Birds of the Pampas Plain

                  Geology and Geomorphology 
                                                                                                                                                                                                                                      

Geology

The described zone is located in the area called “Chaco-Pampeana Plain”. It consists of a very large morphologic unit, and covers a great part of South America, from the plains of Colombia and Venezuela in the North, to the extensive Argentinean Pampas.

Its main characteristic is the lack of relief. It coincides with a large zone of the crust with a big stability, of negative tendency where marine and continental sediments have been accumulated in the geologic past, making wide sediment series, which come from higher areas in the west continent.

The most modern deposits have a wind origin made of Pampas loess and chestnut muddy-sandy sediments, inserted with light sandstones. It is not uncommon to find chalky and gypsum concretions. The fluvial sediments are only in a few river basins of the zone. The presence of volcanic ashes as a result of eruptions in the Andes Mountain Range, make the quality of drinking water not to be optimum because of the presence of several quantities of arsenic in it. 

In the south, we can find the Northern Hills of Buenos Aires or Tandilia System. It is a discontinued mountain range which faces North-West to South-East and covers 300 km from Olavarria until Corrientes Cape in the Mar del Plata coast. Its maximum width is 60 km and its height is 524 meters above sea level in “La Juanita”. They are precambrian rocks made of granite where different sedimentary formations settle in a discordant way (Metapelites, cuarcita, clays, limestones and Dolomities) with different development according to the area. Some of them allow commercial exploitation especially in Olavarria zone. 

In the north, the plain is interrupted by the presence of the Australes Hills in the province of Bs.As. or the Ventania System. It is a mountain range which faces North-West to South-East and it extends 180 km with a maximum width of 60 km. These hills are exclusively made of Precambrian granite. The relief - rougher than in the system of Tandilia and with heights that reach 1,247 m.a.s.l. in Tres Picos Hills – stands out in the landscape because of the heights of the hills.

Geomorphology

    From the geomorphologic point of view, this area is made up of the “Raised Pampas” - foothill zone which extends from the Pampas Hills to the East, up to the Depressed Pampas - which divides it from the “Waved Pampas” in the Argentinean river coast as it happens with the “Salada Basin”. The “Sandbank Pampas” is located in the west and the “intermountain Pampas” is located in the south.

     It is supposed that this higher zone within the Pampas, is influenced by the greater altitude of the crystal clear basement, broken in sub-southern way relative to the subsoil of the Depressed Pampas, meanwhile in the south a new fault has raised the basement generating the Waved Pampas and debility zones where great rivers flow. 

The Raised Pampas is characterized by the presence of countless basins of wind deflation that generate sub-circle depressions, without communication, bad drainage and currently occupied by lakes.

The isohyet of 500 mm divides a zone with more humidity, located in the east – which covers the south of Santa Fe, part of the south of Cordoba and the northwest of Bs.As.- from a more arid zone to the west – La Pampa, south of Cordoba and San Luis. These zones are characterized by the presence of lakes in the east and saltpeter beds, salt marshes and sandbanks in the west. 

The Depressed Pampas is lashed by constant floods which cause a great damage to the agriculture and people who are based in that area. This is a lower zone, flat relief with a smooth slope to the southeast and a bad drainage. 

The Waved Pampas with its undulations have ravines, low lands and fluvial terraces caused by a smooth slope of the crystal basement and the erosion of the rivers.

The ravines of Parana, in the city of Rosario, the flooded areas with elevations under the Rio de la Plata level (Lujan, Riachuelo y Reconquista Rivers) and the terraces which decorate the riversides, are clear examples. 

The Sandbank Pampas in the west of Bs.As. Province is a sandbank flat land without natural gradient and with waterproof subsoil. These characteristics make the rainwater not to drain forming an areic basin.