SEDIMENTARY PROCESSES/Depositional Sedimentary Structures 597 Figure Reactivation surface within a tabular cross bedded set produced by the migration of a low relief sand wave The discontinuity reflects modification of the lee side of the bedform during emergence during falling water discharge Tana River, Norway Compound Bedforms and Bars Figure Examples of cross bedding in sandstones (A) Coset of small sets with slightly asymptotic foresets The bounding surfaces between sets are themselves inclined downstream, sug gesting deposition on the downstream side of a larger compound bar form Westphalian, Northumberland, England (B) Tabular sets with thin units of ripple lamination between sets The foresets are essentially planar Namurian, Staffordshire, England tabular sets vary in shape This reflects the association between trough sets and lee-side scour pits and the fact that steep, three-dimensional dunes, which commonly have such pits, are associated with stronger currents Tabular sets record the migration of dunes or sand waves without scour pits, probably with straight or only gently curved crest lines Changes in the shape of foresets along a set record changes in flow strength during the life of the bedform They are a particular feature of tidal sediments and the deposits of rivers with large discharge variations Wave action on emergence during the falling stage may create low-angle erosion surfaces, truncating the foresets Succeeding foresets re-establish the higher angle dips above these ‘reactivation surfaces’ (Figure 8) Flow reversals in subtidal settings produce similar structures and tidal slack water can cause mud drapes to be interbedded with the foresets In exceptional cases, the spacing of drapes can be interpreted in terms of spring-neap tidal cycles (‘tidal bundles’) In many wide, sandy rivers, the large repetitive bedforms described above are organized into compound forms, which commonly appear to scale with the channel itself Some of these features are attached to channel banks, whilst others are located in mid-channel Some examples have a clear asymmetry to their streamparallel profile with a slipface at the downstream end In other cases, dunes descend a more gently inclined lee side These compound features are commonly referred to as ‘bars’, of which there are many types The important point, in terms of internal structure and its interpretation, is the relative movement directions of dunes and of the accretion of the bar Where the bar advances downstream, in the same direction as the dunes, either a large, cross-bedded set will be present beneath smaller cross-beds, or the bounding surfaces between sets will be inclined downstream Where bars move laterally as a result of dunes migrating along their flanks, bounding surfaces dip in a direction highly divergent from that of the cross-bedding (lateral accretion surfaces) Upper Flow Regime Bedforms and Lamination In situations in which water flow is very rapid and/or very shallow, saltating sand close to the bed moves continuously over an essentially flat surface This phase of movement is known as ‘plane bed transport’ with the moving grain layer termed a ‘traction carpet’ When the flow is particularly strong, the bed develops gentle undulations, forming ridges transverse to flow or three-dimensional domes When this happens, the water surface also takes on a wave form which parallels that on the sediment surface These undulations are ‘standing waves’ (Figure 9) Where the standing waves on the water