Carbonate Rocks: Origin, Occurrence and Classification (Developments in Sedimentology 9A)

DEVELOPMENT S IN SEDIMENTOLO GY

9A

DEVELOPMENTS IN SEDIMENTOLOGY 9A

CARBONATE ROCKS Origin, Occurrence and Classi$cation

EDITED BY

GEORGE V. CHILINGAR Professor of Petroleum Engineering University of Southern California, Los Angeles, Gal$ (U.S.A.)

HAROLD J. BISSELL Professor of Geology Brigham Young University, Provo, Utah (U.S.A.) AND

RHODES W. FAIRBRIDGE Professor of Geology Columbia University, New York, N. Y. (U.S.A.)

ELSEVIER PUBLISHING COMPANY Amsterdam London New York 1967

ELSEVIER PUBLISHING COMPANY

335 JAN

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ELSEVIER PUBLISHING COMPANY LIMITED RIPPLESIDE COMMERCIAL ESTATE, BARKING, ESSEX

LIBRARY OF CONGRESS CARD NUMBER

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131 ILLUSTRATIONS AND 20 TABLES

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

CONTENTS

CHAPTER l. INTRODUCTION R. W. FAIRBRIDGE (New York, N .Y., U.S.A.), G. V. CH!UNGAR (Los Angeles, Calif. U.S.A.) and H. J. BISSELL (Provo, Utah, U.S.A.) . . . . . . . . . . . . . . . . . . CHAPTER 2. MODERN CARBONATE SEDIMENTS W. H. TAFT (Tampa, Fla., U.S.A.) . . . . . . . . . .

29

CHAPTER 3. PETROLOGY AND PETROGRAPHY OF CARBONATE ROCKS Y. GUBLER, J.P. BERTRAND (Rueil-Malmaison, France), L. MATTAVELLI, A. RrzzrNI and R. PASSEGA (Milan, Italy) . . . . . . . . . . . . . . . . . . . . . . . . .

51

CHAPTER 4. CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS H. J. BISSELL (Provo, Utah, U.S.A.) and G. V. CHILINGAR (Los Angeles, Calif. U.S.A.)

87

CHAPTER 5. ORIGIN AND OCCURRENCE OF LIMESTONES J. E. SANDERS (Dobbs Ferry, N.Y., U.S.A.) and G. M. FRIEDMAN (Troy, N.Y., U.S.A.) .

169

CHAPTER 6. ORIGIN AND OCCURRENCE OF DOLOSTONES G. M. FRIEDMAN (Troy, N.Y., U.S.A.) and J. E. SANDERS (Dobbs Ferry, N.Y., U.S.A.) .

267

CHAPTER 7. CARBONATE OIL RESERVOIR ROCKS J. W. HARBAUGH (Stanford, Calif., U.S.A.) . . . . . . .

349

CHAPTER 8. CARBONATE ROCKS AND PALEOCLIMATOLOGY IN THE BIOCHEMICAL HISTORY OF THE PLANET R. W. FAIRBRIDGE (New York, N.Y., U.S.A.)

399

APPENDIX

433

REFERENCES INDEX

435

SUBJECT INDEX . . .

444

Chapter 1

INTRODUCTION RHODES W. FAIRBRIDGE, GEORGE V. CHILINGAR, AND HAROLD J . BISSELL

Columbia University, New York, N. Y. (U.S.A.) University of Southern California, Los Angeles, Calif. (U.S.A.) Brigham Young University, Provo, Utah (U.S.A.)

INTRODUCTION

Limestones are those rocks composed of more than 50% carbonate minerals, of which 50 % or more consist of calcite and/or aragonite. Limestones may be white, gray, dark gray, yellowish, greenish, blue, and, sometimes, black in color. A small admixture of clay particles or organic matter imparts gray color to limestones. Greenish coloration is due to the admixture of clayey material, glauconite, or finely-dispersed ferrous compounds; whereas reddish or brownish coloration is due to the presence of ferric iron. As pointed out by RUKHIN(1961, p.173), coarse-grained limestones usually have lighter colors (associated with well-oxidized conditions) than the fine-grained varieties (often indicative of deeper, possibly reducing conditions in early diagenesis). Impurities in limestone include a spectrum of magnesium carbonates, dolomite, silica, glauconite, gypsum, fluorite, siderite, sulfides, iron and manganese oxides, phosphates, clays, and organic matter. A rock with somewhat under 50% CaC03 is described as a calcareous sandstone, shale, etc. Its carbonate fraction is often subject to differential weathering, so that it appears like a limestone; the insoluble quartz grains, etc. are often unobtrusive and are easily released during weathering processes. Some calcareous eolianites with less than 30 % CaC03 develop karst features and other “characteristics” of limestones. Dolomites (or “dolostones”) are rocks composed mainly of the mineral dolomite (CaMg(C0s)z). Pure dolomite mineral is composed of 45.7 % MgC03 and 54.3 % CaC03, in weight % (or 47.8 % COz, 21.8 % MgO, and 30.4 % CaO) (see Fig. 1). Impurities in dolomite rock (dolostone) include gypsum, anhydrite, iron sulfides, celestite, opal, chalcedony, iron oxides, magnesite, fluorite, and organic matter. Dolomites are quite similar to limestones in appearance and, therefore, it is difficult to distinguish between the two with a naked eye. (The term “dolostone” is used by some authors to specify the rock composed of the mineral dolomite, in large measure at least; the present authors recognize certain utility in this term. The word is not widely used, however, because the meaning is normally clear from the context.)

2

R. W.

FAIRBRIDGE, G.

V. CHILINGAR AND H. J. BISSELL

20 -n

-s v

l5

Magnesium enriched limestones

Magnesium carbonate

( O h )

Fig.1. Graph showing MgC03 concentration in Paleozoic carbonates (data after CHAVE,1954), ar:d two distinct peaks of “magnesium enrichment”. Note that high-magnesium calcites having 15-35 % MgC03 are notably absent due to their preferential dolomitization, leaving only the limestones with MgC03 content of less than 15 %. There is a progressive dolomitization of all ancient limestones having above 15 % MgC03. (After FAIRBRIDGE, 1957, p.128, fig.1.)

Sedimentary carbonate rocks are, for the most part, products of deposition in marine realms and the principal repositories are the paralia- and autogeosynclines, in neritic environments. Calcium carbonate can accumulate also in deep water, or may be found in coral atoll structures surrounded by an abyssal region, but definitely not on the same scale as those of banks, platforms, and shelves. Little or no evidence is available suggesting that areally-extensive carbonates ac cumulated in deep water prior to the Cretaceous. Data compiled more than 40 years ago by VAUGHAN (1924, p.313) from the “Challenger” Report on modern deep-sea sediments, however, indicate that calcium carbonate comprises on the average 32.2 % of these sediments, largely as Globigerina and pteropod oozes, and as coral mud and sand. As pointed out by TWENHOFEL ( I 943, p.285), “This concentration in the deeper water sediments must be balanced somewhere in other deposits; the sediments with a deficiency of lime may be those of the continental and delta environments.” Probably the deep-sea sediments of the Paleozoic and Precambrian eras were low or deficient in CaC03, because the lime-secreting plankton and nekton were rare or absent. As pointed out by STRAKHOV et al. (1954, p.720), the evolution of carbonate rocks in many respects is different, for example, from that of iron ores (Fig.2). According to STRAKHOV et al. (1954, p.721), inasmuch as carbonate rocks occur preferentially on platforms rather than in orthogeosynclines and with time the shallow platform areas were growing at the expense of deeper geosynclinal zones, the processes favorable to carbonate formation were more intensive during the

3

INTRODUCTION

later periods. It is uncertain, however, whether the rate of carbonate withdrawal has changed greatly through time or not. There are proportionately fewer carbonates in exposed Precambrian sequences, but it should be remembered that the latter are largely preserved today in their deeper geosynclinal facies, whereas their shallow neritic equivalents have had the maximum opportunity to be destroyed by erosion. Over geologic time there have been stages of broad epicontinental seas, favorable to neritic carbonate development (e.g., Ordovician, Cretaceous), and other stages A

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Fig.2. Schematic diagram showing evolution of carbonate rocks. Absolute time in millions of years is plotted on the ordinate. A . Normal-saline seas and oceans: I = chemical calcite formation; 2 = biogenic calcite formation; 3 = primary chemical dolomite formation; 4 = diagenetic dolomite formation. B. Carbonate lagoons: 5 = primary dolomite formation is a necessary stage during the history of lagoons; 6 = dolomite formation as an optional stage during the history of the lagoon, requiring existence of definite physicogeographical conditions. C. Lakes of arid zone: 7 = lakes of magnesium carbonate type; 8 = lakes of sodium type. (After STRAKHOV et al., 1954, p.720.)

4

R. W.

FAIRBRIDGE, G . V.

I

-7

SITE OF RESTRICTED 1ARRONATE SEDIMENTATION

SITE OF MAXIMUM CARBONATE SEDIMENTATION

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CHILINGAR AND H. J. BISSELL

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Fig.3. Diagram illustrating maximum distribution of carbonates during the past at times when the sea level was universally high (thalassocratic stages), and poor development of carbonate sediments 1957, p. 157, fig. I I .) when sea level was low (epeirocratic stages). (After FAIRBRIDGE,

of general withdrawal that were unfavorable for the formation of carbonate rocks (e.g., parts of Upper Carboniferous, Triassic) (see Fig.3). Precambrian carbonate rocks, where preserved from obliteration by intense metamorphism, very frequently contain traces of calcareous Algae (such as Collenia, etc.). Even where the carbonates are totally replaced by silica, it is still possible to detect the unmistakable patterns of calcareous Algae; no tectonic process can duplicate their distinctive “cauliflower” growth forms (FAIRBRIDGE, 1950). Precambrian marbles commonly contain graphite in some sections which perhaps suggests an organic association. It is possible that a high Mg/Ca ratio and lower pH in Precambrian sea waters prevented the formation of hard protective and skeletal structures of organisms, or largely hindered their formation (CHILINGAR and BISSELL,1963; FAIRBRIDGE, 1964). Thus, during Precambrian time the carbonate rocks apparently owed their origin not to the secretion of shells by organisms, but to biochemical control of pH in lagoons by Algae and to direct chemical precipitation out of sea water. Algae became important lime-precipitating organisms quite early in Precambrian times (they are found, for example, in the oldest Precambrian rocks of Rhodesia); but only in Cambrian time did organisms of the animal kingdom begin to participate in significant manner in extracting carbonates out of sea water. Consequently, chemical or biochemical precipitation was over-shadowed by biogenic carbonate formation with time. Accumulations of carbonates in modern seas are produced by mechanical, organic, chemical, and inorganic processes; widespread accumulations of pure to relatively pure limestones, and in particular fine-textured varieties, represent moderately shallow to very shallow marine realms. Some of these deposits have

.I

5

INTRODUCTION

resulted from chemical processes only, whereas others owe their origin to physicochemical and biological processes. Coquinas of hard-parts of organisms are forming at present and there are substantial thicknesses and widespread occurrence of bioclastic carbonates. Dolomite, dolomitic limestone, and magnesium-rich carbonates are not accumulating as thick deposits in present-day seas; and the few occurrences that have been reported by ALDERMAN and SKINNER (1957, pp.561-567), SABINS(1962, pp.1183-1196), H. C. W. SKINNER et al. (1963, pp. 335-336), TAFT (1961, pp.561-562), TAFTand HARBAUGH (1964, pp.58-66), VONDER BORCHet al. (1964, pp.lll6-1118), and by others all indicate shallow marine waters and require very special physicochemical conditions, such as high Mg/Ca ratio of medium of deposition. Recent observations, techniques, and analyses have made the picture of dolomite formation much clearer; however, much work still remains to be done. It is still uncertain whether direct chemical precipitation of dolomite (primary dolomite) under “natural”, low-temperature and low-pressure conditions in the Paleozoic seas was possible or not. Many questions concerning secondary dolomitization, such as the relationship between porosity and degree of dolomitization are still unanswered. CHILINGAR et al. (1966) recently discussed some of these problems in detail, and recognized very early diagenetic, early diagenetic, late diagenetic, and epigenetic (subsequent to lithification) dolomites. FAIRBRIDGE (1 966) recognized syn-, ana-, and epidiagenetic stages. (See Fig.4, 5, 6.) STRAKHOV (1956) recognized the following four types of dolomite facies: ( I ) dolomites forming in marginal, supersaline parts of large marine basins (mainly platforms); (2) shallow, supersaline central parts of seas on platforms;

RAINFALL

pH 7

I

Fig.4. Diagenetic stages: syndiagenesis, anadiagenesis, and epidiagenesis. (After FAIRBRIDGE, 1966).

