- Detail of morphostructural interpretation of seabed (location in Figure 3). Note the presence of clastic lobes adjacent to semi-cicular scarps, and toe-of-slope deposits developed at the end of a channelised body fed from the N. Dashed lines indicate most recent erosional episode overprinting the clastic lobes 

Contexts in source publication

Context 1

... Yapen island shows a rather more complex stratigraphy, dominated by volcanic and deep water carbonate deposits belonging to the Auwewa and more recent formations (McAdoo and Haebig 2006). Most of the previous studies addressing basin development in the area have been based on analysis of surface geology, satellite imagery (or aerophoto surveys), regional geotectonic data derived from plate motion vectors, GPS and dating of oceanic seafoor (Hall 2002 and 2012; Van Ufford and Cloos, 2005; Pubellier and Ego, 2002). The recently acquired offshore data (seabed and sub- seabed) in the basin give us the opportunity to ground-truth these models and propose new ideas on the current tectonic setting of the basin and ultimately, its petroleum system. This article focuses on the results obtained from the analysis of high-resolution multibeam data acquired by TGS in a multi-phase campaign (2007-2008) including bathymetry, backscatter, shallow seismic profiling, piston coring, gravimetry and magnetic data acquisition, in conjunction with a regional 2D seismic survey. After the award of the Cendrawasih Bay II PSC, the joint venture acquired over 2600 km of 2D seismic data, alongside with gravity and magnetic data and a new piston coring campaign. These new data are currently being evaluated to better define the exploration potential of the area. The multibeam bathymetry and backscatter data are here analysed to provide a structural and depositional interpretation of the recent Biak Basin. The high resolution of the multibeam data allows the analysis of the morpho-structures at the seabed and the identification of the depositional bodies, using classic geomorphological techniques. The detailed observations collected have been used to interpret the genetic mechanisms at the origin of the morpho-structural features, and their triggering events. In such a tectonically active area, it is critical to fit the seabed geomorphological analysis with the structural setting. Therefore, we used earthquake focal mechanism data from public domain sources (Elkstrom, 2006) and DEM data for extending the tectonic and morphological interpretation in the nearby onshore areas. The Biak basin is a semi-enclosed basin covering an area of ca. 4000 sq km, with water depth reaching over 1100 m in its central part (Figure 3). The basin is connected to the Yapen Strait through a narrow corridor to the south-east, and to the Pacific Ocean to the west, through a broader seaway, between Biak and Numfoor Islands (Figure 3). Recent carbonates develop on its margins and outcrop in raised terraces in the nearby islands (Masria et al., 1981). A dominantly siliciclastic shelf to basin system is currently developing off the cost of Papua mainland, to the east of the Biak Basin. The presence of a sedimentary basin and ultimately, similar deposits in the Biak Basin was unknown till the recently acquired seismic data highlighted the occurrence of a thick sedimentary section (several kilometers). The location of the Biak Basin to the north of the SYFZ, and the geology observed in the nearby Yapen and Biak Islands, suggest that this basin is geologically analogous to the sedimentary basins of the northern part of the New Guinea Island. Therefore, an oceanic (or transitional) affinity is postulated for the basement of the basin, as described in the onshore area (e.g. North Irian Basin, McAdoo and Haebig, 2000). Based on long- distance correlation with the wells drilled in the area, and the location seawards of the main Mamberamo shelf system (prograding westwards from the Papuan shoreline off the Niengo area), we expect that the recent (Plio-Pleistocene) stratigraphy of the Biak Basin is laterally equivalent to the Mamberamo Formation. On the basin flanks, carbonates are likely to have developed, similarly to those mapped on the nearby emerged land. The siliciclastic sediments of the Mamberamo formation have not been encountered on Biak or Yapen islands. This is probably due to the continuous uplift of these islands in the few past million years, and a more distal position with respects to the New Guinea mainland, the most likely main source of sediment supply. The flanks of the basin have a variable slope angle, ranging from 1 to 3 degrees in the W and SW, to relatively steep (5 to 15 degrees) on the NE and E. Sediment accumulation and erosional patterns in the basin vary accordingly, and we describe and classify the related morpho-structural features based on their genesis and location. For the purposes of this study, we divide the basin in the following morphological areas: basin and slopes, including the Supiori, Biak, Yapen and Num-Numfoor slope, adjacent to the respective islands (Figure 3). Multibeam data present an excellent coverage and resolution on the slope or the