Classification - Clastic sedimentary particles are classified in terms of size
Name of Particle
|Boulder||>256 mm||Gravel||Conglomerate or Breccia (depends on rounding)|
|Cobble||64 - 256 mm||Gravel|
|Pebble||2 - 64 mm||Gravel|
|Sand||1/16 - 2mm||Sand||Sandstone|
|Silt||1/256 - 1/16 mm||Silt||Siltstone|
|Clay||<1/256>||Clay||Claystone, mudstone, and shale|
The formation of a clastic sedimentary rock involves three processes:
When sediment is transported and deposited, it leaves clues to the mode of transport and deposition. For example, if the mode of transport is by sliding down a slope, the deposits that result are generally chaotic in nature, and show a wide variety of particle sizes. Grain size and the interrelationship between grains gives the resulting sediment texture. Thus, we can use the texture of the resulting deposits to give us clues to the mode of transport and deposition.
|Sorting - The degree of uniformity of grain size. Particles become sorted on the basis of density, because of the energy of the transporting medium. High energy currents can carry larger fragments. As the energy decreases, heavier particles are deposited and lighter fragments continue to be transported. This results in sorting due to density. |
If the particles have the same density, then the heavier particles will also be larger, so the sorting will take place on the basis of size. We can classify this size sorting on a relative basis - well sorted to poorly sorted. Sorting gives clues to the energy conditions of the transporting medium from which the sediment was deposited.
|Rounding - During the transportation process, grains may be reduced in size due to abrasion. Random abrasion results in the eventual rounding off of the sharp corners and edges of grains. Thus, rounding of grains gives us clues to the amount of time a sediment has been in the transportation cycle. Rounding is classified on relative terms as well.|
Cherts - chemically precipitated SiO2
Evaporites - formed by evaporation of sea water or lake water. Produces halite (salt) and gypsum deposits by chemical precipitation as concentration of solids increases due to water loss by evaporation.
Biogenic Sediments and Sedimentary Rocks
Limestone - calcite (CaCO3) is precipitated by organisms usually to form a shell or other skeletal structure. Accumulation of these skeletal remains results in a limestone.
Diatomite - Siliceous ooze consisting of the remains of radiolarian or diatoms can form a light colored soft rock called diatomite.
Coal - accumulation of dead plant matter in large abundance in a reducing environment (lack of oxygen).
Oil Shale - actually a clastic sedimentary rock that contains a high abundance of organic material that is converted to petroleum during diagenesis.
Features of Sedimentary Rocks That Give Clues to the Environment of Deposition
Stratification and Bedding
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A sedimentary facies is a group of characteristics which reflect a sedimentary environment different from those elsewhere in the same deposit. Thus, facies may change vertically through a sequence as a result of changing environments through time. Also, facies may change laterally through a deposit as a result of changing environments with distance at the same time.
Common Sedimentary Environments
- Non-marine environments
- Stream sediments
- Lake sediments
- Glacial (ice deposited) sediments
- Eolian (wind deposited) sediments
- Continental Shelf sediments
- Estuarine sediments
- Deltaic sediments
- Beach sediments
- Carbonate shelf sediments
- Marine evaporite sediments
- Continental slope and rise sediments
- Deep Sea Fans
- Sediment drifts
- Deep Sea Sediments
- Deep -Sea oozes
- Land-derived sediments