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La Brea Pitch Lake

Date de soumission : 17/08/2011
Critères: (vii)(viii)
Catégorie : Naturel
Soumis par :
Trinidad and Tobago National Commission for UNESCO
Coordonnées N10 15 W61 37
Ref.: 5645
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Description

The Pitch Lake is found in southwest Trinidad in the village of La Brea. The lake measures approximately one hundred (100) acres (41 hectares), and is estimated to be two hundred and fifty (250) feet (76 metres) deep in the centre. It holds about ten million (10,000,000) tons of pitch. It is situated about twelve hundred (1200) yards from the sea, in a depression immediately south of a 140 feet high hill, from the summit of which the ground slopes gently northwards to the sea.

The asphalt is an emulsion of water, gas, bitumen and mineral matter, the latter consisting largely of fine silica sand and a lesser amount of impalpably fine clay. In some parts of the Lake there is a small influx of soft material. This is accompanied by a stronger evolution of gas consisting principally of methane with a considerable proportion of carbon dioxide, and which also contains hydrogen sulphide. This influx gradually hardens and becomes like the main deposit of the asphalt. As this occurs fresh material breaks out elsewhere.

Although quiescent the asphalt still moves with a natural slow "stirring" action. Not only can the flow lines be seen on the surface of the asphalt, but prehistoric trees and other objects have been known in the past to have appeared, disappeared and reappeared.

In appearance the surface of the Lake is a uniform expanse of asphalt which is intersected by areas of water, the extent of which naturally varies according to the season.

Justification de la Valeur Universelle Exceptionelle

This is the largest and most significant pitch lake in the world, the natural phenomenon of Pitch Lakes is an important aspect of earth's history; it relates to the formation of oil and gas. This renowned "Pitch Lake" is the largest, commercial deposit of natural asphalt in the world. It has fascinated explorers, scientists and common folk since its re-discovery by Sir Waiter Raleigh in the year 1595. Raleigh himself used asphalt from the Lake to caulk his battered ship. Since then, there have been numerous research investigations into the use and chemical composition of this material.

In 1888 the government gave a contract to work the "lake" and from this time began the large-scale export of pitch. In 1978, Lake Asphalt of Trinidad and Tobago (1978) Limited (LATT), was incorporated. Its object was to extract and process asphalt from the Lake. LATT now produces a wide range of products with asphalt from the Lake being the base ingredient. Among these are an anti-corrosive black paint, seam sealant, underbody coating for automobiles, a pipe and metal guard and bitumen emulsion.

The La Brea Pitch Lake is considered a tourist attraction and attracts about 20,000 visitors annually. People occasionally swim in the waters of the pitch lake which some say is therapeutic because of the sulphur content. It is also employed in a variety of applications from high-quality road construction to insulating compounds for the cable and electrical industries.

The site also has significance to the indigenous people of Trinidad and Tobago. They had their own ideas as to how the lake was formed. After a victory over a rival tribe, the tribe got carried away with its celebration, cooking and eating the sacred hummingbirds they believed held the spirits of their ancestors. Thus, legend has it, their winged god opened up the earth and conjured up a lake of nasty pitch to swallow the entire village. Amerindian pottery has been found, along with remains of prehistoric animals.

Apart from being a natural wonder, the lake has great social and economic value to residents of La Brea in particular and citizens of Trinidad and Tobago. However, its benefit is far reaching, including both to the Caribbean community and to other countries of the world.

Déclarations d’authenticité et/ou d’intégrité

Experts have, over the past century, put forward conflicting opinions as to the origin of this unique deposit. An early theory was that of a mud volcano liberating an enormous volume of petroleum and mineral matter in a state of protracted agitation during which the volatile constituents were dissipated and the heavy bituminous residue became uniformly homo-genized with the fine clay and silica minerals, water and gas has now been superseded by the conclusions of Dr. Kugler and others as a result of later researches and more advanced environmental knowledge.

During Laramian folding a gentle anticline of upper Cretaceous and Paleocene rocks was formed below the present Lake. This led to the separation of gas, oil and water inside the structure. The Upper Eocene transgression removed the Navet and part of the Chaudiere formations which resulted in a dispersal of most of the gas and lighter oils. Renewed folding movements and additional fracturing took place prior to the transgression of the Nariva Sea when asphaltic viscous oil, heavier than water: started to flow onto the shallow muddy sea floor.

Sediments with their foraminiferal fauna covered repeated flows of the oil kept in agitation by a continuous flow of gas, thus mechanically mixing detrital matter with the inspissated oil and water to an emulsion. Due to its viscosity, the mass of oil held together and it gradually grew to an inverted cone surrounded by the increasing thickness of the sediments. After the deposition of about two thousand (2000) feet of sediments, it may be assumed that the mass of asphalt was probably buried and little seepage of oil took place. During the later Oligocene structural movements, a further dispersal of Cretaceous oil took place. Every crack and fissure was injected by plastic bitumen that subsequently lost its volatile parts and formed the pencillate bitumen (Manjak) dikes partly found on the Upper Cretaceous itself. During the latest Pliocene movements, it appears that the mass of asphalt at the base of the Nariva beds had an opportunity to ascend along the intersection of two major faults that met near the centre of the present Lake. The asphalt flowed into the shallow mangrove lagoon of Upper La Brea in time and grew again as an inverted cone during the deposition of the youngest sediments.

Later, the Gulf of Paria broke in through the Northern Bocas (the narrow sea passages separating Trinidad from the adjacent islands and the mainland of Venezuela) and the erosive waves reached the Pitch Lake area with its large surface sheet of asphalt covering perhaps three thousand (3000) acres. The asphalt flowed down the forming cliffs and protected them, but large masses now lie in the sea off La Brea village. The erosion swept inland and the peninsula of La Brea was formed, protected by the asphalt covering.

Comparaison avec d’autres biens similaires

The La Brea Pitch Lake is the largest and most significant pitch lake in the world. The natural phenomenon of Pitch Lakes is an important aspect of earth's history as it relates to the formation of crude oil and natural gas.