MORPHOLOGY OF IMPACT GLASS EJECTA ASSOCIATED WITH THE CHICXULUB ASTEROID IMPACT: NEW DATA FROM GORGONILLA ISLAND, COLOMBIAN PACIFIC

The discovery of a Cretaceous/Paleogene (K/Pg) marine bathyal section on Gorgonilla Island, Pacific of Colombia, has revealed the presence of a virtually unaltered 2-cm-thick spherule-rich bed separating the Maastrichtian from the Danian. This bed represents Chicxulub asteroid impact ejecta deposits based on detailed mineralogical, geochemical, and stratigraphic studies. The Chicxulub impact is thought to be responsible for the mass extinction at the K/Pg boundary. Although there have been many studies of ejecta deposits around the globe, relatively few have focused on spherule morphology. Taking advantage of the exquisite preservation of the spherules (up to 90% of are pure glass) from Gorgonilla Island, a detailed morphological analysis of 1000 individual spherules has been carried out. Our results indicate all spherules correspond to spin or spin-induced forms with no evidence of ablation-form spherules. The spin-induced spherule morphology can be subdivided into four main categories: I) Splash-forms (the most abundant type); II) Fused-forms (that includes any combination of 2 or more splash-forms); III) Deformed-forms (that includes splash-forms with evidence of deformation by rotation or impact), and IV) Irregular forms (undefined morphologies including ground impact effects). Regarding the splash-forms (Category I), we propose a novel classification scheme based on the axial ratios of ellipsoids, which include a diverse array of shapes ranging from spheres, rods, ovoids, ovoid disks, spherical disks, dumbbells, bowties, oval bowls, circular bowls, teardrops, teardrop bars, ovoid teardrops, and teardrop disks. These different shapes are interpreted as a result of spinning while cooling and solidifying, which indicates that surface tension and centrifugal forces associated with the rotation of fluid droplets are the main factors determining the shapes of this type of Chicxulub's spherules. On the other hand, the statistical analysis confirms that most spherules correspond to spheres or ovoids (especially abundant in the upper part of the K/Pg bed), where rotation does not seem to play an important role in forming these morphologies. A plausible interpretation of the ovoid and spherical spherules is that they are vapor condensates formed in the impact plume. These results represent the first morphological classification of the type of ejecta produced during the Chicxulub asteroid impact and provide direct evidence of glass spherules formation mechanisms during asteroid impacts.