Stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy includes two related subfields: lithologic or lithostratigraphy and biologic stratigraphy or biostratigraphy.

Historical development

Rock layers were studied since the time of Avicenna (Ibn Sina), a Persian geologist who wrote "The Book of Healing" in 1027. He was the first to outline the law of superposition of strata:cite web|author=Munim M. Al-Rawi and Salim Al-Hassani|title=The Contribution of Ibn Sina (Avicenna) to the development of Earth sciences|publisher=FSTC|url=http://www.muslimheritage.com/uploads/ibnsina.pdf|date=November 2002|accessdate=2008-07-01]

The theoretical basis for the subject was established by Nicholas Steno who re-introduced the law of superposition and introduced the principle of original horizontality and principle of lateral continuity in a 1669 work on the fossilization of organic remains in layers of sediment.

The first practical large scale application of stratigraphy was by William Smith in the 1790s and early 1800s. Smith, known as the "Father of English Geology", created the first geologic map of England, and first recognized the significance of strata or rock layering, and the importance of fossil markers for correlating strata. Another influential application of stratigraphy in the early 1800s was a study by Georges Cuvier and Alexandre Brongniart of the geology of the region around Paris.

Lithologic stratigraphy

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. Carbon and oxygen isotope ratios vary with time and are used to map subtle changes in the paleoenvironment This has led to the specialized field of "isotopic stratigraphy".

"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

Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in the rock layers. Strata from widespread locations containing the same fossil fauna and flora are correlatable in time. 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. It provides strong evidence for formation (speciation) of and the extinction of species. 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. In essence, chronostratigraphy seeks to understand the geologic history of rocks and regions.

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

Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. 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. This is possible because when very fine-grained magnetic minerals (< 17 micrometres) fall through the water column, they orient themselves with Earth's magnetic field. Upon burial, that orientation is preserved. The minerals, in effect, behave like tiny compasses.

Oriented paleomagnetic core samples are collected in the field; mudstones, siltstones, and very fine-grained sandstones are the preferred lithologies because the magnetic grains are finer and more likely to orient with the ambient field during deposition. 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. If the data indicate that the North Magnetic Pole was near the South Rotational Pole, the strata exhibit Reversed Polarity.

Results of the individual samples are analysed by removing the Natural Remanent Magnetization(NRM) to reveal the DRM. Following statistical analysis the results are used to generate a local magnetostratigraphic column that can then be compared against the Global Magnetic Polarity Time Scale.

This technique is used to date sequences that generally lack fossils or interbedded igneous rocks. The continuous nature of the sampling means that it is also a powerful technique for the estimation of sediment accumulation rates.

Archaeological stratigraphy

In the field of archaeology, soil stratigraphy is used to better understand the processes that form and protect archaeological sites. The law of superposition holds true, and this can help date finds or features from each context, as they can be placed in sequence and the dates interpolated. Phases of activity can also often be seen through stratigraphy, especially when a trench or feature is viewed in section (profile). As pits and other features can be dug down into earlier levels, not all material at the same absolute depth is necessarily of the same age, but close attention has to be paid to the archeological layers. The Harris-matrix is a tool to depict complex stratigraphic relations, as they are found, for example, in the contexts of urban archaeology.

ee also

* Harris matrix
* Important publications in stratigraphy
* International Commission on Stratigraphy
* Key bed
* Sedimentary basin analysis
* Sequence stratigraphy

References

External links

* [http://strata.geol.sc.edu/ University of South Carolina Sequence Stratigraphy Web]
* [http://snobear.colorado.edu/Markw/Mountains/03/week9.html Front Range stratigraphy]
* [http://www.stratigraphy.org International Commission on Stratigraphy]
* [http://www.uga.edu/~strata/ University of Georgia (USA) Stratigraphy Lab]
* [http://www.stratigraphy.net Stratigraphy.net]