Offshore Tidal Sands

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.

Table of Contents:
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.