ماکروفسیل‌های گیاهی سازند شمشک در مقطع چینه‌شناسی رودبار، البرز غربی

مجموعه‌ای از ماکروفسیل‌های گیاهی با حفظ‌شدگی بسیار خوب، در دو افق 1 (منطقۀ کوشک) و افق 2 (شمال رودبار)، از نهشته‌های سازند شمشک در مقطع چینه‌شناسی رودبار (البرز غربی) وجود دارد. در اجتماع گیاهی مذکور، 32 گونه (متعلق به 22 جنس) از ماکروفسیل‌های گیاهی و تعدادی چوب فسیل (fossil wood) شناسایی شده است. نمونه‌های افق 1، در تناوب شیل و سیلتستون، به تریاس پسین (رتین)؟ ژوراسیک پیشین نسبت داده شد و با زون گیاه‌دار شمشک اشتر منطقۀ شمشک، زون گیاهی پایینی درۀ نور و زون گیاه‌دار a البرز مرکزی مقایسه می‌شود. مجموعۀ گیاهان مذکور، احتمالاً نشان‌دهندۀ جلگه‌ای با پوشش گیاهی، شامل سیکادوفیت‌ها، مخروطیان، سرخس‌ها و اسفنوفیت‌هاست. ماکروفسیل‌های گیاهی افق 2 نیز که در تناوبی از ماسه‌سنگ سیلتستون و میان لایه‌هایی از زغال‌سنگ دیده می‌شود، مبین سن ژوراسیک پیشین؟ میانی (آالنین) و هم‌ارز زون‌های گیاهی شمشکاشتر تا حد زیرین زون گیاهی وسک گاه ناحیۀ شمشک، زون گیاه‌دار a تا بخشی از زون c البرز مرکزی و زون‌های گیاهی ii, iii البرز شرقی در نظر گرفته می‌شود. فرم‌های گیاهی افق 2 معرف پوشش گیاه، شامل مخروطیان، سرخس‌ها، سیکادوفیت‌ها، ژینکوفیت‌ها و اسفنوفیت‌ها در یک دشت با آب‌وهوای گرم معتدل است. در فلورای ژوراسیک منطقۀ رودبار cladophlebis roesserti, otozamites parallelus, ginkgoites yimaensis دیده شد که برای اولین بار از ایران گزارش می‌شود.