6

R. W. FAIRBRIDGE, C . V. (’IIILINGAR AND H. J. BISSELL

~~~

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Fig.5. Dolomites belonging to different stages of diagencsis, from an idealized sketch of conditions observed in the Eifel district of Germany. (After FAIRBRIDGE, 1957, p.162, fig.13.) ( I ) Devonian Givetian Dolomite-syndiagenetic; (2) Couvinian Limestone (Middle Devonian) - affected by epidiagenesis corresponding to the pre-Ti iilssic land surfaces (a former “hard pan”), but to a limited depth only; (3) Triassic basal conglomerate, containing two types of dolomitic boulders, plus limestone boulders; and ( 4 ) hydrothermal dolomite, possibly enriched by anadiagenetic (deep-seated) connate solutions moving upward along fault plane.

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Fig.6. Diagram of an oceanic atoll with a dolomitized core. During thc periods when the lagoon is largely cut off from the ocean, high-density alkaline solutions enriched in Mg content seep downward and dolomitize reef core. Porous and pcrmeable reef walls permit continued refluxing of Mg-rich solutions until dolomitization is complete. (Aftcr FAIRBRIDGE, 1957, p.148, fig.8.)

INTRODUCTION

7

(3) normal marine dolomites, such as the dolomitized reefs of Upper Triassic of Eastern Alps; and (4) dolomites forming in lagoons and in marine bays of the arid zone, but characterized by freshened (by river) waters. The following important types of dolomite deposits were recognized by TEODOROVICH (1958): ( I ) normal-marine calcareous dolomites and replacement dolomitic limestones; (2) primary “pelitomorphic” (fine-textured) dolomites and anhydrite-dolomite rocks; (3) calcareous dolomite deposits of supersaline seas, reef lagoons and periodically drying intertidal calcareous muds; and (4) primarychemical, fine-textured calcareous-dolomitic muds, which form in continental lakes having dry and hot sometimes seasonal) climates. Dolomites of type 2 have formed in supersaline lagoons and may either be interbedded with strata and lenses of anhydrite-gypsum, or they may be primary more of less homogeneous sulfatedolomite rocks. In reviewing the distribution of Paleozoic dolomites, FAIRBRIDCE (1 957) pointed out that in the offshore direction in a non-evaporite facies as one proceeds seaward the dolomitized sediments are generally replaced by nondolomitized limestones of colder and deeper water character. Where the two alternate, one visualizes a eustatic cyclicity. The near-shore dolomitized carbonate is richer in terrigenous insolubles and initially Mg-rich algal nuclei, whereas the non-dolomitized layers lack such nuclei. The correlation between dolomitization and clastics showing nearness to the shore was demonstrated by many authors as exemplified in Fig.7 (see also Fig. 8, 9). This is a very important criterion when preparing paleogeographic maps. Much work, however, still remains to be done on this subject. In certain deep-water limestones, one may also note a rise in the insoluble fraction, but these rocks do not become so readily dolomitized. The insoluble residue is likely to be authigenic quartz or chert that is secondarily derived from fine-grained siliceous dusts or pelagic sources such as Radiolaria or diatoms (CHILINGAR, 1953). Modern seas are noticeably sparse in carbonate environments. There is no single area where sedimentation of various marine limestones, with a spectrum of evaporites and dolomitic materials, can be seen occurring contemporaneously. For this reason certain lacustrine sites have been examined for some of the answers of “evaporite-suite” sedimentation. Great Salt Lake and the Salt Flats of Utah in the western United States have been the subject of recent studies (GRAFet al., 1959, p.1610; 1961, pp. 219-223; GRIMet al., 1960, pp.515-520; BISSELL and CHILINGAR, 1962, pp.200-210). Although this area represents only present-day and Pleistocene carbonate deposition under lacustrine environmental conditions, it nevertheless gives an insight into some of the chemical, physical, physicochemical, and organic processes which may have operated, with certain modifications and variations, in marine realms in the geologic past. For example, thick deposits of salts of various chemical composition can originate in marine basins of different depths. Inflowing

8

R. W. FAIRBRIDGE, G. V. CHILINGAR AND H. J. BISSELL

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Fig.7. Relationship between the amount of insoluble residue and dolomite content in carbonate portion of clayey, calcareous dolomites and dolomitic limestones. (After ZELENOV, 1956, p.42.) Fig.8. Relationship between the amount of insoluble, terrigenous grains (near-shore indicators) and the degree of dolomitization. Purer limestones with less insoluble material have lower MgCO3 content, suggesting offshore facies. Based on a Lower Ordovician section near Harrisburg, Pennsylvania. (After FAIRBRIDGE, 1957, p.155, fig.9; analyses by LESLEY, 1879.)

oceanic waters across shallow shelves reach a level of saturation for the various dissolved salts at widely distant points, and, under certain Eh and p H conditions, can create overlapping facies. Above a very shallow shelf, which tectonically may be also rather stable, progressive saturation can lead to a high degree of fractional sedimentation. Under a particular set of conditions, calcium carbonate may not extend into the repository where saline to highly saline salts are accumulating; calcium sulfate may also be negligible or present in rather small amounts. Variation in degree of tectonism can, however, bring about an entirely different set of sedimentological conditions, and alternation as well as interdigitation may follow; for example, one may observe a sequence of micritic and algal limestones, dolomicrites,

9

INTRODUCTION TOP

E D HOI

0

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EXPLANATION M3C0,

0caco, INSOL

Fig.9. Rhythmic alternation of dolomitized and nondolomitic limestones of a sequence of Lower Ordovician rocks west of Harrisburg, Pennsylvania. Dolomitization always coincides with the beds containing maximum amount of terrigenous (insoluble) material, and, therefore, presumably closer to the original coastline. (Analytic data by LESLEY, 1879; after FAIRBRIDGE, 1957, p. 156, fig.10.)

soft algal dolomicrites, bituminous limestones and dolomites, argillaceous limestones and dolomites, a spectrum of anhydrites and gypsum (gypstone), rock salt, and other sediments which can accumulate in restricted to semirestricted environments. At times there may be an influx of detrital materials from the adjacent land areas, as shown for example by admixtures of red mud, red silt and red-colored NaC1-rich sediments. Large volumes of ocean water are circulating over depositional sites upon shelves over which water depth is more than a few centimeters, possibly some tens of meters, and also as a rule over those shelves of more unstable (subsiding) tectonism. Passage of water across a marginal shelf, rise, reef, ridge, or other shoal area can promote saturation, thus initiating a sequential order of precipitation of dissolved salts. Commonly calcium carbonate will be deposited first; continued fractionation of salts can occur in sequential fashion with progressively but slowly increasing water depths. The latter depends mostly on the ratio of the depth of water lost by evaporation to total depth of water. This is not to infer that the process operates in open deep-seas but rather over thresholds of parts of the neritic zone on which topographic irregularities occur. Deepening of the closed basin may prevent the most concentrated solutions from mixing with those of the lesser

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R. W.

FAIRBRIDGE, G. V.

CHILINGAR AND H. J. BISSELL

depths and thereby trap the salt. Stagnation could dominate in the deep parts of the basin, and deposits of darker-colored (locally bituminous) rock salts could attain thicknesses of 100 ft. to asmuch as 1,000ft. (See also SCRUTON, 1953, pp.2498-2512; and ADAMSand RHODES,1960, pp.1912-1929.) Students of carbonate petrology and petrography long suspected that there was a difference between the chemistry of modern sea water and that of the barred basin seas of the known Paleozoic and Mesozoic salt deposits, which also contain interlayered and interfingered carbonates. Seemingly, in deep-basin deposits the calcium carbonate and calcium sulfate constituents dominate over the sodium chloride accumulations, and locally gypsum-anhydrite deposits are present almost to the total exclusion of carbonates. In shallow-shelf and platform areas, however, sodium chloride may comprise the most extensive deposits; on the other hand, environmental conditions may have prevailed which permitted deposition of thick massive gypsum (gyprock), which can become altered to anhydrite under moderate burial pressure. Bituminous limestone and/or dolomite and “red beds” may also be present. The red-bed material, however, was derived from a terrigenous source and merely represents admixture of detritals into the depositional site. Potash salts are absent or negligible in such platform deposits. The only logical conclusion that can be arrived at when the details of ancient evaporite suites are carefully investigated is that none of these salt deposits represents complete evaporation of a sea water basin (BORCHERT and MUIR,1964). Thus, it would seem that most of the salt solutions that reached a concentration beyond that of saturation for sodium chloride were transferred back to the ocean at depth as return currents (see Fig. 10). This rule also applies to the sodium chloride solutions in the partly closed basins which deposited Ca and Mg salts such as CaC03, MgC03, and CaSO4. Various platforms and shelves, particularly those which are controlled by mild tectonism (subsidence) contain evaporite suites which were evidently precipitated one after another in the inverse order of their solubility, that is, CaC03, CaMg (CO3)2, CaSO4.2H20, NaCl, MgC12 and KClz, etc. It must be realized, however, that the locus of deposition of each of the salts is diachronic, so that the maximum thickness of each salt formation can only be measured in a profile that allows for lateral displacement through time; in short, the salt beds contain lateral and verticolateral facies variants. Over a shallow shelf where evaporation is high in relation to depth of water, saturation is reached first in the inflowing water near the shelf margin, resulting in a calcium carbonate accumulation which normally forms a belt or band parallel to the shelf boundary. It is commonly succeeded landward by massive gypsum deposits while the heavy chloride solutions move out basinward in bottom currents. Progressive landward evaporation of inflowing waters may also be concomitant with density stratification of various salt solutions in deeper portions of the basin (or unstable shelf), and separation of salts from solution should be according to the ionic ratios of the solution. This would result in chemical carbonate sedi-

11

INTRODUCTION

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Fig.10. Four standard types of barred basins, showing denser water (cross-hatched). Only type AZ appears to be suitable for the formation of chemical (evaporite) carbonates. (Modified after DIETRICH, 1963.)

mentation along the outer border, sulfates in a marginal zone, and the chlorides in the basin center. Such an ideal distribution is seldom attained, however, and neritic facies often display deposition cyclicity of most, if not all, of the saltswhich occur in that environment; even in the center of the basin all phases can be present in one cycle. In the deeper portions of some basins, the carbonates and sulfates exhibit finely laminated characteristics and may be bituminous, especially in the beginning phases of accumulation. These texture and composition are in a marked contrast to those of the reef carbonates, the “primary” and other dolomites, and the mottled and wavy-laminated gypsum of the shallow-water environment. Some of these concepts are summarized in Table I as an attempt at reconstruction of paleogeologic facies. The Permo-Triassic sedimentary sequence in a platform-to-shelf-to-basin repository in northwestern and southern Arizona of the western United States seems to illustrate the concepts advanced above. They also seem to be compatible with the environment reconstructed by MCKEE(1938, pp.129-132) for the Toro-

12

R. W. FAIRBRIDGE, G . V. CHILINGAR AND H. J. BISSELL

TABLE I PALEOGEOLOGIC DISTRIBUTION OF CARBONATES I N EVAPORITE-SUITE FACIES

Light carbonates

I

Dark carbonates

Light-colored limestone and/or dolomite, interbedded with calcarenites and/or red beds. Dolomite, anhydrite, algal- and coarse-textured oolitic limestones or dolomites. Algal dolomite; algal-plate limestone; bioclastic limestone; coarse to fine oolitic limestones and dolomites. Mottled anhydrite or gypsum with thininter-beds of micrite, dolomicrite, and/or algal limestones. Massive anhydrite with clouds and knots of dolomite. Wavy-laminated anhydrite with dolomicrite and micrite lentils and thin inter-beds; may be bituminous. Knotty and “augen” anhydrite, spongy dolomite and limestone. Thin-bedded anhydrite and dolomite or limestone in an interlayered sequence; bituminous. Varved anhydrite, dolomite, and bituminous limestone. Limestone with thin anhydrite layers. Sediment is bituminous. Bituminous carbonates with little or no anhydrite or gypsum.

Shelf I

I I

J.

Basin

weap Formation and Kaibab Limestone of northern Arizona and southern Utah, where there are excellent cyclothems of carbonate, gypsum, and red beds in the sedimentary sequences. McKee’s ideal cyclothem in these Permian strata is as follows: (5) Red beds and gypsum1 ( 4 ) Chemically precipitated limestones (3) Marine limestones with mollusks or brachiopods (2) Chemically precipitated limestones (1) Red beds and gypsum Workers concerned with petrology and petrography of sedimentary carbonate rocks, including modern accumulations of these sediments particularly in the shallow marine realm, have erected a complex nomenclature of descriptive terms many of which give a suggestion as to environment of sedimentation. Although it is not the purposein this introductorypreface to review all these names, itis noteworthy that researchers no longer must hasten to a glossary of terms to determine the meaning of such now well-known names as calcarenite, calcisiltite, tuffaceous carbonate, coralgal limestone, bryalgal limestone, grapestone, hard algal limestone, micrite, dolomicrite, dolarenite, coquinite, and many others. Nomenclatural terms and rock names are the trade-marks of sedimentary petrographers and it behooves the young worker to familiarize himself with this language. 1McKee showed these with 5 at bottom and I at top of list.

INTRODUCTION

13

Inherent in any study of the carbonate rocks is an attempt at classification and “pigeon-holing”. Today numerous schemes of classification of limestones and dolomites are available in published form, and each plan has particular merit. It is quite evident that each of the proposed classifications has resulted from a personal viewpoint which arose from research in a particular province or geographic area. Geologists, like other scientists, often have a tendency toward provincialism and, therefore, each worker is inclined to consider that his proposed scheme lacks the weaknesses of the proposals which preceded it. The student must learn to recognize at the outset the difficulties he will encounter in future research if he clings with undue passion to only one method of classifying his materials. Experienced petrologists and petrographers have long realized that a classification of modern carbonate sediments cannot follow one rigid outline, nor can a single carbonate classification now in vogue take into account all the possible ramifications of origins, environments, diagenesis and other sedimentary processes of ancient sedimentary carbonate rocks. Seven different chapters have been written by foremost experts on carbonate rocks in this book (Carbonate Rocks, part A ) , and are reviewed by the editors in order of their appearance. Physicochemical aspects of carbonate rocks are stressed in Carbonate Rocks, part B.

MODERN CARBONATE SEDIMENTS

Reviewing the sites and characteristics of modern carbonate deposition, W. H. Taft (Chapter 2) commented that it is possible to find them in almost all types of depositional environment (except for the deepest ocean). The relative abundance and preservation of such sediments, however, is highly variable, subject to rigid physical and chemical limitations. Most modern marine carbonates are demonstrably organogenic; some doubtful cases occur where very fine grain sizes make identification difficult. Results of certain isotopic studies, however, have suggested that some of the calcilutites, at least, are also organogenic. On the other hand, most of the intertidal and continental carbonates, beach rocks (and pelagosite), soil crusts (calcrete or caliche), spring deposits (travertine, tufa), and evaporites are of inorganic origin. The key minerals represented in modern carbonate environments are (a) low-magnesium calcite (characteristic of pelagic Foraminifera and most deep-sea deposits; also present, but not dominant, in warm neritic areas); (b) high-magnesium calcite (commonly found in calcareous Algae and in many invertebrate exoskeletons, especially those of warm habitats); (c) aragonite (typical of scleractinian corals and certain molluscan shells, notably of warm habitat; also occurs as inorganic cement in intertidal beach rocks); and ( d ) dolomite (inorganic product in superheated intertidal localities; also as rare scattered crystals at abyssal depths).

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R. W. FAJRBRIDGE, G. V. CHILINGAR AND H. J. BJSSELL

Diagenesis of modern carbonates is rapid if they are exposed to fresh water. Thus, beach rock cements that are several thousand years old are converted to calcite. Modern neritic and bathyal carbonates are largely uncemented, but slow changes are taking place, which require further investigation.

PETROLOGY A N D PETROGRAPHY OF CARBONATE ROCKS

The petrology and petrography of the carbonate rocks has been discussed in Chapter 3 by an experienced group of French and Italian geologists: Yvonne Gubler, along with J. P. Bertrand, L. Mattavelli, A. Rizzini and R. Passega. This authorship reflects two interesting trends: ( I ) the steadily growing importance of research by the Institut FranGais de Petrole and AGIP (the Italian National Corporation), and (2) the major role played by carbonates in oil and gas basins of western Europe (notably Aquitaine and Sicily), North Africa and the Middle East. Petrology deals with the geometry of the carbonate rock, its sedimentology, bedding, intraformational structures, and field appearance. In contrast, petrography involves description of features observable on a smaller, microscopic scale: the sedimentary micro-textures, cement and granular detail. Petrographic studies provide feed-back of information to complete the petrologic picture. Statistical integration can be achieved through the method worked out by IMBRIE and PURDY (1961). Carbonate rocks are polygenetic; they are composed of chemical, biochemical and clastic components, generally in various mixtures. The dominant cations in major carbonate minerals are Ca2+, Mgz+, and Fez+; minor components include particulate clays, silica, phosphates, oxides and sulfides. Three fundamental features of carbonate rocks are recognized: (a) Role of orgunisms. Being able to modify the local pco2 directly or indirectly, organisms (especially the photosynthetic Algae and bacteria) play a dominant role in carbonate petrology. Thus, the paleoecology of these organisms is critical t o the understanding of petrology of carbonate rocks. (b) Metastable minerals. Metastable minerals, which are common amongst the newly formed carbonates, control much of the diagenetic evolution of these rocks. High-magnesium carbonate leads the way to stable dolomite; aragonite soon inverts to low-magnesium calcite. Distinguishing primary from diagenetic crystals is not always easy, and the time of diagenesis may range from the earliest t o the latest phase of the rock's history (FAIRBRJDGE, 1967). (c) Past distribution. Past distribution of carbonates was not always the same as today. The fundamental dichotomy took place in the Cretaceous, with an appearance of widespread pelagic organisms (Foraminifera and Coccolithophoridae). Prior to this time the dominant facies were neritic and organisms were

INTRODUCTION

15

mainly benthonic. Phylogenetic position of the principal carbonate rock-builders has changed notably from time to time. As established by GRESSLY, already in 1838, the description of the rock in its total aspect may be termed facies, both lithofacies and biofacies. The interpreted facies is called the petromodel by our European colleagues. On the micro-scale, each individual lamina possesses a characteristic aspect. This is what CUVILLIER (1951, 1952) has termed the microfacies (both the mineral grains and the fossil fragments), which in thin-section or polished (or etched) surface may be employed for correlation (see FAIRBRIDGE, 1954), or may serve to work out the genetic history (CAROZZI, 1950, 1958, 1959, 1961). A succession of microfacies, that is a series of laminae, represents a micvosequence. Brief breaks in sequence (diastems) are recognizable. Micro-crossbedding gives directional indication. Rhythmic sequential repetition, reflecting climatic or tectonic events, may be traced by the geochemical, granulometric or paleontologic studies. Diagenetic changes may appear at any stage of the lithogenesis. They may be due to organic activity (bioturbation, ingestion, etc.), to biochemical effects, or to physical processes. The latter are numerous and include compaction, crystallization, etc. The changes may be brought about by any of three types of waters: by connate waters, by hydrothermal waters, or by meteoric waters. New minerals may form (“neogenesis” or “authigenesis”) and particle size and shape may change (“epigenesis”), with the consequent changes in the porosity and permeability of rocks. Time of tectonism in part controls the time when the various phases are instituted. Inasmuch as the alternation of microfacies leads to anisotropies, marked variations in porosity, etc. are observed; accordingly, the examination of randomly oriented thin-sections is very important. The recent history of carbonate petrography rests virtually in the hands of two men. First, there was CAYEAUX (1935), who, on the basis of vast numbers of thin-sections of samples from all parts of France, and elsewhere, compiled a comprehensive monograph, almost an encyclopedia, of all types of carbonates known to him. (He also prepared comparable works on the phosphatic and siliceous sediments of France and her sphere of influence, interpreted in the broad sense). Secondly, there was SANDER (1930) in Austria, who, on adding stereographic projection to the thin-section technique, first studied carbonate petrofabrics in an attempt to understand the original organization of these rocks, and to distinguish primary from diagenetic features. Sander’s studies opened the way for an evolution from the purely descriptive petrography to interpretive petrology integrated with field study of environments, ancient and modern. Modern development of petrographic techniques resulted in a number of most helpful refinements, extensions, or simplifications. CUVILLIER (1951, 1952) studied the role of micropaleontology in understanding microfacies. CAROZZI (1950, 1958, 1961) applied graphic and statistical methods to the study of distri-

16

R. W. FAIRBRIDGE, G. V. CHILINGAR AND H. J. BISSELL

bution and frequency of particles and organisms. FRIEDMAN (1959) and others brought out the value of staining methods for mineral recognition. SCHMITT (1963) applied microradiography t o identifying dolomite in fine-grained dolomitic lime(1963) injected colored stones. I n studying porosity and permeability, ETIENNE synthetic resins into carbonate rocks in order to examine the geometry of pore spaces. Anadiagenesis and tectonism, still very little known, result in the reorientation of crystals in a rock; some recent laboratory work in this area has been done by D’ALBISSIN (1963). Evidently the use of conventional microscopes is inadequate for a complete understanding of this phenomenon and developments now are being made in the areas of electron microscopy, X-ray diffractometry and emission spectrography. Statistical data are being processed by digital computers. Yvonne Gubler and her colleagues described a number of field studies in detail. First, they presented studies on the “Pisolitic Limestone” of Vigny, France. They rightly avoided the temptation of allocating a special rock name to every unique rock, but urged the use of Folk’s textural geometric (granulometric) method, e.g., “biosparitic calcirudite”. A second example deals with facies variations in the Triassic-Jurassic reef and basin formations of the Ragusa area of southern Sicily. Much of the primary texture is destroyed by dolomitization. An interesting feature of the area is the persistence of upward growth by a reef, with its shallow platform facies, for a total thickness of 2,500 m (8,200 ft.). This exceeds any measured Cenozoic subsidence of modern Pacific atolls or quasicratonic submarine plateaus. The authors stressed that conventional petrographic examination provides an adequate rock description only in exceptional cases. On the scale of basin dimensions, a serious problem in the way of quantitative integration exists because of inadequate data. Systematic core drilling and standardized data sampling are particularly important to establish comparable profiles and to integrate field descriptions and information obtained on studying thin-sections. This integration is the petromodel of our authors.

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

The question of classification of carbonate rocks has been a long and vexed one, especially when compared with the classifications of other sedimentary rocks. The reason is not difficult to appreciate: other sediments for the most part are fairly easily disaggregated into component grains, which may then be sorted for size analysis; also they respond to fairly straight-forward compositional or mineralogical analysis. On the other hand, calcite, dolomite, aragonite and high-magnesium calcite are all minerals that are highly susceptible to diagenetic change, to partial or total recrystallization, and to selective leaching and replacement. Fossils and distinctive primary syn-sedimentary structures of calcareous sediments may be

INTRODUCTION

17

totally destroyed or largely masked during diagenesis, which may include dolomitization or other fundamental modifications. Many rock types may be classified as biogenic and inorganic, but limestones often constitute a complex mixture, for example, an organogenic framework with an inorganic cement. In Chapter 4, H. J. Bissell and G. V. Chilingar considered the many alternative classifications that have been offered, with special attention to the flood of schemes that have appeared over the last decade or so. They stressed the importance of four parameters in particular: ( I ) Composition. Most authorities subscribe to a basic classification close to that of Cayeaux (1935, interpretation of data) for limestone-dolomite mixtures: limestone (( 5 % dolomite mineral), magnesian limestone (5-10 % dolomite), dolomitic limestone (10-50 % dolomite), calcitic dolomite (50-90 % dolomite), dolomite rock (( 10% calcite). Little support is given to the term “dolostone” although it is attractive; geologists are mostly content to see from the context whether “dolomite” refers to the mineral or to the rock. CHILINGAR (1957) rather favored a system of Ca/Mg ratios for compositional definition: normal dolomites (Ca/Mg = 1.5-1.7), calcitic dolomite (Ca/Mg = 2.03 3 , etc. TEODOROVICH (1958) provided further for a clay fraction: clayey dolomite (35-90 % dolomite mineral, 0-45 % calcite, 30-10 % clay), etc. (2) Texture. Bissell and Chilingar recommended a system of classification broadly based on those of LEIGHTON and PENDEXTER (1962) and on PLUMLEY et al. (1962). Three gross texture categories are identified on the basis of their genesis: (a) Accumulated in place (chemical and biochemical)-non-granular, colloform, etc., lack organic traces, e.g., travertine. ( b ) Accumulated in place (organic frame-builders)-e.g., coral, bryozoan, etc. limestone ( )90 % grains), micritic coralline limestone (50-90 % grains), or coralline micritic limestone (( 50 % grains). (c) Clastic and wave-moved-clastic limestones are further subdivided into those with abraded grains (detrital, skeletal, and pellets in part) and those having accretion-aggregation grains (some pellets, lumps and coated grains). Under each of these categories, Bissell and Chilingar recognized: ( i ) )90 % grains (detrital limestones, including calcarenite, etc., from pre-existing rocks only; skeletal limestone, pelletal limestone, lump limestone, oolitic limestone, etc.); (ii)90-75 % grains (micritic, detrital limestones, etc.); (iii)75-50 % grains (micritic-detrital limestones etc.; (iv) 50-25 % grains (detrital-micritic limestone, etc.); (v) 25-10 % grains (detrital, micritic limestone, etc.). Most of these criteria can be established with the aid of a hand lens and a dropper with dilute HC1. (3) Grain-micrite ratio ( G M R ) . GMR shows the relative amounts of coarse and fine textured carbonate material, theoretically related to wave or current action. For rocks having 90% grains, GMR is 9 / l ; for 50% grains, l / l ; and for 10% grains, 119.

18

R. W. FAIRBRIDGE, G. V. CHILINGAR AND H. J. BISSELL

(4) Energy index (EI). EI is a quantitative index for the appraisal of the rock and its environment. )90 % grains indicate strongly agitated waters; 75-90 %, moderately agitated; 50-75 %, slightly agitated; 25-50 %, intermittently agitated; and 10-25 %, gently agitated. With regard to secondary dolomites, most of the limestone classifications may be applied if the original texture is not obliterated. Limestones altered to cryptogenic stages may contain “ghost” relics and granular traces. Totally crystalline phases (aphanic or phaneric) can be classified only on basis of simple grain dimensions. Possibly, oxygen isotope studies may enable one to distinguish between primary (direct precipitates out of sea water) and diagenetic-epigenetic dolomites. Some criteria may be used to distinguish between the latter two (diagenetic or epigenetic) dolomites also (CHILINGAR et al., 1967).

ORIGIN AND OCCURRENCE OF LIMESTONES

The origin and occurrence of limestones is analyzed rather exhaustively by John E. Sanders and Gerald M. Friedman (Chapter 5). They have reviewed the major literature and from personal travels were able to report first hand on many examples of both ancient and modern environments of limestone formation. As to origin, they recognized: (a) primary “stony precipitates” (i.e., skeletons, reef structures), (b) lithification of carbonate sediments, and (c) replacement deposits, such as calcite replacing gypsum, with bacterial assistance. The occurrence of ancient limestones, and their modern prototypes is enormously widespread over the earth in both marine and continental settings. They have received in the past certainly more than their fair share of attention as compared with shales and sandstones, although they comprise only 10-15%1 of the continental sedimentary crust. The explanation is partly to be found in the fact that limestones frequently carry a far richer record of organic life than do the noncarbonates; there are also economic aspects, such as the widespread oil reservoirs in carbonate rocks and their world-wide use for cement manufacture. Limestones are being formed today, anywhere from the deep ocean basins to the interiors of continents. In nonmarine environments, limestones are found sometimes in lake depressions and basins; they may develop as primary precipitates or as derived terrigenous fragmentals. Eolian sediments near the coast are frequently rich in carbonate clastic material and readily lithify to form dune calcarenites or eolianite; interdune paleosols often contain calcrete or travertinous calcareous duricrust, comrnonly called caliche in the American southwest. In shallow seas of the warmer continental shelf regions and in analogous lSome estimates go as high as 25 %.

INTRODUCTION

19

seas from offshore platforms (e.g., the Bahamas, the Maldive-Laccadives, the South China Sea and the Coral Sea) in regions of quasicratonic intermediate crust, there are perhaps the greatest diversity of carbonate occurrences. The shallow environment renders them susceptible to rapid facies change under changing energy conditions, due to shore-line changes associated with eustasy, tectonics, or sedimentary processes. Biofacies of great variety here contribute to the lithologic heterogeneity. On the continental shelves there is potentially a considerable mixing with noncarbonate terrigenous material. Pure carbonate deposits are more likely to occur on the intermediate platforms; but here also there is an admixture of clastic, pre-existing terrigenous limestone debris. In the shelf or intracratonic environments, there are numerous cases of mixed evaporite-carbonate sequences; this is particularly true of the past, but not today, because the present sea-level regime is only about 6,000 years old. In deep-sea environments of today, and back to Cretaceous times, there are enormously widespread pelagic carbonates, coccolith ooze, Globigerina ooze; pteropod ooze, etc. The rock equivalents were mainly the chalks and chalky marls. Prior to Cretaceous these were rare or absent, a phenomenon of geologic history closely related to the sudden flowering of pelagic carbonate organisms in the Late Mesozoic, which in turn is probably related to gross changes in paleogeography and marine geochemistry. Particular attention is drawn to the authors’ treatment of the Capitan Limestone of the classic Permian section of West Texas, U.S.A., on which they do not agree. Friedman supports the widely accepted barrier-reef hypothesis; whereas Sanders advocates many of the views of his former student, G. W. Moore, who has raised doubts on many important points of the barrier-reef interpretation and has substituted the rather astonishing alternative that the fine-grained, massive, high-calcium, unfossiliferous limestone of the Capitan Formation is not recrystallized reef rock, but rather originated by wholesale replacement of massive Castile anhydrite. Moore’s alternative contains numerous important stratigraphic and paleogeographic corollaries, which are of more than academic interest to the geology of this petroliferous province. Another subject worthy of comment is the treatment accorded gravitydisplaced, calcareous, deep-sea deposits. In addition to their discussion of calcareous “turbidites”, the authors emphasize the paleogeographic importance of limestone-pebble “Wildflysch” accumulations in defining the location of ancient steep continental slopes. European readers, in particular, may be interested in this subject in view of the ideas about “thrust conglomerates” which have persisted in the tectonic literature of the Alpine chains. A description of true limestone thrust conglomerate of nonmarine origin-a fan deposit described by C. R. Longwell from southern Nevada, U.S.A.-is included by way of comparison. Another feature of this and the succeeding chapter will also be welcomed by European readers: the metric system has been employed.

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R. W. FAIRBRIDGE, G. V. CHILINGAR AND H. J. BISSELL

ORIGIN AND OCCURRENCE OF DOLOSTONES

As pointed out by G. M. Friedman and J. E. Sanders in Chapter 6, the “dolomite question” has defied solution until the recent advent of X-ray spectroscopy, which made dolomite identification simple, fast, and definitive. As a result of the X-ray and stable isotope (carbon and oxygen) analysis and field studies of Recent and ancient carbonate sediments, researchers seem to be on the verge of a breakthrough in this field. ADAMS and RHODES (1960) proposed that most dolomite (dolostone) deposits in the geologic record owe their origin to hypersaline brines, and must, therefore, be considered as related to evaporite deposits. The “capillary concentration” and “seepage refluxion” seem to our authors also to be the two main processes responsible for dolomite formation. The former is the effective process in supratidal or intertidal zones or along the margins of basins, whereas the latter is operative under deeper water conditions. By capillary concentration, when sea level is low, the interstitial waters in the sediments transpire upward through the porous tidal flat and coastal sediments and evaporate below the sediment-air interface; this process is similar to that under which some caliche is formed. Refluxion evaporation increases the salt concentration and the density of the water in any restricted lagoon or intermontane basin where evaporation periodically exceeds precipitation. This produces a heavy brine that migrates to the lowest possible topographic depressions, and seeps slowly through the underlying sediments resulting in a progressive doloniitization. Dolomite may also form directly from the brine, with aragonite as a possible transitional phase. The Mg/Ca ratio of the brine, which should be higher than that of the normal sea water, may be raised by the removal of calcium as a result of deposition of gypsum in very shallow water pools where the supply of oxygen is ample. Gypsum tends to be broken down to H2S and iron sulfide by bacteria where the oxygen supply is less. On the floors of such pools near the Persian Gulf today, layers of knife-edged crystalline gypsum can be lifted to expose interfingering layers of black mud rich in H2S. Consequently the layered dolomite forming under these conditions is indicative of a somewhat deeper reflux environment, with gypsum being deposited along the shore. Both capillary concentration and refluxion may operate in the same basin. Friedman and Sanders recognized three different types of mineral dolomite: ( I ) syngenetic, (2) diagenetic, and ( 3 ) epigenetic. These terms imply a genetic sequence and have been used by these authors as a basis of classification of dolomites. Syngenetic dolomite was defined by our authors as a dolomite that has formed penecontemporaneously in its environment of deposition as a micrite or as fine crystals. According to them, diagenetic dolomite forms by replacement of preexisting calcium carbonate sediments following consolidation of sediments or

21

INTRODUCTION

coincident with it. Diagenetic dolomite also may have formed penecontemporaneously by replacement of grains and cement of calcium carbonate sediments. Thus, in borderline cases the distinction between syngenetic and diagenetic dolomites becomes difficult or impossible. FAIRBRIDGE (1966) puts both types into subcategories of syndiagenetic dolomite. Epigenetic dolomite of Friedman and Sanders formed by replacement of limestone, and is localized by post-depositional structural elements. It is closely related to fractures and faults in carbonate rocks; and the term “epigenetic” is synonymous with such terms as “structurally-controlled’’ or “tectonic” dolomites. Many, but not all, epigenetic dolomites are genetically associated with metallic deposits, notably of lead and zinc minerals. These dolomites are the anadiagenetic rocks of FAIRBRIDGE (1967). He recognized a distinct fourth type, related to meteoric water circulation, which he has named epidiagenetic. Dolomite formed after burial, which includes all “epigenetic” and most “diagenetic” dolomites, owes its origin to subsurface brines. These subsurface formation waters are depleted in both so42- and Mgz+ ions, which probably indicates that sulfates are involved in the dolomitization process. Dolomites may revert back to limestones by the process of dedolomitizationl during “epidiagenesis”. A genetic relationship appears to exists between the presence v l dispersed anhydrite and dedolomitization, with MgS04 being carried away (in solution) as a by-product, according to the following reaction: Ca.Mg(CO3)z

+ CaS04 + 2CaC03 + MgS04

CARBONATE OIL RESERVOIR ROCKS

In Chapter 7, role of carbonates as oil reservoirs is discussed by John Harbaugh. This is an aspect of physical petrology that depends largely upon porosity and permeability (mass properties of rocks). The ratio of total pore space to the bulk volume of rock is the absolute porosity, which is of little value economically if the pores are not interconnected. Obviously, it is the ratio of the interconnected pores to the totalvolume of rock (the eflectiveporosity)that is of greatest interest. Inasmuch as the fluids have various viscosities, and the connecting passages have different degrees of tortuosity, a calculation of effective permeability is also required. When two or more immiscible fluids are present in a porous solid, the permeability to any of the fluids is termed the effective permeability of the solid to that fluid. It is often convenient t o convert effective permeabilities into permeabilities relative to some standard base (often the single phase liquid permeability), in which case they are called relative permeabilities. Relative permeability curves for a carbonate reservoir rock are also presented in Chapter 7. 1

Some authors prefer the term “calcitization”.

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R.

W.

FAIRBRIDGE, G . V.

CHILINGAR AND H. J. BISSELL

Primary porosity is that ielated to the initial sedimentary features that are only slightly modified by diagenesis, etc. Several varieties of primary porosity include: (a) Framework porosity, which is controlled by the presence of water-filled cavities in the initial sediments (organogenic, e.g., corals, sponges, calcareous Algae, etc.). (b)Mudporosity, which is due to the original presence of vast numbers of microscopic pores filled with connate water, that was not lost entirely during consolidation. The pores are so small that they normally do not contribute to any economical reservoir. (c) Sand porosity, which owes its origin particularly to the presence of oolites, pellets and fragmented skeletal calcarenites, is often very important. Its relative value is related to the sand/mud ratio, which in turn depends on the rate of deposition and the energy of the regime. Cementation is a negative feature as far as reservoir potential is concerned, because the extent to which cements (carbonate, silica, gypsum, etc.) have filled the voids is inversely related to the porosity and permeability. Secondary porosity usually refers to voids (either primary, or due to fracturing) enlarged by solution. Some of this solution may occur early during syndiagenesis, with the removal of aragonitic shell parts, etc., by the acid waters being generated as a result of bacterial activity. Further solution occurs during epidiagenesis when the entire sequence is subject to circulation of acid meteoric waters. Inasmuch as during the deep, anadiagenetic stage the interstitial waters are normally alkaline, they are saturated with respect to CaC03. Solution is restricted to the early and late phases of diagenesis. Calcite is considerably more soluble than dolomite under comparable conditions. In most reservoir rocks one can observe signs of both solution and reprecipitation, sometimes i n a multiple generation sequence. Circulation of solutions is greatly aided by the presence of unconformities and bedding planes between beds having variable porosities, and the growth of joints and fractures. Particularly the latter process, sometimes called “diaclastic revival”, may occur repeatedly during the later phases of the evolution of a basin. This leads to widespread fluid motion, both from uprising connate waters and downworking meteoric waters. Dolomitization has long been recognized as a valuable potential source of porosity, because a molecular replacement of limestone by dolomite would lead to about 12-1 3 % loss of mineral volume. Some investigators, however, believe that the replacement may also occur on a volumetric basis, leading in fact to reduced porosity. Different sedimentary components, e.g., an easily dolomitized, magnesium-rich algal lime mud may be variably graded or interstratified with pure calcitic skeletal fragments that do not dolomitize as easily. Generally, it may be said that dolomitization increases the porosity; this conclusion is based on extensive statistical studies. Consequently, plotting of isopleths of equal Ca/Mg ratios and directions in which Ca/Mg ratios decrease are of valuein exploration for oil and gas (CHiLImAn, 1953, 1956). At low percentages, the dolomite occurs as scattered euhedral rhombs, producing little porosity change

23

INTRODUCTION

up to 50 % of the original porosity. At higher concentrations of dolomite, porosity rises notably. In one instance studied, however, above about 80 % dolomite, the porosity decreases sharply again. Apparently this is due to the filling of the pore spaces by the precipitates out of late-stage dolomitic solutions. In another case, large calcitic fossil fragments have not been dolomitized, but were later removed by solution, increasing porosity correspondingly. Pore geometry is important and is also subject to systematic dimension analysis. The nature of the liquid (especially its viscosity) is critical in evaluating the permeability. In most reservoirs the pores are lined by a continuous thin envelope of water, whereas the oil occurs as globules in the middle. High interfacial tension between oil and water often prevents the passage of the oil through pore interconnections (“throats”). Migration will occur when the buoyancy and hydrodynamic forces exceed this interfacial tension (also modified by the throat dimensions). These characteristics may be tested in the laboratory by mercury injxtion technique. When oil, gas and water are present in rocks, the nature of movements of fluids becomes a complex problem, as shown in Fig.11. Although Fig. 11 applies best to sandstones, it does show the complexity of the problem. Pore geometry is subject to further study in terms of petrogenesis and GAS, 100 O/o

WATER, 100°/o

% WATER

SATURATION

O I L , 100%

Fig.11. Areas of one-: two-, and three-phase flows in a porous medium. (EREMENKO, 1960, p.530.)

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R. W. FAIRBRIDGE, G. V. CHILINGAR AND H. J. BISSELL

paleoecology. Original sedimentary facies variations often had profound effects on subsequent development of porosity and permeability. Consequently, the paleogeographic studies have been stressed, particularly during the last decade or so. Analysis of original environment, e.g., basin, shelf, bank, reef, back-reef, barrier beach, lagoon, etc., is of great help in identification of porosity characteristics and the prediction of porosity trends.

CARBONATES AND PALEOCLIMATOLOGY

In the final Chapter (8), Fairbridge reviews the carbonates as climatic indicators, first at the present-day and then at various stages of the geologic record. “Mineral thermometers” are commonly used by petrologists for appraising the temperature level of metamorphic and igneous events in geologic processes. By the same token, calcite, aragonite and related carbonates can be employed in judging thermal levels of former low-temperature environments; certain element ratios (Ca/Mg, Sr/Ca, Ca/Fe/Ti) and certain isotopes may also be used to help bring a degree of precision to these estimates. The degree of confidence in results decreases as one goes back in time, because there are large numbers of variables; and few, if any, unique solutions arc offered. For Cambrian times, for example, the diameter of the earth is not known, nor is the length of the day, the salinity, alkalinity or pH of sea water; and little is known of the ecologic habits of the organisms prevalent in Cambrian seas. Such looseness might easily lead t o an atmosphere of despair among researchers, yet in fact there are many aspects where the experienced geologist feels intuitively on rather safe ground. He knows the limits of metabolic tolerance in organisms and the stability fields of many sedimentary minerals such as the evaporites. The mean temperature of the earth’s surface may be stated reliably as falling in the vicinity of 20 & 10°C over the last 3 billion years. Paleontologically he can judge with a high degree of reliability certain other environment characteristics from the locomotory habit of organisms (sessile or nektonic, benthonic or pelagic), by studying their appropriate swimming, flying or attachment appendages and by the lithofacies status of the enclosing sediments. Each deduction can be made and appraised on its own basis. The results can then be synthesized so that a “general impression” can be achieved, and certain key indicators can be specified. Of great philosophic as well as practical geologic interest is the fact that a series of critical periods has been worked out, representing great biogeochemical revolutions in the history of the planet earth. These critical stages do not represent any sort of deus ex machina. There is no abandonment of the basic methodology of actualism or the philosophy of uniformitarianism; the past continues t o be judged by the methods and observed processes of today, but a new dynamic approach has t o be adopted. This planet is known to be a member of a

INTRODUCTION

25

solar system and although governed by laws of celestial mechanics, is subject to a systematic evolution through time. Furthermore it is periodically modified by relatively small changes in mass relations and in its energy fluxes, many of which are astronomically predictable, and, if not yet quantitatively, at least qualitatively established. The nature of early carbonate sedimentation can be approached in two ways: (a) by taking a working model of the proto-earth and deductively following through its most probable evolutionary steps; and (b) by taking the known parameters of the present and working backwards. The one may be constantly checked and conpared with the other. Series of models along these lines have already been attempted by various writers: Daly, Kuenen, Conway, Rubey, Urey, Holland, Rutten, Fischer, Berkner and Marshall, and others. A high degree of concordance has now been reached about the general character of the earliest earth and the gradual evolution of our hydrosphere, and the atmosphere of Nz, 0 2 and COZ.In this study, Fairbridge uses the carbonates to demonstrate the threshold stages of this evolution, each major step being designated as a “biogeochemical revolution”. They are: Revolution I. “First Life” (about 3.8 0.3 * lOQyears ago), based on the knowledge that the lime-fixing Algae are isotope-dated in the Bulawayan Dolomite Series as not less than 2.7 x lo9 years old; such Cyanophyte Algae are known today to be quite highly evolved, and as ELSASSER noted at National Academy of Sciences Symposium (1965, p. 1214), even the most simple organisms living today secrete upwards of 2,000 separate enzymes. A long time-span must have extended between their appearance and that of the first life. The latter is visualized as the first self-reproducing organic molecules, possibly of the virus type. During this phase the atmosphere is believed to have been close to the Schmidt-Urey model: CH4, NH3 and HzO with traces of other components notably HzS. The proto-sea was a reducing agency and erosion was essentially limited to mechanical processes. More HzO with COZ,SO2 and HC1 gradually evolved as volcanic gases. Revolution IT. “First Photosynthesis” (about 2.9 i- 0.2 . lo9 years ago) was marked by the beginning of photosynthetic use of COz, with 0 2 as a by-product of immensevital potential. To begin with, all the 0 2 liberated was used up immediately for the oxidation of surface minerals. Lacking a thick atmospheric blanket, as pointed out by BERKNER and MARSHALL (1964), solar UV transmission had split many of these OZ molecules close to their source and produced ozone at ground level. Inasmuch as the latter is a powerful oxidation agent, it contributed to the rapid change of the earth’s superficial environment from a reducing to an oxidizing one. World-wide deposits of SiOz-Fez03 (silica-ironstone) formed during this phase, but never again on this vast scale. Revolution 111.“First Carbonate Shells” (about 6 f 0.3 . 108 years ago) has long been recognized as one of the great enigmas of the geologic record. It is here suggested that the Precambrian atmosphere was richer in COZ (from very active

26

R. W. FAIRBRIDGE, G. V. CHILINGAR AND H. J. BISSELL

volcanicity) while thepo, built up only slowly. Several authors believe the “Pasteur Level” (poZ at 1 % of the P.A.L. or “present atmospheric level”) was reached at the time of this revolution. If the p H passed from the acid side to the alkaline at this time, the first organisms to form shells simply responded to the difficulty of removing alkali waste products from the body fluids into an environment of high ionic strength and high alkalinity, a serious problem in osmosis. Study of the crustacean carapace formation and the enzyme control of body fluid pH during moulting is a rewarding and illuminating experience. Revolution IV. “Great Coal Age” (about 2.5 i0.3 108 years ago) represents the culmination of the Paleozoic history with its continuous record of vast carbonate removal from the ocean, as limestones, followed by the coal burial of Carboniferous and Permian ages (the latter in the Southern Hemisphere). The result was a lowering of pcoz and achievement of a peak in po2 which favored the mammalian and land reptile respiration during Mesozoic time. RUTTEN(1962, 1965) has suggested that this high 0 2 level is necessary for the respiration and metabolism of the great flying insects of the late Paleozoic, when ancestral dragon flies achieved wing-spans of 3 ft. and more. Revolution V. Marking the first appearance of world-wide pelagic Foraminifera and coccoliths (about 1.0 & 0.2 108 years ago), Revolution V is the last of these great thresholds. As pointed out by KUENEN(1950), this event shifted the fundamental site of carbonate sedimentation from shelves to ocean basins (above the “calcium carbonate compensation depth”). From Cretaceous onwards, the carbonates, instead of being limited to shallow epicontinental basins and in part continuously recycled, began to accumulate with more or less continuous burial. It would appear that the Cretaceous Ocean was more extensively saturated or supersaturated with respect to CaC03 than that of today. The precipitation of CaC03 releases C02 (from the bicarbonate ion); and during the Cenozoic the ocean appears to have slowly become less alkaline, and also cooler. Inasmuch as the recharge rate of CaC03 is very high (it can be totally replaced in about one million years), there is little danger of reverting to an acid ocean. Increased atmospheric CO2 content from combustion of industrial fuel will not achieve this possibility, because there is more than adequate reserve of limestone in the ocean floor to buffer any such C02 imbalance. To conclude, there is a very important “moral)’ that may be drawn from this analysis. It is that a thorough appreciation of the nature of biologic metabolism is essential in the training of the thoughtful geologist. Without fairly intimate knowledge of the nature of metabolic limits, photosynthesis, shell calcification, body fluids, osmosis, enzymes, genetics and other aspects of living organisms, the history of the earth‘s atmosphere, ocean and sediments becomes meaningless. Whereas the biological substrate is certainly an inorganic surface, and organic evolution has certainly been in part a response to the physicochemical environment, that environment itself is essentially the product of biological activity.

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INTRODUCTION

27

REFERENCES

In reviewing various chapters, in many instances the editors used the same references used by the author of a particular chapter; these references are not repeated here.

M. L., 1960. Dolomitization by seepage refluxion. Bull. Am. Assoc. ADAMS, J. E. and RHODES, Petrol. Geologisfs,44: 1912-1920. ALDERMAN, A. R. and SKINNER, H. C. W., 1957. Dolomite sedimentation in the southeast of South Australia. Am. J. Sci., 255: 561-567. BISSELL, H. J. and CHILINGAR, G. V., 1962. Evaporite type dolomite in salt fiats of western Utah. Sedimentology, 1 : 200-210. BORCHERT, H. and MUIR,R. O., 1964. Salt Deposits. Van Nostrand, London, 338 pp. CHAVE, K. E., 1954. Aspects of the biogeochemistry of magnesium, 2. Calcareous sediments and rocks. J. Geol., 62: 587-599. CHILINGAR, G. V., 1953. Use of Ca/Mg ratio in limestones as a geologic tool. Compass, 30: 202-209. CHILINGAR, G. V., BISSELL, H. J. and WOLF,K. H., 1966. Diagenesis of carbonate rocks. In: (Editors), Diagenesis in Sediments. Elsevier, Amsterdam, G. LARSEN and G. V. CHILINGAR pp. 179-322. DIETRICH, G., 1963. General Oceanography. Interscience, New York, N.Y., 588 pp. EREMENKO, N. A., 1960. Petroleum Geology (Handbook), Principles of Petroleum Geology. Gostoptekhizdat, I: 592 pp. FAIRBRIDGE, R. W., 1950. Pre-Cambrianalgal limestone in Western Australia. Geol. Mag., 87 (5): 324330. FAIRBRIDGE, R. W., 1954. Stratigraphic correlation by microfacies. Am. J. Sci., 252: 683-694. FAIRBRIDGE, R. W., 1957. The dolomite question. In: R. J. LEBLANC and J. G.BREEDMG (Editors), Regional Aspects of Carbonate Deposition ( A Symposium)-Soc. Econ. Paleontol. Mineral., Spec. Publ., 5 : 125-178. FAIRBRIDGE, R. W., 1964. Theimportance of limestone and its Ca/Mg content to paleoclimatology, In: A. E. M. NAIRN(Editor), Problems in Paleoclimatology. Wiley, New York, N.Y., pp. 431-530. FAIRBRIDGE, R. W., 1966. Phases of diagenesis and authigenesis. In: G. LARSEN and G. V. CHILINGAR (Editors), Diagenesis in Sediments. Elsevier, Amsterdam, pp, 19-89. FRIEDMAN, G. M., 1959. Identification of carbonate minerals by staining methods. J. Sediment. Petrol., 29: 87-97. GRAF,D. L., EARDLEY, A. J. and SHIMP,N. F., 1959. Dolomite formation in Lake Bonneville, Utah. Bull. Geol. SOC.Am., 70: 1660 (abstract). GRAF,D. L., EARDLEY, A. J. and SHIMP,N. F., 1961. A preliminary report on magnesium carbonate formation in glacial Lake Bonneville. J. Geol., 69: 219-223. A. V., 1960. Clay mineralogy of the sediments of the GRIM,R. E., KULBRICKI,G. and CAROZZI, Great Salt Lake, Utah. Bull. Geol. SOC.Am., 71: 515-520. LESLEY, J. P., 1879. Notes on a series of analyses of dolomitic limestone rocks of Cumberland County, Pa. Second Geol. Surv., Penna. (1876-1878), MM: 311-362. MCKEE,E. D., 1938. The environment and history of the Toroweap and Kaibab Formations of northern Arizona and southern Utah. Carnegie Znst. Wash. Publ., 492: 268 pp. RUKHIN,L. B., 1961. Principles of Lithology. Gostoptekhizdat, Leningrad, 779 pp. F. F., Jr., 1962. Grains of detrital, secondary, and primary dolomite from Cretaceous straSABINS, ta of the Western Interior. Bull. Geol. SOC.Am., 73: 1183-1196. SCRUTON, P. C., 1953. Deposition of evaporites. Bull. Am. Assoc. Petrol. Geologists, 37: 2498-2512. SKINNER, H. C. W., SKINNER, B. J. and RUBIN,M., 1963. Age and accumulation rate of dolomitebearing carbonate sediments in South Australia. Science, 139: 335-336. STRAKHOV, N. M., 1956. About types and genesis of dolomite rocks (status of knowledge). In: and G. I. BusmNsmY (Editors), Types of Dolomite Rocks and their Genesis N. M. STRAKHOV -Tr. Geol. Inst., Akad. Nauk S.S.S.R., 4: 5-27.

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STRAKHOV, N. M., BRODSKAYA, N. G., KNYAZEVA, L. M., RAZZHIVINA, A. N., RATEEV, M. A., D. G. and SHISHOVA, E. S., 1954. Formation of Sediments in Recent Basins. SAPOZHNIKOV, Izd. Akad. Nauk S.S.S.R., Moscow, 791 pp. TAFT,W. H., 1961. Authigenic dolomite in modern carbonate sediments along the southern coast of Florida. Science, 134 (3478): 561-562. TAFT,W. H. and HARBAUGH, J. W., 1964. Modern carbonate sediments of southern Florida, Bahamas, and Espiritu Santo Island, Baja California: A comparison of their mineralogy and chemistry. Statlford Univ. Publ., Geol. Sci., X(2): 133 pp. TEODOROVICH, G. I., 1958. Study of Sedimentary Rocks. Gostoptekhizdat, Leningrad, 572 pp. W. H., 1932. Treatise on Sedimentation. Williams and Wilkins, Baltimore, Md., TWENHOFEL, 926 pp. W. H., 1942. The rate of deposition of sediments: a major factor connected with alTWENHOFEL, teration of sediments after deposition. J . Sediment. Petrol., 12: 99-1 10. T. W., 1924. Oceanography in its relations to other earth sciences. J . Wash.Acad. Sci., VAUGHAN, 14: 307-313. VONDER BORCH,C . C., RUBIN,M. and SKINNER, B. J., 1964. Modern dolomite from South Australia. Am. J. Sci., 262: 1116-1 118. ZELENOV, K. K., 1956. Dolomites of Lower Cambrian deposits of northern slope of Aldan Mountain Range and conditions of their formation. In: N. M. STRAKHOV and G. I. BUSHINSKIY (Editors), Types of Dolomite Rocks and their Genesis-Tr. Geol. Inst., Izd. Akad. Nauk S.S.S.R., 4: 28-74.

Chapter 2 MODERN CARBONATE SEDIMENTS WILLIAM H. TAFT

Department of Geology, University of South Florida, Tampa, Fla. (U.S.A.)

SUMMARY

Modern carbonate sediments are accumulating in almost all depositional environments except for the very deep oceans. Chemical and physical conditions, however, govern preservation and abundance of carbonate relative to non-carbonate material. Most modern marine carbonate sediments owe their origin to accumulation of bioclastic debris. Rarely can inorganic precipitation be actually proven. Mineralogical analysis of these sediments has shown that deep cold-water carbonates are predominantly low-magnesium calcites; whereas shallow, warm-water shelf deposits are composed predominantly of metastable carbonates (aragonite and high-magnesium calcite) with minor amounts of low-magnesium calcite. Significant quantities of supratidal dolomite has been reported in modern sediments. Diagenesis is taking place in modern sediments; however, only rarely can mineralogical or chemical changes be demonstrated in sediments that have not been exposed to fresh water. If modern shelf sediments are good examples of ancient sediments, almost all limestones have recrystallized. INTRODUCTION

Study of modern marine carbonate sediments gained momentum after World War I1 as a result of discoveries of huge accumulations of petroleum in limestones and dolostones in the Middle East. As a result of these discoveries, the petroleum industry, recognizing a general lack of basic knowledge in the fields of shallowwater carbonate deposition, diagenesis and lithification, sponsored and encouraged an extensive 10-year investigation of modern carbonate depositional environments. To date, these studies have concentrated on modern marine carbonates of the Bahama Banks, Florida Bay, Gulf of Batabano, the Great Barrier Reef of Australia, the Persian Gulf, and many specific areas of more limited extent. The purpose of this chapter is to broadly summarize the more significant results of these investigations and, hopefully, to encourage additional investigators to enter the study of modern carbonates. For a useful summary of modern carbonates, one may refer to a review by GRAF(1960).

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W. H. TAFT

DISTRIBUTION OF CARBONATE ACCUMULATIONS

Accumulation of carbonate sediments in the oceans is controlled areally and vertically by physical and chemical parameters such as temperature, pH, pressure, and carbon-dioxide content of sea water (REVELLE and FAIRBRIDGE, 1957; RODGERS, 1957). In addition, the abundance of non-carbonate material determines whether or not the final sediment will be a limestone or a clastic sediment with some carbonate material. Modern sediments with more than 30% calcium carbonate tend to be concentrated between 30 “N and 30”slatitude (Fig. 1). Variations from these somewhat arbitrary limits are common, particularly where warm oceanic currents (such as the Gulf Stream) saturated or supersaturated with respect to calcium carbonate are deflected away from the equator. Four general categories of carbonate sediments are recognized: (1)deep-sea oozes; (2) carbonate turbidites; (3) shelf accumulations of lime sands, silts, and muds; and ( 4 ) organic reefs and reef debris.

DEEP-SEA OOZES AND TURBIDITES

Deep-sea oozes are composed predominantly of Globigerina tests with minor quantities of heteropods, pteropods, rhabdoliths and coccoliths. The preservation of these deep-water carbonate sediments (to a depth of approximately 15,000ft.) is controlled more by the solubility of calcium carbonate in deep cold water than by the abundance of pelagic organisms. The so-called “calcium carbonate compensation depth” occurs at a depth of about 4,000-4,500 m. The continual rain of pelagic Foraminifera tests such as Globigerina through the water mass appears to be rather uniform; however, as a result of increased solubility of calcium carbonate in cold waters rich in carbon dioxide these tests are dissolved, leaving only red clays as the dominant sediment. As pointed out by RODGERS (1957, p.3) this gradation from Globigerina ooze (> 50 % calcium carbonate) to red clay (10 i % calcium carbonate) is very abrupt with increasing depth of water. In the East Indian basins, massive slumps have sometimes carried layers of carbonate oozes well below the normal “compensation depth”. Slumping on both large and small scales is rather common in deep-sea areas and often interrupts the regular sequence of layers in core samples. In some places there are gaps; in others, duplication. Unlike typical deep-water oozes that are situated great distances from accumulations of shelf sediments and are composed of fairly distinct assemblages of tests of plants and animals, carbonate turbidites are mixtures of typical deep-sea oozes and shelf sediments. Carbonate turbidites have been described by BUSBY (1962) and by RUSNAKand NESTEROFF (1964). These sediments are mixtures of

