Soft Sediment Deformation Structures in the Coastal Sediments of Bay of Bengal, Talsari, Odisha, India – A Possible Signature of Earthquake

ABSTRACT Trenches excavated within the finer heterolithic beach sediments of a tidal flat at Talsari Coast, Odisha, expose a 12 to 30 cm thick layer containing soft-sediment deformation structures. The layer features structures such as convolute laminations with broad troughs and sharp crests between them, along with upward-piercing sand dykes. Both the overlying and underlying layers consist of undeformed, planar laminated heterolithic tidal bundles. This layer with soft-sediment deformation structures extends for more than 5 km. The smoothly deformed tidal bundles, absence of micro-faults, and significant thickness variations of the individual laminae forming the convolute structures suggest that the sediment was still soft during deformation. Consequently, liquefaction and fluidization are considered responsible for the formation of these structures. The meta-depositional characteristics of the convolutes indicate their formation just below the sediment-water interface, implying the absence of a sediment load during their formation. In the absence of overlying sediment load, combined evidence of pounding waves, high-density mass flow deposits above, and the expulsion of groundwater suggest that cyclic loading due to vibratory ground motion from earthquake waves was responsible for the origin of these soft-sediment deformation structures. The study area is located near the Bengal Basin marginal fault, and over the last 60 years, seven earthquakes with magnitudes between 3 and 5.2 M have occurred. Considering the relative age of the host sediments, the 1964 Calcutta Earthquake may be responsible for the development of the observed soft-sediment deformation structures. This finding also underscores the vulnerability of the terrain to future earthquakes.