New Mexico Geological Society Annual Spring Meeting — Abstracts

The Paleocene Nacimiento Formation in the southern San Juan Basin can be divided into three members best exposed between Nageezi and Cuba. The lowest member is about 175 m thick and is dominated by laterally extensive, black and gray claystones. Extensive channel sandstones also are common in this member. The middle member is about 65 m thick and displays much more variegated colors than the lower member. Thus, the middle member is mostly greenish siltstones that contain brown, ferruginous concretions, channel sandstones and minor amounts of black and red-mottled claystones. The upper member is about 85 m thick and is composed primarily of sandstone and black claystone but contains a high percentage of thin (< 1 m thick), laterally extensive, highly resistant silcretes. The upper member of the Nacimiento Formation thins eastward as it is progressively removed at the unconformity at the base of the overlying Cuba Mesa Member of the San Jose Formation.

In Kimbeto and Betonnie Tsosie Washes, the lower member of the Nacimiento Formation contains the Puercan (earliest Paleocene) Ectoconus zone near its base overlain by about 55 m of strata devoid of fossil vertebrates followed by a sparsely fossiliferous interval which produces Torrejonian mammals. The middle member contains the Deltatherium and Pantolambda zones. The upper member of the Nacimiento Formation lacks fossil vertebrates.