Geophysical survey of the Dunedin district
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Abstract:
The largest residual Bouguer anomaly in the Dunedin District is a 145 μN/kg amplitude (based on a second-order regional surface anomaly), 15 km wavelength feature, centred at Hoopers Inlet, GR290 805. The anomaly is due principally to a 478 km3 intrusive body with an average density of 2.90 g/cc buried 500±50m below sea-level. Igneous bodies are responsible for antiformal doming of Tertiary sediments observed at the surface and in seismic reflection profiles.
Transgressive and regressive, Late Cretaceous to Middle Miocene sediments lie unconformably over the Haast Schist basement and unconformably below the Dunedin Volcanics. The sedimentary rocks produce a -120 μN/kg amplitude (relative to a second-order regional surface anomaly) anomaly surrounding the western part of the positive gravity feature.
A -45 μN/kg residual anomaly occurs on the Taieri Plains near Mosgiel and is probably due to 120m of alluvial gravels (density=1.90 g/cc). The gravels appear to be deposited unconformably on schist basement, which dips gently southeast, and are bounded to the southeast by the Titri Fault, with an apparent dip of 70°SE. Gravity observations are compatible with the Titri Fault bending around to 030-040° east of gridline 080E, nearly converging with the Leith Valley Fault Zone.
On the southeast, uplifted side of the Titri Fault, a positive residual anomaly of 45 μN/kg occurs due to the presence of rocks more dense than schist, possibly igneous rock.
Transgressive and regressive, Late Cretaceous to Middle Miocene sediments lie unconformably over the Haast Schist basement and unconformably below the Dunedin Volcanics. The sedimentary rocks produce a -120 μN/kg amplitude (relative to a second-order regional surface anomaly) anomaly surrounding the western part of the positive gravity feature.
A -45 μN/kg residual anomaly occurs on the Taieri Plains near Mosgiel and is probably due to 120m of alluvial gravels (density=1.90 g/cc). The gravels appear to be deposited unconformably on schist basement, which dips gently southeast, and are bounded to the southeast by the Titri Fault, with an apparent dip of 70°SE. Gravity observations are compatible with the Titri Fault bending around to 030-040° east of gridline 080E, nearly converging with the Leith Valley Fault Zone.
On the southeast, uplifted side of the Titri Fault, a positive residual anomaly of 45 μN/kg occurs due to the presence of rocks more dense than schist, possibly igneous rock.
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52, [24] p. : ill., maps ; 30 cm.
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1990Evans
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Location (WKT, WGS84):
POLYGON ((170.76669114419326 -45.705039625283263,170.754761064589445 -45.938785183754753,170.246875575161909 -45.924694269178637,170.25904786286543 -45.694576984427115,170.76669114419326 -45.705039625283263))
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Citation
Evans, Craig, 1968-, “Geophysical survey of the Dunedin district,” Otago Geology Theses, accessed July 20, 2026, https://theses.otagogeology.org.nz/items/show/242.