Seismic Channels Geomorphology

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Journal of Sedimentary Research, 2007, v. 77, 433446

Research Article

DOI: 10.2110/jsr.2007.042

SEISMIC GEOMORPHOLOGY AND EVOLUTION OF SUBMARINE CHANNELS FROM THE ANGOLANCONTINENTAL MARGIN

M.J.R. GEE,1,* R.L. GAWTHORPE,1 K. BAKKE,2 AND S.J. FRIEDMANN,3,{1 Basin Studies and Petroleum Geoscience, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester M13 9PL, U.K.

2 Norsk Hydro Research Centre, Sandsliveien 90, Bergen, 5020, Norway3 Department of Geology, University of Maryland, College Park, Maryland 20742, U.S.A.

e-mail: [email protected]

ABSTRACT: Three-dimensional seismic data from the shallow subsurface of the continental margin offshore Angola reveal twoend-member morphological styles of submarine channel: (1) high-gradient, low-sinuosity, narrow channels with gull-wingedlevees, and (2) lower-gradient, deeply incised systems with moderate- to high-sinuosity channel axes. A third, and rare, channelform has moderate incision, low to medium sinuosity, and a moderate long-profile gradient. Based on channel parameters(incision depth, long-profile gradient, channel-axis sinuosity) and crosscutting relationships, we suggest that the channelsevolved from initially steep and straight, with low sinuosity, to highly sinuous and deeply incised with lower channel-axisgradients. Correlation of long-profile gradient with both incision and sinuosity suggests that incised channels appear to removeconvex-up curvature from the original slope as the channel axis evolves toward an equilibrium profile. Localized changes inchannel planform, gradient, sinuosity, and incision reflect the complex morphology of the slope associated with growth of salt-related structures. Linear, high-amplitude seismic features, which correspond to weakly incised striations, or rills, on the openslope are considered to be precursors of submarine channels.

INTRODUCTION

Submarine channels respond to sea-level change, sediment flux,

tectonics, and climate, and have a significant impact on the sedimentaryarchitecture of continental margins (e.g., Satterfield and Behrens 1990;

Damuth et al. 1988). They are also important oil and gas exploration

targets along continental margins (e.g., Kolla et al. 2001). Despite this,

many aspects of channel evolution and the processes that control their

geometry are still poorly constrained.

The use of 3D seismic technology along continental margins has

revealed turbidite channels in unprecedented detail (e.g., Roberts and

Compani 1996; Kolla et al. 2001; Mayall and Stewart 2000; Sikkema and

Wojcik 2000; Abreu et al. 2003; Deptuck et al. 2003; Samuel et al. 2003;

Posamentier and Kolla 2003; Posamentier 2003; Saller et al. 2004).

Numerous turbidite channels have been described in the shallow

subsurface of the Angolan continental margin that are exploration

targets, or analogues, for deeper hydrocarbon reservoirs (e.g., Kolla et al.

2001; Sikkema and Wojcik 2000; Mayall and Stewart 2000). The Angolan

continental margin has been strongly influenced by salt tectonics (e.g.,

Lavier et al. 2001; Valle et al. 2001; Hudec and Jackson 2002, 2004),

resulting in channels with an extraordinary range of geometries (e.g.,

Anderson et al. 2000; Mayall and Stewart 2000; Abreu et al. 2003;

Broucke et al. 2004; Gee and Gawthorpe 2006). Some submarine channels

are simple, with a straight planform, whereas others are highly complex,

and consist of broad, deeply incised canyons filled with numerous highly

amalgamated sinuous channels. Channels may be erosional or aggrada-tional, with low or high sinuosities, and with or without well developed

levees (e.g., Mayall and Stewart 2000).

We quantify the geometry of a number of Angolan deep-watersubmarine channels imaged from the shallow subsurface in terms of

present-day long-profile gradient, channel-axis sinuosity, and channeldepth. Many of the channels observed have wide (. 3 km) channel

valleys, which are incised by more than 200 m. At the base of each ofthese incised channel valleys is a highly sinuous channel axis. Otherchannels on the slope are smaller, typically 4060 m wide and tens of

meters deep, and are characterized by low-sinuosity channel axes. Incontrast to many other studies, which document the fill of channel

complexes, we focus specifically on the three-dimensional morphology ofthe master erosion surface bounding channel complexes. By examiningand quantifying key characteristics of channel systems with different

incisional geometries, it may be possible to gain a clearer understandingof the controls on channel geometry, and of aspects of channel initiation

and evolution.

GEOLOGICAL SETTING

The study area is located on the Angolan continental slope, about100 km offshore, in water depths of approximately 1.5 km (Fig. 1). The

present-day shelf break is located approximately 60 km to the NE.During the late Miocene, a large network of submarine channels was

active, transporting clastic sediment into deeper water towards thesouthwest (Fig. 1). These submarine channels flowed across a seabed that

was actively deforming due to movement of a mobile salt layer at depth.The salt is Aptian in age and in many places has pierced through the

* Present address: Lukoil Overseas, Ltd. Moscow, 115035, Russia

{ Present address: Energy and Environmental Directorate, Lawrence Livermore

National Laboratory, Livermore, California 94550, U.S.A.

