the island, from whch they are separated by one-fourth mile of salt marsh. The rise of sea level during the Recent has reflooded the Silver Bluff salt marsh behind Sapelo Island. The meandering tidal channels of the marsh have reworked most of these sediments.

SEDIMENTS

Barrier islands

The sediments of the Pleistocene barrier islands are predominantly fine to coarse sand. Eolian deposits overlie littoral and shallow neritic sediments in most areas. Pleistocene sediments are recognized generally on the basis of their partial induration, coarseness,

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J. H. HOYT, R. J. WEIMER AND V. I. HENRY JR.

soil profile development, and highly oxidized ferruginous color. The generaIized soil profile consists of three zones: ( I ) Surficial gray to black highly organic sand 6-18 inches thck containing oyster shells (kitchen midden) and artifacts. (2) White, leached, fine grained sand 3-6 ft. thick. (3) Dark brown to black sand enriched with mineral and organic material 2-4 ft. thick. The Pleistocene soil profile overlies dark yellow to brown sand, whch varies from fine to very coarse in grain-size. The soil profile is not always developed or has been removed, in which case the sand is commonly massive, fine grained, well sorted, and yellow to brown in color. Surficial Pleistocene deposits are practically devoid of strat!fication. The barrier island sediments interfinger, and to some extent, overlap the salt marsh sediments on the landward side of the island. Tsland width, however, increases primarily by deposition of material on the seaward side. Numerous advances and retreats and directional changes of the shoreline are recorded by dune ridges that developed parallel to the then existing shoreline (Fig.3). These features are conspicuous in some

Fig.3. Aerial photographs of the north end of Blackbeard Island showing dune ridge development. Northeast end (upper right) of island is being eroded by southward shifting channel. Width of photograph equals approximately 3.5 miles. (U. S. Department of Agriculture aerial photographs, 1953.)

Recent deposits, and are not believed to indicate any major (more than a foot or two) sea level fluctuation. The shifts in shoreline are attributed to changes in sediment supply. waves, and currents. Recent sand deposits are composed mainly of fine, angular grains and are well stratified. Eolian stratification is characterized by divergent, truncated, and commonly, steeply dipping bedding as contrasted with parallel, low angle (2-6 degrees), continuous beds of the beach and shallow neritic.

LATE PLEISTOCENE AND RECENT SEDIMENTATION ON GEORGIA COAST

175

Tidal inlet

In general, the barrier islands of the study area have been shifting southward by erosion of the north ends (Fig.3) of the islands and deposition on the south ends. This has resulted in the deposition of a rather large wedge of sediments influenced by the composite environment of the migrating channel inlet. The sediment on the depositional side of the inlet (north) is mainly sand which is characterized by greatly increased stratification slopes compared to neritic sediments along the island front. The channel margin slopes into the channel at angles as high as 12 degrees. Superimposed on this slope are asymmetrical, transverse, megaripples with amplitudes as high as 2 ft. and lengths of 2 0 4 0 ft. The steep slopes of these ripples approach the angle of repose of 30 degrees. Based on the configuration of the depositional interface along the inlet margin, the stratification should dip in the direction of inlet migration, commonly with a component toward the mainland. Thus, the stratification slopes resulting from the southward migration of the inlet represent a marked departure in both strike and dip from those along the island front. Landward of the channel inlet, the sands interfinger with silts and clays of the salt marsh. Salt marsh The area between Sapelo Island and the mainland is now occupied by an extensive salt marsh. cut by numerous meandering tidal channels. The sediment consists primarily of the following: (I) Silts, clays, and very fine sand brought in by tidal action. (2) Organic debris resulting from the attrition and decay of marsh grasses and plants, primarily Spartitiu riltcrnrJlora. (3) Very fine to medium sand originating from the barrier islands as washover fans and from erosion of the remnants of Pleistocene marsh within the modern marshes. (4) Medium to coarse sands within the sounds and larger tidal channels derived from Pleistocene sediments of the mainland and barrier islands. ( 5 ) Indigenous fauna which includes Uca, Nussa, Littorina, Modiolus, Rctusa, and Ostrea.

In general, the surficial sediment is homogeneous gray to black silty clay ranging in thickness from several centimeters along the island and mainland margins to over 2 rn in the mid-marsh areas. Only thin laminae of sand may be found in this layer. Beneath the silty clay, however, the material grades, in some places gradually, in others sharply, into laminated sands, silts, and clays. In addition to the laminations which range from a' fraction to several centimeters in thickness, flow or slump structures and burrows are very commonly found. The total thickness of the recent marshes is quite variable, being dependent on the relief of the underlying Pleistocene surface. Core and pile-driving information indicate this surface is irregular and ranges from 10-16 ni or more in depth in the mid-marsh areas.

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J . H. HOYT, R. J. WEIMER A N D V. J. HENRY JR.

SUMMARY

Coastal sediments of czntral Georgia and their environments of deposition are described. The geologic history of the area is related to the Late Pleistocene to Recent sea level fluctuations. [f) During the middle and late Sangamon interglacial stage of the Pleistocene, a series of barrier islands was developed along coastal Georgia. These island-lagoonsalt marsh complexes are successively younger and lower oceanward and represent stillstands at 20-25 ft. (Pamlico), 10-15 ft. [Princess Anne), and 2-5 ft. [Silver Bluff) above present sea level. (2) Following the lowering of sea level during the Wisconsin glaciation and the subsequent rise of sea level to near its present position, a Recent barrier island formed in front of the Silver Bluff barrier island. The areas between the Recent island and the Silver Bluff island and between the Silver Bluff and the mainland are occupied by Recent salt marsh. (3) The sediments associated with these four shorelines are those characteristic of the barrier island environments along the Georgia coast. The typical open lagoon behind many barrier islands of other areas is replaced in this area by a Spartinu salt marsh composed of clay, silt, and commonly high percentages of sand. The sediments of the migrating channel inlet are characterized by a steeply dipping depositional interface which strikes approximately perpendicular to the island trend and is inclined in the direction of inlet migration.

REFERENCES

BROECKER, W. S., EWING,M. and HEEZEN, B. C., 1960. Evidence for an abrupt change in climate close to 11,000 years ago. Am. J . Sci., 258 : 429448. COOKE, C. W., 1944. Geology of the coastal plain of Georgia. U. S. Geol. Surv. B i d . , 941 : 121. J. A., 1960. Quaternary surface formations of southern part of Atlantic coastal plain. DOERING, J . Geol., 40 : 182-202. FAIRBRIDGE, R. W., 1961. Eustatic changes in sea level. In: L. H. AHRENS, F. PRESS,K. RANKAMA and S. K. RUNCORN (Editors), Physics and Chemistry ofrhe Earth. Pergamon, London, pp. 99-1 85. FLINT,R. F., 1940. Pleistocene features of Atlantic coastal plain. Am. J. Sci., 238 : 757-787. J. R., 1958. Surf statistics for the coasts of the United States. Beach Erosion Board Tech. Metn., HELLE, 108 : 22pp. MACNEIL,F. S., 1950. Pleistocene shorelines in Florida and Georgia. U. S. Geol. Surv., Profess. Papers, 221-F : 95-106. MCFARLAN, E., 1961. Radiocarbon dating of Late Quaternary deposits, south Louisiana. Bull. Geol. SOC.Atpi., 72 129-157. ROYALNETHERLANDS GEOLOGICAL and MININGSOCIETY,1954. Symposium Quaternary changes in level especially in The Netherlands. Ceol. Mvnbouw, 16 : 147-267. J. 0. and STEPHENSON, L. W., 1911. Geology of the coastal plain of Georgia. Georgia Depr. VEATCH, Mines, Mining Geol., Geol. Sirrv. Bull., 26 : 463 pp. WEIMER, R. J. and HOYT,J. H., 1962. Cul/iunassu major burrows, geologic indicators of littoral and shallow neritic environments (abstract). Geol. SOC.Am., Spec. Papers, 68 : 321. J. M., 1959. Origin of the sea islands of the southeastern United States. The Geogruph. Rev., ZEIGLER, 49 : 222-237.

SEDIMENT TRANSPORT I N PART O F THE LOWER PENNANT MEASURES OF SOUTH WALES G. K E L L I N G

Departtilent of Geology, University College, Swansea (Great Britain)

INTRODUCTION

The Lower Pennant Measures of the South Wales coalfield are rocks of Morganian age. As defined by the Geological Survey (WOODLAND et al., 1957) they comprise the strata between the top of the Upper Cwmgorse Marine Band below and the Hughes coal and its equivalents above. Three formations are recognised in this sequence, namely, in ascending order, the Llynfi Beds, Rhondda Beds and Brithdir Beds. The following account is based on a regional study of the sedimentary features and petrology of the Rhondda Beds, east of the Loughor Valley (Fig.2), and is concerned principally with the lower part of this sequence involving the interval between the 110.2 Rhondda coal-seam at the base and the no.1 Rhondda seam above. Detailed analysis of the overall results of this study must await the more extended account now in preparation. The present report is intended to outline those results which refer to current-data and lithological variation and to interpret such results in the broad context of environment.

LITHOLOGICAL FRAMEWORK

Two main lithofacies are developed within the Rhondda Beds. Throughout most of the coalfield the formation comprises a thick sequence of cross-bedded subgreywackesandstones, together with thin coals and shales. However, east of the Taff Valley (Fig. 2) the representatives of the Rhondda Beds are orthoquartzitic in character, with lenticular bands of quartz-conglomerate near the base. These quartzitic members also pass upwards, transitionally, into subgreywackes. On the eastern margin of the coalfield the 110.2Rhondda coal cannot be identified and is probably absent. Recognition of the Rhondda Beds in this area is therefore difficult. Moreover there is, at the least, very drastic eastward thinning of the entire Lower Pennant Measures. Indeed MOORE (1948) and BLUNDELL (1952) claim that around the eastern and northeastern margins of the coalfield most of the Lower Pennant sequence is progressively eliminated by an eastward-developing unconforrnity. Despite the lack of published data on proved thicknesses of the Rhondda Beds,

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G. KELLING

several personal measurements combined with thickness-data compiled from the sixinch maps and memoirs of H. M. Geological Survey (based on seam-correlations given in WOODLAND et al., 1957, p1.2) suggest that in the 110.2 Rhondda to no.1 Rhondda seam interval the (tentative) isopachytes have a general east-west trend, the

N

rJ

-0

-

BASE OF RHONDDA BEDS No 2 - No 1 RHONDDA ISOWCHYTES CONTROL POINTS 0

5

lomiles

Fig.1. Tentative isopachytes from the lower Rhondda Beds of South Wales (no.2-110.1 Rhondda seam interval). Dots and circles show location of control points for isopachytes. Solid dot represents data derived from Geol. Surv. publications; open circle represents a measured section. A = Ammanford; R = Resolven; D = Dunvant; S = Swansea; N = Neath; M = Margam; T = Tondu; MG = Maesteg; L Llanharan; A N = Abercynon; TW = Taffs Well. :

maximum thickness being attained in the Margam-Tondu region (Fig. I). The thickness appears to decrease rapidly northwards and more slowly eastwards from this Margam culmination. In this connection it is not without interest that the massive subgreywacke lithology of the Llynfi Beds is developed earliest in the southwest, around Swansea Bay (WOODLAND et al., 1957), and is represented to the east by a thinner sequence of shales with orthoquartzites and pebbly grits which occur at least as far west as Maesteg (Caerau). There is apparently a general pattern in Lower Pennant times of eastward thinning and possible unconformity, of an eastward decrease in sand-shale ratio and of an eastwards movement with time of the facies-boundary between the western subgreywackes and the easterly orthoquartzites. Typically the Rhondda Beds comprise a number of cyclic sequences of normal Coal Measure type (cf. TRUEMAN, 1954, p.10) but dominated by sandstone and lacking marine horizons. The arenaceous members are generally thick, grey-blue, feldspathic subgreywackes, often micaceous and usually medium- or coarse-grained. Except where affected by channeling the thicker units are parallel-bedded although wedging of sandstone units is locally conspicuous, especially in the orthoquartzite facies. Small

SEDIMENT TRANSPORT IN THE LOWER PENNANT MEASURES

179

carbonaceous flecks are very abundant in the subgreywackes and large mineralised casts of logs (up to 12 ft. long and 2 ft. wide) are common in the thicker beds. Closely associated with the sandy members are rather irregular conglomeratic bands which range in character from subgreywackes carrying numerous pebbles and granules of vein-quartz, rolled ironstone-nodules and clay-galls, to “rubble-bands” which consist of a mtlange of twisted shale-pieces, lenticular coal-rafts, rolled and broken ironstone nodules and carbonaceous or mineralised plant-stems up to 18 inches long, all embedded in an ill-sorted sandy matrix. Rounded and angular pebbles of grey, green and red sandstone and angular pieces of coal (often with cleat) are common in the rudites. These conglomerates vary from 1-8 ft. in thickness and are characterised by irregular, eroded bases. The pebbles and plant-stems within some of these bands lie in a steeply dipping attitude which is construed as evidence for rapid deposition from a high-energy current-m?dium. No marine bands have been recognised within the Rhondda Beds, nor indeed throughout the entire Pennant Measures. However, impersistent plant-beds and nonmarine mussel-bands are frequently encountered.

DIRECTION AND MECHANISM OF TRANSPORT

Channels and cross-bedding are the most common directional structures i n the Rhondda Beds but other orientated structures such as ripple-mark, mega-ripples, riband-furrow, grooves and flutes and aligned plant-remains have been used to determine the direction and nature of the medium of transport. The form and orientation of the Rhondda Beds channels have been described elsewhere ( BLUCK and KELLING, in press). The channel-structures observed and recognised as such show considerable variation in width (10-200 ft.) and depth (3-50 ft.). However, the presence of roughly planar or irregular surfaces of erosion which may be traced across the entire rock-face of even the largest exposures (quarries, railway- or road-cuttings) suggests that channels of much larger dimensions may exist in these sediments. The recognition of channels is further complicated by the general similarity in lithology of the channel-fill and the adjacent sediments. Nevertheless the form and behaviour of the 62 channel-structures recognised and amenable to measurement is generally consistent with a fluvial origin. Approximately 60 % of the sandstones examined are cross-bedded in some degree (cf. Fig.3). Varieties of the simple and planar types of cross-bedding (MCKEEand W E I R , 1953) are common but trough or festoon bedding is relatively rare. Steep, wedging planar foresets are typical of the orthoquartzite facies whereas thick units with long simple foresets predominate in the subgreywackes of the central and southern parts of the coalfield. Trough-bedded units usually occur as thin intercalations of anomalous source and are most frequent along the north, northeast and east margins of the basin. The directions of transport in the sandstones of the lower Rhondda Beds have been

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determined from a regional study of cross-bedding. For this purpose the total outcrop was divided into 54 sectors, chosen randomly, which have yielded a total of 1,613 observations. Eachsector is about six square miles in areal extent. An extended analysis of the results cannot be given here but the basic data are presented in Fig.2.

Fig.2. Modal cross-bedding directions from the lower Rhondda Beds of the South Wales coalfield.

In this diagram each arrow represents the modal orientation (mid-point of the modalsemi-octant class) of the cross-bedding directions within a single sector. Sectors possessing two strong cross-bedding modes have been assigned two cross-cutting arrows with appropriate orientations. Lateral variation

The overall current-pattern obtained from the lower Rhondda Beds is complex and subject to widespread local as well as regional variations. The general picture is one of dominant derivation from the octant between southeast and south but with strong evidence of northerly derivation along the north side of the coalfield. Moreover the broadly centripetal distribution of currents in the quartzitic facies of the eastern part of the basin indicates derivation from eastern and northeastern sources. Superimposed upon this general pattern there may be discerned a tendency for a general westward drift of material in the west-central part of the coalfield, around Cymmer and Neath. Consideration of this current-evidence suggests the following conclusions: (I) The present structural basin of South Wales partly occupies the site of a basin of deposition and of internal drainage in which the Rhondda Beds were laid down. If we may take the “axis” of this sedimentary basin to be coincident with the zone of maximum current-mixing, then in Rhondda Beds times that axis lay near the present North Crop of the coalfield and had an east-west trend. (2) The eastern end of this basin was closed and was receiving quartzitic sediment from an actively rising source-area not greatly distant from the present eastern outcrops.

181

SEDIMENT TRANSPORT IN THE LOWER PENNANT MEASURES

(3) Considerable mixing and redistribution of sediment occurred in the central part of the basin but there was an overriding tendency for the westward, longitudinal movement of material. It should be emphasized that the coherent pattern of current-movement from which these conclusions have been drawn is based ultimately on a framework of crossbedding directions which show considerable variability at within-outcrop and between-outcrops levels. Stratigraphic control

Determination of the mean cross-bedding orientation at intervals within a number of measured sections through the lower Rhondda Beds reveals that, in addition to the lateral variability described above, there also exist marked vertical changes in currentdirection. SIODW

NK~E

A

INDOC.

GAP

8

Q vl

VEWICAL

Lu GAP

SCALE

40

Fig.3. Measured section through the Rhondda Beds and upper part of the Llynfi Beds with compassdiagrams showing mean cross-bedding directions at several levels within this sequence. Numbers within compass-circles denote the number of observations from which mean direction is obtained. From old quarry and road-cuttings, Earlswood, Briton Ferry, Neath (Grid Ref. 728939-731944).

One vertical profile of this type is detailed in Fig.3, which includes data from the upper part of the Llynfi Beds. Study of this profile suggests that at this locality the

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supplv was mainly from the southeast or south but the lowest exposed Llynfi Beds show strong northerly components. Similar north-derived currents recur in the Rhondda Beds but these do little to affect the general pattern of supply from the southeast. This sporadic, recurring near-reversal of direction is common to most of the profiles examined and may be attributed to some feature of the depositional environment (seep. 183). tn this instance there appears to be little correlation between lithology and current-direction. The further point may be made that some of the observed between-outcrop variability may be due to sampling of slightly different stratigraphic levels which exhibit vertical changes in current-direction similar to those described above.

THE PENNANT “DELTA”

The broad framework of Rhondda Beds sedimentation is now reasonably clear. Deposition took place in an enclosed basin similar in general form to the present structural trough but of greater dimensions. At the closed eastern end of this basin sedimentation was controlled by the slow, continuous uplift of a tectonically active ridge of malvernoid trend which, judging from the coarseness of the conglomerates, lay a little to the east of the present East Crop. In this region erosion and scouring almost kept pace with deposition. producing a thin sequence of well sorted, clean orthoquartzites and quartz-conglomerates. Further to the west ill-sorted sediment was being shed north from a rapidly rising ridge which occupied the site of the present Bristol Channel. This ridge appears to have been in existence at least since Basal Coal Measure times (BLUCK,1958) and its rapid uplift in Lower Pennant times may represent the accelerated rate of tectonism which later culminated in the intense folding of the rocks of Devon and Cornwall and the less severe deformation of the South Wales basin. It is interesting to note that the material derived from this ridge includes fragments of Lower or Middle Coal Measures lithology (cf. HEARD,1923), suggesting a form of cannibalism due to constriction of the southern margin of the South Wales basin of deposition. The molasse-like detritus supplied by this ridge spread further north and east with time, producing the progressive eastward migration of the subgreywacke facies. A little later in Pennant times the influence of the eastern axis appears to have diminished completely and deposition of ill-sorted sandstones occurred over the whole coalfield arza. Study of lithology, thickness-variation and sediment-petrography suggests that the influence of the northern source at this time was small. A puzzling feature is the fact that the zone of maximum thickness of the lower Rhondda Beds does not coincide with the basin “axis” (zone of maximum current-mixing). It is tempting to see in the Margam-Maesteg culmination (Fig. 1) the influence of a localised delta-fan building northwards into the main trough but the lack of a radial current-system in this area 1955; MOORE,1959) provides little support for this idea (Fig.2). (cf. WALKER, The dominant lateral s ~ ~ p p lthe y , relative coarseness of the succession, the lack of

SEDIMENT TRANSPORT IN THE LOWER PENNANT MEASURES

183

distinct levee or inter-distributary sediments, the absence of marine intercalations or interdigitations, these features distinguish the Lower Pennant basin of deposition from all known deltaic deposits. Nevertheless the lithological features of the Rhondda Beds are identical with those of ancient alluvial deposits described by YABLOKOV et al. (1958). The abundance, form and orientation of the channel-structures suggests that the Rhondda Beds were formed within the meander-belt of a westward-flowing riversystem and the lateral and vertical variability in current-direction may also be due to meandering. The abundance of lag-deposits (conglomerates and "rubble-bands") and the evidence of contemporaneous reworking lend additional support to the fluvial origin of these rocks. The coarseness and relative angularity of the sands suggests relatively short travel of the detritus and possible deposition in an alluvial valley comparable to that of the present Mississippi (FISK,1952, fig.27). Sediments similar to those which occur in modern deltas (mainly birdfoot) may exist in the earlier Coal Measures of South Wales but their Lower Pennant equivalents, if they exist at all, must be sought to the west of the area under consideration.

SUMMARY

Consideration of the variations in thickness, lithofacies and current-structures (notably cross-bedding and channels) within the Morganian Rhondda Beds of South Wales leads to the conclusion that the present structural trough originated as an elongate basin of deposition, closed to the east but open westwards. Derivation of the ill sorted subgreywacke facies was mainly from the south-southeast with lesser supply from the north. Orthoquartzites and quartz-conglomerates were derived from an axis to the east and represent a much thinner, reworked sequence compared to their western equivalents. The Rhondda Beds sediments were probably formed in the alluvial tract of a westward flowing, meandering river system.

REFERENCES

BLUCK, B. J., 1958. The Sedinientary History of the Rocks between the Horizons C.subcrenatuni arid the Carw Coal in the South Wales Coa!/?eld.Unpublished thesis, University of Wales. G., 1963. Channels from the Upper Carboniferous Coal Measures of BLUCK,B. J. and KELLING, South Wales. Sedimentol?yy, 2 ( I ) : 29-53. BLUNDELL, C. R. K.! 1952. The succession and structure of the northeastern area of the South Wales coalfield. Quart. J . Grol. SOC.London, 107 : 307-333. FISK,H. N., 1952. Geo10,~icalInvestiyation of the Atchafalaya Basin and the Probleni of Mississippi River Diversion, 1. U.S. Corps of Engineers, Waterways Experiment Station, Vicksburg, Miss. ( U S A . ) , 145 pp. A., 1922. The petrology of the Pennant Series, east of the River Taff. Geol. M q . , 59 : 83-92. HEARD, MCKEE,E. D. and WEIR,G. W., 1953. Terminology for stratification and cross-stratification. Bull. Ceol. SOC.Am., 64 : 381-390. MOORE,D., 1959. The role of deltas in the formation of some British Lower Carboniferous cyclothems. J . Ceol., 67 : 522-539.

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G . KELLING

Mch M F . L. R., 1948. The sequence and structure of the southern portion of the East Crop of the South ‘ 4 . t k s Coalfield. Quart. J. Geol. SOC.London, 103 : 261-300. TRuthi.\k. 4 E., 1954. The Coalfields of Great Britain. Edward Arnold, London, 396 pp. C . T 1955. Current-bedding directions in sandstones of Lower Reticuloceras age in the WALKER. Millstone Grit of Wharfedale, Yorkshire. Proc. Yorkshire Geol. Soc., 30 : 115-132.

WOODLAND, A. W., EVANS,W. B. and STEPHENS, J. V., 1957. Classification of the Coal Measures of South Wales with special reference to the Upper Coal Measures. Bull. Geol. Surv. Gt. Brit., 13 : 6 1 3 . YABLOKOV, V. S., BOTVINKINA, L. N. and FEOFILOVA, A. P., 1958. Sedimentation during the Carboniferous and the significance of Alluvial deposits. U.S.S.R. Acad. Sci., Geol. Inst. Moscow [Preprint issued for 4th Intern. Congr. Carbonif. Stratigraph. Geol., Heerlen).

THE RECENT CARBONATE SEDIMENTS NEAR HALAT EL BAHRANI, TRUCIAL COAST, PERSlAN GULF D . J . J . KINSMAN

Department of Geology, Royal School of Mines, London (Great Britain)

INTRODUCTION

The general physiography of the area to be described has been given in the paper by EVANS et al. (1963). Herein the dashed rectangle of fig.3 indicates the area concerned. The more detailed physiography can be seen from Fig. 1 of this paper; here, the high water mark is indicated by a solid line, with the land areas ornamented within. Low water mark is indicated by a simple dashed line, the inter-tidal areas inside it being marked by a vertical ornament. Bathymetric countours of 1, 3 and 5 fathoms are also indicated. All depths are expressed in fathoms below Admiralty Datum which is approximately the level of low water spring tides. The maximum tidal range is 6-7 ft. and mean sea level is 4 ft. above datum. All depths indicated on the maps and profiles (Fig. 1, 3) have been corrected for tidal difference and are related to datum. The tidal range is at a maximum on the open seaward coasts and decreases landwards across the shoal areas and channels. The dominant wind and wave approach is from the northwest. On reaching the delta edges, which coincide almost exactly with the 1 fathom contour line, the waves steepen and break; the seaward limit of the deltas is characteristically marked by a line of breakers. Waves affect the entire subaqueous delta surfaces as far back as the channels. In the channels the tidal currents reach speeds of up to 2 knots and effectively damp out the waves which have crossed the delta tops. Thus the areas lying inside the channels are little affected by wave action at all. The waters of the flood tide flow in along the channels and also in over the shoal delta areas, whereas on the ebb tide the drainage is mainly through the channels. Asymmetric “megaripples” orientated across the channel floors, together with scour features all indicate the ebb currents to dominate over the flood currents. Where the two main branches of the channel meet turbulence has effected a deep scouring of the channel floor to a depth of 7 fathoms. Seawards the channel becomes shallower (Fig.3, profile 2) until at the delta edge it is present only as a 1-2 ft. feature. The mouths of some channels become completely infilled as for example that lying parallel but immediately northeast of the main channel. Seawards of the 1 fathom contour the sea bed falls rapidly to 3 fathoms; this is considered as the delta slope zone. In comparison with the wide shoal areas the zone below

Fig.1. The Halat el Bahrani-Abu Dhabi region of the Trucial Coast. showing land areas, (solid line), inter-tidal areas (simple dashed line), and detailed bathyrnetry. All depths are accurate to within I ft. For location, see inset of fig.3 in accompanying paper by EVANS et al. (1963).

RECENT CARBDNATE SEDIMENTS I N THE PERSIAN GULF

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3 fathoms may be considered as relatively deep. Below this depth the sea bed slopes gently seawards; this foreset zone of the delta is developed to a depth of 5 fathoms and this may be readily seen from Fig. I , where the parallelism of this contour with the delta edge is most obvious. Deeper than 5 fathoms evidence of the deltas is entirely lacking and the sea b:d deepens irregularly to 20 fathoms. Between the two deltas which impinge upon the northeast and southwest tips of Halat el Bahrani stretches a poorly developed coral reef behind which relatively deep water (1-2 fathoms) extends almost to the coast. This arrangement of deltas, in response to channels which drain seawards between the islands, and coral reefs and inshore deeper water i n the inter-delta positions is typical of much of this region.

SEDIMENTARY SUB-ENVIRONMENTS

The sedimmtary sub-environments arc closely controlled by depth as may b: seen from a comparison of Fig. 1 and Fig.2. They are also biological sub-environments. In general, the sediments are produced within the area; the only materials which originate beyond the local arza of accumulation have a windblown origin. These windblown additions are considered to cover the entire region. Some fine carbonate as well as non-carbonate material will have a windblown origin as the rocks exposed inland are largely calcareous in nature. A description of the several sub-environments of deposition follows. Deep water sub-cwvirotirncnt This sub-environment is limited on its inner margin by the 3-fathom line; it represents the fore-set area of the delta and slopes gently seawards from the base of the deltaslope. The sediments, although no doubt subject to wave action under storm conditions, are not as continually affected by wave activity as are those of the delta top. The sediments are generally medium to dark grey in colour and are usually somewhat foetid. They are neither well sorted nor rounded, consisting of fine and very fine sands with an admixture of medium and coarse shelly debris The medium and coarse grades of the sediment consist almost entirely of molluscan and echinoderm debris; many lamellibranch valves are still articulated, or if separated they show little sign of abrasion; unbroken, extremely thin-walled shells are commonly present; broken shells nearly always have angular edges. Other shelly materials such as Foraminifera, fragile Polyzoa and ostracods all occur unabraded. The fine and very fine grades of the sediment contain appreciable quantities of non-carbonate materials; one total sample showed 1 5 % carbonate and 25 non-carbonate. Many of the very fine sand and silt sized non-carbonate grains are angular, although occasional fine or medium sand sized, well rounded, typical aeolian quartz grains do occur. Some composite grains are found, consisting of the finer grades of the sediment cemented together by granular aragonite. Many of the finer grains also have a n irreg-

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ular and incomplete coating of such granular aragonite. The deposition of aragonite is common inside many shells; lamellibranchs, gastropods, Foraminifera and Polyzoa may all show an infilling of small detrital grains cemented together by granular aragonite. The deposition of aragonite in this specialised micro-environment may be influenced by organic agencies. A mixed zone of deep water and delta top sediments occurs in the delta slope zone between I and 3 fathoms. Delta top sub-en vironment

The deltas are limited seawards by the 1 fathom line and extend inwards either to the main ebb channels or to the island coasts. The waves, approaching dominantly from the northwest, build long, en tchelon bars of the delta sands, sub-parallel to the deltaedges. These bars are several hundred feet in wavelength and 2-4 ft. i n amplitude. They may be seen in fig.2 of the earlier paper by EVANSet al. (1963) and Fig.3 of the present paper. The bars parallel the northwesterly facing delta edges but extend across the delta tops where they swing southwest in towards the island coasts. This southwestward movement of the delta sands has resulted in the addition of a large spit t o the northern tip of Halat el Bahrani since 1958 when the aerial photographs were taken. The delta top sediments are always well ripple marked, and in places seaweed growth is prolific. The sediments are predominantly medium and fine grained oolite sands, with subordinate skeletal fragments, but b x o m e coarser grained towards the delta edge. The sands of the intertidal banks north and southwest of Halat el Bahrani are slightly finer in grain-size. The oolite content of these sediments is generally greater than 60%. The skeletal debris consists dominantly of rounded, thick shelled material. Little thin shelled debris occurs except for the occasional rather abraded Foraminifera and small lamellibranch shells. Noncarbonate grains are present, some showing the first stages of oolitisation. Small abraded spines of the burrowing echinoid Echinodiiscus bisperforatus are fairly common together with occasional very abraded spine fragments of the reef echinoid Echinometra mathaei. I n general, there is a lack of the lighter and more fragile organic remains such as Foraminifera, Ostracoda and Polyzoa; those materials which are present are all heavily abraded. Inter-delta sub-environment and ojshore coral reefs

Coral reefs are typically developed in the inter-delta areas. Seaward of the reef extends the deep water zone. The reef is composed'mainly of Acropora although much of the coral is dead; Platygyra, Porites and other massive corals also occur, together with the reef echinoid Echinometra mathaei. Landwards of the coral reef extends the inter-delta zone; here the water is typically 1-2 fathoms in depth. The sediment surface is gently ripple-marked and littered with coral and coarse shell fragments. The sediments of this zone have many characters in common with those of the deep water zone together with the addition of fine oolite and coral sand. They are usually grey in colour and somewhat foetid.

RECENT CARBONATE SEDIMENTS IN THE PERSIAN GULF

191

Inner coral reef sub-environment This sub-environment occurs in shoal areas sheltered from wave action by the channels. It can be seen from Fig.1 that the area lies above datum and thus dries during very low tides. Acropora is the dominant coral and along the channel sides, which mark the edge of this sub-environment, is extremely prolific. On the reef flat itself, coral growth is rather more patchy and wide areas of sands occur between the coral heads, many of which are dead and bear luxuriant growths of seaweed. Acropora is joined on the reef top by massive corals such as Platygyra. The reef echinoid Echinometra mathaei is often extremely abundant, as also are many species of crabs, molluscs, foraminifera and fish. On the reef top the coral heads grow up to 2 ft. above the level of the sediment. Near the base of the coral clumps, the calcareous Alga Lithothamnium is abundant. The broken coral and calcareous algal debris form the main components of the coral sands, although unabraded spines of Echinonietra are also fairly abundant. The sands are extremely angular and ill sorted; fragments may range in size from entire coral heads to silt and clay sized particles. The reduction of debris to small particle size is considered t o be largely biological; much breakdown is probably effected by the activities of Echinometra. An area of mixed coral and oolite sands is found to the north of the main channel; coral growth here is rather poor and the sediments are better sorted and rounded than the true coral sands. Channel sub-environment The channel walls, except for the seaward 1-2 miles, bear a prolific growth of the branching coral Acropora. The channel deposits comprise components from several sub-environments and are thus of an extremely heterogeneous character. The surface is littered with a coarse lag deposit of coral and shell material; finer coral and oolite sand occupy an interstitial position. The finer materials are carried seawards by the ebb currents and where these currents slacken, become gradually deposited; thus the deposits of the main channel, just in from its mouth, are generally medium and fine-grained in character and at its mouth oolites predominate. Beach, island and inner sub-environments The beach deposits bear a close relationship to the offshore sediments. Thus a beach inland of an oolite sand area is composed largely of oolites, one inland of a coral sand area, largely of coral debris. The sands of the seaward coast of Halat el Bahrani are mixed, containing oolites and much medium and coarse shell debris, derived from the inter-delta area. The sands of the upper beach and berms become winnowed by wind action and moved inland to the frontal dune ridge. Much of the finer fraction is blown further inland, across the wind stripped central area, finally to the shoal inner banks between Halat el Bahrani and Jazirat Ftaisi. Thus the sediments of the extensive crab

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flats (Fig.2) are fine and very fine sand but have largely originated in other sub-environments. Both inner coasts of Halat el Bahrani are characterised by a succession of spits and wide crab flats; the latter lie just above the level of mean high water; seaward of the crab flats, where spits enclose especially sheltered environments, a gastropod flat is developed which is covered by every tide. Here, the sediments often contain some silt and clay grade materials. The finest sediments of the region are those of the tidal swamps; an area of tidal swamps lies behind the frontal dune ridge of Halat el Bahrani and a much larger area extends from the southeastern tip of the island almost to the mainland. The sediments comprise precipitated aragonite muds together with coarser wind blown materials. It is readily apparent from the two papers which have described the Trucial Coast region that the detailed physiography is rather complex. This complexity is mirrored in the sedimentary sub-environments. It is the physiography which has largely given rise to the complex sediment pattern, but the reverse is also in part true, the sedimentary and biological sub-environments having given rise to the complex physiography.

ACKNOWLEDGEMENTS

Grateful acknowledgement is made to D.S.I.R. for financing this project, to the Hydrographer of the Royal Navy, Captain and crew of H.M.S. “Dalrymple” for much assistance and cooperation, to the British Museum of Natural History for identification of fauna and flora, to the Oil Companies working in the area, to G. Evans and D. J. Shearman for assistance in the field and laboratory respectively and to Mrs. F. Kelk for assistance in the preparation of the figures.

SUMMARY

A description is given of the detailed physiography of an area of recent carbonate szdimentation. The relationship of tidal waters to the area is discussed and the several sedimentary sub-environments are described.

REFERENCE

EVANS,G.. KINSMAN, D. J. J. and SHEARMAN, D. J., 1963. A reconnaisance survey of the environment of recent carbonate sedimentation along the Trucial Coast, Persian Gulf. In: L. M. J . U. VAN STRAATEN (Editor). Deltaic and Shallow Marine Deposits.. Elsevier, Amsterdam, pp. 129-1 35.

SEDIMENTARY FACIES IN BAY OF FUNDY INTERTIDAL ZONE, NOVA SCOTIA, CANADA GEORGE D E VRIES KLEIN

Deparfmenfof Geology, University of Pennsylvania, Philadelphia, Pennsyh'ania (US. A. )

INTRODUCTION

The Bay of Fundy of eastern Canada has long been renowned for extreme vertical tidal fluctuations reportedly ranging up to 50 ft. (JOHNSON, 1925; GOLDTHWAIT, 1924). Despite extensive interest in the tidal phenomena of the Bay of Fundy, little work has b x n done on the Bay's hydrography ( KLEIN, 1963). This paper summarizes a study of the sedimentary facies of the Bay of Fundy intertidal zone and briefly compares such features with that of the Dutch Wadden Sea.

SEDIMENTARY ENVIRONMENTS

Bay of Fundy intertidal zone sedimentation occurs in 4 sedimentary facies. These facies are: (I) wave-cut benches, (2) estuarine clay flats, (3) lee of bedrock islands, and ( 4 ) tidal marshes. Aproximately 75 % of intertidal sediments in the Bay of Fundy are deposited on wave-cut benches. The remaining depositional areas of note are mouths of estuaries and tidal marshes. Wove-rut benchfacirs

The Bay of Fundy coast is flanked by variable bedrock into which wave action has excavated broad wave-cut benches by undermining of sea cliffs. The width of these benches is controlled by bedrock lithology. Igneous and metamorphic bedrock coastlines are characterized by narrow wave-cut benches, whereas the softer sedimentary rocks of Late Paleozoic and Triassic age are flanked by broad benches. Thin sediment veneers occur on the wave-cut benches. Along the western shore of the Minas Basin (see Fig.1 for locations cited in text), this veneer was found not to exceed 2 ft. in thickness. Coring disclosed thicker sediment zones at the mouths of distributary streams which extend across this evironment. The properties of wave-cut-bench-veneer sediments are controlled by the lithologies of underlying benches and adjacent seacliffs. The tidal flats of the south shore of the Bay of Fundy are flanked by Triassic basalt. Associated intertidal zone sediment

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Fig. 1 . Map of Bay of Fundy showing localities cited in text and limit of low tide (dotted line flanking shoreline).

veneers consist of basalt fragments and minerals derived from the basalt. Texture of these sediments is controlled by intergranular breakdown. Color and mineral composition is identical to bedrock. Along the Minas Basin shore, the sediments on the wave-cut benches consist of debris derived from Triassic redbeds and Paleozoic sediments. These veneer sediments are characterized by the same color, texture and composition as the bedrock. Lateral changes in bedrock properties are also expressed by lateral changes in veneer sediments. Thus, in the southern Minas Basin where Triassic redbeds overlie unconformably Upper Paleozoic rocks, local changes in tidal flat sediment texture, color and composition are controlled directly by lateral changes in the composition of the wavecut benches and sea-cliff lithologies. Similarly, the tidal flat sediments of the south shore of St. Mary’s Bay and of the New Brunswick Coast are characterized by the same color and composition as the metamorphic rocks below. Few streams traverse the sediment veneers. Where such streams occur, a braided drainage pattern is common. Along the western Minas Basin, meandering streams draining adjacent tidal marshes and glacial terrains assume a braided pattern where extended across the sediment veneer flats. Such streams commonly merge into a sheet. The braided streams channel no more than 2 inches into the tidal flats. Consequently, during rising tide, wave action reworks the sediments and destroys most traces of fluvial excavation. During tidal ebb, the braided system extends across the flats once again. The banks of Bass Creek, a stream draining the western Minas Basin shore, were

SEDlMENTARY FAClES IN BAY OF FUNDY INTERTIDAL ZONE

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staked to determine the amount of lateral channel shift. During a 3-day period, the lateral shift ranged from 0-1 1.2 ft. Maximum shifts were observed at the seaward end of the streams because prolonged wave and current action during tidal submergence destroyed channel traces. Closer to land, almost no lateral displacement was observed because wave action and tidal currents were unable to remove the channel traces during shorter tidal submergence. Thus, closer to shore, Bass Creek and similar streams reoccupy partially-destroyed channels during tidal ebb. As few braided streams flow across the intertidal veneer sediments, the major processes influencing sedimentation are the action of waves and of tidal currents. AS the tide rises, wave action churns bottom sediments, reworks it and removes finer-grained material, leaving a coarser-grained residue. During high tide, clay settles out over the entire flat and becomes mixed with the lag sediments. During tidal retreat, some of this clay is again removed by wave action although clay settled close to shore remains mixed with the coarser sediments. Thus, poorly-sorted sediment occurs near the upper and shoreward limits of high tide because less time is available for clay removal. The sediment veneer in the middle and lower parts of the intertidal zone is better sorted because prolonged wave and current action removes the finer material. A similar distribution of clay in near-shore tidal areas was described by VAN STRAATEN and KUENEN(1958) and WHITEand NORTHCOTE (1962, p.406). As a result of combined wave and stream action, the sediment veneers on wave-cut benches contain a wide variety of primary structures. Both oscillation and current ripple marks are common. These ripple marks are flat-topped, scoured and striated. Ripple troughs contain relatively coarser sediment than the crests. Rhomb-shaped interference ripple marks ( KLEIN, 1963, fig.5), rhombic ripple marks, current lineation, scour pits on the down-current side of pebbles and animal tracks are characteristic. The origin of the flat-topped ripple marks is attributed to scouring by current action. Similar flat-topped ripple marks have been described from the intertidal zone of Florida by TANNER (1958) and from Maine by TREFETHEN and Dow (1960). The writer is not at present aware of the occurrence of such decapitated ripple marks in other depositional environments. The distributary channels impart their own characteristic suite of primary structures including rounded current ripple marks, current lineation, flutes, grooves, imbricate boulders and lenticular cross-stratification. Such structures can be observed forming during stream flow but are later destroyed by wave action during rising and falling tide. Megaripples, consisting of coarse sand, are the most characteristic structures associated with bench veneer sediments. Megaripple wave lengths range from 4-10 ft. (average 6 ft.) and amplitude ranges from 8 inches to 3.5 ft. (average 2 ft.). On the Minas Basin south shore, megaripples contain a subordinate set of interference and current ripple marks (KLEIN, 1963, fig.7, 8). Interference ripples are confined to the top of the megaripples, whereas perpendicularly-oriented current ripple marks occur on the steep, down-current face. Whereas general agreement exists that the megaripples are produced by high-

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velocity tidal currents, the origin of the subordinate current and interference ripple marks is not completely understood. KINDLE(1917, pp.21, 29) in a study of Bay of Fundy intertidal zone ripple marks, observed that the smaller ripples are later in origin than the megaripples and were therefore formed by tidal ebb. Current and interference ripples on the down-current steep slopes and megaripple crests were formed by later wave action and shifting current directions. Some current ripples are perpendicular to the megaripples because wave action, being directed sub-parallel to the shore, is perpendicular to the major tidal current directions which formed the megaripples. The current ripple marks occurring on the down-current slope were observed to he formed differently. As ebbing tide progressively exposes different parts of a megaripple, water collects and flows as a channel in the megaripple troughs. This water flows both in the direction of the outgoing tide and the direction of slope. Such flow parallels megaripple crests. Current ripple marks can be observed forming in the troughs perpendicular to megaripple crests during this Iocal channel-type flow. Such low-tide megaripple channels are responsible for accumulation of coarser sediment in the troughs. Channel velocities diminish as tidal ebb continues. In the earlier stages of such channel flow, velocities are high enough to flush out fine- and medium-grained detritus, leaving coarser sediments in the troughs. Estuarine cla-vJIatfacies

In the western Minas Basin where three rivers enter the Bay of Fundy the tidal flat sediments are different. The sediments consist of mixed silt and clay. A thin strip of sand occurs at the uppermost limits of these fats where the flats flank bedrock or tidal marshes. This strip has an average width of 5 ft. The top 3 inches of the clay flat sediments are brown. A black zone occurs below, due to increased quantities of organic material. In contrast to the wave-cut bench environment where animal colonies are absent, the estuarine clay flats are a haven for a varied invertebrate fauna. Estuarine clay flat sediments are unlaminated. Oscillation ripple marks occur in the sandier zone adjacent to sea cliffs. The flats are drained by meandering creeks. Lee of bedrock island facies

The only area of tidal sedimentation in the lee of bedrock islands occurs on the Minas Basin north shore at Five Islands, Nova Scotia (Fig.1). The sediments here are sufficiently unique to warrant recognition as a special facies. The sedimentary profile of the Five Islands tidal flats is the only one in the entire Bay of Fundy which is analogous to the tidal flats of the Dutch Wadden Sea (VAN 1961). I n the higher tidal flat, one finds a sandy area drained by braided STRAATEN, streams. A lower tidal flat, accounting for 95 of the exposed tidal flat area at Five

SEDIMENTARY FACIES IN RAY OF FUNDY INTERTIDAL ZONE

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Islands, consists of silt and clay and gravel. Lower tidal flat sediments are poorly sorted, whereas the higher tidal flat sediments are better sorted. The low tidal flats are drained by creeks which meander laterally across the mud flats during low tide. During rising tide, water works its way upstream along channels before flooding the flats. The lateral migration of the tidal channels in the lower flats is ( I 950). Whereas in the Dutch Wadden Sea similar to that described by VANSTRAATEN one finds a shell lag concentrate on the channel floors, channel-bottom lag concentrates in the low tidal flats at Five Islands consist of mixed medium- to coarse-grained sand, gravel and convex-upward oriented clam shells. As the sediments at Five Islands were derived from an adjacent upland area underlain by bedrock, the texture and composition of the channel floor lag concentrate is more variable. In the Dutch Wadden Sea, no such adjacent upland area exists. Hence the difference in types of channel floor lag concentrates between the two areas.

Tidul marsh facies Tidal marshes are extremely common in Chignecto Bay (Fig.1) and the Minas Basin shore adjacent to estuarine clay flats. GOLDTHWAIT (1924, pp. 135-136) and JOHNSON (1925, pp.562-572) observed that the tidal marshes of the Bay of Fundy are different from those of New England. Whereas New England tidal marshes consist of a mixture of clay and silt and plant debris, Bay of Fundy marshes consist almost exclusively of a silt bank below, covered by a thin veneer of plant material above. These observations were confirmed by coring in the Minas Basin north shore and at Bass Creek. A thin (6 inch) veneer of plant debris and marsh grass which trapped clay was found at the top of the core, whereas the underlying material consisted exclusively of silt and clay. The uppermost 6 inches of silt was brown, whereas the remainder of the silt was gray and blue gray due to reduction of organic material.

COMPARISON TO DUTCH WADDEN SEA TIDAL FLATS

KLEINand SANDERS (1963) have recently reviewed the common features and difl'erences between Bay of Fundy and Dutch Wadden Sea tidal flats in greater detail than space permits here. As noted by these authors, the tidal marsh and lee of bedrock islands environfients of the Bay of Fundy are almost identical in processes and sediment products as salt marshes and the total depositional plan of tidal flats in the Wadden Sea. The estuarine clay flat environment has some features in common with the higher tidal flat environment of the Wadden Sea summarized by VANSTRAATEN ( 1 961). No counterpart to the wave-cut-bench facies of the Bay of Fundy exists in the Dutch Wadden Sea simply because the geological setting of both regions is different. The Bay of Fundy tidal flats are surrounded by a bedrock highland area which sheds considerably more coarse sediment onto the tidal flats, whereas in the Wadden Sea the

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flats are flanked by a soft-sediment, broad flat coast. These differences in coastal geology and geomorphology account for much coarser and more variably-textured sediments in the tidal flats of the Bay of Fundy. As probably most of the littoral sediments of the recoverable stratigraphic record were deposited under coastal regions more similar to the Netherlands coast or the Gulf Coast of the southern United States, few ancient counterparts to the wave-cutbench facies are probably preserved. Two possible examples where such conditions could have prevailed during the past may have been the world-wide marine transgression of Cambrian shelf sediments on Precambrian crystalline rocks or the shallow marine transgression of Upper Llandovery age in the Welsh borderland of Great Britain. Other basal marine unconformities may have been formed by intertidal zone excavation of wave-cut benches followed by intertidal zone deposition and subsequent marine submergence.

ACKNOWLEDGEMENTS

This paper is partly extracted from the writer's doctoral dissertation submitted to Yale University under the supervision of J. E. Sanders. Financial support for this study came from the Geological Society of America, the Nova Scotia Department of Mines, the Nova Scotia Research Foundation and the Hewitt Fund, Yale University.

SUMMARY

Four sedimentary facies occur in the intertidal zone of the Bay of Fundy. 75% of intertidal zone sedimentation occurs in a wave-cut bench environment. Such benches are overlain by a thin sediment veneer which is sorted and modified by a combination of wave action and braided streams. The remaining facies of intertidal zone deposits include a salt marsh facies (silt and clay deposits capped by a thin plant debris zone), an estuarine clay flat facies (silt and clay banks traversed by meandering creeks) and a lee of bedrock islands facies, which can be subdivided into high tidal flat, low tidal flat and channeI facies. Comparison of the intertidal zone of the Bay of Fundy with that of the Dutch Wadden Sea suggests that differences in sediment properties in each are controlled by coastal geomorphology and coastal geology.

REFERENCES

GOLDTHWAIT. J. W., 1924. Physiography of Nova Scotia. Geol. Surv. Canada Mem., 140. JOHNSON, D. W., 1925. The New Eqland-Acadian Shoreline. Wiley, New York. KINDLE, E. M . , 1917. Recent and fossil ripple marks. Geol. Surv. Canada Museum Bull., 25. KLEIN,G. DE V., 1963. Bay of Fundy intertidal zone sediments. J . Sediment. Petrol., in press.

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KLEIN,G. DE V. and SANDERS, J. E., 1963. Comparison of sediments from Bay of Fundy and Dutch Wadden Sea tidal flats. J . Sedinrcnt. Petrol., in press. TANNER. W. F., 1958. An occurrence of flat-topped ripple marks. J . Sediment. Petrol., 28 : 95-96. TREFETHEN, J. M. and Dow, R. L., 1960. Some features of modern beach sediments. J . Sedinrenf. Pefrof.,30 : 589-602. VANSTRAATEN, L. M. J. U.,1950. Environment of formation and facies of the Wadden Sea sediments. Ttjdschr. Koninkl. Ned. Aardrrjkskundig Genoof.,61 : 354368. VANSTRAATEN, L. M. J. U., 1961. Sedimentation in tidal flat areas. J . Alberta SOC.Pefrof.Geologists, 9 : 203-226. VANSTRAATEN, L. M . J. U. and K U E ~ EPH. N , H., 1958. Tidal action as a cause of clay accumulation. J . Sediment. Perrol., 28 : 40&413. WHITE,W. H. and NORTHCOTE, K. E., 1962. Distribution of metals in a modern marine environment. Econ. Ceol., 57 : 405-409.

ASPECTS GENERAUX DE LA SeDIMENTATION ARGILEUSE DANS LES FACIkS LITTORAUX DU PALEOGGNE NORD-AQUITAIN A . KLINGEBIEL

et

C. LATOUCHE

Insrirrrt de Gc‘olqyie rlu Bassin d‘Aqrritaitie, Taletice (France)

INTRODUCTION

Au cours de nos etudes stdimentologiques des stdiments paltogknes du Bassin NordAquitain, nous avons porte notre attention sur les variations de composition du materiel argileux dont I’interpretation prockde de trois id& directrices, compltmentaires.

ASSOCIATIONDE M I N ~ R A U XARGILEUX ET LITHOSTRATIGRAPHIE

A I’tchelle locale, un premier inventaire des divers types d’associations de mintraux argileux provenant de coupes voisines et lithologiquenient comparables, permet de dresser des schtmas chronologiques des dkpbts. Par exemple, en Bordelais, de nonibreuses coupes de forages fournissent u n enregistrement continu des divers aspects de la stdimentation en milieu marin; les grandes unites stratigraphiques se caracttrisent ici, par des proportions particulikres des trois grandes faniilles d’argiles: montmorillonite, illite et kaolinite. Un exemple en est donne dans Fig.1 oh est utilisi un mode de representation par diagramme triangulaire (PRYORet GLASS, 1961). II apparait nettenient que la kaolinite domine B I’Eoctne infkrieur; le mtlange montrnorillonite, illite et kaolinite semble caracttriser l’eockne moyen; l’abondance de la montmorillonite et de I’illite (souvent assocites ;i de la chlorite) individualise 1’Eockne suptrieur. L’illite est prtpondtrante B I’Oligoctne oh la montmorillonite a disparu. Quelques niveaux semblent echapper a cette loi statistique. Ce sont toujours des rtcurrcnces de facits constituant les termes initiaux ou terminaux des sequences mineures formant des “niveaux de transition” entre les grandes subdivisions stratigraphiques adopttes ici. Ces rtsultats statistiques presentent donc un inttr&tlithostratigraphique limitt, mais sont cependant susceptibles de recevoir une interprttation gtnitique, si on les intigre dans u n contexte paltogiographique plus vaste. Ces associations de mintraux argileux rtsultent en effet de la combinaison des phtnomtnes d’htritage et de ntoformation (MILLOT, 1952) dont les modalitis ont varit dans le temps et dans l’espace.

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Fig. I . l h d e statistique de la fraction argileuse des sidinients palkogenes du Bordelais.

R ~ P A R T I T I O NDES ARGILES DANS UNE FORMATION D ~ T E R M I N ~ E

Dans une certaine mesure, en effet, les associations de mintraux argileux apparaissent likes a certains facibs lithologiques. Nous avons tentt d’appliquer la “mtthode d’analyse dynamique” aux formations de l’gocbne moyen du Nord de I’Aquitaine. Le cycle 1962) dtbute par stdimentaire majeur correspondant ici a cette pPriode (KLINCEBIEL, le dtp8t d’importantes masses sableuses, issues du Massif Central qui subit une intense erosion. Les apports dttritiques proviennent non seulement du dtmanttlement de la croiite d’alttration lattritique qui recouvrait ce massif, mais encore de I’trosion directe dii sock cristallin, comme en ttmoigne l’apparition des feldspaths dttritiques et de mintraux lourds peu rtsistants. Dans les sediments dtposts au cours de la transgression lutttienne I’abondance de la kaolinite est lite ;i celle des Cltments dttritiques. Les mintraux de ntoformation tventuelle, I’illite et surtout la montmorillonite y sont associts en proportions d’autant plus fortes que les tltments sableux sont moins abondants. Dans la Fig.2 a t t t sclitmatiste la composition moyenne de la fraction argileuse des formations de 1’8ockne moyen en divers points du Bassin. On y observe que I’illite souligne le caracttre marin de la stdimentation daiis la zone occidentale, directement ouverte sur le domaine octaiiique (VEILLON et VIGNEAUX, 1961a). La kaolinite garde une place prtpondtrante dans la zone de stdimentation dttritique de Bordeauxet VICNEAUX, 1961b). De sensibles proportions de montmorillonites Coutras (VEILLON et parfois de chlorites se rencontrent dans les intercalations d’argiles et de marnes correspondant i des tpisodes de stdimentation lagunaire ou les apports dttritiques sont rtduits. AU sudest, au contraire, dans le fond du golfe a stdimentation plus argileuse, la rnontmorillonite domine. Ce mintral y est soit ntoformt dans des faciks Iagunaires recevant des eaux riches en magnesium, soit directemelit htritt de la zone continentale voisine oh se trouvent des formations lacustres 2 montmorillonite et attapulgite (LAPIERRE,1962). Cette esquisse paltogeographique de la rtpartition des argiles a I’6ocbne moyen repose encore en partie, sur une conception statistique de I’ttude stdimentologique. 11 convient de pousser plus avant l’analyse.

202

A. KLINCEBlEL ET C. LATOUCHE

Fig.2. Composition moyenne de la fraction argileuse des sediments lutetiens.

ARCILES ET S~QUENCESLITHOLOGIQUES

Dam le detail des difftrentes sequences lithologiques d’une mCme strie, on observe des modifications de la composition des argiles q u i apparait lite a la fois l’tvolution du cadre paltogtographique, et a des facits de stdimentation particuliers. Un example est donnt dans Fig.3. Dans la coupe de forage observte Bordeaux-La Benauge sont compartes les variations d’abondance (log lithologique) et de composition de mattriel argileux dans lequel nous avons examint un certain nombre de critkres chiffrts par les donntes suivantes.

Abondunce relative drs minkraux urgileux Les courbes de la colonne 2 mettent en t v ’ ience la diminution progressive, de la base au sommet de la strie, des mintraux deux couches pauvres en alcalins et alcalinoterreux et leur remplacement par des mintraux a 3 ou 4 couches de structure plus complexe et plus fragile: illites, montmorillonites et chlorites. Ce phtnomtne semble

203

SkDIMENTATION ARGILEUSE DANS LES F A C I h LITTORAUX AQUITAlNS

m d i t r i t i q u e s gmssiers

m d b t r i t l q u e s fins & tds fins

-

RAPPORT

NATURE PES llLlTE5 TRAl

-

niveaux b nummulites dolomite

k-TT+VT l

silts argiles carbonates

Fig. 3. Miniraux argileux et sequences lithologiques: forage de Bordeaux-La Benauge.

correspondre i la disparition progressive de la couverture d’tltrnents dttritiques rtsiduels, grossiers et colloidaux, formte par lattritisation du continent tmergt A la fin du Crttact. La part de ces Cltments rtsiduels dans 1 stdimentation marine littorale apparait relativement plus importante au debut de chacune des trois sequences majeures correspondant a l’Eockne inftrieur, moyen et suptrieur. Au contraire, les proportions d’illite et de rnontmorillonite apparaissent systtmatiquement plus importantes dans

204

A. KLINGEBIEL ET C . LATOUCHE

les ptriodes de stdimentation carbonattes que dans les ptriodes de stdimentation dttritique. Ce phtnomtne est mis en relief (colonne 3) par les variations du rapport: ”/, montmorillonite illite -

+

-~

~

”/, kaolinite q u i nous a paru ici plus significatif que le rapport f / K propose par BURST(1959), en raison de I’abondance de la montmorillonite. Dans le cadre des trais sequences majeures ainsi bien individualisees par une Cvolution cyclique de ce rapport, des stquences de second ordre sont simplement marqutes par les inflexions de cette courbe. Cela suggtre une Cvolution analogue du phtnomtne A plus petite tchelle. La derive gtntrale de la courbe vers la droite traduit la modification qualitative de I’litritage oh la kaolinite est de moins en moins reprtsentte. On constate par ailleurs que la stabilitt d u rapport (I 4-M ) / K traduit dans une certaine mesure la stabilitt des conditions de stdimentation. A I’Eoctne suptrieur, le caracttrz alternant de la stdimentation littorale et souvent lagunaire, se marque par des phtnomhes de ntog&itse et de diagtntse sporadiquement prtpondtrants dans des dep8ts alternativement sursales ou dessalts, oh le renouvellement ionique est tres irrtgulier. Le comporteinent des minkraux argileux A I’egard des divers facteurs gtochimiques alors mis en jeu rtgit leur gtnese ou leur conservation. Nature des illites

On peut considtrer que les intensitCs de diffraction aux Rayons X relatives aux plans 00 1 et 002 des illites, traduisent leur composition cristallographique et en particulier leur richesse en tlkments alcaliiis. Les variations du rapport:

I001 A=fOE sont ttroitement concommitantes non seulement des variations lithologiques (KLINGEBIEL et LATOUCHE. 1962) mais tgalenient du rapport (I t M ) / K .Leur considtration apparait utile lors de la dtfinition des stquences de second ordre, car elks seniblent refltter assez fidtlement les vicissitudes de la stdimentation marine. A I’Eoctne inferieur, aprts u n bref tpisode dttritique, s’instaure dans le synclinal de Bordeaux un rtgime marin littoral A apports sableiix trts rtduits. I1 se depose alors des marnes kaoliniques plus ou moins dolomitiques, extraordinairement riches en tests de foraminiftres. L’absence d’apports terrigtnes et I’abondance des eltments alcalins suggkrent une ptriode biostasique. A la kaolinite issue du substratum de la transgression sont assocites des illites ‘‘alumineuses” qui ne peuvent Ctre htrittes d’un continent lessivk. Ces micas ne pouvant conserver leurs cations alcalins dans un milieu dtpourvu de ces elements (GARRELS et H O W A R D , 1959) sont ici probablement neoformts, ou tout au moins sont-ils le produit d’une diagtntse prtcoce dans ce milieu sedimentaire. A la fin de 1’Eoctne inftrieur, la rtgression s’annonce par l’arrivte de materiel dttritique, la disparition dzs calcarinites et l’apparition des oxydes de fer. L’appauvrissement du milieu en ions alcalins s’accompagne de “l’ouverture” des mineraux rnicacts. A des modifications rtduites d u corttge argileux correspondent des

S~DIMENTATION ARGILEUSE DANS LES FACIE% LITTORAUX AQUITAINS

205

transformations notables du milieu biologique et de brusques changements de la composition des illites qui permettent ainsi de dtfinir des stquences de second ordre, comparables aux stquences faunistiques et lithologiques. Des faits analogues s'observent i l'eoctne moyen; en particulier au cours de la transgrzssion lutttienne. A I'Eoctne suptrieur, l'irrtgularitt des conditions de dtpBts s'observe avec une particulitre nettett. La presence des micas ouverts, dans les milieux dktritiques grossiers, rksulte d'une iluviation des illites soit au cours de I'htritage, soit a p r b le dtp8t par suite de la circulation d'eaux agressives dans ces lits permtables ( G R I M , 1958). Dans les niveaux rnarneux, la nature des illites parait dtpendre assez ttroitement, tant de facteurs gtochimiques, comme la prtcipitation d'tvaporites, que de I'tvolution biochimique de la vase au cours de la lithogtntse. Des observations similaires pourraient Etre notees i propos des chlorites. En I'absence de modifications notables du cadre paltogiographique au cows de cette ptriode, on peut admettre que la composition des apports terrigtnes reste sensiblenient la rnCme. Les variations qualitatives du niattriel argileux observtes au sein des stquences lithologiques mineures, paraissent alors rtsulter essentiellement de phtnomtnes de ntogtntse et de diagtntse q u i peuvent Ctre ainsi dtcelts.

CONCLUSION

A I'tgal des divers autres crittres lithologiques avec lesquels ils sont parfois lits, les mintraux argileux reflttent les ttapes de I'tvolution stdimentaire dans une strie. Toutefois, I'interprttation statistique et analytique de leur analyse reste toujours dtlicate. I1 convient, en effet, d'apprtcier, dans la rnesure du possible, la part respective des phlnonitnes d'htritage, de ntoformation et de diagtntse. A la lumitre des premiers rtsultats que nous avons obtenus dans le cadre d u Bassin Nord-Aquitain, sur les plans stratigraphique, paltogtographique et stquentiel, il sernble que Yon puisse y parvenir.

Au sein d'une strie stdimentaire locale, I'etude statistique des associations de mine-

raux argileux amtne des conclusions lithostratigraphiques. Dans un contexte paltogeographique plus vaste, la ripartition des argiles i une tpoque donnte illustre les parts respectives des phtnonitnes d'htritage, de ntoformation et de diagtntse, dont la combinaison doit Ctre prtciste dans le cadre des stquences lithologiques.

206

A. KLINGEBIEL ET C. LATOUCHE SUMMARY

Statistical studies of clay mineral associations in local sedimentary series lead to lithostratigraphical conclusions. The distribution of clay minerals over a wider area, in sediments of one particular stage, illustrates the parts played by heritage, neo-formation and diagenesis.

BIBLIOGRAPHIE

BURSTJK., J. F., 1959. Post-diagenetic clay mineral environment relationships in the Gulf-Coast Eocene. Proc. Natl. Con! Clays Clay Minerals, 6th, 2 : 321-341. P., 1959. Reactions of feldspar and mica with water at low temperature GARRELS, R. M. and HOWARD, and pressure. Proc. Natl. Conf. Clays Clay Minerals, 9th, 6 : 68-88. GRIM,R. E., 1958. Concept of diagenesis in argillaceous sediments. Bull. Am. Assoc. Petrol. Geologists, 42 : 246-253. KLINGEBIEL, A.. 1962. Analyse skquentielle et lithostratigraphique du Paleogkne Nord-Aquitain. Conipt. Rend., 254 : 2035-2031. C., 1962. Etude cristallographique des illites dans les series eocknes du KLINOEBIFL, A. et LATOUCHE, Bordelais. Cottipt. Rend., 255 : 142-144. F., 1962. Etude sidimentofqyiqrie et lithoJ/rat{yraphique des Formations palPogenes de la LAPIERRE, bordure nordest dii Bossin ~I'Aqriitainr.These Geologie Approfondie, Univ. de Bordeaux, 81 pp. MILLOT,G., 1952. Heritage et neoformation dans la sedimentation argileuse. Coqyr. Giol. Intern., Conpt. Rend, 19e, Alyer, 1952, 18 : 163-175. PRYOR, W. A. and GLASS,H. D., 1961. Cretaceous-Tertiary clay mineralogy of theupper Mississippi enibayment. J. Sediment. Petrol., 31 : 38-51. M. et VIGNEAUX, M., 1961a. La limite de la transgression marine du Lutetien dans la zone VEILLON, nordest du bassin nummulitique girondin. Compt. Rend. Sanim. Soc. Ge'ol. France, 4 : 94-95. M., 1961b. Les phases stdimentaires du Lutetien niarin dans le nord de VEILLON, M. et VIGNEAUX, I'Aquitaine occidentale. Compt. Rend. Sorrmi. Sor. GEol. France, 5 : 129-1 30.

PIVOTABILITY STUDIES OF SAND BY A SHAPE-SORTER P H . H. KUENEN

Geoloc~icatInstitute, Univ. of Groniqyen, G r o n i n L p (The Nctherlonrls)

PIVOTABlLITY

SHEPARD and YOUNG(1961) have recently introduced a useful concept in sedimentology, which they call “pivotability”. It is here suggested to define pivotability as: the tendency to start rolling on a slope. It is then a shape characteristic unrelated to weight, size or density. The antonym is “shape stability” (see Fig.]). Pivotability should influence transportation by traction (rolling and saltation, both under water and by wind). In relation to transportation in suspension pivotability is, generally speaking, a negative property. Pivotability is not an independent property, for it is related to both roundness and general shape (Zingg values, sphericity, etc.). For instance tabular shapes are less pivotable than cubes. But an angular cube is less pivotable (more stable) than a semi-rounded one. A rounded disk call have the same Pivotab ility

increase

Fig. I . Pivotability. Pairs of shapes with the more pivotable (= less stable) one o n the right. A = rounding has increased pivotability of identical shapes; B = the same; C = angular shapes of which the flatter one is less pivotable; D = the same; E = identical shapes, one in more pivotable position; F = the same; G = rounding predominates over shape in causing higher pivotability; H = shape predominates over rounding in causing higher pivotability.

208

PH. H. KUENEN

pivotability as an angular cube and therefore pivotability can only be ascertained very roughly by visual methods. Actually the scale used by Shepard and Young. which they claim to be based on pivotability, is a roundness scale. It is only in consequence of the part played by roundness in pivotability that in general this property will be higher for the rounded grains.

ROCK-AND-ROLL SHAPE-SORTER

The only known way in which roundness can be measured is by visual inspection, whereas pivotability must be established by some mechanical device. The writer has developed a “rock-and-roll” shape-sorter which is based on the principle of rocking a half-cylindrical trough on its axis, mounted at an angle of 1 3;’ (Fig.2). The grains of the sample zigzag along the trough to the lower end, the travel speed being higher for greater pivotability. At the far end the grains are funneled into a bottle, replaced at

a

F

End view

Fig.2. Diagram of the rock-and-roll shape-sorter.

PIVOTABILITY STUDIES OF SAND BY A SHAPE-SORTER

209

given intervals, so that 12 fractions of successively lower pivotability are separated from each other. The last fraction (number one) is what remains in the trough after stopping the machine. Each sieve fraction has to be tested separately. The size of the sample depends on the grain-size and is 250 mg for 200-300 p. To gain greater accuracy the average of four saiiiples is determined. Although the actual operation only takes about six minutes, the complete treatment of a single sieve fraction lasts more than a n hour. The speed of travel of a separate grain varies strongly from moinent to moment and in a sample the particles hinder each other. If a sample is tested twice the shape fractions are almost equal in amount, but they are composed of different individual grains. In consequence of this mutual hindrance there is a strong tendency to smoothing of the histogram and a low kurtosis results. Hence the histograni is uncharacteristic. On the other hand the median pivotability can be ascertained with an accuracy of about two to three tenths of a fraction and between the coarser grains of a weathered granite and of a well rounded desert sand, the machine is able to distinguish about 30 degrees of median pivotability. As travel time in the machine is influenced by grain-size, the difficulty is of ensuring that the time allowed for a given pivotability fraction for each size grade results in the same shapes. For example, fraction 5 should mean the same pivotability for 100-150 p, the smallest size that can be used in this machine, as for 750-1,000 p, the largest size that has been treated. As pivotability cannot be determined visually, except for very well rounded grains, inspection of the fractions is of little use. A satisfactory solution was found by testing crushed calcite. Assuming that the various shapes of the cleavage fragments will be divided equally over all size classes the sorting times can be adjusted so as to produce about 50% in the last shape fraction. The time to be allowed for the first class, which is not represented in crushed material, was ascertained on the basis of glass spheres combined with visual inspection of very well rounded sand grains. It is then found that there is a remarkable similarity between the analyses (= histograms) of all calcite size grades between 0.15 and 1.0 mm. Crfished feldspar is slightly less pivotable, but when the same travel times for the same size fractions are allowed as for calcite, the histograms of the various fractions of feldspar are almost identical to each other up to 0.6 mm. Taking each size grade separately, the settling velocity in water increases regularly with pivotability as measured by the rock-and-roll sorter. The average settling times plot in straight lines from fraction one to twelve, and this independent test shows that the shape sorter works satisfactorily. However. it must be admitted that the spread in settling time in tach pivotability fraction is wide.

RESULTS

Seven grain-sizes of a few dozen sands have been tested. Results for separate samples appear to be rather confused and general rules or trends are not readily deduced. The

210

PH. H. KUENEN

part played by pivotability in transportation is evidently not very marked. This is probably partly due to the greater influence of weight and to variations in competency of most transporting mechanisms. Whereas high pivotability must favour transportation by rolling, it is unfavourable to travel in suspension. Hence a grain lagging behind during low water stages of a river because of low pivotability will be raised in suspension during high water stages and outrun the more pivotable grains which continue to move along the bottom. Weight does not have this contrasting influence. The averages of results to date for various environments are shown in Fig.3. Source materials It is assumed that the bulk of non-volcanic and non-organic sands is derived from weathered coarse crystalline rocks, mainly granites and gneisses. The graph shows that such weathered rocks produce sand of low pivotability and that the covering soils are slightly less stable. This difference is mainly the result of removal of mica and chemical attack of feldspar. 11

10

9

8

7 x

.--

+I

.-

6

n

m

- 5 0

.-> n.4

3

Z

1 0.75-1.0

0.6-0.75 0.5-0.6 0.4-0.5 0.3-0.4 0.2-0.3 0.15-0.2 Grain - s i z e

Fig.3. Relation between size and pivotability in various environments. Average of several sands.

