PROPAGATION OF ENVIRONMENTAL SIGNALS AND SEA LEVEL VARIATIONS IN FLUVIO-DELTAIC ENVIRONMENTS: INSIGHTS FROM A DUAL MOVING BOUNDARY FRAMEWORK

Fluvial deltas are dynamic features of the world’s coastlines and are rich in natural resources such as water and hydrocarbons. Additionally, their subsurface architecture holds clues to past climate and sea level change because of their position at the intersection of the mainland and the ocean. This record can provide us valuable information on how fluvio-deltaic environments might respond to external forcing in the future, including an acceleration in sea-level rise.

In order to reverse engineer the processes that led to the present alluvial architecture of fluvial deltas, theories for stratigraphic interpretation need to detangle the signal of allogenic (external) forcing in the sedimentary record, from autogenic (internal) dynamics of the depositional system such as the dynamics of the fluvio-deltaic surface. This project aims to fill this knowledge gap with a one-dimensional forward model, previously applied to the evolution of sedimentary basins under eustatic sea-level cycles, to also account for variations in water discharge, assuming normal flow, and sediment supply. The domain of this model includes a sedimentary prism, which evolves on top of a linear and non-erodible (bedrock) basement, and is delimited by two moving boundaries: the shoreline and the alluvial–bedrock transition.

We use this model to analyze the statistical properties of elevation series in time and space and their relationship with the allogenic signal. Under cycles in water discharge and sediment supply, points along the fluvial surface experience oscillations from high to low rates of sedimentation, or even erosion when the amplitude of the environmental signal is large enough. Sedimentation rate oscillations dampen as we move downstream, making the alluvial-bedrock transition a better environmental recorder than the shoreline. Under sea level cycles, point elevations also shift periodically between sedimentation and erosion, but the sea-level signal attenuates upstream. Consequently, as we might expect, the shoreline is a better recorder of sea-level variations than the alluvial-bedrock transition.

In summary, our framework, developed to solve moving boundary problems, can provide quantitative insight into how environmental signals and sea level variations propagate in fluvio-deltaic environments.