Named after old Sevilleta land grant in Socorro Co, NM, Southwestern Basin-and-Range region, within bounds of which rhyolite occurs. No type locality designated. Forms 5-mi-long belt, from 0.5 to 1.5 mi wide, south and southeast from Hwy 60 across Los Pinos Mountains. Base consists of 500 ft of more or less compact, sericitic rhyolite which varies from light buff to reddish brown. Groundmass is extremely fine-grained with a silky luster. Contains phenocrysts of pink orthoclase ranging from 1-15 mm in length. Basal part grades upward into progressively less-metamorphosed, dark-red to black rhyolite with phenocrysts of orthoclase and quartz. Thickness is 4,500 ft. Disconformably overlies pre-Cambrian White Ridge quartzite (new). Intruded by pre-Cambrian Los Pinos granite (new). Geologic map. Measured cross sections. Cross sections. Pre-Cambrian age [presumably based on regional geology and stratigraphic position].

Source: GNU records (USGS DDS-6; Denver GNULEX).

Pg. 25, geologic time scale (inside front cover). Sevilleta Rhyolite of Stark and Dapples (1946). Rhyolite sample from Los Pinos Mountains, [approx.] Lat. 34 deg. 28 min. N., Long. 106 deg. 30 min. W., Becker quadrangle, Valencia County, New Mexico, yielded a Pb-alpha age of 1,500 +/-250 Ma (zircon) [Precambrian X to Y]; reconnaissance age. Age calculated using decay constants of Steiger and Jager, 1977 (Earth Planet. Sci. Letters, v. 36, p. 359-362).

Source: Publication.