New Zealand Journal of Geology and Geophysics abstracts
Quaternary stratigraphy, structure, and deformation of the Upper Hutt Basin,
Wellington, New Zealand
ANNE MELHUISH
JOHN BEGG
STEPHEN BANNISTER
TERRY MUMME
Institute of Geological & Nuclear Science
P.O. Box 1320
Wellington, New Zealand
Abstract The Upper Hutt Basin is one of three onshore
Quaternary basins in the Wellington region, New Zealand. This
northeast-trending basin is bounded along its northwest margin by the
predominantly strike-slip Wellington Fault, northwest of which upthrown
Torlesse Terrane basement rocks crop out. Quaternary sediment filling the basin
rests unconformably on the basement rocks, onlapping the sloping basement
surface at the east side of the basin. Interpretation of the seismic and
gravity data indicates a total sediment thickness of c. 360-480 m in
the main Upper Hutt Basin and 225 m in the Witako Basin. Gravity modelling
indicates a dip of c. 60deg. to the northwest for the Wellington Fault,
and has defined a basement high within the basin, which is the subsurface
continuation of a NNE-trending topographic spur. The basement high separates a
sub-basin, the Witako Basin, from the main Upper Hutt Basin. A drillhole near
the basement high penetrated interbedded 10-50 m thick gravel and
silt/peat units and reached basement at 204 m. Other drillholes indicate
that this stratigraphy, at least in the upper 80 m, is consistent in the
southwest area of the basin. There are few constraints on the chronology of the
sequence in the basin, but a unit of silt and peat between 51.5 and 68 m
depth in one drillhole yielded cool climate pollen assemblages and is of early
Last Glacial age (50-80 ka).
A high-resolution seismic reflection profile revealed two major sedimentary
units in the basin. The lower unit (B), which is positively identified only
west of the basement high, is conformably overlain by unit A, which is present
across both the Upper Hutt and Witako Basins. The two seismic units have
different reflective character, suggesting a difference in lithology or
depositional environment. Thickening of unit B towards the west, within
c. 500 m of the Wellington Fault, suggests localised westward tilting
of the basin in its initial stage of formation and sediment deposition in a
fault-angle depression situation. By the time unit A was deposited, this
tilting had stopped, and unit A is virtually flat lying across the basin.
Deposition of unit A is considered to have been controlled by localised uplift
and hence drainage restriction at Taita Gorge. Elevated Last Glacial terraces
at the northeast and southwest ends of the basin dip below Holocene gravels
towards the middle of the basin, indicating relative longitudinal basin
subsidence.
Keywords NZMS 260 R27; Wellington; Upper Hutt Basin;
Quaternary; sediments; seismic reflection; gravity; deformation; Wellington
Fault; active basin development
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 19-29
0028-8306/97/4001--0019 $2.50/0 (c) The Royal Society of New Zealand
1997
PDF file of entire paper: medium quality (2057K); (scanned from paper original: notes about this process)
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