CHRONOSTRATIGRAPHY OF THE UPPER CRETACEOUS–EOCENE TORNILLO GROUP, WEST TEXAS: INTEGRATING DETRITAL ZIRCON U-PB MAXIMUM DEPOSITIONAL AGES WITH MULTI-PROXY AGE MODELS

High resolution stratigraphic age models can be built using various age proxies such as magnetostratigraphy, chemostratigraphy, and fossil assemblages. However, age models are substantially improved by absolute age constraints, such as zircon U-Pb dating on volcanic ash. Recently, detrital zircon maximum depositional age (MDA) analysis has been used to provide absolute age constraints in stratigraphic sections that lack volcanic tuff. The Upper Cretaceous–Eocene Tornillo Group in west Texas preserves robust fossil assemblages and a history of tectonics and sedimentation spanning the K-Pg boundary and the Paleocene–Eocene Thermal Maximum (PETM). The age model of the Tornillo Group has been incrementally improved with various chronostratigraphic proxies but still lacks a comprehensive set of absolute age constraints. We present an MDA analysis of 28 new and 8 previously published samples and integrate these results with magnetostratigraphic, carbon isotope, Ar-Ar geochronologic, and North American Land Mammal Age datasets to refine the age models of the two most studied stratigraphic sections of the Tornillo Group. The detrital zircon MDAs are presented as the youngest single grain and the youngest statistical population. Ranked date plots are used to identify negatively skewed tails and possible Pb loss. The detrital zircon MDAs can be sorted into two groups. The first group, from the Javelina Formation through the upper Black Peaks Formation, spans the Maastrichtian-Paleocene including the PETM. These MDAs are characterized by robust youngest age modes and YSG ages that overlap within error of YSP ages and provide valuable new age constraints on the stratigraphy. The second group spans the lower–middle Eocene Hannold Hill and Canoe Formations and is characterized by disparate YSG and YSP ages and long negatively skewed tails of youngest dates, suggestive of Pb loss. Detrital zircon MDAs bracket the K-Pg boundary to the uppermost Javelina Formation at one locality and the lowermost Black Peaks at another locality and support placement of the PETM horizon in the upper Black Peaks. Detrital zircon MDAs also reveal the presence of a basin-wide earliest Paleocene ~3-4 m.y. hiatus in the lowermost Black Peaks Formation, possibly indicating an early phase of Laramide uplift that disrupted the basin in the Eocene.