Art. XL.—Preliminary-Note on the Uplifted East Coast of Marlborough.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 46, 1913, Page 286

Art. XL.—Preliminary-Note on the Uplifted East Coast of Marlborough. By C. A. Cotton M.Sc., Victoria College, Wellington. [Read before the Wellington Philosophical Society, 22nd October, 1913.] Plate XI. Introduction and General Statement. The previous record of uplifts along the coast of Marlborough and its continuation towards Canterbury is limited, so far as the writer is aware, to the mention by Hutton* F. W. Hutton, Geol. Surv. of N.Z., Rep. Geol. Expl. during 1873–74 (1877), p. 55. and McKay† A. McKay, ibid., 1874–76 (1877), p. 177. of raised shore-terraces at a few points on the coast, and the description of cut platforms northward of the mouth of the Waipara River by Speight.‡ R. Speight, Trans. N.Z. Inst, vol. 44 (1912), pp. 223–24. There are three cut platforms on Kaikoura Peninsula, the highest of which forms the flat top of the peninsula at a height of 330 ft., and there is a small remnant near Cape Campbell. In comparison with other evidence, that afforded by raised shore-lines is, however, remarkably slight for the greater part of the length of the coast of Marlborough. In Marlborough there are also other unequivocal proofs of uplift of two kinds—namely, (1) reju venation of topography, and (2) the presence of an uplifted delta. Notes on these are presented in this paper, and also on (3) residuals of a slightly elevated wave-cut platform near Flaxbourne that have previously escaped notice, and (4) a long strip of strand-plain of recent origin which points also to uplift as its probable cause. No pretence of completeness of treatment can be made, as the writer's observations, though spread over a considerable period, were made during the course of other work; and no conclusion can at present be arrived at on the important questions whether the uplift has been uniform throughout the region examined or-is of the nature of warping, whether it is entirely distinct from, or a posthumous continuation of, the orogenic movements by which the Kaikoura Ranges were formed,§ See C A Cotton, Geogr. Journ., vol. 42 (1913), p 227. and what is the relation of the uplifted area to the neigh bouring downthrown areas—namely, those lying off the east coast and in Cook Strait, and also the Marlborough Sounds district. Fig. 1.—Locality Map of Eastern Marlborough.

With regard to the last point, however, it may be noted that, while along the greater part of the length of the eastern coast subsidence of the region to the east almost certainly preceded the uplifts here recorded, for a short distance southward of Kaikoura off-shore subsidence was more recent, resulting in the formation of a strip of fresh fault coast. The writer hopes to describe this fault coast in another paper. The boundary between the depressed Marlborough Sounds area and uplifted eastern Marlbborough is obscured by the flood-plain of the Wairau River, though possibly it may be seen in the Upper Wairau Valley. It may be here noted that, while there is proof that in early Quarternary times the region stood much lower than at present, no evidence has been observed by the writer in eastern Marlborough of a Pleistocene movement of depression* See P. Marshall, “New Zealand,” “Handbuch der regionalen Geologie,” vii, 1, p. 47. of any consequence. It would seem that the Kaikoura orogenic movement was followed by a period of rest, and that afterwards the discontinuous movement of uplift began, which is still in progress. It may be true that during the course of this uplift there have been some downward oscillations; but, if so, they must be of relatively slight importance in the geological history. 1. Rejuvenated Topography. The Weak Rocks. Along the shore of Cook Strait, from the mouth of the Awatere River to Clifford Bay, unconsolidated mudstones and sandstones form the coast. The lower valleys of the Awatere River and the neighbouring smaller streams, the chief of which is the Blind River, are excavated in these beds, which extend up the Awatere a distance of twenty-five miles. The rocks of this area are generally known as the Awatere beds, and are of Tertiary (some of them certainly of late Tertiary) age. The extent of this area of very weak rocks is shown in fig. 2. Fig. 2.—Map showing the Areal. Distribution of the Awatere Beds (After McKay). A strip of lithologically similar rocks† Not necessarily of the same age as the Awatere beds.(nearly all mudstone) forms Cape Campbell and the coast for some distance southward, and there is a considerable area of similar weak mudstone† at the mouth of the Kekerangu River and southward to the mouth of the Clarence. Here the beds stand in a nearly vertical altitude; in the lower Awatere Valley, on the other hand, they lie nearly horizontally or dip at low angles. The topographic forms developed by erosion are, however, independent of the dip and strike, for the rocks are uniformly very weak, and are in this respect in marked contrast with all the other rocks of the Marlborough coast. In a period barely sufficient to allow streams working on moderately resistant rocks to grade their channels the surface of these beds is reduced to mature or even senile relief, and their topography consequently preserves admirably the history of a rapid succession of movements of uplift separated by pauses. Erosion would, however, during a relatively prolonged period of standstill efface the whole record.

