Art. XXIX.—The Older Gravels of North Canterbury.
[Read before the New Zealand Institute, at Christchurch, 4th—8th February, 1919; received by Editor, 24th February, 1919; issued separately, 16th July, 1919.]
|General Description of the District where the Beds are best developed||269|
|Description of Typical Localities||270|
|Grey River, East Branch||270|
|Grey River, West Branch||273|
|Okuku River and Mairaki Downs||274|
|Kowai River, North Branch||274|
|Kowai River, South Branch||274|
|Lower Warpara Gorge||278|
|Other Canterbury Localities||279|
|General Conclusions as to the Origin and Age of the Beds, and Relation to the Gravels of the Canterbury Plains||280|
Widely distributed along the base of the Southern Alps lies a series of unfossiliferous sedimentary beds, consisting for the most part of well-stratified gravels, sands, and clays, with occasional lignite, whose position has hitherto been somewhat doubtful. Haast (1879, p. 316, and map, p. 370) included them in his Pareora formation, and mentioned the occurrence of lignite-beds (p. 318) in the “Moeraki” Downs, at the mouth of the Waipara River, and in the Broken River basin, but hardly mentions the locality where they attain their maximum development—viz., the Mount Grey Downs and the vicinity of the two branches of the Kowai River. Hutton (1885, p. 211) considered them as equivalent to the Wanganui system of the North Island, but remarked that they were difficult to distinguish from the upper gravels of the Pareora system. Park (1910, p. 252) considered them older fluvio-glacial drifts. Thomson (1917, p. 411) refers to them more fully, but is extremely doubtful whether they shall be assigned to his Notocene or Notopleistocene set of deposits.
Owing to the practical absence of fossils it is difficult to determine their position accurately, but they nevertheless represent an interesting series, and the following account is intended to bring out their chief features. In addition to the difficulty noted by Hutton, there is the additional one that in their lithological content as well as to some extent the conditions under which they were laid down they resemble the beds that overlie them, and this makes it at times impossible to separate them from subsequent gravel and sandy beds.
General Description of the District Where the Beds Are Best Developed.
The chief area where they are developed lies to the south-east and south of Mount Grey, between the Waipara and Okuku Rivers, but they attain their greatest development in the basins of the Southern Kowai and the Grey Rivers. Important outliers also occur to the west of the Okuku, on
the lower slopes of Mount Thomas, and south of the Ashley River, where they form the Mairaki Downs (= Moeraki Downs of Haast). The beds form a kind of frontal apron to the higher greywacke hills, such as Mount Grey and Mount Karetu; but still they rise in places to well over 1,000 ft. above sea-level. The downs country has been dissected to some extent, and on the front facing south-east consequent streams have cut deep narrow channels, with high precipitous banks, whereas in the north-eastern portion the tributaries forming the Northern Kowai tend to develop valleys along the strike. The same is also true of the east branch of the Grey. The character of the drainage points to recent and rapid uplift, perhaps in agreement with that of which there is distinct evidence on the coast farther north (McKay, 1877, p. 177; Hutton, 1877, p. 55; Speight, 1918, p. 99). On the sides of the steep banks, especially those running with the dip, numerous good sections are exposed; and it will be best at this stage to give a more detailed description of typical sections, preferably those illustrating the relationship between the underlying Tertiary beds and the overlying gravels. Although the beds are typically developed in the basin of the Kowai River, and I have selected the name of that locality as the one most appropriate to designate the series, yet the most instructive sections are to be seen in the basin of the Grey River, and these will therefore be taken first.
Descriptions of Typical Localities.
Grey River, East Branch.
The eastern or chief branch of the Grey River rises in the country between Mounts Grey and Karetu, flows south therefrom in a deep wooded gorge, and then gradually turns to the south-west and follows along the north-western edge of the Mount Grey Downs till it enters on the plains and joins the Okuku River in the neighbourhood of White Rock Station. The first part of its course has been cut in greywacke, but on leaving the higher country it crosses the marginal fringe of Tertiary sedimentaries at an angle of about 45° with their strike, so that when the stream runs in the direction of the dip the cross-section of its channel is narrow and trench-like, but when it runs along the strike the valley opens out somewhat, with dip slopes bordering the stream on its north-western side and steep scarp slopes on its south-eastern side. The latter are in places very bold and precipitous, and show clear-cut sections. Especially is this the case at the Horseshoe Cliff, about a mile below the gorge, where the north-western slope of the downs has been scored by a deep washout, and the strata are clearly exposed for 500 ft.