w

c

Fig.1, Distribution of modern marine carbonate sediments; solid black : organic reefs; lined: areas of shallow-water carbonate sediments; 1957. Printed by permisstippled: areas of other sediments, containing more than 30 % CaC03 (especially Globigerina ooze). (After RODGERS, sion of McKinley Publishing Co. and Society of Economic Paleontologists and Mineralogists.)

32 W. H. TAFT

v)

e ‘cf

4

L

W W

Fig.3. Pelletal sand from western margin of Andros Island, Bahamas. Material smaller than 0.062 mm in size has been removed.

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W. H. TAFT

planktonic Foraminifera and pteropods in the ungraded sections of cores, and pelagic and reef-derived sediments in the graded sections.

SHELF SEDIMENTS

Carbonate sediments accumulating on continental shelves may be distinguished on the basis o f ( I ) size distribution, (2) amount and composition of organic constituent, and (3) origin. Facies maps based on these criteria have been used successfully to show distribution of modern carbonates in the Gulf of Batabano (DAETWYLER and KIDWELL, 1959), on the Great Bahama Bank (IMBRIE and PURDY, 1962; PURDY,1963b), and on the Heron Island reef (MAXWELL et al., 1964). In addition, carbonate sediments may be distinguished on the basis of shape of particles.

Fig.4. Hand-picked specimens of grapestone from Yellow-Bank, Bahamas. Note progressive increase in degree of cementation and loss of grapestone texture from top to bottom of photograph.

MODERN CARBONATE SEDIMENTS

35

ILLING(1954) emphasized the importance of distinguishing between the skeletal and non-skeletal sediments, as did GINSBURG (1956). A common approach has been that of identifying the relative percentages of skeletal debris. ILLING’S (1954) results showed that there is an abundance of non-skeletal debris in the form of lumps, grains, ooids, aggregates and pellets, the most common of which were lumps. Similar deposits (bahamites) were described by BEALES (1 958). Non-skeletal sand particles also include ooliths. Ooliths tend to form local deposits concentrated in areas where water rises from appreciable depths and spreads out over a shallow bank, such as along the west side of the Andros platform in the vicinity of Cat Cay (NEWELL et al., 1959), and between Pleistocene islands along the western edge of Exuma Sound. Most modern carbonates owe their origin to accumulation of bioclastic debris. If particle grain size is much smaller than 50 p, however, it is extremely difficult to see identifiable organic structure or outlines. Moreover, as grain size approaches 10 p, or smaller, the particles are commonly aragonite needles, which, based on shape alone, cannot be distinguished from aragonite needles artificially precipitated from sea water (REVELLE and FLEMING, 1934). The difficulty of determining origin of modern carbonates is not restricted solely to fine particles. Although the size of ooids, grapestones, pellets and composite grains may be as large as 1 mm, they are generally made up of thousands of fine particles ( < 10 p). These particles owe their origin to organic or inorganic precipitation of CaC03 (principally as aragonite). The physical aggregation of these particles produces sand-sized grains. Examples of some typical carbonate sediments include: (I) foraminifera1 sand (Fig.2), (2) pelletal sand (Fig.3), (3) grapestone sand (Fig.4,5), ( 4 ) skeletal calcareous sand (Fig.6), and (5) sand composed of ooliths (Fig.7).

CHEMICAL A N D MINERALOGICAL COMPOSITION OF CARBONATE SEDIMENTS

Chemical and mineralogical studies of modern shallow-water carbonate sediments (1960, 1962), SIEGEL(1961), STEHLI and HOWER made among others by CHILINGAR (1961), CLOUD(1962), and TAFTand HARBAUGH (1964), demonstrate that these sediments are composed predominantly of metastable carbonate minerals (aragonite and high-magnesium calcite) with minor amounts of stable low-magnesium calcite. Most of the magnesium present in these sediments is substituted for calcium in the calcite structure by solid solution, and occurs in the calcareous skeletons of Algae, Foraminifera, Porifera, Coelenterata, sea urchins, starfishes, Ophiuroidea, Crinoidea, Brachiopoda, Mollusca, Ostracoda, and Cirripedia. Very high Ca/Mg ratios of the skeletons of madreporarian corals, pelecypods, gastropods and cephalopods are due to the fact that aragonitic organisms contain very small amounts of magnesium (seldom over 1 % MgC03).

Fig.5. Grapestone and bioclastic particles from Yellow Bank, Bahamas.

W

-4

Fig.6. Sand-sized bioclastic sediment from Florida Bay, Florida. Note angular nature of carbonate fragments and general absence of whole particles.

w

00

Fig.7. Ooliths from Wide Opening, Bahamas (west side of Euxuma Sound).

MODERN CARBONATE SEDIMENTS

39

Detailed chemical and mineralogical studies of the calcareous skeletons of marine organisms (CHAVE,1954a,b, 1962a, b; LOWENSTAM, 1954a, b, 1961, 1963, 1964; KRINSLEY, 1959, 1960a, b; KRINSLEY and BIERI,1959; PILKEYand HOWER, 1960; PILKEYand GOODELL, 1963) show a marked dependence of chemistry on the polymorphic form of calcium carbonate precipitated. For example, the two polymorphs of calcium carbonate, aragonite and calcite, have an inherent preference for certain cations. Strontium substitutes readily for calcium in aragonite up to 4.08 % SrO (NOLL,1934), whereas SrO rarely exceeds 1.O % in calcite. Magnesium, on the other hand, is most commonly found in the calcite structure (up to 30 mol. % M ~ C O ~ , C H A V 1954a); E , little magnesium is found in aragonite. LOWENSTAM (1954b) demonstrated the role of temperature in controlling the polymorphic form of calcium carbonate precipitated in carbonate skeletons. Four animal phyla (Coelenterata, Bryozoa, Annelida, and Mollusca) show a relationship between the calcite/aragonite ratio of their skeletal structures and the temperature of the water in which they are secreted; aragonite skeletons suggest warm water, whereas calcite skeletons-cold water. Because of the effect of temperature on the polymorphic form of calcium carbonate secreted by organisms, this effect also controls the strontiumlcalcium ratios in these organisms. This is principally due to the fact that strontium is concentrated in aragonite skeletons and calcium and magnesium in calcitic structures. DODD(1963) has shown a lack of dependence of Sr content on temperature for young individuals of the pelecypod Mytilus culiforniunus, which exhibits differences in physiology at different growth stages. LOWENSTAM (1964) in a study of the strontium/calcium ratio of the marine biota from Palau, South Pacific, where temperature and strontium/calcium ratio of the waters are constant throughout the year, demonstrated that crystal chemistry plays the primary role in controlling the strontium/calcium ratio. He also showed that the physiologic control is a second order effect controlling the strontium/calcium ratio. CHAVEet al. (1962) demonstrated solubility relationships between aragonite, high-magnesium calcite, and low-magnesium calcite. They determined the following order of decreasing solubility: high-magnesium calcite > aragonite > low-magnesium calcite. Chave and co-workers suggested that because of these solubility relationships high-magnesium calcite and aragonite may be dissolving in the water mass or on the sea floor in many areas, leaving only the more stable polymorph (low-magnesium calcite) as the predominant mineral. JANSEN and KITANO (1963), however, reported a decrease in the solubility of magnesium carbonate in MgC12 solutions. PILKEY and GOODELL (1964) have emphasized the importance of using caution, particularly with reference to recrystallization, in attempting to utilize chemical composition of carbonate skeletons as geochemical and paleoecologic indices. They have stressed the importance of ion migration and stability of ions in skeletons of carbonate-secreting organisms. LOWENSTAM (1963) reviewed the chemical composition of sediments and its relationship to the mineralogic com-

40

W. H. TAFT

position. He confirmed that temperature and chemistry of sea water in which the organisms live affect the aragonite/calcite ratio of the skeletons secreted, and in turn the magnesium and strontium contents as well as the l 8 0 / 1 6 0 ratio of these carbonates. (For details see WOLFet al., 1967.) One of the most significant results of modern carbonate studies in the last 10 years has been the discovery that dolomite in significant quantities is forming in modern supratidal carbonate environments. These supratidal environments are closely associated with evaporitic environments, and should help to further our understanding of ancient deposits that may have formed in a similar manner. Reports of dolomite formation in modern sediments include those by ALDERMAN (1959), ZEN (1959), MILLER (1961), TAFT (1961), DEFFEYES and MARTIN(1962), WELLS(1962), CURTISet al. (1963), SKINNER (1963), ILLING(1964), LUCIAet al. (1964), and SHINNand GINSBURG (1964).

RELATIONSHIP BETWEEN MINERAL AND CHEMICAL COMPOSITION, AND SEDIMENT GRAIN SIZE

CHAVE(1954b) reviewed chemical analyses of modern carbonates by VAUGHAN

(19 18), BRAMLETTE (1 926), and GOLDMAN (1926); he also recalculated and deter-

mined the chemical composition of carbonate sediments from Australia, the Bahamas, and Pago-Pago. The techniques used by all writers were quite similar. The sediments were separated into biological groups under microscope, weighed, and chemical composition assigned to them from data by CLARKE and WHEELER (1922). The chemical composition thus calculated was then compared with the actual chemical analysis. The results of analyses by Bramlette and Vaughan show fairly close agreement between calculated and determined chemistry, whereas Goldman’s results suggest a loss of magnesium to sea water, especially in the finersize fractions. To test Goldman’s results, CHAVE(1954b, p.596) compared the chemistry of eight mechanically analyzed carbonate bottom samples collected from Alaska, Maine, Florida, Bermuda, and Palau. Based on these analyses, Chave concluded that there was no evidence for exchange of magnesium ions between the sediment and sea water for the samples tested. TAFTand HARBAUGH (1964) reported (1) chemical data for size-sorted sediment samples from Key West, Florida Bay, and the Ten Thousand Island areas in Florida, (2) chemical and mineralogical data for sized samples from the Bahamas, and (3) mineralogical data for two samples from Baja California, Mexico. These samples were analyzed to test for gain or loss of magnesium relative to calcium and variations in proportions of high-magnesium calcite in different grade sizes. Any systematic change of chemistry with grain size might be useful in explaining variations of calcium/magnesium ratios as a function of sediment sorting, or susceptibilityof metastable carbonates to recrystallize to the stable mineral calcite.

MODERN CARBONATE SEDIMENTS

41

Results of these analyses suggest a progressive decrease of the calcium/magnesium ratio with decreasing grain size in Florida sediments. Samples from the Bahamas show that the coarse fraction (> 0.062 mm) tends to have the highest proportion of high-magnesium calcite and the lowest Ca/Mg ratio, whereas the finest fraction ( < 0.001 mm) has the highest percentage of aragonite and the lowest percentage of high-magnesium calcite, and tends also to have the highest Ca/Mg ratio. Baja California samples, on the other hand, show that the proportion of high-magnesium calcite tends generally to increase as the grain size decreases. These analyses suggest that local conditions of biogenic productivity and susceptibility to mechanical breakdown play a more important role in controlling chemistry and mineralogy of fine-grained carbonate particles than do loss or gain of magnesium after burial of the sediments.

ISOTOPIC COMPOSITION

The relative abundance in sea water of the two stable isotopes of carbon (13C and W)and two of the three stable isotopes of oxygen (lSOand160) has been shown to be fairly constant in open-ocean environments. Nearshore environments, on the other hand, ai-e known to be deficient in I3C relative to I2C and in 1 8 0 relative to 1 6 0 , as compared to mean ocean water. These differences are attributed to: ( I ) higher degree of volatility of 12C and 1 6 0 during evaporation, and (2) dilution by freshwater runoff deficient in 1 8 0 and enriched in IZC. UREYet al. (1951) concluded that l 8 0 / 1 6 0 ratios of carbonates are temperature dependent and are independent of species biochemistry or carbonate mineralogy. Marine carbonates precipitated in equilibrium with the surrounding sea water, therefore, should have carbon and oxygen isotope ratios diagnostic of their depositional environments. Unfortunately, like most other geologic tools, a number of problems arise on attempting to use oxygen isotope ratios, which include the following: ( I ) Although it is fairly well established that carbonates are precipitated by organisms in equilibrium with the surrounding sea water, one must establish that neither recrystallization nor diffusion has altered initial isotope ratios. (2) Temperature at which calcium carbonate is precipitated by carbonatesecreting organisms may be appreciably different from the average yearly temperature of the depositional environment. The precise growth-time temperature is significant. EPSTEIN and LOWENSTAM (1953) demonstrated substantial oxygenisotope ratio variations in different carbonate growth increments of some species of Bermuda mollusks. Some species precipitate CaC03 year-round, whereas others precipitate CaC03 selectively during certain periods of the year. (3) Tolerance of organisms to unfavorable environmental changes, which may either force an animal to migrate to a more favorable environment or, if not

42

W. H. TAFT

mobile, may cause its death, must be considered. Obviously, the mere presence of an organism in sediments is no indication that isotopic ratios of its shell provide absolute indication of temperature conditions of the depositional environment. Organisms, permanently attached to the sea bottom and possessing only slight tolerances to changes in chemical and physical environment, provide ideal working material for determination of isotopic composition in order to arrive at the temperature of depositional environment. Although there are a number of problems associated with interpreting stable isotope ratios of carbonate skeletons, nevertheless this technique is in its infancy and already a number of worth-while applications have been found. For example, LOWENSTAM and EPSTEIN (1957) studied the origin of aragonite needles on the Great Bahama Bank and concluded that the isotopic composition of these needles would best be explained as having originated by organic precipitation. This interpretation, however, was challenged by CLOUD(1962, p.97). GROSS(1 964) compared ls0/16O and I 3 C / l 2 C ratios of diagenetically altered limestone in the Bermuda islands with those of modern sediments in the surrounding marine environment, and concluded that alteration occurred in the freshwater zone. TAFTand HARBAUGH (1964) correlated the marl underlying brackishwater sediments in Whitewater Bay, Florida, with that presently accumulating in the Everglade swamps on the basis of their mineralogy and enrichment in 13C. Isotope ratios are being used as indicators of paleotemperatures (EMILIANI and MAYEDA, 1961) and although some controversy surrounds interpretation of analytical results, judicious application of stable isotopes appears to be a powerful tool for future use.

DIAGENESIS OF CARBONATE SEDIMENTS

Physical and chemical changes are taking place in modern carbonate depositional environments. Diagenesis of carbonates is generally more rapid but may continue longer than is the case with most other sediment types. GINSBURG (1957) called attention to some of these processes in Florida Bay sediments. He noted the following: ( I ) Presence of abundant fecal pellets at the water-sediment interface, but loss of their individuality a few feet below the surface. (2) Importance of organisms in reducing particle size by chemical and physical processes. (3) Reworking of sediments by organisms that bring buried sediments to the surface and obliterate depositional features. (4) Activity of Bacteria in producing hydrogen sulfide, oxidation of organic matter, conversion of organic nitrogen to ammonia, and precipitation of iron. (5) Marked contrasts in p H between the sediments and the overlying sea

MODERN CARBONATE SEDIMENTS

43

Fig.8. Underwater photograph of a 36-inch long vertical exposure of modern carbonates on Yellow Bank, Bahamas. Overlying water is 17 ft. deep. Note large blocks in cliff face and near base of photograph. Ruler rests on apparent horizontal layer 36 inches below water-sediment interface (used for 14C dating). Probe is 4 ft. long and extends 3 ft. into the underlying sediments. Tube is a device for removing fines during excavation.

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water are common. In general, sediment p H is at least one full p H unit lower than that of sea water. (6) Compaction, as measured by loss of moisture content, is most prevalent in the first 6-12 inches below the water-sediment interface. (7) No conversion of metastable carbonates to stable forms is evident; lithification is also not noticeable, except in the intertidal zones where alternate wetting and drying allows beach sands to be cemented by aragonite. An excellent petrographic study of modern carbonates by PURDY(1963a,b) demonstrates recrystallization of metastable carbonates on the Great Bahama bank. Recrystallization, however, does not give rise to a more stable mineral species, but rather results in a change from a micro- to a cryptocrystalline variety of the same metastable mineral. According to PURDY(1963a,p.348), “there can be no doubt that cryptocrystalline grains are a recrystallization product, for all stages were observed in the transition of various non-skeletal and skeletal constituents to cryptocrystalline carbonate”. Since GINSBURG’S (1957) excellent paper, a number of papers bearing on the problem of diagenesis in modern carbonate sediments have appeared (SIEGEL, 1960, 1961; STEHLIand HOWER,1961; CLOUD,1962; INCERSON 1962; PURDY, 1963a,b; PILKEY, 1964; TAFTand HARBAUGH, 1964). In general, these authors have shown that modern, warm-, and shallow-water carbonate sediments are composed predominately of aragonite and high-magnesium calcite. There is little or no evidence to suggest either recrystallization to more stable polymorphs since particles were deposited, or lithification of sediments that have never been exposed to fresh water, other than infilling of tests by aragonite and formation of grapestone lumps (ILLING,1954) and flakes (PURDY,1963a, p.348). A notable exception to lack of lithification in modern carbonates is presently being investigated on Yellow Bank, Bahamas, by the author. Fig.8 is a 36-inch vertical exposure of modern carbonates on Yellow Bank in 17 ft. of water. Large blocks (up to 1 cubic ft.) of partially lithified, nonstratified carbonate material underlie a thin veneer of grapestone sediment. These blocks are held together by worm tubes, encrusting Foraminifera such as Homotrema rubrum (LAMARK), and by aragonite (Fig.9). Comparison of mineralogic composition of surface blocks (Table I) and unconsolidated surface samples with the horizontal layer 36 inches below the watersediment interface, however, suggests that these samples have not been exposed to fresh water and have not recrystallized to more stable carbonate minerals.

PROBLEMS

At least three problems continue to plague investigators of modern carbonates:

MODERN CARBONATE SEDIMENTS

45

M

3

.-

f

C L

TABLE I CARBONATE MINERAL SAMPLES AND RADIOCARBON AGE DETERMINATIONS FROM YELLOW BANK, BAHAMAS

Sample number

64-81 64-81 64-81 64-81 64-81 64-8 1 64-8 1 64-8 1

Depth beneath

Mineral fractions (%)

sediment-water interface (inches)

aragonite

IoW-Mg calcite

high-Mg calcite

surface 1 4 10

61 67 64 65 86 19 94 98

trace 1 trace 1 trace 1 trace

33 32 35 34

20

23 32 36

1

14 20 6 1

Mol. % Mgco3 in high-Mg calcite

Type of sample

15 17 15 16 15 17 14 13

loose sediment rock rock rock loose sediment rock rock rock

Age (years before present)

2,411 & 120

4,810 i 130

MODERN CARBONATE SEDIMENTS

47

( 1 ) There is no known continuous shallow-water sediment record from Recent through the Pleistocene to Pliocene sediments. (2) Widespread recrystallization of unexposed Holocene metastable carbonates to more stable forms has not been found. (3) There is a general absence of lithification in modern (Holocene) carbonate sediments. It seems improbable that a continuous record of shallow-water carbonates, uninterrupted by exposure during the Pleistocene, will ever be located. Owing to eustatic oscillations, the neritic zone boundaries have migrated 50-500 km across continental shelves and back again, resulting in a widespread erosion and displacement of facies. Lack of a continuous record of shallow-water carbonates possibly could account for the apparent absence of widespread lithification of modern carbonate sediments. It is puzzling that depositional environments with 8-10 ft. of modern sediments and 6-10 ft. of overlying water show no evidence of cementation. Inasmuch as most modern carbonates are less than 6,000 years old, possibly this is an insufficient time for the process of cementation to take place. Degree of recrystallization of metastable carbonates to stable forms is probably a function of the magnesium concentration in interstitial waters (TAFT, 1967), and time necessary for solid state recrystallization.

ACKNOWLEDGEMENTS

The lithification study on Yellow Bank, Bahamas, was sponsored by Grant No GP 2527 from the National Science Foundation. (Fig.2-6 are reprinted from 1964, with the permission of the publishers, Stanford TAFTand HARBAUGH, University School of Earth Sciences, 0 1964 by the Board of Trustees of the Leland Stanford Junior University.)

REFERENCES

ALDERMAN, A. R., 1959. Aspects of carbonate sedimentation. J. Geol. SOC.Australia, 6: 1-10. F. W., 1958. Ancient sediments of Bahaman type. Bull. Am. Assoc. Petrol. Geologists, BEALES, 42: 1845-1880. BRAMLETTE, M. N., 1926. Some marine bottom samples fromPago Pago Harbor, Samoa. Carnegie Inst. Wash., Papers Dept. Marine Biol.,23: 1-35. BUSBY,R. F., 1962. Submarine geology of the Tongue of the Ocean, Bahamas. U.S. Naval Oceanog. Ofice, Tech. Rept., 108: 1-84. CHAVE, K. E., 1954a. Aspects of biogeochemistry of magnesium. 1. Calcareous marine organisms. J. Geol., 62: 266-283. K. E., 1954b. Aspects of the biochemistry of magnesium. 2. Calcareous sediments and CHAVE, rocks. J. Geol., 62: 587-599.

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CHAVE,K. E., 1962a. Factors influencing the mineralogy of carbonate sediments. Limnol. Oceanog., 7: 218-223. CHAVE, K. E., 1962b. Processes of carbonate sedimentation. In: N. MARSHALL (Editor), Symposium on the Environmental Chemistry of Marine Sediments. Narragansett Marine Lab., Kingston, R. I., pp.77-85. K. S., WEYL,P. K., GARRELS, R. M. and THOMPSON, M. E., 1962. CHAVE,K. E., DEFFEYES, Observation on the solubility of skeletal carbonates in aqueous solutions. Science, 137: 33-34. CHILINGAR, G. V., 1960. Ca/Mg ratio of calcareous sediments as a function of depth and distance from shore. Compass, 37(3): 182-186. CHILINGAR, G. V., 1962. Dependence on temperature of Ca/Mg ratio of skeletal structures of organisms and direct chemical precipitates out of sea water. Bull. Southern Calif: Acad. Sci., 61(1): 45-60. CLARKE,F. W. and WHEELER, W. C., 1922. The inorganic constituents of marine invertebrates. U.S., Geol. Surv., Profess. Papers, 124: 1-62. CLOUD,JR., P. E., 1962. Environment of calcium carbonate deposition west of Andros Island, Bahamas. U S . , Geol. Surv., Profess. Papers, 350: 1-138. CURTIS,R., EVANS,G., KINSMAN, D. J. J. and SHEARMAN, D. J., 1963. Association of dolomite and anhydrite in the Recent sediments of the Persian Gulf. Nature, 197: 679-680. DAETWYLER, C. C. and KIDWELL, A. L., 1959. The Gulf of Batabano, a modern carbonate basin. World Petrol. Congr., Proc., 5th, N. Y., 1959, Sect.& pp.1-21. DEFFEYES, K. S. and MARTIN,E. L., 1962. Absence of carbon-14 activity in dolomite from Florida Bay. Science, 136: 782. DODD,J. R., 1963. Paleoecological implications of shell mineralogy in two pelecypod species. J. Geol., 71: 1-11. EMILIANI, C. and MAYEDA,T., 1961. Carbonate and oxygen isotopic analysis of core 241 A. J. Geol., 69: 729-732. EPSTEIN,S. and LOWENSTAM, H. A., 1953. Temperature-shell growth relations of recent and interglacial Pleistocene shoal water biota from Bermuda. J. Geol., 61 : 424438. GINSBURG, R. N., 1956.Environmental relationships of grain size and constituent particles in some south Florida carbonate sediments. Bull. Am. Assoc. Pelrol. Geologists, 40: 2384-2427. GINSBURG, R. N., 1957. Early diagenesis and lithification of shallow-water carbonate sediments in southern Florida. In: R. J. Le BLANCand J. G. BREEDING (Editors), Regional Aspects of Carbonate Deposition-Soc. Econ. Paleontologists Mineralogists, Spec. Publ., 5: 80-100. GOLDMAN, M. I., 1926. Proportions of detrital organic calcareous constituents and their chemical alteration in a reef sand from the Bahamas. Carnegie Znst. Wash. Publ., 344: 39-65. GRAF,D. L., 1960. Geochemistry of carbonate sediments and sedimentary carbonate rocks, Part I, carbonate mineralogy, carbonate sediments. Illinois State Geol. Surv., Circ., 297: 1-39. GROSS,M. G., 1964. Variations in the 180/160 and 13C/1zCratios of diagenetically altered limestones in the Bermuda Islands. J. Geol., 72: 170-194. ILLING,L. V., 1954. Bahaman calcareous sands. Bull. Am. Assoc. Petrol. Geologists, 38: 1-95. ILLING,L. V., 1964. Penecontemporary dolomite in the Persian Gulf. Bull. Am. Assoc. Petrol. Geologists, 48: 532-533. IMBRIE,J. and PURDY, E. G., 1962. Classification of modern Bahaman carbonate sediments. In: Classification of Carbonate Rocks-Am. Assoc. Petrol. Geologists, Mem., I : 253-272. INGERSON, E., 1962. Problems of the geochemistry of sedimentary carbonate rocks. Geochim. Cosmochim. Acta., 26: 815-847. JANSEN, J. F. and KITANO,Y.,1963. The resistance of Recent marine carbonate sedinients to solution. J . Oceanog. Soc. Japan, 18: 42-52. KRINSLEY, D., 1959. Manganese in modern and fossil gastropod shells. Nature, 183: 770-772. KRINSLEY, D., 1960a. Magnesium, strontium and aragonite in the shells of certain littoral gastropods. J. Paleontol., 34: 744-755. KRINSLEY, D., 1960b. Trace elements in the tests of planktonic foraminifera. Micropaleontology, 6: 297-300. KRINSLEY, D. and BIERI,R., 1959. Changes in the chemical composition of pteropod shells after deposition on the sea floor. J. Paleontol., 33: 682-684.

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LOWENSTAM, H. A., 1954a. Environmental relations of modification compositions of certain carbonate secreting marine invertebrates. Proc. Natl. Acad. Sci. US.,40: 3948. LOWENSTAM, H. A., 1954b. Factors affecting the aragonite/calcite ratios in carbonate-secreting organisms. J . Geol., 62: 284-322. LOWENSTAM, H. A., 1961. Mineralogy, 180/160 ratios, and strontium and magnesium contents of recent and fossil brachiopods and their bearing on the history of the oceans. J. Geol., 69: 241-260. LOWENSTAM, H. A., 1963. Biologic problems relating to the composition and diagenesis of sediments. In: T. W. D ~ N N E L(Editor), LY The Earth Sciences: Problems and Progress in Current Research. Univ. Chicago Press, Chicago, Ill., pp.137-195. LOWENSTAM, H. A., 1964. Sr/Ca ratio of skeletal aragonites from the Recent marine biota at Palau and from fossil gastropods. In: H. CRAIG,S. L. MILLERand G. J. WASSERBURG (Editors), Zsotopic and Cosmic Chemistry. North-Holland Publ. Co., Amsterdam, pp.114132. LOWENSTAM, H. A. and EPSTEIN,S., 1957. On the origin of sedimentary aragonite needles of the Great Bahama Bank. J. Geol., 65: 364-375. LUCIA,F. J., WEYL,P. K. and DEFFEYES, K. S., 1964. Dolomitization of Recent and Plio-Pleistocene sediments by marine evaporite waters on Bonaire, Netherlands Antilles. Bull. Am. Assoc. Petrol. Geologists, 48: 535-536. MAXWELL, W. G. H., JELL,J. S. and MCKELLAR, R. G., 1964. Differentiation of carbonate sediments in the Heron Island Reef. J. Sediment. Petrol., 34: 294308. MILLERJR., D. N., 1961. Early diagenetic dolomite associated with salt extraction process, Inagua, Bahamas. J. Sediment. Petrol., 31 :473476. NEWELL, N. D., IMBRIE, J., PURDY, E. G. and THURBER, D. L., 1959. Organism communities and bottom facies. Great Bahama Bank. Bull. Am. Museum Nut. HiJt., 97: 58-69. NOLL,W., 1934. Geochemie des Strontium, mit Bemerkungen zur Geochemie des Barium. Chem. Erde, 8: 507-560. PILKEY,0. H., 1964. Mineralogy of the fine fraction in certain carbonate cores. Bull. Marine Sci. Gulf Caribbean, 14: 126-139. PILKEY,0. H. and GOODELL, H. G., 1963. Trace elements in Recent mollusk shells. Limnol. Oceanog., 8: 137-148. PILKEY,0. H. and GOODELL, H. G., 1964. Comparison of the composition of fossil and Recent mollusk shells. Bull. Geol. Soc. Am., 75: 217-228. PILKEY,0. H. and HOWER,J., 1960. The effect of environment on the concentration of skeletal magnesium and strontium in Dendraster. J. Geol., 68: 203-214. PURDY,E. G., 1963a. Recent calcium carbonate facies of the Great Bahama Bank. 1. Petrography and reaction groups. J. Geol., 71: 334-355. PURDY,E. G., 1963b. Recent calcium carbonate facies of the Great Bahama Bank. 2. Sedimentary facies. J. Geol., 71: 472497. REVELLE, R. and FAIRBRIDGE, R. W., 1957. Carbonates and carbon dioxide. In: J. W. HEDGPETH (Editor), Treatise on Marine Ecology and Paleoecology. I . Ecology-Geol. SOC.Am., Mem., 67: 239-295. REVELLE, R. and FLEMING, R. F., 1934. The solubility product constant of calcium carbonate in sea water. Proc. Pacific Sci. Congr. Pacific Sci. Assoc., 5th, Victoria, Vancouver, 1933, 3: 2089-2092. RODGERS, J., 1957. The distribution of marine carbonate sediments; a review. In: R. J. LE BLANC and J. G. BREEDING (Editors), Regional Aspects of Carbonate Deposition-Soc. Econ. Paleontologists Mineralogists, Spec. Publ., 5 : 2-1 3. RUSNAK, G. A. and NESTEROFF, W. D., 1964. Modern turbidites: terrigenous abyssal plain versus bioclastic basin. In: R. L. MILLER(Editor), Papers in Marine Geology-Shepard Commemorative Volume. Macmillan, New York, N.Y., pp.488-507. SHINN,E. A. and GINSBURG, R. N., 1964. Formation of Recent dolomite in Florida and the Bahamas. Bull. Am. Assoc. Petrol. Geologists, 48: 547. SIEGEL,F. R., 1960. The effect of strontium in the aragonitexalcite ratios of Pleistocene corals. J. Sediment. Petrol., 30: 297-304.

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SIEGEL, F. R., 1961. Variations of Sr/Ca ratios and Mg contents in Recent carbonate sediments of the northern Florida Keys. J. Sediment. Petrol., 31: 336-342. SKINNER, H. C. W., 1963. Precipitation of calcian dolomites and niagnesian calcites in the southeast of south Australia. Am. J. Sci., 162.: 449472. STEHLI,F. G. and HOWER, J., 1961. Mineralogy and early diagenesis of carbonate sediments. J. Sediment. Petrol,, 31; 358-371. TAFT,W. H., 1961. Authigenic dolomite in modem carbonate sediments along the southern coast of Florida. Science, 134: 561-562. TAFT,W. H., 1967. Physical chemistry of formation of carbonates. In: G. V. CHILINGAR, H. J. BIssELLand R. W. F A I R B R I D G E ( E ~Carbonate ~ ~ ~ ~ ~ )Rocks. , Elsevier, Amsterdam, B: 151-167. TAFT,W. H. and HARBAUGH, J. W., 1964. Modern carbonate sediments of southern Florida, Bahamas, and Espiritu Santo Island, Baja California: a comparison of their mineralogy and chemistry. Stanford Univ. Publ., Univ. Ser., Geol. Sci., 8(2): 1-133. UREY,H. C., LOWENSTAM, H.A., EPSTEIN,S. and MCKINNEY,C. R., 1951. Measurement of paleotemperatures and temperatures of the Upper Cretaceous of England, Denmark and the southeastern United States. Bull. Geol. Soc. Am., 62: 399416. VAUGHAN, T. W., 1918. Some shoal-water bottom samples from Murray Island, Australia, and comparisons of them with samples from Florida and the Bahamas. Carnegie Znst. Wash. Publ., 213: 235-288. WELLS,A. J., 1962. Recent dolomite in the Persian Gulf. Nature, 194: 274-275. WOLF,K. H., CHILINGAR, G. V. and BEALES, F. W., 1967. Elemental composition of carbonate H. J. BISSELL and R. W. FAIRskeletons, minerals and sediments. In: G. V. CHILINGAR, BRIDGE (Editors), Carbonate Rocks. Elsevier, Amsterdam, B: 23-149. ZFN, E-AN, 1959. Mineralogy and petrography of marine bottom sediment samples o f fthe coast of Peru and Chile. J . Sediment. Petrol., 29: 513-539.

Chapter 3

PETROLOGY AND PETROGRAPHY OF CARBONATE ROCKS YVONNE GUBLERl, J. P. BERTRANDI, L. MATTAVELL12, A. RIZZINI’ AND R. PASSEGA3

French Petroleum Institute, Rueil-Malmaison (France) Geologist, AGIP, Direzione Mineraria, San Donato, Milan (Italy) Consultant, San Donato, Milan (Italy)

SUMMARY

This paper is basically a synthesis and literature review of the major features of petrologic and petrographic knowledge on carbonate rocks. The authors have devoted special attention to the construction of a petrologic model based on the concept of “facies” as an integration of qualitative and quantitative data situated at different scalar levels.

INTRODUCTION

General

Sedimentary petrology and petrography are two divisions of lithostratigraphy that cover this fundamental branch of stratigraphy. Petrology commonly deals with a rock as a geometric body of varying constitution, modified by sedimentological features such as stratification, orientation, alteration (diagenesis) and resedimentation. Petrography deals with detailed particulars, e.g., constituents and texture. Both disciplines proceed essentially from descriptive science, but are situated at different scalar levels of organization of the rock (GLANGEAUD, 1962). Thus, sedimentary petrology is based on numerous field investigations, regional and local studies, that lead to a more scientific and useful synthesis during petro1961), that is to say, in the laboratory, graphic studies (BISSELLand CHILINGAR, largely by means of the petrographic microscope, and other aids which enable detailed grain-by-grain examination. The final goal is to attempt to construct a petrologic description and explanation from the progressive differentiation of the relationships of all the data collected; the validity of such a petrographic model depends on the techniques employed and the quantitative significance of the data. The usefulness of multidimensional or stereographic diagrams has been pointed out (KRUMBEIN and SLOSS,1951) in drawing up multiple-paleogeographic

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isopach maps at different scales, as an approach to lithologic models. Most of these maps take into account only a limited number of parameters. As new physical methods are developed, other than the optical ones ordinarily usedin routine work, more and more variables are recorded and quantified; this introduces more selective parameters for identifying a rock. The petrologic model must be worked out in grouping all sets of data, for both field and laboratory investigations. In the case of carbonate rocks, IMBRIEand PURDY(1961) have proposed a simple statistical scheme for identifying discrete sample groupings in a set of data that may be gathered partly in the field and partly in the laboratory. This information is treated by representative vectors; equal weight is given to various properties. Carbonate rocks

Carbonate rocks are a polygenetic group, some of which are true clastics, whereas others are chemical or biochemical precipitates. Most of them are a mixture of clastic and chemical carbonates whose interrelationship is an important parameter of their diagenesis. The major mineral components are calcite, aragonite, dolomite and, less commonly, siderite; the minor constituents are non-carbonate and include clays, quartz, and phosphate, oxide and sulfide minerals. The three prominent and specific features of the group comprise: ( I ) organic content, which can be considered as primary genetic; (2) the crystallinity, either crypto- or diagenetic (SHVETSOV, 1958), or other textural forms of the carbonate particles such as pellets or oolites; and (3) geographic distribution in the past. (I) In regard to the primary petrogenesis, either in marine or lacustrine environments, organisms are providers of carbonates, because they are able to modify, directly or indirectly, the partial pressure of carbon dioxide. Algae, by photosynthesis, are the most important community associated with carbonate precipitation (WOLF,1962). Thus the relationships between organic populations and their distribution (paleoecology) have to be considered as an important set of data in the petrologic model. The organisms fix Ca2+ in the shell, and contribute to the elaboration of stromatolitic or other structures, or they may be represented by transported material (NEWELL and RIGBY,1957; TAFT,1962). (2) Because of the metastability of its crystalline structure, high-magnesium calcite is converted into stable dolomite, and aragonite is transformed into calcite. These transformations can take place very shortly after the sediments are buried. The original structures of carbonate rocks, both mineralogical and organic, thus often become obscure. How, when and where this diagenesis takes place is one of the most difficult problems of the petrology of carbonate rocks. Most dolomites seem to have originated this way (FAIRBRIDGE, 1957; DE CHARPAL et al., 1959). The problem of diagenesis involves the crystalline “sparite” which is considered to be depositional by FOLK(1962), and to be the product of recrystalli-

PETROLOGY AND PETROGRAPHY OF CARBONATE ROCKS

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zation by BATHURST (1959) and POWERS (1962). Time of crystallization is largely dependent upon the movements of fluid through the rock. (3) The most common carbonate rocks in marine and lacustrine deposits are limestones and dolomites which constitute about 14 % of the sedimentary rocks in the geological column (KUENEN,1941). The problem of their distribution in Recent sediments has been outlined by RODGERS(1957) and in the ancient sediments by FAIRBRIDGE (1961, 1964). In Recent sediments, carbonate oozes are located on the bottom of oceans at depths of less than 3,000 fathoms at low latitudes. Clayey carbonates are known to exist in deeper water or close to coasts in places where amounts of terrigenous deposits are high (ARRHENIUS, 1952). Organic reefs between N 30” and S 30” latitude constitute large carbonate-shelf deposits associated with communities of colonial organisms. Through geologic time the distribution of carbonate rocks has been in large measure dependent upon the evolution of lime-fixing organisms. In the Early Paleozoic they are most often associated with benthonic shelly fossils, characteristic of rather shallow water; whereas in the Cretaceous, pelagic oozes characteristic of greater depths appear with the explosive evolution of pelagic Foraminifera. Many hundreds of publications deal with problems of carbonates, either in Recent sediments or ancient deposits, generally treating one or another of the afore-mentioned three problems. Very few deal with the whole subject. Mostly because “the world over they are familiar host rocks for oil, water and ore bodies, providers of economy” (CLOUD,1962), special attention is devoted to reef environments. The latter contain 90 % calcarenite (NEWELLand RIGBY,1957), which originally had many of the same characteristics as quartzose sandstones, i.e., high porosity and varying degrees of sorting. A brief report such as this chapter on the petrography and petrology of carbonate rocks can be expected to outline only the general fields of research with emphasis on recent work and current ideas, as well as to mention some of the important unsolved problems and to suggest promising directions for the solution of some of them. OBJECTIVES

Establishment of a lithological record As already seen, sedimentary petrology and petrography are complementary. This was the conclusion of CAYEUX(1935, p. 111) in his memoir on “Carbonate Rocks” when he wrote: “. . .j’ai fix6 leurs caract&resminkralogiques, organiques et chimiques, en quoi j’ai fait oeuvre de pktrographe, j’ai donnk A l’analyse de plusieurs roches un diveloppement qui ne cadre nullement avec le r6le qu’elles jouent dans la nature. . ”,

.

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Indeed, the integration of the many megascopic variables contributing to the development of a carbonate rock is part of the original account of the concept of facies expressed in these terms as early as 1838, by Gressly (in: WEGMANN, 1963) with regard to the carbonate rocks of the Swiss Jura: . . . “Le faciks . . . concerne I’aspect pktrographique et l’aspect palBontologique d’une formation . . Une description pttrographique qui traitera de la composition mineralogique et des caractkres pktrographiques de nos roches tels que la structure, la cassure, les couleurs, le ciment, la piite, etc. . . Ces caractkres . . . n’en sont pas moins d’une haute importance dans la dCtermination des divers faciks que prksentent une formation entikre, un groupe, un terrain, ou m i k e une simple couche. . . Une description gtognostique qui traitera des phknombnes de nos roches dans leur ensemble, tels que leur stratification, leur puissance, etc. . . Ces phknomknes indiquent d’une manihe B peu prks constante les divers faciks. La palBontologie nous offrira les principaux caractkres, les grandes divisions des faciks”. The first description deals more with granular feature, the second with the gross characteristics of the rocks that ought to be precisely described and grouped for the purpose of constructing the petrologic model. Through Gressly’s definitions the term “facies” is used on different scales, including in particular the lithofacies.

Lithofacies Lithofacies may be defined in terms of the extent of variations of micro- and mega-characteristics of the particular features introduced during the origin, and other properties of the rock considered as a whole. It reflects a succession of homogeneous or heterogeneous primary deposits (Fig.1) modified by diagenesis during the compaction stage and very often also afterwards. Dirnensional fabrics Microfacies Each primary deposit is the product of the hydrochemical, hydrodynamic and thermodynamic factors and organic materials of the particular environment; these factors and components also control the speed of deposition. A “primary microfacies” can be defined as consisting of laminae, thickness of which is no greater than the largest diameter of the coarsest grain (DOEGLAS, 1962). It seems necessary to point out the geometrical characteristics because the term “microfacies” is used and defined in different ways: ( I ) For the purpose of stratigraphic dating by correlation of the qualitative elements observed in thin sections, paying special attention to the organisms (CWILLIER,1951, 1952; FAIRBRIDGE, 1954). (2) From a genetic point of view, CAROZZI (1950, 1958,1959, 1961) proposed 1

“Lithofacies” and “biofacies” (Editors).

PETROLOGY AND PETROGRAPHY OF CARBONATE ROCKS

55

this term to express the vertical variations of minute elements (minerals or microorganisms) that reflect successions of primary microfacies. For the petrologic model, the term “primary microfacies” must be reserved for the observations made in sections parallel to the plane of deposition, whether it is flat or undulating. Thus, this term is applied to an oriented section in reference to a geometrical plane in which, in the case of clastic limestones, preferred orientation can be measured by means of petrofabrics (SANDER, 1936). An analysis of the primary microfacies situated in the plane of stratification reveals the hydrodynamic and organic processes responsible for the nature, size, shape, sorting and orientation of the particles. All this can be quantified and treated statistically. The microfacies at the time of deposition is defined by the ratios of its particular components, as seen in Plate IE. Microsequences The vertical succession is a natural sequence that records all the variations during deposition of time fractions, including the period of any diastem (non-deposition; LOMBARD, 1956) or alteration. It coincides with the succession of geometrical planes which reflects the successivevariations of microfacies. These are visiblewhen achange appears in the microfacies, as shown in Fig.1, and can be observed in thin sections or peels perpendicular to the planes of deposition. In the case of deposition of clastics as cross-laminations, these thin sections enable directional measurements to be made of bottom currents (POTTER and PETTIJOHN, 1963). The microsequences, as shown by CAROZZI (1950, 1958,1961),reflectthe history of filling-in of the basin. Whether the thickness of the layer is a few centimeters or about 1 m, the term microsequence can be utilized to define a natural succession of microfacies constituting the layer. No matter what the scale may be, the problem is the same. The succession of microsequences in a geological sequence constitutes large units that may be rhythmic. This rhythmicity can be a function of the deformation or any major climatic changes registered in time in geochemical components (LOMBARD, 1956; STRAKHOV, 1957), granular components, or organic components (LECOMPTE, 1958). Only quantitative analysis of successions of both microfacies and microsequences (FONDEUR, 1964) enables the physical properties of original clastic carbonates to be defined, even if they have been largely obliterated by diagenesis. Diagenesis in carbonate rocks is the most complex problem. As is observed in Recent sediments, it commences immediately after deposition (GINSBURG, 1957; CLOUD,1962). Diagenesis can also appear much later during the history of a sediment and disturb the original composition and structure or grain texture (BATHURST,1958; POWERS, 1962); it can occur even after lithification of the rock (DECHARPAL et al., 1959). FAIRBRIDGE (1967) recognizes “syndiagenesis”, “anadiagenesis” and “epidiagenesis”. Diagenesis is produced by: ( I ) various organic processes (GINSBURG, 1957), the textural and structural aspects of which include

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aggregation, bioturbation, and particle-size reduction; (2) biochemical changes; and (3) physical processes such as compaction, orientation and deformation of crystals and grains (D’ALBISSIN, 1963). The physico-chemical agents include connate water, hydrothermal solutions and downward percolating vadose solutions (DECHARPAL et al., 1959). Diagenesis is accompanied by changes in the nature (authigenesis), size and shape of carbonate particles (epigenesis). Thus it modifies the original pattern of the rock and, therefore, the porosity and permeability (BATHURST, 1959; POWERS,1962). GARRELS et al. (1960) emphasized that reconstruction of diagenetic stages is possible only if the geological history of the area (tectonic history, time of faulting, etc.) is well known. Thus, a complete petrographic record of carbonate rocks must include considerations of all the important scalar phenomena. As an example, Fig.1 and Plate I show a heterogeneous succession of laminae (primary microfacies), each of which has its own characteristics (size, shape, particle and void distribution; in Plate IE the pores are intergranular). These characteristics can be evaluated under a microscope by using a point counter and an integrating Zeiss stage. Each succession of microsequences records the variations in these characteristics. For example, the sample presented in Fig.1, and which is made up of a series of microseyuences, shows a series of anisotropies that are due to an alternation of fine and coarse laminae. These, in turn, are linked to the physical properties of the rock (porosity, permeability, elasticity, etc.). It is known that anisotropy of a rock depends on the type of porosity (intergranular, vuggy, fissural, etc.) and, in the special case of carbonate rocks, on the proportions of matrix and allochems (Plate IE, C). The term “allochem” may be defined as the transported particles: pellets, skeletal detritus, oolites, etc. The relative contents of allochems and matrix are capable of causing considerable changes in the physical properties of a rock. It is, therefore, not without importance that all of the petrographic measurements be made at random on non-oriented samples. This is illustrated in Fig. 1 and Plate I where the components (organisms, oolites, pellets-see Plate IA, B, C) or depositional structures (Plate ID) are true reflection of the depositional environment. On the other hand, the sparite and dolomite between the allochems, of which the matrix is composed, reflect the diagenetic factors (Plate IE) which have modified the original features and properties of the deposit.

TRENDS AND METHODS

Scattered descriptions of carbonate rocks in texts dealing with sedimentary rocks date back to the 19th century. It was during the period between the two World Wars, however, that a comprehensive sedimentary petrographic encyclopedia came into being in France as a result of the research done by CAYEUX (1935), and a pet-

57

- .

Fig.1. Bioclastic limestone in the reef complex of Wolayersee, Carnia, Austria (Middle Devonian). General aspect of a composite layer having thickness of 23 cm. It comprises a succession of microsequences composed of calcarenite and calcilutite; the rock is fissured and scattered dolomite appears in sequences I-2 and 5. MI-MI’ corresponds to a section parallel to the plane of stratification, giving a detailed microfacies. Numbers I-5 correspond to a continuous succession of thin sections showing detailed structure of the rock.

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et al.

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59

rofabric system for carbonate rocks was introduced in Austria by SANDER (1930). CAYEUX (1935) made use of thousands of isolated field samples collected by many workers to prepare his treatise on carbonate rocks, drawing attention to the most varied types in nature and very wisely presenting many problems of their genesis without attempting to solve all of them. This monumental work, based on the microscopic and qualitative examination of rocks using thin sections, is a unique reference book. As Cayeux said himself, his work is confined to the science of petrography. On the basis of systematic field surveys in limited areas of the Alps, SANDER (1936) also using his own thin sections, was able to distinguish all the microstructural characteristics of particles helpful for explaining the original organization of the rock. He introduced methods of stereographic projection which enable the spatial relations of particles (petrofabric) to be determined quantitatively. Sander’s method enables the greatest number of diagnostic characteristics of a rock to be recorded for purposes of description and correlation. His aim was to show how a comprehensive petrographic survey could clarify and add to field data. In doing this, he opened up the path for modern sedimentary petrology. Since then, a considerable amount of research has been done, mainly stimulated by practical considerations of oil finding and oil-reservoir studies (CLOUD, 1962), as mentioned above. The writers have referred to a number of publications which suffice to indicate trends and improvements, some of which contribute to the development of the petrographic side and others to studies of the petrologic fabrics. Petrographic fabrics Particle identijication Organisms. Important qualitative and quantitative advances have been made during the last decade or so in the identification of microorganisms in thin sections (CUVILLIER, 1951, 1952), their frequency (CAROZZI, 1950), and in the idenPLATE I Bioclastic limestone in the reef complex of Wolayersee, Carnia, Austria (Middle Devonian). Thin sections A, B, C, D perpendicular to MI-MI’ (Fig.1). A. Thin section in the middle part of section 5 (Fig.1). It shows from bottom to top: coarse sparite probably filling a “stromatactis” structure, altered surface, and a thin layer of dolomite with structural reliefs (Algae). The upper part is altered too and is covered by a calcarenite made up of crinoidal detritus; x 10. B and C . Both thin sections are in the lower part of section 3 (Fig.1). One can see: difference in the amount of matrix between pellet and oolitic limestones, compaction and deformation of pellets by oolitic deposits (B and C ) and orientation of pellets around an original structure of organism (B); x 10. D. Shows details between sections 2 and 3: recrystallization of oriented calcite and secondary microfolding with stress joints filled with clay minerals: X 10. E. Parallel to MI-MI’ (Fig.1); is made up of allochems (skeletal detritus and pellets), sparite (dolomite or calcite), micrite and intergranular voids, which constitute a “primary microfacies”; x 10.

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tification of macroorganisms (NEWELL and RIGBY, 1957) or cryptoorganisms (BLACK,1956). This enables a better interpretation to be made of the paleogeography of a deposit.

Mineralparticles. Calcite, dolomite, magnesite, and ankerite are difficult to identify under the microscope without the help of more precise analytical methods, because their optical properties are very close. X-ray diffractometry gives only quantitative data but does not locate the mineral in the rock. The use of staining (1959), is very helpful. Radiomethods, such as those developed by FRIEDMAN graphic methods for identification and localization of dolomite have been described by SCHMITT (1962). Schmitt’s technique is most useful in distinguishing between different carbonates, especially in the case of dolomitic limestones. His method involves study of thin sections of dolomites (including those with variable Mg, Ca, and Fe contents) by contact microradiography, as it is possible to separate carbonates on the basis of their partition efficiency. The values of the mass coefficient of absorption for various carbonates were calculated from the tables for Xrays of V, and the values for Ca from Johnson’s universal function. In microradiography with the radiation K of V, a dolomitic zone appears darker than a calcitic one; the opposite is true with the radiation K of Ca (Fig.2). The contrast is even stronger for magnesite and calcite. The sample studied by SCHMITT (1962) was a dolomite traversed by a calcified fracture and with impurities (Fig.2). The microscopical examination either in natural (Fig.2A) or polarized light (Fig.2B) revealed no optical differences between the crystals. On the other hand, the microradiography with V and Ca showed the localization of calcite crystals, which appeared light-colored in an X-ray of V (Fig.2C), and dark-colored in an X-ray of Ca (Fig.2D). Thus one can observe two types of calcite: one filling up the vein and the other scattered through the rock as relics remaining in the dolomite. Structure identijication Structure identification deals with the orientation of particles and pore spaces with respect to reference planes (Sander’s method of petrofabrics). Pore shape and size can be studied in injecting colored synthetic resin (ETIENNE, 1963). This is of great importance in any study of the physical properties of rocks. In this chapter one should also consider the deformation of crystals which, after a certain degree of pressure, show some response to the stresses applied to them. The crystals become reoriented so that one of their optical axes assumes approximately the same direction as the greatest stress. D’ALBISSIN (1963) used physical methods to study this phenomenon in calcite and to explain the mechanism. In the scale either of an aggregate of calcite crystals or of single crystals, the effect of plastic deformation under great pressure produces intracrystalline slippage by distortion of the crystalline system. At the scale of the aggregate, there is also an

PETROLOGY AND PETROGRAPHY OF CARBONATE ROCKS

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Fig.2. Application of contact microradiography. Sample IFP A-604, Robion B.A.; x 30. A. Micrography in natural light. B. Micrography in polarized light. C . Microradiography with vanadium. D. Microradiography with calcium. (After SCHMITT, 1963.)

intergranular slippage, which is accompanied by other phenomena acting on the same scale, such as sliding and stylolitic joints. On either scale, solution and recrystallization phenomena can occur before, during or after deformation. This structural study opens up a new line of research on the physical properties of carbonate rocks. In analyzing the components of a rock, it is evident that microscopic examination is no longer sufficient and that new and more accurate physical methods have to be used to investigate the crystalline structure itself.

Interrelationship between rock coinponents After the components have been identified, it then becomes necessary to establish characteristics for each rock on the scale of thin sections. For clastic carbonate

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rocks, CAROZZI(1950, 1958, 1961) using 250 sq. mm thin sections has made statistical investigations of the microscopic components, based on the clasticity index, crystallinity and frequency of the detrital components (either organisms or minerals). The values computed were then plotted graphically as variables expressing a sequence of microfacies that he considered to be indicative of bathymetric changes in the environment. A similar approach was made by PASSEGA (1957, 1960) who considered that the hydrodynamic characteristics of the environment are reflected in the distribution of grain size as represented by two parameters: i.e., C, 1 percentile approach value of maximal diameter; and M , the median. The location of C and M on a logarithmic diagram is indicative of the type of current which prevailed during deposition. In this chapter the authors have not discussed either genetic or descriptive classifications which are dealt with elsewhere in this book; however, it is important to mention here the practical classification introduced by FOLK(1959) whose aim was to establish the spectrum of the textural types which reflect the physical energy of the depositional environments. It is based on the relationship between transported particles (allochems) and chemical particles (micrite). FOLK(1959) developed a carbonate terminology consisting of only a handful of labels for major rock types determined by descriptive and quantitative methods. This system provides a framework within which the sedimentary petrographer may introduce as many variables as necessary, based on field work, e.g., color, hardness, bedding, sedimentary structures, slumping, detailed paleontological data, trace elements, stromatolitic structures, birdseye textures, vugs, etc. Special attention must be devoted to the relationship between certain carbonate minerals, such as dolomite, and minor constituents, such as clay and organic matter (DECHARPAL et al., 1959), which appear to play a role in diagenesis. This conclusion is supported by the studies of Recent sediments by CLOUD ( I 962) and TAFT(1963). The main purpose of this research is to relate any carbonate rock to its depositional environment and thus to reconstruct the original paleogeography.

Petrologic fabrics (sequence analysis) On a megascopic scale, i.e., on the scale of a layer or of a succession of layers, LOMBARD (1956, 1963) introduced the idea of stratonomic analysis based on direct field observations with particular attention to the stratification, bedding characteristics, and the boundary planes between strata. Correlation can be established between the stratum thickness curve and the lithology curve. The rhythmic character of any sequence led Lombard to reach general conclusions cancerning the depositional conditions, i.e., to relate the gross shape and size parameters of carbonate beds to the tectonic deformation of the basin.

Fig.3. Correlations based on lithologic sequences of the “Black Marble” of Golzinne (Middle Frasnian, Belgium); the Golzinne Formation is divided here into nine groups of lithologic sequences and the base of the FalnuQ formation into four. s.v.Z.- s6rie virtuelle locale= local virtual series (Lombard): O = calcareous shales; 1 = shaly limestones; 2 = nodular limestones; 3= biomicrites; 4= pure micrites (“black marbles”); 5= dolomitic limestones; A = quarry of Villeret; B= quarry Marchand; C= quarry Arthur-Etienne; D = quarry Marbres-Pierres-Granites; E= quarry JosephEtienne; F= quarry Merbes-Sprimont; g = quarry Dejaiffe; H and I = quarry of 1’Agasse; J= quarry Deffense; K = quarry “Des Polissoirs”; L= well Puits des Isnes; M = quarry van Rompaye; N = quarry Artoisenet; 0 and P= quarry at Huccorgne. (After MAMET,1963.)

cn w

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Lombard’s method was applied by MAMET(1963) who complemented it with petrographic studies. It enabled him to define and correlate a well-known Carboniferous formation, the “Marbre Noir” of Golzinne, Belgium (Fig.3).Using a different method, i.e., the zonation of macroscopic organisms (Rugosa and Tabulata, Porifera, Brachiopoda, Bryozoa, Crinoidea, and Stromatoporoidea) as ( 1958) established the sequences of well as mean grain-size distribution, LECOMPTE carbonate reef environments in the Devonian of Belgium. He interpreted them as a reflection of the progressive “subsidence” of the basin. Lecompte also emphasized that these Devonian reef environments differ in several ways from Recent reefs. He believed that there were constructional phases, detrital and also chemical phases, each related to the turbulence of the respective environments. The utilization of such a large number of parameters in the final model, “petromodel”, is possible only in case of direct and continuous recording of all the data in the form of logs and symbols, such as proposed by BOUMAand NOTA (1960) and by GUBLERand BERTRAND (1965).

Relationship bet ween petrographic fabric and physical propertics No petrographic study can ignore any of the factors which determine the structure of the rock; in particular, grain-size distribution, diagenesis, void distribution and, consequently, the inter- and intragranular porosity. A comparison between the results of granulometric analyses on using thin sections and loose samples discloses a problem, because thin sections are not absolutely representative of the original sediment. In general, it can be assumed that the granulometry deduced from a thin section is closer to that of a loose sample when (f’-f)l is lower as shown in the first example given below. This has led to the devclopment of the technique of making systematic and statistical measurements on thin sections with a point counter and a Zeiss particle analyzer (FONDEUR, 1963). The physical properties of the rock will be different depending on whether the rock has a constant granulometry or not (MURRAY,1960). Relationship between petrology and petrography The above brief discussion shows that the major problem is to identify on a lifesize scale, the original paleogeographic setting of a rock, establishing its origin and any modifications it may have undergone during geological time. This has led to fundamental niineralogical research which is described in other chapters. It includes experimental laboratory studies on physico-chemical processes ( R I V I~RE, 1939; GRAFand GOLDSCHMIDT, 1956; BARON.1960; GRAF, 1960) and on biochemical processes (NESTEROFF, 1955; LALOU,1957). Extensive ~lf’

= porosity

in thin section, andf = porosity of loose sample.

PETROLOGY AND PETROGRAPHY OF CARBONATE ROCKS

65

fieldinvestigations on the Recent carbonate sediments of the platforms in the Bahamas (ILLING,1954; GINSBURG, 1956, 1957; NEWELL and RIGBY,1957; TAFT,1961, 1962; CLOUD,1962), the Gulf of Batabano (DAETWYLER and KIDWELL,1959), and the Persian Gulf (HOUBOLDT, 1957; EVANSet al., 1964) have produced a general life-size model of the organic communities and mechanical accumulation processes found in such areas. These comprise climatic, hydrodynamic, biochemic and geochemic (including isotopic), and diagenetic processes. On the basis of petrographic and petrologic analyses, and taking into account all the basic data provided by these fundamental studies, the authors of this chapter hope to present a clearer picture of how to identify the former geographic setting of carbonate rocks and their physical properties. Using only standard field and microscopic equipment, the authors tried in two examples to apply in the field the results of quantitative analysis obtained from thin sections. In both cases they used continuous field recording (“petrolog” in the first case and electric log in the second case).

EXAMPLES

