Mudrocks are a class of fine grained siliciclastic sedimentary rocks. The varying types of mudrocks include: siltstone, mudstone and shale. Most of the particles of which the stone is composed are less than 0.0625 mm and are too small to study in the field. At first sight the rock types look quite similar. There has been a great deal of disagreement involving the classification of mudrocks. There are a few important hurdles to classification, including: Mudrocks are the least understood, one of the most understudied sedimentary rocks to date It is difficult to study mudrock constituents, due to their diminutive size and susceptibility to weathering on outcrops And most there is more than one classification scheme accepted by scientistsMudrocks make up fifty percent of the sedimentary rocks in the geologic record, are the most widespread deposits on Earth. Fine sediment is the most abundant product of erosion, these sediments contribute to the overall omnipresence of mudrocks. With increased pressure over time the platey clay minerals may become aligned, with the appearance of parallel layering.
This finely bedded material that splits into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due either to the original texture or to the disruption of layering by burrowing organisms in the sediment prior to lithification. From the beginning of civilization, when pottery and mudbricks were made by hand, to now, mudrocks have been important; the first book on mudrocks, Geologie des Argils by Millot, was not published until 1964. Literature on this omnipresent rock-type has been increasing in recent years, technology continues to allow for better analysis. Mudrocks, by definition, consist of at least fifty percent mud-sized particles. Mud is composed of silt-sized particles that are between 1/16 – 1/256 of a millimeter in diameter, clay-sized particles which are less than 1/256 millimeter. Mudrocks contain clay minerals, quartz and feldspars, they can contain the following particles at less than 63 micrometres: calcite, siderite, marcasite, heavy minerals, organic carbon.
There are various synonyms for fine-grained siliciclastic rocks containing fifty percent or more of its constituents less than 1/256 of a millimeter. Mudstones, shales and argillites are common qualifiers, or umbrella-terms; the term "mudrock" allows for further subdivisions of siltstone, claystone and shale. For example, a siltstone would be made of more than 50-percent grains that equate to 1/16 - 1/256 of a millimeter. "Shale" denotes fissility, which implies an ability to part or break parallel to stratification. Siltstone and claystone implies lithified, or hardened, detritus without fissility. Overall, "mudrocks" may be the most useful qualifying term, because it allows for rocks to be divided by its greatest portion of contributing grains and their respective grain size, whether silt, clay, or mud. A claystone is lithified, non-fissile mudrock. In order for a rock to be considered a claystone, it must consist of up to fifty percent clay, which measures less than 1/256 of a millimeter in particle size.
Clay minerals are integral to mudrocks, represent the first or second most abundant constituent by volume. There are 35 recognized clay mineral species on Earth, they able to flow. Clay is by far the smallest particles recognized in mudrocks. Most materials in nature are clay minerals, but quartz, iron oxides, carbonates can weather to sizes of a typical clay mineral. For a size comparison, a clay-sized particle is 1/1000 the size of a sand grain; this means a clay particle will travel 1000 times further at constant water velocity, thus requiring quieter conditions for settlement. The formation of clay is well understood, can come from soil, volcanic ash, glaciation. Ancient mudrocks are another source, because they disintegrate easily. Feldspar, amphiboles and volcanic glass are the principle donors of clay minerals. A mudstone is a siliciclastic sedimentary rock that contains a mixture of silt- and clay-sized particles; the terminology of "mudstone" is not to be confused with the Dunham classification scheme for limestones.
In Dunham's classification, a mudstone is any limestone containing less than ten percent carbonate grains. Note, a siliciclastic mudstone does not deal with carbonate grains. Friedman and Kopaska-Merkel suggest the use of "lime mudstone" to avoid confusion with siliciclastic rocks. A siltstone is a non-fissile mudrock. In order for a rock to be named a siltstone, it must contain over fifty percent silt-sized material. Silt is any particle smaller than sand, 1/16 of a millimeter, larger than clay, 1/256 of millimeter. Silt is believed to be the product of physical weathering, which can involve freezing and thawing, thermal expansion, release of pressure. Physical weathering does not involve any chemical changes in the rock, it may be best summarised as the physical breaking apart of a rock. One of the highest proportions of silt found on Earth is in the Himalayas, where phyllites are exposed to rainfall of up to five to ten meters a year. Quartz and feldspar are the biggest contributors to the silt realm
In geology, fissility is the ability or tendency of a rock to split along flat planes of weakness. These planes of weakness are oriented parallel to stratification in sedimentary rocks. Fissility is differentiated from scaly fabric in hand sample by the parting surfaces’ continuously parallel orientations to each other and to stratification. Fissility is distinguished from scaly fabric in thin section by the well-developed orientation of platy minerals such as mica. Fissility is the result of metamorphic processes. Planes of weakness are developed in sedimentary rocks such as shale or mudstone by clay particles aligning during compaction. Planes of weakness are developed in metamorphic rocks by the recrystallization and growth of micaceous minerals. A rock’s fissility can be degraded in numerous ways during the geologic process, including clay particles flocculating into a random fabric before compaction, bioturbation during compaction, weathering during and after uplift; the effect of bioturbation has been documented well in shale cores sampled: past variable critical depths where burrowing organisms can no longer survive, shale fissility will become more pervasive and better defined.
