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1 Majdifarad M.

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Biostratigraphy of the Middle and Upper Jurassic of northeastern IranMajdifarad, M.

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Majdifarad M.

tom Vol. 4, no. 1

182-183

The Middle and Upper Jurassic sedimentary successions of Alborz in northern Iran and Koppeh Dagh in northeastern Iran comprise five formations: Dalichai, Lar (Alborz), Kashaf Rud, Chaman Bid, and Mozduran (Koppeh Dagh). They constitute a more or less continuous sequence, being confined by two tectonic events, one at the base, in the uppermost part of the Shemshak Formation (Bajocian), the so-called Mid-Cimmerian Event, the other one at the top (Early Cretaceous), the so-called Late-Cimmerian Event. Altogether, 84 species of ammonites from the Dalichai and Chaman Bid formations belonging to 30 genera and 16 families are described. Among the taxa Phylloceratidae are most abundant, followed by Ataxioceratidae, Perisphinctidae, and Cardioceratidae. Pachyceratidae are the least common. The ammonite fauna is of low diversity and is concentrated in several levels. Some of the ammonite genera and species are recorded from Iran for the first time. These include Pachyceras lalandei, Cardioceras praecordatum, Microbajocisphinctes sp., Geyssantia geyssanti, Larcheria schilli, Passendorferia sp., Sequeirosia sp., Phanerostephanus subsenex, Nothostephanus sp., Nannostephanus cf. subcomutus, Parawedekindia callomoni, Physodoceras sp., Extrenodites sp. Biostratigraphically, thirty ammonite zones have been recognized for the Middle and Upper Jurassic successions at the study area. Based on ammonites, the Dalichai and Chaman Bid formations range from the Upper Bajocian to Lower Tithonian, and based on microfossil the Lar Formation ranges from the Tithonian to (?)Neocomian, and the Mozduran Formation from the Kimmeridgian to (?)Neocomian.