“Pisolitic limestone” of Vigny, France In the first example, the authors (Bertrand and Gubler) started with a model considered from the point of view of its reservoir characteristics. Field and laboratory analysis brought out the parameters that control these properties. The final objective was to gain a better understanding of a type of clastic carbonate body from the practical point of view, i.e., location and extent of reservoir properties. The following petrologic and petrographic parameters were considered: (1) composition; (2) stratification, sedimentary structures and granulometry that are direct result of hydrodynamic conditions; (3) color, degree of induration, and minor mineralogical constituents that reflect the environment of deposition and diagenesis; and ( 4 ) biological constituents, trails and burrows that reflect environment and increase primary porosity. The vertical succession of the above characteristics makes up part of the sequential analysis. The field data were collected on a “petrolog” using a scale of 1 : 50 (Fig.4). Vigny is situated 25 km northwest of Paris, France. It is famous for its quarries of the so-called “pisolitic” limestone, whose origin and age have long been controversial. The structural model of the reef complex (Lower Eocene: Montian), about 3 kmzin area, has been described by DESMIDT (1960). There is a superposition of two reef complexes composed of biolithite (Algae and corals), bioclastic limestones (calcirudite and calcarenite), as well as lime-mud (chalk). The present study is only concerned with the section no.2 (C.2 in Fig.5).

67

PETROLOGY AND PETROGRAPHY OF CARBONATE ROCKS E

W +lOOrn,

c.4

r *'O0

c3

C.6 c 2

+

80

+ 80

+

70

* 70 * 60

60

R;g,)Reef

0

Bioclastic ~lmestone D C h a l k

BrokenbiDck

Basement

Fig.5. Section of reef complex showing:distribution:of lithofacies,_Vigny,France.

Although detailed investigations were made in different sections (organic content, granulometry, and cementation: Fig.4), only limited data are given here in an effort to: ( I ) compare the results at different scales of observation (formation, hand specimen, and thin section), with special emphasis on granulometry; and (2) present a rapid examination of the relationships between physical and lithological characteristics. Nomenclature. The writers feel that there is no value in giving a special name to a rock that is specifically determined by its own parameters (Fig.6). In the case of sample No.1253, it could fall within the category of biosparitic calcirudite. CARBONATE ROCKS MICTOSC~PIC data Texture biosparitic General sorting(al1ochems+ micrite) Door jeneity of matrix good

Macroscopic data Structure fine cross-bedded Color: white Fracture: irregular hems Cora Is. echinoids,A Alloct Nature

~~

~~

small coral

algal detritus Iasycladace

Observations

coated sparite detritic particles (7)

normal

broken, round

v

1

0

m

echinoids detritus

lithic fragments (chalk)

normal

micritic deposition in intergranular voids(2)

round

Fig.6. Form for recording petrologic and petrographic data of carbonate rocks.

Fig.4. "Petrolog" of the quarry A. Pisolitic limestones of Vigny, France. Symbols used for apparent granulometry: CG= coarse granular; VC= very coarse; C= coarse; M= median; F= fine; VF= very fine.

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PLATE I1

Grain size

Granulometric data of sample 1253 in section C.4 (see Fig.5). A. Histograms based on hand specimen (unbroken line) and on thin section (broken line). B. Cumulative curves based on hand specimen (unbroken line) and on thin section (broken line). C . Hand specimen. D. Thin section.

PETROLOGY AND PETROGRAPHY OF CARBONATE ROCKS

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Discussion of granulometric results at difSerent sample scales Point examination. Point examination was made on thin sections (Plate IID) prepared from a hand specimen (Plate IIC). The slide was cut perpendicular to the plane of stratification in this case, and can be considered as a typical microfacies because the deposit is homogeneous. The granulometric data are presented on Plate IIA, B. The two curves differ slightly. The hand specimen shows ( I ) a larger size of the grain having maximum frequency, and (2) the best distribution, with good symmetry. It should be noted, as indicated on Plate IIC, that the measurement was made of the same sample from which the thin section was prepared. A zonal extension of measurements would have given different results. Section examination. Section C.4 (Fig.5) has been studied on using microscopic samples (thin sections) and some random hand specimens, in conjunction with M

VC

C

sample! 1254 1253

I

1252

1251

&

!

1250 11249

/

CG

I

1248 1247

1246 1245

1242 1241

Size of most frequent grain

-7

---__

Fig.7. Variations of grade-size of most frequently occurring grain in section C.4, M= median; C = coarse; VC= very coarse; CG= coarse granular; I = measured on macroscopic samples; 2 = measured on slides; 3= based on field data.

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continuous observations in the field. Only the most representative grains (most frequent) were studied, and the results are illustrated in Fig.7. The granulometric curves differ as given below. ( I ) The curves of the random hand specimens differ from the continuous granulometric log recorded in the field. This may be explained by non-represen-

C4 C6

C2

Fig.8. Porosity variation in “pisolitic” limestone of Vigny, France.

U 0

c c

<

+75n

+70n

+65n

I

20

I

40

I

%

‘rnm

Fig.9. Variation of porosity (A) and most frequent grain size (B) with depth in sections C.2, C.4 and C.6, I = section C.2; 2= section C.4; 3 = section C.6.

PETROLOGY AND PETROGRAPHY OF CARBONATE ROCKS

71

tative sampling; yet the general shape of the two curves is more o r less the same. (2) It appears that the distribution of size of most frequently occurring grains on using the hand specimens is not the same as the one obtained on using the thin sections. The fabric of the rock is composed of coarse grains 2-7 mm in size, which appear in very few thin sections because of the small surface area (and thickness) considered. On the other hand, thin sections reveal numerous particles from 200-800p in size. Thus, there is a superposition of two granulometric methods. I n this case, micro- and macroscopic analyses are complementary, and illustrate the genetic a n d textural characteristics of the rock. From this, one may conclude that there are three main points to be considered in any petrologic study: ( a ) the need to orient all thin sections, a n d to relate each one to a single lamina and not to a “microsequence”; (b) how dangerous it wouId be to make a granulometric interpretation of a section based on only one oriented thin section; and (c) the problem of samples being representative. Physical characteristics

In general, porosities decrease laterally from west to east (from section C.4 to C.2, as shown in Fig.8).

Fig. 10. Comparison of two samples having different amounts of cement. A. Sample 1243; section C.4; x 10. Porosity = 24.8 %; sorting (Trask) = 1.55; micritic lime: 5 %. B. Sample 1202; section ‘2.2; X 10. Porosity = 33.6%; sorting (Trask) = 1.53; micritic lime: trace.

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Nevertheless there are two distinct layers: a lower part, $65 to f 6 9 m, and an upper part, +69 m to the top of the quarry (Fig.8). This is not true in case of section C.3, however, as the rock there is composed of biolithite with Iarge vugs.

Infruence ofclasticity. The sorting is about the same in the upper part of sections C.6, C.4, and C.2; the grading of porosities is in the same direction as the clasticity (Fig.9). It is different for the lower parts which calls for further explanation. Iq¶uence of cementation. In two samples presented on Fig. 10, the granulometric characteristics (grain size, sorting) are nearly equal, and the physical properties are subordinated to the greater or lesser development of the matrix. Carbonate lime is particularly unfavorable as far as porosity is concerned, because it fills the intergranular spaces. The bioclastic constituents are coated with chemically precipitated calcite. It can be concluded that the cleanliness (absence of clay fractions) of the original carbonate sediment is as important as it is in quartzose sandstones. The penetration of carbonate solutions into this calcarenite and subsequent precipitation, at different times during diagenesis, destroy a considerable portion of the original porosity. Petrography of Triassic and Jurassic reefs, northwest Gela area, southeast Sicily, Italy1

In the second example, the lateral facies variations of Triassic and Jurassic reef and basin formations in an area northwest of the Gela oil field, southeast Sicily, are discussed by the authors (Mattavelli, Rizzini and Passega), Fig. 11. The facies variations were studied mostly by making petrographic analyses of cores and cuttings from the Gela and Cammarata fields and from well Pozzillo I (see location on Fig. 11j. Electric log correlation was also used. The Gela field is situated on the Ragusa Plateau. A discussion of the Triassic and Jurassic facies will be presented first and will be followed by an interpretation of the paleogeography. Triassic and Jurassic formations, Ragusa Plateau A complete description of these formations, named by RIGODE RIGHIand BARBIERI (1958), can be found in papers by KAFKAand KIRKBRIDE (1959) and by 1

Published by permission of AGIP, Direzione Mineraria.

Fig.11. Location map and cross-section showing electric log correlations; oil fields in southeast Sicily, Italy.

u 3

PETROLOGY AND PETROGRAPHY OF CARBONATE ROCKS

75

SCHMIDT DI FRIEDBERG (1962). The general geology of the Ragusa Plateau and the Gela oil field are discussed by Rocco (1959). The Triassic and Jurassic formations on the Ragusa Plateau are described below.

Taormina Formation. Middle and Upper Triassic. Thickness more than 3,000 m. Generally massive algal dolomite. Streppenosa Formation. Upper Triassic. Thickness 300-400 m. Interbedded dark shales and microcrystalline limestones and dolomites. Villagonia Formation. Lower Jurassic (“Liassic”). Thickness 200-400 m. Interbedded microcrystalline limestones and marls. Giardini Formation. Middle and Upper Jurassic (“Dogger-Malm”). Thickness 50-200 m. Microcrystalline limestones and mark with basalt and tuff flows.

Lithofacies, north west of Gela $field Lithofacies of Triassic and Jurassic formations in the area under consideration are shown on two cross-sections through wells Pozillo 1, Caminarata 1, Gela 37, and Gela 26 (Fig.11 and 12). The first cross-section (Fig.11) shows the electric log correlation. Petrographic characteristics are presented in Fig. 12. The lithofacies of the Taormina Formation, deeply penetrated only in Gela, is shown by a petrographic log of well Gela 32 (Fig.13). In the Gela field, the Taormina, Streppenosa, and Villagonia Formations are defined by fossils and lithologic characteristics. Correlations between Gela, Caininarata 1, and Pozzillo 1 were made with the aid of electric logs (Fig.11). Petrographic descriptions of carbonate rocks follow FOLK’S (1 959) classification and are represented on petrographic logs by the symbols shown in Fig.12. The construction of petrographic logs was discussed by RIZZINIand MATTAVELLI (1964). The terms used by FOLK (1959) are well known, and are only briefly discussed here. Micrite is a microcrystalline limestone deposited as a calcareous mud. Intramicrite and biomicrite are micrites containing intraclasts, that is erosional fragments, or fossil fragments. Intrasparite and biosparite are intraclasts or fossil fragments, cemented by sparry calcite; the fragments on the average are smaller than 1 mm in diameter. Intrasparrudite and biosparrudite are similar to intrasparite and biosparite, but are formed by fragments on the average larger than 1 mm in diameter. Rocks forming reef cores are named biolithites. Fig.12. Cross-section showing the lateral variations and petrographic characteristics (see Fig.1 l), I = grey, green shale; 2= black shales; 3= intraclasts; 4= macrofossils; 5= microfossils; 6 = chert; 7= breccia fragments; 8= sparry calcite; 9 = dolomite; 10= micrite; iI= biolithite.

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Petrographic analyses, where possible, were made on cores. In three wells of the Gela field, the Taormina Formation was ccred continuously. Sections not cored were studied from cuttings. The different formations are discussed in ascending order.

0

50

100m

Fig.13. Petrographic log of Taormina Formation, well Gela 32. Lithologic symbols as in Fig.12. Crosses indicate basalts.

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Triassic. The Taormina Formation can be subdivided into three units on the basis of its facies (Fig.13). The lower two units were reached only in the Gela field. The upper unit was partially penetrated also by the Cammarata and Pozzillo wells. The lowest unit is a greyish algal dolomite, continuously cored in the Gela field. The maximum thickness of this unit penetrated by Gela wells is 500 m. Its maximum thickness, however, is much greater (exceeding 2,500 m in the Ragusa field). The dolomite is massive, fractured and vuggy. Bedding cannot be recognized. The maximum size of the dolomite crystals is about 250 p. Algae include stromatolites, Codiaceae and a few Solenoporae. Shale is almost completely absent. This unit, called the “Algal Dolomite”, is overlain by the second unit, a finely crystalline dolomite named for its color the “Tan Dolomite”. The latter, 50-100 m thick in the Gela field, is fairly well bedded and includes a few paperthin streaks of greenish grey shale. Dolomite crystals have a fairly constant size ranging from 20 to 100 p. A small amount of anhydrite crystals is also present. This unit is not vuggy and has only fracture porosity. Its lithologic characteristics are quite uniform in the Gela field. The uppermost unit of the Taormina Formation is the “Brecciated Dolomite”, which resembles the Algal Dolomite. It also contains algal remains and is formed by dolomite crystals up to 300 p in size. This unit includes a number of beds of breccia formed by fragments of Algal and Tan Dolomites up to a few centimeters in diameter. In Cammarata 1, the Brecciated Dolomite contains fragments of green Algae. The Streppenosa Formation overlies the Taormina Formation and consists of black fossiliferous shales interbedded with black micrites; the clastic ratio being on the average 0.6. Micrites contain fragments of Mollusca and Algae (particularly of green and blue-green Algae) and very few ostracods. Micrite forms beds ranging in thickness from a few centimeters to several meters. Good electric log correlation shows that these micrite beds are persistent laterally. The pyrite and organic matter contents of shales and micrite are high. In the Gela field, the micrite beds gradually increase in thickness and content of fossil fragments to the northwest, as shown by the petrographic logs of wells Gela 26 and Gela 37 (Fig.12). In the Cammarata well the equivalent of Streppenosa Formation studied on cuttings is argillaceous biomicrite. Fossils in fragments are Algae, ostracods, and rare sponge spicules. Algal fragments, which are abundant, range in size from 100 p to 2 mm. In a few places the fossil fragments are well rounded. The electric log of Cammarata shows a few low resistivity streaks of highly argillaceous biomicrite, that form cycles similar to those formed in Gela by the shale beds of the Streppenosa Formation (see Fig. 11). Doloniitization of the Streppenosa equivalent is only slight, but is somewhat greater near the contact with the Taormina Formation.

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I n the Pozzillo well, the equivalent of the Streppenosa Formation is a biolithite, consisting mostly of Algae in position of growth. These are mostly Solenoporae, Codiaceae and in part stromatolites. The cavities between organisms are filled with fossil fragments cemented by sparry calcite. A number of cores show that this is a massive reef core and not a talus formed by debris. There is only a partial dolomitization in the biolithite; however, it is greater than in the biomicrite of Cammarata.

Lower Jurassic (“Liassic”). In most of the Gela field, the Lower Jurassic is represented by the Villagonia Formation. In well Gela 26 this formation can be subdivided into two units. The lower unit is formed by grey or tan argillaceous cherty micrites interbedded with marls. The micrites contain Radiolaria, a few ostracods, and algal fragments. I n the same well, in fairly sharp contact with the lower unit, the upper unit which is much richer in fossils, is a biomicrite. Rounded algal fragments are much larger than those of the lower unit and have a maximum size of 5 0 0 , ~Intraclasts . formed by micrite may be present but are difficult to distinguish from the rounded algal fragments. Fossils and intraclasts form approximately 50 % of the biomicrite. I n well Gela 37, the Villagonia Formation has facies similar to those of the upper unit of Gela 26. The greatest lateral facies variation in the Villagonia Formation occurs between Gela 37 and Cammarata 1 wells (see Fig. 12). In the Cammarata well, the formation consists mostly of biosparrudites and biosparite containing some intraclasts. Fossil fragments are mostly Algae (Solenoporae and partly Codiaceae), crinoids and mollusks. The upper part of the formation forms the reservoir of the Cammarata field. In Pozzillo 1 well, the presence of an equivalent of the Villagonia formation is uncertain. A reef identical to the Streppenosa equivalent is directly overlain by the Giardini Formation and, possibly, is the equivalent of the lower part of the Villagonia Formation. The upper part of the formation is probably represented by a diastem. Middle and Upper Jurassic (“Dogge-Malm”). In contrast to the formations described above, in the Gela area, the Giardini Formution does not show any lateral facies variations. It consists of cherty reddish and greenish, fossiliferous, argillaceous micrites with streaks of shale. Fossils are pelagic: Tintinnidae and Radiolaria. Contact with the underlying Villagonia Formation is sharp. The principal characteristics of the formations discussed here are summarized in Table I. Paleogeographic evolution A n interpretation of the Triassic and Jurassic facies relationships and a tentative reconstruction of the basins can be presented as follows.

79

PETROLOGY AND PETROGRAPHY OF CARBONATE ROCKS

TABLE I THE PRINCIPAL CHARACTERISTICS OF VILLAGONIA AND STREPPENOSA FORMATIONS ..- ...

.~

Wells Gela 26 Villagonia Formation and equivalents clastic ratio 65

percentage of fossils and intraclasts

Cela 37

22

50 %

100-200 p

estimated maximum diameter of fossils and intraclasts

500 p

1,000 p

Streppenosa Formation and equivalents clastic ratio 0.81

8.48

0-5 %

125-300

p~

100-150 p

100-150

estimated maximum size of fossils and intraclasts

250

400 p

/L

Pozzillo I

0

biolithite

60 %

l0-15%

estimated average size of fossils and intraclasts

p~

0

50 %

estimated average size of fossils and intraclasts

percentage of fossils and intraclasts

Cammarata 1

500-1,000

,U

biolithite

2 cm

biolithite

0

0 biolithite

40 % 125-250 2 mm

,U

biolithite biolithite -

The Algal Dolomite was most probably a reef covering the whole of the Gela field as well as most of the Ragusa Plateau. In Gela offshore, the upper part of the Algal Dolomite is laterally replaced by the Streppenosa Formation, which represents the basin facies. The sedimentary features of the reef are almost completely obscured by dolomitization, but in the massive dolomite a number of algal and bioclastic structures can still be seen. The growth of this reef was so rapid that it succeeded in maintaining a relatively shallow platform, by building a thickness of more than 2,500 m (in Ragusa) of algal dolomite in an area of considerable subsidence. Petrographic characteristics give some indications about the depth of the sea that covered the platform. The platform was sufficiently shallow to permit algal life, but was sufficiently deep to be swept by waves that kept it free of clay and micrite. Only occasionally did waves break at the edge of the platform. This explains why breccias are rare and back-reef facies is absent. The absence of quartz sand indicates that the platform was isolated from the continent.

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The end of the construction of the Algal Dolomite platform is attributed by the writers to a diminution in the rate of subsidence. This is shown by the considerable difference in thickness between Algal Dolomite and the overlying formations. The Brecciated Dolomite, largely similar to the Algal Dolomite, was subjected to strong wave action that formed the breccia. This strong wave action was probably a consequence of shoaling of the sea because of the decreased rate of subsidence. The reefs continued to grow and formed a barrier reef at the edge of the platform, where feeding conditions were particularly favorable. Protected by the Brecciated Dolomite reefs, the Tan Dolomite was deposited as back-reef facies. Parallel laminations, scarcity of reef organisms, and streaks of shale are evidence for a low-energy environment. The microcrystalline dolomite, probably a dolomitized micrite, and the anhydrite indicate a restriction of the water circulation between the platform and the open sea which resulted in an almost evaporitic environment. The extensive deposition of the Tan Dolomite buried and killed the platform-building organisms. The end of the Tan Dolomite sedimentation was marked by a marine transgression. The Brecciated Dolomite as a transgressive reef gradually moved over the back-reef facies of the Tan Dolomite. In turn, the basin facies of the Streppenosa Formation gradually transgressed over the Brecciated Dolomite. By using as a datum the top of the Streppenosa Formation i n the Gela field, where control is abundant and electric log correlation is good, it can be seen that the top of the Brecciated Dolomite formed a fairly irregular submarine toppgraphy; the high points were as much as 100 m above the depressions. The Streppenosa Formation, consisting in the Gela field of stratified shales and micrites well correlated by electric logs, indicates a deepening of the sea. This formation becomes thicker toward the south of Gela field where it is in part equivalent of the Taormina Formation. The Streppenosa Formation is rich in pyrite and organic matter indicating little bottom-water circulation. Variations in the geochemical characteristics of this formation on the Ragusa Plateau, as discussed by LONGet al. (1964), seem to indicate that this formation was the source of the oil accumulated in the dolomite. While the deeper Streppenosa sea invaded the Gela area, Pozzillo area was a high on which reef construction persisted. The very gradual lateral transition from basin to reef facies is well shown by the data presented in Fig.12. The typical Streppenosa Formation containing few fossil fragments is found only in Gela 26 well. In Gela 37, lithologic characteristics are largely the same; however, carbonate beds are thicker, and fossil fragments are more abundant and larger than in Gela 26 well. In Cammarata 1 well, shales are absent and are replaced by argillaceous limestones. Micrites contain abundant fossil fragments, larger than those of Gela area, that belong to the talus of the Pozzillo reef. In Lower Jurassic, the paleogeographic conditions largely remained unchanged. The Villagonia sea probably was regressive and somewhat shallower

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than the Streppenosa sea. This is indicated by the abundance of Algae in the Villagonia Formation, and the eastward advance of the Pozzillo reef talus. The lower part of the Villagonia Formation still has a fairly deep-water facies in Gela 26 well, where it consists mostly of cherty shales containing Radiolaria and sponge spicules. In this well, the upper part of the formation, which is a biomicrite, contains, as compared to the lower part, less shale, larger fossil fragments, and a greater percentage of algal fragments. The sea, therefore, became progressively shallower during deposition of Villagonia Formation. In well Gela 37, the Villagonia Formation, which consists mostly of biomicrites containing more than 50 % fossil fragments with practically no Radiolaria or siliceous sponge spicules, has a shallower water facies than the Streppenosa Formation. In Cammarata area the Villagoniabormation was deposited in a very shallow and turbulent water environment. The formation is a reef talus formed by coarse bioclastic fragments partially cemented by sparry calcite. Fragments are well rounded and are as much as a few centimeters in size; micrite is absent. In the Pozzillo area, reef construction probably continued during a part of Villagonia, but during Upper Villagonia the regressive sea caused the emergence of the Pozzillo area. The Giardini Formation marks a complete change in paleogeographic conditions and the beginning of a new sedimentary cycle with deposition of widespread pelagic sediments. Thus, in the area northwest of Gela oil field, the combined use of petrographic and subsurface methods resulted in a fairly precise reconstruction of the evolution of the sedimentary basin during the deposition of Triassic Taormina and Streppenosa and Lower Jurassic Villagonia Formations. Of particular interest is the thick (several thousand meters) Algal Dolomite (Taormina), a reef that formed a broad shallow-water platform in an area of rapid subsidence. The southern part of the platform was separated by a slope from the deep sea floor on which argillaceous and calcareous muds of the Streppenosa Formation were deposited. A change in sedimentation conditions and the end of the Algal Dolomite reef formation occurred as a result of considerable decrease in subsidence. Basin facies transgressed over the reef. Gradual lateral changes from basin to reef facies, that took place during transgression of the Streppenosa sea and regression of the Villagonia sea, are illustrated accurately by petrographic logs. In Gela, the oil is trapped in the Algal, Tan, and Brecciated Dolomites of the Taormina Formation and in Cammarata area, in the bioclastic limestones of the Villagonia Formation. Closure is formed by the Streppenosa Formation in Gela and by the Giardini Formation in Cammarata. Shales and micrites of the Streppenosa Formation are probably the source rocks.

Y. GUBLER

et al.

CONCLUSIONS

Exceptional progress has been made during the last ! 5 years in the field of petrography of carbonate rocks due to the introduction of new methods of laboratory analysis. In particular, new techniques of physical analysis (electronic microscopy, X-ray diffractometry, fluorescent X-ray diffractometry, emission spectrography, etc.) have made it possible to make a rigorous and more thorough identification of the carbonates making up a rock. In addition, methods such as digital data processing by integrating instruments for establishing characteristic parameters for each rock have proved to be of great value. Depending on the parameters taken into consideration, and taking into account the different aspects of the data provided by current sedimentation research, a petrographic examination can resuIt in a recognition of the features of the original depositional environment. It also enables one to evaluate some of the textural characteristics which influence the physical properties of the rock. The writers have also tried to demonstrate that a mere petrographic examination (by thin section) is only rarely satisfactory in giving a representative picture of the rock. Limestones and dolomites usually constitute heterogeneous bodies whose anisotropies can only be discerned on a field scale, and cannot be perceived by means of any more thorough petrographic analysis. This presents a difficult scalar problem. Reservoir engineers and oil field geologists have attempted to approach this scalar problem from the point of view of mathematical theory. One of the causes of the difficulties they have encountered stems from the fact that the life-size data (i.e., the petrologic information) they have to deal with are often incomplete and too qualitative. Consequently, it is imperative that geologists supply them with data which are as complete and quantitative as possible. The writers feel that the only way to do this is by systematic and standardized sampling and presentation of data. It is only under such conditions that measurements can be compared so that correlations can be established from one profile to another and from the field to hand samples and to thin sections. The results thus obtained will be complementary and would enable geologists to construct a “petromodel”. As for the environments considered, efforts have thus far mainly concentrated on reef complexes (particularly Paleozoic); and relatively little is as yet known about the petrology of pelagic limestones including chalks, which, nevertheless, are of considerable regional importance since the Cretaceous time. The scattered data available on these younger carbonate rocks appear to indicate that the same methods would be applicable in approaching the problem. The solution of the fundamental problems of diagenesis requires geochemical and thermodynamic methods just as it does petrographic and petrologic procedures. As far as the mineralogical “replacements” during diagenesis are concerned, one should be aware of the important part played by minor elements

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(i.e., both in solid solution and as ionic impurities in interstitial waters). Consequently, the necessity of applying the above-mentioned methods of analysis for studying them becomes evident. One should also be aware of the effects of pressure and temperature which always influence crystallization. Based on observations made on all scales (from the field study to thin sections), the role of geologists is thus to detect the effects of pressure and temperature, their magnitudes and variations. Taken all together, these data can only be synthesized on the scale of a “petromodel”, which is the basic unit that the sedimentary petrologist has to consider.

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et al.

STERNBERG, R. M. and BELDING,H. F., 1942. Dry peel technique. J. Palaeontol., 16: 135-136. STRAKHOV, N. M., 1957. MCthodes d’6tude des roches sedimentaires. Bur. Rech. GPol., Gkophys. MiniLres, Ann. Sew. Inform. Gtof., 35 (1): 542 pp., 35 (2): 535 pp. TAFT,W. H., 1961. Authigenic dolomite in modern carbonate sediments along the southern coast of Florida. Science, 134: 561-562. TAFT,W. H., 1962. UnconsolidatedCarbonate Sediments of Florida Bay, Florida. Thesis, Stanford University, Stanford, Calif., 60 pp. G. I., 1958. Study of Sedimentary Rocks. Gostoptekhizdat, Leningrad, 572 pp. TEODOROVICH, VINOGRADOV, A. P., 1953. The Elementary Chemical Composition of Marine Organisms. Yale University Press, New Haven, Conn., 647 pp. WARNE,S., 1962. A quick field or laboratory staining scheme for the differentiation of the major carbonate minerals. J. Sediment. Petrol., 32: 29-38. E., 1963. L‘expost original de la notion de facibs par A. Gressly (1814-1865). Sci. WEGMANN, Terre, 9 (1): 85-97. WOLF,K. H., 1962. The importance of calcareous algae in limestone genesis and sedimentation. Neues Jahrb. Geol. Palaeontol., Monatsh., 5: 245-261.

Chapter 4 CLASSIFICATION O F SEDIMENTARY CARBONATE ROCKS HAROLD J. BISSELL AND GEORGE V. CHILINGAR

Brigham Young University, Provo, Utah (U.S.A.) University of Southern California, Los Angeles, Calif. (U.S.A.)

SUMMARY

The objective of this chapter is to evaluate some of the proposed classifications of sedimentary carbonate rocks, and to present suggestions for naming and describing them. It is not an historical review of existing schemes of classification alone, although a critical appraisal has been made of the various systems that have been published in the past 60 years in an attempt to arrive at a svstematic petrologic and petrographic plan of study.

INTRODUCTION

It should be noted, that more than three-fourths of the surface of the earth is covered by water, and certain modern-day carbonates are forming. About threefourths of the total land area is directly underlain by sedimentary rocks, and approximately one-fifth of these consists of carbonate rocks. Thus, it is desirable to study present-day environments of carbonate sedimentation and carefully to investigate indurated equivalents in the geologic record, in order to group carbonate rock types into a single classification or, more likely, multiple classifications having value to both field and laboratory investigators. No single scheme of classification appears to have universal appeal or utilitarian value to geologists, as is evinced by the blizzard of nomenclatural and classification proposals in recent years; and any proposal must be meaningful if it is to be applicable to the tremendously variable and areally extensive carbonate suites. As a beginning, the field classification should be workable to the investigator equipped with no more than a hand lens and acid bottle. This same scheme should be expandable to the degree necessary for the worker whose laboratory contains higher power magnification, as provided by various binocular, petrographic, and electron microscopes, X-ray equipment, and analytical physical and chemical apparatuses. Perusal of the literature indicates that geologists normally define carbonate rocks as those containing more than 50 % of carbonate minerals; investigators

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customarily recognize limestone and dolomite (or dolostone) as the two compositional types. Some workers prefer to treat the two divisions separately, whereas others demonstrate that a scheme of classification can be devised to include both. Rocks containing as little as 10% CaC03 often weather like limestones. Nevertheless, it seems that limestones are best defined as sedimentary rocks containing more than 50% of the minerals calcite (plus aragonite) and dolomite (possibly including ankerite), with calcite dominant. Dolomite (or dolostone) is a sedimentary rock containing more than 50 % of the minerals dolomite (perhaps including ankerite) and calcite (plus aragonite), with dolomite more dominant. In the discussion which follows, limestones are treated separately from dolomite rocks (dolostones), largely because numerous dolomites are the result of diagenetic alteration of limestones. The dual classification, therefore, may result in a more objective approach in the investigation. It will be pointed out, however, that a classification which includes both rock types is nonetheless workable, but of necessity must be handled by a trained petrographer.

CLASSIFICATION OF LIMESTONES

General stutemenl

At least 60 years ago, GRABAU (1904) realized that the general term “limestone” was inadequate to identify correctly the numerous species which geologists allocate to this group. Accordingly, he (GRABAU, 1904, 1913) classified these carbonates genetically under the groupings of hydroclastic, bioclustic, and biogenic (or organic). Most workers since Grabau’s pioneer efforts have agreed that composition (or mineralogy) and texture are compelling parameters in carbonate-rock classification; some of the more recent investigators also included the parameter of environmental energy. In a combined or composite classification, degree of diagenesis and epigenesis (or alteration) is also a parameter of tremendous significance. From the standpoint of composition alone, certain subdivisions are made by most petrologists and petrographers whether in the field or laboratory. For example, pure limestone is regarded by many as the rock containing 90% or more of calcite (possibly with some aragonite). Magnesian limestone could be considered a variety, if appreciable magnesium is present, but not as the mineral dolomite. This would be difficult, if not impossible, to determine in the field with hand lens and acid bottle. Dolomitic limestone is that variety in which both calcite and dolomite are present, but calcite is more abundant. Calcitic dolomite, by definition, is that carbonate rock containing both dolomite and calcite with the former more abundant. The end-member dolomite (without particular qualification) contains more than 90 % of the mineral dolomite (possibly with ankerite). These subdivisions have limited utility in the field, but can find certain acceptance by laboratory workers,

89

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

TABLE I QUANTITATIVE SCHEME OF CALCITE-DOLOMITE-CLAY

SERIES

(After TEODOROVICH, 1958, p.299) ~

Content (%)

Name of rock

~-

Clayey limestone Slightly-clayey dolomitic limestone Slightly-clayey limestone Limestone Slightly-dolomitic limestone Dolomitic limestone Highly-dolomitic limestone

~~

~~

clay

calcite

30-10 5-10 5-10 0-5 0-5 0-5 0-5

35-90 9045 95-85 100-90 95-80 80-65 6547.5

dolomite

045 547.5 0-5 0-5 5-20 15-35 30-50

particularly if the petrologist (field geologist) supplements his investigations by his own petrographic (laboratory) studies. Composition

PETTIJOHN (1949, pp.289, 313) pointed out that limestones are a polygenetic group of rocks, and proposed a chemical scheme of classification that shows intergradaand KENNER (1955, pp.46-48) also proposed a tions of carbonate rocks. GUERRERO classification of limestone-dolomite series on the basis of CaO/MgO molar ratio, more or less a modification of Pettijohn’s quantitative scheme. On the basis of relative amounts of calcite (CaCOs), dolomite (CaMg(CO&), and clayey material, TEODOROVICH (1958, p.299) proposed the divisions given in Table I. Several methods of classification of carbonate rocks on the basis of chemical composition were reviewed by CHILINGAR (1960). Table I1 gives types of limestones that can be recognized on the basis of Ca/Mg (weight) ratios. TABLE 11 CLASSIFICATION OF LIMESTONES ON BASIS OF

ca/Mg RATIO

(Modified after CHILINGAR, 1957a, p.187; see also CHILINGAR and BISSELL, 1963a, p.1) ~-

__

._

- - ~ _ _ _ _ _ _ _ _

.~

Rock name

Range in CalMg ratio

Highly dolomitic limestone Dolomitic limestone Slightly dolomitic (or magnesian) limestone Calcitic limestone

4.74-16 16-60 60-105 >105

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H. J. BISSELL AND G . V. CHILINGAR

Students of carbonate petrology and petrography recognize at least seven different forms of CaC03 and MgC03 as follows: ( I ) calcite; (2) aragonite; (3) dolomite; (4) magnesite; (5) solid solution between calcite and dolomite, such as in skeletons of organisms; (6) nesquehonite; and (7) hydromagnesite. The latter two forms are rare, but have been shown by FROLOVA (1955) to be present in the carbonate-sulfate rocks of the Kuybyshev area of the U.S.S.R. Details of carbonate mineralogy are provided by GRAF(1960a). In addition to detrital quartz, precipitated silica, siderite, ankerite, clay, and glauconite, various minor constituents are present in many limestones. These include detrital and/or authigenic feldspar euhedra, chalcedony, pyrite, and bituminous (organic) matter. If present in amounts less than 10 % of the total bulk, they hardly justify consideration in the compositional classification. They may, however, comprise 10% or more of the rock and belong in the nomenclature. There should be some rational plan of designating them in the rock name, and this can be done easily with the use of the hyphen (-). The usage is systematic, however, and should be consistent. For example, a carbonate rock having slightly more than 50% calcite (possibly with aragonite) and slightly less than 50% (or, 30-50%) dolomite (perhaps with ankerite), is termed a dolomitic-limestone. If the rock is definitely a limestone, but contains more than 10% but less than 30% dolomite it is termed a dolomitic limestone. If noncarbonate clay is present in an amount greater than 10 % (e.g., 15 %), and dolomite constitutes 29 ”/, with calcite cornprising the remaining 56 %, the rock is named a clayey, dolomitic limestone. The term argillaceous may seem more useful than clayey. Usage of the hyphen is best reserved for those compositional classification schemes of limestones that are arranged in tabular form. It has utility in limestone classifications which also involve texture as one of the parameters (MOSHERand PINNEY,1963, pp.219-222). In his scheme of classification of limestones, MOLLAZAL (1961, pp.9-18) utilized composition as an important parameter. As limestones he considered those rocks that contain 50% or more of calcite and possibly aragonite. He noted that any specific limestone may contain up to but not in excess of 50% “adulterant” materials that are composed singularly or in combination of dolomite, silica (commonly quartz sand and silt), silicates, clay, bituminous materials, and others. He utilized a triangular diagram with three end-members to demonstrate the composition of the spectrum of carbonates that he studies. The end-members consist of ( I ) calcite, as the major constituent at the top of the triangle, (2) dolomite, which is placed at the lower right, and (3) silica and clay, on the lower left (see Fig.]). In his triangular arrangement by composition, Mollazal divided the side of his triangle into a five-fold horizontal arrangement, with each subdivision representing 20 % of the corresponding constituents or end-members. The surface of the triangle is divided into one triangle and 20 trapezoids by extending lines downward from the top horizontal bar to intersect equally-spaced points along the base of the triangle. The trapezoids are numbered from upper left to lower right in

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

91

CALCITE

S I L I C A and C L A Y

DOLOMITE

Fig. 1. Triangular diagram illustrating classification of carbonates on the basis of composition. (After MOLLAZAL, 1961, fig.2, p.18; see also MIS~K,1959.)

numerical sequence, and each number shows percentage of the end-members that are representative of the composition of the rock. The closest number to each member is approximately 100 % of that end-member. For example, in Mollazal's diagram, 1 means essentially pure calcite (or calcitic limestone), 21 is dolomite, and 5 approximates 75% (60-80%) calcite, 18% (16-24%) dolomite, and 7 % (4-16%) silica and clay. Thus, it .is possible to demonstrate the composition of a carbonate rock, and in particular limestones, by one number only. Texture

As pointed out by LEIGHTON and PENDEXTER (1962, p.33, most limestones are characterized by the types and relative amounts of textural components, of which four types are dominant: (1) grains, (2) lime mud (micrite), (3) cement, and ( 4 ) pores. Based on their experience, as well as that of scores of other workers in research and petroleum companies with which they were affiliated, these four components form the basis for describing and classifying a large proportion of limestones. They pointed out that grains are discrete particles capable of forming a rock framework, and are, therefore, similar to sand and silt grains in a sandstone

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H. J . BISSELL AND ti. V. CHILINGAR

or siltstone. As examined under the binocular microscope, an arbitrary lower size limit of 0.03 mm was taken (division between medium and coarse silt). Micrite is used for those particles that were once mud-like, of either chemical or mechanical origin, and have an arbitrary upper size limit of 0.03 mm. Cement forms the clear crystalline component that fills the spaces between the grains; sparry calcite or dolomite cement in limestones, dolomites, and some restricted environment arenites fit this category. Porosity is difficult to place quantitatively in routine examination of limestones, but pore spaces are of significance when carbonates are viewed from the standpoint of oil genesis, migration and ultimate storage. Pore space varies with such characteristics as packing, sorting, shape, and size of discrete particles in limestones and type and disposition of cement. The structures of pore spaces of carbonate rocks have been classified by TEODOROVICH (I 943 ; see also CHILINGAR, 1957b, and ASCHENBRENNER and ACHAUER, 1960) into six types (Fig.2).

TYPE \

TYPE I\

TYPE III

TYPE I V

Fig.2. Classification of pore spaces of carbonate rocks. Type Z. The pore spaces of this type consist of pores and of rather isolated more or less narrow conveying canals. Commonly the narrow canals (inner diameter of 0.01-0.005 mm) which connect the pores of this type are not visible in a thin section. If the minimum diameter of a canal is larger, however, the canal can be detected in a transparent thin section. Type ZZ. The communicating ducts of the pore spaces of this type consist merely of constrictions in the pore space which become wider and pass gradually into the pores proper. Type ZZZ. This type of structure is characterized by the presence of pores connected by finely-porous broad canals, which are observed in a thin section in the form of branches. Occasionally, the conveying canals may consist of coarser pores, which sharply increases the permeability. The pore-space configuration of this type is usually found in dolomites; less frequently, it is observed in dolomitic limestones. Type ZV. This type of structure is characterized by a system of pores distributed between the grains and near the grains of the main mass of the dolomite rock or of its cement, reflecting the outlines of the greatest part of these grains (intergranular pores). The interrhombohedral porosity in dolomites serves as an excellent example. Type V. The pore space is formed by fractures. Type VZ. The pore space is characterized by two or more elementary types of pore-space configuration. Legend: 1 = fine conveying canals between pores; 2= pores clearly observable on thin sections; 3= fine and extremely fine-grained conveying branches; 4 = intergranular pores. (1 957b) and ASCHENBRENNER Illustration after TEODOROVICH (1943); see also CHILINGAR (1960). and ACHAUER

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

93

It is to be noted that the four textural groups proposed by LEIGHTON and PENDEXTER (1962), which are grains, lime mud, cement, and pores, contain the subdivisions for grain types as follows: ( I ) detrital grains, (2) skeletal grains, (3) pellets, ( 4 ) lumps, and (5) coated grains. These, however, do not adequately describe the reefoid (or reefal) limestones and their dolomitized equivalents. LEICHTON and PENDEXTER (1962, p.35) term these “in-place organic structures”. The rigid in-place framework of such limestones may be constructed of corals, bryozoans, Algae, sponges, and perhaps others, as well as their combinations. Micritic and/or sparry carbonate cement and infilling may also be present. Not to be overlooked as a textural type, though of relative insignificance in many carbonate rocks, is that resulting from overgrowth, authigenic mineral development, diagenetic crystallization, and recrystallization. This is not necessarily restricted to dolomitization, but may be no more than syndiagenetic (= early diagenetic) crystallized calcite, such as forms in encrinal limestones and as zoned overgrowth in finely crystalline calcitic limestones, and development of authigenic calcite (and at times albite) in the matrix (i.e., interstitial) material of “hashy” limestones, reef trash debris, etc. Various categories of grain types have been erected by petrographers in their efforts to classify objectively the detrital, mechanical, or clastic carbonates. GRABAU (1904, 1913) was among the early workers to attempt this, and some of his terms are still applicable and widely used, such as culcirudite, calcarenite, calcisiltite, and calcilutite. Later workers have added textural terms, and have modified some of those proposed by Grabau; terms such as autochthonous, allochthonous, calcarenitic, bioaccumulated, bioarenite, biocalcirudite, intraclastic, lumpal, pellet (or pelletal), calclithite, biolithite, skeletal, and others have been used with varying degrees of success. FOLK(1959, 1962) modified some of Grabau’s terms, and added others such as micrite, intrasparrudite, intraclast, and others. BRAMKAMP and POWERS (1958), and POWERS (1962) succesfully applied such textural terms as fine-grained limestone, calcarenitic limestone, calcarenite, and coarse carbonate clastic. Mention is made of these few examples (because the list is much longer) to point out the utility of objectively applying texture as a parameter in carbonate rock classification. Among the grain types, the following are of prime importance to the petrologist and petrographer in carbonate-rock study: ( I ) detrital grains, (2) skeletal grains, (3) pellets and oopellets, ( 4 ) lumps, and (5) coated grains (see LEIGHTON and PENDEXTER, 1962, pp.35-36). Detrital grains are lithoclastic (i.e., fragmental) and thus include debris derived from pre-existing rocks. Part of this may be from material washed into the depositional basin or depocenter and thus be allogenic or allochthonous; or it may be carbonate debris within the basin. In the latter case, it is termed intraclastic by FOLK(1959, 1962). Within the repository this detrital material may consist of disrupted weakly consolidated penecontemporaneous sediment that is later indurated, or it may have originated from pre-existing rigid

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H. J. BISSELL AND G. V. CHILINGAR

rock. Various types of calcirudites, biocalcirudites, edgewise conglomerates, flatpebble conglomerates, polymictic and oligomictic conglomerates, calcarenites, biocalcarenites, reef-front breccias, calcisiltites, calcilutites, dismicrites, and others originate in this manner. Skeletal grains may be fragmental, or nonfragmental. Some workers prefer to reserve the term “skeletal limestones” for those varieties which are in-place framebuilders, or bioaccumulated with little or no abrasion. Some fossiliferous limestones and biostromes fit this category. Perhaps a more realistic approach to application of the term skeletal grain is to view it as an intra-basin (or any type of sediment depositional repository such as shelf, platform, etc.) deposit, and, therefore, of either fragmental or nonfragmental nature. The division between a fragmental skeletal limestone and an intraclast composed of abraded and water-worn fossil debris (= fossiliferous-fragmental), however, may be arbitrary. Workers apply the (1904, term “bioclastic” today, not necessarily in the sense suggested by GRABAU 1913), but more commonly to the fossiliferous-fragmental limestones. THOMAS (1960, pp.1833-1834) objected to the latter practice, but geologists have evidently made the slight departure from GRABAU’S earlier (1904) usage and the term is more or less ingrained in the language. Even GRABAU (1913) later modified his first suggestion. Skeletal grains, whether fragmental or nonfragmental, may consist of Foraminifera, Algae, crinoids (or crinoidal debris), brachiopods, molluscs, and others. These grains may be combined with detrital grains (whether allogenic or intraclastic, or both), and/or with micritic material, pellets, lumps, etc. MOSHER and PINNEY (1963, pp.219-222) devised an excellent scheme in their textural classification of limestones (which is a modification of that by LEIGHTON and PENDEXTER, 1962), by applying the hyphen ( - ) and the comma ( , ). For example, skeletal-micritic limestone indicates approximately equal amounts of skeletal and micritic material, but skeletal, micritic limestone is one composed mostly of micrite in which 10-25 % of skeletal elements are embedded. This scheme was utilized earlier by CHILINGAR and BISSELL(1963a) in a classification of limestones. DUNHAM (1 962) classified limestones according to depositional texture, and suggested terms such as mudstone, wackestone, packstone, grainstone, and boundstone. PRAYand WRAY(1963) utilized some of these textural terms in their study of Pennsylvanian porous algal facies exposed along Honaker Trail and the canyon walls of the San Juan River in southeastern Utah. By contrast, BAARS(1963) pointed out that unaltered limestones are composed of lime mud (“micrite” or “matrix” of some writers), particles or grains, cement, and pore space. He foland PENDEXTER (1962) and subdivided particles lowed the suggestion of LEIGHTON into: ( I ) skeletal, (2) detrital, (3) composite grains, ( 4 ) coated grains, and (5) pellets. FERAY et al. (1962) indicated in their classification that skeletal material is biochemical in origin and is secretionary in nature, and stated that: “Skeletal material is produced by organisms progressively secreting calcium carbonate in

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

95

order to provide progressive enlargement of the skeletal structure in keeping with the growth of organisms” (FERAYet al., 1962, p.25). It is to be noted, therefore, that geologists are not in full agreement as to how the term skeletal should be applied in limestone nomenclature. Arguments have been advanced by both sides in defense of their usage; the term has utility, and perhaps each worker should indicate whether he connotes fragmental or nonfragmental texture. NELSONet al. (1962) proposed a utilitarian skeletal limestone classification, and devoted considerable attention to various usages of such terms as reef, bank, bioherm, and biostrome. Thus, their skeletal limestones are in-place accumulations that consist of, or owe their characteristic to, essentially bioaccumulated skeletal matter. These rocks are formed through biologic processes, and are in contrast to fragmental limestones which are formed by mechanical processes of transportation, abrasion, sorting and deposition (NELSON et al., 1962, p.234). In their limestone classification (which has four parameters), skeletal limestones include reefs and banks (both of which are biohermal or biostromal); but the fragmental limestones include the shell debris (coquina), calcarenites (or granular limestones),calcirudites, and sedimentary breccia. The textural terms pellet, pelletal, and pelletoid (or pelletoidal) appear to have utility in limestone (and dolomite) nomenclature and classification. LEIGHTON and PENDEXTER (1962) and BAARS(1963) pointed out that pellets may be fecal debris, or grains of micrite. Modern fecal pellets formed by brine shrimp in sediments of Great Salt Lake of Utah have been described and illustrated by EARDLEY (1938), and many marine examples have been cited. Similar pellets in ancient carbonate rocks have been considered by workers to be of fecal origin. Pellets may be no more than grains of micritic material that lack discernible internal structure, and may be ovoid to subround in shape; these may be of silt to sand (or even granule) size. Perhaps some formed through a process of accretion during transportation within the depocenter. In algal limestones (particularly those of the Osagia and Mizzia types) in which a certain amount of disruption has occurred, algal “dust” may subsequently accumulate as pellets through accretion. Oopellets appear to be intermediate pelletoid grains, and display features suggestive of an origin in an agitated environment wherein particulate pelletal material receives additional material through accretion. Vestiges of concentric, radial or axiolitic features may be shown in such oopellets. Lump limestones, according to geologists of Jersey Production Research Company (see LEIGHTON and PENDEXTER, 1962), are those composed of composite grains that possess surficial irregularities and are believed to have formed by a process of aggregation. WOLF(1960) observed that lumps are aggregates of one or more types of grains such as composite oolites and (or) composite pellets. MOSHER and PINNEY(1963) also considered lumps significant in limestone nomenclature, but applied such names as “lumpal limestone,” “lumpal-micritic limestone,’’ and others to limestones in which lumps form a significant constituent. LEIGHTON

96

H. J. BISSELL AND G. V. CHILINGAR

and PENDEXTER (1962) pointed out that lumps may range in size up to algal “biscuits”. Workers may encounter difficulties in distinguishing lumps from grapestones (see IMBRIE and PURDY,1962, p.266); or from bits of lime mud torn from the sea floor, that through agitation are rolled around and increase in size through aggregation and composite clustering. Lime ooze in the littoral zone may become disrupted, rolled around and shaped into lobate or irregular masses, and ultimately become indurated. Such material could be termed lumps. Furthermore, flocculated lime ooze particularly through mingling of brackish and saline waters may form lumpal limestones, lumpal-micritic limestones, etc. ; this would result in a “glomeroclastic” texture. Thick and areally extensive limestones that contain numerous lumps, and in fact are to be termed lump limestones, occur in Cenozoic deposits of the western interior of the United States. These limestones are largely of the lacustrine environment although many may have accumulated in saline to penesaline waters, and others are evidently strictly of fresh-water origin. Numerous Permian algal limestones in parts of the eastern one-half of Nevada and western one-half of Utah contain lumps; it is believed that many of these lumpal limestones are composed of disrupted algal colonies some of which were broken and re-shaped to form lumps through current and wave action in shallow marine waters. Algal “dust”, as pointed out herein, may have become organized into pellets, intermediate pelletoid grains, or micritic grains; these may be termed the “grains of matrix” of ILLING (1954). The textural term oolitic has been applied to limestones for many years; WOLF(1960, p.1415) preferred the term coated grains for “ooids” or “oolites” to include concentrically formed materials up to the size of pisolites. Wolf’s classification is identical in most respects to that of Jersey Production Researchcompany (LEIGHTON and PENDEXTER, 1962). Thus, coated grains are those having concentric or enclosing layers of calcium carbonate around a central nucleus, and include oolites, pisolites, Algae-encrusted or Foraminifera-encrusted skeletal grains. These coated grains fall into three principal categories, as follows: ( I ) OoZitessmall spherical or subspherical accretionary grains generally less than 2.0 mm in diameter that in thin section display concentric and/or radial structure. A variety in this group is the superficial oolite in which the thickness of the accretionary coating is less than the radius of the nucleus. (2) Pisolites-grains similar to but larger than oolites, and less regular in form (commonly crenulated); they are generally 2.0 mm or more in diameter. (3) Algae- or Foraminifera-encrustedgrains -these are carbonate grains having a nucleus (generally a skeletal or rock fragment) about which Algae or Foraminifera have formed encrustations. It is herein suggested that the textural term “axiolitic” (radial-cylindrical) should be added, either as a subdivision within (I) and (2) above, or as a fourth category of the oolitic class. The term was applied originally to igneous rocks (ZIRKEL,1876), but has utility for sedimenta. carbonate rocks if no attempt is made to assign a particular genetic significance it. Axiolitic is a textural term

Fig.3. Textural classification of limestones. (After TEODOROVICH, 1958, p.291; see also BISSELL and CHILINGAR, 1961, fig.1, p.612.) W

4

98

H. J. BISSELL AND G. V. CHILINGAR

referring to spherical or subspherical coated grains which have a type of acicular structure extending at right angles from a central axis or rod rather than from a point. Concentric structure may be present, but it is subtle. Axiolites range in size from micro-oolitic to pisolitic. As pointed out by BISSELLand CHILINGAR (1961, p.612), this term was applied by TEODOROVICH (1958, p.291) in his structural (i.e., textural) classification of limestones to a variety of spherulitic ray-aggregate limestones in which the main mass of the rock consists of grains, and cementing material does not exceed 10 % of the bulk (see Fig.3). Useful generalizations are not easy to make concerning texture typical of limestones composed of protective and skeletal structures of organisms. GRABAU (1904, pp.229-230) proposed the term “endogenetic” for sedimentary rocks that owe their origin chiefly to chemical agents or agents acting from within, and thus are intimately associated with the formation of rocks. One of his four groups of endogenetic rocks is termed “biogenic” or organic; also termed bioliths. According to GRABAU (1913, p.280), these are the only true organic rocks that are due directly to the physiological activities of organisms; if the rock has definitive structure and texture formed during transportation, sorting, and deposition, the dimensional terms rudaceous, arenaceous, lutaceous, etc. are applicable. By contrast, if the organic limestone is reefal, such textural terms may not be of value except to infilling or matrix materials. The bulk of limestones that have organic structures was formed through secretion; the organisms responsible for construction of this framework may include corals, stromatoporoids, Algae, bryozoans, and others. Fossil remains are still in growth position, or an approximation thereof. These fossils may be closely packed, or have more of an open-work structure (Le., lack interstitial matrix, and are cemented only at points of contact); interskeletal (nonfragmental type) spaces commonly are occupied by micrite (including dolomitized micrite), skeletal material (fragmental), lumps, pellets, and intraclasts. This interstitial “paste” or matrix material may consist of comminuted algal “dust”, coralgal detritus, calcarenitic material, lutite, and the like. The name of essential framework-building organisms (which may have opposed waves) is necessary and not merely accessory in the nomenclature; coralline, algal, bryozoan, coralgal, and bryalgal are terms commonly utilized in the classificatory schemes. Some plans of limestone classification incorporate the terms “biochemical”, “physicochemical”, and “mechanical”, and subdivide these to include the terms “skeletal”, “nonskeletal”, “secretionary”, “accretionary”, “particulate”, etc. FERAYet al. (1962, p.24) developed such a scheme of limestone classification. Limestones that are largely, if not wholly, in-place accumulations of organisms should be defined in terms of the ecological potential of the organisms responsible for forming this important group of sedimentary carbonate rocks. A reefis a nonfragmental skeletal limestone deposit formed by organisms possessing the ecologic potential to construct a wave-resistant framework that is more or less rigid and has definitive topographic structure. The term “bank” has received a

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

99

certain amount of attention from geologists; LOWENSTAM (1950) indicated banks to be the product of organisms which could not raise their own substrate very high above the surrounding bottom. NEWELL et al. (1953) considered banks as accumulations of bioclastic debris as well as in-place accumulations of shells, e.g., oyster banks. The definition advanced by NELSON et al. (1962, p.242) is as follows: “Bank-A skeletal limestone deposit formed by organisms which do not have the ecologic potential to erect a rigid, wave-resistant structure”. It is to be remembered that NELSON et al. (1962, p.234) regarded skeletal limestones as those accumulations which consist of, or owe their characteristics to, virtually in-place calcareous skeletal matter. These rocks, they argued, formed through biologic processes and they contrasted them with the fragmental limestones that formed through transportation, abrasion and sorting. They followed the definitions of CUMMINGS (1932) for bioherm and biostrome, which in essence are as follows: Bioherm“. . . a reef, bank, or mound; for reeflike, moundlike, or lenslike or otherwise circumscribed structures of strictly organic origin, embedded in rocks of different lithology.” (CUMMINGS, 1932,p.333). Biostrome- “. . . purely bedded structures, such as shell beds, crinoid beds, coral beds, etc., consisting of and built mainly by sedentary organisms, and not swelling into moundlike or lenslike forms,. . ., which means alayer or bed.” (CUMMINGS, 1932, p.334). GRABAU (1913, pp.384-457) presented a rather extensive discussion on the biogenic rocks, with a detailed treatment of reefs. He applied the names “zoogenic” and “phytogenic” for animalformed and plant-formed deposits, respectively. If the term biogenic has utility today in limestone nomenclature, then biogenic limestones can conveniently be divided into these two groups: (I) reefs and (2) banks. Reefs, therefore, owe their origin to dynamic growth upward and outward of framework-building organisms, in opposition to waves and currents. These organisms are capable of surviving in high-energy environments, and include certain corals, Algae, bryozoans, and rudistids. Varieties of bioherms and biostromes which accumulate through this process and owe their origin to these organisms are then reefs in the true sense. Banks, on the other hand, are in situ accumulations of skeletal material which are largely, if not wholly, nonfragmental but do not have the structural framework of reefs. Some bioherms and biostromes fit into the category of banks; some accumulations of Foraminifera, crinoids, brachiopods, bryozoans, bryalgal and coralgal materials, and molluscs are best classified here. A discussion of textural types and groups among limestones (and dolomites) is incomplete without reference to some of the terms extant in the literature. The names calcirudite, calcarenite, calcisiltite, calcilutite, calcargillite, and many others are established in geologic literature, and need no further discussion here. FOLK (1959, 1962) has extended or modified this list to include such terms as “intrasparrudite”, “intramicrudite”, “oosparrudite”, “pelmicrite”, and others. Such terms as course-grained,$ne-grained, medium-crystalline, aphanitic, etc. are particleand crystal-size terms for which there is no general concensus. LEIGHTON and

100

H. J , BISSELL AND G. V. CHILINGAR

TABLE 111 SIZE CLASSIFICATION

(After WENTWORTH, 1922; modified by LEIGHTON and PENDEXTER, 1962) ~~

Grade-size scales (mm) --

~

~ _ -

8.0

_

_

_

_

_

~~

~

WENTWORTH (1922) -~

._

-

~~

LEIGHTON and PENDEXTER (1962) _

Pebble gravel

4.0

_

_

_

_ ~

~

.

~ -~

.~

Very coarse sand ~Coarse sand

1 .o 0.5

~

