14-8 Clast-derived late Artinskian conodonts and fusulinids from the Las Delicias Formation, Coahuila Terrane, Mexico: A trans-Marathon faunal connection

Abstract:

Mississippian to Guadalupian (Middle Permian) strata of the Las Delicias Formation, southwestern Coahuila, Mexico, record heterogeneous and partly chaotic forearc-to-intra-arc deep water gravity flow sedimentation and coeval arc volcanism. Deposition was associated with the Marathon salient of the Appalachian-Ouachita-Marathon orogenic belt and occurred adjacent to the southern margin of the Marathon accretionary complex. The most continuous exposures of the Las Delicias Formation crop out approximately 100 km northeast of Torreón, Mexico, in the area between Acatita Valley (west) and Las Delicias Valley (east) where the upper Paleozoic units are unconformably overlain by Lower Cretaceous strata. Here, the Cisuralian (Lower Permian) Las Sardinas beds are exposed in the western part of the outcrop belt and consist of a lower conglomeratic unit and an upper shale and minor graywacke unit (the Perrinites shale of King et al., 1944). For this study, cobble-sized limestone clasts from the lower Las Sardinas beds were individually processed for both conodont elements and fusulinids. Based on the presence of the Kungurian-Roadian ammonoid Perrinites hilli in the overlying shale unit, the upper Artinskian limestone from which the studied clasts were sourced was evidently reworked in short order from shelf deposits and transported downslope. The conglomerate clast-derived assemblages comprise several distinct late Artinskian taxa, including the following fusulinid and conodont occurrences/co-occurrences: 1) Schwagerina hessensis; 2) Paraskinnerella skinneri, "Sweetognathus" ex gr. clarki; 3) Skinnerella diabloensis; 4) Skinnerella cylindrica, Neostreptognathodus ruzhencevi, Neostreptognathodus exsculptus sensu Henderson and McGugan, 1996. Several of the recovered taxa have also been reported from the Midland-Delaware basin complex where they occur in exposures of shelf units in the Glass Mountains, Baylor Mountains, and Sierra Diablo, and equivalent slope deposits of the subsurface “Wolfcamp shale.” The occurrence of “Sweetognathus” ex gr. clarki in cobbles from the Las Sardinas beds is particularly noteworthy, as this morphotype was previously known only from the subsurface Wolfcamp B-Wolfcamp A transition in the Midland Basin. Consequently, the phylogenetic position of “Sweetognathus” ex gr. clarki within the Artinskian-Kungurian sweetognathid lineages is still poorly understood, but the trans-Marathon occurrence reported herein provides new information on the paleogeographic distribution of this conodont taxon.

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