Channel-levee complexes, terminal deep-sea fan and sediment wave fields associated with the Toyama Deep-Sea channel system in the Japan Sea

Abstract

The Toyama Deep-Sea Channel (TDSC) in the Japan Sea is one of the most prominent deep-sea channels in rifted margins. This study revealed the course and morphology of the entire TDSC system for the first time, based on high-resolution and air-gun seismic reflection profiles, and sediment cores. The channel starts from Toyama Bay and extends for 750 km through the Toyama Trough, a Miocene rift, to the Miocene Yamato and Japan back-arc basins. The TDSC system is fed with sediment from the Northern Japan Alps, through tributary canyons on the narrow shelf, which are directly connected with rivers, even during sea-level highstands. Levee complexes border the channel in the Toyama Trough and Yamato Basin, and in the Japan Basin, the channel feeds the terminal Toyama Fan. Levees in the confines of the Toyama Trough are accompanied by an over-bank turbidite plain, whereas levees in the Yamato Basin are characterized by prominent sediment waves. Toyama Fan consists of a channel-levee complex with sediment waves in the upstream part and of lobes at the channel end. The course and morphology of the channel-fan system are primarily controlled by basin morphology. Thick, sheet-like sediments, deposited from ponded turbidity currents, have accumulated in the narrow, elongate Toyama Trough, whereas extensive levees and the Toyama Fan have formed in the more open basins. Distribution of sediments and consequent morphology of the channel-levee complexes are also controlled by Coriolis and centrifugal forces. Preferential development of the levees on the right-hand side is attributed to Coriolis-force tilt effects in the Northern Hemisphere. Centrifugal forces at the meander loops or bends of the channel result in flow stripping, causing levees to build on the outer bends. The distribution, form and orientation of sediment waves are consistent with the extent and direction of inferred spill-over turbidity currents, and with consequent levee growth. Fluvial-like features such as meanders, terraces, levee slumps and a crevasse splay are developed along the TDSC. Unlike other submarine channels, sinuosity seems to be controlled by bedrock structures rather than by valley slopes. Channel avulsion has not been recorded in the TDSC system. Active clastic deposition on the uppermost lobe during the past 1 ka suggests recent active sediment transportation through the channel. Sediment transportation, however, may have ceased during the Holocene in the cut- and fill-tributaries developed in the Quaternary succession on the slope to the trough, where a relatively wide shelf separates canyons from rivers in the eastern margin of the drainage area.