New Mexico Geological Society Annual Spring Meeting — Abstracts

The early Permian DeChelly Sandstone represents an erg that covered much of the Four Corners region of the American Southwest. Its southeastern edges are in New Mexico, and new stratigraphic data from near Placitas in Sandoval County document its rapid, local pinchout and transition into arid coastal plain deposits of the Arroyo de Alamillo Formation. In New Mexico, the lower lithosome of the Yeso Group was long termed the Meseta Blanca Member of the Yeso Formation, but encompasses two distinct lithofacies—to the north, eolian sandstones (dune and interdune deposits) properly termed DeChelly Sandstone (the type Meseta Blanca Member section in the Jemez Mountains is a section of the DeChelly) and the arid coastal plain deposits of the Arroyo de Alamillo Formation to the south. Prominent outcrops of the DeChelly Sandstone are mostly crossbedded sandstone of eolian origin at: (1) Jemez Mountains (T16N, R2E, Sandoval County), 82 m thick; (2) Zuni Mountains (T9N, R11W, Cibola County), 77 m thick; and (3) Lucero uplift (T7N, R3W, Valencia County), 70 m thick. These are the southernmost outcrops of the DeChelly Sandstone. Lower Yeso strata to the south/southeast are of the Arroyo de Alamillo Formation, a mixture of siltstone and sandstone, the latter often gypsiferous with some thin, local eolian sandstones and minor beds of dolomite and gypsum. These outcrops are at: (1) Mesa del Yeso, type section of the Arroyo de Alamillo Formation (T2S, R2E, Socorro County), 107 m thick; (2) Abo Pass (T3N, R6E, Torrance County), 60 m thick; (3) Fra Cristobal Mountains (T12S, R2W, Sierra County), 81 m thick; and (4) Caballo Mountains (T17S, R3W, Sierra County, 74 m thick.

On the northern end of the Sandia uplift north of Placitas (T13N, R5E, Sandoval County), two stratigraphic sections document over a distance of ~ 1 km the southward gradation and pinchout of the DeChelly Sandstone into the Arroyo de Alamillo Formation. To the north, the lower Yeso lithosome is 59 m thick and almost totally eolian sandstone, and thus assigned to the DeChelly Sandstone. But, about 1 km to the south, the lower Yeso lithosome is 24 m thick and about 50% siltstone and 50% sandstone, the latter often gypsiferous and including some eolian sandstones. To the south of the Sandia uplift, at Tijeras Canyon (T10N, R5E, Bernalillo County), the lower Yeso is strata of the Arroyo de Alamillo Formation. To the east/northeast, on Glorieta Mesa at Rowe (T15N, R12E, San Miguel County), the lower Yeso is a mixture of sandstone of fluvial origin and siltstone with minor beds of conglomerate and dolomite. These rocks differ from strata of the De Chelly or Arroyo de Alamillo formations and may merit a new formation name. Earlier workers, who called the entire lower Yeso lithosome from the Jemez Mountains to the Caballo Mountains the Meseta Blanca Member obscured a substantial facies change from the DeChelly eolian (dune and interdune) deposits to the north that grade southward into arid coastal plain deposits of the Arroyo de Alamillo Formation to the south.