1. Sedimentary ProcessLecture 2#

2. Sediment Transport and DepositionMedia:1. Water Overland flow, channel flow Waves, tides, ocean currents2. Air (small particle)3. Ice4. Gravity Rock falls (no transport mediuminvolved) Debris flows, turbidity currents(water involved) 3. Different kinds of mass movements,variable velocity (and other factors) 4. From weathering to deposition:sorting and modifification of clastic particles 5. Effects of transport on rounding, sorting 6. Three types of flow:i. Laminarii. Turbulentiii. Transitional 7. Laminar flowTurbulent flow 8. Flow regimes in a stream 9. Laminar flow vs Turbulent Flow Two modes of flow dependent upon: 1. Velocity2. Fluid viscosity3. Bed roughness Laminar Flow: streamlined, uniform current.Requires: Low fluid velocity or High viscosity or Smoothbeds Turbulent Flow: discontinuous, distorted, floww/considerable motion perpendicular to primary flow direction.e.g: Eddies - highly turbulent flow (water and air)Eddy Viscosity - internal friction at a larger scale 10. Laminar flow vs Turbulent Flow Laminar flow occurs at low flow velocities Turbulent flow occurs at higher velocities Reynolds number (Re) is used to determine whetherflow is laminar or turbulentRe =ULWhen dominates, Re is small (2000)U = mean velocity, L = water depth, =density, = fluid viscosity 11. Surface Waves & Froude NumberFluid Flow variables: Viscosity (ignore for water) Inertial forces Gravity & depth (surface wave velocity)Froude # (Fr) - ratio of inertial forces to gravity,Fr= U , U= Mean velocity, g = gravity, L= water depthgLFr= 100 cm/s9.8m / s2 * 50cmFroude #< 1, max wave velocity exceeds current, tranquil (sub-critical)>1, current velocity exceeds max wave velocity (supercritical) 12. DeeperSubcritical(Fr1) 13. Critical velocities are different for sedimententrainment and deposition, especially in the finerfractions Fluid density and viscosity play a key role indetermining which particle sizes can be transported The amount of sediment transport is not only related toflow velocity (or bed shear stress) and grain size, butalso to:i. Grain densityii. Grain shape 14. Sediment transport and depositionStokes Law (settling velocity in a static fluid)( ) 2 18g fvg= settling velocity; D=grain diameter; g = graindensity; f= fluid density; =dynamic viscosityStokes Law only applies to fine ( 0.7 mm) lower-stage plane beds developinstead of current ripples At high (but still subcritical) flow velocities upper-stageplace beds are formed in all sand grain sizes Supercritical flow conditions (Fr1 or higher) enable theformation of anti-dunes, characterized by bedformaccretion in an upstream direction 24. Waves Waves are wind generated oscillatory motions of water Wave height is dependent on wind strength and fetch The depth to which the oscillatory motion due to waveaction extends is known as the wave base; shallow waterleads to breaking waves Wave ripples are distinct from current ripples due to theirsymmetry, and include low energy rolling grain ripplesand high energy vortex ripples 25. TidesTides are formed by the gravitational attraction ofthe moon and sun on the earth, combined with thecentrifugal force caused by the movement of theearth around the center of mass of the earth moonsystem Semi-diurnal or diurnal tidal cycles Neap-spring tidal cycles Annual tidal cycles 26. Ocean currents The circulation of sea water in the worlds oceans isdriven by wind and contrasts in density due tovariable temperature and salinity (thermo-halinecirculation), combined with the Coriolis effect Ocean currents transport clay and silt insuspension, and sand as bed load, and their effectsare especially important in deep waters, wherestorms and tides are less important. 27. Gravity flows Debris flows have a high (> 50%) proportion of sediment to water canbe both subaerial and subaqueous low Reynolds numbers Turbidity currents have a higher proportion of water, are alwayssubaqueous and move due to density contrast higher ReynoldsnumbersRe = vL = vL where:v is the mean velocity of the object relative to the fluid (SI units: m/s)L is a characteristic linear dimension, (travelled length of the fluid;hydraulic diameter when dealing with river systems) (m) is the dynamic viscosity of the fluid (Pas or Ns/m or kg/(ms)) is the kinematic viscosity ( = / ) (m/s) is the density of the fluid (kg/m). 28. Ripple marks Fine silt to fine sand Slow to fast flow velocity 29. Dunes Fine to coarse sand Medium to high flow velocities Sand loves to make dunesModern Ancient 30. Lower plane beds Fine to coarse sand, low flow velocity Sand grains like to make ripples Really slow velocity 31. Upper planar beds (laminae) Fine silt High flow velocity Hard to tell apart from lower planar beds(Use grain size) 32. Anti-dunes Fine silt to coarse sand High flow velocity Rarely seen in rocks 33. Flow direction From looking at the rock structures, flow direction canbe determined even if the rock is millions of years oldWhen the flow has changed direction,the structure known as crossbeds 34. Conclusion Planar stratification is primarily the product of aggrading planebeds Cross stratification is formed by aggrading bedforms Planar and trough cross stratification are the result of straightcrested (2D) and linguoid (3D) bedforms, respectively- Small scale cross stratification (current ripples)- Large scale cross stratification (dunes)-Wave cross stratification (wave ripples)- Hummocky cross stratification [combined flow] (mounds andswales) A single unit of cross stratified material is known as a set;multiple stacked sets of similar nature form co-sets.