GLACIATED CONTINENTAL MARGINS
EDITED BY
Thomas A. Davies, Trevor Bell, Alan K. Cooper, Heiner Josenhans, Leonid Polyak, Anders Solheim, Martyn S. Stoker, Jay A. Stravers
CHAPMAN & HALL London· Weinheim . New York · Tokyo · Melbourne· Madras
lllustrations
Frontispiece: The west coast of Svalbard in early summer. (Photograph by Thomas A. Davies)
Front cover: Larsen Ice Shelf, West Antarctica. (Photograph by Lawrence A. Lawver)
Back cover: Sirnrad EM 100 multibearn bathymetry showing an ice terminal, morainal bank at the northern end of Emerald Basin on the eastern Canadian continental shelf. The area of seafloor shown is about 5 km wide, with water depths deepening from about 150 m at the ridged, iceberg scoured moraine (white) to about 230 m at the flat lying, pockmarked surface of Holocene clay (green). The pockmarks are likely formed by escaping gas from biogenic or petrogenic origin. (Graphic prepared by Robert Courtney, Geological Survey of Canada (Atlantic).
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TABLE OF CONTENTS
List of Contributors
PART ONE SEISMIC CHARACTER AND VARIABILITY
Seismic Methods and Interpretation Martyn S. Stoker, Jack B. Pheasant, Heiner Josenhans
Iceberg Scours: Records from Broad and Narrow-Beam Acoustic Systems Julian A. Dowdeswell, Robert J. Whittington, Heinrich Villinger
Effects of Shallow Gas on Seismic Reflection Profiles Gordon B.J. Fader
Simultaneous use of Multiple Seismic Reflection Systems for High Resolution and Deep Penetration Heiner Josenhans
PART TWO FEATURES FOUND IN GLACIMARINE ENVIRONMENTS
1 Subglacial Features
Overview Alan K Cooper, Paul R Carlson, Erk Reimnitz
A Glacial Trough Eroded in Layered Sediments in a Norwegian Fjord DagOttesen
Structures in Scorseby Sund, East Greenland Gabriele Uenzelmann-Neben
Glacially Overdeepened Troughs and Ice Retreat "Till Tongue" Deposits in Queen Charlotte Sound, British Columbia, Canada Heiner Josenhans
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Glacial Erosion of Sediments in the Alfjord, Western Norway Inge Aarseth
Glacial Unconfonnities on the Antarctic Continental Margin, an Example from the Antarctic Peninsula Philip J. Bart, John B. Anderson
Glacial Sole Markings on Bedrock and Till in Hudson Bay, Canada Heiner Josenhans
Drumlins in Lake Ontario c.F. Michael Lewis, Larry A. Mayer, Gordon D.M Cameron, Brian J. Todd
A Seabed Drumlin Field on the Inner Scotian Shelf, Canada Gordon B. J. Fader, Rudolph R Stea, R C. Courtney
Drumlin Field on the Ross Sea Continental Shelf, Antarctica Stephanie Shipp, John B. Anderson
Lineations on the Ross Sea Continental Shelf, Antarctica Stephanie Shipp, John B. Anderson
Submarine Glacial Flutes and DeGeer Moraines Anders Solheim
Glacial Flutes and Iceberg Furrows, Antarctic Peninsula Carol J. Pudsey, Peter F. Barker, Robert D. Larter
Subglacial Features Interpreted from 3D-Seismic Tor Helge Lygren, Mona Nyland Berg, Kjell Berg
Subglacial Channels in Hudson Bay, Canada Heiner Josenhans
Subglacial Channels, Southern Barents Sea Valery Gataullin, Leonid Polyak
Buried Sub- and Proglacial Channels: 3D-Seismic Morphostratigraphy Daniel Praeg, David Long
Buried Tunnel-Valleys: 3D-Seismic Morphostratigraphy Daniel Praeg
Glaciotectonic Features, Southeastern Barents Sea Valery Gataullin, Leonid Polyak
Glacial Tectonism and Defonnation of Marine Sediments in the Central Chilean Fjords Jay A. StraYers
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Overview Anders Solheim
Younger Dryas Moraines in the Nordfjord, the Norddalsfjord and the Dalsfjord, Western Norway Inge Aarseth
Ice-contact Deposits in Fjords from Northern Norway Astrid Lysa, Tore 0. Vorren
Morainic Ridge Complex, Eastern Barents Sea Valery Gataullin, Leonid Polyak
Submarine End-Moraines on the West Shetland Shelf, North-West Britain Martyn S. Stoker
Submarine Lateral Moraine in the South Central Region of Hudson Strait, Canada Brian MacLean
Thick Multiple Ice-contact Deposits Adjoining the Sill at the Entrance to Hudson Strait, Canada Brian MacLean
Lobate Stacked Moraines: Lake Melville, Labrador James P. Syvitski
Muir Inlet Morainal Bank Complex, Glacier Bay, S.E. Alaska Keith C. Seramur, Ross D. Powell, Paul R Carlson, Ellen A. Cowan
A Late Glacial Readvance Moraine in the Central Chilean Fjords Jay A. StraYers, John B. Anderson
Grounding Zone Wedges on the Antarctic Continental Shelf, Antarctic Peninsula Philip J. Bart, John B. Anderson
Grounding Zone Wedges on the Antarctic Continental Shelf, Weddell Sea John B. Anderson
Grounding Zone and Associated Proglacial Seismic Facies from Bransfield Basin, Antarctica Laura A. Banfield, John B. Anderson
Grounding Zone Wedges on the Antarctic Continental Shelf, Ross Sea Stephanie Shipp, John B. Anderson
Paleo-ice Streams and Ice Stream Boundaries, Ross Sea, Antarctica Stephanie Shipp, John B. Anderson
Glaciomarine Deposits on the Continental Shelf of the Ross Sea, Antarctica Laura De Santis, John B. Anderson, Giuliano Brancolini, Igor Zayatz
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Overview Jay A. StraYers
Submarine Debris Flows on Glacier-Influenced Margins: GLORIA Imagery of the Bear Island Fan Julian A. Dowdeswell, Neil H. Kenyon, Jan Sverre Laberg, Anders Elverhei
Glacigenic Mudflows on the Bear Island Trough Mouth Fan Kathleen Crane, Peter R. Vogt, Eirik Sundvor
Debris Flow Deposits on a Glacier-Fed Submarine Fan off the Western Barents Sea Continental Shelf Jan Sverre Laberg, Tore O. Vorren
Debris Flows on a Glacial Trough Mouth Fan, Norwegian Channel and North Sea Fan Edward L. King
Submarine Debris Flows on a Glacially-Influenced Basin Plain, Faeroe-Shetland Channel Martyn S. Stoker
A Cross-Section of a Fjord Debris Flow, East Greenland Robert J. Whittington, Frank Niessen
Synsedimentary Faulting in an East Greenland Fjord Frank Niessen, Robert J. Whittington
Staircase Rotational Slides in an Ice-proximal Fjord Setting, East Greenland Robert J. Whittington, Frank Niessen
Glacially-influenced Debris Flow Deposits: East Greenland Slope Andrew B. Stein, James P.M Syvitski
4 Ice Keel Scouring
Depth-Dependent Iceberg Plough Marks in the Barents Sea Anders Solheim
Deep Pleistocene Iceberg Plowmarks on the Yennak Plateau Kathleen Crane, Peter R. Vogt, Eirik Sundvor
Buried Ice-scours: 2D vs 3D-Seismic Geomorphology David Long, Daniel Praeg
Iceberg Turbate on Southeastern Baffin Island Shelf, Canada Brian MacLean
Strudel-Scour Craters on Shallow Arctic Prodeltas Vlll
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Erk Reimnitz Ice-Wallow Relief in the Beaufort Sea
Erk Reimnitz Outcrop Morphology of Overconsolidated Mud in the Beaufort Sea
Erk Reimnitz Arctic Ice Gouging and Ice Keel Turbates
Peter W. Barnes, Erk Reimnitz Iceberg Gouges on the Antarctic Shelf
Peter W. Barnes
5 Other Features
Water-Escape Sea Floor Depressions James P. Syvitski
Buried Fluvial Channels: 3D-Seismic Geomorphology Daniel Praeg
Buried Periglacial Drainage Channels on the New Jersey Outer Continental Shelf Thomas A. Davies, James A. Austin, Jr.
