Abstract Many of the concepts presented in this volume evolved from work by the U.S. Geological Survey on the origin of mineral matter in coal. This work, sponsored primarily by the U.S. Environmental Protection Agency, was undertaken in response to The Clean Air Act of 1970, which was implemented, in part, to regulate emissions from coal-burning power plants. Because coal composition (coal quality) varies within and among coal reserves, there was a need for geologic models that explain and predict these variations. Using the depositional-environment models that were popular at the time, we attempted to delimit and predict the local and regional variations of mineral matter in an upper Middle Pennsylvanian coal deposit in the bituminous coal fields of the northern Appalachian Basin. According to the available depositional models, coal beds were derived from peat that formed in environments such as delta plains or fluvial systems, all of which involve movement of large amounts of siliciclastic sediment. In the broadest sense, these depositional settings were the autocyclic environments of Beerbower (1964) . On the basis of depositional models, many workers believed that allochthonous processes controlled the distribution of mineral matter in coal. Consequently, aluminosilicates and quartz were thought to be mainly detrital, and the sulfur content of coal was attributed to the degree of marine influence during peat formation. Our application of these depositional models failed to explain either the local or the regional distribution of mineral matter in the initial study area, and in Pennsylvanian coal beds in the Appalachian Basin in general.

Abstract Part 1 of this volume provides background information on the concepts of autocyclic and allocyclic processes as controls on stratigraphy, including the importance of climatic controls on sedimentation (Cecil). The first paper contains a classification for temporal climate change and a classification of seasonality of rainfall (in warm climates) based on monthly precipitation. Empirical models that relate seasonality of rainfall to fluvial transport of solids, fluvial transport of solutes, peat formation, and eolian transport are developed in the second paper (Cecil and Dulong). In addition, Part 1 presents data in support of the empirical sediment-supply models by discussing differences in solid and solute sediment supply in response to differences in seasonality of rainfall in selected river systems in Indonesia (Cecil et al.). Finally, Perlmutter and Plotnick relate variations in sediment supply to changes in climate induced by precession cycles of the earth’s orbit, and the presence or absence of polar ice, a pioneering concept they first presented in 1996 during a USGS sponsored workshop titled “Predictive Stratigraphic Analysis: Tectonic, Eustatic, and Climatic Controls on the Occurrence and Quality of Fossil Fuels”.

Abstract The concepts of autocyclicity and allocyclicity ( Beerbower, 1964 ) are extremely powerful tools in stratigraphic analysis. Unlike other approaches to genetic stratigraphy, autocyclicity and allocyclicity integrate sedimentary geochemical and physical processes through time and account for changes in both energy and materials in sedimentary systems. As used herein, energy refers to the physical energy by which sediment is transported and deposited whereas materials refers to transport and deposition of both solids and solutes. The term “cycle” in sedimentary geology refers to recurrent events ( Glossary of Geology, 1997 , p. 159), which may or may not be periodic. Given this definition of “cycle”, it follows that autocyclic and allocyclic events are recurrent, but they may or may not be periodic. According to Beerbower (1964) , autocycles are produced by processes within sedimentary systems. Responses to autocyclic processes tend to be local and may range from millimeter-scale ripple migration to regional-scale events such as delta switching. Autocyclic processes also include phenomena such as stream avulsion and meandering, and fluvial point-bar migration or lateral migration of beach-barrier bars. Unlike allocyclic processes, autocyclic processes tend to be instantaneous geologic events that are random in both time and space, and they contain few interregional feedback mechanisms. As a result, autocyclic processes are aperiodic. Because effects are local and mainly involve changes in energy, autocyclic processes generally result in changes in physical sedimentology and, more often than not, they do not appear to result in significant changes in chemical sedimentology.

