l>Occurrence & Mineralogy that Sedimentary Rocks
Geology 212

Petrology

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

Rivers, oceans, winds, and also rain runoff all have the capability to carry the corpuscle washed turn off of eroding rocks. Such material, referred to as detritus, consists of fragments of rocks and minerals. When the energy of the transporting existing is not solid enough to carry these particles, the particles drop out in the process of sedimentation. This type of sedimentary deposition is described as clastic sedimentation. Another type of sedimentary deposition wake up when product is dissolved in water, and chemically precipitates indigenous the water. This type of precipitate is described as chemical sedimentation. A third procedure can occur, wherein life organisms extract ions dissolved in water to do such points as shells and bones. This type of precipitate is called biogenic sedimentation. Thus, there room three significant types of sedimentary rocks: that deserve to be grouped by the type of particle uncovered in the rocks.
Siliclastic sedimentary rocks type by the build-up of greatly silicate mineral fragments. These include most sandstones, dirt rocks, conglomerates, and breccias.

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Biochemical sedimentary rocks consist of of fragments of particles created by precipitation from once living organisms. Most of this rocks space limestones and cherts. chemistry sedimentary rocks are created by direct chemical precipitation from water. While some limestones and also cherts may form in this manner, evaporite store consisting the halite, gypsum, and also other salts room the many common.
95% of every sedimentary rocks is composed of sandstones (made up of sand size fragments), mudrocks (made increase of silt and clay sized fragments), and also carbonate rocks (made increase of mostly calcite, aragonite, or dolomite). That these, the mudrocks are many abundant, do up about 65% of every sedimentary rocks. Sandstones consist of 20 to 25% of all sedimentary rocks, and carbonate rocks account for around 10 come 15% the all carbonate rocks.

The occurrence of Sedimentary Rocks

Sedimentary rocks have an typical thickness of around 1800 m top top the continents. This thickness is quite variable, however, with some areas, like the Canadian Shield having no sheathe of sedimentary rocks, and other areas, choose the Louisiana and also Texas Gulf coasts, having more than 20,000 m the sedimentary absent cover. Still, about 66% of all continental areas have a sheathe of sedimentary rocks.

Because sediment gets buried, and if exposed it s okay eroded, enlarge sedimentary rocks show less exposed outcrop area 보다 younger sedimentary rocks. End 40% that the exposed sedimentary rocks room 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, dubbed a basin, is required. The biggest of containers are the ocean basins, which right now cover about 70% of the Earth"s surface. In the past, however, sea level has frequently changed, bring about episodes where also the continent were covered by shallow seas, referred to as epiric or epicontinental seas. When sea level rises to invade the continents it is described as a transgression. Major transgressions emerged during the Cretaceous, and also from the beforehand Cambrian with Mississippian (See your message for a more detailed discussion).

It is because that these factors that many of the sedimentary rocks maintained in the geologic document are maritime sediments.

Basins an outcome from plate tectonics, and also even the huge transgressions show up to be pertained to tectonic factors, as increased spreading prices in the ocean containers can an outcome in an altering the configuration of ocean basins that result in flooding of the continents. For a classification of the various varieties of basins see her text.

The Mineralogy that Sedimentary Rocks

Because of their detrital nature, any type of mineral can take place in a sedimentary rock.Clay minerals, the leading mineral developed by chemistry weathering the rocks, is the many abundant mineral in mudrocks.Quartz, since it is steady under problems present at the surface ar of the Earth, and because that is also a product of chemical weathering, is the most abundant mineral in sandstones and also the 2nd most numerous mineral in mudrocks.Feldspar is the most common mineral in igneous and also metamorphic rocks.Although feldspar eventually breaks down to clay minerals and also quartz, that is quiet the third most plentiful mineral in sedimentary rocks. Lead carbonate minerals, either precipitated directly or by organisms, make up most biochemical and chemical sedimentary rocks, but carbonates are also common in mudrocks and sandstones.

Mineral Composition

Mudrocks %

Sandstones %

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

Minerals found in sedimentary rocks deserve to be divided into 2 classes: Allogenic minerals - these are formed elsewhere and transported into the area of deposition. Authigenic minerals - These space minerals the are developed at the website of deposition, either by direct chemical precipitation or by later on diagenetic processes.
Any mineral can be an allogenic mineral, but some are more stable under the problems present at the Earth"s surface ar than space others. Conditions that are current at the Earth"s surface and differ indigenous those where most minerals type are: short Temperature Low press High totally free oxygen concentration High quantities of cost-free liquid water

Because these problems differ from those under which many rocks form, allogenic minerals deserve to be classified based on their stability under near surface conditions. Such a classification, through minerals noted in bespeak of raising stability is together follows:

Stability Under conditions 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 actually an asterisk (*). Note that the order in which they occur is in the exact same order that happen in Bowen"s reaction series. Igneous minerals that crystallize at the highest possible temperatures are most out that equilibrium in ~ the Earth"s surface, and are therefore the most unstable.
Minerals the are very stable in ~ the Earth"s surface space minerals that either form as a an outcome of chemical weathering, or crystallize in ~ the lowest temperatures.

See more: Classifying Matter With No Definite Volume Or Shape, States Of Matter Presentation

Authigenic mineral can also be allogenic minerals, yet some are formed during diagenesis however not very stable in the transportation cycle either because they dissolve readily in water or since they are easily abraded during transportation. For this reason we deserve to divide authigenic minerals into those that room stable during diagenesis and also transportation, and those that are unstable throughout transportation.