Geology of Good Sight Mountain and Uvas Valley, southwest New Mexico

The 435-sq-mi area described by this report includes the Good Sight Mountains and the Uvas Valley in Luna, Dona Ana, and Sierra Counties, New Mexico; the geologic map covers the western part of the Good Sight-Cedar Hills volcano-tectonic depression of Oligocene age. The Sierra de las Uvas on the east and the Good Sight Mountains on the west border the north-plunging Uvas Valley syncline, near the western side of the depression. Rock units exposed in the area range fr om Eocene to Holocene. The Good Sight Mountains are composed chiefly of Rubio Peak Formation rocks, which include latite and andesite tuffs, breccias, flows, dikes, plugs, and stocks with minor interbedded conglomerate and volcaniclastic beds. The source o f these rocks was the Good Sight Mountains vent zone, which trends north for about 30 mi and is 4 mi wide. The Rubio Peak is correlative, in part, with the Palm Park Formation to the east, but may include some younger units. The Rubio Peak is unconformably overlain by tuff 6 and the upper sedimentary member of the Bell Top Formation on the eastern slopes of the Good Sight Mountains. Tuff 4, tuff 5, and the middle sedimentary member pinch out under the eastern part of the Uvas Valley. North of Nutt Mountain, Rubio Peak is overlain by Kneeling Nun Tuff or Tenaga Canyon (new name) andesites and latites, which intrude and overlie the Kneeling Nun. Uvas Basaltic Andesite flows are interbedded and overlie the upper sedimentary member of the Bell Top Formation on the west flank of the Sierra de las Uvas, under the Uvas Valley, and on the eastern flank of the Good Sight Mountains. Tenaga Canyon rocks are intruded and overlain locally by Uvas Basaltic Andesite. Nutt Mountain is a flow-banded rhyolite plug that intruded rocks as young as Tenaga Canyon formation. Quaternary clastic deposits, locally more than 300 ft thick, fill the Uvas Valley and extreme western part of the map area (eastern edge of the Mimbres Basin). Numerous water wells have penetrated these deposits and many of these wells bottom in the underlying Uvas and upper Bell Top Formation strata. Outcrops of the lower aquifers in the Sierra de las Uvas probably provide some recharge to the NuttHockett Underground Water Basin.