Coniacian
The Coniacian ( often shortened in the German language to Coniacian ) is in Earth's history, a chronostratigraphic stage of the Upper Cretaceous. It is geochronological dating to the period of about 89.7 to about 86.3 million years. The Coniacian following the Turonian and is detached from the Santonian.
Naming and history
The typical for this stage rock units are the Saintonge area near the town of Cognac ( Charente, France). Henri Coquand and led stage name in 1857 in the scientific literature.
Definition and GSSP
The lower boundary of the stage is defined by the first appearance of inoceramids Type Cremnoceramus rotundatus (identical to Cremnoceramus deformis erectus ), the upper limit undulatoplicatus by the first appearance of inoceramids Type Cladoceramus. A GSSP (global type locality and type profile) for the Coniacian has not yet been ratified concluded.
Subdivision
The Coniacian is divided to sub- intermediate level in the lower, middle and Oberconiacium. The Coniacian once formed together with the Santonian the stratigraphic level of the Emscher.
In Tethysbereich biostratigraphically three ammonite zones are excreted for the Coniacian:
- Paratexanites serratomarginatus
- Gauthiericeras margae
- Peroniceras tridorsatum
In the boreal region occurs at the bottom must be added the Forresteria petrocoriensis zone.
Magnetostratigraphy
The entire Coniacian magnetostratigraphisch belongs to the C34 anomaly, the so-called " Cretaceous magnetic quiet zone " with normal polarity.
Sequence Stratigraphy
After reaching the maximum absolute sea level in Unterturon makes in the Coniacian, a gradual decrease again in sea noticeable. This cycle first order, however, superimposed by a kurzperiodischeren sea level rise ( cycle 2nd order ), which culminates in a transgression Mittelconiacium. It follows on the regression Co1 ( 87.0 Ma BP ) separating means and Oberconiacium another. A 3rd order cycle also causes a new transgression Oberconiacium.
Oceanic anoxic event
From the Mittelconiacium it comes in the Atlantic region with the formation of black shale sediments at an oceanic anoxic event, called the OAE -3, which continues into the Mittelsantonium ( 87.3 to 84.6 Ma BP). It is the longest lasting and the same time last such event during the Cretaceous period.