Marker horizons (also referred to as chronohorizons, key beds or marker beds) are stratigraphic units of the same age and of such distinctive composition and appearance, that, despite their presence in separate geographic locations, there is no doubt about their being of equivalent age (isochronous) and of common origin. Such clear markers facilitate the correlation of strata, and used in conjunction with fossil floral and faunal assemblages and paleomagnetism, permit the mapping of land masses and bodies of water throughout the history of the earth.[1] They usually consist of a relatively thin layer of sedimentary rock that is readily recognized on the basis of either its distinct physical characteristics or fossil content and can be mapped over a very large geographic area.[2] As a result, a key bed is useful for correlating sequences of sedimentary rocks over a large area. Typically, key beds were created as the result of either instantaneous events or (geologically speaking) very short episodes of the widespread deposition of a specific types of sediment. As the result, key beds often can be used for both mapping and correlating sedimentary rocks and dating them. Volcanic ash beds (tonsteins and bentonite beds) and impact spherule beds, and specific megaturbidites are types of key beds created by instantaneous events. The widespread accumulation of distinctive sediments over a geologically short period of time have created key beds in the form of peat beds, coal beds, shell beds, marine bands, black shales in cyclothems, and oil shales. A well-known example of a key bed is the global layer of iridium-rich impact ejecta that marks the Cretaceous–Paleogene boundary (K–T boundary).
Tephra horizons in Iceland
Palynology, the study of fossil pollens and spores, routinely works out the stratigraphy of rocks by comparing pollen and spore assemblages with those of well-known layers—a tool frequently used by petroleum exploration companies in the search for new fields. The fossilised teeth or elements of conodonts are an equally useful tool.
The ejecta from volcanoes and bolide impacts create useful markers, as different volcanic eruptions and impacts produce beds with distinctive compositions. Marker horizons of tephra are used as a dating tool in archaeology, since the dates of eruptions are generally well-established.[3]
One particular bolide impact 66 million years ago, which formed the Chicxulub crater, produced an iridium anomaly that occurs in a thin, global layer of clay marking the Cretaceous–Paleogene boundary.[4]Iridium layers are associated with bolide impacts and are not unique, but when occurring in conjunction with the extinction of specialised tropical planktic foraminifera and the appearance of the first Danian species, signal a reliable marker horizon for the Cretaceous–Paleogene boundary.[5]
Fossil faunal and floral assemblages, both marine and terrestrial, make for distinctive marker horizons. Some marker units are distinctive by virtue of their magnetic qualities. The Water Tower Slates, forming part of the Hospital Hill Series in the Witwatersrand Basin, include a fine-grained ferruginousquartzite which is particularly magnetic. From the same series a ripple marked quartzite and a speckled bed are used as marker horizons.
On a much smaller time scale, marker horizons may be created by sedimentologists and limnologists in order to measure deposition and erosion rates in a marsh or pond environment. The materials used for such an artificial horizon are chosen for their visibility and stability and may be brick dust, grog, sand, kaolin, glitter or feldspar clay.[6]
^H.V. Cabadas-Báez, S. Sedov, S. Jiménez-Álvarez, D. Leonard, B. Lailson-Tinoco, R. García-Moll, I. Ancona-Aragón, L. Hernández. (2017). "Soils as a Source of Raw Materials for Ancient Ceramic production in the Maya region of Mexico: Micromorphological insight" (PDF). Boletín de la Sociedad Geológica Mexicana. 70: 21–48. doi:10.18268/BSGM2018v70n1a2.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, J.A., eds. (2005) Glossary of Geology (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN0-922152-76-4
^Westgate, J. A.; Gorton, M. P. (1981). "Correlation Techniques in Tephra Studies". In Self, S.; Sparks, R. S. J. (eds.). Tephra Studies. Boston: Kluwer. pp. 73–94. ISBN90-277-1327-8.
^BBC (17 September 2014). "What Really Killed the Dinosaurs?". Retrieved 17 December 2019.
^Evans, Kevin R. (2008). The Sedimentary Record of Meteorite Impacts. Geological Society of America. p. 174. ISBN9780813724379.
^USGS (February 1, 2010). . Archived from the original on October 10, 2012. Retrieved December 17, 2019.
Further readingedit
Truswell, J. F. (1970). Historical Geology of South Africa. Cape Town: Purnell. ISBN0-360-00104-1.
