Architectural complexities and morphological variations of the sediment waves of Plio-Pleistocene channel levee backslope of the Indus Fan

The architecture of the turbidity current sediment waves exhibits intricate morphologies and patterns on the Indus Fan channel levee backslope. The sediment waves are present on the channel levee of Plio-Pleistocene age and are absent in the deeper sections of the study area. The architecture of channel levee backslope on the Indus Fan is poorly understood. We used seismic interpretation techniques and modelling by utilizing high-resolution seismic data to approach this problem. The morphological variations in wavelength, crest dimensions and potential wave formation patterns suggest the autogenic and allogenic processes associated with wave development. Wavelengths reach up to 1473 m with an average of 486.84 m and the height of the levee ranges between 10 m and 60 m (average 30 m). The angle of the channel levee and dimension of the sediment wave here are independent of each other. Low angle levees have accommodated high dimension sediment waves and vice versa at multiple points downslope. Characteristically, the waves have formed on the outer levee (usually left) of the channels marked by steep margins suggesting that flow overspill caused the development of the waves. Generally, the younger sediment waves followed the patterns of older sediment waves, but the varying trends are often observed in the study area. The patterns of the sediment waves towards the younger sections of the levee indicate the modified and varying architectural style of growth. Sediment waves are generated by downslope turbidity currents. However, the deformation features have also possibly triggered the development of sediment waves.


Introduction
The development of sediment waves has intrigued deep marine geologists for many years and yet no conclusive process of wave formation has been identified. Although the features have been described and efforts have been made to constrain the processes involved for the formation and the environment of sediment waves, no conclusive mechanism of formation has been proposed (Wynn et al., 2000;Wynn and Stow, 2002). These waves are commonly associated with Turbidity Current (TC) levee deposits and along-slope bottom current processes Wynn et al., 2000). It has been postulated that sediment waves either travel upslope or downslope and develop parallel, perpendicular, or oblique to the flow. Similar features are described on the Monterey Fan that were developed on the levee backslope (Damuth, 1980;Faugères and Stow, 1993;Normark et al., 1980). Likewise, similar features are described in the Toyama deep sea channel system on the levee backslope (Nakajima et al., 1998). Sediment waves deposited on the preexisting slope failure deposits (Howe, 1996) and generated by turbidity currents in other deep-sea environments have also been documented before. Previously, the sediment waves have been referred to as mud waves, sand waves, giant ripples, etc. Four different processes have been identified for the formation of sediment waves: (a) sediment waves generated through the downslope processes of turbidity currents; (b) sediment waves generated through along-slope bottom current; (c) sediment waves generated through slope failure and deformation processes; (d) sediment waves generated through the interaction of down-slope and along-slope processes (Gong et al., 2012;Wynn and Stow, 2002).
Turbidity current sediment waves with upward concave features are generated by overspill of confined, channelized, and unconfined flows. Unconfined flows generally occur near the channel lobe transition zones, whereas the channelized and confined sediment waves generally develop on the levee backslope (Gong et Normark et al., 1980;Wynn and Stow, 2002). These sediment waves are generated beneath the turbidity currents possibly due to turbulent flows or the substrate wavy morphology (Nakajima and Satoh, 2001;Wynn and Stow, 2002). The finegrained sediment waves developed on the levee backslope generally occur on one side of the channel because of the Coriolis force (Nakajima et al., 1998;Wynn and Stow, 2002). Contrary to this idea, McCave (2017) suggests that in the northern hemisphere, the sediment waves occur on both sides of the channel (looking downslope). Sediment waves are usually concentrated on the outside bend of a meandering channel as an effect of centrifugal forces on the turbidity currents (Normark et al., 2002).
This study focuses on the development of sediment waves and their process of formation on the Indus Fan. Studies have contributed to the understanding of the process of formation of sediment waves. However, the internal morphology and the effect of underlying strata have not been discussed in detail and need to be taken under careful consideration. The objectives of the study are to: 1) understand the morphology and internal structure of the sediment waves on the levee backslope; 2) understand the growth patterns and development phases of sediment waves during the development of the levee; 3) understand the effect of deformation features and underlying strata on the development of sediment waves.

