Geology of the Rakaia Gorge District.
[Read before the Canterbury Philosophical Society, 4th June, 1924; received by Editor, 30th July, 1924; issued separately, 6th March, 1926.]
The district dealt with in this paper is that immediately surrounding the gorge through which the Rakaia River passes before issuing from the mountainous region of the Southern Alps on to the Canterbury Plains—or that part of the valley of the Rakaia River which lies between the Mount Hutt Range, Fighting Hill, and Round Top, the southernmost peak of the Rockwood Range. The general structure of the area is that of an aggraded, glaciated valley, into the floor of which the river has re-entrenched itself. In the central part of the district the river in thus lowering its bed has encountered a barrier of resistant Tertiary and pre-Tertiary rocks, and has cut through it a winding gorge.
References to the Rakaia Gorge district are found in many of the reports of the early geologists of this country. The most comprehensive accounts of the stratigraphy and general geology of the area are those of Haast (1871), in a report on the Malvern Hills, and Cox (1884), in a report on the Selwyn and Ashburton Counties. Since the work of Cox, however, there has been no further investigation. An attempt is there fore made in this paper both to give a more detailed account of the district than has hitherto been published, and to examine some of the problems which the outcrops present in the light of the newer conceptions of the geological history of New Zealand which have evolved during the last forty years. [ unclear: ]
Thanks are due to Professor Speight for assistance both in the field and in preparation of this paper; to Mr. P. G. Morgan for his kindness in obtaining for me the rock analyses included below; and to Mr. S. Sylvester and Mr. T. A. Phillips for facilities while engaged in the field-work.
2. Physiographical Description.
The map (fig. 2) indicates the arrangement of the drainage of the district. The river-bed is flanked on both sides by extensive series of
terraces. Above and below the gorge these terraces are cut out of the Pleistocene deposits, and may be of considerable longitudinal extent. About the gorge they may be cut from older rock or may be of composite structure; these terraces are ill general of much more restricted extent and of more complex arrangement than those above or below the gorge.
The valley where it issues on to the plains is broadly U-shaped in cross-section. To the south the slopes of Mount Hutt (6,810 ft.) show the typical smoothed surfaces with truncated spurs of a glaciated region. On the other side of the valley Round Top (2,917 ft.) is also smoothed and rounded, and Fighting Hill (2,393 ft.), lying to the north of the district, forms a typical roche moutonnée. Smaller roches moutonnées in this district are Bryant's Hill, to the north of the lower end of the gorge, and a low hill composed of rhyolite lying about a mile to the northwest of Bryant's Hill.
The topmost terrace, which forms the main valley-floor, is throughout this district covered with irregular masses of morainic material. In
places these moraines, which are composed of fluvio-glacial conglomerate with occasional large angular erratics of greywacke, are not more than a few feet in thickness; while elsewhere, as against the south-eastern slopes of Fighting Hill, they give rise to undulating country which indicates a thickness of 50 ft. or more. A prominent effect of these morainic accumulations is the formation of swamps and lakelets, such as Lake Constance, where erosion has failed to establish a complete drainage. The farthest extent of the moraines to the south-east is shown by an arc of deposits of somewhat greater thickness than those immediately behind it, which stretches from about the Glenroy Saddle, through Woolshed Hill, to the south-eastern end of the Mount Hutt Range.
A prominent physiographical feature of this district is what is known locally as “The Railroad,” which consists of two roughly parallel ridges running from Bryant's Hill in a north-westerly direction to the river-bed west of Fighting Hill. Since a full description and discussion of the origin of this feature has been given by Speight and Dobson (1923), further mention of it will be omitted from this paper.
The following table shows the succession of rocks observed in the Rakaia Gorge district, with their approximate age correlations :—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|7. River silts, gravels, loess, &c.||Recent||Recent.|
|6. Conglomerates, silts, boulder-clays||Pleistocene||Notopleistocene.|
|5.||Teschenite and allied rocks||Tertiary||Notocene.|
|4. Coal-measures||Upper Senonian||Piripauan.|
|3.||Andesites Rhyolites and pitchstones||Cretaceous||Pre-Notocene and post-Hokonuian.|
|1. Greywacke, shales, &c.||Permian or Triassic||Maitai or Hokonuian.|
(a.) Permian or Triassic Sediments.
The oldest rocks exposed in the Rakaia Gorge district consist of intensely folded and faulted beds of greywacke, indurated grits, and dark-coloured finely-laminated slaty shales. They outcrop on both banks of the river at the upper end of the gorge. On the left bank they are overlain by the lower members of the Pleistocene deposits (Plate 18, fig. 1), and meet the rhyolite in an almost vertical junction—possibly due to faulting. On the right bank they are overlain by both rhyolites and Pleistocene deposits with marked unconformity. The rocks near this junction show intense weathering, which has resulted in the formation of masses of limonite irregularly distributed within a zone of about 10 ft. thickness. Fighting Hill and the Mount Hutt Range are also composed of these rocks.
The average strike of these beds throughout the eastern part of the alpine area of Canterbury appears to be in a north-easterly direction. In the small outcrops of the Rakaia Gorge inlier, however, folding has been intense, and small overthrust faults further obscure any general orientation of the strata. The dip is always at a steep angle, and the following represents a series of observations of the strike taken at intervals of about 20 ft. along the left bank of the river: E. 18° S.; E. 61° S.; E.; E. 10° S.; N. 20° E.
The only organic remains found in these rocks in the Rakaia Gorge are some indistinct annelid tubes similar to Torlessia McKayi Bather. They consist of straight or slightly curved and usually flattened tubes; jointing of the body-segments is visible with a pocket-lens, each segment being about 2 mm. long. The width of the tubes varies from 2 mm. to 4 mm. These occur in a grey coarsely-laminated shale in the outcrop on the left bank, as noted by Haast in a geological map of the Malvern Hills, dated 1871, and now in the Canterbury Museum, Christchurch.
With the criteria at present available it seems probable that these rocks should be referred to the restricted Maitai series—i.e., should be approximately correlated with the Maitai rocks of Nelson, which Trechmann (1917) has shown to be of Permo-Carboniferous or Permian age. Jaworski's (1915) opinion, however, should be noted—that Torlessiia McKayi should be referred to the genus Terebellina and indicate Triassic age.
(b.) Cretaceous Igneous Rocks.