Fissility is used by some geologists as the defining characteristic which separates mudstone from shale. However, some professions, like drilling engineers, continue to use the two terms interchangeably. Fabric
Grain size is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may be applied to other granular materials; this is different from the crystallite size, which refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from small colloidal particles, through clay, sand and cobbles, to boulders. Size ranges define limits of classes that are given names in the Wentworth scale used in the United States; the Krumbein phi scale, a modification of the Wentworth scale created by W. C. Krumbein in 1934, is a logarithmic scale computed by the equation φ = − log 2 D D 0, where φ is the Krumbein phi scale, D is the diameter of the particle or grain in millimeters and D 0 is a reference diameter, equal to 1 mm; this equation can be rearranged to find diameter using φ: D = D 0 ⋅ 2 − φ In some schemes, gravel is anything larger than sand. ISO 14688-1:2002, establishes the basic principles for the identification and classification of soils on the basis of those material and mass characteristics most used for soils for engineering purposes.
ISO 14688-1 is applicable to natural soils in situ, similar man-made materials in situ and soils redeposited by people. An accumulation of sediment can be characterized by the grain size distribution. A sediment deposit can undergo sorting when a particle size range is removed by an agency such as a river or the wind; the sorting can be quantified using the Inclusive Graphic Standard Deviation: σ I = ϕ 84 − ϕ 16 4 + ϕ 95 − ϕ 5 6.6 where σ I is the Inclusive Graphic Standard Deviation in phi units ϕ 84 is the 84th percentile of the grain size distribution in phi units, etc. The result of this can be described using the following terms: Feret diameter Martin diameter Orders of magnitude Soil texture Substrate Unified Soil Classification System R D Dean & R A Dalrymple, Coastal Processes with Engineering Applications W C Krumbein & L L Sloss and Sedimentation, 2nd edition. Udden, J. A.. "Mechanical composition of clastic sediments". Geological Society of America Bulletin. 25: 655–744. Bibcode:1914GSAB...25..655U.
Doi:10.1130/GSAB-25-655. Wentworth, C. K.. "A Scale of Grade and Class Terms for Clastic Sediments". The Journal of Geology. 30: 377–392. Bibcode:1922JG.....30..377W. Doi:10.1086/622910. JSTOR 30063207
Petrology is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous and sedimentary petrology. Igneous and metamorphic petrology are taught together because they both contain heavy use of chemistry, chemical methods, phase diagrams. Sedimentary petrology is, on the other hand taught together with stratigraphy because it deals with the processes that form sedimentary rock. Lithology was once synonymous with petrography, but in current usage, lithology focuses on macroscopic hand-sample or outcrop-scale description of rocks while petrography is the speciality that deals with microscopic details. In the petroleum industry, lithology, or more mud logging, is the graphic representation of geological formations being drilled through, drawn on a log called a mud log; as the cuttings are circulated out of the borehole they are sampled and tested chemically when needed. Petrology utilizes the fields of mineralogy, optical mineralogy, chemical analysis to describe the composition and texture of rocks.
Petrologists include the principles of geochemistry and geophysics through the study of geochemical trends and cycles and the use of thermodynamic data and experiments in order to better understand the origins of rocks. There are three branches of petrology, corresponding to the three types of rocks: igneous and sedimentary, another dealing with experimental techniques: Igneous petrology focuses on the composition and texture of igneous rocks. Igneous rocks include plutonic rocks. Sedimentary petrology focuses on the texture of sedimentary rocks. Metamorphic petrology focuses on the composition and texture of metamorphic rocks Experimental petrology employs high-pressure, high-temperature apparatus to investigate the geochemistry and phase relations of natural or synthetic materials at elevated pressures and temperatures. Experiments are useful for investigating rocks of the lower crust and upper mantle that survive the journey to the surface in pristine condition, they are one of the prime sources of information about inaccessible rocks such as those in the Earth's lower mantle and in the mantles of the other terrestrial planets and the Moon.
