Sed structures Flashcards
Trough Cross Bedding
Commonly recognised type of cross-stratification
3D dune bedform
High(est) energy
Caused by high energy flow - fluid (water)
Found in channel or upper shore environments
Sediments with more negative skewness (Tucker, 2001)
Highest water velocity
Coarsest grain sizes
What is a bedform?
A bedform is a feature that develops at the interface of fluid and a moveable sediment dominated bed
The result of bed material being moved by fluid flow. Examples include ripples and dunes on the bed of a river or sea floor.
Bedforms are often characteristic to the flow parameters, and may be used to infer flow depth and velocity, and therefore the Froude number.
Geopetal
Filled in cavities (such as shells) act as sprit levels (Paleohorizontal)
- Original calcium carbonate shell of living animal
- Mud that infilled shell after animal had died
- Crystals that grew in empty space as shell was buried
- Contact between mud and crystals show the orientation of the ancient seafloor
Pebble Clusters
Example in how drag can influence what is entrained in to a flow and also be used as a palaeocurrent indicator
o Bedflow armouring – boulder or oversized clast moved in rare weather event – regular flow packs smaller clasts behind it – larger ‘v’ on upstream, smaller fine grained sediments on downstream - stabilizes river system
o Larger pebbles on opposite flow direction side
Offers stability to gravel river beds
Delays entrainment and movement of bedforms
Allows an accumulation of finer sediment in the wake - indicated palaeocurrent
Stoss Wake
Flow direction –> oo0OOoo…
Flaser bedding
• Troughs between sandy ripple crests
• Fillled with mud during slack water
created when a sediment is exposed to intermittent flows, leading to alternating sand and mud layers
typically form in tidal environments, they can (rarely) form in fluvial conditions - on point bars
Post-depositional features:
- Desiccation: the evaporation of water and drying up causes shrinkage and cracks to develop delimiting polygons
- Flame structures: Fluid pressurized by deposition and vertical effective stress (burden) – finds cracks and bursts through
Right way up structures
Hummocky cross-stratification:
o Produced on the shallow sea floor during storms from current and oscillatory flow action
o Sediment is dropped irregularly on a scoured surface leading to ‘hummocks’
Hummocky bedforms are generated under moderate to high oscillatory velocities and low unidirectional velocities.
Is an indicator of deposition in unrestricted, open-water conditions.
Wave ripples
- The to-and-fro bottom motions caused by shallowwater waves cause shear stresses on an initially planar sand bottom is to cause grain movement by rolling and saltation
- causes formation of symmetrical wave-formed ripples , crests are usually very persistent laterally but bifurcate in a characteristic manner
To-and-fro flow over orbital ripples causes the formation of flow separation vortices on either side of the well-defined symmetrical crestline.
Wave ripples are initially 2D in plan view, but as oscillatory speed is increased, they become increasingly 3D.
Current ripples (wind ripples)
Asymmetric
Gently flow up stoss side and rapidly down lee side
- Shows palaeocurrent
Wavelength varies with grain size
Grains move with the flow up the stoss side and flow over the top after large amounts of deposition - causing the asymmetric shape
Scour increases with flow
Main citation
Mike Leeder, 2011
Dunes
The flow pattern over dunes is similar to that over current ripples but in addition large-scale advected eddy motions (termed boils or kolks)
Large, 3D ripples essentially
Herringbone cross stratification
Herringbone cross stratification indicates bipolar flow directions, but it is rare
In tidal areas, which have bidirectional flow, structures are formed with alternating layers of cross-beds dipping in opposite directions that reflect the alternating paleocurrent
Indicative of aggradation during alternate tidal flow
General cross stratification formation
Sand grains saltate up the upstream side of the dune, collecting at the peak until the angle of repose is reached. At this point, the crest of granular material has grown too large and will be overcome by the force of the depositing fluid, falling down the downstream side of the dune. Repeated avalanches will eventually form the sedimentary structure known as cross-stratification, with the structure dipping in the direction of the paleocurrent
Where everything forms - marine
Marine
Tidal - bi directional flow - herringbone cross strat
In storm wave base transition zone - hummocky, cross Strat - caused by storm events
Fair weather wave base - trough and planar cross Strat
Fore-shore - wave ripples, going into desiccation cracks
Backshore - terrestrial - current (wind) ripples and dunes
