s 14. GS: CIR 3&1 c. a &\tj&t SuJbO^q STATE OF ILLINOIS WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION VERA M. BINKS, Director Geochemistry of Carbonate Sediments and Sedimentary Carbonate Rocks Part I Carbonate Mineralogy Carbonate Sediments Donald L Graf DIVISION OF THE ILLINOIS STATE GEOLOGICAL SURVEY JOHN C. FRYE, Chief URBANA CIRCULAR 297 1960 ILLINOIS GEOLOGICAL SURVEY LIBRARY JUL 19 taoO I^'NO'S STATE GEOLOGICAL SURVEY 3 3051 00004 1222 GEOCHEMISTRY OF CARBONATE SEDIMENTS AND SEDIMENTARY CARBONATE ROCKS Part I Carbonate Mineralogy Carbonate Sediments Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/geochemistryofca297graf FOREWORD Detailed knowledge of the chemical and mineralogical varia- tions that exist in the carbonate rocks limestone and dolomite and of the processes responsible for this diversity is fundamental to the Il- linois State Geological Survey's program of furthering the practical utilization of these natural resources of the state. Chemical com- position is particularly important when the rocks are used as agricul- tural limestone and fluxing stone or in the manufacture of dolomite refractories, lime, calcium carbide, sodium carbonate, glass, and other products. The invitation extended to Dr. Graf by the United States Geo- logical Survey to prepare the chapter on sedimentary carbonates for their revision of F. W. Clarke's "Data of Geochemistry" has afford- ed a valuable opportunity for the state and federal geological surveys to cooperate in a basic review of selected topics in carbonate geo- chemistry. The resultant material is presented in five Illinois State Geological Survey Circulars and subsequently will serve as the basis for a condensed treatment in the revised "Data of Geochemistry. " Part I, published as Circular 297, includes an introduction and sections on carbonate mineralogy and carbonate sediments. Part II, Circular 298, includes the section on sedimentary carbonate rocks. Part III will deal with the distribution of minor elements. Part IV will present isotopic composition, present chemical analyses, and also will contain the bibliography for the first four circulars . Part V, concerned with aqueous carbonate systems, will be published at a later date. GEOCHEMISTRY OF CARBONATE SEDIMENTS AND SEDIMENTARY CARBONATE ROCKS Part I : Carbonate Mineralogy — Carbonate Sediments Donald L. Graf ABSTRACT The distribution of major and minor elements in sedimentary carbonate rocks and the mechanisms responsible for this distribution are considered on the basis of published information contained in geo- logic studies, and in studies of present-day environments of carbon- ate deposition, isotopic composition of carbonates, and experimental aqueous and nonaqueous carbonate systems. There are five parts in the series,and an extensive bibliography appears in Part IV. INTRODUCTION The discussion which follows is concerned with carbonate rocks, chiefly limestone and dolomite and their unconsolidated equivalents, which have not been subjected to hydrothermal or metamorphic environments. Some discussion of sed- imentary magnesite-, siderite-, and rhodochrosite-bearing materials is also in- cluded. Limestone is defined for convenience as a consolidated sedimentary rock containing more than 50 percent of the minerals calcite (plus aragonite) and dolo- mite (including ferroan dolomite) in which calcite (plus aragonite) is more abundant than dolomite (including ferroan dolomite) (see Rodgers, 1954). Dolomite (the rock) is similarly defined, but with dolomite (including ferroan dolomite) more abundant than calcite (plus aragonite). The arbitrary 50 percent limit obviously excludes some rocks in which calcite and dolomite together constitute the most important single constituent. There appears to be no general agreement on quantitative systems of nomen- clature for impure carbonate rocks, for rocks made up of mixtures of calcite and dolomite, and for dolomites (the mineral) containing various amounts of Fe in solid solution. Descriptive terms for these materials are used here in a qualitative sense. For example, an argillaceous limestone is one containing an appreciable amount, but less than 50 percent, of clay minerals. Whenever possible the terms are re- ferred to chemical and mineralogic analyses for further definition in specific contexts. Similarly, the use of such adjectival modifiers as ferroan and magnesian in describing carbonate solid solutions is qualitative. Terms such as "ankerite" and "mesitite" for arbitrary compositional ranges have but little theoretical justi- fication in the absence of evidence for structural discontinuities in these solid solution series. Perhaps even more important, the use of these terms has not con- tributed significantly to petrologic understanding as have, for example, the arbi- trary divisions of the albite-anorthite series. Carbonate sediments considered as a whole are noteworthy in that material produced through the activity of organisms appears to be the greatest single com- [5] 6 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 297 ponent. The character of carbonate sedimentation is governed more forcibly by to- pography and the consequent detrital contribution, and by climate and other factors that affect water chemistry and biologic activity, than it is by its position in an epeirogenic or geosynclinal area. Indeed, if the rate of deposition is sufficient to keep up with the rate of subsidence of geosynclines, great thicknesses of shal- low-water limestones may accumulate. Sloss (1947) contrasts the 26,000 feet or more of Oquirrh Limestone of Pennsylvanian-Permian age in western Utah with the few hundred feet of its platform equivalent in central Utah. Dunbar and Rodgers (1957) noted that most of the sediments of the Appalachian geosyncline are shallow- water, indicating that deposition kept up with the rate of sinking. The platform- type carbonate sediments of the present-day Persian Gulf also are accumulating in a geosynclinal series. The carbonate portion of carbonate rocks consists of mixtures in various pro- portions of detrital terrigenous carbonate particles, reworked fragments of pene- contemporaneous carbonate sediments, oolites, fecal pellets, and skeletal material, all cemented by microcrystalline ooze and coarser grained, pore-filling cement (see Folk, 1959). The rocks may contain replacive dolomite in addition to that found in some places as an originally precipitated ooze. A distinction as to the vigor of waves and currents in the depositional environment, based on the amount of inter- stitial microcrystalline carbonate matrix remaining, appears to be generally valid (Bramkamp and Powers, 1958; Carozzi, 1960, p. 226; Nelson, 1959), although it has been suggested that in some rocks this matrix has recrystallized to clear calcite cement. Noncarbonate minerals may be detrital or organically or inorganically formed during or after deposition. Terrigenous contributions probably dilute to an unusual degree carbonate sediments now being formed, because of the greater-than-average topographic relief of the continents. Limestones are particularly susceptible to post-depositional recrystallization and carbonate cementation that may obscure evidence regarding the manner of their formation. Illing (1954) has noted that recrystallization to microcrystalline lime- stone, obliterating fossils and individual oolites, is already taking place in the Bahamas early in diagenesis. Considerable depletion or enrichment of CaC03 may occur as well, so that it is somewhat arbitrary to consider carbonate sediments con- taining more than 50 percent carbonate minerals to be the unconsolidated equivalents of carbonate rocks. In the sections that follow, the composition and distribution of carbonate sediments from various present-day environments are described, and an attempt is made to relate lithified carbonate rocks to their original environments of dep- osition. This treatment leads to a considerable emphasis upon the processes and mechanisms responsible for the required 50 percent or more carbonate content, for the presence in some carbonate rocks of other major constituents such as silica and bituminous matter, and for the observed minor-element distributions. The useful- ness of geochemical data for other than gross averages must ultimately rest upon