- Alamo bolide impact
The Alamo bolide impact occurred 367 million years ago, when one or more
hypervelocity objects fromspace slammed into shallow marine waters at a site that is now theDevonian Guilmette Formation of theWorthington Mountains and Schell Creek Range of southeasternNevada ; the event is named forbreccia s of metamorphosed crushed rock deposits, found as far as the town ofAlamo, Nevada (the "Alamo Breccia"). This catastrophicimpact event resulted in what is one of the best-exposed and has become the most accurately dated impact events; it occurred within theFrasnian age of the Devonian at about 367 Ma, a moment in time that was about 3.5 Ma prior to the Frasnian/Famennian extinction events, which it is unlikely to have affected. The actual impact site has not yet been significantly documented—even its precise diameter and its internal structural features are not yet clear enough to make speculations on the mass and trajectory genuinely useful. The "tectonic overprint", the subsequent geologic modifications since the event, have distorted the picture; the distribution of the breccia has been compressed and skewed by west-to-eastthrust fault s across the area. Insufficient detail does not permit a precise reconstruction of the Devonianpaleogeography , beyond the fact that it was a "wet-target" impact in a reef front wherecarbonate s were being built up in marine shallows.The impact site contains megabreccia of gigantic displaced blocks, together with several attributes familiar from other impact sites:
shocked quartz , elevatediridium levels, and sphericallapilli . John E. Warme, one of the geologists who first recognized the geologic anomalies as the results of a bolide, estimates that the total volume of limestone reef deposits and bedrock that was smashed, deformed, partially melted or shifted during the Alamo event at 1,000 cubic kilometers [http://www.geotimes.org/jan04/feature_Alamo.html] . "Ensuingtsunami s rearranged much of the debris" he adds.After initial resistance from the
Geological Society of America , the first paper published concerning the Alamo Breccia was co-authored by John Warme, Brian Ackman, Yarmanto, and Alan Chamberlain in the 1993 "Nevada Petroleum Society Field Conference Guidebook".A
volcanic plain a hundred miles wide marks at least one of the impact sites,Lunar Crater National Natural Landmark . As the picture becomes clearer, thebolide ,meteor storm ,asteroid and its effects are becoming a faint road map to the exploitation of a putative giantoil reservoir underlying much of eastern Nevada and westernUtah .See also
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Pahranagat Valley References
* [http://www.geotimes.org/jan04/feature_Alamo.html John E. Warme, "The Many Faces of the Alamo Impact Breccia" "Geotimes",] January 2004
* [http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2453.pdf J.A. Pinto and J.E. Warme, "Alamo impact crater documented", 2006.] in "Lunar and Planetary Science" 37 (pdf file)
* [http://geoscience.unlv.edu/GSN/trips/Alamo_Impact.html Brian Ackman, "The early history of the Alamo Breccia"] [http://geoscience.unlv.edu/GSN/trips/alamoref.htm Bibliography]
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