Scientific Quarterly Journal of Geosciences

Scientific Quarterly Journal of Geosciences

Sarchahan Formation

Document Type : Original Research Paper

Author
Mohammad Ghavidel-Syooki
Exploration Division of National Iranian Oil Company, P. O. Box, 1065. Tehran/Iran
Abstract
The Paleozoic sequence has an outcrop at Kuh-e- Gahkum. The sequence has been sub- divided in ascending order into the Barut Formation, Silurian Shales, Faraghan and Dalan Formations (Szabo and Kheradpir, 1978). So far, the Silurian deposits have been used to be called as "Silurian Shales". In this study, the use of "Silurian Shales" is dropped and the Sarchahan Formation is introduced for the Silurian sediments in the Zagros basin. The name of Sarchahan Formation originates from the Sarchahan village where is located approximately 120 Km northern Bandar Abbas city. The type section of Sarchahan Formation is at Tang-e-Abzagh of Kuh-e- Gahkum where is located, approximately 10km northern Sarchahan village. This Formation is well-developed and accessible at Kuh-e-Gahkum and Kuh-e-Faraghan as well. The Sarchahan Formation is 102m and 66m thick, respectively at Kuh-e-Gahkum and Kuh-e- Faraghan. Likewise, this Formation is found with a thickness of 75m and 70m, respectively in exploration wells of Persian Gulf (G-3) and Zirreh No.1.
The Sarchahan Formation mainly consists of clastic sediments with subordinate limestone stringers. The lowermost beds of the Sarchahan Formation comprise of buff calcareous conglomerates interbedded with dark-gray shales and sandstones. Overlying beds of these conglomerates change into olive-gray shales with thin- interbeds of limestones and sandstones. The lower contact of Sarchahan Formation is disconformable with the Barut Formation at Kuh-e-Gahkum and is graditional with the Seyahou Formation at Kuh-e-Faraghan. The upper contact of this formation is disconformable with the Faraghan Formation at both Kuh-e-Gahkum and Kuh-e- Faraghan. The Sarchahan Formation contains brachiopod, bryozoa, graptolite, acritarch and chitinozoan species. Therefore, it has been deposited in a shallow marine environment. Some graptolite species such as Monograptus intermedius, Monograptus socialis, and Climacograptus scalaris have been recorded from the Sarchahan Formation at Kuh-e- Faraghan (Kherad pir and Nicol, 1973). Based on the above-mentioned graptolites, the lower Silurian has been suggested for the Sarchahan Formation. Moreover, all samples of Sarchahan and Faraghan Formations at Kuh-e- Gahkum were treated and studied for palynomorph taxa. A total of 54 palynomorph taxa were encountered in this study (Fig.2). These have been arranged in six local ascending zones; the zones I and II appear in the Sarchahan Formation and are characterized by presence of Visbysphaera pirifera, Visbysohaera microspinosa, Visbysphaera oligofurcata, Dictyotidium perlucidum, Dictyotidium faviformis, Ancyrochitina longicollis, Ancyrochitina longicornis, Clathrochitina sylvanica and so on (Fig.2). So far, the above- mentioned palynomorph species have been reported from lower Silurian sediments of Jordan (Keegan, Rasul, and Shaheen, 1990), Saudi Arabia (McClure, 1988), Sweden (Le Herisse, 1989) and U. S. A (Cramer and Diaz, 1972).
    The samples of Sarchahan Formation at Kuh-e- Faraghan were also treated and studied for palynomorph species. All samples contain well-preserved and abundant palynomorph taxa.
    At Kuh-e- Faraghan, the Sarchahan Formation is marked by presence of Dactylofusa estillis, Dactylofusa neaghae, Dactylofusa maranhensis and other diagnostic species. So far, the above-mentioned acritarch taxa have been recorded from the lower Silurian sediments of Jordan (Keegan, Rasul and Shaheen, 1990), Norway (Smelror, 1987), Sweden (Le Herisse, 1989), Saudi Arabia (McClure, 1988) and U. S. A (Cramer and Diez, 1972). Thus, based on diagnostic acritarch taxa, the Sarchahan Formation is related to lower Silurian at Kuh-e- Gahkum and Kuh-e- Faraghan as well, equating to age assignment of graptolite species. At Kuh-e- Gahkum, the Faraghan Formation has also been investigated for palynomorph species. All samples contain well-preserved and abundant miospores which have been arranged in zones III through VI (Fig.2).
    The zones III and IV occur in the lower part of Faraghan Formation and suggest the Givetian- Frasnian for the Zakeen Member of this Formation.
The zones V and VI appear in the upper part of Faraghan Formation and reveal lower Permian for the Chal-i-Sheh Member of this Formation.
     Based on palynological data, there are two hiata, respectively at the base and top of Sarchahan Formation. The first hiatus includes the Cambrian and Ordovician strata whereas the second encompasses the middle-upper Silurian and part of Devonian sediments in the Gahkum area. These hiata possibly coincide with the Caledonian Orogeny in the studied area. Likewise, there is a major hiatus within the Faraghan Formation and includes the Famennian and the whole Carboniferous deposits. This hiatus probably reveals the Hercynian Orogeny in the Zagros basin.
Keywords
Sarchahan Formation
Subjects
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Scientific Quarterly Journal of Geosciences
Volume 4, 15-16
Spring & Summer 1995, Vol. 4,No. 15-16
Summer 1995
Pages 74-89