Paraglacial Coastal Landsystems

The concept of paraglacial coasts is dominated by the progressive release of a vast store of in situ and fluvially reworked glacigenic sediment into the nearshore and offshore sediment budget. This happens in two ways. First, the supply of reworked glacigenic sediment by rivers enhances sediment influx in estuarine and fjord-head locations. Second, where glacigenic sediments crop out in the littoral zone they are eroded directly by waves and tidal currents, introducing reworked sediment that strongly influences coastal landforms and nearshore sediment transfer. Forbes and Syvitski (1994) pointed out that the timing and duration of sediment delivery or availability is largely determined by the disposition of sources of glacigenic sediment relative to the coastline and particularly by changes in relative sea level. On coasts where glacigenic sediments are reworked at the shorefront, for example, rising seas allow wave action to tap pristine sources of sediment, prolonging sediment reworking; conversely, falling seas may isolate glacigenic sediment sources from wave action, terminating sediment reworking. In the account below the dynamics of paraglacial coasts are described in terms of three subsystems: fjords, barrier coasts and glaciated shelves.

17.8.1 Fjords

Fjords have immense sediment storage capacity, and because the mouths of many fjords are crossed by a shallow sill of bedrock or glacigenic deposits, they are often effectively closed basins from which sediment escape is limited. Many fjords therefore contain a record of sediment accumulation that spans the last glacial-interglacial cycle (Gilbert, 1985; McCann and Kostaschuk, 1987; Syvitski and Lee, 1997). Syvitski and Shaw (1995) identified five stages in the deglacial sedimentation sequence:

1. occupance by glacier ice

2. proximal or ice-contact sedimentation associated with retreating tidewater or floating glacier termini

3. distal proglacial sedimentation after glacier termini have become land-based

4. postglacial sedimentation following disappearance of glacier ice, and

5. complete infill of fjord basins.

Paraglacial sedimentation effectively commences when retreating ice becomes land-based (Powell and Molnia, 1989; Forbes and Syvitski, 1994) and has four main elements (Fig. 17.13):

• progradation of a fjord-head delta

• reworking of sediment by subaqueous mass movement and turbidity currents

• settling of suspended sediment on the fjord floor, and

• in some fjords, reworking of glacigenic or older marine sediment around the fjord margins.

Paraglacial Mass Movement

Bedrock

Figure 17.13 Paraglacial sedimentation in fjord basins (schematic). Sedimentation is dominated by: (1) deposition of sand and gravel foreset beds at the front of a prograding fjord-head delta; (2) localized failure of the fjord-head delta front, which generates turbidity currents and results in deposition of turbidite layers on the fjord floor; (3) settlement of fine-grained suspended particles from a surface sediment plume, and associated accumulation of hemipelagic bottom sediments; (4) reworking of shoreface bluffs by wave action. Submarine failure of sidewall sediments may also generate turbidity currents and turbidite deposition.

Bedrock

Figure 17.13 Paraglacial sedimentation in fjord basins (schematic). Sedimentation is dominated by: (1) deposition of sand and gravel foreset beds at the front of a prograding fjord-head delta; (2) localized failure of the fjord-head delta front, which generates turbidity currents and results in deposition of turbidite layers on the fjord floor; (3) settlement of fine-grained suspended particles from a surface sediment plume, and associated accumulation of hemipelagic bottom sediments; (4) reworking of shoreface bluffs by wave action. Submarine failure of sidewall sediments may also generate turbidity currents and turbidite deposition.

Aeolian reworking of outwash sediments may also supply silt and fine sand to some fjords, but is quantitatively significant only in arid high-arctic environments (Gilbert, 1983).

In most fjords, paraglacial sedimentation has been dominated by the influx of reworked glacigenic sediment carried by the trunk stream entering the fjord head. Such rivers may reach the fjord as a single or braided channel, or tidal channels cut through mudflats. The sediment load separates into two components at the river mouth: suspended sediment is carried out into the fjord, but bedload settles on to the front of a prograding fjord-head delta (Kostaschuk, 1985). The foreset beds of fjord-head deltas typically dip seawards at angles of 5-30° to depths of 10-50 m, then prograde along bottomset beds at much gentler gradients. Slope failures are common along steep delta fronts, forming chutes 10-30 m wide (Kostaschuk and McCann, 1983; Syvitski et al., 1988). The failed sediments liquefy as they move, forming turbidity currents that sweep down channels in the prodelta zone before depositing sediment on the fjord floor. Similar sediment slumping and turbidity current formation may also occur on sidewall slopes, side-entry deltas and fjord-mouth sills (Syvitski and Shaw, 1995).

