Holocene sediment deposition on the Poverty-slope margin by the muddy Waipaoa River, East Coast New Zealand

Abstract

The margin off the Raukumara Peninsula, East Coast North Island of New Zealand, is characterised by high terrigenous sediment flux, the dramatic effects of historic land-use change, the rapid uplift of Mesozoic and soft Tertiary sediments, and by complex sediment-tectonic interactions on a steep and unstable continental slope. The Poverty Bay shelf and slope indentation are adjacent to the Waipaoa River, which delivers ∼15 Mt year −1 of mud. This study presents new data from the Poverty slope, the seaward extension of the Waipaoa dispersal system. A 3-km 3 lobe of postglacial sediment emerges from the Poverty Gap and feathers down onto the upper slope. Downslope, the Poverty indentation has received a significant Holocene flux of terrigenous sediment, accumulating in two mid-slope basins: the Paritu Trough and a smaller lower-slope basin below the South Paritu ridge. Hemipelagic sediment has accumulated on the mid-slope at around 0.05 g cm −2 year −1 (0.06 cm year −1) since the mid–late Holocene. Seismic-reflection echo type and backscatter imagery indicate mud deposits on the mid-slope, debris at the base of regularly spaced upper-slope gullies and a large avalanche deposit in the upper Paritu Trough. The consistency of the tephrostratigraphy indicates that despite slope instability, hemipelagic sedimentation is the dominant background process operating at millennial time scales, at least since the mid-Holocene. The Poverty slope should be included in any budget calculation of post-colonisation increased sediment yield for the margin. The Poverty Canyon has probably been largely inactive during the Holocene highstand.