Sand and gravel provenance of the Te Arai River, Waipaoa River, and associated quaternary alluvial terrace deposits, North Island, New Zealand

The Waipaoa River System (WRS) is located on the east coast of North Island, New Zealand, an area of active forearc deformation and erosion of Mesozoic to Cenozoic sedimentary sequences. Tributaries in the headwaters are the major source of sediment influx into the Waipaoa River. A pebble count at 96 km downstream shows that the gravel clasts consist mostly of sandstone, mudstone, and claystone. Petrographic analyses of size subfractions of three sand samples from the lowermost reaches and mouth of the Waipaoa River indicate a dominance of noncalcareous sedimentary lithic fragments, with monomineralic quaiiz and feldspar content increasing in the finer sand subfractions. These results are similar to and complement those of James (2003) who studied sand and gravel in the middle and upper reaches of the Waipaoa River. Additional sand samples were collected and subfractions petrographically analyzed from the Te Arai River, which is the major fine sediment contributor to the lower reaches of the WRS. Te Arai sand is more similar in composition to sand from the Waimata River than the Waipaoa River, in that it contains a high percentage of volcanic lithic fragments and a moderate percentage of calcareous sedimentary lithic fragments. Compositional differences among the Waipaoa, Te Arai, and Waimata sand samples can be linked to source lithologies, thus distinguishing Allochthon, Miocene, and Pliocene sedimentary provenances. Five quaternary alluvial telTace deposits of the Waipaoa River, four dated at c. 14.7 ka (Q2a) and one dated at c. 28 ka (Q3a), were sampled for comparison with the modern WRS sediments to provide a historical perspective of river sediment composition. Pebble counts and petrographic analysis of sand subfractions were performed on samples from all five sites. The tenace sand and gravel deposits are composed predominantly of sedimentary lithic fragments, with monomineralic quartz, and feldspar confined to the finer sand fractions. They are similar in composition to sediment from the modern Waipaoa River, except that the Q2a deposits contain higher percentage of sandstone clasts suggesting that outcrops of this lithology may have been more prevalent at 14.7 ka.