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