Cryptic eruption of Mount Ruapehu revealed by deposits of sediment laden streamflow in a steep mountain valley: The 4 ka Kiwikiwi Formation, Whangaehu Valley, NZ

A distinctive deposit of unaltered reworked volcaniclastic sediment, here designated the Kiwikiwi Formation, is described from the upper Whangaehu Valley at Mount Ruapehu, New Zealand. The grain-size distribution and sedimentary features of the unit suggest deposition from a sheetflow-dominated alluvial system, but simple sediment entrainment calculations show that the unitʹs granulometry is inconsistent with normal streamflow deposition at the observed steep depositional slope. Streams have produced deposits at Mount St. Helens and Mount Pinatubo under similarly anomalous conditions following debris avalanche and/or volcanic eruptions, when highly charged with sediment and subject to common lahars (debris flows and hyperconcentrated flows). Reported sediment transport rates from Mount St. Helensʹ Toutle River, and Pinatuboʹs Pasig–Potrero River, show that a virtually unlimited supply of relatively fine-grained (sand-range) tephra renders some established sediment transport formulae inapplicable because the volume of available material effectively exceeds the transport capacity of the system. We use simple equilibrium flow calculations to demonstrate that this sedimentary unit, which has open porosity, framework grain support and bedding characteristics compatible with alluvial sheet flows, would not have been deposited under normal streamflow conditions. This approach provides a means to confirm the role of capacity limitations on stream behaviour during the post-eruptive sedimentary response phase in catchments overwhelmed by pyroclastic material, and is particularly useful given the lack of clear depositional fingerprints for hyperconcentrated flow. We further infer that because the material in the Kiwikiwi Formation is uniform, unaltered and minimally abraded, differs from older sediment in the valley, overlies a 4.6 ka debris-avalanche deposit, and was emplaced from an over-capacity stream by hyperconcentrated flows or extremely thinned ones, and without evidence of interruption, it represents the previously unidentified proximal products of an unstudied Holocene eruption of Ruapehu.