Contourites

By Elsevier Science

Contourites are sediments deposited or substantially reworked by bottom currents. The study of contourites is crucial for several fields of fundamental and applied research: paleoclimatology and paleo-oceanography, since these fairly continuous and relatively high-resolution sediments hold the key for priceless information on the variability in circulation patter, current velocity, oceanographic history and basin interconnectivity; hydrocarbon exploration, since accumulation of source rocks may be favored by weak bottom currents, whereas "clean" deep-sea sands may be formed by robust flows; and slope stability, since low-permeability fine-grained contourites facilitate the formation of overpressurized gliding planes when fresh contourites with a high pore-water content becomes rapidly loaded, or when their rigid biosiliceous microfabric collapses due to diagenetic conditions.
Despite its significance, this group of sediments is poorly known by the majority of non-specialists. Notwithstanding the growing interest and the intensified research in contourites, a textbook that might also serve as a reference book on contourites was missing until now. This book addresses all aspects of the knowledge in the field of contourites and provides an authoritative and comprehensive coverage of the subject. It also can serve as a standard reference work for non-specialists, and in particular postgraduate students, university teachers and lecturers, researchers and professionals who are seeking an authoritative source of information about contourites.

* reviews both theoretical topics and case histories
* provides practical advice on multidisciplinary research techniques
* provides also nonspecialist users with an intuitively-accessible, cross-referenced, and comprehensive coverage of the knowledge in the field.
* provides a helpful tool for research in the preparation of classroom lectures

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 25, 2008
• ISBN: 9780080931869

Book preview

Developments in Sedimentology

Contourites

M. Rebesco

A. Camerlenghi

ISSN  0070-4571

Volume 60 • Number Suppl (C) • 2008

Table of Contents

Cover image

Title page

Copyright Page

Preface

List of Contributors

Chapter 1: Contourite Research

1.1 Bottom Currents

1.2 Contourites

1.3 Drifts

1.4 Sedimentary Structures

1.5 Prospects

Chapter 2: Personal Reminiscences on the History of Contourites

2.1 Introduction

2.2 Not All Deep-Sea Sands are Turbidites

2.3 Turbidophiles and Turbidophobes

2.4 Erosional Unconformity Misinterpreted

Chapter 3: Methods for Contourite Research

3.1 Introduction

3.2 Oceanographic Measurements

3.3 Geophysical Methods

3.4 Sampling Strategies

3.5 Analytical Methods

3.6 Onshore Studies of Ancient Sequences

3.7 Summary of Multidisciplinary Techniques

Chapter 4: Abyssal and Contour Currents

4.1 Introduction

4.2 Abyssal Currents in the Global Thermohaline Circulation

4.3 Contour Currents

4.4 Conclusion and Summary

Chapter 5: Deep-water Bottom Currents and their Deposits

5.1 Introduction

5.2 Thermohaline-Induced Geostrophic Bottom Currents

5.3 Wind-Driven Bottom Currents: The Loop Current

5.4 Deep-Water Tidal Bottom Currents

5.5 Internal Waves and Tides (Baroclinic Currents)

5.6 Conclusions

Chapter 6: Dynamics of the Bottom Boundary Layer

6.1 Introduction

6.2 Spectral Windows

6.3 Characteristics of the Bottom Boundary Layer

6.4 Analytical Approach of the BBL

6.5 Conclusions

Chapter 7: Sediment Entrainment

7.1 Introduction

7.2 Benthic Storms (Transportation and Resuspension of Particles)

7.3 Gravity Flows

7.4 Bioclastics-Forming Processes in Contourites

7.5 Volcaniclastics

7.6 Glacial Activity

7.7 Resuspension of Particles by Burrowing Activity of Benthic Organisms

7.8 Concluding Remarks

Chapter 8: Size Sorting During Transport and Deposition of Fine Sediments

8.1 Introduction

8.2 Size Analysis of Fine Sediments

8.3 Unsorted Delivery to Deep Current Systems: Pelagic Flux and Down-Slope Transport

8.4 Controlling Factors for Input and Transport

8.5 Sorting in Suspension Transport

8.6 Processes of Deposition from Turbulent Boundary Layers

8.7 Deposits from Currents

8.8 Some Examples of Palaeoflow Inferred from Sortable-Silt records

Acknowledgements

Chapter 9: The Nature of Contourite Deposition

9.1 Introduction

9.2 Bottom-Current Characteristics

9.3 Sedimentation rates and budget

9.4 Drift Deposition and Erosion

9.5 Bottom-Current Bedforms

9.6 Contourite Facies and Features

9.7 Contourite Cyclicity

9.8 Summary

Chapter 10: Traction Structures in Contourites

10.1 Introduction

10.2 The Traction-Structures Controversy

10.3 Setting the Stage for Sedimentary Structures in Contourites

10.4 Sedimentary Structures

10.5 Sedimentary Structures in Facies Models

10.6 Conclusions

Chapter 11: Bioturbation and Biogenic Sedimentary Structures in Contourites

11.1 Introduction

11.2 Effects of Contour Currents on Benthic Habitats

11.3 Examples of Bioturbation in Contourites

11.4 Discussion and Conclusions

11.5 Perspective

Acknowledgements

Chapter 12: Some Aspects of Diagenesis in Contourites

12.1 Introduction

12.2 Some Examples of Sedimentary Rhythms Associated with Changes in Colour

12.3 Diagenesis of Fine-Grained Contourites

12.4 Diagenesis of Coarse-Grained Contourites

Acknowledgements

Chapter 13: Contourite Facies and the Facies Model

13.1 Introduction

13.2 Historical Context

13.3 The Range of Contourite Facies

13.4 Contourite Facies Model and the Contourite Sequence

13.5 Lamination Versus Bioturbation in Contourites

13.6 Contourite-Related Facies

Acknowledgements

Appendix 1 Details of Contourite Facies Recovered from Modern Drift Systems Using Conventional Coring Techniques as Published Over the Past 15 Years

Chapter 14: Contourite Drifts

14.1 Introduction

14.2 Drift Distribution and Characteristics

14.3 Growth History of the Blake Outer Ridge Drift System

14.4 Factors Controlling Drift Location, Morphology and Depositional Pattern

14.5 Contourite-Drift Types

14.6 Discussion

Acknowledgements

Chapter 15: Sediment Waves and Bedforms

15.1 Introduction

15.2 Location, Morphology and Genesis of Fine-Grained Sediment Waves

15.3 Location, Morphology and Genesis of Coarse-Grained Sediment Waves

15.4 Related Large-Scale Features Generated by Bottom Currents

15.5 Applications to Bottom-Current Reconstruction: A Case Study From the NW UK

Chapter 16: Seismic Expression of Contourite Depositional Systems

16.1 Introduction

16.2 Seismic Identification and Characteristics of Contourites

16.3 Seismic Methods and Interpretation Concepts in Contourite Studies

16.4 Summary

Chapter 17: Identification of Ancient Contourites: Problems and Palaeoceanographic Significance

