What is the difference between detrital chemical and organic rocks

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. Processes on the ocean floor cause the water in the deepest parts of the ocean to become more acidic.

This puts a lower limit on how deep in the ocean calcite and aragonite can accumulate, because they dissolve under acidic conditions. Calcite can form chemical sedimentary rocks on land in a number of environments. Tufa forms at springs. The tufa towers in Figure 9. Travertine which is less porous forms at hot springs. Similar material precipitates within limestone caves to form speleothems mineral deposits in caves, Figure 9. Dolostone also referred to as dolomite is the carbonate rock made of the mineral dolomite CaMg CO 3 2.

Dolomite forms through dolomitization , a process thought to involve chemical reactions between magnesium-rich water percolating through rocks, and sediments containing calcite. Calcite and dolomite can be distinguished from one another by applying a drop of weak acid to the rock; calcite will react with weak acid, whereas dolomite will not. Also, when dolomite weathers, it tends to turn buff tan in colour, whereas calcite tends toward grey and white. Chert is made of silica SiO 2. It has the same chemical formula as quartz, but is cryptocrystalline , meaning that the quartz crystals comprising chert are so small that it is difficult to see them even under a microscope.

Chert can be a chemical sedimentary rock, often forming as beds within limestone Figure 9. It can also be biochemical. Some tiny marine organisms e.

When they die their tiny shells or tests settle slowly to the bottom of the lake or ocean, where they accumulate and are transformed into chert. Some ancient chert beds — most dating to between and Ma — are also part of a rock known as a banded iron formation BIF.

It is a deep sea-floor deposit of iron oxide that is a common ore of iron. These rocks consist of alternating layers of dark iron oxide minerals magnetite and hematite and chert stained red by hematite Figure 9. In arid regions, lakes and inland seas typically have no stream outlet, and the water that flows into them is removed only by evaporation. Limestone frequently gets converted into dolomite or dolostones during stages of compaction, dewatering, and lithification of limestone.

This process is known as dolomitization. This process involves the removal of calcium from limestone by magnesium-containing solutions, and this involves the replacement of calcium with magnesium. Another type of chemical sedimentary rock is cherts. This is a hard and glassy sedimentary rock containing silica that precipitates from water.

This rock is formed in pockets or voids that contain gas or organic matter that has been removed or decomposed with time. In addition, this rock type can occur as continuous layers in sedimentary rocks. Detrital sedimentary rocks are a type of sedimentary rocks containing pre-existing sediment pieces that come from weathered bedrock. These are also known as clastic rocks. Most sediments in these rocks are mechanically weathered sediments. However, some detrital sedimentary rocks are pieces of chemical sedimentary rocks.

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. This biochemical extraction and secretion is the main process for forming limestone , the most commonly occurring, non-clastic sedimentary rock. Solid calcite reacts with hydrochloric acid by effervescing or fizzing.

Dolomite only reacts to hydrochloric acid when ground into a powder, which can be done by scratching the rock surface see Chapter 3 , Minerals. Limestone occurs in many forms, most of which originate from biological processes. Entire coral reefs and their ecosystems can be preserved in exquisite detail in limestone rock see figure. Fossiliferous limestone contains many visible fossils.

A type of limestone called coquina originates from beach sands made predominantly of shells that were then lithified.

Coquina is composed of loosely-cemented shells and shell fragments. You can find beaches like this in modern tropical environments, such as the Bahamas.

Chalk contains high concentrations of shells from a microorganism called a coccolithophore. Micrite , also known as microscopic calcite mud, is a very fine-grained limestone containing microfossils that can only be seen using a microscope. Biogenetic chert forms on the deep ocean floor, created from biochemical sediment made of microscopic organic shells.

This sediment, called ooze, may be calcareous calcium carbonate-based or siliceous silica-based depending on the type of shells deposited. For example, the shells of radiolarians zooplankton and diatoms phytoplankton are made of silica, so they produce siliceous ooze. Under the right conditions, intact pieces of organic material or material derived from organic sources are preserved in the geologic record.

Although not derived from sediment, this lithified organic material is associated with sedimentary strata and created by similar processes—burial, compaction, and diagenesis.

Deposits of these fuels develop in areas where organic material collects in large quantities. Lush swamplands can create conditions conducive to the coal formation. Shallow-water, organic material-rich marine sediment can become highly productive petroleum and natural gas deposits.

See Chapter 16 , Energy and Mineral Resources, for a more in-depth look at these fossil-derived energy sources. In contrast to detrital sediment, chemical, biochemical, and organic sedimentary rocks are classified based on mineral composition.

Most of these are monomineralic, composed of a single mineral, so the rock name is usually associated with the identifying mineral. Chemical sedimentary rocks consisting of halite are called rock salt. Rocks made of Limestone calcite is an exception, having elaborate subclassifications and even two competing classification methods: Folk Classification and Dunham Classification [ 11 ; 21 ].

The Folk Classification deals with rock grains and usually requires a specialized, petrographic microscope. The Dunham Classification is based on rock texture, which is visible to the naked eye or using a hand lens and is easier for field applications. Most carbonate geologists use the Dunham system. Udden, J. Mechanical composition of clastic sediments.

Wentworth, C. A scale of grade and class terms for clastic sediments. Folk, R. Petrography of sedimentary rocks. Texas, Hemphill, Austin, Tex , Dickinson, W. Interpreting detrital modes of graywacke and arkose. Affolter, M.