Accueil > Documentations scientifiques > Revues récentes > Madagascar : revue de géographie > Archives > Volume 8, janvier-juillet 1966 > Contribution à l’étude du milieu fluvio-marin sur la côte occidentale de (...)

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  Contribution à l’étude du milieu fluvio-marin sur la côte occidentale de Madagascar
  Madagascar : revue de géographie, Volume 8, janvier-juillet 1966 pp:11 - 66

  Auteur(s) : Hervieu J.

  Auteur correspondant :

  Mots-clés : GEOGRAPHIE PHYSIQUE/ESPACES NATURELS/LITTORAL OUEST/ESTUAIRES/DELTAS/ALLUVIONS/SEDIMENTS (GEOLOGIE) / MADAGASCAR
  
  

  Résumé de l’article

  [FR]

  [MG]

  [EN] The fluvio-maritime ‘milieu’ is particularly developed in the West of Madagascar, especially on the outer fringe of the numerous deltas (Mangoky, Tsiribihina, Manambolo, Besalampy plain, Betsiboka, Mahajamba, Sofia, Sambirano and Ambilobe plain). There is no marked limit between the estuaries and deltas, the estuaries often being associated with a more internal deltaic zone. The dynamic tide, in the dry season, with its spring flow reaches 10 to 80 km, and the salt tide 5 to 30 km according to circumstances. The mangrove forest, as befits a ‘high slikke’ enjoys a fine expansion. These appears no characteristic zoning in the distribution of the mangrove species, but only preferential sites. A barren zone frequently exists behind mangroves, linked with the seasonal aridity of the climate. As a rule, in West Madagascar, the typical fluvio-maritime deposits often present a facies close to logarithmic one defined by A. Rivière. They are well-picked and evolved sediments, deposited through a surfeit of charge. They contain, as often as not, more than 90% of dust and colloids. Yellow or red-brown slimes are most frequent ; they are poor in organic matter and rich in iron hydroxides, the active ‘milieu’ playing a lesser part than in temperate countries. Most of its mud sedimentation is carried out in flood periods, through, a surfeit of charge and a flocculation of the elements in suspense. Adherence also acts, especially in the dry season during slack tides. Almost all the littoral and estuary sands of the West coast are characterized by the fineness and great homometry ; in over 80% of the samples, the average values are comprised between 0,125 and 0,32mm and no heterometry index oscillates round 0,25. The alluvial deposits are generally both hydromorphous and salted. The presence of a Gley horizon in the depths is a customary feature. The clay fraction is formed by a montmorillinite, kaolinitic, illite or mica conglomerate, in which the first element predominaties.