Document Type : Original Research Paper


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


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.   


An, T. X., Zhang, F., Xiang, W. D., Zhang, Y. Q., Xu, W. H., Zhang, H. J., Jiang, D. B., Yang, Ch.Sh., Lin, L. D., Cui, Z. T. & Yang, X. Ch., 1983- The conodonts of North China and the adjacent regions: Beijing, P.R.C., Science Press, 223 p.
Apollonov, M. K., 1991- Cambrian-Ordovician boundary beds in the U.S.S.R., in Barnes, C.R., and Williams, S.H., (eds.), Advances in Ordovician geology: Geological Survey of Canada Paper 90-9, p. 33–45.
Cooper, R. A., Nowlan, G. S. & Williams, S. H., 2001- Global stratotype section and point for base of the Ordovician system. Episodes 24: 19 – 28.
Druce, E. C. & Jones, P. J., 1971- Cambro-Ordovician conodonts from the BurkeRiver structural belt, Queensland: Australian Bureau of Mineral Resources, Geology and Geophysics, Bulletin 110, 158 p.
Dubinina, S. V., 1991- Upper Cambrian and Lower Ordovician conodont associations from open ocean paleoenvironments, illustrated by Batyrbai and Sarykum sections in Kazakhstan, in Barnes, C. R., and Williams, S. H., (eds.), Advances in Ordovician geology: Geological Survey of Canada Paper 90-9, p. 107–124.
Ethington, R. L. & Clark, D. L., 1981- Lower and Middle Ordovician conodonts from the Ibex area, western illard County, Utah: Brigham Young University Geology Studies, v. 28, pt. 2, 155 p., 14 pls., 12 tables [range charts of taxa].
Henningsmoen, G., 1973- The Cambro-Ordovician boundary: Lethaia, v. 6, p. 423–439.
Landing, E., 1980-Late Cambrian - Early Ordovician macrofaunas and phosphatic microfaunas, St. John Group, New Brunswick. Journal of Paleontology, 54; no. 4; (752 – 761).
Lindstrom, M., 1984- Baltoscandic Conodont life environments in the Ordovician: Sedimentologic and paleogeographic evidence. Geological society of America, Special paper, 196, (33 - 42).
Lofgren, A., 1993- Conodont from the Lower Ordovician at Hunneberg, south-central Sweden. Geol. Mag. 130, (215 - 232).
Lofgren, A., 1994- Arenig (lower Ordovician) Conodonts and biozonation in the eastern Siljan District, central Sweden. J. Paleont. 68, (1350 - 1368).
Lofgren, A., 1996- Lower Ordovician Conodonts, reworking, and biostratigraphy of the Orreholmen quarry, Vastergotland, south-central Sweden. GFF 118, (169 - 183).
Lofgren, A., 1997- Conodont faunas from the upper Tremadoc at Brattefors, south – central Sweden, and reconstruction of the Paltodus apparatus. GFF 119, (257 - 266).
Lofgren, A., 1997- Reinterpreatation of the lower Ordovician Conodont Apparatus Paroistodus. Palaeontology 40, (913 - 929).
Miller, J. F., 1984- Cambrian and earliest Ordovician Conodont evolution, biofacies, and provincialism. Geological society of America, Special paper 196, (43 - 68).
Muller, K. J., 1973- Late Cambrian and early Ordovician Conodonts from northern Iran. Geological Survey of Iran, Report No. 30, (1 - 78).
Ross, R. J. Jr., Hintze, L. F., Ethington, R. L., Miller, J. F., Taylor, M. E. & Repetski, J. E.,1997- The Ibexian, Lowermost Series in the North American Ordovician, in Taylor, M. E., 1997. (eds.) Early Paleozoic Biochronology of the Great Basin, Western United States. U.S. Geological Survey professional paper 1579.
Ruttner, A., Nabavi, M. H., Hajian, J., Bozorgnia, F., Eftekharnezhad, J., Emami, K. S., Flugel, E., Flugel, H. W., Haghipour, A., Iwao, S., Kahler, F., Ruttner-Kolisko, A., Sartener, P., Stepanov, D. L., Valeh, N., Walliser, O. H. & Winsnes, T. S., 1968- Geology of the Shirgesht Area (Tabas area, East Iran). Geological Survey of Iran, Report No. 4, (1 - 140).
Shergold, J. H. & Nicoll, R. S., 1992- Revised Cambrian-Ordovician boundary biostratigraphy, Black Mountain, western Queensland, in Webby, B. D. and Laurie, J. R., 1992, Global perspectives on Ordovician geology: Rotterdam, Netherlands, A. A. Balkema, p. 81–92.