In the lower part of the river-gorge there is a most interesting occurrence of the lower members of the Cretaceo-Tertiary series, analogous to that seen in the Waipara and Weka Pass sections. These beds when followed along the strike run in the direction of the greywacke mass of Mount Grey; and unless they turn round on approaching it, as they do on the north-east slope of the mountain, the junction between the two sets of beds will in all probability be a fault contact. To the west of the gorge, however, the junction between the two sets of beds is a normal unconformity.
The following is a description of the beds here exposed, the sequence being in ascending order:—
Greyish sands and sandy shales, glauconitic, concretionary in places, and stained with sulphur; succeeded by light-coloured argillaceous and slightly glauconitic sands—all striking north-east and dipping south-east at 45°.
Greenish glauconitic sand passing up into glauconitic limestone, the glauconitic material being disposed in irregular patches and lenses, giving the rock a somewhat streaky appearance; it is also full of worm-borings filled with glauconitic material. This passes up into
Amuri limestone, 25 ft. thick, with less glauconite than 2. The passage beds between this and the lower bed consist of fragments of Amuri limestone in a greensand matrix, the limestone finally taking on the facies of the typical Amuri stone, being white and jointed into quadrangular blocks. The strike is as before, but the dip is less, being about 30°.
Glauconitic limestone, 20 ft. thick, comparable with the Weka Pass stone as it approaches a shore-line (Speight and Wild, 1918, p. 77), but passing up into a more sandy facies.
Marl, slightly sandy, with concretionary layers and rounded concretions. This is the stratigraphical equivalent of the “grey marl” in the Weka Pass district. It has the same strike and dip as the limestone, and its thickness is about 70 ft.
Thus far the sequence is quite clear and conformable, but for a time the exposures are obscured and the relations to the underlying beds are not plain.
Just below the gorge there is a well-marked bed, striking north-north-east, with slightly flatter dip than the limestone, and containing numerous specimens of Ostrea ingens. Farther down-stream, but higher in the series, is a sandy conglomerate followed by sands with broken shells. These pass up into sands with a layer of oyster and other shell fragments, and then follow the beds of the Kowai series.
These are first exposed at the mouth of the gorge, just above the site of the old sawmill. They consist of sands and sandy gravels containing shell-fragments and showing intraformational unconformities, but no clear evidence, given by sections, of an unconformity between the Kowai series and the lower Tertiaries. Almost everywhere in the case of gravels resting on sands or other finer detrital beds the upper surface of the latter has suffered some erosion, but in no case in this branch of the Grey does this, in my opinion, amount to sufficient to be considered a major unconformity.
On the next bluff down-stream, and higher in the sequence, the beds exposed consist of greenish-grey sands (weathering light-brown) and sandy gravels, with sandy carbonaceous shales and impure lignite. These are capped unconformably by terrace-gravels belonging to the early history of the Grey River. This sequence is repeated on the next bluff, but the gravel beds of the Kowai series become more important, one very heavy band of gravel near the top of the cliff being divided into two parts by a layer of carbonaceous shale. In the bed of the river, at the base of this cliff, is a section which shows an unconformable junction between a greenish sand and an overlying bed of gravel. After a careful consideration of the circumstances of this case I have come to the conclusion that it must be considered only as an intraformational unconformity, due to the erosion of the bed of sand by marine currents in the interval between its deposition and that of the succeeding layer of gravel.
These beds strike north-east, and dip south-east at an angle of 20°.
As the sections are followed down-stream their character does not change except that the gravels become increasingly important, a feature that is well exemplified at the Horseshoe Cliff, on the face of which gravels greatly predominate, some layers being from 50 ft. to 70 ft. in thickness. Well-defined sandy layers also occur. The regular stratification of the beds
towards the base of the cliff points to their having been deposited in shallow water in close proximity to a shore-line, and not on a land-surface; but at the higher levels the stratification becomes more indistinct and the pebbles become coarser and more subangular in shape, so it is almost certain that the closing beds of the series were laid down on a land-surface. The presence of lignite in the lower beds clearly indicates estuarine or deltaic conditions.