Following upon the orogenic movements of early Cretaceous times, and preceding the deposition of the coal-measures in this district, there was a period of volcanic activity which gave rise to flows of rhyolite, pitchstone, and andesite, and some andesitic fragmental deposits. The intrusion of a dyke of andesitic character into the older sedimentaries probably accompanied these eruptions.
At the upper end of the gorge rhyolites occur on both banks of the river, lying with marked unconformity upon the denuded surface of the above-described shales and greywacke. On the left bank they are overlain by andesite-flows and andesitic breccias; on the right bank by the lower beds of the coal-measures. The relation of the andesites on the left bank to the coal-measures and rhyolites on the right bank is somewhat obscure; this is probably due to an abrupt thinning-out of the former, such as is illustrated in fig. 3.
Fig. 3.—Cross-section at upper end of Rakaia Gorge (line AB in fig. 1). 3, rhyolites; 4, andesites; 5, coal-measures; 7, Pleistocene deposits.
Separated from the junction of the rhyolite with the older sedimentaries on the left bank by a few feet of normal rhyolite, there is a mass of pitchstone extending 2 chains along the river-bank. On the same side of the river there is a smaller mass of pitchstone about 6 ft. thick lying between the rhyolite and the andesite.
On the right bank no outcrop of pitchstone was found, though fragments of that rock occur amongst the talus derived from the slope on which the rhyolite meets the Maitai rocks; this probably indicates an extension of the pitchstone mass which lies near to that junction on the opposite bank.
The andesites overlying the rhyolites on the left bank are overlain farther down-stream by the coal-measures.
At the lower end of the gorge rhyolites occur on both banks, and are penetrated by pitchstones near their junction with the andesites. Lying between the pitchstone and the andesite there is a narrow layer of much-weathered rhyolite. The base of the rhyolite is hidden by the Pleistocene gravel deposits, but on both sides of the river there are clear sections showing it to be overlain by andesite-flows with interbedded andesitic breccias. The andesites, in turn, on both sides of the river are overlain by coal-measures.
An intrusion which was doubtless associated with the eruption of the andesites occurs at the upper end of the gorge on the left bank of the river, in the form of a dyke from 1 ft. to 1 ft. 6 in. in width, which penetrates the Maitai rocks. It is clearly shown in a section exposed about 7 chains up-stream from the junction of the older rocks with the rhyolites, and extends up the cliff with very uniform thickness, though with somewhat
irregular walls. The beds on either side do not show any displacement, and there is little sign of contact metamorphism.
Besides these outcrops exposed in the gorge itself, there are numerous smaller occurrences of the Cretaceous volcanic rocks in the immediate neighbourhood. In Camping Gully rhyolite has been exposed in several places below the Pleistocene deposits. Bryant's Hill, and a lower ice-worn hill lying about a mile to the north-west of it, are also composed of rhyolite. Another outcrop of this rock occurs on the terrace slope leading up to the main level of the valley-floor about 20 chains to the west of the upper end of the gorge.
In Round Top, of the neighbouring Rockwood Range, a mass of andesites and associated breccias form the upper and north-western part of the hill. These rocks apparently form a capping with a north-westerly dip of about 15°, and overlie the rhyolites of which the south-eastern slopes are composed. Here, in the gully marked P on the map (fig. 1), pitchstones also occur with the rhyolites.
Rhyolite.—A hard, compact rock, usually jointed into massive rectangular blocks. Through the action of weathering agents the rock generally presents a surface which is stained to a light-brown colour. When fresh it is either white or may vary in colour from green to grey or black. Megascopic crystals of quartz and garnet (almandine) are locally abundant.
Microscopic examination of specimens of this rock taken from various parts of the gorge showed that, while the texture of the groundmass varied considerably, both the mineral content and the structure of the rock was very uniform throughout, the chief mineralogical difference between the various specimens being the presence or absence of garnet. Moreover, apart from that shown in a single flow as described below, there seems to be no progressive change throughout the mass of the rock either in degree of crystallinity of the groundmass or in abundance of garnet. Almost holocrystalline flows are irregularly interstratified with flows whose groundmass may be almost completely glassy. Types rich in garnet also appear apparently irregularly arranged amongst types poor or lacking in these phenocrysts.
The minerals present as phenocrysts are quartz, orthoclase, plagioclase (andesine to oligoclase with occasional albite), garnet, and biotite. Minerals of the groundmass are quartz, feldspar, apatite, magnetite, and rarely zircon. The groundmass varies in texture in different slides, and in different parts of the same slide, from a dark-brown glass to a microfelsitic (Iddings, 1909) matrix. In a few slides indistinct micrographic intergrowths of quartz and feldspar are apparent in the more crystalline parts of the groundmass, together with patches showing microspherulitic structure. In general, however, spherulitic intergrowths are absent. Flow-structure is nearly always present to some extent, and may be very prominent. A feature of nearly all the rhyolites examined was the development of “flow-breccia” structure (Iddings, 1909, p. 331).
Pitchstone.—A brittle, easily-weathered rock, showing either (as in the outcrop at the upper end of the gorge) very perfect rectangular jointing, or (as at the lower end of the gorge) massive outcrops with little jointing. The rock is pitch-black in colour, with the characteristic vitreous lustre and conchoidal fracture of volcanic glasses. Phenocrysts of clear or slightly discoloured quartz are visible, and garnets are abundant in all of the outcrops in this district.
Under the microscope the pitchstones are markedly uniform in character and closely resemble the more glassy types of rhyolite; the occasional presence of small phenocrysts of hypersthene is the only difference in mineral content, and, except that no microspherulitic intergrowths were observed in the pitchstones, their micro-structure is similar. Perlitic structure, especially in the rock of the outcrops at the lower end of the gorge, is perhaps more marked than in the glassy rhyolites.
Andesite.—The rock of which the flows are composed is a hard dark-grey or black rock, which, however, is very susceptible to the action of weathering agents. Fresh specimens of the rock are difficult to obtain except where erosion has been so rapid that the products of decomposition have not been able to remain in place. In the field the rock generally appears as a reddish or greenish mass, presenting a somewhat incoherent and crumbling surface. The only crystals visible megascopically are of feldspar, and these occur as numerous, evenly-distributed, rectangular prisms. The presence of these phenocrysts is very characteristic of the rock, and they remain visible to the naked eye or under a pocket-lens even in most weathered specimens. A very general feature of this rock is the abundance of secondary silica present as quartz of different varieties—chalcedony, amethyst, agate, and opal. These minerals, together with chloritic decomposition-products derived from the ferromagnesian minerals of the rock, are found filling veins, druses, steam-vesicles, and other cavities throughout the whole mass. In places the rock becomes scoriaceous, and the vesicles are filled with bright-green amygdaloids composed chiefly of chlorite. The presence of these minerals has led to the popular supposition that copper-ores and gold may be found in these rocks, but prospecting has failed to detect any minerals of economic value in payable quantities. The amethysts, which have attracted some attention, are of a pale colour, usually occurring in druses as clusters of small hexagonal prisms terminated by pyramids. Specimens have been collected from the face opposite the Mount Hutt homestead up to 4 in. in length, but these are exceptionally large for this locality
Calcite is also common as a vein-mineral in the andesites. On the southern slopes of Round Top there are veins varying in size up to 2 ft. which have been partially filled with calcite, quartz being subsequently deposited as a coating on the calcite, and giving rise to negative pseudomorphs.
The breccias associated with the andesite-flows are typical volcanic breccias consisting of angular fragments of the andesite rock varying in diameter up to 2 in. or 3 in. with occasional larger blocks cemented together in a matrix of finer material. There is no apparent regularity in arrangement of the flows and fragmental deposits, but the former are in far greater abundance. The breccias occur merely as occasional strata, never more than a few feet in thickness, interbedded between the flows. The secondary minerals associated with the andesite-flows are also abundant in the breccias.
The microscopic character of the andesites is very uniform in all specimens collected from this district. The chief variations observed were in the proportion of ferromagnesian minerals present and in the texture of the groundmass. Plagioclase (acid-labradorite to andesine), hypersthene, and augite occur as phenocrysts in a groundmass which is typically composed of minute feldspar laths, some pyroxene in granular masses, magnetite, and a brown glass. The texture of this matrix varies in different slides: in some there is little glass and the feldspar laths attain a larger size, when they may be recognized as andesine or labradorite; in other slides the glassy material predominates, and the feldspars appear as scattered microlites. The amount of pyroxene present in the groundmass also varies, being in some slides apparently absent. In general the structure is hyalopilitic as defined by Rosenbusch, the “felted” character being prominent in the more crystalline
varieties. Throughout the rock there is much secondary quartz and opal, filling small veins and cavities. Small round vesicles frequently show a border of radiating, fibrous, chalcedonic quartz, with the centre filled in with opal.
The rock forming a dyke in the greywacke at the upper end of the gorge is much weathered, and presents a brown sandstone-like appearance. Plagioclase (acid-labradorite), augite, ilmenite, apatite, and a large amount of secondary quartz and calcite are recognizable under the microscope. The texture is of a fine, even grain, the bulk of the rock being composed of feldspar laths with occasional patches of glassy residuum. It would seem more probable that the intrusion of this dyke was associated with the Cretaceous volcanic activity than with the Tertiary basic intrusions, solely on account of the andesitic character of the rock.
The following are the results of analyses made in the Dominion Laboratory of specimens of each of the effusive rocks of this group, with their classification according to the C.I.P.W. system :—
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|Water lost above 105°||C.||0.66||4.98||1.23|
|Water lost below 105°||C.||0.58||2.16||2.60|
Rhyolite: the island, lower end of Rakaia Gorge.
Pitchstone: lower end of Rakaia Gorge, north side.
Andesite: lower end of Rakaia Gorge, south side.
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In (1) and (2) the analyst reports that the carbon dioxide is not present as calcite, as it is liberated only on heating to about 100° C with dilute hydrochloric acid.
I 3(4) 1″ 2″ (Magdeburgose).
I 3(4) (1)2 4 (Alsbachose).
I (II) 4 3 (3)4 (Yellowstonose).
The chief points of interest regarding the composition of these rocks arise out of a comparison of the rhyolite and pitchstone. As is suggested below, the pitchstone may represent merely certain of the acidic flows of these eruptions, which, either owing to rapid cooling or other causes by which free molecular movement was hindered, have consolidated in a more vitreous form. If this is the case the rocks should show similar chemical compositions. The difference in relative proportion of the alkalis is, however, marked: the potassic nature of the rhyolite shows itself mineralogically in the abundance of orthoclase relative to the soda-bearing feldspars, while in the pitchstone the reverse occurs. The chemical similarity of the two rocks, however, seems sufficient to admit of their being successive flows of the same eruption, a conclusion to which the field evidence points.
Comparison of these analyses with those given by Speight (1922, p. 79) of Banks Peninsula rhyolites and pitchstones shows a general similarity.
Owing to the abundance of secondary silica contained in the andesite, the C.I.P.W. quantitative classification, which the authors definitely limit to fresh specimens, does not illustrate the character of the rock. Classified as “yellowstonose,” it is grouped with dacites and rocks corresponding in composition with quartz-diorites, whereas its microscopic character clearly shows that it belongs to a more basic group.
The Origin of the Pitchstones.—Haast (1871 and 1879) considered the pitchstone to be merely a facies of the rhyolite—the first flows to be erupted on to a cold land-surface. Cox (1884, p. 40), however, considered the pitchstone to be dykes belonging to the same system as the dolerite and basalt intrusions found penetrating the coal-measures and older rocks. The following evidence seems to confirm Haast's view that in this and neighbouring districts they are merely glassy facies of the rhyolite :—
1. No instances have been reported of the pitchstones associated with the volcanic rocks here considered penetrating any rocks other than the rhyolites.
2. In every outcrop observed the directions of the pitchstone masses conform to those of the apparent flows of the rhyolite. The presence of jointing frequently makes it difficult to determine the orientation of the flows of the rhyolite, but where determination is possible the pitchstone is found to be in parallel arrangement. This is especially true of the occurrence at the lower end of the Rakaia Gorge, where the direction the rhyolite-flows is clear.
3.There are certain undoubted rhyolite-flows which may be considered as intermediate in character between the normal pitchstone and rhyolite types. They are black in colour, show a somewhat vitreous lustre, and microscopically appear as very glassy rhyolites. Other flows which are chiefly composed of normal rhyolite at the margin grade into a pitchstone type as shown above.
