"Paleoclimate and Mineral Deposits."

GEOLOGICAL SURVEY CIRCULAR 822

Paleoclimate and Mineral Deposits Paleoclimate and Mineral Deposits

Edited by Thomas M. Cronin, William F. Cannon, and Richard Z. Poore

GEOLOGICAL SURVEY CIRCULAR - 8 2 2

1983 United States Department of the Interior JAMES G. WATT, Secretary

Geological Survey Dallas L. Peck. Director

Library of Congress Cataloging in Publication Data Main entry under title Paleoclimate and mineral deposits Geological Survey Circular: 221 Bibliography: p. . 1. Paleoclimatology-United States. 2. Mines and mineral resources-United States. 1. Cronin. Thomas M. Cannon. William F. Poore. Richard Z. IV. Series

Free on application to Distribution Branch, Text Products Section, U. S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 PREFACE

[COULD NOT BE CONVERTED TO SEARCHABLE TEXT] Thomas M. Cronin William F. Cannon Richard Z. Poore Editors

III CONTENTS Page

Preface

Continental aluminous eathering sequences and their climatic implications in the United States, by Harry A. Tourtelot

Effect of sea-level fluctuations on porosity and mineralogic changes in coastal aquifers, by William Back and Bruce B. Hanshaw------6

Pliocene and Pleistocene calcitic veins as indicators of paleohydrologic, paleoclimatologic, and neotectonic events, southern Great Basin: An initial appraisal, by Isaac J. Winograd, Barney Szabo, Tyler B. Coplen, and Gene C. Doty 8

The oxygen minimum zone of continental margins, by Thomas M. Cronin------10

Cenozoic paleoceanography. by Richard Z. Poore------13

The U.S. Atlantic Contine Margin: Eustasy, paleoceanography, and mineral deDosits, by John S. Schlee

Building biostratigraphic framework around an anoxic-kenoxic event, by Joseph E. Hazel------18

Inorganic geochemistry of deep-sea black shales, by Walter E. Dean

Some climatic and oceanographic controls on the time and place of mineralization, by W. F. annon------

Influences of ocean anoxic events on manganese deposition and ophiolite-hosted sulfide reservation, by Eric R. Force, . F. Cannon, Randolph A. Koski, Keith T. Passmore, and Bruce R. De

Late Cretaceous and Paleogene nonmarine climates in North Aerica, by Jack A. Wolfe

Volcanic manganese deposits in the Western Cordillera: Lithologic associations and Daleoceanographic settings and economic deposits associated with biogenic siliceous rocks, by James R. Hein and Randolph A. Koski

Organic carbon, sulfur, and iron relationships as an aid to understanding depositional environments and syngenetic metals in recent and ancient sediments, by Joel S. Leventhal

Paleozoic atmospheric circulation and oceanic upwelling, by Judith Totman Parrish

Paleoclimatic controls on the occurrence and quality of coal, by C. B. Cecil, R. W. Stanton, S. G. Neuzil, F. T. Dulong, and L. F. Ruppert------40

V ILLUSTRATIONS

Page

FIGRE 1. Map showing selected aluminous weathering sequences in the United States------

2. Graph showing inferred temperatures for Tertiary time

3. Diagram showing interface between freshwater and seawater flow systems that forms zone of dispersion------

4. Graph showing upward concavity of activity coefficient as a function of ionic strength

5-7. Diagrams showing:

5. Diagenetic processes known to occur in mixing zone

6. Hypothesis that some manganese ores may be deposited in zone of dispersion

7. Characteristics of oxygen minimum zones 8. Chart showing Tertiary sea levels and climatic trends

9. Graph showing observed range and geometric mean of concentrations of major-element oxides and trace elements in samples of red and green claystone and black shale from DSDP Hole 530A

10. Plots showing carbonate-free concentrations of certain elements versus depth in samples of black shale from DSDP Site 530A

11. Diagrams showing how certain processes may contribute to accumulation and redeposition of sediments containing various amounts of organic carbon------

12-13. Graphs showing:

12. Relative rates of accumulation of marine sedimentary deposits of manganese, iron, and phosphate on a worldwide scale

13. Tons of iron in known deposits of marine sedimentary type as a function of time in comparison with accumulations of crude oil in source rocks as a function of time

14. Profile showing dissolved manganese and depositional model for anoxic event

15. Eh-pH diagram showing oxidation path of anoxic water mass

VI 16. Stratigraphic oolumn showing compositeshallowmarinemanganese deposit ------27

17. Chartshowingagecorrespondenceofoceananoxicevents,shallow marinemanganesedeposits,andophioliticsulfides------28

19. Diagramcomparingaxialridgeventsduringandafteranoxic events------28

19. StratigraphicrecordofwaninganoxiceventatCyprus------29

20. Graph showing relationship of carbon to sulfur in Black Sea marine sediments deposited below oxygenated water and H2S- containing water… 35

21. Diagram showing geochemical relationships in oxic. noxic, and euxinic sedimentary environments 36

22. Map showing atmospheric circulation in the Late Cambrian northern winter 39

23. Inferred paleoclimate changes and generalized stratigraphic column from the Upper Mississippian to the Lower Permian central Appalachian Basin 42

24. Chart showing distribution pattern for values of H S and sulfide minerals when sulfate percent is reduced by various mechanisms------45

25. Map of North America showing contours of approximate average D values of meteoric water------46

26. Graph showing behavior of D and &18O in meteoric water------47

27. Plot of D of fluid inclusions versus calculated values of hydrothermal fluids for various epithermal ore deposits 48

VII PALEOCLIMATE AND MINERAL DEPOSITS

CONTINENTAL ALUMINOUS WEATHERING SEQUENCES AND THEIR CLIMATIC IMPLICATIONS IN THE UNITED STATES

By Harry A. Tourtelot

Introduction highest on the dome underwent further weathering in Pennsylvanian time to the extent that pure kaolinite. Weathering sequences in the geologic record are along with much diaspore and boehmite, was formed. characterized by accumulations of minerals that form Clays in somewhat topographically lower sinkholes chiefly by weathering processes. Weathering processes were not as intensely altered after deposition, and result in the mechanical disintegration of rocks and clays in beds in equivalent sedimentary sequences still the chemical alteration of minerals at and near the lower were hardly altered at all. These Missouri clays surface of the Earth. eathering goes on all the time, are contemporaneous with other refractory clays of and the relative intensities and effectiveness of Early Pennsylvanian age in Kentucky (Patterson and mechanical and chemical processes are controlled in Hosterman, 1962), in Pennsylvania (Bolger and Weitz. large part by climate as well as other variables. Both 1952; Foose, 1944), and elsewhere in the eastern mechanical and chemical weathering lead to the midcontinent region (Keller, 1975). primary formation of mineral resources and soils. Patterson (1981) provides an excellent review of Sequences of Cretaceous age weathering processes and some resulting mineral deposits. Bauxite, coal, and evaporites are regarded in In Colorado, kaolinitic clays in the Dakota themselves as being indicators of warm climates Formation were eroded from a weathered hinterland (Frakes, 1979: Habicht, 1979). This paper is concerned and incorporated in fluvio-deltaic sediments in basal with aluminous mineral resources. the circumstances Cretaceous strata in the Pueblo-Canyon City area of their origin in relation to climate, and their (Waage, 1953) and in the Golden area (Waage, 1961). implications for some other mineral resources. In the Golden area, volcanic ash beds in marine strata equivalent to the fluvio-deltaic rocks consist of Weathering sequences smectite and can be traced into the fluvio-deltaic sequence where the ash beds consist of kaolinite About 20 high-alumina clay deposits, which the (Waage, 1961, p. 25). Postdepositional processes thus present author calls weathering sequences, are may be responsible for the formation of some of the discussed by Patterson (1967, p. 131-151) as possible kaolinite in the thick nonvolcanic clay beds. aluminum resources. Six of these sequences as well as At nearly the same time, a kaolinite-rich a residuum and two paleosols not included by Patterson residuum was being formed on Precambrian and some (1967) as otential aluminum resources will be Paleozoic rocks in the western part of the central described briefly in order of geologic age. General craton in Minnesota (Austin, 1970; Parham, 1970), in locations are shown in figure 1. North Dakota (Moore, 1979), and in South Dakota (Ludvigson and others, 1981). At some places, Cheltenham clays of Pennsylvanian age kaolinite was eroded from the residuum and concentrated in sedimentary rocks of Cretaceous age The refractory clays of Missouri, grouped under where further weathering in Cretaceous time formed the name Cheltenham," are interpreted by Keller minor amounts of gibbsite and boehmite (Parham, (1968) (see also Keller and others, 1954) to represent 1970, p. 56-60). the influx of, kaolinite-rich weathering products into the early Pennsylvanian depositional system. Some of Sequences of Cretaceous-Eocene age these products were trapped in sinkholes on the northern flank of the Ozark Dome, and some were Cretaceous to middle Eocene.-A weathering concentrated in strata of Pennsylvanian age flanking sequence in central Georgia and western South the Ozarks on the north. The clays in the sinkholes. Carolina is implied by derived deposits of Late [COULD NOT BE CONVERTED TO SEARCHABLETEXT]

FIGURE 1.-Selected aluminous weathering sequences in the United States. Ch, Cheltenham clays, contemporaneous with similar clays in Kentucky (OH, Olive Hill) and Pennsylvania (M, Mercer clays). C, Colorado clays. Mi, Minnesota residuum. G, Georgia-South Carolina kaolin district. SEB, Southeastern bauxite deposits and related occurrences of kaolin, together smbolized by dark patterns, attributed to the same weathering sequence. SC, Southern California paleosoi. NGP, Northern Great Plains paleosol. SL, SDokane-Latah clays. OW, Oregon-Washington ferruginous bauxite. S, saprolite or soil that contains gibbsite.

