The Andes Mountain Range
The current geography of NOA is a consequence of the geological history of one of the most impressive chains in the world: The Andes Mountains.
That is why they deserve an important place here, where they tell about their origin and evolution, and how they built, over 10 million years ago, the varied and wonderful landscape of the Argentine Northwest that we see today.
In his "Journey Around the World," Charles Darwin wrote in 1833:
"I have seen in the Andes Mountains the evident evidence that huge mountains have been broken into a thousand pieces, how a crust of bread can be broken, and that the different layers that made it up, horizontal as they were, have become vertical."
The Andes of the Argentine Northwest, show clear evidence of what Darwin affirmed.
The Andean chain extends along the western coast of South America, along some 8,000 km and reaches its maximum height on the Aconcagua hill (6,959 m) located in the province of Mendoza. Its origin is a consequence of the crinkling of the continental crust due to the thrust produced by the Pacific Ocean plate. As is known, the terrestrial globe is formed by a mosaic of continental and oceanic plates. The continental plates, lighter, are of granitic composition, while the oceanic plates, heavier, are of basaltic composition. Both the bottom of the Atlantic Ocean and the Pacific, run underwater mountain ranges, where molten material is pouring from the depths.
That magma hell is pushing the plates, which, when converging, deform giving birth to the mountains. The Pacific slab is known worldwide as "Nazca Plate" and its sinking and thrust of the continent is responsible for the rise of all the NOA hills. The permanent friction of the plates on the continental edge gives rise to tensions that are released in the form of earthquakes and hence their greater intensity on the Pacific coast.
When the plate, which in the Andean zone of the Argentine Northwest sinks with 30 ° of inclination, reaches a certain depth, it sinks and the magma rises giving birth to the volcanoes that we see along the Argentine-Chilean limit. Thus were born some of the most important volcanoes on the planet such as the Ojos del Salado (6,880 m) and the LLullaillaco (6,723 m), and the calderas (giant craters) of La Pacana and Galán, with 60 km and 40 km in diameter respectively.
On the other hand, the vapors released from the magma and richly mineralized gave rise to the formation of valuable metallic deposits, such as those of copper in the Chilean desert. The central portion of the Andes is the most spectacular for its geographical features and geological phenomena. The central Andes are the most vivid expression of how a mountain chain is formed and evolves. Its roots are located 70 km deep, so together with the Himalayas, it is the part where the bark reaches its maximum thickness.
Likewise, the Andes register one of the most important topographic highlights, since in a short distance, between the Peruvian-Chilean trench more than 7 km deep and the mountain ranges that touch the 7 km high, a drop of 14 is reached kilometres.
The Andes, seen figuratively, are like a gigantic wave whose crest coincides with the highest part of the mountain range and extends at the foot of the Chaco-Paraná plain. The wave is advancing, as evidenced by the migration of the volcanic chains that over millions of years were changing from the Pacific edge itself to the international limit where we see it today. In addition, the mountains are moving more and more to the east, due to a complex phenomenon of shortening and stacking of the land, so in some millions of years the NOA will rise and then be swallowed by the Andean devouring machine.
The Yacoraite Formation, a "rib" of the Andes
The Yacoraite Formation is one of the "ribs" of the Andes. Its name is due to the fact that it was defined and studied in the Yacoraite River in the Quebrada de Humahuaca, by Dr. Juan Carlos Turner; It was then studied by the largest specialist in the field: Dr. Rosa A. Marquillas of the National University of Salta (UNSa).
The Yacoraite Formation covers parts of Peru, Bolivia, Chile and northern Argentina and was formed between 65 and 70 million years ago, long before the Andes rose.
At that time, the NOA must have looked a little like The Bahamas, with a warm and humid climate, islands with tropical vegetation and a body of clean and temperate waters where crocodiles lived and a great variety of fish, among them the curious flying fish. On the wide beaches, the dinosaurs that left their prints for posterity were printed and that we can see today in the Tonco Valley and other places. The algae also left their testimony of carbonaceous horns, known as stromatolites.
The calcareous muds deposited at that time, over millions of years, hardened to form limestone, and preserved inside the bones of fish and crocodiles, algal structures, dinosaur footprints and even deposits of uranium, vanadium, copper and even that precious black liquid called oil. Not for nothing the wise German geologist Luis Brackenbusch when he visited Salta, at the end of the 19th century, called it the Oil Formation.
The Cuchuma well, in the limits of the Capital and General Güemes departments of the province of Salta, extracts oil precisely from levels of the Yacoraite Formation as well as the Caimancito wells in Jujuy and Palmar Largo, in Formosa.
About 65 million years ago and in coincidence with the extinction of dinosaurs, Yacoraite limestones were horizontal and at sea level. Then, within the last 10 million years, the tremendous forces of the crust that formed the Andes would come, that broke the layers of rocks, tilted them to the vertical, lifted them in some parts and sank them in others. Today, remains of the yellow limestones of Yacoraite appear at almost 5,000 m high in the Puna and 6,000 m or more deep in the Pichanal region, a vertical difference of 11,000 meters.
