Art. XXXIV.—On the Discovery of Permo-carboniferous Rocks at Mount Mary, North Otago.
[Read before the Otago Institute, 13th October, 1903.]
Plates XXXV. and XXXVI.
In connection with my examination of the Lower Mesozoic rocks of New Zealand I deemed it of the highest importance to discover, if it were possible, the source of certain fossiliferous boulders found in the terrace drifts on the south aide of the Waitaki River by Mr. A. McKay in the course of his geological survey of Waitaki County in the summer of 1880.*
Mr. McKay made a collection of fossils from the boulders. The fossils, he considered, indicated a Lower Trias or Permian age. He traced the boulders up the course of the Awahokomo to the foot of the Kurow Mountains, which ha found were composed at that place of phyllites and altered sandstone, at that time believed to belong to the Kakanui or Walter and Cecil Peak series of supposed Devonian age.
On reaching the “Kurow schists” Mr. McKay desisted in the search, conceiving that the fossiliferous boulders could not have been derived from that formation, and subsequently, when discussing the probable source of the fossiliferous boulders, hazarded the opinion that they had been transported from some part of South Canterbury by the agency of glacier-ice.†
Thus the matter stood from 1880 till the present year. Early in March of this year I visited the Waitaki Valley, making my headquarters at Kurow. I searched the river-terraces between Kurow and Awahokomo, and also the lower courses of the Big and Little Awakino.
I found, as reported by Mr. McKay, that the greatest number of fossiliferous boulders occurred in the bed of the Awahokomo, and subsequently I confined my search to the course of the stream. Boulders containing fossils were fairly abundant in the first three miles—that is, up to the point where the stream cuts into the schistose rocks. Beyond that point, as found by Mr. McKay, fossiliferous boulders seemed to be absent, and for a time I was uncertain which course to pursue. After a time I decided to continue up the bed of the stream. For some distance I met with no success, but after proceeding less than half a mile I began to again find traces of fossiliferous boulders, but only at wide intervals. The traces
[Footnote] * Reps. Geol. Expl., 1881, p. 77.
[Footnote] † L.c., p. 77.
occurred so sparingly that I was compelled to search all the side branches of the main stream lest I should pass the parent rock to the right or left. This work occupied much time, but satisfied me that I was on the right track in following up the main stream.
I had now reached back to the flanks of Mount Mary. The descent of the stream was rapid, and, instead of a wide boulder-strewn bed, the course of the stream was a narrow rock-cut channel. The elevation was over 3,000 ft., and I was now convinced that I had reached a position beyond all possible influence of ancient glacier-ice from South Canterbury. Huge masses of fossiliferous rock occurred in the channel at intervals, and at the big waterfall, at 4,000 ft. above the sea, I found indistinct traces of fossils in highly indurated greenish-grey altered sandstones. Blocks of fossiliferous rock were now more plentiful, and, having the basin at the source of the stream immediately above me, I was at last satisfied that the fossiliferous outcrop was not far distant.
The evening was now approaching, and, being alone and far from my headquarters, I was compelled to retrace my steps without visiting the main outcrop, which I afterwards found was within view of the waterfall.
Three weeks later I again visited the Waitaki, on this occasion accompanied by Mr. A. Hamilton, who was greatly interested in the discovery.
We directed our way at once to the big waterfall near the source of the Awahokomo, and, after a stiff scramble, at a height of 5,160 ft. reached the main outcrop from which the fossiliferous boulders had been shed.
The outcrop lies on the left or west side of the basin at the source of the stream, at some 500 ft. below the summit of the range. Fossils were found in three zones in a thickness of 50 ft. of strata. The two lower zones are altered slaty shales and the upper a bed of conglomerate. In the two lower zones fossils are very abundant and generally well preserved. The Athyris, Spirifera, and Spiriferina are especially large and fine, although many specimens are much crushed and distorted.
The fossiliferous rocks are distinctly altered and closely associated with altered sandstones, and with slates which are almost identical with the slates at Otepopo. They follow the phyllites, quartzites, and altered rocks of the “Kurow schists” in a direct stratigraphical succession.
The strike of the strata from the foot of the range to the summit is N.W.—S.E., and the dip S.W. Excepting in one, or two places, where the strata exhibit minor folds or corrugations, the general strike and dip are remarkably uniform throughout, and in the rock-cut course of the Awahokomo the
succession and relationship of the different rocks are seen to great advantage.
