Recent sedimentation patterns and facies distribution on the Poverty Shelf, New Zealand

Abstract

Modern sediment dispersal and accumulation on the Poverty Shelf off the Waipaoa River, New Zealand, is investigated using X-radiographic, radio-isotopic and physical property analyses of nearly 200 box and Kasten cores collected in January 2005 aboard the R/V Kilo Moana. The high-yield, small mountainous Waipaoa River empties onto a tectonically active, narrow margin and represents an important analog for shelf sedimentation in similar environments worldwide. X-radiographs and bulk-density measurements from a multi-sensor core logger show three distinct facies on the shelf where physical and/or biological factors dominate strata formation. Spatial distribution of these facies delineates a radial pattern with distance from the river mouth, from areas dominated by stratified layers on the inner shelf to mottled muds on the outer shelf, demonstrating a continuum of controls from wave reworking to episodic and extreme wet event sedimentation, despite a tectonic framework that supports rapid accumulation in two recently identified shelf depocenters. Analysis of short-lived 7Be ( t 1/2 = 53 days) reveals a broad spatial distribution across- and along-shelf in surface sediments, suggesting rapid transport from the Waipaoa source. However, beryllium-7 inventories are consistent with centennial-scale trends observed by other researchers using longer-lived 210Pb analyses, with highest inventories in the shelf depocenters. Box core bulk-density analyses show lower than average bulk densities within these depocenters, and higher than average bulk densities on the inner shelf and between the depocenters. These observations confirm rapid deposition of low-density muds in the depocenters, with higher bulk-density, possibly physically reworked sediments, remaining off the mouth of Poverty Bay and between the depocenters. Based on this single observational period, there appears to be no fundamental difference between seasonal and longer-term accumulation patterns on the Poverty Shelf.