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-Perspective view of Niger Delta slope, showing typical shale-based above-grade slope with curvilinear shale-cored ridges and adjacent plunging syncline lows. Zoomed seafloor image comes from the survey area indicated in lower left inset. Study area is outlined by dashed line with OML 134 block. Line of section shown in Fig. 5 follows the X channel.
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This submarine apron is an analog for the stratigraphic architecture of shallow ponded basins common to stepped, above-grade slopes, where late-stage bypass valleys and channels did not form. Deposition of this apron began within shallow ponded accommodation. Sediment gravity flows entering the basin pass through a leveed channel that incises under...
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... objective of this study is to demonstrate how geomor- phology of the slope, such as gradient, entry-point position, and accommodation, controls patterns of deposition within a shallow ponded basin located across a "step" on the upper slope offshore Nigeria (Fig. 1). Together with work by Pirmez et al. (2000), Fonnesu (2003), Deptuck et al. (2003); Deptuck et al. (2007), Deptuck et al. (this volume), and Adeogba et al. (2005), this study extends our knowledge about the evolution of stepped slopes, and the deposition of submarine aprons perched on the slope that are unaltered by formation of ...
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... stepped slopes, and the deposition of submarine aprons perched on the slope that are unaltered by formation of bypass channels and submarine valleys. We focus this study on the details that characterize the stratigraphic evolution of the intraslope basin located in the southeastern portion of an area designated as Oil Mining License (OML) 134 ( Fig. 1) where conventional three-dimensional seis- mic data that cover an area of 225 km 2 (15 km x 15 km) can be integrated with giant piston cores and a sub-bottom profiling survey (Fig. ...
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... study area in OML 134 is located on the upper Niger Delta slope west of the present location of the Niger River in 1100-1400 meters of water (Fig. 1). The Niger Delta covers an area of about 75,000 km 2 and extends for more than 300 km from its apex to its shoreline (Whiteman 1982;Doust and Omatsola 1990). The Niger Delta continental slope extends westwards for another 150 km from the present-day shelf-slope break. The delta and the slope together comprise a sedimentary wedge that ...
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... in recent years ( Iunio et al., 1998;Friedmann et al., 1999;Pirmez et al., 2000;Prather and Pirmez, 2003). The aprons occupy healed-slope accommodation created across steps on the slope profile as shale-cored structural features rose during slope evolution (Fig. 5). The steps link across ramps at present via submarine valleys and channels ( Fig. 1; Pirmez et al., 2000). Pirmez et al. (2000) believe that the distinct rim and onlapping geometry in the subsurface below the steps suggest they were closed intraslope basins in the past (Fig. 5). The most downdip perched submarine apron in the OML 134 area passes updip into lower-gradient unconfined slope deposits that bury earlier ...
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... near-seafloor submarine apron that is the focus of this study occupies a topographic low formed between localized uplifts of mobile shale within the underlying slope (Fig. 5). Evidence of movement of underlying shale is expressed in the near-seafloor geology as a mud volcano, ridges, and linked faults ( Fig. 1). An intraslope basin forms in the hanging wall of the northwest-striking fault across the crest of the buried shale ridge. A series of down-to-the-north faults forms the north flank of the shale ridge. An east-striking shale-cored ridge bounds the intraslope basin to the south, as does the regional Niger Delta slope to the east. A ...
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... channels cross the seafloor in the study area; Pirmez et al. (2000) designate them X, Y, and Y', respectively (Fig. 1). These channels link the upper slope to a shelf margin sourced from relatively small, updip incised coastal river systems unlike the large incised valleys associated with the Opuama or Afam valleys, located on eastern and western Niger deltas respec- tively. The X channel originates below a shelf-margin delta and incised valley in the ...
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... re-emerges downslope of a knickpoint at the distal end of the intraslope basin, where it joins with a large submarine valley (the Y channel of Pirmez et al., 2000) that cuts through several shale-cored ridges as it trends westward, down- slope of the study area, eventually feeding a large submarine apron located at the present base of slope ( Fig. 1). At least six smaller channels or slope gullies flank the X channel ( Fig. 8). Together with the X channel they form a distribu- tary-channel pattern emanating from a point along the X channel several kilometers updip of the apex of the submarine apron. Hanging U-shaped incisions associated with these gullies along the walls of the X ...
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... broad downdip-oriented tributary drainage pattern is evi- dent near the exit point of the basin (Fig. 10). Since this area is buried by submarine apron deposits and is updip of knickpoint erosion created during later bypass of the basin, it evidently developed early in the history of the basin fill, probably by erosive flows bypassing the area of ponded accommodation. The upper reaches of these tributary scours end at the downdip limit of ...
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... the last and far- thest downdip of the intraslope basin fill sequences along the X channel in OML 134 (Fig. 4). The apron occupies the intraslope basin located in the southwest corner of the three-dimensional survey below where the X channel disappears into the apron and updip of where it reemerges from the apron before linking with the Y channel (Fig. 1). The submarine apron in OML 134 consists of at least three units -a thin low-relief ponded apron (?) and two perched aprons which downlap the underlying unit and pro- grade across the intraslope basin floor (Fig. 11). The blue horizon separates the perched apron into lower and upper units (Fig. 11). Discontinuous, highly reflective ...
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... below where the X channel disappears into the apron and updip of where it reemerges from the apron before linking with the Y channel (Fig. 1). The submarine apron in OML 134 consists of at least three units -a thin low-relief ponded apron (?) and two perched aprons which downlap the underlying unit and pro- grade across the intraslope basin floor (Fig. 11). The blue horizon separates the perched apron into lower and upper units (Fig. 11). Discontinuous, highly reflective seismic facies characterizes the lower apron, whereas more continuous highly reflective seismic facies characterizes the upper apron (Fig. 11, 12). This is also the deepest seismic event that connects the basin entry ...
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... from the apron before linking with the Y channel (Fig. 1). The submarine apron in OML 134 consists of at least three units -a thin low-relief ponded apron (?) and two perched aprons which downlap the underlying unit and pro- grade across the intraslope basin floor (Fig. 11). The blue horizon separates the perched apron into lower and upper units (Fig. 11). Discontinuous, highly reflective seismic facies characterizes the lower apron, whereas more continuous highly reflective seismic facies characterizes the upper apron (Fig. 11, 12). This is also the deepest seismic event that connects the basin entry point directly to the basin exit point (Fig. ...
