Lithostratigraphy

Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general a stratum will be primarily igneous or sedimentary relating to how the rock was formed.

Sedimentary layers are laid down by deposition of sediment associated with weathering processes, decaying organic matters (biogenic) or through chemical precipitation. These layers are distinguishable as having many fossils and are important for the study of biostratigraphy. Igneous layers are either plutonic or volcanic in character depending upon the cooling rate of the rock. These layers are generally devoid of fossils and represent intrusions and volcanic activity that occurred over the geologic history of the area.

There are a number of principles that are used to explain the appearance of stratum. When an igneous rock cuts across a formation of sedimentary rock, then we can say that the igneous intrusion is younger than the sedimentary rock. The principle of superposition states that a sedimentary rock layer in a tectonically undisturbed stratum is younger than the one beneath and older than the one above it. The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds.

SEUtahStrat
The Permian through Jurassic lithostratigraphy of the Colorado Plateau area of southeastern Utah that makes up much of the famous prominent rock formations in protected areas such as Capitol Reef National Park and Canyonlands National Park. From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically-jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone. Picture from Glen Canyon National Recreation Area, Utah.
Quebrada de Cafayate, Salta (Argentina)
Strata in Salta (Argentina).

Types of lithostratigraphic units

A lithostratigraphic unit conforms to the law of superposition, which state that in any succession of strata, not disturbed or overturned since deposition, younger rocks lies above older rocks. The principle of lateral continuity states that a set of bed extends and can be traceable over a large area.

Lithostratigraphic units are recognized and defined on the basis of observable rock characteristics. The descriptions of strata based on physical appearance define facies. Lithostratigraphic units are only defined by lithic characteristics, and not by age.

Stratotype: A designated type of unit consisting of accessible rocks that contain clear-cut characteristics which are representative of a particular lithostratigraphic unit.

Lithosome: Masses of rock of essentially uniform character and having interchanging relationships with adjacent masses of different lithology. e.g.: shale lithosome, limestone lithosome.

The fundamental Lithostratigraphic unit is the formation. A formation is a lithologically distinctive stratigraphic unit that is large enough to be mappable and traceable. Formations may be subdivided into members and beds and aggregated with other formations into groups and supergroups.

Stratigraphic relationship

Block diagrams stratigraphic relations
Diagrams showing stratigraphic relations: A: an angular unconformity; B: a disconformity; C: a nonconformity.
EdwardsDisconformity
Disconformity with the Lower Cretaceous Edwards Formation overlying a Lower Permian limestone; hiatus is about 165 million years; Texas.

Stratigraphy investigates the spatial and temporal relationship between geological layers. Two types of relationships: conformable stratigraphic relationship and unconformable stratigraphic relationship. Conformity – no time interruption in deposition of sediments. Unconformity – there was time interruption in deposition of sediments. The surface, which separates sediments deposited at different time, is the surface of unconformity.

Four types of unconformity:

  • Angular unconformity: younger sediment lies upon an eroded surface of tilted or folded older rocks. The older rock dips at a different angle from the younger.
  • Disconformity: the contact between younger and older beds is marked by visible, irregular erosional surfaces. Paleosol might develop right above the disconformity surface because of the non-deposition setting.
  • Paraconformity: the bedding planes below and above the unconformity are parallel. A time gap is present, as shown by a faunal break, but there is no erosion, just a period of non-deposition.
  • Nonconformity: relatively young sediments are deposited right above older igneous or metamorphic rocks.

Lithostratigraphic correlation

In lithostratigraphy to find the unconformity in the sequence of layers the geologists refer to the geometry of different sections of the sequence, namely, to the geometrical relations between the borders of the layers. The geometry of the layers can be extracted from the correlation scheme. The lithological correlation is based on the assumption that

1) During the deposition the surfaces of the layers are parallel or quazy-parallel.

2) The interruption in deposition is caused by significant and quite long tectonic movements.

3) To the moment, when a new cycle of sedimentation starts, the geometry of existent layers is modified by the tectonic movements – the layers are tilted or folded.

As a result appears the case of angular unconformity (Diagram A).

  • Direct correlation: based on lithology, color, structure, thickness...
  • Indirect correlation: electric log correlation (gamma-ray, density, resistivity...)[1]
  • Computer correlation[2]

See also

References

  1. ^ Ricardo A. Olea and Robert J. Sampson (2003), CORRELATOR, an interactive computer program for high-resolution, lithostratigraphic, well-log correlation. http://www.kgs.ku.edu/Publications/OFR/2002/OFR02_51/ManualOFR2002-51.pdf
  2. ^ Guberman S. (2008) Unorthodox Geology and Geophysics: Oil, Ores and Earthquakes. Polimetrica. pp. 123 – 168. https://books.google.com/books?id=aXcpwlU4bDgC&pg=PR2&lpg=PR2&dq=Guberman+%22Unorthodox%22&source=%20bl&ots=rGZnDDEVQu&sig=ACfU3U3EWSYA2tJ7OEgSZkZnw-cjE6IK8w&hl=en&sa=X&ved=2ahUKEwjp--6f0MvjAhXFJzQIHZePB_AQ6AEwAXoECAoQAQ#v=snippet&q=correlation&f=false

