Ancient Rivers Flowed West
Like vacationers taking a pit stop on a long road trip, zircon mineral grains from the northern Appalachians may have stopped off in Michigan before ending up on the Colorado Plateau, a new study suggests. The finding, reported in the June Geology, is a major boost to the notion that a continent-spanning, Amazon-like river system once carried sediments west across North America.
sciencenewsA large proportion of the zircons found in Jurassic-era sandstones throughout a Texas-sized portion of the Colorado Plateau originated in the Appalachians (SN: 8/30/03, p. 131), previous analyses have shown. Those erosion-resistant mineral grains were carried westward by an immense river, deposited on floodplains and then stirred back up innumerable times before ending up in massive dune fields that later solidified into western sandstones, says William R. Dickinson, a geologist at the University of Arizona in Tucson.
The new find by Dickinson and his colleagues is the first to identify any of the Appalachian zircons’ rest stops on their long journey west. Rocks of the proper age are rare in the Midwest and the northern Great Plains, where the river presumably ran. Those rocks either eroded away long ago or are covered by thick layers of soil and glacial debris scraped south from Canada during recent ice ages, says Dickinson. The only easily accessible outcrops of such rocks in central North America are in central Michigan, where quarries reach deep enough to expose mid-Jurassic sandstone.
In their attempt to trace the transcontinental river system, the researchers took a 20-kilogram hunk of Michigan sandstone from one of those quarries and then extracted and analyzed its zircons. About 40 percent of zircons in the sample were between 905 million and 1.3 billion years old, and about 10 percent were between 285 and 510 million years old. Most if not all of these zircons probably eroded from the northern Appalachians, the researchers suggest. The overall age distribution of zircons from the Michigan sample is strikingly similar to those found in the sandstones of the Colorado Plateau, a hint that all of the zircons eroded from the same sources.
“It’s not as good as a barcode, but the match is really quite good,” says Scott D. Samson, a geochemist at Syracuse University in New York who was not involved with the work.
The new study notes one prominent difference between the Michigan and Colorado Plateau sandstones: While the western samples contain abundant zircons ranging between 510 and 725 million years old, zircons of that age range are completely missing from the Michigan sample.
Most likely, the researchers note, zircons of that age originated in the southern Appalachians, and tributaries draining that area would have fed into the large river that carried material west to the Colorado Plateau but not as far north as Michigan.
To garner more evidence for a Jurassic-era, continent-crossing river, researchers could scour ancient sandstones for other erosion-resistant minerals such as monazite, Samson says. Bits of that mineral are rarer than zircons but have a distinct chemical signature that could help researchers pin down their origins, he notes.