MAP 238

GEOLOGIC MAP OF THE GRANITE PEAK AND SAPPHIRE MOUNTAIN AREA, U.S. ARMY DUGWAY PROVING GROUND, TOOELE COUNTY, UTAH
Donald L. Clark, Robert F. Biek, Grant C. Willis, Kent D. Brown, Paul A. Kuehne, J. Buck Ehler, and Carl L. Ege

This area is located in west-central Utah within the eastern Basin and Range Province, near the southern margin of the Great Salt Lake Desert, and within the confines of Dugway Proving Ground.  Granite Peak consists of a granitic intrusion that is Late Jurassic in age (149 million years old).  The upper part of the intrusion was altered and intruded by numerous pegmatite dikes.  Metamorphic rocks of likely Paleozoic or Proterozoic protoliths are exposed at the far south end of the mountain.  These granitic and metamorphic rocks were exhumed during Basin and Range extension, likely from about 15 to 5 million years ago.  Sapphire Mountain is a Miocene-age rhyolite flow that erupted 8 million years ago.  Quaternary surficial map units include lacustrine, alluvial, and eolian deposits, and desert mudflats.  Three of the four primary shorelines of the Bonneville lake cycle are preserved on the mountains’ flanks.

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