Extensive contourite drifts (SFA 6) that form along tectonic relief associated with the Sea Gap and Davie Ridge (Figure 4) dominate the deep-marine sedimentary system in the Cenomanian. Slope channel complex fills (SFA 1) that are confined by drift deposits (SFA 5) and tectonic relief (Figures 4B,E) extend into high amplitude lobe complexes (SFA 2) north and south of the Sea Gap pop-up structure (Figure 4E, Figure 5A–C). In the north, the lower slope is dominated by low amplitude sediment waves (SFA 7) (maximum thickness of ∼100 m and 3 km width) that follow the curved contours of the slope. Lobe complexes fill the topographic lows of sediment waves represented by onlapping high amplitude reflectors onto the low amplitude reflectors of the sediment waves (Figures 6A,B). Lobes are narrow and elongated with average widths of 3 km, length of 19 km and thicknesses up to 100 m (Figure 6B zoom). Sharp amplitude contrasts of RGB colour blends along the fringes indicate steep confinement and poorly-developed thin bedded lobe fringe facies (Figure 5C). Narrow, up to 500 m wide, high amplitude/frequency responses connect each crescent-shaped lobe complex and are interpreted as sediment entry points indicating frontal erosion and sediment transport from one topographic low to another (Figure 5C). Southwest stacking of sediment waves cause a lateral offset of confined lobes over time and governs the lobe complex stacking pattern (Figure 6B, zoom II). The southern depo-centre is dominated by ∼ 5 km wide drift deposits (SFA6) that step up the slope and are pinned to the tectonic relief of the Sea Gap (Figures 5A,B,C, Figure 7A). High amplitude reflectors show well-developed lobate shapes (length > width) that onlap and drape onto the relatively gentle dipping flanks of contourite drift deposits. High amplitude distributary channels feed into individual high amplitude lobes that dim gradually away from the lobe axes indicating well-developed lobe-off-axis and lobe fringe facies (Figure 5C). 2D seismic lines display the extension of small drifts and sediment wave fields into the basin low between the Sea Gap and Davie Ridge, indicating similar confinement styles along the distal part of the sedimentary system (Figure 4A, Figure 6C). Spectral decomposition images of the eastern 3D seismic survey displays elongated northwest striking, high amplitude reflectors between straight crested, low amplitude sediment waves in the west of the Davie Ridge, that represent the most distal part of the turbidite system (Figures 8A,C). The dimming of amplitudes suggests a decrease in grain-size towards the distal end of the sedimentary system.

High amplitude seismic reflectors in the Cenomanian interval are targeted by four wells (Well A, B, C and D in Figure 5, Figure 6A, Figure 7). Low, blocky gamma ray log responses of Well A to D of coarse-grained sediments correspond to the high-amplitude reflectors forming the deep-marine sedimentary system (Figures 3, 9). Frontally confined lobe complexes in the north, penetrated by Well B, show blocky low GR signatures, indicating about 100 m thick sandstones deposited in the topographic lows between the sediment waves (Figures 6A,B). A total of 33 m of core (blue box in Figure 6A, zoom I) shows a basal interval of hybrid beds that represent lobe off-axis deposits (Fa 2) along the topographic confinement (Figure 9A). This interval is overlain by thick amalgamated, poorly-sorted, medium-grained high density turbidites that are deposited in an axial position (Fa 1) (Figure 5C). Individual beds are tentatively identified by increased mud content at the base that transitions into banding and abundant dish and pillar structures towards the top (LF 1). Sandstone beds clean towards the top of the succession and develop low to high angle cross bedding that steepens over an interval of 10–30 cm (LF 2). Mud caps are generally absent from the upper part of the succession.

