LARGE-SCALE GRAIN-SIZE SORTING ON INNER SHELVES: FROM GRAB SAMPLES TO NUMERICAL MODELING

Orrin Pilkey was one of the first to document the puzzling phenomenon of large-scale grain-size sorting far from shore. Based only on a widely-spaced grid of surface grab samples, MacIntyre and Pilkey (1969) showed that domains of coarse sand to gravel and shell hash can lie immediately adjacent to, and interfingered with, domains of fine sand on the North Carolina inner shelf. More recently such features, sometimes called 'rippled scour depressions,' have been observed with sidescan sonar on numerous inner-shelf locations and appear to be pervasive features continental shelves. Coarse sediment domains, which typically extend kilometers from shore and have widths on the order of 100s of meters, contain wave-generated ripples on the order of a meter in wavelength. Ripples in the fine sediment domains are much smaller. Early explanations invoked some form of offshore-directed, localized currents intense enough to winnow the fine sediment, leaving coarse lag deposits (e.g. Cacchione, Grant and Tate, 1984). However, such currents have not been observed.

We hypothesize that large-scale grain-size sorting arises from a feedback involving interactions between wave motions, wave-generated bedforms, mean currents, and sediment transport. We further hypothesize that the patterns observed on inner shelves evolve through self-organization, under the influence of spatially homogeneous, generally alongshelf currents. A simple numerical model designed to test this hypothesis robustly produces large-scale sorting. While the details of the patterns in the model depend on the wave, current, and sediment conditions, the scales and main profile and plan view characteristics are consistent with observations.