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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Africa
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Central Africa
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Angola (1)
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West Africa
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Nigeria
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Niger Delta (2)
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metamorphic rocks
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turbidite (1)
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Primary terms
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Africa
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Central Africa
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Angola (1)
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West Africa
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Nigeria
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Niger Delta (2)
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Atlantic Ocean
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Equatorial Atlantic (1)
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North Atlantic
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Amazon Fan (1)
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Gulf of Guinea (1)
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Gulf of Mexico (1)
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Northeast Atlantic (1)
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Mesozoic
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Ocean Drilling Program
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ocean floors (6)
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Giant meandering channel evolution, Campos deep-water salt basin, Brazil
The stratigraphic evolution of a submarine channel: linking seafloor dynamics to depositional products
How do basin margins record long-term tectonic and climatic changes?
High curvatures drive river meandering
Abstract: Using a script that automatically calculates sinuosity and radius of curvature for multiple bends on sinuous channel centerlines, we have assembled a new data set that allows us to reevaluate the relationship between latitude and submarine channel sinuosity. Sinuosity measurements on hundreds of channel bends from nine modern systems suggest that there is no statistically significant relationship between latitudinal position and channel sinuosity. In addition, for the vast majority of submarine channels on Earth, using flow velocities that are needed to transport the coarse-grained sediment found in channel thalwegs, estimates of the curvature-based Rossby number are significantly larger than unity. In contrast, low flow velocities that characterize the upper parts of turbidity currents in submarine channels located at high latitudes can easily result in Rossby numbers of less than one; this is the reason why levee deposits are often highly asymmetric in such channels. However, even in channels with asymmetric levees, the sinuosity of the thalweg is often obvious and must have developed as the result of an instability driven by the centrifugal force. Analysis of a simple centerline-evolution model shows that the increase in channel curvature precedes the increase in sinuosity and that low sinuosities are already associated with large curvatures. This suggests that the Coriolis effect is unlikely to be responsible for the low sinuosities observed in certain systems.
High-resolution, millennial-scale patterns of bed compensation on a sand-rich intraslope submarine fan, western Niger Delta slope
Three-Dimensional Numerical Modeling of Eustatic Control On Continental-Margin Sand Distribution
Development of cutoff-related knickpoints during early evolution of submarine channels
Stratigraphic rule-based reservoir modeling
Stratigraphic evolution of intraslope minibasins: Insights from surface-based model
Rapid Adjustment of Submarine Channel Architecture To Changes In Sediment Supply
Key Future Directions For Research On Turbidity Currents and Their Deposits
Global (latitudinal) variation in submarine channel sinuosity: COMMENT
Seismic Stratigraphy of a Shelf-Edge Delta and Linked Submarine Channels in the Northeastern Gulf of Mexico
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.
Stratigraphic Response to Evolving Geomorphology in a Submarine Apron Perched On the Upper Niger Delta Slope
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 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 Field and simulation studies indicate that channel architecture and the presence of channel-base drapes (CBDs) can have a significant impact on oil recovery and represent key uncertainties in the understanding of a turbidite channel reservoir. Accordingly, understanding the frequency and distribution of CBDs provides valuable insights into reservoir performance. Core and dipmeter data contain information that can be used to recognize channel-base disconformities and associated CBDs. By comparing the observed number of channel-base disconformities to the observed number of disconformities overlain by mudstone, a statistical assessment of their frequency and distribution can be made. In a spatial sense, the fraction observed in the wells represents the average percentage of the channel elements within the reservoir that are overlain by a drape.
