Ferricretes in Tamil Nadu, Chennai, South-Eastern India: From Landscape to Micromorphology, Genesis, and Paleoenvironmen
Ferricretes located in the coastal plain around Chennai, southeastern India, were analysed in thin sections using the concept of pedo-sedimentary sequence of events based on the hierarchy of sedimentary and pedological features and organisations. Field ch
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Abstract Ferricretes located in the coastal plain around Chennai, southeastern India, were analysed in thin sections using the concept of pedo-sedimentary sequence of events based on the hierarchy of sedimentary and pedological features and organisations. Field characteristics of investigated ferricretes are similar to those described in the literature. From top to bottom, they consist of a pisolithic crust, lying on hard ferruginous crust merging into a plinthite developed on mottled clays. Frequently the hard crust is hosted on ferruginous gravels. A sandy clay layer covers the ferricrete in one of the surveyed site. These ferricretes appear as inliers above the Late Pleistocene and Holocene riverbeds. Micromorphological analysis lead to conclude that the genesis of ferricretes studied have a complex succession of phases and attributes which were overlapping. Some of these phases are purely pedogenic, characterised by an intense iron oxide accretion, which occurred during episodes of landscape stability characterised by high ground water levels. Other phases reveal two different modes of erosion that have been recognised as, (i) deep erosion of plinthitic soils followed by a local deposition in the form of ferruginous gravel, (ii) surface erosion of hard ferruginous crust, long distance transportation and sedimentation in the form of a nodular layer. The sandy clay layers, which were found in one site, also confirmed the aeolian input in the area. A reconstruction of the local past landscapes is also proposed as well as a discussion on the environmental significance of the ferricretes studied. Keywords Abrupt events · aeolian sedimentation · climate fluctuations · ferricrete · laterite · pedo-sedimentary sequences
Hema Achyuthan Department of Geology, Anna University, Chennai 600 025, India N. Fedoroff Institut National Agronomique, 78850 Thiverval-Grignon, France, e-mail: [email protected]
S. Kapur et al. (eds.), New Trends in Soil Micromorphology, © Springer-Verlag Berlin Heidelberg 2008
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H. Achyuthan and N. Fedoroff
1 Introduction Laterites and ferricretes have been investigated mainly by geologists (e.g. Harrassowitz 1926, Maignien 1966, McFarlane 1976, Boulangé 1984, Bourman 1993b). Their studies have been somehow neglected by pedologists, probably because of their insignificance for agriculture. Consequently soil classifications (FAO-WRB 1998, US Soil Taxonomy 1999) are not well suited for these materials. The terms ferricrete and laterite are not mentioned in these classifications. FAOWRB (1998) proposes only the soil group of Plinthosols in which a petroplinthite or a plinthite starts within 50 cm from the soil surface. The plinthite, a diagnostic soil characteristics in US Soil Taxonomy (1999), corresponds only to not hardened laterites, where vesicular and pisolithic ones are excluded from the definition of plinthite. Consequently we have followed geologists (Tardy 1993, Delvigne 1998) for describing and analysing the profiles. According to Tardy (1993), the typic lateritic iron
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