PALYNOLOGICAL CHARACTERIZATION OF THE PETM HYPERTHERMAL EVENT IN CENTRAL TEXAS

Ongoing palynological and sedimentological investigations of Wilcox Group outcrops in Central Texas, have led to 1) refined interpretation of the expression of superimposed sea-level change and hyperthermal events in this area, 2) revised age ranges of pollen species previously regarded as robust indicators of Paleocene and Eocene ages.

Significant evidence points to the presence of a hyperthermal event, possibly the PETM, in the Dark Band bed which overlies the Sabinetown Formation. A super-abundance of Apectodinium homomorphum within the dinocyst assemblages (up to 90% of the overall assemblage), a lack of Eocene-restricted species, and a spike in Cicatricosisporites sp., as well as a last occurrence of Spinaepollis spinosa together suggest the PETM. The pollen species Platycarya platycaryoides may actually have its first stratigraphic occurrence in the latest Paleocene as rare specimens have been identified in the Red Bluff outcrop, stratigraphically below the Dark Band. P. platycaryoides has not been identified from other outcrop localities in the area, suggesting it may be environmentally sensitive. Published data of an Eocene first appearance in the central U.S., with our evidence of a latest Paleocene first appearance in Texas Gulf Coast, suggests it may be diachronous.

Compound-specific isotope analyses of samples from the Copperas Creek locality are equivocal at this time, and additional analyses are being undertaken. Initial results show a small negative carbon isotope excursion (CIE) at the level of super-abundant A. homomorphum. However, the magnitude of this negative excursion is not as great as that documented from other onshore PETM localities. Whether the combination of up-dip shallow shelf and marginal paleoenvironments muted the larger global CIE in the Bastrop area, or if the smaller excursion is due to sampling bias, remains to be resolved.

The abundance of evidence points to distinct paleoenvironmental conditions in Central Texas quite different than those documented from the eastern side of the Mississippi Embayment Paleocene-Eocene strata, and further different from global PETM localities. The broad shallow shelf and mesotidal conditions, coupled with the unique paleoclimatic conditions of this time period, all indicate that the exact character of the PETM in Central Texas will also be unique.