Seismic Channels Geomorphology

Journal of Sedimentary Research, 2007, v. 77, 433–446 Research Article DOI: 10.2110/jsr.2007.042

SEISMIC GEOMORPHOLOGY AND EVOLUTION OF SUBMARINE CHANNELS FROM THE ANGOLAN CONTINENTAL MARGIN 1, 1

1

2

M.J.R. GEE, * R.L. GAWTHORPE, K. BAKKE,

3,{

AND

S.J. FRIEDMANN,

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, Norway 3 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 two end-member morphological styles of submarine channel: (1) high-gradient, low-sinuosity, narrow channels with gull-winged levees, and (2) lower-gradient, deeply incised systems with moderate- to high-sinuosity channel axes. A third, and rare, channel form 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 channels evolved from initially steep and straight, with low sinuosity, to highly sinuous and deeply incised with lower channel-axis gradients. Correlation of long-profile gradient with both incision and sinuosity suggests that incised channels appear to remove convex-up curvature from the srcinal slope as the channel axis evolves toward an equilibrium profile. Localized changes in channel planform, gradient, sinuosity, and incision reflect the complex morphology of the slope associated with growth of saltrelated structures. Linear, high-amplitude seismic features, which correspond to weakly incised striations, or rills, on the open slope 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 sedimentary architecture 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; Mayal l and Stewa rt 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 * Present addres s: 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)

amalgamated sinuous channels. Channels may be erosional or aggradational, with low or high sinuosities, and with or without well developed levees (e.g., Mayall and Stewart 2000). We quantify the geomet ry of a number of Angolan deep-water submarine channels imaged from the shallow subsurface in terms of present-day long-profile gradient, channel-axis sinuosity, and channel depth. Many of the chann els observed have wide ( . 3 km) channel valleys, which are incised by more than 200 m. At the base of each of these incised channel valleys is a highly sinuous channel axis. Other channels on the slope are smaller, typically 40–60 m wide and tens of meters deep, and are characteri zed by low-sinuosity channel axes. In contrast to many other studies, which document the fill of channel complexes, we focus specifically on the three-dimensional morphology of the master erosion surface bounding channel complexes. By examining and quantifying key characteri stics of channel systems with different incisional geometries, it may be possible to gain a clearer understanding of 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, about 100 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 the southwest (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

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FIG. 1.—Map of study area sh owing the nearsurface salt-dome tops and channels interpreted from the upper 500 ms (twtt) of the 3D seismic volume. Channels trend from NE to SW through complex salt structures on the slope. Gray shaded interval shows isochore thicknesses for the stratigraphic interval containing the channels. Darker shades 5 thicker sequenc es. Inset map shows the study area offshore Angola. Dotted lines show 2 4000 m and 22000 m seafloor contours.

overlying stratigraphy to the current seafloor (e.g., Uchupi 1992; Valle et al. 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 this paper, indicates that the slope had a complex geomorphology comprising intra-slope basins and salt-cored growth folds and related faults (Fig. 1). Deformation of the seafloor caused by these growth folds and faults resulted in complex channel geometries that are highly variable over distances 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 and Stewart 2000; Gee and Gawthorpe 2006). This pattern of sedimentation is similar to the ‘‘fill and spill’’ of salt-withdrawal minibasins described from the 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 by salt tectonics is inferred to be relatively low compared to deeper in the subsurface. Figure 2A shows evidence for some postdepositional tilting and erosion of the area in the form of reflection events that are truncated at, or near, the seafloor. Local channel and slope gradients may have undergone some postdepositional modification due to later salt-related deformation, and the absolute gradient values quoted, therefore, comprise a combination of srcinal, deposition al gradient and postdepositional tilting. However, planform geomet ries (e.g., channel sinuosity and width) and the amoun t of incision are unlike ly to be modified by later deformat ion. We have studied particular structu ral domains on the slope where internal deformation is minimal and consistent across the study area. Measurements of channel-axis gradients appear to vary consistently with channel-axis sinuosity and channel-valley incision. 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. This suggests that, over the area where the channels have been measured, any post-channel deformation has mainly affected the entire stratigraphic interval 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 12.5 m and seismic interpretation resolution of objective 60 m horizontally , 15 m of vertically. Thea seismic was to defineand 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 involved a 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), toget her 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.

