Nature and relations of the lower Maitai group at West Dome

Stratford, James M. C. (James Matthew Campbell), 1966-

Late Permian Maitai Group sediments at West Dome overlie volcanic rocks of the ?Lower Permian Livingstone Subgroup, part of the Dun Mountain Ophiolite Belt. All the units of the lower Maitai Group are present.
A new formation and member are proposed. The Waitaramea Formation (new name) is 120m thick and overlies the Wooded Peak Limestone. It is characterised by coarse red and green sandstone, and interbedded mudstones. The sandstones are quartz-poor (<5%) with up to 15% detrital pyroxene, amphibole and opaque minerals. The Waitaramea Formation was derived from a mafic rich volcanic source very similar to that which supplied the sandstones of the Wooded Peak Limestone. Laumontite-prehnite tuffs found within this unit are the only tuffs from the lower Maitai Group at West Dome.
The Oswald Member (new name) of the Tramway Formation overlies the Waitaramea Formation. It consists of 60m of the green sandstones and minor quartz-poor sands and muds. The quartzo-feldspathic composition (Q13:F51:L36) of most sandstones from the Oswald Member is similar to sandstones from the Tramway Formation (Q16:F48:L36). This Member represents the transition from the volcanic-rich sandstones of the Waitaramea Formation to the quartzofeldspathic sandstones of the Tramway Formation.
Two major sandstone petrofacies occur in the lower Maitai Group. The quartzo-feldspathic sandstones of the Tramway Formation (Q15:F47:L38) and the volcanogenic sandstones of the other units (Q2:F38:L50). The volcanogenic sandstones are further sub-divided into mafic mineral (Waitaremea Formation and Wooded Peak Limestone) and volcanic lithic (Little Ben Sandstone and Greville Formation) associations.
The volcanogenic sandstones were derived from an undissected volcanic source. The more quartz-rich Tramway Formation derived from an active margin continental source, assumed to be either plutonic, or metamorphic. The volcanogenic sandstones indicate a calc-alkaline/?tholeiitic source. The composition of these sandstones differs from any of Permain volcanic arc suites presently exposed in the South Island and these volcanic suites are unlikely to be the source for the Maitai Group volcanogenic sandstones. Mudstones from the volcanogenic sandstone units are chemically similar to the continentally derived Tramway Formation sediments. It is proposed that the continental source which supplied the Tramway remained in association with the depositional basin throughout all of the lower Maitai Group sedimentation.
Prehnite-pumpellyite to lawsonite-albite-chlorite fades metamorphism affects these sediments. Structurally the Maitai sediments at West Dome comprise a thick, homoclinal south east facing sequence. Cross faults trending 160 to 200° with a predominantly conjugate displacement area seen to offset the strata.
The lower Maitai Group sediments were deposited by turbidites on a fan system envisaged to be an elongate arrangement of coaslescing fans. Deposition occurred close to both volcanic and continental sources in either a back-arc or fore-arc basin. The back-arc position is favoured.

181 p. : ill. (some col.), maps ; 30 cm.

1990Stratford

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