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Flexurally Influenced Eustatic Cycles in the Pottsville Formation (Lower Pennsylvanian), Black Warrior Basin, Alabama

By
Jack C. Pashin
Jack C. Pashin
Geological Survey of Alabama, P.O. Box O, Tuscaloosa, Alabama 35486-9780
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Published:
January 01, 1994

Abstract

The Lower Pennsylvanian Pottsville Formation in the Black Warrior foreland basin of Alabama contains abundant coal and coalbed-methane resources, and because of numerous data from geophysical well logs, offers one of the best opportunities to evaluate causes of cyclicity in Carboniferous coal-bearing strata through subsurface mapping and facies analysis. Twelve regionally mappable transgressive-regressive cycles are present in the Pottsville Formation, which is of Morrowan age. Individual cycles accumulated in an average of 0.2 to 0.5 my and thus represent high-frequency fluctuations of relative sea level.

The Black Warrior basin underwent a rapid tectonic evolution related to progressive deformational loading of the Alabama promontory as the Appalachian-Ouachita orogen developed. Subsidence rate averaged approximately 15.3 cm/1,000 yr (0.5 ft/1,000 yr) in the structurally deepest part of the basin, and disregarding sediment influx or sea-level change, could account for an increase of water depth of more than 75 m (250 ft) during deposition of some cycles. This rapid subsidence evidently imparted pronounced asymmetry to relative sea-level variation by amplifying marine transgression and suppressing marine regression. Whether or not extremely rapid subsidence in response to the introduction of new load elements onto the continental promontory caused regional transgression is unclear, but episodes of enhanced loading probably resulted in at least local inundation as some cycles were deposited.

Although rapid flexural subsidence may have amplified marine transgression, no tectonic causes of regional marine regression were identified that operated at the time scale of deposition of a single Pottsville cycle. For this reason, glacial eustasy is considered to have been the dominant cause of cyclicity in the study interval. Consistent distribution of fluvial-deltaic sandstone and coal in each cycle mapped indicates that, despite rapidly changing subsidence patterns, a northwest- to west-dipping coastal plain and a single sediment-dispersal system persisted in Alabama. Hence, tectonism and eustasy operated faster than sediment could be dispersed from evolving sources in the advancing orogenic belt, and the resulting paleogeography was much more sensitive to eustatic sea-level variation than to flexural changes of basin geometry.

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Contents

SEPM Concepts in Sedimentology and Paleontology

Tectonic and Eustatic Controls on Sedimentary Cycles

John M. Dennison
John M. Dennison
University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
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Frank R. Ettensohn
Frank R. Ettensohn
University of Kentucky, Lexington, Kentucky
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SEPM Society for Sedimentary Geology
Volume
4
ISBN electronic:
9781565762275
Publication date:
January 01, 1994

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