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Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

The Mount Torlesse Formation

The Mount Torlesse Formation.

Under this name I include all that vast assemblage of rocks, of great thickness, which forms a number of huge folds, reaching in some instances from the East Coast to within 20 miles of the West Coast, and by which more than half of the Province of Canterbury is covered. Although the attempt has been made, both by Captain Hutton and Dr. Hector, to separate this formation in Canterbury into several divisions belonging to different periods very distant in age from each other, both basing their sub-divisions not only on the scant palseontological evidence we possess, but also on lithological character and stratigraphical position, a closer study of the sections has proved convincingly that such divisions were based upon erroneous observations and deductions. The following formations are said by Captain Hutton to be represented in Canterbury—namely, the Kaikoura, the Maitai, and the Putataka formations.* First as to his Kaikoura formation Being acquainted with the district from which the name has been taken, I can state, judging from the lithological character and sequence of the beds, that his Kaikoura is the same as my Mount Torlesse formation, both mountain systems being in that respect alike. Captain Hutton correctly states that all the higher ranges— Mount Torlesse, the Thirteen-mile-bush Range, and Mount Hutt

* Report on the Geology of the north-east portion of the South Island, from Cook's Straits to the Raiaia. By Captain F. W. Hutton, F.G.S., in Report of Geological Explorations during 1872-73. Geological Survey of New Zealand.

page 267included—belong to this formation, which he considers to be of palæozoic age, but he excludes from it all the lower ranges lying east of the former, to which he assigns a much younger age.

Thus to his Matai formation, which he now considers to be triassic, he assigns— although no fossils characteristic of that formation have ever been found in the whole Province of Canterbury or east of the Kaikouras—the western portion of the Malvern Hills, the Oxford Hills, and all the smaller ranges east of the sources of the Ashley river, notwithstanding that many of the rocks in those districts have sometimes a far more altered texture than those which compose the higher ranges lying to the west of the zone in question. And finally, the eastern portion of the Malvern Hills, the small ranges near Heathstock, and the isolated Black Hills, he places in his Pntataka formation, to which he now assigns a lower Jurassic age, relying principally on some impressions of ferns found in the Malvern Hills and in some other localities, which he considers identical with those obtained from some beds of Port Waikato, Auckland, and Waikava, Otago.

Now, considering the age of the Black Hills, where no fossils have ever been found, and judging from the lithological character of the rocks alone, they clearly belong to Captain Hutton's Kaikoura, or my Mount Torlesse formation, as they contain the same chertose altered beds as we find in the very heart of the Southern Alps. In fact, a series of rocks from that locality, and another from the River Cass (Upper Waimakariri) placed in the position of their sequence side by side, cannot be distinguished from each other, except by the labels attached to them. In regard also to the Malvern Hills, his whole arrangement cannot be accepted if we carefully study the sections in the district. These sections clearly show, first, that his so-called Putataka beds underlie his Matai beds, of which, amongst many other localities, a clear section is exhibited on the right bank of the Selwyn. On the northern or left bank of that river, the fern beds (his Putataka formation) are separated by an outlier of cretaceo-tertiary age (Hart's coal measures) from the higher ranges to the west, but crossing the river they can be followed on the southern side without any break till they disappear below that vast assemblage of rocks consisting of cherts, marbles, and diabasic ashes, alternating with large beds of sandstones, slates, and shales, overlying them conformably; consequently the fern beds cannot belong to Captain page 268Hutton's Putataka formation, as they are actually older than his so-called Matai formation, which he places to the west of it. The attached sections—No. 1 and 2 on section plate No. 2—offer all necessary details in illustration of this point.