It should be noted that on the high banks of the Grey River there is a still more recent series of gravels belonging to the history of the stream. They are similar in lithological features to the gravels of the Kowai series, but they are neither so well stratified nor so well cemented. They are undoubtedly river and not sea deposits. Where contacts can be seen they are easily differentiated, but elsewhere, especially on the lower slopes of the downs, it is difficult to separate them from the upper members of the lower set of beds, which were also laid down on a land-surface.
Grey River, West Branch.
The general stratigraphy of the beds in the basin of the western branch of the Grey River is similar to that in the eastern. The following is a general description of the strata exposed above the greywacke as disclosed on the sides of the gorge of the stream:—
Sands and greensands.
Limestone, full of bryozoan remains, but only a few feet thick in the gorge of the stream, thickening, however, to the east and to the west. There is a marked difference in the features of this limestone as compared with that in the eastern branch, and as they are in apparent continuity it might be assumed that the stone in the western branch represents a shallower-water facies. I am by no means certain that this is the true explanation, and the question of the identity as regards their stratigraphical position must be reserved for further investigation.
Marls, greenish in colour, with rounded concretions and concretionary bands, passing up into greyish sands with fragmentary fossil shells.
In the river these beds strike E. 25° S. and dip south at an angle of 30°, but they have suffered some deformation, and the strike changes to north-east on the ridge between the two branches of the Grey, and also as the beds are traced round to White Rock and the Okuku River. The upper surface of the sands was distinctly eroded before the next bed was laid down. This consists of a heavy band of cemented gravel. The following beds are then encountered, in ascending order:—
Gravel bed just referred to.
Sandy clays and gravels.
Sandy clay and carbonaceous shale, repeatedly alternating. One bed of shale is from 12 in. to 18 in. thick.
Sandy gravel, well cemented with iron oxide.
Greenish-grey sands, sandy shales, and gravels, rapidly alternating, totalling over 200 ft. in thickness, the strike gradually becoming east-north-east, and the dip flattening out from 30° to 10°.
Gravels, sandy and with occasional thin layers of sandy clay, lying flat or with slight dip to the south-east. These are at least 500 ft. thick, and are well exposed on the ridge between the western Grey and the stream near the White Rock Station.
The section in this river thus shows that there is a distinct series in which gravels are the dominant beds lying unconformably on marine Tertiaries. It should be noted that in the western branch there are no gravel beds below the unconformity. Either they have never been deposited or they have been removed by erosion. There is a strong suggestion from the eastern branch that gravel beds are present among the higher members of the underlying marine series, so that their presence cannot be taken as decided evidence that beds containing gravels in this locality necessarily belong to the Kowai series.
Okuku River and Mairaki Downs.
Similar gravels occur on the banks of the Okuku, especially on the western side, where they form low hills fringing the base of Mount Thomas, and stretching westward towards the Garry River and Glentui. Towards the Ashley they are masked by more-recent gravels, but they reappear on the south bank of the river, forming the Mairaki Downs. The strata here consist of thick sandy gravels, sandy clays, and occasional layers of carbonaceous shale. Opposite the mouth of the Garry they strike north-east and dip north-west at an angle of 20°, forming the south-eastern wing of a syncline which is developed farther west, while farther east, towards Rangiora, the structure is anticlinal. The country directly between the Mairaki and Mount Grey Downs is probably a syncline, but the surface is completely masked by recent gravels and clays belonging to the Ashley and Okuku Rivers and to the lower course of the Grey and Makerikeri Rivers, the latter draining a considerable area on the south-western flank of the Mount Grey Downs.
Kowai River, North Branch.
