چكيده لاتين :
Introduction
Takht mine, SE Minoodasht contains well-preserved plant macrofossils belonging to 27 species allocated to 22 genera of various orders viz., Equisetales, Osmundales, Filicales, Peltaspermales, Bennettitales, Ginkgoales and Coniferales. The plant macrofossils in this area are studied for the first time. Based on the occurrence of Equisetites muensteri, Clathropteris meniscoides, Dictyophyllum exile, Anthrophyopsis crassinervis, Scytophyllum persicum, Pterophyllum bavieri, and Baiera muensteriana a Rhaetian age is suggested for this assemblage. Since, there was no differentiation between formations in geological map these flora emphasized spreading the Kalariz Formation in this area. The Minoodasht flora is correlated to the plant macrofossil assemblages of Zirab, Tazareh, Narges-Chal, Hiv, Jajarm (Alborz), Parvadeh mines (Tabas), and Darbid-Khun (Kerman Basin). Therefore, there were close floristic relationships between North and Central-East Iran (i.e. Kerman Basin and Tabas Block) and two areas were palaeogeographically closely related, probably forming a uniform paleoenvironment. On the basis of relative abundances of taxa, Filicales, Bennettitales, Ginkgoales and Pinales were 42.02%, 38.11%, 7.17% and 6.51%, respectively. It is noteworthy that variety and relative abundance of the species of Filicophyta and Bennetittales were high within Iran during the Late Triassic epoch. Therefore, a warm wet climatic regime dominated. In addition, a more uniform climate in continental scale is suggested. On the basis of similarity indices and relative abundance of taxa, Iran located within Eurasia climatic belt, Euro-Sinian Region, and the Middle Asia Province of Vakhrameev’s subdivisions and south-western region of Dubroskina’s subdivisions during this epoch.
Material, methods and geological setting
The plant fossils derive from the Takht mine, Minoodasht, E Alborz. A total of 127 rock slabs have been collected from two sites in the coal mine. Some rock slabs yield more than one fossil. Takht mine is located at the east of Gorgan city, 37? 8´North, 55? 26´East. There is a coal mine which mined widely. The flora from this locality is here described for the first time. Material cited in this work (prefixed FJPM; acronym for Fatemeh, Javadi, Parvacideh, and Minoodasht) is held in the collection of the Palaeobotany Laboratory of the Faculty of Geology at University of Tehran.
Systematic palaeobotany
Takht mine, SE Minoodasht contains well-preserved plant macrofossils belonging to 27 species allocated to 22 genera. These fossils are: Equisetites muensteri Sternberg 1833; Neocalamites sp. cf. N. hoerensis (Schimper 1869) Halle 1908 emend. Harris 1961; Todites princeps (Presl 1838) Gothan 1914; Lobifolia iranica Fakhr 1977; Lobifolia rotundifolia Corsin and Stampfli 1977; Clathropteris meniscoides (Brongniart 1825) Brongniart 1828; Dictyophyllum exile (Brauns 1862) Nathorst 1878; Dictyophyllum sp.; Phlebopteris muensteri (Schenk 1867) Hirmer and Hoerhammer 1936; Cladophlebis denticulata (Brongniart 1828) Nathorst 1876; Taeniopteris tenuinervis Brauns 1862; Scytophyllum persicum (Schenk 1887) Kilpper 1975; Anthrophyopsis crassinervis Nathosrt 1878; Anomozamites minor (Brongniart 1824) Nathorst 1878; Otozamites ashtarensis Barnard 1967; Otozamites furoni Boureau 1950; Pterophyllum bavieri Zeiller 1903; Pterophyllum tietzei Schenk 1887; Weltrichia sp. cf. W. primaeva Schweitzer and Kirchner 2003; Williamsonia sp.; Zamites persica Boureau 1950 emend. Barnard 1967; Nilssoniopteris sp. cf. N. musafolia Barnard 1965; Baiera muensteriana (Presl 1838) Heer 1876; Stachyopitys sp. cf. S. preslii Schenk 1867; Chamaetaxus orientalis Schweitzer and Kirchner 1996; Podozamites distans (Presl 1838) Braun 1843, and Podozamites schenki Heer 1876.
Geographic and stratigraphic distribution of taxa
This assemblage is widespread in Central and Eastern Alborz Mountains such as Tazareh, Parvadeh mines (Tabas Block, eastern) and restricted in the Kerman Basin, Central Iran during the Rhaetian age. On the basis of presence of Equisetites muensteri, Scytophyllum persicum, Anthrophyopsis crassinervis, Pterophyllum bavieri, and Baiera muensteriana, the Rhaetian age is suggested for this plant macrofossil assemblage.
Majority of species, such as Equisetites muensteri, Dictyophyllum exile, Phlebopteris muensteri, Pterophyllum bavieri, P. tietzei, are widespread throughout northern hemisphere and some others, such as Scythophyllum persicum, Lobifolia iranica, Otozamites ashtarensis, and Zamites persica occur more regionally. This is an indication of more uniform climate in continental scale and as such, it could be concluded that, during deposition of the Shemshak Group and the Nayband Formation (Ghadir Member), there were close floristic relationships between North and Central-East Iran and two areas were palaeogeographically closely related, probably forming a uniform paleoenvironment.
