The architecture of the lower parts of submarine canyons on the western Nigerian continental margin

Multi-beam,sub-bottom and multichannel seismic data acquired from the western Nigerian continental margin are analysed and interpreted to examine the architectural characteristics of the lower parts of the submarine canyons on the margin.The presence of four canyons： Avon,Mahin,Benin,and Escravos,are confirmed from the multi-beam data map and identified as cutting across the shelf and slope areas,with morphological features ranging from axial channels,moderate to high sinuosity indices,scarps,terraces and nickpoints which are interpreted as resulting from erosional and depositional activities within and around the canyons.The Avon Canyon,in particular,is characterised by various branches and sub-branches with complex morphologies.The canyons are mostly U-shaped in these lower parts with occasional V-shapes down their courses.Their typical orientation is NE–SW.Sedimentary processes are proposed as being a major controlling factor in these canyons.Sediments appear to have been discharged directly into the canyons by rivers during the late Quaternary low sea level which allows river mouths to extend as far as the shelf edge.The current sediment supply is still primarily sourced from these rivers in the case of the Benin and Escravos Canyons,but indirectly in the case of the Avon and Mahin Canyons where the rivers discharge sediments into the lagoons and the lagoons bring the sediments on to the continental shelf before they are dispersed into the canyon heads.Ancient canyons that have long been buried underneath the Avon Canyon are identified in the multichannel seismic profile across the head of the Avon Canyon,while a number of normal faults around the walls of the Avon and Mahin Canyons are observed in the selected sub-bottom profiles.The occurrence of these faults,especially in the irregular portions of the canyon walls,suggests that they also have some effect on the canyon architecture.The formation of the canyons is attributed to the exposure of the upper marginal area to incisions from erosion during the sea level lowstand of the glacial period.The incisions are widened and lengthened by contouric currents,turbidity currents and slope failures resulting in the canyons.

Submarine canyons on the north of Chiwei Island:influenced by recent extension of the southern Okinawa Trough

Based on new multibeam bathymetric data and about 300 km long single seismic profiles, three topographic units were identified： the canyons, fractural valley and submarine terrace on the north of Chiwei Island where is a structural transition zone between the southern trough and the middle trough. The Chiwei Canyon and the North Chiwei Canyon are two of the largest canyons in the East China Sea （ECS） slope. Topographic features and architectures of them are described. The study shows that both of them are originated along faults. The evolution and spatial distribution of topographic units in the study area are controlled mainly by three groups of faults which were formed and reactive in the recent extensional phase of Okinawa Trough. The Chiwei Canyon was initia- ted during the middle Pleistocene and guided by F4 that is a N--S trending fault on the slope and F1, a large NW--SE trending fault on the trough. The pathway migration from the remnant channel to the present one of Chiwei Canyon is the result of uplift of tilted fault block that is coupled to the recent extension movements of the southern trough. The submarine terrace is detached from the ECS slope by the NEE -trending fault. The North Chiwei Canyon, developing during the late Pleistocene, is guided by FS, a N-S trending fault, diverted and blocked by the submarine terrace.

Gravity tectonics controls on reservoir-scale sandbodies:Insights from 3D seismic geomorphology of the canyons buried in the upper slope of the Eastern Niger delta basin

High-resolution 3D seismic data analysis was integrated with a calibrated well and biostratigraphy data to present the first overview of a buried Pleistocene canyon system on the upper slope of the eastern Niger Delta,the Galabor Canyon.Attribute maps of specific horizons allow documenting the changing morphologies and infill lithologies of two main branches of the canyon through two stages of activity separated by a reference horizon dated at 0.99 Ma based on well calibration.At the upper slope,growth faults dissect the canyon heads,the catchment of which encroaches a network of valleys incised on the outer shelf.The canyon fill is composed of muddy channels and mass-transport deposits,largely derived from the collapse of canyon walls and sand-rich bodies forming a tract sourced by shelf-edge deltas at the outlet of the incised valleys.High-density turbiditic processes likely control the distribution of sand bodies along the canyon,ranging from tributary fans on the upper slope to 6 km-wide meander belts on the middle slope.The sandy deposits accumulate in minibasins formed along the canyon path,downstream of the subsiding hanging wall of the growth faults and upstream of shale ridges that damp the flow in the canyon.These results show that canyons can be major targets for sand reservoir exploration on the upper slope of large muddy deltas.

