l>Occurrence & Mineralogy the Sedimentary Rocks
Geology 212

Petrology

Prof. Stephen A. Nelson
Occurrence, Mineralogy, Textures, and also Structures of Sedimentary Rocks

Rivers, oceans, winds, and also rain runoff all have the capacity to carry the corpuscle washed turn off of eroding rocks. Together material, dubbed detritus, is composed of pieces of rocks and minerals. When the power of the transporting present is not strong enough to lug these particles, the corpuscle drop the end in the process of sedimentation. This type of sedimentary deposition is referred to as clastic sedimentation. Another kind of sedimentary deposition occurs when product is dissolved in water, and chemically precipitates native the water. This kind of sedimentation is described as chemical sedimentation. A third process can occur, wherein life organisms extract ions liquified in water to do such points as shells and bones. This type of sedimentation is called biogenic sedimentation. Thus, there are three significant types that sedimentary rocks: that deserve to be grouped by the kind of particle uncovered in the rocks.
Siliclastic sedimentary rocks type by the accumulation of mostly silicate mineral fragments. These incorporate most sandstones, dirt rocks, conglomerates, and breccias.

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Biochemical sedimentary rocks consists of pieces of particles produced by precipitation from when living organisms. Most of these rocks are limestones and also cherts. chemical sedimentary rocks are formed by direct chemical precipitation native water. While part limestones and also cherts may kind in this manner, evaporite deposits consisting the halite, gypsum, and also other salts space the many common.
95% of every sedimentary rocks is composed of sandstones (made increase of sand sized fragments), mudrocks (made increase of silt and also clay sized fragments), and carbonate rocks (made increase of largely calcite, aragonite, or dolomite). The these, the mudrocks are many abundant, make up about 65% of all sedimentary rocks. Sandstones make up 20 to 25% of all sedimentary rocks, and carbonate rocks account for around 10 come 15% of all lead carbonate rocks.

The incident of Sedimentary Rocks

Sedimentary rocks have an average thickness of about 1800 m ~ above the continents. This thickness is quite variable, however, with some areas, prefer the Canadian Shield having actually no covering of sedimentary rocks, and other areas, like the Louisiana and also Texas Gulf coasts, having an ext than 20,000 m of sedimentary absent cover. Still, around 66% of every continental locations have a cover of sedimentary rocks.

Because sediment it s okay buried, and also if exposed gets eroded, enlarge sedimentary rocks present less exposed outcrop area 보다 younger sedimentary rocks. Over 40% the the exposed sedimentary rocks are younger 보다 Cretaceous in age.
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Depositional Basins

In order because that sediment to it is in deposited, a low lying area, called a basin, is required. The biggest of basins are the s basins, which right now cover about 70% of the Earth"s surface. In the past, however, sea level has often changed, bring about episodes where also the continents were covered by shallow seas, referred to as epiric or epicontinental seas. When sea level rises to invade the continent it is described as a transgression. Major transgressions arisen during the Cretaceous, and from the at an early stage Cambrian v Mississippian (See your text for a more detailed discussion).

It is because that these factors that most of the sedimentary rocks maintained in the geologic record are marine sediments.

Basins result from key tectonics, and also even the huge transgressions show up to be concerned tectonic factors, as raised spreading rates in the ocean basins can an outcome in an altering the construction of ocean containers that result in flooding that the continents. Because that a category of the various varieties of containers see her text.

The Mineralogy that Sedimentary Rocks

Because of their detrital nature, any mineral can occur in a sedimentary rock.Clay minerals, the leading mineral developed by chemistry weathering the rocks, is the many abundant mineral in mudrocks.Quartz, because it is steady under conditions present in ~ the surface ar of the Earth, and also because that is also a product of chemistry weathering, is the many abundant mineral in sandstones and also the 2nd most abundant mineral in mudrocks.Feldspar is the most usual mineral in igneous and metamorphic rocks.Although feldspar eventually breaks under to clay minerals and quartz, the is still the 3rd most plentiful mineral in sedimentary rocks. Carbonate minerals, one of two people precipitated straight or by organisms, consist of most biochemical and chemical sedimentary rocks, however carbonates are also common in mudrocks and also sandstones.

Mineral Composition

Mudrocks %

Sandstones %

Clay minerals 60 5
Quartz 30 65
Feldspar 4 10 - 15
Carbonate minerals 3

Minerals discovered in sedimentary rocks deserve to be divided into 2 classes: Allogenic minerals - this are developed elsewhere and also transported right into the area the deposition. Authigenic minerals - These space minerals the are formed at the site of deposition, one of two people by direct chemical precipitation or by later on diagenetic processes.
Any mineral can be an allogenic mineral, but some are much more stable under the conditions present at the Earth"s surface than space others. Problems that are present at the Earth"s surface and differ indigenous those where many minerals kind are: low Temperature Low press High totally free oxygen concentration High quantities of cost-free liquid water

Because these problems differ from those under which most rocks form, allogenic minerals have the right to be classified based on their security under close to surface conditions. Such a classification, with minerals provided in bespeak of boosting stability is as follows:

Stability Under problems Present in ~ Surface Mineral
Unstable Olivines*
Pyroxenes*
Ca-rich Plagiocalse*
Hornblende*
Andesine - Oligoclase*
Less Unstable Sphene
Epidote
Andalusite
Staurolite
Kyanite
Sillimanite
Magnetite
Garnet
Very Stable Muscovite*
Albite*
Orthoclase/Microcline*
Clay Minerals
Quartz*
Tourmaline
Zircon

In this list, the igneous minerals have an asterisk (*). Keep in mind that the stimulate in which they happen is in the same order that take place in Bowen"s reaction series. Igneous minerals that crystallize at the greatest temperatures are most out of equilibrium at the Earth"s surface, and also are therefore the most unstable.
Minerals the are really stable at the Earth"s surface room minerals that either form as a an outcome of chemical weathering, or crystallize at the lowest temperatures.

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Authigenic minerals can also be allogenic minerals, yet some are formed during diagenesis however not an extremely stable in the transport cycle either because they dissolve easily in water or since they are conveniently abraded during transportation. Therefore we deserve to divide authigenic minerals into those that space stable during diagenesis and transportation, and those that space unstable throughout transportation.