Copyright E 2007, SEPM (Society for Sedimentary Geology) 1527-1404/07/077-433/$03.00


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overlying stratigraphy to the current seafloor (e.g., Uchupi 1992; Valle etal. 2001; Hudec and Jackson 2002, 2004).

The isochore for the near-surface seismic interval (, 500 ms (twtt)below seafloor), which contains the submarine channels studied in thispaper, indicates that the slope had a complex geomorphology comprisingintra-slope basins and salt-cored growth folds and related faults (Fig. 1).

Deformation of the seafloor caused by these growth folds and faultsresulted in complex channel geometries that are highly variable overdistances of just a few kilometers. Many channel systems deviate around,or converge between, growth anticlines and salt walls, and deposit

sediment within intervening intra-slope basins (Fig. 1) (e.g., Mayall andStewart 2000; Gee and Gawthorpe 2006). This pattern of sedimentation issimilar to the fill and spill of salt-withdrawal minibasins described fromthe Gulf of Mexico (e.g., Winker 1996; Prather et al. 1998; Twichell et al.2000; Beaubouef and Friedmann 2000; Badalini et al. 2000).

This study concentrates on channels buried a few hundred meters

below the present-day seafloor, where postdepositional deformation bysalt tectonics is inferred to be relatively low compared to deeper in the

subsurface. Figure 2A shows evidence for some postdepositional tiltingand erosion of the area in the form of reflection events that are truncatedat, or near, the seafloor. Local channel and slope gradients may have

undergone some postdepositional modification due to later salt-relateddeformation, and the absolute gradient values quoted, therefore,comprise a combination of original, depositional gradient and post-depositional tilting. However, planform geometries (e.g., channelsinuosity and width) and the amount of incision are unlikely to bemodified by later deformation. We have studied particular structural

domains on the slope where internal deformation is minimal andconsistent across the study area. Measurements of channel-axis gradientsappear to vary consistently with channel-axis sinuosity and channel-valleyincision. In fact, comparison of two channel systems (Channels 3 and 4),one near the base and one near the top of the interval studied (Fig. 2),

shows similar channel-axis gradients and long-profile curvatures. Thissuggests that, over the area where the channels have been measured, anypost-channel deformation has mainly affected the entire stratigraphicinterval around the channel systems in a similar way.

DATABASE AND METHODOLOGY

The 3D seismic data used in this study were acquired for the purposes

of oil and gas exploration, offshore Angola. The dataset has an inline

spacing of 12.5 m and a seismic resolution of, 60 m horizontally and

, 15 m vertically. The seismic interpretation objective was to define the

geometry of the master channel incision surface, to define the channel

valley, and to define the geometry of lowest (oldest) channel axis within

each channel valley (Fig. 3). The channel axis is seismically defined as

a geobody comprising amplitude anomalies recognized at the lowest point

along the base of each channel valley. Seismic interpretation involveda combination of manual, line-by-line interpretation, autotracking, and

voxel-growing techniques. The master channel valley surface, channel

axis, and erosional and depositional features associated with the channel

were imaged using random seismic lines along, and perpendicular to, the

channel axis (long profile), together with a variety of maps and 3D

perspective views of the channel-valley morphology and seismic attributes

(mainly amplitude, dip, and coherence). The channel axis can be

recognized on the basis of its very high amplitude and generally lower

dip relative to the regional slope.

For quantitative analysis, x, y, z data generated from the 3D seismic

interpretation were gridded at ,15 m using GMT software (Wessel and

Smith 1991). Gradient and sinuosity were measured along each channel

axis at 2 km intervals from the gridded data. Gradients were measured in

milliseconds (twtt)/km along the channel axis, and sinuosity was

measured as the ratio of channel-axis length to channel-valley length.

Channel incision was measured by fitting an artificial surface (least

squares, spline curvature) across the top of the channel valley (Fig. 3),

taking into account any levee development, and resampling along the

same coordinates as used for the channel axis below. Incision was thus

calculated as the vertical difference between the contemporaneous slope

and the channel axis. Channel axis and incision profiles were sampled

from the seismic grid at 50 m intervals. Two-way travel times were

converted to depth using a seismic velocity of 2000 m/s (unpublished well

data) for the first several hundred milliseconds of stratigraphy below the

seabed.

FIG. 1.Map of study area showing the near-surface salt-dome tops and channels interpretedfrom the upper 500 ms (twtt) of the 3D seismicvolume. Channels trend from NE to SW throughcomplex salt structures on the slope. Grayshaded interval shows isochore thicknesses forthe stratigraphic interval containing the chan-

nels. Darker shades 5 thicker sequences. Insetmap shows the study area offshore Angola.Dotted lines show 24000 m and 22000 mseafloor contours.

434 M.J.R. GEE ET AL. J S R


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A total of 13 channels were analyzed within the study area, and we

present our results by describing four of these channels in detail to show

the range of channel sinuosity, gradient, incision and the degree of levee

development. The channels are located on a segment of the slope that

strikes NWSE and is affected by a series of NWSE-striking normal

faults, salt-cored folds, and salt diapirs (Fig. 1). The channels are

associated with a suite of distinctive seismic facies, and are recognized by

their V- or U-shaped geometry with up to 200 ms (twtt) of erosional relief

(Fig. 2B). The four channels that represent the focus of this study are

numbered according to the amount of channel-axis sinuosity and valley

FIG. 2.Seismic sections or ientated approximately along strike of the paleoslope, illustrating the cross-sectional seismic characteristics of channel systems and slopedeposits (see Fig. 1 for location). A) Longer seismic line showing cross-sectional geometry of channels, synsedimentary normal faults, and the broad, near-surfaceanticline truncated at the seabed. B) Detail of the four channels analyzed in this study. See text for description of the channels and seismic facies.

