Abstract The study of near-seafloor deepwater landscapes and the processes that form them ( Normark, 1970 ; Piper, 1970 ; Coleman and Bouma, 1984 ) are as important to the understanding of deeply buried marine depositional systems as the study of modern fluvial environments is to our understanding of ancient terrestrial depositional systems. In fact, these near-seafloor studies follow in the great tradition established by earlier clastic sedimentologists (e.g., Fisk, 1947 ; Lowman, 1947 ; Shepard et al., 1960 ; Shepard, 1964 ; Bernard and LeBlanc, 1965 ; Le Blanc, 1975 ) in using modern systems to understand ancient environments. The acquisition of exploration 3D seismic surveys over the last few decades represents a significant advancement in data quality and density that can be used to push forward our community’s general understanding of slope and base-of-slope systems. High-frequency content near the seabed in 3D seismic volumes allows the display of seafloor geomorphology with a spatial resolution comparable to most deepwater multibeam bathymetric tools ( Mosher et al. 2006 ). Although near-seafloor depositional systems are imaged at a lower resolution than outcrops, they provide 3D information typically lacking from surface exposures. Near-seafloor seismic data allows the recognition of surfaces related to episodes of starvation, bypass, and/or erosion that control both reservoir bed-length and connectivity.

Abstract The present-day continental slope offshore Brunei Darussalam (NW Borneo) displays several networks of submarine channels possessing planform attributes similar to those observed in better-studied river systems. We use shallow 3D seismic data to study one tributary network in detail. This network is located directly downslope from the shelf-edge Champion Delta and encompasses an area approximately 8 km by 24 km in the strike and dip directions. The channels in this network initiate 1–2 km down dip of the shelf edge and are not directly linked to a terrestrial river system. Mapping of shallow seismic horizons reveals that the tributary channel network is an aggradational feature constructed on top of a relatively smooth slide plane associated with a large mass-failure event. This observation highlights differences between network construction in submarine settings compared to terrestrial settings where tributary networks are net erosional features. The smooth slide plane provides us with the simplest possible initial condition for studying the deposit architecture of an aggradational submarine-channel network. An isopach map between the seafloor and the slide plane is used to unravel sedimentation trends, particularly relative rates of levee and overbank sedimentation as a function of channel relief, lateral distance from the nearest channel centerline, and distance from the shelf edge. We observe an anti-correlation between channel relief and deposit thickness, which suggests that the degree to which currents are confined within channels exerts a first-order control on local deposition rates. We also find that over 80% of the deposit volume associated with the aggradational network is within levees. Observations suggest that this channel network was constructed from turbidity currents that initiated at the shelf edge as sheet flows prior to transitioning down slope into weakly confined flows through the construction of aggradational channels. Thicknesses of channel-forming turbidity currents are estimated using the distance between channel heads and the ratio of channel to overbank deposit thickness. These two methods yield estimates for flow thicknesses that are between 1.1 and 3 times the mean relief of channels in the network.

Abstract The Pleistocene Fuji–Einstein system in the northeastern Gulf of Mexico consists of a shelf-edge delta that is directly linked to and coeval with two submarine channel–levee systems, Fuji and Einstein. There is a continuous transition between the channel fills and the delta clinoforms, and the seismic reflections of the prodelta are continuous with the levee deposits. Five smaller delta lobes within the Fuji–Einstein delta formed through autocyclic lobe switching that was superimposed on a single falling-to-rising sea-level cycle. The corresponding stratigraphic complexity is difficult to interpret in single downdip seismic sections, especially where elongated mudbelts are attached to some of the delta lobes. The two slope channel systems, Fuji and Einstein, deeply incise the shelf-edge delta. However, late-stage delta progradation was coeval with slope-channel development, and, as a result, there is no easily mappable, single erosional surface separating channel deposits from deltaic sediments. During early delta-lobe development, a gully field forms on the upper slope, directly downdip from the delta lobe. As the delta progrades, one of the larger gullies in the middle of the field captures most of the denser flows and gradually evolves into a sinuous channel. The larger delta-related slope channels source 2–4 km-wide submarine aprons where they encounter areas with lower gradients. If the slope gully or channel remains active for a long enough time, its corresponding submarine apron smooths out the slope and becomes incised by the later bypassing flows. The well-preserved and mappable 3D shelf-edge architecture provides a rare opportunity to understand relationships between deltaic and slope depositional systems.

