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

SEQUENCE STRATIGRAPHY AND
ARCHITECTURAL ELEMENTS OF THE
GIANT FORESETS FORMATION,
NORTHERN TARANAKI BASIN, NEW
ZEALAND
Rochelle J. Hansen & Peter J.J. Kamp
Department of Earth Sciences, The University of
Waikato, Private Bag 3105, Hamilton
(rjhansen*waikato.ac.nz)
The modern continental margin, northern Taranaki
Basin, is underlain by a thick, mud-dominated,
Pliocene and Pleistocene succession (Giant Foresets
Formation, GFF) clearly imaged in seismic
reflection datasets. A study focusing on the
geometry and internal reflection character of the
Giant Foresets Formation has revealed structural,
sedimentological, and eustatic controls on its
accumulation.
Isopach maps prepared for northern Taranaki Basin
show shifts through time in the location of sediment
accumulation of the Mangaa Formation and Giant
Foresets Formation. During the Early Pliocene
(Opoitian Stage) deposition was focused in the
southern part of the Northern Graben. The
progradational front moved into the vicinity of
Arawa-1andTaimana-1ontheWesternPlatform
during the early-Late Pliocene (Waipipian and
Mangapanian Stages), forming large mounded
slope fans. Through the latest Pliocene
(Mangapanian - lower Nukumaruan Stages) the
progradational front moved rapidly to the north and
west through and across the Northern Graben to
form a distinct shelf-slope depositional front.
During the Pleistocene (upper Nukumaruan Stage –
Recent), the progradational front straightened out
reaching the present position of the shelf-slope
break. Even during the Pleistocene, broad
subsidence persisted in the Northern Graben,
trapping a proportion of the sediment flux being
delivered to this part of the basin.
The Late Pliocene part of the GFF, particularly
where it prograded on to the Western Platform,
displays classic clinoform profiles, with over
steepening having resulted in mass-failure of
paleoslopes. Major degradation of the shelf edge
and slope occurred during the Early Pleistocene,
reflecting a change in the calibre and flux of
sediment sourced to the continental margin.
Detailed examination of part of the GFF not
significantly affected by mass-failure indicates that
small-scale channel levee and overbank deposits
dominate slope deposition, while basin floor
deposits are characterised by slope-disconnected
muddy and silty basin floor fans, with little lateral
continuity between systems. In a sequence
stratigraphic context, many of the dominant
components of each seismic unit (slumps, fans, and
channel-levee complexes) were deposited during
the falling (RST) and low (LST) sea level parts of a
relative sea level cycle, resulting in highly
asymmetric sequences. While the Giant Foresets
Formation is considered to have minor potential in
terms of containing sandstone-dominated
stratigraphic traps, it does afford the opportunity to
study in detail how deep-water clastic systems
respond or evolved in response to the various
factors that control depositional architectures,
particularly in a rapidly prograding muddy
continental margin system.
POSTER
THE MEDIAN TECTONIC LINE INITIATED
AS A LATE CARBONIFEROUS
CONTINENTAL TRUNCATION ZONE
H.J. Harrington
16 Hobbs St., O’Connor, Canberra ACT 2602;
(harringt*pcug.org.au)
For a 50th anniversary meeting a little preliminary
history might be permitted in an abstract. In 1941
two topics were important in the Geological Survey
tea-room in Wellington, one being Fyfe’s Line and
the other the Alpine Fault. The evidence for the
Alpine Fault was properly published in writing, but
that for Fyfe’s Line was not, and so it gradually lost
prominence. The Alpine Fault continued to be
discussed with fervour after Wellman announced at
the Pan-Pacific Science Congress in 1949 that there
had been major strike-slip on it. He was brief and
hesitant, and did not publish in writing until 1953.
Fyfe’s Line was re-recognised, completely
independently in 1967, by Landis and Coombs who
named it the Median Tectonic Line (MTL).
In the 1930s Benson, Bartrum, Keble and King
correlated the graptolite zones of the Western
Province of New Zealand with those of the
Bendigo-Ballarat goldfields in Australia. Rocks
similar to those between Brook Street and Dun
Mountain were recognised far to the north at
Gympie by mining engineer T.R. Hacket in the
1860s (Mike Johnston, pers. comm. 2005) and were
again recognised in 1972 by Harrington. He
attempted to draw a schematic map (the poster)
showing the Brook Street rocks passing through
Lord Howe Rise and Norfolk Ridge to Australia. A
severe problem appeared immediately. It is that in
eastern Australia there is a space of 1400 km
between Bendigo-Ballarat and Gympie, but in New
Zealand there is a space of only 40 km between the
Takaka Terrane and the western side of Brook
Street. The gap of 1400 km in Australia is
occupied by the eastern half of the Lachlan Foldbelt
and the whole of the New England Orogen. Most
of those rocks are missing in New Zealand. The
working hypothesis offered to explain that situation
is that the missing rocks were removed tectonically
50 th Kaikoura05 -34- Kaikoura 2005