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Fine-Grained Versus Coarse-Grained Wave Ripples Generated Experimentally Under Large-Scale Oscillatory Flow

¹ Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, K7L 3N6 Canada; present address: Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada; dcumming{at}nrcan.gc.ca
² Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, K7L 3N6 Canada; present address: Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
³ Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, K7L 3N6 Canada

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 ~ 10 cm, heights < 1 cm) formed exclusively in very fine sand at low 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°) cross stratification resembling hummocky cross stratification, whereas the large ripples in coarse sand would deposit high-angle (15–25°) cross stratification that might be 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.