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Late Cenozoic glaciation directly affected sedimentation on more than half the Earth's continental shelves. Ice continues to be a dominant influence on sedimentation around Greenland and Antarctica, and on the shelves facing the Arctic Ocean. The features of these shelves include true glacimarine features, i.e. those found in a marine environment in proximityto, or strongly under the influence of, ice, such as iceberg scours and pits, ice gouges and incisions, subglacial outwash deposits, and diamictons resulting from ice rafting. Also seen, because large areas of the shelves were exposed during the Pleistocene lowering of sea level, are terrestrial glacial and periglacial features, e.g. fluvial outwash valleys and associated deposits, tunnel valleys, drumlin fields and lodgement till, which have subsequently been submerged and modified by marine influences.
Glaciated Continental Margins: An Atlas of Acoustic Images illustrates the complexity of features found in glaciated and formerly glaciated marine environments. The volume was assembled by an international Editorial Committee, led by Thomas A. Davies (University of Texas), from records gathered in the course of recent research and contributed by members of the scientific community from around the world. These include seismic sections, side-scan maps, and 3-D seismic data, supplemented in some cases by bottom photographs and core data, with accompanying text. The work is scientists at 40 institutions in 10 countries is represented.
This book will be an invaluable resource for students, Quaternary scientists, glaciologists, marine geologists and geophysicists, geotechnical engineers, and surveyors teachers working in universities, research institutions and government agencies with interests in polar and subpolar regions, as well as those in industries with offshore interests.

Inhaltsverzeichnis

Frontmatter

Introduction

Introduction

It is now more than 150 years since geologists became convinced that Scandinavian and Alpine glaciers had once extended far beyond their present limits, and more than 80 years since the same situation was recognized in the Antarctic. These observations lead to the realization that in the recent geologic past, ice, now mainly associated with the polar regions, had been much more widespread, and was, in fact, responsible for the formation and shaping of many features of the now ice-free landscape in the higher latitudes of Europe and North America, as well as parts of southern South America and New Zealand [see historical summary in Flint, 1971] Indeed, the dominant climatic event of the late Cenozoic was the Ice Age, during which most areas of the Earth’s surface within 30° of the poles were repeatedly covered by ice sheets, by grounded or floating glacier ice, or at least under the influence of sea ice (Fig. 1). Though most apparent at high latitudes where the morphology of both the land and the continental shelves is dominantly shaped by glacial erosion and deposition, the impact of the late Cenozoic glaciations can be recognized, and is still being felt, worldwide, in the effects of changes in sea level, the distribution patterns of vegetation and related animal species, the distribution of marine plankton, and in other ways. Furthermore, formations in the stratigraphic record extending back into the Precambrian have also been attributed to ancient ice ages [Hambrey and Harland, 1981; Anderson, 1983]. Thus, glaciation and its effects, rather than being unique phenomena confined to the polar regions, are of considerable global geologic and socio-economic significance.

Thomas A. Davies

Seismic Character and Variability

Frontmatter

Seismic Methods and Interpretation

A great deal of literature exists on the basic concept of the seismic method, seismic reflection systems and the interpretation of seismic records. The aim of this atlas is to provide a practical guide focusing on the connection between geology and seismic sections, with an emphasis on interpretation. Whilst this necessitates a brief summary of some of the most commonly used seismic techniques and interpretive methods (this chapter), it is not our intention for the atlas to be a textbook in reflection seismology or seismic interpretation. Instead, we have included a number of selected references that will enable the reader to enhance their understanding of these disciplines. Additionally, italicised text words (excluding headers) in this chapter are expanded upon in the glossary at the back of the atlas.

Martyn S. Stoker, Jack B. Pheasant, Heiner Josenhans

Iceberg Scours: Records from Broad and Narrow-Beam Acoustic Systems

Broad-beam acoustic profiling systems operating at 3.5 kHz have been used in numerous studies of marine and glacimarine sediments. However, the depth-variable footprint diameter of 3.5 kHz systems is relatively large, producing a low horizontal resolution (Fig. 1). By contrast, acoustic systems which use a so-called “parametric principle” have significantly smaller beam-widths. In the case of the Krupp-Atlas Parasound system, beam-width is 4°, producing a footprint diameter of only 7% of water depth (Fig. 1). In 250 m of water, the Parasound system has a footprint diameter of about 15 m compared with 150 m for a conventional broad-beam 3.5 kHz system.

Julian A. Dowdeswell, Robert J. Whittington, Heinrich Villinger

The Effects of Shallow Gas on Seismic Reflection Profiles

Shallow interstitial gas within marine and lacustrine sediments restricts seismic reflection imaging of sub-gas events. The effect is sometimes termed “gas blanking, acoustic turbidity or acoustic masking” [Hovland and Judd, 1988]. The gas scatters or attentuates the acoustic energy, preventing penetration. Coherent reflection events (sediment bedding horizons) that border the areas with high gas content are often deflected downward (pull downs): the result of reductions in the speed of acoustic propogation through the gassy sediments. Examples are presented from Atlantic Canada (Figure. 1).

Gordon B. J. Fader

Simultaneous use of Multiple Seismic Reflection Systems for High Resolution and Deep Penetration

It is possible to resolve the facies architecture of unconsolidated bottom deposits and deeper structures within underlying bedrock by using a combination of seismic reflection profiling systems. Data from Hudson Bay Canada, was concurrently collected using a Huntec DTSTM high resolution subbottom profiler, a.6 litre (40 cubic inch) airgun with single channel seismic eel, and a 16 kilojoule sparker with multichannel seismic array. No detrimental “cross talk” was recorded due to carefull adjustment of the trigger pulses and descriminate filtering for each system. Individual seismic sequences can readily be correlated between the various systems. The finest resolution (30 cm) is obtained with the Huntec DTSTM system (with a centre frequency between 3–4 kHz) [Parrott, et al 1980]. This data reveals detailed structure within the surficial sediments as well as within bedrock which is helpful in interpreting the depositional setting. The low angle bedding and regional consistency of the bedrock seismic horizons within Hudson Bay allows detailed structure revealed by the Huntec DTSTM high resolution system, to be extrapolated to depths where they are visible on the multichannel data. This approach has led to the recognition in Hudson Bay, of sedimentological structures such as fluvial point bars, reefal structures, microfaults and salt collapse structures as well as the facies architecture of the overburden above bedrock.

Heiner Josenhans

Features Found in Glacimarine Environments

Frontmatter

Subglacial Features

Overview

Subglacial features are those features that form or accumulate beneath the basal part of a grounded glacier, which may vary in size from a fjord glacier to a continental ice sheet. Such features are technically not considered of glacimarine origin, but are included in this volume because they are commonly observed in the present marine environment, and record the history of grounded glaciers into the marine realm. The following general descriptions are summarized from Denton and Hughes [1981], Molnia [1983], Solheim and Pfirman [1985], Drewry [1986], Dowdeswell and Scourse [1990], Josenhans and Zevenhuizen [1990], Anderson et al. [1992], Hambrey [1994], Menzies [1995], and Cooper et al. [1995]. These references contain specific information, some regional examples, and numerous literature citations on subglacial features.

Alan K. Cooper, Paul R. Carlson, Erk Reimnitz

A Glacial Trough Eroded in Layered Sediments in a Norwegian Fjord

Only a few decades ago, the general opinion was that during a glaciation almost all sediments both on land and in the fjords in Norway, were eroded, transported and deposited outside the coast, on the continental shelf [e.g. Holtedahl 1960]. However, seismic investigations during the latest years have revealed that glaciers overran unconsolidated sediments without removing them, or only partly affected them. This is well known from the Norwegian shelf, but so far poorly documented from Norwegian fjords. During reconnaissance seismic surveying in Trondheimsleia, Central Norway (Fig. 1), a glacial trough was detected which had first eroded into laminated sediments and then been infilled with younger sediments (Fig. 2).

Dag Ottesen

Structures in Scoresby Sund, East Greenland

The Scoresby Sund, located in the central part of East Greenland (Fig. 1), is one of the largest fjord systems of the world. Several narrow deep fjords open into the wider shallower parts supplying a large number of icebergs. The sedimentary distribution and structures in this fjord system are strongly influenced by former ice shelf/sheet configurations and recent iceberg movements. Scoresby Sund is characterised by two distinct seismic sequences: a thin Quaternary layer on top of probably Mesozoic sedimentary rocks [Uenzelmann-Neben, 1993].

Gabriele Uenzelmann-Neben

Glacially Overdeepened Troughs and Ice Retreat ‘Till Tongue’ Deposits in Queen Charlotte Sound, British Columbia, Canada

The continental shelf of Queen Charlotte Sound, British Columbia, was inundated by piedmont glaciers which extended to the shelf edge by way of the shelf transverse troughs [Josenhans et al., 1992; Luternauer et al., 1989]. Seismic reflection profiles indicate that glacial erosion overdeepened these troughs and developed a smooth glacial unconformity on bedrock. Glacial and deglacial sediments comprising basal till, multiple ice-contact sequences, and stratified glaciomarine sediments overlie the glacial unconformity and typically attain 50m in thickness. The offshore banks, which separate the troughs, appear to be devoid of glacial deposits and may have remained ice free during the last glaciation. Regional seismic profiles from the trough axes reveal multiple generations of channel fill deposits in channels cut into the Plio-Pleistocene bedrock on the inner shelf. The upper surface of the channel deposits is truncated by the glacial erosional unconformity and overlain by till. These channel deposits have not been sampled and may represent a preglacial subaerial drainage system or possibly subglacial channel sequences that were deposited prior to the last glacial advance.

