NEW ZEALAND COAL MEASURES.

Otago Daily Times, Issue 19706, 5 February 1926, Page 3

NEW ZEALAND COAL MEASURES.

VARIOUS FIELDS REVIEWED A paper by Mr M. Ongley, of the New Zealand Geological Survey, placed before the Mining Conference yesterday had reference to sections of the coal measures on the kaitangata coalfield. The writer stated that coal was worked on the seashore at Coal Point, three miles south-east of Kaitangata, in 1858, and a smal coalmine was opened at Kaitangata in 186-1. After 1876, when railway communication with Dunedin was established, the Kaitangata mines were worked on a larger scale'. The oldest rocks of the south-eastern part of Otago were schists, supposed to be of early Palaeozoic age. They were followed by moderately fine-grained sedimentary rocks, generally of bluish-grey colour, which might appropriately be termed greywackes. After the deposition of the greywackes the region went through many vicissitudes, and at last, late in the Cretaceous period, thick gravel beds, formed mainly of greywacke and schist pebbles, together with thin, finer-grained beds and numerous layers of vegetable matter, were deposited on the much-eroded, near!}' level land surface of what was now the Kaitangata district. Other beds, later lost through erosion, might have followed. As time went on the gravels became conglomerates and the vegetable layers coalseams. A period of elevation, accompanied by tilting or folding and probably by faulting, came soon after. The elevated parts of the Kaitangata coal measures were naturally more or less worn away, but in due course of time a series of conglomerates, sandstones, and shales containing several thick layers of plant remains (now coal-seams) were deposited on the land surface. These beds formed the Taratu coal measures. Depression of the land was going on at this time, for a layer of sandstone in the uppei part of this series contained marine fossils of early Tertiary age. Further, other marine beds, now removed by erosion, were deposited above the fossiliferous sandstone. In places the Taratu coal measures lay unconformably (in the strict sense of that term) on the Kaitangata coal measures, in places “disconformably,” that was, without marked departure from parallelism. . , , Since the Eocene, geological changes ot various kinds had taken place in the south-east Otago region. The two sets of coal measures, in common with the underlying rocks, had been elevated, depressed, tilted, faulted, eroded, etc., and thus finally the present-day conditions indicated by the sections had been reached. At half a mile to nearly a mile from the north bank of the Molyncux several coal seams were seen. The quartz conglomerates of the Taratu coal measures next appeared here dipping conformably, or almost so, with the Kaitangata beds, and with the associated beds continued to the end of the section. Nortn-east of Caird Creek there were several small coal seams. At Coal Point, three miles from the beginning of the section, was a 4tt seam. Three-quarters of a mile north-east of Coal Point was a 15ft coal seam, followed by several smaller seams. Prom here to Measly Beach the strata, mainly conglomerates, were almost flat. Near the south end of the Tokomainro Plain, five miles south-west ot Milton, a fault with a considerable downthrow to the west brought the Taratu conglomerates, dipping 20 degrees west to the level of the plain. The coal was not exposed here, but was seen at Elhotvale, a mile to the south. East of the fault the Kaitangata or greywacke conglomerates appeared, and continued to Waronui, nearly four miles to the east. Throughout they seemed to dip almost uniformly 20 degrees west, but at Howard Stream the dip was less. On Mount Misery a patch of quartz conglomerate unconformably overlay the greywacke conglomerate. Near Howard Stream fine conglomerate, sandstone, shale, and very thin coal seams outcropped. At Waronui the greywacke conglomerate was intrxlded by a basalt neck. East of this the ground rose, and quartz conglomerate and sqnda containing a thick coal seam outcropped. These beds were part of the Taratu coal measures, and dipped gently south-eastward. HUNTLY COALFIELD. Dr J. Henderson (New Zealand Geological Survey) read a paper dealing with a section of the coal measures and associated beds in the western part of the Huntly coalfield. The writer stated that the Huntly coalfield was the most important in the North Island, and yielded an excellent brown coal largely used for domestic and industrial purposes. Coal was first mined in the neighbourhood of Huntly, and for many years all the coal was obtained from that locality, but now the principal collieries—namely, Rotowaro, Pukemiro, Glen Afton, and Waipa, were in the western part of the coalfield. Some years ago the Mines Department purchased an area of coalbearing land (Waikokowai) adjacent to the first three above-mentioned collieries, but this had not yet been worked. The coal measures were laid down on weathered Mesozoic rocks, which in places graded upward into the “fireclays.” In the part of the coalfield traversed by the section one workable seam was present which ranged up to 25ft, but was commonly from 10ft to 15ft thick. In some places the seam was thin or altogether absent, and in others it contained many dirt bands or was represented by carbonaceous shale, though close by thick, clean coal might occur. There was usually a few inches of “bone” coal or carbonaceous shale on the roof and floor of the scam. The bores in the Glen Afton and Waikokowai areas indicated that the underlying claystone ranged in thickness from 60ft to 120 ft, but, owing to the difficulty of separating the limestone from the claystone, the variation in thickness was probably more apparent than real. THE OHAI FIELD. In the course of a paper on the Ohai coalfield Messrs G. S. Langford and J. T. Mosley stated that the approximate area held by the existing coal-raining companies covered about four and a-half square miles. Besides the Black Diamond mine, which was working an extraordinarily thick seam, the companies in active operation were the Black Lion, Mossbank, Wairaki, Linton, Birchwood, and Ohai. On the western side was the Morley colliery, at present in the development stage. These mines were all situated in the Ohai Valley, and all the seams outcropped on the banks of the river. The present workings of the Mossbank Company were in the valley itself. Besides these there were three mines in the Orawia watershed, the Wairio, the old Mossbank (now closed down), and the New Brighton. At Nightcaps the old Nightcaps colliery was recently closed down through exhaustion after a profitable life of over 40 years. Professor Park reported that the field was traversed by a major fault following the course of the Ohai, and by many minor faults, running more or less at right angles to the Ohai Valley. The presence of the faults and the frequent change in dip were great hindrances to the systematic working of the mines. Other disabilities were the occurrence of stone backs and patches of muddy coal. Another form of impurity consisted of a network of foreign matter throughout the coal, which might be up to 2ft or more in thickness, usually thickest at the floor of the seam. In many places the coal was coated with a whitish clay, and where this happened it was of an, inferior quality, containing a very high proportion of ash. In one part of the field the whole of the Kosin seam had been burned out. The fire, in some places extended into the main seam, and where this happened the coal had been completely destroyed.

