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ABSTRACT

Fifty-three samples from D.S.D.P. Sites 127, 128 (Hellenic Trench), 130 (Mediterranean Ridge) and 131 (Nile Cone) ranging in size from clayey silt to sand were submitted to grain-size and compositional analyses. The former confirmed the evidences obtained from the visual observation of the sedimentary features as to the presence of turbidites (Sites 127, 128, 131) and fluxoturbidites (Site 131). The cumulative frequency distributions of the latter are closely similar to those obtained from ancient deposits of the same type.

The X-ray quantitative analyses of the total samples showed that at both Site 127 and Site 128 calcite and phyllosilicates make up together, on average, about two-thirds of the total composition. Quartz averages about 20%. Dolomite, plagioclase and K-feldspars are present subordinately in this order; the former, observed in all samples, is both detrital and authigenic. In the Nile Cone quartz comprises, on average, about 70% of the composition. Phyllosilicates, plagio-clases and K-feldspars were observed in minor quantities.

The four samples of the Mediterranean Ridge, although Nile derived, show a composition closer to the Hellenic Trench samples than to the Nile Cone. This is due to the strong control exerted by the grain size on the quantitative mineralogical composition. The microscopic analyses carried out on both light and heavy minerals of the sand fraction and the clay mineralogy clearly differentiate between the Hellenic Trench and all the Nile derived sediments (Sites 130 and 131) and exclude an intermixing of even the finest fractions. Each group is well clustered in the compositional diagrams. The Hellenic Trench sediments could be classified as lithic arenites (Crook, 1960) or greywackes (Malesani & Manetti, 1970) and the Nile Cone sediments as sublabile lithic feldspathic arenites or feldspathic subgrey-wackes.

The semi-quantitative clay mineralogy showed that, on average, almost 50% of the Nile derived clay sized sediments is montmorillonite. Mixed layer minerals make up a relevant part of the remaining fraction. In contrast, the Hellenic Trench sediments show lower quantities of montmorillonite (about 10%) and prevailing percentages of illite and chlorite. Kaolinite is present only in the latter samples and not in the Nile Cone sediments.

The lack of relevant changes in the composition of the Nile Cone sediments along the cored sections of the holes suggests that the present hydrography of the Nile River dates from at least the Lower Pleistocene.