PIVOTABILITY STUDIES OF SAND BY A SHAPE-SORTER

21 1

The average of some entirely angular beach sands, stream bed sands on granite, and one weathered granite soil containing mainly quartz, together present a smooth curve. It is assumed this is close to the condition of detrital quartz grains at birth. The marked decrease of pivotability with smaller grain-size is apparently due to the presence of more thin flakes. The original quartzes are somewhat chunky, but when splintered they tend to produce thin angular blades. The crushing of large quartz grains in nature will produce even less pivotable material, but on the other hand the decomposition of finer grained metamorphic rocks should supply grains of higher pivotability in the smaller sizes. The writer imagines that the drop to the right of the quartz line is somewhat too strong for the true average. Beaches A number of beach sands from California and Oregon are combined as “beaches

young” in Fig.3. Presumably these are composed largely of material in its first sedimentary cycle. Mica and feldspar are much reduced. Rounding is slight. The reason for greater pivotability than in the source material is probably partly rounding but mainly loss of stable flakes out to sea. As it happens only a few beaches with a significant amount of shell detritus have been tested so far (not included). In such cases the most stable fraction is large because shelly matter is flat and of low pivotability. Coastal dunes

For most of these beaches the adjoining dunes were analysed and the results are seen to be rather similar. However, there is a distinct tendency for the grains above 0.4 mm in the dunes to be more pivotable. The distance of transportation averages only a few dozen metres and cannot have resulted in any mechanical abrasion. SHEPARD and YOUNG(1961) have shown that dunes tend to contain more rounded grains of about 0.1 mm than the adjacent beaches. The explanation they offer is that the wind picks up the more pivotable (and rounder) grains. But this explanation is too simple and perhaps even wrong. Their fig.4 shows that the relation holds for grains below 0.1 mm but disappears at about 0.3 mm. This is confirmed by the present pivotability results for Californian beaches, for the smallest fraction is more pivotable in the dunes, but the fractions 0.24.4 mm are less pivotable. The Dutch beaches contain a large amountof re-worked dune material and in the average of beaches and dunes the higher pivotability of the smallest fractions is again shown. Above 0.4 mm the relation is again reversed. Although this may result from arbitrary irregularities the writer is much inclined to believe the changes with size have real significance, because a logical explanation can be offered. Dunes and sand deserts contain very few grains below 0.05 mm, owing to the fact that silt sizes are carried away in suspension. Shepard and Young’s samples (exclusive

212

P H . H. KUENEN

of Lake Geneva) have about 1/6 of silt. The smallest fraction remaining in a wind deposit should, therefore, consist of a selection of pivotable grains, as these are less easily raised in suspension. Hence, the writer suggests that the greater roundness found by Shepard and Young is not due, as they believe, to the more angular grains remaining on the beach, but to these grains being carried further inland than the round ones of the same size. This is in accordance with their finding that the silt of the dunes contains more heavy minerals and less mica. I n other words mica and light silt minerals are removed. Fig.3 shows that for sizes of 0.2-0.4 mm the dunes are slightly less pivotable. This indicates that all grains of this size are easily moved into the dunes but few are carried away in suspension. Possibly those with lower pivotability are moved more efficiently, but there is little sorting for those sizes. However, beyond 0.4 mm the dunes are again more pivotable. This can be explained in two ways. It may be that the coarser the sand the more it moves by surface creep and that pivotability favours this kind of transportation. Possibly saltation is also favoured by roundness. This would mean that the less pivotable grains are left on the beach. But it is also possible that the less pivotable grains travel faster and are therefore removed further inland. The latter view is supported by the marked drop in pivotability from dunes adjacent to the beach to those further inland, apparently due to more efficient travel of the less pivotable grains. However, a different explanation can also be offered which the author is inclined to favour, namely that the more pivotable grains roll further down lee slopes to form the lower parts of the dunes. As it is mainly the upper parts that will be eroded as a dune is pushed inland, the more stable grains will thus be selected and moved furthest inland. More data are required to solve this problem. Desert sands

Half a dozen desert sands, partly well rounded, partly quite angular, are averaged in Fig.3.The points fall on a remarkably straight line. The pivotability is much higher than for quartz at birth. This holds even for the finest fractions which show hardly any rounding. Again the most logical explanation is the removal of the more stable grains of the finer fractions. In the medium and especially in the coarser fractions, eolian abrasion presumably plays a dominant part in producing the high pivotability. River sandy

Too few samples have yet been analysed to allow any conclusions to be drawn. Samples along beaches Ten samples were collected along a stretch of beach in southern California, 50 km long. The largest grain-size available shows a low pivotability in the middle part of the area, with a distinct increase northward and a more marked increase southward. The

PIVOTABILITY STUDIES OF SAND BY A SHAPE-SORTER

South 1

213

North 2

3

L

5

Sample

6

7

E

9

‘1

p o ~ n t s

Fig.4. Pivotability of four size fractions of samples collected by Oostdam on the beach between Dana Point and La Jolla.

three smaller sizes analysed all show a similar low but for each successively finer size this trough is found further south, until for the smallest size it lies close to the southern end of the series of samples. Whether the secondary maxima and minima have true significance is doubtful, although they move along in conjunction with the main low. As the beach drifting is from the north all the way it may be that the low in the middle area is due to the river debouching there, but possibly the grains coarser than those available have a low further north yet (see Fig.4). The writer is inclined to assume that the least pivotable grains of the smallest size fraction have travelled the greatest distance south. This would mean that the principal mode of travel for the fine sand is in suspension, and for the coarsest fraction examined by rolling. Hence, the least pivotable of the small grains and the most pivotable of the large grains have travelled furthest. It is also possible that a stable condition has developed with supply in the north and equal loss in the south. This would mean that where pivotability is high the stable grains are travelling swiftly and are therefore scarce and vice versa. Finally the sorting process may have been active mainly perpendicular to the coast. Samples taken along a line at right angles to the beach would then show large differences, but the average of all lines would be about equal.

THE SIGNIFICANCE OF REPOSITORIES

In sorting processes a fundamental part is played by “repositories” and this is an aspect of sedimentation that has generally been ignored. Where there is complete

214

PH. H. KUENEN

transportatioli, for instance in a barchan or in a narrow V-shaped valley, n o sorting can result, however efficient the transporting agent may be potentially to select certain sizes or shapes. A deposit of sorted material requires that the matter which has been left out has accumulated elsewhere, or that the matter which has become relatively concentrated is more diluted elsewhere. This twin accuniulation with opposite characteristics constitutes the repository. It may be large and diluted or small and concentrated. For instance, the lack of fine material in a desert dune is only possible because the clay and silt sizes have been removed in suspension and dropped over a wide area outside the desert, a region acting as repository. It should be noted that in the rounding of pebbles on a beach no sorting process need be involved. Hence there is no repository however well “sorted” (in the statistical sense) as regards to roundness the deposit may be. All sorting requires movement and actually the transporting mechanism causes lor contributes to) two or more deposits which differ from each other. These can show an abrupt or gradual transition or even lie widely separated. Geologists are usually concerned with one of these at a time, and the other must then be looked upon as the repository. Of a pair of deposits the larger one will show the smallest divergence from the original composition. A repository can lie either up-current or down-current. below, above or beside the deposit under consideration. A beach can have a coastal dune as repository for its sand-sized material, the dune has the beach as repository for the coarser fractions and the far inland as repository for silt and dust carried further away. The significance of a repository can be demonstrated by the case of coarse grained shore dunes. If it is postulated that the wind selects the rounder of these large grains to carry inland, then the beach will soon be covered by a lag deposit of angular material, a very small repository. From that time on, if no reworking intervenes, any further transport will have to carry the angular material. The small capacity of the repository excludes any significant sorting in the dune. However, if the waves continually rework the beach and shoreface, the wind will find a fresh supply of round grains and be abIe to build the dunes by selection of this rounded material. The repository is the whole volume of the beach and shoreface. In another locality a small beach may produce a large dune area while being continuously fed by a river. In this case the dunes must have about the same composition as the river load, because the beach is too small to function as a repository. The results of the shape-sorter show that the pivotability in dunes tends to decrease inland. The seaward dunes and/or the basal parts of the dunes form the repository for the more pivotable material. In the case of laminated beds the laminae differ from each other by selection and they act as so many repositories in relation to one another. The bulk composition of a laminated deposit may or may not differ from that of the original source material. If there has been no loss the composition of the source must lie between that of the contrasting laminae. When any deposit is studied as to its sorting (in the statistical meaning), one point to

PIVOTABILITY STUDIES OF SAND BY A SHAPE-SORTER

215

be considered is whether a sorting action is involved, and if that is the case the question arises what can be deduced concerning the repository. Moss (1962) claims that various environments are characterised by certain relations between size and shape. Presumably this is due to sorting and to render the picture complete one should know where the repositories are and what their composition is. Is the shoreface the repository for the beach or do laminations in the beach fulfil this function? In the latter case a bulk sample will not show the relation Moss discovered. The former case would create obvious difficulties in sampling ancient coastal deposits, for one would have to distinguish between a shoreface or a beach.

ACKNOWLEDGEMENTS

The writer is indebted to Drs. A. M. Winkelmolen of Shell Exploration and Production Laboratory in The Netherlands for much help and advice. Dr. G. Mezzadri of Parma helped develop the instrument. F. P. Shepard and B. L. Oostdam provided samples.

SUMMARY

Pivotability of sands has been studied with a shape-sorter, each size grade between 0.15 and 1 mm being tested separately. Pivotability plays a modest part in transportation causing sorting. The influence is opposite in traction and in suspension transportation. Source materials (weathered granite) show low pivotability. It increases by abrasion, especially in the desert. Coastal dunes are more pivotable than the feeding beaches in the fine and coarse sand grades and less pivotable in medium sizes. The pivotability decreases inland. Minimum pivotability is found at different localities along the south Californian beach for different grain-sizes. In sorting processes the role of repositories is important.

REFERENCES

SHEPARD, F. P.and YOUNG, R., 1961. Distinghuishing between beach and dune sands. J . Seditlinrenl. Petrol., 31 : 196-214. Moss, A. J., 1962. The physical nature of common sandy and pebbly deposits. Am. J . Sri., 260 : 337-373.

TYPICAL FEATURES OF A FLUVIOMARINE OFFLAP SEQUENCE R. LACAAIJ

and

F. P. H .

w.

KOPSTBIN~

KaninklijkelShell Exploratie en Produktie Laboratorium, Rijswijk (The Netherlands)

INTRODUCTION

Much information has become available in recent years on the marine sediments and fauna associated with the RhBne delta, France. The first data were those assembled by KRUIT(1955) from the inshore zone. Later surveys explored a greater part of the submarine delta slope and extended into deeper water (VAN STRAATEN, 1959, 1960a, 1960b). Research at the Koninklijke/Shell Exploratie en Produktie Laboratorium (K.S.E.P.L.) has centered on the sediments and the microfauna, and on the classification of marine environments in this area.

DISTRIBUTION PATTERNS OF FAUNAL REMAINS IN THE R H ~ N EDELTA MARINE AREA

Fig.1 and 2 show how the various parts of the RhBne delta marine area differ with regard to the concentrations of faunal remains and the number of species found in them. i n one respect these maps are strikingly simiIar, i.e., in the absence or scarcity of animal remains in front of the mouth of the Grand RhBne, and to a lesser extent in front of the mouth of the Petit RhBne. Two reasons are advanced by VAN STRAATEN (1960b, pp.117, 121) to explain t h s phenomenon: ( I ) a dilution of the quantity of organic remains by inorganic sediment; (2) a direct or indirect adverse effect of the rapid sediment supply itself on the development of the bottom fauna, which makes it impossible for all but the hardiest forms to survive. Moreover, the extent to which the four groups of benthonic organisms studied react unfavourably to an increase in clay deposition differs from group to group. On a sensitivity scale, the Bryozoa would rank first, the Foraminifera last and the Ostracoda and Mollusca would tie for second place.

SEDIMENTARY SEQUENCE I N THE R H ~ N EDELTA AND ASSOCIATED MICROFAUNA

To these horizontal distribution patterns core drilling by K.S.E.P.L. enabling the study of sediment and fauna in vertical sequences, has now added the third dimension. Present address: Shell-BP Petroleum Development Compmy, Port Harcourt (Nigeria).

217

FEATURES OF A FLUVIOMARINE OFFLAP SEQUENCE

TABLE I STRATIGRAPHY OF CORE HOLE 105, R H ~ N EDELTA ~

~

~~

Environnrent of deposition

‘1 I

-

topsets

1

~

I

(7) coastal plain (undifferentiated) (6) distributary channel

1

(5) fluviorrlarine barrier face ( 4 ) proximil-fluviomsi ine

~

~

sand

-

foresets

I

I

b3ttomsets

1

1

1

-.

~~~

(3) distal-fluviom?rine (2) moderate deposition ( I ) slow deposition

alluvial valley (Pleistocene)

marine clay middle-neritic

sand and gravel

I

i

These core holes have supplied much new information on the structure and on the depositional history of the delta. Core hole 105, drilled close t o the present mouth of the Grand Rh6ne, may be cited as a typical example. Its 56.5 m long continuous core illustrates the offlap of the fluviomarine fan over marine shelf sediments. The stratigraphical information gathered from this core hole is summarised in Table I. The sequence has been subdivided into seven intervals, each representing a distinct phase in the depositional history of the delta. Fig.3 shows the position in the core hole of these seven sedimentary units, and their relative thickness. Photographs of the types of sediments which best characterise each unit are also shown in this figure. The top of the Pleistocene sands and gravels in this core hole is located at 58 m on electrical log evidence. These sands and gravels are alluvial valley deposits, formed at a time when sea level stood considerably lower than at present; they were subsequently flooded by the Late Pleistocene marine transgression. Since our cored section ends at 56.5 m, we have no record of the earliest, lagoonal or inner-neritic, deposits of this transgression, but these, if present at all, are bound to be very thin. Our basal core starts, rather abruptly, in the middle-neritic deposits of unit 1. Units 3-5 together constitute the fluviomarine offlap sequence. They show a gradual change from dominantly clayey to dominantly sandy sediments. It follows from QUIT’S (1955, p.451) reconstruction of the RhBne delta’s growth in historical times that it took no more than a few centuries to deposit this entire sequence. It is interesting to realise that objects associated with Greek or Roman shipwrecks might well have been found at approximately the 55 m level in our core hole. Quite recently a RhBne distributary channel has cut and filled the upper 13 m of the original sequence at this location (unit 6). This is, of course, accidental. In a neighbouring core hole the fluviomarine barrier-face interval (unit 5) extends virtually all the way up to the surface.

218

R. LAGAAY AND F. P. H. W. KOPSTEIN

3000 - 10,000

Nu

umber of specimens >450

Fig.1. Bottom fauna: number of specimens in the RhBne delta marine area. b. After Van de Fliert (1960b). and Ter Keurs (unpublished K.S.E.P.L.report). d. After VANSTRAATEN

219

*E

:.xx2.:.:.:. :.:............:

.:.:::::I:: ......_ :

........_ ........ ..-.

220

R. LAGAAY A N D F. P . H. W. KOPSTEIN

Fig.2. Bottom fauna: number of species in the Rhone delta marine area. b. After Van de Fliert and Ter Keurs (unpublished K.S.E.P.L.report). d. After data supplied by L. M. J. U. van Straaten.

FEATURES OF A FLUVJOMARINE OFFLAP SEQUENCE

22 1

222

R . LAGAAY AND F. P. H . W. KOPSTEIN

PHOTO LOG

! N L

numt -

TYPE OF SEDIMENT

7 COASTAL-PLAIN (undifferentiated) 6

DISTRIBUTARYCHANNEL

1

5

FLUVIOMARINE B A R R I E R - FACE

i

VEA RAP1

PROXIMALFLUVIOMARINE

41.4

1 I

TRANSITION-ZONE

RAP

DISTALFLUVIOMARINE 0

-L

_____---

-

M100LE-NERlTIC.MD0ERATE DEPOS

I----~

1

MIDDLE-NERITIC, SLOW DEPOSITION

SLO

-M

A L L U V l AL-VALLEY (PLEISTOCENE 1

Rh6ne delta, c o r e hole 10

Fig.3. Microfaunal development in a fluviomarine offlap sequence.

FEATURES OF A FLUVIOMARINE OFFLAP SEQUENCE

223

The description of a very similar fluviomarine offlap sequence in the Mississippi delta has recently been published by SCRUTON (1960, p.89). From each 70 cm length of core in core hole 105, a 100 g sample (dry weight) was taken for microfaunal analysis. All Foraminifera, Ostracoda' and Bryozoa > 150 p in size were removed from the washed residues or from a known fraction thereof obtained by means of the Otto microsplit. We thus obtained a very detailed record of the total numbers of specimens and species per 100 g and of the percentage composition of the various assemblages from the basal beds (units 1-2) upwards throughout the fluviomarine offlap sequence (units 3-5). Four trends are apparent in the development of the microfauna (Fig.3): (a) The successive disappearance of entire groups of organisms in the regressive (shoaling) phase. Bryozoa and Foraminifera > 450 p disappear first (in unit 3), followed by the Ostracoda (virtually absent in unit 4). Only Foraminifera < 450 p persist as a group throughout the entire sequence. (6) A general decrease in the number of species in each group towards the top of its range. (c) A tremendous decrease in the number of specimens of the various groups per 100 g sample. Notice that the scale of the corresponding graph in Fig.3 is logarithmic. ( d ) A distinct shift in the percentage composition within the various faunal groups. Unfortunately, this cannot be further illustrated in the scope of this article. Let it suffice to state that, as far as the most persistent group, the Foraminifera, is concerned, units 1 and 2 are marked by a great diversity of species, that in unit 3 Va'alvulineria fubinnii (KICINSKI) becomes numerically the most important single species, and that throughout units 4 and 5 the assemblages are dominated by Streblus beccarii (LINNAEUS) . These trends are due to the steadily increasing rate of deposition; the progressive shoaling as the delta builds up its foreset cone; the variations in salinity of the shallow inshore waters near the river mouth, caused by fluctuations in the river's discharge; etc. The most important of these influences is the rate of deposition. VAN STRAATEN (1960b, p. 109) has estimated that, in the present-day proximal-fluviomarine environment off the mouth of the Grand Rh6ne, the rate of deposition amounts to approximately 40 cmlyzar (i.e., 40 mlcentury) at a depth of 50 m and to even more in shallower water. A similar order of magnitude must be assumed for the proximal-fluviomarine interval (unit 4) in core hole 105. It is not surprising that under such extreme conditions only the most hardy species of the most hardy group manage to survive, nor that their actualxumbers in 100 g of sediment are severely reduced. Consequently such fluviomarine clays may not always be easily recognisable as marine sediments when they occur in ancient rocks. This difficulty may be aggravated by post-depositional solution of calcareous microfaunal remains in those basins which - unlike the Rh8ne delta -do not benefit from the buffering action of a generous fluviatile supply of clastic carbonate (see KRUIT,1955, p.406). The Ostracoda were identified by D. Ter Keurs of K.S.E.P.L.

224 R. LAGAAY AND F. P. H. W. KOPSTElN

FEATURES OF A FLUVIOMARINE OFFLAP SEQUENCE

225

The scarcity of microfauna in the rapidly deposited upper half or two thirds of the sequence contrasts markedly with the great abundance of microfauna in the thin basal “Bryozoa Bed”, which took several thousand years to accumulate. This kind of development, from an abundant microfauna at the bottom to a scarce microfauna at the top, seems to be characteristic of certain transgressive-regressive marine cycles and may thus help us to recognise them in older strata.

THE ONLAP COMPLEX A N D THE OFFLAP DELTA

A final point of interest is to consider core hole 105 in wider context. Fig.4 shows it in key position at the southern end of a section, based on a series of seven core holes, which extends roughly north-south across the Rh6ne delta. This figure illustrates the fact that the marine transgression caused by the late Pleistocene - Middle Holocene rise in sea level is manifested at successively shallower depth as the location of the holes moves inland. It can be seen that the point farthest inland reached by this transgression must lie somewhere between core holes 108 and 110, i.e., some 15 km beyond the present coast line. The lines, connecting the various core holes in Fig.4 represent an attempt to draw a time-correlation through this generalised north-south section. In the absence of 14C measurements, this correlation must necessarily remain somewhat tentative. Our approach to the problem of age determination has been based on the fact that present-day coastal-plain lagoonal deposits are all formed approximately at sea level. This must also have been true of our ancient lagoonal sediments, irrespective of the depth at which they are now found in the core holes. Their approximate age can therefore easily be deduced from diagrams showing the post-Glacial rise in sea level in stable areas, such as have been published in recent years by various authors. The extent to which subsidence and compaction may have occurred in the Rhihe delta area during this period has been neglected; it may safely be assumed that their influence must have been minor in comparison with the influence of sea level rise (cf. K R U I T , 1955, p.447). Since such processes will undoubtedly have taken place, however, it n u s t be admitted that there is an element of uncertainty in the ages assigned to the various deposits. For the time being, this is unavoidable. We have submitted a number of samples from various levels in the holes to Prof. Dr. H. de Waard in Groningen for 14Cmeasurements, to serve as a check on the age correlations tentatively given here. The correlations.presented in Fig.4 show convincingly that the Rh6ne delta, in the broadest sense of this term, can be broken down into two parts, each having an entirely different depositional history: the Late Pleistocene-Middle Holocene onlap complex, and the Late Holocene-Recent offlap delta. Core holes and suggested correlations in Fig.4 present a picture from which an interesting conclusion may be drawn. This is that during the Late Pleistocene-Middle Holocene rise in sea level the delta was built up almost entirely of thick coastal-plain lagoonal deposits, contemporaneous marine deposits constituting no more than a

226

R. LAGAAY A N D F. P. H. W. KOPSTEIN

thin veneer. When, after Middle Holocene times, the sea had reached a stable level that at which it stands today - a delta began to develop that was built up entirely of marine sediments. This association of thick coastal-plain deltaic deposits with rising sea level (or, for that matter, with basinal subsidence), and of thick fluviomarine deltaic deposits with stable sea level cannot be termed a new discovery: it was postulated long ago by BARRELL(1912). Nevertheless this confirmation of Barrell’s theoretical views by the specific case of the Rhdne delta is encouraging. particularly since it is supported by ample evidence.

SUMMARY

A 56.5 m deep core hole, drilled close to the mouth of the main Rhdne distributary, illustrates the typical sequence of sediments and microfaunas resulting from offlap of a deltaic fluviomarine fan over marine s!ielf sediments. The larger part of the sequence was deposited in historical times. The spectacular changes in numbers and in composition of the microfauna are credited primarily to the increase in the rate of deposition as the delta advances. The scarcity of microfauna in the rapidly deposited upper half or two thirds of the sequence contrasts markedly with the great abundance of microfauna in the thin basal “Bryozoa Bed”, which took several thousand years to accumulate. This kind of development, froin an abundant microfauna a t the bottom to a scarce microfauna a t the top. seems to be characteristic of certain traiisgressive-regressive marine cycles. This core hole is situated a t the southern end of a section, based on a series of seven core holes, which extends roughly ncrth-south across the Rhdne delta. It appears from this section that the Rhdne delta sensu lato can be broken down into two parts: the Late Pleistocene-Middle Holocene onlap complex. and the Late Holocene-Recent offlap delta.

REFERENCES

BARRELL, J., 1912. Criteri? for the recognition of ancient delta deposits. .&I//. Geol. SOC.Am., 23 : 377446. KRUIT,C., 1955. Sediments of the RhBne delta. Grainsize and microfauna. Verliatidd. Ned. Gcd. Mijithotrwk. Getroot., 15 : 357-514. SCRUTON, P. C., 1960. Delta building and the deltaic sequence. In: F. P. SHEPARD, F. B. PHLEGER and TJ. H. V A N ANDEL(Editors), Recent Serlinietits, Nor/hwe.rt Gulf of Mexico. Am. Assoc. Petrol. Gedogists, Tulss, Okla., pp. 82-102. VANSTRAATEN, L. M. J. U., 1959. Littoral and submarine nnrphology of the RhBne delta. In: R. J. RUSSELL (Editor), Pror. Coastal Gcograpli. Cotif: Louisiom State Univ., 2nd. Natl. Acad. Sci., Natl. Res. Council, Baton Rouge, La.. pp. 233-264. VAN STRAATEN, L. M. J. U., 19601. Some recent advances in the study of deltaic sedimentation. Liverpnol Mntirfiesfer GPO/.J . , 2 : 41 1-442. VAN STRAATEN, L. M. J. U., 1960b. Marine mollusc shell assemblages of the RhBne delta. G c d . Miitih., 22 : 105-129.

RHAETIC-JURASSIC-LOWER CRETACEOUS SEDIMENTS FROM DEEP WELLS IN NORTH JYLLAND. DENMARK G U N N A R LARSEN

Geological Survey of Deninurk, Hellerup (Denmark )

INTRODUCTION

The knowledge of the Rhaetic-Jurassic-Lower Cretaceous formations of Denmark (except for the island of Bornholni) is of rather recent date; up to about 25 years ago the Senonian White Chalk was the oldest known bed in this part of the country. The extension of knowledge of older formations since that time is due to explorations by the Danish American Prospecting Co., which went on until the year 1959 when the company gave up the concession. The exploration has proved that the Upper Cretaceous overlies widely extended and thick beds of Lower Cretaceous, Jurassic, Triassic and Permian age. The sequence is not uniformly developed all over the country. The most complete sequence is found in the Danish Emhayment (Fig. 1). a trough which towards the northeast borders on the Fennoscandian Border Zone. Towards the southwest the Ringkrabing-Fyn High partly separates the trough from the North German Basin. The geological and geophysical material is now in the possession of the Geological Survey of Denmark. In 1962 this institution cominenced a n extensive examination of the material, led by professor Th. Sorgenfrei. The following presents the first results of the sedimentological part of this examination. DEEP WELLS 1N NORTH JYLLAND

In North Jylland a great number of boreholes have been completed. A few have penetrated deeply into or through Jurassic and Rhaetic beds. Of these deeper holes the following four will,be treated in some detail: Skagen no.2, Frederikshavn City no. I , Berrglum no. 1 and Haldager no. 1 (Fig. I). Part of the data of some of the holes have previously been published (GREGERSEN and SORGENFREI, 1951; N 0 R V A N G . 1957).

LITHOLOGICAL PROPERTIES

The lithological properties of the sequences have been classified from studies of Samples and Schlumberger logs.

228

G. LARSEN

Fig. 1.

Limitation of the sequence

In North Jylland the Rhaetic, Jurassic, and part of the Cretaceous consist of a series of light coloured sand and dark clay beds which lithologically can be clearly distinguished from the older and younger formations. The underlying formation is preRhaetic Keuper developed as reddish claystones and arkoses. The overlying Upper Cretaceous sediments consist of whitish to grayish limestones and marls. The lithological boundary: highly calcareous deposits over clay and sand sediments, is not identical with the chronostratigraphical boundary between Upper and Lower Cretaceous. According to provisional determinations it seems to correspond approximately to the transition Turonian-Cenomanian. Hence the Cenomanian has been treated together with the Lower Cretaceous during the sedimentological studies. Subdivision of the sequence

The clay of this Rhaetic to approximately Cenomanian series is usually pyritic and

229

MESOZOIC SEDIMENTS FROM DEEP WELLS I N DENMARK

micaceous; it is often silty, and transitions to argillaceous silt occur frequently. In certain horizons in the Cretaceous and Upper Jurassic the sediments have a greenish tinge due to the content of glauconite. Marine fossils, e.g., Mollusca and Foraminifera, occur especially in argillaceous facies. Small amounts of lignite have been noted in some of the sand beds. The distribution of sand and clay facies is so characteristic that by comparing the four well-sections it is natural to subdivide the sequence in three cycles of sedimentation, each containing a lower, mostly sandy and an upper, mostly argillaceous member. This subdivision is shown in Table I. It is to be noted that the dating of the members is provisional. The cycles are not uniformly developed in all the wells; e.g., there are variations in thickness. This is shown in the schematic northeast-southwest cross section through the wells, Fig.2. In the Border Zone the total thickness is considerably less than in the Embayment. However, the sand phase of the third cycle has its greatest thickness in the Border Zone; at Skagen the argillaceous member of this cycle seems to be missing entirely. As mentioned in the table, the argillaceous phase of cycle 2 is rather sandy in the upper part. This is especially so in the wells at Skagen and Frederikshavn where a rather gradual transition to cycle 3 is actually seen. In these wells the boundary between cycles 2 and 3 has been placed where glauconitic fine grained sand is overlain by light coloured non-glauconitic sand.

sw

NE HALDAGER 1

k-L

"

EERGLUM 1

DANISH

EMBAVHENT

FREDERIKSHAVN CITY 1

G.01 Surv d Oenm 1962

SKAGEN 2

'BE

Gunnor Loram

230

G. LARSEN

TABLE I SUBDIVISION OF THE WELL-SECTION I N CYCLES ~~~

Cycle riirniher

~

~~~~~

Grrirrnl litholq:y

Appr0.r. age

clay, dark, in part glauconitic; with silty and sandy beds; fossils 3

~~~

~~

~~

~

~~~~~~

Cenoiiianian and

~

sand, light; siiiall occurrences of lignite

Lower Cretaceous

clay, dark, in part glauconitic; with silty and sandy beds especially in the top part; fossils

Malm

~

-~

~~~

-

sand, light, with clay beds and thin lignite seams -.

~

~~

~

~~~~~

~~~

Dogger ~

~

~~

~

clay, dark, silty, with fossils

Lias

sand, light, with few clay beds and some lignite

Rhaetic

Within each of the larger sedimentary units sandy and argillaceous beds are interchanging; thus every cycle may be divided into a number of sub-cycles.

PETROGRAPHIC FEATURES

In Frederikshavn City no. 1 about 75 % of the sequence here treated has been cored. A total of 300 samples was selected from the cores for analysis: i.e., chemical determination of carbonate content and petrographic exmanination of the grain-size fraction 75-250 p. The Frederikshavn City no.1 log is shown in Fig.3, which present an extract of about 1/5 of all examined samples. The difference between the later Cretaceous deposits and the sediments dealt with here appears clearly in the carbonate curve. Besides, this curve shows that within the Rhaetic to Cretaceous sequence an alternation occurs between calcareous and non-calcareous units. Furthermore it is seen that as a rule calcareous deposits are also glauconitic, but the maximum value of glauconite coincides only in a few cases with the carbonate maximum. In addition, it appears that the highest concentration of glauconite is found in the Lower Cretaceous-Cenomanian and the lowest concentration in the Rhaetic-Lias cycle. Intervals without glauconite often contain plant remains. The curve showing the quartz-feldspar ratio correlates rather well with the division in cycles. In the upper two cycles the coarse sandy members have very high values of the quartz-feldspar ratio, while the argillaceous members have much lower values. There is, however, a pronounced difference between the second and the third cycle in that the argillaceous phase of the latter is on the whole more feldspatic than the corresponding phase in cycle 2. The lowermost cycle differs from the two others by the rather high content of feldspar in the sandy beds at the base.

23 1

MESOZOLC SEDIMENTS F R O M DEEP WELLS IN D E N M A R K

~~

FREDERIKSHAVN CITY 1 Lithology

1 Corbonotecontent

~

C ,-

RHAETIC-JURASSIC

0

c

LOWER CRETACEOUS SEDIMENTS

1

Mineral

in t o t a l sample

I

-

content

in

fractlon

75-250~.

j

U

*

U Q

' ' 1 - --

500 +

I

light san d

b 000-

dork

light sand

WO --PRE-RHAETIC I KEUPER

jEOLOGlCAL SURVEY OF D E N M A R K 1962

GUNNAR LARSEN I

Fig.3.

232

G. LARSEN

All through the sequence examined, the same types of heavy minerals are found. However, with respect to the frequency of the different minerals rather large variations occur. From Fig.3 it is seen, that the correlation between the frequency distribution of the heavy minerals and the lithological cycle subdivision is rather good. The most outstanding features of this correlation may be summarized as follows: The association of the first cycle is rich in garnet; in cycle 2 stable minerals are quite dominating; and in cycle 3 epidote is the characteristic mineral, except in sections with low content of feldspar, where stable minerals again predominate. To this may be added that a few analyses of pre-Rhaetic formations seem to show that the clastic mineral association of the pre-Rhaetic Keuper is of almost the same composition as that of the Rhaetic-Lias cycle. Remarks on some of the minerals

Up to about half of the glauconite grains in the upper glauconitic horizon contain remains of organisms or prove to be casts of organisms, among which radiolarians (Fig.4) are rather abundantly represented. Similar organic structures have not been noted in the older glauconitic horizons. In coarse sands some of the quartz grains consist of orthoquartzite fragments, i.e., well rounded quartz grains surrounded by secondary overgrowths of quartz; the outer contours of the grains are often subrounded.

Fig.4. Grains of glauconite, one with inclusion of a radiolarian.

MESOZOIC SEDIMENTS FROM DEEP WELLS IN DENMARK

233

Fig.5. Grain of tourmaline with secondary overgrowth.

Similar secondary overgrowths are found in some of the tourmaline grains (Fig.5). Evidently the well rounded core represents a clastic sand grain. The mantle is probably authigenically formed in a sedimentary stage older than the present sediments. The outer contours of the grains generally show mechanical wear. Such “two-generation” tourmaline has been found throughout the sequence.

INTERPRETATION

On account of the above mentioned sedimentary properties some preliminary interpretations of the geological conditions during deposition of the sequence will be given. Environment of deposition

During Rhaetic to earlier Cretaceous times marine and non-marine conditions have evidently alternated in the present part of the field of accumulation. Environments of marine type, as indicated by the content of glauconite and marine fossils, have mainly existed during the deposition of the argillaceous formations; but also some of the sandy beds may be counted as marine. However, the main part of the sands seems to be predominantly non-marine according to the presence of lignite and the lack of the above mentioned marine evidences. As the non-marine formations have great thick-

234

C . LARSEN

ness and wide extension it seems obvious to classify them as deltaic formations. Cnnsequeiitly each of the three cycles apparently comprises a lion-marine deltaic stage succeeded by a mainly marine stage. Of the three transgressions, in the Lias, the Malm and the Lower Cretaceous respectively, the first has probably been the most extensive, as marine sediments from this period are found all over the area, mainly as argillaceous deposits. In the Malm the sea likewise has covered the whole area, but the considerable content of fine grained sand and silt, especially in the Fennoscandian Border Zone seeins to indicate a closer position of the coast during this period than during the Lias. The Lower Cretaceous transgression has been the least extensive in this area; marine deposits have not been found at Skagen. Furthermore the distribution of carbonate, glauconite and plant remains in the Frederikshavn profile reflects oscillations in the extension or’ the sea during the Lower Cretaceous. The appearance of radiolarians in the top of the sequence possibly marks the beginning of the more highly marine conditions which characterized the area during the following Upper Cretaceous time.

Origin oj‘the clastic inaterial Evidently the clastic minerals originate from older sediments and from metamorphic complexes. The orthoquartzite fragments and the “two-generation” tourmaline are referred to the first group, but a considerable part of the other stable heavy minerals and quartz grains may probably be referred to this group as well. Epidote and a good deal of the feldspar grains must be regarded as representatives of metamorphic rocks. Representatives of the two types of source material are found in all parts of the sequence, but the mutual frequency varies from cycle to cycle. The fact that the mineral association of Rhaetic-Lias fairly well corresponds to that of the pre-RhaeticKeuper may lead to the conclusion that they have had almost the same type of source material, or possibly that the Rhaetic-Lias material in part consists of re-deposited Keuper. The Dogger-Malm cycle has apparently had another type of initial material. Probably this consisted of older “mature” sediments. In the Lower Cretaceous, metamorphic complexes have constituted the principal part of the source area, which was probably situated in Fennoscandia. The fact that the transition from one mineral association to another does not occur gradually but rather abruptly near the boundary between the cycles, may indicate that “sudden” changes of the denudation area have taken place at these times; such changes have probably been caused by tectonic movements. This again may lead to the idea that the cyclic development of the whole discussed sequence in North Jylland has a background of tectonic nature.

SUMMARY

This paper presents some results of lithological investigations of the Rhaetic-Jurassic-

MESOZOIC SEDIMENTS FROM DEEP WELLS IN DENMARK

235

Lower Cretaceous sequence in four deep holes in North Jylland, together with petrographic data from one of the well sections (Frederikshavn City no. l ) . According to these investigations the sequence may naturally be subdivided into three sedimmtary cycles, each comprising a lower more sandy and an upper more argillaceous nieniber. Apparently the sandy members are mainly non-marine, probably of deltaic origin, while the argillaceous phases seem to represent mainly marine environments. The clastic material is not of the same type in the three cycles. The lower cycle (RhaeticLias) is rich in garnet; in cycle 2 (Dogger-Malm) stable minerals are dominating and in cycle 3 (Lower Cretaceous and Cenomanian) epidote is the most characteristic mineral. Because of these differences in clastic niaterial it is assumed that the cyclical development of the sequence is caused by tectonic movements.

REFERENCES

GRECERSEN, A. and SORGENFREI, TH.,1951. Efterforskningsarbejdet i Danmarks dybere undcrgrund. Medd. Dritisk Ceol. Foren., 12 : 141-151. NBRVANG, A,, 1957. The Foraminifera of the Lias Series in Jutland, Denmark. Medd. Dairsk Gcol. Foreti.. 13 : 275414.

REFLEXIONS SUR LA SYST~MATIQUEET LA GENGSE DES BASSINS D E SEDIMENTATION N. LLOPIS LLADO

Instiiuto “Lucas Mallada”, Madrid (Espagne)

BASSINS ET

PALCOBASSINS

Nous faisons la distinction entre bassins marins actuels (octans et mers) et “paltobassins”; l’existence de ces derniers a eti dtduite des stries stratigraphiques. Des bassins, nous connaissons l’ttendue, la morphologie, le volume d’eau ainsi que les caracttristiques chimiques, physiques et dyiiamiques de cette eau; on commence i connaitre les caracteristiques et la rtpartition des sediments. En revanche, nous connaissons trbs bien les dtp6ts des paltobassins mais aucun de leurs autres caractbres. Nous considirons comrne bassin actuel une gtodtpression quelconque occupte par la rner; en revanche, les paliobassins doivent Etre caracttrists comrne suit: (I) 11s occupent une zone effondrte de la voCite sialique. (2) 11s sont soumis i une subsidence. (3) 11s sont un niveau debase gentral. ( 4 ) L’accumulation d’epaisseurs importantes de stdiments. La prtsence de cette tttralogie gtophysique, tectonique, morphologique et stratigraphique est, & notre avis, le minimum indispensable pour caracttriser un “paliobassin”.

LA SYST~MATIQUE

Les connaissances des paltobassiiis ont conduit i chercher une systimatique; DANA ( I 873), SCHUCHERT ( I 923), HAUG(1 900), STILLE (1 926), TERCIER ( I 940), KAY (1944, 1945, 1947), KRUMBEIN et SLOSS(1951), UMBGROVE ( I 950), PEYVE et SINITZYN (1950), LOMBARD (1956) et AUBOUIN(1959) ont tentt de classer les paltobassins. Comme rtsultat, la notion de gtosynclinal, synonyme de bassin sedimentaire avant Schuchert, a Ctt diviste en plusieurs tltments, dont la systimatique de STILLE (1 926) semble la plus gtntraliste, puisque dans l’ensemble, toutes les classifications convergent vers celle-18. La division en orthogtosynclinaux et paragiosynclinaux est trts nette, et aussi les miogtosynclinaux et les eugto-synclinaux cornme tltments des premiers; AuBor.lIN (1959) l’accepte et va encore plus loin en admettant un “couple” mio-eugtosynclinal cornme gtneral dans tous les paltobassins.

SYST~MATIQUEET

GENBSE

DES BASSINS DE S~DIMENTATION

237

Ntanmoins, il ne faut pas ntgliger du tout les idtes de Schuchert et sa systtmatique morphologique, puisque d’abord elk n’est pas incompatible avec celle de Stille et d’autre part il tient compte de la paltobathymttrie, essentielle pour la comprihension de la paltozonographie des stdiments (LLOPISLLADO,1953). Mais quand on fait une ttude comparative des difftrentes systtmatiques, on voit qu’elles ont t t t faites en prenant comme base des caractkres difftrents (Schuchert, la morphologie; Stille, la gtodynamique; Umbgrove, la structure; Krumbein, la stdimentation, etc.); les bassins qu’on a pris pour construire la systtmatique ont aussi tti difftrents, tloignts les uns des autres dans l’espace et le temps. En revanche, tous les auteurs acceptent implicitement la permanence d’un mEme type de palgohassin, sans songer a la possibilitt d’un changement de type pendant son tvolution. Seul KAY(1945) avec ces tpieugtosynclinaux admet implicitement leurs variations. Pourtant le changement de type est frequent dans les paragtosynclinaux et dans les bassins paraliques et tpicontinentaux en gtntral. C’est pourquoi les classifications de Tercier et de Krumbein, bastes sur les lithotopes, sont peut-&treplus prudentes. D’autre part on remarque, surtout en Europe, que la plupart des observations sur la systtmatique ont t t t faites dans les paltobassins alpins, au mtso-tertiaire en gentral, qu’on a pris comme modkles. Mais il faut se demander si la stdimentogenbe alpidique est un modkle de stdimentogenkse, puisque, malgrt le principe de I’uniformitarianisme, il n’est pas dtmontrt que l’tvolution stdimentaire alpidique soit absolument comparable a l’hercynienne ou celle-ci k la caledonienne ou aux archtennes. Peut-&tre faudrait-il envisager de parler non seulement de paltobassins, mais aussi d’archtobassins ou de bassins primitifs, surtout si I’on songe a l’extension, ri la tectonique, au metimorphisme . . . Le tableau comparatif que nous ajoutons, nous parait encore incomplet. Peut-Etre faudrait-il songer a d’autres caractkres systtmatiques tels que: (I) le dtveloppement longitudinal et transversal du bassin (2) la genkse (3) la cintmatique, en considtrant le bassin conime une unid gtophysique et sedimentologique (4) l’ige (5) le concept d’unitt spatiale et gtographique. Nous ne pourrons dtvelopper ici que les deux premiers points.

LE D~VELOPPEMENTLONGITUDINAL ET TRANSVERSAL

La stratigraphie comparee nous apprend que les caracttristiques bathymttriques varient le long d’un mEme bassin. Les variations se manifestent surtout dans les lithofacihs, ainsi que d a m le style tectonique rtsultant du plissement du bassin (LLOPIS LLADO,1948, 1953). Plusieurs colonnes stratigraphiques, prises le long du bassin sudpyrtnten par exemple, considtrtes indtpendamment, nous donneront chacune une idte trks difftrente sur le tvr de bassin. Nous pouvons conclure de mEme pour la

-

N

TABLEAU I

w

00

CLASSIFICATIONDES P A L ~ ~ A S S I N S

SCHUCHERT

HALL-DANA

(1910)

(1873)

HAUG

STILLE

(1918)

( 1926)

TERCIER ( I 940)

Monogeosynclinal

Ort hogeosynclinal

Polygeosynclinal

Eugeosynclinal Miogeosynclinal

Sedimentation oceanique

Geosynclinaux

Geosynclinal

Sedinientation geosynclinale Mesogiosynclinal ~~ ~

~

~~

Nomefrclorirre

Aires d'ennoyage

~-

Parageosyncli nal

Parageosynclinal

Sedimentation paralique Sedimentation epicontinentale

Fosses __

.

-

~~

Subsidence CarnciPres systhofipres

cpaisseur des sedinlents

~

-

~~

Paleobathymetrie et niorphologie du fond

___________

~

Position gtographique

Cpaisseur des sediments Magmatisme Metamorphisme

Subsidence Metarnorphisme

Styles tectoniques resultant du plissement

______

Milieux sedimentaires

zr

s57

m

r r

TABLEAU I (continue)

PALCOBAWNS

CLASSIFICATION DES

PEYVE

KRUMBEIN

KAY (1947. 1951)

UMBGROVE

(1948; 1950)

ET SINITZYN

( 1950) ~

~~

(1950) ~~

Fosses marginaux

~

-

__

-

Geosynclinaux primaires Geosynclinaux

GCosynclinal

- -

-

Bassin intermontagneux

-

Zeugeosynclinal Paraliagkosynclinal Idiogeosynclinal Taphrogeosynclinal Epieugeosynclinal Au togeosynclinal

-

-

Arriere-fosst intra-fosse avant-fosst

Geosynclinaux secondaires S0cl.ls mobiles

Bassins marginaux non orogtniques Sillons infragranitiques

Geosynclinaux residuels Plateformes stables et oscillantes cratogenes

~

Sccles stables Passins intracratogenes voir CaractPres (1926) sysfir7:atiyrws STILLE

Geosynclinaux (sensu stricto) forrnes de: Sillons miogeosynclinaux Sillons eugeosynclinaux

~

Exogeosy nclinal Nomenclatrtre

( 1959)

(1956)

Eugeosynclinal MiogCosynclinal

AUBOUIN

LOMBARD

Sediments (lithotopes et tectotopes) Tectofacies

Bassin s

Fosses

Bassins nucleaires

Rapports entre bassin et structure du sock

Evolution des bassins Position geographique N

w

\D

240

N. LLOPIS LLADO

plupart des paltobassins, ce qui nous amtne ;i nous demander si les classifications actuelles peuvent s’appliquer a la totalitt d’un paleobassin ou simplement a une simple coupe transversale. Reprmant I’exemple pyrtnten, on voit que pendant la stdimentogentse mtsozolque, la bordure occidentale dans les Asturies n’a gutre t t t qu’une plateforme continentale, alors qu’il existait dans le pays basque une zone i subsidence differentielle oh se sont dtposts plus de 12.000 m de stdiments. Sur les 400 km des Pyrenees isthmiques nous en trouvons encore d’autres exempks. On constate les mCmes faits en comparant des colonnes stratigraphiques prises en sens transversal, ce qui nous apprend que les caractcres d’un paltobassin varient en long et en travers et nous trouvons trts couramment qu’un mCme paltobassin peut appartenir ;iplusieurs types acceptts dans les difftrentes classifications. D’aprb ses caracttristiques morphologiques un paltobassin a t t t le sikge de domaines stdimentologiques t r h difftrents dans l’espace et le temps, et a notre avis, seules ]’extension et la persistance d’un domaine dttermint permettront d’arriver i une dtcision sur l’ttablissement de bases systimatiques.

LA

GENBSE

La gentse d’un bassin doit Ctre considtrte comme une constquence de I’orogtnbse, car l’tpirophorese engendre des forces convergentes o u divergentes qui, dans les deux cas, conduisent k la formation de gtodepressions, soit par compression, soit par distension. A partir du moment oh la gtodtpression existe, la stdimentogenbe peut commencer ou non; dans ce dernier cas nous aurons la phase de vacuitt soutenue par AUBOUIN (1959). Mais ces considCrations ont pour base les idtes classiques sur la formation des bassins. Ntanmoins, il faut rtfltchir sur la possibilitt de distinguer entre thalassogentse et angueiogenhe. Nos mers actuelles ne peuvent pas se rapprocher entitrement des images des paltobassins que nous reconstruisons a l’aide de cartes d’isopaques, de lithofacih et de tectofaciks. I1 faut signaler que le paltobassin pendant la stdimentogentse a t t t dtformt par la subsidence et, dans bien des cas, aprts la stdimentogentse, par l’orogentse. Notre reconstruction ne pourra reproduire que trts lointainement la vraie image du paleobassin lorsqu’il ttait vivant. D’autre part, dans nos octans actuels, la stdimentation se concentre surtout sur les bords alors que d’tnormes ttendues de fonds de mer sont sans dtpcts, ou ;ipeu prts. I1 a dO en Ctre ainsi dans le passt gtologique et, par constquent, notre reconstruction ne pourrait pas s’identifier A un paltooctan mais simplement a la partie occupte par des stdiments. C’est ici que rtside la principale difficult6 quant a mettre en rapport nos octans actuels avec les paltobassins. Peut-Ctre faut-il songer la possibilitt qu’ils ne puissent pas s’identifier totalement. C’est peut-Ctre dans les octans que s’engendrent les bassins; dans ce cas un paltobassin ne serait qu’une partie d’un palto-octan. Les recherches actuelles des octanographes et des pttroliers, rtalistes pslr des mtthodes difftrentes, paraissent conduire 2 la conclusion que la sedimentation se borne aux bords des continents et que des ttendues trts larges des fonds de mer restent sans sediments

SYST~MATIQUEET G E N ~ E DES BASSINS DE S~DIMENTATION

24 1

ou avec des couches trks minces a facic?s sptciaux. Daprks ces travaux la stdimentation peut se diviser en trois grands ensembles: (I) Stdimentation sur la plateforme continentale, a strates a section triangulaire avec un front abrupt vers la mer. (2) Stdimentation da m les fosses marginales, i stdiments a section lenticulaire classique. (3) Stdimentation dans les fossts engendrts par dtformation des plateformes continentales, 8 stdimentation lenticulaire classique. Dans les paltobassins nous trouvons trois types de stdimentation qui, sans grands efforts, peuvent &tre paralltlists avec les trois types actuels: La stdimentation de platefomie est la moins rtpandue mais le paralltlisme palto-actuel n’est pas douteux. En revanche, il est fort probable qu’on peut comparer la stdimentation dans les fosses marginales a celle des gtosynclinaux; dam ce cas les miogtosynclinaux pourraient s’tgaler aux cuvettes dtvelopptes sur les plateformes. Une telle cuvette pourrait

IIO

Fig.1. Thalassogenbe. I. Bloc crustal non dtfonnt. IIa. Genese d‘une gbdtprzssion par compression. IIb. Gentse d’une gtodepression par distension. Angueiogenbe. I. Genese d’une plateforme continentale et ebauche d’un bassin marginal. 11. G e n k d’un bassin marginal. IIIa. Genese d’un ensemble eu-miogeosynclinal monomarginal. IIIb. Genkse d’un eugiosynclinal bimarginal.

242

N. LLOPIS LLADO

s’engendrer aux dtpens d’une plateforme continentale, par subsidence d’une partie de cette plateforme, et, a son tour, la plateforme continentale peut rtsulter de l’immersion d‘une ptntplaine. Peut-&treces idtes peuvent-elles expliquer: (I) le dtplacement des bassins stdimentaires vers l’exttrieur des orogtnes; (2) l’abondance des socles ptntplanists. L’avant-pays d’un orogtne est toujours une ancienne aire-source de stdiments pentplaniste; I’tpaisseur de la crofite sialique est trts faible et peut &tre facilement dtformte. L‘orogenbe exige une translation de l’octan vers la ptriphtrie de l’orogkne et par constquent une transgression sur l’avant-pays ptntplanist et cratonist. D’aprts ces idtes, l’angueiogenbse succtderait a la thalassogentse et a partir d’une orogenbse on pourrait envisager les phases suivantes: (I) Transgression cratoptte et orofuge synorogtnique. (2) Mer ptricontinentale a socle ptntplanist. (3) Dtformations Cpirogtniques du socle et engendrement de bassins. Cette tvolution pourrait arriver a former un bassin d’un seul bord continental ou de deux bords quand la dtformation est si intense qu’elle permet I’immersion d’une partie du socle Cloignt du continent. Dans le premier cas nous aurons des bassins rnonomarginaux”, dans le deuxitme, des “bimarginaux”. Les bassins mtditerrantens seraient un cas sptcial parmi ces derniers, rtsultant du dtveloppement de gtodtpressions entre des orogtnes trts proches. Les bassins rntditerrantens, seraientils peut-&tredes gtosynclinaux par excellence ? Cette conception du bassin dtrivt d’une ptntplaine immergte s’accorde trts bien avec les observations stratigraphiques et paltogtographiques: (I) Abondance de socles ptntplanists. (2) Ripartition des stdiments: (a) lithofaciks terrigknes, et ntritiques a rythmes, et cycles de stquences puissants dans la plateforme ou dans le miogtosynclinal. Puissances totales modtrtes. (6) Lithofaci&s thalassogtnes et rythmes acycliques trts minces dans l’eugtosynclinal. Puissances totales trts grandes. (3) Lacunes et diasthtmes nombreux dans le miogtosynclinal, series coniprthensives dans l’eugtosynclinal. (4) Stdimentation “tchelonnte” a strates a section en coin, amincis vers le bord dans la plateforme, strates lenticulaires sections classiques dans les eugtosynclinaux. Des courants de turbiditt et des tboulements sous-marins mettent en rapport les deux domaines stdimentaires. “

CONCLUSIONS

I1 faut envisager la possibilitt d’introduire d’autres Cltments systtmatiques dans la classification des paltobassins tels que le dtveloppement longitudinal, la genbe, la cintmatique, l’ige et le concept d’unitt spatiale et gtographique. I1 faut songer A la possibilitt de distinguer entre bassins et mers dans la nature actuelle et dans le passt gtologique.

SYST~MATIQUEET

CENBSE

DES BASSINS DE S~DIMENTATION

243

Les paltobassins peuvent avoir dtrivt de la deformation de plateformes continentales dtvelopptes sur d’anciennes ptntplaines submergees.

On fait la distinction entre bassins - actuels - et paltobassins - dam le passl gtologique. Dans la systimatique de ces derniers on introduit des nouveaux tltments: ( I ) le diveloppement longitudinal; (2) la gentse; (3) la cintmatique; (4) I’ige; (5) le concept d’unitt spatiale et giographique. On fait des rtflexions sur les possibilitts d’tvolution d’un type dttermint de paltobassin et son passage a d’autres types. On se demande aussi si les bassins alpins sont un prototype et s’il ne faut pas songer a la possibiliti d’une Cvolution difftrente au cours des temps gtologiques. Quant h la genese, on distingue entre thalassogentse (gentse de l’octan) et angueiogenese (gentse du bassin). Le bassin peut &treune partie de l’octan, placte surtout sur la plriphtrie continentale oh se produit le maximum de stdimentation. Certains paliobassins peuvent deriver de la dtformation et de la subsidence d’anciennes pintplaines transformtes en plateformes continentales. On tente un essai comparatif entre la stdimentation dans les bassins et dans les paltobassins.

SUMMARY

It is pointed out that a difference exists between recent basins - in the morphological sense - and paleobasins - in the geological sense. A classification of the latter is presented, in which new elements are introduced: ( I ) the longitudinal development, (2) the genesis, (3) the kinematics, (4) the age, (5) the concept of spatial and geographical unity. It is considered in what ways certain types of paleobasins can develop, and how they can change into other types. Tt is also considered whether the alpine basins represent a prototype and whether one should not reckon with the possibility of a different evolution in the course of geological times. As to the genesis, a distinction is made between thalassogenesis (origin of the ocean) and anguieogenesis (origin of the basin). The basin can be a part of the ocean, generally situated on the continental border where the maximum sedimentation takes place. Certain paleobasins can originate by deformation and subsidence of old peneplains that became continental shelves. A tentative comparison is made between deposition in the recent depressions and in the paleobasins.

BIBLIOGRAPHIE

AULIOUIN, J., 1959. A propos d’un centenaire: Les aventures de la notion de gtosynclinal. Rev. Giogruph. Phys. GioL Dyn., 2 (3) : 135-188.

244

N. LLOPIS LLADO

DANA, J. D., 1873. On some results of the earth's contraction from cooling, including a discussion of the origin of mountains and the nature of the earth interior. Am. J . Sci., 5 : 4 2 3 4 3 ; 6 : 6-14; 104-115, 161-171.

D4NA, J. D., 1873. On the origin of mountains. Am. J. Sci.. 5 : 347450. EWING, M. and PRESS, F., 1955. Geophysical contrast between continents and ocean basins. Geol. SOC. Am., Spec. Papers, 62 : 1-6. HAUG,E., 1900. Les gtosynclinaux et les aires continentales. Contribution A l'etude des rtgressions et des transgressions marines. Bull. SOC.GPol. France, 28 : 617-711. HAUG,E., 1908. TraitP de GPologie. Colin, Paris. HEEZEN, B. C., THARP,M. and EWING,M., 1959. The floors of the oceans. I. The North Atlantic. Geol. SOC.Am., Spec. Papers, 15 : 122 pp. KRUMBEIN, W. C. and SLOSS,L. L., 1951. Stratigraphy and Sedinientation. Freeman, San Francisco, 497 pp. KAY,G. M., 1944. Geosyncline and continental development. Science, 99 : 461462. KAY,G. M., 1945. North American geosynclines: their classification. Bull. Geol. SOC.Am., 56 : 1172. KAY,G. M., 1947. Geosynclinal nomenclature and the craton. Bull. Am. Assoc. Petrol. Geologists, 31 : 426 - 450. LOMBARD, A,, 1956. CPologie se'dimentaire. Masson, Paris, 722 pp. LLOPIS LLADO,N., 1947. Contribucidn a1 conocimiento de la niorfoestruc:ura de 10s Catalanides. Barcelona, 372 pp. LLOPIS LLADO,N., 1954. Types de chaines alpidiques du littoral franco-espagnol et leurs rapports avec les Alpes franwises. Congr. GPol. Intern., Compt. Rend., 19e ,A{yiers, 1952,4 : 261-279. PEYVE, A. V. et SXNITZYN, V. M., 1950. Certains problkmes fondamentaux de la doctrine des gkosynclinaux. Izv. Akad. Nauk. S.S.S.R., Ser. Ceol., 4 : 28-52. SCHUCHERT, C., 1923. Sites and nature of the North American geosynclines. Bull. Geol. SOC.Am., 34 : 151-260. STILLE, H., 1919. Alte und junge Saumtiefen. Nachr. Ges. Wiss. Gottingen, Math. Physik. Kl., 1919 : 327-372. STILLE, H., 1926. Crundfragen der vergleichenden Tektonik. Borntraeger, Berlin, 443 pp. TERCIER, J., 1940. DepBts marins actuels et sCries gblogiques. Eclogue Geol. He/v., 32 :47-100. UMBGROVE, J. H. F., 1933. Verschillendetypen van Tertiaire Geosynclinalen in den Indischen Archipel. Leidsche Ceol. Mededeel., 5 (I) : 3 3 4 3 . UMBGROVE, J. H. F., 1947. The Pulse ofthe Earth. Nghoff, The Hague, 327 pp. UMBGROVE, J. H. F., 1950. Symphony of the Earth. The Hague, 220 pp. WORZEL, J. L. and SCHUBERT, G. L., 1960. Gravity interpretation from standard oceanic and continental crustal sections. Geol. SOC.Am., Spec. Papers, 62 : 87-100.

SEDIMENTOLOGY OF BEACHES ON THE NORTH COAST O F T H E SEA OF AZOV N.

v.

LOGVINENKO

and

1. N. R E M I Z O V

National Committee of Geologists in the U.S.S.R., Moscow (U.S.S.R.)

INTRODUCTION

The sea of Azov is a shallow gulf (max. depth about 14 m), covering an area of 37,600 km2. The predominant winds over this sea are from eastern and northeastern directions. They generate currents (maximum velocity 0.36 m/sec) and waves (maximum height 2.5 m, length 25 m). During storms the sea level along the coast may be raised as much as 4.0 m. The resultant direction of swells is from the east. The north coast of the sea of Azov stretches for about 450 km from east-northeast to west-southwest. It consists of unconsolidated Tertiary and Quaternary sediments. Littoral sediments are of various types. They make up spits and barriers, and they are formed in the mouths of rivers and in lagoons and limans (Fig.1). Raised beaches are also present.

Fig.1. The north coast of the sea of Azov. 1 . Spit Beglickaja. 2. Spit Kryvaja. 3. Spit Yelanchikskaja. 4. Spit Bezymjannaja. 5. Accumulative terrace and incipient spit near Shyrokino. 6. Spit Ljapina. 7. Spit Belosarayskaja. 8. Spit Berdjanskaja. 9. Spit Obitochnaja. 10. Accumulative terrace and incipient spit without name. 11. Spit Fedotova.

The beach deposits consist of inorganic clastic material, shells and shell detritus. The beaches are 5-25 m wide and 1-2.5 m high, with gradients ranging from 1/10 to 1/25. Both profiles with, and without sand ridges are found. The shoreface is sometimes partly uncovered when the sea water is driven away by offshore winds. The

246

N.V. LOGVINENKO AND I. N.

REMIZOV

subaqueous slope is very gentle, depths of one or two metres being found at distances of 150 m from the shoreline. Its gradients vary between 1/50 and 1/200.

THE BEACH SEDIMENTS

The sand of the beaches is of medium to fine grain and contains 0.2-0.4% heavy minerals. Coarse sand (almost free of heavy mineral grains), gravel and pebble deposits are rare. Where present, they lie normally near the waterline. Storm built beach ridges on the highest parts of the backshore often show layers of fine sand (more than 8 5 % of the 0.25 mm size grade) in which heavy minerals are sometimes strongly concentrated (up to 30-90 %), see Table I and 11. At these places, the layers containing heavy mineral concentrates occupy from 10-30% of the total sediment. On the beach surface one may see wind ripples, rill marks, wave ripple marks, traces of life activity of amphipods, birds and other animals, accumulations of pelecypod shells (Cardium edule, Venus, Monodacna), eel grass (Zostrra marina), Medusae, crabs, and isolated specimens of fishes and birds. On the submerged slope one finds medium grained and fine sands (predominantly of the fractions 0.5-0.25 and 0.254.1 mm) with shell detritus and shells. Muddy sediments seem to occur mainly at a depth of 0.7-0.5 ni in areas protected from wave action. The sand surface is covered with wave and current ripple marks. In shallow water, at a depth of 10-1 5 cm, flat-topped ripple marks are formed under the influence of surf (Fig.2). These sands are rich in heavy mineral grains (up to 11 ”/,). The beach deposits are often distinctly laminated owing to varying proportions of sand, shell detritus and shells. The laminations or beds show different inclinations

Fig.2. Flat-topped ripple marks on lower foreshore, near Berdjansk.

TABLE I GRANULOMETRIC COMPOSITION OF BEACH SEDIMENTS WITH ORE MINERAL CONCENTRATIONS, WEST OF JDANOV IN -

~~

Location of sample Fraction limits in niin

76'

~-

Storm built beach ridge

Middlepart of beach ~~

Sand with ore slick

Sand without ore slick

-

-

Lower part beach

Sea bottom

Near sea level

at conditions

I0 nijroni shore 50 rn f r o m shore depth 0.4 ni depth 0.7 ni

~~~~

> 10 1&7 7-5 5-3 3-2 2-1 1-0.5 0.5-0.25 0.25-0.05 Median size Coefficient of sorting Coefficient of rounding by Chabakov, light fraction Idem, heavy fraction

-

-

-

-

0.1 14.1139.4 85.8194.4 0.174 1.77

0.9 40.610.3 58.712.8 0.324 2.29

2.96 1.oo

2.29 1.65

4.4 16.3 37.41~01.gr. 42.110.25 0.289 2.82 -

Denominator: content of heavy minerals in % by weight of corresponding fractions.

sol. gr. 0.4 1.3 49.513.5 48.816.0 0.252 2.88 2.99 1.96

15.4 6.5 10.4 26.9 19.0 14.0 6.5

O.B/sol.gr 0.5 3.64 3.64 gravel is not rounded

-

2.2 0.4 0.2 0.3 0.2 0.2 1.9 5 1.610.3 43.014.0 0.275 1.90

3.5 1.o 0.5 0.5 1.o 0.5 1.5 39.510.7 52.011.2 0.246 1.90

2.20 1.55

2.51 1.86

m

F %

TABLE I1 GRANULOMETRIC COMPOSITION OF BEACH SEDIMENTS WITH ORE MINERAL CONCENTRATIONS.

Location of sanlple Frnctiorl limits in nrm

Upper part of beach

Middle part of beach

> 10 1&7 7-5 5-3 3-2 2- 1 1-0.5 0.5-0.25 0.25-0.05 Median size Coefficient of sorting Coefficient of rounding by Chabakov, light fraction Idem, heavy fraction

-

5.3/sol. gr 94.7135.O 0.166 1.63

0.2 0.3 0.5 3.8 41.2/1.0 54.016.7 0.234 2.34

2.62 1.54

2.34 1.75

Denominator: content of heavy minerals in % by weight of corresponding fractions.

15

K M WEST OF BERDJANSK IN

Near sea level, calm it.enther condirions 21.3 8.7 9.8 29.1 18.5 8.7 3.3/sol. gr 0.611.0 4.26 5.25 gravel is not rounded

%’

Sea bottom depth 0.1 M , 5 nifiorii erast

-

0.2 0.1 0.5/sol. gr 8.114.0 91.1111 .O 0.182 1.82 2.15 I .38

2

<

.

BEACH STUDIES ON THE NORTH COAST OF THE SEA OF AZOV

249

Fig.3. Cross section of a beach cross bedding, east from Jdanov.

towards the sea and towards the land (varying between 1-3" and 15-28'). Orientation diagrams of the cross beds often show two maxima: a pronounced one for beds that dip gently towards the sea and a secondary maximum for beds with steep inclinations towards the land (Fig.4). The latter are absent on beaches without ridges. This type of cross bedding may be named a beach cross bedding (Fig.3, 5: 6, 7). The pebble material consists of calcareous concretions. The gravel is made up of angular fragments of igneous rocks, quartz, feldspar, fragments of sedimentary rocks,

250

N.V. LOGVINENKO AND I . N. REMIZOV

-1

____2

......3

Fig.4. Diagram of cross bedding of the beach sediments. 1 = Emission of azimuths of inclination by sectors of 30" in %. 2 = The angles of inclination in %. 3 = Middle angles of inclination in degrees.

Fig.5. Cross bedding of beach, spit Obitochnaja.

Fig.6. Cross bedding in barrier beach east of Berdjansk.

BEACH STUDIES ON THE NORTH COAST OF THE SEA OF AZOV

1 , 2, 3

=

25 1

Fig.7. Cross bedding o f beach sediments (diagram). Sets formed by storm and strong waves. 4 , 5 = Sets formed by weak waves.

shell detritus and shells (up to 20 %). The sand is composed of grains of quartz, feldspar (up to 10 %), shell detritus, (up to 5-6 in fine and to 20 % in coarse sands) and heavy minerals (up to 30 species), among which ilmenite, garnet, hornblende are the most important. The prevailing grain-size of the heavy minerals is 0.254.05 mm. The sand grains are well rounded, especially those of the light minerals.

SOURCES AND TRANSPORTATlON OF THE SEDIMENTS

Most of the minerals and rock fragments are derived from the crystalline massif of Azov. Under the present circumstances no gravel or sand is brought into the sea by the rivers. The supply of detrital material from the massif took place in two stages. First, in Late Pliocene and Early Quaternary times the massif was eroded and denudated and sand, gravel and pebble deposits were formed. At the present time these deposits are eroded by the sea, whereby bluffs are produced. Some clastic material comes from Quaternary loams (calcareous concretions, sand), as well as from Pontian and Maeotian limestones and sands. The shells are washed on to the shore from the sea bottom. The transport of clastic material is due to the work of the waves. Waves coming from easterly directions approach the shore at angles that are close to optimum (cp) for longshore movement and beach drift from east to west. Displacement of sediment from west to east under the influence of waves coming from the west takes place in the areas protected from easterly waves. The gravel is transported over distances of up to 3 5 4 0 km, the sand over distances of up to 50-60 km. The storm waves on the shoaly sand coast produce the most gentle profiles and form the laminations which dip gently towards the sea or steeply towards the land. Approaching the shore at different angles, the waves of low to moderate energy scour or raise the lower or middle parts of the beach in such a manner, that beach ridges are formed, with beds both inclined towards the sea and towards the land. During the following storm these ridges undergo abrasion and may be covered with new layers, gently pitching towards the sea. In this way the beach cross bedding is formed. The process and place of concentration of heavy minerals depend on the wave regime, on the size of the grains and on their physical properties. Concentration takes place where the water movement up the beach is stronger than in the opposite direction. With each advancing wave both light and heavy particles are carried in landward

252

N . V . LOGVINENKO A N D I. N . REMlZOV

direction while the weaker backwash transports relatively less of the heavy material towards the sea.

SUMMARY

Beach sediments on the north coast of the sea of Azov consist of sand, shells and shell detritus. Gravel and pebbles are present only near the water line. Cross bedding of the beach deposits is due to alternation of layers or sets of layers with inclinations both towards the sea (relatively low angles) and towards the land (relatively steep). Medium grained sands with small contents of heavy minerals occur in the middle and low parts of the beaches. The storm built ridges at the back of the beach consist of fine sand, often with rich heavy mineral concentrations. The transport and concentration of heavy minerals depend on the wave regime, on the size of the grains and on their physical properties.

Z U R G E R O L L M O R P H O M E T R I E VON TRANSGRESSIONSKONGLOMERATEN GERD LUTTIG

Niedersachsisches Landesarnt fur Bodenforschug, Hannover (Deutschland)

Der Begriff des Transgressionskonglomerates scheint - vergleicht man die Angaben dariiber in der Literatur - so gut mit dem Begriff der marinen Flachwasserfazies verkniipft, die genetische Wechselbeziehung daher zwischen “Transgression” und “marinem Kies” so eng zu sein, dass gewohnlich angenommen wird, die Eigentiimlichkeiten des Konglomerates selbst seien damit vollig klar umrissen. Unter einem Transgressionskonglomerat stellen sich Geologe und Sedimentologe, verwenden sie diesen Ausdruck, in den meisten Fallen einen durch das Meer deutlich und in typischer Weise geformten und spezifisch klassierten Kies vor. Dass diese Assoziation einer Fiktion gleichkommt, wird sofort klar, wenn man sich die Fiille der Eigenarten der im marinen Milieu heute anzutreffenden, um es ganz generell zu sagen: von Meerwasser bedeckten rezenten Kiese vor Augen fuhrt. Da sind einmal die Unterschiede in Korngrossenverteilung, Petrographie des den Kies zusammensetzenden Materials, Bindemittel und Schichtungsmerkmale. Vie1 bedeutender sind aber die Unterschiede in der Form der Gerolle. Hier liegt der kritische Punkt in der im geologischen Denken offensichtlich bereits fest verankerten Gedankenkette: Transgressionskonglomerat - mariner Kies - gut gerundet. Auch die jiingste Literatur, sogar die gerollmorphometrische, ist noch voll von Ausdriicken wie “gut gerundeter, typischer mariner Kies”, ‘‘marines Konglomerat mit typischer, extrem guter Rundung” etc. Bei einer positiven Strandverschiebung - der Ausdruck Transgression trifft, da zu eng gefasst, nicht die Gesamtheit dieser Vorgange - entstehen nicht nur Konglomerate, sondern sie werden auch vorgefunden. Das die Transgression abbildende Konglomerat wird daher nicht nur durch die positive Strandverschiebung geformt, sondern es kann bereits mindestens einen Formungsabschnitt hinter sich haben. Praktisch kann das ansteigende Meer vorfinden: (I) Den blanken Fels. (2) Eine Zersatzzone auf diesem Fels. (3) Einen oder mehrere Bodenhorizonte auf dem festen Gestein. (4) Einen Schuttfuss oder ahnliche detritische Gebilde auf oder vor dem Festgestein. (5) Lockerablagerungen, die in Bezug auf die grobe Komponente verschiedene Formungsschicksale hinter sich haben, z.B. durch: (a) Schwerkraft-Transport (ubergang zu 4), (6) Solifluktion. (c) glaziaren Transport, (d) glazifluviatilen Transport,

254

G. LUTTlG

TABELLE I ZUSAMMENHANCE ZWISCHEN VORZEICHNUNG UND MARINER FORMUNG VON GEROLLEN

Reihe der rnarjnen Forrnung

Reihe der Vorzeichnung 'h c-,,\

P o s i t i v e S t r a n d v e r s c h i e b u n g rnit : kaum merk- vorwiegend b a r e r wos - horizontale serbewequnq Strornung S c flach -

T u r bul enz

'

h

o r steil

r

e sehr steil

Tabelle I. Einige der Zusammenhange zwischen Vorzeichnung und mariner Formung.von Gerollen. Die Reihe der marinen Formung umfasst nur einige der fur die Formgebung wichtigen Zusammenhange. Die Reihe der Vorzeichnung ist nicht ganz komplett. Die eingetragenen lndizes betreffen Kalkgeriille. die nach der K-4-dMethode (LUTTIG,1955) vermessen worden sind. Die fett gedruckten Werte geben hohen Indizes, die halbfetten normale, die mageren niedrige Werte an. Q d bedeutet z.B.: besonders hohe Werte von (Abplattungsgrad, degree of flatness) d.h. sehr dicke Gerolle; niittlere Werte von 8. (Zurundungsgrad, degree of roundness), massig gerundete Gerolle; niedrige Werte von d (Grad der Stengeligkeit, degree of elongation), extrem stengelige Gerolle. Die Pfeile rechts neben dem betreffenden Index geben die Richting an,'inder der betreffende Weft zunimmt.

( e ) fluviatilen Transport (mit einer Fiille von Untertypen, vgl. LUTTK, 1962), ( f ) limnischen Transport, und andere Transportarten mehr (z.B. Halofluktion). Im Falle I formt das transgredierende Medium allein die transportierte Komponente. Eine Fiille von Vorgangen ist auch hier moglich. Sie sind abhangig von der Wasserbewegung (diese von Stromung, Form der Kiiste), Petrographie des iiberfluteten Gesteins, Windrichtungen und vielen anderen Dingen mehr. Allein die Form der Kiiste ist von entscheidender Bedeutung, z.B. wegen der Lage der Zone, in der sich die Orbitalwellen brechen, dem Vorhandensein oder Fehlen von Strandversatz, der

GER~LLMORPHOMETRIEVON TRANSGRESSIONSKONGLOMERATEN

255

Situation von Flussmiindungen, der Gestalt und Vegetation des Kiistenlandes und vieler anderer Faktoren. Es liegt auf der Hand, dass nicht nur die Sortierung des Transgressionskonglomerates, nicht nur seine Zusammensetzung verschieden sein konnen, sondern vor allem auch, dass die Gerolle, je nach dem Einfluss des marinen Transportes auf die vorgezeichnete, sehr mannigfaltige Altformung, in ganz unterschiedlicher Weise neugeformt sein kannen. Die These von dem “gut gerundeten marinen Gero11” ist eine Mar, die dringend der Beseitigung bedarf. Transgressionskonglomerate sind, gerollmorphometrisch gesehen, sehr verschiedenartige Gebilde, und ihre Merkmale streuen in morphometrischen Diagrammen iiber einen relativ weiten Bereich. Dennoch sind dieser Streubereich von denen anderer Gerollform-Gemeinschaften abgrenzbar, die marine Formung erkennbar, auch wenn eine altere Formung iiberdeckt ist. Zwischen Vorzeichnung und mariner Formung besteht ein ganzes Biindel von Zusammenhangen (vgl. Tabelle I), da beide Komplexe in sich eine breite, die Formgebung beeinflussende Variation bergen. Ihre Verkniipfung liefert gerollmorphometrische Moglichkeiten, die wir erst zu erfassen beginnen. Bis zu einer verbindlichen Beschreibung der typischen Merkmale mariner Konglomerate ist noch ein weiter Weg, der, anfangend mit aktuogeologischen Untersuchungen und endend in Bestimmungen an fossilen Konglomeraten, mit unzahligen Messungen gepflastert sein wird.

