General Account of the Locality.
(See Map and Plates 23 & 24).
About a mile from the sea-coast at the mouth of the Hurunui, the river runs through a somewhat shallow gorge incised in a spur of greywacke reaching south-west from the mass of Mt. Seddon (984ft.), and both above and below the gorge the Tertiaries are developed on both sides of the river. The road crosses from the north to the south bank by a bridge at the narrowest part of the gorge. Above and below the bridge the stream flows over a gravel bed considerably wider than near the bridge, and has on its upstream side bordering banks with a general height of about 40 feet above stream-level, while downstream the banks on the south side rise to a high cliff (500ft.) and then sink in steps formed of well-defined wave-cut platforms to sea-level (see Plate 23). On the north side of the river below the bridge the terrace continues on the same level as upstream of the bridge, but when it reaches nearly to the coast-line it swings round
and runs for nearly half a mile parallel with the shore, with a narrow lagoon or channel at its base formed by the diversion of the river in a northerly direction by the strong set of the sea waves and currents from the south (see map and Plate 24). This lagoon continues north for a distance of about half a mile before the stream breaks through the bank, and the position of the outlet varies owing to the relative influence of floods in the river and of storms from the sea which pile the shingle across the river-mouth. During such storms the waves may break across the lagoon against the face of the cliff on its inner margin. The height of this cliff is approximately 40 feet, but half way along it rises to a higher terrace, and this continues with slightly increasing height till its surface merges into the southerly slopes of Mount Seddon. The general arrangement of the Tertiaries is in the form of a marginal fringe to the greywacke mass of this hill, for they swing round its base continuously from the coast just north of the mouth of the Hurunui past the south end of the bridge, and then upstream and on over the ridge to the north-east, and finally reach the sea-coast again at Port Robinson and Gore Bay.
Succession of Beds. (See Sections Nos. 1, 2, 3; also Map).
The succession of beds is substantially the same as that given on page 214 of our paper, but it may be amplified slightly from combinations of the sequence displayed upstream from the bridge and also on the face of the cliff near the lagoon. Resting on a surface of greywacke is the following:—
1. Sandstone and Shaly Beds, repeatedly interstratified, the former about one foot in thickness, the latter about three inches, exposed in the river bank above the bridge for about 80 yards. They strike N.-S. approx. and have a westerly dip of 10°. At their base is a conglomerate containing greywacke fragments.
On the lagoon face or just beyond it the beds in a corresponding position resting on greywacke are composed of non-glauconitic concretionary layers of sandstone, with interstratified layers containing glauconite, and sulphur sands with white efflorescence. They strike N. 50° E. and dip to the south-east at angles of 15°. They are exposed for 35 yards at the base of the cliff, and their thickness is probably in excess of 30 feet. They are in all probability the stratigraphical equivalent of beds of similar lithological character exposed in the lower part of the Jed River before it enters Gore Bay.
2. Greensands, very glauconitic near the base, less glauconitic at higher levels, and passing up into a greensandy marl. Upstream on the south bank they have a layer of greywacke pebbles at the base. They appear on the opposite bank both above and below the bridge, where the contacts are obscured, and also on the face of the lagoon cliff, where they show for a distance of 40 yards. (See Sections 1, 2, and 3).
3. Marl (= Chalk Marl). In the lower parts it is definitely glauconitic and sandy, but it soon shows harder layers, white in colour, of more calcareous material, with a flaky fracture. Higher still it loses its definite stratification, and becomes grey in colour,
Plate 25—Amun Lemestone at the base covered with Weka Pass Stone shown in the right-hand corner; the top of the prctune is hlled with Mount Brodn beds when contam large tragments of Amul Limestone and Weka Pass Stone A tault slopes diagonalls up the point from ngth to left Thes putune is taken looking appros N W at the chff tace from a point close to water-level in the lagoon
and after that it takes on a white, flaky, argillaceous limestone facies with interstratified layers of softer, more marly material till it passes up into the Amuri Limestone. This is developed on both sides of the river upstream. On the south bank it strikes nearly N.-S. with slight variation in direction owing to local deformational movements, and a dip to the west at low angles. It is much faulted and shows slickensided surfaces and local puckering and crumpling. Across the river it strikes more to the east, the beds swinging round the western base of Mt. Seddon.
This bed is exposed on the lagoon face for 270 yards, and where it is solid it has a N.-S. strike and a dip to the west of 15°, but the whole face here is much slipped and its relations are obscure. It is terminated towards the south by a fault, with an apparent over-thrust from the south, but the fault may be an ordinary normal fault with a downthrow to the south. The marl also occurs below the bridge on both sides of the river. On the north side the exposure is very much obscured with slips except in the higher horizons, where it looks like the higher horizons on the lagoon face. This bed strikes towards this face under the plantation of the reserve, and it occurs in the gully between the plantation and the bank upstream with the same strike, but it is uncertain whether it reaches the face of the lagoon cliff in this part of the section. A marl does appear there but it is reasonable to think that it is the Hurunui Marl, as will be shown later.
Similar beds occur at river level on the south bank under the high cliff where they run out in submerged reefs, striking E. 25° N. and dipping to the south at an angle of about 5°. The beds are also exposed a few feet above river level. They show at this point hard calcareous bands, about one foot thick, with flaky chalky bands interstratified. These are just opposite the exposure on the north bank. (See Section No. 2).
Further upstream towards the bridge, but separated from the occurrences downstream, the marl occurs lying on greensand-marl, which in turn lies on greensand. The marl can be traced nearly halfway up the hillside in a line with the bridge, and it apparently strikes in the direction of the beds below the bridge on the north side of the stream as if they were connected up beneath its bed. (See Section No. 1).
4. Amuri Limestone. Apart from the beds transitional from the Chalk Marl, which are thick, the Amuri Limestone as typically developed has a thickness of only a few feet. It shows the quadrangular fracture, and lenses of calcareous greensand which are characteristic of the beds near Kaikoura. It outcrops on both sides of the river above the bridge, on the south bank striking N.-S. and dipping at an angle of 20° to the west. It swings round across the river so that the strike is more easterly. The only place where it occurs in position on the lagoon face is about 280 yards from the corner of the road along the cliff, where as the result of folding it shows for a few feet in the core of an anticline just above water. (See Plate 25 and Section 3). The summit of the arch is faulted,
the hade of the fault being 65° to the north-east, and the downthrow is also in that direction; the amount of displacement cannot be more than a few feet. The angle of the contact of the Amuri Limestone with the overlying Weka Pass Stone is 30°. The fault at the junction of the Chalk Marl, about 200 yards farther north, apparently cuts out the Amuri Limestone, so that it cannot be seen, though fragments occur in a breccia alongside the fault. These are the only two occurrences that are visible, and to account for their absence we can only suggest that downstream from the bridge the limestone either lies beneath the river-bed and cannot be seen, or that it has been removed by erosion. We have come to the conclusion that the masses on the south bank near the point, large though they undoubtedly are, are merely floaters.
5. Nodular Layer.—This is well developed upstream in its normal position, but it is not so definite on the lagoon frontage, the nodules being distributed freely through several feet of the Weka Pass Greensand, and not concentrated into a definite layer.
Unconformity.—Here comes a break in the stratification, the evidence for which will be considered in connection with the next overlying bed.
6. Calcareous Greensand (= Weka Pass Stone).—This occurs upstream on the south side of the river in its normal position, but the greatest development in the locality occurs in the lagoon cliff, where it extends for fully 200 yards along the front. (See Section No. 3). The strike and dip vary. At the northern boundary of the occurrence the beds have a strike N.E.-S.W. and a south-easterly dip of 25°. Just past the point to the south of this, near a large cave, the strike is N. 5° E. and the dip easterly at an angle of 15°. The beds are here bent up into an arch with a steep south-westerly limb. The strike again lies parallel to the shore, its direction being N.E. and the dip south-east at an angle of 35°, while a little farther on the strike becomes N. 70° E. and the dip south-easterly at an angle of 20°. A chain further on the strike is practically E.-W., but this is immediately succeeded by a syncline, and then an anticline arching over the occurrence of Amuri Limestone, with an E.-W. strike and a dip to the south on the upstream side. The overlying Mount Brown beds now come down to water level, but there is another small exposure of the bed about a chain further on. The bed therefore shows here an irregularity in dip and strike due to some slight deformational movement. From the point last mentioned it disappears from view.
