Facies, biostratigraphy, diagenesis, and depositional environments of Lower Cretaceous strata, Sierra San José section, Sonora (Mexico)

We used petrofacies analysis, carbon, oxygen and strontium isotope data to interpret the isotopic variations in the carbonate rocks of the Mural Formation of Sonora (Sierra San Jose section), Mexico. The petrographic study reveals a range of lithofacies from wackestone to packstone. The analyzed limestones show significant negative δ18O values (-18.6 to -10.9 VPDB) and δ13C values ranging from negative to positive (-2.6 to +2.5‰ VPDB). The absence of correlation between δ13C and δ18O values suggests a primary marine origin for the δ13C values of limestones from the Sierra San Jose section. The limestones have large variations in 87Sr/86Sr values (0.707479 to 0.708790). Higher 87Sr/86Sr ratios in various levels of the studied section suggest that most of the sediments were derived from the Proterozoic basement of the Caborca block during Early Cretaceous time. A decrease in 87Sr/86Sr ratios at certain levels indicates an influx of lesser amounts of radiogenic Sr that could have been caused by contribution of sediments from the Triassic and Jurassic volcanic rocks.