Suspended sediment influx moves seawards at the water surface in a buoyant plume (Kostaschuk and McCann, 1983), and sedimentation rates tend to decrease exponentially down-fjord. Most paraglacial fjord-bottom sediments thus comprise dominantly fine-grained hemipelagic deposits, often fining down-fjord, intercalated with turbidite deposits derived from subaqueous sediment failures. Reworking of Pleistocene deposits by wave action may also contribute to fjord infill, but tends to be significant only in shallow fjords where sills are absent or deep. The Holocene sediment infill of such 'wave-dominated' fjords in eastern Canada consists largely of such directly reworked sediment (Syvitski and Shaw, 1995), which currently accumulates on the fjord floor at up to 3 mm year-1 (Piper et al., 1983).

The effectiveness and duration of the above processes may be significantly influenced by changes in relative sea level. A fall in sea level may reduce the accessibility of glacigenic deposits to wave attack, but the associated lowering in base level causes incision of fjord-head deltas and outwash deposits, thus prolonging the period of terrestrial paraglacial sediment influx. A rise in sea level may result in the submergence of paraglacial deltas and shoreface platforms. In fjords along the south coast of Newfoundland, for example, paraglacial deltas formed during the Early Holocene have been drowned by rising seas (Shaw and Forbes, 1992, 1995).

Data on fjord sedimentation rates suggest a radical decline since deglaciation. Stravers et al. (1991) calculated that in Cambridge Fjord on Baffin Island, paraglacial sedimentation during and after deglaciation (c. 10-6 ka BP) was an order of magnitude greater than current sedimentation rates. In Sanguenay Fjord in eastern Canada, present rates of sediment delivery account for only 7 per cent of overall sediment accumulation (Syvitsky and Praeg, 1989). If representative, these data suggest that in many fjords deglaciated at the end of the Pleistocene, the effects of paraglacial resedimentation are now much diminished.

17.8.2 Barrier Coasts

Barrier coasts are those characterized by nearshore sediment accumulations such as beaches, baymouth bars, spits and barrier islands. On most paraglacial barrier coasts the main source of sediment in barrier structures is derived from reworking of glacigenic sediment by coastal erosion, such as occurs on the southern Baltic coast and the Beaufort Sea coast of Alaska. Paraglacial barrier coastlines exhibit great variations in the size, configuration, sedimentary characteristics and stability of barrier structures, reflecting the complex interaction of several controls, including antecedent coastal configuration and relief, coastal compartmentalization (interruption by bedrock headlands), sediment availability and texture, wave and tidal energy, and relative sea level change. A classification of paraglacial barrier systems by FitzGerald and van Heteren (1999) is based primarily upon compartmentalization and recognizes a progressive transition from small barriers in isolated rocky inlets to large barriers fed by abundant sources of glacigenic sediment. In the latter situation, sediment release may keep pace with sea level rise, producing progradational barriers, but both progradational and retrogradational behaviour is possible on all paraglacial barrier systems. Within their general typology, barriers may be characterized by morphology (spit, baymouth bar, etc), texture (sand, mixed sand-gravel, gravel- and boulder-dominated), and stratigraphy (progradational, aggradational, retrogradational or complex). Most paraglacial barriers are coarse-grained, except where the parent glacigenic sediment sources are themselves fine-grained, and stratigraphic complexity is typical (Forbes and Taylor, 1987; van Heteren et al., 1998).

The evolution of paraglacial barrier coasts is often dominated by the balance between rates of sediment input and rates of sea level change (Boyd et al., 1987; Forbes and Taylor, 1987; Carter et al., 1989; Shaw et al., 1990; Forbes et al., 1995a, b). Where the net sediment balance is positive, barrier systems may continue to prograde seaward. Under conditions of diminishing sediment supply, however, barrier retreat, erosion and destruction occurs, and a new barrier system may develop landward of the original (Fig. 17.14), nourished not only by new onshore glacigenic sediment sources, but also by sediments reworked from the original barrier and associated back-barrier sediments. Barrier evolution is also influenced by inherited submarine morphology and sediment supply. At St George's Bay in Nova Scotia, for example, paraglacial barrier structures rest on large subaqueous sandy platforms that are partly composed of sediment reworked from glacigenic sources during an early Holocene lowstand (Shaw and Forbes, 1992). Under steady-state conditions, paraglacial barrier systems exhibit gradual evolution involving littoral cell development, beachface realignment, crest build-up and progressive sediment sorting. Changes in relative sea level, sediment supply, storm intensity or wave regime, however, may trigger rapid destabilization, barrier breakdown, sediment remobilization and a cascade of secondary effects both alongshore and in back-barrier embayments (Forbes et al., 1995a, b). On paraglacial coasts where erosion of boulder-rich till forms the main sediment source, boulder lag deposits may accumulate, forming boulder barricades, boulder pavements and boulder-strewn beaches, tidal flats and rock platforms (Lauriol and Gray, 1980; Dionne, 1981; Hansom, 1983; Forbes, 1984). Boulder barricades may armour an eroding shoreface, reducing the sediment supply to barrier structures, and thus contributing to their eventual segmentation and destruction.

