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The sand and gravel within the Waipaoa River system of North Island, New Zealand, elucidate the provenance and sedimentary delivery system of an actively deforming continental forearc. In this region, Mesozoic to Cenozoic forearc basin sedimentary successions are being uplifted, eroded, and recycled into younger deposits. The identification of specific characteristics of the drainage basin bedrock can be linked to sediment composition, which in turn can document the processes of provenance mixing and dilution.

Despite the proximity of the Waipaoa River to an active volcanic arc (∼250 km), its sediment load includes a relatively small proportion of volcanic debris. Sand detrital modes and clast count data indicate that Waipaoa River sediments are dominated by the crushed and sheared sedimentary rocks of the East Coast allochthon, which is located in the upper portion of the catchment. Higher volcanic percentages in the Waimata and Te Arai Rivers suggest that the volcanic contribution in the Waipaoa River has likely been diluted by the great volume of sediment brought into the system from the large gullies in the catchment headwaters.

Petrographic analysis of sand subfractions (very fine to very coarse) of Waipaoa River system sediment samples indicates that there is a distinct relationship between the size of the grains and their respective composition. Quartz and feldspar grains are concentrated in the finer fractions, reflecting the grain size of detrital sand liberated from Mesozoic to Cenozoic sandy source rocks. Thus, their distribution is at least in part an inherited feature related to sediment recycling within the system. Most of the sediment (gravel and sand) consists of mudstone and claystone lithic fragments.

In the Waipaoa River, the sand is dominated by noncalcareous mudstone lithic fragments, some of which are smectitic. In the Waimata River, the sand grains are mainly calcareous mudstone lithic fragments. This difference in rock-fragment type reflects differences in mud rock source lithologies. Similar trends are observed in the gravel fractions of the Waipaoa River, with noted increases in calcareous gravel components in the river where it (or tributaries) traverse calcareous lithologies.

These compositional differences suggest that in future studies of older alluvium and offshore sedimentary sections, it may be possible to distinguish coarse sediment derived from a purely Cenozoic source from that derived from Mesozoic rocks in the upper catchment of the Waipaoa River system, allowing us to interpret drainage evolution and denudation patterns.

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