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Late Devonian Alamo Impact, southern Nevada, USA: Evidence of size, marine site, and widespread effects

By
Jared R. Morrow
Jared R. Morrow
Department of Earth Sciences, University of Northern Colorado, Greeley, Colorado 80639, USAjared.morrow@unco.edu
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Charles A. Sandberg
Charles A. Sandberg
U.S. Geological Survey, Box 25046, MS 939, Federal Center, Denver, Colorado 80225-0046, USA
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Anita G. Harris
Anita G. Harris
1523 East Hillsboro Boulevard #1023, Deerfield Beach, Florida 33441, USA
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Published:
January 01, 2005

The early Late Devonian (early Frasnian) Alamo Impact targeted an oceanic, off-platform site in southern Nevada, excavating a crater with a final diameter of 44–65 km. The original crater is now dismembered and buried beneath younger rocks. Consequently, its size and site must be deduced through multiple converging lines of geological and paleontological evidence. Previous and new evidence includes the catastrophically emplaced Alamo Breccia, tsunamites, shock-metamorphosed quartz grains, carbonate accretionary lapilli, an iridium anomaly, sub-Breccia clastic injection, deep-water Breccia channels, and ejecta material. We now demonstrate, on the basis of conodont microfossils in carbonate ejecta clasts within lapillistone blocks and widely distributed shocked-quartz and lithic-clast ejecta within the upper part of the Breccia, that the Alamo Impact excavated down at least into Upper Cambrian strata, at a depth of 1.7 km, and possibly into the underlying Proterozoic–Lower Cambrian Prospect Mountain Quartzite, ∼2.5 km beneath the Late Devonian seafloor. Distal tsunamites and probable ejecta are now documented as far north as Devils Gate, northern Nevada, and as far northeast as the Confusion Range, western Utah. A charcoal-bearing, early Frasnian estuarine deposit in the Bighorn Mountains, Wyoming, may provide the first evidence of an Alamo Impact fallout-generated forest fire. Our new data further document the widespread effects and size of the Alamo Impact, and constrain the likely present position of the tectonically transported crater to an area between the Timpahute and Hot Creek Ranges, southern Nevada.

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Contents

GSA Special Papers

Large Meteorite Impacts III

Thomas Kenkmann
Thomas Kenkmann
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Friedrich Hörz
Friedrich Hörz
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Alex Deutsch
Alex Deutsch
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Geological Society of America
Volume
384
ISBN print:
9780813723846
Publication date:
January 01, 2005

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