PART THREE GLACIMARINE ENVIRONMENTS/GEOMORPIDC PROVINCES
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6 Fjords 173
Seismic and Side-Scan Sonar Investigations of Recent Sedimentation in an Ice-Proximal Glacimarine Setting, Kongsfjorden, North-West Spitsbergen 175
Robert J. Whittington, Carl Fredrik Forsberg, Julian A. Dowdeswell Seismic Signature of Glaciomarine Fjord Sediments from Central Norway 179
Dag Ottesen, Kare Rokoengen Typical Sections Along a Transect ofa Fjord in East Greenland 182
IX
Frank Niessen, Robert J. Whittington Seismic Account of Ice-Proximal Sediments in a Small Glacial Inlet: Vikingebugt, Central East Greenland
Kris Vanneste, Gabriele Uenzelmann-Neben The Seismic Record of Glaciation in Nachvak Fiord, Northern Labrador
Trevor Bell, Heiner Josenhans Growth of a Grounding-Line Fan at Muir Glacier, Southeast Alaska
Keith C. Seramur, Ellen A. Cowan, Ross D. Powell, Paul R Carlson Glacial Marine Seismic Facies in a Southern Chilean Fjord
Jana L. DaSilva, John B. Anderson
7 Continental Shelves
Glacigenic Sedimentation and Late Neogene Climate Pattern Allen Lowrie, Karl Hinz
Glacigenic Features and Shelf Basin Stratigraphy of the Eastern Gulf of Maine Tania S. Bacchus, Daniel F Belknap
Glacial and Glaciomarine Sedimentation: Halibut Channel, Grand Banks of Newfoundland K. Moran, G.B.J. Fader
Morphology and Stratigraphy Related to the Nearshore Boundary of the Stamukhi Zone Peter W. Barnes, Erk Reimnitz
Larsen Shelf, Eastern Antarctic Peninsula Continental Margin Benjamin J. Sloan, Lawrence A. Lawver
Iceberg Plough Marks, Subglacial Bedforms and Grounding Zone Moraines in Prydz Bay, Antarctica P.E. O'Brien, G. Leitchenkov, P. T. Harris
Current and Glacial Erosion on the Shelf off Mac. Robertson Land, East Antarctica P. T. Harris, P.E. 0 'Brien
Till Sheets on the Ross Sea Continental Shelf, Antarctica Stephanie Shipp, John B. Anderson
Seismic Correlation Between CIROS-I and MSSTS-l Drill Holes, Ross Sea, Antarctica Giuliano Brancolini, Franco Coren
8 Glacial Troughs
Bering Trough: a Product of the Bering Glacier, Gulf of Alaska
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Paul R. Carlson, Terry R Bruns Glacially Overdeepened Troughs on the Labrador Shelf, Canada
Heiner Josenhans Ice Stream Troughs and Variety of Seismic Stratigraphic Architecture from a High Southern Latitude Section: Ross Sea, Antarctica
Louis R Bartek, Janel Anderson, Todd Oneacre
9 Continental Margins (Outer Shelf and Slope)
Seismic Signature of a High Arctic Margin, Svarlbad Anders Solheim, Espen Sletten Andersen
Long-Range Side-Scan Sonar (GLORIA) Imagery of the Eastern Continental Margin of the Glaciated Polar North Atlantic Julian A. Dowdeswell, Neil H. Kenyon
Seismic-Stratigraphic Record of Glaciation on the Hebridean Margin, North-west Britain Martyn S. Stoker
Large-Scale Stratigraphy of Major Glacigenic Depocenters Along the Polar North Atlantic Margins Kris Vanneste, Friedrich Theilen, Heinz Miller
The Antarctic Peninsula Continental Margin Northwest of Anvers Island R.D. Larter, P.F Barker, c.J. Pudsey, L.E. Vanneste, A.P. Cunningham
Trough-Mouth Fans: Crary Fan, Eastern Weddell Sea, Antarctica Marc De Balist, Philip J. Bart, Heinz Miller
Seismic and Downhole Log Signatures of Glacial Deposits from Prydz Bay, Antarctica Alan K Cooper
10 Deep Sea
Glacimarine Drainage Systems in Deep-Sea: the NAMOC System of the Labrador Sea and its Sibling Reinhard Hesse, Ingo Klaucke, Saeed Khodabakhsh, William B.F Ryan
Glacially-Influenced Sediment Drifts in the Rockall Trough Martyn S. Stoker, John A. Howe
Sediment Drifts on the Continental Rise of the Antarctic Peninsula Michele Rebesco, Angelo Camerlenghi
PART FOUR GLOSSARY OF GLACIMARINE AND ACOUSTIC TERMINOLOGY Trevor Bell, Alan K Cooper, Anders Solheim
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IngeAarseth Department of Geology University of Bergen Alltgt. 41, N-5007 Bergen Norway
Espen Sletten Andersen Dept of Geology University of Oslo Oslo Norway
Janel Andersen Department of Geology University of Alabama Tuscaloosa, AL 34587 USA
John B. Anderson Department of Geology and Geophysics Rice University 6100 S. Main St Houston, TX USA
James A. Austin, Jr. Institute for Geophysics The University of Texas at Austin 8701 N. MoPac Expressway Austin, TX 78759 USA
Laura A. Banfield Department Geology & Geophysics Rice University 6100 S. Main St Houston, TX 77005 USA
List of Contributors
P.F. Barker British Antarctic Survey High Cross Madingley Road Cambridge CB3 OET UK
Peter W. Barnes Marine and Coastal Program U. S. Geological Survey 915 National Center Reston, VA 02192 USA
Philip J. Bart Department of Geology & Geophysics Rice University 6100S. Main Houston, TX USA
Louis R. Bartek Department of Geology University of Alabama Tuscaloosa, AL 34587 USA
Trevor Bell Department of Geogrnphy Memorial University of Newfoundland Alexander Murray Bldg. St John's, NF AlB 3X5 Canada
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Kjell Berg Norsk Hydro ASA P.O. Box 200 N1321 Stabekk Norway
Mona Nyland Berg Norsk Hydro ASA P.O. Box 200 N1321 Stabekk Norway
Giuliano Brancolini Osservatorio GeofIsico Sperimentale P. O. Box 2011 34016 - Trieste Italy
Terry R. Bruns U.S. Geological Survey 345 Middlefield Rd. MS:999 Menlo Park CA 94025-3591 USA
Angelo CamerJenghi Osservatorio GeofIsico Sperimentale P. O. Box 2011 34016 - Trieste Italy
Gordon D. M. Cameron Cameron Geoscience Research Dartmouth Canada
Paul R. Carlson U.S. Geological Survey 345 Middlefield Rd. MS:999 Menlo Park CA 94025-3591 USA
Alan K. Cooper Marine and Coastal Geology U. S. Geological Survey 345 Middlefield Road Menlo Park, CA 94025-3591 USA
Franco Coren Osservatorio Geofisico Sperimentale P. O. Box 2011 34016 - Trieste Italy
R. C. Courtney Geological Survey of Canada (Atlantic) Bedford Institute of Oceanography P. O. Box 1006 Dartmouth Nova Scotia B26 4A2 Canada
Ellen A. Cowan Dept. of Geology Appalachian State University Boone, NC 28608 USA
Kathleen Crane Marine Geosciences Division Naval Research Laboratory Washington, D.C. 20375 USA
A. P. Cunningham British Antarctic Survey High Cross Madingley Road Cambridge CB3 OET UK
Jana L. DaSilva Department of Geology and Geophysics Rice University 6100S. Main Houston, TX 77005 USA
Thomas A. Davies Institute for Geophysics The University of Texas at Austin 8701 N. MoPac Expressway Austin, TX 78759 USA
Marc De Batist Renard Centre of Marine Geology University of Gent Krijgslaan 281 S.8 B-9000Gent Belgium
Laura De Santis Osservatoria Geofisico Sperimentale P.O.Box 2011 34016 Trieste Italy
Julian A. Dowdeswell Centre for Glaciology Institute of Earth Studies University of Wales Aberystwyth SY 23 3 DB UK
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Anders Elverhflli Department of Geology University of Oslo Oslo Norway
Gordon B. J. Fader Geological Survey of Canada (Atlantic) Bedford Institute of Oceanography P. O. Box 1006 Dartmouth Nova Scotia B26 4A2 Canada
Carl Fredrik Forsberg Norwegian Polar Institute P. O. Box 5072 Majorstua, N0301 Oslo Norway
Valery Gataullin NIl Morgeo Riga Latvia
P.T. Harris University of Tasmania GPO Box 252c Hobart, Tasmania 7001 Australia
Reinhard Hesse Department of Earth and Planetary Sciences McGill University 3450 University St. Montreal, QUE, H3A 2A 7 Canada
KarIHinz Hanover Federal Republic of Germany Gennany
John A. Howe British Antarctic Survey High Cross Madingley Road Cambridge CB3 OET UK
Heiner Josenhans Geological Survey of Canada (Atlantic) Bedford Instiblte of Oceanography P. O. Box 1006 Dartmouth Nova Scotia B26 4A2 Canada
Neil H. Kenyon Southampton Oceanography Centre Empress Dock Southampton S014 3ZH UK
Saeed Khodabakhsh Department of Earth and Planetary Sciences McGill University 3450 University St Montreal, QUE, H3A 2A 7 Canada
Edward L. King Department of Geology University of Bergen Allegt. 41 N-5007 Bergen Norway
Ingo Klaucke Department of Earth and Planetary Sciences McGill University 3450 University St Montreal, QUE, H3A 2A 7 Canada
Jan Sverre Laberg Department of Geology Institute of Biology and Geology University of Tromso N-9037 Tromso Norway
Robert D. Larter British Antarctic Survey High Cross Madingley Road Cambridge CB3 OET UK
Lawrence A. Lawver Institute for Geophysics The University of Texas at Austin 8701 N. MoPac Expressway Austin, TX 78759 USA
G. Leitchenkov VNIIOkeangeologia St Petersburg Russia
C.F. Michael Lewis Geological Survey of Canada (Atlantic) Bedford Institute of Oceanography P. O. Box 1006 Dartmouth Nova Scotia B26 4A2 Canada
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David Long British Geological Survey Murchison House West Mains Road Edinburgh EH9 3 LA Scotland UK
Allen Lowrie Consultant 230 FZ Goss Road Picayune MS 39466 USA
Tor Helge Lygren Norsk Hydro ASA P. O. Box 200 N1321 Stabekk Norway
AstridLysa Department of Geology Institute of Biology and Geology University of Tromso N-9037 Tromso Norway
Brian MacLean Geological Survey of Canada (Atlantic) Bedford Institute of Oceanography P. O. Box 1006 Dartmouth Nova Scotia B26 4A2 Canada
Larry A. Mayer Ocean Mapping Group University of New Brunswick Fredericton Canada
Heinz Miller Alfred Wegener Institut fiir Polar-uod
Meeresforschung Postfach 120161 27515 Bremerhaven Germany
K.Moran Geological Survey of Canada (Atlantic) Bedford Institute of Oceanography P. O. Box 1006 Dartmouth Nova Scotia B26 4A2 Canada
Frank Niessen Alfred Wegener Institut fiir Polar-und
Meeresforschung Colombusstrasse Bremerhaven 27568 Germany
