Palaeoproterozoic seismites in damtha group, lesser Himalaya, India

Abstract

Palaeoproterozoic Damtha Group of the Garhwal Lesser Himalaya, an argillite-siliciclastic shallow marine succession associated with syn-sedimentary mafic volcanics, possibly represents the oldest sedimentary rocks exposed in the Himalayan Orogen. It constitutes of argillite dominated Chakrata Formation overlain by siliciclastic dominated Rautgara Formation. The soft-sediment deformation structures (SSDS) observed at different levels and are sandwiched between undeformed sedimentary layers. SSD structures are both syn- and meta- depositional types with stages from incipient to advance phases. The SSD structures mainly include ball and pillows of varied dimensions; deformed mud/sand balls with dislocated laminae and flame features are noticed in abundance along the Damtha (DM) section; however, in the Chakrata (CHK), Shivpuri (SH) and Satpuli (SP) sections the SSD structures are not in abundance. SSDS predominantly observed at fine sand and mud interface and are conspicuously absent in single grain size facies. Size of the structures also varies with the bed thickness at the interface. Petro-textural behavior of the SSDS layers also show micro- lobes and micro- silty dykes particularly in the two components i.e. clay and silt rich sediments. Widespread spatial and temporal distributions of SSDS imply repetitive nature of causative processes in the depocentre. The SSD structures in Palaeoproterozoic Damtha Group show distinct similarity with the liquefaction induced deformational structures observed with those generated in the laboratory. Further, the widespread occurrence of SSD structures (seismites) in the comparable sections, over large spatial distances in association with mafic volcanism during Palaeoproterozoic era points towards a regional triggering agent, like frequent earthquakes.