Friday, September 7, 2012

The Rising of South America—Part III

Anciently, to the east of the present day Andes, a sea ran from Colombia to the Parana-Paraguay Basin, almost to the archipelago of Tierra del Fuego, which was called the Pebasian Sea in the north and the Paranan Sea in the south. Further east, south of the Chaco foreland basin was the Paranense Sea covering a wide area in northern Argentina and Uruguay and except for a small coastal strip of present day Chile, covered what is known as Patagonia and was what we call the Atlantic Ocean today.

Map of the ancient seas (dark blue) covering much of  South America (light green), including the highland areas, depressions, and the Purus Arch (red) that later blocked the Amazon Sea’s outflow into the Atlantic.
Note the Thrust Front (above) along the West Coast of South America. The Nazca tectonic plate moved eastward and subducted beneath the South American tectonic plate, causing the latter to rift, tilting the South American plate upward along the east coast.
(Image B – When the sea covering Amazonia receded, the Pebasian Sea became a river. This Pebas River today runs past the city of Pebas where it empties into the Amazon River. This is all that is left of the once gigantic Pebasian Sea, the northern arm of the sea that covered most of South America from Patagonia to Colombia
According to Lovejoy, Albert and Crampton, South America foreland basins (depressions) were areas of constant marine incursions, which occurs when saltwater is drawn-in from the Sea into freshwater aquifers and basins that are below sea level. There were, of course, a few eustatic sea-level rises to break up the open sea. According to Lundberg, these incursions occurred along several lowland areas of foreland basins adjacent to the Andean Chain.
Some of these shallow marine (sea) incursions spread into southeastern Bolivia and into several of the intracontinental basins in South America’s Pebas formation in Brazil, Peru, Ecuador, Colombia, and Venezuela. The loading (rising) of the Andes was responsible for the marine transgression of the Paranense Sea that invaded most of the Andean foothills between 40º South and the Maracaibo area. This transgression—the spreading of the sea over land as evidenced by the deposition of marine strata over terrestrial strata—shows a relative rise in sea level resulting in deposition of marine strata over terrestrial strata. The sequence of sedimentary strata formed by transgressions and regressions provides information about the changes in sea level during a particular time.
According to neotectolnics expert, Allaoua Saadi, along the core area of the Brazilian shield there are also enclaves of tectonically developed lowland areas or depressions that were not directly related to the evolution of foreland basins but to the constant tectonic reactivation events undergone by the complex system of faults of the crystalline basement. Examples of such areas are the Araguala and Tocantins depression 
The map shows sedimentary basins in yellow, with the highlands in red. The sediment in the yellow areas was laid down from marine coverage (seas), which extended beyond this map of Brazil into Bolivia, Paraguay and eastern Peru

Ramos and Aleman claim an Amazon Sea stretched from the present day Andes to the Atlantic Ocean, along what they termed the Amazon Arm, (see top map) then along the Tethys Arm to the Paranense Sea that was open to the Atlantic along some 2000 miles of the east coast of Patagonia (Argentina).
In 1909, C. H. Eigenmann, in his Reports of the Princeton University to Patagonia 1896-1899, was the first to recognize the cis-Andean (Amazonia and Guiana) foreland basins and associated lowlands in South America as possessing an ichthyofauna (fish life of the region) distinct from the river systems draining shield areas. His “Amazon Province” is the combination of the lowlands of the Orinoco, Amazon, and La Plata basins, in his words “the most extensive and intricate fresh water system in the world…a network of rivers practically uninterrupted, extending from the mouth of the Orinoco through the Cassiquiare, Rio Branco, Rio Negro, Rio Madeira, Rio Guapore, Rio Paraguay, Parana and La Plata to Buenos Aires.”
He also recognized the relative youth of the lowlands when compared with the highlands, as well as the ichthyofaunistic similarities between the Amazon and La Plata basins. However, most subsequent authors failed to appreciate the distinction between the ichtyhyofaunas from the lowlands and shield areas.
In addition, he analyzed sediments from two cores taken northeast of the mouth of the Amazon River and concluded that the Amazon River formed through a three-stage sequence, which conflicts, in part or wholly, with virtually every other recent proposal regarding the evolutionary history of the Amazon Basin. And Figueiredo described the onset phase as being prompted, in part, by global sea-level fall. Laurens showed that global sea level is important because a sea level of 150-feet below mean sea level is the threshold value required before sediment can reach the Amazon Fan, as opposed to being distributed northwestward along the coast by the North Brazilian Coastal Current.
Finally, sampling core drilling and finds along the eastern slope of the Andes where Darwin found sea shells samples at 13,000 feet, and the work in the Argentine Puerto Madryn formation shows numerous sea fossils at high altitudes, now far above sea level, but obviously once beneath the sea.
The Madryn Uplift where sea shells and numerous marine fossils have been found far above sea level, showing this entire area was once underwater. Top: Puerto Madryn formation in Argentina: LtoR, top to bottom 1) Fosssiliferous layers; 2) Bivalves; 3) Cirripeds; 4) Bryozoa; 5) Ostreids; 6) Gasteropods; 7) Oysters; 8) Diatom; and 9) Radiolarian
All of this shows conclusively that 1) most of South America east of the Andes was at one time covered by the sea, and 2) the numerous depressions (lowlands) of the continent are near, at or below sea level even today.

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