Paper No. 7
Presentation Time: 9:00 AM-6:00 PM
CLIMATIC AND FAUNAL CHANGES AT THE END OF THE MAASTRICHTIAN: AMMONITE BIOSTRATIGRAPHY AND ISOTOPIC ANALYSES AT THE TYPE LOCALITY OF THE OWL CREEK FORMATION, MISSISSIPPI
Investigations of the Owl Creek Formation at its type locality, Tippah County, Mississippi, reveal insights into climatic and faunal changes at the end of the Maastrichtian. We examined the biostratigraphic distribution of ammonites in the upper 10 m of the section spanning the Discoscaphites minardi and D. iris Zones (duration ≈ 500 ky). The shells were analyzed for oxygen and carbon isotopes. The ammonite fauna consists of D. iris, D. minardi, D. sphaeroidalis, Sphenodiscus lobatus, Eubaculites carinatus, E. latecarinatus and Baculites sp. C of Cobban and Kennedy (1995). There is a change in the ammonite fauna upsection reflecting the zonation. The lower part of the section is characterized by D. minardi, D. iris, and Baculites sp. C whereas the upper part of the section is characterized by D. iris, E. carinatus, and E. latecarinatus. The shell samples selected for isotopic analysis are well preserved, as determined by SEM (Preservation Index = 3.5--4.5, according to Cochran et al., 2010). The oxygen isotope values range from – 1.67 to 0.15‰ and the carbon isotope values range from -3.41 to 1.76 ‰. Converting the oxygen isotopic values to temperatures, using dw of Cretaceous sea water = -1.0 ‰, yields a range of temperature from 16.4°C to 24.9°C. This range is nearly constant throughout the stratigraphic section suggesting a relatively stable climate through the last 500 ky of the Maastrichtian. Five samples were analyzed on the phragmocone and body chamber of a single specimen of E. latecarinatus, yielding d18O values from -0.91 to -0.73‰, which may indicate seasonal variation in temperature from 20.5°C to 21.4°C. Our results suggest that changes in the ammonite assemblages were not related to changes in climate (water temperature) but may have been caused by migration and competitive replacement of species or other environmental factors such as changes in salinity.