Stratigraphy and paleontology of Upper Cretaceous-Paleogene strata in northern Coahuila, Mexico

Upper Cretaceous strata in the Big Bend region of Texas have been studied for over a century, and have yielded the southernmost sequence of fossil vertebrate faunas of Campanian through Maastrichtian age in North America. Exposures of these strata extend southward from Texas into northern Coahuila, Mexico, and have also yielded several important vertebrate fossils there in recent years, but because this area is so remote and inaccessible, it has not been possible to determine how strata there may relate to the well-known sites in Texas. The present study provides a basic stratigraphic framework for Upper Cretaceous strata in the Ocampo region of northern Coahuila, and places the occurrence of vertebrate fossils found in this area within that framework. Two broad exposures in the Ocampo area are included in the present study, an area west of the southern extension of Mariscal Mountain (Sierra la Harina in Mexico), and an area east of Sierra San Vicente. The Aguja Formation in the Ocampo region is subdivided into six informal stratigraphic units. An initial progradational coastal sequence is marked by the paralic Del Mar sandstone member and overlying Pantano shale member. A second series of paralic deposits in the San Miguel sandstone member records a brief marine transgression that occurred during deposition of the middle part of the Aguja Formation. Fluvial floodplain deposits in the overlying Homero shale member and in the Javelina Formation were deposited during a final regressive phase. The prevailing paleoflow direction in the Ocampo area throughout deposition of the Aguja was to the southeast. Only the upper part of the Aguja Formation in Big Bend records a similar trend, influenced by the regional onset of Laramide tectonism during Late Campanian time. This suggests that the basal paralic sandstone of the Aguja Formation in the San Miguel region (Del Mar sandstone member) is correlative with the Terlingua Creek sandstone in Big Bend. If this correlation is correct, the Aguja Formation at San Miguel is correlative with only the upper part of the formation in Big Bend, but is nearly twice as thick. This may reflect much higher sedimentation rates in the Ocampo region, or erosional truncation of the upper Aguja prior to deposition of the Javelina Formation across much of the Big Bend region. Vertebrate fossils are common in both the Pantano shale member and the Homero shale member of the Aguja Formation in the Ocampo region. Although minimal stratigraphic data have been presented in previous studies, it seems likely that the significant vertebrate fossils recovered in this region were obtained in one or the other of these two units. Most of the identifiable vertebrate fossils observed during the course of the present study, and those housed in the Hernandez-Juarez collection at the township of San Miguel, are from the Homero shale member. Isolated limb bones and vertebrae of hadrosaurian dinosaurs are very common in this interval, and as in the correlative strata in Big Bend, hadrosaurs comprise the most abundant large vertebrate in the Aguja fauna. The occurrence of troodontid and centrosaurine dinosaurs in the Ocampo fauna, which are absent in the Big Bend fauna, may indicate that these strata record an interval of the Aguja Formation that is either lacking or poorly exposed in the Big Bend region.