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The formation of large mudwaves by turbidity currents on the levees of the Toyama deep-sea channel, Japan Sea



The development of mudwaves on the levees of the modern Toyama deep-sea channel has been studied using gravity core samples combined with 3·5-kHz echosounder data and airgun seismic reflection profiles. The mudwaves have developed on the overbank flanks of a clockwise bend of the channel in the Yamato Basin, Japan Sea, and the mudwave field covers an area of 4000 km2. Mudwave lengths range from 0·2 to 3·6 km and heights vary from 2 to 44 m, and the pattern of mudwave aggradation indicates an upslope migration direction. Sediment cores show that the mudwaves consist of an alternation of fine-grained turbidites and hemipelagites whereas contourites are absent. Core samples demonstrate that the sedimentation rate ranged from 10 to 14 cm ka−1 on the lee sides to 17–40 cm ka−1 on the stoss sides. A layer-by-layer correlation of the deposits across the mudwaves shows that the individual turbidite beds are up to 20 times thicker on the stoss side than on the lee side, whereas hemipelagite thicknesses are uniform. This differential accretion of turbidites is thought to have resulted in the pattern of upcurrent climbing mudwave crests, which supports the notion that the mudwaves have been formed by spillover turbidity currents. The mudwaves are interpreted to have been instigated by pre-existing large sand dunes that are up to 30 m thick and were created by high-velocity (10°ms−1), thick (c. 500 m) turbidity currents spilling over the channel banks at the time of the maximum uplift of the Northern Japan Alps during the latest Pliocene to Early Pleistocene. Draping of the dunes by the subsequent, lower-velocity (10−1ms−1), mud-laden turbidity currents is thought to have resulted in the formation of the accretionary mudwaves and the pattern of upflow climbing. The dune stoss slopes are argued to have acted as obstacles to the flow, causing localized loss of flow strength and leading to differential draping by the muddy turbidites, with greater accretion occurring on the stoss side than on the lee slope. The two overbank flanks of the clockwise channel bend show some interesting differences in mudwave development. The mudwaves have a mean height of 9·8 m on the outer-bank levee and 6·2 m on the inner bank. The turbidites accreted on the stoss sides of the mudwaves are 4–6 times thicker on the outer-bank levee than their counterparts on the inner-bank levee. These differences are attributed to the greater flow volume (thickness) and sediment flux of the outer-bank spillover flow due to the more intense stripping of the turbidity currents at the outer bank of the channel bend. Differential development of mudwave fields may therefore be a useful indicator in the reconstruction of deep-sea channels and their flow hydraulics.