While it thus seems more likely that the pitchstones do not form dykes, yet Haast's inference that they represent those flows resting directly upon the older rocks, and therefore cooled more rapidly than the succeeding flows, requires some modification. The presence of normal rhyolite between the pitchstone and the older (Maitai) rocks shows that, whatever may have been the cause of the glassy facies, it was not due to conditions attendant only upon the first flow to be put out on to the land-surface. Furthermore, the pitchstones occurring near, but here also not next to, the junction with the andesites, may only be due to this rapid cooling of the first flow if the andesites represent the older of the two volcanic series. As shown below,
however, the weight of evidence seems to point to the andesite being of younger origin than the rhyolite. Thus while it is possible that the first flows of the rhyolitic series in places may have assumed a glassy character, yet the pitchstone facies is by no means confined to that horizon.
In the Rakaia Gorge it must be considered as accidental that each of the pitchstone outcrops occurs close to, though, as careful examination shows, not in actual contact with, the junction of the rhyolites with other rocks.
Order of Eruption of the Rhyolites and Andesites.—Haast, in treating of this and the adjacent Malvern Hills and Mount Somers districts, always considered the andesitic eruptions to have preceded the rhyolitic. Cox, who was the last to discuss at all fully the mutual relations of these two series of rocks, agrees with Haast that the andesites were the first erupted rocks, but in view of inconsistencies arising from an adoption of this theory in the Malvern Hills and Rakaia Gorge he postulates a special type of eruption for the rhyolites. These rocks he considered to have been erupted in a very viscous condition, the accumulation taking place by a process of endogenous growth, as described by Judd (1881, p. 134). Of recent years, however, there has been some doubt cast on this hypothesis, and a close examination of the Rakaia Gorge district, supplemented by more rapid observations in the Malvern Hills and Mount Somers districts, would seem to show that there is a balance of evidence in favour of the rhyolite being the first erupted rock.
Haast does not state definite reasons for considering the andesites to be older, but Cox bases his endogenous-growth theory upon the following observations :—
Micro-structure of the rhyolite indicates a viscous lava.
On the flanks of Mount Somers the andesites dip inwards towards the main rhyolite mass.
“The fact … that the melaphyres are lying on the liparites [in the Rakaia Gorge] at the angle of dip which they assume is in itself a proof of greater age, when we consider that beds of tufa are interstratified with the solid floes.” (Cox, 1884, p. 39.)
Of these lines of evidence, (1) is of merely accessory value; (2) and (3) may both be criticized in the light of later advances in the science of New Zealand geology. At the time of Cox's report the conception of a Pliocene period of crustal movements had not been developed. The work of McKay, Cotton, and others, however, has since shown that the present topography of New Zealand is independent of that of pre-Notocene times. Thus the fact that the andesites flanking Mount Somers dip towards the centre of that mass must be considered as possibly due to the presence of faults. The third argument is also open to criticism. The coal-measures overlie the andesites in this place with a dip of not less than 35°; so that, taking Cox's angle of dip of the andesites (45°), and allowing that no movements have disturbed the igneous rocks relative to the coal-measures, the angle of slope at which the tuffs and breccias were laid down could not have been greater than 10°. This occurrence therefore seems insufficient as “a proof of greater age” of the andesites.
A further point which doubtless influenced both Haast and Cox in considering the relative age of these rocks is that they supposed that there were beds of rhyolitic tuffs interstratified with the coal-measures at Mount Somers and the Rakaia Gorge. As shown below, however, the coal-measures in the Rakaia Gorge are probably entirely of cataclastic origin.
With regard to the possible objection that if the andesites overlie the rhyolites the solutions which deposited the silica, calcium carbonate, &c., in them would be expected to have penetrated the underlying rhyolites, two suggestions may be made. Such solutions were probably closely connected with the magma itself, and deposition of the secondary minerals would follow closely upon the consolidation of the rock, and be to a large extent confined to it. Secondly, the rhyolite, being of a compact nature, would not easily be penetrated by these solutions. The fact that small amethysts, with some veins of chalcedony, occur in the rhyolite on the right bank at the lower end of the gorge shows that these solutions did to some extent affect this rock.
The evidence in favour of the rhyolites having preceded the andesites is—
The andesites in each outcrop of the Rakaia Gorge occur between the outcrops of rhyolite and coal-measures, and in Round Top rest upon the rhyolite. The possibility of faulting having caused an inversion of the older rocks over the younger is unlikely.
The nature of the contact of the andesites with the rhyolites at the lower end of the gorge. The rhyolite which lies between the pitchstone and the andesite is much weathered and decomposed to an easily eroded white mass.
Where the rhyolite rests upon the older Maitai rocks there is little evidence of alteration of the erupted rock. This seems to indicate that the weathered rhyolite represents an old land surface upon which the andesites were erupted.
The supposition that either the andesites or the rhyolites of the Malvern Hills and Rakaia Gorge were not contemporaneous with those of the Mount Somers district would clear away the chief difficulties involved in this problem. The rhyolites of both districts, however, in megascopic and microscopic character are almost identical, and both have similarly associated pitchstones. The andesites of both districts are also of a closely similar nature, and since both series are definitely of Cretaceous age it must be considered very improbable that the eruptions of each type were not contemporaneous in both districts.
(c.) Cretaceous Sediments.
The coal-measures overlying the rhyolites and andesites in the Rakaia Gorge are the only members of the Notocene group of sediments exposed in this district. They outcrop as part of a basin-shaped fold which is tilted towards the south-west, so that one-half of the basin is obscured below the Pleistocene deposits on the south side of the river. The maximum thickness exposed is about 1,000 ft. At the upper end of the gorge on the right bank they rest on a denuded surface of the rhyolite, and strike W. 30° N. with a southerly dip of 30°. On the left bank the coal-measures occur in the central part of the gorge, resting on the andesites. At the upper end of the outcrop the strike is roughly north and south, but in Chasm Creek it has swung round to N. 15° E., and the beds dip at 30° to the west. Following the outcrop down-stream, the strike continues to swing round, following the edge of the basin, until, where again the beds may be seen resting on the andesites, it has assumed a N.N.E.-S.S.W. direction, and the beds dip at an angle of 35° to the W.N.W. On the right bank, below the Mount Hutt homestead, these beds occur again resting on the
andesites with a north-east strike and dipping at 35° to the north-west. Thus the southern wing of the exposed segment of the basin is completed; further evidence of the form of the beds to the south-west is buried beneath the later deposits of gravel and silts lying between the river and the foot of the Mount Hutt Range.
