Abstract
Ghandour, I.M.; Al-Washmi, H.A., and Haredy, R.A., 2013. Gravel-sized mud clasts on an arid microtidal sandy beach: example from the northeastern Red Sea, South Al-Wajh, Saudi Arabia. Gravel-sized mud clasts (GMCs) of clay-grade material are abnormally documented from the microtidal Red Sea sandy beach at the mouth of Wadi Al-Hamd some 55 km S of Al-Wajh, Saudi Arabia. The area of study represents an asymmetric delta flanked to the N by a sandy linear shoreline and to the S by a heterolithic muddy tidal flat. Shape, roundness, and mineralogical composition of 200 randomly collected grains were determined to infer their source and depositional mechanism. The diameters of GMCs vary from 1.25 to 6.7 cm (long axis), with a mean diameter of 2.89 cm. The clasts are mostly rounded to well rounded, and the bladed forms are the most dominant shape, followed by elongated, platy, compact platy, and very rare compact bladed shapes. The GMCs are mostly unarmoured; however, large elongated forms have single-grain thick external armour of sands, shell fragments, or both. The mineralogical composition consists mainly of quartz, halite, plagioclase, and calcite, along with traces of phyllosilicates, feldspars, hornblende, mica, gypsum, anhydrite, pyrite, and dolomite. The documented GMCs are the end product of various processes that started with deposition of muddy source materials within shallow and small pools on the lower tidal flat. This muddy source was then subjected to desiccation, cracking, and detachment by repeated dryness and inundation during summer and winter, respectively. The detached clay-rich mud clasts were submerged during winter sea level rise and then transported to the beach either as bedload or lifted in the water column by high-energy waves and currents during times of sea breeze, some clasts being differentially armoured by adhering loose sand grains to the surface. The findings of this study will help to understand the environmental and climatic conditions required for the formation of GMCs on microtidal coasts in arid regions.