Unraveling weathering episodes in Tertiary regoliths by kaolinite dating (Western Ghats, India)

Abstract

Secondary minerals in soils can record climatic changes affecting continental surfaces over geological times. Their dating should refine our present knowledge about their potential periods of formation as well as their relations with the ongoing change of climate and erosion/weathering regimes. In the present study, twenty kaolinite samples from two lateritic profiles of the Karnataka plateau, an intensively studied area in the southern India, have been dated using electron paramagnetic resonance (EPR) spectroscopy. Kaolinite ages vary between 0.229 ±0.24 Ma to 40.73 ±15.37 Ma. Four different groups of age can be identified with ages clustered around 1.0, 3.5, 9.0 and 39.0 Ma. These groups of age indicate local preferential weathering periods that coincide with distinct Indian climatic events described in independent studies, such as monsoon strengthening. Thus, regional or subcontinental factors likely prevailed over global forcing in the imprint of climatic events in the regolith profiles. These results confirm that despite their simple mineralogy, laterites can contain several relictual and coexisting generations of secondary minerals and that EPR dating of kaolinite contributes to unraveling the complex history of continental surfaces over geological periods.

Tectono‐morphological evolution of the Cauvery, Vaigai, and Thamirabarani River basins: Implications on timing, stratigraphic markers, relative roles of intrinsic and extrinsic factors, and transience of Southern Indian landscape

“…All the modern rivers flow on the antecedent valley and show five stages of terraces–evolutionary history, the relics of which could be recognized in the field (Figures ) as well as DEM (Figure ). While the inheritance of first stage could be envisaged from the occurrence of 53‐ to 50‐Ma old Mn‐rich laterite, the remaining four different stages (1, 3.5, 9, and 39 Ma that fall within a range of 0.229 ± 0.24 Ma to 40.73 ± 1.537 Ma) of kaolinitic contents in lateritic profiles of the study area reported by Mathian et al () also correspond to the developmental stages of landscapes as enumerated in the model (Figure a,b). Active continuance of landscape evolution as a result of tectono–climatic factors could be gauged from the occurrences of organic‐rich layer within flood deposits as young as 1,250–830 years BP ± 30.…”

“…Depending on the parent rocks, these regions are represented by bauxite, laterite, calcrete, and ferruginous‐lateritic regolith. Recent studies have documented significant weathering episodes during 53–50 Ma in laterites of Dharwar Plateau during Palaeocene–Eocene thermal maximum and 37–23 Ma in the highlands during late Oligocene warming and during 2.5 Ma (Bonnet et al, , ) besides four stages of kaolinite formation in Western Ghats, namely, 1, 3.5, 9, and 39 Ma ± 0.24–1.537 ago (Mathian et al ), indicative of the occurrences of inherited regoliths as well as active periods of weathering since the formation of modern/younger drainage basins (i.e., precollision and postcollision).…”

“…It also led to the physiographical diversity and the youthful character of its mountain ranges (Sharma & Rajamani, , ; Singh & Rajamani, ; Sensarma et al, ). Despite such activism, there are regions that preserve the inherited regoliths (Bonnet et al ) as well, as evidenced by significant weathering episodes during 53–50 Ma in laterites of Dharwar plateau during Palaeocene–Eocene thermal maximum and 37–23 Ma in the highlands during late Oligocene warming (Bonnet et al, , ) followed by four stages of kaolinite formation in Western Ghats, namely, 1, 3.5, 9, and 39 Ma ± 0.24–1.537 ago (Mathian et al ) and another weathering during 2.5 Ma (Bonnet et al ) indicative of the occurrences of inherited regoliths as well as active periods of weathering since the formation of modern/younger drainage basins (i.e., precollision and postcollision). Based on these numerical ages and documentation of basin‐scale distribution of specific types of lithologies/weathering profiles/palaeosols, provisionally we recognize the laterite/ferricrete–bauxite–calcrete (Figure a,b,g,h) to be of oldest (53–50 and 37–23 Ma) weathering profiles, palaeosols (I generation palaeosols) of alluvial fans with texturally immature parent rock clasts (Figure f extreme bottom left) to be of 23–9 Ma, palaeosols with intensive calcitized rootlets (Figure f extreme right) that are found to occur in fluvial terraces of palaeo‐floodplain/natural levee (II generation palaeosols) to be of 3.5–2.5 Ma, and relict alluvium that are documented in inland river terraces and older deltaic plains and calcareous sandstones (Figure g,h,i) that are documented in coastal terraces to be ~1 Ma old.…”

“…From this review, five stages of landscape development could be envisaged: Gondwanan basin during Late Jurassic‐Early Cretaceous, drainage reversal and initiation of neo drainage basins during early Cenozoic, significant denudation and pedogenesis during Miocene–Pliocene, significant incision following sea‐level fall during Pleistocene, and general rise of sea level and significant progradation of fluvial systems during Holocene–Anthropocene. Recent publications (Bonnet et al ; Mathian et al, ) have reported the occurrences of four different weathering episodes, namely, 1, 2.5, 3.5, 9, and 39 Ma (±0.24–1.537) ago. The organic carbon layer found sandwiched between flood deposit and floodplain deposit in the Vaigai River bank (Figure ) has returned catastrophic floods and channel shifting during 1,250–830 years BP ± 30.…”

“…Previous geomorphological and seismic studies on the Western Ghats documented periodic uplift since Middle Mesozoic until Holocene along Archean and Proterozoic faults and shear zones (Radhakrishna, 1993;Valdiya & Rajagopalan, 2000;Valdiya, 2001aValdiya, , 2001bValdiya, , 2008 and significantly since the commencement of Himalayan Orogeny. It also led to the physiographical diversity and the youthful character of its mountain ranges (Sharma & Rajamani, 2000a, 2000bSingh & Rajamani, 2001;Sensarma et al, 2008 (Bonnet et al, 2014(Bonnet et al, , 2016 followed by four stages of kaolinite formation in Western Ghats, namely, 1, 3.5, 9, and 39 Ma ± 0.24-1.537 ago (Mathian et al 2019) Although the reversal of drainage flow directions was initiated by the evolution of WGE (Fainstein et al, 2012) Most recent studies, involving multiproxies (Singhvi & Krishnan, 2014) and deep-sea high-resolution data (Pandey et al, 2015), opined that this SW monsoon became intense since 10-8 Ma. It signifies the attainment of orographic barrier effect of WGE during 10-8 Ma, FIGURE 21 (a,b).…”