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Deltaic Turbidites, Mam Tor Cliff

You quite often see cyclic deposits comprising alternating layers of sandstone fining into shale. Often they are produced by turbidity currents and are called turbidite deposits. Turbidites are a type of sub-aqueous density flow, where unconsolidated sediments sitting on a steep slope (at the end of a delta or the edge of the continental shelf, for examople) becomes unstable and as the material falls down the slope sufficient water is entrained to produce a dense turbulent mixture of water and sediment. This turbulent flow can be quite strongly erosive so individual turbidite beds can have erosive bases, sometimes even cutting their own channels but as the flow dissipates sediment is deposited, starteing with th elargest and heaviest particles and fining upwards into the smaller particles which settle more slowly. Between each turbidity event normal sedimentation resumes, producing shales. Here the turbidites are transitional between Edale shales, which were deposited far from land and the Millstone Grit and coal measure sequences which were produced in coatal/lagoonal environments. So the overall sequence is one of marine regression (shallowing sea level), with the deltaic turbidites marking the transition between deep marine and shallow marine conditions.