The contourite depositional system along the base of slope (east of the Sea Gap) changed during the Santonian. The northern depocentre is dominated by northwest-southeast striking, straight crested, sediment waves (SFA 7) with wavelengths of up to 5 km (width ∼2.5 km and a height of ∼100 m) (Figures 5E,F, Figure 7A). Associated with their topographic lows are high amplitude reflectors that onlap and drape over the topography (SFA 8). Through the Santonian and early Campanian, these sediment waves transition into large drift deposits of parallel, continuous, low amplitude reflectors that extend towards the Davie Ridge with maximum thicknesses of ∼1,650 m in Well B (total drift thickness from Santonian to base Paleogene) (Figure 4A, Figure 5D). A northwest-southeast striking, distal channel complex (SFA 1) interacts with this drift (SFA5) (Figure 5F, Figure 7A). High amplitude reflectors extend from the channel and dim towards the northwest where they interfinger with low amplitude reflectors of the large drift (Figure 7A). Smaller drift deposits of 10–15 km width are developed further south where they are pinned to the eastern side of the Sea Gap pop-up structure, and form a moat along the topographic high (Figure 7A, zoom I). Along the southern depocentre these drift deposits are onlapped and draped by high amplitude reflections of a lobe complex (SFA 2) with a length of ∼30 km and a width of ∼19 km (Figure 4E, Figure 5F). Well-developed internal seismic morphologies show up to 500 m wide distributary channels that connect individual lobes (Figure 5F). Amplitude dimming indicates well-developed lobe off-axis and fringe facies where turbidite beds thin and drape over the gentle relief. High amplitude “fingers” extend from the south and north into a topographic low associated with the moat along the Sea Gap with amplitudes dimming away from the sediment entry points and towards the onlap surfaces (Figure 5F, Figure 7A). These elongate, laterally confined lobate bodies are about 5 km wide and 19 km long. Drift morphology flattens towards the basin low between the Sea Gap and Davie Ridge (Figure 4A). Spectral decomposition maps of the eastern 3D seismic survey show elongate northwest-southeast striking high amplitude reflectors between low amplitude, straight crested sediment waves covering the whole seismic survey (Figures 8A,D).

Although no core data are available for the Santonian, gamma ray log responses of Well B, C and D confirm the sand-rich character of the high amplitude bodies forming the lobe complexes. Well D targets high amplitude lobe complexes in the south of the Sea Gap pop-up structure, and displays thick, blocky low gamma ray responses of about 60 m (Figure 7A, zoom I). This interval is interrupted by high gamma ray peaks in a spacing of about 20 m that are interpreted to represent individual coarse grained lobes. Well C, in the east of the Sea Gap shows a spiky log response at the base of the Santonian that correlated with the distal dimming of amplitude reflections along the distal end of the lobe complexes (Figure 7A, zoom I). Similar GR signatures are recorded in the Campanian system where core control supports the fine grained character of these lower amplitude reflections.

The mid-Campanian reflector marks a regional unconformity (Data Rep. Table 1; green seismic horizon). Bottom current-related sedimentation ceased along the upper slope and existing topography was filled by flat, parallel, high to low amplitude reflectors interpreted as ponded turbidites (Figure 4B). Along the lower slope, east of the Sea Gap, large drift deposits continue to grow, extending further into the basin centre towards the Davie Ridge (Figures 4A,B). Mounded drifts flatten towards the basin centre into sheet-like morphologies (Figure 4A) and extensive, NNE-SSW striking sediment wave fields (wavelengths of up to 5 km) formed along the western side of the Davie Ridge (Figures 8B,E). High amplitude lobe complexes (SFA 2) fill the gentle relief of the large drift deposits in the northern and southern depocentre, east of the Sea Gap (Figures 5G–I). Seismic amplitudes indicate lobe complex sizes of up to 30 km long and 23 km wide, and dim gradually over long distances (∼5 km). This transitional dimming of amplitudes suggests a low gradient and limited confinement with well-developed lobe fringe facies (Figures 7B,C). High and medium amplitude reflectors extend into the basin centre where they are confined by the topographic lows of large sediment waves (Figure 6C). Although the interpretation is hampered by the lack of 3D seismic reflection data in this area, the thickness (∼300 ms [TWT]) and internal seismic facies suggest laterally confined lobe complexes in the basin centre (Figure 6C)—similar to the Santonian systems. In the most distal part of the system, medium amplitude reflectors (SFA 8) onlap and drape over the crest of large sediment wave fields east of the Davie Ridge (Figures 8A,B).

Well D targets a ∼75 m thick northward confined lobe complex that drapes onto the gentle relief of low amplitude drift deposits in the south of the Sea Gap (Figure 5I, Figure 7C). Spikey gamma ray signatures and ∼36 m of core display a heterogeneous succession of sandstones and muddy siltstones (Figure 7C; zoom I, 9B). Medium to fine grained, normally graded sandstones up to 1.5 m thick are commonly massive to parallel- and ripple-laminated with intervals of distorted, convolute lamination (LF 3, Figure 3E). Some beds show well developed banding (LF 4, Figures 3E,F) and are often reworked by bottom currents (LF 7, Figures 3K,L). These reworked beds consist of up to fine-grained sandstone and starved ripples (often showing opposing paleo-flow indicators than the underlying beds) that are interbedded with bioturbated, fine-grained (mud- and siltstones) drift deposits (LF6, Figures 3L,M) and local debrites (LF 5, Figures 3I,J). The succession is interpreted to represent stacked lobe off-axis and lobe fringe deposits that are partly reworked by bottom currents (Fa 2 and 3) and interfinger with the toes of contourite drift deposits (Fa 4).

