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PETROLEUM EXPLORATION AND DEVELOPMENT Volume 44, Issue 2, April 2017 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2017, 44(2): 183–216.

RESEARCH PAPER

Contourites: Physical oceanography, process sedimentology, and petroleum geology SHANMUGAM G* Department of Earth and Environmental Sciences, The University of Texas at Arlington, Arlington, TX 76019, USA

Abstract: The purpose of this critical review is to address fundamental principles associated with contourites and other bottom-current deposits. The four basic types of deep-marine bottom currents are: (1) thermohaline-induced geostrophic contour currents, (2) wind-driven bottom currents, (3) tide-driven bottom currents, mostly in submarine canyons, and (4) internal wave/tide-driven baroclinic currents. Contourites are deposits of thermohaline-driven geostrophic contour currents. Contourites can be muddy or sandy in texture, siliciclastic or calciclastic in composition. Traction structures are common in deposits of all four types of bottom currents. However, there are no diagnostic sedimentological or seismic criteria for distinguishing ancient contourites from other three types. The Gulf of Cadiz is the type locality for the contourite facie model based on muddy lithofacies. However, this site is affected not only by contour currents associated with the Mediterranean Outflow Water (MOW) but also by other factors, such as internal waves and tides, turbidity currents, tsunamis, cyclones, mud volcanism, methane seepage, sediment supply, porewater venting, and bottom topography. IODP (Integrated Ocean Drilling Program) 339 cores from the Gulf of Cadiz do not show primary sedimentary structures, which are necessary for interpreting depositional processes. Therefore, the contourite facies model is sedimentologically obsolete. Bottom-current reworked sands of all four types have the potential for developing petroleum reservoirs. Modern sandy carbonate contourites have a measured maximum porosity of 40% and a maximum permeability of 9881 mD due to the winnowing away of muds from the intergranular primary pores by vigorous contour currents. These carbonate contourites are hemiconical-shaped bodies that are up to 600 m in thickness and nearly 60 km in length. Empirical data of modern contourites also show potential for seal and source-rock development. Therefore, future petroleum exploration and development should focus attention on these often overlooked siliciclastic and calciclastic deep-marine reservoirs. Key words: bottom currents; contourite reservoirs; Gulf of Cadiz; Florida Current; traction structures; process sedimentology

1.

Introduction

G. Wust[1] (a German oceanographer), B. C. Heezen[2] (an American marine geologist) and his student C. D. Hollister[3] (an American marine geologist) are the three pioneers of contourite research in the 20th Century. The domain of deep-marine bottom currents and their deposits (Fig. 1) has a long history of contributions on both physical oceanography and process sedimentology[434]. Shanmugam discussed various aspects of deep-marine sedimentation, which include bottom currents, during the past 42 years[3578]. Unlike downslope processes, contour currents flow parallel to regional slope (Fig. 2), with implications of sand distribution and reservoir development. Therefore, contourite research is of significance in understanding the economic potential of bottom-current reworked sands[7678, 81]. In general, most of what we know about modern-day contourites is based primarily on large-scale features observed on seismic and bathymetric data, with some sediment core data. On the other hand, the

literature on ancient contourites offers ample details on smallscale sedimentary features based on outcrop and conventional core data[12, 50, 76, 8286], but with only limited information on paleo-oceanography and on large-scale depositional features. This disparity in conjunction with other issues normally associated with deep-water processes and facies[50] has resulted in a multitude of challenges in interpreting ancient contourites and related deep-water deposits[5863]. In advancing contourite research, a rigorous scrutiny of all basic problems is imperative. Therefore, the primary purpose of this review is to provide a basic understanding of physical oceanography and process sedimentology of bottom currents and their deposits with an emphasis on contourites. An equally important objective is to present empirical data on the potential of contourites as seal, source, and reservoir rocks in petroleum geology. In representing global examples, critical case studies of 21 modern and ancient systems by other researchers (Fig. 3, locations A to U) are considered. In addressing the economic significance of bottom-current

Received date: 26 Jun. 2016; Revised date: 18 Jan. 2017. * Corresponding author. E-mail: [email protected] Copyright © 2017, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

SHANMUGAM G / Petroleum Exploration and Development, 2017, 44(2): 186–216

Fig. 1. Schematic diagram showing complex deep-marine sedimentary environments occurring at water depths deeper than 200 m (shelf-slope break), with (1) alongslope contour currents (wide red arrows), (2) tidal bottom currents in canyons, and (3) wind-driven bottom currents. See Fig. 14B for baroclinic currents. After Shanmugam[42], with permission from Elsevier.

Fig. 2. Comparison of downslope mass flows and their deposits (i.e., debrites, left map) (Embley[79] with alongslope contour currents and their deposits (i.e., contourites, right map) (Flood and Hollister[80]) on the U.S. Atlantic Margin.

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Fig. 3. Map showing the locations of case studies used in this review, which include 21 critical case studies by other researchers (locations A to U). Note 35 locations of core and outcrop descriptions of deep-water sandstones with traction structures that were interpreted by the present author as products of bottom-current reworking (Table 1).

reworked sands[76, 87], descriptions of deep-water strata from 35 case studies worldwide that include 7832 meters of conventional cores from 123 wells, representing 32 petroleum fields are considered (Fig. 3, Table 1). This contribution is a follow-up to my previous attempt to inform the readership of Petroleum Exploration and Development on "New perspectives on deep-water sandstones: Implications" (Shanmugam, 2013c)[55]. Although the primary focus of this study is on deep-marine environments (Fig. 1), some shelf-edge settings affected by thermohaline currents (e.g., Agulhas and Kushiro) are considered for completeness.

2. 2.1.

Ocean circulation and bottom currents Global thermohaline circulation

Wüst[1] was the first scientist to recognize the importance of abyssal deposits generated by deep-marine bottom currents. Historically, the concept of contour currents has been attributed to global thermohaline circulation[8]. Aspects of thermohaline circulation are discussed by Zenk[24] and Talley[27]. The thermohaline circulation and related deep-marine bottom currents in modern oceans became popular when Heezen et al.[8] reported deep-water masses and related contour currents along the continental rise in the U.S. Atlantic margin. An example of such deep-water mass is the Antarctic Bottom Water (AABW) (Table 2). AABW was first identified by Brennecke (1921)[84] in the northwest corner of the Weddell Sea in the Antarctic region (Fig. 4). The deep-water masses in the world’s oceans are caused by differences in temperature and salinity. When sea ice forms in the polar regions due to freezing of shelf waters, seawater experiences a concurrent increase in salinity due to salt rejection and a decrease in temperature. The increase in the density

of cold saline (i.e., thermohaline) water directly beneath the ice triggers the sinking of the water mass down the continental slope (Fig. 4) and the spreading of the water masses to other parts of the ocean. These are called thermohaline water masses. Stommel[4] first developed the concept of the global circulation of thermohaline water masses and the vertical transformation of light surface waters into heavy deep-water masses in the oceans. Broecker[18] presented a unifying concept of the global oceanic “conveyor belt” by linking the wind-driven surface circulation with the thermohaline-driven deep circulation regimes. The large-scale horizontal transport of water masses, which also sink and rise at select locations, are known as the “thermohaline circulation”(THC). The term THC, which refers to a driving mechanism by high-latitude cooling, is a physical concept and not a measurable quantity[118]. The global conveyor belt system in the North Atlantic originates near Greenland and Iceland where the sea-ice formation produces cold and salty North Atlantic Deep Water (NADW). The NADW sinks and flows southwards along the continental slope of North and South America toward Antarctica where the water mass then flows eastwards around the Antarctic continent. According to Talley (2013, p.81)[27], "Description of the pathways and energetics of the global overturning circulation (GOC) is central to understanding the interaction of different ocean basins and layers as well as the interplay of external forcings." Aspects of the global overturning circulation (GOC) have been discussed in some detail by other oceanographers[119122]. For example, Schmitz[90] illustrated meridional sections of inter-basin flow with their global linkages among the Indian, Pacific, and Atlantic Oceans using Antarctica as the core of global circulation (Fig. 5).

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Table 1.

Summary of 21 locations of published case studies on deep-marine bottom currents by other researchers that are used in this

article (Locations: A to U shown by filled squares, see Fig. 3). Note conventional core and outcrop description carried out by the present author worldwide (Locations: 1-13, filled circles, see Fig. 3). Traction structures of bottom-current origin are common in all 35 case studies carried out by the author. Case studies

Data: Thickness of core and outcrop described by the author (Not applicable to studies by other researchers)*

Modern Contour currents

Echo sounding, bottom photographs, sediment cores

B. Case study: Straits of Florida[88]

Modern sandy carbonate contourites

Seismic profiles, cores, rocks recovered by dredging and in-situ sampling

B. Case study: Straits of Florida[89]

Bottom currents

C. Case study: Argentine Basin[90]

Modern muddy contourites

Location symbol and number in Fig. 3 A. Case study: Blake Plateau and BlakeBahama Outer Ridge[8]

D. Case study: Eirik Drift[91]

Deep Western Boundary Current (DWBC)

E. Case study: Weddell Sea[93]

Cyclonic circulation of the Weddell Gyre

F. Case study: Gulf of Cadiz[16, 17, 94, 95]

F. Case study: Gulf of Cadiz[93, 95]

Current-meter records

F. Case study: Gulf of Cadiz[26]

Modern Cadiz Channel

Modern carbonate mounds Sheet-like sediment waves Eirik Drift, South of Greenland Velocity: 24 cm/s

Discussion of problematic contourite facies model (Discussed in this article) Discussion of complex Seismic profiles, bottom photographs, origin of erosional feasediment cores tures (Discussed in this article) Discussion of problematic contourite facies Sediment cores, grain-size analysis, model in terms of velocthin-section studies ity (Discussed in this article) Discussion of problem2 gravity cores atic origin contourite and over 3000 submarine sands (Discussed in this [26] photographs article) Discussion of problematic internal-wave and 1 Outcrop section[96] internal-tide deposits[53,

Modern Faro contourite drift


Introduction of basic concept of contour currents Poroisty and permeability data (Table 4)