~~~

~

Coarse-grained ~

Medium sand

Medium-grained

Fine sand

Fine-grained

~-

~~

0.125

~.

Very fine sand

0.0625

~~

Very coarse-grained

~

0.25

____~-

Breccias and conglomerates

~~

Granule gravel

2.0

0.0312

Reference

~.

~-

~

~

Very fine-grained ~~

-

Silt

Coarselv

Micrograined

0.016

0.002 0.001

Clay

Cryptograined

PENDEXTER (1962, p.52) modified the Wentworth grade scale, that was proposed for “clastic sediments”, to apply to carbonate rocks, as shown in Table 111. The above textural terminology may find a certain application for petrologists and petrographers; it is hardly scientific when an author’s description of limestone states that the rock is “fine-grained’’ when in reality it is micritic, (1959, 1962) also modified the Wentfinely crystalline, or cryptocrystalline. FOLK worth scale, but used a crystallinity scale for the “authigenic constituents”. A comparison of some of the prevailing particle size scales is presented in Table IV. It is herein pointed out that certain terminology of DEFORD(1946) has utility, and has generally been overlooked by petrographers. The term aphanitic is still used by numerous sedimentary petrographers, although the term aphanic is preferred. MOLLAZAL (1961, pp. 14-18) discussed crystallinity of limestones, particularly those that have been diagenetically altered. In following usage of DEFORD (1946), he applied the term aphanic to those limestones which have a crystalline

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

101

(and/or grained) texture, the discrete particles of which are smaller than 0.004 mm in size. Microcrystalline and cryptocrystalline textures are also included here; but aphanic is a term to apply in the field when magnification no greater than 10 x hand lens is available. Subsequent work in the laboratory with thin sections and polarizing microscope will determine if it is micro- or cryptocrystalline. In following the usage of DEFORD (1946) and the variation of MOLLAZAL (1 96 1, p. 1S), the writers believe that the textural term aphanic should replace the term aphanitic for sedimentary carbonate rocks, and that the upper limit of particles should be considered as 0.01 mm. Both grained and crystalline textures are included in Table IV; micrograined and microcrystalline, and for the smaller particles, cryptograined and cryptocrystalline are appropriate terms, defined in the laboratory. Aphanic is an excellent field term, particularly for micritic limestones and dolomites, lithographic limestones, etc. On the basis of extensive research work, KHVOROVA (1958, p.1 I ) proposed the following useful classification: ( I ) very coarse-grained (or crystalline)--> 1 mm: (2) coarse-grained-0.5-1 mm; (3) medium-grained-0.25-0.5 mm; ( 4 ) finc-grained -0.1-0.25 mm; (5) very fine-grained-0.01-0.1 mm; (6) micrograined - <0.01 mm; and (7) cryptograined (pelitomorphic, cryptocrysta11ine)- <0.005 mm. Additional proposals that are made at this time include revisions in size limits of the phaneric sedimentary carbonate-rock particles. Inasmuch as textural classification includes grained as well as crystalline particles, it is important that the petrographer should distinguish between these in both field and laboratory studies. The term macrocrystalline is used, with slight modification, after that of HOWELL(1922); this also includes macrograined textural types if they are macroclastic. Limits of size grades are identical to those given by DEFORD (1946) for megagrained rocks. Perhaps the terms megagrained and megacrystalline are more appealing to some geologists than the terms adopted by the writers. The terms mesocrystalline and mesograined are valuable, and although d o not correspond precisely to the lower limit set by DEFORD (1946) are, nonetheless, within the limitations of normal routine field and laboratory investigations. It is not necessary that the medium- and coarse-textured sedimentary carbonate textural types correspond precisely with the size-grade limits of the WENTWORTH (1922) sandstone textural groups. Sands are segregated into textural types and sizes commonly by sieving, whereas carbonates are examined with the hand lens and microscope and, therefore, lend themselves to study by the scales indicated herein. Aphanic has utility in both field and laboratory studies as a scale size; in the laboratory it can be ascertained if the carbonate has a microcrystalline or micrograined (i.e., microclastic) texture, or should be termed cryptocrystalline or cryptograined. It is possible to determine finely crystalline or finely grained textures in the field with the 10 x hand lens, and more precisely establish size limits later in the laboratory. With the above information available, it should be possible to classify more objectively the sedi-

102

H. J. BISSELL AND G. V. CHILINGAR

TABLE IV PARTICLE SIZE SCALES

Kr nine (1$48)

2 oarsely :rystalline grained) 4.0

2.0

1.0

Howell (1922)

DeFord (194 6)

Petti'ohn (194d)

Macrocrystalline

Megagrained

Calcirudite

Medium :rys t alline grained)

Tinely xystalline grained)

Calc-. arenite

MesoCrystalline

Mesograined

Microcrystalline

Paurograined

0.5

0.25

0.1

0.062

Calcilutite

0.05 0.025

0.016

0.01

0.004

Cryptocrystalline Micrograined

0.002i

0.001

Crypto:rained

103

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

I

I

I

(1-961)

Folk The present (1959,1962) authors

stalline

Extremely coarsely crystalline

Mollazal

' MacrocryCoarse Medium Fine

1

Coarse

4.0

Very coarsely Crystalline

2.0

1.0

Coarsely crystalline

0.5

Medium Medium crystalline

Fine

0.25

lI

0.1

Fine

fine

Finely crystalline

crystalline

Size (mm)

crystalline Finely

0.062 0.05

(grained)

0.025

I

Very finely crystalline Microxystallinc

0.016

Microcrystalline (grained)

0.01

0.004

Aphano crystalline

0.0025

Cryptoxystalline

Cryptocrystalline (grained)

0.001

104

H. J. BISSELL AND G. V. CHILINGAR

mentary carbonate rocks, particularly by those workers who investigate the problems of diagenesis and permeability-porosity. Any scheme of sedimentary carbonate-rock classification should take into account, if at all possible, the energy level of the depocenter (= depositional environment). PLUMLEY et al. (1 962) have provided detailed information concerning this parameter in the classification and stated that: “The depositional energy level, which is a function of wave and current action, varies in space and time and leaves its record in the rocks” (PLUMLEY et al., 1962, p.86). Their classification plan includes five major limestone types and fifteen subtypes based upon interpretation of the energy level, and in part upon the biota. Sedimentary petrologists are more aware than ever that fossils are part of the rock and are not primarily a tool for the paleontologist. In many instances fossils provide the data necessary for environmental interpretation where other evidence might be open to subjective appraisal. Algae in particular fit this category. and POWERS In their classification of Arabian carbonate rocks, BRAMKAMP (1958, pp.1305-1317) indicated two major energy types: ( I ) quiet-water deposits, and (2) current-washed deposits. Coarse carbonate clastics (calcirudites) and some calcarenites, as would be expected, were classified as current-washed deposits; whereas calcarenitic limestones and fine-grained limestones (calcilutites) were assigned to the quiet-water group. CHILINGAR and BISSELL(1963a, table 3, p.9) utilized a somewhat similar plan in their classification of limestones. RICH(1963, 1964) expanded the carbonate-rock classification scheme of BRAMKAMP and POWERS (1958), but did not stress the energy-level factor. It is herein recommended that petrologists and petrographers devote considerable time to the objective study of this parameter in sedimentary carbonate-rock classification. The study of turbidites, for example, is not limited to noncarbonate rocks, and the field worker can map and plot the major depositional energy levels; he can also enhance these studies objectively with detailed laboratory studies. Serious consideration should be given, it is contended, to the Energy Irzdex (EI) classification of PLUMLEY et al. (1962). They proposed the following five water; 11-intermittently agitated water; 111-slightly major types: I-quiet agitated water ;IV-moderately agitated water; and V-strongly agitated water. As they pointed out, each type is a pigeonhole with boundaries that, although arbitrary, can be determined by semi-quantitative and qualitative analysis of the primary textural properties. BOUMA(1962) demonstrated that an objective study is possible with noncarbonate sedimentary rocks; it is herein contended that an equally valid scientific and objective approach is possible with all sedimentary carbonate rocks. Field work is an obvious prerequisite to detailed laboratory investigations; one discipline must complement the other. PLUMLEY et al. (1962) have provided detailed information relating to criteria by which a semi-quantitative interpretation of agitated-water environments can be made; their paper is generally available, and that information need not be repeated here. Geologists are devoting

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

105

considerable time and energy mapping environments, and any worthy tool must be given serious consideration (see BOUMA, 1962; ~ M B R I Eand PURDY,1962; and G. E. THOMAS, 1962). LEIGHTON and PENDEXTER (1962, p.50) suggested that determination of the grain/micrite ratio has value in their textural classification of limestones. This ratio appears along the vertical axis of their chart, and is the sum of the percentages of grains divided by the percentage of mud-like material (micrite). Essentially, this grainlmicrite ratio (GMR) is equal to:

% (detrital grains mineral grains

+ skeletal grains + pellets + lumps + coated grains + % micrite

As pointed out by LEIGHTON and PENDEXTEK (1962, p.50), the following ratio may be of value in studying limestones with organic structures: (framebuilder grains)/micrite ratio. Both micrite and matrix appear in the G M R ratio; micrite is the lime mud having an aphanic t o finely crystalline (or grained) texture, whereas matrix consists of interstitial material between grains that may be difficult to assign to one of the followi~igcategories: detrital or skeletal grains, pellets, coated grains, or lumps. Matrix should not be confused with micritic material. PLUMLEY et al. (1962, pp.86-87) noted that matrix is defined as the material in which any sedimentary particle is embedded, and may be microcrystalline or granular. In his studies of the limestones of the Bird Spring Group (CarboniferousPermian) in southern Nevada, RICH(1963, 1964) proposed a classification which is a modified form of the classification of BRAMKAMP and POWERS (1958) combined with some of the features of the carbonate classifications of FOLK(1959), CAROZZI (1960), and WOLF(1960); see also LEIGHTON and PENDEXTER (1962). RICH(1963, pp. 1662-1 665; 1964) showed percentages of clastic noncarbonate particles and of allochemical grains in his charts. Emphasis is placed by the writers on objectivity in determining the relative proportions of the textural components of sedimentary carbonate rocks. A limestone may originate through the induration of lime ooze (mud), and thus be termed micrite; or it may consist of a major proportion of grains with only very minor amounts of micritic or sparry cement. A grain/micrite ratio serves the purpose of assigning a numerical value which denotes the gradations between the two extremes. These divisions, it is true, are arbitrarily drawn and if for no other reason than semantics allocate rock types to a nomenclatural system. Semantics, however, is not the ultimate reason for carbonate classification, although a language must be available to permit the exchange of ideas and concepts. Thus, a high GMR (> 1) indicates that the rock contains more than 50 % grains, and one with a GMR of 1 defines a rock with about 50% grains and 50% micrite material; finally if

+

106

H. J. BISSELL A N D G. V. CHILINGAR

the GMR is < 1 the rock contains less than 50 % grains. This is shown in the classification of limestones by MOSHER and PINNEY (1963) as modified after the one by L EIGHTON and PENDEXTER (1 962). In any tabular form of rock classification, various “pigeonholes” are usually set up arbitrarily for the grain types, and many rocks consist largely of one textural type, such as micrite, calcarenite, etc. Seemingly, most limestones contain two or more grain types. Where two or more grain types are present in approximately equal amounts in the rock, it is suggested that the two names be hyphenated, such as detrital-micritic limestone. Most generally, however, one grain type predominates over the other. In this case, the limestone is given the name of the predominating grain type with the other as an adjectival modifier, or essential prefix. For example, the carbonate may contain 60% skeletal grains and 40% detrital grains; this would be termed detrital-skeletal limestone. If the rock is composed of 15 % detrital grains and 85 % skeletal grains, however, it is termed detrital, skeletal limestone. In other words, the contents of grains (in %) are arranged in tabular form in the order of increasing percentage in the classification scheme. A rock composed of 10 % detrital grains and 70 % skeletal grains, all embedded in micritic material composing 20 % of the whole, could be named detrital, micritic, skeletal limestone. If detrital grains and micrite are present in about equal amounts, such as 15 % each, and skeletal material comprises the remaining 70%, the rock is named detritalmicritic, skeletal limestone. The name should be euphonious, and pronounceability will dictate the order of arrangement where the hyphen is used. WOLF(1960; see also LEIGHTON and PENDEXTER, 1962) applied a plan of pigeon-holing limestones according to textural types; the vertical and horizontal lines in their charts are arbitrarily assigned for guidance, not limitation. For example, a carbonate may consist of approximately equal amounts (say 30% each) of detrital, skeletal, and pelletal material, cemented with micrite; it could, according to the suggestions advanced by the present writers, be termed detrital-skeletalpelletal limestone cemented with micrite. It may be more pleasing to the ear of a listener to term the rock a detrital-pelletal-skeletal limestone cemented with micrite. It would not be termed micrite; perhaps preference would dictate that it should be identified as a micritic, detrital-pelletal-skeletal limestone. This differs in no fundamental fashion, other than constituents involved, from the naming of an igneous, or a metamorphic rock; for example, quartz-muscovite-albite schist, etc. CLASSIFICATION OF DOLOMITES

General statement

Dolomites have been defined as carbonate rocks composed of more than 50 % by weight of the mineral dolomite (LEIGHTON and PENDEXTER, 1962, p. 53). Some

107

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

TABLE V CLASSIFICATION OF ROCKS INTERMEDIATE IN COMPOSITION BETWEEN PURE LIMESTONES AND DOLOMITES

(After data by CAYEUX, 1935; CAROZZI,1960, p.264) Content (%)

Rock name

Limestone Magnesian limestone Dolomitic limestone Calcitic dolomite Dolomite

calcite

dolomite

>95 90-95 50-90 10-50 <10

<5

5-10 10-50 50-90 >90

geologists, however, prefer the term dolostone (cf. RODGERS, 1954) for the rock in which dolomite content exceeds 50%. It is a truism that most geologists regard any carbonate rock that contains less than 50 % calcite as a dolomite. As pointed out by PETTIJOHN (1957, p.416), despite the possible ambiguity arising from the use of the same term (dolomite) for both the mineral and the rock, this term will probably continue to be used for both. CAROZZI(1960, p.264) stated: “Dolomites are carbonate rocks primarily composed of the mineral dolomite.” The context invariably shows which is which. CAYEUX (1935) classified the rocks intermediate in composition between pure limestones and dolomites as given in Table V. TEODOROVICH (1958, p.299) recognized several groups of dolomite as given in Table VI. Some dolomites contain magnesite in addition to calcite and dolomite, TABLE VI TEODOROVICH’S CLASSIFICATION OF DOLOMITES

(After TEODOROVICH, 1958, p.299) Group name

Content (%) dolomite

Clayey dolomite Slightly-clayey calcitic dolomite Slightly-clayey dolomite Dolomte Slightly-calcitic dolomite Calcitic dolomite Highly-calcitic dolomite

35-90 90-45 95-85 100-90 95-80 80-65 65-47.5

calcite

045 5-47.5 0-5 0-5 5-20 15-35 30-50

clayey material

30-10 5-10 5-10 0-5 0-5 0-5 0-5

108

H. .I. BISSELL AND G . V. CHILINGAR

TABLE VII FROLOVA’S CLASSIFICATION OF DOLOMITE-MAGNESITE-CALCITE

SERIES

(After FROLOVA, 1959, p.35) Name

Content (%) dolomite

Limestone Slightly dolomitic limestone Dolomitic limestone Calcitic dolomite Slightly calcitic dolomite Dolomite Very slightly magnesian dolomite Slightly magnesian dolonlite Magnesian dolomite Dolomitic magnesite Slightly dolomitic magnesite Magnesite

5-0 25-5 50-25 75-50 95-75 100-95 100-9 5 95-75 75-50 50-25 25-5 5-0

-

calcite

95-100 75-95 50-75 25-50 5-25 0-5 -

~

_

_

_

magnesite

-

0-5 5-25 25-50 50-75 75-95 95-100

CaOIMgO ratio

> 50.1 9.1 -50.1 4.0 -9.1 2.2 -4.0 1.5 -2.2 1.4 -1.5 1.25-1.4 0.80-1.25 0.44-0.80 0.18-0.44 0.03-0.18 0.00-0.03

and thus FROLOVA (1959, p.53) proposed a new classification of dolomite-magnesite-calcite series (see Table VII). On the basis of Ca/Mg weight ratios, CHILINGAR (1957a) recognized the following groups of dolomites: ( I ) magnesian dolomites (Ca/Mg = 1.0-1.5), (2) dolomites (Ca/Mg = 1.5-1.7), (3) slightly calcitic dolomites (CaIMg = 1.7-2.0), ( 4 ) calcitic dolomites (Ca/Mg = 2.0-3.5). The upper limit of calcitic dolomites was chosen because the Ca/Mg ratio of 3.44/1 is the lowest ratio known in skeletal structures of organisms (Goniolithon, a calcareous Algae, has a Ca/Mg ratio of 3.44/1). A pure dolomite has a ratio of 1.648/1, and some dolomites contain an excess of magnesium (magnesian dolomites). Before discussing the classification of dolomites, it is important that attention should be directed to some of the prevailing concepts regarding types of dolomites. For example, VISHNYAKOV (195 1,p. I 12) recognized the following major genetic types: (I) epigenetic, (2) diagenetic, and (3) primary. He considered those dolomites as epigenetic which resulted from the alteration of completely lithified limestones either by downward percolating meteoric solutions or by rising hydrothermal solutions, mainly along fractures. These dolomites have obscure stratification, patchy distribution, non-uniform grain size (clear-grained), relic structure and are cavernous. Diagenetic dolomites are considered to be those of great extent and volume; they were formerly limestones. Fossil relics are commonly present in diagenetic dolomites. Primary dolomites result from direct chemical precipitation out of water, and are recognized when associated with other primary sediments. They are well stratified and are pelitomorphic ( ~ 0 . 0 1or ~0.005-0.003mm, with

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

109

coagulating centers in some), possess characteristic microlayering, and very rarely contain fossils. Primary dolomites lack appreciable porosity (in fact are normally rather dense) and are commonly interlayered with evaporites, clays, marls, micrite with suspended oolites, gypsum (commonly containing pseudomorphs after gypsum), and oopelletal to micritic limestones with bladed and/or spherical to subspherical algal bodies. TEODOROVICH (1958, p.303) does not believe in very widespread occurrence of epigenetic dolomites because of difficulty in explaining the source of huge amounts of magnesium which is necessary to effect dolomitization. MCKINLEY(1951, pp. 169-1 83) categorized the theories relating to marine replacement of limestones to form dolomites as follows: ( I ) penecontemporaneous replacement, (2) syndiagenetic replacement, and (3)post-diagenetic (i.e., epidiagenetic) replacement. He defined penecontemporaneous as that type of replacement that takes place immediately after deposition of the sediment, before consolidation or lithification into rock, and prior to superposition of any great thickness of additional material. According to him, syndiagenetic dolomitization takes place during diagenesis of calcareous material, and should be considered a factor in transformation of the sediment into rock. Post-diagenetic replacement (which McKinley indicated as occurring in the marine realm) involves alteration of the completely lithified limestone to dolomite by the action of sea water, or of static connate waters. He pointed out that many dolomitized reef limestones appear to have been altered after lithification by continued exposure of the calcareous reef rocks to sea water in an environment conducive to replacement. SKEATS(1918), who studied coral reefs in the south Pacific, concluded that marine dolomitization took place in shallow water (and as deep as 150 ft.), under slightly increased pressure, with abundant COZ,and in porous limestone allowing free circulation of sea water. The present chapter is not a treatise on processes of dolomitization, but rather it relates to problems of classification. The foregoing is, therefore, but a brief discussion serving as a prelude to an organization of these carbonates into a rational scheme of classification. According to LEIGHTON and PENDEXTER (1962, pp. 53-57), the different types can best be accomodated by a descriptive system of nomenclature, one based primarily on compositional grouping and with appropriate modifying textural terms. They recognized two groups: (1) calcareous dolomitesthose containing 50-90 % dolomite, and (2) dolomites -those containing 90 % or more dolomite. For each type, description in parentheses follows the major name to designate the precursor; for example, a rock may be termed calcareous dolomite (calcareous skeletal fragments with calcareous matrix and cement). Schemes of carbonate-rock classification, which include both limestones and dolomites, by their very nature assume that the latter resulted from alteration of the former. This works very well for dolomites which originated through diagenesis af the various types of limestones; the classification scheme then requires only spaces in the table to indicate the degree of alteration. BRAMKAMP and POWERS

110

W u)

2 E 0

5

n W

LL

_I

4 IW

I 0

F z 5

W

0

Clear-grained relic

Micro-grohed relic

Cleor -grained dolomites

Micro-grained dolomites

relics

negofive

wilh

wifh structure

with negative sfructure

Wifh relic. chemical or bio-chemical strucfure

With relic-organic structure

micro-groined

pisolitic

plsolltlc

-

and

pisolitic

bean-shoped

e,

13

-

] 5C8x

8 ._ v

cr

a

3

1 'f

I cd

v

F

18

0

15

1 V6

3

cn

m

m"

lc! a

rn

'0

0

1 .^ %

3

U

-

Fa

9

I A 1 %

19

H. J. BISSELL AND G. V. CHILINGAR

Other

Cryptograined-pelitomorphic from irregular groins

Detrital

Nodules, onlltes, etc

or

Detrital and biomorphic Biomorphic

Oolitic Detrital Biornorphic-detritol

or

Biornorphic oollllc Detrifal

breccia

Biomorphic - detrital

Biomorphic

SlltY Sandy

etc.

Conglomerate and

Radiolife,

Oolitic-like

and

Lumpy (or clotted or nodular1 and micro- lumpy

Oolitic

5

+

3

8 Bio-dctrital Biomorphic - d e t r i l a l

.-

a

Biomorphic

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

111

(1958), FOLK(1959, 1962), MOLLAZAL (1961), POWERS (1958), TEODOROVICH (1962), CHILINGAR and BISSELL(1963a), RICH (1963, 1964), and others have and POWERS(1958), for successfully demonstrated this possibility. BRAMKAMP example, were among the first to allocate “pigeonholes” in their chart for those limestones that are not visibly altered, those that are moderately altered, and those that are strongly altered, and for those whose original texture has been obliterated. RICH(1963, 1964) modified the classification of BRAMKAMP and POWERS (1958) by assigning percentage parameters to the various degrees of alteration, and by adding such identifying names as “porphyroblastic” and “granoblastic” to two of the columns. MOLLAZAL (1961) indicated relative degree of diagenetic alteration of limestones by assigning objective data relating to size and fabric of crystals. TEODOROVICH (1958, p.295; also in: BISSELL and CHILINGAR, 1961, p.614) devised a plan of classification (Fig.4) which indicates whether the relics are positive (former fossils and minerals identifiable) or negative (former fossils and minerals discernible but not readily recognizable). In his discussion of carbonate rocks, CAROZZI (1960, pp.264-284) was concerned with the petrography (and minor petrology) of dolomitic sediments, and categorized them into the following groups: magnesian limestones and magnesian chalks, dolomitic limestones, bioconstructed dolomitic limestones, bioaccumulated dolomitic limestones, fine-grained dolomitic limestones, dolomi tic chalks, dolomitic lithocalcirudites and lithocalcarenites, dolomitic biocalcirudites and biocalcarenites, dolomitic oolitic calcarenites, dolomitic calcilutites, and dolomites. This approach to the study of these carbonates has merit, and obviously could be a guide to the orderly-minded investigator. It is obvious that dolomites are susceptible to classification utilizing two major parameters, viz.: (I) relic textures (and structures), and (2) crystallinity (which is a function of degree of alteration in diagenetic dolomites, but is commonly lacking in primary types). It may be pointed out that some dolomites which have a high degree of crystallinity (including medium- and coarse-crystalline varieties), lack all vestiges of relics, positive or negative. It is not to be inferred, however, that in all cases high degree of crystallinity is indicative of complete obliteration of relics. It is possible that primary dolomites of the evaporitic and otherwise restricted environments may lose their aphanic to fine-textured character and become medium- to coarsely-crystalline during diagenesis just as limestones do. By utilizing the particle size grade chart (see Table IV), this objectivity in assigning degree of crystallinity can be realized. Furthermore, if the original carbonate was a variety of limestone and has been diagenetically dolomitized, the charts devised by BRAMKAMP and POWERS (1958), POWERS (1962), or the modified version of RICH(1963, 1964) can aid in assigning a percentage parameter to this type of dolomitization. Eventually, petrographers may be able to state more objectively the stage of diagenesis that was attained (such as early or penecontemporaneous, medial, and late).

112

H. J. BISSELL AND G. V. CHILINGAR

LEIGHTON and PENDEXTER (1962, p.57) stated the following concerning the genetic implications in naming of dolomites: “In spite of attempts to avoid genetic implications in naming dolomites and dolomitic rocks, it is impossible to do so completely.” They cited the example of “dolomitic limestone”, and also pointed out that the association of finely micrograined dolomites, laminated dolomites, and dolomitic breccias with anhydrite, chert, and microcrystalline micritic limestone, has led t o the use of the term primary dolomites in evaporitic sequences. Some TABLE VIII CHARACTERISTICS OF DOLOMITE GROUPS~

(After LEIGHTON and PENDEXTER, 1962; modified by the present authors) _ _ _ _ _ ~ ~ Dolomites in evaporitic sequences

Dolomitized rocks

~.

~~~

-

Generally finely micrograined.

May be very coarse grained (sucrosic).

Commonly aphanic t o finely crystalline with uniform texture.

May be meso- to macrocrysfalline. Associated with limestone sequences.

May be laminated or brecciated.

May contain fossil molds.

May be interlayered with dolarenites.

May contain relic limestone textures and belong to gradational sequences showing increased development of dolomite.

May have admixed clay andfine silt. Associated with anhydrite, chert, and microcrystalline micritic limestone.

May be associated with transgressive or regressive deposits that display truncation of other beds.

May be associated with micrific limestones. (some of which may contain “floating” oolites and pellets).

Positive and negative relics of fossils, pellets, detrital particles, lumps and coated grains in dolorhombic mosaic.

Associated with red shales and siltstones.

Reef and bank deposits retain topographic .form but are nodular, lumpy, vary-grained (and may be cavernous, andlor brecciated) .

Associated with gypsum and anhydrite that contain scattered dolorhombs. Contain no relic limestone texture.

Areally extensive, moderate to large volume, subtle facies changes.

May be interbedded with dololutites some of which are bituminous.

May correlate with tectonic features, fault zones, anticlines, etc.; or with former land surfaces.

Collapse limestone- and dolomite-breccias may be present.

Variation in porosity and permeability, from low to very high values. ~

ICharacteristics in italics were added by the present authors.

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

113

petrographers avoid the word “primary” when referring to the dolomites in such a sequence, but this appears to be no more than a play on words. If the entire sequence is an evaporite sequence or suite, then the dolomite is possibly an evaporite; if it is “primary” then no justification can be found to avoid the term. It should be remembered, however, that some very early diagenetic dolomites are classed as “primary” by many geologists (see BISSELL and CHILINGAR, 1962). After making a survey among various geologists concerning the terminology of carbonate rocks, RODGERS (1954, p.232) stated: “ . . . common American usage calls dolomite primary only if the particles were dolomite when first formed, as by direct precipitation from sea water; otherwise they are secondary.” SANDER(1936) termed dolomite primary if the particles were dolomite when they reached their present position in the rock fabric, secondary only if they are replacements of some material that occupied the same position. Rodgers has made an excellent point, in that if the terms “primary” and “secondary” are to be used, their meaning must be defined. LEIGHTON and PENDEXTER (1962, pp.57-58) pointed out that each of their two groups of dolomites has particular textural characters and rock associations; these are listed in Table VIIi, with additions of the present authors.

PROPOSED LIMESTONE AND DOLOMITE CLASSIFICATlONS

The proposed classification of limestones in this chapter (Table IX) is intended to demonstrate the utility of application of these parameters: ( I ) composition, (2) texture, (3) grain/micrite ratio (GMR), and (4) energy index (Ei). i f properly applied, these will also be of great value in dolomite classification, particularly for the diagenetically altered limestones. Of utmost importance is the identification of types and relative amounts of carbonate grains (detrital, skeletal, pelletal, lumpal, coated, etc.) in the field with no more than 10 x hand lens, dilute (3 %) HCI, and eye-dropper bottle with glycerin and water (the latter is merely a wetting medium). The field worker (petrologist) should also be able to make a semiquantitative estimate of grain/niicrite ratio, and/or grain/matrix ratio. The classification should also have utility in the laboratory where microscopes, chemicals, and other materials and equipment (such as X-ray, DTA, etc.) can establish greater precision and thereby provide adequate definition. it is argued that any classification should not be a mere play on words which would result in a semantic struggle; the present authors believe that, after certain basic definitions, if a word is not self-explanatory it has no real value to the sedimentary petrologist and petrographer. The grain/micrite ratio shows the relative amounts of coarse and fine-textured carbonate material; this theoretically is controlled by wave or current action and thus gives an insight into the degree of agitation. If the worker can arrive at

114

H. J. BISSELL AND G. V. CHILINGAR

TABLE IX CLASSIFICATION OF

LIME ST ONES^

(Modified after LEIGHTON and PENDEXTER, 1962; PLUMLEY et al., 1962; and others) Energy index (EI)

Graiiz/ Percentage Moved and deposited by waves micrite of grains and currents ratio abraded grains

detrital

skeletal

Detrital limestone2

Skeletal limestone

-

Strongly agitated (growth and deposition in strongly agitated HzO)

Moderately agitated (deposition in moderately agitated HzO)

9/1

75

Slightly agitated (includes to-andfro HzO action)

Intermittently agitated (alternately agitated and quiet HzO) Relatively quiet (deposition in quiet HzO not necessarily stagnant-may be gently agitated)

90

1/1

50

25

119

10

________

Micritic, detrital limestone3

Micritic, skeletal limestone

Micriticdetrital limestone

Micriticskeletal limestone

Detritalmicritic limestone

Skeletalmicritic limestone

Detrital, micritic limestone

Skeletal, micritic limestone

Micritic limestone

Micritic limestone

Horizontal combinations are useful, as detrital-skeletal, oolitic-micritic, etc. If size connotation is deserved, such terms as calcirudite, calcarenite, calcisiltite, calcilutite, etc. may be used instead. If the particles in the rock are of different orders of size magnitude, and are larger than micrite, the term matrix is used for the smaller individual units that fill the interstices between larger grains.

1 2

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

115

Accumulated in place _.

~~

accretion-aggregation grains ~

organic frame builders

pellets

lumps

coated grains

Pelletal limestone

Lump limestone

Oolitic, pisolitic, oopelletal, algal, etc. limestone

Colonial corals, stromatoporoids, colonial Algae, bryalgal, bryozoan, foraminiferal, etc. limestone

Micritic, pelletal limestone

Micritic, lump limestone

Micritic, oolitic, pisolitic, etc. limestone

Micriticpelletal limestone

Micriticlump limestone

Micriticoolitic pisolitic, etc. limestone

Micritic coralline, algal, bryalgal, etc. limestone (use appropriate comma and/or hyphen for correct combination)

Pelletalmicritic limestone

Lumpmicritic limestone

Ooliticpisolitic etc. micritic limestone

Pelletal, micritic limestone

Lump, micritic limestone

Oolitic, pisolitic, etc., micritic limestone

Micritic limestone

Micritic limestone

Micritic limestone

Corailine, algal, bryozoan, bryalgal, etc. micritic limestone (use appropriate comma and/or hyphen for correct combination)

chemical, biochemical

a

s

116

H . J. BISSELL AND G. V. CHILINGAR

some measure, even semi-quantitatively, of the level of energy, he can ultimately set up a more objective energy index. Consequently, some measure of success will result in identifying turbulence, energy level, and, of course, the most sound interpretation of particular environments within the depocenter. The grain/micrite ratio (GMR) and energy index (EI) can be used as indicators of the amount of physical (mechanical) energy necessary to transport and deposit the carbonate sediment. These are shown in the left-hand column of the proposed classification (Table IX). The remainder of the chart contains the various compositional-textural groups, and their combinations. Various sedimentary carbonate-rock classifications have been published within the past quarter-century, and many of these are worthy of careful analysis by the student of sedimentary petrology and petrography. The present authors have tested some of these classifications in recent field and laboratory investigations; perhaps certain classifications appear to have more value than others, largely because some proposals relate to a certain province and, therefore, represent the geologist’s particular experience in that area. The classification proposed in this chapter is, therefore, a reflection of the present authors’ researches in the limestone provinces in the western United States. The classification of FOLK (1959, 1962) was proposed for Beekmantown (Ordovician) rocks of Pennsylvania; it has been applied successfully in certain other regions. The classification of LEIGHTON and PENDEXTER (1962) was set up originally in Jersey Production Research Company’s Carbonate Rock Manual and was tested in Montana and contiguous areas, and later was given a rigorous test on Paleozoic carbonates in the Great Basin, Rocky Mountains, and Colorado Plateau regions of western United States. Subsequently, it has been shown to have utilitarian value in many other places. MOSHER and PINNEY (1963), for example, slightly modified this classification, and demonstrated its value for most carbonate rocks. PLUMLEY et al. (1962) developed a scheme of limestone classification which incorporates the parameter of energy index (EI), thereby providing for a certain objectivity in environmental interpretation. The present authors have added this parameter in the classification given in this chapter. DUNHAM (1 962) classified carbonate rocks according to depositional texture; these textures are: ( I ) mudstone (less than 10% grains), (2) wackestone (more than 10% grains), (3) packstone, ( 4 ) grainstone, and (5) boundstone. I and 2 are mud-supported, 3 is grain-supported, and all three contain mud; 4 lacks mud and is grain-supported. These four textural groups d o not show evidence of the original components having been bound together during deposition. Boundstones, by contrast, are those limestones in which the original components were bound together during deposition, as shown by intergrown skeletal matter, lamination contrary to gravity, and other features. If the depositional texture is not recognizable, it was recommended that this type of limestone be termed a crystalline carbonate (DUNHAM, 1962, p.121). PRAYand WRAY(1963) indicated that the classification of DUNHAM (1962) proved

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

117

useful in their studies of the porous algal limestones of Pennsylvanian age along Honaker Trail in the canyon walls of the San Juan River, southeastern Utah. BRAMKAMP and POWERS(1958) and POWERS(1962) demonstrated the utility of their classification of limestones and diagenetically altered limestones for Arabian Upper Jurassic reservoir rocks. G. E. THOMAS (1962) tested his classification of carbonate rocks on selected Paleozoic carbonate cycles and reef complexes in a western Canada basin; in his scheme of classification, carbonate rocks can be described and grouped into textural and porosity units for mapping purposes. NELSON et al. (1962) used the term skeletal limestones for rocks which consist of, or owe their characteristics to, the in-place accumulations of calcareous skeletal material. Their limestone classification is, therefore, one in which the rock is classified according to the organism primarily responsible for its formation. These workers reviewed usage of the terms reef, bioherm, biostrome, and bank, and related terms, as well as making certain recommendations as to correct usage. Attempts have been made to classify the modern Bahamian carbonate sediments; one such classification is that of IMBRIE and PURDY(1962). Their scheme identifies five discrete sample groups, as follows: ( I ) oslitic, (2) grapestone, (3) coralgal. (4) oolite, and (5) lime-mud facies. They tested their scheme of classification together with that of FOLK(1 959), with substantial success. Their shelf lagoon sands, for example, include the oolite, oolitic, and grapestone facies, and are comparable to the oosparites and intrasparites of Folk. Outer platform sands of the Bahamas embrace their coralgal facies, equivalent to Folk's biosparites and biopelsparites. Their muddy sands of the shelf lagoon are the lime-mud facies, thus equivalent to oomicrites, intramicrites, biomicrites, biopelmicrites, and pelmicrites of Folk. RICH(1963, 1964) proposed a limestone classification, based on his studies of rocks of Late Paleozoic age in southern Nevada; his classification is, however, mostly a modification of the scheme of BRAMKAMP and POWERS (19x9, and of others. Many of the classifications just mentioned were published by the American Association of Petroleum Geologists in 1962 as a symposium Classification of Carbonate Rocks (HAM,1962). This work is generally available, and the charts which it contains need not be reprinted here. BAARS(1963, p.101) pointed out that this ". . . symposium is a monument to the advancement of our understanding of the carbonate rocks and is highly recommended as an introduction to the study of carbonates. However, no particular classification was found to be mutually agreeable to all authors. And so it goes. For every carbonate petrographer there is a unique system of classification." Although BAARS(1963) did not propose a scheme of classification of limestones, he did discuss them in terms of these compositional types: lime mud (= micrite or matrix), particles or grains, cement, and pore space. He recognized five kinds of particles (which are the ones suggested by LEIGHTON and PENDEXTER, 1962), as follows: ( I ) skeletal particles, (2) pellets, (3) coated grains, (4) detrital particles, and (5) composite particles.

118

H. J. BISSELL AND G. V. CHILINGAR

TABLE X CLASSIFICATION OF DOLOMITES

(Modified after SHVETSOV, 1958) Main genetic groups of dolomite h Y

9 9 2

a

Y

.i;

.e Y

a

v

z Y

E .-

Major part organic

_

Reefs

Coralline, algal, bryozoan, etc.

Banks

Biostromes, layered, coquinites

-~

_ -~

Major part detrital

_

_

_

Pellets -

2;

~

~

~

~.

-

~

Lumps

Moderately to strongly altered (but preserving vestiges of texture and composition of original rock)

~

~~

~

--

~~

Positive pelletal texture

-

~

-

Major part chemical or biochemical

~

~

Dolarenite (original dolomite sand) Dolomitized lithocalcarenite

Lumps and composite

0

~

Dolomitized chalk

_

~

_

~

~

Micrograined

Detrital

-~

-~ ___ -Foraminiferal, sparry criquinites, brachiopods, molluscs, algal

-

Skeletal

(u

n

Characteristics, features, examples, varieties

~~-~~-

Y

-

Main textural types

.~

Megalumps to microlumps

~-

Algal lumpal dolomite

concentric rings

grains ~

-

-~

Superficial oolites and pisolites

Sutureddolorhombssurrounding skeletal-detrital particles Brecciated lumpal dolomite

Dolomitized spherical to subspherical grains with few concentric rings Skeletal, detrital, pelletal, etc. elements as positive relics

-~

-~

Matrix may be micrograined calcite ~

_-

__

Vary-grained, with mosaic of Matrix-micromosaic of dolorhombs sutured incompletely rhombic dolomite surrounding positive Sucrosic dolomites, including sandstones being ieplaced; relics skeletal, detrital material ~

--

Cryptogeniccompletely altered to complete obliteration. Few negative relics may be present, suggesting texture and composition of original carbonate

_-

+

~~~

Reef and bank limestone deposits originally; lumpy, cavernous, patchy, vary-crystalline

~-

~~

Fully dolorhombic; vary-grained Uniformly crystalline; relics rare -

Micro- to finely crystalline

~-

Dolomitized reefs, negative relics

-~

Dolomitized chalks and tuffs -~

Vary-grained lumpal dolomites Nodular, lumpy, brecciated, __ may be cavernous; some rocks are meso- to macro-grained Completely replaced micrites sucrosic, having granular, Calcarenites and quartz sands origitranslucent appearance; nally; dolarenites, sucrosic relics absent

119

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

Main genetic groups of dolomite

Main textural types

Characteristics, features, examples, varieties

Crystalline (relics completely destroyed, or were never present)

Phaneric

___-

Aphanic

Sparry dolomite (dolosparite)

Micri tic dolomite or dolomicrite (some types are primary dolomicrite, and may be laminated)

Finely crystalline Microcrystalline __________ Cryptocrystalline

___ ~

(uniform textures may be primary; some contain fine silt and clay.

- 0.01

mm

- 0.001

mm

TAFTand HARBAUGH (1964) made a detailed study of the mineralogy and physical chemistry of the modern carbonate sediments of southern Florida, the Bahamas, and Espiritu Santo Island, Baja California; they did not classify the sediments according to any prevailing scheme nor did they present a new classification, but their data contribute in a very substantial manner to an understanding of some of the processes of sedimentation and diagenesis of modern carbonate materials. If judiceously utilized, this information can assist any petrographer working with modern or ancient sedimentary carbonates. Soviet geologists have made excellent contributions to the knowledge of sedimentary carbonate rocks, and particularly to their classification. The two classifications recommended by the present writers include the one by TEODOROVICH (1958, p.291), which is presented in Fig.3 of this chapter, and the one by SHVETSOV (1958, p.292). These classifications were published in English by BISSELL and CHIUNGAR(1961, pp.612 and 615, respectively). KRUMBEIN and SLOSS(1963, pp. 179-1 SO) also called attention to the classifications of these Soviet workers. Obviously, no single classification of the sedimentary carbonate rocks and particularly the limestones will immediately suit taste and fit the needs of each

120

H. I. BISSELL AND G . V. CHILINGAR

sedimentary petrographer. The suggestions made in this chapter are advanced at a time of expanding knowledge of sedimentary petrology and petrography, and should serve as guides for future studies. Some classification plans, such as the ones by BRAMKAMP and POWERS (1958), FOLK(1959, 1962), POWERS(1962), and RICH(1963, 1964), include space for altered limestones and dolomitized limestones. These are herein recommended with the suggestion that each worker, or team of workers, may seek to discover the utilitarian values in each; however, they may have to be modified for particular provenances and perhaps also for certain sedimentary suites. Some have real value for surface work, some for well cuttings and cores, and others may assist the investigator in field and laboratory research (see MOLLAZAL, 196I). The proposed classification of TEODOROVICH (1958, p.295; see also BISSELLand CHILINGAR, 1961, p.612) is recommended for critical analysis and testing; the present authors have discovered that it is of great value. The classification of dolomites proposed in this chapter (Table X) is the outgrowth of field and laboratory work by the present authors with strata of Paleozoic age in the western United States. This classification embodies certain elemental parts from published classifications, which were rigorously tested. Principal parameters are crystallinity, high degree of alteration, and low degree of alteration. No effort was made to allocate particular types to one environmental type only, because this is unrealistic. Careful examination of the two classifications (one for limestones and one for dolomites), proposed by the present authors, however, reveals their utility. If tested in both the field and the laboratory, their strengths and weaknesses can be ascertained, and evaluated accordingly. One of the most comprehensive reviews of various classifications of carboK nate rocks up to the year 1959 is found in the excellent article by M I ~ (1959). A glossary or terms considered necessary for petrologic and petrographic work of sedimentary carbonate rocks is also included in this chapter; this is in no way exhaustive, but does embrace numerous terms that are now scattered in geologic literature. Finally, various plates of photomicrographs illustrate the rock types mentioned in the proposed sedimentary carbonate classifications in this chapter (Plates I-XVI).

CLASSIFICATION OF SEDIMENTARY CARBONATE ROCKS

121

EXPLENATION OF PLATES PLATE I

A. Detrital limestone (calcarenite). Composed of sand-size particles of limestone, pelletal mate-

rial, and quartz sand grains in finer-grained matrix. Derryan age Oquirrh Formation, Blue Spring Hills, Oneida County, Idaho; x 3. B. Detrital limestone, composed of silt- and sand-size limeclasts, lumps, algal material, and some quartz sand grains. Bird Spring Formation (Pennsylvanian), Lee Canyon area, Clark County, Nev.; x 3 . C. Detrital limestone, composed of silt- and sand-size limeclasts, with some skeletal grains, in finer-grained matrix. Ely Limestone (Pennsylvanian), Burbank Hills, Millard County, Utah; x 3. D. Detrital limestone, composed of oolites, pellets, limeclasts, and some quartz grains, with minor interstitial lime mud. Morrowan age part of Morgan Formation, Duchesne River Valley, Duchesne County, Utah; x 4.6. E. Calcarenitic limestone, composed of sand-size limeclasts, crinoid ossicles, some algal material, and subangular to subrounded quartz grains, embedded in matrix of finer-grained material. Derryan age portion of Wood River (?) Formation, Arc0 Hills, Butte County, Idaho; x3. F. Calcarenite showing graded bedding. Rock is composed of limeclasts, algal material, pellets, and micro-lumps, with quartz sand grains. Cedar Fort Member @esmoinesian age) of Oquirrh Formation, Cedar Mountains, Tooele County, Utah; x 3. PLATE I1 A. Micritic limestone. Rock is composed of very fine-textured to micritic limestone with minor amount of skeletal material. Ely Limestone (Pennsylvanian), Pancake Range, Nye County, Nev.; x 30.8. B. Detrital limestone, composed of abraded and broken brachiopod shells, crinoid ossicles, algal material, lumps, and bryozoan fragments in finer textured particulate lime material and micritic material. Hall Canyon Member (Morrowan age) of Oquirrh Formation, Oquirrh Mountains, Utah County, Utah; x 7.7. C. Skeletal-detrital limestone. Fusulinids (Triticitessp.) in silt- and sand-size particulate limestone, algal material, pelletal material; and quartz silt and sand grains. Missourian portion of Oquirrh Formation, Hobble Creek Canyon, Utah County, Utah; x 7.7. D. Skeletal-detrital limestone, composed of disarticulated brachiopod tests, limeclasts, in matrix of calcilutite containing some quartz silt and sand grains. Garden Valley Formation (Permian), Diamond Range, White Pine County, Nev.; X 3. E. Detrital limestone, composed of sand- and silt-size limeclasts, pelletal material, and some quartz silt and sand grains. Ely Limestone (Pennsylvanian), Leppy Range, northeast of Wendover, Tooele County, Utah; x 3.85. F. Detrital limestone, composed of interlayered calcilutite and calcarenite, with fractures filled by sparry calcite. Ely Limestone (Pennsylvanian), Pancake Range, Nye County, Nev.; x 7.7.

PLATE I11

A. Calcarenite, slightly quartzose. Rock is composed of limeclasts, algal and pelletal material in fine-grained quartzose-calcareous material, and with subrounded quartz grains also present. Derryan age portion of Weber Formation at Pullem Creek, Wasatch County, Utah; x3.85. B. Micritic limestone (calcilutite), composed of lime mud in which quartz silt grains and some silt-size limeclasts are embedded. Virgin Limestone Member (Lower Triassic) of the Moenkopi Formation, Blue Diamond Mountain, Clark County, Nev.; x 61.6. C. Calcisiltite with calcilutite layers. Rock consists of silt-size limeclasts and lime mud, with silt-size and very fine-grained sand-size quartz particles. Summit Springs Member (Medial

122

H. J . BISSELL AND G. V . CHILINGAR

Leonardian age) of Pequop Formation, Southern Butte Mountains, White Pine County, Nev.; x23.1.

D. Dolomicrite with very fine-grained quartz and with few negative fossil relics. Wolfcampian age portion of Weber Formation, Morris Ranch area on south side of Uinta Mountains, Uinta County, Utah; x 61.6. E. Dolosiltite with admixed quartz silt grains. Rock consists of cross-stratified particulate siltsize dolomite, and some interlayered dololutite. Leonardian age portion of Spring Mountains Formation, west side of Kyle Canyon in Spring Mountains, Clark County, Nev.; x 30.8. F. Primary dolomicrite with interlayered fine-grained dolosiltite and some silt-size quartz grains. Incipient diagenesis has occurred in some layers. Leonardian age portion of Spring Mountains Formation, Love11 Wash area in Spring Mountains, Clark County, Nev.; x 61.6

PLATE IV A. Biocalcarenite (= criquinite, or encrinal limestone). Rock consists of crinoid ossicles, bryozoan fragments, and some bladed Algae, with interstitial material (= matrix) composed of sparite and calcisiltite. Riepe Spring Limestone (Wolfcampian age), Moorman Ranch area, White Pine County, Nev.; ~ 4 . 6 . B. Skeletal-detrital limestone, consisting of abraded fragments of crinoid ossicles, bryozoan fragments, pelletal material, and algal material; interstitial material is calcisiltite. Virgilian age portion of Wood River Formation near Hailey, Blaine County, Idaho; x 7.7. C. Reefal limestone with admixed skeletal and detrital material. Rock consists of bryalgal material enclosing tests of schwagerinid fusulinids, crinoid ossicles, algal pellets, and minor limeclasts. Lime mud fills open spaces. Pequop Formation (Leonardian age), Diamond Range, White Pine County, Nev.; ~ 7 . 7 . D. Skeletal-detrital limestone with admixed quartz sand grains. Rock consists of pelecypod shell fragments, few crinoid ossicles, limeclast lumps, and algal material. Virgin Limestone Member (Lower Triassic) of Moenkopi Formation, Bird Spring Range (Cottonwood Pass area north of Goodsprings), Clark County, Nev.; x 7.7. E. Skeletal-detrital limestone, composed of the alga Mizzia sp. with intertwined bryozoans and bladed Algae (= bryalgal limestone), and with admixed silt-size limeclasts. Pequop Formation (Leonardian age), Cherry Creek Range, Elk0 County, Nev.; X 23.1. F. Diagenetically-altered biocalcarenite. Rock consists of crinoid ossicles and intertwined bryozoans with interstitial calcilutite. Rim cement, impingement, and secondary overgrowth are present. Toroweap Formation (Permian), Blue Diamond Mountain, Clark County, Nev. ; x 7.7. PLATE V A. Skeletal-detrital limestone. Rock is composed of fossiliferous-fragmentalmaterial, consisting of lithoclastic carbonate detritus admixed with bioclastic detritus of bryozoans, encrinal material, and algal fragments. Garden Valley Formation (Permian), Diamond Range, White Pine County, Nev.; ~ 3 . 8 5 . B. Skeletal-micrite. Tests of Triticites sp. and some crinoid ossicles (some replaced by sparite), embedded in micrite. Ferguson Mountain Formation (Wolfcampian, Permian), Ferguson Mountain, Elko County, Nev.; x 7.7. C. Calcarenite. Rock consists of crinoid ossicles (some of which are slightly altered diagenetically), with interstitial space filled with calcilutite and calcareous cement. Bridal Veil Falls Member (Morrowan age) of Oquirrh Formation, Provo Canyon, Utah County, Utah; x 3.85. D. Detrital limestone. Rock consists of slightly abraded tests of the fusulinid Pseudoschwugerina sp. in a matrix of organic-rich calcisiltite. Mid-portion of the Ferguson Mountain Formation (Wolfcampian, Permian), Ferguson Mountain, Elko County, Nev.; x 4.6. E. Skeletal limestone. Consists mostly of “tailings” of pelecypod valves, with interstitial space filled by calcarenite and sparite cement. Thaynes Formation (Triassic), Aspen Range, Caribou County, Idaho; X 1.1.

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F. Skeletal-pelletal limestone. Rock consists of brachiopod shells, crinoid ossicles, fragments of bryozoans, and other skeletal detritus, along with pellets; all in sparite. Brazer Formation (Mississippian), Wellsville Mountain, Boxelder County, Utah; x 3.85. PLATE VI A. Skeletal-detrital limestone, consisting of particulate bryozoans, encrinal material, and probable algal fragments, in part surrounded by sparite and in part filled by sparite. Toroweap Formation (Permian), Spring Mountains, Clark County, Nev.; x 3 . B. Skeletal limestone, consisting of bioclastic detritus, algal and lithic carbonate pellets, brachiopod shells, bryozoan fragments, and minor quantity of millerellid Foraminifera. Hall Canyon Member (Morrowan) of Oquirrh Formation, east of Manning Canyon in Oquirrh Mountains, Utah County, Utah; x 7.7. C. Skeletal-detrital limestone, composed of fossiliferous-fragmental (i.e., bioclastic) detritus, mostly crinoid ossicles and bryozoan material. Calcarenite and calcilutite fill interstitial space. Ely Formation (Pennsylvanian), North Burbank Hills, Millard County, Utah; x 7.7. D. Pzlletal bryalgal limestone. Rock consists of algal and other pelletal debris enmeshed in framework of bryozoans and thread-like to blady Algae which form a rigid framework. Pequop Formation (Leonardian, Permian), northern Cherry Creek Range, Elko County, Nev.; x 7.7. E. Skeletal limestone, consisting of bioclastic debris, largely lioclemid bryozoan fragments, some crinoid ossicles, and minor algal detritus. Dead oil is present in part of the interstitial space; micrite comprises the remainder of the rock. Gerster Formation (Guadalupian, Permian), Medicine Range, Elko County, Nev.; x 7.7. F. Micrite with skeletal material. This rock consists of bryozoans, ostracods, algal pellets, algal filaments, small brachiopods, and lithic carbonate debris in oil-rich micrite. Ferguson Mountain Formation (Wolfcampian, Permian), Dolly Varden Mountains, Elko County, Nev.; x 7.7. PLATE V11

A. Skeletal limestone, consisting of pelecypod shell fragments, some algal filaments and pellets, and calcarenitic interstitial material. Sparry calcite fills some interstitial space. Thaynes Limestone (Triassic) on Dry Ridge, Lanes Creek quadrangle, Caribou County, Idaho; x 3. B. Calcarenite with skeletal material. Rock consists of silt- and sand-size limeclasts and some quartz sand grains, surrounding tests of schwagerinid fusulinids and some crinoid ossicles. Carbon Ridge Formation (Permian), Carbon Ridge, Eureka County, Nev.; ~ 4 . 6 . C. Pelletal-skeletal limestone, composed of algal pellets, limeclasts, bryozoan material, and some brachiopod shells (filled with sparite). Gerster Formation (Permian), Medicine Range, Elko County, Nev.; x 3.85. D. Lumpal (= lump) limestone, consisting of limeclasts, few crinoid ossicles, algal pellets, and Foraminifera (including fusulinids); in calcisiltite matrix. Bird Spring Formation (Pennsylvanian), Spring Mountains west of Mountain Pass, Clark County, Nev.; x 7.7. E. Micrite with skeletal material. Rock consists of silty micrite containing brachiopod shell fragments that have sparite fillings. Hall Canyon Member (Morrowan age) of Oquirrh Formation, Fivemile Pass area, Utah County, Utah; ~ 2 3 . 1 . F. Sparite after skeletal limestone. Rock consists of straparollid gastropods and some crinoid ossicles, with replacement and infilling of sparite. Gerster Formation (Permian), Currie Hills, Elko County, Nev.; x 3. PLATE VIII A. Pelletal calcarenite, consisting of algal pellets, limeclasts, and silt- to sand-size quartz grains. Wolfcampian age portion of Oquirrh Formation, Right Fork of Hobble Creek Canyon, Utah County, Utah; x7.7.

