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Abstract: Incised Valley-fill Deposits in Middle and Upper Pennsylvanian Systems Tracts, Eastern Shelf of Midland Basin, North Central Texas

CLEAVES, Arthur W., School of Geology, Oklahoma State University, Stillwater, OK

Incised valley-fill fluvial systems occur in 29 of 30 fourth order glacial-eustatic cycles of the post-Brannon Bridge Strawn, Canyon, and Cisco groups on the Eastern Shelf in North-Central Texas. The erosional base to these systems represents an Exxon Type 1 sequence boundary, which forms during the period of maximum rate of sea level drop for a forced regression. Individual channels downcut 10m to 50m into underlying highstand deltaic facies and open shelf siliciclastics, as well as into transgressive marine carbonate and black shale of the preceding cycle. Headward erosion of tributaries into an alluvial fan apron bordering the Ouachita Mountains enabled coarse-grained detritus to be transported out onto the exposed lowstand shelf, but not as far as the Delta plain of shelf-margin delta lobes. Most of the chert gravel from the mountain front was trapped and permanently stored in the valleys during the ensuing transgression, as stream gradients became lower and transport competence abruptly decreased.

Valley-fill conglomerates were typically superimposed on late highstand delta lobes. Forced regression caused a basinward extension of one or more distributaries in the youngest active lobe of the delta system. As new, perched delta subsequently became established at the shelf edge upon attainment of stable lowstand conditions. The feeder stream that connected the two deltaic fairways downcut to adjust for the higher gradient, converting the original highstand distributary into an incised-channel system of the lowstand coastal plain. Early transgressive deposits, predominantly chert conglomerate, comprise the best hydrocarbon reservoirs and entrap hydrocarbons in incised meander loops.

AAPG Search and Discovery Article #90936©1998 AAPG Southwest Section Meeting, Wichita Falls, Texas