Abstract

The Chloride quadrangle includes the southern two-thirds of the Cerbat Mountains, a range composed mostly of presumably Precambrian crystalline rocks. The oldest rocks are quartzite, mica schist, hornblende-diopside schist, and amphibolite. Younger and larger in amount are granite and granite gneiss. Gabbroic and dioritic gneiss, pegmatite, aplite, diabase, and mylonite also occur as part of the basement complex. The earliest rocks were strongly folded, but igneous intrusion and granitization have obliterated most of the folds. The rock types and the history of the basement complex compare closely with the Archean in the Grand Canyon.

The area contains no Paleozoic rocks and probably no Mesozoic rocks. Intrusions of granite porphyry associated with mineralization are believed to be Tertiary. Dikes of granite porphyry, pegmatite, aplite, and lamprophyre are associated with these intrusions.

Small areas of andesitic and rhyolitic extrusive rocks of Tertiary (?) age occur along the flanks of the range. Quaternary basalt caps these rocks in some places.

Several periods of faulting and erosion are visible in the volcanic rocks. The latest faulting has produced the present mountains. The greatest displacement has been on the west side, and the Cerbat Range is an eastward-tilted fault block, modified in part into a horst. This mountain building seems to have occurred in late Tertiary and Quaternary time. Recent fault-block movement is suggested by the presence of canyon terraces and by the dissection of the pediment at the west base of the range.

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