An excellent idea of the structure and general features of the northern part of the downs area can be obtained by examination of the high banks of the North Kowai, and especially of a tributary which rises in Mount Brown itself and flows in a south-easterly direction across the strike of the beds, thus exposing all the members of the series present in this locality. The following is a general description of the beds encountered, starting with the Mount Brown beds and following up to the highest members of the series:—
At the contact with the upper members of the Mount Brown series the latter consist of sands, and marine gravels with shells, striking north-east and dipping south-east at an angle of 10°. The Mount Brown beds are here capped unconformably with sandy gravels containing rounded and sub-angular greywacke pebbles, and belonging in all probability to the high-level terrace-gravels of the present Kowai River. Lower down sands, sandy clays, and sandy gravels dipping south-east at very low angles are exposed on the banks of the stream and in the deep gullies on the northern side. There is certainly a disagreement in dip between these beds and the underlying Mount Brown beds, suggesting an unconformity, but nowhere could I see an actual contact in order to determine this point precisely. The slight escarpment of the downs which faces Mount Brown at this point is determined by the presence of the gravel beds which occur at this horizon. It is noteworthy that there is an entire absence of the gravel beds with broken-shell layers which cover the Mount Brown beds in the vicinity of Weka Pass, a point which increases the probability that the beds forming the downs rest on the Mount Brown beds unconformably.
Farther down-stream the beds lie almost flat, with an east-south-east strike and a dip to the north-north-east at very low angles (less than 5°). On a high bluff a series of well-stratified sands and sandy gravels is exposed. Near the base of the cliff, under a sandy bed cemented in its lower part with iron oxide, lies a narrow band of sandy carbonaceous shale, 6 in. to 8 in. thick, containing pieces of lignified wood, and passing down into sandy clay with interstratified irregular lenses of lignite. Under these lie sands and sandy gravels, and then bluish-green and brown sands. A little below this the strike swings round to north-north-east, with an easterly dip, and in a narrow gully on the south side of the stream an interesting section is exposed. Here both the bluish-green sands and the sandy gravels have been eroded, and on the eroded surface have been deposited sands and sandy gravels containing fragments of the lower beds. A similar occurrence is to be observed on the face of a cliff in the main stream, the lower beds dipping 10° and the upper lying flat across them. A thin layer of broken-shell fragments was observed high up on the face of the cliff in an inaccessible position.
Higher in the series are rapidly alternating sands and fine sandy gravels, in layers down to 1 in. in thickness, and these are succeeded by sands, sandy-gravel beds, and bluish-grey and brown sands, the former with broken-shell layers. In the gravels are numerous fragments of limestone, which must have been derived from a surface of the Amuri limestone exposed to decided erosion. The fragments are generally less than 2 in. in length, but are sometimes longer, and are usually flattened like beach shingle. There is no doubt as to the interstratification of these beds in the series under consideration, as the same feature was observed in a deep gully to the north of the stream in its proper stratigraphical position. The presence of these fragments is positive proof of the presence of an unconformity between these beds and the Amuri limestone, and supports the stratigraphical evidence from the Grey River. In the bluish-green sands there are occasional shell-fragments.
For some distance below this spot there are no clear sections, but sand is probably the major constituent of the beds. At the junction with the Kowai River, however, there are high cliffs on the northern side, where the strata are clearly visible for half a mile. The lowest beds exposed in this locality are sands with interstratified gravels, in which limestone-fragments form a most important constituent. The beds with the limestone-fragments are at least 50 ft. thick, and may be thicker. Higher up the limestone constituent gets less and less, and the pebbles are entirely of greywacke. No other included material, such as fragments of Mount Brown limestone, was noted at this spot, which might indicate the date of the break between the Amuri limestone beds and those under consideration. It is possible that these gravel beds are unconformable to the greenish marine sands, since for some distance no exposures are visible which enable their relations to be precisely determined, and there is evidence from other parts of the area that these upper gravels are unconformable to greenish sands—e.g., in the Grey River (see p. 272) and also in No. 2 Creek (see p. 276). These gravel beds are fairly well stratified, with occasional beds of sandy clay and thin carbonaceous shales, their total thickness being about 1,000 ft., and the whole thickness of the series from the junction of the Mount Brown beds upward being about 1,500 ft., though this may include two series—viz., the Motunau and the Kowai series.