3.5 kHz seismic profiles, sediment cores


F. Case study: Gulf of Cadiz[31]

G. Case study: NE Spain[96]

Hydroacoustic data, high resolution multichannel seismic reflection data, CTD casts and sampling High-resolution seismic records (3.5 kHz echograms) Conductivity, temperature and depth (CTD) and Lowered Acoustic Doppler Current Profiler (LADCP) measurements

Comment (This paper)

Ricla Section, Upper Jurassic

54]

H. Case study: Southern Adriatic Sea[97] I. Case study: Israel[83]

J. Case study: Southeast of South Africa[98]

K. Case study: China[99]

Dense water Ancient sandy carbonate Contourite (Cretaceous Talme Yafe Formation)

Various measurements and sampling Outcrop and core

Velocity of bottom currents: 4050 cm/s Sediment prism

69.7 Sverdrups (1 Sv or Mooring deployment off Fort Edward Sverdrup = 106 m3/s) from coast out to 203 km offshore; at 31°S. maximum depth of measurement: Poleward vekocity is > 100 cm/s at or above 100 2 200 m m depth on mooring B. Discussion of problemSeveral outcrop sections[99] atic internal-wave and internal-tide deposits[51, 57]

Agulhas Current

Ningxia, Middle Ordovician

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Continued L. Case study: South China Sea[100] M. Case study: West Phillipine Sea[101]

Modern contourites

Bottom simulating reflectors

Gas hydrates

Kuroshio and Luzon Undercurrent

Field experiment

The annual Kuroshio transport is 16 ± 4 Sv

N. Case study: Makassar Strait[102]

Kutei Basin, Miocene

2 wells?[102104 ]

N. Case study: Makassar Strait[106]

Kutei Basin, Miocene

2 wells[102104]

Deep-marine tidal bottom currents

Regional bathymetry and multibeam echo sounding

Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW)

ODP 1119

Planar-bedded units up to several metres thick

Modern sandy volcaniclastic contourites

Seismic and side-scan sonar data, Seafloor photo, grab samples, piston core

Sheet contourites

Thermohaline Circulation (THC)

Seismic data

Thickness: 1800 m Length: > 1000 km Width: ~ 350 km

O. Case study: Off Fraser Island, SE Australia[107] P. Case study: Canterbury Drifts, SW Pacific Ocean[108] Q. Case study: Offshore of the Pennell Coast, Antarctica[109] R. Case study: Meiji Drift Emperor seamount chain[110] S. Case study: Horizon Guyot, Mid-Pacific Mountains[111]

Baroclinic currents reworking sediments on flat tops of towering guyot terraces.

T. Case study: Equatorial Pacific[112]

Large-scale sediment redistribution by bottom currents

U. Case study, Monterey Canyon, U.S. Pacific Margin[113]

Deep-marine tidal bottom currents

1. Gulf of Mexico, U.S.[72]

1. Mississippi Fan, Quaternary, DSDP Leg 96

Discussion of deep tidal currents[105] Reply to a discussion on the reservoir quality of bottomcurrent reworked sands[57] Highstand transport of coastal sand to the deep ocean

Narrow-beam echo-sounding system, a pair of side-looking sonars, Asymmetrical dunes and a 3.5 kHz seismic profiler and a ripples. Bathymetry of proton magnetometer, and bedforms: 16301632 m. deep-sea cameras Digital seismic reflection data and Bottom-current induced wireline logging data resedimentation Velocity of bottom currents: 30 cm/s Current meter (both up-canyon and down-canyon) ~ 500 m DSDP core (selected intervals Modern submarine fan described)

1. Gulf of Mexico, U.S.[76-78, 71]

2. Green Canyon, late Pliocene 3. Garden Banks, middle Pleistocene 4. Ewing Bank 826, Pliocene-Pleistocene 5. South Marsh Island, late Pliocene 6. South Timbalier, middle Pleistocene 7. High Island, late Pliocene 8. East Breaks, late Pliocene-Holocene

1067 m Conventional core and piston core 25 wells

Sandy mass-transport deposits and bottom-current reworked sands common

2. California[43, 50, 67]

9. Midway Sunset Field, upper Miocene, onshore

650 m Conventional core 3 wells

3. Ouachita Mountains, Arkansas and Oklahoma, U.S.[68]

10. Jackfork Group, Pennsylvanian

369 m 2 outcrop sections

Sandy mass-transport deposits and bottomcurrent reworked sands Sandy mass-transport deposits and bottomcurrent reworked sands common

4. Southern Appalachians, Tennessee, U.S.[35, 64, 69, 70]

11. Sevier Basin, Middle Ordovician

2152 m 5 outcrop sections

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Ancient submarine fan


Continued 12. Lagoa Parda Field, lower Eocene, Espirito Santo Basin, onshore 13. Fazenda Alegre Field, upper Cretaceous, Espirito Santo Basin, onshore 14. Cangoa Field, upper Eocene, Espirito Santo Basin, offshore 15. Peroá Field, lower Eocene to upper Oligocene, Espirito Santo Basin, offshore 16. Marlim Field, Oligocene, Campos Basin, offshore 17. Marimba Field, upper Cretaceous, Campos Basin, offshore 18. Roncador Field, upper Cretaceous, Campos Basin, offshore


Sandy mass-transport deposits and bottomcurrent reworked sands common

6. North Sea[74]

19. Frigg Field, lower Eocene, Norwegian North Sea 20. Harding Field (formerly Forth Field), lower Eocene, U.K. North Sea 21. Alba Field, Eocene, U.K. North Sea 22. Fyne Field, Eocene, U.K. North Sea 23. Gannet Field, Paleocene, U.K. North Sea 24. Andrew Field, Paleocene, U.K. North Sea 25. Gryphon Field, upper Paleocenelower Eocene, U.K. North Sea



7. U.K. Atlantic Margin[74]

26. Faeroe area, Paleocene, west of the Shetland Islands 27. Foinaven Field, Paleocene, west of the Shetland Islands

5. Brazil[43, 50]

8. Norwegian Sea and vicinity[73]

9. French Maritime Alps, Southeastern France[40, 42]

28. Mid-Norway region, Cretaceous, Norwegian Sea. 29. Agat region, Cretaceous, Norwegian North Sea

30. Annot Sandstone, Eocene-Oligocene

Thickness included in Sandy mass-transport the N. Sea count deposits and bottom1 well Current reworked sands Conventional core common; contourites 1 well have been reported[114] 500 m Conventional core 14 wells


Sandy mass-transport 610 m ** deposits and bottom1 outcrop section current reworked sands (12 units described) common (deep tidal currents) Sandy mass-transport deposits and bottomcurrent reworked sands common (deep tidal currents)

10. Nigeria[37, 43, 50]

31. Edop Field, Pliocene, offshore


11. Equatorial Guinea[43, 50, 115]

32. Zafiro Field, Pliocene, offshore 33. Opalo Field, Pliocene, offshore



12. Gabon[43, 50]

34. Melania Formation, lower Cretaceous, offshore (includes four fields)



13. Bay of Bengal, India[75]

35. Krishna-Godavari Basin, Pliocene


Sandy debrites and tidalites common

* The rock description of 35 case studies of deep-water systems comprises 32 petroleum-producing massive sands worldwide. Description of core and outcrop was carried out at a scale of 1:20 to 1:50, totaling 11 463 m, during 1974-2011, by G. Shanmugam as a Ph.D. student (1974-78), as an employee of Mobil Oil Corporation (1978-2000), and as a consultant (2000-2011). Global studies of cores and outcrops include a total of 7832 meters of conventional cores from 123 wells, representing 32 petroleum fields worldwide[55, 56]. These modern and ancient deep-water systems include both marine and lacustrine settings. ** The Peira Cava outcrop section was originally described by Bouma (1962)[116], and later by Pickering and Hilton (1988, their Fig. 62)[117], among others.

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Table 2.

Acronyms of deep-water masses

Acronym

Water mass

Acronym

Water mass

AABW

Antarctic Bottom Water (Fig. 4)

MOW

Mediterranean Outflow Water

ABW AAIW

Arctic Bottom Water Antarctic Intermediate Water (Brazilian margin)

MUC NADW

Mediterranean Undercurrent North Atlantic Deep Water (Fig. 6)

ACC AW

Antarctic Circumpolar Current (Antarctica) Atlantic Water (Mediterranean Sea)

NAdDW NPDW

North Adriatic Dense Water North Pacific Deep Water (Japan)

BC BICC

Brazil Current Brazil Intermediate Counter Current

NSDW PDW

Norwegian Sea Deep Water Pacific Deep Water

Circumpolar Deep Water (Fig. 4) Deep Gulf Stream Return Flow

SACW SOW

South Atlantic Central Water (Brazilian margin) Sea Overflow Water

CDW or CPDW DGSRF DWBUC or DWBC IDW ITF LCDW LIW

Deep Western Boundary Undercurrent Indian Deep Water

UCDW WBUC or WBU

Upper Circumpolar Deep Water Western Boundary Undercurrent

Indonesian Throughflow Lower Circumpolar Deep Water

WDW WSBW

Warm Deep Water (Antarctica) Weddell Sea Bottom Water (Antarctica)

Levantine Intermediate Water (Mediterranean Sea)

WSDW

Weddell Sea Deep Water (Antarctica)

Fig. 4. A conceptual model of the Southern Ocean showing three vertical segments, composed of the upper surface currents, the middle deep-water masses, and the lower bottom currents, forming a vertical continuum (left). Note the origin of AABW by freezing of shelf waters (right). As a consequence, the increase in the density of cold saline (i.e., thermohaline) water triggers the sinking of the water mass down the continental slope and the spreading of the water masses to other parts of the ocean. SC = Surface currents; DWM = Deep-water masses; BC = Bottom currents. Modified after Hannes Grobe, April 7, 2000, http://en. wikipedia.org/wiki/File: Antarctic_bottom_ water_hg.png. Accessed May 18, 2011. Figure from Shanmugam[50], with permission from Elsevier.