Now, when we examine the relations of the beds which he assigns to his Maitai formation, with those of his Kaikoura formation, situated to the west of the former, including the whole of Mount Torlesse,. we observe that exactly the same sequence and character are met with in both; that, in fact, they both are portions of a series of folds in which a succession of chocolate-coloured slates and cherts form a good horizon for recognition. Thus, between Russell's Hills, (northern side of the Malvern Hills) and Mount Torlesse, a huge anticlinal occurs, the same rocks appearing on both sides, but dipping in opposite directions.—(See Section-plate No. 2, section No. 3.) Consequently, also, in this case, Captain Hutton's classification, based upon his observations made in a hasty visit to the district, does not hold good, and the whole series has to be included in the same formation, which I named the Mount Torlesse formation, from the large mountain system where I first studied it attentively. In several other localities I have been able to examine the relations of the different beds to each other in detail, always obtaining the same results. Amongst them, the most interesting one is situated in the Clent Hills, where I found, as far back as 1861, a series of beds, containing numerous impressions of ferns, and in the upper course of the Rangitata in Mount Potts, where I made a good collection of fossil shells. These collections were sent by me to Professor M'Coy in Melbourne for description, and I was informed by that gentleman that the former, the ferns, were of Jurassic, and the latter, mostly brachiopods, of upper Devonian or Lower Carboniferous age, both being identical with exuviae found in the New South Wales Coal-fields. Although at the time I believed this Clent Hill series somewhat younger in age than the Spirifera beds, I demurred at this definition, owing to the fact that the position and sequence of the strata and the character of the rocks of which both are composed, are alike. Since then it has been shown, and as I think with conclusive evidence, that both these fossiliferous strata—the Spirifera and Pecopteris beds—occurring together in the New South Wales coalfields, are of the same age and alternate with each other. The occurrence of Tæniopteris, which hitherto had been considered only of secondary age, seems to speak against a palæozoic origin; however, I page 269may point out that the same objection was made to the Glossopteris in Astralia, which has by overwhelming evidence been shown to be of palæozic age. I do not think that the fragment of a leaf, however distinct, can unsettle all that stratigraphical geology has proved to be correct. *

There is some difference in the character of these old sedimentary strata, going from east to west. It appears that whilst in the ranges near the plains we generally observe true littoral beds-the deposits of large rivers entering here the palæozoic sea-further to the west, as for instance, on the banks of the Upper Rangitata, we find their horizontal equivalents as rocks of a different character They consist of shales and shaly sandstones with marine shells, mostly brachiopods showing that they were deposited in deeper water than the Clent Hill beds, and some distance from the palæozoic coast-line.

In confirmation of my statements, I shall now proceed to give the usual sequence of the lowest sedimentary rocks as they appear in the Clent Hills district in an ascending order.—(See Section-plate, No. 2, section No. 4.)

The lowest beds are usually:

  • 1. Slates greyish, sometimes very siliceous, alternating with sandstones, the latter gradually becoming of a coarser grain, so as first to assume the character of a grit, and afterwards of a pebble-bed.

Upon them repose:

  • 2. Thick bedded conglomerates, in the Clent Hills several hundred feet thick In these conglomerates occur large sometimes fucoid-like carbonaceous markings, as if from drift trees. Smaller beds of sandstone are interstratified, partly ferruginous, partly ful of abscure remains of plants. Well-rolled boulders of greyish or greenish coarse sandstone, from the size of a child's head to that of a bean form the principal portion of the rocks, mixed sometimes with boulders or pebbles of chert and lydian stone Occasionally a few pieces of quartz, porphyry, and gneiss, which I found only after some search, connect the Clent Hill series with the beds of the Puddingstone Valley. In the latter, porphyry and gneiss occur in nearly equal

    * The Rev. W.B. Clarke the veteran geologist of New South Wales, has given an excellent and exhaustive resumé on this quection in his various publications on the Sedimentary Formation of New South Wales, to which I wish to refer the reader.

    page 270proportions with boulders of sedimentary origin, whilst the sections of Trinity Hill show the gradual change from the one series into the other. These conglomerates, like the whole series to which they belong, are much jointed, often a system of four distinct joints beins well developed, crossing each other at various angles, and cutting with sharp planes through the hardest boulders. The palæozoic outlier in the Gawler Downs belongs, without doubt, to this series.
  • 3. Shales alternating with gritty sandstone, coal-sandstone and bands of clay-ironstone now follow. Some of the sandstones are full of the impressions of fossil ferns, others full of those of roots or of drift timber. Flattened stems of trees are also enclosed, often of considerable size, the bark altered to a scaly powdery coal (culm)* the interior filled with sandstone, often of a much finer grain than the surrounding rocks. Amongst the fossil flora I observed, Pecopteris (two or three species), Camptopteri?, Tæniopteris, Otopteris, Cyclopteris Sphenopteris, Cycadites? Palæozamia? Taxites.? Equisetites The Pecopteris beds are usually distinct from the Tæniopteris beds Some of the shales consist almost entirely of the leaves and stems of ferns Amongst them the beds of sandstone are generally the most important some of them having a thickness of over twenty feet, although generally the arenaceous and carbonaceo-argillaceous strata alternate rapidly with each other, and are occasionally only a few inches thick; the whole thus having a ribboned appearance.

This series is together also several hundred feet thick.

  • 4. No. 3 is covered by conglomerate beds like No 2 but of smaller dimensions.

    * The following analysis of two specimens of this culm was made in the laboratory of the Geological Survey in Wellington:—

    •             1, and 2, Bituminous Coals (Culm).
    •      Colour intensely black, lustrous, very fragile.
    •      Colour of powder black, that of ash light buff.
    •      No. 1 cakes freely, the other even more so.
    Approximate Analysis.
    No. 1 No. 2.
    Water 2.27 2.12
    Fixed carbon 55.10 50.04
    Hydro-earbon 11.95 11.44
    Ash 30.68 34.40
    100 100
    • Evaporative power of No. 1 =7.1
    • Evaporative power of No. 2 = 6.5

    Dr. Hector has compared fossil plants found near Reefton, which also contain [unclear: Tæniopis], with the Rajamahal Flora of India.

    page 271
  • 5 The last series is overlaid by beds of shales and sandstones the latter having sometimes a semi-crystalline structure, and being of considerable thickness.
  • 6. Upon them repose very thick beds of a fine-grained sandstone, which has the peculiarity that its matrix or cementing medium generally decomposes to a white powder. Many years ago, comparing this rock with similar rocks in Europe, I applied to it the name of Kaolin Sandstone with which it has a close resemblance, at least in outward appearance. Gradually this rock becomes more argillaceous, and is largely interstratified with shales.
  • 7. The next horizon is formed of several beds of chocolate-coloured or brick-red slates. They are of various thicknesses, and alternate with greyish, greenish, or purple slates They are more or less arenaceous, of a very distinct character, and have been traced by me over a great deal of ground in several localities in this province as well as to the more northern portions of this island. From their lithological character and position, I believe them to be the horizontal equivalent of the cherts and diabasic beds which occupy the same horizon in the Malvern hills and elsewhere.

These beds are again overlaid by a series of beds consisting of clay slates often of yellowish or greyish tints, semi-crystalline sandstones, changing into graywacke, shales alternating with bluish gray and greenish sandstones, cherts, and many other varieties of sedimentary rocks too numerous to mention. Comparing these beds with those in Mount Potts—the only locality in this province where, as far as my researches have gone, the fossil brachiopods alluded to occur-we obtain the following results:—The beds 1, 2, 3 and 4 of the Clent Hills series are here represented by a great thickness of dark shales, often becoming so slaty that they may be termed clay slates, alternating with thinner layers of sandstone, sometimes with a ferruginous or calcareous matrix.

Amongst these deposits occur a few beds of conglomerate, which fairly may be termed bone beds, as in addition to boulders and pebbles of light coloured slates, they consist of great quantities of well rounded pieces of bones and broken shells. The bones are often of considerable dimensions. I was able to measure the proximal end of what was probably a humerus which I found to be eight inches across, and some other bones of similar dimensions; however, the bones, as before observed were so much rolled, and the cementing medium of such page 272considerable hardness that I was unable, with the tools at my command, to procure any characteristic specimens, but I have no doubt that they are of saurian origin. No teeth were visible amongst this bone breccia Some of the bones appearing to have been much rolled, resemble the vertebræ of Ganocephalous reptiles of the carboniferous period; as for instance, those of Dentrorpeton, Hylonomus, &c. I fail to see any resemblance between these vertebra and those of Ichtyosaurus, to which Dr. Hector has referred one of the vertebra collected by me, in his paper * "On the Fossil Reptiles of New Zealand," and upon the strength of which he concludes the beds in question are of Triassic age.

