DR ADRIAN TUITT
Around 8,000 years ago the Amerindians came to the shores of Trinidad and were greeted by a verdant landscape teeming with life. It was apparent even back then that the topography on the island varies from north to south. Indeed, the Northern Range, Northern Basin, Central Range and Southern Basin have a unique tectonic history that has affected not only the surface topography, but the structure of the underlying rock.
The geological history of any country can give rise to different rocks. Rocks can be categorised into three groups – sedimentary, igneous and metamorphic.
Sedimentary rocks are formed as the result of the deposit of grains or the precipitation of minerals. Sandstone, for example, is formed when quartz grains are deposited in terrestrial or marine environments. These grains are eventually compacted upon burial to form rock. Igneous rocks, however, are formed by the cooling of magma. Molten magma is formed by the melting of rock within the earth. When the magma is cooled, different igneous rocks such as basalt or granite are formed, depending on the magma’s chemistry and the cooling rate. Metamorphic rock is formed when pre-existing rocks undergo changes due to an increase in pressure or temperature. Limestone, for example, is a sedimentary rock formed by the deposition or precipitation of calcium carbonate. When limestone undergoes an increase in pressure, marble is formed.
In Trinidad, the Northern Range consists of metamorphic rocks, with a small section around San Souci consisting of igneous rock. Sedimentary rock makes up the majority of the land mass that is Trinidad and is found directly south of the Northern Range in the Northern Basin, Central Range and the Southern Basin.
The Northern Range is the highest terrain on the island and consists of quartzites (metamorphosed sandstone), phyllites (metamorphosed shale) and low-grade marble (metamorphosed limestone). In the Late Jurassic to Cretaceous time (about 150- 65 million years ago), sediments were deposited on the ocean floor. These sediments were uplifted and metamorphosed from the Early Oligocene (around 40 million years ago) as the Caribbean Plate collided with the South American plate, the boundary of which was marked by the El Pilar Fault.
The advancing orogenic belt placed a weight on the South American crust to the south, resulting in subsidence in the Southern Basin. Furthermore, as the orogenic belt eroded, the eroded sediments were deposited into the Southern Basin until Mid Miocene time (around 12 million years ago).
In the Mid Miocene, the plate boundary between the Caribbean and South American plate changed from one of convergence (collisional) to strike-slip (plates slide past each other). The strike-slip fault developed in the area of the Central Range and is called the Warms Springs Fault.
[caption id="attachment_1021206" align="alignnone" width="1024"] A map of of the Northern, Central and Southern ranges. -[/caption]
The orientation of the Warms Spring Fault facilitated compression and the
