ZIRCON U-PB AGE SPECTRA FROM THE TERTIARY MISSISSIPPI DELTA: INSIGHTS INTO WILCOX PALEOGEOGRAPHY AND EROSION OF THE ROCKY MOUNTAINS

Reconstruction of the Cenozoic erosional history of the interior western United States can elucidate the processes that built the high-standing, high-relief tectonic elements in the region. However, evidence for significant bypass and excavation of proximal Rocky Mountain depocenters suggests that the Cenozoic sediment budget of the region remains unclear and that the Gulf Coast contains a key sedimentary archive. We present 106 – 177 replicate single-grain Zircon U-Pb ages for each of 10 outcrop samples from Paleocene to Upper Miocene sandstones from a ~10,000 km² swath of central Louisiana corresponding to the Mississippi Delta, the key Cenozoic depocenter in the central Gulf Coast. Sample depositional ages derive from biostratigraphy and/or regional lithostratigraphic correlation, and typically overlap minimum U-Pb ages, indicating that a) significant volcanic material was incorporated into Tertiary Gulf Coast strata and b) depositional ages are accurate. Populations of U-Pb ages correspond to the various geologic terranes that underlie the modern Mississippi River drainage basin. However, the prominence of various populations changes systematically through time in a way that appears to define three distinct stratal packages. Both Upper Paleocene-Lower Eocene Wilcox Formation and Oligocene-Miocene strata exhibit prominent age populations with Rocky Mountain affinity, whereas the most prominent population in Middle Eocene strata can be tied to the eastern United States. The packages with Rocky Mountain affinities correspond to the two long-lived Cenozoic episodes of rapid sediment influx and continental shelf progradation in the central Gulf Coast, suggesting two discrete episodes of major Rocky Mountain sediment transfer to the depocenter. Thus, in addition to supporting the long-held view that the Laramide highlands shed significant sediment to the Gulf Coast during the Upper Paleocene-Early Eocene, our data may also hint at significant excavation of the southern Rockies starting in the Oligocene. Furthermore, comparison to recent provenance studies in the western Gulf Coast indicates that the southern Rocky Mountains sourced the Wilcox Formation from South Texas to central Louisiana and suggests that the formation may exhibit a broadly similar genetic history across this area.