Geol 135 Sedimentation
J Bret Bennington
Bedforms: features in sedimentary rocks formed by the interaction between flow and the sediment composing the bed. Examples include ripples in beach sand and dunes in deserts.
Free stream - region of unimpeded flow above the boundary layer
Boundary layer - region of slower flow caused by interaction with substrate
Viscous sublayer - region of slow flow where viscous forces predominate
The viscous sublayer varies in thickness depending on flow velocity. At velocities great enough to transport sediment the sublayer is always rough if the grain diameter exceeds .6 mm.
Ripples form because irregularities on the substrate surface cause localized regions of high flow rate and regions of boundary layer separation. Grains move up the stoss side of an irregularity, accumulate at the crest, and avalanche into the trough on the lee side. a roller vortex in the trough pushes grains up against the steep lee face as they avalanche, creating cross laminae. Through this process a ripple grows and migrates through time.
Viewed from above ripples may have continuous straight or sinuous crests, or unconnected sinuous forms called linquoid ripples.
Straight ripples produce planer cross lamination.
Sinuous ripples produce trough cross lamination.
Starved ripples - lack of sand such that ripple is eroded from the stoss side as it is deposited on the lee side. Starved ripples may be preserved if blanketed by mud.
Ripples - deposition of sand results in incomplete erosion of trough, preserving a layer of truncated cross laminae.
Climbing ripples - deposition of sand is so rapid that entire ripple is preserved as arriving ripples migrate up the stoss side of departing ripples.
Ripples form at moderate flow velocities over a hydrodynamically smooth bed. They only form when grain diameter is less than .7 mm because bed roughness inhibites the boundary layer separation required for ripple formation.
Dunes are not simply larger ripples (although they are sometimes called "megaripples"), even though they share the same basic morphology as ripples. In general, dunes form by the migration of sediment up the stoss slope and down the lee slope, forming cross beds that are homologous to cross laminations. Eddies on the lee side form a distinctly scoured trough.
Dunes range in height from 5 cm to over 10 m and form in fine to very coarse sand and gravel. Straight crested dunes produce planar cross beds. Sinuous and lenticular dunes produce trough cross beds.
Sand waves are large, linear bedforms found in sandy areas of the seafloor. Unlike dunes, sandwaves do not show scouring in their troughs. Dunes are often observed migrating up the stoss side of sandwaves. Typically, sandwaves are 1 - 8 meters high (but can exceed 10 m) and have straight or moderately sinuous crests. Often they are seen in tidally influenced estuaries and shelves.
Plane beds and planar lamination
Beds that are deposited in even, horizontal sheets are called plane beds and are composed of planar laminae, typically 5 - 20 grains in thickness. Plane beds form at high flow velocities, beyond those capable of forming dunes. Also typical of plane beds are current lineations, which are furrows and ridges a few grains in height that are oriented parallel to flow direction.
At supercritical flow rates (Froude number > 1) standing waves can form and begin to migrate in the upflow direction. These standing waves produce antidunes that also migrate against flow. However, antidunes are rarely preserved in the sedimentary record because they are reworked as flow slows and becomes subcritical.
Waves have an oscillatory motion (water particles move back and forth) and this motion creates circular orbits of water motion that decrease in size with increasing depth, eventually disappearing at wave base (the depth below which waves do not affect the bottom). In relatively shallow water the bottom is above wave base and waves roll sediment to create wave ripples.
Wave ripples differ from current ripples:
Alternating flow from two different directions can produce a complex pattern of criss-crossing, discontinuous ripple crests.
Mud - Sand mixtures
Varying flow rates in a depositional setting can lead to both sand and mud being deposited at different times.
Mud > sand = lenticular bedding (isolated ripples of sand surrounded by mud)
Sand > mud = flaser bedding (sand ripples with mud drapes)
Sand = mud = wavy laminations
Post-depositional and erosional sedimentary structures
As clay dries on exposure to air it shrinks, forming polygonal regions bounded by dessication cracks. These cracks taper downward and may be preserved if they are later filled in with windblown or water-born sediments.
Trace fossils - trackways, depressions, and marks made by the activities of organisms moving across the bed surface.
All of the above features are often observed as casts on the bottom surface of beds created by infilling of overlying sediment into the depression on the surface of the underlying bed.