SEISMIC GEOMORPHOLOGY OF SUBMARINE CHANNELS 435J S R


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incision. Channel 1 is relatively straight and weakly incised, whereasChannels 3 and 4 are highly sinuous and deeply incised. There is not,however, a simple stratigraphic relationship between the complexity ofthe channel system and its relative age, with Channel 4 the oldest andChannels 2, 1, and 3 progressively younger.

SEISMIC FACIES AND GEOMORPHOLOGY

Seismic Facies

Three seismic facies were used to identify the channel systems, on thebasis of reflection amplitude, geometry and lateral continuity (Fig. 2B).The three seismic facies represent three specific depositional environ-ments: (1) channel-axis deposits, (2) internal channel fill sediments, and

(3) levees.Seismic Facies 1 is of low to moderate amplitude, with discontinuous to

chaotic and discordant reflections, and is confined within channels. It is

best developed within the upper 140 ms (twtt) of Channel 3 and the upper45 ms (twtt) of Channel 2 (Fig. 2B). Seismic Facies 1 is interpreted as

fine-grained channel fill. A proportion of this fill was probably derivedfrom local slumping of the valley wall, especially at bends in the channelvalley (e.g., Mayall and Stewart 2000; Samuel et al. 2003).

Seismic Facies 2 consists of high-amplitude, discontinuous seismicreflections or couplets at the base of, and sometimes within, theinternal fill of channel valleys (Fig. 2B). It occurs at the base of allchannels analyzed in this study. Within the deeply incised Channel 4system, Seismic Facies 2 occurs at the base of the channel valley, and alsohigher up within the fill (e.g., Fig. 2B). Seismic Facies 2 is interpreted ascoarse-grained lithologies deposited in the channel axis.

Seismic Facies 3 is characterized by high- to low-amplitude,

continuous, parallel to subparallel reflections that are immediatelyadjacent to channels. These reflections typically dip away from the

channel axis, and they also decrease in amplitude away from the channelaxis (e.g., Fig. 2B). Seismic Facies 3 is interpreted as overbank depositsthat form gull-wing levees adjacent to the submarine channels (e.g., Kollaet al. 2001; Mayall and Stewart 2000). The small, straight, slightly incisedsubmarine channels, such as Channel 1, typically have well-developedlevees that are 12 km wide and are continuous along the length of thechannel (Fig. 2B). In contrast, levees are generally absent in the moredeeply incised channels such as Channel 3 and 4.

Profiles along the axes of the four representative channels illustrate the

distribution of Seismic Facies 1 and 2 (Fig. 4A, B). Channel 1 has a simpleand relatively continuous, high-amplitude reflection event at its base thatis characteristic of Seismic Facies 2. This channel system is too narrow

and shallow for any channel fill to be clearly resolved. Channel 2 displayslimited, and highly variable, development of Seismic Facies 2 along its

channel axis. Amplitudes of Seismic Facies 2 degrade over high-gradientreaches of the channel axis, and increase in strength and continuity in

lower-gradient reaches (Fig. 4B; Channel 2). The highest amplitudescharacterize areas of lower channel gradient, immediately downslope of

high-gradient reaches. The degradation of amplitudes may be related toflow acceleration and bypass in regions of high gradient, compared toflow deceleration and bedload deposition in lower-gradient sections of the

channel farther downslope. Thus the amplitude change may reflecta change in lithology from coarse- to finer-grained deposits (e.g., Sullivan

et al. 2000; Morend et al. 2002).

Qualitative Geomorphology of Channel Valleys and Channel Axes

The four channel systems are shown in Figure 5A in plan view

superimposed on an isochore map of the stratigraphic interval whichcontains the channels, and in Figure 5B as dip attribute maps that

illustrate the morphology of the channel-valley walls and channel axis. All

four channel valleys appear to have similar gross linear to low-sinuosityplan-view geometry. They range from narrow, straight channels a fewtens of meters deep (e.g., Channel 1), to deeply incised channels, up toseveral kilometers wide, with a more irregular valley-wall morphology

(e.g., Channel 4) (Figs. 5, 6, 7).

Channel 1.This channel has a very low sinuosity (, 1.1) and the

steepest gradient of the four channels (Figs. 4, 6, 7A). Channel 1 is mildlyincised (, 15 m) into the underlying substrate and has a simple, low-sinuosity channel axis up to 4060 m wide marked by a ribbon-like high-

amplitude anomaly with broad, sinuous bends with a wavelength ofapproximately 1 km (Figs. 5B, 6, 7A). The channel has well developed

levees characterized by relatively continuous, high-amplitude reflectionsthat diminish in seismic amplitude and relief over a distance of 12 km

from the channel axis. Similar small channels occur throughout theinterval studied, although they are more abundant in the upper part of the

section.

