Coal Fields.

The coal fields in a new country are generally soon found, and this is especially the case in New Zealand where the seams, lying usually among hills, crop out to the surface, and are easily recognised. The first published notice of coal in Otago was by the the Hon. W. Mantell, who mentioned in 1850, the occurrence of coal at Matakea Point (Shag Point). In 1855 Dr. C. Forbes of H.M.S. Acheron, described the coals at Saddle Hill and Preservation Inlet. The Wairaki coal field was known to the settlers in 1850, and during the same year the mine at Coal Point was opened in the Clutha field; while the Green Island and Tokomairiro fields were opened in 1861. So that all our coal fields were known 13 years ago; consequently we cannot expect any important additions to them, except possibly in the Te Anau district, between the Lake and the Mararoa River, and also west of the Lower Waiau River, and in both these cases the coal, if it exists at all, will be found under the tertiary rocks near their junction with the slates. It is also, of course, possible that a thick seam may be found in the page 102Hokanuis or Waikawa district, but this could hardly be considered as a new coal field, as coal has been known in these localities for a long time although no workable seam has been yet discovered. It is not possible to give any approximate estimate of the total quantity of coal in the Province until topographical maps on a scale of six inches to a mile, have been prepared of all the coal fields, and the geology filled in on them in detail. This would be a very long and expensive process, and the results obtained would be entirely for the benefits of future generations, for we already known of sufficient coal to last for many years, as well as the most accessible places for working it.

Shag Point Coal Field.—The coal at Shag Point was already known, and used in 1848, but the mine was, I believe, first opened by Mr. Hutchinson, in February, 1863. This coal formation extends inland from the north head of Shag River for about four miles to where the north road crosses the Horse Range. To the northwards it stretches as far as the south branch of the Otepopo river, a distance of twelve miles, and a small outlier occurs across the north branch of the Otepopo; the whole area occupied is about 26 square miles, The only previously published account of this field is by Dr. von Haast, who examined it in the autumn of 1871, and his report was published in the "Reports of the Geological explorations during 1871-2″ (Geological Survey of New Zealand).

On its western margin this formation forms hills more than 1000 feet high cut by deep ravines, but towards the east the hills are much lower; the whole country, however, occupied by it is much broken. On the south the beds are considerably disturbed, and sometimes inclined at high angles, but towards the north they get more horizontal, and pass under the tertiary clays of Moeraki and Hampden. The lower part of the formation consists chiefly of conglomerates passing upwards into sandstones, which on the south are interstratified with beds of conglomerate that thin out northward. Coal has hitherto only been found in the southern and more disturbed part of the district, but I know of no reason why it should not also extend further north.

Along the north bank of the Shag River, at least six workable seams can be seen, varying in thickness from 3 to 7 feet, and at the coal mine, near Vulcan Point, there are two seam, the upper one being 7 to 10 feet, and the lower 4 feet in thickness, and separated by 4 feet of shale. It is the upper of these seams that is at present being worked. Both these seams are probably identical with some of those previously mentioned, so that at present we cannot feel sure that more than six workable seams exist, the total thickness of which is probably about 25 feet. It will, therefore, be seen that a considerable quantity of coal exists in this field, but as we do not know how far the seams extend north, we cannot say how much. Dr. page 103von Haast estimates that in the 70 acres surrounding Shag Point, there are "about 1,600,000 tons of coal, which, in order to allow for possible disturbances or other causes by which this quantity of coal might be diminished, reduced by more than one third, would still leave us at least 1,000,000 tons of workable coal." I quite agree with Dr. Haast that this is a moderate estimate.

This coal is the best of our brown coals. It contains little or no resin, but rounded pebbles of white quartz are sometimes found embedded in the middle of it, a peculiarity found also in the coals at Pakawau, near Nelson, and at Green Island.

The formation may perhaps, also underlie the tertiary rocks at Bushy Park, south of the Shag River. But this is uncertain, for the conglomerates interstratified with the coal and shale at Shag Point, prove that the coast line composed of palæozoic rocks, along which the formation was deposited, extended much farther south than at present, and may have occupied the whole of the country south of the Shag River. But be this at it may, it is certain that if the Shag Point coal series underlies Bushy Park, it must be at a considerable depth; for as has been pointed out (p. 89), a large river occupied the Shag Valley in lower eocene times, when the land stood much higher than now, and this river must have excavated a deep valley through the coal series, which was afterwards filled up by tertiary rocks, as shown in Section VIII., which is an ideal representation of what probably exists under the mouth of the Shag River. Consequently I am of opinion that any attempt to find this coal south of Shag River, would not at present prove remunerative, even if it were successful

Green Island and Saddle Hill Coal Field.—This coal field occupies the valley of the Kaikorai stream, near Dunedin. On the west it is bounded by the mica schist ridge that runs from the Chain Hills, south of Saddle Hill, to the mouth of the Otokaia. On the north it is overlaid, and probably cut off by the volcanic rocks of Flagstaff Hill, and on the east it is cut off by the basalts of Dunedin and Forbury. It has a length of about five miles with a breadth of two, and a superficial area of about 9 square miles.

