FACIES DEVELOPMENT AND DEPOSITIONAL FRAMEWORK OF THE MIDDLE MIOCENE CARBONATES (MARMARICA FORMATION) BETWEEN RAS EL-HEKMA AND MATRUH, NORTHWESTERN EGYPT

The Middle Miocene Marmarica Formation represents a wide carbonate platform that occupies the larger part of the northern plateau of the Western Desert of Egypt.The main objectives of the present study include the recognition of the major sedimentary environments and the main diagenetic processes of the Middle Miocene carbonates exposed between Ras El-Hekma and Matruh, northwestern Egypt. Detailed facies analysis led to the recognition of ten carbonate microfacies that are grouped into five groups: floatstone/wackestone, wackestone, packstone, rudstone/packstone and grainstone. Only two clastic rich units were recognized and are differentiated into glauconitic calcareous sandstone and fossiliferous calcareous quartz wacke. The carbonate facies contain rich and diversified fossil assemblage dominated by bryozoans, small and larger benthonic foraminifera, molluscs, echinoderms, coralline algae, ostracods, balanids, serpulid worms and minor planktonic foraminifera.

Deposition took place on a carbonate ramp as indicated by the absence of genuine reefs and the seaward thickening of the Marmarica Formation which suggests a wedge shape geometry. The recorded fauna suggest a fully marine environment within the photic zone. According to the zoarial growth of the bryozoa, the bottom of the Mamarica sea was not deep (15-45 m optimum depth) and the climate was moderate to warm. The Ti/Al ratio of the Marmarica limestone varies between 0.076 and 0.0141, which suggests that terrigenous input dominated by aeolian dust. Hence it is possible to assume that a relatively arid climate prevailed during the Middle Miocene. Such arid climatic conditions are normally associated with lower riverine run off and enhanced aeolian dust production. Early marine intergranular cements are not clearly visible due to the high content of muddy matrix in most facies. However, the effect of early marine cementation is mostly inferred by the scarcity of compactional features. The filling of the biomoldic porosity by blocky and drusy cements suggest diagenesis in a meteoric phreatic setting. Other features of meteoric diagenesis include syntaxial overgrowth on echinoderm plates and spines. Neomorphosis of the micritic matrix is common event affecting the carbonates of the Marmarica Formation.