DISTINCTIVE TYPES OF RIPPLE‐DRIFT CROSS‐LAMINATION

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Abstract
From work on two Upper Carboniferous formations in southwest England, three morphologically distinct types of ripple‐drift cross‐lamination have been recognised. Type 1 is characterised by strong erosion of laminae on the stoss (or up‐current) side of the ripples, and absence of grading. Type 3 is characterised by an absence of erosion on the stoss sides, concentration of mud in the ripple troughs and an upward gradual decrease in grain size and amplitude of rippling. Type 2 is an intermediate form with some characteristics in common with types 1 and 3. An examination of the illustrated literature has shown that types 1 and 3 occur frequently, and that type 2 has not pre‐ viously been recorded.It is suggested that type 1 is formed in fluvial and shallow water environments at times of net deposition of sediment, and that type 3 is formed by deposition from a tur‐ bidity current. Type 2 suggests hydrodynamic conditions intermediate between fluvial or shallow water traction currents, and turbidity currents.Since all current ripples move forwards, or “drift”, it is suggested that the term “ripple‐drift” should be used to describe ripple cross‐lamination where the ripples can be seen to climb onto the stoss slope of the ripple immediately downstream, there having been a net deposition, and not merely a forward drift of sediment.