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... which downlap the underlying unit and pro- grade across the intraslope basin floor (Fig. 11). The blue horizon separates the perched apron into lower and upper units (Fig. 11). Discontinuous, highly reflective seismic facies characterizes the lower apron, whereas more continuous highly reflective seismic facies characterizes the upper apron (Fig. 11, 12). This is also the deepest seismic event that connects the basin entry point directly to the basin exit point (Fig. ...
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... the perched apron into lower and upper units (Fig. 11). Discontinuous, highly reflective seismic facies characterizes the lower apron, whereas more continuous highly reflective seismic facies characterizes the upper apron (Fig. 11, 12). This is also the deepest seismic event that connects the basin entry point directly to the basin exit point (Fig. ...
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... submarine apron that occupies the OML 134 intraslope basin is generally circular in planform (Fig. 13B), reflecting the infilling of nearly circular ponded and healed-slope accommoda- tion (Fig. 13A, C, E). Although we are not able to isolate and independently map a seismic event associated with a ponded submarine apron, the presence of demonstrable ponded accom- modation of about ~ 25 m (Fig. 11, 13A) and a corresponding single ...
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... submarine apron that occupies the OML 134 intraslope basin is generally circular in planform (Fig. 13B), reflecting the infilling of nearly circular ponded and healed-slope accommoda- tion (Fig. 13A, C, E). Although we are not able to isolate and independently map a seismic event associated with a ponded submarine apron, the presence of demonstrable ponded accom- modation of about ~ 25 m (Fig. 11, 13A) and a corresponding single baselapping seismic event that onlaps the intraslope floor below the spill point (Fig. 11) suggests the ...
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... intraslope basin is generally circular in planform (Fig. 13B), reflecting the infilling of nearly circular ponded and healed-slope accommoda- tion (Fig. 13A, C, E). Although we are not able to isolate and independently map a seismic event associated with a ponded submarine apron, the presence of demonstrable ponded accom- modation of about ~ 25 m (Fig. 11, 13A) and a corresponding single baselapping seismic event that onlaps the intraslope floor below the spill point (Fig. 11) suggests the existence of a low-relief ponded apron (sensu Prather et al., this volume). Healed-slope accommodation makes up the remaining ~ 150 m of sediment thickness (maximum) in the basin. Maximum thickness of the ...
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... accommoda- tion (Fig. 13A, C, E). Although we are not able to isolate and independently map a seismic event associated with a ponded submarine apron, the presence of demonstrable ponded accom- modation of about ~ 25 m (Fig. 11, 13A) and a corresponding single baselapping seismic event that onlaps the intraslope floor below the spill point (Fig. 11) suggests the existence of a low-relief ponded apron (sensu Prather et al., this volume). Healed-slope accommodation makes up the remaining ~ 150 m of sediment thickness (maximum) in the basin. Maximum thickness of the basin fill is offset from the deepest point of the basin towards the entry point. The basin fill thins to the southeast ...
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... the entry point. The basin fill thins to the southeast towards the basin exit point. Thinning results in part from down- lap towards the basin rim and in part from erosional truncation beneath a knickpoint at the seafloor that cuts progressively downward into the large submarine valley that represents the downdip extension of the X channel (Fig. 11). The exit point of the basin corresponds to the structural saddle located in the south- western corner of the study area (compare Fig. 6 and Fig. 13A). Such erosional truncation of the downdip portion of the basin fill is a typical characteristic of perched ...
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... the basin rim and in part from erosional truncation beneath a knickpoint at the seafloor that cuts progressively downward into the large submarine valley that represents the downdip extension of the X channel (Fig. 11). The exit point of the basin corresponds to the structural saddle located in the south- western corner of the study area (compare Fig. 6 and Fig. 13A). Such erosional truncation of the downdip portion of the basin fill is a typical characteristic of perched ...
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... isochore thickening of strata symmetrically distributed around the X channel (Fig. 14B) and "gull-wing" cross sections suggest the presence of levees near the basin entry point. Three curvilinear thins that run parallel to the levees suggest that the levees are incised by small slope gullies. The gullies overtop the levees at a break in slope a few kilometers updip of the apron entry point where the channel crosses a ...
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... seafloor, which is the top of upper apron in OML 134, has a prominent knickpoint that connects downdip to a submarine valley (Fig. 13E). This submarine valley merges with a larger valley to the east (termed the Y channel by Pirmez et al (2000) that cuts across the entire slope, connecting the shelf-slope break to a well-defined submarine apron at the toe of slope (Fig. 1). Slope profiles, both down the feeder channel axis and through the aprons at several levels, show ...
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... the top of upper apron in OML 134, has a prominent knickpoint that connects downdip to a submarine valley (Fig. 13E). This submarine valley merges with a larger valley to the east (termed the Y channel by Pirmez et al (2000) that cuts across the entire slope, connecting the shelf-slope break to a well-defined submarine apron at the toe of slope (Fig. 1). Slope profiles, both down the feeder channel axis and through the aprons at several levels, show gradually flattening gradients (Fig. 7D). The gradient change between the feeder channel and the baselap surfaces takes place within ponded accommodation and represents a break in slope that is perched well above the ultimate toe of slope ...
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... surface of the lower apron has an asymmetrical radial-fan shape with its apex slightly off of center to the north relative to the basin entry point (Fig. 13C). The gradient across the proximal part of the lower apron is lower than the gradient across the same region in the upper apron (Fig. 7). Thinning, multiple knickpoints, and irregular topography suggest that the asymmetry of the apron is related to a broad area of erosion on its southern flank. The depocenter for the lower apron is also ...
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... has been stripped away for viewing the hemipelagics at the bottom of the basin. Note that the X channel ends at the top of ponded accommodation whereas the downdip termini of slope gullies, indicated by red triangles, occur at higher levels. underlying ponded accommodation, suggesting that the bulk of the deposition did not occur within the pond (Fig. 14A). Stratal slices through a "seismic texture" volume shows that the lower apron consists of complexes of small distributary channels and lobes (Fig. 15A). Channels emanating from the basin entry point fan out laterally across the prograding apron toward the basin exit point and are confined to the thickest part of the lower apron (Fig. ...
