Andesite Ashflow Tuffs

A sequence of andesitic ash-flow tuffs occurs mostly southeast of the Telephone Road in the northern part of the Hot Springs Mountains. The little-studied sequence consists principally of plagioclase-hornblende ash-flow tuffs which lie with angular unconformity on sedimentary rocks interpreted as the Truckee, Desert Peak, and Chloropagus Formations. Folding and faulting preceded and in part controlled deposition of the andesite unit. In general the thickest andesite accumulations occur in the troughs of synclines and along scarps created by downdropped blocks prior to eruption of the andesitic tuffs (fig. 34). The extent of erosion is unknown but it appears that the unlithified top portion of at least one cooling unit has been removed, creating a stripped structural surface (E. C. Bingler, personal commun., 1978). The original thickness is therefore unknown, although geologists of the Southern Pacific Company measured about 500 feet of section east of the mapped area.

FIGURE 34. Photograph of the andesite ash-flow tuffs lying in synclinal troughs in the Truckee Formation, NE/4 S27,T22N,R26E. The photo shows the westernmost occurrence of the ash flow in a synclinal trough composed of limestone. The ash-flow tuff appears as a small brown cliff.

Three discordant radiometric K-Ar age determinations have been obtained from mineral separates in the andesite sequence. A private laboratory dated plagioclase at 11.2 m.y.; the USGS dated hornblende at 4.6 m.y. and plagioclase at 2.3 m.y. (N. Voegtly, personal commun., 1978). The discordance of the radiometric determinations raises a potentially severe problem. If the sedimentary rocks beneath the andesite unit are correctly correlated with the Truckee, Desert Peak, and Chloropagus Formations, and if the 11.2 m.y. date is correct, then the age index for Nevada's Tertiary nonmarine fossil record needs revision. However, a 2.3 to 4.6 m.y. age for the andesite sequence

' is compatible with the correlations and fossil evidence. The latter interpretation is considered to be correct; if so, the andesite may be the youngest ash flow in Nevada. This sequence needs further radiometric evaluation.

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