Geological Survey Professional Paper 160 Geologic History of the Yosemite Valley

The Devils Postpile quadrangle straddles the crest of the Sierra Nevada southeast of Yosemite National Park and contains the Devils Postpile National Monument (fig. 1). The pre-Cenozoic rocks of the quadrangle include Paleozoic and Mesozoic metasedimentary and metavolcanic rocks. The metamorphic rocks have been intruded by various granitoid igneous rocks, which form a part of the composite Sierra Nevada batholith (Bateman and others, 1963; Huber and Rinehart, 1965a). By late Tertiary time the Sierra Nevada had attained approximately its present overall configuration, except for later modifications caused by faulting, uplift, and increased dissection. Regional volcanism began during the late Tertiary and has continued spasmodically to the present.

FIGURE 1.—Location of the Devils Postpile quadrangle and the adjacent Mount Morrison quadrangle. (click on image for an enlargement in a new window)

The Cenozoic volcanic rocks of the Devils Postpile quadrangle resulted from several types of eruption throughout late Tertiary and Quaternary time; the forms of eruption include domes, lava flows, ash flows, and extensive pumice falls. The volcanic rocks range in composition from basalt to rhyolite, the oldest being basaltic to andesitic and the youngest, rhyolitic; however, most mafic and felsic rocks alternate haphazardly without regard to stratigraphic position. For this reason and because many of the volcanic units remain only as scattered erosional remnants, correlations are difficult. Interpretation of the volcanic history has been greatly assisted by potassium-argon age dating.

Mapping in the Devils Postpile quadrangle (Huber and Rinehart, 1965a) and in the Mount Morrison quadrangle to the east (Rinehart and Ross, 1964) allows the division of the Cenozoic volcanic rocks of the Devils Postpile quadrangle into 11 mappable units. Scattered erosional remnants of uncertain stratigraphic position have tentatively been assigned to these units, although some of the remnants may represent additional episodes of volcanic activity not otherwise recognized. Table 1 lists the 11 units and their radiometric ages where available, and plate 1 shows their distribution.

TABLE 1.—Ages of Cenozoic volcanic rocks of the Devils Postpile quadrangle
[See text for source and other data regarding individual age determinations)

In order to provide a background for description of the individual volcanic units, a general summary of petrologic and chemical data is presented for the entire volcanic suite. Following this, field relationships and general characteristics of each volcanic unit are described in order of decreasing age, rather than by petrologic type, to provide a clearer picture of the sequence of events.

We wish to thank G. B. Dalrymple for making his potassium-argon age determinations available to us in advance of their being published and Gerhard Schumacher for the use of some of his excellent photographs of the Mammoth Lakes region. We also express our appreciation to R. J. Janda for many stimulating discussions of the interrelations of volcanic and glacial events.