Biostratigraphy and palaeogeography of Late Ordovician and Early Silurian chitinozoans from the Zagros Basin, Southern Iran

Investigation of 200 surface samples from the Kuh-e-Faraghun area of the Zagros Basin has permitted recognition of 42 chitinozoan taxa, upon which six successive assemblage zones are based. Assemblage zones I-III occur in the Late Ordovician Seyahou Formation, and IV-VI occur in the Early Silurian Sarchahan Formation. The chitinozoans Plectochitina persica sp. nov. and Plectochitina khosravii sp. nov. are described from the Sarchahan Formation. A hiatus, encompassing the Middle and Late Silurian and possibly corresponding to the Caledonian Orogeny, is recognized between the Sarchahan Formation and the Faraghan Formation (Devonian).

Diverse chitinozoan and acritarch assemblages suggest a marine environment for the two formations. Comparison of chitinozoans from the Zagros Basin with those from other parts of the world indicates that the Zagros Basin was part of North Gondwanaland during the Late Ordovician and Early Silurian.

The chitinozoan material reported and illustrated in this paper was isolated from shale, siltstone, and sandstone by standard palynological procedures. This included treatment of the residues from each sample with 30 ml of saturated zinc bromide to separate the organic residues from the inorganic materials, and screening of the organic residues through 20-micron nylon mesh sieves. Three microscope slides were prepared from each sample for palynological investigation. Slides used in this study are on file in the palaeontological section of the Exploration Division of the National Iranian Oil Company under the sample numbers MEK-115 to MEK-345. ​