The Conditions of Deposition of Greymouth Coal
[Read before the Wellington Branch, Royal Society, August 8, 1946; received by the Editor, October 20, 1947; issued separately, May, 1949.]
The Late Cretaceous seams of the Greymouth Coalfield are thicker, more extensive, and more numerous at eight chief horizons. Six of these are believed to mark the trace, to and fro across the coalfield, of the alternately advancing and retreating shore of a persistent deformational fresh-water lake. The elongated lake basin continued to subside, but it was from time to time temporarily filled with waste worn by subaerial erosion from adjoining, repeatedly elevated areas. The highest horizon originated during a period of diastrophic calm when the region became part of the Cretaceous peneplain, and is succeeded by marine beds due to a transgression that heralded a phase of marine deposition lasting until mid-Oligocene.
Knowledge of the initial constituents of Greymouth coals is limited, but there is evidence that widely different types of plant debris characterise the various horizons.
A certain amount of carbonaceous material drifted into areas of accumulation, but most of the coal-forming substance was derived from generations of plants growing where the coal deposits are now found.
The Greymouth bituminous coalfield covers about seventy square miles on the West Coast of the South Island. It is important as being the only significant present source in New Zealand of low-sulphur bituminous coal, and as a producer of an exceptionally wide range of coal grades. It is geologically interesting in that the coal-bearing strata are locally of exceptional thickness, that the thicknesses of formations vary systematically, and that the various ranks of coal are distributed according to an orderly pattern.
The chief source of geological information is from Morgan, 1911. The present paper is largely drawn from the author's manuscript of parts of N.Z. Geol. Surv. Bull. 45, “The Greymouth Coalfield,” now in course of publication, reproduced here with the permission of the Director of the New Zealand Geological Survey. Attention will be confined to the conditions under which the coal-forming substance accumulated, and to the little that is known of the nature of that substance. The processes by which it was converted to various ranks of bituminous coal are not considered.
Manner of Occurrence.
Coal-measures from less than 50 ft. to more than 3,500 ft. thick rest unconformably upon unfossiliferous greywacke and argillite, presumably early Palaeozoic, and are overlain disconformably by a thick marine Tertiary sequence, the lowest member of which is Middle Eocene. The palaeobotany of the coal-measures has not advanced appreciably since von Ettingshausen (1887) described material sent
to him from Greymouth. On indirect evidence they are now assigned to the Late Cretaceous.
Individual coal beds range from a few square yards to a square mile in area, and from a fraction of an inch to more than 30 ft. in thickness. In cross-section some seams are lenticular, but the majority interdigitate at the margins with non-carbonaceous sediments so that in effect the coal strata grade laterally into stone.
At eight well-marked stratigraphic horizons, seams are more extensive, more closely spaced vertically and laterally, and generally thicker than elsewhere in the section. Five of these horizons have been mined, and they have yielded all but 1 ½ per cent. of the coal produced up to the present. The intervening strata are either entirely barren, or contain merely thin and irregular seams.
The accompanying table shows the succession and grouping of Greymouth coal-measures now adopted, as well as the system of naming of the coal horizons.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Upper Paparoa||Dunollie coal-measures Goldlight mudstone||Duuollie (Lower Dunollie* )|
|Middle Paparoa||Rewanui coal-measures||Upper Rewanui Lower Rewanui|
|Lower Paparoa||Waiomo mudstone Morgan coal-measures and volcanics Ford siltstone Jay coal-measures and basal breccia * Not mined.||Morgan (Lower Morgan*) (Jay*)|
The following summary of events during the coal-forming period, inferred from the lithology and thickness-distribution of the formations, is given as the background for an attempt to picture the conditions under which the coal-forming substance accumulated. In the late Cretaceous, the Greymouth area was part of a terrain of Palaeozoic rocks having considerable relief. The nearest sea-coast at that time is thought to have been some scores of miles to the east, but the extent of land to the west is unknown. Before erosion could destroy the relief, the area was depressed and partially submerged beneath a fresh-water lake or lakes, in which fine silts (Ford siltstone) accumulated above the fanglomerate and alluvial deposits of the submerged valleys (Jay formation). A ridge rose above lake-level along the east side of the area, and waste from it spread westwards, across the lake basin, completely obliterating the old topography (Morgan formation). At the same time. basaltic lava was locally extruded.
