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During the last 10 years several marine geological/geophysical surveys conducted on the Nile deep-sea fan have provided much new information about the large fluid seepage phenomenon, which characterizes this passive margin segment. Among the data, more than 60 sediment cores have been recovered in this area. Rock clasts were collected from eight of them as well as during deep-sea dives, notably in the area of Isis, Osiris and Amon mud volcanoes. In few areas, especially in the Western and the North-eastern provinces of the Nile fan, some clasts consist of crust fragments resulting from cementation of muddy sediment by precipitation of high-Mg calcite. These clasts correspond to a simple cementation of the mud by precipitation of high-Mg calcite: as a consequence no obvious textural or faunal difference with the ambient matrix mud can be observed. These clasts are made of autochthonous crusts derived from microbial anaerobic oxidation of methane and release of bicarbonate and sulphide in the surrounding pore water. These clasts also lack aragonite. In most of the other cases (Menes caldera, Isis, Osiris, and Amon mud volcanoes), allochthonous clasts characterized by low-Mg calcite cement, siderite or dolomite mixed with expelled mud outcrop on the seafloor. Nannofossils have allowed us to date some of these clasts. Most of the ages range from Pliocene to Pleistocene; one clast is Lower Cretaceous in age. The lithological facies often indicate coastal or deltaic environments. Some clasts are closely linked to an evaporite-rich environment (halite, gypsum, and analcime); they all provide useful information on the nature of buried sedimentary series covering the margin. Their presence on the seafloor is due to the expulsion of a pressurized mixture of sediment, water, and gas. This process is believed to have used pre-existing tectonic conduits. |
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