Ludfordian
| Ludfordian | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
 Paleogeography of the Ludfordian, 425 Ma | |||||||||||||
| Chronology | |||||||||||||
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| Etymology | |||||||||||||
| Name formality | Formal | ||||||||||||
| Name ratified | 1980[4] | ||||||||||||
| Usage information | |||||||||||||
| Celestial body | Earth | ||||||||||||
| Regional usage | Global (ICS) | ||||||||||||
| Time scale(s) used | ICS Time Scale | ||||||||||||
| Definition | |||||||||||||
| Chronological unit | Age | ||||||||||||
| Stratigraphic unit | Stage | ||||||||||||
| Time span formality | Formal | ||||||||||||
| Lower boundary definition | Imprecise. Near FAD of the graptoliteSaetograptus leintwardinensis | ||||||||||||
| Lower boundary definition candidates | None | ||||||||||||
| Lower boundary GSSP candidate section(s) | None | ||||||||||||
| Lower boundary GSSP | Sunnyhill,Ludlow,England 52°21′33″N2°46′38″W/ 52.3592°N 2.7772°W | ||||||||||||
| Lower GSSP ratified | 1980[4] | ||||||||||||
| Upper boundary definition | FAD of the graptoliteMonograptus parultimus | ||||||||||||
| Upper boundary GSSP | Požáry Section,Prague-Řeporyje,Czech Republic 50°01′40″N14°19′30″E/ 50.0277°N 14.3249°E | ||||||||||||
| Upper GSSP ratified | 1984[5][6] | ||||||||||||
In thegeologic timescale,theLudfordianis the upper of twochronostratigraphicstages within theLudlow Series.Its age is the lateSilurianPeriod,and within both thePalaeozoicEraandPhanerozoicEon.The rocks assigned to the Ludfordian date to between 425.6 ± 0.9Maand 423.0 ± 2.3 Ma (million years ago). The Ludfordian Stage succeeds theGorstianStage and precedes thePridoli Epoch.It is named for the village ofLudfordinShropshire,England.The GSSP for the Ludfordian is represented as a thin shale seam, coincident with the base of theLeintwardine Formation,overlying theBringewood Formationin England.
Paleoclimate
[edit]TheLau eventis a rapid pulse of cooling during the Ludfordian, about424million years ago;it is identified by a pulse of extinctions and oceanic changes. It is one of the series of fast sea-level and excursions in oxygen isotope ratios that signal fast switches between warm and cold climate states, characteristic of the Silurian climatic instability. The Lau Event occurred during an extended period of elevated seawater saturation state, explained by reservoirs of the planet's fresh water being locked up in massive polar ice caps. The sudden reappearance in normally saline marine environments ofstromatolitesand a mass occurrence of oncoids during the event suggested that minor extinction events like the Lau Event also resulted in periods of reduced grazing pressures on surviving "disaster biota", which can be compared to the aftermath of the more catastrophic end-Ordovician and end-Permian mass extinctions.[7]
References
[edit]- ^Jeppsson, L.; Calner, M. (2007). "The Silurian Mulde Event and a scenario for secundo—secundo events".Earth and Environmental Science Transactions of the Royal Society of Edinburgh.93(02): 135–154.doi:10.1017/S0263593300000377.
- ^Munnecke, A.; Samtleben, C.; Bickert, T. (2003). "The Ireviken Event in the lower Silurian of Gotland, Sweden-relation to similar Palaeozoic and Proterozoic events".Palaeogeography, Palaeoclimatology, Palaeoecology.195(1): 99–124.doi:10.1016/S0031-0182(03)00304-3.
- ^"Chart/Time Scale".stratigraphy.org.International Commission on Stratigraphy.
- ^abHolland, C. (1982)."The State of Silurian Stratigraphy"(PDF).Episodes.1982(3): 21–23.doi:10.18814/epiiugs/1982/v5i3/004.Retrieved11 December2020.
- ^Lucas, Sepncer (6 November 2018)."The GSSP Method of Chronostratigraphy: A Critical Review".Frontiers in Earth Science.6:191.Bibcode:2018FrEaS...6..191L.doi:10.3389/feart.2018.00191.
- ^Holland, C. (June 1985)."Series and Stages of the Silurian System"(PDF).Episodes.8(2): 101–103.doi:10.18814/epiiugs/1985/v8i2/005.Retrieved11 December2020.
- ^M. Calner, "A Late Silurian extinction event and anachronistic period"Geology33, pp. 305–308;Geology On-line Forum: response to a reply
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