The Miocene "Doumsan fan-delta" in the southwestern margin of the Ulleung Back-arc Basin comprises elements common to both Gilbert- and slope-type deltas, forming a composite fan-delta system. It contains an alluvial fan with laterally-discontinuous breccia and gravelstone, deposited by sheet floods, debris flows and stream flows. The Gilbert-topset is characterized by horizontal, cross-bedded and wedged gravelstones and sandstones, formed in shallow braided channels and bars, whereas the foreset (initial slope of ca. 20 degrees ) is dominated by stratified gravelstones formed by gravity sliding and avalanching. At the toe, slope failure caused deposition of mainly disorganized and graded gravelstones which are mostly penecontemporaneously deformed. Further downslope, deposition of well-sorted, disorganized and graded gravelstones are indicative of high-density turbidity currents and debris flows. Here, the bed geometry is suggestive of submarine channels and lobes. On the basin slope, high- or low-concentration turbidity currents, slide/slump and debris flows form debris aprons. On the basin plain, hemipelagic sedimentation was dominant with scarce low-concentration turbidity currents. The Doumsan fan-delta sequence shows a radial, lateral facies change, progressively fining and thinning away from the point of origin. It contains an entire spectrum of sedimentary environments that are characteristic of rapidly-subsiding intraslope basin formed by multi-lateral strike-slip faults.

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