Dataset and Methodology
This study utilizes industrial data consisting of 2D and 3D seismic surveys from the Offshore Indus Basin, Pakistan. Seismic data was acquired in 2001 and 2002 at a water depth of ~2700m. The standard seismic processing includes 2D and a 3D Prestack Time Migrated (PSTM) volume. 2D seismic data includes a grid with 5*5 km spacing and covers an area of 75000 km 2 . The recorded length of seismic data is 12 sec TWT with a 160-fold data coverage sampled at 2ms. The zero phased seismic data is displayed at normal polarity at SEG a standard (Ologe et al., 2014), characterized by white, brown, and black reflection patterns signifying positive acoustic impedance (increasing downward) and negative acoustic impedance (decreasing downward), respectively. The dominant frequency of the data is 45Hz with approximately 25m vertical resolution.
The workflow started with identifying the key channel levee zones that accommodated the development of sediment waves using the available seismic data. The unavailability of core data and well cuttings inhibited the direct control for the identification of the lithologies. Seismic attributes improve the visualization of sediment waves in a two-dimensional view that augments the recognition of small-scale waves. To envisage the patterns and depositional nature of the sediment waves, time slices were generated to image the planform geometry. The Root-Mean-Square (RMS) seismic attribute was applied to estimate the square root of amplitudes to describe the internal architecture of the channel levee com- The white circles suggest the migration pattern of the sediment waves.

Characteristics and Formation process of turbidity Current sediment waves
Sediment waves in the study area display a wavelength of up to 1473 m and a height of 30 m with a slope gradient of 1 0 -3.1 0 (see Table 1 and 2). The dimension of the sediment waves depends on the sediment type, slope gradient, levee height and the distance from the source as it is observed that the wavelength and height of the sediment wave progressively decrease downslope away from the channel margin (Carter et al., 1990;Normark et al., 2002). It is observed in this study that the sediment wave height and wavelength must be directly related to turbidity flow and the volume of turbidity current (see Figure 1).
With a similar levee slope, variations in the height and wavelength of sediment waves have been observed (see Figures 1, 2, 3 and 4). Wave crests are generally perpendicular as they are aligned parallel to subparallel to the slope.
Sediment waves in the study area are generally asymmetrical upslope. However, as the wave progresses downslope, the symmetry increases (see Figures 2 and  3). It has also been observed that sediment waves may show unusual symmetry, as often there is no clear pattern visible. This indication shows that no clear relationship is observed between the wavelength and channel dimensions whereas he suggests that a positive correlation exists between wavelength and wave height (Normark et al., 2002). Sediment waves evolve through time as the parallel developing waves merge upslope (Wynn and Stow, 2002). Sediment waves become more aggrading upslope (see Figure 3) in time because of the ceasing of migration (Normark et al., 2002).