Cretaceous and middle Eocene ages. It has provided bauxite deposits in the southeastern United States the largest kaolin resources in the country. These (Bridge, 1950; Overstreet, 1964). aolinized bedrock strata have long been regarded as being entirely of in Massachusetts (Kaye. 1967) and clay deposits in Cretaceous age (Lang and others, 1965, p. J4), and the central Pennsylvania (Hosterman, 1972) also are complexity of their depositional history is shown by attributed to this weathering sequence. These Techudy and Patterson (1975) as well as others (Hurst, widespread occurrences may constitute an example of 1979). The kaolinite was eroded from a weathered a widespread system by which aluminum and perhaps hinterland developed on pre-Cretaceous rocks other mineral resources are formed. generally to the northwest of the present outcrop of Residual bauxite deposits cap partly buried hills Cretaceous and younger strata. Some of the resulting of svenite in Arkansas (Gordon and others, 1958, p. deposits are in nonmarine strata of Cretaceous age, 136) and were the source for transported kaolinite and and others are in nonmarine strata of middle Eocene gibbsite in the overlapping strata of late Paleocene age. A marine sequence separates the two nonmarine and early Eocene age. Residual deposits formed in sequences and records both Paleocene and Eocene early Tertiary time may also be represented by time, but the record is very compressed and different kaolinitic and bauxitic weathered strata of presumed from that of other parts of the Gulf Coastal Plain. Cretaceous age in Alabama (Bergquist and Overstreet, Local surface weathering in Cretaceous time produced 1965) and in western Georgia (White, 1965). minor amounts of gibbsite in some of the kaolin A weathered hinterland on Paleozoic and older deposits of Cretaceous age. Postdepositional rocks in the Appalachian region is not directly alteration of the centers of kaolinite lenses of middle recognizable but is implied by deposits of clay trapped Eocene age has produced pisolitic clay that contains in sinkholes formed in rocks of Paleozoic age and clay small amounts of gibbsite and that grades in all incorporated in strata of Eocene age. Sinkhole directions into the kaolin (Hurst, 1979). deposits in Virginia (Warren and others, 1965), Paleocene-Eocene age.-A weathering sequence Tennessee (Dunlap and others, 1965), Alabama (Cloud, of Paleocene-Eocene age is related to the origin of 1966, 1967; Denson and Waage, 1966), and Georgia

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3 [COULD NOT BE CONVERTED TO SEARCHABLE TEXT] FIGURE 2.-Inferred temperatures for Tertiary time (modified from Wolfe, 1978). Data for North Dakota plotted from Hickey (1977). Dotted lines indicate inferred temperatures. climate and mineral resources (fig. 2). Mean annual the hinterland in which the kaolinite was formed by temperature varied from high to low at intervals of weathering. about 10 m.v. Expected latitudinal variation also is indicated, as temperatures from the Mississippi The highest mean annual temperatures shown on Embayment are higher than those from southern figure 2 range from about 12 to 13C for the Miocene Alaska. Estimates of mean annual temperature for a to about 30C for the Paleocene and Eocene, but highly kaolinitic transported clay unit of Paleocene bauxitic mineral resources seemingly could be formed age in western North Dakota and for the directly throughout this range. Suitable arent material and overlying earliest Eocene strata also are plotted on other favorable characteristics of the weathering figure 2 (Hickey 1977. p. 87-88). This clot does not regime, including geologic settings conducive to the match the other curves very well. The temperatures preservation of the weathering record, may have an seem somewhat low in comparison with those importance equal to that of climate in influencing the estimated for the middle Eocene of northern Present distribution of mineral resources. California and the Pacific Northwest, which are Given this broad range of temperatures, it is similar in latitude to North Dakota. The climate uncertain whether discrete segments of the suggested by these temperatures may have been a temperature variation cycle are closely related to relatively local feature developed because of specific occurrences of mineral resources. The orographic changes taking place in the Paleocene correspondence between the times of generally high landscape. Because the kaolinite was transported from temperatures during Paleocene and Eocene times and an unknown hinterland (Freas, 1962; Hickey, 1977), the the ages of weathering sequences and related mineral climatic parameters are not necessarily applicable to resources is, however, striking.

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5 PALEOCLIMATEANDMINERALDEPOSITS

EFFECT OF SEA-LEVEL FLUCTUATIONS ON POROSITY AND MINERALOGIC CHANGES IN COASTAL AQUIFERS By William Back and Bruce B. Hanshaw

The ocean is the ultimate base level for ground- thereby cause additional solution of the mineral of water regimes. Climatic changes that affect the interest in order to maintain an activity equal to that relationship between freshwater head and sea level can in either of the original solutions. This effect can be have pronounced effects on ground-water flow pattern, quite pronounced in carbonate aquifers, and it is rate of round-water discharge, position of the possible for a water that is in equilibrium with calcite freshwater-saltwater interface in coastal aquifers, and to be mixed with ocean water that is supersaturated amount of mixing within the zone of dispersion (fig. with respect to calcite to generate a mixture that is 3). Because part of the water discharged in coastal undersaturated with calcite but supersaturated with areas is brackish owing to mixing of freshwater with respect to dolomite (Hanshaw and Back, 1980). ocean water, discharged saltwater must be replenished On the basis of these relationships, we from the ocean. This discharge enerates a flow developed a mixing-zone model (Hanshaw and Back, system within saltwater in the aquifer that is related 1979) for the formation of dolomite in the zone of to, but distinct from, the flow system in freshwater. - dispersion and hypothesized that the boulder zone of The zone of dispersive mixing is a highly Florida that has conventionally been interpreted as a reactive chemical system. This reactivity results from paleokarst feature may be undergoing dissolution and differences in significant chemical parameters such as dolomitization at the present time. Badiozamani temperature. pH. PCO9, and ionic strengths of the two (1973) named this model the Dorag model and used it water bodies. For example, the nonlinearity of the effectively to explain the origin of Ordovician activity coefficient as a function of ionic strength dolomite in the midwestern part of the United States. (fig. 4) shows that mixing two waters will yield an Land and Epstein (1970) independently developed the activity coefficient less than what the value would be same hypothesis to explain the dolomitization of if the relationship were linear. Holocene deposits in Jamaica. The petrologic work of Because activity equals activity coefficient Folk and Land (1975) tended to substantiate the multiplied by molality, molality must increase and mineralogic changes in the mixing zone. nauth (1979) [COULD NOT BE CONVERTED TO SEARCHABLE TEXT]

Ionic Strength

FIGURE 3.-Diagram showing the interface between FIGURE 4.-Graph showing the upward concavity of freshwater and seawater flow systems that forms activity coefficient as a function of ionic the zone of dispersion. strength.

6 FIGURE 5.--Diagram showing diagenetic processes known to occur in the mixing zone. [COULD NOT BE CONVERTED TO SEARCHABLE TEXT]

FIGURE 6.-Diagram demonstrating the hypothesis that some manganese ores may be deposited in the zone of dispersion. [COULD NOT BE CONVERTED TO SEARCHABLE TEXT] 7 PALEOCLIMATE AND MINERAL DEPOSITS

PLIOCENE AND PLEISTOCENE CALCITIC VEINS AS INDICATORS OF PALEOHYDROLOGIC, PALEOCLIMATOLOGIC, AND NEOTECTONIC EVENTS, SOUTHERN GREAT BASIN: AN INITIAL APPRAISAL By Isaac J. Winograd, Barney Szabo, Tyler B. Coplen, and Gene C. Doty

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9 PALEOCLIMATE AND MINERAL DEPOSITS

THE OXYGEN MINIMUM ZONE OF CONTINENTAL MARGINS By Thomas M. Cronin

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FIGURE 7.-Characteristics of oxygen minimum zones. Nonupwelling margin from data by Cronin (1983). Silled basin and upwelling margin model adapted from Ingle (1981) and Pisciotto and Garrison (1981); published with permission from the Society of Economic Palcontologistsand Mineralogists. Sediment-water interface data adapted from Demaison and Moore (1980); published with permission from the American Association of Petroleum Geologists. Sediment characteristics data adapted from Diester-Ilaass (1978);published with permissionfrom Springer-Verlag. input also controlled the depositionofnorthernNorth and other factors.Theformationof organic Atlantic organic shales in turbidite sequences (Dean shales in Mesozoic and Cenozoic sedimentary and others, 1977). cycles along continental marginscanbemost The following observations must be taken into strongly influenced by peculiar marine biological account in models designed to explain and (or) predict and chemical conditions or by terrigenous input the occurrence of sediments deposited in the 2 from the adjacent continent. minimum zone: 4. Understanding the evolution of the 02 minimum 1. The 2 minimumzonevariesinintensityand zone and bottom anoxic conditions in general depth throughout the world's oceans; it is not requires high-resolution climatic records for necessarily synonymouswithanoxicconditions. specific marine basins and more accurate 2. Temporal changes in the development of oxygen methods of paleoenvironmental reconstruction minimum zone and anoxic condition occur at that allow paleodepth estimation, time scales ranging from 101 to 10 yr in the paleogeographic reconstruction, and estimation geologic record. of chemical and hvsical conditions at the sediment-water interface. Benthic organisms 3. Different combinations of environmental having limited tolerances for specific physical conditions can cause anoxia; conversely, several and chemical conditions - in particular, certain sedimentary processes can produce cyclic or ostracodes (Pevpouquet. 1979; Cronin, 1983) and rhythmitic sediments. Anoxic conditions can foraminifera Ingle, 1981 - hold promise for result in the concentration of hydrocarbons and accurately and quickly delineating bottom metals during relatively warm or cool climatic paleoceanographic conditions along continental conditions, depending on local paleogeography margins.

12 PALEOCLIMATEANDMINERALDEPOSITS

CENOZOIC PALEOCEANOGRAPHY

By Richard Z. Poore

Marine geological and geophysical studies in One of the most important recent developments concert with more than a decade of studying cores has been the integration of biostratigraphy. raised by the DeeD Sea Drilling Project (DSDP) have magnetostratigraphy, and isotope stratigraphy with led to increased understanding of the evolution of radiometric dates to formulate reliable high-resolution ocean basins and paleoceanography during the time scales for correlating and dating marine Cenozoic. At the start of the Cenozoic, a sequences. Accurate correlations mean that time- circumglobal ocean, which allowed latitudinal flow of slice maps showing the distribution of microfossil both surface and deep water, was present at low assemblages or lithologies for specific time intervals latitudes. Changing ocean-basin geometry during the can be constructed with confidence and used to infer Cenozoic, however, resulted in restriction of oceanographic conditions. Under optimum conditions, equatorial circulation and concomitant development of correlations over portions of the Cenozoic are circum-Antarctic circulation. In the Paleogene, approaching resolution on the order of 100,000 yr. northward movement of India and Australia greatly Similarly, time-series studies done in different regions restricted circulation in the Indian Ocean and the can be compared with one another with a minimum of western Pacific. Northward movement of Africa ad hoc "curve matching." Another important aspect of caused restriction and finally complete closing of the the development of high-resolution time scales is the eastern Mediterranean by the early Miocene. Later, in ability to detect relatively brief unconformities and to the Neogene, uplift of the Isthmus of Panama first determine sediment accumulation rates in pelagic restricted and then terminated circulation between the sequences. For example, Keller and Barron (1982) Atlantic and the eastern Pacific. The net effect of identified and mapped the geographic extent of seven these events was to greatly enhance the meridional Miocene deep-sea unconformities. These aspect of low-latitude circulation. unconformities are interpreted to be the result of The separation of India and Australia from increased bottom-current activity associated with the Antarctica. coupled with the opening of the Drake expansion of Antarctic glaciation. Passage between Antarctica and South America, led to Mapping the distribution of planktonic development of a circum-Antarctic current. This microfossil assemblages can show the distribution and current isolated Antarctica and allowed extensive migration of surface-water masses and thus delineate formation of an Antarctic watermass, which provided the distribution and migration of climatic belts. For a source for large quantitites of very cold bottom example, Haq and others (1977) demonstrated several water through formation of sea ice. poleward migrations of tropical microfossil Another important development during the assemblages in the Atlantic Basin during the Paleocene Cenozoic was the opening of the Norwegian-Greenland and Eocene. Moreover, individual species and Sea, followed by the sinking of the Iceland-Faeroe assemblages of microfossils are indicative of upwelling Ridge below sea level. Major breaching of the Iceland- conditions. For example, the abundance of the diatom Faeroe Ridge during the Miocene provided a route for genera Thalassiomena and Thalassiothrix is a reliable cold Arctic water to enter the world's oceans and indicator of upwelling intensity in Neogene sediments probably initiated formation of significant amounts of in low and middle latitude areas of the Pacific Basin. North Atlantic deep water. It has been documented for some time that Knowledge of major changes in ocean-basin benthic foraminifers can be used to identify low geometry has allowed construction of broad features environments at the sea floor (Ingle, 1981) of paleoceanography through the Cenozoic (Haq, 1981; Quantitative studies have revealed that benthie Kennett, 1978; Berggren and Hollister, 1977). Within foraminifers can also be used to identify deep-water this framework, details of Cenozoic paleoceanography masses (Lohmann, 1978). In addition, several distinct are being developed primarily from paleontologic, large-scale vertical migrations of benthic geochemical, and sedimentologic study of DSDP cores. foraminifers, which probably reflect major changes in

