REGARDING THE OCEANIC CRUST/UPPER MANTLE GEOCHEMICAL SIGNATURE AT THE K-PG BOUNDARY: IF NOT FROM CHICXULUB CRATER, THEN WHERE DID IT ORIGINATE?

Oceanic crust/mantle geochemical signatures in K-Pg boundary ejecta (1,2,3,4) may be inconsistent with Chicxulub Crater origin given the crater’s location on ~35 km thick continental crust of the Yucatan Platform. This geochemical evidence, along with ejected terrestrial chromites (5) suggest the impact sampled mafic and/or ultramafic rocks not known to exist in the Chicxulub target area. Working hypotheses include: 1) an unknown suture in Yucatan Platform basement, 2) an additional small impact site on oceanic lithosphere, 3) detrital shocked chromite from an unknown older mafic/ultramafic ejecta producing impact, 4) unknown chromite bearing sediments in the Chicxulub target area, or 5) an additional large impact on oceanic lithosphere or continental margin transitional to oceanic lithosphere. Regarding the last hypothesis we note: 1) Ophiolites nearest Chicxulub crater are in Cuba and were obducted in latest Cretaceous/earliest Danian times (6), and not during the Eocene collision of Cuba with the Bahamas platform; 2) the thickest known K-Pg boundary deposits are in Cuba (7,8,9); and 3) Displaced ophiolite belt segments of opposite curvature sense suggest Greater Antilles ophiolites mark the rim of a ~700 km diameter impact basin dismembered from an originally circular form by at least 50 million years of known left-lateral shear along the North American-Caribbean transform plate boundary.

1) DePaolo et al. 1983 EPSL, 64(3):p358-373; 2) Hildebrand & Boynton1988. LPI Cont., 673; p. 78; 3) Hildebrand & Boynton Science, 1990. 248(4957): p. 843; 4) Montanari et al. Geology, 1983. 11(11): p. 668; 5) Olds et al. (submitted to Lithosphere); 6) Garcia-Casco et al. IGR, 2008. 50(9): p. 781-809; 7) Tada et al. 2003. AAPG Memoir 79: p. 583-604; 8) Kiyokawa et al. 2002 GSA Spec. Paper 356, p.125-144; 9) Iturralde-Vinent 1992. EPSL 109(1-2): p. 225-228.