Abstract Sediment discharge in fluvial systems is primarily a function of climate. In tropical and subtropical climates, discharge of solid sediment is nearly zero under perhumid climatic conditions whereas maximum discharge occurs under dry subhumid condition when there is a pronounced dry season and seasonality in rainfall is at a maximum. Discharge of solid sediment also approaches zero under arid conditions when there is little if any rainfall throughout the year. Solute discharge is also largely a function of climate. Solute discharge appears to be greatest under semiarid to subhumid climatic conditions and is nearly zero under hyperarid and perhumid conditions. The effects of tectonics and eustasy on sediment discharge are negligible, particularly at intermediate and short-term time scales. The potential for the production and accumulation of terrestrial organic matter as peat in a given basin is also a function of climate. The effects of tectonics and eustasy as controls on peat formation are minimal, but they are very important in burial and preservation of peat as coal. In addition, it is well known that the potential for eolian transport is also a function of climate, with maximum transport occurring under arid conditions. With increasing wetness, including seasonal rainfall, dune fields begin to stabilize and eolian transport diminishes and finally becomes zero. Climate is also one of the primary controls on weathering, pedogenesis, and soil formation. However, there is not always a strong relation between the genesis of soils and annual rainfall. Therefore, we relate the genesis of soil orders that form under relatively warm conditions to specific degrees of seasonality of rainfall when other factors such as parent material and relief are relatively constant.

Abstract Factors that influence fluvial sediment discharge in warm climates (catchment-basin size, relief, gradient, tectonic setting, bedrock lithology, and rainfall) can readily be evaluated in fluvial systems of Indonesia. In equatorial Sumatra and Seram, rainfall, catchment-basin size, relief, and gradient are similar, whereas bedrock geology and tectonic setting differ. The relief and rainfall in equatorial Borneo is similar to that of Sumatra and Seram, but gradient, catchment-basin size, and tectonic setting differ. All factors, except rainfall, are very similar for Timor and Seram. A pronounced dry season in Timor and Java distinguish those islands from the wet climates of Seram and Sumatra, respectively. The nature of stream channels (braided or meandering), stream bed materials, the degree of fluvial estuarine fill, deltas, and the nature of coastlines were used to evaluate sediment discharge. In addition, reconnaissance-level stream sampling was conducted for solid-suspended-sediment concentrations, solute concentrations, and pH in rivers in equatorial regions in Sumatra and Borneo, in Seram at 3° S, in Irian Jaya at 4° S, and in West Timor at 10° S. Rainfall in Sumatra, Borneo, and Seram exceeds evapotranspiration for all months of the year (> 100 mm/month and > 2.4 m/yr, perhumid climate). In contrast, in Timor 85 percent of all rainfall (1.4 m/yr) occurs during a four-month rainy season (dry subhumid climate). The absence of a fluvially derived bed load, river-mouth deltas, the lack of fluvial fill of estuaries, and mud-dominated coastal zones in the perhumid regions are indicative of a very low fluvial sediment discharge. Very low sediment concentrations (10 mg/l suspended and 10 mg/l solute) in modern rivers in the perhumid equatorial region of Indonesia are consistent with this observation. In contrast, sediment discharge in dry subhumid climates of Indonesia is very high, as indicated by coarse-grained braided-stream bed materials with cobbles transported to the coast, the complete fluvial fill of estuaries, the formation of river-mouth deltas, and coarse-grained beaches. Very high sediment concentrations (2100 mg/l suspended and 340 mg/l dissolved) during rainy-season discharge in modern rivers in dry-subhumid regions of Indonesia (Timor) are consistent with this observation. The dominant variable affecting fluvial sediment discharge among the islands of Indonesia, therefore, appears to be the degree of seasonality in rainfall regardless of tectonic setting, relief, or catchment-basin size. Solute concentrations in humid and perhumid climates are indicative of bedrock geology. Chemical weathering of massive Miocene limestone thrust sheets in high mountainous areas of Seram and Irian Jaya results in solute concentrations that approximate the solubility of calcite (∼ 50 mg/l). Humid and perhumid areas without significant limestone bedrock geology have solute concentrations that approximate that of rainwater (∼ 10 mg/l).