ZUSAMMENFASSLJNG

“Transgressionskonglomerat” und “gute Rundung” sind Begriffe, die zu Unrecht eng mit einander verkniipft sind. Es wird betont, dass den bedeutenden Unterschieden in der Form der Gerolle bisher zu wenig Aufmerksamkeit gewidmet wurde. Der Verband zwischen der Altformung, vor der Transgression, und der Neuformung, unter der Einwirkung der Wellen des ansteigenden Meeres, ist noch zu wenig untersucht worden.

SUMMARY

“Transgression conglomerate” seems to be a term closely connected to the term “good roundness” of pebbles. The author tries to point out that this is one of the errors of plain geological thinking, as the form of marine pebbles can be enormously different. There is a connection between original shaping, in the time before the transgression in various facies areas, and the second shaping under the influence of waves of the rising sea, but until now this connection is not completely studied and understood.

256

G. LU’ITIG

LITERATUR

BLENK,M., 1960. Ein Heitrag zur morphometrischen Schotteranalyse. Z. Geornorphol., 4 (3/4) : 202-242. CAILLEUX, A,, 1945. Distinction des galets marins et fluviatiles. Bull. SOC. GPol. France, I5 ( 5 ) : 375404. CAILLEUX, A,, 1952. Morphoskopische Analyse der Geschiebe und Sandkomer und ihre Bedeutung fur die Palaoklimstologie. Ceol. Rimdschau, 40 : 11-1 8. Lu-rrrc, G., 1956. Eine neue, einfache gerdlmorphometrische Methode. Eiszeitalter Gegenwart, 7 : 13-20. LUTTIG,G., 1962. The shape of pebbles in the continental, fluviatile and marine facies. Publ. Ass. Intern. Hydrol. Sci., 59 : 253-258. POSER,H. J. und HOVERMA”, J., 1952. Beitrage zur niorphometrischen und morphologischen Schotteranalvse. Abhandl. BrarrnschweiF. Wiss. Ges.. 4 : 12-36. RICHTER,K., 1952. Morphometrische Gliederung von Terrassenschottern. Eiszeitalter Gegenwart, 2 : 12C128. RICHTER,K., 1954. Gerollmorphometrische Studien in den Mittelterrass-nschottem bei Gronau an dei Leine. Eiszeitalter Gecgenwart, 415 : 216-220. RICHTER,K., 1958. Bildungsbedingungen pleistozaner Sedimente Niedersachsens auf Grund morphometrischer Geschiebe- und Gerollanalysen. Z. Deut. Geol, Ges., I 10 : -35. RICHTER,K., 1961. Die geologische Gellndeaufnahme. In: A. Bentz (Redakteur), Lehrbuch der angewnndtten Geologie. E k e , Suttgart, 1 : 1-1 17. TRICART, J. et SCHAEFFER, R., 1950. L’indice d’emousse des galets, moyen d’ttude des systtmes d’erosion. Rev GPomorphologie Dyn., 1 (4) : 151-179.

SEDIMENTARY ENVIRONMENTS OF THE WEALD CLAY OF SOUTHEASTERN ENGLAND J. D.

s.

MACDOUGALL

and

J . E. P R E N T I C E

Sir John Cass College, London; University of London, King's College, London (Great Britain)

INTRODUCTION

The Weald Clay is a predominantly argillaceous formation lying above the Tunbridge Wells Sandstone (Hastings Beds) and below the Atherfield Clay (Lower Greensand) in the Cretaceous succession of southeastern England. On the evidence of plant spores, the age of the formation is regarded as Barremian and Lower Aptian (HUGHES, 1958), although recent amendments to the stage boundaries (CASEY,1961, p.490) may place the whole within the Barremian. Its thickness varies from 360 m in the centre of the Wealden anticline, to 70 m in the south; in the north it is overstepped by the base of the Lower Greensand. It is generally conceded that the Weald Clay shows stronger indications of marine conditions than do the preceding Hastings Beds. Undoubtedly marine fossils, including Ostrea, Nemocardium and Cassiope, are found in the highest beds of the Weald Clay (DINESand EDMUNDS,1933); but it is to be noted that the living relatives of the two former show a wide tolerance of variations in salinity, whilst the third is only regarded as semi-marine (MELENDEZ, 1944). Moreover, the known occurrences of these fossils are limited to the topmost beds and to one horizon near the base of the formation (HOLMES, 1958, 1959). The fauna of the majority of the Weald Clay consists largely of the gasteropod Viviparus, the lamellibranch Filosina and abundant Cyprid ostracodes. Viviparus occurs commonly throughout the Purbeck and Hastings Beds, and is regarded as being a fresh-water form (PRASHAD,1928). A significant difference between the Hastings Beds and the Weald Clay is the replacement of the lainellibranch Neomiodon in the former by the superficially similar Filosina (CASEY,1955); this genus is a member of the family Corbiculidae, whose modern representatives are fluvial and estuarine, but whose Mesozoic ancestors may have been marine. The Jurassic Eocallista, regarded as ancestral to Filosina, is not known to be associated with the fully marine ammonite faunas, but is found with oysters and Trigonia in various Jurassic horizons, whilst Filosina occurs with Protocardium in Japan (HAYAM,1960), with a rich lamellibranch fauna in the Aptian of the Lebanon, and with Exogyra in the Aptian of Dorset (CASEY,1961, p.587). The ostracodes occur either alone, or in association with Viviparus and Filosina; they mostly belong to genera that are of fresh-water type,

258

J. D . S. MACDOUCALL AND J.

E. PRENTICE

though some euryhaline forms occur. More significant perhaps is the total absence of Foraminifera, except in the oyster-bearing beds at top and bottom. The evidence in total suggests that these latter represent the only truly marine incursions into the Weald Clay environment, and that for the majority of the time access of the sea was strongly limited. Certainly it seems that marine influence was so slight as to preclude the possibility that it had much effect upon the processes of sedimentation. The Weald Clay has been divided by REEVES (1948, 1958) into three groups on a lithological basis. Until recently the only available exposures were shallow, and deeply weathered, but recent extensions of claypits to greater depths, and extensive borehole investigations have provided the authors with unweathered material for the first time. For the purposes of description, the various lithologies are described separately below, using colour', grain-size and faunal content as discriminatory features.

LITHOLOGICAL TYPES

Greenish-gray clays A very high proportion of the Weald Clay consists of this lithology. Two elements are involved : greenish-black (5GY2/1) clay of illite-kaolinite composition, and pale greenish-gray (5GY6/1) argillaceous silt. Rarely the clay occurs on its own, when it is homogeneous and unlaminated, and there is every gradation from this through clays with rare silt laminae to beds consisting almost entirely of silt with thin laminations of clay. The typical rock consists of alternate laminations, 1-2 mm thick, of the two types; the silt laminae are always intensely disturbed by the activity of burrowing organisms. These appear to have burrowed only to a depth of 3 4 mm, and the rare silt layers which exceed 10 mm in thickness are not noticeably disturbed. Apart from the ubiquitous burrows, the organic remains of this group are very sparse, consisting of isolated ostracodes, fish scales, and Filosina. An overall rhythmic deposition, in which groups of beds with more silt alternate with those with more clay, in units of about 1 m, can be discerned. Olive-gray and brownish-gray clays These occur as subordinate thin bands usually less than 100 cm in thickness, within the main mass of the greenish-gray clays, and are restricted to the lower half of the formation. In them even laminae of about 1 mm in thickness of colour ranging from olive-gray (5Y4/ 1) to brownish-gray (5YR4/ I ) alternate with laminae of greenish-gray clay. The laminations are rarely disturbed by burrows, but carry traces of short straight markings which may be of plant origin. Undoubted plant fragments occur in masses on some bedding planes. Sometimes the clay is mottled by flattened blotches of bluish-gray clay up to 15 mm i n diameter. Colours identified by the Rock-Color Chart of the Geological Society of America.

SEDIMENTARY ENVJRONMENTS OF THE WEALD CLAY

259

Green and red clays

The red clays of the Weald have been noted since TOPLEY( I 8 7 9 , and were used by REEVES (1948, 1958) as stratigraphical marker bands. It can be shown however, that the red colour is very variable (5R4/6-5R3/4) and examination of borehole specimens shows quite clearly that it is secondary. The clays which are stained red however, are always originally of a bright green (5G5/2) hue, quite different from the normal greenish-gray clay. They are homogeneous, unlaminated and silty, and show no signs of burrowing organisms, but these green clays occur associated with the thinner sandstones in units of 10 m or more, sometimes containing abundant plant debris. These units have very sharp and clear-cut tops and a gradational base. Ostracode clays

The abundant ostracodes are always associated with clays which are low in silt content, high in disseminated pyrite, and which have a fine shaly lamination undisturbed by burrowing organisms. The clay is greenish-gray (5GY4/1) or light olive gray (5Y6/1) in colour. A high proportion of the ostracode tests are present as separate valves, and current alignment is not uncommon. Viviparus, small Filosina, and fish fragments frequently occur, and these beds may often be seen to pass laterally into shelly limestone bands. Limestones

Calcium carbonate is in general not a common constituent of the Weald Clay. It is confined to certain thin beds of unusual lithology mainly in the middle of the formation. Normally the limestone contains either Filosina or Viviparus as its dominant component, although rarely the two occur together; they are accompanied by ostracodes, fish fragments and fossil wood. Most of the lamellibranchs are in the form of detached valves. In any one band they all lie with convex or all with concave sides uppermost. The matrix is in general sparse, consisting of comminuted shell fragments and recrystallized calcite; the limestones are distinctively blue-black (5PB3/2) in colour. These limestones occur as extensive sheets which pass laterally into ostracode-, gasteropod- and lamellibranch-bearing clays. Locally conglomeratic limestones occur; they consist essentially of fish teeth, shell fragments, lignite and faecal pellets with clay fragments in a matrix of either calcarous sandstone or clay. They are strictly limited in lateral extent and appear to fill channels cut into the underlying clays; the width and depth of the channels seem to be related. One of these pellet limestones passes upwards into a coarse friable sandstone. Siltstones

The siltstone horizons of the Weald Clay grade from the greenish-gray clays by inter-

260

J. D. S. MACDOUGALL AND J. E. PRENTICE

calation of silt bands within the clay. The silt bands over 10 mm thick are noticeably undisturbed by burrowing organisms, though organic structures occur on certain bedding planes. The thickest beds occur as lenses within the clay and appear to be channel deposits; the basal beds are sole-marked and often exhibit load casting, whilst higher horizons within the siltstone show interference ripples. At one horizon isolated basin-like structures have been observed (PRENTICE, 1962). Some comparatively thin beds are of greater lateral extent and are undisturbed by organisms and exhibit interference ripples; the soles of these beds are rarely grooved and organic remains are sparse, though fish and plant fragments do occur. Sandstones

The sandstones bear the same relation to the clays as do the siltstones. They grade from the greenish-gray clays by the replacement of the silty laminae by sand, and then by increase in thickness of the sand layers into sandstone with clay laminations. The sandstones (unweathered) are gray (N6-7) or pale greenish-gray (5GY6/ 1) with conspicuous laminations. They consist essentially of quartz grains with fragments of carbonaceous material and large conspicuous mica flakes; they may be either crossbedded, ripple-bedded or evenly laminated. Pebble horizons occur within the sandstones; most frequently the pebbles are clay galls but quartzite pebbles have been identified as well as plant and fish fragments. Occasional clay beds occur within the thicker sandstones and at the contact between them load and groove casts have been observed. Ironstones

Concentrations of iron in the Weald Clay occur either as bands of clay-ironstone nodules, or as accumulations of sphaerosiderite. The clay-ironstones are associated with and pass laterally into olive-gray and brownish-gray clays; they frequently contain abundant plant remains. Sphaerosiderite normally occurs within the clay but sometimes in sandstone; it is not of common occurrence.

ENVIRONMENT OF WEALD CLAY DEPOSITION

The most striking single fact about the Weald Clay Formation as a whole is the great abundance of silt. Most of the “clays” contain between 40 and 70% of silt-sized quartz, concentrated in silty laminations, whilst the sandstones contain 30-40 % of this material. VAN STRAATEN (1959) mentioned that in tidal and marsh sediments in The Netherlands a silt ratio of 33 % is normal, and that the higher percentages in the Flevo lake deposits in the former Zuiderzee area are attributed to the low degree of flocculation in scarcely brackish waters. This same explanation may perhaps be held for the Weald Clay, the low salinity allowing the escape of the finer sediments to other

SEDIMENTARY ENVIRONMENTS OF THE WEALD CLAY

26 1

regions. If this is so the conspicuous lamination may be due to variations in salinity, the more argillaceous layers representing the higher salinities of summer and the silty laminations the lesser salinity of the winter period. By contrast sand-sized quartz is in low proportion; even the conspicuous sandstones rarely contain more than 50 % of fine sand, and they form only a very small part of the total thickness of the formation. It seems likely, then, that the normal silt-clay supply of sediment was only occasionally augmented by an influx of sand. The origin of this very considerable amount of fine-grained sediment presents some difficulty of interpretation. Presumably it came from the destruction of the LondonBrabant land-ridge; ALLEN(1959) has indicated a southward flow of several major rivers in Hastings Beds times, and evidence from the Weald Clay seems to support this. However, marine Barremian sediments are known from Norfolk, so that this land-ridge cannot have been more than 100 km across at this point. It is difficult to imagine this supporting a major river system. Another feature characteristic of the Weald Clay as a whole is the absence of any distinctive signs of emergence, or of vegetation growth in situ. A horse-tail rootlet bed has been recorded (KIRKALDY and BULL,1948) but the authors find the present exposures of this unconvincing; apart from this desiccation marks, footprints, rainprints etc. seem to be totally absent. Deposition under a shallow but continuously present sheet of only slightly brackish water seems to have been the persistent environment. The greenish-gray clays, with their strongly disturbed lamination, indicate the continuous presence of a very abundant bottom fauna. This in turn suggests slow deposition and an abundance of oxygen. A comparable environment in the Mississippi delta is found in the pro-delta silty clays. The absence of disturbed lamination in the olivegray clay suggests conditions inimical to organisms although the mottling may be of organic origin. The reasons for the colour differences have yet to be found, but the red-staining associated with the green clays suggests a near-shore, perhaps nearly emergent condition. The significance of the ostracodes is not clear. Working in the top of the Weald Clay in the Isle of Wight, where presumably conditions were more nearly marine, JONES (1959) has found that the ostracodes are of mixed provenance, brackish, freshwater and marine genera being found together. This, together with the evidence of current movement of the carapaces, suggests that these ostracode beds represent transported accumulations. The ostracode clays are closely associated with the limestones, which are a similar monogenetic accumulation. It seems that none of these fossil accumulations in fact represent a life environment, but that they were transported into the depositional area. The fact that they normally consist of one type and size of shell only can then be explained by hydrodynamic rather than biological principles. The siltstones and sandstones indicate a local increase of sediment supply, and are thus more easily explicable by movements of the land than by changes in depositional environment. They frequently have sharp channelled bases, showing that the inrush of sediment was rapid; whilst they contain internal evidence of rapid deposition. Much detailed investigation needs to be carried out before the Weald Clay environ-

262

J. D. S. MACDOUGALL A N D J. E. PRENTICE

ment can be properly interpreted. At the moment, however, the evidence points to a fairly static environment of deposition, whose major changes have been induced by movements and climatic events of the adjacent landmass.

SUMMARY

Eight different lithological types are recognized within the Weald Clay Formation (Barremian). These are: ( 2 ) Greenish-gray clay with a characteristic contorted lamination. (2) Olive-gray and brownish-gray clays, evenly laminated and mottled. (3) Green, often red-stained clays. (4) Ostracode-bearing clays. (5) Limestones with lamellibranch and gasteropod shells. (6) Siltstones. (7) Sandstones, and (8) Ironstones. It is concluded that the environment was relatively stable, consisting of shallow waters only slightly saline; and that the different environments reflect largely climatic and eustatic changes of the adjacent landmass.

REFERENCES

ALLEN,P., 1959. The Wealden environment: Anglo-Paris Basin. Phil. Trans. Roy. SOC.London. Ser. B, 242 : 283-346. CASEY,R., 1952. Some genera and subgenera, mainly new, of Mesozoic heterodont larnellibranchs. Proc. Malac. SOC.,29 : 121-176. CASEY,R., 1955. The pelecypod family Corbiculidae in the Mesozoic of Europe and the Near East. J . Wash. Acad. Sci., 45 : 366312. CASEY, R., 1961. Stratigraphical paleontology of the Lower Greensand. Paleontology, 3 : 487421. DINES,H. G. and EDMUNDS, F. H., 1933. The geology of the country around Reigate and Dorking. Geol. Surv. Gt. Brit. Mem., 286 : 204 pp. I., 1960. Pelecypods of the Jusanharna Group (Purbeckian or Wealden) in the Hastriwa area, HAYAMI, northeastern Japan. Japan J. Geol. Geograph. Trans., 31 : 13-22. S. C. A., 1958. Geological survey borehole. Sum. Pro,e. Geol. Surv. Gt. Brit. 1957,29. HOLMES, HOLMES, S. C. A., 1959. Geological survey borehole. Sum. Prqy. Geol. Surv. G t . Brit. 1958,29. HUGHES,N. F., 1958. Paleontological evidence for the age of the English Wealden. Geol. Mar., 95 : 41-49. JONES, G. R., 1959. The Ostracodes of the Wealden Shales ofthe Isle of Wcrht and their Relatiomhip to Sedimentation in the Wealden Basin. M . k . thesis, University of Wales, Aberystwyth. 246 pp. KIRKALDY, J. F. and BULL,A. J., 1948. Note on the section of weald clay exposed at the Clork House Brick Works, Capel, Surrey. Proc. Geologists' Assoc. Engl., 59 : 8C83. B. M., 1944. Las formaciones del infracretacea de Asturias. Notas Comun. Znst. Geol. MELENDEZ, Minero EspaAa, 13 : 183-216. PRASHAD, B., 1928. Recent and fossil Viviparidae; a study in distribution, evolution, and paleogmgraphy. Mem. Indian Museum, 8 : 153-252. J. E., 1962. Some sedimentary stuctures from a Weald Clay Sandstone at Warnham Brick PRENTICE, Works, Horsham, Sussex. Proc. Geologists' Assoc. En& 13 : 171-186.

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REEVES, J. W., 1948. Surface problems in the search for oil in Sussex. Proc. Geologists' Assoc. Engl., 59 : 234-269. REEVES, J. W., 1958. Subdivision of the Weald Clay in Sussex. Proc. Geologists' Assoc. Engf.,69 :1-16. TOPLEY, W., 1875. Geology of the Weald. Geol. Surv. Gt. Brit. Mem., 1875 : 503 pp. VANSTRAATEN, L. M. J. U., 1959. Minor structures of some recent littoral and neritic sediments. Ceol. Mijnbouw, 21 : 197-216.

SEDIMENTATION DES FACtkS “MARBRES NOIRS” DE LA BELGIQUE ET DU NORD DE LA FRANCE B. MAMET

Laboratoire de GPologie, Universiti libre de Bruxelles, Bruxelles (Belpiqiie)

INTRODUCTION

Les “marbres noirs” sont des sediments calcaires tr&sfins, d’un noir profond, faciles a polir; de ces qualitb rtsultent leur inttr&t au point de w e marbrier et leur intense exploitation entre le moyen 2ge et la premitre guerre mondiale. Les “marbres noirs” ne sont pas exceptionnels parmi les facib calcaires paltozoiques et c’est m&me l’abondance et la dispersion gtographique de leurs affleurements qui a motivi leur Ctude. Ce faciks ttant homotaxe, il apparait a plusieurs reprises dans la succession stratigraphique de la plateforme divono-dinantienne (KAISIN, 1935) de la Belgique et du nord de la France. Nous nous sommes particulitrement attach6 au “marbre noir” de Golzinne d’2ge Frasnien moyen, au “marbre noir” de Dinant du Visten inftrieur et au “marbre noir” de Baskles appartenant au Visten moyen. Le premier affleure au bord nord du Synclinal de Namur, le deuxit?me dans la partie centrale du Synclinorium de Dinant et le dernier dans la partie occidentale du Synclinal de Namur.

S~~DIMENTATION DES

FACIBS

“MARBRES NOIRS”

Avant de passer a l’itude des sequences sidimentaires, nous devons formuler deux remarques concernant la teneur en carbone organique et Ie r81e du potentiel d’oxydoriduction dans la stdimentation. La teneur en carbone organique

Les teneurs en carbone des “marbres noirs” ne sont pas difftrentes de celles d’autres faciks carbonatts du Dinantien. Le “rnarbre no?’ de Dinant, dtcrit comme “saproptlien” par tous les auteurs, posshde moins de carbone (0,15-0,20 %) que la moyenne des calcaires du Carbonif6re inftrieur. I1 en est de mCme pour le “marbre noir” de Golzinne (0,20 %). Seul le “marbre noir” de Baskles a une teneur particulihement tlevte en carbone. L’analyse de plusieurs centaines d’ichantillons de calcaires de la plate-forme

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dinantienne, nous a d’ailleurs montrt que la matikre organique (comme le Nombre de Trask) est like aux dttritiques argileux: nous rejoignons ainsi les constatations de RONOV(1958), faites pour les calcaires de la plate-forme russe. La teinte sombre d’un stdiment n’est pas directement lite la teneur en carbone organique; c’est ainsi, qu’il y a deux fois moins de carbone dans les “rtcifs” gris que dans les “marbres noirs” synchrones; il y a par contre dix fois plus de matitre organique dans les calcschistes gris que dans les calcaires noirs oh ils sont interstatifits. Dans le cas des calcaires, teneur en carbone tgale et pour un m&mettat de rtduction, la dimension des cristaux de calcite est responsable de la teinte du sediment. Les “marbres noirs” sont constituts d’une mosaique d’infinitt de cristaux de micrite trks clairs (FOLK,1959), conduisant a une absorption importante de la lumikre. Les sparites, par contre, ont des cristaux qui atteignent plusieurs centaines de microns; la lumikre incidente n’est plus complttement absorbte et la teinte semble grise. Enfin, pour les sparites a trts grands cristaux, la roche est blanche, quoique la teneur en carbone n’ait gutre varit. Le potentiel d’oxydo-rkhction

Si les “marbres noirs” ne sont pas des saproptlites, ils ne sont pas davantage des roches ultra-rtductrices. La moyenne des rapports Nombre de Trask/carbone. oscille entre 0,8 et 1,3, ce qui les place parmi les roches ltgtrement rtductrices 5 rtductrices. I1 faut de plus rappeler que ce n’est pas tellement l’intensitt du potentiel d’oxydoreduction qui caracttrise le facits, mais bien sa position par rapport A l’interface sMimentt (KRUMBEIN et GARRELS,1952). Ainsi, une barrikre de potentiel ltgbement oxydant, situte a quelques millimktres au-dessus du banc en voie de formation, sera ntanmoins favorable P la prtservation d’une partie du carbone organique; par contre, une barrikre d’un potentiel trts rtducteur situte 5 quelques millimttres sous l’interface n’empCchera pas l’oxydation du plancton. I1 n’est donc pas possible de classer les “marbres noirs” de Dinant et de Bastcles parmi les faciks sulfuriques; la barrikre Redox a fluctut au voisinage immtdiat de la surface stdimentte; c’est ce qui explique partiellement les brusques variations qu’on observe entre les sparites avec leur bios abondant et les micrites noires qui en sont dtpourvues. Pour le “marbre noir” de Golzinne, par contre, un dtficit d’oxygkne a exist6 durant tout le dtp8t et il se traduit par une suppression quasi totale de la faune. Siratigraphie

Ces deux remarques ttant prtcistes, passons aux rtsultats de l’ttude lithostratigraphique de ces dtpats, oh furent appliqutes les mtthodes d’analyse stquentielles de LOMBARD (1956): les raccords stratigraphiques s’y font entre rythmes stdimentaires et non plus de faciPs a facits. C’est donc bien le crittre de stratification, de succession de strates, qui sert de !ild’Ariane pour les corrtlations. La strie virtuelle des “marbres noirs” comprend six lithofacits classts grosso mod0

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par ordre de dtcroissance des dttritiques argileux; soit des calcschistes aux dolomies pentcontemporaines, en passant par les sparites, grumites et micrites (MAMET, 1962). Les “marbres noirs” de Golzinne et de Bastcles prtsentent des stquences ma1 tranchtes, reconnaissables lattralement sur des dizaines de kilomttres sans variations apprtciables. A Bastcles il est m&mepossible de suivre les phases positives actives qui gardent leur individualitt sur toute I’ttendue du bassin carrier (MAMET,1958). La dtlimitation de l’aire de formation de ces “marbres” n’est pas due a une barrikre gtographique, mais a la rapiditt de la subsidence compenste par une stdimentation rapide, monotone, som faible tranche d’eau. Tout autre est le cas du “marbre noir” de Dinant. La formation de ce marbre dtpend de l’isolement du lithotope par une ligne de hauts-fonds et des zones de turbulenze. Ces derniers donnent au lithotope du “marbre noir” le caractere d’un lagon peu profond, mais ouvert aux influences marines: la rtpartition tant horizontale que verticale des plaquettes de “marbre noir” interstratifites dans les calcaires a foraminiftres ou a bryozoaires, les calcaires i empreintes ntrtitiformes nombreuses, les graviers ii ostracodes ou algues calcaires, les biostromes, les calcschistes a hachis vtgttaux et 5 crustacts, y semble de prime abord e xtrhe me nt capricieuse; il s’agit 18, d’une stdimentation tpicontinentale a caractkre tminemment variable; toutefois, les stquences de carrieres stpartes par une centaine de kilomktres ne fluctuent pas davantage que d’autres distantes de deux cents mttres a peine; les changements d’tpaisseur ou de polarite et les variations de facib sont le fait de facteurs locaux, mais les stquences obtissent a des rythmes d’ordre suptrieur qui contr8lent toute l’aire sidimentte; le Visten s’y prtsente comme une grande bistquence, qui se rtsoud dans le detail, en treize termes marquts par l’accroissement des “polystquences” par rapport aux monostquences”. Par “polystquence”, nous entendons un ensemble de microstquences extension lattrale locale et par “monostquence” une suite monotone, sans tvolution verticale apprtciable de strates d’un mCme lithofacits. La dualitt de ces deux styles stdimentaires se retrouve dans tous les affleurements observes; cette rkpttition n’apparait pas comme lite aux variations de courants, ni aux variations de gradient de pente; elle n’est pas contr8lte par la barrikre de hauts-fonds; elle doit donc correspondre a des mouvements tpirogtniques affectant I’ensemble de la plateforme. Elle s’explique, si l’on considtre la dualitt existant entre les “dttritiques argileux”, externes aux conditions locales et dont la rtpartition est contr8lte par les barritres ptriphtriques au lithotope et les “facits calcaires” dont la constitution dtpend de facteurs purement locaux. A l’observation de ces stquences, vient s’ajouter un certain nombre de crittres confirmatifs lithologiques qui fome nt des “marqueurs” a petite tchelle: ils n’en ont pas moins leurs utilitt pour les corrtlations de procheenproche; nous citerons pour mtmoire les joints de stratification, les surfaces d’abrasion, les zones a slumping, les interptnttrations de microfacits, etc. C’est d’affleurement en affleurement que nous sommes parvenus a Ctendre lattralement la succession verticale des rythmes stdimentaires; cette constance nous parait inconciliable avec la migration des stquences par rapport aux isochrones; I’obser“

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vation de stquences bien caracttristiques qui recoupent les facibs postule pour nous l’existence d’un synchronisme absolu entre les rythmes. Le terme de ,,bassin de stdimentation” restant trop vague pour qualifier le type de stdimentation qui nous occupe, il nous parait utile d’introduire un terme nouveau qui tienne compte de l’analogie gtographique des stquences. Par dtfinition, nous qualifierons de “stquotope”, le lieu des points prtsentant une suite de stquences lattralement semblables. Un stquotope comprend une somme de lithofaciks varits; puisqu’il dtpend du mode de stdimentation, on peut prkvoir que son extension gtographique pourra Ctre difftrente de l’ttendue des lithotopes qui le compose. CONCLUSION

Nous avons montrt que le complexe stdimentaire des “marbres noirs” de Dinant cornporte des rythmes superposts, dont chacun est synchrone en chacun des points d‘observations; on peut, d&slors, parler de 1’“Bgz” de ces stquences, chaque rythme reprtsentant un intervalle de temps bien dtfini. I1 apparait d&slors ntcessaire de c r k r une quatrikme sub-division de la stratigraphie classique, subdivision qui joint le concept lithologique au concept chronologique; c’est la stquostratigraphie qui Ctudie la corrtlation des stquences.

L’analyse des stquences stdirnentaires de calcaires paltozoiques fins, dtposts A trts faible profondeur, montre l’existence de rythmes indtpendants du faci&s.Ces rythmes permettent d’ttablir des correlations sur des distances apprtciables et conduisent a la dtfinition de la stquostratigraphie. q u a t r i h e division de la stratigraphie classique.

SUMMARY

Analysis of sedimentary sequences in fine-grained paleozoic limestones, formed in very shallow water, reveals the existence of rhythms that are independent of facies. On the base of these rhythms correlations can be made over appreciable distances. Thus, the three divisions of classical stratigraphy can be supplemented with a new one: sequostratigraphy.

BIBLIOGRAPHIE

FOLK,R., 1959. A practical petrographic classification of 1imesIones. Bull. Am. Assoc. Petrol. Geologists, 43 : 1-32. KAISIN,F., 1935. Le facits “marbre noir” dam le Paleozoique de la Belgique. Mein. Inst. Ghol. Univ. Louvain., 8 : 81-131.

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KRUMBEIN, W. and GARRELS, R., 1952. Origin and classification of chemical sediments in terms of pH and oxidation-reduction potentials. J . Geol., 60 : 1-33.

LOMBARD, A., 1956. GPologie sgdimentaire. Vaillant-Carmanne, Paris, 122 pp. MAMET,B., 1958. Donnks nouvelles sur la stratigraphie, la paleontologie et la sedimentdogie du V i k n moyen et supkrieur de la rCgion de Baskcles. Bull. SOC.Be& GPol. Palhontol. Hydro/., 66 : 368-381. MAMET,B., 1962. Reflexions sur la classification des calcaires. Bull. SOC.Be@ Ge'ol. PalPontol. Hydrol., I0 : 48-64. RONOV,A., 1958. Organic carbon in sedimentary rocks (in relation to the presence of petroleum). Geochemistry ( U . S . S . R . , English Transl.), 1958 ( 5 ) : 51C-536.

LA SEQUENCE-UNITfi ET LES SERIES SEDIMENTAIRES J. PH. MANGIN

Laboratoire de Gdologie, Universite‘de Dijon,Dqon (France)

INTRODUCTION

Les dtp8ts sldimentaires s’ordonnent en successions pour lesquelles le terme de “stquence” a t t t propost depuis longtemps; son acception vtritable a t t t prtciste trts clairement par LOMBARD (1 956, p.270) dont les dtfinitions en cette matitre mtritent d’&treappliqutes i la lettre. Classiquement, la stquence est envisagte sous son aspect macroscopique au moins, sinon mtgascopique: l’analyse des stquences a l’tchelle microscopique n’est appliqute qu’aux successions de varves ou parfois aux dCp8ts a charbon ou i tvaporites. Classiquement encore, il est couramment admis que la stquence est granoclasste ou polaristk et qu’elle dtbute par les termes grossiers pour s’achever avec les dCp8ts de prkipitation. La courte note qui va suivre rtsume des observations qui s’opposent quelque peu i ces conceptions traditionnelles, observations conduites depuis quelques anntes par les chercheurs de moil tquipe et moimeme.

LA S~QUENCE-UNITI!

Les stquences d’ordre macroscopique et mtgascopique (“rnoyennes” et “grandes” de LOMBARD, 1956) apparaissent facilement l’ttude de terrain et figurent seules dans presque tous les travaux gtologiques: “alternances marno-calcaires”, “masse calcaire de 40 m de puissance”, “couche mariieuse de 8 m d’tpaisseur passant i des calcaires marneux en petits bancs” etc. sont des descriptions courantes. Or, dans la majeure partie des cas, pour ne pas dire dans tous les cas, ces “mitts” sont des stries compostes d’un nombre variable de stquences-unitts d’ordre microscopique, n’apparaissant souvent qu’en plaques minces. L’exeniple le plus facile est celui du facibs flysch qui a ttt, en fait, le point de dtpart de nos travaux. M&medans les notes les plus rtcentes, le flysch est considtrt comme une alternance de bancs durs et tendres, tpais ou minces, dont un bin8me forme une sequence: i la base, un banc dur, puis un banc tendre. En fait, l’analyse dttaillte montre que chacun de ces termes est h i - m h e compost d‘une succession de lits trbs fins pour lesquels j’ai propost le terme de “feuillet” (J. PH. MANCIN,1962) et qui montrent, sous une tpaisseur parfois millimttrique, I’ordonnance d’une vtritable

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stqueiice qui est donc dans le cas examine la stquence-unite. Elle comprend gtntralement un dtpbt granoclasst, allant de la dimension des sables fins I celle des ptlites, au sommet duquel sont plaquts des debris de micas et de vtgttaux et parfois des restes de tests minces (foraminifkres etc.) facilitant d’ailleurs le “clivage”. L’tpaisseur de cette siquence-unit6 est trts variable et dans les divers types de flysch qu’il m’a ttt donnt d’ttudier elle va de 1 - 800 nmi. Un oeil i peine exerct remarque fort bien ces feuillets d a m les bancs durs lorsque la coupe observte est un peu altkrte en surface: ils n’apparaissent que bien ma1 dans les couches tendres ofi seule la plaque mince permet leur ttude. 11s ne peuvent etre confondus tvidemment avec la schistositt secondaire ou les figures de stdimentation entrecroiste, de plages par exemple. I1 est clair que chacun de ces feuillets est le rtsultat d’un Cpisode de dtpbt et c’est au fond ramener la stdimentation d’une telle strie de stquences-unitts i celle des systkmes varvaires, ce qui ne peut entrainer aucune opposition logique, les faits ttant assez tvidents en eux-memes. Les sequences-unites s’ordonnent en une succession d‘ordre macroscopique qui reprtsente seulement en fait un des termes de la stquence communtment dtcrite. Ainsi 10-30 stquences-unitts composent gtntralement un banc dur (ou tendre) du flysch authentique. Seuls varient les termes extremes de la granulomttrie de chaque feuillet et son tpaisseur et ces variations entrainent l’allure du facits. Ainsi se notent les trois ordres classiques de stquences: ( I ) SPquence microscopique ou stquence-unitt dont I’assemblage forme une strie donnant un banc (ou une couche) dur ou tendre. (2) Shguence macroscopique composte d’un binbme de couches dure et tendre, donc de deux stries de stquences-unitts. (3) Shquence mPgascopique incluant ce facits dans un ensemble: binbme flysch-barre calcaire, par exemple. I1 est possible aussi de parler des rythmes de dtpbts relatifs ii ces stquences en leur appliquant les termes de “rythme unit?, “rythme niineur” et “rythme majeur”. Mais cette ordonnance si visible dans le faciks flysch peut s’observer dans la quasitotalitt des autres dtp8ts et je me contenterai de fournir ici trois exemples tirts de travaux conduits dans la perspective de la mise en tvidence de la stquence-unit6 et dont les rtsultats dttaillts seront exposts ailleurs. Le facigs marneux, pris par exemple dans la serie du Lias moyen des environs de Dijon (GAUTHIER, 1962), montre a l’analyse stquentielle fine (tchantillonnage continu sur les 30 m de faCade apparemment homogkne d’une exploitation de carriere) une variation alternante des termes extrEmes de la granulomttrie allant de pair avec l’association microfaunistique (probablement thanatocoenique)’. La courbe lithologique d’tchelle centimttrique fait apparaitre la stquence-unitt rtpttte plusieurs centaines de fois avec des variations d’ensemble traduisant la stquence macroscopique; de t r b faibles difftrences dans le contenu carbonatt changerait en une alternance manifeste cet ensemble d’aspect tris homogkne: c’est d’ailleurs ce qui se produit lattralement. Cette remarque vaut, semble-t’il, pour la quasi-totalitt des faciks maineux On retrouve ici les altemances Ctudiees dijh dam la coupe des marnes bleues de Biarritz (M. MANGIN, 1956).

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27 1

classiques qu’il suffit d’observer avec le degri souhaitt de finesse dans l’analyse. Le faciGs calcaire lui aussi rtvtle la succession de stquences-unitts. Certes, de nombreuses ttudes ont dtja t t t consacrtes au litage et au “rubannage” des calcaires et encore rtcemment; toutefois, I’indicateur le plus prtcis du rythnie unit6 pourrait bien Ctre la variation de tel ou tel dement chimique. L’exemple citt iciest celui des calcaires chailles du Bajocien des environs de Dijon (BURTIN, 1962) dans lesquels les lits de chailles, lorsqu’ils existent, traduisent h I’tvidence les stquences-unitts (tpaisseur moyenne 300 mm). Mais, lattralement, en l’absence de lits de chailles individualists et pour un facits calcaire apparemment homogene, l’analyse de la frtquence de Si 0, dans un tchantillonnage continu fait apparaitre egalement un rythme-unitt dam lequel les dtcharges siliceuses sont autant de “marqueurs”. Mtcaniquement, il en irait de meme pour les alternances (trks ma1 visibles en dehors de l’examen microscopique) dans le calibre des trts rares quartz clastiques observts dans ce facits calcaire ou dans l’abondance relative des rtsidus insolubles. Ici, encore, le “banc” calcaire lorsqu’il semble homogtne sans diasthtme ni feuilletage est en fait une strie de stquences-unitis. Les sables et les argiles pliocknes du Val de Sa6ne ont fait l’objet aussi d’analyses fines qui ont fait ressortir le caracttre “varvaire” riel de ces formations qui pourtant apparaissent a l’oeil comrne des couches argileuses d’une dizaine de metres de puissance alternant avec des lits sableux mttriques en une succession reprtsentant en fait la stquence macroscopique. Les stquences-unitts ont ici environ 1 cm d’tpaisseur en moyenne. Bien d’autres exemples pourraient Ctre citts; notamment dans les dCp6ts de galets et de sables des alluvions anciennes et modernes lorsque leur tpandage s’est fait dans un bassin assez vaste pour tliminer les incidents trop locaux: des stquences-mitts apparaissent sous une Cpaisseur dtcimttrique formant des ensembles caillouteux ou sableux, soit des “stries” considirtes A tort comme reprtsentant la stquence tltmentaire. Les conclusions de ce bref expost des rtsultats obtenus (qui seront complttts par une dizaine d’ttudes en cours sur le mCme sujet pour I’ensemble des facits stdirnentaires) sont semble-t’il assez claires et posent un probltme de gtnttique. En fait, la stquence “tltmentaire” gtntralernent considtrte (alternances macroscopiques) n’est pas celle qui correspond h l’tpisode tltmentaire de dtp6t proprement dit et il me parait sans but de proposer une explication du phtnomtne de stdimentation a partir de ces bases qui correspondent ri des groupes d’tpisodes. Certes, le rythrne de stdimentation de ces groupes de sequences-unitts est inttressant A connaitre pour tvaluer les variations dans le temps des conditions gtntrales d’apport rnais il ne montre pas le phtnomtne de dCp6t lui-mCme. Exemple: dire que chaque bin6rne banc dur-couche tendre du flysch est le rtsultat d’un seul Cpisode de dtp6t (courant dense ou autre) implique la mkconnaissance des 20-60 feuillets qui cornposent cette “stquence” et dont’le mtcanisme de dtp6t est alors inexplicable! Autre exernple: parler d’un seul tpisode de dtp6t pour une strie marneuse d’une cinquantaine de metres d’tpaisseur fait envisager des conditions perrna-

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nentes de stdimentation durant tout le temps de ce dtp8t alors qu’il est en fait composite et qu’il a requis des conditions fort peu difftrentes de celles qui ont amen6 ailleurs une Claire alternance marnes-calcaires ! Ceci revient a dtgager un contr6le fondamental rtglant toute stdimentation en de petites unites, contrble aide ou contrarit par les autres facteurs locaux. Apparemment le seul contr6le qui prtsente un tel critere d’universalitt est de nature climatique, les autres facteurs n’ayant qu’un rble spatial plus limitt. Au reste, lorsqu’il s’agit d’alternances tvidentes comme les varves ou les cyclothtmes molassiques on admet sans difficultt un processus de dtpbts influenct par les alternances climatiques tandis que des mtcanismes fabuleux ou accidentels sont requis pour expliquer les autres dtpbts dont l’analyse dttaillte montre qu’ils sont au fond de mCme composition tltmentaire ! Pratiquement chaque stquence-unitt pourrait Ctre due a une sorte de “crue” au sens universe1 du terme naturellement.

LA S ~ R I EDE S ~ Q U E N C E S - U N I T ~ S

Si un tel mkanisme de dCp6t tltmentaire est admis pour une bonne part des formations lithologiques connues, on peut concevoir une sorte d’alternance “saisonnikre” rtglant le granoclassement au sein de la stquence-unitt; mais l’association en strie de ces unitts “saisonnikres” montre que les conditions ne se sont maintenues qu’un certain temps: dts qu’elles changent (pluviositt accrue par exemple, d’oh la correspondance avec le cycle solaire dtjaexposte) les alternances demeurent, mais le calibre des apports varie. Ce serait une explication a la succession bancs durs-couches tendres du flysch et aux rythmes observts macroscopiquenient. Quant aux rythmes mtgascopiques ils peuvent proctder de changements climatiques d’ordre majeur, surtout dans les zones de climats mixtes ou temptrts et, dts lors, des bouleversements peuvent affecter la region, ses sols, sa couverture vtgttale: c’est le cycle biorhexistasique d’ERHART (1956). Pour cet auteur, toute stdimentation est rtglte par la nature des sols de la province distributrice; sans gtntraliser cette ingtnieuse notion, force nous est de constater qu’une province distributrice forestikre fournira d’abord des tltments solubles pouvant donner naissance a des roches d’tlaboration (calcaires, bancs siliceux) et que la destruction de cette couverture vtgttale entrainera la libtration des clastiques qui, parvenus enfin i l’aire de stdimentation, se mettront en place sous forme de marnes et de grts superposts aux calcaires prktdents. Au fond, il s’agit d’un bin6me majeur couvrant un nombre imposant de “saisons” et faisant apparaitre successivement un terme d’tlaboration puis un terme dttritique. Mais il s’agit d’une notion arbitraire: le renouvellement d’une telle alternance peut dtbuter par l’apport solide ou par l’apport soluble et la sequence par les calcaires ou par les grts ! Toutefois, l’analyse des stquences macroscopiques montre que, bien souvent, les termes calcaires dtbutent brusquement sur les stries dttritiques tandis que ces dernieres viennent progressivement “ttouffer” l’tlaboration calcaire: statisti-

SBQUENCE- UNIT^ ET LES SBRIES

S~DIMENTAIRES

273

quement, la stquence rnacroscopique (et peut-Ctre la stquence migascopique) serait donc polariste le plus souvent dans le sens elaboration puis apports dttritiques, ce qui n’empkhe nullement, c’est tvident, la sequence-unite de suivre le granoclassement “normal” impost par le mkanisme normal de dtpbt qui parait invariable dans son processus quels que soient les mattriaux qu’il met en jeu. I1 faut doiic observer encore un grand nombre de stries aussi diverses que possible pour tenter de dtterminer, selon les cas, le sens de l’tvolution des stquences macroscopiques et mtgascopiques et ne pas conclure arbitrairement que toutes les stries sont granoclasstes dans un sens ou dans l’autre. Encore moins faire un dogme de I’etalement dans le temps et l’espace et dans le mCme sens de la succession tltments grossiers-tltments fins ou solubles. Exprimtes en termes frappants, donc abusifs, les conclusions prtliminaires d’une telle ttude sont les suivantes: Tous les dtpbts sont composts d’une succession, visible ou non, de “stquencesunitts”, souvent microscopiques qu’il est indispensable de mettre en tvidence. La stdimentation de la stquence-mitt correspond h un tpisode unique, peut-Ctre trts rapide par rapport au temps total de dtpbt de la strie rtsultante, tpisode dont le mtcanisme gtntral s’apparente a celui d’une crue. Si les stquences-unites sont gtneralement granodtcroissantes, leur succession dans le temps n’obtit pas fatalement a une tendance a I’affinement des tltments mais souvent les stquences macroscopiques et mtgascopiques prtsentent les dtp8ts de prkipitation au dkbut et se terminent par les stdiments plus grossiers. Le contrele clirnatique semble prtpondtrant dans presque tous les cas. La nature des sols et la couverture vtgttale qui dtcoule de ce contrde est un facteur dominant rzlativement a la composition de la stquence-mitt et au facits de la strie rtsultante. La sequence-unitt pourrait alors trouver son origine dans les variations climatiques “saisonnieres” et les stquences d’ordres plus tlevts traduiraient - toutes choses tgales d’ailleurs - des changements plus notables, cycliques ou non, dans les conditions climatiques rtgissant le domaine oh s’exerce le couple erosion-stdimentation ttudit. II me semble souhaitable de vtrifier ces conclusions A la lurnitre des faits connus jusqu’ici en les analysant h l’tchelle qui convient. En attendant, il parait prtmaturt de risquer des hypotheses relatives a la stdimentation en se basant sur des observations n’ayant pas la prtcision nkcessaire.

Cette note dtfinit la stquence-unit6 comme la plus petite stquence dtposte par un episode isolt dans le temps; I’exemple le plus frappant en est la “varve” mais il est facile de constater que presque tous les stdiments du flysch aux calcaires - prtsentent une structure feuillette. Chaque feuillet pourrait Ctre le rtsultat d’un seul apport et I’effet de grano-dtcroissance amenant le style alternant entre les feuillets (ou les -’

274

J. PH. MANGIN

“laminations”) traduirait en de nombreux cas les variations de compttence des cours d’eau transporteurs. La conservation de ces structures implique un mode de dtpBt particulier, trks semblable i celui des varves prtcistment.

SUMMARY

This paper defines the “sequence-unit” provided by an elementary depositional process (rise or similar). Such a sequence (frequently millimetric and inapparent to eye) appears in almost all facies from flysch to so-called homogeneous marls; the grain-size decreases from bottom to top in each sequence-unit but an assemblage of them, or series, can present an opposite polarity and the macroscopical sequence may be composed by a succession of sandstones on limestones as assumed by Erhart’s theory of biorhexistasie. These facts probably traduce the strong control of climatic variations on erosion-sedimentation processes and the sequence-unit could be a “seasonal” deposition.

BIBLIOGRAPHIE

BURTIN,M., 1962. Une M&hode d’Analyse sPrliinentologique appliqirPe au Bathonien iifkrieur de C6te &Or. DiplBme d’etudes Superieures, 1962. Faculte de Sciences de Dijon, 54 pp. ERHART, H., 1956. La GenPse des Sols en tant que PhinoniZnegiolq~ique.Masson, Paris, 90 pp. GAUTHIER, J., 1962. SPdimentologie et Micropaliontologie dir Lias bourguignon. These, Universite de Dijon, 227 pp. A,, 1956. CPologie siditnentaire. Les Siries riiarines. Masson, Paris, 722 pp. LOMBARD, MANGIN, J. PH.,1962. Le flysch, sddiment clirnatique? Compt. Rend. SOC.CPol. France, 2 : 34-37. MANGIN,M., 1956. Etude sur les associations de rnicroforaminiferesde I ’ w n e de Biarritz (Basses Pyrknkes). Bull. Sci. Bourgogne, 15 : 173-1 86.

SEDIMENTARY FRAMEWORK OF THE DROWNED PLEISTOCENE DELTA OF RIO GRANDE DE SANTIAGO, NAYARIT, MEXICO' DAVID G . MOORE

and

JOSEPH

R.

CURRAY

U.S.Navy Electronics Laboratory, San D i g 0 52, C a l q ( U . S . A . ); Siripps Institution of Oceanogwphy, University of California, La Jolla, CaliJ ( U . S . A . )

INTRODUCTION

Internal structure of sediment wedges at the margins of continents is one of the major open problems in marine geology. Prior to instrumental developments of the last few years, it was possible to delineate the upper and lower surfaces of those wedges by echo sounding and seismic refraction work, respectively, but it was not generally possible to determine their internal framework. As a result, the geological literature is fraught with speculation on this subject. With the recent development of deep-penetrating, continuous-recording, acoustic reflection surveying systems, it is no longer necessary to speculate on the subject: one can measure it in detail. Deltaic structure of Pleistocene rivers has been found to be important in the structural framework of the sedimentary wedge off west central Mexico and with further surveys may prove to be one of the more common types of continental margin structures. As apart of the Scripps Institution of Oceanography project of study on the geology and oceanography of the Gulf of California, the writers have surveyed the continental shelf and upper continental slope of the mainland coast of the western side of Mexico between Mazatlan, Sinaloa, on the north and San Blas, Nayarit, on the south (Fig. 1). This is adjacent to the coastal plain typified by regressive Holocene sands reported elsewhere in this volume (CURRAY and MOORE,1963).

METHODS

Two cruises have been conducted over this continental margin during the past two years with the objectives of collecting sediment samples, surveying by echo sounder for detailed bathymetry, and surveying with bottom penetrating acoustic reflection systems for determining thicknesses of sedimentary units and the sedimentary framework beneath the continental shelf and upper slope. For this latter purpose, two types of instruments have been utilized, the Sonoprobe and'the Sonic Profler (Arcer). Contribution from the Scripps Institution of Oceanography, University of California, La Jolla, Calif.

/

COSTA DE WAYAROT

Fig.1. Bathymetric and physiographic chart of the continental terrace and coastal plain, Costa de Nayarit, Mexico, Locations of Sonoprobe and Arcer records of Fig.2 are indicated. Dashed, dotted, and heavy solid lines show respective positions of oldest, intermediate, and youngest fossil shelf edges associated with three periods of delta advance.

THE DROWNED PLEISTOCENE DELTA OF RIO GRANDE DE SANTIAGO

277

The Sonoprobe (MCCLUREet al., 1958) is a continuous-recording, high power, low frequency, echo sounder capable of measuring both depth of water below the survey vessel, and depth to acoustic reflecting horizons below the bottom. Frequencies in the range 3,000-6,000 cycles/sec are generally used; short pulses are emitted 12 times per second, and coupled to the water on both the outgoing signal and the returning reflections by means of magnetostrictive and piezoelectric transducers. Reflections from beneath the bottom may be produced by changes in lithology, density, or porosity of the sediment or rock. This instrument is most useful for determining Holocene sediment thickness and details of shallow internal structure (Fig.2a). The “Arcer” utilizes a high power (3,000 wattfsec) electric arc discharged into the water as a sound source, and a towed streamer of 10 crystal hydrophones as a receiving array (MOORE,1963). Signal processing is essentially similar to that of the Sonoprobe but the fundamental frequencies utilized are lower (200-1,000 cycles/sec) and the pulse rate is one or two per second. Higher power and lower frequency makes the “Arcer” well suited to the study of grosser structure at greater depths beneath the sea floor (Fig.2b).

BATHYMETRY

The section of the continental shelf which was studied off the Costa de Nayarit is 140 nautical miles in length and about 15 miles wide (shoreline to shelf break) at its northern and southern extremities. It is most distinctly characterized by a pronounced broadening of its central part which reaches a width of 40 miles at a relatively narrow protuberance marking the maximum width. Bathymetric details shown in Fig. 1 are based Largely on soundings made during the present study with some utilization of echo sounding traverses made in the seaward part of the area by other Scripps vessels during tracks into and out of the region. A few spot soundings from the U. S. Hydrographic Office chart of the region were also used. The unusual lobate morphology of this shelf surface is of primary interest as it will be shown to be directly the result of deltaic sedimentation during the Pleistocene.

HOLOCENE SHELF SEDIMENTS

Detailed studies of the sediments collected from this area are still in progress at this writing, but sufficient data are available to allow a generalized description of distribution of major facies and t o tie this distribution in with the three dimensional data of the Sonoprobe survey. Sediments of the inner shelf are neritic sands of the present sedimentary cycle. These grade rather abruptly into silty clays and clays of the inner and central part of the shelf which are the distributed products of the present suspended load of the Rio Grande de Santiago. Seaward of these Holocene shelf deposits o n the broad central section of the shelf is a zone of marginal sand-silt-clays. These

278

/

COSTA DE WAYAROT

D. G. MOORE AND J. R. CURRAY

Fig.1. Bathymetric and physiographic chart of the continental terrace and coastal plain, Costa de Nayarit, Mexico, Locations of Sonoprobe and Arcer records of Fig.2 are indicated. Dashed, dotted, and heavy solid lines show respective positions of oldest, intermediate, and youngest fossil shelf edges associated with three periods of delta advance.

THE DROWNED PLEISTOCENE DELTA OF RIO GRANDE DE SANTIAGO

279

are believed to be the result of mixing the fines of the river-derived shelf deposits with residual basal Holocene transgressive sands, which form a 5-8 mile wide band on the outer shelf on the region. On the extreme northern and southern ends of the area the fine shelf deposits believed to be associated with the Rio Grande de Santiago are missing and the seaward sequence of sediments grades from neritic sands to marginal, mixed sands and silty sands to the relict shelf edge basal transgressive sands. Sonoprobe records across the wide central shelf (Fig.1, 2) show these same sedimentary facies in section. Holocene shelf deposits are thickest on the inner shelf where they overlie a surface which appears to have been subject to subaerial erosion with resulting irregularities in the profile. Immediately above the erosional horizon is the basal Holocene sand and superposed above this thin basal member are up to 25 ft. of interfingered open shelf facies, neritic sands and marginal mixed sand-silt-clays. The basal sand member can be followed with the Sonoprobe to the point of its outcropping on the central shelf as the shelf deposits thin to seaward as shown in the figure record.

PLEISTOCENE SEDIMENTS AND DELTAIC STRUCTURE

Pleistocene alluvium is exposed at the base of the foothills of Tertiary volcanics and has been traced westward across the coastal plain, by means of borings, as it dips beneath regressive Holocene sands which cover much of the low-lying central and seaward portion of the plain (CURRAY and MOORE, 1963). The upper surface of this alluvium is believed to correspond to the erosional surface recorded on the Sonoprobe records and shown in Fig.2a. Deeper penetration into the sea floor was achieved by the Arcer which recorded internal structure beneath the probable Pleistocene surface to depths of over 600 ft. The Arcer records show, with amazing clarity, a cross section of classic delta topset and foreset beds which have prograded across their ancestral basin (Fig.2b) and now form the face of the upper continental slope. In detail, records of individual deltaic sequences show a transition from gently seaward-dipping, discontinuously-bedded, probable alluvial and marsh deposits to steep-dipping, arcuate, concave upward marine foreset beds. Examination of the records from all parts of the study area reveals that the deltaic sedimentation was not contemporaneous throughout the area. Some sections show one sequence of deltaic beds superimposed over another, and in most of the complete profiles across the central and outer shelf and upper slope there are recorded buried morphological features which are strikingly similar to the present day shelf edge. By plotting the location of these probable fossil shelf edges and their corresponding sequences of delta deposits, it can be seen that at least three major periods of delta building are preserved in the body of the continental shelf. Each of these has been separated by time and by the level of the contemporary sea. Fig.1 has plotted a postulated sequence of delta growth. The youngest of the recorded foreset slopes is exposed as the face of the present continental slope in one area subject to deltaic

280

D. G. MOORE AND J. R. CURRAY

sedimentation. From material on the upper edge of this slope, a radiocarbon date of about 19,000 years B.P. has been obtained using marine mollusc shells identified by R. H. Parker [personal communication) as intertidal inhabitants. Although a single date is not conclusive, the depth of recovery of the dated shells corresponds very closely with the sea level of 19,000B.P., shown on a curve as proposed by CURRAY (1961) which was based on many radiocarbon dates from several shelf and slope localities. If the measured date of 19,000 B.P. is accepted, it is strong evidence favoring the youngest of the deltaic sequences recorded being Late Wisconsin (Wiirm 11). These two youngest sequences both appear to have been deposited during falling sea level as evidenced by the seaward slope of topset beds. ,

COMPOSITE SHELF STRUCTURE

Although deltaic sediments are an important part of the body of the Costa de Nayarit, it should not be inferred that the marginal sedimentary wedge is entirely deltaic in origin. As on the present shelf, locally thick sections of shelf sediments composed of neritic sand facies and open shelf silty clays were accumulated in the past and can be detected as important components within the sedimentary framework of the shelf body. Alluvial deposits are similarly important particularly in the make-up of the landward half of the sediment wedge. The sequence of development of this complex structure requires concurrent deposition of coastal plain alluvium, marine delta front facies, neritic sands and open shelf silts and clays; the latter being essentially the bottom set beds of the delta complex. As these various deposits accumulate, the sea is either stationary or receding from the land with delta building keeping pace. When sea level again rises, basal sands are formed over the old marsh and alluvial deposits. With yet another still-stand, the stage is set for a new cycle of delta building and marine deposition, with the old delta mass forming the platform on which to build.

SUMMARY

Recent equipment developments make possible the mapping of internal structure of sediment wedges of continental margins. In this study of the continental shelf and upper slope off west central Mexico, conventional sampling techniques are combined with bottom penetrating acoustic reflection surveying to delineate major sedimentary facies and to record the structural framework of the sediments forming the continental margin. A lens of Holocene shelf sediments, thickest on the inner shelf, wedges out on the central shelf to expose basal Holocene transgressive sands. These basal sands blanket the outer shelf of the entire area. Beneath the basal sands on the inner shelf is a complex of Pleistocene alluvium which grades seaward into well developed deltaic facies

THE DROWNED PLEISTOCENE DELTA OF R10 GRANDE DE SANTJAGO

28 1

showing classic foreset beds that have prograded the old shoreline. In at least three distinctly recognizable cycles of deposition, the Late Pleistocene Rio Grande de Santiago built a vast deltaic wedge across its ancestral basin and prograded its contemporary shorelines more than 20 miles seaward to form, upon the inundation of the delta marshes, the present platform of the outer continental shelf. A radiocarbon date of 19,000 B.P. from the youngest delta front suggests the two youngest of the recorded sequences to be Late Wisconsin (Wiirm 11).

REFERENCES

CURRAY, J. R., 1961. Late Quaternary sea level: a discussion. Bull. Geol. SOC.Am., 72 : 1707-1712. CURRAY, J. R. and MOORE,D. G., 1963. Holocene regressive littoral sand, Costa de Nayarit, Mexico. In: L. M. J. U. VAN STRAATEN (Editor), Deltaic and Shallow Marine Deposits. Elsevier, Amsterdam, pp. 7 6 8 2 . H. F. and HUCKABAY, W. B., 1958. The marine sonoprobe system, new MCCLURE,C. D., NELSON. tool for geologic mapping. Bull. Am. Assoc. Petrol. Geologists, 42 : 701-716. MOORE,D. G., 1963. Acoustic reflection reconnaissance of continental shelves, eastern Bering and Chukchi Seas. In: Papers in Marine Geology, Shepard Commemorative Yolitme. Macmillan, New York, in press.

DIE K0RNGRt)SSENVERTEILUNG I N DEN REZENTEN SEDIMENTEN DES GOLFES VON NEAPEL’) GERMAN MULLER

Mineralogisch-Petrographisches Institut, Tiibingen (Deutschland)

EINLEITUNG

Fruhere Arbeiten des Verfassers uber die rezenten Sedimente im Golf von Neapel befassten sich mit der mechanischen und mineralogischen Analyse der Sedimente des Golfes von Pozzuoli ~ U L L E R 1958), , einern Teilgebiet des Golfes von Neapel, sowie mit den Mineral-,New und Umbildungen durch Halmyrolyse in den Bodensedimenten innerhalb des gesamten Golfbereiches (MULLER, 196la). Die folgenden Ausfiihrungen sind die Ergebnisse einer Erweiterung der bereits irn Golf von Pozzuoli durchgefuhrten granulometrischen Untersuchungen auf den gesamten Golf von Neapel, wobei das Untersuchungsgebiet zum offenen Meere hin durch die Verbindungslinie Ischia-Capri begrenzt wird. Untersuchungen von SINDOWSKI (1957), in die bereits unsere Ergebnisse aus dem Golf von Pozzuoli eingearbeitet wurden, beschranken sich auf Teile des nordlichen Golfes. Da Sindowski selbst “die Positions- und Tiefenangaben ungenau und die Probenahme mit dem Schopfeimer primitiv” bezeichnet, wurde von einer Ubernahrne der Daten abgesehen. Wie bei den vorangegangenen Untersuchungen beziehen sich unsere Resultate auf die obersten 10-15 crn der Sedimentschicht, die bei der hohen Sedimentationsrate irn Golf einen Zeitraum von max. 500 Jahren umfassen diirfte. Die Probenahme erfolgte mit einem Sedimentgreifer bzw. einem Fall-Lot, mit dessen Hilfe bis 1 m lange Kerne entnomrnen werden konnten. Fur vielfache Hilfeleistungen und Anregungen bin ich Herrn Dr. Peter Dohrn, Zoologische Station in Neapel, sowie Herrn Prof. Dr. W. v. Engelhardt, Tubingen, zu grossem Dank verpflichtet. Der Deutschen Forschungsgemeinschaft danke ich fur eine finanzielle Beihilfe.

GEOGRAPHIE; STRUKTUR DES MEERESBODENS

Der Golf von Neapel ist eine nach dem Siidwesten offene Bucht, die von den Inseln Ischia und Procida, den Phlegraischen Feldern, dern Sornma-Vesuv, der Halbinsel Teil 3 der Untersuchungen iiber “Die Rezenten Sedimente im Golf von Neapel” (MULLER,1958, 1961a).

KORNGROSSENVERTEILUG I N REZENTEN SEDIMENTEN

283

Abb. 1 . Der Golf von Neapel; no. 1-86 Proben-Entnahmepunkte. Zeichenerklarung: 1 = vonviegend leucittephritische Gesteine; 2 = vonviegend trachytische Gesteine; 3 = mesozoische Karbonatgesteine; 4 = terrigener Flysch.

Sorrent und der Insel Capri umrahmt wird. Grossere Bache oder Flusse miinden nicht in den Golf ein (Abb. 1). Der Meeresboden des Golfes gehort in den Bereich des kontinentalen Schelfs, in den von der Tiefsee des Tyrrhenischen Meeres her tiefe Graben eingeschnitten sind: der Magnaghi-Graben, ubergehend in die Procida-Rime und der Anton DohrnGraben mit seiner Fortsetzung in der Ammontatura- und Walther-Rime. Zwischen den Graben ist ein Teil der ehemaligen Schelfplatte erhalten geblieben: die Secca di Bocca Grande oder Lobianco-Platte. Auf dem Golfboden treten mehrere unterrneerische Banke auf, die z.T. Vulkanbauten darstellen: z.B. die Tauben-, Nisida-, Misenound Ischia-Bank. Die in der Tyrrhenis von Siid nach Nord verlaufende nichtperiodische kiistenparallele Oberflachenstromung mit Geschwindigkeiten von weniger als 25 cm/sec kann zwischen Capri und Sorrent in den Golf eintreten und diesen wieder bei Procida verlassen, eine der Oberflachenstromung (bis 100 m Wassertiefe) gleichgerichtete Stromung im Zwischenwasser (100-600 m Wassertiefe) vermag durch den Anton DohrnGraben in den Golf zu gelangen (SINDOWSKI, 1957; dort auch weitere hydrographische Daten).

284

G. MULLER GEOLOGIE U N D PETROCRAPHIE DER GOLFUMRANDUNG

Die Umrandung des Golfes wird vorwiegend aus vulkanischen Gesteinen, insbesondere aus Tuffen aufgebaut. Lediglich der Hauptteil der Halbinsel Sorrent und die Insel Capri bestehen aus mesozoischen Kalken und Dolomiten. Untergeordnet treten dort auch terrigene Flyschsedimente auf. Die vulkanischen Gesteine der Golfumrandung gehoren den drei grossen Vulkanprovinzen Ischia, Phlegraische Felder und Somma-Vesuv an. Es kann angenommen werden, dass vulkanische Tuffe iiber 90 % des gesamten vulkanischen Materials in der Golfumrandung ausmachen. Sie bestehen zu ca. 85-95 % aus vulkanischem Glas rnit einer Lichtbrechung von 1,50-1,58, den Rest bilden meist idiomorphe Einsprenglinge aus Feldspaten, Pyroxenen, Leucit, Magnetit und Biotit (MULLER, 1958).

DIE ERGEBNISSE DER MECHANISCHEN ANALYSE

Die wichtigsten Ergebnisse der mechanischen Analyse sind in Tabelle I (nach zunehmender Wassertiefe geordnet) enthalten. Die Auswertung zeigt, dass zwischen den einzelnen Parametern und der Wassertiefe und damit (in unserem Falle) auch von der Entfernung vom Ufer einfache Beziehungen bestehen. Die durchschnittliche Korngrosse (Median) der Sedimente Bereits aus Tabelle 1, deutlicher jedoch aus Abb.2, geht hervor, dass der Median mit zunehmender Wassertiefe abnimmt. Die hochsten Werte (max. 1,82 mm) liegen im

.. :\ . * \ 9001-

\

1

-m

.

Abb.2. Der Median in Abhangigkeit von der Wassertiefe. E-E Proben aus dern ostlichen Golfteil.

KORNGR~SSENVERTEILUNGIN REZENTEN SEDIMENTEN

285

TABELLE I ABHANGIGKEIT DER KIES-, SAND-, SILT- UND TON-FRAKTIONEN SOWIE DES MEDIANS UND DES SORTIERUNCS-

KOEFFIZIENTEN VON DER WASSERTIEFE~ .

~~

~

Wassertiefe m .

~~

Probe No.

Sand

Silt

Ton

Median

%

%

%

mm ____

~

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0

0

0 0 5 9 10 10 12 14 14 15 18 18 19 21 25 27 30 33 35 35 37 38 40 42 43 50

6 23 24 25 26 27 28 29 31 39 40 47 50 51 53 54 74 76 82 84 85 86 16 30 5 38 7 17 22 1 4 48 19 46 8 37 44 3 21 75 20 49 41 43 45 81

so

100,o 99,7 99,8 98,2 98,7 96,O 99,8 99,7 60,8 98,l 99,I 95,s 85,8

99,4 99,8 96,5 99,4 97,4 87.2 99,s 94,9 97,2 53,7 97,3 29,7 89,O 84,3 90,9 82,s 50,2 52.8 46,8 60.9 91,9 31,9 91,5 76.9 39,O 98,7 23,4 55,7 20,3 42,l 64,o

38,3 39,7

0,350 0,290 0,270 0,780 0,380 0,950 0,320 0,310 0,380 0,280 0,420 0,300 0,570 0,340 0,370 1,220 0,380 0,360 0,600

0,230 0,830 0,300 1,820 0,440 0,O I6 0,120 0,220 0,I20 0,780 0,210 0,028 0,070 0,078 0,300 0,038 0,160 0,I50 0,028 0,500 0,180 0,570 0,330 0,033 0,100 0,048 0,050

1,20 1,32 1,31 1,20 1,85 1944

1,16 1,18 1,41 1,23 1,18 1,15 I ,57 1,23 1,27 1,30 1,30

1.48 1.48 1,27 1,47 1,30 I ,90 1,41 2,49 1.38 1,61 1,33 2,30 2,22 2,03 2,45 1,62 1,61 4300 1,34 1,76 1.74 1,33 3,5? 1,41 2,74 3,16 2,45 7,90 2,o1

_______

Hier benutzte Korngrossenklassifikation: Kies < 0,002mm.

mm, Ton

> 2,O mm, Sand 2,&0,063 mm, Silt 0,0634,002

286

G . MULLER

TABELLE I (Fortsetzug) ABHANGIGKEIT DER KIES-, SAND-, SILT-, U N D TON-FRAKTIONEN SOWIE DES MEDIANS U N D DES SORTIERUNGSKOEFFIZIENTEN VON DER WASSERTIEFE

Wassertiefe 1)1

50 53 55

56 62 65 65 75 76 77 80 83 91 92 98 98 100 103 105 110 120 120 125 135 140 140 140 150 170 190 210 230 250 255 300 300 450 500 650 800

Probe No. 83 2 9 32 36 42 10 12 35 11

33 18 15

52 13 34 60 14 80 79 71 77 70 78 58

68 73 56 72 69 61 67 55

64 57 62 63 65 59 66

Kies O/

/O

Sand

Silt

01

01

10

/O

35.7 9,7 36,8 13,8 16,3 17,8 45,3 9, I 8,6 37,3 24,2 11,7 63 971

52,3 72,9 42,9 74,5 71,9 66,9 37,9 67,3 75,9 48,s 61,9 69,4 69,3

10,6

63,9 68,7 68.8 60,4 18,7 30,s 41,7 16,l 35,4 39.9 37.9 36,2 41,3 43,l 31,l 34,7

13,l 392 3.4 60,9 57,O 34,1 16,2 30,3 44.8 23,9 33,6 27,l 32.8

55,l 27,l 6,O 22,5 6.7 830 23,9 11,9 1,7 3s 230 1,6

4095

51,O

35,2 45,3 45,O 31,6 32,l 42,3 37,9 37,9 21,l

Ton YO

10,o

17,l 15,4 11s 5,6 13,l 14,2 23,4 I4,2 11,s 13.1

18,6 24,2 50,4 25,l 17,6 28,O 36,2 892 12,3 21,2 72 34,2 15,2 38,l 30,l 31,4 23,l 13,2 38.2 43.0 42,2 47.8 47,O 441 56,O 56,O 59.0 60,l 77,3

Median mtn

so

0,030 0,02 1 0,300 0,022 0,026 0,023 0,020 0,012 0,017 0,019 0,024 0,025 0,012 0,002 0.01 2 0,020 0,010 0,0049 0,340 0,090 0,025 0,160 0,008 0,028 0,005 0,017 0,007 0,012 0,080 0,0055 0,0022 0,0038 0,0024 0,0026 0,0037 < 0,001 < 0,001 < 0,001 < 0,001 < 0,001

6,33 3.24 2,84 1964 1,58 2900 2,93 3,14 1,79 2.18 2.07 3.32 3.79 7.75 4.58 1,73 5.10,

4,80 4,90 3.00 640 132 1,35 6,08 6,33 10,00 7,28 7,90 2,83 > 10,oo 8,12 7.15 > 10,oo 8,45 10.95 6,33 6,48 6,oo > 10,00 > 10,00

unrnittelbaren Strandbereich, die niedrigsten (< 0,001 mm) in uferfernen Tiefen von unterhalb 300 m Wasserbedeckung. Die Abweichungen von der diese Regel wiedergebenden ausgezogenen Linie der Abb.2 sind z.T. betrachtlich; dies gilt insbesondere fur die Proben SO, 77, 79, 72, 71, 78 und 68 (in Abb.2 durch die Linie E-E hervorgehoben), die ausnahmslos im ost-

KORNGR~SSENVERTE~LUNG IN REZENTEN SEDIMENTEN

287

lichen Golfteil in einer Wassertiefe von 105-170 m vorkommen. Im Vergleich zu den aus ahnlichen Wassertiefen stammenden Proben des westlichen Golfbereiches sind diese bedeutend grobkorniger: Im Extremfall kann der Unterschied nahezu zwei Zehnerpotenzen ausmachen (Probe 14, 103 m, Md = 0,005 mm; Probe 80, 105 m, Md = 0,34 mm). Der Sortierungskoeflzient: So

=

dQ,/Q,

Der Sortierungskoeffizient zeigt eine deutliche Abhangigkeit vom Median. Mit abnehmendem Median wachst der Sortierungskoeffizient an, d.h., der Sortierungsgrad wird schlechter. Extrem schlechte Sortierung (So = > 10) ist in den uferfernen feinstkornigen Sedimenten zu beobachten, die Kustensedimente selbst und die ufernahen Proben zeigen sehr gute bis gute Sortierung. Die Fraktion

< 0,002 rnm

Abb.4 zeigt die Verteilung der Tonfraktion im Golfbereich. Die hochsten Gehalte (max. 77,25 % im Anton Dohrn-Graben bei 800 m Wassertiefe) liegen in den tiefsten Bereichen des Golfes, zum Ufer hin nehmen die Werte kontinuierlich ab, wobei die Zonen gleicher Prozent-Intervalle im Ostteil wesentlich breiter als im Westteil des Golfes sind. Dies bedeutet, dass die ostlichen Golfsedimente deutlich tonarmer (= grobkorniger) als die westlichen sind (vgl. hierzu die Medianwerte). Die Verteilung

Wass e r tie fe

I 0-70 ll 1 0 - 5 0 lZl 50 -1OU

\ /'' I

N

\

700-250

M500-750

Silt Abb.3. Das Sand (und KiestSilt-Ton-Verhaltnisin den Sedimenten.

