Near-seafloor core and seismic reflection-data from the western Niger Delta continental slope document the facies, architecture, and evolution of submarine channel and intraslope submarine fan deposits. The submarine channel enters an 8-km-long by 8-km-wide intraslope basin, where more than 100 m of deposits form an intraslope submarine fan. Lobe deposits in the intraslope submarine fan show no significant downslope trend in sand presence or grain size, indicating that flows were bypassing sediment through the basin. This unique data set indicates that intraslope lobe deposits may have more sand-rich facies near lobe edges than predicted by traditional lobe facies models, and that thickness patterns in intraslope submarine fans do not necessarily correlate with sand presence and/or quality.
Core and radiocarbon age data indicate that sand beds southward during the late Pleistocene, resulting in the compensation of at least two lobe elements. The youngest lobe element is well characterized by core data and is sand rich, ∼2 km wide × 6 km long, and >1 m thick and was deposited rapidly over ∼4000 yr, from 18 to 14 ka. Sand beds forming an earlier lobe element were deposited on the northern part of the fan from ca. 25 to 18 ka. Seafloor geomorphology and amplitudes from seismic reflection data confirm the location and age of these two compensating lobe elements. A third compensation event would have shifted sand deposition back to the northern part of the fan, but sediment supply was interrupted by rapid sea-level rise during Meltwater Pulse 1-A at ca. 14 ka, resulting in abandonment of the depositional system.
Sand-rich bodies accumulating in unconfined submarine depositional environments are referred to generically as “lobes” (e.g., Deptuck et al., 2008). The progressive stacking of lobes and channels builds submarine fans (Normark, 1970), which are volumetrically the largest sediment accumulations on Earth (Covault, 2011) and host vast hydrocarbon reserves (Piper and Normark, 2001). The traditional facies model for submarine lobe deposits is based on observations from the modern seafloor (Normark, 1970), outcrops of lobe deposits (Bouma, 1962; Mutti and Ricci-Lucchi, 1972; Walker, 1978; Mutti and Normark, 1987; Smith, 1987), and flume tank experiments (Luthi, 1981; Parsons et al., 2002; Cantelli et al., 2011; Fernandez et al., 2014). These observations indicate that lobes display a downstream and axis to off-axis decrease in thickness, grain size, sand content, and sand bed amalgamation. This model has been validated with the recent acquisition of high-resolution seismic reflection and core data (e.g., Deptuck et al., 2008; Jegou et al., 2008).
This facies model, however, was derived from terminal lobes on basin floors with smooth bathymetric profiles. Variations to this facies model have been considered in areas where complex slope and seafloor morphology is present (Mutti and Normark, 1987; Piper and Normark, 2001; Smith, 2004). Complex slope morphology due to tectonics or mobile substrates can result in lobe deposition in areas of intraslope accommodation (Prather et al., 1998; Adeogba et al., 2005; Sylvester et al., 2015). Lobe deposition in intraslope settings has been well documented with seismic reflection data (Pirmez et al., 2000; Adeogba et al., 2005; Prélat et al., 2010; Pirmez et al., 2012; Prather et al., 2012a, 2012b; Sylvester et al., 2012), but few examples have lithologic (i.e., core) and/or age calibration. Using three-dimensional (3-D) seismic data and 35 piston cores with radiocarbon ages, this study documents the facies, architecture, and millennial-scale bed compensation patterns of intraslope lobe deposits on the western Niger Delta continental slope (Fig. 1).
