Art. XXXVIII. —The Tuamarina Valley: A Note on the Quaternary History of the Marlborough Sounds District.

By C. A. Cotton, Victoria College, Wellington.

[Read before the Wellington Philosophical Society, 23rd October, 1912.] Plates XV, XVI. Introduction.

Immediately after leaving Picton the railway to Blenheim climbs a low saddle, and afterwards follows a comparatively straight valley—that of the Tuamarina—for its full length of ten miles, emerging on the Wairau Plain at the junction of the Tuamarina River with the Wairau.

Fig. 1.—Locality Map, Central New Zealand. P, Picton; B, Blenheim; T, Tuamarina River; Q, Queen Charlotte Sound; TC, Tory Channel; PS, Pelorus Sound.

The writer was able recently to make a few observations which may be of some value, inasmuch as they throw light on points connected with the physical geography of this portion of Marlborough to which little attention has hitherto been directed.

Our previous knowledge of the physiography of the Marlborough Sounds and adjoining country may be summed up as follows:—

1. The Sounds are submerged river-systems.^*

2. Subsidence to the extent of 5 ft. in the neighbourhood of Blenheim was noted at the time of the earthquake of 1855.^†

3. The line of the Wairau Valley has been recorded as an “active” fault marked by earth quake-rents.^‡

Geological Structure.

Various authors^* have recorded their observations of the geology in the neighbourhood of Tuamarina.

A brief general statement will here suffice. The Sounds district is composed almost entirely of sandstone or greywacke and schists, all much deformed. Practically nothing is known of the structure, but it is certainly complex.^†

According to McKay,^‡ the boundary between the sandstone (his Upper Devonian) and schist (his ? Silurian) coincides very nearly with the line of the Tuamarina Valley. He regards it as probable that the junction is the line of a fault,^§ and it is certainly true that at the low saddle, mentioned above, between the Tuamarina Valley and the stream at the head of Picton Bay, there is present a small outlier of Tertiary mudstone which probably owes its preservation to its having been faulted in.

Fig. 2.—The Low Saddle (250 ft.) between the Tuamarina Valley and the Stream flowing into Picton Bay. Elevation Railway-station is on the saddle, and the railway to Blenheim passes around to the left. View looking north-east.

The low pass, an unusual feature (see fig. 2), is accounted for by the presence of the weak mudstone, and it is possible that the position of the valley is determined by the fault, but it is by no means certain that a fault exists for the full length of the valley. Nor is it at all necessary to assume its existence in order to account for the position of the valley, for other parallel valleys are present. It is possible that the drainage-lines are determined by systems of master joints, if not by bedding.

Sculpture of The Sounds Block.

The Tuamarina Valley differs from its neighbours only in having, at its head, a low pass. In other respects it is a typical example of the valleys which, before the subsidence, drained this mountainous district.

The present Sounds area had then been, so far as one can now judge, dissected to the early mature stage by normal agencies in a single erosion cycle. There are many outrunning spurs both in the Tuamarina Valley and in Queen Charlotte Sound which show very even and apparently horizontal crest-lines suggesting successive cycles in the sculpture of the valleys; but it is significant that flat-topped remnants are absent. The writer prefers to ascribe these even-crested spurs, and the apparent coincidence of level of neighbouring spurs which is sometimes observed, to subequal spacing of parallel streams determined by systems of master joints,^* and to assume, in the absence of other evidence to the contrary, that the Sounds valleys were the work of a single erosion cycle.

The relief was strong (2,000 ft. to 4,000 ft.), and hill-slopes and valleysides were steep. The valley-floors, however, were graded and broadly opened, flood-plains being developed nearly to their heads. The amount of subsidence which ensued has not been sufficient to submerge the floodplains entirely. The town of Picton, for example, has been built upon one unsubmerged remnant at the head of a tributary of Queen Charlotte Sound, while another tributary with its flood-plain still unsubmerged opens upon the Sound a little farther east as Waikawa Bay (see fig. 3).

The subsidence which, allowed the sea to enter this valley-system was local. The extent of the block affected by it has not been investigated, but it is important to note that it did not extend beyond the broad valley of the Wairau, for southward and eastward there is abundant evidence of recent uplift. Possibly movement took place along a fault-plane situated somewhere in the Wairau Valley, or perhaps the explanation is to be found in the formation of a flexure along the same line.

Amount of Subsidence.

The amount of subsidence in the Sounds block may be arrived at to a rough approximation. The observer who enters one of the Sounds by steamer may be tempted to produce the steep sides downward, in imagination, until they meet at the bottom of a V-shaped gorge thousands of feet in depth—a veritable canon. As mentioned above, however, flood-plains exist at the heads of the valleys, indicating that they were graded, flatfloored, and broadly opened. Into such valleys a moderate amount of subsidence will allow the sea to enter and penetrate far. A rough estimate, based on probable grade, places the subsidence at between 250 ft. and 500 ft.

It is possible, also, to arrive at a rough estimate from a consideration of the present depth of water shown by soundings. The method can, however, be applied only if it can be shown that the accumulation of marine sediment in the valleys since subsidence is probably not great.

The amount of deposit will be small if—(1) subsidence took place recently; (2) tidal currents have sufficient strength to keep the finer sediment in suspension and carry it out to sea; and (3) the area of deposition is large compared with the area supplying sediment.

by soundings near the mouths should therefore indicate only a little less than the total amount of subsidence.

For the following reason Pelorus Sound (PS, fig. 1) is selected for the application of the test. The soundings in Queen Charlotte Sound^* present irregularities, the chief of which is a bar of shallow water (11 fathoms) just within the main entrance (Q, fig. 1). This is accounted for by the fact that the flood tide enters, not by the broad, main entrance, but by a smaller opening, Tory Channel (TC, fig. 1), on the south-east side and then flows down the Sound, causing permanent slack water at the mouth, which has thus become a locus of deposition for fine sediment. This anomaly is a result of the northward flow of the flood tide through Cook Strait.

Pelorus Sound, on the other hand, exhibits no similar irregularity. There is general agreement of the depth of water in the arms of Pelorus Sound with that within Queen Charlotte Sound, and in the former there is a gradual increase in depth from the head to the mouth. The tide runs with sufficient strength to carry the bulk of the sediment out to sea.

In the mouth of Pelorus Sound soundings indicate a depth of 40 fathoms, suggesting that the subsidence is about 300 ft. This estimate agrees fairly well with that based on normal river grade, but the whole argument is ineffective if at some stage of the subsidence a very long pause occurred, allowing of the accumulation in the Sounds of a great thickness of sediment.

The Tuamarina Valley, unlike those opening on Queen Charlotte Sound, is not now occupied by an arm of the sea. Clearly, however, it has been affected by the same subsidence, and invaded by the sea. In fact, the railway traveller cannot fail to note its resemblance to the Sound left behind at Picton. It is occupied now by flat land and swamp.

The Old Shore Of Cloudy Bay.

Again, after Tuamarina Railway-station is passed and the valley left behind, an old abandoned coast opens out, bold and straight, while at the foot of the cliffs, instead of the sea, the Wairau Plain is spread out.

Fig. 4.—-A View along the old shore North-Eastward from Tumarina.

The straightness of this fossil coast, coupled with the fact that it truncates rock structures indifferently, suggests that its course was determined

by a fault which separated a heaved Sounds block from a thrown block which underlies the Wairau Plain and the extension of Cloudy Bay which formerly occupied its site. It will be noted that the fault which it seems necessary to postulate here, while following approximately the line of the hypothetical fault or flexure along which the Sounds block has recently subsided, had its downthrow in the opposite direction. It must also be supposed to be of earlier date, antedating the dissection of the Sounds block.

The bold scarp, part of which is shown in fig. 4, and the line of which is indicated in fig. 3 as “Old Shore,” cannot be regarded as preserving any remnants of the original fault-scarp. The line of cliff-facets might be produced—(1) by recent faulting, (2) by lateral cutting by the Wairau River, or (3) by marine erosion. The continuity of its line with the actual sea-coast a few miles to the north-east indicates with a fair degree of certainty that it is marine erosion which has been responsible for the cutting of the actual facets. The rock platforms of the old shore are buried beneath alluvium.

Changes Brought About By The Advance Of The Wairau Plain Intc Cloudy Bay.

The Wairau Plain has been built forward along the old shore either as an ordinary delta or, as suggested by Mr. L. J. Wild,^* filling a great lagoon, a portion of Cloudy Bay enclosed by a spit of gravel supplied by the Awatere River.

The advancing alluvial deposits closed the mouths of the Tuamarina and neighbouring valleys, and at Tuamarina the surface of the plain is now about 20 ft. above sea-level.

Deposits In The Tuamarina Valley.

The Tuamarina Valley was thus converted from an arm of the sea into a lake or lagoon. Into the lake the main stream and its tributaries would continue to pour their loads of waste. There would be now, however, no tide to aid in the distribution of the waste and to carry the finer material to sea. Before the mouth of each small stream, therefore, a delta of the coarser waste would be built, while the finer would be spread over the floor of the lake. To the local supply would be added during floods a generous contribution from the fine silts of the Wairau as the Wairau Plain was built up from sea-level to its present height of 20 ft. The result would be that the lake would be filled, forming a flat at sea-level, above which would rise the sloping fan-like surfaces of the deltas of the main and tributary streams. After that stage was reached the waste brought in would all be dropped as the streams emerged upon the flat, fans would be built forward, and farther up each stream aggradation would take place, the fluviatile beds all being laid down parallel to the adjusted grade of each stream, and gradually extending outward over the sea-level flats.

At the valley-month the surface would slope gently up to the level of the Wairau Plain.

If the above reasoning is correct, the deposits in the valley should have the structure indicated in fig. 5, from which, for the sake of clearness, the delta and fluviatile deposits of side streams are omitted. They should have a similar structure to that of the deposits of the main stream.