Abstract The Minas Viejas Formation consists of carbonates and sulfates that are the first evidence of marine incursion into northeastern Mexico during the Late Jurassic (Oxfordian). In the area southwest of Galeana, Nuevo Leon, this evaporite sequence is intensively deformed, but a consistent stratigraphic succession and separation of two members is recognizable. In addition to the Las Minas Member that was defined by Götte (1988), we introduce the La Primavera Member. Our data suggest that only one largely evaporitic succession exists in the region and that the terms Minas Viejas and Olvido are synonyms for the same stratigraphic unit. Lateral and vertical changes of facies in the Minas Viejas Formation are the result of syndepositional normal faulting and relate to the onset of sea-floor spreading in the Gulf of Mexico. Alkaline volcanic rocks occur in the La Primavera Member of the Minas Viejas Formation. This Oxfordian volcanism is hitherto undescribed in the area and links the tectonostratigraphic evolution of northeastern Mexico to early sea-floor spreading in the Gulf of Mexico. In addition, barite deposits in the Galeana area likely are related to this Late Jurassic volcanism. Barite mineralization is restricted mainly to stratigraphic levels older than the alkaline volcanism in the Minas Viejas Formation and is not the result of magmatism of Tertiary age. Apparently, carbonatite magmatism that provided the source for barium by hydrothermal activity was associated with Late Jurassic volcanism.

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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 3‰) of both δ 13 C and δ 18 O was recognized in an interval of pelagic mudstone corresponding to the upper Valanginian. A similar δ 13 C 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 ( Föllmi 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 δ 13 C 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. This late Valanginian δ 13 C excursion is generally interpreted to be the first episode of Cretaceous greenhouse conditions. It reflects a major change in the global carbon budget that could have resulted from increased tectonic/volcanic activity, such as the Paraná continental flood basalts leading to increased atmospheric CO 2 and, consequently, greenhouse conditions. Oxygen-isotopes data in the Mexican sections trend toward higher values, but with a significant temporal lag, as compared with the δ 13 C curve. This indicates seawater and/or climatic cooling, which can be linked either to negative feedback (e.g., increase in weathering) leading to a decrease in atmospheric CO 2 or to intense oceanic spreading and a subsequent global sea-level rise.