Distribution and Geometry of Sediment Waves
The distribution of sediment waves in the Indus Fan is limited to the levee back-slope of Plio-Pleistocene age in the study area, with the exception of a few sediment wave occurrences on the slope. The sediment waves are absent in the older succession of the Indus Fan in the study area. These sediment waves have developed on the outer bends of the channel levee complex. The wavelength of sediment waves is as small as 122 m and extends to 1463 m. The sediment waves on the levee flanks are generally high relief upslope or near the channel margin and decrease in wavelength and height downslope of the levee (see Figures 1, 2, 3, 4, 5 and 6). The NW-SE oriented sediment waves on the regional slope suggest that the flank on the stoss side is generally smaller in contrast to flank on lee side (see Figure 2). It is observed that the sediment waves on the regional slope increase in wavelength downslope, unlike the sediment waves on the levee backslope. The lee side appears to be much steeper than the stoss side. These waves have developed on the sediments deposited due to mass failure.
The geographical positioning of the sediment waves on the levee back-slope is also unique here on the Indus Fan. Often, the sediment waves have developed in proximity to the channel, but it is also noted that the development of sediment waves sometimes have also occurred more towards the distal side while the proximal part lacks the sediment waves (see Figures 4, 7 and 8). In another case, the wave height in the proximal part is smaller, grows higher towards the middle part of the levee and continues to reduce again distally (see Figures  2, 3 and 6).
Sediment waves show two varying patterns of wave development in the proximal portion of an older levee and the distal portion of a younger levee (younger in time) (see Figure 8). The older proximal part exhibits more pronounced sediment waves with dimensions decreasing distally. The wave pattern on the overlying levee is very mild as the reflectors are well stratified parallel to subparallel. Towards the top of the levee in the younger succession, the sediment waves grow to be thicker and bigger in dimension, but the wave height is highly irregular. Although, the wavelength appears to decrease gradually downslope. The rate of upslope migration is approximately similar with a higher aggradation component in both segments of sediment waves, i.e. the sediment waves developed on the bottom and the top of the levee. The sediment waves are deficient of a clear growth pattern in the older to the younger section of the levee. Although, from the proximal to distal side, the waves appear to be symmetrical overall with decreasing dimensions. However, the vertical synchronicity is lacking. Initially, the waves do not follow the underlying undulation, but in the later phase of development, they follow the shape of the older units. A sudden significant decrease in wave dimensions is visible towards the top of the levee but maintains the upslope migration (see Figure 2). Similarly, the initial phase of development of sediment waves becomes irregular towards the younger successions of the levee (see Figure 3). Sediment waves in the proximal portion are half crest but become more pronounced distally (see Figures 3 and 6).
An inconsistent pattern of sediment wave development inside a single levee suggests the varying depositional process (see Figures 8, 9 and 10). Here, the older section of the levee accommodated the development of the sediment waves, but a sudden disappearance of waves is observed as the levee amplitude decreases from high amplitude to a significantly transparent geometry (see Figure 4). On top of the levee, the sediment waves have developed again towards the distal side with a distinct upslope migration style (see Figure 4).
In Figure 6, the sediment waves are parallel to subparallel with normal wave dimensions and growth with a sudden amplitude increase towards the distal part of the levee. Highly chaotic, discontinuous reflections grade into more continuous, parallel sediment waves downslope (see Figure 10). The overlying hemiplegics strata is affected by the wavelength and height of the sediment waves. The symmetry of the wave decreases downslope as well, with stoss side getting steeper. This unique behaviour does not affect the upslope migration of the sediment waves that cease in the proximal part of the levee where the sediment waves become aggradational (see Figures 2 and 10).
In the study area, sediment waves have developed on both flanks of the channels (see Figure 9) as suggested by McCave (2017) but generally on the outer bend of the channel. Although the levees that host the sediment waves are different, the wave dimensions and growth patterns are similar on both levees irrespective of the direction of the flanks. The thickness and the extension of the levees have no impact on the development of the sediment waves. One important fact is that the orientation of sediment waves that have developed on both the flanks is different than those sediment waves that have developed on either side of the levees. Only the seismic profiles-oriented NE-SW have waves on both flanks in the study area (see Figure 9). The older sediment waves have influenced the development of younger sediment investigations, it is observed that the influence of slope angle of the levee, thickness and multiple phases of levee development have minimal impact on the sediment wave growth. High dimension sediment waves lie on low angle levees (see Figures 1 and 9) whereas it is also observed that sediment waves with higher dimensions have also developed on high angle slopes (see Figures 5  and 8). Similarly, high wavelength and low height sediment waves have also developed on high angle levee slopes (see Figure 10). The surface extension of the levee has accommodated the growth of sediment waves without apparently influencing their dimensions. One important element of sediment waves is that often during the entire episode of the levee build up, the formation of the sediment waves has not been terminated (see Figures 1, 2, 3 and 8). The dimensions may have varied but the abrupt seizure has not occurred evidently due to limited control of the levee.