Abstract Large-scale glacial meltwater discharges have long been recognized as important sedimentological agents on the eastern Canadian continental margin. Previous studies in Eastern Valley of Laurentian Fan and Orphan Basin have elucidated aspects of processes and timing of glacial discharges, principally from seismic-reflection profiles and deep-water sidescan sonar. New multibeam bathymetry and piston cores show evidence of important meltwater processes seaward of all transverse troughs on the continental shelf, from Hudson Strait to the Scotian margin. Meltwater cuts broad flat-floored valleys and sculpts residual buttes, depositing thick-bedded gravel and sand turbidites, and builds submarine fans. Based on morphology, a wide range of scales of meltwater discharge may take place. Meltwater is intimately linked with supply of fluid glacial diamict (till) that on gentler slopes (< 2.5°) creates glacigenic debris flows but on steeper slopes breaks up, entrains water, and transforms to create erosive turbidity currents. Three end-member processes are recognized on submarine fans seaward of transverse troughs that were occupied by ice streams: glacigenic debris flows, turbidity-current deposition of channel–levee complexes, and blocky mass-transport deposits resulting from debris avalanches. The relative importance of meltwater appears greater at lower than at higher latitudes, whereas the formation of glacigenic debris flows is dependent on gradient. Pleistocene processes have resulted in slopes that are graded, implying that most sand deposition was on the continental rise.

Abstract The Brazos–Trinity depositional system consists of four linked late Pleistocene intraslope basins (I–IV) located on the upper slope, offshore Texas, U.S.A. Conceptual understanding of the fill history in these basin include “fill and spill” models where basins fill sequentially in the seaward direction, to models which invoke coeval basin filling with the coarse fraction retained preferentially in the up-dip basins and synchronous early bypass of the fine fraction to down-dip basins. Integration of recent coring results with nearly complete 3D seismic coverage has improved age dating and reconstruction of infill history. Initially sediment gravity flows bypassed the upper basins, as they had not yet formed, depositing a basal sandy unit in Basin IV. Higher net/gross sands in Basin II deposited from mixed flows, with mud suspended high within the flows exiting Basin II through a tributary-like flow-gathering zone near the basin exit point. The muddy parts of these flows preferentially accumulated in the lower part of Basin IV, which was a three-dimensionally closed basin with deep ponded accommodation. In contrast, the upper fill in Basin IV comprises a submarine apron that is sourced by a continuous channel system that extends directly from a shelf-margin delta located in Basin I. Within this apron, the observed seaward tapering is controlled by lower-efficiency sandy sediment gravity flows of relatively small volume with respect to basin size. These observations allow us to distinguish perched aprons from ponded aprons, with direct implications for reservoir continuity. We further recognize that low-relief ponded aprons have lower ratios of sand net/gross than either high-relief ponded aprons or perched aprons.

Abstract A series of four intraslope basins linked by submarine channels in the northwestern Gulf of Mexico form part of a source-to-sink depositional system that starts in the headwaters of the Brazos and Trinity Rivers and terminates in a ponded intraslope basin offshore Texas—the Brazos–Trinity depositional system. The system is well imaged with 3D seismic data, and two of the basins have been drilled, with three Integrated Ocean Drilling Program wells and two geotechnical wells. Using an integrated approach, we have combined seismic-litho-bio-tephro-stable-isotope-radioisotope stratigraphic methods, using both new and published results, to generate a millennial-scale-resolution chronostratigraphy for this system. Basins I through IV are infilled with about 62 km 3 of sand-rich sediments (∼ 1.6 ×10 11 metric tons) transported by sediment gravity flows since the last interglacial (Oxygen Isotope Stage 5e). The bulk of the sediments, about 49 km 3 , were deposited within a short time period within Oxygen Isotope Stage 2, starting at 24.3 ka at the latest and ending at ∼ 15.3 ka, before meltwater pulse 1A. Sediment accumulated in the slope basins at rates which varied over time between 1.4 and 55 million tons per year. Except for a short time interval when the Brazos River was diverted to the shelf edge at the head of Basin 1, sediment flux to deepwater was on average less than the present-day sediment discharge of the Trinity–Brazos–Sabine Rivers combined. In the period 24-15 ka the sediment sinks comprising the slope basins and shelf-margin delta can be balanced against the fluvial sources if their discharges are somewhat lower than present day, and if the contribution from incised-valley erosion was relatively small. The history of sedimentation on the slope basins is modulated by sea-level changes, but it is strongly influenced by basin topography and by the dynamics of delta development on the shelf. During peak high stands of sea level the slope area receives only pelagic sediments; during low sea-level stands, the sedimentation in each basin results from a complex combination between fluvial input at the head of the first basin, and the rate of subsidence/sedimentation causing basin topography. The ages of sediments in separate basins show that sedimentation occurs at the same time in multiple basins with trapping of sand in updip basins, while mud is preferentially deposited in downdip basins.