17.1 Introduction

17.2 Examples of Fossil Contourites

17.3 Discussion

Chapter 18: Abyssal Plain Contourites

18.1 Introduction

18.2 Terminology

18.3 Case Studies of Abyssal Contourites

18.4 Oceanic Gateways

18.5 Principal Oceanographic and Sedimentary Processes

18.6 Main Characteristics of Abyssal Plain Contourites

18.7 Final Considerations

Chapter 22: High-Latitude Contourites

22.1 Introduction

22.2 Neogene and Quaternary Changes in Deep-Water Circulation of the Northern North Atlantic and in the Southern Ocean

22.3 Northeast Atlantic Margin Contourites

22.4 Antarctic Margin Contourites

22.5 Conclusions

Chapter 23: Economic Relevance of Contourites

23.1 Introduction

23.2 Major Implications of Contourite Studies

23.3 Contourites and Petroleum Exploration

23.4 Discussion

Chapter 24: Palaeoceanographic Significance of Contourite Drifts

24.1 Introduction

24.2 Oceanographic Settings of Contourites

24.3 Methods and Approaches

24.4 Palaeoceanographic Themes Addressed by Contourite Research

24.5 Summary

Chapter 25: The Significance of Contourites for Submarine Slope Stability

25.1 Introduction and Objective

25.2 Background

25.3 Case Studies of Contourites and Submarine Slope Stability

25.4 Discussion

25.5 Summary

Acknowledgements

References

Index

Color Plates

Copyright Page

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Preface

Michele Rebesco, Angelo Camerlenghi

Contourites are sediments deposited or substantially reworked by bottom currents. The study of contourites is nowadays crucial for several fields of fundamental and applied research:

1. palaeoclimatology and palaeoceanography, since these fairly continuous and relatively high-resolution sediments hold the key for priceless information on the variability in ocean circulation patterns, current velocities, oceanographic history and basin interconnectivity;

2. hydrocarbon exploration, since accumulation of source rocks may be favoured by weak bottom currents, whereas clean deep-sea sands may be formed by robust flows;

3. slope stability, since fine-grained, low-permeability, high pore-water content contourites facilitate the formation of overpressurized gliding planes when rapidly loaded, or when their rigid biosiliceous microfabric collapses due to diagenetic alteration.

Despite its significance, this group of sediments is poorly known by the majority of non-specialists. Notwithstanding the growing interest and the now intensive research in contourites, a textbook that serves as a reference book on contourites was missing until now.

This book addresses all aspects of the knowledge in the field of contourites and provides a comprehensive and cross-referenced coverage of the subject. It can serve as a standard reference work for non-specialists, and in particular postgraduate students, university teachers and lecturers, researchers and professionals who are seeking an authoritative source of information about contourites. It reviews both theoretical topics and case histories, and it points to approaches that may help tackle open problems. Divided into a wide and interdisciplinary spectrum of topical sections, it provides practical advice on multidisciplinary research techniques. The ample use of illustrations, diagrams, photographs and maps, complemented by a CD-ROM including all illustrations (as many as possible in colour), provides a helpful tool for researchers in the preparation of classroom lectures and training courses, journal articles and meeting presentations.

The figures presented in this book are partly new, partly adapted from previous works, and partly reproduced from previous works. Wherever appropriate, permission has been obtained for the reproduction or adaptation, but not all copyright holders could be traced. We would appreciate if such copyright holders would be so kind as to contact us.

The authors invited to contribute to this book are distinguished specialists in the field. They have been invited not simply to produce a research paper on the topic, but to critically review the current state of knowledge. They have been asked to accomplish their review in a clear and concise way, including as much factual information as possible. They have been encouraged to submit as many tables and illustrations (diagrams, photographs and maps) as are necessary to complement the text of their chapter to present the information in the best possible way. The authors have been recommended to keep the language simple, and those whose native language is not English have been encouraged to choose co-authors whose mother tongue is English and/or to propose native English reviewers for their manuscripts.

The editors and the authors are grateful to the reviewers who contributed to improve the scientific quality of each chapter and to the book as a whole: A. Arche, C. Bjerrum, A. Camerlenghi, A. Cattaneo, X.D. de Madron, G. Evans, F. Eynaud, J.-C. Faugères, M. Frenz, Z. Gao, M. Gardner, J. Jones. D.W. Kirkland, P. Knutz, J.S. Laberg, R. Larter, E. Llave, D. Long, J. Lo´pez-Go´mez, L. Lo¨wemark, L. Masse´, T. Mulder, H. Nelson, M. Rebesco, C. Silva, P. Talling, D. Thornalley, R. Urgeles, A.J. van Loon, M. Vanneste, A. Wiewio´ra.

List of Contributors

A. Camerlenghi

ICREA, c/o Universitat de Barcelona, Departament d’Estratigrafia, Paleontologia i Geociències Marines, GRC Geociències Marines, C/Martí i Franquès, s/n,

E-08028 Barcelona, Spain.

[e-mail]

B. Chacón

Fachbereich Geowissenschaften, Universität Bremen, D-28334 Bremen, Germany.

[e-mail]

A. Crise

Istituto Nazionale di Oceanografia e Geofisica Sperimentale, B.go Grotta Gigante

42/c, I-34010 Sgonico (TS), Italy.

[e-mail]

T. Duan

Marathon Oil Company, Houston, Texas, USA.

[e-mail]

J.-C. Faugères

Departement de Géologie et Océanographie, Université Bordeaux1, UMR CNRS 5805 EPOC, Avenue des Facultés, F-33405 Talence cedex, France.

[e-mail]

M.A. Fregenal-Martínez

Department of Estratigrafía Facultad de Ciencias Geológicas, Universidad

Complutense, E-28040 Madrid, Spain.

[e-mail]

Z. Gao

School of Geosciences, Yangtze University, Jingzhou, Hubei province, 434023, China.

[e-mail]

P. Giresse

Laboratoire d’Études des Géo-Environnements Marins, Université de Perpignan,

52, Avenue Paul Alduy, F-66860 Perpignan, France.

[e-mail]

E. Gonthier

Département de Géologie et Océanographie, Université Bordeaux1, UMR CNRS 5805 EPOC, Avenue des Facultés, F-33405 Talence cedex, France.

[e-mail]

Y. He

School of Geosciences, Yangtze University, Jingzhou, Hubei Province, 434023, China.

[e-mail]

F.J. Hernández-Molina

Facultad de Ciencias del Mar, Universidad de Vigo, E-36200 Vigo, Spain.

[e-mail]

J.A. Howe

Scottish Association for Marine Science & UHI Millennium Institute-Dunstaffnage Marine Laboratory, Oban, Argyll, Scotland, PA37 1QA, UK.

[e-mail]

K.J. Hsü

Oakcombe, Marley Common Haslemere, Surrey, GU27 3PT, UK.

[e-mail]

H. Hüneke

Institute of Geography and Geology, University of Greifswald, D-17487

Greifswald, Germany.

[e-mail]

S. Hunter

National Oceanography Centre, Southampton (NOCS), Waterfront Campus,

Southampton, SO14 3ZH, UK.

[e-mail]

P.C. Knutz

Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350

Copenhagen, Denmark.

[e-mail]

A. Kuijpers

Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350

Copenhagen, Denmark.

[e-mail]

J.S. Laberg

Department of Geology, University of Tromsø, N-9037 Tromsø, Norway.

[e-mail]

E. Llave

Instituto Geológico y Minero de España, Ríos Rosas, 23, E-28003 Madrid, Spain.

[e-mail]

A. Maldonado

Instituto Andaluz de Ciencias de la Tierra. C.S.I.C./Universidad de Granada,

Campus de Fuentenueva, s/n, E-18002 Granada, Spain.

[e-mail]

J. Martín-Chivelet

Department of Estratigrafía Facultad de Ciencias Geológicas, Universidad

Complutense, E-28040 Madrid, Spain.

