NEW 40AR/39AR AND PLANKTONIC FORAMINIFERAL DATA INDICATE A KPB AGE FOR THE CHICXULUB-LINKED SPHERULE BED AT GORGONILLA ISLAND, PACIFIC OF COLOMBIA

Bathyal sediments previously dated biostratigraphically to approximately Cretaceous-Paleogene boundary (KPB) age at Gorgonilla, Colombia, include a 20 mm-thick bed composed almost exclusively of glassy spherules (Bermudez et al., 2016). The spherule bed displays clear normal grainsize sorting indicating settling from the water column with minimal subsequent disturbance, and the relationship of the spherule bed to surrounding sediments is unambiguous. The stratigraphic position of the Gorgonilla spherule bed coupled with preliminary biostratigraphic and geochemical data (Bermudez et al., 2016) suggest that these spherules are correlative with those found in many circum-Caribbean locations closely associated with the KPB and ascribed to impact melt produced by the Chicxulub impact. For micropaleontological analysis, 73 disaggregated samples were studied. Planktonic foraminifers are absent in Cretaceous deposits, except for scarce specimens identified in two samples, which includes the Maastrichtian index-species Pseudoguembelina palpebra. Planktonic foraminifers are more abundant in the first meter above the spherule bed, whose assemblages belong to the Zone Pα (basal Danian). The first Danian biozone (Zone P0) is absent, suggesting a small hiatus of no more than 10 ka (Arenillas et al., 2006). To test whether the spherules are Chicxulub-derived tektites, we employed ⁴⁰Ar/³⁹Ar dating. Incremental heating of 25 individual spherules yielded plateau ages for all, with 19/25 yielding 100% concordant plateaux and the remainder comprising >85% of the ³⁹Ar released. The weighted mean of all plateau ages is 66.051 ±0.031 Ma (1 sd, analytical uncertainties only). This age is indistinguishable from the ⁴⁰Ar/³⁹Ar ages (66.038 ±0.025 Ma) of the Haitian tektites, and from the age (66.043 ±0.010 Ma) of the KPB (Renne et al., 2013). Thus we conclude that the Gorgonilla spherules are tektites produced by the Chicxulub impact at the KPB.