POTENTIALITY OF CHEMICAL-WEATHERING UNDER ARID CONDITIONS OF BLACK SHALES FROM EGYPT

The Upper Cretaceous-Lower Tertiary succession is widely distributed i n Upper Egypt and consists of black shales-phosphorites-chert-doloston es association. The present work is confined to study the black shales of Dakhla (Maastrichtian-Danian) and Duwi (Campanian) Formations, fro m the association. Quantitative mineralogy is performed on more than 2 00 whole black shale samples and their clay size fractions by means of infrared spectroscopy and X-ray diffraction with the support of chemi cal analysis and scanning electron microscopy. Although the prevailing climatic conditions are classed as arid proper, evidence of the exten sive effect of chemical weathering is conspicuous. The weathering of t he heterocompounds and asphaltine creates an acidic medium, causing ac celeration of mineral and chemical changes. The breakdown of organic m atter and pyrite increases fissility. The average mass loss due to che mical weathering is estimated to be about 45%, where organic matter, p yrite, dolomite, ankerite and calcite are found to be the main losses. Smectite converts into illite, passing through a smectite/illite mixe d layer. This transformation requires K+ and H+ to liberate Na+, Ca2and Si4+. In the weathering profile the gained illite and quartz are a lmost equal to the smectite lost. Distinction between the black shales of Dakhla Formation and the underlying Duwi Formation, on the basis o f their clay mineralogy, is very possible and it suggests a possible r eadjustment of the stratigraphic boundary between the two formations. (C) 1996 Academic Press Limited