DEPOSITIONAL INTERPRETATION OF A FOSSIL-BEARING STRATUM USING INTEGRATED SEDIMENTOLOGICAL AND PALEONTOLOGICAL DATA

Sediments of a fossil-bearing stratum within the Blackwater Draw Formation are exposed in Plainview, Texas. These sediments are part of the ancient buried channel of Running water Draw, and are currently exposed in a caliche quarry wall. Sediments composed mostly of quartz sands are associated with Pleistocene fluvial deposition, and exhibit lateral changes in texture. As texture changes laterally, so does fossil mollusc diversity. Thus, sediments from three locations (PS-1, 2, and 3) were sampled horizontally for grain size analyses and fossil mollusc abundances. PS-1 and 2 show variable grain size distributions, coarse-grained cross-beds, and a diverse mollusc fauna. PS-3 A and B represent changes in vertical texture. Grains associated with PS-3A contain carbonate-coated quartz grains, low biodiversity, but high abundances of Gyraulus parvus and Sphaerium transversum. G.parvus and S. transversum are tolerant of stressed water conditions. The presence of these taxa and carbonate-coated grains suggest deposition from isolated bodies of concentrated water. PS-3B possesses finer grain sizes and no fossil content. These depositional characteristics of the fossil-bearing stratum were compared to the modern South Fork Brazos channel in west Texas. A hypothesis test was used to compare sedimentological data between the South Fork Brazos and ancient Running Water Draw. Statistically, sedimentological data between the modern and ancient channels compare well. No fossils were recovered from the South Fork Brazos, but a hypothesis test was used to compare mollusc abundances within the ancient Running Water Draw channel. These results indicate that despite high abundances of terrestrial taxa, sampled molluscs represent the accumulation of shells in an aquatic environment. Lateral and vertical changes in the abundances of fossil molluscs are apparently controlled by microenvironments within the channel. Therefore, this study demonstrates the necessity to integrate multiple sources of qualitative and quantitative data for a clearer understanding of depositional characteristics.