NADON, GREGORY C.¹, MARK COBB², and JOSEPH P. SMITH^1
1Dept. of Geological Sciences, Ohio University, Athens, OH
²Sverdrup Technology, ASGMS, Building No. 9101, Stennis Space Center, MS

Abstract: Partitioning the Controls on Rates of Formation of Accommodation Space in a Distal Foreland Basin

Sequence stratigraphic models of the fluvial deposits in foreland basins focus on proximal settings where the maximum rate of formation of accommodation space occurs. The distal foreland basin setting is an ideal location to investigate the interaction of all three variables related to the formation of accommodation space: tectonic, eustatic, and climatic. The Allegheny, Conemaugh, and Monongahela Groups (Desmoinesian-Virgilian) in the distal Appalachian foreland basin consist of a heterolithic assemblage 420 m thick. The Allegheny and lower Conemaugh contain both marine and fluvial facies; the upper Conemaugh and Monongahela Groups sediments are nonmarine. Incised valley systems occur throughout the section, however, they are most prevalent in the middle of the Conemaugh where, paradoxically, the most extensive marine limestone in the basin also occurs. Glacioeustatic variations in Pennsylvanian have been invoked to explain the high frequency variations in facies (cyclothems), but the prevalence of incised valley deposits in the middle of the Conemaugh is also a function of subsidence rate. Backstripping shows a change in medium-term subsidence rates from the 34 m/m.y. (Allegheny) to 20 m/m.y. in the Conemaugh Group to 38 m/m.y. in the upper Monongahela Group. Petrographic analysis of the sandstones from all three units suggest at least three higher frequency tectonic events that also affect facies patterns. The absence of marine facies in the upper Conemaugh and Monongahela Groups, despite a doubling of the subsidence rate, is interpreted to be primarily a result of a climate-induced increase in sediment supply.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas