50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
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distal alluvial plains where large population centres<br />
and infrastructure are typically concentrated.<br />
ORAL<br />
PAST HIGHSTANDS <strong>OF</strong> LAKE ROTORUA:<br />
IMPLICATIONS FOR TECTONISM AND<br />
PALAEOGEOGRAPHY IN THE TAUPO<br />
VOLCANIC ZONE<br />
V. Manville 1 ,R.Marx 2 ,J.D.L.White 2<br />
1 Institute of Geological & Nuclear Sciences, Private<br />
Bag 2000, Taupo, New Zealand<br />
2 Geology Department, University of Otago,<br />
PO Box 56, Dunedin, New Zealand.<br />
(v.manville*gns.cri.nz)<br />
Lake Rotorua currently partially occupies a nearcircular<br />
20 km diameter volcano-tectonic<br />
depression formed at 220 ka by eruption of the<br />
voluminous Mamaku Ignimbrite. Two distinct<br />
lacustrine littoral terraces that fringe much of the<br />
lake basin, defined on the basis of contrasting<br />
geomorphology and field relationships and<br />
separated by tephrostratigraphically dateable<br />
unconformities, are here correlated with former<br />
highstands of the lake. The first and highest<br />
elevation terrace (up to 415 m), corresponds to<br />
accumulation of a lake in the newly created basin in<br />
the immediate aftermath of the Mamaku Ignimbrite<br />
eruption. Considerable uncertainty surrounds the<br />
direction of overflow of this level, but the lake may<br />
have extended southwards through the Hemo<br />
Gorge, a v-shaped notch in the topographic rim of<br />
the caldera, through the Ngakuru Graben and into<br />
the Waikato River drainage. A highstand at this<br />
level would be contiguous with Lake Huka in the<br />
Taupo-Reporoa area and require a blockage in the<br />
Ongaroto Gorge, possibly related to volcanism at<br />
the Ohakuri or Maroa volcanic centres. At some<br />
later time a northeasterly outlet became established<br />
at a lower level through the tectonically subsiding<br />
Tikitere Graben into the Haroharo caldera from<br />
where it flowed into the Bay of Plenty via the<br />
Kawerau Canyon. The second and most extensive<br />
littoral terrace (370-380 m), the post-55 ka Rotoiti<br />
alloformation, is the product of a durable highstand<br />
produced by blockage of this drainage path by<br />
eruption of a voluminous unwelded ignimbrite from<br />
the adjacent Okataina Volcanic Centre. Lake level<br />
was maintained by stable overspill across a welded<br />
ignimbrite sill west of the Tikitere Graben into the<br />
north-flowing Kaituna River until accumulation of<br />
Mangaone Subgroup tephras in the graben could no<br />
longer keep pace with active subsidence. Lake level<br />
initially dropped rapidly by c. 20 m as the<br />
unconsolidated Mangaone tephras were flushed<br />
away, before impingement of the falling lake level<br />
on the underlying more consolidated Rotoiti<br />
pyroclastics produced a short-lived stillstand at c.<br />
350 m marked by a third alloformation comprising<br />
cryptic fluvial strath terraces and shoreline deposits<br />
in the Rotorua basin. Resumption of down-cutting<br />
triggered an apparently catastrophic break-out<br />
through the Haroharo route prior to the 26.5 ka<br />
Oruanui eruption and a fall in lake level to below<br />
260 m. Episodic growth of resurgent dome<br />
complexes between 21 and 9 ka progressively<br />
blocked this drainage path, forcing Lake Rotorua to<br />
rise to a level where it could overtop a drainage<br />
divide on the northern rim of Lake Rotoiti to reoccupy<br />
the former Kaituna River course,<br />
establishing the current outlet channel.<br />
POSTER<br />
PALEOENVIRONMENTAL<br />
RECONSTRUCTION <strong>OF</strong> A WELL-<br />
PRESERVED STAGE 7 FOREST SEQUENCE<br />
CATASTROPHICALLY BURIED BY<br />
BASALTIC ERUPTIVE DEPOSITS,<br />
NORTHERN <strong>NEW</strong> ZEALAND<br />
M. J. Marra 1 ,B.V.Alloway 2<br />
&R.M.Newnham 3<br />
1 Department of Geological Sciences, University of<br />
Canterbury, NZ<br />
2 GNS-Science, Gracefield Research Centre, PO<br />
Box 30368, Lower Hutt, NZ<br />
3 School of Geography, University of Plymouth,<br />
Plymouth PL4 8AA, UK<br />
(Maureen.marra*canterbury.ac.nz)<br />
The well-preserved remnants of a forest sequence,<br />
catastrophically inundated by proximal to medial<br />
phreatomagmatic deposits, are identified on the<br />
shores of the Manukau Harbour. In this study the<br />
stratigraphy and age of this forest succession was<br />
examined in detail along with palaeoecological<br />
proxies (palynology and beetle assemblages) from<br />
carbonaceous muds associated with the forest<br />
sequence. Optically Stimulated Luminescence<br />
dating of the phreatomagmatic deposit together<br />
with palaeoecological evidence for interglacial<br />
climate suggests deposition in late Marine Isotope<br />
Stage (MIS) 7. This extends the known age of<br />
Auckland volcanism by up to 40 ka.<br />
Ninety-eight fossil beetle taxa were identified. All<br />
but two of the fossil taxa occur in the local modern<br />
fauna. Based on an extensive survey of the local<br />
modern fauna, the fossil beetle fauna represents<br />
48% of families, 20% of genera, and 13% of<br />
species in the local modern fauna. The fossil<br />
assemblage comprises taxa from forest, wetland<br />
and beach habitats.<br />
Both beetle and pollen assemblages indicate a<br />
kauri/podocarp forest growing adjacent to a<br />
wetland on or near a coastal plain. The pollen<br />
record shows Agathis australis- dominance<br />
between two phases of Dacrydium cupressinum<br />
50 th <strong>Kaikoura</strong>05 -49- <strong>Kaikoura</strong> <strong>2005</strong>