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B. Skeletal calcisiltite. Rock is composed of tests of triticitid fusulinids, few limeclasts, and rare algal pellets, all in matrix of petroliferous calcisiltite. Ferguson Mountain Formation (Wolfcampian, Permian), Ferguson Mountain, Elko County, Nev.; x 7.7. C. Encrinal limestone (= criquinite), composed of abraded crinoid ossicles, rare limeclasts, in matrix of calcisiltite that contains few quartz grains. Encrinal material has been diagenetically altered to a slight degree. Morgan Formation (Pennsylvanian), Weber Canyon, Morgan County, Utah; ~ 7 . 7 . D. Skeletal calcisiltite, consisting of brachiopod “tailings” (= fragniental) that are in part replaced by sparite, in organic rich calcisiltite that contains few fine-textured quartz sand grains. Bridal Veil Falls Member (Morrowan age) of Oquirrh Formation, Provo Canyon, Utah County Utah; ~ 7 . 7 . E. Diagenetically altered skeletal limestone. Rock is composed of algal (?)pellets, crinoid ossicles, bryozoan fragments, and abraded brachiopod shells, all in various stages of replacement by sparite. Toroweap Formation (Permian), Star Range, Beaver County, Utah; x 7.7. F. Calcisiltite with Algae. Rock consists of calcisiltite enclosing the alga Solenopora sp., and smaller algal pellets. Upper Member (Late Leonardian age) of Pequop Formation, Southern Butte Mountains north of Moorman Ranch, White Pine County, Nev.; x 7.7. PLATE IX

A. Sparite with skeletal material. Rock consists of medium- to coarsely-crystalline sparite enclossing crinoid ossicles, brachiopod fragments, bryozoan material, and millerellid Foraminifera. Morgan Formation (Pennsylvanian) at type locality in Weber Canyon, Morgan County, Utah;

x23.1. B. Pelletal-skeletal limestone, composed of fossiliferous-fragmental material, algal pellets and limeclasts, Foraminifcra, in sparite matrix. Brazer Formation (Upper Mississippian), Wellsville Mountain near Deweyville, Boxelder County, Utah; X 7.7. C. Pellet limestone, composed of algal pellets and limeclast pellets, encrinal material, bryozoan fragments, and silt- to sand-size quartz grains. Morrowan age portion of Oquirrh Formation, Gilson Mountain area, Juab County, Utah; x 7.7. D. Skeletal limestone showing effects of diagenesis. Rock consists of fragments of bryozoans, rare crinoid ossicles, and few pellets, with interstitial material composed of calcisiltite. Kaibab Limestone (Permian), Gold Hill District, Tooele County, Utah; x 7.7. E. Coated grains. Rock consists of oolites, pellets, algal plates, and limeclasts in micrite. Virgin Limestone Member (Lower Triassic) of Moenkopi Formation, east of Blue Diamond Mountain, west of Las Vegas, Clark County, Nev.; ~ 7 . 7 . F. Coated grains in sparite. Rock is composed of oolites showing both radial and concentric structure, skeletal material with coatings, and few limeclasts in coarsely crystalline sparite. Lodgepole Limestone (Mississippian), Western Judith Mountains, Fergus County, Mont.; x 7.7,

PLATE X A. Pelletal-detrital limestone, consisting of algal pellets, limeclast pellets and limeclast detrital material, lumps, and scattered abraded skeletal material; much of the material has been diagenetically altered. Morgan Formation, west end of Uinta Mountains, Wasatch County, Utah; x 7.7. B. Lump limestone in sparite. Rock consists of organic-rich limeclast lumps (and possibly algal material) surrounded by coarsely-crystalline sparite. Toroweap Formation (Permian), Bird Spring Mountains west of Arden, Clark County, Nev.; X 15.4. C. Coated grains in sparite. Rock is composed of skeletal and limeclast grains with oolite overgrowths, all in medium-crystalline sparite. Brazer Formation (Mississippian), Weber River Canyon, Morgan County, Utah; ~23.1. D. Pellet limestone, composed of spindle-shaped and rod-shaped pellets (for the most part filled with sparite) in interstitial material of Algae and calcarenite. Pequop Formation (Permian), central part of Cherry Creek Range, White Pine County, Nev.; X23.1.

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E. Coated grains in micrite. Rock consists of oolites showing both concentric and radial structure; some are distorted. Interstitial material is largely micrite, with local patches of sparite. Hannah Formation (Mississippian), Saypo quadrangle in Sawtooth Mountains, Teton County, Mont.; % 30.8.

F. Lump limestone in sparite. Rock consists of limeclast lumps, possible algal material, algal pellets; in medium-crystalline sparite. Leonardian age portion of Spring Mountains Formation, northern Spring Mountains, Clark County, Nev.; x 3.85. PLATE XI A. Biogenic limestone, consisting of organic frame-building alga, Kumia sp. (by some referred to the stromatoporoids). Interstices consist of algal pellets and limeclasts. Derryan age portion of Wells Formation, South Schmid Ridge, Dry Valley quadrangle, Caribou County, Idaho; x 7.7. B. Diagenetically altered skeletal limestone, consisting of schwagerinid fusulinid tests, algal filaments and plates, bryozoans, and encrinal material, altered in varying degrees to sparite. Pequop Formation (Leonardian age), west end of Leppy Range, Elko County, Nev.; x 7.7. C. Algal limestone (biogenic) in micrite. Rock consists of the alga Mizzia sp. (some surrounding skeletal elements), in micritic limestone in which fine quartz silt grains and occasional crinoid ossicles are present. Pequop Formation (Leonardian age), low hills east of Lund, White Pine County, Nev.; x 15.4. D. Organic-rich micritic limestone with Strornatactis (?).Rock consists of petroliferous limestone containing elongate bodies referred with query to Stromatactis. Riepe Spring Limestone (Wolfcampian, Permian), Rib Hill near Ruth, White Pine County, Nev.; ~ 2 3 . 1 . E. Biogenic-skeletal limestone, consisting of alga (or stromatoporoid) Komia sp., fusulinid tests, algal pellets, crinoid ossicles and limeclasts. Interstitial space partially filled with sparite and remainder by organic-rich calcilutite. Derryan age portion of Ely Limestone, South Schell Creek Range south of Patterson Pass, Lincoln County, Nev.; x 15.4. F. Biogenic limestone in sparite, consisting of the alga Osugia sp., surrounding limeclasts, bryozoans, and other material; in sparite. Some sparite has filled open spaces in skeletal material. Rogers Spring Limestone (Redwall? Limestone), Star Range, Beaver County, Utah; x 15.4. PLATE XI1 In place, organic frame-builders (biogenic). A. Reefal limestone, composed of coraIs, Algae, and sponges, with micrite in interstitial space. Permian of Guadalupe Mountains, Texas; x 7.7. B. Bryalgal limestone, consisting of various bryozoans intermeshed with Algae, and infilled with lime mud. Leonardian age portion of Pequop Formation, Maverick Spring Range, White Pine County, Nev.; x 77. C. Reefal limestone, consisting of brachiopods, Algae, and bryozoans, enclosed in micrite. Garden Valley (?) Formation, east side of Diamond Range, White Pine County, Nev.; x7.7. D. Reefal limestone, composed of bryozoans, sponges, spicular material, and few brachiopod fragments enclosed in lime mud that is organic rich. Leonard Formation, Glass Mountains, Texas; x6.2. E. Reefal limestone, composed of lioclemid bryozoans, Algae, Foraminifera, and crinoid ossicles. Largely fore-reef in situ accumulation. Pequop Formation (Leonardian age), Spruce Mountain area, Elk0 County, Nev.; x 3.85. F. Reefal limestone, consisting of bryalgal material, some of which has undergone diagenetic change to sparite and microsparite. Gerster Limestone (Guadalupian age), Medicine Mountains, Elk0 County, Nev.; x 7.7.

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PLATE XI11 Diagenesis. A. Dolomitized calcilutite; rock consists of dolomite secondary after calcilutite. Original sediment contained variable amount of organic matter and was stratified. Summit Springs Member (Medial Leonardian age) of Pequop Formation, west end of Leppy Range, Elk0 County, Nev.; x 30.8. B. Crystalline criquinite; rock consists of coarsely crystalline material, the fabric having resulted from diagenesis of an encrinal limestone, with the original texture almost completely obliterated in the process. Some algal material is relatively unaltered. Lower part of Kaibab Limestone, low hills east of Ruby Marshes, Elko County, Nev.; ~ 7 . 7 . C. Dolomitized biomicrite, consisting of dolomitized algal filaments and plates (now resembling a form of Stromutuctis), in partially dolomitized micritic matrix. Leonardian age part of Spring Mountains Formation, west side of Kyle Canyon, Clark County, Nev.; x 30.8. D. Dolomitized micrite, consisting of porous rock that originally was a micrite and now is a dolomite with substantial porosity and permeability. Pakoon Formation, east side of Frenchman Mountain, Clark County, Nev.; ~ 4 6 . 6 . E. Dolomitized skeletal limestone, consisting of medium- to coarsely-crystalline dolosparite, secondary after bryozoans, encrinal material, algal filaments and pellets, and possibly Foraminifera. Both positive and negative relics are present. Toroweap Formation, south of Garnet R.R. Siding, Clark County, Nev.; x 15.4. F. Dolosparite, consisting of mosaic of dolospars, secondary after limeclast limestone. Thin unit in Guilmette Limestone (Devonian), Toana Range north of Whitehorse Pass, Elko County, Nev.; X 15.4.

PLATE XIV A. Dolosparite, consisting of cloudy sparry dolomite, secondary after calcarenitic limestone with impure matrix. Lower Kaibab Formation (Permian), Maverick Spring Range, northern White Pine County, Nev.; x 30.8. B. Diagenetically-altered skeletal limestone; rock consists of calcite spar developed in a stratified skeletal-detrital limestone. Unnamed Wolfcampian age (Permian) carbonate sequence, Arc0 Hills, Butte County, Idaho; x 23.1. C. Incipient dolomitization of a calcarenite. Rock is composed of subhedra to imperfect euhedra of dolorhombs and less-altered sparry calcite, with relatively unaltered calcarenite matrix. Loray Formation (Permian), Dead Horse Wash, west of Egan Range, White Pine County, Nev.; x 23.1. D. Sparite, some of which is diagenetically altered to subhedra of dolospar. Rock originally was a calcarenite with impure matrix, and now is a mosaic of sparite and dolosparite. Wolfcampian age part of Weber Formation, Duchesne River area, south flank of Uinta Mountains, Duchesne County, Utah; x23.1. E. Diagenetically dolomitized skeletal limestone, consisting of vary-grained (vary-crystalline) sparry calcite and sparry dolomite, anhedral to subhedral, with patches of less altered material. Original texture strongly altered, some obliterated, but with negative skeletal relics (= crinoid ossicles and bryozoans). Plympton Formation (Permian), Gold Hill district, Tooele County, Utah; x15.4. F. Coarsely crystalline sparite, consisting of calcite spar; no dolomite. Chilliwack Group (Permian), Cascade Mountains, Wash.; ~ 2 3 . 1 .

PLATE XV A. Diagenetically-altered calcilutite, showing development of “eyes” of silicified material (possibly replaced algal material) in dololutite. Some quartz sand grains are present. Park City Group (Permian), near “The Thumb”, 10-12 miles north of Knolls, Tooele County, Utah; x15.4. B. Slightly altered skeletal-detrital liniestone, showing positive relic of fusulinid test, negative

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skeletal material, and limeclasts set in altered matrix. Upper Member (Late Leonardian age) of Pequop Formation, west end of Leppy Range, Elko County, Nev.; x 15.4. C. Dolomitized skeletal-detrital limestone, showing almost complete obliteration of original fabric; spots or “eyes” of dolospar are the lighter colored areas, and darker colored spots may be relics of algal pellets. Matrix is dololutite, secondary after limestone. Kirkman Limestone (Wolfcampian age), South Tintic Mountains, Juab County, Utah; ~ 2 3 . 1 . D. Dolomitized skeletal-detrital limestone, showing negative and positive relics of skeletal material, in matrix of calcisiltite and dolosiltite. Grandeur Member of Park City Formation (Permian), Southern Wasatch Mountains, Juab County, Utah; x 15.4. E. Highly-altered diagenetic dolomite, showing negative relics of material not determinable as organic or inorganic, in matrix of less altered dolosiltite. Few relatively unaltered quartz sand grains are present. Plympton Formation (Permian), Granite Mountain area of Northern Confusion Range, Juab County, western Utah; x 15.4. F. Moderately advanced diagenesis in skeletal limestone, showing positive relics of bryozoans, and the stromatoporoid Komia sp. Ely Limestone (Pennsylvanian), west side of Pequop Mountains, southwest of Shafter, Elko County, Nev.; x 15.4. PLATE XVI

A. Sucrosic dolomite, diagenetic after limestone. Rock shows considerable porosity and dead oil in vugs. Wolfcampian age portion of Weber Formation at canyon of Duchesne River, Duchesne County, Utah; x 30.8. B. Diagenetically-altered skeletal-calcarenite, showing first-stage crystallization, but not dolomitization, of limeclasts and algal to pelletal material. Wolfcampian age part of Oquirrh Formation at South Mountain west of Stockton, Tooele County, Utah; x23.1. C. Advanced diagenesis including partial dolomitization of a skeletal-detrital limestone. Rock shows sparite and dolosparite in mosaic of subhedra and euhedra. Vuggy porosity has been developed and dead oil is present. Lower Member of Kaibab Formation, south of Cottonwood Wash and east of Keystone Thrust, Clark County, Nev.; ~ 2 3 . 1 . D. Diagenetic dolomite, secondary after limestone. Rock shows mosaic of anhedra and subhedra of dolomite, with remnants of sparite. Toroweap Formation (Permian), Northern Muddy Mountains just southwest of Glendale Junction, Clark County, Nev.; x 7.7. E. Dolosparite. Rock shows advanced-stage diagenetic dolomitization of an earlier-formed limestone. Well-developed dolorhombs (some by impingement) have formed a fabric having low to moderate intercrystalline porosity. Fairly high permeability. Wolfcampian age part of Weber Formation on south flank of Uinta Mountains near Morris Ranch, Uinta County, Utah; x 38.5. F. Diagenetic dolomite, secondary after encrinal limestone. Rock shows mosaic of anhedra and subhedra of dolomite, with remnants of calcarenite. Suturing has occurred at crystal-grain boundaries. Summit Springs Member (Medial Leonardian Permian) of Pequop Formation, Ferguson Flat, Elko County, Nev.; x 7.7.

PLATE 1

Legend see p.121.

PLATE I1

Legend see p.121

129

PLATE 111

Legend see p. 12 1.

PLATE IV

Legend see p.122.

131

PLATE V

Legend see p.122.

PLATE V1

Legend see p.123.

133

PLATE VII

Legend see p. 123.

PLATE VIII

Legend see p.123.

135

PLATE IX

Legend see p.124.

PLATE X

Legend see p.124.

137

PLATE XI

Legend see p . 125.

PLATE XI1

Legend see p.125.

139

PLATE XI11

Legend see p.126.

PLATE XLV

Legend see p.126.

141

PLATE XV

Legend see p.126.

PLATE XVI

Legend see p.127.

143

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FOLK, R. L., 1959. Practical petrographic classification of limestones. Bull. Am. Assoc. Petrol. Geologists, 43: 1-38. FOLK,R. L., 1962. Spectral subdivision of limestone types. In: W. E. H.4M (Editor), Classification of Carbonate Rocks-Am. Assoc. Petrol. Geologists, Mem., 1 : 62-85. FRIEDMAN, G. M., 1959. Identification of carbonate minerals by staining methods. J. Sediment. Petrol., 29: 81-97. FROLOVA, E. K., 1955. Magnesite in Lower Permian deposits of Kuybyshev and Saratov Transvolga Region. Izv. Akad. Nauk S.S.S.R., Ser. Geol., 1955(5): 89-96. FROLOVA, E. K., 1959. On classification of carbonate rocks of limestone-dolomite-magnesite series. Novosti Neft. Tekhn., Geol., 3: 34-35. GINSBURG, R. N., 1956. Environmental relationships of grain size and constituent particles in some south Florida carbonate sediments. Bull. Am. Assuc. Petrol. Geologists,40: 2384-2427. GINSBURG, R. N., 1957. Early diagenesis and lithification of shallow-water carbonate sediments (Editors), Regional Aspects of in south Florida. In: R. J. LE BLANCand J. G. BREEDING Carbonate Deposition-Soc. Econ. Paleontologists Mineralogists, Spec. Publ., 5 : 80-100. R. N. and LOWENSTAM, H. A., 1958. The influence of marine bottom communities on GINSBURG, the depositional environment of sediments. J . Geol., 66: 310-318. GORSLINE, D. S., 1963. Environments of carbonate deposition, Florida Bay and the Florida (Editors), Shelf Carbonates of the Paradox Basin, Straits. In: R. 0. BASSand S. L. SHARPS A Symposium-Four Corners Geol. SOC.,Field Conf., 4th, pp.130-143. GRABAU, A. W., 1904. On the classification of sedimentary rocks. Am. Geologist, 33: 228-247. GRABAU, A. W., 1913. Principles of Stratigraphy. Seiler, New York, N.Y., 1185 pp. GRAF,D. L., 1960a. Geochemistry of carbonate sediments and sedimentary carbonate rocks. Parts 1 and 2. Illinois State Geol. Surv., Circ., 297: 39 pp.; 298: 43 pp. GRAF,D. L., 1960b. Geochemistry of carbonate sediments and sedimentary carbonate rocks. Part 4-B. Bibliography. Illinois State Geol. Surv., Circ., 309: 55 pp. A. J. and SHIMP, N. F., 1959. Dolomite formation in Lake Bonneville, GRAF, D. L., EARDLEY, Utah. Bull. Geol. SOC.Am., 70: 1610. J. and SHIMP,N. F., 1961. Apreliminary reporton magnesiumcarbonate GRAF,D. L., EARDLEY,A. formation in Glacial Lake Bonneville. J. Geol., 69: 219-223. J. T., 1960. Introduction to the petrology of the Oil-Shale Group limestones of GREENSMITH, west Lithian and southern Fifeshire, Scotland. J. Sediment, Petrol., 30: 553-560. GREINER, H. R., 1956. Methy Dolomite of Northeastern Alberta: MiddleDevonian reef formation. Bull. Am. Assoc. Petrol. Geologists, 40: 2057-2080. GUBLER, Y., 1959. Problttmes des dolomies. Rev. Znst. FranG. Pe'trole Ann. Combust. Liquides, 14 (4-5): 474. R. G. and KENNER, C. T., 1955. Classification of Permian rocks of western Texas by GUERRERO, a versenate method of chemical analysis. J . Sediment. Petrol., 25: 45-50. HALLA, F., CHILINGAR, G. V. and BISSELL,H. J., 1962. Thermodynamic studies on dolomite formation and their geologic implications: an interim report. Sedimentology, 1: 296-303. HAMW. E. (Editor), 1962. Classificationof CarbonateRocks. Am. Assoc. Petrol. Geologists, Tulsa, Okla., 279 pp. HAMBLETON, A. W., 1962. Carbonate-rock fabrics of three Missourian stratigraphic sections in Socorro County, New Mexico. J. Sediment. Petrol., 32: 579-601. HARBAUGH, J. W., 1959. Small scale cross-lamination in limestones. J . Sediment. Petrol., 29: 30-37. HOBBSJR., C. R., 1957. Petrography and origin of dolomite-bearing carbonate rocks of Ordovician age in Virginia. Bull. Va.Polytech. Inst., 50(5): 128pp. HOWELL,J. B., 1922. Notes on pre-Permian Paleozoics of the Wichita Mountain area. Bull. Am. Assoc. Petrol. Geologists, 6: 413425. ILLING,L. V., 1954. Bahaman calcareous sands. Bull. Am. Assoc. Petrol. Geologists, 38: 1-95. IMBRIE,J. and PURDY, E. G., 1962. Classification of modern Bahamian carbonate sediments. In: W. E. HAM(Editor), Classification of Carbonate Rocks- Am. Assoc. Petrol. Geologists, Mem., 1: 253-272. JENKINS JR., M. A., 1954. On the origin of dolomite. The Compass of Sigma Gamma Epsilon, 31: 296-302.

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JOHNSON, J. H., 1949. An introduction to the study of organic limestones. Quart. Colo. School Mines, 44(4): 139 pp. JOHNSON, J. H., 1961. Limestone-building Algae and algal limestones. Quurt. Colo. School Mines, 56: 297 pp. KHVOROVA, I. V., 1958. Atlas of Carbonate Rocks of Middle and Upper Carboniferous of Russian Platform. Geol. Inst. Akad. Nauk S.S.S.R., Moscow, 170 pp. KORNICKER, L. S . and PURDY,E. G., 1957. A Bahamian faecal-pellet sediment. J . Sediment. Petrol., 27: 126-128. KRUMBEIN, W. C. and SLOSS,L. L., 1963. Stratigraphy and Sedimentation, 2 ed. Freeman, San Francisco, Calif., 660 pp. KRYNINE,P. D., 1948. The megascopic study and field classification of sedimentary rocks. J. Geol., 56: 130-165. LALOU,C., 1957. Studies on bacterial precipitation of carbonates in sea water. J. Sediment. Petrol., 27: 190-195. LAPORTE,L. F., 1962. Paleoecology of the Cottonwood Limestone (Permian), Northern MidContinent, Bull. Geol, Soc. Am., 73: 521-544. LEIGHTON, M. W. and PENDEXTER, C., 1962. Carbonate rock types. In: W. E. HAM(Editor), Classification of Carbonate Rocks-Am. Assoc. Petrol. Geologists, Mem., 1: 33-61. H. A., 1950. Niagaran reefs of the Great Lakes area. J . Geol., 58: 430487. LOWENSTAM, H. A., 1955. Aragonite needles secreted by Algae and some sedimentary implications. LOWENSTAM, J. Sediment. Petrol., 25: 270-272. S., 1957. On the origin of sedimentary aragonite needles of the LOWENSTAM, H. A. and EPSTEIN, Great Bahama Bank. J. Geol., 65: 364-375. LUCIA,F. J., 1961. Dedolomitization in the Tansill (Permian) Formation. BufI. Geol. Soc. Am., 72: 1107-1 110. LUCIA,F. J., 1962. Diagenesis of a crinoidal sediment. J. Sediment. Petrol., 32: 848-865. MARCHER, M. V., 1962. Petrography of Mississippian limestones and cherts from the Northwestern Highland Rim, Tennessee. J. Sediment. Petrol., 32: 819-832. W. G. H., DAY,R. W. and FLEMING, P. J. G., 1961. Carbonate sedimentation on the MAXWELL, Heron Island Reef, Great Barrier Reef. J . Sediment. Petrol., 31: 215-230. MCKINLEY,M. E., 1951. The replacement origin of dolomite- a review. The Compass of Sigma Gamma Epsilon, 28: 169-183. MJS~K,M., 1959. Entwurf einer einheitlichen Klassifikation und Terminologie von gemischten karbonatischen Gesteinen. Geol. Prcice, Zpravy, 16: 61-78. Y . , 1961. Petrology and petrography of Ely Limestone in part of Eastern Great MOLLAZAL, Basin. Brigham Young Univ. Res. Studies, Geol. Ser., 8: 3-35. MOORE,R. C., 1957. Mississippian carbonate deposits of the Ozark Region. In: R. J. LE (Editors), Regional Aspects of Carbonate Deposition-Soc. BLANCand J. G. BREEDING Econ. Paleontologists Mineralogists, Spec. Publ., 5: 101-124. F. J., 1957. Observations on limestones. J. Sediment. Petrol., 27: 282-292. MORETTI, MORRIS,R. C. and DICKEY,P. A., 1957. Modern evaporite deposition in Peru. Bull. Am. Assoc. Petrol. Geologists, 41 : 2467-2474. MOSHER, L. C . and PINNEY,R. I., 1963. Limestone nomenclature. The Compass of Sigma Gamma Epsilon, 40: 219-222. MURRAY,R. C., 1960. Origin of porosity in carbonate rocks. J. Sediment. Petrol., 30: 59-84. MURRAY, R. C., 1964a. Origin and diagenesis of gypsum and anhydrite. J . Sediment. Petrol., 34: 512-523. MURRAY,R. C., 1964b. Preservation of primary structures and fabrics in dolomite. In: J. IMBRIE and N. D. NEWELL(Editors), Approaches to Paleoecology. Wiley, New York, N.Y., pp. 388-403. NELSON,H. F., BROWN,C. W. and BRINEMAN, J. H., 1962. Skeletal limestone classification. In: W. E. HAM(Editor), Classification of Carbonate Rocks-Am. Assoc. Petrol. Geologists, Mern., 1: 224-252. NEWELL,N. D., 1955. Depositional fabric in Permian reef limestones. J. Geol., 63: 301-309. NEWELL,N. D. and RIGBY,J. K., 1957. Geological studies on the Great Bahama Bank: In. R. J.

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GLOSSARY]

Accretionary: added through overgrowth upon a pre-existing grain and/or organic structure or framework; may be concentric, and may originate through rolling. Aggregation: added through coagulation, flocculation, adherance or other method to result in a composite grain or structural mass. May be algal, foraminiferal, algal-dust, or other materials aggregated together or to another mass, fossil, or grain. May be a method of forming lumps. Algal: relating in any manner to algal limestones or dolomites; more commonly the Rhodophyta or red Algae and some genera of the Cyanophyta or blue-green Algae. Algal dust: a term designated to describe micro-textured to finely textured dark-colored (usually brown and brown-gray) micritic and matrix material in carbonates which also contain discernible algal remains. Allochthonous: an accumulation of transported material; for sedimentary carbonate rocks this amounts to transported grains, fossil fragments, rock fragments, and organic matter into the depocenter from adjacent area(s). Allogenic: term meaning generated elsewhere; in a manner similar to allochthonous, in that the constituents came into existence outside of, and previous to, the rock of which they are now a part. For example, quartz sand blown from land into an evaporite sequence; gravel washed into a basin and mixed with lime mud. Anadiagenetic: this term applies to dolomitization (secondary) that occurs under considerable burial, including most tectonic dolomitization (cf. syndiagenetic and epidiagenetic). (After FAIRBRIDGE, 1966.) Anhedral: individual crystals devoid of crystal boundaries or faces; particularly applicable to dolomites and dolomitized limestones of certain varieties. Aphanic: term to describe the texture of most micritic limestones and dolomicrites; individual crystals and grains less than 0.01 mm in size. A useful field term to describe lithographic and sub-lithographic carbonates. To be used in lieu of uphunitic. Articulate: refers to fossils having two or more parts joined together in their natural relationship; for example, valves of brachiopods or molluscs, fronds of bryozoans, columnals of crinoids, etc. Authigenic: generated on the spot; refers to those constituents that came into existence with or after the formation of the host rock. For example, albite that forms in some limestones, calcite or dolomite rhombs over grains, etc. Autochthonous: pertaining to objects that originated in the places where they now occur, and therefore are in situ; examples are frame-building organisms such as corals, algal masses, various bioherms, etc. Autoclastic breccia: refers to a common structure observed in dolomites; a result of diagenetic shrinkage followed by recementation. 1 To

be used as an aid in describing sedimentary carbonate rock terms. It is not intended to cover all definitions of the rock names.

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Axiolitic: a type of elongated or subspherical oolite or pisolite in which the radial structure(usually acicular needles) develops toward the periphery at right angles to a central axis; may have superimposed concentric structure. Bahamite: name of granular limestones that closely resemble the present deposits of the interior of the Bahama Banks. The texture varies from calcisiltites to calcirudites, in which the grains are accretionary and commonly are composite, consisting of smaller granules bound together by precipitated material into aggregate grains.

Bank: an in situ skeletal limestone deposit formed by organisms which do not have the ecological potential to erect a rigid, wave-resistant structure. Beach-rock: a friable to well-cemented beach sediment consisting of calcareous debris (detrital and/or fragmental skeletal) cemented by calcium carbonate. Bioaccumulated: applied to limestone deposits formed by sedentary but noncolonial organisms and their related ecologic communities. Characterized by a predominance of unbroken fossils, diverse organic components, lack of (or poor) sorting, and scarce fine-grained matrix. Biocalcarenite: special group of calcarenites, mechanically deposited carbonate detritus of sand size (1 /16-2 mm in diameter); predominantly composed of organic fragments of any kind, the products of organic activity which have an internal structure such as foraminifera], algal, and faecal pellets, and recognizable fossil detritus worn to sand size. Numerous encrinal limestones fit in this category. Biocalcirudite: rudaceous, sedimentary carbonate rock, the discrete particles of which are composed of fragmental fossiliferous material; individual particles are larger than 2 mm in diameter. Almost any type of reef-building organisms such as stromatoporoids, branched corals, and calcareous Algae can be broken away, worn to a variable degree, and concentrated into biocalcirudites. Bioclastic: a clastic sedimentary carbonate rock which owes its essential character to organisms (GRABAU, 1913). In view of disparity of definition, it is herein suggested that this term be expanded to embrace fossil detritus that is largely intraclastic, whether of rudaceous, arenaceous, or lutaceous texture, and which originates (or originated in the past) largely by being broken and transported by water currents and waves before coming to rest. Bioclastic limestone, therefore, may be poorly sorted, moderately sorted, or well sorted; and it may be clean or have a matrix of finer detritus. Bioconstructed: term applied to limestone deposits resulting from the vital activities of colonial and sediment-binding organic communities. Algal, bryozoan, bryalgal, stromatoporoidal, coralline, and coralgal colonies are predominant. Biogenic: sedimentary carbonate rock, a deposit of organic material or materials formed through the physiological activities of the organisms. Bioherm: an organic reef or mound built by corals, stromatoporoids, gastropods, echinoderms, Foraminifera, pelecypods, brachiopods, Algae, and other organisms. It is a reef, bank, or mound that is reeflike, moundlike, lenslike or an otherwise circumscribed structure of strictly organic origin, embedded in rocks of different lithology.

Biolithite: a limestone characterized by an organic framework of carbonate laminae that bind grains and skeletal elements as a rigid framework; typical of the cores of some organic reefs.

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Biomicrite: a major group of biogenic limestones containing a significant admixture of fine-textured carbonate material filling the spaces between organic tests and fragments. Biosparite: a foraminifera1 limestone composed largely of tests of bottom-dwelling and floating organisms, and lacking a fine-textured matrix; mostly crystalline. Biostrome: a term for stratiform deposits, such as shell beds, crinoid beds, and coral beds, consisting of, and built mainly by, organisms or fragments of organisms (mostly sedentary), and not swelling into moundlike or lenslike forms. Birdseye: spots or tubes of sparry calcite in limestones (and some dolomites). These “calcite eyes” are common to pelsparites, and may have resulted from one of the following (or certain combinations thereof): ( I ) precipitation of sparry calcite in animal burrows, or in worm tubes; ( 2 ) soft-sediment slumping or mud cracking; (3) precipitation of sparry calcite in tubules resulting from escaping gas bubbles; (4) re-working and rapid deposition of soft sediment containing semicoherent clouds of calcareous mud and spar; (5) recrystallization of calcareous (or dolomitic) mud in patches; and ( 6 ) “arrested” dolomitization. Boundstone: applies to most reef rock, stromatolites, and some biohermal and biostromal rocks in which the original components were bound together during deposition, and remain substantially in position of growth. Breccia (sedimentary) : a rock composed of consolidated angular fragments, most of which are larger than 2 mm in diameter, plus matrix and/or cementing material. CAROZZI (1960) mentioned a crystallization breccia that resulted from the differentiation in place of a homogeneous calcilutite. Crystallization began at numerous points scattered throughout thc rock but was incomplete, and as a result the crystallized patches appear as fragments In a groundmass that was spared by the process. Bryalgal: a term for limestones composed largely of materials constructed in situ by organic frame-building bryozoans and Algae; the word is a contraction of bryozoan-algal framebuilding organisms. Resultant deposits range in thickness from thin units to biostromes, bioherms, patch-reefs, and larger reefs. Calcarenite: a mechanically deposited carbonate rock consisting of sand-size carbonate grains (1 /16-2 mm in diameter); the particulate material in this rock may be of lithoclastic and/or bioclastic derivation, and comprises 50 % or more of the rock. Calcilutite: by decrease in grain size, a calcarenite grades through a calcisiltite into a calcilutite, thus forming a rock composed of 50% or more of clay- (plus some silt-) size carbonate particles; includes biocalcilutites and lithocalcilutites. Calcirudite: the term is used as a general designation for mechanically deposited carbonate rocks that are composed of 50% or more of angular to rounded fragments over 2 mm in diameter, and have matrix and/or cementing material. Culcisiltite: a rock type intermediate between calcarenite and calcilutite, in that it consists mostly of silt-size carbonate detritus that comprises 50% or more of the rock; includes biocalcisiltites and lithocalcisiltites. Calclithite: a limestone containing 50 % or more of fragments of older limestone(s) that experienced erosion and redeposition. The individual particles are termed extraclasts. Calcsparite: see sparite.

Caliche: it is a lime-rich deposit found in soils and is formed by capillary action drawing the lime-

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bearing waters to the surface where, by evaporation, the lime is precipitated. In bajadas, intermonts, alluvial fans and colluvium of parts of the Great Basin of the western United States some of the caliche deposits are dolomitic due to presence of extensive dolomite rubble. Caliche, whether calcareous and/or dolomitc, also cements alluvial fans to form Janglomerale. Carbonate: rocks composed of more than SO%, by weight, of carbonate minerals. For practical microscopic work, area percentages, which approximate weight percentages, are used because they are easier to estimate and measure. Cement: clear to opaque, crystalline material occurring in the interstices between grains and matrix material, or between grains. It may be sparry calcite and/or dolomite, and thus is termed sparite; but more commonly, it is smaller than 0.03 mm in crystal size. Cement commonly is chemically precipitated material into voids and in situ onto the surfaces of the hostframework. The calcareous cement in limestones may be of different crystal size-grades: micrite (often mistaken for detrital matrix), microsparite, and sparite. The morphologic and textural types are cryptocrystalline, microcrystalline, granular, fibrous, blady, and drusy. Carbonate cement often resembles products formed by recrystallization and grain growth.

Chalk: a porous, fine-textured material, light colored, friable to subfriable, largely to wholly calcareous. It may be slightly tuffaceous. Commonly finely grained, not crystalline, and may be composed largely of foraminifera1 tests and/or comminuted remains (notably of Coccolithophoridae). Chalk can also be of partly chemical origin, although it normally represents the “flour” formed by break-down of skeletal, nonskeletal, and pelletoid grains and algal “dust”. It is largzly micro-textured (about 0.01 mm or smaller). ClaJt: an individual constituent of detrital sediment or sedimentary rock produced by the physical disintegration of a larger mass either within or outside the depocenter of accumulation. A limeclast, therefore, may be an intraclast of the limestone particle, or a fragment disrupted from partially consolidated lime mud on the sea floor or lake bottom. Particulate material may also be doloclasts (see extraclast and intraclast). Clasts of all dimensions are recognized in the older literature in part as penecontemporaneous intraformational detritus.

Clastic: particles of either fragmental or chemical origin that have been rolled around and abraded before coming to rest in a sediment. The variety intraclast originates in the depocenter of sedimentation. Coated grains: grains possessing concentric or enclosing layers of calcium carbonate (or dolomitized remnant); for example, oolites, pisolites, superficial oolites, and algal-encrusted skeletal grains. Composite grain: aggregation grains (detrital, skeletal, pelletal, algal, coated grains, etc.) formed from clustering of two or more discrete particles. It may also result from aggregation of lumps. Some resemble grapestones. Compound-pellet: a pellet of silt-, sand-, or granule-size or larger originating from pelletal 01’ pelletoid limestone with micritic or sparry cement, and may also have matrix or interstitial material.

Coquina: carbonates consisting wholly or largely of mechanically sorted fossil debris, weakly to moderately cemented but not completely compacted and indurated; interstitial material does not necessarily fill all interstices. Commonly applied to shell debris. For the finer shell detritus of sand size or less, the term mesocoquina may be applied; microcoqarina usually implies a variety of chalk.

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Coquinite: for the most part, indurated equivalent of coquina. Coquinites are carbonates which are nearly all fossil debris, mechanically sorted in large measure, and may have finertextured matrix together with cement. It is a compact and well-indurated, cemented rock. Most discrete particulate fossil material is larger than 2.0 mm in size, and some fossils may still be articulated. This carbonate is the indurated equivalent of loose shell debris. For the finer shell detritus of sand- to silt-size, the term mesocoquinite is used, and microcoquinite applies to still smaller size fossil debris. Coquinoid (limestone) :a distinction should be made between coquina, coquinite, and coquinoid limestone. Coquina is loosely compact, poorly cemented, and weakly indurated (shell debris), whereas coquinite is its indurated, firm rock equivalent. Coquinoid limestones are autochthonous deposits consisting of coarse shelly materials which have accumulated in place and generally have a finer grained matrix, or may be enclosed in micritic limestone. Coquinas and coquinites experienced substantial to considerable abrasion and transit before reaching the depositional site, but coquinoids have formed largely in situ, and under certain conditions can build up to biostromes. Coralgal: intergrowth of Algae (particularly coralline types) and corals, to form a firm carbonate rock. This may result in a reef rock, a bank deposit, or a biolith which has a lesser degree of framework. Normally, this rock is composed in large measure of frame-building organisms arranged in an interwoven to interlaced arrangement. It is an excellent sedimentbinder. Criquina: coquina of crinoidal debris. Criquinite: indurated equivalent of criquina. Commonly this rock is an encrinal limestone composed wholly or largely of disarticulated crinoid stems and/or plate fragments, is firmly cemented, is compact, and is matrix-bounded. Cryptoclastic: micritic limestone (or dolomicrite) having an aphanic clastic or microgranular texture, discrete particles of which are less than 0.001 mm in size, and under high-power magnification display little or no crystallinity. This is “rock flour”, or extremely finely comminuted carbonate “dust”. Cryptograined is essentially synonymous, although this type of texture could result from chemical precipitation and/or biochemical to physicochemical precipitation or flocculation. Cryptocrystalline: micritic limestone (or dolomicrite) having an aphanic crystalline texture, discrete subhedra and euhedra of which are less than 0.001 mm in size. Some varieties of cryptocrystalline dolomites display a translucent “sheen” when broken, and may granulate rather than flake. Cryptograined: a size term for micritic sedimentary carbonate rocks referring to particles of cryptoclastic detritus (or flocculated, or precipitated), discrete grains of which are less than 0.001 mm in size (some workers prefer an upper limit of 0.004 mm). Crystalline: refers to a texture characterized by interlocking crystals in a mosaic, or to discrete crystals whether juxtaposed against each other or against grains, fossils, or matrix. It may also refer to discrete crystals in finer grained matrix, or in niicrite. Crust (algal): a deposit of algal “dust”, filamentous or bladed Algae, or clots of Algae on larger particulate rocks or fossils, arranged due to accretion, aggregation, or flocculation. May form “biscuit-like’’ encrustations on rocks, fossils, grains, pellets, etc. It is common to fresh-water, lacustrine, and marine deposits, and may form large, bulbous masses or “heads”, such as those formed in sediments of the Lake Bonneville Group (Pleistocene).

Dense: compact, having various parts crowded together. Its use is not restricted to aphanic and

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finely textured rocks, and may be used for phaneric rocks as well because it defines the degree of compactness. Micrites and dolomicrites (in particular the primary dolomicrites) are dense. Depocenteu: basin or other repository of sedimentation; it may be lacustrine, paludal, or fluvial, but more commonly is marine to terrestrial-marine for carbonate sedimentary rocks. Derrital: formed from debris of pre-existing rocks; detrital limestone is one formed from the detritus of older carbonate rocks, whether derived from an extra-depocenter or intra-depocenter source (or both). Petroclastic limestone is considered synonymous with detrital limestone. If composed of discrete particulate fossils or of older fossil fragments, the term skeletal-detrital may apply for the rock name.

Diagenesis: all those processes which change a fresh sediment into a stable rock of substantial hardness, under conditions of pressure and temperature not widely removed from those existing on the earth’s surface in various depocenters. Diagenesis refers primarily to the processes and reactions which occur within a sediment between one mineral and another or among several minerals and the interstitial fluids. It includes all those processes leading up to final induration of the rock but just before incipient metamorphism. Syndiugenesis occurs penecontemporaneously, at the interface in lime ooze or mud, and during early stages of compaction, cementation, and water-expulsion, but before deep burial. Deepburial diagenesis, but still in the realm of temperature-pressure conditions normal to the depocenter, is late diagenetic. Diagenetic dolomite: dolomitized limestone, or dolomitized lime ooze or mud while still in an uncompact (possibly watery) state, and prior to complete lithification. The process involves all those changes leading up to final dolomitic limestone or dolomite (= dolostone), with positive and negative relics still discernible. Dolarenite: dolomite sand, and thus largely “primary” in the sense of being reworked and abra. ded pre-existing rock. Some dolarenite results from clotting, coagulation, and aggregation of dolomite mud, with concomitant and later rolling and shaping into sand-size particulate material. The rock has a sucrosic or “sugary” appearance. Dolurenaceous is a term describing the texture of dolarenites, or dolomite “sand” derived from crystalline dolomite. Dolocast: casts in a dolostone, dolomitic limestone, or in gypsum or anhydrite, indicating former presence of dolorhombs. Dolobtite: dolarenites range downward in size-grade through dolosiltites to dololutites, which are silt-size and silty-clay to clay-size dolomitic rocks, respectively. Dolosiltites and dololutites are common in evaporitic sequences and may be interlayered with dense primary dolomites. Dolomicvite: aphanic to finely crystalline and grained micritic dolomite, resulting from induration of magnesium-rich mud, or diagenetic dolomitization of micritic limestone. Dolomicrites are common to evaporitic sequences. Dolomite: the hexagonal rhombohedra1 mineral, CaMg(C03)~;it may be used by some workers to define a carbonate rock composed of more than 50 % by weight of the mineral dolomite (other workers may prefer the term dolostone for the rock). For practical microscopic work, areal percentages are used instead of weight percentages. Dolomitic: where used in a rock name, “dolomitic” refers to those rocks that contain 5-50 % of the mineral dolomite. Dolomitic can also be used as a general term applying to those rocks which are dolomite-bearing.

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Dolomitic mottling: arrested dolomitization, or arrested (or incomplete) dedolomitization. Common to limestones that have large particulate skeletal or non-skeletal material embedded in finer textured matrix that, under the effects of dolomitization, preferentially replace or alter the matrix but not the large particles. Also common to more or less homogeneous textured limestones that have been incompletely dolomitized, leaving patches, blotches, “birdseyes”, laminae, or other structures and textures unaffected. Dolomitized: refers to rocks or portions of rocks in which limestone (and sandstone) textures are discernible, but which have been converted wholly or largely to dolostone or dolomite rock. If a rock has been dolomitized, positive and negative fossil or grain relics commonly are recognizable.

Dolomolds: molds or partially filled molds in dolomite, dolomitic rock, or in gypsum and anhydrite, indicating former presence of dolorhombs. Dolorudite: as dolarenites pass upward in size grade, they are called dolorudites. They can consist of older rock fragments, subangular to round, of mtraclasts, particularly if reworked, and of dolomitized reef “trash” detritus in the fore-reef tract. Dolomite mud (ofearly diagenetic types) can be disrupted from the floor of the depocenter, reworked and indurated to form dolorudite, such as edgewise conglomerate or flat-pebble conglomerate. Dolosparite: see sparite. Druse (drusy): sparry calcite (or sparry dolomite) lining or filling shells, open spaces such as voids, pore spaces, interstices, cavities, etc. The druse is crystalline. Drusy coating: calcarenite grains, regardless of origin, may be surrounded by a thin layer of needlelike calcite (or dolomite) crystals that grow normal to the grain surface. The coat, composed of tightly packed scalenohedral or rhombohedra1 crystals projecting outward into the intergranular pore space, forms a rind generally not more than 100 ,LA thick. Earthy: refers to a variety of argillaceous to slightly argillaceous carbonate (limestone or dolomite) with earthy texture, generally closely associated with chalky deposits and commonly showing similar porosity values. Contrasts in some respects with the term dense. It is microtextured (0.01 mm and less in size). Some primary dolomites have an earthy texture. Encrinal (encrinite) : containing crinoid stem and/or plate fragments in the carbonate. If the content of crinoidal fragments is more than 10 % but less than 50 % of the bulk, the rock is an encrinal limestone (or dolomite); whereas if there is more than 50 ”/o of such material, it is an encrinite or dolomitized encrinite. Endogenic: refers to components derived from within the depocenter. Energy Zndex: inferred degree of water agitation in the depositional environment. Energy level: the kinetic energy that exists in the water at the depositional interface and a few feet above. This energy of motion may be due to either wave or current action, or to surf surge. Eolianite: sedimentary accumulation formed by wind action. Oolites, oopellets, pellets, and some other particulate material that originally formed in a water environment may subsequently be transported by wind action (such as across a tidal flat, beach area, etc.) and ultimately heaped into dunes and other deposits. Discrete particles may be carbonates, gypsum, and some other materials. Wind-drifted oolite sands, pelletal sands, and gyparenites adjacent to Great Salt Lake and in the Salt Flats of Utah are such examples.

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Epidiagenetic: lithologic changes of a rock under ground or artesian water influence. Applies especially to topographic dolomitization (FAIRBRIDGE, 1966). Epigenesis: this includes all processes at low temperature and pressure that affect sedimentary rocks after diagenesis and up to metamorphism in the depocenter. This gradation may occur: syngenesis +-diagenesis + epigenesis + metamorphism. Some geologists reserve the term “epigenesis” to mineral replacements near the earth’s surface. Equant: equidimensional crystalline fabric, whether aphanic or phaneric. Equigranulur: equidimensional grained fabric, whether aphanic or phaneric.

Euhedral: refers to individual crystals with well-developed crystal boundaries or faces; these can be equant, or inequant. Evaporite-solution breccia: solution breccias are created when intervening soluble evaporites (salt, anhydrite, gypsum, etc.) are dissolved away, letting the carbonate beds crush under the weight of overlying sediments. This rock type is an extremely angular collapse breccia, and the matrix commonly is of the same material as the rock fragments. These chaotic breccias normally are associated with evaporites, and may also be adjacent to reef limestones, which, upon removal of the evaporites, collapse and may be “healed” or cemented by calcareous and/or dolomitic material. Exogenic: referring to components derived from outside, i.e., from either above or below, the sedimentary formation and from an extra-depocenter provenance. Extraclast: fragment(s) of calcareous sedimentary material produced by erosion of an older rock outside of the depocenter in which it accumulated. Fabric: arrangement of discrete particles (grains), crystals, and cement relative to each other in a sedimentary carbonate rock. Fibrous: see cement. Fore reef: the seaward side of the reef trend. The fore-reef sediments, composed primarily of reef detritus, interfinger with the reef and basin sediments. The terms fore reef and back reef apply only to linear reef trends, in contrast to reef core and reefflank, which apply to all types of reefs. The back reef is the landward side of the reef trend, and its sediments are largely reef-derived fossil debris, calcarenite, and calcilutite, which may interfinger with both the reef and lagoonal facies. Fragmental: refers to broken or detached debris. Detrital fragments and those derived from the skeletons of organisms, are included under this term. Fragmental limestone is, therefore, a mechanically-formed rock; fossiliferous-fragmental limestone is common in stratigraphic sections, and commonly is termed bioclastic limestone. Framework: rigid, wave- and current-resisting structures bioconstructed by sedentary organisms capable of erecting a limestone upward and outward in a high-energy environment. Framework-building organisms include sponges, stromatoporoids, corals, bryozoans, Algae, and combinations (such as coralgal, bryalgal, etc.).

