Edited by J.A. Dowdeswell, M. Canals, M. Jakobsson, B.J.Todd, E.K. DowdeswelL. K.A. Hogan

In the past two decades there have been several advances that make the production of an atlas of submarine glacial landforms timely. First is the development of high-resolution imaging technologies: multibeam echo-sounding or swath bathymetry that allows the detailed mapping of the seafloor at water depths of tens to thousands of metres across con-tinental margins, and 3-D seismic methods that enable the visualisation of palaeo-continental shelves in Quaternary sediments and ancient palaeo-glacial rocks (e.g. Late Ordovician of Northern Africa). A second technological develop-ment is that of ice-breaking or ice-strengthened ships that can penetrate deep into the ice-infested waters of the Arctic and Antarctic to deploy the multibeam systems. A third component is that of relevance – through both the recognition that the polar regions, and especially the Arctic, are particularly sensitive parts of the global environmental system and that these high-latitude margins (both modern and ancient) are likely to contain significant hydrocarbon resources. An enhanced understanding of the sediments and landforms of these fjord-shelf-slope-rise systems is, therefore, of in-creasing importance to both academics and industry. We are editing an Atlas of Submarine Glacial Landforms that presents a series of individual contributions that describe, discuss and illustrate features on the high-latitude, glacier-influenced seafloor. Contributions are organised in two ways: first, by position on a continental margin – from fjords, through continental shelves to the continental slope and rise; secondly, by scale – as individual landforms and assem-blages of landforms. A final section provides discussion of integrated fjord-shelf-slope-rise systems. Over 100 contri-butions by scientists from many countries contain descriptions and interpretation of swath-bathymetric data from both Arctic and Antarctic margins and use 3D seismic data to investigate ancient glacial landforms. The Atlas will be pub-lished in late 2015 in the Memoir Series of the Geological Society of London.

An Atlas of Submarine Glacial Landforms:Modern, Quaternary and Ancient

ATLAS OF SUBMARINE GLACIAL LANDFORMS

Overview maps showing the geographic distribution of some contributions to the Atlas of the Submarine Glacial Landforms

From: DOWDESWELL, J.A., CANALS, M., JAKOBSSON, M., TODD, B.J., DOWDESWELL, E.K. & HOGAN, K.A. (eds) Atlas of Submarine Glacial Landforms: Modern, Quaternary and Ancient. Geological Society, London, Memoirs, xx, 1-2. © The Geological Society of London, 2014. DOI: xx.xxxx/xxx.x.

De Geer moraines on German Bank, southern Scotian Shelf of Atlantic Canada

B.J. TODD1*1Geological Survey of Canada, Natural Resources Canada, P.O. Box 1006, Dartmouth, Nova Scotia, Canada B2Y 4A2

*Corresponding author (e-mail:Brian.Todd@NRCan.gc.ca)

Numerous, regularly-spaced, parallel, linear to curvilinear ridges of sediment are recognized as forming at or close to the position of the grounding lines of water-terminating glaciers (Lindén & Möller 2005).These ice-flow transverse ridges, sometimes known as De Geer moraines (De Geer 1889), have modest heights and widths and variable lengths. De Geer’s original description of Swedish moraines with these characteristicsinvoked an annual cycle. Present use of ‘De Geer moraine’ refers to closely spaced subaqueous moraine ridges which may or may not be annual. The distribution and pattern of ubiquitous De Geer moraines on German Bank provides insight into the direction and timing of regional deglaciation of the southern Scotian Shelf (Todd et al. 2007).

DescriptionAcross a bathymetric range of over 100 m, southern German Bank ismantled by swarms of ridges; each ridge displays a quasi-linear or curved line in planform. In topographic depressions, the ridges are subdued in relief or are absent (Fig. 1a). The strike of the ridge crests is generally west-southwest–east-northeast. Individual ridges can be traced horizontally from short segments of a few hundred metres up to almost 10 km. Most ridge crests are simple lines in planform, but in some areas the crests are complex with bifurcating crest lines and (or) short, en echelon crest segments. The longer ridge crests are formed of multiple curved sections 2 to 4 km in length with the crests slightly convex toward the southeast. In places on German Bank, individual ridges traverse bathymetric ranges of up to 25 m.

The horizontal distance between ridges, normal to strike, is variable. In some areas, well-defined parallel ridges have a roughly regular spacing of approximately 150 to 200 m. In other areas, sub-parallel ridges are as closely spaced as 30 to 50 m. The ridges vary in height and width. Ridgesdisplaying a subtle appearance on the large-scale multibeam-bathymetricimage (Fig. 1b) are approximately 1.5 m high and 40 m wide. The more prominent ridges are approximately 5 to 8 m high and 100 to 130 m wide(Fig. 1 c, d). The ridges are generally symmetrical in cross section with some of the larger ridges displaying asymmetry with a gentle slope to the north and a steeper slope to the south (Fig. 1d). Cobbles and boulders were observed and recovered from ridges and gravel was recovered from smooth seafloor in troughs (Fig. 1b).

InterpretationThe ridge swarms on German Bank are interpreted to be subaqueous, ice-flow transverse De Geer moraines formed at or close to the grounding lineof the southeastern margin of the marine-terminating Laurentide Ice Sheet.Subaerially exposed De Geer moraines have been interpreted as marking former ice-margin positions where sediment is deposited or pushed up during brief stillstands or minor readvances (De Geer 1889; Blake 2000).