Heiner Josenhans

Glacial Erosion of Sediments in the Ålfjord, western Norway

In western Norway pre Late Weichselian sediments (older than 15,000 B.P.) are limited except at a few locations. Ålfjord (Fig. 1) is a coast parallel fjord with up to 250 ms TWT thick sediment sequences. Ice movement was towards the Norwegian Channel [Holtedahl 1975] and the area has been ice free during Mid Weichselian [Sejrup & Larsen 1991].

Inge Aarseth

Glacial Unconformities on the Antarctic Continental Margin, an Example from the Antarctic Peninsula

The Antarctic Peninsula margin extends from Bransfield Basin to Adelaide Island. The Peninsula is one of the most tectonically complex regions of the Antarctic continental margin. Development of the margin has been influenced by a series of ridge-trench collisions between the Pacific and Antarctic plates. Major fracture zones segment the deep sea floor. The continental shelf break averages 400 m. Foredeepened topography and a series of deeply scoured troughs dominate the bathymetry. On the inner shelf, bathymetry is extremely rugged. The Peninsula region has undergone several episodes of glacial expansion [Bart and Anderson, 1995].

Philip J. Bart, John B. Anderson

Glacial Sole markings on Bedrock and Till in Hudson Bay, Canada

Huntec DTSTM seismic reflection profiles and sidescan sonograms are interpreted to indicate subglacial and ice marginal sole markings [Josenhans and Zevenhuizen, 1990]. The profiles reveal a thin (<5 metres) veneer of undulating, acoustically unstratified sediment in central Hudson Bay, interpreted as till overlying a smooth bedrock unconformity. The sidescan sonogram covers an area of seafloor 1.2 × 5.5 km where the surface of the till is moulded into parallel flutes which trend 340 degrees. They are thought to have been formed at the base of a moving ice mass flowing in a northwesterly direction. The direction of these flute marks is regionally variable with flow trending toward the deep basins. The regional variations in flow direction, can be used to infer the pattern of ice break up and the location of ice domes. Comparative flute marks from terrestrial areas northwest of Hudson Bay are illustrated by the aerial photograph. Note that on a local scale, the (marine) features are mostly parallel throughout and change only slightly along track, presumably in response to an eroding ice base. We have visually observed similar changes in iceberg scour marks from submersibles where abrasion of the ice keel results in modified scour morphology along track. Similar glacial sole marks have been reported from the northern Barents Sea [Solheim et al., 1989].

Heiner Josenhans

Drumlins in Lake Ontario

Lake Ontario (74 m above sea level) is located 150–250 km north of the Late Wisconsinan southern limit of the Laurentide Ice Sheet in North America (Figure 1). Multibeam bathymetric mapping in deep eastern Lake Ontario (Figures 2, 3) revealed a set of parallel, straight, narrow ridges trending 235±5° which commonly rise 10–20 m above the surrounding lakefloor. The mapped ridges range in length to 6 km and from 60 to 600 m in width.

C. F. Michael Lewis, Larry A. Mayer, Gordon D. M. Cameron, Brian J. Todd

A Seabed Drumlin Field on the Inner Scotian Shelf, Canada

The interpretation of offshore glacial landforms is difficult using conventional marine geological survey tools. Multibeam bathymetric data offers new opportunities for recognition and interpretation, and has been collected on the Scotian Shelf, by the Canadian Hydrographic Service (Figure 1). A Simrad EM 1000 mapping system [Loncarevic et al., 1994] was used to produce shadowgram images. Elliptical and linear-shaped relief elements occur on the image (Figure 2b), which are similar in shape to a nearby land field of drumlins (Lunenberg Drumlin Field, Figure 2a). The seabed features are interpreted as drumlins and are elongated in a northwest-southeast direction with their steepest sides facing the northwest, and are overlain by two to five metre high ridges (Figure 2b and d), interpreted as linear moraines. The ridges bifurcate, are discontinuous, and vary in both width and height along their length.

Gordon B. J. Fader, Rudolph R. Stea, R. C. Courtney

Drumlin Field on the Ross Sea Continental Shelf, Antarctica

Ross Sea is a broad embayment, approximately 1500 km wide and 900 km long, on the Antarctic coast (Fig. 1). Water depths range from less than 300 m to greater than 1200 m and average in excess of 500 m. The regional bathymetry is dominated by a series of roughly northeast-southwest ridges and troughs. The continental shelf is foredeepened; the inner shelf is deeper than the outer shelf due to a combination of glacial scour and isostatic loading. Repeated expansion of the East and West Antarctic ice sheets is interpreted to have modified the continental shelf. The records of the most recent glacial expansions are preserved in the surficial features and sedimentary deposits of the Ross Sea floor.

Stephanie Shipp, John B. Anderson

Lineations on the Ross Sea Continental Shelf, Antarctica

Ross Sea is a broad embayment, approximately 1500 km wide and 900 km long, on the Antarctic coast (Fig. 1). Depths range from less than 300 m to greater than 1200 m and average in excess of 500 m. Bathymetry is dominated by a series of roughly northeast-southwest ridges and troughs. The continental shelf is foredeepened; the inner shelf is deeper than the outer shelf due to a combination of enhanced glacial scour and isostatic loading. Repeated expansion of the East and West Antarctic ice sheets is interpreted to have modified the continental shelf. The records of the most recent glacial expansions are preserved in the surficial features and sedimentary deposits of the Ross Sea floor.

Stephanie Shipp, John B. Anderson

Submarine Glacial Flutes and DeGeer Moraines

A marine geophysical survey of the northern central Barents Sea (Fig.1) revealed glacial land-forms with major implications for the understanding of the deglaciation history of the Late Weichselian Barents Sea ice sheet [Solheim et al., 1990]. The landforms consist of glacial flutes and associated transverse ridges, that have been identified over an area of roughly 4000 km2. Acoustic equipment used include a 30 in3 airgun towed at 1 m depth and a single channel streamer, a 3.5 kHz echo sounder and a 50 kHz side scan sonar. Sediment coring was done with vibro- and gravity corers.

Anders Solheim, Anders Elverhøi

Glacial Flutes and Iceberg Furrows, Antarctic Peninsula

Sidescan and 3.5kHz profiler data from the Antarctic Peninsula continental shelf (Fig. 1) show good examples of glacial flutes and iceberg furrows. The Antarctic Peninsula shelf environment is described by Larter et al. [this volume], Larter and Barker [1989] and Pudsey et al. [1994]. The sidescan sonar was a hull-mounted system, stabilised for roll, with operating frequencies of 31–33 kHz. A horizontal range of 1500m or 2500m was used in water depths of 200–700m.

Carol J. Pudsey, Peter F. Barker, Robert D. Larter

Sub-Glacial Features Interpreted from 3D-Seismic

In the Norwegian Channel offshore Western Norway (Fig.1) lineated surfaces are observed on 3D-seismic (Figs. 2, 4 and 5) within the Pleistocene sediments. The lineated pattern is interpreted to be sub-glacial features, reflecting icemovement.

Tor Helge Lygren, Mona Nyland Berg, Kjell Berg

Subglacial Channels in Hudson Bay, Canada

Huntec DTSTM seismic reflection profiles and sidescan sonograms from Hudson Bay Canada, show the presence of large anastomosing channels eroded into sediments interpreted to represent till.

Heiner Josenhans

Subglacial Channels, Southern Barents Sea

Seismic reflection records from the southern Barents Sea have revealed a system of deep (20–50 m, locally reaching 200 m) and narrow (2–5 km) linear channels cut into the Mesozoic bedrock, and covered by Quaternary sediments. These channels are particularly abundant near the Kola Peninsula, where they are roughly subparallel to the coastline (Figs. 1–2).

Valery Gataullin, Leonid Polyak

Buried Sub- and Proglacial Channels: 3D-Seismic Morphostratigraphy

The geological record of glacial drainage includes both sub- and proglacial channel systems. The distinction of such types has been proposed as a guide to former ice margins in the central North Sea, based on interpretation of seismic profiles [Jansen, 1976]. However, channel systems may be spatially overlapped by ice-margin advance and retreat. 3D-seismic methods allow horizontal access to the subsurface and interpretation by geomorphological analogy [Brown, 1991]. Here, a single time-slice from the central North Sea (Fig. 1) reveals superimposed networks of buried channels over a 400 km2 area (Figs. 2, 3).

