Sedimentary evolution of early rift troughs of the central Red Sea margin, Jeddah, Saudi Arabia

Abstract

Field work, sedimentological investigations and bio- stratigraphical data from the Jeddah area provides new information on the Tertiary depositional history of the Red Sea in the central coastal area of western Saudi Arabia.

Sedimentary rocks outcropping in the coastal plain around Jeddah are subdivided into two major units which both unconformably overlie the Precambrian crystalline basement: the Usfan Formation probably of late Cretaceous to Eocene age and the Shumaysi Formation of late Oligocene to early Miocene. The pre-rift Usfan Formation outcrops in the Haddat Ash Sham area north-east of Jeddah and the Shumaysi Formation overlies the granodioritic basement rocks in Wadi Shumaysi, east of Jeddah. The Usfan is represented by a succession of fluviatile sandstones and siltstones (Nubian type), which pass upward into littoral marine facies (phosphatic and carbonate lithologies), and supratidal facies (dolomitic marl and shales with gypsum) indicating shoreline sedimentation. These are followed by a thick red-bed sequence of fluvial sands and conglomerates of the lower Shumaysi Formation.

The presumed syn-rift Shumaysi Formation in Wadi Shumaysi is divided into three units. The lower is a section of fluviatile sandstones and conglomerates. The middle beds are fluvio-lacustrine silts and shales with two oolitic ironstone deposits. The upper unit comprises shales, siltstones, sandstone tuffs and freshwater-marine cherty limestones. Igneous activity contributes a lava at the top of the upper Shumaysi and tuffs to the basin-fill. These two sequences occur in separate troughs in the Jeddah region and their stratigraphic range from the late Cretaceous-Eocene through to the early Miocene and provides information about the early rifting history of the central Red Sea coastal margin. They represent some of the earliest rift sediments of the Red Sea basin.