Basin-floor Fans Research Articles

A record of Eocene (Stronsay Group) sedimentation in BGS borehole 99/3, offshore NW Britain: Implications for early post-break-up development of the Faroe–Shetland Basin

Synopsis A punctuated Eocene succession has been recovered in British Geological Survey borehole 99/3 from the Faroe–Shetland Basin. The borehole was drilled close to the crest of the Judd Anticline and penetrated 110.5 m into the post-break-up Stronsay Group. The borehole proved 23.8 m of Ypresian–earliest Lutetian paralic to shallow-marine deposits, unconformably overlain by 43.85 m of mid-Lutetian lower shoreface to shallow-marine shelf deposits, in turn unconformably overlain by 42.85 m of late Bartonian–Priabonian mass-flow (slope apron) and basinal deposits. At the borehole site, the Ypresian–earliest Lutetian sediments were subaerially exposed in early Lutetian times; these deposits were subsequently transgressed by the mid-Lutetian shelf – itself eroded in the late Lutetian with reworked material transported northwards and re-deposited in the Mid-Eocene basin-floor fan complex in Quadrants 213 and 214. The eroded mid-Lutetian shelf was subsequently downlapped and buried beneath prograding late Bartonian–Priabonian slope apron-to-basinal sediments as the borehole site subsided. Integrating the borehole data with the regional seismic-stratigraphy indicates three major intervals of regression – early/mid-Lutetian, late Lutetian and late Bartonian – which, it is suggested, coincide predominantly with tectonic activity, particularly compressional deformation, possibly linking the early post-break-up development of the Faroe–Shetland Basin to the evolution of the adjacent Norwegian Basin.

Pliocene basin-floor fan sedimentation in the Bay of Bengal (offshore northwest Myanmar)

This study addresses reservoir development and hydrocarbon occurrence of the late Pliocene basin-floor fan deposits in the northeastern Bay of Bengal. The G-series turbiditic sandstones host dry gases, biogenic in origin, of three gas fields that are juxtaposed on the western flank of the NW–SE anticline. The gas reservoirs are stacked in back-stepping fashion being sourced from northwest as part of the Bengal fan. The Shwe field (Shwe fan) has an elongate fan-shaped morphology (12 km long and 4 km wide) and occurs about 30 km off the base of slope.The Shwe field reservoirs consist of two contrasting types of turbidite deposits, lower G5.2 and upper G3.2 reservoirs. The G5.2 is characterized by stacked lobe elements in which amalgamated and layered sheet sandstones provide an excellent vertical connectivity and high net-to-gross ratio (avg. 86%). On the contrary, the G3.2 represents overbank deposits, which is characterized by thin-bedded sandstones with low net-to-gross ratio (avg. 33%) as well as low-resistivity pays.Aside from the primary depositional framework, post-depositional modifications appear to have greatly affected Shwe fan, adding complexity in establishing a geological model. A line of evidence suggests that G5.2 lobe sands were injected into overlying G3.2, mostly in the form of sills. The injected sand beds measured from G3.2 cores account for 10 m, more than half of the total net sand. The sand injection is thought to be triggered by slumping that overpressured G5.2 lobes. Post-G3.2 erosional channel complexes incised into G5.2 and G3.2, ultimately diminishing gas pool extent as well as dividing Shwe fan into multiple compartments.

Connectivity estimation between turbiditic channels and overbank deposits from the modelling of an outcrop analogue (Pab Formation, Maastrichtian, Pakistan)

Abstract The facies associations of the overflow deposits associated with turbidite channels were studied in the outcrops of the Maastrichtian Pab Sandstone, in SW Pakistan. In this area, a basin-floor fan was preserved from the platform to the deep basin setting. In the mid-fan setting, a channel complex crops out in three dimensions, and consists of a dozen channels and their overflow deposits, including levees, crevasse lobes and spill-over lobes. The overbank deposits can be directly in contact with the channel-fill, but, in many cases, matrix-supported debris-flow deposits made lenses close to the channel base. Heterolithic drapes of thin-bedded turbidite deposits were also preserved along the channel margins. A three-dimensional (3D) static model describing the heterogeneity distribution within the turbiditic channel complex was compiled using both stochastic and deterministic approaches in a site where two channels were laterally connected by overbank deposits. Petro-acoustic properties derived from a subsurface database were then assigned to the facies to perform seismic simulations. The synthetic seismic simulations showed that the channel base can easily be misinterpreted compared to the geological model. Dynamic modelling, such as well test and streamline simulations, was also performed using the model to assess the transition between the channel and overbank deposits from a dynamic point of view. As a result of the streamline simulations, the overbank deposits connecting the channel homogenized the pressure regime in the reservoir. However, the sweeping efficiency of water injection can be affected by the heterogeneity distribution along the channel margins. A significant volume of oil could also be by-passed because of the occurrence of early water breakthrough through the spill-over lobes, or because the flow slowed down when it reached the heterogeneity along the channel margin.