In both outcrops of coal-measures on the right bank the topmost bed exposed is overlain by the teschenite rock of a sill; elsewhere the Pleistocene deposits follow unconformably.
The coal-measures consist for the most part of coarse conglomerates with interstratified beds of shale and sandstone and seams of brown coal. Current-bedding is a prominent feature of the deposits wherever exposed. Associated with these sediments, and more especially with the coarser conglomerates exposed on the right bank near their junction with the teschenite sill, there is a considerable quantity of fossil wood which has been preserved by partial petrifaction. Large pieces of tree-trunks and boughs often several feet in length occur associated with limonite masses, and usually containing radiating tufts of fibrous haematite at the nodes.
In two places, marked S on the map (fig. 1), there are deposits of siliceous sinter together with silicified grits and sandstones. These deposits occur at about the same level on opposite banks of the river, and probably represent a period of thermal activity following upon the above-described volcanic activity. The partial silicification of the wood-remains and the formation of haematite are doubtless largely due to the action of waters associated with the deposition of this sinter.
There is little regularity in the arrangement of the beds of different texture throughout the sequence. Conglomerates are the predominant type, and occur throughout. In general the lower beds exposed are of finer material than those of the higher parts of this inlier. The seams of coal are also distributed irregularly, the thickest seams occurring in the upper half of the sequence.
1. The Coarse-grained Beds.—The conglomerates consist of closely packed pebbles of rhyolite varying in diameter up to 6 in. Together with the rhyolite debris there are occasional pebbles which may possibly be composed of much-altered greywacke, but close searching failed to show the presence of any andesite pebbles. In view of the close association of the andesites with the rhyolites in the district, this feature of the deposits requires some explanation. The possibility of the andesites being of later eruption than the deposition of the coal-measures is precluded by their undoubted mutual position. That the andesite pebbles, being more easily decomposed than the rhyolite, have been completely removed by weathering agents, either during deposition or subsequently, is also unlikely. An explanation is more probably to be found in a consideration of the locality from which the waste forming these beds was derived. The well-worn and rounded condition of the rhyolite pebbles suggests that they have been transported for a considerable distance. It is possible that the rhyolites may have extended to the east of the present outcrops over a wide tract of country, from which the pebbles of the coal-measures were derived, whereas the andesites were confined to within the area of deposition. This tract of rhyolite country would now be buried beneath the gravels of the Canterbury Plains, which (Speight, 1915) doubtless occupy the position of an infilled, down-faulted block. The facts that parts of the greywacke surface (now “fossil peneplain”) are found to the
west of this district with coal-measures lying directly upon them, and that rhyolite pebbles are found in the Notocene rocks of the Broken River and Big Ben outliers, supports the idea of an early Notocene land-surface to the east rather than to the west of the Rakaia Gorge district.
2. The Fine-grained Beds.—The finer sediments of the coal-measures consist of normal sandstones, grits, mudstones, and shales. Being doubtless derived from the same landmass that supplied the rhyolite pebbles of the conglomerates, these rocks are usually light in colour and of a siliceous character. In many cases there is much finely-divided carbonaceous matter scattered throughout.
There does not seem to be any evidence of volcanic tuffs in these coal-measures, as assumed by Haast and Cox. The section referred to by Cox (1884) as showing tuffs at the upper end of the gorge shows the normal succession with no brecciated material, volcanic bombs, or lapilli, and the sandstones and mudstones are interbedded with conglomerates of well-worn pebbles. The firmly-cemented character and light-grey or white colour of the finer-grained beds certainly give rise to a superficial resemblance to some volcanic tuffs, but the presence of finely-comminated plant-remains scattered irregularly throughout many of them disposes of this possibility. The colour, moreover, is readily understood, since these beds doubtless represent the detritus from a landmass which was largely composed of rhyolite.
3. The Coal.—The coal-seams vary in thickness from a few inches to about 10 ft. The thickness of any one seam may also vary considerably when traced laterally, the masses of coal being usually in the form of lenses of slight convexity. The coal itself when unaltered is a brown, hydrous coal, very similar in character to that being worked at Homebush, White Cliffs, Glenroy, Mount Somers, and elsewhere in Canterbury. On the right bank, near the junction of the coal-measures with the teschenite sill, it assumes a semi-anthracitic character, due to thermal metamorphism induced by the intrusion.
In the Rakaia Gorge the coal has been worked privately by the various owners of the land for their own consumption. On the left bank, in Chasm Creek, drives have been put in to a distance of about 100 ft. at two levels, to work a seam of 10 ft. thickness. Farther down-stream, on the same bank, a shaft was sunk near the junction of the coal-measures with the andesites, in order to pick up this seam again. Though doubtless it was passed through, it had here thinned out, and nothing of profitable thickness was found. These workings, which were carried out by Mr. George Gerard, of Snowdon, have been discontinued for the last twenty years, since all the easily-accessible coal has been removed. There is no record of the amount of coal extracted.
On the right bank of the river certain of the holders of the Mount Hutt Station have extracted small quantities of coal from the outcrops near the junction of the coal-measures with the sill described below, and near the junction with the andesite below the homestead. In these two places drives have been put into seams of from 4 ft. to 6 ft. in thickness for a distance of about 80 ft.
Age.—Since the fossil wood referred to above is the only recognizable organic material as yet found in the coal-measures of the Rakaia Gorge, the age of these beds can only be inferred by correlation with neighbouring districts. The lithological similarity and the identity of relations with the rhyolite leave no doubt, however, that the Rakaia Gorge coal-measures are the correlatives of those of the Malvern Hills, which (Trechmann, 1917; Woods, 1917; Wilckens, 1922) have been determined as Upper Senonian (Piripauan).
(d.) Tertiary Igneous Rocks.
Overlying the topmost beds of the coal-measures exposed on the right bank in the central part of the gorge is a mass of igneous rock in the form of a sill. There are two outcrops of this rock: one, forming a cliff with a maximum height of 200 ft., extending in an east-and-west direction to the north of the Mount Hutt homestead; the other a bank varying up to 10 ft. high and about 5 chains long which lies about 20 chains south of the homestead. The junction of this rock with the underlying coal-measures is clearly visible; near it the normal sediments have been altered to a hard spilosite-like rock, and in an adjacent coal-seam the normal brown coal has been changed to a semi-anthracite analogous to similar altered coals in the Acheron River and Malvern Hills. From the nature of the contact and the lithological character of the whole mass it is evident that this body of rock represents a sill intruded into the coal-measures. The upper part of the intrusion and any overlying Notocene sediments have been either removed by erosion or buried beneath the deposits of Pleistocene silts and conglomerates.