Architecture of a Deep-water Levee Avulsion, Silla Ojo Mesa, Parque Nacional Torres del Paine, Chile
Extensive outcrops of coarse-grained channel to fine-grained levee deposits of the Campanian Cerro Torro Formation are present throughout the Torres del Paine National Park in southern chile ( Figure 1 ; Fildani et al, chapter 33, this volume). The outcrop panel in Figure 2 represents part of one face of a nearly continuous exposure mapped in this paper that is present on all four faces of a mesa within the Silla Ojo Syncline ( Figure 1 ). The depositional architectures consist predominantly of sheetlike, tabular elements comprising interbedded sandstone and shale bedsets, onlapping older levee deposits (described by Barton et al., chapter 39, this volume). Isolated channel elements and scour features are also present. The vertically stacked, tabular architectural elements observed in the outcrop panel are interpreted to represent a phase of partially confined to unconfined deposition outboard of a major levee avulsion site, analogous in many respects to the avulsion deposits described by Hiscott et al. (1979). Overlying and truncating the tabular elements of the avulsion deposits is a thick, multistory channelized conglomerate ( Figure 2 ) with internally organized and chaotic bedding and impressive debris flow deposits. A similar vertical facies transition from levee to avulsion to channel deposists is also described by O’Byrne et al. (chapter 30, this volume) and Arnott (chapter 29, this volume) from the Isaac Formation, Canada.
Abstract Understanding the origin and geometry of largescale erosional surfaces in fluvial and channelized submarine depositional settings is critical for interpreting reservoir architecture and connectivity, as these surfaces strongly influence reservoir heterogeneity. We use simple and fast-running forward stratigraphic models to investigate the geometry and the relative age of complex erosional surfaces that form in both the subaerial and submarine domain. Because low-sinuosity systems tend to have relatively simple incisional and aggradational geometries, we focus on high-sinuosity systems. Fluvial deposits are commonly preserved on terraces that form during incision, and the basal erosional surface is highly time transgressive. Terraces can form without any external influence as a result of cessation of incision at channel cutoff locations. Similar processes and geometries can be observed in systems containing incising submarine channels. However, extensive deposition of fine-grained sediment in the overbank area of submarine channels tends to result in draping and long-term preservation of terrace geometries. This is in contrast with fluvial systems, as the incisional terrace morphology can be quickly buried after valley filling initiates. Once incision ceases and aggradation begins, erosional surfaces become less continuous and form an intricate network inside the larger and longitudinally more continuous valley surface. Depending on the rate of aggradation and local rate of lateral migration, the internal erosional surfaces can be similar in vertical extent to a single channel depth, or to multiple channel depths and one channel bend in plan view. Phases of low aggradation cause these scallop-shaped surfaces to connect in the downslope direction and form an extensive erosional surface, without any significant re-incision. As relatively fine-grained deposits (e.g., shale drapes, slides, and debris-flow deposits) are primarily distributed along geomorphic surfaces, differentiating time-transgressive erosional surfaces from geomorphic ones results in a better prediction of reservoir compartmentalization and fluid flow. Understanding the origin and geometry of valleys and their deposits informs the controls on the sequence stratigraphy of basin margins. That is, most erosional surfaces are time transgressive and some of them reflect the autogenic dynamics of valley formation, rather than external forcing.
Autogenic and Allogenic Controls on Deep-Water Sand Delivery: Insights from Numerical Stratigraphic Forward Modeling
Abstract Allogenic and autogenic processes interact to regulate sediment distribution in sedimentary basins. Depositional systems can respond in a complex manner to these processes, complicating interpretation of the controls on the stratigraphic record. Here we used published and constant eustatic curves in a stratigraphic forward model to examine the effects of sea-level variation on deep-water sand delivery on a passive continental margin. We found that: (1) models with constant sea level and those with eustatic fluctuations deliver similar volumes of sand to deep water; (2) both large and small eustatic variations result in similar magnitudes of fluctuations in deep-water sand delivery; and (3) deep-water sand delivery signals show similar periodicities for all models. These results suggest that the characteristics of the imposed eustatic curve may not have a significant impact on the total volume of sand delivered to deep water. We propose that the equilibrium state of the shelf-edge delta, where no net deposition or erosion occurs, could explain the similarity in deep-water sand volumes. We posit that such a state could be induced by the progradation of an initial shelf-edge delta that creates a slope which maximizes the efficiency of sediment delivery across the shelf. Because our models show that autogenic and allogenic processes can result in similar deep-water sand volumes, we conclude that other characteristics of sediment-routing systems, such as sediment supply, must exert strong controls on deep-water sand volume.