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Volume 41 Issue 4
Aug.  2020
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Mengtian Gao, Shang Xu, Haiteng Zhuo, Yuxuan Wang, Shaobo Wu. Coupling Relationship between Shelf-Edge Trajectories and Slope Morphology and Its Implications for Deep-Water Oil and Gas Exploration: A Case Study from the Passive Continental Margin, East Africa. Journal of Earth Science, 2020, 31(4): 820-833. doi: 10.1007/s12583-020-1288-8
Citation: Mengtian Gao, Shang Xu, Haiteng Zhuo, Yuxuan Wang, Shaobo Wu. Coupling Relationship between Shelf-Edge Trajectories and Slope Morphology and Its Implications for Deep-Water Oil and Gas Exploration: A Case Study from the Passive Continental Margin, East Africa. Journal of Earth Science, 2020, 31(4): 820-833. doi: 10.1007/s12583-020-1288-8

Coupling Relationship between Shelf-Edge Trajectories and Slope Morphology and Its Implications for Deep-Water Oil and Gas Exploration: A Case Study from the Passive Continental Margin, East Africa

doi: 10.1007/s12583-020-1288-8
More Information
  • Both the shelf-edge trajectories and slope morphology are indicative of deep-water sedimentation, but previous studies are relatively independent from each other in the two dimensions. An integrated investigation can enhance the understanding of deep-water sedimentary systems and enrich reservoir prediction methods. Based on the bathymetry data and seismic data published, this study identified ten slope areas at the continental margin of East Africa and classified the clinoforms into three types:concave-up, sigmoidal and planar. Combined with the distribution of main modern rivers in East Africa, nine modern source-to-sink systems were identified and the catchment area is positively correlated with the size of the shelf-edge delta. It is found that the slope morphology of East Africa is closely related to the geological setting, sediment supply and sediment transport pathway in submarine canyon of passive continental margin. When the sediment supply is stable, the concave-up slopes are dominated by the river-associated and shelf-incising canyons and the sigmoidal slopes are determined by the headless canyons. There exists a strong coupling relationship between the shelf-edge trajectories and slope morphology. In general, concave-up slopes correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories and high-quality reservoirs developed on the basin floor under the influence of river-associated and shelf-incising canyons which have bright prospects for oil and gas exploration. Additionally, sigmoidal slopes usually correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories at times of relative sea-level fall and the reservoirs mostly developed on the upper slope under the influence of headless canyons. Moreover, the planar slopes correspond to high-angle ascending trend trajectories which are hardly potential for exploration. The coupling model built in this study will provide an insight for oil and gas exploration in deep-water areas with limited data and low exploration degree.
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Coupling Relationship between Shelf-Edge Trajectories and Slope Morphology and Its Implications for Deep-Water Oil and Gas Exploration: A Case Study from the Passive Continental Margin, East Africa

doi: 10.1007/s12583-020-1288-8

Abstract: Both the shelf-edge trajectories and slope morphology are indicative of deep-water sedimentation, but previous studies are relatively independent from each other in the two dimensions. An integrated investigation can enhance the understanding of deep-water sedimentary systems and enrich reservoir prediction methods. Based on the bathymetry data and seismic data published, this study identified ten slope areas at the continental margin of East Africa and classified the clinoforms into three types:concave-up, sigmoidal and planar. Combined with the distribution of main modern rivers in East Africa, nine modern source-to-sink systems were identified and the catchment area is positively correlated with the size of the shelf-edge delta. It is found that the slope morphology of East Africa is closely related to the geological setting, sediment supply and sediment transport pathway in submarine canyon of passive continental margin. When the sediment supply is stable, the concave-up slopes are dominated by the river-associated and shelf-incising canyons and the sigmoidal slopes are determined by the headless canyons. There exists a strong coupling relationship between the shelf-edge trajectories and slope morphology. In general, concave-up slopes correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories and high-quality reservoirs developed on the basin floor under the influence of river-associated and shelf-incising canyons which have bright prospects for oil and gas exploration. Additionally, sigmoidal slopes usually correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories at times of relative sea-level fall and the reservoirs mostly developed on the upper slope under the influence of headless canyons. Moreover, the planar slopes correspond to high-angle ascending trend trajectories which are hardly potential for exploration. The coupling model built in this study will provide an insight for oil and gas exploration in deep-water areas with limited data and low exploration degree.