toe-of-slope, where most of the morpho-structures have been identified, and hereafter described. A series of incisional features cut across the slopes in the different areas of the basin (Figure 4). Two geo-seismic sections show the end-members of these incisional features (Figure 5): leveed channels and canyon/gullies. These end members are defined on the basis of the relative erosional/depositional ratio, where the channels with constructional levees are mainly associated with sediment deposition while the canyons and gullies with prevailing erosion. At the toe of the slope, we observe well-defined mounded bodies with lobate shape in plan view, either in isolated or coalescent configuration. On the Biak slope, these bodies are located where the slope gradient decreases abruptly from the high angle slope to the basin. Figure 6 shows a belt of adjacent lobes forming a linear, slope-parallel apron, 20km in width, reaching from 3km (lateral) to 8 km (central) in length from the base of slope. Slope aprons are usually fed by a virtually continuous linear source. The western-most of these features has the character of a buried mass transport deposit (MTD, or submarine-mass failure, sensu Shipp et al., 2011) (Figure 5c). MTD is a general term that refers to all slumps, slides, and debris flows generated and emplaced by gravity-controlled processes except for turbidity currents. MTDs include movement and emplacement by brittle deformation (slides), plastic deformation (slumps), and plastic or laminar flow (debris flows) (Nelson et al., 2011). The described slope apron is directly associated with a series of parallel linear features evident on the seabed. These morphostructures are interpreted as terraced fault scarps and represent the likely linear source for the apron. The overlapping and coalescing series of lobes composing the apron suggest different phases of fault scarp activity that seems to be concentrated in the central part of the apron. The easternmost lobes seem to overlap the western mass transport complex and therefore the overall pattern suggests that the bodies are progressively younger towards the east. This observation is important in order to establish the relative movement of the faults associated with the scarps. On the Supiori slope, similar lobate bodies are observed (Figure 4). The shape and direction of the lobes suggests they are fed from the north, where they are bounded by a series of semi-circular submarine NW-SE directed scarps, concave to the SW (Figure 3). The scarps represent major erosional features in the basin, and the mounded bodies are interpreted as formed by associated resedimentation processes. To the E and W of these scarps, further lobate and mounded bodies are located downslope of channels. In this case, the lobes (Figure 4) are constructional clastic depositional bodies developed downslope of points where laterally confined flows from the feeder channels expand (e.g. Stow et al., 1999). Repeated and increased erosional activity is documented by re-incision of the lobes by the channels, and large blocks deposited in their most marginal part (blue dashed line in Figure 4). A unique morpho-structure is observed on the flat- lying seabed (ca. 500m depth) nearby the Yapen slope. This is a well-defined circular crater-like feature (Figure 6) with an arcuate, crescent-moon rim, open to the N and bounded at the flank by small scale arcuate and concentric scarps. The maximum relief observed in the centre of this crater is ca. 300m, and diameter reaches nearly 10km. The typical concave-upwards subcircular shape observed on seismic data, and associated arcuate scarps, suggest that this feature formed as a collapse structure (e.g., Stewart et al., 1999). Analogous arcuate shaped escarpments interpreted as repeated slope failure episodes at the edge of a carbonate platform have been described by Ten Brink et al. (2006) offshore Puerto Rico. This structure is possibly associated to subcropping carbonate units and differential compaction of overlying deep-water sediments. A series of straight slope gullies, V- shaped in section, is observed on the slope to the west of the circular collapse structure. The last feature here described is associated with a subtle wavy surface morphology, evident on the slope of the basin, nearby Numfoor Island (Figure 3). This morphology is the typical expression of seabed sediment creep. This is a process of slow mass movement driven by the downslope weight of sediment, and it occurs in a region characterized by low slope angles. Although creep might be a precursor to creep rupture and slope failure, normally it suggests the predominance of plastic deformation. To the west of Biak Basin, in the Bird’s Head area, the analysis of present day seismicity together with GPS data suggest that this region is moving at the same pace as the Caroline Plate at a rate of 7.5–8.0 cm/yr in a direction of 252° relative to northern Australia (Stevens et. al, 2002, Decker et. al, 2009, Figure 1). Considering these kinematic indicators, the Bird’s Head area is interpreted to be a product of escape tectonics resulting from the oblique collision between ...

