South American geology

South America can be divided into three major geological provinces. The oldest rocks occur in the east of the continent, in the plateaus of the Guyana and Brazilian Shields, separated by the Amazon basin. To the west and north is a Palaeozoic area of broad plains, underlain by Palaeozoic rocks, and to the west again is the Andean mobile belt, lying along the length of the Pacific coast and extending along the northern Caribbean coast. This belt is of Mesozoic to Tertiary age.

The Guyana Shield This is a high plateau area of old rocks, occupying Guyana, Suriname, French Guiana, and parts of Venezuela, Colombia, and Brazil. The oldest rocks are highly metamorphosed basement plutonic complexes, mainly of middle to late Precambrian age, but including some remnants of early Precambrian complexes, together with a variety of less highly metamorphosed sedimentary and volcanic successions. These represent the remains of mobile belts which reached the final stages of their activity 2,000 million years ago. These old assemblages are unconformably overlain by a flat-lying sedimentary formation known as the Roraima Formation. This comprises several thousand metres of sandstones, conglomerates, and shales, some containing detrital diamonds. The sediments are intruded by large masses of basic igneous rock, forming thick sheets of gabbro, sills, and dykes. These were intruded in late Precambrian times, 2,000–1,800 million years ago.

The Brazilian Shield This shield area underlies the whole of Brazil and stretches through Uruguay and into the adjoining states to the west. Although covered in part by younger sediments, the old Precambrian crystalline basement is well exposed in northeastern Brazil. Most of the rocks are of late Precambrian age, and represent deposits laid down in geosynclinal zones parallel to the present Atlantic coast. A basement of older rocks has been largely reworked during late Precambrian mobile belt activity. Two major cycles can be seen; during the first cycle the rocks of the Pre-Minas Series were laid down, consisting of sediments, volcanics, and some thick ironstone formations. These were folded, metamorphosed, and invaded by granites during an orogenic phase (see orogeny) about 1,350–1,100 million years ago. The second cycle includes the rocks of the Minas Series, and makes up the wide coastal zone known as the Brazilides. The Minas Series consists of psammites, pelites, laminated ironstones, manganiferous beds, and quartzites. These were folded and metamorphosed 650–500 million years ago, and late granites were then intruded. This belt in turn was covered by an unconformable series of sediments.

In many parts of the Brazilian Shield the Precambrian metamorphic rocks are overlain by glacial deposits of very late Precambrian age, termed Infracambrian. These are similar to glacial deposits of the same age found in other continents.

Palaeozoic area Rocks of Palaeozoic age cover a large area between the old shield areas of the east and the Andean belt to the west. They consist of shallow-water sediments laid down on a platform fringing the ancient shield areas. Deposition appears to have started in the west, where Cambro-Ordovician sediments are seen in Bolivia; elsewhere the sedimentary sequence begins with rocks of Silurian or Devonian age. The Devonian sediments include glacial tillites in a number of areas, indicating a further period of glaciation in Devonian times.

Towards the end of the Upper Palaeozoic, important changes took place in the super-continent of Gondwanaland, of which South America was still a part. Beds of a continental facies accumulated in new basins of deposition at the margins of the old shield and within the shield. At the base of this succession are glacial deposits of Permo-Carboniferous age. In the Santa Catarina system of the Paraná basin these terrestrial glacial deposits reach a maximum thickness of 1,600 m/5,250 ft, and they are followed by coal measures, shales, sandstones, and volcanics, spanning a period from late Carboniferous to late Triassic times. The whole sequence forms a pile of continental sediments 4 km/2.5 mi thick.

The first phase in the break-up of Gondwanaland is seen in late Jurassic to early Cretaceous times, when plateau basalts outpoured over much of the Paraná and Amazon basins. Associated with the continental disruption are the marine evaporites and carbonates deposited in mid-Cretaceous times along the Atlantic coast. At the same time the diamond-bearing kimberlite pipes were intruded in the Minas Gerais area of the Brazilian Shield. From late Mesozoic times onwards the southern continents moved apart by a process of seafloor spreading in the South Atlantic.

The Andean mobile belt This belt at the western margin of the continent was active through most of Phanerozoic time. Thick accumulations of Palaeozoic and Mesozoic rocks built up in a geosynclinal environment, and there were episodes of orogenic activity in mid- and late Palaeozoic times, and again in late Mesozoic times, when huge granite masses were intruded. This was followed by widespread volcanic activity and uplift of the mountain chain of the Andes. The continued westward movement of South America is evident from the deep-seated earthquakes occurring below the Andes, and the deep ocean trench off the coast. Late orogenic volcanic activity continued from late Cretaceous times up to the present day, with the eruption of andesite lavas. Copper deposits are found in Chile associated with granites intruded in Palaeogene to Neogene times. In the late Tertiary, uplift was accompanied by explosive volcanic activity, resulting in vast sheets of ignimbrite and acid pyroclastic rocks, especially in eastern Chile.

Outside the Andean belt, thick continental sandstones and conglomerates were deposited from the rising mountain chain during Tertiary and Quaternary times. In the extreme north, adjoining the Caribbean mobile belt, thick deposits of marine Cretaceous, Tertiary, and recent sediments were laid down in the eastern Venezuela basin. This contains important oil deposits.