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... the downdip termini of slope gullies, indicated by red triangles, occur at higher levels. underlying ponded accommodation, suggesting that the bulk of the deposition did not occur within the pond (Fig. 14A). Stratal slices through a "seismic texture" volume shows that the lower apron consists of complexes of small distributary channels and lobes (Fig. 15A). Channels emanating from the basin entry point fan out laterally across the prograding apron toward the basin exit point and are confined to the thickest part of the lower apron (Fig. 15A). Thickening apron deposits, immediately downdip of the ba- sin entry point, reflect the location of multiple erosional features (Fig. 11). Only the ...
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... pond (Fig. 14A). Stratal slices through a "seismic texture" volume shows that the lower apron consists of complexes of small distributary channels and lobes (Fig. 15A). Channels emanating from the basin entry point fan out laterally across the prograding apron toward the basin exit point and are confined to the thickest part of the lower apron (Fig. 15A). Thickening apron deposits, immediately downdip of the ba- sin entry point, reflect the location of multiple erosional features (Fig. 11). Only the shallowest of these erosional features can be mapped outside of the proximal part of the lower apron, because deeper erosional cuts are only partially preserved. The latest of these ...
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... channels and lobes (Fig. 15A). Channels emanating from the basin entry point fan out laterally across the prograding apron toward the basin exit point and are confined to the thickest part of the lower apron (Fig. 15A). Thickening apron deposits, immediately downdip of the ba- sin entry point, reflect the location of multiple erosional features (Fig. 11). Only the shallowest of these erosional features can be mapped outside of the proximal part of the lower apron, because deeper erosional cuts are only partially preserved. The latest of these erosional features modifies the top of the lower apron surface where the entry channel merges with the proximal apron, forming a dip-elongated ...
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... 11). Only the shallowest of these erosional features can be mapped outside of the proximal part of the lower apron, because deeper erosional cuts are only partially preserved. The latest of these erosional features modifies the top of the lower apron surface where the entry channel merges with the proximal apron, forming a dip-elongated scour (Fig. 16). Channels exit the scour area and shallow as they extend down the apron, before they link up with a knickpoint that leads to the basin exit point. The scour is possibly analogous to the "plunge pools" reported from the continental slope off California and from outcrops of the Gres d'Annot, SE France ( Lee et al., ...
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... surface of the upper apron has a symmetrical, radial fan shape with its apex centered downdip of the of the entry-point channel (Fig. 13E). However, there is a lateral (eastwards) shift in thickening in the upper apron compared to the lower apron (Fig. 13F). The upper apron depocenter corresponds to the location of the broad, eroded area at the top of the lower apron. Isochore and trace shape patterns suggest the presence of dis- tributary lobes within the upper apron. ...
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... surface of the upper apron has a symmetrical, radial fan shape with its apex centered downdip of the of the entry-point channel (Fig. 13E). However, there is a lateral (eastwards) shift in thickening in the upper apron compared to the lower apron (Fig. 13F). The upper apron depocenter corresponds to the location of the broad, eroded area at the top of the lower apron. Isochore and trace shape patterns suggest the presence of dis- tributary lobes within the upper apron. The lobes emanate from the basin entry point, switch laterally in a compensating fashion as they prograde downslope ...
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... of the broad, eroded area at the top of the lower apron. Isochore and trace shape patterns suggest the presence of dis- tributary lobes within the upper apron. The lobes emanate from the basin entry point, switch laterally in a compensating fashion as they prograde downslope across the top of the lower apron, and then switch to the southeast (Fig. 15B). Sediment waves, levees, and scour flutes are also present at the seafloor and represent depositional environments in the uppermost part of the upper apron (Fig. 9). This surficial expression suggests that the levee and the sediment waves probably formed towards the end of upper-apron deposition, possibly coeval with deposition of the ...
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... waves, levees, and scour flutes are also present at the seafloor and represent depositional environments in the uppermost part of the upper apron (Fig. 9). This surficial expression suggests that the levee and the sediment waves probably formed towards the end of upper-apron deposition, possibly coeval with deposition of the southernmost lobes (Fig. 15B). The scour is ~ 200 m wide and connects to the entry point through a broad (1.8 km wide), shallow trough on the top of the apron just inside the outer levee (Fig. ...
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... expression suggests that the levee and the sediment waves probably formed towards the end of upper-apron deposition, possibly coeval with deposition of the southernmost lobes (Fig. 15B). The scour is ~ 200 m wide and connects to the entry point through a broad (1.8 km wide), shallow trough on the top of the apron just inside the outer levee (Fig. ...
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... sediment waves are deflected around and over the low- relief levee, modifying their shape just downdip of the entry point (Fig. 8). These bedforms appear to have steep lee sides, with a total amplitude of approximately 1.5 m or less and an average wavelength of ~ 62 m across a slope ranging from 1.0° to 0.5° (Fig. 17). The sediment waves disappear approximately 10 km downdip of the entry-point channel. Comparison with measurements of bedforms compiled by Wynn et al. (2002) suggests that they may be sandy. Their relatively high acoustic impedance, and location extending from the channel floor into the proximal apron apex just downdip of the basin ...
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... a flow velocity of the order of 3 m/s, which is sufficient to carry granules and pebbles in the bedload. e es s sc c ca a ap p pe e e f f fe e ea a at t tu u ur r re e es s s e en n nt t tr r ry y y p p po o oi i in n nt t t c ch h ha a an n nn n ne e el l ls s s N N Horizon slice is extracted a few milliseconds below the blue horizon (see Fig. 12 for mapping horizons); gray contours are from the time isochore map of the same interval (Fig. 14A). B) Trace-shape map of seabed reflector showing compensating lobes, levees, and exit-point submarine valley. Note the close correspondence of the trace shape and the thickness of the upper apron (gray contour representation of Fig. 14B). ...
