Upper Cretaceous stratigraphy and biostratigraphy of south-central New Mexico
— Stephen C. Hook, Greg H. Mack, and William A. Cobban

Upper Cretaceous rocks exposed at isolated outcrops in south-central New Mexico contain a marine molluscan record that spans at least 30 biostratigraphic zones from the middle Cenomanian to the middle Coniacian. Formational names applied to these rocks are, in ascending order: Dakota Sandstone (main body and one tongue), Mancos Shale (four tongues), Tres Hermanos Formation, Gallup Sandstone, and Crevasse Canyon Formation. Five small-scale graphic sections ranging from Doña Ana County on the south to Socorro County on the north and Lincoln County on the east are presented with generalized lithologies and depositional environments. Key faunal collections from each section are tied to their stratigraphic positions in order to facilitate correlation from one area to another.

The index fossils for 20 of the 30 biostratigraphic zones used in south-central New Mexico have been recovered from the five representative sections although there is no one section that contains all 20. The remaining 10 zones for which there are no fossils recorded in the study area are probably present as well; most of these zones are known from the Upper Cretaceous to the north and west of the study area. If these 10 zones are present, they are represented by a rock record that is barren of megafossils in the study area.

The detailed biostratigraphy presented in this study documents the time-transgressive nature of the rock-stratigraphic units, which was a response to three cycles of shoreline movement across this part of New Mexico. The two earliest cycles of transgression and regression (T-1, R-1 and T-2, R-2 of Molenaar, 1983) are present throughout the study area. However, only Socorro County has a complete third cycle (T-3, R-3).