Mengtian Gao, Shang Xu, Haiteng Zhuo, Yuxuan Wang, Shaobo Wu. Coupling Relationship between Shelf-Edge Trajectories and Slope Morphology and Its Implications for Deep-Water Oil and Gas Exploration: A Case Study from the Passive Continental Margin, East Africa. Journal of Earth Science, 2020, 31(4): 820-833. doi: 10.1007/s12583-020-1288-8
Citation: Mengtian Gao, Shang Xu, Haiteng Zhuo, Yuxuan Wang, Shaobo Wu. Coupling Relationship between Shelf-Edge Trajectories and Slope Morphology and Its Implications for Deep-Water Oil and Gas Exploration: A Case Study from the Passive Continental Margin, East Africa. Journal of Earth Science, 2020, 31(4): 820-833. doi: 10.1007/s12583-020-1288-8
  • Deep-water sedimentary system is a hot and frontier area in sedimentology research. However, restricted by high exploration cost, limited data and harsh technical conditions, there are still plenty of key issues to be solved in deep-water sedimentation theory and application in reservoir prediction. Since the 1970s, deep-water sedimentology and favorable reservoir prediction have been rapid developing with the constant innovation of the slope morphology (Covault et al., 2011; Harris and Whiteway, 2011), shoreline and shelf-edge trajectory analysis (Henriksen et al., 2011, 2009; Helland-Hanson and Hampsom, 2009; Helland-Hanson and Martinsen, 1996; Helland-Hanson and Gjelberg, 1994) and source to sink system theories (Pechlivanidou et al., 2017; Liu et al., 2016).

    The study of the slope morphology has a long history, and a number of scholars all over the world are still strengthening these researches, with topics mainly focusing on the following aspects: (1) slope morphology classification and formation mechanism (O'Grady et al., 2000); (2) slope morphological evolution numerical simulation (Salazar et al., 2018, 2015; Salazar, 2014); (3) relationship between slope morphology and development of deep-water fan (Chen et al., 2018; Carvajal et al., 2009; Lin et al., 2009).

    The trajectory analysis is a research hotspot in recent years, which mainly includes: (1) high-resolution sequence stratigraphic framework recognition based on trajectory trends (Gong et al., 2015a; Henriksen et al., 2011; Helland-Hanson and Hampsom, 2009; Helland-Hanson and Gjelberg, 1994); (2) the relationship between shelf-margin delta stacking patterns and shelf-edge trajectories (Liu et al., 2018; Chen et al., 2016; Gong et al., 2015a); (3) the coupling of deep-water fan and shelf-edge trajectories (Gong et al., 2015b; Johannessen and Steel, 2005); (4) application of the trajectory analysis theory in non-marine setting (Xu et al., 2018).

    However, previous studies on the slope morphology and shelf-edge trajectories are relatively independent from each other. Accompanied by the acceleration of deep-water oil and gas exploration around the world, comprehensive studies based on the integration of slope morphology and shelf-edge trajectories will provide a highly efficient method to identify deep- water sedimentary system, to predict favorable reservoirs and to optimize exploration strategies. Here, based on satellite- bathymetric and geomorphic data of continent margin of East Africa, we aim to solve two questions: (1) To clarify the coupling relationship between shelf-edge trajectories and slope morphology; (2) To investigate the significance of shelf-edge trajectories and slope morphology for deep-water reservoir prediction in oil and gas exploration.

  • Trajectory analyses emphasize the lateral and vertical geomorphological characteristics of breaks-in-slope systems and the associated sedimentary environment changing rules, and focus on the path and the direction of the trajectory migration especially. This method is mostly applied to the migration of trajectories both at shoreline (Helland-Hanson and Hampsom, 2009; Helland-Hanson and Martinsen, 1996) and shelf-edge scales (Johannessen and Steel, 2005; Steel and Olsen, 2002), which can reflect the process of both vertical and lateral migration of the breaks-in-slope of shoreline or shelf-edge, and to provide evidences for understanding the evolution of depositional systems. With a steady sediment supply, the migration patterns of shorelines and shelf-edge can be determined by the successive positions of the migrating shelf break and be subdivided into four types of trajectories: descending trend trajectory, flat trajectory, ascending trend trajectory and retreating shelf-edge trajectory (Zhuo et al., 2018; Henriksen et al., 2011; Helland-Hanson and Hampsom, 2009; Helland-Hanson and Martinsen, 1996; Helland-Hanson and Gjelberg, 1994). Among them, the descending trend trajectory and the flat trajectory reflect the long-term fall and stability of relative sea level, respectively, both of which mean that the clinothems have obvious characteristics of progradation towards the basin. Furthermore, there is a positive correlation between the ascending trend trajectory and relative sea level. The low-angle ascending trend trajectory reflects progradational series of clinothems, while the high-angle ascending trend trajectory indicates the aggradational series of clinothems. Moreover, the retreating trajectory suggests a rapid rise in relative sea level and the strata stacking patterns are characterized by retrogradation.