P. E. O'Brien Antarctic CRC Australian Geological Survey Organisation Canberra ACT 2601 Australia
Todd Oneacre Department of Geology University of Alabama Tuscaloosa, AL 34587 USA
DagOttesen Geological Survey of Norway P.O. Box 3006 N-7oo2 Trondheim Norway
Jack B. Pheasant British Geological Survey Murchison House West Mains Road Edinburgh, EH9 3LA Scotland UK
Leonid Polyak Byrd Polar Research Center 1090 Carmack Rd. Columbus OH43210 USA
Ross D. Powell Dept of Geology Northern Illinois University DeKalb, IL 60115 USA
Daniel Praeg Geology and Geophysics University of Edinburgh West Mains Road Scotland EH93JW UK
Carol J. Pudsey British Antarctic Survey High Cross Madingley Road Cambridge CB3 OET UK
Michele Rebesco Osservatorio Geofisico Sperimentale P. O. Box 2011 34016 Opicina Trieste Italy
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Erk Reimnitz U.S. Geological Survey 345 Middlefield Rd. MS: 999 Menlo Park CA 94025-3591 USA
Kare Rokoengen Norwegian University of Science and
Technology Trondheim Norway
William B. F. Ryan Lamont-Doherty Earth Observatory Palisades, NY 10964 USA
Keith C. Seramur Dept of Geology Appalachian State University Boone, NC 28608 USA
Stephanie Shipp Department Geology & Geophysics Rice University 6100 S. Main St. Houston, TX 77005 USA
Benjamin J. Sloan Institute for Geophysics The University of Texas at Austin 8701 N. MoPac Expressway Austin, TX 78759 USA
Anders Solheim Norwegian Polar Institute P. O. Box 5072 Majorstua, N0301 Oslo Norway
Rudolph R. Stea Nova Scotia Department of Natural Resources Dartmouth Nova Scotia Canada
Andrew B. Stein Institute of Arctic and Alpine Research University of Colorado Boulder, CO 80309 USA
Martyn S. Stoker British Geological Survey Murchison House West Mains Road Edinburgh, EH9 3LA Scotland UK
Jay A. Stravers Dept. of Geology Northern Illinois University DeKalb, IL 60115 USA
Eirik Sundvor Naval Research Laboratory Washington D.C. 20375 USA
James P.M. Syvitski Institute of Arctic and Alpine Research University of Colorado Boulder, CO 80309 USA
Friedrich Theilen Institut ffir Geophysik University of Kiel Keil Germany
BrianJ. Todd Geological Survey of Canada Ottawa Canada
Gabriel Uenzelmann-Neben Alfred-Wegener-Institut ffir Polar und
Meeresforschung Postfach 120161, 27515 Bremerhaven Germany
Kris Vannesle Renard Centre of Marine Geology University of Gent Krijgslaan 281 S8 B-9000Gent Belgium
L. E. Vanneste British Antarctic Survey High Cross Madingley Road Cambridge CB3 OET UK
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Heinrich Villinger Department of Geology University of Bremen D-2800 Bremen Germany
Peter R. Vogt Marine Geosciences Division Naval Research Laboratory Washington, D.C. 20375 USA
Tore O. Vorren Department of Geology Institute of Biology and Geology University of Tromso N-9037 Tromso Norway
Robert J. Whittington Centre for Glaciology Institute of Earth Studies University of Wales Aberyslwytb SY23 3 DB UK
Igor ZayalZ Joint Stock Company Marine Arctic Geological
Expedition Murmansk Russia
Editorial Committee
Thomas A. Davies (Chair) * Institute for Geophysics, University of Texas at Austin 8701 Mopac Expressway, Austin, TX 78759
Trevor Bell Department of Geography, Memorial University of Newfoundland, St John's, Newfoundland AlB 3X9, Canada
Alan Cooper U.S. Geological Survey, Office of Marine Geology, 345 Middlefield Road, MS-999, Menlo Park, CA 94025
Heiner Josenhans Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, Box 1006, Dannouth, NS B2Y 4A2, Canada
Leonid Polyak Byrd Polar Research Center, 108 SCOll Hall, 1090 Cannack Rd., Columbus, OH 43210-1002
Anders Solheim Norwegian Polar Institute, P.O. Box 5072, Majorstua, N-0301 Oslo, Norway
Martyn S. Stoker British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, United Kingdom
Jay A. Stravers Department of Geology, Northern Illinois University, Dekalb, IL 60115
* Present address: Ocean Drilling Program, Texas A&G University Research Park, 1000 Discovery Drive, College Station, TX 77845
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Acknowledgements
This book could not have been assembled without the enthusiastic participation of the 88 contributors from 40 organizations in 10 countries who contributed data gathered on numerous expeditions to various remote parts of the world.
All the contributions in the volume have been critically reviewed and we are grateful to the following colleagues who freely gave of their time and expertise to assist the Editorial Committee with this task: Sarah 1. Aavang, Maxine Akhurst, Espen Sletten Andersen, Inge Aarseth, Dennis Ardus, Peter Barnes, Donald Blankenship, Reidulv Boo, Paul Carlson, Ellen A. Cowan, Ian Dalziel, Jiirgen Ehlers, Anders Elverooi, Stephen Eittreim, Dan Evans, Gordon Fader, Jan Inge Faleide, Carl Fredrik Forsberg, Allan Grant, Steinar Thor Gudlaugsson, Michael Hambrey, Richard Hiscott, Richard Holmes, John Howe, Rod Klassen, David Liverman, David Long, Oddvar Longva, Kevin Mackillop, Brian MacLean, Russell Parrott, Douglas Peacock, David Piper, Ross D. Powell, Ruud Schuttenhelm, Hans-Petter Sejrup, Keith C. Seramur, John Shaw (Geological Survey of Canada, Atlantic), John Shaw (University of Alberta), Steve Solomon, Gary Sonnichen, Ralph Stea, Alan Stevenson, James Syvitski, Robert Taylor, Erling Vagnes, Peter Vogt, and Philip Walker.
Funding for editorial and organizational tasks was provided by the U.S. National Science Foundation Office of Polar Programs, under grant OPP- 9526459 to The University of Texas Institute for Geophysics. Additional travel and subsistence funds for committee members to attend Editorial Committee meetings were provided by the U.S. Office of Naval Research (LP), Memorial University of Newfoundland (TB), the British Geological Survey (MSS), the Norwegian Polar Institute, Oslo (AS), and the U.S. Geological Survey (AKC). The financial support provided by all these organizations is much appreciated and gratefully acknowledged.
Finally, we would like to acknowledge: the encouragement of the COLDSEIS Working Group, notably its founder and de facto leader James Syvitski, and the cooperation of the publisher, Chapman and Hall, especially Ian Francis and Jane Plowman, who patiently worked with us to bring the project to fruition.
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SIGNIFICANCE OF GLACIMARINE ENVIRONMENTS
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
INTRODUCTION Thomas A. Davies
University of Texas Institute for Geophysics, Austin, Texas U.S.A.
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.
Although northern hemisphere glaciation apparently did not commence until late Miocene time, extensive glaciation in the Antarctic is inferred as early as the beginning of the Oligocene [Barron and others, 1991], with a major expansion of the Antarctic ice sheets in Miocene time [Kennett, 1982]. Maximum glaciation was reached in the Plio-Pleistocene when both northern and southern high latitudes were covered by ice [Kennett, 1982]. The Pleistocene geologic record and the effects of late Cenozoic climatic changes have also been studied extensively in temperate and low latitudes, both onshore and in the marine environment. A global summary has been provided by Nilsson [1983]. The Pleistocene geology of North America and adjacent regions is described by Fulton [1989], Morrison [1991],
1
and Ruddiman and Wright [1987], while a summary for the Antarctic is provided by Webb [1990].
Detailed study of glacimarine processes and the features of sedimentation characteristic of higher latitudes on a large scale has begun only within the past 25 years, stimulated by the availability of new tools and techniques adaptable to these environments [e.g. Domack, 1983; Eyles, and others, 1985; see Anderson and Molnia, 1989, for a broad summary of present knowledge of glacimarine sedimentation]. Although Antarctica, with its remoteness and inhospitable environment, has captured much of the public imagination, most of the offshore seismic work has in fact been done in the Arctic and sub Arctic regions, where the ice-covered Arctic Ocean is bordered by extensive continental shelves of the major landmasses of North America and Eurasia. Nearly 30 per cent of the Canadian landmass is covered by ocean or inland sea. All of these areas have glacimarine deposits or have been directly impacted by glaciation. By contrast, Antarctica, which presently holds much of the Earth's glacial ice, is an isolated landmass surrounded by the Southern Ocean and consequently exhibits a number of unique features which are not found elsewhere. For example, the role of meltwater in the transportation and distribution of glacially-
Figure 1. Maximum extents of major late Cenozoic ice sheets (solid line) and ice rafting (broken line) for Northern Hemisphere (left) and Southern Hemisphere (right) (based on Anderson [1983], and Nilsson [1983])
2
derived sediment is now vastly reduced in the Antarctic, where the ice sheets terminate in a marine environment and mechanical break-up is a limiting factor, compared to the more temperate Arctic and sub Arctic regions, where many of the glaciers and ice sheets terminate on land or in fjords and are limited by melting. Indeed the Antarctic ice margins might be regarded as representing only one end member of a continuum of glacimarine environments, another end member of which might be represented by the temperate tidewater-glaciers [see Syvitski, 1994, for further discussion].
