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Sedimentary Petrology > Lesson 2: Grain size > Flashcards

Lesson 2: Grain size Flashcards

1
Q

what does sedimentary texture encompass?

A

the three fundamental properties of sedimentary rocks

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2
Q

what are the 3 fundamental properties of sedimentary rocks?

A
  1. grain size
  2. grain shape
  3. fabric
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3
Q

what do you mean by grain shape?

A

form, roundness, and surface texture of grains

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4
Q

what do u mean by fabric?

A

grain packing and orientation

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5
Q

of the 3 fundamental properties of sedimentary rocks which are of individual grains and which are of grain aggregates?

A

individual
-grain size
-grain shape
aggregates
-fabric

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6
Q

why are logarithmic or geometric scales the most useful grade scales for expressing particle size?

A

the key is the scales having fixed ratio between successive elements in the series, because particles have a wide range of sizes

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7
Q

what is the most widely used grain size scale by sedimentologists?

A

Udden-Wentworth scale

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8
Q

describe the scale in the Udden-Wentworth

A

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

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9
Q

what is the range of size in the U-W scale

A

<1/256 mm to >256 mm

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10
Q

what are the major size categories in the U-W scale?

A

mud (clay, slit), sand, gravel

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11
Q

subclasses under silt

A

very fine, fine, medium, coarse

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12
Q

subclasses under sand

A

very fine, fine, medium, coarse, very coarse

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13
Q

subclasses under gravel

A

granule, pebble, cobble, boulder

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14
Q

what class measures from 1-2 mm

A

very coarse sand

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15
Q

what class measures about 1.0 phi to 2.0 phi

A

medium sand

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16
Q

what is the problem with U-W scale

A

does not do well for graphical plotting and statistical calculations

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17
Q

what is the solution for U-W scale’s problem

A

plotting the logarithm to base 2

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18
Q

logarithmic scale for sediment size

A

kumbrein Phi Scale

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19
Q

describe the kumbrein Phi scale equation

A

Φ = log2 (𝐷/𝐷o)

D – Sediment size in diameter
Do – reference diameter equal to 1mm (constant)

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20
Q

what is the significance of grain size analysis?

A

> gives us a clue on how strong the transportation agent is (like of the fluid)
for geotechnical analysis (soil analysis, esp for liquefaction analysis)

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21
Q

how many filters are there in a mechanical sieve with vibrating platform, what is the size of the largest mesh

A

8 and one collecting pan; 4mm

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22
Q

for unconsolidated loose clay to silt sized sediments, what are the 5 methods are used for grain size analysis?

A
  1. pipette analysis
  2. sedimentation balances
  3. sedigraph
  4. laser diffractometry
  5. electro-resistance size analysis, ie. Coulter Counter)
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23
Q

for unconsolidated loose silt to granule sized sediments, what are the 3 methods used for grain size analysis?

A
  1. seiving
  2. settling-tube analysis
  3. image analysis
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24
Q

for unconsolidated loose pebble to boulder sized sediments, what is the method used for grain size analysis?

A

manual measurement of individual clasts

25
Q

for consolidated clay to silt sized sediments, what is the method used for grain size analysis?

A

electron microscopy

26
Q

for consolidated silt to granule sized sediments, what are the 2 methods used for grain size analysis?

A
  1. thin-section measurement
  2. image analysis
27
Q

for consolidated pebble to boulder sized sediments, what is the method used for grain size analysis?

A

manual measurement of individual clasts

28
Q

what do you call the old measuring tool is used in measuring individual clasts one by one

A

Wolman template

29
Q

methods that involve measuring the fall time of particles through water, explain

A

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

30
Q

for most methods for measuring fine sized sediments, what are they based on? expound.

A

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

31
Q

what is the standard conventionally way of measuring particle size that is based on stoke’s law

A

pipette analysis

32
Q

method where fine sediments are stirred into a known volume of water

A

pipette analysis

33
Q

explain pipette analysis

A

-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

34
Q

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

A

sedimentation balances

35
Q

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

A

sedigraph

36
Q

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

A

laser diffraction size analyzer

37
Q

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

A

electro-resistant size analyzers

38
Q

the conventional method for measuring sand and coarse silt size consolidated grains

A

thin-section measurement

39
Q

explain thin-section measurement, explain difficulty

A
  1. petrographic microscope fitted with an ocular micrometer: section diameter of randomly oriented grains that commonly measure smaller than max diameter
40
Q

phenomenon where randomly oriented grains in thin section commonly measure smaller than max diameter, smaller apparent diameters

A

Corpuscle effect

41
Q

explain results from thin-section measurement and sieve analysis

A

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

42
Q

what is a more automated and sophisticated way of measuring grains in thin section

A

image processing

43
Q

explain image analysis

A

-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

44
Q

method that isn’t routinely used for grain sized measurement, but still viable for fine silt and clay sized particles

A

electron microscope

45
Q

T or F: image analysis also be used for loose sediments

A

T

46
Q

in laser-diffraction size analyzer, what is the relationship between the angle of diffraction and particle size

A

increasing angle means decreasing size

47
Q

in electro-resistant size analyzers, what is the relationship between pulses and grain sizes?

A

the stronger the pulse the bigger the particle

48
Q

what could be done to help with the corpuscle effect

A

collect as much samples as possible, or like a really big rock sample

49
Q

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)

A

histograms

50
Q

what are the 3 mathematical measured used to describe average size of grains in sediment sample

A
  1. mode
  2. median
  3. mean
51
Q

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

A

mode

52
Q

represents the midpoint, where half of the population is smaller, while the other half is larger

A

median

53
Q

the arithmetic average of all the particle size of the sample

A

mean

54
Q

since the true arithmetic mean cannot be calculated, what must be done?

A

average of the 16th, 50th and 84th percentiles

55
Q

a measure of the range of grains sizes present and the magnitude of the spread/scatted of these sizes around the mean size

A

sorting of a grain population

56
Q

2 ways to measure sorting

A
  1. graphical -thru visual analysis using a hand lens/microscope referencing a visual estimating chart
  2. statistical -using the phi values from calculating standard deviation
57
Q

what are the phi standard deviation values corresponding to verbal sorting under statistical sorting method

A

<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

58
Q

how does sorting related to maturity

A

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

Sedimentary Petrology (11 decks)

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