Valentin Zuchuat , Miquel Poyatos-Moré , Bjorn Nyberg, Rachel A. Nanson, Stephen Sagar, Leo Lymburner, Robbi Bishop-Taylor

The long-term development of ancient and modern coastal distributary distributive fluvial systems (DFSs) during periods of relative sea-level highstand or fall usually drives net-progradation of shorelines. Such systems often develop in periods of relative sea-level highstand or fall and typically record annual to millennial-scale deviations in coastal trajectories. A new continental dataset (Digital Earth Australia Coastlines: DEA Coastlines) provides an opportunity to examine such variations in coastal behaviour over annual to decadal scales (1988-2019) at local to continental spatial scales. This dataset is herein applied to the 655 km coastline fronting Australia’s largest amalgamated coastal distributary distributive fluvial systems, which is situated in the epicontinental seaway of the Gulf of Carpentaria in the north of the continent. Despite the overall forced regressive conditions (i.e. progradation during relative sea-level fall), only 54% of this coastlines length net-prograded, whereas 47% was eroded. Though temporal cyclicity in progradation and erosion is evident along segments of this coast, these patterns could not be correlated with either the Southern Oscillation Index (R2 = -0.20) or rainfall (R2 = 0.24). Instead, short-term coastline dynamics appear to be the result of complex interactions between fluvial, wave, longshore current, and tidal processes. The high-resolution DEA Coastlines dataset highlights the diachronous, heterochronous, composite, and amalgamated nature of net-progradational stratigraphic strata that can develop in shallow-marine environments where hinge-points between progradating and retrograding coastal segments are dynamic features that migrate with time. Our conclusions show that shorelines display granular temporal and spatial deviations in coastal trajectory, with contemporaneous progradation and erosion occurring over 1-100 km length scales. This is significantly more heterogeneity than previously envisaged, thereby suggesting the need for updating models of coastal systems.