External links

Buntsandstein

The Buntsandstein (German for coloured or colourful sandstone) or Bunter sandstone is a lithostratigraphic and allostratigraphic unit (a sequence of rock strata) in the subsurface of large parts of west and central Europe. The Buntsandstein predominantly consists of sandstone layers of the Lower Triassic series and is one of three characteristic Triassic units, together with the Muschelkalk and Keuper that form the Germanic Trias Supergroup.

The Buntsandstein is similar in age, facies and lithology with the Bunter of the British Isles. It is normally lying on top of the Permian Zechstein and below the Muschelkalk. In the past the name Buntsandstein was in Europe also used in a chronostratigraphic sense, as a subdivision of the Triassic system. Among reasons to abandon this use was the discovery that its base lies actually in the latest Permian.

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.

Keuper

The Keuper is a lithostratigraphic unit (a sequence of rock strata) in the subsurface of large parts of west and central Europe. The Keuper consists of dolomite, shales or claystones and evaporites that were deposited during the Middle and Late Triassic epochs (about 220 million years ago). The Keuper lies on top of the Muschelkalk and under the predominantly Lower Jurassic Lias or other Early Jurassic strata.

The Keuper together with the Muschelkalk and the Buntsandstein, all 3, form the Germanic Trias Group, a characteristic sequence of rock strata that gave the Triassic its name..

Landen Group

The Landen Group is a lithostratigraphic unit (a "group" of rock strata) in the Belgian subsurface. The group consists of two formations of Thanetian (late Paleocene) to Ypresian (early Eocene) age. The Landen Group is named after the town of Landen in Flemish Brabant.

Late Jurassic

The Late Jurassic is the third epoch of the Jurassic period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age. In the past, Malm was also used to indicate the unit of geological time, but this usage is now discouraged to make a clear distinction between lithostratigraphic and geochronologic/chronostratigraphic units.

Lias Group

The Lias Group or Lias is a lithostratigraphic unit (a sequence of rock strata) found in a large area of western Europe, including the British Isles, the North Sea, the Low Countries and the north of Germany. It consists of marine limestones, shales, marls and clays.

Lias is a Middle English term for hard limestone, used in this specific sense by geologists since 1833. In the past, geologists used Lias not only for the sequence of rock layers, but also for the timespan during which they were formed. It was thus an alternative name for the Early Jurassic epoch of the geologic timescale. It is now more specifically known that the Lias is Rhaetian to Toarcian in age (over a period of c. 20 million years between 200 to 180 million years ago) and thus also includes a part of the Triassic. The use of the name "Lias" for a unit of time is therefore slowly disappearing.

London Clay

The London Clay Formation is a marine geological formation of Ypresian (early Eocene Epoch, c. 56–49 Ma) age which crops out in the southeast of England. The London Clay is well known for its fossil content. The fossils from the lower Eocene rocks indicate a moderately warm climate, the tropical or subtropical flora. Though sea levels changed during the deposition of the clay, the habitat was generally a lush forest – perhaps like in Indonesia or East Africa today – bordering a warm, shallow ocean.

The London Clay is a stiff bluish clay which becomes brown when weathered. Nodular lumps of pyrite and crystals of selenite (sometimes called "waterstones") frequently exist in the clay, and large septarian concretions are also common. These have been used in the past for making cement. They were once dug for this purpose at Sheppey, near Sittingbourne, and at Harwich, and also dredged, off the Hampshire coast. The clay is still used commercially for making bricks, tiles, and coarse pottery in places such as Michelmersh in Hampshire.

Maastricht Formation

The Maastricht Formation (Dutch: Formatie van Maastricht; abbreviation: MMa), named after the city of Maastricht, the Netherlands, is a geological formation in the Netherlands and Belgium whose strata date back to the Late Cretaceous, within 500,000 years of the Cretaceous–Paleogene boundary, now dated at 66 million years ago. The formation is part of the Chalk Group and is between 30 and 90 metres (98 and 295 ft) thick. It crops out in southern parts of Dutch and Belgian Limburg and adjacent areas in Germany. It can be found in the subsurface of northern Belgium and southeastern Netherlands, especially in the Campine Basin and Roer Valley Graben. Dinosaur remains are among the fossils that have been recovered from the formation.

Muschelkalk

The Muschelkalk (German for "shell-bearing limestone"; French: calcaire coquillier) is a sequence of sedimentary rock strata (a lithostratigraphic unit) in the geology of central and western Europe. It has a Middle Triassic (240 to 230 million years) age and forms the middle part of the tripartite Germanic Trias, that give the Triassic its name, lying above the older Buntsandstein and below the younger Keuper. The Muschelkalk ("mussel chalk") consists of a sequence of limestone and dolomite beds.

