Abstract

Depositional models for incised valley-fill systems in Pennsylvanian siliciclastic rocks of North-Central Texas should be constructed based on the nature of the regional sediment transport pathway and on the sedimentation pattern present within the specific systems tracts of a Type 1 sequence. Specific sedimentary characteristics such as texture, mineral composition, and overall reservoir quality depend on four principal controls. These controlling factors include the length of the sediment dispersal system, nature of the drainage hinterland (either coastal plain or piedmont), channel stacking characteristics, and the rapidity of relative sea level rise associated with deposition of the transgressive systems tract. Very likely, these Pennsylvanian incised channel systems all developed under conditions of high-frequency, glacio-eustatic cyclicity that generated rapid absolute sea level increases during major transgressive episodes.

Orthodox sequence stratigraphic concepts stipulate that incised valleys are cut at the time of maximum rate of sea level drop. The valleys are then backfilled as deposition of the transgressive systems tract proceeds. During both forced regression and the lowstand of any given eustatic cycle, sediment is transported through the valleys, yet does not accumulate within them to any significant extent. This situation contrasts greatly with the pattern seen for the Strawn, Canyon, and Cisco groups on the Eastern Shelf of the Midland Basin. Incised valley-fill chert gravels preserved in the three groups were transported part of the way through the eroded depressions, but ultimately were stranded in the upper and middle parts of the valleys. The coarse-grained chert detritus never reached the lowstand shoreline or deep-water facies. Finer-grained sediment fractions such as mud and sand outran the gravel during forced regression. They accumulated as lowstand basinal, slope fan, and perched deltaic systems.

The sediment dispersal system for Pennsylvanian Eastern Shelf fluvial-deltaic transport networks is characterized by five distinct segments as seen in map view. These segments are: (1) lowstand (unincised) delta distributaries, (2) lower coastal plain meandering streams, (3) distal incised-valley channels superimposed on the highstand marine shelf, (4) incised-valley channels eroded into highstand delta lobes, and (5) proximal trunk incised valley-fill channels and their tributaries updip from the highstand shoreline and delta plain. Further to the east, a piedmont region fringing the Ouachita Mountains (not preserved) was composed of coarse grained alluvial fans that fed sediment into the transport network during periods of high gradient stream flow. Only segments 3-5 contain chert conglomerate, as each episide of forced regression was too brief to allow for gravel transport across the entire highstand shelf. All five fluvial segments of the terrestrial dip-transport network have distinct hydrocarbon trapping mechanisms that can be delineated by means of cross sections, subsurface mapping, and geophysical surveys.