The Jurassic-Cretaceous boundary in Northeastern Mexico. Confrontation and correlations by microfacies, clay minerals mineralogy, calpionellids and ammonites

Abstract

4 profiles, 3 of them in the Sierra Madre Oriental and one 400 km farther NW in the Durango Basin have been measured and sampled in detail, in order to ascertain the stratigraphic position of the boundary between the La Casita and the Taraises Fms. Calpionellids show that this boundary is diachronous, situated in the lower Berriasian. Late Tithonian mediterranean faunal elements are however largely ( Crassicollaria in the case of calpionellids) or completely missing ( Saccocoma, Chitinoidella)). Calpionellids are still more or less sporadic in the lower part of Zone B, but become frequent in its upper part. This indicates an important turnover in oceanographic conditions (currents, and/or temperatures), which appears also clearly in a change of the composition of clay-mineral associations (increase of chlorite, decrease of kaolinite). First analyses of stable oxygen and carbon isotopes point in the same direction, as well as the simultaneous appearance of mediterranean ammonite genera. Calpionellid Zones C to D1 are reduced or incomplete in this part of the Sierra Madre Oriental. The succession of calpionellid faunas corresponds perfectly to that known from the Mediterranean realm. This allows precise transatlantic correlations. It appears that the ammonite genera Durangites and Hildoglochiceras range at least up into the uppermost Tithonian; whereas Kossmatia, Parodontoceras and Substeueroceras extend up into the Berriasian.