During times of reduced turbidity current activity, bottom current sedimentation dominated the margin east of the Sea Gap (t1 and t4 in Figure 11). The low seismic amplitudes, morphologies and regional extent contourite drifts and sediment waves indicate fine-grained deposits (mud- and siltstone) that were deposited by relatively persistent bottom currents (Faugères et al., 1999; Wynn and Stow, 2002a). Along the lower slope, large south-west (upstream) migrating sediment waves started to develop in the Cenomanian that are aligned with the contours of the tectonic relief of the Sea Gap and show an overall northwest-southeast orientation in the basin centre. Their basin-wide extent indicates a deposition by the extension of intermediate waters that flowed northwards into the Mozambique Channel and East African Basins offshore Tanzania (Figure 2) (Fischer and Uenzelmann-Neben, 2018a; Thiéblemont et al., 2020). Sediment wave size, orientation and fine-grained character suggest deposition under lee-wave conditions by relatively weak bottom currents (0.15–0.25 m*s⁻¹) (Flood, 1988; Flood and Shor, 1988; Stow et al., 2009). The relatively weak bottom currents travelled over gentle relief and deposited fine-grained (up to silt sized) sediment on the lee-side of sediment waves and thus the waves migrated in, or oblique to the up-current direction (Flood, 1988; Flood and Shor, 1988). Local variations in sediment wave shape, such as the crescent shaped wave crests in the Cenomanian may be caused by the ongoing growth of the large pop-up structure on the Sea Gap or flow interference with local dilute turbidity currents. However, the northwest to southeast striking trend of sediment waves continues throughout the Turonian to Santonian as distal as the Davie Ridge (Figure 8). This supports regionally constant northwards flowing currents that increased in strength during the Cretaceous Thermal Maximum Event in the Turonian (Poulsen et al., 2001, 2003; Friedrich et al., 2012; Murphy and Thomas, 2012). Larger drift deposits of the Cenomanian to mid-Campanian nucleated along the growing tectonic relief of the Sea Gap with moats and mixed turbidite-contourite channel complexes dominating the upper slope. Contourite drifts along the lower slope flatten towards the basin centre and transition into sediment wave fields along the Davie Ridge. The sediment incorporated in the extensive contourite depositional system is likely sourced from the south where northward-flowing bottom currents caused erosion, and limited drift deposition during the Upper Cretaceous (Fischer and Uenzelmann-Neben, 2018a; Thiéblemont et al., 2020). Additional fines could have been stripped from local turbidity currents (Fossum et al., 2019), local erosion events (Hollister and McCave, 1984; McCave and Tucholke, 1986; McCave et al., 2017; Thran et al., 2018) or from paleo-Rovuma River in the south (Key et al., 2008; Smelror et al., 2008). During the mid-Campanian, bottom current sedimentation ceased along the upper slope and sediment wave orientation along the Davie Ridge changed from SSE-NNW to NNE-SSW. Time equivalent onset of bottom current related sedimentation along the Mozambique Channel and Mozambique ridge indicates a change in the palaeo-oceanographic setting that was likely caused by a deepening of the Aghulas Passage and re-orientation of major currents (Figures 2A,B) (Fischer and Uenzelmann-Neben, 2018a; Thiéblemont et al., 2020). This variability in bottom current sedimentation in the Upper Cretaceous caused the evolution of complex margin topography and the deposition of distinctive turbidite lobe complexes.

