The potential for generation of dangerous and damaging lahars and floods in response to tephra deposition from volcanic eruptions is now widely appreciated. The style and tempo of this response varies both from eruption to eruption and from catchment to catchment for single eruptions, and an understanding of factors controlling this variation is needed for precise hazard assessment. For the 1.8 ka Taupo eruption, various studies have already addressed the intracaldera response, including the refilling of Lake Taupo, and the extra-caldera re-establishment of the Waikato River, the natural outlet, which involved the catastrophic release of 20 km3 of water. Far-field effects of the eruption have also been investigated in mountainous catchments draining eastwards into Hawke' s ยท Bay. The topography of the upper Rangitaiki catchment, the subject of this study, differs in that the Taupo ignimbrite buried an essentially flat land surface inherited from a 320 ka welded ignimbrite sheet. Previous researchers have inferred that the Rangitaiki River was immediately re-established as a meandering stream, implying sediment loading and transport rates similar to those of the present. My mapping in contrast reveals that a low gradient area of hundreds of square kilometres was initially flooded and washed by hyperconcentrated and sheet flows, resulting in shallow reworking. In higher gradient areas, re-integration of drainage systems was accompanied by incision of deep channels and gullies, interspersed with ephemeral lakelets formed in response to localised damming by pyroclastic material. Ephemeral braided streams and meandering rivers eventually developed as stable rill and gully systems were established and overland flow across the remaining unconsolidated deposits diminished. This pattern of landscape response has implications for the timing and nature of the remobilised pyroclastic flux to downstream depocentres and the Bay of Plenty coast more than 100 km to the north.
Introduction There is a curious geological pattern pointing to the fact that silicic volcanism was active during the Late Cretaceous in the South Island of New Zealand. Small localised 110 - 100 M a ago silicic volcanic deposits are found outcropping in Nelson-North Westland, in the Naseby district of North Otago and in the Shag Valley district of NE Otago. This study investigates the Otago silicic volcanic rocks with the purpose of seeing how they compare with each other. Emphasis has also been placed on clarifying the field relations and documenting the lithology of the Shag Valley volcaniclas'1c sediment. 1.1 Geological Setting In both North and East Otago small amounts of Cretaceous silicic tuff are found intercalated within r r the Kyebum and Horse Range formations respectively. These formations consist of thick sequences of breccias and conglomerates which have accumulated in fault depressions that formed under an extensional regime during the Mid to Late Cretaceous. Lower Cretaceous to Early Tertiary marine deposits overWthe Horse Range Formation in East Otago. In North Otago marine transgression reached up into the Maniototo but not as far as the Naseby field locality. A shift to a compressional regime, which has affected the South Island since the early Tertiary, has reactivated Cretaceous normal faults in the reverse sense, producing uplift of basement rocks and the overlying sequences along the NW/SE trending Waihemo Fault Zone (WFZ). The Horse, Kakanui and Ida ranges were formed and these now act as the northern border of the Shag Valley 1 and the M~miototo plains. The NW/SE trending faults mark a change in the Haast schist basement geology where low grade TZ I-ll schists have been uplifted on the northern side above higher grade TZ Ill-IV schist. Figure 1 is a locality map, this shows the location of the two field localities and their relation to the Cretaceous sediments and major faults. The Shag Valley Locality Note that grid references (GR) referring to the Shag Valley locality are from Sheet 143 (NZMS, 1980). The field area is five minutes drive north of Palmerston. It lies within farmland at the foot of the QC Horse Range on the northern side of the lower Shag Valley. A ~trip 1.5 km wide and 2.5 km long, trending NW has been mapped. Nineteen days were spent in the field, the majority of which were spent at two localities. 1.2.1 The Geology On the northern margin of the Shag Valley schists of grade no higher than TZ 11 have been pushed up along faults of the WFZ, forming the Horse Range. Cretaceous sediments of the Horse Range Formation lie north of the WFZ within the Horse Range. These sediments have been uplifted above Horse Range Formation to the south. The latter sediments lie within a fault bounded strip trending NW -SE. These in turn have been uplifted, juxtaposing Cretaceous sediments against Late Tertiary marine sediments. The silicic volcanics outcrop only in this fault bounded strip. As you move outside the field area, within the infaulted strip towards the coast, you move up through the local Tertiary marine sequence. 2 In the first chapter the stratigraphy and structure is briefly presented. It should be noted that lab and field work has not been focussed on these surrounding rocks.
THE AREA MAPPED COVERS A 64 SQUARE KILOMETRE AREA BETWEEN TOKOROA AND THE SOUTHERN EDGE OF THE MAMAKU PLATEAU. SIX RHYOLITIC PYROCLASTIC UNITS ARE MAPPED AND DESCRIBED.
