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CHARACTERIZATION AND RECOGNITION OF DEEP-WATER CHANNEL-LOBE TRANSITION ZONES

Russell B Wynn, Neil H Kenyon, Douglas G Masson, Dorrik A V Stow and Philip P E Weaver

The channel-lobe transition zone (CLTZ) is an important, but often overlooked, element of many deep-water turbidite systems. Recognizing this zone is difficult in both modern and ancient environments, and depends largely upon the quality and resolution of the data obtained. In this study three case studies of modern CLTZ’s are presented, largely based upon high-resolution sidescan sonar imagery. These data are then compared to other well-defined CLTZ’s, both modern and ancient, and the common characteristics identified.
CLTZ’s occur at canyon/channel mouths and are commonly associated with a break-in-slope. Most sediment bypasses this zone and consequently only coarse sands and gravels are deposited, although these are often patchily distributed and extensively reworked. The CLTZ is characterized by abundant erosional features, including isolated spoon- and chevron-shaped scours up to 20 m deep, 2 km wide and 2.5 km long. In areas of more widespread erosion these merge to form amalgamated scours several kilometers across. Depositional bedforms include sediment waves with wavelengths of 1-2 km and wave heights up to 4 m. The presence or absence of a CLTZ has important implications for hydrocarbon exploration and development, especially in terms of the connectivity between sandy channel-fill and lobe facies.


TURBIDITY CURRENT SEDIMENT WAVES IN SUBSURFACE SEQUENCES

Russell B. Wynn, Douglas G. Masson, Dorrik A. V. Stow and Phillip P. E. Weaver

Two sediment wave fields on the submarine slopes of the Canary Islands display wave heights up to 70 m and wavelengths up to 2.4 km. Wave sediments consist of fine-grained turbidites and pelagic/hemipelagic sediments. The sediment waves are formed beneath unconfined turbidity currents, and are similar to sediment waves found on channel-levee backslopes.
Sediment wave morphology is resolvable on high-resolution seismic profiles. In areas lacking high-resolution seismic data, analysis of dipmeter readings may provide a useful tool for recognizing buried sequences of migrating waves. Thick sequences of sediment waves will impart a marked heterogeneity to a potential reservoir, leading to complications during reservoir production.


CANARY ISLANDS LANDSLIDES AND TSUNAMI GENERATION: CAN WE USE TURBIDITE DEPOSITS TO INTERPRET LANDSLIDE PROCESSES?

Russell B Wynn and Douglas G Masson

The Cumbre Vieja volcano, on La Palma in the western Canary Islands, is an unstable area that may develop into a future landslide, generating a tsunami that could cause damage far from the source. However, volcaniclastic turbidites that are directly correlated with the two most recent Canary Islands landslides, show stacked sub-units within a single turbidite bed. This may indicate multiple stages of landslide failure. Similar findings have previously been reported from volcaniclastic turbidites linked to Hawaiian landslides. Consequently the potential tsunami hazard from such failures may be lower than previously predicted.


THE NORTHWEST AFRICAN SLOPE APRON: A MODERN ANALOGUE FOR DEEP-WATER SYSTEMS WITH COMPLEX SEAFLOOR TOPOGRAPHY

Russell B Wynn, Douglas G Masson, Dorrik A V Stow and Phillip P E Weaver

The Northwest African slope apron is an interesting modern analogue for deep-water systems with complex seafloor topography. A sediment process map of the Northwest African continental margin illustrates the relative roles of different sedimentary processes acting across the entire margin. Fine-grained pelagic and hemipelagic sedimentation is dominant across a large area of the margin, and is considered to result from ‘background’ sedimentary processes. Alongslope bottom currents smooth and mould the seafloor sediments, and produce bedforms such as erosional furrows, sediment waves and contourite drifts. Downslope gravity flows (debris avalanches, debris flows and turbidity currents) are infrequent but important events on the margin, and are the dominant processes shaping the morphology of the slope and rise. The overall distribution of sedimentary facies and morphological elements on the Northwest African margin is characteristic of a fine-grained clastic slope apron. However, the presence of numerous volcanic islands and seamounts along the margin leads to a more complex distribution of sedimentary facies than is accounted for by slope apron models. In particular, the distribution and thickness of turbidite sands are controlled by the location of the break-of-slope, which is itself controlled by the pre-existing submarine topography.


