ABSTRACT

In the Sierra Madre Oriental of northeastern Mexico, two sections (La Huasteca and San Lucas) spanning Berriasian to lower Hauterivian rocks were analysed and are correlated by mean of calpionellid and ammonite ocurrences, microfacies, and stable isotopes (bulk rock). A major isotopic excursion (approximately 3o/oo) of both ¹³C and ¹⁸O was recognized in an interval of pelagic mudstone corresponding to the upper Valanginian. A similar ¹³C excursion was also observed in coeval strata of the southern Italian Alps and Appennines (Weissert and Channell., 1985; Weissert et al., 1989; Weissert and Lini, 1991; Lini et al., 1992); the northern Tethys margin (Fllmi et al. 1994); the Gulf of Mexico (Patton et al., 1984); and the North Atlantic (Robertson and Bliefnick, 1983) and Pacific (Douglas and Savin, 1973) Oceans. The ¹³C shift is independent of changes in microfacies, contents in organic matter, and mineralogical composition of the sediment. This stable isotopic pattern was also identified in the Vocontian basin in France (Hennig et al., 1999), and calibrated to the Campylotoxus-Verrucosum zones of the early/late Valanginian. Integration of our biostratigraphic and isotopic data indicates the presence, at San Lucas, of a complete Valanginian sequence in terms of European ammonite and calpionellid zones, whereas at La Huasteca some of the zones may be absent.