AbstractMarl is a type of mixed carbonate–silicate sediment and contains clay and carbonate minerals deposited in different environments. The terms marl and marlstone are still imprecisely used in geology. I n this study, to revise the nomenclature and sedimentary classification of marl deposits, 99 samples were taken from different terrestrial and marine marl formations. The texture and percentage of carbonate and salt contents in the marls were measured. According to Folk's classification, the majority of the samples are in the range of sandy silt, sandy mud, silt, muddy sand, and silty sand. Based on the electric conductivity parameter (EC), all samples have a high content of salt and are classified in saline and very saline classes. The results based on the classification method of Haldar and Tisljar also confirm that most of the samples are in the category of "calcite clayey siltstone", clayey calcite siltstone, and “calcite-silt clay” and a few examples are "calcite – silt clay” and clayey- silt limestone, except for one sample of the Kond Formation, which is within the marl field. The main samples are not classified by the Pettijohn method and samples are mostly silty, and muddy and the name chalk-salt siltstone and mudstone were found to be more suitable for the terrestrial marl deposits under investigation in this research.Keywords: Continental marl, Carbonate content, Marine marl, Marl Classification, Salinity IntroductionIn geology, the term marl refers to chemical-detrital sediments, which have between 35% to 65% carbonate or clay content. Various definitions have been presented by different researchers about marl, despite relatively different descriptions in all of them, and marl nomenclature has been established on carbonate and clay contents. There is no unanimity regarding the nomenclature of marl sediments and many of the definitions are not precise, and in some of them, different origins are mentioned for marl, such as Terzaghi and Peck (1967), exclusively considered marine origin for marl sediments. Mitchell (1993) related these sediments to the biochemical process, while there is a wide range of marl deposits from terrestrial to marine in nature. Investigating the sedimentary characteristics and granulometry of evaporite marl samples in Iran by Farzami et al. (2014), Hatmian Zarami et al. (2014), Shaban et al. (2015), Haqiqhat et al. (2006) Abazade Chavandarqh et.al. (2006) and Samadi Tabrizi et al. (2013) all point to the fact that among the components of clay, silt and sand, the dominant texture of the samples is silty; in addition, carbonate is not the major chemical component of these samples and instead chalk and salt minerals are dominant. All of them have preferred the terms chalky and salty siltstone and mudstone rocks rather than marl.Marls of Iran can be divided into two categories: terrestrial and marine marls. Terrestrial marls have evaporite particles consisting of silt and clay and chemical substances such as calcite, halite, carbonate and sulfate salts, gypsum and anhydrite or one of these minerals. Marl deposits belong to two groups of Paleogene and Neogene marls in Iran. The origin of these marls are mostly salty lakes and they do not contain marine fossils. They are often red to reddish and pea colored. The surface of the soil has the effects of salt particles, salt crust and the effects of puffiness (Puffy Soil). These marls are often younger and belong to the Neogene period. From a geochemical point of view, they have harmful substances for plant growth, and for this reason, they show great sensitivity to various forms of surface, rill, gully and tunnel erosion (Peyrowan et al. 2014). Marine marls consist of clay and silt and calcite chemical substances. They have little or no evaporite minerals with high solubility such as halite, carbonate and sulfate salts, gypsum and anhydrite. Their origin is an ancient marine environment with normal salinity. They are often green in color. These marls in Iran are pre-Neogeneic in terms of age, i.e., Paleozoic and Mesozoic eras with more vegetation, and erosion mainly surfaces and shallow rills (Peyrowan et al. 2014).The terms marl and marlstone are still used imprecisely in geology (Donovan 2006). According to Donovan and Pickerill (2013), more precise lithological terms should be used for these deposits attributed to marl. This point is noticed by Picard (2010) and Alvarez (2009) stating that the interchangeable use of the terms "marl" and "marlstone" has continued into the 21st century. Picard (1953) has been concerned about the imprecise use of the term marl since almost seven decades ago. Prominent authors such as Alvarez (2009) and Tucker (2011) have also faced certain inaccuracies in the definition of marl or marlstones, and this problem still continues with its effects on published texts of geosciences. The two words "marl" and "marl stone" belong to the past, and surely it is time to be more careful in using such words. These terms have had more of an economic aspect than a precise geological scientific term and have been used for the remedial uses of agricultural soils that have been faced with carbonate deficiency (Neuendorf et al. 2005). As such, the nomenclature of marl sediments in Iran and other countries does not have a correct scientific classification, and this problem is clearly visible even in the maps produced by the Geological Survey and Mineral Exploration of Iran. The problems in the nomenclature of marls in previous research and studies are based on the fact that the amount of carbonate and clay are the classification criterion, while many of these deposits, which are among the marls of the detrital kind, have a silty and muddy texture instead of a clay texture. Also, they have a low amount of carbonate; instead, they