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.