Daniel Praeg, David Long

Buried Tunnel-Valleys: 3D-Seismic Morphostratigraphy

Tunnel-valleys are characteristic elements of glaciated sedimentary basins that record meltwater drainage beneath ice sheets [e.g. Ehlers and Wingfield,1991]. 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, formed during the mid-Pleistocene Elsterian glaciation [Cameron et al., 1987]. A 3D-seismic volume across a 660 km2 portion of a wider study area (Fig. 1) provides information on their form and fill character (Figs. 2–4). The results illustrate the utility of high spatial resolution for Quaternary morpho-stratigraphical analyses.

Daniel Praeg

Glaciotectonic Features, Southeastern Barents Sea

Seismic and borehole data show that Quaternary deposits in the southeastern Barents Sea (Fig. 1) overlie the Mesosoic bedrock with a prominent erosional unconformity, known from the western Barents Sea as the Upper Regional Unconformity (URU) [Solheim and Kristoffersen, 1984].The Quaternary sequence is subdivided into three seismic and lithologic units: I, glacial diamicton with chaotic seismic signature; II, laminated glacimarine mud (in shallow areas, with a sand layer at the bottom); and I, homogenous marine mud/sand (Fig. 2A) [Gataullin et.al, 1993; Polyak et al., 1995].

Valery Gataullin, Leonid Polyak

Glacial Tectonism and Deformation of Marine Sediments in the Central Chilean Fjords

Canal Errazuriz lies just west of the central Andean Cordillera of southern Chile (Fig. 1). During the last glaciation it was the approximate geographic location of the ice divide for an expansive Andean ice cap and was also the site of late glacial residual ice which persisted until the earliest Holocene. Remnant cirque glaciers are present today to the east of the region, however Holocene sedimentation in the fjords at this location has been dominated by non-glacial fluvial and hemipelagic sedimentation.

Jay A. Strayers

Ice-marginal and Ice-Contact Features

Overview

Ice-marginal features form an important group of glacial features, in that they are widely used to reconstruct the extent of past ice masses. The type and character of these deposits also often bear evidence of factors such as the thermal regime and dynamic behavior of the glacier or ice sheet. The ice-marginal and ice-contact features treated in this chapter are formed at the margins of glaciers, ice streams or ice sheets, either at the front, or along the edges of these ice masses. Ice-contact features formed subglacially are treated in the preceeding Chapter 1, “Subglacial features”.

A. Solheim

Younger Dryas Moraines in the Nordfjord, the Norddalsfjord and the Dalsfjord, western Norway

The Younger Dryas (Y.D.) moraine, (c. 10,500 B.P.) can be traced more or less continuous along the western Norwegian coast, crossing many fjords, Fig. 1 [Aarseth et al. 1996].

Inge Aarseth

Ice-Contact Deposits in Fjords From Northern Norway

During the retreat of the Fennoscandian Ice Sheet several ice-contact systems were deposited on the shelf and in the fjords along the norwegian coast (Fig. 1). The most conspicuous system was formed by the glacial readvance during the Younger Dryas chron (10,000–11,000 BP). This system is termed the Tromsø-Lyngen moraine in northern Norway [Andersen 1968]. A less distinctive ice-contact system which often is found a few kilometres ice-distal of the Tromsø-Lyngen moraine, the Skarpnes moraine, is dated to 12,500–12,000 BP [Andersen 1968, Vorren & Elvsborg 1979]. The submarine parts of these moraines have been mapped by seismic investigations in fjords and sounds [Vorren et al. 1988] and detailed investigation on emerged outcrop data from the Tromsø-Lyngen moraine has been done by Lønne [1993].

Astrid Lyså, Tore O. Vorren

Morainic Ridge Complex, Eastern Barents Sea

The seafloor of the eastern Barents Sea west of Novaya Zemlya is characterized by a rough topography with a system of elongated holes and ridges reaching 200–300 km in length and 50–100 m in relative height (Figs. 1–2). Most ridges are concentrated in several large, arcuate belts subparallel to the Novaya Zemlya coastline [Epshtein and Gataullin, 1993], similar to morainic ridge belts around the continental margins of Norway and Svalbard [e.g., Vorren and Kristoffersen, 1986]. The structure and morphology of the ridges suggest that they are marginal moraines, presumably formed by the retreating ice sheet centered over Novaya Zemlya during the deglaciation of the Barents Sea [e.g., Polyak et al., 1995].

Valery Gataullin, Leonid Polyak

Submarine End-Moraines on the West Shetland Shelf, North-West Britain

The most conspicuous glacio-morphological depositional features on the continental shelf off north-west Britain are prominent ridges of late Pleistocene age on the West Shetland Shelf (Fig. 1). These features, which are well imaged on 1 kJ sparker profiles (Table 1), are large end-moraines up to 50 m high, 8 km wide, and can be traced for up to 60 km [Stoker and Holmes, 1991]. Five separate ridges have been identified, trending sub-parallel to the shelf edge (Fig. 1).

Martyn S. Stoker

Submarine Lateral Moraine in the South Central Region of Hudson Strait, Canada

Huntec DTSTM high resolution and single channel (655 cm3 air gun) seismic reflection profiles show the occurrence in south central Hudson Strait of a late glacial moraine up to 70 m in thickness. This is a constructional feature composed of acoustically unstratified, and relatively consolidated material considered to be ice-contact sediments deposited subglacially and adjacent to the margin of a glacial ice sheet. A small till tongue or local debris flow deposit occurs at the foot of the moraine.

Brian MacLean

Thick Multiple Ice-contact Deposits Adjoining the Sill at the Entrance to Hudson Strait, Canada

Hudson Strait has been a major route for drawdown and discharge of Laurentide ice from Hudson Bay and adjoining Ungava and Baffin land masses onto the continental shelf and into the North Atlantic during the Quaternary [see e.g. Dyke and Prest, 1987; Hughes, 1987].

Brian MacLean

Lobate Stacked Moraines: Lake Melville, Labrador

In Lake Melville, Labrador (Fig. 1), Quaternary sedimentary deposits reflect retreat of the Laurentide Ice Sheet during the early Holocene, based on acoustic properties of sediment packages, distribution (isopach) patterns, and information from dated cores. Coastal gravel pits from isostatically-raised moraines have rounded boulders >10 m in diameter, and the large volume of deposited sediment together suggest a high discharge cf water through the ice sheet and into the Lake. During ice-retreat, there were a four small (a few kilometres) ice advances. Seismic units 3/4, 5, 7 and 9 appear as evidence of readvances of the ice margin. Stacked moraines formed and account far over 400 m of glacimarine sediments (Fig. 2).

James P. Syvitski

Muir Inlet Morainal Bank Complex, Glacier Bay, S.E. Alaska

High-resolution seismic-reflection profiles illustrate the geometry and seismic characteristics of sedimentary facies within the Muir Inlet morainal bank complex. The interpreted sedimentary facies include a grounding-line fan, two stratified ridges, debris flow/turbidity current deposits and a field of push ridges (Figs. 1 and 2). This morainal bank complex was deposited on a shallow sill at the mouth of Muir Inlet, a fjord in Glacier Bay, between 1860 A.D. and 1899 A.D. during retreat from the Little Ice Age maximum. Profiles of the morainal bank complex were collected with a single channel 1.2 kJ uniboom (600 and 1700 Hz) system by the U.S. Geological Survey, Menlo Park, CA.

Keith C. Seramur, Ross D. Powell, Paul R. Carlson, Ellen A. Cowan

A Late Glacial Readvance Moraine in the Central Chilean Fjords

During the 1993 cruise of R/V Polar Duke, seismic data were collected from Seno De Las Montañas (Fig. 1), a fjord that is isolated from outflow of the South Patagonia Ice Cap by a prominent bedrock ridge and contains significant deposits from late glacial readvances which flowed into the fjord from the south.

Jay A. Stravers, John B. Anderson

Grounding Zone Wedges on the Antarctic Continental Shelf, Antarctic Peninsula

The Antarctic Peninsula accommodates a small volume of ice but ice-volume fluctuations were sufficiently large to repeatedly advance the grounding line and associated grounding zone wedges across the shelf. Grounding zone wedge stratigraphy records the history of glaciation on the shelf. The continental shelf is approximately 150 km wide and the shelf break averages 400 m. On the shelf, a series of glacially scoured troughs dominate the bathymetry.

Philip J. Bart, John B. Anderson

Grounding Zone Wedges on the Antarctic Continental Shelf, Weddell Sea

During the 1991 R/V Polar Duke cruise, data were collected along the Weddell Sea margin of the Antarctic Peninsula (Fig. 1). Approximately 2000 km of seismic data were acquired using a 100 in3 water gun, fired at a 6 second shot interval, and an oil-filled streamer. The data were collected in analog format on an electrostatic plotter, and in digital format using Elics® single channel seismic acquisition and processing software. The low-cut filter was set to 20 Hz; the high cut filter was set to 4000 Hz. Ship speeds ranged between 4 to 7 knots. Navigation relied on the GPS, with brief periods of satellite navigation.

John B. Anderson

Grounding Zone and Associated Proglacial Seismic Facies from Bransfield Basin Antarctica

Bransfield Basin separates the South Shetland Islands and the western margin of the northern Antarctic Peninsula (Fig. 1). The basin is the youngest of several convergent margin basins formed on the Pacific margins of South America and Antarctica. The study area is characterized by elevated levels of precipitation and temperature relative to the rest of Antarctica [Reynolds, 1981].

