Tertiary rocks of the Candelaria-Mina region of the western Great Basin (Fig. 1) occur in five distinct sequences, each separated by a known or probable regional unconformity. One sequence, the upper Oligocene tuff of the Candelaria Hills, is of particular interest because of its remarkable lateral variations in constitution and thickness in the Candelaria Hills. Such variations point clearly to local differential subsidence during deposition about 25 to 22 m.y. B.P. The Oligocene-Miocene boundary is assumed to be 22.5 m.y. and the Miocene-Pliocene boundary, 5.1 m.y. (Berggren, 1972).

The succession, ages, and general characteristics of the five Cenozoic sequences are set forth in Table 1. Principal emphasis was placed on the upper Oligocene tuff sequence in the central Candelaria Hills (Figs. 2 and 3). This area had previously been mapped in reconnaissance by Ferguson and others (1954).

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