Using the traditional marine geophysical survey tools of seismic reflection profiling and side-scan sonar, De Geer moraines have beenrecognized in the Barents Sea (Solheim et al. 1990) and western Norway (Larsen et al. 1991). In the marine environment, technological constraints precluded small-scale (i.e. regional) mapping of glacial landforms like De Geer moraines until the advent of multibeam sonar in the early 1990s.Applying this technology, De Geer moraines have also been mapped in the Irish Sea (Van Landeghem et al. 2009) and in the North Sea (Bradwell et al. 2008).

The distribution and pattern of De Geer moraines provides insight into the subtleties of deglaciation of German Bank. After the Last Glacial Maximum, the margin of the Laurentide Ice Sheet retreated to the northwest, punctuated by many small readvances that formed De Geermoraines. The pattern of segmented, southeast-convex moraines suggests that the ice sheet margin was gently scalloped in nature. De Geer moraines are absent on exposed bedrock but are numerous on thin deposits of streamlined lineations of till (Fig. 1a); in swales between the lineations the moraines are partly to completely buried by sediment.

The observations on German Bank are consistent with De Geer moraine formation at the grounding line of a tidewater glacier as a result of deposition and (or) pushing during minor readvances and stillstands. The generally regular horizontal spacing of De Geer moraines suggests that the rate of ice-front retreat was consistent from one cycle of formation to the next. Ottesen & Dowdeswell (2006), in a study based on the interpretation of multibeam sonar imagery and historical aerial photographs of a tidewater glacier in a Svalbard fjord setting, provide compelling evidence that De Geer moraines formed there when the glacier underwent a minor readvance each winter during its general retreat. However, sedimentological studies from Scandinavia suggest that several annual cycles may in some cases contribute to the formation of a single De Geermoraine (Lindén & Möller 2005); thus, such moraines do not always represent annual formation. The larger De Geer moraines on German Bank (Fig. 1a) may be the product of several years of ice-front stability or major readvances.

ReferencesBLAKE, K.P. 2000. Common origin for De Geer moraines of variable composition

in Raudvassdalen, northern Norway. Journal of Quaternary Science, 15, 633–644.

BRADWELL, T., STOKER, M.S., et al. 2008. The northern sector of the last British Ice Sheet: Maximum extent and demise. Earth-Science Reviews, 88,207–226.

DE GEER, G.F. 1889. Ändmoräner I trakten mellan Spånga och Sundbyberg. Geologiska Föreningens i Stockholm Förhandlingar, 11, 395–397.

LARSEN, E., LONGVA, O. & FOLLESTAD, B.A. 1991. Formation of De Geer moraines and implications for deglaciation dynamics. Journal of Quaternary Science, 6, 263–277.

LINDÉN, M. & MÖLLER, P. 2005. Marginal formation of De Geer moraines and their implication on the dynamics of grounding-line recession. Journal of Quaternary Science, 20, 113–133.

OTTESEN, D. & DOWDESWELL, J.A. 2006. Assemblages of submarine landforms produced by tidewater glaciers in Svalbard. Journal of Geophysical Research, 111, F01016.

SOLHEIM, A., RUSSWURM, L., ELVERHØI, A. & NYLAND BERG, M. 1990.Glacial geomorphic features in the northern Barents Sea: direct evidence for grounded ice and implications for the pattern of deglaciation and late glacial sedimentation. In Dowdeswell, J.A. & Scourse, J.D. (eds.): Glacimarine environments: processes and sediments. Geological Society of London, Special Publication, 53, 253–268.

TODD, B.J., VALENTINE, P.C., LONGVA, O. & SHAW, J. 2007. Glacial landforms on German Bank, Scotian Shelf: evidence for Late Wisconsinan ice-sheet dynamics and implications for the formation of De Geer moraines. Boreas, 36, 148–169.

VAN LANDEGHEM, K.J.J., WHEELER, A.J. & MITCHELL, N.C. 2009. Seafloor evidence for palaeo-ice streaming and calving of the grounded Irish Sea Ice Stream: Implications for the interpretation of its final deglaciation phase.Boreas, 38, 119–131.

B.J. TODD

Fig.1. Multibeam swath bathymetry and geophysical profiles of De Geer moraines on German Bank, Scotian Shelf. (a) Sun-illuminated multibeam-bathymetric image showing De Geer moraines. Acquisition system Kongsberg EM1000. Frequency 95 kHz. Grid-cell size 5 m. Single-headed arrow denotes younger, larger moraine overriding and cross-cutting older, smaller moraines. Trend of streamlined lineations of till indicated by double-headed arrow. (b) Detailed multibeam image of De Geer moraines indicating location of geophysical profiles. Yellow dots denote samples; white dashed line denotes video transect (Todd et al. 2007). (c) Side-scan sonar image of De Geer moraines. Acquisition system Simrad MS992. Frequency 120 kHz. (d) Seismic reflection profile of De Geer moraines. VE x 23. Acquisition system Huntec Deep-Tow Seismic. Frequency 0.5–8 kHz. (e) Location of study area (red box; map from GEBCO_08).