13 deep-water circulation patterns. have been found in 1980) suggest significant Correlation between the Miocene of the Pacific (Woodruff nd Douglas, paleoceanographic and sea-level events. Figure 8 1981). correlates terrestrial and marine paleoclimate records from Oxygen and carbon stable isotope studies of the Atlantic Basin with global sea-level calcareous microfossils also contribute to our fluctuations during the Paleocene, Eocene, and understanding, of Cenozoic paleoceanography. Oligocene. Cold intervals correlate with sea-level were initially interpreted as a temperature index. minimums, whereas warm intervals correlate with sea- However, it is now clear that the signature of level maximums. These correlations suggest that marine calcites is a complex signal that contains some, if not all, of the second-order sea-level information on a number of factors such as the fluctuations observed in the Cenozoic may be isotopic composition of seawater and the life processes glacioeustatic. Therefore, oxygen isotope records, of microfossils. Planktonic foraminifers show isotopic which contain information on global ice volume, may evidence of depth stratification in the water column as be used eventually to quantify the magnitude of sea- far back as the Mesozoic (Douglas and Savin, 1978), level changes seen in seismic records. and benthic foraminifers show evidence of consistent In summary, the ability to understand Cenozoic offsets from presumed equilibrium values back to at paleoceanography has reached the point where these least the Oligocene (Graham and others, 1981; Poore data can make significant contributions to and Matthews, 1983). Interpretation of data is investigations concerning sedimentary mineral more complex, but values obtained from some taxa resources. Times of intensified oceanic circulation appear to correlate with the dissolved-oxygen content can be identified, and regional variations in of seawater (Belanger and others, 1981). accumulation rates and areas of submarine erosion can be delineated. Areas of high productivity and oceanic The identification of sea-level cycles in seismic climate belts can be mapped. Major oceanographic records has important implications for interpreting the events can be identified, and the dissolved-oxygen geologic history of continental margins (Vail and content of deep water can be traced by both faunal others, 1977). The timing and magnitude of these and isotopic techniques. Increased understanding of cycles, however, are open to question. A number of the magnitude, frequency, and multiple causes of studies (Keller and Barron, 1982; Poore and Wolfe, eustatic sea-level fluctuations appears near.

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FIGURE 8.-Tertiary sea level and climatic trends (modified after Wolfe and Poore, 1982). P and N refer to standard planktonic foraminiferal zones for the Paleogene and the Neogene, respectively.

15 PALEOCLIMATE AND MINERAL DEPOSITS

THE U.S. ATLANTIC CONTINENTAL MARGIN: EUSTASY, PALEOCEANOGRAPHY AND MINERAL DEPOSITS By John S. Schlee

The U.S. Continental Margin is a series of Sea-level trends for five stratigraphic holes elongate sedimentary basins and shallow platforms drilled off the eastern United States reveal a pattern that originated during the latest oening of the of relative sea-level rise during the Jurassic and Atlantic Ocean. In the basins, as much as 16 km of Cretaceous that reached a maximum in the Late sedimentary rock accumulated under marine shallow- Cretaceous and early Tertiary and dropped water or nonmarine conditions. Although the thickness thereafter. Superimposed on this broad trend are of Mesozoic-Cenozoic sedimentary rocks changes from much shorter shifts in sea level that appear to be basin to basin, the vertical succession and seaward nearly synchronous from basin to basin and to have changes in facies appear remarkably similiar and point formed interregional unconformities. Major breaks are to uniform paleoenvironmental conditions over a large evident in the Cenomanian, in the late Turonian, in the area of the Continental Margin. Campanian-Santonian, at the Cretaceous-Tertiary The sedimentary sequence has been most boundary, at the Paleocene-Eocene boundary, in the closely studied in approximately 75 holes drilled late Eocene-early Oligocene, in the early Miocene, and mainly off Nova Scotia. They reveal an older rift in the Pliocene and Pleistocene. Older interregional sequence of Late Triassic red beds, evaporites. and unconformities also probably exist, but faunal remains carbonate rocks that overlies Paleozoic(?) in rocks older than Albian are scarce. metasedimentary rocks and granite. A conspicuous The occurrence of petroleum source beds along Dostrift unconformity, best developed toward the the Atlantic margin has been linked to paleoceanic landward edge of the basins, separates the rift circulation factors and to source factors by Tissot and sequence from a more extensive "drift" sequence of others (1980) and Hunt (1981). Anoxic events leading carbonate and siliclastic rocks of Jurassic age and to the formation of organic-rich shales in the younger. which accumulated following the separation Cretaceous are thought to have occurred because of of North America and est Africa. The postrift the latitudinal elongation of the North Atlantic Ocean section of Jurassic age is mainly limestone and basin, a warm global climate in which warm seawater dolomite (with some interhedded anhydrite in the was depleted in oxygen, and a widespread oceanic Lower Jurassic part of the section) that interfingers transgression onto the continents that created shallow, with deltaic clastic-rock facies toward the uppermost warm seas having high biologic productivity. In terms part of the Jurassic. Except for the Blake Plateau, of paleoceanography and sources of kerogen, these most of the carbonate platform buildup ceased in the conditions translated into a low total hydrocarbon Early Cretaceous as substantial amounts of siliclastic potential for rocks of Cretaceous age and a humic type sediment overspread the margin to build a sedimentary of kerogen most likely to produce gas (if maturity, blanket across the shelf and onto the Continental timing of migration, traps, and reservoirs were also Rise. Clastic sedimentation continued at a diminished favorable). In the Upper Jurassic rocks, Continental rate throughout the Cretaceous and Cenozoic, marked Offshore Stratigraphic Test (COST) wells drilled off by short-term regressions across the shelf and the northeastern United States showed a similar low deposition of thin, widespread transgressive concentration of total extractable hydrocarbons of the limestones. Beginning in the Tertiary, the Blake humic type III (Miller and others, 1980; Miller, 1982). Plateau was affected by periodic erosion and reduced Phosphorite deposits on the Blake Plateau sedimentation as flow of the ancestral Gulf Stream (Manheim and others, 1980) are the continuation shifted into the area. Farther north, a series of offshore of Miocene sediments that probably marine regressions in the Oligocene, Miocene, and originated in a cool, upwelling, shallow-water anoxic Quaternary resulted in a cutback of the slope to environment. Beneath the shelf, the phosphate is a lag expose rocks as old as Early Cretaceous and build up gravel that formed from the erosion of phosphatized the ntinental Rise wedge. carbonate-rich sediment. Further seaward where the

16 unit crops put Blake Plateau, the boulders and pnosphatic fragments. Its occured mainly in cobbles of phosphateareimpregnatedbyironand the area of lagcarbonateandphosphate gravels and manganese oxide crusts (todorokite and goethite). The iron-manganese mineralization is a later phase in two under conditions where the rate of oxide deposition modes--as a replacement of preexisting phosphorite exceeded the rate of sediment deposition (Manheim and carbonate and as a nodular coating over detrital and others, 1982).

n d e d e e

17 PALEOCLIMATE AND MINERAL DEPOSITS

BUILDING A BIOSTRATIGRAPHIC FRAMEWORK AROUND AN ANOXIC-KENOXIC EVENT By Joseph E. Hazel

In the, late Cenomanian and e. Turonian (at and frequency of events associated with times of about 9xl0 yr). sediments with high organic carbon anoxic-kenoxic seas would be prevented. and metal values were deposited at many places in the The integration of various types of world. The climate was mild and equable at this biostratigraphic data into one standard model is the time. arm temperate and subtropical climatic zones key to understanding the timing of events associated extended into high latitudes, and the tropical zone was with OAE-2 in North America. Such integration is expanded. As a result, bottom-water conditions were necessary because precise correlation between the sluggish, and the 02 minimum zone expanded various areas in North America where Cenomanian- vertically. A rapid transgression of the seas over the Turonian seas transgressed requires using a variety of continents occurred (perhaps in response to an increase fossil groups. For example, in many parts of the in volume of the midocean ridge system): the amount Western Interior and the Rio Grande and East Texas of epicontinental sea less than 300 m deep was twice Embayments, ammonites, inocerami, and planktic as extensive as it is today. These drowned low-lying foraminifers occur in rocks associated with OAE-2. areas contributed much plant organic matter to the However, these are more rare in the subsurface of the oceans (Schlanger and Jenkyns, 1976; Jenkyns, 1980; Atlantic Coastal Plain, where pollen, calcareous Rvan and Cita, 1977; Arthur and Schlanger, 1979). nannofossils, and ostracodes are the most useful This Cenomanian-Turonian event has been tools. Thus, for example. it is important to know how termed an oceanic noxic event (OAE), and it is the datums of these latter groups of organisms correlate second of three such events in the Cretaceous-thus, it with foraminiferal or molluscan datums. is referred to as OAE-. The event was short lived but A modeling technique based on the graphic intense. It has been widely recognized. However, in correlation method of Shaw (1964) (see also Miller, Dart because of the problems inherent in dealing with 1977; Murphy and Edwards, 1977) has proved successful sediments deposited under minimum conditions, in building robust biostratigraphic models (or precise biostratigraphic correlation of many sections is frameworks) for several Geological Survey projects. poorly understood. Biostratigraphic frameworks that Preliminary results suggest that it can be used to integrate first-appearance datums (FAD) and last- produce a biostratigraphic framework for OAE-2. appearance datums (LAD) from microfossils, both Graphic correlation is a technique that uses calcareous and organic walled, and megafossils are two-dimensional successive approximations to produce only now beginning to be developed (for example, a scaled solution. That is, the FAD's and LAD's for Robaszynski and others, 1982). fossils are placed in sequence, and the amount of The sediments of OAE's are or have the spacing, expressed in either rock thickness or in a time potential to be source rocks for much of the world's sense, is apparent. If radiometric dates can be hydrocarbons (Arthur and Schlanger, 1979) and other obtained for events in the scaled solution (usually natural resources. Exploration for such resources will termed the standard reference section), the arbitrary depend in part on a cogent biostratigraphic framework, scale can be converted to time, and the position of developed within the context of paleoclimatic biostratigraphic datums and sedimentary and other fluctuations and tectonic-paleogeographic events. geologic events observed in the studied sections can be Without accurate correlations, determining the timing expressed in easily understood numbers.

