Geology of Bolivia

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Map of the large-scale geology of Bolivia
Map showing the relief of Bolivia
Major geological units (left) in Bolivia coincide with topography (right)
Geological units:
  Quaternary deposits
  Quaternary volcanics
  Tertiary deposits
  Cretaceous-Tertiary volcanics
  Cretaceous age rocks
  Devonian age rocks
  Silurian age rocks
  Ordovician-Silurian age rocks
  Precambrian undifferentiated

The geology of Bolivia comprises a variety of different lithologies as well as tectonic and sedimentary environments. On a synoptic scale, geological units coincide with topographical units, to begin the country is divided into a mountainous western area affected by the subduction processes in the Pacific and an eastern lowlands of stable platforms and shields. The Bolivian Andes is divided into three main ranges; these are from west to east: the Cordillera Occidental that makes up the border to Chile and host several active volcanoes and geothermal areas, Cordillera Central (in some contexts also called Cordillera Oriental) once extensively mined for silver and tin and the relatively low Cordillera Oriental that rather than being a range by its own is the eastern continuation of the Central Cordillera as a fold and thrust belt. Between the Occidental and Central Cordillera the approximately 3,750-meter-high Altiplano high plateau extends.[1] This basin hosts several freshwater lakes, including Lake Titicaca as well as salt-covered dry lakes that bring testimony of past climate changes and lake cycles. The eastern lowlands and sub-Andean zone in Santa Cruz, Chuquisaca, and Tarija Departments was once an old Paleozoic sedimentary basin that hosts valuable hydrocarbon reserves. Further east close to the border with Brazil lies the Guaporé Shield, made up of stable Precambrian crystalline rock.

Andes[edit]

6,542 meter high Nevado Sajama is one of several volcanoes along Cordillera Occidental

The Andes of Bolivia began to rise about 200 million years ago (mya) during the Jurassic. The western margin of what is now South America had been the place of several other orogenies before the Andes rose. It has been hypothesized that the central Andes gained its great height 26 to 14 mya as result of a compressive failure of the lithosphere beneath Bolivia and neighboring areas.[2] The great heights of the Altiplano, Codillera Occidental and Cordillera Oriental are isostatically compensated by an up to 70 km deep crust. It is not known to which extent climate changes induced by the rise of the Andes caused the extreme aridity of Atacama Desert and adjacent parts of Bolivia or if a preexisting desert climate and associated low erosion rates allowed the mountains to build up to their current heights.[3][4][5] A hypothesis holds that scarce sediment supply to the Atacama Trench caused by arid climate induced high shear stresses in the subduction process that enhanced the Andean mountain building.[6]

Western Cordillera[edit]

The Western Cordillera is made up by a series of active and extinct volcanoes rising from the western edge of the Altimplano plateau. This range divides the Chilean watersheds of Salar de Atacama and the Pacific from the endorheic Altiplano basin. The western volcanoes of Bolivia are part of the Central Volcanic Zone of the Andes, a major upper Cenozoic volcanic province.[7] Volcanic eruptions in Bolivia are scarce, the latest one occurred in Irruputuncu in 1995. Volcanic hazards do not represent any threat to the major populated centres which are all in the eastern Altiplano or further east, far away from the volcanic centers. Although the Western Cordillera concentrates most active volcanoes (volcanoes active in the last 10000 years) many old large stratovolcanoes rises up to 100 km away from the main line of the Western Cordillera.

Altiplano plateau[edit]

Laguna Colorada in the southern Altiplano lies just east of Cordillera Occidental's volcanoes (background mountain).

The Altiplano plateau or Meseta del Collasuyu to differentiate it from other Andean high plateaux is a wide and long-lived intermontane sedimentary basin with no outlet; it is endorheic. The Altiplano is an infill of sedimentary material from both Cordillera Occidental and Cordillera Oriental. The origin of the Altiplano and its great height has long been major question among geologists. Today the Altiplano is believed to have been an early foreland basin of the "proto-Andes" that got uplifted by crustal shortening in the Late Miocene.[8]

The Bolivian Altiplano hosts the world's largest reserves of lithium. These reserves are in the salt-saturated waters of Salar de Uyuni and Salar de Coipasa, neither of which are currently being commercially exploited for lithium. Salt is extracted from the saltars in minor amounts to be sold as table salt after being added iodine from Chile.