[e-mail]

D.G. Masson

National Oceanography Centre, Southampton (NOCS), Waterfront Campus,

Southampton, SO14 3ZH, UK.

[e-mail]

I.N. McCave

Department of Earth Sciences, University of Cambridge, Downing Street,

Cambridge, CB2 3EQ, UK.

[e-mail]

T. Mulder

Département de Géologie et Océanographie, Université Bordeaux1, UMR CNRS 5805 EPOC, Avenue des Facultés, F-33405 Talence cedex, France.

[e-mail]

T. Nielsen

Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350

Copenhagen, Denmark.

[e-mail]

M. Rebesco

Istituto Nazionale di Oceanografia e Geofisica Sperimentale (OGS), Borgo Grotta

Gigante 42/C, I-34010 Sgonico (TS), Italy.

[e-mail]

S. Salon

Istituto Nazionale di Oceanografia e Geofisica Sperimentale, B.go Grotta Gigante

42/c, I-34010 Sgonico (TS), Italy.

[e-mail]

G. Shanmugam

Department of Earth and Environmental Sciences, The University of Texas at

Arlington, Box 19049 – Arlington, TX 76019 USA.

[e-mail]

M.S. Stoker

British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9

3LA Scotland, UK.

[e-mail]

D.A.V. Stow

National Oceanography Centre, Southampton (NOCS), Waterfront Campus,

Southampton, SO14 3ZH, UK.

[e-mail]

F. Trincardi

ISMAR-CNR, Via Gobetti 101, I-40129, Bologna, Italy.

[e-mail]

A.J. Van Loon

Geological Institute, Adam Mickiewicz University, Maków Polnych 16, 61-606

Poznan, Poland.

[e-mail]

T. van Weering

Royal Netherlands Institute for Sea Research (NIOZ), P.O.Box 59, Texel, 1790 AB Den Burg, and Department of Paleoclimatology and Geomorphology, Free University, de Boelelaan 1085, 1081 HV Amsterdam, The Netherlands.

[e-mail]

G. Verdicchio

ISMAR-CNR, Via Gobetti 101, I-40129, Bologna, Italy and EDISON SpA, Foro

Buonaparte 31, I-20121, Milano, Italy.

[e-mail]

A.R. Viana

Petrobras, Research Center, R&D Exploration, Rio de Janeiro, Brazil.

[e-mail]

F. Werner

Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40-60, D-24118

Kiel, Germany.

[e-mail]

A. Wetzel

Geologisch-Paläontologisches Institut, Universität Basel, Bernoullistrasse 32,

CH-4056 Basel, Switzerland.

[e-mail]

D. Wilkinson

National Oceanography Centre, Southampton (NOCS), Waterfront Campus,

Southampton, SO14 3ZH, UK.

R.B. Wynn

National Oceanography Centre, Southampton (NOCS), Waterfront Campus,

Southampton, SO14 3ZH, UK.

[e-mail]

W. Zenk

Leibniz Institute of Marine Sciences at the University of Kiel (IFM-GEOMAR), Düsternbrooker Weg 20, D-24105 Kiel Kiel Germany.

[e-mail]

Chapter 1 Contourite Research

A Field in Full Development

M. Rebesco¹, A. Camerlenghi², A.J. Van Loon³,

---

¹ Istituto Nazionale di Oceanografia e Geofisica Sperimentale (OGS), Sgonico (TS), Italy

² ICREA, c/o Universitat de Barcelona, Departament d’Estratigrafia, Paleontologia i Geociències Marines, Barcelona, Spain

³ Geological Institute, Adam Mickiewicz University, Poznan, Poland

Abstract

Publisher Summary

This chapter focuses on the role and future development of contourites. The process of contourite deposition involves multi-phase entrainment, long-distance transport, and interaction among depositional processes induced by various types of bottom current. Contourites are generally not easy to recognize because of a lack of simple and unambiguous diagnostic criteria. Contourites should be identified on the basis of an analysis of the characteristics of their facies and facies associations. Contour currents are important transport and sedimentary phenomena that control much of the deep-sea sedimentation. Bottom currents refer to the water-mass flows that control the deposition of contourites. Drift is a process for sediment transport by deep-contour currents to the end product of the process––the sedimentary deposit. An insight into the role of contourites will certainly benefit from input by new researchers within the fairly restricted circle of the current specialists in the area of research.

Contourites were first recognized and described only 40 years ago when photographs of the deep-sea bottom showed distinct current ripples (Heezen and Hollister, 1964; Hollister, 1967); much progress has been made since, but the current status in this field shows clearly that this relatively new research topic is still in full development: a surge of new research results (reflected in, for instance, the publication of several special volumes of scientific journals) go still hand in hand with uncertainties and with insufficient knowledge (there is, for instance, a lack of indisputable diagnostic criteria). New techniques are continuously developed and implemented (e.g., swath bathymetry and 3-D seismics). All this indicates a great potential for further research, in the same way as the now mature research on turbidites developed in the 1960s. Comparing the research on turbidites with that on contourites, it seems that the latter is presently facing a transition from adolescence to maturity.

One might consider such a period of transition not to be the right moment to compile a reference book, which is aimed at providing both experts in the field a state-of-the-art overview, and the non-specialist readers an introduction that may help starting research in this field. However, we consider it just crucial that this transitional stage of contourite research is documented. It is the right moment to increase the recognition of contour currents as important transport and sedimentary phenomena that control much of the deep-sea sedimentation; in addition, this seems the right moment to bring new forces into play.

The aim of this book is therefore not to present an inventory of the fossilized knowledge of a fully developed field, but rather to provide a basis for future development, a platform for ample exchange of complementary expertise, ideas and experience. The insight into the role of contourites will certainly benefit from input by new researchers within the – so far – fairly restricted circle of the current specialists in this area of research.

A good understanding of contourites and of the sedimentary processes behind them is often complicated by a lack of clear, unambiguous and commonly accepted diagnostic criteria, and by discussions about conflicting views about the sedimentary structures. The different views that still exist are particularly confusing for non-specialists and students, who may easily become disoriented by such complex situations. With this book, and with this introduction in particular, we want to offer to non-specialist readers a simplified, often descriptive, overview of contourites. We are aware of the risk implied in any simplifications. For this reason, we neither hide the problems nor provide oversimplified explanations. We asked the authors of the various chapters to accomplish in a clear and concise way a critical review of the current state of knowledge regarding the subject of their chapter. This implies that the book does not contain contributions reflecting new research. Nor is it our intention to present, in this introduction, new syntheses or untested new hypotheses. In contrast, we provide a kind of glossary that may help the reader to find his way, with special reference to key terms. These terms include the generator (bottom currents), the sediments (contourites), the types of accumulation (drifts) and the types of sedimentary structures (traction, bioturbation).

A comprehensive text on contourites is, in our opinion, justified also because of the incomplete coverage of the subject by reference books, to which the non-specialist readers are referred before entering the details of specific journal articles: even the Encyclopedia of Sediments and Sedimentary Rocks (Middleton et al., 2003) does not include Contourites among its entries! The Glossary of Geology (Bates and Jackson, 1987) lists 43 entries for drift, of which the only one referring specifically to sedimentology lists wind or river currents as main generators of sedimentary drifts.

1.1 Bottom Currents

Bottom current is our preferred term to refer to the water-mass flows that control the deposition of contourites. Many other terms are often used, for which reason we need to clarify the meaning.

