Geology of Takitimu group rocks in the Nugget Hill region, western Southland, New Zealand

Abstract

An area approximately 31 km2, centered around Nugget Hill in western Southland was mapped in detail. The geology in this area is dominated by Takitimu Group lithologies which are intruded by the White Hill Intrusive Suite. To the northeast of Nugget Hill, Barretts Formation unconformably overlies Takitimu Group and the White Hill Intrusive rocks. The Barretts Formation has been overthrust by the Murihiku Supergroup along the Letham Ridge Thrust. A sliver of Wairaki Melange occurs between the Barretts Formation and the Murihiku Supergroup.

The Takitimu Group consists of volcanic and volcaniclastic lithologies which were deposited in sedimentary basins flanking the volcanoes of an island arc system. The Group in the Nugget Hill area is divided in to two formations, the Heartbreak Formation consisting primarily of volcanic rocks with rare volcaniclastic material and the MacLean Peaks Formation which is dominated by intercalated volcaniclastic rudites, arenites and lutites with the occasional flow of basalt and basaltic-andesite lavas. The White Hill Intrusive Suite includes gabbros and microgabbros of very similar compositions to the Takitimu Group. Barretts Formation conglomerates and sandstones contain exotic clasts with respect to the Takitimu Group. These include granites, siliceous volcanics and ignimbrite.

A newly recognised southeastward plunging asymmetrical anticline has deformed the Takitimu Group in the region. The Telford Fault which strikes approximately north-west separates approximately east-west striking, south facing units from north-west striking, east facing units of the Takitimu Group. About 1 km of displacement is inferred at the south end and about 7 km of displacement is inferred at the north end of the fault. Southeast of Nugget Hill a southeastward dipping thrust fault is mapped and an oblique fault is mapped down the Wairaki River south of the Telford Burn confluence. These are splayed faults which accommodate offset at the southern end of the Telford Fault.