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Lithostratigraphy

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.
Strata in Salta (Argentina).

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.

Types of lithostratigraphic unitsEdit

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 relationshipEdit

 
Diagrams showing stratigraphic relations: A: an angular unconformity; B: a disconformity; C: a nonconformity.
 
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 correlationEdit

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 quasi-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 alsoEdit

ReferencesEdit

  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 linksEdit