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

few normal faults visible especially in the coastal
Levin area. The reverse faults agree well with the
notion of crustal downwarp in a region of
geodetically-determined, mild compression. Three
possible explanations for the normal faults are a) a
local second order effect due to dextral strike slip
motion along the major faults, b) tensional forces
have been or are still active in this area or c)
flexural bending stresses superimposed on an
overall compressive stress field.
We have started to create models to test for the
origin of the –150 mgal Bouguer/isostatic gravity
anomaly that dominates Wanganui Basin. The
anomaly can only partially be explained by the
sediment fill and it is one of the most compelling
geophysical observations in the Wanganui area, if
not the whole North Island. Initial 3D models show
that the gravity anomaly associated with the basin
is generally consistent with a downwarp model of
the entire crust. However, the downwarp of the
Moho has to be significantly greater than the
downwarp of the sediment-basement interface to fit
the observed gravity anomaly. Hence we propose a
model of lithospheric shortening where thickening
increases with depth.
POSTER
WHAT SOURCE ROCK POTENTIAL DOES
THE WAIPAWA FORMATION HAVE
BEYOND THE EAST COAST BASIN? AN
OUTCROP-BASED CASE STUDY FROM
NORTHLAND
B.D. Field, C.J. Hollis, E.M. Crouch & R. Sykes
GNS Science, PO Box 30 368, Lower Hutt
(b.field*gns.cri.nz)
The Paleocene Waipawa (Black Shale) Formation
is a known source of oil in the East Coast and north
Taranaki basins. It is likely to be a significant
source rock in adjacent basins in Northland and
Canterbury and the Great South Basin. In order to
determine the extent and character of the formation
in northern Northland, we studied Waipawa-like
dark-grey mudstones exposed in Price’s and
Black’s quarries in the allochthonous sequence at
Ohia, west of Taipa, and a shore platform exposure
in the autochthonous sequence at Whatuwhiwhi,
Karikari Peninsula. The Ohia and Whatuwhiwhi
exposures are inferred to record distal and proximal
deposition, respectively, off the northeastern
margin of Northland within the so-called North
Slope Basin.
The quarry outcrops are highly deformed, making
determination of stratigraphic orientation and
thickness difficult. Dinoflagellate biostratigraphy
shows that the sequence in Price’s Quarry is
overturned and ranges in age from Late Cretaceous
to late Paleocene. Although this age range
encompasses that in which the Waipawa Formation
was deposited, the distinctive geochemical
signature of the formation is not evident in any of
the quarry samples. Despite an oily smell and
sulphurous efflorescence, all samples have low
TOC and light bulk � 13 C values (less than 1% and -
26‰, respectively). Based on these features, the
entire sequence is correlated with the siliceous
mudstone facies of the Whangai Formation. In
contrast, the typically high TOCs and heavy � 13 C
values (> -24‰) of the Waipawa Formation are
found in petroliferous mudstone exposed in Black’s
Quarry, 1.5 km to the east. Dinoflagellates confirm
a Paleocene age. Although a stratigraphic thickness
of 40 m is suggested, it is difficult to confirm due to
poor bedding and numerous faults. The formation
here is immature (Tmax 421–427 °C) and TOC and
HI values of 10–11% and 426–445 mg HC/g TOC,
respectively, indicate good potential for both oil
and gas generation. At Whatuwhiwhi, the wellbedded
Haumurian–Teurian Waiari Formation
includes a lower chert-dominated facies and an
upper mudstone-dominated facies. The former has a
Whangai-type geochemical signature whereas the
latter has a Waipawa-type geochemical signature.
However, a lack of palynomorphs and almost
completely exhausted petroleum potentials (S2 <
0.15 mg HC/g rock) indicate that the Waipawa-like
interval has been thermally altered by nearby
Miocene intrusives to an extent that makes
correlation with the Waipawa Formation uncertain.
The heavy isotopic signature may be a result of this
thermal alteration.
The results of this study indicate that good quality
Waipawa Formation source rock is likely to be
present in the sedimentary succession offshore
northeastern Northland. However, its thickness is
uncertain and it is likely to grade laterally and
vertically into comparatively organic-lean siliceous
facies of the Whangai Formation. Seismic facies
mapping and paleoenvironmental reconstructions
are recommended to better define the extent and
quality of this important source rock.
POSTER
DISTRIBUTION AND SEISMIC
CHARACTERISATION OF GAS HYDRATES
IN FIORDLAND, NEW ZEALAND
Fohrmann, Miko 1 ; Gorman, Andrew R. 1 ;
Pecher, Ingo A. 2 , Jarvis, Kevin D. G. 3
1 Department of Geology, University of Otago, P.O.
Box 56, Dunedin, New Zealand
2 Institute of Geological & Nuclear Sciences (GNS),
P.O. Box 30 368, Lower Hutt, New Zealand
3 Fugro-Jason Australia, 69 Outram Street, West
Perth, WA 6005, Australia
(fohmi830*student.otago.ac.nz)
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