Downstream elimination of gravel in multiple distributary channels on the Selenga River delta topset, Russia: a morphodynamics case study of fan delta at the Baikal Rift basin margin
Dong, Tian Yang
Nittrouer, Jeffrey A.
Master of Science
The Selenga River delta, Lake Baikal, Russia, is approximately 600 km² in size and contains multiple distributary channels that receive varying amounts of water and sediment discharge. This delta is positioned along the deep-water (~1600 m) margin of Lake Baikal, a half-graben styled rift basin, qualifying it as a modern analogue of shelf-edge system. This study provides a detailed field survey of channel bed sediment composition, channel geometry, and water discharge. Our data indicate that the delta exhibits downstream sediment fining, ranging from predominantly coarse gravel and sand near the delta apex to silt and sand at the delta-lake interface. An analytical framework is developed utilizing field data to evaluate the downstream elimination of gravel within the multiple distributary channels. Our major findings are: 1.) the Selenga River delta consists of at least eight orders of distributary channels, 2.) with increasing channel order, channel cross-sectional area, width-depth ratio, water discharge, boundary shear stress, and sediment flux all decrease downstream, 3.) the downstream elimination of gravel in distributary channels is caused by reducing boundary shear stress downstream where water discharge is partitioned among bifurcating channels, 4.) Gravel is trapped on the delta topset due to frequent basin subsidence, thus preventing coarse material from being delivered to the axis of the rift basin. The distribution of sediment in deltaic channels and the associated sediment transport processes that construct stratigraphy, combined with the active tectonic setting, allow sedimentary systems like the Selenga delta to be preserved in the long-term geologic record. Therefore, the location of the Selenga River delta along the active Baikal Rift margin renders the opportunity to explore the influence of tectonics and sedimentation on deltaic processes that produce basin stratigraphy.