The world’s largest river basin, the Amazon, once flowed from the Atlantic Ocean to the Pacific – opposite to its present direction – according to research by a geology graduate student and his advisor at the University of North Carolina at Chapel Hill. Graduate student Russell Mapes set out in 2004 to study the speed at which sediment in the Amazon travels from the Andes mountains, in the present headwaters of the river, to the Atlantic. While studying sedimentary rocks in the river basin he discovered something else – ancient mineral grains in the central part of South America that could only have originated in now-eroded mountains in the eastern part of the continent. If the Amazon had continuously flowed eastward, as it does now, Mapes would have found much younger mineral grains in the sediments from the Andes. Mapes explains that these sediments of eastern origin were washed down from a highland area that formed in the Cretaceous Period, between 65 million and 145 million years ago, when the South American and African tectonic plates separated and passed each other. That highland tilted the river’s flow westward, sending sediment as old as 2 billion years toward the center of the continent. A relatively low ridge, called the Purus Arch, which still exists, rose in the middle of the continent, running north and south, dividing the Amazon’s flow – eastward toward the Atlantic and westward toward the Andes. Toward the end of the Cretaceous, the Andes started growing, which sent the river back toward the Purus Arch. Eventually, sediment from the mountains, which contained mineral grains younger than 500 million years old, filled in the basin between the mountains and the arch, the river breeched it and started its current flow. Previous research has identified a reverse flow, but only in segments of the river. Mapes traversed about 80 percent of the Amazon basin.

Abstract: Evidence For A Continent Scale Drainage Inversion In The Amazon Basin Since The Late Cretaceous

Diagram