288

G . MULLER

des Tonanteils spiegelt in groben Ziigen die Morphologie des Untergrundcs wider. Ein Tonanteil von 20-50 % ist iiber die grosste Flichc des Golfes verbreitet.' Sedimente von der Zusarnmensetzung eines Tonsilts bis sandigen Tonsilts sind somit det wichtigstc Sedirnenttypus im Golfgebiet. An nachster Stelle folgen zum Ufer hin die

289

KORNGR~SSENVERTEILUNGIN REZENTEN SEDIMENTEN

Sedimente mit einem Tonanteil von 5-20 % (hauptsachlich toniger Silt bis toniger Siltsand und Sandsilt) und zur Golfmitte hin feinkornige Sedimente mit einem Tonanteil, der grosser als 50% ist (vorwiegend Siltton). In der Uferzone tritt der Tonanteil vollig zuriick (vorwiegend Sand und Kiessand). Eine Ausnahme stellt die in der Ufernahe von Procida liegende Probe 52 mit 50,5 % Tonanteil dar. Das Sand-Silt-Ton- Verhaltnis In Abb.3 ist das Sand-Silt-Ton-Verhaltnis in den Bodensedimenten des Golfes dargestellt, wobei die fast ausschliefllich in den Strandproben auftretende Kiesfraktion (> 2,O mm) zur Sandfraktion (0,063-2,0 mm) zugerechnet wurde. Bemerkenswert ist jedoch, dass auch in tiefen uferfernen Proben z.T. noch geringe Kiesanteile auftreten. Die Abweichungen von den Grundlinien Sand-Silt und Silt-Ton sind z.T. betrachtlich. Es ist besonders augenfallig, dass eigentliche Silt-Sedimente vollig fehlen. Die Sedimente lassen sich nach Tiefenzonen geordnet in Gruppen einordnen, die im Stoffdreieck voneinander getrennte Felder einnehmen. Es ergibt sich damit die Moglichkeit einer natiirlichen Gliederung der Sedimente nach der Wassertiefe (Tabelle 11).

TABELLE I1 SEDIMENTTYPEN IM GOLF VON NEAPEL (BENENNUNG NACH -~

Crrrppe

Wassertiefe

I

& 10m

I1 I11 IV

5&100 m

V

VI

+ VII

MULLER,

1961b)

-

Vorherrschendes Sediiaient _ _ ~ _

1& 50m

1W250 m 25&500 m 500-750 m

Kiessand bis Sand Sand bis Sandsilt tonig-sandiger Silt bis toniger Sandsilt (E-Golf) Sandsiltton und Tonsilt bis toniger Siltsand (E-Golf) Siltton bis sandiger Siltton Siltton

Innerhalb der einzelnen Gruppen kann die Zusammensetzung noch betrachtlich variieren (in Gruppe 111 und IV bilden wiederum Sedimente aus dem ostlichen Golfteil die Extremwerte), die Schwerpunkte der einzelnen Gruppen liegen jedoch auf einer Linie, die vom Ufer zu grosseren Tiefen hin von Sand iiber Sand-Silt-TonMischtypen mit Siltvormacht in Richtung auf Ton verlauft. Der Rundungsgrad Der Rundungsgrad der einzelnen Korner in samtlichen Kornklassen ist schlecht (angular bis subangular). Die in der Strandzone angereicherten idiomorphen Einsprenglinge der Tuffe zeigen keine merkliche mechanische Beanspruchung.

290

G. MULLER DIE MINERALOGISCHE ZUSAMMENSETZUNC DER SEDIMENTE

Die Sedimente des Golfes bestehen zu iiber 85 "/o aus pyroklastischern Material (vorwiegend Glas, Feldspate, Pyroxene, Magnetit, Leucit) und den hauptsachlich aus dem Glas hervorgegangenen Mineral-, Neu- und -Umbildungen (Analcim, Kaolinit, Illit, Opal, Quarz; MULLER,196 1 a). Untergeordnet, jedoch lokal angereichert, treten Karbonat-Detritus (aus den mesozoischen Karbonatgesteinen) und Quarzsand (aus dem terrigenen Flysch) auf. Der organogene sedirnentbildende Anteil besteht vorwiegend aus Kalkalgen und Gehausen anderer kalkabsondernder Organismen. Dieser Anteil liegt irn Durchschnitt bei 8-120/, und ist in den uferfernen Proben hoher als in den ufernahen. Infolge der mechanischen Separierung sind die Einsprenglinge der Tuffe irn Strandbereich, die Neubildungsprodukte im kiistenfernen Bereich angereichert (MULLER, 1958, 1961a und in Bearbeitung). Aus der mineralogischen Zusarnmensetzung der Sedimente ergibt sich, dass der weitaus grosste Sedirnentanteil aus den Gesteinen der Golfurnrand ung entstammt.

DEUTUNG DER ERCEBNISSE

Der Golf von Neapel stellt einen abgeschlossenen Sedimentationsraum dar, der seine Materialzufuhr in ersten Linie aus der Zerstorung des Kiistenbereithes (insbesondere der aus relativ weichen Tuffen bestehenden Steilkiisten) durch die Tatigkeit des Meeres erhalt. Der grobklastische Anteil verbleibt im unmittelbaren Kiistenbereich und wird hier rollend weiter aufgearbeitet. Der feinklastische Anteil kann in die uferfernerer, Gebiete verfrachtet werden. Mit zunehrnender Wassertiefe und Entfernung vom Ufer nimmt der Median und der Sortierungsgrad ab, der Tonanteil zu. Die Sedimente des ostlichen Golfes sind bedeutend grobkorniger als die entsprechenden im Westteil. Wir mochten hier den Einfluss der zwischen Capri vnd Sorrent vom Siiden her in den Golf eintretenden Oberflachenstromung sehen, die im Ostteil des Golfes sicherlich weit starker als irn Nordwesten ist, wo sie durch die submarinen Schwellen zwischen Ischia und Procida (25 rn tief) und Procida und den Phlegraischen Feldern (10 m tief) sowie evtl. durch die von der Kiiste her komrnenden nordlichen Fallwinde stark gebremst wird. Diese irn Ostteil starkere Strornung kann die pelitische Sedimentation zumindest zu einem Teil verhindern. Die starke Abweichung von den Sand-Silt- und Silt-Ton-Grundlinieii irn Konzentrationsdreieck sowie das Fehlen des eigentlichen Silts deuten auf Storfaktoren hin die wahrend oder unmittelbar nach der Sedimentation wirksam waren. Wir glauben,, dass zwei Faktoren wesentlich hierfiir verantwortlich sind: ( I ) Die glasigen Anteile der anstehenden Tuffe haben wegen ihres verschieden hohen, nicht rniteinander in Verbindung stehenden Porenraumes verschiedene Dichten. Bei der mechanischen Aufbereitung und beirn Transport kann dies zu einer Vergesellschaftung von kleinen-schweren und grossen-leichten Kornern rnit gleicher

KORNGROSSENVERTEILUNG IN REZENTEN SEDIMENTEN

29 1

Sedimentationsgeschwindigkeit fuhren. Bimsstein im Kies-Korngrossenbereich kann so iiber den ganzen Golf verfrachtet werden und erklart den z.T. vorhandenen Kiesanteil auch der pelitischen Sedimente. (2) Von der halmyrolytischen friihdiagenetischen Umwandlung der Glassubstanz sind in erster Linie die Partikeln der feinkornigeren Fraktionen betroffen (MULLER, 196 la). Da die chemische Veranderung der Glassubstanz auch gleichzeitig mit einer Zerstorung des urspriinglichen Teilchens verbunden ist, zerfallt ein Siltkorn im Laufe der Zeit in mehrere Teilchen von Ton-Korngrosse. Aus einem relativ gut sortierten Sand-Silt-Gemisch kann so ein schlechter sortiertes und im Ganzen feinkornigeres Sand-Silt-Ton-Gemisch entstehen. In einigen wenigen Gebieten des Golfes tritt zu der halmyrolytischen Umwandlung eine weit starkere chemische, durch Austritt von heissen Losungen und Gasen aus dem vulkanischen Untergrund des Golfes in die Sedimente, die zur Bildung von Alunit und Schwefel fiihrt. Der hohe Tonanteil der Probe 52 in einem submarinen Krater vor Procida kann auf derartige Umwandlungen zuriickgefiihrt werden.

ZUSAMMENFASSUNG

Der Golf von Neapel stellt einen Sedimentationsraum dar, der seine Materialzufuhr in erster Linie von den im Uferbereich anstehenden Gesteinen, und hier insbesondere von den haufig Steilkiisten bildenden vulkanischen Tuffen, durch marine Erosion erhalt. Die Korngrosse der Sedimente nimmt vom Strand bis zu den uferfernen tiefsten Golfbereichen hin kontinuierlich ab, der Sortierungskoeffizient zu. Die Sedimente des ostlichen Golfes sind bedeutend grobkorner als die entsprechenden im Westgolf. Wahrscheinlich sind Oberflachenstromungen, die zwischen Capri und Sorrent in den Golf eintreten, hierfiir die Ursache. Im Sand-Silt-Ton-Dreieck zeigen die Sedimente eine von den Grundlinien SandSilt und Silt-Ton stark abweichende Zusammensetzung. Als Ursache kommen zwei Faktoren in Frage: (I) Die Glaspartikeln der Tuffe haben infolge verschieden hoher Porositat verschiedene Dichten, was zu einer Vergesellschaftung von grossen-leichten und kleinen-schweren Partikeln mit gleicher Sedimentationsgeschwindigkeit fiihren kann. (2) Bereits wahrend der Ablagerung beginnt die Umwandlung der Glaser durch Halmyrolyse. Hiervon werden insbesondere die feinkornigeren Partikeln betroffen. Durch den mit der chemischen Unwandlung gleichzeitig verbundenen mechanischen Zerfall eines Siltkorns konnen mehrere Partikeln im Ton-Korngrossenbereich entstehen. Dies erklart das Fehlen eigentlicher Silt-Typen.

SUMMARY

The sediment-accumulating basin of the Gulf of Naples is mainly supplied with clastic

292

G. MULLER

material eroded by wave action from the outcropping rocks, specially from pyroclastic rocks forming the cliffs, along the coast of the gulf. The grain-size distribution and the sorting depend on the depth of the water and the distance from the coast. The median decreases, the sorting coefficient and the clayfraction increase with increasing depth. The medians of the sediments in the eastern Gulf are lower as compared with the corresponding sediments in the western Gulf. This probably can be attributed to surface currents from the south entering the gulf between Capri and Sorrento. The field of mechanical composition strongly departs from the sides sand-silt and silt-clay in the sand-silt-clay-triangle. Two factors are believed to have caused this deviation: (I) The pyroclastic detritic material shows variations in its density due to the varying porosity of the glass forming the main part of the tuff-material. This may have led to an association of small-heavy and large-light particles with about the same velocity of sedimentation. (2) Already during the deposition of the pyroclastic glass the finer grains are affected by halmyrolytic processes which cause in the long run a decay of a silt-size grain into several grains of clay-size. This might explain why pure silt-size sediments do not occur in the Gulf of Napels.

LITERATUR

MULLER,G., 1958. Die rezenten Sedimente im Golf von Neapel. I. Die Sedimente des Golfes von Pozzuoli. Ceol. Rundschau, 47 : 1 17-1 50. MULLER,G., 1961a. Die Rezenten Sedirnente im Golf von Neapel. IT. Mineral-Neu- und -Umbildungen in den rezenten Sedimenten des Golfes von Neapel. Ein Beitrag zur Umwandlung vulkanischer Gliiser durch Halrnyrolyse. Beitr. MineraL Petrog., 8 : 1-20. MULLER,G., 1961b. Das Sand-Silt-Ton-Verhaltnis in rezenten marinen Sedimenten. Neues Jahrb. Mineral., Monatsh., 1961 : 148-163. SINDOWSKI, K. H., 1957. Bodengestalt und Bodensediment im Nordteil des Golfes von Neapel. Geol. Jahrb., 73 : 595-612.

UNTERSCHIEDE I N DEN CHEMISCHEN UND PHYSIKALISCHEN EIGENSCHAFTEN VON FLUVIATI LEN, BRACKISCHEN UND MARINEN SEDIMENTEN

w.

MULLER

Niedersachsisches Lnndesanrt fur Bodenforschung, Hanriover (Deutschland)

EINLEITUNG

Wie bekannt, unterscheiden sich fluviatile, brackische und marine Wasser durch unterschiedliche Salzkonzentration und Zusammensetzung der in diesen Wassern gelosten Salze. Im Flusswasser iibenviegen bei Salzgehalten unter 0,3O/, die Calzium-Ionen. Der Anteil der Natrium-Ionen ist nur wenig geringer, wahrend Magnesium- und Kalium-Ionen nur zu kleinen Anteilen im Flusswasser vorhanden sind. Zum marinen Wasser hin steigt der Salzgehalt um mehr als das hundertfache auf iiber 30°/, an. Hier dominieren bei weitem die Natrium-Ionen, gefolgt von den Magnesium-Ionen. Die relativen Anteile der Calzium- und Kalium-Ionen bleiben dagegen im Seewasser gering. Bekanntlich hangt die Gesamt-Artenzahl der Organismen sehr stark von diesen Salzgehaltsverhaltnissen ab, wie die entsprechende Kurve von Remane (Abb. I a) zeigt. Die Schwankungen der Artenzahl der Organismen wurden zu Gliederungen der verschiedenen Wasserarten verwendet. In Abb. l a ist eine solche Gliederung nach noch unveroffentlichten Mitteilungen von Hiltermann dargestellt.

DIE KATlONENSORPTlON AN SEDIMENTEN U N D BODEN

Weniger bekannt diirfte sein, dass die Kationen-Sorption der Sedimente ebenfalls stark von den erwahnten Veranderungen des Salzgehaltes und der Salzzusammensetzung in den See-, Brack- und Siisswassern beeinflusst wird. In Abb. 1 b ist die Kationensorption in .% der Sorptionskapazitat der Sedimente bei unterschiedlichen Salzgehalten dargestellt. Im Siisswasserbereich bei Salzgehalten um 0,3°/00 iiberwiegt die Sorption von Ca-Ionen, gefolgt von den Mg-lonen. Die einwertigen Na- und K-Tonen nehmen nur eitien geringen Anteil der Sorptions,:apazitat ein. Die Siisswassersedimente sind somit durch eine iiberwiegende Calzium-Svption und eine geringe Sorption der einwertigen Kationen charakterisiert. Zum Brackwasser hin - mit zunehmender Salzkonzentration - steigt die Sorption

w. MULLER

294

1

1-

<

SuG.\*r{_

-1-

-

I 1

Abb. la. Gesamt-Artenzahl der Organismen nach Remane (HILTERMAN,1949) in log. DarsteUung und Klassifikation der Suss-, Brack- und Seewasser nach HILTERM A N N (1963).

1

c. Kationensorption an Boden aus typischen

1 U

I

_

kackfl 01 g3h

I

II

1

-i

Tfl~

r -

~~

~

--

~~

Bra~b,ma-.:h

BF k n A ~ i n ~

p_

I

~~

5ezrarsrb

I:"ilElnR!

Kfi

Fluss-, Brack- und Seewassersedimenten nach Entwbserung, Entsalmng und Iangerer Durchluftung und Bezeichnung der Boden (Typen und Subtypen).

IS

---as

~.

d. Einige Grunddaten iiber Flockung (1) und Dispergierung (2) sowie die Lagerungsdjchte bzw. Durchlassigkeit (3) von Sedimenten des Fluss-, Brack- und Seewassers. (1) Gesanit-Kationen in mval/l. (2) Dispergierter Ton in yL Gesamtton. (3) Durchlaufzeit von 50 ml Fluss-, Brack- und Seewasser in Min. durch einen tonigen Schlick (45,3% Ton < 2 p).

.

e. Sinkstoffgehalt des Ernswassers in Abhangigkeit von der Salzkonzentration. (1) nach WILDVANG, 1938, (Probenahme nach stiirniischem Wetter), (2) nach DECHEND (1950). (Probenahmenach ruhigem Wetter). -

/

1

'

,y/

'I

/ /',/ , !.)

/

I','

,/' :,J -

'

/

,a

-4

,I

1

3

1

L 5

A b .?;J

I

1

L-

5 ' 6 .-So/zgeholtim " g ~ v j r nd A J T p i g s s e d i r e o f e

f. Wasserdurchlassigkeit ungestorter normaler Marschbodenschichten aus typischen, schluffigtonigen-tonigen Fluss-, Brackund Seewassersedimenten (nicht starker durchwurzelte Schichten und fossile Bodenbildungshorizonte sowie fast reine Sande und Schluffe).

UNTERSCHIEDE IN FLUVIATILEN, BRACKISCHEN UN D MARINEN SEDIMENTEN

295

der Mg-Ionen und die der einwertigen Ionen stark an, wahrend die Ca-Sorption entsprechend zuriickgeht. Bei Salzgehalten um 5O/,, liegt das Maximum der Mg-Sorption. Der gesamte brackische Sedimentationsraum ist charakterisiert durch eine erhohte Magnesium-Sorption; bei Salzgehalten etwa zwischen 1 und 1 lo/,, iiberwiegt die Mg-Sorption absolut. Die Sorptionsanteile der iibrigen Kationen lassen sich zu einer weiteren Unterteilung des brackischen Sedimentationsraumes heranziehen. Zum Seewasser hin mit nun kraftig zunehrnender Salzkonzentration steigt die Sorption der einwertigen Ionen auf Kosten der zweiwertigen Ionen stark an. Das Natrium-Ion dominiert nun bei weitem, wahrend die Anteile der sorbierten KaliumIonen fast die des Magnesiums erreichen. Die Sedimente des Seewassers sind somit durch eine iiberwiegende Natrium- und eine starke Kalium-Sorption charakterisiert. Die unterschiedliche Kationensorption in den verschiedenen Sedimentationsraumen l a s t sich auf Grund der bekannten Eintauschgesetze erklaren. Auf diese theoretischen Grundlagen sol1 jedoch hier nicht eingegangen werden. Von Bedeutung fur die chemische Charakterisierung der Sedimente ist noch die unterschiedliche Calzium-Karbonat-(CaCOJAblagerung. Der Kalkgehalt der kiistennahen marinen Sedimente ist hoch, er nimmt bei natiirlichen Faziesverhaltnissen im allgemeinen zum brackischen Sedimentationsbereich ab und wird ab etwa 3-5O/,, Salzgehalt praktisch bedeutungslos, wenn nicht kalkreiche Sedimente aus dem Hinterland der Fliisse antransportiert werden. Infolge Deichbauten und Fahrtrinnenvertiefungen, also infolge menschlicher Eingriffe, wird allerdings heute die Zone der Kalkablagering in den Aussendeichlandereien flussaufwarts verschobeii. Welche Veranderung der Kationenbelegung der Sedimente erfolgt nun durch die Bodenbildung? Wie ein Vergleich der Abb. l b und lc zeigt, bleiben bei den kalkfreien Siisswassersedimenten und dem grossten Teil der brackischen Sedimente die Relationen der sorbierten Kationen praktisch unverandert. Die unwesentlichen Verschiebungen sind durch die Entwasserung und Entsalzung bedingt. Bei den zunehmend kalkreichen brackisch-marinen und marinen Sedimenten steigt mit der Bodenbildung nach Entwasserung und Entsalzung und mit beginnender Kalkauswaschung die Sorption der Calzium-Ionen vor allem auf Kosten der sorbierten Natrium-[onen stark an. Wie Abb. lc zeigt, lassen sich eine ganze Reihe von Bodeneinheiten auf Grund der Kationenbelegung unterscheiden und verschiedenen Sedimentationsraumen zuordnen. Die Darstellung stiitzt sich auf die Durchschnittswerte von mehreren 10.000 Untersuchungsergebnissen. Zur Kontrolle wurden zahlreiche Diatomeenuntersuchungen durchgefuhrt.

DIE BEEINFLUSSUNGDER SINKSTOFFE IN VERSCHIEDENEN SALINITATSBEREICHEN

Die unterschiedlichen Salzgehalte und Salzzusammensetzungen der verschiedenen Sedimentationsraume fiihren nicht nur zu einer unterschiedlichen Kationenbeleguug der Sedimente und der aus ihnen entstehenden Boden, sondern auch zu deutlich unterscheidbaren physikalischen Eigenschaften dieser Substrate. Wie Abb. 1d, Kurve

296

w. MULLER

1 zeigt, steigt ab Salzgehalten von etwa 5 O / , an die Salzkonzentration stark an. Dies fiihrt zu einer zunehmenden Flockung der im Wasser mitgefiihrten Schwebstoffe. Umgekehrt tritt mit abnehmender Salzkonzentration bei der im brackischen Sedimentationsraum vorherrschendeii Sorption von Magnesium-Ionen und der starken Sorption von Natrium-Ionen a b Salzgehalten unter 5°/00 eine zunehmende Dispergierung auf. Die Kurve 2 auf Abb. 1d zeigt diese zunehrnende Dispergierungsneigung eines tonigen Substrates bei den jeweiligen Salzkonzentrations- und Sorptionsverhaltnissen. Die Flockung bei hohen Salzkonzentrationen fuhrt zur lockeren Lagerung der marinen Sedimente, wahrend die dispergierten brackischen Sedimente mit hoher Mgund Na-Sorption dagegen eine grol3e Lagerungsdichte aufweisen. Wir werden spater sehen, dass die dispergierten plattchenformigen tonigen und schluffigen Feinbestandteile dieser brackischen Sedimente sogar eine horizontale Orientierung, d.h. die grosstmogliche Lagerungsdichte, aufweisen. Die Siisswassersedimente zeigen dagegen infolge der hier iiberwiegenden Ca-Ionen-Sorption wieder eine geringere Lagerungsdichte ohne Orientierung der abgelagerten Feinbestandteile. Die unterschiedlichen Lagerungsverhaltnisse fiihren zu einer unterschiedlichen Wasserdurchlassigkeit der Sedimente. In der Kurve 3 auf Abb.ld ist die Durchlaufzeit einer bestimmten Menge von See-, Brack- und Siisswasser durch sonst vergleichbare See-, Brack- und Siisswassersedimente als Mass fur die erwahnten Lagerungsverhaltnisse dargestellt. Die Durchlaufzeit von Seewasser ist gering, sie steigt im Brackwasserbereich a b etwa 5°/00 Salzgehalt stark an und durchlauft etwa zwischen 3 und 0,5°/00Salzgehalt ein gewisses Maximum. Hier miisste also die Lagerungsdichte der Sedimente am grossten sein. Mit zunehmender Aussiissung nimmt die Durchlaufzeit dann wieder ab. Diese experimentellen Beobachtungen iiber die Dispergierungs- bzw. Flockungsneigung der verschiedenen Sedimente und ihre Lagerungsdichte kbnnen durch praktische Erfahrungen bestatigt werden. Abb. le zeigt den Sinkstoffgehalt in der Unteren Ems bei Ebbe in Abhangigkeit von der Salzkonzentration des Wassers und damit naturgemass auch von der Kationensorption an diesen Sinkstoffen. Die dargestellten Sinkstoffmessungen sind bei verschiedenen Witterungsperioden vorgenommen worden. Die Daten der Kurve 1 entstammen Probenentnahmen nach stiirmischer Nordwestwind-Wetterlage. Die Kurve 2 fusst auf Probenentnahmen nach langerer ruhiger Witterung. Die vorangegangene Sturmperiode hat die Zonen gleichen Salzgehaltes weit flussaufwarts verschoben. Aus dem Vergleich des Verlaufes beider Kurven lassen sich folgende Schlussfolgerungen ziehen: Der erste Anstieg der Sinkstoff kurve im brachyhalinen Seewasserbereich ist nicht von der ortlichen Salzkonzentration abhangig, sondern von den ortlichen Verhaltnissen. Bei der Ems liegt dieser erste Anstieg der Sinkstoffkurve im ostfriesischen Gatje in der Trichtermiindung der Ems. Nach diesem ersten Anstieg bleibt die Kurve iiber langere Strecken praktisch auf gleicher Hohe. Die logarithmische Verkiirzung der Abzisse venvischt diesen grossen Abstand etwas. Ab Salzgehalten von etwa 5°/00

UNTERSCHIEDE I N FLUVIATILEN, BRACKISCHEN UND MARINEN SEDIMENTEN

297

an steigen beide Sinkstoffkurven stark an und erreichen um lo/, das Maximum. Dieser Anstieg erfolgt bei beiden Kurven gleichsinnig in Abhangigkeit vom Salzgehalt, obwohl die Orte gleichen Salzgehaltes sehr weit auseinander liegen. Weiterhin decken sich beide Kurvenverlaufe sehr gut mit der zunehmenden Dispergierungsneigung, die in Abb.Id, Kurve 2 dargestellt ist. Der Schluss liegt somit sehr nahe, dass dieses Sinkstoffmaximum abhangig ist von der Salzkonzentration des Wassers und von der Kationensorption der Sinkstoffe. Es ist etwa gebunden an den oligohalinen und meiohalinen bzw. oberen und mittleren Brackwasserbereich mit Salzgehalten ungefahr zwischen 0,6 und 5O/,,, in dem bei den Sinkstoffen die Sorption der Magnesium- und Natrium-Ionen mehr als 50% der Sorptionskapazitat einnimmt. Die Wirkung sorbierter Natrium- und Magnesium-Ionen auf das Verhalten von Suspensionen ist, wie sich nach unseren Untersuchungen herausgestellt hat, in vieler Hinsicht vergleichbar. Diese Ionen rufen u.a. eine Stabilisierung von Suspensionen bzw. eine verlangsamte Sedimentation von Sinkstoffen hervor, wodurch es zu deren relativen Anreicherung, d.h. zu der Entstehung des Sinkstoffmaximums, kommt. Die iiberwiegende Calzium-Ionen-Sorption im Siisswasserbereich sowie die hohe Salzkonzentration im Seewasser fuhren dagegen zu Ausflockungen und damit zu einem schnelleren Absetzen der Sinkstoffe. Auf die kolloidchemische Erklarung dieser Vorgange soll hier nicht eingegangen werden.

LAGERUNGSDICHTE U N D LAGERUNGSART VON BODENBILDUNGEN

Es wurde bereits erwahnt, dass diese Erscheinungen - Dispergierung und Flockung - zu einer unterschiedlichen Lagerungsart der Sedimente fiihren. Diese Lagerungsart

bewirkt eine unterschiedliche Wasserdurchlassigkeit der Sedimente bzw. der aus ihnen entstehenden Boden. In Abb. If ist die Wasserdurchlassigkeit der aus verschiedenen Sedimenten hervorgegangenen Marschboden dargestellt. Es handelt sich um Durchschnittswerte aus rund 1.OOO Untersuchungen rnit mehreren 1.000 Messungen von Einzelproben. Der unterschiedliche Tongehalt der Boden ist bei dieser Darstellung nicht beriicksichtigt. Allerdings sind die Werte fast reiner Sande und Schluffe nicht aufgefiihrt. Ferner sind die Messergebnisse starker durchwurzelter Bodenschichten (mit jeweils hoherer Durchlassigkeit) und iiberschlickter subfossiler Bodenbildungshorizonte (sog. Dwoghorizonte mit jeweils geringer Durchlassigkeit) hier nicht dargestellt. Es zeigt sich, dass die Wasserdurchlassigkeit der Boden sehr stark von den erwahnten chemischen und physikalischen Eigenschaften der Sedimente und damit vom jeweiligen Sedimentationsraum abhangen. Die Korngrossenzusammensetzung der Sedimente spielt gegeniiber diesen Einfliissen nur eine untergeordnete Rolle. Die Boden, die aus brackischen Sedimenten hervorgegangen sind, weisen gegeniiber denen mariner oder fluviatiler Herkunft eine wesentlich geringere Wasserdurchlassigkeit auf. Abschliessend soll die unterschiedliche Lagerungsart der marinen, brackischen und

298

w. MULLER

fluviatilen Sedimente bzw. der daraus entstehenden Boden anhand von Diinnschliffbildern demonstriert werden. Es wurde bereits erwahnt, dass im brackischen Sedimentationsbereich bei Salzgehalten zwischen 0,5 und 5O/oo die sich dort einstellende Kationenbelegung zu einer Dispergierung der Sinkstoffe fiihrt. Hierdurch kommt es bei langsamer Sedimentation zu einer vorwiegend horizontal orientierten Ablagerung der plattchenformigen Feinstbestandteile und zur Entstehung eines relativ dicht gelagerten Einzelkornverbandes. Da die plattchenformigen feinsten Mineralteilchen das Licht nur in einer Schwingungsebene durchlassen, leuchten im folgenden Diinnschliffbild in polarisiertem Licht alle horizontal eingeregelten Teilchen auf. Es handelt sich hierbei um die erwahnten brackischen Ablagerungen bzw. um einen sog. Knickboden. Im marinen und fluviatilen Sedimentationsbereich ist diese Einregelung infolge Flockung bei der Sedimentation nicht vorhanden.

Abb.2. Dunnschliffbild eines brackischen Sedimentes (Knickboden), x 290; Aufnahme in polarisiertem Licht. Die horizontal ausgerichteten mineralischen Feinstbestandteile leuchten hell auf.

ZUSAMMENFASSUNG

Die unterschiedliche Salzkonzentration und die verschiedenartige Zusammensetzung der Salze in Suss-, Brack- und Seewassern verursachen unter anderen auch eine sehr unterschiedliche Kationen-Sorption an den jeweiligen Sedimenten. Im Susswasser werden iiberwiegend Calzium-Ionen, im Brackwasser Magnesium-Ionen und im Seewasser Natrium-Ionen sorbiert. Weiterhin ist die abnehmende Kalkablagerung vom marinen zum brackischen Sedimentationsraum von Bedeutung. Die Kationensorption von fluviatilen und brackischen Sedimenten verandert sich durch Bodenbildungsvorgange nur wenig. Bei den marinen Sedimenten verdrangen dagegen bei der Boden-

UNTERSCHIEDE I N FLUVIATILEN, BRACKISCHEN UND MARINEN SEDIMENTEN

299

bildung die Calzium-Ionen die iibrigen sorbierten Ionen - vor allem die NatriumIonen - sehr stark. Die unterschiedliche Salinitat in Suss-, Brack- und Seewassern beeinflusst nicht nur die chemischen, sondern damit im Zusammenhang auch die physikalischen Eigenschaften der Sedimente und der daraus entstehenden Boden. Bei hoher Salzkonzentration erfolgt eine Flockung der Sinkstoffe, was eine lockere Lagerung und eine hohe Wasserdurchlassigkeit der Sedimente und Boden zur Folge hat. Im brackischen Sedimentationsbereich tritt vor allem bei Salzgehalten zwischen 5 und 0,5O/, eine Dispergierung der Sinkstoffe auf. Hierdurch kommt es zu verlangsamter Sedimentation und zur Entstehung eines Sinkstoffmaximums im Wasser, weiterhin zu einer horizontal orientierten Ablagerung der feinsten plattchenformigen mineralischen Sinkstoffteilchen. Derartige Sedimente und Boden weisen daher eine grosse Lagerungsdichte und eine geringe Wasserdurchlassigkeit auf. Die Calzium-Sorption im Siisswasser fuhrt dagegen wieder zu einer gewissen Flockung, wodurch Sedimente und Boden eine geringere Lagerungsdichte und eine grossere Wasserdurchlassigkeit zeigen.

SUMMARY

The variable salinity and the different composition of salts dissolved i n fresh water, brackish water and sea water are, among other things responsible for great differences in the cation adsorption in the particular sediments. In fresh water calcium ions are predominantly adsorbed, in brackish water magnesium ions and in sea water sodium ions. Another factor of importance is the decrease of calcium carbonate deposition from the marine to the brackish water environments. The cation adsorption of fluviatile and brackish sediments is only insignificantly affected by soil forming processes. In the case of marine sediments the soil formation leads to a strong base exchange, whereby calcium ions replace to a large extent the other adsorbed ions, especially those of sodium. The variable salinity in fresh, brackish and marine waters influence the chemical properties of the sediments and the soils originating therefrom and, in consequence, also their physical properties. In the case of high salt concentrations a flocculation of the sedimentary material takes place, resulting in a loose bedding and a high permeability of sediments and soils. In the brackish zone a dispersion of the sedimentary material occurs, particularly at salinities of 5 - 0,5°/0,. This leads to a lowered rate of deposition, and to the formation of a maximum of suspended material in the water. It also brings about, after deposition, a pronounced horizontal orientation of clay flakes and other small flat or elongate particles. Such sediments and soils show a high degree of compaction and a low permeability. In fresh water, on the other hand, the calcium adsorption again leads to a certain flocculation, so that sediments and soils here show a lower degree of compaction and a higher permeability.

300

w. MULLER LITERATUR

DECHEND, W., 1950. Die geologischen Untersuchungen in der Ems 1948149. Ber. Forschunpsstelle Norderney . HILTERMANN, H., 1949. Klassifikation der natiirlichen Brackwasser. E r d d Kiihle, 2 :4-8. HILTERMANN, H., 1963. Erkennung fossiler Brackwasser-Sedimente unter besonderer Beriicksichtigung der Foraminiferen. Forrschr. Geol. Rheinland Westfalen, 10 : 49-53. MULLER,W., 1954. Untersuchungen uber die Bildung und die Eigenschaften von Knickschichren in Murschbiiden. Diss., Universitat Giessen, 21 3 pp. MULLER,W., 1959. Besonderheiten der Marschboden durch den Einfluss des Meeres wahrend ihrer Entstehung. 2.Pjanzenernahr. Dung. Bodenk., 84, 129 (1-3) : 289-296. H., 1963. Der Einfluss sorbierten Magnesium auf die Eigenschaften MULLER,W. und FASTABEND, von Sedimenten und Boden. Ceol. Jahrb., zur Presse. VAN SCHUYLENBORGH, J. en VEENENBOS, J. S., 1951. Over de invloedvan magnesium op de struktuur van sedimenten. Landbouwk. Tijdschr. 63 (1 1) : 709-714. WILDVANG, D., 1938. Die Geologie Ostfrieslands. Abhandl. Prerrs. Geol. Landesnrnres, 181 : 21 1 pp.

THE TECTONIC FRAMEWORK O F CARBONIFEROUS SEDIMENTATION IN SOUTH WALES T.

R.

OWEN

Departtiretit of Geology, University College of Swansea, Swansea (Great Britain)

INTRODUCTION

The major structural elements in South Wales are shown in Fig. I . The most important is the east-west downfold of the South Wales Coalfield and its western continuation, the Pembrokeshire Coalfield. The main basin is separated by the Usk anticline from the Forest of Dean Coalfield. Numerous caledonoid belts of folding and faulting occur in South Wales. They comprise the Towy anticlinorium, the Neath disturbance and the Careg Cennen disturbance. South of the Bristol Channel lie the more intensely folded Devonian and Carboniferous areas of north and central Devon respectively. The majority of the structural elements listed above were the products of the Armorican (Hercynian or Variscan) Orogeny which reached its climax in South Wales, as in many other parts of the British Isles, at the close of Carboniferous times. This climax was however preceded by numerous preliminary movements, and the main purpose of this brief survey is to show that almost all the above-mentioned elements were gradually growing during these pre-climax phases. These major structures were growing structures. Moreover they influenced sedimentation as they grew, and the sedimentation in turn affected the ultimate form of the structures.

THE DEVONIAN FRAMEWORK

The earliest setting for the development of these structures can be assumed to be the broad Old Red Sandstone cuvette of South Wales and the Welsh Borderland. This area of essentially non-marine sedimentation merged southwards into the Devonian sea of southwest England, with apparently no major intervening barrier. Faint signs in Pembrokeshire, however, of what was to come, are the probable northward carriage of pebbles in the Ridgeway Conglomerate (PRINGLEand GEORGE,1948, p.48) and the rapid thickness changes in the Cosheston Beds (GEORGE,1962, p.26). Whereas in Upper Devonian times greater thicknesses of sediments were accumulating beyond the Bristol Channel, by Dinantian times the greatest deposition had moved northwards towards South Wales (Fig.3~).Deposition admittedly was of a highly calcareous character, but some mud was by now being transported northwards,

302

T.

BRISTOL

Scale

R. OWEN

CHANNEL

(Devonion)

Carboniferous

Fig.1. The major structural elements of South Wales. 1 = the main coalfield basin. 2 = the Pembrokeshire Coalfield. 3 = the Forest of Dean Coalfield. 4 = the Usk anticline. 5 = the Towy anticlinorium. 6 = the Neath disturbance. 7 = the Careg Cennen disturbance. 8 = the Lower Severn axis.

occasionally as far as southernmost Penibrokeshire (GEORGE,1958, p.251) and more continuously into north-central Devon (GEORGE,1958, p.252). The northern margin of the South Wales depositional area in Dinantian times is well indicated by the increasing terrigenous character of the sediments of the North Crop, especially in its eastern half, and by the “pulsatory uplifts of a St. George’s Land” (GEORGE,1962, p.29) causing marked mid-Dinantian breaks, particularly in the west (GEORGE,1958, p.259), where the southward rate of thickening was also greatest. Apart from slight volcanic activity during this time there is no hint of Early Dinantian growth along the Usk fold. By mid-Visean times, however, there was (a) increasing subsidence, with sandy infilling, along troughs immediately to the east of the upfold (GEORGE,1958, fig.9, p.260) and (b) probably actual upwarping and erosion along the eastern flanks of the present South Wales Coalfield with sandy and pebbly “Millstone Grit”-type material coming in, from northeasterly directions, well before the end of Dinantian times (OWENand JONES, 1961, p. 248). By the very beginning of Upper Carboniferous (ix., Silesian) times, the form of the Usk upfold had been appreciably completed (GEORGE,1956, fig. I ) and moreover the South Wales and Forest of Dean basins had already, by folding and erosion, been separated. The growth of the Neath disturbance can certainly be traced back to mid-Dinantian times, as shown by George (1954, fig.9). Fig.2a summarises these major thickness

TECTONIC FRAMEWORK OF CARBONIFEROUS SEDIMENTATION IN WALES

a

I

I 1Thickness

northwest

. Base of chanaes of

Carboniferous

DP

303

I /

limestone

1

south -

east

Fig.2. The early growth of the Neath disturbance. a. Thickness changes in Dinantian zones, near Brynmawr (after GEORGE, 1954, fig. 9). b. Thickness changes in D, limestone, near Glynneath. G . Glynneath. c. Contrast in Upper Visean and Namurian successions across the disturbance, near Glynneath.

variations in mid-Dinantian zones near the disturbance. OWEN(1954, fig.9) has shown that this caledonoid belt was also active in late-Dinantian times, but more recent studies (OWENand. JONES, 1961, p.248) tend to show that these thickness changes reveal a contemporaneous gravel shoal or shallow southwestward developing headland rather than one period of warping and erosion ( O m and JONES, 1955, fig. 3). Fig. 2b and 2c illustrate the changes which occur near and across the Neath disturbance in the vicinity of Glynneath. Some features of the grit-limestone contact even suggest a contemporaneous, seismic shuddering along the main fault of the disturbed belt. Further contemporaneous movement along the disturbance is suggested by the contrast in the Lower Namurian successions on either side of the belt (Fig.2~).

304

T. R. OWEN THE NAMURIAN FRAMEWORK

The continued growth of St. George’s Land to the north of the South Wales area of deposition during Namurian times is revealed by the coarser and arenaceous character of much of the North Crop succession, in contrast to the more muddy sequences of the South Crop. The line of maximum thickness had by now however spread very close to South Wales and moreover some of the grits in Cower and in south Pembrokeshire show evidence of northward carriage. This kind of deposition differs from that of Devon where the E-R, sequence is mostly extremely thin (about 100 ft.) and consists of slowly accumulating black muds and pale siltstones (PRENTICE, 1960, p.264). There is thus a possibility of some (perhaps slight and temporary) barrier between South Wales and north-Central Devon. Appreciable uplift and erosion of an unknown area to the south of central Devon towards the upper part of Namurian times is indicated by the incoming of the post-R, Instow turbidites. Because of lack of information it is difficult to reconstruct thickness variation diagrams for the Namurian of South Wales. Nevertheless, the marked thickening which takes place towards the South Crop and the thin developments along the eastern flank of the coalfield indicate that the isopachyte pattern might not be very different from that of the overlying basal Coal Measures (Fig.3a). Contemporaneous movements east of the coalfield were responsible for this attenuated sequence along the East Crop.

THE WESTPHALIAN FRAMEWORK

It was during the lower portion of Westphalian time that the structural form of the South Wales Coalfield was to be really anticipated. The line of maximum thickness had now moved to lie almost along the South Crop. Moreover “a close parallelism exists between the trend of the isopachytes and the present rim of the coalfield on its eastern and northwestern flanks” (LEITCHet al., 1958, p. 482). The Usk anticline was now probably a shoal of relatively slow subsidence and sedimentation over this area, when it occurred, was very slow during much of Namurian and Ammanian times. Continual scouring and winnowing of sediment to other areas maintained a constantly attenuated sequence on this eastern flank of the coalfield. Detailed studies of the sediments of these basal Coal Measures further reveal this early “moulding” of the coalfield basin. Sandy material came in from different directions and a contemporaneously active Towy anticline may have been a contributing source. Recent (as yet unpublished) work by B. J. Bluck has shown that the palaeocurrent directions point inwards around the coalfield (Fig.3a) and that during one particular cyclothem an important source of supply lay not far to the south of the present southern margin of the coalfield. It was probably the rise of this “kraton” (presumably along the present Bristol Channel) that caused a marked change of sedimentation, with material coming in from the north, in central Devon (PRENTICE, 1962, p.107).

TECTONIC FRAMEWORK OF CARBONIFEROUS SEDIMENTATION IN WALES

North Crop I

2

South Crop I

North-Central

305

Devon

I

I

Y

I I

*"T

Fig.3. The early growth of the South Wales Coalfield. a. Isopachyte diagram for the basal Coal Measures. Thicknesses in feet. Arrows indicate palaeocur-

rent directions. b. Isopachyte diagram for the Middle Coal Measures. c. Thickness variations in the Upper Devonian-Lower Westphalian successions of South Wales and northxentral Devon. The arrows indicate direction of transport. T = turbidites. L.C,M. = Lower Coal Measures. The letters V to Z indicate the position of the maximum thickness as Carboniferous times progressed. Y = Maximum thickness of Middle Coal Measures. Z = Maximum thickness of the Upper Coal Measures.

306

T. R. OWEN

Even the southern side of the South Wales Coalfield was by now beginning to be defined. The pattern was continued during the remainder of Ammanian times with one important difference. The centre of maximum subsidence seems to have moved even further northwards ( Y in Fig.3b). The presence of a southward-tilted palaeoslope for turbidites of Communis Zone or higher age in Devon (PRENTICE, 1962, p. 107) means that the South Wales and Devon basins of deposition continued to be separated by a belt that was either emergent or loathe to subside. By Upper Coal Measure times, important changes in sedimentation occurred and the molasse-like Pennant sandstones began to accumulate, not only in South Wales but in the Bristol and Kent coalfields also. This surely indicates the rise of a mountain front not too far to the south of these areas. The Armorican Orogeny may in fact have already reached its climax in southwest England (OWEN, 1961, p.487). Studies by G. Kelling are showing that the lower portion of the Pennant Sandstone of South Wales was derived largely from southerly (and along the East Crop from easterly) sources. Thickness changes in the highest Coal Measures of South Wales are difficult to reconstruct, but there are indications that the centre of greatest subsidence was even nearer to the northwestern margin of the coalfield. It had in fact moved into the great anthracite belt and the possibility of appreciable thicknesses of Stephanian deposits having accumulated here must not be overlooked. Late Carboniferous deposition may have spread itself northwards on to St. George's Land, just as it did (southwards) over the north Midlands.

SUMMARY

Some of the main structural elements in South Wales probably grew during Carboniferous times and controlled sedimentation. This development, and control, is traced through Avonian, Namurian and Westphalian times, the final structural form being produced by the Armorican Orogeny.

REFERENCES

GEORGE, T. N., 1954. Pre-Seminulan Main Limestone of the Avonian Series in Breconshire. Quart. J . Geol. SOC.London, 110 : 283-322. GEORGE, T. N., 1956. The Narnurian Usk Anticline. Proc. Geologists' Assoc. Enyl., 66 : 297-3 16. T. N.. 1958. Lower Carboniferous palaeogeography of the British Isles. Proc. Yorkshire GEORGE, Geol. SOC.,31 : 227-318. GEORGE, T. N., 1962. Devonian and Carboniferous foundations of the Variscides in northwest Europe. In: K . COE(Editor), Some Aspects ofthe Variscun Fold Belt. Manchester Univ. Press,ManChester, pp. 1 9 4 7 . LEITCH,D., OWEN,T. R. and JONES, D. G., 1958. The basal Coal Measures of the South Wales Coalfield from Liandybie to Brynmawr. @cart. J . Geol. SOC.London, 113 : 461483. OWEN,T. R., 1954. The structure of the Neath disturbance between Bryniau Gleision and Glynneath, South Wales. Quart. J . Geol. SOC.London, 109 : 333-359.

TECTONIC FRAM!WORK

OF CARBONIFEROUS SEDIMENTATION IN WALES

307

OWEN,T. R., 1961. Age of the orogeny and granites in southwest England. Nuture 191, (4787): 486-487. OWEN.T. R. and JONES,D. G., 1955. On the presence of the Upper Dibunophyllum Zone ( D J near. Glynneath, South Wales. Geol. Mag.392 : 457464. OWEN,T. R. and JONES, D. G., 1961. The nature of the Millstone Grit-Carboniferous Limestone junction of a part of the North Crop of the South Wales Coalfield. Pro?. Geo/qpists’ Assor. Engl., 72 : 239-249. PRENTICE, J. E., 1960. The Dinantian, Namurian and Westphalian rocks of the district southwest of Barnstaple, north Devon. Quurf.J. Geol. SOC.London, 1 1 5 : 261-289. PRENTICE, J. E., 1962. The sedimentation history of the Carboniferous in Devon. In: K. COE(Editor). Some Aspects of the Vuriscan Fold Belt. Manchester Univ. Press, Manchester, pp. 93-108. PRINGLE, J. and GEORGE, T. N., 1948. British Regional Geology. Geol. Surv. Gt. Brit., London, 100 pp

THE FRIO SEDIMENTATION O F THE RAYNEFIELD O F SOUTHWEST LOUISIANA W . R. P A I N E

University of Southwestern Louisiana, Lafayette (U.S..4.)

The Frio Formation is one of the most varied and prolific producing sections in southwest Louisiana. Even though some 5,000 wells in Louisiana have penetrated this section, little is known of its exact lithology and sedimentational history except from the electric logs, cuttings, and cores. In 1954 the discovery of the Rayiie field stimulated interest in the Frio and its stratigraphy and sedimentation. As a result, Continental Oil Company cored nearly all of the 20 wells in this field in some interval of the Frio Formation. The Frio consists of 5,000-10,000 ft. of alternating marine sands and shales which were deposited in a regressioiial condition and which can be divided into two facies, the updip one which thickens at a relatively uniform rate, and the downdip embayment section which thickens across growth faults (see below). At first, the lithology and thickness of the two facies appear totally different, but detailed study now indicates that only the thickness changes markedly. The two major “embayment” areas recognized in southwcst Louisiana are the Hackberry embayment, Jefferson Davis and Calcasieu Parish, and the Nodosaria ernbayrnent, Acadia and Lafayette Parishes. The Rayne field is located in the latter. Fig.1 shows an isopachous niap of the Nodosaria embayment and Fig.2 shows a cross-section of this embayment through the Rayne field area. From this it can be seen that the thickness in the northern part is relatively uniform, but south of a line through North Crowley, Northwest Branch, and South Lewisburg fields, thickening increases rapidly. This line has been termed by OCAMB (1961) as “the updip limit of growth faults” (Fig.3). The concept of contemporaneous faulting and sedimentation or growth faulting and GRIGG (1954), and later expanded by was first recognized in the area by OCAMR GRIGG (1956), PAINE(1956) and OCAMB (1961). Growth faults are defined by OCAMB ( 1961) as those faults which have a substantial increase in throw with depth and across which there is a great thickening of correlation section, from the upthrown to the downthrown block. These faults generally are arcuate and outline the edge of the depositional basin. As the depocenter shifts southward, the zone of fault activity shifts southward also. Therefore each successively farther downdip fault block reaches its maximum tectonic activity and sedimentation higher in the geologic column. The concept of growth

311

mottled brown grey green = light = black dark =

Hardness:

verypoor poor = fair =good

FS FH H VH

s.

VP P F G

Sorting:

soft fairly soft = fairly hard = hard = very hard

Other columns:

Trace minerals:

angular sub angular subrounded

A SA

Roundness:

=

Glauconite Ferromagnesian = Pyrite = Hematite = Mica = Limonite = Garnet = =

= =

=

very =fine nied = medium cse =coarse gnd =grained gns = grains qtz = quartz lg = large

f

v

7

1 2 3 4 5 6

SR

nonporous verypoor =poor = fair =good :

=

NP VP P F G Porosity :

(pp.312-315).

Fig.2. Cross-section through the Rayne field area.

=

=

=

7

~

=

=

~~

~

~

~

noncalcareous slightly calc. calcareous = highly calc.

mott bn gY gn It blk dk

Calcareous content: NC SC C HC

Color:

ABBREVIATIONS TO TABLE I

faulting implies fault development in an area of rapid, fairly shallow water deposition of the inner to middle shelf. Therefore careful examination of cores from these growth fault intervals should give additional information pertaining to the depositional environment of these intervals. Although many intervals have been cored, the following description of 61 ft. of the Homeseeker “C” sand interval from Continental Oil Co., Marie Navarre no. 1A, Rayne field, was chosen as typical of the growth fault

Fig.3. Location map.

THE FRlO SEDIMENTATION O N THE RAYNE FIELD OF LOUISIANA

E

THE FRlO SEDIMENTATION O N THE RAYNE FIELD OF LOUISIANA

311

Fig.3. Location map.

faulting implies fault development in an area of rapid, fairly shallow water deposition of the inner to middle shelf. Therefore careful examination of cores from these growth fault intervals should give additional information pertaining to the depositional environment of these intervals. Although many intervals have been cored, the following description of 61 ft. of the Homeseeker “C” sand interval from Continental Oil Co., Marie Navarre no. 1A, Rayne field, was chosen as typical of the growth fault ABBREVIATIONS TO TABLE I

Color:

mott bn gY gn It blk dk

Calcareous content: NC SC C HC Sorting:

Hardness:

VP P F G

s. FS FH H VH

mottled brown grey green = light = black dark =

(pp.312-315).

Porosity :

~

~

~

~~

noncalcareous slightly calc. calcareous = highly calc.

Roundness:

NP VP P F G

nonporous verypoor =poor = fair =good

A SA

=

SR

=

:

=

= =

angular sub angular subrounded

= ~

Trace minerals:

verypoor poor = fair =good 7

=

soft fairly soft = fairly hard = hard = very hard =

=

Other columns:

1 2 3 4 5 6

7

Glauconite Ferromagnesian = Pyrite = Hematite = Mica = Limonite = Garnet

v

=

= =

very =fine nied = medium cse =coarse gnd =grained gns = grains qtz = quartz lg = large

f

Fig.2. Cross-section through the Rayne field area.

W I

N

TABLE I DETAIL LITHOLOGY OF THE HOMESEEKER SAND INTERVAL, RAYNE FIELD

_ Po,.. Hard. Calc. atid Sortiny Rorrtitfties~ Perm. ~

~

-

-

11,785 clayey silt 86 missing 87 missing 88 missing 89 sand coarse grain 11 -790 sand coarse grain 91 sand coarse grain 92 sand 93 silt 94 95 96

sand sand sand

97 98 99

sand sand sand

-~

-

~-

~

Hyrlrocarb.

Sed. strixtirwr arid

stain

reinarky

-

-~-

~~

niott bn

H

HC

VPnone

P

A

mottled mudstone with clay bodies

gy

VH

HC

P

G

SA

orthoquartzite with calcareous cement, no clay or silt

gy

VH

HC

F

F

SA

some silt and clay

3 P P

P gy

H

HC

VP

VP

SA

disseminated organic material

coarse grain but pores filled with clayey silt

P VP

SA A SA SA SA

in shale partings disseminated and in shale partings not abundant not abundant abundant in two shaly zones

SA SA SA - -

clay galls and laminated silt banded, clayey silt with abundant organic material rudely bedded v f gnd lenstruncated by laminated silt v f gnd, silty and clayey silty and clayey with organic material and pyrite v f gnd, silty and clayey med gnd silty and clayey graded bed, clay galls

gy

H

gY

VH

NC NC

P VP

gy

VH FH H

NC NC NC

VP NP

VP P VP

VH H

NC C SC

NP P P

VP VP VP

gy gy gy

gy gy

H

P

rare 1,2, 3,4 rare, organic films in 1,2, 3 , 4 finer zones -

-.

~

~

-

~~

~

-

TABLE I (continued) -

Depth

Lith.

ft.

Color

~

POI. Hard. Calc. arid Sorting Rorindnes~ Perm.

Trace miiierals

olganic r,ratrlial

Hydrocarb. stain

Serl. striictitres and I ernar X s

~~~ ~~~

~~~

11,800 silt

~~

Itgygn H

NC

NP

VP

SA

I , 2, 5

disseminated-rare

Itgygn H

NC

NP

VP

SA

I , 2, 5

disseminated-rare

FH FH FH FH FH FH FH FH FH

NC NC NC NC NC NC NC NC NC

NP NP NP NP NP NP NP VP NP

VP VP VP

SA SA SA

3, 5 , 6 3, 5 , 6

11 silt

FH

NC

NP

VP

SA

3, 5 , 6

very abund, both carbon films and replaced (qtz) root fibers

banded, clayey silt

12 missing 13 silt

FH

NC

NP

VP

SA

3, 5, 6

abundant in clayey portion

14 silt

FH

NC

VP

VP

SA

1,2, 3, 5 abundant in organic films and in qtz replaced fibers

channel in clayey organic silt with silt filling clayey silt with very high organic content

15 16 17 18

sand missing silt sand

H

C

VP

VP

SA, SR

1, 2

organic films

uniform f gnd, silty and clayey

H H

NC

sc

VP P

VP VP

A SA, SR

192 192

clayey med gnd, slightly silty

19

sand

H

NC

P

VP

SA, SR

192

rare disseminated concentrated in shale partings rare

01 02 03 04 05 06 07 08 09 11,810

silt clay clay clay clay clay clay silt silt silt

L "

L

2 2 L

2

~~

disseminated, in shale partings very abundant both carbon films and replaced (qtz) root fibers

uniform dense cldyey silt with some lg qtz gns unifoini dense clayey silt with some lg qtz gns dark organic shale dark organic shale dark organic shale dark organic shale dark organic shale dark organic shale uniform clayey silt clayey, highly organic

clayey and silty with clay bodies

4

2

n

?? 0

w

e

TABLE I (continued) ~~

~~~

P

~~~~~~

~~

~~

Dept'l

Por. Trace arid Sorting Roundness minerals Perm.

Organic ,,,aterial

Lith.

Color

sand sand

It gy ltgy

FH H

SC C

FP P

VP VP

SA, SR SA, SR

22 sand

Itgy

H

C

P

VP

SA, SR

1,2

rare

23

sand

It gy

H

NC

F

VP

SA, SR

1, 2, 3

rare

24

ltgy ltgy Itgy Itgy gngy

VH VH VH VH VH

SC SC SC SC NC

G G G G NP

P P P P VP

SA, SR SA, SR SA, SR SA, SR SA

1, 2, 3, 5 disseminated 1, 2, 3, 5 disseminated

27 28

sand sand sand sand silt

1,2, 3 , 5 disseminated 1,2, 3, 5 disseminated 1,2 in shale bands

29

sand

It gy

S

C

VG

P

SA, SR

132

disseminated

missing silt

ltgy

H

NC

P

VP

SA

1, 2, 4

VH

NC

NP

VP

SA

1,2, 3

VH

NC

VP

VP

SA

1,2, 3

VH

SC

VP

VP

SA

2

disseminated and in clay streaks very abundant in clay streaks qtz replaced fibers common very abundant in thin bands and as disseminated films disseminated, in replaced fibers disseminated

Hard. Calc.

ft.

Hydrocarb. stain

Sed. structiires and remarks

-

11,820 21

25 26

11,830 31

'

32

silt

33

silt

34

sand

Itand dk gY ltand dk gY ltgy

35

sand

Itgy

H

C

G

F

SA. SR

1, 2, 5

36

sand

Itgy

H

NC

P

P

SA, SR

1, 2, 5

disseminated and scattered films

banded, clayey and silty pore spaces filled with clay and silt f gnd, poorly sorted clayey and silty green clay streak, that is 10% pyrite mica abundant, clay balls clayey few Ig qtz gns some clay in pores clayey, banded silt with organic clay streaks med gnd, silty sand with irregular clay bodies graded bedding, organic material in finer sizes small lens or channel fill material, not organic sandy silt, highly organic clay streaks graded bedding? med cse qtz gns graded bedding? med cse qtz gns graded bedding? med cse qtz gns _______

_

. .

~

_

TABLE I (continued) ~

~~

Depth

Por. Litli.

Color Hurd.

Calc. and

ft.

Sortit1g Rorrtidrress

Trace rnitierals

Perni.

Olga,lic

Hydrocarb. stain

Sed. striictrrres and retilarks

~~

11,837 sand

NC

VP

VP

SA,SR

1,2,5

disseminated

38

sand

H

NC

VP

VP

SA,SR

1,2,5

very abundant

Yes

39

sand

H

NC

VP

VP

SA, SR

1,2, 3

very abundant

Yes

H

NC

VP

VP

A

1.2

41 silt 42 silt 43 clay 44 sand 45 sand 46 sand

H H H H FH H

NC NC NC

VP VP VP F F P

A

sc sc sc

VP VP NP G G F

disseminated in silt and concentrated in clayey streaks lg particles abundant lg particles abundant Yes abundant

SA, SR SA, SR

132 1,2 1, 2 1,2 1, 2, 3, 4 rare rare 1,2,6

47

sand

H

NC

F

VP

SA,SR

1,2, 6

disseminated

Yes

48 49

sand sand

H FH

NC NC

F F

P P

SA, SR

SA,SR

1, 2,6,7 disseminated 1, 2,6,7 rare

Yes Yes

H

NC

F

P

SA, SR

1 1,840 silt

11,850 sand

A SA,SR

51

sand

H

NC

F

P

SA, SR

52

silt

FS

NC

VP

VP

A

53

clay

FS

NC

1, 2, 6, 7 disseminated organic film and qtz replaced fibers 1, 2,6,7 disseminated organic film and qtz replaced fibers 1, 2,6,7 disseminated organic film and qtz replaced fibers disseminated organic film and qtz replaced fibers

very strongly cross bedded laminated, f gnd, silty clayey sand silty sand with organic shale streaks silt with dk green clayey highly organic streaks clayey silt with clay galls clayey silt with clay galls silty clay with clayey silt lenses lg angular qtz gns, iron stain lg angular qtz gns, iron stain square piece of wood replaced by qtz gns clayey and silty sand clay bodies med cse gnd silty sand silty and clayey with some lg qtz gns silty and clayey with abundant replaced fibers clay galls with sharp comers not much clay, some lg qtz gns organic shale, lg qtz g n s common

316

W. R. PAINE

intervals (Table I; Fig.1). The environmental conditions in this Rayne fault block are typical of the Upper and Middle Frio and much of the Lower Frio in the northern, older fault blocks. The core is uniformly grey in color with two exceptions, the shale zones which are dark or black, and several thin silts which have a greenish tinge. The black color of the shales is due to the high organic content of these sediments. The greenish silts probably owe their color to sedimentary pyrite and its products of decomposition. The entire section generally shows very poor sorting of grain-sizes. Only very minor intervals (1 1,789’, 11,790’, 1 1,835’, and 11,84447’) are composed of sediments which can be classified as having fair or good sorting. All of the silts and fine sands, and most of the medium and coarse sands, are composed of angular grains; however, some larger sand grains are somewhat rounded. Very few well-rounded grains are noted. This means the grains are rather immature and probably have experienced only one cycle of deposition. The overall carbonate content is less than one generally expects in so-called “dirty” sands. The calcareous material is almost entirely limited to the sands and one thin bed of silt. None of the shales are calcareous. This would seem to-signify that the carbonate is secondary and may have resulted from the solution and redeposition of shell material in the porous sands. Organic material is quite common in the cored interval, particularly in the shales and silts. Although some organic matter is disseminated in the sand, it shows a great tendency to concentrate in the shale partings and a lesser affinity for the silts. Organic remains are present in a variety of forms. Thin organic films are the most common. These occur throughout the section. In one case, at 1 1,846’, a block of wood was found which has been largely replaced by granular quartz. Root fibers or veinlets which have been replaced with silica are abundant in some intervals, especially the interval 11,850‘-54’. The intervalsfrom 11,809-1 5’ and from 1 1,832’-34’ also have this characteristic. The heavy mineral suite is similar to the Missisippi River suite. The ferromagnesian minerals, particularly the members of the hornblende family are most abundant. Mica is plentiful in some samples, and pyrite is very abundant in some of the organic zones. In fact a thin clay streak at 1 1,823’ contains probably at least 10 % pyrite. The pyrite is not detrital since it occurs as euhedral or subhedral crystals. Hematite and limonite are present, but the limonite probably results from the oxidation of pyrite. It is impossible to tell whether or not this alteration occurred in the cores or in the sedimentary history of the rock. The well was drilled in late 1958 and considerable oxidation of the pyrite may have taken place since that time. Several types of sedimentary structures were noted. Lamination is quite common in the silts. One sand is cross-bedded and several silts exhibit cut and fill structure. Another very small rounded lens of sand, at 11,795’, exhibits sharp truncation at the top by laminations of silt. Clay bodies are fairly common in the sands and silts. Most of the clay bodies in the silts are lenticular, but some are very irregular in shape. For these reasons it may be assumed that they were deposited in place. On the other hand,

THE FRIO SEDIMENTATION ON THE RAYNE FIELD OF LOUISIANA

317

Shoreline Relationship (Deltaic) (Fluvial Marine)

Fig.4. Environment interpretation of Homeseeker “C” interval, Frio Formation.

the clay bodies in the sands were round and are probably true galls. The contact with the sand is quite sharp. One core, at I 1,85 I ’, exhibited clay bodies that are best referred to as chunks. They are quite large and have very sharp corners. These must have been ripped from a clay streak and then redeposited with a minimum of transportation. This set of cores clearly indicates cyclic conditions with the environment fluctuating between marine near-shore conditions i n close proximity to the delta stream mouth and non-marine fluviatile, deltaic and interdeltaic conditions (Fig.4). The clay and silt were deposited first along with the abundant organic material in relatively stable, low energy conditions. The organic content diminishes upward with a corresponding increase in the sand content. Lncluded within the sand are chunks of clay which were torn up and deposited with the sand. Also blocks of wood are encountered which would indicate the close proximity to the delta. These sands are sometimes strongly crossbedded, indicating high energy conditions. This sand grades into silts and clays which are laminated and finally into clay with abundant organic material and pyrite. Because of the poor sorting and near-shore and fluviatile facies of the sediment, the tectonic history would seem to indicate both rapid sedimentation and rapid subsidence, but in such balance that the area remained in the inner neritic or littoral zone throughout the Frio.

318

W. R . PAINE

SUMMARY

It has been postulated that the sedimentation of the Frio in the growth fault interval of the Nodosaria embayment was mostly deposited along the inner and middle shelf, with a balance between deposition and subsidence keeping the shelf area almost in grade. In order to establish the nature of this sedimentation, a careful examination of all available Frio cores was conducted and 61 ft. of a nearly complete set of cores from the Homeseeker “C” sand interval of the middle Frio was carefully examined and herein described. Within this interval four cycles are observed. These cycles contain a non marine portion, which consists of highly organic clays with abundant pyrite which grade upward into silty clay and silts with abundant wood fragments, mica and clay galls. Overlying this are slightly fossiliferous sands which contain varying amounts of calcareous carbonate cement in the upper portions and are probably of near-shore (littoral) origin deposited in close proximity to the delta stream mouth. Most of the fossils are small unidentifiable fragments of shells. The poor sorting, abundant mica, wood fibers and cyclic nature of the sediments indicate deltaic, interdeltaic and fluvial marine environments. Most of the Frio cores from the Nodosaria embayment seem to agree with this set of cores, thus indicating primarily shallow neritic and deltaic environment, with a close balance between sedimentation and subsidence.

REFERENCES

GRIGGJR., R. P., 1956. Key to Nodosaria enibaynient of south Louisiana. Trans. Guy Coast Assoc. Geol. SOC.,6 155-62. OCAMB, R., 1961. Growth faults. Trans. GulfCoast Assoc. Geol. Soc., I 1 : 139-175. OCAMB,R. and GRIGGJR., R. P., 1954. The Lewisburg Field area, Acadia and St. Landry Parishes, Louisiana. Trans. GiilfCoast Assoc. Grol. Soc., 4 : 183-200. W. R., 1956. The Nonion strunia “Lower Frio” wedges of Acadia Parish. Trans. Gulf Coast PAINE, ASSOC.G e ~ lSOC., . 6 1 153-160. PAINE, W. R., 1958. Frio sedimentation patterns in Acadia and Jefferson Davis Parishes of Louisiana. Trans. Gulf Coast Assoc. Geol. SOC.,8 : 101-103. J. W., 1960. Structure and stratigraphy of Rayne field. Trans. Gulf Coast Assoc. Geol. SOC., SHIRLEY, 10 : 77-85. WARREN, A. D., 1957. The Anahuac and Frio sediments in Louisiana, Trans. CulfCoast Assoc. Geol. SOC.,7 : 221-237.

SHALLOW-WATER ORIGIN OF EARLY PALEOZOIC OOLITIC IRON ORES JAN PETRhNEK

ostredni listav cgeoloc+ky, Prague (Czechoslovakia)

INTRODUCTION

The shallow-water origin of oolitic iron ores deposited during the younger geological periods is more or less obvious, and generally accepted. The older ores are another matter. The earliest (i.e., Precambrian) ores mostly lack typical oolitic texture and are considered by many authors to be of deep-water origin. They also differ i n composition, textural and structural features, and in general development and bulk. The causes of the differences may be sought in the special conditions of Precambrian times, particularly, the higher atmospheric content of CO,, the substantially lower pH values of river-, lake- and sea-water, and the lower salinity of seawater in general. Thus the mobility of iron was much greater, the element easily leaching out during intensive weathering and also being supplied by extensive submarine volcanic activity. Living organisms, still primitive and thinly distributed (especially benthos), produced only minor amounts of organic matter. Hence diagenetic reduction did not change any ferric ores deposited so profoundly as in later periods. Owing to increased mobility, most ofthe iron was deposited under fairly deep water, probably on the deep-sea bottom. Subsequent changes in physico-chemical conditions led to marked changes in the ores deposited. Primarily responsible was the expansive evolution of continental floras, resulting in a large reduction in atmospheric CO,. This increased the pH of the hydrosphere generally. Migration of iron and its compounds, therefore, became restricted, and precipitation and coagulation (helped by the increased salinity of seawater) occurred closer inshore. Under these conditions wave action prevented the formation of the f i n t banding so characteristic of many Precambrian ores. Indeed coarse bedding, even lack of evident bedding, became usual. According to STRACHOV (1960) the above changes took place at the Precambrian/Paleozoic boundary. According to others (e.g., FORMOZOVA, 1962) they occurred around the Early Paleozoic/Late Paleozoic boundary, the Early Paleozoic ores being deep-water formations like the Precambrian. DISCUSSION

The more important genetic features of the Early Paleozoic oolitic iron ores are discussed below and their depositional environments interpreted. The Ordovician ores

320

J.

PETRANEK

are especially interesting, for the Ordovician was one of the most important periods of ironstone accumulation. They are also close enough to the Precambrian to assist in defining the differences between the Precambrian and Early Paleozoic processes of ironstone deposition. (I) The ore bodies are lenticular and elongated. I n exceptional cases they may be very elongated, extending over considerable areas (e.g., Silurian Clinton ores of North America). Normally they do not exceed a few kilometres or (rarely) tens of kilometres. In width the majority is relatively narrow, only widening locally. Combined with their commonly small or medium size this does not suggest a deep-water origin. (2) In rich ore-bearing districts the lenses occur at several stratigraphical levels, rather than being restricted to a single extensive horizon (e.g., Ordovician ores of Bohemia and Morocco, Silurian Clinton Group of western and central New York, etc.). (3) The paleogeographical setting of the oolitic ores reveals that most of them were deposited close to the shore on shallow flats, in narrow offshore depressions, in bays that were separated by submarine elevations or by islands from the main sea basin, or even in true littoral or deltaic environments. The deposits thus followed old coast lines. ( 4 ) Certain of the medium-sized ore bodies (e.g., Ejpovice, Ordovician of Bohemia) and groups of smaller bodies (e.g., Thuringian Ordovician) are petrographically zoned. Near the ancient shores ferric (hematitic) ores occur, while further out there are hematite-siderite ores containing more admixed clay, and further out still, lowgrade pelosiderite ores with scattered iron-silicate ooids. Lower in grade, the latter form lateral transitions to argillaceous “basin” facies (shales or marly shales). Seawards also, clastic quartz decreases in amount and coarseness and finally disappears almost completely. Such a zonation of petrographical facies is unlikely to have developed in relatively uniform deep-sea environments. On the other hand it is easily understood i n terms of near-shore conditions. (5) Many of the oolitic ores show only coarse bedding or none at all. This suggests shallow-water deposition under the influence of wave action and continual reworking (cf. fine bedding (banding) in many Precambrian ores). (6) Broken ooids are in some cases very common. They often served as nuclei for second generations. Third generations may form through further abrasion and deposition (HETZER,1958). Such re-working, re-deposition and re-lithification are most likely to have occurred in shallow-water environments. An analogous history may be attributed to the Clinton ores of Alabama (hematitized debris of Bryozoa, shells, etc.). (7) Individual laminae consisting only of ooids are known to occur in some orebearing formations: in shales and sandstones (e.g., Newfoundland, HAYES,1915), in cross bedded psammitic tuffs (e.g., Bohemia) or in certain deltaic-type quartzites (e.g.. Thuringia). Some ore beds may even occur inside the sequences of deltaic origin. (8) As a result of the striking development of organic life since the Precambrian, especially that of the benthonic fauna, many marine deposits of Paleozoic age are

SHALLOW-WATER ORIGIN OF OOLITIC IRON ORES

32 1

distinctly richer in organic matter than older sediments. Such admixtures, potentially highly reducing, accumulated chiefly in the quieter and rather deeper parts of the basins. The fact that a large part of the Early Paleozoic oolitic iron ores belong to the oxidic type shows that these ores were deposited in relatively well aerated shallowwater environments, where the reducing activity of the organic matter was restricted. (9) Whereas many of the Precambrian iron ores show more or less distinctly a genetic relationship to volcanism of the geosynclinal type, no such relationship is found for most Early Paleozoic oolitic ores (e.g., Thuringia (Ordovician), Newfoundland (Wabana), Morocco (Ait Amar), northern Portugal, France (Normandy, Brittany), Bohemia, north Wales, U.S.A. (Silurian Clinton Group)). (10) Barren clastic rocks associated with the ores are chiefly shales, siltstones and orthoquartzitic to subgraywacke-sandstones. Arkoses, typical graywackes and bedded cherts, do not normally occur. This especially suggests shelf environments, at most marginal zones of miogeosynclines, certainly not eugeosynclinal conditions.