Laura A. Banfield, John B. Anderson

Grounding Zone Wedges on the Antarctic Continental Shelf, Ross Sea

Ross Sea is a broad embayment, approximately 1500 km wide and 900 km long, on the Antarctic coast (Fig. 1). Water depths range from less than 300 m to greater than 1200 m and average in excess of 500 m. A series of roughly northeast-southwest ridges and troughs dominate the bathymetry. The continental shelf is foredeepened; the inner shelf is deeper than the outer shelf due to a combination of enhanced glacial scour and isostatic loading. Repeated expansion of the East and West Antarctic ice sheets is interpreted to have modified the continental shelf. The records of the most recent glacial expansions are preserved in the surficial features and sedimentary deposits of the Ross Sea floor.

Stephanie Shipp, John B. Anderson

Paleo-Ice Streams and Ice Stream Boundaries, Ross Sea, Antarctica

During the RVIB Nathaniel B. Palmer 1994–01 and 1995–01 cruises, approximately 7141 km of 50 in3 airgun seismic profiles and 12,150 km of 3.5 kHz trackline data were collected from the Ross Sea continental shelf. Ross Sea is a broad embayment, approximately 1500 km wide and 900 km long, on the Antarctic coast (Fig. 1). Water depths range from less than 300 m to greater than 1200 m and average in excess of 500 m. Bathymetry is dominated by a series of roughly northeast-southwest ridges and troughs. The continental shelf is foredeepened; the inner shelf is deeper than the outer shelf due to a combination of enhanced glacial scour and isostatic loading. Repeated expansion of the East and West Antarctic ice sheets is interpreted to have modified the continental shelf. The records of the most recent glacial expansions are preserved in the surficial features and sedimentary deposits of the Ross Sea floor.

Stephanie Shipp, John B. Anderson

Glaciomarine Deposits on the Continental Shelf of Ross Sea, Antarctica

The Ross Sea is part of a large embayment of the west Antarctic coast, and is characterized by continental rift basins filled by thick sequences of Cenozoic glaciomarine sediments [Hayes and Frakes et al., 1975; Cooper et al., 1991; Anderson and Bartek, 1992; Hambrey and Barrett 1993; Brancolini et al., 1995a; De Santis et al., 1995].

Laura De Santis, John B. Anderson, Giuliano Brancolini, Igor Zayatz

Debris Flows and Slumps

Overview

Sediment gravity flows and slumps of glacimarine sediments have been documented in a variety of modern geographic locations and environmental settings. They are also widely recognized or inferred from seismic and core data collected from previously glaciated fjords, continental shelves, and adjacent continental slopes. This overview summarizes the key environmental and sedimentological aspects of sediment failure and the following examples presented in this atlas are a representative sample of the wonderful variety of glacimarine environments in which debris flow and slumps occur.

Jay A. Stravers

Submarine Debris Flows on Glacier-Influenced Margins: GLORIA Imagery of the Bear Island Fan

Long-range side-scan sonar (GLORIA) and acoustic studies of the 240,000 km2 Bear Island Fan, Polar North Atlantic, show a pattern of elongated debris flows characterised by a low backscatter (Fig. 1) and a transparent signal on 3.5 kHz records (Fig. 2). These debris flows are 5–20 km wide and some can be traced over distances of >200 km, from the upper fan to the Knipovich Ridge (Fig. 1).

Julian A. Dowdeswell, Neil H. Kenyon, Jan Sverre Laberg, Anders Elverhøi

Glacigenic Mudflows on the Bear Island Trough Mouth Fan

SeaMARC II sidescan imagery casts new light on glacigenic mass-wasting on the Bear Island Trough Mouth Fan (Figs 1 and 2), whose surface geology is evidently much more complex than the regular bathymetric contours had indicated [Sundvor et al., 1990; Vogt et al., 1991; Crane et al., 1992; Vogt et al., 1993]. The Sea-MARC II system [Shor, 1990], returns both sidescan and swath bathymetry. The sidescan operates at 11 and 12 kHz for port and starboard transducer banks and returns a 10 km wide swath. The system was operated variously at 8 and 10 second repetition rates at tow speeds of 6-10 knots. At the water depths on the Bear Island Fan (1,000–2,500 m) the imagery has a resolution of a few tens of meters.

Kathleen Crane, Peter R. Vogt, Eirik Sundvor

Debris Flow Deposits on a Glacier-fed Submarine fan off the Western Barents Sea Continental Shelf

The large glacier-fed submarine fan off the western Barents Sea continental shelf, the Bear Island Trough Mouth Fan [Vorren et al., 1989] covers an area of about 240,000 km2. As revealed from both high resolution sparker and 3.5 kHz records, gravity cores and long-range side scan data, the Mid and Late Pleistocene succession is dominated by debris flow deposits, separated by thin interglacial/interstadial hemipelagic sediments [Damuth, 1978; Vorren et al., 1989; Vogt et al., 1993; Laberg and Vorren, 1995, 1996a; Dowdeswell et al., this volume]. A similar stratigraphy characterizes other high-latitude fans [King, this volume; Laberg and Vorren, 1996b].

Jan Sverre Laberg, Tore O. Vorren

Debris Flows on a Glacial Trough Mouth Fan, Norwegian Channel and North Sea Fan

The North Sea Fan is a major Quaternary depocentre at the mouth of the Norwegian Channel, a marginal trough repeatedly sculpted by ice streams draining the southern Fennoscandian Ice Sheet (Figure 1.) Since the mid-Pleistocene at least five sequences of stacked debris flow deposits have been preserved on the fan (Figure 2). Each is related to a shelf-edge position of the ice stream which deposited between 700 and 1500 km3 of sediment thus contributing to 80% of the fan construction since that time (King et al., 1996). Giant trans- lational slides have periodically removed part of the fan sequence. Regional and site-specific air gun surveys, together with shallow cores have enabled a geometric and lithologic characterization of the flows. The seismic source was a tuned array of four-40 in3 sleeve guns towed at a depth of 2 m, digitally recorded through a single Channel, 50 element 7.5 m streamer, and replayed at a compressed horizontal scale with TVG and filtering at 70 to700 Hz.

Edward L. King

Submarine Debris Flows on a Glacially-Influenced Basin Plain, Faeroe-Shetland Channel

The Faeroe-Shetland Channel is a narrow deep-water basin separating the West Shetland and Faeroe shelves (Fig. 1), which acted as a trap for glacigenic sediment during the mid- to late Pleistocene [Stoker et al, 1991]. At the south-west end of the basin, debris flows are preserved within the basinal succession; acoustic profiling with a 6 kV deep-tow boomer (Table 1) has provided extremely well-imaged examples of their seismic facies, depositional features, and stratal geometries and relationships (Fig. 2).

Martyn S. Stoker

A Cross-Section of a Fjord Debris Flow, East Greenland

The steep slope angles commonly found in fjords promotes gravity driven downslope movement of the infilling sediment [Syvitski et al.,1987; Prior and Bornhold, 1990]. Numerous debris flows occur in the long, narrow, deep Kejser Franz Joseph and Kong Oscar fjords (Fig. 1). The Parasound profile in Fig. 2 is a cross section of a debris flow deposit in the outer region of Kejser Franz Joseph Fjord. The debris flow (A, Fig. 2) cuts through a well stratified sequence (B) interpreted to be composed mainly of turbidites and sediments deposited by iceberg meltout and from glacial stream input to the fjord. Of particular note are the abrupt edges of the flow and the high angle of contact between the flow and host sediment which is a function of the extreme vertical exaggeration of this profile (approximately 1:60). The mushroom head of the flow is the result of the flow overtopping the stratified sediments by about 10 milliseconds TWT (about 8 m) and spreading out over the stratified sediments to about a kilometre on each side. Within the mushroom head of the flow, short, chaotic segments of reflectors may be remnants of the original stratified sediments which were incorporated into the flow. The flow surface is characteristically hummocky. The debris flow is overlain by a poorly stratified, mainly homogeneous unit (C).

Robert J. Whittington, Frank Niessen

Synsedimentary Faulting in an East Greenland Fjord

Synsedimentary faulting is one aspect of the deformation of sediments which has received little attention, particularly for glacimarine sediments [cf. Maltman, 1995]. Sexton et al., [1992] show synsedimentary faulting in front of a Little Ice Age moraine in Lilliehook Fjord, Spitsbergen.

