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Tiny Teeth Forecast Ancient Comet Showers
Released: 6/12/1998

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Heidi Koehler 1-click interview
Phone: 303-236-5900 x302

Minuscule fossil animal teeth, known as conodonts, indicate that a 370-million-year-old comet that slammed into southern Nevada could be as much as five times larger than scientists initially suspected.

Evidence provided by conodonts on the depth of the Alamo Impact crater, 130 miles north of Las Vegas, and on the size of the Alamo comet will be presented for the first time to a European audience at the meeting of the Seventh International Conodont Symposium Held in Europe (ECOS VII) meeting at the Università di Bologna, Bologna, Italy, on Thursday, June 25, by Sandberg and his co-author Jared R. Morrow, University of Colorado. Their talk will also discuss conodont evidence that independently led to recognition of the resulting Alamo Breccia as an impact breccia and to dating the impact as having occurred three million years before a Late Devonian mass extinction.

"From conodonts discovered in impact-fallout debris, I calculated the depth and size of the impact crater to have been at least 1.5 kilometers deep and 75 kilometers wide," said Charles A. Sandberg, geologist emeritus with the U.S. Geological Survey, Denver, Colo., USA.

Thus, the ancient Nevada comet could have been as large as five kilometers in diameter. It may have been only part of the first of a series of at least three comet showers that led to a mass extinction of many forms of life three million years later, but 300 million years before the younger comet showers that led to extinction of the dinosaurs.

"Unexplained breccias in the Carnic Alps along the Italian-Austrian border and in the Rheinische Schiefergebirge of Germany are the same age as the Alamo Impact Breccia and may have had a similar origin. Therefore, the ancient comet showers may have also struck parts of western Europe," Sandberg said.

Conodonts are the microscopic teeth of primitive, boneless, eel-like animals. The shape of these animals is similar to that of modern hagfish, best known from the fjords of Norway. Conodont animals lived in many of the world’s oceans from the Cambrian through Triassic Periods of geologic time (550 to 210 million years ago). The largest known conodont teeth, found near the town of Alamo in southern Nevada, measure nearly a centimeter in length, but most are not much larger than the head of a pin.

Preliminary evidence presented by Sandberg and his colleagues John Warme and Jared Morrow in October 1997 at a Geological Society of America meeting at Salt Lake City, Utah, showed only that impact-related phenomena, such as grains of shocked quartz and higher-than-usual levels of the element iridium, occupied a circular area 200 kilometers in diameter in southern Nevada. Iridium is an element found in asteroids and comets but not common on Earth. Shocked quartz grains are produced by the force of an impact on sandstone rocks.

The Alamo Impact occurred during the Devonian Period of geologic time, 370 million years ago, when an ancestral Pacific Ocean covered most of Nevada. The impact occurred offshore from a carbonate platform, very much like the modern Australian Barrier Reef or the Bahamas Bank. Shock waves from the impact and ensuing 300-meter-high super-tsunami waves crashed against the carbonate platform and coastline. As the carbonate platform collapsed, blocks of rock hundreds of meters 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 other ejecta from the impact were deposited over the carbonate platform and high-water deposits were stranded in a semicircular band along the coastline to the east.

Small blocks of impact-fallout debris recently found within the breccia of large jagged blocks contain carbonate spherules formed from limestone fragments that recrystallized within a superheated cloud, shocked quartz grains, bits and fragments of rocks blasted from the crater, and most importantly, conodonts derived from rocks of Ordovician age that lay at least 1.5 kilometers below the Devonian sea floor at the time of impact.

The timing of the Alamo Impact coincides with the demise of some Late Devonian reefs in Belgium and Germany and with unusual breccias in Germany and Austria. Earlier work by Sandberg and his co-authors has already shown the possibility of two other times of impacts between that of the Alamo Impact and the time of the Late Devonian mass extinction.

Following the ECOS VII meeting, a joint investigation of breccias in the Carnic Alps will be conducted by Sandberg and Dr. Hans-Peter Schönlaub, Direktor, Geologisches Bundesanstalt, Vienna. Thereafter, together with Prof. Willi Ziegler, Direktor Emeritus, Forschungsinstitut Senckenberg, Frankfurt am Main, Sandberg, who is a corresponding member of the Forschungsinstiut Senckenberg, will study Late Devonian breccias and reefs in Germany and Belgium.

For additional information contact:

Charles A. Sandberg, Geologist Emeritus
E-mail: sandberg@usgs.gov
Phone: 303.236.5763
Fax: 303.236.0459

Jared R. Morrow
E-mail: morrow@ucsu.colorado.edu
Phone: 303.492.7025

After June 15, Sandberg, c/o Willi Ziegler:

E-mail: wziegler@sngkw.uni-frankfurt.de or
Phone or fax: 49-6402-1421

For the October 1997 press release on the Alamo Impact, go to:


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