    The continental slope is a slope zone between the continental shelf and the abyssal plain, which is composed of the upper slope and the lower slope (continental rise). And the seismic data and satellite-bathymetric data are usually used to qualitatively and quantitatively characterize the paleogeomorphology or modern slope morphology. Although there are various classification schemes for submarine slope in the world (Carvajal et al., 2009; Yu and Chang, 2009; Schlager and Adams, 2001; Adams and Schlager, 2000; O'Grady et al., 2000; Pratson and Coakley, 1996; Ross et al, 1994; Emery, 1980; Hedberg, 1970), we adopt the scheme of concave-up, sigmoidal, and planar slopes in this paper (Adams and Schlager, 2000). The morphology of slope is determined by the sediments accumulation state. When the shelf margin sediments as turbidities are transported through the submarine canyon to the bottom of the slope to form the basin floor fans, it usually shows a concave-up shaped profile. On the other hand, if the deposits of the shelf margin pile up on the upper slope, the sigmoidal shaped profile is formed. But in the case of insufficient sediment supply, the morphology of the slope may show planar shaped profile.

    The shelf-edge trajectory can indicate the striatal stacking patterns of the shelf margin (progradation, aggradation or retrogradation), and is closely related to the morphology of the slope. Published studies have shown that the descending trend, flat and low-angle ascending trend trajectories correspond to the sea level fall, stability or slow rise (Henriksen et al., 2011; Helland-Hanson and Martinsen, 1996; Helland-Hanson and Gjelberg, 1994). In these periods, the accommodation of the continental shelf is little, and the sediment flux is high at the shelf margin, so it is easy to form the shelf-edge delta. Based on the distance extent of deposits transported to the basin floor (deposits mainly accumulated on upper slope or basin floor), the morphology of the slope always shows concave-up (Figs. 1a, 1b, 1c) or sigmoidal geometry (Figs. 1d, 1e, 1f). High-angle ascending trend trajectories correspond to fast sea level rise (Helland-Hanson and Hampsom, 2009; Helland-Hanson and Martinsen, 1996; Helland-Hanson and Gjelberg, 1994) and the deposition supply rate is relatively high and slightly higher than the capacity space growth rate, resulting in the formation of the sigmoidal slope with thick topset, thin foreset and almost no bottomset (Fig. 1g). Retreating trajectories correspond to the sharp rise of sea level (Helland-Hanson and Hampsom, 2009; Helland-Hanson and Martinsen, 1996; Helland-Hanson and Gjelberg, 1994) and the deposition supply rate is significantly lower than the capacity space growth rate, resulting in the formation of the sigmoidal slope with thinner topset and almost no foreset and bottomset (Fig. 1h).

    Figure 1.  Correlation between the types of shelf-edge trajectories and slope morphology (modified after Zhuo et al. (2018), Henriksen et al. (2011), Catuneanu et al. (2009), Helland-Hanson and Hampsom (2009), Plink-Björklund and Steel (2002), Plink-Björklund et al. (2001), Helland-Hanson and (1996), Helland- Hanson and Gjelberg (1994), Ross et al. (1994)). (a) Concave-up slopes corresponding to the descending trend trajectories in a setting of fall of relative sea level. (b) Concave-up slopes corresponding to the flat trajectories when the relative sea level is stable. (c) Concave-up slopes corresponding to the low-angle ascending trend trajectories during slowly rising sea level. (d) Sigmoidal slopes corresponding to the descending trend trajectories in a setting of fall of relative sea level. (e) Sigmoidal slopes corresponding to the flat trajectories when relative sea level is stable. (f) Sigmoidal slopes corresponding to the low-angle ascending trend trajectories in a setting of slowly rising of relative sea level. (g) Sigmoidal slopes corresponding to the high-angle ascending trajectories during slightly rapid rising sea level. (h) Sigmoidal slopes corresponding to the retreating trajectories during when sea level rises sharply. RSL. Relative sea level.

