138-3 Diving into the first minutes to years after the Chicxulub Impact: Insights from Micro–X-ray Fluorescence (µ-XRF) Analysis of a Terrestrial Cretaceous-Paleogene (K-Pg) Boundary

Session: The Cretaceous-Paleogene (K-Pg) Boundary Interval: From Large-Scale Geological Events to Mass Extinction Mechanisms

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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 µ-XRF quantitative integrated-area line scans, together 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 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 hour after impact at Starkville South. In the overlying lignite, pronounced enrichments 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 layer containing the 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 platinum group elements, including Ir), probably deposited <20 years after impact following slow atmospheric settling. These new high-resolution, geochemical observations of the K-Pg boundary serve as input parameters to refine existing impact models, which shed more light on the timing and mechanisms of ejecta processes after large impact cratering events.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8360

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