Continental shelf sediments of northern Portugal

Abstract

Recent research provides the first significant information about sedimentation on the continental shelf of northern Portugal. This paper presents data and interpretations obtained from textural and compositional studies of surface sediment. The shelf primarily contains sand-sized particles of lithogenic quartz (<80m water depth) and biogenic carbonate (80m). A nearshore deposit (<30m) is actively forming from fine sand composed of immature quartz particles (subangular, hyaline) which are transported southward by prevailing waves and currents from fluvial sources in the northern part of the study area. Near the northern rivers (e.g., Minho, Lima, Porto rivers), mica also is found nearshore in high concentrations (20 %). The mid-shelf region (30 to 80 m) is dominated by coarse sand and gravel composed of mature quartz particles (rounded, frosted, iron-stained), which probably were excavated during the Holocene transgression from terrestrial deposits and redistributed by littoral processes. During low stands of sea level, lithogenic sediment was carried off the shelf through submarine canyons, resulting in reduced contribution of lithogenic particles to the outer shelf region. This has caused biogenic carbonate to dominate the sediment there. Grain size decreases across the outer shelf from medium to very fine sand, as the predominant carbonate material changes from mollusk fragments to foraminifera tests. Glauconite has formed in many of the foraminifera tests, and glauconite concentrations are high (20%) in some localities near the shelf break.