18 PALEOCLIMATE AND MINERAL DEPOSITS

INORGANIC GEOCHEMISTRY OF DEEP-SEA BLACK SHALES

By Walter E. Dean

Cretaceous strata containing organic carbon in plankton are, known to concentrate trace elements- the Atlantic Ocean provide considerable insight into especially Ba. Pb, Ni, Cu. Zn, Mn, and Fe (Boyle and the general problem of incorporating trace metals into Lynch, 1968; Knauer and Martin, 1973; Chester and sediments rich in organic carbon. Such marine others, 1978: Leinen and Stakes, 1979)-and the sediments and rocks are known to be significantly suggestion is that bioconcentration is a potentially enriched in a variety of redox-sensitive trace elements important mechanism for incorporating certain trace (Vine and Tourtelot, 1970) relative to average marine elements into marine sediments rich in organic sediment and shale, but the degree of enrichment carbon. However, the great effectiveness of trace- varies greatly. Although few systematicstudies have metal adsorption by clay minerals and organic matter been made of major- and trace-element concentrations and the coprecipitation of trace metals, particularly as in Cretaceous deep-sea lithologies rich in organic sulfide minerals, suggests that these processes may carbon, such lithologies may be important as potential play a more important role in the removal of trace sinks for organic carbon, sulfur, and trace elements, metals from seawater (Tourtelot, 1964b; Holland. particularly As, Ba. Cd, Co, Cr, Cu, Mio, Ni, Pb, V, and 1979). Detailed trace-element profiles from thick Zn. During the deposition of these sediments, it is sequences of organic-rich strata suggest that there is possible that face elements, organic carbon, and differential mobility of trace elements, accompanied sulfur are extented from the oceans at rates greater by diffusion of some elements over distances of at than steady state and that the chemical and isotopic least tens of meters (fig. 10). The sequence of trace- balances of the ocean are thereby perturbed. Average element mobility, from highest to lowest, is deep-sea accumulation rates of organic carbon were as approximately Ba, Mn. Pb, Ni, Co, Cr, Cu, Zn, , Cd. much as times greater during the peak of Aptian- and Mo. Albian black-shale deposition than they are today Although bottom-water anoxia may have (Dean and others. 1983). The reasons for the enhanced occurred during periods when strata rich in organic Dreservation of organic carbon in the Cretaceous deep- carbon were accumulating, it was not necessarily a sea basins, however, are still a matter of debate. cause of accumulation of these strata. The main The enrichment of certain trace elements- reason for the accumulation of these strata was an especially Cu. Zn, Mo. V. Ni. and Cr-in sediments and increase in the relative amount of organic debris being rocks enriched in organic matter has been reported by deposited. This organic debris was derived from many investigators (Tourtelot, 1964; Calvert and continental margin areas where the production and Price, 1970: Vine and Tourtelot, 1970; Chester and accumulation of organic matter from marine. others, 1978: Leinen and Stakes, 1979; Brongersma- terrestrial, or mixed sources were increased; the Sanders and others. 1980; Dean and Gardner, 1982; debris was then transported to slope and basinal sites Dean and Parduhn. 1983) (fig. 9). The association of by turbidity currents (fig. 11). But why was the supply high trace-element concentrations with organic matter of organic debris so much greater during the middle may be the result of concentration of these elements Cretaceous? Local pockets of accumulated organic by organisms bioconcentration) or by chemical matter in continental margin areas today are the sorption and recioitation under anoxic conditions result of local circulation patterns (for example, created by a flux of excess organic carbon. Marine Walvis Bay off southwest Africa and Santa Barbara

19 [COULD NOT BE CONVERTED TO SEARCHABLE TEXT] FIGURE 9.-Observed range (length of bar) and geometric mean (symbol) of concentrations of major-element oxides and trace elements in samples of red and green claystone and black shale from Deep Sea Drilling Project (DSDP) Hole 530A. cores 89-105 (adapted from Dean and Parduhn, 1983).

20 [COULD NOT BE CONVERTED TO SEARCHABLE TEXT] FIGURE 10.-Plots of carbonate-free (cf) concentrations versus depth for Fe203, MnO, Ba, Co, Ni, Pb, Cr, Cu, Cd. Mo. V. and Zn in samples of black shale from Deep Sea Drilling Project (DSDP) Site 530A in the southern Angola basin. (Concentrations of Fe2 O3 and MnO are in percent; all other concentrations are in parts per million. The Fe:AI ratio in black-shale samples, the percentage of black-shale beds in each 9.5-m cored interval,andthemassaccumulationrate(MAR)(ingramspersquarecentimetermillionyears)of organic carbon for each 9.5-m cored Interval also are plotted. The vertical line through each plot is at the Geometric mean concentration for that element or oxide. All values greater than the geometric mean are shaded. The numbers (89-105) within the plot for percentage of black-shale beds are the numbers of each core. The two horizontal lines through all plots are drawn at the top of core 97 and the bottom of core 98 and mark the zone of maximum black-shale bed concentration (from Dean and Parduhn, 1983).

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FIGURE 11.-Diagrams showing how the influx of terrigenous organic matter, surface-water organic productivity, upwelling, and turbidity currents may contribute to the accumulation and redeposition of sediments containing various amounts of organic carbon and, therefore, ranging in color from red to black. .A. Period of moderate supply of organic detritus from autochthonous marine and terrigenous sources and moderate uwelling resulting in accumulation of green sediment in shelf areas, sediment with high organic- carbon concentrations (perhaps black in color) within the 2 minimum zone, and dominantly red clays in the deep basin. Such conditions might be representative of the later Cretaceous (for example, Santonian- Coniaciani in the Atlantic Ocean. B. Period of rapid rate of supply of organic detritus and stronger upwelling resulting in an expanded and more intense 2 minimum zone that may include much of the water mass and allow sediment rich in organic carbon to accumulate over a much greater area and provide a greater source area for supply of such sediment to the deep basin by downslope transport. Such conditions might be representative of periods of accumulation of strata rich in organic carbon in the Atlantic Ocean (Aptian-Albian and Cenomanian-Turonian) (from Dean and others, 1983).

22 [COULD NOT BE CONVERTED TO SEARCHABLE TEXT] 23 PALEOCLIMATE AND MINERAL DEPOSITS

SOME CLIMATIC AND OCEANOGRAPHIC CONTROLS ON THE TIME AND PLACE OF MINERALIZATION

By W. F. Cannon

In the past decade, substantial advances have their promise is great. For instance, knowing when been made in understanding secular variations in oceans underwent major climatic-controlled changes in climate and in the nature of the oceans and the Eh and pH and what the effects of those changes on interplay of the two. Examples include work by Arthur the solubility of metals were is an obvious application and Schlanger (1979), Degens and Stoffers (1976), of the principals to mineral exploration. Anintriguing Demaison and Moore (1980). Jenkyns (1980), and possibility is that relatively discrete regionwide or Schlanger and Jenkyns (1976). In general, eustatic even worldwide time lines for high mineral favorability sea-level rise favors climatic warming because the sea can be defined. surface generally has a greater capacity to retain solar energy than the land surface does. Warm climate and The present-day Black Sea offers a good and low pole-to-equator thermal gradients tend to diminish relatively well studied scale model of former anoxic deep oceanic circulation and favor the development of oceans. It is anoxic below about 200 m. The oxidized widespread stratified oceans that are anoxic below the surface layer has an Eh of about +0.4 v and a pH of zone of surface mixing. Hence, there is both a about 8.3. The deeper anoxic water has an Eh of about theoretically predictable as well as an observed -0.2 v and a pH of about 7.5 (Grasshoff, 1975). These correlation between periods of ocean stagnation and changes place surface water and deep water on anoxia. The recognition in recent years of "ocean opposite sides of a major solubility boundary for anoxic events," eriods when oceans became largely manganese minerals. The dissolved manganese in deep anoxic on a regional or perhaps even a worldwide water is about 500 times greater than that in surface scale, has been a major advance both scientifically and water (Brewer and Spencer, 1974). Where deep anoxic practically and is now a widely alied principal in water is welling up onto a flanking shelf, a manganese petroleum exploration because the products of anoxic deposit is forming just above the anoxic-oxic oceans-black organic-rich shales-form the source interface. Such conditions during major ocean anoxic beds for petroleum. Such considerations of events may account for the formation of important paleoclimate and paleoceanography have not yet had marine sedimentary manganese deposits such as as substantial an impact on mineral exploration, but Groote Eylandt (Cenomanian) in Australia:

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FIGURE 12.-Relative rates of accumulation of marine sedimentary deposits of manganese, iron, and phosphate on a worldwide scale approximated from resource figures compiled from many sources. Horizontal (rate) axis is sketched on logarithmic scale.

24 iron and phosphate. so there is predictabledirect record. Those that formed duringanoxicevents,on relationshipbetweenthetimesofformation the other hand, would not oxidize and would have important marine sedimentary deposits of iron, much higher probability of persisting until they could manganese. nd phosphate. Figure 12 shows at least a be buried and preserved. crude correlation between the times of formation of Warm, humid climate also plays an obvious role deposits of these three commodities on a worldwide in forming many continental deposits, including scale. Many deposits formed in the early Paleozoic bauxite, nickel aterites, residual and supergene (until the end of the Devonian), very few formed from deposits of iron and manganese, and heavy-mineral early Capporifenous until the beginning of the Jurassic placers. Identifying periods of exceptionally warm (with the excoption of a sharp worldwide phosphate climate in the relatively complete Mesozoic and peak in Late Permian), and then very extensive major Cenozoic marine record should allow a more efficient deposits throughout the world formed from Early search for these deposits in the less complete Jurassic to the present. This temporal distribution is continental record. also remarkably similar to that for petroleum source beds. Figure 13 compares the temporal distribution of The above are only a few examples of how the marine sedimentary iron deposits and petroleum source genesis of mineral deposits might be linked to beds and also illustrates that both were favored when fluctuations in climate and oceanographic conditions. sea level was considerably higher than it is at present. Improving our ability to interpret paleoclimate and There may also be a correlation between ocean paleoceanography from the rock record and anoxic events and the temporal distribution of massive understanding the interplay of the two and their effect sulfide deposits, particularly deposits formed at on the physical and chemical processes that form ore spreading centers. Such deposits forming today are promise to greatly improve our ability to predict and oxidizing rapidly on the sea floor, and it appears that discover deposits of many types of ores.

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FIGURE 13. Tons of iron in known deposits of marine sedimentary type in Europe, the Soviet Union, the Middle East, and North Africa (total of IIIdeposits) as a function of time in comparison with accumulation of crude oil in source rocks as a function of time (from Tissot, 1979). Age shown is age of petroleum source rock.

