1. Sedimentology Grain Parameters Reading Assignment: Boggs, Chapter 3
Medium grained turbidite sandstones with crinoid stems in the Brushy Canyon Fm., West Texas
Sedimentology Grain Parameters Reading Assignment: Boggs, Chapter 3

2. Key Concepts Grain-size analysis & statistical parameters
Sediment / Rock textural classification
Grain shape
Grain roundness
Textural maturity
Grain surface features
Sediment / rock fabric

3. Why do Grain-Size Analysis?
Sediment / rock description = communication
Sediment / rock name = classification = communication
Information on source area, transport & depositional environment

4. Grain Size: The Udden-Wentworth Scale
Weathering products in transport include:
Sediment from physical weathering
Sediment from chemical weathering
Ions from chemical weathering
Wentworth Scale addresses sediment
Sediment ranges from clay to boulder
Uses geometric scale because grain size ranges from <0.004 mm to m
Establishes size-classes (cobble, sand, silt, etc.)
Standard sieve sizes for silt to pebble
Also uses a Phi Scale to make units of equal value
F = -log2D (D=grain diameter)

5. How do we measure grain size?

6. Gravel Sand Mud F = -log2D Size range (metric) Wentworth Size Class
<−8
>256 mm
Boulder
−6 to −8
64–256 mm
Cobble
−2 to −6
2–4 mm
Pebble
-1 to -2
2 - 4
Granule
0 to −1
1000–2000 µm
Very coarse sand
1 to 0
500–1000  µm
Coarse sand
2 to 1
250–500  µm
Medium sand
3 to 2
125–250 µm
Fine sand
4 to 3
62.5–125 µm
Very fine sand
8 to 4
3.9–62.5 µm
Silt
20-10
0.98–3.9 µm
Clay
Gravel
Sand
Mud

7. conversion of unconsolidated sediments into rock
Lithification
conversion of unconsolidated sediments into rock
F = -log2D
Size range (metric)
Wentworth Size Class
<−8
>256 mm
Boulder
−6 to −8
64–256 mm
Cobble
−2 to −6
2–4 mm
Pebble
-1 to -2
2 - 4
Granule
0 to −1
1000–2000 µm
Very coarse sand
1 to 0
500–1000  µm
Coarse sand
2 to 1
250–500  µm
Medium sand
3 to 2
125–250 µm
Fine sand
4 to 3
62.5–125 µm
Very fine sand
8 to 4
3.9–62.5 µm
Silt
20-10
0.98–3.9 µm
Clay
Conglomerate
(rounded clasts)
Breccia
(angular clasts)
Gravel
Sand
Sandstone
Mud
Mudstone

8. How To Present The Grain-Size Data
Record weights
Always plotted as Coarse to Fine on Phi scale
Histogram
Frequency
Curve
Cumulative
Semi-Log
Plot

9. Statistical Treatment of Grain-Size Data
“Average” grain size
Mode - most frequently occurring size class
Median – 50%
Mean – arithmetic average
Sorting
Skewness
Kurtosis

10. “Average” Grain Size
Mode
Median
f84
f16

11. Why Sample at 16 & 84%; 5 & 95%? 84% 16% 68% 1 sd 1 sd 95% 5% 90% 2 sd

12. Significance of Average Grain Size
Source Area – What size did the grains start at? Weathering history?
Transport History
Grain size directly correlates to energy level
Selective transport
Proximal to Distal fining
Depositional Environment
Energy level
Calculation of flow speed in water & air from grain size
Depositional process of sediment layers
Segregation of grain populations on bedforms
Bedload vs. suspended load

13. Sorting – How Uniform Is The Population?

14. Sorting in
Real Sediment

15. Visual Estimates of Sorting
Fig 3.3, p. 58.

16. Significance of Sorting
Reflects source area – original range in population
Transport history
Selective sorting
Depositional environment
Rate of deposition (e.g., rapid vs. slow)
Type of processes (e.g., glacial till vs. beach sand)

17. Skewness – Population Symmetry

18. Normal & Skewed Populations
p. 59, fig 3.4, fig. 3.5 p. 60.
Symmetrical
population
Positive-Skewed
Fine-Skewed
Negative-Skewed
Coarse-Skewed

19. Significance of Skewness
How does the grain population depart from a normal population?
Usually reflects depositional processes in removing/concentrating fine or coarse fraction
Beach sand is typically coarse-skewed as wave reworking removes finer sediment and carries it offshore
River sand is typically coarse-skewed because fine sediment is carried in suspension distally
Lag surfaces (as in deserts) are coarse-skewed as fine sediment is deflated
Suspension deposits (lakes, offshore, aeolian loess) tend to be fine- skewed as they represent most distal transport

20. Kurtosis

21. Significance of Kurtosis
Similar to sorting in showing dominance of center or ends of population
As with sorting – product of source area, transport history & depositional processes
Leptokurtic sediment are usually well sorted with concentration of population at mean
Platykurtic sediments are usually poorly sorted with coarse & fine ends of population over-represented
Bimodal or Polymodal distributions are platykurtic, typically reflecting mixing of environments