Abstract Research on cyclic sedimentation in the Pennsylvanian System of the United States began with Udden (1912) , who delineated cyclic patterns in Pennsylvanian strata in the Eastern Interior Basin (Illinois, U.S.A.). Udden described a stratigraphic succession associated with four upper Middle Pennsylvanian (Desmoinesian Stage) coal beds in western Illinois where similar lithologic sequences were repeated above each coal bed. The stratigraphic succession, as described by Udden, consists of the following lithologies:

Abstract Middle Pennsylvanian tectonics of the United States were dominated by the Laurasia–Gondwana collision and assembly of the Pangea supercontinent. The resulting Alleghanian and Ouachita orogenies along the eastern and southern continental margins as well as the Ancestral Rocky Mountain orogeny in the western interior have typically been linked to this collisional event. Large syntectonic clastic wedges spread westward and northward from the Appalachian and Ouachita highlands into the foreland basins and contributed large volumes of clastics to the midcontinent. Broad intracratonic basins such as the Illinois and Michigan were subsiding at the same time but during the Middle Pennsylvanian probably were not receiving any significant sediment contribution from the east. Although there is general agreement that the tectonic and depositional activities along the margins can be related to the construction of Pangea, the mechanism and underlying cause of the coeval mountain building and basin filling in the western interior are still enigmatic. Analysis of reflection seismic data along the Uncompahgre Uplift–Paradox Basin margin documents Desmoinesian and Wolfcampian thrust faulting with only minor lateral offset. The results of this and related studies indicate a principal stress direction of northeast–southwest, rather than northwest–southeast as would be expected if it had been the result of compression along the southeastern margin. Recent results from a number of workers suggest that the western and southwestern continental margins were significantly more active in the Pennsylvanian than previously thought and may hold the key to resolving some of the apparent inconsistencies.

Abstract The existence of a southern polar ice cap through geologic time can be established by identifying asymmetries in the marine stratigraphic records of the Northern and Southern Hemispheres produced by the interaction of precession-scale sediment-yield cycles and glacially controlled sea-level cycles. Asymmetries should be manifested in the clustering of bed thicknesses and other stratigraphic properties. Hemispheric asymmetry should not exist in an ice-free world. To demonstrate the concept, this paper establishes the foundations for the hypothesis and presents the results of a statistical analysis of the synthetic stratigraphy of a tropical monsoonal area.

Abstract A middle Pennsylvanian (Desmoinesian) conodont fauna was used to identify the interval represented by the Lower Kittanning cyclothem across the North American continent. The conodonts show both a succession through the cyclothem and a geographic variation that is related to the water mass and depositional environment. The faunas can be divided into normal-marine, high- and (or) variable-salinity, and high-productivity assemblages. Normal-marine faunas are dominated by Idiognathodus , which can be further divided into low-diversity and high-diversity faunas. The low-diversity Idiognathodus faunas characterize the marine deposits of the nearshore, marginal environments of the Appalachian Basin, and also the bulk of the black and gray shale and carbonate depositional environments of Midcontinent cyclothems. The high-diversity Idiognathodus faunas characterize fossiliferous limy shale and shaly limestone depositional environments typical of the shale–limestone transition in Midcontinent cyclothems and the richly fossiliferous limestone deposition in carbonate sequences in much of the western U.S., within the dolomite beds associated with sandstone deposition of the Tensleep Sandstone, and in fossiliferous limestones between sandstones and carbonate mudstones of the Paradox Basin. High-salinity and (or) variable-salinity faunas are dominated by Neognathodus and (or) Adetognathus , and characterize carbonate deposition in the Bird Spring–Ely platform, southwestern U.S., and shallow-water, typically algal facies within the Midcontinent cyclothems. High-productivity faunas are dominated by Gondolella , are extremely rare, and characterize horizons of surface substrate accumulation within black-shale and mud depositional environments. Maximum flooding within the cycle, as indicated by the highest diversity of normal-marine macrobiota and conodont fauna, does not occur in black shales but at the shale–limestone transition. Sixteen species of conodonts in eight genera are recognized in the cyclothem. The following four species are new: Idiognathodus crassadens, I. ignisitus, Neognathodus intrala , and Hindeodus calcarus. Two of these new species, N. intrala and I. crassadens , together with I. robustus , define an assemblage zone that allows correlation of the marine portion of the Lower Kittanning cyclothem across North America.