Drumlin coasts represent a subset of paraglacial barrier coasts, with individual drumlins acting as discrete sediment sources. In areas of Holocene marine transgression, such as eastern Nova Scotia and western Ireland, paraglacial barriers fed by coastal erosion of drumlins tend to experience a distinctive cycle of growth then destruction as sediment supply fails to keep pace with rising sea level, leading to shoreward movement of sediment and re-establishment of barriers (Boyd et al., 1987; Forbes and Taylor, 1987; Carter and Orford, 1988; Fig. 17.14). Individual drumlins may be eroded down to sea level then submerged by transgressive seas, forming offshore shoals of lag boulders (Piper et al., 1986; Carter et al., 1990). Where drumlin islands occur in broad, shallow bays, cuspate spits form at sites of longshore drift convergence, often forming tombolos that link drumlin headlands

Figure 17.14 Evolution of paraglacial barrier systems associated with marine transgression across drumlin headlands. 1) Coastal reworking of glacigenic deposits into a prograding barrier system. 2) Erosion of barriers as sediment supply diminishes. 3) Destruction of the original barrier. 4) Growth of a new barrier system, fed by new sources of glacigenic sediments and by sediments reworked from the earlier barrier and associated back-barrier deposits. (Adapted from Boyd et al. (1987).)

Figure 17.14 Evolution of paraglacial barrier systems associated with marine transgression across drumlin headlands. 1) Coastal reworking of glacigenic deposits into a prograding barrier system. 2) Erosion of barriers as sediment supply diminishes. 3) Destruction of the original barrier. 4) Growth of a new barrier system, fed by new sources of glacigenic sediments and by sediments reworked from the earlier barrier and associated back-barrier deposits. (Adapted from Boyd et al. (1987).)

458 GLACIAL LANDSYSTEMS

(Rosen and Leach, 1987). Where inter-drumlin valleys have been invaded by the sea, barriers evolve at the mouths of individual inlets (Carter et al., 1990, 1992). Back-barrier sedimentation in larger enclosed basins is dominated by flood-tidal sand sheets and basin muds, but open estuaries are characterized by incipient beach and barrier formation, lateral flood-delta expansion and saltmarsh accretion (Carter et al, 1989, 1992; Shaw et al., 1993).

Active outwash coasts form a further subset of paraglacial barrier coasts, and occur where unconfined outwash (sandur) plains fronting present-day glaciers meet the sea. Examples occur in northwest Svalbard, the Gulf of Alaska and the coast of southeast Iceland. In such locations, coastal configuration and barrier formation reflects the delivery of sediment by braided meltwater streams and the reworking of such sediment by waves, tidal current and longshore drift. In southeast Iceland, for example, outwash sediments reworked by longshore drift form a predominantly sandy barrier complex of beaches, spits and islands that increases westwards in width from about 200 m to 700 m. Under conditions of stable relative sea level, the shoreline has locally prograded up to 1 km during the past century (Nummedal et al., 1987).

17.8.3 Glaciated Shelves

The stratigraphy of Quaternary deposits on glaciated shelves comprises five major elements:

1. till or ice-contact sediments, overlain by

2. glacimarine sediment, sometimes subdivided into a lower ice-proximal unit and 3 an upper ice-distal unit

4. a paraglacial sequence, and

5. an uppermost postglacial unit, comprising lag deposits, basin fill, estuarine muds and pelagic oozes.

Syvitski (1991) suggested that deposition of paraglacial sediments typically occurred during a period of falling relative sea level, and reflects high rates offluvial sediment influx associated with widespread reworking of terrestrial glacigenic sediment. He characterized the paraglacial sequence as rapidly prograding deltaic wedges comprising '... a mixture ofsediment gravity flow and hemipelagic deposits with syndepositional mass flow deposits, buried channels and shear planes related to submarine landslides' (Syvitski, 1991, p. 910). The paraglacial sediments ofthe eastern Canadian shelf(Syvistski and Praeg, 1989) and Alaskan shelf (Carlson, 1989; Powell and Molnia, 1989) are typical examples. On the latter, where paraglacial re-sedimentation is still active, recent sediment fines seawards from littoral sand to clayey silts 75 km offshore, and the total thickness ofHolocene sediment accumulation on the shelf averages ~55 m (Molnia et al., 1978). On many glaciated shelves, the distinction between 'paraglacial' and 'postglacial' sediment accumulation (sensu Syvitski, 1991) is rather arbitrary, as the latter may comprise a substantial component of reworked glacigenic sediment carried seaward from estuaries or the littoral zone; present sediment accumulation on many glaciated shelves is thus essentially paraglacial in the sense that the greater part of the sediment ultimately derives from glacigenic deposits.

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