Startigraphy and provenance of Tertiary sediments at Lake Hauroko, with reference to dextral strike-slip movementon the Moonlight Fault system.

Constantine, AF

Eocene - Oligocene sediments in the Lake Hauroko area, Western Southland, occur in three fault-bounded basins known as the Monowai Sub-basin, the Waiau Basin, and the Fiordland Shelf Basin. The Hauroko Fault (southern segment) trends north-east across the lower reaches of Lake Hauroko and splays into three faults; the Hauroko Fault (northern segment), the Blackmount Fault, and the Oblong Hill Fault near Steves Point. The Monowai Sub-basin lies between the two former faults and is bisected by the latter. The two halves are referred to as the West Monowai Sub-basin and the East Monowai Sub-basin.
Four regional formations are recognized: (1) the Blackmount Formation (conglomerate and sandstones), (2) the Tunnel Burn Formation (limestone), (3) the Lill Burn Formation (calcareous sandstone), and (4) the Waicoe Formation (mudstone), which are internally subdivided into lithogenetic members according to the basin of deposition. The Blackmount Formation is subdivided into: (i) the Kaherekoau Member, (ii) the Hump Ridge Member, and (iii) the Caroline Member. The Tunnel Burn Fom1ation is subdivided into: (i) the Hill Member, (ii) the South Arm Member, and (iii) the East Arm Member. The Lill Bum Formation is subdivided into: (i) the Eel Member, and (ii) the Oblong Hill Member. The Waicoe Formation is everywhere uniform in lithology.
The stratigraphic order of litho-units in the West Monowai Sub-basin begins with the Kaherekoau Member, overlain by the Waicoe Formation. The Hill Member, occurs as a fault-bounded sliver within the Waicoe Formation. In the Eastern Monowai Sub-basin, the oldest stratigraphic litho-unit is the South Arm Member, overlain by the Lill Burn Formation and then the Waicoe Formation. The stratigraphic order of-the Waiau Basin begins with the Hump Ridge Member at the base, stratigraphically overlain by the East Arm Member. The Caroline Member is the basal litho-unit of the Fiordland Shelf Basin.
The Eocene- Oligocene sediments in the Lake Hauroko area are interpreted as having been derived from immediately adjacent Fiordland basement. West Monowai Sub-basin litho-units are derived from mixed source terrain of probable Eastern Fiordland provenance and granitoids of uncertain affinity (Southwest Fiordland?). East Monowai Sub-basin sediments are derived from a mixed source terrain of granitoids of uncertain affinity (Southwest Fiordland?) and Eastern Fiordland basement lithologies. Waiau Basin sediment is derived from basement lithologies in the Green Lake region at Tangney Bend, Lake Monowai. A point source has been positively identified for detrital aegerine and arfvedsonite as being the Monowai gneissic granite. Fiordland Shelf Basin sediments are derived from a mixed source tenain of granitoids of uncettain affinities (Southwest Fiordland?) and possible Western Fiordland.
A minimum of 18 kms of dextral strike-slip has occuned on the Blackmount Fault - Hauroko Fault (southern segment) part of the Moonlight Fault System. Approximately 3 - 5 kms of dextral strike-slip has occuned on the Oblong Hill Fault and Hauroko Fault (northern segment). Between 0.76 and 1 km of strike-separartion is inferred to have occurred on each of three north-north-west striking faults in the Hump Ridge region, obliquely intersecting the Blackmount Fault - Hauroko Fault (southern segment). Reconstruction of the original geographic positions of the Monowai Sub-basin, the Waiau Basin, and the Fiordland Shelf Basin indicates that the southeasterly extension of the Grebe Fault Zone may be the Hump Ridge Fault, a major fault separating the Waitutu Basin to the southwest from the Monowai Basin the the north east. Folding is interpreted as pre-dextral strike-slip.

v. 176 p. ill. (some col). Photos. Map (folded in pocket). 30 cm.

1987Constantine

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