Medium grained turbidite sandstones with crinoid stems in
the Brushy Canyon Fm., West Texas
Sedimentology
Grain Parameters
Reading Assignment: Boggs, Chapter 3
Key Concepts
I. Grain-size analysis & statistical parameters
II. Sediment / Rock textural classification
III. Grain shape
IV. Grain roundness
V. Textural maturity
VI. Grain surface features
VII. Sediment / rock fabric
Why do Grain-Size Analysis?
• Sediment / rock description = communication
• Sediment / rock name = classification = communication
• Information on source area, transport & depositional environment
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)
How do we measure grain size?
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
Lithification
conversion of unconsolidated sediments
into rock
F = -log2D
Size range
(metric)
Wentworth Size Class
<−8
>256 mm
Boulder
Conglomerate
−6 to −8
64–256 mm
Cobble
(rounded clasts)
−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
Breccia
(angular clasts)
Sand
Mud
Sandstone
Mudstone
How To Present The Grain-Size Data
Histogram
Frequency
Curve
Record weights
Cumulative
Frequency
Curve
Semi-Log
Plot
Always plotted as Coarse to Fine on Phi scale
Statistical Treatment of Grain-Size Data
• “Average” grain size
• Mode - most frequently occurring size class
• Median – 50%
• Mean – arithmetic average
• Sorting
• Skewness
• Kurtosis
9
“Average” Grain Size
Mode
f84
Median
f16
Why Sample at 16 & 84%; 5 & 95%?
16%
84%
68%
1 sd 1 sd
5%
90%
2 sd 2 sd
95%
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
Sorting – How Uniform Is The Population?
Sorting in
Real Sediment
14
Visual Estimates of Sorting
Fig 3.3, p. 58.
15
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)
Skewness – Population Symmetry
Normal & Skewed Populations
p. 59, fig 3.4, fig. 3.5 p. 60.
Symmetrical
population
Positive-Skewed
Fine-Skewed
Negative-Skewed
Coarse-Skewed
18
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 fineskewed as they represent most distal transport
Kurtosis
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
Sediment/Rock TEXTURAL Classification
22
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
Grain Shape & Environment
Beach gravel – concentration of blades
& disks
River gravel – concentration of
rollers
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
Grain Roundness
Rounded
Subangular
Angular
Textural
Maturity
Consider:
- Clay %
- Sorting
- Roundness
Textural Maturity Flow Chart Used in Lab
Clay
content
> 5%
Immature
< 5%
Sorting
(σφ)
> 0.5φ
Submature
< 0.5φ
Roundness
Angular
Mature
Rounded
Supermature
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)
Class ended here.
Grain Surface Features
• Common descriptions:
unmodified, polished,
frosted, pitted,
scratched
• Origin in transport,
chemical weathering,
precipitation
(diagenesis)
• SEM-scale
• Distinctive frosting of
sand with aeolian
transport
Large field of
grain surface
textures
Frosted Aeolian Sand
Sediment/Rock Fabric
• Grain orientation
• Current orientation of elongate (rod-shaped)
grains
• Imbrication – most stable configuration
• Random / chaotic patterns with suspension
settling
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
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.
Deposit Porosity = (1 – volume concentration of sediment) = void space fraction
Porosity (%)
Porosity of a granular deposit is primarily a function of grain sorting, NOT grain
size.
Permeability = measure of a deposit to transmit interstitial fluid (e.g., fluid flow
through the pore network)
9.87  10 -9 cm2
Beard, D.C., and Weyl,
P.K., 1973, Influence of
texture on porosity and
permeability of
unconsolidated sand:
AAPG Bulletin, v. 57, p.
349-369.
Permeability controlled by both grain size & sorting.
Questions You Should be Able to Answer
1.
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?
2.
What are the 3 basic weathering products? What range of these does the Wentworth Size Scale address?
3.
Why is a geometric scale used in the Wentworth Chart?
4.
What is the phi scale?
5.
What are the basic size classes in the Wentworth Scale? What is the mm size break for each of these?
6.
What are the 4 basic graphic forms in which to display grain-size data?
7.
What is the mode, median and mean as used in grain-size data?
8.
Why is a grain population typically sampled at the 5, 16, 50, 84 and 95% points?
9.
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?
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?
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?