UPPER CRETACEOUS COMPOSITE REFERENCE SECTIONS ON THE EASTERN FLANK OF THE MISSISSIPPI EMBAYMENT: ESTABLISHING A HIGH-RESOLUTION FRAMEWORK

High-resolution event stratigraphy involves the combination of information from many measured sections into a single chronostratigraphic framework. This information includes paired events, such as the first and last occurrences of fossils, as well as single events. CONOP (COnstrained OPtimism) is a computer system that seeks the optimum ordering of all events. Although individual measured sections can be input into CONOP, correlating sections into composite reference sections (CRSs) produces pseudo-sections that can provide large frameworks into which smaller sections (or sections with fewer samples) can be projected. CRSs can preserve stratigraphic thicknesses, as well as large-scale stratigraphic ordering of events.

In the early 2000’s, two large CRSs were constructed by the author that included all the marine Upper Cretaceous strata at the surface of the eastern flank of the Mississippi Embayment, one on eastern Mississippi and the other in central Alabama. These two CRSs were correlated based on ostracods, planktonic foraminifera and event beds using graphic correlation. Comparison of the results of graphic correlation and CONOP shows the greatest disparity is at the ends of the sections, which is due to the lack of points to define the line of correlation in the extremities in graphic correlation, a common occurrence. Three other CRSs have now been constructed in this area: one around Demopolis, western Alabama, that includes the Demopolis Chalk (mid- to late-Campanian); one in Lowndes County, AL, that includes the upper Ripley Formation and Prairie Bluff Chalk (Maastrichtian); and one in eastern Alabama that includes Santonian through Maastrichtian sediments. Structure contour maps of key surfaces, principally the top of the Eutaw Formation and the top of the Arcola Limestone Member of the Mooreville Chalk, are used to correlate the individual measured sections. Elevations of these key surfaces was observed mainly in geophysical logs and sample descriptions, and structure contour maps were made using 3DField software and a radial basis function. These pseudo-section CRSs, as well as particularly reliable events, can be weighted to produce the most confident range charts and correlations. These ranges can be used to calculate diversity through time, among other applications.