thorough understanding of process. The chemical information available at present is seldom adequate to define the depositional environment of a given carbonate rock, particularly if it is a very pure limestone. The isotopic compositions of coexisting sedimentary min- erals and the concentrations of minor elements such as boron in accessory minerals are most useful, but this information is not yet available for many samples. In most cases where it seems that the depositional environment can be defined with some confidence, heavy emphasis has been placed upon such criteria as texture GEOCHEMISTRY OF SEDIMENTARY CARBONATES - I 7 and fossil content. In spite of the use of such additional nonchemical criteria, the degree to which environmental identification can be made varies greatly, and a mixed classification in the discussion of sedimentary carbonate rocks has resulted. Thus the headings "cave deposits" and "shallow-water marine limestones" (oolites, reefs, coquinas, chemically precipitated CaC03) have rather definite environmental con- notation, but "fine-grained limestones" and "phosphatic limestones" refer to tex- tural and chemical types of carbonate rocks that probably come from a variety of environments. At the present state of knowledge, one value of a summary of this sort is the mere listing of carbonate rocks whose depositional environment is believed to be known. They then may be studied further and an attempt made to formulate geo- chemical criteria for recognition of environments. Although most carbonate rocks probably were formed in relatively shallow, well aerated marine waters receiving but little terrigenous detritus, it is vitally important, for an understanding of the range of operative processes, that the environments of formation of less common carbonate rock types, such as some of those discussed here, be investigated. CARBONATE MINERALOGY The most important rock-forming carbonate minerals are two rhombohedral compounds, calcite (CaC03) and dolomite (CaMg^OgK), that are found widely dis- tributed in sedimentary, metamorphic, and hydrothermal environments . Dolomite is a 1:1 ordered compound — that is, its crystal structure differs from that of calcite in having successive basal cation planes populated exclusively by each of the two kinds of cations in turn. Three other calcite-type rhombohedral carbonates, mag- nesite (MgCOg), siderite (FeC03), and rhodochrosite (MnCOg), occur only in re- stricted types of sedimentary rocks but are found rather commonly in hydrothermal assemblages. Kutnahorite (CaMnfCOj),) has the dolomite-type structure and has been described by Frondel and Bauer (1955) from the Mn-rich orebody at Franklin, New Jersey, and from two localities in Czechoslovakia. It is of interest principally because its behavior in experimental studies has furthered understanding of the properties of carbonate solid solutions. Aragonite, the orthorhombic polymorph of CaCOg, is a common constituent of geologically young materials, including oolites, cave and spring deposits, and invertebrate skeletal remains. Compilations of chemical analyses of carbonate minerals such as those made by Bilibin (1927) and Ford (1917) are of limited usefulness because they do not con- sider composition-temperature relationships and because the materials analyzed may have contained more than one phase. Palache et al . (1951) discussed the physical properties of the carbonates, Graf and Lamar (1955), the physical properties of the Ca and Mg carbonates only. Goldsmith (1959) has reviewed recent work on the phase relations of the anhydrous carbonates. CALCITE-TYPE STRUCTURES The cations Fe ++ , Mg ++ , and Mn ++ have closely similar ionic radii, and chemical analyses of supposedly single-phase materials suggest that extensive solid solution between MgC03 and MnCOg and complete solid solution between the other two pairs is possible (Palache et al., 1951; Frondel and Bauer, 1955). Rel- atively pure end members may be formed, of course, in natural environments that are essentially free of possible substituting ions. Thus, a magnesite concentrate from the anhydrite zone of the Middle Evaporite Bed, in northeast Yorkshire, contains only 0.17 percent Fe and 0.02 percent Mn (D. L. Graf, unpublished data). 8 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 297 Ca ++ is sufficiently larger than Mg ++ , Mn"*" 1 ", and Fe ++ so that solid solu- tion between CaCOo and the carbonates of these three cations is incomplete at earth-surface temperatures. Goldsmith and Graf (1957) found that a complete series of solid solutions exists between calcite and rhodochrosite above approximately 550 "C, but that at lower temperatures a gap exists in the Mn-rich half of the system. At 450 "C, the lowest temperature at which equilibrium could be reached in experi- mental runs, the gap extends from about 5 2 to about 80 mol percent MnCO . This range is in reasonable agreement with the gap observed by Frondel and Bauer (1955) in their frequency distribution of Ca-Mn carbonates from Franklin, New Jersey, based upon chemical analyses and optical determinations. Goldsmith and Graf precipitated the complete series of Ca-Mn carbonate solid solutions at room temperature, and at least those compositions lying below the solvus described above must have been meta stable. At 500 °C, approximately 14 mol percent FeC03 can be taken into solid solu- tion by calcite and about 5 mol percent CaCOg by siderite (Rosenberg and Harker, 1956). At 700°C some 37 mol percent FeC03 can be taken up by calcite (Goldsmith, 1959). The solubility of FeCOg in calcite is thus between those of MgCOg and MnC03- Information on natural materials is limited to chemical analyses "(see, for example, Palache et al., 1951), which indicate that substitutions of a few mol per- cent of either carbonate for the other are not uncommon. Larger substitutions are suggested by a few analyses. The immiscibility gap in the system CaC03~MgC03 between magnesite and dolomite is virtually complete. At 900 °C magnesite will take only about 2 wt per- cent CaC03 into solid solution (Harker and Tuttle, 1955), and dolomite will take only about 1 percent excess MgC03 (Goldsmith, 1959). Dolomite will, however, hold about 2 mol percent excess CaC03 at 800 °C and 4 mol percent excess at 900 °C (J. R. Goldsmith, unpublished data). The allowed substitution at a given temper- ature of a greater excess of the larger Ca ++ ion in dolomite, compared with the allow- ed excess substitution of Mg ++ , cannot be explained by simple considerations of ionic size. The solubility of MgCOo in calcite in equilibrium with dolomite rises from 5.5 mol percent at 500°C to 17.5 mol percent at 800°C (Harker and Tuttle, 1955; Graf and Goldsmith, 1955, 1958). The equilibrium solubility of MgC0 3 in calcite in equilibrium with MgO + COo, which is a function of both temperature and partial pressure of CO2, also has been measured by Graf and Goldsmith. The Mg content of a number of naturally occurring calcite s has been deter- mined (Chave, 1954a; Goldsmith et al . , 1955; Graf and Goldsmith, 1958). Tufa from Mono Lake, California, contains up to l\ mol percent MgC03 in solid solution, and calcite from the low temperature hydrothermal magnesite orebody at Currant Creek, Nevada, contains 6 mol percent. The magnesian calcites making up the hard parts of some invertebrates contain as much as 18 mol percent MgC03 and are thus clearly meta stable after the death of the organism, whatever the biochemical con- ditions were within the organism when the magnesian calcite was laid down. Also meta stable at the conditions under which they formed are the magnesian calcites reported by Alderman and Skinner (1957) to be precipitating in ephemeral lakes and in the Coorong in southeastern Australia, and similar synthetic materials prepared at room temperature (Graf and Goldsmith, 1956; Brooks et al., 1950) and at elevated temperatures (Graf and Goldsmith, 1955, p. 124; Harker and Tuttle, 1955, p. 278). Because magnesian calcites in sedimentary environments typically lose their Mg rather quickly with geologic time, the calcites from Mesozoic and older rocks are virtually free of Mg (Chave, 1954b; Goldsmith et al . , 1955). Meta stable mag- nesian calcite fossils may in rare instances persist over long geologic time if sealed GEOCHEMISTRY OF SEDIMENTARY CARBONATES - I 9 in relatively impermeable rocks. Heinz Lowenstam (personal communication) found calcitic Pennsylvanian coelenterates containing as much as 5.2 wt percent MgC03 and a Cretaceous coelenterate containing about 7 wt percent MgC03. Calcites in marbles have been observed to contain up to l\ mol percent MgC0 3 (Goldsmith et al., 1955; Graf and Goldsmith, 1958). When the rock con- tains dolomite in equilibrium with magnesian calcite, the amount of solid solution constitutes a geologic thermometer; thus l\ mol percent MgC03 corresponds to a temperature of about 600 °C. If there is no dolomite present with the calcite, the temperature indicated is a minimum one, inasmuch as there may not have been enough Mg in the environment to saturate the calcite. It is not possible to specify a temperature of formation for predazzites, some of which have calcites containing up to l\ mol percent MgCOj, because the calcite was in equilibrium with MgO + CO2 and actually may have increased in Mg content during cooling (see Goldsmith et al., 1955). Some marbles contain dolomite plus two magnesian calcites of differing composition, and in a number of such rocks (Goldsmith et al., 1955; Goldsmith, 1956, 1957, and unpublished data) it has been possible to show by single-crystal X-ray diffraction methods that the dolomite and poorly magnesian calcite occur to- gether in tiny blebs that have the same crystallographic orientation as do enclosing host crystals of more highly magnesian calcite, from which the blebs must have exsolved during cooling. In some cases such dolomite inclusions in milky meta- morphic calcites are too small to be visible optically. It is probable that at least some coarser dolomite-calcite intergrowths and independent grain assemblages of these two minerals in marbles may have originated by exsolution and subsequent recrystallization (J. R. Goldsmith, personal communication). Oriented exsolved dolomite has been produced experimentally by heating to 500 °C a single-crystal, echinoidal, magnesian calcite host containing some 10 mol percent MgC03 (Gold- smith, 1956). Fine or even submicroscopic intergrowths from Franklin, New Jersey, con- sist of kutnahorite and either a more Ca-rich or a more Mn-rich carbonate, and single-crystal X-ray photographs show the crystallographic orientation of the two phases to be the same (Goldsmith, 1957; J. R. Goldsmith and D. L. Graf, unpub- lished data). These materials, if interpreted as exsolution products, indicate the presence of an immiscibility gap in the Ca-rich half of the system CaC03~MnC03, which it has not yet been possible to demonstrate experimentally because of slow reaction rates in the system at low temperatures. The extent of cation substitution observed in natural calcites is a function not only of the supply of such ions in various environments and the extent to which they can be accommodated in the calcite structure, but also of the partial pressures of CO2 required to maintain the solid solutions. Thus the high partial CO2 pressures required to maintain ZnC03 at elevated temperatures (see Harker and Hutta, 1956) indicate that calcites containing significant amounts of ZnC03 would decompose at moderate temperatures to yield a more nearly pure calcite unless CO2 pressures were high (Goldsmith, 1959). High-temperature calcites typically contain more Mg ++ than Fe , although the solubility of Fe ++ in calcite at a given temperature has been shown to be greater than that of Mg ++ . Goldsmith suggests that decomposition re- lations may be the controlling factor here. The equilibrium thermal decomposition curve of siderite is not yet accurately known. 10 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 297 DOLOMITE-TYPE STRUCTURES The formation of the 1:1 cation-ordered carbonate, dolomite, appears to be favored by a large difference in the ionic size of the two cations, and thus the fail- ure to synthesize the compound CaFe (C0 3 ) 2 (Rosenberg and Harker, 1956; Gold- smith, 1959) or to find it in nature is puzzling. The Fe ++ found commonly in ferroan dolomites substitutes, in Mg ++ positions because of the similar ionic sizes of these two ions. Thus the formula Ca-i Q25~ M 90.626 Fe 0.330 Mn 0.022 ^ C0 3h .997 ma Y ^ e calculated from the analysis of a single-phase ferroan dolomite with an enlarged unit cell from a bed of nonmarine lamellibranchs in the Lancashire Coal Measures (Howie and Broadhurst, 1958; Broadhurst and Howie, 1958). Similarly, it may be noted from Frondel and Bauer's (1955) analyses that substitution of Fe ++ and Mg++ in kutnahorite is accompanied by a greater decrease in Mn ++ than in Ca ++ . A number of more complex dolomite solid solutions have been described. Hurlbut (1957) gives analyses of five dolomites from Tsumeb, Southwest Africa, that show up to 8.74 wt percent ZnO and up to 4.96 wt percent PbO. The results of dif- ferential thermal analysis can be correlated with change in composition of these ma- terials, but there is a much less regular variation of indices of refraction, specific gravity, and unit cell dimensions. Above about 650°C a complete series extends between CaMn (COo)2 an d CaMg(C03)2 (Goldsmith and Graf, 1960), and order reflections are observed in X-ray powder patterns of compositions containing more than about 50 mol percent dolomite. The single phase that is stable at high temperatures is replaced at lower temperatures by two or more phases. Replacement of Mg ++ by Mn ++ in this series obviously can- not be nearly as extensive at low temperatures as the replacement of Mg" 1-1 " by Fe" 1-1 " in ankerite. Because Mn ++ is significantly larger than Fe ++ or Mg" 1 "" 1 ", it is possible than Mn ++ may go into Ca ++ positions to some extent. The temperatures at which the several 1:1 compounds disorder vary greatly. Dolomite remains ordered at the temperatures up to 900 °C from which it has thus far been quenched. Heating experiments using natural (ordered) kutnahorite single crystals show that the ordering temperature for CaMn(C03)o is at or below 450°C. The ordering reflections of this material are too weak to be detected with cer- tainty in powder X-ray diagrams, and thus it is not known whether any of the synthetic materials of this composition prepared over a range of temperatures were ordered (Goldsmith and Graf, 1960). Although the considerable amount of FeC03 that has been experimentally substituted into calcite suggests that a disordered CaFe (003)2 could be produced at a sufficiently high temperature and partial pressure of CO2, this material would, on cooling, apparently break up into a CaC03~rich phase and a FeC03~rich phase rather than ordering. As part of an experimental study of the subsolidus relations in the system CdC03-MgC02, J. R. Goldsmith (personal communication) has determined the ordering behavior of compositions near CdMg(C03)2- These materials, not en- countered in nature, have ordering temperatures intermediate to those of dolomite and kutnahorite, so that Goldsmith has been able to put together an unusually complete picture of ordering relations that is invaluable in understanding dolomite and kutnahorite . As demonstrated in the CdC03~MgC03 system, the 1:1 compounds may de- part significantly from ideal composition and still show the extra X-ray reflections resulting from cation order. Natural kutnahorites from Franklin, New Jersey, con- GEOCHEMISTRY OF SEDIMENTARY CARBONATES - I 11 taining approximately 10 mol percent excess CaC03 in their structures still show some order when examined by single-crystal techniques. Goldsmith and Graf (1958) determined by X-ray diffraction the mol percent CaC03 in dolomites from various rocks, after verifying from emission spectrographic analyses that the amount of Fe and Mn in the samples was too small to affect significantly the unit cell size. The metamorphic and hydrothermal dolomites examined and most of the dolomites associ- ated with evaporite deposits have essentially the 1:1 molar ratio of CaCO-^MgCO,. Dolomite occurring as scattered rhombs in Ordovician and younger limestones not uncommonly contains CaC03 above this ratio, reaching values as high as 56 mol percent CaCO„. A number of Cenozoic rocks from Florida made up almost entirely of dolomite show the same effect, as do dolomite from a Searles Lake horizon having a radiocarbon date of 10, 000 years and the