Composition and diagenesis of ancient Shali city buildings of evaporite stones (kerchief), Siwa Oasis, Egypt

Abstract

Shali is an ancient fortress, built in the XII–XX century in Siwa, Northwest Egypt. It is built on two Middle Miocene limestone and marl hills, between the wadi plain. The architecture of the fortress buildings is composed of local materials derived from the Miocene, Quaternary, and recent salt lake deposits. The framework blocks of the city are mainly composed of salt (Kerchief), limestone, and bentonite, wood particles, dry date seeds, and bones derived from ancient tombs. In this study, the modification and mineral phases that developed during the diagenetic alteration and cementation of the salt and clay mortars with different framework in the walls of the ancient houses and settlements will be followed and interpreted. Diagenesis included transportation of salt materials from the nearby Fetnas Lake and mixing with some sand and clays as mortar pressed into the voids between the frameworks (Kerchief blocks). This resulted in dehydration, gypsum crystallization, and halite cementation. The continuous crystallization through the epitaxial growth of halite in both cement and framework blocks results in strong adhesion and binding of the framework. At the end of the process the kerchief blocks and the cement will be completely homogenous to a point that they apparently no longer be distinguished.