Monday, September 23, 2013

The Amazon River Changed Direction

For the past few years I have been selling the Jequitinhonha Agates from Brazil.  Two previous blog postings describe these amazing agates that are proof that South America used to be directly connected to Africa (July 22, 2011 and Sept 24, 2012).  These agates were naturally polished by the diamond dust contained in the sand travelling down the river currents.  The diamond dust was deposited in the Brazilian rivers during the time of Pangea, which was the last supercontinent assembled by the powers of plate tectonics.

Pictures of some Jequitinhonha River agates are below.
Previous studies have shown that some segments of the Amazon Basin once flowed opposite their present course.  Back in the 1960s geologists discovered proof that the Brazilian rivers flowed in the opposite direction.  They also found diamonds in the rivers, later proving that the source of the diamonds was in Africa. Pictures of the Amazon are below.
The diamond fields of Africa are shown in yellow below.  Not only did the Amazon River change direction, but so did the Jequitinhonha River where the diamond dust-polished agates were found (shown as red dot below).

In 2004 researchers were studying the speed at which sediment in the Amazon travels in its current west to east direction -- from the Andes mountains to the Atlantic. While studying sedimentary rocks in the river basin they discovered ancient mineral grains in the central part of South America that could only have originated in now-eroded mountains located in the eastern part of the continent.  This proved that at some time in the past the Amazon River had to flow in the opposite direction -- from east to west. 
Scientists determined that the sediment washed down from a highland area that formed during the Cretaceous Period, between 65 million and 145 million years ago.  This period occurred during the breakup of Pangea, when the South American and African tectonic plates separated and passed each other. That highland tilted the river's flow westward, sending ancient continental sediment as old as 2 billion years toward the center of the continent. Toward the end of the Cretaceous, the Andes mountain chain formed.  As these mountains grew, the higher elevation caused the Amazon River, as well as other Brazilian rivers, to reverse direction and flow back toward the east.  This progression is diagrammed below.


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