Abstract Many upper Middle Pennsylvanian coal beds can be readily identified and correlated by palynological methods on both an intrabasinal and an interbasinal scale. Coal palynology is also very useful in helping to reconstruct the vegetational composition of ancient mire floras, because the parent plants of most palynomorphs are known. As such, palynological analyses allow us to positively identify and trace upper Middle Pennsylvanian coal beds over large distances, while at the same time documenting changes in mire floras geographically. This paper discusses the criteria by which upper Middle Pennsylvanian coal beds can be palynologically identified and correlated. The role of palynology in helping to reconstruct the ecology of the ancient mires are also discussed, because plants are very sensitive indicators of local climatic, hydrologic, and edaphic conditions. One bed, the Lower Kittanning coal of the northern Appalachian Basin, is discussed in detail. The Lower Kittanning coal bed has been correlated by both palynologic and lithostratigraphic methods across three coal basins (Appalachian, Eastern Interior, and Western Interior). Results indicate that the Lower Kittanning is equivalent with the Princess #6 of northeastern Kentucky, the No. 5 coal of southeastern Ohio, and the No. 6 Block of southern West Virginia. In the Eastern Interior (Illinois) Basin, it is equivalent with the Colchester (No. 2) coal, and in the Western Interior Basin it is equivalent with the Croweburg/Henryetta coal of Oklahoma, Kansas, and Missouri, and the Whitebreast coal of Iowa. The palynofloras of the Lower Kittanning coal and correlative coals in the Eastern Interior and Western Interior Basins are similar in overall composition. Lycospora and tree fern spores co-dominate the palynofloras, with calamite and small lycopsid spores, and cordaite pollen, being locally abundant. Temporal patterns in all three basins are similar as well. Lycospora typically dominates the bottom third of the bed and is co-dominant with tree fern spores in the middle third. In the top third of the bed increased percentages of accessory taxa are seen, which include Calamospora, Laevigatosporites, Endosporites, Florinites , and Densosporites . Geographical differences include more abundant Densosporites in the Lower Kittanning coal of Ohio and Pennsylvania, and higher percentages of Florinites in the Croweburg coal of Oklahoma.

Abstract This paper evaluates the paleoceanography of marine strata within a Middle Pennsylvanian (Desmoinesian) cyclothem. Paleoecological data are used to interpret spatial and temporal changes in energy, salinity, and turbidity across the North American craton. Using mid-continent terminology, the marine part of the study interval commenced at the top of the Croweburg coal and continues up through the Verdigris Limestone. Cores from four boreholes, in Kansas, and exposures of the interval at 21 outcrops in 18 states were measured, described, and evaluated. Body fossils of marine invertebrates, some vertebrates, plant debris, and trace fossils were identified and evaluated in terms of their environmental and paleoecological significance. These biotic data are presented for eight geographic areas, as follows: (1) Appalachian Basin (Ohio, Pennsylvania, West Virginia, and eastern Kentucky); (2) Eastern Interior Basin (Illinois, Indiana, and western Kentucky); (3) Western Interior Basin (Missouri, Iowa, Kansas, outcrops and cores, and Oklahoma); (4) subsurface of northwestern Kansas; (5) Hartville Uplift (Wyoming) and Black Hills (South Dakota); (6) Fort Worth Basin (Texas); (7) Pedregosa Basin (Arizona); and (8) Paradox Basin (Utah and Colorado), Great Basin (Arrow Canyon, Nevada), and Death Valley (California). In the eastern and central United States the biota occurs in gray mudrocks and black, platy to fissile shales. Fossiliferous limestones occur at the top of the cycle in the central area and become better developed westward into Nevada and California, where the entire interval is dominated by limestone. Comparison of overall diversity and number of taxa versus lithology, mode of life, and feeding type in these eight areas indicates that the greatest diversity is in the Western Interior Basin. In this central region, calcareous skeletons in dark gray shales are commonly replaced by pyrite. Pyritization occurs in such environments because environments of dark gray mud are slightly more reducing than the more oxygenated depositional environments of lighter gray mudrocks and limestones. For ease of further comparison, the eight study areas are grouped into three regions, as follows: East, Central, and West. Comparison of these three regions indicates the following: (1) fragmentation of skeletons is greatest in the East, (2) epifaunal suspension feeders (typical of Paleozoic sedimentary sequences) dominate all three regions, and (3) there is some similarity between the general biotic diversity and total number of taxa in the black mudrocks and limestones in the Central region. Eustatic or tectonic sea-level change has traditionally been invoked to explain the differences documented in this stratigraphic interval and thus along our transcontinental transect. These two processes were active and, no doubt, may have been contemporaneous at some scale. Our data indicate, however, that temporal changes in water depths were similar in most basins across the North American continent. The biotic and lithologic changes, therefore, appear to be principally a function of differences in energy and salinity in response to temporal and spatial changes in climate and tidal conditions across a broad, shallow, island-studded epicontinental sea. Eustasy and tectonics controlled accommodation space, but temporal and spatial climate change was the primary control on sediment supply, wind-driven energy, and seawater chemistry.

