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Pleistocene to Holocene Transgressive System (TST) of the Western Arabian Gulf, Saudi Arabia


Alternating layers of unconsolidated sand and well cemented beachrock that transition to hardgrounds are found along the western margin of the Arabian Gulf in central eastern Saudi Arabia. The deposits extend from Half-Moon Bay (N 26.023148°, E 50.032806°) southward to the Salwa beach (N 24.747859°, E 50.761675°) near the Saudi - Qatar border. The parallel to shoreline extensions of these cemented deposits range from hundreds of meters to tens kilometers and extend eastward (seaward) from a few hundred meters to over 1 kilometer and appear to continue to deeper parts of the basin. The cemented layers are 4 to 6 cm thick and alternate with unconsolidated sand consisting of carbonate bioclasts and fine quartz sand. The topmost beachrock layer is found in places onlapping to sabkha deposits or interfingering with the beach sand. Beachrock is in most places continuous and exposed at low tide up to 300 m eastward from the high tide mark, and technically becomes a "hardground" as it is subtidal extending as much as 1 km from the high tide surf mark. The surface cemented layer is covered by a thin veneer of sand less than 1 cm thick and underlained by unconsolidated sand at the shoreline. A second cemented layer is penetrated twenty to thirty meters from the shoreline at a depth of 20 to 25 cm below the surface of the topmost cemented layer and is followed by a layer of unconsolidated sand. These deposits are similar to those documented in late 60’s by Shinn on the western shorelines of Bahrain and Qatar and in the open Arabian Gulf (between Bahrain and Qatar and Saudi Arabia) at depths of up to 30 m. Near the shoreline the surficial cemented surface is covered by blue-green (cyanobacteria) in the 20 to 30 m proximal to the high tide surf line, and in some areas transitions to a rhodolith covered zone than extends for 30 meters seaward, in most places it is followed by a surface covered by brown algae, encrusting sponges, and bryozoans. Bivalves and barnacles encrust the upper surface of most the layers in first 100 meters from shoreline. The underside of the cemented sediments has an abundant microbial cover, and serpulid worm tubes, and often an EPS cover. In the studied localities water has been observed to rise through drill holes to nearly 0.05 cm above the uppermost surface of the beachrock sequence and from 0.5 to 1.0 m above advancing sea level and 50 to 100 m inland. Suggesting that tidal pumping through the poorly cemented or uncemented layers maybe driving cementation and replenishing the narrow sabkhas along the coast of the study area. The striking similarity to the deeper water sequencesdocumented by Shinn in the late 60’s suggests the topmost surface is the maximum flooding surface of the Pleistocene to Holocene transgressive system tract. Detailed mapping, systematic coring and fluid sampling, and hydrodynamics measurements is being conducted to document the evolution of the system.