The geomorphology of monoclinal scarps associated with interstratal-dissolution fronts in evaporite formations, illustrated with the Upper Jurassic Arab and Hith formations in Ar Riyadh and Central Saudi Arabia

Abstract

The downdip migration of dissolution fronts in gently tilted evaporite formations produces updip-facing monoclinal folds and adjacent synformal troughs (depositional basins) in the supra-evaporite strata. These gravitational deformation structures, up to 1000 km long, can be expressed in the landscape as laterally migrating fold escarpments and linear depressions, forming the largest karst features on Earth. Despite their large dimensions and numerous associated practical implications (hydrocarbon, salt, geostorage, water quality, sinkhole hazards), the scientific publications dealing with these interstratal evaporite karst features are rather scarce. This work reviews the available literature on dissolutional edges and associated features developed on dipping salt and gypsum/anhydrite formations. It also analyses the >800 km long dissolution and subsidence belt associated with the updip edge of the Upper Jurassic Arab and Hith anhydrites in the Interior Homocline of central Saudi Arabia, with special focus on its striking geomorphic features. This is the largest Ca-sulphate karst feature in the world, despite the aridity of the region, and is also the example in which the associated landforms and deformation structures are best displayed. It displays striking monoclinal scarps with an aggregate length of 420 km, affected by crestal extensional structures and punctured by numerous giant caprock collapse sinkholes. The increased sinkhole hazard and risk documented in the Ar Riyadh area in recent times can be attributed to adverse human activities (localized artificial water input) and the expansion of the urban area across the dissolution front and monoclinal scarp.