Evidence for the Paleocene–Eocene Thermal Maximum in Outcrops, Onshore Texas

Evidence for the Paleocene–Eocene Thermal Maximum (PETM) in outcrops near Bastrop, Texas includes a -2.3‰ carbon isotope excursion (NCIE) in bulk organic δ13C carbon within an informally named “Dark Band” at the top of the Sabinetown Formation of the Wilcox Group, and an acme of the dinoflagellate Apectodinium homomorphum immediately overlying the NCIE. An unconformity separates PETM deposits from the overlying sandstone-rich Carrizo Formation of the Claiborne Group. This is the first time that the PETM NCIE is documented in Texas outcrops, providing a new and robust, multidisciplinary dataset for regional and global correlations.

Coarsening-upward Sabinetown deposits record progradation of tidally modified delta front to delta plain paleoenvironments. Sabinetown biostratigraphy indicates a Thanetian to Thanetian–Ypresian age for deposits that likely correlate with the uppermost Middle Wilcox.

The lower 0.5 m of the Dark Band comprises weakly bioturbated organic-rich mudstone and coal with a low abundance of Apectodinium homomorphum correlating with the -2.3‰ NCIE. The upper 0.6 m of the Dark Band preserves a heavily bioturbated, organic-rich siltstone with a super abundance of Apectodinium homomorphum consistent with the PETM acme, and a return to heavier δ13C carbon values. The Dark Band is interpreted as partially equivalent to the Yoakum Shale known from down-dip, deeper-water deposits.

The unconformably overlying Carrizo Formation preserves tidal delta deposits containing Schizosporis spp. and common Tetraporina spp., recording an influx of freshwater algae above the post PETM hiatus (lacuna) of unknown duration. A single specimen of Thomsonipollis magnificus indicates that Carrizo deposits are likely Early Eocene, Ypresian.