ELSEVIER Sedimentary Geology 119 (1998) 239–252 Seismites in a Proterozoic tidal succession, Singhbhum, Bihar, India H.N. Bhattacharya L , Sandip Bandyopadhyay Department of Geology, Presidency College, Calcutta 700073, India Received 28 August 1997; accepted 20 February 1998 Abstract Early Proterozoic metasediments of the Chaibasa Formation (Galudih–Ghatsila–Dhalbhumgarh region, Singhbhum, Bihar, India) comprise a number of cyclic fining-upward prograding successions of tidalites. The tidalites show indications for earthquakes in the form of synsedimentary deformation features, apart from the structures due to high-energy wave action. Deformed cross-bedding, convolute laminations, synsedimentary faults, graben-like structures, sandstone dykes, pseudonodules and slump folds record the seismic activity. A gradual decline in the frequency of seismites and tsunami-related depositional features, in combination with an upward increase in thickness of the tidal cycles, are attributed to gradual diminishing of tectonic activity within the basin.  1998 Elsevier Science B.V. All rights reserved. Keywords: Proterozoic; North Singhbhum Fold Belt; seismites; liquefaction; fluidization 1. Introduction Soft-sediment deformation structures provide a record of processes that affected unconsolidated sed- iments prior to, or soon after, burial. They are fre- quently used as top=bottom criteria and for this reason attention has been focused on the mechan- ics of their formation. The significance of such soft-sediment deformation structures for unravelling synsedimentary basin tectonics has, however, not yet been fully explored. The Proterozoic tidal succession (Chaibasa Formation) at Singhbhum, Bihar, which abounds with such structures, is no exception in this respect. Faulting along basin margins is known to trig- ger seismic shocks in fault-controlled extensional basins (Mastalerz and Wojewoda, 1993). Vibratory L Corresponding author. Fax: C91 33 241 2100; E-mail: acecopc@giascl01.vsnl.net.in ground motions thus generated lead to liquefaction and fluidization in unconsolidated sediments, caus- ing penecontemporaneous deformation (Seilacher, 1969; Sims, 1973, 1975; Lowe, 1975; Seth et al., 1990). Deposits with such shock-induced formations have been termed seismites (Seilacher, 1969, 1984). The main objective of the present paper is to as- sess the synsedimentary basin tectonics based on the study of the soft-sediment deformation structures in the Chaibasa Formation (Proterozoic), as exposed between the Galudih and Dhalbhumgarh areas of Singhbhum, Bihar (Fig. 1). 2. Geological setting The North Singhbhum Fold Belt is now rec- ognized as a Proterozoic rift basin evolved under a regime of continued crustal stretching (Gupta et al., 1980; Mukhopadhyay, 1984; Sarkar et al., 0037-0738/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S0037-0738(98)00051-7