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... the bedload. e es s sc c ca a ap p pe e e f f fe e ea a at t tu u ur r re e es s s e en n nt t tr r ry y y p p po o oi i in n nt t t c ch h ha a an n nn n ne e el l ls s s N N Horizon slice is extracted a few milliseconds below the blue horizon (see Fig. 12 for mapping horizons); gray contours are from the time isochore map of the same interval (Fig. 14A). B) Trace-shape map of seabed reflector showing compensating lobes, levees, and exit-point submarine valley. Note the close correspondence of the trace shape and the thickness of the upper apron (gray contour representation of Fig. 14B). Although seabed cores in the proximal apron were short and recovered no sand, cores downdip of the ...
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... horizon (see Fig. 12 for mapping horizons); gray contours are from the time isochore map of the same interval (Fig. 14A). B) Trace-shape map of seabed reflector showing compensating lobes, levees, and exit-point submarine valley. Note the close correspondence of the trace shape and the thickness of the upper apron (gray contour representation of Fig. 14B). Although seabed cores in the proximal apron were short and recovered no sand, cores downdip of the sediment waves show that sands in the granule and pebble size range are present in this area (Fig. 18). The sediment waves described here are similar to cyclic steps found in bedrock rivers (e.g., Wohl, 2000) and created in flume ...
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... submarine valley. Note the close correspondence of the trace shape and the thickness of the upper apron (gray contour representation of Fig. 14B). Although seabed cores in the proximal apron were short and recovered no sand, cores downdip of the sediment waves show that sands in the granule and pebble size range are present in this area (Fig. 18). The sediment waves described here are similar to cyclic steps found in bedrock rivers (e.g., Wohl, 2000) and created in flume experiments with cohesive and erodible beds (Sawai 1977;Koyama and Ikeda 1998). Cyclic steps occur in net-erosional (Parker and Izumi, 2000) or net-depositional (Sun and Parker 2005) flows. Fildani et al. ...
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... preferred interpretation for the stratigraphic evolution of this shallow-ponded intraslope basin is built on the assump- tion that there are ponded deposits onlapping the intraslope basin floor in OML 134 and that these are overlain by a subma- rine apron that downlaps the top of ponded deposits (Fig. 11). In this scenario deposition began following subsidence of the intraslope basin and creation of ponded accommodation. The presence of older apron units above the dsp horizon suggests that subsidence occurred relatively early and the basin was at least partially filled. Thickening of unconfined slope units above this older apron unit ...
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... alternative interpretation for the stratigraphic evolution of this shallow intraslope basin is built on the assumption that a single ponded apron does not cover the entire intraslope-basin floor, but rather there are multiple ponded aprons, resolved by seismic as the toesets of the prograding clinoforms, that charac- terize the apron (Fig. 19). Under this scenario deposition begins as an apron progrades into ponded accommodation of ~ 30 m depth. The apron clinoform builds angle until bypass begins on its proximal part. Denser parts of sediment gravity flows bypass the upper part of the clinoform and pond in front of the apron and behind the downdip basin sill. Progradation ...
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... If there was any ponded ac- commodation, this phase of deposition would produce a low- relief ponded apron in the basin. If the alternative scenario was the case and no ponded accommodation existed in the basin, then deposition of a perched submarine apron would occur. In either case incisions between the rim of the basin and the basin exit point (Fig. 10) suggest an episode of early sediment bypass as sediment gravity flows downcut the basin sill after available accommoda- tion was filled (Fig. 20A). Early bypass produced truncation at the exit point similar to that seen in the shallow ponded Brazos- Trinity Basin II (Prather et al., this ...
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... tional distributary channels and lobes with multiple plunge-pool scours at the entry point, to an apron characterized by laterally shifting distributary lobes, levees, and sediment waves, suggests a change in the character of sediment gravity flows entering the basin. A channel to lobe transition in the lower apron within the area of the step, (Fig. 15) and an upper unit consisting of small sandy (?) lobes also within the area of the step (Fig. 8), demon- strates that flows entering the step throughout apron deposition were small enough to have sand runout distances less than the slope length across the step. Decrease in the size of lobes and a change from sandy (?) lobes to a muddy ...
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... percent of producing deepwater reservoirs occur in slope aprons ( Prather et al., 2009), of which perched aprons make up a significant proportion (28%; Fig. 21). The OML 134 system is therefore a useful analog for a significant subset of producing deep-water reservoirs. If the fundamental control on reservoir architecture and distribution across varied slope pro- files is the interaction between local gradient change and grain size of sediment gravity flows, regardless of absolute position ...
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... interaction between local gradient change and grain size of sediment gravity flows, regardless of absolute position along the slope, then the architecture of the OML 134 perched apron should also be analogous to submarine aprons elsewhere regardless of absolute slope position and to as much as 79% of producing reservoirs from deep-water globally (Fig. ...
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... field (Booth et al., 2003), located at the exit point of the Auger intraslope basin in the Gulf of Mexico. The OML 134 analog shows that sands at exit- point position have the potential of occupying isolated channels associated with early bypass of the step followed by further stratigraphic isolation due to erosion from the overlying knickpoint (Fig. ...
Citations
... However, Prather et al. (2012) suggested that in an "abovegraded" slope such as the Niger Delta slope (i.e., where seafloor "possesses downslope high profile, and shows subtle changes in depositional gradient resulting in low-relief stepped or terraced topography"), sand deposition might take place mainly on the midslope. Some sand could be deposited on the upper slope but with poor preservation potential. ...
... Some sand could be deposited on the upper slope but with poor preservation potential. In Prather et al. (2012), the possibility of trapping sand in the slope was not considered as the result of canyon processes but rather the occurrence of ponded basins formed in the piggyback of gravitational thrusts. Steffens et al. (2003) demonstrated that shale-based stepped slopes (e.g., Niger Delta slope) had little ponded basins. ...
... Reynaud, V. Delhaye-Prat et al. Energy Geoscience 5 (2024) 100293 with gravity-flow sediment (Damuth, 1994;Morgan, 2004;Bilotti et al., 2005;Corredor et al., 2005;Wiener et al., 2010;Prather et al., 2012). The eastern Niger Delta slope has a dynamic equilibrium because the movements of mobile shale create constant structural highs smoothed by gravity-flow sediment that accumulates in the adjacent intraslope basins (Bakare, 2006;Prather et al., 2012;Clark and Cartwright, 2012;Chima et al., 2019). ...
