| Popis: |
The South Dunedin suburb of Dunedin city lies on a coastal plain, which was a salt marsh prior to urban development, that began in 1840. Now one of the more densely populated areas in New Zealand, South Dunedin is within 2 m of current sea level, has groundwater close to the surface and is vulnerable to flood hazards that are expected to be worsened by sea-level rise. In addition, soft sediments that underlie much of South Dunedin provide poor foundation conditions and may potentially be susceptible to seismic shaking amplification and liquefaction. In 2019, various subsurface investigations were undertaken to gain better information on the South Dunedin groundwater regime, geotechnical conditions and the depth and geometry of subsurface geological materials. Cone Penetration Tests (CPT) were done at 16 sites, with shallow piezometers installed at eight of these sites. Three deeper drill holes sought to determine sediment thicknesses and deeper-level groundwater conditions. The new information, along with available existing drill hole and CPT data, have been collated into a database. Aided by the drill hole and CPT data, together with geophysical survey information collected as part of University of Otago student research projects, an interpretive three-dimensional (3D) model of the subsurface geology was developed. It is the latest iteration of a succession of previous geological interpretations of the South Dunedin subsurface. Bedrock beneath South Dunedin comprises a weak, marine sedimentary rock, Caversham Sandstone, overlain by a variety of igneous rocks comprising the Dunedin Volcanic Group. The contact between these geological units appears to become approximately flat-lying under South Dunedin. An irregular erosion surface cut across the bedrock under South Dunedin is interpreted to be the now-buried former valley floor of the Water of Leith and its tributaries. The former valley is filled with sediment of Quaternary age (younger than 2.6 million years). The sediments are interpreted to comprise two distinctive depositional packages. A package of older deposits (Pleistocene sediment) overlies the bedrock valley floor. It generally comprises sands, silts and some gravels that are interpreted to be predominantly river or stream (alluvial) deposits but includes hillslope-derived sediments (colluvium) near the hilly margins of the coastal plain. A younger package (Holocene sediment) was deposited during and following the post-glacial sea-level rise and is composed predominantly of soft silt-clay deposits of marine to estuarine origin, containing some shells and sand layers. The 3D model delineates two former valley geometries associated with the Quaternary sediments. The Pleistocene sediment is interpreted to lie in an older valley cut in bedrock, extending at least 60 m below present sea level, passing south under the St Clair area. The Holocene sediment is interpreted to lie in a valley cut into the Pleistocene sediments and locally onto bedrock highs, passing south between St Kilda and Tainui, with a floor as much as 25 m below present sea level. Both interpretations are based on geological inference from limited data and need testing in future investigations. In particular, some interpretations of geophysical data suggest that the Pleistocene valley floor may run much deeper than 60 m. If so, it would imply long-term subsidence of the land that may be relevant to future planning. The 3D geological model is intended to inform future modelling of the impacts of sea-level rise scenarios on groundwater and flooding and other engineering or planning initiatives. (auth) |