Seismic geomorphological analysis and hydrocarbon potential of the Lower Cretaceous Cromer Knoll Group, Heidrun field, Norway

The Heidrun field, located on the Halten Terrace of the mid-Norwegian continental shelf, was one of the first giant oil fields found in the Norwegian Sea. Traditional reservoir intervals in the Heidrun field lie within the Jurassic synrift sequence. Most Norwegian continental shelf fields have been producing from these Jurassic reservoirs for the past 30 yr. Production has since declined in these mature fields, but recently, exploration for new reservoirs has resurged in this region. The Jurassic rifted fault blocks form a narrow continental shelf in Norway, thereby greatly reducing the areal extent for exploration and development within existing fields. As the rift axis is approached farther offshore, these Jurassic reservoirs become very deep, too risky to drill, and uneconomical. This risk has prompted exploration in more recent years of the shallower Cretaceous, postrift stratigraphic succession. Cretaceous turbidites have been found in the Norwegian and North Seas, and the discovery of the Agat field in the Norwegian North Sea confirms the existence of a working petroleum system capable of charging Cretaceous reservoirs. These Cretaceous reservoirs were deposited as slope- and basin-floor fans within a series of underfilled rifted deeps along the Norwegian continental shelf and are thought to be sourced from the localized erosion of Jurassic rifted highs. We use three-dimensional seismic and well data to document the geomorphology of a deep-water, Lower Cretaceous wedge (Cromer Knoll Group) within the hanging wall of a rift-related half graben formed on the Halten Terrace offshore mid-Norway. Seismic attribute extractions taken within this Lower Cretaceous wedge reveal the presence of several lobate to elongated bodies that seem to cascade over fault-bounded terraces associated with rifted structures. These high-amplitude, elongated bodies are interpreted as deep-water sedimentary conduits that are time equivalent to the Cretaceous basin-floor fans in more distal parts of the basin to the west. These half-graben fills have the potential to contain high-quality Cretaceous sandstones that might represent a potential new reservoir interval within the Heidrun field.

Modern internal waves and internal tides along oceanic pycnoclines: Challenges and implications for ancient deep-marine baroclinic sands

Thus far, the subject of deep-marine sands emplaced by baroclinic currents associated with internal waves and internal tides as potential reservoirs has remained an alien topic in petroleum exploration. Internal waves are gravity waves that oscillate along oceanic pycnoclines. Internal tides are internal waves with a tidal frequency. Internal solitary waves (i.e., solitons), the most common type, are commonly generated near the shelf edge (100–200 m [328–656 ft] in bathymetry) and in the deep ocean over areas of sea-floor irregularities, such as mid-ocean ridges, seamounts, and guyots. Empirical data from 51 locations in the Atlantic, Pacific, Indian, Arctic, and Antarctic oceans reveal that internal solitary waves travel in packets. Internal waves commonly exhibit (1) higher wave amplitudes (5–50 m [16–164 ft]) than surface waves (2 m [6.56 ft]), (2) longer wavelengths (0.5–15 km [0.31–9 mi]) than surface waves (100 m [328 ft]), (3) longer wave periods (5–50 min) than surface waves (9–10 s), and (4) higher wave speeds (0.5–2 m s–1 [1.64–6.56 ft s–1]) than surface waves (25 cm s–1 [10 in. s–1]). Maximum speeds of 48 cm s–1 (19 in. s–1) for baroclinic currents were measured on guyots. However, core-based sedimentologic studies of modern sediments emplaced by baroclinic currents on continental slopes, in submarine canyons, and on submarine guyots are lacking. No cogent sedimentologic or seismic criteria exist for distinguishing ancient counterparts. Outcrop-based facies models of these deposits are untenable. Therefore, potential exists for misinterpreting deep-marine baroclinic sands as turbidites, contourites, basin-floor fans, and others. Economic risks associated with such misinterpretations could be real.