... For instance, it is documented that structural growth can control submarine channel behaviour, including confinement, deflection, diversion and incision (e.g., Ashiru et al., 2020;Clark & Cartwright, 2009, 2012a, 2012bDeptuck et al., 2007;Don et al., 2020;Gee & Gawthorpe, 2006;Huyghe et al., 2004;Jolly et al., 2017;Mayall et al., 2010). Sedimentation rates, architectures and facies, including ponding of sand have all been directly linked to evolving structure at or near the seabed (e.g., Clark & Cartwright, 2011;Doughty-Jones et al., 2017;Fonnesu, 2003;Hansen et al., 2017;Hay, 2012;Howlett et al., 2021;Kane et al., 2010;Mitchell et al., 2021bMitchell et al., , 2022Prather et al., 2012). In many cases, evidence for structural movement (i.e., changes in sea-floor topography) is inferred from stratigraphic relationships such as onlap or rapid thinning of stratigraphic units (e.g., Clark & Cartwright, 2012b;Don et al., 2020;Krueger & Grant, 2011) and/or changes in sedimentary geometries such as channel orientation, channel width or depth (e.g., Clark & Cartwright, 2011;Deptuck et al., 2012;Ferry et al., 2005;Hansen et al., 2017;Oluboyo et al., 2014;Pirmez et al., 2000) and channel-lobe transitions which are interpreted to be due to structurally influenced factors (Hay, 2012;Howlett et al., 2021). ...
In submarine settings, the growth of structurally‐influenced topography can play a decisive role in controlling the routing of sediments from shelf‐edge to deep water, and can determine depositional architectures and sediment characteristics. Here we use well‐constrained examples from the deep water Niger Delta, where gravity‐driven deformation has resulted in the development of a large fold and thrust belt, to illustrate how spatial and temporal variations in the rate of deformation have controlled the nature and locus of contrasting depositional styles. Published work in the study area using 3D seismic data has quantified the growth history of the thrust‐related folds at multiple locations using line‐length‐balancing, enabling cumulative strain for individual structures over time and along‐strike to be obtained. We integrate this information with seismic interpretation and facies analysis, focusing on the interval of maximum deformation (15 to 3.7 Ma), where maximum strain rates reached 7 %/Ma. Within this interval, we observe a vertical change in depositional architecture where: (1) leveed‐confined and linear channels pass upward in to (2) ponded lobes with erosionally‐confined channels and finally (3) channelized sheets. Our analysis demonstrate that this change is tectonically‐induced and diachronous across the fault array, and we characterise the extent to which structural growth controls both the distribution and the architecture of the turbidite deposits in such settings. In particular we show that leveed‐confined channels exist when they can exploit strain minima between growing faults or at their lateral tips. Conversely, as a result of fault linkage and increased strain rates submarine channels become erosional and may be forced to cross folds at their strain maxima (crests), where their pathways are influenced by across‐strike variations in shortening for individual structures. Our results enable us to propose new conceptual models of submarine channel deposition in structurally complex margins, and provide new insights into the magnitude of fault interaction needed to alter depositional style from leveed to erosionally confined channels, or to deflect seabed systems around growing structures.
... Turbidity current re-channelization systems are increasingly being documented in disparate settings around the world by multibeam bathymetric and seismic data (Prather et al., 1998(Prather et al., , 2012Gee and Gawthorpe, 2006;Stevenson et al., 2013) and also in rock outcrop studies (Mutti and Normark, 1987;Mutti, 1992;Posamentier and Walker, 2006;Brooks et al., 2018;Tinterri et al., 2020). First, depositional-to-erosional transitions have been recognized as a consequence of salt-tectonism resulting in confinement/un-confinement of channels and abrupt changes to slope gradient (e.g. ...
... Where no channel exists, they can be incised by turbidity currents. Third, depositional-to-erosional transitions have been shown to develop where there are significant changes to continental slope gradients; lower gradients resulting in turbidity current transport capacity being reduced and therefore deposition before subsequent gradient increases result in turbidity current acceleration and channel incision (Prather et al., 2012;Brooks et al., 2018;Jobe et al., 2017). Here, we identify an additional turbidity current rechannelization mechanism. ...
... Our data show that pre-existing topography, such as seamounts, located on abyssal plain floors are capable of causing sufficient re-focusing of turbidity currents to enable erosion and re-channelization to occur (Fig. 11). It also shows that this can occur unaccompanied by increases in seafloor gradient which have been commonly been identified elsewhere (Prather, 2003;Gee and Gawthorpe, 2006;Prather et al., 2012;Stevenson et al., 2013;Brooks et al., 2018). Most published deepsea examples of turbidity current re-channelization occur in small-scale submarine aprons or intra-slope mini-basins of the continental slope (Gee and Gawthorpe, 2006;Prather, 2003;Prather et al., 2012;Brooks et al., 2018), which are very different from our study of an open abyssal plain. ...
Turbidity currents can be characterized as net-erosive, net-depositional or net-bypassing. Whether a flow is erosive, depositional or bypasses depends on the flow velocity, concentration and size but these can also be impacted by external controls such as the degree of confinement, slope gradient and substrate type and erodibility. Our understanding of the relative importance of these controls comes from laboratory experiments and numerical modelling, as well as from field data due to the proliferation of high-resolution 3D seismic and bathymetric data, as well as the outcrop and rock record. In this study, based on extensive multibeam and seismic reflection surveys in combination with International Ocean Discovery Program cores from the South China Sea, we document a new mechanism of turbidity current transformation from depositional to erosive resulting in channel incision. We show how confinement by seamounts and bedrock highs of previously unconfined turbidity currents has resulted in the development of seafloor channels. These channels are inferred to be the result of confinement of flows, which have traversed the abyssal plain, leading to flow acceleration allowing them to erode the seafloor substrate. This interpretation is further supported by the coarsening of flow deposits within the area of the seamounts, indicating that confinement has increased flow competency, allowing turbidity currents to carry larger volumes of coarse sediment which has been deposited in this region. This basin-scale depositional pattern suggests that pre-established basin topography can have an important control on sedimentation which can impact characteristics such as potential hydrocarbon storage.