Late Cenozoic glaciation directly affected sedimentation on more than half the Earth's continental shelves [Syvitski, 1991]. Ice continues to be a dominant influence on sedimentation around Greenland and Antarctica, and on the shelves facing the Arctic ocean. Elsewhere, glacimarine sedimentation is of particular significance on the shelves of North America, north of 40oN, and northwest Eurasia, which were strongly influenced by the Plio­ Pleistocene glaciation, though now ice is a minor factor. The features of these shelves include true glacimarine features, i.e. those formed in a marine environment in proximity to, 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 found, since 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, lodgment till, which have subsequently been submerged and modified by marine influences. Syvitski [1991]
has divided the sediments of glaciated continental shelves into five categories representing different depositional environments: (1) ice-contact (ice-loaded) diamicton, (2) ice proximal (outwash) sands, diamictons and muds reflecting the dynamics near an ice margin, often influenced by glacifluvial discharge, (3) ice-distal, recording a strong marine influence (including ice-rafting), (4) para-glacial nearshore deposits that record terrestrial ablation of ice sheets, and (5) post­ glacial deposits which record the present marine environment. Recognizing and understanding the features of glaciated shelves and continental margins will become increasingly significant as the Earth's population continues to grow and turns increasingly to higher latitudes and offshore regions in search of resources. Better understanding of present-day glacimarine environments will also enhance interpretation of the stratigraphic record in terms of past environments and the possible global climate changes implied by these interpretations.
FEATURESOFGLAC~
ENVIRONMENTS
Typical features found in Arctic glacimarine environments are illustrated schematically in Fig. 2 and their formation is described in Hambrey [1994]. Generally speaking, two broad types of glacimarine environments can be recognized, although these share some features in common. Fjord environments are characterized by tidewater glaciers, terminating in fjords, or land-based glaciers terminating in outwash plains at the fjord head. Fjord environments are best known from Arctic and
3
sub-Arctic regions, although they are also found in the southern hemisphere, notably in Chile and New Zealand. The glaciers themselves provide ice-contact deposits, glaciofluvial (meltwater) sediments and ice-rafted debris. The land furnishes fluvial, rockfall, aeolian, and gravity­ flow deposits, and the marine environment contributes suspended sediment and biogenic materials. The second type of glacimarine environment, most common in the Antarctic, is characterized by floating or grounded ice sheets extending onto, or across, the continental shelf. Meltwater or other fluvial processes are relatively minor. On the other hand, erosion of the shelf and the accumulation of lodgment till, waterlain till deltas, and gravity-flow deposits are common. The shelf is usually tilted landward as a result of ice-loading and erosion of the inner shelf and accretion of glacial debris on the outer shelf and continental slope. Seaward of the ice front in both types of environments, iceberg scouring, and debris flows and slumps from oversteepened accumulations of unconsolidated sediment are found, while in deeper water of the continental slope and rise turbidites and drifts of fine sediment transported by bottom currents accumulate.
COLLECTED EXAMPLES FROM GLACIATED CONTINENTAL MARGINS
The idea of a compilation of typical seismic images of marine glacigenic features, similar in concept to seismic atlases of other regions or environments published recently by AAPG and other professional societies [e.g. Bally, 1987; Berryhill, 1987], developed at a meeting of the COLDSEIS group in Columbus, Ohio, in April
Debris flows, slumps
Glacier-fed marine delta
Ice shelf grounded below sea level
Figure 2 Schematic diagram showing features typical of an Arctic glaciated continental margin (modified from EyIes, et al. 1985)
4
1993. COLDSEIS is an informal, international working group, initially affiliated with the International Quaternary Association (INQUA), devoted to "Seismic Facies Analysis of Marine Glacigenic Sequences" principally of Arctic regions. The group was formed under the leadership of James Syvitski (University of Colorado) and Martyn Stoker (British Geological Survey). In the development of the present volume, COLDSEIS collaborated with the ANTOSTRAT (Antarctic Offshore Acoustic Stratigraphy) Project, which is part of the International Committee on Antarctic Research (SCAR) [Cooper and Webb, 1994].
The principal objective of the present volume is education in the broadest sense. It is intended to serve as both a teaching tool in universities and professional "short courses", and a reference for students, teachers, and practicing professionals working in glacimarine environments. In this regard, the present volume complements other books (e.g., Hambrey, 1994) which address the nature and formation of the various features found in all types of glacial environments.
The concept for this book was initially proposed by the Arctic community to illustrate the complexity and high resolution detail of Quaternary glacial features seen in Arctic marine sedimentary sections, which are commonly ubiquitous, but thin (less than 200 m). In expanding the book to include the Antarctic, via collaboration with ANTOSTRAT, it has been possible to incorporate different environments (e.g. deeper-water shelf) and thicker and older Cenozoic sediments, where however, detailed resolution of glacial features is generally lower. Nevertheless, from the
comparative study of the various polar environments, we hope that this book will be useful to Quaternary scientists, glaciologists, marine geologists and geophysicists, geotechnical engineers, and surveyors working in universities, research institutions and government agencies "ith interests in polar and subpolar regions, as well as those in industries with offshore interests in the Arctic where an understanding of the local and regional geologic setting is of critical importance [see Ardus, 1980, especially papers by Fannin, Gunsleikrud and Rokoengen, King, and, Ploessel and Campbell].
This project was coordinated by Thomas A. Davies (University of Texas), with the advice and guidance of an international Editorial Committee broadly representative of the membership and interests of the COLDSEIS Working Group. For economy and speed, we adopted the approach of using author-prepared camera-ready copy. Inevitably, this has resulted in some inconsistency between contributions which hopefully will not detract from the substance of the book. The page size, "landscape view" orientation, though not universally popular, was a compromise aimed at producing a book which is small enough to be handled easily but still large enough to show illustrations in reasonable detail. We have also adopted, particularly in the Features section, the somewhat unconventional approach of beginning each contribution on a left-hand page. This enables the reader to see both text and illustrations without turning the page.
The book was assembled from records gathered in the course of recent research by members of the scientific community. It includes
5
seismic sections, side-scan maps, 3-D data, supplemented in some cases by bottom photographs and core data, with accompanying explanatory text. Coverage is somewhat uneven, since some features are more easily recognized or more thoroughly studied than others. Our aim, however, was not encyclopedic coverage, with ideal "textbook" examples of all types of features from all glaciated regions, so much as to present a representative selection of features, as typically observed, and with reasonable geographic coverage. Features of Arctic, Antarctic, and fjord environments are included, and we have incorporated examples from regions which are presently glaciated and regions which have undergone glaciation in the recent geologic past (i.e. Quaternary).
The book is arranged in four parts. The first part, Seismic Character and Variability, contains a general description of seismic methods and interpretation followed by some brief contributions illustrating the effects of source frequency and sub-surface structures on seismic records. The second part, Features Found in Glacimarine Environments, consists of numerous short contributions illustrating specific features found in these environments. Contributions are grouped and arranged in a progression from features formed under the ice, to features formed at the ice margins, and finally to features formed beyond the ice margin but which owe their formation to glacial influences (debris flows and slumps, iceberg scours, fluvial channels). Glacimarine Environments! Geomorphic Provinces, the third section, has longer contributions to illustrate glacimarine features as they relate to one another in the larger context of geomorphic provinces. Here,
the progression is from fjord and coastal environments, across the continental shelf to the slope and deep sea. Finally there is a Glossary of commonly used terms as these are generally understood.
We recognize that the present volume is not comprehensive and may not contain the "best" examples of different features. However, we hope that these representative examples will help the reader better understand the varied glacial environments of both polar regions. And if the book serves to inform, and to stimulate new studies, it will have done its job.
REFERENCES
Anderson, J.B., Ancient glacial marine deposits: their spatial and temporal distribution, in Molnia, B.F. (ed.) Glacial-marine sedimentation. Plenum Press, New York, p. 3-92, 1983.
Anderson, 1.B., and Molnia, B.F., Glacial-Marine Sedimentation. AGU Short Course in Geology, v. 9, pp. 127,1989.
Ardus, D.A., (Ed.), Offshore site investigation. Graham & Trotman Ltd., London. 291 pp, 1980.
Bally, AW., Atlas of Seismic Stratigraphy. AAPG Studies in Geology, no. 27 (3 vols.), 1987.
Barron, J., Larsen, B., and Baldauf, J. G., Evidence for Late Eocene to Early Oligocene Antarctic glaciation and observations on Late Neogene glacial history of Antarctica: results from Leg 119. In: 1. Barron, B. Larsen, and others (Eds.), Proc. ODP, Sci. Results v. 119: Ocean Drilling Program, College Station, 'IX. pp. 869-891, 1991.
Berryhill, H.L., Jr., Late Quaternary Facies and Structure, northern Gulf of Mexico. AAPG Studies in Geology, no. 23, 1987.
Cooper, AK., and Webb, P.N., The ANTOSlRAT Project: an international effort to investigate
Cenozoic Antarctic glacial history, climates, and sea-level changes. Terra Antarctica, v. I, p. 239- 242,1994.