The basin continued slowly to subside, so that after a time fresh water re-invaded it and deposited another bed of fine, even, dark mud and silt (Waiomo mudstone), but was again expelled by floods of gravel and sand (Rewanui formation) eroded from newly elevated
[Footnote] * In a palaeographic map, Benson (1923) shows land extending right acress the Tasman Sea.
granite mountains west of the present coast. As the mountains were lowered by erosion, the supply of waste to the basin declined once more, and as it continued to deepen, the fresh-water lake re-formed. Deposits of silt and mud in this lake (Goldlight mudstone) attained a thickness of 600 ft. The maximum thickness of this and the succeeding terrestrial and marine formations, deposited during portions of three geological periods, all lie within a belt 0·5 to 1 5 miles wide, oriented about north-north-west, marking a miniature geosyncline that persisted until Oligocene times.
A slackening of the rate of subsidence of the lake basin resulted in its becoming filled with well-bedded sands and siliceous and aluminous clays (Dunollie formation). The tempo of events was becoming slower and these sediments were produced, transported, and deposited more and more slowly from the denuded stumps of the western mountains. The basin at this stage is visualised as an extensive shallow lake or swamp. A temporary sharp local uplift along the east side of the lake exposed to erosion the lower members of the basin beds near its east margin, and a strip of basement greywacke beyond, so that a quantity of coarse, imperfectly rounded greywacke waste was discharged into the basin from the east near the south end of the area. At points from forty to fifty miles to the north, more intense movements occurred at this stage, and angular unconformity interrupts the succession.
Following this temporary disturbance, all relief was practically climinated during a long period of comparative diastrophic calm. The area at this stage was part of a peneplain that probably extended over all the South Island at least.* A greatly reduced supply of erosion products, thoroughly leached, rich in quartz, and many times resorted, continued to accumulate slowly in the basin. The adjoining areas bore a veneer of the end-products, mainly quartz, of prolonged chemical weathering.
The final event with which we are at present concerned was a transgression by mid-Eocene seas, possibly from the east, so that the terrestrial beds are succeeded by littoral sediments (Island sandstone). Differential subsidence of the earlier lake basin was renewed, and probably was most rapid while the upper Eocene Kaiata siltstone accumulated thickly in it. In late Oligocene, the geosynclinal phase came to a close. The contents of the tectonic furrow were compressed, clevated, and partly eroded. Thereafter, a ridge persistently tended to rise on the site of the geosyncline; but it was marginally or completely flooded by mid-Miocene sea, and at the same time a new trough formed alongside and to the east of the first. Recurring upward pulses of the anticlinal ridge culminated at the end of the Pliocene in the orogenic spasm that gave the main elevation to the present-day fectonic mountains.
Composition of the Coals.
Before the genesis of the coal deposits is examined in the light of the foregoing historic account, it is desirable to mention the existing information, or rather, the deficiency of information, regarding the materials constituting the Greymouth coals. It is mostly a matter
[Footnote] * “Cretaceous peneplain”; Benson. 1935.
of inference from chemical analyses, macroscopic examinations, and the lithology of the containing sediments, for there is regrettably little direct knowledge.
Penseler (1933) studied in microsection samples of the James Seam, an extreme type of coal from the Brunner formation, showing it to be made up largely of plant cuticle fragments in a groundmass derived from cell contents. Fragments of vascular tissue, including some recognised as from a fern, were also present. Apart from this, nothing has been published regarding the microstructure of Greymouth coals. A few coals were sectioned in the laboratory of the Coal Survey Division of the Dominion Laboratory, but the proposed comprehensive study of all types was abandoned during the war.
Hundreds of proximate analyses of Greymouth coals, representing all known samples for which reasonably accurate localities are available, have been assembled and studied in relation to their geological setting by Mr. H. W. Wellman, of the New Zealand Geological Survey. He shows that accumulations of different types of vegetal debris gave rise to fundamentally different types of coal related to stratigraphic position, and that these types may be traced through successive metamorphic stages until the original composition differences disappear at low-volatile bituminous rank. *
Mr. M. Te Punga, of Victoria University College, Wellington, has recently examined preparations of a few Greymouth coals intended to show spores and pollen grains. He informed me that they contain an interesting range of pollens, suggesting that with further study characteristic pollen associations with some stratigraphic value might be recognized and that even identification of certain plant genera might be possible. It is to be hoped that Mr. Te Punga will have the opportunity to follow up his initial work with a comprehensive study of the fossil pollens of our coal-measures.
Conditions of Coal Accumulation.
