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

IS THE LATE CRETACEOUS WARD
COASTAL SUCCESSION
ALLOCHTHONOUS?
M.G. Laird 1 & P. Schiøler 2
1 Dept. of Geological Sciences, University of
Canterbury, PB 4800, Christchurch.
2 Geol. Surv. of Denmark and Greenland, Ø.
Voldgade 10, 1350 Copenhagen, Denmark.
(malcolm.laird*canterbury.ac.nz)
Over most of NE Marlborough, the Haumurian
sedimentary rocks consist of a lower portion,
comprising the fine-grained clastic Whangai
Formation (locally Herring Formation) typically
comprising a monotonous succession of massive or
finely-laminated dark silty or very fine sandy
mudstone, which passes upwards abruptly or with
rapid transition into the siliceous limestone of the
Mead Hill Formation, occupying the upper portion.
In the coastal area east of Ward, however, the
Whangai Formation includes massive sandstone
bodies, a thick turbidite unit, and debris flow
deposits.
A large proportion of the lower half of the exposed
Haumurian succession, which contains
dinoflagellates of late early Haumurian (I.
korojonense Zone) age, is occupied by a body of
massive well-sorted fine sandstone, up to 120 m
thick. The lower and upper contacts of the
sandstone body with the enclosing Whangai
Formation are sharp, with large, rounded blocks
and rafts up to 2 m thick of laminated dark
mudstone of the underlying Whangai Formation
present in the basal 15 m, suggesting erosion and
incorporation in a sandy debris flow. The younger
part of the succession is largely fine-grained, with
scattered intervals of matrix-supported
conglomerate or graded sandstone up to 5 m thick,
interpreted to represent debris flows, slumps and
turbidites, and occupying shallow channels. Softsediment
slumping occurs at several horizons. The
fine-grained succession passes upwards either
directly into the Flaxbourne Limestone, a ~20 m
thick siliceous/calcareous unit of late Haumurian
(P. granulatum Subzone) age, or into channelled,
massive, well-sorted fine sandstone bodies up to 20
m thick, which in turn pass upwards into the
Flaxbourne Limestone. A 220+ m thick unit of
turbidite sandstones occupies the stratigraphic
position between the Flaxbourne Limestone and the
latest Cretaceous and Paleocene Mead Hill
Formation. The thick sandstone bodies are inferred
to occupy channel systems, and the turbidite unit
represents either the infill of a depression or a
laterally-migrating submarine fan.
With the exception of scattered sandstone beds in
the otherwise fine-grained Woodside Creek
succession, none of the coarser units are traceable
to the west of the London Hill Fault, a major
Neogene structure forming the western boundary of
the Ward coastal area. A limited amount of dextral
strike-slip movement (~4 km), as well as
overthrusting, was inferred to have occurred across
the fault during the Neogene (Audru, 1996), but
this is unlikely to account for the marked difference
in sedimentary facies across it. There are, however,
similarities between the Haumurian Ward coastal
succession and the equivalent succession at Tora in
SE Wairarapa, where similar channels, slumps and
coarse clastic deposits also occur in the upper part
of the sequence. The Late Cretaceous succession at
Tora represents the southernmost segment in the
eastern North Island of the partly allochthonous
Eastern Sub-belt, and it is possible that the Sub-belt
extended further south to include the Ward coastal
area, with the London Hill Fault marking the
tectonic boundary of a well-travelled block.
Reference.
Audru, J-C. 1996: De la subduction d’Hikurangi à la
Faille Alpine, region de Marlborough, Nouvelle Zélande.
Ph.D. thesis, University of Nice, France.
ORAL
FAULT CHARACTERISATION AND
EARTHQUAKE SOURCE IDENTIFICATION
IN THE OFFSHORE BAY OF PLENTY
Geoffroy Lamarche &PhilipM.Barnes
National Institute of Water and Atmospheric
Research (NIWA) Ltd,
P.O. Box 14-901, Wellington
(g.lamarche*niwa.co.nz)
We identify and map active faults within 100 km of
the Bay of Plenty coast, from interpretation of
>8,000 km of high-resolution seismic reflection
profiles, >11,000 km 2 of multibeam bathymetric
data, and archived side-scan sonar imagery. Active
normal faulting in the central Bay of Plenty is
associated with continental back-arc extension in
the offshore NNE-trending Taupo Volcanic Zone
and southern Havre Trough. In the eastern Bay of
Plenty, the faults include N-S-trending components
of the North Island Dextral Fault Belt (NIDFB),
and NW-SE striking reverse faults related to the
deformation of north-western Raukumara
Peninsula. The data set enables the recognition of
fault displacements