Sanctuaire Agoa

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Chapter 3. SITE DESCRIPTION

a - General features of the site

Terrestrial surface under sovereignty, excluding wetlands:

0 sq. km

Wetland surface:

0 ha

Marine surface:

138000 sq. km

b - Physical features

Brief description of the main physical characteristics in the area:

The marine and coastal environments of the French Antilles are home to various habitats, estuaries and lagoons, mangrove, beaches, coral reefs and seagrasses, high seas and deep zones. The available knowledge at the moment in French Antilles does not allow us to determine with certainty and exactitude the use of all the environments by marine mammals.

 

Even if the preservation of the integrity of the coastal ecosystems and their functional relations is a major stake for the sanctuary Agoa, it is also essential for the preservation of marine mammals, in particular some species which feed in deep waters, to discover the features of the habitats situated beyond 50 meters deep.

 

These areas are used by marine mammals, or by some of them during their life cycle for the reproduction, breeding, resting, feeding... To define the use of theses areas by the marine mammals and to know the relations of MM with the ocean food web are necessary to determine effective conservation actions which will guarantee a favorable conservation status of marine mammals.

Geology:

The Caribbean Plate is a mostly oceanic tectonic plate underlying Central America and the Caribbean Sea off the north coast of South America.

The eastern boundary is a subduction zone, the Lesser Antilles subduction zone, where oceanic crust of the South American Plate is being subducted under the Caribbean Plate. Subduction forms the volcanic islands of the Lesser Antilles Volcanic Arc from the Virgin Islands in the north to the islands off the coast of Venezuela in the south. This boundary contains active volcanoes as, for the French Antilles : la Montagne Pelée on Martinique; la Soufrière on Guadeloupe.

There are two West Indian Arcs. The oldest, in the East, arises from a first volcanic episode, 55 million years ago. The islands which were born were eroded then covered with thick-layered limestones of coral; it is the case of Saint-Martin and Grande-Terre in Guadeloupe. The second arc, more on the West, dates less than 5 million years and testifies of a volcanism always active, as on the Basse-Terre.

- Ores: even if these areas conceal a big variety of ores (copper, sulfur, iron, manganese, etc.) the extractable quantities would be too much limited to value an exploitation.

- Active volcanoes are present on certain islands in the form of warm waters or of release of sour gas in Basse-Terre and in Martinique. From time to time, one of the volcanoes erupts. This was the case in 1902: the Montagne Pelée destroyed the city of Saint-Peter in Martinique.

Topography:

 

According to all spatial scales, the uneven topography of the Lesser Antilles leads to a great lot of biotopes. So, from species to landscapes, through phytocenoses and ecosystems, numerous types of floristic composition occur. Despite an important man-made pollution, this archipelago belongs to one of the Hotspots of biodiversity: the Caribbean.

 

Saint-Martin :

The coastline of St. Martin is divided into numerous bays lined with thirty white sandy beaches but only some bays are lined with pebbles.
There are many brackish water ponds :
Grand Étang de Simsonbaai, Great bay, Étang aux poissons, Étang de Grand-Case, Étang Chevrise, Étang Guichard, Grand Étang des Terres Basses etc.. which were formed in the bottom of bays with cords of coastal sands.
The entire coastline is often steep with cliffs up to forty feet high.
There are ten islets around the island.
Apart from several coastal alluvial plains and valley floors, the environment is mountainous with steep slopes. The ridges have an average altitude of 300 meters intersected by several mountain passes. The highest peak is the peak paradise, culminating at 414 meters.

Saint-Barthélemy :

Saint-Barthélémy is located approximately 230 kilometers North-west of Guadeloupe and 25 kilometers South-east of Saint-Martin. It is separated from Saint Martin by the Saint-Barthélemy Channel.

This is a mountainous island consists mainly of volcanic rocks of approximately 21 km2 (24 km2 with its islets). It has 32 km of coastline.

The climax of the island is Morne de Vitet, 286 meters.