Among geologists, the most adhered to definition of contourites is that they are sediments deposited or substantially reworked by the persistent action of bottom currents (e.g., Stow et al., 2002c; Rebesco, 2005). However, the term bottom currents is not usually employed by physical oceanographers. It is obvious that the time span of the processes involved in most geological processes is quite different from that analysed by physical oceanographers. In fact, the semi-quantitative inferences derived by geologists from the impact of ocean currents on the sea-floor sediments represent an integral over a poorly defined time interval.

A detailed discussion about bottom currents from the point of view of physical oceanographers is given by Zenk (2008). However, we state as a general simplification that any persistent water current near the seafloor may be called a bottom current. Such a current has the capability to affect the sea floor by re-suspending, transporting and/or controlling the deposition of sediments. This type of current is influenced by a number of processes (tides, internal waves, barotropic waves, dynamic instabilities) that modulate the speed and the instantaneous direction. Though typically affecting the sea floor, episodic flows that are not in equilibrium conditions do not belong to this category. Bottom currents do therefore not include turbidity currents, which are – in contrast to contour currents – rapid, dense (loaded with suspended sedimentary particles) currents driven by gravity, which fail to develop equilibrium conditions, even if they perhaps have more impact on the sea floor than bottom currents.

For the same reason, the specific type of thermohaline circulation (THC) called submarine overflows (e.g., Denmark Strait Overflow Water) or cascading currents, that are generated by transients in water-mass distribution in response to the local effect of evaporation, cooling or freezing in the surface layer over the continental shelf (Shapiro et al., 2003; Ivanov et al., 2004), should not be strictly considered as a bottom current. These currents are produced by the sinking – in relatively confined flow pipes – of important volumes of sea water, typically with high speed and intermittent flow; they occur every few years. The flow is controlled by morphological elements such as sills, submarine canyons and narrow straits. They are included among the engines of the THC at a basin-wide scale because the intermittent flow is – in the long term – compensated by the persistence of the process over very long time intervals, often coinciding with long-term climatic cycles. Known as purely physical oceanographic processes, these currents are now known to have an effect on the sea-floor morphology and to be able to transport important quantities of sediment in suspension (Canals et al., 2006; Trincardi et al., 2007). Future work should address the sedimentological importance of these currents. Shallow-water motion produced by surface waves, storms and tides (which are intermittent and not in equilibrium conditions), does not typically develop long-lasting bottom currents, though they affect the sea floor of continental shelves.

Nevertheless, bottom currents can be affected by a number of distinct forces acting at different water depths. The term bottom current should be considered as a generic term that embraces different types of current. For clarity, we list here the (sometimes coexisting) types of current included within this term:

• wind-driven currents, which originate by horizontal movement of the superficial layers due to wind shear stress, and subsequent propagation of the motion through the water column down to greater depth;

• thermohaline currents, driven by gravity (uneven density distributions due to variable temperature and salt content of water masses), which are the most common type of bottom current; the predominantly horizontal large-scale transport of a water mass (advection) is called THC or meridional overturning circulation (MOC) in the Atlantic Ocean and determines the so-called oceanic conveyor belt;

• geostrophic currents, which are characterized by long-lasting equilibrium conditions between the horizontal pressure gradient and the Coriolis force. By definition they have zero vertical velocity, which implies that the flow is forced to follow the bathymetric contours (therefore these currents are also named contour currents);

• contour currents, which are currents that have a net flow along-slope, sub-parallel to the topographic isobaths (contours); in spite of that, for certain parts of their path they can also flow upslope, down-slope, around and over topographic obstacles or irregularities;

• boundary currents, which are currents the direction of which is controlled by sea-floor morphology (a wide canyon, continental slopes, but also flanks of major submarine mountain chains); due to the meridional distribution of continents and the Coriolis force, they are typically intensified along the western boundaries of the oceans (western boundary currents);

• abyssal currents (sometimes also indicated as ocean currents), which may be considered synonyms of large-scale bottom currents flowing in the deep sea (below the edge of the continental shelf) where topography and Coriolis force play a major role in determining the pathway of the current.

Two more processes are important in the context of the processes related to contourite formation:

• downwelling, which is the downward transport of cooled water masses from the surface; it occurs in restricted regions, mainly at polar latitudes, resulting in bottom currents; note that downwelling does not necessarily occur at the ocean margin, so it not necessarily affect the sea floor;

• upwelling, which involves – in contrast – deep-water masses, and which produces bottom currents against topographic barriers and continental margins, mainly at low latitudes.

Bottom currents can also be forced by deep-water tidal currents (in submarine canyons), long-wave baroclinic currents (internal waves and tides, solitary waves) and tsunami-related traction currents (Shanmugam, 2008).

In conclusion, most (though not all) bottom currents start as density currents sinking to their equilibrium level (quasi-geostrophic balance). They are predominantly unidirectional subsurface currents in contact with the sea floor. They show a quasi-steady flow, though possibly affected by tides, seasonal changes and/or migrating eddies. They are controlled by topography, and when their direction largely parallels the topography, they are called contour currents.

1.2 Contourites

The term contourite was originally introduced to define the sediments deposited in the deep sea by contour-parallel thermohaline currents. Early pioneering work documented the strong influence of the deep Western Boundary Undercurrent along the continental margin of eastern North America (Heezen and Hollister, 1964; Heezen et al., 1966; Schneider et al., 1967). However, a rigorous restriction to this definition would prevent the application to ancient deposits, where both depth and direction of the currents can rarely be precisely reconstructed.

The definition was consequently widened to embrace a larger spectrum of sediments that are affected to various extent – and in a wide range of water depths – by different types of current. According to a now widely accepted recent definition, contourites are sediments deposited or significantly affected by bottom currents (Stow et al., 2002c; Rebesco, 2005; Stow and Faugères, 2008). Yet, the different types of bottom current are known to influence to a greater or lesser extent many depositional environments. They affect various types of sediment, both during and after deposition. This implies the risk of an excessively wide application of the term contourite, and consequently of a loss of significance.

On the contrary, where the dominant action of a bottom current is ascertained, the presence of other types of sediment is not excluded. Turbidites, for example, occur frequently even where the bottom-current influence is large enough to control the overall geometry of the deposits and to generate a (contourite) sedimentary drift. It is therefore not possible to restrict the use of the term contourites to the sediments contained in a sediment drift. In contrast, it is commonly accepted that contourite sedimentary facies also include sediments, usually interbedded, that are not deposited under the influence of bottom currents. This is especially true where the persistence of bottom-current action has not been enough to determine the geometry of the deposit.

Another way to restrict the use of the term contourites is to set a minimum depth above which contourites cannot reliably be distinguished from shallow-water shelf-current deposits. This water depth is suggested to be around 300 m, according to the definition by Stow et al. (2008), even though this limit should not be applied rigidly. Shallow-water contourites may reflect also other hydrodynamic factors (shelf currents, tides and waves, storms) capable of impinging the sea floor, but the influence of which is still negligible compared to a dominant (though not absolutely steady) bottom current. As discussed in detail by Verdicchio and Trincardi (2008a), the processes involved in the formation of shallow-water contourites are generally more varied and less steady than in the case of deep-water contourite deposits. Shallow-water bottom currents derived from a stable geostrophic circulation can nevertheless form contourite drifts resembling the typical deep-water contourite drifts in morphology, internal geometry and sedimentary facies. Therefore, water depth alone does not seem to be an effective criterion to restrict the use of the term contourites.

