ABSTRACT

Upper Cretaceous rocks in west Texas and northeastern Chihuahua, Mexico, are divided into three formations, the Ojinaga Formatios, the San Carlos Formation, the El Picacho Formation.

A quantitive zonation based on morphologic changes in stratigraphically successive Placenticeras samples is proposed in order to supplement the established Upper Cretaceous collignonicerid zonation. The correlation coefficients of eight stratigraphically unrelated samples are statistically compared with the established Placenticeras evolutionary sequence and time correlations are made.

The results of the biostratigraphic analysis suggest that the Senonian strandline is Presidio and Jeff Davis Counties, Texas, and northeastern Chihuahua moved in a southeasterly direction during an extensive marine regression.