Stages of sedimentary prism development on a convergent margin — Eocene Tyee Forearc Basin, Coast Range, Oregon, USA

Architecture of ancient forearc basin successions can be difficult to reconstruct because of the widespread syn-depositional and post-depositional deformations experienced by many forearc basin-fills. For this reason various techniques have been used for reconstructing forearc basin-infill geometry, including geochemical correlation. The Tyee Basin succession exposed in Coast Range of Western Oregon, USA, is an Eocene forearc-fill that includes genetically related non-marine, shallow marine and deepwater clastic deposits and is gently deformed. Reconstruction of the depositional geometry of the Tyee Basin succession from detailed outcrop and subsurface data reveals two distinct stages of development for this active basin-margin. These stages are characterized by two different basin-margin clinoform architectures and also by a pronounced change in the character of the associated deepwater deposits. During the initial stage, the basin-margin clinoforms are smaller (<250m clinoform height) and strongly progradational, with clinoform topset dominated by fluvial deposits. At this stage thick sand-rich unconfined turbidite beds accumulated on the slope segment of the clinoforms and extended out onto the basin-floor. Large scale slope conduits such as slope channels or canyons, are notably absent in this stage. The second stage is characterized by larger clinoform height (>500m), a greater degree of topset aggradation with repeated fluvio-deltaic cycles on the shelf, and well-organized, large turbidite channels on the slope. The turbidite channels supplied medium-grained sands to the extensive, stacked basin-floor fans. The first stage described above marks the early development of a shelf-slope prism on the Tyee continental margin, and has been interpreted by some earlier workers as an unique category of basin-margin architecture, termed as a ‘submarine ramp’. However, this was only the initial stage of development of the Tyee margin and it was followed by a period of basin-filling when repeated fluvial and shallow marine shelf-transit cycles fed well-organized turbidite channels on the slope as well as Tyee Basin floor fans. The large volume of sediment deposited during the initial stage, resulted from of the unique geometry of the Tyee Basin, as influenced by the presence of pre-existing topography on the accreted oceanic basement underlying the Tyee succession.

Paleoclimatology and Paleoecology across the Eocene/Oligocene boundary, Thrace Basin, Northeast Aegean Sea, Greece

The detailed biostratigraphic and ichnological analysis of the Upper Eocene/Lower Oligocene forearc sedimentary sequence in the southern Greek part of the Thrace Basin on Lemnos Island reveals a relationship between eco-sedimentary factors and the base-level dynamics influencing calcareous nannofossils and trace fossil assemblages. The lower part of the ~350m thick succession was deposited in a sand-rich submarine fan system and upper part in a shelf environment. It contains nannofossil assemblages and distributions typical of the NP19–NP21 biozones. The Eocene/Oligocene (E/O) boundary (top of NP20 biozone) is clearly distinguished within the upper parts of the turbidite system where a decrease in water temperature, salinity, nutrient supply and oxygenation level is also revealed. Additionally, 19 ichnospecies, mostly graphoglyptids typical of deep-sea fan deposits, have been distinguished. The trace fossil diversity and abundance decrease upward reflecting the impact of the changing depositional environments from basin floor fans, to slope systems and then shelfal environments. The regional basin-fill history, rather than global climatic changes, was probably the fundamental controlling factor on the distribution of graphoglyptids and other trace fossils across the E/O boundary.

