The geological evolution of Otago Harbour: a high-resolution seismic reflection study

Fletcher, Patrick T.

The sedimentary units that line the floor of Otago Harbour contain a rich geological record of past environmental changes and catastrophic events, like earthquakes and floods that may have affected the region. While the surface geology of the region around Otago Harbour has been well studied, very little is known about the units and structures that lie beneath. These subsurface features hold vast amounts of geological evidence which can be used to determine the origin of and processes controlling, the harbour, offering insight into possible hazards the future might hold.
A total of 23 harbour-crossing seismic lines were collected between Taiaroa Head an the Leith Canal covering the majority of Otago Harbour. These lines were processed and refined to produce cross sections of the harbour, imaging from the upper sediments right down to the bedrock. The resulting data were then used to produce a 3D model of the paleovalley’s bedrock contact, as well as allowing the identification of major depositional horizons within the sediment.
From this modelling and analysis, interpretations related to sedimentary time scales, erosional processes and faulting were made. The paleovalley 3D model showed far deeper depths (upto 160m) than imaged by previous studies, as well as providing evidence to support a time scale for the erosional valley’s formation. Evidence such as the merging of multiple small channels and shallowing paleovalley depths near the centre of the harbour, indicated a paleovalley formed by the erosion of two rivers out from a paleowatershed, situated above today’s Portobello Peninsula and Quarantine and Goat Islands. The 3D model also allowed a calculation of a stored sediment volume which exceeded previous estimates by ~37%. The harbour cross sections supported correlation of past depositional events with major sea level rise on two possible time scales (Pleistocene and Post Glacial). These same depositional horizons also exhibited possible offsets and slumping which sets the foundations for future studies into possible faulting in the harbour.
Many of these findings provide insight into the possible future of Otago Harbour, constraining possible transgressional rates as well as identify indicators of a possibly active fault, both which could have a significant impact in terms of hazard assessments and the overall sustainability of Dunedin’s coastal setting.

145 pages A4

2016Fletcher

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