Abstract

The Uinkaret Plateau, a major block of the Colorado Plateau province in northern Arizona, is bounded on the west by the Hurricane Ledge, developed along the Hurricane fault, and on the east by scarps developed along the Toroweap and Sevier faults. Over the southern part there are extensive outpourings of lava which fall into four age groups.

The oldest eruptions preceded earliest movement along the Hurricane fault and antedated cutting of the present Grand Cayon. These lavas poured out on a relatively even surface believed to have been a large pediment or series of pediments extending at least 16 miles north from the Colorado River. Extensive dissection of the oldest lavas preceded the next eruptions, which took place at considerably lower elevations. The Inner Gorge of Grand Canyon was cut to approximately its present depth and later filled to a height of at least 600 feet and perhaps 1,200 by lava floods from Toroweap Valley. The lava dams were entirely removed before later flows, pouring into the Inner Gorge from the rim of the Esplanade, again dammed the river, which has removed all but small portions of these dams. Repeated movements along the Toroweap fault have occurred in post-lowan-Wisconsin time. In the very recent past, renewed volcanic activity formed a single small cone and lava flow.

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