Abstract
Mineralogical and geochemical characteristics of the shallow subsurface sediments retrieved from a short sediment core (2.05 m long) collected from the tidal flat south of Al-Kharrar Lagoon, Rabigh area, Saudi Arabia, are presented to determine the impact of temporal change of depositional environment and distinguish the principal control(s) on their chemical composition. The sediments are dominantly siliciclastic, consisting of two vertically stacked sedimentary fades: lagoonal (LG) gray silt-rich mud to argillaceous very fine sand at the base and intertidal flat (TF) brown mud and argillaceous very fine-grained sands at the top. The sediments of the two facies are similar in their mineralogical and chemical composition except for slight vertical variations in the relative abundance of minerals and concentrations of major oxides and trace elements. The mineralogical composition is dominated by quartz, feldspars, and plagioclase with less abundant clay minerals and hornblende, all of detrital origin. In addition, traces of high and low Mg-calcite, dolomite, and gypsum were recognized. Geochemically, the sediments are first-cycle and compositionally immature. The Al2O3/TiO2 ratios range from 9.4 to 17 and from 13 to 15.4 for the sediments of the LG and TF facies, respectively. These values suggest an intermediate igneous source rock of the Birak group that belongs to the oceanic island arc of the Hijaz Terrane, western Arabian Shield. The average values of paleoweathering indices CIA, CIW, and PIA are <50, suggesting unweathered to poorly weathered source rock that is consistent with the late Holocene arid climate. Although the redox-sensitive V and Cr in the sediments of LG facies are slightly enriched relative to the average shale, the values of the V/Cr ratio are generally <2, suggesting deposition under oxic conditions. The chemical characteristics of the sediments are primarily controlled by the composition of source rock and to a lesser extent by hydraulic sorting during transportation and deposition.