Sedimentation and Stratigraphy Ch. 4

Rock fall

geologic mass-wasting when piles of sediment effected by gravity move to the bottom of a slope

Scree

loose rocks and boulders on mountains that are rarely preserved in the stratigraphic record

Talus

any debris on surface of mountains

Angle of rest

the steepest slope that a sediment can remain stable at without collapsing

What are the main types of transport media?

gravity, water, ice, and air

The capacity of wind to transport material is limited by?

the low density of air

What is a debris flow?

when a very high concentration of sediment in the water forms a slurry (think wet concrete texture)

What is fluid dynamics

the behavior of fluids (liquid, gas)

Laminar flow

when all molecules move parallel to each other in the same direction; almost no mixing occurs

Turbulent flow

fluid moves in all directions with net movement in the transport direction; thoroughly mixed

What does the Reynolds number do?

quantitatively distinct laminar and turbulent flows

What is the equation for the Reynolds number?

Re = u(l/v) where u = velocity, l = diameter of pipe/body, v = viscosity

At what values is the flow laminar in Re?

<500

At what values is the flow turbulent in Re?

>2000

Particles of any size may be moved in a fluid by one of what three mechanisms?

rolling, saltation, suspension

What is rolling transport?

classy move along bottom of air or water flow without losing contact to bed surface

What is saltation transport?

particles move in a series of jumps, periodically leaving bed surface and carried short distances before running to bed again

What is suspension transport?

turbulence in flow suspends sediments in the moving fluid more-or-less continually

Particles carried by rolling and saltation are called?

bellows

Particles carried by suspension are called?

suspended load

What causes rolling grains?

frictional drag

What is the Bernoulli effect?

phenomenon that allows birds and aircraft to fly. the pressure of air/water above the object is less than that below the object, allowing it to be lifted up

Critical velocity

at which a particle becomes entrained in the flow

What relationship does the Hjülstrom diagram represent?

water flow velocity & grain size

What causes the initial steep slope on the erosion line in the Hjülstrom diagram?

clay minerals are cohesive, so it requires extra force to move after initial deposition

Normal grading

pattern of overall decrease in grain size from base to top

Reverse grading

pattern of overall increase in grain size from base to top

What does Stokes Law describe?

the relationship between the density of the particle, the fluid, and fluid velocity to determine settling velocity

What does Stokes Law imply?

larger diameter class reach higher settling velocities

Why do mica grains take longer to settle than quartz grains?

because the longer shape slows it down in the medium from an upwards force

What conditions result in reverse grading?

increase in flow velocity over time

What is grading?

a trend in grain size within a single bed and trends in grain size that occur through a number of beds

Fining-upward

a pattern of several beds that start with a coarse class size in the lowest bed and finer material in the highest

Coarsening-upward

coarsest bed on top, finest bed on bottom

Important implication of Stokes Law

forces acting on a grain are a function of the viscosity and density off the fluid medium as well as the mass of the particle

Bedform

a morphological feature formed by the interaction between a flow and cohesion less sediment on a bed

Sedimentary structures generated by bedforms provides information about?

1. current strength

2. flow depth

3. transport direction

Free stream

the portion of flow unaffected by boundary effects

Boundary layer

velocity begins to decrease due to friction within the bed

Viscous sublayer

region of reduced turbulence

The thickness of the viscous sublayer decreases with…?

increasing flow velocity (independent of flow depth)

Hydraulically smooth

only if all particles are contained within the viscous sublayer

Hydraulically rough

if there are particles which project up through the viscous sublayer

Turbulent sweeps

irregulars within the viscous sublayer which move grains by rolling or saltation to create clusters of grains

Streamlines

fluid, imaginary lines to help visualize direction of flow

Current ripples

small bedrooms formed by the effects of boundary layer separation on a bed of sand

Stoss side

the shallower side of a bedform parabola

Lee side

the steeper side of a bedform parabola

Cross-laminae

tiny, inclined layers of sediment within a rock bed that form at an angle to the main horizontal layers

Cross-lamination

a sedimentary structure that is a build up of cross-laminae

Linguoid ripples

a pattern of unconnected arcuate forms

Planar cross-lamination

perfectly straight ripples generate cross-laminae that dip all in the same direction and lay in the same plane

Trough cross-lamination

a sedimentary structure formed by curved or sinuous ripples, characterized by trough-shaped, non-parallel cross-laminae bounded by erosional surfaces

Climbing ripples

when the rate of added sand is high → no net removal → each ripple migrates up the stops side of the ripple form in front

