Sedimentary Textures

Question 1

What aspects are included in the texture of sedimentary rocks?

Answer 1

1. Grain Size
2. Sorting
3. Grain shape
4. Grain surface texture
5. Rock fabric

Question 2

Classifications of grain size:

Answer 2

a. Clay (0.00006-0.0020mm)
b. Silt (0.0039-0.053mm)
c. Sand (0.0625-1.68mm)
d. Granule (2.0-3.36mm)
e. Pebble (4.0-16.0mm)
f. Cobble (>16-256mm)
g. Boulder (>256)

Question 3

In a cumulative curve, what are mean, mode and median?

Answer 3

Mean: Arithmetic average of grain size
Mode: Most frequently occurring size.
Median: Midpoint of grain size distribution.

Question 4

What 3 factors control particle shape?

Answer 4

a. Particle form (Sphericity)
b. Particle Roundedness
c. Particle surface textures

Question 5

What shapes of grains settle most rapidly, and are transported more easily by traction and rolling?

Answer 5

Spherical grains.

Question 6

Which conditions will lead to more rounding?
a. Quartz grains
b. limestone
c. pebbles
d. sands
e. water
f. wind
g. rivers
h. beaches

Answer 6

a, c, f, h

Question 7

What is textural maturity? Give an example of an immature and a mature environment.

Answer 7

Textural maturity is the amount of sorting/reworking a sediment has undergone. Measured by removal of clay/matrix, sorting of framework grains and rounding of grains.
Immature: Alluvial Fan
Mature: Beach or sand bar.

Question 8

What is fabric mostly concerned with?

Answer 8

Orientation of grains. Most obvious in non-equant grains. Fabric has both magnitude and direction.

Question 9

How do porosity and permeability differ?

Answer 9

Porosity is a proportional volume unoccupied by grains, and permeability is the ability to transmit fluids (pore connectedness).

Question 10

What is the difference between matrix- and class-supported rocks?

Answer 10

In a matrix supported rock, larger clasts do not touch. In a class-supported rock, the clasts are in contact and make up a larger proportion of the volume than matrix.