The texture of this rock varies markedly at different levels. At its lower margin, where it is little weathered, it appears as a dark, hard rock of fine grain. Proceeding upwards it becomes coarser in texture, until at the highest point of its exposure it appears as a coarse-grained rock much weathered to a dark-green incoherent mass. In this part of the sill there are numerous small veins, varying from ½ in. to 2 in. in thickness, and arranged roughly parallel with the floor. The rock of which these veins are composed is of lighter colour and finer grain than the enclosing country.
The following are the chief types which occur as various facies of this intrusion :—
Coarse-grained or teschenite type. Coarse even-grained rock containing plagioclase (labradorite), titan-augite, olivine, analcite, prehnite, biotite, ilmenite, and apatite. Ophitic structure was not observed.
Non-porphyritic or dolerite type. Even-grained but of much finer texture than (1). Plagioclase (acid-labradorite), titan-augite, aegirine-augite, olivine, biotite, apatite, and ilmenite present. Ophitic structure slightly developed.
Porphyritic or basalt type. Very fine grain with small phenocrysts of olivine and a slightly greenish augite. The groundmass consists of a fine-grained though holocrystalline matrix of small labradorite laths together with some granular augite and a little olivine. Ilmenite is also abundant.
Vein rock. Of typically granulitic texture and containing orthoclase. nepheline, analcite, pyroxene (both titan-augite and aegirine), barkevicite (occasional granular crystals), biotite, and ilmenite. Slides made from the margin of these veins show an abrupt transition, though with no definite line of division, into the normal country rock by decrease in the nepheline and orthoclase with increase in the mafic elements.
Of these types, (1), (2), and (3) grade insensibly into each other, and depend upon distance from the margin of the intrusion. The whole group
is a typical suite of teschenitic rocks, and bears a close resemblance to many of the Scottish analcite rocks (Tyrrell, 1923; Walker, 1923).
The vein rock, which corresponds in mineral composition to a nepheline syenite, is of interest as showing a leucocratic differentiate of the magma from which the intrusion was derived. The process of differentiation was doubtless of the type in which the veins represent the product of a fluid residuum which has been squeezed along lines of weakness in the mass and has crystallized after the consolidation of the main bulk of the rock. The rock may thus be compared with the aplites of acidic rocks; the almost panidiomorphic structure of the vein-rock of this sill is noteworthy in making this analogy. Tyrrell (1923) shows that differentiation of a teschenitic magma in a syenitic direction will result in a theralite. The vein-rock of this sill does not, however, contain plagioclase, and thus represents a still further stage in the differentiation process—a nepheline syenite.
The age of the intrusion of this sill cannot be determined within close limits. It is post-Senonian and pre-Pleistocene. Probably it was intruded contemporaneously with the numerous other similar basic intrusions of Canterbury, and may have been associated with the Kaikoura orogenic activity, which reached a maximum in Pliocene times. It is, however, involved in the same folding and faulting that was then induced in the Notocene sedimentary rocks.
(e.) Pleistocene Sediments.
Overlying the Notocene and older rocks of the Rakaia Gorge inlier there are deposits of more or less unconsolidated sediments, including fluviatile conglomerates, lacustrine silts, glacial and fluvio-glacial deposits. These sediments surround the inlier, and form the material with which the Rakaia Valley is aggraded. They extend from the base of the Mount Hutt Range to Fighting Hill and the Rockwood Range, and form a continuous sheet between these two elevations into the High Peak basin. Upon the slopes of Round Top and the adjacent members of the Rockwood Range greywacke boulders representing glacial deposits of this age occur up to a height of 1,340 ft. above the level of the water at the lower end of the Rakaia Gorge. To the south-east of this district these Pleistocene deposits extend outwards to form the Canterbury Plains. In this district these deposits attain a great thickness above and below the gorge, where the river has eroded to a depth of 700 ft. below their surface without reaching the underlying rock.
The chief types of sediments represented in this series are:—
(1.) Conglomerate: coarse to medium grained, the pebbles having an average diameter of 3 in. or 4 in. Greywacke is the chief rock of which the pebbles are composed, but limestone, dolerite, gabbro, and, below the gorge, rhyolite and andesite, are sparsely represented. These gravel-conglomerates form by far the greater part of the Pleistocene deposits, and of them two distinct types may be recognized: one is stained to a light-brown colour through oxidation of the iron-bearing minerals under the action of weathering; the other is of the normal grey colour of the greywacke pebbles.
(2.) Silts: fine-grained, of a yellow or white colour. Usually show “varve” banding of very fine and slightly coarser material, each band
being from ¼ in. to ½ in. thick. Occasional pebbles of greywacke are scattered throughout the deposits of this silt, suggesting transportation by floating ice during the time of deposition.
(3.) Fluvio-glacial conglomerate: composed of angular and striated boulders together with normal rounded pebbles of fluviatile gravels. Large angular boulders varying up to 4 ft. or 5 ft. in diameter occur in these beds. Greywacke, as in the normal fluviatile conglomerates described above, is the chief rock of which the boulders and pebbles are composed. The whole is cemented together with a very fine glacial silt.
(4.) Boulder-clay: a typical glacial boulder-clay consisting of angular boulders of variable size, some of which are smoothed and scratched, embedded in a very fine-grained silt. The boulders in this case also are almost exclusively composed of greywacke.
Correlation of the various sections of these deposits exposed in and about the gorge is rendered difficult by lack of continuity of the beds. The following represents the general succession observed:—
Loess, sands, and other recent superficial deposits.
Upper fluvio-glacial conglomerate.
Grey fluviatile conglomerate.
Brown fluviatile conglomerate.
Lower fluvio-glacial conglomerate.
