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Geological Society, London, Special Publications; 2009; v. 323; p. 301-318;
DOI: 10.1144/SP323.15
© 2009 Geological Society of London

Articles

Palaeoproterozoic seismites (fine-grained facies of the Chaibasa Formation, east India) and their soft-sediment deformation structures

R. Mazumder1,*, J. P. Rodríguez-López2, M. Arima3 & A. J. van Loon4

1 Geological Studies Unit and Fluvial Mechanics Laboratory, Indian Statistical Institute, 203 B.T. Road, Kolkata 700108, India
2 Grupo de Análisis de Cuencas Sedimentarias UCM-CAM, Departamento de Estratigrafía, Instituto de Geología Económica UCM-CSIC, Facultad de Ciencias Geológicas, Universidad Complutense, Ciudad Universitaria, 28040 Madrid, Spain
3 Department of Environment and Natural Sciences, Graduate School of Environment and Information Sciences, Yokohama National University, 79-7, Tokiwadai, Hodogaya Yokohama 240-8501, Japan
4 Institute of Geology, Adam Mickiewicz University, Maków Polnych 16, 61-606 Poznan, Poland

* Corresponding author (e-mail: mrajat{at}isical.ac.in)

Metamorphosed shales, heterolithic deposits and sandstones build up the Palaeoproterozoic Chaibasa Formation in east India. The shales (referred here to as the fine-grained facies) comprising mudstone (clay and silt size) with some minor amounts of very fine to fine sandstone were deposited below storm wave base in a deep marine basin that simultaneously underwent tectonic activity. This fine-grained facies contains strongly deformed layers, intercalated between undeformed layers. Sedimentological analysis of the deformations indicates that they formed while still in an unconsolidated or slightly consolidated state, partly during and after sedimentation, but before being covered by younger sediments. The types of deformation structures indicate an earthquake-induced origin. Thus, they should be considered as seismites. The soft-sediment deformation structures in the seismites show a wide variety of shapes and other characteristics that appear to depend on their relative position to the epicentre of the earthquake.