Context 2

... section (several kilometers). The location of the Biak Basin to the north of the SYFZ, and the geology observed in the nearby Yapen and Biak Islands, suggest that this basin is geologically analogous to the sedimentary basins of the northern part of the New Guinea Island. Therefore, an oceanic (or transitional) affinity is postulated for the basement of the basin, as described in the onshore area (e.g. North Irian Basin, McAdoo and Haebig, 2000). Based on long- distance correlation with the wells drilled in the area, and the location seawards of the main Mamberamo shelf system (prograding westwards from the Papuan shoreline off the Niengo area), we expect that the recent (Plio-Pleistocene) stratigraphy of the Biak Basin is laterally equivalent to the Mamberamo Formation. On the basin flanks, carbonates are likely to have developed, similarly to those mapped on the nearby emerged land. The siliciclastic sediments of the Mamberamo formation have not been encountered on Biak or Yapen islands. This is probably due to the continuous uplift of these islands in the few past million years, and a more distal position with respects to the New Guinea mainland, the most likely main source of sediment supply. The flanks of the basin have a variable slope angle, ranging from 1 to 3 degrees in the W and SW, to relatively steep (5 to 15 degrees) on the NE and E. Sediment accumulation and erosional patterns in the basin vary accordingly, and we describe and classify the related morpho-structural features based on their genesis and location. For the purposes of this study, we divide the basin in the following morphological areas: basin and slopes, including the Supiori, Biak, Yapen and Num-Numfoor slope, adjacent to the respective islands (Figure 3). Multibeam data present an excellent coverage and resolution on the slope or the toe-of-slope, where most of the morpho-structures have been identified, and hereafter described. A series of incisional features cut across the slopes in the different areas of the basin (Figure 4). Two geo-seismic sections show the end-members of these incisional features (Figure 5): leveed channels and canyon/gullies. These end members are defined on the basis of the relative erosional/depositional ratio, where the channels with constructional levees are mainly associated with sediment deposition while the canyons and gullies with prevailing erosion. At the toe of the slope, we observe well-defined mounded bodies with lobate shape in plan view, either in isolated or coalescent configuration. On the Biak slope, these bodies are located where the slope gradient decreases abruptly from the high angle slope to the basin. Figure 6 shows a belt of adjacent lobes forming a linear, slope-parallel apron, 20km in width, reaching from 3km (lateral) to 8 km (central) in length from the base of slope. Slope aprons are usually fed by a virtually continuous linear source. The western-most of these features has the character of a buried mass transport deposit (MTD, or submarine-mass failure, sensu Shipp et al., 2011) (Figure 5c). MTD is a general term that refers to all slumps, slides, and debris flows generated and emplaced by gravity-controlled processes except for turbidity currents. MTDs include movement and emplacement by brittle deformation (slides), plastic deformation (slumps), and plastic or laminar flow (debris flows) (Nelson et al., 2011). The described slope apron is directly associated with a series of parallel linear features evident on the seabed. These morphostructures are interpreted as terraced fault scarps and represent the likely linear source for the apron. The overlapping and coalescing series of lobes composing the apron suggest different phases of fault scarp activity that seems to be concentrated in the central part of the apron. The easternmost lobes seem to overlap the western mass transport complex and therefore the overall pattern suggests that the bodies are progressively younger towards the east. This observation is important in order to establish the relative movement of the faults associated with the scarps. On the Supiori slope, similar lobate bodies are observed (Figure 4). The shape and direction of the lobes suggests they are fed from the north, where they are bounded by a series of semi-circular submarine NW-SE directed scarps, concave to the SW (Figure 3). The scarps represent major erosional features in the basin, and the mounded bodies are interpreted as formed by associated resedimentation processes. To the E and W of these scarps, further lobate and mounded bodies are located downslope of channels. In this case, the lobes (Figure 4) are constructional clastic depositional bodies developed downslope of points where laterally confined flows from the feeder channels expand (e.g. Stow et al., 1999). Repeated and increased erosional activity is documented by re-incision of the lobes by the channels, and large blocks deposited in their most marginal part (blue dashed