Art. XXXV.—Unconformities in the Stratified Rocks of the West Coast of the South Island.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 46, 1913, Page 270

Art. XXXV.—Unconformities in the Stratified Rocks of the West Coast of the South Island. By P. G. Morgan [Read before the Wellington Philosophical Society, 22nd October, 1913.] Introduction. The area in which occur the unconformities described in this paper lies to the westward of the Southern Alps, and is in shape a drawn-out triangle having its base to the north and its extremely acute apex to the south. Both by reason of its physical isolation and because of other peculiarities this region forms a geological unit, the history of which may to some extent be considered separately from that of other parts of New Zealand. Since only a comparatively small portion of the area has been surveyed in detail, no complete statement regarding the unconformities that occur in the stratigraphical succession can yet be made, and the present account is confined to those that have been observed during the geological surveys of the Westport district and of that part of North Westland which lies to the west of the Southern Alps. The order of succession in the sedimentary rocks of these districts is shown by the following table:— Formation or Series. Subdivisions. Approximate Age. Marine gravels and sands Recent and Pleistocene Fluviatile gravels Recent and Pleistocens. Glacial gravels Unconformity. Moutere Gravels Pliocene. Upper—Blue Bottom, &c. Oamaru series Middle cobden Limestone Port Elizabeth beds} Miocene. Lower—Omotumotu beds Unconformity. Bituminous-coal measures (Mawheranui or Waima-ngaroa series) (Kaiata beds (including Islands Sandstone) Brunner beds Eocene. Paparoa beds; Hawk's Crag and Brighton breccias Great Unconformity. Aorere (Greenland)series Ordovician.* The Greenland series of New Zealand Geological Survey Bulletins Nos. 6 and 13 is here provisionally correlated with the Aorere series of Bulletins Nos. 3 and 11. Decisive proof of this correlation has not been obtained, but the age of the rocks in question does not materially affect any statement in this paper.

I. The Post-Miocene Unconformity. The first unconformity shown in the table of formations is typically a comparatively short break between Pliocene and Pleistocene, and represents a time during which the whole westerly part of the South Island was being rapidly uplifted. Elevation, however, had begun at an earlier epoch—probably before the close of the Miocene-and with the exception of several periods of rest and one well-marked period of depression (near the end of the Pleistocene) has continued till Recent times. Moreover, besides being spasmodic, elevation has been differential in the extreme, block-tilting and faulting being prevalent, so that whilst some areas—for example, the Paparoa and Brunner Ranges—were probably uplifted 5,000 ft. or even more,* The total uplift since the Eocene is known to have reached 6,500 ft. in places, but some of this may be ascribed to pre-Miocene elevation: see a later page. If there is no unconformity between Eocene and Miocene, the maximum uplift approaches 10,000 ft. others remained almost stationary. These differential movements are roughly reflected by the varying extent of unconformity. Thus in many places Recent gravels rest on Upper Miocene claystones (Blue Bottom), or on still older rocks. At Cape Foulwind horizontal Pleistocene sandstone of marine origin overlies an eroded surface of gently dipping Miocene rocks. In various parts of the Greymouth district unconformity between Moutere or Old Man gravels of Pliocene age and Pleistocene gravels is generally indicated merely by the latter resting on an eroded surface of the former without any noticeable discordance of dip. The plane of contact is usually marked by a layer of large stones, evidently residual from the denuded portion of the Moutere gravels. On the other hand, in a few places a decided discordance of dip may be seen, the most notable instance being at Healy's Gully, north of Blackball, where nearly horizontal Pleistocene gravels rest on steeply dipping Pliocene beds, which are involved in a great fault.† P. G. Morgan: “Geology of the Greymouth Subdivision, North Westland,” Bull. No. 13 (n.s.), N.Z. Geol. Surv., 1911, p. 74. II. The Post-Eocene Unconformity. The unconformity between the bituminous - coal measures of Eocene age and the Miocene or Oamaru rocks presents peculiar features, and will therefore be fully discussed. It was announced by the writer in 1909,‡ Third Annual Report of N.Z. Geol. Surv., Parliamentary paper C.–9, 1909, p. 13. (See also Bull. No. 13 (n.s.), 1911, pp. 66–66, and earlier reports by McKay cited on later pages.) but since then its existence has been questioned by Marshall,§ “New Zealand and Adjacent Islands” (reprinted from “Handbuch der regionalen Geologic”), Heidelberg, 1912, p. 68. (See also “The Younger Rock-series of New Zealand,” by Marshall, Speight, and Cotton, Trans. N.Z. Inst., vol. 43, 1911, pp. 392–93.) partly on the ground that no direct stratigraphical evidence of unconformity has been discovered, and partly for other reasons. The problem is one of great importance, particularly from an economic point of view, for its solution bears directly upon the question of boring for bituminous coal beneath Oamaru rocks near Westport and elsewhere. The evidence for the existence of a post-Eocene unconformity, including data recently obtained, may be classified as follows:— A. The Miocene rocks contain water-worn pieces of coal and carbonaceous shale almost certainly derived from the bituminous-coal measures.