In the past, the time span in which the Muschelkalk was deposited could also be called "Muschelkalk". In modern stratigraphy, however, the name only applies to the stratigraphic unit.

Series (stratigraphy)

Series are subdivisions of rock layers based on the age of the rock and formally defined by international conventions of the geological timescale. A series is therefore a sequence of strata defining a chronostratigraphic unit. Series are subdivisions of systems and are themselves divided into stages.

Series is a term defining a unit of rock layers formed during a certain interval of time (a chronostratigraphic unit); it is equivalent (but not synonymous) to the term geological epoch (see epoch criteria) which defines the interval of time itself, although the two words are sometimes confused in informal literature.

Stage (stratigraphy)

In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.

Rock series are divided into stages, just as geological epochs are divided into ages. Stages can be divided into smaller stratigraphic units called chronozones. (See chart at right for full terminology hierarchy.) Stages may also be divided into substages or indeed grouped as superstages.The term faunal stage is sometimes used, referring to the fact that the same fauna (animals) are found throughout the layer (by definition).

Stratigraphy

Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.

Stratigraphy has two related subfields: lithostratigraphy (lithologic stratigraphy) and biostratigraphy (biologic stratigraphy).

Tectonostratigraphy

In geology, tectonostratigraphy is stratigraphy that refers either to rock sequences in which large-scale layering is caused by the stacking of thrust sheets, or nappes, in areas of thrust tectonics or to the effects of tectonics on lithostratigraphy.

Thickness (geology)

Thickness in geology and mining refers to the distance across a packet of rock, whether it be a facies, stratum, bed, seam, lode etc.

Thickness is measured at right angles to the surface of the seam or bed and thus independently of its spatial orientation. The concept of thickness came originally from mining language, where it was used mainly to indicate the workability of seams. It has since become an established term in earth science, for example in geology, for the depth of sedimentary rocks, in hydrogeology for the vertical extent of groundwater – i.e. the distance from the base of the groundwater layer to its surface – or in soil science for the vertical extent of soil horizons.

Voort Formation

The Voort Formation or Voort Member (Dutch: Formatie van Voort or Laagpakket van Voort; abbreviation in Belgium: Vo or in the Netherlands: VEVO) is a stratigraphic unit in the subsurface of north Belgium and the south of the Netherlands. The unit has the status of a formation in Belgium but is seen as a member of the Veldhoven Formation in the Netherlands. The Voort Formation consists of shallow marine sands with a late Oligocene age (Chattian stage, about 25 million years old).

Wealden Group

The Wealden Group is a group (a sequence of rock strata) in the lithostratigraphy of southern England. The Wealden group consists of paralic to continental (freshwater) facies sedimentary rocks of Berriasian to Aptian age and thus forms part of the English Lower Cretaceous. It is composed of alternating sands and clays. The sandy units were deposited in a flood plain of braided rivers, the clays mostly in a lagoonal coastal plain.The Wealden Group can be found in almost all Early Cretaceous basins of England: its outcrops curve from the Wessex Basin in the south to the Cleveland Basin in the northeast. It is not found in northwest England and Wales, areas which were at the time tectonic highs where no deposition took place. The same is true for the London Platform around London and Essex. Offshore, the Wealden Group can reach a thickness of 700 metres.

Wenlock Group

The Wenlock Group (Wenlockian), in geology, is the middle series of strata in the Silurian (Upper Silurian) of Great Britain. This group in the typical area in the Welsh border counties contains the following formations: Much Wenlock Limestone Formation, 90–300 ft.; Wenlock Shale, up to 1900 ft.; Woolhope or Barr Limestone and shale, 150 ft.

Wetterstein limestone

Wetterstein limestone (German: Wettersteinkalk) and Wetterstein dolomite (Wettersteindolomit) are the most common names for a carbonate rock from the Middle Triassic epoch of the Ladinian stage, comparable to the German stage in which Muschelkalk rock strata were formed.

The rock gets its name from the Wetterstein Mountains, because the Wetterstein limestone is the bedrock of large mountains here. The center of its distribution, however, is in the Karwendel Mountains. It occurs in the Northern and Southern Limestone Alps and in the Western Carpathians.

Zechstein

The Zechstein (German either from mine stone or tough stone) is a unit of sedimentary rock layers of Middle to Late Permian (Guadalupian to Lopingian) age located in the European Permian Basin which stretches from the east coast of England to northern Poland. The name Zechstein was formerly also used as a unit of time in the geologic timescale, but nowadays it is only used for the corresponding sedimentary deposits in Europe.

The Zechstein lies on top of the Rotliegend; on top of the Zechstein is the Buntsandstein or Bunter.

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