Characterizing the Paleocene turbidites of the North Sea: the Mey Sandstone Member, Lista Formation, UK Central Graben

This paper presents an integrated seismic, petrophysical and core facies study of the Mey Sandstone Member of the Central North Sea Lista Formation. Seismic mapping and attribute analysis reveal that the Mey Sandstone Member is composed of distinct axial and lateral routing systems. In turn, the axial system can be divided into coeval western and eastern fairways defined by the underlying graben topography in a similar manner to the overlying Sele Formation (Forties) sandstones. These trends are confirmed by petrophysical analysis, which also reveals that the lateral systems are not as important as previously proposed and that the cycles of the Mey Sandstone Member prograded over time before a late stage of backstepping. These variations can be related directly to published sea-level curves. Core analysis reveals that mean grain size is the main control on sandstone quality and that similar proximal (channelized) to distal (sheet-like) changes in sedimentological facies occur to those described in the Sele Formation. It is argued that these deposits cannot be described as simple basin floor fans due to the impact of topography on turbidite flow routing and the existence of multiple entry points of sediment into the basin.

Seismic Facies in a Deepwater Area of a Marine Faulted Basin: Deepwater Area of the Paleogene Lingshui Formation in the Qiongdongnan Basin

Abstract:In recent years, deep water areas have become popular exploration fields because of their abundant hydrocarbon resource potential. There are only relatively poor planar seismic profiles and no wells for deepwater areas of the Lingshui Formation in the Qiongdongnan Basin. A lot of faults developed and strata are fragmented due to high temperatures and high pressure, and this has resulted in dim sequence boundaries. Based on seismic data of the deepwater area and well data of bordering shallow water areas, Lingshui Formation was divided into four third class sequences; namely SI, SII, SIII and SIV, and the three‐dimensional isochronous stratigraphic framework of the Lingshui Formation in the studied area was shaped. Based mainly on seismic attributes such as amplitude, continuity, internal structure and external shape, six typical seismic facies were identified, including mat‐shaped, filling, wedge‐shaped, foreset, moundy‐shaped and lenticular‐shaped, and a seismic facies distribution map was subsequently drawn. With studies on wells of bordering shallow water areas, regional sedimentary characteristics, and isopach map as references, sedimentary planar distribution features were analyzed. The deepwater area of the Lingshui Formation has mainly developed littoral and shallow sea. Sandstone bodies of fan delta, braided river delta, slope fan, basin floor fan, and turbidite fan are at an interdigitate junction to marine hydrocarbon source rocks and thus are favorable prospecting targets.

Confined to unconfined: Anatomy of a base of slope succession, Karoo Basin, South Africa

Two contemporaneous weakly confined deepwater systems form Unit B of the Permian Laingsburg Formation are sufficiently well-exposed to allow investigation of the down dip passage from channelized base of slope to distributive deposits over a 25km dip section, with strike control over some 20km. A high resolution stratigraphy was established over a 1200m strike section in the proximal Skeiding locality and extended regionally at a coarser scale. Analysis indicates that Unit B comprises 3 depositional sequences which, at a regional scale, thin towards the N and E. The lowstand systems tract of sequence 1 comprises weakly confined high-aspect ratio, vertically stacked channels cut into a basal frontal lobe system, and are overlain by a regionally correlated condensed hemipelagic mudstone interval (combined TST/HST) that shows evidence of remobilisation in up-dip areas The LST of sequence 2 includes two superimposed channel complexes of different styles that become deeply entrenched 5km down dip and pass basinward into tabular, distributive lobe sandstones. Sequence 3 is marked by a regional 100m thick levee complex related to a lower slope channel system and marks a basinward shift in facies. Channel fills in the lowstand systems tracts of all 3 sequences commonly include a thin drape of mudstone-clast conglomerate over the basal erosion surface, onlapped by thin-bedded sandstones with tractional structures that exhibit a characteristic axis to off-axis transition in facies. These deposits accumulated during periods of sediment bypass and are overlain by amalgamated structureless sandstones which thin from the axis into characteristic wings that extend laterally up to 200m. Levees are absent in the lower two sequences and flows appear to have been only weakly confined by basal erosional keels. The resultant succession is extremely sandstone rich (90%), a much higher percentage than in underlying basin floor fan and overlying slope channel-levee complexes. The levee deposits of sequence 3 are much less sandy (30%), consistent with a more proximal, slope setting. In the 1400m thick Karoo deepwater succession the base of slope channel sandstones of Unit B represent the maximum sandstone content and connectivity, which has predictive implications for hydrocarbon reservoir development in weakly confined deepwater channel systems.

