The Continental Shelf System

12.1.1 Diamicton Formation on Continental Shelves

The most widespread sediments on glaciated continental shelves (Fig. 12.1) are diamictons. There has been, and still is, much discussion on the origin of these diamictons. The correct

Figure 12.1 World map showing continental margins that were glaciated once or several times during the Late Cenozoic.

interpretation of the continental shelf diamictons is vital for understanding the glacial history of the shelf in question. In this chapter the possible glacial and glacimarine origins of the diamictons will be elucidated. In addition to the glacial and glacimarine diamicton-forming processes it should be kept in mind that other processes, particularly debris flows, might generate diamictons on shelves.

12.1.1.1 Grounded Glaciers

The same type of glacial basal deposits are to be expected in the marine as in the terrestrial environment, for example different types of basal tills (lodgement tills, melt-out tills, deformation tills; Fig. 12.2A). Discoveries of a deforming till layer beneath Ice Stream B in the Ross Sea region (e.g. Blankenship et al., 1987; Alley et al. 1987) demonstrate that sediment deformation offers a means for efficient transportation of debris at the base of an ice sheet towards the grounding line.

In addition to these, a group of deposits called waterlain tills may occur (Fig. 12.2A). Dreimanis (1979) suggested three mechanisms for the deposition of waterlain tills; by grounded icebergs, as subaquatic flow tills and as waterlain basal melt-out tills. Basal melt-out of glacial debris can occur below floating glacier termini, ice shelves or in subglacial basins. Evidently for floating ice it is a matter of definition where to draw the line between waterlain till and glacimarine sediments. Dreimanis (1979) described waterlain till as being deposited either in direct contact with glacier ice or from glacier debris without substantial disaggregation or sorting. A priori, however, it is hard to see how the melt-out process will not disaggregate the sediments, unless they occur as a frozen or compacted block, and then it is an ice-rafted clast.

Subaquatic flow till was first described from a lacustrine environment (Evenson et al., 1977), but was later identified in marine environments (e.g. Hicock et al., 1981; Lenne, 1995). The sediment in this type offacies comprises proglacial cones ofinterbedded diamictons ('subaquatic flow tills') and outwash with glacimarine fine sediments occurring as discontinuous lenses between cones (Fig. 12.2B). Related to the subaquatic flow tills, are the 'till tongues' found on the Canadian and Norwegian continental shelves (King et al., 1991), which are wedge-shaped deposits of sediments interbedded with stratified glacimarine sediment, and they constitute discrete stratigraphic units laid down near margins ofmarine ice sheets. According to the model, the till tongues "are formed through the accumulation of glacial debris by subglacial processes proximal to the grounding line, together with a penecontemporaneous, proglacial contribution from sediment gravity flows" (King et al., 1991). Stravers and Powell (1997) interpret till-tongue-like deposits on the Baffin Island shelf originating from sediment failure and debris flows at the terminus of a temperate ice sheet. A better distinction regarding processes as well as terminology between 'subaquatic flow tills', 'till tongues' and 'debris flows' on glaciated shelves needs further work.

12.1.1.2 Ice Shelves

Deposition beneath ice shelves may take place by meltwater deposition near the grounding line, basal melting with deposition of basal and englacially transported debris, or basal melting with deposition of 'frozen-on sediments'. In addition to the siliclastic sediments, there may also be biogenic components from advection of plankton underneath the shelf.

In cases where the ice sheet proximal to the grounding line is at the pressure melting point, the sediments may be transported by subglacial meltwater. A priori, a shift in the grounding line

The Glacial System
Figure 12.2 Models of glacigenic sedimentation on continental shelves. See text for further explanation. (Adapted from Vorren et al., 1983).

could give rise to a complex stratigraphy of interbedded glacimarine diamictons, subglacial meltwater deposits and basal tills mixed with ice-rafted debris. However, no such processes have yet been described at modern ice-shelf grounding lines.

Orheim and Elverhoi (1981) suggested a process by which ice shelves can cause redeposition. If an ice shelf grounds (due to lower sea level and/or thicker ice) it may freeze to the bed. When the shelf then refloats it will contain frozen-on debris. Later melting releases this material (Fig. 12.2D). This process has actually been observed at a pinning point on a glacier tongue floating in Antarctica (Powell et al., 1996).

