Quaternary stratigraphy, structure, and deformation of the Upper Hutt Basin, Wellington, New Zealand

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. 60 degrees 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.