Lower Miocene oyster shells from the Southeast Zagros Basin (Zade Mahmud area, Iran) and their paleoecology
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ORIGINAL PAPER
Lower Miocene oyster shells from the Southeast Zagros Basin (Zade Mahmud area, Iran) and their paleoecology Fatemeh Hosseinipour & Mohammad Dastanpour
Received: 29 August 2011 / Accepted: 25 November 2011 / Published online: 24 December 2011 # Saudi Society for Geosciences 2011
Abstract The Guri limestone Member of the Mishan Formation in the Zagros Basin consists of thick bedded limestone bearing benthic foraminifera and oyster shells. Seven species of Ostreidae and Gryphaeidae were identified as belonging to four genera (Crassostrea, Cubitostrea, Ostrea, and Hyotissa), i.e., Cubitostrea frondosa, Ostrea (Cubitostrea) dubertreti, Cubitostrea digitalina, Crassostrea gryphoides, Hyotissa virleti, Ostrea vesitata, and Ostrea plicatula. These fossils are reported for the first time from Iran. Miocene deposits in the studied area contain a rich benthic foraminiferal fauna dominated by Pseudotaberina, Meandropsina, Miogypsina, Flosculinella, Borelis, and other larger benthic foraminifera. We assign these sediments to a Burdigalian age based on Borelis melo curdica–Borelis melo melo Assemblage Zone. Paleoecological considerations also revealed that the beds were deposited in a nearshore non-agitated and shallow-water environment with moderate to low sedimentation rate. The studied oysters are located in one of more important paleogeographic settings in the world and very similar to many other Tethyan regions. The presence of these oysters suggests that the Zade Mahmud area located at margins of a sea way that connected the north and south of Zagros Basin during Burdigalian. Keywords Guri member . Oyster . Miocene . Paleoecology . Iran
Introduction The Cenozoic rocks of Zagros Basin have been extensively studied. These beds contain most of Iran’s recoverable oil F. Hosseinipour (*) : M. Dastanpour Department of Geology, Shahid Bahonar University of Kerman, Kerman, Iran e-mail: [email protected]
reserves (Hull and Warman 1970; Mc Quillan 1985). The Mishan Formation (Early to Middle Miocene) was named after the village of Mishan in Khuzestan Province by James and Wynd (1965). This formation at the type section along the Golden Staircase road on the southwest flank of the Gachsaran oilfeild consists of 710 m of limestone and marl. The basal (61 m) is composed of shelly “worm bed” limestone alternating with grey marls, and the upper (649.1 m) consist of mainly soft low-weathering grey marls alternating with bands of more resistant shelly limestones deposited under marine conditions along a northwest southeast linear trough. The development of these Marine conditions was less persistent. Toward Lurestan, southeast Iraq, Kuwait, and Saudi Arabia, the Agha Jari clastics gradually replace laterally the marls and limestone of this formation (Fig. 1). The northeastern depositional limit of the Mishan Formation is obscured because of erosion (Stocklin and Setudehnia 1991). In Khuzestsan, coastal and sub-coastal Fars, the Mishan overlies the Gachsaran Formation, and in the inland Fars, it conformably overlie
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