Mass movements in small canyons in the northeast of Baiyun deepwater area, north of the South China Sea

The process of mass movements and their consequent turbidity currents in large submarine canyons has been widely reported, however, little attention was paid to that in small canyons. In this paper, we document mass movements in small submarine canyons in the northeast of Baiyun deepwater area, north of the South China Sea （SCS）, and their strong effects on the evolution of the canyons based on geophysical data. Submarine canyons in the study area arrange closely below the shelf break zone which was at the depth of -500 m. Within submarine canyons, seabed surface was covered with amounts of failure scars resulted from past small-sized landslides. A complex process of mass transportation in the canyons is indicated by three directions of mass movements. Recent mass movement deposits in the canyons exhibit translucent reflections or parallel reflections which represent the brittle deformation and the plastic deformation, respectively. The area of most landslides in the canyons is less than 3 km2. The trigger mechanisms for mass movements in the study area are gravitational overloading, slope angle and weak properties of soil. Geophysical data indicate that the genesis of submarine canyons is the erosion of mass movements and consequent turbidity currents. The significant effects of mass movements on canyon are incision and sediment transportation at the erosion phases and fillings supply at the fill phases. This research will be helpful for the geological risk assessments and understanding the sediment transportation in the northern margin of the SCS.

A buried submarine canyon in the northwestern South China Sea: architecture, development processes and implications for hydrocarbon exploration

High-resolution multichannel seismic data enables the discovery of a previous,undocumented submarine canyon(Huaguang Canyon)in the Qiongdongnan Basin,northwestern South China Sea.The Huaguang Canyon with a NW orientation is 140 km in length,and 2.5 km to 5 km in width in its upper reach and 4.6 km to 9.5 km in width in its lower reach.The head of the Huaguang Canyon is close to the Xisha carbonate platform and its tail is adjacent to the Central Canyon.This buried submarine canyon is formed by gravity flows from the Xisha carbonate platform when the sea level dropped in the early stage of the late Miocene(around 10.5 Ma).The internal architecture of the Huaguang Canyon is mainly characterized by high amplitude reflections,indicating that this ancient submarine canyon was filled with coarse-grained sediments.The sediment was principally scourced from the Xisha carbonate platform.In contrast to other buried large-scale submarine canyons(Central Canyon and Zhongjian Canyon)in the Qiongdongnan Basin,the Huaguang Canyon displays later formation time,smaller width and length,and single sediment supply.The coarse-grained deposits within the Huaguang Canyon provide a good environment for reserving oil and gas,and the muddy fillings in the Huaguang Canyon have been identified as regional caps.Therefore,the Huaguang Canyon is a potential area for future hydrocarbon exploration in the northwestern South China Sea.The result of this paper may contribute to a better understanding of the evolution of submarine canyons formed in carbonate environment.

A buried submarine canyon in the northwest South China Sea:architecture,development processes and implications for hydrocarbon exploration

High-resolution multichannel seismic data enables the discovery of a previous,undocumented submarine canyon(Huaguang Canyon)in the Qiongdongnan Basin,northwest South China Sea.The Huaguang Canyon with a NW orientation is 140 km in length,and 2.5 km to 5 km in width in its upper reach and 4.6 km to 9.5 km in width in its lower reach.The head of the Huaguang Canyon is close to the Xisha carbonate platform and its tail is adjacent to the central canyon.This buried submarine canyon is formed by gravity flows from the Xisha carbonate platform when the sea level dropped in the early stage of the late Miocene(~10.5 Ma).The internal architecture of the Huaguang Canyon is mainly characterized by high amplitude reflections,indicating that this ancient submarine canyon was filled with coarse-grained sediments.The sediment was principally scourced from the Xisha carbonate platform.In contrast to other buried large-scale submarine canyons(central canyon and Zhongjian Canyon)in the Qiongdongnan Basin,the Huaguang Canyon displays later formation time,smaller width and length,and single sediment supply.The coarse-grained deposits within Huaguang Canyon provide a good environment for reserving oil and gas,and the muddy fillings in Huaguang Canyon have been identified as regional caps.Therefore,Huaguang Canyon is potential area for future hydrocarbon exploration in the northwest South China Sea.Our results may contribute to a better understanding of the evolution of submarine canyons formed in carbonate environment.