The course of the main stream above its junction with the tributary follows almost along the strike of the beds, so that the structure is not so well displayed. The beds exposed consist of marine sands, which are remarkably
current-bedded, and loose and cemented sandy gravels with numerous fossils similar to the beds exposed in the Lower Waipara Gorge. The highest bed of this series exposed in the valley of the stream consists of greenish sandy clay, which weathers a light brown, and contains fossil shells. Its upper surface has been distinctly eroded, and on it rests a heavy layer of cemented gravel, and following this are sandy clays and gravel beds dipping south-east at angles of 10°. These beds pass upward into the gravel beds exposed on the cliffs of the river below the junction with the tributary. In the tributary mentioned above I could find no indication of an eroded surface analogous to that in the main stream, and so it may be an unconformity of local character similar to those recorded elsewhere, but it may indicate a decided unconformity between the Motunau and the Kowai series.
It is owing to the typical development of these gravels and the beds associated with them in the Kowai River, not only in this locality but in the south branch as well, that I have called them the Kowai series. It is possible, however, that the lower part of this group of beds may be equivalent to the upper part of the Motunau series, and subsequent investigation may show the term to be unnecessary.
In No. 2 Creek, a southern tributary of the North Kowai, there is a very important section. Just below the high bluff on the north side, about four miles above the junction, the stream has exposed the following beds:—
Greenish sands, becoming more clayey in the upper portions and passing up into sandy shale.
Lignite, very impure, 10 in. thick, striking east-north-east and dipping north-north-west at 5°.
Argillaceous sands, decidedly clayey above the coal but becoming more sandy and greenish in colour higher up. The thickness exposed is about 6 ft., but it is eroded, and sandy gravels rest on it unconformably. An eroded surface appears just below this in the bed of the stream, with an angular mass of green sandy clay embedded in the gravel.
In close proximity to the erosion surface there is another section showing the same features, but with only 3 ft. of bed 3 interposed between the coal and the gravels.
Just over the dividing-ridge between this and the South Kowai River there are high cliffs, facing south, composed of similar beds, with gravels more strongly developed in the higher levels, and dipping south at angles about 5°. Thus an anticlinal axis runs east-north-east along the ridge in close proximity to the road which runs along the crest.
Just at the point where the stream turns after leaving the steeper slopes of Mount Grey, and assumes a north-easterly course, coal and associated beds are exposed in its actual channel and in the bank of a small gully on the southern side. They consist of—
Greenish sands, passing up into sandy clays.
Clays succeeded by greenish sandy clays.
Gravels, mixed with sand, cemented with iron oxide.
Greenish sandy clays.
These beds strike north-east, and dip north-west at an angle of 5°.
When followed up-stream there is an alternation of sands and gravels, apparently conformable to the beds just enumerated, exposed in the slips
on the river-banks; but the dip becomes steeper till, on the face of a high bluff below the bush, it reaches 20°. Here are alternating sandy clays and gravels, the former greenish-yellow in colour, which are capped unconformably by somewhat irregular sands and gravels, lying almost horizontally across the denuded edges of the lower set. The upper series evidently forms the distinct ridge which leads down-stream past the point where the undoubted unconformity described above was observed.
Similar beds are observed in places on the banks of the stream higher up, but the covering of bush and soil is too complete to attempt a correlation with those lower down. Owing to this covering it is likewise impossible to say whether the junction between the greywackes of Mount Grey is a simple unconformity or a fault contact.
Kowai River, South Branch.
The high banks of this river rise in places to a height of 500 ft. above its bed, frequently with precipitous faces, and thus excellent sections are exposed. The strata are also folded into gentle anticlines and synclines, so that in the cores of the former the lower beds are exposed. They consist of the following in ascending order:—
Sands with concretionary layers, with broken-shell beds in the lower part, at least 80 ft. thick.
Green sandy clays and gravel beds, the latter finer in grain and thicker in the lower part, and cemented with iron oxide.
At higher levels there are rapidly alternating gravel and sandy beds, the former composed of subangular pebbles, which point to deposit either on a land-surface or on a shore-line in close proximity to the source of supply. No limestone pebbles were seen in these beds. In places they exhibit intraformational unconformities, such as one would expect when rapid changes in the conditions of deposit take place, especially when the change is from a sand to a gravel, and vice versa. The gravel beds frequently form steep cliffs; and their hard bands determine the dividing-ridges between the tributary streams running into the main river, especially on the south side, and they also determine an important reach of the river itself, although its direction is primarily across the strike, and therefore of consequent character.
