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RESEARCH PRODUCT

Marine Ostracods of the Upper Miocene of the Well Ashtart 1 (Gulf of Gabès, Southeastern Tunisia)

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We studied the ostracod fauna from a Late Miocene-Early Pliocene interval 600 m thick, in Ashtart 1, an offshore well in the Gulf of Gabes, southeastern Tunisia. Most of the 131 samples analyzed contained rich and much diversified populations of marine ostracods. In fact, 212 species have been found, belonging to 70 genera. Among them, eight genera, 122 species and subspecies, considered as new, will be described elsewhere. In spite of contamination, usual in oil wells due to caving from higher levels previously drilled, some stratigraphical and palaeoenvironmental features have been established. The last occurrence datum of a selected group of species has been defined with accuracy, and some assemblages have been tentatively recognized. The Tortonian-Messinian boundary did not show any important biostratigraphic event for the species taken into consideration. In fact, the Tortonian “Sands of Somaa.”, a mollassic forma-tion, are practically azoic; the apparently rich ostracod assemblages found in the samples of this unit are only contaminations from the overlying beds. Three abundance maxima have been noticed within the “Melqart limestones” formation, a lithostratigraphic unit considered of Early Messinian age. The Upper Messinian, with the gypsiferous clays of the “Oued bel Khedim” formation, showed the rather abrupt extinction of most of the marine species, the survival of some euryhaline taxa and the development of a brackish environment colonised by Cyprideis. The environment becoming too hostile, all forms of life disappeared and the upper 70 metres of the formation are azoic. Thus, in this area of the Tunisian shelf, the Messinian sediments perfectly registered the pro-gressive deterioration of the environment, certainly related to the “salinity crisis” which affected the whole Mediterranean Sea at the same time. The first Early Pliocene sediments are characterised by the re-establishment of a normal marine environment, thus showing a sharp contrast with the underlying Upper Miocene series. During the Lower Messinian, the palaeobathymetry, as deduced by the ostracod genera repre-sented, seemed to. be rather constant, with weak oscillations between 70 and 100 metres. This palaeoenvironmental interpretation suggests a subsidence phenomenon of more than 300 m during the Late Miocene.