SEDIMENTOLOGY AND GEOMORPHOLOGY OF AN ARID SILICICLASTIC COAST, AL QAHMAH, SE RED SEA, SAUDI ARABIA

Sedimentary and geomorphologic characteristics in Al Qahmah, Saudi Arabia, provide insights into processes controlling sediment type, transport and environmental evolution in an arid siliciclastic coast. 97 samples were compositionally and texturally analyzed and landforms were classified. The coast contains wadi, delta, beach, tidal inlet, washover fan, sabkha, dunes and rocky headlands with pocket beaches. Wadi sediments from igneous and metamorphic highland provide copious sediments for reworking; and are medium and fine sands (Mz=1.97, 2.47φ) and poorly and moderately well sorted (δI=1.57, 1.03φ), respectively. They were transported by flash floods. Delta-front sands are redistributed by southward longshore currents to form spit and beach. Beach is composed of fine sand (Mz=2.55φ) with minor gravel (Mz=-1φ), which are well and moderately sorted (δI=0.4, 0.9φ). Wind and storm surges rework the berm and back beach. The high concentration of heavy minerals (74.4%) in the beach reflected strong winnowing processes. Sabkha occurs extensively (46 km²) in depressions behind beach, is flooded by marine and rain storms and tides, and contains a 5-cm-thick firm crust of halite, quartz, albite (67, 23, 7%, respectively), and minor gypsum and biotite (2, 1%, respectively). Halite occurs as a thin layer; gypsum as nodules of a chickenwire structure. The clastic fraction ranges from coarse silt to coarse (Mz=4.97-0.8φ, average 3.4φ), well to poorly sorted (δI=0.35-1.23φ) sand, being washed in probably by tide. Dunes and sand sheets cover 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 occur north and south of wadi due to variable wind directions. Dune sands are fine (Mz=2.32φ) and moderately well sorted (δI=0.6φ); but sheet sands are coarser (Mz=2 φ), indicating stronger wind in a different morphdynamic condition, and are poorly sorted (δI=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, and chemical processes affect the stability of coastal landforms and sediment distribution along the arid Al Qahmah coast.