Seismic and oceanographical aspects of lower Otago Harbour

Cournane, Steve

Current meter experiments on the inside of Harington Bend (Lower Otago Harbour) during a neap tide demonstrated that the mean flow was ebb dominant, concentrated in an east-southeast direction with a maximum velocity of 0.58 m/ s. Flood values reached 0.47 m/ s and were concentrated in a west to southwest direction. Sediment entrainment was likely to occur only at the height of an ebb tide for the duration of the experiment. Current meter determined velocities were supported by the geomorphology of the Aramoana tidal flats. Water temperature varied between 5 95-6.97°C, the water column being cooled by the tidal flats irrespective of the diurnal atmospheric temperature cycle. Salinity, and density ranges were 34.865-35.377 %a and 1.0273-1.0278 g/ cm3 respectively, values that appear anomalously high when compared with ocean data and other salinities obtained in the harbour.
The bedforms in the Lower Otago Harbour are summarised on map one in the map pocket. Bedform wavelength ranges from ripples, approximately (27 cm) up to long period (60 m) sandwaves with heights seldom reaching above 1 m.The majority of sandwaves in the harbour are flood orientated at all stages of the tide and show little evidence of reorientation within a tidal cycle. Regions of three dimensional, scoured areas are noted as well as plane beds on the outside of shipping channel bends.
High resolution sub bottom profiling equipment has enabled the recognition of a number of (Holocene?) reflectors distinguished by their height below mean sea level. A reflector (R2) at 20 m below M.S.L. is relatively continuous occurring from Taiaroa Head to Pulling Point, with occasional areas of localised acoustic opacity and is correlated with an 8 m thick gravel layer (in the northern end of the field area). In the vicinity of Dowling Bay, the R2 reflector becomes noticeably discontinuous and is more likely to represent the bottom of a mud wedge. There was no definitive seismic evidence for Dowling Bay Formation (Tertiary) under the harbour, during this study. Deep volcanic (basement) reflectors are reported from near Port Chalmers (80 m below M.S.L.) and also in the cross channel area close to Aramoana (85 m below M.S.L.). Slightly offshore from Port Chalmers, the Port Chalmers Breccia appears as a rough channelised topographical surface overlain by young (Holocene) reflectors. The contact between Careys Bay Basalt and the Port Chalmers Breccia appears as a sharp transition.
Borehole data reveal the presence of significant sub surface muds, possibly exposed to erosion in the shipping channel, at depths greater than 12-15 m. The environment of formation of these muds is interpreted as a barrier enclosed lagoon, as at Hoopers Inlet today. The discovery of extant crab concretions containing Macrophthalmus hirtipes, (Jacquinot) a burrowing mud crab and Cancer novae zealandiae in Port Chalmers muds at a depth of 15-18 m below mean sea level suggests a rapid burial event during barrier enclosure. The burial was probably the result of a catastrophic rainfall event and the resulting high levels of terrestrial derived sedimentation.

vi, 216 p. : ill. (some col.), maps ; 30 cm.