Geometry of the Levees
Sediment waves have generally developed on the outer bank of the channels that suggest an overspill of the turbidity currents (see Figures 1, 5, 6, 7, 8 and 9). Sediment waves have developed on the levees that are steeply dipping (see Figure 5, 8 and 9). 2D seismic profiles (see Figure 13a), a line drawing (see Figure 13b) and 3D seismic attributes (see Figure 13d) endorse the fact that sediment waves have developed perpendicular to oblique to the flow direction and the slope gradient. In   waves although the entire levee is devoid of sediment waves, except the portion that is overlying the sediment waves of the older levee (see Figure 7). the presence of irregular morphology or deformation features (see Figure 12), the spill of turbidity flow may still lead to the generation of sediment waves even if no internal lee wave is generated. Wave initiation will take place over the deformed features which may be followed by upslope or downslope migration of the sediment waves.

Wave forming process
The continuous growth and temporal persistence of sediment waves originated from the turbidity current, which indicates the existence of their continual equilibrium profile with the host turbidite system (Lewis and Pantin, 2002). As the slope steepens beyond 0.1 0 , the sediment waves begin to develop (Lee et al., 2002). Several studies have formulated the process of initiation of sediment waves and concluded that waves form as antidunes with waves generated internal to the turbidity current flow (Ercilla et al., 2002; Wynn et al., 2000). It is also suggested that the waves may have been generated by internal lee waves (Lewis and Pantin, 2002). In this study, it is likely that two major processes have influenced the generation of sediment waves simultaneously: 1) the generation of internal waves in the turbidity currents; 2) the morphological irregularities underneath the turbidity currents have influenced the sediment waves formation. Evidence for the process of formation of sediment waves through internal waves is that, despite the variations of slope angles of the levees and the turbidity current flow, the waves have generated with higher dimensions. The flow velocity may not always be an imposing factor as slow, diluted and continuous gravity flows pos-

Wave migration
Generally, wave sediments are fine grained turbidites comprised of mud/silt with pelagic and hemipelagic interbeds (Wynn and Stow, 2002). Beds become generally thicker upslope near the channel margin. A similar feature has been observed on the Amazon Fan during Ocean Drilling Program (ODP) i.e. the drilling of sediment waves (Flood, 1994;Normark et al., 2002). Three different types of migration have been observed in the study area: (a) upslope wave migration; (b) vertical aggradation of sediment waves; (c) upslope or downslope wave set migration.

a) Upslope wave migration
Sediment waves display an upslope migration pattern from the distal to the proximal part of the levee that ceases near the channel margin where the waves become thicker. On the contrary, different wave sets on a single levee that develop with different turbidity current flows display dissimilar patterns of growth (see Figure 10). The older sediment wave set displays progressive upslope migration overlain by a more aggradation display of a younger wave set at a similar distance from the channel margin. Lateral wave migrations are reported to be as high as 20m/1000 years (Lewis and Pantin, 2002).

b) Vertical aggradation of sediment waves
Over time, sediment waves witness aggradation as the upslope migration ceases (see Figures 2, 3 and 6). Here it is noted that older sediment waves are thicker in amplitude, which decreases upwards towards the younger sediment waves. The patterns of migration and aggradation are limited to individual wave sets which vary with the development of younger sediment waves. The sediment wavelength also increases towards the top of the levee (see Figure 2). An example from Amazon drilled

c) Upslope or Downslope Wave Set Migration
The migration of an entire wave set is observed in the study area where the multiple set of sediment waves dis-play distinguishing behaviours (see Figure 6). The sediment waves developed on the left side of the channel (looking downslope) (see Figure 8). Here, the wave is grouped into two sets. The older wave set is migrating upslope with a significant aggradation component. The younger set, which was deposited in the later phase of channel development, shows a predominant aggradation component with slight upslope migration as well. Pe-   lagic / hemipelagic deposits fill the hiatus or break in levee development. The younger sediments waves are affected by the older sediment waves only in the distal section of the levee (see Figure 7).

