APTIAN-ALBIAN INTRAFORMATIONAL CONGLOMERATE IN THE SIERRA LOS CHINOS, EAST-CENTRAL SONORA, MEXICO

Most of the Lower Cretacous rocks exposed in Sonora are related to the Bisbee Group stratigraphy which is an extension of the Gulf of Mexico into Sonora and Arizona. Nonetheless, the Lower Cretaceous rocks exposed in the Sierra Los Chinos are related to the Chihuahua trough stratigraphy, and also related to the Lampazos stratigraphy, 30 km to the northwest of Sierra Los Chinos. In this area, located in east-central Sonora, 20 km northeast of the town of Sahuaripa, the Lower Cretaceous rocks exposed include the Agua Salada, Lampazos and Los Picachos Formations. Lower Cretaceous rocks of the Sierra Los Chinos area are strongly folded and faulted, with kilometer-scale structures (overturned folds and thrusts) being the most conspicuous. In addition, the area was affected by at least two episodes of normal faulting. The compressional structures in the Los Chinos area are similar to deformation of coeval rocks in the states of Chihuahua, Coahuila, and Nuevo León. In the Sierra Los Chinos the Picachos Formation, which is separated into5 lithic packages, is mainly made up of limestone, limestone conglomerate and sandstone with rudists, gastropods, orbitolinids, miliolids and corals. The presence of rudists (Coalcomana sp) included in the limestone clasts of the conglomerate beds, as well as the orbilolids in the limestone beds are evidence that the Los Picachos Formation is middle to upper Albian in age. The textural characteristics of the limestone beds as well as its faunal content indicate a marine shallow-water neritic environment in a carbonate platform with local development of ooidal banks and bioherms. The limestone conglomerate beds near the base of this formation, which is a marine intraformational conglomerate, is evidence of a significant tectonic episode at the end of the Albian with reactivation of the shallow-water continental platform. Finally, the Sierra Los Chinos succession is paleogeographically and tectonically related to the Chihuahua tectonic belt and is unrelated directly to the Bisbee Group of Sonora.