Frontally-confined lobe complexes developed dominantly in the Cenomanian, in the northern depocentre, where transverse sediment gravity flows filled topographic lows between crescent-shaped sediment waves (Figure 11 t2b, Figure 12A). Channelised, high amplitude reflectors are interpreted as sediment entry points into each topographic low, indicating filling of topography in a fill-and-spill fashion, similar to steeply confined mini-basins (Prather et al., 2012, 2016) or other small scale sea-floor bedforms (Maselli et al., 2019). These confined structures are dominated by aggradational lobe stacking and distinct phases of flow ponding, flow stripping and bypass (Sinclair and Tomasso, 2002). Core data in Well B represents the progressive infill of such a steeply confined structure. Initial flows that become confined rapidly decelerate along the frontal confinement leading to the development of muddy, hybrid beds and slumps (Lf 2 and 4) that represent lobe off-axis to proximal fringe deposits along onlap surfaces (Fa. 2) (Figure 2B) (Al Ja’Aidi et al., 2004; Haughton et al., 2009; Bakke et al., 2013; Patacci et al., 2014, 2015; Soutter et al., 2019). With ongoing sediment progradation, highly concentrated turbidity currents enter the confined structure and decelerate along the frontal confinement, which causes frontal erosion and rapid deposition of massive amalgamated high-density turbidites, and entrainment of a wide grain size range (Lf 1) (Lowe, 1982; Soutter et al., 2021). This rapid deposition and large grain-size range promotes the trapping of pore waters along low permeability laminae that eventually burst with increasing fluid pressure, and form the well-developed banding and dewatering structures (dishes and pillars) (Lowe, 1975; Porten et al., 2019). Ongoing infill and erosion along the frontal confinement allows sediment-laden flows to bypass into the next topographic low (Figure 11 t3b). With increasing sediment bypass, high-density turbidites are dominantly deposited by continuous deposition, which is represented by a decrease in dewatering structures and the increase in steepening cross bedding (Lf 2) (Figure 3C, Figure 9A) (Hiscott, 1994; Kneller and Branney, 1995). The steepening cross bedding in the upper part of the core may be associated with long sediment waves or the backfilling of large scours in the proximal location of the lobe axis (Fa 1) (Arnott and Al-Mufti, 2017; Pohl et al., 2019). Increased flow velocities are associated with narrow erosional corridors through which flows are focussed and accelerated (Soutter et al., 2021), that support locations of sustained sediment bypass (Amy et al., 2000; Stevenson et al., 2015). The lack of bottom current influence on axial lobe deposits is interpreted to result from strong confinement and high sediment concentration of the gravity currents in which grain-grain interaction in the high density layer at the base of the flow dominate the transport over dilute turbulent flow (Lowe, 1982; Talling et al., 2012). Bottom current influence in these low velocity bottom current systems (0.1–0.3 m*s⁻¹) is inferred to be pronounced along the fine grained lobe fringe where dilute, super-elevated cloud of turbidity currents strip away or locally re-distribute finer grains in the down-current direction (in respect to the bottom current) (sensu de Castro et al., 2020). High sediment concentration and long reoccurence times of turbidity currents at terminal lobes (Jobe et al., 2018) in combination with steep contourite relief may favour a passive interaction of these processes-similar to other confined systems (Figure 12). During times of long quiescence (of turbidity currents), sediment waves migrate southwest and cause a re-routing of sediment gravity flows and a modification of lobe complex stacking pattern (Figure 11 t4).

Lateral confinement is observed in the Santonian along the Sea Gap and the mid-Campanian in the west of the Davie Ridge, where sediment gravity flows are confined along straight-crested sediment waves. Core was not recovered, but high seismic amplitudes and gamma ray log data from Well B (similar to the responses along cored sections) indicate coarse-grained sediments in the topographic lows between the sediment waves (Figure 5, Figure 7, Figure 8). Sediment is inferred to have been sourced from the northwest and entered the confinement axially. This lateral confinement increases the run out length of turbidity currents and the deposition of elongated lobes (Salles et al., 2014; Soutter et al., 2021). Asymmetrical draping of high amplitude bodies out of the confinement and dimming of amplitudes towards the northeast suggest an increasingly active interaction of bottom currents with confined turbidity currents similar to “mixed” contourite-turbidite channels (Fuhrmann et al., 2020; Miramontes et al., 2020b). Flow stripping and reworking by bottom currents causes a facies asymmetry and the deposition of bottom current reworked sands (Fa 3) on the down current side of the sediment waves in respect to the bottom current similar to other systems (Shanmugam et al., 1993; de Castro et al., 2020; Fonnesu et al., 2020) (Figure 12B). Away from the sediment entry point the turbidity currents wane, grain size decreases (represented by the dimming of seismic amplitudes) and bottom-current influence increases. During the shutdown of turbidity currents, margin-parallel bottom currents, acted with an angle of up to 45° (in respect to the elongated deposit), reworked and deposited fined-grained material on the bottom current upstream facing side of the sediment waves (Hopfauf and Spieß, 2001; Wynn and Stow, 2002b). This interplay of processes suggests a stacking of fine grained drift deposits (Fa 4) and reworked lobe fringe deposits (Fa 3) on the up-current facing side of the sediment waves and coarse grained turbidite facies (Fa 1 and 2) in the centre of the structure. With increasing distance from the margin, bottom currents dominate and facies ratios changes towards drift deposits and bottom current reworked deposits (Fa 4 and Fa 5). This transition is seen along the Davie Ridge in the Cenomanian to mid-Campanian where large sediment-wave fields show dim amplitudes along topographic lows that are draped towards the northern wave crests (Figure 8).

