Wm Record Copy

A.1,219 meter (4,000 ft) drill hole in Crater Flat shows the absence of burled Pliocene or Quaternary volcanic rocks, and penetrates a section of Timber Mountain, Paintbrush, and the upper part of the Crater Flat Tuffs, similar to that exposed adjacent-to-Crater Flat. A prominent negative aeromagnetic anomaly between the drill hole and Bare Mountain is attributed to a westward thickening.section of a reversely magnetized Miocene basalt.. The relatively shallow-depth of this basalt in the west-central part of Crater Flat'-indicates that no large amount-of tectonic movement has occurred in approximately the last,10-m.y.Massive brecciated wedges of Paleozoic rocks. are penetrated in two stratigraphic intervals n the drill hole;,.the older, one, between the Tiva Canyon Member of the Paintbrush Tuff and the Rainier Mesa Member of the Timber Mountain Tuff, correlates with the time of maximum faulting east of Crater Flat in the Yucca Mountain area. The younger slide masses are correlated with a large slide block of probable late Miocene age exposed along the southwestern rim of Crater Flat. The structural pattern and style buried beneath central ad western Crater Flat is deduced to be similar to that exposed at Yucca Mountain, but less developed. The major fault system controlling the steep east face of Bare Mountain, though probably still active, is believed to have developed mainly as a result of caldera collapse between 13 and 14 m.y. ago. Relations between faulting and four episodes of basalt eruption n the Crater Flat area strongly suggest contemporaneity of the two processes.