Jura (geology)

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< Triassic | J ura | Cretaceous >
201.3–145 million years ago

Position of the continental plates at the time of the Jurassic
Atmospheric O 2 share
(average over period) 		approx. 26 vol .-%
(130% of today's level)
Atmospheric CO 2 share
(average over period) 		approx. 1950 ppm
(5 times today's level)
Floor temperature (average over period) approx. 16.5 ° C
(2 ° C above today's level)
system series step ≈ age ( mya )
higher higher higher younger
law Upper Jurassic Tithonium 145
⬍
152.1
Kimmeridgium 152.1
⬍
157.3
Oxfordium 157.3
⬍
163.5
Middle Jurassic Callovium 163.5
⬍
166.1
Bathonium 166.1
⬍
168.3
Bajocium 168.3
⬍
170.3
Aalenium 170.3
⬍
174.1
Lower Jurassic Toarcium 174.1
⬍
182.7
Pliensbachium 182.7
⬍
190.8
Sinemurium 190.8
⬍
199.3
Hettangium 199.3
⬍
201.3
deeper deeper deeper older

The Jura is the middle chronostratigraphic system (or period in geochronology ) of the Mesozoic Era in geological history . The Jurassic began about 201.3 million years ago and ended about 145 million years ago. It therefore lasted around 56.3 million years. The Jura is underlain by the Triassic and overlaid by the Chalk .

History and naming

The name "Jura" was introduced into scientific literature by Alexander von Humboldt in 1795 for rock layers in the Jura Mountains , and in 1829 Alexandre Brongniart expanded it to include the current system name. The Jura consists mainly of the deposits ( sediment ) during the system on the edge of the former of the Jura Tethys have been deposited.

Definition and GSSP

The beginning of the Jurassic is defined by the first appearance of the ammonite species Psiloceras spelae . A final definition of the GSSP (corresponds roughly to a type profile and a type locality ) took place in 2010 at the Kuhjoch in the Karwendel in Tyrol near the border with Bavaria . The upper limit of the Jura or the lower limit of the Chalk (and thus that of the Berriasium level) has not yet been conclusively defined. It is expected to be placed near the first appearance of the ammonite species Berriasella jacobi .

Subdivision of the Jura

A scene from the Upper Jura of Northern Germany. The sauropods in the center of the picture belong to the species Europäischeaurus holgeri . In the foreground two Compsognathus can be seen, in the background a herd of Iguanodont animals passes by.

As can be seen in the info box, the Jura system is divided into three series and a total of eleven levels .

The terms Schwarzer Jura , Brauner Jura and Weißer Jura or the Quenstedt divisions in Lias , Dogger and Malm should no longer be used as names for the chronostratigraphic series of the Jura. However, they are used as lithostratigraphic units in the southern German Jura (Black Jura, Brown Jura and White Jura) or in the North German Jura (Lias, Dogger, Malm; provisional names). The boundaries of these units are purely lithostratigraphic; H. only defined by a change in the rock features. They therefore only roughly correspond to the chronostratigraphic units, since the lithostratigraphic limits do not always exactly match the system and series limits.

The most important key fossils in the Jurassic are the ammonites . These distant relatives of today's octopus , which occur exclusively in the marine environment, were almost extinct by the end of the Triassic. A significant number of new genera developed again during the hettangium, making them one of the most common invertebrates of this time. You can find them e.g. B. in southern Germany in the Posidonia schist and in limestone banks of the Swabian Alb, the Franconian Alb and in the Alps together with the belemnites to be placed with the squids .

Paleogeography

In the early Jurassic, the supercontinent Pangea continued to disintegrate ; this process had already started in the Upper Triassic with the formation of rift fracture systems. Pangea was divided into Laurasia (North America, Europe) and the southern continent of Gondwana . The early Atlantic and Tethys Sea were still narrow. In the course of the Jura, Gondwana also began to break apart. This resulted, with a main phase in the Middle Jurassic, in the formation of several volcanic centers of activity in the area of ​​South Africa and Proto-Antarctica in the form of the Karoo-Ferrar magma outflows, associated with a greatly increased ocean floor spread rate .

climate

Arrangement of the continents in the Middle Jura

In the geoscientific literature, the Jura has long been described as a relatively calm, uneventful and climatically stable epoch. This assessment has changed fundamentally in the last few decades. According to this, the middle section of the Mesozoic Era was a time of extensive tectonic processes, marked sea level fluctuations and rapid warming and cooling phases with a duration of 0.5 to 1.0 million years each. For the chronostratigraphic levels callovium (166.1 to 163.5 mya) and oxfordium (163.5 to 157.3 mya), several studies, after evaluating a series of proxy data, determine a significant cooling, the decrease in carbon dioxide concentration from 700 ppm below 500/400 ppm and a related glaciation of the polar regions of the northern hemisphere. Other publications assume a moderate cooling and assess the existence of larger ice caps as unlikely. An important indicator for the occurrence of icing processes in the Jura are the pronounced rise and fall of the sea level, which, due to their very rapid succession, exclude tectonically induced changes in the ocean basin volume in many cases. A comprehensive analysis of the oceanic trends comes to the conclusion that the significant sea level fluctuations (mainly in the range of 25 to 75 meters) can hardly be explained without the temporary existence of large ice sheets.

