Nonmarine Stratigraphy of Latest Cretaceous and Early Tertiary Age, Southwestern New Mexico: ABSTRACT

Abstract

Recent fossil collections from nonmarine strata at localities in southwestern New Mexico indicate that the Ringbone formation, as originally defined, comprises units separated by a major hiatus that is represented in the field by an angular unconformity. The lower unit has yielded (NMMNH locality 298) two anterior caudal vertebral centra, morphologically and metrically indistinguishable from those of the Late Cretaceous tyrannosaurids Albertosaurus and Daspletosaurus. These dinosaur fossils establish a late Campanian or Maastrichtian age for the unit, indicating its equivalence with the McRae Formation of south-central New Mexico and the Fort Crittenden Formation of southeastern Arizona. The unit is composed of approximately 1,000 m of sedimentary-clast conglomerate, arkose, volcanic litharenite, and gray shale; it thus appears to contain detritus from several different source areas. The younger unit has yielded a low-diversity ostracod fauna of Paleocene to early Eocene age. Diagnostic taxa from the assemblage include Pseudoeocypris pagei and Cypridea arvadensis. The age of this fauna suggests equivalence with the Love Ranch and Lobo Formations of southern New Mexico. The unit consists of approximately 350 m of interbedded red siltstone and boulder conglomerate derived from Lower Cretaceous strata overlain by a sequence of laminated shale and subordinate sandstone with a preservedmore » thickness of 150 m.« less