Occurrence and morphometric analysis of sorted bedforms on the Alentejo continental shelf, Southwestern Portugal.

Abstract

The distribution of detrital sediments across continental shelves can provide important information on mineral concentrations, geological processes, and ecosystem substrates. On the southwestern Portuguese continental shelf, this distribution is governed by a complex interplay of hydrodynamic energy gradients, seafloor morphology, and eustatic sea-level changes. In particular, the strong energy gradient from the coastline to the shelf break, combined with open-ocean-wave dynamics, tidal regimes, and active continental uplift, determines the specific nature and grain size of submarine deposits. To better understand these dynamics, this study employed continuously recording geophysical methods, including multibeam echosounder bathymetry and backscatter, complemented by ultra-high-resolution seismic profiling and sediment sampling. Our results reveal four distinct depositional environments: homogeneous fine-grained sediments near the shelf break, sorted bedforms on the flat outer shelf, isolated sorted bedforms on the mid-shelf, and a homogeneous sand sheet near the coast. Notably, the presence of isolated bedforms at depths exceeding 80 meters suggests they are generated by wave action, reflecting the long-term impact of the Holocene sea-level rise over the last 8,000 years and possibly active mobilization under modern hydrodynamic conditions. Consequently, these findings highlight how high-resolution mapping can reveal sediment patterns and environmental histories that remain hidden through traditional discrete sampling.