12.1.1.3 Icebergs

Deposition from icebergs can occur by release from floating as well as grounded bergs. The extensive Heinrich layers in the North Atlantic are well known examples of deposition from floating icebergs (e.g. Heinrich, 1988; Bond et al., 1992). Floating icebergs can release debris in several ways:

• by dumping when the iceberg overturns, fragments or tilts

• by small mudflows

The textural and petrographic composition of the iceberg debris is almost identical to the till deposited by the source glacier of the iceberg (e.g. Anderson et al., 1980 a). However, modification to this till composition will occur, depending on current activity (which causes winnowing), and the amount of fines added from suspension. The resulting sediment may be coarser or finer ('residual glacimarine' and 'compound glacimarine sediments, respectively; Anderson et al., 1980b). The geometry of these sediment facies would be sheet drape and/or basin fill, and the stratification mostly massive or crudely stratified.

Redeposition by icebergs may be caused by iceberg ploughing and by adfreezing. The magnitude of iceberg-ploughing depends on the size of the iceberg, water depth, drifting velocity, bottom relief and shear strength of the bottom sediments. Iceberg ploughing causes deformation, reworking and some sorting/winnowing of the bottom sediments. Belderson et al. (1973) and King (1976) indicate that the berm of the furrow is composed of coarser sediments than the base of the furrow. Deformation structures beneath iceberg scours and sub-horizontal thrust faults related to lateral displacement and stacking of clay slabs are observed within the berms (Woodworth-Lynas and Guigne, 1990). The stirring action must cause turbulence and resuspension. Thus in the long run the sediments as a whole are probably depleted in fines; how much depends on the bottom current regime.

The composite geometry of this type of iceberg-redeposited sediment is sheet drape. The resulting sediment is a diamicton that is mostly coarser than the normal glacimarine sediments. Fossils in life position seldom exist. Vorren et al. (1983) suggested naming this sediment type 'iceberg turbate' (Fig. 12.2F). Barnes et al. (1988) suggested the term 'ice-keel turbates' to include sediments disturbed by sea ice as well.

Iceberg till (Dreimanis, 1979) is a type of waterlain till deposited by grounded icebergs (Fig. 12.2G). The most favourable situation for deposition of iceberg till would be a sudden decrease in water depth in the iceberg environment. The textural and petrographic composition should be similar to that of the till from the source glacier, but differ geometrically from the till being deposited as lenses.

Several observations confirm sea ice as a debris-transporting medium (e.g. Carlson, 1975; Pfirman et al., 1989; Nürnberg et al., 1994; Stein and Korolev, 1994). Sea ice may acquire debris by adfreezing in the shore zone, by infreezing of suspended sediments, by settling of aeolian sand and silt, and from rivers flowing onto the sea-ice in spring.

Sediments may be deposited from sea ice in much the same way as from icebergs. The sediments may differ from iceberg-rafted debris with regard to clast roundness; most of the sea-ice clasts are derived from rounded littoral gravel (Lisitzyn, 1972). Another factor to consider is the subduing effect of sea ice on wind- and wave-generated currents, the consequence of which is reduced winnowing of the sediments. The nearshore sea-ice environment has been described in detail by Reimnitz et al. (1978), and Fig. 12.2H shows sea-ice zonation as found in the Alaskan Beaufort Sea. Between the seasonal pack ice and stamukhi zone, Kovac and Sondhi (1979) have described a zone with a floating fast ice extension. It should be noted that the ice gouging on the underlying sediments may be heavy (e.g. Rearic et al., 1990), particularly in the stamukhi zone. The draft of the sea ice in pressure ridges may be as much as 47 m (Reimnitz et al., 1972). Thus sea ice may cause bulldozing, resuspension and adfreezing at depths shallower than about 50 m.

