Presenter: Wen-Shu Koo
Date: 2016/10/13
Abstract
The Poverty continental margin, New Zealand, is a segment of the Waipaoa Sedimentary System (WSS), and also a MAGRINS Source-to- Sink focus site. It had chosen for examine the terrestrial and marine sedimentary response to natural and human impacts on dispersal system. Grain size, composition, sorting and shape were used to identify the potential source on Poverty shelf. The modern Waipaoa River is dominated by mudstone fragments, mainly products of gullying in river headwater region. In contrast, the terrace deposits are enriched in sandstone lithics. Both the terrace and modern rivers show a compositional dependency on grain size, with increasing lithic components in coarse sand fraction. However, mean QFL (Quartz-Feldspar-Lithic fragments) values indicate no significant compositional difference among shelf sand fractions like that seen in the fluvial and terrace sands of Waipaoa River, whereas the shelf samples are generally more enriched in volcanic and metamorphic lithic. In addition, the petrographic analysis of sand from Poverty shelf reveals that the sand is mainly derived from intra- and potentially extra-basinal (several) sources, but surprisingly, the Waipaoa River is not the dominant source of fluvial system that drains into Poverty Bay. Moreover, the coarser, poorly sorted and angular sands distribute along the outer shelf, while finer, well-sorted sands characterize the middle shelf. These findings suggest a shorter transport history for the material near the outer-shelf bathymetric high area. The clast (sediment) shape is indicated by anomalous prolate and equant-shaped greywacke (Torlesse), which is distinctly different from the terrestrial segment of WSS and the clasts on beach of Hawke Bay. Rather, these sediments are more similar to Torlesse stream gravels. Seismic and multi-beam data support the possibility that during the recent sea-level lowstand, the Hawke Bay fluvial system flowed into Poverty Canyon, bringing these unique greywacke (Torlesse) onto the Poverty outer shelf. It also highlights the WSS was not a closed sedimentary system in the past.
Reference
Parra, J.G., Marasaglia, K.M., Rivera, S.K., Dawson, A.T., Walsh, J.P. (2012) Provenance of sand on the Poverty Bay shelf, the link between source and sink sectors of the Waipaoa River sedimentary system. Sedimentary Geology, 280, 208-233.
Marasaglia, K.M., DeVaughn, A. M., James, D. E., Marden, M. (2010) Provenance of fluvial terrace sediments within the Waipaoa sedimentary system and their importance to New Zealand source-to-sink studies. Marine Geology, 270, 84-93.