A branch of geology concerned with the study of sequences of layers of rock, usually sedimentary. It aims to unravel changes in their depositional environment, and to correlate rocks of the same age in different places by their rock type and fossil content.
Stratigraphy, a branch of geology, is basically the study of rock layers and layering (stratification).
Stratigraphy includes two related subfields: lithologic or lithostratigraphy and biologic stratigraphy or biostratigraphy.
Lithologic stratigraphy
See also: Lithostratigraphy
Lithostratigraphy, or lithologic stratigraphy, is the most obvious. It deals with the physical lithologic or rock type change both vertically in layering or bedding of varying rock type and laterally reflecting changing environments of deposition, known as facies change. Key elements of stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries mean in terms of depositional environment. One of stratigraphy's basic concepts is codified in the Law of Superposition, which simply states that, in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence.
Chemostratigraphy is based on the changes in the relative proportions of trace elements and isotopes within and between lithologic units.
Cyclostratigraphy documents the often cyclic changes in the relative proportions of minerals, particularly carbonates, and fossil diversity with time, related to changes in palaeoclimates.
Biostratigraphy
See also: Biostratigraphy
Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in the rock layers. Biologic stratigraphy was based on William Smith's principle of faunal succession, which predated, and was one of the first and most powerful lines of evidence for, biological evolution. The geologic time scale was developed during the 1800s based on the evidence of biologic stratigraphy and faunal succession. This timescale remained a relative scale until the development of radiometric dating, which gave it and the stratigraphy it was based on an absolute time framework, leading to the development of chronostratigraphy.
One important development is the Vail curve, which attempts to define a global historical sea-level curve according to inferences from world-wide stratigraphic patterns. Stratigraphy is also commonly used to delineate the nature and extent of hydrocarbon-bearing reservoir rocks, seals and traps in petroleum geology.
Chronostratigraphy
Chronostratigraphy is the branch of stratigraphy that studies the absolute age of rock strata.
Chronostratigraphy is based upon deriving geochronological data for rock units, both directly and by inference, so that a sequence of time relative events of rocks within a region can be derived.
The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of the Earth.
Magnetostratigraphy
When measurable magnetic properties of rocks vary stratigraphically they may be the bases for related but different kinds of stratigraphic units known collectively as "magnetostratigraphic units" ("magnetozones"). The magnetic property most useful in stratigraphic work is the change in the direction of the remanent magnetization of the rocks, caused by reversals in the polarity of the Earth's magnetic field. Such reversals of the polarity have taken place many times during geologic history.They are recorded in the rocks because the rocks may record the direction of the Earth's magnetic field at or near the time of rock formation (see paleomagnetism). The direction of the remanent magnetic polarity recorded in the stratigraphic sequence can be used as the basis for the subdivision of the sequence into units characterized by their magnetic polarity.
Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic stratigraphic sections. The samples are analyzed to determine their Detrital Remanent Magnetization (DRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited.
If the ancient magnetic field was oriented similar to today's field (North Magnetic Pole near the North Rotational Pole) the strata retain a Normal Polarity.
Sampling Procedures
Oriented paleomagnetic core samples are collected in the field using a Pomeroy Drill. Spacing of the sample sites within a stratigraphic section depends on: 1) the type of depositional environment: The farther away from the orogenic front, the closer the sample spacing due to generally lower rates of deposition;
Analytical Procedures
Samples are first analyzed in their natural state to obtain their Natural Remanent Magnetization (NRM).
DRM orientations of all samples from a site are then compared and their magnetic polarity is determined with Fisher statistics. The latitudes of the Virtual Geomagnetic Poles from those sites determined to be statistically significant are plotted against the stratigraphic level at which they were collected. Ages
Because the polarity of a stratum can only be Normal or Reversed, variations in the rate at which the sediment accumulated can cause the thickness of a given polarity zone to vary from one area to another.
Because the age of each reversal shown on the GMPTS is relatively well known, the correlation establishes numerous time lines through the stratigraphic section. These ages provide relatively precise dates for features in the rocks such as fossils, changes in sedimentary rock composition, changes in depositional environment, etc.
Sediment Accumulation Rates
Perhaps the most powerful application of these data is to determine the rate at which the sediment accumulated.
These data are also used to model basin subsidence rates. Knowing the depth of a hydrocarbon source rock beneath the basin-filling strata allows calculation of the age at which the source rock passed through the generation window and hydrocarbon migration began.
Another application of these results derives from the fact that they illustrate when sediment accumulation rates changed.
Archaeological stratigraphy
See also: Archaeological stratigraphy
In the field of archaeology, soil stratigraphy is used to better understand the processes that form and protect archaeological sites.
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