Domack, E.W., Facies of Late Pleistocene glacial­ marine sediments on Whidbey Island, Washington: an isostatic glacial-marine sequence, in Molnia, B.F. (ed.) Glacial-marine sedimentation. Plenum Press, New York, p. 535-570, 1983.
Eyles, C.H., Eyles, N., and Miall, A.D., Models of glaciomarine sedimentation and their application to the interpretation of ancient glacial sequences. Paleogeography, Paleoclimatology, Paleoecology, v. 51, p. 15-84, 1985.
Flint, R.F., Glacial and Quaternary Geology. John Wiley, New York, 892 pp, 1971.
Fulton, R.J. (ed.), Quaternary geology o/Canada and Greenland (DNAG Volume K-1). Geological Survey of Canada, Ottawa. 839 pp. plus plates, 1989.
Hambrey, M.J., Glacial Environments. UCL Press Ltd., London. 296 pp, 1994.
Hambrey, M.J., and Harland, W.B., (eds), Earth's pre-Pleistocene glacial record. Cambridge University Press, Cambridge, 1981.
Kennett, J.P., Marine Geology. Prentice-Hall, Englewood Cliffs, NJ, 813 pp, 1982.
Kennett, J.P., and Barron, J.A., Introduction. in Kennett, J.P., and Warnke, D.A. (eds.) The Antarctic Paleoenvironment: a perspective on global change, Part 1. Antarctic Research Series, v. 56, p. 1-6, 1992.
Morrison, R.B. (ed.), Quaternary non-glacial geology: coterminous United States (DNA Volume K-2). Geological Society of America, Boulder, CO. 672 pp. plus plates, 1991.
Nilsson, T., The Pleistocene. Reidel Publ. Co., Boston, pp. 651, 1983.
Ruddiman, W.F., and Wright, H.E., Jr. (eds.), North America and adjacent oceans during the last glaciation (DNAG Volume K-3). Geological Society of America, Boulder, CO. 501 pp. plus plates, 1987.
6
Syvitski, J.M.P., Towards an understanding of sediment deposition on glaciated continental shelves. Continental Shelf Res., v. 11, p. 897-937, 1991.
Syvitski, J.M.P., Glacial sedimentary processes. Terra Antarctica, v. I, p. 251-253, 1994.
Webb, P.N., The Cenozoic history of Antarctica and its global impact. Antarctic Science v. 2 (pt. 1), p. 3-21,1990.
University of Texas Institute for Geophysics, 8701 Mopac Expressway, Austin, Texas 78759
PART ONE
Martyn S. Stoker!, Jack B. Pheasane and Heiner Josenhans2
1 British Geological Survey, Edinburgh, Scotland, UK, and 2Geological Survey of Canada (Atlantic) Dartmouth, Nova Scotia, Canada
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.
As the emphasis of this atlas is on Late Cenozoic glacimarine environments, the sections relating geology to seismic reflection profiles and sidescan-sonar interpretation will focus on the types of material and features characteristic of such depositional settings, and their typical acoustic response. In order to highlight some of the techniques and interpretive methods referred to in this chapter, we have cross-referenced, where appropriate, with examples from contributions included within the atlas. In addition, several specialist contributions follow this chapter and provide specific examples directed at the use of different acoustic imaging systems and their bearing on interpretation
[Josenhans, this volume; Dowdeswell et al., (a), this volume].
SEISMIC TECHNIQUES
A knowledge of the geology of the sea bed and underlying strata is basic to the study of glacimarine environments on continental margins around the world, as is an understanding of the methods, and their limitations, used to acquire this knowledge. Acoustic methods are the most widely used surveying techniques; they are fundamental to studies of sea-bed morphology using echo-sounder and scanned sonar, as well as the investigation of both shallow and deep sub-bottom layers in seismic reflection profiles. The most commonly used methods for the study of Quaternary glaciated-margin successions are summarised below, with emphasis on high­ resolution investigations. A more complete description of these seismic techniques can be found in Belderson et al. [1972], McQuillin and Ardus [1977], McQuillin et al. [1984] and Evans et al. [1995].
Shallow Seismic Reflection Profiling
Seismic reflection methods are those which depend on the generation and detection of acoustic waves. In shallow-marine seismic profiling, the acoustic source generates a short pulse of sound (shot) which passes through the
9
water and penetrates the sea bed (Fig. 1). Reflection of energy takes place at boundaries between sediment/rock layers of differing acoustic impedance, the reflection strength depending on the impedance contrast. Reflected energy is detected by a hydrophone and processed electronically to improve the signal/noise ratio. Returning signals from each shot are displayed against time as one line across a line-scan record, time zero being at the shot instant. Successive shots are displayed as adjacent lines/scans on the recorder, building up a profile as the survey vessel moves through the water.
Most studies of Quaternary glaciated margins have, to date, used high-resolution single­ channel seismic systems because of the relatively shallow (up to 2 km) target zone, hence the term shallow-seismic reflection profiling. The resulting profile is referred to as an analogue record where the reflected signals are translated directly into paper records by the graphic recorder. The choice of which acoustic source to use is primarily dependent upon the objective of the survey, and the requirements for resolution and penetration. However, as these types of studies are undertaken primarily by university and governmental organisations factors such as cost, survey vessel capability, ease of operation and maintenance have to be taken into consideration. Technical details of data acquisition specific to the seismic data presented
in this atlas are summarised with each contribution.
.,
' ...
technique depends on digital acqUIsitIOn and processing techniques, which have only recently begun to be used with the single-channel system [Evans et at., 1995]. Multi-channel methods using high-resolution systems have been employed on a limited scale for the past 20 years in support of regional structural mapping programmes [McQuillin and Ardus, 1977]. However, over the last few years, multi-channel seismic reflection profiles of medium to low resolution have also begun to be collected specifically for glaciated-margin studies from areas with excessively thick glacigenic
------....,------ Airgun Hydrophone
Deep-Tow Boomer
Figure I. The basic elements and towing-configuration of a shallow-seismic survey.
10
sedimentary successions. For the most complete interpretation such data should be complimented by shallow-seismic profiles.
Acoustic source, resolution and penetration. The resolution obtained on a seismic profile is dependent on the frequency of the acoustic energy: the higher the frequency the better the resolution (Fig. 2). The attenuation of sound in sediments/rocks is also dependent on frequency, the higher frequencies being attenuated much more quickly than low frequencies. Therefore, to achieve the best possible range of information it is necessary to use a variety of seismic sources. The high-frequency boomer, pinger and parasound systems give very detailed information in the near-surface region, penetrating up to 100 m below the sea bed. The medium-frequency, larger energy, sparker penetrates to a depth of about SOOm whilst maintaImng good resolution. Airgun and sleevegun pulses are of even higher energy and lower frequency, and penetrate to a depth of 1 to 2 Ian but with a corresponding reduction in resolution. All of these shallow-seismic sources are designed to produce intense, short-duration, bursts of sound. Sources for use in multi-channel, deeper-penetration, surveys are designed to have a higher total acoustic-energy output, the bulk of which occurs at low frequency. A combination of these seismic systems can often be operated at the same time [Josenhans, this volume; Dowdeswell et al., (a), this volume] , their firing and reading cycles being controlled in a programmed sequence by a sophisticated firing control unit which reduces interference between systems.
Multiples. Not all reflections on the seismic profile are primary reflections. Many combinations of multiple reflections (essentially
reflections that have undergone more than one bounce) are possible from reflecting horizons within the sub-sea-bed sediments/rocks and the sea surface. They can be divided into two main categories: short-path and long-path (Fig. 3). Short-path multiples arrive so soon after the primary reflection that they merely extend the duration of the pulse or primary signal; long-path multiples arrive later as distinct events [Badley, 1985; Evans et al., 1995]. Such artefacts are common both to single- and multi-channel profiles, and, as is exemplified in this atlas [e.g. Stoker, this volume; Vanneste et al., this volume; Anderson et al., this volume], one of the most important of these is the long-path, sea-bed multiple (Figs. 3b and 4). The signal reflected from the sea bed may be reflected back to the sea surface and then downwards again, to act as a false outgoing signal. The sea surface is an excellent reflector due to the high impedance contrast between air and water, and so most of the energy is reflected to produce a second, delayed, downgoing signal. The amplitude of the multiple is a function of the acoustic reflectivity of the sea-bed material. Rock, gravel and sand produce strong reflections and thus strong multiples, whereas mud is a poor reflector thus generating a weak multiple. On continental shelves, where the sea bed can be strongly reflective, a train of closely-spaced multiples (reverberation) will be produced which can obscure sub-bottom reflections below the first multiple. The strength of the sea-bed multiple is commonly greater than the deeper sub-bottom reflectors, and it becomes impossible to interpret reflectors beneath the arrival of the first sea-bed multiple. This effect is exemplified in Fig. 4 where a significant portion of a prograding shelf­ margin succession is obscured by a series of sea-
bed multiples. This can result in serious correlation problems between the shelf and the slope, as reflections cannot often be traced with confidence through the area of the record affected by the multiples.
As the sea-bed multiple presents one of the major problems in interpreting continental shelf surveys, it is essential to distinguish correctly between primary and multiple reflections. This is particularly important on analogue records as no processing parameters can be changed once the profile is recorded. In areas of sloping sea bed, and particularly on the upper slope, the multiple will have a steeper gradient than the sea bed (Fig. 4). Additionally, the multiple is often of higher amplitude and frequency than the sub-sea-bed reflection arriving at an equivalent time, as it has undergone less attenuation. On digital records, many of the problems caused by multiples are removed by reprocessing of the data [Badley, 1985].