Fusulinal: a term denoting presence, in minor to major (and even dominating) amourlth of fusulinid tests in a carbonate sedimentary rock. These may be in a micrite, or other iirriestone, or may remain as relics in dolomitized rock particularly if silicified prior to dolomitization.

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Glomeraclustic: a textural term applied to sedimentary carbonate rocks in which lumpal particles are grouped together in clusters. Grain growth: this process acts in monomineralic rocks of low porosity. The intergranular boundaries migrate causing some grains to grow at the expense of their neighbors. The reaction takes place in the solid state, ions being transferred from one lattice to another without solution. Larger grains tend to replace smaller ones, and a fine mosaic is gradually replaced by a coarser one. As grain growth proceeds, many of the enlarged grains are themselves replaced by their more successful neighbors. In limestones grain growth appears to affect only the very fine mosaics with grain diameters from 0.5 to 4.0 mm. These include calcitemudstones, the walls of Foraminifera, algal frameworks, Bahamite particles, and ooliths (BATHURST, 1958,1959). Grain-supported: rocks in which grains are so abundant as to support one another, with little or no interstitial mud-matrix, but with various cement types. Grains: discrete particles larger than 0.01 mm (for most routine work), but technically particles can be cryptograined and micrograined. They may form the rock framework, similar to sand grains in a sandstone, or they may be subordinate to smaller particles in the rock. Grains include detrital (lithoclastic) particles, skeletal grains (bioclastic), pellets, coated grains, oopellets, and glomeroclastic grains (many of which may have formed by clotting, coagulation, flocculation, aggregation, etc.).

Grainstone: mud-free carbonate rocks, which are necessarily grain-supported, are termed grainstone; some are current laid, some are the product of mud being by-passed while locally produced grains accumulate, or of mud washed out. Granular: applied to sedimentary rocks made up of grains, usually larger than 2.0 mm in diameter. Grapestone: composite grains, clusters of pellets, or irregularly-shaped grains having protuberances resulting from overgrowths, aggregation, flocculation, clotting, coagulation, etc. of lime mud. May be of silt-size, but more commonly are of sand- and granule-size. Grumous: textural feature seen in limestones that experienced pervasive crystallization. Such crystallization (and recrystallization) develops patches of coarsely crystalline carbonate which invade, in an irregular way, shell debris, oolites, and matrix alike. The uncrystallized areas remain dark, dense, and finely textured, and are ultimately surrounded by waterclear coarse crystalline calcite (sparite). Such a rock has a clotted or grumous texture; and in some respects resembles spotted dolomite. This texture is common in diagenetic dolomites. High-energy: the environment of lithoclastic and bioclastic carbonate working and accumulation, mostly in a zone of turbulence created by waves, currents, and surf-surge. Has the highest Energy Index.

Hydroclust: lithoclastic and bioclastic carbonate detritus that is transported, worked, and deposited in a water environment. Hydrolith is the resulting rock. Impingement: a mechanism or process in dolomitization in which dolomite crystals replace limestones, commonly skeletal particles such as crinoid ossicles and plates, but not in optical continuity with the calcite of the original particle. Interface: depositional boundary condition separating two different physicochemical regions. When particles come to rest on the bottom of a depocenter, they form a solid matrix having water-saturated pores. The water has the same composition as the medium above, but marked changes occur once it is sealed from free circulation by confinement in the pores.

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As deposition continues, the lamina of sediment passes from the water-sediment interface to successively lower positions and enters a realm of greater pressure, higher temperature, and of changed chemical and biologic conditions. These new conditions promote the consolidation or lithification of the sediment into a sedimentary rock.

Intergranular porosity: void space between grains, whether bioclastic or Iithoclastic. In sedimentary carbonate rocks the term granular commonly refers to the grains, whether skeletal or nonskeletal. Some geologists, however, regard a granule as a size-grade textural term, with discrete particles larger than 2.0 mm in diameter. Internal sedimentation: allochthonous clastic and/or chemical sediment derived from the surface or from within the rock framework and accumulated in cavities within the sedimentary rock formation. It is a collective term including both mechanical and chemical internal sediments. Interstitial: of, pertaining to, existing in, or forming an interstice or interstices (Standard), In sedimentary carbonate rocks, interstitial denotes the space between grains and/or crystals. Intraclast: fragments of penecontemporaneous, generally weakly consolidated carbonate sediment that have been eroded from adjoining parts of the sea bottom and redeposited to form a new sediment. The particles have been reworked within the area of deposition and within the same formation. Inversion: the process by which unstable minerals change to a more stable form of the same chemical composition (except for a possible change in content of trace elements and/or isotopes) but with a different lattice structure. Limestone: pertains to a carbonate rock composed of more than 50 % by weight of the mineral calcite. For practical purposes in microscopic work, area percentages are used instead of weight percentages. Lithocalcarenite: that variety of calcarenite in which detrital and/or intraclastic fragments predominate; these are devoid of organic structures originating from aggregation processes. This is the lithic calcarenite, or lithoclastic variety of calcareous arenite. Lithocalcarenites contain a predominant number of sand-size carbonate grains, angular to well rounded, which are fine-grained to coarse-grained and are devoid of any internal structure. The name bahumite includes deposits varying from calcisiltites to calcirudites, in which the grains are accretionary and commonly are composite, consisting of smaller particles bound together by precipitated material into aggregate grains. Lithocalcilutite: fine-grained equivalent of Iithocalcarenite, and also the fine-grained equivalent of practically all the allochthonous types of calcareous deposits, particularly of the lithocalcarenites of aggregation origin (bahamites). Many of the original lithocalcilutites appear now entirely crystallized; they have a crystallinity coarser than 0.002 mm and consist of a finely crystalline mosaic of interlocking anhedral to subhedral crystals of calcite or of dolomite (dololutite). Lithoculcirudite: composed of uniform to nonuniform, or composite nonskeletal particles of microcrystalline calcite generated by aggregation processes (bahamites), pisolites, composite oolites, and coarse particles disrupted and torn from any pre-existing autochthonous and allochthonous limestones. Lithodolorudite, like lithodolarenite and lithodololutite, is of CaMg(CO& composition. Lithocalcisiltite: companion term for silt-size carbonate lithic fragments; lithodolosiltite is the dolomitic equivalent.

H. J. BISSELL AND G. V. CHILINGAR

Lithoclastic: autochthonous or allochthonous carbonate detritus; mechanically formed and deposited carbonate clasts, derived from previously formed limestone and/or dolomite, within, adjacent to, or outside the depositional site. Some linieclasts are intraclasts and were derived from particulate material torn from the sea bottom (or lake bottom) and incorporated in the new unit. Lithographic: pertaining to a compact carbonate rock having about the same particle size and textural appearance as the stone used in lithography. Characterized by conchoidal fracture, extreme smoothness of texture (aphanic), and uniformity of grains. Internally, the rock may be entirely micro- or cryptograined, entirely micro- or cryptocrystalline, or variously combined. Lithographic is a term to describe the appearance, fracture habit, utility in lithography, etc. and applies (in part at least) to the micrites and dolomicrites. Sub-lithographic is a term designating a minor degree of the above features. Lump: in recent sediments, “Iumps” are composite grains typically possessing superficial reentrants and believed to have formed by a process of aggregation; or lumps may result from disruption of partially indurated lime mud or dolomite mud in the depocenter. In ancient sediments, the “composite grain character,” may not be easily distinguished, but “lumpal” material of larger dimensions will be readily discernible. The following criteria are useful in recognizing lumps: ( I ) lobate outline, reflecting superficial re-entrants, (2) grains texturally similar to the material in which they occur, and (3) rock associations. Lumps may originate through clotting, flocculation, aggregation, and through disruption of newly-deposited lime mud or dolomite mud. In the latter case the fragments are reworked and redeposited within the unit from which they were disrupted.

Luster-mottling: sandstones of various compositional types may become cemented with calcite and/or dolomite which assumes crystallinity; each pore may be filled with a single crystal or with several crystals, some of which are up to a few millimeters in size. Freshly broken arenites so cemented by sparry calcite or dolomite display luster-mottling when turned in the light. Many dolomitic quartz sandstones illustrate this phenomenon. Luiite: this is mud, and is thus a combination of clay- and silt-size particles with the former predominating; the term can carry prefixes such as calci-, dolo-, lithocalci-, biocalci-, etc. Mad: semifriable mixtures of clay materials and carbonates. The better-indurated rocks of like composition are marlstones or marlite, which can be considered more correctly as earthy or impure limestones rather than shales. Marl contains 30-70 % of carbonates and a complementary content of clay. Most commonly, the term marl has been used to denote certain friable carbonates (usually earthy) which accumulated in Recent or present-day fresh-water lakes. It may result from precipitation, flocculation, or physical settling out of the water; photosynthesis of plants in lakes, shallow seas, etc. can hasten the precipitation of marl. Certain microorganisms likewise can aid in precipitating, flocculating and settling of the sediment. Matrix: the natural material in which any fossil, rock fragment, crystal, grain, etc. is embedded. In a rock in which certain grains are much larger than the others, the grains of the smaller size comprise the matrix. If the particles in a rock are of different orders of magnitude of size, the term matrix applies to the smaller individual units that fill the interstices between the larger grains. Sand-, silt-, and clay-sized material which is resolvable only by size and shape analysis is included in the definition of carbonate matrix. Micritic material may also be called matrix when it encloses grains or fills interstices between them. Maturity (sediment) :the extent to which clastic carbonate material approaches the end product to which it is driven by the formative processes that operate on it.

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Mechanical: pertaining to particles of sediment brought to their place of final deposition by agents such as water currents, wind currents, or gravity. Megalump: most lump (= lumpal) limestones contain silt-size and sand-size clots, coagulated irregularly-shaped grains, and aggregate grains (bahamite); some, however, contain megalumps, which are coarser-textured and lie in the granule- to boulder-size classes. These are “lithocalcibreccia-” and “lithodolobreccia-” like masses, and may originate through tearing up by waves, currents, and surf-surge of very high energy-index values, and possibly by turbidity currents. Partly indurated lime mud or dolomite mud could be ripped up into fragments which are shaped to resemble rudites, and then incorporated within the unit from which they were derived. In the early literature, these are known as penecontemporaneous intraforniational mud-pebble conglomerates or breccias. Metusomatism (dolomitic) :diagenetic differentiation is the redistribution of the materials within a sediment, leading to segregation of the minor constituents into nodules, concretions, and related bodies. Diagenetic metasomatism involves introduction of materials from without, leading to replacement. Micrite: consolidated or unconsolidated ooze or mud of either chemical or mechanical origin. FOLK(1959) originally stated that the term should be reserved for those rocks that, under the petrographic microscope, are seen to consist almost entirely of microcrystalhne calcite. LEICHTON and PENDEXTER (1962) defined the micritic material as that consisting of particles less than approximately 0.03 mm in diameter. In the present chapter, micrite is employed for material, whether crystalline or finely grained, that is 0.05 mm or smaller in diameter or across faces. Micrite is lime mud or its indurated equivalent, and dolomicrite is dolomite mud or its indurated equivalent. Micritic limestone: a limestone which consists of 90 % or more micrite. Microclastic and microcrystalline limestones are two varieties of micritic limestone (LEIOHTON and PENDEXTER, 1962), the former possessing a clastic texture and the latter a texture of microscopic size interlocking crystals. Aphanic (aphanitic of some authors) limestone, matrix limestone, calcilutite, and lithographic limestone are practically synonymous with micritic limestone. Microcrystalline: usage varies (see Table IV), but it refers to crystallinity in limestones and dolomites between 0.001 and 0.01 mm across crystal faces. Some petrographers prefer the limits of 0.004 to 0.062 mm.

Micrograined: clastic carbonate particles between 0.001 and 0.01 mm in diameter, or it may he preferred (see LEIGHTON and PENDEXTER, 1962) to have an upper limit of 0.0625 mm and a lower limit of 0.004 mm. They apply the terms coarsely micrograined (0.03 to 0.06 mm) and finely micrograined (0.004 to 0.03 mm), with cryptograined particles being smaller than 0.004 mm in diameter. Micropelfetoid: particles of pellet nature, or possibly true pellets (faecal or otherwise), of a fine to very fine grade size, possibly smaller than 0.01 mm in diameter. FOLK (1962) places all pellets in the size range of 0.03 to 0.15 mm.

Mosaic: a textural term, more applicable to dolomites than to limestones (except the non-dolomitized, but diagenetically altered varieties). Secondary overgrowth of dolomite on rhombs produces mosaic-like texture. This destruction of original intercrystalline porosity by continuing growth of dolomite is analogous to the “cementation” phenomena of wellsorted skeletal and nonskeletal limestones, such as encrinal limestones and the like. Mud: silt-clay mixture (the latter size commonly predominant) in water without connotation as to composition. Lime mud dolomite mud, etc. identify the variety.

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Mud aggregate: any aggregate of mud grains, commonly having the size of a sand or silt particle, and usually mechanically deposited. Initially the aggregate may have been a faecal pellet, or a rounded, subspherical aggregate of mud grains cemented originally by aragonite with no signs of organic control, or a fragment of algal precipitate, or a spherical or ovoid growth form of a calcareous alga (BATHURST, 1959). Mud-supported: muddy carbonate rock which contains more than 10% grains, but not in sufficient amount that they support one another; such grains are “floating”, and thus they are mud-supported. Mudstone: muddy carbonate rocks containing less than 10% grains (grain/bulk ratio being 0.1); the name is synonymous with calcilutite, except that it does not specify mineralogic composition, and does not specify that the mud is of clastic origin (DUNHAM, 1962). Nodular limestone: a variety characterized by nodules, lumps, clots, and grapestone accumulations within argillaceous and micritic limestones. Nodules are not necessarily concretions, but may represent lumps, flocculated material, and round to subround aggregations, and similarly-shaped very large coated grains. Most commonly nodules are composed of the same type of material that encloses them. Nonclastic: having a texture showing no evidence that the sediment was deposited mechanically. Nondetrital: minerals that are precipitated from solution by chemical, physical, physicochemical, biochemical, or biologic means. Accumulation occurs at, or generally close to, the site of precipitation. This group also includes the authigenic minerals formed in the sediment after deposition. Nonskeletal limestone: lithocalcarenites and lithocalcisiltites containing subrounded to rounded grains devoid of internal structure, organic or otherwise. These grains have been termed pellets, granules, false oolites, oopellets, intermediate pelletoid grains, pseudo-oolites, etc. Olistolith: exotic blocks of older strata apparently transported by gravity sliding: “calcolistoliths” are limestone exotic blocks of this type, whether transported by turbidity currents or by gravity sliding. “Olistostrome” is an entire formation of slumps and exotic blocks. Oolite (or ooid): spherical or subspherical accretionary grain generally less than 2.0 mm in diameter. In section, oolites display concentric structure, and may also exhibit radial structure. Oolite is a coated grain, and may or may not have a nucleus. Superficial oolite is a type of oolite in which the thickness of the accretionary coating is less than the radius of the nucleus. Some workers prefer the name oii[ith for the rock, and oolite (or ooid) for the discrete coated grain; some petrographers use the two terms interchangeably, regardless of whether it is a rock or a grain. Oolitoid: similar shaped and sized bodies to oolites and ooliths, but which lack the internal structure normally found in oolites; they consist of a fine-grained aggregate of Fe-rich dolomite (GREENSMITH, 1960). Oopellet: spherical or subspherical grain displaying characters of both an oolite and a pellet, and should not be confused with superficial oolite. The internal part is pelletoidal, and thus may be ovoid in shape, but it has an accretionary coating, the thickness of all layers being equal to or slightly greater than the diameter of the pellet which they enclose. Open-space structures: they are structures in carbonate rocks which formed by the partial or complete occupation with internal fillings composed of internal sediments and/or cement of one to several generations.

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Organic lattice: reef-building framework, in situ. Organic linzestone: biogenic limestone owing its origin directly to physiological activities of plants and animals. Organic structures limestones: the major framework of these limestones has been secreted by organisms such as Algae, stromatoporoids, sponges, corals, bryozoans, and combinations thereof (bryalgal, coralgal, etc.); and the fossil remains are still in their approximate growth position. Orthochemical: carbonate sediment or rock equivalent which is of straight or direct chemical origin. Flocculated and precipitated lime mud may form micrites; and primary dolomites are orthochemical because they have been precipitated directly out of sea or lake water. Primary dolomite is, therefore, an orthodolomite. Ovoid grains: pellet-shaped grains having a length two or more times as long as the diameter; these are “football-shaped” grains, and are commonly 0.1-2.0 mm long (though some are up to 5.0 mm in length). They commonly lack an internal structure, although weakly-

developed radial structure, and rarely concentric structure, can be seen.

Packstone: a limestone in which the grains are arranged in a self-supporting framework, and yet contains some matrix of lime mud. If no mud is present the rock is called grainstone; and if grains comprise less than 10% of the rock mass, it is a mudstone. Paragenesis: a general term for the order of formation of associated minerals, textures, and structures in time succession, one after another. Purticulate: discrete particles, grains, fossils, fragments, skeletal material, and crystals. faurocrystalline: lowest size-grade group of the phaneric crystalline carbonate rocks; subhedra and euhedra lie in the size range of 0.01 to 0.1 mm. The termpaurograinedis the clastic or grained equivalent. Pelagosite: this is a deposit (generally white, gray, to brownish with a pearly luster) composed of CaC03 with higher MgC03, SrC03, CaS04.HzO and SiOz contents than those found in normal limy sediments. It is restricted to intertidal spray-formed incrustations a few millimetres thick (see REVELLE and FAIRBRIDGE, 1957). felite: size-grade of lutite, and is of clay- to silt-size material (the former predominant). Pelitomorphic is an all-embracive term for carbonate particles of this size; but more commonly it connotes anhedra to subhedra. Pellet: a grain composed normally of micritic material, lacking significant internal structure and generally ovoid in shape; it may also be sub-ovoid. Most pellets in limestones are of siltsize to coarse sand-size (some are slightly larger). In some respects pellets are pseudooolites, for they are spherical to subspherical to oval bodies with distinct boundaries, and resemble oolites; however, they do not possess comparable internal structure, for example, faecal pellets. Carbonate muds are commonly pelleted, pelletal, or pelletoid, displaying rounded or ellipsoidal aggregates of “grains of matrix” material. These muds are thought to be pelleted either by faecal activity, gas bubbling, or by algal “budding” phenomena. Penecontemporaneous: a term used in connection with the formation of sedimentary rocks, and implies “formed at almost the same time”. fhaneric: textural term for carbonates, particularly limestones, which are crystalline (and/or grained), and the discrete particles of which are larger than 0.01 mm. faurograined (0.01

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-0.1 mm), mesograined (0.1-1 .O mm), and megagrained (1 .O-10.0 mm) are the three textural subdivisions. The term phaneritic is a term applied to texture in igneous rocks in which individual crystals are visible to the unaided eye, and should not be used as a textural term for carbonate rocks.

Pisolite: a grain type similar to an oolite, and generally 2.0 mm or more in diameter. The term pisolite is restricted to crenulated, rounded or semi-rounded, commonly composite carbonate grains or bodies thought to have been formed by biochemical algal-encrustation processes. They could, therefore, become enlarged by accretion to form “algal-balls”, “algal-biscuits”, and the like. Porphyroblastic: textural term to describe limestones and in particular dolomites, in which large crystals (porphyroblasts) are scattered through a crystalline matrix of finer textured materials. Crystallized crinoid ossicles in micrite or in fine-textured matrix would display this texture in limestones; and sparry dolorhombs embedded in finer crystalline or dolomicrite material would be one example among dolomites. Preferred fabric: preferential crystallinity, particularly in diagenetically altered limestones and dolomites; and also a primary depositional fabric in mechanically deposited !ithodastic and bioclastic sedimentary carbonate rocks. Preferred fabric is characteristic in primary dolomites, as deduced with the universal stage and thin sections. For example, the C-axes of crystals in primary dolomites lie parallel to the plane of bedding. Pressure solution: a preferential solution takes place on the higher stressed parts of a grain (or crystal) and deposition of matter 011 surfaces with lower potential energies. The pressure is supplied by the overburden and should result in recognizable grain fabric, with the grains flattened at right angles to the pressure. Regarded as perhaps the most important process in closing the original pore spaces of sediment (BATHURST, 1958, 1959). Primary dolomite: resulting from direct precipitation out of sea water or lake water. It may have a preferred fabric, and is aphanic to finely textured (crystalline and grained). Dolomite “sand”, regarded by some workers as a primary deposit of first-cycle dolarenite, is hardly categorized as primary dolomite, but is a derived clastic carbonate. Pseudobreccia: masses of grain growth mosaic which lie in a “matrix” of less altered limestone: most of these are visible to the naked eye. The “fragments” are irregularly shaped lumps of coarse calcite mosaic usually between 1.O mm and 20.0 mni in diameter, and are dark gray in hand specimen. They lie in the finer, pale-gray, “ground mass” of calcite-mudstone. In thin section, the “fragments” appear light and the “ground mass” dark. (See Bathurst, 1959.) Pseudomorphic replacement: a diagenetic process whereby the original character of a limestone is altered during dolomitization; skeletal material for example (and specifically crinoidal material) is replaced in such a manner that single crystals of dolomite are in optical continuity with the calcite of the original crinoid fragment. The process contrasts with that of impingement, in that in the latter case there is lack of optical continuity of dolomite crystals with the original crinoid fragment. Pseudo-oolites: some varieties of pellets and oopellets. Recrystallization: a term signifying a process wherein original crystals of a particular size and morphology become converted into crystal units with different grain size or morphology, but the mineral species remains identical before and after the process occurs. First-stage crystallization is not to be termed recrystallization. This latter term is usually used today loosely for a number of processes that include inversion, recrystallization sensu stricto, and grain growth, all of which may result in textural and crystal-size changes. Recrystalli-

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zation proper occurs when nuclei of new unstrained grains or crystals appear in or near the boundaries of the old, strained ones. These nuclei grow until the old mosaic has been wholly replaced by a new, relatively strain-free mosaic with a nearly uniform grain (or crystal) size. Recrystallization fabric: mosaic or other crystalline textural features which identify the pattern of any sedimentary carbonate that has undergone recrystallization. This is not the fabric of metasomatically replaced limestones (by dolorhombs). Replacement crystallization is not recrystallization if the mineral species are no longer the same, such as dolomite after calcite. Reef: a structure erected by frame-building or sediment-binding organisms. At the time of deposition, the structure was a wave-resistant or potentially wave-resistant topographic feature. Reefs and the heterogeneous reef-derived materials form the reef complex. The following types of reefs can be recognized: (1) fringing reefs, for those veneering types that lie or were ad,jacent to the pre-existing land; (2) barrier reefs, for sublinear structures that are or were separated from nearby older land by a lagoon; (3) atolls, for composite structures with ring-like outer reefs that surround or once surrounded a central lagoon devoid of pre-existing land; ( 4 )patch reefs, for small, sub-equidimensional or irregularly shaped reefs that are parts of reef complexes; ( 5 ) table reefs, for flat-topped, isolated, characteristically small reef-mounds of the open ocean; (6)pinnacle reefs, which have a very small area and grow almost vertically; ( 7 ) bank reefs, that grow over submerged highs of tectonic or other origin-these reefs are large and have an irregular shape, and the marine bottom water surrounding them is too deep to support growth of reef-forming organisms; and (8) shoal reefs, that grow on the shoals of the fore-reef and back-reef areas; these are smaller in area than the bank or platform reef types, and generally grow on the debris of a larger reef. Reefal: this is purely a descriptive and not genetic term having reference to carbonate deposits in and adjacent to any of the numerous varieties of reefs, and to any or all of their integral parts. Reef milk: matrix material of the back-reef facies, consisting of microcrystalline white and opaque calcite ooze, and derived from abrasion of the reef core and reef flank. Reef tufa: fibrous calcite which forms thin to thick deposits, layered or unlayered, in the myriads of voids in reefs and other organic frame-builders; the fibrous calcite is prismatic in structure and is radial in respect to the depositional surfaces. The fibrous calcite or reef “tufa” is deposited directly upon the framework of the reefs and within the various voids and interstices, from supersaturated water. The mechanism may be largely physicochemical, or, aided by profuse algal growth to extract CO? from the water, may also be biological to biochemical deposition. Development of reef tufa follows and/or accompanies growth of organic frame-builders, and precedes infilling of detritus such as lime mud, calcarenite etc. Relic: vestige(s) of skeletal and nonskeletal material in a sedimentary carbonate rock, commonly dolomites. Crystallization, recrystallization, impingement, and other diagenetic alteration has not completely obliterated these features if they are to be termed relics. There can be two types of relics: ( I ) positive-the skeletal or nonskeletal element can be identified, though it is altered; (2) negative (“dissolved out”)-it is known that there was a skeletal or nonskeletal item present, but it cannot be identified as having been a part of an organic or non-organic species. Rim cement: cement which grows into interparticle voids and is optically continuous on single crystal particles such as crinoid fragments, etc. Thus, the host is a single crystal and the cement forms a single rim in lattice continuity with it. The overgrowth is a continuation of this crystal, and the overgrowth can form by filling the pore space.

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Saccharoidal: a descriptive term meaning “sugary” texture. More specifically it is a result of diagenetic process (or result of dolomitization) in which crystallization or recrystallization gives rise to a new texture. It may be first-stage crystallization, but more commonly is recrystallization that occurs early in a newly-deposited carbonate mud; it does not alter gross primary structures of the sediment such as ripple marks, thin bedded to laminated layers, etc., but does tend to destroy minor structures such as shells of organisms. Saccharoidal texture is recognized by the well-developed rhombs of dolomite of approximately uniform size resting one against the other with point contact and, likewise, commonly separated by exceptionally large as well as small pore openings. The fabric displays loose packing, and suggests that dolomitization occurred when the grains were loose and before compaction altered the original texture (i.e., a packing typical of loose beach and shore-line sands). Recrystallization of the original smaller dolomite grains, or replacement crystallization of the original calcite grains destroys the original particle-size distribution, and substitutes a new, highly restricted, crystal-size distribution ranging from medium- to coarse-sand dimensions. Secondary: a general term applied to minerals and rocks formed as a consequence of alteration. This term is too all-inclusive and ambiguous in detailed studies and should be used only as a very general colloquial term when misinterpretation is absolutely impossible. Ske/etal: pertaining to debris derived from organisms that secrete hard parts and hard material around or within organic tissue. NELSON et al. (1962) defined skeletal limestones as those which consist of, or owe their characteristics to, virtually in-place accumulation of calcareous skeletal matter. These rocks, formed through biologic processes, are contrasted with fragmental limestones. LEIGHTON and PENDEXTER (1962) considered the term bioclastic to be synonymous with skeletal. The term skeletal is thus also used to indicate faunal (or floral) fragments or wholecomponents of these organisms that are not in place of origin. Solution transfer: this is a translation of the German Losungumsatz. It refers to the solution of detrital particles around their points of contact where elastic strain and solubility are enhanced (pressure solution), followed by redeposition on less strained particle surfaces (BATHURST, 1959). Sparite: a contraction of, and therefore synonymous with, sparry calcite. Sparite is a loose descriptive term applied to any transparent or translucent crystalline calcite and aragonite. It can occur in numerous morphologic forms, viz. granular, drusy, fibrous, and blady. Three possible origins are recognized: ( 1 ) physicochemical precipitation, (2) recrystallization, and (3) grain growth. The first is distinguished by adding the genetic prefix ortho-, and the latter two by adding prefix pseudo-. Sparite is larger than 0.02 mm in diameter. Petrographers who prefer to use also the term nzicrosparite set its size limits at 0.005-0.02 mm. The prefix dolo- is used to indicate sparry dolomite crystals, i.e., dolosparite and tlolomicrosparite. Some workers prefer the prefix calc- to distinguish calcsparite from the dolo’ mitic variety, but to some the term sparite is automatically understood to mean the calcareous variety. Sparry: refers to clear, transparent, or translucent, readily cleavable, crystalline particles generally having an interlocking mosaic texture. FOLK (1959) referred to sparry calcite cement which forms grains or crystals lop or more in diameter. The name spar alludes to its relative clarity both in thin section and in hand specimen. Speleothem: a carbonate cave deposit of any sort (also “speleal” limestone). Spergenite: a coquinite (to niicrocoquinite), and/or a biocalcarenite composed of sorted fossil debris, including bryozoan fragments, and Foraminifera (possibly endothyroids), together with carbonate detritus of various types, cemented by sparry calcite; a biogenic calcarenite.

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SpheruZite: a textural term, applicable to limestones and dolomitized equivalents, in which rounded and subrounded, spherical to subspherical ooids are present. These may have either a concentric, radial, or axiolitic structure, or a combination of all three. Spherulitic limestone contains 50 % or more of these spherules; they range in size from about 0.5 to 2.0 mm, although some are up to 5.0 mni in diameter. No particular genetic significance is attached because they may be (1) detrital, (2) coated grains, (3)algal spherules, and ( 4 ) authigenic. According to some workers only a radial structure is indicated; and according to some, a sgherulite is a small spherical or spheroidal particle composed of a thin dense calcareous outer layer with a sparry calcite core. According to PETTIJOHN (1957), spherulites are minute bodies of oolitic nature in which only a radial structure is visible. Stinkstein: “stink-stone”, or smelly rock; among carbonate rocks, both limestones and dolomites, stinksteins are common. Normally these are of three types, or some combination thereof: ( I ) “sweet” or hydrocarbon odor; (2) fetid, or foul odor (common to most criquinites), or (3) “sweet-and-sour”, typical of carbonates rich in organic-phosphatic material, particularly detectable if dilute HCl is applied to freshly-powdered rock.

Stromutuctis: these are open-space structures with horizontal flat to nearly flat bottoms, and are filled by internal sediments and/or cement. Their genesis has been variously interpreted as being caused by the burial of soft organisms which upon decomposition left an open space. More recent studies, however, show that they are most likely syngenetic voids in calcareous sediments, which are or are not changed by subsequent corrosion and corrasion. Algae are only indirectly responsible by overgrowing surface pits and channels, and thus form an internal cavity system. It seems that Strornaiuciis are most common to micritic limestones formed by calcareous Algae, that left little or no evidence in most occurrences thus far reported from Great Britain, North America, etc.

Stromutolite: laminated sediment formed by calcareous Algae, which bind fine detritus and/or precipitate calciumcarbonatebiochemically. The deposit may form irregular accumulations or structures that may remain fairly constant in shape, for example, Colleniu. Subhedrul: refers t o individual crystals exhibiting a few crystal boundaries; the term unhedrul defines those with no well-defined crystal boundaries, and the term euhedral indicates excellent crystallinity. Subhedrul is midway between these latter two varieties. Sucrosic: contraction of saccharoidal, thus meaning “sugary” texture. The term is commonly applied to certain types of dolomites, e.g., dolarenites.

Syneresis cracks or vugs: cracks or vugs formed by a spontaneous throwing off of water by a gel during aging. In some carbonates, precipitation evidently occurs as a colloidal gel encrusting leaves of sea plants (photochemical removal of carbon dioxide from sea water by the plants causes precipitation). The end-result may be the production of cryptotextured limestone which contains “syneresis” cracks and associated contraction vugs. Syngenetic: originating at about the same time; in sedimentary carbonate rocks, it refers to concretions, authigenic minerals, nodules, and other bodies which form at identically, or approximately, the same time as the rock which encloses them. Syndiugenetic dolomitizution, for example, refers to diagenetic dolomitization which occurs at the interface while lime mud is accumulating.

Syntaxid rims: a mechanism of replacement overgrowth, which develops during diagenesis as a syntaxial extension of a detrital single crystal (e.g., a crinoid fragment). Not to be confused with drusy mosaic, which is a convenient term for grain mosaics which have been deposited on the walls of cavities. Whereas drusy mosaic and granular cement are entirely chemically deposited, the mosaic formed by rim cementation processes consists of grains each of which has a core composed of the detrital host. The textural relationships between lime mud and

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calcite overgrowth in some limestones suggest that rim cementation is the dominant process, and commonly dolomitization occurs after rim cementation. Travertine: a massive to often finely layered colloform type of chemical limestone, often with porous interlayers. In the type area, at warm carbonate springs at Tivoli near Rome, Italy, afine type of sound-absorbing building stone is worked and exported all over the world. The Tivoli River of Italy deposits travertine very rapidly because of agitation. Travertine commonly forms about hot springs, faults, in caves, in soil crusts, as soil nodules, etc. Tufa: a spongy, porous rock which forms a thin surficial deposit about springs and rivers, or may form thick, bulbous or otherwhise swelling features in lacustrine environments, particularly around the shore. It has a reticulate structure, and is weak and semifriable. Travertine, by contrast, is fairly dense, banded CaC03, having tan, cream, and white colors. Turbidity limestones: some litho- and biocalcarenites display features indicating resedimentation by turbidity currents; exotic limestones, particularly relatively large blocks (“calcolistoliths”), are now known to have formed by agents other than waves and currents, possibly by gravity-sliding and turbidity currents. Winnow: eolian sorting; should not be used to describe sorting in water, which is “washing”.

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Chapter 5

ORIGIN AND OCCURRENCE OF LIMESTONES JOHN E. SANDERS AND GERALD M. FRIEDMAN

33 Sherman Avenue, Dobbs Ferry, N. Y. (U.S.A.) Department of Geology, Rensselaer Polytechnic Institute, Troy, N. Y. (U.S.A.)

SUMMARY

Limestones originate by three main processes: ( I ) precipitation of calcium carbonate in an initially stony condition, as in travertine and organic reefs; (2) lithification of calcium carbonate sediments, which includes various steps beginning with changes of grain mineralogy, and includes addition of concentric coatings to grains, selective dissolution of matrix and/or grains, precipitation of mineral cement in pore spaces, and may end with recrystallization; and (3) replacement of calcium sulfate or quartz by calcium carbonate. Of these, the second process is by far the most important. Limestones occur in most parts of the world and throughout the geologic column, but they are notably more abundant in Cambrian and younger rocks, reflecting the increased abundance of marine shell-secreting invertebrates during and after the Cambrian Period. Grains of carbonate sediments mgy come from older limestones, but chiefly they originate as first-cycleparticles within the waters of the depositional basin, as skeletal remains of lime-secreting organisms, ooids, superficial ooids, pseudooids, aggregate grains, faecal pellets, and so forth. Carbonate grains are deposited by various vertical and/or lateral sedimentation processes to build up stratigraphic units, distinctive features of which are controlled by the interplay of physiographic environment of deposition and crustal subsidence. “Pure” limestones and limestones mixed variously with terrigenous sediments from Recent environments and the geologic record are grouped broadly into three major environmental suites: ( I ) nonmarine environments from large inland structural basins (lakes and alluvial fans); (2) shallow-water marine and/or marginal marine environments (beaches, lagoons, bays, dunes, and so forth); and (3) open-sea environments. Work on this chapter commenced in the Department of Geology, Yale University, but was completed after the senior author became a Senior Research Associate, Columbia University. Contribution No. 66-5 of the Department of Geology, Rensselaer Polytechnic Institute.

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INTRODUCTION

The origin of limestones, though a complicated subject, may be stated simply: limestones originate in part by direct inorganic or organic precipitation of calcium carbonate, in part by replacement of other substances by calcium carbonate, but chiefly by the lithification of calcium carbonate sediments. The complexities are due to the great variety of textural characteristics and mineralogical and chemical compositions of calcium carbonate sediments, and the varying degrees to which these original variable features are preserved or modified during lithification. The possibilities range from imperceptible changes in original characteristics to complete loss of original characteristics by solution, redeposition, deposition of new material, to thorough recrystallization. The discussion of the origin of limestones in this chapter presupposes the reader’s acquaintance with the chemical and mineralogic composition of the particles in calcium carbonate sediments and the petrology and petrography of limestones (GUBLER et al., 1966), the characteristics of modern calcium carbonate sediments (TAFT,1966a), the physical chemistry of the formation of carbonate rocks (TAFT, 1966b), and the classification of carbonate rocks (BISSELLand CHILINGAR,1966). All these contributions are found in the present two-volume work on carbonate rocks. Beyond these, the origin of limestones embraces the subjects of the processes and products of lithification of calcium carbonate sediments and of replacement of other materials by calcium carbonate, subjects which are properly restricted to this chapter. The occurrence of limestones is here discussed in terms of the stratigraphic principles that govern the relationships of the different kinds of calcium carbonate sediments to each other and to non-carbonate sediments, omitting the less common types of limestones. The stratigraphic principles which apply to calcium carbonate sediments include most of those that govern noncarbonate terrigenous sediments; in addition, calcium carbonate sediments involve certain unique principles which apply only to themselves, owing to the fact that calcium carbonate sediment particles may be derived from the water of the depositional basin and, further, that most of these particles are of biogenic origin. In view of the foregoing, the logical point of departure for a discussion of the origin of limestones would appear to be the calcium carbonate sediments from which most limestones have been derived. Because this topic is discussed by TAFT (1966a), it would be repetitious to organize this chapter in that manner. Accordingly, the arrangement used here is somewhat different, and follows three major headings: ( I ) review o f the classic literature on limestones; (2) origin of limestones, including original stony precipitates, lithification of calcium carbonate sediments, and replacement processes; and (3) occurrence of limestones, including stratigraphic relationships of different kinds of calcium carbonate sediments with each other

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and with noncarbonate sediments, citing examples botn from Recent calcium carbonate sediments and ancient limestones.

REVIEW OF THE CLASSIC LITERATURE ON LIMESTONES

Limestones have long attracted the attention of geologists, perhaps initially because of their early use as building stones and also because of the inherent fascination prompted by the fossils in limestones. LYELL’S classic textbook, which appeared in 1830 and passed through numerous editions for nearly 3 decades, contains extensive descriptions of the Italian travertine deposits and comparisons between recently elevated coral-reef limestones, as in Sicily, with modern living reefs. Publication of DARWIN’S “Journal” (1845) of his voyage on “H.M.S. Beagle” and other writings on coral atolls (DARWIN, 1837, 1889) emphasized the problems connected with the origin of Indo-Pacific coral atolls. Many studies resulted from Darwin’s stimulus; their culmination was in the drilling of many test borings by the U.S. Geological Survey in the decade after World War I1 (LADD et al., 1953). Laboratory study of limestones by means of thin-sections and the petrographic microscope, and also field study of polished surfaces prepared with an abrasive stone or file and then moistened before study, was inaugurated by SORBY(1879). Sorby’s Presidential Address to the Geological Society of London is one of the most penetrating studies of limestones ever made; he applied these methods to a study of the mineralogy and textures of limestones and Recent carbonate sediments with great success. He concluded that limestones are mainly derived from cementation of broken up and decayed shells and corals; hence, a knowledge of the mineralogy and structure of shells is fundamental to understanding limestones. Sorby employed careful specific gravity measurements to distinguish aragonite powder (s. g.=2.93) from calcite (s. g.=2.72); in addition he used optical properties (aragonite is biaxial; calcite, uniaxial) and hardness (aragonite is harder than calcite and will scratch Iceland spar crystals in a direction parallel to the line of the shorter diagonal of the rhombic face, whereas calcite will not do so). Using these techniques he determined the mineralogy of various shell groups and set forth the principles of the different behavior of calcite and aragonite, and discussed how these factors influenced preservation of shells and textural characteristics of limestones. Sorby examined travertine and tufa, oolites, and a suite of specimens collected systematically from the principal limestones of Great Britain. He recognized three kinds of ooids: (1)those with concentric structure; (2) those with radial structure; and (3) those that have been recrystallized. He remarked that “unoriented

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granular matter” (lime mud, etc.) occurs with the oriented crystal layers in the first two kinds. Sorby’s descriptions of the British limestones include numerous references to sand-sized particles of lime mud and lime-mud coatings on shells and as layers within ooids. The full significance of these mud particles and coatings was not realized until 1954, when ILLING’S work on the Bahamian calcareous sands was published. Other important contributors to the petrographic study of limestones include CAYEUX(1897, 1935) in France; HADDING(1941, 1950, 1956, 1958) in Sweden; and SANDER (1936, 1951) in Austria. For the most part, these descriptive studies concerned only ancient limestone rocks; only SORBY (1879) made the all-important transition between Recent carbonate sediments and ancient limestones. Walther, one of the stalwarts of geology of the late 1800’s and early part of this century, wrote a three-volume textbook and devoted the third volume, nearly 500 pages, to “Lithology of the Recent” (WALTHER, 1893-1894). In this volume considerable space is given to Recent carbonate sediments. Walther presented an excellent review of the subject through 1893, and it is surprising how much knowledge had already accumulated by that time. He was an outstanding synthesizer and shrewd observer who had travelled extensively on four continents (Europe, Asia, Africa, and North America) to make geological observations. WALTHER’S study (1888) of the Recent and Pleistocene carbonate sediments and rocks of the Red Sea is many decades ahead of his time. It was Walther who took the term “diagenesis”, which he had borrowed from Von Guembel (WALTHER, 1893-1 894, p.693), and gave it the meaning presently used by most geologists. In his book, Walther made many suggestions for the solution of geological problems and was an early advocate of the experimental approach to problems of carbonate diagenesis. GRABAU (1904, 1932) recognized most of the varieties of limestone that are known today. His insights became obscured, however, because few geologists bothered to penetrate beyond his elaborate genetic terminology, or to follow his scattered discussions of limestone, which are found in eight different chapters of his textbook (GRABAU,1932, pp.272-273, 280-285, 294-297, 331-347, 384-476, 573-578, 692-693, and 645, respectively). One major change from Grabau’s views on limestones, occasioned by subsequent research, is the recognition of the importance of wave working of chemical deposits of calcareous material formed in the sea. Although GRABAU (1932, p.645) indicated that such limestones were “not known”, he did remark that “chemically formed marine oolites when worn by waves may come under this head”. GRABAU(1932) divided all rocks, including “limestones”, into two major categories: ( I ) endogenetic, formed by agents within the rock mass itself, such as solidification from molten material or precipitation out of solution; and (2) exogenetic, formed by agents acting outside of already existing rock masses, as clastic rocks. Endogenetic limestones form in water, hence are hydrogenic; they are called

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calcareous hydrolitlzs in Grabau’s terminology. Many endogenetic limestones have formed by organisms, thus are biogenic; the rocks were called calcareous bioliths and further subdivided into zoogenic types if formed by animals, and phytogenic, if by plants. Endogenetic limestones include the calcareous hydroliths such as: chemical limestone (mostly oolites in the sea, tufa and onyx marble in lakes, rivers, and springs), stalactitic deposits, and calcareous tufa; and calcareous bioliths: organic limestones, coral rock, and shell beds (organic oozes and the like). Exogenetic limestones include varieties broken by the action of water (hydrocalcilytes, or hydroclastic limestones), by organisms (biocalcilytes, or bioclastic limestones), or by the wind (anemoclastic limestones). Grabau further classified the exogenetic rocks into textural groups based on grain size: rudaceous (coarser-grained than sand-size), areizaceous (sand-size), and lutaceous (finergrained than sand-size). He offered a new classification of marine deposits (1932, p.645), in which marine elastic limestones, the hydroclastics and bioclastics, were further subdivided into terrigenous types, if the material composing them was derived from the land, or thalassigenous types, if derived from the sea. It is important to note that in Grabau’s usage bioclastic is not a synonym of biogenic, as many geologists have supposed. This term refers specifically to fragments of older rocks that were broken or fragmented by the mechanical action of organisms, such as parrot fish feeding on corals and nullipores in a reef, grinding up the calcareous material, and excreting it back onto the bottom. Although Grabau’s monumental textbook did not include much discussion based on observations with the petrographic microscope, his treatment of the other aspects of limestones is so thorough that this omission seems relatively minor by comparison. Grabau’s bibliographies are cosmopolitan, but somehow he omitted Sorby’s work on limestones (SORBY,1853, 1855, 1861, 1879), possibly because it is petrographic. Much of the so-called “new” research on limestones that occurred in the United States after 1950 would have been accomplished far earlier had the study of limestones commenced with a combination of the material available in the published works of both Sorby and Grabau. In the 1920’s and early 1930’s, KLAHN (1922, 1923a, b, 192% b, c, 1926a, b, 1928, 1932a, b) was an active worker in the field of carbonate sedimentation and petrology, yet his extensive studies have been largely ignored by modern workers. Most of KYahn’s observations were concerned with fresh-water carbonate sediments, and he was one of the first workers to attempt the experimental approach. Another prodigious student of carbonate sedimentation and petrology was Pia, who between 1912 and 1933 made many contributions to this field. In 1933, in a 420-page book on “Recent Carbonate Sediments”, he summed up the entire field as then known to him (PIA, 1933), and ranged across the spectrum of carbonate sedimentology and petrology. It is still one of the most comprehensive studies on this subject, although now obsolete in many areas. PIA(1933) devoted more than half of his book to the study of Recent nonmarine carbonates, an effort not since equalled.

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On the whole, these early pioneers in the study of carbonate rocks made little impact on the present generation of carbonate petrologists. A breakthrough in this field came during the last 15-25 years. The source of inspiration, energy, and progress, which culminated in this advance, was provided by the major oil companies of the United States. The enormous petroleum reservoirs discovered in carbonate rocks of the Middle East, in the Devonian of western Canada, and in the Pennsylvanian of west Texas prompted the oil companies to increase their efforts in carbonate research. In 1959, a Carbonate Rock Subcommittee was established as a division of the American Assoication of Petroleum Geologists, and a memoir (HAM, 1962) was published on the classification of carbonate rocks which is a major milestone and turning point in the history of the study of carbonate rocks. An annotated bibliography of papers on carbonates from the United States and Canada published in 1953-1958 and referenced by subjects was prepared by SANDERS (1960). ORIGIN OF LIMESTONES

Introduction

Limestones originate by three processes: ( I ) crystallization of calcium carbonate in an initially stony condition by means of inorganic, organic, or combined inorganic and organic processes to form caliche, nari, calcrete, hard skeletal reefs, travertine, and speleothems; (2) lithification of calcium carbonate grains in a manner analogous to the lithification of quartz grains to form sandstones; and (3) replacement of other materials, such as calcium sulfate or quartz, by calcium carbonate. Most limestones have been formed by lithification of calcium carbonate sediments; those formed by the other two processes, though of theoretical interest, are quantitatively insignificant by comparison. Crystallization of calcium carbonate in an initially stony condition

Limestones in which the calcium carbonate crystallized as stony material originate by inorganic and organic precipitation, or by a combination of both types of precipitation. In this discussion the term “inorganic” refers to crystallization outside the tissues of living organisms and “organic” to crystallization within the tissues of living organisms. Some organisms, such as Algae or bacteria, may play a significant role in changing the chemical conditions within the water and thus cause precipitation of calcium carbonate ‘(HASSACK, 1888; CHAMBERS, 19 12). Though such precipitation might appropriately be considered as “organic” in view of the large part played by organisms in it, this type will be treated here as “inorganic” because it took place outside the living tissues and was not a part of the metabolic processes of the organisms.

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Caliche, nari, and calcrete are examples of limestones that originate by inorganic crystallization processes; reef rock and stony biogenic incrustations, of limestones that originate by organic crystallization processes; and travertine, tufa, speleothems, and so forth, of limestones that originate by a combination of inorganic and organic crystallization processes. Caliche, nari, and calcrete Caliche, nari, and calcrete are limestones precipitated from evaporating soil moisture. Caliche forms in a semiarid climate where the predominant direction of movement of soil moisture is upward, owing to the excess of evaporation over rainfall. As carbonate-bearing waters are evaporated from the soil, calcite is precipitated between the soil particles. The term nari (from the Arabic word “nar” which means fire, alluding to its use in limekilns) was first introduced by BLANCKENHORN (1905) to designate a special variety of caliche that forms by surface, or near-surface, alteration of permeable calcareous rocks. Nari occurs in the drier parts of the Mediterranean climatic region, through the combined effects of dissolution and redeposition of calcium carbonate, Nari is distinguished by a fine network of veins, which surround unreplaced remnants of the original rock. The nari often contains clastic rock particles such as flint, hard limestone or dolostone and, locally, shells of sub-Recent terrestrial gastropods. The nari crust in Israel, which reaches a maximum thickness of at least 6 m, develops preferentially on Cretaceous and Tertiary chalks, but is also found on the chalky cementing material of conglomerates. It is thus widespread in Galilee, Mount Carmel, the Shefela, and the Judean and Shomron Mountains. i n regions of arid climate a surface soil crust may form, which is called duricrust (WOOLNOUGH, 1928). The term calcrete has been used where this crust consists of calcium carbonate (LAMPLUGH, 1902). Reef rock and stony biogenic incrustations Reef rock and stony incrustations represent deposits whose initial condition was solid and coherent, owing to the cementing effects of the original organic precipitation of calcium carbonate. i n modern reefs, corals, coralline Algae, and Foraminifera are the chief organic precipitaters of solid limestones. Other organisms, such as modern oysters and serpulid worms and fossil stromatoporoids, sponges, rudistids, and certain brachiopods, also cemented themselves to the substratum to form an initally stony mass (NEWELL,1955). Limestones formed by organic precipitation comprise the constructional limestones of J. H. JOHNSON (1957), the biolithites of FOLK(1959) and WOLF (1960), the residual organic class of R. W. POWERS(1962), and the incrustate limestones of SCHLANGER (1964). In addition to reef-building corals, Schlanger

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listed the Algae Lithothamnium and Lithophyllum, and the Foraminifera Carpenteria, Homotrema, and Gypsina as important genera of incrusting organisms. As noted by R. W. Powers, residual organic limestones are the most valuable paleoecological indicators. H. F. NELSONet al. (1962) referred to this type of limestone as “skeletal limestone”, but the term skeletal has been widely used to refer to the hard parts of organisms, whether in place or not, so that their usage. if adopted, would generate confusion.

