SEDIMENTARY FACIES, DEPOSITIONAL ENVIRONMENTS, AND MAJOR CONTROLLING PROCESSES IN AN ARID SILICICLASTIC COAST, AL QAHMAH, SE RED SEA, SAUDI ARABIA

Facies and environments of an arid siliciclastic coast are investigated in Al Qahmah, Saudi Arabia. Petrographic study of sediment composition and texture of 97 samples and facies mapping delineate facies distribution and mechanisms of sediment transport and deposition. The coast has alluvial, marine, and eolian facies associations (FAs) in 30x2 km². Alluvial FA includes wadi and delta deposits from igneous and metamorphic highland, providing copious sediments for reworking; and are medium and fine sands (Mz=1.97, 2.47φ) and poorly and moderately well sorted (δ_I=1.57, 1.03φ), respectively. Wadi sediments were transported by flash floods. Delta-front sands are redistributed by southward longshore currents to form spit and beach. Marine FA includes beach, tidal inlet and channel, washover fan, and sabkha. Beach is composed of fine sand (Mz=2.55φ) with minor gravel (Mz=-1φ), and are well and moderately sorted (δ_I=0.4, 0.9φ). Crabs and other burrowers destroy primary sedimentary structures and mix sediments in fore and back beaches, while wind and storm surges rework the berm and back beach. Sabkha facies occurs extensively (46 km²) in depressions behind beach, is flooded by marine and rain storms and tides, and contains a 5-cm-thick firm top crust of halite, quartz, albite (67, 23, 7%), and minor gypsum and biotite (2, 1%). Halite occurs as a thin layer; gypsum as nodules of a chickenwire structure. The clastic fraction ranges from coarse silt to coarse sand (Mz=4.97-0.8φ, average 3.4φ) with moderate sorting (δ_I=1.23φ), resulting probably from tidal fluctuations. Eolian association includes sand dunes and sheets covering 2.3 km². Dunes include low (1.5 m) N-NW-trending foredunes along beach, tall (7 m) NW-SE barchan and tall (5 m) W-NW seif dunes. They are located north and south of wadi due to variable wind directions. Dune sands are fine (Mz=2.32φ) and moderately well sorted (δ_I=0.6φ); but sheet sands are coarser (Mz=2 φ), indicating stronger wind in a different morphdynamic condition, and are poorly sorted (δ_I=1.12) due to vegetation baffling. All eolian sands are sourced from beach. Tidal inlet assumes abandoned wadis. Washover fans spill over beach crests. A suite of complex riverine, wave, tidal, current, wind, chemical, and biological processes form the facies mosaic in the arid Al Qahmah coast.