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Report on the Geology & Gold Fields of Otago

Kakanui Formation. — Tuamarina Formation (Hutton)

Kakanui Formation.
Tuamarina Formation (Hutton).

Distribution.—This formation extends in an almost continuous belt round the western, southern, and eastern margins of the Wanaka formation. Commencing in the Forbes Mountains, north of Lake Wakatipu, it includes the northern half of that lake and, passing between Kingston and Athol, it bends gradually to the east to form the northern portions of the Garvie and Umbrella Mountains. East of the Pomahaka it is interrupted by an overe lapping of the Kaikoura formation in the Tapanui, or Blue Mountains, but appears again on the eastern side of the Clutha, forming the southern part of the Tuapeka gold fields from Gabriel’s Gully to Mount Stuart and Waihola Lake. On the north-eastern side of the Tokomairiro Plain it re-appears, forming the low hills near the sea from the Akatore River to Saddle Hill. From the latter place a narrow band runs northward, forming the Chain Hills, until it disappears under the basalts of Flagstaff Hill. Commencing again north of the Taieri Plain, it forms the southern part of the Silver Peak Mountains, and runs out to a point at the south-west corner of Waikouaiti Harbour. Commencing again in the’ Shag Valley, it runs in a north-westerly direction, and gradually increases in breadth until it covers the country from Hyde and Hamilton to the Kakanui River, including a large part of the Kakanui Mountains. Further to the north-east a small detached portion is found at Awamoko. North of the Kakanui Mountains it splits into two bands, one of which runs north to the Waitaki River at Kurow, and the other runs north-west through Kyeburn Hill, Mount Ida, the Hawkdun Ranges and Blackstone page 33Hill, Dunstan Peak, Mount St. Bathans, and the Upper Lindis, to the north-east corner of Lake Hawea.

Rocks.—The principal rock composing this formation is a grey phyllite, or argillaceous mica schist, in which the mica is but very feebly developed, but beds of clay slate, mica schist, and occasionally of quartzite, also occur.

Position of Strata.—At the head of Lake Wakatipu the rocks belonging to this formation dip 65° S.W. on the east side, and 15° S.S.W. on the west side of the lake. At Fews Creek the dip is 45° to 60° W. East of Athol the dip is 45° E.S.E., but more to the west it alters to W.N. W. In the Upper Pomahaka I observed the dip to be 50° to 70° N.E., which shows some irregularity in the structure of this district. In the neighbourhood of Lawrence the general dip is southerly, but the lay of the rocks is not uniform. I observed here one dip of 30° S.E., and others from 10° to 20° S.W. At the head of Weatherstone’s Gully it is 25° S.E. At Saddle Hill the dip is 20° to 30° W.S W., but in the tunnel through the Chain Hills it varies considerably, being 50° E.N E. on the west side, and 35° to 43° S. by E. on the east side, and it is probable that the southern branch of the main anticlinal passes between the tunnel and Saddle Hill. In the north-east part of the province the dip is much more regular, being at Morven Hill station and Lindis Pass 45° N.E., at St. Bathan’s 45° N.E., at Mount Ida 30° N.E., and at Hamilton 5° to 17° N.N.E. North of the northern syncline the dip at Kurow is 45° W. At Maruawhenua the beds are vertical, with a S.E. by E. strike; at Awamoko they dip 50° S.W., and in the Otepopo district the dip is pretty constant to the west and south-west.

Relation to underlying formation.—This formation is quite conformable to, and passes insensibly into, the Wanaka formation. The distinction between them is quite arbitrary, and I should not have attempted to separate them if it had not been necessary to divide such an enormous thickness of rocks in order that the map might display somewhat of the geological structure of the district. But the difference between them is simply owing to the older and lower formation having undergone more metamorphism than the upper.

Thickness.—The thickness of this formation can be estimated far more accurately than either of the two former; for in the Upper Lindis the rocks dip pretty steadily at an angle of 45° for a distance of 14 miles. This would give a thickness to the formation of 10 miles, or 52,800 feet.

Fossils.—In my report on the geology of the north-east district of the South Island,* I stated that some obscure fossils had been found near Picton in a blue argillaceous sandstone belonging

* Reports of Geological Explorations 1872-3, p. 31.

page 34to this formation, but Dr. Hector has since informed me that a a considerable number of fossils have been lately obtained from this locality by Mr. A. McKay, and that some of them are identical with tertiary fossils in the Colonial Museum; so that at present no fossils have been found in these rocks, although they may probably exist in some of the clay slates. If found they will, I expect, prove to be identical with the graptolites, &c., of the auriferous slates of Victoria.

Eruptive rocks.—I know of no eruptive rocks in this formation, except some tertiary basalts which will be mentioned further on in the report.

Minerals.—Gold is generally distributed through this formation, but not so abundantly as in the last. The richest diggings where the gold has been exclusively derived from these rocks is at Naseby, for the alluvial deposits worked at Nokomai, Switzers, Tuapeka, Hamilton, &c., have been in great part derived from the Wanaka formation.

Nomenclature.—This formation is identical with Dr. Hector’s Kakanui series*, and also with the Tuamarina formation of my report on the geology of the north-east district of the South Island, and as that name was given by me, because at the time I was not able to identify it with Dr. Hector’s Kakanui series, I now abandon it and revert to Dr. Hector’s previous name.

* Quar. Jour. Geo. Soc., 1865, p. 128, VIII. 1 (i).

Reports of Geological Explorations, 1872-3, p. 31.