Mexico is usually considered to have formed the western end of the
Tethys during Late Jurassic and Early Cretaceous times. The
circumstances of the opening of the Gulf of Mexico Basin towards the
Tethys and the exact stratigraphic timing, however, are not clear. Four
sections covering this time interval, located in northeastern Mexico,
have been measured and sampled in detail, in order to clarify their
stratigraphic position during the Late Jurassic to Early Cretaceous time
interval and the paleogeographic and oceanographic changes that
accompanied this opening. Our studies include microfacies, micro- and
macropaleontology, whole rock and clay-mineral x-ray diffraction and
stable isotopes analyses. Our data indicate that the Jurassic-Cretaceous
boundary, as defined by the Lyon-Neuchatel Colloquium of 1973, cannot be
determined precisely in northeastern Mexico due to the near-absence of
calpionellids and endemism of ammonite taxa. In the lower and upper
Berriasian sediments, we detected Mediterranean ammonite taxa so far
unknown from Mexico, corresponding to the appearance of typical
calpionellid-rich facies. These faunas allow direct biostratigraphic
correlation with European ammonite and calpionellid zones.
We propose that a major oceanographic change occurred in the upper part
of calpionellid Zone B of the Early Berriasian. At this time, sediments
in northeastern Mexico present increasingly pelagic facies, a dramatic
appearance of Tethyan microfossils (calpionellids) and ammonites,
changes in stable isotopic values, whole rock and clay-mineral
mineralogy. We suggest that these changes are due to a global sea-level
rise that connected directly northeastern Mexico to the European Tethys
and ended the endemic, semi-restricted and anoxic environment of the
Late Jurassic La Casita and equivalent La Caja and La Pimienta
Formations. (C) 1996 Academic Press Limited