Lithostratigraphy
Encyclopedia
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.
, 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 that are characteristic for 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. The formation is a lithologically distinctive stratigraphic unit that is large enough to be mappable and traceable.
Hierarchy of terms: Supergroup – Group – Formation – Member - Bed/Bed sets
Two types of contact: conformable and unconformable.
Conformable: unbroken deposition, no break or hiatus (break or interruption in the continuity of the geological record). The surface strata resulting is called a conformity.
Two types of contact between conformable strata: abrupt contacts (directly separate beds of distinctly different lithology, minor depositional break, called diastems) and gradational contact (gradual change in deposition, mixing zone).
Unconformable: period of erosion/non-deposition. The surface stratum resulting is called an unconformity
.
Four types of unconformity:
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
, the geological
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...
associated with the study of strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
or rock layers. Major focuses include geochronology
Geochronology
Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating methods are used by geologists to achieve this, and schemes of classification and terminology have been proposed...
, comparative geology, and petrology
Petrology
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....
. In general a stratum will be primarily igneous or sedimentary relating to how the rock was formed.
Sedimentary
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
layers are laid down by deposition of sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
associated with weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
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
Biostratigraphy
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period...
. 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
Law of superposition
The law of superposition is a key axiom based on observations of natural history that is a foundational principle of sedimentary stratigraphy and so of other geology dependent natural sciences:...
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 units
A lithostratigraphic unit conforms to the law of superpositionLaw of superposition
The law of superposition is a key axiom based on observations of natural history that is a foundational principle of sedimentary stratigraphy and so of other geology dependent natural sciences:...
, which state that in any succession of strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
, not disturbed or overturned since deposition
Deposition (geology)
Deposition is the geological process by which material is added to a landform or land mass. Fluids such as wind and water, as well as sediment flowing via gravity, transport previously eroded sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of...
, younger rocks lies above older rocks. The principle of lateral continuity
Principle of lateral continuity
The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous...
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
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
. 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 that are characteristic for a particular lithostratigraphic unit.
Lithosome: Masses of rock of essentially uniform character and having interchanging relationships with adjacent masses of different lithology
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, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
. e.g.: shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
lithosome, limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
lithosome.
The fundamental Lithostratigraphic unit is the formation. The formation is a lithologically distinctive stratigraphic unit that is large enough to be mappable and traceable.
Hierarchy of terms: Supergroup – Group – Formation – Member - Bed/Bed sets
Stratigraphic relationship
Conformable: unbroken deposition, no break or hiatus (break or interruption in the continuity of the geological record). The surface strata resulting is called a conformity.
Two types of contact between conformable strata: abrupt contacts (directly separate beds of distinctly different lithology, minor depositional break, called diastems) and gradational contact (gradual change in deposition, mixing zone).
Unconformable: period of erosion/non-deposition. The surface stratum resulting is called an unconformity
Unconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
.
Four types of unconformity:
- Angular unconformity: younger sediment lies upon an eroded surface of tilted or folded older rocks. The older rock dipsStrike and dipStrike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented...
at a different angle than the younger. - Disconformity: the contact between younger and older beds is marked by visible, irregular erosional surfaces. PaleosolPaleosolIn the geosciences, paleosol can have two meanings. The first meaning, common in geology and paleontology, refers to a former soil preserved by burial underneath either sediments or volcanic deposits , which in the case of older deposits have lithified into rock...
might develop right above the disconformity surface because of the non-deposition setting. - Paraconformity: the bedding plans below and above the unconformity are parallel. A time gap is present but there is no erosion, just a non-deposition period.
- Nonconformity: relatively young sediments are deposited right above older igneousIgneous rockIgneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
or metamorphic rockMetamorphic rockMetamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s.
Lithostratigraphic correlation
To correlate lithostratigraphic units, geologists define facies, and look for key beds or key sequences that can be used as a datum.- Direct correlation: based on lithology, color, structure, thickness…
- Indirect correlation: electric log correlation (gamma-ray, density, resistivity…)