Sedimentation and tectonism in the Laramide tornillo basin of West Texas

The Tornillo Basin of west Texas and adjacent Mexico is the southernmost Laramide-age intermontane basin in the North American cordillera. The basin is bounded on the west by the Chihuahua Tectonic Belt, a southern extension of the Cordilleran Fold and Thrust Belt, and on the east by the Marathon Uplift, a Laramide-style basement-cored uplift. Sedimentological evidence indicates that tectonism began in this region during Late Cretaceous time (ca. 70 Ma) and ended during middle Eocene time (ca. 50 Ma). The basin experienced two major pulses of fluvial sedimentation, the first during middle to late Maastrichtian time, and the second during late Paleocene to early Eocene time. High sedimentation rates during these periods correspond to the influx of coarse extra-basinal detritus, high mudstone content in the basin alluvium, and thick tabular fluvial sand bodies. These episodes probably reflect pulses of tectonism. A lull in sedimentation during early Paleocene time resulted in a thin sequence of alluvium with very high mudstone content, thin lenticular fluvial sand bodies, and well-developed paleosols. The basin experienced left-lateral transpressive deformation during the second phase of tectonism. The sedimentary sequence in the Tornillo Basin is compatible with the two-phase model of Laramide tectonism developed for the central and southern Rocky Mountains.