12.1.2 First-Order Morphological Elements

The first-order morphological elements on a glaciated shelf are banks and large depressions (troughs/channels). Most banks on glaciated shelves reflect the bedrock morphology, but some are accentuated by shelf diamictons. Depressions are ubiquitous on glaciated continental shelves. In general there are two types of depressions, transverse troughs and longitudinal channels (Figs 12.3, 12.4 and 12.5). Examples oflongitudinal channels are found offNorway (Holtedahl, 1958), Labrador (Josenhans, 1997), Alaska (Carlson et al., 1982) and Antarctica (Anderson, (1999). Off Labrador a longitudinal channel runs parallel to the coast for more than 400 km, with depths of 600-800 m (Holtedahl, 1958). Another example is the Norwegian channel that encircles the southern part of Norway. It runs parallel to the coast, and the largest depths in the east are more than 700 m whereas it shallows to about 220 m in the middle before deepening to about 400 m at its mouth (Holtedahl, 1993). The longitudinal channels generally follow boundaries between sedimentary rocks on the shelf and the older crystalline troughs towards the coast, or faults and other zones of weakness (Holtedahl, 1958; Vogt, 1986; Josenhans, 1997; Anderson, 1999; Fig. 12.5).

Transverse troughs are normally overdeepened in their inner reaches. Off Norway the depths reach about 400-500 m (Fig. 12.5), but in Antarctica they may be deeper than 1000 m (Fig. 12.4). Most often the transverse troughs represent seaward extensions of fjord/glacial valleys. There is a tendency for troughs to be less than 20 km wide where the continental shelf is narrow. On broader shelves and in epicontinental seas, transverse troughs are much wider. Bear Island Trough in the epicontinental Barents Sea, is 170 km wide and 600 km long. The very deep inner parts of the troughs on the Antarctic shelves have been attributed partly to proglacial isostatic depression. However, this can only account for a minor part, and the great depth must primarily be due to glacial erosion (Anderson, 1999).

In recent years channels and troughs have often been demonstrated to be the drainage routes for ice streams (e.g. Anderson, 1999; Sejrup et al., 1996; Vorren and Laberg, 1997; Ottesen et al., 2001; Figs 12.6 and 12.7). At the front of many of these channels and troughs, trough mouth fans are deposited (see below).

12.1.3 Second-Order Morphological Elements

The main glacigenic morphological elements and lithofacies on a glaciated continental margin are shown in Fig. 12.8. Often glacigenic sediments are bounded below by a pronounced unconformity (e.g. Dekko, 1975; Solheim and Kristoffersen, 1984; Vorren et al., 1986, 1990; Josenhans, 1997). Glacigenic sediments on a shelf are characterized by various types of diamictons

Glacial Lithofacies Pictures

with internal sub-horizontal conformable boundaries. In the Barents Sea, buried glacigenic conformable subsurfaces often have glacial lineations on them (Rafaelsen et al., 2002). Angular unconformities and irregular boundaries may also occur, in particular related to subglacial glacifluvial drainage. The thicknesses of stratiform glacigenic sequences on continental shelves are normally less than a couple of hundred metres. On the Norwegian shelf proper, thickness varies between 0 and ~ 300 m, but at the shelf edge thicknesses often increase substantially (Vorren et al, 1992).

The banks are often covered by palimpsest sediments (i.e. a mixture of the underlying diamictons and Holocene skeletal remains). The trough fills comprise tills and various types of glacimarine sediments (e.g. Vorren et al., 1984; Sejrup et al., 1996).

12.1.3.1 Moraine Ridges and Grounding Line Features

On many continental shelves, end-moraine systems or morainal banks as they are called by some (e.g. Powell, 1983; Powell and Molnia, 1989) are identified by acoustic methods. West of

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Figure 12.4 Representative bathymetric profiles from the Antarctic continental shelf illustrating the great depth and landward sloping profile of the shelf. (After Anderson, 1999).

Shetland are large end moraines up to 50 m high, 8 km wide and which can be traced for up to 60 km (Stoker, 1997). The moraine ridges are arranged in a series, and each ridge probably marks a stillstand during eastward glacial retreat. The proximal flank of each ridge was systematically overlain by a veneer of successively younger acoustic layers (Fig. 12.9A). Vorren and Kristoffersen (1986) and Gataullin and Polyak (1997) described moraine ridges in the southwestern and eastern Barents Sea, respectively, being 1.5—4 km broad and up to 40 m high. Slowly advancing or halting ice sheets probably deposited these moraines.

