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

achiopod orders that had survived into the
Triassic, had finally disappeared, and brachiopod
faunas in New Zealand, as elsewhere, became
dominated by a variety of ribbed rhynchonellides
with a few species of generally smooth
terebratulides making up the balance. Little is
known of New Zealand Cretaceous brachiopods,
but from the early Cenozoic to the present day, in
New Zealand seas as globally, terebratulides have
become by far the most abundant and diverse group
of brachiopods, greatly outnumbering
rhynchonellides in terms of both generic and
species richness, and in total biomass. What caused
this major shift or changeover in brachiopod faunas
since the Mesozoic? Was competition between
brachiopods and bivalves a factor? Did the
endopunctate terebratulides come through the
Mesozoic Marine Revolution more successfully
than impunctate rhynchonellides because of
superior morphological, physiological and
ecological characteristics? We will examine the
morphological differences between the two groups,
and indicate how these may have enabled the two
brachiopod clades to cope with various
environmental changes (ocean chemistry,
temperature and sea level fluctuations, varied
substrates) and biotic filters (competition, feeding
efficiency, predation) over the past 200 million
years. Our analysis uses examples from New
Zealand Mesozoic and Cenozoic strata, and the
modern New Zealand brachiopod fauna.
ORAL
SOUTH ISLAND OLIGOCENE
UNCONFORMITIES – EVOLUTION OF AN
IDEA
Helen Lever
Department of Geological Sciences, University of
Canterbury, Christchurch.
(helen.lever*canterbury.ac.nz)
Early mapping in the Canterbury and Otago regions
quickly generated controversy about the
interpretation of the contact between the Amuri and
Weka Pass Limestones and their stratigraphic
equivalent. Mappers were divided on whether the
contact was conformable or unconformable.
In publications (up to the 1960s) the contact is well
described, but is never referred to as an
unconformity. In the Oamaru region the two
limestones are often separated by a unit of
calcareous glauconitic sandstone, and the faunal
differences between the underlying limestone
(correlated with the Amuri in Canterbury), the
glauconitic sandstone and the overlying limestone
(correlated with the Weka Pass Limestone) led to
the subdivision of the Oligocene into the New
Zealand stages Whaingaroan, Duntroonian and
Waitakian.
It was not until the biostratigraphy of the Oligocene
was much better defined that a definite age break
between the Amuri and Weka Pass Limestones was
proved, and by this time the correlation of these
limestone units was so well established that it was
an easy logical step to assume that the age of the
unconformity must therefore be the same wherever
the two limestones occur.
At this time the Tertiary succession in the South
Island was being interpreted as a transgressiveregressive
sequence, with the boundary between the
two limestones representing starvation due to
maximum sea-level. Alternatively, global studies
were connecting the change from Greenhouse to
Icehouse climates with the development of a strong
current circling Antarctica, at around the beginning
of the Oligocene. Both these theories were
advanced to explain synchronous development of
unconformities across the South Island and into the
offshore regions.
However, the unconformity itself is quite different
across the South Island, and may not be present in
some successions. Biostratigraphic studies have
confirmed the presence of multiple unconformities
within many successions, although only one at
others. Other dating methods are more difficult to
apply, and only strontium isotope dating has been
attempted at what has been described as the type
section of the regional unconformity, at Squires
Farm in South Canterbury. This date proved to be
slightly inconsistent with earlier interpretations and
some biostratigraphic results for other successions,
but correlates well with some offshore dating from
ODP Leg 181.
The current status is impasse, with interpretation of
the unconformities as being caused by a single
event still the dominant explanation, but
insufficient data available to either prove or
disprove this hypothesis.
POSTER
DEPOSITIONAL PROCESSES AND
SEDIMENTARY FRACTIONATION IN THE
FORMATION OF TEPHRA BEDS FROM
PYROCLASTIC ERUPTIONS: A STUDY OF
POTAKA AND CORRELATIVE KAIMATIRA
PUMICE SAND DEPOSITS
1 Lewis B., 1 White J.D.L., 2 Manville V.,
& 1 Palin J.M.
1 Dept. of Geology, University of Otago, P.O Box
56, Dunedin
2 Institute of Geological & Nuclear Sciences,
Wairakei Research Centre, Private Bag 2000,
Taupo
(grumpsnz*yahoo.co.nz)
50 th Kaikoura05 -44- Kaikoura 2005