The complete succession is not, however, shown in any single section. The following may be taken as representative exposures illustrating the stratigraphical relations of the several deposits:—
(1.) At the upper end of the gorge, resting upon the Maitai rocks on the left bank (Plate 18, fig. 1):—
|7. Blown. sands, river-gravels, &c.||10 ft.|
|3. Fine-grained lake-silts||30 ft.|
|2. Silts with interstratified fluviatile conglomerates; conglomerates increasing with depth||30 ft.|
|Fine silts with occasional boulders||10 ft.|
|1. Boulder-clay||10 ft.|
(2.) At the lower end of the gorge, on the right bank, up-stream of the bridge. The sequence of beds, though somewhat obscured below the road by talus, appears to be as follows:—
|7. Superficial deposits||2 ft.|
|5. Grey fluviatile conglomerate||30 ft.|
|3. Lacustrine silts||10 ft.|
|Conglomerate with limonite||3 ft.|
|Lacustrine silts||80 ft.|
|2. Fluvio-glacial conglomerate||50 ft.|
|1. Boulder-clay||10 ft.|
The chief point of interest in this section is the presence of an over-thrust fault which has involved all the beds except the veneer of gravel forming the top of the terrace. In the road-cutting the effect of this fault is clearly shown where the silts have been thrust across the grey conglomerate; in the latter a bed of sandy material shows considerable distortion in the neighbourhood of the fault-plane. Below the road the line of fault becomes less clearly marked, and probably resolves itself into a series of parallel displacements; where, however, it has been thrust across the fluvio-glacial conglomerates the rhyolite shows a typical slickenside surface. The line of fault has an approximately N.N.E.-S.S.W. direction,
and the beds to the east have been tilted so that they dip at an angle of about 15° to the south-east. The throw of this fault cannot be estimated with precision owing to the absence of any recognizable horizon on both sides of the line of movement. On the west side of the fault there is no sign of the easily recognizable bed of limonitic conglomerate which appears about half-way up the whole section, so that the throw cannot have been less than 40 ft. Estimating from an average level of the junction of the silts with the fluvio-glacial conglomerate, the throw would appear to be of about 70 ft. This junction, however, is not a definite line, since the silts pass gradually down into the conglomerate.
(3.) Below the gorge the Pleistocene deposits are represented by grey fluviatile conglomerates (5), and upper fluvio-glacial beds (6), overlying the brown conglomerates (4). Sections exposed on the right bank show à thickness of 150 ft. of brown conglomerate overlain by 350 ft. of grey conglomerate, which is overlain by fluvio-glacial beds varying in thickness up to 50 ft.
Structurally the Rakaia Gorge inlier is an intermontane basin analogous to the Trelissick basin, Big Ben outlier, High Peak basin, or any of the numerous remnants of Notocene beds preserved amongst the oldermass blocks of the alpine regions of Canterbury. Owing to the presence of the Pleistocene deposits, however, the exact positions of the bounding fault-lines cannot be accurately determined. Some such major fault doubtless passes in a north-westerly direction between the gorge inlier and the Mount Hutt Range. In the northern part of the district there is also probably some continuation of the southern fault boundary of the High Peak basin, extending in a roughly south-west direction in front of Fighting Hill.
Two minor faults are apparent in the beds of the inlier:—
(1.) As described above, the rhyolite at the lower end of the gorge has been thrust across the Pleistocene deposits. This line of displacement probably continues across the river, so that the cliff forming the north-western boundary of the old ferry reserve may be a fault-line scarp. No definite evidence of displacement of the beds on the left bank of the river is, however, available. A similar example of Recent overthrusting is that described by Morgan (1908, p. 72) in North Westland; he attributes the movement to ice-pressure in Pleistocene times.
(2.) As shown on the map (fig. 1), faulting has occurred between the rhyolite at the upper end of the gorge and the adjacent rhyolite, coal-measures, and andesites. The downthrow side is to the south, and there is no evidence of its extension on either side of the gorge. The throw is indeterminable, but probably not great.
Small earthquakes, not recorded by the seismograph at the Christchurch Observatory, are frequently felt in this district. The shocks are of a sharp character, short in duration, and without appreciable preliminary tremors. Their intensity is about IV on the Rossi-Forel scale. These movements are probably due to settling-down of the loosely compacted sediments of the Canterbury Plains, which causes slipping at their junction with the older rocks rather than to disturbances of a deeper-seated nature.
The presence of Pleistocene silts and conglomerates above the gorge to a depth of at least 500 ft. below the level of the top of the barrier of resistant rocks now forming the gorge inlier requires that either (1) the bed of the river before the deposition of these beds was eroded to that depth, and therefore the barrier cannot have occupied its present position; or (2) this basin in the older rocks represents a trough hollowed out by the glacier which at one time occupied the valley, and whose deposits form the base of the Pleistocene series.
That glacial troughs analagous to the latter do occur has been amply shown by De Martonne (1911) and others. Such troughs, however, show gently sloping floors in an up-stream direction near the termination of the trough. In this case, allowing for later erosion by the river (which it should be noted would not greatly tend to steepen this face), the floor must have risen 300 ft. in 10 chains—i.e., possessed a grade of 1 in 2.
It would therefore seem that the later geological history of the district has been somewhat as follows:—
1. After emergence caused by the Kaikoura orogenic movements, erosion continued until a topography little less mature than that of the present was produced.
2. An extension, followed by retreat of the Rakaia valley glacier, took place, and the above-described boulder-clay was deposited.
3. Through upward movement of the block which now forms the gorge inlier, at a rate that was quicker than the river could keep pace with in cutting down its bed, a lake was formed in which were deposited the silts and conglomerates now overlying the lower boulder-clay. It should be noted that the part of the barrier now forming the south-eastern part of the inlier—i.e., the rhyolite of the island and surrounding parts—at one time must have been the highest part of the barrier, since the silts are found as far down-stream as Pipeclay Gully.
4. Aggradation continued until silts and gravels filled the valley to about the level of the present highest terrace.
5. About the time when this point of maximum aggradation was reached the glaciers again advanced, and the second glaciation of the district as described above took place. Upon the retreat of the glacier, erosion of the valley-floor set in, and the present gorge of the river was carved out with the terraces as described below.
A feature of morphological interest shown in the gorge is the influence of the structure of the beds upon the form of the course of the river. At the upper end it runs parallel with the strike of the coal-measures along the line of their junction with the older volcanics. Upon leaving the igneous rocks it makes a right-angle turn and follows the dip of the coal-measures until diverted by the less easily eroded igneous sill; from here it follows roughly the strike of the coal-measures as it swings round in the basin-shaped fold until a point is reached where it breaks across the volcanic rocks forming the lower wing of the fold.
