Lower Cretaceous rocks of Sierra Los Chinos, east-central Sonora, Mexico

Abstract

Most Lower Cretaceous rocks in Sonora are part of the Bisbee Group of southern Arizona, deposited within a basin considered to be an extension of the Gulf of Mexico into Texas, New Mexico, Arizona and Sonora. However, the Lower Cretaceous rocks of the Sierra Los Chinos are more closely related to the Lampazos rocks of eastern Sonora, and consequently, to Chihuahua stratigraphy. In the Los Chinos area, the exposed Lower Cretaceous units include the Agua Salada, Lampazos and Los Picachos Formations. The Upper Cretaceous sequence is made up of volcanosedimentary rocks overlain by Tertiary intrusive, volcanic, and sedimentary rocks. Lower Cretaceous rocks of the Sierra Los Chinos area are strongly folded and faulted, with kilometer-scale overturned folds and thrusts. The area was affected by at least two episodes of normal faulting. The Sierra Los Chinos sequence is paleogeographically and tectonically related to the Chihuahua tectonic belt and is unrelated to the Bisbee Group of Sonora. The marine conglomerate at the base of the Los Picachos Formation represents a significant active tectonic episode at the end of the Albian, possibly related to re-activation of strike-slip faulting due to tectonic transpression. The compressional structures in the Los Chinos area are similar to structures of the same age in the states of Chihuahua, Coahuila, and Nuevo León.