More than 2 billion years ago a massive hard crystalline sandstone plateau was formed. This was part of the supercontinent PANGEA. Within Pangea the present day Guiana Shield was formed. 400 million years ago Pangea split into the northern continent Laurasia and the southern continent Gondwana. GONDWANA composed the present day Americas and Africa. The AMERICAN plate was moving west and the AFRICAN plate was moving north as the Atlantic Ocean forced its way in between. About 70 million years ago the South American and the African plate were separated by the Atlantic ocean.
When the earth was in the cycles of the ice-ages there was no major effect on the environment in the shield region. Although not so severe as in the northern hemisphere: the periods where dryer and cooler resulting the rain forests splitting from a single extended forest into forest islands separated by bigger savannas; new species evolved on these forest-islands. When the wet and warm periods came, the rain forest islands rejoined ....... this continuous cycle resulted in an increased diversity of plant and animal species.
The Guiana Shield refers to a geological unity formed during the precambruim period (4,500- 570 million years ago). Rocks on the basis of the shield are known as the Greenstone belts and form the Imataca complex which were developed 2,250 to 2,100 million years ago. About this period the Guiana Shield, the Brazil Shield (now in the south) and the west African Shield (now in the east) were formed.
The basis of the Guyana and Brazil Highlands where formed.Today the Guiana Shield covers: North East Brazil with the northern part of the Amazone Craton (parts of the states of Amapa, Para and Roraima), and includes parts of French Guiana, Suriname, Guyana, Venezuela and Colombia. Comparative regions in age and surface in the northern hemisphere are Scandinavia in Europe and North-East Canada in North America
1,900-1,500 million years ago the largest parts of the present Roraima plateau were formed as a huge, massive hardcrystalline sandstone plateau. Now these rocks are geologically known as the Roraima group. The Roraima group consists of a silica cemented quarts (red) sandstone which has a variable but mostly strong resistance to weathering and erosion. Nowadays we find this plateau as series of small hills, plateaus and table mountains (Tepuis) rising up from 300 to 2,800 m and sloping to the west, forming the Guiana Highlands and scattered over an area of some 500,000 square kilometers.
The rivers emerging from these mountains drained off via a basin to the west and into the Pacific ocean. When the first biological plant life appeared, about 570 million years ago in the Precambrian period, these rocks were already 1.4 billion years old.
During the precambruim period the Guiana Shield was part of the supercontinent Pangea. Today we find black weathered sandstone cathedral-like rock formations on top of the tepuis and huge caves and water-falls. 400 million years ago - during the cambrium period - Pangea split into the northern continent Laurasia and the southern continent Gondwana. About 200 million years ago the opening of the Atlantic seafloor began and the Atlantic ocean forced its way in between. About 135 million years ago (at the end of the Jurassic period) the break up of the former continent Gondwana took place.
The drifting of the continental plates (the American plate was moving to the west and the African plate was moving to the north) caused fissures and fractures in the Roraima plateau. Erosion occurred in these weak spots in the earth's crust and enlarged them. Functioning as waterways for the rivers in the Roraima plateau. The mountains and hills in the Guiana Highlands are separated by river valleys and marshy depressions. Until now the rivers are draining the huge amount of yearly rainfall. At the end of the Jurassic period the rise of the Andes mountains in the West part of the continent began.
During the cretaceous - 75 million years ago - seas invaded the larger part of the South American continent, with the Precambrian shields remaining as islands. About 70 million years ago the South American and the African plate really were separated from each other by the Atlantic ocean.
During the end of the Cretaceous period (65 million years ago) the rise of the Andes reached its first maximum. The seas regressed from the Amazon basin at this time and sedimentation took place in the huge fresh water lake. And the Amazon flowed west to the Pacific Ocean in a channel between the Brazil and the Guiana Shield.
Access to the Pacific Ocean via Ecquador closed when the Andes rose some 40 million years ago. The trapped water became a sluggish lake. The Amazon river, now blocked by rising rock, shifted it's flow to the Atlantic Ocean, carrying with it the rich sediments of the new range 7 million years ago (end of the Eocene epoch) the gate for the Amazon basin to the west was finally closed. The volume of water carried by the present Amazon riverbed, passing between the Guyana and Brazil Highlands, comprising a fifth of all the flowing water of the world.
South America remained a completely isolated (ecologically isolated continent) continent from the rest of the world during the whole Teriary period from 65 until 5 to 2.5 million years ago. During this long period only moderate climate changes occurred. In the ice-ages the South American continent had less colder climate changes then in the northern regions. This explains why the landmass of South America together with Australia, has the most original biological life in the world. One could say that the biological life is frozen in time, since the extinction of the dinosaurs. Nowadays Mountain Lions and Lowland Tapirs can be found living in the area.
Finally the landbridge of Panama, from South America to North America was formed when volcanic activity caused one of the uplifts of the Andes 5 to 2.5 million years ago. Once the landbridge was realized old inhabitants of the South American continent like the Marsupial Wolf became extinct.
During the Pleistocene epoch (2.5 million to 10,000 years ago) the earth was in the climatic cycle of ice-ages with shorter warmer periods. Ice caps covered the Andes and glacial terraces covered the high plateaus. In this period much less deposition took place.
The climate got temporarily cooler and dryer. Resulting in bigger parts of the Amazon region covered with Savannah vegetation. The rain-forests were retreating to small wet areas (refugia) where the rainforests could maintain to exist. When the wet and warm periods came, the rain forest islands rejoined. Finally about 12,000 -10,000 years ago the first settlements of first South American people were build at Tiama-Taima in Venezuela, at the Goias sites in South-East Brazil and Monte Verde in Chili.
EPILOGUE
Next to the warm and wet conditions of the rainforest, it is important to realize that age is another factor contributing to the abundance of tropical life-forms in the ecosystems of the Shield. When the earth was in the cycles of the ice-ages there was no major effect on the environment in the tropical regions. Although not so severe as in the northern hemisphere, the periods where dryer and cooler. Due to these climate circumstances the rain forests split from a single extended forest into forest islands separated by bigger savannas; new species evolved on these forest-islands. When the wet and warm periods came, the rain forest islands rejoined and so on.......the net result was more plant and animal species.
More than 90 % of the shields region remains in a pristine state. Within the area there exist tropical forest, highlands, Savannah's, swamps and wetlands. In the shield the landscapes contain valuable natural resources as well as an ecological heritage.
The Guiana Shield faces unprecedented changes as we approach the end of the millennium. The human and natural environments of the region are under the mounting pressure of logging, mining drug-trafficking, road building, dam construction, agriculture and cattle ranching, hunting and poaching. Now investments and activities of the international community have reached such levels that they pose serious threats to the values of the region.
Republished from: Geology - Guiana Shield Media Project
Written by: Carlos Huijser, Petra Kreule & Ray Kril - October 1997
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