At the foot of the mountains the dip of the rocks is very high, but ascending the Awahokomo it gradually decreases, and at the foot of Mount Mary is as low as 35°. Ascending Mount Mary the dip gradually increases, and at the fossiliferous outcrop is about 60°
The total thickness of strata exposed in this section is not less than 10,000 ft. The lowest rocks are pale-grey and blue phyllites, generally much crushed and often drossy. The phyllites are followed by thin-bedded quartzite and altered claystone and a great thickness of pale-green altered grey-wacke and breccia, the former occurring sometimes in thin and sometimes in thick bands, and frequently seamed with a network of small quartz veins.
Above the upper forks of the Awahokomo blue silky slates are interlaminated with thin layers of quartz, which vary from a mere thread to 4 in. thick.
Above an altitude of 3,000 ft. altered argillaceous rocks become less abundant. At 3,200 ft. there is a considerable development of thin-banded siliceous sandstone, almost quartzite, and dark-blue claystone, occurring in thin laminæ varying from 1 in. to 2 in. thick.
From this onward to the summit of Mount Mary the bands of sandstone, greywacke, and breccia become more massive and less altered, and here we meet a few narrow bands of red and green slaty shale which are often streaked with jasperoid segregations. The pale-green aphanitic sandstones and greywacke, which are so prominent in the lower part of the succession, are now absent. The sandstone bands are coarse and gritty, but the argillaceous beds, lying between two bands of sandstone or greywacke, are still considerably altered.
The rocks just described are highly altered and schistose near the bottom of the series, and become gradually less and less altered in passing upward towards the top erf the sequence. This is partly, but not altogether, explained by the circumstance that the more easily altered argillaceous strata predominate in the lower part, and the more siliceous and consequently less easily altered rocks in the middle and upper parts.
The rocks in the immediate neighbourhood of the main fossiliferous outcrop are as follows in ascending order:—
(a.) Grey indurated sandstones, often coarse and gritty.
(b.) Blue fissile slates, like slates at Otepopo.
(c.) Slaty and flaggy claystones slightly micaceous,con-
taining two zones of fossils about 20 ft. apart, the lower 6 ft. thick and the upper 3 ft. thick.
(d.) Bed of pebbly conglomerate and sandstone varying from 3 ft, to 6 ft. thick, and separated from the upper fossiliferous zone by several feet of flaggy claystone.
(e.) Grey indurated sandstones, and flaggy claystones.
Fossils are very abundant in the two lower zones, and, although many are flattened and deformed through the alteration which the rocks have suffered, distinct and well-preserved casts are numerous, and easily broken out.
Among the genera identified in a small collection were Spirifera (two sp.), Spiriferina (two sp.), Athyris, Epithyris, Rhynchonella, Edmondia(?), Allerisma(?), Schizodus, Ostrea, Turbo, Patella, Pleurotomaria, and a nautilord shell.
Captain Hutton examined the collection in Dunedin, and subsequently, after looking over the Permo-carboniferous fossils of New South Wales in the Canterbury Museum, informed me that he was pretty sure the Mount Mary collection included Spirifera vespertilio, G. Sow., Spirizfera subradiata, Sow., Eurydesma, Morris, and Platychisma—forms characteristic of the Permo - carboniferous of New South Wales.*
Mr. W. S. Dun, of Sydney Museum, to whom a small collection of these fossils was sent, in a letter to Mr. A. Hamilton confirmed the identification of the spirifers made by Captain Hutton.
A collection of these fossils is now in the hands of Professor Boehm, of Frieburg, for identification and description. His determinations will be awaited with much interest by students of New Zealand geology.
Subdivision and Correlation.
The grey phyllites, banded quartzites, aphanitic sandstones, and greywacke at the base of the succession comprise the “Kurow schists” of McKay, which were rightly referred by that geologist to the Kakanui series of Hector. It has generally been admitted that the Kakanui series of Hector is the equivalent of the Kakanui (Tuamarina) formation of Hutton† and the “Waihao formation of Haast.‡
The rocks forming the upper part of the succession correspond closely to the description given of the Kaikoura formation of Hutton§ and the Te Anau series of Hector,
[Footnote] * Hutton, Letter to author, 29th April, 1903.
[Footnote] † Hutton, “Geology of Otago,” 1875, p. 32.
[Footnote] ‡ Hutton, “Geology of Canterbury and Westland,” 1879, p. 260.
