Facies of Cisco-Equivalent Slope Deposition Systems and Their Role in Construction of Eastern Shelf: ABSTRACT

Abstract

Uppermost Pennsylvanian rocks of the Eastern shelf, a paleogeographic feature of the eastern flank of the Midland basin, consist of 3 depositional systems. (1) The Cisco fluvial-deltaic system extends across the updip part of the shelf and consists of fluvial to deltaic facies flanked by interdeltaic embayment facies. (2) Downdip, along the margin of the shelf, the Sylvester shelf-edge bank system separates relatively flat-lying shelf units from slope deposits. (3) The Sweetwater slope system is composed of numerous broad, coalescing to restricted wedges of sediment 800 to 1,200 ft thick. The terrigenous wedges are bounded by carbonate aprons that extend upslope into the shelf-edge banks. Sandstone distribution within the slope wedges forms fan-shaped maxima that are elon ate perpendicular to the shelf edges and grades from relatively narrow, restricted belts upslope to irregular, broad patches downslope. Principal facies include shelf-margin sandstone, which caps the slope wedges, slope-trough sandstone, which extends through the body of the slope wedge, and distal-slope sandstone. Slope-trough facies include thinly interbedded sandstone and mudstone and medium to thick-bedded, graded sandstone units. Sedimentary structures and textures indicate sporadic deposition, probably by turbidity currents and associated traction carpets, and by subsequent reworking. The slope system was fed from distal parts of the largest delta distributaries that prograded to the shelf edge, or where tidal or storm currents swept sediment through local breaches in the bank system. Sediment moved downslope by gravity transport mechanisms. The Eastern shelf was constructed by both simultaneous upbuilding (fluvial-deltaic and shelf-edge bank deposition) and outbuilding (slope deposition). Sites of deposition shifted widely across the shelf and slope in response to subtle variations in subsidence and consequent abandonment of active delta lobes. Petroleum production is limited to lower slope-trough, distal-slope, and shelf-margin sandstones. The small number of fields in the slope system may be the result of (1) low permeability, characteristic of slope sandstones, (2) updip migration of petroleum through slope sandstone facies into facies of other systems, or (3) insufficient drilling. End_of_Article - Last_Page 153------------