Lithostratigraphic and depositional framework, near-surface Upper Pennsylvanian and Lower Permian strata, southern Brazos Valley, North-central Texas.
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Seals, Mary J.
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The purpose of this study was to interpret the depositional framework of the original Cisco Group, Upper Pennsylvanian and Lower Permian, in the shallow subsurface of North-central Texas, Stratigraphic interpretation was based on several subsurface mapping methods—(1) stratigraphic cross sections, (2) isopach maps of limestone-bounded intervals, (3) sandstone percentage maps, (4) structure contour maps, (5) channel sandstone isopach maps, (6) paleotopographic maps, (7) well sample study, and (8) various special maps and cross sections. Several classifications of the Upper Pennsylvanian and Lower Permian strata have been proposed; however, Plummer and Moore's (1922) classification, although not completely suitable, applies best to the study interval. The problem of the Pennsylvanian-Permian boundary was not a concern of this study. The strata of the Cisco Group in the study area consist, in order of abundance, of shale, sandstone, limestone, and coal. Rock characteristics and interpretation of depositional environments was of necessity based primarily on electrical responses and geometric distribution. Little is known about shales within the study area except that they may be highly fossiliferous, commonly carbonaceous, and probably represent transitional environments. Relatively thin, widespread limestones persist throughout the study interval and represent decrease in clastic influx. The persistent limestones are important in subsurface correlation. Sandstone deposits can be divided into sheet sandstones of probable marine origin and linear channel-fill sandstone bodies. Topographic and structural lows were apparently controlled to some degree by differential sand-shale compaction and by compaction of shales underlying massive sandstone bodies. These local compactional features are superimposed on a broad, regional monocline. Interpretation of channel trend relationships to structure and underlying strata has been an important aspect of this investigation. The dominant orientation of channel trends is in a west-southwest direction, which probably reflects a source area to the east and northeast of the study area. Channels are best developed in structural lows which apparently were also paleotopographic lows. In non-channel areas differential shale compaction created topographic lows on which a succeeding channel would probably develop. Subsidence due to shale compaction beneath massive sandstone bodies also created topographic lows, which are commonly areas where an upper channel crosses a lower channel. As many as five intersections were observed in a local area. Continued surface and subsurface research is necessary to describe sufficiently the depositional history of the Cisco Group of North-central Texas. Suggestions for further study include (1) extension of the study downdip and along strike, (2) more dense well control on a larger map scale, (3) mineralogic, petrologic and sedimentary structure studies, and (4) paleotopographic surface and subsurface studies.