UPPER MAASTRICHTIAN AMMONITE BIOSTRATIGRAPHY OF THE NORTHEASTERN GULF COASTAL PLAIN

Although the upper Maastrichtian ammonite biozonation has been well documented on the Atlantic Coastal Plain (ACP) (Landman et al. 2004), very little has been done with respect to the Gulf Coastal Plain (GCP). Initial investigations by Kennedy and Cobban (1995) on the GCP suggested that the Discoscaphites conradi Zone represents the top of the Maastrichtian sequence followed by the slightly older Discoscaphites iris Zone. This differs from Landman’s proposed ammonite biozonation for the ACP, where the D. iris Zone represents the uppermost part of the Maastrichtian sequence, followed by D. minardi and D. conradi below. The current study evaluates the proposed GCP ammonite biozonations using high resolution biostratigraphic sampling of D. minardi, D. iris, D. conradi, Eubaculites carinatus, E. latecarinatus, and Baculites sp. C (Cobban and Kennedy, 1995) from several sites in Mississippi and Alabama. These sites expose the upper Maastrichtian Owl Creek Formation and Prairie Bluff Chalk. The Owl Creek Formation is the highest unit of the Cretaceous sequence in Mississippi and is unconformably overlain by the Clayton Formation of Paleocene age. The Owl Creek Formation extends northward into Tennessee and Missouri and southward into Mississippi, grading laterally into the Prairie Bluff Chalk. The Owl Creek Formation consists of fossiliferous, micaceous, muddy silt to muddy very fine sand. The Prairie Bluff Chalk consists of light to medium gray glauconitic micaceous silty to sandy chalk. In Mississippi, initial findings show that D. iris is most abundant at the top and D. minardi at the base of the Owl Creek Formation in Tippah County, and in the Prairie Bluff section in Chickasaw County. At these two localities, dinoflagellates Deflandrea galeata, Palynodinium grallator, Piercites pentagonum occur within the top of the sequence along with abundant D. iris. This suggests that the D. iris Zone in the Gulf Coast correlates with the P. grallator Zone representing the uppermost part of the Maastrichtian CC26b nannozone. This interpretation is supported by the co-occurence of the nannofossil Micula prinsii. The abundance and associated fauna in Mississippi and Alabama suggest that the ACP zonation is also present in the GCP and consists of three zones, from bottom to top: D. conradi Zone, D. minardi Zone and D. iris Zone.