STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENTS OF THE LOWER PART OF THE RINGBONE FORMATION, LITTLE HATCHET MOUNTAINS, SOUTHWESTERN NEW MEXICO

D. A. Wilson

Department of Earth Science, New University, Las Cruces, NM, 88003

Approximately 1800 meters of the Ringbone Formation is exposed in the Little Hatchet Mountains in southwest New Mexico. The Ringbone Formation is composed predominantly of Cretaceous terrestriaI sediments. Mack and Seager (1986) inferred the Ringbone to be Late Cretaceous based on its position between the overlying Tertiary volcanics and the underlying Mojado Formation of Early Cretaceous age. However, angular unconformities separate the Ringbone from both the Tertiary volcanics and the Mojado Formation. Recently, two anterior caudal vertebral centra of a theropod dinosaur were recovered in the Ringbone Formation. These centra are believed to be from Late Cretaceous tyrannosaurids (Albertosaurus or Daspletosaurus), thus indicating a late Campanian or Maastrichtian age. The dinosaur fossils indicate a time equivalence with the deposits of the Fort Crittenden Formation of southeast Arizona (Lawton et al., in press).

The lower part of the Ringbone Formation consists of, in ascending stratigraphic position, 1) a conglomeratic unit, 2) arkosic sandstone unit, 3) shale with alternating arkosic sandstone unit, 4) gray shale with freshwater fossils. Beds within the conglomeratic unit consists predominantly of lower Cretaceous clasts which include limestone clasts from the U-Bar Formation, and quartz arenite clasts from the Hell-to-Finish Formation. However, conglomeratic beds in the middle of the unit contain significant amounts of volcanic clasts and Paleozoic limestone clasts. The upper conglomeratic beds contain Precambrian granitic clasts. The conglomeratic unit thus represents an unroofing sequence of fluvial channel-fill deposits. The arkosic sandstone unit consists of upward fining beds, approximately one meter thick, with thin interbedded shales, crossbeds, plant fragments, rip-up clasts, and scour-and-fill structures. This unit represents a fluvial system. Taken together, units one and two show dramatic thickness changes from section to section and are interpreted to represent infilling of a paleovalley system. The sandstones of unit three occur in upward coarsening packages. The shale and sandstone beds contain plant fregments, freshwater fossils, syndepositional tuff beds, hummocky cross stratification, a large box fold, and other soft sediment deformation stuctures. The box fold is believed to represent unstable sediments at the toe of a prograding delta. A clean quartz arenite is present at the top of unit three, and is interpreted to represent a beach environment. The gray shale unit contains abundant freshwater molluscs and ostracods, plant fragments, and syndepositional tuff beds. This unit is interpreted to represent a lacustine environment.

Sediments of the Ringbone Formation rapid subsidence of the basin and basement involved uplift during Late Cretaceous time. Therefore, Ringbone Formation is a key unit for more precise interpretation and reconstruction of Laramide paleogeography and deformation in southwestern New Mexico.

Keywords: stratigraphy

pp. 16 1989 New Mexico Geological Society Annual Spring Meeting April 7, 1989, Macey Center