Sediment waves in the Bismarck Volcanic Arc, Papua New Guinea

In the Bismarck Volcanic Arc in Papua New Guinea, six fi elds of sediment waves were imaged with sonar. Sediment structures observed in seismic data and swath bathymetry are not unique and can result from predominantly continuous (bottom) currents, or episodic (turbidity) currents, or from deformation of sediment. Two of these wave fi elds overlap and appear to be of turbidity-current origin and modifi ed by bottom currents, with one fi eld unconformably overlying the other fi eld. A fi eld off the coast of Dakataua caldera displays an arcuate morphology, and a series of enclosed depressions within the fi eld suggests creation by extensional deformation of rapidly deposited sediment. Scour features in side-scan imagery suggest turbidity-current activity, which also likely modifi es the sediment waves. The wave fi eld is isolated from hyperpycnal currents, however, suggesting that in the absence of a shelf, coastal erosion and small landslides can produce semiregular gravity-driven sediment fl ows that deposit in deep (>1400 m) water. In Kimbe Bay a fourth sediment-wave fi eld also displays arcuate morphology and enclosed depressions within the fi eld. This wave fi eld is found within a bay >40 km from shore and also appears to have been formed by a combination of extensional deformation of sediment and energetic current activity. Two additional fi elds in Hixon Bay are fed by small and medium rivers (<̃450 m ³ /s mean annual discharge) draining volcanoes and mountainous regions. One small fi eld appears within a slide scar, suggesting that the initial topography of the scar provided the conditions for early sediment-wave growth. A much larger fi eld is best explained by repeated hyperpycnal currents originating from the Pandi River. We cored a series of upward-fi ning, graded sequences consistent with a turbidity-current origin. Ages from these cores and measurements of relative thickness in sub-bottom imagery of the field constrain deposition rates for the field and suggest that a large part of the pandi River discharge must be bypassing the shelf and depositing on the sediment-wave field in deep water (>1200m). These findings suggest that the sedimentary record in arc collision zones will be dominated by mass-wasting deposits very close to vol-canoes, and by river discharge deposting in select, extent regions far from shore. Because sedimentation rates can vary by a factor of 2 between the two flanks of a sediment wave, care must be taken when comparing bed thickness across an entire sedimentary siction.