HAFNIUM ISOTOPE DATA FROM THE UNITED STATES GULF OF MEXICO BASIN REVEAL CORDILLERAN TECTONIC INFLUENCES ON PALEOGENE DEPOSITIONAL SYSTEMS

The Paleocene-Eocene Wilcox Group and Oligocene Frio Group comprise an expansive set of fluvio-deltaic, shelf, slope, and deep-water depositional sequences within the central and western Gulf of Mexico (GOM) Basin. Derived primarily from the Central and North American Cordillera, these strata contain a long record of Laramide orogenic onset and evolution, continental-scale drainage network reorganization, and derivative clastic sedimentation in the GOM. We employ detrital zircon hafnium isotope data to identify the genetic character of crustal material entering the GOM Paleogene source-to-sink system to improve sediment source terrane discrimination and highlight the role of tectonic factors in system evolution.

Data from Paleogene sand bodies contain three distinct tectonic signals: a period of widespread crustal thickening (~82-72 Ma; ε-Hf: -30 to -15), and likely paleotopographic buildup, associated with the Laramide Orogeny; a strong juvenile crustal signal of magmatic arc origin (~65-53 Ma; ε-Hf: +5 to +15); and a steady background signal (ε-Hf: -15 to +5) from sustained erosion of recycled Proterozoic crust in the hinterland. The arrangement of ε-Hf trends within GOM Paleogene strata demonstrates that hinterland orogenic growth far preceded resultant siliciclastic sedimentation in the deep-water sink and was a primary control on system depositional timing and scale. Zircon derived from thickened crustal regions first enter the GOM with the initial Wilcox Group depositional pulse ca. 61 Ma, indicating a mean depositional lag time of ~15 m.y. for sediments from plutonic terranes. The system then carried a proportional mix of first-cycle and recycled zircon for a few million years before the appearance of a basin-wide increase in the relative proportion of zero-lag zircon ca. 58 Ma. This signal coincides with a bulk shift towards positive ε-Hf values and is attributed to advanced hinterland denudation, which allowed catchments to reach further westward in the NA Cordillera and tap into juvenile crustal terranes within the magmatic arc. All Frio Group zircon have a characteristic narrow range of ε-Hf values (-5 to +5), indicating derivation from a stable, homogenous source region that did not intersect any of the mid-system sediment caches that supplied distinct tectonic signals to the Wilcox Group.