Terminology and Hierarchy of Depositional Elements
Lobe deposits are hierarchical due to compensational stacking, and this study follows the hierarchy developed by Prélat et al. (2010). In order of increasing dimensions and complexity, the hierarchy consists of beds/bedsets, lobe elements, lobes, and lobe complexes (Prélat et al., 2010). Beds are deposited by turbidity currents and have internal sedimentary structures (e.g., Bouma or Lowe divisions; Bouma, 1962; Lowe, 1982). Beds and bedsets stack to form lobe elements; lobe elements are generally a few meters thick and a few kilometers in length and width (Prélat et al., 2010). One or more lobe elements stack to form a lobe, which is fed by a single channel. Avulsion or significant migration of the channel creates a new lobe, and thus a lobe complex (Prélat et al., 2010). This terminology has typically been used to describe base-of-slope lobe deposits. Various other terms have arisen to describe intraslope lobe deposits: “transient fan” (Adeogba et al., 2005), “intraslope lobe” (Flint et al., 2011), and “perched slope apron” (Prather et al., 2012a). We will use the term “intraslope submarine fan” to avoid confusion between lobe dimensions and stacking patterns and the more general term “fan” (Normark, 1970), with the understanding that lobe deposits that constitute an intraslope submarine fan may have different morphology and facies architecture due to their intraslope setting.
DATA SET AND METHODS
An area of ∼100 km2 of 3-D seismic reflection data was interpreted for this study (Figs. 1 and 2). The 3-D survey consisted of prestack, time-migrated, 90°-phase-rotated (quadrature) data, with a bin spacing (i.e., horizontal resolution) of 12.5 m (x) × 18.75 m (y). The dominant frequency was 60 Hz, resulting in a vertical resolution, or tuning thickness, of ∼8.3 m. All reported thicknesses were converted from time to depth using a compressional velocity of 2000 m/s, characteristic of shallowly buried deep-marine sediments (e.g., Flood et al., 1997). Thirty-five piston cores were collected in the study area (Fig. 2) with an average length and recovery of 4.4 m and 72%, respectively. Figures DR1 and DR2 show detailed core descriptions, photos, X-rays, radiocarbon ages, and more than 100 grain-size samples analyzed using a Malvern Mastersizer 2000 particle size analyzer.1 In order to attain radiocarbon ages, samples were taken from 6-cm-thick muddy intervals in the cores and trimmed to avoid contamination from core edges. These samples were wet sieved, and the residue was picked to obtain ∼10 mg of the near-surface dwelling planktonic foraminifera Globigerinoides ruber. Picked foram samples were pulse-sonicated in methanol to remove clays trapped inside the shell and then analyzed at the Center for Accelerated Mass Spectrometry at the Lawrence Livermore National Laboratory. The ages were calibrated and reservoir corrected using Calib 7.0 (Stuiver et al., 2005) and the MARINE13 calibration data set (Reimer et al., 2009). A standard 400 yr marine reservoir age was applied to all ages, as no local refinements are available. All quoted ages are given in yr B.P. (see Table DR1 [see footnote 1]).
Modern Niger Delta
The Niger Delta is one of the largest sediment accumulations in the world (∼140,000 km2 and 12 km thick) and is a prolific hydrocarbon province (Allen, 1964, 1965; Evamy et al., 1978; Doust and Omatsola, 1989). The Niger River has a large but semiarid drainage area of 1.2 × 106 km2, providing the delta with an annually averaged discharge of 6140 m3/s and sediment load of 1270 kg/s (Mulder and Syvitski, 1995). Rapid Neogene sedimentation and associated progradation created gravity-induced tectonism that has resulted in significant intraslope accommodation (Damuth, 1994). The study area is located in ∼1200 m water depth on the continental slope of the western Niger Delta, where shale diapirs and ridges are common features that create intraslope accommodation (Fig. 1).
