Biostratigraphy of the First and Second Members of Type Section of the Shirgesht Formation in North of Tabas with Special Emphasis on Conodonts

Ghaderi, A.; Aghanabati, A.; Hamdi, B.; Miller, J. F.

doi:10.22071/gsj.2009.57826

Document Type : Original Research Paper

Authors

¹ Center of Excellence for Paleontology, Ferdowsi University of Mashhad, Iran

² Research Institute for Earth Sciences, Geological Survey of Iran, Teharan, Iran

³ Missuri University, USA

https://doi.org/10.22071/gsj.2009.57826

Abstract

The Shirgesht Formation is one of the fossiliferous units of Central Iran which in the type section is defined (informally) in the terms of 3 members. This formation is mainly composed of limestone, marl, siltstone, shale and dolomite, conformably overlay on the upper part of the Derenjal Formation with a gradual contact, composed of brachiopods limestone bearing (Billingsella limestone). The Shirgesht Formation is overlain by sandy limestone and volcanic rocks of the Niur Formation.
To do biostratigraphy studies on the first and second members of Shirgesht Formation in the type section, totally 99 rock samples were selected. The conodont fauna of lower part of its first member contain some species of Westergaardodina, Furnishina and prooneotodus which show an age of late Cambrian. Several species of Cordylodus, Oistodus, Drepanodus, Proconodontus and Paroistodus in distance of middle part of the lower member to the end of the second member represent an age of Tremadocian – Arenigian. According to conodont content of the first and second members of Shirgesht Formation, 7 conodont assemblage zones were determined in the type section which show an age of Late Cambrian – Early Ordovician. The recognized biozones are as follow:

1. Furnishina - Westergaardodina Assemblage Zone.
2. Proconodontus - Oneotodus Assemblage Zone.
3. Cordylodus proavus - Cordylodus oklahomensis Assemblage Zone.
4. Cordylodus lindstromi Assemblage Zone.
5. Cordylodus angulatus Assemblage Zone.
6. Drepanodus - Paltodus deltifer Assemblage Zone.
7.Paroistodus proteus Assemblage Zone.
The mentioned conodont assemblage zones are equivalent to Cordylodus spp., Deltiferand Proteus global standard conodont biozone. In contrast with the previous reports, the Cambrian – Ordovician boundary is located in the first member of the Shirgesht Formation. Thus, the Shirgesht Formation in type section is diachronous formation.

Keywords

20.1001.1.10237429.1387.17.67.15.6

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Scientific Quarterly Journal of Geosciences

Biostratigraphy of the First and Second Members of Type Section of the Shirgesht Formation in North of Tabas with Special Emphasis on Conodonts

References

References

Volume 17, Issue 67
February 2008
Pages 150-163

Biostratigraphy of the First and Second Members of Type Section of the Shirgesht Formation in North of Tabas with Special Emphasis on Conodonts

References

References

Volume 17, Issue 67February 2008Pages 150-163

Volume 17, Issue 67
February 2008
Pages 150-163