Shelf sedimentation on a tectonically active margin: A modern sediment budget for Poverty continental shelf, New Zealand

Abstract

The combination of high sediment yields and the prevalence of tectonically controlled accommodation on collision margins such as that adjacent to the Waipaoa River, NZ, create the potential for these areas to contain high-resolution records of natural and anthropogenic signals. This study describes modern (100-yr) sedimentation patterns off the Waipaoa and quantifies a sediment budget for the continental shelf, which is compared to long-term Holocene trends. 210Pb and 239,240Pu geochronologies reveal three main shelf depocenters. Two of these depocenters are located in subsiding synclinal basins on the mid-shelf, landward of the actively deforming Ariel and Lachlan anticlines. The depocenters exhibit accumulation rates ranging from 0.75 to 1.5 cm yr − 1 and display both steady-state and non-steady-state 210Pb activity profiles. Textural characteristics of the non-steady-state cores indicate the possible preservation of flood event layers. The third depocenter is located near the shelf break and has accumulation rates as high as 1.0 cm yr − 1 . The inner shelf and central mid-shelf are characterized by low, uniform 210Pb activity profiles and low accumulation rates, indicating that sediment is bypassing the inner shelf region and being deposited on the mid- to outer shelf. The modern sedimentation patterns seen in this study are similar to those for the Holocene, suggesting that regional tectonics are the major influence on Poverty shelf sedimentation. A modern, bulk sediment budget estimates that 3.6 ± 0.9 × 10 6 t yr − 1 of sediments remains on the shelf, amounting to only ~ 25% of the 15 Mt of sediments discharged from the river per year. This indicates massive export of sediments from the study area to the adjacent slope or along the shelf. In contrast, studies of the Mid-Late Holocene sediment budget in the same area indicate that the sediment input and shelf trapping have been roughly in balance. When compared with the modern budget, this suggests an extraordinarily rapid shift from shelf trapping to shelf bypassing, most likely driven by increasing sediment discharge in response to deforestation.