What is the difference between clastic chemical and biochemical sedimentary rocks

A less common carbonate mineral that is still important for forming carbonate rocks is dolomite, which has the formula CaMg CO 3 2. It is similar to calcite and aragonite, except that some of the calcium is replaced with magnesium. Dolomite is more common as a replacement mineral, which has replaced calcite in carbonate rocks. Limestone is comprised of calcite and aragonite. It can occur as a chemical sedimentary rock, forming inorganically due to precipitation, but most limestone is biochemical in origin.

In fact, limestone is by far the most common biochemical sedimentary rock. Almost all limestone forms in marine i. Today continental shelves are relatively narrow zones along the margins of continents, but for large parts of geologic history sea-level was much higher, and large parts of the interiors of continents were flooded.

Reefs are highly productive ecosystems populated by a wide range of organisms, many of which use calcium and bicarbonate ions from seawater to make carbonate minerals especially calcite for their shells and other structures. These include corals as well as green and red algae, urchins, sponges, molluscs, and crustaceans. Some of micro-organisms use CaCO 3 to build tiny tests shells which accumulate on the ocean floor when these organisms die.

Erosion can break all of these carbonate materials apart, scattering fragments throughout surrounding region Figure 9. Figure 9. Reefs tend to form near the edges of steep drop-offs because the reef organisms thrive on nutrient-rich upwelling currents. As the reef builds up, it is eroded by waves and currents to produce carbonate sediments that are transported into the steep offshore fore-reef area and the shallower inshore back-reef area. Reef-derived sediments are dominated by reef-type carbonate fragments of all sizes, including mud.

In many such areas, carbonate-rich sediments also accumulate in quiet lagoons, where mud and mollusc-shell fragments predominate Figure 9. Limestone also accumulates in deeper water, from the steady settling out of the carbonate shells of tiny organisms that lived near the ocean surface. Limestone can form as either a chemical rock made of calcite or as a biochemical rock made of fossils that are, in turn, made of aragonite or calcite. Chalk is a biochemical sedimentary rock formed by the accumulation of microscopic marine calcite fossils.

Sedimentary rocks are classified based on how they form and on the size of the sediments, if they are clastic. Clastic sedimentary rocks are formed from rock fragments, or clasts; chemical sedimentary rocks precipitate from fluids; and biochemical sedimentary rocks form as precipitation from living organisms. The difference between chemical and biochemical sedimentary rocks is that in biochemical sedimentary rocks, organisms play a role in turning the ions into sediment.

In chemical sedimentary rocks, the process is inorganic, often resulting from a body of water evaporating and concentrating the ions. Sedimentary rocks are formed from pieces of other existing rock or organic material.

There are three different types of sedimentary rocks: clastic, organic biological , and chemical. Clastic sedimentary rocks are made of sediments. Mudstone is a general term for rocks made of sediment grains smaller than sand less than 2 mm. Rocks that are fissile , meaning they separate into thin sheets, are called shale.

Rocks exclusively composed of silt or clay sediment, are called siltstone or claystone , respectively. These last two rock types are rarer than mudstone or shale.

Rock types found as a mixture between the main classifications may be named using the less-common component as a descriptor. For example, a rock containing some silt but mostly rounded sand and gravel are called silty conglomerate. Sand-rich rock containing minor amounts of clay is called clayey sandstone.

Chemical sedimentary rocks are formed by processes that do not directly involve mechanical weathering and erosion. Chemical weathering may contribute to the dissolved materials in water that ultimately form these rocks.

Biochemical and organic sediments are clastic in the sense that they are made from pieces of organic material that are deposited, buried, and lithified; however, they are usually classified as being chemically produced.

Inorganic chemical sedimentary rocks are made of minerals precipitated from ions dissolved in solution, and created without the aid of living organisms. Inorganic chemical sedimentary rocks form in environments where ion concentration, dissolved gasses, temperatures, or pressures are changing, which causes minerals to crystallize. Biochemical sedimentary rocks are formed from shells and bodies of underwater organisms. The living organisms extract chemical components from the water and use them to build shells and other body parts.

The components include aragonite, a mineral similar to and commonly replaced by calcite, and silica. Organic sedimentary rocks come from organic material that has been deposited and lithified, usually underwater.