X Channel and Intraslope Submarine Fan
Pirmez et al. (2000) first studied the modern turbidite depositional systems in the study area, identifying the X, Y, and Y′ channels (Fig. 2). The X and Y′ channels are tributaries to the Y channel (Fig. 2). Seafloor bathymetry and core data were used as inputs for a 3-D numerical model that simulated turbidity currents for the Y channel (Abd El-Gawad et al., 2012a) and X channel (Abd El-Gawad et al., 2012b). The Y channel shows distinct temporal changes in stratigraphic architecture related to variations in sediment supply and tributary activity (Jobe et al., 2015). This study focuses on the X channel, which flows southwest for ∼80 km from the shelf edge and crosses a complex slope profile (Fig. 2; Fig. 4 in Prather et al., 2012a). The X channel terminates at ∼1200 m water depth at an abrupt decrease in slope caused by shale diapirism that creates an intraslope basin (Fig. 2; Pirmez et al., 2000; Prather et al., 2012a). The intraslope submarine fan occupying this intraslope basin (hereafter the X fan) has high seafloor amplitudes and a roughly circular shape with a diameter of ∼8 km and an area of 76 km2 (Fig. 2C). The deposits of the X fan are more than 100 m thick, and Prather et al. (2012a) subdivided the fill of the X fan into two units (Fig. 3). A core on the muddy edge of the X fan has been the focus of a West African paleoclimate study by Parker et al. (2016). Moving downslope from the X channel terminus and onto the X fan, seismic reflection character becomes increasingly continuous (Fig. 3C). The increasing slope gradient at the distal edge of the X fan causes the formation of multiple knickpoints that coalesce into a steep, short channel segment that then joins the Y channel (Fig. 2C). The presence of this “exit” channel and a sharp decrease in seismic amplitude indicate that flows are bypassing the X fan and eroding its distal edge (Figs. 2C and 3C).
X CHANNEL FACIES AND ARCHITECTURE
The X channel has an average width of ∼360 m (calculated from cross sections taken every 500 m along the channel reach) and has low sinuosity (1.25) with dominant meander half wavelengths of 1–2 km, although a few tight bends are evident (Fig. 2). Piston cores were taken in the X channel ∼20 km upstream of its terminus (Fig. 4; Fig. DR1 [see footnote 1]). High seafloor seismic amplitudes suggest sand deposition in the thalweg and near-channel overbank areas (Fig. 2B). Piston cores recovered sand, gravel (up to 5 mm in diameter), and chaotic muddy units in the thalweg of the X channel and interbedded sand and mud deposits in the overbank areas (Fig. 5). Sand deposition occurred from at least 19 ka until ca. 14 ka, and an overlying Holocene muddy drape characterizes the upper 3–4 m of every core (Figs. 4 and 5). The X channel was probably active long before 19 ka, but shallow core penetration does not allow characterization and dating of older channel-related deposits (Fig. 5B). Sand beds are thicker (average 5.4 cm) in the X channel thalweg than in overbank areas (average 3.1 cm; Fig. 4). Visual core descriptions indicate that the thicker thalweg sand beds are also coarser grained, with one bed having granules 5 mm in diameter (inset photo in Fig. 4B). Grain-size analyses from a laser particle size analyzer (histograms in Fig. 4) confirm this trend, with average D10/D50/D90 values in the thalweg cores of 62/175/315 μm versus 77/129/216 μm in the overbank cores. Thick Tabc sand beds show normal grading and an improvement in sorting from base to top (Fig. 4), suggesting deposition from turbidity currents (Bouma, 1962; Lowe, 1982). A muddy, ∼1-m-thick muddy unit with sheared fabric and discontinuous silty laminae (Fig. 4B, core bend 4) suggests deposition by mass failure. A radiocarbon age in this mass transport deposit (Fig. 4B) is older than 50 ka, suggesting this unit was derived from older sediments, perhaps a nearby bank collapse or updip slope failure.
The overbank areas on the outer and inner bends of the X channel have very different architecture (Fig. 5). The levee on the outer bank thins and dims away from the X channel on seismic data (Fig. 5B), and cores show sand-bed thinning (e.g., cores bend 2 to bend 7; Fig. 5A), consistent with observations of other external levees (e.g., Hansen et al., 2015). The deposition rate for the distal part of the outer levee is 67 cm/k.y. (core bend 5; Fig. 4A); proximal levee deposition rates are likely much higher (e.g., core bend 2), but poor core recovery prevented sampling. The overbank area on the inner bend of the X channel (Fig. 5B) does not display levee morphology, but rather is a low-elevation terrace, likely caused by lateral migration of the X channel (Figs. 5A and 5B; cf. Babonneau et al., 2010; Maier et al., 2013; Hansen et al., 2015). Low seismic resolution prevents detailed imaging of the internal architecture of the terrace, but core data (core bend 6 in Fig. 5A) indicate a sand-rich environment. The lower elevation of the inner-bend terrace as compared to the outer-bend levee allowed for more than 2 m of amalgamated sand beds to be deposited (core bend 6 in Figs. 4C and 5A). Radiocarbon age correlations indicate that sand deposition on the inner levee terrace was concurrent with outer levee sand deposition and emplacement of intrathalweg sand, gravel, and mass transport deposits (Fig. 5A).
