AGE, BIOSTRATIGRAPHY, GEOCHEMISTRY AND MINERALOGY OF CHICXULUB IMPACT SPHERULES ON GORGONILLA ISLAND, COLOMBIA

The age of the Chicxulub impact is still controversial. Impact spherules found at or near the Cretaceous-Paleogene boundary (KPB) in the North Atlantic, Caribbean, Belize and Guatemala are reworked in early Danian sediments above a KP hiatus [1-3]. But, in NE Mexico and Texas where impact spherules are beyond erosion by the Gulf Stream, they are found in late Maastrichtian sediments with the primary spherule layer predating the KPB by ~100 ky [4,5]. Recent discovery of pristine Chicxulub impact glass spherules on Gorgonilla Island, Colombia [6], and re-sampling of the section sheds new light on this controversy. Sediments consist of radiolarian-rich marls alternating with volcanic-rich turbidites depleted in calcite. Three closely spaced ~2 cm-thick spherule beds, two discontinuously present, overlie a late Maastrichtian erosion surface and have a similar mineralogical composition as the turbidites, suggesting spherule re-deposition marked by decreasing grain size and repetition of beds due to slumping. Spherule beds contain reworked Maastrichtian species and clay clasts with early Danian planktic foraminifera typical of zone P1a(2) about 150-200 ky after the KPB indicating re-deposition during the early Danian. Study of radiolaria is in progress. δ¹³C_org.carb values show no negative shift near the spherule bed, confirming a KPB hiatus, as also indicated by absence of significant Ni and Co enrichments. δ¹⁸O by laser ablation of spherules have a wide range of values (8.5 to 10.4‰) typical of andesitic-dacitic (~8‰) and rhyolitic glass (9 to 10‰), suggesting melt contaminated with sedimentary material. While these analyses confirm an impact origin, spherules with higher SiO₂ content also have higher d¹⁸O values, indicating crystal fractionation processes or assimilation of sedimentary material. Preliminary ⁴⁰Ar/³⁹Ar dating of Gorgonilla impact spherules is currently underway at two laboratories.

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