Argentina Geology Part 2

By | January 11, 2022

The great Cenozoic uplift of the Andes is in chronological order the last purchase of the Argentine territory towards the west. The lands of the Mesozoic itself, from the Trias, were affected. Lias and Jura up to the Cretaceous, with the eruptive expansions interspersed in the same series, in a first phase that falls in the ancient Tertiary, perhaps a continuation of the previous uprising of the Patagonids. After this, which was the main phase, two more were recognized, very recent because Pliocene, manifested even better than with foldings, with carriageways similar to the Alpine ones, preceded, accompanied and followed by eruptive manifestations with very extensive tuffs, activities that are still in place today. efficiency in the numerous volcanoes of the Andes.

The main chain is formed, in its northernmost trunk, from the Bolivian border almost to Neuquén (39 ° lat. S.), by three well-characterized longitudinal zones, two of which eruptive on the sides, and an intermediate sedimentary one. The eastern area, of quartz porphyry, reaches S. as far as the Río Atuel; the sedimentary, Mesozoic, is lost further to the South. still in the plateaus of Patagonia. Finally, the western one extends far beyond the previous ones with its porphyrites, so as to form the eastern edge of the Patagonian mountain range. This last cordillera is formed by a bundle of folds, the northern extension of which plunges into the Pacific, parallel but flanked in the west to that of the Chileno-Argentine Andes. Continuing towards S., the Patagonian Cordillera, however, abandons the meridian direction preserved by the system for such a long stretch in two hemispheres, and curves towards the E., until it settles on the margins of the Strait of Magellan in the direction of the parallels. Also in its constitution the Patagonian Cordillera differs from the main one; although it also consists of Mesozoic strata, these are strongly metamorphosed, and therefore very different from the Andean Mesozoic proper.

The grandiose Andean movement was not accomplished without having repercussions on the pre-existing structures to the east, and it determined. in them the dislocations, fractures and overthrows, already mentioned, similar to those that occurred in the Alps. Furthermore, arching would have been produced in the more distant masses.

However, in the Cenozoic the great Chaco-Pampa lowlands remained a region of negative movements and therefore of accumulation of sediments, mainly of a purely continental nature. In Patagonia, on the other hand, various epeirogenetic movements had the final result of a block lifting. In the two territories, however, and especially in the second, where they are best visible, powerful and extensive Tertiary sediments were deposited in the Cenozoic, starting with the upper Eocene, in which the fossil remains of those singular mammals whose discovery and whose study constituted the glory of Florencio Ameghino. This tertiary series, which includes both the Paleogenic and the Neogenic, thanks to the alternation of faciesmarine and terrestrial, it was able to subdivide into numerous floors and sub-planes (Patagonico, Notostylopense, Santacrucense, Entreriano, Rionegrense, Araucanense, Calchaquense) and gradually passed to the Quaternary. Less rich in development is the Tertiary of the foot of the Andes (Subandino), more influenced by local conditions.

In Patagonia, a particular formation known as the Patagonian rodados (pebbles) is widely spread over the undisputed Tertiary, a subject of divergent interpretations, which was also meant to refer to the Quaternary Glacial, although it goes far beyond the limits of truly ascertained glacial expansions. The opinion that collects the most votes is that it is the product of a Quaternary reworking of a similar previous Pliocene formation.

About two thirds of the surface of Argentina belongs to the Quaternary. It appears mainly in the two forms of glacial and aeolian deposit. In the high areas of the Andes, moraines are found due to multiple glaciations, which descending towards S. gradually become more impressive, until in Patagonia they also invade the eastern plateaus, from Neuquén downwards. However, these are always mountain glaciers with an outlet on the plain; we can speak of a true Quaternary continental ice only in the extreme southern limb, in Tierra del Fuego and in the vicinity of the Strait of Magellan.

An incomparably larger area is covered by the other Pleistocene formation, the Pampeana, the Argentine formation par excellence, which as a whole is considered as a loess, of Aeolian origin. It spreads its uniform mantle over an area of ​​much more than one and a half million square kilometers. The complex, being a deposit, even better than a continental one, terrestrial in the proper sense, is formed not only by the predominant aeolian types, but also by alluvial types of each species, depending on the different geographic climatic conditions of the large covered area, which from the arid climate of its southern and western parts, to the areas of tropical rains. The characteristic fossil fauna contained therein, which has become famous especially because of the great toothless like the Megatherium, the Glyptodon, etc., descendants of the Patagonian tertiary faunas already mentioned above, was widespread from the southernmost regions to the borders of Bolivia, where in Tarija it is mixed with forms typical of North America (Mastodon, etc.), evidently emigrated after the meeting of the two continents and which were spreading towards the south. All these gigantic forms, both of the indigenous Toothless and of the emigrated Proboscidians, died out in the Pleistocene, except one that must have been preserved until relatively close times, the jemi š (Glossotherium) of the Patagonian tradition, which seems to have been kept in domesticity by the ‘man.

During this period, and thanks to the climatic variations that distinguish it, the hydrographic systems were determined, and the soil took on the aspect under which we see it. They are widespread, especially along the marine and fluvial lítorale and in the arid and desert parts of the lowlands as well as the plateaus of the Puna, the dunes (medanos), other continental Aeolian formation.

The carbonic formation (Antracolitico) proper is known in several localities of the Precordillera, in the provinces of San Juan and Rioja with coal banks, which have given rise to industrial attempts. Coal banks are also found in the upper part of the Gondwana formation, corresponding to our Rhaetian (upper Trias) and also in more recent soils. However, coal production has so far been very small.

A thriving industry in full and promising development is based on the oil fields. The oil fields discovered in Argentina in different soils, due to the migration of the liquid, are however always linked to the presence of formations from the Middle and Upper Mesozoic (Lias, Jurassic, Cretaceous) which have therefore received the epithet of oil. They were found under more recent ground covers, in Comodoro Rivadavia, in Plaza Huincal in Neuquén, in Mendoza, and in Salta near the Bolivian border. Although the petroleum indications had been known for a long time in various locations, today’s industry was born in 1907 from a happy accidental discovery in Comodoro Rivadavia, on the shores of the Gulf of S. Giorgio in Chubut, where there was no evidence whatsoever, due to drilling for drinking water. From that point began the development of the extraction of the precious liquid, whose production in 1928 exceeded 1,400,000 cubic meters.

Argentina Geology 2