... In modern or near modern turbidite systems, distributary channels have been imaged within lobate deposits in some cases (O'Connell et al., 1991;Twichell et al., 1992;Kidd, 1999;Posamentier and Kolla, 2003;Hadler-Jacobson et al., 2005;Clark and McHargue, 2007;Hadler-Jacobson et al., 2007;Bourget et al., 2010;Bakke et al., 2013;Doughty-Jones et al., 2017;Howlett et al., 2020). However, even in modern submarine fan systems, detailed bathymetric records and sidescan sonar recordings often do not produce clear images of distributary channel networks within lobate deposits (Bonnel et al., 2005;Gervais et al., 2006;Jegou et al., 2008;Dennielou et al., 2009;Bourget et al., 2010;Hanquiez et al., 2010;Migeon et al., 2010) even though incisional transient fan channels, when present, may be well imaged (Johann et al., 2001;Adeogba et al., 2005;Gamberi and Rovere, 2011;Maier et al., 2011;Barton, 2012;Maier et al., 2012;Prather et al., 2012a;Maier et al., 2013;Yang and Kim, 2014). ...
... The areas of low gradient (i.e., steps of Prather et al., 1998) occur on the landward sides of the thrust ridges. LE1 accumulated within a sediment wedge on one of these steps in what has been called a slope apron (Gorseline and Emery, 1959;Barton, 2012;Prather et al., 2012a) within healed slope accommodation (Prather, 2000;Prather, 2003;Barton, 2012;Prather et al., 2012a;Sylvester et al., 2012). Figure 5). ...
... The areas of low gradient (i.e., steps of Prather et al., 1998) occur on the landward sides of the thrust ridges. LE1 accumulated within a sediment wedge on one of these steps in what has been called a slope apron (Gorseline and Emery, 1959;Barton, 2012;Prather et al., 2012a) within healed slope accommodation (Prather, 2000;Prather, 2003;Barton, 2012;Prather et al., 2012a;Sylvester et al., 2012). Figure 5). ...
Lobate deposits in deep-water settings are diverse in their depositional architecture but this diversity is under-represented in the literature. Diverse architectures result from multiple factors including source material, basin margin physiography, transport pathway, and depositional setting. In this contribution, we emphasize the impact of differing source materials related to differing delivery mechanisms and their influence on architecture, which is an important consideration in source-to-sink studies. Three well imaged subsurface lobate deposits are described that display three markedly different morphologies. All three lobate examples, two from intraslope settings offshore Nigeria and one from a basin-floor setting offshore Indonesia, are buried by less than 150 m of muddy sediment and are imaged with high resolution 3D reflection seismic data of similar quality and resolution. Distinctively different distributary channel patterns are present in two of the examples, and no comparable distributaries are imaged in a third example. Distributary channels are emphasized because they are objectively recognized and because they often represent elements of elevated fluid content within buried lobate deposits and thus influence permeability structure. We speculate that the different distributary channel patterns documented here resulted from different processes linked to source materials: 1) a lobate deposit that is pervasively channelized by many distributaries that have branched at numerous points is interpreted to result from comparatively mud-rich, stratified, turbulent flows; 2) an absence of distributaries in a lobate deposit is interpreted to result from collapse of mud-poor, turbulent flows remobilized from littoral drift; and 3) a lobate deposit with only a few, long, straight distributaries with few branching points is interpreted to be dominated by highly viscous flows (i.e., debris flows). We propose a conceptual model that illustrates the relationship between the proportion of mud in contributing flows and the relative size and runout distance of lobate deposits. We conclude that reconciling 3D seismic morphologies with outcrop observations of channels, scours, and amalgamation zones, and simple application of hierarchical schemes, is problematic. Furthermore, when characterizing unconfined deep-water deposits in the subsurface, multiple models with significant differences in predicted permeability structure should be considered.
... Over the last two decades, significant progress has been made in the understanding of (1) the large-scale stratigraphic architecture and evolution of salt-withdrawal minibasins (Mahaffie, 1994;Pratson and Ryan, 1994;Winker, 1996;Prather et al., 1998;Badalini et al., 2000;Beaubouef and Friedmann, 2000;Booth et al., 2000;Winker and Booth, 2000;Meckel et al., 2002;Sinclair and Tomasso, 2002;Booth et al., 2003;Smith, 2004;Beaubouef and Abreu, 2006;Mallarino et al., 2006;Madof et al., 2009;Pirmez et al., 2012;Prather et al., 2012a, b) and (2) the behavior of turbidity currents entering the basins and the resulting fine-scale depositional features (Brunt et al., 2004;Lamb et al., 2004Lamb et al., , 2006Smith and Joseph, 2004;Toniolo et al., 2006a, b;Khan and Imran, 2008;Viparelli et al., 2012). However, these studies have largely focused on sedimentary processes, and the effects of basin subsidence on the stratigraphic architecture have received limited attention. ...
Intraslope basins, or minibasins, are topographic features of the continental slope that can be filled with sediment transported by submarine flows. These deposits may contain important hydrocarbon reservoirs. Here we present results of two-dimensional numerical simulations of multiple turbidity currents entering two linked minibasins. The numerical model accounts for the non-uniformity of sediment grain size in the flow and the resulting deposit. Model results reasonably reproduce the evolution of linked minibasins illustrated in the field based “fill-and-spill” conceptual model. The conceptual model was developed for the Brazos–Trinity system from field observations. Further, simulations of two linked minibasins show that the upstream basin traps most of the coarse sediment. This material is deposited in the proximal zone of the basin and fine sediment is transported farther downslope, resulting in the formation of a weak pattern of downstream fining. Model results with different initial and boundary conditions reveal that minibasin geometry and turbidity-current characteristics are important controls on the deposit shape and grain-size distribution.
... The classification scheme of Prather (2003) describes the western Niger Delta slope profile as a 'graded slope' characterised by a seafloor whose downslope profile is elevated above the level of theoretical concave-upward, smoothed graded profile. Shale tectonics on the Niger Delta slope give rise to shale/thrust-cored structural highs flanked by mobile shale withdrawal intraslope basins that interact with sedimentgravity flows (Doust and Omatsola, 1990;Damuth, 1994;Cohen and McClay, 1996;Connors et al., 1998;Graue, 2000;Morgan, 2004;Bilotti et al., 2005;Corredor et al., 2005;Wiener et al., 2010;Prather et al., 2012;Fig. 1D, E). ...
