The Cretaceous-Paleogene boundary deposit in the Gulf of Mexico : oceanic basin response to the Chicxulub impact and geomechanics of reservoir-scale sand injectites, Panoche Hills, California
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Hydrocarbon exploration in the last decade has yielded sufficient data to evaluate the Gulf of Mexico basin response to the Chicxulub asteroid impact. Given its passive marine setting and proximity to the impact structure on the Yucatán Peninsula, the Gulf is the premier locale in which to study the near-field geologic effect of a massive bolide impact. We mapped a thick (dm- to hm-scale) deposit of carbonate debris at the Cretaceous-Paleogene boundary that is ubiquitous in the Gulf and readily identifiable on borehole and seismic data. We interpret deposits seen in seismic and borehole data in the distal deepwater Gulf to be predominately muddy debrites with minor turbidites based on cores in the southeastern Gulf. Mapping of the deposit in the northern Gulf of Mexico reveals that the impact redistributed roughly 1.05 x 10⁵ km³ of sediment therein, and over 1.98 x 10⁵ km³ Gulf-wide. Deposit distribution suggests that the majority of sediment derived from coastal and shallow-water environments throughout the Gulf via seismic and megatsunamic processes initiated by the impact. The Texas shelf and northern margin of the Florida Platform were primary sources of sediment, while the central and southern Florida Platform underwent more localized platform collapse. Crustal structure of the ancestral Gulf influenced post-impact deposition both directly and indirectly through its control on salt distribution in the Louann salt basin. Nevertheless, impact-generated deposition overwhelmed virtually all topography and depositional systems at the start of the Cenozoic, blanketing the Gulf with carbonate debris within days. The Panoche Giant Injection Complex (PGIC) in central California is a complete injectite system. We measured hundreds of injectites over ~600 m of stratigraphic thickness in two locales in order to determine geomechanical controls on injection. We document an injectite architecture in the PGIC that we interpret to reflect a reversal in paleostress state from reverse to strike-slip or normal with proximity to the paleoseafloor. We demonstrate that injectite aperture and bulk strain decrease with distance from the injection source. We model this behavior with three hydraulic fracture geometries and conclude that injectites formed via radially propagating hydraulic fractures. We document a northeast-southwest paleo-orientation preference of subvertical injectites, which reflects the control of Farallon plate subduction on stress state at the PGIC. We estimate that the PGIC was complete and active for ~1 Ma and punctuated by ~20–150 ky-long periods of quiescence based on thickness and spacing of extrudites in the PGIC.