On the Antarctic continental shelf several so-called grounding zone wedges are observed (Bart and Anderson, 1997; Anderson, 1997; Shipp and Anderson, 1997b; Shipp et al, 1999). The wedges average 35—75 m in thickness and extend for several tens of kilometres in length and width (Bart and Anderson, 1997; Fig. 12.9B). These features are what Alley et al. (1989) termed 'till deltas'. They suggested that unconsolidated, water-saturated sediment is transported at the base of the ice stream and deposited at the grounding line as a subglacial delta, with

Map Spread Islam Through Malaysia

Figure 12.5 Part of the shelf off northern Norway illustrating transverse and longitudinal troughs and banks. Note the landward slope of the troughs and the steep headwall aligned along the boundary between crystalline and sedimentary bedrock. Malangsdjupet trough represents a seaward continuation of the Malangenfjord system and Malselv Valley.

Figure 12.5 Part of the shelf off northern Norway illustrating transverse and longitudinal troughs and banks. Note the landward slope of the troughs and the steep headwall aligned along the boundary between crystalline and sedimentary bedrock. Malangsdjupet trough represents a seaward continuation of the Malangenfjord system and Malselv Valley.

topset beds composed of till and foreset and bottomset beds composed of sediment gravity flow deposits.

Marginal moraines deposited from surging glaciers are found off the modern ice cap Austfonna in the Svalbard archipelago. Typically the cross-sectional shape of a surge moraine is asymmetrical with a smooth distal slope of 1-3°, a proximal slope of 3-6°, and relief varying between 5 and 20 m. The width may exceed 1 km. The distal part of the ridge is associated with acoustically transparent slump lobes that drape the iceberg-ploughed seafloor. The morphology on the proximal side of the ridge is dominated by linear sediment ridges forming a rhombohedral cross pattern (Solheim, 1991; Fig. 12.9C; see also Chapter 11).

MacLean (1997) described an interesting lateral moraine from Hudson Strait (Fig. 12.9D). The moraine was probably deposited at the lateral margin of an ice sheet, the position of which was at

Figure 12.6 Late Pleistocene glacial reconstruction for the Ross Sea. Shaded areas in the troughs are where major ice streams existed, and the cross-hatched areas designate the location of the Last Glacial Maximum grounding zone. (After Anderson, 1999).

least in part controlled by water depth. The preservation of overridden sediments and the grounding depth relationships suggest that the ice sheet responsible for the ice-contact sediments was only lightly bearing on the seabed at the time of deposition (MacLean, 1997).

12.1.3.2 Linear Forms

Submarine drumlin fields are described from the inner Scotian shelf, Canada (Fader et al., 1997), where the drumlins extend up to 35 m high and 800 m long, and range from 39 to 300 m wide. These drumlins typically have a flat upper surface. Drumlins from the Ross Sea in Antarctica are described by Shipp and Anderson (1997a). These average 2 km wide, range from 2 to 5 km long, and are tens of metres high. Some have well-developed hairpin-shaped scours rimming them. Down-glacier they merge with features identified as megaflutes by Shipp and Anderson (1997b).

A detailed swath bathymetry of the inner Norwegian channel (Longva and Thorsnes, 1997; Fig. 12.10) shows many linear forms. On the upstream part, crag and tails and drumlins occur. Flutes and drumlins from different ice movement directions are preserved. In the middle part of the channel, flutes 2—7 m high reflect the last ice movement in the area. The same succession of morphological elements is observed in the Ross Sea (i.e. a transition from striated basement rocks to drumlins (and crag and tails) to megascale lineations). Anderson (1999) interprets this

298 GLACIAL LANDSYSTEMS

Continental Shelf Glacier Drawing
Figure 12.7 Inferred ice flow pattern of the Fennoscandian and Barents Sea Ice Sheets during Last Glacial Maximum on the Norwegian continental shelf. (Redrawn from Ottesent et al. (2001), south of Lofoten; Vorren and Laberg 1996, north of Lofoten).

