G433Review of sedimentary structures

September 1 and 8, 2010

Fluid Parameters

The three main parameters that determine the stable bedform in unidirectional flow conditions are:

grain size flow velocity flow depth

Several other parameters are equally important, though for most pure fluid flows on Earth, these parameters can be assumed to be constant. They include:

m = fluid viscosity rf = fluid density rs = grain density g = gravitational constant

Cohesive vs. non-cohesive sediments

Hjulstrom Diagram

Bedform phase diagram and hysteresis

2-D vs. 3-D structures

Secondary flow created by bed roughness

Aggradationvs. migration of

bedforms

Examples of climbing bedforms

(unidirectional ripples)

Dunes Dunes are similar to ripples, but dynamically distinct. Dune wavelengths commonly range from 0.6 m to hundreds of meters; heights range from 0.05 -10.0 m.

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

Parting Lineation

flow migration

Antidunes occur in flows with sufficiently high Froude numbers. AntidunesTypically migrates upstream and shows little asymmetry. The water surface isstrongly in phase with the bed. Commonly seen as train of symmetrical surface waves.

Shoot and pool structures: Trains of cyclic steps occur in very steep flows with supercritical Froude numbers. The steps are delineated by hydraulic jumps (immediately downstream of which the flow is subcritical).

flowhydraulic jump

Bedforms in cohesive sediments

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

Gutter castssubaqueous, usually associated with storms

Shrinkage cracks

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

Shrinkage crackssubaerial desiccation

Bedforms generated by surface waves• Surface waves transfer little mass but considerable energy• Surface waves define orbitals in fluid that have decreasing

diameter 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 reaching

bottom create a shear stress that oscillates back and forth as waves pass overhead

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

Wave orbitalsdeep water wavesshallow water waves

Movement of sediment by wave orbitals

Unidirectional, combined flow, and oscillatory bedforms

Wave ripples

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

Physical features characteristic of HCS

– hummocks (concave up features) and swales (concave down features)

– psuedo-parallel laminations within hummocks and swales (although laminae may thicken into swales and thin over hummocks)

– low angle (<15°) truncation surfaces

HCS

Eolian Dunes

Sediment dynamics on dunes

Grain flow depositsGrain fall depositsWind ripple deposits

Sediment gravity flows

Turbidity currents:• particles are kept aloft in the body of the flow by turbulent suspension • density of flow greater than that of ambient fluid • both high density and low density turbidity currents exist

Turbidite in flume

Flume turbidite 2

Turbidites

Liquified flows:• very concentrated dispersions of grains in fluid • usually result from shock of granular sediment (e.g. seismic shock) • grains kept in suspension by fluid pore pressure and from upward movement of expelled fluid

Grain flows:•characterized by grain-grain collisions. •Little reduction of friction occurs in such flows, so they can only occur on steep slopes where the angle of initial yield has been exceeded.

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 flow • matrix serves to lubricate grain irregularities so debris flows may occur on very gentle slopes

Debris flow deposits