Some small satellite islets belong to St. Barthélémy including Île Chevreau (Île Bonhomme), Île Frégate, Île Toc Vers, Île Tortue and Gros Îlets (Îlots Syndare). A much bigger islet, Île Fourchue, lies on the north of the island, in the Saint-Barthélemy Channel.

 

Guadeloupe :

The department of Guadeloupe comprises five islands: Guadeloupe island composed of Basse-Terre Island and Grande-Terre (separated by a narrow sea channel called salt river) and the dependencies composed by the adjacent French islands of La Désirade, Les Saintes and Marie-Galante.

 

Basse-Terre Island is the name of the western-half of Guadeloupe proper, in the Lesser Antilles. Basse-Terre Island has a land area of 847.82 km² (327.35 sq. miles).

La Grande Soufrière, the tallest mountain in the Lesser Antilles, is located on the island. It rises to 1,467 m (4,813 ft) high. It is an active volcano. The surrounding area of Basse-Terre has a rough volcanic relief fertile land. The surrounding areas of Basse-Terre consists of forest, grassland and sugar, coffee, banana and cocoa plantations. The island has a land area of 847.82 km² (327.35 sq mi).

The eastern coast of Basse-Terre Island, known as the Windward Coast, is exposed to the trade winds and receives a lot of rain, whereas the western coast of the island, known as the Leeward Coast is protected from the winds by the Soufrière volcano and is thus much drier.

 

Grande-Terre is smaller than Basse-Terre Island. Its indented coastline is surrounded by coral reefs and the island itself is a limestone plateau. Its surface is a series of rolling hills, white sand beaches and cliffs. The island's beaches consist of both white and black sands, as well as beaches of golden sand. The island has a land area of 586.68 km²

 

La Désirade is a French West Indies island located at the eastern of Guadeloupe, in the Lesser Antilles.

It has a land area of 20.64 km² (7,97 sq. miles). La Désirade is 11 km long and 2 km wide; the interior of the island forms a central plateau.

 

Les Saintes is a volcanic archipelago fully encircled by shallow reefs. It arise from the recent volcanic belt of the Lesser Antilles from the Pliocene Epoch. It is composed of rocks appeared on the Tertiary age between (4.7 to 2 million years ago). By origin, it was a unique island that the Tectonic and Volcanic earthquakes separate to create an archipelago due to the subduction zone between the South American plate, the North American plate and the Caribbean plate.

Les Saintes is composed of two very mountainous inhabited islands, Terre-de-Haut Island and Terre-de-Bas Island, with seven other uninhabited îlets: Îlet à Cabrit, Grand-Îlet, la Coche, les Augustins, la Redonde, le Pâté, les Roches Percées.

The total surface is 12.8 km2 (4.9 sq mi). The archipelago has approximately 22 km (14 mi) of coast and its highest hill, Chameau, reaches about 309 meters.

 

Marie-Galante : the island is more commonly known as "La grande galette" due to its round shape and almost flat surface (its highest peak, the hill Morne Constant, rises to 670 ft). The island is undulating substrate calcareous, fanned by the trade winds and subject to cyclones and earthquakes.

The northern coast is characterized by a high cliff. A fault called the "Bar" separates the northern quarter from the remainder of the island. To the west beaches and mangroves extend along the Caribbean Sea. The rivers of Saint-Louis and the Vieux-Fort run out there after having crossed the insular plate originating at the center of Marie-Galante. In the east and the south, the plate becomes dull to rock inclined towards a littoral plain. This one skirts the Atlantic from which it is protected by a coral barrier.

 

Martinique :

With the total area of 1100 km2 Martinique is the 3rd largest island in The Lesser Antilles after Trinidad and Guadeloupe. It stretches 70 km in length and 30 km in width. The highest point is the volcano of Mount Pelee (1397m). The last two major eruptive phases occurred in 1902: the eruption of May 8, 1902 destroyed Saint-Pierre and took 28,000 dead in 2 minutes; that of August 30, 1902 caused nearly 1,100 deaths, mostly in Morne-Red and Ajoupa-Bouillon.

The coast of Martinique is difficult for navigation of ships. The peninsula of Caravelle clearly separates the north-Atlantic and South Atlantic coast.

