Grain Size Flashcards
- A widely used grade scale for expressing particle size
- A logarithmic and geometric scale with a fix ratio between successive elements
Udden-Wentworth Scale
Udden-Wentworth Scale extends from
< 1/256 mm to >256 mm
logarithmic scale for sediment size
Kumbrein Phi Scale
Increase in abs value negative phi =
Increase in mm size
- Increase in positive phi =
decrease in mm size
particles that are settling will pass through a diffracted light, the angle of the diffracted light increases with a decreasing particle size. A detector senses the angular distribution of scattered light energy.
- Laser Diffraction size analyzer
sediments are slowly poured into an electrolyte solution, where sediments will pass through, where changes in the electrical field will produce pulses, translating into the grain size
- Electro-resistant size analysers
Φ < 0.35
VW Sorted
0.35 < Φ < 0.5
W Sorted
0.5 < Φ < 0.7
MW Sorted
0.7 < Φ < 1
M Sorted
1 < Φ <2
P Sorted
2 < Φ <4
VP Sorted
0.4 > Φ
EP Sorted
- based on a constant ratio of two between successive class boundaries together with terms for the classes
- A Geometric Scale
Udden Wentworth Scale
Kumbrein introduced an ?? Measured in ?? uunits
Kumbrein; Phi
clast size of conglomerates and breccias can be measured directly with a ?
tape measure
With poorly cemented sandstones and unconsolidated sands, ???is the most popular technique
sieving
- Medium silt to small pebble
- ?? min sieving time
- ?? gram samples
15; 30
show the frequency of grains in each size-class and usefully give an immediate impression of the grain-size distribution
histogram, smoothed frequency curve
shows the percentage frequency of grains coarser than a particular value
cumulative frequency curve
Most sediments are ??, i.e. one class dominate
Unimodal
If grain-size distribution is perfectly normal
then the median, mean and mode values are the same
grain size decreasing away from the source area
- relates to selective sediment transport
- fluvial and deltaic
- turbidites in deep-sea basin
- offshores, wave and current energy decrease
a measure of the standard deviation, spread of the grain-size distribution.
- Sorting
Several Factors that Determine Sediment Sorting
Sediment Source
Grain
Depositional Mechanism
A general rule about sorting
improves along the sediment transport path
a measure of the symmetry of the distribution
- Skewness
If the distribution has a ????, i.e. excess coarse material, then the sediment is said to be ????
Coarse tail; Negatively Skewed
If the distribution has a ????, i.e. excess fine material, then the sediment is said to be ????
Fine tail; Positively Skewed
If the distribution is symmetrical, then there is ?????.
no skew
Skewness > 0.30
S fine skewed
Skewness 0.3>0.1
fine skewed
Skewness 0.1>–0.10
Near-symmetrical
Skewness -0.1>–0.30
Coarse Skew
Skewness >-0.30
S Coarse Skew
River sands are usually ???? skewed
positive
Beach sands, for example, tend to have a ??? skew
negative
n general, sediment becomes more ???? skewed (and finer grained) along its ?????/
Negative; Sediment Transport Path
????????? becomes more positively skewed and relatively coarse
source sediment (lag?
- section diameter of randomly oriented grains are usually smaller than their maximum diameter
- grains cut marginally in a thin-section plane have smaller apparent diameters
Corpuscle Effect
- sharpness or peakedness of a grain-size frequency curve
Kurtosis
- Sharp peaked curves
- better sorting in the central portion of the grain-size distribution
Leptokurtic
- flat peaked curves
- poor sorting in the central portion of the grain-size distribution
Playkurtic
A method of direct calculation of mean size, std dev, skewness, kurtosis w/o cumulative curve plots
Moment Method
- when size distribution of natural sediment, such as dune sand, is plotted on log–log plots and a curve is fitted to these data, the resulting curve actually has the shape of a ??????
hyperbola
