Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations
All the coalfields situated on both sides of the central chain, belong to the Waipara and Oamaru formations, those in the former being most valuable and extensive. Beginning with the Waipara formation page 451on the western slopes of the Southern Alps, the first locality where coalfields—one of the best in the Colony—exist is situated about six miles above Greymouth. The principal seam of these coal measures was discovered by me in 1860, and described in my report on the western districts of the Nelson Province, published by the Provincial Government in 1861. Owing to the many faults existing, and the density of the forest, it then appeared that besides the principal seam, several others were in existence. However, a careful examination of the Geological Survey Department, extending over a number of years, has proved that there are only two workable seams, an upper seam of about three feet, and a lower or main seam of 16 feet in thickness, separated from the upper by about 35 feet of grit. The principal and most accessible portion of this coalfield, although considerably faulted, is situated on the northern side of the Grey river, the coal on the southern or Westland side being more limited in extent. Several shafts, of which the principal one is 635 feet deep, have here been sunk, the coal having been reached at various depths. It is a fine bituminous coal, clear and homogeneous, burning to a fine metallic coke, about 66 degs. in the average, and very useful for steam, manufacturing, and home purposes. From the Table of Analyses of coal made at the Colonial Laboratory it will be seen that it is in every respect a very superior coal, the value of which together with that of the Buller coal measures cannot be overrated.
It is a remarkable fact that this coal, although not being of greater age than the principal brown coal seams on the eastern side of the dividing range, has nevertheless undergone such an enormous alteration throughout, that it has the character of coal of far greater age, and in some respects is superior to coals from New South Wales. It is, therefore, evident that some abysso-dynamic agencies have been at work to accomplish this metamorphism throughout the whole Grey and Buller coalfields, which did not extend to the eastern side of the Alps, where the coal is only locally altered, the cause of the alteration being always traceable to volcanic rocks in close proximity, erupted during or after the formation of the coal seams. The seams existing near Lake Kanieri, at the Paringa, and near Jackson's Bay, although containing coal of good quality, are too thin for practical use, but I have no doubt that the extensive Waipara beds near the Paringa river will yield in years to come workable seams, by means of which the central portion of Westland will be settled by a population more stationary than a gold mining community generally is.page 452
Passing over to the eastern slopes of the Alps, the most important district in Canterbury where coal-bearing strata of some extent have been proved to exist, is situated some 30 to 40 miles west from Christchurch. This district, called the Malvern Hills, was first examined by me in 1861-62, but more systematically during 1870-71 the results of my surveys having been published in the Reports of Geological Explorations during 1871-72. Since that report has appeared, a few more discoveries have made us acquainted with several new localities where coal crops out, of which the principal one is that where the Springfield Company has opened up a colliery a few years ago. In the publication referred to, I have shown that only those portions of the coal seams which were subjected to igneous action, have undergone metamorphism in the immediate neighbourhood, and to such a local extent, that sometimes the upper portion of large coal seams over which a basaltic lava-stream has been flowing, is altered to an anthracite coal, whilst the lower portion has remained an unaltered brown coal. The same limited effect has been produced where the volcanic rocks in the form of more or less vertical dykes have ascended through the carboniferous strata, the coal seams being affected in a similar manner on both sides of the dyke, for a short distance only. The surveys of the coal measures under consideration have proved that during the sinking of the land the material for the formation of brown coal seams was accumulating all over the district under review, only those portions having been preserved where favourable circumstances were existing, such, for instance, as the coulées of basaltic rocks having flowed over them. In other cases, hard fossiliferous sandstones have acted in the same manner. Consequently we find that the coal seams not only are fringing the slopes of the palæozoic ranges, appearing in that case as marginal seams, and at altitudes from 800 to 1500 feet, but outl ers of more or less extent occur up to 3500 feet above the present sea level. Of the marginal coal seams, those in which the Canterbury Colliery (Jebson's), the Homebush Colliery (Deans'), and the Wallsend Colliery on the northern bank of the Selwyn, are situated, have not been altered by wolcanic eruptions, the large coulées of anamesite by which the upper series of the Waipara formation is here covered, being too far distant to have had any appreciable effect upon the coal seams.
Beginning with the Canterbury Colliery, we observe that the whole series from the first seams worked in 1861 to the ferruginous sandstone forming its base, stretches 12½ chains along the banks of the river, dipping on the average at an angle of 19 degrees to the page 453south-east, and thus possessing a considerable thickness. In descending order there occurs, first a set of two seams, consisting of
These two seams were worked in 1861. Three and a half chains from this drive, now abandoned for some time, Mr Jebson first began to work, extracting coal from three seams of 11 inches, 14 inches, and 20 inches respectively. These seams were separated by small bands of fire-clay and micaceous shales. Afterwards Mr Jebson went lower down into the series, and six and three-quarter chains from the first workings, a new mine was opened, from which brown coal of fair quality is now extracted. This set consists of two seams, each about two feet thick, dipping 18 to 19 degrees to the E. S. E., with two feet of fire-clay between them.
The next locality where brown coal of good quality crops out is in Surveyor's Gully, a small tributary of the Selwyn, in which I observed in 1870 two seams of coal, the lowest one being six feet thick. On this seam repose 2 feet 6 inches of shale, upon which another workable seam of three feet in thickness follows.