This gives a clear idea of how the rocks of northern Argentina were deformed, how the Andes split in two and why Darwin was so impressed on his South American trip. Like the German traveler Antony Zacharías Helms, who passed through northern Argentina in 1789 and was so impressed by the Tectonic configuration of the Andes, he was so impressed, and he left it written:
"Mountains as irregular and broken as in this part of the Cordilleras, with so much alternation of their component parts, I have not seen them in Hungary, in Saxony or in the Pyrenees. Nowhere does a revolution of nature seem to have been so general as in South America, there is evidence of it everywhere. "
The aridization of the Andes
The Andean building has changed its vegetation cover both in quality and quantity, from extensive forests that extended from the Atacama Desert to the Calchaquí Valleys some 40 million years ago to total aridity in some cases or a mantle of natural bonsai as those who occupy the Puna today. The evolution of aridity must be understood from the depth of geological time.
The Andean region in which we live has changed its vegetation cover over time in response to the progressive orogenic uprising. The Andes have suffered a progressive geological aridity. Until 40 million years ago, the Puna was a tropical region with large rivers that drained from the interior of what is now Chile to the Atlantic. The landscape conformation was different. In the middle of the present desert of the north of Chile there was a volcanic chain, whose ashes were carried by the winds of the east towards the Pacific. These volcanoes disappeared with the passing of millions of years, destroyed by erosion, and their roots today constitute the rich copper deposits of that country.
The Puna at that time was almost at sea level, and its climate was warm and humid. The discovery of fossil remains of crocodiles and turtles, without going any further a few kilometers from San Antonio de los Cobres, prove this in a restrictive way. In addition to these reptiles lived a whole range of rare mammals, obviously all extinct, consistent with the climatic conditions prevailing at that time.
But that paradise was not going to last forever. The Andean movements began to raise bark segments that gradually abolished the Atlantic drainage of the rivers and turned the Puna into an endorheic basin, that is, with internal drainage. This began to occur at least 15 million years ago, when the first remains of salt lakes appeared, expressed in deposits of salt and plaster.
The elevation of mountainous reliefs began to curb the wet winds that came from the Atlantic, making the Puna gradually become what is known as "a desert in the shadow of the rains." Meanwhile the region that today occupies the Calchaquí Valleys was a spectacular jungle, with rivers that were born on the edge of the Puna and ran freely towards the Atlantic Ocean, without reliefs that interposed. Paleontologists found fossil fish remains with representatives of the scianid and cynodontid groups that currently inhabit rivers and lakes in low regions, in Neotropical Brazil.
Thus, while the puna progressively increased its aridization as the Andean spine broke and the humidity was retained more and more to the east, the western regions became increasingly arid until they reached the extreme degrees of hyperaridity that are recorded in the Atacama Desert in Chile. The aridity is so great that not only the exotic nitrogen minerals (nitrates) are formed but also the neighboring ocean fossa is clear of sediments as there are no rivers that transport them, and reach over the coast of Antofagasta more than 8,000 m deep
The Andes continued to grow, raising its tortuous crest of a prehistoric monster that stretches in the crust of South America, changing day by day, century to century and millennium to millennium, the nature of the landscape.
About 6 million years ago, the jungles still prevailed in the Calchaquí Valleys while in the Puna powerful mantles of rock salt, gypsum and borates were deposited. The presence of the Calchaquí jungles is demonstrated by the findings of fossil trunks and leaves discovered in Angastaco by Luisa Anzótegui of the Universidad del Nordeste (Corrientes).
There, the paleontologist of fossil flora discovered numerous evidences of large trees, various types of ferns, plants related to fig trees, fossil vines, to which we must add remains of crocodiles and turtles, all of which give an environment of high rainfall (more than 1,000 mm), high temperatures, low altitude above sea level (less than 500 m); that is a warm and humid subtropical environment, which has nothing to do with the current one. As you can see, the phytogeography of the place was very different a few million years ago, with a jungle of high-rise trees that reached the buttress of the Puna.
As more and more mountainous reliefs were formed eastward by the gradual elevation and thrust of the Andean orogenic forces, more and more the eastern humid winds slowed down and therefore more and more vegetation was removed. Today the Salta jungle, well represented in the Baritú National Park, is hundreds of kilometers east of where it was a few million years ago.
The Puna reached almost 4 km high above sea level and its low xerophytic vegetation recalls a small world of bonsai. Aridity has progressively progressed, without haste or pause, to the inexorable rhythm of geological time. A new change would take place with the arrival of sedentary man and the implantation of his animals that triggered the rhythm of erosion and aridization as proved by Ing. Carlos Saravia Toledo.
Today the issue of aridization is a matter of state, so it is good to know that for a holistic analysis we must incorporate the dimension of time. Why? Because time is the way that nature prevents everything from happening at the same time.