The Las Delicias basin in northeastern Mexico is defined by a thick sequence of mid(?)-Pennsylvanian through Permian marine strata, which forms most of the basement rocks of the Coahuila island, a terrane that stood above sea level during the Late Jurassic and Early Cretaceous. Most of the strata accumulated as mass-gravity deposits derived from an active volcanic arc. Principal components of these deposits are (1) andesitic and dacitic debris, (2) pelagic sediment that underwent postdepositional movement, and (3) limestone debris from the basin margin. Facies patterns and macroscopic soft-sediment structural relations indicate a source to the south; volcanic rocks at Cañon Rosillo may be part of the arc. The Las Delicias area is probably not part of the Ouachita-Marathon system; stratigraphic relations and structural setting suggest that the Coahuila island may be an allochthonous stratigraphic terrane.