As is often stressed in literature, the process of contourite deposition is not a simple one: it often involves multi-phase entrainment, long-distance transport, and interaction among depositional processes induced by the various types of bottom current described above (e.g., He et al., 2008; McCave, 2008; Stow et al., 2008). In addition, contourites are generally not easy to recognize because of the lack of simple, unambiguous diagnostic criteria. A composite triple-stage approach (e.g., Faugères et al., 1999; Rebesco and Stow, 2001; Nielsen et al., 2008) is recommended for the identification of sediments deposited by bottom currents as can be determined beyond reasonable doubt from seismic-reflection data: the analysis must include the overall architecture of the deposit (gross geometry and large-scale depositional units), the internal architecture (structure and sub-units) and seismic attributes and facies in each sub-unit.

When samples are available, the problem of the lack of universally recognized unequivocal diagnostic criteria for the sedimentary characteristics of contourites must be faced. Though most workers suggest that pervasive bioturbation is the most diagnostic criterion (Stow and Faugères, 2008; Wetzel et al., 2008), others suggest a combination of traction structures (Martín-Chivelet et al., 2008; Shanmugam, 2008). Distinctive characteristics that are not diagnostic by themselves may be provided by early diagenesis (Giresse, 2008) and physical properties (Laberg and Camerlenghi, 2008). We therefore suggest that contourites as such should be identified on the basis of analysis of the characteristics of their facies and facies associations (cf. Stow and Faugères, 2008).

Considering the above, we suggest that the well-established term contourite should be used as a generic term, in the same way as, for example, mass-wasting deposits or gravity-flow deposits. These generic terms may be considered family names that simply describe a process that affects a certain kind of sediment: bottom currents in the case of contourites, down-slope mass transport of sediment and water for mass-wasting deposits. These family names include several kinds of sediment that have more specific names (e.g., turbidites, debris-flow deposits or mudflow deposits). In the case of contourites, such specific names do not exist; they should be defined in the time to come on the basis of research aimed at distinguishing between the various types of transport and depositional processes involved.

1.3 Drifts

In sedimentology, drift is a general term used to describe unconsolidated rock debris transported from one place and deposited to another (Bates and Jackson, 1987). Its use is taken from one of the many definitions that can be found in a dictionary: motion or action under external influence. Its use has traditionally been related to sediment transport by river currents, wind and glaciers. It is important to note that the drifting implied by the term does not refer to the deposit, but to the particles that move, transported by a flow, before settling. The first to use the term in association with bottom currents were Heezen and Johnson (1963), who stated that scour and drift of sediments due to current activity are the most reasonable explanation of sediment moats that were identified at that time next to seamounts and other striking morphological structures in the Atlantic and Pacific. The drift of sediment particles by the geostrophic circulation was later recognized as responsible not only for sediment moats, but also for sediment knolls (Heezen et al., 1966) as the drift could focus sedimentation in certain areas of the continental rise.

The use of the term drift then shifted from that of a process for sediment transport by deep contour currents, to the end product of that process: the sedimentary deposit. The Blake–Bahama Outer Ridge was defined by Heezen and Hollister (1971) as a migratory sediment drift built by bottom currents. The sediments deposited by the action of bottom currents, whether or not forming a sediment drift, were termed contourites by the same authors. Because the flow responsible for the drift of particles from one place to another and the resulting sedimentary deposit is implicitly a steady one, mass-flow deposits resulting from surges of density currents were distinguished in principle from contourites since the very beginning (see the early definitions by Hollister and Heezen, 1971, p. 421).

Bottom currents are capable of building thick and extensive accumulations of sediments not only on the continental rise, but also elsewhere along continental margins, from the abyssal floor to the outer shelf, if a significant sediment input is available (Faugères and Stow, 2008). These sediment bodies have received various names, including outer ridges, sedimentary ridges, sedimentary mounds, sediment drifts and contourite drifts. In our view, all these terms are synonyms, referring to accumulations of sediment deposited – or significantly affected – by bottom currents. Strictly speaking, the term contourite drifts should be specifically used for sediment accumulations deposited by currents flowing along the contours. However, we highlighted already above that different types of bottom current exist, and that even the so-called contour currents do not always follow the contours. An additional complication is provided by sedimentary accumulations for which the current direction is inferred only indirectly. We hence think that the term contourite drifts should be used for sediment accumulations deposited by bottom currents in general. Further, because of the presence of contourite in this term, we think that this term is to be preferred.

In many cases, especially where mixed turbidite/contourite systems are involved, terms that not belong to the contourite terminology in a strict sense (e.g., mounds, levees, fans, lobes, channels) are used in the literature for deposits that have been significantly affected by the interaction with bottom currents. We recommend for such situations that these terms be preceded by the prefix contourite (e.g., contourite levees).

Not only depositional structures are produced by bottom currents: there are a number of erosional and non-depositional structures. The classification and precise terminology of these structures that are related to bottom currents have not been fully developed yet; only some attempts have been made (Hernández-Molina et al., 2008a, b).

1.4 Sedimentary Structures

What are the diagnostic sedimentary structures of contourites? The scientific community has not come yet to complete agreement. There are two viewpoints, with contrasting ways of reasoning: one is in favour of traction structures, whereas the other one is in favour of bioturbation.

The initial finding in the 1950s of current ripples and other traction structures in deep-marine deposits affected by bottom currents (Hollister, 1967; Hollister and Heezen, 1967, 1972;Bouma, 1972a, b, 1973; Bouma and Hollister, 1973) suggested that contourites show lamination as a result of fluid-flow processes and depositional sorting mechanisms. Distinctly laminated deposits are still interpreted as contourites by some authors (see Hüneke and Stow, 2008; Martín-Chivelet, 2008; Shanmugam, 2008). These authors stress that traction structures are abundant on modern ocean floors that are influenced by bottom currents; they also interpret ancient coarse-grained deposits in some large outcrops as contourites. They wonder whether the general absence of such structures in the case studies based on sedimentary cores may result from a bias imposed by the impossibility of larger-scale observations. They also suggest that bioturbation in contourites should not be considered a diagnostic criterion, since bioturbated mud is equally abundant in areas unaffected by bottom currents and since turbidites may be extensively bioturbated as well.

According to most contourite workers (e.g., Stow and Faugères, 2008), cross-lamination is only rarely described from modern contourites, whereas extensive bioturbation is generally dominant (Wetzel et al., 2008). Their observations are derived from a huge number of data – published over the past 15 years – from modern drift systems using conventional coring techniques. They suggest that the lack of clear lamination in contourites is due to several reasons, including the commonly low current velocity that is insufficient to produce primary lamination, the relatively low accumulation rate allowing bioturbation to destroy primary lamination, and the small sediment input that is insufficient to allow a sorting mechanism to develop lamination.

The controversy regarding diagnostic criteria (traction structures versus bioturbation) for the identification of contourites is still a problem. It was not possible to provide here a solution, considering the present state of the knowledge. We therefore purposely took care that both views are adequately expressed in this volume by inviting authors belonging to the two schools. In our opinion, they honestly offered their own evidence, and we hope that the comprehensive scenario provided by this scientific, non-personal confrontation will provide a sound basis for a new step forward in the ongoing struggle to clarify this controversial issue.

1.5 Prospects

More research is obviously needed to define a universally acceptable set of diagnostic criteria for contourites. Such research should be aimed at obtaining a much deeper insight into the processes involved. Simultaneously, the existing terminology for all aspects related to contour currents (deposits, processes, morphology, etc.) needs more consistency and logic, and new terminology has to be developed for aspects for which no real terminology exists as yet. This is a prerequisite for progress, because more precise terms and a stricter application of the terminology are needed for unambiguous interpretation of descriptions, as well as for better comprehension of the numerous processes involved. Improved terminology is, as mentioned before, particularly important to distinguish between the various types of contourite, and for classification of erosional structures that are related to bottom currents.