INTRASLOPE SUBMARINE FAN ARCHITECTURE
The X channel terminates into an intraslope basin that contains a sediment body that is 8 km × 8 km × 120 m thick, termed the X intraslope submarine fan (X fan; Fig. 2C). Bathymetric cross sections spaced every 500 m (Fig. 6) and three seismic cross sections (Fig. 3D) show mounding of the proximal area of the X fan (near the channel mouth), while the distal area is relatively flat. The proximal mounding is roughly symmetrical and emanates from the X channel terminus (Figs. 6 and 7A). Slope gradients on the X fan are low (Fig. 7A; average gradient 1.4°, and 82% of values are <2°). The mouth of the X channel has sediment waves (Fig. 17 in Prather et al., 2012a), and the northern levee continues onto the X fan as a ridge that curves and tapers to the south (Fig. 7A). Slope gradient (i.e., dip magnitude) and aspect (i.e., dip azimuth) maps (Fig. 7) clearly delineate this ridge and show the continuity from the X channel to the southern part of the X fan. Immediately downstream of the termination of the ridge, there are two large (up to 1000 m long) scours, the larger of which is cored (core fan 11, Figs. 7A and 8). This seafloor morphology suggests that the most recent flows exiting the X channel were directed onto the southern portion of the X fan, and the aspect (i.e., dip azimuth) map (Fig. 7B) reveals a lobate feature, 2 km × 6 km, emanating from the X channel. In contrast, the northern part of the X fan is underfilled (left side of Fig. 6) and consequently has lower seafloor amplitudes (Fig. 2C). The presence of sediment waves suggests deposition from alternating segments of supercritical and subcritical flow, with hydraulic jumps in between (cf. Covault et al., 2014.). The sediment waves are developed close to the channel mouth, where the supercritical channelized flow becomes subcritical due to the sudden decrease in slope gradient and loss of confinement. At the distal edge of the X fan, multiple knickpoints coalesce into an exit channel (Fig. 7A) that eventually joins the Y channel (Fig. 2C). The presence of this exit channel indicates that flows entering the X fan from the X channel were bypassing at least some volume of sediment. The presence of large seafloor scours (Fig. 7A) supports the interpretation of bypassing flows (Kane et al., 2009).
Sand Distribution and Characterization from Core and Seismic Data
Core data indicate that areas with high seafloor amplitudes are sandy (e.g., X channel and fan), while areas with low amplitudes are predominantly muddy (Fig. 2). To compare core data from the study area, we used the net-to-gross ratio (hereafter N:G), calculated from each core as the summed thickness of sand beds divided by the total “gross” thickness of the core (Fig. DR2 [see footnote 1]). We excluded the hemipelagic drape in each core from the N:G calculation in order to more accurately delineate lithology when the system was active. There is no significant proximal to distal trend in N:G (see bubble plot in Fig. 7A). However, a lateral increase in N:G is evident from north to south (Fig. 7A). Nine cores, mostly from the southern X fan, have N:G values larger than 0.9, while in the northern part of the X fan, core N:G values are less than 0.5 (Fig. 7A). The southern cores also have greater mean sand bed thicknesses than the northern cores (16 vs. 7 cm; bubble plot in Fig. 7B). Sand beds are normally graded and exhibit structureless bases (Ta/S3 divisions) with parallel laminated (Tb) and current ripple cross-laminated tops (Tc), interpreted as the deposits of turbidity currents (Fig. 8; Bouma, 1962; Lowe, 1982). Bed amalgamation and mud clasts are common (Figs. 8A–8B). Grain-size analyses show a clear progression of increasing sorting and decreasing grain size from the base to the top of thick-bedded turbidites (Fig. 8D) and confirm the presence of amalgamation surfaces described in the core (Fig. 8B). Generally, sand beds deposited on the X fan are coarser grained than in the X channel; grain-size analyses demonstrate that the average D10/D50/D90 on the X fan is 76/160/444 μm (n = 118), while the X channel is 75/130/221 μm (n = 88). The difference in the coarse fraction (e.g., D90) is especially apparent when comparing grain-size histograms between channel and fan (cf. Figs. 4 and 8). This indicates that most of the coarser-grained fraction is likely bypassing the X channel, with only a coarse-grained bypass lag deposited in the channel thalweg (e.g., Fig. 4B). Interestingly, there is no significant trend in sand grain size across the X fan, indicating that unconfined/nonchannelized flows carried even the coarsest grains in the flow for 8 km to the distal edge of the intraslope basin (Fig. 7). Supporting this inference are high am plitudes in the exit channel downdip of the X fan (Fig. 2C) and thick-bedded, corase-grained sand beds in the most distal X fan core (core fan 7, Fig. 7).