  • The continental margin of East Africa is located in the western side of the Indian Ocean (Fig. 2), which is a typical passive continental margin formed during the break-up of Gondwana continental plate (Bosellini, 1986). The coast of East Africa has been in a long-term extensional tectonic regime since the rifting of Gondwana in Carboniferous, resulting in the formation of slope area at the continental margin of the East Africa (Salman and Abudula, 1995). From the Middle to Upper Jurassic (160–130 Ma), followed by the rifting and southward motion of Madagascar, the continental margin of the Somalia Costal Basin and the northern part of the Lamu Basin gradually evolved. During the drifting phase, Madagascar drifted parallel to the present position along the Davie fracture zone and ended by the late Lower Cretaceous (Lawver et al., 1992), inducing the development of the continental margin systems in the southern Lamu Basin, the Tanzania Coastal Basin and the Rovuma Basin, and the different sedimentary patterns in the Rovuma Basin and the Mozambique Coastal Basin. Subsequently, at the end of the Lower Cretaceous, the structure of the continental margin of East Africa was basically formed (Salman and Abudula, 1995), and the stable passive continental margin composed of continental shelf, slope and rise developed until the Neogene (Cui, 2015; Burke et al., 2003) (Fig. 3).

    Figure 2.  Map of the study area, distribution of the basins and location of the typical bathymetry profiles in East Africa.

    Figure 3.  Geological events summary chart for the East Africa margin (geologic events are modified from Cruciani et al. (2017), Davison and Steel (2017), Klimke et al. (2016)) and global eustatic sea-level curves (modified from Miller et al. (2005)).

    The East Africa coastline is about 6 100 km, including Somalia Coastal Basin, Lamu Basin, Tanzania Coastal Basin, Rovuma Basin and Mozambique Coastal Basin. The narrow continental shelf is a typical feature of the continental margin of East Africa, and the water depths of the shelf-break on shelf are generally less than 200 m, and the water depths at the bottom boundary of the slope are 2 600–5 000 m, becoming gradually shallower from north to south.

  • The bathymetry and digital elevation data were obtained from the National Oceanic and Atmospheric Administration (NOAA) and global topographic database can be available through the ETOPO1. This dataset is a 1 arc-minute (1 nautical mile=1.852 km) global relief model with bathymetry derived from numerous sources of sea-surface satellite altimetry measurements and ocean sonar systems.

  • Based on the Global Mapper 14.1 software, 157 bathymetric profiles were constructed from the northern to southern part of the continent of the East Africa along dip (Fig. 2), and relevant parameters of the slope were counted (Table 1; Fig. 4). In order to compare the slope topography, the slope profiles were normalized by taking the shelf break on shelf as the origin (Zhuo et al., 2018) (Fig. 5).

    Profile Shelf width
    (km)
    Shelf break bathymetry (m) Bottom of slope bathymetry (m) Slope width (km) Upper slope average slope (°) Lower slope average slope (°)
    P1 11.37 51.65 4 786 121.36 1.25 3.79
    P2 2.19 30.92 2 978.61 88.51 2.74 1.13
    P3 23.47 32.77 3 023.06 214.61 0.6 0.93
    P4 29.3 49.71 2 713.15 126.73 1.9 0.86
    P5 21.2 14.08 3 133.49 71.16 1.93 3.98
    P6 14.67 53.24 2 462.11 92.95 2.46 0.92
    P7 52.83 95.81 1 788.56 67.01 1 2.18
    P8 1.94 31.05 1 415.87 62.43 3.7 0.78
    P9 20.24 94.21 2 996 117.81 0.98 2.01
    P10 5.05 105.76 2 876.97 59.19 8.02 1.89

    Table 1.  Main parameters of typical bathymetry profiles of continental margin of East Africa. The locations of the profiles are shown in Fig. 2

    Figure 4.  Typical bathymetry profiles selected in this study that were captured along the deposition-dip in East Africa margin. The locations of the bathymetry profiles are shown in Fig. 2.

