Identification and description of the upper Neogene palustrine sediments in the east of Damghan

Abstract

Abstract The studied area is a silty-clayey sedimentary landform, which is located on the border of Alborz and Central Iran sedimentary-structural units. It has 13 km long in the vicinity of the road Damghan to Shahroud. An integration of sedimentological and palynological studies is used for palaeoenvironmental interpretation, origin investigation, and age dating of the landform sediments. The presence of transparent and angular quartzes and unstable minerals and the abundance of organic matter in the landform sediments indicate an aquatic environment in which a series of erosion events to sedimentation has taken place in a short period of time. A combination of sedimentological investigations and palynological studies such as the organic matter preservation factors and the presence of index dinocysts such as Operculodinium cf. eiricianum and Bitectatodinium tepikiense which are characteristic of the shallow aqueous environment, a palustrine environment with brackish water in with an age of Late Neogene is assigned for the studied sediments. This relatively vast aqueous environment was related to the Haj Ali Qoli salt playa (Chah Jam playa) in the south of the studied region. These sediments were developed on a mud flat zone on the northern margin of this playa. Uplifting and exposure of this sedimentary landform could be due to the tectonic activity of the region and the movement of the left-lateral strike-slip of the eastern part of the Damghan Fault. Keywords: Palaeoenvironment condition, Palustrine, Palynomorph, Late Neogene Introduction Palynology and sedimentology are common and applied methods for paleoenvironment interpretation, especially for Neogene and younger sediments. From the combination of palynological and sedimentology studies, useful information on palaeoclimate and palaeoenvironment is available to researchers in the fields of archaeology, palaeogeography, etc. (Asikainen et al. 2007; Head 1996). The main aim of this study is to identify depositional palaeoenvironments in detail and investigate the origin and age of landform sediments. The studied area is located on the border between Alborz structural state and Central Iran, approximately 30 km east of Damghan (coordinates with E 54° 36΄09 ˝ to 54° 45΄37˝ and N 36° 15΄43 ˝ to N 36° 16΄ 26 ˝). Material & Methods Field and laboratory methods were used in this research. After the field studies, samples were collected from three study stations for palynological, sedimentological and microfacies studies. These samples were prepared in the palynology laboratory. The preparation method of Traverse (2007) was used. Cold hydrochloric (20 %) and hydrofluoric acids were used to dissolve carbonate sand silicates, respectively. The residue was neutralized and centrifuged in ZnCl2 with specific gravity (1.9 g/cm3). The remaining residue was then sieved through a 20 µm nylon sieve prior to mounting on slides. A count of 500 particles was recorded for each sample using transmitted light microscopy as is necessary for the differentiation of palyofacies type and the palaeoenvironmental interpretations (Tyson 1995). Discussion of Results & Conclusions A combined investigation of sedimentary and palynology was performed on a silty-clayey sedimentary landform, which is located on the border between the Alborz structural state and central Iran to define its palaeoenvironment and investigation of the origin as well as the age of landform sediments. Sedimentology results: The presence of transparent and angular quartzes, unstable minerals such as feldspar, muscovite and the abundance of organic matter in the landform sediments indicate an aquatic environment in which a series of erosion events to sedimentation has taken place in a short period of time. The vertical fresh profiles were mostly composed of a mixture of distal alluvial and palustrine facies (the latter is particularly dominated by mottling colors and the dominance of carbonate muds) (Mángano et al. 1994). Palynology results: Organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come from the feces and remains of organisms such as plants and animals. Terrestrial palynomorphs are the most abundant and diverse group of palynomorphs in the study area, and the largest percentage of them are pollens. Among the studied pollens, the most pollen belongs to Poaceae (55%), Pinus (25%), Quercoide (15%), Salix (3%) and Glochidion (2%). Based on the presence of some index dinoflagellate species such as Operculodinium cf. eiricianum and Bitectatodinium tepikiense and comparing it with the world standard biozones of Williams et al. (2004), the age range of Late Miocene to Pliocene can be considered for the landform sediments (Head 1996; Hennissen et al. 2014). The presence of marine dinoflagellate cysts such as Bitectatodinium tepikiense is most probably because of the erosion of extensive outcrops of the Late Miocene to Pliocene marine deposits (Head 1994). Dinocysts maybe were simply reworked from older formations. But it is not impossible that during exceptional short-lived episodes of highstands lake in the Pliocene, we could have some of these dinoflagellates (Operculodinium sp.) because the hypersaline lake water or brine groundwater could have been diluted into the brackish water conditions. Organic matter preservation (Lability) factors; palynomacerals derived from higher plants and generally become less abundant, smaller, and more oxidized in a distal direction. Palynomacerals are mostly transported the same way as silt or sand and are thus preferentially deposited in such sediments (nearshore, higher energy or turbidites) (Waveren and Vischer 1994; Zonneveld and Lange 1997). Palynomacerals can be translucent or opaque. A higher percentage of brown palynomacerals is usually found close to the fluvial source, prodelta facies or estuaries may be very dark in color due to oxidation or selective degradation (Van der Zwan 1990; Batten 1996; Bombardiere and Gorin 2000). Opaque palynomacerals can be produced by either the oxidation of brown phytoclasts during prolonged transport or even post-depositionally. Therefore, the ratio of brown to opaque palynomacerals decreases in section number one and two because the brown palynomacerals is lost by selective degradation in distal alluvial and palustrine facies (Waveren and Vischer 1994; Zonneveld and Lange 1997). The ratio of transparent to opaque amorphous organic matter (TAOM/OPAOM); The Amorphous organic matter consists of all particulate organic components that appear structureless at the scale of light microscopy. The main sources of AOM are the degradation of phytoplankton and terrestrial organic matter by heterotrophic bacteria (Tyson 1995; Mendonça Filho et al. 2010). Amorphous materials are divided into two categories: transparent and opaque. The fluorescent intensity of the AOM is controlled by the redox conditions under which it was deposited (Waveren and Vischer 1994; Zonneveld and Lange 1997). An increase in the brown palynomacerals and fluorescent amorphous organic matter in section 3, indicated the proximity of the depositional site to a coastal conditions in a stagnant environment in the palustrine. Combining all information mentioned above, the depositional environment is most probably a transitional fluvial-hypersaline lake environment. The modern analog of such deposits is currently found in the vicinity of the active playas of central Iran including the Haj Ali Qoli playa in the south of Damghan. This interpretation is supported by sedimentological and paleontological data. But the most important question arises where was the studied area located in this palaeolake and why did it rise. The Alborz Mountains in the north of the Iranian Plateau are an active tectonic region with numerous faults. The activity of faults in the region has been studied and investigated by various researchers in the field of geosciences (Berberian 1976; Krinsley 1970). Fault analysis in the region shows that the studied landform is located exactly on the eastern section of the North Damghan Fault. This fault is located ten kilometers north of Damghan city and it represents a Pleistocene to Holocene fault due to the cutting of the same age deposits. The studied landform has been uplifted due to the tectonic activity of the region and the movement of the left-lateral strike-slip of the eastern part of the Damghan Fault (Berberian 1976; Krinsley 1970). The neotectonic activity caused the later upward displacement of those older fluvial-lacustrine/wetland facies to current altitudes creating those beautiful outcrops.

Keywords

• palaeoenvironment condition
• palustrine
• palynomorph
• late neogene