Paper No. 76-42
Presentation Time: 9:00 AM-5:30 PM
EVIDENCE FOR ECOLOGIC COLLAPSE ACROSS THE K/PG ALONG THE GULF COASTAL PLAIN, LOWNDES COUNTY ALABAMA
Examining environmental changes leading up to the End Cretaceous mass extinction event can provide insight into the mechanism of ecological collapse. A new K/Pg exposure of late Maastrichtian Prairie Bluff Chalk and Early Paleogene Clayton Formation was analyzed in south central Alabama in order to link faunal distribution to paleoenvironmental changes. Three distinct parasequences (denoted A, B, and C) are exposed composed of muddy to sandy marls capped by fossiliferous limestones. Each marl differs lithologically representing varying depositional environments possibly related to Milankovitch cycles. Faunal, taphonomic, and sedimentological analyses were conducted on each marl and limestone horizon. Biodiversity was calculated using Simpson’s Diversity Index (SDI) across the three intervals. The base of the sequence, Marl A, was a micaceous fine sand to silty marl. The marl was dominated by small bivalves such as Pecten simplicius with an SDI of 0.17 and a species richness of 19. Marl B was a muddy to silty, very micaceous marl. The SDI was 0.05 and species richness was 30 exhibiting a more diverse fauna that included Legumen ellipticum, Eubaculites latecarinatus and Discoscaphites iris (representing the latest Maastrichtian D. iris ammonite zone). The uppermost Marl C was less micaceous and more sandy then those below. Only 4 species were recovered. The boundary with the overlying limestone was heavily bioturbated, typical of K/Pg sites throughout the Gulf Coastal Plain. This limestone was heavily dominated by one species, Gryphaeostrea vomer. SDI was calculated at 0.91, evidence of an extremely low diversity. We interpret this to be the post extinction recovery community. This sequence gives unique insight into the changing environment leading up to and following the End Cretaceous mass extinction event.