Abstract

Sedimentologists commonly rely on selected mineral phases in shelf sediments to establish sediment sources and transport patterns. Unfortunately, selective sorting processes in the wave-dominated nearshore can strongly bias mineral composition of shelf sands, leading to misinterpretation of mineral assemblage analyses. However, if regional variation in sediment grain size and grain density is properly interpreted, then sorting processes and associated mechanisms of shelf sediment transport can be established. Such interpretations of nearshore placer deposits have been used by the authors to document wave-dominated northward transport of shelf sediments along the Oregon coast during Holocene and Pleistocene times.

Along the central Oregon coast, exposed placer deposits rich in ilmenite (density 4.7 g/cm³) and chromite (density 5.2 g/cm³) have been extensively sampled, together with quartz-rich sands from a series of modern beaches and the Seven Devils terrace, a transgressive Pleistocene shelf sequence. Grain size and grain density relationships between quartz, pyroxene, and the placer oxide minerals in microlaminations demonstrate flow shear mechanisms for the selective enrichment of the dense oxide minerals. On a regional scale (tens of kilometres), enrichment of the fine-grained heavy minerals occurs at the northern end of littoral cells defined by major headlands, for both modern beach deposits and for the Seven Devils shelf sequence. Large winter surf from the south drives strong longshore currents to the north, carrying both light and heavy minerals to the northern sections of modern littoral cells. Swell direction reverses during the summer months and weak southward-trending currents transport only less dense quartz and feldspar grains southward. A similar transport mechanism is envisioned for the enrichment of heavy minerals in the Seven Devils terrace deposits. Interestingly, the extreme enrichment of heavy minerals (>80% by weight) in the lower facies is reflected to a lesser degree throughout most of the shelf section, indicating the importance of the initial selective transport process in determining local shelf sediment composition.