Autogenic shrinkage and channel destabilization of an overexpanded downstream alluvial system under steady rise of relative sea level: An experimental study.

• During sea-level rise, an alluvial-deltaic system has a critical spatial dimension. • The over-expanded system is subjected to non-deltaic transgression with shrinkage. • Shrinkage can be normalized by autostratigraphic length and time scales. • During shrinkage, the alluvial channels change their intrinsic behavior. • The shrinkage concept can account for some alluvial features on Earth and Mars. The dynamics of downstream alluvial systems undergoing relative sea level (RSL) rise exhibit significant variations depending on spatial size. According to autostratigraphy theory, such systems cannot sustain deltaic sedimentation under a steady RSL rise (rate R slr), and are prone to non-deltaic transgression once their plain area A exceeds the critical limit A crt. During this transgression, A decreases asymptotically toward A crt , while the overall alluvial aggradation rate R agg_overall increases toward R slr. In this study, we investigated the behavior and morphodynamics of an overexpanded system through physical modeling, with a focus on the inherent area scale of the depositional system. In the early stages of non-deltaic transgression (A >> A crt , R agg_overall << R slr), the alluvial channels experience minimal aggradation rate R agg_channel (< R slr), leading to stabilization and inundation. As the transgression progresses, the channels aggrade more rapidly, increasing the likelihood of lateral migration and avulsion. Eventually, a state of morphodynamic equilibrium is reached, characterized by A ∼ A crt , R agg_overall ∼ R slr , and R agg_channel ∼ constant > R slr despite sustained non-deltaic transgression. During this equilibrium stage, the channels undergo rapid aggradation, resulting in complete destabilization with continuous lateral migration and frequent avulsion, forming system-wide subaqueous steps. The implications of overexpansion and autogenic shrinkage in response to RSL rise extend to stratigraphic convergence during RSL cycles and provide insights into Holocene non-deltaic transgression. Moreover, when combined with decelerating RSL rise, this process could have facilitated marine delta development. However, projected accelerated RSL may lead many modern marine deltas to transition into non-deltaic transgressive systems. Additionally, natural alluvial channels may exhibit varying behavior depending on the spatial size of the non-deltaic transgressive system. Furthermore, the autogenic shrinkage model may offer an explanation for the stepped surfaces observed on alluvial fans and deltas on Mars. [ABSTRACT FROM AUTHOR]