Abstract
A complex (petrographic, micropaleontological, and X-ray diffraction) investigation of the sedimentary cover on the northwestern slope of the Okushiri Ridge in the Sea of Japan revealed that its basal layers are of Oligocene age and composed of terrigenous silty-clayey sediments, which were deposited in coastalmarine environments with calm hydrodynamics and low sedimentation rates. The relative sea-level rise combined with regional tectonic processes at the early-middle Miocene transition resulted in widening and deepening of sea basins and accumulation of a thick diatomaceous-clayey sequence of middle-upper Miocene sediments. Tectonic activation in the Pliocene was responsible for development of the ridge and exhumation of rocks formerly occurring at depth of 500–1000 m.
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