. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography FIGURE 26. Generalized sand transport paths around the British Isles and France, based on the velocity asymmetry of the tidal ellipse and the orientation and asymmetry of bed forms. From Kenyon and Stride (1970). istics. The most obvious is that allochthonous shelves tend to be floored by fine sands, fine muddy sands, or muds that have escaped from adjacent river mouths: autochthonous shelves in contrast are generally covered by coarser grained sand
. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography FIGURE 26. Generalized sand transport paths around the British Isles and France, based on the velocity asymmetry of the tidal ellipse and the orientation and asymmetry of bed forms. From Kenyon and Stride (1970). istics. The most obvious is that allochthonous shelves tend to be floored by fine sands, fine muddy sands, or muds that have escaped from adjacent river mouths: autochthonous shelves in contrast are generally covered by coarser grained sand of local origin. Although sur- faces of allochthonous shelves are constructional in nature, they tend to be smooth and featureless; their fine materials have traveled primarily in suspension, and the effective underwater angles of repose of the sediment may be too low to result in such large-scale bed forms as sand waves or sand ridges. However, such features are not totally unknown. Allersma (1972) has reported "mud waves" from the Venezuelan shelf that appear to be very similar to the shoreface-connected ridges of the Middle Atlantic Bight. Transport on Allochthonous Shelves Mechanisms of sediment transport on allochthonous shelves have been generally described by Drake in Chapter 9. Since this chapter stresses regional transport patterns, it seems worthwhile to summarize Drake et al.'s (1972) study of river-dominated sedimentation on the southern California shelf. This carefully docu- mented, real time study of the dispersal of flood sedi- ment is probably the most detailed report on the nature of allochthonous sediment dispersal available at the time of writing. In January and February of 1969, southern California experienced two intense rainstorms which resulted in a record flood discharge. The freshly eroded sediment was a distinctive red-brown in contrast to the drab hue of ZONE I BEDROCK & GRAVEL ZONE II SAND RIBBONS ZONE III SAND WAVES ZONE IV SMOOTH SAN
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