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.

Clastic Sedimentary Rocks Made from the cementation and lithification of particles (sediments or fragments). 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.

Clastic sedimentary rocks are composed of two parts the clasts (or sediment) and cement (glue that holds the clasts together. Examining the individual clasts that make up clastic rocks, we can infer a lot of information.

Clastic Sedimentary Rocks: Grain SizeIndividual grains in a sedimentary rock can range from boulder size (256 mm diameter) to clay size

(0.0039mm). For our purposes know the following: Boulders, Cobbles and Pebbles Sand (very coarse, medium, fine, very fine) Silt Clay (smallest grain sizes)

Boulders>Cobble>Pebble>Sand>Silt>Clay>Dissolved Stuff

Grain Size tells the ENERGY involved in transporting material. As a general rule, grain size can tell us • 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.

Clastic Sedimentary Rocks: ColorGenerally 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).

BLACK sediments usually mean the opposite a lack of oxygen.Stagnant water bodies do not mix with the air at the surface. Dead and decomposing organic matter

are acted upon by bacteria which subsequently remove oxygen from the water for respiration.

Black color usually represents lots of decomposing organic matter in the sediments.What are some examples of where we might find black sedimentary rocks?

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.

WHITE sediments may lack any iron or water (to be rusted) such as in sand on a beach or wind blown desert sand dunes.

We might not be able to indicate oxygen levels through white sediments, however, we can apply the principle of uniformitarianism.

Where do we see white sediments accumulate? Sandy beaches, dunes, deserts.

Clastic Sedimentary Rocks: SortingThe 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?

Poorly sorted sediments are deposited rapidly without being selectively separated into sizes.Fast moving water (carrying many grain sizes) may abruptly stop, dumping its sediment load. (i.e.

alluvial fan).Glacial ice can carry ALL different sizes and when the ice melts, all the sediments are deposited

together.

Clastic Sedimentary Rocks: RoundnessRoundness 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.

Chemical Sedimentary RocksRocks 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).

Precipitates• Chemical sedimentary rocks that form by precipitation generally develop in the water due to supersaturation.• When we discuss the limestone caves (groundwater), the structures (stalactites and stalagmites) are made from dripping water containing calcium ions and carbonate ions.• When water drips down into the cave, the water drips down a calcite icicle—precipitating the calcite mineral, layer after layer.• Calcite also directly precipitates out of water in warm, shallow marine environments forming crystalline limestone.

Biogenic Sedimentary RocksForm from the remains of a living organism.Sedimentary rocks 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. As other elements are squished out, CARBON is left behind. The more squishing, the more CARBON, the higher grade coal. Peat-lignite-bituminous-anthracite are all coal with peat being the least pressurized and anthracite the best quality and highly pressurized coal.

Sedimentary Rock Rules• 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).