GE0-3112 Sedimentary processes and products

1. Lecture 10. Estuaries and coasts GE0-3112 Sedimentary processes and products Geoff Corner Department of Geology University of Tromsø 2006 Literature: Leeder 1999. Ch. 21 Estuaries. Ch. 23, ’Linear’ clastic coastlines.

2. Contents • Coastal depositional systems • Estuaries • Barrier-beach coasts

3. Coastal depositional systems • Controlling factors: • Sediment supply (rivers, coastal cliffs, alongshore transport). • Wave energy • Tidal range • Deltas vs. other coastal systems • Local dominance of point-source sediment supply  deltas. • Dominance of basinal processes  linear clastic coasts, estuaries, etc. • Estuaries a special case where fresh and saltwater interact.

4. Coastal types • Deltas • Beaches • Barriers • Strandplains • Tidal flats • Estuaries Galloway & Hobday 1996

5. Variation in coastal morphology as a function of tidal range

6. Estuaries • Definition • Processes (mud suspension) • Circulation types (type A, B, C, D) • Facies: tide- and wave-dominated estuaries • Estuaries and sequence stratigraphy • Ancient estuarine facies

7. Estuary definitions Walker & James, 1992 Various definitions: • Hydrologic (oceanographic): stratified water mass with riverine water (fresh - brackish) overlying or mixed with marine water (brackish - salt). • Geomorphologic: coastal inlet, usually a drowned river valley. • Geologic: semi-enclosed coastal body of water having free access to the sea and containing seawater measurably diluted by water from land drainage.

8. Manukau and Waikato estuaries, NZ (mesotidal)

9. NB. Estuary type (circulation) varies in space & time: • From inner to outer • From neap to spring phase of tidal cycle • Seasonally with variation in river discharge Tidal/fluvial discharge ratio vs. fluvial discharge

10. Estuarine processes • Fluvial and marine sediment flux. • Wave- and tidal reworking. • Fresh, brackish and marine water. • Sand, mud and bioturbation.

11. Mud suspension and resuspension • Flocculation enhances settling. • High particle concentration reduces settling. • Layers of suspended form in response to tidal pumping.

12. Suspended sediment concentration • Hindered settling and flocculation produce a lutocline at high sediment concentrations. Dilute Concentrated

13. Estuarine circulation • Four types: • Type A: well stratified • Type B: partly stratified • Type C: well mixed • Type D: homogenous (theoretical end member)

14. Type A estuaries • Well stratified, river dominated. • Salt wedge below buoyant plume • Low tidal/river discharge ratio (<20)

15. Type A estuaries • Cf. e.g. Mississippi, Fraser and Tana rivers. • Deposition at tip of salt wedge; sediment flushing as salt wedge migrates. Fraser River

16. Type B estuaries • Partially stratified, moderate tidal turbulence. • Salt wedge degraded; gradual salinity gradient. • Moderate tidal/river discharge ratio (20 – 200). • E.g. Tamar, nr. Plymouth.

17. Type B estuaries • Coriollis: up-estuary flow shallowest and strongest to the left in N. hemisphere. • Turbidity max. (in suspended particulate matter) most prominent in upper estuary on ebb and flood tides (low on slackwater). Tamar estuary

18. Type C estuaries • Well-mixed; strong tidal currents. • Salinity gradient downstream and laterally (Coriolis) but not vertically. • High tide/river discharge ratio (>200). • E.g. Severn, UK, Gironde, France, Weser, Germany.

19. Type C estuaries • Repeatable hysteresis of suspended matter concentration (C) with tidal velocity (u): - deposition during slackwater. - resuspension during ebb and flood. • Clayey silt, sandy mud.

20. (Type D estuaries) • Fully mixed; transitional to shelf. • No vertical or lateral salinity gradients. • Sediment movement by tides; no internal sediment trap.

21. Modern estuarine facies • Sedimentological classification of estuaries: • Tide-dominated • Wave-dominated Walker & James, 1992

22. Basic model Walker & James, 1992 • Subenvironments • Bayhead delta (A) • Central basin (B) • Estuary mouth (C) • Sediment types • Alluvial sands and gravels (A) • Bay silts and muds (B) • Marine sands (C) • NB. Upward-fining here reflects transgression C B A

23. Tide-dominated estuaries • Macrotidal (and megatidal). • Well-mixed. • Funnel-shaped, open-ended. • E.g.Gironde, Severn, Bay of Fundy.

