Guyana Guyana ( or ), officially the Cooperative Republic of Guyana, is a country on the northern mainland of South America. Guyana is an indigenous word which means "Land of Many Waters". The capital city is Georgetown. Guyana is bordered by the ...

belongs to the

Guiana Shield The Guiana Shield (french: Plateau des Guyanes, Bouclier guyanais; nl, Hoogland van Guyana, Guianaschild; pt, Planalto das Guianas, Escudo das Guianas; es, Escudo guayanés) is one of the three cratons of the South American Plate. It is a ...

, a massif extending north of the Amazon, between the

Andes Mountains The Andes, Andes Mountains or Andean Mountains (; ) are the List of mountain ranges#Mountain ranges by length, longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range i ...

and the Atlantic, made up of

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...

rocks dated to between 2.7 and 3.5 billion years ago. The shield consists of several geological units: the southern peneplain, the Inini synclinorium, the central

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

massifs and the northern

synclinorium In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimpose ...

. The southern peneplain consists of a set of granito-gneiss consisting of metagabbros, metagranodiorites and metagranites dating from 2075 ± 7 Ma from a

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

of metamorphosed mantle origin. To the north, the southern peneplain straddles the Inini synclinorium, mainly composed of the Paramaca series, which corresponds to a series of micaschists, aluminous paragneisses and black

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

s whose total thickness is about 1 km. The central granite massifs consist of granite plutons, granodiorites and

quartz diorite Quartz diorite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less potassium feldspar. Quartz is present at between 5 and ...

s dating from 2.05 to 2.15 billion years ago. The north of French Guiana corresponds to a synclinorium composed of the Paramaca series on which

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

and

fluviatile In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvio ...

conglomerates lie

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

. Shield rocks in Guyana are all dated between 1.6 and 2.5 billion years ago. The

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

lands are located in the northern part of Guyana, near the coast, unconformably on the

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

lands. The coastal plains consist of thin layers (between 8 and 15 meters in the old plain) of

marine clay Marine clay is a type of clay found in coastal regions around the world. In the northern, deglaciated regions, it can sometimes be quick clay, which is notorious for being involved in landslides. Marine clay is a particle of soil that is dedica ...

, sand and pebbles. The newest fields contain organic matter such as

peat Peat (), also known as turf (), is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem covers and is the most efficien ...

or shells. Fluvial

alluvia Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...

of continental origin are mainly located in the lower reaches of the rivers, they are mainly sandy products. They come from the erosion of the Precambrian

basement A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...

of the Guyanese Shield.

References

*Jean Dercourt, Géologie et géodynamique de la France : Outre-mer et européenne, Paris, Dunod, coll. « Sciences Sup », 2002 (réimpr. 1998), 3e éd. (1re éd. 1996), 336 p., 19,5 cm x 27 cm (, présentation en ligne) {{South America topic, Geology of