SEDIMENTARY PROCESSES/Deposition from Suspension 15 Bedforms from Suspension Bedforms from sandy suspensions are covered in (see Sedimentary Processes: Depositional Sedimentary Structures) Some structures seen in the deep sea, but rarely preserved in mudstones, are worth noting Mud Waves Mud waves are regular undulations of the sediment surface with wavelengths of about 0.5–3 km and heights of 10–100 m Most mud waves are very nearly symmetrical, but they contain subsurface layering that indicates migration Observed migration usually is up-slope and up-current, but instances of downcurrent migration have been observed Under a simple flow, maximum shear stress is expected on the upstream face of a wavy bedform, and lower shear stress, with a greater deposition rate, on the downstream side This would give downstream migration of the wave Commonly observed upstream migration has suggested to some that mud waves are analogous to fluvial antidunes developed under a supercritical flow An alternative is that the mud waves form under internal lee waves initially triggered by an upstream topographical disturbance, without the necessity for a supercritical flow speed Temperature data over mud waves show that such an upstream phase shift does occur, that the implicit internal-wave phase velocity is 0.05 m s 1, and that this is very similar to the measured flow velocity required for the internal wave to be stationary The flow pattern over the waves has widely spaced stream lines, giving a small velocity gradient and shear stress (¼ high deposition rate) on the upstream slope and the opposite on the downstream slope This would give the observed upstream migration Figure Photographs of bedforms in mud from the deep sea (A) Longitudinal ripple Scale bar has penetrated the mud; width of view, $40 cm; relief of ripple, $12 cm (B) Barchan ripples Both from the Nova Scotian Rise at 4800 m depth in deep-sea photographs These structures have not been recorded from shallow marine or estuarine muds, although closely spaced features up to a few tens of centimetres apart have been seen on tidal flats Longitudinal Ripples Smaller, Current-Controlled Bedforms Longitudinal ripples are elongated features parallel to the depositing flow, probably with helical secondary circulation involved in their formation In the deep sea, they are 5–15 cm high, 0.25–1 m wide, spaced at 1–5 m apart, and up to 10 m long, and have a generally symmetrical cross-section with sides slightly concave upwards (Figure 7A) In many cases, the ripples have a mound of biological origin at the upstream end Surface markings on some ripples demonstrate the action of oblique flows, with flow separation and a zone of helical reversed flow on the lee side Dating by 234Th (half-life, 22 days) suggests that longitudinal ripples form by deposition from suspension, occurring in a few episodes of very rapid deposition following deep-sea storms Subsequently, the ripple is scoured by flows that may be oblique to its trend, giving the surface markings seen Smaller, current-controlled bedforms are also revealed by deep-sea photography The photographed features of the seabed can be arranged in a sequence indicative of increasing flow speed The progression is from tranquil seafloor (biological mounds, tracks, trails, and faecal pellets), through increasing overprinting by current effects, to features showing clear evidence of erosion Biological activity is almost ubiquitous, so that a smoothed surface is indicative of an appreciable current, sufficient to remove the surface effects of biota The most common features are actually biologically produced faecal mounds, tracks and trails, and pelleted surfaces Mounds are often modified by current activity, the most frequent structure being mound-and-tail formed by lee-side deposition As suggested above, it may be that longitudinal ripples are very large tails on mounds Both structures are