پالینوستراتیگرافی، آب و هوا و محیط دیرینه سازند دلیچای در برش چینه‌شناسی بشم (البرز شرقی)

توالی کاملی از سازند دلیچای در برش چینه‌شناسی بشم در البرز شرقی مورد مطالعه قرار گرفت. این مجموعه حاوی پالینومورف‌ها‌ی متنوع با حفظ‌شدگی نسبتاً خوب بوده که سیست‌های داینوفلاژله و میوسپورها (اسپور و پولن‌ها) از اجزای غالب آن است. اسپور قارچ و اکریتارک‌ها با فراوانی کمتر، از دیگر اجزای این پالینوفلورا هستند. نهشته‌های ژوراسیک میانی در این برش متشکل از مارن، سنگ آهک رسی و مقدار کمی از میان لایه‌های سنگ آهک چرتی است. در گردنه بشم سازند دلیچای به طور ناپیوسته روی سازند شمشک قرار گرفته و به طور پیوسته و هم‌شیب توسط سنگ آهک‌های ضخیم سازند لار پوشیده شده است. بر اساس گسترش زمانی داینوفلاژله‌های کلیدی سه پالینوزون cribroperidinium crispum total range biozone (باژوسین پسین)، dichadogonyaulax sellwoodii interval biozone (باتونین کالووین پیشین)، ctenidodinium continuum interval biozone (کالووین پیشین میانی) معرّفی شد. به علاوه با توجه به حضور داینوسیست‌های شاخص مانند mendicodinium groenlandicum و systematophora areolata در قسمت‌های بالایی برش مطالعه شده سن سازند دلیچای در این برش چینه‌شناسی، باژوسین پسین کالووین پسین تشخیص داده شده است که با سن تعیین شده توسط آمونیت‌ها در همین برش مطابقت دارد. پالینوزون‌های ارائه شده نیز، با پالینوزون‌های ژوراسیک میانی در شمال‌غربی اروپا و مناطق همجوار آن انطباق دارد. داده‌های پالینولوژی حکایت از غلبه یک محیط دریایی نسبتاً کم‌عمق و کم‌اکسیژن دارد که در فواصلی از پایین به سمت بالای برش چینه‌شناسی با افزایش نسبی در عمق حوضه همراه بوده است. به علاوه بر اساس داده‌های موجود، آب و هوایی معتدل تا گرم برای سازند دلیچای در برش چینه‌شناسی بشم پیشنهاد شد.

Palynostratigraphy, paleoclimate, and paleoenvironment of the Dalichai Formation in the Bashm stratigraphic section (Eastern Alborz)

