The “Grey Marls” and Mount Brown Beds.
The beds following the Weka Pass stone have long been known as the “grey marls” and the Mount Brown beds, and the conformity or unconformity of these two sets of beds has been much canvassed, but there has been no close definition of what is to be included in these two series. “Grey marls” by common consent include any mudstone between the Weka Pass stone and the overlying limestones, which also by common consent are included in the Mount Brown beds; but between these two limits there is also a considerable thickness of sands and sandstones; and, moreover the upper limit—viz., the lowest limestone of the Mount Brown series—is not a persistent lithological horizon in the district. It will therefore be convenient to describe these two “series” together.
Five limestones must be distinguished in the Mount Brown series, and may be conveniently indicated by the letters A, B, C, D, and E. The lowest, A, forms a cuesta on the Ram Paddock, and also on the watershed between Boby's Creek and the Kowhai River, towards Mount Grey. It is a white polyzoan impure limestone containing in places an abundance of large cup-shaped Polyzoa, and is the “white and yellowish calcareous sandstone” of Hector (1869), and the “Bryozoa beds” of Haast (1871). The succeeding limestones, except the last, are mostly reddish-brown rubbly arenaceous limestones, the calcareous matter being largely comminuted shells of various marine organisms. Polyzoa, barnacles, or brachiopods in places constitute the greater part of the limestones, and there are also molluscan shell-beds. The second, B, forms the lower of the two limestone cuestas on the south-east side of the Weka Pass, and contains few fossils except small cup-shaped Polyzoa and barnacles. It may possibly be the same as the third, C, which forms the lower band on the cliffs overlooking the Waipara River below the limestone gorge, and is characterized by the presence of the brachiopod Magadina waiparensis Thomson. The fourth, D, is the main band throughout the district, occupying the
top of the ridge overlooking the Weka Pass, the greater part of the skyline between the Weka Creek and the Waipara River, and the summit of Mount Brown. It contains a rich brachiopod fauna, the commonest species being Magadina browni Thomson, Pachymagas parki (Hutton), and species of Rhizothyris. The uppermost limestone, E, forms the cuesta southeast of the main band, D, at the approach to the Weka Pass, and is characterized by the brachiopods Neothyris novara (von Ihering) and Stethothyris sufflata (Tate).
Middle Waipara, South of Boby's Creek-Fault.—Starting at the western end of the district, in the tributary of Boby's Creek rising near Mount Grey, the Weka Pass stone passes up gradually into grey mudstones, the typical “grey marls,” which are here apparently 200 ft. to 300 ft. thick. They yielded Verconella costata, Malletia australis, Limopsis aurita, Pecten huttoni, and Diplodon zelandica (Gray)?. Above these the section is not clear, but there are sands containing Turritella and Malletia. The watershed between Boby's Creek and the Kowhai River is here occupied by a cuesta of the lowest Mount Brown limestone, A, which is a white polyzoan limestone, about 50 ft. thick. It yielded Pecten huttoni, fragments of a ribbed Pecten, and Pachymagas clarkei n. sp. The higher Mount Brown limestones were not here studied.
In Mount Brown two bands of reddish-brown limestone may be distinguished. The lower, B, is not richly fossiliferous, but has yielded Anomia trigonopsis and Pecten palmipes. Bed C has not been identified, but the upper band, D, forming the summit, is thicker than usual. Fossils are scarce near the summit on either side, but in the cliffs overlooking the Waipara River, where over 100 ft. of limestone is exposed, there is a very persistent band, formed mainly of Magadina browni, near the top. From some holes at the base of the cliff I obtained an abundance of Bouchardia minima Thomson, besides Magadina browni, Pachymagas McKayi n. sp., Anomia trigonopsis, Pecten williamsoni Zittel (?), P. zelandiae, and Lima colorata.
Middle Waipara, North and North-east of Boby's Creek Fault.—On the northern side of the Boby's Creek fault, in the north branch of Boby's Creek, the Weka Pass stone passes up gradually into grey mudstones, about 60 ft. thick. These are followed by a considerable thickness of soft sandstones, separated into upper and lower divisions by a thin bed of mudstone containing Mopsea sp. and Foraminifera. The upper sands are cut off by the fault.