CONCLUSIONS

I n the main, the Early Paleozoic oolitic iron ores are thus shallow-water deposits. They differ substantially from the Precambrian ores. Attempts to trace the Early Paleozoic ore materials from their present sites (“centres d’accumulation” of CAYEUX, 1909) to their places of birth (“milieux gtntrateurs” of Cayeux) suggest origins close inshore. This is particularly true of beds near the bases of transgressive formations (e.g., in the Ordovician of Bohemia). Some of the younger ores (e.g., Oligocene of Aral lake region) may have been formed in rivers (FORMOZOVA, 1959). It is uncertain whether any of the Early Paleozoic oolitic ores were formed in this way. Underlying causes for the occurrence of major sedimentary ore deposits and important metallogenetic epochs (e.g., Ordovician) tend nowadays to be sought i n major transgressions (e.g., BROCKAMP, 1942;and others). Some of the Early Paleozoic (especially Ordovician) ores seem, however, to have originated during regressions (e.g., in northwestern France, Bohemia, Thuringia, etc.). Where such regressions were accompanied by uplift of the nearby land masses, an increased transport of weathering products, often very rich in iron must have taken place. This supply of iron compounds must have been especially important where older ferruginous sediments of near-shore origin had been raised above sea level. Extensive oolitic iron ores may of course have formed in shallow water during the later stages of regression. But they (especially the latest) will have been very susceptible to erosion. Hence regressive ores are less common than transgressive ores.

SUMMARY

The environmental conditions of deposition of the Early Paleozoic oolitic iron ores

322

J.

PETRANEK

are discussed. The interpretation of several genetic features, such as shape and size of the ore bodies, zonation of the ore types, textural and structural features, relation to volcanism, associated rocks. etc., suggests shallow-water origin. In this respect, the Early Paleozoic iron ores differ substantially from the majority of the Precambrian ores and are on the other hand similar to those of later periods. The interpretation of the tectonic framework shows that the ore deposits originated not only during the transgressive phases but also during the regressive oscillations of the sea level. Economically, the ore deposits of the regressive type are considerably less important than those of the transgressive type.

REFERENCF3

ACARD, J., DESTOMBES, J. et VANLECKWIJCK, W., 1952. Fer. En: E. RAGUIN (Redacteur), Ce'ologie des Cites Mine'raux Marocains - Notes Me'ni., Service CZol. Maroc., 87 : 103-1 32. BROCKAMP, B., 1942. Palaogeographische Stellung der Eisenablagerungen. In: B. BROCKAMP (Redakteur), Zur Enfsfehungdeurscher Eisenerz1agerstatr:n -Arch. Lq~erstattenforschun~, 75 : 1 8 I- 186. CAYEUX, L., 1909. Les Minerais de Fer oolithique de France. I. Paris, 344 pp. FORMOZOVA, L. N., 1959. Zheleznye rudy Severnogo Priaralia. Tr. Ceol. Inst., Akad. Nauk. S.S.S.R., 20 : 1 4 6 . FORMOZOVA, L. N . , 1962. Usloviya obrazovaniya oolitovych zheleznych rud v nizhnem paleozce i dokembrii. In: I. M. VARENCOV and L. N. FORMOZOVA, Osadochnye rudy zheleza i niarganca Tr. Geol. Inst., Akad. Nairk S.S.S.R., 70 : 65-1 18. GiLLErrE, T., 1947. The Clinton of western and central New York. Bull. N . Y. State Muserrrii, 341 : 1-191. HAYES, A. O., 1915. Wabana iron ore of Newfoundland. Geol. Surv. Can., Meni., 78 : 1-165. HETZER, H., 1958. Feinstratigraphie, Sedimentationsverhaltnisseund Palaogeographie des hoheren Ordoviciums am Sudostrand des Schwarzburger Sattels. Geologie (Berlin), 7 (23) : 1-96. PETRANEK, J., 1960. Origin of the Ordovician iron ores of Bohemia. Intern. Geol. Congr. Z l s t . . Copenhagen 1960, Abstr., 156 : XXI. P E T R ~ N EJ.,K ,1964. Gemeinsame Merkmale der Eisenerzlager ini bohmischen und thiiringischen Ordovicium. Deut. Akad. Wiss. Berlin, F. Drrrbel Festband, in press. STRACHOV, N. M., 1947. Zhelezorudnye facii i ich analogi v istorii zemli. Tr. Insf. Geol. Nauk S.S.S.R., 73 : 1-267. STRACHOV, N. M., 1960. Osnovy Teorii Litogeneza. IT. Izd. A. N. S.S.S.R., Moskva, 574 pp. STRAHAN, A,, et al., 1920. Iron ores. Pre-carboniferous and Carboniferous bedded ores of England and Wales. Ceol. Surv. Gt. Brit., Mem. Ceol. Surv., Spec. R e p . Mineral Resources C t . Brit., 13 : 1-123.

ORIGINE ET CONDITIONS DE SGDIM€NTATION DES DfiPoTS SABLEUX ET ARGTLEUX DANS LE GOLFE BARTONIEN D U BASSIN DE PARIS C H A R L E S P 0 ME: R 0 L Lahoraroire dc G i o l o g i e S.P.C.N., FacrrltP des Sciences rle Paris, Paris ( F r a n c e )

INTRODUCTION

Nous envisagerons dans cette note quelques modalites de la stdimentation dttritique dans le golfe bartonien du Bassin de Paris, en particulier l’origine des mattriaux dttritiques et les conditions de dep6t de divers facits. Prtcisons tout de suite que nous entendons par Bartonien le cycle stdimentaire de I’Eoctne suptrieur (POMEROL, 1961a) qui comporte trois transgressions bien marquees: celle des sables d’Auvers et de Beauchamp (zone I = Auversien), celles des sables de Cresnes et de Marines (zone I1 = Marintsien) et enfin celle des sables et marnes a Pholadomya ludmsis (zone III = Ludien).

ORIGINE DES MATBRIAUX

DBTRITIQUES

A u versien

La granulomttrie des sables auversiens, considtree d’une manitre statistique, montre une tltvation rtgulitre de la mtdiane de l’ouest (0,15 mm dans le Parisis), vers l’est (0,25-0,30 mm dans la Brie et le Tardenois) (Fig. I). En m&metemps que cette granocroissance, l’htttromttrie des sables dtposts dans les lits non perturbts passe de 0.50 rnm dans le Parisis, a 1 , l mm dans le Tardenois, tandis que la courbe de friquence devient le plus souvent bimodale (0,25 et 0,16 mm). Ces observations signifient vraisemblablement que rapport dttritique venait de I’est, et non du sud. D’ailleurs, depuis la fin du Lutttien, et par constquent a l’Auversien, le Bassin de Paris ttait bordt au sud par une ceinture laguno-lacustre ou lacustre, facits calcaire, qui exclut uiie dtcharge ditritique massive en provenance du Massif Central. Cette opinion concorde avec celle de DEMARCQ (1955) pour qui les dtp6ts dttritiques attributs au Sparnacien dans le sudest du Bassin de Paris, comme les poudingues de Nemours, appartiennent a tout l’Eoctne inftrieur, Thanttien, Sparnacien et Cuisien, s’interrompent a I’fioctne moyen pour reprendre 2 l’fiocine terminal et a I’Oligoctne (Stampien). Les associations de mintraux lourds des sables auversiens rivklent un fond de mint-

324

CH. POMEROL

-0.5

0.a

1

1.2

1.5

HCJ

Fig. 1. Diagramme midiane-heteromttrie des sables auversiens dans le Parisis (a l'ouest) et le Tardenois (a l'est). La mediane Md est exprimee en mni. L'hCteromCtrie Hv =

Q2-Q1,

2

Q, et Q,ttant le

second et le premier quartile, exprimes en unites a, cologarithme des dimensions des tamis en progression geomttrique de raison YlO # 1,259.

raux de mttamorphisme et de miniraux ubiquistes communs a tout I'eoctne du Bassin de Paris sur lequel se projette une surcharge en tourmaline nettement plus accusCe A l'est qu'a l'ouest (Fig.2). Afin de rechercher l'origine du fond et celle de la surcharge, j'ai effectut des analyses de sables cuisiens, thanttiens, albiens et cinomaniens du Pays de Bray, de grks quartzites du PalCozoique de 1'Ardenne et de leurs artnes d'alttration. Je me suis aussi rCfCrt aux analyses mintralogiques du Trias du Palatinat (SINDOWSKI, 1957), de la Sarre (HENRICH, 1961), des Vosges (PERRIAUX, 1961) et du Carbonifere et Permien de la Sarre (SCHNEIDER, 1958).

PARISIS

TARLIENOIS

-

andalousite disthkne staurodite epidote hornblende

Fig.2. Diagramme des associations moyennes de miniraux lourds dans le Parisis et le Tardenois. Noter dans cette dernikre rigion la disparition de la hornblende, de l'tpidote et du grenat et l'augmentation des taux de tourmaline (fraction 0,16-0,08 mm). (I1 faut lire disthkne et staurotide au lieu de disthene et staurodite).

ORIGINE DES

D ~ P G T SSABLEUX DANS LE BASSIN DE PARIS

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-Limites de la mer Auversienne

Echelle

Fig.3. Origine probable des rnintraux lourds caracteristiques de I’Auversien (Bartonien infkrieur ou zone I) dans le Bassin de Paris.

Je suis alors arrivt aux conclusions suivantes (POMEROL, I96 1b): le fond mintralogique provient du dtmantblement des assises sableuses crttactes et thanttiennes de l’aurtole nord et nordest du Bassin de Paris. Quant i la surcharge en tourmaline, il faut probablement en chercher la source dans le Trias et le PalCozoYque du Massif ardenno-rhtno-vosgien. On y trouve en particulier dans les arknes actuelles provenant des g r b quartzites d’Haybes des teneurs extraordinaires en tourmaline, atteignant 95 ”/, de la fraction lourde, oh la forrne bleue (indicolite), retrouvte dans l’Auversien, est bien reprtsentte (Fig.3). D’ailleurs la dtcouverte, i la base d’une carribre de la For&td’Halatte, i Apremont (Oise) de galets de quartzites micactes i filonnets de quartz semblables i ceux de I’Ardenne apporte un autre argument en faveur de l’origine ardennaise d’une partie du mattriel dttritique des sables moyens. Cela ne signifie pas que cet apport oriental ait t t t exclusif. En effet, la prksence de hornblende et d’tpidote uniquement dans le Vexin, le Parisis, le Valois et le Laonnois tvoque cette fois une influence nordique des mers de la Belgique et de 1’Angleterre oh ces mintraux ttaient plus abondants. MarinPsien

La mer marintsienne n’a pas recu, comme la mer auversienne, d’apport dttritique notable venant de l’est. A l’inverse des sables auversiens, les dtpBts sableux marintsiens sont en effet de plus en plus fins au fur et a mesure qu’on va vers l’est. Leur mtdiane de 0,18 mm dans le Vexin n’est plus que de 0 , l l mrn i Champlbtreux, au sud de Luzarches, et 0,07 mm dans le sondage de Survilliers (VERNIER, 1962) ri l’extrtmitt du golfe. Manifestement la source dttritique orientale ttait tarie, ce qui n’a rien

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CH. POMEROL

d’itonnant, puisque, vers I’est, les sables de Cresnes passent lattralement aux calcaires de St Ouen (FEUGUEUR, 1952). Ainsi, suivant les modalitts de l’apport en fonction des actions continentales, les sables peuvent Etre de plus en plus grossiers (Auversien), ou de plus en plus fins (Marintsien) au fur et a mesure qu’on s’approche du rivage. Dans le second cas il est d’ailleurs difficile de parler de “rivage”, car on observe le pissage IatCral insensible des sables marins a des marno-calcaires laguno-lasustrzs. D’autre part, le cortkge mineralogique des sables de Cresnes (MarinCsien) compare a celui des sables d’Auvers (Auversien) montre une diminution de la teneur en tourmaline, une augmentation du taux de zircon, rutile, grenat et une trts nette prtdominance de la staurotide parmi les mintraux de mttamorphisme. Ces modifications pourraient bien Ctre imputables au decapage de l’aurtole cuisienne du Pays de Bray, ou mCme a la reprise de sables albiens dtja a I’affleurement au coeur de l’anticlinal.

CONDITIONS DE S~DIMENTATION

Stratifications entrecroisbes Les gtologues du Bassin de Paris ont gintralement considtrt que les stratifications entrecroistes ttaient le rtsultat d’un dtp6t par des courants violents: courants estuariens, ou courants marins capables, d’aprts MUNIERCHALMAS (1900) de transporter des galets de silex sur 200 km depuis le Vexin jusqu’a la Brie! De IZi le nom de “facits charrib” attributs a ces stratifications particulitrement frtquentes dans les sables d’Auvers et de Cresnes. Or nous avons pu constater, notamment 5 Ronquerolles (CAVELIER et POMEROL, 1962), que les lits obliques renferment en abondance des coquilles trts fragiles de ptltcypodes fouisseurs, aux valves accoltes telles que Corbula lamarcki, Meretrix elegans, Donax parisiensis et Pprna lamarcki, et des tubulures gtntralement rapporttes 2 des terriers de vers ou de mollusques. De telles stratifications sont connues sur des c6tes plates oh les dCpBts obliques s’effectuent chaque marte, a l’abri de petits cordons littoraux. La prtsence de galets n’est pas rare au voisinage des fleuves. Quant aux coquilles ustes, elles peuvent provenir des plages avoisinantes. Cette conception, conforme aux donnees actuelles de l’observation et de l’exptrimentation (MCKEEet STERRETT, 1960), permet une interprttation plus correcte des stratifications obliques de trks faible puissance, alternant avec des lits parfaitement horizontaux. En fait, la bathymttrie variait peu. II suffisait de dtplacement de rides ou de cordons pour provoquer la formation des dtp6ts obliques qui ont souvent fonctionnt comme des “pitges” a coquilles. La granulomttrie globale de ces lits obliques rtvtle une mtdiane plus tlevte, (0,204,30 mm au lieu de 0,154,16 mm) ainsi qu’une plus forte htttromttrie, (0,85-1,32 mm au lieu de 0,54,65 mm). En rtgle gtntrale ces deux indices varient d‘ailleurs dans le mCme sens. En ce qui concerne les mintraux lourds, le fait le plus frappant est l‘enrichissement

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en rutile et zircon, c’est i dire en mintraux de forte densitt, alors qu’on observe gtntralement l’inverse dans les sables grossiers lorsqu’ils sont en lits horizontaux. On assiste donc alors, dans les stratifications obliques, par suite de la reprise d’un sable d t j i stdimentt, a une dtsolidarisation du groupe de densitt moyenne (tourmaline, andalousite) qui est entraint, et du groupe de forte densitt (zircon-rutile) qui se restdimente. ThanatocPnoses Dans les sables auversiens les nummulites ( N . variolarius) sont trbs souvent endommagtes. D’autre part on les rencontre dans certains bancs grtsifits, en quantitt prodigieuse, i l’exclusion de toute autre faune. C’est ainsi que, dans un nouveau gisement auversien, a Hervillers (Seine et Marne), 1 g de grbs ii nummulite renferme en moyenne 650 individus, chacun d’eux pesant a peine plus d’un milligramme! Les nummulites sont cimenttes par du sable grossier (mtdiane 0,20 mm) et ma1 class6 (htttromttrie 1,55; courbe de friquence trimodale 0,40,0,25 et 0,125 mm), la fraction fine ttant la mieux classte (asymttrie -0,65). Corrtlativement on note des anomalies de rtpartition dans les associations de mintraux lourds: c’est ainsi que le zircon prtdomine dans la fraction grossibre (0,31-0,16 mm) tandis que la prtsence de quelques grains d’augite verte et de biotite alttrtes traduisent la proximiti d’un rivage. Ces grbs rtsultent de la cimentation de “graviers” de nummulites, vtritables thanatoctnoses, dtp6ts mi-organiques mi-dttritiques rtsultant du triage, du transport et de l’accumulation de tests d’animaux morts. Herbiers Dans certaines zones du golfe bartonien se sont dtveloppts des facibs d’herbiers, en particulier dans le “niveau” du Gutpelle, facibs lathal des sables d’Auvers. Ces facibs, rencontrts par exemple ii la carribre de Ver (Oise), se prtsentent sous forme de couches mauves trbs fossilifbres. Ce sont des sables fins, a faible htttromttrie, ii distribution symttrique, contenant de 2-10°/0 de carbone organique. Cette substance provient probablement de la carbonisation de debris vtgttaux dont il est impossible, jusqu’i prtsent, de prtciser la nature. La faune est abondante par le nombre des individus mais pauvre en espbces: Cyrbnes, Mtrttrix et Corbules trbs bien conservtes, auxvalves accoltes, parmi les lamellibranches: Miliolidtes parmi les foraminifbres. L‘abondance et la rtpartition des fossiles tvoque un biotope marin, littoral, peu profond, aux eaux assez chaudes, saltes et claires comme celles des herbiers de la Miditerranee actuelle. C’est aussi un biotope analogue qu’on retrouve fossilist dans les plages monastiriennes des c6tes de Tunisie. Les facibs d’herbiers, contemporains a l‘Auversien des stratifications obliques de plage et des thanatocoenoses, montrent la diversitt des facibs dans une zone nkritique, disposition pleine d’embQches pour ttablir une stratigraphie correcte de ces dCp6ts.

328

C H . POMEROL CONCLUSION

Ces observations stdimentologiques, comme les multiples variations lattrales de faciks, comme, a d’autres moments, la grande extension de faciks trks peu tpais (calcaire de Ducy, sables de Mortefontaine, marnes a Plioladomya ludensis) permettent de conclure a l’existence, a u Bartonien, dans le Bassin de Paris, d’un golfe a fond plat, faiblement subsident et temporairement ouvert aux influences miridionales ou nordiques. Ce golfe, limit6 par une bordure lagunaire bien dtveloppte a u sud dts 1’Auversien, puis a l’est et au nordest au Marintsien, recevait surtout des mattriaux dktritiques en provenance des assises paltozoiques, triasiques, crttactes et mCme thanttiennes et cuisiennes des pays du nord et de l’est du Bassin de Paris, en voie de soulhement.

Les sables auversiens du Bassin de Paris ont une origine nordique et orientale depuis l’aurtole thanetienne jusqu’au Trias et au Paltozoique ardenno-rhtnan. A l’appui de cette hypothkse on note une augmentation de la mtdiane, de l’htttromttrie, de la teneur en mintraux ubiquistes, en particulier tourmaline, en allant de I’ouest vers l’est et le nordest, ainsi que la prtsence de galets de quartzites micacts du Paltozoique de I’Ardenne. Au contraire, la mer marintsienne ttait bordte i l’est par des lagunes sans apport dttritique grossier. La plupart des stdiments a stratifications entrecroistes sont interprktts comme des dtp6ts d’eaux calmes, de faible profondeur et non comme des “facib charriks”. L‘existence de thanatodnoses a Nummulites variolarius, gasttropodes et d’herbiers de type mditerranten, complkte les particularitts stdimentologiques de ce golfe peu profond mais de grande extension.

SUMMARY

It is believed that the Auversian sands of the Paris Basin were derived from the north and east, from an area including the Thanetian aureole and extending to the Trias and Paleozoic of the Ardenno-Rhenish massif. In support of this hypothesis it is noted that the median and the quartile deviation increases from west to east and northeast, as does the proportion of the ubiquitous heavy minerals, especially tourmaline. The distribution of micaceous quartzites from the Paleozoic of the Ardennes varies in the same way. By contrast, the Marinesian sea was bordered by lagoons without inflow of coarse detritus. Most of the cross bedding has been interpreted as the result of deposition in calm shallow waters, and not as a “faciks charrie”. The existence of thanatoctnoses of N . variolarius and gastropods, and of submarine plant growths are other sedi-

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mentological characters of the shallow but extensive gulf which occupied the Paris Basin in those times.

BIBLIOGRAPHIE

CAVELIER, C. et POMEROL, CH., 1962. Le Bartonien de Ronquerolles (Seine et Oise). Bull. Sor.GPol. France, 7 (4) : 17cL181. DEMARCQ, G., 1955. Le problerne du Sparnacien dans le sudest du Bassin parisien. Bull. Sor. GCol. Franre, 6 (5) : 155-165. L., 1952. Presence de caillasses a Potamides tricarinatus au sornrnet des calcaires de St. FEUGUEUR, Ouen A Harblay (Seine et Oise). Reflexions sur la valeur stratigraphique et la synchronisation de certains termes bartoniens. Bull. SOC.CPol. France, 6 (2) : 373-378. HEINRICH, H. W., 1961. Sedinientpetrographisrhe Untersuchungen im Buntsandstein des Saarlandes rind der angrenzenden Gebiete. Diss., Univ. von Saarland, Saarbriicken, 96 pp. MCKEE,E. D. and STERRET,T. S., 1960. Laboratory experiments on form and structure of longshore and J. C. OSMOND(Editors), Geonietry of Sandstone Bodies. bars and beaches. In: J. A. PETERSON Am. Assoc. Petrol. Geologists, Tulsa, pp. 13-28. M., 1900. Sur les caracttres generaux du Bartonien dans le Bassin parisien. Bull. MUNIER CHALMAS, Sor. GcJol. France, 3 (28) : 11-1 3. J., 1961. Contribution A la geologie des Vosges griseuses. MPm. Serv. Carte GPol. AlsacePERRIAUX, Lorraine, 18 (1) : 235 pp. POMEROL, CH., 1961a. Les Sables de I’Eocdne sip‘rieur (Bartonien-LcJdien) des Bassins de Paris et de Bruxelles. These, Univ. de Paris - Serv. Carte Geol. France, a paraitre. POMEROL, CH., 1961b. Sur I’origine des niineraux lourds des sables de l’eocene suptrieur du Bassin de Paris. Conipt. Rend., 253 : 887-889. K. H., 1957. Schiittungsrichtungen und Mineral-Provinzen im westdeutschen BuntsandSINDOWSKI, stein. Ceol. Jb., 73 : 277-294. SCHNEIDER, H. E., 1958. Geologisch-sedirnentologische Untersuchungen in1 Bereich der KarbonPerm Grenze des Saargebietes. Ann. Univ. Saraviensis, Sci., 7 ( 3 4 ) : 349400. VERNIER, C., 1962. etude granulornttrique et rninkralogique de sables cuisiens et lutetiens du Vexin francais et du Parisis. D.E.S., Paris, 70 pp.

COUCHES INTRAFORMATIONNELLES A GALETS PRIMITIVEMENT MOUS DANS L’ORDOVICIEN MOYEN DE LAREGION DE CAEN JACQUES PONCET

Laboramire de GPologie (C.N.R.S. J, UniversitC de Caen, Caen (France)

INTRODUCTION

Dans les synclinaux paltozolques de la region de Caen (synclinaux de May sur Orne, d’Urville et de Ranville), le niveau des Schstes a Calymene (Synhomalonotus) tristani BRONGNIART qui reprtsente le Llandeilien est essentiellement constitut de schistes bleuitres rarement ardoisiers. Ces schistes trbs fossilifkres rtsultent de dtp6ts marins stdimentts en eau peu profonde. En effet, rappelons bribvement qu’a la base de ces schistes se situe une couche de minerai de fer oolithique et que dans les synclinaux de May sur Orne et de Ranville, au mur de la couche de minerai, a rtcemment it6 dtcouvert un poudingue (DORE,1962). Au-dessus des Schistes a Calymbnes les sediments grtseux caracttristiques de I’Ordovicien moyen et suptrieur bas-normand prennent une grande importance avec le “Gres de May”. Ainsi cet tpisode schisteux, encadrt par des sediments d‘eau peu profonde et m&metrbs peu profonde, doit s’&trevraisemblabement stdimenti dans des conditions bathymttriques assez voisines de celles ayant prtsidt au dCpBt des niveaux encaissants. I1 en est bien ainsi car les Schistes a Calymknes ont livrt des brachopodes inarticults et de nombreuses traces d’organismes fouisseurs (DANGEARD et RIOULT,1959; DELPEY,1942) assocites des fonds bosselts, des ripplemarks et des couches a nodules gris-clair accompagnts d’une lumachelle. Ce sont les couches a nodules gris-clair que nous nous proposons d‘ttudier dans cette note. Dans les Schistes a Calymbnes se rencontrent tgalement des couches a petits nodules noirs t r h phosphatts; elles ne seront pas traittes dans le cadre de cette communication.

~ T U D EDES COUCHES k NODULES GRIS-CLAIR

A notre connaissance ces couches a nodules ont t t t signaltes pour la premikre fois en 1904 par BIGOT.CAYEUX (1939), mentionne ces nodules comme ltghrement phosphates et en donne quelques analyses. Dans une note parue en 1955, DANCEARD et DORE remarquent que ces nodules sont souvent entourts par une lumachelle de dkbris de trilobites. Nous avons ttudit ces couches i nodules sur des carottes provenant de sondages

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effectuts pour la reconnaissance du minerai de fer dans les trois synclinaux rnentionnts plus haut. Cette ttude sur carotte a eu l’avantage de nous permettre de travailler sur du materiel frais et de montrer le passage de ces couches avec les couches encaissantes. L’tpaisseur des couches a nodules gris-clair va de quelques centimbtres a une vingtaine de centimttres; cette dernitre puissance ttant maxima. Etude des nodules

Les parois des carottes permettent d’observer ces nodules en coupe, rtvtlant ainsi leur texture et leur nature pttrographique. La plupart, de couleur gris-clair, offrent des forrnes trbs varites: ovo‘ides, sub-sphtriques, aplaties et festonntes. Les dimensions sont tgalement variables et oscillent entre 2 et 6 cm. Sur les plus gros nodules gris-clair, en section et aprbs polissage, on peut observer j usqu’a trois zones grossitrernent concentriques: une zone externe gris-fonct, une mtdiane gris-clair et une interne gris-clair ii fonct avec de grandes plages de pyrite disposkes de faFon i reproduire assez grossibrernent le contour du nodule. Sur les nodules de r n h e couleur mais de taille moyenne ces trois zones ne se discernent plus, il y a seulement de la pyrite en plages rtparties un peu partout dans la masse du nodule avec, tout de mCme, une certaine prtftrence pour la ptriphtrie. Quelques nodules gris-clair laissent voir, plaquts i leur surface ou faisant saillie de leur masse, des tests d‘organisrnes. Ces nodules sont “arrnts” par les fragments de tests qui proviennent surtout de brachiopodes et de trilobites et en plus faible proportion de lamellibranches. Ces fragments se retrouvent tgalement dans la masse des nodules. Les dtbris de tests sont t r b pyritists, parfois mCme compltternent tpigtnists par ce mintral. Ces nodules gris-clair font effervescence ii l’acide. En plaque mince se voient des petites plages de calcite grenue ainsi que de la pyrite en perles ou en granules sur un fond constitut par une pdte argileuse. Dans la rtaction au molybdate d’ammonium, ils fournissent un trouble jaune qui prtcipite lentement en donnant des cristaux de phosphomolybdate d’arnrnonium. Ce sont donc des nodules calcarto-argileux phosphatts. De rnCrne que CAYEUX (1939, 1941, 1950) nous n’avons pu dtceler sur les plaques minces le moindre tltment phosphatt. Pour expliquer la prtsence d’acide phosphorique dans ces nodules nous nous rallions a la thbse de cet auteur qui k i t : . . . A la vtritt, la prtsence de restes de trilobites suffit pour assurer aux nodules une teneur en acide phosphorique . . .”. Ajoutons que les tests des brachiopodes sont tgalernent susceptibles d’avoir fourni une partie de l’acide phosphorique dtcelt. Associts a ces nodules gris-clair se rencontrent, en moindre quantitt, des nodules franchernent schisteux de la m h e couleur bleu-noir que celle des Schistes a Calymknes. 11faut Cgalement mentionner la presence de nodules schisteux forrnts de feuillets finement stratifits. Les nodules peuvent Ctre dans la lurnachelle sans contact les uns avec les autres ou bien, en contact tangentiel. Parfois on observe des phtnomknes d’impression. Certains nodules aplatis sont ployts et semblent bien s’&tre dtformts sur les “

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nodules sous-jacents. Le plus souvent, les nodules se prtsentent avec leur plus grand axe parallkle a la stratification locale donnte par les couches encaissant les niveaux a nodules. Certains, toutefois, ont des positions qui orientent leur grand axe a 30",45" et mCme 90" par rapport au plan de stratification. l h l e de la lurnachelle accompagnant les nodules

Dans toutes les couches oh prtdominent les nodules gris-clair, ces derniers apparaissent accompagnts d'une lumachelle de fragments de tests de brachiopodes et de carapaces de trilobites. Associts A ces debris s'observent des petits fragments calcartoargileux phosphatts ou argileux plus ou moins roults. La pyrite de fer est prtsente dans cette lumachelle sous forme de granules ou bien comme un enduit soulignant les fragments de tests; parfois elle s'infiltre dans les canalicules des tests de brachiopodes ou bien les tpigtnise totalement. Passage des couches a nodules aux couches encaissantes

Au voisinage et au contact des couches a nodules observtes les couches encaissantes sont formtes de straticules de schiste alternativement noires et gris-fonct, souligntes de place en place par une trainte ou une tache de pyrite. Traittes au molybdate d'ammonium, ces couches ne donnent pas de prtcipitt jaune de phosphomolybdate d'ammonium. La limite du mur de la couche a nodules avec les couches encaissantes est tvidente et bosselte par les nodules qui dtforment les straticules sous-jacentes. Ceci est bien visible sur la figure accompagnant cette note. Parfois les straticules sousjacentes peuvent Ctre Itgkrement ravintes.

ESSAI D'INTERPR~TATIONDE LA

GENBSE

ET DU PROCESSUS DE S~DIMENTATION DES NODULES

Les observations faites prtctdemment tant sur les couches a nodules que sur les stdiments encaissants peuvent se rkcapituler comme suit: (I) Nodules aux dimensions et formes variables armis par des fragments de tests. (2) Nodules se dtformant ou s'impressionnant mutuellement. (3) Nature pttrographique des nodules difftrente de celle des couches encaissantes. (4) Majoritt des nodules disposts avec leur plus grand axe parallkle A la stratification. ( 5 ) Base de la couche a nodules dtformant les straticules sous-jacentes. (6) Sommet de la couche A nodules nivelt progressivement par les straticules susjacentes. (7) Stdimentation identique de part et d'autre de la couche a nodules. De I'examen de cette rtcapitulation, certaines conclusions se dtgagent. Par Ieurs formes arrondies, lobtes, et le fait qu'ils se moulent les uns sur les autres, au moment de leur stdimentation ces nodules devaient Ctre constituts par un mattriel encore

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plastique, imprtgne d’eau. De plus, ttant donne qu’un certain nornbre sont arrnts par des fragments de tests, ceci indique qu’ils ont r o u k sur un fond couvert de coquilles et de carapaces bristes. La prtsence, parmi ces nodules, de certains constitues de feuillets finernent stratifits rappelant en tous points la stratification fine observable en plusieurs niveaux des Schistes a Calymknes, conduit a regarder ces nodules cornrne des fragments roults a partir de couches ayant t t t plus ou rnoins Crodtes. La nature pttrographique des nodules ttant differente de celle des couches encaissantes, la base de la couche a nodules deformant Ie toit de la couche encaissante sous-jacente, tout ceci traduit indiscutablernent I’allochtonie des couches ci nodulps d a m les schistes straticu/&s encaissants. Dks lors, on est amen6 regarder ces nodules et les fragments de tests cornme des eltments remanits et transportts par un courant dans des zones oh se faisait une sedi-

Fig. I . Schema dessine sur uiie section longitudinale de carotte perpendiculaire a la stratification. 1 = Straticules encaissantes sous-jacentes. 2 = Galets mous. 3 = Lumachelle. 4 Straticules encaissantes sus-jacentes. La pyrite de fer est reprksentk par les taches noires.

mentation calme, vaseuse, crtant un milieu franchement rtducteur tres prks de l’interface stdimentleau. L’arrivte des nodules et des debris de tests apparait comrne un hiatus dans la sedimentation calme qui ne cesse d’ailleurs pas car elle nivelle peu a peu les nodules. 11 convient d’ouvrir ici une parenthese. Apres cette description des nodules grisclair, le terrne de “nodule” apparait impropre car, de toute evidence, on a affaire a des galets qui ttaient mous au moment de leur sedimentation. Ce disant, nous confirmons une hypothese h i s e par DANGEARD et RIOULT (1959). Les difftrentes coupes completes obtenues dans les Schistes a Calyrnhes par carottage continu r t d l e nt a plusieurs niveaux la presence de ces couches 2 galets mous. Quelle interprttation pouvons-nous donner ce phtnornene? Dans la nature actuelle les galets mous, encore appelts galets de boue, sont bien connus et prennent naissance sur les estrans partir de bancs d’argile ou de vase, disstques par l’erosion. Nous citerons pour rntrnoire ceux de la Mer du Nord dkrits par 1956) et par VANSTRAATEN (1957) sur le littoral du R. Richter (citt dans LOMBARD, delta du RhBne et dans la mer des Wadden.

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A l’aide de ces donntes, il semble que l’on puisse, avec assez de vraisemblance, donner l’interprttation suivante de la genbe et du processus de sedimentation de ces galets. Sur un estran, des bancs de vase carbonatte plus ou moins consolidtes se niorctlent sous l’action des vagues et donnent des mottes qui s’arrondissent par roulage. Comme, trts souvent, sur les estrans existent des cordons formes de fragments de coquilles et de carapace d’organismes, certains galets mous s’arment en passant et repassant sur ces debris. Les diffhents courants c6tiers s’emparent de ces galets et des fragments de tests puis les ttalent sur d’autres estrans ou bien les emmtnent vers le large. Des observations, faites actuellement, ont montrt que les courants pouvaient transporter les galets mous a quelques kilomttres d’un rivage. I1 semble bien que ce soit le cas pour les couches a galets mous des Schistes a Calymenes. En effet la stdimentation de type calme des schistes encaissants parait devoir s’&tre optrte A quelqce distance d’upe rtgion tmergte. Ceci est bien en accord avec les rtsultats d’exptriences effectutes par CAILLEUX (1936) qui voit dans un fond vaseux tranquille un environnement trts favorable a la conservation et i la fossilisation des galets mous. I1 n’est pas impossible que de tels courants, dirigts vers le large, assez compttents pour transporter ces galets, aient trodt sur leur trajet des bancs toujours immergts de vase argileuse dont les fragments arrachts puis roults et associts aux galets mous, ont donne, aprts lapidification, des galets de schiste argileux bleu-noir. Les petits fragments, plus ou moins arrondis, visibles dans la lumachelle, proviennent trhs certainement de galets mous dissocits au cours du transport. Nous avons suppost possible la genhse de galets mous partir de vases argileuses toujours immergtes. Comme la formation de galets mous a partir de telles vases n’est possible que si cellcs-ci offrent un certain degrt de compaction, pour que notre hypothtse soit fondte il nous faut une preuve de cette compaction. Une observation nous la fournit. Sachant que les organismes fouisseurs recherchent des sediments ayant d6ji une certaine compaction et, comme nous avons dtcouvert un terrier laminaire partant de la limite lumachelle-straticules sous-jacentes, et ptnttrant dans ces dernieres, nous avons bien ainsi la preuve que ces straticules posstdaient un certain degrt de compaction lors du dtp6t de la couche galets mous. Cette etude sur carotte a son revers car elk ne nous permet pas de connaitre l’extension horizontale de ces couches, et nous place dans l’impossibilitt momentante de repondre A la double question: le transport des galets mous, suppost vers le large, se faisait-il sur un front assez large ou bien, au contraire, ttait-il canalist par des chenaux ?

I1 est tout d’abord rappelt que les Schistes A Calymhnes sont considtrts comme des dtp6ts d’eau peu profonde. Les observations faites sur des couches A nodules gris-clair apparaissant a plusieurs niveau dans la coupe des Schistes A Calymhnes conduisent a

COUCHES INTRAFORMATIONNELLES DANS L’ORDOVICIEN DE CAEN

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regarder ces “nodules” comme Ctant, en rCalitC, des galets, primitivement mous. Un essai d’interprttation de leur mode de sidimentation laisse supposer l’existence d’un rivage ou d’un haut fond tmergt assez proche et celle des courants les ayant vehicults vers le large.

SUMMARY

The Schistes a Calymtnes are considered as shallow water deposits. Investigations of the layers with light-grey coloured nodules, several of which are intercalated in these Schistes, lead to the conclusion that the nodules were originally mud pebbles. It may be supposed that they were formed on a shore or on an emerged bank, and that they were deposited in the vicinity of these places after transport by currents.

BIBLIOGRAPHIE

BIGOT,A., 1904. Reunion extraordinaire de la SOC.Geol. France en Basse-Normandie en 1904. Birll. SOC.CPol. France, SPr. 4 , 4 : 861-953. CAILLEUX, A., 1936. Galets et grains mous. Bull. Sor. CCol. France, Ser. 5 , 6 : 321-330. L., 1939, 1941, 1950. Les Phosphates de Chaux sidimentaires de France, Ehdes des Cites CAYEUX, niinPraux de la France. Serv. Carte GCol. France. Imprimerie Nationale, Paris, 1 : 350 pp.; 2 : 310 pp.; 3 : 360 pp. DANGEARD, L. et DORE,F., 1955. Silurien du synclinal d’Urville (Calvados). Dkouvertes paltontologiques et observations nouvelles de sedinientologie. Compt. Rend., 241 : 1323-1325. L. et RIOULT,M., 1959. Observations sur les traces des organisrnes fouisseurs dans ]’OrDANGEARD, dovicien normand. Bull. SOC.GPol. France. SPr. 7. 1 : 271-276. Calyrnenes. Conlpt. Rend. SOC.BiqyPograph. Paris, 19 (162DELPEY, G., 1942. Le bios des Schistes I 163) 35-39. DORE,F., 1962. Passage du Carnbrien Il’ordovicien dans la coupe du synclinal de May (vallee de I’Orne). Compt. Rend., 255 : 325-326. LOMBARD, A., 1956. G~ologiesPdimentaire. Les Shies marines. Masson, Pans, 722 pp. VANSTRAATEN, L. M. J. U., 1957. Recent sandstones on the Coasts of The Netherlands and of the RhBne delta. Ceol. Mijnbouw, 19 : 196213.

MESOZOIC AND CENOZOIC DELTAS O F TIAN-SHAN AND PAMIR V. I. POPOV,

S . D.

MACAROVA, A. A . PHILLIPOV, A. and R . Y . M U Z A P H A R O V A

A.

BOGOIAVLENSKY

Moscow Geological Prospecting Institute, Moscow (U.S.S.R.)

In this report a reconstruction is given of certain ancient environments on the basis of depositional facies studies, following principles expounded by 0. V. Navlikin and A. P. Pavlov. The separate environments were distinguished from each other by one of us (V. I. Popov). The major environmental units are complexes, belonging to four types: (I) elevations of the land; (2) land-valleys; (3) submarine valleys; (4) submarine rises. In these complexes of environments we distinguish smaller units called subenvironments. Each is characterized by certain dominant processes of sediment transportation. The continental sediments which have been studied in detail in the region of AmuDarya can be divided into: (a)channel sands, deposited in wash-outs (coarse grained material with trough-like cross-bedding), or on point bars (fine material without distinct laminations), (h) sediments of river banks (laminated aleuropelites) and (c) those formed in stagnating basins of the flood plain. The last type includes swamp deposits (dark, unlaminated aleuropelites with plant remains) and lake sediments (laminated and unlaminated aleuropelites, characterized by homogeneous composition and presence of ostracode and mollusc remains). One often encounters also indications of old soils (swamp, meadow, dry steppe and takyr types). The subaqueously formed delta deposits frequently show, both in the continental and in the marine parts, contorted structures due to slumping, and sand dykes. The marine deltaic sediments may be subdivided into the following types arranged in the order of decreasing distances from the original shore: (a) Coastal barrier deposits, composed of the coarsest material in the whole marine complex: various sands and gravels with concave and irregular bedding (Fig. 1. 4). (6) Sands and aleurites deposited in submarine channels extending from the coast into the marine basin (Fig.1. 5). (c) Sands and aleurites of subaqueous bars in front of the delta shore (Fig. 1. 6, 7), characterized by the frequent occurrence of series of unidirectionally inclined laminae with subparallel surfaces (1-5 mm thick) (Fig.2). (rl) Pro-delta sediments formed at a greater distance from the shore: horizontally bedded clays (Fig. 1. 8). In the transition zone from the submarine sand bar area (Fig. 1. 6) to the pro-delta clays (Fig.1. 8) one often finds horizontal, irregular, somewhat

MESOZOIC AND CENOZOIC DELTAS OF TIAN-SHAN AND PAMlR

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Fig.1. Theoretical diagram of paleo-delta environments, as studied in the Turonian of the depressions of Central Kizil-Kum and Bucharo-Chivin. 1 = Subaerial part of delta. 2 = Channels of subaerial part of delta. 3 = Shallow delta bays (or estuaries), on landward side of coastal barrier (coarse silts). 4 = Barriers (with intei ruptions, curvatures etc.) (unsorted laminated coarse sands). 5 = Submarine channels (unsorted sands and aleurites). 6 = Area of submarine bars (irregular alternations of aleurite and clay laminae, with many plant remains). 7 = Id. (regular, even laminations). 8 = Pro-delta area (with fine clays). 9 = Id. (with pyrite). 10 = Id. (clays and mads with many shells).

Fig.2. Diagram of unilateral cross laminations as found in zone 5 of Fig.1.

nodular laminations [laminae 1-10 mm thick), due to the alternation of sandyaleuritic and clayey material. ( e ) Deposits of stagnant water in the more remote pro-deltaic environments are clays with carbonaceous faunal remains and with pyrite and marcasite (Fig. 1. 9). (f) Only locally, one observes more or less discontinuous beds of clays and mark with many shells, which perhaps formed shoals leading to or caused by the breaking of waves. They originated by sediment supply from the delta and usually represent the last stage of development of rhythmoseries of submarine deltaic deposition. The ripple marks in these sediments are mainly of two kinds: current ripples, 6-8 and 11-12 cm long and wave ripple marks, 5-10 cm long. The latter usually have their

338

V. 1. POPOV ET AL.

Fig.3. Paleo-deltas of Early Albian of Tian-Shan. Delta 1 = Paleo-narin. Delta 2 = Paleo-tara. Delta 3 = Paleo-gulchy. Delta 4 = Paleo-phairama. Delta 5 = Paleocaratag. Delta 6 = Paleoamudarya. Delta 7 = West Kizil-Kumian River. a = Sea coast (after A. G. Babaer).

crests more or less parallel to the shores. Interference patterns are also found, but they are rather rare. On the whole, the submarine deltaic deposits are rather uniform in composition over great distances and they show little variation in colour (yellow-green in the exposures of this arid region). Locally they contain a fauna of Foraminifera. Glauconite and phosphorite are rather commonly present. 63'

66'

200 100

69'

750

100 ZOOkm

Fig.4. Pdleo-deltas of the end of the Early Turonian. Delta 1 = Paleo-arisy. Delta 2 = Paleo-kelesa. Delta 3 = Paleo-angrena. Delta 4 = Palm-narina. Delta 5 = Paleo-tara. Delta 6 = North Pamirian River. Delta 7 = Dzam-Darya. Delta 8 = Paleo-zeravshan. Delta 9 = Nura-Darya. Delta 10 = West Kizil-Kumian River.

The first investigator who pointed out the probability of a wideydistribution of Cretaceous deltas in Middle Asia was Borneman. His interpretationuof - the Paleoze-

MESOZOIC AND CENOZOIC DELTAS OF TIAN-SHAN AND PAMIR

339

rarshan delta was confirmed by the works of E. P. Bruns and the present authors. This area has been repeatedly the site, during the Mesozoic and Cenozoic, of deltaic sedimentation (Fig.3, 4).

SUMMARY

In the Mesozoic and Cenozoic delta formations of Tian-Shan and Pamir nine different facies are distinguished. The areal relations of the corresponding environments of sedimentation are indicated in the theoretical diagram (see Fig. 1). The maps (Fig.3, 4)show the location of these paleo-deltas.

A REVIEW O F THE FACTORS AFFECTING THE SEDIMENTATION OF THE MILLSTONE GRIT (NAMURIAN) IN THE CENTRAL PENNINES H. G. R E A D I N G

Department of Geology, University Museum, Oxford (Great Britain)

INTRODUCTION

The principal interest in a study of the Central Pennine Namurian Millstone Grit centres upon the demonstration of the passage from a sea with moderately deep clay basins and limestone shallows, margined by intermittent deltas and coastal plains in Visean times, into a region of widespread coastal swamps and marshes with rare marine incursions in the Westphalian Coal Measures. It is the type area of Millstone Grit, having a low structural dip and good stratigraphical control because of the work on goniatite zones by BISAT(1924), BEAT and HUDSON(1943), HUDSON(1945), MOORE(1950) and HODSON(1957). Detailed mapping by the Geological Survey of Great Britain over a large part of the area has provided a fine basis for more specialised work. The study of Millstone Grit sediments of Namurian age began with SORBY(1859) who showed that they had been derived from a crystalline terrain lying to the northeast. GILLIGAN(1920) measured the cross bedding and described the sediments as the accumulation of deltaic deposits of a large river flowing from the northeast. For the next 30 years little work was done specifically on the sedimentology of the Millstone Grit, though WRIGHTet al. (1927) noticed the rhythmic character and described a four-fold cycle of marine band, mudstone, sandstone and coal. In the last ten years, interest in the sedimentation history has revived with the work of TROTTER (195 1) and MOSELEY (1954). WALKER (1955) measured the cross bedding directions of R, sandstones south and east of Skipton and showed that current flow was to the southwest and southeast. STEPHENS (1953) did the same for R, and G, sandstones between Sheffield and Derby, finding that they have a northerly direction. Recently, ALLEN(1960) has described the R,, Mam Tor Sandstones as a turbidite facies and SHACKLETON (1962) has suggested that the Rough Rock (GI) was deposited on a broad coastal plain, the sediment being distributed by a number of rivers, perhaps aided by flash floods, flowing in general from the northeast. The present study has arisen from a compilation of literature, and field work which the author undertook during 1960 for Shell International Research, to whom the author is grateful for financial support and permission for publication.

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SEDIMENTATION FACIES

The distribution of sedimentation facies is shown in Fig. 1 and 2. Four units have been established, beside the areas of non-deposition or erosion shown in Fig.2. ( I ) Regular cyclic units of paralic coastal plain and near-shore sediments composed of pebbly sandstones, sandstones, shales, coals and frequent marine horizons. (2) Thick accumulations of coastal plain and near-shore sediments composed of pebbly sandstones, sandstones and shales with rare coals and no marine horizons. (3) Off-shore delta front sediments composed of sandstones, siltstones and shales, some of which were deposited by turbidity currents, with a very sparse marine fauna. (4) Marine basinal shales with an abundant lamellibranch-goniatite fauna.

LATE VISEAN

By Late Visean times marine goniatite~lamelljbranchshales were being deposited in the northern part of the Central Pennine trough. In places limestones were developed, black limestones in the deeper parts and back-reef, reef and fore-reef limestones on the Derbyshire massif. To the north the deltaic and the coastal plain deposits (Yoredale facies), which were confined to the Northumbrian trough in Early Visean times, had spread over the North Pennine massif to the Craven hinge line. To the south sediments thinned against the Welsh-Brabant massif. Thus by Late Visean times the sedimentary influence of North Atlantis (the gneissic terrain lying across the north of Scotland and extending eastwards to Scandinavia, and whose uplift provided most of the clastic material for the Carboniferous of northern England and Scotland) reached the Craven hinge line; the Central Pennine trough was established as a negative area of rather deep water sedimentation, with the Derbyshire massif as a local topographical high and the Welsh-Brabant massif as a further major positive area to the south.

NAMURIAN

Namurian E,a-b The passage from Visean to Namurian is conformable in the Central Pennine trough, except towards the edges, where in the north there is a wide area of erosion in early El times (Fig.2a) and around the Derbyshire massif, where non-deposition with relatively little erosion prevailed until late R, times and which perhaps was not finally covered by sediments until R, times. Ela-b time is characterised by deposition of marine shale of no great thickness (about 300 ft.). Around Skipton, however, STEPHENS et al. (1953) found the Upper Bowland Shale differed from the Visean Lower and Middle Bowland Shale in being more sandy and having the marine faunas confined to calcareous bands. They con-

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.

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MILES

Fig.1. Diagrammaticcross section of the Namurian of the Central Pennine trough, showing sedimentationfacies.

SEDIMENTATION OF THE MILLSTONE GRIT

343

sider that the sandy non-marine layers show the start of the northern deltaic influence. Further south, north of Castleton and west of the Derbyshire massif, siliceous mudstones and siltstones, known as Crowstones, are found. Their siliceous nature contrasts with the typical quartzo-feldspathic Millstone Grit sandstones and they were probably derived from the Welsh-Brabant massif which at this time was the provenance for the largely quartzitic Cefn-y-Fedw Sandstones of North Wales. Namurian El, In El, times (Fig.2b) the first great flood of coarse clastic material swept over the Craven hinge line with deposition of the Skipton Moor Grits, which expand from 3W 2,500 ft. in less than 5 miles. These Grits consist largely of very coarse pebbly quartzofeldspathic sandstones, but include flaggy sandstones, siltstones and shales and occasional fireclay and coal. Few beds continue far laterally and MOSELEY (1954) has described channels. No marine bands are known. To the south the strata are hidden, but in the Alport Borehole there is no sign of coarse clastics, marine basinal shales persisting around the Derbyshire massif until R,, times. The stage is brought to a close by a widespread marine horizon, the E . bisulcatum-Edge Marine Band which initiates a set of conditions which remain more or less constant until R,, times. Namurian E2-R,b Throughout the period from E, to early R,, coarse non-marine clastics do not reach far beyond Skipton, for to the south marine Sabden Shales occur (Fig.2~). North of Skipton there are several cyclothems; some have a wide extent (such as the single one which embraces H stage and two in E,) and can be traced right onto the North Pennine massif; others in E, and R, vary from place to place and present difficulties in correlation between the Lancaster Fells and Skipton and even within the Skipton area. Pebbly sandstones are rare between E, and R,, and are almost confined to E,, the H stage sandstones being non-feldspathic. Namurian R,, In R,, times, the close of which is shown in Fig.2d, there is a repeat of the conditions of the Skipton Moor Grits with a great influx of clastic sediment, over 1,500 ft. thick in places compared with less than 200 ft. for R,a-b. The Kinderscout Grit reaches further south than the Skipton Moor Grits, extending to the northern part of the Derbyshire massif. It is preceded by a considerable thickness of sediments of a type which has not been recognised below the Skipton Moor Grits although it may occur in the unexposed centre of the trough, and it may be partly represented by the Upper Bowland Shale. This series of sediments begins with the turbidite Mam Tor Sandstones (ALLEN,1960) fronting the main delta advance and continues through the Shale Grit and Grindslow Shale, a complex sequence of sandstones and shales with very rare marine fossils, which appears to have been deposited on the upper slopes of an advancing delta front. The Kinderscout Grit usually has a sharp erosive base, and it is made up of two or

344

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F i g 2 Sedimentation sketch maps of the Central Pennine trough and surrounding areas. Legend as for Fig. I .

SEDIMENTATION OF THE MILLSTONE GRIT

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more leaves of very coarse pebbly sandstone separated by shale and finer sandstone. To the north the Kinderscout Grit passes into a sequence of regular paralic cycles of non-marine sandstones, shales and marine-bands, which are difficult to match with the southern Kinderscout Grit because the conditions of deposition are essentially different. The Upper Kinderscout of Hebden Bridge appears to be of this type and thus while through the whole of R,, times regular units of paralic sediments were deposited above the Skipton Moor Grits, this facies spreads southwards in late R,, times. Namurian R,-G, Finally in R, and GI times, virtually the whole area is covered by paralic sediments, and the classical Millstone Grit cycles described by WRIGHT et al. (1 927) are developed. There are five major cycles in the north, four in R, and one in G,, but towards the south and west the non-marine portions of the lower cycles disappear and in the west (Flintshire) marine shales persist into G, times. Difficulties in the correlation of some marine horizons and the lateral variation of fauna suggest local movements of the coast line caused by river and distributary diversion. A new development arises in the southeast where southerly thickening and northwestward pointing cross-stratification (STEPHENS, 1953) in the upper two cycles indicate further complications. It is evident that by this time the broad coastal plains of the Coal Measures were already established, the Rough Rock (SHACKLETON, 1962) covering an area of over 4,000 square miles yet maintaining a fairly constant average thickness of only 200 ft.

CONCLUSIONS

Whilst much detailed work remains to be done before the sedimentology of the Millstone Grit is fully understood, it is apparent that sedimentation is governed by several factors. (I) Uplift in and proximity to the area of erosion, North Atlantis. This is the dominant factor in Carboniferous sedimentation in northern Britain and its uplift is shown to be episodic by the coincidence of coarse pebbly sandstones with the pulsatory fill of the basin southwards. (2) Separate provenance and minor supply from the Welsh-Brabant massif shown by the Crowstones and possibly other clastics west of the Derbyshire massif. (3) Variations 'in subsidence in the area of deposition giving variations in thickness, as for example across the Craven hinge line. (4) General relative rises of sea level providing thin but widespread marine horizons which cross local tectonic lines such as the Craven hinge line. These must be due either to sudden regional subsidence over a very wide area or else to eustatic rises of sea level. The latter seems the more likely. (5) Local geographical variations causing intermittent marine horizons which cannot be traced or correlated very far.

346

H. G . READING SUMMARY

The sedimentation history of the Central Pennine trough is described and it is suggested that the sedimentary pattern is the response to a variety of factors, including proximity to and episodic uplift of the source area, differential subsidence in the area of deposition, variations in provenance, widespread changes of sea level and local fluctuations in supply. It is emphasized that there are two pulses of basin fill, the Skipton Moor Grits and the Kinderscout Grit, whose environment and facies are essentially different from the more regular cyclic paralic sediments which make up the rest of the Millstone Grit.

REFERENCES

ALLEN,J. R. L., 1960. The Mam Tor Sandstones: a “turbidite” facies of the Namurian deltas of Derbyshire, England. J. Sedinient. Petrol., 30 : 193-208. BISAT,W. S., 1924. The Carboniferous goniatites of the north of England and their zones. Proc. Yorkshire Geol. SOC.,20 : 40-124. BISAT,W. S. and HUDSON,R. G. S., 1943. The lower Reticuloceras (R,) goniatite succession In the Narnurian of the north of England. Proc. Yorkshire Geol. SOC.,24 : 3 8 3 4 0 . A., 1920. The petrography of the Millstone Grit of Yorkshire. Quart. J . Geol. Soc. London, GILLIGAN, 15 : 251-294. HODSON,F., 1957. Marker horizons in the Namurian of Britain, Ireland, Belgium and western Germany. Publ. Assoc. I?tud. PalPontol., 24 : 1-26. R. G. S., 1945. The goniatite zones of the Namurian. Geol. Mar., 82 : 1-9. HUDSON, MOORE,E. W. J., 1950. The genus Sudeticeras and its distribution in Lancashire and Yorkshire. J. Munchester Geol. Assoc., 2 : 31-50. F., 1954. The Namurian of the Lancaster Fells. Quart. J . Geol. Soc. London, 109 : 423454. MOSELEY, J. S., 1962. Cross-strata of the Rough Rock (Millstone Grit Series) in the Pennines. SHACKLETON, Liverpool Munchester Geol. J . , 3 : 109-1 18. SORBY,H. C., 1859. On the structure and origin of the Millstone Grit in South Yorkshire. Proc. Yorkshire Geol. Polyiech. Soc., 3 : 669-615. E. A,, 1953. On the Rough Rock and Lower Coal Measures near Crich, Derbyshire. Proc. STEPHENS, Yorkshire Geol. Soc., 28 : 221-227. J. V., MITCHELL, G. H. and EDWARDS, W., 1953. Geology of the country between Bradford STEPHENS, and Skipton.Geol. Surv. Gr. Brit., Mem. Geol. Sitrv. Gt. Brit. Engl. Wales, 1953 : 180 pp. TROTTER. F. M., 1951. Sedimentation facies in the Namurian of northwestern England and adjoining areas. Liverpool Manchester Geol. J., 1 : 71-1 12. WALKER, C. T., 1955. Current bedding directions in sandstones of Lower Reticuloceras age in the Millstone Grit of Wharfedale, Yorkshire. Proc. Yorkshire Geol. SOC.,30 : 115-132. WRIGHT,W. B., SHERLOCK, R. L., WRAY,D. A., LLOYD,W. and TONKS,L. H., 1927. The geology of the Rossendale anticline. Geol. Surv. Gt. Brit., Mem. Geol. Surv. Gt. Brit. E y l . Wales, 1927 : 182 pp.

LE FLYSCH: DGFINITION; DEPOT DE FAIBLE PROFONDEUR? M A R G U E R I T RECH-FROLLO

Centre National de la Recherche Scientifique, Toulouse (France)

INTRODUCTION

En 1827, STUDERexposait dans un Mtmoire des Annales des Sciences Naturelles “le rtsultat d’observations faites au cours de plusieurs voyages dans les Alpes”. I1 employait alors pour la premihe fois le terme de Flysch pour un certain groupe de formations qu’il dtcrivait ainsi: “Le fond de la vallte depuis Erlenbach jusqu’ri Zweisimmen, tout le Hundsrucken et le fond de la vallte d’Ablenschen, les Saanenmoser, le fond de la vallte de Rougemont et de Chiiteau d’Oex, la vallte tlevte des Mosses jusqu’a Stpey, toute cette ligne parallble ri la direction des Alpes est occupte par une formation qui se montre en gtntral sous la forme de schistes et grbs noiritres, rnais qui prend un caractbre trbs compliqut par la prtsence de blocs et de couches calcaires subordonntes, de grandes masses de brkches calcaires, des couches de quartz et de silex pyromaque noir et vert de poireaux. Les roches oh la structure schisteuse prtdomine sont appelltes Flysch dans le pays et nous pouvons sans inconvtnient ttendre cette dtnomination toute la formation. Les roches ressemblent tellement ri celles de la chaine du Niesen et par constquent celles de la chaine de Glaris que je n’htsiterai pas A rtunir ces formations si le gisement le permettait, seulement -ajoutait Studer le mica est assez rare dans le Flysch tandis qu’il est fort commun dans les grks du Niesen”. Par la suite il a t t t Ctabli que le Flysch des regions tnumtrtes ici par Studer appartient i plusieurs unites paltogkographiques et tectoniques difftrentes: Flysch de la nappe de la Simme (Hundsrucken, Rougemont, rtgion de ChBteau d’Oex), Flysch de la nappe de la Brkhe (Saanenmoser), Flysch de la nappe des Prtalpes mtdianes (rtgion de Chiteau d’Oex tgalement), Flysch de la nappe du Niesen (vallte tlevte des Mosses et S6pey)’etm&meFlysch de la nappe ultrahelvktique (col des Mosses). I1 est curieux de remarquer que ce terme de Flysch, introduit par Studer dans la terminologie des formations alpines et qui tout de suite a connu une fortune considtrable n’a t t t utilist ri l’origine par l’auteur qu’accidentellernent. I1 ne lui a donnt aucun sens explicatif ni aucune valeur d’entitt pttrographique encore moins celle d’entit6 gtologique. Aussi dks le dtbut il s’est c r t t une confusion sur son vtritable sens: bien des auteurs s’en emparent et l’appliquent ri toute formation embarassante qualifier tant par son Bge que par sa nature, tant et si bien que 50 ans plus tard STUDER(1872,

348

M. RECH-FROLLO

p.82-83) voit la ntcessite d’tnumtrer i nouveau les formations auxquelles il avait applique le terme de Flysch, et d’insister pour que ce terme leur soit uniquement rtservt. Mais si cette fois encore I’auteur ne donne pas une dtfinition compltte des caracttres du Flysch, il exprime leur originalitt, bien que le terme ne soit “localement donnt dans le Simmenthal qu’a une roche se dtbitant en feuillets” (STUDER,1872, p. 22) et que les diversitts pktrographiques et paltogtographiques n’aient tt6 reconnues que beaucoup plus tard. Lorsque j’ai entrepris le travail de dCfinir et d’interpreter les formations dtsigntes sous le terme de Flysch par Studer je me suis tout d’abord appliqute i analyser ce double aspect d’hornogtntitt et en mCme temps de diversitt qu’elles prtsentaient. J’ai t t t ainsi amente en premier lieu i faire la part des caractkres qui se montraient essentiels dam la dkfinition du Flysch et des caractkres qui ttaient particuliers, pouvaient varier ou rnCme manquer d’une unitt de Flysch des Prtalpes suisses A une autre (RECH-FROLLO,1955). Un premier fait certain m’apparaissait immtdiatement: les caractkres essentiels du Flysch pris stpartinent ou mCme par groupes de trois ou quatre se rencontraient aussi dans des formations autres que le Flysch et ce n’ttait que leur coexistance dans une formation, leur totalitt qui permettait de dksigner correctement cette formation du terme de Flysch (RECH-FROLLO, 1955l, p. 1589).

LES C A R A C T ~ R E SESSENTIELS DU FLYSCH

Dans des notes successives, partir de 1955, j’ai fait connaitre l’tvolution de mes observations qui s’enrichissaient mesure que rnes recherches sur la nature des caracttres essentiels du Flysch s’approfondissaient. Voici en l’ktat actuel des connaissances les caractkres essentiels des formations des Prtalpes suisses auxquelles Studer donnait l’appelation de Flysch; ces caractkres sont d‘ordre gtologique, paltogkographique, pttrographique, palto-ockanographique et palkontologique. CaractGres ge‘ologiques

( I ) Les Flyschs des Prtalpes suisses sont concordants avec leur substratum (Flysch de la Sirnme, Flysch de la Brtche, Flysch des MMianes) ou ltgtrement transgressifs, sans que soit intervenu avant cette transgression des modifications paltogtographiques rnajeures (Flysch du Niesen, Flysch ultrahelvttique, Flysch helvktique). (2) Les Flyschs des Prtalpes suisses reprtsentent un “facies de fermeture” : le dernier terme d’un dtp6t avant son plissement lorsqu’il s’agit d’unitts charrites ou le dernier dCpBt avant celui d’une Molasse marine lorsque l‘exondaison se fait sur place insensitdement (Flysch helvttique autochtone p.e.). Cette observation essentielle a C t t par la suite reprise par d’autres auteurs, sans que toutefois, janiais, j’en trouve mentionnee l’origine.

LE FLYSCH

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Caracteres pale‘oge‘ographigues Les reliefs a l’origine du mattriel dttritique du Flysch des Prtalpes suisses n’ont laisst aucune trace; ils sont inconnus, comme demeurent aussi certains massifs granitiques qui ont fourni au Flysch un mattriel correctement appelt exotique (granite 8 feldspath rose du Flysch ultrahelvttique p.e.). Caractires pe‘trographiques Le Flysch des Prtalpes formt de conglomtrats, microbrkhes, grks, vases calcaires ou microgrks calcaires est un faciks essentiellement dttritique. Exceptionnellement Wildflysch du Pessot, p.e., - il renferme des lentilles de calcaires organiques qui cependant demeurent toujours subordonntes aux faciks dttritiques, conglorntrats ou microbrkches. Le mattriel dttritique est form6 d’tltrnents frais, non alttrts, - feldspaths et biotite p. e. - c’est un mattriel jeune, non tvolut. I1 cornprend aussi de la glauconie, dgalement non alttrte, et des dtbris charbonneux en abondance. Les feldspaths sont trks souvent corrodts, le quartz aussi. Ce dernier dtpourvu d’accroissement secondaire quartzitique, posskde des contours anguleux ou subanguleux; il est htttromttrique et disperse dans le ciment (“cirnent originel” au sens de Lucien Cayeux). Le ciment i quelques exceptions prks certains grks et microbrkches du Flysch ultrahelvttique notamrnent - est entikrement calcaire, extrkmement pauvrz en argiles mais souvent riche en limonite. Les ptlites du Flysch ont une constitution identique ti celle du ciment des grks ou microbrkhes entre lesquelles elles sont intercaltes: ptlites calcaires entre les termes grtseux 8 ciment calcaire, ptlites calcarto-ferrugineuses 18 oh le cirnent des termes grtseux est calcarto-ferrugineux, glauconieuses, entre les termes grtseux ii ciment calcaire chargt de glauconie. Elles deviennent mCme calcarto-argileuses l a oh exceptionnellement le ciment des termes grtseux renferme une proportion plus ou moins importante d’argiles. Certaines ptlites calcaires sont trks riches en grains de quartz de diamktres en-dessous de 20 p, ce sont alors des microgrks. Les ptlites-microgrks calcaires reprtsentent le type le plus frtquent. Des ptlites calcaires pauvres en quartz dttritique laissent apparaitre une forte proportion de Coccolithophoridtes, d’autres, sur de grandes tpaisseurs, renferment une quantitt notable de spicules calcaires de mollusques opisthobranches (Flysch i Helmintho’ides).

-

Caractires palPo-ocganograph iques Le Flysch des Prtalpes suisses est un dtp8t rythmique formt de sequences A l’inttrieur desquelles, sur de larges ttendues et de grandes tpaisseurs, apparait une structure varde‘, improprernent appellee schistositt (les “straticules” de FOURMARIER (1 956), Nous donnons ici au terrne vurve son sens gtntral qui en Scandinave veut dire succession d’tvtnements ou de faits.

350

M. RECH-FROLLO

ou les “laminites de deuxikrne ordre” de LOMBARD (1960). Le passage des termes pClitiques aux termes grossiers se fait brusquement, sans transition aucune, ttmoignant de ruptures d’tquilibre ptriodiques et trts frtquents pendant la phase de dCpBt des apports. CaractPrespalkontologiques

Les organismes du Flysch des Prealpes suisses possbdent des caracttres trts particuliers. 11s peuvent Etre classts en deux groupes distincts: ( I ) Un premier groupe, homogtne, est trts rtpandu dans tous les terrnes du Flysch, aussi bien dans les termes ptlitiques que dans le ciment des termes grossiers. (2) Un second groupe renferme des organismes varits, localists seulement dans certains niveaux. Le premier groupe, rtunit uniquement des foraminiftres, sans valeur stratigraphique, qui gtntralernent passent inapercus et qui possbdent les caracttres d’organismes ayant v k u dans des conditions dtfavorables: tailles exigues (dtpassant rarement 20 p), loges disproportionntes, nombre incomplet de loges, dtgtnkrescence ferrugineuse (RECH-FROLLO,1962d). Les organisrnes de la seconde cattgorie se groupent par ordre de tailles correspondant a celles des tltments dttritiques mintraux qui les accompagnent: foraminifkres normaux, dtbris de rntlobtsites, prismes d’inoctrames, fragments de bryozoaires et d’echinodermes, dtbris charbonneux. 11s possMent en comrnun un caractbre fondamental: ils sont tous susceptibles d’Ctre flottts (RECH-FROLLO, 1962d). Les calcaires du Flysch, lorsqu’ils sont largement dtveloppts, en ttendue et en tpaisseur, renferment des ttmoins de rnollusques opisthobranches (spicules calcaires et cordons de ponte). De tels calcaires sont inconnus ailleurs que dans le Flysch (RECH-FROLLO, 1962b). LE FLYSCH DE LA C H A ~ N EALPINE

Les caracttres particuliers du Flysch des Prtalpes suisses - nornbre et tpaisseur des termes d’une stquence, prtsence ou non d’un granoclassement des Cltments dttritiques, prtsence d’une schistositt vraie, prtsence d’tltments volcaniques, Bges des difftrents Flyschs - varient ou mCme nianquent d’une m i t t de Flysch a une autre et ils ne doivent pas figurer dans la dtfinition du Flysch; leur mention, mtlangte a celle des tltments essentiels ne contribue qu’i crter la confusion. PrPsence des caracterrs essentiels

Les caracttres essentiels qui dtfinissent les formations que Studer dtsignait sous le terme de Flysch se retrouvent-ils inttgralement dans les formations de la chaine alpine qui par la suite ont Ctt grouptes sous un mCme terme de Flysch? De longues ttudes dans les Alpes franCaises, 1’Apennin septentrional, les Carpates et les Pyrentes rn’ont permis de constater qu’un certain nombre seulement des forma-

LE FLYSCH

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tions qualifikes de “Flysch” posstdaient, en effet, la totalitt des caracttres essentiels du Flysch des Prlalpes suisses et que d’autres par contre s’en kartaient foncibrement; ces dernibres formations rtunissaient en outre, en totalitt, les caractkres essentiels, gtologiques, paltogtographiques, pktrographiques, palto-octanographiques et paltontologiques de certaines Molasses marines (RECH-FROLLO, 1960). En d’autres termes l’analyse approfondie des terrains que Studer avait groupis sous le terme de Flysch m’avait donc permis d’une part de dtfinir leur originalitt et d’autre part de montrer qu’ils pouvaient &trereconnus avec certitude aussi dans d’autres segments de la chaine alpine mais avec une frtquence plus limitte que celle qu’on leur avait assignee. Le Flysch dtfini en Suisse, prenait ainsi la valeur d’entitt gtologique universelle. I1 ressortait en outre que le passage insensible entre un Flysch et une Molasse ttait un fait incontestable, certains Flyschs - tels le Flysch du versant sud des PyrPlntes ou celui des Carpates Orientales de Roumanie p.e. - ktant trts dtmonstratifs a cet tgard.

LES CONDITIONS DE DfiPi)T ET DE CONSOLIDATION

Les caracttres descriptifs suffisent i eux seuls a dkfinir un Flysch, a le reconnaitre et le distinguer d’autres formations qui, en apparence, lui ressemblent. Cependant l‘interprttation du Flysch a t t t tentte depuis longtemps. Les tentatives d’interprttation ayant p r k t d t toute ttude dttaillte comportent forctment des erreurs. Le cadre de cette note ne permet pas de faire le point de tout ce qui a t t t h i t h ce sujet. Les progrts rtalists dans l’ttude des skdiments actuels sont remarquables et certains caracttres essentiels du Flysch - tels p.e. les caractbres pal~ontologiques,un certain nombre de caracttres pktrographiques - apportent des donntes fondamentales pour l’ttablissement des conditions de dCp8t et de consolidation. Cependant, l’interprttation complbte de la totalitt des caracttres essentiels du Flysch demeure difficile, voire m&me impossible, en l’ttat actuel des connaissances; les conditions de dipst et de consolidation du calcaire du Flysch, de sa glauconie, les causes de sa rythmicitt, reprtsentent des phtnombnes complexes, lits des mtcanismes encore peu connus sur un plan gtntral. Le 6e Congrts International de Stdimentologie propose comme thkme principal I’ttude des dkp8ts de faible profondeur (shallow water deposits). Je tlcherai d’analyser plus bas le probltme de la bathymttrie du Flysch afin de vtrifier si les formations qui, i mon avis, lui correspondent peuvent Ctre en effet classtes parmis ces dtp8ts. Dans ce but nous allons examiner successivement les arguments en faveur d’une bathymttrie profonde du Flyseh et les arguments en faveur de son classement parrni les dtp6ts de faible profondeur. Le Flysch, skdiment de merprofonde? Nombreux sont les auteurs pour qui le Flysch reprtsente un stdiment de profondeur. Quelles sont les preuves fournies? Un bon nombre relbvent uniquement de l’hypothtse. Elles ont h l’origine la con-

3 52

M. RECH-FROLLO

ception de HAUG(1900) des dtp6ts gtosynclinaux, dont le Flysch serait une des phases, et qui par dtfinition sont profonds. Mais si on analyse en dCtail la notion de gtosynclinal, notion qui synthttise la relation entre les divers phtnomknes de dtp6t et de plissement, on s’apercoit avec GOCUEL(1952, p.270) qu“’e1le devient d’un vague insaisissable” et qu’elle mCme, pour avoir un sens prtcis, rtclame d’abord “l’analyse sans id& priconcue de la nature des stdirnents” et “la dtduction de leur conditions de dCpGt”. D’autres auteurs ont estimt que le Flysch ttait un stdiment de profondeur parce qu’il pouvait Ctre homologut aux dtp6ts actuels, supposts Ctre mis en place par les courants de turbiditt, entourant les Iles de la Sonde, ou encore de I’Atlantique ou de la Mtditerrante (les “sables profonds” actuels). Une ttude approfondie de ces dtp6ts comparte i celle que j’avais faite des differents caracttres essentiels du Flysch m’a permis de montrer que les difftrences ttaient telles entres ces cattgories de dtp6ts actuels et ceux du Flysch qu’il apparaissait immtdiaternent coinme Cvident que leur histoire n’ttait pas la m&me(RECH-FROLLO, 1962~).SONDER (1946), pour qui le probleme de la bathymttrie d’un dtp6t “revCt une importance secondaire faute de preuves scientifiques siires” et qui confere l’alluvionnement le r6le essentiel dans la difftrenciation des facies, avait deja fait remarquer, avec juste raison, que en dehors de toutes autres considtrations des dtp6ts des mers actuelles homologuts 5 un Flysch ou a une Molasse marine prtsentent un taux d’alluvionnement insuffisant - quelques metres pour une colonne d’eau d’un kilometre - ce qui ne pouvait Ctre le cas d’un facib oh la rtserve en mattriel dttritique de la colonne d’eau ne pouvait Ctre qu’infiniment plus grande. La seule preuve directe en faveur d’une bathymttrie profonde des mers du Flysch a paru rtsider pour certains gtologues dans la prtsence, parmi les termes ptlitiques du Flysch, de genres et d’esptces de foraminiferes vivant actuellement a de tres grandes profondeurs. Dans une note rtcente (RECH-FROLLO,1962c) j’ai rappelt, preuves nouvelles a l’appui, que la rtpartition bathymttrique des foraminifkres n’ttait pas sptcifique pour un genre et une espke donnte et qu’elle dtpendait de nombreux facteurs physiques et chimiques parmi lesquels la profondeur n’intervenait pas directement. Faible profondew des mers du Flysch Les arguments en faveur de cette seconde hypothkse revetent souvent la valeur de preuves directes. LUCAS(1942, pp.358421), apres une ttude gtologique et pttrographique de formations jurassiques rappelant le Flysch des reliefs formant la partie occidentale des Monts de Tlemcen, a t t t le premier a ttablir une analogie entre le micanisme de dtp6t d’un Flysch et celui de certains dtp6ts actuels de faible profondeur (p.421) oh l’alluvionnement est puissant. Moi-mCme, a peu p r b a la mCme tpoque, j’homologuais les formations des Carpates Orientales de Roumanie grouptes sous le terme de Flysch, B des dtpdts de faible profondeur (RECH-FROLLO, 1942, pp.174-176).