The north of the island is mountainous and lushly forested. It features four ensembles of pitons (volcanoes) and mornes (mountains): the Piton Conil on the extreme North, which dominates the Dominica Channel; Mount Pelée, an active volcano; the Morne Jacob; and the Pitons du Carbet, an ensemble of five extinct volcanoes covered with rainforest and dominating the Bay of Fort de France at 1,196 meters.

The highest of the island's many mountains, at 1,397 metres (4,583 ft), is the famous volcano Montagne Pelée. Its volcanic ash has created gray and black sand beaches in the north (in particular between Anse Ceron and Anse des Gallets), contrasting markedly from the white sands of Les Salines in the south.

Bathymetry:

 

The seabed of the insular shelf of Guadeloupe and Martinique.

Surface deposit maps

Catherine SATRA, Claude AUGRIS
Ifremer, GM, BP70, 29280 Plouzané

The seabed of the insular shelves (from 0 to 100 m) of the archipelago of Guadeloupe and Martinique have been the subject of a detailed geological reconnaissance, using imaging techniques such as sonar sidescan, bathymetry, analysis of sediment samples and shooting underwater. Maps of two islands, initially published on a scale 1/25 000, were consolidated into a single document to scale 1/100 000.

Maps

Carte des travaux
Carte des travaux

Small Caribbean islands are characterized by the fragility of their environment, particularly in relation to their small size. Part of their development depends on knowledge of the seabed surrounding, especially with regard to the exploitation and sustainable management of their resources. The area of ??island shelves being at least equal to that of the islands themselves, their mapping is even more interest.
The French Antilles is part of the Lesser Antilles which is made up of about twenty islands between the Caribbean Sea from the Atlantic Ocean.

Topography and geology

Relief et Géologie
Relief et Géologie

 

These islands, earth and submarine relief high-contrasted, in relation to their geological nature, fall into two types: islands in low relief and recovery limestone (St. Martin, Grande-Terre, La Désirade, Petite Terre and Marie-Galante in Guadeloupe) and in high relief and volcanic islands (Martinique, Basse-Terre and Les Saintes in Guadeloupe).
The relief of the coastal seabed presents a clear asymmetry between the western and eastern fronts of the two islands. The depth increases rapidly on the Caribbean seaboard. The seabeds of 100 m are reached at a short distance from shore: 1 km for Guadeloupe and 400 m for Martinique. Conversely, on the Atlantic seaboard, it takes 25 km in Martinique to achieve the same depth of 100 m.
The seabed is mainly composed of coral reefs and soft sediments.

Nature of coastal seabeds

Nature des fonds marins côtiers
Nature des fonds marins côtiers

 

The large shallow submarine areas are favorables for the development of coral formations, scattered in Guadeloupe and more concentrated in Martinique. Coral reefs are found in various shapes and sizes. The most common form is the reef flat (planar form), whose extension can reach several tens of square kilometers.

Sediments mobility

Mobilité des sédiments
Mobilité des sédiments

During trade winds, sediments are immobile except near coral reefs, bays and areas between the shore and coral.
In times of storm, and especially cyclone, waves can mobilize sediments to depths of about 50 to 60 m.
This mapping of the insular shelf of Guadeloupe and Martinique shows the baseline status of the submarine landscape. The different information can be useful for resource assessment, planning and environmental protection.

References :

Augris et al. (2003) - Les fond marins du plateau insulaire de la Guadeloupe et de la Martinique. Carte des formations superficielles. Échelle 1/100 000. Édition Ifremer - Conseil Général de la Martinique.
Augris et al. (2000) - Carte des formations superficielles du plateau insulaire de la Martinique. 4 feuilles. Échelle 1/25 000. Édition Ifremer - Conseil Général de la Martinique.
Augris et al. (1992) - Carte des formations superficielles du plateau insulaire de la Guadeloupe. 10 feuilles. Échelle 1/25 000. Édition Ifremer - Conseil Général de la Guadeloupe.

Volcanic formations:

Active volcanoes : La Montagne Pelée in Martinique and la Soufrière in Guadeloupe.