Abstract 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 underlying slope muds. Flows spread, becoming depositional once reaching lower-gradient area within ponded accommodation. Incisions at the distal end of the basin suggest that gravity flows downcut the basin sill as they bypass the basin during filling of ponded accommodation. A channelized apron downlaps the ponded deposits, healing the stepped topographic profile formed after ponded accommodation fills. Collapsing flows exiting the entry-point channel create plunge-pool scours in the proximal part of the apron. Sediment gravity flows exiting the plunge-pool scour accelerate over the steeper face of the apron, eroding bypass channels as healing progresses. Avulsion takes place as the height of the lower apron unit builds, forcing flows to bypass and erode the southwestern flank of the lower apron. Avulsion leads to deposition of an upper apron unit. Throughout deposition of the aprons, flows leave the basin through a gather zone at the exit point of the basin, forming a tributary scour pattern. Acceleration of these flows as they top the basin sill forms a deeply incised submarine valley. Erosion of the sill progresses by headward-migrating knickpoints that truncate apron deposits.

Abstract This paper documents the architecture of basin fill and depositional evolution along a stepped, above-grade profile in the Kwanza Basin, Angola Block 21. Detailed mapping of a well-imaged, near-seafloor seismic dataset reveals the influence on preserved depositional architecture of evolving slope gradient, along corridors flanked by pronounced lateral confinement. Sediment gravity flows entered the upper-slope to mid-slope basin system via an incised valley and interacted with the above-grade stepped topography. Effects of slope topography are most prominent during the early stages of deposition, such that flow contraction occurs over highs and flow expansion occurs in topographic lows. Evidence of sediment bypass, knickpoint migration, and erosion immediately outboard of areas of deposition supports the interpretation of an open stepped slope profile; there is only limited ponded accommodation in the study area. Initial flows into the system were deposited in both updip and downdip basins, suggesting a weakly confined stepped profile with flow stripping and/or bypass between basins. Ongoing deposition healed the slope profile, with subsequent flows bypassing to downdip basins. Preserved depositional architecture is dominated by distributary channel–lobe complexes that comprise offlapping, shingled lobes deposited downdip of slope breaks, progressively wrapping around the salt topography. Lobes are more numerous, decrease in size, and increase in degree of channelization up through the section. Bypass initially occurred through numerous narrow channel stories that converge to a wider, single through-going system. With time, and sediment progradation, the locus of deposition shifted progressively basinward, away from basinal highs. Fan apron stacking patterns are dominantly progradational, but fan packages are retrogradational prior to feeder-channel avulsion. Initial flows from each of the channels produced large-volume, highly channelized lobes. Prior to abandonment of the channel system, the flows are smaller and possibly muddier, and deposits are less channelized. Channel avulsion and local tectonism had a first-order control on the locus of deposition in the area, and caused an incomplete depositional cycle that deviates from predictions according to classic stepped-slope models.

Abstract A high-resolution 3-D seismic dataset from the offshore Niger delta slope was utilized to study the stratigraphic architecture and evolution of a near-seafloor intraslope apron that overlies an abrupt break in slope. Elements that constitute the apron are from oldest to youngest: (1) a package of prograding lobes, (2) a complex of laterally offset stacked channels, and (3) a sinuous deeply incised bypass channel. Apron evolution reflects the adjustment and response of sediment gravity flows to an evolving slope gradient. Lobes are deposited as flows enter the basin and encounter an abrupt decrease in slope, decelerate, and lose confinement. As the step is healed, flows remain confined and form channels. Eventually, the apron becomes a site of erosion and bypass as down-dip basins become linked by a common graded profile. A comparison with published examples of slope aprons suggests that the geometry of the step may impact the architecture of the apron. Aprons formed above mild breaks in slopes should be thinner, more channelized, and potentially more dissected then aprons formed above severe breaks in slope.