The coal was very strong, requiring little timber in the bords. The great thickness of the coal made the clean extraction of the pillars impossible, and, where there was a strong roof, grinding and crushing of the stumps was liable to cause gob fires.

The Ohai coal was a high-grade brown coal suitable for household and steam pur poses. Its great disadvantage, in common with all brown coals, was its rapid deterioration on exposure to the weather. Owing to the short grain of the coal and the rapid action of permitted ex plosives large quantities of dross were made, amounting usually to abour 33 per cent, of the gross output. This dross contained n fair percentage of good small coal, but the public were too sophisticated tv use small coal .and hydro-elect.rio power wa<? cutting out the smaller steam planrs which were formerly consumers of dross The consequence of this was that the general public was complaining about the cost of coal, while the coal companies, had thousands of tons of good fuel going to .waste. Tlie heaps of small coal, more-

over, formed a menace to health and property, as it was only a matter of time till they would fire* Through their representatives the people of Southland had not acted in the best interests of the province, for, from the waste coal of the Ohai district they could have derived cheap power until the population and industries of the province warranted the ambitious hydro-electric scheme now in progress. Professor Park stated that he knew the JJ.untly district very well, having made the first geological survey of it. In those days conditions were different, and there’ were no roads and no means of easy transport. He was glad to find that the results of Dr Henderson’s survey of the field coin cided with his own. He had much pleasure •in proposing a hearty vote of thanks to Dr Henderson. This was carried by acclamation. Mr P. G. Morgan congratulated Messrs Langford and Mosley on their paper, and said that he knew the coal from this field, and it was a good household coal. JJnfortunately, as the authors had stated, fully 33 per cent, went to waste in dross which was nobody’s fault. The calorific value of this slack was still high, but it was impossible to use it until some cheap method of briquetting had been found. The Ohai coal was a brown coal, the moisture in which precluded the using of pressure as a satisfactory means —a good binder was really ne-cessary. Ori the motion of Professor Park, a hearty vote was accorded the joint -authors of the paper.