Maitai and Murihiku rocks (Permian and Triassic) in Nelson, New Zealand

Owen, Stuart Richard.

Murihiku and Maitai strata in the Wairoa-Lee and Wai-iti rivers areas form two parallel NE trending sedimentary belts; the Murihiku occurring as a number of imbricated fault blocks, the Maitai synclinally folded (Nelson Regional Syncline) and imbricated by SE dipping reverse faults. Eight formations ranging from upper Etalian to Otamitan (Middle to Late Triassic) are mapped in the Richmond Group, Murihiku Supergroup, including one new unit, the Church Valley Formation (Oretian). Nine formations are mapped in the Maitai Group, including one new unit, the Chrome Creek Formation. and a three-fold subdivision is recognised, comprising the Barrington Subgroup (new), Port Hardy Subgroup (redefined after Waterhouse 1987), and the Stephens Subgroup. Mappable Late Permian atomodesmatinid limestone and sandstone bodies up to 1-2 km dimension within the Stephens Subgroup are allochthonous and informally referred to as Martins olistoliths. The highest in situ Perrnian fauna occurs in the Tramway Formation. Petrologic correlation of Martins olistoliths and Wooded Peak Limestone suggests a Dorashamian age for the latter. Durvilleoceras woodmani Waterhouse 1973 is assigned to the family Flemingitidae, and together with cf. Hypophiceras sp., suggests an Induan (Early Triassic) age for the Greville. A new ammonoid fauna collected from limestone olistoliths in the lower Stephens Subgroup is described, and indicates a maximum early Olenekian age for the Stephens Subgroup. The Perrnian fauna is characterised by atomodesmatinids,locally bryozoans and brachiopods, the Triassic faunas by ammonoids,locally echinoderms and sessile foraminifera. Age considerations and rudite petrology suggest a hiatus within or between the lowermost Maitai and underlying Dun Mountain Ophiolite Belt. Murihiku rocks belong entirely to zeolite mineral facies. Three metamorphic zones are mapped in the Maitai (laumontite, lawsonite, prehnite-pumpellyite). The sporadic occurrence of lawsonite in the lawsonite zone probably largely reflects volatile activities, nature of replaced phases, and partially bulk composition. The Maitai Group was derived from a largely calc-alkaline continental volcanic arc and deposited mainly by sediment gravity flows in a deep, elongate eastward sloping basin. Turbidite paleoflow was mainly obliquely along the basin with locally perpendicular to opposing flows suggesting point sources along the western margin. Small scale (up to ea. 40 m thick) coarse tuff turbidite sequences reflect episodes of explosive volcanism. Formation scale (hundreds of metres thick) laterally extensive (hundreds of kilometres) repetition of sandstone dominated and mudstone dominated units must be extrabasinally controlled and is attributed to fluctuations in relative sea-level. Contourites (largely terrigenous with a very small hydrogenous component) and direct fallout tuffs are locally evident. Sedimentary olistoliths/clasts in the Stephens Subgroup were formed in a shallow to marginal marine environment to the west of and largely contemporaneously with the Maitai. The geochemistry of igneous conglomerate clasts, tuffs, and probably also epiclastic sediments has been significantly modified during diagenesis and/or prehnite-pumpellyite and lawsonite-quartz-albite facies metamorphism of the Maitai Group in Nelson. Colour of Maitai sediments reflects primarily oxidation state of iron and abundance of carbonaceous matter, and secondarily metamorphic mineralogy. Red (hematitic) mudstones are possibly slightly enriched in iron. The hematite is probably largely detrital but some diagenetic to metamorphic effects are locally recognised. Three petrological suites are distinguished in the Maitai Group: (1) Barrington Subgroup (lower Maitai); characterised by atomodesmatinid debris and dacitic tuffs in addition to ubiquitous mafic volcanic detritus. The Tramway Formation also includes quartz- and phyllite-rich litharenites indicating additional plutonic and low-grade metasedimentary provenance. (2) Port Hardy Subgroup (middle Maitai); comprising largely epiclastic mafic volcanogenic sediments. (3) Stephens Subgroup (upper Maitai); characterised by andesitic to rhyolitic tuffs in addition to the ubiquitous mafic volcanic detritus. Reworked shell fragments are common in Stephens rudites. The Richmond Group (Murihiku Supergroup) was derived from a largely calc-alkaline continental volcanic arc and is probably entirely marine. Shellbeds are common and largely worked by marine processes. Bottom current deposits are locally evident. Conclusive sediment gravity flow deposits (paleoflow ranging from SE-NE directed) are only recognised in the Te Arowhenua-Wantwood and Church Valley-Saxton formations which formed as submarine channel-overbank complexes. Although probably shallow marine the depositional environment and processes of the other units remains unclear. Te Arowhenua-Wantwood (Etalian to Kaihikuan) and Garden-Problematica-Max (upper Oretian to Otamitan) formations are largely volcanogenic,locally tuffaceous. The Wells Formation (Kaihikuan) had a mixed granitic and tuff provenance, and Church Valley-Saxton formations (lower Oretian) largely granitic provenance recording unroofmg of shallow volcanic-arc plutons and temporary cessation of explosive volcanism.

1 v. (various pagings) : ill. (some col.), maps ; 30 cm.

1995Owen

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