Abstract: Eustatic and Possible Tectonic Controls Over Depositional Architecture of Pennsylvanian Basin Filling on Northern Midcontinent Shelf and Basin Margin

Philip H. Heckel

During deposition of major Pennsylvanian cyclothems, glacial-eustatic sea level rise was rapid enough to deposit only thin transgressive limestone and shale over the entire northern midcontinent shelf. During highstand, so much accommodation space existed in relatively deep dysoxic to anoxic water that only thin, dark, phosphatic, conodont-rich shale was deposited across the entire region that is presently preserved. Thicker carbonate and siliciclastic sediments undoubtedly prograded at the highstand shoreline, but these have been erosionally removed north of the Iowa outcrop. Only during sea level fall did the sea bottom become reoxygenated, warm and sunlit enough for the shallowing-upward regressive limestone to form over most of the shelf. It would have initiated on higher areas (p rticularly near the highstand shoreline), and it prograded into lower areas as bottom conditions progressively improved while sea level continued falling, eventually exposing the limestone on higher areas. Occasional interruptions of general regression by minor sea level rises resulted in an en-echelon succession of shallowing-upward carbonate wedges prograding basinward. If a siliciclastic source became available, a wedge of deltaic sediment prograded into the decreasing accommodation space. This either smothered the regressive limestone, or just mantled its thinner basinward end, depending on where the accommodation space was accessible. During sea level lowstand, either type of regressive deposit continued to prograde basinward, and each typically could form a thick mass of sediment i the remaining accommodation space. Sea level rise, however, drowned both types of deposits, and their wedge-shaped forms were preserved by draping of thin transgressive and highstand sediment-starved deposits over them. A succession of regressions that took shoreline to similar positions on the middle to lower shelf caused both upbuilding and outbuilding of a sedimentary shelf edge composed of these thick wedges of regressive/lowstand sediment. Greater regressions accompanied by abundant detrital influx caused rapid filling of more of the basin margin, covered previous depositional topography, and resulted in formation of a younger sedimentary shelf edge farther basinward of the previous ones.

Tectonic subsidence provided the general area of accommodation space, and increasing rates of subsidence provided progressively more space toward the foreland basin of central Oklahoma. This would have contributed to general upbuilding (i.e., stacking) of the more basinward depositional shelf edges in Marmaton strata of northern Oklahoma, compared to general outbuilding of depositional shelf edges in Missourian strata of eastern Kansas. The major immediate control over basin-filling architecture, however, was how far basinward regression took the shoreline before the next sea level rise, as most of the sediment on the preserved higher to mid-shelf was deposited during middle to late regression, and most of that on the lower shelf was deposited during late regression and lowstand, when it formed the depositional shelf edges.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994