Lithostratigraphy of the Takitimu Group, central Takitimu Mountains, western Southland, New Zealand

Abstract

Strata of the Takitimu Group form a 14 km thick homoclinal sequence in the central Takitimu Mountains. The sequence consists of Lower Permian interbedded marine volcaniclastic sediments and basaltic to rhyodacitic volcanic rocks.

The importance of the predominantly mass-flow emplaced sediments which comprise over 75% of the sequence is stressed, and it is suggested that the group accumulated principally as an apron of volcaniclastic detritus in an arc-related basin or basins. Five new formations are proposed and described: Brunei (oldest), Chimney Peaks, Heartbreak, MacLean Peaks, and Elbow (youngest).

Basaltic volcanism predominated throughout the period when the Takitimu Group accumulated, but andesite and rhyodacite are locally significant. The latter is recognised here for the first time.

dBrunei Formation consists principally of hemipelagic and distal basic volcaniclastic sediments with subordinate basaltic lava flows, hyaloclastites, and rare rhyodacitic lava flows. The Brunei Formation formed on the flank of a calcalkaline volcanic chain, probably close to the axis of a foreor back-arc basin.

dChimney Peaks Formation consists of redeposited volcaniclastic arenites and subordinate rudites and basaltic and rhyodacitic rocks. The formation is interpreted in part as a proximal turbidite association which accumulated adjacent to an active volcanic chain.

dHeartbreak Formation, consisting of a succession of microgabbro and basaltic lava, pillow lava and breccia, is distinctive because it contains little or no volcaniclastic sediment. The formation is interpreted as a near-vent submarine facies deposited at principal or parasitic vents within the volcanic chain.

Mass-flow emplaced volcaniclastic arenites and rudites and submarine lava flows characterise dMacLean Peaks Formation. Andesite first appears in this formation, and there is a continuous range of silica content from basalt to andesite. The redeposited sediments, like those of the Chimney Peaks Formation, represent a proximal turbidite association thought to have formed as a wedge or apron of sediments adjacent to the volcanic chain.

Rudites are more abundant in dElbow Formation, the youngest unit described. Well-rounded framework-supported rudites of probable shallow water origin are restricted to this unit. Lavas range in composition from basalt to andesite. Deposition of Elbow Formation is thought to have been characterised by shallowing of the arc-related basin prior to limited uplift and a major decline in volcanic activity in middle Permian times.

The White Hill Intrusive Suite is defined as a suite of moderately to coarsely crystalline gabbroic to granophyric rocks which intrudes the Takitimu Group in the central Takitimu Mountains. The intrusives, which are generally concordant, are probably of Late Permian age.

Keywords:

• Takitimu Group
• Permian
• volcaniclastic
• lithostratigraphy
• arc-related basin
• Brunei Formation
• Chimney Peaks Formation
• Heartbreak Formation
• MacLean Peaks Formation
• Elbow Formation
• new stratigraphic names