Disentangling Impact Ejecta Dynamics Using Micro–X-Ray Fluorescence (μ-XRF): A Case Study From the Terrestrial Cretaceous-Paleogene (K-Pg) Boundary

This study presents a non-destructive geochemical and petrographic workflow to generate high-resolution chemostratigraphic records across key stratigraphic intervals, exemplified by a terrestrial Cretaceous-Paleogene (K-Pg) boundary sequence preserved at Starkville South (Raton Basin, Colorado, USA). To fingerprint specific Chicxulub impact ejecta and to unravel their mode and timing of deposition, we combined high-resolution (25 μm) micro-X-ray fluorescence (μ-XRF) mapping and quantitative integrated-area linescans with scanning electron microscopy. A complex microstratigraphy is observed at Starkville South in which additional sublayers are identified in contrast to the classic “dual-layer” succession described in the literature for US Western Interior K-Pg sites. First, a basal claystone occurs with abundant glassy impact spherules that were altered over time to kaolinite and jarosite due to acidic and reducing conditions in a local swamp. This first lithology is followed by a carbonaceous shale rich in ejected quartz grains. These two ejecta intervals are interpreted to have formed primarily by ballistic transport from the Chicxulub impact structure and were likely emplaced within ∼1 hr after impact at Starkville South. In the overlying lignite, pronounced enrichment in zirconium and chromium are detected, hinting at an ejecta sequence containing three distinct sublayers with a large part of the siderophile element anomaly being likely preserved at the base of this coaly interval, including the famous iridium anomaly. These enrichments are attributed to fine-grained impact dust composed of pulverized granitoid basement (Zr) and an admixture of meteoritic material (Cr, Ni, and likely Ir), probably deposited <20 years after impact following slow atmospheric settling.