Stratigraphy, paleontology, depositional framework, and nomenclature of marine Upper Cretaceous rocks, Socorro County, New Mexico
— Stephen C. Hook

During Late Cretaceous time, New Mexico was covered by part of the epicontinental seaway that extended from the Arctic Ocean to the Gulf of Mexico and was as much as 1,600 km wide. The western shoreline of this seaway advanced and retreated across New Mexico many times and left a complex record of intertongued marine and nonmarine sediments. These clastic sediments once covered most of New Mexico; subsequent uplift led to erosion of much of the Upper Cretaceous deposits. The rock record that remains is sufficient to document five major cycles of transgression and regression of the western shoreline across New Mexico (Molenaar, 1983a). The Upper Cretaceous rocks preserved in Socorro County record only the two earliest of these five cycles of transgression and regression.

The earlier of these cycles, which began in middle Cenomanian time and lasted until middle Turonian, is called the Greenhorn Cycle (Hattin, 1964; Kauffman, 1969). The latter cycle, which lasted from middle Turonian until early Coniacian time in New Mexico, is herein called the Carlile Cycle (see Hook and Cobban, 1979). The Carlile Cycle is equivalent to the early part of the Niobrara Cycle of Kauffman (1967, 1969). The regressive phase of the Carlile Cycle—the Gallup regression—is unique to New Mexico and northeasternmost Arizona (Molenaar, 1983a). Figure 1 shows the relationship of these cycles to the biostratigraphic/radiometric age framework for the middle Cenomanian through Coniacian stages. Absolute dates with an asterisk are from Fouch and others (1983); those without asterisks are estimates at the beginning of each half cycle of deposition.

Figure 2 shows the approximate positions of the western shoreline at maximum transgression and maximum regression for both cycles. The transgressive and regressive portions of each cycle are named for prominent rock units in New Mexico representative of that subcycle. Two of these units—the Tres Hermanos Formation and the D-Cross Tongue of the Mancos Shale have their type sections in Socorro County.

The major rock units in Socorro County associated with the two cycles consist of the following formations: Dakota Sandstone, Mancos Shale, Tres Hermanos Formation, Gallup Sandstone, and Crevasse Can- yon Formation. Each formation will be discussed in the sections that follow.