An Occurrence of Upper Cretaceous Sediments in Northern Sonora, Mexico

Abstract

Sediments of Upper Cretaceous age, unconformable on Lower Cretaceous and older beds, and containing both fresh water and marine invertebrates and dinosaur remains, occur in the Cabullona basin, Sonora, Mexico. No beds of this age have been reported previously in southern Arizona and Sonora and this occurrence not only fills in a wide gap but also throws new light on the Upper Cretaceous shore line. These sediments were deposited on land and on the shallow margin of the Upper Cretaceous sea which probably deepened toward the north and east. The nature of the sediments gives much information regarding climatic and physical conditions in this region during the later part of the Upper Cretaceous. The northeastern boundary of the basin in which they now occur is marked by a strong zone of thrusting along which Lower Cretaceous and older beds were thrust, from the northeast, over the Upper Cretaceous. Pyroclastics occur in the Upper Cretaceous, and there are many Tertiary intrusions in the region.