A specially good section is to be seen where the river makes a right-angle turn, and changes from the subsequent to the consequent direction. The beds are here bent up into a rather sharp anticline with a north-east strike and a dip to the north-west at an angle of 50°. On the seaward side of the anticlinal axis the dip is much less, the angle being about 10°, the succession being similar. But farther down-stream the strike swings round till it is north-north-east, then north-north-west, and finally north-west, following for a time the direction of the main river. This allows the lower members of the series to be exposed again in the bed and banks of the river. They consist here of sands and sandy clays, greenish-blue in colour and weathering brown, containing fossils, some of the sands with concretionary layers and associated with thin gravel beds. The shells consist of Siphonalia, Glycymeris, and Ostrea, but in a fragmentary condition.
Farther down-stream the greenish-blue beds are still exposed, but the strike gradually becomes north-east with a dip to the south-east, and gravel beds form the greater part of the high bluffs which face the river on the north above the Mount Grey Station.
Opposite the right-angle turn of the river referred to above, and immediately to the west of the axis of the anticline, there is distinct evidence of the presence of an unconformity between the gravels just referred to and a higher series. The beds here consist of sands and sandy gravels which are lithologically indistinguishable from the higher members of the lower series. On a bluff facing the river the beds in contact with the gravels of the lower series are exposed lying across their denuded edges at low angles, and immediately to the north-west they show a reversal of dip and are inclined to the south-east at angles of from 5° to 7°. This dip is in agreement with that which can be observed in sections in the upper basin of the Southern Kowai, and notably to the south of the anticline which runs to the south-east of No. 2 Creek and parallel with it. Thus there is evidence of an unconformity in the Southern Kowai in close proximity to that in the No. 2 Creek in the drainage area of the Northern Kowai.
It will be noted that the lowest members of the series present in the two branches of the Kowai consist of sandy beds with marine fossils, whereas these do not appear with certainty in the Grey River. The gravel beds are, however, equally developed in each area. This difference is perhaps of no special stratigraphical importance, since the gravel beds in both areas are undoubtedly marine, and in the Grey area conditions may not have been favourable for the preservation of fossil remains. As far as I can see at present, there is no evidence of a major unconformity between the gravel beds and the lower marine beds, although minor, intraformational unconformities undoubtedly exist. There is, however, distinct evidence of a discordance at a higher level in the Kowai series between beds of similar lithological character both above and below the unconformity, but not such a discordance as necessitates the higher beds being placed in another distinct series. The whole area and its vicinity no doubt experienced a fairly rapid elevation, probably of a differential character, so that erosion went on in one part of the area while deposition was continuous in an adjacent part. If subsequent deposition over the denuded area then ensued this special stratigraphical feature can be satisfactorily explained.
In Fox's Creek, the next stream south of the Kowai, there is an excellent section of the gravels forming the great mass of the downs area. This stream rises in the centre of the area and flows east, being bounded on the north for the middle part of its course by precipitous banks, in places up to 500 ft. in height above the stream. The beds here exposed consist of sandy gravels, sands, and sandy clays, with occasional thin, discontinuous layers of sandy carbonaceous shale. The sands are frequently blue-green in colour, and without fossils as far as I could see. In this part of the course of the stream the beds lie flat, with a slight dip to the north, but on the eastern margin of the downs the dip increases to 10° and its direction becomes south-east. In the adjacent valleys to the north and south there are similar beds with similar dip.
Lower Waipara Gorge.
Just where the river crosses the western end of the Limestone Range, beds of the Motunau series are exposed, consisting of marine sands, sandy clays, and sandy gravels frequently cemented with calcareous material and containing numerous fossil shells (Speight, 1912 and 1914). They are
involved in anticlines and synclines, and sometimes dip at steep angles—as high as 55° to 60°. They do not contain, as far as my observation goes, any limestone-fragments such as might have been shed from the Weka Pass or Amuri limestone beds, and this suggests that they are not unconformable to the beds containing those limestones, a conclusion which is supported by general stratigraphical evidence. These Motunau beds are capped unconformably by gravels containing numerous fragments of limestone, and from their lack of distinct stratification it may be concluded that they are high-level terrace-gravels of more recent date. Similar gravels occur on the downs just east of the Amberley—Waipara Railway, covering a considerable extent of country, as they are occasionally exposed in the sides of deep washouts and cap the cliffs cut by the river in making its gorge.
