The Mountain Plateau hayl as a Specific Form of Karst Depression in the Al-Hajar Mountains (South-Eastern Arabia)

This contribution investigates the formation of karst depressions containing metres of fine sediment and prehistoric artefacts at an elevation of around 1000 m a.s.l. in the central part of the Al-Hajar Mountains in south-eastern Arabia. Through field reconnaissance, satellite imagery analysis and laboratory methods, we highlight the origin of their thick sediment deposits and the factors enabling their retention in an otherwise erosional landscape. Comprehensive analyses of heavy minerals and microfossils reveal that the sediment deposits accumulated in the depressions of the Sint Karst Plateau (local term: huyul, sg. hayl) contain alluvially redeposited aeolian materials rather than in situ weathering products. The basal terra rossa incorporates dust from surrounding rock formations, while the overlying silt originates from more distant sources, presumably coastal shelves exposed during marine regression (for instance, from the Arabian Gulf). The sediment aggradation and flooding observable at huyul is attributed to clogging by suspended silt particles during the infiltration of floodwater – a process similar to siltation in modern recharge dams, although occurring at a much different temporal scale and hydraulic pressure. Satellite imagery confirms that similar clusters of karst depressions exist elsewhere in the Al-Hajar Mountains at elevations between 700–1500 m a.s.l. and that they are specifically associated with near-horizontal karst plateaus with a distinctive criss-cross pattern of fractures. These mountain plateau huyul represent valuable palaeoclimatic and palaeoenvironmental archives and have preserved aeolian dust no longer present in steep terrain of equal elevation.