FINDING A SPECIFIC NEEDLE IN A HAYSTACK: TRACING EJECTED ZIRCON FROM THE CHICXULUB IMPACT IN PROXIMAL K-PG BOUNDARY SECTIONS

Numerical modeling of the Chicxulub impact event on the Yucatán Peninsula in Mexico suggests that shock-induced ejection and vaporization of target lithologies into the stratosphere caused severe environmental stress, culminating in the Cretaceous-Paleogene (K-Pg) mass extinction at 66 Ma. However, the exact timing and mechanisms of the impact cratering ejection processes are not yet fully understood. To better refine particle transport physics in impact ejecta models, it is crucial to incorporate additional physical and chemical proxy data from within the Chicxulub impact structure and from the global K-Pg ejecta layer to bolster the current proxies (grain size, ejecta thickness and content, and the global iridium anomaly). We performed U-Pb detrital zircon (DZ) analyses and electron backscatter diffraction imaging on material from four K-Pg boundary sites to test for the presence of "tracer" ages linked to distinct pre-impact and impact lithologies, which can be used as an input parameter for ejecta models. Chicxulub target rocks include a newly dated 334.3 ± 2.3 Ma magmatic arc uplifted within the peak ring and the well-known ~550 Ma and ~420 Ma Yucatán basement ages, previously determined from ejecta and target rock clasts within the crater suevite. Fully reset or neoformed 66 Ma grains are also considered a “tracer” age if no contemporaneous regional volcanism exists. We investigated the K-Pg stratigraphy at multiple depositional environments and distances from the crater ranging from very proximal (~300 km) to intermediate (~1300 km). In the K-Pg sites, we discovered 66 Ma grains, the most abundant “tracer” age in the sites, and grains of Chicxulub-derived basement ages. These Chicxulub-derived grains display a variety shock textures such as fractures, microtwins, planar deformation bands, and granular textures indicating shock pressures ranging from <20 GPa to >50 GPa within a single sample. Pairing of the zircon ages and microstructures suggest mixing of heterogeneously shocked material in the ejecta curtain. The careful investigation of these ages and microstructures is critical to understanding the ejecta processes that govern the K-Pg stratigraphy at each site, including ejecta emplacement and mixing with siliciclastic Upper Cretaceous sediment which complicate the DZ signatures.