The growing attention for these sediments – because they can be hydrocarbon seal rock and reservoirs, because they form a significant part of the palaeoceanograpic record, and because they form a source for potentially hazardous submarine landsides – will hopefully give rise to an ever increasing amount of high-quality data sets that will improve the understanding of these sediments, especially on the basis of their sedimentology and their seismic characteristics. An intensified access to industrial-quality hydrocarbon-exploration data in particular may play an essential role in an increased understanding of contourites, just like was the case in the 1960s for turbidites.

References

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R.L. Bates, J.A. Jackson. Glossary of geology, 3rd ed. Alexandria, Va: Am. Geol. Inst.; 1987. 788 pp

A.H. Bouma. Contourites in Niessenflysch, Switzerland. Eclogae Geol. Helv.. 1973;66:315–323.

A.H. Bouma. Recent and ancient turbidites and contourites. Trans. Gulf Coast Assoc. Geol. Soc.. 1972;22:205–221.

A.H. Bouma, C.D. Hollister. Deep ocean basin sedimentation. In: G.V. Middleton, A.H. Bouma, eds. Turbidites and deep-water sedimentation. Anaheim, SEPM Pacific Section Short Course. Oklahoma: Tulsa; 1973:79–118.

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J.-C. Faugères, D.A.V. Stow. Contourite drifts: Nature, evolution and controls. M. Rebesco, A. Camerlenghi, eds. Contourites. Developments in Sedimentology, 2008;60:257–288. (this volume)

J.-C. Faugères, D.A.V. Stow, P. Imbert, A.R. Viana. Seismic features diagnostic of contourite drifts. Mar. Geol.. 1999;162:1–38.

P. Giresse. Some aspects of diagenesis in contourites. M. Rebesco, A. Camerlenghi, eds. Contourites. Developments in Sedimentology, 2008;60:203–222. (this volume)

B.C. Heezen, C.D. Hollister. The Face of the Deep. New York: Oxford University Press; 1971. 659 pp

Y. He, T. Duan, Z. Gao. Sediment entrainment. M. Rebesco, A. Camerlenghi, eds. Contourites. Developments in Sedimentology, 2008;60:99–120. (this volume), forthcoming

B.C. Heezen, G.L. Johnson. A moated knoll in the Canary Passage. Dent. Hydrograph. Z.. 1963;16(6):269–272.

B.C. Heezen, C.D. Hollister, W.F. Ruddiman. Shaping of the continental rise by deep geostrophic contour currents. Science. 1966;152:502–508.

F.J. Hernández-Molina, E. Llave, D.A.V. Stow. Continental slope contourites. M. Rebesco, A. Camerlenghi, eds. Contourites. Developments in Sedimentology, 2008;60:379–408. (this volume)

C.D. Hollister, B.C. Heezen. Contour current evidence from abyssal sediments. Trans. Am. Geophys. Union. 1967;48:142.

C.D. Hollister, B.C. Heezen. Geological effects of ocean bottom currents: Western North Atlantic. A.L. Gordon, ed. Studies in Physical Oceanography, Gordon and Breach, New York, 1972;Vol. 2:37–66.

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Hollister, C.D., 1967. Sediment distribution and deep circulation in the western North Atlantic. Unpublished Ph.D. thesis Columbia University, New York, 467 pp.

H. Hüneke, D.A.V. Stow. Identification of ancient contourites: Problems and palaeoceanographic significance. M. Rebesco, A. Camerlenghi, eds. Contourites. Developments in Sedimentology, 2008;60:323–344. (this volume)

V.V. Ivanov, G.I. Shapiro, J.M. Huthnance, D.L. Aleynik, P.N. Golovin. Cascades of dense water around the world ocean. Prog. Oceanography. 2004;60:47–98.

J.S. Laberg, A. Camerlenghi. The significance of contourites for submarine slope stability. M. Rebesco, A. Camerlenghi, eds. Contourites. Developments in Sedimentology, 2008;60:537–556. (this volume)

G.V. Middleton, M.J. Church, M. Coniglio, L.A. Hardie, eds. Encyclopedia of sediments and sedimentary rocks. Encyclopedia of earth sciences, Kluwer, Academic, Dordrecht, The Netherlands, 2003:821.

I.N. McCave. Size sorting during transport and deposition of fine sediments: Sortable silt and flow speed. M. Rebesco, A. Camerlenghi, eds. Contourites, Developments in Sedimentology, 2008;60:121–142. (this volume)

J. Martín-Chivelet, M.A. Fregenal-Martínez, B. Chacón. Traction structures in contourites. M. Rebesco, A. Camerlenghi, eds. Contourites. Developments in Sedimentology, 2008;60:159–182. (this volume)

T. Nielsen, A. Kuijpers, P. Knutz. Seismic expression of contourite depositional systems. M. Rebesco, A. Camerlenghi, eds. Contourites, Developments in Sedimentology, 2008;60:301–322. (this volume)

M. Rebesco, D.A.V. Stow. Seismic Expression of Contourites and Related Deposits: A Preface. Mar. Geophys. Res.. 2001;22:303–308.

M. Rebesco. Contourites. R.C. Selley, L.R.M. Cocks, I.R. Plimer, eds. Encyclopedia of Geology, Elsevier, Oxford, 2005;Vol. 4:513–527.

D.A.V. Stow, S. Hunter, D. Wilkinson, J. Hernández-Molina. The nature of contourite deposition. M. Rebesco, A. Camerlenghi, eds. Contourites. Developments in Sedimentology, 2008;60:143–156. (this volume)

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A. Wetzel, F. Werner, D.A.V. Stow. Bioturbation and biogenic sedimentary structures in contourites. M. Rebesco, A. Camerlenghi, eds. Contourites. Developments in Sedimentology, 2008;Vol. 60:183–202. (this volume)

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Chapter 2 Personal Reminiscences on the History of Contourites

K.J. Hsü

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Oakcombe, Marley Common Haslemere, Surrey, UK

Abstract

Publisher Summary

This chapter presents the personal reminiscences of K. J. Hsü from the United Kingdom on the history of contourites. He undertook an investigation on the Pliocene sandstone reservoirs of the Ventura Field, California. He found that the existence of groove casts as bottom markings clearly indicatedthat the flow depositing the laminated bottom layer is not turbidity current; rather it consists of debris in viscous motion. He also carried out experiments to show that the bedform of suspension deposits can be rippled. He identified four lithofacies in the Pliocene of the Ventura Basin that represent four environments of deposition—namely, a mudstone facies deposited on the slope of the basin, a conglomerate facies in the canyons cutting across the slope, a turbidite sand facies in the deepest part of the basin trough, and a thin-bedded facies on the fringes of the turbidite sand. However, his interpretation of the genesis of the cross-laminated or thin-bedded sand represents little competitive value to Shell. He gave a comparison of the sedimentary structures of the deposits from the two very different environments.

2.1 Introduction

I studied metamorphic petrology at the University of California (UCLA) in the early 1950s, and my ambition was to become a teacher of petrology at an American university. That was 10 years after the repeal of the Chinese Exclusion Act and there were not yet voices in the Congress on equal employment. I had little luck in finding a job. The nearest I came was a chance to become an acting lecturer at the Oregon State University as a 1-year substitute for somebody with sabbatical leave. However, my application was rejected.

