Philip H. Heckel

Origin of Phosphatic Black Shale Facies in Pennsylvanian Cyclothems of Mid-Continent North America

Continued lithologic and paleontologic considerations of Kansas Upper Pennsylvanian megacyclothems more firmly establish the basic ascending sequence of: outside (nearshore) shale--middle (transgressive) limestone--core (offshore) shale--upper (regressive) limestone--outside (nearshore) shale, as representing a single transgressive-regressive sequence; this sequence is thus considered simply a cyclothem. Lateral facies change along the 500-km Iowa-Kansas outcrop belt is greatest in upper parts of upper limestones and outside shales, as would be expected in shallow-water to shoreline deposits; it is least in middle limestones, core shales, and bases of upper limestones, as would be expected in deeper water deposits. The widespread phosphatic black shale facies that commonly accompanies maximum transgression in the core shale is explained by water becoming deep enough to develop a thermocline strong enough to prevent bottom oxygenation by wind-driven vertical circulation. Pennsylvanian position of Mid-Continent North America, in the trade-wind belt north of the paleoequator along the Appalachians, allowed establishment of large-scale quasi-estuarine circulation in the Mid-Continent epicontinental sea. Cold, deep, oxygen-poor, phosphate-rich water from the western ocean was drawn in along the bottom through the basins of West Texas eventually to upwell in the eastern Mid-Continent and replace the surface water moved westward out of the sea by the prevailing winds. Upwelling greatly increased surfac -water production of organic matter, which continually settled (while being carried westward) into the deeper incoming current, where it decayed and depleted the remaining oxygen while continually enriching the already high phosphate in a circulatory trap. In this way substantial organic matter and phosphorite were deposited on the anoxic sea bottom to produce the phosphatic black shale facies. This model for offshore phosphatic black shale deposition obviates the difficulty of explaining in shallow tropical water the combination of nonskeletal phosporite production, and widespread lateral uniformity of a quiet anoxic environment between two marine limestones. It supports large-scale Pennsylvanian transgressions and regressions in the Mid-Continent sea, but remains compatible with the local cyclic sedimentary process of delta outbuilding and abandonment along the shoreline. In fact, large-scale marine transgressions and regressions account for the widespread distribution of delta-shoreline deposits from the Appalachians to Kansas. The offshore black shale model can be expanded to a more general depositional model that not only explains the lateral variation in black-shale-be ring Pennsylvanian cyclothems from the Appalachians to West Texas, but also accounts for the scarcity of black shales in younger Pennsylvanian and Permian Mid-Continent cyclothems by suggesting that water depths at maximum transgression during that time were generally too shallow to establish an effective thermocline.

Glacial-eustatic sea-level curve for early Late Pennsylvanian sequence in north-central Texas and biostratigraphic correlation with curve for midcontinent North America

At least 30 transgressive-regressive cycles of deposition are recognized from the upper Desmoinesian East Mountain Shale to the mid-Virgilian Wayland Shale in north-central Texas. Maximum regressive deposits are typically paleosol mudstones and fluvial sand- stones; maximum transgressive deposits are typically widespread, ammonoid-bearing, conodont-rich, dark phosphatic shales in more major cycles, and persistent fossiliferous shales or limestones overlying terrestrial deposits in more minor cycles. Delta complexes dominate the regressive sequences of many cycles. Using biostratigraphic criteria of first, last, sole, or acme occurrence of ammonoid, conodont, and fusulinid taxa, we correlate 17 cycles in the Texas sequence directly with 17 glacial-eustatic cycles of similar magnitude in the northern midcontinent. This correlation suggests that glacial eustacy was the basic control over the cyclic sequence in Texas, that tectonic masking of the eustatic signal by nearby orogenic movement in Texas was negligible, and that delta shifting, though conspicuous, was only a secondary control over the cyclicity there. Minor cycles recognized between the correlated cycles also match well enough between Texas and the midcontinent to further discount potential tectonic or deltaic masking of glacial-eustatic cyclicity. This strengthens the likelihood of correlating glacial-eustatic events across larger parts of North America, and perhaps with other parts of the world.