Turbidite Bed Stacking at the Millennial Scale
The 28 piston cores provide excellent lithologic calibration for lobe deposits in the X fan (Fig. DR2 [see footnote 1]). Because cores average 4.3 m in length, the 3-D seismic data could not be used for correlation due to ∼8 m vertical resolution (Fig. 3). Consequently, extensive radiocarbon dating of the piston cores provides the basis for intercore correlation and calculation of sedimentation rates across the X fan. Given that most cores have >1 km spacing, correlation of individual sand beds is sometimes uncertain, but bedsets (i.e., time-equivalent packages of sand beds) are easily correlated (Fig. 9). Time lines for 14, 18, and 25 ka (shown as dashed lines in Figs. 9 and 10) were calculated by assuming a linear sedimentation rate between ages in each core and that the top of each core has an age of 0 ka. Strike-oriented core cross sections in Figure 9 show a prominent younging of sand beds and bedsets from north to south. No sand was deposited on the northern X fan younger than 17 ka, while sand beds as young as 13.9 ka are present on the southern X fan (Fig. 9). In the most proximal strike section, sand beds are thicker and younger on the southern X fan (Fig. 9A). The youngest sands on the northern and southern X fan are 17.0 ka (core fan 15) and 15.2 ka (core fan 1), respectively, while X channel mouth sands have ages similar to the southern X fan (e.g., 13.9 ka in core fan 16 3 inch; Fig. DR2 [see footnote 1]). Core fan 17 consists of mud throughout this time interval (Fig. 9A), demonstrating the pinchout of sand against the southern basin margin (see also Figs. 2C and 3D). Core fan 17 was also used to constrain longer-term deposition rates using oxygen isotope methodologies (see Parker et al., 2016). Radiocarbon ages from the medial strike section (Figs. 9B) indicate that sand deposition shifted southward at least 4 km in ∼4 k.y., from 18.0 ka to 14.1 ka. The distal strike-oriented core cross sections display the same southward-younging trend (Figs. 9C–9D), although poor core recovery (likely due to the presence of amalgamated, thick-bedded sands) prevents full characterization. The southward-younging trend is also observed in dip-oriented core cross sections, where the youngest sand on the northern X fan is ca. 16.9 ka (Fig. 10A), while sand beds on the southern X fan are as young as 13.9 ka (Figs. 10C–10D).
Back-stepping is also observed in deposits on the X fan, best shown in dip-oriented core cross sections (Fig. 10). The central dip-oriented core cross section most clearly shows the back-stepping, with progressively younger sand beds deposited toward the mouth of the X channel (19.6 ka to 13.9 ka; Fig. 10C). The southern dip section shows a similar back-stepping trend, with distal ages of ca. 16 ka and proximal ages of 13.9 ka (Fig. 10D). The shorter duration of back-stepping on the southern X fan is further evidence that it was most recently active (Figs. 7B, 9, and 10). Back-stepping has been observed in other lobe deposits on the modern seafloor (Deptuck et al., 2008; Prather et al., 2012b) and in flume tank experiments (Cantelli et al., 2011; Fernandez et al., 2014).
