Palynology and Palaeoecology of the Zakeen Formation, Kish Gasfield, northern Persian Gulf

• Main Authors: Hossein Hashemi, Nima Nezam Vafa
• Format: Article
• Language: fas
• Published: University of Isfahan 2014-11-01
• Series: Journal of Stratigraphy and Sedimentology Researches
• Subjects: Palynology; Palaeoecology; Late Devonian; Zakeen Formation
• Online Access: http://uijs.ui.ac.ir/jssr/browse.php?a_id=360&slc_lang=en&sid=1&ftxt=1

Introduction During Middle Devonian and contemporaneous with the global rise of sea-level, the westward transgression commenced from north and northeastern Iran and culminated in early Late Devonian. Accordingly, in many parts of Iran the Devonian strata disconformably succeed older rocks. The Devonian rocks of Zakeen Formation at the subsurface Kish #2, Kish Gasfield, 53°56' E and 26°30' N, nearly 3 km southwest of Kish Island, northern Persian Gulf, consisting ca. 98 m of argillaceous sandstones and lithic sandstones, precede the Faraghan Formation (Permian). In Kish #2, the contact between the Zakeen and Faraghan formations is defined on the first introduction of mono- and bisaccate pollen occurring in the latter. Lower boundary of the former in subsurface section studied, however, is unknown as the drilling terminated at the depth of 4408 m, i.e., somewhere in the Zakeen Formation. Material & Methods Ten rock samples collected from the Zakeen Formation at the subsurface Kish #2 are processed for palynomorphs. Reasonably diverse, variably preserved palynofloras occur therein especially in samples collected from 4312-4408 m interval. The palynofloras include both marine and terrestrial elements; viz., prasinophytes, acritarchs, monolete and trilete spores, cryptospores, chitinozoans, scolecodonts, and foraminiferal test linings; the latter occur only as caving palynomorphs from the overlying Faraghan Formation. Discussion of Results & Conclusions In the samples studied diversity and relative abundance of marine palynomorphs is less than that of spores. In other words, 40 species of spores (distributed among 26 genera) and 11 species of microphytoplankton cysts (assigned to 7 genera) are identified. Additionally, scolecodonts are relatively abundant while chitinozoans rarely occur. Furthermore, foraminiferal test linings occurring in the palynofloras examined are believed to be caved palynomorphs coming from the overlying Faraghan Formation. Stratigraphic distribution of spores allows for identification within the Zakeen Formation of an informal biozone, here termed assemblage zone A. it is characterized with the co-occurrence of Retusotriletes distinictus, R. rugulatus, R. triangulatus, Verrucosisporites polygonalis, Dictyotriletes submarginatus, Emphanisporites rotatus, Emphanizonosporites sp. cf. E. radiatus, Stenozonotriletes minus, Cristatisporites triangulatus, Densosporites rotatus, Ancyrospora ancyrea, A. langii, Auroraspora macra, Geminospora lemurata, Grandispora sp. cf. G. owensii, G. cornuta, G. inculta, Laevigatosporites ovatus, Quadrisporites granulatus. The associated microphytoplankton cysts include Chomotriletes vedugensis, Gorgonisphaeridium sp. cf. G. tabasense, Lophosphaeridium granulosum, Multiplicisphaeridium ramispinosum, Veryhachium trispinosum. It is noteworthy that some index species of some northern hemisphere Devonian biozones including polygonalis-emsiensis, velatus-langii, lemurata-magnificus, flexus-cornuta Assemblage Zones (Richardson & McGregor 1986) also occur in the material investigated. Szabo & Kheradpir (1978) initially attributed the Zakeen Formation to Early Permian (Asilian-Kungarian). However, Ghavidel-Syooki’s (1988, 1998, 2003) and Amadeh’s (2012) palynological examinations favored an Early-early Late Devonian and an early Late Devonian age, respectively. Such stratigraphically important microphytoplankton species as Chomotriletes vedugensis and spores as Cristatisporites triangulatus, Retusotriletes rugulatus, R. distinctus, R. triangulatus, Geminospora lemurata, Ancyrospora ancyrea, A. langii, Grandispora cornuta, G. inculta, Verrucosisporites polygonalis, Emphanisporites rotatus occur in the material examined. Based on the presence of such distinctive taxa and taking into account the absence of Retispora lepidophyta (Kedo) Playford, 1976, globally occurring at the Devonian-Carboniferous boundary, suggest an early Late Devonian age for the Zakeen Formation investigated. The Zakeen palynofloras closely resembles, at the species-level those ±co-eval reported from the Zagros and Alborz Ranges. Beyond Iran, the Zakeen palynofloras share 12 and 11 species with those of Saudi Arabia and Australia, respectively. This might well indicate their palaeoproximity during Late Devonian. As previously indicated, land-derived palynomorphs are far more diverse and abundant than marine elements, acritarchs (sensu lato) and scolecodonts rarely occurring in the assemblages studied. The former include spherical forms bearing both short and long, solid processes. Such diverse palynological composition suggest accumulation of the host strata in a nearshore, marginal marine setting. Reconnaissance of spore-plant relationship of dispersed spores of the Zakeen palynofloras indicates that the contemporaneous surrounding vegetation was dominated by Zosterophyllopsida, Filicopsida, Progymnospermopsida, Equisetopsida, Lycopsida, Rhyniopsida, Barinophytopsida, Cycadopsida, Ginkgoopsida, and Sphenopsida.