CLEAVES, ARTHUR W.
School of Geology, Oklahoma State University, Stillwater, Oklahoma

Abstract: Genetic Stratigraphy and Incised Valley-Fill Deposits of Middle and Upper Pennsylvanian Depositional Sequences, North-Central Texas

Incised valley-fill fluvial systems are preserved in 25 of 26 fourth order glacialeustatic cycles in the stratigraphic interval between the top of the middle Strawn Brannon Bridge Limestone and the base of the middle Cisco Saddle Creek Limestone. The sharp base beneath each of these deeply eroded, chert-conglomerate-bearing channel complexes indicates the presence of an Exxon type 1 sequence boundary. Each disconformity developed during the part of a relative sea-level change cycle when forced regression was causing the maximum rate of sea level decline. These channels typically downcut 60 to 180 feet into underlying delta-plain and marine-shelf facies and in all instances truncate marine transgressive limestone and black shale units laid down earlier during the cycle. Each interval bracketed by incised-valley disconformities constitutes a regionally mappable Depositional Sequence across the entire shelf.

Pennsylvanian incised valley-fill systems of the Eastern Shelf comprise sediment dispersal conduits that transported sediment from the Ouachita Mountains westward to the Midland Basin. Each system involves four distinct dip-oriented segments defined on the basis of associated facies. These components include an updip alluvial-fan element, incised delta lobe element, a "naked" channel element superimposed on marine shelf deposits, and an outer shelf, meandering, trunk channel that supplies sediment to the lowstand, perched delta complex. Each of these segments contains different reservoir facies and distinct stratigraphic trapping mechanisms for hydrocarbons. Alluviation within the three more distal segments takes place largely during the ensuing marine transgression.

AAPG Search and Discovery Article #90918©1999 AAPG Southwest Section Meeting, Abilene, Texas