Tuatara: Volume 17, Issue 2, October 1969
Tilted Marine Beach Ridges at Cape Turakirae, N.Z.*
Tilted Marine Beach Ridges at Cape Turakirae, N.Z.*
The Axis of the growing Rimutaka Range reaches the coast of Cook Strait 1 km east of Cape Turakirae. The east flank is steeply dipping and faulted by the Wairarapa Fault. The west flank dips gently for 14 km to the Wellington Fault.
Six uplifted marine beach ridges here named A, B, C, D, E, and F are exceptionally well preserved for 5 km east of the Rimutaka Axis. Beach ridge A has grown since the 1855 Earthquake and is poorly developed: B was growing until elevated during the 1855 Earthquake; C is the largest and is the oldest containing pumice erupted in A.D. 200; D is the next largest; and F, the highest and oldest, is considered to have formed immediately after the post-glacial rise in sea level, and to be about 6,500 years old.
The six uplifted beach ridges dip westward. Ridge F is 25 m above the mean sea level 4 km north-east of Turakirae Head, 23.5 m at the head itself, and 13 m at 4 km north-west of the head. From the right angle in the coast at the head, the direction of tilting is determined as being about 270° and at right angles to the crest of the growing Rimutaka Range. The average rate of tilting of the gentle flank of the anticline is 0.03° per 1,000 years, and the rate of uplift at the anticlinal axis 4m per 1,000 years.
Since there is a complete absence of ridges intermediate between A, B, C, D, E, and F, and because of the sudden uplift that took place during the 1855 Earthquake, the growth of each ridge is thought to have been started and terminated by sudden earthquake uplift. The following uplift sequence is inferred from the size and elevation of the ridges at the anticlinal axis:
F 3 m, (5.6); E 6m, (4.9); D 9m, (3.1); C 6.5m, (0.6); B 2.5m, (0.1); A, successive uplifts being given in metres and time of uplifts in thousands of years ago. Within the sequence the amount of each uplift is closely proportional to the length of time from the previous to the following uplift, and the next uplift of the Rimutaka Axis is expected to take place about 500 years hence.page 83
The common intersection of ridges B, C, D, E, and F (but not A and B) on a line between the Rimutaka and Wellington anticlinal axes indicates a long continued balanced uplift of both fold axes. Ridges A and B intersect west of the Wellington Axis, indicating that movement during the 1855 Earthquake was unbalanced and took place on the Rimutaka Axis only. Balance would be restored by uplift of the Wellington Axis only provided that it takes place before Ridge A becomes well developed.
Regional Setting (Fig. 1)
The tilted marine beach ridges of Turakirae Head lie 17 km S.E. of Wellington City in the S.W. corner of the North Island of New Zealand. The region is crossed by two active major faults — the Wellington and the Wairarapa. Both are dextral and both are page 84 upthrown to the N.W. The region is crossed by two anticlines, an ill-defined one marked by the crest of the Wellington Peninsula, and a better defined one marked by the crest of the Rimutaka Range. The beach ridges lie on the west flank of the Rimutaka Anticline, and being Holocene in age, they indicate that the anticline is still growing. The dextral faulting and the anticlinal growth doubtless have a common cause but its nature is uncertain.
Nature of the Beach Ridges
The ridges at Turakirae Head are by far the best example of tilted beach ridges in New Zealand and among the best in the world (Fig. 5). There are six beach ridges in all: A, B, C, D, E and F; A being that of the present day and F the highest and oldest. Each old beach ridge is a distinct bank of gravel that slopes inland as well as seaward. The upper ridge is largely covered by screes from the Rimutaka Range but the lower four can be traced continuously for 5 km. The present day beach ridge is poorly developed and cannot be traced continuously. The ridges rest on a platform cut across steeply dipping ‘greywacke’ of Triassic age that is littered with boulders up to 2 m high. The platform and boulders extend seaward for an unknown distance. The boulders are older than the beach ridges and represent the most resistant part of the greywacke that was eroded when the platform was cut. Being one of the few sources of cheap resistent rock near Wellington City, the boulders are now being extracted, the coast is being appreciably changed, and its scientific interest diminished.
Much of the energy of the waves that sweep towards the expose coast is absorbed by the boulders and the seaweed that grows on them, and the beach ridge of the present day is forming at the remarkably low level of about 1 m above M.H.W.M. The tidal range is only 1.1 m. Conditions are thus ideal for recording progressive uplift and tilting of the land.
The height of the beach ridges was determined by levelling along the crest of each beach ridge with an automatic level, levels being taken at points about 100m apart. Irregularities, largely due to the number and size of the boulders, were smoothed out by averaging over each 500 m length, the mean difference between each observation and the average being about 0.3 m. Towards the mouths of the Orongorongo and Wainuiomata Rivers the boulders are buried beneath the flood of gravels from the rivers, the waves are not absorbed, and the ridges were formed 1 m to 2 m higher than where sheltered by the boulders. (See left hand side of Fig. 3).page 85
Fig. 2: Diagram showing heights of past sea levels relative to the present day sea level for localities with uplift rates ranging from 0 to +7 mm per year relative to the Netherlands. Diagram is based on data from the Netherlands, New Zealand, and other localities for which three or more sea level heights that are well distributed in time are known for the last 10,000 years. It should be noted that the lines for the various uplift rates are not independent. If one is known the others can be calculated. The diagram shows that submergence was followed in regions with uplift rates of more than about 2 mm per year (relative to the Netherlands) by emergence. The total emergence given by the emergence height of the Highest Holocene Shoreline (H.H.S.) is used to date the oldest beach ridge. It should be noted that the diagram is not applicable to localities undergoing isostatic or other non-uniform uplift, and that the uplift rate of most parts of the world is about + 1.3 mm per year relative to the Netherlands.
Direction of Tilt of Ground
All that can be determined, if a coast line is straight, is the tilt component in the direction of the coast. It is fortunate that there is a right angle bend at Turakirae Head. By using the heights of the ridges around the right angle, the direction of tilt on that part of the coast can be reliably determined. Directions of page 86 265° ± 1° and 266° ± 1° determined from Ridges D and E are in excellent agreement. An east-west plane was chosen for the cross section. The chosen direction is not appreciably different from the tilt direction and has the advantage that positions can be defined on it from the grid lines shown on existing maps. It is assumed that the direction of tilt does not change appreciably away from Turakirae Head.
Intersection of Ridges and Proportionality of Uplift
Fig. 3 shows the height of the six ridges on the east-west projection plane. The dots represent average heights, and if lines are drawn through the dots and extended to the west it will be found that ridges B, C, D and E (but not A and B) intersect (within the limit of accuracy of the measurements) at a point that lies on the southern extension of the south-east side of Wellington Harbour. The intersection of the lines at a common point indicates that the axis of rotation was fixed, and that successive uplifts have been proportional at all places.
The intersection of ridges A and B as determined by the uplift that took place during the 1855 Earthquake lies 25 km to the N.W. of the intersection of the other ridges. The reason for the difference in position is discussed later.
Ages of Ridges
The absence of material for radio-carbon dating makes it impossible to determine the age of the older beach ridges directly. At Turakirae Head ridge C contains pumice that was erupted in A.D. 200, and at Putangirua Stream 30 km east of Turakirae Head wood from one of the higher of a similar series of beach ridges gave a radio-carbon age of about 4,000 years (Grant-Taylor and Rafter, 1963). It is thus reasonably certain that the oldest of the ridges at Cape Turakirae is more than 4,000 years old. Indirect methods are used to get better age. It is assumed that during the last 10,000 years sea level was at the same ‘height’ at all places in the world. That is to say that its level was not appreciably affected by changes in the rate of rotation of the earth or by changes in the salinity (density) of its water. It is further assumed that during the last 10,000 years the average rate of uplift or submergence has been uniform everywhere outside the regions of isostatic uplift. It is impossible to be sure that any particular region is stable, but relative height changes can be determined provided that the height of sea level is known at corresponding times. In Fig. 2 the height of sea level relative to present sea level is shown for the last 10,000 years for regions that are rising uniformly at rates ranging from 0 to 7 mm per year relative to the Netherlands. The Netherlands is chosen as datum because sea level page 87 changes from six to three thousand years ago are better known there than anywhere else. Levels are from the Netherlands, New Zealand, Mississippi Hinge Line, and a few other places where at least three past sea levels are known.
Fig. 3: Cross section plotted on east-west plane showing heights of the six beach ridges at Turakirae Head. Heights are given in metres above approximate M.H.W.M. The vertical lines are the north-south grid lines, the numbered lines being 1,000 yards apart. The mouth of the Orongorongo River is at 453, Turakirae Head at 468, and the axis of the Rimutaka Range at about 495. Beach ridges defined by closely spaced levels are shown by solid lines, those less well defined by dashed lines. The dots are average values for the best defined parts of the ridges and the dotted lines give the best indication of uplift and tilting. By extending the dotted lines it will be seen that ridges B, C, D and E intersect near grid line 400, whereas ridges A and B intersect much further to the west.
The curves have been constructed by assuming that the rate of uplift has been uniform, but in general different, at all localities. All samples not more than a few kilometres apart are considered to belong to a single locality. Height was then plotted against time on a separate sheet of tracing paper for each locality, the height scale and time scale being the same for each locality.
The sheets of tracing paper were then superposed and adjusted to give the best fit for the points representing the radio-carbon-dated past sea levels. For most localities there are two degrees of freedom. One corresponds to the unknown rate of uplift and the other to the unknown height above or below sea level of the dated samples at the time they formed. The uplift rate is allowed for by tilting the sheet of tracing paper, and in order to prevent the tilt from seriously page 88 shortening the time scale, the time scale was made long relative to the height scale. The unknown height was allowed for by moving the corner of the paper with the origin point on it up or down.
Fig. 4: Diagram showing stages of ridge formation and uplift near axis of Rimutaka Range. Duration of beach formation is based on relative cross sectional area of ridges and is given an 10 arbitrary units for C, the largest ridge. It will be seen that the points midway along the times representing ridge formation (still-stand) lie on a straight line that gives the average rate of uplift. The amount of each uplift has been proportional to the total time from the previous to the following uplift, and the time of the next uplift is forecast by the dotted lines in the top right hand corner.
Fig. 5: Oblique air photograph of rock platform near Turakirae Head. The four beach ridges are B, C, D, and E. A is poorly defined and is hidden by surf, and F is buried beneath the scree at the foot of the hill at the back. Boulders and rock outcrops up to 3 m high are scattered over the whole width of the platform. Note the absence of any trace of intermediate beach ridges.
Rate of Uplift and Rate of Tilting
As mentioned, the average rate of uplift of the Turakirae coast relative to the Netherlands, or any other place, can be determined by comparing the differences in height between present day sea level and the sea level at any particular time in the past at the two places, and then calculating an average relative rate of uplift for the time interval. In order to determine the true rate of uplift, comparison has to be made with an area that is known to be stable. But no place is known with certainty to be stable and the best that can be done is to assume that the places that are modal with respect to sea level changes are stable, and those non-modal unstable. For the last 50 years mareograph (tide guage) measurements can be used, and it is commonly accepted that sea level has been rising at a rate of 0.8 mm per year for the last 50 years (Jakubovsky, 1966). It is of interest that this is also true for Wellington and Auckland, the two places in New Zealand where we have long-term mareograph records.page 90
In the Netherlands the mareographs are all tied in to Amsterdam which has an unrivalled record that extends back to 1670 A.D. (Waalewijn, 1966.). For the last 50 years sea level has been rising there at the rate of 2.1 mm per year, 1.3 mm per year faster than the modal rate of 0.8 mm per year, and it is thus reasonably certain that the Netherlands is subsiding at 1.3 mm per year. It is more difficult to obtain modal values for earlier sea levels but those available indicate that the Netherlands has been subsiding at about the same rate for at least the last 7,000 years. By making the correction of 1.3, the rate of uplift of the Turakirae coast can be determined from Fig. 2. It is about 4 mm per year on the coast at the axis of the Rimutaka Anticline decreasing almost uniformly to zero at the south-eastern side of Wellington Harbour at the point of intersection of the uplifted beach ridges. Tilt rate is more easily determined than uplift rate, and depends merely on knowing the tilt and the age of a tilted surface. For the Turakirae coast the average rate of tilting for the last 7,000 years is 0.03° per 1,000 years.
In spite of conditions being ideal for their formation and preservation, there is no trace of any shoreline features between the six well defined ridges (Fig. 5). It is inferred that the ridges represent periods of still-stand, and the spaces between the ridges periods of rapid uplift. The last uplift took place during the 1855 Earthquake and was sudden, and it is inferred that the other uplifts took place during earlier earthquakes and were sudden also.
The oldest beach ridge is estimated to have formed about 6,500 years ago and the youngest cannot have started to grow until after the 1855 Earthquake. The relative time of the still-stand for the intermediate beach ridges is estimated from their relative cross sectional area by assuming that their rate of growth was constant at any one part of the coast. As the age of the oldest ridge and the uplift values are known a diagram has been constructed (Fig. 4), showing the amount of each uplift and the duration of each still-stand periods. It will be seen that the mid points of each ‘still-stand’ line lie on a straight line which defines the average rate of uplift at the crest of the Rimutaka Anticline (the part of the coast chosen for illustration). The amount of any one uplift is thus proportional to the total time from the previous to the following uplift and it is estimated that the next uplift will take place in 500 years time and will be at least 1.5 m.page 91
Fig. 6: Marine rock platforms on the spur between the Orongorongo and Wainuiomata Rivers. The Holocene bench is at the bottom right corner. The lowest high level bench is considered to represent the Last Interglacial and to be about 100,000 years old. It is covered by several metres of scree and rises towards the crest of the Rimutaka Range which forms the sky-line of the photograph. There are two higher high level benches. The lower of the two is well defined and the higher on the extreme left hand side of the photograph less well defined. The higher is 250 m above sea level.
Common Intersections, Proportionality of Uplift, and the 1855 Earthquake
The difference in position between the line of intersection of ridges A and B and the common intersection of the older ridges B, C, D, E, and F requires explanation. When projected on to a cross section in the direction of tilt the older beach ridges are almost straight lines and successive uplifts have been proportional at all places. The intersection of ridges A and B being well to the west of the common intersection of the older beach ridges indicates that the uplift that took place in 1855 was not proportional to the earlier uplifts.
The difference can be explained if the uplift that takes place during particular earthquakes is related, for example, to the parallel folds of the south end of the North Island. If folding, as is likely, is a summation of sudden movements, then they must average themselves out in some way to produce the folds. If they were all centred on a single point they would produce, not the parallel anticlines that exist, but a simple dome. It is inferred that the averaging out is done by earthquake uplift taking place first here and then there, and never in the same place until a stage in the uplift of the anticlines of a region is complete. In the long term the anticlines will grow steadily each keeping pace with the other, but the short term uplift during individual earthquakes will be anomalous with respect to the long term uplift pattern.
The apparently anomalous uplift of the 1855 earthquake is thus expected. It will be rectified if uplift takes place on the axis of the Wellington Anticline before Beach Ridge A has become sufficiently well developed to become an obvious beach ridge in the future.
Geological Significance of Tilting and Uplift Rates
The values given for the uplift rate at the axis of the Rimutaka Range at the coast of 4 mm per year, and the tilt rate for the dip slope of the range of 0.03° per thousand years, appear to be small and have little meaning for geologists until they are expressed in geological time intervals. They then become 4 km per million years and 30° per million years, and it is clear that the deformation cannot have continued for a million years nor can it continue for a million years into the future. It is thus important to know if the deformation that has taken place during the last 7,000 years is merely a short-lived phase with little tectonic importance, or whether it has continued long enough to have tectonic importance. The higher benches on the Turakirae coast provide the answer. High level rock platforms that are analogous to the low level platform on which the beach ridges are situated, are conspicuous coastal features west of the Orongorongo River (King, 1930). The lowest is almost page 93 certainly that of the last interglacial and some 100,000 years old. From the higher platforms which continue almost to the surface of the west flank of the range (Fig. 6) it is thought that the formation of the range itself is part of the present phase of deformation. A maximum age of a mere half a million years for the Rimutaka Range is thus probable.
In modifying my original account of the Turakirae beach ridges I have been guided by suggestions made by the Ph.D. Students of the Geology Department of Victoria University. I wish to thank them for their help and interest.
Grant-Taylor, T. L. and Rafter, T. A., 1963. New Zealand Natural Radiocarbon Measurements I-V Radiocarbon, Vol. 5, p. 122 (N.Z.-24).
Jakubovsky, O., 1966. Vertical movements of the Earth's crust on the coasts of the Baltic Sea. Ann. Acad. Sci. Fennicae, Ser. A, III, No. 90: 479-488.
King, L. C., 1930. Raised beaches and other features of the south-east coast of the North Island of New Zealand. Trans. N.Z. Inst. Vol. 61, pp. 498-523.
Waalewijn, A., 1966. Investigations into crustal movements in the Netherlands. Ann. Acad. Sci. Fennicae, Ser. A, III, No. 90. Helsinki.
Wellman, H. W., 1967. Tilted Marine Beach Ridges at Cape Turakirae, N.Z. Journal of Geosciences, Osaka City University. Vol. 10, Art. 1-6.
* A somewhat shorter account under the same title was given at the Eleventh Pacific Science Congress in Tokyo during August 1966 and was published in Japan (Wellman, 1967).