Paleocene depositional history of the Cretaceous-Paleogene impact basin, Yucatan Peninsula, Mexico

Abstract

In the spring of 2016, the International Ocean Discovery Program set out on Expedition 364 to recover core from the peak ring of Chicxulub Impact Crater at Site M0077. In total, 829 m of core was collected spanning granite to Paleogene sedimentary rocks. From this core, we have a well-preserved record of the Paleocene, which represents ~10 million years post impact in just under 10 m of sedimentary rock record. This has presented an incredible research opportunity, as we have gained invaluable information on how the environment responded and recovered from the global catastrophe that was the Chicxulub Impact. The Paleocene at M0077 is highly condensed and comprised of predominantly pelagic carbonate rocks. High resolution core logging and thin section analysis were used to identify facies in the Paleocene. Facies include marlstone, argillaceous wackestone, foraminiferal wackestone, and rare coarser grained lithologies such as packstone and grainstone. Overall, the Paleocene exhibits a succession of rhythmically bedded cycles composed of marlstones grading to argillaceous wackestones and capped by foraminiferal wackestones. Coarser grained lithologies only exist in the lower and uppermost portion of the core. In total, 72 cycles that ranged from 5-30 cm thick are identified and grouped into six larger packages based on pattern similarities in color, lithology, ichnofabric indices, and geochemical data. These cycles are interpreted as parasequences, and show predictable stacking patterns that allow us to make sequence stratigraphic analyses. Each package represents one to two systems tracts, and some can be correlated to eustatic sea level change. Recorded in this core is the progression of an initial sea level lowstand immediately post impact, and the fluctuation between highstands, lowstands, and transgressive systems tracts that follow. Major and trace elements were analyzed throughout the core, as well as delta¹³Corg and delta¹⁵Nbulk values. Three sets of geochemical proxies (paleoredox, detrital input, productivity) were used to provide insight into paleoecological conditions. Initial conditions in the crater show a period of high productivity, which tapers off within a million years post impact. Redox conditions vary, and show one major anoxic event, with other enrichments likely representing periods of pore water euxinia or increases in stratification leading to a more robust redox gradient.