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A study was made of bathymetric charts of those coast lines characterized by large vertical tidal ranges (greater than 10 feet). In these areas tidal currents are strong (1-5 knots) and may significantly affect sedimentation. Two characteristic types of sand accumulation were found which appear to be formed by these tidal currents. Both exhibit wave-like profiles, and are of a scale significant to oil exploration.
The first type is here called "tidal current ridges." These are rhythmic series of ridges oriented parallel with tidal currents. They are 25-100 feet high, 5-40 miles long, and spaced 1-6 miles apart. Most are composed of sand, but some may be mud or silt. Their spacing is proportional to the depth of water. Their origin appears to be related to the similar problem of the hydraulic geometry of stream channels. Although best developed in the Bay of Korea and the Gulf of Cambay, these ridges appear to be present wherever tidal current velocities range between 1 and 5 knots and a supply of sediment is available.
The second type is sand waves. These are large ripple marks oriented perpendicular to the current direction. Recent evidence by European oceanographers has indicated that, whereas in rivers these waves are fairly small-scale features, in the open ocean they commonly have heights exceeding 25 feet. Cartwright and Stride have shown a wide distribution of sand waves of this size, particularly in the North Sea. Their relation to tidal current ridges is not known, although they appear to occur in the same environment.
Since tidal currents are now significant in shallow ocean areas, their effect should be visible in a large percentage of the shallow-water deposits of the geologic past. In particular it is suggested that some of the lenticular sands of the Chester Series of Illinois, of the Cardium Formation of Canada, and of the Clinton sands of Ohio show tidal current effects. The rhythmic pattern of tidal current ridges and sand waves should be considered in the study of the distribution of these and other shoestring sands.
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