1.4 GRAIN SIZE DISTRIBUTIONS

Question 1

What reference scale is used for grain size?

Answer 1

Udden-Wentworth scale

• uses ratio scale where grade boundaries differ by a factor of 2
• Gravel-sand boundary at 2mm
• Sand-silt boundary at 0.062mm
• Silt-clay boundary at 4microns, but sometimes 2microns
• uses Phi

Question 2

What are the different ways to classify grain size distributions?

Answer 2

Ternary diagram
Histogram
Frequency distribution curve
Cumulative frequency distribution curve

Question 3

How would you spot a mix of two sediment populations (size distributions) in a probability plot?

Answer 3

It will have distinct straight-line segments (more than one line) including equal mixture of 1 and 2.

Question 4

How would you identify grain size sorting using sigma?

Answer 4

• well sorted = small sigma

- poorly sorted = large sigma

Question 5

What is Kurtosis?

Answer 5

The degree of “peakedness” of distribution

- degree of concentration of grains relative to average

Question 6

Describe the shape of Platykurtic and Leptokurtic grain distribution.

Answer 6

platykurtic = better sorted in tails than in central part; flatter
leptokurtic = better sorted in central part than in tails; pointed

Question 7

What are the parameters for sand grain sizes?

Answer 7

Very coarse = 1-2mm
Coarse = 0.5-1mm
Medium = 0.25-0.5mm
Fine = 0.125-0.25
Very fine = 0.062-0.125