have a lot of gypsum and salts. Even marls of marine origin, such as the Qom Formation, do not have the range of marl in terms of the amount of carbonate and clay, except for a few cases. The present research was conducted in the marl areas of Tehran province to determine the sedimentological characteristics of the marls in the study area in order to provide a suitable classification method regarding the sedimentological nomenclature of these deposits. Materials & MethodsThe studied area is located in the south and east of Tehran city, which includes Pakdasht, Varamin, Ivanki and Hassanabad. In general, the region has a dry semi-desert (desert) climate with little annual rainfall. Marl deposits are exposed in the form of hills at the foot of the Alborz highlands and in the southern parts of Tehran and Varamin plains. Due to their high erodibility, these deposits have different forms of surface, rill and gully erosion. Also, they are exposed in the form of single and witness hills during the severe erosion cycle. On the surface of the slopes of these sediments, there are many gypsum crystals, salt crust and sodium fatty stains (slicken slide). Due to the severe soil erosion of these slopes, vegetation is difficult to establish. In the parts where these slopes are covered by Quaternary gravel cover, erosion is less and plant establishment is improved.In this research, different marl units including Pliocene marls, Qom Formation marl and three Upper Red marl members and two Lower Red chalk and salt members were selected for investigation and the following steps were done:1- Sampling of 99 samples from Varamin, south of Varamin, south of Hassanabad, Ivanki and Pakdasht from Upper Red marl members (M1, M2, M3, Lower Red members (OLg, OLS), Eocene marl units of Kond Formation (Ek), marl unit Pliocene (PlM), marl unit of Qom Formation (OlM) and marl alluviums (QtM);2- Determining physical characteristics including granulometry through three steps: Sieve analysis, hydrometer and a combination of both;3- Determining the percentage of carbonate content in marls of the region by acid neutralization method;4- Determining the rate of salinity of the samples based on the electrical conductivity (EC) criterion. Discussion of Results & ConclusionIn this research, with the aim of revising the nomenclature and sedimentary classification of marl deposits from South Varamin, South Hassan Abad, Ivanki and Pakdasht areas, samples were taken from different continental and marine marl units. Granulometry, carbonate percentage and salinity were measured. Sediment classification (Folk 1974) has been used to name the marl deposits of the study area. The samples are plotted in six fields of Folk's triangle, which show predominantly sandy silt, sandy mud, silt, mud, muddy sand and silty sand respectively. The examination of marl samples in the present research shows that the amount of carbonate in the marl samples of marine formations such as the Qom Formation is higher than the continental marl formations and even reaches 68.53%, but in the Neogene continental marl formations of the region including the Lower and Upper Red formations, it ranges from 5.1 to 33.34%, with an average of 20.98%. In these formations, the amount of clay varies in the range of 2.5 to 49%, and the average clay in all samples is 25.49%. The chemical part is not limited to carbonate; chalk and salt are also present in addition to carbonate. Therefore, most of the samples are not classified as marl sediments.The results of the classification method (Haldar and Tisljar 2014) also confirm that the application of marl nomenclature to the studied fine-grained deposits does not have scientific precision and accuracy. Except for one sample of the Kond Formation, which is within the marl class, most of the samples are in the category of "calcite clayey siltstone", clayey calcite siltstone, and “calcite-silt clay” and a few examples are "calcite – silt clay” and clayey- silt limestone.The salinity of the samples was estimated by measuring the electrical conductivity (EC) of an extracted solution. The salinity of the studied marl samples showed that all the mentioned samples have high degrees of EC, which indicates the presence of high salts. The presence of abundant gypsum crystals, white salt crust on the surface of marl, dark spots of sodium fatty color and puffiness of the soil surface (Puffy Soil) are the field evidence of the presence of salts in the chemical composition of the samples. Since the samples are more silty and muddy in terms of texture and have a percentage of carbonate content less than 35% (the threshold for naming marl in the Pettijohn classification) and on the other hand, they are rich in salts, thus the names of “chalky – Salty” siltstone and mudstone is more suitable for the studied marl deposits. Based on the results of the present study, the studied marls physically have more silt than clay, and chemically, more gypsum and salt content than carbonate. This is in line with the findings of other researchers including Abbasi and Amini (2008), Hatmian Zarmi et al. (2012), Shaban et al. (2012) and Farzami et al. (2015). The results of this research are also in line with other researchers including Alvarez (2009) and Neuendorf et al. (2005), Picard (2010), Donovan and Pickerill (2013). Based on the results, it is suggested to revise the nomenclature and classification of the deposits attributed to the marly formations of Iran. The texture of the sediment, the type of mineral composition present in terms of the abundance of calcite or evaporite minerals, as well as the color and marine or continental origin, should be used as criteria, and naming should be done based on the two destructive and chemical components of the sediments with accurate measurements.