Larger-Scale Stacking Patterns
While this study focuses on millennial-scale event bed/bedset stacking patterns, we briefly describe here the larger-scale stacking patterns on the X fan. Prather et al. (2012a) interpreted two main phases of deposition and mapped lower and upper units (Fig. 3). Both units were deposited in the X intraslope basin (Fig. 3) and are relatively thick for their areal extent (e.g., “confined lobes” of Prélat et al., 2010). The lower unit covers an area of 6 × 8 km and has a maximum thickness of 75 m (Fig. 3). The lower unit is thickest in the center of the X fan (Fig. 3A). The upper unit is areally larger (8 × 8 km) but thinner, with only ∼40 m maximum thickness (Fig. 3B). Covault and Romans (2009) observed a similar trend of successively larger and thinner lobate deposits in the California Borderland. Assuming roughly constant sediment supply, this trend is the consequence of filling a bowl-shaped intraslope minibasin (Sylvester et al., 2015). The upper unit is thickest where the lower unit is thin (Figs. 3A–3B), indicating large-scale compensational stacking of the lower and upper units of the X fan. Linear extrapolation from oxygen isotope age data (core Fan 17; Parker et al., 2016) suggests that the age of the base of the upper unit is ca. 130 ka, roughly coincident with low sea level during marine isotope stage (MIS) 6 (Lisiecki and Raymo, 2005), and the base of the lower unit is ca. 630 ka, coincident with low sea level during MIS 16. These ages assume constant sedimentation rate through time and are thus speculative.
Compensation of Beds and Architectural Elements
This study interprets the character and timing of bed and bedset compensation on a modern intraslope submarine fan. While other studies have described bed and bedset compensation in modern submarine lobe deposits (e.g., Vittori et al., 2000; Deptuck et al., 2008; Jegou et al., 2008; Romans et al., 2009), this study is the first to document higher-resolution, bed-scale compensation in an intraslope basin setting. Our interpretations rely on shallow core penetrations, and while the patterns of compensation are convincing, they are not unequivocal. Additional data from deeper stratigraphy are needed to support our interpretations of bed-scale compensation during the late Quaternary. Our correlations indicate a 4 km southward shift in sand deposition that occurred over an ∼4 k.y. period. This scale of compensation most likely represents the emplacement of two “lobe elements” (terminology of Prélat et al., 2010) on the northern and southern parts of the X fan. The older lobe element was deposited on the northern part of the X fan from ca. 25 to 18 ka (Fig. 9). The most recent lobe element on the southern part of the X fan is ∼2 km wide and 6 km long and can be seen on core cross sections (Fig. 9) and the seafloor aspect map (Fig. 7B). The thickness of the most recent lobe element is at least 1 m but is limited by core penetration; scaling from Prélat et al. (2010) suggests a thickness of 3–10 m. The seafloor geomorphology (Fig. 6) and back-stepping of beds in the most recent lobe element (Fig. 10) suggest that if sand deposition had not been interrupted at 14 ka, a third lobe element would have been deposited on the northern X fan (cf. Cantelli et al., 2011). These lobe elements stack in a compensational manner to form a larger depositional element termed a lobe (cf. Prélat et al., 2010).