Mounded seismic units in the modern canyon system in the Shenhu area,northern South China Sea:Sediment deformation,depositional structures or the mixed system?

The canyon system,including 17 small slope-confined canyons in the Shenhu area,northern South China Sea,is significantly characterized by mounded or undulating features on the canyon flanks and canyon heads.However,the mechanism underlying the formation of these features has yet to be elucidated.In previous studies,most of them were interpreted as sediment deformation on the exploration seismic profiles.In this paper,we collected high-resolution bathymetric data,chirp profiles and geotechnical test data to investigate their detailed morphology,internal structures,and origin.The bathymetric data indicated that most mounded seismic units have smooth seafloors and are separated by grooves or depressions.The distance between two adjacent mounded units is only hundreds of meters.On chirp profiles,mounded seismic units usually exhibit chaotic reflections and wavy reflections,of which the crests migrate upslope.The slope stability analysis results revealed that the critical angle of the soil layers in the study area tends to be 9°,indicating that most mounded seismic units on the canyon flanks and heads are stable at present.The terrain characteristics and seismic configurations combined with the slope stability analysis results indicated that most mounded seismic units are not sediment deformation but depositional structures or mixed systems composed of deformation and depositional structures.

Deposition processes of gas hydrate-bearing sediments in the inter-canyon area of Shenhu Area in the northern South China Sea

The Shenhu Submarine Canyon Group on the northern slope of the South China Sea consists of 17 slope-confined canyons,providing a good example for investigating their hosting sediments.Three drilling sites,including W07,W18,and W19,have proven the occurrence of gas hydrate reservoirs in the inter-canyon area between canyons C11 and C12.Whereas,variations of the geomorphology and seismic facies analyzed by high-resolution 3D seismic data indicate that the gas hydrate-bearing sediments may form in different sedimentary processes.In the upper segment,a set of small-scale channels with obvious topographic lows can be identified,revealing fine-grained turbidites supplied from the shelf region during a very short-term sea-level lowstand.In the middle part,gas hydrate units at Site W07 showing mounded or undulation external configuration are interpreted as sliding sedimentary features,and those features caused by gravity destabilization were the main formative mechanism of gas hydrate-bearing sediments that were sourced from the upper segments.In contrast,for the canyon transition zone of lower segments between C11-C12 inter-canyon and C12 intra-canyon areas,where W18 and W19 sites are located,the gas hydratebearing sediments are deposited in the channelized feature in the middle to lower segment and slide erosive surface.Gas hydrate-bearing sediments of the lower segment were migrated through channelized features interconnecting with the middle to lower slope by gravity-driven flows.The majority of deposits tended to be furtherly moved by lateral migration via erosive surface created by sediment failed to intra-canyon area.The conclusion of this study may help better understand the interaction between the formation mechanism of gas hydrate-bearing sediments and the geomorphologic effects of inter-canyon areas.

Formation of Hengchun Accretionary Prism Turbidites and Implications for Deep-water Transport Processes in the Northern South China Sea

Located at the end of the northern Manila Trench,the Hengchun Peninsula is the latest exposed part of Taiwan Island,and preserves a complete sequence of accretionary deep-sea turbidite sandstones.Combined with extensive field observations,a’source-to-sink’approach was employed to systematically analyze the formation and evolutionary process of the accretionary prism turbidites on the Hengchun Peninsula.Lying at the base of the Hengchun turbidites are abundant mafic normal oceanic crust gravels with a certain degree of roundness.The gravels with U-Pb ages ranging from 25.4 to23.6 Ma are underlain by hundreds-of-meters thickness of younger deep-sea sandstone turbidites with interbedded gravels.This indicates that large amounts of terrigenous materials from both the’Kontum-Ying-Qiong’River of Indochina and the Pearl River of South China were transported into the deep-water areas of the northern South China Sea during the late Miocene and further eastward in the form of turbidity currents.The turbidity flow drastically eroded and snatched mafic materials from the normal South China Sea oceanic crust along the way,and subsequently unloaded large bodies of basic gravel-bearing sandstones to form turbidites near the northern Manila Trench.With the Philippine Sea Plate drifting clockwise to the northwest,these turbidite successions eventually migrated and,since the Middle Pleistocene,were exposed as an accretionary prism on the Hengchun Peninsula.

