The Alamo bolide impact occurred 377–378 million years ago, when one or more

hypervelocity Hypervelocity is very high velocity, approximately over 3,000 meters per second (6,700 mph, 11,000 km/h, 10,000 ft/s, or Mach 8.8). In particular, hypervelocity is velocity so high that the strength of materials upon impact is v ...

objects from

space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consi ...

slammed into shallow marine waters at a site that is now the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...

Guilmette Formation The Guilmette Formation is a geologic formation in Nevada. It preserves fossils dating back to the Devonian period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainmen ...

of the Worthington Mountains and Schell Creek Range of southeastern

Nevada Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...

; the event is named for

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

s of metamorphosed crushed rock deposits, found near the town of

Alamo, Nevada Alamo is an unincorporated town in Lincoln County, Nevada, United States, about north of Las Vegas along U.S. Route 93. Its elevation is . As of the 2010 census it had a population of 1,080. History A post office has been in operation at Alam ...

(the "Alamo Breccia"). This catastrophic

impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or ...

resulted in what is one of the best-exposed and has become the most accurately dated impact events; it occurred within the

Frasnian The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage. Major reef-building was under way during th ...

age of the Devonian at about 377-378 Ma, a moment in time that was about 5.9 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-east

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

s across the area. Insufficient detail does not permit a precise reconstruction of the Devonian

paleogeography Palaeogeography (or paleogeography) is the study of historical geography, generally physical landscapes. Palaeogeography can also include the study of human or cultural environments. When the focus is specifically on landforms, the term pal ...

, beyond the fact that it was a "wet-target" impact in a reef front where

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

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, elevated

iridium Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density o ...

levels, and spherical

lapilli Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range ...

. 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 John E. Warme, "The Many Faces of the Alamo Impact Breccia" ''Geotimes'', January 2004 "Ensuing

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater exp ...

s rearranged much of the debris" he adds. After initial resistance from the

Geological Society of America The Geological Society of America (GSA) is a nonprofit organization dedicated to the advancement of the geosciences. History The society was founded in Ithaca, New York, in 1888 by Alexander Winchell, John J. Stevenson, Charles H. Hitch ...

, 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''.

References

J.A. Pinto and J.E. Warme, "Alamo impact crater documented", 2006.
in ''Lunar and Planetary Science'' 37 (pdf file)

{{Impact cratering on Earth Geology of Nevada Geology of Utah Devonian impact craters Impact craters of the United States Landforms of Lincoln County, Nevada Landforms of Utah

See also

References