To the NW of Channel 1 there is a series of subparallel striations,spaced 100300 m apart, that trend downslope and are characterized by

negative relief (, 15 m) and higher amplitudes than the surroundingslope sediments (Figs. 6, 7A). We interpret these striations as open slopeerosion by either unconfined turbidity currents or mass-wasting events.

These open slope striations are morphologically similar to the sloperills reported from the New Jersey continental slope by Pratson et al.(1994).

Channel 2.The overall morphology of the channel valley of Channel2 is similar to Channel 1, but it has more irregular valley walls, especially

on the SE flank (Figs. 5B, 7B). Channel 2 is incised 50100 m into slopesediments and, despite the overall straight form of the channel valley, its

axis has low to moderate sinuosity (approximately 1.3) (Figs. 5B, 7B).The higher amplitudes at the base of the channel valley, interpreted as

channel-axis sediments (Seismic Facies 2), are highly variable andrelatively discontinuous compared to Channel 1 (Fig. 7B).

Channel 2 is cut by three synsedimentary, down-to-the-basin normalfaults, over a 34 km reach (Figs. 5B, 7B). Across these faults there is an

increase in channel-axis gradient and a general widening of the channelvalley (Fig. 5B). Some of the largest channel-axis loops occur in this

faulted region (Figs. 5B, 7B). Up-dip of the fault scarps the channel valleyis narrower (e.g., Fig. 5B) and more deeply incised.

FIG. 3.Schematic diagram of a channel system, showing an incised channelvalley containing a highly sinuous channel axis, together with a graphicaldefinition of the terms used to describe the channel systems in this paper.



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The paleo-seafloor immediately adjacent to Channel 2 is characterized

by discontinuous low to moderate amplitudes (Fig. 7B). Areas of higheramplitude flanking Channel 2 occur localized, immediately adjacent toouter bends along the valley walls (Fig. 7B). These observations suggestthat Channel 2 has poorly developed levees compared to Channel 1.

Channel 3.Channel 3 is deeply incised, up to 200 m, and has thehighest channel-axis sinuosity of the four channels (maximum 5 3.2;mean 5 2; Figs. 5B, 7C). The channel axis is recognized by its sinuous,ribbon-like high seismic amplitude which contrasts with the adjacent low-amplitude channel valley fill (Fig. 7C). The channel valley is relativelystraight with scalloped walls that are not as irregular as Channels 2 and 4.The radius of the valley-wall scallops is similar to channel-axis loops.

Outside of the channel valley, high amplitudes are preferentially

developed on the SE side of the channel, especially where there isa prominent embayment in the downstream part of the channel valley(Fig. 7C).

Channel 4.This channel is deeply incised, up to 200 m, and hasa moderate- to high-sinuosity channel axis, with a sinuosity up to 1.8(Figs. 5B, 7D). Although the channel valley is overall straight, the valley-wall morphology is irregular and asymmetric, with the SE wall beinghighly arcuate or scalloped in plan view (Figs. 5B, 7D). In contrast to theother channels, the channel axis in Channel 4 has an asymmetric positionwithin the valley, being preferentially located towards the SE side of thevalley (Figs. 5B, 7D). In many cases the position of the scallops on the SE

FIG. 4.AD) Long-profile seismic sectionsalong the four channel axes (Channels 14). SeeFigure 5 for location of the channels.



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valley wall corresponds to large loops of the channel axis (Fig. 5B). In

contrast, loops on the NW side of the channel axis tend to be locatedwithin the channel valley and not against the NW wall. The distal portion

of the channel system is generally less incised than the proximal region

and is characterized by a broad area of high amplitudes over 1 km wide

(Fig. 7D). This distal part of the channel also shows a prominent swingfrom the regional SW dip of the slope to flow west-northwest, parallel to

the axis of an intra-slope basin (Figs. 1, 7D; Gee and Gawthorpe 2006).

The contemporaneous slope, outside of Channel 4, is generallycharacterized by low to moderate amplitudes and no levee development.Localized, moderate to high amplitudes do occur on the SE flank of

Channel 4, especially downslope of large scallops in the valley wall,suggesting limited channel overspill (Fig. 7D). The asymmetry in valley-

wall morphology, channel-axis position, and amplitude anomalies on theadjacent slope suggest that the whole channel system was progressivelytilted to the east-southeast during deposition (see Fig. 2).

Quantitative Analysis of Channel Parameters

Channel-axis long profiles for all 13 channel systems studied showa distinct grouping into either high-gradient or low-gradient systems(Fig. 8A). The high-gradient systems, for example Channel 1, tend to

have smoother long profiles, whereas the low-gradient systems, such asChannel 4, have short-wavelength roughness at a length scale of, 4 km(Fig. 8A). Present-day channel-axis long profiles, gradient, sinuosity, and

incision depth for the four case-study channels are summarized inFigure 9. For all channel systems, apart from Channel 1, sinuosity isnegatively correlated with present-day channel-axis gradient. The lowest-

sinuosity example, Channel 1, has the steepest gradient. The steeperchannels (e.g., Channels 1 and 2) are also less incised, in contrast to thehighly sinuous systems (e.g., Channels 3 and 4; Figs. 8B, 9). Usinga reasonable velocity of 2000 m/s, the maximum channel-axis gradient

observed is approximately 3.4u(Channel 1), and the minimum channel-axis gradient is approximately 0.4u (Channel 4). For all channels, exceptChannel 4, the amount of incision decreases down channel (Fig. 8B, 9).

