Because they contain numerous bioclasts i. The concept of "support" assume continuity of either the mud matrix or that of the grains. If the carbonate is mud-supported the grains float into a continuum of mud matrix. In grain-supported carbonate rocks the grains from an interconnected skeleton in which the mud fills the gap. Examples from Murrumbidgee Here is a nice example of grainstone from Shark Bay WA.
Matrix - The matrix of carbonate rocks consists of either fine grained carbonate mud, called micrite. Or coarser grained calcite crystals formed during diagenesis, called sparite. The micrite results from recrystallization of carbonate mud during diagenesis or from direct precipitation of calcite, and causes lithification of the sediment. The micrite gives the dull opaque appearance of most limestones as seen in hand specimen.
If the rock consists entirely of fine-grained mud matrix, it implies deposition in a low energy environment just like in siliclastic mudstones. Some of the mud may start out as aragonite needles 5 to 10 m m in length produced by calcareous algae. But, again this becomes recrystallized to a microspar 5 to 15 m m in diameter during diagenesis. Larger sparry calcite matrix results from diagenesis in the same way that calcite cement originates in sandstones.
Insoluble Residues - While minor amounts of clay minerals and quartz occur in limestones, most of the insoluble residues, so called because they do not dissolve in HCl are grains of nodules of chert. Such chert mostly originates from the shells of silica secreting organisms. These include diatoms, radiolarians, and some sponges. Individual grains of chert result from recrystallization of the shells of these organisms. Chert nodules can range in size from centimeters to meters in length.
Many nodules are concentrated along bedding planes and probably resulted from dissolution of the siliceous debris and reprecipitation of the microcrystalline quartz at centers of nucleation located along zones of migration of the fluids, such as along bedding planes.
Current-Generated Structures. Structures like cross-bedding, ripple marks, dunes, graded bedding, and imbricate bedding are common in carbonate rocks, although they may not be as evident as in siliclastic rocks because of the lack of contrasting colors of individual beds in carbonates. Since many shells of organisms have curved outlines in cross-section brachipods, pelecypods, ostracods, and trilobites, especially , when the organism dies it may settle to the bottom with the outline being concave downward, and latter become filled with carbonate mud.
The most common type of lamination in carbonate rocks is produced by organisms, in particular blue-green algae that grow in the tidal environment.
These organism grow as filaments and produce mats by trapping and binding microcrystalline carbonates, as incoming tides sweep over the sand. This leads to the formation of laminated layers that consist of layers of organic tissue interbedded with mud.
In ancient limestones, the organic matter has usually been removed as a result of decay, leaving cavities in the rock separated by layers of material that was once mud. Another type of lamination occurs as bulbous structures, termed Stomatolites for photo, see Figure , p. These are produced in a similar fashion, i. Stylolites are irregular surfaces that result from pressure solution of large amounts of carbonate.
In cross-section they have a saw tooth appearance with the stylolites themselves being made of insoluble residues or insoluble organic material. The principal carbonate depositional environments are as follows: Carbonate Platforms and Shelves. Warm shallow seas attached the continents, or in the case of epiric seas, partially covering the continents, are ideal places for carbonate deposition.
Other shelves occur surrounding oceanic islands after volcanism has ceased and the island has been eroded these are called atolls. Carbonate platforms are buildups of carbonate rocks in the deeper parts of the oceans on top of continental blocks left behind during continent - continent separation. Reef building organisms from the framework of most of these carbonate buildups.
Tidal Flats. Tidal flats are areas that flood during high tides and are exposed during low tides. Carbonate sands carried in by the tides are cemented together by carbonate secreting organisms, forming algal mats and stromatolites. Deep Ocean. Carbonate deposition can only occur in the shallower parts of the deep ocean unless organic productivity is so high that the remains of organisms are quickly buried. This is because at depths between 3, and 5, m largely dependent on latitude - deeper near the equator and shallower nearer the poles in the deep oceans the rate of dissolution of carbonate is so high and the water so undersaturated with respect to calcium carbonate, that carbonates cannot accumulate.
This depth is called the carbonate compensation depth CCD. The main type of carbonate deposition in the deep oceans consists of the accumulation of the remains of planktonic foraminifera to form a carbonate ooze.
Upon burial, this ooze undergoes diagenetic recrystallization to form micritic limestones. Since most oceanic ridges are at a depth shallower than the CCD, carbonate oozes can accumulate on the flanks of the ridges and can be buried as the oceanic crust moves away from the ridge to deeper levels in the ocean. Since most oceanic crust and overlying sediment are eventually subducted, the preservation of such deep sea carbonates in the geologic record is rare, although some have been identified in areas where sediment has been scraped off the top of the subducting oceanic crust and added to the continents, such as in the Franciscan Formation of Jurassic age in California.
Non-marine Lakes. Carbonate deposition can occur in non-marine lakes as a result of evaporation, in which case the carbonates are associated with other evaporite deposits, and as a result of organisms that remove CO 2 from the water causing it to become oversaturated with respect to calcite. Hot Springs. When hot water saturated with calcium carbonate reaches the surface of the Earth at hot springs, the water evaporates and cools resulting in the precipitation of calcite to form a type of limestone called travertine.
Although there used to be a common perception that the abundance of dolostones increased with age of the rock, it is now recognized that although no primary dolomite bearing rocks are being directly precipitated in modern times, dolostones have formed throughout geologic time. This is true despite the fact that modern sea water is saturated with respect to dolomite. Still, most dolostones appear to result from diagenetic conversion of calcite or high-Mg calcite to dolomite, after primary deposition of the original calcium carbonate bearing minerals.
Dolomite, and therefore rocks containing large amounts of dolomite, like dolostones, is easily distinguished by the fact that it only fizzes in dilute HCl if broken down to a fine powder.
Other Sedimentary Rocks Evaporites Evaporite minerals are those minerals produced by extensive or total evaporation of a saline solution. Shallow arid coasts or sabkhas. Along shallow arid coastlines where input of fresh water is rare and evaporation increases the salinity of the marine water, evaporation may increase the salinity of the water to a point where evaporite minerals like halite and gypsum are precipitated.
Cherts Chert is a mineralogically simple rock consisting of microcrystalline quartz. There are three common occurrences of chert.
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