Andre C. Klein¹

(1) Department of Earth Science, Rice University, Houston, TX, 77005-1892

ABSTRACT: Variations in Detrital and Authigenic Comonents in Paleogene Sediments, South Alabama: A Sequence-Keyed Petrographic Look at Gulf Coastal Plain Sequences

A quantitative analysis of detrital and authigenic minerals within two cores from south Alabama reveals distinct cycles of recurring sedimentary fabrics and components. These cycles correlate with previously defined unconformity-bound sequences.  The fact that these cycles can be correlated between areas of differing sediment supply and subsidence rates supports eustasy as the dominant control on third-order sequence development.

The cored intervals investigated at St. Stephens Quarry (SSQ) and Bay Minette (BM) range from the Hatchetigbee Formation (lower Eocene) to the Bucatunna Clay member of the Byram Formation (Oligocene).  Petrographic (point count, grain size, sorting, rounding) analysis of this time interval, in conjunction with faunal ages and lithologic core analysis, suggests the presence of at least seven robust cycles.  The cycles are clearest where carbonate and siliciclastic sediment components are subequal.  At SSQ, the lower part of each cycle is commonly calcareous, whereas the upper part of each cycle is dominated by argillaceous and micaceous sediments.  Glaucony and foraminifera are concentrated between the upper and lower parts of each cycle.  Quartz, feldspar, and glaucony are common at the boundaries between cycles.  At BM, the interval is nearly all carbonate, but cycles are separated by abundance peaks of quartz and glaucony.  Each cycle at BM can also be distinguished by differing amounts of carbonate matrix. 

When integrated with previous mesoscopic sequence stratigraphic studies of these cores, this detailed microscopic method can potentially (1) provide a more detailed and objective criteria for identifying third-order depositional sequences and their component systems tracts; (2) increase correlation precision between cores; and (3) better constrain depositional models for this part of the Gulf of Mexico basin for the interval investigated.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado