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Paleozoic Partner Highlight- Utah Geological Survey

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Cambrian Fossils in Utah’s West Desert, Millard County

Utah is recognized for having the longest and most diverse dinosaur record in the nation. Yet, the Cambrian rocks in Utah’s West Desert contain one of our nation’s best records of the early evolution of life on Earth. View a slideshow of rocks and fossils from the West Desert here. During the Cambrian Period, North America straddled the Equator and the continent was oriented nearly 90 degrees clockwise of its present position. The Cambrian coastline extended north-south across Utah shifting southward (our east) with rising sea level. This resulted in a nearly complete sequence of Cambrian rocks preserved in Utah’s West Desert on what was the northern coastline of early North America. When Tertiary extension forces formed the Basin and Range Geological Province throughout the last 20 million years, these Cambrian rocks became well-exposed across western and central Utah, revealing the extraordinary fossil record within. Nowhere is this geology better exposed than in Millard County, Utah. Refer to Hintze and Davis (2003) (17 MB PDF) for a detailed discussion of the county’s geology. The Cambrian is best known for the “Cambrian Explosion” (or “Cambrian Radiation”) , when a great diversity of multicellular animals first appears. The first scientific report on these fossils was a description of Elrathia kingii in 1860, probably the world’s most well-known trilobite species.

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MAP 238

GEOLOGIC MAP OF THE GRANITE PEAK AND SAPPHIRE MOUNTAIN AREA, U.S. ARMY DUGWAY PROVING GROUND, TOOELE COUNTY, UTAH
Donald L. Clark, Robert F. Biek, Grant C. Willis, Kent D. Brown, Paul A. Kuehne, J. Buck Ehler, and Carl L. Ege

This area is located in west-central Utah within the eastern Basin and Range Province, near the southern margin of the Great Salt Lake Desert, and within the confines of Dugway Proving Ground.  Granite Peak consists of a granitic intrusion that is Late Jurassic in age (149 million years old).  The upper part of the intrusion was altered and intruded by numerous pegmatite dikes.  Metamorphic rocks of likely Paleozoic or Proterozoic protoliths are exposed at the far south end of the mountain.  These granitic and metamorphic rocks were exhumed during Basin and Range extension, likely from about 15 to 5 million years ago.  Sapphire Mountain is a Miocene-age rhyolite flow that erupted 8 million years ago.  Quaternary surficial map units include lacustrine, alluvial, and eolian deposits, and desert mudflats.  Three of the four primary shorelines of the Bonneville lake cycle are preserved on the mountains’ flanks.

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