THE OLDEST EXPOSED FORMATION IS THE WHAKAMARU IGNIMBRITE. STRATIGRAPHICALLY ABOVE ARE THREE NEW PYROCLASTIC DEPOSITS, ONE OF WHICH IS BEING PROPOSED FOR FORMATIONAL STATUS.
THE OLDEST OF THESE THREE DEPOSITS IS A THIN PLINIAN FALL WITH AN OVERLYING TWENTY FIVE METRE, NON-WELDED* CRYSTAL POOR, PUMICE RICH, FINES CONCENTRATED, IGNIMBRITE THAT SITS UNCONFORMABLY ON A THICK PALEOSOL ABOVE THE WHAKAMARU IGNIMBRITE. THIS DEPOSIT IS HERE INFORMALLY NAMED THE CHIMP FORMATION. THE NEXT DEPOSITS ARE A SIX METRE THICK GROUP OF PUMICE AIRFALL UNITS, EACH SEPARATED FROM THE OTHER BY A PALEOSOL, WHICH SITS UNCONFORMABLY ON THE CHIMP IGNIMBRITE, AND HERE ARE INFORMALLY NAMED THE GALAXY TEPHRAS.
THE POKAI IGNIMBRITE IS THE NAME FORMALLY PROPOSED FOR THE NEXT DEPOSIT, WHICH COMPRISES A PLINIAN FALL AND IGNIMBRITE UNIT. A TYPE SECTION IS SET UP FROM A 30 METRE SECTION IN POKAI ROAD. THE PLINIAN FALL CONSISTS OF MEDIUM TO COARSE LAPILLI SIZE PUMICE INTERBEDDED WITH FINE RAINFLUSHED ASH BEDS. THE PLINIAN FALL IS SHARPLY OVERLAIN BY THE IGNIMBRITE WITHIN WHICH CREAMY WHITE TO YELLOW PUMICE, IN A PALE BROWN MATRIX AT THE BASE PASS PROGRESSIVELY TO ORANGE-BROWN PUMICE IN A DARK GREY MATRIX AT THE TOP. THIS PUMICE RICH, CRYSTAL POOR IGNIMBRITE FORMS A THICK SINGLE COOLING UNIT1 WITHIN WHICH THREE FLOW UNITS ARE RECOGNISED.AND NORMAL THERMAL ZONATION IS DISPLAYED. A NORTH EASTERLY SOURCE IS POSTULATED FOR THIS DEPOSIT.
SIX METRES OF MAMAKU IGNIMBRITE LIES UNCONFORMABLY ABOVE THE POKAI FORMATION. THE LAST ERUPTIVE DEPOSIT EMPLACED IN THE STUDY AREA WAS THE AD186 TAUPO IGNIMBRITE WHICH FORMS DISTINCTIVE VALLEY PONDED DEPOSITS OF VARYING THICKNESS WITHIN THE AREA.
*TO MODERATELY WELDED
The mapped region encompassing the Haunui Valley covers 32 km2 and is situated on the western margin of the Taupo Volcanic Zone, 12 km southwest of Rotorua. Six formations are identified. The oldest formation is the densely welded, quartz poor Waiotapu Ignimbrite which is stratigraphically overlain by phreatoplinian, pyroclastic flow and plinian eruptives of the non-welded, crystal poor Onuku Breccia. The oldest phase of Haparangi Rhyolite extrusion follows these, and is represented by the Horohoro Rhyolite Dome. Onlapping and overlying this, is the Korokoro Ignimbrite, an ignimbrite formation newly proposed in this thesis. The youngest pyroclastic flow formation is the voluminous quartz-rich Mamaku Ignimbrite forming the Mamaku Plateau. A period of lake sedimentation produced the Huka Group in the Guthrie Graben from 42 000 yrs B.P. until 20 000 yrs B.P. after which a series of plinian tephra formations were deposited.
Laboratory work involves several aspects. Firstly, correlations of ignimbrite formations to areas outside the Haunui Valley are presented, based on mineralogy and stratigraphy. Secondly, grainsize analyses and scanning-electron-microscope data are used to investigate the phreatoplinian nature of the Onuku Breccia. Thirdly, previously unestablished characterisation of ignimbrite formations in terms of recrystallization zones are presented. Finally, petrographic modal analyses are compared to XRF major element chemical analyses in vertical sequences of ignimbrite formations. The results are inferred to reflect compositional zonation of the magma chambers involved, by convection-driven thermogravitational diffusion.
The Haunui Valley is delineated by a tectonic rim, inferred to be associated with Holocene anticlinal warping of the Mamaku Plateau. Two other major faults are present. The oldest is the northerly trending Pokaitu Fault, which represents post-Korokoro Ignimbrite tectonism. The youngest fault is the probably recent active, northeasterly trending Horohoro Fault and its subparallel, discontinuous derivatives.