IntroductionThe Middle Jurassic strata (Dalichai Formation) is widespread across the Alborz Range. It is chiefly characterized by alternating shale beds, bluishgrey thin to medium bedded limestone, and marl including ammonites, belemnites, and sponge (Steiger, 1966). Various mega and microfossil faunas are recorded in the formation, including fairly rich ammonites, spores, pollen grains, dinoflagellate cysts, belemnites, bivalves, brachiopods, echinoderms, sponges, bryozoans, and foraminifera. The 212mthick Dalichai Formation in the Bashm stratigraphic section, Eastern Alborz, mainly consists of marl, argillaceous limestone, and a few intercalations of cherty limestone. With distinct disconformity, it succeeds the Upper TriassicLower Middle Jurassic Shemshak Formation and gradationally underlies the thickbedded limestone of the Upper Jurassic Lar Formation. This study aimed to make and correlate a palynostratigraphic framework for the studied interval and interpret the palynofacies and paleoclimate of the Dalichai Formation based on palynological data at the Bashm stratigraphic section, Eastern Alborz, northern Iran. Materials and MethodsIn the studied section, twentyfour samples were collected for palynological analysis. All samples were prepared following standard palynological processing procedures (e.g., Phipps Playford, 1984), comprising treatment with hydrochloric (1050%) and hydrofluoric (ca. 40%) acids for dissolution of carbonates and silicates, respectively. Thereafter, hot 50% HCl was used to dissolve silicagel formed during the HFsilicate reaction. Then the solution was neutralized and centrifuged in ZnCl2 (specific gravity 1.9) for density separation. Neither oxidative nor alkali treatments were applied. Residues were sieved through a 20μm mesh and strew mounted on glass slides using liquid Canada balsam. All the samples, slides, and figured specimens were stored at the Laboratory of Paleontology of the Department of Geology, Faculty of Science, Imam Khomeini International University, Qazvin, Iran. DiscussionThe 24 samples examined yield wellpreserved, diverse palynofloras consisting predominantly of dinoflagellate cysts and miospores (spores and pollen grains). Fungal spores and acritarchs occur as minor constituents. The presence of some index species of dinoflagellate cysts leads to the identification of three of the Northwestern European biozones (Woollam Riding, 1983; Riding Thomas, 1992; Poulsen Riding, 2003), i.e., Cribroperidinium crispum Total Range Biozone (late Bajocian), Dichadogonyaulax sellwoodii Interval Biozone (Bathonian to early Callovian) and Ctenidodinium continuum Interval Biozone (early to middle Callovian). Moreover, the presence of such dinoflagellate species as Mendicodinium groenlandicum and Systematophora areolata has been used to attribute a late Callovian age to the upper part of this studied section. Therefore, the late Bajocian late Callovian age is collectively attributed to the studied section. This age assignment based on palynological data is consistent with the age presented by the ammonites (Hossein Zadeh, 2011) in this stratigraphic section. The erected palynozones correspond largely to those established in Northwestern Europe and reveal the marine connection between North of Iran with Northwest Europe and the Northwestern Tethys during the late Bajocian to late Callovian ages. Quantitative analyses of the palynological data indicate the dominance of a relatively shallow, lowoxygenated marine environment with a relative increase in depth at some intervals upward in the studied section. Such spore species as Klukisporites, Cyathidites, Dictyophyllidites, and Gleicheniidites known to have been produced by Pterophyta dominate the palynofloras. Thus, the contemporaneous coastal vegetation was characterized by the predominance of Pterophyta and minor components of Ginkgophyta, Pteridospermophyta, Coniferophyta, Lycophyta, and Cycadophyta. Moreover, this reconstruction accompanied by the dominance of warm and intermediate dinoflagellate cysts collectively indicates a mild to warm climate for the Dalichai Formation in the studied section during the Middle Jurassic (late Bajocian late Callovian). Indeed, according to the paleogeographic maps, in the Middle Jurassic (Middle Callovian), the Alborz Mountains of northern Iran occupied lower latitudes between 20 to 30 degrees north (Thierry, 2000; SeyedEmami et al., 2008; Wilmsen et al., 2009, 2010). This position confirms this paleoecology for the Dalichai Formation. ConclusionAll samples examined yield wellpreserved, diverse dinoflagellate cyst assemblages. 26 identified dinoflagellate cyst species led to the identification of three biozones: Cribroperidinium crispum (late Bajocian), the Dichadogonyaulax sellwoodii (Bathonian to early Callovian), and Ctenidodinium continuum (early to late Callovian). Due to such dinoflagellate species as Mendicodinium groenlandicum and Systematophora areolata, a late Callovian age is attributed to the upper part of the studied section. This age assignment is consistent with the age presented by the ammonites in this stratigraphic section. Palynological data designate the dominance of a relatively shallow marine environment with a relative increase in depth at some intervals upward the studied section. The abundance of intermediate and warm water dinoflagellate cysts and ferns in the surrounding area of the Dalichai Formation depositional setting and also fungal spores collectively indicate a mild to warm climate for the Dalichai Formation in the studies section. ReferencesHossein Zadeh, K., 2011. Ammonitesbased Biostratigraphy of the Dalichai Formation in Tuye Darvar (west of Damghan), eastern Alborz. M.Sc. thesis, Payame Noor University of Tehran, 125p. (In Persian).Phipps, D., Playford, G., 1984. Laboratory techniques for extraction of palynomorphs from sediments. Department of Geology. University of Queensland. Papers, 11(1): 123.Poulsen, N.E., Riding, J.B., 2003. The Jurassic dinoflagellate cyst zonation of Subboreal Northwest Europe. In: Ineson, J.R., Surlyk, F., (eds.), The Jurassic of Denmark and Greenland. Geological Survey of Denmark and Greenland Bulletin, 1: 115144.Riding, J.B., Thomas J.E., 1992. Dinoflagellate cysts of the Jurassic System. 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