A seismic and sedimentological investigation and evolution of the Otago continental shelf and the Saunders Ridges demonstrating the role of longshore drift

Author:

McLachlan, Christine Jane

Year:

Project type:

Abstract:

A seismic survey was collected between March and October 2013 using the RV Polaris II on the inner‐mid Otago continental shelf. The survey included ~200 km of high resolution single channel boomer seismic, CHIRP and side scan sonar as well as piston cores which were collected on the October cruise. The study area is ~25 x 10 km and is located ~10 km off the city of Dunedin where the shelf is ~40 km wide with a low gradient to the shelf break at ~120 m water depth. The majority of the profiles collected are shore normal with three tie lines linking them.

The survey was set up to investigate the origin of the Saunders Ridges, which are an anomalous set of ridges in the study area. It had been previously suggested that they are the result of an overstepping event during a rapid sea level rise and meltwater pulse 1A has been proposed as the causal event. The results of this study are inconclusive; however the preliminary evidence suggests that an overstepping event is the probable cause of the ridges and the depth at which the ridges occur suggests that the sea level rise event may have been meltwater pulse 1b, which occurred ~9.5 ka.

The evolution of the last three sea level cycles on the Otago continental margin was determined and shows a regressive transgressive stratigraphic stacking pattern. Sequence 2 is only observed as an incised channel in two of the profiles; otherwise it is unknown in the study area.

A difference in the stratigraphic stacking pattern over the profiles in the study area and a previously collected profile ~30 km to the north, Osterberg’s (2001) profile, has been investigated. Sequence 3 in the study area has an unusually thick transgressive systems tract when compared to the equivalent sequence in Osterberg’s profile, which has a thick regressive systems tract. The difference in stratigraphic stacking patterns has been attributed to a change in the hydraulic regime and sedimentation patterns between over the course of the Late Quaternary.

The Holocene highstand sand wedge migrates northeastwards up the littoral zone ~190 km from the Clutha River by longshore transport. However, on the mid shelf a proposed eddy on the down‐current side of the Otago Peninsula that was operational during the highstand is the most likely cause for the limited extent of the sand wedge on the mid shelf, which is restricted to ~60 m north of the Clutha River mouth. During lowstand this would not be present allowing for sediments to be transported unimpeded north on the Otago continental shelf, creating a sediment supply for Osterberg’s region during the highstand‐regressive phase.

During the transgressive phase of sequence 3 a coastal plain would have been present in the study area draining the ancient Water of Leith which could have provided the increased sediment supply for the thick transgressive systems tract. This shows that the investigation of hydraulic regime and longshore transport as well as point source of sediment play an important role in determining the development of the stratigraphic stacking patterns.

Thesis description:

xiv, 192 pages, A4, some A3 foldouts, some bigger foldouts

Department:

OU geology Identifier:

2015McLachlan

Author last name:

OURArchive access level:

Open Access

Location (WKT, WGS84):

POLYGON ((170.614049517661641 -45.949554100034938,170.760494851073446 -46.043012833490131,170.725929991760438 -46.135203730806282,170.541156819792349 -46.207579980975247,170.41619407450213 -46.092897465911165,170.614049517661641 -45.949554100034938))

Abstract PDF File

http://download.otagogeology.org.nz/temp/Abstracts/2015McLachlan.pdf

Collection

Citation

McLachlan, Christine Jane, “A seismic and sedimentological investigation and evolution of the Otago continental shelf and the Saunders Ridges demonstrating the role of longshore drift,” Otago Geology Theses, accessed October 23, 2018, http://theses.otagogeology.org.nz/items/show/606.