Planorbis zone

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The Planorbis zone is a biostratigraphic zone of the lower Hettangian . It follows the Tilmanni zone and is in turn replaced by the Liasicus zone .

Definition

Psiloceras planorbis

The Planorbis zone, also called Psiloceras planorbis zone, English Planorbis Chronozone , was established in 1856 by Albert Oppel , who named it after the key fossil characteristic of the lower hettangium - the neoammonite Psiloceras planorbis J. de C. Sowerby, 1824 .

Originally the Planorbis zone defined the beginning of the Hettangium and thus the Jura . In the course of time, however , further ammonites were discovered in the lying area of the Planorbis zone, which were fundamentally different from the last ammonites of the Triassic - the Ceratites with the genus Choristoceras . A further zone, the Tillmanni zone, is now inserted below the Planorbis zone in the so-called preplanorbis layers .

Breakdown

In order to achieve a stratigraphically better resolution, the Planorbis zone is further subdivided into two sub-zones (English subchronozones ), five bio- zones (English zonules ) and ten horizons (English biohorizons ). The following sequence results from hanging to lying :

The two subzones are separated from each other in a faunistic way, so the genera Psiloceres and Neophyllites occur only in the Planorbis subzone and the genus Caloceras only in the Johnstoni subzone. The oldest is the horizon of Psiloceras erugatum , the youngest the horizon of Caloceras intermedium .

Schill on the underside of a lime bench in the psilo tone, Sampsoni biozone

The Planorbis subzone begins with the Planorbis biozone, which in turn is divided into four horizons: the ribbed Psiloceras taxon Psiloceras erugatum at the base, followed by the smooth-skinned neophyllites Neophyllites imitans and Neophyllites antecedens and finally the smooth-skinned Psiloceras planorbis in the Hangendenas planorbis . The following Sampsoni biozone - often also referred to as Psilonotum biozone (see Psilonotenton formation ) - consists only of the horizon of Psiloceras sampsoni α . The final Plicatulum Biozone contains only the horizon of Psiloceras plicatulum . Note: The plicatulum biozone was previously also known as the plicatulus horizon or biozone. Axel von Hillebrandt would even like to identify it as a separate sub-zone, as everything in south-west England indicates the existence of further psilocera horizons.

The Johnstoni subzone (named after Caloceras johnstoni ), dominated by early representatives of the genus Caloceras, is made up of two biozones, the Johnstoni biozone in the horizontal and the Belcheri biozone in the hanging wall. The Johnstoni Biozone leads in ascending order the horizons of Caloceras sp. 1 , Caloceras sp. 2 and Caloceras johnstoni . The final Belcheri biozone contains only the horizon of Caloceras intermedium .

history

In 1922 Arthur Elijah Trueman established the Planorbis subzone . As a stratotype, Warrington and colleagues then proposed the outcrops at St. Audrie's Bay (near Watchet ) along the west coast of Somerset in England in 1994 . The sub-zone is here at the base of Bed 8 or A 18 .

The Johnstoni Subzone that followed was also established by Trueman in 1922, but Urban Schlönbach had already designated it as a separate zone in 1863 .

Age

It is difficult to give an absolute age for the Planorbis zone. According to Guex and colleagues (2012), the genus Psiloceras with the taxon Psiloceras spelae evolved from the Phyllocerataceae 201.5 million years ago .

The genus Neophyllites appeared very early in the Unterhettangian by 201.3 million years, followed by the genus Caloceras around 201.0 million years. At the beginning of the middle hettangian around 200.7 million years ago, the genus chamber carites appeared, but it was already part of the liasicus zone. Overall, therefore, the period 201.3 to 200.7 million years can be given for the Planorbis zone.

Zone equivalents

The Planorbis zone was developed in the area of ​​the North West European Shelf Sea and therefore only applies to countries such as Germany , France , Benelux and the United Kingdom . In the Tethys area of the Alps it is represented by the Calliphyllum zone with the subzones of Psiloceras calliphyllum , Psiloceras costosum and Psiloceras naumanni . In North America , however, the following zones apply: Minutus zone in the horizontal, above the Pacificum zone and then the Polymorphum zone in the hanging wall.

Individual evidence

  1. ↑ Kevin N. Page: The Lower Jurassic of Europe: its subdivision and correlation . In: Geological Survey of Denmark and Greenland Bulletin . tape 1 , 2003, p. 23-59 .
  2. ↑ W. Lange: The ammonite fauna of the Psiloceras level of northern Germany . In: Palaeontographica . A 93, 1941, p. 186 .
  3. ^ Gert Bloos and Kevin N. Page: The proposed GSSP for the base of the Sinemurian Stage near East Quantoxhead / West Somerset (SW England) - the ammonite sequence . In: RL Hall and PL Smith, Advances in Jurassic research 2000. Proceedings of the 5th International Symposium on the Jurassic System (Eds.): GeoResearch Forum . tape 6 , 2000, pp. 13-26 .
  4. ^ Axel von Hillebrandt, A .: The Triassic / Jurassic boundary in northern Chile . In: Cahiers de l'Université Catholique de Lyon, Série Science . tape 3 , 1997, p. 27-53 .
  5. ^ R. Mouterde and M. Corna: Hettangien . In: E. Cariou and P. Hantzpergue, Biostratigraphie du Jurassique ouesteuropéen et méditerranéen: zonations parallèles et distribution des invertébrés et microfossiles (eds.): Bulletin du Center Recherches Elf Exploration Production Mémoire . tape 17 , 1997, pp. 7-8 .
  6. ^ AE Trueman, AE: The Liassic Rocks of Glamorgan . In: Proceedings of the Geologists' Association (London) . tape 33 , 1922, pp. 266 .
  7. ^ G. Warrington, JCW Cope and HC Ivimey-Cook: St Audries Bay, Somerset, England: a candidate Global Stratotype Section and Point for the base of the Jurassic System . In: Geological Magazine . tape 131 , 1994, pp. 191-200 .
  8. ↑ Jean Guex et al .: Geochronological restraints on post-extinction recovery of the ammonoids and carbon cycle perturbations during the early Jurassic . In: Palaeogeography, Palaeoclimatology, Palaeoecology . tape 346-347 , 2012, pp. 1–11 , doi : 10.1016 / j.palaeo.201204030 .