Potentiality of chemical weathering under arid conditions of black shales from Egypt

The Upper Cretaceous–Lower Tertiary succession is widely distributed in Upper Egypt and consists of black shales–phosphorites–chert–dolostones association. The present work is confined to study the black shales of Dakhla (Maastrichtian-Danian) and Duwi (Campanian) Formations, from the association. Quantitative mineralogy is performed on more than 200 whole black shale samples and their clay size fractions by means of infrared spectroscopy and X-ray diffraction with the support of chemical analysis and scanning electron microscopy. Although the prevailing climatic conditions are classed as arid proper, evidence of the extensive effect of chemical weathering is conspicuous. The weathering of the heterocompounds and asphaltine creates an acidic medium, causing acceleration of mineral and chemical changes. The breakdown of organic matter and pyrite increases fissility. The average mass loss due to chemical weathering is estimated to be about 45%, where organic matter, pyrite, dolomite, ankerite and calcite are found to be the main losses. Smectite converts into illite, passing through a smectite/illite mixed layer. This transformation requires K+and H+to liberate Na+, Ca2+and Si4+. In the weathering profile the gained illite and quartz are almost equal to the smectite lost. Distinction between the black shales of Dakhla Formation and the underlying Duwi Formation, on the basis of their clay mineralogy, is very possible and it suggests a possible readjustment of the stratigraphic boundary between the two formations.