Plant macrofossils from the Shemshak Formation, Rudbar, western Alborz

AbstractPlant macrofossils from the Shemshak Formation, Rudbar, western Alborz, collected at two distinct horizons, including horizon 1 (Koushk area) and horizon 2 (northern Rudbar), are examined to determine the relative age and sedimentary environments of the host strata. The assemblage includes 32 species (distributed among 22 genera) of plant macrofossil; variably preserved petrified wood could also be found in some of the studied samples. The Horizon 1 consists of siltstones and shales. Horizon 1 consists of siltstones and shales is dated to the Late Triassic (Rhaetian?)–Early Jurassic based on the known stratigraphic ranges of various taxa.  It is comparable with the Shemshak–Ashtar zones in the Shemshak area, the lower floral zone of Noor valley, and floral zone A of central Alborz. Representatives of cycadophytas, coniferophytes, pteridophytes, and sphenophytes dominate the assemblages examined. The contemporaneous coastal vegetation apparently flourished in a vast coastal plain, under a warm climate. Horizon 2 comprises alternation of sandstones, siltstones and shales with few coal seams includes coniferophytes, cycadophytes, ginkgophytes, sphenophytes, and filicophytes. This assemblage suggests the Liassic–Middle Jurassic (Aalenian)? age that is comparable with the Shemshak–Ashtar zones, up to the lower part of Vasekgah of Shemshak, floral zone A–C (partial) of central Alborz, and plant zones II–III of eastern Alborz. The sediments that host plant macrofossils of the horizon 2 seemingly accumulated in river systems and probably under temperatewarm climate with variable humidity of this plant assemblage, Cladophlebis roesserti, Otozamites parallelus, Ginkgoites yimaensis are recorded for the first time from Iran.Keywords: Plant macrofossil, Shemshak Formation, Late Triassic (Rhaetian), Early Jurassic, Western Alborz.   IntroductionAs a result of the Early Cimmerian tectonic event, Iran is divided into two geologically independent zones; one in the northeast (comprising central Iran, northern and northeastern Iran) and the other in the southwest (Zagros area) of the country. These two distinct sedimentarystructural zones include the Jurassic strata of notably variable lithofacies and biofacies complexion. The Jurassic deposits in the former zone are represented by a single sedimentary cycle known as the Shemshak Group (a.k.a. Shemshak Formation) bounded by two unconformities. The rock unit and its equivalents in the Alborz Ranges and central Iran (especially around Kerman and Tabas) include coal layers/seams, thus extensively investigated by both geologists and paleobotanists. The study of plant macrofossils not only affords information about the past plant communities, their temporal/spatial distribution and their evolution but also allows for the palaeoecological and palaeoclimatological inferences.Notably, thick exposures of the Shemshak Formation widely distributed in the vicinity of Rudbar (36˚32 ́ 37˚7 ́N, 49˚ 11 ́50˚ 05 ́), south of Gilan Province (western Alborz), comprise mainly alternation of dark green sandy, locally coalbearing black shales, graygreen siltstones and sandstones, and thin intercalations of limestones (Aghanabati 1998). The upper and lower contacts of the rock unit at this locality are both faulted against the Eocene–Quaternary and Triassic strata, respectively. The rich, diverse, and reasonably wellpreserved plant macrofossils sporadically occurring in the Shemshak Formation especially at two distinctive horizons; horizon 1 (Koushk, south of Rudbar) and horizon 2 (north of Rudbar) are examined to derive biostratigraphical and palaeoecological inferences. Material & MethodsThe plant macrofossils elaborated upon herein, 320 rock samples, are collected from outcrops of the Shemshak Formation around Rudbar, western Alborz. The specimens occur as reasonably preserved impressions on bedding surfaces of shales and finegrained sandstones. All of the samples collected and used in the current study are housed in the Palaeontology Collection, Kharazmi University, Tehran, Iran. Discussion of Results & ConclusionsHorizon 1 (Koushk area), consisting of siltstones and shales, includes plant remains representing sphenophytes, filicophytes, cycadophytes and coniferophytes. The most diverse taxa in this assemblage, in an descending order, are ferns, conifers, sphenophytes, and cycadophytes. Based on the known stratigraphic ranges of such various species as Ctenis, Clathropteris, Equisetites, Neocalamites, a Late Triassic (Rhaetian?) –Early Jurassic age is suggested for the host strata. The latter accumulated in a vast coastal plain under a warm, moist climate with adjacent highlands drained by rivers. The plant macrofossils occurring in horizon 1 generally seem comparable with those retrieved from the Kalariz (partial) and Javaherdeh formations of northern Alborz.Horizon 2, occurring in the Shemshak Formation of northern Rudbar, comprises alternation of sandstones, siltstones, shales as well as few coal seams and limestones intercalations. The horizon includes various representatives of sphenophytes, filicophytes, cycadophytes, coniferophytes, ginkgophytes, and petrified wood specimens. The plant macrofossils collected from this interval, Cladophlebis roesserti, Otozamites parallelus, and Ginkgoites yimaensis are reported for the first time from Iran. Horizon 2 can be divided into three units (A, B, C) based on the relative abundance of the plant macrofossils. Unit A is characterized by, in ascending order of abundance, Marattiopsis, Cladophlebis, and Podozamites. In unit B, preceding unit C, Podozamites, Cladophlebis, Nilssonia, Pterophyllum, Ginkgoites, Phlebopteris, Anomozamites, Otozamites, Baiera, and Pagiophyllum occur in an ascending order of profusion. Unit C is differentiated from unit B by the exclusive occurrence, in the former, of Annulariopsis and Coniopteris. The plant macrofossil assemblages of horizon 2 indicate an Early–Middle Jurassic (Aalenian)? age for the host strata. This horizon seems comparable with the plant fossil assemblages previously reported from the Javaherdeh Formation of northern Alborz, Shemshak–Ashtar zones, the lower part of Vasekgah of Shemshak, Floral zone A–C (partial) of central Alborz,  and Plant zones II, III from eastern Alborz. The host strata of horizon 2 seemingly accumulated in a sizeable river system and probably under temperatewarm climate and variable humidity.