Dunes

large bedforms

Cross-beds

the construction of a succession of sloping layers formed by the avalanching on the lee slope

Roller vortex

flow separation creates this in front of the lees slope where reverse flow can form

Tabular cross-bedding

cross-beds bound by horizontal surfaces

Counter-flow ripples

generated when roller vortex is well developed, creating a counter-current

Trough cross-bedding

formed by a migration of sinuous subaqueous dunes

Bars

bedforms that are of a larger scale than dunes

Plane bedding

horizontal layering in sands deposited from a flow

Planar lamination

sedimentary structure characterized by thin, flat beds produced by plane bedding

Ripples only form when…

grains are smaller than the thickness of the viscous sublayer

Plane beds

a near-horizontal surface of sediment where sand or gravel is transported by water without forming large ripples or dunes

Primary current lineation (pcl)

elongated ridges a few grain diameters high separated by furrows oriented parallel to the flow direction in plane beds

What is the Froude number (Fr)?

the relationship between the flow velocity and the flow depth with acceleration due to gravity

When Fr < 1, what is the flow described to be?

subcritical

If Fr > 1, what is the flow described to be?

supercritical

What does it mean for a flow to be supercritical

flow is too fast for a wave to propagate upstream

What does it mean for a flow to be subcritical?

wave can propagate upstream because it is travelling faster than the flow

Hydraulic jump

this is a sudden change in the height of the surface of a natural flow at the transition from thin, supercritical flow to thicker, subcritical low

What happens when Fr ≈ 1?

standing waves may temporarily form on the surface of the water before steepening and breaking in an upstream direction

Antidune

a sediment bedform that forms on the bottom of a stream or channel under fast, shallow, supercritical water flow

Bedform stability diagram

indicates the bedform that is likely to occur for a given grain size and velocity and has been constructed from experimental data

What structures are stable in a lower flow regime?

ripples, dunes, and lower plane beds

What structures form in an upper flow regime?

plane beds and antidune

Wave

a disturbance travelling through a gas, liquid, or solid which involves the transfer of energy between particles - oscillary motion

Fetch

the expanse of water across which the wave-generating wind blows

Wave base

the depths to which surface waves affect a water body

Rolling grain ripples

form at low energies; characterized by broad troughs and sharp crests

What rounds bedforms if, at low energies, sharp peaks occur from rolling grains?

vortex ripples occur higher energies, kicking up the grains in sharp crests

What is the difference between wave ripples and current ripples?

the former only form in shallow water without strong currents whereas the latter may form as the result of water flow in any depth

Sediment gravity flows/Mass flows

mixtures of detritus and fluid that move under gravity

Debris flow

dense, viscous mixtures of sediment and water in which the proportion of sediment exceeds that of water (tends to be laminar)

Where do debris flow occur?

predominantly on land in arid environments where water supply is sparse, in submarine environments, and some coarse-grained delta slopes

When does deposition occur?

when internal friction becomes too great and the flow “freezes”

Turbidity currents

gravity-driven turbid mixtures of sediment temporarily suspended in water (relatively lower density + relatively high Re)

Turbidite

the deposit of a turbidity current; characterized by grain size

Bouma Sequence

the idealized succession in medium-grained turbidites with five distinct divisions

Describe the Bouma Sequence

1. bottom part is poorly sorted sand

2. laminated sand

3. cross-laminated medium to fine sand (moderate flow)

4. fine sand & silt

5. top part fine-grained silt & clay

Turbidity currents are waning flows. What does this mean?

they decrease in velocity through time as they deposit material

Fine-grained turbidites

found as distal equivalents of medium-grained turbidites, deposited by turbidity currents that may travel far out to the sea floor - ONLY sand base with silt and mud sediment

Coarse-grained turbidites

occur under conditions of higher density and coarser material (thicker base)

Grain flows

mechanisms of mass transport down a steep slope

Kinetic sieving

a physical process where smaller sediment grains slip down through open spaces in a moving mix of larger grains, pushing the coarser material upward → produces slight reverse grading

Desiccation cracks

polygonal paternity of cracks that develop when muddy sediment dries out completely in subaerial conditions

Mud-chips/Mud-flakes

the edges of desiccation cracks that are easily removed by later currents and may be preserved as these in the overlying sediment

Syneresis cracks

shrinkage cracks that form under water in clayey sediments - as the clay settles, it compacts and shrinks to form cracks in the surface

How to visually identify the difference between desiccation cracks and syneresis cracks?

the former is polygonal, the latter is straight lines