ABSTRACT

Modal analyses of the sand-sized fraction of 200 surface samples provide significant information on the origin and distribution of continental shelf sediments of northern Portugal. About 80% of the samples are polymodal, and the highest detected polymodal character (number of modes of the sample) was heptamodality (7 modes).

The areal distribution of principal modes reveals the general depositional pattern on the shelf. Coarsest modes delineate the presence and general surficial geometry of relict deposits, and the finest modes reflect accumulation zones or areas where the sediments are in adjustment to shelf processes. The latter type sediments tend toward unimodality. Finer modes are more affected by distributor processes, while coarser modes develop during regressions and persist during transgressive periods. Relict modes are thought to persist where bottom energy is low and sediment supply is low, or where sediment bypassing occurs.

The grouping of all modes leads to the definition of six modal classes, the genesis of which is related to either ancient or recent processes. Areal distribution of these classes of modes delineates two paleo-littoral zones and suggests that some fine sand from the nearshore is bypassing the shelf to accumulate near the shelf-break, Although not as powerful as the decomposition of multimodal curves, results indicate the utility and efficiency of modal analysis in deducing relevant aspects of ancient and modern processes.