Travertine, tufa, and speleothems Travertine and tufa refer to calcareous deposits of lakes, rivers, and springs, in which precipitation of calcium carbonate is a combination of inorganic and organic, chiefly algal, activity. Speleothems are limestones in which the precipitation, essentially inorganic, took place from cave waters. In each type the texture and shape of the deposit are governed by the detailed morphology at the site of precipitation; growth of new calcium carbonate takes place upon the surfaces of the former deposit. Texture, though variable, is characterized by indications of crystal growth on free surfaces and by a complete absence of indications of mechanical shifting of particles prior to final deposition. Travertine is the genetic term for all organic-inorganic nonmarine limestone accumulations formed in lakes, rivers, springs, and caves. It normally is colloform or concretionary, extremely compact in individual layers, but highly porous between layers. It may be wholly or partly hydrothermal. Surface deposits are commonly associated with calcareous Algae. Tufa, or calcareous sinter, is a highly porous spring variety. Onyx marble is a colorful banded variety. Travertine has been extended locally to include caliche and calcrete, but this usage is not recommended. The term travertine is derived from the Italian word, travertino, a corruption of tiburtino, “the stone of Tibur”, which is a former name of the locality now called Tivoli. The Italian travertine from near Rome is known by several local names, which include tufa litoide, the main building stone and material from which the catacombs have been excavated, and tufa granolare. LYELL(1 830, pp.207-210) reported three kinds of travertine here: hard and compact, cellular, and botryoidal-mammillary. Deep gorges at Tivoli show that the travertine is at least 150 m thick and that the diameter of some of the large spheroids with concentric structure reaches 2-2.6 m. Some authors distinguish travertine, as a fairly massive deposit that originated chiefly from deposition from hot spring waters, from calcareous tufa, as a spongy material deposited from ordinary spring and stream waters. According to HATCH et al. (1938, p.179), calcareous sinter (calc-sinter) is a synonym of travertine; both are regarded as products of precipitation from spring waters, especially those in volcanic regions. These authors followed I. C . RUSSELL’S (1885) usage of tufa for the deposits of Pleistocene Lake Lahonton, Nevada.

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GROUT (1940, p.170), on the other hand, defined travertine as “the precipitates from waters on plants”. He used “Mexican onyx” to designate the layered type of calcium carbonate precipitate in which the crystallization took place on flat surfaces. PIRSSON and KNOPF(1947, p.265) defined travertine as “the rock formed by precipitation of calcium carbonate at the orifices of springs”. They mentioned that deposition of carbonate was increased by the action of Algae. Classic localities for such calcareous deposits include Karlsbad, Czechoslovakia, where the famous Sprudelstein includes aragonite pisolites (COHN,1862); Tivoli, Italy; Vichy, France; Mammoth Hot Springs, Yellowstone Park, U.S.A. (WEED,1889);Pleistocene Lake Lahonton, Nevada (1. C . Russell, 1885); and Searles Lake, California (I. C. RUSSELL,1887; SCHOLL,1960). These and other deposits were reviewed by PIA(1933), whose work also includes an extensive bibliography. Dripstone and flowstone refer to speleothems made by dripping and flowing water, respectively. The deposits made by flowing and dripping are texturally distinct and can be easily recognized. Dripstone is limited to the central zones of stalactites; it is characterized by crystals oriented with their long axes parallel to the long axis of the stalactite. Flowstone surrounds the dripstone cores of stalactites and forms the entire mass of stalagmites; it consists of concentric layers of variable thickness in which the long axes of crystals are radially arranged with respect to the long axis of the stalactite or stalagmite (G. W. MOORE,1962). Where the dripping occurs into shallow pools, cave pearls are formed, analogous to ooids (BAKERand FROSTICK, 1947). Most speleothems consist of calcite, but aragonite does occur in warmer climates. Lithijkation of calcium carbonate sediments Lithification of calcium carbonate sediments is by far the most important factor in the origin of limestones. Conversion of the calcium carbonate sediments into limestone rock involves a large number of reactions and processes, which may leave original depositional textures essentially unaltered, on the one hand, or which may so completely rearrange the constituents that original depositional textures are completely obscured or obliterated, on the other. The subject of lithification, which is onlypart of the broader topic of diagenesis, has been much studied recently, both from the point of view of the chemical reactions involved (TAFT,1966b) and of changes in porosity and permeability (HARBAUGH, 1966). Both these contributions are found in the present two-volume work on carbonate rocks. The following discussion traces some of these changes in a sequence that proceeds from the stage of imperceptible alteration of original features of the sediment to the stage of complete obliteration of original features. The subject is conveniently, even if somewhat arbitrarily, subdivided into two parts: (I) changes of individual grains, and (2) changes within the grain mass.

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Changes of individual grains Many of the changes that influence the individual grains in calcium carbonate sediments do not strictly involve lithification of these grains to form limestone rock; but because these grain changes almost invariably take place during this transformation, they are properly included in this general summary. Two kinds of changes occur: ( I ) changes in mineralogy of the grains without external modification of the texture; and (2) addition of external concentric coatings to the grains. Changes in grain mineralogy. Recent nearshore marine calcium carbonate sediments contain the minerals aragonite, high-magnesian calcite, and low-magnesian calcite. I n ancient limestones, however, of these minerals, only low-magnesian calcite is at all common; the other two occur but rarely (CHAVE,1954a, 1962; STEHLIand HOWER,1961; FRIEDMAN, 1964; see also TAFT, 1967b). An order of LOW- M CALClTir

A

ARAGONITE

HIGH-Mg

CALCITE

@

REEF SEDIMENTS

6

SKELETAL SANDS

Fig.1. Mineralogy of Recent carbonate sediments from Bermuda. (After FRIEDMAN, 1964, p.782; by permission of Journal of Sedimentary Petrohgy.)

179

ORIGIN AND OCCURRENCE OF LIMESTONES LOW-Mg C ALCl TE

ARAGONITE

HIGH-Mg CALCITE SUBMERGED IN SEA WATER

0

FROM MARINE SPRAY ZONE

A

BEYOND REACH OF MARINE SPRAY

Fig.2. Mineralogy of Pleistocene skeletal sands from Bermuda. Note that Pleistocene carbonate sands submerged in sea water or exposed to marine spray contain high-magnesian calcite and that those beyond the reach of the sea water are, with one exception, devoid of magnesium. The absence of magnesium is due to removal by fresh-water leaching at a post-depositional stage of subaerial exposure. (After FRIEDMAN, 1964, p.782; by permission of Journal of Sedimentary

Petrology.)

increasing stability among these minerals is: high-magnesian calcite, aragonite, low-magnesian calcite (STEHLIand HOWER,1961; CHAVE,1962; FRIEDMAN, 1964). The loss of magnesium from reef sands in the Bahamas was first reported by GOLDMAN (1926), long before the mineralogical varieties of calcite were recognized. The exact mechanism by which this change takes place is not known. It may consist of removal of magnesium from high-magnesian calcite to yield low-magnesian calcite, or a solution-and-deposition reaction on a micro-scale, in which high-magnesian calcite is removed and low-magnesian calcite deposited without textural changes in the grains involved (FRIEDMAN, 1964). Coralline Algae, for example, which secrete particles of high-magnesjan calcite, commonly persist in

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limestones with original texture preserved (Plate IB, C ) ; but after diagenesis, in limestones, these grains consist of low-magnesian calcite. The changes in grain mineralogy, as shown by FRIEDMAN (1964), are well illustrated by comparing Recent reef sediments and skeletal, and oolitic sands from Bermuda, the Bahamas, Red Sea, and other areas, with nearby Pleistocene limestones that consist of the lithified equivalents of these sediments. Recent reef sediments consist of aragonite in greater abundance than highinagnesian calcite (Fig. 1), indicating a preponderance of coral material. With only one exception, low-niagnesian calcite is absent in the Bermuda samples studied by FRIEDMAN (1964). The Recent skeletal sands from Bermuda contain these same minerals, but also include a variable amount of low-magnesian calcite, ranging from 10 to I5 %, the variation being due to the presence of terrigenous carbonate grains from Pleistocene limestones on the island (Fig. 1). The mineralogy of Pleistocene limestones formed by lithification of skeletal sands varies according to whether their postdepositional history involved complete subaerial exposure or partial subaerial exposure and partial exposure to sea water (Fig.2). Specimens of limestone that have been partially exposed to sea water contain variable amounts of high-magnesian calcite. Recent oolitic sands from the Bahamas contain abundant aragonite and small quantities of low-magnesian calcite, whereas in Pleistocene oolitic limestones, the abundance of aragonite decreases and that of low-magnesian calcite increases PLATE I Photomicrographs of Pleistocene limestones from Bermuda, which originated by modification and cementation of skeletal sands. (After FRIEDMAN, 1964, p.785; by permission of Journal of Sedimentary Petrology). A. Lime-mud envelopes outline former tests of organisms originally composed of aragonite, such as pelecypod shells or Halimeda, which were subsequently dissolved and the resulting void (mold) was later filled with a mosaic of drusy calcite. Original interparticle porosity has been completely occluded by drusy calcite mosaic. Pleistocene eolianite, Ferry Road, St. George’s Island, Bermuda. B. Skeletal fragments, which were originally composed of aragonite, are outlined by lime-mud envelope. The interior of these organisms has been lime-mud infilled by a mosaic of drusy calcite. Organism in extreme upper left of photograph is Halimedu. Note micritic lining of the internal tubes of Halimedu. Fossils which were originally made up of high-magnesian calcite, as the coralline algal fragment in upper left of photograph, have retained their original texture even though magnesium has been removed from this rock. The original interparticle porosity has been occluded by drusy calcite mosaic. Pleistocene eolianite, Ferry Road, St. George’s Island, Bermuda. Notice difference in texture between “rim cement” on exterior of “lime-mud envelope” and coarser mosaic of the cement. C. Molds with infilled drusy calcite mosaic in upper left of photograph are probably those of Halimeda. Note the well-preserved textural details of the coralline algal fragment (in right center of photograph) which was originally composed of high-magnesian calcite. Removal of magnesium from this rock has not changed the textural characteristics of this fragment. The original interparticle porosity has been occluded by drusy calcite mosaic. Pleistocene eolianite, Ferry Road, St. George’s Island, Bermuda. Notice difference in texture between “rim cement” on exterior of “micritic envelope” and coarser cement mosaic.

ORIGIN A N D OCCURRENCE OF LIMESTONES

PLATE I

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(Fig.3, 4). These and other mineralogic changes associated with lithification are discussed by FRIEDMAN (1964). Paramorphic replacement of aragonite by low-magnesian calcite commonly occurs where the aragonite exists in the form of structureless lime mud, cryptocrystalline grains, or ooids (FRIEDMAN, 1964). Presumably this replacement of aragonite by low-magnesian calcite occurs with an intervening solution-deposition stage on a micro-scale. The result is a complete change of mineralogy of the grains but a retention of their original depositional texture. Thus, for example, ooids and aggregate grains of non-skeletal sand-size particles, which are deposited originally as aragonite in Recent calcium carbonate sediments (ILLING,1954; NEWELLet al., 1960; RUSNAK,1960b), almost always consist of low-magnesian calcite in limestone (LINCK,1903; FRIEDMAN, 1964). This paramorphic replacement contrasts markedly with the large-scale dissolution of aragonite skeletal debris and subsequent deposition of low-magnesian calcite in the empty space formerly occupied by the shells. (See section on Changes within the sediment mass.) The mineralogic composition of calcium carbonate in Recent deep-sea sediments differs from that of nearshore carbonate sediments in containing abunLOW-Mg CALCITE

A

Fig.3. Mineralogy of Recent ooids from Browns Cay, South Cat Cay, Joulters Cay, and Bird Cay (Berry Islands), Great Bahama Bank. (After FRIEDMAN, 1964, p.790; by permission of Journal of Sedimentary Petrology.)

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Mg CALCITE

Fig.4. Mineralogy of Pleistocene oolites from New Providence Island, Hog Island (near New Providence Island), Andros Island, Long Rock Island (near Andros Island), and North Bimini Island, Great Bahama Bank. (After FRIEDMAN, 1964, p.790; by permission of Journal of Sedimentary Petrology.)

dant low-magnesian calcite and little or no high-magnesian calcite and/or aragonite (STEHLIand HOWER, 1961;FRIEDMAN, 1965b). Carbonate pelagic sediments of the Red Sea at depths of 1,000-2,000 m consist of high-magnesian calcite and lowmagnesian calcite with aragonite being sporadic or absent, whereas those from much greater depth (3,676 m) in the Indian Ocean contain only low-magnesian calcite (FRIEDMAN, I965b). Asample from the East China Sea at a depth of 1,370 m also contained only low-magnesian calcite (STEHLIand HOWER, 1961). The mineralogy of deep-sea carbonate sediments is controlled in part by sedimentation and is a reflection of the composition of the planktonic skeletal remains that are the chief contributors to the sediment, and in part by dissolution of high-magnesian calcite and aragonite. In the deep-sea sediments studied Globigerina is one of the most abundant constituents. Inasmuch as Globigerina tests consist of low-magnesian calcite, that mineral species predominates in the material which falls to the bottom. Coccoliths, which constitute other important planktonic sources of calcium carbonate in deep-sea sediments, also consist of low-magnesian calcite. Removal of aragonite and high-magnesian calcite by dissolution, as a function of

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water depth and grain size, may be an important diagenetic effect. Under deep-sea conditions low-magnesian calcite is the most stable of these carbonate minerals and high-magnesian calcite appears to be more stable than aragonite (FRIEDMAN, 1965b).

Additions of concentric coatings to grains. Grains of calcium carbonate and other minerals, such as quartz or feldspar, found in environments of Recent calcium carbonate sedimentation, commonly show external concentric coatings of calcium carbonate. These occur as envelope(s) of unoriented cryptocrystalline aragonite crystals (“lime mud”, “paste”, or “micrite” of different authors), as layers of clear, oriented needle-like aragonite crystals with concentric or radial structure, or as various combinations of these. It should be added that some grains consist entirely of the same material found as coatings. In such cases, grains are composed either entirely of one type or of combinations of two or more types of coating material. The addition of coatings to individual grains is not strictly a lithification process; in fact, coatings are added while the grains are still in the final stages of sedimentation. Because the fate of original textural features during subsequent lithification processes is greatly influenced by the presence of coatings, particularly of “lime mud”, however, the subject of coatings is included here. A . Envelopes of unoriented cryptocrystalline aragonite (“lime mud”). One of the commonest types of grain coatings consists of unoriented cryptocrystalline aragonite (SORBY,1879; ILLING,1954; NEWELLet al., 1960; RUSNAK,1960b; BATHURST, 1964; FRIEDMAN, 1964; and others). The coatings have been called “micrite envelopes” (BATHURST, 1964)or “micritic envelopes” (FRIEDMAN, 1964), but these imply “rock” by the connotation inherent in “micrite”. These structureless coatings (Plate IA-C) are acquired in the depositional environment, but the mechanism of their formation is not fully understood. Many of them originate by the same accretion process that is involved in the aggregation of tiny aragonite crystals to form sand-size grains, as in the Recent non-skeletal carbonate sands of the Bahamas (ILLING,1954). Other coatings may owe their origin to the perforating Algae or the lime-mud infilling of peripheral zones of grains that have been bored by organisms. The grains may have been the substrate on which microorganisms lived, so that the envelopes formed as a result of physiological processes of these organisms (FRIEDMAN, 1964, p.808). Crusts may also be due to Algae(WoLF, 1965a). Whatever their origin, these envelopes are commonly responsible for the preservation of original shapes of aragonite grains, for they are typically more resistant to dissolution than an aragonite skeletal particle. Thus, they preserve the shape of the coated particle, even if the aragonite of this particle has been removed by dissolution. If the entire grain consists of lime mud, it may be a faecal pellet, intraclast (FOLK,1959), or pseudooid (CAROZZI,1957). The latter is a more general term that can be applied when the origin is unknown; it simply refers to a grain without a clear coating of concentric or radial aragonite crystals. This is a slightly different

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definition than the one thought to be the original (as “pseudo-ooliths”) and which referred to clastic or worn calcite grains or shell fragments that resemble “ooliths” (GRABAU,1932, p.284, after J. G. Bornemann, reference not cited, but possibly BORNEMANN, 1885). B. Clear layers of oriented aragonite crystals with concentric or radial structure. Grains that have acquired a coating of clear aragonite crystals with concentric or radial structure in the arrangement of the crystals have been called superficial ooids or normal ooids (or “true ooids”, or simply “ooids”, which is the term preferred to oolith; oolite is the rock composed of cemented ooids), depending on the number of coating layers. SuperJicial ooids contain only one shell (of variable thickness) with concentric or radial structure, and normal oiiids contain two or more shells (of variable thickness) having concentric or radial structure (CAROZZI, 1957). This definition of superficial ooid is preferred to those based on thickness of the shells, such as “only thin external oolitic coating” (ILLING,1954, p.36) or “thickness of the coating envelope being less than the radius of the nucleus” (LEIGHTON and PENDEXTER, 1962, p.38). Though originally composed of aragonite, these coatings nearly always consist of calcite in limestones. Inasmuch as more than one coating may be present and one or more of the different types may alternate on the same grain, great variation is possible. A central nucleus of foreign material may be surrounded by alternating envelopes of structureless lime mud and clear layers of oriented aragonite crystals with either concentric or radial structure. In other grains, a foreign nucleus may be lacking and the entire grain may consist of material that otherwise forms coatings. The different types of texture of the aragonite have been attributed to the rate of precipitation of aragonite and degree of agitation (RUSNAK,1960b). According to this explanation, rapid precipitation produces envelopes of unoriented cryptocrystalline aragonite; slower precipitation combined with weak agitation produces radially arranged crystal aggregates; and very slow precipitation in combination with vigorous agitation produces a preponderance of mechanical attrition that re-orientates the aragonite crystals from an initial radial attachment to a tangential (concentric) growth position. Whatever their origin, the sequence of coating and particle types may be of great paleogeographic significance in defining ancient shorelines. In Great Salt Lake, Utah, U.S.A., pseudooids form a continuous belt just offshore from the zone of breaking waves. In the breaker zone the particles may consist entirely of normal ooids; if so, then a belt of mixed ooids (and superficial ooids, not separately recognized) and pseudooids intervenes between the normal ooids and pseudooids offshore. In other localities where normal ooids occur in the breaker zone, a narrow landward belt of mixed ooids and pseudooids exists. In still other localities, there is no belt of pure ooids, but only a shoreline zone of mixed ooids and pseudooids (CAROZZI,1957).

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Changes within the sediment mass Changes within the sediment mass involve selective dissolution, which increases pore space; precipitation of mineral cement in pore spaces, which decreases pore space; and recrystallization, the effect of which on pore space is not established. Precipitation of mineral cement between grains or mineral infilling of molds generally does not disturb the texture, but selective dissolution and recrystallization tend to destroy original textures. Selective dissolution. Selective dissolution is the opposite of lithification, but it is included here because it so commonly occurs during or before lithification has been completed. Aragonite, the most important constituent of nearshore marine calcium carbonate sediments, may be dissolved away completely or break down into a whitish powder if the sediments come into contact with meteoric ground water. The relationships are especially well illustrated by the material from the deep borings on Eniwetok and Bikini atolls, in the Pacific Ocean. Unlithified calcium carbonate sediments in which aragonite still persists are found at levels in the borings where the calcium carbonate sediments may be inferred to have been continuously exposed to sea water. Evidently time alone, for spans at least as long as the Cenozoic, is not a relevant factor in the stability of aragonite as long as it remains in contact with sea water. At levels where aragonite has been dissolved away, the calcium carbonate sediment has always been converted to calcite-cemented limestone rock; such levels coincide with indications of subaerial exposure and contact with meteoric ground water (SCHLANGER, 1963, p.997). Locally, aragonite has been preserved even in material that has been subaerially exposed, but, typically, aragonite has been dissolved and a calcite cement precipitated where such exposure has occurred. The dissolved aragonite doubtless provides the material which precipitates as calcite cement (SORBY,1879; see also section on Precipitation of mineral cement). Where skeletal remains consisting of aragonite have acquired a lime-mud envelope, the aragonite fragment on the interior may be dissolved away completely. This leaves an empty mold preserved within the lime-mud envelope, which itself may have undergone paramorphic replacement from aragonite to calcite (FRIEDMAN, 1964). The empty space thus created has been called “moldic porosity”, or “secondary void porosity” (R. W. POWERS, 1962, p.140, pl.IV, fig.3). It may later be filled in with mineral matter (Plate IA-C), the textural arrangement of which indicates that crystallization took place into an empty space, the crystals having grown inward from the marginal walls (BATHURST,1964). On the other hand, the original depositional lime-mud matrix between grains rather than the interior of a lime-mud-coated grain may be selectively dissolved. In the subsurface Devonian crinoidd limestones of the Andrews South oil field, Texas, visible porosity exists in the spaces between crinoid ossicles where an original interparticle lime-mud matrix has been selectively dissolved away. The

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presence of the lime mud inhibited precipitation of clear calcite, which cemented nearby originally mud-free sediments into a non-porous rock (LUCIA,1962). Selective dissolution may also affect the lime-mud matrix of a mixture of lime-sand and lime-mud particles after the growth of a framework of dolomite crystals (R. W. POWERS, 1962, p.185). Soft algal tissue may be removed after initial cementation of the framework grains, leaving empty spaces that may later be filled by mineral cement or internally sedimented clasts (WOLF,1965b). Precipitation of mineral cement in pore spaces. Precipitation of mineral cement in the pore spaces within a mass of calcium carbonate sediments is the most important single lithification process. The pore space may have existed originally between the essentially unaltered grains of the sediment, or it may have been created by selective dissolution of the lime-mud matrix between grains or of the soluble centers of grains within lime-mud envelopes, as previously discussed. The cement commonly begins as clear crystals that grow radially outward from the grains (or inward toward the center of pore spaces) (ILLING,1954; GINSBURG, 1957; R. W. POWERS, 1962; and others). Where crystal growth does not occupy all the available space, the cementation is incomplete and the cement resembles a grain coating; it has been called “drusy coating” (R. W. POWERS, 1962) or “rim cement” (GINSBURG, 1957). Recrystallization of a lime-mud coating, or of original lime-mud matrix between grains, may produce an end product the appearance of which is similar to mineral cement. The cementing minerals closely reflect the chemical environment within the interstitial waters, the composition of which may range from fresh to supersaline. Mineral cements generally have been interpreted in terms of exposure of aragonitic marine sediments to meteoric ground water, but marginally evaporating sea water may diffuse upward through porous intertidal and supratidal sediments and precipitate “evaporitic” cements. Through-flowing connate waters of varying salinity and commonly having high pH, which are released during compaction, may also introduce cements of varying composition. If aragonite grains come in contact with meteoric water, they dissolve forming calcium bicarbonate, according to the reaction: CaC03

+ HzO + CO2 ~2Ca(HC03)z

Precipitation of low-magnesian calcite as a drusy mosaic in pore spaces takes place when the solution becomes saturated and then loses carbon dioxide or water, either of which may easily occur under subaerial conditions where evaporation is possible (FRIEDMAN, 1964, p.809). Plate I1 illustrates the progressive development of calcite cement in pore spaces. As noted previously, at levels in the Eniwetok and Bikini borings where dissolution of aragonite has occurred, calcite cement has always precipitated. By contrast, the Cretaceous chalk, which consists of tiny skeletal fragments

PLATE 11

Photomicrographs of Pleistocene skeletal sands (limestones) from Bermuda. (After FRIEDMAN, 1964, p.785; by permission of Journal of Sedimentary Petrology.) A. Calcite cements skeletal fragments only near grain-to-grain boundaries. Interparticle porosity has been completely preserved except near grain-to-grain boundaries. Somerset Eolianite, Wellington Hill, St. George’s Island, Bermuda. B. Drusy calcite mosaic cements skeletal fragments only near grain-to-grain boundaries retaining interparticle porosity where grains are not in close juxtaposition. Pembroke (?) Eolianite, Grape Bay, Bermuda. C. Drusy calcite mosaic cements skeletal fragments and partially occludes interparticle porosity. Southampton or Somerset Eolianite, top of Park Gates, St. George’s Island, Bermuda.

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that are composed almost exclusively of calcite, has remained uncemented where aragonite was initially absent. In the so-called “hard chalks”, which include larger proportions of organisms with originally aragonitic skeletal debris, however, the aragonite has been dissolved so that only molds remain and a calcite cement is present (HATCHet al., 1938, p.168). Aragonite, or the “evaporite” minerals, such as dolomite, anhydrite, gypsum, barite, celestite, or halite, have also been found as the mineral cement between grains of calcium carbonate deposits; in addition, the “cement” may replace the grains. Such cements may be a great deal more common than is generally realized; they generally do not persist on the outcrop and are preserved in bore-hole samples only where saline drilling fluids are used. One explanation for the presence of these minerals has recently been provided by KINSMAN(1965), who studied the minerals found in the intergranular spaces of calcium carbonate sediments exposed on marginal supratidal flats along the Persian Gulf. Owing to extreme aridity, sea water transpires upward through these porous sediments. With evaporation, the concentration of dissolved salts progressively increases in the interstitial waters, and eventually evaporite minerals precipitate. Kinsman has found that a systematic sequence of evaporite cements exists in a landward direction from aragonite, which precipitates at lower salinity, to halite, which precipitates at higher salinity. This may be visualized as a kind of “marine caliche” process, for the mechanism of upward-moving pore water is exactly the same as that of meteoric ground water, which is the pore water where caliche forms. The difference is that the pore water in the sediments on the supratidal flats is sea water. Deposition of evaporite minerals within the sediment pores may occur even in the absence of associated bedded evaporites, indicating that the salinity of the adjacent body of sea water did not reach extremely high values. An absence of halite from most beach rocks and many littoral flat deposits suggests tidal flushing, a process which involves deposition of halite at low tide and its removal (owing to high solubility) during the next high cycle or seasonal high-water stage. There is a difference in cements within beach rock reported from different localities; this may be explained by considerations of the type of water present within the pores of the beach sediment prior to deposition of the cement. Aragonite has been found to be the cement in many beach rocks (GINSBURG, 1953, 1957; REVELLE and FAIRBRIDGE, 1957; STODDART and CANN,1965), whereas calcite has been claimed as the original cement in others (R. J. RUSSELL,1963a, b). Both aragonite and calcite have been found in Bahamian beach rock (ILLING,1954, p.70). As with other types of aragonite precipitates, however, initial aragonite cement in beach rocks changes eventually to calcite when exposed to meteoric ground water, so that calcite may be the final product even if the initial material was aragonite. Gypsum is found as a beach rock cement in Somaliland today (R. W.

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Fairbridge, personal communication, 1965). Anhydrite is the local intergranular cement in the Queens, Grayburg, and San Andres Limestones (Permian) of west Texas (G. M. Friedman, personal observations), and in the Arab-D Limestone (Upper Jurassic) of Saudi Arabia (R. W. POWERS,1962, p.137, ~1,111,fig.4). Gypsum, barite, and celestite constitute local sulfate cements in some carbonate deposits (G. M. Friedman, personal observations); halite is also known as a cement. Dolomite may also be precipitated as a mineral cement within the pores of calcium carbonate sediments (SHINNand GINSBURG, 1964); this topic is considered separately in Chapter 6. Recrystallization. Recrystallization is the name applied to the vaguely understood process in which original textures are obliterated and a mass of large interlocking crystals forms. The process may operate in either fine-grained or coarse-grained sediments, and it may be selective, involving only parts of the material, or complete, involving all materials. Selective replacement commonly occurs where lime mud is present. The result is a transformation of aragonite particles, which are not easily seen as individuals in thin-sections of standard thickness and consequently appear “turbid”, into a mosaic of coarser, clearer, transparent sparry calcite (BATHURST, 1958, 1959; FRIEDMAN, 1964). The possibility that this transformation occurs is important for limestone classification, so the evidence for it will be considered with additional examples beyond those cited by Bathurst. The Middle Ordovician flysch exposed on the north shore of the Gasp6 Peninsula, Canada, includes a great variety of rocks, among them calcareous wackes (ENOS,1964). The wackes are typically graded, and the lower parts of the graded beds consist of coarser grades of terrigenous fragments set in a mosaic of coarse sparry calcite. The grain size of both terrigenous fragments and sparry calcite decreases upward until at the top the bed becomes a typical calcisiltite composed of silt-size grains of terrigenous (recycled) carbonate fragments in a fine-grained lime-mud matrix. The sparry calcite in the lower part of the bed clearly represents the end product of selective recrystallization of an initial lime mud matrix that was deposited mechanically along with the coarser terrigenous grains. Recrystallization has been impeded near the top of the bed, where the permeability was less owing to the finer grain size. Lime mud has also been thought to have recrystallized to sparry calcite in the Tertiary limestones of Guam (SCHLANGER, 1964, p.D-10). In these rocks an initial lime-mud envelope has been inferred to have recrystallized to form a halo of clear calcite, with crystals oriented perpendicular to the rim of the fragment; first recrystallization took place next to skeletal fragments and other larger grains. An alternative, not discussed by Schlanger, is that the calcite represents a rim cement. This illustrates the problem and the difficulties encountered in trying to formulate a clear-cut interpretation. The most satisfactory results are achieved by following

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the criteria of grain-support and mud-support set forth by R. J. DUNHAM (1962). The indications that a primary lime mud can recrystallize to form coarsegrained sparry calcite, complicates petrographic classification of limestones in which major subdivision is based on the assumption that mosaics of clear sparry calcite always indicate cleanly washed original sediments with numerous primary voids into which the sparry calcite was deposited as a mineral cement (FOLK,1959). Although this assumption may be valid in many cases, it should always be used with caution; a better alternative is to follow R. J. DUNHAM’S (1962) suggestions about major subdivision on the basis of grain-support or mud-support, rather than use sparry calcite alone. Recrystallization of skeletal material in certain limestones from Lau, Fiji, was discussed by CRICKMAY (1945); from Saipan, by J. H. JOHNSON (1957); and from Guam, by SCHLANGER (1964). The following order of decreasing susceptibility to recrystallization was observed: corals (least resistant), Halimedu, mollusks, pelagic Foraminifera, beach-type Foraminifera, larger Foraminifera, echinoids, and calcareous red Algae (most resistant). On Saipan, green Algae showed a more variable susceptibility (J. H. JOHNSON,1957). In the Saipan limestones, recrystallization generally began in the matrix and later attacked skeletal remains, both from inside and outside. The end product is a crystalline limestone with only “dust” outlines, color bands, or textural differences to indicate former skeletal debris (J. H. JOHNSON,1957). In the Guam limestones, the originally aragonite coral fragments have completely recrystallized into a calcite mosaic of varying grain size. The original coral shape is suggested by traces of dark mud in the interseptal spaces, by fine dark lines that preserve the coral outline, and by algal coatings on fragments, which preserve the original outline of the fragments (SCHLANGER, 1964). PURDY(1963) has shown that ooids, corals, and Foraminifera, particularly Peneroplids, recrystallize in the marine environment to cryptocrystalline aragonite. All stages of recrystallization have been observed in the transition of various skeletal and non-skeletal constituents into cryptocrystalline aragonite. Although both the original grains and final recrystallization product are aragonite, recrystallization has destroyed original textures. Experimental work Experimental work planned to duplicate the mineralogical changes that take place during lithification of calcium carbonate sediments was initiated by TAFT(1962) and FRIEDMAN (1964). TAFT (1963) demonstrated that replacement of saline by fresh water played an important role in aragonite conversion to calcite. FRIEDMAN (1964) showed that both the p H and salinity of the waters are important factors which control the mineralogical and, by inference, textural changes in calcium carbonate sediments. According to his experiments, p H is a more important controlling factor than salinity.

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Replacement of other materials, clziefly calcium sulfate and quartz, by calcium carbonate

A few limestones have originated by replacement of other materials, chiefly calcium sulfate and quartz grains. Such limestones are probably not common, but they are important in that their origin differs so drastically from that of other limestones, and because those formed by replacement of calcium sulfate may be associated with free sulfur deposits. The two most common reactions are bacterial conversion of calcium sulfate to calcite and carbonate replacement of quartz grains. i Bacterial conversion of calcium sulfate to calcite Sulfate-reducing bacteria, which feed on organic matter, obtain oxygen by breaking down sulfates, such as calcium sulfate (gypsum or anhydrite). Calcite is formed when carbon dioxide, either liberated in the bacterial reaction from the organic matter, or derived externally, combines with calcium oxide released from the calcium sulfate by the bacteria. A somewhat simplified reaction may be presented as follows:

+ 2(CH20) -+ 2 Ca0 + 2 s + 2 COz + 2 H20 + + 2C02 -+ 2CaC03

2CaS04 2Ca0

0 2

This process is thought to have been responsible for the production of limestone cap rocks atop salt plugs, as in the Gulf Coast region of the United States (FEELY and KULP,1957). It may also have caused the replacement of the Castile Anhydrite (Permian) along faults in the Delaware Basin, west Texas, which has given rise to local limestones that form topographically resistant landforms called “castiles” (ADAMS,1944, pp.1606-1607). Replacement of Castile Anhydrite resulted in a laminated limestone; where massive Castile Anhydrite has been replaced, a massive limestone has resulted (G. W. MOORE,1960). In addition, bacterial processes may have been responsible for the calcite-rich layers in the calcite-aragonite couplets in Recent sediments of the Dead Sea (NEEV, 1963, 1964; see also section on Calcium carbonate lake sediments). A similar bacterial replacement origin has been proposed as an explanation for the thick-bedded, sparsely fossiliferous parts of the Capitan Limestone (Permian) of southeastern New Mexico (G. W. MOORE,1960). According to this interpretation, the main body of the Capitan Limestone is the lateral equivalent of the upper massive Castile Anhydrite, and is not entirely an older barrier reef, as is generally supposed (NEWELLet al., 1953). The massive, unfossiliferous parts of the Capitan Limestone have been explained as the product of recrystallization of the reef rock, but it retains no vestiges of organic remains; rather, it is indistinguishable petrographically from the massive limestone formed on the “castiles” in the Delaware Basin, where massive anhydrite has been replaced.

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Carbonate replacerncnt of quartz grains Carbonate minerals, such as calcite, dolomite, siderite, or ankerite, commonly replace quartz grains; a favored site of replacement is near the upper contact or updip pinch-out of sand bodies, particularly lenticular ones. In the Cardium Sandstone (middle Upper Cretaceous) at Pembina, Alta., Canada, siderite-bearing calcareous sandstone forms an impervious zone at the upper contact of the sandstone where it underlies a conglomerate. The calcium carbonate content of analyzed samples ranges from 10 to 50 %. The quartz grains have been markedly corroded and replaced by the calcite cement. Replacement of the quartz grains proceeded from the outer edges inward; locally, calcite ramified through quartz. Several stages of replacement were traced from peripherally corroded and embayed quartz grains to residual quartz embedded in a calcite pseudomorph (G. M. Friedman, unpublished observations). Where replacement has been complete, no traces of the former quartz may be found, or, at best, only a few corroded quartz relicts may remain. WALKER (1 960) noted that the replaced grains are preserved as carbonate pseudomorphs, which can be mistaken for grains of clastic carbonate. Thus, sandstones could, by replacement, be converted into limestones. Although regarded as “uncommon” (NICHOLAS, 1956, p.6), this type of replacement may be rather common, and limestones formed in this manner may be more widespread than is generally realized.

OCCURRENCE OF LIMESTONES

Introduction Limestone occurs in all parts of the world and at all levels in the stratigraphic column, though probably it is less common in ancient Precambrian strata. Limestones have been estimated to comprise 19-22% of available measured stratigraphic sections (LEITHand MEAD, 1915; SCHUCHERT, 1931; PETTIJOHN, 1957). Slightly lower values for total limestone abundance in the continental crust have been calculated from geochemical material balance analyses, ranging from 5 to 14% according to MEAD(1907), CLARKE (1924), or HOLMES (1937). Although outranked by “shales” (44-58 % of measured sections, whereas geochemically calculated values range from 70 to 82 %) and “sandstones” (22-37 % of measured sections and 12-1 6 % based on geochemical calculations), limestones perhaps have received proportionately more than their share of geologic study from the earlier generations of paleontologically-oriented stratigraphers, who were particularly concerned with the great abundance of marine fossils in limestones. Figures for limestone in the deep-sea crust are not yet available. In more recent times, the interest in limestones has continued to expand partly for their fossil content, partly because many limestones are petroleum

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reservoirs (HARBAUGH, 1967), and partly because it is possible to make rather exact environmental comparisons between limestones which preserve information about original calcium carbonate sediments and their Recent analogues (TAFT, 1967a); both of these contributions are found in the present volume. The occurrence of limestones may be considered from many points of view. One view, which became particularly common in the United States in the first half of the 20th century, was to assign all calcium carbonate rocks to the single group, “limestones”, and to contrast them with various kinds of noncarbonate deposits. Thus, “limestone” tended to denote a “chemical” rock, in contrast with “terrigenous” (or "elastic") rocks, which were subdivided on the basis of grain size and/ or mineralogic composition. This approach essentially ignored the variations within limestones that reflect initial variations in the calcium carbonate sediments from which most limestones have been derived. Another approach, which also tended to obscure the variations within the original carbonate sediments, was to subdivide limestones according to different gross tectono-stratigraphic “facies”, such as “platform” limestones, “basin” limestones, and “geosynclinal” limestones (SLOSS,1947). “Platform” limestones, for example, typically are extensive, thin blankets of light-colored rock that commonly contain abundant fossils and have been extensively dolomitized (especially those of Paleozoic age). “Basin” limestones may resemble “platform” limestones lithologically, but locally they are much thicker and pass laterally into reefs and/or the deposits of barred basins. “Geosynclinal” limestones are identified by their occurrence in thick sequences, siliceous composition, dark color, and interstratification with deep-sea shales. More recently emphasis has been placed on analysis of limestones in terms of the different kinds of carbonate sediments from which most of them have been derived, and to use primary lithofacies and biofacies as a basis for paleoecologic reconstructions. This paleoecologic approach is followed and extended here. The occurrence and distribution of limestones is considered to be a stratigraphic problem. Such sediments result from the interplay between the instantaneous lateral pattern of geographic environments of deposition and the vertical distribution of these environments through time, which governs the build-up of the stratigraphic record. Such an environmental-stratigraphic approach applies generally to all sediments; not only to pure carbonate sediments, but also to noncarbonate sediments and mixtures of these two types. Before pursuing this approach further, it is necessary to review the kinds of particles found in calcium carbonate sediments and to explore the interplay between the types of sedimentation processes that influence these particles and that are responsible for creating stratigraphic units.

Kinds of calcium carbonate particles in carbonate sediments Calcium carbonate particles in carbonate sediments consist of two major types:

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(I) recycled particles eroded from older carbonate rocks, and (2) first-cycle particles that originate from the water in the depositional basin itself. Recycled, or allochthonous, calcium carbonate particles belong in the category of ordinary terrigenous grains; they have been derived by erosion of older carbonate rocks on land and delivered to the site of deposition from an external source. Considerable confusion has grown up in connection with the nomenclature of such grains and rocks composed of them. The terms “mechanical” (FERAYet al., 1962, p.41), “detrital” (LEIGHTON and PENDEXTER, 1962, p.35), and “clastic” have all been applied in the same sense as the term “terrigenous”, which is used here; but these other terms are not recommended because they have also been used to refer to the “physical” transportation of carbonate grains by currents as opposed to the “chemical” deposition out of the water. All kinds of carbonate particles, no matter what their origin, behave as quartz grains in a water current, and this behavior has been expressed by applying the terms “mechanical”, “detrital”, “clastic”, or “fragmental”. As pointed out by FOLK (1959), use of the term terrigenous for recycled particles avoids this possible ambiguity. Limestones formed of recycled grains have been called “calclithites” (WOLF,1960). First-cycle, or autochthonous, calcium carbonate particles (allochemical coiistituents of FOLK, 1959; 1962) include skeletal remains of organisms, ooids, superficial ooids, pellets, lumps, aggregate grains, pseudooids (CAROZZI,1957) and intraclasts (FOLK,1959, 1962), all of which owe their origin to various processes from the water or bottom of the depositional basin. Only distinctive skeletal remains are restricted to marine environments; the other types of first-cycle particles may originate in lake water as well as sea water. For further details on the kinds of particles in calcium carbonate sediments the reader is referred to TAFT (1966a) in this volume. In the case of first-cycle carbonate sediment particles, further complication arises due to the fact that they may be deposited in approximate environmental equilibrium, that is, near the place where they originated. They also may be displaced by gravity down subaqueous slopes that typically are steep at the margins of banks, where marine carbonate sediments are forming, and ultimately may be deposited far from their place of origin, in the same manner as recycled terrigenous grains. A remarkable and unique feature of marine calcium carbonate sediments is that first-cycle particles can be produced in enormous abundance from sea water within a very short time. Hence, thick accumulations of pure calcium carbonate sediments may originate even in the absence of any external source of terrigenous grains. This permits sedimentation of calcareous material to keep pace with subsidence of the basin bottom, and this balance may produce thick piles of shallowwater carbonate sediments. For example, approximately 1,500 m of carbonate sediments and limestones derived from calcium carbonate sediments were drilled in hole F-1 on Elugelab Island, Eniwetok Atoll (1 l”40’ N 162”12’ E), the age of

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which ranges from Eocene to Recent. In the Funafuti boring (8 “30’ S 179” E), approximately 330 m of carbonate deposits range in age from Pliocene to Recent; and in the Kita-Daito-Jima boring (26” N 132” E), 435 m of carbonates range in age from Miocene to Recent (SCHLANGER, 1963). In the Bahamas, the well drilled by Superior Oil Company and others at 26’52‘37.2’’ N 78”01’54.7” W, at Stafford Creek, Andros Island, Bahamas, penetrated 5,000 m of carbonates the age of which ranges from Late Cretaceous to Recent (Marie Spencer, in: EARDLEY, 1951). Depositional processes involved in accumulation of marine carbonate- and mixed carbonate- and noncarbonate sediment units In spite of their distinctive origins, first-cycle biological carbonate grains (or other types) respond to various common physical processes of sedimentation just as terrigenous (recycled) carbonate and noncarbonate particles do. Accordingly, they follow the same principles that govern accumulation of almost all stratigraphic units. The physical depositional processes involved may be classified according to the predominance of vertical or lateral sedimentation activities and also on the basis of “energy”. Vertical sedimentation processes. Vertical sedimentation processes involve the more or less simultaneous vertical accumulation of particles that have been derived ( I ) from fallout of skeletal material of planktonic organisms distributed in the water, or suspended load carried by the water; (2) from growth of benthonic shell beds on the bottom, scattered or in the form of in situ biostromes; or (3) from vertical upgrowth of reefs or bioherms. Each of these vertical processes is characterized by upward accretion at the top of the growing layer of sedimentary material. The lateral spreading out or diffusion of the materials involved is not directly included in the depositional process; but rather takes place previously within the water as a result of migration or spreading of living organisms or a mixing of fine-grained sediment particles by turbulence within the water. Lateral sedimentation processes. Lateral sedimentation processes involve the sideways shift along the bottom of sediment already present, or the change in position of environments in which particular kinds of sediments may be restricted. Lateral shifting of sediment along the bottom may be due to bottom currents within the water mass; or it may be due to gravity displacement by turbidity currents, fluidized layers of flowing grains (SANDERS, 1965), or slumps, which cause sediment to move along the bottom independent of water currents. Gravitydisplacement processes require subaqueous slopes, and consequently their effects are more commonly found in sediments deposited in “deep” rather than in “shallow” water. Change of position of environments may take place as a normal consequence

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of sediment reworking, or it may reflect major geographic changes. An example of a change of position of an environment as a normal consequence of reworking is the lateral displacement of creeks on tidal flats due to undercutting of the concave bank and accretion on the opposite convex bank (VANSTRAATEN, 1951; 1952). Examples of changes of position of environments that are due to major geographic changes include those brought about by changes of sea level or changes in abundance of sediment supplied. The shoreline and associated sediment belts may move seaward as a result of seaward progradation of nearshore sediments on beaches or reefs, or because of emergence; these sediment belts may move landward as a result of submergence. These lateral shifts of nearshore sediment belts produce offlapping (“regressive”) and onlapping (“transgressive”) stratigraphic patterns, respectively, in which the patterned vertical sequence eventually formed obeys Walther’s Law of Facies (WALTHER,1893-1894). According to this law the vertical succession at any point is a reflection of the lateral distribution of the adjacent sediment belts at any instant in time.

Physical processes based on “energy”. Another approach to physical depositional processes is to subdivide them on the basis of “energy” into two groups: ( I ) “highenergy” and (2) “low-energy”. Depositional areas of “high energy” are inferred to produce clean, well-sorted, coarse-grained carbonate sediments with a maximum content of broken skeletal remains; whereas areas of “low energy” are held to be responsible for “muddy” deposits that are entirely fine-grained or contain abundant fine-grained matrix between coarser particles, with broken skeletal remains being less common. For further discussion of this approach with several intermediate categories, the reader is referred to the work of PLUMLEY et al. (1962). This analysis is certainly valid to a first approximation; however, the choice of the word “energy” is perhaps unfortunate, for what is implied is not the total energy of the system (which includes also chemical and biological components), but the mechanical effects on the bottom of turbulent motion in the water. In general the surf zone and bottoms subjected to constructed tidal flow (zone of maximum mechanical breakage) are sites of lag concentrates of whatever coarse material is present; any fines that arrive or are generated here by abrasion of coarse particles will remain in turbulent suspension and are moved elsewhere. The interpretation of bottoms subjected to less turbulent water motion is more complex. In the first place, fine-grained sediment may be absent. Inasmuch as particle supply is chiefly dependent on organisms for the most part, only coarse-grained sediment may be available. This produces an anomalous result: coarse-grained sediment in the absence of current turbulence. In addition, particles may be broken by several biological processes, including activity of boring and burrowing organisms, so that breakage need not always signify “high energy ’. Finally, grasses or sediment-binding organisms may trap sediments of finer grain sizes, which may be produced locally by several processes, and thus cause mud

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accumulation even where some currents are present. Examples illustrating these various alternatives have been described from the Florida reef tract by SWINCHATT (1965). Thus, although the “energy” scheme is useful, it should be applied with great caution.

Environmental settings of calcium carbonate deposits Recent calcium carbonate sediments occur in stratigraphic successions that are accumulating in three main environmental settings: ( I ) nonmarine environments, commonly within large inland structural basins; (2) shallow-marine and intermediate-marine platform environments; and (3) open-sea basins. Although certain attributes are common to all the carbonate sediments formed in any of these environmental settings, other attributes are more restricted and hence may be diagnostic characteristics of the environment in which they occur. For example, certain particle types, such as ooids, have been found in deposits from all of these environmental settings; other particle types, such as coccolith remains, however, are generally restricted to open-sea sediments, although they also occur in reef lagoons around the