Turonian microfossil assemblage of the Eagle Ford Formation from the La Bolita locality, Coahuila, Northwest Mexico

Late Cretaceous carbonate rocks of the Eagle Ford Formation are widely distributed in north-western Mexico (Nuevo Leon, Coahuila, Chihuahua) and southern USA (Texas). Around the La Bolita locality, Castanos town (Coahuila State), this unit consists of approximately 31 m of marine deposits, characterized by thin-bedded, slightly clayey limestones. Within the scope of a microfacial analysis, 20 hand samples were collected from selected limestone levels. Identified microfossil remains are mostly represented by planktonic foraminifers, among which: Planoheterohelix moremani, P. reussi, P. globulosa, Globigerinelloides bollii, Globigerinelloides spp., Muricohedbergella delrioensis, M. planispira, Muricohedbergella spp., Whiteinella archaeocretacea, W. aprica, W. baltica, W. brittonensis, W. inornata, W. paradubia, Whiteinella spp., Clavihedbergella moremani, C. simplex, Marginotruncana renzi, M. sigali, M. schneegansi, M. pseudolinneiana, Marginotruncana spp., Praeglobotruncana algeriana, and Dicarinella canaliculata. There are also abundant calcispheres, such as Risserella rablingae, Pithonella ovalis, and Calcisphaerula innominata; as well as roveacrinoid ossicles (genus Roveacrinus), and unidentified radiolarians. Based on the stratigraphic range of both the genus Marginotruncana and the species Praeglobotruncana algeriana, an early–middle Turonian age is assigned to the studied section. The pelagic microfossil assemblage and the lithological features all suggest deposition of the studied sediments in an open marine platform, with poorly oxygenated, nutrient-rich waters.