B. In one place, there is discordance in dip between Miocene and Eocene rocks. C. In various localities Miocene strata are in apparently unconformable contact with Eocene strata. D. The Miocene rocks exhibit a strong overlap on the pre-Tertiary land-surface. A. Water-worn Coal, &c., in Miocene Rocks. The chief evidence for unconformity between the Eocene and the Miocene adduced by the writer in 1909 and 1911 consisted in the presence of waterworn coal in the Omotumotu beds near Greymouth. In the valleys of Kaiata and Omotumotu Creeks, over an area of several square miles, are found mudstones, sandstones, and grits containing innumerable grains and lumps of coal.* N.Z. Geol. Surv. Bull. No. 13 (n.s.), 1911, pp. 65–66. These rocks were originally discovered by McKay in 1873, and were described by him in the following terms: “Much of the sandstone [loose blocks in a small creek, probably a branch of the Omotumotu] contained streaks of coaly matter, sometimes as a coal sand, and at other times being rough enough to be called a conglomerate, some pieces thus included being 1½ in. diameter. I went up the creek, and some distance up found beds of this sandstone, and, in one place, the bed whence this coal conglomerate comes. At this point where I observed it it was not more than 6 in. thick, and the coal pebbles small. I, however, saw blocks in the creek-bed that would prove that this coal conglomerate sometimes reached a thickness of a foot. Other beds of the sandstone became coaly, so as to give it a dark colour and a flaggy appearance, but nowhere, except in the one particular bed, did it approach the conditions of a conglomerate…. The coal conglomerate and sandstone beds lie all along the western face of the range to the east of Omotumotu Creek.”† “Reports relative to Collections of Fossils made on the West Coast District, South Island.” Rep. Geol. Surv. during 1873–74, No. 8, 1877, pp. 78–79. In 1900 McKay examined a branch of Kaiata Creek in which coal-seams had been reported to occur, and as a result of his observations stated, “The carbonaceous deposit where examined is about 2 ft. in thickness, and consists of rolled pieces of coal, the largest of which are about 6 in. in diameter, the lesser a fine gravel passing into grit and carbonaceous mud, mixed in varying proportions with arenaceous” sands.”Dagger; “Report on Supposed Coal-seams in Kaiata Range, Greymouth.” Mines Report, 1901, Parliamentary paper C.–10, p. 7. In both of the reports quoted McKay regarded the presence of waterworn coal in the Omotumotu beds as proof of unconformity with the bituminous-coal measures. In the earlier report McKay correctly placed the Omotumotu beds below the Cobden limestone, but in 1901 he reversed his opinion as to their relative position, thus avoiding the necessity of having to admit an unconformity in the Cretaceo-tertiary formation.§ Op cit., p. 8. (See also N.Z. Geol. Surv. Bull. No. 13 (n. s.), 1911, p. 63.) In the Westport district the lower Miocene beds of two localities contain pebbles of water-worn coal and of carbonaceous shale apparently derived from the bituminous-coal measures. In February, 1911, when traversing Hodge's Creek, a small stream entering the Mokihinui River near Seddonville, the writer and Dr. J. Henderson found a few water-worn coaly fragments in a sandy mudstone which is probably of estuarine origin, and lies near the base of the Miocene strata as developed in the Mokihinui district.