dolomite described by Alderman and Skinner (195 7) as precipitating today in ephemeral lakes and in the Coorong in south- east Australia. From the results of laboratory studies, sedimentary dolomites containing more than a fraction of one percent excess CaCOo must be metastable at earth-sur- face conditions. These dolomites are similar to the poorly ordered Ca-rich proto- dolomites produced synthetically by Graf and Goldsmith (1956) at temperatures be- low 200 °C and in experimental runs of short duration at higher temperatures. The formation and persistence of protodolomites has been interpreted (Goldsmith, 1953; Graf and Goldsmith, 1956) as a consequence of the similarity in terms of crystal energy of the non-equivalent Ca and Mg positions. As a consequence, it is difficult, particularly at rapid rates of crystallization, either to produce initially a perfectly ordered array or to move cations later from wrong positions to stable positions of lower energy. TERNARY SYSTEMS The subsolidus relations in the system CaC03"MgC03-MnC03, as deter- mined from 500° to 800 °C by Goldsmith and Graf (1960) using a squeezer-type ap- paratus, are summarized in figure 1. Phase diagrams at 600° and 700°C lie between those shown for 500° and 800 °C, with two- and three-phase fields progressively decreased in area from those at 500 °C. The existence of stable, sedimentary, three- phase rhombohedral carbonate assemblages in this system is indicated by the in- crease in area of the three-phase field with decreasing temperature, provided that hydrous phases do not intervene. At experimentally investigated temperatures below 600 °C, manganoan dolomites contain less than 50 mol percent CaC03. Rosenberg (1959) reported the synthesis at 450 "C of ferroan dolomites having up to 75 percent of the magnesium positions filled with iron, a figure in good general agreement with the maximum observed in natural samples. Dolomitic phases with 75 to 85 percent substitution were found by Rosenberg to lie in a three-phase region in equilibrium with calcite and siderite solid solutions. J. R. Goldsmith, D. L. Graf, and David Northrop (unpublished data) found no three-phase region in the sys- tem between 700° and 800°C. ARAGONITE-TYPE STRUCTURES Jamieson (1953) computed the pressure for calcite-aragonite equilibrium at 25°C, using available data, and then determined the equilibrium curve as a function of temperature and pressure over the range 25 to 80 °C and several thousand kg/cm^, using the electrical conductivity of aqueous solutions of each form. MacDonald (1956) studied the equilibrium in the range 250 to 600 °C and 6 to 30 kb, using the simple squeezer. Clark (1957) used an experimental apparatus in which pressure 12 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 297 was generated by compression of nitrogen and measured with a manganin coil. The data obtained by these several methods are in good agreement. Aragonite is the high pressure polymorph of calcite and is metastable at the temperatures and pres- sures near the earth's surface. Sedimentary aragonites in rare instances may be preserved for geologically long times if sealed in asphalt or highly impermeable rocks, Stehli (1956) described aragonite fossils from the Lower and Middle Penn- sylvanian. The geothermal gradient lies completely in the calcite field, so only in peculiar local conditions of high hydrostatic pressure and moderate temperature would aragonite be stable at depth. CaCO, MgCO CaMg(C0 3 ) 2 /„ ~ Zf /- VCaMn(C0 3 ). MnCO, Fig. 1 - Subsolidus relations in the system CaC03-MgCC>3-MnC03, adapted from Goldsmith and Graf (1960). Heavy lines and numbers delimit the one-, two-, and three-phase regions at 500 "C; lighter lines show the analogous situation at 800°C. In this temperature range, at a pressure of 10 kb, all phases are rhombohedral. The suggestion has been made (see Johnston et al., 1916) that the presence of foreign ions such as Pb ++ and Sr ++ in solid solution might stabilize aragonite with respect to calcite at earth-surface conditions. MacDonald (1956) assumed that CaC03 forms an ideal solid solution with other components and used the ex- perimentally determined value of 200 ±100 cal/mol for the difference in Gibbs GEOCHEMISTRY OF SEDIMENTARY CARBONATES - I 13 free energy between aragonite and calcite at 25 °C to show that at least 30 percent of components other than CaCOo would be needed in aragonite to stabilize it rela- tive to calcite. Palache et al . (1951) observed a maximum of 9 percent solid solu- tion in natural aragonites. Mg is even less soluble in the orthorhombic aragonite structure than in cal- cite. Goldsmith (1959) noted that even at 800 °C, in an experimental system in which the calcite is saturated with MgCO-j, aragonite takes no significant amount of Mg into solid solution. Both skeletal and inorganically precipitated marine ara- gonites contain only insignificant amounts of Mg; Chave (1954a) noted that the former seldom contain more than 1 percent MgCQo • Larger cations substitute in aragonite, however; Palache et al. (1951) cited chemical analyses indicating Sr:Ca as high as 1:25 and Pb:Ca as high as 1:12. John Jamieson (personal communication) has prepared a complete series of orthorhombic solid solutions between CaC03 and SrC03 by rapid mixing of . 1 M (Ca,Sr)Cl2 and Na2C03 solutions at 82.9°C. By heating these materials to temperatures above the aragonite-calcite transition in this sytem and then quench- ing them, Jamieson obtained single-phase calcite-type structures containing from to 70 mol percent SrCOg . Faivre (1946) described synthetic orthorhombic calcium- barium carbonates containing up to 65 percent Ba and rhombohedral ones with up to 40 percent Ba. Some of the latter are anion-disordered. MIXED-LAYERING Both ordered and disordered rhombohedral carbonate solid solutions are fre- quently observed to have abnormal c-axis progressions, which Graf et al. (1957) have interpreted by using mixed-layer theory. The effect is characteristic of Ca- rich dolomites and highly ferroan dolomites. It also occurs in supposedly ideal disordered solid solutions involving cations differing considerably in size, such as Ca-Mn and Ca-Mg, in which the amount of substitution is considerable and some segregation of cation types into discrete planes or regions has taken place. These mixed-layered materials obviously do not fit into existing carbonate mineral classifications . CRYSTAL ZONING Crystal zoning parallel to rhomb faces is not uncommon in the rhombohedral carbonates. The angle between the rhomb faces and the basal planes involved in the mixed-layering just described is about 45°, and the two phenomena are quite distinct. Grout (1946) described zoned carbonates from hydrothermal environments, and a number of workers, using optical measurements and differential etching with acid, have reported calcite zones in sedimentary dolomite crystals. Staining of Illinois Paleozoic limestones with K ferricyanide solution has shown in some instances that the Fe ++ content of dolomite rhombs is concentrated in a number of sharply defined zones and is essentially absent in the others. The total amount of iron involved is not great; six concentrates of dolomite rhombs from these limestones contained from 0.46 to 1.06 percent Fe and from 0.032 to 0.120 percent Mn (Goldsmith and Graf, 1956). Magnesite crystals described from a Permian halite bed of northeast York- shire have centers with markedly lower refringence than the rest of the crystal (Armstrong et al., 1951). Well formed zoned crystals of calcite, 0.1 to 0.2 mm in diameter, are reported in many samples from the North Atlantic deep-sea cores (Bramlette and Bradley, 1940). 