Microbial and physical sedimentary structures in modern evaporitic coastal environments of Saudi Arabia and Egypt

Abstract

Field and petrographic studies of recent supratidal sabkhas and ponds, and a solar salt works, in Saudi Arabia and Egypt have documented the formation of distinctive surface sedimentary structures that have resulted from microbial activity and abiological physical processes. Microbially induced sedimentary structures (MISS) dominate the permanent and ephemeral parts of supratidal ponds of Al Zeeb sabkha, Saudi Arabia and halite crystallization ponds in salt works, west of Alexandria, Egypt. They are varied and include gas bubbles, blisters, wrinkles, pinnacles, cones, and polygonal folds (petees) induced by epibenthic microbial mats. Physically induced sedimentary structures dominate the emergent areas surrounding the ponds, as well as the supratidal sabkhas in Al Zeeb and Ras Shukeir areas. They include polygonal cracks and different types of tepees. The sediments of the microbial-induced structures are composed of green and brown microbial filaments that entrap and bind lenticular and clastic gypsum, or form nucleation sites for halite and/or grass-like gypsum crystals. The sediments of the physically induced structures are composed of halite-cemented siliciclastic sand and mud, or bottom-nucleated chevron and cornet halite crystals. The results of this study indicate that microbial and physical structures co-exist due to local factors, especially topography, brine recharge, salinity, microbial activity, and history of the supratidal sabkha and pond. The importance of the local interplay of these conditions indicates that it will be difficult to interpret sedimentary successions in fossil sabkhas and their general depositional environment if only limited sections are available for study.