Paleogeography of Eustatic Model for Deposition of Mid-Continent Upper Pennsylvanian Cyclothems: ABSTRACT

Abstract The hypothesis that eustatic sea level changes produced Upper Pennsylvanian cyclothems in Midcontinent North America has been supported by recent documentation of many episodes of Mississippian through Permian glaciation in Gondwanaland (Crowell, 1978). Changes in Midcontinent paleogeography and sedimentary facies during a single eustatic advance and retreat of the sea are described in 6 phases: 1. At maximum transgression, deep water promoted development of a thermocline, quasi-estuarine circulation cell, upwelling, and anoxic bottom conditions, all leading to widespread deposition across the Mid-continent of phosphatic black shale, which graded in shallower peripheral areas to gray marine shale and carbonates. 2. Shallowing during early regression destroyed the thermocline, which restored bottom oxygenation and changed deposition from black to gray shale, then to skeletal calcilutite as benthic algal carbonate production became established across the Midcontinent. Deltas began prograding from Oklahoma and the Appalachians, and shoreline carbonates began prograding southward from the Dakotas. 3. During late regression, shoal-water calcarenites developed over most of Kansas, carbonate shoreline facies prograded into southern Nebraska and Iowa, and deltas of Appalachian origin prograded across Illinois. 4. At maximum regression, the sea became nearly confined to the deep basins of west Texas and Oklahoma, while the exposed carbonate terrain to the north underwent formation of karst, caliche and residuum, and the extensive deltaic deposits to the east underwent channeling, alluviation, and soil and coal-swamp formation. 5. Expansion of the sea during early transgression restored shoal-water calcarenite deposition across western Kansas, caused gray shale deposition in embayments and lagoons along the inundated deltaic terrain to the east, and impounded Appalachian-derived streams flowing westward across the immense alluvial plain to form widespread coal swamps in Illinois. 6. Deepening during late transgression restored skeletal calcilutite deposition across the Midcontinent, caused marine shell accumulations over coals in Illinois, and shifted transgressive coal-swamp formation eastward into the Appalachian region. In Texas the same sea level changes resulted in much less lateral shifting of shoreline and associated facies because of greater depositional relief, narrower shelves, and closer detrital sources. Thus individual units there are less widespread, and eustatic effects are less noticeable than in the Midcontinent.

Cyclothem [“digital”] correlation and biostratigraphy across the global Moscovian-Kasimovian-Gzhelian stage boundary interval (Middle-Upper Pennsylvanian) in North America and eastern Europe

The long-standing difficulty of correlating Pennsylvanian strata among provincial faunal regions is resolved by effecting “digital” correlation of major glacial-eustatic cyclothems that represent high-stands when certain species achieved more global distribution than usual. In the late Moscovian–early Gzhelian (late Desmoinesian–early Virgilian) succession in the midcontinent United States, several major cyclothems are correlated, by both conodont species in common and cyclothem scale, with cyclothems in Russia (Moscow Basin) and Ukraine (Donets Basin), and the remaining cyclothems fit into the framework by position and scale. In this way the suggested event marker for the global Kasimovian-Gzhelian stage boundary (first appearance of Idiognathodus simulator ) is supported, while possible event markers for the Moscovian-Kasimovian boundary await further evaluation.

Cyclostratigraphy - concepts, definitions, and applications

Cyclostratigraphy is the subdiscipline of stratigraphy that deals with the identification, characterization, correlation, and interpretation of cyclic variations in the stratigraphic record and, in particular, with their application in geochronology by improving the accuracy and resolution of time-stratigraphic frameworks. As such it uses astronomical cycles of known periodicities to date and interpret the sedimentary record. The most important of these cycles are the Earth’s orbital cycles of precession, obliquity, and eccentricity (Milankovitch cycles), which result from perturbations of the Earth’s orbit and its rotational axis. They have periods ranging from 20 to 400 kyr, and even up to millions of years. These cycles translate (via orbital-induced changes in insolation) into climatic, oceanographic, sedimentary, and biological changes that are potentially recorded in the sedimentary archives through geologic time. Many case studies have demonstrated that detailed analysis of the sedimentary record (stacking patterns of beds, disconformities, facies changes, fluctuations in biological composition, and/or changes in geochemical composition) enables identification of these cycles with high confidence. Once the relationship between the sedimentary record and the orbital forcing is established, an unprecedented high time resolution becomes available, providing a precise and accurate framework for the timing of Earth system processes. For the younger part of the geologic past, astronomical time scales have been constructed by tuning cyclic palaeoclimatic records to orbital and insolation target curves; these time scales are directly tied to the Present. In addition, the astronomical tuning has been used to calibrate the 40Ar/39Ar dating method. In the older geologic past, “floating” astronomical time scales provide a high time resolution for stratigraphic intervals, even if their radiometric age is subject to the error margins of the dating techniques. Because the term “sedimentary cycle” is used in many different ways by the geologic community and does not always imply time significance, we propose using “astrocycle” once the cycle periodicity has been demonstrated by a thorough cyclostratigraphic analysis. Authors’ addresses: André Strasser, Department of Geosciences, University of Fribourg, CH-1700 Fribourg, Switzerland, e-mail: andreas.strasser@unifr.ch; Frederik J. Hilgen, Faculty of Earth Sciences, University of Utrecht, 3584 CD Utrecht, The Netherlands, e-mail: fhilgen@geo.uu.nl; Philip H. Heckel, Department of Geoscience, University of Iowa, Iowa City, Iowa 52242, U.S.A., e-mail: philip-heckel@uiowa.edu DOI: 10.1127/0078-0421/2006/0042-0075 0078-00421/06/0042-0075

