Beschreibung
In the early 1970s a start was made on a broad review of what was known or could be surmised about sedimentation by strong tidal currents on modern continental shelves. This task was initiated because of the need to define the next phase of research in this field by the Marine Geology Group of the Institute of Oceano graphic Sciences. Related indications of the longer term evolution of the deposits were sought by close reference to the nature of modern tidal currents and the supposedly offshore tidal deposits of ancient seas. As the review grew in completeness it became of increasing relevance to a wider audience so it was amalgamated with the new results and shaped as a book. The fruits of the long-continued discussions within and outside the Geology Group have served to improve understanding of the processes and products of offshore tidal current sedimentation. On the other hand, the discussions have blurred the parts played by the people concerned. This applies to all chapters in varying degrees, but is especially true for Chapters 3, 4 and 5. The authorship attributed to each chapter therefore seeks to reflect those who were most con cerned with it.
Autorenporträt
Inhaltsangabe1. Background and outline.- 1.1 Introduction.- 1.2 History of research on modern offshore tidal current sedimentation.- 1.2.1 Early work.- 1.2.2 Post-1950 advances.- 1.3 A depositional surface for late Holocene deposits.- 1.4 Limits and outline of the book.- 2. Tidal currents of the continental shelf.- 2.1 Introduction.- 2.2 Tide generating forces and the ocean's response.- 2.2.1 Tide generating forces.- 2.2.2 Spring-neap cycles.- 2.2.3 Peak astronomical tides.- 2.2.4 Relative amplitudes of daily and twice-daily tides.- 2.2.5 A computation of ocean tides.- 2.3 Tidal currents in shelf seas.- 2.3.1 Amplification due to decreasing depth and width.- 2.3.2 Resonance.- 2.3.3 Progressive and standing waves.- 2.3.4 Effects of the Earth's rotation.- 2.3.5 Some effects of continental shelf width.- 2.3.6 Tidal range at the coast.- 2.3.7 Tidal current speeds on the continental shelf around the British Isles.- 2.3.8 Tidal ellipse.- 2.4 Net sand transport caused by tidal current asymmetries.- 2.4.1 Distortions to the tide.- 2.4.2 Combination of the principal and its first harmonic.- 2.4.3 Net sand transport by tidal currents.- 2.4.4 Tidal current patterns in the vicinity of sand banks.- 2.5 Flow near the sea floor.- 2.5.1 Constant stress layer.- 2.5.2 Ekman layers.- 2.5.3 Effects of the oscillatory nature of tidal currents.- 2.5.4 Drag coefficient and bottom stress.- 2.5.5 Current profiles above the logarithmic layer.- 2.6 Internal tides.- 2.6.1 Nature.- 2.6.2 Causes.- 2.6.3 Measured currents of internal tides.- 2.7 Tides past.- 2.7.1 Effects of tidal friction.- 2.7.2 Effects of changes of bathymetry.- 2.8 Main conclusions.- 3. Bedforms.- 3.1 Introduction.- 3.2 Relevant flume bedforms.- 3.2.1 Lower flow regime flume bedforms (sand ripples and sand waves).- 3.2.2 Transition bed conditions.- 3.2.3 Upper flow regime flume bed states.- 3.2.4 Paucity of longitudinal bedforms in flumes.- 3.2.5 Note on theory of transverse bedforms in flumes.- 3.3 Transverse bedforms of the continental shelf 34.- 3.3.1 Unlikelihood of antidunes occurring on the continental shelf.- 3.3.2 Sand ripples.- 3.3.3 Sand waves.- 3.3.4 Transverse sand patches.- 3.4 Longitudinal bedforms of the continental shelf.- 3.4.1 Scour hollows.- 3.4.2 Longitudinal furrows.- 3.4.3 Obstacle marks.- 3.4.4 Sand ribbons and longitudinal sand patches.- 3.4.5 Tidal sand banks.- 3.5 Relationship between bedforms.- 3.6 Aeolian equivalents.- 3.7 Main conclusions.- 4. Sand transport.- 4.1 Introduction.- 4.1.1 Availability of sand for offshore transport.- 4.2 Relation of sand transport rate to tidal current speed.- 4.2.1 Sand transport rate in flumes and rivers.- 4.2.2 Relative sand transport rate over the sea bed.- 4.2.3 Lag effects in tidal current sand transport.- 4.2.4 Transport of sediments with two or more modes.- 4.3 Geographical variation in sand transport rate.- 4.3.1 Relative sand transport rate shown by mean spring peak tidal current speed.- 4.3.2 Relative sand transport rates shown by bedforms.- 4.4 Net sand transport by tidal currents.- 4.4.1 Net sand transport direction predicted from mean spring peak tidal currents.- 4.4.2 Field evidence of net sand transport directions.- 4.4.3 Regional net sand transport directions around the British Isles.- 4.4.4 Net sand transport paths on other continental shelves.- 4.4.5 Bed-load partings and bed-load convergences.- 4.4.6 Origin of bed-load partings and convergences.- 4.4.7 Bed-load partings and convergences with non-tidal currents and in deserts.- 4.5 Temporal variations of sand transport rate and direction in a tidal sea.- 4.5.1 Variations due to the tidal cycles.- 4.5.2 Variations due to sea surface waves.- 4.5.3 Variations due to non-tidal currents.- 4.6 Growth, migration and decay of sand waves in the Southern Bight of the North Sea by total water movements.- 4.7 Local sand transport on modern sand banks.- 4.8 Main conclusions.- 5. Offshore tidal deposits: sand sheet and sand bank facies.- 5.1 Introduction.- 5.2 Late Holocene sand and gra
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