line in Figure 4). A unique morpho-structure is observed on the flat- lying seabed (ca. 500m depth) nearby the Yapen slope. This is a well-defined circular crater-like feature (Figure 6) with an arcuate, crescent-moon rim, open to the N and bounded at the flank by small scale arcuate and concentric scarps. The maximum relief observed in the centre of this crater is ca. 300m, and diameter reaches nearly 10km. The typical concave-upwards subcircular shape observed on seismic data, and associated arcuate scarps, suggest that this feature formed as a collapse structure (e.g., Stewart et al., 1999). Analogous arcuate shaped escarpments interpreted as repeated slope failure episodes at the edge of a carbonate platform have been described by Ten Brink et al. (2006) offshore Puerto Rico. This structure is possibly associated to subcropping carbonate units and differential compaction of overlying deep-water sediments. A series of straight slope gullies, V- shaped in section, is observed on the slope to the west of the circular collapse structure. The last feature here described is associated with a subtle wavy surface morphology, evident on the slope of the basin, nearby Numfoor Island (Figure 3). This morphology is the typical expression of seabed sediment creep. This is a process of slow mass movement driven by the downslope weight of sediment, and it occurs in a region characterized by low slope angles. Although creep might be a precursor to creep rupture and slope failure, normally it suggests the predominance of plastic deformation. To the west of Biak Basin, in the Bird’s Head area, the analysis of present day seismicity together with GPS data suggest that this region is moving at the same pace as the Caroline Plate at a rate of 7.5–8.0 cm/yr in a direction of 252° relative to northern Australia (Stevens et. al, 2002, Decker et. al, 2009, Figure 1). Considering these kinematic indicators, the Bird’s Head area is interpreted to be a product of escape tectonics resulting from the oblique collision between the Australian plate and the remnants of volcanic belts and accreted terranes carried by the Pacific plate (Pubellier and Ego, 2002). The escape movement is accommodated by two broad left lateral strike-slip fault zones, the SYFZ to the North, and the Tarera-Aiduna Fault Zone to the South (Figure 1). The analysis of computed focal mechanisms of earthquakes (Dziewonski, et. al, 1981) shows that some of the sidewall fault systems bounding the Biak Basin are active at present day, and are consistent with a strike-slip motion (Figure 7). The earthquake data also points out that contractional deformation to the North of Biak Island is then partitioned into strike-slip deformation to the south, towards the SYFZ (Figure 1). In addition to the dominant strike-slip and contractional earthquake solutions, we observe a few of normal extensional earthquakes in the vicinity of Biak Island. This fact may be due to stress perturbations caused by pre- existing structures. As the crustal configuration of the area is composed of several fragments, fault block rotations, and local extension, may exist because changes of slip rates between different crustal blocks. Deposition in the Biak basin appears to be dominated by slope and deep water clastic sediments. Depositional and erosional features are consistent with the different slope angles and with fault activity. The interplay between recent seismic activity and deformation, erosional and depositional patterns of the Biak Basin is illustrated by the analysis of the circular shaped crater-like collapse located in the southern edge of the basin (Figure 6 and 7). This collapse is associated with a strike-slip focal mechanism solution, and this allows us to speculate that the feature is controlled by recent fault activity. A second clear example of this interplay is how the fault scarps associated with base-of-slope depositional lobes are consistent in orientation and location with nearby strike-slip earthquake focal mechanisms (Figure 6 and 7). It is well known that instability and resedimentation processes are favoured by high slope angles and repeated cyclical stress. We consider that this is more likely to be caused by the documented seismic activity, rather than high sediment supply, in this relatively starved basin. Division between seismic and aseismic areas of the basin can be made based on the presence or absence of focal mechanisms, fault scarps and spatially associated sediment lobes: predominantly aseismic on the Num-Numfoor slope, and seismic-controlled in the Supiori, Biak and Yapen slopes. This is consistent with the location of the recent focal mechanisms (Figure 7). Various theories have been formulated on the geodynamics of the area. One theory proposes that the basins along the SYFZ, were created at the interaction of collisional and strike slip movements, and may have travelled 100s of kilometres from its original location (e.g. Hill and Hall, 2003; Hall, 2012). ...

