Marker horizon
Encyclopedia
Marker horizons or chronohorizons 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
.
The ejecta from volcano
es 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.
One particular bolide impact 65 million years ago, Chicxulub
, is controversially held to have led to a major extinction event
and produced an iridium anomaly
that occurs in a thin, global layer of clay marking the Cretaceous–Tertiary boundary. 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 K–T boundary
.
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.
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
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
Isochronous
Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...
) and of common origin. Such clear markers facilitate the correlation 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...
.
The ejecta from volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
es and bolide impacts create useful markers, as different volcanic eruptions and impacts produce beds with distinctive compositions. Marker horizons of tephra
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....
are used as a dating tool in archaeology
Archaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...
, since the dates of eruptions are generally well-established.
One particular bolide impact 65 million years ago, Chicxulub
Chicxulub Crater
The Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named...
, is controversially held to have led to a major extinction event
Extinction event
An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...
and produced an iridium anomaly
Iridium anomaly
The term iridium anomaly commonly refers to an unusual abundance of the chemical element iridium in a layer of rock strata, often taken as evidence of an extraterrestrial impact event because of the case of such an anomaly at the Cretaceous–Tertiary boundary...
that occurs in a thin, global layer of clay marking the Cretaceous–Tertiary boundary. Iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
layers are associated with bolide impacts and are not unique, but when occurring in conjunction with the extinction of specialised tropical planktic foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
and the appearance of the first Danian
Danian
The Danian is the oldest age or lowermost stage of the Paleocene epoch or series, the Paleogene period or system and the Cenozoic era or erathem. The beginning of the Danian age is at the Cretaceous-Tertiary extinction event...
species, signal a reliable marker horizon for the K–T boundary
K–T boundary
The K–T boundary is a geological signature, usually a thin band, dated to 65.5 ± 0.3 Ma ago. K is the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period...
.
Fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
faunal and floral assemblages
Faunal assemblage
Faunal Assemblage is the archaeological or paleontological term for a group of associated animal fossils found together in a given stratum.The principle of faunal succession is used in biostratigraphy to determine each biostratigraphic unit, or biozone...
, both marine and terrestrial, make for distinctive marker horizons. Some marker units are distinctive by virtue of their magnetic
Rock magnetism
Rock magnetism is the study of the magnetic properties of rocks, sediments and soils. The field arose out of the need in paleomagnetism to understand how rocks record the Earth's magnetic field. This remanence is carried by minerals, particularly certain strongly magnetic minerals like magnetite...
qualities. The Water Tower Slates, forming part of the Hospital Hill Series in the Witwatersrand Basin
Witwatersrand basin
The Witwatersrand Basin is a geological formation in South Africa holding the world's largest known gold reserves and having produced over 1.5 billion ounces. The basin straddles the old provinces of Transvaal and the Orange Free State and is of the same period as the Vredefort impact of 2.023 Ga...
, include a fine-grained ferruginous
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
quartzite
Quartzite
Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...
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.
External links
- http://books.google.co.za/books?id=l03YnbnoxzUC&pg=PA15&lpg=PA15&dq=Marker+horizons+stratigraphic+units&source=bl&ots=RO1WgIPm3i&sig=tsLian2kVsqew4S21LdT7V00oAY&hl=en&ei=W1-kTLu4LcOh4Qb7l-39DA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CBgQ6AEwAQ#v=onepage&q=Marker%20horizons%20stratigraphic%20units&f=falseInternational Stratigraphic Guide - Geological Society of America (1994) - Amos Salvador (editor)]