    Figure 5.  Numerical fitting and normalization of the slope topographical geometries of East Africa margin.

  • The ETOPO1 bathymetric grid was interpreted by the Arc toolbox of Arcgis (ArcMap 10.5), successively carrying out the operations of fill, flow direction, flow accumulation, stream order, stream link, and extracting flux cell with a volume of more than 3 000 to restore the major rivers in East Africa (Fig. 6). Afterwards, watershed extraction was carried out, and raster to polygon was conducted for them. Then, the sinusoidal coordinate was used for projection, and the catchment areas of major rivers in the projected watershed area layer in continental slope area of East Africa were calculated (Table 2).

    Figure 6.  Map of the source-to-sink systems of East Africa. S2S. Source-to-sink.

    Slope area Major rivers Age Typical delta Catchment area
    (×104 km2)
    Progradational distance of typical delta (km)
    AB Intermittent stream None None 19.22 None
    BC Intermittent stream None None 58.83 None
    CD Tana River Recent Tana delta 12.27 ~284
    DE Rufiji River Recent Rufiji delta 23.61 ~280
    EF Rovuma River Recent Rovuma delta 15.48 ~219
    FG Zambezi River Recent Zambezi delta 173.21 ~1 800
    GH Limpopo River Recent Limpopo cone 42.08 ~400
    HI Mfolozi River None None 3.17 None
    IJ Tugela River Recent Tugela cone 3.90 ~250
    JK None None None 0 None

    Table 2.  Main parameters describing characteristics of slope morphology for the various source-to-sink systems discussed in this study (progradational distances of typical shelf-edge deltas are from Castelino et al. (2017), Davison and Steel (2017), Sømme et al. (2009), some parts use Global Mapper software to estimate the distance according to the contour line of the delta position). The locations of the slope areas are shown in Fig. 6

  • Based on the 157 bathymetric profiles in shelf margin of East Africa, we divide the shelf margin into ten slope regions, named as AB, BC, CD, DE, EF, FG, GH, HI, IJ, JK slope areas, respectively (Fig. 2). Combined with the typical bathymetric profiles normalized contrast results (Fig. 5), the morphology of continental slopes of East Africa was divided into three types: concave-up, sigmoidal and planar.

  • The concave-up slope region is composed of BC, DE, FG and HI slope areas, represented by profile P2, P4, P6 and P8 (Fig. 2). The angle of the upper slope is bigger, with an average slope of 1.9°–3.7°, while the lower slope angle decreases gradually, with an average slope of 0.78°–1.13° (Table 1), displaying the characteristics of concave-up curves (Figs. 4 and Fig. 5). The slope is relatively long, with a width between 62.43 and 126.73 km, and the shelf break is sharply obvious.

  • The sigmoidal slope region is composed of AB, CD, EF, GH and IJ slope areas, represented by profiles P1, P3, P5, P7 and P9 (Fig. 2). The upper slope is relatively flat, with an average slope of 0.6°–1.93°, while the lower slope angle increases, with an average slope of 0.93°–3.98° (Table 1), showing convex curves (Figs. 4 and 5). The slope width is between 67.01 and 214.61 km, which is the largest among three types of slope, and the water depth at the bottom of the slope can reach 4 786 m. The slope is in smooth contact with the shelf and the shelf break is not significant.

  • Planar slope region is mainly in JK slope area, represented by profile P10 (Fig. 2). The gradients of planar slope are the largest among the three types of slopes, with the upper slope up to 16.3°, average slope up to 8.02°, and with the average slope of the lower slope of 1.89° (Table 1). As a result, the shelf-break is visible, with significant turning point clearly seen in the junction between the upper slope and lower slope. The slope is narrow, with width of about 59.19 km.

  • The source-to-sink system contains all regions associated with erosion, transportation and deposition of sediments within an erosional-depositional system from catchment headwater to deep-marine basin floor fan (Sømme et al., 2009; Allen, 2008) and the amount of sediment supply depends on the catchment area (Sømme et al., 2009). The catchment area in all studied source-to-sink systems is considered the plateau landscape in western East Africa. Numerous rivers are developed in the extensive floodplain areas between the plateau and the Indian Ocean, providing channels for sediments to bypass the shelf margin and eventually accumulate on the shelf and slopes to form shelf-edge deltas (Fig. 6).