LE FLYSCH

353

Ces analogies dans les deux ouvrages ttaient l’aboutissement convergent d’ttudes pttrographiques et palto-octanographiques de rtgions tres difftrentes. Mais la pttrographie des roches stdimentaires, magistralement exposte par Lucien Cayeux, est restte et reste encore ignorte en dehors d’un nombre restreint de chercheurs. De nombreuses preuves directes de faible profondeur d’un stdiment ancien tirtes de I’etude des structures et compositions des roches consolidtes demeurent ainsi mkonnues par un grand nombre. Les conditions de formation d’une roche sont extrkmement multiples, elles varient dans le temps du fait mEme qu’elles dtpendent ttroitement de “certaines conditions physiques de la terre que tout dtmontre Etre variables” (CAYEUX, 1897, p.537). L’tpoque actuelle peut souvent donner un tquivalent de la modalitt de formation d’une roche ancienne lorsque les conditions de formation deduites de la constitution de cette roche ancienne se retrouvent rtunis, localement, dans leurs parties essentielles du moins, en un point quelconque de la terre. Le Flysch nous en fournira une bonne illustration: dans le cas du Flysch il est avtrt que le taux d’alluvionnement du dtpdt ttait extrkmement elevt ce qui supposait en mEme temps une forte agitation des eaux. Or on a chercht a homologuer les mers du Flysch aux bassins marins oh le taux d’alluvionnement de la colonne d’eau est trks faible comme SONDER (1946) l’a fait remarquer. Actuellement on ne trouve d’alluvionnement intense, en milieu marin, qu’au voisinage de certaines chtes, oh des conditions particulitres d’apport, de c h a t et d’agitation de l’eau sont rtalistes (p.e. GLANGEAUD, 1938). Le caractkre d’alluvionnement intensif et d’agitation des eaux reprtsentt par les roches du Flysch s’accompagne d’autres caractkres non moins instructifs. La constitution pttrographique des roches du Flysch, nous enseigne que les apports dttritiques ttaient faits de sables fortement vaseux; l’tcologie des organismes melangts a ces sables et vases nous apprend que d’une part, les individus rtpandus uniformtment dans toutes les roches du Flysch prtsentaient des caractkres de dtgtntrescence et que, d’autre part, les organismes ou les dtbris d’organismes qui n’apparaissaient que sporadiquement prtsentaient le caractkre unique d’avoir t t t flottts. Or des stdiments qui offrent de tels caractkres se rencontrent actuellement dans les baies peu profondes, les vases d’estuaires (dtgtntrescence ferrugineuse-Darse de Villefranche), dans les baies a alluvionnement intense oa aprks des vagues de tempEte des organismes flottts du large se mtlangent au mattriel terrigkne (Baie d’Alger) et se dtposent en structure granoclasste (criques de la Costa Brava); ou encore dans les plateformes sousmarines de 180 m maximum de profondeur (sables vaseux des mers de Chine) Est-ce i dire que ces rtgions seraient des bassins actuels de Flysch? Certainement non. Mais dans ces rtgions seules, qui ont comme trait commun un caractkre unique, celui de n’Etre pas profondes, se trouvent rtunis un certain nombre des caractkres essentiels du Flysch. D’autre part rien ne s’oppose, bien au contraire, ;i ce que sous des conditions physiques difftrentes de la terre, la totalitt des caractkres essentiels du Flysch n’y soit rtunie. En dehors du problkme de l’alluvionnement, SONDER(1958, p.715) a montrt que les lois gtomtcaniques de la terre prouvent qu’au moment du dtpat du Flysch il existait un rtgime de “tremblement de terre bordiers avec des ptriodes de durte de quelques dix mille ans et d’amplitudes qui peuvent Ctre irrtgu-

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M. RECH-FROLLO

litres mais qui d’aprts le montant des estimations calculables peuvent atteindre 100 m” (la moyenne serait de 50 m). Une variation si grande dans la valeur des durtes et des intensitts d’un facteur gtophysique qui influence directement la stdimentation peut avoir des constquences fondamentales sur l’tpaisseur et l’ttendue d‘un dtpet de faible profondeur oh les caractkres du Flysch apparaissent localement aujourd’hui fidtlement reproduits. Ceci d’autant plus qu’aux variations de facteurs gtophysiques s’ajoutaient aussi l’influence de facteurs physiques diffirents (ttendue et nature des reliefs, climat different). Avant de terminer il convient de signaler qu’en dehors des preuves pttrographiques, des preuves directes de faibles profondeur des sMiments du Flysch ont CtC apporttes rtcemment par MANGIN (1962a, b) par la dkcouverte d‘empreintes de pattes d‘oiseaux ii la surface de certaines couches de Flysch. En outre cet auteur montre l’impossibilitt de la formation de la structure varvte du Flysch, en dehors d’eaux marines peu profondes. Moi-m&me (MANGIN,1962a, p.36) j’ai mentionnt la prtsence d’empreintes de pattes d’oiseaux dans le Flysch ultrahelvttique des Prtalpes suisses; une ttude approfondie ii ce sujet est en cours.

Une ttude, li partir des formations des Prtalpes suisses dtsigntes par Bernard Studer en 1827 sous le terme de Flysch, m’a permis de dtfinir d’une manitre prtcise le sens du terme Flysch par un ensemble de caractkres essentiels, gtologiques, paltogtographiques, pttrographiques, palto-octanographiques et paltontologiques exposts ici. I1 s’est rtvtlt en outre que beaucoup d’autres formations dtsigntes sous le terme de Flysch ne possidaient pas I’ensemble des caracttres essentiels d’un Flysch mais au contraire la totalitt des caracttres de certaines Molasses marines. Les interprttations du Flysch ont prtctdt toute ttude dktaillte; un certain nombre renferment par constquent des erreurs. I1 est analyst ici uniquement le probltme de la bathymttrie du Flysch dans un expost des arguments pour et contre le dtp6t en profondeur des sediments anciens des roches du Flysch.

SUMMARY

In 1827 Bernard Studer published a study of aformation in the Swiss Prealps which he called “Flysch”. This study made it possible for the present author to define accurately the significance of the term “Flysch”, using a complex of essential characters, geological, paleogeographical, petrographical, paleo-oceanographical, all of which are treated in this Paper. It is shown that many other formations known as “Flysch” do not possess the

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essential complex of Flysch-characters but on the contrary show a complex of characters typical of certain Molasse deposits. As every detailed Flysch-study is preceded by an interpretation of “Flysch”, consequently some of them contain errors. Here only the bathymetrical aspect of “Flysch” is analysed by giving arguments for and against Flysch deposition at great depth.

BIBLIOGRAPHIE

CAYEUX,L., 1897. Contribution A I’ttude micrographique des terrains sedimentaires. M6m. SOC. Gdol. Nord France, 4. FOURMARIER, P., 1956. L‘imprkision d’un t e r m usuel du langage gtologique. Compt. Rend., 243 : 695498. GLANGEAUD, L., 1938. Transport et sedimentation dans I’embouchure de la Gironde. Bull. SOC. Gdol. France, 8 (7-8) : 599430. GOGUEL, J., 1952. Trait6 de Tectonique. Masson, Paris, 383 pp. HAUG,E., 1900. Les gtosinclinaux et les aires continentales. Bull. SOC.Ciol. France, 28 : 617-71 1. LOMBARD,A., 1960. Les larnites et la stratification du Flysch. Arch. Sci. Geneva, 13 (4) : 567-570. LUCAS,G., 1942. Description gtologique et pktrographique des Monts de Char Rouban et du Sidi el Abed. Bull. Serv. Carte GPoI. Algdrie, 1, 2. J. PH., 1962a. Le Flysch, sediment climatique? Compt. Rend. SOC.Gdol. France, 2 : 34-38. MANGIN, MANGIN, J. PH., 1962b. Traces de pattes d’oiseaux et flutecasts associk dans un “facib flysch” du Tertiaire pyrtnten. Sedimentology, 1 : 163-166. RECH-FROLLO, M., 1942. Etude pitrographique des SPries sddimentaires de la Vallie du Bicaz, Carpates orientales. T h b e , Paris, 190 pp. RECH-FROLLO, M., 1955. Caractires essentiels et caractires particuliers du faciis Flysch. Compt. Rend., 241 : 1589-1592. RECH-FROLLO, M., 1960. Flysch et Molasse. Bull. SOC.Gdol. France, 2 : 752-757. RECH-FROLLO, M., 1962a. L’originalitt des organismes du Flysch et les conditions d’environnement qui pourraient I’expliquer. Compt. Rend., 254 : 708-710. RECH-FROLLO, M., 1962b. Une nouvelle hypothbe sur l‘origine des Helminthoides du Flysch. Compt. Rend., 254 : 894-896. RECH-FROLLO, M., 1962c. Quelques aspects des conditions de dtpbt du Flysch. Bull. SOC.Giol. France, 4 : 4 1 4 8 . RECH-FROLLO, M., 1962d. Arret de dtveloppement et dtgtnerescence ferrugineuse parmi les Foraminifires du Flysch. Compt. Rend., 256 : 465-468. SONDER, R., 1946. Meerestiefen und lithologische Fazies. Eclogae Geol. Helv., 39 : 2W263. SONDER, R., 1958. Uber die Fazies von Schichtverbanden. Eclogae Geol. Helv., 51 : 742-753. STLJDER,B., 1827. Remarques gtognostiques sur quelques parties de la chaine septentrionale des Alpes. Ann. SOC.Hist. Nut. Paris, 1 (1) : 5 4 7 . STUDER, B., 1872. Index der Petrographie und der Stratigraphie. Berlin, 1,2.

CONTRIBUTION A L'ETUDE DE LA SGDIMENTOLOGIE DES SeDIMENTS CARBONATES A.

R I V I B R E et s . V E R N H E T

Faculte' des Sciences de I'lJniversite' de Paris; Laboratoire de Se'dinientologie d'Orsay (Seine-et-Oise),Paris (France)

Des herbiers de Ruppia occupent une part importante des fonds des ttangs littoraux de la c8te languedocienne restis en communication permanente ou semi-permanente avec la mer. Arrachtes par les vagues de tempete, ces plantes viennent s'tchouer sur le rivage en d'tnormes cordons de varech, isolant des bassins Ctroits et peu profonds qui persistent souvent pendant plusieurs mois. Surtout pendant les saisons chaudes, des fermentations s'y ttablissent rapidement et le milieu devient t r b vite rtducteur (Fig. 1). Alors que le p H des eaux libres de ces ttangs est gtniralement nettement alcalin, pouvant m&me atteindre des valeurs suptrieures 9, nous avons mesurt dans ces bassins bordiers, des pH nettement inferieurs, descendant jusqu'a 6,5, tandis que le rH prenait des valeurs trts basses, comprises entre 7 et 4,8 (mesures faites in situ). TABLEAU I PH ET TH DANS L'EAU DES BASSINS LIITORAUX SULFO-RBDUCTEURS ET DANS LES SfDIMENTS SOUS-JACENTS ~.

~

~~

~

eau libre 5 10 20 30 40 50 60

1.5

_ _ _ _ _ _ ~

~~

6,5

7,0

6,2 6,O 6,4 7,O 7,4 68

6,2 5,3 4,15

~

~~

7,8

8,6

5,8 3,4

63

5,O

5,O'

70 80 90 100 ~

~

6,5 6,O 62 --

_

_

~

_

~

4,8

6,O 28.2

3,8 3.0 3,4 4,O 5,o 3,o 270

3,8 1,6 1,6 1,0

30,6

475

3,o 4,o ~

~

.

~

_

_

_

_

I , 2, 3: coupes verticales diverses; 4 : eau de l'ttang i l'exttrieur des bassins littoraux sulfo-rducteurs; 5: eau de I'ttang, partie centrale. Niveau de vase dure.

_

_

_

S~DIMENTOLOGIEDES S~DIMENTSCARBONAT~S

357

D’autres mesures, faites directement, au moyen d’tlectrodes-sondes, dans l’eau de ces bassins et dans les sediments sous-jacents ont donnt les rtsultats que les chiffres de le Tableau I, relatifs ii deux stries de mesure rtsument assez bien. Si le pH varie assez peu, par suite de la rtserve alcaline et put-Ctre aussi par suite de la prtsence de carbonates (d’origine organogtne) dans les sediments du fond, les valeurs du pH voisines de 5 semblent tout de mCme assez exceptionnelles en milieu naturel pour devoir Ctre mentionntes. Cependant, les variations du rH sont beaucoup plus importantes et plus signi-

Fig.1. Bassin littoral sulfo-riducteur en bordure de l’ktang de Leucate.

ficatives. Nous avions constatt anttrieurement ( RIVIBREet VERNHET, 1959; R I V ~ R E 1959), dans tous les milieux lagunaires ttudits par nous, que des rH inferieurs 12 accompagnaient trts gCnCralement les fermentations sulfhydriques anatrobies. Or ici, toutes les valeurs trouvtes sont inftrieures ii ce chiffre, rtvtlant un milieu excessivement rtducteur. Elles prennent toute leur signification si l’on considkre que le rH atteint normalement 28-31 dans les eaux atrtes normales des ttangs et ne peut descendre au dessous de ztro sous la pression atmosphtrique (RIVIBRE et VERNHET, 1959; RIVIBRE,19591, cette valeur de ztro, obtenue par l’un d’entre nous dans des recherches trts anciennes pour un milieu experimental strictement anatrobie, Ctant celle pour laquelle de l’hydrogkne commencerait A se dtgager. Les valeurs 1 et 2 obtenues, qui correspondent ii des valeurs du pH voisines de 5, sont les plus basses que nous ayons effectivement rencontrtes dans le milieu naturel et sont tvidemment en liaison avec la teneur en SH, de celui-ci. Des contingences mattrielles n’ayant pas permis de suivre l’tvolution du milieu d’une manikre continue, il peut Ctre considtrt comme trts probable que des valeurs encore plus basses et voisines du ztro thtorique puissent Ctre observtes lorsque l’tpaisseur des saiments, leur richesse en matikres organiques et en

358

A.

RIVIBRE

ET

s. VERNHET

collo‘ides argileux ainsi que l’ensemble des conditions locales favorisent l’anatrobiose stricte. Les eaux rtcolttes dans ces petits bassins ont t t t I’objet d’analyses faites, les unes (les plus simples) presqu’immtdiatement, avec le mattriel transportable dont nous disposions, les autres plus tard, au Laboratoire de Stdirnentologie d’Orsay pour les plus complexes. Nous donnons (Tableau 11) les rtsultats principaux relatifs i l’tchantillon no. F 361qui a Ctt le plus coniplktement ttudit. TABLEAU I1 ANALYSE COMPARATIVE DE L’EAU LIBRE DE L‘ETANG ET DES BASSINS L I T O R A U X SULFO-RiDUCTEURS

Dates EcchantiIIons

des

pH

mesures

Eau libre de l’ttang Eau lagunaire no. F 361

2.6.61 29.5.61 2.6.61 27.6.61 1.3.62

rH ____

8,40 6,50 7,60

28 4.8

Rgserve Teneur Teneur alen en M
20

3,9

SOa2-

324,32 724,91

1.80

21

20,25

20

17,50 328,20 789,7 4,50 125,lO 816,2l

1,21 1,28

8,20

8,40

Teneur iotis

en

La salinitk est exprimtie en grammes par litre d’eau, la rberve alcaline en cm3 d’acide N par litre d’eau, les teneurs en Ca et M g en mg par litre d’eau et la teneur en ions SO,Z-en grammes par litre d’eau. L‘augmentation de la teneur en Mg provient probablenient de la mineralisation de la matiere organique (chlorophylle).

Au moment des premikres mesures, le liquide, riche en SH,, laissait ce gaz se dtgager i la pression atmosphkrique. Les teneurs totales en Ca et Mg, telles qu’elles ont pu Ctre recalcultes i partir des mesures ulttrieures, ne difftraient pas sensiblement, compte-tenu de la prtcision des mesures, de celles des eaux libres de l’ttang presentant la mCme salinitt. Par contre, la rtserve alcaline, voisine de 20 (ndlitquivalents par litre), ttait de trks loin suptrieure i celles (dtji tlevtes par rapport i l’eau de mer) que l’on observe habituellement dans les ttangs et qui sont gintralernent voisines de 3-3,5 millitquivalents par litre. Cette Cltvation de la rtserve alcaline ne peut tvidemment s’expliquer que par la rtduction des sulfates par les bacttries anatrobies et leur transformation en d f u r e s et en carbonates suivant un mkanisme A peu prts Cvident. Les flacons de polytthylkne contenant les tchantillons ont ensuite t t t abandonnts au laboratoire pendant une longue ptriode. Le polytthyltne utilist s’est montrt permtable au gaz et SH, a Ctt progressivement tlimint par diffusion, le liquide devenant parfaiternent inodore. De nouvelles analyses ont alors rtvtlt une profonde tvolution du milieu. Le pH a remontt, ce qui s’explique facilernent par l’tlimination de I’acide sulfhydrique et d’une certaine quantitt de CO,, le rH n’a pas t t t mesurt, car il n’aurait eu aucune signification aprts la mise en contact du liquide avec I’atmosphkre. La

S~DIMENTOLOGIE DES S~DIMENTS C A R B O N A T ~ S

359

Fig.2. Precipitt cristallin obtenu au c o w s de l’exptrience dkcrite; x 450.

rtserve alcaline passant de 20,25 a 4,50 a considtrablement diminut, mais cette valeur est encore suptrieure a celle qui serait normale pour des eaux de mCme salinite. On a tgalement recueilli une petite quantitt d’un precipite (environ 0,7 g pour un litre de liquide). Ce prtcipitt, de couleur brune, prtsente une forte odeur de Poisson qui temoigne de la presence de matikres organiques entraintes lors du dep6t du prtcipitt, alors que le liquide n’a plus aucune odeur. Examint au diffractornktre X, par la mtthode de Debye-Scherrer, ce prtcipitk s’est rnontre surtout constitut d’aragonite et de calcite (reconnaissables a l’ensemble de leurs raies caracttristiques) et semble-t-il, d’une petite quantitt de sidtrite dont quelques raies sont prtsentes. Le m&meprtcipite, montt au baume entre lame et larnelle quelque temps aprks, et exarnint au microscope polarisant s’est rnontrt comme formi de grains d’apparence vaguement rhombokdrique, de dimensions comprises entre 15 et 75 p, de birtfringence trks Cltvte. Les figures de lumikre convergente, lorsqu’elles sont observables, sont uniaxes et negatives. Ce serait donc surtout de la calcite. La disparition de l’aragonite s’explique par le temps tcoult et peut-Ctre par l’tchauffement subi par le mattriel lors du montage de la prtparation. Les dosages de Ca, Mg, Fe dans le prtcipite conduiraient a considtrer qu’il est form6 de 84,80 %‘de CO,Ca, de 9,38 % de C0,Mg et de 1,73 % de Fe dont il est difficile de dire s’il est prtsent a I’ttat de sidtrite ou d’hydroxyde, mais la prtsence d’une quantitt relativement considtrable de carbone organique (analyse micro-chimique du C.E.A.) donne penser qu’une faible partie des cathions est prtsente dans le prtcipit6 l’ttat de sels d’acides organiques, parmi lesquels, probablement des humates qui contribueraient a expliquer la teinte brunltre de prtcipitt. Les carbonates constituant la rnajeure partie du prtcipitt correspondent a la diminution de la rtserve alcaline a u cours de l’tvolution spontante de l’tchantillon. La

360

A.

RIVIERE ET s. VERNHET

faible augmentation du magntsium total semble pouvoir s’expliquer par la mintralisation de la matikre organique. Le fait que la rtserve alcaline restante soit suptrieure a celle que l’on trouve habituellement pour les eaux de mCme salinitt s’explique par la diminution des ions Ca2+lits des radicaux acides forts (notamment SO,,-) et par le jeu de la loi d’action de masse suivant un micanisme ttudit anttrieurement ( R I V I ~ R E et VERNHET, 1957, 1958) ii propos des mtcanismes de la sedimentation calcaire.

CONCLUSIONS ET

CONSBQUENCES ~BDIMENTOLOGIQUE~

Les faits qui viennent d’Ctre exposts conduisent aux conclusions suivantes: (I) Dans les milieux naturels plus ou moins isolts de la masse des eaux libres des lagunes et dans lesquels la prtsence de debris organiques diminue l’agitation et la circulation - et par suite la diffusion de l’oxygbne - les fermentations anatrobies peuvent produire la rtduction d’une forte proportion, sinon de la totalitt des sulfates (comme semble I’indiquer une ancienne observation inkdite de L. R. Lafond). (2) Dans les stdiments sous-jacents i de tels milieux, le rH peut s’abaisser i des valeurs qui ne sont sans doute pas loin du ztro thtorique, c’est-i-dire de conditions telles qu’il y aurait dtgagement spontant d’hydrogkne (ou de mtthane), dtgagement que nous avons parfois observt. Ces faits impliquent ntcessairement qu’en profondeur et pour des pressions plus grandes, les conditions soient encore plus rtductrices (RIVI~RE, 1959) et, sans doute capables de donner naissance a des hydrocarbures. (3) Le depart de SH,, par diffusion, entraine, comme nous l’avons constate, une prtcipitation de carbonates. Ce phtnomkne doit ntcessairement se produire i la limite superieure des zones euxiniques caractkrisant les bassins profonds privts de circulation (comme c’est, par exemple, le cas dans la Mer Noire). L’entrainement par adsorbtion des colloi’des organiques qui s’est produit dans nos exptriences nous semble une explication de la tres grande richesse en constituants organiques de certains stdiments carbonatks. Le phtnombne a sans doute jout un certain r61e dans la constitution des sCries pttrolifkres calcarko-dolomitiques. ( 4 ) Les phtnomknes dtcrits peuvent tvidemment jouer un r61e direct dans la stdimentation lagunaire carbonatte, mais le seul rntlange des milieux sulfo-rtducteurs, tvoluant dans les conditions prtcistes ci-dessus, avec les eaux libres des ttangs, a ntcessairement pour constquence une tltvation de la rtserve alcaline de celles-ci, facilitant la prtcipitation du calcaire par le jeux des mkanismes complexes etudits anttrieurement ( R I V I ~ R etEVERNHET, 1958). L’importance et la frtquence des milieux sulfo-rMucteurs qui se constituent en bordure des ttangs littoraux que nous avons Ctudits permet de penser que ce phtnomkne a une importance non ntgligeable dans la genkse des carbonates que nous avons vu s’y prtcipiter (RIVIBRE et VERNHET,1961) et cela d’autant plus que les conditions sulfo-rtductrices peuvent a l’occasion s’ttendre A des surfaces Cnormes, comme ce fut le cas pendant 1’Ctk de 1958 oh elks rtgnaient dans l’ensemble du golfe des Roquettes qui constitue la corne sudouest du grand ttang de Leucate-Sakes.

S~DIMENTOLOGIE DES S~DIMENTSCARBONAT~S

36 1

Les auteurs ttudient en milieu lagunaire des phknomknes de sulfo-rtduction, entrainant l’augmentation de la rtserve alcaline et, constcutivement, la formation des carbonates. La prtcipitation de ceux-ci entraine une proportion relativement importante de matikre organique. I1 parait possible que ces phtnomknes aient jout un certain r d e dans la genkse des roches-mkres des stries pttrolifkres carbonattes lagunaires ou marines.

SUMMARY

The authors studied phenomena of sulfate-reduction in a lagoonal environment, which lead to the increase of the alcaline reserves and consequently to the precipitation of carbonates. The latter leads to relatively high contents of organic material. It seems possible that these phenomena have played a part in the formation of the source rocks of oil in carbonate series of lagoonal or marine origin.

BIBLIOGRAPHIE

LALOU, C., 1962. Formation des carbonates dans des vases prises a diffkrents niveaux dans une carotte du fond de la baie de Villefranche-sur-Mer (Alpes-Maritimes. France). Conf. Centre Rech. fhdees OcPunog., 4 (4) : 11-18. R L V I ~ RA., E , 1959. Sur une technique nouvelle de determination directe du rH et sa signification sedimentologique. Compt. Rend., 248 : 717-719. RIVI~RE, A. et VERNHET, S., 1957. Contribution a l’etude physicochimique de la sedimentation calcaire. Compt. Rend., 244 : 208CL2083. R I V I ~ RA. E ,et VEmmT, S., 1958. Contribution a I’etude geochimique des mkanisnies de la stdimentation carbonatee en milieu lagunaire. Compt. Rend., 246 : 2784-2787. R I V I ~ R A. E , et VERNHET,S., 1959. etats d’oxydo-reduction dans les milieux naturels. Technique de determination directe du rH. Quelques rtsultats en milieu lagunaire. Cuhiers Ocianog. C.O.E.C., 11 ( 5 ) : 309-314. R I V I ~ R A. E , et VERNHET,S., 1961. Sur la sedimentation calcaire en milieu lagunaire. Rapporfs et Procds-verbaux des RPunions de la C.I.E.S.M.M.,16 (3) : 854-858.

TH E PENECONTEMPORANEOUS DEFORMATION OF HEAVY MINERAL BANDS IN THE TORRIDONIAN SANDSTONE OF NORTHWEST SCOTLAND R.

C.

SELLEY

Department of Geology, Imperial College, London (Great Britain)

INTRODUCTION

The Torridonian Formation crops out for a distance of some 320 km along the northwest coast of Scotland. It is of Precambrian age, and is composed essentially of red arkoses and feldspathic sandstones with subordinate conglomerates and shales. The maximum observed thickness is some 4 km. Many of the sandstones carry thin but conspicuous bands and laminae of heavy minerals the petrography of which has been described by PEACHet al. (1907). These heavy mineral bands commonly show disturbed bedding. The purpose of this paper is to describe the various types of contortion which occur, and to suggest that the deforniation was caused by quicksand activity of the enclosing sediment shortly after the deposition of the beds. The same mechanism has recently been attributed to larger scale sedimentary structures which are common throughout much of the Torridonian (SELLEY et al., 1963). The structures described in this paper have been observed on the islands of Skye, Scalpay and Raasay and at Stoer and elsewhere on the mainland. These places extend over a distance of some 120 km.

DESCRIPTION OF HEAVY MlNERAL BANDS

The heavy mineral bands are black, purple or grey green in colour and are composed of magnetite, haematite, ilmenite and leucoxene with accessory epidote, garnet and zircon etc. These grains are of approximately the same order of size as the quartz and feldspar of the host sandstone. These are generally fine to medium in grain-size. Where the heavy mineral bands are undisturbed they are seen to consist of alternating layers of heavy minerals and quartz-feldspar sand. The separate laminae are seldom more than 2 mni in thickness, but the bands may be up to 1 m thick depending on the number o f laminae present. The bands are inipersistent and can seldom be traced laterally for more than a few metres.

PEN'ECONTEMPORANEOUS DEFORMATION OF HEAVY MINERAL B A N D S

363

TYPES OF DEFORMATION OF HEAVY MINERAL BANDS

i n a high percentage of the examples the laminae and often the whole bands are seen to have undergone violent contortion and disruption. In some cases the tops of the resulting structures are seen to have been truncated by penecontemporaneous erosion. Thus the origin of these contortions is not tectonic. In vertical section a wide variety of deformational shapes occur but the following four modal types can be distinguished:

Type 1. The heavy mineral laminae are drawn up into sharp crested anticlines separated by broad flat synclines. Amplitudes of up to 1 m have been observed and adjacent peaks may be between 1.5-2 m apart (Fig.lc). Such structures are more commonly developed in Torridonian sandstone devoid of heavy mineral laminae and have been termed Streamers (SELLEY et al., 1963). Type 2. The heavy minerals are concentrated into droplets, a centimetre or so in diameter, each of which has two thin tails which die out upwards (Fig.2b, c). Intermediate forms [Fig.2a) suggest that the individual laminae broke up at intervals of between 5-10 cm along their length. As these segments sagged down into the underlying sand the heavy minerals coalesced to form droplets. Type 3. This kind of deformation also starts with rupturing along the length of a lamina or of several closely spaced laminae. The parts of the laminae adjacent to the ruptures are upturned and occasionally curl over onto themselves (Fig. I a, b). This gives rise to a series of shapes comparable to pseudonodules ( M A C A R , 1948). Although this similarity is morphological rather than lithological there may be some genetic connection (see next section). Type 4. The heavy minerals occur in balled-up masses some of which are 3 0 4 0 cm in diameter. In some the original laminae are still clearly deiined although highly contorted and of irregular thickness. In others the grains of the heavy minerals and quartz feldspar laminae are so intermixed as to be indistinguishable. Usually when this occurs the boundary between the balled-up mass and the enclosing sand is diffuse (Fig.3b). An example was found from which a tail of the original heavy mineral band could be traced out into the surrounding sediment (Fig.3a).

POSSIBLE MECHANISM OF DEFORMATION A N D GEOLOGICAL SIGNIFICANCE

In all the structures described above the field evidence indicates that the movements within the sands were dominantly vertical and that lateral movement was of only minor importance. Clearly the disturbance of the bedding was not produced by slumping of the sediment down a slope. As already mentioned (p.362) a variety of large scale sedimentary structures occur in the Torridonian rocks which are attributed to

3 64

R. C. SELLEY

a

I

100 crn-

b

I

50 cm

4

C

Fig.1. a. Heavy mineral laminae deformed to look like pseudonodules. 500 m west-northwest of Mullach nan Cam, Scalpay island, Inverness-shire. b. Heavy mineral laminae deformed to look like pseudonodules. North side of Clachtoll Bay, Stoer, Sutherlandshire. c. Heavy mineral laminae deformed to streamer structure. West coast of Raasay, 3 km north of Bagh an Inbhire, Inverness-shire.

PENECONTEMPORANEOUS DEFORMATION OF HEAVY MINERAL BANDS

365

I

'15 cm

b

I

Fig.2. a. An early stage in the deformation of laminae to droplet structure. 500 m up Allt na Criche, Scalpay, Inverness-shire. b. Droplet structure with well developed tails. West coast of Raasay, Inbhire allt Manishmore. c. Deformed heavy mineral laminae withvery well rounded droplets. North coast of Raasay, 1 km east of Manish point.

366

R. C. SELLEY

I

\

A

I

7 /

A

I 7-

I

/j.,”,

. . ,’

.

,

.

’

.I..

_ :,

..

.

__ .. .. . 30

_J-

b

CII

-

Fig.3. a. Balled up heavy mineral band showing laminae protruding from the main mass. West coast of Raasay, 400 m south of Manish island. b. Balled-up heavy mineral band. 1,100 m north of Camas na Fisteogh, east Coast of Scalpay. Note droplets at the top of the ball and the diffuse core of intermixed heavy minerals and ordinary sand (stippled).

quicksand activity within the beds shortly after deposition. Similar structures have been produced on a small scale in the laboratory (SELLEYand SHEARMAN,1962). Vessels of loosely packed water saturated sand were vibrated. This resulted in a tightening of the packing of the sediment. As the excess pore water escaped vertically it dragged up the sand laminae into shapes analogous to those seen in theTorridonian. The deformed heavy mineral bands and the large scale structures are associated in the field and show many features in common. 1t.k thus reasonable to suppose that they were produced by a similar mechanism. However in the case of the former loading must have played a significant part, the heavy minerals sinking down through the surrounding quicksand by virtue oftheir greater density. It is curious to note theway in which the heavy mineral laminae do not always disintegrate by the diffusion of the heavy mineral grains into the adjacent sand as one might expect. On the contrary the laminae typically preserve their individuality despite contortion and often even thicken as a result of the deformation. This indicates that the heavy mineral grains

PENECONTEMPORANEOUS DEFORMATION OF HEAVY MINERAL BANDS

367

were often able to retain their cohesion whilst the quartz and feldspar grains were completely disorganized by the escaping water. This cohesion might perhaps be ascribed to the magnetic attraction of the iron ore which is still retained in the rock today. Disturbed heavy mineral bands can also be observed in Holocene beach sands at Sanna bay, Ardnamurchan, and elswhere along the northwest coast of Scotland. Apart from indicating past quicksand activity in sediments, these structures could be used as criteria of order of deposition when found in rocks that have undergone a high degree of structural deformation. Furthermore they could also probably survive in metamorphic grades at which other useful sedimentary structures had been obliterated.

ACKNOWLEDGEMENTS

The author gratefully acknowledges receipt of a Department of Scientific and Industrial Research maintenance award during the period of this work. Thanks are also due to Dr. J. Watson and Mr. D. 3 . Shearman for critically reading the manuscript.

SUMMARY

Heavy mineral bands have been known from the Torridonian sandstone of Scotland for many years. Recent study of this formation shows that the heavy mineral bands often exhibit disturbed bedding. Four characteristic deforniational shapes are described. In the light of recent work on other sedimentary structures in the Torridonian and on laboratory experiments the deformation is attributed to ancient quicksand activity, the heavy mineral bands sinking down through the enclosing quicksand by virtue of their greater density.

REFERENCES

MACAR,P., 1948. Les pseudonodules du Fammenien et leur origine. Ann. SOC.GPol. Be&., Bull., 12 : B 41-15. PEACH, B. N., HORNE, J., CLOUGH, C. T. and HINXMAN, L. W., 1901. GeologicalStructureof the Northwest H&hlands of Scotland. H.M. Stationery Office, Glasgow, 285 pp. SELLEY, R. C. and SHEARMAN, D. J., 1962. Experimental production of sedimentary structures in quicksands. Proc. Geol. SOC.London, 1599 : 101-102. SELLEY, R. C., SHEARMAN, D. J., SUITON, J. and WATSON,J., 1963. Underwater disturbances in Torridonian sediments of Skye and Raasay. Geol. Map. In press.

ON THE PENECONTEMPORANEOUS DISTURBANCE OF BEDDING BY “QUICKSAND” MOVEMENT IN THE DEVONIAN ROCKS OF NORTH DEVON D. J. SHEARMAN

Departnient of Geology, Imperial College, London (Great Britain)

The sedimentary structures which are discussed in this paper occur in sandstones in the lower part of the Ilfracombe Beds (Middle Devonian) of north Devon. They are exposed along the coast section east of Ilfracombe in the cliffs and seastacks from Widmouth Head to Hamator Rock. The sandstones in which the structures are developed are false bedded, with long low angle foresets. Individual beds, as defined by master bedding planes, taper when traced laterally indicating that these master bedding planes are in fact gently inclined erosional backslopes. At various levels within the sandstones the bedding as seen on vertical joint faces is balled up into bowl-like masses with incurved rims. The bedding laminae can be traced unbroken around the base and the sides of each mass, but they are always truncated by a plane of erosion across the top. Commonly the bedding laminae become strongly crumpled towards the centre of each “bowl”. In some examples the structures are essentially symmetrical but others show a marked asymmetry. At any one level, the symmetrical and asymmetrical structures are intermixed, and the direction of asymmetry is not constant. Individual structures range up to 1 m or more in length, and up to 30 cm in thickness. Although they are well dispIayed on the joint planes, bedding surfaces are poorly exposed, but the evidence suggests that the structures are subcircular in plan. In the majority of occurrences there is no continuity of bedding between adjacent structures which are separated by an apparently structureless sand. Occasionally, however, this sand shows vague vertical streaming lines. Around the base of the bowls there is often a thin clay film which may or may not continue around the sides. The truncation of the deformed bedding laminae by the base of the overlying sandstone demonstrates conclusively that the structures are not tectonic, but that they were developed before the deposition of the overlying bed. Superficially they resemble “slump balls”, but the opposing asymmetry which the individuals show at any one level mitigates against an origin by slumping. In general appearance they resemble the pseudo-nodules described by MACARand ANTUN( 1950), but these latter are balled-up masses of sand in a clay groundmass. KUENEN(1958) produced structures similar to pseudo-nodules by causing sand to sink down into a hydroplastic clay. Although a

PENECONTEMPORANEOUS DISTURBANCE OF BEDDING

369

thin clay film occurs at the base of most of the structures described in this paper, the amount of clay is far too small to allow the suggestion that they were formed in the same way as the pseudo-nodules, produced by Kuenen. It is evident in the field, that the structuies were formed by sand sinking down into sand. Such structures are unlikely to have been caused by downward acting forces, such as local differential loading, because these would tend to increase the rigidity of the sand. The most likely mechanisms which can cause a bed of sand to lose its rigidity are those which lead to a decrease in the friction at grain contacts. This could be achieved by increasing the pore-fluid pressure, or by inducing an upward movement of fluid through the sand. Under these conditions the sand will become quick and flow almost like a Iquid. Such a mechanism is adequate to explain the origin of the stt uctures observed in the Ilfracombe Beds. The thin, almost inconspicuous, clay layer which is preserved at the base of the deformed masses of sand may have been an essential requirement, because this would have provided a water-tight seal between what was to become the deformed bed and the underlying quick sand. It is suggested that the underlying sand was deposited with a loose packing, and that subsequent to the deposition of the overlying sand some disturbance caused this loose sand to fall into a tighter packing. By virtue of the clay seal between the beds, the now excess pore water could not be expelled and the sand became quick. The overlying sand sagged down into this virtually fluid foundation, and the quicksand flowed into the areas between the sags and burst up through the overlying bed. Once the clay seal between the beds had been ruptured, the pore przssure would be released, but the excess pore fluid would continue to flow upwards through the breaks in the overlying bed. This upward movement of water would keep the sand grains lubricated, so that the sand too would continue to flow upwards with the water until all the excess pore water had escaped. Once this has been achieved the sand which remained would become rigid. In effect, It is suggested that the deformed bed was the passive agent, and that the active material was the underlying bed. All that now remains of this active bed, is the structureless sand which occurs between the balled-up masses. The asymmetry shown by many of the sandstone “bowls” and their frequent opposing asymmetry are readily explained by virtue of the fact that the structures were moulded on and have deformed original false bedding. SELLEY and SHEARMAN (1962) were able to produce similar structures experimentally in the laboratory by simulating the conditions suggested in this paper.

SUMMARY

Balled up masses of sand in sand are described. It is suggested that the structures were produced by sand sinking down into an underlying sand which had become “quick” as a result of the grains falling into a tighter packing, and thus causing an excess of pore water.

370

D. J. SHEARMAN

REFERENCES

KUENEN, PH. H., 1958. Experiments in geology. Trans. Geol. SOC.Glasgow, 23 : 1-28. MACAR, P. et ANTUN,P., 1950. Pseudo-nodules et glissement sous-aquatique dam 1’Emsien inferieur de I’Oesling. Ann. SOC.GPol. Belg., Bull., 73B : 121-150. R. C. and SHEARMAN, D. J., 1962. Experimental production of sedimentary structures in SELLEY, quicksands. Proc. Geol. SOC.London, 1599 : 101-102.

NEAR-SHORE AND SHALLOW-WATER DEPOSITS O F APTIAN AND ALBIAN AGE I N THE MOSCOW REGION M. S . SHVETZOV

Moscow Geological Prospecting Institute, Moscow (U.S.S. R.)

INTRODUCTION

Carbonate sedimentation, which predominated in the Moscow region during the Paleozoic, was replaced, beginning with the Jurassic, by the accumulation of sandyargillaceous rocks. During the Aptian mostly continental quartz sands were deposited, reaching thicknesses of 20 m, partly altered to quartzites, with imprints of plants (alluvium, dunes, deltas).

DESCRIPTION OF THE DEPOSITS

North of Moscow there are exposures of the upper part of these Cretaceous rocks (Plate Ia, bed 2), white cross-bedded quartz sands and silts and thin bands of black clay (2 m) (river-bed deposits). Higher up (Plate Ia, bed 3) the bands of black clay become more numerous, marking the transition to a layer with alternations of undulating laminae of clay and white silt, interrupted by micro-wash-outs and animal traces. The rock gradually changes into an irregularly laminated (thickness of laminae 0.1-0.02 mm) mixture of sooty clay and black silt with innumerable animal tracks and plant remains, as well as spores, plant tissues, bitumen and marcasite. Detrital grains (0.005-0.02 mm) are represented by corroded quartz. There is less than 20 % of feldspar, micas, etc., which are often kaolinized. The clay consists of hydromicas with kaolin. The layer has a thickness of 2.5 m (deposits of a swampy floodplain). Above it (Plate Ta, layer 4; Plate Ib) less organic matter is present. The rock changes into a quartzy silt with mica, friable when dry, with a peculiar, laminated structure (petering out, wash-outs, tracks). Very often only shapeless patches are left of the thin bands ( 1 4 . 1 cm) of black clay owing to the activity of mud-eaters. The clay contains many spores and fragments of coalified plant tissue. Feldspars are present in quantities of up to 10-20 %, micas, often kaolinized, up to 6 % and clastic glauconite, up to 5 The rock is friable, not distinguishable from a recent sediment and represents deposits formed in basins with sluggish currents or sometimes even stagnant water, in a delta area, which partly received its supply from a new source. The thickness is 1.5 m.

x.

PLATE I

m Aptian

AIGian

16 15

rn

t4

29

13 7 12

28

ii

26

10

25

77

24

-6 87 -

23

5

22 21

-4

5-

20

4 -

f9

:1,

f8 17

IG

L

0

keyend

E l clay andsilt4 Mixture of

. . .

...

.....

Admixture

sand oryrffvei

See explanation p.374

374

M . S. SHVETZOV

The composition and structure of the rocks described here and below is well discernable in dry sequences. In moist sections they have the appearance of a homogeneous black “clay”. The overlying layer (Plate Ia, layer 5) differs considerably from the above described sediments. It consists of a mixture of silt and clay with small amounts of sand. There are no animal traces and no distinct bedding; the distribution of material is irregular (Plate Ic). There is a sharp decrzase upwards of the content of feldspars and micas. Three parts can be distinguished in this layer: (I) The lower silty-clayey part (30 cm) which is enriched in iron and shows very thin (2-0.1 mm) wavy bands of clay (Plate la, bed 5, bottom). The main mass includes randomly distributed angular (1 x 0.4 cm) fragments of clay and single granules of quartz (Plate Ic). (2) The middle part (80 cm) has the same composition, but the clay fragments are larger, just as the clay streaks, which create an irregular wavy lamination. Superimposed on it are funnel-like and other peculiar structures, apparently due to infiltration (Plate la, layer 5 ; Plate Id, e). ( 3 ) In the upper part (90 cm) the percentage of clay drops, while the amounts of sand and gravel increase. The rock looks like a breccia of laminated silt and black clay (Plate If). This structure is obviously of primary origin, as the rock is pierced by vertical burrows beginning from the overlying bed, up to 80 cm in length and filled with gravel and clay (Plate If). At the upper boundary pieces (10 cm) of loose silty-gravel rocks are locally found. The layer has been deposited during a stage of increased water movements by supply of material differing from that which was deposited previously, probably under conditions of alternating periods of flooding and drying, and in the absence of burrowing organisms. The next bed (Plate Ia, bed 6), 1 m thick, resting on a washed-out surface of the former, was deposited as result of a very strong incrzase in water movements, which was soon followed by stagnation and reworking of the supplied material. It is characterized by paradoxical features: (a) combination of abundant gravel and clay, (b) clay not in beds, but as lumps (Plate Ig) up to 10 cm big, i.e., deposited as rudites and not as lutites, (c) a good parallel orientation of clay particles (simultaneous extinction under crossed nicols) despite an admixture of gravel. The composition of the clay is different from that in the underlying beds: hydromica with montmorillonite. Charac-

PLATE I (pp.372-373) a. Lithological section with legend. b. Structure of layer 4; about l/5 natural size. c. Structure of layer 5; about 1/3 natural size. d. Seconddry structure of the middle part of layer 5; about natural size. e. Secondary structure of the middle part of layer 5 ; about 36 natural size. f. Structure of the adjacent parts of layers 5 and 6; about 1/11 natural size. g. Layer 6. Lumps of clay and gravel: about 1/3 natural size. h. Structure of layer 7; natural size. i. Structure of layer 9; about 119 natural size. j. Layer 9, secondary structure due to infiltration; about 2/3 natural size. k. Layer 1 I . Lumpy structure; about l / 5 natural size. 1. Glauconitisation of feldspars. m. Phosphatisation of a glauconite grain. Phosphate“intrusions” (black and white) on its borders, and spherules inside. n. Spherulitic structure of phosphate inside a glauconite grain (m).

APTIAN AND ALBIAN SHALLOW MARINE DEPOSITS IN THE MOSCOW REGION

375

teristic is the appearance of sideritic concretions (1 5 x 7), apparently the result of a stronger supply and decomposition of biotite and glauconite (amounting sometimes to as much as 50 % in the Jurassic of the Moscow region) and due to the formation of chlorite. From a mixture of the latter with clay, as seen in thin sections, there has been a segregation of spherical (0.03) excretions of chlorite, which changed into microspheres of siderite enclosed in the still isotropic groundmass, a mixture of clay and chlorite. Later-on, spheres of siderite became partly oxidized and partly well crystallized, especially around quartz grains which were thereby corroded. No remains of organisms are found here except deep (about 80cm)vertical burrowsof 1-2 cmdiameter. The transition to ordinary conditions of sedimentation was established gradually (Plate Ia, bed 7). In the lower part of bed 7 there is a band (50 cm) consisting of a fine indistinctly laminated mixture of silt and clay penetrated by vertical animal burrows. Above it there follows first a thin zone with fragments of clay, and then the main mass, consisting of fine clay and silt, which have been deposited as separate flakes or grains, their structure reflecting the conditions of transportation by stable streamlets. One of these components can be distinguished on the background of another as points, branching filaments, lumps, flexed lamellae, coiled whirls etc. (Plate Ih). Apparently, there are no traces of mud-eaters here. This structure is preserved throughout the entire 7-meter layer, the top of which is a truncation surface produced by the Middle Albian transgrzssion. At three levels enrichment in iron took place which is disseminated through the sediment (Plate Ia, beds 5,7) or concentrated into large (up to 50 cm diameter) spherosiderites (Plate Ia). The rare (heavy) minerals of layers 6 and 7 differ from the minerals of the overlying and underlying beds. This sequence has been described since the middle of the last century by prominent Russian geologists, such as Trautschold, Nikitin and others. In another section two fragments of ammonites have been found near layer 6, which were determined as Aptian Hoplites deshaisi LEIM.However, despite repeated studies of these sections by individual visits and excursions of geologists during 80 years, not one single other marine form has been found here and it has been concluded (by Arkhangelsky and others) that these beds (1-7) belong to a chiefly continental Aptian, in contrast to the overlying rocks which contain a marine fauna, of Middle and Upper Albian age. Up till now they had not been studied petrographically. The first petrographic description, given here, indicates that the Aptian sediments have been deposited mainly in an alluvial nearshore-deltaic region, which could be covered by sea water for a short time (bed 6), owing to a storm surge or other movement of sea level. Middle Albian marine deposits (Plate Ia, bed 8) are petrographically very different from Aptian deposits. They were deposited under conditions of an extremely shallow marine basin undergoing a slow relative subsidence, interrupted by regressions of the sea and rewashing of the sediments. Traces of the first stage of an Albian sinking - Lower (?) Albian - are preserved only in the basal conglomerate (15 cm). This consists partly of lumps of Aptian silt. The main elements, however, are phosphoritic nodules of various shapes. The latter are for the most part sandy, and contain remains of ammonites, fishes, fossilized wood,

376

M. S. SHVETZOV

but almost no glauconite. A minor part of the nodules is argillaceous and fissile, and free of animal remains or glauconite. The conglomerate is covered by a layer (1.5 m), consisting of a mottled mixture of silt (55 %) and clay (35 %) with sand (10 %). The arenaceous material is predominantly composed of quartz grains. Authigenous glauconite forms up to 50-70% of the rock. The lamination is partly obscured by the density of animal tracks and burrows (diameter 2-3 mm). In small (3 x 4 x 6 cm) phosphorites rare traces of radiolarians, diatoms and zeolites are noticeable. These are sediments of a quiet bay. Higher up (Plate la, bed 9) a cross-laminated deposit (3 m) is present indicating a change in the environmental conditions. It consists mainly of sands and silt with an admixture of glauconite (25-30 %). Clay occurs only in streaks between thin bands of sand and silt. In the middle part there is a band of coarse sand with gravel and large (1 5 cm) rounded sandy phosphorites (currents, rewashing). The layer is characterized by a structure of trough-like sets of cross laminae with a uniform angle of inclination, partly concealed by funnel-like structures of infiltration (Plate Ia, layer 9; Plate Ii, j), which occasionally pass into grooves up to 25 cm deep filled with crumpled laminae. In layers that are very rich in such funnels, complicated crumpling structures (Plate Ii) are formed. No animal tracks or burrows are seen. In phosphorites there are sometimes traces of radiolarians, spicules, Algae and zeolite crystals. These sediments have been formed in very mobile waters of an open shallow-water basin. Layer 10 of Plate Ta contains small phosphorites at the base. The layer is composed of alternating horizontal thin bands of loose sandy silt and bands that are very rich in dark clay and glauconite (50%), with an admixture of clay clots (1 cm) and with tracks of mud-eaters (0.5-1 cm). Perhaps this is a biogenic breccia. It points to a repeated alternation of mobile- and quiet-water conditions in a partly isolated basin. Bed 11 on Plate Ia (3.5 m) is separated from the underlying bed by the undulating surface of a wash-out. It is of a darker colour despite the content of glauconite, owing to a smaller amount of sand and a greater percentage of clay (40-50 %). However, the clay is partly present in the shape of irregularly distributed pebbles (20-30 mm). In consequence the lamination is not noticeable in small samples (Plate Ik). In the lower part there are many rounded phosphorite fragments (P,O, up to 15 %) ranging from silt size to pebbles of 20 cm diameter. The latter are strongly bored by animals (Plate Ia, bed 11). Contrary to phosphatic elements in other beds they are partly clayey and sandless, white (weathered ?) and sometimes without distinct outlines. Characteristic are also other chemical replacements (yellow, red and white patches of iron, sulphur and other compounds). No organic remains are macroscopically visible, but under the microscope, especially in phosphorites, there is an abundance of radiolarians, diatoms and other organisms accompanied by accumulations of zeolites. A comparison of the 10th and 1 Ith beds shows that the material of the 10th bed, deposited in a detached part of the basin, was later covered by clays with clayey phosphorites. After a new connection with the open basin had been formed, the phosphorites have been washed out and redeposited together with freshly supplied sand material. The transition to the next layer (Plate Ia, layer 12), belonging to the Upper Albian,

APTIAN A N D ALBIAN SHALLOW MARINE DEPOSITS IN THE MOSCOW REGION

377

is gradual; in the exposure their boundary is not distinct. Perhaps for this reason it would be more correct to draw the boundary between Middle and Upper Albian at the base of layer 11. As a whole, the difference between the rocks of layers I 1 and 12 is quite pronounced. Tn layer 12 there is no gravel and sand, the main components being: silt, clay and glauconite which are unevenly distributed in different parts of the samples and which are intersected by innumerable tracks and burrows of small animals (1-2 mm). These tracks create a quaint pattern on the background of the clayey-glauconitic mass. Here, just as in Cretaceous deposits described by Bushinsky, nearly all the sediments passed through the alimentation track of small animals. There are no phosphorites, but many radiolarians and zeolite crystals. The thickness is more than 4-5 m.

CONCLUSION

After deposition of these sediments their minerals were subjected to reworking and decomposition. Better preserved large grains of quartz acquired by partial solution a “resorbed” surface and, as seen in thin sections, a queerly corroded shape. Alumosilicate minerals in different beds were subjected to different kinds of reworking into micaceous-clay minerals, which often indicate not a primary composition of the sediment, but the conditions of the environment of sedimentation. In continental sediments the formation of minerals was reduced to the formation of clays and the segregation of iron as marcasite, chlorite, siderite, hydro-goethite etc., which accumulated in separate beds. In marine sediments it resulted in the formation of glauconite, phosphates, opal, zeolites etc. In marine deposits glauconite makes up sometimes up to 50% of the rock. For a small part this is “clastic glauconite”. The bulk is of authigenous origin and occurs not only as rounded and cracked grains, but also often as fresh grains, which still preserve the shape of the mother mineral. It should be stressed that the abundance of an obviously diagenetic glauconite still preserving the shape of feldspars (Plate Ik, I) and of feldspars nearly altered by diagenesis to clay-micaceous minerals, indicates to which errors a formal approach to the mineralogical analysis can lead. Phosphate minerals have been found in the investigated exposures only in the marine Albian. They replace glauconite, first of all, but also other minerals. The replacement proceeds in stages with the formation of different phosphate minerals. Sometimes partial replacement takes place by amorphous phosphate, which penetrates the grains from the exterior like minute intrusions (Plate Im). Further inside the glauconite we find hexahedral concentric and spherulitic excretions of phosphate (0.0 15 mm, Plate In). More often pore walls and sand particles are covered by intruding coatings of phosphate minerals - first isotropic, then crystallized, sometimes with a concentricspherulitic texture (0.05 mm). Authigenous opal appears in the Albian deposits as skeletons of radiolarians, diatoms, more rarely sponges and other organisms. It is abundant in Upper Albian clays.

378

M. S. SHVETZOV

Zeolites. Together with siliceous organisms zeolites are present. They are most abundant ( 10 % and more) in the Upper Albian which might suggest a connection between the two, especially as the zeolites occur as aggregates filling out cavities in the rock and particularly those left by dissolved radiolarians. This general presence of zeolites, with only rare exceptions, since the Mesozoic, is rather enigmatic. Apparently it is impossible to explain it by a chemical evolution of sedimentation. It seems more plausible to look for a cause in the development of the organic world. Information on the composition of radiolarian skeletons is not exact even for the modern forms. Only some of them have a siliceous skeleton. I n many fossils no skeleton has been preserved. The skeleton of recent acantharians, according to some authors, contains celestine while according to others it consists of an “alumocalcium silicate” (i.e., possibly zeolite?). The question deserves a special study on the basis of extensive material.

SUMMARY

The report deals briefly with continental Aptian and shallow marine Albian deposits in the Moscow region which were briefly described by geologists a long time ago, but not studied petrographically. A description is given of their specific petrographic features, which change from layer to layer and reflect the conditions of their formation and thus the history of the region. (a) Aptian. ( I ) Lengthy continental rzworking. (2) Deposits of the channel area of a river. (3) Sediments of a swampy floodplain. (4) Sediments of a high floodplain densely inhabited by animals, which disturbed the depositional structures by burrowing. (5) Sediments formed after a change in conditions, resulting in a simultaneous supply of both clayey and rudaceous material and absence of animal traces. (6) After a drastic change in conditions (fluctuations in level, formation of a wash-out, supply of gravel, etc.) a decomposition in a stagnant water environment, leading to concentration of iron in chlorites and siderite. The appearance of new organisms (deep burrows). (7) Re-establishment of the conditions under which sedimentary matter is transported by a steady stream. (6) Interval. Wash-out. Middle and Upper Albian. (8) Main conglomerate with phosphorites, ammonites, etc. Layer of glauconitic, sandy-clay with many burrows from mud-eaters. Sediments of calm waters. (9) Predominantly sandy layer with uniformly directed cross laminations. Admixture of glauconite. Phosphorites. Sediments formed by currents. (10) Layer of clayey-glauconitic material with indistinct horizontal laminations and tracks of mud-eaters (bay sediments). (ZI) After an interval and formation of a wash-out re-establishment of marine conditions in an environment with strong water motion. Clay pebbles. (12) In calm deeper waters the deposition of a mixture of clay, glauconite, and silt. Abundance of mud-eaters. Many radiolarians and zeolites. (c) Finally some data are given on phosphorites, glauconite and zeolites.

PRIMARY STRUCTURES I N A PART OF THE NILE DELTA SAND BEACH SOLIMAN M. SOLIMAN

Department of Geology, Faculty of Sciences, Ain Shams University, Cairo (Qypi)

INTRODUCTION

The sand beach of the Nile delta, between the Rosetta and Damietta distributaries, has a length of about 160 km. It forms, for the most part a barrier damming off a large lake (Lake Brollos) and small ponds. The present paper gives some results of a study of the primary structures in this sand beach. No previous investigations of this kind have been made in this area, or anywhere else along the Egyptian coast. A 6 km long strip of the beach, 3 km to the east of the Rosetta distributary, was selected for the study of primary structures in beach sands. It has workable black sand concentrations. This part of the beach, which has a northwest-southeast direction, was examined at twelve stations regularly spaced on the foreshore and the backshore (Fig.1). Other sections were made on the beach scarp. At each station, L-shaped trenches were dug to the water-table or to the clay bottom. They were so oriented that sections both parallel with and at right angles to the strand were exposed. Diagonal

30'

Fig.1. Location map.

380

S.

M. SOLIMAN

sections were also examined to show the inter-relation between both sides. The depth of sections ranges between 20-40 cm i n midforeshore, 43-50 cm in the upper foreshore, and 30-75 cm in the backshore stations. Clay was met with at the bottom of several stations. The structures are clearly visible owing to the alternation of dark laminae (mainly of black sands with magnetite, ilmenite, monazite, rutile, zircon, etc.) and light-colored laminae. Thanks are due to the authorities of the Egyptian Black Sand Company for the facilities offered during the visits to these localities.

FORESHORE STRUCTURES

The foreshore is characterized by its smooth nature, the presence of swash and backwash marks, and the interm.i'ttent sharp crests at its top. At low tide, the foreshore beach is approximately 6 8 m wide, and its surface slopes seaward with angles of

'12-6O. The foreshore is made up of quartz sand with black sand concentrations. The analyses of representative samples (Table I) indicate differences in grain-size percentTABLE I MECHANICAL ANALYSIS OF BEACH SANDS ~~

