Biostratigraphic, Chronostratigraphic, and Stratigraphic Sequence Analysis of Lower Tertiary Marine Sediments of Alabama for Indicators of Sea-Level Change

THOMPSON, PETER R., ARCO Oil and Gas Company, Plano, TX, and GERALD R. BAUM, ARCO Alaska, Inc., Anchorage, AK

Early Eocene to late Oligocene marine sedimentary units in southwestern Alabama were sampled at closely spaced intervals to derive a precise time-stratigraphic framework and to determine the paleoecological and mineralogical responses to fluctuations in sea level. Paleontologic control consisted of planktonic, smaller and larger benthonic foraminifera, calcareous nannofossils, dinoflagellates, and megafossils. Paleomagnetic reversals were delineated in two boreholes which, when supplemented by strontium isotope dates and the biostratigraphic control, provided a robust in situ chronostratigraphy for the Gulf Coast lower Tertiary.

Paleoecologic trends in regression and transgression can be clearly correlated across major regional facies changes. Using the chronostratigraphy developed here, the second-, third-, and fourth-orders of Vail's global sea-level cycles can be recognized and demonstrate the influence of sea-level change on sedimentation.

Stratigraphic systems tracts (SSTs) and bounding surfaces in outcrop were determined by lithologic variations and paleoecologic trends, and additionally by gamma logs in the cores. The lower sequence boundary occurs at a contact where an older, relatively fine-grained, deep-water, fossiliferous unit was abruptly succeeded by a coarse-grained, shallow-water, poorly fossiliferous unit (lowstand SST downdip or transgressive SST updip). The transgressive surface occurs at the base of a fining- and deepening-upwards unit (transgressive SST) that was commonly glauconitic and very fossiliferous. Transgression culminated with a pulse of planktonic microfossils in a bed having reduced clastic sedimentation (condensed interval); on the log the surface of maximum starvation was marked by a gamma spike. The late stage of the sequence was a shallowing-upwards marine unit (highstand SST) commonly showing progradation.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)