J. A. DOWDESWELL ET AL.

ice-sheet retreat after the LGM occasionally protruding through a fine-grained basin-fill (Dowdeswell & Vásquez 2013). This interpretation is confirmed by the presence of draping and ponded acoustically laminated sediments reported from a numbers of Chilean fjords (Fig. 2c). DaSilva et al. (1997) noted that the inner fiords of central Chilean Patagonia contained 200-400 ms-thick acoustically laminated ice-proximal sediments that relate, in part, to deglaciation from the LGM,augmented by continuing fine-grained sedimentation from turbid meltwater where Holocene tidewater glaciers and glacier-fed rivers are present at fjord heads. Similar processes can be inferred from a sub-bottom profile of Ainsworth Fjord in Tierra del Fuego (Boyd et al. 2008). Holocene basin fill is of mainly weakly-laminated acoustic character with scattered chaotic facies inside a recessional moraine(Fig. 2c). Radiocarbon dates suggest accumulation rates of up to a few tens of millimetres per year (Boyd et al 2008). Crater-like features that break the smooth fjord floor in some areas are interpreted as pockmarks, probably produced from the escape of shallow gas from the underlying sediments. In addition, very few iceberg ploughmarks are observed on the relatively smooth floors of Chilean fjords. This is a result of relatively deep water combined with the calving of only small, irregular-shaped icebergs from tidewater glacier termini.

Large transverse ridges, found along and at the mouths of a number of Chilean fjords, are of similar dimensions to the terminal moraine in inner Iceberg Fjord (Fig. 1b). They are interpreted as recessional moraines formed as grounded tidewater glaciers retreated through the Chilean fjords after the LGM. The ridges indicate former still-stand positions, probably on the order of decades. This interpretation is supported by the location of several moraine ridges at the mouths of fjords; likely pinning-points points where water deepens by tens to hundreds of metres immediately distal of the ridges.

Submarine channel systems (Fig. 2a, b), formed beyond major glacifluvial deltas in Iceberg and Bernardo fjords (Fig. 1e, f), are interpreted to be turbidity-current channels eroded by dense underflows (Syvitski et al. 1987). These dense submarine currentsprobably formed when: (i) strongly seasonal meltwater flow deliveredlarge quantities of coarse-grained sediment from the braid plain: and/or (ii) slope failures on glacifluvial delta fronts, areas of high and very variable rates of sediment delivery, translated downslope into turbidity currents. The delta-fronts appear from swath bathymetry to be dominated by down-slope sediment-transfer processes that produce channels and chutes (Fig. 1e, f). At some other delta-fronts, with more lobate relief (Dowdeswell & Vásquez 2013), mass-wasting processes in the form of sediment slumps and slides probably predominate.

Discussion: landform assemblage for Chilean fjords

The assemblage of glacial, glacifluvial and glacimarine landforms found in the fjords of Chile is summarised in the schematic model in Figure 3. The three principal components of the landform-assemblage model for these relatively mild, meltwater-dominated fjords are (Dowdeswell & Vásquez 2013):

(a) Moraine ridges, indicating still-stands during ice retreat down fjords (Figs. 1d, 3), including the limit of recent Little Ice Age advance (Fig. 1b). The ridges are orientated transverse to past ice-flow, are asymmetrical with steeper ice-distal sides, and sometimes have debris-flow lobes on their ice-distal flanks. Smaller transverse-moraines ridge, found inside Little Ice Age moraines, mark ice recession over approximately the last century (Fig. 1b).

(b) Glacifluvial and fluvial deltas, formed through sedimentationfrom braided rivers from glaciated or fluvial catchments (Fig. 1e-g). The deltas have small channels and chutes, and sometimes debris-flow lobes, on their submarine faces.

(c) Relatively deep, sinuous turbidity-current channels, often found within predominantly smooth fjord floors (Fig. 2a, b).

Both glacifluvial and fluvial deltas, together with turbidity-current channels, are active sites of debris transfer today in the Chilean fjords.The glacial imprint in the landform-assemblage model, however, is often limited and related to recession from the LGM, except where tidewater glacier termini have advanced within the past few centuries (Figs. 1b-c). Apart from these three landform-components, the floor of

many Chilean fjords is mainly smooth, although bedrock ridges break through occasionally. Landforms that are not sustained by contemporary processes are likely to be buried by relatively rapidfine-grained sedimentation from meltwater plumes (Fig. 2c), and, in progressively more ice-distal settings, by Holocene marine or pelagic sediments (Fig. 2d) (Fernández et al. 2013).

Subglacial and ice-contact landforms, similar to those imaged in Arctic and Antarctic fjords and shelves, are likely to have developedat the base and margins of Chilean glaciers under full-glacial and deglacial conditions (Dowdeswell & Vásquez 2013); they can be identified in fjord acoustic stratigraphy (Fig. 3). High rates of snow-accumulation and ice-surface melting on Chilean glaciers led, however, to rapid meltwater and fine-grained sediment delivery to thefjords from tidewater glaciers and glacifluvial sources subsequent todeglaciation. Most pre-existing glacial landforms on the fjord floor, with the exception of major retreat moraines (Fig. 1b, d) and protruding streamlined bedrock, are largely buried and obscured bythis subsequent glacimarine and glacifluvial sediment delivery (Figs. 2c, 3); it is predominantly submarine landforms that remain in an active process-environment today, such as turbidity-current channelsand those relating to neoglacial ice advance, which are observed on our swath-bathymetric imagery (Fig. 2a, b).

The fjords of Chile represent an environmental setting similar to the high mass-throughput glacial system of SE Alaska in the northern hemisphere, where sedimentation rates in fjords are also very high (Powell 1990). In progressively colder polar settings, ice-surface melting is reduced, slowing the rate of sediment delivery to fjords. Here, subglacial landforms produced during the LGM and subsequent deglaciation can still be observed in swath-bathymetric imagery because they remain at or just below the modern sea floor.