    According to ETOPO1 data, following the division plan of slope areas, the East Africa continental slope areas can be divided into 9 source-to-sink systems: AB, BC, CD, DE, EF, FG, GH, HI and IJ source to sink systems (Fig. 6). Among them, the catchment areas of the CD, DE, EF, FG, GH and IJ source-to-sink areas are lager, ranging from 3.9×104 to 173.21×104 km2 (Table 2), all of which have major rivers flowing into the continental margin and developing shelf-edge deltas. As Fig. 7 shows, the scales of deltas tend to be proportional to the catchment areas. In comparison, the catchment areas of AB, BC and HI source to sink systems are smaller, ranging from 3.17×104 to 58.85×104 km2, mainly developing intermittent rivers or small-scale rivers, without the development of shelf- edge deltas. The JK slope area has no catchment area, hence no rivers and shelf-edge delta have developed there (Figs. 6 and 8).

    Figure 7.  Plot of catchment area versus progradational distance of modern delta in continental margin of East Africa.

    Figure 8.  Bar graph of catchment area for sigmoidal, concave-up and planar slope areas in continental margin of East Africa. Data are listed in Table 2.

  • Structure is the primary control factor affecting the shape of the clinoform (Zhuo et al., 2018; Pratson et al., 2007). The shelf, slope and rise of continental margin of East Africa are formed following the break-up of Gondwana and rifting of Madagascar from Africa (Fig. 3). After the continental margin remains tectonically stable, the clinoforms continue to develop and evolve into diverse morphologies in the context of falling relative sea level. Secondly, regional tectonics moments play an important role in controlling the width of continental slope and sediment flux. We hypothesize the salt diapirism in AB-DE slope areas, mud shale diapirism in EF slope area and basement uplifting in FG-IJ slope areas (Davison and Steel, 2017; Hicks and Green, 2016; Salman and Abudula, 1995; Rabinowitz et al., 1982) all extended the width of the slopes, which is conducive to the evolution of gravity flow from hyperpycnal clastic flow to low-density turbidity in the process of sediment transport, and the formation of deep-water reservoirs with good physical properties. According to Davison and Steel (2017), under the influence of the uplifting of the East Africa rift shoulder (30–0 Ma BP), the degree of denudation increased and a large amount of sediments were provided to the AB-IJ slope areas, which promoted the formation and development of the Rufiji delta, Rovuma delta, Zambesi delta, Limpopo cone and Tugela cone, greatly influencing the morphology of the slopes.

  • Eustasy normally affects the sediment flux to the shelf margin (Posamentier et al., 1988). The global relative sea level displays a long-term continuous falling trend in Neogene to Quaternary times, thus the rate of deposition could possibly be greater than the rate of accommodation increase (Fig. 3), which is favorable for the sediment to be advanced towards the sea and deposited in the shelf edge and slope areas. What is more, as there are differences in catchment areas in the continental margin, the difference in sediment supply could also significantly affect the slope topography (Table 2; Figs. 68).

    The concave-up slope areas are supposed to have the most abundant sediment supply, and generally develop large scale progradational shelf-edge deltas, such as the Zambezi delta and Rufiji delta in the study area, which is conducive to the formation of gravity flow and its associated erosion seascape (e.g., submarine canyon) and creating a better conditions for sediments bypassing the shelf break to be transited to the lower slope and basin floor (Figs. 7 and 8). In the sigmoidal slope areas, the sediment supply is less than that in the concave-up slope areas, and the scale of the shelf-edge delta is also relatively smaller, such as the Tana delta, Rovuma delta, Limpopo cone and Tugela cone (Figs. 7 and 8), and the sediments are mainly deposited on the upper slope. In the planar slope areas, there is little or no sediment supply although accommodation is enough for sediment to fill (Table 2; Fig. 8). Therefore, the slope is in the state of sediment-starving for a long time and does not have smooth characteristics at the junction of the upper and lower slope either. In addition, parts of the planar shaped slopes were produced by action of faulting or gravity landslides (Zhuo et al., 2018; Helland-Hanson and Martinsen, 1996).