[Footnote] § Hutton, “Geology of Otago,” 1875, p. 35.
and they occupy the same relative position with respect to the “Kurow schists” that the Te Anau series does to the Kakanui series.
In his “Outlines of New Zealand Geology” Sir James Hector recognised the close association, of the Te Anau and Kakanui series, and referred them to the Devonian period, as under:—*
“(a.) Te Anau series.
“(b.) Kakanui series.”
In the latest classification of the Geological Survey,† dated the 30th June, 1887, the Kakanui series disappears from the table of formations, and is apparently, but without explanation, included in “XIII. Lower Devonian— Reefton beds,” which had no existence in the classification of the previous year (“Outlines of New Zealand Geology,” p. 40).
Briefly summarising the foregoing, we find that the highly altered phyllites and quartzites of the lower flanks of the Kurow Mountains are succeeded conformably by the less altered Mount Mary fossiliferous slates and sandstones. The former are included in the Kakanui series of Hector and Hutton, and the latter in the Kaikoura formation of Hutton And Te Anau series of Hector.
I am inclined to agree with the early opinion of Hector and Hutton that the “Kurow schists,” notwithstanding that they pass insensibly into the Mount Mary series, are sufficiently distinctive to be separated from the upper series.
For the lower and more altered group of Kurow rocks I propose to revive the old “Kakanui series” of Hector, in which they were originally included. For the upper and less altered group I do not think there is a more appropriate name than Mount Mary series.
The Mount Mary series undoubtedly includes the Te Anan series of the Geological Survey, of supposed Devonian age, but there are several reasons why the name “Te Anau” should be abandoned. In the first place, we have it on the authority of Mr. McKay that the rocks typical of the Te Anau series do not occur at Lake Te Anau; and, further, the Te Anau series has always been difficult to distinguish from rocks supposed to belong to the Maitai series of; the Geological Survey.
On the other hand, the rocks at Mount Mary are fossiliferous, and perhaps this is the best reason for attaching the name of that mountain to the new series. The proposed subdivisions of this succession of rocks are as follows:—
[Footnote] * Hector, “Outlines of New Zealand Geology,” 1886, p. 40.
[Footnote] † Reps. Geol. Expl., 1886–87, Appendix, p. 256.
Mount Mary series.
(Syn.: Te Anau series, Hector; Kaikoura formation Hutton; Lower part Mount Torlesse formation, Haast.)
(Syn.: Kakanui series, Hector and Hutton; Tuamarina formation, Hutton; and Waihao formation, Haast.)
Relations to Adjoining Rock-formations.
The Waitaki River, between the Kurow and Awahokomo, runs along the axis of an anticline of claystones and sandstones, in which Mr. McKay discovered numerous remains of Inoceramus, including an almost perfect example found near the mouth of the Hakataramea.*
Mr. McKay shows these rocks to be unconformable to the altered Kurow rocks, and refers them to the Maitai series of the Geological Survey, of supposed Carboniferous age.
As the “Kurow schists” have been shown to be associated with rocks of Permo-carboniferous age, it necessarily follows that the Waitaki “Valley rocks, which overlie these unconformably, must be of subsequent date; and the presence of the Secondary genus Inoceramus clearly indicates that they must be referred to the Jurassic system, a position quite in harmony with my conclusion respecting the Jurassic age of the Maitai series in Nelson.
The Mount Mary series fills a gap in the New Zealand geological record, and, furthermore, furnishes for the first time reliable data by means of which the age of the Kurow phyllites can be ascertained.
The classification which my investigation of the Lower Mesozoic and Upper Palæozoic rocks in the past three years has led me to adopt is as follows:—
Triassic—Shaw Bay series—
(a.) Athyris (or Clavigera) beds.
(b.) Mytilus and Monotis beds.
(c.) Trigonia beds.
(d.) Halobia lommeli beds.
(e.) Spiriferina beds.
(f.) Nugget Point plant beds.
Permo-carboniferous-Mount Mary series.
[Footnote] * Reps. Geol. Expl., 1881, p. 78.
Explanation of Plates XXXV. and XXXVI.
Plate XXXV.—Section from Waitaki River to Mount Mary.
(a.) Claystones and sandstones.
(b.) Sandstone conglomerate.
(d.) Banded phyllite and quartzite.
(e.) Altered sandstones, &c.
(f.) Sandstones, gritstones, &c.
(g.) Slates and flaggy sandstones.
(i.) Grey sandstones, and flaggy claystones.