We thank the Servicio Hidrografico y Oceanografico de la Armada de Chile (SHOA) for allowing use of their swath-bathymetric data.

References

BOYD, B. L., ANDERSON, J. B., WELLNER, J. S. & FERNANDEZ, R.A. 2008. The sedimentary record of glacial retreat, Marinelli Fjord, Patagonia: regional correlations and climate ties. Marine Geology, 255, 165-178.

DaSILVA, J. L., ANDERSON, J. B. & STRAVERS, J. 1997. Seismic facies changes along a nearly continuous 24° latitudinal transect: the fjords of Chile and the northern Antarctic Peninsula. Marine Geology, 143, 1-3-123.

DOWDESWELL, J. A. & VÁSQUEZ, M. 2013. Submarine landforms in the fjords of southern Chile: implications for glacimarine processes and sedimentation in a mild glacier-influenced environment. Quaternary Science Reviews, 64, 1-19.

FERNANDEZ, R., GULICK, S., RODRIGO, C., DOMACK, E., LEVENTER, A. & SAUSTRUP, S. 2013. The seismic record of late glacial variations in the Strait of Magellan: new clues for the interpretation of the glacial history of Magallanes. Bollettino di Geofisica Teorica ed Applicata, Supp. 2 - Geosur, 54, 257-259.

MASIOKAS, M., RIVERA, A., ESPIZUA, L. E. VILLALBA, R. DELGADO, S. & ARAVENA, J. C. 2009. Glacier fluctuations in extratropical South America during the past 1000 years. Palaeogeog., Palaeoclim., Palaeoecol., 281, 242-268.

OTTESEN, D. & DOWDESWELL, J. A. 2006. Assemblages of submarine landforms produced by tidewater glaciers in Svalbard. Journal of Geophysical Res., 111, doi:10.1029/2005JF000330.

OTTESEN, D. & DOWDESWELL, J. A. 2009. An inter-ice stream glaciated margin: submarine landforms and a geomorphic model based on marine-geophysical data from Svalbard. Geological Society of America Bulletin, 121, 1647-1665.

POWELL, R. D. 1990. Glacimarine processes at grounding-line fans and their growth to ice-contact deltas. In Dowdeswell, J. A. & Scourse, J. D., (eds.), Glacimarine Environments: Processes and Sediments. Geological Society Special Publications, 53, 53-73.

SYVITSKI, J. P. M., BURRELL, D. C. & SKEI, J. M. 1987. Fjords: Processes and Products. Springer, Berlin.

Additional contributions are welcome. Deadline is January 30, 2015. Please contact Julian Dowdeswell, jd16@cam.ac.uk

J.A. Dowdeswell1, M. Canals2, M. Jakobsson3, B.J. Todd4, E.K. Dowdeswell1, K.A. Hogan5.

1University of Cambridge, UK2University of Barcelona, Spain3University of Stockholm, Sweden4Geological Survey of Canada, Dartmouth, Nova Scotia, Canada5British Antarctic Survey, Cambridge, UK

Editors

Multibeam imagery of drumlins and eskers in the Gulf of Bothnia, Sweden. The data are from a bathymetric survey contracted by the Swedish Maritime Administration. This imagery is shown in the Atlas contribution: “Drumlins in the Gulf of Bothnia”, M. JAKOBSSON1, S. L. GREENWOOD1, B. HELL2 & H. ÖIÅS

TheGeologicalSociety

To be published in 2015 by the Geological Society of London as a Memoir within the Lyell Collection

The Atlas is sponsored by the following companies:

From: DOWDESWELL, J.A., CANALS, M., JAKOBSSON, M., TODD, B.J., DOWDESWELL, E.K. & HOGAN, K.A. (eds) Atlas of Submarine Glacial Landforms: Modern, Quaternary and Ancient. Geological Society, London, Memoirs, xx, 1-2. © The Geological Society of London, 2014. DOI: xx.xxxx/xxx.x.

Corrugation ridges in the Pine Island Bay glacier trough, West Antarctica

M. JAKOBSSON1* & J. B. ANDERSON2

1Department of Geological Sciences, Stockholm University, Svante Arrhenius väg 8, 106 91 Stockholm, Sweden 2Department of Earth Sciences, Rice University, 6100 Main Street, Houston, Texas 77005, USA

*Corresponding author (e-mail: martin.jakobsson@geo.su.se)

Glacial landforms with dimensions smaller than the imaging capability of the first generation multibeam sonars will become a more frequent topic as high-resolution seafloor mapping technology advances. In Pine Island Bay (PIB) glacial trough, West Antarctica, small regular ridges, only a few metres high from trough to crest, were mapped in water depths around 700 m. The small size of these ridges is on the limit for what modern deep-water multibeam sonars are capable of mapping. They are interpreted to have been formed at the trailing end of mega-icebergs moving up and down in response to tides while ploughing the seafloor (Jakobsson et al.2011). The mega-icebergs in PIB were produced by an ice shelf break-up and associated grounding-line retreat.

