ABSTRACT

The Tew Lake Marker (TLM) beds are recognized on electric logs throughout east-central Louisiana by an uncommon, resistive unit within the Wilcox delta complex. This study explores how the wide distribution of this marker sequence is linked to its paleoenvironmental history. Petrographic and foraminiferal data derived from continuous, conventional cores from two wells located one mile apart are used to identify paleoenvironments and to interpret paleoenvironmental trends recorded in the TLM beds. The main discoveries of this investigation are:

1. The resistive TLM beds consist of a shale sequence interbedded with very thin, calcareous, fossiliferous layers. The TLM beds were deposited in bay/lagoon paleoenvironments having salinities that ranged from hyposaline to normal marine.
2. The TLM beds are divisible into lower and upper depositional sequences that are related to paleoenvironmental changes in the region.
3. The lower depositional sequence fines upward and records normal marine and hyposaline-normal marine bay/lagoon deposits. This lower depositional sequence rests unconformably upon freshwater lignite, documenting a marine transgression inland over a freshwater swamp of the Wilcox lower delta plain.
4. The upper depositional sequence coarsens upward and contains hyposaline-normal marine bay/ lagoon deposits that grade upward into hyposaline bay/lagoon deposits, suggesting regression in the lower delta plain. Muddy, fresh waters from nearby delta-distributary streams and stream crevasses filled lakes, inner bays and inner lagoons with sediment and reduced the salinities of water bodies and wetlands of the lower delta plain.
5. Lateral facies between the two wells indicate marine influence during TLM deposition was greater and more persistent toward the east.