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Description: The Ouarzazate Basin is the southern foreland basin to the High Atlas Mountains in Morocco. The sedimentary fill records a sequence from the Eocene to Pleistocene that records the interplay between tectonics and climate. This study presents the first stable isotope and facies analyses of the Middle to Late Miocene Aït Ibrirn lacustrine Member (Aït Kandoula Formation). These data test whether the basin was internally draining and enable the development of palaeoenvironmental models for the Middle to Late Miocene. Five sedimentary facies of lacustrine and palustrine limestones are interbeddded with extensive sequences of palaeosols and fluvial sandstones and conglomerates, often associated with evaporite (gypsum) development. These facies can be divided into two facies associations related to water depth and sub-aerial exposure within the basin. In the Serravalian and Tortonian shallow water successions dominate the stratigraphy, typical of underfilled foreland basin settings. Furthermore, carbonate δ18O and δ13C isotopes from the sections show covariance confirming that these carbonates were deposited within a hydrologically closed basin. However, late Tortonian to Messinian carbonates do not demonstrate the covariance typical of endorheic basins. Additionally, the facies association indicates the presence of deeper water lake systems demonstrating that the basin was externally draining at this time. These results question the established view of tectonic stagnation in the Late Miocene and suggest that the Cenozoic sediments of the Ouarzazate Basin contain a rich and untapped record of climate change and tectonic evolution on the edge of the Sahara desert.