I. Sedimentary rocks form from
the interaction of rocks at Earth's surface with water and atmosphere.
The deposition of mechanically and chemically weathered particles
following erosional processes leads to the formation of detrital
or clastic sedimentary rocks. The second major type
of sedimentary rock is chemical sedimentary rock.
- A. Detrital sediments become sedimentary rocks through the
process of lithification. Lithification occurs through two processes.
- 1. Compaction reduces the pore space between sediments
by the overburden, or weight of overlying sediment.
- 2. Cementation occurs when H2O solutions precipitate
minerals in the pore spaces. Some common cements:
- a. calcite
- b. quartz/silica
- c. iron oxide
- B. Detrital sedimentary rocks can be classified by grain
size
| Size Range |
Particle name |
Sediment name |
Detrital rock |
>256mm
64-256
4-64
2-4 |
boulder
cobbles
pebbles
granules |
gravel
|
breccia or
conglomerate |
| 1/16-2
|
sand |
sand |
sandstone |
1/256-1/16
<1/256 |
silt
clay
|
mud |
shale or
mudstone |
- C. Grain texture gives clues to the energy present in the
environment of deposition.
- 1. Sorting means % of grains with a given particle
size. If more grains in a rock are of the same size, the rock is better
sorted. For instance:
- a. 40% pebbles, 30% sand, 30% silt is a poorly
sorted rock
- b. 100% sand is a well-sorted rock
- 2. Grain shape refers to the roundedness of the grains
- a. Angularity -- how rounded are the angular
fragments?
- b. Sphericity -- how spherical are the grains?
- D. What are common sedimentary features found in
sedimentary rocks?
- 1. Layering
- a. Bedding planes = depositional surface
- b. Ripple marks from waves or currents
- c. Crossbeds = sloping layers inclined to bedding
planes
- d. Graded beds = larger fragments on the bottom of
a layer, fining upward.
- d. Mudcracks from drying of the depostional surface
- 2. Fossils of organisms that live on or in the sediment
layer
|