... 1D, E). The western Niger Delta slope is in constant dynamic equilibrium as the movements of mobile shale create structural highs which, in turn, are smoothed by sediment-gravity flows that preferentially accumulate within adjacent intraslope basins (Deptuck et al., 2003;Adeogba et al., 2005;Bakare et al., 2007;Prather et al., 2012;Clark and Cartwright, 2012;Chima et al., 2019;Fig. 1D, E). ...
... The distribution of shale/thrust-cored structures on the western Niger Delta slope significantly controls the morphology of submarine channels, depositional patterns, reservoir architecture and distribution (e.g. Adeogba et al., 2005;Bakare et al., 2007;Jobe et al., 2015Jobe et al., , 2016Clark and Cartwright, 2012;Deptuck et al., 2012;Prather et al., 2012;Jolly et al., 2016;Hansen et al., 2017;Chima et al., 2019). The present study was conducted in the translational zone of the western Niger Delta slope at the transition between the extensional and the contractional domains ( Fig. 1A, C). ...
Although climate proxy (δ18O) across the world ocean basins reveals that orbital forcing significantly controlled the Pliocene and the Pleistocene sediment deposition, and has been demonstrated in seismic and outcrop studies on the continental shelves of many margins, few or no seismic stratigraphic studies have investigated orbital forcing on deep-water sediment records. In this study, we combined detailed seismic stratigraphy and 3D geomorphological analysis of a high-resolution 3D seismic block in a detailed study of the stratigraphic evolution of the western Niger Delta intraslope basins over the last 5.5 Ma. Two mega seismic units named MSU 1 and MSU 2 were identified.
The change in sedimentary architecture from (i) mass flows and turbidite sequences to (ii) hemipelagic and turbidite sequences at the MSU 1/MSU 2 transition coincides with a significant (x3) increase in sedimentation rates and a transition from dominant 400 ka eccentricity cycles (from 5.3 Ma-0.8 Ma) to dominant 100 ka eccentricity cycles, at the Middle Pleistocene Transition (MPT) (circa 0.8‐–0 Ma). The timing of these changes was estimated based on a detailed analysis of seismic facies succession, correlation of seismic markers with high-resolution sea-level and oxygen isotope curves, and estimation of sequence duration. Further changes in the sedimentary record, characterised by turbidite-dominated sequences at the lower part of MSU 1 to mixed mass flows and turbidite sequences at the upper part of MSU 1, were respectively correlated with changes that occurred in the early Pliocene (circa 4.9 Ma) and in the early Pleistocene (circa 2.6 Ma).
The depositional sequence on the western Niger Delta intraslope basin is usually characterised by a falling stage erosional surface (FSES) at its base and top (sequence boundary), and by (i) basal MTDs/bypass facies (where preserved), (ii) turbidite feeder channels/aggrading or meandering channel levee complexes and/or MTDs (slides/slumps) and (iii) hemipelagic drapes that successively document the falling stage, lowstand to early transgressive and late transgressive to highstand transits of the shoreline.
The Pliocene and Pleistocene sedimentary records of the western Niger Delta intraslope basins were controlled by interplay between allocyclic forcing linked to glacio-eustatic sea-level oscillations and basin tectonics associated with mobile shale movements.
... Regions with mobile salt substrate and deep-water systems include well-studied basins in the Gulf of Mexico (Smith, 2004;Prather et al., 2012), eastern Mediterranean Levant Basin (Clark & Cartwright, 2009;Niyazi et al., 2018), offshore Brazil, such as the Santos or Espirito Santo Basins (Gamboa & Alves, 2015;Rodriguez, Jackson, Bell, Rotevatn, & Francis, 2020), as well as along the west African margin, such as the Lower Congo or Kwanza Basins (Broucke et al., 2004;Oluboyo et al., 2014). Large-scale tectonic studies often utilise regional 2D seismic reflection data to understand structural development (Marton, 2000;Valle, Gjelberg, & Helland-Hansen, 2001;Tari et al., 2003;Hudec & Jackson, 2004), and these have developed useful methods to analyse the geometry of stratal packages deposited next to mature passive diapirs in local studies (Giles & Rowan, 2012;Pichel, Jackson, Peel, & Dooley, 2020;Rojo & Escalona, 2018). ...
Understanding the evolution of submarine channel‐lobe systems on salt‐influenced slopes is challenging as these systems react to subtle, syn‐depositional changes in sea‐floor topography. The impact of large blocking structures on individual deep‐water systems is well documented, but our understanding of the spatial and temporal evolution of extensive channel‐lobe systems on slopes influenced by relatively modest salt structures is relatively poor. We focus on Late Miocene deep‐water depositional systems contained within a c. 450 ms TWTT thick interval imaged in 3D seismic reflection data from the contractional salt‐tectonic domain, offshore Angola. Advanced seismic attribute mapping, tied to seismic facies analysis and time‐thickness variations, reveal a wide range of interactions between structurally‐induced changes in slope relief, deep‐water sediment routing, geomorphology and sedimentology. Five seismic units record a striking tectono‐stratigraphic within eight minibasins. We observe gradual channel diversion through lateral migration during times of relatively high structural growth rate, as opposed to abrupt channel movement via avulsion nodes during times of relatively high sediment accumulation rate. Our models capture the response of deep‐water depositional systems to the initiation, maturation, and decay of contractional structures on salt‐influenced slopes. The initiation stage is defined by small, segmented folds with deep‐water depositional system being largely able to transverse multiple minibasins. In contrast, the maturity stage is characterised by large, now‐linked high‐relief structures bounding prominent minibasins leading to ponding and large‐scale diversion of channel‐lobe systems and the emplacement of MTCs derived from nearby highs. The decay stage is expressed by structures that are shorter and more subdued than those characterising the maturity stage; this leads to a more complicated array of channel‐lobe system, the evolution of which is still influenced by bypass, diversion and ponding. During the decay stage, remnant structures still exert a subtle but key control on the development and positioning of avulsion nodes.