The bed is really a calcareous greensand, becoming more sandy at higher levels. It is no doubt the local equivalent of the Weka Pass Stone, though it may grade up to the horizon of what is called the Grey Marl, itself definitely sandy and slightly glauconitic in places. It is important to note that it contains angular fragments of Amuri Limestone. They are most common near the base of the bed, but they also occur at higher levels. Their presence indicates a break with the Amuri Limestone. This may be local only, as the general evidence from other parts of the North Canterbury district appears to indicate conformity.
It may be noted that it contains “fucoids,” and that its absence from the river section below the bridge may be explained in the same way as the absence of the Amuri Limestone from that locality.
The fault which bounds it on the north is associated with blocks of sandstone, fragments of Amuri Limestone and phosphatic nodules, which are in places no doubt reassorted.
Unconformity.—A stratigraphical break occurs at this stage.
7. Hurunui Marl and Breccia (see Sections Nos. 2 and 3).—This bed presents serious difficulties and the authors are still not entirely satisfied with the correctness of their interpretation, though it is given here as being the most reasonable in their opinion. The beds are typically developed on the south bank of the river in the lower part of the cliff face extending from the point for a distance of about 400 yards upstream. This is the least accessible part of the section, the river swirling round the base of steep cliffs. The beds consist of marls with lenses and masses of sandstone, and harder bands which contain fragments of either Amuri Limestone or Chalk Marl, probably of both, thus proving the existence of a break between the Hurunui Marl and the underlying beds. Associated with the marl is a breccia composed of fragments of Amuri Limestone, Chalk Marl, and Weka Pass Stone, and of another calcareous bed not seen anywhere in the locality. This last may be the equivalent of the Grey Marl or a higher part of the Weka Pass Stone, but it is not glauconitic and it is composed of fragments. The major fragments are of large size, some 25 feet in length, and frequently showing the effect of crushing. The marly facies is the bed marked 2 in Section No. 2 of our former paper (1928, p. 215), and correlated there with the Chalk Marl, which it closely resembles in places, but the presence of included fragments of Chalk Marl and Amuri Limestone rules that contention out. A similar marl occurs on the lagoon face, in a line with the Chalk Marl on the north bank below the bridge, and they were naturally taken to be identical, but the fragmentary nature of some of the bands appears to negative their identity. The occurrence on the south bank therefore appears to be connected with the marls just across the stream. All these beds lic flat or with slightly wavy inclination. The presence of large floater blocks themselves composed of smaller fragments implies that a consolidated bed occurred or still occurs covered up, between the top of the Amuri Limestone and the breccia just referred to. This may be the Grey Marl, which in this locality must be unconformable to the beds near the base of the section, seeing that the Weka Pass Stone, which underlies the Grey Marl, is itself unconformable to the Amuri Limestone and the Chalk Marl.
The relation of the large blocks to the finer marly facies is not clear, but they apparently underlie it and form the base of the bed. This can be seen near the point on the south side of the river. In our section just referred to (1928, p. 215) one of these blocks is represented as being faulted into the marl. This gives a correct representation of what appears, and the block may be pushed up, but this may not be due to faulting, and it may be attributable to
the settlement of weaker beds of marl around a more resistant solid mass. In any case the disturbance does not affect the overlying Mount Brown beds.
Just around the corner upstream from this point occurs a faulted mass of Amuri Limestone, with an accumulation of phosphatic nodules along the fault plane. This may have been faulted in position, in which case the fault does not reach the Mount Brown beds; in fact it cannot be detected passing through the large block formed of fragments which is associated with the Amuri Limestone block on the lower side of the fault plane. It is possible, too, that the faulting may date from the time anterior to the erosion which formed the block, that is, to the time when the beds they belonged to were still in position.