Again, in the Lower Buller Valley for some miles eastward from Hawk's Crag, the conglomerates, grits, and sandstones of the lowest Miocene horizon contain numerous pebbles and grains of coal and carbonaceous shale. In places carbonized fragments of wood, some of which are more or less rounded, are present, so that caution is necessary in considering the origin of individual pieces of coal-like material. There need, however, be no doubt but that the pebbles of carbonaceous shale and most of the coaly fragments are derived from coal-bearing strata, and, unless evidence to the contrary can be obtained, the natural conclusion is that the bituminouscoal measures are the source. The following analyses show the composition of the fragmental coal in the Miocene rocks:— — 1. 2. 3. 4. 5. 6. Fixed carbon 40·70 35·58 39·33 41·96 17·37 29·08 Volatile matter 46·61 47·28 49·00 46·19 29·38 10·82 Water 7·37 6·06 8·15 8·10 4·55 0·66 Ash 6·32 12·08 3·52 3·75 48·70 59·44 100·00 100·00 100·00 100·00 100·00 100·00 Total sulphur 0·96 1·46 0·91 0·21 Specific gravity 1·36 1 and 2. Omutumotu Ridge. Samples collected by Geological Survey (probably by A. McKay in December, 1873). Analyses by W. Skey (Lab. Rep. No. 10, 1875, pp. 9–10). 3. Upper part of Kaiata Creek. 4 and 5. Hodge's Creek. Bright coal and impure coal. 6. Pebbles in conglomerate, close to Blackwater Bridge, Buller Gorge Road. (Analyses 3–6 by Dr. J. S. Maclaurin and staff, Dominion Laboratory.) From these analyses it appears that the samples, with the exception of that from the Buller Gorge, have a fairly uniform composition. Volatile matter predominates over fixed carbon, and moisture in the purer material averages a little over 7 per cent. The pebbles from Blackwater Bridge consisted of very impure material, and were taken from porous conglomerate, so that weathering may be wholly or in part the cause of the low volatile content. Unfortunately, no other sample from the Buller Gorge has been analysed, and therefore it is not certain that the analysis given is fairly representative of the fragmental coal in that locality. There are Eocene coal-seams in the Greymouth district similar in composition to samples 1–4, and the more altered portions of the Miocene coalseams in the Inangahua district also exhibit some resemblance. The latter, however, cannot have been the source of the carbonaceous fragments in the Buller Gorge conglomerates, which are, without much, doubt, in a lower horizon. To this statement the thin seams near Hawk's Crag are an exception, since they occur interbedded with the coal-pebble conglomerates. In general the Oamaru coal-seams are much higher in water than the carbonaceous fragments, and this furnishes another reason why the latter are not likely to have been derived from the former. On the other hand, at the beginning of the Miocene period the Eocene coal-seams had been in existence for a long time, and, according to the writer's belief, had been uplifted or otherwise associated with earth-movements, so that their

chemical composition would probably be much the same as that of the Oamaru brown coals of to-day. During the period that has elapsed since the Miocene, further chemical change has doubtless taken place both in the Eocene coal-seams and in the pebbles derived therefrom. The extracts from McKay's reports of 1877 and 1901, as well as the writer's own observations, completely dispose of the suggestion that “the coal fragments were wood when entombed in the mud and… have been changed into coal in situ.”* Marshall, op. cit., p. 68. It is true, as already mentioned, that fragments of carbonized wood are associated with coal pebbles in the Buller Gorge conglomerates, but these may nearly always be distinguished from the true coal by their shape, by their more rotten character, and in most cases by distinct traces of woody structure. Marshall has also suggested contemporaneous erosion of neighbouring coal-seams as a possible cause for the presence of water-worn coal in the Lower Oamaru beds. It has, however, already been stated that the coalpebble conglomerates of the Buller Gorge are older than the chief Miocene coal-seams, and that the water-worn coal does not agree in composition with them. There is, moreover, no independent evidence of contemporaneous erosion of Oamaru coal-seams, and in any case, at the time when