Along the north bank of the river below the Teviotdale Bridge there is also a series of Motunau beds, consisting of sands, sandy clays, and gravels, some of which are very fine and smooth and are evidently of marine origin. These beds contain fossil shells at various levels, which point to the age being Mio-Pliocene or Pliocene (Speight, 1914). On the south bank of the river the beds are much obscured by slip-material and vegetation, so that in no place is the contact clearly displayed. On the terrace near the mouth of the river the following section is exposed:—
Yellowish sands, exposed at river-level and for 6 ft. upwards.
Sandy lignite, with well-marked woody structure and containing crystals of gypsum in stellate and columnar groups, 4 ft. in thickness.
Sandy clay (fireclay?), with occasional pieces of bituminized wood.
Lignite, full of bituminized wood, 6–8 in.
Sand and sandy clay, with pieces of wood, 4 ft.
Gravel, 6 ft.
Farther up-stream the lignite-beds are exposed in similar stratigraphical position, succeeded by yellowish clays and sandy gravels, and at one place there is an exposure under the lignite of well-stratified and rounded marine shingle.
These beds all strike east-south-east and dip west-south-west at very flat angles, and it is impossible to tell on stratigraphical grounds whether the gravels overlying the lignites are conformable or not, or whether they belong to the Kowai series or to recent terrace-gravels. The locality furnishes evidence of the ease with which conformity may be simulated under certain circumstances. If level beds are planed by the sea, and no irregularity left on their being depressed and covered with a veneer of sediments, apparent conformity may occur over considerable distances, and especially will this be the case if the beds in contact are of a sandy or gravelly nature. A suggestion of unconformity is given in this case by the presence in the lowest layer of gravel of large pebbles, up to 8 in. in diameter, and more or less subangular, indicating strong currents on a land-surface or on a sea-bottom in close proximity to land, and that the beds were deposited under conditions entirely dissimilar from those obtaining when the better-rounded gravels were laid down.
Other Canterbury Localities.
There are other localities in this part of the South Island where similar gravels occur, among which may be cited the Isolated Hills in the Culverden
Basin; the cliffs at Gore Bay, where well-cemented gravels are involved in a syncline; the western side of the Trelissick Basin, specially in the Hog's Back Creek; between the Pudding Stone and the North Ashburton River; and in South Canterbury between the Tengawai and Pareora Rivers. It is probable, too, that the deeper gravels encountered in the bore at Chertsey belong to this series, the indications of petroleum coming from plant-remains which elsewhere have formed lignite.* Just below the Rakaia Gorge, where great thickness of gravels has been exposed, there is an underlying set of beds which are more strongly oxidized than the covering strata, and they may perhaps be assigned to a series older than the prevailing shingle beds of the plains. Although this criterion is perhaps an unsatisfactory one on which to base a determination of relative age, yet it has been applied in Switzerland in order to differentiate the gravels of the older glacial series of that region.
General Conclusions as to the Origin and Age of the Beds, and Relation to the Gravels of the Canterbury Plains.
The materials of which these gravels are composed have been derived almost entirely from original greywackes. No limestone was noted among them except in the case of the beds in the North Kowai. Occasional pebbles of basalt also occur, such as might have been derived from areas where such rocks are known to exist. A siliceous sandstone, white in colour and forming rounded masses, which could not be traced to its source, also occurs freely in the gravels of the Mount Grey district. These are perhaps masses of sandstone which have been loosely cemented by processes analogous to those which have formed the sarsen stones, or “Chinamen,” as they are called by miners, of the schist areas of Central Otago.
The subangular nature of the pebbles shows that the greywacke land must have been in close proximity to the area of deposit. The absence of large pebbles suggests that it was of moderate relief, though it might have been the outlying portion of a more elevated tract. The gravels contain, however, no suggestion of a glacial or fluvio-glacial origin; they are just such gravels as might have been brought down by the present Ashley or Waipara Rivers, which have no connection with glaciers. The origin of the limestone constituent can be traced exactly, as exposures of limestone of similar nature occur within a short distance of the area where they have been deposited; but these limestone pebbles occur low down in the series and disappear at higher levels, so that the uppermost beds must have been derived from areas where limestone does not exist. Although the lower members of the series are undoubtedly marine, the upper members were in all probability deposited under estuarine conditions or actually on a land-surface.
