Tidal rhythmites in offshore shale: A case study from the Palaeoproterozoic Chaibasa shale, eastern India and implications L. Mallik a , R. Mazumder b, c, * , B.S. Mazumder c , M. Arima d , P. Chatterjee e a Department of Geological Sciences, Jadavpur University, Kolkata 700032, India b Geological Studies Unit, Indian Statistical Institute, 203, B.T. Road, Kolkata 700108, India c Fluvial Mechanics Laboratory, Physics and Applied Mathematics Unit, Indian Statistical Institute, 203, B.T. Road, Kolkata 700108, India d Graduate School of Environment and Information Sciences, Yokohama National University, 79-7, Tokiwadai, Hodogaya, Yokohama 240-8501, Japan e Geoinformatics & Remote Sensing Cell, West Bengal State Council of Science & Technology, Department of Science & Technology, Bikash Bhavan, Kolkata 700091, India article info Article history: Received 26 July 2011 Received in revised form 6 October 2011 Accepted 11 October 2011 Available online 25 October 2011 Keywords: Internal-tide deposit Turbidite Harmonic analysis Semidiurnal tide Palaeoproterozoic Chaibasa Formation abstract Tidal rhythmites have been documented from modern continental shelves and distal offshore settings. In significant contrast to their modern counterparts, sedimentological studies of ancient tidal rhythmites formed in distal offshore settings and quantification of tidal rhythms encoded therein are, however, scarce. This paper presents sedimentological analysis of vertically accreted tidal rhythmites from the Palaeoproterozoic Chaibasa shale facies in India, and quantification of their tidal rhythms. The facies characteristics of the Chaibasa Shale coupled with harmonic analysis of the laminae thicknesses of the sandstone inter-layers corroborate a distal shelf palaeogeography for the generation of the vertically accreted tidal rhythmites. The latter are the distal counterparts of laterally accreted, tidal “bundle” deposits, well preserved within the shallow marine Chaibasa sandstones. The number of lunar days in a synodic month (w23) estimated from the Chaibasa shale facies is, however, considerably lower than that estimated from the Chaibasa sandstone facies (w32) suggesting that the tidal record encoded from the Chaibasa shale facies is incomplete. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Tidal rhythmites are laterally and/or vertically accreted, lami- nated to thinly-bedded sedimentary deposits of tidal origin that form in estuarine to upper and lower delta slope settings and in distal, offshore tidal deltas (Williams, 1991; Kvale et al., 1999; Eriksson and Simpson, 2000; Mazumder and Arima, 2005). Sedi- ment deposited in the distal, offshore setting, is relatively undis- turbed by weak flood tide currents. Low wave energy and relatively rapid deepening beyond the terminal lobe of the ebb tidal delta promote better preservation of the vertically accreted laminae in such distal offshore settings (Williams, 2000). These vertically accreted tidal rhythmites represent the distal counterpart of later- ally accreted, tidal “bundle” deposits of cross-bedded sand boun- ded by mud drapes, which are common in migrating sandbars in modern tidal environments (Visser, 1980; Williams, 2000). In contrast to their modern analogues, sedimentological studies of ancient tidal rhythmites formed in a distal offshore palae- ogeography, and quantification of the tidal rhythms encoded in them, are scarce (Williams, 2000; Mazumder and Arima, 2005). This paper presents sedimentological analysis of vertically accreted tidal rhythmites from the Palaeoproterozoic Chaibasa shale facies in India, as well as quantification of the tidal rhythms therein. 2. Geological setting The tidal deposits described in this paper are part of the 6e8 km thick Late Palaeoproterozoic Chaibasa Formation and are best exposed in the Ghatshila-Dhalbhumgarh area (Fig. 1), Jharkhand, eastern India. The formation rests partly upon an Archaean granitic basement, partly on the terrestrial Dhanjori Formation, and is overlain by the Dhalbhum Formation (Bose et al., 1997; Mazumder, 2005)(Fig. 1). The precise age of the Chaibasa Formation has not been established yet, but the underlying Dhanjori mafic volcanics are w2100 Ma (Roy et al., 2002a), whereas the minimum age of the Dalma Lavas that conformably overlie the Singhbhum Group (cf. Bhattacharyya and Bhattacharya, 1970) to which the Chaibasa Formation belongs, are w1600 Ma (Roy et al., 2002b). The rocks underwent several post-depositional deformation phases and * Corresponding author. Geological Studies Unit, Indian Statistical Institute, 203, B.T. Road, Kolkata 700108, India. Tel.: þ91 (0) 9477589086; fax: þ91 (0) 3325773026. E-mail addresses: mazumder.rajat@gmail.com, mrajat2003@yahoo.com (R. Mazumder). Contents lists available at SciVerse ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2011.10.005 Marine and Petroleum Geology 30 (2012) 43e49