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

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 NW–SE and is affected by a series of NW–SE-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

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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 simple and relatively continuous, high-amplitude reflection event at its base that is characteristic of Seismic Facies 2. This channel system is too narrow and shallow for any channel fill to be clearly resolved. Channel 2 displays limited, and highly variable, development of Seismic Facies 2 along its channel axis. Amplitudes of Seismic Facies 2 degrade over high-gradient reaches of the channel axis, and increase in strength and continuity in lower-gradient reaches (Fig. 4B; Channel 2). The highest amplitudes characterize areas of lower channel gradient, immediately downslope of high-gradient reaches. The degradation of amplitudes may be related to flow acceleration and bypass in regions of high gradient, compared to flow deceleration and bedload deposition in lower-gradient sections of the channel farther downslope. Thus the amplitude change may reflect a 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

FIG. 3.—Schematic diagra m of a channel system, showing an incised channel valley containing a highly sinuous channel axis, together with a graphical definition of the terms used to describe the channel systems in this paper.

incision. Channel 1 is relatively straigh t and weakly incised, whereas Channels 3 and 4 are highly sinuous and deeply incised. There is not, however, a simple stratigraphic relationship between the complexity of the channel system and its relative age, with Channel 4 the oldest and Channels 2, 1, and 3 progressively younger. SEISMIC FACIES AND GEOMORPHOLOGY

Seismic Facies Three seismic facies were used to identify the channel systems, on the basis of reflection amplitude , geometry and lateral continuity (Fig. 2B). The three seismic facies represen t three specific depositio nal environments: (1) channel-ax is 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 upper 45 ms (twtt) of Channel 2 (Fig. 2B). Seismic Facies 1 is interpreted as fine-grained channel fill. A proportion of this fill was probably derived from local slumping of the valley wall, especially at bends in the channel valley (e.g., Mayall and Stewart 2000; Samuel et al. 2003). Seismic Facies 2 consists of high-amplitude, discontinu ous seismic reflections or ‘‘couplets’’ at the base of, and sometimes within, the internal fill of channel valleys (Fig. 2B). It occurs at the base of all channels analyzed in this study. Within the deeply incised Channel 4 system, Seismic Facies 2 occurs at the base of the channel valley, and also higher up within the fill (e.g., Fig. 2B). Seismic Facies 2 is interpreted as coarse-grained lithologies deposited in the channel axis. Seismic Facies 3 is characterized by high- to low -amplitude, continuous, parallel to subparallel reflections that are immediately adjacent to channels. These reflections typically dip away from the channel axis, and they also decrease in amplitude away from the channel axis (e.g., Fig. 2B). Seismic Facies 3 is interpreted as overbank deposits that form gull-wing levees adjacent to the submarine channels (e.g., Kolla et al. 2001; Mayall and Stewart 2000). The small, straight, slightly incised submarine channels, such as Channel 1, typically have well-developed levees that are 1–2 km wide and are continuous along the length of the channel (Fig. 2B). In contrast, levees are generally absent in the more deeply incised channels such as Channel 3 and 4.

