Lithology

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition.[1][2][3] Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock.[4] Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7.

SEUtahStrat
Stratigraphy as seen in southeastern Utah

Rock type

ItalyPillowBasalt
A basalt, showing the 'pillow' lava shape characteristic of underwater eruptions, Italy

The naming of a lithology is based on the rock type. The three major rock types are sedimentary, igneous, metamorphic.

Sedimentary rocks are further classified by whether they are siliciclastic or carbonate. Siliciclastic sedimentary rocks are then subcategorized based on their grain size distribution and the relative proportions of quartz, feldspar, and lithic (rock) fragments. Carbonate rocks are classified with the Dunham or Folk classification schemes according to the constituents of the carbonate rock.

The name of an igneous rock requires information on crystal size and mineralogy. This classification can often be performed with a QAPF diagram.

Metamorphic rock naming can be based on texture, protolith, metamorphic facies, and/or the locations in which they are found. Naming based on texture and a pelite (e.g., shale, mudrock) protolith can be used to define slate and phyllite. Texture-based names are schist and gneiss. These textures, from slate to gneiss, define a continually-increasing extent of metamorphism. Metamorphic facies are defined by the pressure-temperature fields in which particular minerals form. Additional metamorphic rock names exist: greenstone (metamorphosed basalt and other extrusive igneous rock) is a classification based on composition and being located Precambrian terranes, while quartzite is based only on composition, as quartz is too stable and homogeneous to change phase at typical metamorphic temperatures and pressures.

Grain/clast size

GLMsed
A claystone, the finest-grained sedimentary rock, deposited in Glacial Lake Missoula, Montana

In igneous and metamorphic rocks, grain size is a measure of the sizes of the crystals in the rock. In igneous rock, this is used to determine the rate at which the material cooled: large crystals typically indicate intrusive igneous rock, while small crystals indicate that the rock was extrusive. As metamorphic reactions progress, the grains in metamorphic rocks can often be broken down into smaller grains.

In clastic sedimentary rocks, grain size is the diameter of the grains and/or clasts that constitute the rock. These are used to determine which rock naming system to use (e.g., a conglomerate, sandstone, or mudstone one). In the case of sandstones and conglomerates, which cover a wide range of grain sizes, a word describing the grain size range is added to the rock name. Examples are "pebble conglomerate" and "fine quartz arenite".

Mineralogy

Iddingsite
An ultramafic mantle xenolith with olivine and pyroxene (altering brown to iddingsite) in a matrix of mafic basalt scoria

In rocks in which mineral grains are large enough to be identified using a hand lens, the visible mineralogy is included as part of the description. In the case of sequences possibly including carbonates, calcite-cemented rocks or those with possible calcite veins, it is normal to test for the presence of calcite (or other forms of calcium carbonate) using dilute hydrochloric acid and looking for effervescence.[5]

The mineralogical composition of a rock is one of the major ways in which it is classified. In general, igneous rocks can be categorized by increasing silica content as ultramafic, mafic, intermediate, or felsic, though more mineral-specific classifications also exist. Likewise, metamorphic facies, which show the degree to which a rock has been exposed to heat and pressure and are therefore important in classifying metamorphic rocks, are determined by observing the mineral phases that are present in a sample.

Colour

The colour of a rock or its component parts is a distinctive characteristic of some rocks and is always recorded, sometimes against standard colour charts, such as that produced by the Rock-Color Chart Committee of the Geological Society of America based on the Munsell color system.[6]

Fabric

The fabric of a rock describes the spatial and geometric configuration of all the elements that make it up. In sedimentary rocks the main visible fabric is normally bedding and the scale and degree of development of the bedding is normally recorded as part of the description. Metamorphic rocks (apart from those created by contact metamorphism), are characterised by well-developed planar and linear fabrics. Igneous rocks may also have fabrics due either to flow or to the settling out of particular mineral phases during crystallisation, forming cumulates.

Texture

Olivine basalt
The lithology of this porphyritic basalt is characterized by olivine and augite phenocrysts.

The texture of a rock describes the relationship between the individual grains or clasts that make up the rock. Sedimentary textures include the degree of sorting, grading, shape and roundness of the clasts. Metamorphic textures include those referring to the timing of growth of large metamorphic minerals relative to a phase of deformation – before deformation porphyroclast – after deformation porphyroblast. Igneous textures include those that refer to the size of the mineral grains; vitreous – glassy, without crystals, aphanitic – grains not visible, phaneritic – grains clearly visible, and porphyritic – large grains in a finer matrix.

Small-scale structures

WaveRipple
Ripple marks from Mongolia

Rocks often contain small-scale structures (smaller than the scale of an individual outcrop). In sedimentary rocks this may include sole markings, ripple marks, mudcracks and cross-bedding. These are recorded as they are generally characteristic of a particular depositional environment and may provide information on paleocurrent directions. In metamorphic rocks associated with the deeper levels of fault zones, small scale structures such as asymmetric boudins and microfolds are used to determine the sense of displacement across the zone. In igneous rocks, small-scale structures are mostly observed in lavas such as pahoehoe versus ʻAʻā basaltic flows, and pillows showing eruption within a body of water or beneath ice.