Channel 1.This channel has the highest average gradient, 44 ms

(twtt)/km, the lowest channel relief, 50 ms (twtt), and lowest sinuosity(, 1.1) of all of the four channels. The axis of Channel 1 has a gradientthat starts at , 35 ms (twtt)/km, at the proximal end of the channel, and

increases to 55 ms (twtt)/km, 9 km down-channel (Fig. 9A). The gradientreduces again to 40 ms (twtt)/km at 15 km, then rises to 60 ms (twtt)/km

at 20 km (Fig. 9A). Gradient and sinuosity appear to be positivelycorrelated across short channel reaches, , 4 km long. For longer

reaches, of approximately 20 km in length, sinuosity fluctuates anddecreases slightly against a gradient increase.

Channel 2.Channel 2 is incised by up to 100 ms (twtt) in its proximalreach, but the amount of incision fluctuates along the length of thechannel, with a general decreases to approximately 50 ms (twtt) in itsdistal part (Figs. 8B, 9B). The channel has an overall low to moderate

sinuosity, with a maximum of approximately 1.6 and an average of 1.3.Syndepositional faults intersect Channel 2 and create higher channel-axisgradients starting at approximately 13 km along the channel-axis profile

(Fig. 9B). The faults are reflected in the sinuosity, which decreases slightlythen increases in the immediate hanging wall of the faults (Fig. 9B).Gradient and sinuosity are negatively correlated overall, although overshort reaches (, 4 km) sinuosity and gradient can show a positive

correlation.

Channel 3.The axis of Channel 3 has the highest sinuosity (. 3), andthe channel valley is also the most deeply incised, being locally over

200 ms (twtt) deep (Figs. 8B, 9C). Channel-valley incision is greatest inthe upper reaches of the channel, where it is . 200 ms (twtt) between 5and 10 km (Fig. 8B). The amount of incision fluctuates markedly up to50 ms (twtt) over just a few kilometers, but there is a gradual decreases to, 150 ms (twtt) downslope (Fig. 8B). The present-day channel-axisgradient is relatively constant at approximately 20 ms (twtt)/km, for the

FIG. 5.Plan-view geometry of Channels 14 (see Fig. 1 for location).A)Detailof the channels in relationship to the isochore of the stratigraphic interval thatcontain the channels. Dotted lines show isochore contours in ms (twtt). B)Interpretations of the master incision surfaces of the channel valleys displayed asdip-attribute maps (dark shades indicate steeper slopes). Thin black lines withinthe channel valleys highlight the channel axis.



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first 11 km, drops slightly to 15 ms (twtt)/km, then increases to

approximately 30 ms (twtt)/km at 30 km distance (Fig. 9C). Sinuosity isrelatively high initially, rising from approximately 2 to a maximum of 3.2

at 13 km. From 1330 km the sinuosity fluctuates, but there is a trend ofdecreasing sinuosity downslope to values of 1.21.6 at the distal end of

the channel (Fig. 9C). With the exception of the interval between 17 and

22 km, which shows a positive correlation, there is a general negativecorrelation between sinuosity and gradient (Fig. 9C).

Channel 4.This is the longest channel interpreted in this study, and ithas a gently convex-up to sigmoidal channel long profile and a relatively

constant amount of incision between 180 and 200 ms (twtt) (Figs. 8B,9D). There is no consistent relationship between the channel-axis

sinuosity and present-day gradient along the length of the channel.

However, there is a general increase in sinuosity and channel-axisgradient from 15 to 27 km (Fig. 9D). Sinuosity shows a marked decrease

from 27 km even though the gradient continues to increase (Fig. 9D).Note that the position at which sinuosity sharply decreases at 27 km

corresponds to where the slope steepened and becomes more convex-up,and does not coincide with the prominent break of slope at 35 km

(Fig. 9D). The channel valley straightens and increases in width from60 m to 150 m downstream of the break of slope at 35 km (cf. Figs. 7D,

9D). This distal part of the channel is characterized by low gradients and

a broad area of very high seismic amplitudes.

Sinuosity, Gradient and Incision Relationships.The measurements of

sinuosity and gradient for all of the four main channels are summarized inFigure 10A. Sinuosity shows wide scatter, but below a gradient of 10

12 ms (twtt)/km (approximately 0.7u), and above 3035 ms (twtt)/km(approximately 2u), the channel axis is essentially straight, with sinuosities

generally below 1.3 (Fig. 10A). Between these low-gradient and high-gradient domains characterized by low sinuosity there is a wide scatter of

sinuosity for a given channel-axis gradient. There is a marked shift from

straight to the highest-sinuosity channel axis (3.2) at a gradient of

, 12 ms (twtt)/km (Fig. 10A). The peak sinuosity for any given gradient

then generally decreases with increasing gradient until the channel axis iseffectively straight above 35 ms (twtt)/km. When the parameters for eachof the four channels are averaged there is a simple inverse correlationbetween channel-axis gradient and both sinuosity and amount of incision(Fig. 10B).