Abstract Continental-scale correlations of a Middle Pennsylvanian fourth-order sequence have provided evidence for the relative importance of allocyclic controls on the formation of Pennsylvanian cyclothems. These correlations (and related studies) indicate that eustatic changes in sealevel were the primary control on accommodation space in most basins. Tectonic subsidence was a secondary control and provided additional accommodation space in a few basins. Temporal and spatial variations in climate, however, were the primary controls on physical and chemical sedimentology. Interpretations of changes in weathering and fluvial and eolian sediment supply suggest that the climate was wetter during lowstands than during highstands and that the physical and chemical oceanography of epeiric seas responded to changing patterns in atmospheric pressure regimes as sea level rose and fell. In addition, the climate was wetter in the eastern part of the tropical regions of what is now North America relative to the west. Temporal and spatial changes in paleoclimate, therefore, appear to be the primary control on lithostratigraphy. Previous studies have suggested that repetitive fluctuations in the sizes of continental ice sheets resulted in repetitive eustatic changes in sea level during the Middle Pennsylvanian (e.g., Wanless and Shepard, 1936 ). Such changes in ice volume and sea level must have been in response to global climate change. The climate model developed herein suggests that repetitive changes in rainfall patterns and surface winds at low latitudes were coincident with the glacial and interglacial intervals. During glacial intervals, a large permanent high pressure cell was associated with a southern-hemisphere ice cap (a nearly stationary polar front). The ice cap minimized annual (summer to winter) thermal variation in the atmosphere (sensible heating) over the southern-hemisphere land mass. As a result, permanent high pressure over the ice cap confined the intertropical convergence zone (ITCZ) to low latitudes, and a permanent low-pressure belt (doldrums) developed in the equatorial region of Pangea. During interglacials, the doldrums belt (low pressure belt) degenerated and was replaced by seasonal swings in the ITCZ in response to seasonal heating of air masses (sensible heating) over both northern-hemisphere and southern-hemisphere land masses. As a result, in low latitudes the climate changed from relatively wet conditions during glacial intervals to drier and more seasonal conditions during interglacial periods. Glacial and interglacial climates are indicated by the following: (1) intense chemical weathering of paleosols, low sediment supply, and peat formation (now coal in the eastern United States) during lowstands in response to a permanent low-pressure rainy belt and wet conditions, (2) deposition of black shale in basin centers during the early stages of transgression in response to low wind speeds and poor wind-driven circulation as the doldrums belt began to deteriorate, (3) transport and deposition of eolian sediments (western United States) in basin margins as sea level continued to rise and the doldrums belt disappeared, and (4) deposition of marine limestone in response to increased wind speeds and wind-driven circulation in epeiric seas coincident with highstands and maximum north–south swings of the ITCZ. All climatic factors (annual rainfall, seasonality of annual rainfall, wind speed, and wind direction) controlled sedimentation in cratonic depositional environments as sea level rose and fell. Although tectonics and eustasy controlled accommodation space, paleoclimate change (coincident with eustatic changes in sea level) controlled the lithostratigraphy of cyclothems at any given paleolatitude in the tropical regions of Pangea. There is no genetic relation between autocyclic delta-plain, back-barrier, or fluvial depositional models and the onset of Pennsylvanian peat formation as a precursor to commercial coal deposits.

