DEPOSITIONAL INTERPRETATION USING INTEGRATED SEDIMENTOLOGICAL AND PALEONTOLOGICAL DATA FROM A FOSSIL-BEARING UNIT WITHIN THE BLACKWATER DRAW FORMATION, PLAINVIEW, TEXAS

A fossil-bearing unit within the Blackwater Draw Formation, previously interpreted as entirely fluvial deposition, is exposed in an abandoned quarry wall inside the city limits of Plainview, Texas. This unit was laterally sampled to investigate the nature of deposition based on composition, grain size analysis, and taxonomic variation of fossil molluscs. Compositionally, grains are predominantly quartz with minor amounts of feldspars and carbonates. All sampled sites (PS 1, 2, and 3) exhibit similar grain size histograms (of variable grain size distributions), and contain bimodal negatively skewed frequency curves that indicate low stream flow velocity. Of these sampled sites, PS 3 exhibited the greatest variability. For example, samples PS 3B1 and 3B2 appear texturally different than others, showing a negatively skewed unimodal frequency curve which is typical of eolian sedimentation. Samples 3A1 and 3A2 contain a higher concentration of carbonate grains, carbonate-coated quartz grains, and Gyraulus parvus. Such characteristics are uniquely different from the other samples and are more representative of conditions and sedimentation in shallow, non-flowing bodies of water. Taxa of fossil molluscs are distributed laterally but with variable proportions. Site 3 is represented by strictly aquatic varieties, while locations 1 and 2 are represented by mostly aquatic taxa with variable semi-aquatic and terrestrial forms. Therefore, current data from the fossil-bearing unit suggest disparity in depositional environments, indicating that sedimentation and taxonomic diversity is not completely controlled by fluvial processes.