as reflecting a transition from a 'sticky bed', where the ice sheet is coupled to the seafloor and basal meltwater is confined to the ice-bed interface, to a deforming bed, where meltwater is incorporated in the bed and the sediments are moulded into megaflutes and megascale lineations.

iceberg grounding zone palimpsest moraine glacial streamlined furrow wedge lag ridge forms marginal moraine iceberg grounding zone palimpsest moraine glacial streamlined furrow wedge lag ridge forms

Bathymetry Moraine Continental Shelf

marginal moraine stratiform glacigenic sequence regional unconformity trough fills buried subglacial channels stratiform glacigenic sequence regional unconformity

SLOPE SYSTEM SHELF SYSTEM FJORD SYSTEM

Figure 12.8 Model showing the main glacigenic morphological elements and lithofacies of a passive continental margin, exemplified by the margin off northern Norway.

Ploughmarks

12.1.3.4 Plough Marks

Iceberg plough marks (Fig. 12.11) form when keels of icebergs exceed the water depth and are therefore able to erode sea floor sediments. As an iceberg moves it ploughs aside sediments to create berms, it scrapes striae inside the furrow channels, and it may create a pile of sediments when the iceberg becomes grounded (Syvitski et al., 2001). Iceberg plough marks have been revealed in many shelf seas by side-scan sonar and other acoustic studies during the last decades. Lien (1983), who studied the Norwegian shelf, found that the iceberg plough marks occurred in current depth ranges of 120-400/500 m. The maximum width and depth of the marks found were 250 and 25 m, respectively. Barrie (1980) observed maximum furrow depths of 17 m on the Labrador Bank; Syvitski et al. (2001) observed a maximum furrow depth of 28 m and a width of 274 m on the east Greenland margin; Rafaelsen et al. (2002) observed a maximum furrow depth of 25 m and a width of 500 m in the Barents Sea (Fig. 12.11); and Polyak et al. (2001) observed up to 30 m deep furrows on the Chucki borderland and adjacent continental margin. Clearly the draft of the iceberg limits the water depth at which this process can operate. On the Greenland and Antarctic margins iceberg scouring down to 550 m is observed (Barnes and Lien, 1988; Dowdeswell et al., 1993). The largest sea depth (850 m) and width (1 km) of iceberg-ploughed sea floor is reported from the Yermak Plateau (Crane et al., 1997).

12.1.3.5 Subglacial Channels

Tunnel valleys are characteristic elements of glaciated shelves that record meltwater drainage beneath ice sheets. They are not particular to marine settings but do occur on and beneath the present continental shelves. The southern North Sea contains large examples, locally up to 500 m in relief, 12 km wide and tens of kilometres long (Cameron et al., 1987; Ehlers and Wingfield, 1991; Praeg, 1997). On the Grand Banks, Moran and Fader (1997) describe channels having a depth of about 25 m and widths of some hundreds of metres (Fig. 12.12). Various mechanisms for the formation of these valleys are commonly evaluated, but most researchers agree that they are formed by subglacial meltwater drainage.

Figure 12.9 Four examples of submarine moraine ridges. A) Geoseismic profile across submarine moraine ridges off northwest Britain showing stratigraphical relationships of the end moraines (dense stipples) and associated acoustically layered, ponded glacimarine/marine (Gm/m.) deposits. The underlying erosive sheetform unit (shaded and dotted) also belongs to the same seismostratigraphical sequence as the morainal complex; the erosional base (e.b.) is attributed to the expansive phase of the ice sheet. As the morainal complex formed during the subsequent gradual decay, this Late Devensian section appears to preserve a well-defined glacial advance-retreat cycle (redrawn from Stoker and Holmes, 1991; Stoker, 1997). B) Geoseismic profile (based on 3.5 kHz records) across the central part of the surge moraine of Brasvellbreen in the northern Barents Sea. Inside the moraine is a rhombohedral pattern of ridges having a relief in the order of 5 m and spacing between individual ridges of 20-70 m; the ridges are interpreted as crevasse fills through squeeze-up of deformable sediments during an early phase of post-surge stagnation (see Chapter 11; redrawn from Solheim, 1997). C) Schematic cross profile of a grounding zone wedge (till delta/diamicton apron). D) Geoseismic profile (based on a single channel seismic reflection profile) from south-central Hudson Strait illustrating a lateral moraine up to 70 m thick and its relationship with underlying relatively undeformed acoustically statified sediment and younger glacimarine and postglacial sequences (modified from MacLean, 1997).

