GEOLOGIC MAP OF THE CHURCH MOUNTAIN 7.5' QUADRANGLE, NORTHERN SACRAMENTO MOUNTAINS, SOUTHEASTERN NEW MEXICO

The Church Mountain quadrangle, located at the north end of the ~38 to 26 Ma Sierra Blanca volcanic field, includes volcanic and volcaniclastic rocks of the Walker Formation (formerly Walker andesite breccia), Church Mountain phonolite (formerly Church Mountain latite), and Nogal Peak trachyte, which are intruded by syenites of the Rialto stock and alkaline mafic to mafic-intermediate plugs. Consistent with the stratigraphy developed on quadrangles to the south, the Walker Formation is composed of a laterally-continuous sequence of (from bottom to top) plagioclase-pyroxene phyric lavas, breccias, and debris flows; plagioclase phyric and aphyric lavas and associated conglomerates; the tuff of Argentina Spring; and plagioclase, plagioclase-pyroxene, and plagioclase-amphibole phyric lavas and associated conglomerates. In contrast, the younger volcanic units (Church Mountain phonolite, Nogal Peak trachyte, flow of Kountz Canyon, flow of Gaylord Peak) are geographically restricted, due at least in part to eruption onto a rugged paleotopography. The tuff of Argentina Spring on this quadrangle generally thickens northward and, along the north flank of and locally within the Church Mountain highlands, contains a medial conglomeratic interval, suggesting episodic eruption. At least five monzonitic to syenitic phases of the Rialto stock crop out in the southeastern corner of the quadrangle, with abundant angular xenoliths of older lavas incorporated into the margins of the stock. The alkaline mafic to intermediate intrusions crop out as relatively small, elongate bodies in the northern portion of the Church Mountain highlands. Alkaline mafic to felsic dikes are prevalent to the area, with a radiating pattern of some mafic-intermediate dikes suggesting an unexposed source intrusion below the pervasively altered rocks of Diamond Peak. Bedding attitudes are moderately (15-20°) eastward-dipping to subhorizontal, consistent with the location of the quadrangle on the western limb of the NNE-trending synclinal Sierra Blanca basin. NE- and NW-striking normal faults associated with the Rio Grande rift offset all volcanic strata. Late Tertiary and Quaternary alluvium derived from the exhumed Church Mountain highlands are also mapped, differentiated by age based on soil development and geomorphology.