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

Abstract 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.