The question arises as to how such blocks may have been able to form a bed associated with a fine-grained marl. The answer to this is provided by the locality itself. From the face of the lagoon cliff large blocks are now being detached, falling down at the base of the cliff, perhaps transported a short distance by the waves, whhile round them accumulates a calcareous mud analogous to the marl. Similarly at the Napenape Limestone cliffs, about five miles south, large blocks of limestone are being embedded in marine deposits and, if the water were still or enclosed, deposits analogous to those at the Hurunui would be laid down. The deposit has been formed at the base of a cliff alongside the sea, and this cliff must have contained beds formed of Chalk Marl, Amuri Limestone, and the equivalent of the Weka Pass Stone and probably the Grey Marl, the evidence for unconformity being the lithological character of the included blocks. The strike and dip are so uncertain in places that it cannot be used as a criterion for conformity or otherwise. We have given the name Hurunui Marl and Breccia to this bed because of the uncertainty of its correlation with beds in other parts of the district. It may be equivalent to the Grey Marl, and in that case it would necessitate the placing of a break between the Grey Marl and the Weka Pass Stone in this particular locality and perhaps elsewhere, although the general consensus of opinion is that the Grey Marl and the Weka Pass Stone are conformable. All the same, both Haast and Park considered the relation to be unconformable. It should be noted that at the base of the cliff south of the river where the Chalk Marl and Hurunui Marl are in contact, it is difficult to say where the boundary lies. Beds without included fragments are assigned to the former, while those with fragments are placed with the latter. (See Section 2).
Unconformity.—This unconformity is the one which was discussed in our former paper, and the evidence given there is endorsed.
8. Mount Brown Beds.—These are as described formerly. They occur upstream on both sides of the river, form the main part of the cliffs south of the river, and a long stretch of the banks below the bridge on the north side of the stream, and also a part of the cliffs on the lagoon front. There are, however, one or two observations to be made.
The occurrence farthest is upstream is developed on both sides of the river. The next occurrence following the stream down also lies across the river. In our former paper the contact of this bed and the Chalk Marl is represented as an erosion contact on the upstream side. Having examined both sides of the river, the most reasonable explanation of the junction of this bed with the marl is that it is a fault contact, with some overthrusting from the north-west. The Mount Brown beds can now be traced along the bank at river level for a considerable distance upstream beyond the point where it was seen by us previously, and the position renders it practically certain that the beds have been overthrust from the north-west. This is supported by the fact that the Mount Brown beds occur in a similar position on the north bank, and this increases the unlikelihood of an erosion contact on the upstream side of the occurrence. It is further increased by the observation that the Chalk Marl is exposed in a cutting on the road to the south of the river, where it would be reasonable to expect Mount Brown beds, were the contact an erosion contact. All the same the steep contacts which are clearly seen in other cases, and our explanation of the erosion along a sea-cliff to account for the inclusion of such large floater masses in the sand, support the possibility of the junction being an erosion contact. The crushing and faulting of the beds in close proximity upstream support the fault hypothesis, and therefore this is accepted as probable.
It must be noted, too, that our original section should be amended so as to show these beds at river level, although there is an isolated occurrence of Chalk Marl at river-level with Mount Brown beds both above and below it upstream and downstream from it. It should be noted that a talus slope has been swept from this point by river erosion since we last saw it.
The occurrence on the south bank downstream from the bridge does not need further comment beyond what was made in our former paper, except to note the very steep contact between the Mount Brown beds and the Chalk Marl on the face of the hill just opposite the end of the bridge. Also some obscure remains of fossils were found on the hillside just above the contact.