14 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 297 A striking example of crystal zoning occurs in the Northampton sand iron- stone of central England. The principal carbonate phase of the ironstone, described earlier on the basis of optical and chemical study as a siderite containing a total of about 20 mol percent MgC03, MnC03, and CaC03 in solid solution, was shown by Cohen (1952), using a microradiographic technique, to consist of zoned crystals having alterate layers of iron carbonate and iron- free carbonate. Although perhaps not generally recognized, such zones deserve to be considered individually, inas- much as they represent responses to distinctly different chemical environments. LUMINESCENCE Certain energy levels in naturally occurring calcite and dolomite crystals can be populated with metastable electrons by exposure to Co°^ gamma radiation or to the naturally occuring radiation in sedimentary environments. The thermo- luminescence which results from heating natural materials previously exposed to Co radiation is released in four temperature regions, 120" to 140°, 150° to 190°, 210° to 250°, and 290° to 310°C (Daniels and Saunders, 1951). The two lower temperature peaks are notgenerally observed for natural materials run as received because ambient temperatures are high enough to drain electrons from the traps involved in this low temperature thermoluminescence. Zeller and Pearn (1960), however, observed the 125°C peak for naturally refrigerated Antarctic limestone samples and estimated a half-life of about 25 hours at 25 °C. Synthetic calcium carbonate with a low impurity level shows no radiation - induced thermoluminescence, unlike similar material crystallized in the presence of such impurities as Fe, Mn, Mg, Sr, or Ba (Zeller et al., 1955). The high- purity synthetic material ground in a mortar or compressed for 12 hours at about 700 kg/cm z (but not irradiated), gives a curve with maxima at 360° and 424 °C (De- benedetti, 1958). Zeller et al . found that for limestone samples older than 100 mil- lion years the thermoluminescence induced by natural radiation is always decreased by application of pressure. Jamieson and Goldsmith (in press) attempted to esti- mate the localized temperatures and pressures generated within calcite during grind- ing, which converts this material to aragonite, the polymorph stable at somewhat higher pressures (see also Burns and Bredig, 1956). Luminescent effects have been studied from calcite single crystals deformed dry at room temperature under a constant confining pressure of 2750 bars at a strain rate of 1 percent per minute (Handin et al., 1955; Lewis et al., 1956). The crystals exposed to 17 megaroentgens of gamma radiation and then deformed with the great- est principal stress paralleling the c-axis, resulting in translation gliding on r J 1011 J , changed in color from amber to blue and exhibited absorption maxima centering around 26000 cm" 1 and 17000 cm -1 _. With the least principal stress paralleling the c-axis (twin gliding on e [0112J ) no color change resulted. Sam- ples deformed by translation gliding show a new thermoluminescence peak, whereas the intensity ratio of the two highest-temperature peaks is altered after twin gliding. Lewis (1956) considered it impossible at present to separate the effects of impurities from those of defect trapping centers for the 120°C thermoluminescence peak, but felt that the work on deformed materials favors a relation between the 240 °C peak and the defect trapping centers. Specific wavelengths of luminescent radiation have been attributed to par- ticular impurity ions in carbonates (see Graf and Lamar, 1955), but the actual mechanisms rarely have been determined. CARBONATE SEDIMENTS EOLIAN SEDIMENTS A sample of the dust carried over the Persian Gulf by prevailing winds from the northwest contained "83 percent calcite" (apparently the weight loss in HC1) (Emery, 1956). This sample is exceptional, however, for the maximum recorded carbonate contents in loess appear to be between 40 and 45 percent (Lamar and Willman, 1934; Leighton and Willman, 1950; Fisk, 1951; Swineford and Frye, 1955). Newell and Boyd (1955) described a very coarse, eolian lag concentrate from the lea Desert of Peru that locally consists of fragments of Eocene mollusks derived from nearby outcrops, the finer fractions having been winnowed out by the wind. Shell dunes on the Sonoran shore (Ives, 1959) have been similarly produced from recently elevated bottom marls. A section of foreset beds in one of the dunes of the Great Salt Lake Desert, Utah, consists of alternating 1-inch layers of granule- sized algal fragments and thick layers of gypsum arenite (Jones, 1953). Repeated intervals of maximum wind velocity are indicated. SOILS A few rudimentary soils (included here for convenience) are known, in areas underlain by limestones, that still contain at least 50 percent carbonates. Analyses of the Hikutavake rocky silt loam from Niue Island in the South Pacific (Schofield, 1959) and its parent limestone are given as analyses 11 and 12 (see Part IV). In- cipient soils in the northern Marshall Islands are calcareous sands containing as much as 32 percent organic matter (Fosberg, 1954). Yaalon (1954) mentioned moun- tain marl soils from the Galilean Hills of Israel that contain more than 50 percent carbonates and are very low in clay minerals. The Galilean soils are really only physically disintegrated, friable limestone. The Lisan marl soils of the upper Jordan Valley are fairly deep and contain 20 to 50 percent carbonates. GLACIAL DEPOSITS Carbonate tills are formed when continental glaciers traverse areas of car- bonate bedrock and subsequently deposit their loads of carbonate sediment before it has been unduly diluted by fragments of noncarbonate rock. Dreimanis (1957) stated that glacial drift in Ontario contains as much as 95 percent carbonate in places. The carbonate content of several glacially derived materials from Ontario, at least two of which would appear to have been redistributed by water after glacial deposition, are given below in table 1 . Lamar and Willman (1934) listed three tills from Illinois that contain 22.41, 25.81, and 35.4 percent CO,. Analyses of the Mankato till in Manitoba (Ehrlich and Rice, 1955) showed up to 65 percent CaC03, and a sample of till from the Iowan drift sheet in southwestern Minnesota lost 50.2 percent weight on treatment with cold dilute HC1 (Kruger, 1937). Most pebble counts of tills reported in the literature that show a majority of carbonate pebbles have not been included in table 1 because one cannot be certain that other particle- size ranges have the same carbonate content. The calcareous content of Wisconsinan till in the Upper Whitewater Basin, Indiana-Ohio, for instance, drops from 70 percent in coarse sand and gravel to 25 percent in medium and fine sand, 40 percent in very fine sand, 30 percent in silt, and 15 percent in [15] 16 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 297 Table 1. - Carbonate Content of Some Glacial Tills in Canada and the United States A - from Dreimanis (1957) Total percent carbonates Probable age of deposit (years) Remarks (Glacial) Lake Algonquin beach, Orilla, Ontario Pebbly gravel, Fanshawe, Ontario (Glacial) Lake Warren beach barrier, Union, Ontario Mitchell moraine, Elginsfield, Ontario, with 90% carbonate pebbles 70 8,000-9,000 80 14,000 50 9,600-12,000 65 No dolomite pebbles More limestone than dolomite pebbles More limestone than dolomite pebbles More limestone than dolomite pebbles B - from Anderson (1957) Sand grains (%) Limestone Dolomite Pebbles (%) Limestone Dolomite Erie lobe (Wisconsinan Stage), 36 northeast Indiana Erie lobe (Wisconsinan Stage), 23 northeast Indiana Lake Michigan lobe (Wisconsinan 4 Stage), Rockdale moraine, northeast Illinois Green Bay lobe (Wisconsinan Stage), 1 Valders moraine, southeast Wisconsin Green Bay lobe (Wisconsinan Stage), Rush Lake moraine, southeast Wisconsin 17 28 63 60 81 50 30 2 13 41 75 93 90 clay (Thorp et al., 1957). Several of Anderson's (1957) samples, for which particle counts are given for both sand and pebbles, are shown in table 1 . The lesser car- bonate content of the sand sizes compared with that of the pebbles in Anderson's study results, at least in part, from a concentration of the Precambrian quartz and feldspar grains in the sand-size range. The Trenton and Black River Limestones of Ordovician age under Lake Erie furnished the limestone particles in the Erie lobe samples. The Niagaran age (Silurian) dolomite and the dolomites in the Prairie du Chien Group and the Galena Formation of Ordovician age were the sources of the dolomite particles in the Lake Michigan lobe and Green Bay lobe samples. STREAM DEPOSITS Stow (1930) described pea- to egg-sized, layered CaC03 concretions, of uncertain derivation but apparently of inorganic origin, which occur near Lexington, Virginia, in streams draining limestone terrain but not in those draining