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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few normal faults visible especially in the coastal<br />
Levin area. The reverse faults agree well with the<br />
notion of crustal downwarp in a region of<br />
geodetically-determined, mild compression. Three<br />
possible explanations for the normal faults are a) a<br />
local second order effect due to dextral strike slip<br />
motion along the major faults, b) tensional forces<br />
have been or are still active in this area or c)<br />
flexural bending stresses superimposed on an<br />
overall compressive stress field.<br />
We have started to create models to test for the<br />
origin of the –150 mgal Bouguer/isostatic gravity<br />
anomaly that dominates Wanganui Basin. The<br />
anomaly can only partially be explained by the<br />
sediment fill and it is one of the most compelling<br />
geophysical observations in the Wanganui area, if<br />
not the whole North Island. Initial 3D models show<br />
that the gravity anomaly associated with the basin<br />
is generally consistent with a downwarp model of<br />
the entire crust. However, the downwarp of the<br />
Moho has to be significantly greater than the<br />
downwarp of the sediment-basement interface to fit<br />
the observed gravity anomaly. Hence we propose a<br />
model of lithospheric shortening where thickening<br />
increases with depth.<br />
POSTER<br />
WHAT SOURCE ROCK POTENTIAL DOES<br />
THE WAIPAWA FORMATION HAVE<br />
BEYOND THE EAST COAST BASIN? AN<br />
OUTCROP-BASED CASE STUDY FROM<br />
NORTHLAND<br />
B.D. Field, C.J. Hollis, E.M. Crouch & R. Sykes<br />
GNS Science, PO Box 30 368, Lower Hutt<br />
(b.field*gns.cri.nz)<br />
The Paleocene Waipawa (Black Shale) Formation<br />
is a known source of oil in the East Coast and north<br />
Taranaki basins. It is likely to be a significant<br />
source rock in adjacent basins in Northland and<br />
Canterbury and the Great South Basin. In order to<br />
determine the extent and character of the formation<br />
in northern Northland, we studied Waipawa-like<br />
dark-grey mudstones exposed in Price’s and<br />
Black’s quarries in the allochthonous sequence at<br />
Ohia, west of Taipa, and a shore platform exposure<br />
in the autochthonous sequence at Whatuwhiwhi,<br />
Karikari Peninsula. The Ohia and Whatuwhiwhi<br />
exposures are inferred to record distal and proximal<br />
deposition, respectively, off the northeastern<br />
margin of Northland within the so-called North<br />
Slope Basin.<br />
The quarry outcrops are highly deformed, making<br />
determination of stratigraphic orientation and<br />
thickness difficult. Dinoflagellate biostratigraphy<br />
shows that the sequence in Price’s Quarry is<br />
overturned and ranges in age from Late Cretaceous<br />
to late Paleocene. Although this age range<br />
encompasses that in which the Waipawa Formation<br />
was deposited, the distinctive geochemical<br />
signature of the formation is not evident in any of<br />
the quarry samples. Despite an oily smell and<br />
sulphurous efflorescence, all samples have low<br />
TOC and light bulk � 13 C values (less than 1% and -<br />
26‰, respectively). Based on these features, the<br />
entire sequence is correlated with the siliceous<br />
mudstone facies of the Whangai Formation. In<br />
contrast, the typically high TOCs and heavy � 13 C<br />
values (> -24‰) of the Waipawa Formation are<br />
found in petroliferous mudstone exposed in Black’s<br />
Quarry, 1.5 km to the east. Dinoflagellates confirm<br />
a Paleocene age. Although a stratigraphic thickness<br />
of 40 m is suggested, it is difficult to confirm due to<br />
poor bedding and numerous faults. The formation<br />
here is immature (Tmax 421–427 °C) and TOC and<br />
HI values of 10–11% and 426–445 mg HC/g TOC,<br />
respectively, indicate good potential for both oil<br />
and gas generation. At Whatuwhiwhi, the wellbedded<br />
Haumurian–Teurian Waiari Formation<br />
includes a lower chert-dominated facies and an<br />
upper mudstone-dominated facies. The former has a<br />
Whangai-type geochemical signature whereas the<br />
latter has a Waipawa-type geochemical signature.<br />
However, a lack of palynomorphs and almost<br />
completely exhausted petroleum potentials (S2 <<br />
0.15 mg HC/g rock) indicate that the Waipawa-like<br />
interval has been thermally altered by nearby<br />
Miocene intrusives to an extent that makes<br />
correlation with the Waipawa Formation uncertain.<br />
The heavy isotopic signature may be a result of this<br />
thermal alteration.<br />
The results of this study indicate that good quality<br />
Waipawa Formation source rock is likely to be<br />
present in the sedimentary succession offshore<br />
northeastern Northland. However, its thickness is<br />
uncertain and it is likely to grade laterally and<br />
vertically into comparatively organic-lean siliceous<br />
facies of the Whangai Formation. Seismic facies<br />
mapping and paleoenvironmental reconstructions<br />
are recommended to better define the extent and<br />
quality of this important source rock.<br />
POSTER<br />
DISTRIBUTION AND SEISMIC<br />
<strong>CHARACTERISATION</strong> <strong>OF</strong> GAS HYDRATES<br />
IN FIORDLAND, <strong>NEW</strong> ZEALAND<br />
Fohrmann, Miko 1 ; Gorman, Andrew R. 1 ;<br />
Pecher, Ingo A. 2 , Jarvis, Kevin D. G. 3<br />
1 Department of Geology, University of Otago, P.O.<br />
Box 56, Dunedin, New Zealand<br />
2 Institute of Geological & Nuclear Sciences (GNS),<br />
P.O. Box 30 368, Lower Hutt, New Zealand<br />
3 Fugro-Jason Australia, 69 Outram Street, West<br />
Perth, WA 6005, Australia<br />
(fohmi830*student.otago.ac.nz)<br />
50 th <strong>Kaikoura</strong>05 -27- <strong>Kaikoura</strong> <strong>2005</strong>