Early Paleogene fluvial regime shift in response to global warming; a subtropical record from the Tornillo Basin, West Texas, USA

The early Paleogene was a dynamic period marked by long-term climatic trends and rapid climate events superimposed upon a hot greenhouse state. The response of the terrestrial hydrological cycle to these climate variations can be investigated at the continental scale by analysis of Paleogene strata in Laramide basins and the Gulf of Mexico. New U-Pb geochronologic data from detrital zircons in Paleogene fluvial strata of the Tornillo Basin in west Texas indicate relatively continuous deposition from 63 to 47 Ma. The combination of detrital zircon ages with existing carbon isotope chemostratigraphic data allows us to revise the age model of these strata and to correlate stratigraphic changes with Paleogene climate variations. Sedimentologic data indicate that, during the Paleocene (63-55.5 Ma), high-sinuosity, fluvial systems meandered on a forested floodplain with well-developed soils and swamps. The fluvial regime changed abruptly to low-sinuosity, braided fluvial systems with floodplains characterized by intermittent wetting at the Paleocene-Eocene thermal maximum (55.5 Ma). Braided fluvial regimes persisted thereafter and culminated during the early Eocene climate optimum (52-50 Ma) with deposition of a coarse sheet of sand and gravel. We interpret the transition from meandering to braided fluvial systems as a shift in discharge and sediment supply associated with the more seasonal precipitation of the hot, subtropical monsoonal climate of the early Eocene. After 54 Ma, this shift in fluvial regime is superimposed by an upward-coarsening grain-size trend probably driven by enhanced regional Laramide tectonism. These data provide the first reconstruction of Paleogene subtropical fluvial dynamics in western North America and correlate with regional and global changes in Paleogene climate and landscapes.