Depositional environments of Late Eocene lignite-bearing strata, east-central Texas

Abstract

Late Eocene lignites in the Brazos River Valley area occur in parasequence-scale cycles containing mostly marine sediments. Lignite is present at the base of the marine interval and forms the base of the cycle in a majority of cases. Less frequently, lignite is underlain by a thin interval of nonmarine deposits that form the base of the cycle. In both situations, the lignite is part of a thin transgressive interval. Cycles are separated by an exposure surface, usually marked by plant rooting and/or development of a paleosol in the top of the underlying sediments. Peat accumulated on top of these exposure surfaces and lignite is preserved where marine flooding allowed ocean waters to cover the peat deposits with fine grained sediments, producing high-ash lignite horizons of great lateral extent but variable thickness and quality. This strandplain model of lignite occurrence provides a better understanding of Late Eocene lignites in Texas than the deltaic or fluvial-deltaic models previously used. The strandplain model of lignite occurrence is appropriate for most of the Yegua-Jackson lignites (Middle-Late Eocene) of Texas and should be considered when evaluating depositional conditions of Paleogene lignites in the northern Gulf Coast region. A re-evaluation of Late Eocene stratigraphy of the Brazos Valley area accompanies the documentation of depositional environments.