Glacial-Eustatic Base-Level--Climatic Model for Late Middle to Late Pennsylvanian Coal-Bed Formation in the Appalachian Basin

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Thin widespread Pennsylvanian black shales of Midcontinent North America: a record of a cyclic succession of widespread pycnoclines in a fluctuating epeiric sea

ABSTRACT Late Middle-Late Pennsylvanian (middle Allegheny and lower Conemaugh) marine units in the Appalachian basin represent the distal edges of glacial-eustatic marine incursions of the Midcontinent sea onto the detrital apron of the Appalachian highlands. Because these marine units typically overlie coal beds, the main environment of formation of this type of coal bed appears to have been that of a coastal swamp migrating ahead of transgression. As base level rose, the water table continually rose, and the swamp was further nourished and surrounding vegetation kept thick enough to inhibit detrital influx by rainfall from the expanding nearby source of moisture. Even in parts of the Appalachian cyclic succession with no marine units (upper Allegheny, Monongahela, Dunkard), the coal beds rep esent the wettest part of the climate cycle of Cecil (1990), sandwiched between detrital units representing seasonal wet-dry climates when sediment was most readily mobilized, and antipodal to nonmarine limestones representing the driest climates. Throughout this time, the Appalachian basin was in the center of the tropical zone of a nearly accreted megacontinent, quite distant ( 4000 km) from permanent oceanic sources of moisture. Climate modeling without considering inland seas or orographic effects on winds shows that the tropical zone of such a large land mass would have suffered from a severe deficit in rainfall and soil moisture. Although further modeling shows that the Appalachian orogenic highlands would have drawn in moisture to ncrease rainfall in their vicinity, they also may have impeded the normal westward flow of moisture,laden easterly winds. The western source for enough moisture to form coal beds was most likely the North American Midcontinent sea. During major highstand, this sea would have covered about 2 million km2 and provided both high base level and abundant nearby moisture for enough continual rainfall to keep the coastal peat swamps widespread, fresh, and relatively detritus-free in the Appalachian basin. In contrast, during major lowstand it would have covered only about 200,000 km2 with its shoreline about 1500 km away, and it apparently provided less consistent moisture to the Appalachian basin then, because most nonmarine limestones and paleosols formed there during the e times contain evidence of drier climates. Therefore, upper Allegheny, Monongahela, and Dunkard coal beds most probably formed when the Midcontinent sea was approaching highstand and stood just west of the presently preserved outcrop. Thus, they likely represent the coastal lowland equivalent of a marine unit at highstand farther west. This constrained time of peat formation and preservation, in a variety of coastal environments prior to and during marine highstand, is responsible for the discrete and widespread nature of the coal-bed interval in a typical depositional cycle (or cyclothem), a pattern that stands in contrast to the complicated relations of overlying and underlying detrital units, which reflect the segregation of sand and mud into a complex mosaic of local deltaic and ter estrial subenvironments that developed when sea level rainfall and the water table were lower.

