Report on the Geology & Gold Fields of Otago
Mattai Formation. — Kaihiku Series, (Hector?); Wairoa series, (Hector); Richmond sandstone, (Hochstetter); Otapiri series, (Hector); Shaw s Bay series, (Lauder Lindsay)
Mattai Formation.
Kaihiku Series, (Hector?); [unclear: Wairoa] series, (Hector); Richmond sandstone, (Hochstetter); Otapiri series, (Hector); Shaw s Bay series, (Lauder Lindsay).
|| Ante p. 23, Fig. 1, C.
Rocks.—The rocks are principally shaly slates, argillites, and green sandstones, with occasional beds of conglomerate and green-stone tuff. These conglomerates consist entirely of subangular fragments of slate and sandstone, and I have never found a single pebble of any eruptive rock in them.
Position of Strata.—Along its north-west margin, this formation is very much disturbed, being thrown into several bold plications. At Mount Hamilton, the dip is 40° to 65° E.; at Centre Hill, 50° W. by S.; and at Coal Hill, (a south-westerly spur from the Eyre Mountains,) it is 75° E. by N. Along the southern part of the Wairaki Hills, near the Lead of the Morely Creek, the dip is 20° N.E.; and at the gorge, where the river enters the plain, 15° S.W. At Castle Rock Station, on the north side of the Moonlight Range, the dip is 20° S.W.; while further south, near Dipton, it is 30° S.S.W. On the eastern side of the Oreti River, above Benmore, these rocks dip 20° S.S.W.; and on the north-west side of the Hokanuis, in the bed of the Waimea Stream, 60° N.E. by E.; while at the head of the Otapiri, they are vertical; and strike south-east and north-west. At Popotunoa Gorge, the rocks dip S.W. at high angles, to vertical; but between Clinton and Balclutha, the dip seems to be generally northerly, and in the Waitepeka I found it to be 55° N. At the Nuggets, these rocks are vertical, and strike N.W. and S.E.
Relation to Under lying Formation.—The junction between this and the last formation is not very clear in the Province of Otago, and I have not been able to find a section showing it resting on the rocks of the Kaikoura formation. But that it does do so is evident from the general geological structure of the district, and from the much less amount of metamorphism that these rocks have undergone, than those of the Kaikoura Formation. In the Clutha district, there appears to be an uniformity between the two, but it is not well marked. In the Province of Nelson, however, complete unconformity exists, as I have elsewhere pointed out.*
Thickness.—I am unable to give any estimate of the thickness of this formation on account of the foldings along its northern border, which will require very careful surveying to unravel; but it is certainly more than 15,000 feet.
* Geological Reports, 1872-3, p. 34.
* N. Z. Exhibition Jurors’ Reports and Awards, p. 265.
Contemporaneous Eruptive Rocks.—In Preservation and Chalky Inlets a considerable area is composed of a coarse grained pink granite, which I consider to belong to this formation. This granite consists of a matrix af red orthoclase and white quartz with small quantities of black mica., and is clearly eruptive and younger than the slates and sandstones of the Kaikoura formation which it pierces, and pieces of which are often seen enveloped in the granite. Fig. 2 represents a junction of the granite with slate on the east side of Isthmus Sound in Preservation Inlet. The junction here is quite abrupt and jagged, and two angular fragments of slate are seen embedded in the granite. The slate near the junction has been considerably altered, and converted into a finely crystalline rock of a dark grey color. The minerals are separatedf and mica and quartz grains can be recognised with a lens, but microscopical investigation is necessary before the changse can be satisfactorily made out. This alteration, however, does not penetrate very far, and the great mass of the slates in the neighbourhood are quite unaltered. The felspar of the granite generally gets white as it approaches the slates, and for about an inch from the junction gets very fine grained, but the mica flakes increase in size. But in some cases the granite preserves its character close up to the slates. This proves clearly the eruptive nature of the granite, and that the eruption took place later than the Kaikoura formation. It also proves that the highly metamorphic character of the gneiss rocks of the Manipori formation is in no way owing to the granite out-burst, as the granite has failed to alter the slate rocks in its immediate vicinity.*
* See ante, p. 28.
In Preservation Inlet the granite extends from Revolver Bay and Isthmus Sound to beyond Lady Bay, but occasional masses of slate are found amongst it. Great Island, in Chalky Inlet, is also entirely composed of it. How far it extends up Long Sound, Cunaris Sound, and Edwardson Sound I do not know, as I have not been up them; and I have filled in this boundary on my map from the remarks on the rocks in Dr. Hector’s narrative of his West Coast exploration. With the exception of a few veins, I saw no granite in any of the other Sounds that I visited, viz., Dusky Sound, Wet Jacket Cove, Breaksea Sound, Doubtful Sound, Bradshaw Sound, Thompson Sound, Bligh Sound, and Milford Sound; and I saw none on the west side of Te Anau Lake. At the Bluff Hill, a broad dyke of syenite, composed of white felspar and crystals of black hornblende, runs nearly parallel with the bedding of the slates, and the line of junction between the two rocks is very complicated, veins of syenite, isolated apparently from the main mass, appearing among the stratified rocks parallel with the bedding, and looking as if the syenite was here a product of metamorphism, and that the argillaceous rocks had been changed into syenite, while the more arenaceous ones had resisted the action; but these appearances are, I am satisfied, fallacious. This syenite sometimes passes into an almost pure hornblende rock; sometimes it is of a green color, caused by the dissemination of small particles of hornblende through the mass, but when exposed to the weather, these minute particles soon disappear from the surface, leaving the larger crystals studding the white felspar base with black spots. Several dykes of a similar rock are also seen at Port William in Stewart Island. Centre Island appears to be entirely composed of it, as also, Captain Fairchild informs me, is Ruapuke. Another dyke also occurs at the east end of Wakapatu Bay, at the base of the Longwood Range, and others probably in the Takitimus. On the east side of the Nuggets a dyke of grey porphyry, with white crystals of felspar, traverses the rocks of the Maitai formation at right angles to the bedding.
* Reports of Geol. Exploration, 1872-3, p. 34.
In a former report on the geology of the north east district of the South Island, I separated the Monotis bearing beds from those with Spirifera and Inoceramus (?), under the name of the Wairoa formation; but a further examination of these rocks in Otago, where they are better displayed than in any other part of New Zealand, has convinced me that this distinction will not hold, and that all must be classed in one formation.