Evidence that the valley of the Rakaia River has during Pleistocene times been occupied by an extensive valley-glacier is afforded by (1) the presence of glacial deposits, (2) the character of the existing topography. Some descriptive account of these features has been given above, and this
section will be confined to reconstructing that part of the geological history of the period which will throw light upon their origin.
As shown when dealing with the Pleistocene deposits, there are two independent glacial deposits in this series which are separated by a considerable thickness of normal fluviatile or lacustrine sediments. It therefore follows that there must have been at least two periods in which ice, extended as far down the Rakaia Valley as the present gorge.
Evidence of fluctuations in the severity of the glaciation in New Zealand is extremely scanty, and it is generally considered that there has been but one great advance of the glaciers, followed by a gradual retreat to within their present limits. Speight (1921) shows that the notched spurs of the Upper Rakaia and Waimakariri regions give evidence of only one period of ice-advance. The extent of this intervening retreat may not have been great, but the thickness of intervening sediments, which may be, as above the gorge, more than 700 ft., shows that the duration of the interglacial phase was not short.
Of the extent of the first advance of the ice little can be said, since its deposits, which are visible only about the gorge (where they have doubtless been elevated to an exposed position by subsequent earth-movements) are the only traces of its existence. Results of the erosion of the ice of this period upon the topography have been completely obliterated by the action of the subsequent extension. The thickness of the lower glacial deposits, moreover, gives little indication of the intensity of that advance, since it is impossible to say whether they represent accumulations at a terminal face during a stationary period of the glacier, or whether they are merely the results of deposition along a steadily-shrinking ice-front.
Of the second extension more may be deduced. As shown by Haast (1879), the ice probably reached, when at its maximum, as far as Woolshed Hill, and spread out in piedmont form across the Canterbury Plains from the neighbourhood of the Glenroy Saddle to the south-eastern end of the Mount Hutt Range.
It is of interest, in considering the character of this glacier in its final extension, to notice how the loosely consolidated Pleistocene deposits which form the floor of the valley have resisted the erosion of this mass of ice. If the erratics found on the north-western slopes of Round Top are due to this extension, the ice in this place must have had a thickness of at least 500 ft. Even if these erratics were due to the earlier advance, a considerable thickness of ice must have been present to cause its extension as far as the Woolshed Hill morainic deposits. Despite this mass, however, the sediments of the valley-floor were not entirely scooped out. This occurrence seems to lend support to the theory that the action of glaciers on the floors of their valleys exerts a protective rather than erosive influence.
The terraces occurring above and in the gorge are most easily explained as due to the slow reduction of the barrier of resistant rock. Their heights roughly alternate on either side of the river, as is characteristic of barrier terraces. Of especial importance in the preservation of the terrace-remnants about the gorge are the effects of bluffs and ridges of the more resistant rock, which have acted as turning-points for the stream in the course of its entrenchment. Illustrations of this preservation are shown in Plate 18, fig. 2.
The terraces above the gorge barriers may thus be explained without the assumption of any change in base-level due to change in position of the
land relative to sea-level; the numerous terraces below the gorge, however, present some difficulties. The explanation advanced by Hutton (1873) and supported by Marshall (1912), that they are normal river-terraces due to emergence, cannot be accepted without question.
Speight (1907) shows that there is a lack of supporting evidence for a theory of recent elevation of the central Canterbury area. Such movement would also necessitate the deposition of the upper beds of the gravels of the plains under marine conditions. As yet, however, there has been no evidence adduced to show that any part of the plains has been formed of delta or marine deposits. Speight, concluding that these terraces are not due to uplift, suggests that the effect of a diminution in the supply of waste would be to cause an entrenchment of the rivers below the levels of their fans at their apices. Whether or not a stream will erode its bed in any place depends, however, upon the gradient of the stream in that place; other factors, such as load carried, merely determine the rate of erosion. In accounting for these terraces it is therefore necessary to explain how a change in gradient occurred by which the river was enabled to erode its own deposits.
In tracing the history of a fan formed by any stream the following two stages may be noted:—
(1.) In its earliest youth the stream, passing down a steep bed, with consequent high velocity and carrying a heavy load, builds up a fan upon reaching the edge of the immature country; as this process goes on the fan is built up until its apex is level with the point where it leaves its rocky bed.
(2.) As erosion continues the fan is extended to a less convex shape, with lower angle of declivity, at the same time the bed of the stream in the alpine area reduces its grade. The point where the stream leaves the rocky bed to pass over the fan will therefore also be lowered. The river must then begin to cut into the top of its own fan-deposits. As this process continues, residual terraces will form at the top of the fan, which will possess the feature of barrier terraces—that the remnant on either side of the river will not correspond in level but will form an alternating series. These terraces, moreover, will decrease in height above the river-level when traced towards the fringe of the fan.
Fig. 4.—Section along left bank of gorge (line CD in fig. 1). 1, Martai beds; 2, rhyolites; 3, pitchstone; 4, andesites; 5, coal-measures; 6, teschenite; 7, Pleistocene deposits; F, fault.
Applying this hypothesis to the terraces of the Rakaia River below the gorge, the observed facts appear to be explained—(1) The terraces are discordant in level on either side of the river: (2) they decrease in height above the river-level when traced towards the sea. In the Rakaia River the terraces have completely disappeared about twelve miles inland from the coast.
The following is an outline of the geological history of the Rakaia Gorge district:—
Permian or Trias: Deposition of greywacke, shales, &c.
Lower Cretaceous: Orogenic activity, with contortion of Maitai beds and emergence of land-surface.
Middle Cretaceous: Period of erosion when land-surface reduced to peneplain; eruption of rhyolites and pitchstones followed by andesites and andesitic breccias.
Upper Senonian: Marine transgression with deposition of coal-measures, and probably followed by more or less continuous deposition until Pliocene times.
Tertiary: Intrusion of teschenite sill into coal-measures.
Pliocene: Orogenic movements characterized by block-faulting and causing emergence of the land-surface.
Pleistocene: Period of erosion giving rise to present topography, and including two periods of extension of the Rakaia Valley glacier as far as this district.
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Speight, R. 1922. The Rhyolites of Banks Peninsula. Rec. Cant. Mus., vol. 2.
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Woods, H. 1917. Cretaceous Fauna of the North-east of the South Island. Geol. Surv. Pal. Bull. No. 4.