Soft-sediment deformation structures from the Paleoproterozoic Damtha Group of Garhwal Lesser Himalaya, India

Soft-sediment deformation (SSD) structures are observed in the Paleoproterozoic Damtha Group in the Garhwal Lesser Himalaya for a lateral extent of over 110 km in comparable litho-sections. The SSD structures are primarily observed in medium- to fine-grained sandstones, siltstones and shale at sand–mud interfaces that were deposited in an inter-tidal to supra-tidal setting of the shallow marine platform. The SSD structures are conspicuously absent in horizons within facies dominated by a single grain size. The SSD structures include load structures, namely drops, pseudonodules and ball-and-pillow structures, along with contorted beds, slumps, cusps and water-escape structures of varying geometry and dimensions. The SSD structures are invariably bounded by undeformed beds and their dimensions depend on the thickness of bed affected by liquefaction. The observed SSD structures show distinct similarities with liquefaction-induced deformation structures in geological records and those produced in the laboratory under reverse and normal density gradient conditions. The recurring nature of SSD structures in the stratigraphic column over large spatial distances points toward a trigger mechanism with recurrent activity that affected the basin on a regional scale such as prolonged rifting of the Damtha basin during the Paleoproterozoic. This would have caused frequent earthquakes and shakes of the basin floor, producing recurrent SSD structures. In view of the similarity with seismites and absence of a viable alternative, rift-related seismicity is preferred as the trigger mechanism for the observed SSD structures in the Damtha Group.