Central Cordillera[edit]

Erosion landscapes like Valle de la Luna near La Paz are common in the Central Cordillera and the Sub-Andean Zone

Cordillera Central is made up of a bended arc of plutons. The bend occurs at the latitude of Cochabamba and corresponds to eastward proyection of the Arica elbow, the bend of south Amicas coastline at the Peru-Chile border. North of the bend Cordillera Centrals batholiths are northwest-southeast oriented while south of it they are north-south oriented.[9] These bodies of granitic to granodioritic composition were formed in two episodes: the Early Mesozoic (199–180 Ma) and the Neogene (19–8 Ma). The emplacement of these igneous bodies have been interpreted as an effect of two temporary decreases in subduction angles of the ancient Farallon Plate.[10] This arc of magmatic material hosts Bolivia's widely known tin belt as well as the famous silver mine of Potosí. These mineralizations are of the porphyry mineralization type that are typical for convergent plate margins. The Cordillera Real form the northern, highly uplifted and eroded, part of the Central Cordillera.

Lowlands and Sub-Andean zone[edit]

To reach the lowlands north of La Paz the Yungas-vegetated Cordillera Oriental has be passed.

The Sub-Andean Zone and the Northern and Eastern Lowlands share a common ancient history but has since the Andean orogeny developed to two distinct zones. While both zones share essentially the same old sedimentary platform strata the strata at the Sub-Andean Zone have been folded and thrusted into a fold and thrust belt with structural traps where hydrocarbon fluids have accumulated. The limit between the fold and thrust belt and the Eastern Lowlands is drawn by the Serranía del Aguaragüe a north-south range representing the Andean thrust front.

Fold and thrust belt[edit]

The relatively low Cordillera Oriental, located east of Cordillera Central, are part of a fold and thrust belt and exposes Silurian and Ordovician age strata some of which are fossil-bearing. The fold and thrust makes up the framework of the hydrographic network of Pilcomayo and other rivers. Currently one of Bolivia's most seismic zones is the Cochabamba Fault Zone located just below the city of Cochabamba and its fertile valley. This fault zone is related to the Arica elbow and the bend in the Andes at this latitude.

Eastern Lowlands[edit]

The geology of the eastern lowlands is dominated by the ancient Paleozoic sedimentary Chaco-Tarija Basin that has considerable hydrocarbon reserves. During the Carboniferous, the Chaco-Tarija Basin was covered by thick sequences that included sandstone filled channels.[11] In the 1970s these sandstone were interpreted as tillite from the late Paleozoic glaciations, but new interpretations consider them to be of marine origin but still glacially influenced.[11] The depositional environment for these channel systems has been likened to that on the floor of the present day Labrador Sea, which was influenced by repeated Pleistocene glaciations.[12] The late Cenozoic deformation associated with the Andean orogeny forced hydrocarbons sourced in Devonian shales to migrate to shallower stratigraphical levels.[13]

Northern Lowlands[edit]

The Northern Lowlands are and have long been a depositional milleau are mostly covered by Tertiary and Quaternary deposits. The bulk of these deposties correspond to laterites. River erosion and sediment transport have created a large number of oxbow lakes and added copious meanders to the rivers.

Guaporé Shield[edit]