The four channel systems are shown in Figure 5A in plan view superimposed on an isochore map of the stratigraphic interval which contains 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-sinuosity plan-view geometry. They range from narrow, straight channels a few tens of meters deep (e.g., Channel 1), to deeply incised channels, up to several 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 mildly ,

incised ( 15 m) into the underlying substrate and has a simple, lowsinuosity channel axis up to 40–60 m wide marked by a ribbon-like highamplitude anomaly with broad, sinuous bends with a wavelength of approximately 1 km (Figs. 5B, 6, 7A). The channel has well developed levees characterized by relatively continuous, high-amplitude reflections that diminish in seismic amplitude and relief over a distance of 1–2 km from the channel axis. Similar small channels occur throughout the interval 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 100–300 m apart, that trend downslope and are characterized by negative relief (, 15 m) and higher amplitudes than the surrounding slope sediments (Figs. 6, 7A). We interpret these striations as open slope erosion by either unconfined turbidity currents or mass-wasting events. These open slope striations are morphologically similar to the ‘‘slope rills’’ reported from the New Jersey continental slope by Pratson et al. (1994). Channel 2.— The overall morphology of the channel valley of Channel

2 is similar to Channel 1, but it has more irregular valley walls, especially on the SE flank (Figs. 5B, 7B). Channel 2 is incised 50–100 m into slope sediments 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 and relatively discontinuous compared to Channel 1 (Fig. 7B). Channel 2 is cut by three synsedimentary, down-to-the-basin normal faults, over a 3–4 km reach (Figs. 5B, 7B). Across these fault s there is an increase in channel-axis gradient and a general widening of the channel valley (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 valley is narrower (e.g., Fig. 5B) and more deeply incised.

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FIG. 4.— A–D) Long-profile seismic sections along the four channel axes (Channels 1–4). See Figure 5 for location of the channels.

The paleo-seafloor immediately adjacent to Channel 2 is characterized by discontinuous low to moderate amplitudes (Fig. 7B). Areas of higher amplitude flanking Channel 2 occur localized, immediately adjacent to outer bends along the valley walls (Fig. 7B). These observations suggest that Channel 2 has poorly developed levees compared to Channel 1. Channel 3.—Channel 3 is deeply incised, up to 200 m, and has the highest 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 lowamplitude channel valley fill (Fig. 7C). The channel valley is relatively straight 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 chann el, especially where there is a prominent embayment in the downstream part of the channel valley (Fig. 7C). Chann el 4.— This channel is deeply incis ed, up to 200 m, and has a moderate- to high-sinuosity channel axis, with a sinuosity up to 1.8 (Figs. 5B, 7D). Although the channel valley is overall straight, the valleywall morphology is irregular and asymmetric, with the SE wall being highly arcuate or scalloped in plan view (Figs. 5B, 7D). In contrast to the other channels, the channel axis in Channel 4 has an asymmetric position within the valley, being preferentially located towards the SE side of the valley (Figs. 5B, 7D). In many cases the position of the scallops on the SE

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(Fig. 7D). This distal part of the channel also shows a prominent swing from 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 generally characterized 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 valleywall morphology, channel-axis position, and amplitude anomalies on the adjacent slope suggest that the whole channel system was progressively tilted to the east-southeast during deposition (see Fig. 2).

Quantitative Analysis of Channel Parameters Channel-axis long profiles for all 13 channel systems studied show a 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 as Channel 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 in Figure 9. For all channel systems, apart from Channel 1, sinuosity is negatively correlated with present-day channel-axis gradient. The lowestsinuosity example, Channel 1, has the steepest gradient. The steeper channels (e.g., Channels 1 and 2) are also less incised, in contrast to the highly sinuous systems (e.g., Channels 3 and 4; Figs. 8B, 9). Using a reasonable velocity of 2000 m/s, the maximum channel-axis gradient observed is approximately 3.4 (Channel 1), and the minimum channelaxis gradient is approximately 0.4 (Channel 4). For all channels, except Channel 4, the amount of incision decreases down channel (Fig. 8B, 9). u

u

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 gradient that 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 gradient reduces 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 positively correlated across short channel reaches, , 4 km long. For longer reaches, of approximately 20 km in length, sinuosity fluctuates and decreases slightly against a gradient increase. ,