The most complete section is that afforded by the banks of the Waipara River below the limestone gorge, and partially repeated in the lower part of Boby's Creek owing to folding. The section is continued in the higher slopes to the east up to the horizon of the main Mount Brown limestone, and includes the following beds:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Main Mount Brown limestone (D)||60|
|Loose yellow-brown sands||80|
|Third Mount Brown limestone (C)||30|
|Bluish muddy sandstones with concretions, passing down into polyzoan shelly beds and a grit at the base||200|
|Mudstones with thin sandstone intercalations||200|
|Whitish sandstones with thin mudstone intercalations||200|
The Weka Pass stone becomes glauconitic at its top, and passes quite gradually into glauconitic mudstones. The succeeding sandstones are in beds of 4 ft. to 10 ft., separated by mudstones 6 in. to 2 ft. thick. Some of the sandstones contain small rounded pebbles of foraminiferal calcareous sandstone, suggesting derivation by erosion from the Weka Pass stone. In the higher beds the sandstones are in thinner layers and the mud-stones thicker. Fossils are scarce throughout, and so tender as to be very difficult of collection. Epitonium zelebori was obtained low down in the sandstones. Two shelly beds were noted on the right bank of the river, below the grit, both containing Pecten huttoni, the higher being similar to the polyzoan beds above the grit. The latter rock contains small pebbles of greywacke. It is succeeded by alternations of thin polyzoan limestones and bluish sandstones, and the latter beds continue to the base of the third Mount Brown limestone (C), and contain poorly defined concretions with shells and plant-remains (Plate XX). Just above the polyzoan beds I obtained Paphia curta and a fine specimen of Pecten beethami var. B Hutt.
Part of the above sequence is repeated in the lower part of Boby's Creek, north-east of the fault, and in the banks of the Waipara River above and below the junction of Boby's Creek. The cuesta of the lowest Mount Brown limestone (A) on the Ram Paddock is composed of a whitish polyzoan calcareous sandstone, consisting chiefly of larger cup-shaped and smaller Polyzoa, and yielding fairly numerous but poor specimens of Pachymagas clarkei n. sp., with rare pectens and echinoids. The limestone thins out rapidly along its strike in both directions, and obviously formed a polyzoan reef or shoal in the Oamaruian sea. To the west-south-west it crosses the Natural Bridge Creek, greatly diminished in thickness, just above the natural bridge, but does not continue to the east-north-east as far as the banks of the Waipara River. It apparently thins out also in the direction of its dip (south-south-east), but is presumably represented by the polyzoan beds near the bottom of Boby's Creek and those above described in the Waipara River.
In the Natural Bridge Creek, and in Boby's Creek below it, there is some gentle folding, so that a continuous section is difficult to trace. The polyzoan beds appear to be the lowest horizon exposed, and are succeeded by bluish muddy sandstones yielding Anomia trigonopsis, Pecten beethami, Pecten huttoni, and Nucula sagittata Sut., the latter species being first described from this locality. These are succeeded by current-bedded sands, on which a cream-coloured sandstone rests unconformably.
At the time of my first visit, in 1912, a recent slip had exposed a very clear unconformity on the side of the bluff facing the Waipara River at the upper corner of the junction between Boby's Creek and the river (fig. 6). The rocks below and above the surface of the contact were of similar nature— viz., bluish muddy sandstone—but those below were not so clearly bedded. The upper beds contained pebbles and boulders of the same nature, and also of grey mudstones and of greywacke, as well as broken shells. This section had become obscure at the time of my visit in 1913. A short distance up the Waipara River, on the same bank, I observed some shell-beds, which must lie above the unconformity, containing casts of Cucullaea, ribbed Pectens, a large Dentalium, and many gasteropods.
