Upper Paleozoic Fluvial-Deltaic, Shelf, and Slope Depositional Systems in a Cratonic Basin, West-Central Texas: ABSTRACT

Abstract

Upper Pennsylvanian and Lower Permian rocks on the eastern flank of the West Texas basin were deposited within fluvial-deltaic, shelf-edge bank, and slope depositional systems. The Cisco fluvial-deltaic system consists of numerous 100- to 200-ft-thick packages of dip-fed facies flanked by strike-fed interdeltaic embayment facies. Fluvial deposits include tabular and belted sandstone bodies characterized by braided and coarse-grained, meander-belt sequences. Fine-grained multistory meander-belt facies exhibiting general fining upward point-bar sequences commonly occur high in the system, pointing to decreasing sediment supply from the Ouachita Mountains and Fort Worth Piedmont. Fluvial channels normally cut subjacent deltaic facies and may lie on rocks of previous depositi nal episodes. Cisco deltaic facies are progradational, coarsening upward prodelta, delta-front, channel-mouth bar, and distributary channel units, and aggradational crevasse splays and delta-plain facies (coal, mudstones, or organic-rich clays); local destructional sandstone bars may fringe the delta. Interdeltaic embayment facies flank delta lobes and include mudstones, sheet sandstones, limestones, and impure detrital coals. Sheet sandstones may reflect strike-fed strandplain accretion or locally reworked delta-front sandstones. The Sylvester shelf-edge bank system occupies a position along the shelf to basin break in paleoslope. The system is composed predominantly of carbonate units that intertongue updip with fluvial-deltaic facies and downdip with slope facies. Individual units range from 100 to 400 ft thick; carbonate facies End_Page 1906------------------------------ thicken upward and basinward because terrigenous sediment supplied to the fluvial-deltaic system diminished. The Sweetwater slope system, studied intensively by W. E. Galloway, is composed of numerous broad, coalescing to restricted wedges of mudstone and sandstone averaging 1,000 ft thick. Slope wedges are bounded by carbonate aprons and the wedges thin upward and basinward in response to decreasing terrigenous sediment supply. Sandstone distribution within slope wedges forms fan-shaped patterns that are elongate perpendicular to shelf edges. Principal slope units include shelf-margin, slope-trough, and distal-slope fan facies. Deposition was sporadic, probably by turbidity currents and associated traction carpets. The slope system was fed by major delta distributaries that prograded to the shelf edge, or by tidal or storm currents that transported sediment through local breaches in the ban system. Destructional processes such as slumping, sliding, and minor canyon development alternated with periods of active slope construction. The eastern flank of the West Texas basin was, therefore, filled by simultaneous upbuilding (fluvial-deltaic and shelf-edge bank deposition) and outbuilding (slope deposition); some slope destructional activity followed episodes of outbuilding. Sites of deltaic and complementary slope deposition shifted widely in response to subtle variations in subsidence and normal avulsion of drainage systems. Fluvial, deltaic, and slope sandstones provide a variety of petroleum reservoirs and stratigraphic traps; early and late compactional and structural traps can be postulated from second-order compactional and structural mapping. End_of_Article - Last_Page 1907------------