HYDRODYNAMIC SIGNIFICANCE OF VARIABLE RIPPLE MORPHOLOGY ACROSS DEEP-WATER BARCHAN DUNES IN THE FAROE-SHETLAND CHANNEL

Russell B Wynn, Douglas G Masson and Brian J Bett

A combination of high-resolution (30kHz) sidescan sonar and seafloor photography has been used to image barchan dunes and sand ripples in the Faroe-Shetland Channel. The barchan dunes display body lengths and horn-to-horn widths of up to 120 m, and the horns point down-current to the south-west. The seafloor surrounding the dunes is variable, with gravel patches, sand with gravel streaks, and uniform rippled sand all observed. The pattern of sand ripples across the dune surface is strongly controlled by the dune morphology. On the lower stoss slopes of the dunes, straight-crested transverse ripples gradually merge upslope into sinuous-crested transverse ripples. Most of the upper dune surface is dominated by short-crested linguoid ripples. This sequence represents an increase in height above seafloor and increasing flow velocity.
The overall pattern of ripples across the dune surface indicates that the bottom current flow is modified by the dune morphology, with flow concentrated over the dune crest and along the inner edge of the horns. It is also possible to estimate flow velocity across the dune field, using published analogues and short-term current meter observations. Generally, published examples of submarine barchans suggest a flow velocity of 40-80 cm/s. This range of values is in agreement with near-bed current meter measurements in the study area, which indicate peak flow velocities of 50-60 cm/s. However, in the longer term higher flow speeds may be possible, especially considering the presence of erosional furrows adjacent to the dune field.

GENERATION AND MIGRATION OF COARSE-GRAINED SEDIMENT WAVES IN TURBIDITY CURRENT CHANNELS AND CHANNEL-LOBE TRANSITION ZONES

Russell B Wynn, David J W Piper and Martin J R Gee

Large-scale sediment waves, composed of gravels and sands, have been studied using deep-water sidescan systems. New data are presented from submarine channels off the Canary Islands and from canyon mouths off Portugal. Data from other areas are briefly reviewed, including a re-interpretation of data from Laurentian Fan, in order to summarise the varied morphology and setting of these bedforms. Coarse-grained sediment waves are found in the proximal, dominantly bypassing areas of deep-water turbidite systems, within canyons, channels and channel-lobe transition zones. Wave heights are in the region of 1-10 m, and wavelengths are up to several hundred metres. The distribution of waves, and sparse sedimentological evidence from modern and ancient sediment-wave fields, suggests that initial transport and deposition of coarse sediment occurs within a high-density turbidity current, and not as a non-Newtonian debris flow. In some cases the development of pronounced wave asymmetry, and evidence of wave disruption and reworking, suggests that the wave morphology is at least partially controlled by a later phase of low-density turbidity flow. Grain size also appears to exert some control on wave morphology, for example, gravel-rich waves have a greater height for the same wavelength than sand-rich waves. Coarse-grained sediment waves are often difficult to recognise on the seafloor because of reworking or burial by younger turbidity currents, and are equally difficult to recognise in outcrop because of their large size.


TURBIDITY CURRENT SEDIMENT WAVES ON THE SUBMARINE SLOPES OF THE WESTERN CANARY ISLANDS

Russell B Wynn, Douglas G Masson, Dorrik A V Stow and Phillip P E Weaver

Two sediment wave fields have been identified on the flanks of the western Canary Islands of La Palma and El Hierro, using a high-quality 2-D and 3-D dataset that includes GEOSEA and TOBI imagery, 3.5-kHz profiles, and short sediment cores.
The La Palma sediment wave field covers some 20,000 km2 of the continental slope and rise, and consists of sediment waves with wave heights of up to 70 m and wavelengths of up to 2.4 km. The wave crestlines have a complex morphology, with common bifurcation and a clear sinuosity. Waves have migrated upslope through time. Cores recovered from the wave field contain volcaniclastic turbidites interbedded with pelagic/hemipelagic layers. The wave field is interpreted as having formed beneath unconfined turbidity currents. A simple, previously published, two-layer model is applied to the waves, revealing that they formed beneath turbidity currents flowing at 10 - 100 cm/s-1, with a flow thickness of 60 - 400 m and a sediment concentration of 26 - 427 mg/l.
The El Julan sediment wave field lies within a turbidity current channel on the southwest flank of El Hierro. The sediment waves display wave heights of about 6 m and wavelengths of up to 1.2 km. The waves are migrating upslope, and migration is most rapid in the centre of the channel where the flow velocity is highest. This wave field has been formed by channelised turbidity currents originating on the flanks of El Hierro.