DISCUSSION

Existing models of deep-water channel complexes offshore Angola arebased on 3D seismic, wireline log, and core data (Mayall and Stewart

2000; Sikkema and Wojcik 2000; Kolla et al. 2001). These models largelydescribe the evolution of the fill of major submarine channel valleys.Typically this fill consists of a basal lag, debris flows, and slumps at thebase of the channel fill, overlain by sandy, straight or sinuous stackedchannels, and finally mud-rich, sinuous channellevee complexes. Aproblem in defining former erosional channels by the deposits that fillthem is that the fill may not be directly related to the processes thatformed the original master erosional valley (Kneller 2003). Mayall andStewart (2000) make a broad distinction between the erosional sinuositythat established the channel, and the constructional sinuosity thatcharacterizes the final channel fill.

The data generated by this study allow us to analyze the three-

dimensional morphology of the master channel valley. In the followingdiscussion we synthesize the seismic geomorphology of the submarine

channels, and the various channel parameters, in order to examine theorigin and evolution of erosional submarine channels and discuss thepossible controls on the variability of their planform geometry, longprofiles, sinuosity, and incision.

Local Controls on Gradient, Sinuosity, and Incision

The steep, linear to convex-up long profiles of the channel-valley axesdocumented in this study reflect, in part, the structural morphology of theAngolan continental slope. In the study area, the morphology of the slopeis complex, with a typical length scale of 1020 km controlled by thegrowth of salt-cored anticlines, salt diapirs and walls, and intra-slope

FIG. 6.Oblique view of seismic amplitudedraped on timestructure map for the base ofChannel 1 looking upslope, towards the NE.Inset is detail of channel and adjacent slope(lighter, red and yellow colors are higher

amplitude). The highly linear channel valleycontains a slightly sinuous channel axis. Leveesare highlighted by high amplitudes adjacent tothe channel. Note the out-of-channel striationson the slope to the west of Channel 1.



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FIG. 7.Oblique views of seismic amplitude draped on timestructure maps for the four channels (Channels 14) looking upslope, towards the NE. The timestructuremaps are for the bases of the channels; lighter, red and yellow colors are higher amplitude. See text for discussion.



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basins (Fig. 1) (Mayall and Stewart 2000; Gee and Gawthorpe 2006). Asa result of this complex sea-floor topography, the long profiles of thechannels in this study are quite different from the concave-up equilibrium

profiles recorded from many submarine channels andcanyons (e.g., Pirmezet al. 2000; Mitchell 2005). It should also be remembered that a , 50 kmreach of the channels is imaged, and neither the proximal nor distalportions of the channels have been studied. Despite these restrictions, thereare several observations that suggest that the studied channels were tendingto approach an equilibrium profile during their evolution.

Overall, the channels are more deeply incised, and the channel axes are

more sinuous, on the steep limbs of anticlines, and they broaden andcontain more sheet-like, aggradational, distributary channel networkswhere they enter intra-slope basins (e.g., Channel 4, Fig. 5A; also see Geeand Gawthorpe 2006). Thus evolution of the channels towards anequilibrium profile is accomplished by erosion of high-gradient reachesand deposition in low-gradient portions of the channel. Furthermore,

there are systematic relationships between channel parameters such assinuosity, channel depth, and gradient (e.g., Fig. 10B), an observationthat has been used by other workers to suggest a system that is in, orapproaching, equilibrium or grade (e.g., Pirmez and Flood 1995, Pirmezet al. 2000).

In addition to the broad folding of the seafloor, across which thechannels formed, there are a number of syndepositional faults that

intersect the channels and cause significant changes in channelmorphology. Across these faults there are local increases in slope and

channel-axis gradient, and associated changes in sinuosity, depth ofincision, and channel-valley width. For example, across the faulted reachof Channel 2 (Figs. 5, 7B, 9B), there is an increase in channel-axis

gradient, and a general widening of the channel valley, most notable inthe immediate hanging wall of each fault (Fig. 5B). Some of the largestchannel-axis loops occur in this region of Channel 2 (Figs. 5B, 7B). Inaddition, up-dip of the faults the morphology of the channel is markedly

different: it is more deeply entrenched, the channel valley is narrow, andsinuosity of the axis is lower and less variable (e.g., Figs. 5B, 9B).

In other slope settings, examples of submarine channels flowing over

steep slope segments appear to respond by either increasing sinuosity orincreasing incision in order to reduce the local gradient anomaly (e.g.,Flood and Damuth 1987; Pirmez and Flood 1995; Pirmez et al. 2000). Assubmarine flows cross a steeper slope they accelerate and become more

erosive. Increased erosion can result in either incision or sinuosityincreaseboth operate, over time, to reduce the local gradient. However,there is a limit to how much gradient reduction can be achieved by

sinuosity increase alone, due to increased probability of loop cutoffs atvery high sinuosity (. 3) (e.g., Pirmez and Flood 1995). For some of theAngolan submarine channels, sinuosity increases where channel-axisgradients increase until sinuosity abruptly decreases (e.g., Channel 4;

FIG. 8.Characteristics of long profiles. A)Long profiles for the channels within the studyarea (straight channels 5 dotted lines; sinuouschannels 5 solid lines). Note grouping intohigh-gradient straight channels and lower-gra-dient sinuous channels.B) Variation of depth ofincision along channel axis for the four mainchannels discussed. Note that low-sinuositychannel (Channel 1) has the lowest depth ofincision, whereas the high-sinuosity channels(e.g., Channels 3 and 4) have the highest depth of

incision. Most channels show a downslope de-crease in depth of incision.



