PREDACIOUS BORING INTENSITIES OF UPPER CRETACEOUS RIPLEY AND DEMOPOLIS FORMATIONS

Upper Cretaceous (Campanian-Maastrichtian) marine deposits in the Prairie Bluff Member of the Ripley Formation and the Bluffport Marl Member of the Demopolis Formation located in Sumter County, southwest Alabama, contain macroinvertebrate fossils with various biogenic borings. The genera Exogrya, Ostrea, and Paranomia represent the bulk of preserved taxa and were selected as grounds for comparison of boring intensity preserved within the units evaluated in this study. At two outcrop sites, one of the Prairie Bluff Member and one of the Bluffport Marl Member, counts of over 100 individual specimens were conducted in order to represent the proportions of fauna present. Only valves considered to be intact, having 50% or more of the shell material preserved, were counted. The Ichnogenus Oichnus is the dominant boring trace observed in the specimens. Raw boring intensity is calculated by dividing the total number of specimens bearing Oichnus traces by the total number of specimens collected. At the Prairie Bluff site, the boring intensity of 133 specimens is 11.27%. The boring intensity of the 123 specimens studied at the Bluffport site is 9.76%. This yields a grounds for comparison of boring intensity for species collected at each outcrop, and total boring intensity at each site. Preliminary measurements of boring hole area were performed using NIS-Elements imaging software. Results reveal average boring hole area is 6.71 mm² for the Prairie Bluff Formation and 2.63 mm² for the Bluffport Member of the Demopolis Formation. These units exhibit significantly different matrix, with the Bluffport Marl composed of a chalky marl and the Prairie Bluff Member composed of chalk mixed with course-grained sand. Because these units are closely related both temporally and spatially, we hypothesize that these units provide an opportunity for comparing the relationship between biogenic boring and matrix composition over time, as well as understanding temporal trends in boring predation and prey. Simultaneous documentation of matrix composition for sites investigated may also contribute to understanding of bioerosive controls on substrate texture.