Channel 2.— Channel 2 is incised by up to 100 ms (twtt) in its proximal reach, but the amoun t of incision fluctuates along the lengt h of the channel, with a general decreases to approximately 50 ms (twtt) in its distal 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-axis gradients starting at approximately 13 km along the channel-axis profile (Fig. 9B). The faults are reflected in the sinuosity, which decreases slightly FIG. 5.—Plan-view geometry of Channels 1–4 (see Fig. 1 for location). A) Detail of the channels in relationship to the isochore of the stratigraphic interval that contain the channels. Dotted lines show isocho re contours in ms (twtt). B) Interpretations of the master incision surfaces of the channel valleys displayed as dip-attribute maps (dark shades indicate steeper slopes). Thin black lines within the channel valleys highlight the channel axis.

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 located within 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

then increase s in the immediate hanging wall of the faults (Fig. 9 B). Gradient and sinuosity are negatively correlated overall, although over short reaches (, 4 km) sinuosity and gradient can show a positive correlation. Channel 3.— The axis of Channel 3 has the highest sinuosity (. 3), and the channel valley is also the most deeply incised, being locally over 200 ms (twtt) deep (Figs. 8B, 9C). Channel-valley incision is greatest in the upper reaches of the channel, where it is . 200 ms (twtt) between 5 and 10 km (Fig. 8B). The amount of incision fluctuates markedly up to 50 ms (twtt) over just a few kilometers, but there is a gradual decreases to , 150 ms (twtt) downslope (Fig. 8B). The present-day channel-axis gradient is relatively constant at approximately 20 ms (twtt)/km, for the

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FIG. 6.—Oblique view of seismic amplitude draped on time–structure map for the base of Channel 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 valley contains a slightly sinuous channel axis. Levees are highlighte d by high amplitud es adjacent to the channel. Note the out-of-channel striations on the slope to the west of Channel 1.

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 is relatively high initially, rising from approximately 2 to a maximum of 3.2

then generally decreases with increasing gradient until the channel axis is effectively straight above 35 ms (twtt)/km. When the parameters for each of the four channels are averaged there is a simple inverse correlation

at 13 km. From 13–30 km the sinuosity there is aend trend decreasing sinuosity downslope to valuesfluctuates, of 1.2–1.6but at the distal of 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 negative correlation between sinuosity and gradient (Fig. 9C).

between channel-axis gradient and both sinuosity and amount of incision (Fig. 10B).

Channel 4.— This is the longest channel interpreted in this study, and it has 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-axis gradient 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 coinc ide with the promi nent break of slope at 35 km (Fig. 9D). The channel valley straightens and increases in width from 60 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 amplitude s. Sinuosity, Gradient and Incision Relationships.— The measurements of sinuosity and gradient for all of the four main channels are summarized in Figure 10A. Sinuosity shows wide scatter, but below a gradient of 10– 12 ms (twtt)/km (approximately 0.7 ), and above 30–35 ms (twtt)/km (approximately 2 ), the channel axis is essentially straight, with sinuosities generally below 1.3 (Fig. 10A). Between these low-gradient and highgradient 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 u

u

,

DISCUSSION

Existing models of deep-water channel complexes offshore Angola are based on 3D seismic, wireline log, and core data (Mayall and Stewart 2000; Sikkema and Wojcik 2000; Kolla et al. 2001). These models largely describe the evolution of the fill of major submarine channel valleys. Typically this fill consists of a basal lag, debris flows, and slumps at the base of the channel fill, overlain by sandy, straight or sinuous stacked channels, and finally mud-rich, sinuous channel–levee complexes. A problem in defining former erosional channels by the deposits that fill them is that the fill may not be directly related to the processes that formed the srcinal master erosional valley (Kneller 2003). Mayall and Stewart (2000) make a broad distinction between the erosional sinuosity that established the channel, and the constructional sinuosity that characterizes the final channel fill. The data genera ted by this study allow us to analyze the three dimensionalwemorphology of the master channel valley.of Inthe the submarine following discussion synthesize the seismic geomorphology channels, and the various channel parameters, in order to examine the srcin and evolution of erosional submarine channels and discuss the possible controls on the variability of their planform geometry, long profiles, sinuosity, and incision.

