We analyzed data from seven piston cores, multi-channel seismic-reflection (MCS) and chirp profiles, and multibeam echosounder (MBES) data to study the distribution, emplacement time, sedimentary facies, and depositional processes of sediment-gravity-flow deposits in the Onnuri Basin, a confined basin in the East Sea. These data reveal that debris flows have traveled ca. 30 km downslope, forming a seismic facies consisting of stacked, wedge-shaped, transparent units separated by high-amplitude continuous reflectors. Analysis of piston cores shows three distinct sedimentary units, throughout the basin. The lowest unit, I, is a debrite containing numerous mud clasts of varying size and color distributed in a mud-rich matrix; it is absent over elevated basinal highs or ridges, such as the Onnuri Ridge, suggesting that local topography controls its distribution. The debrite forms a recognizable acoustically transparent layer on subbottom chirp profiles (av. 7 m thick), covers approximately 500 km2, and has an estimated volume of ∼ 3.5 km3.The overlying unit, II, contains normally graded beds composed of massive sand, laminated and cross-laminated sand and silt, and a thick cap of structureless mud. This unit is interpreted to be a megaturbidite deposited from turbidity currents that originated from the flow transformation of debris flows on the upper continental slope. The megaturbidite covers the entire basin (at least 650 km2), and has an average thickness of 2.8 m (maximum thickness of 4.35 m), and comprises a volume of 1.8 km3. Variations in grain size and sedimentary structures suggest that the megaturbidite was deposited by progressively waning flows that reflected off basin flanks and ridges. The thick (up to 3.65 m) structureless mud cap further indicates deposition in a confined basin. The sharp basal contact, together with the lack of hemipelagic sediments between debrite and overlying megaturbidite, suggest that both were deposited during the same flow event, likely to have originated from a single catastrophic slope failure. Collapsing slide material evolved into a debris flow, from which a turbidite formed by dilution of the debris flow. Radiocarbon dates suggest that the slope failure occurred about 13–11 ka, a time when sea level was ca. 50 m lower than at the present day. Hemipelagic sediments in the topmost unit, III-2, above the megaturbidite indicate that the basin has been stable since ca. 11 ka.We provide robust evidence that submarine slope failures evolve downslope into slides, debris flows, and finally, thick megaturbidites. This contribution highlights the importance of seafloor morphology on the distribution and stratigraphy of submarine flows in confined basins.

中文翻译：

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密闭盆地中的深水碎屑和相关的巨浊体：以韩国东海Onnuri盆地为例

我们分析了来自七个活塞核心，多通道地震反射（MCS）和线性调频剖面以及多波束回声测深（MBES）数据的数据，以研究沉积物重力流沉积的分布，沉积时间，沉积相和沉积过程。 Onnuri盆地，东海的一个受限制的盆地。这些数据表明，泥石流已经传播了大约10年。下坡30公里，形成地震相，由堆叠的楔形透明单元组成，并由高振幅连续反射器隔开。活塞芯的分析显示了整个盆地中三个不同的沉积单元。最低的单位I是一个碎屑，它包含许多分布在富泥基质中的大小不一，颜色各异的泥屑；在高架的盆地高点或山脊（例如Onnuri山脊）上不存在，提示当地地形控制着它的分布。碎屑在底部bot声波剖面（厚约7 m）上形成可识别的透声层，覆盖约500 km2，估计体积约为3.5 km3。上覆单元II包含通常由大块沙子组成的梯度床，夹层和交叉夹层的沙子和淤泥，以及厚厚的无结构泥浆盖。该单元被解释为是由浊流沉积的巨浊体，该浊流源自上大陆坡上泥石流的流动转换。巨浊体覆盖整个盆地（至少650 km2），平均厚度为2.8 m（最大厚度为4.35 m），体积为1.8 km3。晶粒尺寸和沉积结构的变化表明，巨浊浊石是由逐渐减弱的流沉积而成的，这些流从盆地的侧面和山脊上反射出来。厚的（最多3.65 m）无结构泥浆盖进一步表明沉积在密闭盆地中。尖锐的基底接触以及碎屑岩和上覆的巨浊云母之间缺乏偏航沉积物，这表明两者都是在同一流动事件中沉积的，很可能是由于一次灾难性的边坡破坏引起的。坍塌的滑片材料演变成泥石流，通过稀释泥石流从中形成了浊石。放射性碳年代数据表明，边坡破坏发生在大约13-11 ka时，此时海平面约为。比今天低50 m。最上层单元III-2的半海岸沉积物 巨浊体上方的高空表明该盆地自大约 11 ka。我们提供了有力的证据，证明海底边坡破坏会导致坡度下降，形成滑坡，泥石流，最后形成厚厚的巨浊体。这一贡献凸显了海底形态对密闭盆地海流分布和地层学的重要性。