50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

QUATERNARY TEPHRA BEDS PRESERVED
ON CHATHAM ISLAND: THEIR
IDENTIFICATION AND SIGNIFICANCE IN
INTERPRETING ASSOCIATED DEPOSITS
AND PALEOVEGETATION
Kat Holt
Institute of Natural Resources, Massey University,
Palmerston North, N.Z.
(k.holt*massey.ac.nz)
The Chatham Islands preserve the easternmost
occurrences of terrestrial Quaternary deposits of the
New Zealand microcontinent. These include dune
sands, loess, extensive blanket peats, and rhyolitic
tephras derived from the TVZ in mainland New
Zealand. At least three separate rhyolitic events are
preserved as macroscopic tephra beds at many
different locations around Chatham Island.
The youngest of these is the well known and well
dated Kawakawa Tephra, which is ubiquitous about
the Island, occurring in peat, sand, colluvium and
loess deposits, providing an excellent marker for
LGM studies on the Island.
The c.340 ka Rangitawa Tephra is present at three
or more sites around the Island. At one of these
locations it occurs less than two metres below the
Kawakawa Tephra, indicating either extended
periods of non-deposition or periods of erosion at
these sites on the Island during the mid to late
Quaternary.
In addition, devitrified tephra beds at three different
localities have been recognised using
ferromagnesian mineral content and geochemistry
of ilmenite grains. At present, their ages are still
unknown, but zircon grains preserved within these
tephras are to be fission track dated which will aid
in identifying them.
The occurrences and stratigraphic significance of
the various tephras will be discussed in relation to
the landscape evolution and vegetation history of
Chatham Island.
ORAL
PETROLOGY OF TRANSGRESSIVE COOL-
WATER LIMESTONE SEQUENCES IN
NORTHERN TE KUITI GROUP, WAIKATO
BASIN, NEW ZEALAND
S.D. Hood, A. Tripathi, C.S. Nelson
& P.J.J. Kamp
Department of Earth Sciences, University of
Waikato, Private Bag 3105, Hamilton
(s.hood*waikato.ac.nz)
The mainly Oligocene Te Kuiti Group is a broadly
transgressive mixed-siliciclastic cool-water
carbonate succession occurring in King Country
and Waikato Basins, central western North Island.
The group consists of six 3rd to 4th order (1 to 5
Ma) sequences. Each of sequences 2 to 6 includes a
major transgressive limestone unit at their base.
The field characteristics and petrology of these
limestones are little known in the Waikato Basin
(from Port Waikato to Kawhia) in contrast to their
correlatives to the south in King Country Basin
(includes Te Kuiti/Waitomo area). Current study of
these northern limestone units is providing new
insights into the evolution of the Waikato Basin
during the Oligocene.
The earliest Oligocene Elgood limestone of
sequence 2, the lowermost of the Te Kuiti Group
limestones, commonly onlaps onto Mesozoic
basement rocks (e.g. Port Waikato). This limestone
is tight, flaggy, well-cemented, c.10-12 m thick,
and comprises pure biosparites, and occasionally
biomicsparites. Skeletons are dominated by
calcareous red algae, bryozoans, benthic
foraminifera, and echinoderms, suggestive of
shallow-shelf water depths. The mid-Oligocene
Awaroa Limestone of sequence 3, onlapping onto
Ahirau Sandstone, is restricted to the Kawhia
region, has a flaggy appearance, and is commonly
10-18 m thick. Rocks are dominated by bryozoan,
benthic foraminiferal, and echinoderm biosparites
suggestive of mid-shelf water depths. The mid-Late
Oligocene Waimai Limestone of sequence 4 onlaps
Kotuku Siltstone and is distinctly tabular crossbedded.
Rocks include a spectrum of biosparites
through to rarer biomicrites. Bioclasts are
dominantly bryozoans, echinoderms, and benthic
foraminifera typical of mid-shelf water depths. The
late Oligocene Raglan Limestone of sequence 5 is
commonly c.10 m thick, locally up to 20 m, and is
restricted to northern Raglan Harbour area. This
limestone transgressed onto Patikirau Siltstone, and
is a slabby, intensely bioturbated, planktic
foraminiferal biomicrite indicative of accumulation
at outer-shelf water depths. The latest Oligoceneearliest
Miocene Otorohanga Limestone or its
equivalent of sequence 6, the uppermost limestone
of the Te Kuiti Group, onlaps the Waitomo
Sandstone and is restricted in occurrence to
Gibson’s Beach. The limestone is a flaggy, dense,
well-cemented, pure limestone c.8 m thick. Rocks
are bryozoan, echinoderm, benthic foraminiferal
dominated biomicsparites indicative of mid to
outer-shelf water depths.
Data suggest that during the Oligocene Waikato
Basin experienced six major transgressive events
characterised by siliciclastic sediment starvation
and the establishment of carbonate factories at
commonly inner- to mid-shelfal depths. Sea level
changes were probably mainly tectonically driven
with a superimposed glacio-eustatic component.
The calcite cementation of these limestones is
consistent with burial-induced cementation
associated with pressure-dissolution of skeletal
50 th Kaikoura05 -38- Kaikoura 2005