The larger-scale stacking patterns in the X intraslope basin seem to be compensational as well (Fig. 3; Prather et al., 2012a). The upper unit is 35 m thick and has a clearly depositional lobate shape (Fig. 3B; Fig. 15B in Prather et al., 2012a), suggesting it can be considered a “lobe” in the scaling and hierarchy of Prélat et al. (2010). In contrast, the lower unit can be considered a lobe complex, as it is thicker (∼75 m), and its thickness pattern is not lobate, but rather mimics the shape and subsidence pattern of the basin (Fig. 3A). This, along with the convergent nature of the seismic reflections (cf. Sylvester et al., 2015), suggests that the morphology of the lower unit records subsidence history rather than depositional morphology. The striking difference between the thickness patterns of the lower (lobe complex) and upper (lobe) units (Fig. 3) demonstrates the concept of the “stratigraphic integral scale” (Sheets et al., 2002; Lyons, 2004; Straub et al., 2009), where individual depositional elements have a process-based thickness pattern (e.g., lobe element in Fig. 7B, and lobe in Fig. 3B), but over longer time scales, the thickness pattern reflects the basin subsidence history (e.g., lobe complex in Fig. 3A). The duration of the lower (∼500 k.y.) and upper (∼130 k.y.) units reinforces the interpretation of the thickness patterns and hierarchical assignments.
Timing of Sand Delivery to the X Channel and Fan
Core data indicate that sand was delivered from the X channel to the X fan from at least 25 ka until 14 ka (Fig. 9). The lack of deeper cores limits knowledge of earlier deposits of the X fan, but we infer that sand was being delivered to the X fan since at least 50 ka, when the nearby Y channel was active (Jobe et al., 2015). Average deposition rates for sandy deposits on the X fan are more than two times higher than muddy deposits (35.8 vs. 13.5 cm/k.y., respectively). The youngest sand beds present in the X channel and fan were deposited at 13.5 ka and 13.9 ka, respectively (Figs. 5C and 9A). The 400 yr difference suggests that the back-stepping trend observed in the youngest lobe element on the X fan (Fig. 10) may have continued up the X channel for ∼400 yr after the end of sand deposition on the fan. This style of progressive updip abandonment has also been demonstrated in nearby turbidite systems (Jobe et al., 2015).
The mechanism forcing the cessation of sand deposition in the study area is interpreted to be allogenic in nature, as no autogenic forcing mechanisms (e.g., submarine channel avulsions) are observed. While we cannot exclude tectonic forcing (e.g., growth fault or shale diapir movement), there is no evidence of tectonic activity at ca. 14 ka. Other allogenic signals, such as relative sea-level and climate change, tend to be propagated very quickly from source to sink (Romans et al., 2009; Covault et al., 2010; also see discussion in Romans et al., 2016). The abrupt cessation of sand delivery to the study area occurred at 14 ka, the same time period during which sea level rose dramatically during meltwater pulse 1-A (∼20 m in 300 yr; Deschamps et al., 2012). From 14.6 to 14.3 ka, global sea level rose from –105 m to –85 m, approximately the current water depth of the Niger shelf edge (Allen, 1964, 1965). This rapid sea-level rise likely caused abrupt flooding of the continental shelf, preventing sand delivery off the shelf edge.
Facies Models for Lobes in Intraslope Settings
Traditional lobe facies models indicate a progressive decrease in sand thickness and N:G away from the lobe axis (e.g., Pyrcz et al., 2005; Deptuck et al., 2008; Prélat et al., 2009). Thickness maps similar to those in Figure 3 are often interpreted as evidence of such trends in internal properties (e.g., Pirmez et al., 2000). This study indicates, however, that the traditional facies model does not apply for lobes deposited in intraslope settings, where flows are often able to bypass large volumes of mud (Fig. 11). Many turbidites on the X fan lack mud caps and well-developed laminated Tb and Tc divisions, probably due to significant bypass of fine-grained sediment. This results in intraslope lobe deposits that are very sand rich (Fig. 11), and thick-bedded, coarse-grained sands are present even in the most distal core location (core fan 7 in Fig. 9D).
While this study documents the sand-rich nature of intraslope lobe deposits (Fig. 11), fine-scale heterogeneity is still present. For example, a scour imaged on Figure 7A affects bed continuity of sand beds inside (core fan 11) and outside (core fan 10) of the scour. These two cores are only 450 m apart, and Figures 8A and 8B show the rapid facies change caused by the scour (cf. Kane et al., 2009). Although we believe that Figure 11 characterizes the differences between intraslope and base-of-slope lobes, exceptions to the sand-rich intraslope fan facies architecture are possible and may include cases where mud-rich flows are dominant or where the intraslope basin is large compared to the flow size (e.g., Prather et al., 2012b). For natural resource prediction and production, the X channel-fan system provides a well-constrained example of subseismic heterogeneity that is vital in understanding interwell connectivity in reservoirs contained in intraslope submarine fan deposits.
