Sedimentary Rocks

Weathering processes break down rock into finer and finer material and such material is often transported and deposited as sediment. The processes of compaction and cementation of this sediment over a long period of time turn the sediment into rock. The formation of rock in this way is termed "lithification".

Sedimentary rocks are divided into two broad classes, detrital sedimentary rocks and chemical sedimentary rocks as described below. Sedimentary rocks make up perhaps only five percent or so of the outer 16 kilometers (10 miles) of the Earth (Lutgens and Tarbuck), but the majority of surface rocks are sedimentary. About 75% of the rock outcroppings on continents are sedimentary rocks.

Detrital sedimentary rocks are those for which the material has been transported as solid particles. The particles themselves may have derived from either physical weathering or chemical weathering. Sedimentation means settling from a fluid, either water or air. The texture of sedimentary rocks is described as "clastic", meaning that they are composed of discrete particles that are compacted and cemented together. An example of a non-clastic texture would be crystalline material.

Detrital Sedimentary Rocks
Sediment name
and particle size
Description
Rock Name
Gravel (>2 mm)
Rounded rock fragments
Conglomerate
Angular rock fragments
Brecia
Sand (1/16 to 2 mm)
Quartz predominates
Quartz sandstone
Quartz with considerable feldspar
Arkose
Dark color, quartz with considerable feldspar, clay and rocky fragments
Graywacke
Mud (<1/16 mm)
Splits into thin layers
Shale
Breaks into clumps or blocks
Mudstone

Chemical sedimentary rocks derive from material that is carried in solution to lakes and seas. If the solute precipitates out of the solution to form chemical sediments, rocks such as limestone can be formed.

Chemical Sedimentary Rocks
Group
Texture
Composition
Rock Name
Inorganic
Clastic or nonclastic
calcite, CaCO3
Limestone
Nonclastic
Dolomite, CaMg(CO3)2
Dolostone
Nonclastic
Microcrystalline quartz, SiO2
Chert
Nonclastic
Halite, NaCl
Rock salt
Nonclastic
gypsum, CaSO4.2H2O
Rock gypsum
Biochemical
Clastic or nonclastic
calcite, CaCO3
Limestone
Nonclastic
Microcrystalline quartz, SiO2
Chert
Nonclastic
Altered plant remains
Coal

The data in both tables above is taken from Lutgens and Tarbuck, Essentials of Geology, 7th Edition.

Examples of sedimentary rock
Grand Canyon sedimentary rock
Index

Reference
Lutgens and Tarbuck
Ch 6
 
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