--> --> Multidisciplinary Characterization Of The PETM Hyperthermal Event In Central Texas: Palynology, Isotope Chemistry and Sedimentology

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Multidisciplinary Characterization Of The PETM Hyperthermal Event In Central Texas: Palynology, Isotope Chemistry and Sedimentology

Abstract

A multi-disciplinary study has led to characterization of the PETM hyperthermal event in Central Texas. Although isotope geochemistry results are equivocal, other geochemical findings suggest unique paleoenvironmental conditions persisted along the Gulf Coast during the Paleocene-Eocene times, such that recognition criteria for the PETM documented elsewhere may not be applicable to the U.S. Gulf Coast. From these new chronostratigraphic and sedimentological datasets, source-to-sink relationships into shelf-edge and deepwater Wilcox strata can be refined. The emerging Paleocene-Eocene chronostratigraphic framework suggests that large time gaps are present in the Central Texas outcrop succession, and shelf-edge deltas and deepwater Wilcox deposits are not coeval with the strata present in these outcrops. Within the palynological assemblages, the super-abundance of Apectodinium homomorphum in the dinocyst population (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, indicate the PETM occurs within a coal seam that occurs in the upper part of a unit termed the Dark Band. The widespread presence of marine dinoflagellates suggests marine influence on deposition, contradicting earlier interpretations of these strata as fluvial/non-marine. Accompanying evidence, including common tidal sedimentary structures, and shallow marine trace fossils, including abundant Ophiomorpha, corroborates marine influence in the vicinity of the Paleocene-Eocene boundary in Central Texas. This overwhelming evidence of shallow marine depositional environments permits reassessment of Wilcox Group paleoenvironments and paleogeography. The associated shoreline was to the northwest during most of the time of deposition, perhaps as far as the Balcones Escarpment. Compound specific isotope data require additional study, although a small negative excursion has been noted from earlier datasets. N-alkanes are not well-preserved in the outcropping sediments, likely due to a combination of the activity of modern bacteria and fungi and petroleum migration through the system, which makes exact identification of any negative isotope excursion problematic. Other geochemical data indicate the presence of unique biomarkers, likely related to algal and bacterial contributors to the sedimentary organic matter and further related to the unique Paleocene-Eocene paleoenvironments of this region.