Structure, stratigraphy, and petrology of mainly Triassic rocks, Hokonui Hills, Southland, New Zealand

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Abstract
About 260 km2 of mainly Triassic rocks on the north limb of the Southland Syncline, Hokonui Hills, Southland, consist of a thick sequence of steeply dipping, mainly marine, volcanogenic sedimentary rocks. Changes in strike and numerous faults with a conspicuous strike-slip component indicate the area has undergone a compressive stress in a NW-SE direction. The compressive stress post-dates the folding of the Southland Syncline. The North Range Group of the Murihiku Supergroup (Campbell & Coombs 1966) varies bewteen 3 and 3.5 km thick and ranges in age from Lower Triassic (Scythian) to Middle Triassic (Anisian). It is subdivided into six new formations: Fairplace Formation, Gavenwood Tuffs, Glenure Formation, Crosshill Gully Siltstone, North Peak Formation, and Stag Siltstone, in order of decreasing age. The Taringatura Group (Middle and Upper Triassic), which overlies the North Range Group, is not subdivided here, but at least one unit, the Bare Hill Tuff Zone, is continuous across the area. The group thins from west to east; constituent rock units are more fossiliferous, less continuous, and contain fewer plant fragments than those of the North Range Group. Conglomerates are common throughout the Triassic sequence. The lensoidal nature of most conglomerate beds indicate they were deposited mainly in near-shore submarine channels, probably by traction currents. Andesites are the predominant components of North Range Group and Otapirian (Upper Triassic, Rhaetian) conglomerates. Rhyolites and dacites are the predominant volcanics in Kaihikuan (Middle Triassic, Ladinian) and Oretian (Upper Triassic, Carnian) conglomerates. Diorite, tonalite, and granodiorite pebbles are associated with the more basic volcanics whereas granites and adamellites are associated with the more acid volcanics. Plutonic pebbles were probably derived from intrusive bodies underlying the volcanics and were genetically and temporally related to them. Rudaceous-size material, predominantly andesitic, becomes generally finer grained from west to east in the GIc-'1ure Formation. Acid volcanic pebbles occur at a lower horizon in the eastern Hokonui Hills than in the western Hokonui Hills. The source area of the acid material is evidently east of the source area of the more basic material. Sandstones are usually massive, poorly sorted, and grading is not common. The distribution of types of volcanic clasts in sandstones parallels that of conglomerate pebbles. In addition, Owmitan (Upper Triassic, Carnian) sandstones contain predominantly rhyolite and dacite fragments and Jurassic sandstones contain predominantly andesite fragments. The pattern is also verified by distribution of detrital clinopyroxene and anhydrous TiO2 + Fe2O3 + FeO + MgO values of 21 newly analysed sandstones. Sandstones with colour mottling on a scale of 3 mm up to 10 mm are widespread but are especially common in the Taringatura Group. Non-resistant siltstones are common throughout the sequence. Thick sequences of resistant siltstones occur mainly in the Taringatura Group. Resistant and non-resistant siltstones frequently contain animal trails and burrows and traction current indicators, but grading is uncommon. The siltstones were probably deposited at similar depths to the sands but during tectonically quiescent times.

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