Palynostratigraphy and Palaeoecology of the Shishtu Formation, Kuh-e-Tir, northwestern Kerman

The triangular-shaped central Iran Microcontinent, being located almost in the middle of Iran, is bounded on the north and south by the Alborz Ranges and Makran Zone, respectively. During Silurian and Devonian, parts of the Iran Plate including Alborz, central Iran, and Sanandaj-Sirjan as well as Afghanistan and Turkey joined Saudi Arabia and Africa, forming the southern shorelines of Palaeotethys along the northwestern margin of Gondwana. The Late Ordovician-Late Devonian palaeogeographic maps of northern Saudi Arabia allude to the existence of a vast stable shelf along the northern margin of the Gondwana, i.e., northern Africa and Saudi Arabia (Berberian and King 1981; Husseini 1991; Sharland et al. 2001; Ruban et al. 2007; Al-Juboury and Al-Hadidy 2009; Torsvik and Cocks 2009).
The Upper Palaeozoic strata of northern Kerman comprise diverse marginal marine sediments embracing, inter alia, the Shishtu Formation. Well-preserved, diverse palynofloras including marine microphytoplanktons (acritarchs and prasinophytes), spores, scolecodonts, and chitinozoans are retrieved from the latter and its correlatives in the Zagros Mountains and central Alborz Ranges (Hashemi & Playford 1998; Ghavidel-Syooki 198; Hashemi & Fahimi 2006; Hashemi & Tabea 2006).
Material and This study is based on palynological analysis of 20 surface rock samples collected from palynologically promising levels of the Shishtu Formation, Kuh-e-Tir as cropped out in northwest of Kerman; i.e., grey-black shales and fine-grained sandstones therein. To prevent/reduce the risk of contamination and/or palynomorphs oxidation, samples collected from ca. 20-50 cm below the surface (Wood et al. 1996). Conventional laboratory methods (e.g., Barss and Williams 1973; Phipps and Playford 1984; Wood et al. 1996) used to retrieve and to concentrate palynomorphs from the host strata. Three strew-slides made from each residue used for the light microscopy that was undertaken at the Department of Geology, Faculty of Earth Sciences, Kharazmi University, Tehran, employing a Zeiss Axioplan 2 microscope equipped with a digital Canon Power Shot G5 camera.
Discussion of Results and Relatively diverse, poor-moderately preserved palynofloras comprising of marine microphytoplanktons acritarchs (8 species distributed among 3 genera), prasinophytes (3 species assigned to 2 genera), spores (52 species attributed to 21 genera), scolecodonts, and chitinozoans occur in surface samples of the Shishtu Formation, Kuh-e-Tir, northwestern Kerman. Of the microphytoplanktons, Lophosphaeridium and Leiosphaeridia are relatively more abundant. The spores are mostly radiosymmetrical and trilete; bilateral, monolete forms such as Laevigatosporites rarely occur. Acavate taxa, namely, Leiotriletes, Punctatisporites, Apiculatasporites, and Planisporites, such cavate forms as Vallatisporites and Grandispora, and the distinctive cingulate taxon, Densosporites, dominate the land-derived palynofloras.
Stratigraphic distribution pattern of the spores allows for the introduction of two informal successive assemblage biozones, viz., assemblage biozones a and b in the Shishtu Formation at the section investigated. The former can be identified with the co-occurrence of such distinct species as Cristatisporites triangulatus, Geminospora lemurata , Punctatisporites stabilis Grandispora inculta, Apiculatasporites sp. cf. A. perpusilus, Grandispora sp. cf. G. eximia, Grandispora sp. cf. G. artusa. It contains such key species of the north American early Late Devonian (Frasnian) as optivus-triangulatus, lemurata-magnificus, ovalis- bulliferus Assemblage Zone (Richardson and McGregor 1986). Characterized by the association of Retispora lepidophyta, Geminospora punctata, Densosporites spitsbergensis, Leiotriletes ornatus, Amicosporites jonkeri, Apiculatasporites adavalensis, Densosporites cassiformis, the assemblage biozone b comprises characteristic taxa of the late Late Devonian (Famennian) pussilites-lepidophyta Assemblage Zone of the previously mentioned biozonation scheme. The absence in the material examined of such distinct marine microphytoplanktons as Unellium lunatum, Unellium piriform, Cymatiosphaera perimembrana, Daillydium pentaster, Chomotriletes vedugensis is noteworthy as they have already been recorded from ±co-eval strata from elsewhere in Iran. A brachiopod referred to as Strophomenid, is the sole palaeontological evidence yet reported from the host strata based on which the rock unit dated as late Famennian-early Tournaisian (Wendt et al., 2002).
In the material investigated terrestrial palynomorphs (spores) dominate the marine microphytoplanktons in terms of both abundance and diversity. The acritarchs and prasinophytes encountered are generally with ±circular vesicle, relatively thin eilyma bearing fairly short sculptural elements or being virtually psilate; such complexion indicates accumulation of the host strata in a shallow marginal marine setting. Spore-parent plant relationships testify to the presence of a contemporaneous coastal vegetation dominated by Progymnospermopsida, Lycopsida, Filicopsida, Equisetopsida; apparently these were accompanied by less abundant representatives of Barinophytopsida, Zoesterophyllopsida, Rhyniopsida, Ginkgopsida, Cycadopsida.