YUCATAN SUBSURFACE STRATIGRAPHY - IMPLICATIONS AND CONSTRAINTS FOR THE CHICXULUB IMPACT

Much of the discussion about the effects of an end-of-Cretaceous impac t by a large extraterrestrial body in northwestern Yucatan has been do ne in the context of limited and partly erroneous published data on th e Mesozoic stratigraphy of that area. Reexamination of cores and geoph ysical logs taken in several Pemex wells has produced improved litholo gic and biostratigraphic correlation of the Jurassic to Maastrichtian section across the northern Yucatan peninsula. These data suggest that major disturbance of strata by an impact would have been confined to within about 100 km of the proposed impact center near Chicxulub. The only unusual lithologic unit is polymict breccia, which apparently was penetrated at or near the top of the Cretaceous section in all the de ep wells of northern Yucatan. This breccia in Pemex wells Yucatan 1, 2 , 4, 5A, and 6 is composed predominantly of detrital dolomite, limesto ne, and anhydrite clasts set in dolomitized carbonate mud matrix, whic h contains upper Maastrichtian foraminifers. These constituents, mixed with fragments of altered glass or melt rock, shocked quartz and feld spar, and basement rock, suggest an impact as the most likely origin f or the breccia. The timing of brecciation is poorly constrained by bio stratigraphic data. There is some evidence, however, that the breccia unit is overlain by about 18 m of uppermost Maastrichtian marls, sugge sting an impact before the Cretaceous-Tertiary boundary. In addition, there may have been more than one episode of breccia deposition.