Sedimentary Processes and External Forces

Fjords are complex systems influenced by a wide range of processes, those processes interacting to produce positive and negative feedbacks on each other and controlling depositional systems. The major processes include glacial, fluvial, oceanic, iceberg, sea ice, aeolian, subaerial mass flow and

Meltout

Meltout

Tsunami DepositsSedimentary Process

Meltwater stream with high concentration of suspended matter

Homogeneous mud with scattered ice-rafted pebbles, and organic varves

Laminated mud /sand deposited from the meltwater stream

Meltwater stream with high concentration of suspended matter

Homogeneous mud with scattered ice-rafted pebbles, and organic varves

Laminated mud /sand deposited from the meltwater stream

KANGERDLUGSSUAQ FJORD

GLACIER

1 500

SIKUSSAK

GLACIER

SIKUSSAK

1 500

TURBIDITES and ICEBERG HEMIPELAGICS

DISTANCE (m)

FSW AIW

TURBIDITES and ICEBERG HEMIPELAGICS

50,000

Glacimarine
Stratified or unstratified shelfstone diamicton

Figure 13.1 Sketches of processes and deposits of fjord landsystems under different climatic regimes; terms are defined and concepts are developed more fully throughout the chapter.

A) Temperate glacimarine setting based on Alaskan examples (from Powell and Molnia, 1989).

B) The warmer end of the sub-polar setting based on Svalbard examples. (After Elverh0i et al., 1980). C) The cooler end of the sub-polar setting based on Greenland examples. (After Syvitski et al., 1996; O Cofaigh and Dowdeswell, 2000). D) The warmer end of the polar setting based on Antarctic Peninsula examples. (After Domack and Ishman, 1993). E) The fully polar setting based on an East Antarctic example. (After Powell et al., 1996).

biological (Fig. 13.2; Table 13.2). Before dealing with these processes and systems in detail we need to discuss controls external to the local fjord regime that influence sedimentary processes, facies geometries and stratigraphies (Fig. 13.2).

Climate, tectonics and sea level exert strong forces on the local processes that create and maintain fjord landsystems. Climate, primarily in the form of annual ranges of precipitation and temperature, as well as their rates of change, has a major influence on glacial regime (mass balance, dynamics, bed condition), bedrock weathering and vegetation all of which feed into erosion rates and sediment fluxes, sediment composition and chemical and carbon content of freshwater

INTERNAL SYSTEM PROCESSES

EXTERNAL FORCES

Debris rainout, grainfall, interflow rockfall

Overflow,

SEA LEVEL

Weathering, vegetation, precipitation

Base level, gradient change

Debris rainout, grainfall, interflow rockfall

Overflow,

Debris rainout, grainfall, rockfall I

..TECTONICS

(+Glacial isostasy)

Bedrock quality erosion

Debris rainout, grainfall, rockfall I

Figure 13.2 The complex system of external forces, sedimentary sources and transportation and depositional process in glacimarine systems. (Modified from Dowdeswell, 1987).

Depositional system

Fjord infilling phase

Bedrock walls and floor

Glacial

Subglacial systems

Glacial

Grounding-line systems

Glacial

Floating glacier-tongue systems

Glacial

Iceberg zone systems

Glacial

Open water systems

Glacial, paraglacial, non-glacial

Sidewall systems

All

Fjord-head systems

Paraglacial, non-glacial

see Glaciated Valley Landsystems

Fjord valley

Table 13.2 Major processes of depositional systems.

Table 13.2 Major processes of depositional systems.

discharges into fjords. The thickness and extent of sea ice and its seasonal and inter-annual variability are important here, as well as the marine biology in terms of species, assemblages, productivity and their spatial and temporal variability. Ultimately, all of these determine the types, composition, geometries and sizes of depositional systems within fjords.

Fjords occur in the full range of tectonic settings from convergent to transform to divergent continental margins. Tectonics has a primary place in sediment production by influencing the bedrock quality relative to its glacial erodability. In active tectonic settings, tectonic uplift and depression are important in terms of snow accumulation and glacial dynamics, of fjord circulation in changing basinal geometries, and by providing the space in which the sediment can be accommodated.

Although internal forces such as sediment accumulation, drive changes in local water depth, changes in relative sea level and eustatic sea level primarily are the results of external forces: tectonics and global climate change, respectively. An exception to the external driving of relative sea level change is glacial isostasy, which plays an important role on facies sequences of shallow water depositional systems (e.g. Andrews, 1974; Bednarski, 1988; Boulton, 1990; Fig. 13.3). Such influences are primarily due to changing wave and tidal current energies that instigate sediment reworking. Another important consequence of fluctuating sea level is its changing of base level and gradients on land in terms of the sediment delivery systems to the fjord. Water

I-1 Distal

I I Proximal

I_Iglacimarine I_Iglacimarine I I Sub9lacial

Figure 13.3 Model to show the effect of combining glacial eustasy and glacial isostasy relative to sea level changes around a glaciated continental margin shown in a spatial and temporal (0-80 ka) space. Variations in relative sea level and broad glacimarine landsystems at the margin of a continental ice sheet are shown with the ice sheet advancing and retreating from and to the right. (After Eyles and Eyles, 1992; who adapted the figure from Boulton, 1990).

Distance (Km) Eustatic sea level (m)

Figure 13.3 Model to show the effect of combining glacial eustasy and glacial isostasy relative to sea level changes around a glaciated continental margin shown in a spatial and temporal (0-80 ka) space. Variations in relative sea level and broad glacimarine landsystems at the margin of a continental ice sheet are shown with the ice sheet advancing and retreating from and to the right. (After Eyles and Eyles, 1992; who adapted the figure from Boulton, 1990).

depth has been found to be important in controlling calving speed of grounded glacial tidewater cliffs (Brown et al., 1982; Pelto and Warren, 1991; Warren et al., 1995), and although presently debated (van der Veen, 1996) has been inferred to influence the terminus position of glaciers ending in the sea. In fact, glaciers themselves may supply enough sediment to their terminus to decrease water depth there and help create their own stability (e.g. Powell, 1991; Fischer and Powell, 1998). Changes in water depth in a fjord also alter sediment accommodation space and change water masses and circulation patterns, which in turn can modify iceberg paths and sediment dispersal patterns.

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Responses

  • Donnamira
    How do glacial processes lead to sedimentary processes?
    3 months ago

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