the Omotumotu beds were formed, it is not likely that any Miocene accumulations of vegetable matter were consolidated to a coal sufficiently firm and free from moisture to withstand ordinary erosion and transportation by water without complete disintegration. Before Eocene coal-seams could be eroded, faulting or folding movements must have elevated portions of the bituminous-coal measures near Greymouth at least 2,000 ft., and more probably 3,000 ft. At Seddon ville an elevation of 1,500 ft. might have sufficed. Of an inter-Eocene movement on this scale there is no trace. Certainly there was no defined folding; and though it is just possible that elevation of part of the bituminouscoal measures may have taken place whilst the remainder continued to subside, and was conformably covered by Miocene strata, it is much more likely that the whole area on the west coast of the South Island containing Eocene strata was more or less affected. In any case, it must be admitted that elevation extended from Greymouth to Seddonville, and for some distance north and south of those points. It would then follow that the Miocene rocks in that area rest unconformably upon the Eocene. B. Discordance in Dip between Eocene and Miocene Rocks. Ordinarily, discordance of dip between the Eocene and the Miocene rocks cannot be proved, and it is evident that no marked folding of the bituminous-coal measures took place prior to the Miocene in areas where beds of the latter age are now present. In all probability, however, there was block-faulting similar to the faulting that has taken place since the Miocene, and this may have been accompanied by tilting, with the production of relatively steep dips near the fault-planes. Such tilting may be responsible for a discordance in dip observed at Brighton, thirty miles by road south of Westport, where Miocene strata, containing a seam of lignite or brown coal near their base, rest on a breccia or breccia-conglomerate similar to the Hawk's Crag breccia in the Buller Gorge. McKay states that the relations of the lignite beds and the breccia are decidedly unconormable,

since the former a short distance up the Fox River are nearly horizontal, whilst the breccia in the same locality dips at 35° to the east-north-east*“Reports relative to Collections of Fossils made on the West Coast District, South Island.” Rep. Geol. Surv. during 1873–74, No. 8, 1877, p. 109. The unconformity was also observed by J. A. Bartrum in December, 1912.† Personal communioation. Since the Hawk's Crag breccia is a slope deposit, it may be maintained that its dip at Brighton is partly or wholly original, and not due to pre-Miocene earth-movement. Even were this admitted, unconformity would still be deducible from the absence of the other members of the bituminous-coal measures. C. Apparently Unconformable Contacts of Miocene and Eocene Strata. If it could be clearly shown that Miocene rocks were anywhere deposited on the middle or lower beds of the bituminous-coal measures, or that the upper part of the Miocene in any locality rests on any part of the bituminous-coal measures, unconformity would in ordinary cases be considered as proved. Such unconformable contacts seem to occur in a number of places, but in most cases faulting introduces an element of doubt. Near the head of Patten's Creek, about two miles west of Seddonville, steeply dipping sandstone, probably of Upper Miocene age, is in contact with Brunner beds. On account of strong faulting here present, unconformity cannot be considered as proved, but the entire absence of the Kaiata beds from the section is significant. On the road to Denniston, not far from Waimangaroa, sandstone, which contains, fossils of Upper Oamaru (Blue Bottom) age, is in contact with Kaiata mudstone. The strata, owing to involvement in the great fault zone (Lower Buller or Kongahu fault) at the western base of the Papahaua and Paparoa Ranges, are dipping almost vertically, but only a slight movement is detectable at the actual contact, which therefore practically represents a plane of deposition. Between Waimangaroa and the Buller River, Upper Miocene beds are found in several places close to Eocene strata, but no actual contact has been seen, and in each case the presence of the Kongahu fault renders interpretation doubtful. For this latter reason uncertainty also applies to a contact of bituminous-coal measures with Upper Miocene sandstone, seen at Moran's water-race, two or three miles north-east of Addison's. At Kotuku, twenty-one miles from Greymouth, below Oamaru strata occurs a conglomerate which lithologically resembles the Brunner conglomerate, and rests upon Aorere rocks. It is quite an open question whether this rock was deposited at the base of the bituminous-coal measures or at the base of the Oamaru series.