Occurrences and origins of soft-sediment deformation structures in the late Pleistocene marine terrace deposits of the southeastern coast of Korea

Abstract

Soft-sediment deformation structures (SSDS) are examined in the late Pleistocene marine terrace deposits of the southeastern coast of the Korean Peninsula. The marine terrace deposits are exposed in thicknesses of more than 10 m, and consist of lagoon, foreshore to backshore, coastal dune, and foreshore to backshore deposits in ascending order. The SSDS are restricted to the coastal dune deposits and the upper part of the underlying foreshore to backshore deposits, and are grouped into ductile and brittle deformation structures. The ductile deformation structures include irregular laminations, compacted laminations in the hinge zone of folds, asymmetric folds of foreset laminae, and water escape structures. The brittle deformation structures include syndepositional faults or deformation bands. The most probable triggering mechanism for the SSDS is liquefaction by moderate-magnitude earthquakes (M = 5–5.5). Likely, the SSDS were generated over time in various stress fields, including those of main shocks, aftershocks, and later seismic events. The evolution of the SSDS-bearing deposits in the study area can be summarised as follows: 1) rising of the groundwater table in the coastal dune deposits as a result of seismic shock, 2) formation of SSDS by liquefaction in the lower part of the dune deposits and upper part of the underlying foreshore to backshore deposits, 3) descending of the groundwater table, and 4) deposition of foreshore to backshore deposits on the SSDS-bearing dune deposits with erosion of the top of the dune deposits.