CONSTRUCTION OF A HIGH-RESOLUTION CHRONOSTRATIGRAPHIC FRAMEWORK OF LATE CRETACEOUS (SANTONIAN-MAASTRICHTIAN) DEPOSITS, ALABAMA AND MISSISSIPPI

Reconstructing paleoenvironmental changes requires reference to some chronostratigraphic scale to determine rates of change. To identify rapid paleoenvironmental changes, a high-resolution chronostratigraphic framework is needed. Typically, age resolution is defined by biostratigraphic zones defined by few taxa, often last a million years or more, and ignores information from other taxa. Graphic correlation is an excellent quantitative tool that combines information from many sections into a single timescale and maximized the ranges of the fossil constituents. One limitation of graphic correlation is the requirement of measuring sample localities relative to some lithostratigraphic bed. It is difficult to correlate sections that do not have common lithostratigraphic horizons.

Constrained optimism (CONOP) is software that combines all types of stratigraphic information, such as the First Occurrence Datums (FADs), Last Occurrence Datums (LADs), species’ co-occurrences, age dates and event beds into a single time scale that is independent of physical distances in the sections, thus sections containing common taxa do not need reference to common lithostratigraphic beds. The software goes through a series of trials that calculate local best solutions with the goal of forming a timescale with the fewest number of contradictions. The fidelity of the solutions is quantified by a penalty, which is a measure the number of contradictions of solutions. Once a good ordinal fit is achieved, dated events, such as the age of key taxon FADs and LADs and radiometrically dated beds, are used to define the absolute ages of the timescale.

For this study, two composite reference sections (CRSs) in the offshore, Late Cretaceous (Santonian-Maastrichtian) areas of the eastern flank of the Mississippi Embayment correlated by graphic correlation serve as the starting point for building a chronostratigraphic framework that includes almost 100 measured sections, 350 taxa of microfossils, and 7000 fossil occurrences. Sections in nearshore deposits contain higher diversities than the offshore, chalky deposits, yet lack key lithostratigraphic markers used to define the line of correlation in the CRSs, thus their inclusion in the model greatly increases the chronostratigraphic resolution.