Thursday, September 6, 2012

The Rising of South America--Part II

According to the Smithsonian National Zoological Park, Amazonia is an area containing the Amazon Drainage System and Amazon rain forest, and occupies as much as 40 percent of Brazil's total geographical area. Geologically, the Amazon River and Basin, are contained by two large stable masses of metamorphosed rocks (protolith), including quartzite, quartz-mica schist, greenstone and metarhyolite, in a cup-like bowl. This cup shape includes the Guiana Shield or Highlands to the north, the Central Brazilian Shield or Plateau to the south; the Andes Mountains to the west, and opens in an eastward flow into the Atlantic Ocean.

Solimoes Basin, the largest of the ten basins that make up the Amazon Drainage System, and as can be seen, as flat as a pancake. Shown is Lago der Coari (Coari Lake) where the Amazon and the Solimoes rivers meet
The Amazon River Basin, actually there are ten different basins involved in the Drainage Area, of which Solimoes and Amazonas are the largest, and collectively they are about 4,195 miles long, covering about 2,720,000 square miles in area, including its 15,000 tributaries and subtributaries (four of which are in excess of 1,000 miles long). It is the largest river basin in the world, with much of northern Brazil drained by the Rio Negro, which joins the Amazon, called the King of the Waters by the aborigines, to give it full strength before flowing into the Atlantic. The flow of the river has an very slow average velocity of 1.5 miles per hour, which increases greatly at flood times. 

This area includes three fourths of Brazil, one half of Bolivia, two thirds of Peru, three fourths of Ecuador, and a portion of Venezuela. Little is understood about this area, even today, since it was not open to foreign observation or investigation until September 1867, and few have traveled the area besides a handful of hearty explorers. In fact, more is known about the so-called “Unknown African Interior” than that of Brazil. In fact few of the interior waterways and tributaries have ever been explored.
The mouth of the Amazon Delta as it empties the 2.7 million square miles of Amazon Drainage Basin into the Atlantic Ocean at the rate of 300,000 cubic meters per second
The powerful discharge at the mouth of the Amazon measures about eight trillion gallons a day, 60 times that of the Nile and eleven times that of the Mississippi. The annual average discharge is 6,350,000 cubic feet per second ("cusecs") into the Atlantic, rising to over 7,000,000 cusecs during a flood. The mouth of the Amazon is more than 250 miles wide.
In this lowland basin, about the size of the U.S., the ground level is barely above sea level and during six months of the year, with rising flood waters of 30 to 40 feet, is inundated with water, which would create an inland sea (as it did anciently) without the opening to the Atlantic along the east coast of Brazil.
The entire Amazon Drainage Basin of 2.7 million square miles is barely above Sea Level, and during six months of the year, is totally flooded during the wet season
According to scientists, geologists and oceanographers, the present Andes Mountains consist of deformed, squeezed up, and dissected beds of  the Tertiary and older rocks, with eruptives--mainly stratified material which was deposited at the bottom of a sea, so that at some former time the highest portions were submerged, probably in consequence, to a certain extent, of subsidence of the sea bottom. Since the latest deposits there has been upheaval and denudation. The range, then, has resulted from the accumulation of sediment on a subsiding area; from the subsequent upheaval of such deposits, which have been increased in height by the ejection of volcanic products; and from the operation of denuding agents. The width of the range varies from about 60 to 300 or more miles, but, as compared with other mountains, the Andes are for the most part narrow relatively to their height, suggesting a very young age.
During the era when mammals were present, the Amazon Drainage Basin split into two tracts along the middle of the continent by the sudden rise of the Purus Arch (a continental rise that caused water to run east and west toward the oceans). This arch ran vertically, north to south, originating as a NNW-SSE–trending graben--a trench of depressed block bordered by parallel faults, and inverted--in the opposite direction. This structural feature alternatingly separated—or connected—the Solimões (western Amazonia) and Amazonas basins (eastern Amazonia). The Solimões Formation is restricted to western Amazonia, therefore a reactivation of the Purus Arch must have happened during this time. Causal mechanism for this reactivation is ascribed to the Quechua I tectonic event which is coincident in age with deposition of the Solimões Formation.
This range is about mid-way through what is now Brazil, parallel but a couple of thousand miles east of the present day Andean Mountain chain. As the submerged land rose above sea level, the Andes mountains also rose, creating a large basin and an inland sea between the Andes and the Purus mountains. The rising of the Andes created a large lake, now known as the Solimões Basin, in Colombia, Peru and Brazil. As the waters rose in this basin it caused a break through the Purus Arch and rejoining the easterly flow toward the Atlantic. The Amazon-Solimões River now runs from Peru through Brazil, and contains Pelvicachromis pulcher, or Rainbow Krib (Kribensis) fish, originally found only in West Africa, southern Nigeria and Cameroon on the eastern side of the Atlantic from Brazil.
Currently, the Amazonas, Paraná, Parnaiba, and Solimões basins span more than 3.1 million square miles, larger than the continguous United States, and almost as large as the U.S., including Hawaii, Alaska, Guam, and other territories. This entire area in South America is a low-lying valley almost entirely covered by tropical rainforest, criss-crossed by rivers including the mighty Amazon itself.
The Amazon rainforest, also known as Amazonia and the Amazon Jungle, is a moist broadleaf forest that covers most of the Amazon Basin and is totally inundated with swamps, ponds, lakes, rivers, and bogs
As the middle and eastern areas of the continent tilted upward from the subduction of the base tectonic plates, the waters of the forming inland sea broke the sandstone to the west, allowing the Amazon to flow eastward. As the inland sea levels dropped, the great Amazon Lake rapidly drained into the Atlantic becoming a river.
The Pebasian Sea was a huge wetland system in western Amazonia, and the elevated tidal range of the Paranan Sea connecting all of western Amazonia 
The Pebasian Sea was a shallow marine environment tens of meters below sea level that extended from the Andean lift to the Purus Arch in central Brazil, covering much of the Western Amazonis. This Basin, along with that of the Paranan Basin to the south, were deformed and uplifted and eventually drained by the subsequent thrust deformation of the Andes.  

