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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compositions associated with caldera structures<br />
along the entire Kermadec arc. Three of the<br />
volcanic centres exhibit the full compositional<br />
range from basalt to rhyolite, three are basaltandesite,<br />
and the remaining two entirely rhyolitic.<br />
In contrast to published results for the southern<br />
Kermadec arc, there is no evidence for<br />
compositional bimodality, with the data set<br />
containing sub-equal proportions of basalt, basaltic<br />
andesite, andesite, dacite, rhyodacite and rhyolite.<br />
Despite the variety of volcanic edifices sampled, all<br />
of the volcanic centres yield tight geochemical<br />
correlations for major and trace elements, which are<br />
interpreted to indicate simple mixing or<br />
fractionation processes. There is a suggestion, yet<br />
to be fully established with trace element or<br />
isotopic modelling, that three of the four southernmost<br />
centres, spanning some 200 km of arc-front,<br />
are genetically related.<br />
ORAL<br />
Volcanic<br />
Centre<br />
Latitude<br />
°S<br />
SiO2 wt%<br />
Minimum Maximum Mean<br />
‘V’ 25.19 47 69 57.4<br />
‘U’ 25.44 50 71 58.6<br />
Monowai 25.95 49 62 52.9<br />
‘T’ 26.39 50 73 65.2<br />
‘S’ 26.81 69 72 71.5<br />
‘R’ 27.20 72 74 73.3<br />
‘P’ 27.86 50 65 58.3<br />
‘O’ 28.59 56 61 57.6<br />
All 47 74 60.4<br />
INCREASED FRESHWATER RUN<strong>OF</strong>F<br />
FROM THE GROWTH <strong>OF</strong> URBAN<br />
AUCKLAND - THE GREATEST CAUSE <strong>OF</strong><br />
ESTUARINE FAUNAL CHANGE<br />
H.R. Grenfell 1 , B.W. Hayward 1 ,A.T.Sabaa 1 ,<br />
M. Morley 1 & M. Horrocks 2<br />
1 Geomarine Research, 49 Swainston Rd, St Johns,<br />
Auckland.<br />
2 Microfossil Research, 31 Mont Le Grand Rd, Mt<br />
Eden, Auckland.<br />
(h.grenfell*geomarine.org.nz)<br />
Microfossils and molluscs from 13 upper<br />
Waitemata Harbour and Tamaki Estuary cores<br />
provide a record of human impact on sheltered<br />
coastal marine to estuarine environments around<br />
the fringes of growing Auckland City. Polynesian<br />
forest clearance had only minor impact compared<br />
with the increasing tempo of changes in European<br />
times. Although sedimentation rate has increased<br />
there is no consistent trend towards increasing mud<br />
or sand. In all sites, marine ostracods and infaunal<br />
bivalves were decimated in European times and<br />
largely disappear from the record, coincident with<br />
increased abundance of large diatoms and<br />
thecamoebians. The foraminiferal microfossil<br />
record is the most complete and documents a<br />
progressive switch from calcareous-dominated<br />
faunas to the agglutinate-dominated faunas of<br />
today.<br />
All observed changes are similar to those<br />
documented in transects passing from the marine<br />
environment up an estuary and suggest that<br />
increased freshwater runoff from the mushrooming<br />
urban areas of Auckland city is the primary driver,<br />
not sediment or pollutants. This is further supported<br />
by the observation that changes occur during late<br />
Polynesian to early European times in the upper<br />
Waitemata Harbour, versus late European times<br />
(post 1950) in the Tamaki Estuary when impervious<br />
surfaces increase dramatically. If increased<br />
freshwater is indeed the culprit, carrying out<br />
remedial measures will be problematic.<br />
ORAL<br />
VOLCANIC GEOLOGY <strong>OF</strong> THE ORTON<br />
BRADLEY FORMATION<br />
S.J. Hampton &J.W.Cole<br />
Dept of Geological Sciences, University of<br />
Canterbury, Private Bag 4800, Christchurch<br />
(sjh163*student.canterbury.ac.nz)<br />
The Orton Bradley Formation was deposited during<br />
the period 9.5 – 8.6 Ma. It is part of the Mt Herbert<br />
Volcanic Group of central Banks Peninsula,<br />
Canterbury, which is considered to be the<br />
intermediate stage of volcanism between Lyttelton<br />
Volcano (11 – 9.7 Ma) and Akaroa Volcano (9.3 –<br />
8 Ma). Prior to the emplacement of the Mt Herbert<br />
Volcanic Group, sector collapse occurred on the<br />
southeast side of Lyttelton Volcano. This collapse<br />
was probably the result of persistent rifting, similar<br />
to that of Valle del Bove, Mt Etna. The sector<br />
collapse destroyed the existing crater rim, and<br />
provided a pathway for renewed / migrating<br />
volcanism in central Banks Peninsula.<br />
This collapse structure would have produced a large<br />
horseshoe-shaped amphitheatre with a hummocky<br />
surface, and would have controlled the deposition /<br />
emplacement of later volcanic and fluvial activity.<br />
The upper sector of the breach contained alluvial<br />
fans, and a topographic low, steep source,<br />
amphitheatre, which constrained the initial<br />
Homestead Lava Member (new name) flows at the<br />
base of the Orton Bradley Formation. Mud deposits<br />
formed in localised depressions, on top of the<br />
topographically controlled lava flows. A period of<br />
50 th <strong>Kaikoura</strong>05 -32- <strong>Kaikoura</strong> <strong>2005</strong>