Surficial lithology

Unconsolidated surficial materials may also be given a lithology. This is defined by grain size and composition, and is often attached to an interpretation of how the unit formed. Surficial lithologies can be given to lacustrine, coastal, fluvial, aeolian, glacial, and recent volcanic deposits, among others. Examples of surficial lithology classifications used by the US Geological Survey are, "Glacial Till, Loamy", "Saline Lake Sediment", and "Eolian Sediment, Coarse-Textured (Sand Dunes)".[7]

See also

References

  1. ^ "Lithology". Earthquake Glossary. US Geological Survey. Retrieved 29 October 2010.
  2. ^ Bates, R. J.; Jackson, J. A., eds. (1984). Dictionary of Geological Terms (3 ed.). American Geological Institute. p. 299. ISBN 0-385-18101-9.
  3. ^ Allaby, Ailsa; Allaby, Michael (1999). Oxford Dictionary of Earth Sciences (2 ed.). Oxford University Press. p. 320. ISBN 0-19-280079-5.
  4. ^ American Heritage Dictionary, ed. (2005). The American heritage science dictionary. Houghton Mifflin Harcourt. p. 364. ISBN 978-0-618-45504-1.
  5. ^ Geology.com. "The Acid Test for Carbonate Minerals and Carbonate Rocks". Geology.com. Retrieved 28 November 2016.
  6. ^ "4 Classification of rocks and description of physical properties of rock". Engineering Geology Field Manual (PDF). 1. US Bureau of Reclamation, Technical Service Center Engineering Geology Group. 1998. pp. 57–90. Retrieved 7 June 2010.
  7. ^ USGS Rocky Mountain Geographic Science Center. "Surficial Lithology: Attribute information". US Geological Survey. Retrieved 15 September 2011.
Banff Formation

The Banff Formation is a stratigraphical unit of Devonian age in the Western Canadian Sedimentary Basin.

It takes the name from the town of Banff, Alberta, and was first described on the north-west slope of Mount Rundle, near Banff by E.M. Kindle in 1924.

Colorado Group

The Colorado Group, also called the Colorado shale, is a stratigraphical unit of Cretaceous age in the Western Canadian Sedimentary Basin.

It was first described in the Rocky Mountains front ranges of Colorado by A. Hague and S.E. Emmons in 1877.

Duvernay Formation

The Duvernay Formation is a stratigraphical unit of Frasnian age in the Western Canadian Sedimentary Basin.

It was first described in well Anglo Canadian Beaverhill Lake No. 2 in LSD 11-11-50-17W4M by Imperial Oil staff in 1950. The Formation was named by Andrichuk and Wonfor in 1954 for Duvernay, Alberta.

According to Canada’s Alberta Energy Regulator, the Duvernay Formation "holds an estimated 443 trillion cubic feet of gas and 61.7 billion barrels of oil (Penny China Institute 2012-12)." Calgary-based, Athabasca Oil Corporation (formerly Athabasca Oil Sands Corporation), holds 640,000 acres, the largest publicly disclosed Duvernay rights.

Eday Group

The Eday Group is a Devonian lithostratigraphic group (a sequence of rock strata) in Orkney, northern Scotland. The name is derived from the island of Eday where the strata are exposed in coastal cliffs.

Fault block

Fault blocks are very large blocks of rock, sometimes hundreds of kilometres in extent, created by tectonic and localized stresses in the Earth's crust. Large areas of bedrock are broken up into blocks by faults. Blocks are characterized by relatively uniform lithology. The largest of these fault blocks are called crustal blocks. Large crustal blocks broken off from tectonic plates are called terranes. Those terranes which are the full thickness of the lithosphere are called microplates. Continent-sized blocks are called variously microcontinents, continental ribbons, H-blocks, extensional allochthons and outer highs.Because most stresses relate to the tectonic activity of moving plates, most motion between blocks is horizontal, that is parallel to the Earth's crust by strike-slip faults. However vertical movement of blocks produces much more dramatic results. Landforms (mountains, hills, ridges, lakes, valleys, etc.) are sometimes formed when the faults have a large vertical displacement. Adjacent raised blocks (horsts) and down-dropped blocks (grabens) can form high escarpments. Often the movement of these blocks is accompanied by tilting, due to compaction or stretching of the crust at that point.

Fox Hills Formation

The Fox Hills Formation is a Cretaceous geologic formation in the northwestern Great Plains of North America. It is present from Alberta on the north to Colorado in the south.

Fossil remains of dinosaurs, including tyrannosaurs, as well as large marine reptiles, such as mosasaurs, have been recovered from the formation.