NO MAJOR STRATIGRAPHIC GAP EXISTS NEAR THE MIDDLE-UPPER PENNSYLVANIAN (DESMOINESIAN-MISSOURIAN) BOUNDARY IN NORTH AMERICA

Abstract Within the Middle-Upper Pennsylvanian cyclic sequence of Midcontinent North America (Iowa, Nebraska, Missouri, Kansas, Oklahoma), there are at least 20 horizons of thin (<1 m) phosphatic black shales that extend up to 600 km along outcrop. They overlie transgressive limestones that range from very thin (1–5 cm) shell beds where they lie above coals, to thin (1 m or less) skeletal calcilutites where they lie above fluviodeltaic deposits and palaeosols. The black shales underlie thicker (2–10 m) regressive limestones that are characterized by various types of shallowing-upward sequences and are typically capped by palaeosols on the northern shelf (Iowa to central Kansas). The black shales represent the deepest-water deposits within the succession of widespread eustatic inundations that were responsible for the major cyclothems. Their frequency in the cyclic sequence is estimated to be close to that of the longer of the earth’s orbital parameters that are considered to control the wax and wane of continental glaciation. They apparently formed beneath pycnoclines that were established during interglacial high-stands of sea level when the water was both warm enough (at the surface) and deep enough to maintain a thermocline for perhaps several thousand years. These thermoclines were probably augmented by haloclines in areas of greater fresh-water run-off and possibly in areas accessible to more saline bottom water.

Phylloid Algal-Mound Complexes in Outcropping Upper Pennsylvanian Rocks of Mid-Continent

Abstract Interregional correlation of the marine zones of major cyclothems between North America and eastern Europe does not support assertions that a major stratigraphic gap exists between the traditional regional Desmoinesian and Missourian stages in North America. Such a gap was previously proposed to explain an abrupt change in megafloral assemblages in the northern Appalachian Basin and by extension across all of North America. Conodont-based correlation from the essentially complete low-shelf Midcontinent succession (distal from the highstand shoreline), through the mid-shelf Illinois Basin, to the high shelf of the Appalachian Basin (proximal to highstand shoreline) demonstrates that all major ∼400 kyr cyclothem groupings in the Midcontinent are recognizable in the Illinois Basin. In the Appalachian Basin, however, the grouping at the base of the Missourian is represented only by paleosols and localized coal. The immediately preceding grouping was removed very locally by paleovalley incision, as is evident at the 7–11 Mine, Columbiana County, Ohio, from which the original megafloral data were derived. At the few localities where incised paleodrainage exists, there may be a gap of ∼1000 kyr, but a gap of no more than ∼600 kyr occurs elsewhere in the Appalachian Basin at that level and its magnitude progressively decreases westward into the Illinois (∼300 kyr) and Midcontinent (<200 kyr) Basins. Thus, while a gap is present near the Desmoinesian–Missourian boundary in North America, it is typically more than an order of magnitude smaller than that originally proposed and is similar to the gaps inferred at sequence boundaries between cyclothems at many horizons in the Pennsylvanian of North America.

Missourian (early late Pennsylvanian) climate in Midcontinent North America

A phylloid algal-mound complex is a local to subregional thickening of limestone attributed chiefly to the presence of a distinctive suite of rock types containing leaflike or phylloid algae. Twenty-three such mound complexes are present at or near the southern ends of most limestone units in Missourian and lower Virgilian (Upper Pennsylvanian) rocks exposed in eastern Kansas, northeastern Oklahoma, and northwestern Missouri. Mound complexes are composed of two facies, (1) the mound, consisting primarily of massive algal calcilutite to algal sparite, and (2) mound-associated facies, consisting primarily of thin- and cross-bedded skeletal and oolitic calcarenite capping and flanking the mound. Overlying shale beds thin across the tops of mound complexes. Most mound complex s grade northward into thinner, more diversely fossiliferous, open marine limestone beds, and grade abruptly southward into thin limestone beds and lenses and thick terrigenous clastic strata. Pennsylvanian phylloid algae are comparable with Holocene calcareous codiacean green algae and coralline red algae, and flourished in shallow sunlit water where they were sediment suppliers and stabilizers. Mounds probably began on topographic highs favorably situated between a region of great clastic influx and the open sea, and grew as the algae proliferated and produced sufficient sediment to compensate for subsidence. Mound growth allowed the algae to continue to flourish in their optimum sunlit environment. Stacking of mound complexes may reflect positive topographic influence of underlying mounds on the sea bottom, and shifts in the stackings probably resulted from shifts in northward extent of the clastic influx.