290 Delta Evolution from Rivers to the Shelf: Past, Present and Future Perspectives for Society

The session focuses on deltas and adjacent depositional environments from fluvial systems to the continental shelf. Research on modern, ancient or extraterrestrial deltas that emphasize their importance to our society is welcome.

Wednesday, 22 October 2025
1:30 PM - 5:30 PM
HBGCC, 303AB

Regional and Global Significance of Evidence for the Paleocene‒Eocene Thermal Maximum in Outcrops, Onshore Texas

Paper number: 290-12
Program: Technical Sessions
Time: 3:51 PM – 4:06 PM

Authors

Abstract

Paleocene-Eocene Thermal Maximum (PETM) deposits record a worldwide 170-ka warm climate period, an analog for modern climate change, and the potential sedimentary response to it. The basal inflection of a negative carbon isotope excursion (NCIE) defines the PETM onset and base Eocene. Related phenomena include an acme of the dinocyst Apectodinium homomorphum, PETM restricted ranges of some palynomorphs and the nannoplankton Rhomboaster spp. and Discoaster spp., and mass extinction of abyssal and bathyal benthic foraminifera.

Evidence for the PETM in Wilcox Group outcrops near Bastrop, Texas includes palynomorph marker species consistent with Gulf of Mexico (GOM) PETM taxa coupled with overall highly negative δ13Corg values. Sabinetown Formation tidal delta front to tidal flat deposits contain age-diagnostic Spinaepollis spinosus, Bagelopollis verrucatus, and Kyandopollenites anneratus but lack abundant Eocladopyxis spp., Caryapollenites, and Momipites spp., a PETM GOM characteristic assemblage. The dinocyst Apectodinium homomorphum is intermittently abundant, including a superabundance interpreted as a PETM acme.

In the uppermost Sabinetown the lower part of an informally named “Dark Band” contains a 2.3‰ NCIE in weakly bioturbated and rooted mudstone and lignite representing the most proximal deposits. In contrast, the upper Dark Band is a heavily bioturbated siltstone dominated by Thalassinoides that records a marine maximum flooding surface (MFS) and offshore deposition with super-abundance of  A. homomorphum.  The Sabinetown is interpreted as the up-dip Yoakum Shale equivalent, with the MFS in the upper Dark Band. As the only PETM interval in south-central Texas outcrops it provides a robust datum for regional correlations.

Sediment bypass to deeper-water upper Wilcox deposystems occurred during an estimated 3-5 my hiatus at a sub-Carrizo unconformity that separates PETM deposits from the overlying Carrizo. The marine Carrizo comprises sand-rich, transgressive estuarine or tidal-delta deposits with abundant Ophiomorpha. Although Carrizo pollen recovery was poor, rare Thomsonipollis magnificus limits the Carrizo to no younger than Ypresian.

Overall geochemical profiles for the Sabinetown and Carrizo Formations are consistent with ecological, climatic, compositional, and mechanical weathering variability across the PETM. Geochemical analysis of fine-grained sediments indicates that the Dark Band records a condensed interval, consistent with the Yoakum Shale flooding surface. All results allow for both regional and global correlations to PETM successions to better understand fluctuating sea level and climatic controls on sediment and stratal characteristics.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413726

© Copyright 2025 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.