Sedimentary
rocks can be categorized into three groups
based on sediment type. Most sedimentary rocks
are formed by the lithification of weathered rock
debris that has been physically transported and
deposited. During the transport process, the particles
that make up these rocks often become rounded due
to abrasion or can become highly sorted. Examples
of this type of sedimentary rock include conglomerate and sandstone.
Scientists sometimes call this general group of
sedimentary rocks clastic.
The remaining types of sedimentary rocks are created
either from chemical precipitation and crystallization,
or by the lithification of once living organic matter. We identify
these sedimentary rocks as being non-clastic.



Conglomerate.





Sandstone.





All sedimentary rocks are lithified into
some collective mass. Lithification is
any process that turns raw rock sediment into consolidated
sedimentary rock. The process of lithification usually
produces identifiable layering in these type of rocks
(Figure 10f-3). Lithification can occur by way
of:

• Drying and compaction.
  
• Oxidation of iron and aluminum.
  
• Precipitation of calcium and silica.
  

Figure 10f-3:
Dipping sedimentary layers of rock, Rocky Mountains,
Canada.



The classification of clastic sedimentary
rocks is based on the particle types found in the rock.
Some types of clastic sedimentary rocks are composed
of weathered rock material like gravel, sand, silt, and clay.
Others can be constructed from the break up and deposition
of shells, coral and other marine organisms by wave-action
and ocean currents. Table 10f-1 describes some
of the main types of clastic sedimentary rocks.


Clastic
sedimentary rocks.


Name
of Rock
Fragment
Type


Breccia (Image Link)
Coarse Fragments
of Angular Gravel and
Rocks


Conglomerate
Coarse Fragments
of Rounded Gravel and
Rocks


Sandstone
Sand Sized
Particles that are 90% Quartz


Arkose
Sandstone
composed of 25% Feldspar Grains


Shale
Clay Particles


Siltstone
Silt Particles


Mudstone
Mixture
of Clay and Silt


Limestone
Mixture
of Shells, Coral, and Other Marine Skeletons




Arkose.


Shale.


] Siltstone.


Limestone.



Earlier it was suggested that there were
two types of non-clastic sedimentary rocks. One group
forms through the chemical precipitation and crystallization
of elements and compounds from solution. Elements such
as calcium, sodium, potassium, and magnesium are commonly
released into the environment through a variety of chemical
weathering processes. These elements can then
become dissolved into aqueous solutions that are often
transported via runoff, stream
flow, or groundwater flow. If this solution
enters a basin environment where evaporation exceeds precipitation and
in-flow, sedimentary evaporites can
form because of the loss of water from the solution.

The oceans are almost saturated with dissolved calcium
carbonate. This compound originates from
the shells of a variety of marine organisms that
use it for the construction of shells and other hard
body parts. Because these organisms are surrounded
in a solution, some of the calcium carbonate dissolves
into the ocean waters. Under the right circumstances
the dissolved calcium carbonate can precipitate out
forming chemically created limestone deposits.
The formation of dolomite involves
the chemical modification of limestone deposits by
a magnesium rich solution.





Dolomite.



Name
of Rock
Precipitate
Type


Halite
Sodium and Chlorine


Gypsum
Calcium, Sulfur, and
Oxygen


Silcretes
Silica


Ferricretes
Iron


Limestone
Calcium Carbonate


Dolomite
Calcium Magnesium Carbonate




Halite.


Gypsum.



Several types of sedimentary rocks are
formed from the lithification of once living organisms. Limestone deposits
can be formed by the direct lithification of coral reefs,
marine organism shells, or marine organism skeletons. Chalk is
a particular variety of limestone that is composed of
the skeletons of marine microorganisms like forminifera. Coal and lignite are the lithified
remains of plants.



Figure 10f-11Chalk.





Figure 10f-12Coal.