The Glacial System

Figure 12.10 The central figure (B) shows a shaded image of processed multibeam echosounding data from the Norwegian Channel between Norway and Denmark (For location, see map (A) at upper left). The locations of the smaller images are shown by boxes. C) Glacitectonic hill-hole features. Slabs of bedrock or sediments were tectonically disrupted and dislocated. This erosion can be identified from the holes where the rock was removed and the hills where it was deposited. D) Plastic forms (p-forms) or grooves with a maximum depth of 20 m eroded into bedrock. E) Flutes and pockmarks. The flutes have a relief of 2-7 m and reflect the ice flow along the axis of the Norwegian Channel. Flutes from two different ice flow phases can be seen. F) Flutes and drumlins from different ice movement directions situated on top of the terrace landward of the Norwegian Channel. G) Crags and tails formed by glaciers moving toward the southwest. H). Flutes, iceberg scour and pockmarks. The iceberg scour marks cut the fluted surface. (Adapted from Longva and Thorsnes, 1997).

Figure 12.10 The central figure (B) shows a shaded image of processed multibeam echosounding data from the Norwegian Channel between Norway and Denmark (For location, see map (A) at upper left). The locations of the smaller images are shown by boxes. C) Glacitectonic hill-hole features. Slabs of bedrock or sediments were tectonically disrupted and dislocated. This erosion can be identified from the holes where the rock was removed and the hills where it was deposited. D) Plastic forms (p-forms) or grooves with a maximum depth of 20 m eroded into bedrock. E) Flutes and pockmarks. The flutes have a relief of 2-7 m and reflect the ice flow along the axis of the Norwegian Channel. Flutes from two different ice flow phases can be seen. F) Flutes and drumlins from different ice movement directions situated on top of the terrace landward of the Norwegian Channel. G) Crags and tails formed by glaciers moving toward the southwest. H). Flutes, iceberg scour and pockmarks. The iceberg scour marks cut the fluted surface. (Adapted from Longva and Thorsnes, 1997).

The Glacial System

Figure 12.11 Heavily iceberg-ploughed sea floor in the Barents Sea at about 72° 30' N and 23° 27' E. The plough marks in this area are 30-500 m wide,l-20 km long and 2.5-25 m deep. The figure is an illuminated time-structure map based on 3D seismic data where the light source is located east of the horizon. (Adapted from Rafaelsen et al., in press).

Figure 12.11 Heavily iceberg-ploughed sea floor in the Barents Sea at about 72° 30' N and 23° 27' E. The plough marks in this area are 30-500 m wide,l-20 km long and 2.5-25 m deep. The figure is an illuminated time-structure map based on 3D seismic data where the light source is located east of the horizon. (Adapted from Rafaelsen et al., in press).

12.1.3.6 Glacitectonic Forms

Ssttem (1990, 1991) observed several types of glacitectonic phenomena on the Norwegian continental shelf. He concluded that glacitectonic deformation of bedrock strata occurs widely. In particular, he described glacitectonic hill-hole pairs (Fig. 12.13), earlier observed in the terrestrial environment. Hill-hole pairs are also observed in the Norwegian Channel (Longva and Thorsnes, 1997; Fig. 12.10). The inferred glacitectonic elements include both in situ deformation and large-scale displacement. Ssttem (1990) concluded that glacitectonism has played an important role in the Cenozoic erosion, first by displacement of bedrock bodies or floes, resulting in instantaneous erosion of an amount depending on the size of displaced bodies, and second, by the transformation of displaced or in situ bedrock to glacitectonite and ultimately a till. It is theoretically elaborated that fluid overpressure and expulsion from the underlying bedrock may have played a role in the glacitectonic bedrock deformation.

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