Glossary:A supplement to “Submarine fans:A critical retrospective(1950-2015)” in the Journal of Palaeogeography (2016,5[2])

In this Gtossary, setected terms and concepts associated with submarine fans are defined.

Submarine fans： A critical retrospective (1950-2015)

When we look back the contributions on submarine fans during the past 65 years （1950 -2015）, the empirical data on 21 modern submarine fans and I0 ancient deep-water sys- tems, published by the results of the First COMFAN （Committee on FANs） Meeting （Bouma eta｜., 1985a）, have remained the single most significant compilation of data on submarine fans. The 1970s were the ＂heyday＂ of submarine fan models. In the 21st century, the general focus has shifted from submarine fans to submarine mass movements, internal waves and tides, and contourites. The purpose of this review is to illustrate the complexity of issues surrounding the origin and classification of submarine fans. The principal ele- ments of submarine fans, composed of canyons, channels, and lobes, are discussed using nine modern case studies from the Mediterranean Sea, the Equatorial Atlantic, the Gulf of Mexico, the North Pacific, the NE Indian Ocean （Bay of Bengal）, and the East Sea （Korea）. The Annot Sandstone （Eocene-Oligocene）, exposed at Peira-Cava area, SE France, which served as the type locality for the ＂Bouma Sequence＂, was reexamined. The field details are documented in questioning the validity of the model, which was the basis for the turbidite- fan link. The 29 fan-related models that are of conceptual significance, developed during the period 1970-2015, are discussed using modem and ancient systems. They are： （I） the classic submarine fan model with attached lobes, （2） the detached-lobe model, （3） the channel-levee complex without lobes, （4） the delta-fed ramp model, （5） the gully-lobe model, （6） the suprafan lobe model, （7） the depositional lobe model, （8） the fan lobe model, （9） the ponded lobe model, （I0） the nine models based on grain size and sediment source, （11） the four fan models based on tectonic settings, （12） the Jackfork debrite model, （13） the basin-floor fan model, （14） supercritical and subcritical fans, and （15） the three types of fan reservoirs. Each model is unique, and the long-standing belief that submarine fans are composed of turbidites, in particular, of gravelly and sandy high-density turbi- dites, is a myth. This is because there are no empirical data to validate the existence of gravelly and sandy high-density turbidity currents in the modern marine environments. Also, there are no experimental documentation of true turbidity currents that can trans- port gravels and coarse sands in turbulent suspension. Mass-transport processes, which include slides, slumps, and debris flows （but not turbidity currenrs）, are the most viable mechanisms for transporting gravels and sands into the deep sea. The prevailing notion that submarine fans develop during periods of sea-level lowstands is also a myth. The geologic reality is that frequent short-term events that last for only a few minutes to several hours or days （e.g., earthquakes, meteorite impacts, tsunamis, tropical cyclones, etc.） are more important in controlling deposition of deep-water sands than sporadic long- term events that last for thousands to millions of years （e.g., lowstand systems tract）. Submarine fans are still in a stage of muddled turbidite paradigm because the concept of high-density turbidity currents is incommensurable.

