Stratigraphy of the northwestern Sierra Blanca volcanic field
— Shari Kelley, Daniel J. Koning, Fraser Goff, Colin Cikoski, Lisa Peters, and William McIntosh

A thick succession of 28–38 Ma alkalic lava flows, volcaniclastic sedimentary deposits, and minor welded trachytic ash-flow tuffs is preserved in the northwestern Sierra Blanca volcanic field (SBVF) on the western flank of the Sierra Blanca massif, on Barber Ridge, and in the Godfrey Hills. Here, we present a stratigraphic framework for this succession based on geologic mapping, geochronology, and geochemistry. Three sequences of volcanic rocks are preserved on the Sierra Blanca massif. The 38 to 36 Ma lower sequence is composed of three units that are relatively continuous (listed from bottom to top): interbedded debris flows and pyroxene-phyric basalt to basaltic trachyandesite flow breccias and lava flows (Hog Pen Formation), plagioclase-phyric basaltic trachyandesite flows (Rattlesnake Member of the Three Rivers Formation), and fine-grained trachybasalt to trachyte flows interbedded with maroon volcaniclastic sediment (Taylor Windmill Member of the Three Rivers Formation). The northward-thickening, 36 Ma rheomorphic Argentina Spring Tuff caps the lowest sequence. The middle sequence consists of laterally discontinuous volcaniclastic debris flows, basaltic trachyandesite, phonotephrite, and trachyte of variable thickness (Double Diamond Member of the Three Rivers Formation, 36–34 Ma). The highest sequence on the Sierra Blanca massif is composed of thick, biotite- and potassium feldspar-bearing, 33–31 Ma flows filling paleovalleys cut into the older lavas. Volcanic rocks on Barber Ridge between Sierra Blanca and the Godfrey Hills are composed exclusively of the 38–37 Ma debris flows and pyroxene-phyric flow-breccias and lavas of the Hog Pen Formation. The Godfrey Hills, on the hanging wall of the north-striking, west-dipping Godfrey Hills normal fault, preserve the younger part of the volcanic succession of the SBVF. The oldest units in the Godfrey Hills are distal equivalents of the Hog Pen Formation, and the Rattlesnake Canyon and Taylor Windmill members of the Three Rivers Formation on Sierra Blanca. A thin lithic-rich tuff with biotite and brown porphyritic clots, the Buck Pasture Tuff, lies above the Taylor Windmill Member; this tuff may correlate with a similar tuff above the Argentina Springs Tuff on Sierra Blanca. The upper succession in the Godfrey Hills is divided into two units, the older (34 to 28.7 Ma) Lopez Spring Formation with lavas of variable composition and the younger Husk Windmill (28.7–28.2 Ma) Formation dominated by trachytic lava and tuff. The northwestern SBVF primarily formed during two widespread pulses of volcanic activity at 38–34 Ma and 29.4 -28.2 Ma and a more localized episode 33–31 Ma. The youngest volcanic rocks dated so far in the northwestern SBVF were erupted at ca. 28.2 Ma; the youngest sill is ~27 Ma.