The influence of sea level and tectonics on Late Pleistocene through Holocene sediment storage along the high-sediment supply Waipaoa continental shelf

We present geophysical and core evidence showing how subsidence caused by forearc shortening has accommodated Late Pleistocene and Holocene sediments supplied to the tectonically active Waipaoa shelf (NZ), limiting off-shelf export during the early sea level highstand. The last glacioeustatic fall and subsequent rise exposed and then flooded a shelf segmented into subbasins separated by zones of uplift, leaving key stratigraphic markers of shoreline regression and transgression that vary strongly in character across the shelf. Highstand sediment isopachs tied to piston cores dated using tephra correlation and a radiocarbon age model provide a sediment budget at ∼ 2000 yr intervals from the mid-Holocene (∼ 5500 cal. yr BP) to present. Sediment load estimates from our shelf budget are in agreement with published model estimates for suspended sediment discharge from the Waipaoa River for the past 3000 yr but, importantly, do not show the 6-fold increase in the Waipaoa's sediment output that began with human settlement 700 yr ago and accelerated with deforestation over the last century. Bypassing of Waipaoa sediment to the slope may therefore be a recent phenomenon caused by unnaturally high sediment loads, a conclusion supported by data reported elsewhere in this volume. Our study also reveals evidence for (1) a relatively thick mid-shelf transgressive section deposited during the last eustatic rise that may correlate to estuarine sequences reported from numerous sites on the modern coastline of the North Island, (2) a slight decrease in total basin filling rates during the highstand, and (3) variability in the partitioning of highstand sediments between individual subbasins that may reflect differing degrees of tectonic accommodation.