Les Brachiopodes du Lias-Dogger: Paléontologie et biostratigraphie

Abstract

Field-trips in the central part of Saudi Arabia, withsearches of B.R.G.M. so as to realize geological maps, have made possible the establishment of the lithostratigraphy and the chronostratigraphy of the Jurassic. Numerous well situated fossils (cephalopods, brachiopods, bivalves, gastropods, echinids, crinoids) have been collected in many sections. Beds between the Marrat Formation (Toarcian) and the top of the Tuwaiq Formation (Callovian) have provided more than 4300 brachiopod shells. The object of the present work is: 1) to makeknown the morphology of the main brachiopod species which follow one another in the platform environments (middle and proximal infralittoral biotopes) during the Lower and Middle Jurassic. The stratigraphical distributions of the species are justified by the representation of shells from different levels; 2) to establish a biostratigraphical scale of brachiopods. The main results are as follow: - an alone bed with brachiopods has been found inthe Toarcian, at the bottom of the Middle Marrat Formation (Lower Toarcian, Serpentinus Zone); - the brachiopod faunas of the Lower part of the Dhruma Formation (Bajocian except the Garantiana and Parkinsoni Zones) are very different to the above faunas. Some species show an european mark ( Monsardithyris ventricosa, Sphaeroidothyris cf. sphaeroidalis), other species have been described already on the south side of the Mesogea, mainly in Morocco ( Burmirhynchia termeirae, Tubithyris chouberti, Rugitela praebullata) or in Egypt and in Sinaï Peninsula ( Eudesia magharensis). The fig. 8 shows for the first time a succession of five species associations; the main species of which were previously referred to a total age Bajocian-Bathonian and even Callovian. Turning to discontinuities, no fauna known elsewhere from the single Middle Bathonian has been observed. A gap of the Middle Bathonian and perhaps of a part of the Upper Bathonian can be looked at the top of the Barrah Member of the Dhruma Formation otherwise characterized by generalized hard-grounds. Effectively, there are only two polymorphic species with a large stratigraphical distribution ( Daghanirhynchia subversabilis and Cererithyris bihinensis) in this Barrah Member (beds with Microphalites and moreover the brachiopod associations 3 and 4 which enclosed it are entirely different. This fact suggests rather important variations of the palaeoenvironments. The most noticeable of these variations preferentially succeed to the sedimentation stoppings