Description:
The Owl Head 7.5' quadrangle and the northern third of its southern neighbor, the
Kofa Butte 7.5' quadrangle, represent an 80 mile' area of the northern Kofa Mountains, a
remote, rugged range of southwest Arizona (Figure 1). The only part of the map area
covered by significant amounts of Quaternary alluvium is an 8 mile' area in the northwest
corner which is part of a narrow, Quaternary or younger Tertiary alluvial-filled valley that
separates the Kofa Mountains from the New Water and Plomosa mountains to the north.
Most of the bedrock exposures are Miocence calc-alkaline volcanic rocks of the
Kofa volcanic field. They occur in three sequences: a series of outflow sheets of ash-flow
tuffs that depositionally overly the crystalline basement rocks, a middle sequence of mafic
to intermediate lava flows, and a younger sequence of basalt lavas. The crystalline
basement rocks are exposed in the footwall of two major south-side-down normal faults
which transect the north-central part of the map area. The basement consists primarily of
Mesozoic(?) granitoids which intrude amphibolite-facies, Proterozoic(?), sedimentary
rocks to the east. To the west, the granitoid has an indeterminate east-northeast striking
contact with low-metamorphic grade argillites of probable Late Mesozoic age which
underlie the northwestern peidmont of the Kofa Mountains, and which are probably
correlative to extensive areas of similar rocks which make up large areas of the Plomosa
Range to the north (Richard and others, 1993).