... Extensive works have been undertaken to address the problems of predicting stratigraphic architecture and patterns of sand deposition on the topographically complex continental slope of Gulf of Mexico Beaubouef, Abreu, & Van Wagoner, 2003;Prather, 2003;Prather et al., 1998;Smith, 2004), Niger Delta (Prather, Pirmez, Sylvester, & Prather, 2012), and offshore Nova Scotia, Canada (Deptuck & Campbell, 2012), all of which are affected by numerous structure-related sub-basins. Some studies (e.g., Badalini et al., 2000;Brunt, McCaffrey, & Kneller, 2004;Prather, 2003;Smith, 2004) termed these sub-basins as "minibasins" or "intraslope basins" and proposed detailed models for succession filling and down-slope spilling of such small-scale depressions. ...
... Delta slope) commonly take the form of long, linear to arcuate, doubly plunging synclines (Prather, 2003;Prather et al., 2012). For such shale-withdrawal minibasins, there is still much to learn, particularly about the associated growth faults and variations of accommodation space. ...
... To the west, the sediment gravity flows were confined by NNE-SSW or N-S trending growth faults and to the south by a frontal slope caused by two mud diapirs (Figure 6a). This geometry, combined with a northern entry and southeast exit point of turbidity flows contributed to an elongated plan shape of Minibasin I and various-shaped lobe complexes (Figure 6), which is very common on the Niger Delta continental slope (Prather et al., 2012). Two infilling scenarios are thought possible. ...
... we focus on the sedimentary processes in intraslope basins, interactions between 481 shale tectonics and submarine channel evolution, depositional patterns and their implications for 482 reservoir distribution. We prefer the term 'filled ponded basins' to 'shallow ponded basins' used 483 byPrather et al. (2012a) in the description of the depositional architecture in the intraslope 484 basins we studied. We think this term is more appropriate as the 3D block we studied is located 485 in a supply-dominated deep-basin between 900-1,150 m, where deep-water processes dominate, 486 and where the high rate of sediment supply filled the accommodation created by shale tectonics. ...
Located on a divergent margin dominated by gravity tectonics above overpressured marine shales, the Niger Delta slope has been described as having a stepped profile characterized by ‘filled ponded basins’ that are prone to erosion and sediment bypass. Previous studies based on 3D seismic data have described the depositional architecture of the western Niger Delta's upper slope, but calibration of the seismic facies is lacking and the timing of major changes in sedimentary record remains elusive. In this study, seismic sequence-stratigraphy, 3D geomorphological analyses of high-resolution 3D seismic data, and bio/chronostratigraphic analyses from four boreholes, enabled the identification and characterization of the depositional architecture in Neogene ‘filled ponded basins’. Seven major seismic units were dated as Chattian, Burdigalian, Serravallian, Tortonian, Middle Pliocene and Middle Pleistocene to the present day. Major changes in the sedimentary record occurred in the Plio-Pleistocene, with the onset of erosive channel levee systems (CLSs) and mass-transport deposits (MTDs) generally capped by a hemipelagic drape. Amalgamated CLSs characterize the Tortonian-Late Miocene while erosive MTDs and CLSs characterize the Plio-Pleistocene units. Thick, laterally extensive MTDs are associated with regional slope instability, while active mobile shale triggered local spatially confined MTDs. Submarine channels evolved from moderate to highly sinuous. The degree of channel confinement generally decreases downstream where they are characterized by abandoned meander loops and avulsion resulting from levee breaching. Channel fills and levees/overbank deposits topped by hemipelagic drapes provide effective reservoir/seal (traps) for hydrocarbons. The alternation of channel deposits and hemipelagic layers indicate that eustasy controlled depositional patterns at a regional scale, while the spatio-temporal switches in submarine channel courses show that shale tectonics locally controlled deposition in intraslope basins.
... Slope breaks tend to occur in ponded basins, on stepped slopes (Prather 2003;Prather et al. 2012a, b;Jobe et al. 2017;Brooks et al. 2018a), and at plunge pools formed at the base of steep active continental margins (Lee et al. 2002;Bourget et al. 2011). When reaching the more gently dipping sea floor, turbidity currents usually switch from bypass to depositional conditions, forming sediment bodies (Mutti & Normark 1987, 1991Amy et al. 2000;Wynn et al. 2000;Lee et al. 2004;Carvajal & Steel 2006;Talling et al. 2007;Prélat et al. 2010). ...
... The Niger Delta slope is an above-grade slope system with a stepped slope topography (Beaubouef & Friedmann 2000;Adeogba et al. 2005;Prather et al. 2012a;Jobe et al. 2017). The stepped slope profile was formed by gravity-induced tectonism in the Neogene (Damuth 1994). ...
... These catchment areas are filled with perched submarine aprons, resulting in a healed-slope profile and disappearance of the steps over time. The perched submarine apron (OML 134) is subdivided into a lower apron and an overlying upper apron ( Fig. 2.10a) (Prather et al. 2012a). At the basin entry point, which corresponds to the slope break, the lower apron is characterized by multiple erosion features (i.e. ...
The principle transport agent in deep ocean environments are turbidity currents, avalanches of sediment and water that travel down the continental slope. Turbidity currents usually flow within deep-marine channels, comparable to terrestrial rivers on land, which can extend for 1000s kilometers across the ocean floor. At the downstream end of these channels are lobe shaped sandy deposits called submarine fans that represent potential reservoirs for hydrocarbons but also a sink for any material transported by the turbidity currents such as microplastic. The internal structure of submarine fans and their location depends on how sediment is deposited by the turbidity currents. Sediment deposition is controlled by the turbidity current dynamics that are strongly affected by changes in the ocean-floor topography across which the turbidity current is flowing. This thesis investigates these effects and links the turbidity current dynamics to the resulting deposition pattern. For this, turbidity currents are physically modeled in the laboratory and exhumed ‘real-world’ deep-marine deposits are investigated in outcrops. Experiments focused on turbidity currents going across a decrease in ocean-floor gradient and explains how these topographic change triggers deposition. A second experiment series dealt with turbidity currents leaving the confinement of a channel and revealed a novel flow mechanism we called ‘flow relaxation’. Flow relaxation describes the lateral spreading and thinning of the flows resulting in erosion and sediment bypass. The experiment results are used to explain sedimentary structures observed in the outcrop and to reconstruct and predict changes of the ancient ocean-floor topography.