In the lower groups, coal beds are prominent in the section near changes of facies, that is, near the boundaries between lake and fluvial sediments. The coal beds of the horizons, up to and including the Lower Dunollie, are thus thought to have accumulated on the threshold of transgressions or recessions of lake waters (Fig. 1). In other words, the continuous coal bed or series of adjoining coal lenses making up each horizon marks the trace of a migrating lake-fringing zone in which plant growth was especially vigorous and preservation of falling plant debris particularly favoured. During lake recessions, the zone travelled with the advancing shoreline, carbonaceous sediment spreading over lake-bed muds, and in turn being covered by growing deltas. Deposition of each individual peat bed terminated when it became smothered with alluvium, or eroded by streams. Conversely, when the area of the lake was increasing owing to unfavourable balance of sediment-supply compared with basin-deepening, the peat zone advanced over earlier lake-shore delta beds, and was in turn covered by lake silts and muds. Peat accumulation ceased at any place as soon as the water became too deep for vigorous growth of marginal marsh
[Footnote] * An account of this investigation is included in N.Z. Geol. Surv. Bull. 45 (in press).
vegetation. Many fluctuations were no doubt superimposed upon the general trend of shoreline migration, for the changes of facies are in most places a transition marked by alternating lake and fluvial beds, and including from two to five prominent coal seams. The most abrupt boundary is that above the Morgan coal-measures, marked over a wide area by a single thick coal seam directly overlain by Waiomo mudstone.
Fig. 1—Diagrammatic column of Greymouth coal-measures showing position of main coal horizons in relation to changes of facies. (A, Jay; B, Lower Morgan; C, Morgan; D, Lower Rewanui: E, Upper Rewanui; F, Lower Dunollie; G, Dunollie; H, Brunner.
Irregular coal seams in the middle of the Morgan and Rewanui formations are truly terrestrial, being formed of vegetal matter that
collected in bodies of water impounded between alluvial fans or locked in deserted stream courses. Many of these have eroded roofs, ascribed to erosion by streams reverting to earlier channels.
The Dunollie horizon and the thin seams in the middle portion of the Dunollie formation are composed of the organic deposits of extensive swamps or shallow impermanent lakes. Thick coal beds are limited to one small area, and their existence at all under the changed conditions of Dunollie times is not explained. The lithology of the enclosing sediments indicates that time taken in transporting waste was greater, and the rate of depression of the basin was slower, than during the period of formation of the lower Paparoa groups. The rocks are thinly bedded and accumulated slowly, and the persistence at one locality of conditions favouring the growth of a thick peat bed was exceptional. Dunollie coal has, rank for rank, the lowest content of volatile hydrocarbons, and is distinguished by an abundance of bright-coal layers up to half-an-inch thick. This can be explained by derivation from a different type of vegetation in which the larger woody plants were prominent, perhaps as a result of a greater degree of stability of the landscape than had prevailed earlier.
The highly specialised Brunner sediments were resorted many times during a long period with a minimum of transportation or addition of fresh material. The final resorting of the uppermost layers was done by waves and currents during the marine transgression that brought the coal-forming period to a close; but except where the formation is very thin, the bulk of it is a truly terrestrial deposit. The supply of sediment to the basin was insufficient to compensate even for the continued very slow subsidence, so that, although most of the Greymouth coalfield area was probably a swampy plain, the neighbourhood of the axis of submergence was for a long time the site of a permanent body of water, in which accumulated the matter composing the thick and extensive Brunner Seam. The soil on land areas would inevitably have been deeply leached, and there would have been mineral deficiencies requiring a special plant ecology adapted to the Brunner environment, and we do indeed find that Brunner coals are of an extreme type, in many places approaching cannel. What is not explained, however, is why they should constitute the opposite extreme from the Dunollie type. The conditions of their deposition differed from those of the lower Paparoa coals in some important respects. The Dunollie conditions had differed in the same respects, but to a smaller degree. If the Brunner environment was mainly an intensification of the peculiarities of the Dunollie environment compared with lower Paparoa times, why, then, are Brunner and Dunollie coals opposite extremes, on either side of a mean represented by lower Paparoa coals? Advances in our knowledge of the coal floras, perhaps most hopefully from pollens, may throw light on this problem.
The high sulphur content of Brunner coal has been taken as an indication that it accumulated in lagoons connected with the sea; but, although marine beds in places closely overlie coal, a small thickness of Brunner beds may represent a considerable lapse of geologic time. An alternative hypothesis to explain the stratigraphic variation of sulphur content recently developed by Wellman (to be presented
in N.Z. Geol. Surv. Bull. 45 (in press) regard the transgressing mid-Eocene sea as the source of the inorganic sulphur, and explains the depth-variation as the result of a balance between the rate of downward diffusion of a sulphate-bearing sea-water beneath the sea-floor and the rate of upbuilding of the sea-floor by sedimentation.
Thickness of Seams.
