APPLICATION OF FOSSIL CONCENTRATIONS IN PALEOENVIRONMENTAL AND STRATIGRAPHIC ANALYSIS OF THE PRAIRIE BLUFF FORMATION IN SOUTH-CENTRAL ALABAMA

Fossil concentrations are a powerful tool for paleoenvironmental reconstruction and for correlation between outcrops. We compared two fossil concentrations in the upper Maastrichtian Prairie Bluff Chalk (PBC) for faunal diversity and taphonomy . The PBC disconformably overlies the well-sorted, glauconitic, quartz sands of the Ripley Fm. The first locality (“Mooseland”), located in Wilcox County, AL, contains a locally continuous, 0.5m thick, shell bed. It occurs within the Discoscaphites conradi biozone just above the base of the PBC. The shell bed represents a sequence bounding unconformity that separates the shoreface deposits of the Ripley Fm from the offshore shelf deposits of the PBC. This fossil concentration is clast-supported and contains a diverse assemblage of fragmented, disarticulated, and encrusted macro-invertebrates, most of which are preserved as internal molds. Biodiversity of the shell bed was calculated using the Simpsons Diversity Index and showed a 1-D value of 0.93 (α = 53). The co-occurrence of nearshore and offshore fauna is evidence of a mixed assemblage, probably a result of extensive reworking during shoreface retreat. The stratigraphic position and taphonomic character of the shell horizon suggest that it was deposited as a transgressive lag that formed during the beginning of a transgressive systems tract. At the second location (“Moscow Landing”), located in Marengo County, AL, there is also a PBC fossil concentration within the D. conradi biozone. It is matrix supported and contains a variety of mollusks preserved primarily as phosphatized internal molds. This concentration displays a lower biodiversity and richness (1-D= 0.87, α = 36), than Mooseland with no evidence of a mixed assemblage. A high degree of epibiont encrustation, fragmentation, disarticulation and reorientation, along with abundant phosphatic nodules suggest they formed during maximum flooding. The presence of D. conradi within both shell beds supports that they formed during a similar timeframe. Paleoecologic, taphonomic and lithologic differences between the fossil concentrations can be attributed to lower sedimentation rates in Mooseland. This resulted in a more condensed and consequently diverse shell bed in which the transgressive lag and maximum flooding surface converged.