Frank Niessen, Robert J. Whittington

Staircase Rotational Slides in an Ice-Proximal Fjord Setting, East Greenland

Vikingebugt is an embayment on the south side of Scoresby Sund (Fig. 1) leading into the Brede tidewater glacier. The fan sediments within Vikingebugt are well stratified and deformed by a sequence of rotational slides which gives a staircase appearance to the fan profile [Dowdeswell et al, 1994]. A Parasound profile from the bedrock ridge at the mouth of Vikingebugt to within about 2 km of the glacier is shown in Fig. 2. Whereas the amount of throw of each rotational fault (maximum about 35 m) and the block width generally decreases down the fan, Fig. 2 shows the complex nature of these rotational slides. Adjacent Parasound profiles and swath bathymetry suggest that each block is of limited lateral extent. Although some of the faults can be seen to be listric, the level of decollement is below the level of penetration of Parasound data. However, the decollement is suggested from other seismic data to occur at about 25 m below the seafloor [Dowdeswellet al., 1994]. At the foot of the fan, the sediments are in local compression against a lateral moraine and a bedrock ridge resulting in minor pop-up blocks which occur just upslope of a sequence of largely undeformed sediments (Fig. 2). There is no evidence of ponding in the staircase steps or thickening into the buried faults. It is suggested that these rotational failures are essentially continuous and contemporary.

Robert J. Whittington, Frank Niessen

Glaciation-Influenced Debris Flow Deposits: East Greenland Slope

Debris flow deposits dominate the acoustic profiles of the East Greenland continental slope (Figures 1 and 2). Huntec Deep Tow Seismic (DTS)® and 10 in3 sleeve gun provided the seismic source signatures. Huntec data recorded by the internal hydrophone were prefiltered between 1.0 and 6.0 kHz and digitized by the Ferranti SE880 at 50 µsec (20 kHz) with a data window of 400 to 500 µsec (i.e. between 8000 to 10000 samples per shot trigger). The boomer source was an ED10F/C powered by 4 kv (560 joules). The sleeve gun data were prefiltered between 100 and 1500 kHz and digitized by the Ferranti SE880 at 200 µsec (5 kHz) with a data window of 1 and 2 sec (at 5000 to 10000 samples per shot trigger). Details of the methodology are well described in Asprey et a 1. (1994).

Andrew B. Stein, James P. M. Syvitski

Ice Keel Scouring

Overview

Ice scouring (plowing) of the sea floor is a common feature in marine areas affected by floating icebergs and/or sea ice [e.g. Reimnitz et al., 1978; Lewis et al., 1980; Barnes and Lien, 1988]. Icebergs, detached from a calving glacier terminus, may be dragged by winds or currents for long distances before finally melting out. On the way, deep protruding parts (keels) of icebergs may come into contact with the sea bed and significantly deform the sea-floor surface by plowing the soft sediment. At present, icebergs commonly touch bottom at depths down to 100 m off Svalbard, 200 m on the Greenland shelf, and over 300 m in the Sub-Antarctic. Occasional icebergs may reach deeper, due to an increase in draft through iceberg rolling. By contrast, sea ice, thickened by the formation of shear and pressure ridges, largely affects only shallower depths of less than 50 m [Barnes and Reimnitz, this volume]. During past glacial epochs, iceberg plowing was especially widespread, and affected wide areas of the North American and Eurasian continental margins [e.g. Josenhans et al., 1986; Solheim et al., 1988]. In some areas, almost the entire the sea floor has been reworked by modern and/or relict floating ice activity.

Leonid Polyak

Depth-Dependent Iceberg Plough Marks in the Barents Sea

Iceberg plough marks are found at all depths in the Barents Sea, down to 450 m. Keels of present-day icebergs observed in the region rarely exceed 100 m. Hence, most ploughmarks found at greater depths are relict. Plough mark degradation is dependent on a number of factors, such as sedimentation rate, degree of benthic activity and strength of bottom currents. Therefore, age determination of plough marks based on morphological characteristics can be dubious.

Anders Solheim

Deep Pleistocene Iceberg Plowmarks on the Yermak Plateau

The southern Yermak Plateau (Fig. 1) at approximately 80°N and 8°E, was investigated with the SeaMARC II [Shor,1990], 11–12 kHz side-looking sonar system [Doss et al., 1991; Vogt et al., 1991, 1994; Crane et al., 1992]. The useful SeaMARC II swath width of backscatter imagery decreases from 10 km at 2,000 m water depth to about 5 km in the shallowest areas (400–600 m), where however, the resolution is better (a few tens of meters). Several distinct sets of relict iceberg plowmarks were discovered at present water depths from less than 450 m to at least 850 m (Fig. 2), perhaps recording the deepest iceberg keels yet known.

Kathleen Crane, Peter R. Vogt, Eirik Sundvor

Buried Ice-Scours: 2D vs 3D-Seismic Geomorphology

Subaqueous ice-scours are mainly known as surficial phenomena, formed by floating or glacier ice, acoustically imaged using sidescan sonar. Buried ice-scours have seldom been recognised in the geological record. Here, a reconstruction of the plan form of buried scours from seismic profiles [Hovland and Judd, 1988] is contrasted with the ready access to subsurface morphological information afforded by a 3D-seismic horizontal section [Gallagher et al., 1991]. The 2D and 3D datasets come from the central North Sea and the mid-Norwegian Shelf (Haltenbanken), respectively (Fig. 1).

David Long, Daniel Praeg

Iceberg Turbate on Southeastern Baffin Island Shelf, Canada

Huntec DTSTM high resolution seismic reflection profiles and core data indicate that glaciomarine sediments on parts of the Southeastern Baffm Island continental shelf have been extensively reworked by the keels of grounding icebergs [Praeg et al., 1986] into an iceberg turbate. This term was first applied by Vorren et al., (1983) to the deformed and reworked sediments resulting from the iceberg ploughing process.

Brian MacLean

Strudel-Scour Craters on Shallow Arctic Prodeltas

Strudel scours are craters in the sea floor, typically 3 m deep and 15 m wide, but ranging to as much as 7.5 m deep and greater than 30 m wide (Figs. 1 and 2). They are excavated by vertical drainage spouts of fresh water during the yearly spring flooding of vast reaches of shore-fast ice surrounding arctic deltas (Fig.3)[Reimnitz and Bruder, 1972; Reimnitz et al., 1974]. Off northern Alaska, the craters form as far as 15 km from river mouths, to water depths of 5 to 8 m. In one particular year, an average of 2–3 craters was estimated to have actually formed every km2 across a shallow prodelta, and filled with sandy, organic-rich sediments within a period of 2 to 3 years [Reimnitz and Kempema, 1983; Alpha and Reimnitz, 1995]. Internal sedimentary structures in vibra-cores from Alaskan Arctic deltas record this history of repeated strudel-scour excavation and filling (cut-and-fill), and show that this filling occurs by bedload transport from the east (Fig. 4). Cut-and-fill is also recorded in high-resol-ution seismic profiles of Arctic prodeltas that cross the outer boundary of a flooded area (Fig. 5).

Erk Reimnitz

Ice-Wallow Relief in the Beaufort Sea

The irregular morphology of the Beaufort Sea shoreface consists of broad, gentle depressions and rounded knolls, as revealed in contour charts prepared from U.S. Coast and Geodetic Survey data (Fig. 1), and a detailed diving-observations diagram (Fig. 2) [Reimnitz et al., 1972; Reimnitzand Barnes, 1974]. In most summers, this shoreface is marked by grounded ice floes, a relationship that has been noted in our earliest investigations of the area [Reimnitz et al., 1972]. Closely spaced, tightly navigated, and repeated bathymetric surveys were used to explain the morphology as dynamic ice-wallow relief [Reimnitz and Kempema, 1982]. The action of grounded ice wobbling on the bottom in a seaway, and of intensified currents scouring around grounded ice keels that present obstacles to flow, produces the characteristic relief (Figs. 3 and 4). Relief formation combines erosion and deposition, which result in complex ripple patterns and internal sedimentary structures, as shown in Figs. 3 and 4.

Erk Reimnitz

Outcrop Morphology of Overconsolidated Mud in the Beaufort Sea

Detailed studies of the Alaskan Beaufort Sea shelf using fathograms, bottom samples, sono-graphs, photographs and diving observations indicate widespread occurrence of outcrops of overconsolidated mud [Reimnitz et al., 1973; Reimnitz and Barnes, 1974]. The locations at which such outcrops had been recorded were later summarized by Reimnitz et al. [1980], and shown to extend from the shoreface to the outer shelf. Chamberlain [1978] and Chamberlain et al. [1978] thought that overconsolidation was the result of de-watering by a seasonal cycle of freeze/thaw, and divers subsequently confirmed surficially ice-bonded sediments at the onset of winter [Reimnitz et al., 1987]. Based on interpretation of borehole data, Lee and Winters [1985] speculate that overconsolidation most likely was inherited from times of shelf-surface exposure to the cold atmosphere during the last glaciation.

Erk Reimnitz

Arctic Ice Gouging and Ice Keel Turbates

Distinctive surficial morphology (Fig. 1) and stratification are generated when sea-ice keels are driven along the seabed in the Beaufort Sea (Fig. 2). The morphology consists of linear ice-gouge furrows that criss-cross extensive shallow shelves in overlapping patterns (Fig. 3). Intense ice gouging in the Arctic is associated with the stamukhi zone [Reimnitz and Barnes, 1974], where sea-ice ridges form and are grounded on the shelf between 15–50 m water depth. Ice gouges in the Beaufort Sea typically are incised 1 m into the sea floor with maximum incisions over 4 m deep, relief of over 7 m and densities greater than 200 km-2 [Barnes et al., 1984]. The orientations and terminations of ice gouges indicate oblique uphill scouring as well as strong shore-parallel movement. Sea-floor morphology linked to studies of ice motion at Barrow, Alaska (Fig. 4) support the idea that gouging occurred during ice break-up [Shapiro and Barnes, 1991]. In contrast, investigations of ice-gouge terminations off Canada indicate ice-push events resulted from ice motion in response to storms during freeze-up [Héquette et al.,1995].