Abstract Part 3 of this volume contains five papers that illustrate some of the various effects of climate on sedimentation, including studies of climatic controls on sediment supply in marine environments and in desert environments, numerical modeling of ancient climates, and climate influences on the occurrence of petroleum. The paper by Edgar and Cecil relates spatial changes in sedimentation in the Peru–Chile trench (off the west coast of South America) to spatial changes in climate along the Andes Mountain chain. They show that the deepest part of the trench is sediment starved adjacent to the zone where the Andes Mountains are highest and the climate is hyperarid. Fluvial sediment supply is climate dependent, regardless of the heights of mountains in provenance regions. As the climate becomes progressively wetter north and south of the sediment-starved region, the trench is progressively filled, even though the height of the Andes decreases. As an example of climate effects on basin fill, the spatial distribution of sediment in the Peru–Chile trench and the climate of western South America are used as an analogue for the late Paleozoic sediment fill of the Ouachita trough of North America. Edgar and Cecil suggest that the onset of filling of the Ouachita trough is related to the northward drift of the North American craton (from the dry tropics into the wet tropics) rather than the onset of tectonism. The article by Edgar et al. presents shallow seismic interpretations derived from the modern Gulf of Carpentaria, in northern Australia. The seismic data suggest that numerous Pliocene to Recent transgressive–

Abstract Traditionally, an abrupt and massive influx of siliciclastic sediments into an area of deposition has been attributed to tectonic uplift without consideration of the influence of climate or climatic change on rates of weathering, erosion, transportation, and deposition. With few exceptions, fluvial sediment transport is minimal in both extremely arid climates and in perhumid (everwet) climates. Maximum sediment transport occurs in climates characterized by strongly seasonal rainfall, where the effect of vegetation on erosion is minimal. The Peru–Chile trench and Andes Mountain system (P–CT/AMS) of the eastern Pacific Ocean clearly illustrates the effects of climate on rates of weathering, erosion, transport, and deep-sea sedimentation. Terrigenous sediment is virtually absent in the arid belt north of lat. 30° S in the P–CT, but in the belt of seasonal rainfall south of lat. 30° S terrigenous sediment is abundant. Spatial variations in the amount and seasonality of annual precipitation are now generally accepted as the cause for this difference. The spatial variation in sediment supply to the P–CT appears to be an excellent modern analogue for the temporal variation in sediment supply to certain ancient systems, such as the Ouachita Trough in the southern United States. By comparison, during the Ordovician through the early Mississippian, sediment was deposited at very slow rates as the Ouachita Trough moved northward through the southern hemisphere dry belt (lat. 10° S to lat. 30° S). The deposystem approached the tropical humid zone during the Mississippian, coincident with increased coarse clastic sedimentation. By the Middle Pennsylvanian (Atokan), the provenance area and the deposystem moved well into the tropical humid zone, and as much as 8,500 m of mineralogically mature (but texturally immature) quartz sand was introduced and deposited. This increase in clastic sediment deposition traditionally has been attributed solely to tectonic activity. However, we contend that the principal control on the introduction of abundant terrigenous sediment was the movement of the deposystem from an arid or semiarid climate into a seasonally wetter climatic regime. The physical and mineralogical maturity of the quartz sand is the result of tropical weathering in provenance areas.

