Grain size changes across the brink zone in gravelly Gilbert-type deltas: examples from the Pleistocene Corinth Rift (Greece)

Gilbert-type deltas present a distinctive tripartite sedimentary architecture consisting of a fluvial delta plain topset, a steeply inclined foreset of subaqueous delta-slope deposits, and a sub-horizontal bottomset consisting of prodelta deposits (e.g., Gobo et al., 2015). A determining component of a Gilbert-type delta is its narrow delta brink, which facilitates the transfer of sediment from the delta plain topset to the delta front. It regulates the overall sediment distribution trend along the slope, based on the available accommodation space and the degree of sediment storage or bypass. The resulting sedimentary architecture is an alternation of sigmoidal and oblique toplap geometries that are preserved at the topset-foreset transition. These geometries are indicative of a base level rise in the former case and of a base level fall or stillstand in the latter case. Additionally, being accumulated below the base level, foreset deposits of Gilbert-type deltas can serve as a continuous archive of changes in sediment supply or accommodation space affecting the delta during its entire lifespan. These architectural features make Gilbert-type deltas valuable archives for documenting recurrent variations in accommodation space (e.g., relative sea-level changes) or sediment supply (e.g., river discharge pulses) both on Earth (Gobo et al., 2015) and on Mars (Mangold et al., 2021). For these reasons, outstanding exposures of Pleistocene Gilbert deltas of the Corinth Rift (Greece) have been selected to unravel the link between sediment distribution across the delta brink zone and relative forcings. Sedimentological and architectural data on sigmoidal foreset geometries, where the genetic relationship between topset and foreset beds is evident, were collected in the field during ground-based field surveys, as well as through the analysis of images and 3D photogrammetric models obtained from drone surveys. Grain size variations in topset and foreset beds were analyzed to establish a relationship between the sediment transport capability of the feeder topset and the sediment trapping efficiency of the associated delta front. Establishing such a relationship would allow the reconstruction of variations in sediment supply to the delta, even where topset beds are not preserved. The results of this study can provide insights into changes in sediment supply dynamics in deltaic systems associated with specific climate variations both on Earth and on Mars.