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ABSTRACT: Geochemical and Mineralogical Evidence for Relative and Eustatic Sea Level Changes in Eocene to Oligocene Carbonates in Alabama

Joan S. Baum, John D. Humphrey, Gerald R. Baum, Peter R. Thompson

Eocene to Oligocene marine shelf carbonates cored in Baldwin County, Alabama, were subdivided into a sequence stratigraphic framework and sampled at closely spaced intervals to determine characteristic geochemical and mineralogical responses to fluctuations in

eustatic and relative sea level. Except for the Eocene/Oligocene boundary interval, the carbonates were deposited landward of their respective depositional shelf edges. Sequence boundaries, transgressive surfaces, and surfaces of maximum starvation were delineated by lithologic and textural variations and gamma logs.

Comparison of the Baldwin County core data with correlative Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) records reveal more clearly defined stable isotopic data trends due to greater sampling density per unit time. In the sequence stratigraphic framework, the ^dgr13C signature mimics relative sea level changes (paleobathymetry) and changes in the O2 minimum; whereas, the ^dgr18O signature records the eustatic sea level history. Because the stable isotopes are recording different aspects of sea level history, it is necessary to evaluate isotopic signatures independently. Although minor diagenetic perturbations in the stable isotopic signals are found, the depositional signal clearly dominates the diagenetic signal. The paleogeographic po ition of this core gives a more accurate sea level record than either more landward sections presently exposed in outcrop that have undergone intense subaerial alteration and erosion or DSDP/ODP records that may be so condensed that the record appears incomplete in conventional sampling schemes.

Authigenic mineralization related to sea level changes can be used to characterize surfaces within the depositional sequence framework. Trace elemental data shows that hydrologic flow, porosity, and cementation trends are subsequently controlled by the sequence framework and early authigenic mineralization.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990