CHAPTER 1 INTRODUCTION

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1 1 CHAPTER 1 INTRODUCTION 1.1 GENERAL Washover sandsheets are often found in low-lying coastal embayment s (Sedgwick and Davis 2003). Washover deposition of marine sand is usually attributed to storm activity or tsunami and the distinction between these agents is often controversial especially in geological sequences that date beyond historical records. Landward tapering sandsheets found in the sedimentary record of coastal embayments are the most common tsunami signature worldwide (Dawson 1999, Bryant 2001). The distinctions between tsunami and storm deposits are related to differences in the hydrodynamics and sediment-sorting processes during transport. Tsunami deposition results landward erosion zone. Tsunamis can have flow depths greater than 10 m, transport sediment primarily in suspension, and distribute the load over a broad region where sediment falls out of suspension when flow decelerates. In contrast, storm inundation generally is gradual and prolonged, consisting of many waves that erode beaches and dunes with no significant overland return flow until after the main flooding. Storm flow depths are commonly < 3 m, sediment is transported primarily as bed load by traction, and the load is deposited within a zone relatively close to the beach from a few high-velocity, long-period waves that entrain sediment from the shoreface, beach. The deposits would have been derived from the shallow neritic zone by the high-energy tsunami

2 2 waves during the back wash, so more emphasis is on palaeotsunami rather than storm deposits. The current research applies many of the sedimentological techniques developed from studies over the last decade for the differentiation and identification of palaeotsunami deposits to the sites along the southeast coast of India. Preliminary investigations of this study identified a number of sand deposits in Vanagiri, Karaikal, Kameshwaram and Pushpavanam showing characteristic sedimentological signatures of washover sandsheets. These deposits are all found in the upper fill of geomorphically diverse coastal environments. They are compared with the characteristics observed in 26th December 2004 Indian Ocean tsunami deposits from South India. As a modern tsunami deposited analog, this study also presents an analysis of washover deposition from 2004 tsunami. 1.2 AIM AND PURPOSE OF THE STUDY To derive criteria for stratigraphic and sedimentological characteristics for washover deposits from 2004 Tsunami deposits, a known depositional origin. To review, investigate and critically assess the evidence for tsunami from the southeast Indian coast with particular focus on the allochtonous sand deposits of Cauvery Delta coast, Tamil Nadu region. To define and characterise the internal sedimentology of a large, raised laterally extensive sand sheets of Cauvery Delta coast. To define the depositional chronology of washover deposits using sedimentological analysis and Optically Stimulated Luminescence dating techniques (OSL).

3 3 1.3 BACKGROUND OF THE STUDY Tsunamis have become more frequent in the recent past, and are taking a heavy toll of life and livelihoods. As this coastal hazard has direct social impacts, it is important to identify paleotsunami deposits to predict recurrence and to conduct hazard-vulnerability assessments. Tsunami events result in washover sandsheets from a train of long-period waves that can rapidly travel long distances from where they were generated by deep-ocean earthquakes, submarine landslides, volcanic eruptions, or asteroid impacts. Tsunami events with extreme effects on sedimentary transport or considerable alterations of the coastal configuration are rather rare in recorded human history, but considering geological timescales they occur frequently. At least 100 mega tsunami in different parts of the world have been recorded in the past 2000 years, but presumably far more have failed to be noticed during historical times and are not mentioned either in written or oral ancient records. Tsunamis occur seldom in the Indian Ocean region and in the last 300 years this region recorded 13 tsunami and 3 of them occurred in Andaman and Nicobar region (Harikrishnan 2006). Tsunamis have been observed to cause extensive coastal erosion and sediment deposition (Dawson and Shi 2000). Evidence of tsunami has been found in coastal as well as marine stratigraphical sequences, but the current study will focus only on coastal deposits. It is important to note that when using geological records to identify and study historical or prehistorical events it is necessary to recognize tsunami deposits and to distinguish them from deposits that originate from storm surges. Tsunami sediments generally have sharp contacts with overlying and underlying deposits. In most cases, they consist of massive sand with abundant wood and other plant detritus. The sediments commonly contain

4 4 marine microfossils, and thin, fine, and rise in a landward direction (Benson et al 1997). Such fossils and architecture indicate that the sediments were deposited by landward surges of water, not by streams. The sedimentology of extensive sandsheets and elevated shell-rich deposits found in coastal mudflats, creeks and estuaries is investigated with the aim of defining the depositional mechanism of each deposit. All study sites for this research are located on the Cavery Delta Coast of Southern India (Figure 1.1). Depositional hypotheses are put forward and tested for each deposit and Optically Stimulated Luminescence (OSL) dating technique is used to investigate the chronology of the deposits. The internal sedimentology of these deposits relates directly to the degree to which the environment is open to oceanic influences. Generally sand deposits consist of structureless quartz sand in the open parts of an embayment that fine landward to a shell-rich muddy sand found in sheltered parts of the embayment (Roy et al 1980) and although both contain considerable marine sand, they exhibit little internal lithological similarity with the sandy back-barrier deposits from this study. All the sand deposits investigated in this study exhibit very subtle lateral change in sedimentology and are almost essentially quartz sand, with few of the deposits containing considerable deep sea shelly fauna. 1.4 STUDY AREA The present study covers 76 km coastal stretch in the Cauvery Delta coast of the Tamil Nadu from Vanagiri to Pushpavanam. The area contains beach rides, paleo-lagoons, paleo-tidal flats and paleo barriers (beach ridges). As the study deals mainly on finding locations for paleotsunami, Vanagiri, Karaikal, Kameshwaram and Pushpavanam were selected which were frequently affected by storms and waves along the Cauvery Delta coast of

5 South India. As Karaikal and Pushpavanam are the most drastically affected area in recent tsunami, the current study concentrated towards the inland for geomorphic changes due to tsunami.

5 5 South India. As Karaikal and Pushpavanam are the most drastically affected area in recent tsunami, the current study concentrated towards the inland for geomorphic changes due to tsunami. Altogether 14 trenches were made and samples were collected at different intervals like 1cm, 2cm and 5cm respectively to analyse washover deposits practically. Figure 1.1 Location map of Cauvery Delta coast of Tamil Nadu 1.5 STRUCTURE OF THESIS The thesis comprises 6 chapters. Chapter 1 deals with introduction, study purpose and hypothesis. Chapter 2 describes the characteristics of largescale washover sedimentation with the assistance of regional reviews of relevant literature along with case studies of modern analogs for the deposition of washover sandsheets in coastal environments. Chapter 3 presents the field methodology and the experimental procedure for analysis of

6 6 the samples and grain size analysis for understanding and the comparison of the textural and compositional characteristics (mineralogy) of sediments from various depositional environments within a particular sedimentary system and also dating techniques and foraminifera analysis. Chapters 4 present the regional setting and physical environment of east coast and also explain about climate and rainfall of the study area. Chapter 5 deals with the results and discussion and Chapter 6 summarises the conclusion about the current study.

THE DEPOSITS OF TSUNAMIS WESLEY PESANTEZ, CATHERINE NIELD, COLIN WINTER

THE DEPOSITS OF TSUNAMIS WESLEY PESANTEZ, CATHERINE NIELD, COLIN WINTER AN OVERVIEW OF OUR SEMINAR WHAT IS A TSUNAMI WHY STUDY TSUNAMIS PROPERTIES OF TSUNAMIS TSUNAMI HYDRODYNAMICS IDEALIZED DEPOSITS SEDIMENT