Depositional Systems and Shelf-Slope Relations on Cratonic Basin Margin, Uppermost Pennsylvanian of North-Central Texas

Abstract

The Eastern shelf was a constructional platform developed on the margin of the sediment-starved Midland basin during Late Pennsylvanian and Early Permian times. A mixed terrigenous-carbonate sedimentary province characterized the shelf during most of its history. Sediments were derived from highlands on the east and northeast. Along the outcrop belt in north-central Texas, uppermost Pennsylvanian beds comprise the Harpersville Formation, a boundary-defined rock stratigraphic unit within the Cisco Group. Harpersville facies extend westward into the subsurface 50-60 mi (80-96 km), where they grade into equivalent shelf-margin carbonate and slope terrigenous facies. Preserved relief between the shelf margin and basin floor ranges from 600 to 1,100 ft (180-330 m) with dips ra ging up to 5 degrees. Three component depositional systems, recognized on the basis of gross lithologic composition and position relative to the shelf edges, are the Cisco fluvial-deltaic system, the Sylvester shelf-edge bank system, and the Sweetwater slope system. The Cisco fluvial-deltaic system is composed of fluvial-deltaic facies and associated interdeltaic embayment facies. Eight deltaic lobes have been mapped. The Sweetwater slope system is composed of several slope wedges, or fans, each of which includes shelf-margin, slope-trough, and distal-slope sandstone facies, as well as slope mudstone facies. Terrigenous sediments were transported across the shelf by prograding fluvial-deltaic channels that locally extended through the Sylvester shelf-edge bank system and onto the slope where deposition in he deeper basin constructed submarine fans. The Eastern shelf prograded into the Midland basin through contemporaneous, local upbuilding by fluvial, deltaic, and shelf-edge bank deposition and outbuilding by slope-fan deposition. Sites of shelf construction and outbuilding shifted through time in response to sedimentary and structurally controlled abandonment of major delta lobes. Extrabasinal controls such as eustatic sea-level changes were of secondary importance in developing the depositional fabric of the Harpersville shelf sequence. Petroleum is found in all three depositional systems. Principal productive facies include fluvial, distributary-channel, and distributary-mouth bar sandstones of the fluvial-deltaic system and distal slope and shelf-margin sandstones of the slope system. The complex, lenticular geometry of these thin deltaic sandstones affords maximum opportunity for development of stratigraphic and combination traps. Coal beds, consisting of detrital plant debris, are present within inter-deltaic embayment facies.