Abstract The Gulf of Carpentaria is a tropical, silled epicontinental sea and may be a modern analogue for ancient cratonic basins. For the purpose of this study, the Gulf of Carpentaria is compared to Pennsylvanian cratonic basins of the United States. During the Pennsylvanian, the North American continent moved from the Southern Hemisphere, through the Equator, into the Northern Hemisphere. Today, the Gulf of Carpentaria–New Guinea region is a few degrees south of the Equator and is moving towards it. During the Pennsylvanian, the world was subjected to major glaciations and associated sea-level changes. The island of New Guinea and the Gulf of Carpentaria have undergone similar processes during the Quaternary. A reconnaissance seismic survey of the gulf conducted by the USGS and the Australian National University (ANU), combined with oil-exploration well data, provided the first step in a systematic evaluation of a modern tropical epicontinental system. During the Cenozoic, the region was dominated by terrestrial sedimentation in a temperate climate. At the same time, carbonates were being deposited on the northern shelf edge of the Australian Plate. During the Miocene, carbonate deposition expanded southward into the gulf region. Then in the Late Miocene, carbonate sedimentation was replaced by terrigenous clastics derived from the developing Central Range of the island of New Guinea, which developed a wetter climate while moving northwards into the tropics. At least 14 basin-wide transgressive–regressive cycles are identified by channels that were eroded under subaerial conditions since about the Miocene. Comparison of the modern Gulf of Carpentaria sequences with those of the Pennsylvanian reveals many similarities.

Abstract The Quaternary stratigraphy of the Chott Rharsa Basin, which is located in southern Tunisia (North Africa) on the northern margin of the Sahara Desert, displays distinct patterns of sediment distribution and stratigraphic accumulation as functions of climatic and tectonic variables. The northern margin of the basin is characterized by terraces of fluvial–lacustrine origin and by alluvial fans that have prograded into the basin from uplifted areas to the north. The center of the basin is occupied by a continental sabkha that lies below sea level, and the southern margin of the basin consists of eolian, lacustrine, and sabkha deposits. The Quaternary stratigraphic record is thicker and spans a longer period of time (at least 160,000 years) on the northern margin of the basin, but available dates reveal that major changes in the stratigraphic record have occurred at a relatively low frequency (tens of thousands of years). In contrast, the Quaternary record on the southern margin of the basin is thinner and spans a shorter period of time (about 13,000 years), but major changes in the stratigraphic record have occurred at a relatively higher frequency (thousand-year intervals). In addition, the northern and southern margins of the basin are out of phase with respect to the timing of deposition. During times of humid climates, the alluvial and fluvial systems on the northern margin of the basin are more active, while eolian systems on the southern margin are less active (i.e., stabilized by vegetation). In contrast, during arid times the alluvial and fluvial systems are less active, eolian systems are more active (i.e., not stabilized), and older alluvial and fluvial deposits tend to be reworked into eolian deposits. Furthermore, the detailed record on the southern margin of the basin reveals a history of creation and destruction of eolian stratigraphic records via temporal and spatial movement of an eolian sequence boundary, with each rise and fall of the water table (and associated climate change). The resulting stratigraphic record is thus the net sum of the positions of sequence boundaries, as a function of climate and water table. Finally, the record from the Chott Rharsa Basin demonstrates that subsidence alone is not sufficient for the creation of a stratigraphic record, and that the role of climate in this matter is more important than tectonic activity.

Abstract During the Middle Pennsylvanian, peat-producing swamps of year-round wetness must have occupied a broad tropical zone based on our knowledge of paleolatitudes of coal deposits formed during this period in North America, Europe, and Ukraine and the absence of growth rings in preserved wood found coexisting in these deposits. Here, we describe a series of global climate model simulations that evaluate the roles of tropical mountains, polar ice, and vegetation in determining the climate over tropical Gondwana. Our results suggest that the tropical mountains played the dominant role in allowing climatic conditions for tropical coal formation. The Central Pangean Mountains acted to impede the July excursion of the Intertropical Convergence Zone (ITCZ) north of the Equator and allowed the tropical everwet band to extend from 10° N to 12° S. The presence of permanent polar ice as a mechanism for increased year-round equatorial rainfall is shown to be less important. Polar ice on the southern supercontinent of Gondwana would have restricted the southward shift of the January tropical precipitation maximum but would have had no effect on the movement back northward in July. Forest vegetation over southern Gondwana would have warmed the southern high latitudes, melting the summer snowcover needed to accumulate into a polar ice sheet. These results define the large-scale boundary conditions necessary for Middle Pennsylva-nian coal formation.

