Calpionellid biostratigraphy across the Jurassic/Cretaceous boundary in San José de Iturbide, Nuevo León, northeastern Mexico

Detailed bed-by-bed sampling on an outcrop of the La Casita and Taraises formations in northeastern Mexico (San José de Iturbide, Nuevo León State) allows the delimitation of the Jurassic/Cretaceous boundary. The Late Tithonian was determined by the presence of the Crassicollaria Zone (Colomi Subzone). Underlying subzones (Remanei and Brevis) were not identified due to the scarcity and poor preservation of calpionellids. The Jurassic/Cretaceous boundary was defined by the acme of the small and spherical form of the species Calpionella alpina Lorenz in sample IT-120. The Berriasian was divided into two zones and five subzones: the Calpionella Zone (Alpina, Ferasini and Elliptica subzones) and the Calpionellopsis Zone (Simplex and Oblonga subzones). The facies studied indicate that deposition during the Tithonian occurred in a toe of slope environment with occasional deep shelf incursions. Near the Jurassic/Cretaceous boundary, a sea level drop is recorded and the facies indicate a slope environment with the occurrence of a breccia level. A general deepening of the environments recorded within the frame of the Elliptica Subzone where the deposits pass into basinal facies