On records acquired from instruments which may require a variable tow-depth, such as the deep-tow boomer (Fig. 1), the sea-surface reflection (source to sea surface to hydrophone) (Fig. 3a) is a common interference problem as the trace of the multiple can meander across the record as the height of the boomer is adjusted. This may result in the appearance of an 'apparent' seismic unit on the record (Fig. 5c).
Noise. Other sources of noise (a term used to cover all phenomena on the seismic profile largely unrelated to the geology) that typically affect high-resolution seismic profiles include diffractions and curvature, as well as other random background noise. Diffractions can emanate from any abrupt interface in the subsurface and, because of their curved shape, can be mistaken for a real structure. They are
11
1 ,5()()
• Spa,k ..
• Airgun
• Pa'.ownd .~
• Ping&<
Figure 2, Characteristics of seismic systems,
generated by features which have dimensions comparable to the wavelength of the acoustic signal (between 0.5 and 3.0m), thereby acting as point sources. At a sharp discontinuity, such as a fault plane, where the end of a reflector acts as a point source, acoustic energy is scattered in all directions and is recorded in the form of a hyperbolic trace with the source of its diffraction at its apex. With a fault plane, only half the hyperbola on the downthrow side is imaged on the seismic profile. Diffractions are also to be expected from areas of rough topography, large sea-bed erratics, sand waves, and, on glaciated shelves, iceberg plougbmarks [Pudsey et al., this volume; MacLean, this volume]. If the point reflector is smaller than the wavelength of the signal, e.g. gravel, the diffracted wave radiates in all directions; this is termed scattering.
Reflectors (especially at sea bed) associated with this phenomenon are chaotic with few well­ defined hyperbolic reflections (Fig. 4). Diffractions can also be produced by features out of the plane of the seismic section, not directly beneath the track of the survey vessel. These are commonly referred to as sideswipe events [Evans et at., 1995], and are most likely in areas of variable relief. Deeply-incised continental slopes and upstanding, strongly-reflective, underwater obstacles, such as rock ridges and slide scarps (Figs. 5a and 5b), commonly produce this type of reflection. Such reflections are distinguishable by their semi-transparent (ghost-like) character which enables the primary reflections to still be differentiated on the record. Interestingly in Fig. 5a, the central part of the upstanding ridge may actually lie within the plane of the seismic section as the primary reflections cannot be traced wholly through the image of the ridge. If the curvature of a reflector exceeds that of
the incidence wavefront, reflections may be generated from more than one point. This is typically associated with features which have a synformal character, such as submarine canyons and channels, including buried, infilled channels, and narrow depositional hollows in areas of slumping (Fig. 5b). As reflections are received both from the flanks and the centre of the syncline, a complex pattern is generated of three reflector branches. When the source is directly above the axis of the synformal feature, reflections from the sides will arrive before that from the deepest point, and the floor of the canyon, channel or depositional hollow may appear shallower than its true depth. Additionally, the synform will appear to be underlain by an antiformal reflector generated by the V -shaped point source at the bottom of the
(a) SHORT -PATH MULTIPLES
=eDt
Figure 3. (a) Short-path multiples produced by reflectors at the sea bed, sea surface, and within sediments. Note the contrast with the ray-path configuration of the direct ray to the sea bed, and the direct reflection from the sea surface. (b) Long-path multiples produced by reflections at the sea bed and the sea surface, with a schematic representation of how a sea-bed multiple appears on a seismic profile. The multiple exaggerates the relief of the sea bed. In areas with a reflective (hard) sea bed, additional multiples with increasingly exaggerated relief may underlie the first multiple. Similar multiples may be generated by deeper, higher-amplitude reflectors.
synform. This group of reflectors gives rise to a characteristic bow-tie effect (Fig. 5b).
Random noise is generated by other boats surveying in the area, waves, fish shoals, wrecks etc. Noise from other instruments being run simultaneously from the same survey vessel can also produce acoustic interference structures on
12
separate records (Fig. 5c) [Stein and Syvitski, this volume]. Seismic data obtained during conditions marginal for data acquisition will be poorer than data acquired from the same area under good conditions. The difference between the two will be the higher level of noise in the data obtained during the bad weather. Although inboard-
NW HEBRIDES SHELF • I - ,
. '.
Figure 4. Single-channel analogue seismic profile (British Geological Survey - BGS - watergun record) from the continental margin off North-West Britain, showing long-path multiple reflections generated by the sea bed, buried bedrock surface, and high-amplitude dipping reflectors (e.g. A) within the overlying prograding wedge. The high number of strong mUltiples (up to 9 are observable) at the south-east end of the profile reflects the hard, reflective nature of the sea bed and bedrock. The latter locally crops out at the sea bed where it consists of Precambrian crystalline basement; the associated irregular, rough topography of the sea bed in this area causes scattering and loss of penetration of the acoustic signal. The reduced number of multiples underlying the outer shelf and slope marks a change to the less-reflective sediments of the prograding wedge. Nevertheless, the strength of the first two multiples is enough to obscure much of the stratigraphical detail on this part of the margin, thus hindering correlation of reflections between the shelf and slope. On this particular record an additional reflection, known as the 'watergun precursor', is an artefact of the type of watergun used. Abbreviations: SBM, sea-bed multiple; BM, bedrock multiple; A, primary high-amplitude reflector; AM, multiple of reflector A; SC, scattering.
13
a
c
T 40ma
lkm
d
Figure 5. (a) BGS airgun profile from the Faeroe-Shetland Channel off North-West Britain, showing a sea-bed ridge most of which is out of the plane of the seismic section. Note the apparent overlap between the primary reflection of the sea bed and the 'ghost-like' sideswipe diffraction from the ridge. The lack of total continuity of the sea-bed reflection through the ridge suggests that the central part of the ridge may lie within the plane of section. (b) Abundant hyperbolic reflections, including 'bowtie' reflections (B), generated on a BGS deep-tow boomer profile across the hummocky surface of Pleistocene slump deposits of the Storegga Slide, off Western Norway. Note also the diffraction from the scarp wall. (c) BGS deep-tow boomer profile across Pleistocene slope deposits off Western Norway, showing interference patterns generated by the sea-surface reflection and by background electrical mains noise (zigzag pattern), which partly overprint the primary reflections from the acoustically-layered sediments. The variation in position of the sea-surface reflection is due to the varying tow depth of the boomer fish during survey. On this part of the profile, the sea-surface reflection gives the impression of a 'pseudo mounded-unit' resting on the layered strata. (d) Acoustic blanking within upper Pleistocene-Holocene sediments from the Forth Estuary, Scotland. BGS surface­ tow boomer. The abrupt lateral termination of the blanking may be due to gas escaping vertically through the sea bed. Abbreviation: SBM, sea-bed multiple.
14
processing and recording techniques [cf. Evans et al., 1995] can help to reduce the effects of noise, such noise cannot be avoided. Whilst this may be a problem initially for new interpreters, the experienced mind will eventually become trained to disregard it in any interpretation.
Acoustic blanking. This occurs where reflectors suddenly become obliterated beneath a near­ horizontal layer within the sediment pile, and is most commonly associated with shallow gas (Fig. 5d). In contrast to water and sediment, gas bubbles are compressible and tend to absorb some of the acoustic energy by compression. This attenuates the energy passing down through the sediment thereby reducing the penetration of the acoustic signal. As gas bubbles behave elastically, they continue to resonate and emit acoustic energy after the initial compression. Consequently, this resonation produces noise which shows up on the seismic profile as incoherent noise beneath the gas-rich horizon. Reflectors underlying this layer become masked by this noise [Fader, this volume]. Escaping gas may produce a pockmark or low-angled sea-bed crater, which are a common feature of some formerly glaciated shelves [Hovland and Judd, 1988]. More violent eruptions of gas may produce larger sea-bed depressions [Solheim, this volume].
2D and 3D surveys. To date, most seismic reflection studies of offshore Quaternary successions, in general, and glaciated margins, in particular, have been undertaken using conventional 2D survey methods. In most cases (as exemplified in this atlas), the nature of the survey has been to establish a better regional understanding of the area under investigation. By implication, this is best achieved through the study of long, regional, survey lines which, when
gridded, enable a quasi-3D picture to be built up of the gross geometry of the sediment bodies. Such an approach has also been the basis for systematic mapping programmes of national exclusive economic zones or EEZ's [Fannin, 1989]. More recently, however, 3D high­ resolution seismic reflection studies have been undertaken in areas of specific interest, including parts of formerly-glaciated margins, in order to improve the level of understanding of the complexities of glacial successions. 3D seismic exploration has been commonplace to the hydrocarbon exploration industry for at least 10 years, and its increasing use in Quaternary studies has been largely (though not exclusively) driven through exploration activity.
Although 3D study areas are much smaller than the regional 2D surveys, the value of these studies lies in their ability to resolve the inherent internal structure of a complex succession. A 3D­ seismic data-set consists of a 3D data volume or cube from which horizontal (plan view) and vertical (profile) sections can be viewed (Fig. 6). In addition to better defining the geometry of sediment bodies or morphological features, such as channels, time-slice and azimuth maps present an image of a palaeo-surface which may produce geomorphologic evidence on the ongm, significance and trend of a relict, buried feature. This atlas shows 3D-seismic examples of relict subglacial, glacimarine and periglacial marine features, such as ice-scoured surfaces, tunnel valleys, iceberg ploughroarks and fluvial channels [Davies and Austin, this volume; Lygren et al., this volume; Praeg,(a) (b) this volume; Praeg and Long, this volume]. To achieve the quality of image presented by these contributors to the atlas requires significant processing of the data. Whilst it is beyond the
15
plan view
Vertical slice: profile
Figure 6. A schematic representation of a 3D data volume illustrating the geometry of the different views. The horizontal (plan) view is also termed a time slice.
scope of this chapter to describe the processing techniques, the reader is referred to Davies et al. [1992] who present a case study of late Quaternary sedimentation using 3D seismic profiles from the continental shelf off New Jersey. A more general account of 3D-seismic exploration techniques is presented by Yilmaz [1987] and Brown [1991].
