Named as basal member of Spears Formation of Datil Group. Type area located about 20 mi northeast of Datil in Dog Springs, Chavez and Old Canyons, S1/2 T2N, R8W and north edge T1N, R8W, western Gallinas Mountains, Socorro Co, NM, Orogrande basin. Known in Datil Mountains too. Previously called andesite breccia and conglomerate unit of Spears. Is a thick (0-3,000 ft) accumulation of volcaniclastic deposits, mudflow deposits, especially tan to brown, crystal-rich, nonpumiceous quartz latite breccias. Contains large exotic blocks of limestone and autobrecciated volcanic rocks as much as 0.5 mi long and 250 ft thick. A few large bodies of autobrecciated andesitic to latitic rocks appear to be intrusives. Minor local accumulations of well stratified tuffaceous pond deposits. Bedding in breccias is contorted and chaotic; bedding in overlying and underlying units is gently dipping and consistent. Contorted bedding is probably penecontemporaneous. Overlies Baca Formation; underlies Chavez Canyon Member (new) of Spears. Yielded 39.6 +/-1.5 m.y. K-Ar date on biotite in lower part and a 38.6 +/-1.5 m.y. Z-FT date on an andesite clast. Assigned to Oligocene.

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