Bol. Soc. Geol. Mexicana. Tomo XXXVIII, No. 1, pp. 2-17, junio 1977

Bioestratigrafía del Cretácico Inferior Basada en Microfósiles Planctónicos

http://dx.doi.org/10.18268/BSGM1977v38n1a1

José Francisco Longoria*

Instituto de Geología, Universidad Nacional Autónoma de México.

Abstract

A biostratigraphic scheme for the mexican Lower Cretaceous is presented. the time span represented by the Lower Cretaceous is subdivided into 15 biozones, from which 13 are Interval Zones and 2 are Total-Range-Zones. The Jurassic/Cretaceous Boundary is placed at the base of the Calpionella elliptica/Calpionellopsis simplex Interval-Zone, corresponding to the first evolutive appearence of C. elliptica Cadisch. The Lower Cretaceous/Upper Cretaceous Boundary is placed at the base of the Thalmanninella ticinensis/Thalmanninella evoluta lnterval-Zone, which also corresponds to the first evolutive appearence of keeled Hedbergelloidea.