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 three related subfields: lithostratigraphy (lithologic stratigraphy), biostratigraphy (biologic stratigraphy), and chronostratigraphy (stratigraphy by age).
Historical development
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The first practical large-scale application of stratigraphy was by William Smith in the 1790s and early 19th century. Known as the "Father of English geology",[1] Smith recognized the significance of strata or rock layering and the importance of fossil markers for correlating strata; he created the first geologic map of England. Other influential applications of stratigraphy in the early 19th century were by Georges Cuvier and Alexandre Brongniart, who studied the geology of the region around Paris.
Lithostratigraphy
Variation in rock units, most obviously displayed as visible layering, is due to physical contrasts in rock type (
Cyclostratigraphy documents the often cyclic changes in the relative proportions of minerals (particularly carbonates), grain size, thickness of sediment layers (varves) and fossil diversity with time, related to seasonal or longer term changes in palaeoclimates.
Biostratigraphy
Biostratigraphy or
One important development is the
Chronostratigraphy
Chronostratigraphy is the branch of stratigraphy that places an absolute age, rather than a relative age on rock strata. The branch is concerned with deriving geochronological data for rock units, both directly and inferentially, so that a sequence of time-relative events that created the rocks formation can be derived. The ultimate aim of chronostratigraphy is to place dates on the sequence of deposition of all rocks within a geological region, and then to every region, and by extension to provide an entire geologic record of the Earth.
A gap or missing strata in the geological record of an area is called a stratigraphic hiatus. This may be the result of a halt in the deposition of sediment. Alternatively, the gap may be due to removal by erosion, in which case it may be called a stratigraphic vacuity.[2][3] It is called a hiatus because deposition was on hold for a period of time.[4] A physical gap may represent both a period of non-deposition and a period of erosion.[3] A geologic fault may cause the appearance of a hiatus.[5]
Magnetostratigraphy
Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout a section. The samples are analyzed to determine their detrital remanent magnetism (DRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. For sedimentary rocks this is possible because, as they fall through the water column, very fine-grained magnetic minerals (< 17 μm) behave like tiny compasses, orienting themselves with Earth's magnetic field. Upon burial, that orientation is preserved. For volcanic rocks, magnetic minerals, which form in the melt, orient themselves with the ambient magnetic field, and are fixed in place upon crystallization of the lava.
Oriented paleomagnetic core samples are collected in the field;
Results of the individual samples are analyzed 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.
See also
References
- ISBN 978-1862392144.
- ^ "SEPM Strata". sepmstrata.org.
- ^ ISBN 978-1-86239-039-3
- ISBN 978-0-14-051494-0.
- ISBN 978-0-444-42165-4.
Further reading
- Christopherson, R. W., 2008. Geosystems: An Introduction to Physical Geography, 7th ed., New York: Pearson Prentice-Hall. ISBN 978-0-13-600598-8.
- Montenari, M., 2016. Stratigraphy and Timescales, 1st ed., Amsterdam: Academic Press (Elsevier). ISBN 978-0-12-811549-7.
External links
- ICS Subcommission for Stratigraphic Information
- University of South Carolina Sequence Stratigraphy Web
- Front Range stratigraphy
- International Commission on Stratigraphy
- University of Georgia (USA) Stratigraphy Lab
- Stratigraphy.net A stratigraphic data provider.
- Agenames.org A global index of stratigraphic terms