Description The glacial trough in central PIB contains a suite of landforms indicative of fast-flowing ice streams and episodic ice-sheet retreat following the Last Glacial Maximum (Anderson et al. 2002; Graham et al. 2010; Jakobsson et al. 2011). The relatively flat 690 to 710 m deep central section of the PIB trough is dominated by linear to curvilinear sets of large furrows aligned parallel to the trough axis with a spacing of 150 m to >500 m (Fig. 1). These large furrows resemble mega-scale glacial lineations, although their more irregular alignment suggests formation from an armada of clustered icebergs rather than from a more intact fast-flowing ice stream (Jakobsson et al. 2011). Within the large furrows, remarkably regular sets of smaller ridges occur oriented at close-to-right-angles to them (Fig. 1a, b). These ridges are separated by ~60–200 m, with spacing generally decreasing progressively in a seaward direction; ridge heights range from 1 to 2 m from trough to crest. The extremely regular appearance of these ridges makes the furrows look corrugated; hence the smaller ridges were named “corrugation ridges” by Jakobsson et al. (2011).

Similar landforms have been mapped in the eastern Weddell Sea in individual iceberg plough marks using side-scan sonar (Lien 1981; Barnes and Lien, 1988). Furthermore, features identical to the corrugation ridges of PIB have been mapped in large iceberg furrows in the northern part of Bjørnøyrenna, Barents Sea (Andreassen et al. 2013), and within mega-scale glacial lineations in the central (Jakobsson et al. 2011) and western Ross Sea (Anderson, 1999).

Interpretation Several formation mechanisms for corrugation ridges have been explored by Jakobsson et al. (2011) and Graham et al. (2013). Their extreme regularity excludes formation as recessional moraines formed near a retreating ice margin. This also eliminates the interpretation that the corrugation ridges are De Geer moraines (Hoppe, 1959; Lindén and Möller, 2005) or corrugation moraines (Shipp et al. 1999). Sediment cores from the ridges contained poorly sorted glacial diamict and glacimarine sandy clays, suggesting that a current-influenced formation process could be dismissed. The discovery of corrugation ridges within individual iceberg ploughmarks points towards a formation mechanism that occurs at the trailing end of a ploughing iceberg, because otherwise the iceberg would itself remove the seafloor imprints from its own impact with the seafloor.

The corrugation ridges in PIB are interpreted to have been generated at the trailing edge of mega-icebergs that broke off at the grounding line of the former PIB ice stream and drifted seaward (Jakobsson et al. 2011). Each ridge is formed when an iceberg, or an armada of icebergs in the case of PIB, rhythmically settles to the sea floor under the influence of tidal motion and squeezes sediments into ridges that are preserved in the wake of the drifting icebergs (Fig. 1d). In PIB, where the corrugation ridges exist in large parallel furrows, the formation model calls for a rather uniformly thick iceberg armada just thick enough to keep the icebergs grounded on the gently upward-sloping glacial trough. This mélange of ice would eventually disintegrate into individual icebergs that drifted away on their own; most of them were probably unstable and thus soon to rotate (Fig. 1d). In PIB, iceberg plough ridges were formed at the end of each large megaberg-induced furrow before the icebergs rotated (Fig. 1a). While corrugation ridges may form in slightly different glacial environments, i.e. behind icebergs or underneath a fractured ice stream, the vertical and rhythmic tidal motion is the common denominator for the formation process.

References ANDREASSEN, K., KARIN ANDREASSEN, WINSBORROW, M.C.,

BJARNADÓTTIR, L.R., RÜTHER, D.C. Accepted. Landform assemblage from the collapse of the Bjørnøyrenna Palaeo-ice stream, northern Barents Sea. Quaternary Science Reviews.

ANDERSON, J.B. 1999. Antarctic Marine Geology, Cambridge University Press, Cambridge.

ANDERSON, J.B., SHIPP, S.S., LOWE, A.L., WELLNER, J.S., MOSOLA, A.B. 2002. The Antarctic Ice Sheet during the Last Glacial Maximum and its subsequent retreat history: a review. Quaternary Science Reviews, 21, 49-70.

BARNES, P.W., LIEN, R. 1988. Iceberg rework shelf sediments to 500 m off Antarctic. Geology, 16, 1130-1133.

GRAHAM, A.G.C., LARTER, R.D., GOHL, K., DOWDESWELL, J.A., HILLENBRAND, C.-D., SMITH, J.A., EVANS, J., KUHN, G., DEEN, T. 2010. Flow and retreat of the Late Quaternary Pine Island-Thwaites palaeo-ice stream, West Antarctica. Journal of Geophysical Research. 115, F03025.

GRAHAM, A. G. C., DUTRIEUX, P., VAUGHAN, D. G., NITSCHE, F. O., GYLLENCREUTZ, R., GREENWOOD, S. L., LARTER, R. D., AND JENKINS, A., 2013, Seabed corrugations beneath an Antarctic ice shelf revealed by autonomous underwater vehicle survey: Origin and implications for the history of Pine Island Glacier. Journal of Geophysical Research: Earth Surface, 115, F03025.

HOPPE, G., 1959. Glacial morphology and inland ice recession in northern Sweden. Geografiska Annaler, 41, 193-212.

JAKOBSSON, M., ANDERSON, J.B., NITSCHE, F.O., DOWDESWELL, J.A., GYLLENCREUTZ, R., KIRCHNER, N., O’REGAN, M.A., ALLEY, R.B., ANANDAKRISHNAN, S., MOHAMMAD, R., ERIKSSON, B., FERNANDEZ, R., KIRSHNER, A., MINZONI, R., STOLLDORF, T., MAJEWSKI, W. 2011. Geological record of Ice Shelf Breakup and Grounding Line Retreat, Pine Island Bay, West Antarctica. Geology, 39, 691-694.

