G433 (2010) review of sed structures_v2.pdf

7/29/2019 G433 (2010) review of sed structures_v2.pdf

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G433Review of sedimentary structures

September 1 and 8, 2010


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Fluid Parameters

The three main parameters that determinethe stable bedformin unidirectional flowconditions are:

grain sizeflow velocityflow depth

Several other parameters are equallyimportant, though for most pure fluid flows onEarth, these parameters can be assumed tobe constant. They include:

m = fluid viscosityrf = fluid densityrs = grain density

g = gravitational constant


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Cohesive vs. non-cohesive sediments

Hjulstrom Diagram


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Bedformphase diagram and hysteresis


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2-D vs. 3-D structures


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Secondary flow created by bed roughness


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Aggradation

vs. migration ofbedforms


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Examples of

climbing bedforms(unidirectional

ripples)


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DunesDunes are similar to ripples, but dynamically distinct.

Dune wavelengths commonly range from 0.6 m tohundreds of meters; heights range from 0.05 -10.0 m.


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Upper plane bed flow: intensive sediment transport over a flat bed

Parting Lineation


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flow migration

Antidunes occur in flows with sufficiently high Froude numbers. AntidunesTypically migrates upstream and shows little asymmetry. The water surface is

strongly in phase with the bed. Commonly seen as train of symmetrical surface waves.


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Shoot and pool structures:Trains ofcyclic steps occur in verysteep flows with supercritical Froude numbers. The steps are delineated

by hydraulic jumps (immediately downstream of which the flow issubcritical).

flowhydraulic jump


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Bedforms in cohesive sediments


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Subaqueous bedforms in cohesive sediments: flutes and toolmarks, including bounce, skip, groove, and chevron marks


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Gutter castssubaqueous, usually associated with storms


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Shrinkage cracks


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Incomplete, non-orthogonal, Ordovician Eureka Quartzite, W. Utah


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Shrinkage cracks

subaerial desiccation


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Bedforms generated by surface waves Surface waves transfer little mass but considerable energy

Surface waves define orbitals in fluid that have decreasingdiameter with depth

Depth below which orbital diameters = 0 is termed wavebase

Deep water waves do not reach bottom

Shallow water waves do reach bottom; orbitals reachingbottom create a shear stress that oscillates back and forth aswaves pass overhead

With sufficient shear stress, sediment grains will move,creating bedforms


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Wave orbitalsdeep water wavesshallow water waves


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Movement of sediment by wave orbitals


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Unidirectional, combined flow, and

oscillatory bedforms


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Wave ripples


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Hummocky cross-stratification (HCS)

Occurs in fine- to medium-grained sand

Produced by combined flow

Typically occurs below fair weather wavebase by larger

waves produced during storms


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Physical features characteristic of HCS

hummocks (concave up features) and swales (concave downfeatures)

psuedo-parallel laminations within hummocks and swales

(although laminae may thicken into swales and thin overhummocks)

low angle (


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HCS


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Eolian Dunes


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Sediment dynamics

on dunes


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Grain flow depositsGrain fall depositsWind ripple deposits


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Sediment

gravity flows


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Turbidity currents:

particles are kept aloft in the body of the flow by turbulent suspension

density of flow greater than that of ambient fluidboth high density and low density turbidity currents exist

Turbidite in flume

Flume turbidite 2


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Turbidites


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Liquified flows:

very concentrated dispersions of grains in fluidusually result from shock of granular sediment (e.g. seismic shock)grains kept in suspension by fluid pore pressure and from upward movement ofexpelled fluid


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Grain flows:

characterized by grain-grain collisions.

Little reduction of friction occurs in such flows, so they can only occur onsteep slopes where the angle of initial yield has been exceeded.

Debris flows:


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Debris flows:

slurry like flows in which large particles (up to boulders) are set in a fine-grained matrix

matrix has yield strength which helps support grains during flowmatrix serves to lubricate grain irregularities so debris flows may occur onvery gentle slopes

Debris flo deposits


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Debris flow deposits

Documents

G433 (2010) review of sed structures_v2.pdf