Intermontane Basins of Canterbury: Part 2.
[Read before the Philosophical Institute of Canterbury, 2nd April, 1924; received by Editor, 31st December, 1924; issued separately, 31st March, 1926.]
The following account is intended to be an appendix to the paper on the “Intermontane Basins of Canterbury” (Trans. N.Z. Inst., vol. 47, 1915, pp. 336–54), wherein the author discussed the question of the origin of certain basins in the mountain region of that province, and came to the conclusion that they were formed initially as the result of deformational movements towards the close of the Tertiary era. This contention was subsequently maintained in another paper, entitled “Notes on a Geological Excursion to Lake Tekapo” (Trans. N.Z. Inst., vol. 53, 1921, pp. 37–46), in which a similar origin was attributed to the inland basin known as the Mackenzie Country.
Since writing the last paper I have had additional evidence in support of this conclusion—viz., the discovery of coal on the Balmoral Run, near the head of Irishman Creek, to the east of Lake Pukaki, as well as a similar occurrence of older gravels in which coal is also reported to the west of the Tasman River, the probability of such discoveries being indicated in the last-mentioned article (loc. cit., p. 44). The main portion, however, of the present contribution deals with an entirely different district, situated in north Canterbury.
Recently, by the kind assistance of Mr. J. E. Strachan, of Rangiora, I was enabled to examine an area concerning which there is little information—viz., that at the head of the South Branch of the Waipara River, and the basin of the Upper Ashley, commonly known now as Lees Valley (see map). These two areas occupy two structural basins, probably not entirely independent of each other, which may be called the Karetu (from Mount Karetu) and the Lees Valley basins. It will be best to consider the Karetu basin first. This lies north of Mount Karetu (3,177 ft.), which is immediately west of Mount Grey (see map) and east of the Okuku Range (2,921 ft.), and extends across the South Branch of the Waipara River. The easiest access to it is obtained by means of the road which passes Whiterock over the Okuku Pass, but it may be approached by pack-tracks from Waikari through Heathstock and the McDonald Downs, or by way of the middle gorge of the Waipara. It forms a basin, ringed on every side by greywacke ranges with an average height of between 2,500 ft. and 3,000 ft., except on the north and north-east, where the ring is lower. It is some five miles in length by two to three in breadth. Its most interesting feature is the presence of small discontinuous areas of Cretaceous beds on its floor, which give an important clue to its mode of origin (see map).
These Cretaceous beds are first seen in Birdseye Creek, a tributary of the Waipara rising in the Okuku Range. They occur much better developed in the branch of that creek which flows past the Karetu Hut, on both sides of the road in the next creek to the west—viz., Nicholls Creek—and again farther west still near the road-crossings of the creek near the Mount Laurie
Hut. The thickness of beds exposed is not great as a rule, since the greywacke floor has been uncovered in many places and a mere veneer left, but it is greatest along the base of the Okuku Range on the south-west of the basin, and especially at its southern end, where the remnants of the Cretaceous cover must be several hundred of feet in thickness.
In the Karetu Hut Creek the beds consist of sands, greensands, and clays, with impure lignite, striking north-west and south-east and dipping south-west at low angles. Some of these beds contain very broken shells, but Mr. Strachan was fortunate in coming across the fragment of a belemnite which enables the age of the beds to be stated with reasonable certainty as being Cretaceous, and they are thus to be correlated with the lower members of the series existing in the classical locality in the middle Waipara and Boby's Creek, and this indicates a much wider extension of these beds towards the west than was formerly believed.
From this locality the beds stretch south-east into Birdseye Creek behind a knob of greywacke, and north-west across the two main branches of Nicholls Creek. In the most easterly branch the beds are developed both above and below the road-crossing. In the latter position they are
sands, greyish-green and brown in colour, concretionary in their lower levels, and associated with beds of sandy clay. The lower members of the sequence are exposed farthest down the creek, where they rest unconformably on greywackes, and their former wider distribution is indicated in the stripped surface which forms the downs extending from the limits of the Cretaceous beds towards the east. The sides of the creek towards the south-west are obscured by scrub and bush, so that the junction with the greywackes in this direction cannot be seen, but it is probably a fault-contact, the line of fault extending north-west along the base of the Okuku Range.
It is probable that the beds once extended from the line of this creek to the north-west, but there are now undoubted exposures of greywackes over large areas, so that the Cretaceous beds form only discontinuous remnants. Where the road crosses the western branch of Nicholls Creek there is a very small remnant of sandy beds bent up into a syncline; but farther west, just at the point where the road leaves a creek to climb out of the basin in the direction of Lees Pass, there is a development of sands and sandy shales with an east-and-west strike and a dip to the north at low angles. Farther up the creek, near the junction with the greywackes, occur lignites, shales, and clays, lower in the sequence, with the same strike but with greatly increased dip. To the south-east of this occurrence there is a fine basaltic dyke, striking north-west and south-east, and standing up as a ridge for several hundred yards, but no contacts with the country rock can be seen on its flanks, even where it crosses gullies. This dyke was not followed beyond the point where it meets the road, but it is possible that it continues some distance farther to the south-east through the scrub-covered country.
