Page 18 - Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico

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The Kneeling Nun Tuff forms conspicuous escarpments in the map area and throughout the southeastern Black Range. It unconformably overlies the Sugarlump Tuff. At most localities, two or more cooling units are present as welded tuff with well- developed columnar joints underlain by poorly welded, slope- forming tuff. The Kneeling Nun was erupted from the Emory cauldron, located immediately west and northwest of the map area, and its eruption was accompanied by subsidence of the cauldron (Elston and others, 1975). The age of the Kneeling Nun is 34.89 ± 0.05 Ma by the 40Ar/39Ar method (McIntosh and others, 1991). The tuff is as much as 230 m thick from Berrenda Mountain to McClede Mountain and about 180 m thick on Sibley Mountain.
The Kneeling Nun Tuff consists entirely of rhyolitic ash- flow deposits. It is crystal rich but poor in lithic fragments and ranges in color from pale red and pinkish gray to medium light gray. Welded zones display well-developed eutaxitic texture. Typically, the tuff contains 35–40 percent broken crystals as large as 4 mm across, dispersed in a devitrified groundmass of ash. Crystals consist of about 18 percent oligoclase (An20–28), 13 percent sanidine, 5–6 percent quartz, and 2–3 percent biotite. Accessory minerals include sphene, magnetite, oxyhornblende, clinopyroxene, apatite, zircon, and secondary calcite. Chemical analysis of the tuff at Berrenda Mountain shows it to be rhyolite (table 1, sample 3; fig. 3A).
Landslide Megabreccia
Megabreccia, as much as 120 m in thickness, forms the lower slopes west and north of McClede Mountain (fig. 5). The megabreccia probably formed as a paleo-landslide along the topographic wall of the Emory cauldron (Elston, 1989). Large blocks of Kneeling Nun Tuff, some measuring hundreds of feet across, are intermingled with smaller and fewer blocks of Sugar- lump Tuff and Rubio Peak Formation. Many of the largest blocks of Kneeling Nun are shattered and brecciated.
The megabreccia overlies Sugarlump Tuff and Rubio Peak Formation and is in turn overlain by tuff of the Mimbres Peak Formation on McClede Mountain. North of McClede Moun­ tain, the top of the megabreccia has been exhumed, revealing a hummocky paleosurface consistent with landslides.
Mimbres Peak Formation
The Mimbres Peak Formation forms rhyolite domes at Town Mountain and directly west of McClede Mountain. The domes west of McClede Mountain are herein called the Deer Hill dome complex, which is named for exposures on both sides of Tierra Blanca Creek immediately west of the mapped area of this report. The Deer Hill complex, of which McClede Moun­ tain is also a part, is interpreted to have been erupted within the ring fracture zone of the Emory cauldron; Town Mountain was intruded along faults outside the cauldron. In ascending order, the domes are composed of tuff, vitrophyre, and rhyolite flow,
A
Figure 3.
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5
SiO2, IN WEIGHT PERCENT
Kneeling Nun Tuff
Trachy­ andesite 1
Trachy­ dacite 5
3 8
Rhyolite 9
Basaltic trachy­ andesite
7
Basaltic andesite
2
6
Dacite
10 11
Andesite
4
9
Alkali olivine basalt
13
Silicic alkalic basalt
12
Basaltic andesite
Tholeiitic andesite
Olivine tholeiite
045 50 55 60
SiO2, IN WEIGHT PERCENT
B
Alkali-silica plots from Lake Valley map area. A, Mid-Tertiary igneous rocks, classified according to I.U.G.S. (Le Bas and others, 1986). B, Upper Tertiary alkali basalts, classified according to McDonald and Katsura (1964). Numbers refer to samples listed in table 1.
make up only 5–10 percent of the tuff. Biotite is conspicuous, but broken phenocrysts of oligoclase (An23–25), sanidine, and quartz are also present. Thin tuffaceous sandstone weathers platy and is locally crossbedded and burrowed; it consists of medium- to coarse-grained reworked pumice, crystals, and lithic fragments.
Breccia fills a vent on O Bar O Peak, and outflow breccia crops out south of Berrenda Mountain (fig. 4). The vent fill con­ sists of massive breccia with angular blocks of andesitic Rubio Peak Formation as large as 1 m across in a densely welded tuf­ faceous matrix. Tuff beds and lenses measuring only a few cen­ timeters in thickness are interspersed in the breccia; these beds dip gently east, in conformity with the regional dip of the sur­ rounding volcanic rocks. The vent fill appears to rise plug-like from, and may intrude, Sugarlump Tuff that makes up the foot of O Bar O Peak. On the lower slopes south of Berrenda Moun­ tain, proximal outflow from the vent at O Bar O Peak consists of moderately welded ash-flow tuff. The proximal outflow resem­ bles other ash-flow tuff of the Sugarlump, except that it contains clasts of Rubio Peak Formation as much as 1 m across. The out- flow is crudely bedded and contains sparse lenses of tuffaceous sandstone a few centimeters in thickness.
Geology of the Lake Valley Area 9
K2O + Na2O, IN WEIGHT PERCENT
K2O + Na2O, IN WEIGHT PERCENT