25 PALEOCLIMATE AND MINERAL DEPOSITS

INFLUENCES OF OCEAN ANOXIC EVENTS ON MANGANESE DEPOSITION AND OPHIOLITE- HOSTED SULFIDE PRESERVATION By Eric R. Force, William F. Cannon, Randolph A. Koski, Keith T. Passmore, and Bruce R. Doe

Periods of widespread warm climate correspond interface itself to sweep up the shelf; the locus of to eustatic transgressions and to ocean anoxic events manganese precipitation shifts landward, and previous (Schlanger and Jenkvns. 1976: Arthur and Schlanger, deposits may be remobilized. The preserved 1979: Jenkyns, 1980). The concurrence is well manganese deposit thus ranges from basal to maximum documented and has implications for sedimentary transgressive in stratigraphic setting. In this model manganese deposits and for ophiolite-hosted sulfides. for "bathrub-ring" manganese, deposits, manganese can be derived from a variety of sources, and anoxic Shallow marine environment seawater serves as ore-forming fluid. The process can also be represented on an Eh- Consider first the situation in shallow seas pH diagram constructed for stratified seas like the during an anoxic event. A surface mixed zone of present Black Sea (fig. 15). The deep anoxic water oxygenated water overlies anoxic water with a sharp mass lies in the stability field of pyrite, where iron is interface at depths of as little as 100 m. Dissolved removed from solution. Oxidation of this water mass manganese concentration below the interface is as takes it through a solution field, a manganese much as 500 times greater than that in the normal carbonate field, and finally into a manganese oxide oxidized seawater above it (fig. 14). Mixing of the field at high Eh in shallow-water environments. Note water masses across the interface (for example, by that both Eh and pH increase from deep anoxic water upwelling) and diffusion cause manganese to shallow oxygenated water. precipitation. The precipitate is preserved in Shallow marine (nonvolcanic) manganese sheltered sites landward of the intersection with the deposits comprise 75 percent of the world's present shelf owing to slow manganese precipitation (fig. 14). manganese reserves and show stratigraphic features Coincident transgression causes the oxic-anoxic (fig. 16) in accordance with our model (Cannon and

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FIGURE 14.-Profile of dissolved manganese (after Brewer and Spencer, 1974) and depositional model for an anoxic event.

26 [COULD NOT BE CONVERTED TO SEARCHABLE TEXT] FIGURE 16.-Composite shallow marine manganese deposit (data from Cannon and Force, in press). FIGURE 15.-Eh-pH diagram (after Hem, 1972) showing oxidation path of an anoxic water mass (data from Grasshoff, 1975).

Force, in press). Transgressive setting is apparent. and from seawater at the bottom of the oxygen The deposits are both clay and carbonate hosted and minimum layer (explaining the gradual change in lead tend to be oxide facies above and landward of isotopes in manganese oxides away from the vent) and carbonate facies. They follow or correspond in age to forms a manganese halo around the vent. Vented anoxic events where this information is available (fig. sulfides are unstable, develop syngenetic corrosion 17). In some deposits, there are coeval black-shale features, and eventually dissolve (unless they are basins nearby or a carbon isotope signature of an rapidly buried by lava). Two products of corrosion are anoxic seawater reservoir. goethite and elemental sulfur. Oxygenated bottom There is no reason why the United States should water support the diverse "vent faunas" that function not have some deposits of this type. We have not without light but that cannot exist in totally anoxic looked for them, and manganese mineralization water. (especiallyMnCO 3) can be inconspicuous. The stratigraphy resulting from this sequence of events is that seen in the Troodos ohiolite of Cyprus (figs. 17, 19). The lower pillow lavas contain simple Deep sea sulfides and no manganese halos. The upper pillow lavas contain manganiferous umbers. The onset of Now consider the situation in the deep sea at a oxidizing conditions on the sea floor is recorded by spreading center. Assume continuous hydrothermal ochers, iron-oxide deposits representing venting during and after an anoxic event (fig. 18). postdepositional corrosion of the sulfides at the top of During the anoxic event, anoxic water reached down to the lower pillow lava. The umber deposits just some, if not all, oceanic rises, and sulfides were postdate the long Aptian-Albian anoxic event (fig. 19). vented into water in which they were stable and thus consistent with our hypothesis that sulfides were were preserved. preserved only during anoxic events. Anoxic events ended when renewed bottom The model' should be subjected to faunal, currents resulted in thinner and weaker oxygen isotopic, and petrographic tests. However, the sulfide minimum zones (fig. 18. The base of the anoxic layer potential of ophiolitic pillow lavas formed during lifted off the bottom and the situation became anoxic events appears to be higher than that of those analogous to the modern-day North Pacific, which has formed at other times. The parts of pillow lava a widespread midwater oxygen minimum layer. sequences containing umbers may be systematically Manganese precipitated both from hydrothermal fluids impoverished in sulfides.

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FIGURE17 Age correspondence of ocean anoxic events, shallow marine manganese deposits, and ophiolitic sulfides. Dashed lines indicate no data; short vertical lines indicate range of uncertainty.

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FIGURE 18.-Comparison of axial ridge vents during and after anoxic events.

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FIGURE 19.-Stratigraphic record of waning anoxic event at Cyprus (relations mostly after Robertson, 1975).

29 PALEOCLIMATE AND MINERAL DEPOSITS

LATE CRETACEOUS AND PALEOGENE NONMARINE CLIMATES IN NORTH AMERICA By Jack A. Wolfe

Nonmarine climates have some parameters that important consideration in dealing with Cretaceous are of major significance and that are unique, or and Paleogene floras that contain representatives of nearly so. to such climates. In contrast to the aqueous many extinct genera (and of even higher level and generally equable marine realm, where the major taxonomic groups that are now extinct). Among the climatic component is mean temperature, a discussion foliar characteristics that can be used for of nonmarine climates must include annual paleoclimatic inferences are leaf size, type of leaf precipitation patterns and temperature extremes. margin, leaf shape, leaf texture, and certain venation Indeed, the present distribution of major vegetation characteristics (Wolfe, 1971, 1981). types is primarily controlled by mean annual Application of physiognomic analyses to temperature mean of the cold month, mean of the Paleogene floras of North America leads to several warm month and precipitation (olfe, 1979). Thus, major conclusions (Wolfe, 1978): the valid recognition of certain vegetation types in the fossil record affords a ready means of inferring the 1. The mean annual temperature fluctuated by to past distribution of climatic parameters on the 8C between the four warm and three cool continents. intervals during the Paleocene and Eocene; these Most paleoclimatic inferences based on plants intervals apparently correlate with similar rely on the accurate identification of organisms in intervals based on Deep Sea Drilling Project work terms of extant relatives and the assumption that the (Wolfe and Poore, 1982). environmental tolerances of the fossil organisms were fundamentally the same as those of their extant 2. A major drop of about 10C occurred in mean relatives (Wolfe, 1971). Although this floristic annual temperature at the end of the Eocene. approach perhaps has some validity for Neogene and Quaternary organisms, it yields only gross 3. Mean annual temperatures in excess of 20 0C Daleoclimatic approximations for older assemblages, occurred at paleolatitudes greater than 60N. which display greater "paleoecologic dissonance" with during some warm intervals (Wolfe, 1977. 1980). increasing age. From evolutionary theory, the floristie aDDroach cannot be rigorously applied because one of 4. Truly tropical (mean annual temperature,25 0C) the major factors driving evolution is the environment; climates extended at times to about latitude 47 that is, a given lineage diversifies in response to N. changing environmental parameters, and thus tolerances must have changed through time. 5. The mean annual range of temperature (that is, The most valid paleoclimatic inferences based the difference between the means of warm on plants rely on the morphologic (physiognomic) months and those of cold months) was much less convergences that result from adaptations to during the Paleocene and Eocene than it is today. particular environments. For example, tropical rain forest in Indonesia is physiognomically similar to 6. The mean annual range of temperature increased tropical rain forest in the Amazon, despite the drastically at the end of the Eocene and has since considerable differences in floristic content. The been gradually decreasing (Wolfe, 1981). broad-leaved evergreen rain forests of North Island, New Zealand. hysiognomically resemble those of 7. The Mississippi Embayment assemblages indicate upland Central America, despite the almost total a pronounced drying trend from the Paleocene floristic differences. Emphasis on the into the late Eocene. physiognomy-particularly foliar-of fossil assemblages will yield the most valid paleoclimatic inferences. The 8. At the time of drying in the southeast, Pacific physiognomic approach is also largely independent of Coast assemblages indicate humid climates (the identifying fossils in terms of extant relatives, an reverse of the present precipitation pattern).

30 Late Cretaceous leaf assemplages generally have not been physiognomicallyanalyzed.Thatthe indicate some major fluctuations,temperature Paleogene fluctuation, in mean annual temperature Cenomanian assemblages from the Alaskan North may have Late Cretaceous analogs is indicated by Slope (a paleolatitude almost at the Pole) indicate pollen analysis of Santonian (84 m.y.) through polar temperatures at least 15C higher than those at MaestrichtiansamplesfromtheAtlanticCoastal Plain Late Cretaceous leaf assemblages are (Wolfe. 1976): these samplesshowthatperiodsduring present. which conifers wereabundantalternatewithperiods obviously in great need of study in order to infer during which inziosperms were abundant additional information on Late Cretaceous nonmarine Physiognomic analyses of some described leaf climates.

31 PALEOCLIMATE AND MINERAL DEPOSITS

VOLCANIC MANGANESE DEPOSITS IN THE WESTERN CORDILLERA: LITHOLOGIC ASSOCIATIONS AND PALEOCEANOGRAPHIC SETTINGS AND ECONOMIC DEPOSITS ASSOCIATED WITH BIOGENIC SILICEOUS ROCKS By James R. Hein and Randolph A. Koski

Preliminary investigations suggest that 3. Deposits in etachert-metavoleanic numerous stratiform manganese deposits in the sequences-for example, late Paleozoic to Western United States were formed by Jurassic strata of the Sierra Nevada and Klamath hydrothermal-volcanogenic processes. These deposits Mountains. These deposits are similar in can be grouped into five categories on the basis of occurrence and general lithology to those in the manganese mineral assemblage, lithologic association, Franciscan and Havallah sequences but have and tectonic settings: undergone a higher grade of metamorphism, so that the manganese mineral assemblage includes 1. Deposits in chert-gravwacke sequences-for rhodochrosite, rhodonite, spessartine, example, parts of the Franciscan Complex of piedmontite, and manganese-rich pyroxene and California. where lenses and beds of manganese amphiboles. carbonate and manganiferous opal are associated with bedded chert within thick sequences of 4 Deposits in limestone-argillite-basalt graywacke and conglomerate. Manganese sequences-for example, the Eocene Crescent minerals from the Buckeye Mine in the Formation on the Olympic Peninsula of Franciscan Complex include rhodochrosite, Washington, where marine carbonate and braunite. bementite, birnessite. and pennantite. soilitized volcanic rocks host a diverse manganese mineralogy dominated by silicates and Deposits in chert-greenstone (ophiolite) oxides. Soren and Gunn (1967) determined the sequences--for example, the late Paleozoic manganese oxides to be braunite, hausmannite, Havallah sequence in Nevada, parts of the jacobsite, and crednerite, whereas the manganese Franciscan Complex of California, and the silicates include alleghanyite, bementite, inesite, Nicova Complex in Costa Rica. Manganese johannsenite, rhodonite, and tephroite. jasper. manganese oxides, and manganese silicates occur in lenses within bedded chert, at 5. Deposits in Miocene and Pliocene sequences of the interface between basalt and chert, and conglomerate-sandstone-tuff-gypsum of the within basalt. Manganese minerals from the Colorado River-Lake Mead area of Nevada and Nicoya Complex in Costa Rica include pyrolusite, Arizona, where high-tonnage low-grade hollandite, romanechite, braunite, -MnO 2 , and stratiform and largely strata-bound deposits bixbyite. Chemically, these ores are rich in Ba consist mainly of poorly crystalline manganese (to .0 percent). Cu (to 0.2 percent), and Ni (to oxide cement in lastic sedimentary rocks. 0.1 percent) but poor in Fe, Co, and Zn (Hein and Manganese minerals from the Lake Mead area others, 1982). The Costa Rica manganese include pyrolusite, todorokite, cryptomelane, and deposits have not been exploited since 1920 and ramsdellite, whereas minerals from the Artillery have Produced about 32,000 tons of ore having an Mountains in Arizona include these minerals plus average grade of 50 percent manganese (Roberts, hollandite, romanechite, and coronadite. 1944). The largest deposits in the Franciscan, Chemically, these deposits are rich in Pb (to 1.5 the Ladd nd the Buckeye, produced 30,000 tons percent), Cu (to 0.2 percent), Zn (to 0.45 and 26,000 tons of high-grade manganese ore, percent), and Ba (to 11.0 percent) but poor in Co, respectively. at grades between 35 and 55 Ni, and Fe. The Artillery Mountains contain percent manganese (Trask, 1950). about 175 million tons of manganiferous rocks