INITIATION AND EVOLUTION OF TURBIDITY CURRENT SEDIMENT WAVES IN THE MAGDALENA TURBIDITE SYSTEM

Gemma Ercilla, Russell B. Wynn, Belén Alonso, and Jesús Baraza

This study describes an extensive sediment-wave field in the Magdalena Turbidite System, which occupies an area of at least 15,000 km2 on the continental slope (3330 - >3800 m). The waves display wavelengths up to 1.9 km, wave heights up to 18 m, and crestlines that are aligned roughly parallel to the regional bathymetric contours. Preferential deposition on the upslope wave flank has led to upslope migration, in the manner of antidunes. The Magdalena sediment waves are interpreted as forming beneath unconfined turbidity currents, which may result from the downslope evolution of slumps and mass flows. The unconfined turbidity currents are derived from several point sources along the continental slope and spread laterally as they flow downslope. This has led to the formation of a laterally extensive sediment-wave field. Simple numerical modelling estimates that the turbidity currents responsible for wave generation were near- or super-critical, with flow thickness and velocity estimated at 40-160 m, and 36-82 cm s-1 respectively. However, later phases of wave growth were not dependent on specific flow conditions.
The most important aspect of this study is that the entire sediment-wave unit, from the basal boundary to the present-day seafloor, has been investigated using ultra high-resolution seismic profiles. The sediment-wave unit rests upon an irregular discontinuity that marks a recent change in the sedimentary regime of the Magdalena Turbidite System, from channelised to unchannelised gravity flows. Above this boundary, the sediment waves display a growth pattern characterised by an increase in wave dimensions. In addition, the wave dimensions appear to become more regular through time. However, breaks of slope in the lower bounding surface of the wave field have produced variations in wave morphology that are still visible at the present-day seafloor. This indicates that there is a close relationship between variations in slope angle and turbidity current flow characteristics, which in turn leads to variations in wave morphology.


TURBIDITY CURRENT SEDIMENT WAVES ON IRREGULAR SLOPES: OBSERVATIONS FROM THE ORINOCO SEDIMENT-WAVE FIELD

Gemma Ercilla, Belén Alonso, Russell B. Wynn, and Jesús Baraza

The Orinoco sediment-wave field covers an area of at least 29,000 km2 on the southern margin of the Orinoco Valley, at a water depth of 4400-4825 m. Wave dimensions are highly variable across the wave field, with wavelengths of 110-2600 m, and wave heights of 1-15 m. Slope gradients are also very variable, with values of 0.14º-0.48º. Overall, the sedimentary sequence on the upslope wave flanks is about 40% thicker than that on the downslope flanks, leading to upslope wave migration in the manner of antidunes. In addition, reflectors on the upslope flanks generally display higher reflectivity than those on the downslope flanks, suggesting that a higher proportion of coarser sediment occurs on the upslope flank. An unconfined turbidity current origin is proposed for the Orinoco sediment waves, based upon detailed analysis of regional stratigraphic/seismic facies, and sediment wave distribution, morphology and dimensions. Sediment waves are not related to flows passing along (or spilling out of) the Orinoco Valley, or to bottom currents flowing parallel to the slope. Turbidity currents responsible for wave generation are interpreted as originating from slope failures on the adjacent Venezuela, Guyana and Surinam continental margins. Simple numerical modelling has enabled turbidity current flow characteristics across the Orinoco sediment waves to be estimated: internal Froude number = 0.7-1.1, flow thickness = 24-645 m, and flow velocity = 31-82 cm s-1.
A key finding of this study is that there appears to be a close relationship between changes in slope gradient and changes in wave dimensions across the wave field. The irregular gradient of the present-day wave field is partly a reflection of the irregular lower bounding surface of the sediment waves, which is represented by mass-flow deposits and associated mud diapirs. The changes in slope gradient along this lower boundary lead to variations in the flow thickness and flow velocity of passing turbidity currents, which in turn controls the wave dimensions. Generally, on lower gradients beyond minor breaks of slope, flow thickness increases and flow velocity decreases, leading to an increase in wavelength and a decrease in wave height.


