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Report on the Geology & Gold Fields of Otago

Section III. — General Geological Structure

page 23

Section III.
General Geological Structure.

The West Coast range of mountains, as far inland as Lakes Te Anau and Manipori, is composed of the oldest known rocks in New Zealand, which consist chiefly of gneiss and syenetic gneiss. At both extremities of the rangy a belt of slates, of the upper palæozoic age, wraps round the gneiss, and at Chalky and Preservation Inlets both the gneiss and slates have been broken through by a mass of eruptive granite, which occupies an extensive area in this district.

The oldest gneiss formation (Manipori formation) appears to be separated from the rocks to the eastward by an immense fault (see Sec. I.): but at present this cannot be fully proved.

Eastward of this supposed fault, that is to say, in the whole of the country between the Waiau and the Waitaki, the rocks are thrown into broad folds, the axes of which run north and south in the north part of the Province, but gradually sweep round to the north-west and south-east, and dip slightly to the south-east, or away from the highest mountains. The main anticlinal axis (fig. 1.A)
Fig. 1.—Gencralised section across Otago:—A. Main anticlinal curve; B. Northern synclinal curvo; C. Southern synclinal curve.

Fig. 1.—Gencralised section across Otago:—A. Main anticlinal curve; B. Northern synclinal curvo; C. Southern synclinal curve.

starts from between the Shotover and Lake Wanaka, and crossing through the Carrick and Knobby Ranges, passes north of Waipori to Outram and Saddle Hill. To the north-east a synclinal curve (fig. 1.B.) runs from the Ahuriri Pass in a south-east direction through Mt. Domett, and along the northern slopes of the Kakanui Mountains (Sections I. III. and IV.) to the Horse Range; while south-west of the main anticlinal another synclinal (fig. 1. C.) runs from the Greenstone River (Section I.) through the Hokanui Mountains to Catlin’s River (Sec. V.)

It will be thus seen that the main part of the Proviuce of Otago is formed by a central anticlinal curve running in an approximate north-west and south-east direction, with a parallel synclinal page 24trough on either side of it. All the rocks of precretacous age take part in these curves, but the tertiary rocks are quite independent of them, and lie, when viewed on a large scale, horizontally, with only local varying dips.

The main anticlinal curve brings up to the surface a broad band of the older mica schists (Wanaka formation), which is the principal gold-bearing formation in New Zealand, and consequently it is along this band that most of the gold fields are situated. On either side of the anticlinal the mica schists are followed by a newer schist formation (Kakanui formation), which also contains gold in many places. Owing to the south-easterly dip of the anticlinal axis, that has been already mentioned, this newer schist formation wraps round the older mica schists on their south-eastern boundary, and the latter disappear below the younger rocks before reaching Dunedin. (Sec. II.)

On either side, again, the newer schist formation is overlaid by a mass of sandstones and slaty rocks of upper palceozoic age, (Kaikoura formation) which, under ordinary circumstances does not contain gold in payable quantities, but may do so exceptionally, as at Orepuki. In the northern synclinal (Fig. 1 B), these slates are the youngest rocks seen, the newer schists rising up again below them towards the Waitaki (Sec. III.) but a band of upper cretaceous rocks (Waipara formation) with coal, rests on the northern slopes of the Horse Range, and reaches the sea at Shag Point (Sec. VI). In the southern synclinal (Fig. 1, C) however, these slate rocks are followed by conglomerates and sandstones of triassic and Jurassic age. These rocks form the Wairaki, Moonlight, and Hokanui hills, and extend down to the sea between the Mataura and Port Molyneux. More to the south-west, the slate rocks (Kaikoura formation) again come up to the surface, forming the Bluff Hill, Longwood Ranges, and Takitimus, and extends in a narrow band, which runs west of Lake Wakatipu, to Martin and Big Bays (Sec. I). Further to the westward neither of the schist formations have as yet been observed, but the slates appear to overlie directly the gneiss formation, previously mentioned as composing the West Coast Range (Sec. 1).

