Sent: 01/28/99
Contact Information: P. Jonathan Patchett 520-621-2070 [email protected]
James Gleason 520-621-7984 [email protected]
Continent 'awash' in Paleozoic mountain sediment
****THE PUBLICATION, SCIENCE, HAS EMBARGOED THE RELEASE OF THIS STORY UNTIL 4 P.M. EST (2 P.M. MST) THURSDAY, JAN. 28, 1999 ****
Geoscientists have discovered that sediments from two ancient major mountain systems were deposited over the entire North American continent between 450 million and 150 million years ago. About 150 million years ago, they add, a new mountain system appeared and became the major source of sediment carried over the continent.
It is the first time scientists have documented with geochemistry the origins of sediment at the scale of a whole continent.
The river-borne sediments deposited over North America for 300 million years came from the Caledonian Mountains that formed in Greenland and the Appalachian Mountains that formed in the eastern United States during the Paleozoic era 450 million years ago. This is when jawless fishes swam the seas and life was just beginning to appear on land.
The geoscientists used a naturally occurring, long-lived radioactive isotope called "Neodymium-143" to find the origin of shale deposits created by the compaction of the sediments over time. They studied sedimentary rocks from all parts of North America, from Texas to far northern Canada.
The shale is evidence that sediment was eroding from the Caledonian and Appalachian Mountains and was carried across North America 450 million years ago. The shale deposits also show that sediment from the same mountains continued to erode and cover North America right up to the Jurassic period of the Mesozoic era, 150 million years ago, when reptiles and conifers dominated life on land.
"We were surprised that sediments were eroded from these mountains so persistently through time and so constantly over all of the continent," said P. Jonathan Patchett, professor of geosciences at The University of Arizona in Tucson. "It is important because it shows that once mountains exist -- even if they are very far away -- they will dominate all the sedimentary detritus on an entire continental region for a very long time."
Patchett, Gerald M. Ross of the Geological Survey of Canada and James D. Gleason of the UA Lunar and Planetary Laboratory report the research in today's issue (Jan. 29) of Science.
Before the Caledonian-Appalachian mountains existed, as far back as 600 million years ago, at the end of the Precambrian era, sedimentary deposits around North America were regional deposits, the scientists report in their journal article, "Continental Drainage in North America during the Phanerozoic from Nd Isotopes." Sedimentary rocks in the Canadian Arctic and also Alberta clearly had eroded from very ancient rocks known as the Canadian shield. These rocks date from between 3 billion to 2.5 billion years old, and from between 1.9 billion to 1.7 billion years old. By contrast, sedimentary rocks in the eastern and southern United States formed from the younger continental crust present under most of the Great Plains.
"The earlier picture is much more regionalized," Patchett said. "The sedimentary material was eroded from rocks in the hinterland, much as the Mississippi River carries material down from the Mississippi Basin today. But, suddenly, at 450 million years, everything changes. Suddenly, all the sedimentary rock has the same signature, and it appears to come from the distant Paleozoic mountains."
Much later, 150 million years ago, there is another sudden change, Patchett and his colleagues report. Suddenly -- at least sudden in terms of geologic time -- the Cordillera begins to grow like a backbone across western North America. (The Cordillera is the system of mountain ranges that includes the Rocky Mountains.)
"Suddenly, new and different kinds of sedimentary detritus spreads across the continent," Patchett said. "That's the paleogeographic aspect of our story, that when you make mountains, you make lots and lots of sedimentary material that dominates the surface of that continent until the next mountains appear."
"What we have discovered is not some incredibly new, revolutionary thing," he added. "But it's a way of thinking about mountains and sedimentary rocks on a grand scale as few people really have before. Earth scientists thinking on the large scale may have thought about the surface of the continent this way, but they didn't have all of these nice data."
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