CLASSIFICATION AND CHARACTERISATION OF DEEP-WATER SEDIMENT WAVES

Russell B Wynn and Dorrik A V Stow

Deep-water sediment waves can be classified using a combination of grain size and wave-forming process, although in some cases one or other of these criteria may be indeterminable. Sediment waves are generated beneath currents flowing across the seabed, either in the form of downslope-flowing turbidity currents or alongslope-flowing bottom currents. Waves formed by either process show varying characteristics, depending on whether they are constructed of coarse- or fine-grained sediments.
Sediment wave studies over the last five decades are reviewed, and clear trends can be discerned. Early descriptive studies in the 1950’s and 1960’s relied almost exclusively on seismic reflection profiles, and the wave-forming process was often a subject of much debate. In the 1970’s and 1980’s the quality of sediment wave datasets increased, with sidescan sonar, deep-sea drilling and numerical modelling all applied to sediment wave studies. Consequently, the wave-forming process became more easily identifiable, and models for the growth of bottom current and turbidity current sediment waves were introduced. Most studies from the 1990’s onwards have focussed on turbidity current sediment waves, in response to the increasing demand for data from turbidite systems from the hydrocarbon exploration and production industry. Studies of bottom current sediment waves during this period have focussed on the applications to paleoceanography, in response to the recent boom in climate change studies.
The main focus of this paper is the characterisation of both fine- and coarse-grained, turbidity and bottom current sediment waves, including the depositional environment, wave morphology, wave sediments and migration, and the wave-forming process. In addition, criteria for distinguishing between fine-grained bottom current and turbidity current waves are discussed, and also for identifying other wave-like features formed by different processes, such as creep folds. Although in many sediment wave studies the dominant wave-forming process is easy to determine, in others it is likely that a more complex combination of processes has occurred. Further studies should concentrate on methods for identifying these processes and how they interact, and also investigate the exact mechanisms for the initiation and evolution of sediment-wave fields.


TURBIDITE DEPOSITIONAL ARCHITECTURE ACROSS THREE INTER-CONNECTED DEEP-WATER BASINS ON THE NORTHWEST AFRICAN MARGIN

Russell B Wynn, Philip P E Weaver, Douglas G Masson and Dorrik A V Stow

The Moroccan Turbidite System (MTS) on the Northwest African margin extends 1500 km from the head of the Agadir Canyon to the Madeira Abyssal Plain, making it one of the longest turbidite systems in the world. The MTS consists of three interconnected deep-water basins, the Seine Abyssal Plain, the Agadir Basin, and the Madeira Abyssal Plain (MAP), connected by a network of distributary channel systems. Excellent core control has enabled individual turbidites to be correlated between all three basins, giving a detailed insight into the turbidite depositional architecture of a system with multiple source areas and complex morphology. Large-volume (>100 km3) turbidites, sourced from the Morocco Shelf, show a relatively simple architecture in the Madeira and Seine Abyssal Plains. Sandy bases form distinct lobes or wedges that thin rapidly away from the basin margin, and are overlain by ponded basin-wide muds. However, in the Agadir Basin the turbidite fill is more complex, due to a combination of multiple source areas and large variations in turbidite volume. A single very large turbidity current (200-300 km3 of sediment) deposited most of its sandy load within the Agadir Basin, but still had sufficient energy to carry most of the mud fraction 500 km further downslope to the MAP. Large turbidity currents (100-150 km3 of sediment) deposit most of the sand and mud fraction within the Agadir Basin, but also transport some of their load westwards to the MAP. Small turbidity currents (<35 km3 of sediment) are wholly confined within the Agadir Basin and pinch out on the basin floor.
Turbidity currents flowing beyond the Agadir Basin pass through a large distributary channel system. Individual turbidites correlated across this channel system show major variations in sand fraction mineralogy, whereas the mud fraction geochemistry and micropalaeontology remain very similar. This is interpreted as evidence for flow separation, with a sand-rich, erosive, basal layer confined within the channel system, overlain by an unconfined layer of suspended mud.
Large-volume turbidites within the MTS were deposited at oxygen isotope stage boundaries, during periods of rapid sea-level change. They do not appear to be specifically connected to sea-level lowstands or highstands. This contrasts with the classic fan model, which suggests that most turbidites are deposited during lowstands of sea level. In addition, the three largest turbidites on the MAP were deposited during the largest fluctuations in sea level, suggesting a link between the volume of sediment input and the magnitude of sea level change.


