Using a Mass Balance Framework to Investigate Downstream Distributary Channel Narrowing in a Terminal Intraslope Mini Basin

Martin, John¹, Julian Clark², Andrea Fildani², Timothy McHargue³, Frank Harris² (1) St Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN (2) Chevron Energy Technology Company, San Ramon, CA (3) ChevronTexaco, San Ramon, CA

Shallow 3D seismic data from offshore Angola reveals a terminal intraslope basin filled by a paleosubmarine distributary system. Proportional edge horizons through the sediment fill illustrate a point sourced, outward expanding distributary channel network. Asymmetry of the basin planform shape forces variable downstream channel narrowing, where the averaged channel width decay rate along the basin's long axis is smaller compared to channel width reduction along its short axis. To correct for variable downstream channel narrowing, we propose a basin coordinate transformation that maps downstream distance into the fraction of sediment supply deposited at that point. The motivation behind this is to investigate the sediment mass balance control over alluvial architecture, where long term sediment extraction to the bed must exert a first order control over the time averaged downstream decrease in turbidity current size. Interestingly, this analysis stems from work on experimental braided distributary networks and offers a scale independent method of investigating channelized stratigraphic organization. By mapping channel widths within this coordinate space, we reduce the range of channel widths and channel width variability between different downstream basin transects by approximately 50%. Thus, the mass balance correction accounts for roughly one-half of the observed differences in downstream channel narrowing. This approach highlights that at an arbitrary downstream radius channels oriented approximately parallel with the maximum basin length were more sediment bypass dominated and consequently larger on average. Additionally, by using mass balance coordinates all downstream basin transect lengths are the same, which allows for basinwide comparative architectural analysis irrespective of basin shape.