Coastal progradation and sediment partitioning in the Holocene Waipaoa Sedimentary System, New Zealand

Over the late Holocene highstand, the shoreline at Poverty Bay, NZ migrated 12 km seaward, fed by sediment from the Waipaoa river. Paleo-shorelines indicate steadily decelerating progradation, possibly signaling changes in forcing on the Waipaoa Sedimentary System. To isolate the cause of this progradation slowdown we reconstruct late Holocene tectonics and stratigraphy over the Waipaoa coastal plain and nearshore from 7 ka-present. We find that decreasing rates of sediment storage by coastal progradation were driven by increasing tectonic storage in the steadily subsiding but rapidly growing coastal plain, such that net terrestrial storage remained constant at ∼ 0.8 Mt/yr. Hence changes in shoreline migration were due to autogenic increases in accommodation rather than allogenic changes in forcing. Furthermore, while the Waipaoa sediment load is primarily mud, reconstructions suggest that progradation was largely controlled by the supply of coarse-grained sediment. Our results suggest that in coastal systems such as the Waipaoa, where progradation is confined and wave energy is high, net accumulation of muds occurs only behind the prograding sandy shoreface, which shelters them from wave attack. Accounting for mud storage in the Waipaoa coastal plain and Poverty Bay suggests that export of muddy sediment to the Waipaoa shelf remained roughly constant at ∼ 2.0 Mt/yr from 7 ka until the onset of anthropogenic deforestation in the 19th century.