The lower part of the “grey marls” is exposed on the back of the cuesta of Weka Pass stone between the limestone gorge of the Waipara River and the saddle north-west of the North Dean. Here 50 ft. of grey mud-stone follows the Weka Pass stone with every appearance of complete
Fig. 1.—Syncline in the main Mount Brown limestone (D), Waipara River, north-east side below Boby's Creek. Fig. 2.—Cliff in Weka Pass Stream, below railway-cutting, 43¾ miles from Christchurch. A fault with downthrow to the left intersects the cliff. 1, grey sandstone (top of “grey mails”); 2, hard calcareous conglomerate with shells 3. lower Mount Brown limestone (B).
conformity. Fossils are fairly plentiful, including corals and Foraminifera, but the molluscs are mostly in the condition of casts. They include Turritella carlottae Watson and Corbula canaliculata Hutt. The succeeding beds are not exposed, but higher up the slope loose sands are seen.
The Middle and North Dean are composed of a yellow calcareous sandstone with many comminuted shells in certain bands, and frequent inclusions of a yellow-brown sandstone, which also forms separate bands. This is probably the second Mount Brown limestone (B). The main band (D) does not here form the crest of the range, but appears in rounded hills about half a mile to the south-east. Between B and D there are sands and further yellowish-white calcareous sandstones containing “fucoids,” barnacles, Polyzoa, and echinoids.
The third Mount Brown limestone (C) is a yellow calcareous sandstone, about 30 ft. thick, containing in places an abundance of Magadina waiparensis. It may be traced from near the South Dean to the cliffs opposite the meander in the Waipara River below the gorge, but appears to pass into a sandstone before the river is reached.
The Main Mount Brown limestone (D) forms the crest of the cuesta on the cliffs near the Waipara River, but higher up the hill, towards the Deans, it falls back behind the crest. It is divided into two parts by a persistent band of sand, 5 ft. thick, which contains occasional specimens of Ostrea, Anomia, and barnacles. The lower part is harder and not so rubbly as the upper, and contains few fossils but barnacles. The base of the upper part consists of a persistent shell-bed, 2 ft. thick, containing Pectem beethami, P. burnetti, Lima colorata, Anomia trigonopsis, and casts of many other-species, including Turritella. The remainder is the usual red-brown rubbly impure limestone, containing an abundance of Magadina browni. The main band is bent into a syncline where it reaches the Waipara River (Plate XXI, fig. 1); the lower part consists of alternating sands and calcareous sandstone, containing Pecten huttoni, while the underlying sands contain Placunanomia sp. and Pachymagas not sufficiently well preserved for specific identification.
On the opposite side of the river the Main Mount Brown limestone (D) is exposed in a small syncline, truncated by the Boby's Creek fault. It is of the usual rubbly character, and yielded Pecten burnetti, Ostrea sp., Magadina browni, Rhizothyris rhizoida, and Pachymagas of the parki series. Cup-shaped Polyzoa are fairly abundant.
Weka Creek.—In the Weka Creek the contact of the Weka Pass stone and “grey marls” is well exposed. Speight and Wild (1918) have noted that the agreement in dip is absolute, and the contact does not show any signs of unconformity, but the Weka Pass stone exhibits on its upper surface a narrow bored zone similar to that on the upper surface of the Amuri limestone. “This is succeeded by 1 ft. of slightly glauconitic sandy marl, then by 12 ft. of slightly glauconitic sandstone, passing up into sandy marl and becoming more argillaceous higher up but still preserving something of its arenaceous nature.”
The thickness of the sandy mudstones is difficult to estimate, as the creek here runs obliquely to the strike, but is about 70 ft. Near the top Amusium zitteli is fairly common, and there are also casts of other bivalves and gasteropods, at least two species of coral, fish-scales, and Foraminifera. Such shells as exist are mostly too fragile to collect. The mudstone is succeeded by a sandstone, and then there are alternations of sandstone and mudstone up to the horizon of the second Mount Brown limestone (B).
Weka Pass.—In the middle part of the Weka Pass Stream from the road-bridge over the stream downwards, and in the railway-cuttings opposite, there are several isolated exposures of the “grey marls,” but no continuous section. The lowest beds, at the bridge, are typical sandy mudstones resting directly on the Weka Pass stone. The actual junction cannot be observed, but only about 3 ft. of beds is not exposed, and each rock appears to be approacheng the other in composition. From these sandy mudstones, which appear to be about 50 ft. thick, I collected casts of Verconella, Crassatelites, Loripes, Nucula, and Nuculana.
Lower down the stream there are two large cliffs of well-bedded soft grey sandstone, and similar beds are exposed in the railway-cuttings above. The thickness of these sandstones does not probably exceed 200 ft. They are again succeeded by a sandy mudstone of unknown thickness, exposed at the first bend of the stream above the cliff of the lower Mount Brown limestone described below. These are succeeded by loose sands, passing into a grey muddy sandstone, together about 50 ft. thick.
In the upper part of the Weka Pass Stream only the lower part of the “grey marls” is exposed, as a typical sandy mudstone near the viaduct. From this rock I collected Limopsis aurita Brocchi (?) and Foraminifera, and McKay's earlier collection included Ampullina miocaenica Suter. A selection of the Foraminifera supplied by Mr. F. Chapman was as follows: Clavulina communis d'Orb., Bulimina inflata Seguenza, Ehrenbergina serrata Reuss, Nodosaria vertebralis Reuss, N. prismatica Reuss, N. consobrina d'Orb., N. longiscata d'Orb., Lingulina costata d'Orb., Cristellaria vortex d'Orb., C. gyroscalprum, Stache, Globigerina triloba Reuss, Truncatulina thiara (Stache), Anomalina ammonoides (Reuss), Pulvinulina karsteni Reuss, and Rotalia soldanii d'Orb. These indicate, according to Mr. Chapman, that the horizon is probably Eocene.
There are two prominent calcareous horizons in the Mount Brown beds on the south-east side of the Weka Pass, lying about 400 ft. and 800 ft. respectively above the Weka Pass stone. The upper horizon (D) forms an escarpment on the crest of the ridge, and the lower (B) presents a less prominent escarpment as a salient half-way down the slope, but in the angle between the Weka Pass Stream and the Weka Creek it forms a separate lower cuesta in front of the cuesta of the main band (D), and it assumes the same physiographic prominence between the Weka Creek and the North Dean, where, as already noted, it forms the summit of the range.
The lower horizon (B) consists, in the railway-cutting 43¾ miles from Christchurch, of upper and lower hard bands, 25 ft. and 20 ft. thick, separated by about 35 ft. of sands. Both bands consist of hard brown arenaceous limestone, with sandstone intercalations, forming cavernous cliffs owing to the weathering-out of included fragments of derived sand-stone. This phenomenon is well displayed in the cuesta between the Weka Creek and the Weka Pass Stream, where the derived fragments ofter show clear bedding oblique to that of the enclosing rock. Fossils are scarce and consist chiefly of small cup-shaped Polyzoa and barnacles, but Anomia sp. and partial valves of Magadina were observed. The lower bands are exposed on a cliff below the railway-cutting, between it and the Weka Pass Stream, where they are intersected by a small fault with downthrow to the north (Plate XXI, fig. 2). The base of the limestone on the northern (downthrown) side consists of a lenticular hard calcareous conglomerate enclosing specimens of Cucullaea, Struthiolaria tuberculata and many other gasteropods, and numerous barnacle (Balanus) fragments. Unfortunately the matrix is too hard to enable satisfactory specimens to be collected. The conglomerate rests upon soft grey sandstones, of which the few feet exposed show no bedding, so that the presence of an unconformity cannot be definitely asserted, but the presence of the derived fragments of sandstone in the overlying limestone makes it probable.
In the Weka Creek the lower limestone (B) flattens out just before reaching the creek-banks and is not exposed on the banks. It seems probable that it is cut off by a fault with downthrow to the south-east.
The third Mount Brown limestone (C) does not appear to be developed in the lower part of the Weka Pass or in the Weka Creek, but is again found not far below the main band (D) on the north-west face of Mount Donald, and for some distance to the south-west, where in a col in the cuesta it reaches the summit. It forms at the last point about 40 ft. of hard calcareous sandstone, in bands of 1 ft. to 3 ft. thick separated by shelly sands containing Magadina waiparensis, Anomia trigonopsis, and Pecten burnetti. A little nearer Mount Donald the bands coalesce to form a shelly limestone containing an exceptionally large number of derived sandstone inclusions, which weather out and give it a very cavernous appearance. It contains Polyzoa, barnacles, and shelly fragments, including Magadina waiparensis and Anomia trigonopsis. On the north-western face of Mount Donald the base contains a shell-bed with many casts of large gasteropods. It is here underlain by sands containing concretions.
The main Mount Brown limestone (D) forms, as already mentioned, the crest of the watershed south-east of the Weka Pass. It is divided by a persistent bed of sand, which outcrops just below the crest on the Weka Pass side, and yielded Pachymagas cottoni n. sp. and Waiparia abnormis. This limestone crosses the railway-line in the cutting 43 miles 21 chains from Christchurch. At the northern end of the cutting there is about 35 ft. of sands exposed below the lowest bed of limestone. These sands contain occasional shells, including Ancilla pseudaustralis, Pecten huttoni, and very fragile shells of Crepidula sp. Immediately below the lowest limestone bed is a thin bed of broken shells, including Anomia trigonopsis and Glycymeris sp. The lowest limestone bed is 5 ft. thick. It is succeeded unconformably by 25 ft. of sands, containing many small derived pieces of sandstone, and yielding Anomia trigonopsis. Then follows 3 ft. of limestone, succeeded by another 25 ft. of sands. These are succeeded by the
main mass of the limestone, 25 ft. thick, containing at the top a shelly band with Pecten huttoni, P. beethami, P. burnetti, and Lima colorata. The limestone also contains many brachiopods, including species of Rhizothyris and Pachymagas, Magadina browni, and Terebratulina suessi. It is followed by 6ft. of sands, and a further 16 ft. of nodular limestone, which contains Bouchardia minima, Magadina browni, and Pachymagas sp. This is followed by 3 ft. of creamy calcareous sandstone, which, as will be seen later, is best regarded as forming the base of the next horizon.
Fossils are abundant in places on the dip-slopes of the main band (D), especially near the top of the small valley entering the Weka Pass Stream between the 43 m. 21 ch. and 43 m. 3 ch. cuttings, and over the saddle at the head of this valley down to the first valley trending to the Omihi Creek. Here the uppermost rubbly band seen in the railway-cutting is well exposed near the foot of the main dip-slope, and yields a rich brachio-pod fauna, besides Pecten burnetti, P. beethami, Pecten sp. nov., Lima colorata, L. paucisulcata, L. lima, Ostrea angasi, O. gudexi Suter (?), Anomia huttoni, A. furcata, Isurus desori (Ag.), I. hastalis (Ag.), small echinoids and fragments of larger species, and numerous cup-shaped and bottle-shaped Polyzoa. The brachiopods identified are Terebratulina suessi, Bouchardia minima, Magadina browni, Rhizothyris scutum n. sp., R. rhizoida, R. elongata n. sp., R. curta n. sp., R. crassa n. sp., R. elliptica n. sp., R. fortis n. sp., R. obesa n. sp., R. pirum n. sp., R. ovata n. sp., R. amygdala n. sp., Pachymagas bartrumi n. sp., P. speighti n. sp., P. haasti n. sp., P. hectori n. sp., P. parki, P. McKayi n. sp., P. morgani n. sp., and P. coxi n. sp.
The uppermost Mount Brown limestone (E) forms a prominent cuesta behind the dip-slope of the main band, and thence crosses the railway in the cutting 43 miles 2–3 chains from Christchurch, and descends into the Weka Pass Stream and Weka Creek a few yards above their junctions. The succession from the main band upwards may be followed without a break in the Weka Pass Stream and the Weka Creek, while parts of the beds are exposed on the escarpment of the cuesta to the east. The total thickness is about 100 ft., the last 35 ft. being formed by the uppermost limestone, which in the railway-cutting is a reddish-brown to yellow arenaceous limestone with numerous small pockets containing small pebbles, up to ¼ in. in diameter, of greywackes and jaspers. It contains many polyzoan and echinoid fragments.
