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olistostrome (olisthostrome) A sedimentary deposit which consists of a chaotic mass of rock and contains large clasts composed of material older than the enclosing sedimentary sequence. The clasts may be gigantic and are then called ‘olistoliths’. Such deposits are generally formed by gravity sliding of material, sometimes into oceanic trenches. Olistostromes have also been called ‘sedimentary mélange’ (see MÉLANGE).