Plant Macrofossils and Biostratigraphy of the Hojedk Formation at the North Kouchekali, West Tabas and its Climate analysis

Hojedk Formation is well-exposed at the Kouchekali area, south western of the Tabas city. It consists of mainly grey-dark shale, siltstone and sandstone intercalating with coal seams. Here, the core number 169 was studied and plant macrofossils were collected. Thirty-two species of plant macrofossil remains allocated to nineteen genera of various orders such as Equisetales, Dicksoniales, Schizaeales, Caytoniales, Bennettitales, Cycadales, Ginkgoales and Pinales were identified. On the basis of the occurrence of index fossils such as Coniopteris hymenophylloides, Elatides thomasii, Klukia exilis and Nilssonia macrophylla, Aalenian-Bajocian age is suggested for this assemblage. One assemblage zone (Klukia exilis-Nilssonia macrophylla) and four sub-biozones were recognized in this formation. These subzones are: I) Sagenopteris nilssoniana-Equisetites columnaris Concurrent range zone, II) Equisetites beanii-Ptilophyllum harrisianum Concurrent range zone, III) Ginkgoites huttonii Taxon range zone and IV- Ferizianopteris undulata-Coniopteris hymenophylloides Assemblage zone. Biozones of the Hojedk Formation in the Kouchekali area were correlated to the similar age sediments of the Mazino area (Tabas Block), the Dansirit Formation in Baladeh and Rudbarak areas (Central Alborz Mountains) and Bazehowz Formation in its type section (south Mashhad, Binalud Mountains). Moreover, on the basis of relative abundance of Filicales and Cycadales in this area, a humid sub-tropical (warm temperate) climate or is suggested for this period of time.
Material, methods and geological setting
The plant fossils have been driven from the borehole number 169, North Kouchekali, W Tabas city, 33˚17´North, 56˚18´East. A total of 86 specimens have been collected from a 405 m stratigraphic-core section. Some specimens yielded more than one fossil. There were several coal seams among this core. The flora from this locality is here introduced for the first time. Material cited in this work (prefixed FJNK; acronym for Fatemeh, Javadi, Namjoo and Kouchekali) is held in the collection of the Palaeobotany Laboratory of the Faculty of Geology at University of Tehran.
Systematic palaeobotany
North Kouchekali core section, W Tabas city contains well-preserved plant macrofossils belonging to 32 species allocated to 19 genera. These fossils are: Anomozamites sp., Cladophlebis aktashensis, Cladophlebis whitbyensis, Coniopteris hymenophylloides, Ctenis sp. cf., Ctenis sulcicaulis, Dictyophyllum sp., Elatides thomasii, Elatocladus zamioides, Equisetites beanie, Equisetites columnaris, Ginkgoites huttonii, Klukia crenata, Klukia exilis, Lobifolia rotundifolia, Lobifolia sp., Neocalamites sp., Nilssonia feriziensis, Nilssonia macrophylla, Nilssonia orientalis, Nilssonia sp., Nilssonia sp. cf., Nilssonia tazarensis, Nilssonia tenuicaulis, Nilssonia undulate, Podozamites distans, Pterophyllum feriziense, Ptilophyllum harrisianum, Rhizomopteris rezaii, Sagenopteris nilssoniana and Sphenobaiera sp.
Biostratigraphy of the North Kouchekali
The Hojedk Formation consists of shale, siltstone and fine-sandy silt alternating with a thick medium-grained sandstone at the base of column. Several coal seams 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 Klukia exilis and Nilsssonia macrophylla respectively. Furthermore, four informal subzones were recognized which are upward: I) Sagenopteris nilssoniana-Equisetites columnaris Concurrent range zone; II) Equisetites beanii-Ptilophyllum harrisianum Concurrent range zone; III) Ginkgoites huttonii Taxon zone and IV) Coniopteris hymenophylloides-Nilssonia feriziensis Concurrent range zone.
