بازسازی محیط رسوبی و نوسانات نسبی سطح آب دریا در نهشته‌های کرتاسه پالئوسن، البرز مرکزی، شمال ایران

نهشته‌های کرتاسه پالئوسن که در پهنه شمالی مرکزی البرز رخنمون گسترده‌ ای دارند، در 45 کیلومتری جنوب ساری از سنگ آهک‌ها و ماسه‌سنگ‌ ‌ها به همراه سیلت سنگ ‌ها و تبخیری ‌ها تشکیل شده‌ اند. ضخامت این توالی در این ناحیه 235 متر است. به منظور بازسازی محیط رسوبی و نوسانات نسبی سطح آب دریا در زمان نهشته شدن این رسوبات، تعداد 70 نمونه سنگی به روش سیستماتیک برداشت شد. مرز کرتاسه پالئوسن در این برش با استفاده از مطالعات دیرینه‌شناسی روزن‌داران و پالینومورف ‌ها تعیین گردید. همچنین به منظور بازسازی نوسانات نسبی سطح آب دریا و محیط رسوب‌گذاری، مطالعات آماری پالینومورف ‌های حاضر در این توالی‌ ها انجام شد. بر مبنای مطالعات فسیل شناسی روزن‌داران و پالینومورف‌ها و با شناسایی tricolpites phillipsii پولن شاخص مرز کرتاسه پالئوسن، همچنین با حضور گونه داینوفلاژله palaeopridinium cretaceoum که گستره سنی آن تا انتهای کامپانین می ‌باشد و نیز وجود روزن‌دار elphidiella multiscissurata که گستره سنی آن مربوط به ماستریشتین می باشد، سن مجموعه توالی ‌های مورد مطالعه از کامپانین تا ابتدای پالئوسن تعیین گردید. با توجه به ظهور tricolpites phillipsii، مرز کرتاسه پالئوسن در افق 133 متری از قاعده این برش قرار دارد. مطالعات آماری بر روی شاخص‌های پالینولوژی و ترسیم نمودار نوسانات نسبی سطح آب دریا بر مبنای آنها نشان داد که در مرز کرتاسه پالئوسن افت نسبی سطح آب رخ داده است، به نحوی که این پدیده با نمودار جهانی سطح آب دریا و نمودار فراوانی نسبی داینوسیست‌ها در مقیاس جهانی کاملاً منطبق می‌ باشد. این موضوع موید ارتباط حوضه رسوبی البرز با آب‌های آزاد اقیانوسی و تاثیر ائوستازی بر حوضه رسوبی مورد مطالعه در زمان کرتاسه پالئوسن می‌ باشد. همچنین بر اساس مطالعات رخساره ‌های رسوبی و پراکندگی داده ‌های پالینولوژیکی بر روی نمودارهای سه‌تایی پالینوفاسیس، محیط رسوبی نزدیک به ساحل در یک سیستم رمپ کربناته برای این توالی‌ها پیشنهاد ‌می‌شود.

Reconstruction of Sedimentary environment and sea-level fluctuations in the Cretaceous-Paleocene deposits, central Alborz, North of Iran

IntroductionAlborz Mountain (with EastWest trend) in North of Iran and South of Caspian Sea has been created by the collision of central Iran and South Caspian plates (Stocklin, 1974). Upper Cretaceous strata in the Alborz area are consist of marine carbonates and marls that are different in western and eastern regions. Tertiary conglomerates and sandstones strata disconformably overlie those. In the Northern Alborz, Cretaceous carbonate strata are continuously overlain by shallow marine lower Paleocene layers (Rezaeian, 2009). CretaceousPaleocene strata in the north flank of central Alborz are consist of 235 meters of carbonate, siltstone, sandstone, and evaporitic layers. For reconstruction of the sedimentary environment in this strata, one stratigraphic outcrop in the south of Sari city has been sampled and studied. In this study, benthic foraminifera, palynomorphs, and palynofacies have been examined.Materials and MethodsSeventy rock samples have been processed using the standard palynological processing method (Traverse, 2007). In this method, carbonate and siliciclastic matters have been removed by HCL and HF (37%), then the residual matter has been sieved and centrifuged (using ZnCl2), and finally, palynological slides have been prepared. Also, seventy thin sections in order to the identification of benthic foraminifera have been prepared. Finally, all data have been plotted on diagrams using Corel Draw, Triplot, Adobe illustrator, and Excel software.DiscussionThe studied section from base to top is consists of carbonate strata with siliciclastic intercalation layers, marl, argillaceous limestone, shale, and evaporate. To the identification of relative age and CretaceousPaleocene boundary in the studied section, fossils content of samples have been studied. Four species of dinoflagellates, ten species of spores, seven species of pollens, and two species of foraminifers have been identified. Based on stratigraphic distribution of Tricolpites phillipsii (Ocampo et al., 2006; Schulte et al., 2010; Vajda and Raine, 2003; Willumsen and Vajda, 2010), Palaeopridinium cretaceoum (Ogg et al., 2016) and Elphidiella multiscissurata, Campanianearly Paleocene age can be proposed for these strata. KPg boundary is identified in the sample (KT 36) based on the first appearance of Tricolpites Phillipsii index species. Thin section studies lead to the identification of five sedimentary facies (evaporite, siltstone, litarenite, sublitarenite, and bioclast packstone/wackstone) in the studied samples that proposed shallow marine carbonate ramp as a sedimentary environment for these layers. Organic matter in palynological slides can be used as a paleontological factor to reconstruct the paleoecology and sealevel changes in the past (de Araujo Carvalho et al., 2006). High abundance of terrestrial palynomorphs (spore and pollens) and plants particles (phytoclasts) and low abundance of marine palynomorphs (dinoflagellate, acritarch, chitinozoans, …) and structureless organic matter (SOM) can prove landward environments and relative sealevel fall event and vice versa (de Araujo Carvalho et al., 2006; Handford and Loucks, 1993; Tyson, 1993). Palynological studies in this section show a relative sealevel rise trend from the base of the section to the middle parts, then a relative sealevel fall trend from the middle parts towards the end of the section. Moreover, there is a relative sealevel fall event across the KT boundary the can be correlated with the global sealevel diagram (eustasy). Based on this fact, we can say that the CretaceousPaleocene basin in this area was connected to the open oceans.ConclusionCretaceousPaleocene strata in the South of Sari (Northern Iran) are composed of carbonate, evaporate, and siliciclastic sedimentary rocks. In this study, based on paleontological investigations and the appearance of Tricolpites phillipsii (index species of Paleocene age), CretaceousPaleocene boundary has been identified in KT36 sample (133 meters from the base of section). Stratigraphical distribution of index fossils proposed a Campanianearly Paleocene age for the studied strata. Facies studies lead to suggest a shallow carbonate ramp as a sedimentary environment for these strata. Statistical analysis on palynological factors shows a deepening upward trend from the base of the section to the middle parts. Then there is a shallowing upward trend from the middle parts towards the end of the section. 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