Semi-confined lobes are present along the southern depo-centre in the Cenomanian, Santonian and the mid-Campanian. Deposits are of elongated, lobate shape with high seismic amplitudes that dim towards the fringes where they onlap and drape over low angle contourite drifts (Figure 11, Figure 12C). Individual architectural elements across lobe complex are well developed in form of ∼300 m wide distributary channel-fills in proximal location, and individual lobes with high-amplitude reflections in the axis that dim towards the fringes (Figure 5I, Figure 7B,C). Amplitude dimming is interpreted to represent well preserved lobe off-axis and fringe deposits developed due to longer flow runouts along the gentle relief (e.g., Hansen et al., 2019; Spychala et al., 2017). Lateral, one sided confinement dominates the lobes along the margins of fine grained, low-angle drift deposits, causing draping onlaps and aggradation, with lateral accretion and compensational stacking observed towards the unconfined flank similar to other systems (Gervais et al., 2006; Deptuck et al., 2008; Hansen et al., 2019). Core data of Well D reveals complex facies stacking along the lobe off-axis and lobe fringes (Figure 9B). Increased flow run-out lengths along and onto the gentle relief allow grain size segregation and well developed, normally-graded turbidite beds (LF 3) that thin away from the focal point and are commonly associated with lobe fringes (Spychala et al., 2017). The locally well-developed banding in individual beds (LF 4) is interpreted to result from transitional flows (switching from turbulent to laminar flows) (Baas et al., 2009; Sumner et al., 2009). Flow transformation was likely caused by the entrainment of mud due to local erosion of muddy contourite drifts and the interaction with local topography (Kneller et al., 1991; Kane and Pontén, 2012). Further indicators for local influence of topography are convolute lamination (Tinterri et al., 2016), slumped deposits and sand injectites that result from remobilisation of unconsolidated sediment along the depositional relief and the build-up of overpressure during burial of sandstones along onlaps (Cobain et al., 2017; Hansen et al., 2019). This topographic complexity may have also caused opposing palaeo-current indicators in form of ripple lamination within individual beds (Bell et al., 2018b).

Additional complexity to the facies stacking is interpreted to result from the influence of bottom currents along the lower slope of the Upper Cretaceous sedimentary systems (Figures 2A,B) (Fischer and Uenzelmann-Neben, 2018a; Thiéblemont et al., 2020). Interbedded starved ripples and streaks of fine-grained sandstone and muddy siltstones that consistently overlie turbidite beds are interpreted to represent reworking by bottom currents (Fa 3) (Ito, 1996; de Castro et al., 2020). The paleocurrent indicators of these laminae differ from the underlying beds, which may either be the result of flow deflection or lateral reworking of bottom currents (Figure 3K; white arrow) (Shanmugam et al., 1993; Martín-Chivelet et al., 2008; de Castro et al., 2020). The regional context and re-occurrence of these beds favour a reworking by temporally variable bottom currents with local velocities of up to 0.3 m*s⁻¹ (Stow et al., 2009 and references therein) (Figure 11t2 to t3). The thick, mottled, muddy siltstones, with faint, up to fine-grained sandstone lenses and streaks (Figure 3I, Fa 5) are interpreted as toes of the drift deposits that migrate onto the turbidite lobes during the absence of turbidity currents (Fa 4) (Figure 11t4). Increased run-out lengths along gentle topography and frequency of turbidity currents control the facies-stacking pattern. Higher sedimentation rates along the proximal lobe fringes promotes the preservation of individual tractional sedimentary structures (Fa3) that are increasingly disrupted by bioturbation towards the most distal parts of the system. These mottled and poorly sorted siltstones resemble commonly described contourite drift facies (Fa 4) (Stow et al., 2008; Stow and Faugères, 2008; Brackenridge et al., 2013).