In the north and east of Santa Cruz Department the crystalline and Precambrian Guaporé Shield (also called Central Brazil Shield)[14] makes up most of the bedrock but is mostly covered by Tertiary laterites and Quaternary alluvial basins.[15] The shield expresses itself topographically as a large elevated hinterland located north of Serranías Chiquitanas towards the Brazilian border. High grade metamophic rocks occur in the Bolivian part of the Guaporé Shiled partly due to the 2000 millions years old "Transamazonian Tectono-Thermal Event" that affected parts of South America. Low grade metamorphic rocks occur as well. The Guaporé Shield is believed to extend beneath the Phanerozoic Tarija-Chaco and Beni Basin sediments into the Andes. Weakenses in the Guaporé Shield have been suggested to be responsible for the formation of the Arica Elbow. The East-West Río Mercedes Line in the Guaporé Shield host several Proterozoic diabase intrusions. As of 1985 the rocks of Guaporé Shiled were considered to be poorly known.[16] Near the Brazilian border the iron deposit of El Mutún stands out as the world's largest iron ore deposit.[17] It is located in the Serranía de El Mutún, also called Cerro Mutún or Serranía de Jacadigo in Brazil. The shield also underlies Serrania Caparuch or Huanchaca which is the inspiration for the Lost World of Conan Doyle.[18]

Proyecto Precambrico, an Anglo-Bolivian Technical cooperation project, explored the area between 1976 and 1986. There are a series of published 1;250 000 scale maps of the area with accompanying bilingual reports, along with more detailed 1;100 000 maps of regions of economic interest. There is a published geochemical atlas. Most of this data is summarised in an "Overseas Memoir" of the British Geological Survey.[19]

See also[edit]

References[edit]

  1. ^ The Use of Solar Energy for Improving the Living Conditions in Altiplano/Argentina
  2. ^ Geology of the Altiplano, Oregon State University
  3. ^ Neogene climate change and uplift in the Atacama Desert, Chile
  4. ^ Tectonics and Climate of the Southern Central Andes, Annual Review of Earth and Planetary Sciences, 2007.
    Cite: "precipitation and erosion may influence the kinematics and locus of tectonic activity in orogens."
  5. ^ David R. Montgomery, Greg Balco and Sean D. Willett. 2001 Climate, tectonics, and the morphology of the Andes
    Cite: "...like tectonics, nonuniform erosion due to large-scale climate patterns is a first-order control on the topographic evolution of the Andes."
  6. ^ Lamb and Davis. 2003. Cenozoic climate change as a possible cause for the rise of the Andes, Nature
  7. ^ Baker, M.C.W. and Francis, P.W. 1978. Upper Cenozoic Volcanism in the Central Andes - Ages and Volumes
  8. ^ Lamb, Hoke, Kennan, Dewey. 1996. Cenozoic evolution of the Central Andes in Bolivia and northern Chile.
  9. ^ Hydrothermal Processes and Mineral Systems. Pages 376–381.
  10. ^ Sandra L. McBride, Ronald C. R. Robertson, Alan H. Clark and Edward Farrar. 1983.Magmatie and Metallogenetie Episodes in the Northern Tin Belt, Cordillera Real, Bolivia. Geologische Rundschau. Vol. 72.
  11. ^ a b Petroleum Basins of South America, Hydrocarbon-Bearing Late Paleozoic Glaciated Basins of South America, page 175. 1995
  12. ^ Petroleum Basins of South America, Hydrocarbon-Bearing Late Paleozoic Glaciated Basins of South America, page 180. 1995
  13. ^ Petroleum Basins of South America, Hydrocarbon-Bearing Late Paleozoic Glaciated Basins of South America, page 178. 1995
  14. ^ Litherland,M. and Bloomfield, K. 1981. The proterozoic history of eastern Bolivia, Precambrian Research, vol 15, issue 2.
  15. ^ METALLIFEROUS ORE DEPOSITS OF BOLIVIA
  16. ^ Jones, J.P. 1985. The southern border of the Guaporé Shield in western Brazil and Bolivia: An interpretation of its geologic evolution
  17. ^ "Serrania Mutun, Chiquitos Province, Santa Cruz Department, Bolivia". mindat.org. Retrieved 2006-10-20. 
  18. ^ The geologic and geomorphic evolution of Serrania Huanchaca, eastern Bolivia: the legendary 'Lost World'. M Litherland and G Power: Journal of South American Earth Sciences, Vol 2, No 1, 1-17, 1989.
  19. ^ The Geology and Mineral Resources of the Bolivian Precambrian Shield. M Litherland and others: Overseas Memoir 9, British Geological Survey, Her Majesty's Stationary Office, 1986.