ASPECT SEDIMENTOLOGIQUE DE LA "FORMATION DU DJEBEL SERRAF" (FINI ORDOVICIEN) AU KSAR D’OUGARTA ET SES ENVIRONS

Abstract

The Late Ordovician (Asghillian) deposits of the "Djebel Serraf" Formation were the subject of a lithostratigraphic and sedimentological study in the Ksar of Ougrata and El Kseib areas. The lithostratigraphic study enabled us to distinguish three members for this formation: 1) - the basal unit is clay-sandstone-conglomeratic (Sandstone of Serraf) which reflects a fluviodeltaic environment attested by the abundance of channel structures; (2) the median member (Clays of El Kseïb) is characterized by the predominance of clays and the presence of the conglomerates in particular in the El Kseïb sector. It indicates a low depth marine environment with glacial influence in some locality; (3) the upper member (Sandstone of El Ksar) is made of sandstone-argillaceous bar rich in Skolithos at the base and sandstone-quartzitic at the top. This unit is also characterized by thin and chenalized intercalations of layers with Plectothyrella chauveli and stromatolithic forms. The lithological and ichnologic characters, as well as the frequency of the hydrodynamic structures (oblique laminations, HCS) indicate a low depth marine environment with tides or storms currents influence. The Sequential stratigraphy shows sequences of deposit of 3rd order, increasingly developed towards the SE, in the proximal to distal direction of the vast detrital platform of the period. They are interpreted like the end of a filling sequence before the Silurian transgression in regressive period of 2nd order. The megasequece consists of several sequences of 3rd order, more or less lacunar towards the SE. It indicates successive ingressions of the sea followed by regressive periods. It corresponds also to progradante or agradante sequence of the regressive period of a regressive/transgressive of 2nd order cycle. Keys-word. Asghillian, Late Ordovician, Djebel Serraf, El Kseïb, Ougarta, Environment of deposits, Sequences. Ichnology, glacial