Biostratigraphy of fusulinids from the lower Permian of central Sonora, Mexico. Paleoenvironmental implications

The central region of Sonora state, Mexico, is recognized by its different upper Paleozoic outcrops, highlighting the La Cueva Limestone and Mina México Formation. The rocks of Cerro Las Rastras and Sierra Martínez are considered among the most important Permian localities in central Sonora. This work studies the fusulinid faunas in two different sections, focusing on their biostratigraphical and paleoenvironmental relevance. Both sections comprise strata from the La Cueva Limestone and Mina México Formation. The fusulinids identified are Parafusulina sp., Paraskinnerella cf. skinneri, Praeskinnerella crassitectoria, Skinnerella cobachiensis, Skinnerella imlayi, and Eoparafusulina cf. linearis. The presence of Pa. cf. skinneri, Pr. crassitectoria, S. cobachiensis and S. imlayi indicates a lower–middle Leonardian (=upper Artinskian–lower Kungurian) age for the Las Rastras Section. The occurrence of E. cf. linearis in the base of the Sierra Martínez 2 Section suggests a youngest age of upper Wolfcampian for the bearing rocks. The microfacies analysis allowed the identification of six facies associations in the Las Rastras Section related to deep shelf, slope, and ramp-margin sand shoals. While three associations were identified in the Sierra Martínez 2 Section, indicating the deposition of open marine and slope environments. Fusulinids from both sections are classified as allochthonous elements in most facies associations (except those of open waters) since they are found in environments that are known to not been inhabited by these foraminifera. The fusulinids identified have a paleobiogeographic affinity with coeval assemblages from the North American Province.