Implications of Sedimentation, Turbidity Currents and Levee Slope
The variation in the wavelength and wave height are influenced by the turbidity current velocity and thickness (Ercilla et al, 2002). Each flow has undoubtedly travelled with different flow regimes that carried varying grain sizes and quantities . It has been observed in the study area that an upward increase in wavelength possibly occurs because of increasing flow velocity and higher sediment input (see Figures 2, 3, 8  and 10). The Indus Fan has witnessed an increased sediment supply between 11 m.y. and 2 m.y. after the uplift of Himalayan and Karakorum ranges that act as the source for the sediment supply to the Indus Fan A break in deposition or decrease in turbidity current flow velocity changes the dimensions of the sediment waves as well (see Figures 4 and 8). Older sediment waves that developed in the proximal section undergo reduced dimension towards the middle of the levee until an increase in dimensions is observed in the youngest succession of the levee towards the distal side (see Figure 8). This is apparently due to the episodic decrease followed by an increase in flow velocity and sediment load of the turbidity current. On the other hand, an opposite pattern is apparent in the study area that sediment waves tend to maintain their size and height toward the top of the levee (younger section) (see Figure 8). This indicates that each overspill of the turbidity current may deposit a significantly different kind of wave set irrespective of the underlying older sediment waves. Upslope migration is the result of increased sedimentation on the upslope flank and erosion on the downslope flank of the wave. Increased sedimentation upslope may cause the waves to be thicker upslope as well as the wave migration. It is not observed in the study area that any inner levee has hosted the sediment waves. Individual reflections are easily traceable in the sediment waves on levee backslope due to the presence of parallel to subparallel seismic reflections.
Levee slope angle in the study area is less influential to the generation of sediment waves of the channel flanks (see Table 2 In the proximal side, the minimum wave height is 10 m with the maximum height reaching up to 12 m which is averaged at 10 m. On the contrary levee slope with low angles have generated the higher wavelengths and heights in the study area. The minimum wavelength of the sediment wave on the low angle levee is 196.88 m whereas the maximum wavelength is 1473.91 m with an average of 832.83 m (see Table 1). The minimum height of the sediment waves is 10 m while it reaches to a maximum of 60 m and an average of 31.66 m. It is important to note the wave height is 60 m at the distal end of the wave field, which is the repercussion of an undulation provided by the underlying morphological feature. This also indicates that the morphological effect on the formation of sediment waves is equally essential. Deformation features significantly impact the sediment wave dimensions, usually in the form of an increase in wavelength and height (Lee et al., 2002). Sediment waves have developed on the proximal side of the levee with low dimensions but the downslope decrease in dimensions pattern followed. Near the distal end of the levee, deformation features abruptly increased the sediment wave height and wavelength that carried its effect into the overlying hemipelagic drapes (see Figure  12). Another morphological impact is apparent where the sediment waves on one levee influenced the development of sediment waves on another levee of a different channel (see Figure 7). No sediment waves are visible in the proximal side of the levee near the channel margins but at the distal end, sediment waves have developed (see Figure 7). In similar scenarios, an internal wave is not essential to initiate a wave but the subsequent flows on the levee may affect the upslope migration of the sediment waves.

A comparison between Sediment Waves and Deformation features
Deformation on a submarine slope is quite prevalent, especially where slope gradients and the sedimentation rates are high (Wynn and Stow, 2002). The fluctuations of sea level are essentially also responsible for slope failure, either due to the retaining of sediments during high sea level or exposure of shelf and slope during falling sea level (Posamentier and Kolla, 2003). A few deformation features that closely resemble the sediment waves have been analyzed in the study area. These features lie on the levee backslope in the Pleistocene age that created confusion unless closely investigated. Some

Migration
Fine grained sediment waves generally migrate upslope opposite to the direction of prevalent current direction. Upslope sediment waves are thicker due to higher sedimentation rate. The variation between lee and stoss side is also unique in the sense that the lee side undergoes erosion and sedimentation occurs on the stoss side. Deformation features do not display similar migration patterns, but rather in opposite direction and the thickness on both the flanks are similar (Lee and Chough, 2001). Here in the study area, no migration trend has been observed as the levee upslope clearly lacks any deformation feature (see Figure 2).

Reflections
Individual reflections can be traced across the sediment wave crests that are easily recognizable between the adjacent waves. On the contrary, the creep folds   clearly display displacement and breakage between the adjacent beds. It is often difficult to track the beds in the deformation structures (see Figure 12).

