Depositional systems and shelf-slope relationships in uppermost Pennsylvanian rocks of the eastern shelf, north-central Texas
The Eastern Shelf was a constructional platform developed on the margin of the sediment-starved Midland Basin during Late Pennsylvanian and Early Permian time. A mixed terrigenous-carbonate sedimentary province characterized the shelf during most of its history. Sediments were derived from highlands to the east and northeast. Along the outcrop in Eastland, Stephens, Young, and Jack counties, uppermost Pennsylvanian beds compose the Harpersville Formation, a boundary-defined rock stratigraphic unit within the Cisco Group. Harpersville facies extend westward into the subsurface 50 to 60 miles, 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 1100 feet with dips of up to five degrees. Three depositional systems are recognized on the basis of gross lithologic composition and position relative to the shelf edges. They are the Cisco fluvial-deltaic system, the Sylvester shelf edge bank systern, and the Sweetwater slope system. The Cisco fluvial-deltaic system is composed of dip-fed fluvial-deltaic facies and associated strike-fed interdeltaic embayment facies. Eight deltaic lobe complexes have been mapped. The Sylvester 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, which locally extended through the shelf edge bank system and onto the slope where submarine fans were constructed into the basin. The Eastern Shelf prograded into the Midland Basin by local upbuilding through fluvial, deltaic, and shelf edge bank deposition contemporaneous with outbuilding by slope fan deposition. Sites of shelf construction shifted through time in response to sedimentary and structurally controlled abandonment of delta lobes. Extrabasinal controls such as eustatic sea level changes were of secondary importance in developing the depositional fabric of the shelf.