LIEN, R. 1981. Seabed features in the Blaaenga area, Weddell Sea, Antarctica, Port and Ocean Engineering under Arctic Conditions, Quebec, Canada, pp. 706-716.

LINDÉN, M., MÖLLER, P., 2005. Marginal formation of De Geer moraines and their implications to the dynamics of grounding-line recession. Journal of Quaternary Science, 20, 113-133.

SHIPP, S., ANDERSON, J.B., DOMACK, E.W. 1999. Late Pleistocene-Holocene retreat of the West Antarctic ice-sheet system in the Ross Sea; Part 1. Geophysical results Geological Society of America Bulletin, 111, 1468-1516.

M. JAKKOBSSON && J. B. ANDEERSON

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From: DOWDESWELL, J. A., CANALS, M., JAKOBSSON, M., TODD, B. J., DOWDESWELL, E. K. & HOGAN, K. A. (eds) Atlas of Submarine Glacial Landforms: Modern, Quaternary and Ancient. Geological Society, London, Memoirs, xx, 1-4. © The Geological Society of London, 2014. DOI: xx.xxxx/xxx.x.

Assemblage of glacial and related landforms in the fjords of southern Chile

J. A. DOWDESWELL1*, E. K. DOWDESWELL1, C. RODRIGO2, & J. DIAZ3

1Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK2Department of Geology, Andrés Bello University, Quillota 980, Viña del Mar, Chile

3School of Ocean Sciences, Catholic University of Valparaiso, Altamirano 1480, Valparaiso, Chile

*Corresponding author (e-mail: jd16@cam.ac.uk)

The fjords of southern Chile (Fig. 1a) are one of the lowest-latitude and mildest regions in which glaciers reach the sea today (Dowdeswell & Vasquez 2013). The Andean ice caps and the glaciers of the Darwin Cordillera are of high mass throughput, with precipitation often metres per year and, to balance this, very high rates of surface melting. These ice masses either reach the sea at fjord heads, or feed braided river systems that flow into the fjord systems. Large volumes of meltwater and glacial sediment are therefore delivered to Chilean fjords either directly from subglacial streams beneath tidewater glaciers, or indirectly by glacifluvial processes. At the Last Glacial Maximum (LGM), the Patagonian Ice Cap expanded to fill these fjords, which were reoccupied by ocean waters as deglaciation proceeded.

Description

Some Chilean fjords have experienced tidewater-glacier advance during the cool Little Ice Age, followed by retreat over the past century or so (Masiokas et al. 2009). In inner Iceberg Fjord the sea-floor contains several sets of features (Dowdeswell & Vásquez 2013):a series of transverse ridges of varying height distributed across the fjord; a number of sub-parallel ridges orientated in the likely direction of past ice flow; and areas of flat, smooth sea floor (Fig. 1b). The outermost transverse ridge is about 100 m high and is asymmetrical, with a steeper ice-distal face (Fig. 1b); a second large ridge is about 50 m in height. A further ten or so transverse ridges are smaller, at about 10 m high. Closer to the modern ice front, a series of sedimentary lineations up to about 0.5 km long trend first just north of west and then northward (labelled s in Fig. 1b). The rest of the floor of inner Iceberg Fjord is predominantly flat and smooth (Fig. 1b).

In inner Eyre Fjord, Pio XI Glacier, which is of surge-type, has undergone advances of up to 5 km during the 20th Century (Masiokas et al. 2009). The sea floor beyond about a kilometre from the modern glacier terminus is largely flat, with a number of hummocks up to 14 m high together with short ridge segments, but otherwise shows few signs of an organised set of submarine landforms (Fig. 1c). These rather chaotic positive-relief features give way to a smooth sea-floor down-fjord (Fig. 1c).

In several Chilean fjords, large sedimentary ridges are present across fjord axes (Dowdeswell & Vásquez 2013). In the 50 km-long Europa Fjord, a prominent ridge is located at the fjord mouth where it joins the 250 m-deeper Wide Fjord. Three other sedimentary ridges are spaced about 8 km apart in the inner half of the fjord (Fig. 1d).Prominent sedimentary ridges of similar geometry are located where relatively shallow tributary fjords enter the 40-km long Farquahar-Bernardo Fjord system and in Ringdove Fjord (49 42’S), usuallyextending the full width of the fjord. On their steeper, outer-fjord slopes, small channels or chutes (up to a few hundred metres long and a few metres wide) are sometimes imaged, along with lobate features that appear to override fjord-floor sediments (Fig. 1d).

Where glaciers from the Patagonian Ice Caps do not reach the seatoday, braided rivers or sandurs link the glacial and marine sedimentary systems. For example, Occidental Glacier reaches outer Iceberg Fjord via an 11 km-long braided channel (Fig. 1e), and a 2km-long braid plain links Bernardo Glacier with the adjacent fjord (Fig. 1f). Sediment delivery from these braided channels has built glacifluvial deltas 1.5 to 2 km wide into the fjords beyond (Fig. 1e, f).

Smaller fluvial catchments also have deltas at their marine margins that have built out into relatively deep fjord waters (Fig. 1g). Swath bathymetry of the large glacifluvial delta fronts images small submarine channels and narrow chutes which appear to organise into single major submarine channels on the fjord floors (Fi. 1e, f). The submarine channels continue for a number of kilometres along the flat floors of Iceberg and Bernardo fjords and into Messier Fjord (Fig. 2a, b). The submarine channel in outer Iceberg Fjord is at least 15 km long, up to about 50 m deep, and with an average width of 275 m and that in Bernardo Fjord extends about 10 km, with a width and depth of about 100 m and 50 m, respectively.