Scanned Sonar Methods
Sidescan sonograms, the marine equivalent of aerial photographs, use sound to generate sonographs which give an indication of surface relief and surface texture of the sea floor. The basic principle is that of detecting echoes of a transmitted pulse in such a way that the time scan can be calibrated in terms of distance (swath width) across the sea bed (Fig. 7). The first echo
in any scan is the bottom echo, with subsequent echoes being reflected from features ranging across the sea bed to the outer limit of the scan. The forward motion of the ship provides scanning in the direction parallel to the track, and the picture is built up line by line as the ship moves forward . The recorder displays the strength of those echoes scattered back towards the array of acoustic transducers, and this backscattering strength depends on the topography and texture of the sea bed. If the sea bed is relatively flat, most of the sound is reflected away from the transducer. However,
Figure 7. Sketch showing how, as the sidescan-sonar fish is towed along the ships track, an image of the sea bed is created from the strength of its backscattering properties; a reflection of surface roughness and sediment texture.
even on a flat sea bed there is usually some roughness due to sediment texture. In general, the rougher the sea bed, the stronger the backscatter will be.
The sidescan-sonar system may be either hull­ mounted or towed behind a survey vessel; the latter reduces the effect of background noise and roll associated with the vessel. To maximise the coverage obtained on each survey line, systems are dual channel so that separate beams are scanned to each side of the ship. Consequently, a picture of the sea bed can be generated ranging from beneath the ship to up to several tens of kilometres either side of the ship's course.
The range of the system is closely linked to the resolution obtainable, and there are many commercial sidescan-sonar systems available which vary in range and resolution. The longest­ range sidescan sonar system called GLORIA (Geological LOng Range Inclined Asdic) [Somers and Searle, 1984] employs a low frequency (6.5 kHz) to acoustically illuminate a swath up to 30 km in width normal to the ships track. The advantage of using a low frequency system is that it can illuminate a large area, and is particularly useful in deep ocean basins; the disadvantage of this long-range capability (a function of wavelength) is a lack of resolution, which in the case of 6.5 kHz is approximately 7 m. In contrast, a high-resolution sidescan sonar employs a frequency of 500 kHz which although illuminating only a swath of 150 m below the towed transducer will attain a resolution of about 5 cm. Such relatively short-range systems are commonly used on shallow continental shelves. Intermediate between the long-range GLORIA system and the high-resolution systems are tools such as SeaMarcII and SeaMARCIII, which use, respectively, a frequency of 11-12 kHz and 27-30 kHz to achieve a maximum range between 10 and 6 km. In addition, some systems combine 100 and 500 kHz transducers and provide concurrent images which display a 300 m range
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from the 100 kHz transducer, and a high­ resolution from the shorter 500 kHz wavelength.
GEOLOGY AND SEISMIC REFLECTION PROFILES
A seismic profile is not a geological cross­ section. Despite their apparent resemblance, the character of the seismic section is dependent upon acoustic impedance contrasts within the geological succession. Lithological boundaries will only be detected if the acoustic impedance changes across the boundary. As the reflection strength depends on the impedance, not every boundary is necessarily imaged. Alternatively, where boundaries are closely spaced, interference may affect the seismic response and further hinder geological interpretation. Another important consideration is that the seismic profile is time-related rather than distance/depth-related. The horizontal axis is scaled in elapsed travel time of the survey vessel, whilst the vertical axis is scaled in two-way travel time. The latter represents the time interval between initiation of the pulse of sound and the reception of the sound wavelets which have been reflected from the acoustic interfaces within the transmitting media; water, sediment and rock. In general, seismic velocity increases with increased density in sediments and rocks (Table 1).
Table 1. Examples of seismic velocities
TYPE OF MATERIAL Water Glacimarine muds Glacial moraine Limestone Granite
VELOCITY 1490 mls 1500-1800 mls 1600-2700 mls 3500-6500 mls 4600-7000 mls
The distortions that this last factor creates, due to both vertical and lateral velocity changes within the sediment/rock sequence, must be considered when linking geology and seismic profiles. Knowledge of the velocity structure of the sediment/rock sequence and the horizontal scale is a necessary pre-requisite before the profile can be converted to a depth section.
Ultimately, the geological interpretation of any seismic profile depends on the ability and skill of the interpreter. Filling the information gap between what is observed on the seismic profile, and the likely geological scenario, requires the interpreter to be able to (a) identify and eliminate all events relating to noise and interference, and (b) to employ considerable geological skill; in the present context, this includes knowledge of glacial and glacimarine environments, sedimentology, stratigraphy, etc., to translate the seismic image into a geological interpretation. Sound geological concepts and models can be used predictively and as a guide to interpretation. Despite local variations in glaciaVglacimarine environments, many processes driven by a common underlying cause will result in a similar end product. An ice-proximal glacimarine environment, for example, will be unique on a local scale but will display many of the larger­ scale features typical of such environments. The recognition of these features on a seismic profile is based on a number of seismic reflection parameters of which character of the single reflection, configuration of reflections within sequences, and external form of facies units or sequences are the most obvious and directly analysed parameters. The main features of these parameters are summarised below. Most general terms used to describe these parameters were originally defined by Mitchum et at. [1977 (a),
(b)] and refined by Berryhill [1986] for Quaternary deposits on continental shelves and slopes.
Reflection Character
This can be described in terms of amplitude, frequency and continuity.
Reflection amplitude. This is a function of the acoustic impedance contrast between strata, and can be described as low, moderate or high. In Quaternary glacigenic successions, high­ amplitude reflectors commonly occur In
interbedded sequences of sand and mud, are associated with peat beds, occur at the interface between glacial diamicton and normal-to­ underconsolidated sediments, and at the interface between bedrock and Quaternary sediments. Whilst lateral changes in amplitude may help distinguish seismic facies, caution must be exercised as many changes in amplitude are due to interference effects.
Reflection frequency. This is largely dependent on bed thickness and imparts a character to a seismic unit in terms of the breadth - broad, moderate or narrow - of the frequency cycle. Vertical changes in thickness can be used to help locate a sequence boundary, whereas lateral changes may be used to infer facies change, although, as with amplitude, the interpretation of thickness or character is susceptible to noise and interference.
Reflection continuity. This is related to the continuity of stratal surfaces and so may be an indicator of the environment of deposition of the sediment facies. High continuity is often characteristic of sediments deposited primarily under tranquil, argillaceous, lacustrine or marine conditions, where no major bedforms disrupt the
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bedding planes. However, although argillaceous sediments may display numerous high-amplitude reflections, not all of the reflections necessarily reflect lithological change; some of the reflections may result from changes in the physical properties of the sediments. Low continuity (discontinuous reflections) is often shown in higher-energy, sandy sequences where bedding planes lack continuity (relative to the horizontal resolution of the seismic system). Such sequences more typically display chaotic to inclined, discontinuous, low-amplitude reflectors against a noisy acoustic background.
A lateral change in lithology mayor may not be accompanied by a change in reflection continuity, and hence precise facies boundaries are difficult to delineate on seismic data alone. Reflectors bounding facies units or sequences may similarly be of variable continuity depending upon the impedance contrast between .the component lithologies of the units. The character of such reflectors may provide information on the nature of the boundary or the existence of a bounding deposit (e.g. basal gravel or weathered crust) to the unit.
Gravel-rich beds and diamictons scatter acoustic energy; these units, regardless of origin, contain many point sources which reflect acoustic energy in a disorganised manner. This produces a sequence with high internal backscatter resulting in structureless to chaotic reflectors. Moreover, acoustic penetration is limited; the higher the clast content, the greater the backscatter effect.
In addition to lithological or facies variability, reflection continuity may also be affected by the presence of shallow gas within the sediment column. The effect on the seismic profile is one of sudden obliteration of reflectors. This effect is
termed acoustic blanking, the cause of which has been described above.
Reflection Configuration
This represents the shape of a reflection or surface, and has implications for bedding patterns, depositional processes, erosion and palaeotopography. Three main types of reflection configuration occur: stratified, chaotic and reflection-free (Fig. 8).
Stratified reflections. Simple parallel and sub­ parallel patterns commonly form sheet or sheet drapes on shelves and slopes, and may locally infill bathymetric depressions. This configuration suggests uniform suspension sedimentation under tranquil conditions. A ponded basin-fill consisting of horizontally-stratified reflectors suggests that deposition was more dynamic and controlled by current activity. A divergent reflection pattern may indicate varying rates of deposition caused by tectonic tilting, or by changing rates of sediment input, or both; or alternatively, differential erosion and sedimentation.
