Slope-stability change from late Pleistocene to Holocene in the Ulleung Basin, East Sea (Japan Sea)

Abstract

A wide variety of mass-movement deposits prevail along the entire margin of Ulleung Basin, overlain by a hemipelagic mud unit (1–2 m thick). The boundary between the lower turbidite sequence and the upper hemipelagic mud on the basin plain is distinct, and approximately coincides with the well-known, basin-wide ash layer (Ulleung ash) of early Holocene age. In the absence of significant terrigenous sediment input, the lithologic change from the turbidite to hemipelagic mud may reflect physical effects of glacio-marine eustatic sea-level change. Sea-level lowering during the glacial period may have generated excess pore pressure within the sediments caused by reducing hydrostatic confining pressure, and facilitated triggering of the deep-water slope failure by earthquakes. Storm waves may also have affected the uppermost slope area to induce small-scale slope instabilities during the lowered sea-level stand. These glacial factors destabilizing the slope appear to have enhanced large-scale slope failures during the late Pleistocene. In contrast, the topmost hemipelagic mud attests to a stable slope phase associated with raised sea-level in the Holocene, consistent with the infinite slope stability analyses.

Calculations of the sedimentation rates based on C-14 dates of several ash layers indicate that the pre-Holocene turbidites on the basin plain had accumulated at an average rate of 40 cm/1000 years with a minimum recurrence interval of 50 years during the late Pleistocene. Mass-flux estimations between the slumps and turbidites suggest a predominance of the nondisintegrative mode of slope failures in the Ulleung Basin. In each event of slope failure, most of the failed mass rested on the slope as slide/slump blocks, and only its meager portions were deposited on the basin plain as turbidite layers.