22. Sediment/Rock TEXTURAL Classification

23. Grain Shape
Determined by relative lengths of long, intermediate & short axis
Zing (1954) classification
Largely inherited from parent mineralogy reflecting crystal form or mineral cleavage
Plutonic Quartz has an spherical shape
Biotite has a disk shape
But is influenced by transport history
Significant in transport – think Mica vs. Quartz

24. Grain Shape & Environment
River gravel – concentration of
rollers
Beach gravel – concentration of blades
& disks

25. Grain Shape and Roundness
Measure of abrasion of grain edges
Reflects transport history – intensity and/or duration
Strongly influenced by mineral hardness and mineral cleavage planes
Grain shape and roundness are independent

26. Grain Roundness
Rounded
Subangular
Angular

27. Textural Maturity
Consider:
Clay %
Sorting
Roundness

28. Textural Maturity Flow Chart Used in Lab
Clay content
Sorting (σφ)
< 0.5φ
Roundness
< 5%
Rounded
> 0.5φ
Angular
> 5%
Immature
Sub-mature
Mature
Super-mature

29. Significance of Textural Maturity
Reflects source material….
BUT primarily reflects type and rate of depositional processes – a natural progression
Is the clay content (suspended load) removed?
How well is the traction load sorted?
Have the grains been in transport long enough to become rounded?
Textural Inversion – mix of textural parameters
Example: well rounded grains in a mud matrix
Usually a mixing of environments (e.g., aeolian sand blown into lagoonal mud)

30. Class ended here.

31. Grain Surface Features
Frosted Aeolian Sand
Large field of
grain surface
textures
Common descriptions: unmodified, polished, frosted, pitted, scratched
Origin in transport, chemical weathering, precipitation (diagenesis)
SEM-scale
Distinctive frosting of sand with aeolian transport

32. Sediment/Rock Fabric Grain orientation
Current orientation of elongate (rod-shaped) grains
Imbrication – most stable configuration
Random / chaotic patterns with suspension settling

33. Sediment/Rock Fabric Clast support vs. Matrix support
Grain packing in sand
26-47 % original porosity determined by grain shape with same grain size
Porosity largely independent of grain size (given the same sorting)
Permeability decreases with grain size as pores get smaller and friction increases
Both Porosity & permeability decreases with sorting as smaller grains fill pores between larger grains

34. Significance of Grain Size & Sorting for Reservoir Characterization
River Bottom
Fine sediment
Coarse
sediment
Field of view 30 m by 10 m
How a fluid moves through a sedimentary body is a function of fabric, which is determined by sedimentary processes.

35. Deposit Porosity = (1 – volume concentration of sediment) = void space fraction
Porosity of a granular deposit is primarily a function of grain sorting, NOT grain size.

36. Permeability controlled by both grain size & sorting.
Permeability = measure of a deposit to transmit interstitial fluid (e.g., fluid flow through the pore network)
Beard, D.C., and Weyl, P.K., 1973, Influence of texture on porosity and permeability of unconsolidated sand: AAPG Bulletin, v. 57, p
Permeability controlled by both grain size & sorting.

37. Questions You Should be Able to Answer
How would you determine the grain size in a river bed of cobbles, aeolian dune sand, or clay sediment from a lake? How would you determine grain size in a sandstone? Shale?
What are the 3 basic weathering products? What range of these does the Wentworth Size Scale address?
Why is a geometric scale used in the Wentworth Chart?
What is the phi scale?
What are the basic size classes in the Wentworth Scale? What is the mm size break for each of these?
What are the 4 basic graphic forms in which to display grain-size data?
What is the mode, median and mean as used in grain-size data?
Why is a grain population typically sampled at the 5, 16, 50, 84 and 95% points?
What determines the average (mean) grain size? What sorts of information can you derive from the mean grain size? Why does sediment typically fine from proximal to distal?

38. 10. What does sorting measure in a sample
10. What does sorting measure in a sample? What factors determine sorting? 11. What does skewness measure in a grain sample? What factors influence skewness? In what settings would you expect to find a fine-skewed population? A coarse-skewed sample? 12. What does kurtosis measure in a grain sample? What factors influence kurtosis? In what settings would you expect to find a leptokurtic population? Platykurtic population? 13. What are bimodal and polymodal distributions? Under what conditions would they occur? 14. What is the basis for the tertiary grain-size classification of sediment/rock? Officially, what is gravel? Mud? Sand? What is the difference between, say, a muddy gravel and a sandy gravel? 15. What determines grain shape? How is it measured? What are the basic grain shapes? What is the shape of typical river cobbles? Beach cobbles? 16. What is grain roundness? What factors determine grain roundness? What does grain roundness tell you? 17. What is the textural maturity of a sediment?

39. 18. What is the typical progression in textural maturity in a sediment
18. What is the typical progression in textural maturity in a sediment? Why does it occur? What is a textural inversion? 19. What surface features are common on grains? How do grain surface features develop? Why are aeolian grains commonly frosted? 20. What is sediment/rock fabric? Why does it develop? What sorts of information does it yield? 21. What is imbrication? How does it indicate flow direction? 22. What is the difference between matrix- and clast-supported fabric? 23. What is porosity? What is permeability? 24. What is the relationship between porosity/permeability and grain size? Why? 25. What is the relationship between porosity/permeability and sorting? Why? 26. What is the range of depositional (original) porosity in sand? What factors influence depositional porosity?