Willsher Group and geology of the Triassic Kaka Point coastal section, south-east Otago, New Zealand

• Published in: Journal of the Royal Society of New Zealand 2003-03, Vol.33 (1), p.7-38
• Main Authors: Campbell, J. D., Coombs, D. S., Grebneff, A.
• Format: Article
• Language: English
• Subjects: albitisation; ammonoids; analcime; Bates Siltstone; biostratigraphy; Dun Mountain-Maitai Terrane; Etalian; geochemistry; heulandite; Kaihikuan; Kaka Point Structural Belt; Kaka Point Volcanic Sandstone; Karoro Formation; Malakovian; Murihiku Terrane; Nelsonian; Pilot Point Sandstone; Port Molyneux Siltstone; Potiki Siltstone; Short Bay Formation; Steney Voe Formation; Tilson Siltstone; Triassic; tuffs; Waituti Siltstone; Willsher Group; zeolite facies
• ISSN: 0303-6758; 1175-8899
• DOI: 10.1080/03014223.2003.9517719

Willsher Group (new) and component formations in the Kaka Point Structural Belt are defined and described: Karoro Formation (Nelsonian, late Early Triassic), Potiki Siltstone (?Malakovian, late Early to early Middle Triassic), Bates Siltstone, Kaka Point Volcanic Sandstone, Tilson Siltstone, and Steney Voe Formation (Etalian, Middle Triassic), Pilot Point Sandstone (Etalian or Kaihikuan), Short Bay Formation (late Etalian or Kaihikuan), Port Molyneux Siltstone and Waituti Siltstone (Kaihikuan, latest Middle to early Late Triassic). Sparse body fossils include ammonoids, bivalves, gastropods, brachiopods, and crustacean, crinoid, and rare bone fragments. Bentonitic and zeolitised ash beds are copiously developed in thick sequences of laminated siltstones with subordinate mostly highly felsic volcaniclastic sandstones; a laharic origin is suggested for some. Sedimentary structures indicate current directions and slope directions from an easterly quarter. Analcime-quartz and albite are progressively advanced dehydration products of heulandite and clinoptilolite replacement of glass with accompanying movement of mobile elements. Some analcime has formed without heulandite precursor. Differences from coeval rocks in the Southland Syncline of the Murihiku Terrane in inferred source directions, sedimentary facies, very low-grade metamorphism, and geochemistry imply distinct provenances and tectonic histories, although sources were probably parts of the same volcanic arc complex. The Kaka Point Structural Belt is fault-bounded and was displaced relative to the Southland Syncline sequence during accretion or later. Its relationship to the Dun Mountain-Maitai Terrane is at present unclear. The belt may be regarded as a displaced splinter of either the Murihiku Terrane or the Dun Mountain-Maitai Terrane, or it may be a fragment of a separate but related suspect terrane.