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continuity between Channels 1 and 2; Fig. 7A, B). In more entrenched

channel systems overspill or levee deposits are restricted to the outer

bends of channels, suggesting localized flow stripping or overspill ofturbidity currents (Piper and Normark 1983; Hiscott et al. 1997; Peakallet al. 2000).

Initiation of Submarine Channels

The process by which submarine channels and canyons initially form is

not well understood. Pratson et al. (1994) describe closely spaced sloperills, on the New Jersey continental margin, that incise inter-canyon

areas. Morphologically, the slope rills are similar to the downslope-directed slope striations observed in this study, which occur on the open

slope, outside of the main channel valleys (e.g., Figs. 6, 7A). We interpret

the slope striations to result either from large, unconfined and erosive

turbidity currents or from mass wasting (see Gee et al. 2005). This may be

one mechanism by which seafloor imperfections are produced that act asseeds from which channels initiate and grow. A similar mechanism for

channel initiation on open submarine slopes was suggested by Elliott

(2000), in which erosional megaflutes may have created the initialconditions for channel formation.

A conceptual model for the initiation of submarine channels can be

developed from a synthesis of the observations of the slope striations andstraight channels (e.g., Channel 1) presented in this paper, together withresults of numerical simulations of submarine-canyon evolution (e.g.,

Pratson and Coakley 1996) and fluvial incised-valley development during

sea-level fall (e.g., Ritchie et al. 2004). Once the slope striations areinitially developed on the open slope by erosion by submarine flows, their

growth can be viewed as a positive feedback process, whereby the amount

of sediment that is captured from sediment flows on the slope, and

generated by failure of the striation walls, increases as they deepen anderode headward. Thus the slope striations that deepen and lengthen the

quickest capture more sediment flows from the surrounding slope, and

also generate more turbidity currents through larger and more frequentcollapse of their walls. Eventually the system of slope striations organizes

so that one or two of the most active striations evolve into submarine

channels, and the others become inactive and may be preserved on theopen slope.

A Conceptual Model for Channel Evolution

In general, our data show that the steeper, less incised channels (e.g.,

Channel 1), have smooth, convex-up long profiles that are similar to thepresent-day seafloor profile. In contrast, the deeply incised channels (e.g.,

Channel 3), have rougher, less convex-up long profiles that have lower

gradient (Figs. 8, 9). Thus it appears that the less incised, and apparently

immature, channels still reflect the curvature of the slope on which theyinitiated, whereas the channels that are deeply incised (e.g., Channel 3)

appear to have evolved and removed the original slope convexity. The

logical extension of this evolutionary trend would move towards theformation of a concave-up equilibrium profile, as observed elsewhere for

many modern submarine channels (e.g., Pirmez et al. 2000; Mitchell2005).

These observations can be summarized in a conceptual model ofchannel evolution (Fig. 11). This model describes three stages in the early

evolution of a channel from an initially straight, high-gradient channel to

a deeply incised, highly sinuous channel with lower gradient. Observa-

tions supporting the idea that deeply incised channel systems evolvedfrom high-gradient, linear channel systems include the geometry of the

mature, incised channel valleys, which, although enlarged, still have an

overall linear channel-valley geometry (e.g., Figs. 5, 7), and thecorrelation of channel parameters (e.g., sinuosity, gradient, incision)

across all types of channels (e.g., Figs. 9, 10). What is not clear from the

data used in this study, however, is what specific flow types and process

were responsible for channel profile evolution.

Stage 1 corresponds to a low-sinuosity, high-gradient channel, several

tens of meters wide (e.g., Channel 1; Figs. 4, 5, 7). Levees are well defined

along the length of the channel, and the channel axis is slightly incisedinto the underlying seafloor. Levee relief contributes up to a few tens of

meters of the total channel relief for Stage 1 channels. The seismic data

(e.g., Figs. 1, 2) indicate that these Stage 1 channels are common in the

study area and are spatially associated with slope striations from whichwe postulate that they develop (Fig. 11).

By Stage 2 (Fig. 11), the channel valley has widened to several hundred

meters and it has incised into the slope by several tens of meters. Theincised axis of the channel valley has developed a low to moderate

sinuosity (e.g. Channel 2; Figs. 4, 5, 7), and the combination of incision

FIG. 10.Summary of relationships between sinuosity, gradient, and incision.A)Variation of channels-axis sinuosity with gradient. Note abrupt increase to peaksinuosity at 1012 ms (twtt)/km, followed by general decrease in sinuosity withincreasing gradient, and largely straight channel axes above a gradient of 35 ms(twtt)/km. B) Average value of incision, gradient, and sinuosity for each of theChannels 14, showing general inverse correlation between channel-axis gradientand both incision and sinuosity.



