ANALYSIS OF TRENDS IN THE COMPOSITION AND ROUNDING OF SAND AND GRAVEL FROM THE WAIPAOA RIVER AND COASTAL REGION, NORTH ISLAND, NEW ZEALAND

The eastern margin of North Island, New Zealand is an actively deforming and eroding forearc. In this area, the Waipaoa River System has developed on older Mesozoic metasedimentary (Torlesse) and uplifted Cenozoic forearc basin sequences. These units are dominated by mudstone lithologies with minor sandstone and limestone. There has been additional periodic input of volcanic ash into the forearc from the Taupo magmatic arc to the west since the Pliocene. The purpose of this study is to petrographically characterize coarse fluvial sediment in the Waipaoa River System as part of the Margins Source-to-Sink program.

Within the exposed Waipaoa River bed, gravel clast size decreases and the relative abundance of sand and mud increases downstream. Gravel clasts are predominantly mudstone with minor sandstone, claystone, and limestone, proportionately reflecting exposed lithologies exposed in the drainage basin. In a similar fashion, the sand is dominantly composed of mudstone lithic fragments with minor volcanic and carbonate components. There is a distinct relationship between grain size and sand composition however, with the finer sand fraction relatively enriched in quartz and plagioclase feldspar grains. This sequestering of monomineralic components in the finer fractions may be characteristic of sediment recycled from siliciclastic sedimentary terranes (e.g., accretionary prisms).

Gravel clasts and sand grains become more rounded downstream within the Waipaoa River. Petrographically distinct sand grain types were selected to document downstream rounding trends, as well as to compare rounding in fluvial versus coastal sand. These grain types include micrite, coarse carbonate, a specific mudstone variety and colorless nonvesicular glass. Grain rounding trends were determined using qualitative petrographic analytical techniques.