Much of the Chilean fjord system, especially with increasing distance from tidewater-glacier margins, is devoid of major sedimentary landforms, with the exception of occasional streamlined features and pockmarks. Streamlined sea-floor lineations, orientated along-fjord, are observed in several areas (Dowdeswell & Vásquez 2013); some appear as streamlined bedrock protruding above the flat, sedimentary fjord floors. Sets of crater-like structures, numbering tens of individual features, are also observed on the floor of several Chilean fjords; median crater depth is between 3 and 5 m and median diameter is 40 to 60 m (Dowdeswell & Vásquez 2013).

Interpretation

The cross-fjord ridges observed in many Chilean fjords are interpreted as moraines produced at the former margins of tidewater glaciers. The largest ridge in inner Iceberg Fjord is a terminal moraine marking the outer limit of a Little Ice Age advance of Tempano Glacier (Fig. 1b). Its steeper distal face is typical of moraines formed at the maximum neoglacial positions of many northern hemisphere tidewater glaciers (e.g. Ottesen & Dowdeswell 2009). The sets of smaller transverse ridges closer to the modern glacier terminus are very similar to those deposited during still-stands, or minor readvances, superimposed on more general glacier retreat in Svalbard fjords (e.g. Ottesen & Dowdeswell 2006). The lineations trending first west and then north in the innermost part of Iceberg Fjord (Fig. 1b)are interpreted as streamlined sedimentary landforms produced beneath active glacier ice (Ottesen & Dowdeswell 2009). They indicate that the tidewater glacier was grounded in the deepest parts of the inner fjord. They also suggest that ice flow included a northward component, perhaps occurring when ice had retreated from the area of transverse ridges to fill only the innermost fjord. The smooth and flat areas of the sea-floor, for example between the transverse moraine ridges (Fig. 1b), are interpreted as sedimentary infill from the rainout of fine-grained debris transported away from the tidewater glacier as a turbid plume of suspended sediment (Powell 1990).

Beyond Pio XI Glacier, in the 35 km-long Eyre Fjord (Fig. 1c), the overall rather flat sea-floor observed proximal to the present ice front is most likely also a result of deposition of fine-grained material from turbid meltwater plumes. Isolated hummocks and ridges were probably formed during a previous surge of the glacier, given that deformed medial moraines on the glacier surface imply recent surge activity (Masiokas et al. 2009).

The floor of Eyre Fjord, in common with many Chilean fjords (e.g. DaSilva et al. 1997; Boyd et al. 2008), is largely smooth, withstreamlined bedrock lineations and recessional-moraine ridges from

J. A. DOWDESWELL ET AL.

Fig. 1. Sun-illuminated multibeam bathymetric images of submarine landforms in fjords of Chilean Patagonia. Acquisition system Atlas Fansweep. Frequency 200 kHz. Grid-cell size 20 m. (a) Location of study area (red box; map from GEBCO_08). (b) Terminal moraine ridge and other transverse ridges produced by Tempano Glacier. S is streamlined lineations. (c) Hummocky but otherwise largely flat sea-floor beyond Pio XI Glacier in Eyre Fjord. H is hummocky glacial debris. (d) Swath image of 100 m-high recessional moraine in Europa Fjord. (e) Glacifluvial delta in Iceberg Fjord and associated braided river. (f) Glacifluvial delta in Bernardo Fjord, showing braided river and adjacent Bernardo Glacier. (g) Small fluvial delta in Eyre Fjord. Images modified after Dowdeswell & Vásquez (2013).

J. A. DOWDESWELL ET AL.

Example of a 4-page contribution describing an assemblage of glacial landforms

Examples of 2-page contributions describing glacial landforms

Accepted papers (25 November 2014)ANDERSON, J.B. & JAKOBSSON, M. Grounding-zone wedges on Antarctic continental shelves (820)

AMBLAS, D. & CANALS, M. Contourite drifts and canyon-channel systems on the Northern Antarctic Peninsula Pacific margin (844)

BARNES, P., PICKRILL, R. & BOSTOCK, H. Thompson and Bradshaw Sounds, Fiordland, New Zealand: a relict, mid-latitude, temperate glacier system (880)

BARNES, P. M., PICKRILL, R. A., BOSTOCK, H. C., DLABOLA, E. K., GORMAN, A. R. & WILSON, G. S. Relict proglacial deltas in Bradshaw and George Sounds, Fiordland, New Zealand (903)

BELLEC, V., PLASSEN, L., RISE, L. & DOWDESWELL, J. A. Malangsdjupet: a cross-shelf trough on the North Norwegian margin (860)

BELLEC, V., RISE, L., BØE, R. & DOWDESWELL, J. A. Glacially related gullies on the upper continental slope, SW Barents Sea margin (861)

BØE, R., BELLEC, V. K., DOLAN, M. F. J., RISE, L., BUHL-MORTENSEN, P. & BUHL-MORTENSEN, L. Cold-water coral reefs in the Hola glacial trough off Vesterålen, North Norway (821)

BROTHERS, L. L., LEGERE, C., HUGHES CLARKE, J. E., KELLEY, J. T, BARNHARDT, W. A., ANDREWS, B. D. & BELKNAP, D. F. Pockmarks in Passamaquoddy Bay, New Brunswick, Canada (904)

