ABSTRACT:The early Pennsylvanian (Morrowan) transgressions in the Illinois Basin can be documented by mapping the depositional limits of the subdivisions of the Mansfield Formation. Initial Morrowan subsidence and/or sea level rises resulted in northeastward onlap in three separate centers of deposition that are associated with paleovalley systems on the sub-Pennsylvanian landscape. In the central and southern parts of the basin most of the sub-Pennsylvanian topography was buried by late Morrowan time. In middle Pennsylvanian (Atokan) time onlap deposition had extended beyond the present outcrop belt of Pennsylvanian rocks in the Illinois Basin.
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