Context 3

... (Masria et al., 1981). A dominantly siliciclastic shelf to basin system is currently developing off the cost of Papua mainland, to the east of the Biak Basin. The presence of a sedimentary basin and ultimately, similar deposits in the Biak Basin was unknown till the recently acquired seismic data highlighted the occurrence of a thick sedimentary section (several kilometers). The location of the Biak Basin to the north of the SYFZ, and the geology observed in the nearby Yapen and Biak Islands, suggest that this basin is geologically analogous to the sedimentary basins of the northern part of the New Guinea Island. Therefore, an oceanic (or transitional) affinity is postulated for the basement of the basin, as described in the onshore area (e.g. North Irian Basin, McAdoo and Haebig, 2000). Based on long- distance correlation with the wells drilled in the area, and the location seawards of the main Mamberamo shelf system (prograding westwards from the Papuan shoreline off the Niengo area), we expect that the recent (Plio-Pleistocene) stratigraphy of the Biak Basin is laterally equivalent to the Mamberamo Formation. On the basin flanks, carbonates are likely to have developed, similarly to those mapped on the nearby emerged land. The siliciclastic sediments of the Mamberamo formation have not been encountered on Biak or Yapen islands. This is probably due to the continuous uplift of these islands in the few past million years, and a more distal position with respects to the New Guinea mainland, the most likely main source of sediment supply. The flanks of the basin have a variable slope angle, ranging from 1 to 3 degrees in the W and SW, to relatively steep (5 to 15 degrees) on the NE and E. Sediment accumulation and erosional patterns in the basin vary accordingly, and we describe and classify the related morpho-structural features based on their genesis and location. For the purposes of this study, we divide the basin in the following morphological areas: basin and slopes, including the Supiori, Biak, Yapen and Num-Numfoor slope, adjacent to the respective islands (Figure 3). Multibeam data present an excellent coverage and resolution on the slope or the toe-of-slope, where most of the morpho-structures have been identified, and hereafter described. A series of incisional features cut across the slopes in the different areas of the basin (Figure 4). Two geo-seismic sections show the end-members of these incisional features (Figure 5): leveed channels and canyon/gullies. These end members are defined on the basis of the relative erosional/depositional ratio, where the channels with constructional levees are mainly associated with sediment deposition while the canyons and gullies with prevailing erosion. At the toe of the slope, we observe well-defined mounded bodies with lobate shape in plan view, either in isolated or coalescent configuration. On the Biak slope, these bodies are located where the slope gradient decreases abruptly from the high angle slope to the basin. Figure 6 shows a belt of adjacent lobes forming a linear, slope-parallel apron, 20km in width, reaching from 3km (lateral) to 8 km (central) in length from the base of slope. Slope aprons are usually fed by a virtually continuous linear source. The western-most of these features has the character of a buried mass transport deposit (MTD, or submarine-mass failure, sensu Shipp et al., 2011) (Figure 5c). MTD is a general term that refers to all slumps, slides, and debris flows generated and emplaced by gravity-controlled processes except for turbidity currents. MTDs include movement and emplacement by brittle deformation (slides), plastic deformation (slumps), and plastic or laminar flow (debris flows) (Nelson et al., 2011). The described slope apron is directly associated with a series of parallel linear features evident on the seabed. These morphostructures are interpreted as terraced fault scarps and represent the likely linear source for the apron. The overlapping and coalescing series of lobes composing the apron suggest different phases of fault scarp activity that seems to be concentrated in the central part of the apron. The easternmost lobes seem to overlap the western mass transport complex and therefore the overall pattern suggests that the bodies are progressively younger towards the east. This observation is important in order to establish the relative movement of the faults associated with the scarps. On the Supiori slope, similar lobate bodies are observed (Figure 4). The shape and direction of the lobes suggests they are fed from the north, where they are bounded by a series of semi-circular submarine NW-SE directed scarps, concave to the SW (Figure 3). The scarps represent major erosional features in the basin, and the mounded bodies are interpreted as formed by associated resedimentation processes. To the E and W of these scarps, further lobate and mounded bodies are located downslope of channels. In this case, the