Eventually I found work for Shell Research, because it was a Dutch company. No American company would employ me because it would be impossible for Orientals to make business trips to the Deep South, where hotels existed only for the white or the coloured. I was not well prepared for my new job, not having taken any course on sedimentology.

I asked for some advice, when I went to say goodbye to Jerry Winterer. Winterer was a classmate, but also he taught sedimentary petrography. I did not know him very well, because we, hardrock guys, had our coffee room on the fourth floor, while the softies congregated on the fifth. Winterer was very pleased to learn that I was going to work for Shell. He just read an article in the AAPG Bulletin by someone there called Nanz, on the geometry of Oligocene sandstone reservoirs of the Seeligson Field in south Texas. It was brilliant, Winterer told me. I should look him up; perhaps he might teach me a thing or two on the art and science of basin analysis.

I did not have to look Nanz up: I was assigned to him as a trainee. Taking into account of my experiences in California, Nanz asked me to make a study of the Pliocene sandstone reservoirs of the Ventura Field, California. There had been rumors that those might be turbidites. Nanz was ready to consider all scientific evidence, while most of his colleagues were downright hostile to the innovative suggestion. I went to Ventura in 1954, and the Pliocene sands are obviously turbidites, as can be deduced from their sedimentary structures.

Applying Nanz’s technique of mapping the geometry of genetic units, I recognized on the electric logs a group of sand layers within a thick succession of shale. With easy correlation, I could map the distribution of this genetic unit. It had been thought, on the basis of the Grand Banks study by Heezen and Ewing (1952), that turbidites were blanket sands. I found, to the surprise of everyone, that the Ventura turbidites are shoestring bodies (Hsü, 1977). In hindsight, the discovery should not have been surprising. The event started from a point, and the loci of points constitute a line. Turbidites originated from a single locality, and should thus be linear sand bodies. My Shell report was internally distributed in 1954, but it was not published for more than two decades (Hsü, 1977), and even then only in part. Nevertheless, the impact in the oil industry was significant.

After the Saticoy Field of the Ventura Basin had been discovered in 1955, the knowledge that the turbidite reservoirs are string-sands has become very helpful to production geologists. The recognition that linear sand bodies have up-dip pinch-out edges has contributed to the discovery of gas fields by the Occidental Company in the Sacramento Valley. I suspected that one of my former assistants at Ventura made the knowledge transfer; he received a generous bonus from his company after he changed employers.

2.2 Not All Deep-Sea Sands are Turbidites

Turbidite soon became a household word in the Shell Oil Company, and the word was synonymous with deep-sea sand. I did not like the word because of the genetic implication of deposition from the turbid waters of a suspension. When I was a post-doc in Switzerland, I found evidence in the Alpine flysch. The existence of groove casts as bottom markings clearly indicates that the flow depositing the laminated bottom layer (Bouma interval A) is not a turbidity current; it consists of debris in viscous motion (Hsü, 1959). Neither is graded bedding (also Bouma A) necessarily an evidence of settling from a suspension. The grading may have resulted from a decrease of the current-transport velocity (Kersey and Hsü, 1976). Finally, the cross-laminated silt (Bouma C), above the parallel laminated Bouma B interval, may or may not be a deposit from suspension. Years later my students and I did carried out experiments to show that the bedform of suspension deposits can be rippled (Hsü et al., 1980).

At that time, however, I was speculating on the possibility that the so-called Bouma C unit has been reworked by a bottom current. Bruce Heezen, who had acquired a fame because of his paper with Ewing on the turbidity-current deposition after the Grand Banks Event, came in 1955 to Houston to give a talk on deep-sea sedimentation. I was his host. We soon found ourselves in agreement that not all deep-sea sands are turbidites. He showed me beautiful pictures of deep-sea ripples. I could tell him, from my observations at Ventura, that those ripples are underlain by cross-laminated sand or silt.

I had a field season, in the summer of 1955, to study the Ventura sediments. Together with Jim Valentine, who was collecting foraminifera samples for ecological studies, we climbed a steep slope covered by a dense growth of sage brush. Below a vertical cliff of a thick turbidite ledge was a mudstone deposit. Intercalated in the pelitic sediment is an ash unit which crops out everywhere in the Ventura Basin; it is the boundary ash between the Pliocene Pico Formation and the Pleistocene Santa Barbara Mudstone. The volcanic ash had settled on a deep-sea bottom, as Valentine identified the typical Uvigerina peregrina assemblage in the pelitic sediments. On a close inspection, we found that the tuff unit consists of two ash layers, with a rippled horizon between the two (Figure 2.1). Instead of a laterally continuously rippled layer, the ripples are starved micro-dune features underlain by a cross-laminated, very well-sorted coarse silt. That silt cannot be a turbidite, because the speed of a turbidity current would have disturbed or eroded away the underlying ash layer.

Figure 2.1 Deep-sea sedimentary facies. Cross-laminated silt deposited by bottom currents is found intercalated in the sediments of thin-bedded sand facies, which is found on the fringe of main turbidite sand bodies (Hsü et al., 1980, reprinted with permission from AAPG, whose permission is required for further use).

2.3 Turbidophiles and Turbidophobes

I wrote up the results of the Ventura Basin first as an internal report. I identified four lithofacies in the Pliocene of the Ventura Basin that represent four environments of deposition (Figure 2.2), namely: a mudstone facies deposited on the slope of the basin, a conglomerate facies in the canyons cutting across the slope, a turbidite sand facies in the deepest part of the basin trough, and a thin-bedded facies on the fringes of the turbidite sand. My interpretation of the genesis of the cross-laminated or thin-bedded sand represents little competitive value to Shell.

Figure 2.2 Cross-laminated silt between two ash falls. When I encountered the cross-laminated silt layer between the ash falls for the first time, I began to doubt if all deep-sea sands are turbidites (Hsü, 1964, with permission from the Society for Sedimentary Geology).

The work was released for publication (Hsü, 1964) after I had presented the results at the 1963 SEPM meeting at Houston. My talk was attended by two persons, among others. Glenn Bartle was the President of the Harpur College; he came to the talk because I was being considered for a staff position at the College, or SUNY Binghamton. He did not make any comment of my interpretation, but he was very critical of my presentation. I did not hold the microphone steadily, and the variable volume of my voice was very irritating to the audience. The other person was Ken Emery, and he was furious. Emery, then teaching at the Southern California University, was a turbidophile. He had convinced himself that all deep-sea sands are turbidites. He was angry because I was reinforcing the doubt of the turbidophobes, and there were many of those in the oil industry. As he said, he had been working for more than a decade to educate the ignorant. And now I was giving them a new excuse. I was thus obstructing the progress of science, while driving a wedge between the academics and the industry.

Ripples are ripples, and the same bedform can be found in different depositional environments. Deep-sea ripples are morphologically not distinguishable from ripples on tidal flats. I gave a comparison of the sedimentary structures of the deposits from the two very different environments (Figure 2.3). Emery was right, my after-dinner talk at the St Louis SEPM was a hit and gave much comfort to the turbidophobes of the oil industry. I was also right, not all deep-sea sands are turbidites. I learned then that Heezen was continuing to develop his idea on deep-sea sand deposition. Eventually, the rippled and cross-laminated sands and/or silts are called contourites, because they have been deposited by deep-sea currents flowing parallel to submarine contours.