Abstract The morphology of a 1250 km 2 portion of the middle slope off the western Niger Delta shows that gradients on the Pleistocene slope vary both spatially and at different stratigraphic levels. In the deeper section, three lower-gradient steps are connected by three higher-gradient ramps, generating a stepped-slope morphology. Through time, preferential accumulation of slope aprons, composed of mass-transport deposits, compensationally stacked lobes, and overbank deposits (wedge-shaped outer levees), helped fill slope accommodation, smoothing over the gradient change across ramps and steps, and vice versa. Consequently at the local scale, the stepped slope evolved into a smoother slope that is nearly graded at the modern seafloor. As in other studies, preferential accumulation of sediment on the slope is believed to reflect in part the deceleration of sediment gravity flows (both turbidity currents and debris flows) as they encountered lower-gradient steps. Down-slope changes in slope morphology also caused variations in the amount, and presumably rate, of erosion along the axes of canyons in the study area—with increased incision depth where knickpoints cut through positive-relief bathymetric structures in an attempt to establish a graded profile. Along the Benin-major Canyon there is an inverse linear relationship between the thickness of deposits that accumulate on the slope adjacent to the canyon and the amount of vertical erosion along its axis. The thickest outer levee deposits coincide with canyon segments that have the shallowest incision, in turn corresponding to slope segments showing a sharp decrease in pre-incision gradient. This implies that the increase in sediment flux to outer levees on some parts of the stepped slope results from a combination of increased overspill from flows passing through shallower canyon reaches, and increased sedimentation caused as mud-dominated flows decelerated on lower-gradient slope segments immediately adjacent to the canyon. Thus there appears to be an intimate relationship between slope morphology, canyon incision depth, and the thickness of overbank deposits adjacent to canyons.

Abstract Apron 1 in the Shallow Auger Fan System records the transition from ponded deposition in the lobe complex to bypass at the top of the channel complex. The lobe complex, at the base of Apron 1, exhibits characteristics typical of ponded apron deposits: it onlaps the basin margin, exhibits a concentric isopach pattern, has a lobe geometry in amplitude extraction, and is composed of continuous seismic reflections that have uniform thickness. The transition from ponded deposition to bypass is recorded with increasing gradients along the four channels in the channel complex. Healed-slope accommodation is filled as these channels aggrade at the sediment entry point. In the proximal reaches, the channels have thick levee deposits and minimal incision. Downdip, the levee deposit thickness decreases and incision increases. Each channel aggrades to a single gradient; once this gradient is achieved, the channel avulses to an area of greater accommodation. As healed-slope accommodation fills, channel gradients in the proximal reaches decrease and sinuosity increases. In the distal reaches of the channel, greater incision depths may compartmentalize the underlying ponded deposits.

Abstract The deepwater fold and thrust belt of the Western Niger Delta provides an ideal natural setting in which to study interactions between coeval sedimentation and deformation. Deformation in this area takes the form of folding due to the up-dip gravitational collapse of the Niger Delta above the overpressured shale detachment of the Akata Formation. The seafloor relief formed by folding is initially oriented perpendicular to the downslope sediment transport direction. This results in a significant barrier to the basinwards transport of material and the creation of accommodation space within the hangingwall and footwall areas of the fold. Coeval sedimentation during uplift results in deposition of a growth sequence composed of a compensationally stacked vertical succession of mass-transport deposits (MTDs), channel–levee systems (CLSs), and hemipelagic drape deposits (HD). Variations in the along-strike structural style and relief of a large-scale fold c. 40 km in length control variations in growth-sequence geometry. These variations in fold style along strike also determine sediment flow pathways around the positive relief formed at the seafloor during fold uplift. Switching of sedimentation between the two structurally induced flow pathways around the fold is related to the compensational stacking patterns within the hangingwall which cause a shift in flow pathways from one fold edge to another. The combined structural–stratigraphic approach to the interpretation of sedimentation in deepwater fold belts can provide a useful method for reconstructing the development of relief during folding.

Abstract A buried Quaternary channel–levee system (CLS) with unique architectural characteristics was identified and studied through 3-D seismic-reflection data on the lower continental slope of the NW Niger Delta. The CLS runs in an elongated topographic depression, created by the edges of two thick (100 m) mass-transport deposits (MTDs). Because of this confinement, the CLS is forced to run over a ridge 20–70 m tall. More than 3.5 km updip from the ridge, the CLS is ∼ 4 km wide, and displays well-developed outer levees and multiple, strongly meandering channel forms (up to 100 m deep). The depth of the channel forms suggests a minimum thickness of 100 m for the largest flows. An erosional valley 3.5 km long and 2 km wide that cuts 50 m down to the substrate develops immediately updip and downdip from the ridge. A plateau 6 km long and 2 km wide is observed at the top of the ridge. On the plateau, the CLS is ∼ 4 km wide, and consists of thin levees and multiple, strongly meandering channel forms. Packets of high-amplitude reflections (HARs) are widespread at the base of these channel forms, and much thicker (up to 80 m) compared to the HAR packets observed in the CLS updip and downdip from the ridge (up to 40 m thick). Gamma-ray logs indicate that these HARs represent thick channel-sand deposits. These observations indicate that updip from the ridge, turbidity currents went through a hydraulic jump and developed a turbulent bore, which prevented deposition and enhanced erosion. The flows were thinned and spread on the plateau, resulting in extensive sand deposition. On the steep downdip flanks of the ridge, the flows accelerated and became erosional. This interpretation is consistent with flume studies showing that flows are able to surmount and transfer their suspended material over an obstruction with relief less than half the height of the flows.