lobes (Figure 4) are constructional clastic depositional bodies developed downslope of points where laterally confined flows from the feeder channels expand (e.g. Stow et al., 1999). Repeated and increased erosional activity is documented by re-incision of the lobes by the channels, and large blocks deposited in their most marginal part (blue dashed line in Figure 4). A unique morpho-structure is observed on the flat- lying seabed (ca. 500m depth) nearby the Yapen slope. This is a well-defined circular crater-like feature (Figure 6) with an arcuate, crescent-moon rim, open to the N and bounded at the flank by small scale arcuate and concentric scarps. The maximum relief observed in the centre of this crater is ca. 300m, and diameter reaches nearly 10km. The typical concave-upwards subcircular shape observed on seismic data, and associated arcuate scarps, suggest that this feature formed as a collapse structure (e.g., Stewart et al., 1999). Analogous arcuate shaped escarpments interpreted as repeated slope failure episodes at the edge of a carbonate platform have been described by Ten Brink et al. (2006) offshore Puerto Rico. This structure is possibly associated to subcropping carbonate units and differential compaction of overlying deep-water sediments. A series of straight slope gullies, V- shaped in section, is observed on the slope to the west of the circular collapse structure. The last feature here described is associated with a subtle wavy surface morphology, evident on the slope of the basin, nearby Numfoor Island (Figure 3). This morphology is the typical expression of seabed sediment creep. This is a process of slow mass movement driven by the downslope weight of sediment, and it occurs in a region characterized by low slope angles. Although creep might be a precursor to creep rupture and slope failure, normally it suggests the predominance of plastic deformation. To the west of Biak Basin, in the Bird’s Head area, the analysis of present day seismicity together with GPS data suggest that this region is moving at the same pace as the Caroline Plate at a rate of 7.5–8.0 cm/yr in a direction of 252° relative to northern Australia (Stevens et. al, 2002, Decker et. al, 2009, Figure 1). Considering these kinematic indicators, the Bird’s Head area is interpreted to be a product of escape tectonics resulting from the oblique collision between the Australian plate and the remnants of volcanic belts and accreted terranes carried by the Pacific plate (Pubellier and Ego, 2002). The escape movement is accommodated by two broad left lateral strike-slip fault zones, the SYFZ to the North, and the Tarera-Aiduna Fault Zone to the South (Figure 1). The analysis of computed focal mechanisms of earthquakes (Dziewonski, et. al, 1981) shows that some of the sidewall fault systems bounding the Biak Basin are active at present day, and are consistent with a strike-slip motion (Figure 7). The earthquake data also points out that contractional deformation to the North of Biak Island is then partitioned into strike-slip deformation to the south, towards the SYFZ (Figure 1). In addition to the dominant strike-slip and contractional earthquake solutions, we observe a few of normal extensional earthquakes in the vicinity of Biak Island. This fact may be due to stress perturbations caused by pre- existing structures. As the crustal configuration of the area is composed of several fragments, fault block rotations, and local extension, may exist because changes of slip rates between different crustal blocks. Deposition in the Biak basin appears to be dominated by slope and deep water clastic sediments. Depositional and erosional features are consistent with the different slope angles and with fault activity. The interplay between recent seismic activity and deformation, erosional and depositional patterns of the Biak Basin is illustrated by the analysis of the circular shaped crater-like collapse located in the southern edge of the basin (Figure 6 and 7). This collapse is associated with a strike-slip focal mechanism solution, and this allows us to speculate that the feature is controlled by recent fault activity. A second clear example of this interplay is how the fault scarps associated with base-of-slope depositional lobes are consistent in orientation and location with nearby strike-slip earthquake focal mechanisms (Figure 6 and 7). It is well known that instability and resedimentation processes are favoured by high slope angles and repeated cyclical stress. We consider that this is more likely to be caused by the documented seismic activity, rather than high sediment supply, in this relatively starved basin. Division between seismic and aseismic areas of the basin can be made based on the presence or absence of focal mechanisms, fault scarps and spatially associated sediment lobes: predominantly aseismic on the Num-Numfoor slope, and seismic-controlled in the Supiori, Biak and Yapen slopes. This is consistent with the location ...

