SHOCKED QUARTZ IN THE ALAMO BRECCIA, SOUTHERN NEVADA - EVIDENCE FOR ADEVONIAN IMPACT EVENT

A transmission electron microscope (TEM) study of quartz grains strong ly implies that the Alamo breccia of southern Nevada resulted indirect ly from a Late Devonian hypervelocity impact event. The Alamo breccia is perhaps the most voluminous marine carbonate megabreccia exposed on land. It covers similar to 4,000 km(2), averages similar to 70 m thic k, and contains more than 250 km(3) of carbonate-platform debris that was deposited by a giant submarine slide. The breccia is a single bed with the characteristics of a chaotic debrite at the base evolving upw ard to a graded turbidite at the top. The bed is anomalous, compared t o other marine megabreccias, because over a large area it is intercala ted with cyclic shallow-water carbonate-platform rocks, rather than wi th deep-water turbidites as expected. Thin sections of peculiar quartz grains, recovered from insoluble residues of the breccia, show one to six sets of imperfect parallel lamellae and other defects suggesting shock metamorphism. When studied by TEM, the grains clearly display pl anar deformation features (PDFs) and other defects from a high-pressur e shock wave. Straight and narrow planar microstructures consist of a high density of dislocations mostly parallel to crystal habit plane {1 012}, but {1013}, {1011}, and {1121} orientations were also detected. The PDFs appear identical to those in quartz grains associated with we ll-known impact structures such as Manicouagan and Manson. We conclude that energy from an impact triggered the epiplatform slide and the co nsequent sedimentary processes that formed the Alamo breccia.