Rock composition and origin of late Cretaceous shales in the Duwi Formation, Upper Egypt.

Abstract

Duwi Formation that crops out between Qena to the north and Idfu to the south is composed of a number of phosphorite beds intercalated with shales, marls, cherts, and limestones. The shale intercalations were subjected to field and petrographic, mineralogical and geochemical investigations in order to examine their rock composition and origin. Shales are mainly fossiliferous and composed essentially of argillaceous materials, quartz, feldspar, phosphatic pelloids and skeletal fragments. The light fraction of the separated sand fraction is composed of quartz, plagioclase and microcline. The heavy fraction consists of opaque minerals, zircon, tourmaline, epidote, garnet, rutile and phosphates. X-ray diffraction analyses of bulk rocks identified quartz, francolite, gypsum, anhydrite, microcline, albite, halite, goethite and limonite as non-clay minerals. The clay minerals are smectite, kaolinite, and illite. The study have revealed that the constituents of these shales were inherited from the weathering of mafic igneous rocks as well as quartzose sedimentary rocks situated to the south and southeast of the study area, transported via a fluvial system into shallow marine environment. The abundance of smectite suggests the deposition of those shales in a period of transgression and arid to semi-arid climatic conditions.