The Murimotu Formation—9500 year old deposits of a debris avalanche and associated lahars, Mount Ruapehu, North Island, New Zealand

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Abstract
The Murimotu Formation comprises three facies, distinguished on the basis of surface morphology and abundance of debris-avalanche blocks: masses of loosely consolidated debris transported nearly intact from the source area Facies 1 and 2 are debris-avalanche deposits. Facies 1 occupies the axial part of the deposit, contains over 50% debris-avalanche blocks (to 36 m in length), and is characterised by large (to 10 m high) hummocks. Lithologic layering in facies 1 preserves the original stratigraphy of the volcano. Facies 2 surrounds facies 1, contains less than 30% debris-avalanche blocks (< 1.0 m in length), and has low (to 1.5 m) hummocks. Facies 3 was deposited by lahars. Facies 3 either overlies facies 1 and 2 or occurs around the margins of facies 2. Facies 3 is matrix supported, ungraded, and has planar upper surfaces. The Murimotu event was triggered by gravitational collapse probably due to intrusion of dikes into hydrothermally weakened Te Herenga Formation. Facies relationships suggest that the collapse occurred near the headwaters of the Whakapapanui Stream. Facies 1 was confined to the Whakapapanui valley while on the volcano and spread laterally over a small distance when flow became unconfined on the ring plain. Greater mobility of facies 2 allowed it to overtop drainage divides on the volcano and spread laterally over larger areas on the ring plain than did facies 1. Lahars postdated emplacement of the debris avalanche. They resulted from subsequent failures in the area of the initial collapse or from the surface of the debris-avalanche deposit