Local Controls on Gradient, Sinuosity, and Incision The steep, linear to convex-up long profiles of the channel-valley axes documented in this study reflect, in part, the structural morphology of the Angolan continental slope. In the study area, the morphology of the slope is complex, with a typical length scale of 10–20 km controlled by the growth of salt-cored anticlines, salt diapirs and walls, and intra-slo pe

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FIG. 7.—Oblique views of seismic amplitude draped on time–structure maps for the four channels (Channels 1–4) looking upslope, towards the NE. The time–structure maps are for the bases of the channels; lighter, red and yellow colors are higher amplitude. See text for discussion.

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FIG. 8.—Characteristics of long profiles. A) Long profiles for the channels within the study area (straight channels 5 dotted lines; sinuous channels 5 solid lines). Note grouping into high-gradient straight channels and lower-gradient sinuous channels. B) Variation of depth of incision along channel axis for the four main channels discussed. Note that low-sinuosity channel (Channel 1) has the lowest depth of incision, whereas the high-sinuosity channels (e.g., Channels 3 and 4) have the highest depth of incision. Most channels show a downslope decrease in depth of incision.

basins (Fig. 1) (Mayall and Stewart 2000; Gee and Gawthorpe 2006). As a result of this complex sea-floor topography, the long profiles of the channels in this study are quite different from the concave-up equilibrium profiles recorded from many submarine channels and canyons (e.g., Pirmez et al. 2000; Mitchell 2005). It should also be remembered that a , 50 km reach of the channels is imaged, and neither the proximal nor distal portions of the channels have been studied. Despite these restrictions, there are several observations that suggest that the studied channels were tending to 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 and contain more sheet-like, aggradational, distributary channel networks where they enter intra-slope basins (e.g., Channel 4, Fig. 5A; also see Gee and Gawthorpe 2006). Thus evolution of the channels towards an equilibrium profile is accomplished by erosion of high-gradient reaches and deposition in low-gradient portions of the channel. Furthermore, there are systematic relationships between channel parameters such as sinuosity, channel depth, and gradient (e.g., Fig. 10B), an observation that has been used by other workers to suggest a system that is in, or approaching, equilibrium or grade (e.g., Pirmez and Flood 1995, Pirmez et al. 2000). In addition to the broad folding of the seafloor, across which the channels formed, there are a number of syndepositional faults that

intersect the channels and cause significant changes in channel morphology. Across these faults there are local increases in slope and channel-axis gradient, and associated changes in sinuosity, depth of incision, and channel-valley width. For example, across the faulted reach of Channel 2 (Figs. 5, 7B, 9B), there is an increase in channel-axis gradient, and a general widening of the channel valley, most notable in the immediate hanging wall of each fault (Fig. 5B). Some of the largest channel-axis loops occur in this region of Channel 2 (Figs. 5B, 7B). In addition, up-dip of the faults the morphology of the channel is markedly different: it is more deeply entrenched, the channel valley is narrow, and sinuosity 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 or increasing incision in order to reduce the local gradient anomaly (e.g., Flood and Damuth 1987; Pirmez and Flood 1995; Pirmez et al. 2000). As submarine flows cross a steeper slope they accelerate and become more erosive. Increased erosion can result in either incision or sinuosity increase—both 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 at very high sinuosity (. 3) (e.g., Pirmez and Flood 1995). For some of the Angolan submarine channels, sinuosity increases where channel-axis gradients increase until sinuosity abruptly decreases (e.g., Channel 4;

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FIG. 9.—Long-profile variations in channel-axis sinuosity and gradient, together with plots of long-pro file geometry and channel-valley depth for Channels 1–4. Long profile of channel axis 5 solid line; projected top of adjacent slope 5 dotted line. Gradient and sinuosity were measured along the channel axis at 2 km intervals. Black triangles 5 sinuosity, open squares 5 gradient. Profiles were sampled every 100 m along the channel axes. See text for discussion.