There remains the occurrence north of the river. About 400 yards downstream from the bridge these beds form the bank of the river, butting against the Chalk Marl, and acting as a mere fringe which continues down till just past the plantation on the reserve. The large size of the floating blocks is noteworthy. The Hurunui Marl then continues for nearly 70 yards, then the Mount Brown comes in for 14 yards, forming a point in the cliff. This is succeeded by 500 yards of Hurunui Marl, after which the Mount Brown beds continue round the cliff face for over 400 yards, capping in places first of all the Hurunui Marl and then the Weka Pass Greensand.
At one place, near the western end just opposite the end of the road, the contact is a fault, although the steepness of the line of junction elsewhere suggests that it too may be an erosion contact.
The fact that the marl overrides the Mount Brown beds is, however, strongly suggestive of a fault contact here. There is another fault on the north-east side of the point some 70 yards from the road, where the evidence is analogous to that in the first case.
It may be noted as well that the lower part of the Mount Brown beds consist of a sandstone usually free from large blocks; these come in at a higher level, and the final facies is one of pure sandstone without the large blocks.
There remains now the final question to consider as to what are the precise relations of the Mount Brown beds to the Hurunui Marls, for there are difficulties which have not been disclosed up to the present. These relate chiefly to the lower level of the Mount Brown beds on the north side of the river below the bridge as compared with the level of the top of the Hurunui Marls in the cliff south of the river. This might suggest that the Hurunui Marls were interstratified with the Mount Brown beds. As against this there is no such interstratification in the occurrence of Mount Brown beds above the bridge. Also, opposite the end of the bridge on the south bank the upper facies of Mount Brown beds rest directly on the Chalk Marl, although with a very steep contact. It is to be observed that the large blocks are more common in the Mount Brown beds on the north side of the river near the plantation, but they also occur on the south side of the river in a position definitely above the Hurunui Marls. For these reasons therefore we consider that the Hurunui Marls are not interstratified with the brown sandstones of the Mount Brown Series. That being so, there is the possibility of the lower level of the occurrence north of the river being due to a downthrow. Of this there is no definite evidence. The remaining explanation is that they have been laid down on a very irregular surface, and that under the river lie concealed either the entire Amuri Limestone beds and the Weka Pass Greensands, or such remnants as have escaped erosion.
9. The closing members of the sequence exposed near the mouth of the river consist of gravels and loose beds. The former cap the cliffs south of the river as a thick veneer, and they also occur on the terraces between the river bank and the slopes of Mount Seddon. The former are perhaps the Kowai Gravels, whereas the latter are in general the deposits of the river when it ran at higher levels, but in one case at least, near the northern end of the lagoon cliff, gravels are capped with loess, also containing thin layers of pebbles, and it is probable that in this case these gravels are pre-glacial, that is, belong to the Kowai Series.
It will be seen from this description that the area furnishes evidence of three unconformities in the Tertiary sequence:—
(1) Between (a) the Weka Pass Greensand and Grey Marl, and (b) the Amuri Limestone.
(2) Between (a) the Hurunui Marl, and (b) the Weka Pass Greensand and Grey Marl, and therefore also with the Amuri Limestone.
(3) Between (a) the Mount Brown beds, and (b) the Hurunui Marl and lower beds.
The question of the conformity or otherwise of the Greta Marl and the Mount Brown beds does not arise.
In deciding on these unconformities no assistance has been received from the fossil content of the beds, since the only place where fossils were discovered was in the Mount Brown beds just opposite the southern end of the bridge.
The last of the three unconformities mentioned above is believed to be widespread, but it is questionable whether the unconformity is only local in the cases of Nos. 1 and 2. The possibility of local unconformity with general conformity can be illustrated from the district, for deposition may be continuous in the bed of the sea off the coast, whereas near the cliffs erosion may be proceeding as local uplift or warping brings a part of the beds concerned within the sphere of action of the waves. These are the circumstances under which fragments of large size may form locally a dominant element in a bed. The bed may pass gradually into a fine-grained sandstone or marl with parallel stratification and without a distinct break offshore, if the elevation has been restricted to a narrow coastal belt and has petered out either suddenly or gradually when traced seaward. This appears to be perfectly possible at Hurunui.