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Lahar sedimentology and spectral fingerprinting: A reassessment of Mount Ruapehu's Onetapu Formation

^^ Brian Perttu^^ , Gabor Kereszturi, Anke Zernack, Jonathan Procter

Abstract

Lahars pose significant hazards to communities surrounding active volcanoes, potentially impacting areas over 100 km from their source. The ability to 'fingerprint' and correlate lahar deposits across different catchments is crucial for understanding their triggers and flow behaviour. This study integrates spectral, XRD, and sedimentological analyses to develop diagnostic signatures for a sequence/series of Late Quaternary lahar deposits in two catchments on Mount Ruapehu, New Zealand. Our research questions focus on distinguishing between collapse-triggered and hydraulically reworked deposits and correlating distinct fingerprints between units. Field mapping documented stratigraphic relationships and sedimentological characteristics for 17 members of the Onetapu Formation across five assemblages. Spectral angle mapping of 108 bulk sediment samples, analysed using Visible-Shortwave infrared (350-2500 nm) reflected light spectroscopy, revealed distinct spectral clusters between units and varying proportions of kaolin and smectite group minerals derived from hydrothermally altered source rocks. This enabled correlation of deposits between catchments and identification of previously undocumented flow paths, as well as distinguishing debris-flow and hyperconcentrated-flow deposits where high clay contents suggest a landslide origin. Results show that at least two debris flows travelled down a previously unmapped catchment and contributed to two of the five main Onetapu Formation assemblages, including deposits comprised of remobilised Taupo ignimbrite rhyolitic material (~1850 yr BP). This fingerprinting approach enhances our ability to trace lahar deposits across complex terrain and provides new insights into the tempospatial distribution of lahars in this active volcanic system, while also improving our understanding of their trigger mechanisms and potential hazard impacts.