Environmental response to a large, explosive rhyolite eruption: sedimentology of post-1.8 ka pumice-rich Taupo volcaniclastics in the Hawke's Bay region, New Zealand

Abstract

The 1.8 ka eruption of the Taupo ignimbrite from the Taupo Volcanic Centre in the central North Island of New Zealand produced a ∼30 km 3 ignimbrite covering an area ca. 160 km in diameter, centred on Lake Taupo. Remobilisation of its pyroclastic detritus triggered major alluviation in the years to decades after its emplacement. In the Hawke's Bay region in the eastern North Island, three large rivers rise in the ignimbrite-impacted mountainous headwaters, in areas that were buried by up to 40 m of loose, unconsolidated pumiceous material. Analysis of preserved volcaniclastic sediments along valleys shows that resedimentation occurred in two main stages characterised by contrasting sediment:water ratios. The first comprised erosion and redeposition of ignimbrite debris by lahars, e.g. debris flows and hyperconcentrated flows. The second was marked by decreasing sediment:water ratios and the re-establishment of braided fluvial systems in the former river valleys, as recorded by such deposits as trough, planar, low-angle and antidune cross-bedded, planar-bedded, and ripple cross-laminated sands and gravels. However, there is no systematic chronological trend towards lower pumice contents in these fluvial facies. Four characteristic sets of co-occurring lithofacies were produced during the sedimentary response: (1) Laharic remobilisation association; (2) Upper-flow regime-dominated stream-flow association; (3) Lower-flow regime-dominated stream-flow association; and (4) Lacustrine association. Individual lithofacies were strongly controlled by the unusual hydrodynamic properties of pumice, particularly its low and variable density. Automated settling tube analysis of water-saturated pumiceous sediment confirms that sieve analysis is misleading with respect to the hydrodynamic behaviour of such sediments. Recasting granulometric data in terms of effective hydraulic grain size shows better sorting of mixed component sediments, but statistical sorting and mean grain-size parameters do not discriminate between the deposits of tractional and overcapacity parent flows. Therefore, particle behaviour is addressed by using both lithofacies and petrofacies in the description and classification of the remobilisation deposits.