The coal at Saddle Hill was mentioned by Dr. Forbes in 1855, and a mine was opened at Fairfield in 1861. It was worked by a drive from a gully. The field, and the nature of the coal in it have been described by Dr. Lauder Lindsay in his lecture delivered in Dunedin in 1862, and afterwards in a paper on the tertiary coals of New Zealand, which was published in the page 104"Transactions of the Royal Society of Edinburgh for 1865. Dr. Hector reported on it in his departmental report of the Geological Survey of Otago in 1864; in his first general report on the coal deposits of New Zealand in 1866; and in the reports of the Geological Explorations for 1871-2.

On the western margin of the field the lower beds of the formation crop out to the surface, and dip gently E.S.E., but towards Caversham they become quite horizontal, consequently it is only on the western margin that the coal seams, all of which belong to the lower part of the series, come near the surface. In the centre of the valley the formation has undergone considerable denudation, and the coal series is here overlaid by clays, grits, and ferruginous conglomerates belonging to the newer pliocene or pleistocene periods. These upper beds also contain small seams of lignite with wood almost unaltered, which have nothing to do with the coal of the older formation. This denudation however was not deep enough to affect any of the coal seams. (See. IX.)

These analyses show that the coal, although very free from ash, is inferior in quality to other brown coals, and it has more the appearance of a lignite than a brown coal.

Clutha and Tokomariro Coal Field.—This field is situated on the sea coast from the Clutha to a little beyond the Tokomairiro River, and is distant from Dunedin between 40 and 60 miles. On the west it is bounded by the alluvial flats of the Clutha and Kaitangata Lake, and on the north by the northern base of Mount Misery, and the slate ranges between the Toko-page 105mairiro and Akatore Rivers. In shape it forms a broad triangle with a base along the sea coast of about 15 miles, and a height from the sea to the Tokomairiro Plains of eight miles, thus having an area of about 60 square miles. It forms a series of round Backed hills, with deep valleys between, and attains its greatest elevation (1098 feet) at the summit of Mount Misery. Another small patch of the same formation lies on the north-west side of the Tokomairiro plain, between the Woolshed Creek and the Tokomairiro River, but it is not yet known to contain any coal.

The first coal mine in Otago was opened in this field at Coal Point in 1859, by Mr. J. G. Lewis, but it has since been abandoned, owing probably to finding coal in a more accessible position at Kaitangata. The coal field at Tokomairiro was opened in 1861. The only report on this field is that by Dr. Hector in his departmental report of the Geological Survey of Otago in 1864, which he afterwards reproduced with some additions in the reports of Geological Explorations in 1871-2.

In the southern part of the field, from Kaitangata to Wangaloa, the beds are considerably disturbed, and occasionally faulted, but in the northern portion, from Mount Misery down both banks of the Tokomairiro River on the edge of the basin, they dip slightly to the south, and then gradually become horizontal. (Sec. VII.)

Fig. 23.—Denuded coal, Tokomairiro. a, Loose conglomerate, and b, coal, Oamaru formation; c, fragments of coal in conglomerate.

It will be thus seen that these coals very nearly equal that from Shag Point in the amount of fixed carbon that they contain while the amount of water in them is slightly less.

Wairaki Coal Field.—This coal field is about. 45 miles distant from Invercargill. The formation here skirts along the southern bases of the Takitimu Mountains, and Wairaki Downs; from Taylor’s Creek on the west, as far as the alluvial plain of the Jacob River on the east, a distance of about 18 miles. Near the Jacob page break

Geology Of Otago, Hutton. Plate IV
Real Mackay Coal Mine, Tokomairiro.

page 107River it has a breadth of about three miles which gradually diminishes westward, thus forming a narrow triangle, the apex of which is at Taylor’s Creek, and the base at the southern spur of Mount Beaumont, called the Night-Cap. This will give an area of about 27 square miles. Another outlier is stated to lie at the base of Mount Twinlaw, but I did not visit it, and the coal may also probably extend under Sharp Ridge and Grassy Creek, towards the Waiau.

This formation never rises to great altitudes, but forms low rounded hills.

This field was reported on by Dr. Hector, in 1869,^* and by myself, in 1872^†

The whole field appears to be considerably disturbed, but the beds have generally a S.W. dip; if, however, the formation is continued under Grassy Creek and Sharp Ridge, the beds may be expected to become flatter.

At Taylor’s Creek, a section is opened on the edge of the basin, and a seam of coal five feet thick is seen to rest on the edges of slates and sandstones (Fig. 24). The coal is here covered with shales and micaceous sandstone, the whole dipping 20°S.W. Following down the creek more black shales are seen to overlie the micaceous sandstone, which probably indicate the presence of a second seam of coal, but the section stops suddenly. As the section here is evidently quite on the edge of the basin, the seam of coal may very likely get thicker in a south-westerly direction.