Paleocene deep-water depositional systems in the North Sea Basin

Interpretation of 3D seismic reflection data supported by well data provides insights into the geometry of early Paleogene depositional systems along the eastern margin of the North Viking Graben. These deposits, which consist mainly of sandstones interbedded with claystones and siltstones, are interpreted to document deposition at the edge of a large base-of-slope to proximal basin floor fan system which was sourced from the eastern basin margin. Individual sandstone bodies are up to 80 m thick and occur within four sandstone-prone packages (DU1–4), and well-to-seismic ties indicate that the thicker sandstones (&gt;10 m) are represented by channelized or sheet-like, high-amplitude anomalies. Both well and seismic data suggest that the sandstones are of limited lateral extent (&lt;1–5 km), implying they were deposited in a series of channels. Channelization and compensational stacking of sandstones may have been at least partly controlled by differential compaction across previously deposited sandbodies. The study reveals that deep-water depositional patterns are more complex than is apparent from previous, lithostratigraphically-driven correlations and from regional isochron mapping. In particular, this study has implications for the controls on the distribution and reservoir architecture of deep-water sandstones.

Multi-scale characterisation of the Paaratte Formation, Otway Basin, for CO2 injection and storage

A resurgent interest in the gas potential of the Gulf of Papua has been enhanced by the recent announcement of developing new LNG facilities in Port Moresby. Petroleum Geo-Services (PGS) has recently acquired a 6,000-km2Multi-Client 3D (MC3D) survey in the gulf. Although most of the discoveries in PNG are from the highland areas, the gulf is a proven hydrocarbon province with gas/condensate accumulations previously discovered in several reef build-ups. It is thought that sediments from the Papuan Fold Belt produced clastic dominated deltas in the Gulf area. Although this play is untested offshore, there is evidence these sandy facies exist and could contain hydrocarbons. On the first phase of acquisition/interpretation, an efficient screening workflow called Prospect Scanner, developed to highlight areas of AVO effects in large 3D seismic datasets, was tested. It uses pre-stack seismic time migrated (PSTM) gathers to extract AVO attributes that are then inverted to derive relative acoustic and shear impedance volumes. Using idealised cross-plots, the relationship between Vp/Vs ratio and the Ip values gives a good indication of the fluid and lithology of the tested interval. The results are loaded into a conventional interpretation package for interpretation/visualisation. Prospect Scanner has highlighted previously undrilled turbidite and basin floor fan prospects in the region. The Vp/Vs versus Ip cross-plot shows clustering ofthe sand anomaly and good separation from the majority of the plot. These points correspond to the idealised location for a gas sand. Past concerns about lateral prediction of reservoir presence and quality associated with the basin floor fans can be addressed through this workflow.

Amplitude-versus-offset (AVO) screening in frontier basins: an example from the Gulf of Papua, Papua New Guinea

A resurgent interest in the gas potential of the Gulf of Papua has been enhanced by the recent announcement of developing new LNG facilities in Port Moresby. Petroleum Geo-Services (PGS) has recently acquired a 6,000-km2 Multi-Client 3D (MC3D) survey in the gulf. Although most of the discoveries in PNG are from the highland areas, the gulf is a proven hydrocarbon province with gas/condensate accumulations previously discovered in several reef build-ups. It is thought that sediments from the Papuan Fold Belt produced clastic dominated deltas in the Gulf area. Although this play is untested offshore, there is evidence these sandy facies exist and could contain hydrocarbons. On the first phase of acquisition/interpretation, an efficient screening workflow called Prospect Scanner, developed to highlight areas of AVO effects in large 3D seismic datasets, was tested. It uses pre-stack seismic time migrated (PSTM) gathers to extract AVO attributes that are then inverted to derive relative acoustic and shear impedance volumes. Using idealised cross-plots, the relationship between Vp/Vs ratio and the Ip values gives a good indication of the fluid and lithology of the tested interval. The results are loaded into a conventional interpretation package for interpretation/visualisation. Prospect Scanner has highlighted previously undrilled turbidite and basin floor fan prospects in the region. The Vp/Vs versus Ip cross-plot shows clustering of the sand anomaly and good separation from the majority of the plot. These points correspond to the idealised location for a gas sand. Past concerns about lateral prediction of reservoir presence and quality associated with the basin floor fans can be addressed through this workflow.