Peter W. Barnes, Erk Reimnitz

Iceberg Gouges on the Antarctic Shelf

Glaciogenic features on the shelf around the margin of Antarctica can be related to iceberg gouging of the seabed to water depths of at least 500 m. The reworking of the seabed by ice produces modern diamicts [ice keel turbates; Vorren et al., 1983] related to the presence and draft of icebergs rather than to immediate glacial proximity [Woodworth-Lynas et al.,1985] Off Wilkes Land in Antarctica, sea-floor gouges caused by iceberg keels are recognized as linear incisions a few meters deep and tens of meters wide (Figs. 1 and 2)[Barnes and Lien, 1988]. Sub-circular depressions 30–150m in diameter and hummocky bed features also represent iceberg resting and dragging sites. The crispness of sea-bed morphology in unconsolidated sediments, adjacent Holocene sediment ponding in similar features, and active hydraulic sedimentary processes suggested by banktop lag deposits, indicate that the sea floor is presently being reworked by ice keels.

Peter W. Barnes

Other Features

Overview

In this section we include a few examples of features which, while not unique to submarine glaciated environments, are nevertheless common features of these environments. These include gas and water escape features and buried fluvial channels which formed in a periglacial setting.

Thomas A. Davies

Gas–Related Sea Floor Depressions

Acoustical profiling revealed a field of semicircular, closed depressions in a local area of the western Barents Sea (Figs. 1 and 2) [Solheim and Elverhøi,1993]. The analogue seismic data were filtered at 50–500 Hz and recorded on a single channel streamer, with a 30 in3 airgun source, towed at 1.5 m depth. Additional acoustic equipment consisted of a 3.5 kHz echo sounder and a 50 kHz side scan sonar, towed approximately 30 m above the sea floor.

Anders Solheim, Anders Elverhøi

Water-Escape Sea Floor Depressions

Water-escape sea floor depressions occur on glaciated shelves in: (1) ice proximal areas where glacial ice becomes buried by sediment and later melts; (2) areas of rapidly loaded sediments; and (3) fjords where ground water driven by large hydrostatic pressure from surrounding mountains enters from beneath the glacimarine sediments (Sadler and Serson, 1980). Seismic imagery was collected in 1982 in McBeth Fiord, Baffin Island, over a 535 m deep sea floor using an air gun (Fig. 1) and Huntec (DTS®) Boomer (Fig. 2). Survey lines were run at speeds of 4–5 kmhr-1, positioned by radar (Figs. 3 and 4).

James P. Syvitski

Buried Fluvial Channels: 3D-Seismic Geomorphology

Fluvial channels potentially occur within the Quaternary stratigraphy of continental shelves world-wide, as a consequence of repeated glacioeustatic lowerings of relative sea level. Buried channels are well-suited to imaging by 3D-seismic methods due to a combination of contrast with surrounding materials and spatial resolution of their distinctive plan form [e. g. Brown,1991; Davies et al.,1992]. This is illustrated using 3D-data from the southern North Sea (Fig. 1). Horizontal seismic sections across a 220 km2 area provide morphological evidence of a fluvial channel system that drained to the north (Figs. 2, 3).

Daniel Praeg

Buried Periglacial Drainage Channels on the New Jersey Outer Continental Shelf

At the time of maximum Cenozoic glaciation, the New Jersey outer continental shelf, lying south of the ice margin (thought to have been along the southern shore of Long Island), occupied a periglacial, shallow water environment. Thick wedges of sediment on the mid and outer shelf are thought to be composed of material delivered via the Hudson River drainage system during melting and glacial retreat.

Thomas A. Davies, James A. Austin

Glacimarine Environments/Geomorphic Provinces

Frontmatter

Fjords

Seismic and Side-Scan Sonar Investigations of Recent Sedimentation in an Ice-Proximal Glacimarine Setting, Kongsfjorden, North-West Spitsbergen.

The sediments and rates of deposition adjacent to the margins of tidewater glaciers have been described from a range of glacimarine settings, including south-east Alaska, Baffin Island and Svalbard [e.g. Elverhoi et al., 1980; Syvitski et al.,1987; Powell and Molnia, 1989]. One purpose of these studies is to present evidence from modern glacier-influenced environments, where the glaciological setting is either observable or relatively readily inferred [Dowdeswell and Scourse, 1990]. This information can then be utilised in the interpretation of Quaternary and older glacimarine sediments, where the depositional setting must be inferred on the basis of seismic stratigraphy and sedimentary facies.

Robert J. Whittington, Carl Fredrik Forsberg, Julian A. Dowdeswell

Seismic Signature of Glaciomarine Fjord Sediments From Central Norway

The fjord areas in the outer Tr∅ndelag region (Fig. 1) contain large volumes of infilled layered glaciomarine sediments closely related to the deglaciation of the area [Oftedahl,1977; Reite, 1994; Ottesen et al.,1995]. A single-channel seismic profile (Fig. 2) illustrates well the glaciomarine sediments in the area. The profile was shot with a 15 in3 air gun with a shot interval of 2 seconds. The returned acoustic pulses, received through one hydrophone streamer, were filtered between 50 and 600 Hz. The active part of the streamer (7 m) was towed in a subsurface position 30 metres behind the vessel. The data were fully analog and recorded on an EPC printer.

Dag Ottesen, Kåre Rokoengen

Typical Sections Along a Transect of a Fjord in East Greenland

Four Parasound seismic sections are presented which typify most of the glacimarine sediments in the long, narrow, deep fjord of Kejser Franz Joseph (Fig.1).

Frank Niessen, Robert J. Whittington

Seismic Account of Ice-Proximal Sediments in a Small Glacial Inlet: Vikingebugt, Central East Greenland

Vikingebugt is the largest of a series of small inlets in the broad outer portion of the Scoresby Sund fjord system in central East Greenland (Fig. 1). It measures 14 km from head to mouth and is c. 6 km wide. A medium-sized glacier, Bredegletscher, drains from a small ice cap into the head of the inlet. A small grid of high-resolution seismic reflection profiles was acquired by RV Polarstern in the summer of 1990. Relevant acquisition and processing parameters of this data set are listed in Table 1.

Kris Vanneste, Gabriele Uenzelmann-Neben

The Seismic Record of Glaciation in Nachvak Fiord, Northern Labrador

Nachvak Fiord is a 45 km long glacial trough in the Torngat Mountains of northern Labrador, Canada (Fig. 1). The fiord is 2 to 4 km wide, increasing gradually eastward to Nachvak Bay, which opens to the Labrador Sea. The sidewalls are generally steep, rising in places 1000 m vertically from sea level. Bathymetry reveals a succession of basins, four of which occur between Tasiuyak Arm and Nachvak Bay (Fig. 1). Maximum water depths in the four basins are 90, 160, 170, and 210 m from west to east. The four basins are separated by shallow barriers between 10 and 180 m below sea level. Two of these, at Kogarsok and Tinutyarvik, have many of the characteristics of riegeln or glacial steps. The fiord threshold at the entrance to the fiord is very shallow with an average depth of <50 m and numerous bedrock-cored shoals.

Trevor Bell, Heiner Josenhans

Growth of a Grounding-Line Fan at Muir Glacier, Southeast Alaska

Two high-resolution seismic-reflection profiles collected 13 years apart along the axis of Muir Inlet show the growth of a grounding-line fan to an ice-contact delta at the terminus of Muir Glacier (Fig. 1). During this time, the position of the terminus was quasi-stable and the submarine fan grew to sea level, changing the glacier terminus from tidewater to terrestrial [cf Powell, 1990].

Keith C. Seramur, Ellen A. Cowan, Ross D. Powell, Paul R. Carlson

Glacial Marine Seismic Facies in a Southern Chilean Fjord

During the Last Glacial Maximum (LGM) ice flowed from the Cordillera Darwin Ice Cap through the study area (Fig. 1) to the Segunda Angostura in the Straits of Magellan [Caldenius,1932; Mercer, 1976; Porter, 1990; Porter, et. al., 1992; Clapperton,1993]. Recent work by Clapperton [1995] indicates that as many as five glacial advances occurred along the Strait of Magellan during the last glacial cycle, with the last advance tentatively occurring between 16,590 – 11,800 14C YBP.

