Abstract: Cretaceous-Tertiary Boundary Sedimentary Breccias from Southern Mexico: Normal Sedimentary Deposits or Impact-Related Breccias?

M. Limon, E. Cedillo, J. M. Quezada, J. M. Grajales, W. Alvarez, A. R. Hildebrand, M. A. Sanchez, C. Rosales, V. Gonzalez

Oil-producing sedimentary breccias constrained to Late Cretaceous-Early paleocene ages from southern Mexico have been previously interpreted as the result of normal geological processes such as formation along fault scarps and platform margins. New data derived from surface and subsurface geology indicate that the breccias have been produced by disturbances caused by a giant Cretaceous-Tertiary boundary impact on the Yucatan peninsula. The Chicxulub crater is believed to be the result of this large body impact (approx.) 65 million years ago and has recognizable ballistic ejecta extending to a radius of several thousand kilometers. Previous work and continuing investigations indicate the presence of sedimentary breccias at the K-T boundary in southern Mexico, nearby offshore areas, and in northern Guatemala. These breccias are much thicker than the thickness of ballistically distributed impact ejecta alone and thus indicate local derivation of most of the breccia components. Impact-produced seismicity and giant ocean waves are plausible causal mechanism for the formation of the boundary breccias around the Yucatan platform. Impact-related origins are the only means that can account for the apparently simultaneous generation of thick breccias, containing shocked quartz and tektites and underlying a cap of Ir-rich clays, across a >1,000 km geographic range. The indicated K-T boundary age for these deposits matches that suspected for the Chicxulub crater. The impact energy released at Chicxulub is sufficient to trigger widespread submarine slumps, although other mechan sms produced comparable slump deposits in some sedimentary sequences in the region so that other origins need to be seriously considered.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994