Fig. 9D). Such a downstream sinuosity increase followed by an abrupt decrease against continued increasing channel-axis gradient suggests that a peak sinuosity threshold has been reached, above which only increased incision is capable of further reduction of gradient. This may explain the

of the channel loops in Channel 4 is their asymmetry, with the channel axis offset to the SE side of the channel valley (Fig. 5B). The morphology of the channel valley mimics the asymmetry in of the channel axis, with more irregular and scalloped walls on the SE side of the channel valley

rapid fall-off in sinuosity with increasing gradient after the peak sinuosity associated with a gradient of c. 12 ms (twtt)/km in Figure 10A. Recent studies of laterally and vertically aggradati onal submarine channels show excellent examples of lateral and downslope migration of channel loops (e.g., Abreu et al. 2003; Posamentier 2003; Deptuck et al. 2003). However, in this study we observed little evidence of significant channel-axis migration. This may be a function of the seismic resolution of the data. Alternatively it may reflect major differences in sinuosity evolution for aggrading and incising systems. For example, the lack of channel-loop migration might simply indicate that the rate of incision was too fast and the channels became entrenched. A high rate of incision relative to the rate of channel widening has been shown to inhibit lateral and downslope channel migration (Peakall et al. 2000). A notable feature

compared to the NW (Figs. 5B, 7D). Such asymmetry is very reminiscent of fluvial asymmetric meander belts (e.g., Alexander and Leeder 1987; Alexander et al. 1994) and is another manifestation of the local structural control on the morphology of these slope channels, in this case tilting subparallel to the long profile. As channels become progressively entrenched, they are able to confine sediment-laden flows more effectively. Rapid incision also establishes the channel location on the slope, inhibiting channel bifurcation or avulsion (Babonneau et al. 2002). Progressive flow confinement would therefore lead to levees becoming smaller and less continuous as channels become more incised. In this study, channel valleys deeper than 50–60 m appear to be able to confine the majority of submarine flows and inhibit levee formation outside the main channel valley (see difference in levee

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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 as seeds 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 initial conditions 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 and straight channels (e.g., Channel 1) presented in this paper, together with results 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 are initially 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 and erode 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 frequent collapse 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 the open 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 the present-day seafloor profile. In contrast, the deeply incised channels (e.g., Channel 3), have rougher, less convex-up long profiles that have lower

FIG. 10.—Summary of relationship s between sinuosity, gradient, and incision. A) Variation of channels-axis sinuosity with gradient. Note abrupt increase to peak sinuosity at 10–12 ms (twtt)/km, followed by general decrease in sinuosity with increasing 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 the Channels 1–4, showing general inverse correlation between channel-axis gradient and both incision and sinuosity.

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 of turbidity currents (Piper and Normark 1983; Hiscott et al. 1997; Peakall et 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 ‘‘slope rills,’’ on the New Jersey continenta l margin, that incise inter-c anyon areas. Morphologically, the ‘‘slope rills’’ are similar to the downslopedirected slope striations observe d in this study, which occur on the open slope, outside of the main channel valleys (e.g., Figs. 6, 7A). We interpret

gradient (Figs. 8, 9). Thus it appears that the less incised, and apparently immature, channels still reflect the curvature of the slope on which they initiated, whereas the channels that are deeply incised (e.g., Channel 3) appear to have evolved and removed the srcinal slope convexity. The logical extension of this evolutionary trend would move towards the formation of a concave-up equilibrium profile, as observed elsewhere for many modern submarin e channels (e.g., Pirmez et al. 2000; Mitchell 2005). These observations can be summarized in a conceptual model of channel 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. Observations supporting the idea that deeply incised channel systems evolved from 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 the correlation 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 incised into 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 which we 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. The incised axis of the chann el valley has developed a low to moderate sinuosity (e.g. Channel 2; Figs. 4, 5, 7), and the combination of incision