Relative abundance of Taxa in Minoodasht area
The relative abundances of taxa, Filicales, Bennettitales, Ginkgoales, and Pinales in Minoodasht area were 42.02%, 38.11%, 7.17% and 6.51%, respectively. Ferns (Clathropteris, Dictyophyllum) most likely had inhabited moist areas. This is further supported as several taxa of Order Bennettitales (Pterophyllum, Otozamites, Zamites, and Nilssoniopteris) are indicators of tropical-sub-tropical climate and occur in this area. The Early and Middle Jurassic epochs featured less differentiation of climate, generally greater moisture content and a minor thermal gradient. These features were inherited from the climate of the Late Triassic epoch whose flora had developed very gradually (Vakhrameev 1991; Ziegler et al. 1996). Based on the relative abundances of taxa, a humid subtropical-tropical climate is suggested for this assemblage during the Rhaetian in the Minoodasht area, Central Alborz. This assemblage is comparable to the Kalariz-Serie of the Rhaetian of Alborz, northern Iran (Schweitzer and Kirchner 2003) and Parvadeh area, Tabas area, East-Central Iran (Vaez-Javadi 2014).
S?rensen’s similarity index is a statistic used for comparing the similarity of two samples. It was developed by the botanist Thorvald S?rensen (1948, Bray & Curtis 1957). Hence, it is noted that species similarity between the Minoodasht and Narges-Chal areas is high during the Rhaetian, using following similarity index equation: (A= Number of species of the Minoodasht area, B= Number of species of the other area, C= Number of common species)
Using similarity index criterion, it is cited that species similarity between the Tabas area and Alborz region (Minoodasht, Narges-Chal, Vaez-Javadi 2006) was also high during the Rhaetian. Similarity indices of the whole flora of various localities in Iran and Eurasia were obtained and comparisons were made at species and genera levels (Tables 1, 2).
Table 1- S?rensen’s similarity index of Minoodasht flora in comparison with the other florizones of Iran.
Localities Narges-Chal Gheshlagh Hiv Sapuhin Kerman Parvadeh
Si index at genus level 72% 36.84% 39.02% 37.84% 54.55% 58.54%
Si index at species level 43% 23.73% 15.38% 24.49% 30.51% 34.48%
Table 2- S?rensen’s similarity index of Minoodasht flora in comparison with the other florizones of Eurasia (Greenland, Sweden, France, N. China, Korea, and Japan).
Localities Greenland Sweden France Viet Nam N. China Korea Japan
Si index at genus level 37.33% 31.25% 31.25% 17.65% 29.73% 30.14% 26.41%
Si index at species level 16.80% 7.69% 8.88% 10.53% 6.56% 10.10% 12.5%
Study of the Triassic flora began about 150 year age in Central Europe. Later, Oishi (1940) identified three flora series in the Mesozoic plant-bearing deposits in Japan. These series were Dictyophyllum Series (Raethian-Liassic), Onychiopsis Series and Angiosperm Series. Consequently, Dobruskina (1994) published a monograph that included stratigraphic position of Triassic plant-bearing beds of Eurasia and correlation charts of these deposits. She divided Eurasia into seven regions: 1- Western Europe, Svalbard and Greenland; 2- the European part of the USSR, Mangyshlak, and the Pamiers; 3- South-western Asia (Iran, Afghanistan); 4- Middle Asia; 5- Eastern Siberia and Kazakistan; 6- Central Asia, and the Far East, and 7- Industan. Here, we prepared some new data on Iran and made a comparison with the Eurasia. However, it is concluded that Iran is located in a humid subtropical-tropical belt and it was comparable with Greenland and Sweden.
Conclusion
Kalariz Formation is well-spread in the Minoodasht region located in the east of Gorgan city (East Alborz). It contains abundant well-preserved plant macrofossils belonging to 27 species allocated into 22 genera of various orders such as Equisetales, Osmundales, Filicales, Peltaspermales, Bennettitales, Cycadales, Ginkgoales, and Coniferales. Based on the occurrence of Equisetites muensteri, Clathropteris meniscoides, Dictyophyllum exile, Scytophyllum persicum, Anthrophyopsis crassinervis, Pterophyllum bavieri, and Baiera muensteriana a Rhaetian age is suggested for this assemblage. On the basis of relative abundances of taxa, Filicales, Bennettitales, Ginkgoales and Coniferales were 42.02%, 38.11%, 7.17% and 6.51%, respectively.
It is noteworthy that variety and relative abundance of the species of Filicophyta and Bennetittales were high within Iran during the Late Triassic epoch. Presence of Equisetales and ferns, as indicator taxa of humid climate with a total index of 46.58% in Minoodasht, in coal deposits indicated and indeed emphasized that quantity of precipitation was greater than the quantity of evaporation. Therefore, a warm wet climatic regime dominated. In addition, a more uniform climate in continental scale is suggested and as such, it could be concluded that, during deposition of the Shemshak Group and the Nayband Formation (Ghadir Member), there were close floristic relationships between North and Central-East Iran (i.e. Kerman Basin and Tabas Block) and two areas were palaeogeographically closely related, probably forming a uniform paleoenvironment. On the basis of similarity indices and relative abundance of taxa, Iran located within Eurasia climatic belt, Euro-Sinian Region, during this epoch.
