Scientific Quarterly Journal of Geosciences

Scientific Quarterly Journal of Geosciences

Investigation of Lower Paleozoic Sediments at Tang-e-Zakeen of Kuh-e- Faraghan and Introduction of Seyahou and Sarchahan Formations in the Zagros basin

Document Type : Original Research Paper

Authors
Mohammad Ghavidel-Syooki
Mohammad Ebrahim Khosravi
Exploration Division of National Iranian Oil Company
Abstract
A thick lower Paleozoic succession is well- developed at Tang-e- Zakeen in Kuh-e- Faraghan, approximately 103 K m northern Bandar Abbas city. The succession has been visited by several geologists in the past decades. However, neither a complete stratigraphic column nor a detailed study has been done on this succession. Thus, it was measured and sampled for detailed study by author and M. E. Khosravi, in 1985. This succession is 807m Thick and mainly consists of clastic sediments and subordinate limestones. So far, this succession has been used to be called "Silurian Shales". In this study, the use of Silurian Shales is dropped, instead, the Seyahou and Sarchahan formations are introduced, respectively for the upper Ordovician and lower Silurian of this succession in the Zagros basin.
    The name of Seyahou formation originates from the Seyahou village where is located approximately 80 K m northern Bandar Abbas city. The type section of this formation is at Tang-e- Zakeen in Kuh-e- Faraghan, where is located 23 K m northern Seyahou village. At type locality, the whole thickness of lower Paleozoic succession is 807 m that from this thickness, 741 m belongs to the Seyahou formation and the rest is the Sarchahan formation. The Seyahou formation has also surface exposure at Kuh-e- Surmeh and Zard- Kuh, respectively with a thickness of 58m and 70m. Likewise, this formation is present in some exploration wells such as Kuh-e- Siah no. 1, Zirreh no.1 and Kabir- Kuh no.1, Darang no. 1, respectively with a thickness of 117m, 156m, 100m, and 250m.
The Seyahou formation mainly consist of variegated shales, siltstones, sandstones and subordinate limestones. Iron compounds such as hematite and limonites occur locally along bedding planes of the Seyahou formation. Thus, color varies from white to light purple, buff, brown, reddish brown and black in the Seyahou formation. This variegated characteristic differentiates the Seyahou formation from the Sarchahan formation which is only dark-black in color. Sedimentary structures such as cross- bedding, graded bedding, symmetrical ripple- mark, micro- ripple, large concretion and trace fossils (trails, tracks and burrows) are common in the Seyahou formation whereas they are absent or rare in the Sarchahan formation. The lowermost beds of Seyahou formation consist of variegated sandy conglomerates which contain some pebbles of black chert and rounded quartzite. Overlying beds of these conglomerates begin with alternation of shales, siltstones, sandstones and subordinate limestones and ends with white and cream-brown sandstones at the top. The lower contact of this formation is not clear with older sediments, but it is probably conformable with the Zard-Kuh formation such as Zard- Kuh and exploration wells of Zirreh no.1, Kabir- Kuh no.1, and Darang no.1. The upper contact of Seyahou formation is gradational with the Sarchahan formation at Tang-e-Zakeen. This contact is characterized by presence of 18 meters of white and cream- brown sandstones which sharply change into the black shales of the Sarchahan formation. The Seyahou formation contains orthoceras, brachiopod, graptolite, crinoid, bryozoa, chitinozoa, acritarch taxa and cryptospores of the earliest land plants. Based on the above-mentioned fauna, and flora a shallow marine environment is suggested for the Seyahou formation, since, the shales which contain the chitinozoa and acritarch assemblages were deposited on a mud dominated platform in pro- delta and off-shore areas which have had a holomarine conditions.
    Before this study, some graptolite species have been identified from the upper part of this succession. This part is now called the Sarchahan formation which includes Monograptus intermedius, Monograptus socialis and Climacograptus scalaris. Based on the above-mentioned graptolite species the Lower Silurian has been suggested for the upper part of the succession, or Sarchahan formation. Later on, the lower Silurian age has been extended to the whole thickness of succession at Tang-e- Zakeen (Mollazal, 1968; Kheradpir & Nicol, 1973). At present time, the graptolite fauna serve to date the Sarchahan formation, not the Seyahou formation.