Context 4

... interpretation in the nearby onshore areas. The Biak basin is a semi-enclosed basin covering an area of ca. 4000 sq km, with water depth reaching over 1100 m in its central part (Figure 3). The basin is connected to the Yapen Strait through a narrow corridor to the south-east, and to the Pacific Ocean to the west, through a broader seaway, between Biak and Numfoor Islands (Figure 3). Recent carbonates develop on its margins and outcrop in raised terraces in the nearby islands (Masria et al., 1981). A dominantly siliciclastic shelf to basin system is currently developing off the cost of Papua mainland, to the east of the Biak Basin. The presence of a sedimentary basin and ultimately, similar deposits in the Biak Basin was unknown till the recently acquired seismic data highlighted the occurrence of a thick sedimentary section (several kilometers). The location of the Biak Basin to the north of the SYFZ, and the geology observed in the nearby Yapen and Biak Islands, suggest that this basin is geologically analogous to the sedimentary basins of the northern part of the New Guinea Island. Therefore, an oceanic (or transitional) affinity is postulated for the basement of the basin, as described in the onshore area (e.g. North Irian Basin, McAdoo and Haebig, 2000). Based on long- distance correlation with the wells drilled in the area, and the location seawards of the main Mamberamo shelf system (prograding westwards from the Papuan shoreline off the Niengo area), we expect that the recent (Plio-Pleistocene) stratigraphy of the Biak Basin is laterally equivalent to the Mamberamo Formation. On the basin flanks, carbonates are likely to have developed, similarly to those mapped on the nearby emerged land. The siliciclastic sediments of the Mamberamo formation have not been encountered on Biak or Yapen islands. This is probably due to the continuous uplift of these islands in the few past million years, and a more distal position with respects to the New Guinea mainland, the most likely main source of sediment supply. The flanks of the basin have a variable slope angle, ranging from 1 to 3 degrees in the W and SW, to relatively steep (5 to 15 degrees) on the NE and E. Sediment accumulation and erosional patterns in the basin vary accordingly, and we describe and classify the related morpho-structural features based on their genesis and location. For the purposes of this study, we divide the basin in the following morphological areas: basin and slopes, including the Supiori, Biak, Yapen and Num-Numfoor slope, adjacent to the respective islands (Figure 3). Multibeam data present an excellent coverage and resolution on the slope or the toe-of-slope, where most of the morpho-structures have been identified, and hereafter described. A series of incisional features cut across the slopes in the different areas of the basin (Figure 4). Two geo-seismic sections show the end-members of these incisional features (Figure 5): leveed channels and canyon/gullies. These end members are defined on the basis of the relative erosional/depositional ratio, where the channels with constructional levees are mainly associated with sediment deposition while the canyons and gullies with prevailing erosion. At the toe of the slope, we observe well-defined mounded bodies with lobate shape in plan view, either in isolated or coalescent configuration. On the Biak slope, these bodies are located where the slope gradient decreases abruptly from the high angle slope to the basin. Figure 6 shows a belt of adjacent lobes forming a linear, slope-parallel apron, 20km in width, reaching from 3km (lateral) to 8 km (central) in length from the base of slope. Slope aprons are usually fed by a virtually continuous linear source. The western-most of these features has the character of a buried mass transport deposit (MTD, or submarine-mass failure, sensu Shipp et al., 2011) (Figure 5c). MTD is a general term that refers to all slumps, slides, and debris flows generated and emplaced by gravity-controlled processes except for turbidity currents. MTDs include movement and emplacement by brittle deformation (slides), plastic deformation (slumps), and plastic or laminar flow (debris flows) (Nelson et al., 2011). The described slope apron is directly associated with a series of parallel linear features evident on the seabed. These morphostructures are interpreted as terraced fault scarps and represent the likely linear source for the apron. The overlapping and coalescing series of lobes composing the apron suggest different phases of fault scarp activity that seems to be concentrated in the central part of the apron. The easternmost lobes seem to overlap the western mass transport complex and therefore the overall pattern suggests that the bodies are progressively younger towards the east. This observation is important in order to establish the relative movement of the faults associated with the scarps. On the Supiori slope, similar lobate bodies are