Although the Karetu basin is now completely shut off from the neighbouring intermontane area, it was in all probability once connected with it, and the covering Cretaceous and Tertiary beds extended over a wide tract of country to the north and west, where the Lees Valley and its north-easterly extension is now situated.
The Lees Valley intermont is of much greater extent. The area proper is certainly fifteen miles in length, and it may extend to the north-east an additional five to seven miles; the maximum width of the floor is about four miles. It lies behind the range extending from the Okuku River, through Okuku Hill (3,743 ft.), Ashley Hill (3,343 ft.), Blowhard (3,439 ft.)—which might well be called the Ashley Range—to Mount Oxford, while the north-west margin reaches up to the slopes of the Puketeraki Range. The basin is occupied chiefly by the upper waters of the Ashley River, including its tributaries the Townshend and Whistler, but the Okuku flows right across the north-east end and breaks through, by a narrow gorge, the range bounding the basin on the south-east. It is divided from the basins in the Upper Waipara by a northerly extension of the Okuku Range, which sinks to quite a low saddle at Lees Pass. The Ashley collects all the drainage from the rest of the basin, and breaks through the rocky barrier near the south-west corner by means of a steep picturesque gorge. In general the slopes to the north-west of the basin are much more gentle than those to the south-east, a feature of many of the river-valleys and intermontane basins of Canterbury. It thus forms an exact counterpart of the High Peak Valley drained by the Selwyn, except that the latter breaks through at the north-east end and not at the south-west end of the basin; in fact, one could quite imagine, if standing in one of these basins,
that he was in the other, so close is the resemblance in their general features.
Although the general form of the area suggests a tectonic origin for it, with a great fault bounding it on the south-east side and extending along the inner base of what I have called the Ashley Range, there is, fortunately, positive evidence of faulting along this line furnished by the circumstances of a small remnant of Tertiaries in the bed of the Ashley River, near the point where it impinges against this range after its course across the basin from the slopes of the Puketeraki Range. My attention was drawn to this by Mr. W. B. Starky, who sent in to the Canterbury Museum a sample of coal from Lees Valley, and this suggested the possibility of a supply of fuel in a district where wood was scarce. I examined the spot very carefully in company with Mr. Strachan. The exposure is of somewhat limited extent, and has to be approached by wading the river; fortunately this was low at the time of our visit, so we saw all that was likely to be visible at any time.
The beds containing the coal outcrop on the southern bank of the river, the total length exposed on the steep bank being about 100 yards, but they are not clearly visible throughout even that small length. On the spot farthest up-stream where the beds are exposed the following sequence occurs:—
Yellowish sands, with fragments of Bryozoa.
Greenish sands, weathering brown.
Greenish glauconitic sands.
Whitish sands, slightly glauconitic.
Carbonaceous grit with quartz-grains.
The beds are capped unconformably with Recent river-gravels. The strike is north-east and south-west, and the dip south-east at an angle of 30°. The total thickness of the exposure is about 50 ft. Some parts show slickensided surfaces.
Down-stream the sequence is obscured, but a little farther on it is again clear but at a slightly higher horizon. At water-level there is a grit, 20 ft. thick, containing quartz pebbles with fragments of bryozoans, echinoid spines and plates, crinoid stems, molluscs, and corals (?), very calcareous in composition, and containing lenses and nests of bright shining coal, crushed and powdery in texture. This is succeeded by 1 ft. of black carbonaceous shale, then by 4 ft. of white sands, and then by 2 ft. of carbonaceous shale (brown coal), and the visible sequence is closed by yellow clay on the line of a thrust-plane, the whole thickness seen being about 30 ft. It is capped by river-gravels, as in the case of the exposure up-stream. The beds strike north-east and south-west, and dip south-east 30°. The grit just referred to is frequently dark in colour, well bedded, with some pieces of coal, looking like stems carbonized in position and not parts of a regular seam. It contains as well numerous rosettes of gypsum crystals. Under the microscope it appears to be composed chiefly of bryozoan fragments.
Just above this occurrence there is a slickensided surface of decomposed black shaly greywacke in contact with whitish sand. About a chain farther down-stream is another exposure of coarse yellowish-white sand, stained brown in places with oxide of iron, derived probably from pyritous matter.
The obvious presence of reversed faulting at this point is very important in its bearing on the origin of the whole valley, and confirms the conclusion
arrived at by inference from physiographic characters. It is possible that there may be evidences of faulting in other places along the northern foot of the Ashley Range, but we could not see any. The fragments of coal have probably been derived from a lower level and brought up by fault-movements, and it is possible; but not probable, that a fair amount exists in position beneath the water-level, and perhaps in other parts of the plain to the north-west, but the exposures of greywacke on mounds penetrating the gravel which covers the floor of the plains, such as that on which the Duckworth Homestead is placed, do not encourage the hope that a payable field exists. If any area does occur it will be beneath water-level, and of limited extent. The cover of Recent gravels prevents a definite answer being given on this point. There may, however, be an area containing coal in the flat on the lower slopes of Blowhard behind Mr. Starky's house.
There is thus little doubt that the basin owes its origin to tectonic movements in which faulting played a part. This area of deformation extends in all probability across the middle course of the Okuku and the uppermost reaches of the three branches of the Waipara towards the Waikari-Hurunui intermont. A most interesting feature of the problem of origin is that connected with the date of the deformation and the relationship of the courses of the Ashley and the Okuku to it. Both rivers present the same anomaly: they rise in the main ridge or the flanking ridges of the Puketeraki Range, flow across the widely opened Lees Valley nearly at right angles to its trend, and then cut through the Ashley Range by means of deep narrow gorges in making their way out on to the plains. What is the reason for this anomaly ? That which comes most readily to mind is that the rivers were there before the Ashley Range was formed: they are thus antecedent rivers, and if they were consequent on the surface as it emerged from the sea at the close of the Tertiary era they might be called ante-consequent streams. The presence of the remnant of marine Tertiary beds in the Lees Valley suggests that the sea extended widely over the area in Tertiary times, and when it emerged the rivers such as the Okuku were established on its surface as consequents, and that then deformation took place and the land occupied by the Lees Valley sank, or the Ashley Range was raised, and that in spite of this movement the rivers were able to cut down their beds and maintain their positions. Such a history could not develop without subordinate changes, and it is likely that the Ashley is, after all, formed by the union of other streams which had independent existence as consequents. The gaps in the Ashley Range at the head of the Garry and Glentui may well have marked the initial course of such streams as the Upper Ashley and Whistler, which, together with the Townshend, were absorbed in the main stream following the line of the Townshend over a gap in the ridge just east of Mount Oxford, thus creating the present Ashley Gorge.
There is another solution of the problem. It is quite possible that the original Ashley drained towards Waikari through the low country at the head of the three branches of the Waipara, and included in its territory the Upper Waipara itself, and that a portion of this area was captured by streams like the Okuku cutting back through the Ashley Range. The former explanation appears to me to be the sounder and more in accordance with the evidence at one's disposal.
It is a possibility that the movements producing this basin have ceased only recently—if, indeed, they are not actually in progress. The course of the Ashley through its gorge suggests recent rejuvenation. Throughout
a considerable portion of its course the river is flowing in deeply-incised meanders on the floor of a wider valley, these meanders producing remarkable scenic effects. It is just possible, therefore, that this is due to movements along an old line, similar to those occurring farther north-east at the present time (note the Christmas Day earthquake of 1922), near Waikari, and but recently at Cheviot. The fault-line is therefore one of the Kaikoura series of Cotton, and is producing surface features characteristic of that group of movements.
The streams such as the Ashley, Whistler, and Townshend show evidences of aggrading in their course across Lees Valley, which is quite in keeping with the fact that the interference with the normal grade of the Ashley is recent in point of time.
Evidences of glaciation were looked for but not found. Truncated and semitruncated spurs and groups of knobs at the ends of spurs, so characteristic of glaciated regions, were seen, specially in the valley of the Whistler, but the entire absence of moraines and other proof of glaciation in places where it might be expected negatives this somewhat uncertain evidence. At high levels in the Puketeraki Range there are hollows suggestive of the former presence of corrie glaciers. This range is placed athwart the course of the rain-bearing winds coming from the west, and lies so far east of the main divide that it is unlikely that the eastern slopes, even in the height of the glaciation, nourished any but the smallest of glaciers, and so evidence of the former presence of valley-glaciers is likely to be wanting.
In conclusion, I should like to make a few remarks on the rocks of the Ashley and Okuku Ranges. The only account of these in print is that furnished by McKay (Rep. Geol. Explor. for 1879–80, 1881, pp. 83–107), which is somewhat involved and difficult to follow. McKay notes a remarkable variation in the direction of the strike of the beds, especially in connection with the Okuku Range; but in all observations for strikes the general direction was north-east and south-west, even in the case of the Okuku Range, so that they seem to conform to that generally obtaining in the Southern Alps region of Canterbury. No doubt there are local variations, but they are not on a big scale. All through the Ashley Gorge this is the regular direction, although there are cases which depart in both senses from this mean. On a point this side of the bridge in the gorge, where a road-cutting gives a clear section, there is a beautiful example of isoclinal folding, the axes of the folds being E. 10° N., with a northerly dip. At the upper end of the gorge the strike is north-east and the dip south-east, with the dip getting steeper and steeper down-stream till the beds are overturned. There is an occurrence of diabase ash at this point, with the beds striking E. 20° S. and dipping north at high angles, the red-and green-coloured beds being interstratified for several chains. Then again there is a reversion to the normal strike and dip. No fossil was seen in these beds but Terebellina mckayi, which occurs in very large numbers in rocks well exposed in a cutting alongside the road just above the point where the river makes the remarkable serpentine meanders near the lower end of the gorge.