Soft-sediment deformation structures in the Earth’s oldest seismites Rajat Mazumder a, * , A.J. (Tom) van Loon b , Makoto Arima c a Department fu ¨r Geo- und Umweltwissenschaften, Geologie, Ludwig-Maximilians-Universita ¨t Mu ¨nchen Luisenstr. 37, D-80333 Mu ¨ nchen, Germany b Faculty of Earth Sciences, University of Silesia, Bedzinska 60, 41-200 Sosnowiec, Poland c Department of Environment and Natural Sciences, Graduate School of Environment and Information Sciences, Yokohama National University, 79-7, Tokiwadai, Hodogaya Yokohama 240-8501, Japan Received 10 October 2005; received in revised form 1 December 2005; accepted 7 December 2005 Abstract The Chaibasa Formation in Eastern India, which was deposited between 2100 and 1600 million years ago, shows deformations that must have formed when the sediments were not yet consolidated. Some of these deformation structures have never been described before. Here they are described, depicted and their origin is analysed. We show that they must be the result of shocks, which can only be explained satisfactorily as triggered by earthquakes. The layers containing these deformation structures are termed bseismitesQ. They are among the earliest records of earthquakes known in the Earth’s history. D 2005 Elsevier B.V. All rights reserved. Keywords: Soft-sediment deformation structures; Seismites; Proterozoic; India; Chaibasa Fm. 1. Introduction The Chaibasa Formation in Eastern India (Fig. 1), which is entirely siliciclastic, is 6–8 km thick. It is underlain by a granitic basement and siliciclastic sedi- ments, and overlain by the Dhalbhum Formation (Saha, 1994; Bose et al., 1997). The formation cannot be dated directly, but the underlying volcanics are 2100 million years old (Roy et al., 2002a) and the minimum age of overlying lavas is 1600 million years (Roy et al., 2002b). The rocks suffered several post-depositional deformation phases, as well as greenschist to amphib- olite facies metamorphism around 1600 Ma ago (Naha, 1965; Mazumder, 2005). Although the rocks described here are metamorphosed, we will refer to them as sandstones and mud/siltstones, to emphasize their char- acter, as it was when the sediment layers described here were deformed. The Chaibasa Formation consists of alternations of sandstones, heterolithic units (very fine sandstone/silt- stone/mudstone) and shales (Fig. 2). Layers with soft- sediment deformation structures are abundant in its upper part. These structures show a wide variety of shapes and occur in units that are separated by unde- formed intervals. Trigger mechanisms for the deforma- tions have been analysed following the approach suggested by Owen (1987), including distinction be- tween syndepositional and metadepositional deforma- tion (Nagtegaal, 1963; Allen, 1982; Owen, 1995), and reconstruction of the deformational mechanisms. Deformation can take place after a bed has been covered by younger layers (postdepositional deforma- 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.12.002 * Corresponding author. Tel.: +49 89 2180 6560; fax: +49 89 2180 6514. E-mail addresses: mrajat2003@yahoo.com (R. Mazumder), tvanloon@ultra.cto.us.edu.pl (A.J.(T). van Loon), arima@ed.ynu.ac.jp (M. Arima). Sedimentary Geology 186 (2006) 19 – 26 www.elsevier.com/locate/sedgeo