Lesson 2: Grain size Flashcards
what does sedimentary texture encompass?
the three fundamental properties of sedimentary rocks
what are the 3 fundamental properties of sedimentary rocks?
- grain size
- grain shape
- fabric
what do you mean by grain shape?
form, roundness, and surface texture of grains
what do u mean by fabric?
grain packing and orientation
of the 3 fundamental properties of sedimentary rocks which are of individual grains and which are of grain aggregates?
individual
-grain size
-grain shape
aggregates
-fabric
why are logarithmic or geometric scales the most useful grade scales for expressing particle size?
the key is the scales having fixed ratio between successive elements in the series, because particles have a wide range of sizes
what is the most widely used grain size scale by sedimentologists?
Udden-Wentworth scale
describe the scale in the Udden-Wentworth
each value is either two times larger or one half as large as the preceding value depending if you’re going from up or down, respectively
what is the range of size in the U-W scale
<1/256 mm to >256 mm
what are the major size categories in the U-W scale?
mud (clay, slit), sand, gravel
subclasses under silt
very fine, fine, medium, coarse
subclasses under sand
very fine, fine, medium, coarse, very coarse
subclasses under gravel
granule, pebble, cobble, boulder
what class measures from 1-2 mm
very coarse sand
what class measures about 1.0 phi to 2.0 phi
medium sand
what is the problem with U-W scale
does not do well for graphical plotting and statistical calculations
what is the solution for U-W scale’s problem
plotting the logarithm to base 2
logarithmic scale for sediment size
kumbrein Phi Scale
describe the kumbrein Phi scale equation
Φ = log2 (𝐷/𝐷o)
D – Sediment size in diameter
Do – reference diameter equal to 1mm (constant)
what is the significance of grain size analysis?
> gives us a clue on how strong the transportation agent is (like of the fluid)
for geotechnical analysis (soil analysis, esp for liquefaction analysis)
how many filters are there in a mechanical sieve with vibrating platform, what is the size of the largest mesh
8 and one collecting pan; 4mm
for unconsolidated loose clay to silt sized sediments, what are the 5 methods are used for grain size analysis?
- pipette analysis
- sedimentation balances
- sedigraph
- laser diffractometry
- electro-resistance size analysis, ie. Coulter Counter)
for unconsolidated loose silt to granule sized sediments, what are the 3 methods used for grain size analysis?
- seiving
- settling-tube analysis
- image analysis
for unconsolidated loose pebble to boulder sized sediments, what is the method used for grain size analysis?
manual measurement of individual clasts
for consolidated clay to silt sized sediments, what is the method used for grain size analysis?
electron microscopy
for consolidated silt to granule sized sediments, what are the 2 methods used for grain size analysis?
- thin-section measurement
- image analysis
for consolidated pebble to boulder sized sediments, what is the method used for grain size analysis?
manual measurement of individual clasts
what do you call the old measuring tool is used in measuring individual clasts one by one
Wolman template
methods that involve measuring the fall time of particles through water, explain
sedimentation methods
-settling-tube analysis
-rapid sediment analyzers
> the weight of the sediments accumulating at the bottom of the tube is, or maybe a change in water column pressure as the sediment settles is recorded on an X-Y plotter/chart recorder as a cumulative curve
when the curve is calibrated, the grain size can be read from it
latest dev:
>the output is then fed to an analyzer directly to a microcomputer thatdigitizes the data, calculating grain-size stats and makes various graphs and charts
for most methods for measuring fine sized sediments, what are they based on? expound.
stoke’s law
D = sqrtV/sqrtC
V – Settling velocity
C is a constant (s – f)g/18
s – density of the settling grains
f – density of the fluid
g – gravitational acceleration
what is the standard conventionally way of measuring particle size that is based on stoke’s law
pipette analysis
method where fine sediments are stirred into a known volume of water
pipette analysis
explain pipette analysis
-measured volume of distilled water in settling tube
-sediment is stirred into a suspension
-aliquots are drawn by a pipette at specified times
–evaporated into dryness in oven
–weighed
=data can be used to calculate diameter using modified stokes law
D =sqrt(x/t) / sqrtC
x – depth in centimeter
t – withdrawal time
•Where x/t=V
a type of automated settling tube for fine sediment that work on the same principle as rapid sediment analyzers for sandy sediment, fine sediment is continuously weighed as it collects on a pan at the bottom of the settling tube
sedimentation balances
an automated particle size analyzer that determines the size of particles dispersed in a liquid by measuring the attenuation of a finely collimated X-ray beam as function of time and height in a settling suspension
sedigraph
method where 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
method where 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 analyzers
the conventional method for measuring sand and coarse silt size consolidated grains
thin-section measurement
explain thin-section measurement, explain difficulty
- petrographic microscope fitted with an ocular micrometer: section diameter of randomly oriented grains that commonly measure smaller than max diameter
phenomenon where randomly oriented grains in thin section commonly measure smaller than max diameter, smaller apparent diameters
Corpuscle effect
explain results from thin-section measurement and sieve analysis
since sieve analysis measures intermediate grains and thin-section results have smaller apparent diameters, they typically do not yield the same results, thus need to be corrected to make them agree with each other better
what is a more automated and sophisticated way of measuring grains in thin section
image processing
explain image analysis
-tv camera with special viewing tube mounted on petrographic microscope
-camera feeds image into high res tv monitor and to video digitizer controlled by a microprocessor
-tv camera views grains thru microscope, projects the image in monitor/tablet
-grain to be measured in encircled
-boundary is detected by pixel units
-pixel units are converted to micrometers
method that isn’t routinely used for grain sized measurement, but still viable for fine silt and clay sized particles
electron microscope
T or F: image analysis also be used for loose sediments
T
in laser-diffraction size analyzer, what is the relationship between the angle of diffraction and particle size
increasing angle means decreasing size
in electro-resistant size analyzers, what is the relationship between pulses and grain sizes?
the stronger the pulse the bigger the particle
what could be done to help with the corpuscle effect
collect as much samples as possible, or like a really big rock sample
diagrams constructed by plotting individual weight percent(frequency) along the ordinate(Y axis) and the phi size of each class along the abscissa(x-axis)
histograms
what are the 3 mathematical measured used to describe average size of grains in sediment sample
- mode
- median
- mean
the most frequently occurring particle size in a population of grains, translated to normal every day strength of transport agent
-lowest during drying or flashfloods
mode
represents the midpoint, where half of the population is smaller, while the other half is larger
median
the arithmetic average of all the particle size of the sample
mean
since the true arithmetic mean cannot be calculated, what must be done?
average of the 16th, 50th and 84th percentiles
a measure of the range of grains sizes present and the magnitude of the spread/scatted of these sizes around the mean size
sorting of a grain population
2 ways to measure sorting
- graphical -thru visual analysis using a hand lens/microscope referencing a visual estimating chart
- statistical -using the phi values from calculating standard deviation
what are the phi standard deviation values corresponding to verbal sorting under statistical sorting method
<0.35, 0.5, 0.7, 1, 2, >4 : very well sorted, well sorted, moderately well sorted, moderately sorted, poorly sorted, very poorly sorted, extremely poorly sorted
how does sorting related to maturity
being well sorted is a clue of reworking, meaning if mature, after deposition, it had been constantly reworked over long periods of time na, enabling it to rearrange, filter out weak minerals, become more uniform, connected to fabric