The work of experimental petrologists has laid a foundation on which modern understanding of igneous and metamorphic processes has been built. Important publications in petrology Ore Pedology Atlas of Igneous and metamorphic rocks and textures – Geology Department, University of North Carolina Metamorphic Petrology Database – Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute Petrological Database of the Ocean Floor - Center for International Earth Science Information Network, Columbia University
Shale is a fine-grained, clastic sedimentary rock composed of mud, a mix of flakes of clay minerals and tiny fragments of other minerals quartz and calcite. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility, it is the most common sedimentary rock. Shale exhibits varying degrees of fissility, breaking into thin layers splintery and parallel to the otherwise indistinguishable bedding plane because of the parallel orientation of clay mineral flakes. Non-fissile rocks of similar composition but made of particles smaller than 0.06 mm are described as mudstones or claystones. Rocks with similar particle sizes but with less clay and therefore grittier are siltstones. Shales are composed of clay minerals and quartz grain, are grey. Addition of variable amounts of minor constituents alters the color of the rock. Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing environment.
Black shale can be referred to as black metal. Red and green colors are indicative of ferric oxide, iron hydroxide, or micaceous minerals. Clays are the major constituent of other mudrocks; the clay minerals represented are kaolinite and illite. Clay minerals of Late Tertiary mudstones are expandable smectites whereas in older rocks in mid- to early Paleozoic shales illites predominate; the transformation of smectite to illite produces silica, calcium, magnesium and water. These released elements form authigenic quartz, calcite, ankerite and albite, all trace to minor minerals found in shales and other mudrocks. Shales and mudrocks contain 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales, which form in anoxic conditions, contain reduced free carbon along with ferrous iron and sulfur. Pyrite and amorphous iron sulfide along with carbon produce the black coloration; the process in the rock cycle which forms shale is called compaction.
The fine particles that compose shale can remain suspended in water long after the larger particles of sand have deposited. Shales are deposited in slow moving water and are found in lakes and lagoonal deposits, in river deltas, on floodplains and offshore from beach sands, they can be deposited in sedimentary basins and on the continental shelf, in deep, quiet water.'Black shales' are dark, as a result of being rich in unoxidized carbon. Common in some Paleozoic and Mesozoic strata, black shales were deposited in anoxic, reducing environments, such as in stagnant water columns; some black shales contain abundant heavy metals such as molybdenum, uranium and zinc. The enriched values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after sedimentation or to slow accumulation from sea water over long periods of sedimentation. Fossils, animal tracks/burrows and raindrop impact craters are sometimes preserved on shale bedding surfaces.
Shales may contain concretions consisting of pyrite, apatite, or various carbonate minerals. Shales that are subject to heat and pressure of metamorphism alter into a hard, metamorphic rock known as slate. With continued increase in metamorphic grade the sequence is phyllite schist and gneiss. Before the mid-19th century, the terms slate and schist were not distinguished. In the context of underground coal mining, shale was referred to as slate well into the 20th century. Bakken Formation Barnett Shale Bearpaw Formation Burgess Shale Marcellus Formation Mazon Creek fossil beds Oil shale – Organic-rich fine-grained sedimentary rock containing kerogen Shale gas Shale gas in the United States Wheeler Shale Wianamatta Shale Media related to Shale at Wikimedia Commons
Sandstone is a clastic sedimentary rock composed of sand-sized mineral particles or rock fragments. Most sandstone is composed of quartz or feldspar because they are the most resistant minerals to weathering processes at the Earth's surface, as seen in Bowen's reaction series. Like uncemented sand, sandstone may be any color due to impurities within the minerals, but the most common colors are tan, yellow, grey, pink and black. Since sandstone beds form visible cliffs and other topographic features, certain colors of sandstone have been identified with certain regions. Rock formations that are composed of sandstone allow the percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs. Fine-grained aquifers, such as sandstones, are better able to filter out pollutants from the surface than are rocks with cracks and crevices, such as limestone or other rocks fractured by seismic activity. Quartz-bearing sandstone can be changed into quartzite through metamorphism related to tectonic compression within orogenic belts.