Core and seismic data on the western Niger Delta continental slope illustrate the facies architecture and temporal evolution of a linked channel-lobe depositional system in an intraslope environment. The X channel has low sinuosity and is ∼360 m wide, with thalweg deposits consisting of thick-bedded sand and gravel interbedded with chaotic muddy units, and overbank deposits consisting of thin-bedded, fine-grained sands and muds. The intraslope submarine fan (X fan) at the terminus of the X channel is 8 km long × 8 km wide and contains >100 m of intraslope lobe deposits sourced by the X channel. There are two distinct large-scale units imaged by 3-D seismic data. The lower unit is 6 × 8 km by 50 m thick and is thickest in the basin center, while the upper unit is 8 × 8 km by 25 m thick and has a pronounced southward shift in its maximum thickness. This compensation is at a large (i.e., lobe or lobe-complex) scale and may be linked to sea-level variation during the last glacial cycle (130–0 ka).
Core data indicate that deposits on the X fan are more sand rich, thicker bedded, and coarser grained than in the X channel, implying bypass on steeper, channelized slope gradients and deposition on lower, unconfined slope gradients. There is no significant downslope trend in grain size or net-to-gross ratio on the X fan, indicating that flows were likely bypassing some sand and most mud through the intraslope basin. This resulted in more sand-rich deposits and partial sediment bypass than predicted by traditional facies models for lobes, particularly near lobe/fan edges. The most recent flows exiting the X channel were directed onto the southern fan, building a lobe element ∼2 km wide by 6 km long, and at least 1 m thick. Deposition rates average 36 cm/k.y., but they are locally greater than 80 cm/k.y. This most recent lobe element was emplaced in only 4000 yr, from 18 to 14 ka. Prior to the deposition of the most recent lobe element, flows from the X channel were directed onto the northern part of the X fan, from ca. 25 to 18 ka. This bed-scale compensation shifted the locus of sand deposition 4 km over millennial time scales (4 ka). Radiocarbon dating, seafloor geomorphology, and 3-D seismic amplitudes confirm the location and age of these two compensating lobe elements. Sand deposition in the X channel/fan was interrupted by rapid sea-level rise during meltwater pulse 1-A at ca. 14 ka, causing abandonment of the system.
This unique data set constrains the facies and stratigraphic architecture of bed- and lobe-scale compensation in an intraslope basin setting. Core data indicate that intraslope lobe deposits can have very different facies architecture than predicted by traditional lobe facies models, and that thickness patterns do not necessarily correlate with sand presence and net-to-gross ratio. Further work aims to incorporate simple analytical modeling to quantify the effects of sediment flux on lobe compensation in intraslope submarine fans.
We would like to thank Shell International Exploration and Production and Shell Nigeria Exploration and Production Company for permission to publish; Fugro for collection and processing of autonomous underwater vehicle data; Mary McGann and Diablo Valley Geological Services for processing core samples, Tom Guilderson at Lawrence Livermore National Laboratory for radiocarbon dating, Texas A&M University for core storage and sampling materials, and TDI-Brooks for collection of core data. Jake Covault, Amandine Prélat, and Larissa Hansen provided insightful reviews that greatly improved the paper. Finally, this paper benefited from discussions with Daniel Minisini, John Martin, Steve Hubbard, Brian Romans, and Neal Auchter.
↵§ Present address: Department of Geology, Colorado School of Mines, Golden, Colorado 80401, USA.
↵# Present address: Chevron Energy Technology Company, Houston, Texas 77002, USA.
Science Editor: Aaron Cavosie
Associate Editor: Henning Dypvik
- Received 6 October 2015.
- Revision received 23 May 2016.
- Accepted 12 July 2016.
- © 2016 Geological Society of America