FINE-GRAINED VERSUS COARSE-GRAINED WAVE RIPPLES GENERATED EXPERIMENTALLY UNDER LARGE-SCALE OSCILLATORY FLOW

Wave ripples were generated in a wave tunnel under large-scale oscillatory flow (orbital diameter 1-4.5 m) using two different grain sizes, very fine sand and coarse sand. The geometry of bed configurations that were produced varied strongly as a function of grain size: small anorbital ripples (wavelengths similar to 10 cm, heights < 1 cm) formed exclusively in very fine sand at loss, oscillatory velocities, whereas large orbital ripples (wavelengths 50-350 cm, heights 7-26 cm) formed in both very fine and coarse sand, but were subdued, sharp-to round-crested, and 2-D to 3-D in very fine sand, and steep, sharp-crested, and 2-D in coarse sand. The large ripples in fine sand, if aggraded, would deposit low-angle (5-15 degrees) cross stratification resembling hummocky cross stratification, whereas the large ripples in coarse sand would deposit high-angle (15-25 degrees) cross stratification that might he mistaken for the deposit of a dune because of its high dip angle and large set thickness (> 5 cm). These results support the hypothesis advanced by Leckie (1988) that large waves generate markedly different stratigraphic signatures in fine-grained and coarse-grained sediment.