SILICEOUS SPONGES AND MICROBIALITE CARBONATE MOUNDS AND THEIR ENVIRONMENT AT THE SINEMURIAN OF NZALA AND FOUM TILLICHT, CENTRAL HIGH ATLAS (MOROCCO)

The Nzala and Foum Tillicht outcrops that occur North of Rich village, are part of the Sinemurian carbonate mounds well exposed all over the Midelt-Errachidia transect in central High-Atlas (Morocco). These mounds constitute the upper part in the upper member of the Idikel formation, dated by ammonites, in the Lower-Upper Sinemurian transition. They locally grow in the Upper Sinemurian and disappear at the bottom of the Lower Carixian. The biosedimentological study of these carbonate mound, allowed the distinction between three different sets: a lower set (25 m) where the carbonate mounds are simple shaped and small sized boundstones, dominated by calcified sponges in a life position, including hexactinellides sponges, lithistids demosponges and non-rigid Demospongiae; a medium set (35 m) which shows complex shaped and big sized constructions including sponges and different thrombolytic fabric (clotted, digitate and dendrolitic). And an upper set, the most developed (150 m) consists of a vertical arrangement of lenticular structures of metric width and decametric extension where the thrombolites and sponges are very rare. This set shows some solitary corals and a hardened surface rich in ammonite of the base of lower carixian. The analysis of an elementary mound type show, essentially two parts: - A massif micritic-dominated core, which is a boundstone dominated by siliceous sponges and microbialites with associated encrusting forms (Radiomura, Serpula and Terebella, Tubiphytes and Bryozoans), beside, brachiopods, pelecypods, gastropods, isolated corals, ostracods, hyaline foraminifera and ammonites. - A well bedded sediments, corresponding to the core lateral synchronous, showing a biocalcarenitic facies a proximal to the core and a biomicritic facies further laterally. Brachiopods and echinoderms are particularly abundant. Complete or fragmented bivalve and gastropod shells are common. The studied outcrops resulted volumetrically dominated by siliceous sponges and microbialites that represent the major constructor and stabilizer, in addition to such benthic assemblage described previously, suggest an open marine depositional environment, infratidal to circatidal and relatively low energy.