Flexurally Influenced Eustatic Cycles in the Pottsville Formation (Lower Pennsylvanian), Black Warrior Basin, Alabama
Jack C. Pashin, 1994. "Flexurally Influenced Eustatic Cycles in the Pottsville Formation (Lower Pennsylvanian), Black Warrior Basin, Alabama", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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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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The collected volume begins with a brief perspective by one of the conveners, followed by articles in order of increasing stratigraphic age. Eustatic sea-level changes and tectonic warpings of basins are competing mechanisms for explaining many stratigraphic patterns. The model for sea-level changes should be developed first for a basin, since it is allocyclic and leads to a series of time bands in the strata. The residual effects should then be modeled for tectonic patterns affecting the depositional processes. Doing the reverse limits time constraints on the tectonic warping models and will blur the resolution of detailed time surfaces in the strata. Case histories of situations with both tectonic warping and time surfaces marked by sea-level events will lead to improved interpretations of earth history.