JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.87, March 2016, pp.345-360 0016-7622/2016-87-3-345/$ 1.00 © GEOL. SOC. INDIA Magnetic Susceptibility Studies of the Upper Albian-Danian Sediments of Cauvery Basin, South India V. PRASANNAKUMAR, P. PRATHEESH, C. VIKAS, RAJESH REGHUNATH and R.S. PRASANTH Department of Geology, University of Kerala, Kariavattom campus, Trivandrum – 695 581, Kerala, India E-mail: pratheeshponline@gmail.com Abstract: The Cauvery basin of the Indian Peninsular shield, formed during the fragmentation of the Gondwana Supercontinent, continued to evolve until the end of Neogene through rift, pull-apart, shelf sag and tilt processes. The basin witnessed many cycles of transgression, regression, erosion and deposition. A more or less complete succession of upper Cretaceous–Paleocene sediments is exposed in the Ariyalur–Pondicherry depression of the basin. Anisotropy of magnetic susceptibility (AMS) technique is applied in an attempt to examine the depositional and eustatic conditions prevailed during the formation of these sedimentary sequences. AMS results point to the sedimentation history dominated by marked sea level changes with several phases of transgression and regression. The sedimentation occurred mainly in a shallow epicontinental sea which has been punctuated with terrigenous supply more often. Greater utility of magnetic fabrics is suggested as a tool to trace fluvial responses to tectonic and climatic changes. Keywords: Anisotropy of Magnetic Susceptibility, Cretaceous, Cauvery basin, Sea level changes. INTRODUCTION Palaeohydrodynamic characteristics of sedimentation are generally analysed by relating the preserved primary fabrics to depositional environments and palaeocurrent directions (Allen, 1982). Following the recent use of magnetic susceptibility (MS) in correlating sediments (Crick et al. 1997, 2002; Ellwood et al. 1999, 2006, 2007; Hladil et al. 2003a, 2004; da Silva and Boulvain, 2002, 2003, 2006; Mabille and Boulvain, 2007), the technique is being widely used to examine the eustatic-sea-level changes and to estimate the associated lithogenic inputs (Davies et al. 1977; Ellwood et al. 2000). The Anisotropy of Magnetic Susceptibility (AMS) is considered as a useful tool to understand sedimentary as well as tectonic characteristics (Ellwood et al. 1979; Joseph et al. 1998, 2004; Hassold et al. 2006; Praveen et al. 2009; Prasannakumar et al. 2011; Pratheesh et al. 2012) and also to estimate the paleodirections of current flow (Liu et al. 2001; Pares et al. 2007; Prasanakumar et al. 2010; Pappanna et al. 2014). The magnetic fabric technique is a well-established approach for defining the orientation of sedimentary grains in three dimensions and hence forms a rapid, cost effective and non- destructive method for specifying the preferred orientation of the constituent grains in sediments. In the past decade, numerous studies have demonstrated the applicability of AMS in evaluating tectonic setting and fabric evolution in various kinds of sedimentary rocks like shale, mudstone, carbonates and sandstone (Sagnotti and Speranza, 1993; Parés et al. 1999; Liu et al. 2001; Aubourg and Robion, 2002; da Silva et al. 2009). Magnetic fabrics are also used to infer the turbulence due to tectonic impulse or climate dynamics (Sangode et al. 1999), which in turn extends its scope in flood-related studies. The late Jurassic to early Cretaceous rifting between India/Australia and India/Antarctica has resulted in the formation of a number of NE–SW-trending basins in the Indian Precambrian basement. One such basin, the Cauvery basin, is located in the south-eastern part of the Indian peninsula between N latitudes 8° 30' and 12° 30' and E longitudes 78° 30' and 80° 20' (Fig.1). This largest sedimentary basin, on the east coast of India, extends from Pondicherry in the north to Tuticorin in the south, stretching into the offshore waters of Bay of Bengal, Palk-strait and Gulf of Mannar. The exposed part of the Cauvery basin contains a more or less complete sedimentary record of the upper Cretaceous-Palaeogene periods (Yadagiri and Govindan, 2000). Numerous studies on stratigraphy, sequence stratigraphy, palaeontology, clay mineralogy, geochemistry, depositional environments and tectonic evolution of the Cauvery basin have been carried out by many researchers (Sastry et al. 1977; Sundaram and Rao, 1986; Ramasamy and Banerji, 1991; Govindan et al. 1996;