Stratigraphic Relationship Between Odontogryphaea Thirsae Beds and Big Shale of Wilcox (Paleocene-Eocene) in Louisiana

Lloyd N. Glawe

The relationship between beds of the oyster Odontogryphaea thirsae, of the surface Wilcox Group exposures in the Sabine uplift area of northwestern Louisiana and the "Big Shale," a well-known subsurface mapping unit in central Louisiana, has been unclear. In a paleontological study of 2,158 ft of continuous Wilcox core from Sabine Parish, planktonic foraminiferal biostratigraphic zones were determined that include: Globorotalia angulata and G. pusilla (P3), G. pseudomenardii (P4), and G. velascoensis (P5). The O. thirsae bed was assigned to the G. pseudomenardii zone.

Electrical-log cross sections were used to demonstrate (1) the correlation between the O. thirsae bed in the core hole and the O. thirsae bed exposed at the surface at Many, Louisiana, a distance of 6 mi and (2) the correlation between the base of the 10-ft thick shale beneath the O. thirsae bed in the core hole and the base of the "Big Shale" as recognized in the central Louisiana subsurface.

Based on its assigned biostratigraphic zone, the O. thirsae bed in the updip Wilcox core hole is equivalent, in part, to the "Big Shale" of the basinal Wilcox of east-central Louisiana, which previously has been included in the G. pseudomenardii zone. Paleontological recognition of a thin marine unit 1 ft above the O. thirsae bed in the core hole on the southern flank of the Sabine uplift provides documentation of the transgressive character of the "Big Shale"-O. thirsae sequence and is evidence of the TP 2.1 cycle in Louisiana.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.