Before the Mississippi, minerals show ancient rivers flowed west
Michigan zircons uncover path of ancient river system across North America
By Sid Perkins
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.
A 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.