32 averaging6.5to4percentmanganeseand tons at -20 percent manganese (Lasky and paleoceanographic environment. Webber. 1949). The Three Kids district near In addition to manganese, biogenic siliceous Lake lead in Nevada produced over 2.225,000 deposits host important accumulations of iron, long tons of ore averaging 18 percent manganese, uranium, barium, phosphates, and petroleum. which yielded more than 600,000 tons of Paleozoic and Mesozoic bedded chert sequences and concentrate averaging 4 percent (Crittenden, Tertiary siliceous deposits, mainly diatomite, 1964). porcellanite, and chert, occur in several circum-Pacific orogenic belts. These siliceous Our preliminary interpretation concerning deposits are derived from biogenic manganesedepositsinthewesternCordillera,based on debris--radiolarians, diatoms, and sponge spicules. lithologic associations, mineralogy, chemistry, and Tertiary siliceous deposits form the source and regional settings, is that most deposits formed in reservoir rocks for petroleum in many places, occur incipient oceanic rifts or at spreading centers close to with important deposits of phosphate, and may be a continental margin. Contemporary settings of this source rocks for uranium accumulation, whereas nature include the type of incipient oceanic rifts and Mesozoic and older bedded chert sequences host iron. back-arc basins found in the Gulf of California and the manganese, and barium accumulations. Laminated Red Sea. This concept can be best tested in the diatomites were deposited in basins on continental Arizona-Nevada deposits, where spreading associated shelves and slopes in areas of oceanic upwelling; with the proto-Gulf of California rift may be the bottom-water conditions were commonly anoxic. source for the sandstone- and conglomerate-hosted The rhythmic bedding in Mesozoic and manganese deposits. Deposition of manganese in Paleozoic bedded cherts probably resulted from shallow-water environments over the rift may be deposition by turbidity currents that carried mainly related to paleoclimatic and paleoceanographic siliceous biogenic debris. Interbedded shales represent factors. Sedimentological characteristics of bedded ambient hemipelagic sedimentation. Some chert cherts and comparisons with open-ocean deep-sea sections in Japan, however, are thought to show an chert sequences support a deep-water near-continent opposite relation; their shale beds are identified as environment for most chert sequences (Hein and Karl, turbidites and their cherts as ambient pelagic 1982) and presumably for the enclosed manganese deposits. As mentioned, on the basis of sedimentary deposits. The bedded cherts formed in areas of structures, chemical compositions, and regional oceanic upwelling, probably similar to the oceanic settings, many bedded chert sequences apparently conditions presently found in offshore Peru, southern were deposited in deep water but near a continental California, or the Gulf of California. In addition, the margin. Bedded cherts show few similarities with red limestones and argillites hosting manganese open-ocean deep-sea cherts recovered from the deposits in the Olympic Peninsula and the red-bedded Pacific.

33 PALEOCLIMATE AND MINERAL DEPOSITS

ORGANIC CARBON. SULFUR, AND IRON RELATIONSHIPS AS AN AID TO UNDERSTANDING DEPOSITIONAL ENVIRONMENTS AND SYNGENETIC METALS IN RECENT AND ANCIENT SEDIMENTS By Joel S. Leventhal

Syngenetic iron sulfide phases in sediments are These carbon and sulfur data from the Devonian generally formed from aqueous sulfide resulting from black shales of the Appalachian Basin and the catabolism of organic matter by anaerobic sulfate- Cambrian Alum Shale of Sweden are used as distinct reducing bacteria. It has been shown that recent prototypes to examine the enrichment controls for marine sediments below oxygenated depositional metals. These samples are contrasted with the waters show a constant relationship of carbon to sulfur Permian Kupferschiefer of central Europe. Major (Berner, 1970; Sweeney, 1972; Goldhaber and Kaplan, controls are a relatively low sedimentation rate and a 1974). When the weight percent of organic carbon is relatively high input of organic matter and metals. plotted on the axis and the weight percent of sulfur Organic carbon and sulfide-sulfur relationships can be is plotted on the v axis and the data are fitted by the used to establish the presence of an H2 S-laden water linear least-squares method to y=a~bxthe S/C slope is column for these ancient shales by analogy with the approximately 0.3, and the line passes through the Recent Black Sea. However, in the Alum Shale, it is origin (fig. 0). New interpretation of data from shown that all of the iron is sulfidized; thus, the recent sediments from euxinic marine environments normal carbon-sulfur relationship is disturbed for example, the Black Sea) also shows a linear (Leventhal, 1981b). relationship between carbon and sulfur, but the slope is variable, and the lines for individual cores can intercept the axis anywhere from 0.7 to 2 percent Trace-element abundances (Vine and Tourtelot, sulfur (Leventhal. 1979). Thus, the presence of an 1970) are closely related to organic carbon (U, Mo) and anaerobic H,S-laden water column leads to an extra (or) sulfide sulfur (As, Hg) in the Devonian shales. increment of sulfide without a corresponding fraction Somewhat different element-association patterns are of residual organic carbon in the sediment. The extra present for the Alum Shale. For the Devonian shale sulfide is probably due to metal sulfide formation in (Swanson, 1961), the type of organic matter is also a the water column (Sweeney and Kaplan, 1980) and at control for the uranium. In particular, the the water-sediment interface: some is formed terrestrially derived organic matter is specific as a diagenetically in the sediment. Data from the two control and residence site of the uranium (Leventhal, most recent sedimentary units of the Black Sea 1981a). These relationships are illustrated in the illustrate the effect of the evolving H2 S layer on diagram in figure 21. Trace element-organic and trace sulfur geochemistry by producing a more positive element-sulfide ratios vary by less than a factor of 2, sulfur intercept for the youngest unit having the whereas the sedimentation rate varies by more than an thickest HS layer. Between-core comparison of data order of magnitude in the Appalachian Basin samples. from several sites shows variable S/C slopes, which Uranium and organic carbon contents for individual can be related to location and (or) deposition rate. stratigraphic units from various parts of the basin The larger SC slope is associated with higher rates of show an inverse relation to the thickness of the unit deposition nearer the edge of the basin at shallower (Leventhal and Kepferle, 1982). deDths (Leventhal. 1983). New data from cores of Upper Devonian black The source of the uranium and certain trace shales from the Appalachian Basin show carbon and elements is in part related to leaching of volcanic ash sulfur relationships similar to those found in modern (near its source, on land), as demonstrated by euxinic environments. The SC slopes for the ancient increased contents of metals in samples shales can be interpreted in relation to deposition rate, stratigraphically above the ash in comparison with availability of iron, and perhaps the type of organic those below the ash (Leventhal and Kepferle, 1982; matter and the thickness of the aqueous sulfide layer. Leventhal and Hosterman, 1982).

34 [COULD NOT BECONVERTEDTOSEARCHABLETEXT] ORGANIC CARBON, IN WEIGHT PERCENT FIGURE 20.-Relationship of carbon to sulfur in Black Sea marine sediments deposited below oxygenated water (normal marine) and HS-containing water (euxinic) (Leventhal, 1983). Patterned area represents about 500 samples from modern normal marine sediments.

These black shales are important because they source of energy for mining and milling metals of the can be the host for Kupferschiefer-type ore deposits or black shale itself when higher grade metal deposits the source of the metals for metamorphic-type ore without such associated energy sources become deposits or higher grade mineralization and because uneconomic (Leventhal and others, 1981; Hatch and their high organic content can provide an in-place Leventhal, 1981).

35 FIGURE 21.-Geochemical relationships[COULD in oxic, anoxic,NOT and euxinicBE sedimentaryCONVERTED environments TO(modified SEARCHABLE from TEXT] Leventhal and others, 1981).

36 PALEOCLIMATE AND MINERAL DEPOSITS

PALEOZOIC ATMOSPHERIC CIRCULATION AND OCEANIC UPWELLING By Judith Totman Parrish

Paleoclimatological models traditionally have different from present ones because of different been based on the latitudinal components of climatic paleogeography. patterns and their effects on the distribution of An example of the use of such models is the organisms and (or) climatically significant sedimentary prediction of the locations of ancient upwelling zones rocks. The latitudinal components of climate includes (that is, zones of high biologic productivity in the an euator-to-poles temperature gradient and a oceans) (Parrish, 1982). Because of the large input of latitudinal distribution of atmospheric pressure, wind, organic matter to the sediment, upwelling zones are and precipitation. The distribution of climatic important settings for the deposition of very organic patterns parallel to latitude results from the rotation rich sediments and of phosphorites. The most of the Earth (that is, the Coriolis effect) and the persistent upwelling currents are wind driven-hence, existence of a temperature gradient. The rotation of the necessity of first modeling the atmospheric the Earth prevents a direct exchange of heat between circulation. Upwelling currents occur in two major the equator and the poles. Three first-order vertical forms-as coastal upwelling zones and as oceanic circulation cells parallel to latitude in each divergences. Coastal upwelling zones occur where hemisphere are required to effect the heat exchange. persistent winds flow parallel to a coastline so that the The resulting surface wind pattern consists of easterly net transport of water, which is at right angles to the winds between about 20 N. and S., westerlies between wind owing to the Coriolis effect, is offshore. At 35 and 500 N. and S.. and easterlies above 65 N. and present, coastal upwelling zones occur primarily along S. High-pressure (descending air) belts occur at the the west coasts of continents (for example, California poles and at about 30 N. and S., and low pressure and Peru) and are driven by winds that compose the (ascending air) occurs at about 60 N. and S. and at the eastern limbs of the subtropical high-pressure cells. equator. Precipitation is higher in the low-pressure Coastal upwelling also occurs off Somalia during the regions and lower in the high-pressure regions. summer monsoon in Asia and off Venezuela. The predominance of west coast upwelling zones is partly an artifact of present geography. Theoretically, zonal The latitudinal model explains only the coarsest coastal upwelling currents, such as the current off components of the global climatic pattern; it Venezuela, are potentially more extensive. Zonal corresponds to the pattern that would be seen if the coasts were more prevalent in the Paleozoic; Earth had a homogeneous surface. The contrasting consequently, the potential for zonal coastal thermal properties (that is, specific heat capacity) of upwellings was higher then (fig. 22). Oceanic land and sea greatly complicate the simple latitudinal divergences have been almost completely ignored by thermal regime imposed by the equator-to-poles geologists because, in the present world, the temperature gradient. Fortunately, the role of land- divergences tend to occur over very deep water and sea circulation in disrupting the latitudinal components leave little mark in the sediment except a higher of atmospheric circulation to create large-scale biogenic silica content. However, the very wide cellular features such as the subtropical high-pressure continental shelves that characterized many past cells at low midlatitudes in the oceans is fairly well geographies, particularly during the early Paleozoic, understood. at least for the present-day global thermal would have permitted the development of oceanic regime. Moreover, global paleogeographic divergences in settings where high biologic reconstructions. especially for the Paleozoic, are productivity could have resulted in the preservation of becoming more reliable, as the convergence in organic carbon and phosphorites as well as of biogenic reconstructions published by different research groups silica (fig. 22). indicates. Therefore, ancient atmospheric and oceanic The following table compares the distribution of circulation patterns can be modeled with some predicted Paleozoic upwelling zones with the confidence, and we can thereby begin to examine some distribution of phosphorites, organic-rich rocks, and of the elements of paleoclimatic patterns that were chert.

