Remarques Sur Les Niveaux a Tasmanacees du Paleozoique Saharien

Abstract

Among the "Acritarcha" belonging to the family of Sphaeromorphitae Downie, Evitt, Sarjeant, the genera Tasmanites Newton and Leiosphaeridia Eisenack are observed in the whole Saharian Paleozoic. However, their occurrence, which is rather inconspicuous in the bulk of the microplancton of these series, may become prevailing at certain particular levels where they constitute up to the 95 per cent of the microscopic population. These levels, moreover, show the interesting particularity of being rich in subcolloidal organic matter, and of having a strong radioactivity (radioactive markers). The main radioactive levels with Tasmanaceae (SOMMER,1956) have been chiefly observed at the base of the argillaceous Gothlandian and at the base of the Frasnian. They thus mark the beginning of these two transgressions. The interest of these Tasmanaceae "palynofacies" is therefore real both from the ecologic and paleogeographic points of view. As well from the tratigraphic point of view, as long as these forms are accompanied by good ronostratigraphic markers, as these Tasmanaceae are so little evolving that may be found unchanged from Cambrian to Mesozoic and perhaps up to the Recent. However, a more thorough observation shows a morphologic evolution which leads in a minority of individuals to diversely ornamented forms or even to conspicuousely modified structural states. Concerning the systematic position of these microfossils Wall supplies some demonstration which however is not universally aceepted. We think nervertheless that an algal nature for these microplancton is an appealing hypothesis.