--> --> Alamo Breccia: a key to unravel thrust duplexes in the southern Nevada portion of the western North American Cordillera fold thrust belt

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Alamo Breccia: a key to unravel thrust duplexes in the southern Nevada portion of the western North American Cordillera fold thrust belt

Abstract

The north-south structural grain of the eastern Great Basin of western Utah and eastern Nevada was caused by compression of a thick wedge of Paleozoic rocks along a portion of the western American Cordillera. Except for the Colorado River Basin, thrust duplexes in the Great Basin thrust belt are mostly concealed under their own erosional debris due to the lack of external drainage. Deep oil wells, erosion by the Colorado River, and internal Paleozoic stratigraphy reveal thrust duplexes. If it were not for erosion of the Colorado River exposing the heart of the thrust belt near Las Vegas the thrust duplex core of thick footwall Jurassic sandstones lying below hanging wall Early Paleozoic carbonates would not be revealed. Several deep oil wells in eastern Nevada and western Utah have cut through thrust duplexes revealing repeated Paleozoic strata. A careful comparison of Paleozoic stratigraphy between thrust sheets is another way to reveal the compressional nature of this part of the Cordillera thrust belt. The chronostratigraphic Upper Devonian Alamo Impact Breccia in southern Nevada provides one of the best stratigraphic tools to unravel the thrust duplexes because it is easily recognized in the field, its thickness and characteristics changes dramatically between thrust sheets, it was not differentially removed by subsequent Paleozoic unconformities, and it represents a geologic instant.