Facies evolution and depositional model of an arid microtidal coast: example from the coastal plain at the mouth of Wadi Al-Hamd, Red Sea, Saudi Arabia

Authors: IBRAHIM MOHAMED GHANDOUR, HAMAD A. AL-WASHMI, RABEA A. HAREDY, AAID G. AL-ZUBIERI

Abstract: This study describes the subsurface sedimentary facies of a modern Red Sea coastal plain at the mouth of Wadi Al-Hamd, northern Saudi Arabia, in an attempt to infer the influence of aridity and limited tidal range on facies characteristics. The study provides criteria to delineate the fluviomarine transition in a setting where tidal signatures and other marine influences are weak. Six manually collected cores (1.4 to 1.75 m long) from the channels at the wadi mouth, beach ridge, intertidal flat, strandplain, and supratidal flat enabled the identification of 23 sedimentary facies. The sediments displayed rapid and tremendous variability in facies both within and among cores. They were deposited generally under low energy conditions occasionally punctuated by short-lived high energy events. The sediments in the channel exhibit features characterizing deposition under an arid and semiarid climate, such as rapid and intermittent discharge and short-lived rapidly abandoned channels. The sediments and depositional settings on both sides of the wadi mouth are different. A low gradient intertidal flat influenced by wave and tidal processes develops to the south, whereas a relatively high energy wave dominated strandplain occurs to the north. The deposition in the supratidal flat was accomplished by a short-lived high energy washover event. The facies stacking pattern indicates that the depositional systems are in a state of transgression, with a regime of gradually increasing accommodation possibly during the rising of relative sea level and diminishing of sedimentation. The turnaround from regressive to transgressive stage that coincides with the fluviomarine transition in channel fill deposits is placed at a level of increased soft sediment deformation because the tidal influence is weak in this microtidal setting. Though the plane view shows a deltaic shoreline, the depositional system is interpreted as estuarine. The findings of this study can be applied to similar recent and ancient settings.

Keywords: Fluviomarine transition, arid microtidal coasts, washover deposits, estuarine vs. deltaic coasts, soft sediment deformation, Wadi Al-Hamd

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