Some Unusual Shore Platforms near Gisborne, North Island, New Zealand
[Read before the Wellington Branch of the Royal Society, November 13, 1947; received by the Editor. August 22, 1947; issued separately, February, 1950.]
Characteristic of the coastline north of Gisborne are some shore platforms of phenomenal width. As Henderson and Ongley (1920, p. 21) record, “In some places the wave-cut benches attain a width of 30 chains, though commonly they are not more than 20 chains.” With a chain tape the writer measured two of these platforms, with the following results:
A. Kaiti Beach, Gisborne (marked A on map): 20 chains (1,320 ft.) from the outer edge of the platform to the seaward edge of the beach. From the latter point to the old sea cliff behind the beach is 3 ½ chains (231 ft.).
B. Tatapouri Headland (marked B on map) [Plate 10, Figs. 1–4]: 1,115 ft. from the outer edge of the platform to the beach, which was 125 ft. at the place of measurement. On the point of the headland the sea reaches the cliffs, but behind the beach there is a stratum of pebbles covered with sand on which grass is growing. The presence of this vegetated beach ridge, and the fact that the sea does not now reach the old sea cliff along quite considerable stretches of the coast (see map), is evidence of a small emergence of probably 2 to 3 ft.
Edge of Shore Platform
Although the shore platforms are so unusually wide, the outer edges are very steep. They drop away directly into comparatively deep water. They are also heavily vegetated with kelp and other marine growth, indicating that they are not now being actively eroded. Chemical effects of the sea-water, and the biological effects of the encrusting flora and fauna are agencies at work, but there is no evidence of mechanical abrasion of these surfaces. Even in platforms 30 chains wide the surfaces are practically horizontal, and there is no apparent evidence of reduction of the outer edges.
Relation of Shore Platforms to Terrain
The remarkable width of the shore platforms makes their presence or absence along the coast very noticeable. They have been mapped by Henderson and Ongley in their geological maps of the Turanganui, Whangara, and Uawa Survey Districts (1920). The platforms are present on the headlands, where they jut far out into the sea, and where the attack of the sea is strongest. Where the coast recedes into bays, and so such structures would be more protected, they are absent. The seas attacking the headlands are widening the platforms by breaking down the cliffs, but not affecting the outer edges of the platforms.
The incidence of shore platforms is clearly correlated with the valleys of the land terrain. Where streams emerge, as at Wainui Beach,
Photographs of shore platform on south side of Tatapouri headland, 64 miles E.N.E. of Gisborne Post Office. taken January 1947. The photographs were taken on a calm day at dead low tide, and at a time of neap tides. Fig. 1—View east along the outer edge of the platform. The summit levels of the dissected platform are 4 ft. 6 in. above low water neap-tide level. Fig. 2–View from seaward edge of platform looking shorewards. The human figure is standing at the junction of the elevated outer part of the platform with the inner planated part. Note the concordance of summit levels of the elevated part. The car in the distance is standing at the junction between the beach and the shore platform. Fig. 3–View of the inner planated two-thirds of the Tatapouri shore platform. The human figure in the photograph is approximately five feet tall. Fig. 4–View from the outer edge of the shore platform showing the point of the headland where the sea is cutting into the cliffs. Note again the outer elevated and differentially eroded part of the shore platform, and the inner low planated part. The car is nearly 4 mile away, on the edge of the beach.
there are no platforms. So likewise where Turehau Stream, Pouawa Stream, Waiomuku River, Pakarae River, and Uawa River emerge, there are no shore platforms.
Hypothesis to Explain Discontinuity of Shore Platforms
Lower eustatic sea-levels during the Pleistocene have been proved. During such times of greater emergence, the consequent rejuvenation led to the cutting of valleys not only in the present land terrain, but also below present sea-level. The submarine contours indicate the continuation of the courses of streams across the continental shelf where the sea now is (see Map). Thus the streams cut valleys through the rocks where the shore platforms are now being formed. This accounts for the discontinuity of the shore platforms opposite the mouths of present rivers and streams.
There are even breaks in the shore platforms opposite quite small streams. This can be seen in the map accompanying this paper, at the mouths of the creeks on each side of the Tatapouri Headland, for example.
Later submergence has resulted in the infilling of the valleys of the main streams near their mouths, and with the formation of beaches
Map of area east and north-east of Gishorne, New Zealand (after Henderson and Ongley, 1920). Shore platforms are in solid black, and beaches are stippled. A and B are the shore platforms of Kaiti and Tatapouri respectively, referred to in the paper.
Note the interesting stream pattern, the incidence of beaches (and absence of platforms) relative to the positions of stream mouths, the continuation of the stream courses under present sea level as revealed by the bathymetric contours, and the wide continental shelf.
on the seaward edges of these alluvial flats. The Poverty Bay flats provide a conspicuous example of this infilling. The bed-rock is below sea-level, and excavations have revealed the presence of marine shell-beds.
Hypothesis to Explain Precipitous Edges
In the period or periods of greater emergence, the former shore platforms would become elevated benches, and their outer edges would become coastal cliffs. They would thus tend to become vertical. This may account for the steep outer edges to be found on them now. On this interpretation, they are homologues of the present sea cliffs. Dr. R. W. Fairbridge, of the University of West Australia, advises me that he came independently to this same hypothesis as an explanation of the steep outer edges of the shore platforms.
Henderson and Ongley have summarized some of the evidences for both emergence and submergence found in this area. This is but part of abundant evidence in the district of such movements. There has been much faulting, and movements are still in progress. But even known eustatic changes of sea-level are adequate to account for the changes described affecting the shore platforms.
Hypothesis to Explain Vegetated Edges
Much remains yet to be determined concerning the extent of submarine erosion, and the degree to which it operates at various depths. However, it is pretty clear that the main part of the active erosion is in the intertidal zone, while below it whatever erosion takes place is comparatively limited and slow. This, in my opinion, explains why a shore platform can be actively planated, while its outer edge remains comparatively free of abrasion, and is covered with a rich marine flora. Movement of the water at the coast causes it to be highly oxygenated, thus favouring biological activity, while the steep outer edge of the shore platform provides an eminently suitable substratum on which the marine vegetation can grow. The cutting of the shore platform is a function of the intertidal processes, but the outer edge is nearly all below that zone.
Hypothesis to Explain Width of Platforms
Henderson and Ongley state, “The width of the benches is in part due to the weakness of the rocks from which they are cut, but a factor of great importance is the small amount of material that had to be removed. Along the shore-line of the Gisborne Subdivision there is ample evidence of a recent elevation of from 10 ft. to 12 ft., an amount which would convert the present platform into dry land. Remnants of the terrace existing previous to the last uplift still occur, and the advance of the strand-line on such a low shelf of weak rock must have been peculiarly rapid.”
The weakness of the rock under subaerial conditions is patent. The “papa” rock of the area rapidly disintegrates when exposed to the air. Thus the shore platforms are firm, but the cliffs readily break away. Every storm causes falls of rock. Even when but a summer breeze is blowing, there is a constant soft rattle of small pieces of rock, dislodged by the wind, running down the cliff. Moreover, as can be seen in the shore platforms, there are hundreds of minor faults cris-
scrossing through the rock, forming minor crush zones where a number of them intersect.
The point made by the authors quoted concerning the amount of material to be removed may not be as important as they thought at the time. It is true that a small emergence has meant that a smaller amount of material has had to be removed than would otherwise have been the case, but the earlier, higher platform was cut just as far (judging by the remnants of it) apparently without such aid. High cliffs occur all along the coast, and these have been formed by the action of the sea. In my opinion, the nature of the country rock is the most important factor in the formation of these wide platforms, and the amount of material to be transported by the waves secondary. The rock so readily disintegrates that even big landslides are soon removed. The sea has no large boulders with which to cope, such as. for example, are often found on a basaltic coast.
A further factor contributing to the width of the shore platforms is the openness of the coast. Powerful breakers sweep in from the open Pacific Ocean, having all the force necessary to sweep away the copious products of subaerial erosion. As the solidity of the platforms shows, the sea would not be able to cut these broad benches if it were not for the weakening of the rock by subaerial agencies. Bartrum (1916) and many others have commented on the importance of subaerial effects, often overlooked in the presence of the dramatic power of a rough sea. The papa of the Gisborne platforms is firm until exposed to the weather.
Thus it would seem that the nature of the bed-rock is the chief factor in the formation of these phenomenally wide platforms. This is in keeping with observations made elsewhere. Platforms in hard, igneous rocks are never very wide, while platforms in more easily attacked rocks are proportionately wider, other factors being equal. The width of a shore platform is thus believed to be chiefly a function of the resistance to erosion (subaerial and marine) of the rock from which it is cut.
A Faulted Shore Platform
A remarkable structure was observed on the shore platform on the Gisborne side of the Tatapouri headland. The outer part of the platform is higher than the inner part (Plate 10, Figs. 1, 2, 4). The beds of the high area, although worn differentially by the action of the sea (Plate 10, Figs. 1, 2, 4), have concordant summit levels, showing that they originally constituted a flat surface. The top of the beds is 4 ft. 6 in. above low-water neap-tide level. The mean range of tide is 4–8 ft., and the spring range 5–4 ft. The raised portion is within the intertidal zone, and that is why it is differentially eroded by the waves into a series of ridges, and is not being removed by the progression of a nip. It is covered by every tide. The area of the elevated part is 315 ft. wide and about half a mile long. The strata dip shoreward at an angle of about 25.
The following are possible explanations that might be offered to account for the phenomenon:
That it is a remnant of an island. Islands on the shore platforms are not unusual on this coast. However, it is of the very nature of
shore platforms that they are formed by the progression of a wave-cut nip. Examination of other islands on the same coast shows that the nip advances until the island becomes a rock stack, and then finally is too small to stand. Furthermore, the concordance of summit level shows that the high area had been previously planated.
That it is a remnant of a shore platform cut by a higher sea-level. Remnants of former shore platforms cut by relatively higher seas are found on this coast, but always at the shoreward end of the present platforms and not at the seaward end. It is difficult to imagine how the sea could pass over and round an area 315 ft. wide by half a mile long to cut a shore platform nearly 1,000 ft. wide and some 4 ft. lower.
That it is due to faulting. This provides a more acceptable explanation of the high outer area of the shore platform. Faults are very common throughout the district (see Henderson and Ongley's maps). In the present year (1947), three tidal waves and a number of earth tremors have been reported in the Gisborne District, these being due apparently to continuing movement on some of these fault planes.
As the change in elevation follows the strike of the bed-rock, the fault, is apparently a dip-slip-strike reversed fault.
Summary of Conclusions
Shore platforms of phenomenal width north of Gisborne are believed to be due chiefly to the nature of the country rock, although there are other contributory factors.
A shore platform with its outer third 4 ft. 6 in. higher than its inner two-thirds is considered to owe this character to recent faulting.
I am indebted to Dr. R. W. Fairbridge for reading the manuscript and making helpful suggestions.
Bartrum, J. A., 1916 High-water Rock-platforms; a Phase of Shore-line Erosion. Trans, and Proc. N.Z. Inst., vol. 48, pp. 132–134.
Henderson, J., and Ongley, M., 1920. The Geology of the Gisborne and Whatatutu Subdivisions, Raukumara Division. N.Z. Geol. Surv. Bull., No. 21 (New Series).