‡ See N.Z. Geol. Surv. Bull. No. 13 (n.s.), 1911, pp. 67, 138. In the former case there is unconformable contact. In the eastern part of the Lower Buller Gorge Miocene rocks are found resting on Hawk's Crag breccia, the lowest member of the bituminous-coal measures as developed in the Westport district. Near Hawk's Crag the contact is obscured by faulting, so that details cannot be ascertained. In the valley of Nada Creek, a tributary of the Blackwater River, contact of Miocene and Eocene is again accompanied by faulting, but at one spot in the bed of a small brook (Lily Creek) Miocene sandstone rests on an irregular, apparently eroded surface of breccia. The exposure, however, is so small that it must be deemed unsatisfactory, more particularly since the

identification of the breccia as the Hawk's Crag rock could be questioned. Elsewhere actual contacts have not been located, but the complete absence of the Brunner and the Kaiata beds from the Buller Gorge section in itself appears to prove unconformity. McKay in 1895 speaks of the brown-coal series resting on the breccia with some appearance of unconformity,*“Geology of the South-west Part of Nelson and the Northern Part of the West-land District.” Mines Report, 1895, Parliamentary paper C.–13, p. 7. and at an earlier date describes a probable unconformity in the rocks of the Buller Gorge.† “Reports relative to Collections of Fossils made on the West Coast District, South Island.” Rep. Geol. Surv. during 1873–74, No. 8, 1877, pp. 103, 104, 105. Apparently, however, he supposes that the stratigraphical break is immediately below the brown-coal horizon, and therefore above the conglomerates containing coal pebbles. It would seem that the Buller Gorge section in itself proves unconformity between Eocene and Miocene, unless it can be shown that the Hawk's Crag breccia does not belong to the bituminous-coal measures. The peculiar nature of the breccia, however, leaves a loophole for doubt. As already mentioned, it is a slope deposit, and was possibly formed far above sea-level, so that the contacts with Miocene rocks at Brighton and in the Buller Gorge may perhaps be cases of overlap rather than indications of true unconformity. D. Overlap of Miocene Rocks on the Pre-Tertiary Land-surface. In many places the Oamaru rocks of the west coast of the South Island are found to rest on the pre-Tertiary land-surface at points only a short distance from bituminous-coal measures. Thus at Kongahu Point and for some miles southward they rest on granite, at Cape Foulwind and Charleston on gneiss, and at Inangahua Junction on greywacke and argillite penetrated by granitic dykes. In Stillwater Creek Valley, at a spot about three miles south of Brunner, is a small outcrop of greywacke surrounded by Miocene rocks. At Kotuku, near Lake Brunner, boring for petroleum has shown that Miocene strata either rest on Aorere rocks or, as previously mentioned, are separated from the latter only by the basal conglomerate of the bituminous-coal measures. In the Collingwood district similar overlap of Miocene strata on pre-Tertiary rocks near bituminous-coal measures is found,‡ Park (“The Geology of New Zealand,” 1910, pp. 310–11) says that the upper coal-measures conformably follow the bituminous-coal measures, a statement which is of course, opposed to the trend of this paper. and probably the extension of detailed geological surveys through Central Nelson will furnish other examples. The deposition of Oamaru strata on a pre-Tertiary surface in localities many miles from known outcrops of the bituminous-coal measures may easily be explained as cases of overlap, and therefore it is not necessary to describe such occurrences. Since, however, the marine beds of the Eocene succession are from 1,500 ft. to 3,000 ft. in thickness, the overlap oi Miocene beds on a pre-Tertiary land-surface not far from areas of the bituminous-coal measures is strongly significant. Though in a fault-rent country like New Zealand strong overlaps do not necessarily prove an unconformity, they at least furnish confirmatory evidence of its existence. III. The Pre-Eocene Unonformity. Owing to the somewhat uncertain age of the Greenland series, the lower time-limit of the unconformity between these supposed Aorere rocks and

the Eocene coal-measures is doubtful; but, in any case, the interval of time represented is long. The general evident discordance in strike and dip, together with the great difference in lithological characters between the Palaeozoic and the early Tertiary rocks, renders recognition of the unconformity an easy matter. Probably–in fact, certainly—some deposition of sediments took place between the Ordovician and the Eocene in the North Westland and Westport districts; but of this there is no direct evidence, all traces of such beds having been removed by pre-Eocene erosion. At Reefton, however, near the districts discussed, Devonian strata are found, and Trias-Jura rocks appear on the eastern side of the Southern Alps. It is noteworthy that though over considerable areas the bituminous-coal measures were deposited on a nearly flat surface, yet in places the overlap of the higher beds on the pre-Tertiary old land is considerable. The nature of the basal conglomerate, and more especially of the Hawk's Crag breccia, is further evidence of an irregular land-surface at the beginning of the Eocene. Although the Kaiata beds, the uppermost member of the bituminous-coal measures, have nowhere been observed to thin out owing to overlap, yet in many places Eocene strata are absent, and Miocene beds are found resting on pre-Tertiary rocks. The question of whether this increased extent of the pre-Eocene unconformity is due wholly to overlap, or in part to erosion of the bituminous-coal measures, has already been discussed, and decided in favour of the latter supposition. Summary and Conclusions. The evidence of unconformity at two horizons, one pre-Tertiary and the other post-Miocene, is clear and unmistakable. In the case of a third unconformity—that between the Eocene and the Miocene—certainty is wanting, but the available data strongly favour a stratigraphical break. In view of the importance of the matter from an economic point of view, it may be as well roughly to summarize the evidence both for and against this pre-Miocene unconformity, and to leave the reader either to form his own conclusion or, if he pleases, to preserve an open mind. Unconformity is supported by the occurrence in Miocene rocks of water-worn pebbles of coal and of carbonaceous shale almost certainly derived from Eocene coal-measures; by marked discordance of dip between Miocene strata and Hawk's Crag breccia at Brighton; by a number of apparently unconform-able contacts between Eocene and Miocene strata; and by the strong overlap of the Miocene rocks on the pre-Tertiary land-surface. On the other hand, it is just possible that the coal pebbles are a product of contemporaneous erosion; that the discordance in dip between Miocene and Eocene rocks at Brighton has another explanation; that the apparently unconformable contacts between Eocene and Miocene strata can be explained by faulting or in some other way; and that the strong overlap of the Oamaru rocks on the pre-Tertiary land-surface is wholly due to the bold relief of the land at the beginning of the Eocene, and not at all to unconformity between two Tertiary formations. Though the evidence for a stratigraphical break may not be regarded as conclusive, it is mainly of a positive character, whilst the opposing evidence is largely negative. It is practically certain that considerable earth-movements took place after the formation of the bituminous-coal seams and before the deposition of the Oamaru series. The point in doubt

is whether these movements were so far concentrated at any time as to produce unconformity. The failure to obtain conclusive evidence may be attributed mainly to the rough bush-clad nature of the country explored, which rendered difficult any access to several localities where Eocene and Miocene strata are in contact, and where, therefore, important data might have been obtained. Careful examination of a wider area, however, may be expected to yield definite results.