Abstract: Eocene Depositional Environments and Related Sulfur Content, Big Brown Lignite Deposit, Freestone County, Texas

R. C. Lentz

The Big Brown lignite deposit consists of three seams of commercial thickness, containing 275 million tons of lignite. The seams formed as thick peats deposited in freshwater swamps and marshes in two interlevee flood basins, separated by a paleochannel complex, low on the alluvial plain of the Mt. Pleasant fluvial system in the Eocene Wilcox Group.

Two coarsening-upward facies sequences can be identified in overburden in the "A" and "B" mine areas. The facies sequences were fed by extended crevasse distributaries, initiated by overbank flooding and crevassing on trunk streams in the northeast and south. (1) Swamp, crevasse-splay, and distributary-channel fill, and swamp facies form the lower sequence. (2) Lacustrine, lacustrine-delta-fill and distributary-channel-fill, and capping-swamp facies form the upper sequence.

Following burial of the sediments, low ferrous iron and sulfate concentrations in fresh ground waters may have been the principal factors limiting the formation of pyrite. Pyrite is concentrated in the lowest 2 cm of crevasse-splay silty mud at the commercial lignite-sediment contact (0.68 to 3.92 percent pyritic sulfur by weight). Hydrogen sulfide was carried in ground waters from permeable peats, where organic compounds for sulfate-reducing bacterial metabolism were available, into adjacent sediments where reactive iron was present. Pyrite content of overburden in the A mine area, near the flood basin axis of deposition, is more than twice that of B area overburden, suggesting brackish waters may have transgressed the flood basin interior during deposition of the sediments.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA