A Definite Break in the Tertiary Sequence in North Canterbury.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 59, 1928, Page 213

A Definite Break in the Tertiary Sequence in North Canterbury. By R. Speight, M.Sc, F.G.S., and Geo. Jobberns, M.A. [Read before the Philosophical Institute of Canterbury, 7th December, 1927; received, by Editor, 9th March, 1928; issued) separately, 10th August, 1928.] Plates 34-37. The question of the conformity or otherwise of the Cretaceous to Upper Tertiary sequence in North Canterbury has been under consideration since the beginning of the geological investigation of the province, and it still presents points for discussion. The general statement of the case has been given in the paper by Marshall, Speight, and Cotton on the Younger Bock Series of New Zealand (1911), and the matter has been referred to by Morgan (1916) and by Thomson in his account of the Notocene Geology of the Middle Waipara and Weka Pass District (1920), so that this need not be recapitulated. The present authors in the course of an examination of the shore-platforms in the vicinity of the mouth of the Hurunui came across an unconformity of which the evidence is so clear as

to leave no room for doubt, and this has prompted them to examine other localities in order to see if there is evidence of a break in a similar stratigraphical position. Such evidence has been found as indicates that the break is not a mere local feature, and the localities where it occurs will be taken in turn, commencing with that where the unconformity was first dearly detected. Mouth of the Hurunui. (Sections 1, 2, and Figs. 1, 2, 3.) The only reference to the geological features of this spot is to be found in the paper by Speight and Wild on the Relationship of the Weka Pass Stone to the Amuri Limestone (1918) where a brief account of the beds occurring there is given. Very fortunately, on the occasion of the last visit of the present authors the river had swung away from the right bank above the bridge and thus they were able to get a clear view of the sequence of beds in the locality (Fig. 1). Lying unconformably on the greywacke are the following beds in ascending order, all striking N.-S. and dipping W. at angles of about 15°. (Section 1.) 1. Calcareous Greensand, very glauconitic and with discontinuous calcareous layers and nodules running through it, resting unconformably on greywacke, and passing up into No. 2. 2. Sandy, Argillaceous Limestone, i.e., a sandy marl, hard, tinted a greyish green, distinctly glauconitic. 3. Amuri Limestone, the top layers showing the characteristic jointed stone passing down into flaky limestone with flaky argillaceous layers of finer texture, occasionally faulted. 4. The ‘Nodular Layer,’ of Speight and Wild (1918). 5. Calcareous Greensand, the glauconitic facies of the Amuri Limestone. Section 1. Hurunui River, South Bank, above lowest bridge. Length—About 100 yards. 1. Calcareous Greensands. 2. Sandy Marl. 3. Amuri Limestone. 4. Nodular Layer. 5. Calcareous Greensands. 6. Sands with Concretionary Layers. Mt. Brown Beds. 7. Terrace Gravels. G. Greywacke. 6. Sands, with concretionary layers and nodules, at times argillaceous, usually brown in colour, but with occasionally greenish layers. The sands are at times finely laminated, and are decidedly glauconitic immediately above the contact with the lower bed, where there are included pebbles and boulders of Amuri Limestone. They are very thick and underlie the greenish marls which form the beds under the Hurunui-Cheviot basin, i.e., the Greta Marls, whether conformably or not could not be determined.

This description of the sequence differs slightly from that of Speight and Wild, chiefly as regards the beds under the Amuri Limestone. What was thought by them to be sands in position at the base of the series resting on the greywacke proves to be a detached mass of No. 6, resting unconformably on No. 2 with, boulders of limestone at its base close down to river level. The present position of the stream has enabled this point to be settled definitely. The same series is exposed on the opposite bank of the river slightly further upstream, but the most interesting section is to be found on the cliffs at the south side of the mouth of the river. The following sequence is exposed here (see section No. 2, also Fig. 2). 1. Marly beds, flaky in texture, the usual facies below the Amuri Limestone in this locality, well stratified, with the beds slightly folded. 2. Sands, brown, in colour, with marked concretionary masses and layers in their upper portion but comparatively free from them in their lower levels. They rest on the denuded edge of the marly beds, and do not show any sign of their folding. About 7 ft. above the break there lies a bed composed chiefly of boulders and fragments Of Amuri Limestone and occasional masses of marly greensand. These beds are overlain by gravels, which attain great thickness a few hundred yards away to the south, and are probably the Kowai Gravels. These are in turn capped by recent terrace gravels. Section 2. Hurunui River, South Bank, at Mouth. Length—About one chain. 1. Marly Beds. 2. Amuri Limestone. 3. Sands with Concretionary layers. Mt. Brown Beds. 3a. Fragments of Limestone and Marly Greensand. 4. Gravels. The length of the section is about a chain, and at the upstream end there is a mass of Amuri Limestone which has been thrust up into the marls so that their ends are bent up by the movement, and the surface of the limestone slickensided. The fault is a reversed one and does not affect to the slightest extent the overlying beds numbered 3. The mass of limestone shows, a bed of phosphatic nodules 18′ thick, but it is quickly cut off by the river. For some little distance upstream the cliffs are difficult of access and the section obscured, but in a gully coming in from the south

there is an excellent section across the strike. The sequence here is as follows:— 1. Limestone, with flaky facies forming the lowest exposed bed. On this rest with an erosion surface. 2. Sands, irregular in thickness varying from 0 up to 5 ft., containing fragments of Amuri Limestone. On this with marked erosion surface rest 3. Sands enclosing fragments, angular boulders, and large masses, up to 20 ft. in length, of Amuri Limestone, very rudely stratified. The fragments are of both facies of the limestone, i.e., those in which the jointing is (i) quadrangular and (ii) flaky (Fig. 3). 4. Sand, irregular in thickness. 5. Fragmental layer. 6. Phosphatic nodular band, very well developed, sometimes splitting into two with a thin layer of sand intercalated. 7. Thick beds of sand, with hard calcareous bands and nodular concretions. 8. Capped unconformably with Kawai Gravels. All the beds No. 1 to 7, strike E.-W. and dip S. at low angles. In the big floaters of No. 3 are fragments of the phosphatic horizon. It cannot be asserted for certain whether or not the bands of phosphatic nodules are in places a re-concentrate of those formed at a lower level as a result of erosion. At the head of the gully is a waterfall, below which there is a fault which affects the two facies of Amuri Limestone but not the sands above. The two levels of sand with interstratified large masses, are a counterpart of the beds seen on the cliff further downstream. Further upstream the banks of the river are inaccessible except by means of a boat, but when viewed from the bed of the river opposite they show a definite erosion contact. The evidence in this case in favour of unconformity may be summarized as follows:—1, The upper set of beds rests on a clear-erosion surface; 2, The upper set of beds contains numerous and large fragments of the lower set; 3, The latter are folded while the former are not; and 4, Faults affecting the lower set stop short on the line of junction of the two sets. This is convincing, and we do not know what more can be demanded except palaeontological evidence, which unfortunately we cannot supply as we saw no fossils in any of the exposures we examined. On the opposite side of the river the evidence is equally clear. Just below the bridge on the bank of the river are marls with harder-bands of limestone interstratified, dipping S.E. at angles of from 5° to 10°, and these are overlaid unconformably by sands, with calcareous, concretionary layers and nodules, the kernels of the nodules being at times formed of Amuri Limestone and occasionally of a greyish marl. There is a cemented band of calcareous sandstone on the contact, which looks in places like a fault, but if so it does not affect the upper set of beds, and the evidence is on the whole against faulting being the cause of the occurrence. A little further downstream near the plantation at the mouth of the river the relationship of the beds is just the same. Sands

with concretionary layers are laid on the top of flaky limestone with an erosion surface; large boulders of Amuri Limestone, up to 10 ft. in length, and of calcareous greensand, occur in sandstone, forming quite commonly kernels to the concretions, and angular pebbles form the layer immediately on the contact. In some places the lower set of beds is disturbed by faulting or folding, the faulting being apparently on a line with that on the south side of the river. Very large floaters of limestone, so large that they might be taken after a casual inspection to be actually in position, occur in sandstone on the bank of the river at the upper end of the plantation. The evidence is thus strengthened by an examination of every exposure of these beds in the locality. The only points of importance to be considered are (1), what is the age of the upper set of beds; (2), what is the highest bed of the lower series involved in the break; and (3), does the break occur elsewhere? In answer to the first question we can only use the test of lithological resemblance and of stratigraphical position. The general character of the beds is the same as that shown by the lower portions of the Mount Brown beds in their type locality. As these are 40 miles apart a correlation based on that alone is perhaps dangerous, but at intervening places, often continuous for miles, there are beds of similar lithological character which have always been correlated by authorities with the Mount Brown beds. Of course similarity in lithological character continuous over a considerable distance does not of itself imply contemporaneity in age, but it establishes a prima facie case for consideration. As far as the stratigraphical evidence is concerned we can only point out that the beds at the mouth of the Hurunui underlie the Miocene Marls of the Hurunui-Cheviot basin which are correlated with the Greta or Motunau beds of the Weka Pass area. Concerning these Thomson has remarked (1920, p. 364): “No pebbles of the underlying Notocene beds have been observed, nor has any clear unconformity with the Mount Brown beds been detected, but a faunal break is such that one may well be suspected, and it is more than probable that outside the area an overlap of these beds on the pre-Notocene will be discovered.” No contact between these two seta of beds was observed in the Hurunui which could help to solve this problem. At the eastern base of Pendle Hill, some six miles W.S.W. of the Hurunui mouth, massive shelly conglomerates of undoubted Mount Brown age are seen in the left bank of the Blyth river to overlie the Grey Marls, though the actual contact is not seen here. These conglomerates pass up into soft brown sands precisely similar to those exposed in the south bank of the Hurunui below the bridge, and these sands in turn are overlain by sandy marls of the Motunau series, exposed in the bed of the stream five miles from the sea. From this point the whole of the lower Blyth valley is occupied by thick beds of stratified gravel dipping south-easterly at low angles and unconformable to the Motunau beds. This exposure must remove any doubts as to the Mount Brown age of the sands at the Hurunui mouth, and it probably enables the gravels to be correlated with those of the Kowai series. Hutton (1877, p. 55) noted the presence of the Pareora formation in the

“north head of the Blyth,” and there can be no doubt that these are the beds referred to by him. In all the exposures at the mouth of the Hurunui the highest bed involved in the erosion-surface is the marly facies of the Amuri Limestone, but boulders of a calcareous slightly glauconitie sand are found above the contact, which proves that the break occurred either after the deposition of the greensand facies of the Amuri Limestone which overlies the ordinary facies, or after the deposition of the “Grey Marl.” It is somewhat difficult to distinguish between fragments derived from these two beds seeing that the “Grey Marl” is frequently slightly glauconitic at its junction with the limestone, and it is possible that the greensandy facies of the limestone in one place may be correlated with the grey marl in another. The last question can be answered by an inspection of other localities where similar beds occur. For this reason a number of places were visited and an account will be given of them in turn. Port Robinson and Gore Bay. This locality is only a short distance from the Hurunui mouth, and similar phenomena were expected to be present. This proved to be the case. The beds at Gore Bay have been examined by Haast (1871, pp. 41-44), Speight and Wild (1918, pp. 79-80), and Henderson (1918, pp. 171-4), but no mention is made of the break about to be described. This was discovered when a low tide enabled the point dividing Gore Bay from Port Robinson to be fully examined. The sea cliff rises here from a shore-platform cut in Amuri Limestone and its associated beds, to a former shore-platform at a height of from 230 to 250 ft. Although the sequence cannot be closely examined on the cliff face, a gully a few chains to the north enables the whole to be closely inspected, and the following is the result of an examination of both places, the sequence being in ascending order. 1. Amuri Limestone of the usual facies. 2. Calcareous Greensand, passing up into layers of less calcareous material, more glauconitic material with phosphatic nodules at the base. 3. Greyish sand, containing small pebbles of Amuri Limestone and of the underlying calcareous greensand, with phosphatic nodules at its lower surface. Its thickness is irregular but with a maximum of 2 ft. It rests on an erosion surface of No. 2. 4. Sands, alternating with argillaceous and calcareous beds, so that some layers are pure sand, others sands cemented with calcareous material, others sandy marl. They are more definitely sandy and cemented towards the top while towards the base they are more argillaceous, and with thin bands of cemented material. Their thickness is 150 ft. and they strike N.E.-S.W. and dip N.W. at angles from 15° to 20°. 5. Gravels or Conglomerate, forming the Gore Bay syncline, composed of rounded pebbles of greywacke, highly oxidized and thoroughly cemented, with a thickness of at least 400 ft. The age of these beds is uncertain, but they have been

assigned to the Pliocene by Haast, while Henderson is doubtful as to their true age. 6. Sandy Clay, with interstratified gravels and sands, the gravels thicker and more important at the base (the beds out of which the so-called ‘Cathedral’ has been eroded), resting unconformably on Nos. 5 and 4. All the beds above the break appear to be more calcareous when traced across the shore-platform towards the north-east, and No. 3 bed contains “fucoids,” so that it may be correlated with the so-called “Grey Marl.” Faulting is quite a common phenomenon in the locality. The exposures on the north-west wing of the syncline on the south side of the Jed are not so satisfactory, but fairly high up on the face of the scarp above the road about half a mile from the beach, there is an occurrence of beds with calcareous concretionary layers containing in places broken shells and small pebbles of Amuri Limestone, so there must be some break between the two sets of beds. No clear contact was seen owing to the surface being masked by slip material, and any conclusion based upon such contacts as are visible under these circumstances must be unreliable. However, such evidence as there is supports the conclusions arrived at from the other wing of the syncline, and thus tends to confirm the conclusions arrived at from a study of the mouth of the Hurunui that there is a definite break between the beds overlying the Amuri Limestone and beds which can be assigned tentatively to the Mount Brown series. Amuri Bluff. This gives no section where a similar contact might be expected. Conway River. In Haast's account of the beds at the mouth of the Conway reference is made to the occurrence of the “Grey Marl” at the mouth of Limestone Creek. This we were unable to confirm although the place was carefully examined. In our opinion the exposure of sand at the mouth of the creek belongs to the beds exposed on the coast near Hawkeswood further south, and is considerably higher in the Tertiary sequence, and certainly unconformable to the Amuri Limestone on which it apparently rests. As far as other reaches of the Conway are concerned we can only say that the area is at present under examination by a student, and we do not wish to comment on it further than to say that there are indications of an unconformity between the Amuri Limestone and overlying marly beds in the vicinity of the Ferniehurst Homestead, as recorded already by Haast (1871, pp. 39-40), and Hutton (1878, pp. 39-48). Motunau. The contact between the Amuri Limestone and the overlying greensand with the equivalent of the Mount Brown series in the Vulcan Gorge of the Motunau wag carefully examined, but no evidence of a break was forthcoming. The conformity of the beds

seems complete, as is recorded in the paper by Marshall, Speight, and Cotton, an opinion which is, however, contrary to that expressed by McKay (1881, pp. 110-3) who places two breaks in the sequence exposed in the Motunau Creek, where we can find none. The only feature which might suggest a break is the fact that some of the faults which affect the Amuri Limestone do not affect the overlying Mount Brown beds,’ but this evidence is of course very slight and not convincing. Boundary Creek. McKay (1881, p. 111) gives a section through the gorge of Boundary Creek, showing an unconformity between Ma Mount Brown beds and his Pareora series. This conclusion must have been based on the difference of dip of the beds in the gorge and near the old ford across the creek. Probably McKay did not trace the series sufficiently far downstream to note that the dip flattens regularly till, in the vicinity of the ford, the beds are bent up into a syncline to be followed immediately by a gentle anticline, below which, they preserve a low south-easterly dip to the sea. There is no apparent unconformity of dip within the Mount Brown, series or the overlying Motunau beds. The exposure of limestone at the top of the gorge is obscured by slips (Speight and Wild, 1918, p. 82) but these do not affect the clear contact shown between a thick layer of marly greensand and the immediately overlying Mount Brown beds. This marly greensand is similar to that exposed in the Motunau River (loc. cit. p. 82) and is the local equivalent of the “Grey Marl” overlying the Amuri limestone, which, however, is not seen in position here. Large angular blocks with abundant fucoids lie in the bed of the stream below the slip on the south side, and the exposure in the cliffs a little to the north indicates that the limestone has thinned out very much in this locality. Near the bed of the stream the soft brown sands (Mt. Brown beds) are separated very abruptly from the underlying greensand by a thin well stratified layer of highly oxidized ferruginous sand forming a layer varying from 4 inches to a fraction of an inch in thickness. This layer is not continuous to the top of the cliff, the brown sands being penetrated by a wedge-shaped mass of, the marly greensand. Nodular fragments of ferruginous material occur sparingly in the upper few inches of the marly greensand- The Mount Brown beds strike N.E.-S.W. (mag.) and dip S.E. at 20°. Motunau River, South Branch. (Sections 3, 4 and Fig. 4.) The South Branch of the Motunau River near the Mount Vulcan Homestead affords further evidence. A section is given by McKay (1881, p. 117), and while we are in general agreement with his interpretation of the locality our opinion is divergent from his in several particulars. The Homestead of the station is placed on a hill composed of Upper Cretaceous sands dipping south-east, but a little to the north-west of the house the dip changes gradually to northwest, i.e., the house is placed near the axis of an anticline.

The dip is gentle at first, but it becomes more and- more steep till at the point where a bed of limestone crosses the creek to the north of the house the beds are overturned as shown by McKay. But he gives no details of the section although it appears; to us a singularly important one. Our description is as follows:— The limestone bed as exposed here has a strike in a direction approximately N.B.-S.W. and it apparently dips to the S.E. at an angle of from 60° to 65°. Parts of it show the Anmri facies and again, other parts approach the Weka Pass Stone in appearance and are decidedly glauconitic. The limestone has apparently been broken Section 3. Near Mt. Vulcan Homestead. 1. Cretaceous Sands. 2. Limestones. 3. Calcareous Green sands. 4. Marls. Section 4. Generalised Section from Black Hills to East of Mt. Vulcan Homestead, across Upper Valley of South Branch of Motunau River North-east of Main Road. Distance—About 2 miles. 1. Cretaceous Sands. 2. Limestones and Calcareous Greensands. 3. Marls. 4. Sands with Concretionary Layers. Mt. Brown Beds. G. Greywacke. Sections in Upper Valley of the Motunau River, South Branch. across by faults running at right angles to the strike so that its northwesterly face forms a series of steps, and slickensided surfaces are common. There has been a considerable amount of local disturbance of the beds as the limestone on the crest of the hills immediately to the north-east and to the south-west has the normal dip, i.e., it forms the north-western wing of a great anticline striking N.E. and S.W. In contact with the limestone and apparently dipping under it conformably on the north-western side is a bed of calcareous greensand with the usual phosphatic-nodule layer on the contact sparingly developed but still distinct. This greensand bed becomes less and less glaucontic and more and more marly as the distance from the limestone increases and passes up without any apparent break into the marl. Both facies contain fucoids, and the marl contains concretionary layers. In a small gully about a chain away from the limestone is a fault running north-east and on its north-

west side the beds dip more steeply. They consist here of loose-brownish sands, and greyish marly beds, i.e., typical “Grey Marl,” occasionally concretionary, diping S.E. at high angles and continuing for half a mile towards the old Montserrat homestead. The marls and the sands are interstratified throughout the distance, mentioned. The cross-faults affecting the limestone present some difficulties. As far as one can see they do not affect the marl, and the upper cross-fault exposed on the face of the spur may after all be an erosion surface. If so, then erosion has taken place after the green-sand facies has been laid down and the conformity is only apparent. If the phenomenon is due to faulting then, since this does not affect the marl and does affect both the limestone and the calcareous greensand, there is some slight evidence for an unconformity between, the calcareous greensand and the marl, although the former apparently passes up gradually into the latter. So much movement, however, has taken place that conclusions based solely on observations at this locality are dangerous, and an accurate inference can be arrived at only in the light of observations elsewhere. Our tentative opinion of the history of the beds at this spot is therefore as follows. The limestones were laid down conformably on the sands forming the heart of the anticline. They were succeeded apparently conformably after some interval by calcareous-greensands. These beds were faulted, perhaps eroded (though this is very doubtful) and marls were deposited on the resulting surface. Then a deformational movement resulted in the beds being folded into an anticline and in the immediate proximity of this spot they were overturned. How far away from the limestone the overturning movement extends we cannot say definitely owing to the absence of exposures. A few hundred yards to the south-west along the strike in the upper basin of the south branch of the Motunau River there is a syncline, and the same is probably true towards the north-east, so that the section where overturning has taken place can only be of short length. There is one small point to note in conection with the marls, viz., at one place underlying a concretionary layer there is a bed of finely-broken shells containing angular fragments of marl, which suggests a minor break within the limits of the marls. This is entirely analogous to the break seen elsewhere under the Mount Brown beds, but it is not so pronounced. The clearest prof of unconformity between the marls and the Mount Brown beds occurs) in the south branch of the Motunau just above Mr. Webley's house and between it and the Montserrat Homestead. As mentioned above the general structure of the upper basin of this branch of the stream is synclinal, the axis of deformation running north-east and south-west approximately. The two peaks Oldham and Montserrat of the Mount Cass Range both formed of limestone, are the two wings of the syncline, and between them the crest of the range is formed of marls and Mount Brown beds. Along the axis of the syncline north-east from the line of the road the beds are involved in minor folds—antieline and synclines—with, occasional faulting. Just opposite the old Mountserrat Homestead there is a well-marked reversed fault with a throw of some 50 feet or more. The general trend of these structural features is parallel

to the direction of the valley, and where the stream cuts across the beds good exposures are to be seen, and especially is this the case between Mr. Webley's house and the old station homestead, and also in the reach of the stream above Mr. Webley's house. The stream has here eroded a deep channel for some distance along the axis of an anticline, so that the contacts between the marl and Mount Brown beds are exposed over a length of about a quarter of a mile. Wherever the contacts between the two sets of beds can be seen their character is the same. In places bands of rock occur with broken shell-fragments, but in all cases there are pebbles of marl, occasionally forming nuclei to the concretionary layers of the upper set of beds, lying as a rule on a definite erosion-surface. The following description of a contact just above the Montserrat Homestead may be taken as typical:— 1. Merl—a sandy calcareous clay, typically grey in colour. 2. Sandy facies of No. 1. 3. Bed consisting of pebbles and boulders of marl, up to 2ft. in length, well consolidated, composed almost entirely of boulders, the bed from 2 ft. to 10 ft. in thickness. 4. Shelly fragments, with occasional fragments of marl, the layer from 2 ft. to 4 ft. in thickness. 5. Alternating sands and concretionary layers. To the north-west of this locality the marls are found to rest on the Cretaceous beds without the intervention of the limestones or greensands, as is the case in the basin of the North Branch of the Motunau just over the ridge from Tipapa. This suggests an unconformity between the marls and the Cretaceous series, but does not affect the contention as regards the conformity or otherwise of the Marls and the Mount Brown beds. The latter form a well-defined escarpment south of the Omihi-Montserrat road to the west of the area just described. This was not closely examined, but the sections across it in the vicinity of Mr. Croft's house near Omibi were investigated in some detail. In three of the gullies of streams running down through his property there are contacts between the two sets of beds substantially the same as those further east, and the sections here are all the more instructive since there is a occurrence of a complete sequence of beds from sands with black oysters and saurian bones, through limestones, calcareous greensands, and marls, with the usual break dividing them from the overlying Mount Brown beds and the closing beds of Greta marl. The correlation of the Mount Brown beds with beds of similar lithological character and in a similar stratigraphical position can thus be made. At one of the contacts in a creek west of the homestead there are numerous sharks' teeth (Isurus desori) associated with the fragmentary layer. In Smothering Gully, further west still, no proper contact was found where it was expected, but the beds are involved in faulting and away from the zone of disturbance the relationships are obscured by the covering of soil and slip material. Weka Pass. In view of our discoveries at Hurunui a careful examination was made of the contact between the Mount Brown beds and the

“Grey Marl” in the classic and much discussed locality in the Weka Pass, and a definite break was discovered at the spot where it was anticipated it might occur. This is situated at the cliff-face above the Weka Pass Stream, just opposite Mr. Archer's house, near the 43¾-mile post on the railway, and is referred to by Thomson (1920, p. 361, and Plate xxi., Fig. 2). He says concerning it, “The conglomerate rests upon soft grey sandstone, of which the few feet exposed show no bedding, so that the presence of an unconformity cannot be definitely asserted, but the presence of derived fragments of sandstone in the overlying limestone makes it probable.” Hutton had placed the break between the “Grey Marl” and the Mount Brown beds at a higher stratigraphical level than we do, at a point about two chains nearer Waipara, but we are of the opinion that the phenomena displayed there are more satisfactorily explained by faulting. Our description of the occurrence on the cliff-face coincides in general with that of Thomson, except that we do not see the necessity to postulate the presence of a fault to account for the irregularity. Our reasons against the fault are as follows:—(1), There is no evidence of dislocation in the railway-cutting in close proximity nor on the face of the cliff upstream, i.e., the beds are continuous above; (2), the underlying marl is continuous and shows no break; (3), there is no appearance of the sandy facies of the marl at the end of the cliff, which should show if the downthrow were to the north; (4), if faulting is used to explain the contact on the northern side then the downthrow must be to the south since sandstone layers of the Mount Brown beds butt against “Grey Marl.” The section exposed in the railway cutting is as follows:— 1. Limestone, sandy, and composed largely of shelly and bryozoan fragments. 2. Sandy layers with angular fragments of “Grey Marl,” up to 8 inches in diameter, getting finer and finer, through from 10 to 12 feet of thickness, one layer, 18′ thick, being composed almost entirely of fragments. These fragments are visible on both sides of the cutting. 3. Sandy beds, with irregular concretionary bands. These horizons can be traced round the face of the spur and the cliff towards the stream in their proper positions. On the cliff face the “Grey Marl” and its upper sandy facies appear and resting on them are Mount Brown beds with inter stratified sandy concretionary bands, the contact being apparently an erosion-surface. At the junction there is a conglomerate composed of rounded and angular fragments of the lower beds, rounded pebbles stained with oxide of iron, and numerous shell-fragments—molluscs, barnacles, bryozoans—as described by Thomson, the whole thoroughly cemented into a hard mass by means of calcium carbonate. The angular fragments are numerous and vary in size up to 4 ft. Part of the face is masked by sinter, and on both sides of the contact are concretionary layers of sand, parallel to the junction, and cemented as similar layers are at the Hurunui.

It seems to us that this occurrence is due first of all to the erosion of the “Grey Marl” and its upper sandy facies, and that a deposit of the upper beds has been laid down on. a shore-line-where there has been considerable wave-action, for the fossils are rounded and abraded. The character of the contact is practically the same for a couple of chains, and apart from the absence of faulting and folding of the lower beds, is exactly analogous.-to that at the mouth of the Hurunui. There is also a suggestion of unconformity from the fact that the sandy facies of the marl shows a much less thickness at this spot than further to the northeast, pointing to the existence of an irregular surface which maybe due to erosion of the soft incoherent layer. The evidence is therefore fairly strong that in spite of the agreement of dip and strike of the Mount Brown beds and the “Grey Marls” there is a physical break, but Thomson has come-to the conclusion on palaeontological grounds that there is no great faunal break between the two sets of beds. Middle Waipara. Gorge. (Figs. 5, 6.) This also is an important locality in view of the attention it has attracted. The places likely to give clear contacts being in the river-bed itself and on its banks, these were examined closely, but the only place where the evidence suggests a break occurs at the junction of Boby's Creek with the main stream and just above-it. About 150 yards above the junction in the bed of the main stream is a contact analogous to those seen elsewhere. Resting-on sandy grey marl is a bed of sandy limestone containing numerous remains of finely-comminuted shells with angular fragments of the underlying marl, up to 8 inches in length. This can be traced downstream for over a chain when the river covers the contact (Fig. 5). The beds here strike N.E.-S.W. (mag.) and when-followed downstream further there is no appearance of this bed in the bank where it should occur in proper alignment, but about a chain to the south-east there is a clear erosion-surface with masses of marl of large size, i.e., yards in diameter, and also irregular fragments of smaller dimensions grading down into pebbles (Fig. 6). The sequence here is as follows:— 1. Grey sandy marl, with occasional concretionary layers, usually in detached masses. 2. Fragmentary layer, composed of round and sub-angular fragments and masses of large size from bed No. 1, the lower portion definitely current bedded and composed of smaller rounded fragments. 3. Calcareous sandy beds with many harder concretionary layers passing up into the Mount Brown sands and Limestone, the Lower Limestone of Thomson (1920, pp. 357 seq.) These beds contain in their lower portions sandy calcareous layers composed of shells with fragments of marl scattered all through, the layer with shells being-exaetly analogous to that seen upstream.

The face of this exposure fronting upstream to the Waipara is over 50 ft. high, and we think it must be the same as that referred to by Thomson (loc. cit., pp. 358-9). There has been a considerable change in the river banks owing no doubt to the recent severe floods, and for this reason Thomson was perhaps unable to determine the, extent of the break. We considered the possibility of this break being due to some local and, geologically speaking, fairly recent erosion-phenomenon, and decided against it; but we are of the opinion that this does apply to a great detached mass of Mount Brown limestone which rests against a marly surface also belonging to the Mount Brown series close to this spot in Boby's Creek. It is remarkable, however, that this mass contains numerous fragments of marl as inclusions, which also emphasizes the break between the marl and the Mount Brown beds. There is thus decided evidence of a break without angular unconformity in the series as exposed in the Waipara River, and in such a position as may fit in with the break below the Mount Brown beds elsewhere. This section, however, emphasizes the difficulty of deciding what beds must be included in the Mount Brown Series and what in the “Grey Marls,” seeing that there is a close similarity in the lithological character of the beds forming the base of the former and the top of the latter. This is after all not so remarkable since the lower set of beds, especially if they are of soft incoherent character, will naturally furnish a considerable quantity of the material for the upper set and will therefore resemble them more or less closely in lithological features. Thomson notes the similarity of the fauna of the two sets of beds and therefore infers conformity, but without questioning the accuracy of his observations or conclusions we should like to be in a position to know whether the fossils from the “Grey Marl” on which he bases his conclusion, were from above or below the break as it occurs in the Waipara. Lower Waipara Gorge. This locality was naturally expected to furnish a contact between the “Grey Marl” and the Mount Brown beds, but an examination of the cliffs below the pronounced loop in the gorge gave no satisfactory evidence. Over beds which were undoubtedly the “Grey Marl,” judging by its stratigraphical position, lithological character, and continuity with the “Grey Marl” beds exposed over the limestone on the Dovedale road, lay sandy beds, with well-defined richly fossiliferous concretionary layers, inter-stratified with which were occasionally sandy marly layers. Owing to the covering of surface-slip the nature of the contact could not be seen. On the upstream side of this loop marls are exposed and apparently involved in some dislocation or discordance, but it is practically certain that these marls represent beds interstratified with the Mount Brown beds, and any conclusion based on their features would be unreliable. However, in a creek coming down from Mount Cass, parallel with the Dovedale road, and entering the Waipara River through a narrow gorge cut across the strike of the beds, there is a very suggestive section.

On the north-western side of this stream along the Dovedale road is the typical development of Amuri Limestone passing up in many places without any definite line of demarcation into the Weka Pass Stone, and over this lies the “Grey Marl” in which the creek has been eroded on the surface of the limestone. On the south-east side of the creek the marl forms cliffs capped by the sandy facies of the marl, and further south-east still occurs the parallel escarpment formed out of Mount Brown beds. Thus the beds show their normal stratigraphical relationship. The creek flows south-west in the marl, practically parallel with the strike, but on approaching the main stream it cuts across the strike, and as the beds are tilted at angles of about 60° a good, opportunity is given to study their relations. On the surface formed out of the sandy facies of the marl lie concretionary gravel beds, showing an abrupt change, with markedly irregular surface, as if eroded on the sandy marls, the contact containing fragments up to 4 ft. in length of the underlying beds. The gravel beds are full of fossils. Of course one must be very careful in postulating unconformity when a gravel bed lies on incoherent material with an irregular under-surface, but a similar contact can be observed in various places along the line of this particular bed, and there is additional evidence of the break from the fact that between this bed and the marl further downstream near the loop of the river referred to above there is a thickness of sands with concretionary fossiliferous layers, amounting to some Hundreds of feet, whereas in the locality mentioned above the gravel bed lies directly on the sandy facies of the marl. This indicates overlap, and the inferential unconformity of the upper set of conformable beds with the lower set. Waikari River. The stratigraphical relationship of the beds exposed in the lower part of the valley of the Waikari Creek was investigated at several points. About a mile above the junction of the stream with the Hurunui River there is the following section:— 1. Greywacke, with granitic conglomerate, striking N.E.-S.W. and dipping at high angles, almost vertical. Lying unconforably over these are:— 2. Sands, yellow to brown, with calcareous concretionary bands. 3. Greensands, very glauconitic. 4. Sands, light-coloured to yellowish. 5. Marls, greyish in colour with harder more calcareous bands, occasionally flaky, more glauconitic in the top layers. 6. Band of oxidized fragments, very well defined and covered by an irregular concretionary layer. 7. Glauconitic marl with occasional fragments of oxidized material 12in. to 18in. thick. 8. Sands, with concretionary layers, i.e., the Mount Brown horizon.

The junction is not so marked as elsewhere and the general texture of the beds suggests somewhat different conditions of deposit, all the beds involved being finer in grain and more marly. However, about half a mile up one of the two creeks opposite the Greta Railway Station, the break is clearly seen. Large rolled fragments of flaky grey marl and of sands from the underlying beds occur at the junction of the Mount Brown beds with the sandy facies of the underlying marl. This is analogous to the phenomena seen elsewhere at this horizon. There is another case of unconformity at a higher level to bo seen in this locality. In the first railway-cutting past the homestead, Greta Marls with fossils (Turritella, etc.) are overlain unconformably with gravel-beds containing abundant shell-remains and fragments of Greta Marl near the zone of contact. At the junction of the creek which runs past the homestead and the Waikari Creek the contact is marked by large boulders and angular blocks of the subjacent beds. A suggested tentative correlation of the gravelly bed is with the Kowai Gravels. Near the Ethelton Railway Bridge. a. Hurunui River. On the left bank of the Hurunui River just above the junction, of the Kaiwarra Stream the following sequence is exposed. 1. Greywacke—at mouth of Kaiwarra Stream. 2. Sands—grey and yellow sands with concretions, greensands. 3. Marls—grey, brecciated and distorted. 4. Marly greensands. 5. Brown and grey sands with hard calcareous bands and concretions (Mt. Brown beds)—these appear 150 yards upstream from the mouth of the Kaiwarra. The upper part of the grey marls is involved in a fault, on which is a small exposure of brecciated limestone—very greensandy. The fault does not disturb the small exposure of marly greensand and the immediately overlying Mount Brown beds, the junction between these being quite clear near the bed of the stream. The lower hard calcareous band of the Mount Brown series shows a flat undersurface pitted with holes from which ferruginous inclusions have weathered. The contact is very similar to that exposed in the lower portion of the Waikari Creek—i.e., the change from the marly greensand to the overlying brown sands is very abrupt and the contact marked by a thin zone of ferruginous matter weathered to a deep brown. Here one large irregularly lenticular inclusion of marly sand occurs 4 ft. above the junction. The sands of the Mount Brown series strike 10° E. of N. (mag.) and dip westerly at 70°, but the dip of the underlying marly greensands, though apparently conformable, cannot be determined with certainty.

Fig. 1.—-View of the Hurunui River from near Lower Bridge looking west, and showing Tertiary Beds on left. The beds on the point are Amuri Limestone, Mount Brown Beds are in the middle of the section, and greensands and greywacke under the bush in the foreground. Fig. 2.—View of the mouth of the Hurunui River, from near bridge, looking east. The limestone and associated marl show up white near river level on both sides of the stream, and the Mount Brown Beds overlying them unconformably are darker in colour. The high cliff is capped with gravel.

Fig. 3.—Mount Brown Beds with included mass of Amuri Limestone on right, view taken in gully on south side of mouth of Hurunui River. Fig. 4.—-Limestones and marls near Mount Vulcan Hd. The limestone is overturned; the marls with concretionary layers show in the gully on the left of the picture. Approximate contact of limestone and marl shown by the line.

Fig. 5—Contact between Mount Brown Beds and Marls in Middle Waipara Gorge above junction with Boby's Creek. The bed with angular fragments of marl is marked X. This does not appear across the river. The grey material in the middle background is marl capped with recent terrace gravels.

Fig 6—Mount Blown Beds lying unconformably on marl just above junction with Boby's Creek. The large angular masses in the middle of the bluff are of grey marl, which is in position on the right of the picture. The bluff is capped with Mount Brown Beds with characteristic concretionary layers, which are also seen on the extreme left.

On the right (south) bank of the river there is no exposure of the beds lying between the greywacke and the Mount Brown beds which here strike a little to the west of north and dip westerly at 70°-75°. They form high bluffs and pass upstream into the grey sands and sandy marls (Greta beds) occupying the main portion of the synclinal basin. b. Kaiwarra Stream. The same series of beds can be traced upstream from its junction with the Hurunui, viz:—(1) Greywacke. (2) Sands. (3) Marls—lower beds brecciated and disturbed, the upper becoming-very greensandy. (4) Mount Brown beds. Some 300 yards upstream from the mouth, the junction between the Mount Brown beds and marly greensands is marked by a thin layer of very ferruginous nodules, the lower calcareous band in the Mount Brown beds being here jointed into quadrangular blocks. Where the first bend of the stream is eroded practically on the strike of the beds, the contact between the Mount Brown sands and the underlying marly greensands is highly irregular. If this area had not been so much disturbed it might have been stated quite definitely that this indicates deposition of the Mount Brown beds on an erosion surface. Summary and Conclusions. It will be seen from the above descriptions that over a wide area there is evidence of a physical break between the Marl and the Mount Brown beds; only in the Motunau Gorge of all the localities cited is there no evidence of such. Perhaps the evidence is not so strong in the later cases as it is in the case first described, but it is still sufficient. The only proof wanting is that dependent on the fossil-content of the two sets of beds. Unfortunately the “Grey Marl” is somewhat poor in fossils; in many cases none have been recorded, and in the most-studied locality, viz., the Weka Pass and Waipara, there is no evidence of a faunal break; in fact, on the fossil evidence Thomson (1920, pp. 356-363 and 386-396) concluded that the two sets of beds should be grouped together. Hutton (1888, pp. 257-9) considered that they should be separated. It is possible, however, that more complete collecting in localities not yet exploited may support the evidence obtained from physical considerations. A specially favourable locality for this work would be the Lower Waipara Gorge and the tributary coming in from Mount Cass. There is one aspect of the case which should be considered, viz., the possibility that the contacts just described may after all be normal when a marl, sandy marl, or soft sandstone changes upward to a bed of similar lithological character. We admit that the change of such a bed to a conglomerate or gravel-layer may be attended by interformational erosion, since the change in lithological character implies the onset of conditions where strong currents are operating, depositing coarse material and at the same time causing erosion of

the underlying incoherent beds, but this hardly operates when, the change is from one soft bed to another of similar non-resistant material. Also, the reason why such breaks have not been definitely recognized, maybe that the material of the upper set of beds must in many such cases be derived directly from the lower, and hence must resemble it in composition and frequently in texture. However, judging from the form of the fragments in the cases under consideration it is clear that the consolidation of the lower set of beds had taken place before they were subjected to erosion, and this implies a considerable time interval. There is also the general absence of angular unconformity between the two sets of beds to be accounted for. In some cases, as at the Hurunui mouth and in the South Branch of the Motunau, this does occur, but in general there is no obvious difference in angle between the two sets of beds. This has been commented on by most writers, although Hutton and McKay have insisted on differences which we cannot confirm. This absence of angular unconformity implies that erosion took place as a rule while the beds were tying flat, and that the upper set was also deposited flat and that tilting took place subsequently. It is unreasonable to assume that had tilting taken place in the interval between the depositions the conformity in angle could have been so general. In cases where there is an angular discordance, as at the Hurunui mouth, it is clear that the break took place near an old sea-cliff, cut in the limestones and marls, it being impossible to account for the very large blocks with gravel-beds and rolled shell-fragments in any other way, and it emphasizes the point that the mere presence of large fragments in older beds does not of itself imply glacial conditions unless other evidence of ice-action is there to confirm the supposition. We refer to the supposed glacial conglomerate in the Hokonui Hills as an illustrative example. Such deposits, composed of large blocks of limestone and marl, may be seen now forming at the base of limestone and marl cliffs on various stretches of the present coastline. Finally, reference has been made to the possibility of a break below the “Grey Marl,” as indicated in the sections at Port Robinson, and perhaps near the Mount Vulcan Homestead and in the Conway near Ferniehurst, but the question has little relationship to the break at the top of the marl, and can be left to future consideration. In advancing the present hypothesis we understand that it will provoke discussion, especially as one of us has maintained the conformity of our Cretaceous and Tertiary sequence. It is possible that some other explanation of the phenomena we refer to may be advanced, and may indeed be substantiated against us, but we feel that such discussion will result in adding to the general knowledge of the stratigraphical geology of a critical area, and that alone justifies us in submitting the question of the conformity or unconformity of our Tertiary sequence for reconsideration.

References. Haast, J., 1871. On the Geology of the Amuri District, in the Province of Nelson and Marlborough, Rep. Geol. Explor. during 1870-1, pp. 25-46. Henderson, J., 1918. Notes on the Geology of the Cheviot District, N.Z. Jour. Sci. Tech. vol. 1., pp. 171-4. Hutton, F. W., 1877. Report of the Geology of the North-east Portion of the South Island, Rep. Geol. Explor. during 1873-4, pp. 27-58. " 1885. On the Geological Position of the Weka Pass Stone, Q.J.G.S., vol. Xli., pp. 266-78. 1888. On Some Railway Cuttings in the Weka Pass. Trans. N.Z. Inst., vol. 20, pp. 257-63. McKay, A., 1881. On the Motunau District, Ashley County, Rep. Geol. Explor. during 1879-80, pp. 108-16. Marshall, P., Speight, R., and Cotton, C. A., 1911. The Younger Rock Series of New Zealand, Trans. N.Z. Inst., vol. 43, pp. 378-407. Morgan, P. G., 1916. Notes of a Visit to Marlborough and North Canterbury with especial reference to Unconformities post-dating the Amuri Limestones, 10th Ann. Rep. (n.s.) N.Z. Geol. Sur., Parl. Paper C.—2B, pp. 17-29. " 1916 (a). Records of Unconformities from Late Cretaceous to Early Miocene in New Zealand, Trans. N.Z. Inst., vol. 48, pp. 1-18. Park, J., 1888. On the Geology of the Waipara and Weka Pass Districts, Rep. Geol. Explor. during 1887-88, pp. 25-35. " 1905. On the Marine Tertiaries of Otago and Canterbury, with special reference of the Relations existing between the Pareora and Oamaru Series. Trans. N.Z. Inst., vol. 37, pp. 489-551. 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. Speight, R., 1912. A Preliminary Account of the Lower Waipara Gorge, Trans. N.Z. Inst., vol. 44, pp. 221-33. Thomson, J. Allan, 1920. The Notocene Geology of the Middle Waipara and Weka Pass District, North Canterbury, New Zealand, Trans. N.Z. Inst., vol. 52, pp. 322-415.