BURTON, D. J., DOWDESWELL, J. A., HOGAN, K. A. & NOORMETS, R. Little Ice Age terminal and retreat mo-raines in Kollerfjorden, northwest Spitsbergen (865)

CHAND, S., THORSNES, T., RISE, L., BRUNSTAD, H. & STODDART, D. Pockmarks in the SW Barents Sea and their links with iceberg ploughmarks (852)

DECALF, C., DOWDESWELL, J. A. & FUGELLI, E. M. G. Seismic character of possible buried grounding-zone wedges in the Late Ordovician glacial rocks of Algeria (915)

DOWDESWELL, J. A. & OTTESEN, D. 3D seismic imagery of deeply buried iceberg ploughmarks in North Sea sediments (893)

DOWDESWELL, J. A. & OTTESEN, D. Current-modified recessional-moraine ridges on the northwest Spitsber-gen shelf (916)

DOWDESWELL, J. A. & OTTESEN, D. Eskers formed at the beds of modern surge-type tidewater glaciers in Spitsbergen (918)

DOWDESWELL, J. A., BATCHELOR, C. L., HOGAN, K. A. & SCHENKE, H.-W. Nordvestfjord: a major East Green-land fjord system (870)

DOWDESWELL, J. A., OTTESEN, D. & NOORMETS, R. Submarine slides from the walls of Smeerenburgfjorden, NW Svalbard (850)

DOWDESWELL, J. A., OTTESEN, D. & RISE, L. Skjoldryggen terminal moraine on the mid-Norwegian Shelf (917)

DOWDESWELL, J. A., SOLHEIM, A. & OTTESEN, D. Rhombohedral crevasse-fill ridges at the marine margin of a surging Svalbard ice cap (908)

ELVENES, S. & DOWDESWELL, J. A. Possible 'lift-off moraines' at grounded ice-sheet margins, North Norwe-gian shelf edge (856)

ELVENES, S., BØE, R. & RISE, L. Post-glacial sand drifts burying De Geer moraines on the continental shelf off North Norway (855)EVANS, J. & DOWDESWELL, J. A. Submarine gullies and an axial channel in glacier-influenced Courtauld Fjord, East Greenland (851)

FLINK, A. E., NOORMETS, R. & KIRCHNER, N. Annual moraine ridges in Tempelfjorden, Spitsbergen (911)FORWICK, M., KEMPF, P. & LABERG, J. S. Eskers in deglacial sediments of three Spitsbergen fjords (881)

GALES, J. A., LARTER, R. D. & LEAT, P. T. Iceberg ploughmarks and associated sediment ridges on the south-ern Weddell Sea margin, Antarctica (831)

GALES, J. A., LARTER, R. D. & LEAT, P. T. Submarine gullies on the southern Weddell Sea slope, Antarctica (832)

GREENWOOD, S. L., JAKOBSSON, M., HELL, B. & ÖIÅS, H. Esker systems in the Gulf of Bothnia (858)

HILL, J. C. Iceberg ploughmarks on the upper continental slope, South Carolina (891)JAKOBSSON, M. & ANDERSON, J. B. Corrugation ridges in the Pine Island Bay glacier trough, West Antarctica (817)

JAKOBSSON, M., GREENWOOD, S. L., HELL, B. & ÖISÅS, H. Drumlins in the Gulf of Bothnia (873)

JAKOBSSON, M. & O'REGAN, M. Arctic Ocean iceberg ploughmarks (838)

JAKOBSSON, M., O'REGAN, M., GREENWOOD, S. L., FLODEN, T., SWÄRD, H., BJÖRCK, S. & SKELTON, A. Post-glacial tectonic structures and mass wasting in Lake Vättern, southern Sweden (901)

LASTRAS, G. & DOWDESWELL, J. A. Terminal and recessional moraines in the fjords of southern Chile (859)

LASTRAS, G., AMBLAS, D. & CANALS, M. Fjord flank collapse and associated deformation in Aysén fjord, Chile (842)

NITSCHE, F. O., LARTER, R. D., GOHL, K., GRAHAM, A. G. C. & KUHN, G. Bedrock channels in Pine Island Bay, West Antarctica (771)

O'BRIEN, P., SMITH, J. & RIDDLE, M. Bouldery debris mantle deposited by cold-based ice offshore of the Vest-fold Hills, East Antarctica (887)

REBESCO, M., CAMERLENGHI, A. & LLOPART, J. Glacigenic debris flow deposits, Storfjorden Fan (864)

RISE, L., BELLEC, V., OTTESEN, D., BØE, R. & THORSNES, T. Hill-hole pairs on the Norwegian Continental Shelf (872)

RISE, L., OTTESEN, D., OLESEN, O. & DOWDESWELL, J.A. Buried mid-Quaternary mega-scale glacial linea-tions in the Norwegian Channel from 3D seismic imagery (867)

RÜTHER, D., ANDREASSEN, K. & SPAGNOLO, M. Aligned glacitectonic rafts on the floor of the central Barents Sea (907)

RYAN, J. C., DOWDESWELL, J. A. & HOGAN, K. A. Three cross-shelf troughs on the continental shelf of South-west Greenland from Olex data (823)

SACCHETTI, F., Ó COFAIGH, C. & BENETTI, S. Iceberg ploughmarks on the Rockall Bank, Northeast Atlantic (826)

SHAW, J. & POTTER, D. P. Anthropogenic modification of a fjord: Bay of Islands, Newfoundland (914)

TODD, B. J. De Geer moraines on German Bank, southern Scotian Shelf of Atlantic Canada (818)