SEDIMENTARY PROCESSES/Depositional Sedimentary Structures 601 Figure 15 A schematic profile through a turbidite sandstone bed showing a complete sequence of Bouma intervals and their hydrodynamic interpretation (commonly graded) sandstone; B, parallel laminated sandstone; C, ripple cross-laminated sandstone; D, horizontally laminated (commonly silty) sandstone; E, mudstone The complete succession of lamination types is known as the ‘Bouma sequence’ (Figure 15) The A interval records rapid deposition from suspension, with graded bedding reflecting sorting within the flow of a mixed grain size population The B interval records upper flow regime plane bed transport, whilst the C interval records ripples moving in the lower flow regime Climbing ripple lamination is quite common, recording rapid rates of bed aggradation, and convolute lamination may also result from rapid deposition The D interval, often poorly displayed in weathered exposures, records fallout from suspension with weak reworking on the bed, whilst the E interval grades into the background sedimentation from suspension The overall succession, however complete or incomplete, records an episodic current, possibly erosional in its early stages, and progressively decelerating thereafter Most commonly, the Bouma sequence is associated with deep-water settings where the episodic flows are turbidity currents (see Sedimentary Processes: Particle-Driven Subaqueous Gravity Processes), but similar decelerating flows occur in shallow-water settings of floodplains, shallow lagoons, and lakes due to Figure 16 Coarse fluvial gravels showing clear pebble imbrica tion The flat surfaces of the clasts dip upstream, indicating that flow, in this case, was from left to right Holocene, Pyrenees, Spain crevassing of river banks during floods Similar sands also occur in shallow marine settings as a result of storms, although the Bouma sequence may then be modified to include hummocky cross-stratification and wave ripples Gravel Forms and Fabric Whilst the preceding section deals mainly with sands and finer grained sediments, powerful water currents also transport gravel, and its deposition can lead to distinctive structures and fabrics (see Sedimentary