New Zealand Journal of Geology and Geophysics abstracts
Stratigraphy, structural evolution, and tectonics of the northern part of the
Tawhero Basin and adjacent areas, northern Wairarapa,
North Island, New Zealand
G. NEEF
Department of Applied Geology
University of New South Wales
Sydney 2052, Australia
Abstract The study area lies within the forearc of the
Hikurangi margin in northern Wairarapa, North Island, New Zealand. Two elements
of the forearc are described: the Tawhero Basin (which lies west of the trench
slope break), and the eastern part of the forearc ridge (which lies west of the
Tawhero Basin).
In the Tawhero Basin the Neogene stratigraphic succession commences with the
turbiditic Whakataki Formation, which is widespread in the east coast of
Wairarapa. The overlying Coast Road Formation, Otaian to early Altonian in age,
comprises Lower and Upper Mudstone Members separated by the Sandstone Member.
In the Altonian, the turbiditic Greenhollows Formation was deposited, followed
locally by deposition of the Tutaekuri Calcarenite. However, in the south, only
the Takaritini Formation formed during the Altonian. The Settlement Road
Formation, of late Altonian to early Lillburnian age, comprises Lower and Upper
Mudstone Members, separated by the Sandstone Member. The formation largely
crops out along the trend of the Waioakura Horst, which lies between the
north-trending Breakdown and Glencoe Faults. During the late Altonian to the
early Tongaporutuan, the turbiditic Tanawa Formation was deposited in two
sub-basins which were separated by the Waioakura Horst. A local break in
deposition during the late Waiauan was followed in the early Tongaporutuan by a
marine transgression, during which the ng1 Sandstone Member of the Ngarata
Formation was deposited. Subsequently, but also in the early Tongaporutuan, the
turbiditic part of the Pakowhai Formation formed near Waihoki.
In the forearc ridge part of the study area, Late Cretaceous and Paleogene
strata comprising the Whangai, Wanstead, and Weber Formation crop out near
Pongaroa, and Early Cretaceous strata crop out near Tiraumea. Generally the
oldest Neogene strata of the forearc ridge are the ng1 Sandstone Member of the
Ngarata Formation (although locally near Pongaroa the Coast Road and Tanawa
Formations are present) followed, in the mid and late Tongaporutuan, by
deposition of the ng4 Mudstone Member. Turbidite deposition resumed during the
Kapitean resulting in the Waihoki Formation. In the early Opoitian the
north-aggrading bathyal Saunders Siltstone was deposited. Tane Sandstone
followed in the late Opoitian, filling the eastern part of the Eketahuna Basin.
Following a regional unconformity, the neritic Makuri Sandstone was followed in
the late Pliocene and the early Pleistocene by the Lower and Upper Pori
Limestones.
The Tawhero Basin was formed by processes related to the subduction of the
underlying Pacific plate. Initially, in Waitakian and Otaian time, subduction
was probably normal to the Hikurangi margin, whereas in late Altonian time and
thereafter subduction was probably oblique, causing periods when there was
dextral faulting on the bounding faults (especially in the Tongaporutuan), but
the amount of dextral displacement on them is unknown. In the late
Tongaporutuan the basin became amalgamated with the forearc ridge, and there
was little deposition after Kapitean time. The forearc ridge is less deformed
than the Tawhero Basin; it is cut by the Saunders Road Fault.
Keywords Tawhero Basin; forearc ridge; forearc basin; oblique
subduction; structure; tectonics; stratigraphy; basin analysis
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 335-358
0028-8306/97/4003-0335 $7.00/0 (c) The Royal Society of New Zealand
1997
PDF file of entire paper: medium quality (6768K); (scanned from paper original: notes about this process)
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