Presentation on theme: "GE0-3112 Sedimentary processes and products Lecture 10. Estuaries and coasts Geoff Corner Department of Geology University of Tromsø 2006 Literature: Leeder."— Presentation transcript:
GE0-3112 Sedimentary processes and products Lecture 10. Estuaries and coasts Geoff Corner Department of Geology University of Tromsø 2006 Literature: Leeder 1999. Ch. 21 Estuaries. Ch. 21 Estuaries. Ch. 23, ’Linear’ clastic coastlines. Ch. 23, ’Linear’ clastic coastlines.
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.
Variation in coastal morphology as a function of tidal range
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
Estuary definitions 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. Walker & James, 1992
► 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
Estuarine processes ► Fluvial and marine sediment flux. ► Wave- and tidal reworking. ► Fresh, brackish and marine water. ► Sand, mud and bioturbation.
Mud suspension and resuspension ► Flocculation enhances settling. ► High particle concentration reduces settling. ► Layers of suspended form in response to tidal pumping.
Suspended sediment concentration ► Hindered settling and flocculation produce a lutocline at high sediment concentrations. Dilute Concentrated
Estuarine circulation ► Four types: Type A: well stratified Type B: partly stratified Type C: well mixed Type D: homogenous (theoretical end member)
Type A estuaries ► Well stratified, river dominated. ► Salt wedge below buoyant plume ► Low tidal/river discharge ratio (<20)
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
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.
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
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.
Type C estuaries ► Repeatable hysteresis of suspended matter concentration (C) with tidal velocity (u): - deposition during slackwater. - resuspension during ebb and flood. ebb and flood. ► Clayey silt, sandy mud.
(Type D estuaries) ► Fully mixed; transitional to shelf. ► No vertical or lateral salinity gradients. ► Sediment movement by tides; no internal sediment trap.
Modern estuarine facies ► Sedimentological classification of estuaries: Tide-dominated Wave-dominated Walker & James, 1992
Basic model ► 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 A B C Walker & James, 1992
Tide-dominated estuaries ► Macrotidal (and megatidal). ► Well-mixed. ► Funnel-shaped, open-ended. ► E.g. Gironde, Severn, Bay of Fundy.
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
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
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
Linear clastic (incl. barrier) coasts ► Depositional coasts away from deltas and estuaries. ► High wave energy (micro- to mesotidal).
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
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
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
Wave processes ► Significant wave height: mean ht of highest 1/3 of waves over a time interval.