Hettangian

The Hettangian ( in German often shortened to Hettang, rarely also modified to Angien Blanchett ) is in Earth's history, a chronostratigraphic stage of the Jura, the geochronological approximately corresponds to the period from 201.3 to 199.3 million years ago and thus about two million years of rebuilding. The Hettangian is the first ( oldest) stage of the Jura (or the Lower Jurassic series) and subsequent to the stage of the Upper Triassic Rhaetiums. On the Hettangian Sinemurian that follows.

• 6.1 dinoflagellates
• 6.2 Calcareous nannoplankton
• 6.3 foraminifera

• 7.1 mass extinction
• 7.2 Fossils

History and naming

The Hettangian was named after the place Hettange -Grande (Dept. Moselle) in Lorraine (France). The stage and the name were proposed by Eugène Renevier 1864.

Definition and GSSP

The beginning of the Hettangiums is defined by the first appearance of the ammonite spelae Type Psiloceras. The GSSP ( = " Global Stratotype Section and Point" is roughly equivalent to a type profile ) is located at Kuhjoch in the Karwendel mountains in Austria. An upper limit of the first appearance of the ammonite genera Vermiceras and Metophioceras has been set.

Subdivision of Hettangiums

The higher part of the stage is divided further into three well-defined Tethysbereich ammonite zones ( from hanging wall to footwall ):

• Angulata zone of Schlotheimia
• Zone of Alsatites liasicus
• Zone of Psiloceras planorbis

The subdivision of the Lower Hettangiums is controversial, especially as the exact basis of the law has not yet been determined. Often, these layers are combined as " Präplanorbis layers ". Other authors excrete between the actual " Präplanorbis layers " yet different biostratigraphic horizons, such as the Neophyllites antecedens and the Neophyllites imitans horizon and the Psiloceras erugatum horizon. There may be under the erugatum horizon as the horizon of Psiloceras tillmanni. Other authors consider here a "transitional horizon " from. But it is quite possible that other horizons can be excreted in the basal region of the Hettangiums. However, the full transition profiles between Triassic and Jurassic are rare worldwide.

Sea ​​level

Immediately at the beginning of Hettangiums a dramatic sea level drop was made, the regression Hey 1 regressive tendencies lasted until the middle of the Hettangiums and then gave way to a gradual increase.

Chemical stratigraphy

Carbon isotopes

The δ13C values ​​for the Hettangian move 1-2 ‰ ( PDB), reaching its maximum in the middle section. They are therefore generally somewhat lower than in the rest of the law, ie the organic carbon production was low or the conditions prevailing something anoxic and restrictive.

Oxygen isotopes

The values ​​of δ18O are at - 2 ‰ (PDB ) with slightly increasing trend. They are, with the exception of Toarciums lower than in the rest of Jura and speak for a rather warm climate in the Hettangian, which gradually but was cooling off.

Strontium isotopes

The ratio 87Sr/86Sr in seawater was very high with an average of 0.7077. It reached at the end of Hettangiums a maximum value and then fell to the beginning of Toarciums steadily. The fall of the strontium ratio for the Hettangian is correlated with increased submarine volcanic activity and is likely the increasing opening of the North Atlantic reflect ( breaking up of Pangaea and reduction of the continental sediment input ).

Biostratigraphy

Dinoflagellates

The Hettangian forms the lower half of the Dinoflagellatenzone Dpr, named after the dominant taxon in the boreal region Dapcodinium priscum. As a zone name for the Hettangian also DSJ1 is in use.

Calcareous nannoplankton

The calcareous nannoplankton was published shortly before the Hettangiums with the endtriassischen transgression for the first time in the world's oceans.

The Hettangian completely belongs to Nannoplanktonzone NJ1 that (FAD ) of the taxon Schizosphaerella punctulata is marked by the first occurrence. This ends with the first appearances of Parhabdolithus liasicus.

Foraminifera

On the basis of foraminifera can identify for the Hettangian three zones:

• Upper zone of the taxa Vaginulina subporrecta and Ychtyolaria xyphoidea
• Mean zone with Lenticulina quadricosta
• Lower zone of the taxa Lingulina striata, Lingulina collenoti and Lenticulina Austro alpina.

Paleontology

Mass extinction

The turn of Triassic / Jurassic is one of the top five mass extinctions of Earth's history. This event fell victim to the conodonts, but also with the Ammonites and bivalves it came to a dramatic decline in biodiversity.

Fossils

The seas were populated during the Hettangiums of the predatory ichthyosaurs with the taxon Ichthyosaurus and of the plesiosaurs with Macro Plata and Rhomaleosaurus. They were the predators of ammonites, which spread after the regression back into the oceans.

On the mainland, the dinosaurs were prevalent, including the bird Beck dinosaur ( Ornithischia ) with the taxa Abrictosaurus, Fabrosaurus, Heterodontosaurus, Lanasaurus, Lycorhinus, Scelidosaurus, Scutellosaurus and Stormbergia.

The air space was dominated by the pterosaurs such as Dimorphodon.

Among the early mammals Haramiya be mentioned.

Protosuchus, an early crocodile appeared in the Hettangian to the first time.

At the Triassic - Jurassic boundary, a large Faunenschnitt took place. How many other creatures were almost completely extinct by external events, the Ammonites.

In the Hettangian, however, developed the " Neoammoniten " relatively quickly, so that even in the middle Hettangian again, there were a greater number of genera and species. In contrast to later periods were obviously the Fossilationsbedingungen not optimal, so that only a few sites with well-preserved Hettangian ammonites are known worldwide.

Occurrence

The Hettangian occurs in the following Sedimentationsräumen on (some examples):

• Paris Basin: It contains the Grès d' Hettange the original ( eponymous ) stratotype. This wedges from the Calcaire à gryphées.
• Aquitanisches pool: Basal Transgressionssequenz of Hettangiums and Sinemuriums.
• England: Blue Lias formation.
• Northwest Germany: Psilonoten layers and above the Angulaten layers.
• Southern Germany: Psilonotenton lineup and in the hanging wall Angulatenton the lineup or the Angulatensandstein Formation ( superseded by the Bamberg - formation and the formation of Bayreuth ).
• Swiss Jura: insects marl and about the Cardinienkalk.
• Northern Limestone Alps: Adnet lime ( Schnöll Formation) - Allgäu layers.
• India: Kota Formation
• People's Republic of China: Lufeng Formation
• Lesotho: Elliot Formation
• Zimbabwe: Forest Sandstone, Vulcanodon Beds
• North America: Newark Supergroup