نوع مقاله : علمی -پژوهشی
1 گروه زمینشناسی، دانشگاه شهید بهشتی، تهران، ایران
2 انستیتو نفت دانشگاه تهران، تهران، ایران
عنوان مقاله [English]
Introduction The eastern Alborz contains a rich assemblage of plant macrofossils studied previously by several authors (e. g. Corsin and Stampfli, 1977; Vaez-Javadi, 2006; Najafi, 2009 and Vaez-Javadi, 2016). Ferns are the most dominant plant macrofossils in the Shemshak Group (Kalariz formation) in the eastern Alborz Mountains. Accordingly, this study focuses on the ferns and the habitat of this group. In addition, paleogeographic distribution and the range of extension of encountered species are discussed, and the composition of ferns in the studied flora was compared to those of other Rhaetian assemblages in other parts of the world.Materials and methodsThe samples containing Triassic fern macro-remains were collected through systematic sampling during the measurement of stratigraphic section in the Ghoznavi area (116 Km NE of Shahroud, 22 Km SE of Azadshahr). The specimens were studied using binocular microscopy. The identification and classification followed Taylor et al., 2009; Nagalingum and Cantrill, 2006; and Vera and Passalia, 2012.Results and discussionsThe aim of this study is to establish a systematic investigation of Triassic fern macrofossils including fronds and reproductive organs from Shemshak Group (Kalariz Formation) in the Ghoznavi area. The encountered specimens comprise of fern macro-remains referable to three orders (Marattiales, Osmundales and Filicales) including 7 genera, 14 species: two Pterophyta incertae sedis species (Korallipteris yipinglagensis and Thainguyenopteris parvipinnulata); three Filicale species belonging to the Dipteridaceae (Dictyophyllum exile, Dictyophyllum nathorsti and Clatheropteris meniscioides); eight species of Osmundales and their reproductive organ (Cladophlebis nebbensis, Cladophlebis australis, Cladophlebis sp., Cladophlebis haiburnensis, Cladophlebis raciborskii, Cladophlebis scoresbyensis, Cladophlebis denticulata and Osmundopsis sturii); and one species of Marattiales (Marattia intermedia).Although certain encountered species are long-ranged, extending through the Jurassic period (i.e. Cladophlebis denticulata and Clathropteris meniscioides), others (i.e. Dictyophyllum exile, and Thainguyenopteris parvipinnulata) are indicative of Rhaetian.The encountered fern species in the studied area show the affinities with other Rhaetic flora from both Laurasia and Gondwana landmasses. The similarities between the fern species of the studied area and those of Korea, Germany and Argentina (each one having 5 common species); China and Japan (each one having 6 common species); and finally, Greenland, Italy and Vietnam (each one having 4 common species) support the cosmopolitan nature of these taxa during the Late Triassic.The presence of large and mature fern fragments, preserved parallel to bedding in gray shale and sandstone, indicates the lowland of deltaic environments (swamp, marsh, levee, flood plain). Most of the encountered Osmundaceae ferns are indicative of wetter lowland environment and warm conditions, although the osmundaceous genus, Cladophlebis denticulata, with narrow pinnules and denticulate margin, suggests drier conditions in drier parts of delta lowland (i.e. levee and flood plain). The large fronds of understory fern family, Dipteridaceae, (three species of Dictyophyllum and Clatheropteris) reflect a humid climate with low levels of light in lowland of delta (i.e. marsh and river bank), and the presence of Marattialean fern species (Marattia intermedia) suggests warm and moist environments in deltaic lowland (i.e. marsh and swamp).ConclusionA comparison was made between the Triassic ferns of the studied area and those of other parts of the World. This indicates broad similarity with the Late Triassic of Korea, Japan, China, Vietnam, Greenland, Sweden, Germany, Italy and Argentina. On the other hand, the co-occurrence of hydrophilic and xerophilic species reveals that these plants lived in the lowland of deltaic environment, but in different microhabitats.