The Awatere Area. A view up the Awatere Valley from a point near the railway bridge shows a succession of broad and high terraces. It seems probable that an accurate survey of the terraces would show that some of them can be correlated on both sides of the river as remnants of dissected flood-plains that mark stationary episodes in a movement of uplift; while, on the other hand, there can be no doubt that a number of the terrace remnants have no such significance, but have a merely local development, and simply mark latcral swings of the river-channel during down-cutting. It will here suffice to refer to the highest of the valley-floor remnants noted and to some lower series of terraces that indicate prolonged standstills. The highest patch of superficial gravel noted by the writer in the lower valley occurs on the top of an isolated, rounded hill, 964 ft. high, situated about two miles south-west of the Awatere railway, bridge. The top and flanks of the hill are thickly covered by fluviatile gravel consisting of pebbles of clastic and igneous rocks similai to those in the bed of the Awatere River to-day. The rock composing the hill being a weak mudstone, the surface of which has been sculptured to mature outlines during later erosion periods, its top does not preserve a flat remnant of the ancient flood-plain, the height of which above present sea-level must, therefore, have been somewhat greater than the present height of the hill. The height at this point, above present sea-level, of the bed of the ancient Awatere, which was no doubt the stream that spread the gravel, may be taken to be about 1,000 ft. Opposite this point the river is now about 200 ft. above sea-level. In the present cycle it is a submature, steep-grade stream, whereas in the ancient cycle in which the high-level gravels were laid down it may have been fully mature and may have had a flatter grade; but, on the other hand, the mouth may have been farther seaward. It is, therefore, difficult to estimate the height above sea-level at which the gravel was originally deposited, but it was probably not more than 200 ft., and its presence where it is now found may be taken to indicate elevation of 800 ft. Farther up the valley of the Awatere terrace remnants are much higher above present sea-level, but their distance from the mouth of the river renders them of little value in estimating the amount of uplift. Fig. 3.—View looking North-west from the East Side of Lake Grassmere, showing the Cliffed Margin of the 350 ft. Surface. The ridge in the background, forming the divide between the Wairau and Awatere Rivers, shows also an even sky-line sloping down to a height of about 900 ft. near White Bluffs. Flat-topped interfluves that appear to be remnants of a suiface of senile relief developed over a considerable area during a later pause in the movement of uplift are now prominent on the seaward portion of the Awatere

area. McKay* A. McKay, Geol. Surv. of N.Z., Rep. Geol. Expl. during 1885 (1886), p. 125. notes the presence of Awatere River gravels on low hills in this vicinity. This ancient surface, now dissected, but indicated by an even sky-line (see fig. 3), slopes gently seaward, and apparently two trigonometrical stations, at heights of 352 ft. and 355 ft., close to the edge of the cliffs bordering Cook Strait, are situated on it. Some flat-topped remnants near the south-east bank of the Awatere River, at a distance of about three miles from the sea, reach a height of 543 ft., but the writer failed to note whether they are accordant with the last-mentioned surface or stand above it. During a later—the last—long pause in the general movement of uplift fairly broad valley-floors were cut by the Awatere and neighbouring smaller rivers upon the weak rocks. In the vicinity of the railway bridge the broad terrace plains on either side of the Awatere River, remnants of the flood-plains of this period, together have a width of three miles, and are at a height of 290 ft. above sea-level and 120 ft. above the present level of the river. The river now wanders in braided channels on the gravel-covered-floor of a trench a few hundred yards in width. Such braided courses are now a common feature of New Zealand rivers, and, of course, indicate excess of waste supply over transporting-power, resulting in aggradation. In the writer's opinion, the present excess of waste in Marlborough is largely, if not wholly, the results of destruction of the original vegetable covering of the land as a result of settlement. The excessive supply of waste-more than balances the effect of a very recent, small movement of uplift in Marlborough, which will be mentioned later, and which, other things being equal, would have resulted in degradation. The writer has been informed by E. A. Weld, Esq., of Flaxbourne, that the bed of the Flaxbourne River has been perceptibly raised, and it is said of the Kekerangu River, which now flows in braided channels on a broad gravel bed, that in the early days of settlement it was a “swamp stream”—that is, had a marshy flood-plain. Speight regards the cause of similar aggradation in Canterbury rivers as climatic.† See footnote to p. 294. With respect to present sea-level as base-level the lower course of the Awatere is graded, but its declivity is still steep, being something like 25 ft. or 30 ft. per mile. The vertical concave banks and the convex banks descending gradually as series of terraces indicate that the curves, of the stream have been enlarged contemporaneously with down-cutting. Considered in connection with the extreme weakness of the rocks in which the river is working, the youth of the present valley indicates that the latest important uplift—that is, the one that interrupted the last long plain-cutting pause—took place very recently. Tributary streams flow for long distances over the surface of the uplifted flood-plain, and rejuvenation has reached but an insignificant distance from their mouths. An example of these is Starborough Creek,‡ This tributary, flow ng for about five miles over the surface of the uplifted flood-plain, parallel with the main stream, affords an example of a “postponed junction.” which enters the Awatere on the south-east side, a short distance below the railway bridge. In the neighbourhood of Flaxbourne (or Ward) there is similar proof of pauses in the general movement of uplift, the pauses being sufficiently long there also to allow of the excavation of extensive valley lowlands by the local streams where these flowed over the weak Awatere beds. In places the intervening ridges were cut right through, continuous lowlands

being developed from stream to stream, and portions of the ridges or spurs were isolated as a kind of monadnocks. The Kekerangu Area. Fig. 4.—The Uplifted Flood-Plan and Revived Valley at the Mouth of the Kekerangu River. Fig. 5.—Map of the Clarence Delta. In the lower valleys of the Kekerangu and neighbouring streams flowing to the east coast across a strip of weak mudstone there is also evidence of revivals of erosion, and in the period of standstill before the latest of these the Kekerangu had excavated a broad lowland. This was quite probably the same standstill in which the broad uplifted flood-plain of the Awatere was developed. The uplifted Kekerangu flood-plain (see fig. 4) slopes gently seaward, ending in a line of wave-cut cliffs nearly 150 ft. in height, and must have reached a somewhat lower level at the former shore-line. There are, however, reasons for believing that the coast has not been retrograded far, and probably the amount of the uplift is not very far short of 150 ft.; it may be stated at about 120 ft. The pause preceding the 120 ft. uplift was sufficiently long to allow one of the neighbouring streams (Deadman's Creek) to erode a mature valley through a ridge of resistant conglomerate (Gréat Marlborough conglomerate), which it crosses near its mouth, whereas the period that has since elapsed is so short that the gorge cut by the revived stream is still very young, and its grade is interrupted by a fall.

The higher terrace remnants in the vicinity of Kekerangu do not preserve a good record of the earlier movements, but more information is to be obtained from the uplifted Clarence delta, a little farther south. 2. The Uplifted Clarence Delta. The convex curve of the shore-line at the mouth of the Clarence suggests that the river is building a delta, and this is found to be the case, although only a small proportion of the projection is a delta built with respect to present sea-level. The remainder is a promontory of high land, an uplifted delta, through which the river makes its way to debouch at the point (see figs. 5, 6, and 7). Fig. 6.—View of the Uplieted and Modern Deltas, Low Terraces, and Floodplain of the Clarence River from A Point on the Surface of the Uplifted Delta on the South Side. The promonotory so formed is flat-topped, the surface sloping gently seaward, and reaches a height of 580 ft. about half a mile within the present cliffed margin, to which the subaerial top-set beds extend. It is impossible to say with certainty how far seaward from the present margin of the platform the strand lay at the close of the period during which the delta was built. The amount of uplift indicated may be taken as 500 ft., which is about the height of the base of the top-set beds. Fig. 7.—View of The Clarence Delta From The North. The least dissected portion of the surface of the ancient delta lies close to the north bank of the Clarence. The even edge of this portion appears in fig. 6. It has escaped dissection because it slopes away from the river. The writer crossed the southern portion, and found that its dissection shows at least three distinct stages. A broadly rolling, mature, upland surface points to a very long period of rest following an early movement of uplift of small amount, though, on the other hand, this dissection might

have resulted from coast-retrogradation normally taking place towards the end of the stage of maturity in the “delta cycle with stationary crust,”* J. Barrell, Bull. Geol. Soc. Am., vol 23 (1912), p. 396. a period that might possibly be correlated with that at which a valley lowland was developed in the Middle Clarence Valley.† C. A. Cotton, “The Physiography of the Middle Clarence Valley,” Geogr. Journ., vol. 42 (1913), p. 228. Deep, dissecting valleys, with broad terraces, some of them at a height of 300 ft., are the work of erosion during the later discontinuous uplift, and here, as elsewhere on this coast, the latest rejuvenation has been a very short period in progress. The fore-set portion of the delta, which, however, does not show stratification, may be examined in the banks of the river near the Clarence Bridge. It consists of gravel of fairly even texture, few pebbles being over 4 in. or 5 in. in diameter, but there is a considerable admixture of finer gravel and sand, and some patches of sand. The material is fairly well cemented. The fore-set portion is distinctly marked off from the topset (or at least from the subaerial top-set) beds by its bluish-grey colour, corresponding to a low state of oxidation of the finer material, the latter beds being oxidized and of a yellow to reddish-yellow colour. The top-set beds begin with a boulder-bed, possibly of subaqueous or beach origin, which is followed by gravel and clay strata, and their average thickness on the delta proper is about 80 ft. There are very thick accumulations of oxidized fluviatile beds on some lower terraces, which may possibly indicate downward oscillations interrupting the general upward movement. 3. The Rock Platform Near Flaxbourne. Well-preserved remnants of a wave-cut platform which indicate uplift of several feet may be seen half a mile north of the mouth of the Flaxbourne River. The rock exposed on the shore is a fine-grained, argillaceous sandstone overlying the Amuri limestone. It is in a vertical attitude, but exhibits no bedding, and is but little jointed. Marine denudation planes it off to a surface which slopes gently seaward, and which from a distance appears absolutely smooth, though closer inspection reveals numerous inequalities with a relief of a few inches (see Plate XI), and the tabular remnants of the slightly higher platform, several of which are shown in the photograph, have an exactly similar surface and seaward slope. Fig. 8.—Diagram Illustrating The Theorem That The Amount of Uplift Indioated By Residuals Of A Wave-cut Platform Is Less Or Greater Than The Height of The Residuals Above Their Base According As The Pre-Uplift Position Of High-Water Mark was Seaward Or Shoreward of Its Present Position. Their surface is several feet above the present level of high tide, and they therefore indicate uplift. The height of the residuals above their bases is 6 ft., but this cannot be taken as an absolute measure of the amount of uplift unless it is assumed that the position of high-water mark (in plan) was the same immediately before the uplift as it is now.

Remnants of A Wave-Cut Rock Platform Half A Mile North of the Flaxbourne River

In fig. 8, A is the present position of high-water mark, and AA' is present high-water level. If the former position of high-water mark was seaward of its present position, as at B, the amount of uplift that has taken place (A'B') is less than 6 ft. If, on the other hand, it was shoreward, as at C, the amount of uplift (A'C') is greater than 6 ft. In this case it is improbable that the former position of high-water mark was shoreward of its present position, for there is not a well-defined raised beach. It may, however, have been some little distance seaward. These residuals, therefore, afford proof of uplift of 6 ft., or somewhat less. 4. The Strand-plain of the Marlborough Coast. From Cape Campbell south-westward to and a little beyond the mouth of the Clarence River the shore is bordered by an almost continuous strand-plain, broken only at one or two points where outcrops of resistant rocks project slightly seaward as rocky bluffs. This coastal strip will be referred to as the Marlborough Strand-plain. Geographically it is of great importance, as it affords the only practicable route between northern and southern Marlborough, and is followed for many miles by the surveyed route of the South Island Main Trunk Railway. Its width varies from a few yards to several hundred yards. Sandhills and dunes form its surface at the wider parts, and the material of the beach varies from coarse sand at the north-eastern end to coarse gravel at the mouth of the Clarence. The bulk of the material is, without doubt, supplied by the Clarence, though smaller streams also contribute a share. The waste supplied is swept north-eastward along the coast by a constant along-shore current, and, the supply being very abundant owing to the mountainous and recently uplifted character of the land, it is not surprising that progradation of the coast has taken place; but it is more difficult to account for the fact that progradation has begun very recently. Proof that this is the case is to be seen in the presence of a line of perfectly fresh-looking wave-cut cliffs at the rear of the strand-plain, and it is clear that the sea reached the base of the cliffs very recently. Some radical change in conditions must have taken place to bring about this sudden change from retrogradation to progradation throughout the length of forty miles of coast. Two possibilities suggest themselves- (1) increase in the supply of waste, and (2) uplift. (1.) Increase in the supply of waste without uplift of the coast is regarded as a sufficient cause in the case of a strand-plain at Ancona, in Italy.* W. M. Davis, Geogr. Journ., vol. 34 (1909), p. 303.In that case the increase is accounted for by rejuvenation following a differential uplift inland. In the case of the Marlborough coast, however, while, as already stated, dissection is now in a very young stage as a result of the recent general uplift, the writer has seen no evidence of more recent differential uplift or warping of the surface that might bring about the change.† At only one place in Marlborough was an indication noted of strong tilting or warping of the surface. This was in the case of the small streams flowing north-westward into Lake Grassmere, the uplifted mature floors of which slope down-stream more steeply than the grade of the revived streams. This tilt is probably in sympathy with, or forms one boundary of, a kind of cauldron subsidence, forming a bay, which has later been cut off from the sea by a bar, and has become Lake Grassmere. Further, the change has taken place so late in Quaternary time that it is difficult to believe that a climatic change of

sufficient magnitude to bring about sudden marked increase in the supply of waste can have taken place.* R. Speight, however, considers that Canterbury streams indicate that “a maximum of erosion [due to a climatic cause] is past, and another cycle of deposition has commenced.” (Trans. N.Z. Inst., vol. 43 (1911), p. 410.) (2.) Since there is other evidence that uplift of small amount has very recently taken place, it is worth while considering its efficiency as a cause of progradation. Rapid uplift, even of small amount, seems to be capable of producing the result if the profile of the neighbouring sea-floor was graded before the movement took place. Slight shallowing of the sea would disturb the equilibrium, subjecting the off-shore deposits to a somewhat stronger wave-action than formerly, and therefore causing them to be reworked and the finer material to go into suspension. Meanwhile the Clarence and the other smaller rivers would continue to pour out their enormous load of waste, the bulk of which would be slightly increased by rejuvenation following the small uplift. The balance previously existing between the supply of waste and the transporting-power of waves and currents having been in this way disturbed, some of the surplus waste would be thrown up along the shore, covering over the rock platforms of the uplifted coast. A similar coastal strip of fiat land at the mouth of the Waipara River, in North Canterbury, has also been explained by Speight† R. Speight, Trans. N.Z. Inst., vol. 44 (1912), pp. 224–26. as the result of uplift. It would seem that, in the case of the Marlborough Strand-plain, after the off-shore profile has become graded with respect to the present level of the sea, and waves and currents are again able to dispose of the waste supplied, the attack of the waves on the shore will probably again become energetic, the strand-plain will be cut away, and the cliffs at its rear will again begin to recede. It is, indeed, possible that progradation has taken place many times in the recent past, continuing for a brief period after each small movement of uplift.