International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

CLAY MINERALOGY OF THE NUBIA FORMATION, WESTERN DESERT (EGYPT)

Pages: 329 - 336

Authors: HASSAN MOHAMED BAIOUMY, ISMAEL SAYED ISMAEL, IBRAHIM HASHEM ZIDAN

Abstract: The term Nubia Formation has been used in a broad range of stratigraphic and sedimentological connotations to designate terrestrial sandstone of Paleozoic to Mesozoic age in Egypt. The Nubia Formation is the oldest exposed rock unit and forms the basal part of the scarp face and the floor of Kharga and Dakhla Depressions. It is overlain by the Qusseir Shale Formation. The Nubia Formation in the studied locality is composed of very thick sandstone intervals intercalated with several thin shale intervals. Bulk samples are composed of clay minerals and quartz with traces of feldspars. Clay fractions separated from the different intervals of the Nubia Formation are dominated by kaolinite, smectite, and illite. Kaolinite is the major constituent in all samples. Smectite represents a major constituent in the shale intervals while it occurs as traces in the sandstone intervals. Illite occurs as traces in some samples. Clay minerals are classified into three assemblages. Based on X-ray and Scanning Electron Microscope analyses, the studied kaolinite is classified into two types. Authigenic kaolinite forms highly crystalline pore-filling books of stacked hexagonal flakes and occurs between quartz grains in the sandstone intervals while detrital kaolinite of lower crystallinity associates smectite and illite in the shale intervals. In the shale intervals occurrence of smectite as poorly crystalline flakes of unclear outlines is suggestive of detrital origin. Authigenic kaolinite formed during diagenesis of the Nubia Sandstone as a result of complete or partial dissolution or replacement of detrital feldspar grains. Abundance of detrital smectite in the shales intervals suggests their formation under arid or semiarid climatic conditions.

Keywords: Egypt, Nubia Formation, diagenesis, kaolinite, smectite, illite

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