Late Pleistocene to Holocene Architecture of a Land-attached Carbonate Platform Lagoon in the African-Arabian Desert Belt (Al Wajh platform, N Red Sea, Saudi Arabia).

Investigation of carbonate platform architecture is a crucial element to understanding the evolution of a platform. Extensive studies have been done on the architectures of various modern carbonate platforms. However, compared to humid climates, detailed studies in arid climates are rare, although many ancient carbonate reservoirs are developed under these conditions.

This study investigates the Late Pleistocene architecture of the land-attached Al Wajh carbonate platform in the Northeastern Red Sea, Saudi Arabia. The platform is enclosed by a coral reef belt and characterized by a large lagoon (1,100 km²). The lagoon reaches 43 meters in depth and hosts more than 90 carbonate islands and numerous pinnacle and patch reefs. We utilize 700 km hydroacoustic data acquired using EdgeTech sub-bottom profiler during two research cruises with KAUST RV EXPLORER. An age model was established by utilizing a recently published Red Sea sea-level curve. Available climate data were used for the reconstructions of depositional environments.

Data analysis reveals five depositional units: U1(Holocene) to U5(Late Pleistocene). Nine hydroacoustic facies are identified to describe the internal architecture, from homogenous reflection-free to wavy laminated facies. The oldest unit (U5) consists of homogeneous facies and reef facies. The unit is overlain by units 4 and 3, with up to five meters thick homogeneous facies and stratified facies. Unit 2 has a maximum thickness of 3 meters and consists of wavy laminated facies. Unit 1 is the youngest unit and consists of several facies, including heterogeneous, homogeneous, stratified, drift, reef, and reef debris facies. During MIS5e (U5), the Red Sea was experiencing a pluvial period, while the sea level was 10 meters higher than the present, leading to total flooding of the lagoon. Most of today's exposed carbonate islands in the lagoon correspond to carbonate accumulation during MIS5e. The depositional environment is interpreted as carbonate-dominated with the frequent siliciclastic influx in the coastal region during heavy rain. In the subsequent periods (MIS 5d to 5a), sea level dropped stepwise and exposed the platform partly. Stratified facies indicate terrestrial sediment input introduced during short pluvial periods. In the following glacial period (MIS 4 to 2), the platform was fully exposed for over 70,000 years. Due to the hyper-arid climate, we interpret unit 2 as an aeolian deposit likely reworked during Holocene transgression. During the platform's flooding in the Holocene, carbonate sedimentation restarted while coastal near stratified facies indicate an increased terrestrial influx during the short Holocene pluvial period (10,000-6000 years ago). The modern Al Wajh lagoon experiences an arid climate, with active carbonate sedimentations and minimal terrestrial input. Although the Red Sea has experienced several humid periods during the last 125,000 years, and extensive diagenetic alteration is recognized in the island's drill cores, no karst morphology has been identified.

Results indicate that climate highly influences Al Wajh lagoon architecture, shown by its unique characteristics, including extensive carbonate deposition, intermittent terrestrial influx including aeolian deposits, and minimum karstification. Insights of this study will improve our understanding of the architecture of carbonate platforms in the subsurface deposited under similar conditions.