Posted on 09/15/2003 11:53:35 AM PDT by blam
Appalachian Sands Covered N. America
Larry O'Hanlon, Discovery News
Navajo Sandstone
Sept. 12, 2003 — The vast and mysterious Sahara-like sea of sand that once covered much of western North America came from an Amazon-like river that brought sand from the far-off Appalachians, not the Rockies, geologists said.
Thick remnants of the 190 million-year-old sand sea are found today in the scenic, buff-colored Navajo Sandstone cliffs of Utah, New Mexico, Arizona and Colorado. By studying durable minerals in the sandstone called zircons, Yale University geologist Jeffrey Rahl and his colleagues have found a chemical signature that reveals their surprisingly distant Appalachian origin. Rahl's report is in the September issue of the journal Geology.
"People had speculated that the early Rockies were the source [of the Navajo Sandstone]," said geologist Jeffrey Rahl of Yale University. "This was definitely not what we were expecting."
The researchers made the discovery by measuring traces of uranium, lead and helium in Navajo Sandstone zircons. Relative quantities lead and helium, which form as uranium decays over millions of years in the zircons, allowed them to determine how long ago the minerals crystallized and then how long they remained hot inside their original mountains. Those two factors are usually quite different from one mountain range to another, Rahl said.
In the case of the Navajo Sandstone, most of the zircons match what's known of the age and cooling time of the granites in the Appalachians, not the early Rockies, Rahl explained.
If correct, the revised history of early North America would require that an Amazon-like river drained the continent from the Appalachians westward — carrying the zircons and the rest of the Navajo sands to somewhere near present-day Idaho, Montana or Wyoming. Trade winds then blew the river sands southward to create a vast dry sand dune sea.
"The big picture is that we really shouldn't be surprised by that," said University of Arizona geologist George Gehrels, who has worked on a separate Navajo Sandstone study with colleague William Dickinson, now in press for the journal Sedimentary Geology.
The Amazon today carries material from the Andes all the way across South America. The same was probably happening with a long lost river in North America, Gehrels said.
Gehrels and Dickinson's study takes the matter a bit further by working out just how much sand came from where. They estimate that about 50 percent of the sand came from the Appalachians, 25 percent came from the early Rockies and the rest from other, long-lost lesser mountains.
The Navajo sandstone is part of the Grand Canyon type section, which was the first group of rocks I studied when in school. I taught the Grand Canyon section as part of the labs for Introductory Geology as well.
The Navajo Sandstone is also an important aquifer for much of the western midwest. Very cool to learn its origin.
Now if you could find the remnants of that river you could be into some interesting digging and mining.
Note: this topic is from 9/15/2003. Thanks blam.
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| GGG managers are SunkenCiv, StayAt HomeMother & Ernest_at_the_Beach | |
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Note: this topic is from 9/15/2003. |
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| GGG managers are SunkenCiv, StayAt HomeMother & Ernest_at_the_Beach | |
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