U-Pb isotopic dating of detrital zircon from supracrustal Proterozoic and Cambrian arenites from the western UnitedStates and northern Mexico reveal three main age groups, 1.90 to 1.62 Ga, 1.45 to 1.40 Ga, and 1.2 to 1.0 Ga. Small amounts of zircons with ages of 3.1 to 2.5

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The Butte Valley and Layton Well Thrusts are major structural features in two adjacent mountain ranges west of southern Death Valley. The Butte Valley Thrust in the southern Panamint Range underlies most of the range and emplaces Proterozoic rocks over strata as young as Jurassic(?) in age. The Layton Well Thrust to the southwest in the Slate Range has been interpreted to have Proterozoic rocks juxtaposed on rocks as young as Jurassic, suggesting that the Butte Valley Thrust and the Layton Well Thrust might be correlative. New information indicates that the allochthonous rocks of the Layton Well Thrust are Mesozoic in age and are not likely part of the same allochthon as that above the Butte Valley Thrust. In addition, the Butte Valley Thrust cuts sharply downward to the north and west across lower plate Paleozoic strata, suggesting that the fault roots beneath the Layton Well Thrust. The Layton Well Thrust probably belongs to the East Sierran thrust system and thus would be in the upper plate of the Butte Valley Thrust. in the southern Cordillera, Geol. Soc. Am. Bull., 104, 80-105, 1992. prominent fault at the south end of the Panamint Range, Inyo County, California, Geol. Soc. Am. Abstr. Programs, Cordilleran Sect., 64, 132, 1968.

STUDIES RELATED TO WILDERNESSThe Wilderness Act (Public Law 88-577, September 3,1964) and related acts require the U.S. Geological Survey and the U.S. Bureau of Mines to survey certain areas on Federal lands to determine their mineral resource potential. Results must be made available to the public and be submitted to the President and the Congress. This report presents the results of a geological survey of the Mazatzal Wilderness (NF 3048) and Contiguous Roadless Area in Tonto and Coconino National Forests, Gila, Maricopa, and Yavapai Counties, Arizona. The Mazatzal Wilderness contiguous roadless area (3-016) was classified as a further planning area during the Second Roadless Area Review and Evaluation (RARE II) by the U.S. Forest Service, January 1979.

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