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FIG. 11.—Model summari zing the initiatio n and evolution of erosional channels. Channels evolve from rills on the open slope into highgradient, low-sinuosity channels that are slightly incised into the slope and largely confined by levees. As channels attempt to develop an equilibrium long profile, they become more incised and increase sinuosity in order to lower long-profile channel-axis gradient. As a result they develop wider, deeply incised channel valleys, with lower gradient and a high-sinuosity channel axis.

and increased sinuosit y has reduced the overall gradient of the channelaxis 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 collaps e, creating an irre gular, scalloped morphology. Channel levees and deposits outside the main channel valley are recognizable, but they are discontinuous and have lower relief

‘‘gull-winged’’ levees, and (2) lower-gradient, deeply incised systems with moderate- to high-sinuosity channel axes. Quantitative analysis of present-day gradient, sinuosity, and depth of incision suggests an inverse relationship between channel-axis gradient and both sinuosity and depth of incision. Sinuosity is highly variable for a given gradient but shows an overall decrease in sinuosity with increasing

and lateral extent compared to Stage 1. By Stage 3 the main channel valley has widened to 2–3 km wide and it has incised to several hundred meters deep (Fig. 11). The axis of the channel valley has become highly sinuous and the combination of incision and 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 probably able 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 the main 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 by repeat 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).

gradient for long-profile gradients between approximately 0.7 and 2 . For gradients above and below these thresholds, channel axes are essentially straight. The long profiles of the channels are typically steep, linear to convex-up and, 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 long profiles as a response to synsedimentary growth faulting and folding of the slope due to salt tectonism. The channels tend towards an equilibrium profile by incising and/or increasing sinuosity across steep channel reaches, associated with anticline limbs and normal faults, and by aggrading in lower-gradient reaches within intra-slope basins. Synsedimentary deformation also has a major effect on the locati on and planform geometry of the channels. Synsedimentary faults cutting across channels at high angles cause marked changes in channel-valley width and channel-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 erosional submarine channels, with an initiation phase linked to the growth of linear striations, or rills, on the open slope, whereby one or two of the striations 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 surrounding slope, are weakly incise d, and are largely confined by levees. These channels evolve by reducing the initial high long-profile gradient by incising and becoming more sinuous. As a result, the channels become progressively more incised and sinuous with maturity, creating channel valleys several kilometers wide and several hundred meters deep, with highly sinuous channel axes. A consequence of progressive increase in the depth of incision is that the channels generally become fixed in position

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 channel valley) 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 of submarine 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 of the channel-valley axis, two end-member channel types have been recognized: (1) high-gradient, low-sinuosity, narrow channels with

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on the slope and submarine flows become increasingly contained within the channel valleys. Thus, as the channels evolve, levee and overspill deposits adjacent to the channels become rare. ACKNOWLEDGMENTS

We are grateful to Total Exploration and Production Angola, Norsk Hydro, Esso Explorati on Angola (Block 17) Ltd., BP Exploration (Angola) Ltd., Statoil Angola Block 17 A.S., and Sonangol for permission to publish the 3D seismic data from offshore Angola. We particularly thank Norsk Hydro for supporting this project, and their long-term support of research in Manchester. We gratefully acknowledge software donations by Schlumberger and Paradigm Geophy sical to the Basin Studies Group. We thank John Gjelberg, Gianluca , Carlosand Pirmez, and slope Lornaprocesses. Strachan for interesting discussionsBadalini about channels submarine REFERENCES

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Received 27 September 2005; accepted 7 December 2006.

Seismic Channels Geomorphology

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