observed (Figure 4). The shape and direction of the lobes suggests they are fed from the north, where they are bounded by a series of semi-circular submarine NW-SE directed scarps, concave to the SW (Figure 3). The scarps represent major erosional features in the basin, and the mounded bodies are interpreted as formed by associated resedimentation processes. To the E and W of these scarps, further lobate and mounded bodies are located downslope of channels. In this case, the lobes (Figure 4) are constructional clastic depositional bodies developed downslope of points where laterally confined flows from the feeder channels expand (e.g. Stow et al., 1999). Repeated and increased erosional activity is documented by re-incision of the lobes by the channels, and large blocks deposited in their most marginal part (blue dashed line in Figure 4). A unique morpho-structure is observed on the flat- lying seabed (ca. 500m depth) nearby the Yapen slope. This is a well-defined circular crater-like feature (Figure 6) with an arcuate, crescent-moon rim, open to the N and bounded at the flank by small scale arcuate and concentric scarps. The maximum relief observed in the centre of this crater is ca. 300m, and diameter reaches nearly 10km. The typical concave-upwards subcircular shape observed on seismic data, and associated arcuate scarps, suggest that this feature formed as a collapse structure (e.g., Stewart et al., 1999). Analogous arcuate shaped escarpments interpreted as repeated slope failure episodes at the edge of a carbonate platform have been described by Ten Brink et al. (2006) offshore Puerto Rico. This structure is possibly associated to subcropping carbonate units and differential compaction of overlying deep-water sediments. A series of straight slope gullies, V- shaped in section, is observed on the slope to the west of the circular collapse structure. The last feature here described is associated with a subtle wavy surface morphology, evident on the slope of the basin, nearby Numfoor Island (Figure 3). This morphology is the typical expression of seabed sediment creep. This is a process of slow mass movement driven by the downslope weight of sediment, and it occurs in a region characterized by low slope angles. Although creep might be a precursor to creep rupture and slope failure, normally it suggests the predominance of plastic deformation. To the west of Biak Basin, in the Bird’s Head area, the analysis of present day seismicity together with GPS data suggest that this region is moving at the same pace as the Caroline Plate at a rate of 7.5–8.0 cm/yr in a direction of 252° relative to northern Australia (Stevens et. al, 2002, Decker et. al, 2009, Figure 1). Considering these kinematic indicators, the Bird’s Head area is interpreted to be a product of escape tectonics resulting from the oblique collision between the Australian plate and the remnants of volcanic belts and accreted terranes carried by the Pacific plate (Pubellier and Ego, 2002). The escape movement is accommodated by two broad left lateral strike-slip fault zones, the SYFZ to the North, and the Tarera-Aiduna Fault Zone to the South (Figure 1). The analysis of computed focal mechanisms of earthquakes (Dziewonski, et. al, 1981) shows that some of the sidewall fault systems bounding the Biak Basin are active at present day, and are consistent with a strike-slip motion (Figure 7). The earthquake data also points out that contractional deformation to the North of Biak Island is then partitioned into strike-slip deformation to the south, towards the SYFZ (Figure 1). In addition to the dominant strike-slip and contractional earthquake solutions, we observe a few of normal extensional earthquakes in the vicinity of Biak Island. This fact may be due to stress perturbations caused by pre- existing structures. As the crustal configuration of the area is composed of several fragments, fault block rotations, and local extension, may exist because changes of slip rates between different crustal blocks. Deposition in the Biak basin appears to be dominated by slope and deep water clastic sediments. Depositional and erosional features are consistent with the different slope angles and with fault activity. The interplay between recent seismic activity and deformation, erosional and depositional patterns of the Biak Basin is illustrated by the analysis of the circular shaped crater-like collapse located in the southern edge of the basin (Figure 6 and 7). This collapse is associated with a strike-slip focal mechanism solution, and this allows us to speculate that the feature is controlled by recent fault activity. A second clear example of this interplay is how the fault scarps associated with base-of-slope depositional lobes are consistent in orientation and location with nearby strike-slip earthquake focal mechanisms (Figure 6 and 7). It is well known that instability and resedimentation processes are favoured by high slope angles and repeated cyclical stress. ...