Abstract
The Rotorua volcanic centre (RVC) forms a well-defined topographic depression ≈ 21 km by 22 km in diameter that is located along, and partly delineates, a structural embayment in the western margin of the central TVZ.
The late Quaternary lacustrine sedimentary record in the Rotorua depression probably began shortly after Rotorua Caldera formed with the eruption at ≈ 220 ka, of the voluminous Mamaku Ignimbrite. Initial sedimentation probably began with debris and hyperconcentrated flows, followed by lacustrine deposition as the new catchment adjusted to the volcanically created accommodation space.
Three littoral terraces, formed during periods of stable high water levels surround Lake Rotorua. Deposits exposed in these terrace outcrops include those of beaches, subaqueous channels, fine-grained delta fronts and well-formed delta foresets. These can be separated by their geomorphology and field relationships, in particular by identifying the unconformities that separate highstand deposits.
Sediment from these deposits was also characterised by Electron Microprobe analysis of glass shards, granulometry, their ferromagnesian mineral assemblage, and XRD and SEM analysis of samples obtained from different lacustrine deposits. This dataset allows construction of an integrated stratigraphic model from individual deposits and outcrops, informally subdividing them into three alloformations based on their bounding unconformities. Allostratigraphy provides an objective means of correlating heterogenous lacustrine deposits that are nevertheless genetically related.
Lacustrine beds were deposited in deep lake waters during three time intervals, when Lake Rotorua was filled to between 65 m and 120m higher than today. Each suite of deposits is informally named for the volcanic event inferred to have initiated the high lake level.
1. Post-Mamaku alloformation (up to ≈ 415 m a.s.l.).
2. Post-Rotoiti alloformation (up to ≈ 380 m a.s.l.).
3. Post-Hauparu alloformation (up to ≈ 349 m a.s.l.).
The first highstand, after the Rotorua depression formed, may have ended when the caldera wall adjacent to the Pohaturoa Dome collapsed, with this breach forming the Hemo Gorge. Subsequent high stands followed eruptions from the neighbouring Okataina Volcanic Centre that deposited large, northwardly dispersed, tephra deposits which formed natural dams across the Rotoiti channel, the northern outlet of Lake Rotorua. Eventually these dams were breached either by headward stream erosion or by land sliding of weak damming material. This channel was finally occluded at ≈ 9 ka and now contains Lake Rotoiti. At very high lake levels, the lake may have spilled across the caldera margin above Mission Bay; this water would have entered the Kaituna River, to drain into the Western Bay of Plenty. At this high level, the lake extended through the Hemo Gorge and flooded the neighbouring Kapenga depression, probably crossing the modem drainage divide and spilling into the Waikato River to the South.