Recent Sedimentation Patterns and Facies Distribution on the Waipaoa River Shelf, NZ

Modern sediment distribution and fine-scale stratigraphy are the keys to understanding changes in sediment transport to, deposition and accumulation on continental margins over time. This work is part of a collaborative investigative effort, MARGINS Source-to-Sink, to characterize sediment transfer from the source waters of the high-yield, small mountainous Waipaoa River, N.Z. to the continental shelf and slope. Spatial distribution of sedimentation patterns and facies from X-radiographs, ⁷Be and sediment bulk density measurements from nearly 100 sediment box cores collected in January 2005 aboard the R/V Kilo Moana highlight modern sediment dispersal and accumulation on the continental shelf. X-radiographs and bulk density measurements from a multi-sensor core logger show three distinct facies occupy the shelf where physical and/or biological factors dominate strata formation. Spatial distribution of these facies delineates a radial pattern from interlaminated muds and sands on the innershelf to mottled muds on the outer shelf. Beryllium-7 (t_1/2 = 53 days) inventories are consistent with centennial scale trends observed by other investigators using longer-lived ²¹⁰Pb analyses, with highest accumulation in two depocenters located landward of two actively uplifting shelf anticlines. Bulk density analyses reveal lower than average bulk densities correspond with the locations of these depocenters, and higher than average bulk density are found closer to the river mouth and between the depocenters. This confirms rapid deposition of low-density muds in the depocenters, with higher bulk density, possibly reworked sediments, remaining near the river mouth and between the depocenters. A disconnect between facies progression across shelf and the depocenters defined by ⁷Be and bulk density patterns reveals multiple factors, beyond sediment supply, influence preservation.