زیست‌ چینه‌نگاری ماکروفسیل‌های گیاهی سازند هجدک منطقه چاه-رخنه، جنوب‌غرب طبس و تحلیل آب و هوای دیرینه

سازند هجدک در منطقه چاه‌رخنه، جنوب غرب طبس دربر‌دارنده ماکروفسیل‌های گیاهی به نسبت خوب حفظ شده‌ای می‌باشد به‌طوری که تعداد 33 گونه ماکروفسیل گیاهی متعلق به 21 جنس از راسته‌های مختلف شناسایی شدند. براساس اولین و آخرین حضور مشاهده شده گونه‌های شاخص، یک زیست زون تجمعی با نامconiopteris hymenophylloides klukia exilis assemblage zone  و سه زیر زیست زون غیررسمی شامل: زیر زیست زون equisetites beanii ptilophyllum harrisianum interval zone  (i)، زیر زیست زونptilophyllum harrisianum elatides thomasii interval zone (ii)  و زیر زیست زون elatides thomasii nilssonia macrophylla interval zone (iii) به ترتیب از پایین به بالا تعیین گردیدند. این زیست زون‌ها با سایر زیست‌زون‌های هم ارز بازه زمانی ژوراسیک میانی در منطقه طبس قابل مقایسه و تطابق می‌باشند. حضور گونه‌های klukia exilis  و elatides thomasii نشان دهنده سن  آالنین باژوسین برای این مجموعه می‌باشد. مطالعات آماری نشان می‌دهد که راسته‌های فیلیکال‌ها، سیکادال‌ها، پاینال‌ها و اکوئی‌ستال‌ها به ترتیب با فراوانی نسبی 46.31 %، 23.50 %، 12 % و 9.26 % و جنس‌های nilssonia، klukia، coniopteris و equisetites به ترتیب با فراوانی نسبی 22.81 %، 13.38 %، 8.40 % و 8.40 % بیشترین حضور را در منطقه چاه‌رخنه داشته‌اند. بدین ترتیب، براساس فراوانی نسبی فیلیکال‌ها و سیکادال‌ها، حضور گونه‌های شاخص آب و هوایی و بوم‌شناختی نظیر equisetites beanii و میانگین عددی تراز گیاهی فلورای چاه‌رخنه (41.81)، آب و هوای مرطوب و نیمه گرمسیری در این منطقه در بازه زمانی آالنین باژوسین غلبه داشته است.

Biostratigraphy of plant macrofossils of Hojedk Formation in the Chah-Rekhneh area, south-west Tabas and its palaeoclimate analysis

IntroductionEarly and Middle Jurassic, continental sediments of considerable thickness are widespread in the northern, central, and centraleastern Iran (Kilpper, 1964, 1968, 1971, Barnard and Miller, 1976, Sadovnikov, 1976, 1984, 1991, Corsin and Stampfli, 1977, Fakhr, 1977, Schweitzer and Kirchner, 1995, 1996, 1998, 2003, Schweitzer et al., 1997, 2000, 2009, VaezJavadi and PourLatifi, 2004, VaezJavadi and MirzaeiAtaabadi, 2006, VaezJavadi, 2008, 2011, 2012, 2014, 2015a, b, 2018, Saadatnejad et al., 2010, VaezJavadi and Abbasi, 2012, 2018, VaezJavadi and Allameh, 2015). For a more detailed study, plant macrofossils from ChahRekhneh were considered here.Material, methods and geological settingThe plant fossils have been driven from a borehole in ChahRekhneh, 62km of SW Tabas city, 33˚17´56ʺ North latitude, 56˚24´01ʺ East longitude. A total of 84 specimens have been collected from a 240 m stratigraphiccore section. Some specimens yielded more than one fossil. The flora from this locality here is introduced for the first time. Material cited in this work (prefixed MJHCh; acronym for Mehdizadeh, Javadi, Hojedk, and ChahRekhneh) is held in the collection of the Palaeobotany Laboratory of the Department of Geology at the University of Ferdowsi, Mashhad.Biostratigraphy of the ChahRekhnehThe Hojedk Formation consists of shale, siltstone, and finesandy silt alternating with thick mediumgrained sandstone at the base of column. Several coal seams are present at the middle and upper part of the core column. One assemblage biozone was established in this section with lower and upper boundaries identified by FOO (First Observed Occurrence) and LOO (Last Observed Occurrence) of Coniopteris hymenophylloides and Klukia exilis, respectively. Furthermore, three informal subzones were recognized which are upward: I Equisetites beanii Ptilophyllum harrisianum Interval zone; II Ptilophyllum harrisianum Elatides thomasii Interval zone, III Elatides thomasii Nilssonia macrophylla Interval zone.Palaeoclimate AnalysisIn order to recognize palaeoclimate of the ChahRekhneh area during the Middle Jurassic, plant macrofossils relative abundance analysis and floral gradient method were used. Ziegler et al. (1996) assigned all Jurassic leaf genera to ten coarser morphological categories (or ’morphocats’). Rees et al. (2000) explained &Floral gradient& analysis. They show how this analysis can be used to interpret phytogeographic patterns based on the axis 1 scores of individual leaf genera and corresponding plant localities, due to their relative degrees of association. Then the climatical distribution in terms of the basic morphological characteristics of individual leaf genera and the palaeogeographic distribution of plant localities can be understood. By calculating the average of the scaled (0 to 100) axis 1 scores of the 32 genera common to all three intervals, a Jurassic ’floral gradient’ has been derived. Floral gradient score of this flora in ChahRekhneh is 41.81. It shows that the flora assigned to the middle part of Floral Gradient table and suggests a humid and subtropical climate during early Middle Jurassic.ConclusionThe Hojedk Formation in the ChahRekhneh area, south west Tabas contains 33 plant macrofossil species (21 genera) of various orders. On the basis of FOO and LOO of index species, one assemblage zone Coniopteris hymenophylloidesKlukia exilis Assemblage zone and three informal subzones were recognized. These subzones are upward: I Equisetites beanii Ptilophyllum harrisianum Interval zone, II Ptilophyllum harrisianum Elatides thomasii Interval zone, and III Elatides thomasii Nilssonia macrophylla Interval zone. The occurrence of index fossils such as Klukia exilix and Elatides thomasii, AalenianBajocian are assigned for this assemblage. Statistical analysis studies reveal that Order Filicales, Cycadales, Pinales and Equisetales with relative abundance of 46.31%, 23.50%, 12% and 9.26%, and Nilssonia, Klukia, Coniopteris and Equisetites were the most abundant genera with 22.81%, 13.38%, 8.40% and 8.40%, respectively in the ChahRekhneh area. Therefore, based on relative abundance of Filicales and Cycadales, the occurrence of Equisetites beanii as index species of climatic and ecologic, and average floral gradient score (41.81), a humid subtropical palaeoclimate was dominant in this area during the AalenianBajocian interval.