Round the coast, where the older rocks have been worn away by the sea, patches of tertiary rocks, sometimes containing thick beds of coal, are found. These younger rocks extend in some cases far up the river valleys, as in the Waitaki, Shag River, Pomahaka, Oreti, and Waiau, where they reach to Lake Te Anau. A small patch also still remains on the east side of Lake Wakatipu, shewing that at one time they must have been far more widely spread than at present.

Volcanic action in tertiary times was chiefly developed in the neighbourhood of the east coast, the district around Dunedin Harbour, having been the principal seat of activity. But volcanic page 25rocks are found in the interior at Hamilton and Hyde, and in the south at Mount Pleasant; and other localities may probably exist, which have as yet escaped notice.

It will be seen from this general sketch that there is very little relation between the outward form of the ground and its internal geological structure. The highest land in the Province, from Mt. Aspiring through Mt. Earnslaw to Mt. Christina, crosses the strike of the rocks, and each of these mountains is situated in a different formation. Both schist and slate rocks form equally high mountains in the interior, and equally low and rounded hills near the coast. And it will be found that the shape and altitude of the hills is of no assistance in mapping the boundaries of all formations that are of pre-tertiary age.

Nearly all the rivers and lakes run obliquely or directly across the strike, the principal exceptions being the rivers in the north, such as the Hollyford, Dart, Shotover, and Lake Wanaka, all of which lie in the direction of the strike of the rocks.

Te Anau is the only lake that has different formations on its east and west sides. The difference in aspect here is striking, but it is caused by one side of the lake being composed of soft tertiary clays, and the other of hard gneiss, in a somewhat similar manner to Lake Geneva. The tertiary rocks are usually easily recognised at a distance, but I have in Otago ridden over a hill composed of tertiary sandstone without even suspecting that I had left the palæozoic slates.

Of the twelve different marine formations which, according to my views, are found in New Zealand, only one (Ahuriri formation), is altogether absent from Otago, and that belongs to the tertiary era. One other, however, of the tertiary formations (Wanganui formation), is not represented by marine rocks, but only by lacrustine deposits. So that the Province of Otago not only presents us with an almost complete epitome of the geology of New Zealand, but the structure of the district is so simple that it offers unequalled facilities for making out correctly the main features of its geological structure.

In the following table, which exhibits these formations, together with their probable age, I have thought it best to include all the known New Zealand formations, distinguishing that not found in Otago by printing it in italics; for to understand properly the geology of the Province it is nearly as important to know what formations are absent, as to know what are present:—

page 26
Table of Sedimentary Formations.
Probable Age. Name of Formation. Remarks.
Quaternary—
Recent Recent dcposits
Pleistocene Pleistocene deposits
Pleistocene Newerglacier deposits (a)
Tertiary—
Newer Pliocene Wanganui formation (b) Marine beds absent in Otago.
Older Pliocene Older glacier deposits (c)
Upper Miocene Pareora formation* (d)
Middle Miocene Ahuriri formation Absent in Otago
Lower Miocene Oamaru formation* (e)
Secondary—
Upper Cretaceous Waipara formation (f)
Lower Jurassic Putataka formation (g)
Triassic Maitai formation* (h)
Palæozoic—
Carboniferous (?) Kaikoura formation* (i)
Upper Silurian (?) Kakanui formation (k)
Lower Silurian (?) Wanaka formation (l) Wanaka formation (l)
Eozoic—
Laurentian (?) Manipori formation (m)

The letters (a, &c.) refer to the sections, and to Plate II.

The most important points in the geology of New Zealand, on which the Province of Otago can throw considerable light, are the following:—

1.The true position of the Spirifera beds (Maitai formation), and their relations to the formations above and below them.
2.The relations between the Wanaka, Kakanui, and Kaikoura formations.
3.The relation between the Waipara and Oamaru formations.
4.The date of the granitic and syenite outburst.
5.The date of the metamorphism of the Schist rocks.
6.The age of the propylites of Dunedin Peninsula.
7.The date of the last great glacier period in New Zealand.

* Contemporaneous eruptive rocks are found in these formations.