Sandstones are clastic in origin. They are formed from cemented grains that may either be fragments of a pre-existing rock or be mono-minerallic crystals; the cements binding these grains together are calcite and silica. Grain sizes in sands are defined within the range of 0.0625 mm to 2 mm. Clays and sediments with smaller grain sizes not visible with the naked eye, including siltstones and shales, are called argillaceous sediments; the formation of sandstone involves two principal stages. First, a layer or layers of sand accumulates as the result of sedimentation, either from water or from air. Sedimentation occurs by the sand settling out from suspension. Once it has accumulated, the sand becomes sandstone when it is compacted by the pressure of overlying deposits and cemented by the precipitation of minerals within the pore spaces between sand grains; the most common cementing materials are silica and calcium carbonate, which are derived either from dissolution or from alteration of the sand after it was buried.
Colors will be tan or yellow. A predominant additional colourant in the southwestern United States is iron oxide, which imparts reddish tints ranging from pink to dark red, with additional manganese imparting a purplish hue. Red sandstones are seen in the Southwest and West of Britain, as well as central Europe and Mongolia; the regularity of the latter favours use as a source for masonry, either as a primary building material or as a facing stone, over other forms of construction. The environment where it is deposited is crucial in determining the characteristics of the resulting sandstone, which, in finer detail, include its grain size and composition and, in more general detail, include the rock geometry and sedimentary structures. Principal environments of deposition may be split between terrestrial and marine, as illustrated by the following broad groupings: Terrestrial environmentsRivers Alluvial fans Glacial outwash Lakes Deserts Marine environmentsDeltas Beach and shoreface sands Tidal flats Offshore bars and sand waves Storm deposits Turbidites Framework grains are sand-sized detrital fragments that make up the bulk of a sandstone.
These grains can be classified into several different categories based on their mineral composition: Quartz framework grains are the dominant minerals in most clastic sedimentary rocks. These physical properties allow the quartz grains to survive multiple recycling events, while allowing the grains to display some degree of rounding. Quartz grains evolve from plutonic rock, which are felsic in origin and from older sandstones that have been recycled. Feldspathic framework grains are the second most abundant mineral in sandstones. Feldspar can be divided into two smaller subdivisions: plagioclase feldspars; the different types of feldspar can be distinguished under a petrographic microscope. Below is a description of the different types of feldspar. Alkali feldspar is a group of minerals in which the chemical composition of the mineral can range from KAlSi3O8 to NaAlSi3O8, this represents a complete solid solution. Plagioclase feldspar is a complex group of solid solution minerals that range in composition from NaAlSi3O8 to CaAl2Si2O8.
Lithic framework grains are pieces of ancient source rock that have yet to weather away to individual mineral grains, called lithic fragments or clasts. Lithic fragments can be any fine-grained or coarse-grained igneous, metamorphic, or sedimentary rock, although the most common lithic fragments found in sedimentary rocks are clasts of volcanic rocks. Accessory minerals are all other mineral grains in a sandstone. Common accessory minerals include micas, olivine and corundum. Many of these accessory grains are more dense than the silicates that
In geology and related fields, a stratum is a layer of sedimentary rock or soil, or igneous rock that were formed at the Earth's surface, with internally consistent characteristics that distinguish it from other layers. The "stratum" is the fundamental unit in a stratigraphic column and forms the basis of the study of stratigraphy; each layer is one of a number of parallel layers that lie one upon another, laid down by natural processes. They may extend over hundreds of thousands of square kilometers of the Earth's surface. Strata are seen as bands of different colored or differently structured material exposed in cliffs, road cuts and river banks. Individual bands may vary in thickness from a few millimeters to a kilometer or more. A band may represent a specific mode of deposition: river silt, beach sand, coal swamp, sand dune, lava bed, etc. Geologists categorize them by the material of beds; each distinct layer is assigned to the name of sheet based on a town, mountain, or region where the formation is exposed and available for study.
For example, the Burgess Shale is a thick exposure of dark fossiliferous, shale exposed high in the Canadian Rockies near Burgess Pass. Slight distinctions in material in a formation may be described as "members". Formations are collected into "groups" while groups may be collected into "supergroups". Archaeological horizon Geologic formation Geologic map Geologic unit Law of superposition Bed GeoWhen Database