Depositional facies of Cambrian Series 2 – Miaolingian transition in the El Sahuaral Hill, central Sonora, Mexico: Paleoenvironments and regional stratigraphic correlation

Studies on lower Paleozoic sedimentary facies are scarce in northwestern Mexico, and the information is mainly restricted to Ordovician sequences in central Sonora. Series 2 – Miaolingian detrital-carbonate deposits of the El Sahuaral Hill located in central Sonora, northwestern Mexico, were analyzed for siliciclastic and carbonate microfacies, and biofacies based on the presence of trilobites. This research fills a gap in the knowledge of Cambrian strata in Mexico and is based on 25 thin sections, where the microfacies analysis resulted in recognition of one siliciclastic (M1), eight carbonate microfacies (M2-M9), and three biofacies (B1–B3). Based on the information obtained, the silicilastic microfacies (M1) for the Proveedora Formation indicates a shallow marine environment in intertidal to subtidal conditions related to the Skolithos ichnofacies. The carbonate microfacies M2, M3, M4, and M5 for the Buelna Formation denote a carbonate platform that varies between a lagoonal environment of open circulation and a restricted platform. Microfacies M6 and M7 for the Cerro Prieto Formation were deposited in carbonate sand shoals and banks within a moderate-energy environment in restricted platform, and open marine to platform-margin sand shoal within a high-energy environment. On the other hand, the faunal associations in the El Gavilán Formation allow to recognize three main biofacies: B1 is characterized by the presence of Euagnostus and abundant of agnostid trilobites; B2 is characterized by the presence of various genera of trilobites of the family Oryctocephalidae; and B3 represents trilobites of the genera Elrathina and Bathyuriscus. In accordance with the above, for the El Gavilán Formation, it is established that it was deposited on an external platform, transiting to a deeper environment towards the slope area. Microfacies M8 and M9 of the El Tren Formation indicate a transition from an open marine platform to a restricted circulation lagoonal environment, possibly at the outer margin of the carbonate platform, exposed to surface waters and mixed with meteoric water. The El Sahuaral Hill strata record environmental and eustatic changes, where abrupt facies change occurred from inner to outer platform during the Cambrian Series 2 – Miaolingian. The above correlate with other deposits in North America (Nevada, California, and British Columbia) which record relatively drastic eustatic changes at the platform edge. Furthermore, the lithological and faunal characteristics of the El Gavilán Formation in the El Sahuaral Hill suggest an affinity with the deposits of Sonora, Clayton Ridge in Nevada and the Burgess Shale Lentil, which is a member of the Stephen Formation in British Columbia, where the paleogeographic identity with the North American craton is confirmed within an outer platform environment.