Some aspects of the geology of the eastern Hokonui Hills

Wright, Edward, 1967-

Fossiliferous strata of the Murihiku Supergroup crop out in the Otamita Downs area, eastern Hokonui Hills, Southland, New Zealand. Detailed biostratigraphic subdivision indicates that this is the thinnest sequence of Triassic rocks yet mapped in the Hokonui Hills. This probably reflects a depositional setting distal from the volcanogenic source. At least three species of Rastelligera coexisted in the uppermost Triassic (Otapirian). The Otapirian (Triassic)-Aratauran (Jurassic) boundary, although not marked by any lithological change, is a pronounced biostratigraphic hiatus indicated by the extinction of Rastelligera and Clavigera .. The Jurassic Spiriferina ongleyi is placed in the genus Mentzelia thus M. kawhiana-M. ongleyi is the only Spiriferinid lineage to survive the boundary. Well preserved ammonite specimens from middle Aratauran beds are identified as Saxoceras sp. Extreme ribbing variation in Otapiria marshalli questions existing species designations. The Jurassic sediments host minor volumes of pebble conglomerate, and the uppermost Triassic Otamita Downs Boulder Conglomerate contains clasts up to 3.5 m in diameter. Clast composition in this unit varies widely and does not mirror the finer grained volcanogenic sediments. Common lithologies include granophyres, tonalites, undeformed greenschist fades volcaniclastics, and amphibolite fades metabasalts, some of arc ankaramite affinity. A common source is proposed for these clasts Rare clasts from the pebble conglomerates include metasedimentary quartzose schists with a complex structural history. The method of deposition of the boulders within the Otamita Downs Boulder Conglomerate is unclear; however the large size suggests minimal transport. If a transport direction is assumed from the south-west for the majority of the Murihiku sediments then the boulder association was derived from a different, probably closer source. A suite of rocks with some petrographic similarities can be found in the silicic association of the Dun Mountain Ophiolite Belt.

17, 24, 37 p., 10 leaves of plates : ill. (some col.), maps (some col.) ; 30 cm.

1990Wright

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