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, here exemplified by a terrestrial Cretaceous-Paleogene (K-Pg) boundary sequence. The geochemical records fingerprint specific Chicxulub related impact ejecta products and thereby further constrain the timeline of ejecta deposition. High-resolution (25 μm) micro-X-ray fluorescence (µ-XRF) mapping and quantitative integrated-area linescans in combination with ESEM-EDS analyses of the Starkville South sequence (Raton Basin, Colorado) reveal a complex microstratigraphy, in which additional sublayers can be identified than the classic ‘dual-layer’ succession, described in literature for US Western Interior K-Pg sites. First, a basal claystone is identified with abundant glassy impact spherules that have been altered over time to kaolinite and jarosite due to acidic and reducing conditions in a local swamp environment. This first lithology is followed by a carbonaceous shale interval rich in ejected quartz grains. These two ejecta intervals are interpreted to have formed by ballistic transport from the Chicxulub crater region and were likely emplaced within ∼1 hour after impact at Starkville. In the overlying lignite layer, pronounced enrichments in both zirconium and chromium are detected, hinting to a triple ejecta layer with a large part of the siderophile element anomaly being likely preserved in this coaly interval, including the famous iridium anomaly. These enrichments are attributed to fine-grained impact dust comprising 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.