sandstone or Precambrian crystallines. Stream-borne carbonate particles are reduced in size and number because they are susceptibile both to abrasion and breakage and to solution. Thus Plumley (1948) found that the percentage of limestone plus sandstone pebbles 16 to 32 mm in size was reduced 90 percent in a distance of 30 miles along Rapid Creek, South Dakota . GEOCHEMISTRY OF SEDIMENTARY CARBONATES - I 17 Along the Virginia and Maryland streams examined by Hack (1957), coarse limestone boulders enter the stream from undercut cliffy slopes and remain near their source as a lag concentrate. Breakage and abrasion reduce the boulders, and pebbles in the bedload persist for five miles or less downstream, depending on the physical properties of the limestone. Thus there is only a very short interval from a given outcrop in which fluvial carbonate sediment can persist. During the summer calcareous tufa forms on the stream bed at riffles in Middle River, so at that time solution does not appear to be even a minor factor. Limestone pebbles persist, however, for much greater distances in the Colorado River of Texas than in the streams mentioned above. Sneed and Folk (1958) found that 68 percent of the peb- bles at the farthest downstream limestone outcrop were limestone, 30 miles from the outcrop 50 percent were limestone, 54 miles from the outcrop, 40 percent, 176 miles, 3 percent. At points sufficiently close to sources of abundant carbonate detritus, such as carbonate outcrops in arid regions or areas of high relief, stream deposits may be actual carbonate sediments. One probable example is a portion of the Colorado River in Texas, described by Sidwell and Cole (1937), along which the pebbles and boulders are primarily limestone. Udden (1914) described gravels of Pleistocene age along the lower Rio Grande River in Texas that consist of white limestone derived from the Pennsylvanian outcrops found in the westernmost part of the state. He described similar gravels now being formed in the wide channels of Tequesquite and other creeks that are tributaries to rivers draining the Edwards Plateau. When flooded, these creeks roll broad sheets of limestone gravel several hundred feet wide. The Molasse beds of southern Germany contain from 25 to as much as 50 per- cent detrital dolomite-rock grains, which become smaller in size away from the Alps, and some 25 percent calcite (Hans Fiichtbauer, personal communication) . The Alpine fans contain up to 80 percent limestone and dolomite, much of it in large, only slightly degraded blocks. By the time rivers that have attained maturity or old age in their lower courses reach the ocean, however, they are transporting practically no carbonate. A com- posite of analyses of Mississippi River silt at the Mississippi delta shows only 1.40 percent CO2 (Clarke, 1924). The fluvial solids contributed to marine carbonate sediments are therefore almost entirely noncarbonate . FRESHWATER LAKE SEDIMENTS The mud in the deep central part of Lake Zurich consists of alternate laminae rich in microgranular carbonates and in organic matter. Nipkow (1927) noted that each summer his plankton nets were clogged by particles of CaCO-, precipitated from the surface waters of the lake. He correlated variations in the diatom size distribution, mud content, and thickness of the top layers of the cores with shore slumps he had observed and with diatom size distributions obtained from eight years of surface collections. The laminae are therefore nonglacial varves. Glacio-lacustrine sediments in northeastern Wisconsin contain appreciable dolomite rock flour believed by Thwaites (1943) to have been mechanically trans- ported from the parent till. Ellsworth and Wilgus (1930) found as much as 25 per- cent of this material in summer layers and as much as 50 percent in winter layers. Soft, argillaceous, CaCOo-rich deposits (marls) that locally are rich in shells occur as postglacial freshwater deposits from western New York through the Middle West in many swamps and lake basins. In their study, Blatchley and Ashley (1901) observed that these deposits are found only in those Indiana lakes occurring in drift areas (see also Stout, 1940), and that the thickest deposits 18 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 297 are in areas where the drift is thickest. Similarly, Thiel (1930, 1933) concluded that the marls are best developed where the basins are surrounded by high morainic hills of coarse-textured outwash gravel sufficiently porous to be effectively leached of carbonate by ground water. This is so even though the clayey-textured moraines have a higher percentage of CaC03 than the sandy-textured ones, and even though the ground water in the former contains an average of 138 ppm CaC03 compared with an average of 59 ppm in the latter. Thiel noted that in chains of deep lakes marl may occur in any of them, but in chains of shallow lakes the greatest amount of marl is in lakes toward the head of the chain. Such lakes gradually become filled with marl until vegetation is able to gain a foothold and a swamp is formed (Smith, 1916; Davis, 1901). Deposits of marl underlying peat deposits in Ohio may be several feet thick and may cover several square miles (Dachnowski, 1912). Teichmuller (1954) described a Swabian peat deposit located in a closed basin between an older (Riss), carbonate-poor moraine and a younger (Wiirm), carbonate-rich one; the freshwater limestones inter- calated in the peat occur only in the part of the basin close to the Wiirm moraine. Lundqvist (1936) showed on a map of Sweden the lakes in which calcareous sediments occur, and Thunmark (1937) indicated that in southern Sweden such cal- careous lacustrine deposits coincide either with areas rich in outcrops of carbonate rock or with areas immediately south of the outcrops in which glacial deposits are rich in carbonate. Marl deposits in British Columbia commonly are found in lakes of the interior where the climate is drier, and in most cases such deposits are near limestone outcrop areas (Mathews and McCammon, 1957). A marked decalcification of the water of the River Susaa is observed after it has passed through two Danish lakes, Tystrop So and Bavelse So, which have sed- iments containing as much as 75 percent calcium carbonate (Berg, 1943; Hansen, 1959). The "marl islands" that rise almost to the surface of Tippecanoe and Winona Lakes in Indiana lie atop knobs or swells of the original bottom (Wilson, 1936, 1938) The thickest marl deposits are in the deep parts of the lakes and the thinnest are on steep slopes and in the zone of wave action near shore. A comparison of sediment volumes and water areas in various parts of Tippecanoe Lake shows that erosion of the sediments forming in the shallows is greater at the leeward end of the lake where the wave action is more vigorous than at the other end of the lake. In the lakes that they studied, Blatchley and Ashley (1901) found a correla- tion in thickness of marl deposits with nearness to spring outlets in the lakes, a relation they attributed to CO2 loss from the spring water as it warmed. In central Pope County, Minnesota, where an impervious till seals porous sands and gravels containing water under hydrostatic pressure, marl deposition is extensive around springs at points where lake basins have cut below the till (Thiel, 1933). More often, however, marl deposition is limited to the shallow waters of the lakes, as was the case for the lakes that Davis (1903) examined. The fine- grained CaC03 deposit in Lake NeuchStel is also thickest in the shallowest waters (Portner, 1951). Kindle (1927, 1929) found lacustrine marls limited to relatively small areas that were shallow and protected from wave action in lakes where altitude and latitude did not keep the temperature of the epilimnion zone too low for the de- velopment of Chara, Potamogeton, and other plants of widely different types that extract CO2 • Higher water temperatures in the shallows aid deposition of CaC03 by decreasing both the solubility of CaC03 and of C0 2 . The waters of the cold, deep zone are rich in CO2 and would dissolve carbonate. GEOCHEMISTRY OF SEDIMENTARY CARBONATES - I 19 Several species of the Charae, nonmarine plants known since early Devonian times, contribute to freshwater limestones and marls by secreting CaCO^ from the