Sequence Stratigraphic Analysis of “XB Field”, Central Swamp Depobelt, Niger Delta Basin, Southern Nigeria

Well logs and biostratigraphic data from six wells in the “XB Field”, central Swamp Depobelt, Niger Delta were integrated to carry out a sequence stratigraphic analysis of depositional systems in the field. The analysis revealed four 3rd order depositional sequences (SEQ1 to 4) bounded by three erosional unconformities interpreted as Sequence Boundaries (SB1 to 3). Transgressive Surfaces of Erosion (TSE1 to 3) that mark the onset of marine flooding and turnarounds from progradational facies to retrogradational facies during sequence build-up were delineated. Three 3rd order Maximum Flooding Surfaces (MFS1, MFS2 and MFS3) characterized by marker shales, high faunal abundance and diversity were also delineated and dated 15.9, 17.4 and 19.4 Ma, respectively. The delineated sequences comprised Lowstand Systems Tracts (progradational packages), Transgressive Systems Tracts (retrogradational packages) and Highstand Systems Tracts (aggradational packages), which reflect depositional systems deposited during different phases of base level changes. The Lowstand Systems Tract (LST) consists of Basin Floor Fans (BFF), Slope Fans and Channel Sands deposited when sea level was low and accommodation space lower than rate of sediment influx. Transgressive Systems Tract (TST) consists of retrogradational marine shales deposited during high relative sea levels and when accommodation space was higher than rate of sediment influx. Highstand Systems Tracts (HST) consisted of shoreface sands displaying mostly aggradational to progradational stacking patterns. The sands of LST and HST show good reservoir qualities while the shales of the TSTs could form potential reservoir seals. The above recognized sequences, were deposited within the Neritic to Bathyal paleoenvironments and are dated mid-Miocene (15.9 - 20.4 Ma) in age.

Porosity-Permeability and Textural Parameters of the Palaeogene Forearc Sedimentary Fill on Lemnos Island, NE Greece

The Palaeogene forearc sedimentary fill on Lemnos Island, NE Greece, was examined to determine reservoir characteristics and textural parameters. During this time interval the studied area was the site of accumulation of submarine fans that underlie shelf deposits, with tectonic activity responsible for the shallowing upward trend. Turbidites were deposited in both inner (slope fan facies) and outer parts (basin floor fan facies) of the submarine fan system and consist of alternating sandstone and mudstone beds. Sandstones occur in both complete and incomplete Bouma sequences while shelf deposits have been interpreted as storm-surge deposits on the deeper parts of shelves. The 'Mercury Porosimetry Technique' was carried out on 20 sandstone samples, while 30 sandstone samples were examined under a polarizing microscope for grain-size analysis. This porosity-permeability study suggests that these rocks can be considered as both oil and gas reservoirs. 'Slope' fan facies generally reveal the most efficient values, making them the most promising sub-environment for further hydrocarbon research. Most samples display two pore-size distributions suggesting major textural heterogeneity. Textural parameter analysis reveals that sorting was of great importance during sedimentation. Rocks are generally well to very well-sorted while samples with moderate sorting are also present. This fact can be both attributed to the restricted grain-size range and their possible great distance from the source area. This generally well-sorted sequence argues well for further hydrocarbon research in the Northeast Aegean Sea, since the higher the sorting the higher the porosity. Selected samples are generally very fine to fine grained, whereas medium-grained sandstones are extremely rare and mostly seen on 'slope' fan facies. The finest grained sandstones are the best sorted.

Controls on the development of valleys, canyons, and unconfined channel–levee complexes on the Pleistocene Slope of East Kalimantan, Indonesia