Jana L. DaSilva, John B. Anderson

Continental Shelves

A Surge Affected, Tidewater Glacier Environment

Austfonna ice cap on the island Nordaustlandet in the Svalbard archipelago (Fig.1) terminates in the open Barents Sea along a nearly 200 km long tidewater glacier front. 28% of the 8,120 km2 ice cap is grounded below sea level. The ice cap comprises 19 drainage basins [Dowdeswell, 1986], of which Bråsvellbreen (1,109 km2) is the second largest. Bråsvellbreen had a major surge between 1936 and 1938, during which the terminus advanced up to 15 km over sea floor with depths varying between 30 m and 100 m. The glacier has experienced a post-surge retreat of up to 5 km, exposing a large area of sea floor that was recently covered by grounded, surged ice. Detailed investigations in this area include single channel 3 kJ sparker seismic, 3.5 kHz echo sounding and 50 kHz side scan sonar profiling [Solheim, 1991]

Anders Solheim

Glacigenic Sedimentation and Late Neogene Climate Pattern

The East Greenland dip seismic section presented here is 70 km (≈ 40 mi) long and extends back 10 million years. The area is prograding with maximal advancement during low sea levels, roughly 50% to 100% greater than during high stands. Deposition generally has been greatest since 2.2 million years ago. The Arctic in general, Greenland in particular, has apparently prograded steadily during the late Neogene, a time characterized by cooling relative to the Paleogene.

Allen Lowrie, Karl Hinz

Glacigenic Features and Shelf Basin Stratigraphy of the Eastern Gulf of Maine

The Gulf of Maine’s topography is atypical of the U.S. Atlantic continental shelf, with a series of deep shelf basins separated by shallow, hummocky interbasin ridges (Figure 1). These shelf basins, which are deeper than 200 m, preserve glacial and glaciomarine sediments left by retreating Laurentide ice during the late Wisconsin.

Tania S. Bacchus, Daniel F. Belknap

Glacial and Glacial and Glaciomarine Sedimentation: Halibut Channel, Grand Banks of Newfoundland

The Grand Banks of Newfoundland primarily consists of thin (<20 m) Holocene surficial sediments overlying well-indurated Cretaceous and Tertiary bedrock. Glaciomarine sediments are generally absent, however, Halibut Channel, on the Central Grand Banks (Fig. 1) provides a unique window of Pleistocene sediments.

K. Moran, G. B. J. Fader

Morphology and Stratigraphy Related to the Nearshore Boundary of the Stamukhi Zone

Sea-bed morphology and stratigraphy changes can be linked to sea-ice zonation on Arctic shelves. The stamukhi zone [Reimnitiz et al., 1978] marks the boundary between the moving arctic pack ice and more stable coastal ice, and it is composed of abundant coast-parallel, shear and pressure ridges of sea ice in water depths of 15–50 m (Fig. 1). Many of the stamukhi zone ice ridges form in the same location year after year. Most of the larger ice ridges of the stamukhi zone are grounded. This is a zone where a large amount of energy is expended building ice ridges and gouging the sea floor [Barnes et al., 1984].Inshore of this zone, smoother and less deformed fast ice extends to the coast. The sea-floor features associated with this boundary include an extensive series of shoals [Reimnitz et al., 1984] and a pronounced break in slope or knickpoint at about 20 m depth [Barnes et al., 1987].

Peter W. Barnes, Erk Reimnitz

Larsen Shelf, Eastern Antarctic Peninsula Continental Margin

Single-channel seismic data from the Larsen Basin reveal an overdeepened shelf with a stratigraphy characterized by many erosional surfaces inferred to be glacigenic. Some of these are overlain by chaotic seismic facies interpreted to be subglacial tills.

Benjamin J. Sloan, Lawrence A. Lawyer

Iceberg Plough Marks, Subglacial Bedforms and Grounding Zone Moraines in Prydz Bay Antarctica

Prydz Bay is located between 068°E and 078°E on the East Antarctic Margin (Fig. 1). It is the downstream end of the Lambert Glacier-Amery Ice Shelf drainage system and has been the site of major ice streams from the East Antarctic Ice Sheet since the late Eocene-early Oligocene [Hambrey et al., 1991]. The bay has a reverse bathymetric gradient typical of the Antarctic Shelf with water depths of 100 to 200 m over shallow banks on the outer shelf and as great as 1400 m in inner shelf deeps (Fig. 1). Mesoscale features on the floor of Prydz Bay have been imaged by multichannel seismic surveys and echo sounder profilers. The images presented here were gathered using a 3.5 kHz echo sounder [Stagg et al., 1983] and a 10 litre airgun with 24 channel, 2400 m streamer (Table 1).

P. E. O’Brien, G. Leitchenkov, P. T. Harris

Current and Glacial Erosion on the Shelf off Mac. Robertson Land, East Antarctica

Bathymetric, sidescan sonar, and high-resolution seismic data indicate that the shelf off Mac. Robertson Land, East Antarctica is deeply eroded by glaciers and currents. Basement is exposed over wide areas. Data available include MCS and 3.5 kHz obtained by Stagg (1985) in 1982 (see O’Brien et al, this volume, for details). In 1995 R.V. Aurora Australis collected bathymetric data using a Simrad EA200 12.5 KHz echo sounder and seismic data were obtained using a Seismic Systems Inc. generator-injector (GI) gun (150 cubic inches) with 4 groups (4 channels) of ten hydrophones. Analogue (uncorrected) sidescan sonar data were recorded using an EG&G 960 tow fish and an EG&G 996 digital modem connected to an EPS 9800 graphic recorder. The sidescan sonar was operated at ranges of 375 to 500 m.

P. T. Harris, P. E. O’Brien

Till Sheets on the Ross Sea Continental Shelf, Antarctica

Ross Sea is a broad embayment, approximately 1500 km wide and 900 km long, on the Antarctic coast (Fig. 1). Water depths range from less than 300 m to greater than 1200 m and average in excess of 500 m. Bathymetry is dominated by a series of roughly northeast-southwest ridges and troughs. The continental shelf is foredeepened; the inner shelf is deeper than the outer shelf due to a combination of enhanced glacial scour and isostatic loading. Repeated expansion of the East and West Antarctic ice sheets is interpreted to have modified the continental shelf. The records of the most recent glacial expansions are preserved in the surficial features and sedimentary deposits of the Ross Sea floor.

Stephanie Shipp, John B. Anderson

Seismic Correlation Between CIROS-1 and MSSTS-1 Drill Holes, Ross Sea, Antarctica

The Ross Sea is part of a wide embayment that continues south beneath the Ross Ice Shelf. The embayment is the result of many continental rifting phases that probably started in Cretaceous time, with the break-up between Australia, New Zealand and Antarctica [Behrendt et al, 1991; Lawyer et al., 1991. Extensional tectonism is still active in the western sector of the Ross Sea, as evidenced by faulting and alkaline volcanism [Cooper et al. 1987; LeMasurier and Thomson, 1990].

Giuliano Brancolini, Franco Coren

Glacial Troughs

Bering Trough: a Product of the Bering Glacier, Gulf of Alaska

The Gulf of Alaska area has experienced glaciation since Miocene time [Lagoe et al., 1993]. Large broad seavalleys that are incised in the continental shelf (Fig. 1) indicate that the extensive glaciers, which presently border the northeastern shore of the Gulf of Alaska, had lobes of ice that extended completely across the shelf [Carlson et al., 1982]. This chapter will concentrate on one of the glacially-carved seavalleys, the 25-km-wide Bering Trough (Fig. +1), that extends about 60 km across the shelf just seaward of the Bering Glacier, which recently re-advanced about 9 km between September, 1993 and September, 1994 [Molnia et al., 1994].

Paul R. Carlson, Terry R. Bruns

Glacially Overdeepened Troughs on the Labrador Shelf, Canada

The continental shelf off Labrador is underlain by a clastic wedge of Early Paleozoic to Cretaceous-Tertiary sandstones, siltstones and limestones which onlap the Precambrian metasediments of the mainland. Regional seismostratigrphic mapping [Josenhans et al., 1986] shows that the contact between Precambrian basement and the clastic wedge is faulted in places. Buried valley fill remnants are recognised on the middle shelf together with faulted sequences at the base of the fluvial sequences. Fluvial down-cutting along fault-weakened zones is suggested to have occured in late Tertiary-Pliocene time [Grant, 1966]. Intense glacial erosion, particularly on the inner shelf near the Precambrian/clastic wedge contact, has down cut the clastic sediments and shaped a marginal trough which connects to a transverse channel (Hopedale Saddle) with depths of up to 800m. The intensity of glacial erosion appears most pronounced at the Precambrian contact and diminishes toward the outer shelf edge as demonstrated by the presence of remnant valleyfill and early glacial progradational deposits. Water depth on the outer shelf of Hopedale saddle/trough is only about 300m suggesting up to 500m of glacial overdeepening on the inner shelf. Note that the upper till lies directly on the bedrock unconformity on the inner shelf although older glacial(?) deposits are preserved on the outer shelf. This is interpreted to result from increased glacial erosion on the inner shelf.

