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The upper Shemshak Formation (Toarcian–Aalenian) of the Eastern Alborz (Iran): Biota and palaeoenvironments during a transgressive–regressive cycle

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Abstract

The siliciclastic, up to 4,000 m thick Upper Triassic–Bajocian Shemshak Formation is widespread across the Iran Plate, especially in the Alborz Mountains of northern Iran. In contrast to its lower, generally non-marine part, the upper part is marine. Based on the Tazareh section of the eastern Alborz, an integrated analysis of this marine interval is presented. The 1,700 m thick marine sedimentary succession records a gradual deepening from inner to mid and outer shelf environments from the Middle Toarcian to early Late Aalenian. During the Late Aalenian–Early Bajocian, the trend was reversed and infilling of the basin by a large delta system occurred. This general facies development reflects a nearly symmetrical transgressive–regressive (T–R) megacycle, terminated by the inter-regional mid-Cimmerian tectonic event. A renewed transgression in the early Late Bajocian initiated a subsequent sedimentary megacycle. The bioturbated mid and outer shelf sediments contain a low to moderately diverse benthic fauna dominated by deep burrowing bivalves, often preserved in the growth position. A hierarchy of four orders of sedimentary cycles can be recognized (parasequences, parasequence sets, unconformity-bounded third-order depositional sequences, and the 13 ma long second-order T–R megacycle). A regional correlation with the Jajarm area (200 km to the east) shows a very similar temporal facies pattern of the upper Shemshak Formation. The eastern Alborz T–R cycle is completely out-of-phase with other (eustatic) sea-level curves, suggesting regional tectonic control. Rough estimates of subsidence rates give an average value of 126 m/ma. However, much higher values for the Aalenian (230 m/ma), particularly the Late Aalenian (700 m/ma), indicate a distinct increase in subsidence rate towards the Early Bajocian mid-Cimmerian tectonic event. These high subsidence rates suggest that the sediments of the Shemshak Formation of the eastern Alborz formed in a (young) rift basin.

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Acknowledgements

The research has been carried out within the framework of the Middle East Basin Evolution programme (MEBE) and within the institutional partnership of the Faculty of Engineering, Tehran University, and the Institute of Palaeontology, University of Würzburg, sponsored by the Alexander von Humboldt Foundation. We would like to thank the University of Tehran (Center of Intelligence) and acknowledge the logistic support by the Geological Survey of Iran and the hospitality of Eng. Sedighifar, head of the East Alborz Coal Company. J. Taheri, GSI Mashad, and M. R. Sheikholeslami, GSI Tehran, assisted us in the field. Helpful reviews by M. Aberhan (Berlin) and W. Piller (Graz) are gratefully acknowledged. Last but not the least we thank M.F. Brunet (Paris) for helpful discussions

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Authors and Affiliations

  1. Institut für Paläontologie der Universität, Pleicherwall 1, D-97070, Würzburg, Germany

    Franz T. Fürsich & Markus Wilmsen

  2. Faculty of Engineering, Tehran University, P.O. Box 11365-4563, Tehran, Iran

    Kazem Seyed-Emami

  3. CNRS-UMR 5143 “Paléobiodiversité et Paléoenvironnements”, Tour 56-46, 5ème étage, case 104, Université Pierre et Marie Curie, 4 place Jussieu, 75252, Paris cedex 05, France

    Fabrizio Cecca

  4. Geological Survey of Iran, P.O. Box 131851-1494, Tehran, Iran

    Mahmoud R. Majidifard

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  1. Franz T. Fürsich
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Correspondence to Franz T. Fürsich.

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Fürsich, F.T., Wilmsen, M., Seyed-Emami, K. et al. The upper Shemshak Formation (Toarcian–Aalenian) of the Eastern Alborz (Iran): Biota and palaeoenvironments during a transgressive–regressive cycle. Facies 51, 365–384 (2005). https://doi.org/10.1007/s10347-005-0051-z

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