Shallow 3D seismic data show contrasting depositional patterns in Pleistocene deepwater slopes of offshore East Kalimantan, Indonesia. The northern East Kalimantan slope is dominated by valleys and canyons, while the central slope is dominated by unconfined channel–levee complexes. The Mahakam delta is immediately landward of the central slope and provided large amounts of sediments to the central slope during Pleistocene lowstands of sea level. In the central area, the upper slope contains relatively straight and deep channels. Sinuous channel–levee complexes occur on the middle and lower slope, where channels migrated laterally, then aggraded and avulsed. Younger channel–levee complexes avoided bathymetric highs created by previous channel–levee complexes. Levees decrease in thickness down slope. Relief between channels and levees also decreases down slope. North of the Mahakam delta, siliciclastic sediment supply was limited during the Pleistocene, and the slope is dominated by valleys and canyons. Late Pleistocene rivers and deltas were generally not present on the northern outer shelf. Only one lowstand delta was present on the northern shelf margin during the upper Pleistocene, and sediments from that lowstand delta filled a pre-existing slope valley complex and formed a basin-floor fan. Except for that basin-floor fan, the northern basin floor shows no evidence of sand-rich channels or fans, but contains broad areas with chaotic reflectors interpreted as mass transport complexes. This suggests that slope valleys and canyons formed by slope failures, not by erosion associated with turbidite sands from rivers or deltas. In summary, amount of sediment coming onto the slope determines slope morphology. Large, relatively steady input of sediment from the Pleistocene paleo-Mahakam delta apparently prevented large valleys and canyons from developing on the central slope. In contrast, deep valleys and canyons developed on the northern slope that was relatively “starved” for siliciclastic sediment.

Architecture of gas-hydrate-bearing sands from Walker Ridge 313, Green Canyon 955, and Alaminos Canyon 21: Northern deepwater Gulf of Mexico

Logging-while-drilling data acquired during the 2009 Gulf of Mexico (GoM) Gas Hydrate Joint Industry Project Leg II program combined with features observed in seismic data allow assessment of the depositional environment, geometry, and internal architecture of gas-hydrate-bearing sand reservoirs from three sites in the northern Gulf of Mexico (GoM): Walker Ridge 313, Alaminos Canyon 21, and Green Canyon 955. The site descriptions assist in the understanding of the geological development of gas-hydrate-bearing sands and in the assessment of their energy production potential. Three sand-rich units are described from the Walker Ridge site, including multiple ponded sand-bodies representing turbidite channel and associated levee and terminal lobe environments within the Terrebonne basin on the lower slope of the GoM. Older units display fewer but greater-reservoir-quality channel and proximal levee facies as compared to thinner, more continuous, and unconfined sheet-like sands that characterize the younger units, suggesting a decrease in depositional gradient with time in the basin. The three wells in the Green Canyon 955 site penetrated proximal levee sands within a previously recognized Late Pleistocene basin floor turbidite-channel-levee complex. Reservoirs encountered in GC955 exhibit thin-bedded internal structure and complex fault compartmentalization. Two wells drilled in the Alaminos Canyon 21 site tested a large, shallow, sand unit within the Diana mini-basin that exhibits steep lateral margins, non-sinuous elongate form, and flat base with hummocky upper surface. These features suggest deposition as a mass-transport deposit consisting of remobilized sand-rich turbidites or as a large basin-floor fan that was potentially eroded and buried by later-stage, mud-prone, mass-transport deposits.

Assessing controls on the distribution of ichnotaxa in submarine fan environments, the Basque Basin, Northern Spain

The Cretaceous to Eocene aged Basque Basin in northern Spain preserves a variety of submarine fan related environments of deposition in well exposed outcrops. Here, quantitative trace fossil data is calibrated to sedimentologically defined environments of deposition. Nine environments of deposition are assigned according to sedimentary facies associations, depositional architecture and stratigraphic context. The preserved trace fossil assemblages are interpreted in terms of extrinsic palaeoecological, and intrinsic taphonomical and depositional environment controls. In channel related environments, diverse pre-depositional dominated assemblages are prevalent in marginal settings. These are replaced by low diversity post-depositional dominated assemblages in more axial positions within the system. Lobe related environments display a higher level of diversity and bioturbation intensity in more distal/off axis (lobe fringe) environments compared to the most axial lobe environments that preserve low diversity, and exclusively post-depositional assemblages (dominated by Ophiomorpha). Diverse pre- and post-depositional assemblages are common in fan fringe deposits, with less diverse assemblages dominated by post-depositional ichnotaxa (particularly Zoophycos) in basin floor fan deposits. The use of ‘sub-ichnofacies’ is found to only aid characterisation of trace fossil assemblages based on a general (proximal–distal) position within a submarine fan system. Tier preservation of trace fossil assemblages is almost exclusively determined by the depth of erosion that a substrate is subjected to. As such, colonisation style (pre- versus post depositional) and ethological groupings are proposed as being the most powerful tool in assisting sedimentological observations in assigning environments of deposition.

