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
Rock)

• 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
(biological)

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

http://www.mtsu.edu/~cdharris/GEOL100/erosion/sed-rk-cycle.gif

• In general most sediments travel downhill or downstream.
• Mode of transportation include:
– Streams, rivers
– Glaciers
– Wind
– 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 sorting .

http://earth.geol.ksu.edu/liu/g100/figures/20_transportation.jpg

http://www.rjmaxwell.com/education/basic_geological_classification/

• 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.

pasadena.wr.usgs.gov/office/given/geo1/lecturenotes/SedRx_files/image010.gif

• 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.

• 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.

http://www.rjmaxwell.com/education/basic_geological_classification/

• There are two types of Clastic rocks:
– 1.Sandstones and fine grained
– 2.Chemical/Biochemical

• 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 lithified.
– 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.

• 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 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 dolomites.

http://www.geologyrocks.co.uk/tut.php?id=14

• 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.

http://www.geo.msu.edu/geo333/images/salt-cycle.jpg

• 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 classified.

• Thanks to Greg Anderson for use of lecture notes.
http://pasadena.wr.usgs.gov/office/ganderson/es10/lectures
ALSO: http://earth.geol.ksu.edu/liu/g100/figures/20_transportation.jpg
http://www.rjmaxwell.com/education/basic_geological_classification/ pasadena.wr.usgs.gov/office/given/geo1/lecturenotes/SedRx_files/image010.gif
http://www.rjmaxwell.com/education/basic_geological_classification/ http://www.geologyrocks.co.uk/tut.php?id=14 http://www.geo.msu.edu/geo333/images/salt-cycle.jpg
http://www.mtsu.edu/~cdharris/GEOL100/erosion/sed-rk-cycle.gif