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SEDIMENTARY ROCKS Sedimentary rocks form from solidification of sediment (particles from pre-existing rocks, chemical or biologic material) “ Sediment” are particles formed at the earth’s surface. Sediment can form in several ways, giving us three main classes of sedimentary rocks: 1. Detrital (or clastic ) sedimentary rocks form from fragments of older rocks. These pieces may be transported some distance by water, wind, or glacial ice, then deposited, buried, and turned into rock. Examples include sandstone and conglomerate . 2. Chemical sedimentary rocks form from chemical precipitation, generally precipitation from a solution of water and dissolved ions. Examples include rock salt and gypsum . 3. Biogenic sedimentary rocks form from the remains of living things. For example, coal is the altered remains of wood, and most limestone and chalk comes from the shells of marine animals.
COQUINA (a form of limestone): an example of a biogenic sedimentary rock. CONGLOMERATE: an example of a detrital sedimentary rock. ROCK SALT: an example of a chemical sedimentary rock.
This figure shows how clastic sediment of various sizes will, after compaction and cementation, form different types of detrital sedimentary rocks . The process of sediment turning into rock is called lithification .
how much ‘weathering’ of minerals has occurred and
How much ENERGY was required to transport and deposit the sediments.
Weathering: Individual clasts that are smaller in size most likely have been broken down more than bigger stuff. This could either be due to a long time in transit (as sediments are grinding against other sediments and rocks they become smaller and smaller) or due to weak minerals (the weaker they are, the easier they are to erode into smaller sizes).
Grain size also tells information about the VELOCITY and ENERGY of the transporting medium. It takes more energy (a higher velocity stream) to move large pebbles compared to clay or silt. Very fine grained (small grain sizes) clastic rocks indicates a very slow moving (if not stagnant) transporting medium.
Generally speaking, color MAY be an indicator of the oxygen concentrations in the environment where sediments were deposited.
RED sediments typically form in well oxygenated environments involving water (assuming iron is present in sediments). i.e. a running stream or floodplain to the stream. Fe in sediments will rust in the presence of water and oxygen yielding a red color in sediments (recall oxidation in weathering).
GREEN sedimentary rocks indicate oxygen somewhere in between. For example, on the continental shelf where in the ocean we have oxygen introduced by wave action at the surface (and by photosynthetic stuff living up there) but we still have animals living in sediments to remove oxygen simultaneously.
The sorting (poorly to well sorted) of the rock refers to the number of different grain sizes present in the rock.
A well sorted rock would contain predominantly one size of clast (i.e. all medium sand).
A poorly sorted rock would contain a range of many different grain sizes.
Sorting tells us some information:
Well sorted sediments occur when water flow slows down gradually (so that each grain size drops out of suspension at different water velocities). Also, wind can pick up small sizes and transport them (i.e. sand) leaving only the large stuff behind.
-> For example, imagine water during a flood—stream velocity is the fastest IN the stream channel. Water eventually tops the banks of the stream and flows onto the floodplain. As the water washes on the floodplain it slows down gradually until it stops. What grain size would be deposited closest to the stream channel? Furthest from the stream channel?
Roundness refers to the degree of rounding of each individual clast. Sphericity is how closely the grain resembles the shape of a sphere.
Roundness increases with an increasing length of transportation (recall abrasion from weathering). Clasts smacking into one another during transport tends to increase the roundness of the grains (i.e. sea glass).
Clasts with rough edges are considered angular and have been in transport for a much shorter period of time.
We call sediments with angular grains IMMATURE as they have been in transit and have not been ‘worked’ a short time.
Well sorted and well-rounded usually but very fine grain sizes.
Shale (left) and siltstone (right) both are made of fine grained sediment (shale is made of clay, the smallest particles, therefore the finest grains). The smaller the grain size, the slower the water must be moving in order for it to settle out of solution (or the air).
Micrite is analogous to shale (only shale is made from silicate minerals and thus doesn’t react with HCl). Small clasts indicates slow moving water.
Coquina is usually made from the shells of clams and snails that live in marine environments. Upon death, the shells become subject to weathering just like any rock. Shells become fragments (clasts) and can be ‘glued’ together with calcite cement.
Crystalline LS below forms by CHEMICAL processes.
All of these rocks would react with acid.
LIMESTONE: Made mostly of calcium carbonate. (Calcite can precipitate from solution, or form by animals). Coquina (left), fossiliferous limestone (right). Fossil LS looks like shale (it does have small grains) but when in doubt, do an ACID test for calcite.
Chemical Sedimentary Rocks Rocks that form chemically; precipitates out of solution or water evaporates leaving solutes behind.
Chemical sedimentary rocks form by directly precipitating out of water or water evaporates leaving the once dissolved minerals behind.
Precipitation occurs when there is so much solutes dissolved in water that they just crystallize right there in the water. For example, when making rock candy, you must boil water and continuously dissolve sugar in the water (this supersaturates the solution). Then you let the water cool down to room temperature. The sugar crystals cannot remain dissolved anymore and ‘precipitate’ from the solution. Oftentimes, direct precipitation of minerals out of water is the ‘cement’ that glues clasts together in clastic rocks.
Evaporation occurs when water has dissolved stuff in it (i.e. salt). Boil the water so that evaporates—the salts that were dissolved in the water are left behind (as a chemical sedimentary rock).
It forms chemically through evaporation. Salt water stranded on land, in a warm environment, will evaporate leaving the salt beds behind.
In a similar situation, fresh water that has been flowing, dissolved materials. If a fresh water body is stranded on land, and evaporates, the silicate mineral rock gypsum will be left behind.
Rock gypsum (left) and rock salt (right). Stranded bodies of salt water in warm climates evaporate and leave the salt behind. Gypsum forms in the same way but from fresh water with different elements dissolved in it.
Note that coquina is a clastic sedimentary rock formed from clasts produced by living organisms (clam and snail shells).
Sedimentary rocks, therefore can have a clastic and biogenic origin.
Biogenic sedimentary rocks are made by biological organisms. Coal is an example of lots of plant material that becomes pressurized. As pressure increases oxygen, water, hydrogen, nitrogen (etc.) is squeezed from the plant material. (Peat left, lignite right).
As other elements are squished out, CARBON is left behind. The more squishing, the more CARBON, the higher grade coal. (Bituminous left, anthracite right). Peat-lignite-bituminous-anthracite are all coal with peat being the least pressurized and anthracite the best quality and highly pressurized coal.
Sedimentary rocks are examined to determine the environment in which they were deposited.
CLASTIC sedimentary rocks form from lithification/cementation of clasts (pieces of rocks). CHEMICAL rocks form from precipitation or evaporation of water. BIOGENIC sedimentary rocks form with the help of some living organism.
In clastic sedimentary rocks:
The rounder the grains, the longer the transportation time.
The smaller the grains, the longer the transportation time.
The smaller the grains, the slower the transport (water/wind).
The largest grain indicates the fastest moving water (big grains, fast moving; need a lot of energy to move big stuff).
Well sorted means deposition velocity was gradually slowing whereas poorly sorted sediments were essentially ‘dumped’; quick deposition.
Color can indicate oxygen content (clastic only). BLACK (low low oxygen), GRAY/GREEN (medium oxygen), RED (high oxygen).