File:Impact breccia (Alamo Breccia, Upper Devonian; southeastern Nevada, USA).jpg

English: Impact breccia from the Devonian of Nevada, USA. (cut & polished surface)

Impact breccias are poorly-sorted, clastic-textured rocks having angular clasts. They form when asteroids or meteoroids impact the surface and pulverize rocks in the target area.

This specimen is from the famous Alamo Breccia of Nevada, which was discovered in the 1990s. Most of the clasts are limestone fragments. The horizon occurs throughout southern Nevada, as well as western Nevada and western Utah.

The impact structure itself is buried and tectonically modified, but is estimated to be about 44 to 65 kilometers in size (see Morrow et al., 2005). The impact site was originally oceanic, with Devonian to Cambrian sedimentary units being excavated from below the seafloor during the event. The current location of the impact structure is somewhere in southern Nevada - it's broadly constrained to being north of Groom Lake ("Area 51").

Stratigraphy: Alamo Breccia, Guilmette Formation, Frasnian Stage, lower Upper Devonian

Locality: unrecorded/undisclosed site in southeastern Nevada, USA

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Reference cited: Morrow et al. (2005) - Late Devonian Alamo Impact, southern Nevada, USA: evidence of size, marine site, and widespread effects. Geological Society of America Special Paper 384: 259-280.

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See info. at:

en.wikipedia.org/wiki/Alamo_bolide_impact