External linksedit
International Stratigraphic Guide - Geological Society of America (1994) - Amos Salvador (editor)
January 01, 1970
marker, horizon, also, referred, chronohorizons, beds, marker, beds, stratigraphic, units, same, such, distinctive, composition, appearance, that, despite, their, presence, separate, geographic, locations, there, doubt, about, their, being, equivalent, isochro. Marker horizons also referred to as chronohorizons key beds or marker beds are stratigraphic units of the same age and of such distinctive composition and appearance that despite their presence in separate geographic locations there is no doubt about their being of equivalent age isochronous and of common origin Such clear markers facilitate the correlation of strata and used in conjunction with fossil floral and faunal assemblages and paleomagnetism permit the mapping of land masses and bodies of water throughout the history of the earth 1 They usually consist of a relatively thin layer of sedimentary rock that is readily recognized on the basis of either its distinct physical characteristics or fossil content and can be mapped over a very large geographic area 2 As a result a key bed is useful for correlating sequences of sedimentary rocks over a large area Typically key beds were created as the result of either instantaneous events or geologically speaking very short episodes of the widespread deposition of a specific types of sediment As the result key beds often can be used for both mapping and correlating sedimentary rocks and dating them Volcanic ash beds tonsteins and bentonite beds and impact spherule beds and specific megaturbidites are types of key beds created by instantaneous events The widespread accumulation of distinctive sediments over a geologically short period of time have created key beds in the form of peat beds coal beds shell beds marine bands black shales in cyclothems and oil shales A well known example of a key bed is the global layer of iridium rich impact ejecta that marks the Cretaceous Paleogene boundary K T boundary Tephra horizons in Iceland Palynology the study of fossil pollens and spores routinely works out the stratigraphy of rocks by comparing pollen and spore assemblages with those of well known layers a tool frequently used by petroleum exploration companies in the search for new fields The fossilised teeth or elements of conodonts are an equally useful tool The ejecta from volcanoes and bolide impacts create useful markers as different volcanic eruptions and impacts produce beds with distinctive compositions Marker horizons of tephra are used as a dating tool in archaeology since the dates of eruptions are generally well established 3 One particular bolide impact 66 million years ago which formed the Chicxulub crater produced an iridium anomaly that occurs in a thin global layer of clay marking the Cretaceous Paleogene boundary 4 Iridium layers are associated with bolide impacts and are not unique but when occurring in conjunction with the extinction of specialised tropical planktic foraminifera and the appearance of the first Danian species signal a reliable marker horizon for the Cretaceous Paleogene boundary 5 Fossil faunal and floral assemblages both marine and terrestrial make for distinctive marker horizons Some marker units are distinctive by virtue of their magnetic qualities The Water Tower Slates forming part of the Hospital Hill Series in the Witwatersrand Basin include a fine grained ferruginous quartzite which is particularly magnetic From the same series a ripple marked quartzite and a speckled bed are used as marker horizons On a much smaller time scale marker horizons may be created by sedimentologists and limnologists in order to measure deposition and erosion rates in a marsh or pond environment The materials used for such an artificial horizon are chosen for their visibility and stability and may be brick dust grog sand kaolin glitter or feldspar clay 6 Contents 1 See also 2 References 3 Further reading 4 External linksSee also editRadiometric dating Relative dating StratigraphyReferences edit H V Cabadas Baez S Sedov S Jimenez Alvarez D Leonard B Lailson Tinoco R Garcia Moll I Ancona Aragon L Hernandez 2017 Soils as a Source of Raw Materials for Ancient Ceramic production in the Maya region of Mexico Micromorphological insight PDF Boletin de la Sociedad Geologica Mexicana 70 21 48 doi 10 18268 BSGM2018v70n1a2 a href Template Cite journal html title Template Cite journal cite journal a CS1 maint multiple names authors list link Neuendorf K K E J P Mehl Jr and J A Jackson J A eds 2005 Glossary of Geology 5th ed Alexandria Virginia American Geological Institute 779 pp ISBN 0 922152 76 4 Westgate J A Gorton M P 1981 Correlation Techniques in Tephra Studies In Self S Sparks R S J eds Tephra Studies Boston Kluwer pp 73 94 ISBN 90 277 1327 8 BBC 17 September 2014 What Really Killed the Dinosaurs Retrieved 17 December 2019 Evans Kevin R 2008 The Sedimentary Record of Meteorite Impacts Geological Society of America p 174 ISBN 9780813724379 USGS February 1 2010 Establishing Marker Horizons Archived from the original on October 10 2012 Retrieved December 17 2019 Further reading editTruswell J F 1970 Historical Geology of South Africa Cape Town Purnell ISBN 0 360 00104 1 External links editInternational Stratigraphic Guide Geological Society of America 1994 Amos Salvador editor Retrieved from https en wikipedia org w index php title Marker horizon amp oldid 1144822109, wikipedia, wiki, book, books, library,