Talley[27] has shown that the overturning pathways for the surface-ventilated North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) and the diffusively-formed Indian Deep Water (IDW) and Pacific Deep Water (PDW) are intertwined (Fig. 6). According to Talley[27], the global overturning circulation (GOC) includes both large wind-driven upwelling in the Southern Ocean and important internal diapycnal transformation in the deep Indian and Pacific Oceans

Oceans (Fig. 6). McManus et al.[123] discussed the link between Atlantic meridional circulation and climate changes. According to Bryden et al. (their Table 3)[98], the Agulhas Current (Fig. 7) is considered to be the largest western boundary current in the world's oceans, with an estimated net transport of 69.7 Sverdrups (1 Sv or Sverdrup = 106 m3/s) at 31°S (Table 1), as western boundary currents at comparable latitudes transport less — Brazil Current (16.2 Sv) at 28°S, East

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Fig. 5. Schematic meridional sections of inter-basin flow with their global linkages among the Indian, Pacific, and Atlantic Oceans using Antarctica as the core of global circulation. Diagram modified after Schmitz (1996)[90].

Fig. 6. Map showing the global overturning circulation (GOC). The location of Gulf of Cadiz is added in this article. This site served as the type locality for the contourite facies model. The global circulation is not important in interpreting the primary sediment provenance at a given site. Modified after Talley[27], with permission from the Oceanography Society.

Australian Current (22.1 Sv) at 30°S, Gulf Stream (29.9 Sv) at 27°N, and Kuroshio (21.5 Sv) at 24°N. The Kuroshio Undercurrent is an example that illustrates the complexity of ocean circulation in the Philippines Sea[101]. 2.2.

Types of deep-marine bottom currents

Southard and Stanley[85] recognized five types of bottom currents at the shelf break based on their origin. These currents are generated by (1) thermohaline differences, (2) wind forces, (3) tidal forces, (4) internal waves, and (5) surface waves. In addition, tsunami-related traction currents have been speculated to occur in bathyal waters[153, 44]. Also, cy-

clone-related bottom currents are common[48]. However, the mechanics of such currents are not yet well understood[48, 52]. In this review, I have selected four major types of deep-water bottom currents[46], namely (1) thermohaline-induced geostrophic contour currents[8], (2) wind-driven bottom currents[154], (3) tide-driven bottom currents, mostly in submarine canyons[113, 42], and (4) internal wave/tide-driven baroclinic currents[53, 111, 155]. Studies have shown that all four types of bottom currents (i.e., thermohaline-induced contour currents, wind-driven bottom currents, deep-marine tidal currents, and baroclinic tidal currents) have produced similar bedforms and traction structures (Fig. 8)[46]. This similarity in sedimentary

 190 


structures stresses the need for a better understanding of all four processes and their depositional mechanics in order to develop criteria for distinguishing their respective deposits. In the context of understanding sediment provenance, it is worth noting that the four types of bottom currents are reworking agents, and as such they are generally not involved in transporting large volumes of coarse detrital sediment (e.g., gravel, coarse and medium sand) from the source to sites of deposition. 2.2.1.

Thermohaline-driven geostrophic contour currents

Thermohaline-driven bottom currents tend to winnow, rework, and deposit sediment on the seafloor for a sustained period of time. They are popularly known as contour currents because they follow bathymetric contours[8]. Maximum current velocities of bottom currents in different parts of the world’s oceans are summarized in Table 3. Measured current Table 3.

velocities usually range from 1 to 20 cm/s[11]; however, exceptionally strong, near-bottom currents with maximum velocities of up to 300 cm/s were recorded in the Strait of Gibraltar[17]. Bottom-current velocities of 73 cm/s were measured at a water depth of 5 km on the lower continental rise off Nova Scotia[128]. Heezen and Hollister[156] suggested that at extremely high bottom velocities of over 100 cm/s, relict pockets of sand and gravel may occur on the barren seafloor. According to Bulfinch and Ledbetter[129], the Deep Western Boundary Undercurrent (DWBUC) flows southwards along the North American continental slope and rise between 1 000 m and 5 000 m. The DWBUC has a 300-km wide high-velocity zone, with a maximum measured velocity of 73 cm/s, which winnows both fine and very fine silt, and results in deposition of medium and coarse silt. Traction structures are common in contour-current deposits (Fig. 9)[116, 12].

Maximum current velocities of bottom currents in different parts of the world’s oceans

4001 400 200 100

Dominant driving mechanism, this article Thermohaline Thermohaline Wind-driven

Maximum current velocity (cm/s) 300 300 204

45

Wind-driven

153

760 4 800 2 0003 000 5 200 3 008 2 0003 000 4 0008 000

Dense water triggered by February 2012 severe cold spell Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Thermohaline Wind-driven[161] Thermohaline Thermohaline

* 1-year vector averaged speed ** Antarctic Circumpolar Current has both wind-driven and thermohaline-driven components (CIMAS, 2015)[152] Note: Velocity measurements for some examples (e.g., Agulhas and Kushiro) were made on shallow-water settings

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40-50 48 33 33 >30 30 30 30 26.5 26 25 24 21.5 21 20 20 20 19 17 17 17* 12 10


Fig. 7. Map showing the Agulhas Current flowing along the southern coast of Africa. Credit: Arnold L. Gordon. Fig. 9. a. Core photograph showing well-sorted fine-grained sand and silt layers (light gray) with interbedded mud layers (dark gray). Note sand layers with sharp upper contacts, internal ripple cross-laminae, and mud-offshoots. Also note lenticular nature of some sand layers. Pleistocene, continental rise off Georges Bank, After Hollister[3] and Bouma and Hollister[12], reproduced with permission from SEPM. b. Core photograph showing rhythmic layers of sand and mud, inverse grading, and sharp upper contacts of sand layers (arrow), interpreted as bottom-current reworked sands. Paleocene, North Sea. Figure from Shanmugam[46], with permission from Elsevier.

Fig. 8. Summary of traction features interpreted as indicative of deep-water bottom-current reworking by all types of bottom currents. Each feature occurs randomly and should not be considered as part of a vertical facies model. From Shanmugam et al.[76], with permission from AAPG.

2.2.2.

Wind-driven bottom currents

The wind-driven bottom current, a product of wind stress (i.e., atmospheric forcing) exerted at the sea surface that causes flows to extend all the way to the sea floor thousands of meters below, is well documented in the world’s oceans. For example, the Gulf Stream is a powerful, warm and swift Atlantic Ocean current that originates at the tip of Florida (Fig. 10a), and follows the eastern coastlines of the United States

and Newfoundland before crossing the Atlantic Ocean. The Gulf Stream proper is a western-intensified current, largely driven by wind stress[157]. The Loop Current in the eastern Gulf of Mexico is a wind-driven surface current[154], and it is genetically linked to the Gulf Stream in the Atlantic Ocean[158]. Velocities in eddies that have detached from the Loop Current have been recorded as high as 200 cm/s at a depth of 100 m[125]. Computed flow velocities of the Loop Current vary from nearly 100 cm/s at the sea surface to more than 25 cm/s at 500 m water depth[159]. Kenyon et al. (2002)[160] reported 25 cm/s current velocity measured 25 m above the seafloor. Such currents are capable of reworking fine-grained sand on the seafloor. Current ripples, composed of sand at a depth of 3091 m on the seafloor, are the most convincing empirical evidence of wind-driven bottom-current activity in the Gulf of Mexico today[154]. Another example of a wind-driven bottom current is the eastward-flowing Antarctic Circumpolar Current (ACC), which has influenced sedimentation on the slope and floor of the western Falkland Trough, where the axis of the current is topographically constrained[161]. This deep-water flow (below 3 000 m) has produced a symmetrical sediment drift on the trough floor, with non-depositional margins indicating higher current velocities at the base of slope. Deposits of the Loop Current have been interpreted in the cores from the Ewing Bank 826 Field, Plio-Pleistocene, Gulf of Mexico. The Ewing Bank Block 826 Field is located nearly 100 km off the Louisiana coast in the northern Gulf of Mexico (Fig. 10b). It contains hydrocarbon-producing clastic reservoir sands that have been interpreted as bottom-current-reworked sands[76, 77]. Cores from the Ewing Bank and adjacent areas exhibit traction structures (Fig. 11), such as horizontal layers,

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low-angle cross laminae, ripple cross-laminae, flaser bedding in ripples, mud offshoots in ripples, eroded and preserved ripples, and inverse grading (see Shanmugam et al. for additional core photographs)[76, 77]. 2.2.3.

Tide-driven bottom currents

In understanding tide-induced bottom currents, Shepard et al.[113] measured current velocities in 25 submarine canyons worldwide at water depths ranging from 46 to 4200 m by suspended current meters, usually 3 m above the sea bottom (Fig. 12a). Shepard et al. (1979)[113] also documented systematically that up- and down-canyon currents closely correlated with timing of tides (Fig. 12b). These submarine canyons include the Hydrographer, Hudson, Wilmington, and Zaire in the Atlantic Ocean; and the Monterey, Hueneme, Redondo, La Jolla/Scripps, and Hawaii canyons in the Pacific Ocean. Maximum velocities of up- and down-canyon currents commonly ranged from 25 to 50 cm/s[113]. Keller and Shepard[162] reported velocities as high as 7075 cm/s, velocities sufficient to transport even coarse-grained sand, from the Hydrographer Canyon.

Fig. 10. a. Sea surface temperature (SST) image showing the Loop Current in the Gulf of Mexico and the axis of the Gulf Stream in the Atlantic Ocean along the U.S. Continental margin on March 12, 2011. Darker orange to red color enhancement represents temperatures in the upper 70°s F (upper 20°s C). See also Fig. 28A. Figure from Shanmugam[50], with permission from Elsevier. b. Location map of the Ewing Bank and adjacent areas in the Northern Gulf of Mexico. Solid dots show locations of cores. After Shanmugam et al.[76], with permission from AAPG.

Fig. 11. a. Core photograph showing rhythmic layers of sand and mud. Middle Pleistocene, Gulf of Mexico. Figure from Shanmugam[50], with permission from Elsevier. b. Core photograph showing discrete thin sand layers with sharp upper contacts (top arrow). Traction structures include horizontal laminae, low-angle cross-laminae, and starved ripples. Dip of cross-laminae to the right suggests current from left to right. Note rhythmic occurrence of sand and mud layers. Middle Pleistocene, Gulf of Mexico. After Shanmugam et al.[76], with permission from AAPG.

