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Jurassic Forum » Impressions and Creativity » Impressions » Jurassic-Cretaceous boundary beds at the Lena lower flows (rich in oceanic ammonoids)
Jurassic-Cretaceous boundary beds at the Lena lower flows
mhornДата: Tuesday, 27.10.2009, 12:50 | Сообщение # 1
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During the August I have participated in the field works held in the Lena lower flows, the little-studied region (especially respect to Upper Jurassic-Lower Cretaceous ammonite faunas and biostratigraphy). Only scattered data concerning this matter were published in small little-known articles.


The section

Some interesting ammonites were obtained from the Oxfordian part of the section, i.e. many Cardioceras, accompanied by phylloceratids resembling Phylloceras alaskanum and Boreiophylloceras (?) subobtusiforme.

Volgian deposits in this region mainly have a small thickness (ca. 8 m) and poor in ammonites. Among the latter some Praechetaites, Craspedites and Boreiophylloceras were collected.

Ryazanian (of the total thickness few hundreds meters) much more rich in fossils. Ammonites preserved mainly as crushed moulds, but numerous.


Borealites constans

Most interesting ammonites, in my opinion, belongs to phyllo- and lytoceratids. Phylloceratids represented by succession of Boreiophylloceras species only, but abound at least in some levels of the Ryazanian Stage. Lytoceratids were encountered only near to the Ryazanian-Valanginian boundary. These lytoceratids are really big - one 40 cm ammonite was collected, accompanied by few pieces of the same size giants. But it generic identification is difficult due to poor data about Arctic lytoceratids, its big size and strange sculpture


"Lytoceras"


Boreiophylloceras densicostatum, Kochi Zone


Boreiophylloceras praeinfundibulum/lenaense, Ryazanian/Valanginian boundary beds

Presence of numerous ocean-related ammonites (phyllo- and lytoceratids) indicates connection between Pacific Ocean and remains of the earliest Cretaceous South Anuy Ocean.


Middle Jurassic - Lower Cretaceous ammonites & aptychi
 
Jurassic Forum » Impressions and Creativity » Impressions » Jurassic-Cretaceous boundary beds at the Lena lower flows (rich in oceanic ammonoids)
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