Fine sediment sequestration on an active inner shelf, Waiapu River, New Zealand

The inner shelf of active continental margins has long been considered a region of sediment segregation and fine-sediment bypassing. The Waiapu River, North Island, New Zealand, presents an opportunity to study fine sediment segregation and strata formation in a spatially constrained, highly energetic, aggradational setting, with one of the highest sediment yields on earth. In this study, we present evidence that the inner shelf of the Waiapu River plays a significant role both in the fate of fine-grained riverine sediments and in the formation of continental margin stratigraphy. Results obtained from seismic profiles ground-truthed by cores show significant spatial variation preserved on the Waiapu inner shelf, likely controlled by varying sediment deposition and resuspension processes as well as antecedent geology. Three distinct depositional regions are interpreted as: 1) hypopycnal, characterized by rock-enclosed, muddy basins; 2) delta dominated, characterized by acoustically transparent seismic profiles and muddy sands; and 3) hyperpycnal, characterized by thickly laminated sediments. A topographic low overlying the paleo-Waiapu channel has likely controlled the fate of hyperpycnal flows via bathymetric steering in the recent (500-700 yrs) past, and may still influence deposition of density-driven flows today. All three regions of the inner shelf are similar in that they sequester considerable amounts of fine sediments and preserve a fining upwards sequence. Volume calculations were performed for each region based on two different chronostratigraphic baselines. Percent fines by volume were calculated for each region and baseline, and results range from approximately 16 percent fines over the last 500-700 years in the delta dominated region to 97 percent fines over the last 100-150 years in the hypopycnal region. When averaged over the entire inner shelf study area, approximately 47 percent of the sediments deposited within the last 500-700 years are fine, compared to 57 percent over the last 50-100 years. It is hypothesized that the high sediment yield of the river, due at least in part to elevated sediment loads related to deforestation-induced erosion, overwhelm this energetic system's ability to segregate and disperse fine sediments. The capture of such a significant amount of fine sediments coupled with the spatial variability of depositional environments casts doubts on the common assumption that fine sediments bypass the inner shelf of high yield continental margins.