Fig. 12. a. Conceptual diagram showing a cross-section of a submarine canyon with ebb and flood tidal currents (opposing arrows). Shepard et al.[113] measured current velocities in 25 submarine canyons at water depths ranging from 46 to 4200 m by suspending current meters commonly 3 m above the sea bottom. Measured maximum velocities commonly range from 25–50 cm/s. Figure from Shanmugam[42], with permission from Elsevier. b. Time-velocity plot from data obtained at 448 m in the Hueneme Canyon, California, showing excellent correlation between the timing of up- and down-canyon currents and the timing of tides obtained from tide tables (solid curve). 3mAB = Velocity measurements were made 3 m above sea bottom. Modified after Shepard et al.[113], with permission from AAPG.

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Deep-water petroleum reservoirs exhibit parallel laminae and double mud layers in offshore Nigeria (Fig. 13a) and in the Bay of Bengal (Fig. 13b). Double mud layers are unique to deposition from tidal currents in both shallow-water[163, 164] and in deep-water environments[42, 84, 165-167]. However, such parallel laminae are commonly mislabeled as Bouma Tb divisions and misinterpreted as turbidites[38, 47, 57, 102]. 2.2.4.

Internal wave-/tide-driven baroclinic currents

Internal waves are gravity waves that oscillate along the interface between two water layers of different densities (i.e.,

Fig. 13. a. Core photograph showing double mud layers (DML), indicative of deposition by deep-marine tidals currents, in a submarine-canyon setting. Pliocene strata, Edop Field, offshore Nigeria. Figure from Shanmugam[42], with permission from Elsevier. b. Sedimentological log showing alternation of sand (lithofacies 3) and mudstone (lithofacies 4) intervals with continuous presence of double mud layers (DML). Note muddy debrite facies (Lithofacies 2) near the bottom. Wentworth grain-size classes: C = clay; S = silt; VFS = very fine sand; FS = fine sand; MS = medium sand. C. Lithofacies 3 core photograph showing rhythmic bedding (rhythmites) and double mud layers (DML, arrows) in sand. N = Neap (thin) bundle; S = Spring (thick) bundle. Note that one could designate the DML intervals as Tb and the massive sand unit (between scale divisions 2 and 4 cm) as Ta using the Bouma Sequence, however, Shanmugam et al. did not[75]. Core photograph from Shanmugam et al.[75], with permission from SEPM.

pycnocline). Gill (1982) illustrated that fluid parcels in the entire water column move together in the same direction and with same velocity in a barotropic (surface) wave, whereas fluid parcels in shallow and deep layers of the water column move in opposite directions and with different velocities in a baroclinic (internal) wave (Fig. 14). Apel[169], Apel et al.[170] and Jackson[171] documented internal waves and tides worldwide. A sedimentologic and oceanographic review of baroclinic currents associated with internal waves and tides is provided by Shanmugam[53]. Internal waves are gravity waves that oscillate along oceanic pycnoclines (Fig. 15a). In a stratified ocean, internal tides are generated commonly above an area of steep bathymetric variation, such as the shelf break, seamount, etc. Empirical data on physical properties of internal solitary waves and tides, which include wave speed, have been compiled for 51 locations in the world’s oceans (Shanmugam[53], his Table 2). Turnewitsch et al.[172] discussed internal tides and sediment dynamics in the deep sea using evidence from radioactive 234Th/238U disequilibria. Brandt et al.[173] reported results of high-resolution velocity measurements carried out by means of a vessel-mounted acoustic Doppler current profiler on the November 12, 2000 in the equatorial Atlantic, at 44°W between 4.5°N and 6°N. The data showed the presence of three large-amplitude internal solitary waves. The pulse-like intense solitary disturbances propagated perpendicular to the Brazilian Shelf, toward the north-northeast. These internal waves were characterized by maximum horizontal velocities of about 200 cm/s and maximum vertical velocities of about 20 cm/s. Shepard[174] suggested that internal waves, which occur in canyon depths of up to 3 500 m, were mostly tidal in origin (i.e., internal tides). In the Suruga Trough in Japan, semidiurnal tidal fluctuations are evident in the current with the total amplitude reaching 50 cm/s at a depth of 1 370 m. These currents have been associated with internal tides[176]. Velocity measurements

Fig. 14. a. Barotropic (surface) wave showing the movement of fluid parcels in the entire water column (H) in the same direction with same velocity (short horizontal arrows). b. Baroclinic (internal) wave showing the fluid-parcel movement in upper (H1) and lower (H2) layers in opposite directions (short horizontal arrows) with different velocities (U1 and U2). Modified after Gill[175]. From Shanmugam (2013a)[53], with permission from AAPG.

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Fig. 15. a. Conceptual oceanographic and sedimentologic framework showing deposition from baroclinic currents on continental slopes, in submarine canyons, and on guyots. On continental slopes and in submarine canyons, deposition occurs in three progressive stages: (1) incoming internal wave and tide stage, (2) shoaling transformation stage, and (3) sediment transport and deposition stage. Continental slopes and submarine canyons are considered to be environments with high potential for deposition from baroclinic currents. In the open ocean, baroclinic currents can rework sediments on flat tops of towering guyot terraces, without the need for three stages required for deposition on continental slopes. In this model, basin plains are considered unsuitable environments for deposition of baroclinic sands. Not to scale. From Shanmugam[53], with permission from AAPG. b. Cross-profile showing asymmetrical dunes and asymmetrical ripples observed from side-looking sonar and photographic evidence obtained from the terrace of the Horizon Guyot, Mid-Pacific Mountains. Bathymetry of bedforms: 1 6301 632 m. Dune heights (H) were estimated from the length of acoustic shadows. Redrawn from Lonsdale et al. (their Fig. 10)[111], with permission from the Geological Society of America.

associated with internal tides in the Gaoping Submarine Canyon off southwestern Taiwan have revealed maximum velocities of more than 100 cm/s[177]. At these velocities, even gravel- grade grains can be eroded, transported, and deposited by baroclinic tidal currents. In fact, Lonsdale et al.[111] documented asymmetrical dunes and asymmetrical ripples observed using side-looking sonar and photographic evidence obtained from the terrace of the Horizon Guyot, Mid-Pacific Mountains at a depth of 1 6301 632 m (Fig. 15b). Despite a great wealth of oceanographic information published on internal waves and tides[170], there is a clear lack of published sedimentological details of baroclinic currents[53]. This knowledge gap hinders distinguishing baroclinites (i.e., deposits of baroclinic currents) from contourites in the ancient stratigraphic record. 2.3.

Oceanographic problems

Recently, Shanmugam[61] discussed details of various oceanographic problems. Selected examples are as follows. 2.3.1.

Complex forcing

The oceanic Thermohaline Circulation (THC) is driven by a combination of wind stress, convection, and (tidal) mixing/turbulence[18, 27, 91, 157, 178]. The paradigm of global ocean

circulation has been the thermohaline forcing of two independent water masses, namely the North Atlantic Deep Water (NADW) or the "great ocean conveyor"[18] and the Antarctic Bottom Water (AABW)[119]. The global ocean circulation is initiated in the Southern Ocean (Antarctica) as the cumulative result of (1) wind-driven (adiabatic) upwelling, (2) surface buoyancy flux, and (3) deep-water formation by cooling and saline rejection (i.e., thermohaline). Both atmospheric forcing (i.e., surface-wind stress) and thermohaline forcing (i.e., bottom-water formation) are necessary to induce and maintain global ocean circulation (Fig. 16)[27]. For example, the Antarctic Circumpolar Current (ACC) is widely-accepted as being dominantly a wind-driven current[161]. In light of the complex forcing of most water masses, it is inappropriate to classify an ancient layer as a contourite routinely, with a skewed emphasis on thermohaline forcing and with a total avoidance of the role of atmospheric forcing. 2.3.2.

The continuum principle

Rebesco et al. (their Fig. 1)[179] begin their review with a ternary diagram with three end members composed of contourites, turbidites, and pelagites. The ternary diagram is based on the continuum principle of these three basic deep-sea sediment types that was advocated nearly 35 years ago by Stow and

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Fig. 16. Schematic diagram showing the wind-driven and thermohaline-driven mechanisms in the Southern Ocean (Antarctica) in initiating global ocean circulation. From Talley[27], with permission from the Oceanography Society.

Lovell[13]. It is difficult, however, to reconcile a process continuum between turbidity currents and contour currents. By definition, the term "continuum" refers to a gradual transition from one end member to the other, without any abrupt changes. The continuum principle is unsustainable because downslopeflowing turbidity currents and along-slope flowing contour currents are almost at right angles with each other (Fig. 17). Even if the two interact with each other, the interaction would be emphemeral and is of no sedimentological significance. 2.3.3.

Nevertheless, other authors have broadened the meaning (Fig. 18), which is confusing.

Controurites and contourite drifts

Contourites, based on a regional study of the continental rise off eastern United States in the Atlantic Ocean, covering the Blake Plateau and Blake-Bahama Outer Ridge (Fig. 3, location A), are defined as deposits of thermohaline-driven geostrophic contour currents in deep-marine environments[3, 5, 8]. Their seminal study was based on a robust dataset composed of echo sounding, bottom photographs, and sediment cores.

Fig. 17. Conceptual model showing the spatial relationship between downslope turbidity currents and along-slope contour currents. This is an unlikely scenario for developing a process continuum between the two types. After Shanmugam et al.[76], with permission from AAPG.

Fig. 18. Four types of bottom currents and their depositional facies. The facies term "contourites" is appropriate only for deposits of thermohaline-driven geostrophic contour currents in deep-water environments, but not for deposits of other three types of bottom currents (i.e., wind, tide, or baroclinic). Note that BCRS represent only sandy lithofacies, but may also be applicable to silty lithofacies. Figure from Shanmugam[59], with permission from Elsevier.

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The term “contourite drift” is used commonly in the geologic literature[180, 25]. The Antarctic Circumpolar Current (ACC) produces drifts at great depths of over 3 000 m[161, 181]. These drift sediments are products of currents that follow bathymetric contours, and therefore they could be classified as contourites. However, these drift sediments are not genuine "contourites" because they are products of mostly wind-driven currents, not thermohaline-driven currents. In other words, contourites could be generated by more than one type of bottom currents. The problem here is that there are no sedimentological criteria for distinguishing deposits of purely winddriven bottom currents from those of thermohaline-driven bottom currents. Therefore, the application of the term "contourites" to the ancient stratigraphic record, with little information on forcing mechanisms, should proceed with caution. A solution is to replace the genetic term “contourite drift” with a non-genetic term "sediment drift". 2.3.4.

Hiatuses in contourites

In deep-marine environments, regional erosion throughout thousands of square kilometers of seafloor has been attributed to bottom currents[182, 183]. In the Gulf of Cadiz, the lower core of the Mediterranean Outflow Water tends to cause more erosion[29]. In the Rockall Trough region, bottom currents associated with the NADW (North Atlantic Deep Water) have caused an erosive area extending over 8500 km2 in water depths of 5002 000 m[184]. This erosive phase, which eroded approximately 150 m of sediment and lasted nearly 35 Ma (Early Oligocene-Holocene), existed through four supercycles (second order) and 23 cycles (third order) of sea-level rise and fall in the global chronostratigraphic chart of Haq et al.[185]. Viana[186] cautioned on the potential dangers of misinterpreting regional unconformities at the base of contourites as “sequence boundaries” on seismic profiles using examples from the Santos Drift, offshore Brazil[187]. Clearly, there is no simple correlation between current-induced erosional surfaces (unconformities) and eustasy. These practical challenges exist because there are no objective criteria to recognize erosional surfaces, caused by deep-marine bottom currents versus by other processes, on seismic profiles[36, 45]. 2.3.5.

Origin of erosional features

In defining the contourite depositional system (CDS), Hernández-Molina et al.[188] state, “An association of various drifts and related erosional features has been termed a ‘‘contourite depositional system’’ (CDS)…” This inclusion of erosional features under the term “contourite depositional system” is conceptually confusing. It is useful to maintain a distinction between erosion and deposition. A solution is simply to group both erosion and deposition under “contourite system” instead. Alternatively, one might adopt two different systems, namely (1) contourite depositional system and (2) contourite erosional system. By nature, erosion does not leave behind any clue in the rock record for establishing the type of process that caused the erosion. Furthermore, modern unfilled submarine channels

and canyons are a testimony to the fact that the processes that created these erosional features in the past are probably not the same processes that will fill them in the future. Therefore, there is a need to develop criteria for distinguishing erosional features cut by contour currents from those cut by other processes, such as turbidity currents. 2.3.6.

Gulf of Cadiz as the type locality

Hernández-Molina et al.[28] characterized the Gulf of Cadiz as “the world’s premier contourite laboratory”. The modern Gulf of Cadiz has served as the center for contourite research activities since the 1970s (Fig. 3, location F). The Gulf of Cadiz, despite its popularity, has its limitations. Although the MOW in the Gulf of Cadiz is a thermohaline-driven water mass[189], it is not a genuine contour current. Zenk[24] characterizes the behavior of MOW as follows: “The warm and salty Mediterranean outflow water (MOW) in the Gulf of Cadiz of the eastern North Atlantic represents an excellent example for the transition between a purely bottom-following current to a genuine contour current…” Empirical data indeed support the transition of the MOW in the Gulf of Cadiz. The MOW undergoes three progressive stages of evolution during its journey from the Strait of Gibraltar where it enters the Gulf of Cadiz to Cape São Vicente where it exits the gulf before entering the Atlantic Ocean (Fig. 19). In other words, genuine contour currents do not operate in the Gulf of Cadiz (Fig. 19). Furthermore, the Gulf of Cadiz site is a highly complex oceanographic location for studying depositional and erosional aspects of genuine contour currents because the deep-sea sediments in this gulf are controlled by the following factors (Fig. 19):  Transitory Mediterranean Outflow Water[24]  Internal waves and tides[190, 191]  Sediment-gravity flows[28]  Pelagic and hemipelagic settling  Tsunamis[192]  Cyclones[192]  Mud volcanism[193, 194]  Methane seepage[195]  Sediment supply[133]  Pore-water venting and hydraulic pumping[196]  Channels and ridges[26]  The Camarinal Sill[197]. Complex localities like the Gulf of Cadiz requires an understanding of all processes in concert with each other because "deep-water" processes are tightly intertwined with "shallowwater" processes by oceanic wave phenomena, such as internal waves and tsunamis. Therefore, the archaic notion of dealing with a particular "deep-water" process (e.g., contour currents) in a vacuum is no longer meaningful. The 21st century necessitates the rigor of holistic process sedimentology. 2.3.7.

Abyssal plain contourites

Hernández-Molina et al.[28] discussed "abyssal plain contourites". Conventionally, the term "abyssal plain" refers to a flat region of the ocean floor, usually at the base of a continental rise, where slope is less than 1:1 000[198]. It represents

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Fig. 19. Schematic diagram showing the location of Gulf of Cadiz and complex transport nature of the Mediterranean Outflow Water (MOW), involving three stages of evolution: (1) channel-current stage, (2) mixing and spreading (i.e., transition) stage, and (3) genuine contour-current stage (see Zenk, 2008, his Fig. 4.10)[24]. Figure from Shanmugam[59], with permission from Elsevier.

the deepest and flat part of the ocean floor that occupies between 4 000 and 6 500 m in the U.S. Atlantic Margin. A more general term “basin plain” is commonly used in referring to ancient examples[199]. However, Hernández-Molina et al.[28] consider abyssal plains or basin plains to include up to 10 distinct morphological elements: (1) continental rise, (2) abyssal plains, (3) oceanic rises, (4) distal fans and their distributary channels, (5) sediments drifts, (6) abyssal hills, (7) seamounts, (8) transfer fracture zones, (9) mid-ocean channels, and (10) oceanic trenches. This reclassification of abyssal plains, ignoring the basic principles of classification of continental shelf, slope, rise, and plain based on the position of seafloor depths, is not only unnecessary but confusing. This reclassification defies the basic concept of "contour currents" that was introduced for contour-following bottom currents along continental slope and rise, not for bottom currents over flat abyssal plains.

3.

flow Water (MOW) in developing the first muddy contourite facies model from the Gulf of Cadiz (Fig. 20). Students[202, 203] and researchers[179] use this model routinely. Nevertheless, the vertical facies model suffers for the following reasons.

Process sedimentology and related problems

3.1.

Process sedimentology

Sanders[200] published the pioneering paper on process sedimentology entitled “Concepts of fluid mechanics provided by primary sedimentary structures.” A combined knowledge of basic physics, soil mechanics, and fluid mechanics is essential for interpreting the mechanics of various fluid-sediment-gravity processes[201]. Related details are discussed by Shanmugam (2006, Chapter 1)[43]. The underpinning principle here is the sustainable observation of primary sedimentary structures and a consistent process interpretation. 3.2.

The contourite facies model

Faugères et al.[16] explained the role of Mediterranean Out-

Fig. 20. a. Revised contourite facies model with five divisions proposed by Stow and Faugères[94]. b. Original contourite facies model by Faugères et al.[16]. Note that the original authors of this model did not include the five internal divisions[16]. The most recent version of this model by Faugères and Mulder[25] contains neither the five internal divisions nor the hiatuses in the C3 division (red arrow inserted in this article). Originally from Faugères et al.[16], with permission from the Geological Society of America.

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3.2.1.

3.2.2.

Five internal divisions [16]

Faugères et al. (1984) developed the original facies model without internal divisions. Stow and Faugères (2008: their Fig. 13.9)[94], however, revised the original model with five internal divisions (C1, C2, C3, C4, and C5) (Fig. 21a), analogous to the Bouma turbidite model (Bouma, 1962). In their most recent version, Faugères and Mulder (2011, their Fig. 3. 18)[25] have reverted back to the 1984 version, without the five internal divisions. Reasons for such back-and-forth fundamental changes to the facies model, by the same group of authors, need to be explained in the literature for the benefit of the international research community. If recognized in the ancient rock record, these five divisions would reveal nothing about deposition from thermohaline-driven geostrophic contour currents in deep-water environments.

Current velocities

The vertical facies model, composed of a basal upwardcoarsening interval followed by an upward-fining interval (Fig. 20b), has been attributed to a successive increase and decrease in contour-current velocity and competency[16]. However, Mulder et al.[31] suggest that the origin of this vertical sequence is much more complex than due to a simple velocity variation. Mulder et al.[31] state that "... the contourite sequence is only in part related to changes in bottom current velocity and flow competency, but may also be related to the supply of a coarser terrigeneous particle stock, provided by either increased erosion of indurated mud along the flanks of confined contourite channels (mud clasts), or by increased sediment supply by rivers (quartz grains) and downslope mass transport on the continental shelf and upper slope. The clas-

Fig. 21. Core photographs showing sedimentary facies of contourites (a, b, c, e), turbidites (d), debrites (f), and slumps (g) recovered during IODP Expedition 339. Note that vertical grain-size variations showing grading are schematic (red arrows), not factual using the Wentworth grain-size class on the abscissa. From Hernández-Molina et al. (2013)[28], with permission from IODP Expedition 339 Scientiic Drilling.

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sical contourite facies association may therefore not be solely controlled by current velocity, but may be the product of a variety of depositional histories." No further explanation is necessary. 3.2.3.

Internal hiatuses

In the original contourite facies model, Faugères et al. (their Fig. 4.)[16] did not include internal hiatuses. However, Stow and Faugères (their Fig. 13.9)[94] included hiatuses in the middle C3 division of their revised contourite facies model (Fig. 20a, see horizontal red arrow). In the most recent 2011 version of the model (Faugères and Mulder, their Fig. 3.18)[25], the hiatuses are absent once again. How can a natural, observed, sedimentary feature (i.e., hiatus) simply vanish? The authors need to explain this puzzle. Wetzel et al.[204] state, “When bottom currents prevent deposition for a considerable time span, and/or erode sediments, submarine hiatuses develop, represented by semi-consolidated firm or hard grounds or stable cohesive partially dewatered muddy substrates.” Because hiatuses occur in the C3 division (Fig. 20b), the lower and upper intervals must represent two different depositional events. Conventionally, a genetic facies model is designed for a single depositional event, without internal hiatuses (e.g., the turbidite facies model)[116]. In fact, Walther’s Law[205] is not meaningful for sequences with internal hiatuses. This is because a hiatus can represent a considerable span of time (spanning millions of years) that is missing in the sedimentary record[184]. 3.2.4.

Bioturbation

A characteristic feature of the contourite facies model is the bioturbation (Fig. 20b), which has generated debates[41, 206]. The genetic link between contourites and bioturbation is based on the belief that active contour currents would increase the oxygen concentration of the water mass[207], and thereby would increase the activity by benthic organisms. Conventionally, a genetic facies model (e.g., the turbidite facies model)[116] is based on vertical disposition of primary physical sedimentary structures. This is because physical structures can be used to interpret a particular physical process in the rock record. But bioturbation cannot be used as a criterion for interpreting deposit of a single process (i.e., contour currents). The bioturbation criterion is defective because ancient deep-water turbidites (e.g., in the Late Cretaceous Point Loma Formation near San Diego, California) are also extensively bioturbated and even contain the trace fossil Ophiomorpha[208]. Furthermore, convincing cases of “contourites” without bioturbation have been documented in the rock record[209]. In describing Canterbury Drifts from SW Pacific Ocean, Carter[108] state that "Bioturbation is moderate and rarely destroys the pervasive background, centimetre-scale, planar or wispy alternation of muddy and sandy silts displayed by Formation Micro-Scanner imagery." In summary, the muddy contourite facies model with emphasis on bioturbation defies the very first principle of process sedimentology, which is to

interpret the fluid mechanics of depositional processes using primary physical sedimentary structures[200]. Therefore, the contourite facies model is sedimentologically obsolete. 3.2.5.

Multiple interactive processes

The Gulf of Cadiz is an extremely complex setting in terms of physical oceanography with multiple processes (e.g., MOW, internal waves, and internal tides) and bottom topography with channels, ridges, and sills (Fig. 19). Deep-water depositional processes are variable in time and space. Furthermore, extensive bioturbation caused by influx of prolific oxygen in deep-sea currents obliterates physical structures. From a practical viewpoint of interpreting ancient deposits as contourites on land, there is no way of knowing the contours of the paleo-seafloor. In summary, the global applicability of the contourite facies model is dubious. 3.2.6. Grain-size data A fundamental aspect of many sedimentological studies is the documentation of detailed vertical grain-size variation that is plotted on a sedimentological log. It is so vital that the present author has allotted the maximum space for grain size (i.e., expanded column widths for silt, very fine sand, medium sand, etc.) in sedimentological logs (see Fig. 13b). But such sedimentological logs illustrating vertical grain-size variations and other sedimentological details for sandy contourite intervals are absent in publications by Stow and Faugères[94] and by Stow et al.[210]. In fact, none of the 19 core photographs (6 from the Gulf of Cadiz, 8 from the Brazilian margin, and 5 from the UK margin) has associated sedimentological logs in Stow and Faugères[94]. Consequently, the reader is left with core photographs of sandy contourites without the fundamental grain-size data. 3.2.7. IODP 339 results During the IODP (Integrated Ocean Drilling Program) Expedition 339, five sites were drilled in the Gulf of Cádiz and two sites off the West Iberian margin[28]. The total length of recovered core is 5447 m, with an average recovery of 86.4% (Expedition 339 Scientists, 2012)[211]. Published results of the IODP 339 core studies, although preliminary, are useful in testing the contourite facies model.  A key element of the contourite facies model is the vertical grain-size variations (Fig. 20b). However, none of the published lithologic columns of drilled intervals contains Wentworth grain-size class on the abscissa.  Core photographs labeled as "bigradational sequences" (Fig. 21a) and "sandy contourite" (Fig. 21c) do not show vertical grain-size variations based on measurements.  Specific sedimentological criteria used for distinguishing base-cut-out contourites with normal grading (Fig. 21b) from turbidites with normal grading (Fig. 21d) are not discussed.  The five internal divisions of the contourite facies model are not evident in any of the published core intervals. Even in the core interval U1390A-8H-6A, which is labeled as “Bigra-

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dational grading”, which presumably represents the entire contourite sequence, the five internal divisions are not evident (Fig. 21a).  Expedition 339 Scientists[211] reported hiatuses in contourites. It is unclear as to how these hiatuses fit into the contourite facies model. Do these hiatuses represent the C3 division in the model (Fig. 20b)?  Unlike turbidites with a sharp or an erosional contact at the base, contourites with gradational bases do not have a precise point of origin (Fig. 21). As a consequence, the starting point of a basal inversely-graded contourite sequence is purely subjective. One of the principal scientific aims of IODP 339 Expedition was to investigate the nature and effects of the bottom currents related to Mediterranean Outflow Water (MOW) on contourite deposition and erosion along the Iberian continental margin[28]. In summarizing the results of IODP 339 cores, Stow et al.[212] reported the following characteristics:  The uniformity in sedimentation of muddy contourites  The dominance of greenish grey colour  The general absence of primary sedimentary structures  The sediment homogenization by bioturbational mottling  The uniformly mixed biogenic-terrigenous composition  The consistent cyclicity of facies  The grain size in bi-gradational units. The two fundamental problems are evident from the IODP 339 cores. First, the absence of primary sedimentary structures, which renders it impossible to interpret depositional processes[200]. Second, thin bigradational muddy units are impossible to recognize in the compacted mudstone intervals in the ancient stratigraphic record. Expedition 339 Scientists[211] report that cored intervals at both Sites of U1390 and U1391 show similar features, such as, bigradational trends, a lack of five internal divisions, and internal hiatuses. The problem is that the Site U1390 is located within the Gulf of Cadiz (36°19.110′N; 7°43.078′W, whereas the Site U1391 is located outside the Gulf of Cadiz, on the southwest Iberian Margin (37°21.532′N; 9°24.656′W). Therefore, the true significance of MOW in developing unique properties of contourite deposits within the Gulf of Cadiz (touted as the premier contourite site) is unconvincing. In IOPD 339 cores, Alonso et al.[213] have identified all five divisions of the contourite acies model, namely C1, C2, C3, C4, and C5, in core photographs, but failed to provide corresponding vertical grain-size variation using Wentworth scale. Instead, each contourite division is shown to exhibit grain-size trend in a vertical column without any scale on the abscissa, which makes it practically impossible to evaluate the true vertical variation in grain size. Even if there are subtle differences in grain size among the five divisions, it would be impossible to recognize these massive contourite divisions without primary sedimentary structures (e.g., ripple cross-laminae) in the ancient rock record due to compaction. The ultimate goal of studying modern analogs, such as the Gulf of Cadiz, is to gain knowledge in interpreting ancient deposits as

contourites for which the information on paleo-current circulation is absent. But the sedimentological features observed in the cores of IOPD 339 sites yet failed to provide that basic knowledge for interpreting ancient strata as contourites. Alonso et al.[213] have also recognized internal divisions, composed of Tc, Td and Te of the now defunct turbidite facies model, known as the "Bouma Sequence"[38]. The problem is that "Tc, Td, and Te turbidite divisions" can also be formed by bottom-current reworking, composed of contour currents (Fig. 22). For example, in areas in which both downslope sandy debris flows and along-slope-bottom currents operate concurrently (Fig. 22a), the reworking of the tops of sandy debris flows by bottom currents may be expected. Such a scenario could generate a basal massive sand division and an upper reworked division, mimicking a partial Bouma Sequence (Fig. 22b). The reworking of deep-water sands by bottom currents has been suggested by other researchers as well[214-216]. But Alonso et al. (2016)[213] ignored this alternative possibility in their interpretation. Genetic facies models are nothing more than a “groupthink”[50] that tends to thrive more on custom and complacency than on intellect and innovation. 3.3.

Traction structures

The presence of traction structures in cores and outcrops

Fig. 22. a. Conceptual model showing reworking the tops of downslope sandy debris flows by along-slope bottom currents. Such complex deposits would generate a sandy unit with a basal massive division and upper reworked divisions with traction structures (ripple laminae), mimicking the ‘Bouma sequence.’ Figure from Shanmugam[43]. b. The turbidite facies model (i.e., the Bouma Sequence) showing Ta, Tb, Tc, Td, and Te divisions. Conventional interpretation is that the entire sequence is a product of a turbidity current[116, 217, 218]. According to Lowe[219], the Ta division is a product of a high-density turbidity current and Tb, Tc, and Td divisions are deposits of low-density turbidity currents. In this article, the Ta division is considered to be a product of a turbidity current only if it is normally graded, otherwise it is a product of a sandy debris flow; the Tb, Tc, and Td divisions are considered to be deposits of bottom-current reworking. Figure from Shanmugam[38], with permission from Elsevier.

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(Fig. 8) have long been recognized as evidence for bottomcurrent reworked sands by contour currents, wind-driven currents, and tidal currents in deep-water strata[3, 6, 7, 9, 46, 76, 77, 82, 86, 214, 220222]. As noted earlier, ripples and dunes have been associated with internal tidal currents[135]. Plus, traction structures have been associated with tsunami-related bottom currents[52] and with cyclone-related bottom currents[48, 52]. In other words, traction structures and bedforms have been associated with all types of bottom currents. The challenge is how to distinguish a traction structure (e.g., ripple or parallel laminae) formed by contour currents from those formed by other types of bottom currents in the ancient stratigraphic record. 3.4.

Shale clasts

In discussing the origin of shale clasts in muddy and sandy contourites, Stow and Faugères[94] state, “The shale clasts are generally millimetric in size, and occur with long axes sub-parallel to bedding and, presumably, also sub-parallel to the current direction.” Alternatively, the planar clast fabric (i.e., alignment of long axis of clasts parallel to the bedding surface) could be interpreted as evidence for laminar debris flow[64, 223, 224]. In short, there are no reliable sedimentological criteria that one can apply in interpreting the ancient rock record as sandy contourites. 3.5.

Bedform-velocity matrix

Van Rooij (2013)[225] used the bedform-velocity matrix (Fig. 23) of Stow et al.[226] in discussing the challenges associated with processes and products of deep-water bottom currents. Problems associated with the bedform-velocity matrix are as follows:

 Stow et al.[226] proposed a bedform-velocity matrix (Fig. 23) for deep-water bottom currents. This matrix diagram is a slightly modified version of Figs. 3.1 and 3.2 in Belderson et al.[227]. Stow et al.[226] applied the bedform-velocity matrix, developed by Belderson et al.[227] for shelf tidal currents, to all types of deep-water bottom currents. But shallow-water tidal currents and deep-water bottom currents are not one and the same hydrodynamically. As mentioned earlier, least four different types of deep-water bottom currents exist[46]. The underpinning assumption of the matrix, which is that all four deep-water bottom currents hydrodynamically behave the same as the shallow-water tidal currents, is incongruous.  Stow et al.[226] acknowledged that (1) although the velocity data presented by them were for near-bottom flow, they did not define the exact height above seafloor; (2) they did not address the variable nature of the benthic boundary layer that will also complicate how flow velocity affects seafloor bedform; (3) for most of their data sets it was impossible to know the precise flow velocity (mean or peak) that created the observed bedform; (4) they rarely had the opportunity of witnessing the development of deep-water bedforms in situ; and (5) they did not consider the effects of sediment supply and bed roughness on bedform development. In other words, the matrix was built without the necessary empirical data.  The concept of bedform-velocity matrix became popular in the 1960s with the advent of matrix diagrams of alluvial sedimentary structures based on empirical data derived from flume experiments[228]. However, the matrix diagram proposed by Stow et al.[226] is not based on experiments; meaning that their “data” are neither verifiable nor reproducible independently.  In commenting on the problems with the bedform-velocity diagram of Stow et al.[226], Dykstra (his Fig. 14.2 caption)[229] states, “Note that this Fig. does not take into account either the duration of a current or sediment availability, both of which are important controls on the development of bedforms…” Given the above uncertainties, it is unreliable to estimate current velocities for modern bedforms using the bedformvelocity matrix. 3.6. Seismic profiles, sonar images, and submarine photographs

Fig. 23. Bedform-velocity matrix for deep-water bottom currents. From Stow et al.[226], with permission from the Geological Society of America.

Nelson et al.[230] interpreted sandy contourites in the Gulf of Cadiz based on seismic data, but without critical sedimentological data. Well-developed wave geometries seen on seismic profiles, interpreted as mega sediment waves formed by the MOW off Southwest Portugal, have been reported[231]. However, seismic wave geometry has also been associated with sand dunes formed by internal solitary waves[232]. Furthermore, no objective criteria exist to distinguish wave geometry created by contour currents from wave geometry created by tidal currents or by turbidity currents on seismic profiles[50]. In their comprehensive review of seismic expression of contourite depositional systems, Nielsen et al.[231] state that “... because the reflections result from changes in the physical parameters through the sedimentary succession, there is no unequivocal correlation between seismic facies and sedimen-

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tary structures within the facies. A seismic facies characterized by a parallel reflection configuration, for example, need not necessarily indicate the existence of fine parallel banding or stratification of the sediments.” In a study of the Faro Drift in the Gulf of Cadiz, Alonso et al.[213] calibrated seismic profiles (Fig. 24) with cores from two Sites U1386 and U1387. Analogous to contourites on the Norwegian Margin[233], muddy contourites in the Gulf of Cadiz also show parallel reflections (Fig. 24). The problem is that cored intervals of muddy contourites do not show any primary sedimentary structures, which are the foundation of process sedimentology. Therefore, these muddy contourites

cannot be recognized in the compacted ancient sedimentary record either on seismic profiles or in sediment cores. Clearly, there are fundamental problems in using seismic facies for interpreting both muddy and sandy bottom-current deposits. Bottom-current-reworked sands are difficult to recognize even from the direct examination of the rocks because of the presence of traction structures in deposits of all four types of bottom currents. Sedimentary bedforms on the seafloor have been documented using side-scan sonar images[235]. Stow et al.[26] state, “Examination of bottom photographs is one of the principal methods by which we can determine the nature of processes

Fig. 24. a. Location map showing Expedition 339 sites (yellow solid circles) in the Gulf of Cadiz and West Iberian margin. Note red line through Sites U1386 and U1387 showing the position of seismic profile shown in Fig. 24b. Map from Hernández-Molina et al.[234]. b. North-South seismic profile of the Pliocene-Quarternary Faro Drift showing parallel reflections, Gulf of Cadiz. See position of this profile in Fig. 24a. See calibrated core intervals in two Sites U1386 and U1387. BQD = Lower Quaternary seismic reflector; M = Messinian seismic reflector. Modified after Alonso et al.[213].

operating at the present day in deep water environments.” Although they have used over 3000 submarine photographs, interpreting a specific process from a bird’s eye view of the submarine photograph is problematical. Photographic images of ripples and other bedforms on the seafloor are useful for inferring current directions, but not current types (i.e., hydrodynamic behaviors). Identical ripple types can be formed by more than one type of bottom current in the deep sea. In the deep Pacific Ocean, for example, ripples and dunes were attributed to internal tidal currents[135] (Fig. 15b). But in the deep Gulf of Mexico, ripples were related to the wind-driven Loop Current[154] at a depth of 3091 m (Shanmugam, his Fig. 4.24)[50]. The problem is that there are no objective criteria to

distinguish ripple types associated with contour currents from those associated with wind-driven bottom currents. In the modern Gulf of Cadiz, where both MOW and internal tides are active, one cannot distinguish the type of ripples formed by MOW-related bottom currents from those formed by baroclinic tidal currents. Turbidity currents and debris flows can develop normal grading and inverse grading, respectively. But such internal features cannot be resolved on submarine photographs of external bedform-surfaces. Internal sedimentary structures are best studied using core and outcrop, which are the key to interpreting fluid mechanics of depositional processes (Sanders, 1963)[92].

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3.7.

Sediment provenance

Commonly, primary sedimentary structures and related current directions are used in deciphering sediment provenance[236238]. However, complex current directions associated with all four types of bottom currents pose immense challenges in inferring the primary sediment source. For example:  Contour currents are global in circultion pattern and flow parallel to the strike of the regional slope (Figs. 1 and 6).  Wind-driven bottom currents are complex in circulation pattern in the Gulf of Mexico (Fig. 10a), which include circular motions (gyres) unrelated to the slope. Such bottom currents have been reported beneath the Gulf Stream Gyre at a depth of nearly 4 km in the northern Bermuda Rise[246]. Laine and Hollister[247] suggest that the Deep Gulf Stream Return Flow (DGSRF) entrains suspended sediment in a deep gyre and may be responsible for the deposition at the base of the continental rise.  Deep-marine tidal currents are bidirectional in nature and they flow up and down submarine canyons (Fig. 12a).  Baroclinic currents are extremely variable in propagation directions with respect to sediment source (Fig. 25).  Because bottom currents are strictly a reworking agent, their sedimentary structures do not reflect the true direction of the primary sediment source. Therefore, the conventional approach of inferring source directions (i.e., sediment provenance), using current ripples

and cross beddings, is unreliable when dealing with deepmarine bottom currents and their deposits. The other important criterion in interpreting sediment provenance is the detrital composition[238, 242]. However, reworking by bottom currents may not alter the original composition of the sediment derived from the primary provenance. For example, in understanding the compositional difference between contourites and turbidites in the Bounty Submarine Fan, New Zealand, cored intervals from the Ocean Drilling Program (ODP) Site 1122 on Leg 181 have been studied. In discussing the results, Shapiro et al.[243] state that "... there are no significant trends among thickness, grain size, composition, and depth of Site 1122 sand samples, except that thicker beds tend to contain slightly more metamorphic rock fragments. The generally homogeneous composition of Site 1122 sand indicates that it may have had a relatively uniform source back into the early Miocene. Thus, the up-section change from sandy contourite to turbidite deposits at Site 1122 is not reflected in sand composition. This suggests that the sand provenance remained constant while the depositional processes of sand at Site 1122 changed." Distinguishing compositional variations caused by variations in deep-sea depositional processes is a potential area of future research on sediment provenance.

4.

Petroleum geology Aspects of petroleum geology regarding bottom-current

Fig. 25. Maps showing the variable directions of propagation of internal waves with respect to shoreline or shelf edge seen as surface manifestations on satellite images. a. Internal waves propagating toward the shoreline of Palawan Island in the Sulu Sea. b. Internal waves propagating away from the shoreline or shelf edge in the Yellow Sea[239]. c. Internal waves propagating nearly parallel to the shoreline of northern Somalia in the Indian Ocean[171, 240]. d. Internal waves propagating parallel to the strait or channel axis in the Strait of Messina. e. Internal waves propagating in the same direction on both sides of the Strait of Gibraltar. Note the position of the Camarinal Sill at the point of origin of internal waves[197]. f. Internal waves propagating in opposite directions from the point of origin, which is a sill in the Lombok Strait[241]. Baroclinic currents, associated with internal waves and tides, are reworking agents and as such they are unrelated to the primary sediment provenance. Features shown are schematic and not to scale. From Shanmugam[53], with permission from AAPG.

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reworked sands and, in particular, contourites were discussed by Shanmugam et al.[76] and by Viana[186]. 4.1.

Seal potential

Muddy sediment waves formed by benthic circulation by Antarctic Bottom Water (AABW) in the Argentine Basin show impressive dimensions with an area of 1.0 x 106 km2, an amplitude of 26 m (average), and a wavelength of 3-7 km[90]. These muddy contourite drifts (Fig. 26) can regionally develop thick accumulations, which might have seal potential for trapping hydrocarbon. 4.2.

Source-rock potential

Published data on total organic carbon (TOC) of muddy contourites are rare. In this study, one muddy contourite sample Off Bermuda Rise, affected by the Gul Stream, shows 0.35% TOC (Table 4). However, an exceptionally high value of TOC of up to 2% has been reported from the muddy siliciclastic contourites of the Great Antilles Outer Ridge[244]. Because active contour currents tend to increase the oxygen concentration of the water mass and would increase the activity by benthic organisms[207], oxic conditions are not favoured in preserving TOC in contourites. However, Yu et al.[100] discussed accumulations of gas hydrates in contourites using bottom simulating reflectors in the northern South China Sea

(Fig. 3, location L). Finally, one should not overlook the importance of terrestrial organic matter in generating oil[245] and their contribution to the deep sea[102]. 4.3.