The whole of these strata form a large anticlinal in Fossil Gully and they are well and clearly exposed in this deep and rocky gome for several miles. They contain the following genera, and probably species:—Orthis spnigera, Spirifera (lineata, lata, oviformis duodecimocostata), Producta, Athyris, Euomplialus, Murclisonia Orthoceras, Encrinites, of which many resemble, or are closely allied to, Australian forms from the New South Wales coal-field series They are covered by the same succession of beds as occur in the Clent Hills beginning with No. 5, and I may here observe that I was able to trace some of these upper beds, as, for instance, the chocolate-coloured slates, all the way from near the summit of the Mount Potts range to the foot of Mount Potts, five miles to the south.

This is therefore good and, as I think, conclusive evidence that the Clent Hills and Malvern Hills plant beds, notwithstanding they

* " On the Fossil Reptiles of New Zealand," by James Hector F.R.S. ''Transaction New Zealand Institute,'' Vol. VI, pages 334 and 336.

I have not the least doubt that in future years, both the shell and plant beds will here be found interstratified with each other, as this has been the case in the coal-fields of New South Wales, and consequently that also in New Zealand, it will be clearly proved that they cannot belong to such distant periods as the carboniferous on the one hand, and the Jurassic on the other. Comparing them with well-defined European formations, they may, like the rhætic formation in the European Alps, be passage-beds, or represent the carbo-permian formation of the western portion of the United States (North America), where it has also been impossible to separate these two formations, which, in Europe, are so clearly and distinctly defined. When speaking further on of the Waipara formation, a similar difficulty will be placed before us, as here also the remains of mezozoic saurians and kainozoic shells are mixed with each other. Consequently, in this case, we have to deal with still another passage formation uniting two well-defined European divisions Thus had the geological nomenclature been based upon the strata of the Southern instead of the Northern Hemisphere, and the whole had been divided into periods, we should doubtless have included sanrians with the lower tertiary division, and the definition of palæozoic forms would have been much different from that Which has been obtained in European nomenclature. Stoliczka has shown also that the carbo-permian cannot be separated from the lower trias in the Himalayas, as these beds-are also closely connected by passage-beds.

page 273contain the remains, of a plant closely allied to Tæniopteris, are nevertheless of great age, and if we adopt Professor M'Coy's conclusions for the Mount Potts fossil shells, at least carboniferous. Moreover, there is no doubt that they are of the same age as the formations which in New South Wales contain the fine coal-fields. The relations of these palæozoic beds are shown, for comparison in section 4, and whilst the more westerly Mount Potts beds exhibit only one huge anticlinal arrangement, those in the Clent Hills have undergone greater flexures, of which in a distance of three miles, six synclinals are clearly exposed in Fern Gruliy. The same sequence of the palæozoic sedimentary beds is shown in all the sections I examined, of which several of the annexed sections give the details, and to which I beg to refer.

As before observed, the more we advance towards the Canterbury plains, the more we are sure to find the conglomerates in exposed positions, having by their hardness, without doubt, resisted most effectually the disintegrating influences here at work for numberless ages. I have mentioned that the chocolate-coloured slates which overlie the fossiliferous beds forming the summit of the Mount Potts range, reach the Rangitata river-bed five miles more to the south. In order to show the huge dimensions of the foldings, I may state that these peculiarly coloured slates, with a series of more silicious beds, are again found on the opposite banks of the Rangitata, and are well developed in Butler's Creek, where it leaves the range's, and, still further to the south, they cross the Forest Creek near its sources. Between these localities and the summit of the Southern Alps, I have been able to trace them five times more (they may occur still oftener), always standing at a high angle, and showing well the clearly defined anticlinal and synclinal arrangement of the strata. Beds of conglomerate and shales are always associated with them forming a lower horizon, and also the texture of the sandstones is always of the same character, having either a white kaolin-like matrix, or being of blueish or greenish colours with a hard semi-crystalline structure. Moreover, these latter are always true graywacke sandstones, whatever their other characteristics may be, small angular pieces of slate being enclosed in them. These pieces are not always of the usual dark bluish tints, but sometimes exhibit reddish and greenish colours, proving that similar slates to those which now alternate with the graywacke sandstones were also existing before this large series of beds was formed.