نوع مقاله : مقاله پژوهشی
نویسندگان
1 گروه چینهشناسی و فسیلشناسی، پردیس بینالملل دانشگاه فردوسی مشهد، مشهد، ایران
2 گروه چینهشناسی و فسیلشناسی، دانشکده زمینشناسی، پردیس علوم، دانشگاه تهران، تهران، ایران
3 گروه زمینشناسی، دانشکده علوم، دانشگاه فردوسی مشهد، مشهد، ایران
چکیده
کلیدواژهها
عنوان مقاله [English]
نویسندگان [English]
Introduction
Early and Middle Jurassic, continental sediments of considerable thickness are widespread in the
northern, central, and central-eastern 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; Vaez-Javadi and Pour-Latifi, 2004; Vaez-Javadi;
Mirzaei-Ataabadi, 2006; Vaez-Javadi, 2008, 2011, 2012, 2014, 2015a, b, 2018; Saadatnejad et al, 2010;
Vaez-Javadi and Abbasi, 2012, 2018; Vaez-Javadi and Allameh, 2015). For a more detailed study, plant
macrofossils from Chah-Rekhneh were considered here.
Materials and Methods
The plant fossils have been driven from a borehole in Chah-Rekhneh, 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 stratigraphic-core 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 Chah-Rekhneh) is held in the collection of the Palaeobotany
Laboratory of the Department of Geology at the University of Ferdowsi, Mashhad.
Results and Discussion
Biostratigraphy of the Chah-Rekhneh
The Hojedk Formation consists of shale, siltstone, and fine-sandy 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; IIPtilophyllum
harrisianum- Elatides thomasii Interval zone, III- Elatides thomasii - Nilssonia
macrophylla Interval zone.
Palaeoclimate Analysis
In order to recognize palaeoclimate of the Chah-Rekhneh 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 Chah-Rekhneh is 41.81. It shows that the flora
assigned to the middle part of Floral Gradient table and suggests a humid and sub-tropical climate
during early Middle Jurassic.
Conclusion
The Hojedk Formation in the Chah-Rekhneh 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 hymenophylloides-Klukia 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,
Aalenian-Bajocian 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 Chah-Rekhneh 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 sub-tropical palaeoclimate was
dominant in this area during the Aalenian-Bajocian interval.
کلیدواژهها [English]