In the western part of the external Rif Mountains, in the areas located between Ouez-zane, Mjara and Mokhisset, the Upper Jurassic and Lower Cretaceous succession crops out in a narrow band 5 to 20 km wide and 70 km long SE-NW. In this zone corresponding to internal Prerif and Mesorif, the Kimmeridgian and the Lower Tithonian form are represented by a limestone bar 10 to 150 in thick; the Upper Tithonian (0 to 30 in) is marly and calcareous; the Berriasian (10 to 100 in thick) is mainly marly. The stratigraphic analysis of the carbonate series led us define three lithological units: a neritic unit of blocks (50 to 150 in thick); a pelagic unit (10 to 40 in thick) constituted by thin mudstone beds and a unit of shallow water shoals corresponding to blocks with average thickness (20 to 80 in), constitued by bioclastic or laminated calcareous beds with abundant micro-fauna and of polygenic or monogenic sedimentary breccias or compact breccia beds. The first unit keeps an external or internal position in the internal Prerif, the second one corresponds to the internal Prerif (s.s.) and the latter, in internal position, corresponds to the Mesorif. These units have been equally recognized in the eastern sectors from Mjara to Msila (central and eastern Prerif and Mesorif). In the western external Rif, blocks of pelagic unit, in comparison with others external Rif sectors are thin, incomplete and very poor of ammonite. Conversely, the blocks of the Mesorif unit are abundant and developed; they show more numerous beds consisting of sedimentary breccias or compacted breccias up to 20 in thick. These breccias often contain centimetric to metric pebbles of volcanic rocks. The strata of Upper Tithonian (zona A of Calpionellids) and at minor degree of the Lower and Middle Berriasian (zona B and C) are rarely preserved; they often are reworked in breccia beds of Lower or Upper Berriasian age. Indeed, during the period from the Lower Kimmeridgian to the Upper Berriasian (zona D of Calpionellids), the Mesorif was situated in a tectonically active sector indicated by: 1 - intercalation of several breccia within the Kimmeridgian-Lower Tithonian carbonate series and within the Berriasian marls; 2 - the almost general Upper Tithonian and Lower Berriasian hiatus, caused by synsedimentary sliding at the top of the Upper Tithonian, at the bottom and at the top of the Berriasian and 3 - the abundance of volcanic rock pebbles at various stratigraphic levels within several outcrops in the Mesorif.
