Resedimentation of the 1.8 ka Taupo ignimbrite in the Hawke's Bay region, North Island, New Zealand

Segschneider, Beate, 1969-

The Taupo ignimbrite erupted from the Taupo Volcanic Centre, North Island, New Zealand, circa 1800 years ago. It has an estimated volume of 30 km3 and covered the topography over a circular area approximately 160 km in diameter, centred on a vent beneath Lake Taupo. Primary deposits of loose pyroclastic material reached thicknesses in the Hawke's Bay region of up to 40 m in valleys and topographic depressions (valley pond ignimbrite; VPI), and 1 m on hill slopes (ignimbrite veneer deposit; IVD), destroying hydrological systems and all vegetation throughout the impacted area. Sedimentary response to such large ignimbrite-emplacing eruptions from rhyolitic calderas is poorly understood. This thesis analyses the resedimentation of debris from this voluminous ignimbrite in the Hawke's Bay region, in order to understand post-eruptive surficial processes and as an aid in the assessment of hazards from future eruptions. Pumice pyroclasts behave in unusual ways in the sedimentary environment due to their low, and variable density. This unusual behaviour was addressed here by systematic sieving and automated settling tube analysis of water saturated pumiceous sediment, confirming that sieve analysis are misleading in respect to hydrodynamic behaviour of pumiceous sediments. This distinctive and different behaviour required the use of both lithofacies and petrofacies in the description and classification of the remobilisation deposits (chapter 2). Analysis of preserved remobilisation sediments along Hawke's Bay rivers showed that resedimentation occurred in two main stages, depending on the sediment:water ratio during the response period. Stage one comprised erosion and redeposition of the ignimbrite by lahars. Stage two was marked by decreasing sediment: water ratios, and the re-establishment of braided fluvial systems in the former river valleys. This sedimentary response pattern was revealed by grouping characteristic sets of eooccurring lithofacies into 4 lithofacies associations. Resedimentation of debris from the 1.8 ka Taupo ignimbrite is described and interpreted separately for each Hawke's Bay river system (chapters 3 to 5). Pumiceous deposits are preserved in a series of terraces in the ignimbrite covered area and beyond. The rivers are divided into headwaters, middle, and lower reaches due to their different sedimentary responses. The most prominent feature in the headwaters was stepwise incision into VPI. Along the middle reaches, partial erosion of VPI was followed by up to 20 m of aggradation of the valley floor by lahar deposits. Temporary lakes formed in tributary valleys that were dammed by aggradation in the main valley. Erosion of the lahar deposits followed, with up to 15 m of fluvial aggradation downstream from the most distal preserved lahar deposits along the Mohaka River. Similar fluvial aggradation reached up to 10 m along the Ngaruroro River, and up to 8 m along the Waiau River. Final degradation brought the rivers back into their pre-1.8 ka bed. Along the lower reaches of all rivers laharic and fluvial aggradation occurred. Fluvial aggradation averaged 2 m on the Heretaunga Plains. Individual catchment areas received between 3 km3 (Mohaka) and 0.07 km3 (Waiau) of primary ignimbrite. Of this initial volume, between 71 and 96 %respectively was removed by post-eruption remobilisation (chapter 6). Estimated times for the duration of the laharic and fluvial resedimentation periods lie between 3 and 17 years. Variations in resedimentation processes, and the duration of the inferred resedimentation periods between the three catchments are interpreted to reflect differences in: 1) initial ignimbrite volumes; 2) gradient of the rivers; and 3) number and size of tributaries unaffected by ignimbrite emplacement. Future volcanic hazards for the Hawke's Bay region from the emplacement of a similar-sized ignimbrite to the 1.8 ka Taupo ignimbrite at the Taupo Volcanic Centre are assessed based on the results from this study (chapter 7). River valleys downstream from their reaches affected by ignimbrite would experience major flooding events and lahars.

1 v. (various pagings) : ill., maps (some folded) ; 30 cm.

2000Segschneider

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