Regularity
Sediment waves are highly regular in terms of reflector spacing and crest and trough dimension . The internal structure of the waves appears to be similar from one bed to another (Lee et al., 2002). On the other hand, creep folds are highly irregular (Lee and Chough, 2001). Such regularity is observed in a study where each reflector on sediment waves can be traced to its entire length (see Figure 1, 2, 6 and 8).

Coherence
Sediment waves display a clear trend in dimensions e.g. a wave dimension decrease downslope as the turbidity current, slope angle, sedimentation decrease away from the source (Wynn et al., 2000). However, the varying nature of dimension patterns is also observed in the study area (see Figures 1, 2 and 10). Contrary to this, creep folds show a hummocky spread of dimension (see Figure 12).

Thickness
Sediment waves decrease in thickness downslope whereas deformation features may display an increased thickness away from the source of origin (Wynn and Stow, 2002). A similar trend in thickness of the deformation features is analyzed away from the source near the distal end of the levee (see Figure 12).
Creep folds modify the strata and morphological surfaces that in turn may act as the initiation surface for the sediment waves to occur. Similar features have provided the geometry for the development of sediment waves in the study area on the Indus Fan (see Figure 12).

Regional slope Sediment waves
An example of slope sediment waves has been observed on the Indus offshore slope which is not related to levee back slope sediment waves (see Figure 11). These layered, concave upwards, subparallel reflectors, sediment waves have formed because of unconfined turbidity flows as antidunes. The NW-SE seismic cross-section displays the upslope migrating sediment waves that die out in the proximity. The wave field is only visible on 2D seismic data hence any curvature and sinuosity quantification are not possible. Slope gradient in the wave area is 2.01 º . Wave dimensions are an irregular form upslope to downslope. Initially, the wavelength is 50 m, and the wave height reaches up to 200 m and the minimum height is 50 m with an average of 90 m. These sediment waves are underlain by mass failure deposits which acted as a platform for wave initiation (see Figure 11). As the turbidity current flows across a rough floor, sediment waves develop and migrate upslope (Lee et al., 2002).
The NW-SE oriented seismic profile reveals that the sediment waves show a sudden increase in negative relief at the proximal side and then continue to be asymmetrical downslope. The sediment waves height decreases normally downslope, but the wavelength gradually increases till the termination of sediment wave field (see Figure 11). This may be the result of increasing velocity of turbidity currents downslope that restrict the deposition of sediment on the slope. Upon changes in environmental conditions, the same turbidity current may not produce the sediment waves (Lee et al, 2002). Lateral thinning and thickening of layers are evident because of erosion on the steeper downslope side and deposition on the gentle upslope flank.

Conclusion
Sediment waves are abundantly present in the Pleistocene channels that suggest the occurrence of highly turbulent and thick sediment carrying turbidity flows. No sediment waves are encountered in the older successions from Eocene-Oligocene to Miocene. The dimensions of waves are often irregular in the study area. Wave dimensions decrease downslope and away from channel margins, but random patterns of wave dimensions are also observed, which is indicative of varying downslope waves formation processes. Not only the downslope processes but also the subsurface morphology can trigger the formation of sediment waves. Wave set migration patterns are indicative of autogenic processes occurring during the wave formation. Each proceeding wave set is influenced by the architecture of earlier sediment waves.

Implications and prospects
Indus offshore is a frontier zone where the hydrocarbon industry has unsuccessfully made attempts for oil and gas discovery. Our study does not focus on the hydrocarbon prospects of the area, but we believe that submarine channels could provide the industry with a maybe, better prospect for future discoveries. We believe that not only channel belts, but channel levees can provide the petroleum play necessary for a successful venture.
In the study area, the sediment waves lie near the sea floor, i.e. Plio-Pleistocene age overlain by a thick succession of hemipelagic deposits that probably deposited during the high stand. Thus, the zone may prove to be a potential reservoir as long as detailed sedimentological analyses are carried out. edge the financial assistance of the China National Natural Science Foundation. Schlumberger is thanked for issuing the license for the use of research purposes and student support. The funding for the study is provided by the National Natural Science Foundation, China under grant number 41672098.