Abstract

Some marl and impure chalk beds of the Thebes Formation (Lower Eocene) at its type-section of Gebel Gurnah, southern Egypt, display, on exposed surfaces, varying degrees of spheroidal weathering. Sepiolite, palygorskite, and smectite clay minerals are frequent in these rocks besides calcite, dolomite, and rare quartz (chert). It is suggested that the spheroidal weathering is due to expansion of smectite and other expandable mixed-layer clays in concentric zones at the exposed free outer surface of the beds. Initially, the affected sediments have a massive homogeneous appearance and most probably have had a random orientation of their clay particles. The concentric structures were initiated by Liesegang diffusion of dew water into bed exposures, starting from spaces between joint and fracture polygons of the rock. Diffusion proceeded inwardly into the cores of the polygons producing alternating concentric zones of different tenacities. Planar exfoliation (papery weathering) is apparently restricted to laminated beds and to beds or parts of beds whose expandable clay particles were shear-oriented by friction slippage along bedding planes during Miocene or later fault tectonics.

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