Sand provenance in the Waipaoa River System, North Island, New Zealand

The Waipaoa River System (WRS) is located on the eastern margin of North Island, New Zealand (NINZ). This region is an actively deforming and eroding forearc where sedimentary successions are being recycled into younger deposits. The WRS has developed on uplifted Mesozoic and Cenozoic forearc basin sequences. Since the Pliocene, there has been periodic input of volcanic ash into the forearc from the Taupo magmatic arc, which is located to the west of the drainage. The size and lithology of bedload clasts were recorded at five sites (n= lOO per site) along the Waipaoa River. The gravel clasts are predominately mudstone/siltstone with lesser amounts of sandstone, claystone, and limestone. The proportion of calcareous clasts increases from 16% to 4 7% at 65 km downstream; this increase is possibly due to calcareous sediment input from tributary streams. Sand samples were collected from exposed sandy bars along the length of the Waipaoa River, along several tributary streams to the Waipaoa River, and down the length of the Waimata River. Stained thin sections prepared from these samples were point-counted using the Gazzi-Dickinson method, with a total of 400 points per thin section. The petrographic analysis of 15 bulk sand samples shows them to be composed predominantly of sedimentary lithic fragments (average QFL%L = 86%; LmLvLs%Ls = 94%) with lesser quartz (average QFL%Q= 11 %) and feldspar (average QFL%F= 3%) and relatively minor volcanic lithic (LmLvLs%Lv = 2%), carbonate/limestone (4% average of total grains) and other components (2% average of total grains) such as mica, glauconite and dense minerals. Petrographic analysis of 5 sand subfractions (very fine to very coarse) for 14 samples, indicate that there is a distinct relationship between grain size and sand composition. Quartz and to a lesser degree feldspar grains are sequestered in the finer fractions, with QFL% Q up to 53% and QFL%F up to 9% in the very fine fraction. The sand data and clast count data show that the Waipaoa River sediments are dominated by the crushed and sheared sedimentary rocks of the East Coast Allochthon located in the upper portion of the catchment. Despite the proximity of the Waipaoa River to an active volcanic arc (- 250 km), its sediment load contains a relatively small proportion of volcanic debris. There is also a relatively distinct compositional difference between the Waipaoa and Waimata rivers, where the proportions of volcanic lithic fragments vary, as do the percentages of specific mudstone lithic fragment types. The Waipaoa River is dominated by non-calcareous mudstone lithic fragments while the Waimata River is dominated by calcareous mudstone lithic fragments. These compositional differences suggest that in older alluvium and offshore sedimentary sections it may be possible to distinguish sand derived from purely a Miocene/Pliocene source from that derived from the upper catchment of the Waipaoa River.