Middle Miocene carbonates from the Northern Plateau of the Western Desert (Egypt): Petrography and geochemistry

Summary An extensive carbonate platform of predominately Middle Miocene age (Marmarica Formation) occupies the larger part of the northern plateau of the Western Desert of Egypt. The Marmarica Formation (up to 150m thick) exposed on the cliffs facing the Mediterranean coast consists mainly of alternating limestones and dolostones. Deposition took place in a shallow and normal marine environment. The limestones are dominated by algal boundstone, crossbedded packstone and bioturbated wackestone facies. The occurrence of the crossbedded packstone facies throughout the Marmarica Formation indicates that a shallow marine environment prevailed. Lithification of the precursor carbonates took place mainly in a meteoric environment. Replacement dolomitization ranged from fabric destructive to retentative and from fabric selective to pervasive. The presence of an abundant open marine fauna, the lack of evaporites, coupled with the contents of Sr and Na suggests that dolomitization took place in solutions more dilute than normal sea water. The limestone and dolostone facies cannot be separated into distinct types based on their major or trace element chemistry. Only Mn and Sr seem to be facially controlled. Both elements are particularly enriched in the lagoonal facies compared with the organic buildup facies. The difference in the Mn content is attributed to their proximity to continental areas at the time of deposition. The differences in Sr values are interpreted by an originally differing mineralogy (calcite versus aragonite) and different rates of diagenesis. Dolomitization did not appear to influence the Mn content as substantially as the Sr content. The amount of the acid insoluble residue is controlled by the distribution of Si, Ti, Al, Fe, k, Rb and Zr.