GLAWE, LLOYD N., University of Louisiana at Monroe, LA; and ALAN M. BAILEY, University of Louisiana at Lafayette, LA

The Tew Lake Marker beds are a thin (10-12 ft or 3-4 m thick), distinctive sequence of calcareous shales that occur within the predominantly noncarbonate Middle Wilcox delta deposits of east-central Louisiana. These beds are named after the Tew Lake Marker (TLM) which is a dense, resistive marker horizon recognized on electric and sonic logs between the Tew Lake and E-2 sands.

Paleoenvironments and early diagenesis of the Tew Lake Marker beds are interpreted from a set of continuous, conventional cores taken from the Hunt A-68 well in the Nebo-Hemphill Field, LaSalle Parish, Louisiana. Analysis and interpretation of foraminiferal assemblages contained in 8 core samples from the Tew Lake Marker beds indicate this sequence represents a series of bay/lagoonal deposits that formed in paleoenvironments that ranged from normal marine outer bay/lagoon to brackish water inner bay/lagoon. At the A-68 well, these bay/lagoonal deposits overlie a bay-fill sequence of shale-sand-lignite and record a marine transgression.

The occurrence of early diagenetic pyrite, siderite, dolomite, and calcite in the Tew Lake Marker beds contributes to an understanding of early diagenetic reactions in this sequence and provides further evidence for paleoenvironmental interpretations. Pyrite appears to form very early, Filling fossils associated with organic fragments, and is fairly abundant, reflecting marine influence. Siderite forms somewhat later in approximately equidimensional to somewhat elongated bodies. X-ray diffraction peaks often do not occur at ideal positions, indicating impure varieties. This and variable proportions of dolomite and calcite within the same body also suggest marine influence and support paleontological interpretations.

GLAWE, LLOYD N., and ALAN M. BAILEY

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