New conodont data related to the western Ouachita-Marathon-Sonora orogen: Age of the autochthonous Laurentian deformation

We report herein new Permian conodont data from the strata of the La Cueva Limestone and the Mina Mexico Formation in central Sonora. These data allow us to characterize the biostratigraphy of the southwestern Laurentian Permian successions and the last stage of the Ouachita-Marathon-Sonora orogen in northwestern Mexico. We studied three sections in central Sonora, which cover the stratigraphic contact between both formations. In the Las Rastras Section (LRS), we recognized a conodont association that includes Neostreptognathodus exsculptus sensu Henderson and McGugan, Sweetognathus “whitei”, Sw. subsymmetricus, and Hindeodus aff. H. minutus at its base and Sw. subsymmetricus, Hindeodus aff. H. gulloides, Gullodus tieqiaoensis, and Mesogondolella siciliensis at its top. These conodonts date the upper part of the La Cueva Limestone as latest early to earliest late Kungurian and the lower part of the Mina Mexico Formation as late Kungurian in the LRS. In the Sierra Martinez 1 and 2 sections (SMS-1 and SMS-2), located 7 km to the east of the LRS, the conodont association points to an age corresponding to the latest early to earliest middle Kungurian for both La Cueva Limestone and Mina Mexico Formation. This association is represented by the species Ns. exsculptus sensu Henderson and McGugan and Sw. subsymmetricus. The stratigraphic contact between both formations has been interpreted as the beginning of the autochthonous deformation of the Laurentian platform. This new conodont data allows us to associate this event to the latest early–earliest middle Kungurian in SMS-1 and SMS-2, and to the late Kungurian in the LRS showing a diachrony between these localities. This diachrony may be related to the continental oblique collision of Laurentia against Gondwana that resulted in progressively younger basins westward along the Ouachita-Marathon-Sonora orogen. This diachrony can also be due to the configuration of the subsiding Mina Mexico foredeep, which would have produced a non-coeval contact in the areas closer to the thrust front and those located far from it.