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Dennis A. Sylvia1, William E. Galloway1
(1) University of Texas at Austin, Austin, TX

Abstract: Reservoir architecture of nested, incised valley fill; A river's response to the Late Pleistocene glacial cycle

The Pleistocene stratigraphic record of the Texas Gulf Coastal Plain consists of coalesced, low-gradient, aggradational fluvial sequences. Late Pleistocene climate change and concomitant glacio-eustatic fall caused the Brazos River to incise deeply into the alluvial plain in response to the changed hydraulic conditions. Cores and geophysical logging were used to examine the Brazos River’s Late Pleistocene incised valley fill. Five boreholes, with depths ranging from 160 feet to 210 feet were drilled across the incised valley. Three boreholes were continuously cored. All boreholes were logged using natural gamma ray and resistivity tools. In addition, four boreholes also were logged using a spectral gamma ray tool. Geophysical measurements were calibrated to the cored boreholes and used to infer lithologies in uncored boreholes and through intervals of poor core recovery. The fill sequence is dominated throughout by overbank facies. Fill lithologies include silts and clays, with lesser amounts of very fine sands. In contrast to the conventional model of valley fill deposits, an increase in sand content toward the base of the fill was not observed. However, thin, very coarse sand was observed, or inferred based upon drilling characteristics, at the base of the latest Pleistocene-early Holocene fill in each of the boreholes. Punctuated, climate-modulated, aggradational cycles of valley filling appear to reach equilibrium in a relatively brief interval of time. Results challenge the current model for the architecture and fill of incised valley systems and document that the Late Pleistocene deposits are more heterogeneous and somewhat thicker than previously believed.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana