The-flows-that-left-no-trace-Very-large-volume-turbidity-cur

The ?ows that left no trace:Very large-volume turbidity currents that bypassed sediment through submarine channels without eroding the sea ?oor

Christopher J.Stevenson a ,*,Peter J.Talling a ,Russell B.Wynn a ,Douglas G.Masson a ,James E.Hunt a ,Michael Frenz b ,Andrey Akhmetzhanhov c ,Bryan T.Cronin d

a

National Oceanography Centre,European Way,Southampton SO143ZH,UK b

Micrometrics GmbH,Rutherford 108,52072Aachen,Germany c

Lukoil Overseas Limited,Charles House,5-11Regent Street,London SW1Y 4LR,UK d

Deep Marine,9North Square,Footdee,Aberdeen AB115DX,UK

a r t i c l e i n f o

Article history:

Received 13October 2011Received in revised form 30January 2012

Accepted 1February 2012

Available online 13February 2012Keywords:

Turbidity currents Submarine channels Bypass

Agadir basin

Moroccan Turbidite system Turbidites

Sea-?oor gradient Madeira Channels Sediment cores

a b s t r a c t

Turbidity currents are an important process for transporting sediment from the continental shelf to the deep ocean.Submarine channels are often conduits for these ?ows,exerting a ?rst order control on turbidity current ?ow processes and resulting deposit geometries.Here we present a detailed exami-nation of the Madeira Channel System,offshore northwest Africa,using shallow seismic pro ?les,swath bathymetric data and a suite of sediment cores.This shallow (<20m deep)channel system is unusual because it was fed infrequently,on average once every 10,000years,by very large volume (>100km 3)turbidity currents.It therefore differs markedly from most submarine channels which have well developed levees,formed by much more frequent ?ows.A northern and a southern channel comprise the Madeira Channel System,and channel initiation is associated with subtle but distinct increases in sea-?oor gradient from 0.02 to 0.06 .Most of the turbidity currents passing through the northern channel deposited laterally extensive (>5km),thin (5e 10cm)ripple cross-laminated sands along the channel margins,but deposited no sand or mud in the channel axis.Moreover,these ?ows failed to erode sediment in the channel axis,despite being powerful enough to ef ?ciently bypass sediment in very large volumes.The ?ows were able to reach an equilibrium state (autosuspension)whereby they ef ?ciently bypassed their sediment loads down slope,leaving no trace of their passing.

ó2012Elsevier Ltd.All rights reserved.

1.Introduction

Turbidity currents are one of the most important ?ow processes for moving sediment across the surface of the Earth.Inpidual events,such as those described in this study,can be extremely large volume,transporting over ten times the annual sediment ?ux for all the world ’s rivers (Mulder and Syvitski,1995).Submarine channels are often conduits for these ?ows,exerting a ?rst order control on turbidity current ?ow processes and resulting deposit geometries.Much of our understanding of submarine channel morphology comes from a number of intensely studied modern deep-water fans (Wynn et al.,2007).The channels found across these fans are generally sinuous (>1.2)and are connected to larger feeder canyons,which cut back into the continental shelf.Within the

upper parts of the fans the channels are relatively deep (100s m)and narrow (2e 20km),becoming progressively shallower (tens of metres)and broader (tens of km)as they progress distally down the fan (Wynn et al.,2007).In terms of depositional architecture,submarine channels and their ?anking levees are commonly referred to as channel-levee systems.Such systems broadly comprises a coarse-grained channel ?ll,such as massive sands and gravels (Babonneau et al.,2010;Bernhardt et al.,2011;Wynn et al.,2007),and ?ne-grained levee deposits that thin and ?ne away from the axis of the channel (Kane et al.,2007).Channel depth may be maintained via a combination of erosion along the channel ?oor and/or from construction of levees along the channel margins.However,many channels are also net aggradational both in the channel axis and across the levees (Janocko et al.,2013;dalla Valle et al.,2013;Wynn et al.,2007).Channel-levee architecture is pervasive across most modern fan systems and has been inter-preted in numerous ancient channel systems (Babonneau et al.,2010;Bernhardt et al.,2011;McHargue et al.,2011;Normark,1978;Normark et al.,1979;Wynn et al.,2007).

*Corresponding author.Tel.:t44(0)23805923562(Of ?ce),t44(0)7748487754(Mobile).

E-mail address:chris.stevenson@625e67446529647d26285226 (C.J.

Stevenson).Contents lists available at SciVerse ScienceDirect

Marine and Petroleum Geology

journal ho mep age:www.elsevier.co m/lo cate/marp

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0264-8172/$e see front matter ó2012Elsevier Ltd.All rights reserved.doi:10.1016/j.marpetgeo.2012.02.008

Marine and Petroleum Geology 41(2013)186e 205

However,not all channel systems?t the deep-water fan model. For example,the Northwest Atlantic Mid-Ocean Channel(NAMOC) consists of a major“basin draining”trunk channel supplied by numerous tributary and satellite channels that are linked up with the continental slope(Hesse and Rakofsky,1992;Hesse,1989; Hesse et al.,2001;Klaucke et al.,1998).Proximally the system is dominated by sandy braid-plains with relatively shallow relief channels(10s m).Little variation in grain size occurs between braided channel and levee elements,with channel axes and levees being sand-rich.As the NAMOC progresses distally(extending over 4000km)it develops into a deep single channel with a coarse-grained channel?ll and thinner,?ne-grained channel levees (Hesse et al.,1987;Klaucke et al.,1997,1998).

Flow processes operating within submarine channels are complex,involving erosive,bypassing and depositional phases (Macauley and Hubbard,2013;Peakall et al.,2000).These processes are governed by sea-?oor/channel morphology and the properties of the?ows passing through the channels(e.g.?ow thickness,grain size,density and velocity).Speci?cally,gradient has been shown to be a fundamental control on the ability of turbidity currents to erode, transport and deposit sediment(Mulder and Alexander,2001; Wynn et al.,in press).Therefore,changes in sea-?oor gradient down ?ow can exert a strong control on channel architecture,particularly in complex slope settings(Adeogba et al.,2003;Ferry et al.,2005). However,there have been very few direct measurements of active ?ows passing through submarine channels(Kripounoff et al.,2003; Vangriesheim et al.,2009;Xu et al.,2004),and sediment concen-trations have never been measured in any channel in the deep ocean. This ensures that major questions remain concerning submarine ?ow dynamics.Thus our understanding rests on analysis of?ow deposits in submarine channels.However,the highly complex,often discontinuous nature of deposition within channels(Di Celma et al., 2011)means our understanding of how inpidual?ows actually behave is limited.A novel aspect of this study is that the deposits of inpidual?ows can be correlated between basins that lie up slope and down slope of the channel system.This correlation enables the number,grain size and volume of?ows that passed through the intervening channels to be well constrained.Hence,the depositional architectures of inpidual turbidites within the channels them-selves can be placed in context and the?ow processes can be better understood.This study aims to:

(1)Document a poorly studied modern channel system and show

how it differs from previously described channel-levee models (2)Document in detail the deposits of inpidual?ows across the

channels

(3)Discuss how differences in?ow properties can affect deposi-

tional architecture across the channels

(4)Discuss the effects of sea-?oor gradient on inpidual?ow

behaviour(e.g.erosion,bypass and deposition)and the resulting channel architecture

2.The Moroccan Turbidite System

Over the past200ka the Moroccan Turbidite System,situated offshore northwest Africa,has been host to some of the largest turbidity currents ever recorded on Earth with volumes exceeding 150km3(Frenz et al.,2008;Talling et al.,2007;Wynn et al.,2010, 2002b).The system spans w2000km comprising three inter-connected sub-basins(Fig.1A):the Seine Abyssal Plain to the northeast,the Agadir Basin situated centrally and the Madeira Abyssal Plain forming the western most extent of the system. Entering the system from three sources are:(1)organic-rich silici-clastic?ows,sourced from the Moroccan Margin;(2)volcanoclastic ?ows,sourced from either the Canary Islands or Madeira and;(3) carbonate-rich?ows,sourced from local seamount collapses (de Lange et al.,1987;Pearce and Jarvis,1992;Weaver et al.,1992; Wynn et al.,2002b).Excellent core recovery throughout the system, coupled with a robust geochemical and chronostratigraphic framework,has enabled inpidual turbidite beds to be correlated between all three sub-basins(Wynn et al.,2002b).A complex series of channels cross the lower continental rise,connecting the Agadir Basin with the Madeira Abyssal Plain.These channels,originally mapped by Masson(1994),are w700km long and comprise sepa-rate northern and southern channel systems.For clarity this study refers to the southern channel system and northern channel system, of Masson(1994),as the Canary Island Channel System and the Madeira Channel System respectively(Fig.1B).The Madeira Channel System itself comprises a northern channel and a southern channel that are initially separated by local seamounts before converging w200km down slope(Fig.1B).This study focuses on the proximal parts of the Madeira Channel System.Herein,the term Madeira Rise will be used to describe the immediate area surrounding the Madeira Channels themselves and is restricted to the area of study as shown in Fig.1B,unless otherwise stated.

The Madeira Channel System is unusual in that it initiates far from the continental shelf,located at the distal end of the relatively ?at Agadir Basin(Fig.1A).Turbidity currents entered the channels obliquely from the northeast,via the Agadir Basin,or perpendicular to the channels,from the Canary Islands to the south(Frenz et al., 2008;Wynn et al.,2002b).Turbidity currents passing into the Madeira Channels from the Agadir Basin were largely uncon?ned and able to spread across the width of the basin(Frenz et al.,2008). Turbidity currents from the Canary Islands were also uncon?ned and able to spread across the entire Madeira Rise(Hunt et al.,2011). This makes the Madeira Channels signi?cantly different from most submarine fan channel systems that are directly fed by?ows that are con?ned within large canyons that cut back into the shelf (Wynn et al.,2007).

3.Methods

The geophysical data used in this study were collected during ‘RRS Charles Darwin cruise CD166’.A dense network of3.5kHz pro?les and continuous EM12multibeam bathymetry(Figs.2and 3)covers the eastern part of the Madeira Channel System. Shallow sediment cores collected from a number of cruises over the past30years,situated in three transects across the Madeira Channel System,are used to‘ground truth’the geophysical data (Fig.1B).Cores were analyzed using a number of methods.First, cores were subject to detailed visual logging.Deposits from turbidity currents were described and categorized into planar laminated sand,ripple cross-laminated sand and mud.Detailed grain size analysis was carried out on turbidite beds using a Mal-vern Matersizer.Samples(1cm3)were taken from turbidites and disaggregated with1%Calgon solution then shaken continuously for w10h.This ensured that inpidual sediment grains,particu-larly clay particles,were not clumped together into larger?ocs. Samples were then analyzed three times and the average grain size distribution calculated.Geochemical analysis was carried out on cores CD166/17and19using an ITRAX XRF core scanner(Croudace et al.,2006;Rothwell et al.,2006).Elemental abundance was measured down core every0.5mm.Cores CD166/15,16,17,18,19, 23and90PCM36,37and39were subject to high-resolution coc-colith biostratigraphic dating,following the method of Weaver and Kuijpers(1983).Smear slides were taken down core every5e10cm, although intervals that were considered likely to be eroded were subject to sampling every1cm.Approximately300coccoliths were counted per smear slide.

C.J.Stevenson et al./Marine and Petroleum Geology41(2013)186e205187

4.Results

4.1.Morphology of the Madeira Channels

The Madeira Channels initiate on the southwest margin of the

Agadir Basin (Figs.1B and 2A ).Two channels are identi ?ed,

developed to the north and south of volcanic seamounts (herein referred to as the northern and southern channels,respectively).Both the northern and the southern channels are shallow (<30m)and relatively narrow (<5km)with low sinuosity (<1.1).The northern channel initiates from a broad shallow gather zone w 5km across and w 12m deep,progressing into a more de ?ned,?at-bottomed channel with braid bar like features down slope (Fig.2A).The more de ?ned segment of the northern

channel Figure 1.Map showing:(A)The Moroccan Turbidite System comprising the Seine Abyssal Plain (SAP),Agadir Basin (AB)and Madeira Abyssal Plain (MAP)highlighted in light grey.Morocco,Madeira and The Canary Islands are coloured black.Sediment cores are marked with black circles.The Madeira Channels connect the Agadir Basin to the Madeira Abyssal Plain and are shaded in light grey.Core sites used to correlate inpidual beds across the length of the channel system (see Fig.4)are highlighted with large grey circles and labelled

(B)the Canary Island and Madeira Channel Systems.The Madeira Channel System has two main channels,referred to as northern and southern in the text.Cores used in this study are marked with large grey circles and core transects 1e 3are labelled.The proximal Madeira Channel System,highlighted with a square box,is the focus for this study and illustrated in more detail in Figs.2and 3.

C.J.Stevenson et al./Marine and Petroleum Geology 41(2013)186e 205

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maintains a depth of w 20m and a width of w 5km,up to w 100km

from the point of channel initiation.The southern channel initiates

from a well-de ?ned nick point,into a narrow (w 1km across),

V-shaped channel up to 28m deep (Fig.2A).A number of smaller

tributary channels occur on the southern margin and converge

with the main southern channel (Figs.2A and 3).

The southern and northern channels converge approximately

180km from the point of channel initiation (Fig.1B).At this

con ?uence the channel is broad,shallow and poorly de ?ned.

GEBCO bathymetry and sea-?oor gradient pro ?les,trending

northwest to southeast across this zone,show an exceptionally

low-relief channel,w 36km across and w 20m deep (Fig.4).This style of low-relief channel evolves down slope into steeper-walled narrower channels before debouching into the Madeira Abyssal Plain (Masson,1994).4.2.Character of Madeira Channels in seismic pro ?les 3.5kHz shallow seismic pro ?les across the northern channel show two distinct acoustic signatures (Fig.5).The ?rst occurs proximal to the site of channel initiation.The 3.5kHz pro ?les show relatively high amplitude re ?ectors on the margins of the channel and lower amplitude re ?ectors in the channel axis (Fig.6A).Deeper acoustic penetration is observed in the channel ?oor compared

with Figure 2.(A)EM12multibeam bathymetric map over the southwest (distal)Agadir Basin and proximal northern Madeira Channels.Bold arrows show main ?ow pathways:organic-rich siliciclastic ?ows sourced from the Moroccan Margin passing through the Agadir Basin (solid arrows)and volcanoclastic ?ows originating from the Canary Islands (dashed arrows).(B)3-D view of EM12backscatter looking upstream along the northern channel towards the Agadir Basin.Lighter grey areas denote zones of low backscatter and darker grey/black areas denote zones of high-backscatter.Core locations are marked with red bars.Note the low backscatter response within the northern channel axis (highlighted with a dashed red line)and the patchy high-backscatter response across the channel margins.The highest and most extensive backscatter response is seen in an area across the southern channel (top right of ?gure).(For interpretation of the references to colour in this ?gure legend,the reader is referred to the web version of this article.)

C.J.Stevenson et al./Marine and Petroleum Geology 41(2013)186e 205189

the surrounding high amplitude margins.The second acoustic signature occurs more distally,where the character of the northern channel changes (Fig.6C).The axis of the channel is ?at-bottomed with high amplitude re ?ectors,similar in strength to those observed on the channel margins.The acoustic penetration is the same inside the channel as it is on the margins.The acoustic change is associated with steeper channel walls and a narrower channel ?oor.The 3.5kHz pro ?les shown in Fig.6A and C (and indeed lines 4e 9,Fig.6)show the walls of the northern channel becoming progressively steeper through time.In contrast,the walls of the southern maintain a relatively constant morphology to the present day sea-?oor (Fig.6D).3.5kHz pro ?les running across the southern channel show high amplitude re ?ectors developed on the channel margins and within the axis of the channel itself (Fig.6D).Re ?ectors appear vertically stacked within the channel axis,maintaining relatively steep channel walls of similar gradients up to modern sea-?oor morphology.The smaller tributaries show very similar acoustic character to the main channel.The axes of both the southern and the northern channels do not appear to migrate laterally.4.3.Character of the Madeira Channels in backscatter images The northern and southern channels are signi ?cantly different in terms of their backscatter response (Fig.2B).The northern channel shows patches of high (dark grey)backscatter along its northern margin and signi ?cantly lower backscatter (light grey)within the channel axis.The backscatter response does increase slightly within the channel axis between 3.5kHz pro ?le lines 7and 9.The southern channel shows a high-backscatter response both inside the channel axis and on the margins.Indeed,the area surrounding the southern channel shows larger,higher backscatter zones compared with the northern channel.4.4.Bed correlations across the Madeira Channels

A robust geochemical and chronostratigraphic framework has been well established across the Moroccan Turbidite System.Within the Agadir Basin and Madeira Abyssal Plain this has been achieved from:

High-resolution coccolith biolithostratigraphy of hemipelagic sediments,which provides an age model as far back as w 500ka (Weaver,1991;Weaver and Kuijpers,1983;Weaver and Rothwell,1987;Weaver et al.,1992).

Detailed geochemical analysis of large-volume volcanoclastic turbidites,which provides aerially extensive marker beds across the system (de Lange et al.,1987;Pearce and Jarvis,1992,1995).

Analysis of coccolith species within turbidite mud caps,which enables inpidual turbidite events and their erosional char-acteristics to be identi ?ed (Weaver,1994;Weaver and Thomson,1993;Wynn et al.,2002b ).

Using the methods outlined above,the detailed inpidual bed correlations across the Agadir Basin and Madeira Abyssal Plain (Frenz et al.,2008;Jones et al.,1992;Rothwell et al.,1992;Talling et al.,2007;Weaver and Rothwell,1987;Weaver et al.,1992;Wynn et al.,2002b )are extended across the Madeira Channel System (Fig.7).Bed numbering from 1to 14follows the nomen-clature of (Wynn et al.2002b )as established for the Agadir Basin.Below bed 14,beds are numbered sequentially down core.The stratigraphy within the Madeira Channel System is penetrated down to w 400ka and comprises 18correlatable turbidite beds.Many of the large-volume turbidites (Wynn et al.,2002b )correlate continuously across the channel system (e.g.Beds 2,5,12and 14)maintaining a relatively constant grain size.Turbidite deposits within the Madeira Channel System are generally thinner than those in the Agadir Basin or Madeira Abyssal Plain,indicating signi ?cant amounts of sediment bypass.Indeed,the silt and mud component of most beds is thickest in distal areas,where it is ponded into the Madeira Abyssal Plain (Jones et al.,1992;McCave and Jones,1988;Rothwell et al.,1992).

4.5.Sedimentary facies across the Madeira Channels

The cores comprise two distinct types of sediment:hemi-pelagic mud and turbidites.The hemipelagic mud has two end member lithologies:(1)Cream coloured carbonate ooze comprising abundant randomly distributed foraminifera tests

and

Figure 3.EM12grey shaded bathymetry showing the position of 3.5kHz pro ?les across the proximal Madeira Channel System.3.5kHz pro ?les are marked with thick white lines and labelled 1e 12.Black circles denote locations of core sites.

C.J.Stevenson et al./Marine and Petroleum Geology 41(2013)186e 205

190

coccolithophores;(2)Dark red/brown mud mainly composed of terriginous clay.These lithologies have been related to changes in the dissolution of carbonate in the ocean bottom waters caused by climatic variations (Crowley,1983).In interglacial periods disso-lution of carbonate is relatively low and allows sedimentation of carbonate-rich ooze.During glacial periods dissolution of carbonate is increased,which produces smooth brick-red clay (Crowley,1983).Turbidites are identi ?ed by a sharp base,often with a distinct change in grain size (i.e.from mud to sand),and changes in the colour of the sediment.Turbidites found within the Madeira Channel System and across the Madeira Rise are either:(1)organic-rich with a green colouration,sourced from the Moroccan Margin or;(2)volcanoclastic with a dark grey/black colouration,sourced from either the Canary Islands or Madeira (de Lange et al.,1987;Pearce and Jarvis,1992;Weaver et al.,1992;Wynn et al.,2002b ).

Turbidites primarily comprise thin (15e 28cm),ripple cross-laminated ?ne sands with modal grain sizes of between 63and 250m m (Fig.8B,D and F).Some turbidites have planar laminated sand at the base or occasionally throughout the deposit (Fig.8A).Contorted lamination occurs in the upper parts of the deposits,often underlain by planar laminated sand and overlain by ripple cross-laminated sand (Fig.8C,D and E).Proximal to the site of channel initiation,deposits have inversely graded bases overlain by an ungraded interval,which progresses into a normally graded top (Fig.8A e D).Inverse to normal vertical

grading

Figure 4.(A)Slope map of the proximal Madeira Channel System.Two scales are used representing the minor and larger magnitude changes in sea-?oor gradient.Gradients from 0 up to 0.15 range in colour from white to dark red,whilst gradients from 0.15 to 2 are shaded brown to dark green respectively.Note the change in scale within the second colour scheme.The channels are highlighted in black.The dashed white line shows the trace of the 2D sea-?oor pro ?le in B.Core transects 1e 3are marked with solid white lines and are labelled.(B)2D sea-?oor pro ?le (blue line)and gradient (red line)following the axis of the northern Madeira Channel along its entire length (see Fig.1A),which passes through core transects 1and 3.Note two distinct zones along the Madeira Channel sSystem:(1)a relatively steep zone with high variability,showing gradients between 0.02 and 0.34 ;(2)a ?atter zone with less variability,showing gradients between 0.01 and 0.06 .(For interpretation of the references to colour in this ?gure legend,the reader is referred to the web version of this article.)

C.J.Stevenson et al./Marine and Petroleum Geology 41(2013)186e 205191

patterns are found but are less frequent in the more distal parts

of the channel system.For the most part,turbidites found in the

con ?uence zone typically have ungraded sandy bases overlain by

normally graded sands (Fig.8E and F).Both proximal and more

distal sandy deposits are typically overlain by a sharp grain size

break and in turn a thin (1e 10cm)turbidite mud cap with modal

grain sizes between 8and 24m m.However,in a number of

proximal core sites the turbidite beds have no mud cap at all (e.g.

Fig.8A). 4.6.Turbidites from the northern channel Transect 1comprises ?ve cores across the northern channel located along 3.5kHz pro ?le line 5(Fig.9).In this transect turbidite deposition primarily occurs outside the channel.Turbidites mainly comprise ungraded,thin (5e 10cm)ripple cross-laminated or planar laminated clean sands overlain by a sharp grain size break and very thin turbidite mud (1e 2cm).Deposits mostly thin and pinch out to the south,toward the channel.However,

some Figure 5.Consecutive 3.5kHz pro ?les along the Madeira Channel System showing channel morphology and acoustic character of the channels (scale of 3.5kHz lines is shown in Fig.6).Dashed black lines highlight the course of the channels.Detail of pro ?le lines 5,8and 10are shown in Fig.6A e D.An EM12bathymetric map is included showing plan form channel morphology (bottom right).

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192

turbidites (e.g.Beds 20,21and 23)are thickest on the northern

channel margin,thinning both towards the south,towards the

channel,and north,away from the channel.Within the channel axis

itself almost no turbidite deposition is recorded.Turbidite Beds 5,7

and 12originating from the Moroccan Margin,passing across the

Agadir Basin and into the Madeira Abyssal Plain (Frenz et al.,2008;

Wynn et al.,2002b ),show no deposition in the channel axis (Fig.9).

The only deposition of turbidite sediments in the channel axis is

that of Bed 14originating from the Canary Islands,and Bed 10.5

from Madeira.In this case the deposits are only subtle traces of strongly bioturbated turbidite mud and are barely visible.These deposits are dif ?cult to detect through grain size analysis or in down core magnetic susceptibility pro ?les.However,deposits of Bed 14can be detected from elevated levels of titanium and potassium within the sediment (Fig.10B).4.7.Turbidites from the southern channel Core Transect 2cutting across the southern channel is limited in that it only has two core sites:one from the channel axis and

the Figure 6.3.5kHz pro ?les across the northern (A e C)and southern (D)Madeira Channels.The horizontal scale is the same for all pro ?le lines.Pro ?le Line 5(A and B)shows higher amplitude re ?ectors under the northern channel margin.The channel axis shows weaker sub-bottom re ?ectors and signi ?cantly deeper acoustic penetration than the margin.

(B)Shows an interpretation of Pro ?le Line 5with re ?ectors manually correlated and estimates of core penetration under each core site (black pillars).Blue pillars Y and Z denote the thickness of the sedimentary sequence outside and inside the channel respectively.The thickness is measured from a continuous re ?ector to the present day sea ?oor.(C)Shows Pro ?le Line 8with high amplitude re ?ectors both inside and outside the northern channel axis.Note the change to a ?at-bottomed channel with steeper channel walls.The southern channel (D)shown in Pro ?le Line 10has high relief,steep channel walls and shows high amplitude re ?ectors both inside and outside the channel axis.The southern channel also has several low-relief tributaries highlighted with strong re ?ectors along its margins.(For interpretation of the references to colour in this ?gure legend,the reader is referred to the web version of this article.)

C.J.Stevenson et al./Marine and Petroleum Geology 41(2013)186e 205193

Figure 7.Core transect (located on Fig.1A)showing correlation of the established Moroccan Turbidite System Stratigraphy across the Madeira Channels.(A)Shows sea ?oor topography (red line)with core locations marked.(B)Shows turbidite bed correlations across the Agadir Basin,Madeira Channel System and the Madeira Abyssal Plain.Note the relatively condensed sequence found in the area of the Madeira Channels.Bold numbers (1e 22)label turbidites that can be correlated across the Moroccan Turbidite System.(For interpretation of the references to colour in this ?gure legend,the reader is referred to the web version of this article.)C.J.Stevenson et al./Marine and Petroleum Geology 41(2013)186e 205

194

other from the channel margin (Fig.11).Turbidites are predomi-

nantly thick (22e 46cm)ripple cross-laminated clean sands over-

lain by a sharp grain size break and a thin mud.Some turbidites

have planar laminated sands developed at their bases.In contrast to

the northern channel,the southern channel shows turbidite

deposition occurring both in the channel axis and on the margins.

Volcanoclastic Beds 8and 10.5,sourced from Madeira (Frenz et al.,2008),show deposition within the channel and thinning/pinching out up to the channel margin.Canary Island Turbidite 2is thicker on the channel margins and thins signi ?cantly into the channel axis.Turbidity currents sourced from the Moroccan Margin,?owing southwest through the Agadir Basin,show a complex range of depositional architectures.These include:deposition located only on the channel margin with deposits pinching out into the

channel Figure 8.Photographs of turbidite deposits in core with accompanying logs and grain size pro ?les (see Fig.7B for key)showing examples of turbidite deposition from the southern channel (A and B),northern channel (C and D)and the con ?uence (E and F).Deposits are located on Figs.9,11and 12respectively.Turbidites mainly comprise ripple cross-laminated and contorted sands.Proximally,subtle inverse grading can be seen in the base of the southern and northern channel deposits (Figs A e D)whilst those at the downstream con ?uence have normal grading (Figs E and F).Note that in all examples there is a sharp grain size break between sand and the overlying mud cap.Sand-to-mud grain size breaks of this nature are pervasive throughout the channel system.In the case of Figure E two grain size breaks are shown,the ?rst has sand overlain by ?ner sand/silt,the second has sand overlain by mud.

C.J.Stevenson et al./Marine and Petroleum Geology 41(2013)186e 205195

axis (Bed 5);deposition occurring both inside and outside the channel axis with no signi ?cant thinning or ?ning of the deposits between core sites (Bed 7);and deposition localized within the channel axis (Bed 12).

4.8.Turbidites from the Madeira Channel Con ?uence

Core Transect 3(Fig.12)cuts across the wide,low-relief channel situated approximately 100km down slope of Transects 1and 2,where the northern and southern channels converge (Fig.4).Turbidite deposition within this transect is dominated by relatively thick (20e 62cm),normally graded,planar laminated and ripple cross-laminated sands overlain by a sharp grain size break and thin turbidite mud.Occasionally grain size breaks occur between planar laminated and ripple cross-laminated sands.Deposits appear thickest within the deepest part of the channel axis and thin to the southeast.Turbidites in the southeastern part of the channel are much thinner (2e 12cm)and comprise only ?ne-grained turbidite silt and mud.

4.9.Turbidites from the Madeira Abyssal Plain

The Madeira Abyssal Plain is situated at the end of the Madeira Channel System (Fig.1A)and records most of the turbidites found in the Madeira Channel System (Fig.7B).The turbidity currents that deposited Beds 1,2,5,7,12and 14deposited ?ne sandy lobes at the Madeira Channel terminuses (Wynn et al.,2000)then ?ne-grained silts and muds across the rest of the Madeira Abyssal Plain (Jones et al.,1992;McCave and Jones,1988;Rothwell et al.,1992).These turbidites have estimated volumes within the Madeira Abyssal Plain of 30km 3(Bed 5),110km 3(Bed 7),190km 3(Bed 12)and 80km 3(Bed 14;Frenz et al.,2008).5.Discussion

The aim of this discussion is to explain the origin of the patterns of turbidite deposition seen in the geophysical and core data across the Madeira Rise and Madeira Channel System.What was the nature of the turbidity currents passing through the

channels?

Figure 9.Core transect 1across the proximal northern channel.(A)exaggerated sea ?oor topography across the channel (red line)with core locations marked.The difference in water depth between core sites is noted underneath the red line.(B)turbidite stratigraphy within the northern channel.Turbidite Beds are labelled after the established Agadir Basin stratigraphy (Wynn et al.,2002b ).Below the base of the established Agadir Basin stratigraphy (Bed 14)turbidite beds are labelled sequentially down core.The core transect shows inpidual turbidite beds and glacial clay layers correlated across the channel axis (refer to Fig.7B for key).Note turbidite deposition is preferentially developed on the northern channel margin with almost no deposition occurring within the channel axis.Two distinct architectures of channel margin deposition can be seen;(1)Turbidites 1e 19,which thin and ?ne towards the channel axis and;(2)Turbidites 20e 23,which thin and ?ne towards and away from the channel axis.(For interpretation of the references to colour in this ?gure legend,the reader is referred to the web version of this article.)

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196

Speci ?cally why is it that turbidites are mostly absent within the northern channel?

5.1.Interpretation of 3.5kHz seismic and EM12backscatter with core transects

Criteria for recognizing areas of coarse sediment deposition from 3.5kHz echograms and EM12backscatter data have been established by a number of studies (Masson,1994).Areas of low 3.5kHz acoustic penetration,often associated with high amplitude re ?ectors,indicates sandy sediment deposition,interpreted here to be turbidites.Turbidite sands interbedded with hemipelagic muds also produce a high-backscatter response on EM12imagery (Droz et al.,1996).These acoustic signatures are often irregular,re ?ect-ing non-uniform deposition of sand by turbidity currents.In this study,ground truthing the geophysical data with a suite of shallow sediment cores allows con ?dent interpretations to be made regarding the depositional architectures of turbidites within the Madeira Channels.Core Transect 1shows deposition of relatively thick turbidite sands on the northern channel margin (Fig.9),corresponding with a patchy high-backscatter response on the

EM12bathymetry (Fig.2B),and high amplitude re ?ectors with shallow acoustic penetration on the 3.5kHz pro ?le line 5(Fig.6A).In contrast,Core Transect 1shows no sand deposition within the channel axis,which is re ?ected in the low backscatter EM12response (Fig.2B)and weak re ?ectors with deeper penetration on 3.5kHz pro ?le line 5(Fig.6A and B).This pattern is pervasive along the northern channel up to w 80km from the site of channel initiation.Beyond w 80km,sandy turbidite deposition is shown in the channel axis from high amplitude re ?ectors with reduced penetration in 3.5kHz pro ?les (Fig.6C).5.2.Northern Channel architecture

The northern channel shows turbidites developed only on the margins of the channel.Almost no turbidite deposition is recorded in the channel axis itself.This could be a result of the turbidity currents not passing through the channels.However,inpidual turbidite beds are correlated across the Madeira Rise,both northern and southern channels,and across the Agadir Basin and Madeira Abyssal Plain.It seems highly unlikely that these turbidity currents would spread across such an area and not pass through

the

Figure 10.Detailed analysis of core CD166-17illustrating the absence of most turbidites in the axis of the northern channel.(A)Core photograph with interpreted stratigraphy and coccolith biostratigraphy down to w 350ka (after Weaver and Kuijpers,1983).Note the lack of erosional hiatuses.(B)A suite of grain size and ITRAX data through the interval where Bed 14should occur (i.e.the glacial clay of oxygen isotope stage 6).Grain size shows no signi ?cant change through the interval but Potassium (K)and Titanium (Ti)show increased levels where a grey bioturbated mud occurs.This is the only trace left behind from Bed 14.

C.J.Stevenson et al./Marine and Petroleum Geology 41(2013)186e 205197

northern channel.Indeed,trace deposition from Bed 14(Fig.10B)

shows that this large-volume ?ow did pass through the channels

yet left minimal trace of its passing.We suggest that the northern

channel was able to con ?ne the basal parts of turbidity currents,

accelerating these ?ows enough for them to become non-

depositional within the channel axis.This allowed parts of the

?ows to become more ef ?cient,bypassing their sediment loads

further down slope (core transect 3,Fig.12).The thickness of turbidity currents passing across the Madeira Rise and through the northern channel will have a profound impact on the depositional architecture inside and outside the channel axes.Relatively thick ?ows will be largely uncon ?ned and able to spread laterally over the entire rise and deposit sediment across the channel margins.Parts of the ?ows will be con ?ned within the channel and become non-depositional,bypassing their sediment load down slope.Due to the subtle nature of the channel relief,the amount

of Figure 11.Core transect 2across the proximal southern Madeira Channel.(A)Exaggerated sea ?oor topography across the channel (red line)with core locations marked.The difference in water depth between core sites is noted underneath the red line.(B)Turbidite stratigraphy for the southern Madeira Channel.Turbidite Beds are labelled after the established Agadir Basin stratigraphy (Wynn et al.,2002b ).The core transect shows both turbidite beds and glacial clay layers correlated across the channel (refer to Fig.7B for key).Note the complex depositional heterogeneity from ?ows with the same ?ow pathway (i.e.through the Agadir Basin);Bed 5deposits on channel margin and is non-depositional in channel axis;Bed 7deposits across both channel axis and margin equally;Bed 12deposition is restricted to the channel axis only.(For interpretation of the references to colour in this ?gure legend,the reader is referred to the web version of this article.)

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198

con ?nement (hence amount of ?ow bypass)will progressively increase from the channel margin towards the channel axis.This produces deposits that are thickest away from the channel and progressively thin and pinch out towards the channel axis (e.g.Beds 5,12,14,18;Fig.9).Thinner turbidity currents extending only a few metres above the sea-?oor will be less able to spread laterally and have a larger proportion of the ?ow con ?ned by the channels.In this case most of the ?ow will be con ?ned and becomes non-depositional within the channel,bypassing sediment down slope.In places the thickness of the ?ow is suf ?cient that the upper parts can overspill the channel and deposit sands.This produces deposits that thin and ?ne both away from the channel axis and towards the channel axis (e.g.Beds 20,21and 23;Fig.9).5.3.Southern Channel architecture

Turbidity currents with similar ?ow pathways between the Agadir Basin and Madeira Abyssal Plain (e.g.Beds 5,7,8and 12)show different depositional architectures inside and outside the southern channel.All of these turbidites are found down slope within the Madeira Channel System (Fig.12)and across the Madeira Abyssal Plain (Wynn et al.,2002b ).High-backscatter signatures (Fig.3B)and high amplitude re ?ectors on the 3.5kHz pro ?les (Fig.6D)indicate turbidity currents deposited sands across the Madeira Rise and within the southern channel.Indeed,Core Transect 2shows turbidite deposits located up to 25m above the axis of the southern channel (Fig.11).Therefore,it is most likely that all the turbidity currents passed through the southern channel.Beds 2and 5show deposition only on the channel margins and non-deposition within the southern channel axis (Fig.11).This suggests a similar process to that operating in the northern channel with con ?ned parts of the ?ows becoming non-depositional within the channel axis and bypassing sediment down slope.Beds 10.5and 12show deposits restricted to the channel axis only,indicating the ?ows were relatively thin and not able to drape above the height of the channel wall.Bed 8has thickest deposits in

the

Figure 12.Core Transect 3across the distal northern Madeira Channel System.(A)Exaggerated sea ?oor topography across the channel (red line)with core locations marked.The channel is extremely low-relief at this point and its boundaries are dif ?cult to distinguish.The difference in water depth between cores sites is noted underneath the red line.(B)Turbidite stratigraphy across the distal northern Madeira Channel System.Turbidite Beds are labelled after the established Agadir Basin Stratigraphy (Wynn et al.,2002b ).The core transect shows both turbidite beds and glacial clay layers correlated across the channel (refer to Fig.7B for key).Note the rippled clean sands deposited within the channel axis and thinner ?ner deposition on the southwest margin.(For interpretation of the references to colour in this ?gure legend,the reader is referred to the web version of this article.)

C.J.Stevenson et al./Marine and Petroleum Geology 41(2013)186e 205199

channel axis and then thinner,?ner-grained deposits on the channel margin.This indicates that the?ow was thicker than for Beds10.5and12with the upper,?ner-grained parts of the?ow able to drape over the channel margins.Whether completely con?ned or mostly con?ned by the southern channel,these?ows remained depositional within the channel axis rather than becoming non-depositional.The differences in depositional architectures between turbidites is probably related to differences in inpidual ?ow properties such as velocity,?ow density,the suspended grain size population,and the thickness of the?ows as they enter the channels.

5.4.What type of?ows were depositing across the channels?

Using a combination of sedimentary structures and detailed vertical grain size pro?les the nature of the?ows passing through the Madeira Channels can be assessed.The dominance of ripple cross-laminated sands(Fig.8A e F)indicates the parent?ows were dilute,and incrementally depositing via tractional reworking of sediment along the bed(Allen,1982;Best and Bridge,1992).The vertical pro?le of an incrementally aggraded deposit re?ects the longitudinal(i.e.temporal)structure of the base of a?ow as it passes over a?xed point(Kneller and Branney,1995;Kneller and McCaffrey,2003).Changes in the velocity of the?ow,and thus the shear stresses near to the bed,will produce variations in grain size vertically through a deposit.Hence,a waxing(accelerating) depositional?ow is capable of producing inversely graded deposits, whilst a waning(decelerating)depositional?ow will produce normally graded deposits(Kneller and Branney,1995;Kneller and McCaffrey,2003).Proximal to the site of channel initiation, inverse-to-normal grading in turbidite sands indicates that the parent?ows were initially accelerating then decelerating.Turbidity currents spilling into the Madeira Channels from the Agadir Basin were most likely accelerated by the break in slope from<0.02 to >0.06 .Although subtle,the relative change is a signi?cant three-fold increase in gradient.Intense ambient mixing at the head (Middleton,1993),and reduced mixing in the body of turbidity currents(Stevenson and Peakall,2010),enables the body of a?ow to maintain its density and move up to30e40%faster than the head (Kneller and Buckee,2000).The velocity increase from the head into the body of the?ow most likely produced the inversely graded deposits found in the proximal Madeira Channels.Eventually the ?ows lost enough sediment and decelerated,depositing progres-sively?ner sediments,resulting in normally graded deposits.

5.5.How erosive were the?ows?

Turbidity currents that are non-depositional must be in one of two states:(1)erosive,where the?ow has suf?cient velocity to entrain more sediment into the?ow(Parker et al.,1986),or(2) bypassing,whereby the?ow is in equilibrium and able to keep its entire sediment load in suspension,yet not powerful enough to entrain any more sediment into the?ow(Sequeiros et al.,2009). Flows entering into the Moroccan Turbidite System can initially be highly erosive,producing large-scale scours and erosional hiatuses around the mouth of the Agadir Canyon(Huvenne et al.,2009; Macdonald et al.,2011;Wynn et al.,2002a).Proximal to the Canary Islands,in parts of the Canary Island Channel System,some turbi-dites are underlain by signi?cant hiatuses representing several meters of erosion(Masson,1994).However,erosion is limited to the mouth of the Agadir Canyon and parts of the Canary Island Channel System(Huvenne et al.,2009;Macdonald et al.,2011;Wynn et al., 2002a).Where turbidity currents enter the Madeira Channel System,either from the Agadir Basin or from the Canary Islands to the south,they are non-erosive and depositional(Talling et al.,2007;Weaver,1994;Weaver and Thomson,1993;Wynn et al., 2002b).

Within the axis of the northern Madeira Channel high-resolution coccolith biolithostratigraphy shows no signi?cant erosional hiatuses over the past w400ka(Fig.10A).Core Transect1 (Fig.9)shows that even thin(5cm)glacial clay layers,correlate from the northern margin,through the channel axis and onto the southern margin without changing thickness.Therefore,the glacial clay layers have not been affected by erosion.If any signi?cant erosion had occurred along the?ow path it would be expected that the distal Madeira Abyssal Plain would record a different mixture of coccolith species within the turbidite mud caps compared with the proximal Agadir Basin(Weaver and Thomson,1993).However,no signi?cant variation in coccolith mixtures is found(Wynn et al., 2002b),indicating a lack of signi?cant erosion within the Madeira Channels.The non-erosive/non-depositional nature of the?ows can be seen in the3.5kHz pro?les,which show a distinct lack of channel migration or lateral-accretion surfaces within the channel axis(Figs.5and6).

Turbidity currents passing into the Madeira Channels appear to reach a non-erosive,bypassing equilibrium state,which is stable enough to be maintained for w80km along the channel.Numerical and experimental modelling has suggested?ows can become autosuspending,whereby the net deposition and entrainment of sediment from the?ow is zero(Pantin,1979;Parker et al.,1986; Sequeiros et al.,2009).Flows can become autosuspending if they are accelerated down a slope yet cannot entrain more sediment into the?ow.With no change in?ow density or slope gradient, autosuspending?ows can run out inde?nitely(Sequeiros et al., 2009).Autosuspension can only be maintained if the?ow is powerful enough to suspend its entire sediment load,yet not powerful enough to erode sediment from the underlying substrate. Flows entering the Madeira Channels are texturally mature,having travelled>250km before reaching the channels.Most of the coarse grain sizes have been deposited further up slope leaving predom-inantly?ne-grained sand and mud in suspension(Frenz et al., 2008).With only a?ne grain size in suspension,?ows would only need a marginal increase in velocity to fully re-suspend their entire sediment load.The break in slope(<0.02 to>0.06 )found at the distal end of the Agadir Basin into the Madeira Channels, coupled with con?nement within the channel axes,most likely caused a marginal increase in velocity.Within the northern Madeira Channel a critical velocity was reached whereby the relatively?ne grain size population within the?ows was fully suspended,yet the?ows were not fast enough to erode sediment from the underlying sea?oor.Autosuspension might explain why turbidity currents bypassing through the northern Madeira Channel can do so for w80km,and this state only changing with another break in slope a few kilometres west of transect1(Fig.4A). The change in slope decelerates the?ows and forces them to deposit within the channel axis(Fig.6C).

5.6.Channel evolution:are the channels maintained by deposition or erosion?

Core Transect1shows the sediment sequence on the northern channel margin to be w1.7times thicker than the equivalent sequence in the channel axis(Fig.9).The difference in thickness is the product of sands being deposited on the margin and?ows bypassing their sediment load within the channel axis.A similar architecture is seen in the3.5kHz pro?le line5,with the northern margin sequence1.7times thicker than the channel axis stratig-raphy(Fig.6B).This suggests that the increased thickness of the northern margin sequence seen on the3.5kHz pro?le is also the product of?ow bypass within the channel axis,and that this

C.J.Stevenson et al./Marine and Petroleum Geology41(2013)186e205 200

asymmetry of deposition has been occurring over a time period much longer than that sampled by the cores.Hence erosion is not required for the channel to be maintained.

The northern channel has no de?ned nick point(Fig.3A),no detectable erosive surfaces either in core(Fig.9)or the3.5kHz pro?les(Figs.5and6),and is maintained by constructive margins rather than erosion in the channel axis.Therefore it seems unlikely that erosion would have generated the channel originally.It is more plausible that existing sea-?oor topography(i.e.an increase in gradient)caused turbidity currents spilling out of the Agadir Basin to begin bypassing sediment down slope whilst allowing some deposition at the margins of the?ows.Over time the?ow pathway became more de?ned,as consecutive?ows constructed its margins and bypassed sediment in its axis,eventually creating the channel as seen today.Considering the northern channel as a purely constructive feature suggests that it is very old.Extrapolating a constant sedimentation rate of1cm per1000years(Weaver and Kuijpers,1983)through an estimated3.5kHz penetration of w20m (Fig.6B)gives an age of at least2million years for the northern channel.This estimate will be conservative,as compaction will effectively decrease the apparent sedimentation rate with depth thus yielding an even older age for the channel.

5.7.Effect of gradient on channel architecture

Initiation of both the northern and southern Madeira Channels coincides with a marked increase in sea-?oor gradient(Figs.4and 13).For the?rst w80km from the site of channel initiation the gradient?uctuates but overall remains relatively high in the northern channel(w0.06 ;Fig.13).This section is characterized by ef?cient ?ow bypass.Beyond w80km the sea-?oor?attens out(<0.06 ) coinciding with deposition of turbidite sands both within the northern channel and along its margins.Moving w100km further down slope to Core Transect3the sea-?oor gradient is even lower (w0.04 ;Fig.4),and here turbidite sands are deposited in the channel and?ner-grained silts and muds on the margins.It appears that for the northern channel,0.06 is the critical sea-?oor gradient above which?ows become bypassing.However,the absolute gradient may not be as important as the relative change from one gradient to another in controlling?ow behaviour(Wynn et al.,in press).In this case the gradient changes relative to the Agadir Basin slope are approximately three fold(w0.02 to>0.06 ).

The southern Madeira Channel has a similar slope pro?le to the northern channel but exhibits considerable depositional complexity,showing variation from the architectures seen in the northern Madeira Channel(Fig.11).Inpidual turbidites have a range of depositional architectures with both sand deposition in the channel axis only and sand deposition on the margins only, bypassing the channel axis.It is dif?cult to explain why turbidity currents with similar?ow pathways(e.g.down the Agadir Basin) and similar grain size distributions(Fig.8)would generate very different depositional architectures across the southern channel. Comparing inpidual turbidite architectures between the northern and southern channels adds further complexity.Some turbidites deposit in a similar manner in both channels.For example Bed5 exhibits bypass in both northern and southern channel axes. However,other turbidites display different depositional architec-tures in different channels.For example,Bed12shows bypass

in

Figure13.EM12bathymetry with corresponding2D sea-?oor pro?les showing gradient control on channel initiation.Averaged sea-?oor gradient is labelled on the2D pro?les and signi?cant breaks in slope are marked with black lines on the2D pro?les and black circles on the EM12bathymetry maps(see Fig.4for detailed sea-?oor gradients).Note that both channels initiate over the break in slope from the southwest(distal)Agadir Basin onto the Madeira Rise.

C.J.Stevenson et al./Marine and Petroleum Geology41(2013)186e205201

the northern channel axis yet con?ned sandy deposition in the southern channel axis.The non-uniformity of depositional archi-tecture from the same?ows following similar sea-?oor slope pro?les suggests further controls are acting upon the turbidity currents as they pass into the channels.It is possible that?ow properties such as density,velocity,grain size and?ow thickness could affect how a?ow might respond to changes in sea-?oor gradient.Perhaps subtle differences in these?ow properties within different parts of the?ows were enough to trigger funda-mentally different responses to similar changes in sea-?oor gradient.

625e67446529647d26285226parison to other submarine channel deposits

The morphology and situation of the Madeira Channel System is signi?cantly different compared to the large sinuous channels found within the Amazon,Bengal,Indus,Mississippi,Nile and Rhone fans.The channels that cross these fans are generally deeper (hundreds of meters),narrower(1e3km)and have higher sinu-osities(Normark,1978;Normark et al.,1979;Wynn et al.,2007) compared to the Madeira Channels.Fan channels are typically connected to larger feeder canyons,which means turbidity currents are already con?ned(channelized)by the canyon before they pass into the fan channels(Babonneau et al.,2002).Events passing through the channels are relatively frequent,in some cases occur-ring more than once a year(Heezen et al.,1964).The Madeira Channels are not directly connected with a feeder canyon.Instead, they are connected with the distal end of the Agadir Basin.This means that turbidity currents passing into the Madeira Channels via this?ow pathway are largely uncon?ned,being as wide as the Agadir Basin(i.e.w100km across).Events that have suf?cient volume to reach the channels are relatively infrequent,w1every 10,000years(Wynn et al.,2002b).

Typical channel-levee architectures are not observed in the Madeira Channels.Coarse-grained turbidite deposition is entirely lacking in the axis of the northern Madeira Channel whilst the axis of the southern Madeira Channel has a combination of thin sandy turbidites and?ow bypass.Neither of the channel axes have any detectable lateral-accretion packages nor any erosive/cross cutting surfaces.Typically,channel levees are generated from the upper parts of turbidity currents periodically or continuously overspilling the con?nes of the channel walls as they pass along the channel axis(Peakall et al.,2000).Levees produced in this manner will thin and?ne laterally,away from the channel axis(Kane and Hodgson, 2011;Kane et al.,2007,2010).Most of the turbidites within the Madeira Channel System enter the Madeira Channel System as uncon?ned sheet?ows resulting in deposition along the margins that progressively thickens away from the channel axis.Hence the channel margin deposits should not be referred to as channel levees,as the process for their formation(and depositional archi-tecture)is quite different from channel overspill and crevasse splay type levee formation.

Some comparisons can be drawn with the smaller scale Brazos-Trinity Turbidite System,offshore Gulf of Mexico.Within this system submarine channels connect a series of mini-basins for w60km across the continental shelf(Badalni et al.,2000). Turbidity currents are interpreted to be channelized until they pass into the?rst mini-basin.Here they spread and eventually spill over into the connecting channel system as uncon?ned?ows,much wider than the channels.Due to the uncon?ned nature of the?ows a number of nick points and smaller tributaries develop that coa-lesce into a main channel(Beaubouef and Friedmann,2000).In terms of regional setting the Madeira Channels are a larger version of the Trinity-Brazos channels,connecting two basins over w700km.Indeed,turbidity currents passing into the Madeira Channels are also largely uncon?ned(>100km wide).This is probably the reason why the southern Madeira Channel initiates from a number of well-de?ned knick points that quickly converge into a main channel(Fig.2A).However,in terms of depositional architecture the Trinity-Brazos System is similar to other channel-levee systems with coarse-grained channel?ll and?ne-grained levees that thin away from the channel axis(Beaubouef and Friedmann,2000).From detailed seismic pro?les the channel axes show signi?cant erosion cutting into underlying levee deposits and lateral channel migration(Beaubouef and Friedmann, 2000),whereas the Madeira Channels are purely constructional with no channel migration.