Controlling factors on the submarine canyon system: A case study of the Central Canyon System in the Qiongdongnan Basin, northern South China Sea

Based on an integrated analysis of high-resolution 2D/3D seismic data and drilling results, this study analyzes the tectonic- sedimentary evolution of the Qiongdongnan Basin （QDNB） since the late Miocene, and discusses the controlling factors on the formation and development of the Central Canyon System （CCS）. The sediment failures caused by the relative sea level falling might have discharged deposits from the slope to the canyon. The two suits of the infillings, i.e., turbidites and mass transport complex （MTC）, were derived from the northwestern source and northern source, respectively. The sediment supplies, which differ significantly among different areas, might have led to the variations observed in the internal architectures. Tectonic transformation around 11.6 Ma had provided the tectonic setting for the CCS and formed an axial sub-basin in the central part of the Changchang Depression, which could be called the rudiment of the CCS. The tectonic activity of the Red River Fault （RRF） at about 5.7 Ma might have strengthened the hydrodynamics of the deposits at the junction of the Yinggehal Basin （YGHB） and the QDNB to trigger a high-energy turbidity current. The MTC from the northern continental slope system might have been constrained by the Southern Uplift, functioning as a barrier for the infillings of the CCS. Thanks to a sufficient sediment supply during the Holocene period and the paleo-seafloor morphology, the relief of modern central canyon with the starving landform in the eastern Changchang Depression might have been accentuated by deposition of sediments and vertical growth along the canyon flanks, where collapse deposits were widely developed. Corresponding to the segmentation of the CCS, the forming mechanisms of the canyon between the three segments would be different. The turbidite channel in the head area had likely been triggered by the abundant sediment supply from the northwestern source together with the fault activity at about 5.7 Ma of the RRF. The formation and evolution of the canyon in the western segment were caused by combined effects of the turbidite channel from the northwestern source, the MTC from the northern continental slope, and the paleo-seafloor geomorphology. In the eastern segment, the canyon was constrained by the tectonic transformation occurring at approximately 11.6 Ma and the insufficient sediment supply from the wide-gentle slope.

Morphology, sedimentary characteristics, and origin of the Dongsha submarine canyon in the northeastern continental slope of the South China Sea

The Dongsha submarine canyon is a large canyon belonging to a group of canyons on the northeastern South China Sea margin Investigation of the Dongsha canyon is important for understanding the origin of this canyon group as well as the transport mechanism of sediments on the margin, and the evolution of the Taixinan foreland basin and the associated Taiwan orogenic belt. In this study, the morphology, sedimentary characteristics, and origin of the Dongsha canyon were investigated by inte- grating high-resolution multi-channel seismic reflection profiles and high-precision multibeam bathymetric data. This is a slope-confined canyon that originates in the upper slope east of the Dongsha Islands, extends downslope in the SEE direction, and finally merges with the South Taiwan Shoal canyon at a water depth of 3000 m. The total length and average width of the canyon are around 190 and 10 km, respectively. Eleven seismic sequence boundaries within the canyon fills were identified and interpreted as incision surfaces of the canyon. In the canyon fills, four types of seismic facies were defined： parallel onlap fill, chaotic fill, mounded divergent facies, and migrated wavy facies. The parallel onlap fill facies is interpreted as altemating coarser turbidites or other gravity-flow deposits and fine hemipelagic sediments filling the canyon valley. The chaotic fill faci- es is presumed to be debrites and/or basal lag deposits filling the thalwegs. The mounded divergent and migrated wavy seismic facies can be explained as canyon levees consisting mainly of overspilled fine turbidites and sediment waves on the levees or on the canyon-mouth submarine fans. Age correlation between the sequence boundaries and the ODP Site 1144 data suggests that the Dongsha canyon was initiated at approximately 0.9 Ma in the middle Pleistocene. Mapping of the canyon indicates that the canyon originated at the upstream portion of the middle reach of the modem canyon, and has been continuously expanding both upstream and downstream by retrogressive erosion, incision, and deposition of turbidity currents and other gravity transport processes. The ages of the sequence boundaries representing major canyon incision events are in good agreement with those of global sea-level lowstands, indicating that sea-level changes may have played an important role in the canyon＇s development. The Dongsha canyon developed in a region with an active tectonic background characterized by the Taiwan up- lifting and the development of the Taixinan foreland basin. However, no evidence suggests that the canyon formation is directly associated with local or regional faulting and magmatic activities. Turbidity currents and other gravity transport processes （includ- ing submarine slides and slumps） may have had an important influence on the formation and evolution of the canyon.