37 Comparisonofdistributionpredictedupwellingzoneswiththe phosphorites, organic-rich rocks, and cherts

[Number in parentheses is the total number of deposits for that time; first number is the number of deposits that fell into the predicted upwelling zones. statistically significant correspondences]

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With the exception of phosphorites in the the present world. However, other Paleozoic cherts Ordovician, wherever sample sizes are large enough to either have undergone so much diagenesis that their give meaningful statistical results, there is a origin cannot be determined or are derived from significant correspondence between upwelling and the sponge spicules. The relation of siliceous sponges to three types of deposits commonly found in upwelling upwelling is not well known. zones. Particularly interesting is the fact that cherts The results presented in the table suggest that, correspond so well, because biogenic silica in modern despite undoubted differences in climatic parameters, upwelling zones is almost entirely derived from such as the absolute value of the equator-to-poles diatoms, which did not evolve until much later. Some temperature gradient, past atmospheric circulation of the Paleozoic cherts are radiolarites, and patterns were governed by the same factors that radiolarians tend to concentrate in upwelling zones in govern them today.

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FIGURE 22.-Atmospheric circulation in the Late Cambrian northern winter (adapted from Parrish, 1982; published with permission from the American Association of Petroleum Geologists). II, high pressure; , low pressure; hatchuring, upwelling zones; dark shading, highlands; medium shading, lowlands; light shading, continental shelf (reconstruction from Scotese and others, 1979). Dark lines re isobars; arrows indicate wind direction. Note the zonal coastal upwelling zones predicted for the north and south paleocoastsofNorthAmerica(leftcenter) and the equatorial divergence over the self in western Gondwana.Solid circles indicate organic-rich rocks with marine organic matter; half- solid circles indicate organic-rich rocks of unknown origin; plus signs represent 5 longitude-latitude tick marks. PALEOCLIMATEANDMINERALDEPOSITS

PALEOCLIMATICCONTROLS ON THE OCCURRENCE AND QUALITY OF COAL

By C. B. Cecil, R. W. Stanton, S. G. Neuzil, F. T. Dulong, L. F. Ruppert

The industrial nations of the world have devoted work by Cecil and others (1982) has shown that the many years of research to understanding coal quality; variation in sulfur is far greater among coal beds of however, the geologic factors that control the origin strictly nonmarine origin than it is for a given coal bed and distribution of mineral matter in coal are still that is overlain by both marine and nonmarine poorly understood. In a broad sense, paleoclimate is sediments. recognized as one controlling factor in the worldwide Geochemical and paleoclimatic models provide distribution of coal. Although the quality of coal a more comprehensive explanation for the systematic deposits varies worldwide, there are three general variation of ash and sulfur in coal beds of the central "families" of coal deposits (Stach and others, 1982): (1) Appalachian Basin. In the Upper Mississippian, coal banded bituminous coals of Carboniferous age of the beds are thin, discontinuous, and generally of poor Northern Hemisphere, (2) banded bituminous coals quality. In the Lower and lower Middle Pennsylvanian. known as Gondwana coals of Permian, Triassic, and coal beds are generally low in both ash ( 10 percent) Jurassic age in India. southern Africa, eastern and sulfur (I percent), whereas, in the upper Middle Australia, South America. Madagascar, and and Upper Pennsylvanian, coal beds contain high Antarctica, and (3) high-moisture lignite and amounts of ash (>1 percent) and sulfur(>l percent). subbituminous coals of late Mesozoic or Tertiary age Sedimentary rocks associated with the low-ash low- in Europe, western North America, and southeastern sulfur coal beds are noncalcareous-sandstone, Australia. The Carboniferous coals formed under a siltstone, and shale. Coal beds that are high in ash and humid tropical and subtropical climate. In contrast, sulfur are associated with calcareous sandstone and climate conditions were cold temperate during the shales as well as with nonmarine limestone. The formation of the Gondwana coals (Stach and others, mineralogy of the low-ash coal beds is dominantly 1982). Climate conditions that revailed during the kaolinite and quartz; the mineralogy of the high-ash formation of coals of Cretaceousage and younger are high-sulfur coal beds is dominantly kaolinite, illite, not well documented. However, paleoclimates directly quartz, pyrite, and calcite. In the sedimentary and indirectly affect and (or) control both the sequence of the upper Middle Pennsylvanian, flint occurrence and the quality of these three broad clays are present that, in places, grade laterally into "families" of coal deposits. nonmarine carbonates; these sediments were On a local or regional scale, attempts to predict subaerially exposed and, as a result, contain the quality and thickness of coal beds are generally microkarst zones that are brecciated and have unsuccessful without data from closely spaced subaerial crusts. The Upper Pennsylvanian also drilling. Models to explain variations in thickness and contains nonmarine carbonates that were subaerially quality are limited to physical depositional exposed; however, flint clays are not present. In environments such as alluvial plain, upper delta plain, addition, red beds as well as paleosols are present. lower delta plain, or back barrier. Coal beds whose The aleosols are somewhat similar to caliches origins are traced to an alluvial plain or upper delta because they contain teepee or pseudoanticlinal plain depositional setting are generally believed to be structures and are highly enriched in nodular carbonate low in ash and sulfur, whereas coal beds of lower delta at the top. plain or back barrier origin are generally believed to The stratigraphic occurrence of coal, changes in be high in ash and sulfur as a result of proximity to coal quality, and changes in the lithologies of marine conditions. High-sulfur coal has been related associated sediments in the Appalachian Basin indicate to superjacent strata of marine origin (White and at least two major climatic changes during others, 1913: Horne and others, 1978). Williams and Pennsylvanian time. In Late Mississippian time, Keith (1963) and Gluskoter and Hopkins (1970) evaporation was high relative to precipitation, as the demonstrated that sulfur content is higher in areas presence of nonmarine carbonates, red beds, and where a coal bed is overlain by marine sediments than paleosols having the characteristics of caliches it is in areas where the same bed is overlain by indicates. In the latest Mississippian or Early sediments of nonmarine origin. In contrast, recent Pennsylvanian, the first climatic change occurred as

40 to late Middle Pennyslvanian time, precipitation was Organic acids maintained the watersat low pit values high relative to evaporition: thus. highly acid because the concentration of dissolved solids waslow, conditions (pH>4.5) were produced in the paleo-peat- and the resulting buffering capacity was also low. In forming environments. Coal derived from peat that the upper Middle and Late Pennsylvanian, rainfall was formed under these acid conditions is in general low in reduced, and the climate became more seasonal both ash and sulfur (Cecil and others. 1981). (White, 1925). ater in the peat-forming environments In the Upper Middle Pennsylvanian, the second was derived chiefly from surface or ground water. climatic change occurred when evaporation increased Interstitial water contained increased concentrations relative to precipitation.Theserelativelydrier of dissolved solids; buffering capacity was increased, conditions resultedinanincreaseofdissolvedsolids and p values increased in interstitial water of the and pH in surface nd near-surface waters and thereby peats. These latter conditions were more conducive to changed the geochemical conditions in peat-forming sulfur and ash fixation in the peat. and associated depositional environments. Coal derived from peat that accumulated under nearly The climate changes and the subsequent shift in neutral conditions is relatively high in both ash and geochemical conditions account for the stratigraphic sulfur (Cecil and others, 1981). Changing climatic variation in the ash and sulfur contents of coal beds in conditions are also indicated by the presence of () the Appalachian Basin. This variation cannot be nonmarine limestones that have shallow-water and explained by physical depositional models. The marine subaerial features and (2) paleosols similar to caliche. influence model generally used to explain the high- The climatic changes interpreted from lithologic and sulfur content of coal (Williams and Keith, 1963) is nothing more than a special ease thin a more coal-quality characteristics are summarized in figure encompassing geochemical model (Cecil and others, 23. The inferred paleoclimatic changes from the Late 1982). Mississippian through the Late Pennsylvanian are consistent with paleobotanical data ( hite, 1925; The continued development and application of Phillips, 1978, Pfefferkorn and Thomson, 1982). paleoclimatic-geochemical modeling will expedite Paleoclimate was one of the principal factors coal-resource assessments and quantify resource that controlled the geochemistry of the Pennsylvanian estimates. Such modeling can also be applied to peat-forming environments. In turn, these exploration, reserve characterization, mine planning, geochemical conditions controlled the quality of the coal preparation, and the successful prediction of coal resulting coal. In the Early and lower Middle properties as related to coking, combustion, and Pennsylvanian. water in the peat-forming synthetic fuels.

41 [COULD NOT BE CONVERTED TO SEARCHABLE TEXT] FIGURE 23.-Inferred paleoclimate changes and generalized stratigraphic column from the Upper Mississippian to the Lower Permian(?) central Appalachian Basin.