Heiner Josenhans

Ice Stream Troughs and Variety of Cenozoic Seismic Stratigraphic Architecture From a High Southern Latitude Section: Ross Sea Antarctica

This series of figures illustrates the significance of gaining a better understanding of the seismic stratigraphy and chronostratigraphy of depositional units in the Ross Sea. Bartek et al., [1991] present Neogene shelf margin aggradation and progradation patterns that appear to be eustatically driven. Comparison to the Hag et al. [1987] Cenozoic sea level chart suggests that waxing and waning of Antarctic ice sheets appears to be synchronous with these eustatic events. We are currently working on refining the coarse chronostratigraphy that currently exists in the Ross Sea (numbered units 1–13 Anderson and Bartek [1992]). We have also identified a number of seismic facies that appear to represent a broad spectrum of glacial regimes ranging from interglacial to temperate glacial to perhaps polar glacial. However, a problem in this area is that, although there are more than 14,000 km of high to intermediate resolution seismic data, very few cores penetrate this material and so many of our interpretations must remain tentative and speculative. It is also interesting to note that some of the features we observe have not been reported in the literature from other locales so there is little basis for comparison.

Louis R. Bartek, Janel Andersen, Todd Oneacre

Continental Margins (Outer Shelf and Slope)

Seismic Signature of a High Arctic Margin, Svalbard

The western continental margin of Svalbard is strongly influenced by Late Cenozoic glaciations, and large sediment volumes have been eroded from the adjacent Svalbard and Barents Sea hinterlands. Off Svalbard, the total thickness of glacial sediments exceeds 1.5 km. Further south along the Barents Sea margin, glacial deposits of up to 4 km thickness have been mapped [Faleide et al., 1996].

Anders Solheim, Espen Sletten Andersen

Long-Range Side-Scan Sonar (GLORIA) Imagery of the Eastern Continental Margin of the Glaciated Polar North Atlantic

The intensification of glaciation from about 2.6 M yr ago [Jansen and Sjøholm, 1991] resulted in increased debris input to the passive continental margins of the Polar North Atlantic. A series of 2 to 4 km-thick sedimentary wedges or fans was built up [Vorren et al., 1991] and major slope failures formed huge submarine slides [Bugge et al., 1988].

Julian A. Dowdeswell, Neil H. Kenyon

Seismic-Stratigraphic Record of Glaciation on the Hebridean Margin, North-West Britain

The record of ice-rafting from the Hebrides Slope (Fig. 1) indicates that the continental margin off north-west Britain has been accumulating glacially-derived sediment since the late Pliocene, about 2.48Ma [Stoker et al., 1994]. The glacial influence was distal in character until the early mid-Pleistocene when ice-sheets expanded across the Hebrides Shelf, and locally deposited vast amounts of sediment onto the adjacent slope [Stoker, 1995]. This contribution illustrates single-channel airgun and sparker profiles (Table 1) supplemented by borehole data from the Hebridean margin (Fig. 1), to show how the seismic architecture reflects the impact of glaciation.

Martyn S. Stoker

Large-Scale Stratigraphy of Major Glacigenic Depocenters Along the Polar North Atlantic Margins

Along both margins of the polar North Atlantic major sediment cones have accumulated off the largest glacial outlets. Prominent seaward-convex bathymetry outlines the shape of two such depocenters in front of Scoresby Sund in central East Greenland, and at the mouth of the Bear Island Trough along the western Barents Sea margin, respectively (Fig. 1). Scoresby Sund represents the largest fjord system in East Greenland, while Bear Island Trough is the dominant transverse trough on the Barents Shelf. Both features are the physiographic expression of converging ice flow during past glaciations [e.g., Vorren et al., 1989].

Kris Vanneste, Friedrich Theilen, Heinz Miller

The Antarctic Peninsula Continental Margin Northwest of Anvers Island

The Antarctic Peninsula has a dissected central plateau at 1500–2000 m elevation, at present overlain by a few hundred metres of ice which drains into transverse glaciers. On the west coast, there is no significant floating ice shelf N of 67°S. Present ice cover is very restricted compared with the inferred extent of the ice sheet during glacial periods. The snow accumulation rate is more than three times the Antarctic average, indicating a potential for rapid ice-sheet growth. The present-day sea floor on the outer continental shelf is 400–500 m deep and water depth increases steadily inshore from the shelf break.

R. D. Larter, P. F. Barker, C. J. Pudsey, L. E. Vanneste, A. P. Cunningham

Trough-Mouth Fans: Crary Fan, Eastern Weddell Sea, Antarctica

The “trough-mouth fan (TMF)” concept was introduced by Vorren et al. [1989] when describing the large cone-shaped glaciogenic depocentre on the Barents Sea continental margin, located near Bear Island at the mouth of a transverse shelf trough of glacial origin. Such TMF’s are composed of glacial debris deposited on the outer shelf and upper slope at the grounding line of ice streams when these extend out to the shelf edge. This glacial debris forms prograding-slope strata that pinch out in basinward direction. A comparative MCS study has allowed Vanneste [1995] to identify three basic types of TMF’s: 1. mostly stable TMF’s characterised by absence of large-scale mass-wasting deposits (e.g. Scoresby Sund TMF off East Greenland); 2. unstable TMF’s characterised by the presence of large-scale mass-wasting deposits (e.g. Bear Island TMF in Barents Sea); and 3. TMF’s associated with deep-sea fan systems in their distal parts (e.g. Crary Fan in Weddell Sea).

Marc De Batist, Philip J. Bart, Heinz Miller

Seismic and Downhole-log Signatures of Glacial Deposits from Prydz Bay, Antarctica

The continental margin of Prydz Bay, Antarctica, is underlain by early Cenozoic and younger glacial deposits that were cored and logged during Ocean Drilling Program (ODP) Leg 119 in austral summer 1987/88 [Barron, Larsen et al., 1989]. A transect of five sites were drilled across the continental shelf and upper slope (Fig. 1). Downhole geophysical logging was done at two sites on the mid- to outer-continental shelf. This report briefly describes the varied signatures of the drilled glacial sections derived from single-channel seismic-reflection data and downhole logs.

Alan K. Cooper

Deep Sea

Glacimarine Drainage Systems in Deep-sea: The NAMOC System of the Labrador Sea and its Sibling.

The continental Pleistocene Laurentide Ice Sheet (LIS) had far-reaching marine influence in shaping the ocean-floor adjacent to ice margin. The basinwide submarine-canyon and deep-sea channel system of the Northwest Atlantic Mid-Ocean Channel (NAMOC) of the Labrador Sea is the submarine continuation of the drainge system of the LIS on land, forming an interconnected land/sea drainage system 6,000 km long, one of the word’s longest drainage systems of Pleistocene age. The submarine portion forms a dual system, consisting of the mud-dominated NAMOC with its tributaries and a submarine sandy braid-plain.

Reinhard Hesse, Ingo Klaucke, Saeed Khodabakhsh, William B. F. Ryan

Glacially-Influenced Sediment Drifts in the Rockall Trough

Sediment drifts are a major depositional product of bottom-current activity in deep-water settings, particularly on slopes and basin plains within, and adjacent to, continental margins. They commonly form positive features on seismic reflection profiles. Bottom-current reworking of sediment, which may be derived from turbidity currents or pelagic/hemipelagic processes, is the major factor controlling the development of sediment drifts. In the vicinity of glaciated margins they are also likely to receive an input of coarse-grained material, including gravel-sized dropstones, derived through ice-rafting processes. Thus, despite subsequent bottom-current reworking, a distinct glacimarine signature will be retained within the sediments. Such deposits form an important component of the late Cenozoic sediment drifts in the northeast Rockall Trough (Fig. 1), where a record of distal glacimarine sedimentation, since the late Pliocene, is preserved [Stoker et al., 1993].

Martyn S. Stoker, John A. Howe

Sediment Drifts on the Continental Rise of the Antarctic Peninsula

The sediment drifts of the Antarctic Peninsula Pacific margin (Fig. 1) are a systematic component of a distinctive, fully glacial pattern of sediment transport and deposition. Terrigenous sediment supply is provided during glacial maxima by grounded ice sheets transporting unsorted tills to the continental shelf edge.

Michele Rebesco, Angelo Camerlenghi

Glacimarine Environments/Geomorphic Provinces: Overview

The glacimarine environment includes all areas where marine sedimentation is influenced by glacial ice (including grounded tidewater ice sheets, floating glacier tongues, ice shelves and icebergs) and sea ice [Powell, 1984]. During the late Cenozoic, this encompassed large parts of mid- to high-latitude continental margins and adjacent ocean basins. Continental shelves in both the Arctic and Antarctic remain important regions of contemporary glacimarine sedimentation, whilst icebergs continue to influence sedimentation, albeit to a volumetrically minor degree, up to a distance of several hundred kilometres from continental landmasses.

Martyn S. Stoker

Glossary of Glacimarine and Acoustic Terminology

Frontmatter

Glossary of Glacimarine and Acoustic Terminology

By its nature, an atlas that portrays acoustic images of glacimarine features and environments must incorporate a specialized vocabulary that covers many earth science disciplines. This glossary contains abbreviated non-technical defmitions of commonly used terms, to help the reader, as needed, understand the Atlas contributions, and to facilitate communications between specialists and others in the broader scientific community.

Trevor Bell, Alan K. Cooper, Anders Solheim
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