CHICXULUB IMPACT EJECTA AT THE CRETACEOUS-PALEOGENE (K-PG) BOUNDARY FROM ODP LEG 207, TROPICAL WESTERN NORTH ATLANTIC: IMPLICATIONS FOR EJECTA COMPOSITION AND DISPERSAL

The ODP Leg 207 from the Demerara Rise, tropical western North Atlantic, has recovered an expanded and stratigraphically complete Cretaceous-Paleogene (K-Pg) sedimentary record that also includes a well-developed Chicxulub ejecta deposit. Therefore, ODP Leg 207 provides insights into the Maastrichtian-Danian deepwater record from low latitudes and ejecta deposition possibly in an upward direction from an oblique (>45°) Chicxulub impact (Morgan et al., 2006, EPSL). We have conducted a high-resolution mineralogical, isotope geochemical, and rock magnetic study to characterize the Chicxulub ejecta deposit and reveal subsequent environmental changes. The single 1.5-2 cm Chicxulub ejecta horizon is composed of a normally graded layer of spherules that decrease in diameter from ~2 to ~0.25 mm. The upper third part of the of spherule layer also includes “round” dolomite clasts and accretionary carbonate clasts as well as shocked quartz and feldspar grains. Spherules are massive, hollow, or vesicle-rich, and some show in situ collapse. The graded nature, the complex composition of the ejecta, and the, in part, good preservation of delicate spherule textures suggests an origin as primary air-fall deposit. Spherules are altered to well-crystallized Na,-Ca-rich smectite and comprise internal globules, schlieren and lath-shaped crystals with a distinct Fe and Mg enrichment. These textures are indicative of silicate-silicate “liquid immiscibility” and quenching from a melt, suggesting a primary origin. Similar Fe-Mg-enrichment has been observed in Chicxulub spherules from Mexico and Texas. The occurrence of the characteristic Chicxulub-derived spherule layer at the base of Biozone P0 in conjunction with the mass extinction of planktonic foraminifera, the strong Iridium-anomaly (MacLeod et al., 2007, GSA Bulletin, v. 119, 101-115) and the sharp negative δ¹³C anomaly strengthens the genetic link between the Chicxulub impact and the K-Pg boundary clay.