Much of the discussion about the effects of an end-of-Cretaceous impact
by a large extraterrestrial body in northwestern Yucatan has been done
in the context of limited and partly erroneous published data on the
Mesozoic stratigraphy of that area. Reexamination of cores and
geophysical logs taken in several Pemex wells has produced improved
lithologic 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 deep wells of northern Yucatan. This breccia in Pemex wells
Yucatan 1, 2, 4, 5A, and 6 is composed predominantly of detrital
dolomite, limestone, and anhydrite clasts set in dolomitized carbonate
mud matrix, which contains upper Maastrichtian foraminifers. These
constituents, mixed with fragments of altered glass or melt rock,
shocked quartz and feldspar, and basement rock, suggest an impact as the
most likely origin for the breccia. The timing of brecciation is poorly
constrained by biostratigraphic data. There is some evidence, however,
that the breccia unit is overlain by about 18 m of uppermost
Maastrichtian marls, suggesting an impact before the Cretaceous-Tertiary
boundary. In addition, there may have been more than one episode of
breccia deposition.