The determination of the age of the Kowai series is a matter of some difficulty. The unconformity in the Grey River shows that it is certainly post-Miocene, and in the Lower Waipara Gorge beds occur under the gravels with a fossil content which shows them to be upper Pliocene (Speight, 1914, p. 300)—that is, beds which form the upper part of the Motunau series. Therefore we may reasonably infer that the Kowai series is either upper Pliocene, if no unconformity exists between two sets of beds, or Pleistocene, if an unconformity is demonstrable. The Pliocene beds of the lower Waipara are perhaps the uppermost beds of a conformable Cretaceo-Tertiary series.
[Footnote] * Pieces of carbonized bark have recently been obtained from a depth of 1,900 ft. in this bore.
Therefore if an unconformity can be proved between the Kowai series and any member of the lower series it will be unconformable to all. As the presence of included fragments of limestone in the gravel beds of the Kowai demonstrates the existence of a clear unconformity between the gravels and the limestones, and the undoubted erosion-surface in the Grey River demonstrates the presence of one at a higher level still, we may therefore infer that the gravels of the Kowai series must be of Pleistocene age. If, however, the Cretaceo-Tertiary series is broken up eventually into subordinate unconformable elements, then this argument fails, and the matter will depend on the relation of the Kowai series to the fossiliferous marine beds of the lower Waipara and the Northern Kowai, as being the highest beds on which the Kowai series undoubtedly rests. The relation of the two sets of beds is somewhat obscure, though, judging from the evidence in the latter locality, probably unconformable. Therefore all that can be definitely stated is that the Kowai series overlies undoubted upper Pliocene beds and must be of a later age, and is most probably Pleistocene.
This must be earlier than the gravels forming the Canterbury Plains, for these have suffered no deformation by folding movements, whereas the gravels of the Kowai series are at times folded somewhat acutely. They would therefore antedate the last period of glaciation to which the region had been subjected.
A point which bears on the conformity of the Tertiary sequence should be noted—viz., that in neither of the two branches of the Grey River are the typical Mount Brown beds developed, thus suggesting an unconformity between the Motunau series, or the Kowai series, and the Mount Brown beds in case the absence is due to erosion, or between the Mount Brown beds and the “grey marls” in case the overlying beds, together with the Mount Brown beds, are part of a conformable series. As there appears to be no evidence of unconformity between the Mount Brown beds and the “grey marls” in the typical locality, whereas there is some evidence of an unconformity between the Motunau series and the Mount Brown beds, it seems more likely that the absence of the Mount Brown facies in the Grey River is due to erosion of these beds after deposition. This, however, is a point which requires further investigation.
Haast, J. von, 1879. Geology of Canterbury and Westland.
Hutton, F. W., 1877. Rep. Geol. Explor. during 1873–74, pp. 27–58.
— 1885. Sketch of the Geology of New Zealand, Quart. Journ. Geol. Soc., vol. 41, pp. 191–220.
McKay, A., 1877. Rep. Geol. Explor. during 1874–76, pp. 172–84.
Park, J., 1910. The Geology of New Zealand, Whitcombe and Tombs, Christchurch.
Speight, R., 1912. A Preliminary Account of the Lower Waipara Gorge, Trans. N.Z. Inst., vol. 44, pp. 221–33.
— 1914. Additions to the List of Fossils from the Lower Waipara, Trans. N.Z. Inst., vol. 46, p. 300.
— 1918. Structural and Glacial Features of the Hurunui Valley, Trans. N.Z. Inst., vol. 50, pp. 93–105.
Speight, R., and Wild, L. J., 1918. The Stratigraphical Relationship of the Weka Pass Stone and the Amuri Limestone, Trans. N.Z. Inst., vol. 50, pp. 65–93.
Thomson, J. A., 1917. Diastrophic and other Considerations in Classification and Correlation, and the Existence of Minor Diastrophic Districts in the Notocene, Trans. N.Z. Inst., vol. 49, pp. 397–413.