PETROLOGY OF WAIPAOA RIVER TERRACE DEPOSITS AND IMPLICATIONS FOR THE EVOLUTION OF THE WAIPAOA RIVER SYSTEM, NORTH ISLAND, NEW ZEALAND

The Waipaoa River System (WRS) is located on the east coast of the Raukumara Peninsula, North Island, New Zealand. The WRS is an area of active forearc deformation and erosion. The drainage basin of the WRS is underlain by Mesozoic and Cenozoic forearc sedimentary sequences consisting of marine muddy to sandy siliciclastic rocks with minor carbonate intervals. Studies of sediment within the meandering and braided rivers of the modern WRS indicate that upstream tributaries are the source of the majority of the gravel clasts. The gravel load is mostly clasts of claystone and mudstone with lesser sandstone. Older river terraces identified and mapped in the upper reaches of the WRS are thought to be indicators of periods of cooler climate and higher erosion rates in the paleo-WRS. Several of these, 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. Pebble counts were conducted in the field. Clasts were categorized by lithology and dimensions recorded. Thin sections were produced from representative clasts. Preliminary data indicate that the Q2a gravels contain a greater proportion of sandstone clasts than both the modern WRS and older Q3a deposits. More detailed study of the sandstone lithic types is in progress. Sandy intervals and matrix within the terrace gravels were also collected and petrographically examined. The terrace sands are similar in composition to the WRS, being composed predominately of mudstone sedimentary lithic fragments. These petrological data will ultimately be linked to the offshore record of clastic sedimentation as part of an ongoing study of this MARGINS source-to-sink focus site.