Offshore Tidal Sands

1. 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 gravel sheet facies.- 5.2.1 Grain size and current speed.- 5.2.2 Gravel sheet form, composition and structure.- 5.2.3 Sand sheet form and texture.- 5.2.4 Structure of a sand sheet in the Southern North Sea.- 5.2.5 German Bight sand to mud sheet.- 5.2.6 Irish Sea sand to mud sheet.- 5.2.7 Regional cross-bedding dip directions within the sand sheet facies.- 5.2.8 Sand patches.- 5.2.9 Sand waves formed by tidal lee waves.- 5.2.10 Facies model of an offshore tidal current sand sheet.- 5.3 Sand bank facies.- 5.3.1 Early Holocene low sea level sand bank facies.- 5.3.2 Late Holocene sand bank facies.- 5.3.3 Internal structure of offshore and estuarine sand banks.- 5.3.4 Facies models of offshore and estuarine tidal sand banks.- 5.4 Sediment and faunal indicators of shape, depth and exposure of continental shelves.- 5.5 Longer term evolution of the deposits.- 5.6 Sand and gravel deposits of non-tidal marine currents.- 5.7 Main conclusions.- 6. Shelly faunas associated with temperate offshore tidal deposits.- 6.1 Introduction.- 6.2 Faunal associations.- 6.3 Bioturbation.- 6.3.1 Depth of disturbance by bioturbation.- 6.3.2 Types of bioturbation.- 6.4 Topics and areas excluded.- 6.5 Temperate water regions studied and their geological importance.- 6.5.1 Carbonate content of sediments on the continental shelf around the British Isles.- 6.6 Faunas in shallow nearshore waters.- 6.6.1 Temperate water calcareous algal gravels.- 6.7 Faunas of the middle and outer continental shelf.- 6.8 Faunas of a bed-load parting.- 6.9 Faunas associated with bedform zones in the Western English Channel.- 6.9.1 Faunas from the gravel sheet.- 6.9.2 Faunas from the sand ribbon zone.- 6.9.3 Faunas from the zone of large sand waves.- 6.9.4 Faunas from the zone of rippled sand.- 6.10 Faunas associated with bedform zones in the Bristol Channel.- 6.10.1 Benthic faunas in relation to tidal bottom stress.- 6.10.2 Faunas from the rock floor.- 6.10.3 Faunas from the sand ribbon zone.- 6.10.4 Faunas from the zone of large sand waves.- 6.10.5 Faunas from the rippled muddy sands in bays.- 6.11 Faunas associated with bedform zones in the Southern North Sea.- 6.11.1 Faunas from the zone of large sand waves.- 6.11.2 Faunas from the zone of small sand waves.- 6.11.3 Faunas from the zone of rippled sand.- 6.11.4 Faunal differences from the sand wave zone to the zone of rippled sand.- 6.12 Faunas associated with bedform zones on the Atlantic continental shelf between Brittany and Scotland.- 6.12.1 Faunas from the gravel sheet zone, Fair Isle Channel.- 6.12.2 Faunas from the rippled sand zone.- 6.12.3 Faunas associated with gravels in weak current areas west of Scotland.- 6.13 Faunas of active sand banks.- 6.14 Faunal evidence for stability of sand waves.- 6.15 Faunas as environmental indicators.- 6.15.1 Faunal differences between adjacent sand transport paths.- 6.15.2 The proximity of the open ocean.- 6.15.3 The edge of the continental shelf.- 6.16 Factors determining the faunal composition of death assemblages in shell gravels.- 6.16.1 Predation on shell bearing invertebrate faunas.- 6.16.2 The role of borers in the breakdown of shells.- 6.16.3 Mechanical breakage and dissolution of shells.- 6.16.4 Differences in faunal composition between living and dead faunas.- 6.17 Age of temperate water carbonates.- 6.17.1 Age of shell gravels on the continental shelf around the British Isles.- 6.17.2 Rates of deposition.- 6.18 Relative proportions of the major carbonate producers in death assemblages of continental shelf carbonates.- 6.18.1 Faunal composition of death assemblages in shell gravels in the strong current areas, Western English Channel and Celtic Sea.- 6.18.2 Faunal composition of death assemblages in shell gravels on the continental shelf west of Scotland.- 6.19 Temporal changes in the faunal composition of shell gravels.- 6.19.1 Faunal evidence of lowered sea level.- 6.20 Long term evolution of temperate shelf carbonates.- 6.21 Applications to the fossil record.- 6.22 Main conclusions.- Apendix 6.1 List of species mentioned in Chapter 6.- 7. Ancient offshore tidal deposits.- 7.1 Introduction.- 7.2 Recognition of ancient offshore tidal current activity.- 7.3 Structures preserved in ancient offshore tidal current deposits.- 7.3.1 Sand waves.- 7.3.2 Sand banks.- 7.3.3 Sand and mud sheets.- 7.3.4 Scoured horizons and bed-load partings.- 7.4 Tidal currents aided by storm processes.- 7.5 Factors controlling the structure and composition of offshore tidal sediments through geological time.- 7.6 Some possible palaeotidal regimes.- 7.6.1 Upper Jurassic gulf of western North America.- 7.6.2 Upper Cretaceous epicontinental seaway of western North America.- 7.7 Sedimentology of a tidal sea: the Lower Greensand of southern England.- 7.7.1 Lower Aptian phase.- 7.7.2 Upper Aptian and Lower Albian phases.- 7.8 Tidal currents through geological time: implications for future studies.- 7.9 Main conclusions.- Apendix 7.1 Possible ancient offshore tidal current deposits.- Apendix 7.2 Estimate of the amplification of the twice-daily tidal wave in the Lower Aptian gulf of south-east England.- REFERENCES.