Stratigraphy of abandoned deltaic distributary channels and implications for assessing avulsion style

Abstract:

Avulsions relocate fluvial-deltaic channel pathways, thereby reorganizing the flow network delivering water and sediment to shorelines. An avulsion may either rapidly cut off sediment and water to the previous channel pathway or retain a tie channel that continuously supplies fresh water and sediment to the abandoned channel. Studies evaluating water and sediment flux between active and inactive channels are primarily informed by modern and ancient river-floodplain settings. As such, little is known about how river process-based understanding translates to abandoned deltaic channels, or how these sedimentary deposits are preserved in the stratigraphic record. This is a critical knowledge gap; fresh water and sediment supplied to abandoned portions of deltas are important for building topography and influencing vegetal cover, both necessary elements for delta resilience. Differences between floodplains and delta plains are expected, since deltaic tie channels are inevitably influenced by receiving-basin hydrodynamics (i.e., waves and tides). Here, the geomorphic evolution of two abandoned channels on the Huanghe delta, China are evaluated to discern the competing influences of fluvial and receiving-basin hydrodynamics. One channel was abruptly cut-off from upstream water and sediment supply via an engineered diversion and lacks a tie channel. The second channel was abandoned naturally and retains a tie channel. Sedimentary cores, measurements of water stage and flow velocity, and remote sensing of the planform evolution record the disparate abandonment histories of each channel. The rapidly abandoned channel exhibits an abrupt transition from fluvial-sourced bed material (coarse) to marine-sourced sediment (fine). In contrast, the channel abandoned naturally, which maintains a tie channel throughout its abandonment history, displays a gradual upward fining trend in sediment grain size, as it synchronously fills with fluvial- and marine-sourced sediment. This study shows that sediment filling an abandoned channel captures information about the style of channel abandonment. This is a valuable tool because avulsion style dictates the delivery of fresh water and sediment to abandoned delta lobes, thereby influencing resiliency to reworking by tides and waves, and geomorphic evolution.