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and increased sinuosity has reduced the overall gradient of the channel-axis long profile. Deepening of the channel valley, together with lateral

erosion of the valley walls by submarine flows, increases instability of the

walls, leading to their collapse, creating an irregular, scalloped

morphology. Channel levees and deposits outside the main channelvalley are recognizable, but they are discontinuous and have lower relief

and lateral extent compared to Stage 1.

By Stage 3 the main channel valley has widened to 23 km wide and ithas incised to several hundred meters deep (Fig. 11). The axis of the

channel valley has become highly sinuous and the combination of incisionand sinuosity has significantly decreased long-profile gradients. The

valley walls have a complex scalloped morphology as a result of

collapse and lateral erosion. These deeply incised channels were probablyable to confine the majority of turbidity currents effectively, focusing

erosion within the channel valley and inhibiting sedimentation adjacent to

the channel on the open slope (e.g., Pirmez et al. 2000). However, the

high-amplitude lobate bodies on the slope, immediately adjacent to themain channel valley of Channels 3 and 4 (Fig. 7), suggest that some of the

larger submarine flows were able to overspill the main channel valley. The

deeply incised valley that forms in Stage 3 is subsequently affected byrepeat infill and reincision, which further modify valley-wall morphology

and create highly variable channel-fill architecture (e.g., Mayall and

Stewart 2000; Deptuck et al. 2003; Samuel et al. 2003).

CONCLUSIONS

Three-dimensional seismic data from the continental slope, offshore

Angola, allow imaging of a wide range of submarine turbidite channels.

We have focused on the master erosion surface (here termed channelvalley) bounding submarine slope channels to determine the variability in

erosional channel morphology and to assess how these channels may

initiate and evolve. This study complements many previous studies ofsubmarine channels that, in contrast, are based largely on analysis of the

channel fill.

Based on analysis of the planform and cross-sectional geometry of the

channel valleys, and the long profile, sinuosity, and amount incision ofthe channel-valley axis, two end-member channel types have been

recognized: (1) high-gradient, low-sinuosity, narrow channels with

gull-winged levees, and (2) lower-gradient, deeply incised systems withmoderate- to high-sinuosity channel axes.

Quantitative analysis of present-day gradient, sinuosity, and depth ofincision suggests an inverse relationship between channel-axis gradientand both sinuosity and depth of incision. Sinuosity is highly variable fora given gradient but shows an overall decrease in sinuosity with increasinggradient for long-profile gradients between approximately 0.7u and 2u.For gradients above and below these thresholds, channel axes areessentially straight.

The long profiles of the channels are typically steep, linear to convex-upand, thus, quite different from the smooth concave-up long profiles of

many submarine channels such as those associated with the Rhone or

Amazon (e.g., Pirmez et al. 2000). We interpret these complex longprofiles as a response to synsedimentary growth faulting and folding ofthe slope due to salt tectonism. The channels tend towards an equilibriumprofile by incising and/or increasing sinuosity across steep channelreaches, associated with anticline limbs and normal faults, and byaggrading in lower-gradient reaches within intra-slope basins. Synsedi-mentary deformation also has a major effect on the location andplanform geometry of the channels. Synsedimentary faults cutting across

channels at high angles cause marked changes in channel-valley width andchannel-axis sinuosity. In contrast, tilting subparallel to the channels

causes marked asymmetry of the morphology of the channel-valley walls,and in the location and morphology of the channel axis.

We suggest a conceptual model for the evolution of erosionalsubmarine channels, with an initiation phase linked to the growth oflinear striations, or rills, on the open slope, whereby one or two of thestriations evolve at the expense of others to become submarine channels.Early in their evolution, the channels are linear, narrow, and steep,typically a few tens of meters wide and deep. They are characterized by

long profiles that mimic the morphology and gradient of the surroundingslope, are weakly incised, and are largely confined by levees. Thesechannels evolve by reducing the initial high long-profile gradient byincising and becoming more sinuous. As a result, the channels becomeprogressively more incised and sinuous with maturity, creating channelvalleys several kilometers wide and several hundred meters deep, withhighly sinuous channel axes. A consequence of progressive increase in thedepth of incision is that the channels generally become fixed in position

FIG. 11.Model summarizing the initiationand evolution of erosional channels. Channelsevolve from rills on the open slope into high-gradient, low-sinuosity channels that are slightlyincised into the slope and largely confined bylevees. As channels attempt to develop anequilibrium long profile, they become moreincised and increase sinuosity in order to lowerlong-profile channel-axis gradient. As a resultthey develop wider, deeply incised channel

valleys, with lower gradient and a high-sinuositychannel axis.



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on the slope and submarine flows become increasingly contained withinthe channel valleys. Thus, as the channels evolve, levee and overspilldeposits adjacent to the channels become rare.

ACKNOWLEDGMENTS

We are grateful to Total Exploration and Production Angola, NorskHydro, Esso Exploration Angola (Block 17) Ltd., BP Exploration (Angola)

Ltd., Statoil Angola Block 17 A.S., and Sonangol for permission to publishthe 3D seismic data from offshore Angola. We particularly thank NorskHydro for supporting this project, and their long-term support of research inManchester. We gratefully acknowledge software donations by Schlumbergerand Paradigm Geophysical to the Basin Studies Group. We thank JohnGjelberg, Gianluca Badalini, Carlos Pirmez, and Lorna Strachan forinteresting discussions about channels and submarine slope processes.

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