The 26.5 ka Oruanui eruption, New Zealand: A review of the roles of volcanism and climate in the post‐eruptive sedimentary response

Abstract

The landscape response to large explosive pyroclastic volcanic eruptions is one of the most dramatic processes in sedimentology and geomorphology. Processes of post‐eruptive erosion and resedimentation are maximised by large erupted volumes, abundant unconsolidated ash‐sized material, destruction of the vegetation cover (particularly by burial by ignimbrite), and inhibition of vegetation regrowth (e.g., by harsh climatic conditions). The 26.5 ka Oruanui eruption from Taupo volcano in the central North Island of New Zealand created optimal conditions for a large‐scale sedimentary response that was influenced and prolonged by the succeeding climatic nadir of the Last Glacial Maximum. About 530 km3 of rhyolitic magma was erupted as 420 km3 of fall deposits, 320 km3 of pyroclastic density current deposits (mostly non‐welded ignimbrite), and 430 km3 of primary intracaldera fill. The eruption, and formation of the Oruanui caldera, destroyed one major lake but created the forerunner to modern Lake Taupo. This lake initially stably overflowed to the northwest before breaking out in a catastrophic flood during establishment of a northeasterly outlet along the line of the modern Waikato River. Suppression of revegetation by the contemporaneous harsh periglacial climate contributed to intense erosion and remobilisation of Oruanui pyroclastic units, triggering massive downstream fluvial aggradation in impacted catchments. In particular, aggradation caused the lower 180 km of the Waikato River to avulse from its long‐established route via the Hauraki Plains into the Hamilton Basin where it was subsequently trapped. Aeolian reworking created localised dune fields, while generation of tephric loess formed deposits over much of the central North Island. The initial perturbation to fluvial sedimentary systems created by the eruption was generally sustained by climatic conditions until c. 17 ka. Climatic amelioration eventually stabilised primary sediment sources through the re‐establishment of forest vegetation, but incision and recycling of alluvial material stored in volcaniclastic fans continued until as late as c. 14 ka. Overall, the scale of the post‐Oruanui sedimentary response was primarily driven by characteristics of the eruption, whereas its duration was controlled by the Last Glacial Maximum climate.