24. Tidal flats, Banks Peninsula, NZ

25. Tide-dominated estuaries - facies • Alluvial • Tidal-fluvial channel • Saltmarsh/mudflats • Upper flow regime sand flats • Tidal sand bars

26. Walker & James, 1992 • Example from Gironde-type estuary. • estuarine point bar sands w. mud A • tidal bar sands B • estuarine muds C • tidal inlet sands D C D B A

27. Wave-dominated estuaries • Micro- to mesotidal. • Stratified to partially stratified/mixed. • Lagoonal to funnel-shaped, semi-enclosed. • E.g.E. coast USA

28. UK NZ

29. Wave-dominated estuaries - facies • Alluvial • Bay-head delta • Central basin • Flood-tidal delta • Barrier/tidal inlet • Shoreface

30. Waikato River estuary, NZ

31. Delaware estuary • Outer estuary: • E.Holocene – tide-dominated outer • L. Holocene – wave-dominated • Turbidity maximum: • Moved up-estuary in Holocene • Mud deposition and tidal wetlands at head of estuary

32. Estuaries and sequence stratigraphy • Incised valley during lowstand. • Estuarine valley fill during transgression and highstand. • Processes and infill are time and space dependent. Galloway & Hobday 1996

33. Ancient estuarine facies • Prograding estuarine succession: • Fluvial (above) • Bay-head delta • Estuarine • Marine nearshore (below) • Complexities due to s.l variation • Criteria for recognition: • Tidal facies • Brackish water biota Galloway & Hobday 1996

34. Linear clastic (incl. barrier) coasts • Depositional coasts away from deltas and estuaries. • High wave energy (micro- to mesotidal).

35. Shoreline types: wave vs. tidal energy

36. Definitions Beach and barrier systems • Beach - narrow strip of sand or gravel attached to a coastline. • Strandplain - broad, composite beach. • Barrier - beach complex enclosing a lagoon. Walker & James, 1992

37. Shoreline type (morphology) • Attached • Detached Walker & James, 1992

38. Coastline variability: tidal range • Moderate wave energy, variable tidal range:

39. Shoreline subenvironments • Attached beaches and intertidal flats. • Partly attached spits. • Detached barriers, tidal inlets and lagoon complexes. • Shoreface slope and shelf transition. Galloway & Hobday 1996

40. Tides • Lunar influence gives semi-diurnal tides (interval 12.42 hours) • Variations in successive semi-diurnal tides give different semi-diurnal to diurnal tide spectrums • Solar influence gives spring-neap cycle (period 14.77 days, 28 tidal cycles for semi-diurnal tides). • Flood and ebb-tide • Slackwater at high and low-tide Walker & James, 1992

41. Tides • Tides develop fully in oceans; smaller seas and lakes show smaller tides • Open ocean tide has amplitude of <1 m • Tidal range increases: • on shallow shelves • along convergent coasts • where resonance amplification occurs (where natural period of water body is close to astronomic period) Corner , 2005

42. Tides Tidal range • Classification of mean tidal range (Davies 1980): • Microtidal: 0 - 2 m • Mesotidal: 2 - 4 m • Macrotidal: > 4 m • (Megatidal: > 8 m) • High tidal range in several areas, e.g: • S and W coast of UK • Maximum tidal range at: • Bay of Fundy (Maine) (16.3 m) Walker & James, 1992

43. Wave processes • Significant wave height: mean ht of highest 1/3 of waves over a time interval.

44. Breaking waves • Spilling • Plunging • Surging

45. Wave refraction

46. Rip currents Backwash/rip-current eddies at Breivikeidet

47. Beach profile • Backshore (>HW) • Foreshore (LW-HW) • Shoreface (<LW...) • Offshore (< wave base) Galloway & Hobday 1996

48. Beach dynamics and sedimentation

49. Beach morphpology and facies • Onshore coarsening - forward obital wave motion powerful compared with seaward return flow. • Beachface steeper in gravel than sand - percolation weakens backflow. • Summer profile with berm – constructive swell waves transport sediment onshore. • Winter profile with offshore bars – destructive steep waves transport sediment offshore.

50. Beach sediment • Onshore coarsening - forward obital wave motion powerful compared with seaward return flow.