Palaeoclimate of the Baghamshah Formation, northeastern Tabas, central Iran, based on terrestrial palynomorphs

Abstract

AbstractThe Baghamshah Formation (Middle Jurassic) at the Senj stratigraphic section, northeastern Tabas, central Iran, consists of diverse and relatively well-preserved palynomorphs, including spore and pollen of land plants, microscopic algae, acritarchs, Tasmanites, fungal spores, foraminiferal test linings and dinoflagellate cysts. The miospores are attributed to the Pterophyta (11%), Lycophyta (1%), Coniferophyta (80%), Cycadophyta (3%) and Pteridospermophyta (5%). Among the miospores, the maximum frequency is related to Classopollis from the Chirolpidiaceae family of conifers. The abundance of Classopollis in the studied samples may represent the abundance of Chirolpidiaceae in the composition of vegetation nearby the sedimentary environment, the abundance of produced pollen, and/or the vicinity of the habitat of the parent plants to the depositional environment. Previous investigations about the environment of the Chirolpidiaceae family indicate the predominance of hot and dry climates. The abundance of Botryococcus algae separately or attached to Classopollis also supports this type of weather. The dominant size of the miospores in the stratigraphic section (25–35 µm) indicates the transport by a wind from highland to lowland. Based on the model of Sporomorph EcoGroups (SEGs), miospores typifying all the six plant communities are present in the studied strata. The palaeoclimate study was determined by the pattern of relative abundance of drier/wetter and warmer/cooler elements. The consequences of these calculations approve the hot and dry climatic conditions during the accumulation of the strata examined.Keywords: Palaeoclimate, Sporomorph EcoGroups, Baghamshah Formation, Tabas, central Iran. IntroductionThe Baghamshah Formation (Stöcklin et al. 1965) is the second formation of the Mago Group, a soft and easily eroded rock unit that has a noteworthy expansion in Tabas (Shotori Mountains), Yazd, Kalmard, Lakarkoh, Naybandan, Shirgesht and Jam areas (east of Semnan). In these areas, the Baghamshah Formation represents Middle Jurassic deposits and is made of shales, marls, and a small amount of sandstones and limestones. Some of the layers contain small amounts of gypsum and limited coal lenses. This formation overlain the Parvadeh Formation conformably and in the upper boundary the Esfandiar Formation, Qaleh Dokhtar Formation or pecten-bearing limestones is present.The marl composition of this formation has caused the outcrops to appear as hills and in many areas, they are covered. In most regions of central Iran, the upper boundary of the Baghamshah Formation is marked by a decrease in the depth of the sedimentary basin and even with a sedimentary interruption and erosion cycles, so that the boundary of the Baghamshah Formation with younger formations is characterized by sandstones and sometimes red horizons. The decrease in the depth of the basin and especially the sedimentation cease after the Baghamshah Formation indicates tectonic movements comparable to the global tectonic event of Nevadan orogeny (Aghanabati 1998, 2004). Ammonites, brachiopods and bivalves are the most abundant fossils of the Baghamshah Formation, among which ammonites have determined the age of the Baghamshah Formation as Bathonian–late Callovian–early Kimmeridgian (Ruttner et al. 1968; Seyed-Emami et al. 1997; Aghanabati 1998, 2004). The only palynological study has been conducted on the Baghamshah Formation (Dehbozorgi 2014) in the stratigraphic section of Virab, Jam area (east of Semnan), which according to the abundance of miospores attributed to ferns, warm and humid climate during the Middle Jurassic (end of late Bathonian–Callovian) has been determined. Considering the very little data that is available about the vegetation and climate of the Baghamshah Formation, in this investigation, the palynology of the stratigraphic section of Senj, northeast of Tabas, was examined. Material & MethodsA total of 44 samples (from palynologically promising intervals within the Baghamshah Formation) were prepared using standard physicochemical processing techniques (e.g. Traverse 2007; Phipps and Playford 1984). Samples were broken into pea-sized lumps (1–2 mm in diameter) and treated with acids (40–50% hydrochloric acid to dissolve carbonates and 40% hydrofluoric acid to remove silicates); followed by application of hot 50% HCl to dissolve silica-gel formed during HF treatment. The residues were then further treated with saturated ZnCl2 for mineral separation by gravity separation. All residues were sieved through a 20μm mesh prior to making strew slides. Three slides of final residues were examined by a transmitted light microscope. The slides are stored in the collection of the School of Geology, Tehran University, Iran. Discussion of Results & ConclusionsDiverse and relatively well-preserved palynomorphs dominated by spore and pollen of land plants, microscopic algae, acritarchs, Tasmanites, fungal spores, foraminiferal test linings and dinoflagellate cysts occur in surface samples of the Baghamshah Formation in the Senj stratigraphic section, northeast of Tabas. The existing miospores belong to Pterophyta (11%), Lycophyta (1%), and groups of spore-bearing plants such as Coniferophyta (80%), Cycadophyta (3%) and Pteridospermophyta (5%). Among the miospores, the maximum abundance is related to Classopollis from the Chirolpidiaceae family of conifers, which can happen for three reasons. First, it can be due to the abundance of Classopollis parent plants in the configuration of vegetation around the sedimentary environment. Secondly, it is due to the plenty of pollen produced by these types of plants and thirdly, it is due to the proximity of the habitat of Classopollis parent plants to the sedimentary environment. The abundance of this type of pollen, along with Botryococcus algae separately or attached to pollen, indicates a hot and dry climate. The small size of all miospores (about 25–35 µm), indicates their transport by a wind from highland to lowlands.SEGs data and associated plant communities are considered as an available approach used to draw palaeoecological inferences for their host strata (Abbink 1998; Abbink et al. 2004). This method was used for the Baghamshah Formation in the Senj stratigraphic section. Miospores typifying all six plant communities are retrieved from the material examined. The smallest and most abundant miospores taxa are those related to river and coastal SEGs, respectively. The pattern of relative abundance of drier/wetter and warmer/cooler elements shows that a hot and dry climate prevailed during the formation of these sediments, which is confirmed by the presence of gypsum in some of the layers of the Baghamshah Formation.

Keywords

• palaeoclimate
• sporomorph ecogroups
• baghamshah formation
• tabas
• central iran