DEPOSITIONAL ENVIRONMENT AND DIAGENESIS OF THE MIDDLE JURASSIC BLACK SHALES AND ASSOCIATED IRON SULFIDS, CENTRAL SINAI, EGYPT

Petrographic studies of selected core samples from the subsurface Middle Jurassic sequence (Bajocian- Bathonian strata) in Central Sinai, Egypt depicts different lithofacies that were deposited in complex environmental conditions. Variations in the depositional settings have resulted in formation of different lithologies that range from strata that were formed in oxic condition (highly bioturbated with depleted pyrite), to euxinic strata (fine lamination, coal layers, and sulphide-rich sediments). The deposition of these Bajocian- Bathonian strata (Middle Jurassic) were controlled by major cycles of rapid sea level changes and sequence of uplift movements. Three major lithofacies were recorded as follows: (1) lagoon and shoreline deposits (highstand system tract); (2) estuarine with tidal flat deposits (transgressive system tract); and (3) deltaic deposits (lowstand system tract).

Inductively Coupled Plasma (ICP-MS), X-ray, and Sulfur isotope analyses of the anoxic organic-rich black shale indicate that it mainly consists of Kaolinite, montmorillonite, chlorite, and iron/sulfur mixed layers. The high TOC (13.8%), Vanadium (370 ppm), high Ba content, low Th/U ratio, the high degree of pyritization (up to 0.86), and the close positive relationships between total sulfur and Cr, Ni, Cu, Mo indicate pervasive euxinic- depositional environment of the black shales.

SEM studies reveal the presence of pyrite as cubes and framboids of different shapes and sizes. Framboidal pyrite is made up of spherical aggregates of submicron-sized pyrite crystals densely packed together. The δ ³⁴S value of the framboidal pyrite ranges from -7.4 to -5.8 ‰ V-CDT and indicates their formation through bacterial reduction in euxinic conditions just below the sediment/water interface in an open system.