PLEASE NOTE:


*
Date sent:        Mon, 27 Oct 1997 09:20:26 -0500 (EST)
From:             Benny J Peiser <B.J.PEISER@livjm.ac.uk
Subject:          Comet Struck Southern Nevada 370 Million Years Ago
To:               cambridge-conference@livjm.ac.uk
Priority:         NORMAL

from: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov
 

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U.S. Department of the Interior U.S. Geological Survey Central Region
Outreach Office PO Box 25046, MS 150 Denver, CO 80225

Contact: Heidi Koehler
Phone: 303-236-5900x302
Fax: 303-236-5882

News Release: October 20, 1997

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Comet Struck Southern Nevada 370 Million Years Ago

An extraterrestrial object, theorized to be a comet at least one
kilometer in diameter, impacted what is now southern Nevada about 370
million years ago. Evidence of the widespread damage caused by this
collision was announced October 20, 1997, at the annual meeting of
the Geological Society of America in Salt Lake City, Utah, by Charles
A. Sandberg, geologist emeritus of the U.S. Geological Survey,
Denver, and his collaborators, Jared R. Morrow, University of
Colorado, and John E. Warme, Colorado School of Mines.

"The impact, which is named the Alamo Impact after the town about 90
miles north of Las Vegas, occurred offshore from a carbonate
platform, very much like the modern Australian Barrier Reef or the
Bahamas Bank," said Sandberg, who has been studying this area since
1991.

At the time of impact during the Devonian geologic Period, also known
as the age of sharks, an ancestral Pacific Ocean covered most of
Nevada. Shock waves from the impact and an ensuing initial
1,000-foot-high tsunami wave caused extensive damage to the carbonate
platform and coastline in a semi-circular area 100 miles from north
to south and 35 miles across. As the carbonate platform collapsed,
blocks hundreds to thousands of feet across were torn from the
seabed, twisted, and transported seaward. As tsunamis of decreasing
intensity reverberated back and forth across the ocean basin,
broken pieces of rock and ejecta from the impact were deposited over
the carbonate platform and high-water deposits were deposited in an
bow-like curve along the coastline to the east.

Three lines of evidence corroborate the extraterrestrial origin of
the triggering mechanism for the unusual phenomena observed in 13
mountain ranges: shocked quartz grains, an iridium anomaly, and
spherical carbonate ejecta. Shocked quartz grains are sand grains
pervasively shattered by the force of an impact, iridium is a
platinum-like element that is rare on Earth, and carbonate spherules
are formed from limestone fragments that recrystallize within a
superheated cloud. All three features have been recorded in
association with other impacts elsewhere on Earth, such as the
impacts that produced the extinction of the dinosaurs hundreds of
millions of years later. The Alamo occurred about three million years
before and may have led to an earlier extinction, one of the five
great mass extinctions recorded in Earth's history, in Late Devonian
time.

Late-breaking field evidence suggests the existence of several
channel deposits filled with broken rock debris in the deeper ocean
basin west of the carbonate platform. Channel-fill deposits found at
four localities show that the area of impact-related phenomena
occupies a circular area at least 120 miles in diameter. Whether
these deposits represent debris that flowed from the crater outward
or slumped back into it has not yet been determined. However,
plotting a point midway between these sites and the possible
eastern crater rim at the west end of the Tempiute mountain range to
the east suggests location of the impact site, now buried beneath a
valley or much younger volcanic flows, north and west of the
settlement of Rachel and about 130 miles northwest of Las Vegas.

The U.S. Geological Survey provides the nation with reliable,
impartial information to describe and understand the Earth. This
information is used to minimize the loss of life and property from
natural disasters and manage water, biological, energy and other
mineral resources in the wisest way possible.



CCCMENU CCC for 1997

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