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Sediments and Sedimentary Rocks

Sediments and Sedimentary Rocks
What are sediments?
Sediments are loose particles of former
rocks. Sediments may be particles in the
form of mineral grains or boulders or they
may be dissolved particles which will later
precipitate from the water and form solid rock.
What is a Sedimentary Rock?
Sediments after they are deposited may
be buried and undergo physical and chemical
change resulting in a solid rock (Sedimentary
Types of Sedimentary Rocks
• Three main types of Sedimentary rocks:
– Clastic-is formed from solid particles which
are eroded from previous rocks.
– Chemical/Biochemical—these sediments
which are precipitate from water either
through chemical or biochemical processes
Sedimentary Cycle
(part of the Rock Cycle)
Sedimentary rocks all go through the
following cycle:
1. weathering from previous rocks.
2. erosion of weathering products.
3. transportation from site of origin to
another site.
4. deposition of the sediments.
5. burial of the sediments
6. diagenesis
Sedimentary Rock Cycle
Transportation of Clastic Rock
• In general most sediments travel downhill or
• Mode of transportation include:
Streams, rivers
Ocean waves/tides/currents
• The size of particle depends on the available
force applied. Example: at flood level a river will
be able to transport larger particles than it would
during a normal river levels. This results in
particles being sorted by size which is called
Transportation of Clastic Rock
• If the particles are being transported they
bang into each other repeatedly resulting
in the grains wearing down and becoming
rounded. This is called Rounding (Dah).
• In general the farther a particle travels
from it source the more rounded it
becomes. A jagged particle is near the
source rock, where the rounded, smooth
particle is farther from the source rock.
Sorting and Rounding
Clastic Rock Deposition
• Sediments will be dropped by it mode of
transportation when the energy is not
sufficient to carry the particle farther. This
is called deposition. A particle may be
deposited numerous times before final
deposition occurs.
Clastic Rock: Burial/Diagenesis
• As the particles are deposited and sorted
they build up layers and become buried by
later sediments. Layer after layer pile up
on top of each other. Those sediments on
the bottom are buried under increasing
pressure, which compact the sediments
and force out the water. The sediments
are undergoing diagenesis (physical and
chemical changes that turn sediments into
sedimentary rocks.
• In addition to squeezing the water out,
new minerals form in the space between
the sediment grains which cements the
grains together.
• Also some sedimentary grain may dissolve
and be replaced by other minerals.
Classification of Clastic Rocks
• There are two types of Clastic rocks:
– 1.Sandstones and fine grained
– 2.Chemical/Biochemical
Sandstones and fine grained
Clastic Rocks
• Sands when buried lithifies to form Sandstone.
Depending on the grain size, shape, and
mineralogy they can be placed in three
additional major categories.
– Siltstones- composed of silt size grains which become
– Shale- composed of silt and clay size grains which
lithified. These tend to break on bedding planes.
Some may contain oil but is difficult to separate from
the rock itself.
– Mudstone- composed of lithified mud. Generally
blocky in appearance and when freshly broken have a
musty smell.
Chemical/Biochemical Rocks
• Ions which were dissolved from pre-existing
rocks go into solution and travel down stream to
lakes and oceans. In the lakes and oceans the
ion precipitate out either by biochemical or
inorganic chemical processes.
• The precipitates form a solid, and go through
biogenesis just like clastic rocks. Examples of
this is aragonite which forms fine grain calcium
carbonate. Halite (NaCL) is another precipitate
that occurs in this way.
Chemical/Biochemical Rocks
• Chemical/Biochemical Rocks can be
broken down also into 3 major categories.
– 1. Carbonates
– 2. Evaporites
– 3. Cherts
• Carbonates are formed from lithifed
carbonate material in ocean waters
through biochemical processes. Marine
organisms extract the carbonate from the
ocean waters and form shell material.
Organism such as foraminifera,
gastropods, corals, etc. make up the bulk
of the material forming limestones, and
Some typical environments that
carbonates can form.
• Evaporates generally form in shallow
brackish lagoons in arid regions where the
water is evaporated away leaving the
precipitates such as Aragonite
(carbonates), rock salt, and gypsum.
• Some large salt deposits also occur, such
as the M zone in the Mediterranean Sea.
• Cherts form in the deep, deep ocean
environments where organism which
secret a siliceous shell have deposited
after death. Diatoms will accumulate and
form a siliceous ooze which will then go
through diagenesis, becoming chert.
• You should know the following:
– What are sediments?
– What are sedimentary rocks.
– What are the major sedimentary rocks.
– Able to label and draw the processes which
make up the sedimentary rock cycle.
– How do sedimentary rocks (clastic and
chemical/biochemical) form and how are they
• Thanks to Greg Anderson for use of lecture
ALSO: http://earth.geol.ksu.edu/liu/g100/figures/20_transportation.jpg
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