~~

Sample no. ~~

~

R 43 R 16 R 9 R 10

~~~

Coarse ~

traces traces traces -

~~

~~

Mediitm -

0.3 1.9 I .5 3.8

~~

Fine -

~

Veryfine ~-

35.5 79.0 73.6 87.1

~~

~

~~

Silt,etc.

~

60.5 16.9 22.7 7.8

3.6 1.1 1.9 0.4 ~-

R 43 = black sand of high concentration, upper foreshore, station 1 . R 16 = lightcolored sand, upper foreshore, station 1 . R 9 = dark sand, backshore, station 4. R 10 = light-colored sand, backshore, station 4.

ages. Generally, however, the sands consist dominantly of fine sand and are wellsorted. Stratification in the upper foreshore is of the same type as found in the foreshore at other investigated localities, e.g., on the California beaches (THOMPSON, 1937; MCKEE, 1957). It is characterized by even, regular, uniform laminae of light-colored sands alternating with dark sand laminae containing up to 80 % heavy minerals (commonly lower concentrations are present). The black sand laminae increase in thickness and abundance near the top of the section (Fig.2). The sequence is usually divided into several sets of strata, corninonly f0u.r in number. Each set has up to six laminae per cm of thickness, on the average 2-3. The laminae are generally horizontal, but the

PRIMARY STRUCTURES OF THE NILE DELTA SAND BEACH

38 1

Fig.2. Regular laminae, upper foreshore, station 5.

ower sets of strata may dip as much as 6" seaward. Some difference in inclination is noticed between the lower and upper sets. They may dip in the same direction, the lower sets with a smaller angle than the upper ones; or they may dip in different directions with the upper sets inclined seaward. In contrast to the general evenness and regularity of the foreshore stratification, some irregularities are also present, particularly in the upper foreshore sections adjoining low lying berms. The irregularities consist partly in thickness variations of series of laminae, or in simple invaginations of strata (21/2cm amplitude) in the underlying sets. More coniplex structures are shown by contortions which are most intense i n a bed about 9 cm thick, overlain and underlain by normal, uniform, regular strata. The irregularities are caused by truncation, and by the presence of current strata, of small clay streaks, dipping 26" westward parallel to the shore, and of asymmetrically filled channels, dipping up to 39" seaward and 12l/," landward. Such irregularities, though very uncommon in the Nile delta upper foreshore, and not recorded in similar localities elsewhere (McKEE, 1957), may throw some light on the relationship and possible interchange of conditions, between the upper foreshore and backshore areas.

BEACH SCARP STRUCTURES

The break in the beach slope is intermittently marked by a scarp of 10-40 cin relief above the general foreshore slope. Waves are noticed to destroy high scarps by excavating their lower parts until the berm crests eventually collapse onto the uppermost foreshore. This leads to relative decrease i n relief of the berm with respect to the general slope of the foreshore.

382

S. M . SOLIMAN

Asection of more than 3 m in length was made just landward of the beach scarp (Fig. 3). On the whole, the revealed structures are intermediate between those of the foreshore and the irregular ones of the backshore. The strata are thicker than those of the foreshore, and by visual comparison of color intensities, the concentration of black sand is generally less. Irregularities in stratification are intense in a 10 cm thick zone, and individual contortions average 3-8 cm in thickness and 5-1 5 cm across. Symmetric and asymmetric channel-filled structures are found. One of the conspicuous features are the tube-shaped

Fg.3. Beach scarp with tube-shaped structures (a) and a cross-section (h).

Fig.4. Two zones of irregularitiesand an unconformity, backshore, station 4.

PRIMARY STRUCTURES OF THE NILE DELTA SAND BEACH

383

structures crossing the strata and filled with non-laminated sand (Fig.3). They are less than 20-37 cm in length with a slight curvature from the vertical; their crosssection is circular, averaging 5 cm in diameter. The cause of such structures has been observed in action. Sea-shore crabs, abundant in these places, usually dig up holes in the sands clear from wave disturbances and pile the excavated sand around the rims of the holes. The type of sand overturned, indicates the presence or absence of black sand in the subsurface. With the advance of waves or tides, these holes are filled from the top with undifferentiated sand. The ring-shaped pile of sand also forms a nonlaminated mass around the upper end of the tube.

BACKSHORE STRUCTURES

The backshore area extends from the beach crest to the farthest point reached by waves which is commonly one of the southern shallow depressions or ponds, or the line of sand dunes. Its width ranges from 1/2-21/2km. It is generally horizontal, with ripple marks, channels, and other shallow surface irregularities. The backshore sand is similar to that of the foreshore with differences in size-grade frequencies (Table I). It is generally lighter in color depending on the smaller abundance of the dark minerals. The strata are thicker, but thin out landward. Within a stratum, the number of laminae is large in the lower sets, but is usually less in the upper part. Sets of cross-strata may be as much as 27 cm thick. Backshore sections, in contrast to those of the upper foreshore, are characterized, in most places, by irregularities in stratification, and locally by layers steeper than 4-20'. In all backshore trenches, the sequences of laminae are divided into two composite sets. The lower set, generally with even regular stratification, dips 1-2" seaward, but sometimes 3" landward. The upper composite set, with many contortions. dips 1-4" seaward or 2l/,4" landward (Fig.4). Both sets show a gradual increase in their black sand content toward their tops. The plane of separation is sometimes a clear angular unconformity (Fig.4). At station 4, the unconformity plane, dipping 6" seaward, could be traced for over 50 m parallel to the strand, and for 20 m approaching the upper foreshore. Structural relationships indicate that irregularities, both above and below the unconformity have no major matchable effect on it. The irregularities are confined to one or two zones within the regular sequence of laminae. If one zone is present, it generally occurs near the top of the sequence. In case of two irregular zones, one is present within the lower set of strata and the other in the upper set. The irregularities take various forms, the most important of which are the following: (I) Cross-stratification: Tabular minor stratification is present in a section of laminae 3 cm thick and dips 1040" seaward. Large-scale wedge-shaped cross-stratification (Fig.5) is also present over a long area. (2) Sand-pockets: These are found within a zone of strata 3-5 cm thick. They are saucer shaped, filled with laminated (normal lamination, or cross-lamination) or non-

384

S. M. SOLIMAN

laminated sand (Fig.6). Some of the sand-pockets are symmetrical, others are asymmetrical and inclined seaward. The overlying strata may lie conformably on or truncate them. (3) Disrupted laminae: A few dark layers (black sand) are disrupted and the chunks lie parallel to the adjoining stratification or may lie athwart it in non-laminated or contorted laminated parts of the sequence. Separate small clay streaks are also found amid a highly deformed zone 27 cm thick. All together, these strzaks have a wavy con-

Fig.5. Cross-strata, faults, fold, and disrupted laminae, backshore, station 10.

Fig.6. Sand-pockets, backshore, station 12.

PRIMARY STRUCTURES OF THE NILE DELTA SAND BEACH

385

Fig.7. Crab-boring, backshore, station 2.

figuration. They are similar to the intraformational conglomerate described by McKee (1957, p. 1712) in the California beach sand. (4) Minor faulting: Faults are not uncommon within large contortions. Normal, reverse, thrust, and step-faults, as well as grabens and horsts are noticed. They are generally parallel to the major deformation whether the latter are contortions, largescale cross-stratification (FigS), or of other types. (5) Tube- shaped structures: Complete tube-shaped structures, or parts of them are also present (Fig.7). Channel-fillings are noticed in station 2, where they have a northwest-southeast direction.

ORIGIN

The sedimentary structures in the Nile Delta beach sands are divided genetically into those related to direct deposition, or “depositional structures” and those related to contemporaneous deformation after deposition, or “deformational structures”. The latter are produced by the action of mechanical and organic agencies. The development of the even, uniform, flat-lying laminae of the upper foreshore or the backshore sections is understood to be related to the waves and tides that spread thin veneers of sorted sand washed from land and sea. The alternation of black and light-colored sand laminae is related to the repeated cyclic changes of the sedimentary agent’s competency, and to the physiography of the beach. The irregular structures in the backshore have a more complicated origin. They have been related to the movements of pockets of trapped air (EMERY,1945) and experimented on by STEWART (1956); or they may be developed in irregular channels and troughs (McKEE, 1957, p. 1707).

386

S. M. SOLIMAN

Other causes of the irregular depositional or deformational structures in the Nile Delta beach sands are one or more of the following: (1) Beach physiography and its irregularities, such as ripples, swash and backwash marks, channels, troughs, etc. caused by wind and tides, may be smoothed out by aeolian deposition. Some of the light-colored loosely packed sand pockets are here considered to be produced by deposition of blown sand on comparatively hardened irregular surfaces. The agent of deposition may develop cross-stratification, truncation of the underlying strata, etc. (2) Sea-shore crabs produce long holes that are filled later with undifferentiated sand, forming tube-shaped structures. (3) Sliding and slumping produce irregularities. Disrupted strata and minor faults parallel to major contortions or other major structural features (channel fillings) were caused by small-scale sliding or collapse of the laminae. Differential compaction among primary differently packed parts of the sand sequence produces slumping. (4) Collapse of parts of the berms may develop irregular structures especially if the fallen masses are buried without much disturbance and resedimentation. This may explain the uncommon irregularities in the upper foreshore sands adjoining high beach scarps.

CONCLUSION

The primary structures in the 6 km long strip of the Nile delta sand beach to the east of the Rosetta distributary, are largely similar to those described elsewhere, as in California. They are either depositional structures or deformational structures. In the localities examined, the foreshore sand is regularly laminated. The structures in the backshore are irregular in one or two zones within regularly laminated strata. Generally, the irregularities are found to be more intense going southeastward, toward the eastern end of the studied strip. The structures in the upper foreshore, beach scarp, and backshore sands change gradually among themselves; and the interchange between land and sea, particularly during winter and summer, produces structures of the foreshore on the backshore when high water covers all of its area.

SUMMARY

A 6 km long strip of the Nile delta sand beach was selected for the study of its primary structures. The structures in the upper foreshore are represented by regular, even stratification of dark and light-colored sand laminae. Locally, irregularities are present adjoining the berm. The beach scarp presents structures of the upper foreshore and of the backshore. The backshore stratification is characterized by irregularities and by the strata being generally divided into a lower set and an upper set. The irregularities are confined to one or two zones within the regular sequence of laminae. Thickening

PRIMARY STRUCTURES OF THE NILE DELTA SAND BEACH

387

of strata, channel-fillings, sand-pockets, tube-shaped structures, minor and large-scale cross-strata, minor folds, minor faults, disrupted laminae, clay streaks, etc., are common structures. These structures are either depositional or deformational structures. The regular laminae are explained by simple wave and tide action. The irregularities are caused by the interaction of physiography, waves and wind, aeolian sedimentation on surface irregularities, differential compaction, crab-boring, slumping, and sliding of berms. Interchange between foreshore and backshore conditions over the backshore areas may clarify the presence of alternating regular and irregular strata within the sequence.

REFERENCES

EMERY, K . O., 1945. Entrapment of air in beach sand. J . Sediment. Petrol., 15 : 39-49. MCKEE, E. D., 1957. Primary structures in some recent sediments. Bull. Am. Assoc. Petrol. Geologists, 41 : 1704-1747. STEWART, R. B., 1956. Contorted sediments in modem coastal lagoon explained by laboratory experiments. Bull. Am. Assoc. Petrol. Geologists, 40 : 153-161. THOMPSON, W. O., 1937. Original structures of beaches, bars, and dunes. Bull. Geol. SOC.Am., 48 : 723-75 1.

DISTRIBUTION AND LATERAL VARIABILITY OF HEAVY MINERALS IN THE ANNOT SANDSTONES DANIEL J. STANLEY

U. S . Army Engineer Waterways Experiment Station, Vicksburg, Miss. (U.S.A. )

INTRODUCTION

Petrographic studies of the Nummulitic Annot Sandstones in the French and Italian Maritime Alps have revealed that both the nlineralogical and the textural composition of these rocks vary extensively laterally. The Annot Sandstones have already been subdivided into three distinct but intergradational lithofacies on the basis of detailed studies in the field and laboratory. These consist of flysch facies in the Northern Zone, a non-flysch facies in the Southern Zone, and a transitional facies in the Intermediate Zone between the Northern and Southern Zones (STANLEY, 1961a, b). A laboratory investigation of heavy minerals was undertaken as part of a petrographic study of larger scope. These laboratory studies were set up to determine whether or not lateral geographic microfacies variations conforming to the three distinct facies defined in the field could be recognized. A distinct lateral variability in the distribution of heavy mineral suites was observed in an area over 5,000 km2. This variability, therefore, is an aid not only in the determination of source areas which supplied sediments to the paleobasin in this part of the Alps, but also in the location of the shoreline and depositional environments onto which the detrital materials accumulated.

METHODS

Over 400 heavy mineral separations were prepared from samples collected in the 60 sections of Annot Sandstone. Generally, 6-12 separations were made per section. The mineral separations prepared by SADOUN (1 957), LEHMANN (1 959) and GAGNIBRE (1960) were re-examined, and their graincount data were incorporated in the present study. Minerals from the sandy samples of the Southern Zone of the basin were separated in bromoform without difficulty. The majority of samples, however, collected from consolidated sandstones and quartzites of the Intermediate and Northern Zones, had to be crushed before separation, which resulted in the breakage of grains and the unfortunate reduction in mineral count accuracy. The relative percentage of heavy minerals in any sample was found to be a function

HEAVY MINERALS IN THE ANNOT SANDSTONE

389

of the grain-size distribution of that sample. Garnet, for example, is larger than zircon and rutile grains in the Annot Sandstones. Garnet, therefore, would be found in relatively greater numbers in the lower portions of turbidites with coarser quartz, feldspar, and lithic fragments than in the upper finer grained portion of graded beds. The relative abundance of zircon and rutile, on the other hand, tends to increase progressively upward in a turbidite. Grain counts performed on 100-450 p sample fractions were found to give the most advantageous results (STANLEY, 1963). For every separation, 100 transparent heavy minerals were counted, mica flakes being excluded from these counts. The 6-12 mineral counts per section were compared, and a rough “average” value was obtained to represent the relative percentage for the particular section. The rough value plotted for each section then represents the approximate relative percentage of heavy minerals that would be obtained from a sample, randomly collected, whose grain sizes fall between 100 and 450 p.

LATERAL DISTRIBUTION OF HEAVY MINERAL SUITES

The lateral variability of the heavy minerals in the paleobasin becomes more clearly apparent when plotted on a geographic map (Fig.1). The following four principal lithofacies provinces are now recognized on the basis of the distribution of heavy mineral assocations’: (a) The Southern Zone, defined by an S-K-G association, comprises the clastic formations in the BarrCme Basin, as well as the sands at Allons, Rouaine, St. Antonin, and Puget-Thtniers eastward to the Pointe des 4 Cantons section (sections 1-12, 48; and 11-IX). (b) The Intermediate Zone, consisting of an S-K-R-A association, is found near the western border of the Argentera-Mercantour Massif. In this region, the relative percentages of staurolite and kyanite have decreased noticeably toward the north (sections 16 and 17), and the resistant and apatite minerals account for over 70% of the mineral suite. ( c ) The Northern Zone consisting of the G-R-A association occupies an area from Le Ruch (section 12) to the northwest border of the Argentera Massif. Garnet makes up more than 15% of the mineral suite in these sandstones. This same heavy mineral province is also to be found in a region extending from the D6me de Barrot toward the Trois EvEchts (section 22) and in a small isolated outcrop at Faudon (section 26) north of the DBme de Remollon and south of the Pelvoux Massif. ( d ) The region between the DBme de Remollon, the Lac d’Allos, and the north and northeast contours of the Argentera Massif contains the R-A association. Over two-

Abbreviations used in reference to these associations are: = Apatite R = Resistant minerals (zircon, tourmaline, rutile) G = Garnet S = Staurolite K = Kyanite A

-GAP

4

I T A L Y ~~

D ~ M E DE REMOLLON

~~

F R A N C E

~

D

L. ~

~ ~

~

-24~~

~

-~ ~

DBME DE

./I W

BARLES

ANNOT SANDSTONE AND RELATED S T R A T I G R A P H I C SECTIONS EXAMINED (.TINLEI 8II8.1 L O C A T I O N OF P E R M I A N S A M P L E S L O C A T I O N OF LOWER TRIASSIC SAMPLES

Fig. 1 . Lateral geographic variability of heavy mineral associations in the Annot Sandstones and related tirne-equivalent formations.

.O

0

D 8 M E DE REMOLLON 0

‘O

0

“p\ 0.

DBME DE BARLES

-

ARGENTERA MERCANTOUR MASSIF c . \

a,

-

N O T E ISOLINE VALUES, VARYING IN GEOMETRIC PROGRESSION, CONNECT POINTS OF EQUAL RELATIVE PERCENTAGES OF STAUROLITE.

21.7.

22.6’ I

SCALE

LATERAL VARIATION OF STAUROLITE IN THE ANNOT SANDSTONES AND RELATED TIME- EQUIVALENT FORMATIONS Fig. 2.

b,

P

D~ME DE REMOLLON 0

\ l o

-i

4

ROT: / ’A,’/, ,5,L

‘!

y>

,

0

5

I0

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I--

Fig. 3.

>/

A,

LEGEND

LATERAL VARIATION OF GARNET IN THE ANNOT SANDSTONES AND RELATED TIME- EQUIVALENT FORMATIONS

HEAVY MINERALS IN THE ANNOT SANDSTONE

393

thirds of the heavy minerals in this association consist of resistant minerals and apatite. Studies by DEB(1938) and BOUMA(1959, 1962) have shown that the S-K-G association should be continued eastward to the outcrops at Tournerait, Peira-Cave, Contes, and Menton. Sands collected by BOUMA (1959) in the southern portion of the Peira-Cave syncline, 25 km southeast of the Pointe des 4 Cantons (section 26), contained minerals in percentages almost identical to those in sands at Ourges, Rouaine, Courlevtras, and Puget-Thtniers (sections 1,2, 3, and VIII). Unlike fractional analyses, relative percentages determined by grain counts imply that as the percentage of one or more mineral types decreases, the percentage of another type (or types) of heavy mineral will have to increase. The regularity with which staurolite, kyanite, and garnet decrease in certain specific directions indicated, however, that a closer look at those mineral distribution was necessary. Equal average relative percentages of staurolite and of garnet were plotted by means of isolines whose values increase in geometric progressions (Fig.2, 3). Staurolite decreases from 32-16 at Rouaine, Ourges, and Courlevtras (sections 1,2, and 3) and to 4 or less in sections only 4 km to the north near the Coulomp River. A decrease to the north and to the east of Puget-Thtniers (section VIII) may also be observed. The sands of Puget-Thtniers and sands of equivalent age at la Rochette (section IX, underlying the younger conglomerates) contain almost identical percentages of the same minerals, including staurolite and garnet. Kyanite, much less abundant than staurolite, also disappears north of an imaginary line connecting the Ruch - Castellet-les-Sausses - Rognone sections (no. 12, 9, and 17, respectively). The relative percentages of garnet are also present in a distinctive pattern. The sandstones in the south, in the west, on the northwest edge of the Argentera Massif, and near Faudon (section 26) are richest in garnet. The regions north of the Argentera Massif, north of the Ddme de Barrot, and southeast of the Dame de Remollon contain the lowest percentages of this mineral. The distribution patterns of zircon, tourmaline, rutile, and apatite, common in all samples, were also plotted individually on maps by means of isolines. The patterns (not figured here) were irregular, and no distinct linear progression laterally was observed. Rounded apatite grains, of probable acid plutonic origin, and resistant minerals become particularly abundant north of the Ruch. Their relative abundance rapidly masks the presence of the S-K-G minerals transported toward the north. Special studies of the different varieties of garnet (clear, rose, striated, nonstriated), zircon (euhedral, rounded, long, short, with or without gas inclusions), and tourmaline (brown, green, bluish subspecies) were undertaken. The lateral variability patterns of these mineral varieties were indistinct. Sphene and monazite, both common in acid plutonic rocks, are minor accessory minerals found in most sections. These and titaniferous minerals such as anatase and brookite comprise less than 5 % of the minerals in the majority of counts. Chloritoid, glaucophane, and andalusite are rare and occur only as trace minerals in the Southern Zone. Epidote, a characteristic mineral of the subalpine chain, is

394

D. J.STANLEY

found only in the Southern Zone, particularly at Allons (section 4) and at la Rochette (section IX) in the St. Antonin syncline. Augite and hornblende are found as minor accessory minerals in the sandstones near the Argentera Massif.

PALEOGEOGRAPHIC IMPLICATIONS AND CONCLUSIONS

The particular lateral distribution of heavy minerals indicates that several source areas must have supplied sediments to the sea during Nummulitic time. SADOUN (1957), Bellair (in KUENENet al., 1957), and BOUMA(1962) have related the S-K-G mineral suites with the micaschist (“meso”) zone of the metamorphic massifs present in the south of this region. The Esterel Massif was exposed to erosion as early as Cretaceous time, according to BORDET(I95 l), and may well have contributed sediments to the north. The sedimentary Permian-Triassic cover of the Maures-Esterel Massifs could also have supplied sands northward. Minerals separated from Permian samples collected on the southern border of the Tanneron Massif near Thkoule and Tremblant (A-M) are listed in order of decreasing frequency in Table I. The sands containing primarily G-R-A and R-A associations in the Northern Zone have a different origin than those containing the S-K-G association in the Southern Zone. Earlier hypotheses stated that the rocks of the Argentera-Mercantour Massif gave rise to Annot Sandstones. However, the heavy minerals listed by FAURE-MURET (1955) in her extensive petrographic treatise on the plutonic and metamorphic rocks of this massif do not support this theory. The heavy minerals most often mentioned are: actinolite, apatite, epidote, garnet, hornblende (both green and brown varieties), pyroxene, sillimanite, sphene, tourmaline, and zircon. These minerals, with the exception of zircon, tourmaline, apatite, and garnet, are uncommon in the Annot Sandstone. The heavy minerals collected from sedimentary Permian and Lower Triassic (werfenian) cover of the Argentera-Mercantour Massif (see Fig. 1) differ from those listed above. The minerals separated from the Permian Bego and Meraviglie Formations are listed in order of decreasing relative abundance in Table TI. The minerals separated from the Lower Triassic (Werfenian) orthoquartzites on the periphery of the massif are given in Table 111. These results partially support the hypothesis, set forth by GUBLER(1959) at the fifth International Congress of Sedimentology, that the Permian-Triassic cover, and not the crystalline rocks of the northern half of the massif gave rise to the Annot Sandstone of the Northern Zone. However, more recent observations indicate that still another important source gave rise to the Annot Sandstones. The progressive decrease of certain minerals from the region of the Pelvoux Massif toward the southeast and from the Dbme de Barles toward the east-northeast may be noted. A tectonic high, oriented north-south between the Pelvoux Massif and Barles, was recognized by GOGUEL(1936). This north-south high area could very well have exposed parent rocks, giving origin to the

395

HEAVY MINERALS IN THE ANNOT SANDSTONE

TABLE I RELATIVE FREQUENCY OF MINERALS O N THE SOUTHERN BORDER OF THE TANNERON MASSIF _

_

_

_

_

Mineral

_

_

_

~- ~

.

-

~

~

Relative amount' _ _ _ _ ~ .~

+ + + + + +

zircon apatite tourmaline anatase staurolite garnet sphene monazite

+ + + + + + +

+ i

Symbols used to represent relative amount in Tables 1-111 are: common present

+ + + abundant

++

+

TABLE I1 RELATIVE FREQUENCY OF MINERALS I N THE BEGO- A N D MERAVIGLIE FORMATIONS O N THE ARGENTERA-MERCANTOUR MASSIF -

~~

Mineral

~~

~~

Relative amount

. ____

_______

+ t + + + + i +

zircon tourmaline rutile apatite garnet anatase monazite augite hornblende

___

~~-

+ + + + + + +

t

TABLE111 RELATIVE FREQUENCY OF MINERALS IN THE WERFENIAN ORTHOQUARTZITES O N THE PERIPHERY

OF THE ARGENTERA-MERCANTOUR MASSIF ~~

.

Mineral -~

zircon tourmaline rutile apatite game! brookite anatase monazite

Relative amount ~~~

+ + + + t + i

i

+ + -1+ +

+

396

D. J. STANLEY

R-A and G-R-A associations. The net decrease in the relative percentage of garnets from 32 in the sandstones at Combe Roranches, south of the Pelvoux Massif, to about 30 in the Faudon area (section 26) and to less than 4 east of the D6me de Remollon is particularly noteworthy in this connection. Abundant information on sedimentary structure orientations, sand-shale ratios, and variation in the number and thickness of sandstone and shale beds, in the number and distribution of conglomerates, and in the number and distribution of slumped deposits has previously been recorded. The lateral variability of the heavy mineral distribution conforms with this trend of data obtained in the field and supports the hypothesis that sediments were brought into the Nummulitic Sea from the south, east, and west (STANLEY,1961b). The rapid disappearance northward of the staurolite and kyanite minerals resulted when sizeable quantities of sand, fed from the east, mixed near shore and at shallow depths with sands supplied from the south. Sands containing R-A and G-R-A suites eroded from the Permian-Triassic sedimentary cover west and east of the paleobasin were transported by torrential rivers to the sea. These sands also flooded the S-K-G containing sands from the south, so that the influence of the “meso”-metamorphic minerals may be traced only as far northward as the Rognone-TCte de Meric outcrops (sections 16 and 17). The S-K-R-A containing sands probably represent this type of clastic mixture near the ancient shoreline. Minerals transported from the south were carried past the D6me de Barrot region, which was still beneath the sea, by tractive currents in the shallow littoral-neritic and possibly deltaic environments. The region north of the Var between the now-exposed limits of the D6me de Barrot and the southwestern border of the Argentera Massif was probably a rather shallow sea during the latter part of Nummulitic time and likely a transitory zone where detrital materials from the south, east, and southeast were brought together. The sands of diverse origin accumulating in this area were then periodically transported toward the northwest by subaqueous gravity currents and redeposited in deeper portions of the flysch basin.

ACKNOWLEDGEMENTS

The writer is indebted to Dr. Y . Gubler and Dr. J. Debyser of the Division de SCdimentologie, Institut Francais du PCtrole, for their guidance during the basic research leading to this paper, and expresses his appreciation to Madame C. Fondeur and M. F. Boyer for their generous assistance in the preparation and analyses of numerous mineral separations. Thanks are due to Dr. C. R. Kolb, Soils Division, U. S. Army Engineer Waterways Experiment Station, for his suggestions improving the manuscript.

HEAVY MINERALS IN THE ANNOT SANDSTONE

397

SUMMARY

The heavy minerals of the marine Annot Sandstones in the Maritime Alps may be grouped into four major mineral associations, each association occupying a distinct geographic position. These include an S-K-G association in the non-flysch southern portion of the formation, an S-K-R-A association near the western edge of the Argentera-Mercantour Massif, and G-R-A and R-A associations in the northern flysch portion of the formation. This distribution of heavy minerals supports the hypothesis that sediments were brought into the Nummulitic paleobasin from the south, east, and west. The clastic materials were derived from crystalline rocks of the Maures-Esterel Massifs and their Permian sedimentary cover, the Permian-Lower Triassic sandstone and quartzitic cover of the Argentera--Mercantour Massif, and sedimentary series to the west-northwest of the basin possibly in a region between the Pelvoux Massif and the Ddme de Barles. The sudden apparent disappearance of staurolite and kyanite minerals toward the north is due in reality to a sudden masking of the S-K-G sands supplied from the south by the G-R-A and R-A bearing sands fed from the east and west of the basin. The S-K-R-A association is found in sands deposited in one of these zones of mixing.

Les mintraux lourds caracttristiques des grbs d’Annot, formation marine des Alpes du Sud, peuvent Ctre classts en quatre associations mintralogiques occupant chacune une position gtographique distincte. Ces quatre associations sont: ( I ) association S-K-G dans la zone non-flysch au sud de la formation, (2) association S-K-R-A p r b de la bordure ouest du massif d’Argentera-Mercantour, (3) et (4> associations G-R-A et R-A dans la zone flysch au nord de la formation. Cette classification confirme l’hypothkse selon laquelle des stdiments auraient Ctt dtposts dans le paltobassin grlce A des apports venant du sud, de l’est, et de l’ouest. La plupart des mattriaux dttritiques ont t t l fournis par les roches cristallines des Maures-Esterel et leur couverture permienne, la couverture permotriassique (grks quartzitiques) de 1’Argentera-Mercantour et des stries stdimentaires venant de l’ouest-nordouest. La dtcroissance apparente de la staurotide et du disthkne vers le nordest est due A l’accumulation soudaine des associations G-R-A et R-A, provenant de l’est et de l’ouest, qui masquent l’association S-K-G venant du sud. L‘association S-K-R-A est constitute par les mattriaux dttritiques dtposts dans une de ces zones de mtlange.

REFERENCES

BORDET,P., 1951. Etude ,yPologiqrre et phtrographique de I’Estercl. Mkm. Sew. Carte Gtol. France, Pans, 209 pp.

398

D.

I. STANLEY

BOUMA,-4. H., 1959. Some data on turbidites from the Alpes-Maritirnes (France). Geol. Mijnbouw. 21 : 223-221. BOUMA, A. H., 1962. Sedimentology of some Flysch Deposits: A Graphic Approach to Facies Interpretation. Elsevier, Amsteidam, 168 pp. DEB,S., 1938. Contribution ii l’etude stratigaphique et petrographique des roches tertiaires des Alpes Maritirnes. Mim. SOC.Giol.France, 36 : 1-114. FAURE-MURET, A,, 1955. EtudesgColqyiques sur le Massif de I’Argentera-Mercantour et ses Enveloppes shdimentaires. Thesis, Mern. Expl. Carte Gkol. France, 336 pp. GAGNIBRE. G., 1959. Contribution d l’L?tirde du Tertiaire du DPportement des Basses-Alpes: Observations stratigraphiques et tectoniques sur le Niinimulitiqire de la Rcyion de Faucon-Gigors. DiplBrne d’etudes SuFrieures, Grenoble, 39 pp. GOGUEL,J., 1936. Description tectonique de la Bordure des Alpes de la Blione au Var. Mem. S ~ N . Carte Geol. France, 360 pp. GUBLER, Y.,1959. etude critique des sources du materiel constituant certaines series detritiques dans le tertiaire des Alpes franqaises du sud: formations detritiques de Barreme, flysch Grks d’Annot. Eclogue Geol. Helv., 51 : 942-911. KUENEN, PH. H., FAURE-MURET, A., LANTEAUME, M. et FALLOT,P., 1957. Observations sur les flyschs des Alpes-Maritimes franpises et italiennes. Bull. Sor. Ghol. France, 6e Sir., 7 : 11-26. LEHMANN, J. P., 1959. Contribution d 1’Etude giologique des Formations tertiaires dans les BassesAlpes: Synclinal d’Esparron-La Batie (Flanc S W ) entre le Sasse et les Monges. DiplBrne d’fitudes Suptrieures, Grenoble, 51 pp. SADOUN, M., 1951. Contribution d I‘Etude du Nummulitique entre Rouaine et le Dime de Barrot. Diplbme d’fitudes Superieures, Grenoble, 38 pp. STANLEY, D. J., 1961a. Etudes sidimentolqpiques des Gr&sd’Annor et de leurs Equivalents latiraux. Thesis, Inst. Franq. Pttrole, Grenoble, 158 pp. STANLEY, D. J., 1961b. J h d e s stdimentologiques des grts d’Annot et de leurs equivalents lattraux. (Resume). Rev. Inst. Franc. Pitrole, 16 : 1231-1254. STANLEY, D. J., 1963. Vertical petrographic variability in Annot Sandstone turbidites. J. Sediment. Petrol., in press.

TRAITS PARTICULIERS MINfk4LOGIQUES-GEOCHIMIQUES DE L'ASSISE TERRIGENE DU CARBONIFERE INFERTEUR DANS LA REGION OURALIENNE-VOLGIENNE G . I . THEODOROVITCH, N . N. SOKOLOVA, E. D . ROSONOVA

et

M. V. BAGDASSAROVA

ComirC National des GPologues de I'U.R.S.S., Moscou (U. R.S.S.)

INTRODUCTION

Au cours des etudes de l'assise terrigkne d'ige Carbonifkre inftrieur dam la majeure partie de Ia rtgion ourdienne-voIgienne, faites en vue de son caracttre petrolifere et houilleux, il fut possible de recomaitre les facits de stdimentogenbe, les faciks mintralogiques-gtochimiques de diagenkse de ces stdiments, le caracttre rythmique et les complexes possibles de la forniation de pttrole. Les auteurs ont tlabore pour la region ouralienne-volgienne le schtma stratigraphique suivant de cette assise, de bas en haut: ( I ) couches Malinov appartenant A la partie suptrieure du Tournaisien, (2) couches (horizon) Radaev de la partie inftrieure du Visten, (3) horizon Bobrikov du Viskn, A deux sous-horizons (I'inftrieur a complexe mixte Radaev-Bobrikov, de spores et le suptrieur B complexe typiquement Bobrikov), (4) paquet terrigkne inftrieur de l'horizon de Toula (Fig.1). L'iige Malinov, dans les limites de la rtgion ttudiee, ttait caractiris6 par des conditions marines. Dans cette mer il se trouvait une zone relativement ttroite et de profondeur plus grande: la dtpression Kama-Kinel, htritie du Dtvonien supirieur. Le passage du Tournaisien au Visten, ou plus exactement du Malinov au Radaev, ttait marqut, dans la zone intercotidale, a l'exception de la dtpression Kama-Kinel, par une interruption brkve de la sedimentation et m e emersion de la plus grande partie. Les iiges Radaev et Bobrikov sont les plus demonstratifs pour l'analyse concrkte de la distribution et de la formation des sediments deltaiques et des dtppbts thalassiques-littoraux. L'ttude des rythmes a permis d'ttablir une certaine rtgularitt dans la distribution des mintraux ferrugineux des premikres ttapes de la diagentse du dip&: de la pyrite, de la sidtroplisite-sidirite, des leptochlorites, de la glauconie, ainsi que de la calcite et de la dolomite. La dtpendance des mineraux ferrugineux de parties dtfinies du

Fig. 1. (pp. 400,401) Coupe du faciks de I'assise terrigine du Carbonifire infirieur de I'aire Elabouga forage no. 12 de Mourzikhin.(Colonne 6: II faut lire carotte au lieu de carottage).

Age

ices de Faune e t f l o r e

:rok e t biturne

ieraux higenes

1

premikes apes de igenbe depdt

53

. .

8.

0

. ..

Fig.1. Ltgende voir p.399.

L

I neraux 1

Iices de

Age

i premiere!

btrole et

Faune e t f l o r e

~ 1 6 mindras

thigeyes ofil du potent IxydationCduction

apes de agenese depdt

' bitume

p

3

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1::

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8 .

.'

I,

:BE

o-

,!

z

H

8: 8:

'

:

.. i '.I

H 8 H

8?

@ H

i

g

n

grhs aleurolite aleurolite argilleuse

e

c

s

R@

calcaire

1

calcaire argilleux

1 0 ]

v

e

n

t

i

o

n

n

e

l

s

brachiopodes segments de crino'ides

@

bitumes

H

sulfides de fer

&chinides

@ carbonates ferrugineux

coralliaires

0

leptochlorites

forarninifkres

argillite

siderite argillite houilleuse e t houilleS nodules de siderite

n

8

@

argillite aleuritique

dolomite

o

-restes

//

spicules de spongiaires

~

a

c

i

h

ostracodes des squelettes de poissons algues tubulaires

0 algues pila 4 passages de vers

ILIIIIIIIIIIIII] 2 sid6rite-sulfide ses analogues a sulfide-sldCrite ses analogues

Stigrnariae

@ spores I

0

assortments dhfinis de spore debris fossites phytogenes

Fig.1 (suite).

a LeptochLoorite analogues au facie5 5 sidfkite

s

402

G.

1. THEODOROVITCH ET AL.

rythme, du type de rythme, d‘horizons stratigraphiques et de rtgions divistes de la rtgion ttudite, a permis de distinguer un nombre de facits mintralogiques-gtochimiques et de suivre leur distribution selon les coupes et dans le sens horizontal sur des distances plus grandes. Gtant inttressts aux caractkres pttrolifkres et houilleux des sediments, nous avons examint principalement les paquets ou complexes argileux et aleurolito-argileux.

S ~ D I M E N T SET M I N ~ R A U XFERRUGINEUX S Y N G ~ N ~ T I Q U E S

Dans les couches Malinov de la partie suptrieure du Tournaisien on trouve des argilites microstratifites. Leur structure est probablement due aux variations saisonnitres des conditions de stdimentation. Ces stdiments appartiennent aux faciks sulfure-sidtrite et sidtrite-sulfure (Fig. I). Les couches Radaev du Visten inftrieur se caracttrisent par une structure macrorythmique, due au renforcement des mouvements tectoniques oscillatoires; leurs faciks mintralogiques-gtochimiques sont un peu plus varits. Le changement de facits gtochimiques des couches Malinov et Radaev dans le sens vertical est moins rtgulier que dans les stdiments terrigtnes Toula. Dans le paquet terrigkne de I’horizon Toula, qui se dtposait sous les conditions d’une transgression gtntrale du bassin, on a ttabli: (a) a la base des rythmes (d’un ou de deux termes) la prtsence de glauconite (dans des rtgions o t ~ il y avait des conditions marines normales, surtout quant a la salinitt) et de leptochlorite (dans les parties du hassin dessaltes I’origine), (b) plus haut dans les unitts rythmiques, ainsi que dans l’assise carbonatte de I’horizon Toula, I’existence principalement de pyrite, qui est survenue dans les horizons profonds du dtpbt, en prtsence de faune benthonique (Fig. 1). Ceci dtmontre l’ttablissement, au cours de la transgression Toula, d’une position stable de la limite d’oxydation-rtduction dans les vases au lieu d’un rtgime riittrt de micro-oscillations, observi au stade initial de cette transgression. En montant dans la coupe stratigraphique on peut observer des changements nettement rtguliers dans la composition mintralogique des argiles. Ainsi, les couches Malinov d’origine rtgressivement marine sont caracttristes par la composition hydromicacte de la masse argileuse avec addition de terre a porcelaine. Leur partie inftrieure contient des lits intercalk de composition a beidellite. Dans les couches Radaev, dans la depression Kama-Kinel, au sud de la rtgion de Kouibychev et sur le versant ouest de I’Oural (argiles marines) et sur le reste du territoire (argiles de formation ptriodiquement continentale) les hydromicas prtdominent sur la terre a porcelaine. En ce qui concerne les argiles ou, en gtntral, le mattriel argileux des stdiments de l’horizon Bobrikov, formtes principalement sous conditions continentales subaquatiques, elks sont caracttristes par une prtdominance de terre a porcelaine, particulitrement dans leur sous-horizon suptrieur. Pour les argiles du paquet terrigkne de l’horizon Toula, qui ouvre la transgression marine, la composition ii hydromicas et a montmorillonite est caracdristique. Un tel changement rtgulier de la composition des argiles dans la coupe de l’assise temgkne du Carbonifkre inftrieur

CARBONIFBRE

I N F ~ R I E U RDE LA R ~ G I O NOURALIENNE-VOLGIENNE

403

s’explique par un passage graduel de conditions marines a l’ige Malinov a des conditions lagunaires-marines A l’ige Radaev, ensuite a des conditions lacustro-palustresalluviales a l’ige Bobrikov et enfin a des conditions marines intercotidales B l’ige Toula inftrieur. Sur la base du contenu des mintraux ferrugineux syngtnttiques (carbonates, sulfures, leptochlorites, glauconites) ainsi que de la faune benthonique, on distingue dans les complexes argileux et argilo-aleuritiques de l’assise terrigtne des facits mintralogiques-gtochimiques de deux series: (a) a rtgime micro-oscillatoire de la division des zones d’oxydation et de rtduction, (6) a situation stable de cette division. Dans la premitre strie de facits les types suivants ont t t t ttablis: B sulfure initial ou gaz puant, a sidtrite-sulfure, sulfure-sidtrite, a sidtrite, leptochlorite-sulfure, B leptochloritesidtrite, a leptochlorite et B glauconite. Nous discernons les stdiments analogues de presque tous ces facits a position stable de la division des zones d’oxydation et de rtduction, quand les mintraux ferrugineux syngtnttiques sont reprlsentis seulement par des sulfures; ceci se fait selon la prtsence et le caracttre de la faune benthonique. Dans la partie vistenne de l‘assise terrigbne nous avons reconnu des types varits de rythmes, caracttristiques pour la dtfinition de certains horizons stratigraphiques. Ces rythmes diffkrent les uns des autres par leur structure: outre les rythmes transgressifs on observe aussi des rhythmes transgressifs~rtgressifsa partie transgressive exprimle plus complttement. De plus, les rythmes difftrent par le dtveloppement dans leurs parties suptrieures de facits mintralogiques-gtochimiques varits. La structure rythmique, observte dans l’assise terrigtne, en partant des couches Radaev, ttmoigne d’un renforcernent des mouvements oscillatoires au dtbut du Visten. Des cartes de facits mintralogiques-gtochimiques ont t t t compostes pour six stades de la formation de I’assise terrigtne dans la partie principale de la rtgion ouralienne-volgienne: pour les parties inftrieure et suptrieure des couches Malinov, pour les couches Radaev et les stdiments Bobrikov inftrieurs et suptrieurs ainsi que pour les dCpBts de Toula inftrieurs. En composant ces cartes on a appliqut une mtthode nouvelle. Les cartes donnent les facits 8 rtgime micro-oscillatoire des zones d’oxydation et de rtduction ainsi que les faciks analogues et ceux ii position stable de la division de ces zones. L’analyse des cartes montre que la disposition des facib gtochimiques est influencte non seulement par la vie tectonique des soultvements de voiite et des dtpressions qui les stparent, mais aussi par l’existence et le developpement d’un Clkment sptcifique comme la depression Kama-Kinel. En relation avec le thtme de cet expost, arrEtons-nous seulement a l’analyse des facits de stdimentogentse et des facits mintralogiques-gtochimiques de diagenese pour les iges Radaev (Fig.2) et Bobrikov.

AGE RADAEV Durant 1’8ge Radaev, qui Ctait caracttrist dans la region de I’Oural et de la Volga par des courtes ingressions superpostes sur une regression plus gtntrale, il existait dans

404

C. I. THEODOROVITCH ET AL.

Echellc 0 2p 40

- -6p-

-

80

100kn

Signes convcntionnels

Limites de la depression Kama-Kinel

...... Limites des t aciis ,‘--

Limites des districts bu ce sidiments sont absents

__ Faci’es i

siderite-suliide

i

sulfide-sidrite

ses analogues

Fig.2. Carte schkmatique des facits mintralogiques-gCochimiques de la partie principale de region Ouralienne-Volgienne pour Ies sediments argileux et argilo-aleurolitiques des couches Radaev. Erratum: ses analogues a trait au facies i siderite-sulfide.

les liniites de la dtpression Kama-Kinel u n dttroit marin itroit, pour la plupart dessali. Au nordest de la rigion, dans les environs de Koungour-Perm-Goubakha, le dttroit Kania-Kinel passait da m le bassin marin du versant occidental de I’Oural. Les stdinients marins d’2ge Radaev, diveloppts sur le versant occidental de I’Oural central, sont analogues iceux que nousavons Ctuditsau sud de l’Oural. 11s sont reprtsentis par des calcaires marins intercotidaux ii riche faune de brachiopodes, foraminiferes et une teneur considtrable en algues calcaires d’espkces variies. Au sud de la rtgion, le dttroit de la depression Kama-Kinel passait dans le bassin marin. dont I’existence est fixbe h un nombre d’endroits de la region Kouibychev (Moukhanovo, Dmitrievka, etc.). Les stdiments marins intercotidaux, dtveloppts ici, contiennent par places des restes de spicules d’tponges dans la partie infirieure des couches Radaev, et de foraminifkres dam la partie suptrieure de ces couches. Les sidiments Radaev reprisetitent ici une lternance rythmique de grks quartzeux a grain

CARBONIFkRE INFiRIEUR DE LA RlkiION OURALIENNE-VOLGIENNE

405

fin, d‘aleurolites i traces de passage d’une sorte de vers (vasophages) et d’argilites A nodules de sidtrite et B restes d’une faune de lingules, de poissons et de spongiaires; on y observe de 2 a 4 rythmes sur une tpaisseur totale des couches de prts de 90-1 10 m. Seulement dans la region du Gorky Ovrag, dans une zone qui est supposte reprbenter la partie sous-marine d’un petit delta, I’tpaisseur de ces couches augmente jusqu’a 220 m. En gtntral, dans les coupes ttudites il y a prtdomiriance des argilites et des aleurites argileuses. Les rythmes se terniinent parfois par un lit intermtdiaire de dolomite grtsiforme. La zone examinte des stdimerits marins intercotidaux d’gge Radaev se caracttrise par un diveloppement du facibs A sulfure-sidtrite. Dans la zone du dttroit marin de la dtpression Kama-Kinel, qui s’est transformte quelquefois dans les limites de la Tatarie en une zone de Rhizophores, on observe dans les couches Radaev une alternance rythmique (3-4 rythmes) de grts ou d’aleurolites grtseuses avec des argilites qui contiennent en Tatarie des fins lits (1-5-10 cm) intermtdiaires et discontinus houilles. A beaucoup d’endroits on y voit des Stigmariae, particulitrement dans la baie Menzelinsk-Aktanysh, oh des mangroves se sont dtvelopptes frequemment et pendant des ptriodes prolongees. Ces couches renferrnent les restes d’une faune d’ostracodes et de poissons. Dans les argilites on observe des nodules et des fins lits intercalks de sidtrite. L‘tpaisseur des dtp6ts Radaev dans cette zone est prts de 100 m. Les argilites et les aleurolites yprtdominent. Quant aux mintraux ferrugineux, le facibs dans la zone du dttroit Kama-Kinel a l’lge Radaev ttait celui A sulfure-siderite. La zone littorale du dttroit, qui portait parfois des mangroves, se caracttrise par une alternance d’aleurolites quartzeuses a traces de passage de vasophages, avec des grks 2 grain fin et avec des argilites stratifites ;i nodules de sidtrites. 11 y a des intercalations fines de houilles et des Stigmariae. La faune est reprbentte par des spicules de spongiaires et par de sostracodes. Dans la coupe dominent des grks et des aleurolites. On y distingue 1-2 rythmes surune tpaisseur gtntrale des couches Radaev de 45-60 m. Aussi dans cette zone un faciks a sulfure-sidtrite Ctait developpt a l‘gge Radaev. Au nord de la dtpression Kama-Kinel, dans la plaine littorale, on rernarque un nonibre de places oh les couches Radaev sont absentes. Les stdiments continentaux lacustres-palustres comprennent une alternance d’aleurolites argileuses a traces de passage de vasophages et d’argilites aleuritiques finement stratifites i lits intercalts de houilles. Ici, on ne distingue qu’un seul rythme sur une tpaisseur gtntrale des couches Radaev de 5-1 8 m. Dans certaines parties de la rtgion Kouibychev, ainsi que sur une partie considtrable de la Tatarie au-deli du fleuve Kama, les couches Radaev de ce type se distinguent par un dtveloppement de stdiments appartenant aux dtp6ts analogues au facits A sidtrite-sulfure. On peut remarquer que certains terrains, d’ttendues limittes, adhtrant comme une zone ttroite A la dtpression Kama-Kine1 (Zagliadino, Bouldyr et Mouslumovo), montrent dans les couches Radaev le facits a sidtrite-sulfure. Dans les confins du sommet sud de la voClte Tatare il y a un nombre de places oh les couches Radaev font dtfaut. Dans les limites de la Bachkirie du nord et de la ptriphkrie sudouest de la rtgion de Perm, l’absence de ces couches est A peu prts gtntrale. II

406

C . I . THEODOROVlTCH ET AL.

semble que cela est dO a leur denudation a 1’8ge prt-Bobrikov. On peut supposer qu’h ces endroits les couches appartenaient aux dtp6ts analogues au faciks de sidtrite-sulfure mintralogique-gtochimique et a ce faciks lui-mCme.

AGE

BOBRIKOV

Anttons-nous maintenant a la distribution des types gtnttiques de stdiments (facies de stdimentogentse) au cours de la dtposition de l’horizon Bobrikov, quand la rtgression de la mer a atteint un maximum stable. A cette tpoque-18 il se formait dans les confins de la rtgion de Perm, entre Sylva et Kyn-Tchussovaya-Goubakha-Kizel, des sediments delta’iques d’une tpaisseur de 100-120 m et mCme jusqu’a 220 m. En mCme temps il se trouvait dans la dtpression Kama-Kinel une vallte fluviale, dans laquelle se jetait un nombre de petites arttres fluviales. La rtgion du d6me sud de la voiite Tatare ttait la plus tlevte et sur une partie considerable les stdiments de l’horizon Bobrikov y sont absents; par endroits ils sont reprtsentts par des dtp6ts lacustrespalustres et fluviatiles. Le cours suptrieur de cette rivitre ttait situt5, semble-t-il, quelque part dans la rtgion de Tchistopol et Alekseevskoe et un des grands affluents dans les parties amont traversait les environs de Koukmor. Entre le d6me sud de la voiite Tatare et la vofite Tokmov il se trouvait, selon toute probabilitt, une selle soulevte. Plus au sud, une autre vallte fluviale commeqait, qui se jetait dans le bassin de la partie sudest de la plate-forme ruse. Ce dernier systkme fluvial se distinguait par des stdiments fort grtseux, et, tvidemment par une grande activitt hydrodynamique. Dans les confins des deux valltes fluviales indiqutes, deux principaux types gtnttiques de stdiments peuvent Ctre discernes: les dCp6ts formts principalement dans le lit et ceux de la plaine d’inondation. Les stdiments du lit sont surtout des grts quartzeux grossiers ou a grains varits, ma1 trits, parfois a stratification transversale, et contenant dans la partie suptrieure de la coupe des intercalations d’argilites a lentilles de houille. Les grts et les aleurolites dominent sur les argilites, notamment dans les stdiments de la rivikre sud. L’tpaisseur des stdiments du lit est de 25-60 m. I1 faut remarquer, qu’en tant que les roches grtseuses pouvaient subir des modifications considtrables du milieu hydrochimique au cours de leur histoire gtologique, on ne peut pas utiliser, a notre avis, des indices mintralogiques-gtochimiques des complexes grtseux pour ttablir les conditions de sedimentation. Parmi les sdiments aleurito-argileux du type examint, on trouve une prtdominance d e stdiments appartenant aux analogues du facits a siderite-sulfure. Les stdiments de la plaine d‘inondation sont reprtsentts par une alternance d’aleurites quartzeuses ma1 trikes (prtdominantes) avec des grts et argilites peu tpais h intercalations de houilles. Parfois on rencontre des aleurolites k lits intermtdiaires de sidtrites et d’oolites remanites d’hydroxydes de fer. I1 y a des Stigmariae. L’tpaisseur des sediments varie de 25-50 m.11s sont dtveloppts au nord de la Bachkirie et au sudouest de la rtgion de Perm (stdiments analogues a ceux des facits sulfure-sidtrite

CARBONIFkRE INFBRIEUR DE LA RBGION OURALIENNE-VOLGIENNE

407

et a sidtrite-sulfure) et le long des bordures des valltes fluviales de la depression Kama-Kine1 en Tatarie et de la rtgion Kouibychev (facibs A sidtrite-sulfure). Les stdiments lacustro-palustres sont diveloppis dans certaines parties des plaines d’inondation, particulibrement dans la rtgion Menzelinsk-Aktansh de Tatarie (facits a sulfure-siderite) et sont Ctablis aussi dans certains districts de la rtgion Kouibychev et au sud de Bougourouslan (facib i siderite-sulfure et analogues). Ces stdiments se caracttrisent par une interstratification d’argilites aleuritiques houilleuses avec des aleurolites, pleines de traces de passage de vers et de rares lits intermtdiaires de g r b et de houille. L’tpaisseur des stdiments de ce type est de 10-24 m. ArrEtons-nous brikvement aux stdiments des ravins d’8ge prt-Bobrikov, dtveloppts dans les confins de la rtgion soulevte des dbmes sud et nord de la votite Tatare et de la rtgion moins soulevte de la Bachkirie nordouest. Parmi ces ravins il y a deux types distincts. Les uns, ttroits, en forme d’encoches, sont principalement remplis par des sediments grtseux et aleurito-grtseux d‘alluvion de l’horizon Bobrikov; ces ravins se dtversaient dam la vallte fluviale qui peut Etre suivie de Kamskoe Oustie (embouchure de la Kama) jusqu’i Sylva. Dans d’autres ravins, dont seule la partie inftrieure des profils est en forme d’encoche, une partie considtrable de la coupe de l’horizon Bobrikov est reprtsentte par des roches aleuritoargileuses et grtso-aleuritiques i tpaisses lentilles de houille (Soultevo et autres endroits de la Tatarie au-deli de Kama, Arlan et Tcheraoul dans la Bachkirie nordouest); ces stdiments sont caracttrists par une prtdominance de stdiments analogues au f a d s B siderite-sulfure. Dans la Bachkirie septentrionale on a ttabli que les parties superieures des tpaisses lentilles de houille (jusqu’i 10-18 m) s’ttendent en lits intermtdiaires de faible tpaisseur (- 0.5 m) dans les vastes aires de la plaine d’inondation. Ceci indique un remplissage rapide des ravins examints, et l’ttablissement dtja au milieu de l’ige Bobrikov, d’une seule surface gtntrale de la plaine d’inondation dam laquelle se dtveloppaient, pendant des ptriodes brkves, de vastes martcages. Ainsi, dans les stdiments aleurito-argileux, observe-t-on une coincidence des types de facits de stdimentogenbe avec les types de facibs mintralogiques-gtochimiques de diagenese, mais dans un nombre de cas il existe aussi des difftrences. A prtsent on avance au premier plan l‘ttude des faciEs mintralogiques-gtochimiques a position stable de la division des zones d’oxydation et de rtduction. Ces facib ont, comme on l’a ttabli. une distribution trks grande.

Conformtment au contenu des mintraux ferrugineux syngtnttiques dans les complexes argileux et argilo-aleuritiques de l’assise terrigene, on Ctablit des facits minbalogiquesgiochimiques de deux series: ( I ) B rtgime micro-oscillatoire de la division oxydationrtduction (a sulfure-siderite, a sidtrite, etc.); (2) avec une situation stable de cctte division (les analogues de ces faciks dans les sediments desquels les mintraux ferrugineux syngtnttiques sont rtprtsentts seulement par des sulfures en prtsence du benthos).

408

G . I. THEODOROVITCH ET AL.

Les stdiments d’iges Radaev et Bobrikov du Visten inftrieur dans la rtgion ouralienne-volgienne sont les plus dtmonstratifs pour l’analyse des dtp8ts littoraux intercotidaux et deltaiques. Au cours de l‘ige Radaev, caracttrist par prtdominance de conditions rtgressives, il existait dans les limites de la dtpression Kama-Kinel un dttroit marin, qui passait au nordest et au sud dans un bassin marin. Les sediments marins y montrent un developpement du facits a sulfure-sidtrite. Localement, des mangroves ttaient prtsentes a ce temps. A l’ouest de la rtgion Kouibychev et dans la Tatarie au-dela de la Kama, des sediments lacustro-palustres se formaient, qui se distinguaient par un dtveloppement de sediments analogues au faciks a sidtritesulfure. Au temps Bobrikov des sediments de deltas s’accumulaient dans la region de Perm, tandis que dans la dtpression Kama-Kinel deux valltes fluviales s’ttaient formtes, divistes par une selle. Dans ces valltes on reconnait deux types de stdimentation: l’un principalement celui du lit fluvial et l’autre ii la plaine d’inondation. Les stdiments Bobrikov sont pour la plupart analogues au facits 2 sidtrite-sulfure. Dant des parties non inondies appartenant i la plaine d’inondation on trouve des dtp8ts lacustro-palustres de facies B siderite-sulfure et B sulfure-siderite et ceux qui leur sont analogues. Quant aux sediments aleurolito-argileux, on y observe principalement une coi’ncidence des types de facies de stdimentogenbe avec ceux de facits miniralogiques-geochimiques de diagenese.

SUMMARY

On the basis of the content of syngenetic iron-minerals in the clayey and clayey-silty deposits of the terrigenous layer, two series of mineralogical-geochemical facies have been established: those where the zones of oxidation and reduction underwent small scale fluctuations, and those where these zones had a constant position. The first ones typically contain sulfide-siderite, siderite, etc., while in the second facies the syngenetic iron-minerals are represented only by sulfides in presence of benthos. The sediments of Radaev and Bobrikov age (Lower Vistan) in the Ural-Volga region best demonstrate the analyses of littoral-marine intertidal and deltaic sediments. During the Radaev stage (characterized by dominantly regressive conditions) a narrow sea strait existed in the area of the Kama-Kinel depression, which opened at the northeastern and southern endsinto marine basins. The marine deposits here show a development of the sulfide-siderite facies. Locally, along this strait mangroves were present during Radaev times. West of the Kouibychev area as well as in Tataria, beyond the Kama, lacustrine to marshy deposits were formed, which show a facies analogous to the siderite-sulfide facies. During the Bobrikov stage delta sediments were accumulated in the Perm area, while in the Kama-Kine1 depression two river valleys were formed, separated from

CARBONIFBRE INFkRIEUR DE LA RkCION OURALIENNE-VOLGIENNE

409

each other by a saddle. In these valleys two types of sedimentation took place: one mainly limited to the river bed itself, the other limited to the flood plain. The Bobrikov deposits show chiefly a facies analogous to the siderite-sulfide facies. In certain parts of the flood plain, lacustrine to marshy sediments are found, with siderite-sulfide, sulfide-siderite and analogues of these. Regarding the siltydayey deposits, it was observed that the depositional facies types coincide for the main part with the mineralogical-geochemical facies of diagenesis.

CRITERES DE SENSIBILITE APPLIQUES AUX INDICES DE FORME DES GRAINS DE SABLE V. T O N N A R D Institut Agronomique de I’Etat, Gembloux (Belgique)

INTRODUCTION

I1 existe actuellement un t r b grand nombre d’indices qui prttendent chiffrer la plus ou moins forte usure subie par les grains de sable. On peut dire que chaque technicien de la stdimentologie a prtsentt son indice personnel pour peu qu’il ait t t t amen6 B developper un travail particulier. On se trouve ainsi devant une quantitt considtrable de rtsultats ttmoins de longs et patients efforts, cependant inutilisables dans un travail de synthtse oh la comparaison est indispensable. I1 nous a semblt que le temps Ctait venu de prtparer systtmatiquement le choix d’un seul indice que, sur les bases d’une ttude systtmatique, nous pourrions conseiller d’utih e r dans un but d’uniformisation des rtsultats (TONNARD, 1960).

INDICES DE FORMES DES GRAINS DE SABLE

Lorsqu’on ttudie de p r h les crittres qui font qu’un indice est un bon indice, on se rend compte que celui-ci doit satisfaire B nombre d’imptratifs: il doit &treparlant, logique, facile de calcul et de manipulation (crittres subjectifs). Mais il doit en plus et surtout &tresensible, stlectif et fidkle (crittres objectifs). La sensibilitt d’un indice est la plus ou moins grande rapiditt avec laquelle il accuse des variations dans le degrt d’usure du mattriau ttudit. La stlectivitt est la proprittt

Les indices sont remanies afin de varier entre des limites communes (de &100) et dans le mZme sens. Ce remaniement n’affecte en rien la sensibilite originelle des indices. Ces indices ont 6tB appliquCs A dix khantillons de sables trts caracttristiques choisis dans la vaste collection du Prof. Cailleux et Venus du monde entier. Termes communs a la deuxieme colonne: r l = plus petit rayon de courbure du pirimbtre du grain. f n = rayons de courbure successifs. N = nombre de rayons de courbure mesures. C = circonference d’un cercle de surface Cgale a celle de la projection du grain. a b c . . n = somme des angles au centre sous-tendant des segments de droite. S = surface planimCtrk de la projection du grain. S, = surface du cercle circonscrit. L = plus grand diambtre du grain. P = phimetre curvimetre de la projection du grain. Si = surface du cercle inscrit. I = plus grande largeur du grain (perpendiculaire A L). R = rayon du cercle inscrit. d = diambtre d’un cercle de surface tgale (S) a celle de la projection du grain.

+ + + +

Pe

Sg

= surface = surface

Sc

S Sc

= -.lo0

du grain du cercle circonscrit

Cox 1927)

A

=

p

=

surface du grain perimttre du grain

TICKELL ( I 93 I )

Si Ti=--l00

sg

Roundness = -

4~= surface du grain si

=

S

Si

surface du cercle inscrit

FISCHER (1933) Angularite

=

a+b+c.. f n 100% 360" ~

CAILLEUX ( I 950) Ca rl L

= plus = plus

2r1 L

=-

. 100

perit rayon de courbure grand diamttre

GOGUEL-KUENEN (1953,1956) 2r I=' 1 rl = plus petit rayon d e courbure I = plus grande largeur (perpendiculaire a L ) TONNARD (1958) S.4.1W P =T D S = surface d u grain P = perimttre du grain D = plus grand diamttre du grain WADELL (1 935)

d q ) Dc d = diamttre d'un cercle de surface tgale a celle de la projection du grain Dc = plus grand diametre du grain (cercle circonxrit)

z -T i

PI-- R N r, R

N

= rayon de courbure des = rayon d u cercle inscrit = nombre de mesures

- -

-

~~

contours

- -~-~ ~~~

-

--

-

-

~

-

-

412

V. TONNARD

de distinguer d a m un lot de sables les grains de difftrentes origines. (La sensibilitt se confond pratiquement avec la stlectivitt). La fidtlitt est cette aptitude a fournir des rtsultats reproductibles dans le temps ( m h e optrateur travaillant a des temps difftrents sur u n mCme tchantillon) et daiis I’espace (deux optrateurs travaillant sur un m&metchantillon). De toutes ces qualitts, la plus importante est, sans contre-dit, la sensibilitt et, en corollaire, la stlectivitk. C’est pourquoi, parmi les indices prtsentts dans la liste ci-dessous et apparaissant le plus frtquemment dans la litttrature, nous avons essayt de dtgager le plus sensible. Nous indiquons dans le Tableau I1 ci-dessous, les coefficients de temps et les coefficients de fatigue des difftrents indices. Les coefficients de temps sont calcults d’aprts TABLEAU I1 COEFFICIENTS DE MANIPULATION

CAILLEUX WADELL1 WADELL 2 GOGUEL-KUENEN

cox TICKELL FISCHER TONN AR D PENTLAND

le temps moyen chronomttrt, nkcessaire i un opkrateur entrain6 pour calculer entitrement un indice (du dessin de la projection au rtsultat chiffrt). Le coefficient de fatigue est une apprtciation personelle baste sur l’exptrience de plusieurs milliers de grains soumis a chacun des indices tnumtrts.

ANALYSE STATISTIQUE

L’analyse statistique des rtsultats, pour les dttails de laquelle le lecteur consultera les publications de la bibliographie (TONNARD,. 1960, 1962), comporte classiquement: Choix du nombre de grains Ptudiks

Dans chacun des dix tchantillons, neuf groupes difftrents de 25 grains ont it6 respectivement soumis aux neuf indices citts. Ce planing a t t t adoptt, plut6t que de reprendre 9 fois les mCmes 25 grains dans un kchantillon de faGon i satisfaire aux conditions thtoriques du test de sensibilitt de SCHUMANN et BRADLEY (1959).

INDICES DE FORME DES GRAINS DE SABLE

413

Nous avons i priori choisi 25 grains parce que l’exptrience montre que, dans l’utilisation de la plupart des indices, la moyenne ne se modifie gutre de plus de quelques pour-cent dts que I’on atteint le vingt-cinquitme grain environ, quel que soit le nombre de grains suppltmentaires ttudits. Un dtveloppement statistique montre le nonibre minimum de grains ii utiliser suivant le but poursuivi et les conditions de travail. Contr6le de la normalite‘ (rkpartition gaussienne) des populations Test d’hgalite‘ des variances II serait peu logique de vouloir comparer par la moyenne une population de valeurs trks disperstes ii une autre population de valeurs trks concentrles. L‘analyse de la variance est d’ailleurs subordonnte i cette hypothkse au m h e titre qu’i l’hypothese de la rtpartition gaussienne. Le test des variances a Ctt rtalisl suivant le schima de HARTLEY (1940) qui envisage le rapport de la variance maximum a la variance minimum. C‘est ainsi qu’8 cBtt de quatre populations normalernent etalees (arrondi de Wadell, roundness de Tickell, usure de Pentland, angularit6 de Fischer), cinq sont a dispersion trop variable (indice de Cailleux, indice de Goguel-Kuenen, indice de Cox, ptrimktrie de Tonnard, sphtricitt de Wadell) et ne peuvent, telles quelles, Ctre utilistes dans des comparaisons statistiques. En passant aux logarithmes, il est possible de ramener les indices de Cailleux et de Goguel- Kuenen dans des limites de variabilitt admissible. Pour les autres indices, (sphtricitt de Wadell, ptrimttrie de Tonnard, indice de Cox), aucune relation n’a pu Etre dtcouverte; si elk existe, elk est trop complexe pour Ctre d’application pratique. Nous considererons donc que ces trois derniers indices ne sont pas aptes 8des manipulations statistiques par suite de leur hypersensibilitt incontr6lable. Remarquons que ces indices font tous trois intervenir des notions de surfaFage et de ptrimttrie du grain. Ana!vse ilr la variance (comparaison des valeurs de F ) Les dix echantillons ttudits au moyen de l’un des neuf indices se distinguent les uns des autres plus ou moins suivant la valeur du paramttre F. Toutes les diffirences entre tchantillons sont hautement significatives et I’application du test de Student ne donne pas de renseignements supplCmentaires. Attribuant, en preniikre approximation, la plus grande sensibilitt A I‘indice obtenant la plus graude valeur de F, les rtsultats obtenus nous conduisent i effectuer le classement, indiqut d a i s le Tableau 111. I1 est assez remarquable de constater que les indices qui se dttachent nettement par leur sensibilitt (arrondi de Wadell, indice de Cailleux et indice de Goguel-Kuenen) sont issus d’une conception de l‘usure faisant intervenir la notion de plus petit rayon de courbure (r) tandis que les indices faisant appel aux notions de plrimetre et de surface (sphlricitt de Wadell, plrimttrie de Tonnard, indice de Cox) sont non seulement peu sensibles mais statistiquerneiit inutilisables par suite de I’inipossibilitl de riduire leur inCgalit6 des variances, mCme aprks passage aux logarithmes.

414

V. TONNARD TABLEAU I11 SENSIBILIT~COMPARBE DES INDICES

Indice

Arrondi de Wadell Indice de Cailleux Log. (indice de Cailleux) Indice de Goguel-Kuenen Log. (indice de Goguel-Kuenen) Indice de Cox Roundness de Tickell Perimttrie de Tonnard Usure de Pentland Angularite de Fischer Sphericitt de Wadell

Valertr de F 26.0 25.0 24. I 22.3 21.2 7.0 5. I 3.8 3.3 3.0 2.8

Remarques

Inutilisable (inegalitk des variances) Inutilisable (inkgalit6 des variances) Inutilisable (inkgalit6 des variances) Inutilisable (inegalite des variances) Inutilisable (intgalitt des variances)

Test de sensibilite‘ Le test de sensibilitt de SCHUMANN et BRADLEY (1959) est incapable de dissocier lequel des trois indices (Indice de Wadell, log. ind. de Cailleux, log. ind. de CoguelKuenen) est le plus sensible. Ce sont donc des considerations pratiques qui nous amtneront ri choisir.

DISCUSSION

Les indices de Cailleux et de Goguel-Kuenen prtsentent tous deux I’inconvtnient d’exiger le passage aux logarithmes. De plus, l’indice de Goguel-Kuenen exige la mesure de la plus grande largeur (1) perpendiculaire a la plus grande longueur (L). Ceci demande un peu plus de temps que la mesure de la plus grande longueur (L).Ce Ieger alourdissement des manipulations ne conduit pas A un accroissement de sensibilitt: c’est pourquoi nous preferons l’indice de Cailleux celui de GoguelLKuenen. Compart a l’arrondi de Wadell, l’indice de Cailleux prtsente deux dtsavantages. L’un, dtja citt, est le passage aux logarithmes. L’autre apparait en comparant les formules des deux indices: Wadell envisage la moyenne de tous les arrondis tandis que Cailleux n’envisage que le plus petit rayon de courbure, ce qui est une cause non ntgligeable d’erreurs accidentelles. Cette cause d’erreur est fortement rtduite, il est vrai, si l’optrateur, comme le conseille I’auteur, effectue toujours la mesure de rl et r2. D’autre part, surtout si on envisage l’expression graphique de l’indice et l’interprttation des accidents de la courbe cumulative, il faut se rappeler que la cible micromttrique de Cailleux ne porte qu’un nombre restreint de cercles et qu’on travaille dans une fraction granulomttrique assez ttroite (valeur relativement constante de L). Ces faits entrainent un classement artificiel des valeurs de l‘indice, d’oa des crochets dans la courbe cumulative qui, loin d’etre des “remaniements” ne sont que des arttfacs de laboratoire.

INDICES DE FORME DES GRAINS DE SABLE

41 5

Par contre, I’arrondi de Wadell est trts long A mesurer et B calculer malgrt des amtliorations techniques possibles; or il nous parait plus utile - dans un travail oh on exprimera les rtsultats par une moyenne d’indices. - d’effectuer plutet des mesures sur un grand nombre d’echantillons d’une m&me provenance que de s’attacher a I’expression rigoureuse de l’usure. C e s t pourquoi nous optons pour l’utilisation intensive de l’indice de Cailleux ramen6 aux logarithmes B condition de ne pas en tenter, sans essais preliminaim, l’expression par courbes cumulatives.

REMARQUE3 FINALES

Le manque de place nous empkhe de dttailler les travaux qui, posttrieurement au choix de I’indice de Cailleux en ont fait une utilisation systtmatique. Signalons seulement que la comparaison de series sableuses horizontales et la comparaison de stries verticales correspondantes, ont pu dernontrer (TONNARD, 1962) qu’en certains cas l’indice d’tmousst des sables peut Ctre un reptre stratigraphique non ntgligeable en l’absence de toute donnte paltontologique. Les etudes en cours tendent A affiner cette signification et a l’ttendre aux grts cohtrents dans le but final d’une application aux monotones stries grtseuses. 11 semble donc raisonnable, dans certaines conditions, d’entreprendre une ttude systtmatique des possibilitts d’application stratigraphique des donntes morphometriques et morphoscopiques issues de l’observation des grains de sable ou des grts. 11 est certain qu’on se trouvera toujours devant la difficult6 des variations lattrales dues A des conditions particulitres de stdimentation. Mais, d h B present, on entrevoit des differences qui sont dues B des conditions plus gtnirales que les ttroites circonstances de dtpbt: peut-Etre faut-il faire appel A des conditions climatiques (TRICART, 1958). Les variations verticales dans les sablieres du Bruxellien, trouvtes plus fortes dans le temps que les variations horizontales ne le sont dans l’espace entre le Cap Gris-Nez et I’embouchure de I’Escaut, sont un argument en faveur de cette hypothtse.

Dans le cadre des travaux morphomttriques et morphoscopiques men& sur une base statistique par le laboratoire de Gtologie de 1’Institut agronomique de ]’&at B Gembloux (Belgique), l‘auteur dtcrit la technique suivie pour dtceler l’indice de forme ou d’usure le plus sensible parmi ceux que presente la litt6rature. Les tests appliquts retiennent l’indice de Cailleux et l’arrondi de Wadell. Des considtrations pratiques choisissent l’indice de Cailleux. Une courte relation des travaux ulterieurs est prCsentte.

41 6

V. TONNARD

SUMMARY

I n connection with statistical studies of the shape of sand grains, carried out at the Laboratoire de Geologie of the Institut Agronomique de I’Etat at Gembloux (Belgium), it became of importance to know which of the indexes of shape or abrasion, mentioned in the literature, is the most sensitive. The present paper gives a description of the method followed to find this out. It appears that two indexes meet the requirements relatively well, viz. those of Cailleux and of Wadell. The index of Cailleux is selected as being the most practicable. Mention is briefly made of further, as yet unfinished studies by the author on this subject.

BIBLIOGRAPHIE

CAILLEUX, A., 1950. L‘indice d’eniousst des grains de sable et grts. Rev. CPomorphologie Dyn., 3 4 : 78-82. Cox, E. P., 1927. A method of assigning numerical and percentage values to the degree of roundness. J . Paleontol., 1 : 179-183. FISCHER, G., 1933. Die Petrographie der Grauwacken. Juhrh. Preuss. Geol. Landesanstult Berlin, 54 : 322-323. GOGUEL, J., 1953. A propos de la mesure des galets et de la dtfinition des indices. Rev. CPornorphol. Dyn., 21 : 115-118. GRAULICH, J. M., 1951. L‘emploi des courbes cumulatives dans 1’0tude de I’indice d’tmoussk des galets. Ann. Soc. GPol. Belg., Bull., 74 : 155-162. HARTLEY, H. O., 1940. Testing the homogeneity of a set of variances. Biometrika, 31 : 249-255. KUENEN, PH.H., 1956. Experimental abrasion of pebbles. Lei& Geol. M e d e k l . , 20 : 131-137. PENTLAND, A,, 1927. A method of measuring the angularity of sands. Proc. Trans. Roy. SOC.Can., Section, 3, 21 (abstracts) : XCIII. D. E. W. et BRADLEY, R. A,, 1959. The comparison of the sensitivities of similar experiSCHUMANN, nients.Biornefrics,15 (3) : 405416. TICKELL, F. G., I93 1. The Examination ofFrugnienta1 Rocks. Stanford Univ. Press, pp. 6-7. TONNARD, V., 1958. Presentation et application de l’indice de ptrinittrie. Eclqpue Geol. Helv., 51 (3) : 779-783. TONNARD, V., 1960. etude comparative des indices niorphornttriques appliquke aux sables. Bull. Itist. Apron. Sfa. Recli. Genihloitx, Hors SPr., 1 (1960) : 397416. TONNARD, V., 1962. Signification stratigraphique de I’indice d’8niousst des sables. Birll. Inst. &on. Sta. Rech. Genrhloitx, 29 ( 3 4 ) : 385400. TRICART, J., 1958. Donnees pour I’utilisation paltogtographique des callloutis. Eclogae Geol. Hell.., 251 (3) : 784-795. WADELL, H . , 1935. Volume, shape and roundness of quartz particles. J. Ceol., 43 : 25C-280.

EFFECT OF THE ORIGIN OF THE LOWER CARBONIFEROUS CLAYS IN THE WESTERN PART O F THE MOSCOW BASIN ON THE ALTERATIONS OF THEIR CLAY MINERALS M. F. VIKULOVA

National Coniniittee of Geologists in the U.S.S.R.,Moscow ( U . S . S . R . )

INTRODUCTION

Data arc found in literature dealing with factors acting upon the formation, alteration and destruction of clay minerals. They are: acid and alkaline solutions of various composition and concentration; reactions of oxidation, reduction and base exchange; bacterial activity, influence of animal organisms and plants; processes of interaction between clay material and the products of decomposition of organic remains, etc. Therefore it is but reasonable to suppose that the physical-chemical conditions of various environments must alter the composition of clay minerals carried into the region of Sedimentation. This problem, however, cannot be definitely solved in every case, due to the lack of sufficiently sensitive methods of identifying fine distinctions of the structure and coniposition of minerals attributed to the same species. It is therefore important to make an attempt to penetrate into the microworld of clays and to try to reveal the peculiarities of their composition dependent on various physical-chemical conditions of their formation. The method of mineralogical analysis affords information principally about the species of clay minerals and their ratios in clays. Yet these data are not sufficient for an adequate facies analysis because the number of species of clay minerals is limited. While using the data about the distinctions of the species of clay minerals, it is not difficult to reveal in vertical sections, changes in the composition of clays, related to variations in the character of the sedimentation, which took place during comparatively long geological periods and were connected with changes in paleogeographic conditions in a given region. As a rule they are explained by changes in rock composition in the source area. However, within the limits of the deposits of a single stratigraphical horizon formed by the material of a single source area, the clays of different facies usually display a similar mineral composition and approximately similar ratios of rock-forming clay minerals. Theclay material of clays, carried into the area of sedimentation, seems to be unaffected by the conditions of accumulation. This observation misleads many investigators, who draw the conclusion that only the rock composition of the source area is responsible for the clay composition.

418

M. F. VIKULOVA

Intensive studies of the mineral composition of clays, important for many branches of geology. require an exact knowledge of the allochthonous and authigenous origin of clay minerals of clays, formed under different environmental conditions. We must also know the features revealing the origin of clays and tlie causes governing the transformation of the clay material during the different stages in the history of the rock. With that purpose in view, special investigations have been started concerning the composition of clays of various formations. This article deals with the first results of a study of clays of the Tula horizon of tlie Lower Carboniferous and its enclosing rocks (Dj. C,t nil ~-lip, C,v hh, Clv a / ) in the western part of the coal-bearing Moscow Basin froiii Ljubitino in the north up to the Safonov coal deposit in the south (Fig. 1).

Fig. I . General n u p of the investigated region.

The author collaborated with A . S. Korzenevskaja (lithology of clays of the southern part of the Moscow Basin), A . A. Borisova (minelalogical analysis of heavy and light fractions), J. S. Diakonov (X-ray analysis), B. H. Zvjagiii and V. A. Shitov (electron

ORIGIN A N D COMPOSITION O F CLAYS IN THE MOSCOW BASIN

419

microscope analysis), 0. F. Safonova. G. T. Volostnykh (description of the cores of some boreholes and shafts), A. K . Frolova and N . V. Kasakova (preparation of rocks for inineralogical analysis).

FACIES O F TULA DEPOSITS

The Tula horizon (Clv I / ) is the upper niember of the coal-bearing Visean Formation occurring on the rough erosional surface of underlying rocks (DJ. C,r) and of the pieVisean kaolin crust of weathering. The area of the western part of the hloscow Basin i n Visean times was a low swampy coastal plain, with abundant tropical vegetation, periodically submerged by a shallow epicontinental sea. Thc Tula horizon is characterized by a complex alternation of shallow marine and coastal continental deposits and consists of interbedded clays. aleurites, aleurolite4. sands, sandstones, coals and, occasionally, limestones. Alternations of niarine and continental deposits resulted in a cyclic structure of the Tula horizon, which can be clearly traced only i n the northern part of the investigated region. In the southern part niarine sedimentation prevailed at that time. Therefore it is only within the limits of the southern half of the western part of the Moscow Basin that we find in the Tula horizon clays typical of sliallow epicontineiital basins with a nornial salinity, i.e., calcareous clays characterized by calcareous skcletons of fauna containing siderite and pyrite. Northward they gradually disappear from the section and arc replaced by non-calcareous niarine clays. occasionally bearing scarce fauna remains of desalted niarine basins. In the southern part of the investigated region clays of the swampy and alluvial type are but rarely encountered i n the Tula horizon and white lacustrine kaolin clays are absent. These facies types of clay occur i n the north of the Borovich-Ljubitino region, where deposits of the Tula horizon are of a more continental character. In the structure of C,v t / of the Borovichy-Ljubitino region two cycles of sedinientation are distinguished. reflecting changes in the conditions of sedinientation connected with a repeated alternation of wider regressive and transgressive movements of the coast line. At the end of the accumulation of the coal-bearing series i n the Visean time the transgressive movements were inore intensive and resulted i n a vast subniergence of the coastal plain and i n the accumulation of marine carbonate sediments of the next C1v n/ stratigraphic horizon. Therefore the sediments Cli?tl of t h e second cycle i n the transgressive part are gradually replaced by the Alcxin sediments (liniestone u1 and its enclosing clays). At the base of the first cycle, i n the depressions of the pre-Viseaii relief of the Borovichy-Ljubitino region we occasionally find deposits of an earlier cycle, belonging to the Bobrikov horizon (C,v hh), widely di\tributed in the southern part of the investigated area. Among the C,v I / deposits, some investigators distinguish continental deposits (alluvial. talus. Ruvial. lacustrine, swampy, deltaic) and littoral-marine sediments from ba4ns of a normal salinity and

15 0 TABLE I MINERALOGICAL COMPOSITION OF FRACTIONS -~

__

~

~

~

~~

~~

-

-

C,v tl

I cycle

_

__ _

< 0,001 mm

_

~-

~

OF CLAYS (RATIO OF CLAY MINERALS I N

:$)

~

~

~

The crust of weathering (low horizons)

Kgj-go H,-,,, sometimes traces of H M

Lacustrine kaolin clays

K,,,, sometimes

C,v bb

+H K l o o ,sometimes

C,v f l I cycle

Clays with kaolin features of various facies replacing kaolin clays along the strike