  • Submarine canyons are channels through which sediments are transported into deep-marine basin (Mo et al., 2019; Lü et al., 2012; Nittrouer et al., 1994). They are generally categorized into two types according to their evolution processes: (1) River- associated and shelf-incising canyons, formed by erosive turbidity flows derived from fluvial, shelf and upper sources, are connected with subaerial rivers and shelf channels and can extend to the bottom of the slope, with strong sediment transporting capacity. (2) Headless canyons, developed by slumping, slope failure and other mass wasting events, whose erosion direction is landward from the slope to shelf (source erosion), having weak sediment transport ability (Ma et al, 2018; Harris and Whiteway, 2011; Piper and Normark, 2009; Pratson and Coakley, 1996; Pratson et al., 1994; Farre et al., 1983; Shepard, 1981).

    River-associated and shelf-incising canyons are most common in the concave-up slope areas compared to the headless canyons (Figs. 9a, 9b). A large amount of sediments on the shelf and upper slope were exported to the base of slope and basin to form thick basin-floor fans complex (Fig. 10a), so the gradients of the upper slope are larger and the gradients of the lower slope are smaller, showing the concave-up stratal stacking patterns. The sigmoidal slope areas are dominated by headless canyons with a few river-associated and shelf-incising canyons (Figs. 9a, 9b), which own relative poor transport capacities. As a result, sediments were mainly accumulated on the upper slope and the gradients of upper slope are smaller while the gradient of lower slope are larger, performing the signomal shaped proflies (Fig. 10b). In the planar slope area, there are a small amount of headless canyons (Figs. 9a, 9b) and sediment supply is insufficient on upper slope with a steep gradient. Furthermore, the slumping, slope failure and other mass wasting events always happened on the lower slope, leading to a relatively gentle slope, which ultimately resulted in showing the planar curve profiles (Fig. 10c).

    Figure 9.  Distribution and statistics of submarine canyons in continental margin of East Africa. (a) Map showing the location of 83 separate large submarine canyons that are resolved by the ETOPO1 bathymetry data (modified after Harris and Whiteway (2011)). (b) Bar graph of two types of submarine canyons for sigmoidal, concave-up and planar slope areas.

    Figure 10.  Coupling model of shelf-edge trajectories and slope morphology of continental shelf margin of East Africa. (a) The model of concave-up slope and trajectories (stable sediment supply). (b) The model of sigmoidal slope and trajectories (stable sediment supply). (c) The model of planar slope and trajectories (lack of sediment supply). RSL. Relative sea level.

  • Combined with high-resolution seismic data published on the continental margin of East Africa (Figs. 11 and 12) and on the basis of the corresponding characteristics between the shelf-edge trajectories and the morphology of the slope, we assume that there is a good coupling relationship between them (Fig. 10). At times of falling and low sea-level stand from the Neogene to present, the passive continental margin of East Africa has a steady sediment supply and a progradational sedimentary system. Descending trend, flat or low-angle ascending trend were observed in the shelf-edge trajectories (i.e., AB-IJ slope areas), and the clinoforms geometries are mainly showing concave-up (Fig. 10a) and sigmoidal (Fig. 10b) stacking patterns.

    The seismic profile AA' is located in the FG slope area (Fig. 6) which belongs to the concave-up slope area. Since the Miocene epoch, the migration path of the shelf-edge break is successively descending, flat and low-angle ascending (Figs. 11a, 11b). The clinothems show progradational features, and sediments were transported to the basin floor for accumulation in large quantities, indicated by thick bottomsets. The thick, stacked deep-marine fans can be seen at the bottom of the lower slope-basin floor, which is characterized by thinner topset and thicker foreset and bottomset. Hence, the upper slope gradients are larger and the lower slope gradients are obviously reduced, corresponding to the concave-up shaped geometry (Figs. 10a and 11a). The seismic section BB' is located in the IJ slope area (Fig. 6) and belongs to the sigmoidal slope area. With an overall falling trend of global relative sea level since the Miocene, three kinds of trajectories have been observed: descending trend, flat and low-angle ascending trend (Figs. 12b, 12c). The section Unit E shows obvious progradational wedge, most of which are deposited on the upper slope (Fig. 12c). Therefore, the clinoform topsets and bottomsets are thinner and the foresets are thicker. So, the upper slope angles are smoother but the lower slope angles are remarkably steeper, corresponding to the sigmoidal shaped topography (Figs. 10b and 12a).