Abstract Outcrop and subsurface examples of vertically stacked, multicyclic Pennsylvanian petroleum systems in the Powder River Basin of Wyoming are discussed. In the Rocky Mountain region, different source rock, reservoir quality, oil and gas composition, and seals among cycles reflect increasingly arid conditions from Desmoinesian to Virgilian time as the North American plate migrated from an equatorial position in early Carboniferous time into the arid trade-wind belt (∼ 20° N) in late Carboniferous time with associated eolian-sand-sea deposits. Here, Pennsylvanian rocks contain prolific hydrocarbon-producing, stratigraphically isolated reservoirs (commonly eolianites or phylloid algal mounds) and closely associated organic-rich black shale source beds. The black shales, containing as much as 80% total organic carbon (TOC), are the sources of Paleozoic oils in the Powder River and Paradox basins. The source beds are in a cyclic sequence composed ideally of black shale overlain by carbonate, clastic, and evaporite sediments grading back to carbonate and culminating in the next younger shale. Differences among the black shales, however, reflect in part climatic fluctuations in the marginal-marine settings where these shales were deposited. Kerogen in the black shale and carbonate marlstone of the Leo Formation in the Powder River Basin range from type II to type IV and are predominantly type III. Different proportions of terrestrial organic matter consistently are present in individual shale beds and most likely correlate with climatic variations; i.e., more humid in the early Pennsylvanian and more arid in the late Pennsylvanian. Transported organic matter may reflect cyclic variations in runoff and (or) terrestrial vegetative cover. A model of wetter climate would explain the combination of high TOC, high proportion of type III kerogen, high carbonate content, and carbon isotope composition (i.e., 12 C-depleted kerogen and 12 C-enriched bitumen) in some Desmoinesian shales and the resulting high associated H 2 S gas composition and higher sulfur content of derived oils. Desmoinesian reservoirs have dominantly carbonate cements. Wet/dry cycles are suggested for Missourian cycles where eolian dune sandstones cemented by both anhydrite and carbonate cement intercalate with very organic-rich, oil-prone (sapropelic) lacustrine black shales. Virgilian cycles in the Powder River Basin reflect even more arid conditions dominated by carbonates and evaporites with relatively minor clastics and a predominance of anhydrite cement in eolian reservoirs. The mechanisms controlling the deposition of the cyclothemic black shales are of great interest because source-rock quality is affected. These results suggest that differences among black-shale cycles should be considered as resulting from climatic fluctuations; a eustatic mechanism alone offers little utility in differentiating or understanding oil/rock differences among various Pennsylvanian cycles. Such variable effects are critical, as demonstrated by completion and production concerns relating to production of toxic associated gases and or formation damage, which can be related to specific cycles. Associated gases are highly variable among cycles, ranging from some with very high sulfur content (e.g., 48% in some Desmoinesian cycles) to others with high nitrogen and low sulfur content. Some of these differences are related to different regions with differing depth of burial and thermal maturity; however, where different oils are produced within a single field, as at Red Bird field, the differences are due to differences in source-rock composition that reflect variations in climate. A “sinking reservoir” accommodation model in an uncompensated basin is proposed to explain ancient coastal eolian reservoirs in the Pennsylvanian of eastern Wyoming. This model contrasts significantly with a “buried topography” model, which requires tectonic subsidence or eustatic rise to accommodate and preserve reservoirs. The “sinking reservoir” model may also be applicable to isolated carbonate (e.g., phylloid algal, thrombolitic, or stromatolitic) reservoirs in the Pennsylvanian of the Rocky Mountains, the Cambrian of Russia, and elsewhere. The repeated exposure of sediments to climatic influences in uncompensated basins provides conditions whereby the progressive loading of eolian, fluvial, deltaic, and carbonate sediments (i.e., potential reservoirs) on an unconsolidated substrate may preserve these potential reservoirs via the “sinking reservoir” model. The concept of subsidence moats around such reservoirs preserving a variety of nearby diachronous sediments, together with so called “disjunctive” (growth) faults created during subsidence, has important implications for interpreting sediment sequences.