Upslope migrating sand waves on sediment-starved shelves: An example from the southeastern continental margin of the Korean Peninsula

An uncharted field of sand waves was discovered in a low-relief submarine canyon incised in the outer shelf on the southeastern continental margin of the Korean Peninsula in water depths of 180–190 m. We characterize the nature and origin of the waves and the sand forming them using sub-bottom chirp profiles, eXpendable bathythermograph (XBT) profile, multibeam echosounder (MBES) data, and sediment samples from four piston cores. Two types of sand waves characterized by distinct height versus wavelength relationships were found in the study area. The sand waves in the upper, narrower part of the shelf-incised canyon are sinuous-crested, with amplitudes of 0.3–2.1 m (mean: ~1 m) and wavelengths of 10–45 m (mean: ~24 m). Their asymmetry indicates migration upslope in a southwesterly direction, opposite to the surface currents. In contrast, the lower part of the canyon that is wider and closer to the margin of the continental shelf hosts nine long (ca. 1 km) curvilinear-crested sand waves with symmetrical crests; these waves likely reflect transient bedforms forming under fluctuating current conditions.

The sediment of the sand waves consists of a variable mixture of siliciclastic and carbonate materials. The carbonate fraction (~22–55%; mean: ~34%) is derived mainly from the remains of bryozoans, bivalves, echinoderms, foraminifers, gastropods, and serpulids. Six bioclasts were dated by the radiocarbon method between ca. 41.3 and 11.8 ka BP. These relatively old ages and palaeontological data supports reworking from a shallow-marine environment during the last glacial transgression and limited sedimentation/sediment supply in the study area. The siliciclastic fraction (~44–79%; mean: ~37%) is composed of rounded to subrounded quartz and feldspar of moderate to good sorting and a mean grain size of ~1.3 phi (medium sand). The uppermost ~30 cm of all the sand wave cores reveals a decrease in the grain size of the siliciclastic fraction coupled with an increase in the carbonate/siliciclastic ratio, suggesting episodic sediment reworking and migration of the sand waves in response to fluctuating bottom currents. The coarser sediment that forms the core of the sand waves records bedload transport during periods of stronger currents. Finer carbonate-rich pelagic sediment (i.e., plankton) accumulated at the top of the sand waves during periods of weaker bottom currents. Significantly, our results show that the grain size and mineralogy of the sediment composing the sand waves are controlled by changes in hydrodynamic conditions. Our study provides novel geomorphological evidence for the influence of SW-flowing cold-water incursions (Korean Strait Bottom Cold Water) on the seafloor sediments.