    Figure 11.  Seismic profiles showing morphology and shelf-edge trajectories of the concave-up slope. (a) Seismic profile of Zambezi delta showing the morphology of slope in FG slope area (modified after Ponte et al., 2019; Davison and Steel, 2017). The location of the seismic profile is shown in Fig. 6. (b) Seismic profile of shelf-edge trajectories of concave-up slope. The location of the seismic profile is shown in Fig. 6.

    Figure 12.  Seismic and bathymetry profiles showing the morphology and shelf-trajectories of the sigmoidal slope area. (a) Bathymetry profile showing the morphology of the position of Tugela Delta in IJ slope area. The location of the bathymetry profile is shown in Fig. 6. (b) Seismic profile of the position of Fig. 12a in IJ slope area. The location of the seismic profile is shown in Fig. 6. (c) Seismic profile of shelf-edge trajectories of sigmoidal slope. Unit C. Campanian to Eocene; Unit D. Oligocene; Unit E. Miocene; Unit F. Pliocene to recent (modified after Hicks and Green, 2016). The location of the seismic profile is shown in Fig. 6. LST. Lowstand systems tract; HST. highstand systems tract; S4. seismic surfaces 4; S5. seismic surfaces 5; S6. seismic surfaces 6.

    A few migratory directions of trajectories along the continental shelf of East Africa display high-angle ascending trends, mainly located in JK slope area. The few influx of sediment in this region (Figs. 6 and 8) could have triggered the aggradational strata stacking pattern, leading to thick topsets and extremely thin or even undeveloped foresets and bottomsets. Therefore, the upper continental slope has sheet-like sedimentary cover and the gradients are extremely steep, and collapsed block resulted from gravitational instability accumulated on the lower slope and the basin floor so that the lower slope is relatively smooth, corresponding to the long-term planar shaped topography (Fig. 10c).

  • The model established for the coupling of shelf-edge trajectories and slope morphology can provide a new idea and an effective evaluation method for the prediction of deep-water oil and gas reservoirs (Fig. 10). The shelf-edge break migration of the concave-up slope areas have the characteristics of descending trend, flat and low-angle ascending trend. In these conditions, the probability of the formation of deep-water fans on the lower slope and basin floor is greater and it may have better physical properties for sand-prone reservoirs, which are conducive to the formation of lithologic hydrocarbon reservoirs. The DE and FG slope areas of East Africa are concave-up slope regions and at present they are in a state of relatively low degree of deep-water oil and gas exploration. We recommend that oil and gas exploration should be more focused and enhanced in the Rufiji delta and Zambezi delta, as well as their surrounding areas.

    Under the background of global sea level falling, the shelf-edge trajectories in sigmoidal slope areas of East Africa also have the characteristic of descending trend, flat and low-angle ascending trend, with the reservoir primarily develops on the upper slope. Up till now a great number of major breakthroughs in oil and gas exploration have been reached in these areas. For example, in EF slope area, large natural gas fields have been discovered in the Rovuma delta's deep-water region of more than 1 000 m in depth, and the recoverable reserves reach 3.6×1012 m3.

  • (1) According to the morphology of the continental slope, the shelf edge of East Africa can be divided into 10 slope zones and three types of slopes were identified: concave-up, sigmoidal and planar.

    (2) Based on the major current river systems in East Africa, nine modern sources to sink systems were identified, and the catchment area is positively correlated with the size of the shelf-edge delta.

    (3) The slope morphology of East Africa is closely related to the geological setting of the passive continental margin, sedimentary supply and sediment volume transported by the submarine canyon. When the sediment supply is sufficient, the slopes of the main river-associated and shelf-incising canyons are characterized by concave-up shaped profiles, and the slopes of the main headless canyons are characterized by sigmoidal shape profiles.

    (4) The concave-up slopes usually correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories. Under the influence of river-associated and shelf-incising canyons, high-quality reservoirs are developed on the basin floor which has bright prospects for oil and gas exploration. The sigmoidal slopes generally correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories in the setting of fall of relative sea-level fall. Under the influence of headless canyons, reservoirs with great exploration potential are mostly developed on the upper slope. Moreover, the planar slopes correspond to high-angle ascending trend trajectories and are hardly potentials for exploration.

  • This research was financially funded by the National Natural Science Foundation of China (Nos. 41690134, 41821002, 41702155, 41690131). The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1288-8.

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