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.