New Mexico Geological Society Annual Spring Meeting — Abstracts

Dinosaurian remains recovered from the Ringbone Formation of southwestern New Mexico indicate a late Campanian to Maastrichtian age for the unit. Previously, the Ringbone was broadly constrained as Late Cretaceous age by a plant fossil and radiometric ages of the Hidalgo volcanics which unconformably overlie the formation. Two anterior caudal vertebral centra found in the lower part of the formation (NMMNH locality 298) are morphologically and metrically indistinguishable from those of the Late Cretaceous tyrannosaurids Albertosaurus and Daspletosaurus. The Ringbone Formation as defined by Zeller is thus equivalent to the Fort Crittenden Formation of southwestern Arizona and the McRae Formation of south-central New mexico.

The Ringbone Formation forms, an upward-coarsening sequence of nonmarine clastic strata locally as much as 2000 m thick. In ascending order the unit is composed of six lithologic units: 1) a lower sedimentary-clast conglomerate and Iitharenite to arkose facies unconformable upon various Lower Cretaceous formations; 2) olive-gray shale: 3) crossbedded litharenite and gray shale 4) and 5) sedimentary-clast conglomerate with gray and red siltstone, respectively; 6) gray shale interbedded with andesite flows and breccias. The relationship the uppermost unit to the others has not yet been firmly established.

Although the conglomerates of the Ringbone are dominated by clasts derived from Lower cretaceous units, granitic cobbles and arkosic sandstone compositions indicate that the formation records rapid unroofing of granitic basement.The basin appears to have been one of several contemorary, yet geographically distinct, depocenters characterized by rapid local subsidence. These depocenters were subsequently dismembered by high-angle reverse faults and low-angle thrust faults. We regard this latest Cretaceous episode of basin formation and uplift as the result of a dominantly strike-slip deformational event prior to well-documented Eocene ("Laramide") deformation in southern New Mexico.