Abstract

A transmission electron microscope (TEM) study of quartz grains strongly implies that the Alamo breccia of southern Nevada resulted indirectly from a Late Devonian hypervelocity impact event. The Alamo breccia is perhaps the most voluminous marine carbonate megabreccia exposed on land. It covers ≈4,000 km2, averages ≈70 m thick, and contains more than 250 km3 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 upward to a graded turbidite at the top. The bed is anomalous, compared to other marine megabreccias, because over a large area it is intercalated with cyclic shallow-water carbonate-platform rocks, rather than with 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 tamorphism. When studied by TEM, the grains clearly display planar deformation features (PDFs) and other defects from a high-pressure shock wave. Straight and narrow planar microstructures consist of a high density of dislocations mostly parallel to crystal habit plane {1012}, but {1013}, {1011}, and {1121} orientations were also detected. The PDFs appear identical to those in quartz grains associated with well-known impact structures such as Manicouagan and Manson. We conclude that energy from an impact triggered the epiplatform slide and the consequent sedimentary processes that formed the Alamo breccia.

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