To form ultimately a 30 ft. seam of bituminous coal, considerably more than 100 ft. of original material must accumulate. It is hard to see how such a thickness of relatively pure carbonaceous sediment could accumulate without a progressive rise of water-level, sufficient to preserve the debris from oxidation by the atmosphere, but slow enough for the upbuilding of the peat bed to keep pace and so prevent drowning of the vegetation. Yet deposits thick enough to form important coal seams accumulated at times of delta advance, when the floor of the lake basin was being built up by sedimentation faster than the rate of tectonic subsidence. It may be concluded, therefore, that the rate of outgrowth of the peat-swamp fringe from the lake shore was comparatively rapid, and sufficient to enable it to keep in advance of the delta. It would normally extend from the shore until checked by reaching the maximum depth of water favourable for plant growth. Peat would accumulate until the deposit was buried beneath the rock debris of the next phase of delta advance. Water-level is recognised as a vital factor, but in some circumstances this need not be the level of the lake surface. In a marginal peat-swamp, the water-holding properties of peat maintains the water-level suspended above local base-level if the rainfall is regular and ample.
The low content of ash in seams of the chief horizons is ascribed to the filtering action of living swamp vegetation, preventing current-borne sediment from penetrating within the swamp fringe. Dirt or shale layers in the coal mark incursions of sediment-laden water during heavy floods, when perhaps the vegetable screen had been breached or overtopped.
A well-marked minimum value of ash-content represents the mineral content of the plant tissue in pure organic sediment. Many seams or portions of seams of exceptional purity have been mined, and it is thought that successive generations of vegetation grew upon the remains of earlier deceased plants, building up peat beds without incorporating extraneous inorganic matter. The plants would have abstracted the mineral matter needed for growth from minute quantities in solution in the swamp water, or from the substratum of plant remains.
“Chuckie-stones” are not known from Dunollie coals, or from Brunner except along the west side of the field. In the other formations, they are common where the containing beds are largely composed of conglomerate. The stones are usually well-rounded, scattered irregularly through the coal, and not necessarily associated with bands of shale or dirt. In places, they are sparsely distributed in distinct bands at uniform distances from roof or floor. The favoured explanation is that they were rafted into the swamps at times of comparatively high water-level, attached to logs or roots of floating trees.
The floor shale underlying most Greymouth seams is part of the coal accumulation itself. The proportion of impurity increases rapidly near the floor, so that there is in effect a transition from coal to stone within an inch or so. Such floor clays are not true underclays, which at Greymouth are known only from the Dunollie and Brunner horizons. Structures identified as fossil roots of herbaceous plants appear in some clays, and the roots of woody plants have been recognized in the clay beneath the James Seam (Brunner formation).
The coal miner uses the term “roll” for ridges of the roof or floor that project into the seam, as well as for small faults and monoclinal folds. Neglecting those of deformational origin, “floor-rolls” may be due to overlap of the coal upon pre-existing ridges of basement rock, as in the Blackball Mine, but “roof-rolls” generally represent fillings of stream channels eroded in the upper surface of the carbonaceous beds, perhaps soon after deposition. Roof protrusions, roughly circular in plan, known as “blisters,” occur in mines at Brunnerton. These appear to be the fillings of stream-bed potholes worn in the upper surface of the peat.
Growth-in-situ versus Drift.
It is necessary to refer to this ancient controversy because in the most recent geological account of the Greymouth coalfield, Morgan (1911) claims that the field evidence favours the drift theory. Both processes undoubtedly have contributed. Chuckie-stones and rounded Jumps of resin in the coal indicate that some material drifted in but it is difficult to explain by drift alone how over 100 ft. thickness of pure organic sediment could have accumulated near land masses of appreciable relief unless the greater part was formed of the remains of many successive generations of plant that once grew where the coal deposits are now found.
Benson, W. N., 1923. Palaeozoic and Mesozoic Seas in Australasia. Trans. N.Z. Inst., vol. 54, p. 51.
—— 1935. Some Land Forms in Southern New Zealand. Austr. Geogr., vol. 2, no. 7, pp. 3–23.
Ettingshausen, E. von, 1887. Beitrage zur Kenntniss der fossilien Flora Neuseelands. Denkschr. Math. Naturv. Cl. Kais. Acad. Wissensch. (Vienna), Bd. liii. Translation by C. Juhl in Trans. N.Z. Inst., vol. 23, pp. 237–310.
Morgan, P. G., 1911. The Geology of the Greymouth Subdivision, North Westland. N.Z. Geol. Surv. Bull. 13.
Penseler, W. H., 1933. The James Coal of New Zealand. Fuel, vol. 12, no. 5, pp. 166–181.