Figure 2.3 Morphological similarity of cross-laminated silts (Hsü, 1964, with permission from the Society for Sedimentary Geology). (a) Cross-laminated silt in the Pliocene, Pico Formation, California. (b) Cross-laminated silt in Holocene tidal-flat deposits of the Wadden Sea, the Netherlands. The same bedform can be present in two different environments.

2.4 Erosional Unconformity Misinterpreted

I left Shell in 1963. While I continued to be interested in sedimentology, I became almost a full-fledged geological oceanographer after I joined the JOIDES Deep Sea Drilling Project. As the Chairman of the South Atlantic Group of the Paleoenvironment Panel, I was an avid reader of reports and cruise proposals on deep-sea circulations, and became acquainted with the latest investigations on the Antarctic Bottom Current (AABW) and the North Atlantic Deep Water (NADW). Particularly interesting were the results of drillings on the West African Margin. The power of the NADW caused deep erosion of the slope sediments. In places, Middle Miocene hemipelagic deposits of the South Atlantic overlie slope deposits as old as the Cretaceous.

When I was, as they called it, on the beach, I taught Alpine tectonics at the Swiss Federal Institute of Technology. I encountered in the geology of the Alps a century-old puzzle that had been called Wang Transgression. Unlike the common transgressive deposits of sand or gravel, the Wang is a Maastrichtian shale formation, containing a deep-sea fauna indicative of marine deposition on a Cretaceous slope. The relation was considered transgressive, because the Maastrichtian formation lies directly upon Campanian pelagic deposits in the North Helvetic palaeogeographic realm. The Wang sediments of the higher ultrahelvetic nappes were deposited farther offshore; they overlie unconformably progressive older (Santonian, Turonian, Cenomanian, Early Cretaceous, Late and Middle Jurassic) hemipelagic deposits (Figure 2.4).

Figure 2.4 The Wang Transgression (Hsü, 1960, with permission from the Geological Society of America). A reconstruction of the paleogeographic relations suggests that the Maastrichtian Wang Formation consists of transgressive deposits overlying sub-aerially eroded older formations. This classic interpretation is wrong. The Wang Formation consists of contourite beds.

The orthodox interpretation postulates uplift, followed by subaerial erosion, followed by subsidence, and finally followed by a shallow-marine transgression. There is in fact no evidence of uplift, of subaerial erosion, of subsidence, or of a shallow-marine transgression. Using the modern West African Margin as an analogue, we see the obvious fact that the Wang transgression was not a transgression: it is an overlap of Maastrichtian contourites above older slope sediments. The unconformity under the Wang Formation signifies a period of very active submarine erosion by contour currents in the Helvetic realm of an Alpine Basin during the pre-Maastrichtian. This erosional progress is comparable to that on the West African Margin by the NADW during the pre-Middle Miocene. An American student, S. Diefenbach, completed a master thesis on the Wang Transgression. She found clear evidence that the North Helvetic realm was a marine slope environment far south of the European continental coast. The gradient was steep enough to have caused widespread slumping of hemipelagic deposits, and the overshore slope was cut by submarine gullies or canyons in which coarse clastics were accumulated.

The Wang is clearly a contourite formation. The interpretation was not published until my book The Geology of Switzerland was printed after my retirement (Hsü, 1995). Meanwhile, I committed an indiscretion when I taught the idea in my class on The Geology of Switzerland. For that, students and colleagues alike chastised me. I was told that I should not have taught Ken Hsü’s crazy ideas to beginning students. I often wonder, if Isaac Newton was similarly reproached when he taught gravity to his Cambridge students before the publication of the Principia. If the academic establishment of the 17th-century England had been as dogmatic as the Swiss geological community of the 20th century, Newton would probably have had to tell his students, contrary to his conviction, that the Sun went around the Earth. I recalled that one of my students had to delete the word mélange when he intended to publish, in the early 1970s, his thesis on the Wildflysch in the Eclogae Geologicae Helvetiae. Now that a book on contourites is published, I am hopeful that the editors of that illustrious journal would permit the use of the word contourite in articles on Alpine palaeoceanography.

References

K.J. Hsü, K. Kelts, J.V. Valentine. Resedimented facies in Ventura Basin, California, and model of longitudinal transport of turbidity currents. AAPG Bull.. 1980;64:1034–1051.

K.J. Hsü. Cross-laminated sequence in graded bed sequence. J. Sediment. Petrol.. 1964;34:379–388.

K.J. Hsü. Paleocurrent structures and paleogeography of the ultrahelvetic flysch basins, Switzerland. Geol. Soc. Am. Bull.. 1960;71:577–610.

B.C. Heezen, M. Ewing. Turbidity currents and submarine slumps, and the 1929 Grand Banks earthquake. Am. J. Sci.. 1952;250:849–873.

K.J. Hsü. Studies of Ventura Field, California. I. Facies geometry and genesis of Lower Pliocene turbidites. AAPG Bull.. 1977;61:137–168.

K.J. Hsü. Flute- and groove-casts in the Prealpine Flysch, Switzerland. Am. J. Sci.. 1959;247:529–536.

K.J. Hsü. The Geology of Switzerland. An introduction to tectonic facies. Princeton, NJ: Princeton Univ. Press; 1995. 250 pp

D. Kersey, K.J. Hsü. Experimental investigation of energy relations of density current flows. Sedimentology. 1976;23:761–789.

To view the full reference list for the book, click here

Chapter 3 Methods for Contourite Research

J.A. Howe

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Scottish Association for Marine Science & UHI Millennium Institute-Dunstaffnage Marine Laboratory, Oban, Argyll, Scotland, UK

Abstract

Publisher Summary

This chapter illustrates the range of techniques that can be employed to investigate contourite sedimentation in the deep sea using hydrographic, geophysical, and sedimentological studies. The range of processes contributing to current-influenced sedimentation in the deep sea can be extremely diverse and at a variety of scales from the localized to the global, such as biogenic fluxes from the water column, changes in ocean chemistry, the localized dynamics of the benthic boundary layer, and the variation of thermohaline flow in response to bathymetry. Bottom-current-influenced sedimentation is a highly significant process in the deep ocean. Geophysical methods, sampling strategies, and analytical methods are described. Deep-ocean technologies and instruments are being developed which are smaller and cheaper, and hence easier to use on research cruises. Satellite data are now widely available, showing the distribution of surface currents and sea-surface heights, thus enabling regions of enhanced bottom-current activity to be better understood. The recognition of contouritic sedimentation in the deep sea and investigative techniques used for small scale, medium scale, and large scale are tabulated.

3.1 Introduction

The physical effect of a persistent bottom current on deep-sea sediment can be studied using a wide variety of oceanographic, geophysical and sedimentological techniques. Contourite workers need to be able to study both the modern and palaeo-deep-sea floor. A definition of the term contourite is provided by Rebesco et al. (2008); this chapter aims to present a summary of the range of techniques that can be employed to investigate contourite sedimentation in the deep sea. The range of processes contributing to current-influenced sedimentation in the deep sea can be extremely diverse, and at a variety of scales from the localised to the global, such as biogenic fluxes from the water column, changes in ocean chemistry, the localised dynamics of the benthic boundary layer and the variation of thermohaline flow in response to bathymetry. Since Wüst (1936) first proposed the idea of thermohaline flow in the deep oceans, bottom current and perhaps more specifically, contourite researchers have needed to take a broad, multi-disciplinary approach when examining the deep sea bed for evidence of contourites.

Early workers have, understandably, been restricted by the variety of techniques