Middle Jurassic palynomorphs of the Dalichai Formation, central Alborz Ranges, northeastern Iran: Paleoecological inferences |
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| Authors: | Afsaneh Dehbozorgi Freshteh Sajjadi Hossein Hashemi |
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| Institution: | 1. Faculty of Geology, College of Science, University of Tehran, Enghelab Avenue, Tehran, 14155-6455, Iran
2. Department of Geology, Faculty of Science and Research School of Applied Sciences, Kharazmi University, No. 49, Mofatteh Avenue, Tehran, 15614, Iran
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| Abstract: | Palynological data are used to draw paleoecological inferences for the Dalichai Formation, northeast of Semnan, northeastern Iran. All samples examined yield well-preserved, diverse palynofloras consisting predominantly of miospores; dinoflagellate cysts, foraminiferal test linings, and fungal spores occur as minor constituents. Fifty-four species of spores (37 genera), 18 species of pollen (9 genera), and 16 species of dinoflagellate cysts (13 genera) are recognized. Co-occurrence of such miospore species with known vertical ranges as Klukisporites variegatus, Sellaspora asperata, Murospora florida, and Concavissimisporites verrucosus suggests a Middle Jurassic (Bajocian-Callovian) age for the host strata, thus corroborating the available paleozoological evidence. Such spore species as Klukisporites, Cyathidites, and Dictyophyllidites known to have been produced by Pterophyta dominate the palynofloras. Thus the contemporaneous coastal vegetation was characterized by predominance of Pterophyta whereas representatives of Ginkgophyta, Pteridospermophyta, Lycophyta, Coniferophyta, and Bryophyta were rarely represented. This reconstruction implies that a moist, warm climate prevailed in northeastern Iran during the Middle Jurassic (Bajocian-Callovian). This is confirmed by occurrence of fungal spores accompanied by such warm water dinoflagellate cysts as Mendicodinium groenlandicum, Pareodinia ceratophora, and Gonyaulacysta jurassica. Abundance of Amorphous Organic Matter (AOM) signifies a shallow, low-energy, dysoxic-anoxic depositional site for the host strata. Furthermore, the ratio of AOM to marine palynomorphs as well as abundance of blade-shaped to eqiudimensional opaque palynomacerals could indicate low sedimentation rate in a shallow, low-oxygenated marine environment. Additionally, occurrence of chorate dinoflagellate cysts (e.g., Adnatosphaeridium caulleryi) and acritarchs bearing relatively long processes (e.g., Micrhystridium) possibly suggests deepening upward in the study section. |
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