The NAMOC system found in the Labrador Sea,offshore Greenland has an extensive network of shallow(5e20m deep) braided and‘yazoo type’channels that progressively converge into a main trunk channel(Hesse and Rakofsky,1992;Hesse,1989; Hesse et al.,1987;Klaucke et al.,1998).These shallow channels show little fractionation in grain size between channel?ll and levee deposition with sandy levees developed along their margins.The sandy levees are interpreted to have formed by deposition from largely uncon?ned,sand-rich turbidity currents sourced from the continental shelf,travelling along the lowest points of the basin (Hesse et al.,2001,1990).This is similar to the depositional archi-tecture of turbidites along the margins of the Madeira Channels. However,the NAMOC system has coarse-grained channel?ll and records signi?cant channel?oor erosion,which contrasts with the bypass dominated,non-erosive Madeira Channels.

Exceptional exposures of deep-marine sediments within the Neoproterozoic Windermere Supergroup,western Canada,and in the Permian Lainsburg Formation,Karoo,South Africa,document ‘poorly-’or‘weakly-con?ned’channel systems(Arnott et al.,in this issue;Brunt et al.,2013).‘Poorly-con?ned’channel architectures are characterized by laterally extensive,rippled sands deposited across the channel margins.These architectures are interpreted to be the product of largely uncon?ned turbidity currents passing across low-sinuosity channels(Arnott et al.,in this issue).The channel margin architectures observed across the Madeira Chan-nels,albeit without any associated channel?ll,are interpreted to the product of a similar process;largely uncon?ned?ows passing across the low sinuosity channels.

6.Summary of turbidity current processes

As turbidity currents passed across the Madeira Rise through the Madeira Channel System,either from the Agadir Basin or the Canary Islands,their ability to transport and deposit sediment was fundamentally affected.The range of depositional architectures seen in the core transects and geophysical data are interpreted to be the product of three types of?ow behaviour,categorized as:(1) Uncon?ned channel bypass,(2)Con?ned channel bypass and,(3) Con?ned channel deposition(Fig.14).Flow behaviour is primarily governed by changes in sea-?oor gradient,and?ow parameters such as:thickness,velocity and suspended grain size.These three types of?ow behaviour are as follows:(1)Turbidity currents entering the channels were uncon?ned and large-volume relative to the channels.The?ows were able to spread across the sea-?oor largely unimpeded,depositing laterally extensive ripple cross-laminated sands across the Madeira Rise.The shallow(w20m deep)channels con?ned only the lowermost parts of these turbidity currents.The con?ned parts of the?ows were accelerated within the channel axis and bypassed sediment down slope.This state of‘uncon?ned channel bypass’dominates across the northern channel(e.g.Beds2,5,12and14;Fig.9)but is also associated with turbidites in the southern channel(e.g.Beds2and5;Fig.11).This state of‘uncon?ned channel bypass’can evolve into‘uncon?ned

C.J.Stevenson et al./Marine and Petroleum Geology41(2013)186e205 202

channel deposition ’with changes in sea-?oor gradient.Flatter areas of sea-?oor decelerate both the con ?ned and uncon ?ned parts of the ?ows triggering deposition of ripple cross-laminated sands across the margins and within the axes of the channels.This depositional architecture is shown by one turbidite in the southern channel (Bed 7in Core Transect 2;Fig.11)and acoustically across the more distal parts of the northern channel (Figs.5and 6C ).(2)Thinner ?ows were more con ?ned by the channels and are unable to spread as extensively.In this case a higher proportion of the ?ow was accelerated and bypassed down slope.The uppermost parts of the ?ow were able to overspill the con ?nes of the channel,which produced laterally restricted lenses of ripple cross-laminated sands along the channel margins.Examples of ‘con ?ned channel bypass ’can be seen within the northern channel (e.g.Beds 20and 23;Fig.9).(3)Some turbidity currents that were con ?ned by the channels were not accelerated enough to become bypassing.In this case ‘con ?ned channel deposition ’occurred with relatively thick ripple cross-laminated sands deposited in the channel axis and thinner,?ner-grained sediment deposited on the channel margins (e.g.Bed 8;Fig.11).Indeed,Beds 10.5and 12in the southern channel demonstrate that some channelized ?ows were completely con ?ned,and unable to deposit on the channel margins altogether (Fig.11).‘Con ?ned channel deposition ’also occurs with increasing distance from the site of channel initiation,across areas of ?atter sea-?oor.This indicates con ?ned (channelized)parts of the ?ows were more ef ?cient than the uncon ?ned (lateral)parts of the ?ows and able to run out for much longer distances.These completely channelized turbidity currents deposited ripple cross-laminated sands within the axes of the channels and ?ner-grained silts and muds on the margins.This ?ow behaviour and

depositional architecture dominates the distal parts of the channels (e.g.all Beds in Core Transect 3;Fig.12).Once channelized the ?ows were highly ef ?cient,able to transport sediment for w 700km down slope before eventually spreading and depositing across the Madeira Abyssal Plain (Fig.7).7.Conclusions

This contribution presents a detailed examination of the Madeira Channel System using EM12bathymetric data,a dense network of 3.5kHz pro ?les,and a suite of shallow sediment cores.This detailed data set provides insights into the response of large-volume turbidity currents entering a shallow (<20m)channel system.Speci ?c conclusions are:

1.Although relatively shallow and low-relief,the Madeira Channel System exerts a major in ?uence on turbidity currents,generating complex across ?ow and down ?ow heterogeneities in turbidite deposition.The channel relief would be extremely dif ?cult to detect in outcrop.

2.Subtle changes in sea-?oor gradient (<0.02 e 0.06 )can change turbidity current behaviour from deposition to ef ?cient bypass of their sediment loads down slope.Relative change in slope (i.e.three fold)could be more important than the absolute slope change (i.e.w 0.04 ).Currently these changes in sea-?oor gradient would also be undetectable in outcrop or within the subsurface.

3.Extremely large-volume (>100km 3),aerially extensive turbidity currents can ef ?ciently bypass sediment leaving no trace of their passing.Indeed,despite large volumes of

bypass

Figure 14.Simpli ?ed cartoon illustration summarizing the three main types of ?ow behaviour and resulting depositional architectures across the Madeira Channel System.The left side shows a plan view of ?ows passing into the channel system.Arrows indicate ?ow direction and size of arrows shows ?ow velocity:the larger the arrow the faster the ?ow.Darker shading represents areas of turbidite deposition whilst lighter shading represents areas of non-deposition.The right side of the ?gure shows corresponding channel cross-sections with inferred ?ow behaviour and associated depositional architectures (see main text for details).Downward pointing arrows indicate zones of deposition.

C.J.Stevenson et al./Marine and Petroleum Geology 41(2013)186e 205203

no discernable erosion occurred in the channels.This means that an absence of a turbidite(or associated erosion)in

a channel axis does not necessarily mean an absence of

a turbidity current passing through the channel.

4.Erosion is not required to build or maintain the Madeira

Channels.Rather it is construction of the channel margins and bypass within the channel axes that builds and maintains channel relief.

Acknowledgements

This work was funded by the Natural Environment Research Council(NERC)and UK-TAPS Consortium(Conoco Phillips,Shell, Exxon Mobil,BHP Bilton and Norsk Hydro).The British Ocean Sediment Core Research Facility(BOSCORF),Southampton, provided access to the cores used in this study and technical support in their analysis.We thank the numerous scientists and technical support crew aboard the scienti?c cruises upon which the data for this study were collected.Finally we would like to thank Michal Janocko and Simon Barker for their thorough and constructive reviews,which improved the structure and focus of the paper considerably.

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