The geological knowledge of Argentina, which begins with the works of D’Orbigny (1842), Darwin and Burmeister, thanks to the interest of the government of the republic as well as the initiatives of private explorers and scholars, has made by now such progress that it is possible to sketch, albeit in broad strokes, a fairly complete general picture of the constitution and structure of the country, and of the progress from remote geological eras to the current state.
The Argentine stratigraphic series is complete, as all known systems are more or less widely represented. However, while for most of the Paleozoic their physiognomy remains generic and without profound differences with those of other countries and continents, it goes instead, starting from the Carbonic, acquiring its own features, which with the progress of time become more and more decisive and characteristic. The first grandiose manifestation occurs in the Permo-Trias, where the particular facies of the two systems specific to today’s southern hemisphere appears, with the flora of Gangamopteris and Glossopteris, and the indisputable glacial deposits, such as were found in southern Africa (Karroo), Australia and India, continental and coastal formations of the great ancient continent of Gondwana or Brazilian-Ethiopian. The regional character is accentuated in the Mesozoic, but it is above all in the Tertiary that it assumes this imprint and multiplicity of aspects, especially for the richness and singularity of the fossil forms, to have acquired universal notoriety.
The oldest continental nucleus of Argentina is the residue preserved in the eastern part of South America, of the Brazilian-Ethiopian continent of Gondwana, already mentioned above, a very important nucleus, designated with the name of “Brasilia”, consisting of a very ancient audience, remained rigid, over which successive transgressions left Paleozoic sediments a first time up to the Devonico, and subsequently those of the Permo-Trias or Gondwana series. Widely outcropping at E. and N. of the South American continent, it remains, as far as it shows itself on the surface, for the most part outside the political borders of the Republic, penetrating only into the territory of Misiones, and the crystalline islet of Martín García at the mouth of Uruguay. But the “Brasilia” it extends widely beyond its visible limit towards the west and south, forming the substratum now more now less close to the surface, of the recent and very recent formations of the eastern and middle part of Argentina, the Chaco and the Pampa. This is demonstrated by some sporadic outcrops in the province of Corrientes, and by drilling, which towards S. met it at 300 and 400 m. deep, respectively in Buenos Aires and La Plata.
The two vast regions just mentioned, the Chaco and the Pampa, completely surround the outcropping part of the “Brasilia” to the West and S. and correspond to a lowered area, covered by recent and very recent formations. The basin thus formed, relatively shallow in the S., as seen with the drilling of Buenos Aires and La Plata, is much more so towards the N. A drilling of 2000 m, in Alhuampa (Santiago del Estero) at about 27 ° of latitude S after 800 m. of floods, crossed for the remaining 1200 m. a red sandstone of uncertain age, without reaching the rocky bottom of the “Brasilia”.
Beyond this huge depressed area, towards S., a second very ancient nucleus has been reported, in particular by the studies of Keidel, which stretches from the Río Colorado to the Gulf of San Matías, consisting of schists, psammites and granites, to which Keidel himself gave the name of “Patagonia”, in the paleogeographical sense, and therefore with a different extension from the current homonymous country. Of this ancient continent, perhaps a fragment of the ancient primitive Gondwana, already detached in remote time, which the Keidel proves to have remained independent from “Brasilia” until the late Mesozoic, the Falkland or Malvinas islands, where the Devonico is known, would also have been part . The oldest “Patagonia” assizes, however, have very limited outcrops, because in most of those plateaus,
As happened for the whole American continent (see America), Argentina was formed, starting from the two positive elements described above, with successive enlargements towards the west, that is from the Atlantic towards the Pacific, thanks to the sediments that were deposited along the contours, both in epicontinental seas, and thanks to the epeirogenetic movements, they invaded the areas that first emerged more or less partially, both in deep geosynclines, which orogenetic movements have repeatedly raised in mountain chains.
With some exceptions, which will be discussed later, most of the orogenic foldings that occurred at long intervals, from the Pre-Paleozoic to the Cenozoic, have an almost meridian direction, that is, close to that of the Andean system, and manifest themselves to the west of the large negative area pit Chaco-Pampa, in order to make the distinction between successive folds very laborious.
The most ancient of all, perhaps Pre-Paleozoic, is found in the extreme NW flap. Argentina, west of the Chaco and east of the very high plateaus of the Puna, in the sub-Andean Sierras of the provinces of Salta and Jujuy, up to Tucumán. The cambrosiluric sediments are in clear discord on a proterozoic and archaic platform, folded and abraded, the only indication of a precambric folding, in soils that, however, unlike the contemporary ones of Brasilia, have not remained immune from the repercussions of the rear foldings, and of the tertiary one of the Andes in particular, which has caused dislocations and imbricated structures.
The so-called Sierre Pampeane, along the western edge of the Pampa, and the Precordigliere, in the provinces of Rioja, San Juan and Mendoza, also made up of exclusively Paleozoic lands, are due to more recent folds, but always Paleozoic. In the Precordigliere in particular, the soils of the Permian (lower Gondwana) are also affected by folding, which has led to the name of the mountains due to this dastrophism Gondwanids ; a later movement than the Hercynian, which instead occurred in the middle Carbonic, and of which it could be a posthumous one. The sierras of the province of Buenos Aires are attributed to the same folds, although not by all explorers, which emerge isolated from much more recent terrains than the Pampa, in two groups: one northern, the Sierra del Tandil, the other southern, the sierras of Pillahuincó and Ventana. The first, where pre-Cambric rocks such as gneiss and mica schists with granite, covered by paleozoic layers predominate, has been interpreted differently, now as a simple reappearance of the substrate formed by Brasilia, now instead connected with the aforementioned Sierre Pampeane (Tandilia). In the south, however, the presence of permic terrain implications with facies glacial, prompted the Keidel to assign them to the Gondwanids. For these sierras of Buenos Aires it is worth noting the ONO.-ESE. Trend, very different from the meridian one of the Sierre Pampeane and the Precordigliere, which according to Keidel refers to relations with similar reliefs in southern Africa. This meant that the Steinmann regarded these structures as leftovers of an ancient folding chain that, coming from the Sierre Pampeane of western Argentina, circled Brasilia at the height of the Tandil-Ventana sierras, and continued across the Atlantic towards southern Africa.
In Patagonia there are signs of a higher Mesozoic (mesocretacic) orogeny, perhaps a precursor movement of the Andean Cenozoic, a religion of which Patagonia which had remained independent until then would have been joined to Brasilia. The resulting chain was called by the Imidel of the Patagonids; in it, above folds formed by the Middle Cretaceous, the Dinosaurus layers of the Upper Cretaceous are undisturbed in discordance.