C,v bb

Kaolin clay altered by marine water

C,v tl

~

Environment of sedinienfation or type of clay

Age of clays D,-C,

_

11 cycle

C,v tl I and I1 cycles C,v a1 C,v bb

Lacustrine-swampy facies Clays replacing lacustrine-swampy clays along the strike Sapropelic clays of various facies: (1) swamp

(2) sea bay, lagoon with fresh water

+H

TABLE I (continued) Age of clays

Environment of sedirnentation or type of clay

Borovichy-Ljubitino region

Southern part of rhe investigated area

~-

~

C,v 11 I cycle C,v bb C,v bb C,v 11 I1 cycle c , v I[ C,v al C,v a1

c,v tl C,t nil

Sea-bay, lagoon: with reduced salinity salinity somewhat lower than normal salinity near to normal

K ~ O - SHm-so O K,o Ha0 Krlo MMlO ( B D Y KBWSO

H4&50

K S Sffm K HM K 6 , H,,, traces M M K50 Hso HM Hloo;H traces K and M M

normal salinity

+ +

+ up K1c-50 HBc-60,sometimes traces HM

D3

K25-40

H’76-80

MMSO H40 KlO H100; HMlOO

4

Lagoon with increased salinity

H+PG;H+HM MM+PC+K

- ~- -~~ ~-~ ~

~

1

“BD”

=

~

~.

intergrowth of kaolinite and montmorillonite of “beidellite” type (Metodicheskoe rukovodstvo, 19571.

__--

422

M. F. VIKULOVA

from desalted basins (marine parts of deltas, lagoons and bays). Their conception is based on the complex of petrographic and other lithologic peculiarities of rocks, on the remains of fauna and flora, the mode of occurrence, the thickness and shape of bodies and on tlie character of the contact with other rocks. Clays are developed aniong all the types of deposits.

THE CLAYS STUDIED

Due to the rapid succession of different rocks in vertical and in horizontal directions, and also to their variable thickness, an accurate deterinination of the facies type of any bed of clay is iiot always possible, the more so, since many clays of a similar appearance and composition are encountered in various facies conditions. However, in different parts of the Clv // section there occur horizons of clay of more or less considerable thickness and extension which, according to the whole complex of their features, may be positively referred to as lacustrine, swampy and littoral marine deposits. The clays of these facies types have been studied in order to come to a clearer understanding of the character of the variations in their composition (clay minerals) in connection with the different conditions of their formation. The clays were studied by means of various methods (mainly using thin sections and X-ray and electron-microscope analyses) and systeiiiatically i n tlie section from the D, up to tlie C,v a/ deposits i n many boreholes in tlie Borovicliy-Ljubitiiio region and also along the strike of some individual clay horizons of the Tula deposits eastward, in the direction where the conditions of sedimentation were of a niore pronounced niarine character. I n the southern part of the region swampy clays (sapropelic) of the Clv ti aiid C,Vu / were studied as well as clays of lagoons aiid marine bays of various degrees of water salinity of the C,r' ti horizon. kaolin clays of the Bobrick horizon (C1v hh) and clays up, C,r ti and Clip a/. from marine basins of normal salinity of tlie C,t mi I n the northern part of the Borovichy-Ljubitino region we studied niarine lagoon clays of D,. clays of the pre-Visean crust of weathering (Da - C,). lacustrine kaolin and seiiii-kaolin clays. lacustrine-swampy plastic clays (sapropelic included) of the Clv tl. littoral-niarine clays of saline and desalted basins and non-calcareous and calcareous clays o f tlie C,I' t / and C,v a/. The following clay minerals are established in the fractions < 0.001 n i n i : Kaolinite ( K ) . hydromica ( H ) , montniorillonite ( M M ) . palygorskite ( P C ) and a mixed-layer mineral: hydromica-moiitiiiorilloiiite ( H M ) . The composition of the investigated clays is shown iii Table I . In most cases the clays of different continental and niarine facies of the C1v /I i n the western part of the Moscow Basin display a siniilar mixed composition of fractions < 0.00 I nun. The rockforming niinerals of these clays are reprssented by kaoliiiite ( K ) and dioctahedral hydromica ( H ) of the I M type, both being of imperfect structure. The quantitative content of each mineral i n different clays varies in most cases

+

ORIGIN AND COMPOSITION OF CLAYS IN THE MOSCOW BASIN

423

from 40-60 (although some deviations are occasionally observed towards the prevalence of one of the minerals). Data about the clay composition of C,v t l coincide with those about the miiieralogical analysis of heavy and light fractions of rocks > 0.01 mm in size made by A. A. Borisova, and BAZHANOWA (1939). They state that during Tula tiiiies, the erosion in the source area of the western part of the Moscow Basin was mainly limited to the sedimentary rocks of the Upper Devonian and to tlie ancient kaolin crust of weathering. According to data in the literature and our personal investigations, the D:,rocks are strongly micaceous, hydromica with an admixture of kaolinite prevailing in their clay fractions. Occasionally an admixture of montmorillonite. a mixed-layer clay mineral and palygorskite may be found there. Thus, the results of mineralogical studies of fractions < 0.001 mm of the C,v t l clays, on the whole. are in conformity with the conception of the initial terrigenous origin of their clay material and the inherent character of their composition. The presence of a larger amount of kaolinite in the C,v t1 clays, as compared to the amount of hydromica in the Devonian clays, may be explained by mixture, during transportation, of the material derived by erosion from the kaolin crust of weathering and the Devonian rocks.

FORMATION OF KAOLIN CLAYS

Though kaolinite-hydromica clays prevail in the C,v t l deposits, clays of another composition also occur there: pure kaoliiiite clays (horizons of lacustrine kaolin and semikaolin clays of the first cycle and of the lacustrine-swampy plastic clays of the second cycle), kaoliiiite and kaolinite-liydrornica clays of different facies with an admixture of a mixed-layer clay mineral ( H M ) . of montmorillonite ( M M ) , or both of them. There is not enough evidence to assume that during tlie formation of kaolinite clays only the kaolin crust of weathering was being washed out, though it is possible, that i n connection with sea rzgressions in the source area vast areas of the crust of weathering could crop out, thus increasing the supply of kaolinite to the area of sedimentation. Along the strike, the kaolinite clays of both horizons (kaolin and kaolinite plastic clays), without intervals in sedimentation, are replaced at short distances by hydromica-kaolinite clays, whereas in the regions of a lower ancient relief near the coast line they are replaced by kaolinite and kaolinite-hydroniica clays with an admixture of a mixed-layer mineral ( N M ) or of montmorillonite. In the latter case K and M M sonietinies form intergrowths of tlie "beidellite" type. described from the marine clays of the Mesozoic and Paleogene age of West Siberia(METOD1CHESKOE RUKOVODSTVO, 1957). It must therefore be supposed that the formation of kaolinite clays i n tlie Clv It deposits was. to a greater degree, due to the specific conditions of their forniation and to tlie subsequent transformation of their composition, than to a possible supply of great masses of kaolinite during the regressive periods of sedimentation cycles.

424

M. F. VIKULOVA

The following factors seem to have been of essential importance for the formation of kaolinite clays. (I) The processes of soil-genesis which took place simultaneously with the process of sedimentation and led to kaolin formation under the conditions of humid tropical climate during the longer periods of regression of the sea. (2) The transformation of the supplied clay material under the influence of the life activity and the products of decay of great masses of plants. It should be taken into account, that the mineral part of plants of the Carboniferous Period, being of kaolinite composition, as a result of the decay of plants served as an additional source of sols of A1,0, and SiO, for stream water and soils. They were involved in the general circulation of the clay suspension components which participated in the formation of clays. Traces of processes of soil formation are encountered in all types of clays of continental facies and in littoral zones with plant growth. They are evident from rootshaped remains of plants (Stigmariae with rhizoids or only rhizoids) in situ, the layers of ancient soils and a lumpy and porous texture, especially of kaolin clays. Thin sections of clays display traces of migration of finely dispersed clay substance along fissures and remains of plant roots and also the appearance of newly formed clay minerals of collomorphic shape with a perfect optical orientation of particles. Such kinds of clay are characterized by various types of collomorphic structures (VIKULOVA and STRUSTEROVA, 1940) observed in modern soils as well. The processes of soil-genesis led to the formation of kaolin clays representing a peculiar type of weathering complex. Its development took place in a humid tropic climate, on clay and sand-clay sediments of the more elevated parts of the coastal plain during the period of prolonged regressions of the sea. These clays are present in the first cycle of the C,v t l sedimentation (Borovichy-Ljubitino region) and in C,v bh (the southern part of the investigated area). The processes of diagenesis favoured the recrystallization of soil colloids and the cementation of kaolin clays. At the same time there appeared crystals of kaolinite in the form of porphyroblasts or of vermiculite-like shape denoting their authigenous origin. This is also evident from the perfect or almost perfect structure of the kaolinite of kaolin-clays. The processes of changes in the kaolin-clays, in connection with the appearance of kaolinite crystals in the pores and fissures of rocks, continued during the epigenesis and are still going on, due to the transportation of kaolinite suspensions by ground waters. Some layers of kaolin clays occurring among sands contain a number of generations of authigenous kaolinite. The earlier neo-formations are of a more perfect structure. The formation of kaolin clays (white and light gray) took place in oxidizing environment, which is shown, among other things, by their colour, by the poor preservation. of the organic substance of plant remains as well as by the composition of the organic substance of the clays. According to Uspensky’s data (VIKULOVA and STRUSTEROVA, 1940) only small amounts of organic substance (0. 09416% of rock) are present in kaolin clays. Its main mass consists of organic carbon, though it comprises also “bitumens” which may be extracted by means of chloroform and small amounts of humic acids.

ORIGIN A N D COMPOSITION OF CLAYS I N THE MOSCOW BASIN

42 5

Pyrite appeared in kaolin clays in reducing environment during the stage of diagenesis as a result of the supply of sulfide waters from peat bogs and bogs which once covered the kaolin-clays. The relics of these bogs are preserved in the form of Carbonaceous rocks i n the roof of these clays. In semi-kaolin clays of the Ljubitino region, siderite, belonging to diagenetic minerals, is of wide distribution. Occasionally we find in these clays lenses of marls containing remains of Stigmariae with rhizoids in situ, testifying to a temporary penetration of sea waters in the basin where clays had been formed. In lacustrine-swampy basins with semi-stagnant waters saturated by humic acids, under reducing conditions, another type of kaolinite clay found its development (gray, dark gray, black, plastic, fat, sometimes bearing the features of kaolin clay). The organic substance of these clays amounts to 5.5 % ofthe rock and consists mainly of humic acids and organic carbon. It also contains a small amount of “bitumens” extracted by means of chloroform. Various plant remains (roots, crushed detritus, cellular tissue, etc.) are preserved in these clays. Thin sections occasionally reveal traces of the decay of plant remains and of spreadings of organic substances in the clay mass. The kaolinite of these clays may be of a perfect and imperfect structure. Mica, hydromica and quartz grains carried into swampy basins dissolved under the action of humic acids. In plastic clays, bearing kaolin clay features, one may sometimes observe new formations of clay minerals of collomorphic shape which replaced plant remains, but there are no typical collomorphic structures, which have developed in kaolin-clays.

CLAYS OF LITTORAL-MARINE BASINS A N D OF SUBMARINE PARTS OF DELTAS

Clays of littoral-marine basins C,v tl, C,v al and C,v 66 are subdivided into two groups. The first group is characterized by a random distribution of rock-forming elements (clay material, aleurite, sand, plant detritus). Often large amounts of mica are present with flakes of a large size (over 0.1 mm in diameter) distributed in various directions. Sometimes traces of plant remains in situ are present, as well as collomorphic new formations of clay substance denoting periodical regressions of the coast line. The second group is represented by clays with a regular laminar arrangement of the same rock forming elements imparting to these clays specific platy and slaty features. Besides, one occasionally finds some remains of fauna and microfauna, and grains and aggregates of calcite and siderite. There are neither root-like remains nor collomorphic new formations of clay minerals in marine clays. Clays of the submarine part of deltas are referred to the first group, whereas clays of calm bays and lagoons of various salinity belong to the second group. In all types of the C,v tl littoral-marine clays the fraction < 0.001 mm consists mainly of K and H, displaying approximately the same ratio as in the case of continental basins. In some clays, however, the presence of a number of K and M M intergrowths

426

M. F. V I K U L O V A

of the "BD" type has been established, as well as a small amount of mixed-layer mineral ( H M ) arid of montmorillonite. especially in basins with a normal salinity of the water. Hydromica clays were found only in the C,t mi 4 up and D, deposits. The origin of clay mineral-admixtures is not as yet sufficiently clear. It could be supposed that the main clay minerals K and H and the admixtures H M , M M and "BD" are of terrigenous origin and appeared as a result of the erosion of local Devonian clays which once contained these minerals (Table 1). But then one would expect a nwch wider distribution of these minerals in all types of the C,v t l clays, as well as i n the types C,v a/ and C,v 66, than is actually found. These clay minerals are also present in clays replacing, along the strike (without any interruption in deposition), kaolinite lacustrine-kaolin clays and lacustrine-swampy plastic clays in those parts of the coastal plain which were once submerged by the sea. These clays also display a platy structure characteristic of marine clays. The following supposition is therefore perhaps more plausible. The formation of H M , M M and "BD" in the C,v ti sediments was taking place under the influence of marine saline waters. The alkaline environment and the composition of the marine saline waters called forth: (I) A partial destruction of the K crystallites and the formation of K and M M intergrowths observed, as mentioned above. in marine clays of Pg and M z of West Siberia. (2) Tlie replacement by niontmorillonite of separate packets in the structure of hydromica. Tlie inechanisni of these transformations is not as yet known.

STRUCTURAL PECULIARITIES OF CLAY MINERALS

In this branch of investigation tlie study of fine structural distinctions of clay niinerals of one and the same species and of the character of their intrastructural changes occurring under various conditions of clay forniation, seems to be promising. As result of electron-microscope studies of the C1v tI clays, B.B. Zviagin and V. A. Shitov managed to establish that kaolinites of these clays differ in the degree of perfection of their structure (perfect structure with an exact c-period, intermediate with a not quite exact c-period and imperfect with an inexact c-period) and also in the shape of tlie unit cell (triclinic with a 92", monoclinic with a = 90" and tricliiiic-iiionoclinic with variable a changing in the interval from 90-92"). These distinctions are due to a dilferent constitution and distribution of laycrs in kaolinite structure. According to Zviagin the peculiarities of the formation of structures of two-sheet kaolinite layers are such that they should forni a unit cell structure with a = n/2. if the layers were without distortions. Kaolinite proper (contrary e.g.. to dickite and nacrite) is composed of layers in such a way that their reflection planes are not parallel to tlie a and 6 axes; therefore in the case of perfect order (perfect structure) it displays triclinic symmetry and, therefore, does not require a = n/3. In most cases, due to concrete distortions of layers, the unit cell of kaolinite is triclinic. a being 9 l"4 I Sometimes a = 90", the unit cell is then I .

ORIGIN AN11 COMPOSITION OF CLAYS I N THE MOSCOW BASIN

427

monoclinic. One finds also intermediate cases where u is not constant for the various unit cells of the structure. This is seen from the "smearing" of reffections wich should split due to the fact that u # n/2. According to experimental observations. we have then to do with an average unit cell which may be conditionally called monoclinictriclinic. In all of the three cases the c-period seems to be exact and we observe reflections in which k # 3k. The kaolinite structure is triclinic independent of the shape of the unit cells, as it is deprived of all elements of symmetry except translation. A similar subdivision is applied to kaolinites with a structure displaying incomplete regularity in the disposition of layers, with a rather inexact or obviously inexact r-period (intermediate or imperfect structures). It should be noted that as a matter of fact, the unit cells here are but pseudo-unit cells. It is only necessary to take into account that if an exact c-period is absent, and if we do not observe in the diffraction picture reflections in which k # 3k, a structure may occur with various alternating layers displaying different directions of the planes of symmetry (in this case the unit cell must be monoclinic). On the average (according t o statistics) the structure possesses planes of symmetry and the corresponding symmetry is pseudo-monoclinic (so are supposed to be species of the "fireclay" type). As seen from our investigations, kaolinites of a perfect structure were formed either in the crust of weathering or in those continental facies in which processes of authigenous formation of kaolinites could take place during sedirnentogenesis, or diagenesis of sediments, or epigenesis of rock, or during all of the stages of their development, i.e., under conditions favourable for the formation of kaolinite crystals. Tn the investigated region kaolinite of a perFect structure and with a triclinic u n i t cell has been established in the kaolin crust of weathering, in fresh-water lacustrine kaolin-clays (the material of which was strongly affected by soil forniing processes). in diagenetic new formations of kaolinite, in kaolin clays and i n carbonaceous rocks. Kaolinite of a perfect structure with a triclinic~moiiocliiiicunit cell has been encountered i n kaolin and kaolinite plastic clays which were formed i n the parts of the continent adjacent to the coast line. Probably the different composition of the water in lacustrine basins played an important role here. Kaolinite of a structure of intermediate perfection with a triclinic unit cell has been found i n young-epigenetic new formations of kaolinite and i n kaolin-clays with clay material incompletely treated by soil-processes. Kaolinite of the same structure with various shapes of unit cells has been found in hydromica-kaolinite clays from basins with an inflow of saline waters. Kaolinite of imperfect structure with a u n i t cell of various shape was carried into the given region from the source area. Tt is characteristic of the majority of kaolinitehydromica clays of the Clv f/ continental and marine environments, where local treatment of the imported material did not take place. Probably a collision of kaolinite particles wlth one another and with particles of other minerals i n the course of transportation, as well as the influence of the composition of running waters cauLe changes in the structure of kaolinite. The peculiarities of the structure of kaolinite have as yet only been studied on a

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limited number of samples. A further systematical study is therefore indispensable in order to be able to define more accurately the significance of differences in facies conditions in the structure of kaolinite. I n this article we consider it important to have pointed out that these differences exist and that they are dependent on the conditions of the formation of clays. Species of hydromica and of other clay minerals in the C,v tl clays have not as yet been established.

SUMMARY

The clay substance of the C,v tl clays is partly allothigenous and partly authigenous. The authigenous formation of clay minerals is the result of transformation processes acting upon the imported material during the stage of sedimentation and of the prediagenetic soil-weathering, as well as of diagenesis and epigenesis, including weathering at depths. The character of the secondary processes is determined by the conditions of formation of the clays. According to the present appearance of clays we are not as yet able to define those mineralogical peculiarities which reflect the environment of their formation. Our data concerning the mineralogical distinctions of clays of a series of facies types from the Lower Carboniferous deposits in the Moscow Basin, together with data about other lithological peculiarities of clays and of the enclosing rocks, afford the possibility of a better understanding of their genesis. For a successful solution of problems connected with the facies analysis of clays, a further investigation of the fine peculiarities of clay minerals structure is of the utmost importance.

REFERENCES

BAZHANOWA, M. M., 1939. Results of the petrographical investigation of sandday rocks in the Selizharovo region. Sb. Lenityr. Geol. U p . , 3. VIKULOVA, M. F. and STRUSTEROVA, M. S., 1940. Composition and morphology of deposits of refractory clays of the Lower Carboniferous Period in the Borovichy-Ljubitino region. Litolo~ i c h e s k ySb., 1. METODICHESKOE RUKOVODSTVO, 1951. (Methodical Textbook on Petrographical-Mineralogical Study of Clays.) Gosgeoltechisdat, Moscow.

PRESENT-DAY PRECIPITATION 0F CALCIUM CARBONATE IN THE PERSIAN GULF A. J. WELLS

and

L . V. I L L I N G

KoninklijkelShell Exploratie en Prodirktie Laboratorbm, Ripwijk (The Netherlands) ; Illing and Partners, Cuddington Croft, Cheani, Surrey (Great Britain)

INTRODUCTION

Large-scale instantaneous precipitation of calcium carbonate in the open waters of the Persian Gulf was first recognised by the authors in early 1961. The phenomenon takes the form of isolated patches of milky water, known as “whitings”, which have since been observed to occur with almost monotonous regularity throughout the year, and appear to be most frequent on the east side of the Qatar Peninsula. Whitings have been seen in many places south of a line joining the northern tip of the Qatar Peninsula to Abu Dhabi on the Trucial Coast, and also off the coast of Saudi Arabia, between Ras Tanura and Ras Safaniya [see sketch map, Fig.1). The phenomenon is therefore known to occur over an area of at least several tens of thousands of square kilometres. The term “whiting” comes from the Bahamas, where similar patches of milky water were observed by the British nautical survey as long ago as 1836. CLOUD(1962, pp. 19-22) has recently published the results of his valuable observations on the whitings of the Great Bahama Bank.

WHITINGS IN THE PERSIAN GULr

In the Persian Gulf, the whitings are commonly in the order of 1 km across, with an irregular shape; larger whitings, as much as 10 km across. have been observed. They may grow from nothing to an area hundreds of metres wide in only a few minutes (Fig.2 and 3). They persist for several hours and may drift for up to a kilometre or two with tidal currents while the minute crystals of which they are composed flocculate and settle out. Visibility in the whiting water is commonly less than half a metre, and may be reduced to only a few centimetres. The average whiting produces about one gram of precipitate for every hundred Iitres of water involved. X-ray analysis shows that the precipitate consists of microcrystalline aragonite, but in addition small quantities of carbonate from pelagic organisms will inevitably be included in any sample. This phenomenon is producing vast quantities of non-skeletal aragonite mud every day over a wide area of the shelf on the south side of the Persian Gulf, and is clearly a process of major geological importance.

430

A. J . WELLS A N D L. V. ILLING

'1"

48OE

I

56OE

I

0

300km S c a l e 1'6,000,000

Fig.1. Sketch map of tile Persian Gulf. showing areas in which whitings liiive been observed. The arcs east of Abu Dh:ibi h:Ls not yet been visited.

R E C E N T C A R B O N A T E S E D I M E N T A T I O N IN THE PERSIAN GULF

Fig.2. Beginning of a whitin:.

43 I

viewed from 50 rn abo\,e sea level. G r o w t h from this sire to I :: kni or

iii3re across may take only a fcw minutes.

It could be argued that these whitings may merely bc suspensions of previously deposited lime mud which have been stirred up by fish. a theory which would refute the sediment-producing role and therefore the significance of whitings. It is true that schools of bottom-feeding fish d o occasionally stir up sediment into suspension. Field evidence, however, leaves no doubt that actual precipitation is also occurring, and some of the more significant observations which lead to this conclusion are outlined below. (I) Formation of a whiting is an almost instantaneous process over an arcs which may sometimes cover many square kilometres. Shoals of fish will not normally S L ~ d a i l y start bottom-feeding over such a wide area; observed patches of turbid water associated with exceptionally hii$ populations of fish of all shapes amd sizcs were roughly circular and up to several hundred nictres across. In contrast, thc precipitated whitings were almost devoid of fish. ( 2 ) Whitings always for111a t t h e surface. Clear wat21- has been observed between the whiting and the sea bottoni shortly after its formation. Fish will stir LIPsedirnent

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A. J . WELLS AND L. V . ILLING

Fig.3. A whiting with characteristic irregular but sharply defined and digitate front, formed of aragonite needles precipitated near the surface.

from the bottom first, so that the relationship between clear and opaque water is reversed. ( 3 ) Formation of whitings is largely independent of water depth. They tend however, to be smaller and very frequent in water less than 10 rn deep, larger and less frequent in water up to about 30 m deep (the maximum depth in this part of the Gulf). (4) The minute crystals of aragonite in whitings rapidly flocculate into aggregates up to a millimetre or more across, and sink to the bottom within the day that they are formed. Mud stirred up from the sea bed apparently does not flocculate like this, and . remains in suspension for several days. (5) Samples of muddy sediment from the sea floor in areas of frequent whitings always contain a significant percentage of calcite [sometimes as much as 50 %) derived mainly from bottom-living organisms. Our analyses show that precipitate from whitings contains very little calcite. (6) Analyses of pairs of filtered water samples collected at the same location, just before and just after a whiting formed there, show that this process causes a very slight drop in calcium content (and in the Ca/Mg ratio), which suggests that calcium

RECENT CARBONATE SEDIMENTATION IN THE PERSIAN GULF

433

had been removed from the water. But the accuracy of analysis is not yet such that proof can be considered conclusive. Fish activity would have no effect on the ratio. Accurate horizontal and vertical profiles of temperature, salinity and pH have been measuredl through a number of whitings. These measurements have confirmed that water stratification is stable, with salinity increasing and temperature decreasing slightly but more or less uniformly with depth. These minor changes are, however. quite independent of the whitings, as similar gradients and similar values ai equal depth were recorded within whitings and in the adjacent clear water. The whitings form almost as commonly in winter when the w:er temperature falls to 19°C as tney do in summer at a temperature of 34 C. Changes of p H associated with the formation of whitings, if they occur, are too small to be detectable, but this is evidently due to the natural buffering effect in sea water. Household ammonia, poured into the open sea on several occasions in attempts to cause a whiting to develop in areas where they were known to be common, momentarily raised the pH of the water sufficiently to produce a local cloudiness, but the precipitate went back into solution within 5 seconds because of the buffer effect. It therefore does not seem likely that whitings are triggered off by actual change in pH of the sea water. Salinity, temperature and pH changes are evidently not critical factors which determine why precipitation should occur in some places and not in others. Following up another line of reasoning, however, plankton droguing was recently carried out under controlled conditions both inside whitings and in the adjacent clear water. A quantitative analysis of the first four samples has revealed that those from within whitings contained between five and ten times as many siliceous diatoms as those from outside. Spectrochemical analysis of trace elements in a sample of precipitate from a whiting confirmed an abnormal concentration of silica. This difference i n order of magnitude is thought to be sufficient to explain the trigger effect with which whiting formation is initiated. Field measurements with the carbonate saturometer (WEYL,1961) indicate that sea water on the shallow shelf forming the southern part of the Persian Gulf is generally saturated with respect to aragonite. Planktonic organisms living in the surface layers consume carbon dioxide by photosynthesis, but at a rate which is normally balanced by replenishment from vaiious sources. At times, however, it appears that abnormally high populations (“blooms”) of diatoms quickly develop in certain areas; the full reproductive cycle of cell-division involved in such a diatomic explosion is known to take no more than a few hours, and in favourable circumstances may be completed in only a matter of minutes. When this happens, carbon dioxide consumption from the water will sharply increase, and the tendency will be for the pH of the water to rise. Sea water is so well buffered against pH changes, however, that this does not happen in the open sea, and eventually the critical level is reached at which equilibrium can Using, among other instruments, a sensitive Temperature-ConductivityBridge designed and kindly lomed to us by the National Institute of Oceanography,England.

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A. J. WELLS A N D L. V. ILLING

only be restored by precipitation of calcium carbonate out of the already saturated water. Whether the diatoms are the only agents in this trigger mechanism is not known; it may be that other still smaller and less easily recognised organisms are important. In either case, however, we are led to the conclusion that carbonate precipitation in the Persian Gulf is apparently initiated by carbon dioxide consumption by phytoplanktonic organisms. Field observations show that in the case of aragonite derived from whitings, rapid flocculation causes deposition of most of the precipitate as lime mud within a kilometre or two of the area of its formation. This means, of course, that the distribution of this type of lime mud will be highly dependent upon its mode of origin. Our observations suggest that spasmodic precipitation in the surface layers of a sea already saturated with aragonite can be a dominant source of lime mud over a broad shallow marine shelf. It has recently been shown by CLOUD(1962, pp.21-22) that at least some of the whitings on the Bahama Banks west of Andros Island appear also to be initiated on local sites of accelerated phytoplanktonic photosynthesis. Cloud calculates that approximately three-quarters of the total sediment in the area is probably derived from chemical precipitation. This process of calcium carbonate Precipitation from sea water, important as it is in modern carbonate depositional provinces, is presumably one which was also active during geological time. It would therefore have played its part in the distribution of lime mud throughout the geological column.

SUMMARY

Instantaneous precipitation of calcium carbonate from open sea water is taking place at the surface over tens of thousands of square kilometres in the shallow southern part of the Persian Gulf, producing isolated irregular opaque milky-coloured patches of water known as “whitings”. These are commonly about a kilometre across, and occur throughout the year. The average whiting produces about one gram of microcrystalline aragonite for each hundred litres of water involved. The precipitate flocculates and settles out on the sea bed within a few hours of its formation. Vast quantities of nonskeletal aragonite mud are being produced every day by this process. Whiting formation is not accompanied by salinity or temperature variations, or by detectable changes in pH. First results of plankton analysis, however, show a five- to ten-fold increase in the siliceous diatom population in whitings. This probably provides the mechanism needed for triggering precipitation. A sudden increase in numbers of phytoplankton will sharply increase consumption of carbon dioxide from the water. This will tend to raise the pH of the water, but natural buffering prevents it. The water is generally saturated with respect to aragonite, so that the critical level is soon reached at which equilibrium can only be restored by precipitation.

RECENT CARBONATE SEDIMENTATION IN THE PERSIAN GULF

43 5

REFERENCES

CLOUD, P. E., 1962. Environment of calcium carbonate deposition west of Andros Island, Bahamas. U.S. Geol. Surv., Profess. Papers, 350 : 138 pp. WEYL,P. K., 1961. Thecarbonate saturometer.J . Geol., 69 ( 1 ) : 3244.

DELTA SEDIMENTATION IN THE GERMAN KEUPER BASIN P A U L WURSTER

Geologisches Staatsinstitut, Hamburg (Germany)

INTRODUCTION

The Schilfsandstein, which covers, as a very thin veil, an extremely large area, consists of fine-grained sandstone, silt and clay. Thickness and facies of the formation change continually over small distances, both in lateral and longitudinal directions. Yet, sandstone specimens from very different regions are very much alike; they are soft, spotted and of subdued colours. The present paper deals with the areal distribution of facies and thickness, as well as with the manner of origin of the deposits. The investigations were sponsored by the Deutsche Forschungsgemeinschaft.

ECOLOGY

Animal remains are rare. Intraformational breccias in the basal part of the formation contain at some localities residual bones of vertebrates, especially teeth of elasmobranchs (Palaeobates,Polyacrodus, and others; SEILACHER, 1943), teeth, skull bones or skeletons of amphibians (Mastodonsaurus, Capitosaurus, Cyclotosaurus, Metoposaurus and others; SCHMIDT,1928, 1938) and reptiles (Phytosaurus, Dyoplex, Nothosaurus and others; SCHMIDT, 1928, 1938). External moulds of entomostracs (Isaura; B I B L E , 1962) are found in intercalated clay lenses. Moulds of pelecypods are very rare. Trace fossils of many different kinds of annelids, entomostracs and limulids occur here and there on the bedding planes (LINCK, 1949). Elasmobranchs, limulids and annelids indicate brackish conditions, perhaps resulting from some kind of connection with the open sea. Isaura is typical of fresh water conditions. Amphibians prove that dry land was adjacent. Plant remains are abundant. This led to the name Schilfsandstein (Schilf = reed), although the flora was composed of Filicales, Equisetales, Cycadophyta and some other Gymnospermae and Coniferae (SCHMIDT, 1928, 1938). Beds with rhizomes are encountered at some places in the upper part of the formation. The drifted plants are badly preserved. At some places they form thin coal seams at the base or in basal parts of the sandstone. The character of the plant remains points to a landscape of plains and shallow streams. Extremely terrestrial or marine environments are improbable.

DELTA SEDIMENTATION IN THE GERMAN KEUPER BASIN

437

GRAIN ASSOCIATION

Thin sections of representative sandstone or siltstone specimens from different regions show angular grains of quartz, remarkable percentages of feldspars with authigenic borders, decomposed and altered mica, ferrosilicates and authigenic minerals. Fig. I shows the grain size distribution of the samples. In a vertical section the mean grain size increases at the base, remains constant in the middle part and diminishes somewhat towards the top layers (Fig. la). Corresponding cumulative curves from even very

.. .......

...~. .. .. . ..

Fig. 1 . Grain-size distribution of Schilfsandstein (Keuper, Triassic, Germany). a. Variation of median grain size (mm) in vertical section (scale in m) (locality see Fig.8.7). b. Cumulative curves of 20 samples from different localities (Fig.8.S) with median diameters (mm). c. Mean cumulative curve with median diameter Mof all samples in (b). d. Dispersion of the separate median diameters m in relation to the mean M .

distant localities differ scarcely in grain size and sorting coefficients (Fig. 1 b-d). The wide distribution of well sorted, but scarcely abraded, always fine-grained sands in a solitary thin veil make the question of their provenance and conditions of sedimentation all the more interesting. In the present investigations this question was approached by studying the sedimentary structures.

BEDDING STRUCTURES

The predominant structures in the sandy and silty rocks are cross-bedding, current ripple lamination and irregular massive layering. The cross-bedded units consist of

438

P. W R S T E R

series of lenticular bodies (width 2-10 m, length 6-30 m, thickness 0.1-1.0 m) with concave, spoon-shaped laminae (WURSTER,1958a). All bodies of each vertical section are arranged in a constant direction (Fig.2). This was shown by measuring the dip directions of all accessable laminae in some hundred outcrops. The maximum angle of dispersion of the dip directions of each outcrop exclusively corresponds to the curved shapes of the different, strictly parallel orientated foresets. The bisection line of the angle of dispersion therefore determines almost exactly the local current direction. The outlines of the mean shape variation served as a contour mask for the graphic

Fig.2. Cross-bedding (or current ripple lamination) of Schilfsandstein. Stream bed: parallel migrating sand bars are piled up to a cross-bedded complex. Longitudinal section with diagonal traces of foreset laminae. Cross section with curved traces of laminae. Horizontal section with traces of laminae and the outlines of cross-bedding (or current ripple lamination) bodies.

determination of the current direction (Fig.3, 4). The tight order of the cross-bedding bodies points to a very uniform process of deposition: unidirectional current systems of great streams, inducing braided, parallel orientated fields of migrating sand bars. The sand bodies corresponding to the migrating bars were piled up to sedimentary complexes at the front of inclined planes (Fig.2, 3). The lenticular bodies of current ripple lamination (WURSTER,1958b; “Schragschichtungs-Bogen”, GURICH,1933, HANTZSCHEL, 1935; “rib-and-furrow structures”, STOKES, 1953; “micro-cross-lamination”, HAMBLIN, 1961) are small (width 5 cm, length 20 cm, thickness 1 cm), about a hundred times smaller than the bodies composing the units of cross-bedding, but in structure and pattern they are of the same type (Fig.2). The current ripple lamination builds up layers of 0.1-1 .O m thickness. Their upper and lower surfaces are smooth planes. They are often separated from each other by thin layers of siltstone or claystone (Fig.3). Their internal structures become more clearly visible by cleaving or grinding the sediment (Fig.2). In this way the structural bisection line can be measured directly; it represents the local current direction. No essential changes are found, when the directions are measured for the successive layers

DELTA SEDIMENTATION IN THE GERMAN KEUPER BASIN

439

1 .-5m

B Fig.3. Graphic determination of the current direction. Evaluation of cross-bedding measurements: c = Circle of 40"inclination in a stereographic net, lower hemisphere. d = Projection point of a directly measured lamina. e = Contour mask of the shape outline of the curved laminae. f = Suggested current direction. Evaluation of current ripple lamination measurements: g = Diagram circle. h = Directly measured current direction (see Fig.2). i Geometrical mean of current directions. Constant current direction in vertical sections (locality see Fig. 8.6). A. Current ripple lamination (b). B. Cross-bedding (a) and current ripple lamination (b).

in vertical sections (Fig.3, 4). Cross-bedding and current ripple lamination are produced by the same, unidirectional currents (Fig.3B, 4), the different scale being due to differences in velocity. Irregular massive layers have a thickness of 0.5-2.0 m. They contain intraformational breccias with sand-, silt- or clay components. In these deposits the vegetable remains received no special orientation. The origin of these layers is still unknown; perhaps they are formed by crevassing of levees. Mud cracks and other common sedimentary structures are rare or absent.

FACIES PATTERN

Thickness and facies change within short distances. The amplitude of thickness varia-

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P. WURSTER

Fig.4. Determination of current directions in Middle Franconia (locality see Fig.8.8). Application of the graphic determination (see Fig.3). Each singular outcrop has a constant and real current direction. The directions alter within short distances.

tion reaches 30-40 m, the corresponding variations of horizontal dimensions amount to some kilometres. Two main facies types were distinguished by THURACH(1900), one mainly sandy, the other predominantly clayey and silty, with few sand layers. The latter he called “Normalfazies”. Locally, some of its layers contain precipitates such as dolomite or gypsum. There are no indications of current action. This facies lies on a complete sedimentary complex of Estheria Beds (Estherienschicliten, the upper part of the Gipskeuper, km,). The lateral change of the “Normalfazies” into the other, or “Flutfazies” proceeds by a quick thickening and merging together of the sandstone intercalations, whereby some of the top layers of the underlying Estheria Beds may

DELTA SEDIMENTATION IN THE GERMAN KEUPER BASIN

441

vanish. Thiirach suggested, that the latter had been eroded by scour of channels. These channels were filled with the sand of the “Flutfazies”. However, according to observations of REIFF(1938) and of the present author, erosion of the underlying beds took place rather exceptionally. REIFF(1938) believed, that a preexistent relief of lakes or lagoons was filled up by sand deposition. The facies pattern of the Keuper escarpment near Beilstein in Wiirttemberg has been investigated by special mapping. It contains 30-40 m thick shoestring sandstone bodies (“Flutfazies”). The measured current directions coincide strictly with the longitudinal sections of the sandstone cords. They apparently represent meandering and anastornosing stream distributaries. The cords are separated from each other by thin sheets of quiet-water deposits (“Normalfazies”). Cross sections of the shoestring sand bodies show them to be of biconvex shape (Fig.5). The base of the underlying

Fig.5. Idealized sections of the facies pattern. A. Mississippi delta(Ffi~,1961,fig.9, p.49): a = Pro-deltasediments with abundant fauna. b = Delta-

front sediments with sparse fauna and mudlumps. c = Bar-finger sands (width 5-10 km, thickness 70 rn). d = Delta-plain sediments. e = Distributary stream with natural levees. f = Marshes. B. Schilfsandstein in Wiirttemberg (South Germany): a’ = Gray Estheria Beds (Graue Estherienschichten, km,); a = “Anatina-Bank”, a dolomite layer with brackish fauna. b’ = Upper coloured Estheria beds (Obere Bunte Estherienschichten, km,), barren. c’ = Sandstone cord (“Flutfazies”) of Schilfsandstein (km,), with cross-bedding in the core, vertebrate fossils, trace fossils,and plant remains (width 1-3 km,thickness 30 m). d’ = Quiet-water deposits (“Normalfazies”) of Schilfsandstein (km,), barren, occasionally with gypsum and dolomite precipitates. el = Stream-bed filling (top layers of Schilfsandstein, km,). f’ = Dark Marl (Dunkle Mergel, krn,), occasionally with fauna.

Estheria Beds curves also downwards below the base of the sandstone cords. Certainly there was not much erosion here. The thinning of the Estheria Beds below the sandstone cords may even be largely or entirely caused by pressing away of the still muddy material under the weight of the newly supplied sand. On the other hand it may be remarked, that in some of the investigated sections the “Anatina Bank”, a dolomite layer of the Estheria Beds, containing a brackish or marine fauna(Dr. 0. Linck, personal communication, 1960) is present only below the sandstone bodies. No explanation of this situation can be offered as yet, but it obviously is the result of primary facies relations (Fig.5). The fossils of the sandstone point to a transitional environment between land and sea. The bedding features prove that sand was transported by very constant currents, of steady streams. Moreover the facies pattern of sheets of quiet-water deposits with a framework of sand cords, and the simultaneity of deposition and settling present many analogies to the modern sedimentation of the Mississippi delta (FISK,1961)

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P. WURSTER

(Fig.5). The major distributaries of this bird-foot delta which have a very constant position, lie on the crests of elongate, sandy “bar fingers” (width 5-10 km, thickness 70 m). Deposition and load settling of the bar sand masses into the delta-front muds take place simultaneously in front of the distributary mouths. The delta fingers grow

Fig.6. Paleogeographic framework and morphogenesis of the Keuper escarpment. Idealized block diagrams illustrating the situation in the area of Beilstein/Wurttemberg. 1 = Estheria-beds (Estherienschichten, km,). 2 = Quiet-water deposits (“Normalfazies”, km,). 3 = Sandstone cords (“Flutfazies”, km,). 4 = Dark Marl (Dunkle Mergel, km,) and Red-beds (Rote Wand, kmj). 5 = Cherty Sandstone (Kieselsandstein, km,,). A. Paleogeographic framework after diagenesis. The different bedding units are separated. Biconvex shapes due to compaction. B. Morphogenesis: the exhumed sandstone cords protect the underlying strata, the quiet-water deposits are excavated by erosion.

quickly. During a hundred years the southwest pass increased about 6 km in length. The convincing similarity of the sedimentation models includes also the underlying Estheria Beds as a pro-delta sediment and the overlying Dark Marl (Dunkle Mergel, FRANK, 1929) as a natural levee and marsh formation.

PALEOGEOGRAPHY

The base of the Cherty Sandstone (Kieselsandstein, km,,), about 30 m above the top

443

DELTA SEDIMENTATION LN THE GERMAN KEUPER BASIN

of the Schilfsandstein, forms a structural high above the sandstone cords. This feature has been caused by different compaction of the sand cords and the muddy clay matrix between them during diagenesis. Erosion exhumed the buried framework. The Keuper .escarpment in south Germany well demonstrates this morphogenesis. The thin quiet-

Fig.7. Hydrographic system of the Schilfsandstein delta (locality see Fig.8.9). a Overlying Keuper sediments, forming the upper escarpments of the Keuper formation. b = Mean current directions, graphic determination as shown by Fig.3.4. c = Reconstructed delta distributaries; occasionally they form nowadays long ridges and mountain chains in the south German landscape. d Underlying Keuper und Muschelkalk sediments, forming a questa-plain. 2

water deposits were first excavated. Owing to their thickness the sandstone cords resisted and form nowadays long ridges and mountain chains (Fig.6B). The coincidence of modern relief and ancient current directions prove these physiographic relations. They help to reconstruct ancient hydrographic systems of the Schilfsandstein delta (Fig.7), consisting of meandering and anastomosing streams in a general northeast-southwest direction. Some irregularities in the Kraichgau, Zabergau and Schonbuch environments are perhaps caused by the local topography of the shelf area or by epirogenic downwarping during sedimentation. A fair idea of the paleogeography of the whole delta system was obtained by investigation of about 500 outcrops in Luxembourg, east France (WURSTER,1963a), Switzerland, Swabia, Franconia, Thuringia, Westphalia and Lower Saxonia. Throughout the total German Keuper basin between the Brabantian and the Bohemian massifs

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P. WURSTER

Fig.8. Paleogeographic outlines of the Schilfsandstein system. 1 = Massifs. 2 = Rudaceous borders of the basin. 3 = Arenaceous and argillaceous delta deposits. 4 = Mean current directions. 5 = Localities of samples for grain size distribution (Fig.1b-d). 6 = Localities of vertical sections of Fig.3. 7 = Localities of vertical section of Fig.la. 8 = Special map Fig.4. 9 = Special map Fig.7.

the delta streams flowed generally towards the southwest (Fig.8). Everywhere they transported the fine-grained and well sorted sands. Only at the borders of the basin small streaks of coarse grained sands and conglomerates accompanied the general advance of the delta towards the open sea. The basin was connected with the Alpine geosyncline through the RhBne basin, the Provence basin and the Western Alps. In all these regions the member of Schilfsandstein has been discovered during the last decade (RICOUR,1952, 1959; RICOUR, HORONand LIENHARDT, 1960). Our results agree with the opinion of RICOUR (1952) and TRUMPY (1960), that there are no reasons to assume a separating “Vindelician Chain” (GIGNOUX, 1955).

DELTA SEDIMENTATION IN THE GERMAN KEUPER BASlN

445

The source area and the source rocks of the fine-grained, well sorted and uniformly transported sands are still unknown. At any rate they came from far northeastern regions. The fact that a similar facies is found in the basin areas of north and east Germany, Denmark and Poland shows that the upstream beginnings of the delta were situated at least at the borders of Fennoscandia. Perhaps the detritus came from the Fennoscandian shield or from the Russian platform. The dimensions of the sketched delta system (lateral extension 300 km, longitudinal extension 1,000 km, thickness 30 m) prevent any simple comparison with modern examples. Though similar processes of sedimentation may be active at the present time, no comparable large delta systems could form, owing to the recent and pleistocene movements of land and sea, and owing to the concomitant changes of climate, vegetation, weathering processes etc. By its small thickness, the Schilfsandstein delta system also differs from many other fossil (pre-Quaternary) deltas which were formed in geosynclinal conditions.

SUMMARY

The Upper Triassic Schilfsandstein of Germany and adjacent countries covers a large area, with a maximum diameter of the order of 1,000 km, but it reaches nowhere great thickness, on the average only some 30 m. In most of the area two facies can be distinguished, one mainly arenaceous, the other chiefly silty and argillaceous. The petrology of the sediment is remarkably uniform over great distances. The arenaceous facies is represented by systems of elongate sandstone bodies (shoestring sands) which are apparently of deltaic origin (river distributaries). Their depositional structures consist of very regular cross-bedding, very regular current ripple laminations and irregular, massive layering. From the orientation of these structures it follows that sediment transport took place predominantly from northeast (Fennoscandian shield or Russian platform) to southwest (Alpine geosyncline). I n some cases the elongate sandstone bodies have subsided during their seaward extension, into the soft, underlying pro-delta muds (Estheria Beds). The relationships appear to be very similar to those found in the modern Mississippi delta.

REFERENCES

FISK,H. N., 1961. Bar-finger sands of Mississippi delta. In: J. A. PETERSON and J. C. OSKCIND (Editors), Geometry of Sandstones Bodies. Am. Assoc. Petrol. Geologists, Tulsa, pp. 29-52. FRANK, M., 1929. Das stratigraphische Verhaltnis zwischen Dunklen Mergeln und Schilfsandstein im mittleren Wiirttemberg. Mitt.Geol. Abt. Wiirttemberg. Statist. Landesamt, 12 : 1-30. GIGNOUX, M., 1955. Stratkraphic Geology. Freeman, San Francisco, 682 pp. GURICH,G., 1933. Schrlgschichtungsbogen und zapfenformige Fliesswiilste in1 “Flagstone” von Pretoria und ahnliche Vorkommnisse im Quarzit von Kuibis, SWA., dem Schilfsandstein von Maulbronn u.a. Z . Deut. Geol. Ges., 85 : 652463. HANTZSCHEL, W., 1935. Fossile Schriigschichtungs-Bogen,“Fliesswiilste” und Rieselmarken aus dem

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Nama-Transvaal-System (Siidafrika) und ihre rezenten Gegenstiicke. Senckenbergiana Lethaea, 17 : 167-177. HAMBLTN, W. K., 1961. Micro-cross-lamination in the Upper Keweenawan sediments of northern Michigan. J. Sedinienr. Perrol., 31 : 3 9 M 0 1 . LINCK,O., 1949. Lebens-Spuren aus dem Schilfsandstein (Mittl. Keuper km,) NW-Wiirttembergs und ihre Bedeutung fur die Bildungsgeschichte der Stufe. Jahresh. Ver. Varerlandische Narurkrrnde Wiirtternberg, 971101 : 1-100. REIBLE,P., 1962. Die Conchostraken (Brachiopoda, Crustacea) der Germanischen Trias. Neues Jahrb. Geol. Palaontol., Abhandl., 114 : 169-244. REIFF,W., 1938. Obere Bunte Estherienschichten, Schilfsandstein und Dunkle Merge1 irn niittleren Wiirttemberg. Tiibinger Geqyraph. Geol. Abhandl., 26 : 200 pp. Rrcom, J., 1952. A propos de la “chaine vindtlicienne”. Compt. Rend. SOC.GcJol.France, 1952 : 242-244. Rr co m , J., 1959. Stratigraphie du Trias du bassin de Paris. Bull. SOC.Ciol. France, 1 : 3-12. 0. and LIENHARDT, G., 1960. Le Trias du Jura, de la Bresse, de la plaine de la RICOUR,J., HORON, SaBne et de la bordure du Massif central. Bull. SOC.GPol. France, 2 : 156167. SCHMIDT, M., 1928. Die Lebewelr unserer Trias. Ferdinand Rau, ohringen, 461 pp. SCHMIDT,M., 1938. Die Lebewelt unserer Trias, Nachtrag. Ferdinand Rau, ohringen, 143 pp. A,, 1943. Elasmobranchier-Reste aus den1 oberen Muschelkalk und Keuper WiirttemSEILACHER, bergs. Neues Jahrb. Mineral. Grol. Palaontol., Monatsh., 1943 : 256271,273-292. STOKES, W. L., 1953. Primary sedimentary trend indicators as applied to ore finding in the Carrizo Mountains, Arizona and New Mexico. U.S . At. Energy Comm., RME-3043 : 1 4 8 . THURACH, H., 1900. Beitrage zur Kenntnis des Keupers in Siiddeutschland. Geognosr. Jahresh. 13 : 7-53. TRUMPY,R., 1960. Hypothesen iiber die Ausbildung von Trias, Lias und Dogger im Untergrund des schweizerischen Molassebeckens. Eclogae Geol. Helv., 52 : 4 3 5 4 8 . WURSTER, P., 1958a. Geometrie und Geologie von Kreuzschichtungs-Korpern. Grol. Rrmdschau, 47 : 322-359. WURSTER, P., 1958b. Schiittung des Schilfsandsteins im mittleren Wiirttemberg. Neries Jahrb. Geol. Palaontol., Monarsh., 11 : 479489. WURSTER, P., 1963a. Probltmes du gres-a-roseaux.Sci. Terre, in press. P., 1963b. Perrologie und Palaogeographie des Schilfandsteins. In preparation. WURSTER,

SEDIMENTOLOGY OF A TERTIARY BEACH SAND IN THE SUBALPINE MOLASSE TROUGH1 WINFRIED ZIMMERLE

Deutsche Erdol-Akticngesellschafi,Hamburg (Germany)

INTRODUCTION

Well established examples of ancient shoreline deposits are rare. Some geologists (e.g., TWENHOFEL, 1955, p.97) have even doubted the existence of “any real shore sediments in the geologic column”. However, a well documented example of an ancient littoral deposit is described in this paper. Object of the sedimentological study is the Lower Glass sand outcropping near Promberg within the Nonnenwald syncline of the subalpine Molasse Trough (Fig. 1). Stratigraphically this sand is a clastic member of the upper “Cyrenenschichten” of

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Fig.1. Sketch map showing the tectonic framework of t h e subalpine Molasse Trough and sample locations. Published with permission of the Deutsche Erdd-Aktiengesellschaft,Hamburg 13, Mittelweg 180.

448

W. ZIMMERLE

Upper Oligocene age, underlain by the middle portion of the limnic-brackish coalbearing “Cyrenenschichten”, and overlain by the brackish Schwaig member of the “Cyrenenschichten” (ZOBELEIN, 1957). During Tertiary time the Molasse Trough was a narrow foredeep north of the rising Alps. Most molasse sediments were derived from the southern land. Previous investigations of the Lower Glass sand by ANDREE (1936) and WIESENEDER (1943) were restricted to routine heavy mineral analyses of grab samples. These authors found that, unlike other molasse sandstones, most of these rocks contain

.

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.. . . . . . :.

.

...

WEST S I D E EAST SIDE

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Clay layer

LEGEND

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OUTCROP N0.9

Sand

Mica c o n c e n t r a t i o n

Conglomerate

Fossils

Heavy mineral streak

a

Concretion

Fig.2. Schematic columnar sections of the Lower Glass sand from outcrop no.8 and 110.9after ZOBELEIN (1957). north of Promberg.

A TERTIARY BEACH S A N D IN THE SUBALPINE MOLASSE TROUGH

Fig.3. Lamination in the lower part of the Lower Glass sand (scale ZOBELEIN (1957), north of Promberg.

=

449

30 cm). Outcrop no.8 after

450

W. ZIMMERLE

abundant opaque minerals, zircon, and andalusite. Because of this distinctive mineral composition they suggested that the Lower Glass sand was derived from both north and south.

SEDIMENTARY FEATURES AND FOSSIL CONTENT

The Lower Glass sand is up to 40 m thick. According to its ratio of width to thickness the sand can be classified as a blanket sand of medium size ( K R Y N I N E , 1948, p.146). The precise position within the sedimentary framework is not well known since the upper and lower contacts are not exposed. The gross sechientary features were observed in the sand pits north of Promberg (Fig.2). Primary bedding features change rapidly in the lateral as well as in the vertical directions indicating turbulent conditions during sedimentation. Irregular bedding types prevail. The most common types of cross-stratification are: (I) lenticular or wedge-shaped sets of small-scale trough crossstratification with concave low-angle cross-strata and (2) tabular sets of medium-scale current-stratification with straight high-angle cross-strata. The individual cross-strata of the latter type differ in grain size and mineral composition but dip uniformly in a southern landward direction. Also common are thinly-laminated beds (Fig.3). The thickness of individual sedimentation units varies from mm-dimensions in laminated layers and heavy mineral streaks to dm-dimensions in conglomeratic and currentbedded layers. Moderately to poorly sorted, medium- to coarse-grained sands predominate. Single granules and pebbles, or lenses and pockets of pebble conglomerate are frequently disseminated in the sand; laterally-persistent conglomeratic horizons also occur sporadically. The maximum pebble size observed was 37 mm. The grain sizes change abruptly across most erosion surfaces and bedding planes. A consistent vertical gradient of increasing or decreasing grain sizes does not exist. The coarser fractions are subrounded to rounded and the finer fractions are subangular to subrounded. Unusually large amounts of silt and clay are found by sieve analyses, and cumulative curves show an uncommon grain-size distribution. However, thin sections reveal that most sand grains are shattered into angular fragments so that sieve analyses give distorted results. The shattering was caused before cementation (Fjg.4) by movements on the thrust fault (Fig. 1). Clay galls and burrows occasionally produce bedding deformation. Carbonaceous plant fragments are locally concentrated in bedding planes. Oval calcareous sandstone concretions of varying size occur in several horizons and often contain concentrations of macrofossils and their detritus. Macrofossil fragments are accumulated in thin bands within the sands not containing concretions; such fragments are also frequently associated with conglomeratic lenses. Fossils collected from outcrop no.8 (after ZOBELEIN, 1957)were identified by 0. Holzl (personal communication, 1962):

A TERTIARY BEACH SAND IN THE SUBALPINE MOLASSE TROUGH

451

Fig.4., a. Fractured quartz grain cemented by calcite, sandstone concretion from the Lower Glass sand. Thin section cut perpendicularly to the bedding plane. Outcrop 110.8 after ZOBELEIN(1957). nicols). b. Fractured quartz grains cemented by fibrous chalcedony. north of Promberg (partially Lower Glass sand from 4th level, Penzberg coal mine, south of Promberg (+ nicols).

+

Molluscs' Congeria basteroti (DESHAYES) Polyrnesoda convexa convexa (BRONGNIART) ? Cavilucina (Gonitnyrtea) cf. intercalata HOLZL Pliacoides sp. Cardiwn sp. Pitaria (Cordiopsis) pol!tropa ANDERSON Pitaria (Paradione) bqyrichi (SEMPER) Psammobia cf. protracta MAYER-EYMAR Psammobia ‘sp . Angulus (Moerella) angusta H O L Z L Sphenia? sp.

++ ++ +

+ ++ + ++ +

Explanation of symbols: known from marine deposits only; -1- main distribution in the brackish facies of the “Cyrenenschichten”; occur in marine as well as in brackish deposits (distribution in the molasse: RupelianBurdigalian).

+

+ +++

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W. ZIMMERLE

+ + Corbula carinata (DUJARDIN) f Corbula (Varicorbula) gibba (OLIVI) + + Tympanotonus margaritaceus (BROCCHI) Angiospermae Porana sp. Myrica? sp. Laurus sp. The molluscs indicate a brackish to marine environment. The plant leaves which were derived from the near-by land are restricted to a few thin layers.

PETROGRAPHY

The very pale orange (10 YR 8/2) to dusky yellow (5 Y 6/4), friable sand is an orthoquartzite to subarkose (after PETTIJOHN, 1957) with minor feldspar (potash feldspar > plagioclase) and siliceous rock fragments, and traces of mica, chlorite, and glauconite. Detrital carbonate grains are absent in the non-concretionary sand. Feldspars are not altered to kaolinite. X-ray determination shows that the powdery silt and clay fractions consist mainly of quartz with minor feldspar, illite, chlorite, and kaolinite. The amount of primary matrix and secondary cement is low and their distribution is irregular. The very light gray (N 8) to yellowish gray ( 5 Y S/ I) hard concretions of calcareous sandstone contain varying amounts o f detrital and organic carbonate concentrated in thin layers. The calcareous cement of the concretions is partly derived from crushed macrofossils. The heavy nuneral content of the Lower Glass sand varies considerably. The lightcoloured sand contains 0.0 1 4 . 6 weight percent of heavy minerals; the black heavy mineral streaks of 1-10 mni thickness (Fig.5), however, carry up to 50 weight percent of heavy minerals. Such heavy mineral concentrations are most pronounced in the upper part of the Lower Glass sand in outcrop 110.9 (Fig.2). At this location heavy mineral streaks persist along bedding planes for more than 10 m, although they occasionally bifurcate. Even current-bedded units show strata mainly composed of heavy minerals. In outcrop no.8 (Fig.2) the heavy mineral streaks have a scalloped outline and extend farther along strike than downdip. Many of the heavy mineral streaks contain rather coarse-grained admixtures of quartz and feldspar. In a single case a lenticular concentration of heavy minerals is highly contorted (Fig.6). Vertical tubes filled with non-laminated sand are sometimes found in this contorted layer. THOMPSON (1937, p.738) and STEWART (1956) observed similar structures in recent Californian beaches, and Stewart attributed them to movement of trapped air. In the black streaks ilmenite and minor magnetite constitute up to 90 volume percent of the total heavy minerals. Transparent heavy minerals include zircon, monazite,

A TERTIARY BEACH SAND IN THE SUBALPINE MOLASSE TROUGH

453 z w"

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LI

N

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W. ZIMMERLE

xenotinie, garnet, andalusite, tourmaline, rutile, staurolite, kyanite, epidote, titanite, hornblende, topaz, dumortierite, chloritoid, brown and green spinel. Based on the dominant constituents the heavy mineral concentrations are ilmenite-magnetiteplacers. SCHNEIDERH~HN (1955, p.211) reported similar recent beach placers from Brazil. Within sporadic lenses biotite, altered biotite, and muscovite comprise up to 5 weight percent of the sand. Also a 5-7 cm thick lenticular layer of moderate yellowish brown (10 YR 5/4) to pale brown (5 YR 5/2) clay is intercalated in the lower part of the Lower Glass sand (Fig.2). The clay consists of quartz, feldspar, muscovite, illite, chlorite, kaolinite, and calcite with coarser-grained admixtures of quartz and feldspar. Microfossils are not present in this clay layer with the exception of undeterminable plant, pollen, and spore remnants (G. von der Brelie, personal communication, 1962). The nearly oligomictic conglomerates are composed of rounded granules and pebbles of quartz (over 80 grain percent) and minor feldspar metamorphic and igneous rocks.

DEPOSITIONAL ENVIRONMENT

The stratigraphic position of the Lower Glass sand near the top of the limnic-brackish “Cyrenenschichten” and below the marine Promberg formation points to a transitional environment of deposition. The observed limited thickness of the sand, the rapid changes in sedimentary structures which are more pronounced downdip than parallel to the strike, thin lamination, and small-scale cross-bedding are typical features of many recent beaches as shown by THOMPSON (.1937) and MARTENS (1955). Also sets of landward-dipping current-stratification are reported from recent shoreline deposits by RUCHIN(1958, p.204), VAN STRAATEN (1959, p.204), and HOYT (1962, p.309), and their origin has been explained experimentally by MCKEEand STERRETT (1961). Likewise poorly sorted coarse-grained sands do occur at recent beaches with a high hinterland. The worn and broken shells of brackish and marine molluscs which are occasionally associated with interspersed leaf remnants of land plants indicate a mixing of fossils derived from various environments. Such a fossil assemblage is also indicative of a beach environment. The orthoquartzitic to subarkosic composition of the sand and the conglomeratic horizons as well as the presence of black heavy mineral streaks and thin mica concentrations prove intense reworking and sorting of light and heavy minerals during deposition; this is typical of the high energy environments of littoral deposition. Such compositional features are unknown from other Tertiary sandstones of the subalpine Molasse Trough. The lenticular highly contorted heavy mineral concentration with occasional airheave structures was caused by movement of trapped air. Similar structures occur in

P

Fig.6. Contorted heavy mineral concentration overlain by a current-bedded layer which is composed of alternating light and heavy mineral strata. Both units show vertical tubes filled with non-laminated sand. Outcrop no.9 after ZOBELEIN (1957), north of Promberg.

Rwl

456

W. ZIMMERLE

recent sediments only in shoreline deposits (STEWART, 1956; VAN STRAATEN, 1959). These features, which are similar to those of recent beaches (THOMPSON, 1937; MARTENS, 1955; MCKEE,1957; PETTIJOHN, 1957; RUCHIN,1958; VANSTRAATEN, 1959) lead to the conclusion that the Lower Glass sand near Promberg was deposited in a littoral environment. Complete interpretation of this sediment must simultaneously account for all the aspects listed above. Hence, the former paleogeographic interpretation of the Lower Glass sand, which was based on heavy mineral data only, needs revision. This sand is the product of intense reworking and sorting in a littoral environment.

ACKNOWLEDGEMENTS

The writer expresses his appreciation to Prof. Dr. F. Hecht for stimulating this paper, to Dr. E. H. T. Whitten for critically reading the manuscript, to Dr. 0. Holzl for identifying the macrofauna and macroflora, to Dr. G. von der Brelie for examining samples of the clay layer for spores and pollen, and to the Penzberg Mine direction for permitting collection of samples.

SUMMARY

Stratigraphic position, specific sedimentary features, fossil content, and mineral composition indicate that the Oligocene Lower Glass sand near Promberg in the subalpine Molasse Trough was deposited along an ancient shoreline.

REFERENCES

ANDR~E, H., 1936. Die Schwermineraliender alteren oberbayerischen Molasse. Neues Jahrb. Mineral. Geol. Palaontol., Abt. A , Beilage Bd., 71 : 59-120. HOYT,J. H., 1962. High-angle beach stratification, Sapelo Island, Georgia. J. Sediment. Petrol., 32 : 309-311. KRYNINE, P. D., 1948. The megascopic study and field classification of sedimentary rocks. J. Geol., 56 : 130-165. J. H. C., 1955. Beaches. In: P. D. TRASK(Editor), Recent Marine Sediments - SOC.Econ. MARTENS, Paleontologists Mineralogists, Spec. Publ., 4 : 207-218. MCKEE,E. D.; 1957. Primary structures in some recentsediments. Bull. Am. Assoc. Petrol. Geologists, 41 : 1704-1747. T. S., 1961. Laboratory experiments on form and structure of longshore MCKEE,E. D. and STERRETT, and J. C. OSMOND (Editors), Geometry of Sandstone Bodies. bars and beaches. In: J. A. PETERSON Am. Assoc. Petrol. Geologists, Tulsa, pp. 13-28. PETTIJOHN, F. J., 1957. Sedimentary Rocks. Harper, New York, 2nd ed., 718 pp. RUCHIN, L. B., 1958. Grundzuge der Lithologie. Akademie-Verlag, Berlin, 806 pp. SCHNEIDERH~HN, H., 1955. Erzlagerstatten. Kurzvorlesungen zur Einfiihrung und zur Wiederholung. Fischer, Jena, 3. Aufl., 375 pp. JR., H. B., 1956. Contorted sediments in modern coastal lagoon explained by laboratory STEWART experiments. Bull. Am. Assoc. Petrol. Geologists, 40 : 153-161.

A TERTIARY BEACH SAND IN THE SUBALPINE MOLASSE TROUGH

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THOMPSON, W. O., 1937. Original structures of beaches, bars, and dunes. Bull. Geol. SOC.Am., 48 : 723-752. TWENHOFEL, W. H., 1955. Discussion to: F. P. SHEPARD and D. G. MOORE.Sediment zones bordering and H. W. MENARD(Editors), Finding the barrier islands of central Texas coast. In: J. L. HOUGH Ancient Shorelines - SOC.Econ. Paleontologists Mineralogists, Spec. Publ., 3 : 97-98. VANSTRAATEN, L. M. J. U.,1959. Minor structures of some recent littoral and neritic sediments. Geol. Mijnbouw, 21 : 197-216. WIESENEDER, H., 1943. Petrographische Analyse der Sedimentationsabfolge in der nordalpinen Saumtiefe Ober- und Niederbayerns. Neues Jahrb. Mineral. Geol. Paliiontol., Abhandl., Abt. B, 88 : 157-175. ZOBELEIN, H. K., 1957. Kritische Bemerkungen zur Stratigraphie der Subalpinen Molasse Oberbayerns. Abhandl. Hem. Landesamt Bodenforsch., 23 : 91 pp.

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INDEX I ACCORDING TO MAIN TOPICS OF PAPERS

Clays and muds, chemistry and mineralogy 93

293 417 399 200

Mangenese contents and sediment transport along Dutch and German North Sea coast (DE GROOT). Cation adsorption, flocculation and permeability of sediments (Ems estuary etc.) (W. MULLER). Clay minerals in Lower Carboniferous of Moscow Basin (VIKULOVA). Clay minerals and iron compounds in Lower Carboniferous of Ural-Volga region (THEODOROVITCH et al.). Clay minerals in relation to Paleogene sedimentary evolution, Aquitaine, France (KLINGEBIEL et LATOUCHE). Organic matter

83 Origin and changes of organic matter in basins off California (DEGENS). Carbonate sedimentation

429 Mechanism of carbonate precipitation in Persian Gulf (WELLSand ILLING). 356 Mechanism of carbonate precipitation in coastal lagoon of southern France (RIVIBREet VERNHET). et al.). 129 Environment of carbonate sedimentation along Trucial coast (EVANS 185 Carbonate sediments along Trucial Coast (KINSMAN). 148 Deposition of Lithotharnniurn material in ria of Brittany (GUILCHER). 48 Devonian biostromes and bioherms in northwestern Spain (BROUWER). Phosphorite formation

62 Formation of phosphorite deposits in shallow seas (BUSHINSKI). Iron ooliteformation

319 Formation of Early Paleozoic iron oolites in shallow seas (PETRANEK). Shape of clastic elements

207 Pivotability of sand grains (KUENEN). 410 Comparison of methods of roundness determination (TONNARD). 253 Shape of pebbles in transgression conglomerates (L~TTIG). Porosity andpermeability

71

Relation between porosity, permeability and grain-size distribution (CHILINGAR). Sediment structures

379 269 136 368 362 123 35

Minor structures of Nile delta beach sands (SOLIMAN). Significance of primary thin laminations (MANGIN). Appearance of trace fossils in relation to deposition and erosion (GOLDRING). Contortions due to quicksand movements in Devonian of Devon (SHEARMAN). Contortions due to quicksand movements in Precambrian of Scotland (SELLEY). Structures due to differential compaction during deposition, Holocene, The Netherlands (ENTE). Giant scale breccia formed by intraformational undermining, Molasse, Switzerland (BERSIER).

460

INDEX I

Recent sedimentation Deltas in general 26 Sedimentology of Niger river delta (ALLEN). 216 Sedimentology of RhBne river delta (LAGAAIJ and KOPSTEIN). Beaches 144 Beach topography and wave observations, western Florida coast (GORSLINE). 379 Sediment structures of Nile delta sand beaches (SOLIMAN). 245 Sediment structures and heavy mineral concentrations on Sea of Azov beaches ( ~ G V I N E N K D and REMIZOV). 76 Beach ridges in relation to coastal prograding, Nayarit, Mexico (CURRAY and MOORE). Tidal f a t areas 193 Sedimentology of Bay of Fundy tidal flat area (KLEIN). 93 Manganese contents and transport of mud along Dutch and German North Sea coast (DE GROOT). Barriers, inlets, salt marsh areas 170 Sedimentology of Central Georgia coast (HOYTet al.). 76 Beach ridges in relation to coastal prograding, Nayarit, Mexico (CURRAY and MOORE). Coastal lakes and lagoons 123 Structures due to differential compaction during deposition, Holocene, The Netherlands (ENTE). 356 Mechanism of carbonate precipitation in coastal lagoon of southern France (RIVIBRE ret VERNHET). 165 Diatom muds in coastal lagoon of north Greenland (HANSEN). Rias 148 Grain-size distributions, transport and deposition in Ria of Brest, France (GUILCHER). Coasts with carbonate sedimentation 129 Environment of carbonate sedimentation along Trucial coast (EVANS et al.). 185 Carbonate sediments along Trucial coast (KINSMAN). Sherf 54 Heavy mineral distribution and sediment transport off Ghana coast (BRUCKNER and MORGAN)-

282 Grain size distribution, submarine weathering and sediment transport, Gulf of Napels, Italy (G. MULLER). Continental slope and - borderland 101 Sand flow from shelf into submarine canyons off California and Mexico (DILL). 83 Origin and changes of organic matter in basins off California (DEGENS). Fossil deposits Mainly or partly deltaic, paralic or formed in inland seas Sedimentology of Lower Carboniferous in Central Pennines, England (READING). Sedimentology and transport directions in Lower Carboniferous of South Wales (KELLING). Tectonic framework of Carboniferous sedimentation in South Wales (OWEN). Trend surface analysis of thickness variations in Carboniferous, east Pennines, England (DUFF and WALTON). 39 Sedimentology of Carboniferous and Permian delta deposits in Donetz Basin and Priuralie, U.S.S.R. (BOTVINKINA and YABLOKOV). 399 Sedimentology of Carboniferous deltaic and shallow sea sediments in Ural-Volga region (THEODOROVITCH et al.).

340 177 301 114

INDEX I

46 1

436 Sedimentology and transport directions in Triassic Schilfsandstein deposits of Germany (WURSTER). 227 Sedimentology of Mesozoic in deep wells of North Jylland, Denmark (LARSEN). and PRENTICE). 257 Sedimentology of Cretaceous Weald clay, England (MACDOUGALL 371 Sedimentology of Cretaceous strata in Moscow Region (SHVETZOV). 336 Sedimentology of Cretaceous and Tertiary paleo-deltas in Tian Shan and Pamir, U.S.S.R. (POPOV et al.). 105 Delta formation in Cretaceous and Tertiary eugeosynclinal belts of Oregon, California and Chili (DoTT). 308 Sedimentology of (Tertiary) Frio deposits in Louisiana (PAINE). 275 Drowned Pleistocene delta off Nayarit, Mexico (MOOREand CURRAY).

Beach and nearshore deposits 441 Sedimentology of Oligocence Lower Glass sand Bavaria, Germany (ZIMMERLE). Mainly sherf and shallow sea deposits Formation of phosphorite deposits in shallow seas (BUSHINSKI). Formation of early Paleozoic iron oolites in shallow seas (PETR~NEK). Devonian biostromes and bioherms in Northwestern Spain (BROUWER). Lithified mud pebble beds in Ordovician of Normandy: France (PONCET). Sedimentation of black limestones in Devonian and Carboniferous of Belgium and northern France (MAMET). 323 Sedimentation of (Eocene) Bartonian deposits in Paris Basin, France (POMEROL). 388 Heavy mineral distribution and transport directions in Eocene Annot Sandstones, Maritime Alps, France (STANLEY). 347 Definition of Flysch; shallow water deposits ? (RECH-FROLLO).

62 319 4s 330 264

Facies change correlations 264 Sedimentation of black limestones in Devonian and Carboniferous of Belgium and northern France (MAMET). 157 Sedimentation of Liassic strata in northwestern Europe (HALLAM). Tectonic control of sedimentation 301 Tectonic framework of Carboniferous sedimentation in South Wales (OWEN). 236 Classification and origin of morphological and geological basins (LLOPIS LLADO).

INDEX I1 ACCORDING TO MAIN AREAS CONCERNED

Belgium

264 Devonian and Carboniferous black limestones (MAMET). Canada

193 Nova Scotia. Sedimentation in tidal flat area of Bay of Fundy (KLEIN). Chili

105 Cretaceous and Tertiary deltaic deposits (DoTT). Denmark

221 Mesozoic marine and continental deposits of Jylland (LARSEN).

%YP' 319 Recent beach sands of Nile delta (SOLIMAN). 330 323 200 388 148 356 216

France Normandy. Ordovician clay pebble beds (PONCET). Paris Basin. Sedimentology of (Tertiary) Bartonian deposits (POMEROL). Aquitania. Paleogene clay mineral distribution (KLINGEBIEL et LATOUCHE). Maritime Alps. Heavy minerals of (Tertiary) Annot sandstones (STANLEY). Brittany. Recent ria sediments (GUILCHER). Mediterranean coast. Carbonate precipitation in recent lagoon of - (RIVIBRE et VERNHET). RhGne delta. Sedimentology of Holocene - (LAGAAIJ and KOPSTEIN). Germany

436 441 293 93

Sedimentology of (Triassic) Schilfsandstein deposits (WURSTER). Bavaria. Sedimentology of (Oligene) Lower Glass sand (ZIMMERLE). Ems river estuary. Cation adsorption in recent deposits of - (W. MULLER). North Sea coast. Mangenese contents and sediment transport along recent - (DEGROOT). Ghana

54 Heavy minerals and sediment transport on recent shelf off - (BRUCKNER and MORGAN). Great Britain

362 136 368 340 111 301 114 251

Scotland. Sedimentary structures in (Precambrian) Torridon sandstone (SELLEY). Devon. Trace fossils in relation to erosion and deposition (GOLDRING). Devon. Sedimentary structures in Devonian sandstones (SHEARMAN). Central Pennines. Facies relations in Carboniferous of - (READING). South Wales. Sedimentology of Carboniferous deposits of - (KELLING). South Wales. Tectonics and sedimentation in Carboniferous of - (OWEN). East Pennines. Thickness variations of (Carboniferous) Modiolaris zone (DUFFand WALTON). Weald district. Sedimentology of (Cretaceous) Weald clay (MACDOUGALL and PRENTICE).

INDEX I1

463

Greenland 165 Recentlagoonal depositsjnInorthern - (HANSEN).

Ira4 282 Gurfof Naples. Sediments of recent - (G. MULLER). Mexico

and CURRAY). 275 Nayarit. Drowned Pleistocene delta off - (MOORE and MOORE). 76 Nayarit. Holocene beach ridges on coast of (CURRAY 101 Baju California. Sand movement in submarine canyons off - (DILL). The Netherlands

123 Differential compaction in (Holocene) Almere deposits (ENTE). 93 Manganese contents and sediment transport along recent North Sea coast (DECiltoo1). Nigeria

26 Sedimentology of Holocene Niger river delta (ALLEN). Persian G u y

429 Recent carbonate precipitation (WELLSand ILLING). Spain

48 Devonian biostromes and biohernis in Cantabrian Mountains (BROUWER). Switzerland

347 Definition of Flysch (RECH-FROLLO). 35 Giant scale breccia in Miocene Molasse deposits (BERSIER). Trucial Oman

129 Recent carbonate sedimentation along coast of - (EVANS et al.). 185 Recent carbonate sediments along coast of - (KINSMAN). U.S.A. 105 105 308 170 144 83 101

Oregon. Cretaceous and Tertiary deltaic deposits (DoTT). California. Cretaceous and Tertiary deltaic deposits (DoTT). Louisiana. Sedimentology of (Tertiary) Frio deposits (PAINE). Georgia. Pleistocene and Holocene coastal deposits (HOYTet al.). Florida. Beach observations on coast of - (GORSLINE). California. Organic matter in basins off - (DEGENS). California. Sand movement in submarine canyons off - (DILL).

U.S.S.R. 417 Moscow Basin. Clay minerals in Carboniferous deposits (VIKULOVA). 39 Donetz Basin. Sedimentology of Carboniferous deltaic deposits (BOTVINKINA and YABLOKOV). 399 Ural- Volpa region. Sedimentology of Carboniferous deltaic and shallow water deposits (THEODOROVITCH et al.).

464 39 371 336 245

INDEX I1

Priuralie. Sedimentology of Permian deltaic deposits (BOTVINKINA and YABLOKOV). Moscow basin. Sedirnentologyof Cretaceous near-shore and shallow-water deposits (SHVETZOV) Pamir and Tian-Shim. Cretaceous and Tertiary deltaic deposits (POPOVet al.). Sea ofdzov. Sedimentology of beaches on north coast of - (LOGVINENKO and REMIZOV).

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