Confrontation of stratigraphic methods to define the Jurassic-Cretaceous boundary in eastern Mexico subsurface

This study determined the Jurassic-Cretaceous boundary over a 1300 km, three-part subsurface section by comparing ammonites, calpionellid microfossils, and radioactive logs from oil wells in east and northeast Mexico. Cores from 26 wells ranging in depth between 400 m and 3000 m established that the top of the Tithonian was defined by ammonites Durangites and Proniceras, and the Berriasian was determined by Berriasella aff. jacobi and Subthurmannia. Both pairs allowed regional characterization of the Jurassic-Cretaceous boundary in the coastal plain of the Gulf of Mexico. These ammonites establish stratigraphic correlations with southern Europe, the Middle East, Cuba, Argentina, and California. Calpionella alpina and C. elliptica appear simultaneously from the upper Tithonian to the Berriasian, making them unreliable fossils for determining this boundary.