Abstract
A terrane boundary was mapped between the Murihiku Supergroup and the Takitimu Group in the Wether Hill area, Western Southland. The two terranes (Murihiku and Brook Street) are juxtaposed along the gently eastward-dipping Letham Ridge Thrust. The rocks to the west of the thrust comprise the steeply-dipping, eastward-younging, volcanoclastic sediments of the Takitimu Group. Pyroxene-phyric basalt and two-pyroxene andesite dykes intrude the Takitimu Group. A diorite plug with associated marginal sulphide mineralisation, and the Early Triassic hornblende andesite sills of the MacKinnon Peak Intrusives, intrude the Takitimu Group. An olivine monzonite dyke outcrops within the Takitimu Group to the north of Wether Hill, it is post Early Triassic in age. The Takitimu Group is unconformably overlain by gently eastward-dipping Jurassic sediment of the Barretts Formation. The Barretts Formation is comprised of conglomerate, sandstone, and siltstone, containing volcanic, plutonic, and metamorphic detritus.
The Speights Limestone Melange outcrops within the Letham Ridge Thrust. Blocks within the melange include Permian limestones, sandstones, and siltstones.
The Murihiku Supergroup is thrust over the Barretts Formation along the Letham Ridge Thrust. The Murihiku Supergroup within the area is comprised of Early to Late Triassic volcanogenic sediment, and two-pyroxene andesite. The sediments record a change from predominantly andesitic detritus in the Early to Mid Triassic to more felsic detritus in the Late Triassic.
Petrographic data from sandstones of the Barretts Formation indicates that the sandstone composition is heterogeneous throughout the unit. The sandstone composition varies from lithic-rich (e.g Q:F:L-7:33:60) to quartzofeldspathic (37:34:29). The variable nature was also noted in the geochemical composition of the sandstones. Immobile trace element and major element concentrations vary between samples. Geochemical and petrographic work confirmed that the Barretts Formation was derived from a continental arc margin. Petrographic and geochemical differences suggests that the Barretts Formation and the Murihiku Supergroup did not share the same source during the Jurassic.
The Late Cretaceous New Brighton Conglomerate overlaps both terranes. Sediment within the New Brighton Conglomerate has a higher proportion of intermediate to mafic volcanic, and sedimentary lithics, than sediment from the Barretts Formation, suggesting significant differences between these units. Small areas of Eocene coal measures of the Nightcaps Group unconformably overlie the Takitimu and Murihiku basement within the field area.