Clinoformal (sloping) patterns are commonly associated with prograding sedimentary systems, which develop when sediment builds-out laterally from source (e.g. deltas). The oblique pattern is generally assumed to represent high-energy conditions, with some combination of relatively high sediment supply, little to no basinal subsidence, and a stillstand of sea level resulting in rapid basin infill and sedimentary bypass of the upper depositional surface. In contrast, the sigmoid (curved) pattern is interpreted to reflect a lower-energy regime, with a relatively low sediment supply, relatively rapid basin subsidence, and/or rapid rise in sea level, which
GENERAL REFLECTOR PATIERNS
Disrupted Convergent! divergent Contorted Chaotic Reflection free
CHANNEL -FILL PATIERNS
PROGRADING CLiNOFORMAL PATIERNS
Sigmoid Oblique parallel
Shingled
Figure 8. Examples of reflection configuration patterns commonly observed on seismic profiles (modified from Mitchum et al. [l977a] and Berryhill [1986]. This is not a complete representation, and names and types of configurat ion should be modified to meet particular needs if necessary.
result in an aggradational topset succession. The sigmoid pattern may, therefore, be associated with predominantly argillaceous sediments. A shingled progradation occurs when low-angled dipping reflectors are constrained between two bounding reflectors with gentler dip, and is
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common in shallow-water environments. Clinoformal configurations are also common in channel-fills, reflecting the often multi-phase (channel-in-channel) nature of the infill. The reflection patterns may be further described by the use of modifying terms such as even, uneven,
wavy, contorted, hummocky/ lenticular, disrupted, convergent/divergent and contorted, which are self-explanatory.
Chaotic reflections. These patterns obviously suggest a chaotic arrangement of reflectors, and may occur in a variety of settings including diamicton-dominated sequences on shelves, mass-flow deposits on slopes, and channel fills of submarine fans. In all cases, the nature of the structures will be apparent from the geometry of the surrounding reflectors.
Reflection-free configuration. This pattern is generally assumed to indicate a uniform lithology, such as massive marine muds. However, it may also characterise mass-flow deposits which, although often poorly sorted, have been homogenised texturally during the reworking process.
External Geometry
The external form and areal association of seismic facies units provides information on gross depositional environments, sediment source and geological setting. The range of three­ dimensional shapes that may characterise individual units or sequences includes sheet, sheet drape, wedge, bank, lens, mound, fan, channel fill, slope-front fill and basin fill [Mitchum et al., 1977a]. The identification of any of these shapes can only be established from a two-dimensional grid of seismic profiles which allows the geometry of the sequence to be built­ up in a quasi-three-dimensional manner.
Seismic Stratigraphy: A Brief Review
The continuous section of the subsurface revealed by a seismic profile can be analysed for
Toplap
Onlap
Downlap Downlap sudace
Figure 9. Diagram showing types of reflection tennination patterns commonly used to differentiate between depositional sequences; discontinuities are underlined.
stratigraphical purposes [Payton, 1977]. This is generally achieved by grouping reflection patterns into packages of relatively conformable or concordant seismic reflections, which are bounded by unconformities or correlative conformities. These packages are called depositional sequences (Fig. 9) and form the basic building block in the construction of the seismic stratigraphy of an area [Mitchum et al., 1977 (b)].
On seismic profiles, discordance of strata is the main criterion used in the determination of sequence boundaries. Discordance is indicated by reflection terminations, which further indicate whether an unconformity results from non­ deposition or erosion. Onlap, downlap and top lap (Fig. 9) are characteristic of non­ deposition although minor erosion may be associated with the latter. Toplap terminations can often be traced downdip along reflectors into downlap. These styles of termination generally
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represent the depositional limit of a stratum. In contrast, truncation indicates an erosional hiatus short of its original depositional limit; differentiating it from top lap depends on the recognition of an irregular erosion surface.
As the main exponent of this technique has been the hydrocarbon industry in the exploration of petroliferous basins, the scale and resolution of the stratigraphical units (equivalent to groups and supergroups) established from the analysis of deep-seismic data is several orders of magnitude below that applicable to the Quaternary. The differentiation of depositional sequences within Mesozoic-Cenozoic sedimentary basins has largely been related to depositional cycles associated with second (10-80 my) and third (1- 10 my) order changes in sea level [Vail et al., 1977]. During the Quaternary, and particularly the mid- to late Quaternary, cycles of sea-level change were significantly shorter. The result is a detailed seismic representation of an already
detailed stratigraphic record, with the assignation of higher-order sequences [cf., Fulthorpe, 1991] applicable to the Quaternary section. Nevertheless, the technique of identifying bounding disconformities and the grouping of reflections into packages is independent of the scale of the analysis.
Application to Quaternary studies. It is in the more detailed realm of facies analysis (using the parameters described above) that seismic stratigraphy will probably have its major impact in Quaternary studies, as it is this scale that is most appropriate for Quaternary problems. This approach may be enhanced by the growth of the new sequence-stratigraphical model [Wilgus et al., 1988] which, although owing its origin to seismic stratigraphy, provides a higher-resolution interpretation. In addition to analysing the geometry of stratal packages, detailed facies analysis and an understanding of the processes operative during different phases of a cycle of relative sea-level change are used to develop a process-orientated framework, within which the complex record of glaciation can be evaluated. The model is hierarchical and, to some extent, independent of time or physical scale. As stratal units range in thickness from millimetres to kilometres, they may be recognised from seismic profiles, well logs or surface outcrops [Wilgus et aI., 1988; Van Wagoner et al., 1990].
Interpretational procedure. A number of steps are generally involved in the interpretation of shallow-seismic profiles, and include the following: (a) recognition and correlation of seismic sequences and facies; (b) interpretation of the seismic facies and depositional systems; (c) construction of a chronostratigraphical correlation chart; (d) integration of groundtruth (and outcrop) information; and (e) dating the
seismic sequences and mapping the lithology and depositional environment.
GEOLOGY AND SIDESCAN-SONAR INTERPRETATION
Superficially, sidescan-sonar images look like photographs, and in areas of high relief the image may appear as a near-analogue of an oblique aerial photograph. However, sonographs suffer from geometric and pictorial distortion which must be considered during any interpretation. Geometrically, the two most obvious are 'slant­ range distortion' and 'anamorphic distortion'. The former is a result of the sonar measuring differences in travel time along the slanting raypath from the transducer to the sea bed, whereas the interpreter is concerned with horizontal range [McQuillin and Ardus, 1977]. Anamorphic distortion is caused by the problem of maintaining a constant speed over the ground due to weather and currents. Pictorial distortions reflect the vagaries of acoustic propagation in an inhomogeneous ocean, which give rise to shadow zones, and, in rough weather, the problem of roll and yaw tend to destabilise the transducer array.
A sonograph consists basically of a sheet of paper marked by shades of varying intensity and resolution. Features with sharp outlines alternate with vaguely-defined areas in which subtle changes of tone may occur. To interpret these various appearances correctly one must be aware of the factors that can· cause changes in tone or intensity on the recording paper. On sonographs produced by high-resolution sidescan sonars, the stronger the returning signal (backscatter) is, the darker will be the mark on the paper (Fig. 7). There are two main sources that may cause darkening of the recording paper. One is purely
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electronical, caused by manipulation of the control settings on the recorder. The second main source is the incoming signal, of which two types must be distinguished. One type is caused by topographic features: slopes facing the transducer are better reflectors than surfaces lying oblique to the sound beam and will consequently plot darker. The second type is caused by sea-bed texture: the reflectivity of the various materials on the sea bed. Rock and gravel are better reflectors than sand and will therefore appear darker. Sand, in turn, is a better reflector than mud. As the sea bed consists of an infinite variety of combinations of mud, sand and gravel, such changes in grain size may be gradual and therefore difficult to define. However, sandy and gravelly areas are seldom smooth and sea-bed features such as sand waves and ripples, gravel waves, and furrows and ridges in gravel beds contrast markedly with areas of mud which are commonly flat and featureless. Large objects, such as boulders, rock pinnacles, ridges and sand waves, are not only good reflectors but also produce an acoustic shadow-zone behind them where nothing is recorded thus leaving white patches on the paper.
It should be noted that on GLORIA sonographs, the tonal contrasts are the reverse of those described above. Dark tones are associated with low backscatter levels, such as those indicative of muds, whereas lighter tones denote coarser-grained material and rock outcrop. This simply reflects the local convention that has always been used by the geophysicists at the Institute of Oceanographic Sciences (lOS), in the UK, where GLORIA was developed (N H Kenyon, personal communication, 1996). Initially, the lOS geophysicists believed that GLORIA was only good for mapping relief
features, and as shadows were black, strong backscatter should be white. This tonal display is particularly effective in studies of the mid-ocean ridges which have been extensively studied using GLORIA since 1971. Consequently, this convention has been adhered to ever since by the lOS. A further point to be aware of in the interpretation of GLORIA data, is that some degree of sub-sea-bed penetration (up to several metres) may be achieved by the acoustic signal. This means that the image that is displayed on the record may not always represent the actual sea­ bed surface, but some reflective, sub-sea bed horizon.
Sidescan-sonar imagery is particularly useful on glaciated continental shelves, where features such as iceberg furrows, morainic ridges and glacially-eroded surfaces [e.g. Barnes, this volume; Barnes and Reimnitz, this volume; Harris and O'Brien, this volume; Josenhans, this volume; Pudsey et al., this volume; Solheim, this volume; Solheim and Elverhoi, this volume; Whittington et al., this volume] are well-imaged on sonographs due to the combination of topographic and textural heterogeneity preserved on the sea bed. To further guide the interpretation of the sonograph, the best available bathymetric data should be used. High-resolution definition of sea-floor relief and texture can now be obtained by the combined use of sidescan sonar with multibeam bathymetric swath mapping. The swath-sounding technique uses an array