Caractères sédimentologiques, morphologiques et paléogéographiques de la Plate-forme Carbonatée de l'avant-pays Rifain Oriental (Maroc) au Jurassique supérieur et à l'Eocrétacé

Abstract

On the northwestern African passive margin, from Upper Kimmeridgian to Lower Berriasian, the Rift platform is characterized by a very thick muddy carbonate sedimentation. This platform grew up between the northward Rift basin and the southward emerged Atlatic domain. The sequential arrangement shows a general regressive evolution but without a true progradation of the sedimentary bodies because sedimentation and subsidence rates are approximately equivalent. Three sedimentary systems are described; each system is defined as a functional unit with elementary patterns, the evolution of which is controlled by a number of factors which interact with each other. The factors include basement tectonics, hydrodynamics, biologic activity and climate. In the first system, the general pattern is a homoclinal ramp that may have exceptionally skeletal sand shoal complexes (northern edge of Beni Snassen mountains). The second system is characterized by instability of the basement: at the beginning the platform appears as a mosaic of low and shallow zones (lagoons) and pellet sand shoals, then between the Rift basin and inner part of the platform the mobility of the basement induces tilting of blocks as those of passive (extensional) continental margins with high or moderated energy sandy deposits or reef-buildups on the block crests; at last the system finds again a situation of relative stability. In the third system, sheltered behind a series of bioclastic or oolitic sand shoals restricted to the platform edge, a very shallow zone develops with confined and supersalted or unsalted lagoons and algmats on intertidal or supratidal shoals and on the margino-littoral area. A number of common features allow us to define a most general pattern of neritic carbonate platform with: (1) a circalittoral muddy outer part with sponges in quiet deep water, (2) a transition zone with a sand bioclastic barrier or not, (3) a sheltered shallow inner part with muddy sedimentation where a more or less important benthic biologic activity (specially algal activity) develops and (4) a margino-littoral area with algmats and evaporitic deposits.