Immediately succeeding the main limestone (D) is a creamy calcareous sandstone a few feet thick, well exposed just above the foot of the dip-slope of the main band, where it yields Stethothyris sufflata and Neothyris anceps n. sp. When followed over the first saddle into the most easterly tributary of the Omihi Stream it forms a sharp V down-stream, and on the far side is 3 ft. thick and yields Pachymagas andrewi n. sp. It is here followed by a hard band 2 ft. thick in turn succeeded by more soft creamy limestone, 3 ft. thick, yielding Lima colorata and Pachymagas cottoni n. sp. This is again followed by another hard band 1 ft. 6 in. thick, and the exposed section here ends with soft calcareous sandstone containing Lima colorata and Cucullaea alta var. B. The above limestone bands are included with the uppermost limestone (E) because of the occurrence in them of Stetho-thyris sufflata.
In the gorge of the Weka Pass Stream the above calcareous sands are succeeded by blue muddy sands, about 50 ft. thick, which contain fossils sparingly throughout, and include two shell-beds. The lower 20 ft. contains Cucullaea alta var. B. and Lima colorata fairly commonly, and also yielded
Turritella concava, Natica australis, Ampullina suturalis, Verconella costata, Ancilla pseudaustralis, Surcula fusiformis, Limopsis zitteli, Pecten william-soni, P. huttoni, Tellina eugonia, and Dosinia greyi. The lower shell-bed lies about 30 ft. below the limestone, and is exposed in the Weka Pass Stream at and below the suspension bridge, and also in the Weka Creek. It yielded Magadina browni, Neothyris novara, Rhizothyris curiosa, Crepidula monoxyla, C. gregaria, C. striata, Polinices gibbosus, Galeodea sulcata, Sigapatella novae-zelandiae, Latirus brevirostris, Verconella costata, V. dilatata, Voluta arabica, Voluta sp. cf. protorhysa Tate, Ancilla novae-zelandiae, Dentalium solidum, Placunanomia incisura, Limopsis zitteli, Pecten crawfordi, P. burnetti, P. huttoni, Crassatellites attenuatus (fragments), Venericardia purpurata, Cytherea sulcata, Protocardia alata, and Thracia n. sp. The upper, or Hinnites, shell-bed occurs at or near the base of the limestone, and is exposed in the railway-cutting, in the Weka Pass Stream and its tributary crossing the railway-line above the cutting, and in the Weka Creek. It yielded Neothyris novara, Stethothyris sufflata, Hemithyris nigricans mut., Dentalium solidum, Pecten crawfordi, P. burnetti, Hinnites trailli, Lima paucisulcata, Ostrea angasi, Cytherea sulcata, Chione stutchburyi, Cochlodesma angasi, and Protocardia alata.
The limestone (E) closing the sequence of the Mount Brown beds contains a fair number of brachiopods and a few molluscs in the Weka Creek, the railway-cutting, and the cuesta leading to the Omihi watershed, and yielded the following species from these localities: Crepidula gregaria, Galeodea senex, Ancilla pseudaustralis, Anomia trigonopsis, Antigona sulcata, Pecten burnetti, P. beethami, P. triphooki Zitt. (?), P. hochstetteri, Lima paleata, Terebratulina sp. cf. cancellata Koch, Stethothyris sufflata, Neothyris novara, N. iheringi n. sp., Rhizothyris curiosa, R. media n. sp., R. scutum n. sp., R. curta n. sp., R. elliptica n. sp., R fortis n. sp., R. obesa n. sp., and Pachymagas hectori n. sp.
As the limestone cuesta is traced from the Weka Creek past the first tributary of the Omihi Creek towards the second it exhibits no longer the characteristic brachiopods and becomes more of a hard shell-bed, the shells being mostly casts at the outcrop. It appears to be continuous past the back of Mount Donald towards the Waikare Valley, but has not been examined in this direction.
The summit of Mount Donald forms an outlier of beds resting on the main limestone band (D). These appear to be the lower beds of the Stethothyris sufflata zone. Park (1905) stated that some mile and a half north of the pass, near the highest part of Mount Donald, the beds were richly fossiliferous, and gave a list of forty-eight species of cetacea, fish, molluscs, brachiopods, cirripedes, and echinoids. I have been unable to rediscover this locality.
Behind the cuesta of the main limestone (D), running from the Weka Creek towards the Deans, the first cuesta is that of a shelly calcareous sandstone containing fine pebbles, which lies about 120 ft. above the main band (D). This is presumably the uppermost band (E). It has not been recognized in the Waipara end of the district.