To reach the coal at a lower level the Homebush Colliery was opened in 1872 about half a mile lower down the valley, when the following strata were passed through (section No. 6 on plate 3), beginning with the lowest bed.
|a. Shale, only exposed in part|
|b. Brown Coal||2||8|
|d. Brown Coal||0||6|
|e. Shale (micaceous)||1||6|
|f Brown Coal||1||10|
|g. Shale (argillaceous)||3||0|
|h. Brown Coal||3||6|
|i. Shale, with small seams, and streaks of coal||30||0|
|k. Carbonaceous Shale||3||6|
|l Light Shale||4||5|
|p. Quartzose Sands, only exposed in part|
The seams f and h are being worked. It will be seen from this list that the large seam of 6 feet observed higher up the valley has not been met with, but I have no doubt that either one of the seams enumerated above enlarges further on, or that two seams will join to form a thicker one. Two remarkable breaks in the otherwise regular seams (sections No. 5 and 6 on plate 9), which I wish to notice here, bear upon this point. In the one case the seam h continues quite regularly for a number of chains in the workings on the left bank of Surveyor's Gully, after which it disappears altogether, and anew seam of about 2 feet thick, separated from the former, by 1 foot 6 inches of shale takes its place, gradually thickening. It is thus evident that both seams have been formed in two separate basins, the lower being of course older than the upper one. A similar and still more clearly defined occurrence was observed on the opposite or right bank of Surveyor's Grully, when the same seam h enlarged at one time to a thickness of 7 feet 9 inches with a band of shale of 2 inches in the centre. However, after 6 yards, the upper seam thins out rapidly, the lower one only continuing.
Of the new ventures lately opened none is of greater interest from a geological point of view than the Springfield Colliery. It is situated at Kowhai corner, where a volcanic eruption of dolerites has taken place, the rock forming on the summit of a nearly isolated hill a crateriform rim, from which a lava-stream has run in a south-easterly direction. This lava-stream, together with a dyke of a fine grained dolerite of considerable thickness, forming a projection above the Waipara beds, has preserved the lower deposits from destruction. The coal has at the same time been partially altered. When I visited the mine in 1876, the following beds were exposed in ascending order:
|Shale, only exposed in part|
I understand that whilst the other seams have diminished, the 4 feet seam has improved in thickness the more it has been opened up, being now 4 feet 6 inches thick. Section No. 5 on plate 3 gives a section of the colliery under consideration, whilst No. 7 on the same plate shows the principal details of the Kowhai corner hill more to the page 455north-west, where.the neck or vent of the yolcanic eruption has been laid open by several trial shafts and adits. As a matter of course, the sedimentary strata have here undergone great changes by faulting crushing, and metamorphisim. In this locality I examined a number of sections, from which it appears that the coal seam nearest to the basic rocks has become so much altered that it consists of a brittle anthracite coal, whilst those seams separated only by 10 to 15 feet of shale contain only a hydrous brown coal.
There are several other localities where coal seams partly altered have been worked, as for instance, Hart's and Hill's (Cordy's) collieries, but either the seams were too thin, or the outlier of too small an extent (Hart's and Kowhai). In other instances, the demand was not sufficient, or the coal seams too difficult of access, other collieries being nearer to the consumers. There is, however, no doubt that in years to come, all these localities where coal seams of workable brown coal exist, will form a nucleus for population and industry.
Of the localities in the valley of the Rakaia, where coal seams occur, I wish to mention two, the first of them not so much on account of the extent of the coal seams, but for the manner in which the latter have become altered to a fine anthracite throughout. In the valley of the Acheron the brown coal measures repose unconformably upon paleozoic rocks without any porphyry conglomerate between them. The former consist first of 20 feet of black shales with several small seams of coal changed to anthracite, and of a thickness of 2 to 14 inches; the main seam of an average thickness of 4 feet 2 inches follows next. It is very pure throughout, and has been, changed by the action of the dolerite, principally close to the channels of eruption, to a fine anthracitic coal. It dips in the locality where Mr. Oakden obtains his supply of coal, N. N. W. 39°., whilst near the dolerite stream it shews together with the shales a false stratification, dipping apparently W. N. W. 70°. This seam is covered in ascendingorder, by about 7 feet of black or iron grey shales; 8 feet of loose sands coloured black by carbon; 24 feet of dark coloured shales, which for the last 10 feet before we reach the dolerite have been greatly altered, so as to assume a greyish white colour and the character of a porcelain, jasper, but still preserving the markings of obscure remains of plants enclosed in them. These beds, following generally the surface outlines of the palaeozoic rocks appear in several localities as isolated patches from below the morainic accumulations or the post-pliocene alluvium page 456both together forming the upper deposits of the whole district. In the upper portion of the valley no large seams are visible, and only three small ones of 1, 3, and 5 inches appear between the shales.
Section No 3 on plate No 4 gives the details of this interesting and instructive section.
|150 feet Alluvium||bluish tint|
|Fine Brown Coal (Pitchcoal)||5||2|
|Conglomerate consisting mostly of pebbles of Quartziferous Porphyry||6||0|
|Shale, only exposed in part|
The last seam consists of a fine hard pitch coal interstratified with numerous small layers of glance coal.
In the more northern portions of the province, coal seams of economic value are of rare occurrence. There is a seam near the source of the Motanau, 4 feet thick, which one day may be useful to the district. Another seam of similar thickness is exposed near the source of a creek falling a few miles north of the mouth of the Motanau' into the sea, but it is difficult of access. In the Waipara and its tributaries the beds corresponding with the coal seams of the Malvern Hills consist, with the exception of a seam in Boby's Creek, only of a very shaly lignite, and of inconsiderable thickness. Of the outliers, containing brown coal of good quality, those behind Big Ben and at Craigieburn might also here be mentioned.