Abstract Three-dimensional visualization techniques were used to determine the presence of a submarine sinuous channel in the Pleistocene foredeep basin of the Adriatic Sea (Mediterranean). The channel system (Adria Channel) drains the upper slope and forms a leveed-channel system and frontal splays out onto the basin floor. Integration of geomorphic and morphometric studies applied to the seismic data set gives insights into the paleoceanography of the buried basin. In particular, the Adria Channel indicates that, besides the channelized flows, bottom currents swept the lower slope, affecting the overgrowth of levees, the deflection of sediment waves, and the progradation of sediment units draping the channel–levee system. The Adria Channel is the longest channel system recognized in the Pleistocene units of the Adriatic Sea (160 km) and one of the few sinuous channels described in a collisional margin.

Abstract The Kramis deep-sea fan extends over 45 km at the base of the western Algerian continental slope between 2000 and 2550 m water depth and covers an area of approximately 1200 km 2 . The Kramis Fan was initiated after Messinian time, evolved during the Plio-Quaternary, and, is still active, as proved by submarine cable breaks during the 1954 Orléansville earthquake. The Kramis Fan is fed by two perpendicular canyons: the Kramis Canyon and the Khadra Canyon, merging in a single E–W-oriented channel confined at the foot of the slope. It is strongly asymmetric with a super-developed levee on the right-hand side of the channel, the Kramis Ridge. Based on recent multibeam, side-scan sonar, and sediment core data (Maradja, 2003 and 2005, Prisma, 2004, and Prisme, 2007 cruises), we describe the morphology and internal structure of the fan and particularly the sediment ridge, showing marked lateral changes in the sediment-wave morphology and their association with a series of large scours in the intermediate part of the ridge aligned in the continuity of the Khadra Canyon direction. Overall, the Kramis Ridge is formed by turbidity currents overspilling the ridge crest, which is 100 m above the channel floor, with two exceptions. In the distal part of the ridge the subdued ridge-crest height probably causes continuous overspill, testified by sediment waves migrating parallel to the channel. The scours occur in the intermediate part of the ridge where the ridge height is only 50–60 m; scours are interpreted as the result of cyclic steps due to flow stripping of currents provided by the intersection of the Khadra Canyon with the Kramis Canyon and Channel system. The scours probably postdate the main growth of the Kramis Ridge and induce the local erosion of the ridge, which could correspond to a new channel initiation cutting the ridge. The superposition or the interaction of flows with different directions is responsible of the amplification of the size of the sediment waves with erosional downside flanks and their transformation in scours. The Kramis Fan provides a clear example of flow interaction to explain the presence of large sediment waves and scours on modern submarine fans.

Abstract The Pleistocene Alaminos Fan in the northwestern deep Gulf of Mexico is a large submarine fan located at the base of the continental slope. High-resolution, near-seafloor 3D seismic data were interpreted to study the evolution of two Pleistocene shallow sequences. Results indicate that there can be considerable variability in the evolution of deepwater systems in the same overall setting, and the significance that seafloor topography and gradient changes can make in the overall development of different architectural elements. Seismic facies indicate that the sequence consists primarily of mass-transport deposits (MTDs), channel-fill sediments, and sheet deposits in the lower sequence, whereas the upper sequence consists of a single channel. The older sequence consists of two distinctly different deepwater systems. To the east, a prominent channel system overlies basal MTDs. The channel system consists of one upfan channel that bifurcates downfan to at least six discrete channels flanked by levees. The evolution of the downdip distributary channel system is the result of deposition in the unconfined setting. To the west, an updip channel fed sheet deposits that developed in the back limb of a fold in a confined setting. The sheet, in turn, is overlain by a channel–levee system. This vertical change is likely the results of the fill-and-spill of sediments in the area. In the near-floor sequence, a single deepwater channel is flanked by low-amplitude levee reflections. Quantifying the dimensions of this younger channel shows that it evolved from a wider, straight channel at the base of the sequence to a relatively narrow sinuous channel with increasing sinuosity upward.