An engineering geological investigation of Waikawa Bay and Whatamango Bay, Marlborough Sounds, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The Waikawa area is a rapidly expanding urban centre within the Marlborough Sounds and as such there is an increasing need for a better understanding of the engineering geological constraints for urban development. The objectives of this study are twofold; firstly to assess the structural characteristics of the area and the nature and extent of the Waikawa Bay Fault, and secondly to complete an engineering geological and hazard assessment in order to determine the development constraints to future urbanisation. The field area extends from Waikawa township to Green Bay and includes The Snout in Picton, Karaka Point and Whatamango Bay. The geology is dominantly Pelorus Group greywacke and weakly foliated Marlborough Schist, which are in faulted contact along the Waikawa Bay Fault.
Structural mapping in the field area identified three distinct deformation phases following the completion of metamorphism. Additionally the area is dominated by a series of reverse thrust faults which generally dip shallowly (approx. 30°) to the west in Picton and steeply (60°-80°) to the east from Waikawa to Green Bay. The Waikawa Bay Fault, an easterly dipping reverse thrust with a dextral strikeslip component, represents a significant structural boundary between the Picton Domain and the eastern rocks of the field area, and is thought to overthrust the Picton Thrust system creating a fault wedge in the Picton area. The Waikawa Bay Fault is also thought to be related to the Green Bay Fault, and tentative correlations are made with the inferred presence of the Waipapa Terrane rocks which may be in contact with Pelorus Group rocks along the Waikawa Bay Fault. Reassessment of the geomorphology of the field area indicates that the age of last rupture of the Waikawa Bay Fault took place between 18-12ka BP, the age estimate being determined from the relative ages of alluvial terraces (Wo-W5) in Waikawa Stream, the Rimu Terrace alluvial fan and the Maori Cemetery debris fan complex.
Engineering geological investigations involved the use of field mapping, aerial photograph interpretation, hydrological monitoring and limited laboratory testing to determine the geotechnical properties and extent of the bedrock and surficial units in the field area. The principal active geomorphic processes are assessed and the data is presented on an engineering geology map at 1:5,000. The data obtained confirmed that the strength of rock material decreases with increasing weathering grade, and that schist at a maximum of 46MPa is noticeably weaker than the maximum strength for greywacke of 71MPa. Problems were encountered with grainsize determinations for regolith and colluvial material due to the presence of clay aggregates which cannot be resolved using the present methods. The principal clay minerals identified using XRD methods are kaolinite and illite together with the presence of an illitechlorite interstratified clay, while imogolite was identified in the red weathered regolith.
The hazard assessment quantifies the hazard potential of the landscape modification processes identified and is also presented in map form as a hazard assessment map at 1:5,000. The processes of slope failure, flooding, stream bank erosion and debris deposition are represented on the map using a colouring system for degree of hazard and a lettering system identifying the principal processes in each area. Hazards are quantified on the basis of their magnitude, their frequency, and the area affected by the process. The final stage in the hazard assessment was the production of a development constraints map, also at 1:5,000, which delineates the degree of geotechnical limitations and the type of investigation required for future urban development. The development constraint map is designed to be used to assess the expected conditions at a site, but does not replace extensive individual site investigations for subdivision planning or for housing construction.