Geological formation

A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain amount of rock strata that have a comparable lithology, facies or other similar properties. Formations are not defined by the thickness of their rock strata; therefore the thickness of different formations can vary widely.

The concept of formally defined layers or strata is central to the geologic discipline of stratigraphy. Groups of strata are divided into formations, which are divided into members.

Horizon (geology)

In geology, a horizon refers to either a bedding surface where there is marked change in the lithology within a sequence of sedimentary or volcanic rocks, or a distinctive layer or thin bed with a characteristic lithology or fossil content within a sequence. In the interpretation of seismic reflection data, horizons are the reflectors (or seismic events) picked on individual profiles. These reflectors represent a change in rock properties across a boundary between two layers of rock, particularly seismic velocity and density.

Interbedding

In geology, interbedding occurs when beds (layers or rock) of a particular lithology lie between or alternate with beds of a different lithology. For example, sedimentary rocks may be interbedded if there were sea level variations in their sedimentary depositional environment.

Knickpoint

In geomorphology, a knickpoint or nickpoint is part of a river or channel where there is a sharp change in channel slope, such as a waterfall or lake. Knickpoints reflect different conditions and processes on the river, often caused by previous erosion due to glaciation or variance in lithology. In the cycle of erosion model, knickpoints advance one cycle upstream, or inland, replacing an older cycle.

Montney Formation

The Montney Formation is a stratigraphical unit of Lower Triassic age in the Western Canadian Sedimentary Basin in British Columbia and Alberta.

It takes the name from the hamlet of Montney and was first described in Texaco's Buick Creek No. 7 well by J.H. Armitage in 1962. The well was drilled 41 kilometers (25 mi) north of Fort St. John, immediately east of the Alaska Highway.

Petrology

Petrology (from the Ancient Greek: πέτρος, romanized: pétros, lit. 'rock' and λόγος, lógos) is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together because they both contain heavy use of chemistry, chemical methods, and phase diagrams. Sedimentary petrology is, on the other hand, commonly taught together with stratigraphy because it deals with the processes that form sedimentary rock.Lithology was once approximately 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 specifically mud logging, is the graphic representation of geological formations being drilled through, and drawn on a log called a mud log. As the cuttings are circulated out of the borehole they are sampled, examined (typically under a 10× microscope) and tested chemically when needed.

Petrophysics

Petrophysics (from the Greek πέτρα, petra, "rock" and φύσις, physis, "nature") is the study of physical and chemical rock properties and their interactions with fluids.A major application of petrophysics is in studying reservoirs for the hydrocarbon industry. Petrophysicists are employed to help reservoir engineers and geoscientists understand the rock properties of the reservoir, particularly how pores in the subsurface are interconnected, controlling the accumulation and migration of hydrocarbons. Some of the key properties studied in petrophysics are lithology, porosity, water saturation, permeability and density. A key aspect of petrophysics is measuring and evaluating these rock properties by acquiring well log measurements – in which a string of measurement tools are inserted in the borehole, core measurements – in which rock samples are retrieved from subsurface, and seismic measurements. These studies are then combined with geological and geophysical studies and reservoir engineering to give a complete picture of the reservoir.

While most petrophysicists work in the hydrocarbon industry, some also work in the mining and water resource industries. The properties measured or computed fall into three broad categories: conventional petrophysical properties, rock mechanical properties, and ore quality.

Petrophysical studies are used by petroleum engineering, geology, mineralogy, exploration geophysics and other related studies.

Rundle Group

The Rundle Group is a stratigraphical unit of Mississippian age in the Western Canadian Sedimentary Basin.

It takes the name from Mount Rundle (itself taking the name from Robert Terrill Rundle), and was first described in outcrops at the northern side of the mountain in Banff National Park by R.J.W. Douglas in 1953.

Seismic inversion

Seismic inversion, in geophysics (primarily in oil-and-gas exploration/development), is the process of transforming seismic reflection data into a quantitative rock-property description of a reservoir. Seismic inversion may be pre- or post-stack, deterministic, random or geostatistical; it typically includes other reservoir measurements such as well logs and cores .

Spirit River Formation

The Spirit River Formation is a stratigraphical unit of middle Albian age in the Western Canadian Sedimentary Basin.

It takes the name from the Spirit River, and was first described in Imperial Oil Spirit River No. 1 well by Badgley in 1952.

Wabamun Formation

The Wabamun Formation is a stratigraphic unit of Late Devonian (Famennian) age in the Western Canada Sedimentary Basin. It takes the name from Wabamun Lake and was first described in the Anglo Canadian Wabamun Lake No. 1 well (located between the Wabamun Lake and the North Saskatchewan River) by Imperial Oil in 1950.

Yesnaby Sandstone Group

The Yesnaby Sandstone Group is a Devonian lithostratigraphic group (a sequence of rock strata) in west Mainland Orkney, Scotland. The name is derived from the locality of Yesnaby where the strata are exposed in coastal cliffs.

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