Decoding tectonic and lithological controls on sediment exported from footwall catchments: Implications for hanging-wall stratigraphy

Hillslope and river channel processes combine to erode topography and release sediments in response to external forcing, such as tectonics. Any forcing that drives these processes to shape topography likely also controls the resulting sediment characteristics; however, the extent to which the forcing translates into exported sediment grain size remains elusive, especially where tectonic and lithological forcings coexist. Here, we focus on two catchments with these characteristics in the Gulf of Corinth, Greece, to probe how tectonics and lithology compete to control exported sediment grain size via hillslope and channel processes. Topographic analysis reveals that both tectonic and lithological forcings can drive hillslope and channel processes to shape topography. In the Sithas catchment, normal faulting older than 600 ka initiates knickpoints that steepen hillslopes and trigger concentrated mass-wasting downstream of the knickpoints. In the Phoenix catchment, which was only perturbed by active faulting for <100 k.y., the landslide-prone bedrock triggers pervasive mass-wasting and steepens hillslopes. Combined with the observation that mass-wasting produces coarser grains, our data therefore show that both tectonic and lithological forcings can coarsen sediment generated on hillslopes. If hillslope sediment is friable, however, coarsened sediment from hillslopes can be rapidly abraded upon entering river channels and lose its coarse signal before reaching catchment outlets (e.g., tectonically induced coarsening in the Sithas catchment). In contrast, coarsened sediment from hillslopes can preserve its coarse signal to catchment outlets if its lithology is resistant to abrasion (e.g., lithologically induced coarsening in the Phoenix catchment). En route to catchment outlets, sediments coarser than the threshold for entrainment are filtered out; however, this selective transport attenuates rather than obliterates the coarse signal because coarse sediment input itself can increase the entrainment threshold by influencing channel steepness. Finally, we explore how continuous unroofing and knickpoint propagation may cause grain size variations in exported sediments and impact hanging-wall stratigraphy. Consequently, our study illustrates that lithology can outweigh tectonics in determining the characteristics of sediment exported from footwall catchments and paves the way for more comprehensive stratigraphic interpretation.