SEDIMENTARY PROCESSES IN THE SELVAGE SEDIMENT-WAVE FIELD, NE ATLANTIC: NEW INSIGHTS INTO THE FORMATION OF SEDIMENT WAVES BY TURBIDITY CURRENTS

Russell B Wynn, Philip P E Weaver, Gemma Ercilla, Dorrik A V Stow and Douglas G Masson

An integrated geophysical and sedimentological investigation of the Selvage sediment-wave field has revealed that the sediment waves are formed beneath unconfined turbidity currents. The sediment waves occur on the lower continental rise, and display wavelengths of up to 1 km, and wave heights of up to 6 m. Wave sediments consist of interbedded turbidites and pelagic/hemipelagic marls and oozes. Nannofossil-based dating of the sediments indicates a bulk sedimentation rate of 2.4 cm/1000 yrs, and the waves are migrating upslope at a rate of 0.28 m/1000 yrs. Sediment provenance studies reveal that the turbidity currents maintaining the waves are largely sourced from volcanic islands to the south.
Investigation of existing models for sediment-wave formation leads to the conclusion that the Selvage sediment waves form as giant antidunes. Simple numerical modelling reveals that turbidity currents crossing the wave field have internal Froude numbers of 0.5-1.9, which is very close to the antidune existence limits. Depositional flow velocities range from <6-125 cms-1. There is a rapid increase in wavelength and flow thickness in the upper 10 km of the wave field, which is unexpected as the slope angle remains relatively constant. This anomaly is possibly linked to a topographic obstacle just upslope of the sediment waves. Flows passing over the obstacle may undergo a hydraulic jump at its boundary, leading to an increase in flow thickness. In the lower 15 km of the wave field, flow thickness decreases downslope by 60%, which is comparable to results obtained for other unconfined turbidity currents undergoing flow expansion.


CONTINENTAL MARGIN SEDIMENTATION, WITH SPECIAL REFERENCE TO THE NORTH-EAST ATLANTIC MARGIN

Philip P.E. Weaver, Russell B. Wynn, Neil H. Kenyon and Jeremy Evans

The north-east Atlantic continental margin displays a wide range of sediment transport systems with both alongslope and downslope processes. Off most of the north-west African margin, south of 26°N, upwelling produces elevated accumulation rates, though there is little fluvial input. This area is subject to infrequent but large-scale mass movements, giving rise to debris flows and turbidity currents. The latter traverse the slope and deposit thick layers on the abyssal plains, whilst debris flows deposit on the continental slope and rise. From the Atlas Mountains northwards to 56°N the margin is less prone to mass movements, but is cut by a large number of canyons which also funnel turbidity currents to the abyssal plains. The presence of a lithospheric plate boundary off SW Iberia is believed to have led to high rates of sediment transport to the deep sea. Even larger quantities of coarse sediments have fed the canyons and abyssal plains in the Bay of Biscay as a result of drainage from melting icecaps. Bottom currents have built sediment waves off the African and Iberian margins, and created erosional furrows south of the Canaries. The Mediterranean outflow is a particularly strong bottom current near the Straits of Gibraltar, depositing sand- and mudwaves in the Gulf of Cadiz. North of 56°N the margin is heavily influenced by glacial and glaciomarine processes active during glacial times, which built glacial trough-mouth fans such as the North Sea Fan, and left iceberg scour marks on the upper slope and shelf. Over a long period, especially during interglacials, this part of the margin has been greatly influenced by alongslope currents, with less influence by turbidity currents than on the lower latitude margins. Mass movements are again a prominent feature, particularly off Norway and the Faeroes. Some of these mass movements have occurred during the Holocene, though high glacial sedimentation rates may have contributed to the instability.


EVALUATING THE LINKS BETWEEN TURBIDITE CHARACTERISTICS AND GROSS SYSTEM ARCHITECTURE:
UPSCALING INSIGHTS FROM THE TURBIDITE SHEET SYSTEM OF PEIRA CAVA

Amy, L. A., McCaffrey W. D. and Kneller, B. C.

Understanding how the characteristics of individual turbidites measured along a 1D (vertical) section may relate to reservoir-scale system geometry remains a significant upscaling problem, yet the ability to make this link is fundamentally important when evaluating turbidite reservoirs. Insights into the key relationships are perhaps best gained from well-exposed outcrops in which bed-to-bed correlations can be established. The Peïra Cava outlier of the Tertiary Annot Sandstones contains sheetform turbidites that were deposited in a confined and ponded basin. They are now exposed over a 10 by 6 km area. Bed-to-bed correlations have been established throughout a selected 420 m stratigraphic interval, allowing the 3D geometry of the system to be constrained. We test the significance of bed sand thickness, mud-cap thickness, sand percentage, grain-size, and the presence or absence of erosional structures and cross-stratification for their value as predictors of up-stream and down-stream bed geometry. The results are compared with models currently used to predict the spatial distribution of sediment properties, such as flow-efficiency concepts and the influence of topographic control.



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November 2005