LIASSIC (UPPERMOST SINEMURIAN UP TO MID TOARCIAN) SEQUENCE STRATIGRAPHY AND GEODYNAMIC EVOLUTION OF THE BENI-MELLAL ATLAS (CENTRAL HIGH ATLAS, MOROCCO)

Biostratigraphical and sedimentological studies carried our in the Ben i-Mellal Atlas lead to a sequence stratigraphy subdivision of the Lias sic record (Uppermost Sinemurian - Mid Toarcian) into 9 third order se quences. These sequences are involved in two second order cycles which tectonically enhanced unconformities are located in the upper Dart of the lower Domerian boundary and the lower part of the mid Toarcian. T he third order sequence organization shows similarities with the local or regional examples. Specific evolution seem to be related to autocy clic processes within the sedimentary supply or to correlation problem s between the platform and basin formations. The second order cycles i nvolve a real influence of local tectonic motions. So, the transgressi ve Uppermost Domerian - Lower Toarcian stage, far from indicating a no rthward extension of the sedimentary realm towards the Beni-Mellal pla tform, shows an <<in situ>> filling up of the southern Tilougguit trou gh, created during the preceeding transtension Mid-Upper Domerian stru cturation through transtension phase. The accomodation changes related to the associated eustatic and tectonic influences induce deep modifi cations in the trough faunic associations (ammonites), specially durin g the Pliensbachian period.