The Layla Lakes (300 km S Riyadh) had been fed by fossil groundwater until they dried up in the 1990s due to agricultural water abstraction, revealing a series of 22 sinkholes. At their walls, well stratified to laminated sediments became exposed, unlocking a hitherto unexplored paleoenvironmental and paleoclimatic archive in the center of the Arabian Peninsula.
In November 2022, sedimentary logs were recorded, including on-site spectral gamma-ray and magnetic susceptibility measurements. From four sinkholes, >600 samples were taken to unravel the geochronological, mineralogical, geochemical, and palynological evolution of the lakes at high resolution. Lithofacies analysis shows changes between laminated lake sediments and weakly stratified sebkha deposits composed of sulfates (gypsum, anhydrite), carbonates, siliciclastic components, and bioclastic remains (shells, chironomidae tubes).
Initial radiocarbon dating indicates that the laminated lake sediments comprise a time interval from recent to 300 a A.D., suggesting at least partly varve-type sediments. The sebkha facies with short lacustrine intervals covers the entire Holocene. Different sections can be stacked together by high-resolution 3D-models, generated from a drone survey (cooperation with KAUST, Saudi Arabia). Based on field observations and first data analysis, a multi-stage paleolake model is suggested: Triggered by faults in the underlying Lower Jurassic Hith, anhydrite converts into gypsum, forming a topographic bulge and a sebkha environment. During fall of the groundwater level, sinkholes collapsed in its center due to phreatic dissolution, forming the groundwater-fed lakes. All sections show a pronounced cyclicity which will be further analyzed by high-resolution multi-proxy analyses.