Le biostrome à Crassostrea gasar (Bivalvia) de l'Holocène du Sine-Saloum (Sénégal); Données nouvelles et interprétation écostratigraphique

Abstract

A biostrome of Crassostrea gasar Dautz was discovered by the authors in 1989 near the village of Bassoul (Senegal) into the present mangroves environment of the Sine-Saloum marine estuary. Following a new campaign it was cartographied (total superficies 10 ha), thickness measured (0,65 m) and age clarified: it begins at −5440 years B.P. and stops suddenly at −4850. A hole allows to note that the oysters are developped in situ in the sandy clayed sediment; they are bivalved, large and thick. First, they are settling down flat, then revolve on one's side, and next stand up to become vertical in the end. Three samples of left valves, during this bioconstruction development phases, allowed the measurement of four morphologic parameters (length, width, dimension of the ligament area, maximal thickness); their bivariate analyses have been studied and interpreted according to the biostrome evolution. The micropaleontologic study (foraminifera, ostracoda, palynology) indicates a marine environment, polyhalin and not hyperhalin as in the present, in a nearly mangroves biotope with herbaceous areas and only one fern (Schizeaceae), and with fluviatil water inflows. These data are in accordance with the humid climate of this middle Holocene period, at the maximum of transgression. Under the biostrome, black peaty clays (0,40 m) correspond to organic muds of classic mangrove (basis of the hole −6590 years B.P.). A temporary balance during about 600 years was realised by the synchronism of this two phenomenon, climatic (pluvial period), and eustatic (transgression), allowing the life of the oysters populations in the form of a biostrome, and not only fixed on the mangroves roots as at present. But its unique localization, comparable to a stable shoal, remains to be explicited. In the time, this episod follows the one of Anadara senilis L. terraces and takes place in a regional context of the Nouakchottian stage (−6800 to −4200 B.P.) whose characters are like this clarified and complemented. Theath of the biostrome had been sudden, owning to a contemporaneous and speedy fall of the marine level and of the raininess: this abrupt event forecasts the approach of the Tafolian, aride and regressive period. A chemical corrosion occurs afterwards because the recent putting up of a new mangrove, with acide soll, dissolving the horizontal bed top on several centimeters.