Sedimentary erosive processes and sediment dispersal in Kaoping submarine canyon

The Kaoping submarine canyon, connected to the Kaoping River in the coastal plain in SW Taiwan, continues the dispersal path of modern Kaoping River sediments, from an active small mountainous drain basin to the receiving basin of the South China Sea. Using seismic reflection sections, Chirp sonar profiles, and bathymetric mapping, we reveal characteristic erosive processes responsible for multiple cut-and-fill features, deeply entrenched thalweg, and sediment dispersal that are closely related to turbidity currents in the canyon. The river-canyon connection setting, along with extreme climatic conditions and active tectonism, is favorable for generation of turbidity currents at the canyon head. The upper reach of the Kaoping Canyon is distinguished into three distinct morpho/sedimentary features. The canyon head is characterized by V-shaped axial thalweg erosion. The sinuous segment of the upper reach is dominated by a deeply incised canyon pathway with trough-like morphol- ogy. Relatively small-scaled features of cut-and-fill associated with the dominant incision process are commonly along the canyon floor, resulting in a flat-floored pathway. Sliding and slumping dominated the steep canyon walls, producing and transporting sediments to canyon floor and partially filling up canyon thalweg. The meandering segment is characterized by erosive features where deeply down-cutting occurs in the outer bend of the major sea valley, forming V-shaped entrenched thalweg. The recurrences of turbidity currents have allowed continuous incision of the canyon head and have kept the connec- tion between the canyon head and the river mouth during Holocene highstand of sea level. The upper reach of the Kaoping Canyon is linked to drainage area and maintains as a conduit and/or sink for terrigenous and shallow marine material. Sediment-laden river plume operates in the Kaoping River-Canyon system, with turbidity currents flushing fiver sediments into the canyon head where the canyon thalweg is the most erosive. Presently, the upper reach of the Kaoping Canyon can be considered as a temporal sediment sink.

Controls on sediment transport from rivers to trenches in passive and active continental margins

Active and passive continental margins exhibit significant differences in sediment transport.However,opportunities to quantify these specific sedimentary processes remain limited.The northeastern South China Sea hosts both active and passive continental margins,offering a distinct opportunity to compare the relative contributions from a large passive continental margin with those from a smaller but more dynamic active margin.We present detrital zircon U-Pb ages from marine sediments in the northeastern South China Sea to decipher their provenance and reveal the control of transport processes in the margins.Our results show varied provenance of sediments from the shelf to the trench.In the continental shelf,sediments were primarily derived from the Jiulong and Gaoping rivers,controlled by the topography and ocean currents in the passive margin.In contrast,the sediments in the submarine canyons predominantly originated from the Gaoping and Zhuoshui rivers in western Taiwan Island.Quantitative results indicate that the Gaoping submarine canyon provided approximately 86.2%of sediments to the Manila Trench by directly linking to the Gaoping River in the active margin.Based on the provenance results,we propose a distinct sedimentary process in the active and passive margins.In passive margins,sediments from drainage systems enter the shelf and are transported by ocean currents,but topographic highs often block these sediments,preventing them from reaching the deep sea.In contrast,active margins feature narrow shelves,which facilitate direct connections between submarine canyons and river mouths,allowing gravity flows to carry sediments directly to the trenches.Our study provides an understanding of the sediments transported from rivers to trenches in the northeastern South China Sea and highlights the controls(ocean currents,dramatic topography,and gravity flows)during the sediment transport in the continental margins.

3D palaeogeographic reconstructions of the Phanerozoic versus sea-level and Sr-ratio variations:Discussion

Verard et el. （2015, Journal of Palaeogeography, 4（1 ）： 64-84） claim that their global geodynamic model allows one to reconstruct the surface features of topography on rand and in adjacent oceans （i.e., pateobathymetry） anywhere on the gtobe and at any geological time during the past 600 miltion years （Ma）. Such a grand model requires a rigorous scrutiny. The purpose of this discussion is to illustrate that the modet suffers from （1） the selective omission of real-world datasets that do not fit the model, （2） the inclusion of datasets without revealing their original sources or without citing relevant peer-reviewed publications, （3） the emphasis on ＇unpublished＇ internal company datasets that disallow open access to the international scientific community, and （4） the use of poorly understood concepts without providing the basic conceptual clarity. These deficiencies undermine the credibility of the heuristic modet.