Middle and upper cambrian type sections of the siberian platform as potential stratotypes for the global stage subdivisions and their basal boundaries
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Author||Pegel, T.V.۱; Shabanov, Yu.Ya.۱; Sukhov, S.S.۱; Abaimova, G.P.۱; Pack, K.L.۱; Lazarenko, N.P.۲; Gogin, I.Ya.۳; Korovnikov, I.V.۴|
|Holding Date||04 October 2008|
Considerable progress has been achieved in the Cambrian global scale developing since the last International Geological Congress in Italy, 2004. The names of the Lower Cambrian series (Terreneuvian) and stage (Fortunian), basal boundaries of the two stages of traditional Middle Cambrian (Drumian and Guzhangian), Upper Cambrian series Furongian and stage Paibian have been already ratified.
In a frame of the project on global Cambrian subdivisions International Subcommission on Cambrian Stratigraphy determined a number of levels of global correlation, which could be treated as possible boundaries of international stages and series. Among these levels the First Appearance Data (FADs) of trilobites Agnostotes orientalis and Lotagnostus americanus are considered to be the basal boundaries of two Upper Cambrian stages. Two variances of the traditional Lower-Middle Cambrian boundary are the most interesting: the levels coinciding with the FADs of trilobites Ovatoryctocara granulata and Oryctocephalus indicus. The first variance is supported by the Russian researchers.
In the official stratigraphic chart of the Siberian Platform the FAD of O. granulata coincides with the bases of the Oryctocara Zone and Amganian Stage compared with the Lower-Middle Cambrian boundary in the Russian standard. This level is well tracing in the open basin Kuonamka Formation on the Siberian Platform. The formation section along the Molodo River (Anabar anticline, Western Yakutia) is proposed as a potential stratotype for the fifth undefined global Cambrian stage and its lower boundary. New stage (named Molodovian) has an entire stratigraphic volume in the section: its lower boundary is defined by the FAD of Ovatoryctocara granulata and the upper boundary - by the FAD of trilobites Tomagnostus fissus. The FAD of the last species corresponds to the FAD of Ptychagnostus atavus used for defining the basal boundary of the ratified global Drumian Stage.
The Lower to Upper Cambrian continuous succession is observed along the Khos-Nelege River (north-eastern outskirts of the Siberian Platform, the Tuora-Sis Range). The Ogon’or Formation section is a biostratigraphic standard for the Upper Cambrian basinal sediments of the Siberian Platform. It is proposed as a potential stratotype for two undefined (the ninth and tenth) Upper Cambrian global stages. The lower boundary of the ninth (proposed Chekurovian) stage is defined by the FAD of Agnostotes orientalis at a level 213.3 m above the base of the Ogon’or Formation. Its upper boundary is determined by the FAD of Lotagnostus americanus at a level 339 m above the base of the formation. This level coincides with the base of the tenth (proposed Nelegerian) global stage.