vegetative cells and within the spiral enveloping cells of the oogonia (Peck, 1957). Davis (1901) found an appreciable amount of calcium succinate in the cell sap of Chara. The CaC03 deposited in the vegetative parts frequently, but not always, disintegrates when the plant dies, and it is the small spiral calcified parts of the oogonia that are recognized in nonmarine limestones that often contain few other fossils. The numerous white, calcareous cakes from 1 to 5 cm in diameter, described by Clarke (1900) from the north shore and nearby bottom of Canandaigua Lake, New York, are made up of concentric layers of travertine, but a soft, spongy, organic re- siduum of the same volume as the original cake remains after treatment with dilute acid. Roddy (1915) and Howe (1932) described similar cakes from streams. Examin- ation of residues and thin sections reveals a mixture of minute plants — diatoms, unicellular and filamentous Myxophyceae (Cyanophyta) , and unicellular Chloro- phyceae. The fine-grained, spongy, arborescent, carbonate masses described by Bradley (1929a) from the shores of Green Lake, New York, were attributed by Rezak (1957) to CaCOo precipitation as a result of algal photosynthesis. Mawson, cited in Twenhofel (1932, p. 312), has described "algal biscuits" from shallow inter- dunal lakes underlain by limestone in South Australia. The marl deposits in Littlefield Lake, Isabella County, Michigan, as des- cribed by Davis (1900, 1901) were 25 to 30 feet thick near shore but thinned mark- edly as the water deepened. The surface of the shallow bottom was covered with growing Chara, and the beaches and upper sediment layers consisted largely of brittle, fragile remains of Chara that broke into fragments at a touch, and pebbles that showed bluish green radiating lines of filamentous algae (Zonotrichta, Schlzo- thrix) when broken open. Marl deposits found in sheltered embayments and marginal areas of five lakes in the Knik Arm area of Alaska contain from 12 to 77 percent CaCOo, depend- ing on the amount of clastic material and organic matter admixed (Moxham and Eck- hart, 1956). The marl completely fills some embayments and has been covered by a layer of muskeg. Chara and possibly some blue-green algae are responsible for the deposit in Edlund Lake. The freshwater lake sediments in Wisconsin contain (Twenhofel and McKel- vey, 1941) proteins, cellulose, lignin, fats, waxes, gums, and resins contributed principally by water plants, together with terrigenous detritus, siliceous diatom tests, shells of freshwater gastropods, and microscopic CaC03 crystals. The gastropods are a significant contribution in only one of the lakes these authors examined. The black sludge in Lake Monona beyond a belt of muddy shore sands contains 8 to 48 percent CaC03 (typically 38 percent) and overlies firm, light colored sediments containing from 51 to 83 percent CaC03 (typically 65 percent), 0.47 to 2 percent MgO, 0.5 to 4.1 percent A1 2 3 , 0.5 to 1.8 percent Fe as Fe 2 0,, and organic matter, clay, and silica present as detrital quartz grains and diatom tests. This sediment would correspond to a generally fine-grained, siliceous, slightly argillaceous limestone. The comparable material in the deeper parts of thermally stratified Lake Mendota (Twenhofel, 1933) contains 60 to 80 percent CaC0 3 , 13 to 25 percent Si0 2 , 0.5 to 0.8 percent Fe as Fe 2 3 , 0. 3 to 3. 3 percent A1 2 3 , and 1 .3 to 5 . 6 percent MgO. Murray (1956) reported an average of 32.2 percent carbonate in the Lake Mendota sludge, 62.7 percent in the underlying marl, and 54 percent in the gray to dark gray marl that it appears is still being deposited in Lake Wingra. 20 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 297 A blue plastic marl containing an average of 77 percent carbonate and a pink clay that contains clastic dolomite pebbles and may have up to 43.6 percent carbonate also are found in Lake Mendota. The percentage of P 2 O s in the Lake Mendota cores, without regard for lithologic type of sediment, ranges from 0.168 to 0.275 percent, averaging 0.20 percent for 15 determinations. Twenhofel stated that some 30 feet of marl has accumulated in hard-water Lake Mendota since the departure of the glaciers,in contrast to as little as 3 feet in soft-water lakes. Murray (1956) found a sharp interface between the sludge and underlying marl in Lake Mendota, with no evidence of diagenetic chemical changes. The total organic content is only slightly different on the two sides of the interface — 12.4 versus 13.2 percent — but the sludge contains more clastic material than does the marl. The black color in the marl results from authigenic ferrous sulfide, whose presence is inferred from the presence of formed by the oxida- tion of organic matter,is swept away. Pratje (1939) found that clay fines had been removed from ridges in the South Atlantic by currents of intermediate depth, leaving calcareous oozes on the ridges as far as 35° south latitude. Bramlette and Bradley (1940) noted that cores from topographic highs in the North Atlantic are more than usually stained by hydrous iron oxide and manganese oxide and contain no pyrite, indicating an oxidizing environment. Trask et al. (1942) attributed the low content of organic matter, only 0.2 to 0.3 percent, in the coarse, carbonate-rich sediments from these ridges to current winnowing of the buoyant organic material. Ericson et al . (1955) published underwater photographs of ripple marks on these rises, and isotopic data that show accumulation rates on the rises are much lower than those in deep water. The leeward concentration of fine-grained material swept off the Muir seamount (33°42' N, 62° 30' W) can be identified in cores (Ericson and Heezen, 1959); on the basis of limited information available from two cores, it appears that current direction, and the consequent direction of sediment drifting, shifted with climatic change. Turbidity current deposits have been described (Ericson et al., 1952, 1955) in a number of cores from depths of more than 4000 fathoms on the flat abyssal plain of the Puerto Rico Trench and from more than 220 fathoms off the Bermuda Islands. The cores contain graded layers increasing in particle size and CaCOg content downward, in one case from 36 percent to 76 percent carbonate, and con- taining shallow-water foraminifera, particles of Halimeda, coarse vegetal detritus, and certain species of clams that must have lived in very shallow water. Absence of the occasional small teeth and rare benthonic foraminifera usually found in deep- sea clays indicates rapid deposition. These authors noted that the frequency of turbidity current deposits encountered in Atlantic cores is higher in sediments rep- resenting the rapid sedimentation of the glacial stages than in either older or younger deposits. The carbonate content of the Globiierina oozes examined in North Atlantic deep-sea cores (Bramlette and Bradley, 1940) ranged from 46.8 to 90.3 wt percent, averaging 68.2 percent. Ericson and Wollin (1956) obtained CO2 values equivalent to from 43.7 to 77.0 percent CaCOo in the foraminiferal lutites of cores A179-4 (off the southwest coast of Haiti) and A180-73 (midway between Brazil and Africa on the gently sloping flank of the mid-Atlantic Ridge). Both cores consisted of materials that appeared to have been deposited particle by particle. Biogenic carbonate in these samples is diluted by fine-grained terrigenous sediment from Africa and South America (see analysis 27, Part IV). Oxygen-isotope determinations of paleotemperatures (Emiliani, 1955a), micropaleontological study of the vertical distribution of cold- and warm- water species (Ericson and Wollin, 1956b), and radiocarbon dating (Ericson et al., 1956) of core A180-74 from an equatorial portion of the mid-Atlantic Ridge establish that a rather abrupt change to a warmer climate took place about 11. 