Reservoir potential

Perhaps the first application of the contourite concept to a major petroleum reservoir was in the Frigg Field, North Sea[248]. These authors interpreted a wavy surface, between wells 25/1-1 and 25/1-5, on a seismic profile as evidence for contour currents. The Frigg field was considered as one of the largest gas fields in the world in the 1970s. However, some petroleum geologists still believe that reservoir quality of bottom-current reworked sands, which include contourites, is poor in comparison to that of turbidites. In discussing the reservoir quality of deep-water Miocene sands in the Kutei Basin, Makassar Strait (Fig. 3, location N), Dunham and Saller (2014)[106] claim that “The key point from the perspective of the Exploration-Geologist is that bottom currents did not transport or redistribute these Kutei basin reservoir-sands from their original-depositional locations. If significant redistribution of sand had occurred, our exploration-model would have failed, and we would not have found thick high-quality reservoir sands in our prospects. We based our interpretations (Saller et al. 2006, 2008b) on evidence from seismic data, cores, and exploration discoveries.” Contrary to the above claim, numerous published examples of bottom-current reworked sands, which include sandy contourites, as petroleum reservoirs exist[43, 50, 57, 76-81, 86, 249, 250]. Sandy contourites have the best reservoir potential. Selected examples are the following: 4.3.1.

Fig. 26. Distribution of muddy contourites, composed of sediment waves, formed by benthic circulation by Antarctic Bottom Water (AABW) in the Argentine Basin[90].

Straits of Florida (Fig. 3, location B)

The Florida Current is an ocean surface current that flows from the Gulf of Mexico to the Atlantic Ocean. The water is forced out from the Gulf of Mexico through the Florida Straits, and flows northward along the east coast of the United States (Fig. 10a), where this current joins the Gulf Stream (Fig. 10a). The surface velocity of the Florida Current reaches a maximum of over 160 cm/s at 27°N[251]. Near-bottom velocities of up to 4060 cm/s have been measured in the Straits of Florida and on the Blake Plateau[252, 253]. Mullins et al. (1980)[88]

Table 4.

Measured properties of modern sandy and muddy contourites

Sample

Location (Bottom Current)

Porosity/ %*

Air Permeability/ mD

Weight loss/%

Total organic carbon/%

C5: Sandy carbonate contourite

Off Great Bahama Bank (Florida Current)

-

-

24

0.66



37.5

4809

84

0.20



35.0

688

85

0.18


26.1

550

89

0.07


39.7

9881

89

0.07

Off Bermuda Rise (Gulf Stream)

33.2

186

87

0.35

C8: Sandy carbonate contourite (top hardground part) C8: Sandy carbonate contourite (bottom main part) C9: Muddy siliciciclasticcontourite

*Samples were dried at 76.7 °C (170°F) for 48 hours. Measurements were made at the Field Research Laboratory, Mobil Research and Development Corporation, Dallas, Texas in 1979. Samples C5-C8: Courtesy of H.T. Mullins[88]. Sample C9: Courtesy of E.P. Laine[246, 247].

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discussed the significance of deep-water bottom currents of the Florida Current in forming a thick prism of calciclastic reworked sands (Middle Miocene to Pleistocene) off Little and Great Bahama Banks in the northern Straits of Florida (Fig. 27). These carbonate sandy contourite drifts, underlain by a basal erosional unconformity, are hemiconical-shaped bodies that are up to 600 m in thickness (Fig. 28). Cross-stratification, indicating traction deposition, is present. Bottom currents in this area reach velocities up to 60 cm/s[88]. Sandy calciclastic or carbonate contourites collected off Great Bahama Bank have a maximum porosity of 40% and a maximum permeability of 9881 mD (Table 4). The high permeability has been attributed to the winnowing away of muds from the intergranular primary pores from foraminiferal sands by vigorous contour currents[88]. Scanning electron microscope (SEM) photographs of sandy calciclastic contourites indeed show the mud-free nature of pores (Fig. 27b). Such bottom-current reworked sands are potential petroleum reservoirs. These carbonate sands were lithified by early submarine cementation, thus providing the potential for preservation. Cemented hardground parts show much lower porosty and permeability (Table 4). However, development of secondary porosity by macro- and micro-borings made by endolithic organisms (Wilbur and Neumann, 1977)[254] considerably enhances the reservoir quality of these cemented carbonate con-

tourites. Other aspects of carbonate contourites in this area have been studied by Anselmetti et al.[255] and by Chabaud et al.[256]. 4.3.2.

Israel (Fig. 3, location I)

The Cretaceous Talme Yafe Formation has been interpreted as a carbonate contourite deposit[83] (Fig. 29). This formation is 3 km thick, 20 km wide, and nearly 150 km long. It is characterized by an erosional unconformity. Traction structures (e.g., parallel and cross-laminae) are common. This huge prism of conturite sediments was preserved in a continental slope and rise environment as a result of fault-controlled sedimentation[83]. Geometrically, the thick prism of carbonate sandy conturites occurring off the Great Bahama bank is considered to be a modern analogue of Talma Yafe Formation in Israel. 4.3.3.

South Africa (Fig. 3, location J)

Fleming[257] studied bedforms (Fig. 30) formed by reworking by the Agulhas Current near the shelf edge, southeastern South Africa (Fig. 30). He documented a variety of bedform types, which include gravel pavements, sand ribbons, comet marks, sand streamers, dunes, and smooth sand sheets. The implication is that siliciclastic sandy and gravelly contourites near the shelf edge can develop important reservoirs with high

Fig. 27. a. Image showing the Great Bahama Bank and the Floria Current (see also Fig. 10a). Source: NASA. SEM photographs showing mud-free nature of primary pores in foraminifera-rich carbonate sandy contourites, northern Straits of Florida. b. Primary intergranular porosity. c. Primary intragranular porosity. Note absence of mud in pores due to winnowing by bottom currents, which results in high porosity and permeability. Samples courtesy of H.T. Mullins.

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ers commonly range in thickness from 5 cm to 10 cm, but the entire unit reached up to 6 m in total thickness. 4.3.5.

Fig. 28. Sediment prism consisting of carbonate sandy contourites, reworked by the Florida Current, northern Straits of Florida. After Mullins et al.[88], with permission from AAPG

Bay of Bengal (Fig. 3, location 13)

In the Bay of Bengal (Fig. 3, core location 13), high-quality Pliocene petroleum-producing reservoir sands formed by deep-marine sandy debris flows and tidal currents have been documented in the Krishna-Godavari Basin. Tidalite sands show measured porosity values of 3441% and permeability values of 5255 977 mD (Shanmugam et al., their Table 4)[75]. Individual tidalite units vary from a few centimeters to nearly a meter in thickness (Fig. 13b). 4.3.6.

Gulf of Cadiz (Fig. 3, location F)

In the Gulf of Cadiz, a 10-m thick sheet sand has been interpreted as "contourites"[258]. 4.3.7.

Fig. 29. A depositional model for the Cretaceous (Albian–Turonian) Talme Yafe Formation interpreted as a carbonate contourite deposit. Figure from Bein and Weiler[8].

In Antarctica, a widespread (3200 km2 volcaniclastic contourite sand, formed within the past 9000 yr, has been reported on the continental shelf and upper slope offshore of the Pennell Coast. The thickness of this massive sand ranges from 10 cm to more than 1 m (Rodriguez and Anderson, 2004)[109]. In summary, bottom-current reworked sands have better reservoir quality than turbidites in many cases[50, 56].

5.

Fig. 30. Depositional model showing various sandy bedforms (e.g., sand waves) and gravel lags formed by reworking by the Agulhas Current near the shelf edge, southeastern South Africa. Note that the zone of potential contourite reservoir (thin red horizontal arrow) is added in this study. Figure modified after Fleming[257].

porosity and permeability. If preserved, these sandy and gravelly contourites may occupy areas covering 10s of km in length (i.e., parallel to the shelf edge) and about 5 km in width (i.e., perpendicular to the shelf edge) (Fig. 30). 4.3.4.

Gulf of Mexico (Fig. 3, location 1)

In the Ewing Bank Block 826 area (Fig. 10b), bottom- current reworked sands (Plio-Pleistocene) show 25-40% measured porosity and 1001 800 mD permeability (Shanmugam et al., 1993a, their Table 1)[76]. Individual reworked sand lay-

Antarctica (Fig. 3, location Q)

Concluding remarks

The four basic types of deep-marine bottom currents are: (1) thermohaline-induced geostrophic contour currents, (2) winddriven bottom currents, (3) tide-driven bottom currents, mostly in submarine canyons, and (4) internal wave/tide-driven baroclinic currents. All four types of deep-marine bottom currents and their deposits are important both sedimentologically and economically. However, the petroleum industry has focused its attention primarily on downslope processes and their deposits (e.g., turbidites) during the past 65 years. Even now, there is a negative view on the reservoir potential of bottom-current reworked sands among petroleum geologists[106]. Empirical data from modern settings show that bottom currents are equally important agents, analogous to downslope processes, in developing petroleum reservoirs, seal, and source rocks. Therefore, future petroleum exploration and development should focus attention on the often overlooked and under-appreciated bottom-current reworked sands.

Acknowledgements I am grateful to Mobil Oil Corporation management for their encouragement of my contourite research during my employment with Mobil (1987-2000) and various Mobil affiliates worldwide for granting permission to publish results (Table 1). I also thank Reliance Industries Limited (Mumbai, India) for granting permission to publish results from the Krishna-Godavari Basin, Bay of Bengal. I thank H. T. Mullins and E. P. Laine for providing contourite samples for porosity

 207 


and permeability measurements. I wish to thank Elsevier for granting permissions to reuse figures. I wish to thank Rajat Mazumder and an anonymous reviewer for their helpful comments. I am grateful to my wife, Jean Shanmugam, for her general comments.

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