Fig. 24—Coal at Taylor’s Creek, Southland. Oamaru formation …. a, gravel; b, micaceous sandstones; c, shaley coal; d, chocolate shales; e, blue shale; f, slates (Kaikoura formation).

At Linton home station, two distinct seams can be made out, but the thickness of neither is known. The lower seam is overlaid by micaceous sandstone like that at Taylor’s Creek, and the upper one is in the sandstone. This latter has a very uneven floor. The dip here is 25° S.S.W.

The whole dipping 25° W. b. S.

Higher up the same creek the coal is found in several places covered by the same micaceous sandstone as at Taylor’s Creek and Linton, the thickness is 10 feet, and the dip 25° S. W.; and at the west side of the Night-Cap the thickness is 11 feet, and the dip, according to Mr. J. R. Thomson, is 11° E.S.E.;^* and at Opio Creek, it is 3 feet thick, with a dip of 15° E, also according to Mr. Thomson.

A tolerably approximate estimate of the quantity of coal in this field cannot at present be given, but it certainly contains 100,000,000 tons.

Near the Morely Creek station, the coal was on fire for about a year in 1858-9, and about ten acres of it have been burnt out.

Orepuki Coal Field.—The Orepuki coal field is much smaller than the others. It occupies a part of the valley of the Waimeamea, which rises in the Longwood Range, and runs into the sea south of the Waiau. It forms a triangle about 3 miles in breadth and one in height, and has consequently an area of about 1½ square miles. The coal seam is only exposed in water-races, some of which have fallen in, and in no place was I able to measure its thickness. It is, however, evident that this is considerable, and, according to the miners, it is from ten to fifteen feet. The coal is situated at about a height of 150 or 200 feet above the sea, and about five miles from the beach. It is covered by dark, very tough shales, and the floor is composed of chocolate coloured shales. The upper part of the formation is soft, green sandstone, and the dip is 10° S.S.W.

Near the beach the coal formation has been washed away, and a thick deposit of sands and gravels of pleistocene age has been deposited in its place. These pleistocene beds also contain seams of lignite, which are exposed in the sea cliff, and which have nothing to do with the brown coal formation. This coal field is about 20 or 25 miles from Riverton. The following is an analysis of the coal made by Dr. Hector:—

Fig. 25.—Coal at Orepuke: a, soft sandstone and clay with lignite (Pleistocene); b, coal formation (Oamaru formation); c, slates (Kaikoura formation).

Water	19.10
Fined Carbon	41.10
Hydro-Carbon	37.20
Ash	2.60

Preservation Inlet Coal Field.—The formation in which this coa field is situated occurs quite at the south west corner of Otago. Owing to its inaccessible position, it has not yet been properly explored, but as well as I could judge from the steamer, appears to commence at the Green Islets, and to extend continuously to Preservation Inlet. It occurs also at Coal Island; at the point between Preservation and Chalky Inlets, and on Chalky Island, forming a strip of low hill, thickly covered by bush, about 13 miles in length by four in breadth at Preservation Inlet, and tapering to a point at Green Islets. Omitting all north west of Preservation Inlet, the area thus mentioned would be about 26 square miles. It is, however, only on the east side of Preservation Inlet that a workable coal seam has been found at present. Here two good seams, each about three feet in thickness, are found between Otago’s Retreat and Puysegur Point. The first of these is just round the point opposite the south end of Coal Island; a road has been made to it from Otago’s Retreat, and the seam has been opened by a small drive. The seam here dips 25° W. (Fig. 26.) The coal is a good brown coal, like that of the Clutha field, and contains a considerable quantity of resin. Above this are several other quite small seams. The other workable coal seam crops out on the shore about half-way between the last and Puysegur Point, and dips about 15° W.

Fig. 26.—Coal at Preservation Inlet: a, coal seam.

I was not able to examine personally the coal strata on Coal Island, but no workable seam has been found there. Dr. Hector reports that on Coal Island the only seam of coal he found dipped 15° E. in one place, and to the west a few hundred yards off. It was only four to nine inches thick, but the coal was of good quality, quite equal to the Sydney coal that he had on board. He also discovered another small seam, six to fourteen inches in thickness, at Price’s Beach, a small bay lying north east of Gulche’s Head; but the quality of coal in this seam was inferior to that on Coal Island. It would be useless to try to estimate the quantity of coal in this field until more is known about it.

This coal field was first mentioned in 1855 by Dr. C. Forbes, of H.M.S. Acheron; afterwards by Dr. Hector in 1863, and again in 1873; also by Mr. A. Johnstone in 1862. A company was formed in Dunedin to work it, and Captain Greig, late Harbour Master at the Bluff, went there to make a report. It was this party that formed the road from Otago’s Retreat to the first workable seam, and made a drive 99 feet in length into it. He informs me that the seam commenced with a thickness of two feet, increased to three feet three inches, and then decreased again to two feet four inches. He excavated 45 tons from this drive. This company has now abandoned the ground.