Rio Parana River, all that is left of the once huge Paranan Sea that stretched from Colombia to Peru just east of the present day Andes
The Parana River is all that is left of the Paranan Sea--the name Parana is an abbreviation of the phrase “para rehe onava,” which in indigenous Tupi language means “like that sea,” that is, "as big as the sea.” This sea was to the south of the Pebasian Sea (mentioned above) and formed a waterway extending from the east of the present day Andes eastward along the several basins surrounding the Brazilian Shield.

Recently, Dr Drew Coleman, a geology professor, and Russell Mapes, a graduate student at University of North Carolina, Chapel Hill, have found geological backing for these theories. They were studying sedimentary rocks in the upper Amazon basin when they found ancient mineral grains in the center of South America, which clearly originated in now-eroded mountains in the eastern South America. According to their findings, if the Amazon had continuously flowed eastward, like it currently does, much younger mineral grains in the sediments, originating from the Andes, should be found.  "We didn't see any," Mapes said of the absence of such younger material. All along the basin, the ages of the mineral grains all pointed to very specific locations in central and eastern South America.
"It just happened in a way that the current Amazon could take advantage of where an old river and ocean basin used to sit," said Mapes. Working with his UNC faculty advisor Drew Coleman, and Brazilian colleagues Afonso Nogueira and Angela Maria Leguizamon Vega, Mapes found that the age of rocks on the South American continent differs between east and west. Sediment from the Andes, containing young mineral grains, filled in the basin between the Andes and the Purus, the river was blocked in the lake and after that forced eastward. The team collected zircon samples, a common mineral easy to date in order to see the age of the sediment's source, from about 80% of the Amazon basin. "The finding,” Mapes said, “helps illustrate that the surface of the earth is very transient. Although the Amazon seems permanent and unchanging it has actually gone through three different stages of drainage in a short period of time geologically speaking."
(See the next post, "The Rising of South America - Part III," for more on this event that lifted the Andes Mountains from their level setting, lifted the eastern half of the continent and formed South America as we now see it)

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