The stratigraphy and environment of deposition of the undivided Wilcox Group (lower Eocene) and Carrizo Formation (Eocene) were studied in an 88 km² (34 mi²) area in northeastern Rusk County, Texas. Seven cores and 300 borehole logs were used in the study. The undivided Wilcox Group is the predominant geologic unit in the study area and consists of poorly lithified, interlaminated sandstones, siltstones, claystones, and lignite seams. Lignite seams range in thickness from 0.1 to 2.2 m (4 in. to 7 ft) and are conformable with the overlying and underlying strata. Subtle coarsening-upward sequences, 1.8-31.3 m (6-103 ft) thick, occur between lignite seams; however, the individual sandstone units, 0.3-1.8 m (4 in.-6 ft) thick, within these sequences fine upward. The fine-grained rocks of the Wilcox Group are overlain unconformably by well-sorted, medium to coarse-grained sandstones of the Carrizo Formation. The small-scale sedimentary structures, fine-grain size, and matrix-rich nature of the undivided Wilcox units are characteristic of fluvial overbank deposits. Peat beds probably accumulated in interchannel swamps on a lower alluvial plain, distal from overbank discharge. As streams meandered across the area, overbank discharge buried the swamps. The coarsening-upward sequences between lignite seams indicate overbank deposition from a prograding stream. Swamps were reestablished as the stream was abandoned or migrated away. Fining-upward trends in grain size and the upward decrease in scale of sedimentary structures indicate the Carriz Formation was deposited in fluvial channels.

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