ABSTRACT: Subsurface Sequence Stratigraphic Analysis of the Eocene-Lower Oligocene of Alabama

KLEIN, ANDRE C., GERALD R. BAUM, and PETER R. VAIL , Department of Geology and Geophysics, Rice University, Houston, TX; PETER R. THOMPSON, Computational Biochronology, Plano, TX

South Alabama contains world-class outcrops of Eocene and Oligocene strata that have been the object of intense scrutiny over the past several years. Despite such thorough examination, these outcrops have failed to assuage controversy as to the nature of the Eocene/Oligocene (E/O) boundary in this region. Some workers have made this faunal boundary coincident with a sequence boundary, while others place it within a stratigraphically condensed section during relative sea level highstand. This problem is exacerbated by the fact that outcrops in Alabama are limited in areal extent, thus large-scale stratal patterns such as onlap and downlap have not been used to place constraints on this matter.

A grid of geophysical wireline logs (SP and resistivity) has been assembled and digitized in the area of interest to examine the nature of the E/O boundary on a seismic scale. The log data are calibrated to lithology and biostratigraphy using two stratigraphic test cores. Preliminary results show the E/O boundary occurs in a relatively fine-grained interval, stratigraphically below a surface of onlap (sequence boundary).

In addition, these well log data have been useful in analyzing other controversial stratigraphic intervals, such as the Lisbon Formation, in the subsurface of Alabama. Preliminary results, again calibrated to test cores, suggest the presence of at least three regional coarsening upward cycles within the Lisbon. One of these cycles may represent the Doby's Bluff tongue of the Kosciusko Formation. It is hoped that large-scale stratal analysis will clear up these lingering controversies.

 

AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas