The Structure of North-eastern Marlborough, New Zealand
[Read before the Wellington Philosophical Society, August 11, 1936; received by the Editor, September 24, 1936; issued separately, June, 1937.]
The Structure of the Clarence-Kekerangu Sector.
The Structural Elements.
The Structure of the Kaikoura Mountains.
List of References Cited.
It was inevitable, when I left New Zealand, that many strands of geological thought, unwoven into any definite pattern, should be rudely severed. Especially was this so regarding Marlborough, where I had spent most of the vacations of 1932–34. This presentation of only partly-tested hypotheses is undertaken, therefore, with full appreciation of their lack of completeness; but in the hope that they may result in stimulation of geologic research in Marlborough, and consequent increase in our understanding of the late Tertiary geology of New Zealand. Particularly is this so with regard to what is described as “the Thrust.” The evidence upon which the account is based is still very meagre and admits of other interpretations; but as I have no opportunity of gaining new material or reëxamining the old, it has seemed best to lay the emphasis upon that hypothesis which seemed to work best in the field. After all, the philosopher has said, “Orthodoxy is my doxy, heterodoxy is the other man's doxy,” and perhaps we may be allowed to believe that either is better than no “doxy” at all. Finally, I may perhaps remark that, in a country where the Table Mountain Sandstone (Devonian) lies horizontally for scores of miles, and where equally featureless Karroo Beds extend from near the coast to the Drakensberg, it does come as a considerable relief to discuss foldings and thrustings and immense dislocations in Miocene strata.
The Structure of the Clarence-Kekerangu Sector.
The Clarence-Kekerangu Sector, with the district north of the Kekerangu River, appears to be the most perfectly preserved remnant known to us of the cover which once extended over the sites of the Kaikoura and Patutu Ranges, and offers a rich harvest to the structural investigator. A detailed study of its nature cannot fail to result in a much more perfect understanding of New Zealand's late Tertiary tectonics.
An account of the stratigraphical succession in this area has already been presented (King, 1937), together with a geological sketch-map, a glance at which is sufficient to show that the structure
is by no means simple. Owing to the scale of the map, it was impossible to represent the more complex areas in detail, and in places the map had to be more or less generalized. Briefly, three sub-districts are readily distinguishable, (a) the broken range of limestone cast of the Clarence Gorge, (b) the large upland near the Clarence Mouth over which the Great Marlborough Conglomerate outcrops, and (c) the more northerly coastal strip in which the various stratigraphical units are more or less intimately folded and faulted together. Attention may here be drawn to the narrow strips of Great Marlborough Conglomerate which are sandwiched in between outcrops of Amuri Limestone in this third sub-sector.
The ridge of limestone has partly a synclinal structure which, at Trig. Jacob* may be overturned, for the upper part of the hill is more flinty than the lower (Fig. 6). Between Trig. Jacob and Trig. Esau, the ridge is breached by a small stream which is probably antecedent, and just north-north-east of Esau it is dislocated from the main limestone mass which extends to the Molehill. After disappearing at Priam's Flat, the limestone ridge crosses to the south bank at Corner Hill and eventually reaches the coast some two and a-half miles south of Clarence Bridge. Possibly a branch extended at one time from Jacob towards Mount Alexander in the Puhi Puhi Valley (Fig. 6).
The upland of conglomerate near the mouth of the Clarence River has a structure which is difficult to unravel because of the constantly changing nature of the conglomerate mass. Hector (see McKay, 1890, p. 173) has reported folding on the north side of the Clarence River; but this the writer could not verify. Dip and strike are unusually difficult to determine owing to the absence of bedding, and the writer had not time to apply other field methods. The upland has been deeply dissected by numerous small streams (e.g., the Kawau-nui) which have carved deep valleys with precipitous sides out of the resistant mass. The thickness of the conglomerate must here be very great, for the valley-walls are, in most cases, composed of this material from top to bottom (Figs. 1, 2).
Most of the features of structural interest occur, however, in the northern sub-sector. North of Washdyke Stream the younger rocks form a general pitching syncline which, especially to the north, has been complicated by later faulting. The western extension of the syncline is also truncated by the powerful fault which forms the eastern boundary of the Limestone Range. These faults are of the high-angle type commonly associated with the “blocking movements” usually spoken of as the “Kaikoura Orogeny.” As such they require little comment, though one may be deemed worthy of special mention. This is the fault which, crossing the Deadman Stream above the waterfall, coincides very closely in strike and dip with the upper surface of the Great Marlborough Conglomerate, causing a repetition of the beds and developing a second ridge of conglomerate in the topography. That this fault should approximate
[Footnote] * For place-names see maps supplied with the writer's paper on The Tertiary Sequence in North-eastern Marlborough, which appears also in this part of the Transactions, pl. 11.
to the bedding, here 45° N.W., seems to be purely fortuitous, no similar features having been found elsewhere in the sub-sector. The fact that the angle of 45° is a common dip with the basal Awatere beds of the Haldon Hills district on both sides of the Haldon Hills Fault, however, may indicate that this is a critical angle for compressive stress, which is relieved by fracturing after the beds have been deformed by that amount.
In various sections in the northern sub-sector, the Amuri Limestone is followed by a mash, a few inches or feet in thickness, of “Grey Marl” and conglomerate fragments, and then by a moderate thickness of Great Marlborough Conglomerate, which is succeeded once more by Amuri Limestone.
Three such occurrences were studied in reasonably clear sections: (i) the small strip of conglomerate north of the Pimple, which may be seen to perfection in the gorge of Washdyke Stream, (ii) the long strip which, commencing near the upper end of the Washdyke gorge, crosses Woodbank Stream below the junction of its two main branches and appears finally at the mouth of Deadman Stream, and (iii) the strip, with a base of conglomerate-marl mash, involved between the two parallel limestone ridges of the Razor Back.
(i) Washdyke Section. This section is difficult to decipher, being complicated by the effects of a fault which strikes almost along the stream. A result of this, or another fault, is that the sequence including the smash-band is repeated, and strong folding may also have occurred since the movement which gave rise to the band. Part of the succession of beds, stripped of details, may be studied in Fig. 3, a view southwards across Washdyke Stream north-north-east from the Pimple, which is just out of view to the right. Outcrops near the sky-line are of Great Marlborough Conglomerate constituting the ridge north of Kawau-nui Stream, and the section shown in the south bank and tributaries of Washdyke Stream, reading from east (left) to west (right) is: “Grey Marl,” Amuri Limestone (white), Great Marlborough Conglomerate (dark) separated from the limestone by the smash-band, “Grey Marl,” Amuri Limestone in a small ridge, and finally “Grey Marl,” beyond which repetition occurs due to later faulting. These alternations of strata occur in too small a compass to be delineated on the map supplied with the writer's other paper referred to above. The western (second) ridge of limestone apparently marks an anticlinal between two outcrops of the “Grey Marl,” and in this connection it may be noted that the writer found blocks of sandstone at about this position in the bed of the stream which appeared “foreign” to the surrounding outcrops and contained fragments of a fibrous shell and sections of bones, indicating that the Cretaceous may be nipped in the anticlinal or at a fault close by. As may be seen in the illustration (Fig. 3), all the beds and the smash-band stand at high angles.
(ii) Woodbank Section. The view shown in Fig. 4 is south-westwards across the gorge of Woodbank Stream, along a tributary valley the floor of which is composed of conglomerate while the ridges on either side are of Amuri Limestone, a relation which also shows in the walls of the Woodbank gorge. This section is related to that
a short distance away in the Washdyke from which it has been separated by later faulting. The anticlinal folding seems to be absent, resulting in a simpler section than in the Washdyke; but this is no obstacle to their correlation.
(iii) The Razor Back. The outcrops here are the poorest of the series, no complete section being available; but the intervention of a narrow outcrop of conglomerate between two sharp ridges of limestone is sufficiently clear to show distinctly in the topography.
From their similar appearance, with the “sandwich” of conglomerate (with or without a margin of “Grey Marl”) between two ridges of Amuri Limestone, the constant presence of the “smashband” between the conglomerate and one limestone ridge, the similar attitude in each case and their relation to the general pitching syncline of the area, allowance being made for later high-angle faulting, these exposures must be regarded as part of a once continuous feature now disrupted by later movements of folding and faulting. Such movements can be abundantly demonstrated in the neighbourhood. The essential condition of the features is the presence of the conglomerate-marl mash between the conglomerate and one of the adjacent developments of limestone. This the writer regards as the crush-zone or “sole” of a powerful, sub-horizontal movement of compression whereby masses of Amuri Limestone and super-incumbent strata have been thrust over similar formations.
The Structural Elements.
Beyond that described above, no evidence of horizontal or low-angle thrusts has yet been recorded from Marlborough: the movements which raised the Kaikoura Mountains to the present elevation had a large vertical component, whether the major factor was folding or faulting. Arguments may be advanced for regarding some of the features of the known exposures as due, in part, to tight folding; but the constant nature and presence of the “smash-band,” its involvement in later folding, and its relation—similar to that of a bedding-plane—to the general synclinal of the district impress the writer as being sufficient to show that the original form of the crush was a simple one. When allowance for later movements is made, the disconnected outcrops of the “sandwich” with its attendant plane of movement in Washdyke Stream, Woodbank Stream, and the Razor Back, and at the mouth of Deadman Creek, become resolved into a single feature, a splendid thrust-plane, which has since been disrupted and assumed generally steep attitudes in its several parts. The movement appears to have taken place at an early stage in the formation of the Kaikoura Ranges, for the feature is affected by both folding and high-angle faulting. That it cannot have followed very closely upon the formation of the Great Marlborough Conglomerate, however, is indicated by (a) the absence of the deformation structures usually found when unconsolidated material is powerfully disturbed, (b) the manner in which pebbles in the conglomerate are sheared in the Woodbank section, and (c) the apparently conformable manner in which the Ericaburn and Upton beds lie on the
conglomerate in neighbouring areas. These facts also tell strongly against the hypothesis, which might otherwise be advanced, that the enormous masses of Amuri Limestone and “Grey Marl” included in the Great Marlborough Conglomerate in some localities were derived from the front of such an advancing, over-riding sheet of these materials.
In the next section it will be shown that great overfolds have played a considerable part in the genesis of the Kaikoura region. To these the “thrust” may be intimately related, possibly in the form of a “strut fault.” Such faults, or low-angle thrusts, are developed where lower, incompetent beds yield to crustal stress by folding, the shortening in the upper, harder formations being taken up by lateral movement, with only minor buckling, along a thrust-plane (Willis and Willis, 1934, pp. 230–231). Such a “sole” may be flexed if the lower folding is uneven. If the underlying Clarentian rocks were to fold as suggested, the Marlborough thrust might well be classed as of the “strut fault” type; but with the great rigidity of the greywacke basement beneath the Clarentian it is difficult to see how crustal shortening of any magnitude could be accommodated there.
The close proximity of the sea, with deep water, to the east and the immediate presence of the Seaward Kaikoura Range behind suggest that, if the latter was formed largely as a result of over-folding, the thrust may be representative of that peculiar type, the “erosion thrust.” Erosion thrusts appear to result from the over-stressing of recumbent folds towards an area where pressure is weak and erosion active. Most of the conditions for the formation of such a fracture seem to be present in the Clarence-Kekerangu sector, and any reëxamination should be made with this possibility in mind.
There remains the further possibility that these remarkably similar occurrences belong not to a single thrust-plane as we have hitherto assumed; but that they represent part of an incipient imbricate structure of curving shears ascending from a still deeper thrust, in advance of the main Kaikoura masses. In this case the features described must again be associated with the great overfolds (see next section) which have occurred along the Clarence and other valleys.
The relative merits of these interpretations are at present difficult to assess; but reëxamination of the district by some unhurried researcher, able to study also the infaulted strips of conglomerate recorded by McKay, Thomson, and Jobberns in the Kekerangu Gorge and Benmore Stream, may result in establishing the relations more clearly. Reference must also be made to the conglomerate beds recorded from near Cape Campbell by several investigators, who have succeeded in giving very diverse accounts of them. McKay (1877, p. 190) considered them to be superficial and referred them to his Post-Miocene Conglomerate; Park (1910, p. 201) alluded to them as Pleistocene moraine. Later work has shown them to be older and to be involved in the major earth-fractures affecting other rocks (Morgan, 1916, pp. 18–19). Jobberns (1935) compared them with the conglomerates of White Bluffs, equivalent to the Great Marlborough. Thomson, however (1916, p. 346) was of opinion that they
were part of the basal Clarentian conglomerate, and the writer has stated (1934, p. 8) that such outcrops as he had examined might “without hesitation be referred to the Clarentian,” though he noted the possibility that outcrops of a second conglomerate might also occur. In view of the peculiar relations between the Amuri Limestone and the Great Marlborough Conglomerate in the Clarence-Kekerangu sector, reëxamination of the Cape Campbell district may reveal the presence there also of “the Thrust.”
Comparison with the “Taitai Overthrust” (Morgan, 1928, pp. 54–55; Ongley, 1930, pp. 376–382) of the Gisborne area reveals little apparent similarity. The “Taitai” involves strata of a very different type and age, and the fault-plane still preserves a relatively simple form, whereas that of Marlborough has been warped and dislocated by later movements. There is probably, therefore, no immediate connection between them. Strong overthrusts may also exist in the region of the Haurangi mountains, and these may be expected to conform to the Kaikoura type rather than to the Taitai.
An extremely important element in the structure of the eastern part of Marlborough, to which insufficient attention has hitherto been directed, is the folding of the Notocene covering beds.
Cotton (1913, p. 227) realized that to account for the present topographic features and the distribution of the Notocene rocks it was necessary to assume some folding of the younger (covering) strata before the major faulting took place, and gave a clear exposition of the pre-faulting site of the Kaikoura Ranges as “portions of two anticlinoria … with an intermediate synclinorium.” The introduction of this conception of folding was due to the obviously impossible height of the mountains before erosion if faulting alone were invoked to account for their origin. Even this conception probably falls far short of the mark so far as the influence of folding is concerned, for not only have gentle anticlinoria and synclinoria existed, but also strongly overfolded and, in some cases, possibly also overthrust flexures.
Hector (1890, p. xxxv) has recorded inversion of the Cretaceo-Tertiary rocks, in part, along the Hapuka River, and in his section across the Middle Awatere Valley (loc. cit., p. xlvi) has included a reversed fold among the Upper Cretaceous rocks on the western side. Thus, in these situations, the faulting which bounds the valleys is to be regarded not as simple dislocation, but as the final phase of a very strong compression which had previously produced overfolds. With regard to the Middle Clarence Valley and neighbouring localities, Thomson (1919, pp. 297, 310, 316–7, 321, 326, 327) has clearly demonstrated that towards either end of the Great Clarence Fault the beds of the cover on the downthrown side are involved in an overturned syncline which is truncated by the fault. There is great probability, therefore, that the fundamental structure along the whole front (east side) of the Inland Kaikoura Range is of this nature; but that in the middle portion, owing to curvature of the fault or arching of the original synclinal axis, the fault transected the lower
half of the overfold and that the upper limb has subsequently been removed by erosion. Forward remmants of such a fold might in some cases show a structure somewhat similar to that of the feature described as the “Thrust.” The appearance of the monoclinal ridge of limestone in the Middle Clarence Valley, developed from the homoclinal series of beds east of the fault-plane, may thus be a falsely simple form derived from more complex conditions. If such be the case, then the mountains owe considerably more of their original height to folding than has been hitherto acknowledged, the final reversed faulting being responsible for only half the amount usually specified.
To this hypothesis the rocks along the eastern base of the Seaward Kaikouras may be expected to yield significant data. The observations of McKay (in Hector, 1890, p. xxxv) are supported by Jobberns's investigations (1932, pp. 341–352) in the Puhi Puhi Valley, where he has demonstrated that the covering beds are involved in an overturned syncline between the Seaward Kaikouras and the Patutu or Coastal Range. * Jobberns's insistence upon folding rather than faulting is well illustrated in the following passage (loc. cit., p. 343): “Examination of these important sections (Puhi Puhi) leads to the conclusion that the area is a long synclinal depression, the syncline being overturned by thrusting pressure from the west. Faulting has, as might be expected, resulted from this thrusting, but it is of relatively a minor order, and its effect on the surface relief features has been overestimated.”
A monoclinal ridge of limestone also exists east of the Patutu Range. In the neighbourhood of the Clarence River this ridge is of simple structure, but on the coast at Waipapa Point it appears to be folded into a close syncline. Here Speight (in Jobberns, 1932, p. 349) has observed that the Amuri Limestone is overridden by the sandstone of the Patutu Range so that in both folding and later fracturing the eastern base of the Patutu Range appears to be similar to both the Seaward and Inland Kaikouras.
Structurally there is probably some connection by way of the Wharekiri Valley between Mount Alexander and the Limestone Range extending from Mole Hill to Jacob along the front of the Seaward Kaikouras in the Lower Clarence (see Fig. 6). The main branch, however, is offset and forms the ridge along the eastern base of Patutu Range. Thomson (1919, p. 301) records that the “Sawtooth Range is flanked on the south-eastern side by Clarentian rocks followed by the Amuri Limestone in the Lady Range.” Here steep faulting does not exist or is of relatively small amount, Sawtooth Range owing perhaps the greater part of its elevation to folding. The writer is not aware, however, whether this is so strong as to be termed an overfold.
The axes of the folds appear to have been gently arched, a feature which has been noted by Thomson (1919, p. 321) along the Quail Flat Fault and deduced by him along the main Kaikoura
[Footnote] * His diagrammatic section on p. 351 is an oversimplification by distortion of the true scale and his “fold lines” are not dislocated by the later faults. A much truer picture would probably be presented by incorporating some of the minor folding of Cotton's diagram which he reproduces on the opposite page.
Range from the heights of the major peaks (loc. cit., p. 300), where it will account for the cutting out of the upper limb of the overturned syncline by the Clarence Fault along the middle part of the valley.
The tectonic features of the third group are typified by steeply-dipping fault-planes, usually of the reversed type. The profound influence of these faults on the topography of this region was first demonstrated by McKay in his series of classic papers published between 1886 and 1892, where he showed that the mountain masses form the higher parts of gigantic, tilted fault-bounded earth-blocks. All later geologists have accepted McKay's interpretation, and Cotton, who expanded the conception in the type-locality (1913, pp. 225–246) and extended it to cover the orogenic movements over the whole of New Zealand in late Tertiary and Recent times, termed the upheaval the “Kaikoura Orogeny” (1916, pp. 314–318). Furthermore, all geologists have admitted that the faulting has been due to the accumulation of compressive stress, the younger rocks at a fault-contact generally appearing to dip under the older. Thus Hector (1890, pp. xxxv-xxxvi), summarizing McKay's data, wrote, “In 1876 Mr. McKay showed that between the Waipapa Mountains and the Seaward Kaikouras or Looker-on Mountains the Amuri and Cretaceo-Tertiary strata dip under the older rocks forming that mountain range. In 1885 he showed that much the same thing has happened along the base of the Inland Kaikouras, and proved that the first dislocations took place not earlier than older Pliocene times…. In 1888 he further showed that a similar fracture runs along the Awatere Valley; and now that all three lines of fracture are of the same date…. All these fracture-lines show decided evidence of lateral thrust from the south-east or east, and a tendency of the strata on the north-western side of the fracture to rise and over-ride the younger rocks that may chance to be involved on the opposite side of the fault.”
Most of the fault-planes are inclined steeply to the north-west, governing the trend of the main mountain chains; but this is not always true of smaller faults or even the ends of some of the larger. The direction of movement has usually been conceived as an overthrust from the north-west, but the writer has given evidence (1934, p. 9) which, supported by the occurrence of a number of earthquake foci off the east coast of the South Island, indicates that Hector's view of underthrust from the south-east is essentially more correct.
That these “blocking movements” are the last of the Kaikoura orogenic cycle is demonstrated by the fact that the fault-planes truncate the folded structures. As to how distinct the movements are, opinions may vary. Along the Middle Clarence folding no doubt largely passed into faulting; but that the two were not always in harmony is shown by the different manner in which the fault transects the fold in different parts of the valley. In all probability folding was succeeded, in general, by faulting wherever it was insufficient to relieve the accumulated stress. The type of rock involved no doubt exercised a considerable influence in each case. Where
the covering strata were thick, e.g., in the Middle Clarence Valley, folding seems to have been of great magnitude; where the covering strata were thin, e.g., in the Medway District, folding has been negligible and all relief has been achieved by faulting alone. Cotton (1916, p. 248) has probably supplied the explanation: “Perhaps owing to the rigidity of the older mass, the strata of which had previously been folded on other lines and would strongly resist the new folding, the region was broken up into a number of ‘blocks’ bounded on one or more sides by faults.” With the continuation of the sentence, “folding being generally subordinate to faulting except in the higher members of the covering strata, where the resistance of the rigid floor had least effect,” we can no longer agree, for Thomson's work has proved that, on the contrary, it is always the lower members of the cover that are affected by strong folding. Possibly a fracture in the greywackes of the undermass may pass into a fold dying out upwards in the cover.
The Structure of the Kaikoura Mountains.
The history of the Kaikoura mountains begins with the deposition of the Clarentian sediments on the planed surface of an older, folded mountain system. Up to the time of deposition of the “Grey Marl” the history is that of a sedimenary cycle, with volcanic interludes in the earlier part. The character of the middle member, the Amuri Limestone, has been taken to indicate a deepening of the water with a cessation of the supply of terriginous material, followed by a progressive shallowing and a renewal of deposition of land waste. At this stage, in the Lower Awatere-Wairau district, emergence took place, the newly deposited beds being stripped from the greywacke basement. The extent of this emergence is limited to the east by the presence of Amuri Limestone in the Cape Campbell-Flaxbourne district. In the shallow seas which followed the Deadmans Creek sandstones and the Medway beds were laid down, with their species of Zeacolpus, Struthiolaria, Polinices, Austrosipho, Baryspira, Glycymeris and Dosinia. Following this, there is again evidence of local deformation, for the Medway beds are commonly lightly truncated and were removed altogether on the site of the head of Upton Brook before the accumulation of that peculiar formation which succeeds, the Great Marlborough Conglomerate. Whatever shades of opinion exist regarding the mode of deposition of this mass, there can be no doubt that a neighbouring land-mass was elevated considerably at this period and that the deposit accumulated very rapidly. The forces which were later to culminate in the uplifting of the Kaikouras were already at work. The location of this land-mass is by no means certain. Some of the constituents of the conglomerate were more or less locally derived (King, 1934, pp. 13–14; and 1937); but the greywacke pebbles, which form the bulk of the deposit, were more travelled. The general character of the formation is that of a Nagelfluh before the face of a rising mountain chain. To the east the country plunges from a steep coast into relatively deep water, and any ancient terrain must here have sunk along profound faults similar to those of the Clarence and Awatere Valleys.
To the west are mountains of a lower order of magnitude, the Spencer and St. Arnaud Ranges, which, so far as is known, preserve no remnant of a Tertiary cover. These mountains may, therefore, be older than the Kaikouras, and may have provided to some extent the materials of the Great Marlborough Conglomerate shortly after their elevation. The conglomerate was consolidated, however, before the final movements which heaved the Kaikoura chains themselves into existence, as is shown by its deep involvement along the major block-making faults and the manner (e.g. at Woodbank Gorge) in which the pebbles are sheared across. In some localities also it is succeeded with apparent conformity by younger beds, though in others the preliminary Kaikoura movements had begun before the deposition of the succeeding strata.
With the early throes of mountain-building on the Kaikoura site (the “Thrust” and the “Overfold”) the primitive Inland and Seaward Kaikoura Ranges and the Patutu Range rose into existence, probably with a low altitude. All later deposits are marginal, and, though involved in the later block-faulting, show no traces of the earlier movements.* Thomson's descriptions of the overturned syncline of the Middle Clarence and Ure Valleys, and Jobberns's account of the Puhi Puhi overfold, clearly show that beds younger than the Great Marlborough are not included in these structures. The later Upton and Starborough beds of the Lower Awatere sector are probably representative of this marginal group. Though involved along the Awatere Fault as far as Jordan they probably at no time extended farther up the valley into the Middle Awatere. If they had done so outliers should be preserved in the fault-angle depression above Jordan. Thus only beds up to the Great Marlborough Conglomerate represent the true covering beds of the Kaikouras.
As the later deposits occur only on the east and north, the western mountains are probably the older (vide supra) and may not have had a marine cover younger than the Amuri Limestone at any time, certainly none much younger than the “Grey Marl.” Thomson (1917, p. 400, footnote) has expressed the contrary opinion: “The Kaikoura movements did not commence everywhere at the same time, and probably the Awatere and Clarence areas were the first to be affected”; but on this point the more recent pronouncement of Marwick (1934, p. 959) on the molluscan evidence is more definite and may be quoted in full: “According to the molluscan evidence, the Kaikoura orogeny consisted of two main phases, an older, western one, and a younger, eastern one. The Otago mountains, the Southern Alps, and the Nelson Mountains have no marine sediments later than Awamoan in their structure and belong to the older, western phase. The Kaikoura, Tararua, and Ruahine chains, on the other hands, have Waitotaran (lower Pliocene) marine sediments deeply involved in the faulting which elevated them.” He refers, in each case, to involvement along faults of the more or less vertical type. Thus the western mountains seem to have undergone “blocking” at much the same time as the Kaikoura region was
[Footnote] * The gently pitching syncline of the Ericaburn is probably not related to the intense overfolds of the Puhi Puhi and Ure Valleys.
subjected to folding, for no rocks younger than Great Marlborough Conglomerate (which is probably late Awamoan) are known to be involved in the fold movements. In both instances the later type of faulting has persisted to the present day, as is shown by the movement on the White Creek (Murchison) and Palliser Bay faults since white settlement, and the rent which opened along the Awatere Fault in 1848 or 1855 (Hector, 1890, p. xli).
The structures of the Kaikoura Ranges may now be considered with respect to the mountain building movements elsewhere in New Zealand. In the Southern Alps traces of three distnct orogenies are recognizable: one in Palaeozoic times, which folded the Ordovician rocks of Preservation Inlet, Westland and North-west Nelson along N. W.—S. E. lines with strong overfolding to the north-east; one in the Lower or Middle Cretaceous, which, acting on a great thickness of late Palaeozoic and Mesozoic clastics, folded them very tightly in a direction almost at right-angles to that of the old rocks; and, lastly, in late Tertiary times an orogeny of the block-mountain type, which, splitting the country into a number of separate blocks, elevated them to different heights. This last, the only orogeny with which we are at present concerned, resulted generally in a back-tilting of the blocks to the east (Morgan and Bartrum, 1915, p. 62; Bell, Clarke and Marshall, 1911, p. 22), the faults separating them being of the reversed type and dipping also to the east. Morgan (1908, pp. 72, 108) records evidence of Miocene rocks being involved in the great reversed faults of the western side of the Alps and appearing to pass under gneiss, while at “Cowhide Creek and at Hende's” crushed dark schist “has been pushed over relatively recent gravels” on the line of an old overthrust to the west.
This direction of thrust is opposed to that in the Kaikoura region, where, since the time of MeKay, the great reversed faults have been recognized as dipping steeply to the west. The opposition of these two ranges is emphasized by the occurrence of the covering strata mainly to the west of the Alps and to the east of the Kaikouras and the similar distribution of the two Nagelfluh deposits, the Moutere gravels and the Great Marlborough Conglomerate. The former extends, probably with some difference in age, from Ross to Golden Bay along almost the whole length of the Alps so far geologically surveyed (Morgan, 1908, p. 113), and, like the Great Marlborough Conglomerate, which it much resembles, is partly composed of Tertiary material that has been more or less locally derived (Morgan, 1908, p. 35; Morgan and Bartrum, 1915, p. 52). In later years Morgan (1928, p. 55) regarded the northern portion of the South Island as “a complex mountain range with a two-sided orogen,” a point of view which permits comparison with some interpretations of the Alpine system of Europe, the Southern Alps (or Zealides) corresponding broadly with the Alpine chain proper and the movements in the Kaikouras (or Kaikourides) comparing with the backwardly directed movements of the Dinaric Alps. In both cases the minor mountain chains seem to be of somewhat later formation.
In the southern portion of the North Island there are again two ranges, the main or Rimutaka chain and the Haurangi Mountains.
The evidence available clearly indicates that the Haurangi Mountains and the eastern margin of the Rimutakas are of the Kaikouride type; but, as yet, no comprehensive statement has been made regarding the direction of movement in the western part of the main range. W. H. Hobbs (1923, p. 751) records having observed “on the shores of Port Nicholson an excellent example of underturned folding, here with easterly dipping axial planes.” Such a feature is obviously of the type of the Southern Alps and opposed to that of the Kaikouras; but, as it was found only in greywacke rocks, it may well belong not to the orogeny we are considering but to the earlier Hokonui orogeny. The Wellington fault probably hades to the west; but, as this occupies the site of a much older fracture, conclusions drawn from it may not be valid. The western margin of the Tararua ranges from Cape Terawhiti to Shannon probably represents an old fault coast, and if this is so it may indicate that here, along the western margin of the Tararua ranges, the movements have been of the western or Zealide type. Farther north the two directions are again apparent, with a certain amount of oscillation between them (see Henderson, 1929, p. 96).
The recognition of the double-sided nature of the central mountain mass carries certain obligations in interpreting the fractures which have given rise to the present topographical features. Henderson (1929, p. 93) has discussed the occurrence of curved shears, concave to the upthrow, and rising from a horizontal sole, as a basis of the structure of New Zealand, and expressed the opinion that “Pressure is relieved by the earth block above the sole moving forward and upward on the upturning shear-zone.” All the movement of translation is attributed by him to the upper segment, above the sole. On the two-sided orogen hypothesis the fundamental movement becomes a double, deep-seated underthrust, with a rising of upper segments along steeply-curving shears, without considerable horizontal movement. Presumably the earliest sector affected would be in close proximity to the median line of the present-day mountain systems, and such an early-formed, wedge-bottomed mass may be expected to reveal older rocks now at its surface than are exposed in the surrounding ranges. As the central mass becomes more tightly compressed, movements tend to take place away from the median line, and the bordering ranges facing in opposite directions rise into existence, the younger ranges being farther from the axis of the twofold system. Probably these also rise with only small horizontal movement, which is taken up (in the reverse direction) in the lower segments.
Such a mechanism may be expected to result in more irregular features at the surface than the simple overthrust block. The case of an earth-block, tilted away from a reversed fault concave to the upthrow will be a dominant form under both over-and underthrust conditions; but the latter permits greater flexibility in the surface layers and accounts better for scissor faults (e.g. the Wairau fault), and faults which, while of a compressional nature, are concave to the downthrow. The Great Clarence Fault itself is of this nature
where it swings to the east at the Ure River,* and the White Creek Fault, Murchison, active in 1929, was also of this type (Ferrar and Grange, 1929, p. 188).
The geological and topographical features of N. E. Marlborough represent one-half of a two-sided mountain system, which rose into existence as a result of deep-seated compressional movements. Pressure was relieved on up-curving shear-planes by lateral movement below and vertical rising of blocks, with small horizontal movement, above. The surface results of this were more iregular than by movement of the upper segments alone. The mountains to the west, the Spencers and St. Arnauds, are probably older than the Kaikouras and are situated more nearly on the median line of the twofold system.
List of References Cited.
Bell, J. M., Clarke, E. DE C., and Marshall, P., 1911. The Geology of the Dun Mountain Subdivision, Nelson, N.Z. Geol. Surv. Bull., no. 12, 71 pp.
Branch, W. J., and Dagger, J. R., 1934. The Conglomerates of the Lower Wairau Valley, Marlborough, N.Z. Journ. Sci. & Tech., vol. 16, pp. 121–135.
Broadgate, F. K., 1916. The “Red Rocks” and Associated Beds of Wellington Peninsula, Trans. N.Z. Inst., vol. 48, pp. 76–86.
Cotton, C. A., 1913. The Physiography of the Middle Clarence Valley, New Zealand, Geog. Journ., vol. 42, pp. 225–246.
— 1916. The Structure and Later Geological History of New Zeland, Geol. Mag., dec. 6, vol. 3, pp. 243–249 and 314–320.
— 1925. Evidence of Late Tertiary or Post-Tertiary Orogeny in New Zealand, Gedenbock Verbeek, pp. 89–109.
Ferrar, H. T., and Grange, L. I., 1929. Geological Reconnaissance in the Murchison Earthquake Area, N.Z. Journ. Sci. and Tech., vol. 11, pp. 185–191.
Fyfe, H. E., 1931. Amuri Subdivision, N.Z. Geol. Surv. Ann. Rept.. no. 25, pp. 5–6.
Hector, J., 1890. Amuri District, Progress Rept., R.G.E., during 1888–9, pp. xxxi-xxxvi.
— 1890. Marlborough District, Progress Rept., R.G.E., during 1888–9, pp. xxxvi–liv.
Henderson, J., 1929. The Faults and Geological Structure of New Zealand, N.Z. Journ. Sci. and Tech., vol. 11, pp. 93–97.
Hobbs, W. H., 1923. The Growing Mountain Ranges of the Pacific Region, Proc. Pacific Sci. Cong., no. 2, pp. 746–757.
Jobberns, G., 1932. The Puhi Puhi Valley and the Seaward Kaikoura Mountains, N.Z. Journ. Sci. and Tech., vol. 13, pp. 341–352.
— 1935. The Physiography of Northern Marlborough, N.Z. Journ. Sci. and Tech., vol. 16, pp. 349–59.
[Footnote] * This fault was correctly mapped by Thomson (1919), but in both Henderson (1929) and the Fault Map of New Zealand, the Geological Survey have reverted to the ideas of MeKay published during the last century.
King, L. C., 1934. The Geology of the Lower Awatere District, Marlborough, New Zealand, N.Z. Dept. Sci. and Ind. Res., mem. no. 2, 49 pp.
— 1937. The Tertiary Sequence in North-eastern Marlborough, New Zealand, Trans. Roy. Soc. N.Z., vol. 67, pp. 21–32.
Mckay, A., 1877. Report on Cape Campbell District, R.G.E., during 1874–6, pp. 185–191.
— 1886. On the Geology of the Eastern Part of Marlborough Provincial District, R.G.E., during 1885, pp. 27–136.
— 1890. On the Geology of Marlborough and the Amuri District of Nelson, R.G.E., during 1888–9, pp. 85–185.
Marwick, J., 1934. The Sequence of Molluscan Life in New Zealand, Proc. Fifth Pacific Sci. Cong., vol. 2, pp. 947–960.
Morgan, P. G., 1908. The Geology of the Mikonui Subdivision, N.Z. Geol. Surv. Bull., no. 6, 175 pp.
— 1961. Notes on a visit to Marlborough and North Canterbury, with special reference to Unconformities Post-dating the Amuri Limestone, N.Z. Geol. Surv. Ann. Rept., no. 10, pp. 17–29.
— 1928. Notes the Geology of the Waiapu Subdivision (in Ongley and Macpherson N.Z. Geol. Surv. Bull., no. 30, pp. 53–55).
Morgan, P. G., and Bartrum, J. A., 1915. The Geology and Mineral Resources of the Buller-Mokihinui Subdivision, N.Z. Geol. Surv. Bull., no. 17, 210 pp.
Ongley, M., 1930. Taitai Overthrust, Raukumara Peninsula, N.Z. Journ. Sci. and Tech., vol. 11, pp. 376–382.
Ongley, M., and Macpherson, E. O., 1928. The Geology of the Waiapu Subdivision, Raukumara Division, N.Z. Geol. Surv. Bull., no. 30, 79 pp.
Park, J., 1910. The Geology of New Zealand, Whitcombe and Tombs, 488 pp.
Thomson, J. A., 1917. Diastrophic and Other Considerations in Classification, and the Existence of Minor Diastrophic Districts in the Notocene, Trans. N.Z. Inst., vol. 49, pp. 397–413.
— 1919. The Geology of the Middle Clarence and Ure Valleys, East Marlborough, New Zealand, Trans. N.Z. Inst., vol. 51, pp. 289–349.
Willis, B., and Willis, R., 1934. Geologic Structures, McGraw-Hill, 544 pp.
Fig. 1.—The dissection of the ancient plateau of Great Marlborough Conglomerate north of the Clarence Mouth. Head of Kawau-nui Stream.
Fig 2—The dissection of the ancient platean of Great Marlborough Conglomerate north of the Clarence Mouth. Head of Rika Stream
Fig. 3.—Part of the geological sequence in the Washdyke Stream From the left (east): “Grey Marl” in the tributary stream, white Amuri Lamestone. dark band of Great Marlborough Conglomerate, strip of “Grey Marl” exposed in slip, ridge of Amuri Limestone followed by “Grey Marl.” The light patch on the skyline is Great Marlborough Conglomerate. View looking south Stream flowing from right to left in the lower portion of the figure.
Fig. 4. The [ unclear: ] strip of Great Marlborough Conglomerate in the gorge of the Woodbank Stream. The strip of conglomerate which may be seen between two exposures of Amuri Limestone (white) in the gerge follows up along the small valley on the left between the two ridges of limestone. Woodbank Stream flowing from right to left below the level of the picture.
Fig 5—Three main features in the topography of eastern Marlborough. Foreground, the Tertiary country between the Lower Clarence and the sea; centre, the greywacke range of the Seaward Kaikouras: beyond this lies the Middle Clarence Valley and the snowcapped Tapuacnuku massif of the main Kaikoura range.
Fig 6—Trig. Jacob View south, showing the limestone hill on which the Trig. is situated, the Clarence River (on right), and Mount Alexander in the Puhi Puhi Valley (beyond the low saddle on left). The small stream in the foreground joins the Clarence River between Trigs. Jacob and Esau (out of view on the right) and is probably antecedent).