Art. XXXIII.—The Geological History of the Westport-Charleston High-level Terraces.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 46, 1913, Page 255

Art. XXXIII.—The Geological History of the Westport-Charleston High-level Terraces. By J. A. Bartrum New Zealand Geological Survey. (By permission of the Director of the Geological Survey. [Read before the Wellington Philosophical Society, 22nd October, 1913.] Introduction The first physiographical feature in the neighbourhood of Westport to arrest the attention of an observer is the abruptness with which the mountains a few miles from the town rise from a comparatively low coastal strip to heights ranging between 4,000 ft. and 5,000 ft. The next noteworthy feature is the regularity and persistence of a prominent high-level terrace nestling not far from the steep lower slopes of the mountains. The general surface of this terrace is about 500. fc. above sea-level, or 250 ft.

above the general level of the great coastward expanse of more or less flat open or “pakihi” country which rises gradually from near sea-level to an altitude of approximately 250 ft. at the basc of the higher terraces near Addison's. McKay* “Geology of the South-west Part of Nelson and the Northern Part of the Westland District,” Parl. paper C.–13, 1895; and see also McKay and Gordon, “Mining Reserves, Westland and Nelson,” Parl. paper C.–9, 1896. alone of many writers upon the geology of the Westport district has given the elucidation of the geological history of these terraces and flats any serious attention, and it is with the hope of enlarging upon the foundation already laid by McKay that these notes have been written. Sketch of General Geology. The various geological formations encountered in the Westport–Charleston district, which forms the area discussed in this paper, in ascending order, are:— (a.) Folded argillites and greywackes of doubtful age, assigned by McKay† See particularly “On the Geology of the Reefton District, Inangahua County,” Rep. Geol. Expl., 1882, pp. 91–163. on evidence from the vicinity of Beefton to his Carboniferous “Maitai” formation, intruded by later granitic rocks and by quartzporphyries, the whole unconformably overlain by (b.) Bituminous coal-measures, commonly considered to be of Eocene age. (c.) Above the coal-measures are Miocene beds. As developed at Cape Foulwind and Charleston, where, as the writer believes, on account of overlap and not unconformity (a question dealt with later), they rest directly upon the old erosion surface of granite or gneiss, these consist of sandstones with shaly bands and with lignite seams, which attain a thickness of over 25 ft. at Charleston. They are followed by Cobden limestone and then by a great thickness of fine-grained blue marine sandstones. (d.) Unconformably overlying the uppermost Miocene blue sandstones, or “Blue Bottom” as the beds are commonly called on the West Coast, are alluvial deposits of gravel and sand, chiefly the former, constituting high-level terraces and beaches extending from sea-level to a height of over 500 ft. Such gravels have no internal evidence of age, beyond the fact that they are undecomposed, though since their formation they have always been above water-level. As, however, there is apparent conformity in the Greymouth‡ “The Geology of the Greymouth Subdivision, North Westland” (Morgan), N.Z. Geol. Surv. Bull. No. 13, 1911, p. 42. and Reefton§ Seventh Annual Report, N.Z. Geol. Surv. Department, Parl. paper C.–2, 1913, p. 119 (Henderson). districts between rotten gravels commonly regarded as Pliocene (“Moutere” or “Old Man” gravels) and the underlying “Blue Bottom,” it can safely be asserted that these are younger—viz., post-Pliocene to Recent. They consist in part of reassorted fluvioglacial gravels, and hence are at least in part post-Glacial. The question of conformity or unconformity between the Eocene coal-mleasures and the Miocene beds is much in debate; in the Westport, as in the Greymouth¶ See “The Geology of the Greymouth Subdivision, North Westland” (Morgan), N.Z. Geol. Surv. Bull. No. 13, 1911, pp. 42, 65. district, there is strong evidence of widespread unconformity in water-worn fragments of coal present in mudstones and sandstones immediately above the bituminous-coal measures. It is pro-

bable, however, that, in much the same manner as has been suggested by Mr. C. A. Cotton* “Geology of the South-west Part of Nelson and the Northern Part of the West-land District,” Parliamentary paper C.–13, 1895, p. 8. in explanation of similar conditions in the Tertiary beds of South Marlborough previously described as unconformity, warping allowed erosion of an upraised portion of the coal-measure beds, whilst deposition continued uninterruptedly in a depressed area. It is a significant fact that wherever both are developed the Miocene series has been found invariably to rest in perfect conformity upon the Eocene. Distribution of Geological Formations. The Westport–Gharleston district is governed topographically by two main earth-blocks, an upraised one forming the Paparoa Range and its northern continuation, and a relatively depressed coastal block forming the wide lowlands extending from the base of the upland block to the coast. The earth-fracture between these blocks determines the western flank of the Paparoa Range and of the elevated country to the north of the Buller River, and has been designated by McKay† Trig. Station U has an additional interest in that it represents a fossil monadnock of the pre-Eocene cycle of erosion. the “Lower Buller” fault. East of it, south of the Buller, granitic and gneissic rocks/and to the north older sedimentary rocks and various acid intrusives, capped by relatively thin Eocene coal-measures, form the mass of the elevated tract. Sections along Lines AB and CD of Locality Map. To the west of the great fault, which has been indicated upon the locality map, soft “Blue Bottom” sandstones extend below Quaternary gravels over almost the whole of the lowland block, and outerop wherever