Development of the fauna

Some invertebrate fossils from Jurassic marine deposits (From Meyers Konversations-Lexikon (1885–90))
Some characteristic vertebrate fossils of the Jura (from Meyers Konversations-Lexikon (1885–90))

The Jura represents the first heyday of the dinosaurs . In Central Europe footprints ( Barkhausen , Münchehagen ) and skeletal remains of dinosaurs from the Jurassic period ( Ohmdenosaurus , Compsognathus ) have been discovered. The only cat-sized Compsognathus von Jachenhausen near Riedenburg was long considered the smallest dinosaur.

The "primeval bird" Archeopteryx was found in the rock layers of the Upper Jura (Malm), so far only in the Franconian Alb , especially near Solnhofen and Eichstätt .

The discovery of a mammal fossil, Hadrocodium wui , which is considered the oldest mammal in the narrower sense, also comes from the Lower Jurassic of China . More recent finds from the Middle Jurassic in northeast China's Jiulongshan Mountains ( Inner Mongolia Autonomous Region , Ningcheng District , Daohugou ) have permanently changed previous ideas about the mammalian world of the Mesozoic Era. The genus Castorocauda lutrasimilis ( Docodonta ), which lived 164 million years ago in the Middle Jurassic, resembled a beaver and already shows the further development of mammals.

Famous "fossil structures" from the Jura of Germany are the sponge stump reefs of the Swabian Alb. Small-scale reef formations also existed in northern Germany.

Development of flora

The flora was dominated by the gymnosperms (including the coniferous plants such as sequoias and pines , but also ginkgo trees and cycads ). The Jura is also known as the age of the cycads (cycads), as these were very common. Ferns and horsetail formed the undergrowth of the forests .

The Jurassic Period in Central Europe

At the beginning of the Jura, the sea, coming from the north, initially transgressed in a relatively narrow area of ​​northern and western Germany to southern Germany. Continental deposits were sedimented in north-eastern and eastern Germany . In the Central Jurassic, the sea then expanded further east. Almost the entire Eastern European platform was flooded. Large parts of Scandinavia and parts of Bohemia and the Rhenish Island remained mainland for almost the entire Jura. The Bohemian Island and the Rhenish Mass became one island during a regression in the Upper Central Jura, separating the North German and South German Jura. At the end of the Jura, southern Germany largely silted up, while northern Germany continued to have marine or brackish deposit conditions.

Fossil sites

A well-known site for fossils of the Lower Jurassic from the Posidonia schist formation (e.g. ichthyosaurs , plesiosaurs , crocodiles , fish, sea ​​lilies , ammonites ) is Holzmaden near Kirchheim , at the foot of the Swabian Alb . The Urwelt-Museum Hauff located there has world renown.

The English name for the Jura - Jurassic - became known to a wide audience through the film Jurassic Park and its successors. However, many of the dinosaurs depicted in the film, including Tyrannosaurus rex and Velociraptor , date from the Cretaceous period .

Web links

Commons : Jura  - collection of images, videos and audio files

literature

Individual evidence

  1. ^ GSSP Table - Mesozoic Era ; Retrieved August 20, 2011.
  2. ↑ Guillaume Dera, Benjamin Brigaud, Fabrice Monna, Rémi Laffont, Emmanuelle Pucéat, Jean-François Deconinck, Pierre Pellenard, Michael M. Joachimski, Christophe Durlet: Climatic ups and downs in a disturbed Jurassic world . (PDF) In: Geology . 53, No. 3, March 2011, pp. 215-218. doi : 10.1130 / G31579.1 .
  3. ↑ Yannick Donnadieu, Gilles Dromart, Yves Goddéris, Emmanuelle Pucéat, Benjamin Brigaud, Guillaume Dera, Christophe Dumas, Nicolas Olivier: A mechanism for brief glacial episodes in the Mesozoic greenhouse . In: Paleoceanography (American Geophysical Union) . 26, No. 3, September 2011. doi : 10.1029 / 2010PA002100 .
  4. ↑ G. Dromart, J.-P. Garcia, S. Picard, F. Atrops, C. Lécuyer, SMF Sheppard: Ice age at the Middle-Late Jurassic transition? . (PDF) In: Earth and Planetary Science Letters . 213, No. 3-4, August 2003, pp. 205-220. doi : 10.1016 / S0012-821X (03) 00287-5 .
  5. ↑ Hubert Wierzbowski, Mikhail A. Rogov, Bronisław A. Matyja, Dmitry Kiselev, Alexei Ippolitov: Middle – Upper Jurassic (Upper Callovian – Lower Kimmeridgian) stable isotope and elemental records of the Russian Platform: Indices of oceanographic and climatic changes . (PDF) In: Global and Planetary Change . 107, 2013, pp. 196-212. doi : 10.1016 / j.gloplacha.2013.05.011 .
  6. ^ Bilal U. Haq: Jurassic Sea-Level Variations: A Reappraisal . (PDF) In: GSA Today (Geological Society of America) . 28, No. 1, January 2018, pp. 4–10. doi : 10.1130 / GSATG359A.1 .
  7. ^ Qiang Ji, Zhe-Xi Luo, Chong-Xi Yuan, Alan R. Tabrum: A Swimming Mammaliaform from the Middle Jurassic and Ecomorphological Diversification of Early Mammals. In: Science . Volume 311, Feb 24, 2006, pp. 1123-1127.