     In order to determine the geological relationships of this succession, more than 200 samples were selected and treated for palynological studies. A total of 100 palynomorph species were identified in this study. These have been arranged in six local acritarch and four chitinozoan assemblage zones (Fig.2). The acritarch zones of I through IV and chitinozoan zones of C1 through C3 appear and disappear in the Seyahou formation. In general, the above-mentioned acritarch and chitinozoan zones are characterized by presence of Veryhachium reductum, Veryhachium subglobosum, Villosacapsula setosapellicula, Leiosphaeridia ketchenata, Leiosphaeridia tenuissima, Orthosphaeridium ternatum, Orthosphaeridium inflatum, Orthosphaeridium bispinosum, Orthosphaeridium chondrododora, Navifusa ancepsipuncta, Actinotodissus crassus, Actinotodissus longilateosus, Baltisphaeridium longispinosum delicatun, Baltisphaeridium perclarum, Rhabdochitina usilata, Conochitina senta, Calpichitina lenticularis, Cyathochitina fistulosa, Desmochitina spp., Armoricochitina nigerica and some tetrade cryptospores. So far, the above-mentioned palynomorph species have been recorded from the upper Ordovician sediments of Saudi Arabia (McClure, 1988), Jordan (Keegan, Rasul & Shaheen, 1990), North America (Loeblich & Tappan, 1978; Leoblich, 1969; Jacobson & Achab, 1985), England (Turner, 1982, 1984, 1985) Libya (Molyneux & Paris, 1985), Czechoslovakia (Vavrdova, 1988, 1989) and Iran (Ghavidel- Syooki, 1994). Based on this comparison, the upper Ordovician (Caradocian- Ashgillian) is suggested for the Seyahou formation in the Zagros basin. Likewise, the acritarch zones of V through VI and chitinozoan zone of C4 occur in the Sarchahan formation. These zones are marked by appearence of Dactylofusa estillis, Dactylofusa neaghae, Dactylofusa maranhensis, Visbysphaera microspinosa, Visbysphaera oligofucata, Visbyphaera brevifurcata, Dictyotidum perlucidum, Dictyotidium faviformis, Dateriocradus monterrosae, Evittia denticulata denticulata, Tunisphaeridum tentaculiferum, plectochitina saharica, Sphaerochitina longicornis, Sphaerochitina longicollis, Ambitisporites avitus, Ambitisporites dilutus and so on. So far, the above- mentioned palynomorph taxa have been recorded from the lower Silurian sediments of Norway (Smelror, 1978), Sweden (Le'Herisse, 1989), the U. S. A (Cramer & Diez, 1972; Cramer, 1969), Saudi Arabia (McClure, 1988), and Jordan (Keegan, Rasul & Shaheen, 1990). Therefore, the lower Silurian age (Llandovery) is suggested for the Sarchahan formation at Tang-e- Zakeen in Kuh-e- Faraghan, equating to the age- assignment of graptolite species.
Keywords
Seyahou Formation
Sarchahan Formation
Subjects
Cramer, F. H., 1969. Distribution of selected Silurian acritarchs. Rev. Esp. Micropaleontoa., num. extr. 203pp.
Cramer, F. H., and Diez M. del, C. R., 1972. North American Silurian Palynofacies and their spacial arrangements. Paleontographica., Abt. B. 138: 107-180.
Ghavidel- Syooki, M., 1993. Palynological study of Paleozoic sediments at Chali sheh area, Southwestern Iran, J. Scien. Islam. Rep. Iran, 4(1): 32-46.
Gorka, H., 1987. Acritarches et prasinophyceae de L'Ordovicien moyen (viruen) du Sondage de Smedsby Gard No. 1 (Gotland, Suede). Rev. Palynol., 52 (4): 257-297.
Hill, P. J., Paris, F. and Richardson, J. B., 1985. Silurian Palynomorphs. J. Micropalaeontology 4 (1): 27-48. Hoffmeister, W. S., 1959. Lower Silurian plant spores from Libya. Micropaleontology, 5(3): 331-334.
Jacobson, S. R. and Achab, A., 1985. Acritarch biostratigraphy of Dicellograptus complanatus graptolite zone from the Vaureal formation (Ashgillian), Anticosti Island, Quebec Canada. Palynology, 9: 165- 198.
Keegan, J. B., Rasul, S. M. and Shaheen, Y, 1990. Palynostratigraphy of the lower Paleozoic (Cambrian to Silurian) sediments of the Hashemite Kingdom of Jordan. Rev. Paleobot. Palynol., 66: 167- 180.
Le' Herisse, A., 1989. Acritarches et Kystes d'algues prasinophycees du Silurien de Gotland, suede. Paleontographica Italica, 76: 57-302.
Loeblich, A. R. Jr., 1970. Morphology, ultrastructure and distribution of Paleozoic acritarch. Proc. N. Amer. Paleontol Conv. Chicago 1969, part G: 705-788.
Loeblich, A. R. Jr. and Tappan, H., 1978. Some middle and late Ordovician microphytoplankton from Central North America. J. Paleontal., 52: 1233-1287.
McClure, H. A., 1988. Chitinozoan and acritarch assemblages, stratigraphy and biogeography of the early Paleozoic of northwest Arabia. Rev. Palaeobot. Palynol., 56 (1/2): 41-60.
Molyneux, S. G. and Paris, F., 1985. Late Ordovician Palynomorphs. J. micropalaeontology, 4 (1): 11-26. Powers, R. W., 1968. Lexique Stratigraphicque International, Volum III, Asie, Fasc. 10b 1, Arabie Saoudi. Setudehnia, A., 1976. The Paleozoic sequence at Zard- Kuh and Kuh-e- Dinar. Bull. Iran. petrol. Inst., 60: 16-33.
Smelror, M., 1987. Early Silurian acritarchs and prasinophycean algae from the Ringerike district, Oslo region (Norway). Rev. Palaeobot. Palynol., 52 (2/3): 137-159.
Szabo, F. and Kheradpir, A., 1978. Permian and Triassic stratigraphy, Zagros basin, southwest Iran. J. Petrol. Geol., 1(2): 57-8.
Turner, R. E., 1984. Acritarchs from the tye area of the Ordovician Caradoc Series, Shropshire, England. Palaeontographica, Abt. B. 190: 87-157.
Turner, R. E., 1985. Acritarchs from the type area of the Ordovician Llandeilo Series, south Wales. Palynology, 9: 211-235.
Vavrdova, M., 1988. Further acritarchs and terrestrial plant remains from the late Ordovician at Hlasna Treban, Czechoslovakia: Casopis, Pro. Mineralogii, a Geologii, 33: 1-10.
Vavrdova, M., 1989. New acritarchs and miospores from the late Ordovician of Hlasna Treban, Czechoslovakia. Casopis. Pro. Mineralogii, a Geologii 34: 403- 420.
Wellman, C. H. and Richardson, J. B., 1993. Terrestrial plant microfossils from Silurian Inliers of the Midland Valley of Scotland. Palaeontology, 36 (1): 155-193.
Scientific Quarterly Journal of Geosciences
Volume 4, Issue 14
Winter 1995, Vol. 4, No. 14
Spring 1995
Pages 2-21