Linking Onshore-Offshore Sediment Dispersal in the Golo Source-to-Sink System (Corsica, France) During the Late Quaternary

Abstract Spatial and temporal relationships between climate, tectonics, and sea level have a primary control on sediment transport, storage, and deposition in onshore and offshore depositional environments. Although many simplistic models have tried to predict onshore system behavior and sediment partitioning between onshore and offshore depositional environments in response to changes in these boundary conditions, the alluvial response to changes in external forcings coupled with autogenic processes can be highly complex and unpredictable. The Golo source-to-sink system, on the eastern margin of the island of Corsica in the Mediterranean Sea, provides an ideal laboratory to study sediment partitioning in both onshore and offshore realms. The terrestrial part of the system consists of the Golo River, which debouches onto a narrow shelf (~ 10 km), which in turn passes into a narrow (~ 45 km) confined basin known as the Corsica Trough, dominated by submarine fan deposits. Estimates on timing of late Quaternary fluvial aggradation are compared with timing of sediment storage on the shelf and on the basin-floor fan. The results indicate that onshore deposition and storage of sediment in the fluvial system may occur both during sea-level highstand, transgression, and lowstand. Volume calculations from the alluvial record show that onshore storage is relatively low (~ 13%) relative to the overall sediment budget. Comparing estimated deposition rates with a sediment prediction model further suggests that deep-sea fan sedimentation rates at times may be up to 50% higher or 25% lower than what is being supplied by the river, indicating temporary storage and release of sediment on the shelf. On average, however, there is good agreement between predicted and calculated sediment volumes in the Golo system. The study demonstrates the value of investigating the entire source-to-sink system in order to obtain a comprehensive understanding of sediment dispersal between onshore and offshore depositional environments over 104 year timescales. Observations suggest that alternating periods of aggradation and degradation of the Golo River are controlled by system thresholds controlled from within the catchment. Comparison with climate proxy data and timing of major alluviation events elsewhere in the Mediterranean region supports the notion that each source-to-sink system has a unique threshold that must be exceeded to induce regional aggradation and subsequent terrace formation. The same is also inferred for timing of sediment dispersal to the deep-sea environment. In addition, preservation of onshore sediment is controlled by local factors such as uplift rate and its impact on the aggradational response. It is, therefore, expected that the onshore record may be diachronous on local, regional, and global scales, making detailed correlation difficult except for the most extreme events. Finally, stream incision rates appear to be one to two orders of magnitude higher than regional hinterland denudation rates, indicating that the Golo system is characterized by increasing landscape relief.

Relationships between shelf-edge trajectories and sediment dispersal along depositional dip and strike: a different approach to sequence stratigraphy

Analysis of shelf-edge trajectories in prograding successions from offshore Norway, Brazil, Venezuela and West Africa reveals systematic changes in facies associations along the depositional dip.These changes occur in conjunction with the relative sea-level change, sediment supply, inclination of the substratum and the relief of the margin. Flat and ascending trajectories generally result in an accumulation of fluvial and shallow marine sediments in the topset segment. Descending trajectories will generally result in erosion and bypass of the topset segment and deposition of basin floor fans. An investigation of incised valley fills reveals multiple stages of filling that can be linked to distinct phases of deepwater fan deposition and to the overall evolution of the margin. In the case of high sediment supply, like the Neogene Niger and Orinoco deltas, basin floor fans may develop systematically even under ascending trajectory styles. In traditional sequence stratigraphic thinking, this would imply the deposition of basin floor fans during a period of relative sea-level highstand. Facies associations and sequence development also vary along the depositional strike.The width and gradient of the shelf and slope show considerable variations from south to north along the Brazilian continental margin during the Cenozoic. During the same time interval, the continental shelf may display high or low accommodation conditions, and the resulting stacking patterns and facies associations may be utilized to reconstruct palaeogeography and for prediction of lithology. Application of the trajectory concept thus reveals nuances in the rock record that would be lost by the application of traditional sequence stratigraphic work procedures. At the same time, the methodology simplifies the interpretation in that less importance is placed on interpretation and labelling of surface boundaries and systems tracts.