Abstract
The present work investigates geochemical and mineralogical signatures preserved in the fluvio-lacustrine sediments of the middle-upper Miocene Bathan Formation at Al-Rehaili area, northern Jeddah, Saudi Arabia to constrain the composition and tectonic setting of the source area and to appraise the influence of weathering. Sixteen sediment samples were collected from the lacustrine delta plain and delta front sandstones (SS I), lacustrine offshore mudrocks, and the younger fluvial channel sandstones (SS II). The sandstones are compositionally immature first cycle. The sandstones (SS I) and the mudrocks of the first depositional stage were derived from an intermediate igneous source rock, whereas the sandstones (SS II) of the second depositional stage were derived mainly from a felsic igneous source rock with subordinate contribution from an intermediate igneous source rock. They indicate an active continental margin to oceanic arc tectonic setting. The values of the Chemical Index of Alteration (CIA) and the Plagioclase Index of Alteration (PIA) as well as the Al2O3/Na2O, Rb/Sr, and Al2O3/(CaO+Na2O) ratios indicate that the source rocks experienced low to moderate degree of weathering consistent with arid to semi-arid climate that prevailed during the Miocene. Subaerially exposed lacustrine offshore mudrocks displayed excessive depletion in the concentration of mobile elements comparing to the bedrock. The low values of the paleoweathering indices are further explained by the enhanced uplift of the Red Sea Escarpment after the cessation of the first stage of the Red Sea floor spreading at 14-15m.y. ago.