Research On The Depositional Architecture And Rupture Mode Of Deep-water Channels In The Continental Slope Area Of ??the Niger Delta, West Africa

As the builder of submarine fan,the largest sedimentary body in the earth,submarine channels,on the one hand,play a role of the conduit to transport terrestrial sediments into the deep sea;their formation,extension and distribution directly control the dispersal of turbidity sands.On the other hand,submarine channels also act as repositories for coarse elastics;they are important hydrocarbon reservoirs in different continental marginal basins.Therefore,studying the architecture and avulsion of submarine channels i5 of great significanee for the future hydrocarbon exploration and development in deepwater.Choosing an avulsion channel network and a typical avulsion phenomenon recognized in southern Niger Delta continental slope as the research objects,this work uses traditional 2D seismic facies analysis as well as the 3D seismic geomorphology approach to characterize and analyze them and in turn gets a series of understanding on channel architecture and avulsion.The studied avulsion channel network is located in Study Area 1 and named as "Abalama Channel System"(abbreviated as "ACS");It overlies a mass transport complex and consists of six observed channel segments,delimited by five avulsion points and one confluence point.As the substrate of ACS,that mass transport complex originates from the sediment failure of upper slope or shelf margin and mainly consists of matrixes and remnant blocks,which respectively show negative and positive relief and provided the initial confinement and the potential flow path for subsequent channels.In areas where the MTC-induced confinement was weak or decreased abruptly,channels tended to develop higher sinuosity,increasing channels instability and ultimately causing avulsions.In addition,through the quantitative characterization and analysis of 5 avulsion events in ACS,three ideal categories of submarine channel avulsions are observed;Type 1 is characterized by parent and avulsion channel having similar maturity;Type 2 is characterised by a high-maturity parent channel and a low-maturity avulsion channel;Type 3,on the contrary,emphasizes the higher maturity of the avulsion channel compared to the parent channel.In the distal part of the study area,a topographic high related to mud diapirs provided lateral confinement that captured flows avulsed at different times resulting in an channel confluence.Generally,this paper divides the evolution of ACS into two phases that are respectively characterized by the landward and basinward shifts of avulsion points.For the typical avulsion phenomenon located Study Area 2,the parent channel and the avulsion channel are respectively named as "Bukuma Channel"(abbreviated as BC)and“Tombia Chatnel"(abbreviated as TC).Such avulsion represents an aborted avulsion process and belongs to the second avulsion(i.e.Type 2)as mentioned above;its sedimentary records have provide a unique opportunity to investigate the occurrence mechanism,detailed process and evolution of avulsions.TC emanated from a slope transition zone of BCS where a sharp bend as well as the channel-floor aggradation increased the instability of BCS and triggered by an outsize flow event,the avulsion happened.The initial flow path of TC appeared to follow discontinuous linear trains of small scours and residual pockmarks;as it evolved,TC developed a very low sinuosity and the specific cross-sectional morphology.Extracted cross-sectional profiles of TC changed from dish,deep-U,V to shallow-U shape downstream,which was mainly controlled by increasing then decreasing background slope.In addition,three channel forms were recognized within TC,forming a fining upward and coursing downstream sequence,which may respectively be associated with the progressive abandonment of BC and the dilute turbidity current tails.This paper generally divided the evolution of TC into four stages:Stage I took place before the occurrence of avulsion and was characterized by the interaction of overspilling flow stripped off from BC with the inherited seafloor topography;Stage 2 refers to the initial period of avulsion and was featured by the full establishment of a channelized flow pathway and then by deposition of early Channel form 1;Stage 3 represents the middle and later periods of avulsion and was dominated firstly by further development of the flow pathway and then by deposition of latter Channel form 2;Stage 4 occurred in the abandonment phase when the avulsion process had broken off and referred to the hemipelagic draping on the residual negative relief,resulting in the formation of Channel form 3.