STRATIGRAPHIC FRAMEWORK OF THE MARINE UPPER CRETACEOUS DEPOSITS OF THE EASTERN FLANK OF THE MISSISSIPPI EMBAYMENT

Four complete and two partial composite reference sections (CRS) were constructed along the eastern flank of the Mississippi Embayment for use in quantitative chronostratigraphy. The complete CRSs are in northern Mississippi and southern Tennessee, eastern Mississippi, central Alabama and eastern Alabama. The two partial CRSs include one near Demopolis, which includes the useful Radotruncana calcarata planktonic foraminiferal range zone, and the other is in Lowndes County, which includes the K/Pg boundary. The contact between the sands of the Eutaw Formation and the overlying marls of the Mooreville Chalk as observed on geophysical well logs was used for most CRSs to construct a structure contour map from which the outcrop measured sections were projected. The top of the Arcola Limestone Member of the Mooreville Chalk, which is an isochronous surface, was used to construct the Demopolis CRS.

The construction of a CRS in northern Mississippi and southern Tennessee has been difficult because the age of the base of the chalk varies drastically along strike, becoming younger into southern Tennessee. In addition, lithostratigraphic units equivalent to the Mooreville Chalk grade northward into siliciclastic units and become difficult to distinguish from the underlying Eutaw Formation. For these reasons, the stratigraphic relationship between the Coon Creek at the type locality in southern Tennessee and at Blue Springs in northern Mississippi has been uncertain. The top of the Coonewah bed, which is an isochronous, two-foot, relatively pure chalk bed mapped by Frederick Mellen in the 1950’s, was used to construct the northern Mississippi CRS. The Coonewah bed occurs about 60 feet below the base of the Campanian R. calcarata zone. Mapping of this horizon shows that the Coon Creek at Blue Springs is lithostratigraphically more than 300 ft higher than the Coonewah bed at the type locality, indicating that the unit is considerably younger in Mississippi than in Tennessee.