High resolution seismic study of offshore southeast Otago
Year:
Project type:
Abstract:
Structural geology within the first 12 km offshore of southeast Otago from Blackhead to Nugget Point, situated to the northwest of the Great South Basin, has been investigated using high resolution sub-bottom profiles and side-scan sonar images. The structures evident in the sub-bottom profiles and side-scan images can be related to those onshore and further offshore in multi-channel seismic profiles collected by Hunt International Petroleum Company (H.I.P.C.O.). The sediments can be correlated with known onshore stratigraphy and offshore well data.
Onshore basement consists of Haast Schist in the north and Murihiku Supergroup in the south. Cretaceous and Tertiary sediments unconfomably overlie the basement. Taratu Formation extends throughout the area studied from just offshore from Brighton to the Clutha River. The Wangaloa Formation and Tertiary sediments are present overlying the Taratu Formation further offshore and south of the Clutha River. Cretaceous and Tertiary sediments are unconformibly overlain by post-glacial modern sand and relict gravels which are derived mainly from the Clutha River.
Basement, Cretaceous, Tertiary and in some places the modern sediment are folded and faulted. The major structures trend at 040° to 055° between Blackhead and Mitchells Point where the major trend changes to 180°, although the Akatore Fault still trends northeast. From south of the Clutha River the trend of folding and faulting changes to 135° with the offshore extension of the Castle Hill Fault Zone which coincides with the northern limb of the Southland Syncline. Most faults within the study area are reverse, some with associated asymmetric synclines and anticlines.
Northeast striking structures within 20km offshore are antithetic, southeast dipping, faults in the offshore extension of a previously described imbricate fault system through Central and Eastern Otago connected along a low angle decollement at mid-crustal depths. This fault system is associated with reverse faulting initiated in the Miocene in response to compression across the Alpine Fault. The Korora Dome and associated reverse faulting (part of the Waipounamou Fault System, a major set of northeast trending faults), situated 40km southeast of Nuggets Point, is thought to be the southeastern continuation of the Central and Eastern Otago fault system.
The northwest trending structures are compressional features which have formed due to reactivation along pre-existing structures.
The Akatore Fault and Fault B, trending parallel to and situated 2-3 km to the east of the Akatore Fault, are both active. The Akatore Fault can be traced from its termination at the Castle Hill Fault Zone northeastwards, trending at 040° to 055°, and continuing onshore south of the Tokomairiro River mouth, and then offshore south of Taieri Mouth and extending north on the landward side of Taieri Island and Green Island before its projected on-land continuation at Waldronville. Fault B lies on the seaward side of Green Island and Taieri Island and has only been traced offshore in the Taieri area. Fault B's anticipated onshore extension in the south is at Akatore Creek and to the north is on the northern side of Blackhead.
Onshore basement consists of Haast Schist in the north and Murihiku Supergroup in the south. Cretaceous and Tertiary sediments unconfomably overlie the basement. Taratu Formation extends throughout the area studied from just offshore from Brighton to the Clutha River. The Wangaloa Formation and Tertiary sediments are present overlying the Taratu Formation further offshore and south of the Clutha River. Cretaceous and Tertiary sediments are unconformibly overlain by post-glacial modern sand and relict gravels which are derived mainly from the Clutha River.
Basement, Cretaceous, Tertiary and in some places the modern sediment are folded and faulted. The major structures trend at 040° to 055° between Blackhead and Mitchells Point where the major trend changes to 180°, although the Akatore Fault still trends northeast. From south of the Clutha River the trend of folding and faulting changes to 135° with the offshore extension of the Castle Hill Fault Zone which coincides with the northern limb of the Southland Syncline. Most faults within the study area are reverse, some with associated asymmetric synclines and anticlines.
Northeast striking structures within 20km offshore are antithetic, southeast dipping, faults in the offshore extension of a previously described imbricate fault system through Central and Eastern Otago connected along a low angle decollement at mid-crustal depths. This fault system is associated with reverse faulting initiated in the Miocene in response to compression across the Alpine Fault. The Korora Dome and associated reverse faulting (part of the Waipounamou Fault System, a major set of northeast trending faults), situated 40km southeast of Nuggets Point, is thought to be the southeastern continuation of the Central and Eastern Otago fault system.
The northwest trending structures are compressional features which have formed due to reactivation along pre-existing structures.
The Akatore Fault and Fault B, trending parallel to and situated 2-3 km to the east of the Akatore Fault, are both active. The Akatore Fault can be traced from its termination at the Castle Hill Fault Zone northeastwards, trending at 040° to 055°, and continuing onshore south of the Tokomairiro River mouth, and then offshore south of Taieri Mouth and extending north on the landward side of Taieri Island and Green Island before its projected on-land continuation at Waldronville. Fault B lies on the seaward side of Green Island and Taieri Island and has only been traced offshore in the Taieri area. Fault B's anticipated onshore extension in the south is at Akatore Creek and to the north is on the northern side of Blackhead.
Thesis description:
119 p. : ill. (some col.), maps ; 31 cm.
Department:
OU geology Identifier:
1990Johnstone
OURArchive handle:
OURArchive access level:
Location (WKT, WGS84):
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Files
Collection
Citation
Johnstone, Tanya, 1967-, “High resolution seismic study of offshore southeast Otago,” Otago Geology Theses, accessed May 15, 2025, https://theses.otagogeology.org.nz/items/show/243.