Late Pleistocene siliceous sinter associated with fluvial, lacustrine, volcaniclastic and landslide deposits at Tahunaatara, Taupo Volcanic Zone, New Zealand

Abstract

The Tahunaatara sinter, Taupo Volcanic Zone, New Zealand, is a ∼17–20-kyr-old hot-spring deposit of opal-A mineralogy. It is interbedded with fluvial, lacustrine and volcaniclastic sediments, some silicified by infusing thermal waters. The exposed sinter (∼4 m thick, 90 m long) was truncated at its southern margin by a landslide, which deposited a conglomerate (up to 2 m thick, 56 m long) of sinter blocks and associated strata nearby. Kaolinite-rich cobbles at the base of the conglomerate indicate a change in the thermal regime and its probable trigger: acid steamcondensate produced alteration. Clasts in the landslide are oriented SW, the same direction as flattened plant reeds entombed in sinter, and as intercalated fluvial beds. Thus, thermal waters, stream flow and the landslide all likely followed the same palaeo-valley, which is similar in terrain and stratigraphy to the Devonian Rhynie hydrothermal system. The plant-rich, layered, in situ sinter contains fossilised microbes and rare stromatolites, and was deposited on mid- to distal slopes adjacent to marshes. Ash falls, fluvial activity and ponding occurred during and after the thermal activity. Unsilicified tephric Ohakea loess (∼26–17 kyr BP) and Taupo Tephra (1·86 kyr BP) blanket both sinter and landslide. Today, the deposits form resistant remnants in a topographically inverted landscape.