Paleoenvironmental reconstruction of the Pleistocene site of Oued Sarrat (Northwestern Tunisia) using mineralogical and geochemical data

Abstract

The mineralogical and geochemical analyses of Pleistocene sediments and mollusks shells (gastropods and bivalves) from the archaeopaleontological site of Oued Sarrat (Tejerouine, NW Tunisia) permitted us to determine the paleoenvironmental conditions and to reconstruct the local depositional environment during the Middle and Late Pleistocene. The mineralogical cortege of the sediments, for all the analyzed samples, records the characteristics of a mixture ofsilica and calcite dominance with a small fraction of gypsum and aragonite. The mineralogical analyses of all mollusk species reflect a cortege dominated by aragonite, associated with low amounts of calcite, silica, hematite and goethite. We consider that the domin- ance of aragonite indicates that the tests have not yet or little undergone mineralogical transformations linked to the phenomena of fossilization, as evidenced by the absence or low calcite content. On the other hand, the low percentages of silica, goethite and hematite are probably related to the existence of impurities and sediments trapped in lodges, or  adhered to the shell surface. The mineralogical data confirm a composition dominated by calcium carbonates, expressed by high contents of CaO and CO2, reflecting a chemical test of organisms’ development in continental environment.This kind of environment is characterized by the absence of MgO, which is usually present in the organisms’ tests developed in marine environment. However, the contents of SiO2, Fe2O3, Al2O3 are related, as it was reported during the mineralogical study, to the lithological impurities trapped mainly in the lodges of helicides. Mineralogical and geochemical data tracing, carried out on sediments and tests, converge to deduce the establishment of paleoenvironment attributable to fluvial deposition of sufficiently high energy where the ultrafine fractions of clay minerals are remarkably lacking. The absence of indicators of marine chemistry such as magnesium in sediments and in the tests of organisms confirms almost total contribution of the continental meteoric water without marine influence. Such environment, however, is affected by intermittent episodes of aridity as attested by the presence of evaporate minerals such as gypsumand aragonite.