the gravel cap is removed. At Cape Foulwind, and for many miles southwards from the mouth of the Waitakere River along the coast, uptilted inliers of granitic or gneissic rock appear, but cover no great area. Close above the old rock in these localities are the lignite-bearing beds prominent near Charleston, and then again, above these latter, is a definite stratum of Cobden limestone (see section AB). This is seen only as a narrow band at Cape Foulwind, east of which it does not outcrop within the district here described. After a gap of many miles in its southerly trend it reappears at the Totara River, not far above the mouth, of this stream, and thence thickens rapidly until near Charleston it attains a thickness of approximately 600 ft. or 700 ft., and, forming a prominent cuesta, with a steep and lofty escarpment facing the sea, continues southward outside the limits of the district studied. The Eocene coal-measures are absent south of the Buller River from the uplands of the area described, and reach the surface only in the upturned edges of the relatively downthrown coastal block at the base of the Paparoa Mountains in outcrops up several branches of the Fox River, ten miles south of Charleston, and at Omanu or Back Creek, about three miles east of Addison's. Later Geological History. Since the date of the “Lower Buller” fault, which has involved the Miocene beds, is evidently post-Miocene, the terraces and flats of the lowlands of the Westport–Charleston district are thus indicative solely of post-Miocene events. For convenience of description the physiographical features of these lowlands may be classed thus:— (1.) An extended series of high-level terraces, ranging in height from about 450 ft. at the seaward edges to 600 ft. against the mountain-slopes. Such have been designated the “500-foot” terraces. (2.) The lower-level terraces and flats, similar to Addison's Flat, northwestward of the “500-foot” terraces. (3.) High-level terraces south of the Little Totara River and east of the limestone cuesta near Charleston. (1.) The “500-foot” Terraces (see Section CD). The terraces that have been designated the “500-foot” terraces have a remarkable development for thirteen or fourteen miles south-westwards from Fairdown to the Little Totara River. They are almost perfect in uniformity of level, regularity of surface, and marginal outline, and consist of a substratum of “Blue Bottom” capped by gravels from 30 ft. to 60 ft: or more in depth. McKay* “Geology of the South-west Part of Nelson and the Northern Part of the West-land District,” Parliamentary paper C.–13, 1895, p. 8. states that east of Addison's “black-sand” beach leads are contained by these gravels. These appear to be not far above the floor of Miocene sandstone. Furthermore, beach leads are found in terraces over 500 ft. above sea-level near Charleston, and again at Trig. Station U† Trig. Station U has an additional interest in that it represents a fossil monadnock of the pre-Eocene cycle of erosion.(759 ft.), three miles south of Charleston, a typical wave-cut platform is preserved in resistant gneiss. These facts all show that north of the Little Totara River wave-action sculptured the surface of “Blue Bottom” on which the gravels of the “500-foot” terraces rest. South

of the Little Totara the uplifted stratum of moderately resistant limestone seen in the cuesta early resisted the advance of the sea and protected the weaker “Blue Bottom” sandstones overlying it to the east. The origin and mode of deposition of these gravels are of interest. The basal members, which are said to contain “black-sand” gold-leads, are beach deposits, and it is probable that a great portion of the gravels above these were brought down by the Buller River and other streams, and distributed by wave-action along the shore-line. If it is assumed, as may reasonably be done, that a moderately constant and abundant supply of waste was maintained from the relatively uplifted inland district, it may be concluded that the marine shelf was cut during a period of slight depression, and that during ensuing standstill the progradation of the coast was effected. Sufficient standstill occurred at this juncture to permit of the widening of the bed of the Totara River at its debouchure from the granite and gneiss mountains to a width of about 200 yards where now the present stream flows in a narrow gorge 100 ft. below the lip of this rock platform. The uppermost members of the gravel series were laid down as fan deposits by the Buller River and mountain torrents from the upfaulted Paparoa Mountains during the early stages of a succeeding movement of elevation. (2.) Lower Terraces and Flats. The terraces and flats forming the extensive lower belt extending seaward from the foot of the higher terraces, and attaining a width of over seven miles between Cape Foulwind and Addison's, are essentially related in origin to the high-level terraces. After the formation of the piedmont plain noted above, elevation of at least 250 ft. ensued, and was followed by a period of standstill, and perhaps of slight depression, causing the re-advance of the sea until the former piedmont plains were cut back and formed an elevated coastal fringe, notched by the Buller River and by the torrential streams from the Paparoa Range, and with high cliffs marking the line of sea-front. South of the Little Totara River the tilted limestone stratum again proved a barrier to wave-advance. At the base of the seacliffs highly auriferous beach-drift was deposited, constituting one of the richest gold-leads of the district, and, prior to the re-elevation which now followed, gravels 30 ft. and more in depth were laid as a covering upon the beach-sands by the larger streams, such as the Okari and Totara Rivers, which had time at this stage to widen their flood-plains enormously by the undercutting of the “500-foot” terraces. An interesting remnant of the former coastward extension of the high-level terraces which has escaped complete destruction by the advancing sea, and later by the lateral erosion of the Totara River, can be seen beside the Charleston Road, about a mile and a half north-east of the Totara River bridge. Several pauses during this latest movement of re-elevation are evidenced by the numerous terraces so well seen on the west bank of the Buller. Lines of auriferous beach sands and gravels were also left to mark periods of more especial concentration of the gold present in the drift, which was deposited in moderate abundance by the retreating sea, and have proved of considerable economic value in many localities. The depth of such old beach material is seldom more than about 12 ft. to 20 ft.; with, it are often associated erratic boulders and blocks, sometimes 8 ft. and more in diameter, which must owe their present position largely to glacial transpoit.

(3.) High-level Terraces South of the Little Totara River and East of the Limestone Cuesta near Charleston. The “500-foot” terraces described lose their distinctive character south of the Little Totara River; some hummocky country of slightly greater elevation, consisting of dissected “Blue Bottom,” intervenes for a short space, and then commences a great belt of level gravel-capped “Blue Bottom” terraces, with an altitude of about 600 ft., occupying a depression between the edge of the gneiss mountains and the coastward limestone ridge, which slopes gently inland along the dip of the beds from the crest of its high escarpment. Against the mountains the “Blue Bottom” beds are steeply tilted by the main fault, and there attain an elevation of over 1,000 ft. Even at this altitude traces of terraces, cut by the streams now flowing 400 ft. or 500 ft. below in deeply cut gorges, are plainly delineated. The depression to which attention has just been drawn continues in a south-south-west direction many miles to the south of the area shown in the locality map illustrating this paper; its drainage is effected by moderately large streams flowing in a general north-west direction from the Paparoa Range and cutting in deep gorges through the coastal limestone ridge to which they are antecedent. Two such streams near Charleston are the Waitakere and Four-mile Rivers. There is evidence in a wind-gap south-east of Trig. Station U in the limestone cuesta, of another large stream having formerly flowed northwest across this ridge. Probably the gneiss floor of the Miocene beds has intervened at a higher altitude here than in the cass of the existing streams, and, owing to failure to corrade the resistant rock sufficiently rapidly, the drainage of this valley inland of the cuesta was captured by the Waitakere and Four-mile Rivers, and thus reversed. McKay* Geology of the South-west Part of Nelson and the Northern Part of the Westland District,” Parliamentary paper C.–13, 1895, p. 23. draws attention to the fact that despite vigorous prospecting no auriferous black-sand or other leads have been discovered in the gravels of these widespread terraces. He noted the inland depression, and, speculating upon the absence there of beach leads, considered that most probably it was due to these having been removed by denudation, although he states at the same time that this absence may possibly have been caused by “inequality of level affecting the areas east and west of the limestone.” The writer believes that the limestone has been an effective barrier to wave-encroachment during and since the period of sculpture of the highest wave-formed terraces or platforms of the district, and that the depression, with its gravel cap, is the work of the streams now transecting and draining it at a period corresponding to that when the “500-foot” terraces were formed, since there was then considerable standstill. Prominent elevated more-recent flood-plains of the Four-mile River, cut in the weak Miocene sandstones beyond the eastward limit of outcrop of limestone, are largely dependent in origin on the control of stream-grade by the local base-level of erosion caused by a resistant gneissic barrier at the Brighton Road bridge across the Four-mile River. At this place, which, though only one mile from the coast, is at an altitude of approximately 350 ft., the stream is slowly lowering its bed below the lip of the low gorge already cut. Further Details of the Charleston District. At Charleston, where everywhere is semi-obliterated record of the remarkable industry of the early gold-diggers, auriferous beach leads were

worked on at least six different terraces. All of these show evidence of former wave-action not only in the character of the covering alluvium, but also in the characteristic planing of the hard gneiss and overlying soft Miocene lignite-bearing beds into remarkably uniform terraces, with here and there a residual undestroyed hummock of gneiss. In nearly all cases gneissic rock forms a protective barrier on the seaward side of the terraces, and where this has been absent—for example, near the mouth of the Waitakere River—the removal of the weak beds underlying the Miocene limestone has been more complete, and beach deposits attain a thickness of at least 150 ft. in the locality mentioned. As has already been suggested, the peneplained surface of gneiss on which the Miocene beds of the Charleston were deposited did not have its inequalities entirely reduced. It appears, however, from various details of evidence that a great part of the present irregularity of the exposed fossil peneplain, forming a large part of the surface of the coastal strip south of Charleston, is due to the displacement of small earth-blocks along unim portant fractures, trending probably in a north-west and south-east direction. These accompanied a tectonic movement in which the beds were broadly arched into a south-eastward plunging anticline. This fact is demonstrated by the uprise of the base of the limestone band, in a direct line between its outcrops in the Waitakere and Fox Rivera, from approximately sea-level at these localities to an altitude of about 500 ft. or 600 ft. near the Four-mile River. Similar tectonic movements are abundantly evidenced in all parts of the Buller Coalfield; it is probable that the earth-stresses giving rise to them have continued in operation to the present day, and are those responsible for the severe earthquakes* Hogben, G.: Paper read before the Wellington Philosophical Society, 24th September, 1913 (see p. 301 of this volume).experienced in the district in February, 1913, which were due to the subsidence of an earthblock several miles south-west of Cape Foulwind. Resume of Post-Miocene Geological, History. From the fact that small quartz-pebble bands are present in the upper “Blue Bottom” near the Paparoa Range, in the valley of the Waitakere River, it appears that the present western slopes of the range must have been not far from the old land of Upper Miocene times. Of the shore-line of this old land nothing definite is known. Orogenic movements, which had already caused warping and unconformity between the bituminous coal-measures and the succeeding Miocene beds over a considerable portion at least of the West Coast area, became again active at a period post-dating the close of Miocene sedimentation, and the uplift of the Paparoa block relatively to the coastal earth-block resulted. The question whether the whole area of Miocene deposition was uplifted in toto before faulting occurred, or whether uplift was relative in the various earth-blocks, need not be discussed here. In post-Miocene times the upper strata of the weak “Blue Bottom” sandstones north of the Little Totara River were planed off by wave-encroachment back to the barrier afforded by the resistant granitoid rocks of the Paparoa Mountains, whilst south of the Little Totara the limestone proved an effective barrier to the advance of the sea. Upon this wave-cut platform the gravels of the “500-foot” terraces were deposited. Prior to the formation of this plain of marine erosion, now buried below the gravels, uplift varied by standstill, and perhaps by slight depression, had been in

progress, and the surface of the monadnock at Trig. Station U was levelled by wave-action at this time. The uplift continued after the deposition of the gravels of the “500-foot” terraces, and the land rose at least 250 ft.; again a period of considerable standstill and probable slight depression intervened: the sea advanced upon the recently formed strand-plain, and wore it back until a bold line of high sea-cliffs marked the edge of the present highlevel terraces. The movement of uplift was renewed, and continued intermittently until comparatively recent times, the various periods of standstill being well illustrated by numerous terraces, such as those well developed where the Buller and Totara Rivers cross the coastal lowlands. It is tolerably certain that depression varied the main movement of uplift, such depression being indicated in particular by the relations of the Pleistocene beach-sands to the Tertiary and older underlying rocks near the mouth of the Waitakere River (see section AB). The covering of drift over the lowlands consists largely of beach deposits left as the sea retreated; but, in addition, a quantity of material has been brought down and deposited by the Buller River on its lateral terraces, deposition having taken place owing to the seaward extension of the delta of that river during the early stages of uplift. The most recent events in the geological cycle have been the formation of the present Buller delta and a general prograding of the coast-line, except where rocky headlands disturb the uniformity of the sea-margin and are being rapidly undereut into steep and lofty cliffs. These events are still in progress. Acknowledgements. In conclusion, the writer wishes to thank Mr. P. G. Morgan, Director of the Geological Survey, for permission to present this paper, and Mr. G. E. Harris, of that Department, for assistance in the preparation of locality map and sections, and to express his indebtedness to Mr. C. A. Cotton, of Victoria College, for much helpful criticism and advice.