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Geometry and Distribution of Fluvial and Deltaic Sandstones (Pennsylvanian and Permian), North-Central Texas (1)
L. F. Brown, Jr.
Upper Pensylvanian and lower Permian rocks of the Eastern Shelf in North-central Texas are composed of 10 to 15 repetitive sequences including open shelf, deltaic, fluvial, and interdeltaic depositional systems. Sediments derived from the Ouachita Mountains and associated piedmont were transported westward across a narrow coastal plain. Fluvial and deltaic sandstone facies define a southwest paleoslope of about 5 feet per mile. Sandstone facies are delta front sheets, distributary mouth bars, distributary and fluvial channels, and destructional bars.
Sandstones displaying distributary patterns represent distal deposition in the upslope area. Belt sandstones, typified by uncommonly thick fluvial channel deposits, prograded far downslope. Composite patterns include distributary and belt sandstones representing complex progradational history. Rocks display one-half degree northwest regional dip; negative structure residuals outline a broad area within which 70 percent of the deltaic facies were deposited.
Elongate sandstones are generally arranged parallel to paleoslope in vertically offset patterns controlled by differential compaction of fluvial and deltaic sands and interdistributary muds. Multistory sandstone bodies were deposited along narrow, structurally unstable belts which were periodically overloaded and later reoccupied by prograding deltas. Initial Cisco deltas followed a paleosurface grain controlled by underlying bank limestones; this orientation was maintained during deposition of 1,200 feet of Cisco strata. Each fluvial-deltaic system inherited its geometry from previous systems and, in turn, provided control for the next deltaic episode. Stratigraphic and structural mapping utilizing mud decompaction techniques confirms the roles played by compaction and structure in controlling the geometry of sandstone bodies.
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