Geographic and stratigraphic distribution of taxa
Floristic association described here is widespread in the Central-East Alborz Mountains, Kerman Basin and Tabas area in the early Middle Jurassic. Similar plant macrofossil assemblages have been distinguished from the Shemshak Group in the Alborz Mountains, the Bazehowz Formation in the Binalud Mountains and the Hojedk Formation in the Kerman Basin and in the Tabas area (e.g. Barnard & Miller, 1976; Fakhr, 1977; Schweitzer et al., 2000, 2003; Vaez-Javadi, 2011, 2012, 2014; Vaez-Javadi & Allameh, 2015).
Relative abundance of Taxa in North Kouchekali
Relative abundances of morphocats was studied in which the relative abundances of Filicales and Equisetales and macrophyllous cycadophytes were 38.5% and 37% respectively. It is noteworthy that variety of the species of Genus Nilssonia (six species) and species of Order Ginkgoales (two taxa), as a macrophyllous cycadophyte was high within the North Kouchekali core column. Therefore, a humid subtropical (warm temperate) climate is considered during this period of time in this area.
Floral gradient analysis of the flora of North Kouchekali
In order to recognized palaeoclimate of the Calshaneh area during the Middle Jurassic floral gradient and correspondence analysis was used. Ziegler et al. (1996) assigned all Jurassic leaf genera to ten coarser morphological categories (or 'morphocats'). These are sphenophytes, ferns, pteridosperms, microphyllous cycadophytes, unassigned (intermediate or morphologically variable) cycadophytes, macrophyllous cycadophytes, ginkgophytes, microphyllous conifers, unassigned (intermediate or morphologically variable) conifers and macrophyllous conifers. These groups parallel the major taxonomic subdivisions which in turn reflect the individual physiognomic strategies of their constituent plants. 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 it can be understood these climatically in terms of the basic morphological characteristics of individual leaf genera and the palaeogeographic distribution of plant localities. By averaging the scaled (0 to 100) axis 1 scores of the 32 genera common to all three intervals, a Jurassic 'floral gradient' has been derived. In this case, microphyllous conifers and microphyllous cycadophytes have low scores, whereas macrophyllous conifers and ginkgophytes have high ones. Ferns and macrophyllous cycadophytes occupy the central portion of the gradient, along with sphenophyte genera such as Equisetites. Using the floral gradient, can assign a value to any Jurassic plant locality simply by averaging the scores of its constituent leaf genera. In this study I established this analysis. Floral gradient score of the North Kouchekali flora is 56.2. This shows that the flora assigned to the middle part of Table and suggests a humid and sub-tropical climate during early Middle Jurassic.

In this study, new data have been provided from the Hojedk Formation at the North Kouchekali, west of Tabas city, central-east Iran. Jurassic deposits in this core-column contains thirty-two species of plant macrofossil remains belonging to nineteen genera of various orders such as Equisetales, Dicksoniales, Schizaeales, Caytoniales, Bennettitales, Cycadales, Ginkgoales and Pinales. On the basis of the occurrence of index fossils such as Coniopteris hymenophylloides, Klukia exilis, Nilssonia macrophylla and Elatides thomasii, Aalenian-Bajocian age is suggested for this assemblage. One assemblage zone (Klukia exilis-Nilssonia macrophylla) and four informal sub-biozones were recognized in this formation. These subzones are upward: I) Sagenopteris nilssoniana-Equisetites columnaris Concurrent range zone, II) Equisetites beanii-Ptilophyllum harrisianum Concurrent range zone, III) Ginkgoites huttonii Taxon range zone and IV) Ferizianopteris undulata-Coniopteris hymenophylloides Assemblage zone.  This assemblage biozone is comparable to other biozones of the same age in Iran. Furthermore, relative abundances of taxa and morphocats were studied and relative abundances of Filicales, Equisetales, macrophyllous and cycadophytes were 38.5% and 37% respectively. It is noteworthy that variety of the species of Genus Nilssonia (six species) and species of Order Ginkgoales (two taxa), as a macrophyllous cycadophyte was high within the North Kouchekali core column. In addition, floral gradient score of this assemblage is 56.2. This shows that the flora assigned to a humid and sub-tropical climate (warm temperate biome) during the Middle Jurassic.