|mhorn||Дата: Monday, 28.09.2015, 22:16 | Сообщение # 1|
|Lopez R., Barragán R., Reháková D. (2015) The Jurassic/Cretaceous boundary by means of calpionellids in San José de Iturbide, Nuevo León, northeastern Mexico // Geological Quarterly, doi: 10.7306/gq.1244 |
A detailed bed by bed sampling on an outcropping section 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). Infayacent subzones (Remanei and Brevis) were not identified due to the scarcity and bad preservation of calpionellids. The Jurassic/Cretaceous boundary was defined by the acme of the small and spherical form of Calpionella alpina Lorenz in sample IT-120. The Berriasian was divided in two zones and five subzones. Calpionella Zone (Alpina, Ferasini and Elliptica Subzones) and Calpionellopsis Zone (Simplex and Oblonga Subzones). Studied facies indicate that the deposition along 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 facies indicate a slope environment with the occurrence of a breccia level. A general deepening of the environment is recorded within the frame of the Elliptica Subzone where sediments shift into basinal facies.
Middle Jurassic - Lower Cretaceous ammonites & aptychi