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[HCG23-P09] The characteristics of the paleosols recorded in the “Indus Molasse”, Ladakh, NW India
Keywords:Molasse, paleoweathering, Paleosols
During the collision between the Indian and Eurasian plates, sediments produced by the uplift and erosion of the southern margin of Asia are known as "Indus Molasse," characterized by coarse-grained deposits [1, 2]. These strata are correlated mainly to the Indus Group, typically distributed in northwestern India. The deposits are considered to have recorded environmental changes associated with the uplift and erosion of the southern margin of Asia. Detrital zircons showing late Oligocene U-Pb ages have recently been reported within these formations, suggesting deposition extending into the late Oligocene [3].
On the other hand, it is considered that paleosol reflects the climatic conditions during the deposition and is useful in reconstructing terrestrial depositional environments. In the Ladakh area of northwestern India, the Indus Group is prominently exposed, with many intercalated layers of paleosols. This study describes paleosols from the Nimu and Nurla formations [3] in the Indus Group, exposed along the Indus River in southeastern Ladakh, and their depositional environments are reported.
The Nurla formation comprises terrestrial deposits overlying the Eocene foraminiferal limestone, with a thickness exceeding 700 meters [3]. It mainly consists of units of coarse-grained sandstone and varicolored alternation of sandstone and mudstone, occasionally intercalates thick, muddy deposits, showing primarily green, reddish-purple, and red, occasionally dark green. The paleosols are frequently found, exhibiting significant soil horizons with root traces exceeding 30 cm in length, and containing carbonate nodules (caliche). Fine-grained layers rich in clay minerals are observed, resembling Alfisols, Ultisols, and Oxisols, with some paleosol exhibiting gleization due to groundwater influence.
The Nimu formation occupies the uppermost part of the Indus Group, with uncertain depositional ages [3]. With a thickness exceeding 600 meters, it comprises mainly units of coarse to medium-grained sandstone and varicolored alternation of sandstone and mudstone, along with varicolored mudstone. The muddy rocks are predominantly greenish-gray and reddish, with occurrences of black and grayish-green. Caliche is scarce, with many containing small nodules of siderite. While often exhibiting gleization, paleosols show clear soil differentiation, accompanied by large-scale traces and root traces covered with carbonate nodules. Some layers contain concentrations of iron minerals. These soils resemble Inceptisols, Oxisols, and Spodosols.
The transition of the paleosols from the Nurla to Nimu formations suggests a change from slightly drier to more humid environments, with a shift from young forest soils to soils characterized by high seasonality. Further investigation is needed to determine whether changes in soil drainage within ancient soils result from changes in depositional topography or fluctuations in groundwater levels.
References
[1] Searle et al., 1990, Tectonophysics, 174, 301-314, [2]Brookfield and Andrews-Speed, 1984, Sed. Geol, 40, 287-291, [3] Zhou et al., 2020, Jour. Geol. Soc., 177, 818-815. [4] Sinclair and Jaffey, 2001, Jour. Geol. Soc., 158, 151-162.
On the other hand, it is considered that paleosol reflects the climatic conditions during the deposition and is useful in reconstructing terrestrial depositional environments. In the Ladakh area of northwestern India, the Indus Group is prominently exposed, with many intercalated layers of paleosols. This study describes paleosols from the Nimu and Nurla formations [3] in the Indus Group, exposed along the Indus River in southeastern Ladakh, and their depositional environments are reported.
The Nurla formation comprises terrestrial deposits overlying the Eocene foraminiferal limestone, with a thickness exceeding 700 meters [3]. It mainly consists of units of coarse-grained sandstone and varicolored alternation of sandstone and mudstone, occasionally intercalates thick, muddy deposits, showing primarily green, reddish-purple, and red, occasionally dark green. The paleosols are frequently found, exhibiting significant soil horizons with root traces exceeding 30 cm in length, and containing carbonate nodules (caliche). Fine-grained layers rich in clay minerals are observed, resembling Alfisols, Ultisols, and Oxisols, with some paleosol exhibiting gleization due to groundwater influence.
The Nimu formation occupies the uppermost part of the Indus Group, with uncertain depositional ages [3]. With a thickness exceeding 600 meters, it comprises mainly units of coarse to medium-grained sandstone and varicolored alternation of sandstone and mudstone, along with varicolored mudstone. The muddy rocks are predominantly greenish-gray and reddish, with occurrences of black and grayish-green. Caliche is scarce, with many containing small nodules of siderite. While often exhibiting gleization, paleosols show clear soil differentiation, accompanied by large-scale traces and root traces covered with carbonate nodules. Some layers contain concentrations of iron minerals. These soils resemble Inceptisols, Oxisols, and Spodosols.
The transition of the paleosols from the Nurla to Nimu formations suggests a change from slightly drier to more humid environments, with a shift from young forest soils to soils characterized by high seasonality. Further investigation is needed to determine whether changes in soil drainage within ancient soils result from changes in depositional topography or fluctuations in groundwater levels.
References
[1] Searle et al., 1990, Tectonophysics, 174, 301-314, [2]Brookfield and Andrews-Speed, 1984, Sed. Geol, 40, 287-291, [3] Zhou et al., 2020, Jour. Geol. Soc., 177, 818-815. [4] Sinclair and Jaffey, 2001, Jour. Geol. Soc., 158, 151-162.