Sed Pet Week II

sediment

Udden-Wentworth scale

Most widely used grade scale by sedimentologists. Each value is either two times larger than the previous value or half as large, depending on direction (clay, silt, sand, gravel)

Sieving

Older technique for measuring grain size of sandy sediment. Sandy sediment can also be measured by fall time in a tube of water

Stoke’s Law

Method for measuring fine-size sediment; through pipette method

Sedimentation balances

fine sediment is
Sediment is continuously weighed as it collects on a pan at the bottom of the settling tube

Sedigraph

Determines the size of particles dispersed in a liquid by measuring the attenuation of a finely collimated X-ray beam as a function of time and height in a settling suspension.

Corpuscle effect

grains cut marginally in a thin-section plane have smaller apparent diameters

Image analysis

a television camera with a special viewing tube is mounted on a petrographic microscope. The camera feeds an image to a high-resolution television monitor

Skewness

additional measure of grain-size sorting that reflects sorting in the tails of the distribution. When plotted as a frequency curve, the grain-size distributions are asymmetrical. The greater the number from zero, the more skewness

Kurtosis

The sharpness or peaked-ness of a grain-size frequency curve

Form

the gross, overall morphology or configuration of particles. Most measures of form consider the three-dimensional shape of the grains.

Roundness

measure of the sharpness of the corners of a grain, and is commonly measured in two dimensions only

Surface texture

refers to microrelief features, such as scratches and pits, that appear on the surfaces of clastic particles, particularly particles that have undergone transport

Fourier analysis

Used analyze grain shape. Breaks down complex functions or signals into a sum of simpler sinusoidal (sine and cosine) waves, revealing their underlying frequencies and amplitudes. Does not work for highly irregular grains

best material to record surface textural features

Quartz. It is stable at the surface and very hard, so it records textural features well.

Few features are specific to one environment, but V-patterns are common in quartz on beaches and striations are common from dragging (glacial activity) or wind (sand)

Textural maturity of sandstones (guide)

1. the amount of clay size sediment present

2. sorting of framework grains

3. rounding of framework grains

Grain contacts

Contact index (average number of contacts/grain) and tight packing index (average number of long, concavo–convex and sutured contacts/grain)

Grain orientation (water)

Particles may be oriented with long or flattened dimensions parallel to bedding but have orientations within the bedding plane that are either parallel to current flow or perpendicular

Primary porosity types

Intergranular (interparticle) - pore space between framework grains, like siliciclastic/carbonate grains

Intragranular (intraparticle) - pore space within particles, like cavities in fossils and open space in clay minerals

Intercrystalline - pore space between chemically formed crystals, like dolomites

Secondary porosity types

Solution porosity - caused by dissolution of cements/framework grains in chemically formed rocks

Intercrystalline porosity - pore space in cements or among authigenic minerals

Fracture porosity - fracturing of any type of rock by tectonic forces

Darcy’s Law

the rate of flow of a fluid through a porous rock is directly proportional to the rock permeability and indirectly proportional to the fluid viscosity.

Permeability decreases with

decreasing particle size (clay sediments pack tightly and let fewer fluids through). Particle orientation also matters; parallel or perpendicular to bedding plane

Sphinx mountain conglomerate

Cretaceous. Active foreland fold-thrust belt feeding sediment to basin; limestone and metamorphics. Modern Madison fault is extensional. Progressive unconformity

Progressive unconformity

Progressive thrust faults consume the strata from previous sequences of deformation. Sediment is layered with each new faulting sequence; older beds begin to tilt upwards

Synorogenic sedimentation

Accumulation of sediment that occurs at the same time as a mountain building event

Erosional unroofing

Eroding a sequence from the top down

Guyot

Underwater seamount volcano. Often topped with limestone (carbonates); flat-topped. Undergo early cementation; walls are prone to collapse. Produce thick carbonate breccias

3 parameters that determine stable bedform in unidirectional flow conditions

grain size, flow velocity, flow depth.

Other parameters include m (fluid viscosity), rf (fluid density), rs (grain density, 2.65), and g (gravitational constant, 9.81)

Fundamentals of flow

1. Laminar v/ turbulent flow

2. Sub-and super-critical flow

3. velocity profiles

Laminar flow

Smooth movement of particles in parallel layers; typically at low flow rates or with high-viscosity fluids. Unlike turbulent flow, laminar flow maintains organized paths for particles

Turbulent flow

chaotic, irregular fluid motions characterized by swirling eddies and vortices, a high Reynolds number, and a significant mixing of fluid layers

Reynold’s number

a dimensionless quantity that predicts whether a fluid flow will be laminar or turbulent

Re > 2000 = turbulent

Re < 500 = laminar

It takes higher velocity flow to

move heavier particles and effectively erode them. Also, a packed, finely sorted bed of silt would be harder to erode than floating sand grains due to grain surface area (static attraction keeps particles together)

Cohesive v non-cohesive sediment

sticky or non-sticky sediment (eg; sand and gravel do not bed together like mud or silt in water).

Dunes

Akin to large ripples. Wavelengths range from 0.6 m to hundreds of meters. Erosion of stoss side and avalanching of lee side

Antidunes

Typically migrate upstream and show little asymmetry. The water surface is strongly in phase with the bed. Commonly seen as train of symmetrical surface waves

Upper plane bed

Intense sediment transport over a flat bed. High velocity with small grains; produces planar-laminated sand. Seen at beaches.

Ripples

current ripples - form in response to unidirectional flow (low velocity)

wave ripples - a flow moving back and forth

Limiting streamlines (skin-friction)

Lines representing the direction of the wall shear stress vector, which arises from the fluid's friction against the surface. Motion vectors meet at the ripple’s crest

Climbing ripples

formed through aggradation of sediment. Aggradation levels vary depending on type. Subcritical, critical, supercritical. Erosional surfaces separate beds

Hysteresis (lag effect)

occurs when the change in the system's response occurs with a time delay to the change of the driving force causing it

Secondary flow

Any flow not in the direction of the main current (edy)

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)

Bedforms in cohesive sediments

Most are erosional; flutes and tool marks (divots), including bounce, skip, groove, and chevron marks

Proximal and distal

Close to and far from the source (near to far). Down gradient movement

Gutter cast

Subaqueous, often associated with storms. Found in marine shelf/lake environments

Shrinkage cracks (desiccation)

Shrinking of a muddy layer exposed to the atmosphere (shallow water)

How surface waves bedforms are formed

Surface waves define orbitals which have decreasing width with depth. Shallow water waves reach the bottom, deep water waves do not. Orbitals at the bottom create shear stress that oscillates back and forth as waves pass overhead. This stress will move sediment

Wave ripples

Sharp crested with broad troughs, symmetrical (often seen in vortex ripples)

Hummocky cross stratification

Hummocks (concave up) and swales (concave down). <15 degree truncation surfaces. Occurs in fine to medium-grained sand. Produced by combined flow, typically occurs below fair weather wave base by larger waves produced during storms

Dune grain flow dynamics

grainflow (along crest of dune), grainfall (side of dune), wind ripple (sand is blown into ripples, backside and base of dune)

laminae coarsen up-dune since larger grains roll faster

Sediment gravity flows

4 types. Based on support mechanism clasts during sediment flow

Turbidity current

Particles are kept aloft in the body of the flow by turbulent suspension. Density of flow is greater than that of ambient fluid

both high density and low density turbidity currents exist

sharp erosional base with upward fining; whole sequence rarely preserved