000 years ago. Broecker et al. (1958) made carbonate analyses and further C measurements on less-than-74-micron size fractions from the same core and found that neither clay GEOCHEMISTRY OF SEDIMENTARY CARBONATES - I 37 nor carbonate deposition had been constant over the time span considered, and that the rate of deposition of both decreased markedly about 11, 000 years ago. Core A172-6 from off Haiti was studied by Yalkovsky (1957) who found no correlation between paleotemperature and the rate of carbonate deposition. Wangersky (1959) recently examined all four of the CaCOg-rich cores de- scribed above, A180-73, A180-74, A172-6, and A179-4. No apparent correlation was found between total percent CaCOg and paleotemperature, but the amount of fine-grained coccolith carbonate shows a negative correlation and the coarser foraminiferal carbonate a positive correlation with paleotemperature. Mg is inversely correlated with paleotemperature (correlation coefficient -0.61, significant at the 0.05 percent level), and a consideration of the SiO, analyses suggests that Mg is deposited chiefly in clays during interglacial periods and principally in carbonate, probably coccoliths, during the glacial periods. The calculated Mg carbonate for 31 samples from North Atlantic deep-sea cores averages only 2.19 percent (Bramlette and Bradley, 1940). In only a few of the 20 precision analyses made was the Ca present insufficient to combine with all the CO2 found, which suggests the presence of dolomite. Limestone and dolomitic limestone pebbles and granules are the most common rock types in the glacial marine deposits, have the least total carbonate (well under 50 percent), and contain the most MgO. Two fractions of almost pure foraminiferal shells from the North Atlantic cores (Bramlette and Bradley, 1940) analyzed by J. J. Fahey contained less than 0.01 percent S0 3 . Phosphate nodules associated with greensand have been dredged from the Agulhas Bank, south of Africa, at depths of 400 to 1500 fathoms (Twenhofel, 1932). Pacific Ocean Bottom water introduced into the Pacific Ocean from the high southern lati- tudes contains 5.0 ml/1 dissolved O2 and has an in situ temperature of 0.9°C (Wooster and Volkmann, 1960). Property changes indicate a northward drift, and the deep water of the eastern Pacific, which has 3.4 ml/1 dissolved O2 and an in situ temperature of 1.6°C, is the most modified and presumably the oldest. Morita and ZoBell (1955) reported that red clays and Glob tier ina-ooze samples from cores in the open ocean are oxidizing at all core depths and at most depths contain an apparent preponderance of aerobic bacteria over anaerobic ones. This oxidizing environment is in direct contrast to the markedly reducing conditions that prevail in nearshore sediments of high organic content. Calcareous oozes are extensive in the South Pacific but generally at less depth than in the Atlantic. Pacific pelagic samples contain 50 percent or more CaCOg above about 3500 meters; Atlantic samples reach that level above about 4900 meters (Revelle, 1944). Revelle et al. (1955) suggested that the difference may be related to differing rates of carbonate accumulation and thus differing periods available for corrosion, or to differing amounts of CO? derived from decomposing organic matter (Revelle, 1944). The larger Pacific Ocean has fewer large rivers and a smaller area of river drainage. Goldberg and Arrhenius (1958) noted that the con- centration ranges observed for Mn, Cu, Ni, Co, Zn, Pb, Fe, and Ti in Atlantic pelagic sediments are lower than those for Pacific pelagic deposits. They assumed that the rate of deposition of marine deposits is inversely related to Mn content and thus concluded that accumulation rates are higher in the Atlantic. The region of more than 75 percent carbonate content in the surface sedi- ments of the east Pacific lies just south of the Equator but swings north at its 38 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 297 eastward end (Arrhenius, 1952). This distribution parallels the surface current pat- tern, for the equatorial countercurrent sends a branch northeastward and dissolves in a series of vortices as it passes over the East Pacific Ridge. Upswelling in the latter area results in high organic productivity, a plentiful supply of organic matter for the sediment, far-reaching dissolution of foraminiferal tests, and a consequently lowered CaCO, content in the sediment. The rate at which Ti settles out in the east Pacific eupelagic area was found by Arrhenius (1952) and Arrhenius and Blomqvist (1956) to be practically constant. The C/N ratio of marine organic matter there is lower in clayey sediments than in highly calcareous ones, and organic N content is higher. A gradual decrease of organic N downward in cores that have essentially constant organic C contents indicates secondary decomposition of organic matter. The unusually high Mn content of sediments of the East Pacific Ridge, com- pared with those of the eupelagic area to the west, indicates (Arrhenius, 1952) that Mn was introduced there by submarine weathering as well as from land. The P con- tent on the ridge is unusually high, and the corrosion and peptization of radiolaria and diatoms is much more severe than in the eupelagic area. Some laminae on the ridge are strongly cemented by silica, indicating silica migration within the sedi- ment. Such cementation does not occur in the eupelagic area, and its occurrence on the ridge may result from rising interstitial solutions heated at lower depths. Revelle et al. (1955) showed that there are several types of CaCO- variation with depth in the Pacific: continuous high CaCOg, high surface CaC03 and lower CaC03 with depth, and low surface CaC03 and higher CaCO„ with depth. Increased oceanic circulation during the Ice Age must have furnished a more abundant nutrient supply for plankton growth, but it also must have enhanced the dissolution of cal- careous plankton tests settling to the ocean bottom. Thus the percentage of carbon- ate in the pelagic sediments at a given locality depends upon the relative importance of the two factors (Arrhenius, 1954), assuming that contributions of detrital minerals and minerals formed inorganically in the ocean remain essentially constant. Siliceous skeletal remains commonly form more than 30 percent of Pacific calcareous oozes, a distinctly higher figure than is found for the Atlantic (see Correns, 1950). Goldberg and Arrhenius (1958) found a steep gradient in the silicon content of near-bottom water at all of the South Pacific stations they investigated, indicating a flow of dissolved silica from the bottom sediments back into the ocean. The contribution of eolian quartz from large arid continental areas is marked in pe- lagic sediments of the Pacific at lower and middle latitudes in the Northern Hemi- sphere, except in the region of the Hawaiian Islands where it is outweighed by an influx of basaltic pyrocla sties (Goldberg and Arrhenius, 1958; Hamilton, 1957). The particle size range most frequently observed for the quartz shards and chips, 1 to 20 microns, is reasonable for eolian transport, as is the observed distribution pattern of the quartz when it is considered with regard to atmospheric wind fluxes and the location of arid regions on the continents (Rex and Goldberg, 1958). Globiierina ooze of Eocene age on Sylvania Guyot, the seamount adjacent to Bikini'Atoll in the Marshall Islands,and on Horizon Guyot 1500 miles to the east has been extensively phosphatized and encrusted with Mn oxides, indicating a marked diminution in the rate of sediment accumulation (Hamilton and Rex, 1959). Areas of turbidity-current deposited graded beds occur in the bottoms of trenches off the northwest coast of North America. GEOCHEMISTRY OF SEDIMENTARY CARBONATES - I 39 HYPERSALINE LAGOONS The northern portions of the Laguna Madre, a bay along the semiarid Texas coast separated from the open Gulf of Mexico by a wide barrier island, have a salinity about twice that of the Gulf (Rusnak and Shepard, 1957) . Gypsum is pre- cipitating on shallow flats, and aragonitic oolites are forming where waves break on the inner shore of the bay. Oyster reefs thrive in brackish water areas of the Laguna Madre. The waters of the long, narrow Bocana de Virrila inlet on the arid Peruvian coast contain progressively less Ca ++ and HCOo - and become steadily more saline toward the head of the inlet. The "white marls" being deposited in the seaward part of the inlet are presumably carbonates, as they are succeeded by gypsum and then by halite deposits lying under higher salinity water farther from the sea (see Morris and Dickey, 1957). Illinois State Geological Survey Circular 297 39 p., 1 fig., 3 tables, 1960 tiinnznzzi CIRCULAR 297 ILLINOIS STATE GEOLOGICAL SURVEY URBANA