Stratigraphy, paleontology and correlation of lower Cretaceous exposures in southeastern New Mexico

Abstract

— In southeastern New Mexico, limited isolated outcrops of Lower Cretaceous strata occur at Lee Ranch, the Black River valley, North Lake and near several of the peaks of the Cornudas Mountains. Preliminary study of these strata has resulted in a better understanding of their stratigraphic and paleontological relationships with better known Lower Cretaceous units to the south (west Texas), west (Cerro de Cristo Rey) and north (east-central New Mexico). Most of these outliers are less than 20 m thick, but the section on the east slope of Washburn Mountain includes more than 130 m of Fredericksburg- and Washita-age (Albian) strata. Both the Campagrande Formation and a relatively thick (41 m) sequence of Cox Sandstone—formations prominent in west Texas—are recognized in the Cornudas Mountains. Overlying middle-upper Washita strata consist of a massive nodular limestone unit and a slope-forming shale/siltstone/calcarenite unit above it. These appear to be eastern outcrops of the Muleros and Mesilla Valley formations, respectively, of Cerro de Cristo Rey. The Muleros-equivalent limestone contains an abundant and diverse fauna, consisting mainly of bivalves (especially a small form of Texigryphaea having affinities to both T. pitcheri and T. washitaensis), gastropods and echinoids. The overlying (Mesilla Valley) shale at the Lee Ranch locality contains the foraminifer Cribratina texana and a small, coiled oyster transitional between Exogyra plexa and I Imatogyra arietina. These faunas, present at Lee Ranch, Cornudas Mountains and Black River valley, are quite similar to faunas of correlative units in west Texas and at Cerro de Cristo Rey and represent the Albian Caribbean faunal province. The North Lake exposure, in contrast, is lithologically similar to the Tucumcari Formation of east-central New Mexico and bears a low-diversity fauna characterized by large numbers of Texigryphaea pitcheri, sparse Ceratostreon texana and Lopha quadriplicata and no echinoids. Strata connecting the Albian outcrops along the New Mexico–Texas border and the North Lake exposure (northern Lea County) are not present on the surface or in the subsurface, thus precluding study of the transition between these two faunas. Tertiary-Qua- ternary sediments without on-site field inspection, additional localities probably will be discovered in the future. Much work remains to be done on the paleontology of Lower Cretaceous marine units in southern New Mexico and west Texas. Systematic studies of relatively few elements of the west Texas faunas have been published and the paleontology of the well exposed, fossiliferous, lithologically heterogeneous sequence at Cerro de Cristo Rey was last comprehensively treated in 1910, by Bose. A more detailed study of the marine faunas of the southeastern New Mexico Lower Cretaceous localities is in preparation; here, only a few of the most common and biostratigraphically important taxa are noted and illustrated. All specimens are catalogued into the paleontology collections of the Department of Earth and Planetary Sciences, University of New Mexico (UNM).