The source materials are plant and animal remains that are transformed through burial and heat, and end up as coal, oil, and methane natural gas. Inorganic chemical sedimentary rocks are formed when minerals precipitate out of an aqueous solution, usually due to water evaporation. The precipitate minerals form various salts known as evaporites.

For example, the Bonneville Salt Flats in Utah flood with winter rains and dry out every summer, leaving behind salts such as gypsum and halite. The deposition order of evaporites deposit is opposite to their solubility order, i. The deposition order and saturation percentages are depicted in the table, bearing in mind the process in nature may vary from laboratory-derived values [ 19 ].

Calcium carbonate-saturated water precipitates porous masses of calcite called tufa. Tufa can form near degassing water and in saline lakes. Waterfalls downstream of springs often precipitate tufa as the turbulent water enhances the degassing of carbon dioxide, which makes calcite less soluble and causes it to precipitate. Saline lakes concentrate calcium carbonate from a combination of wave action causing degassing, springs in the lakebed, and evaporation.

In salty Mono Lake in California, tufa towers were exposed after water was diverted and lowered the lake levels.

Cave deposits like stalactites and stalagmites are another form of chemical precipitation of calcite, in a form called travertine. Calcite slowly precipitates from water to form the travertine, which often shows banding. This process is similar to the mineral growth on faucets in your home sink or shower that comes from hard mineral-rich water.

Oxygenation of the atmosphere and oceans caused free iron ions, which are water-soluble, to become oxidized and precipitate out of solution. The iron oxide was deposited, usually in bands alternating with layers of chert. Chert , another commonly found chemical sedimentary rock, is usually produced from silica SiO 2 precipitated from groundwater.

Silica is highly insoluble on the surface of Earth, which is why quartz is so resistant to chemical weathering. Water deep underground is subjected to higher pressures and temperatures, which helps dissolve silica into an aqueous solution. As the groundwater rises toward or emerges at the surface the silica precipitates out, often as a cementing agent or into nodules. For example, the bases of the geysers in Yellowstone National Park are surrounded by silica deposits called geyserite or sinter.

The silica is dissolved in water that is thermally heated by a relatively deep magma source. Chert can also form biochemically and is discussed in the Biochemical subsection. Chert has many synonyms, some of which may have gem value such as jasper, flint, onyx, and agate, due to subtle differences in colors, striping, etc. Oolites are among the few limestone forms created by an inorganic chemical process, similar to what happens in evaporite deposition. When water is oversaturated with calcite, the mineral precipitates out around a nucleus, a sand grain or shell fragment, and forms little spheres called ooids see figure.

As evaporation continues, the ooids continue building concentric layers of calcite as they roll around in gentle currents. Biochemical sedimentary rocks are not that different from chemical sedimentary rocks; they are also formed from ions dissolved in solution. However, biochemical sedimentary rocks rely on biological processes to extract the dissolved materials out of the water. Most macroscopic marine organisms use dissolved minerals, primarily aragonite calcium carbonate , to build hard parts such as shells.

When organisms die the hard parts settle as sediment, which becomes buried, compacted, and cemented into rock. Sedimentary rock is classified into two groups based on how they form. They are clastic and chemical. Clastic sedimentary rock is formed as bits of weathered rock become cemented together. Because all kinds of rock are subject to weathering many different minerals can make up this group of rocks. How do clastic sedimentary rocks differ from chemical and biochemical sedimentary rocks? Clastic sedimentary rocks are dominated by components that have been transported as solid clasts clay, silt, sand, etc.

In contrast, chemical and biochemical sedimentary rocks are dominated by components that have been transported as ions in solution e. Organic sedimentary rocks form from the action of organisms. Examples include fossiliferous limestone and coal.

Fossiliferous limestone and coal are two examples of organically-formed sedimentary rocks. This fossiliferous limestone left is from Giles County, and was formed from the shells of many aquatic organisms. Clastic sediments or sedimentary rocks are classified based on grain size, clast and cementing material matrix composition, and texture. An example of clastic environment would be a river system in which the full range of grains being transported by the moving water consist of pieces eroded from solid rock upstream.

Clastic sedimentary rocks are produced by the compaction and cementation of rock and mineral fragments that were derived from weathering or preexisting rock.

These fragments are called clasts. Clastic rocks are classified by clast size.