Late Holocene Evolution of Fluvial System in Tamilnadu, India: Implications to Climate and Tectonics

Mahadev, . (2020) Late Holocene Evolution of Fluvial System in Tamilnadu, India: Implications to Climate and Tectonics. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Paleo-flood is one of the extreme events, also an essential fluvial archive preserving signatures of intense precipitation linked with climate changes. However, it remains a puzzle to be taken as a proxy for intense humid phases. Usually, humid periods are associated with high discharge, high-frequency flooding, whereas dry climate is associated with low discharge and low frequency of such extreme events. However, even dry periods are related to extreme events making it complicated. The study of paleo-flood requires a robust paleoflood record and precise geochronology beyond historical or instrumental records. The current work proposes exploring these events and thus constructing past flooding history by analysing the various fluvial deposits. The Palar, Gingee, Then-Pennai, Vellar and Lower and Kaveri River and its tributary as Amravati River in Southern India, Tamilnadu was chosen for the study. These rivers receive winter monsoon as a significant source of precipitation. Being as a low discharge river in rocky terrain, these are sensitive to high precipitation events. Luminescence dating is a widely accepted and popular method for estimating the age of sediment burial. However, sediments' partial or heterogeneous bleaching poses a problem in dating the young sediments (< 2 ka). Gingee, Vellar, Amaravati and lower Kaveri River form a fluvial system in eastern Tamil Nadu. Except for the Kaveri River, all others are ephemeral river systems, flowing for a few hundred kilometres through the rocky terrain and having a high chance of mixing unbleached weathered sand grains. Bleaching and mixing can be further enhanced by short transportation distance. Application of various luminescence age models is required to get the best age estimate for such extreme events. This study aims to test luminescence dating feasibility in a system consisting of high energy fluvial deposits, where partial bleaching could be high. All the samples were processed for OSL dating; the quartz grains are showing relatively low sensitivity in Gingee, Vellar and Amaravati rivers and high sensitivity in the lower Kaveri River, equivalent to the calibration quartz. However, most of them suffer from partial bleaching. With the application of various luminescence age models, three sets of OSL ages were obtained. The first set (100 to 200 years BP) indicates a drastic change in climatic conditions associated with the recurring tropical cyclonic systems, which have resulted in high-frequency flooding events in recent time. The second set of OSL ages suggests a large scale flooding event between 700-800 years BP coinciding with the change of Medieval Warm Period (MWP) to the Little Ice Age (LIA). The third set of OSL age (2ka to 3.5ka) shows a shift in the Inter-Tropical Convergence Zone (ITCZ). The study suggests increased flooding in a transition zone and not in a prolonged phase of either dry or humid periods. Earlier the southern part of the Indian Peninsula has been thought to be a tectonically stable shield area. However, the research carried out during the past three decades indicates possible unstable tectonism, at least since the Jurassic. The fluvial system in continental records response swiftly to tectonics or climate changes. The tectonic and geomorphic processes are related to each other, and the effect of change in tectonic or climate can be easily seen in the drainage systems. The current study focuses on reconstructing the Holocene evolutionary history of the Gingee and the Vellar River basin. Physiography, drainage patterns, geomorphic features, and structural controls of the Gingee and the Vellar rivers were evaluated to reconstruct the drainage basin's evolutionary history. The morphometric approach was applied to get the tectonic signal from the Gingee and Vellar River basins. The study helped in understanding the role of tectonic elements in the evolution of the basins. The literature survey and the morphometric analysis results have been applied to obtain information about the tectonic elements and the possible reconstruction of their activity in recent times. The analysis indicates the southward tilting of the Gingee drainage systems and northward tilting of the Vellar drainage systems, strong asymmetry in some reaches, pronounced elongation of certain tributaries. All these analyses and the previous studies point towards active tectonism in the area. Because of the tilting, the Gingee River is migrating towards the south and the Vellar River towards the north, leaving their paleochannels towards the north and south, respectively. The Gingee River migrated clockwise (towards south) to its current position during the mid-Holocene period ~3.5 ka. The Vellar River got shifted in an anticlockwise (towards north) direction during 1.28 ka BP. Luminescence ages of the paleochannels also suggest that both the rivers are migrating with the same rate of ~4.5km/ka.

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