How Does Subsidence Affect Deltaic River Systems?

Abstract:

Although it is clear that delta topset surfaces can maintain an average long-term parity between subsidence and sedimentation, the ways in which subsidence information is transmitted to the fluvial system and its effects on the rivers remain unresolved. Here we argue that the direct, local effect of subsidence on river systems is transmitted through spatial gradients in subsidence, and that the overall spatial mean subsidence communicates only indirectly with the river system, through its effect on relative sea level. With this idea in mind, we first express the local spatial variation of subsidence by taking the (vector) gradient of the local (scalar) subsidence rate expressed as a downward speed. This vector field represents the rate and maximum direction of rotation induced by subsidence gradients. We then compare this rotation-vector field to the surface-slope field, and decompose the subsidence-induced surface rotation into slope-normal and slope-parallel components. The slope-normal component is associated with lateral steering of the river channels, tending to direct them towards areas of maximum subsidence. The slope-parallel component acts directly to induce local erosion and deposition by changing the surface slope. The effect of the slope-normal component is also modulated by floodplain deposition and other out-of-channel effects. We apply these ideas to experimental data and to the Holocene Ganges-Brahmaputra-Jamuna (GBJ) Delta, Bangladesh, and compare the resulting predictions to stratigraphic and other observations.