42 PALEOCLIMATE AND MINERAL DEPOSITS

CARBON DIOXIDE, GEOCHEMICAL MODELING, AND PALEOENVIRONMENTAL RECONSTRUCTIONS By Eric T. Sundquist

Paleoclimate is sometimes linked to other thousands of years. Another geological concern is the paleoenvironmental factors by means of geochemical potential for significant geochemical effects, such as speculation on a regional or global scale. For example, enhanced marine carbonate dissolution. These effects several investigators have suggested various are implied by geochemical principles that will operate relationships among past climate, atmospheric C02, even if there are no climatic effects. Finally, the carbonate dissolution, organic carbon burial, and geological record contains abundant evidence for past biological productivity. In order to develop nonanthropogenic variations in the global carbon geochemical models of these relationships, it is useful cycle. Many of these variations appear to have been to draw uon recent improvements in modeling associated with changes in climate and some strongly atmospheric CO. influenced the deposition and preservation of fossil During the past 25 years, atmospheric CO 2 fuels. The sedimentary record of past fluctuations concentrations have increased from about 315 to about also necessitates a careful reexamination of the 340 ppm. Extrapolation to future levels of fossil-fuel steady-state assumption that underlies all predictive consumption suggests that atmospheric CO, will be C02 and climate modeling. twice its preindustrial level before the year 2050. The best climate modeling available indicates that this Geochemical modeling is a owerful and increase will enhance atmospheric absorption of essential tool for approaching these geological aspects of the C2 problem. Geochemical models provide a infrared radiation enough to raise the mean global way of integrating numerous kinds of geological surface temperature by about 20 to 30C and the polar evidence into paleoenvironmental reconstructions and surface temperature by about 50 to 80C. Concern for predicting how geological processes will respond to these potential climatic effects has stimulated a broad future C02 emissions. The models are invariably international effort to understand the factors that rooted in the fundamental principles of chemical control atmospheric CO concentrations. equilibrium, chemical kinetics, and mass and energy The measured atmospheric increase accounts conservation. If they are to be applied to the for only about half of the CO produced by burning integration of stratigraphic evidence or future effects, fossil fuels. Accounting for the other half is very they must be time dependent. These characteristics difficult because it requires understanding the oceans are shared by many predictive carbon-cycle models and the biosphere, the two major reservoirs that developed by the CO2 research community. Predictive exchange carbon rapidly with the atmosphere. The C02 models provide a valuable starting point for problem is further complicated because it is not clear geochemical modeling. whether the biosphere is a net CO2 sink or an additional source; thus, the fossil-fuel CO2 problem is However, the development of geochemical a fundamental carbon-cycle problem. carbon-cycle models from C02 models is not Because of their long-standing interest in straightforward. Three very important considerations geochemical cycles, geologists can offer unique and are (1) the interdependence of processes and time important perspectives on the CO2 problem. One scales, (2) the multiplicity and unreality of steady particularly geological concern is the long time scale states, and (3) continuity with the present. These of response to anthropogenic CO2 production. considerations imply very specific constraints on the Whereas most public attention has focused on effects form of geochemical carbon-cycle models and the way that may occur during the next few decades or that the models are applied. They also have important centuries, geochemical models suggest that some implications for predictive CO2 models and for other effects may persist for thousands or even tens of geochemical models.

43 PALEOCLIMATE AND MINERAL DEPOSITS

ISOTOPIC RESEARCH, CLIMATE, AND THE GENESIS OF MINERAL DEPOSITS

By Bruce R. Doe

There are several fundamental climatic factors reliable as paleotemperature indicators. Increasing that we need to study to document the history of past salinity tends to result in heavier oxygen isotopic climates and to understand the relationship of compositions, so that taking the lightest values from paleoclimates to the genesis of mineral deposits. several analyses can help compensate for this These climatic factors include temperature, problem. The oxygen isotopic composition of ocean temperature distribution, humidity (including water is itself a roblem because It is controlled by cloudiness and indiness), oxidation state of waters, the growth and decay of icecaps, local evaporation, or pH. salinity, and saline species. In addition, several precipitation, large-scale atmospheric circulation, and factors that may be indirectly related to climate and nearby continental runoff. Berger (1979) gives a useful the influence of climate on mineralization are source review of the use of stable isotopes in foraminifers and materials (mountains versus plains of various materials how they reflect various hydrologic and climatologic such as volcanic rocks or sediments) and plate tectonic conditions. configurations. A number of other factors that may be In the case of carbon isotopes in carbonate important as mechanisms for climate change include rocks, the thermal fractionation effect is very small changes in solar luminosity, variations of the Earth's (about 0.035 permil/0C) and, in rocks or sediments magnetic field, and a change in the Moon's distance poor in carbon, carbon isotopes are less susceptible to from the Earth. Isotopes have played a large role in diagenetic effects than oxygen isotopes are (Scholle resolving many of these paleoclimatic problems. The and Arthur, 1980). Carbon is concentrated into two following is a survey of some of these applications. main reservoirs: carbonate and organic carbon. Although carbon isotopes do not vary as much in the Oceans geologic marine record as oxygen isotopes do, Cretaceous variations have been found to correlate Because temperature exerts a primary control with ocean anoxic events (Scholle and Arthur, 1980). on the isotopic composition of benthic and planktonic Because organic matter contains very light carbon foraminifers, oxygen isotopes have been the workhorse (--256 1 'C), carbonates formed in the oxic layer for determining paleotemperatures in the oceans. The during anoxic events can be expected to trend toward use of oxygen isotopes of benthic and planktonic heavy values. Recent studies confirm this trend in the foraminifers for Daleothermometry culminated in heavy carbon isotopic ratios found in the Albian and delineating the temperature history of the oceans over Turonian stages of the Cretaceous. Carbon isotopes the last 100 m.y. (Savin, 1977). Belemnites have also are also governed by the following secondary been important because of the ease in dentifying reservoirs: carbonate from the oxidation of old recrystallization in thin section. and foraminifera have organic carbonate, sedimentary and marine sulfates shown broad applicability for the Cenozoic. Several formed from oxidation of sulfide, ferric oxides formed factors influence the application of oxygen isotopes to from oxidation of iron, and the Earth's atmosphere. paleothermometry: diagenesis, disequilibrium, oxygen Sulfur is thought to be partitioned into an isotopic composition of the water in which the animals oxidized inorganic sulfate reservoir and a reduced, grow, water density in the case of planktic biologically mediated sulfide reservoir containing the foraminifera, and salinity. Identifying the contribution lighter values of . Secular variations in the 6 of each parameter is essential in isotopic studies. For in sulfate are well documented (Claypool and others, example, the extent of diagenetic recrystallization can 1980). The reduced sulfur in mineral deposits depends often be determined through thin-section study. Also, on several parameters, some of which are shown in some foraminifera secrete their tests out of figure 24. There is a tendency for sulfate-sulfur and equilibrium with seawater, so that some taxa are more carbonate-carbon isotopes to be inversely correlated

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FIGURE 24.-Distribution pattern for 6 34S values of H2 S and sulfide minerals when sulfate 6 34S=+20 percent is reduced by various mechanisms. (From Ohmoto and Rye, 1979.) (Reproduced with permission as modified Geochemistry of Hdrothermal Ore Deposits, copyright 1979 by John Wiley and Sons, Inc.)

with each other (Veizer and others, 1980), and this Continents tendency has led to hypotheses involving control of both isotopes by biologic means. There is also a Deuterium (fig. 25) and oxygen isotopes in correlation of sulfur with B'Sr/o Sr in marine fossils. continental surface water vary in a systematic For example, strontium in the oceans is radiogenic at fashion. There is a tendency for deuterium to the present time and was also in the Mississippian but decrease northward because precipitation is enriched was nonradiogenic in the Jurassic and Cretaceous in the light isotope, and the remaining vapor becomes (Peterman and others, 1970). Sulfate-sulfur is depleted (Taylor, 1979). A decreasing temperature relatively heavy at the present time and was in the trend also tends to make the precipitation heavier. Mississippian but lighter in the Jurassic and This trend is well illustrated in the midcontinent from Cretaceous. Nonradiogenic strontium and light sulfur the Gulf of Mexico northward. On the east coast, the are compatible with a young igneous component trend is modified by the Atlantic Ocean and mountain because of either submarine activity or erosion from ranges. The trends caused by jet-stream-related magmatic arcs (George Claypool, oral commun., precipitation coming eastward across mountains in the 1981). The inverse correlation of sulfate-sulfur with western United States add additional complexities. carbonate-carbon may therefore be more complex than For example, as clouds moving up the mountains cause current accounts indicate. precipitation, the vapor becomes depleted in the heavy Lead isotopes in sedimentary manganese isotope. Increasing elevation is accompanied by a deposits might indicate the importance of volcanism in temperature drop that accentuates the isotopic providing manganese to anoxic seas because, as Doe change. East of the highest elevations, the sotopic and Zartman 1979) pointed out, there is a large composition of surface water begins to become heavier isotopic difference between mantle lead and the lead because sinking clouds become warmer, and there is in seawater and pelagic sediments. The Cretaceous is mixing with precipitation from the Gulf of Mexico. a period of unusually great volcanic activity and also Because precipitation is a process approaching hosts some of the best-documented oceanic anoxic equilibrium, oxygen varies systematically with events. Radiogenic isotopes such as those of lead and deuterium isotopes (fig. 26), deuterium being about strontium may help distinguish climatic effects from 8 6 104 10 (in permil). This line is called the nonclimatic effects. meteoric water line (Taylor, 1979). Most surface

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FIGURE 25.-Map of North America showing contours of the approximate average values of meteoric water. (From Taylor, 1979.) (Reproduced with permission from Geochemistry of Hydrothermal Ore Deposits, copyrightl979,by John Wiley and Sons, Inc.)

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FIGURE 26.-Behavior of D and 6 180 in meteoric water. Crosses are values for precipitation at various places throughout the world. Values for Antarctic lakes can be more negative. Squares and triangles represent data from two lakes in the northwestern Sahara that are typical of evaporating water. Note reversal of data on lake represented by squares, caused by the effect of dissolved salts on the activity of free water. (From Pearson and Coplen, 1978.) water lies to the right of this line (fig. 26) because of data, coupled with data on fluid inclusions or surface evaporation and evaporation of the falling alteration minerals on magmatothermal ore deposits, precipitation in arid areas. These relationships are have been used to identify a major meteoric water expressed as a D of about 6 18O+10 (in permil). At component in many hydrothermal mineralizing very high salinities, the trend is reversed because of systems. The full significance of this important absorption effects on ions. In general, meteoric water finding is not yet completely realized in ore genesis or can also be distinguished from magmatic or as a guide to ore, but a number of economic geologists metamorphic water except in the case of highly are now considering topography and climatic evaporated systems at high latitudes. However, in the conditions during igneous activities. eastern Triassic basins, which were near the equator at Detailed analysis of the light stable isotope the time they were formed, the meteoric water trends geochemistry of lakes is highly complex (Pearson and probably followed heavier isotopic path than that Coplen, 1978) but has the potential to yield significant shown for primary magmatic water in figure 27. These paleoenvironmental information. It is possible to

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FIGURE 27.-Plot of D of fluid inclusions versus calculated 8 180 values of hydrothermal fluids for various epithermal ore deposits. Offset to the left of the primary magmatic water field shows influence of meteoric water. SMOW, standard mean ocean water. (From Taylor, 1979.) (Reproduced with permission as modified from Geochem - of Hdrothermal Ore Deposits, copyright 1979, by John Wiley and Sons, Inc.)

determine parameters such as temperature, pH, and fluids. Two such radionuclides currently under salinity of lake waters in some cases through studies of investigationare36C1whichhasahalf-lifeof0.31 fluid inclusions, fossils. travertine, chert, sanidine, and m.y., and I, which has a half-life of 15.7 m.y. zeolite-type minerals formed in many lakes (Cerling and others, 1977). In conclusion, the use of stable isotopes, both Preliminary investigations of cosmogenic radiogenic and nonradiogenic, should play a major role radionuclides other than radiocarbon indicate that we in unraveling the importance of climate in the may be able to date some oilfield brines and other old formation of many kinds of mineral deposits.

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