Art. VIII.—Geology of the Oamara-Papakaio District.
[Read before the Wellington Philosophical Society, 19th September, 1917, received by Editors, 31st December, 1917; issued separately, 24th May, 1918.]
Description of the Area.
Aim of this Paper.
Description of the Sections.
I. Description of the Area.
The area (about twenty square miles in extent) with which this paper is mainly concerned is the north-eastern corner of the Province of Otago. It is bounded on the north-east by the valley-plain of the Waitaki River, on the south-east by the sea, on the south by a line drawn east and west through one mile north of Oamaru, and on the west by a line drawn due south through Peebles (see fig. 1).
A considerable portion of the district, particularly between the township of Papakaio and Oamaru, is capped by heavy river-gravels and silts; these are evidently the remnant of a formerly extensive plain which sloped to the south-south-east, as the general trend of the streams that drain the area is in that direction. This former surface has been sculptured into well-rounded ridges and hills, attaining an elevation of 650 ft. near Papakaio in the north, and falling to the 350 ft. level in the south-east. In the north-western part of the district the Ngaparan coal-grits, mudstones, and green-sands crop out, and a ridge of quartz gravels in this locality attains a height of over 1,000 ft. Noticeable features of the country south of this ridge are the two well-marked depressions in the vicinity of Tabletop Hill and on the Ardgowan Estate. The Oamaru Creek in flowing over the softer greensands and tuffs of the former locality has by rapid lateral erosion widened its basin considerably, leaving, however, two limestone-capped flat-topped hills standing prommently above the surrounding low-lying area. The stream on leaving this open tract of country becomes much constricted as it flows over the limestone, and at the Devil's Bridge it passes beneath a natural bridge of limestone before it reaches the Ardgowan area, where the softer Hutchinsonian and Awamoan rocks have undergone extensive denudation, forming the second depressed area. The stream then enters the volcanic area north of Oamaru, and its narrow bed is flanked by precipitous cliffs of basaltic rock.
II. Historical Summary.
Although the older Geological Survey did a considerable amount of work in the Oamaru and Waitaki districts of North Otago, little reference has been made to the geology of the Papakaio district.
In 1874 Traill collected fossils at Pukeuri, and Hector (1882, p. 123) assigned the mudstones there to the horizon of the nummulitic beds of the Upper Eocene—that is, he placed them below the Hutchinson Quarry beds. McKay in 1876 made collections at the Devil's Bridge and at Ardgowan. He also examined the limestone at Landon Creek and referred it to the Cretaceo-Tertiary. The volcanic rocks in the watershed between Oamaru and Landon Creek he referred to a horizon higher than the Waiareka tuffs, and rightly ascribed them to the second period of vulcanicity. Park (1905, p. 519), on palaeontological grounds, placed the Pukeuri beds below the limestone, and stated that the limestone at the Devil's Bridge overlay the Hutchinson Quarry beds. Marshall and Uttley (1913, p. 303), on palaeontological and stratigraphical evidence, placed the Pukeuri beds above the limestone—that is, above the Oamaru beds—and the Hutchinson Quarry beds at the Devil's Bridge also above the limestone.
III. Aim of this Paper.
In 1916, in a paper on the geology of the Kakanui district, I gave a detailed succession of the beds of the Oamaru system east of the Waiareka Valley, and that paper gave some of the observations on which the sequence was based. The Waitaki stone of Professor Park was shown to be the Ototara stone in the locality where he had described it.
It is the aim of the present paper to produce further evidence of post-Waiarekan volcanic activity, to give an account of some hitherto undescribed sections in the Oamaru and Papakaio districts, and to show the relationship of the beds to those in the south of the Oamaru district.
IV. Description of the Sections.
It has already been mentioned that gravel deposits form the surface rock over a great part of the country, but the Tertiary beds crop out in the basins of Oamaru and Landon Creeks, and at several places on the Oamaru-Kurow Road. The sequence is usually clear, but at Papakaio the beds are faulted, and the continuity of an otherwise excellent section is broken.
In the Devil's Bridge area the deposition of the limestone appears to have been continuous from the close of the Waiarekan period to the commencement of the Hutchinsonian; in the other localities to be described, deposition was interrupted by a recrudescence of vulcanicity. It is proposed, therefore, to give an account of the Devil's Bridge section, to be followed by descriptions of sections in the district that show important departures from the normal sequence as represented in that area.
1. Devil's Bridge. (Fig. 2.)
Fig. 2 represents a section from the Devil's Bridge in a west-north-west direction to a point about a mile beyond the area mapped, so as to include the Waiarekan beds.
The tuffs (a) are very fine and tachylytic, and are interbedded with bands of diatomaceous earth. Dykes and sills intersect the tuffs and the
diatomaceous earth, and the latter is altered in places to a hard flinty rock. There are inclusions of quartz in the dolerite, this being a noticeable feature of the earlier intrusive rocks associated with the Waiareka tuffs. The quartz has probably been incorporated during the passage of the molten rock through the quartz grits that lie at the base of the Tertiary series. The limestone is poor in fossils, and is similar to the building-stone, but in parts it becomes chalky. A peculiar nodular surface marks the junction of this rock with the overlying greensand, although it is not so conspicuous in this locality as in other parts of the district. The green-sand (c) overlying is glauconitic, casts of Foraminifera being plentiful. Some distance above the base Pachymagas parki (Hutt.) occurs in abundance, other fossils being scarce. The brown sands (d) are also glauconitic and very fossiliferous. The nodular band contained Terebratula sp., Aetheia gualteri (Morris), Hemithyris sp., and stems of Isis. The greensand (c), in addition to Pachymagas parki (Hutt.), contained Pecten huttoni Park. The fossils from the brown sands (d) have been recorded by Marshall and Uttley (1913, p. 303) and clearly indicate that the beds are Awamoan.
Fig. 2.—Section (diagrammatic) W.N.W.-E.S.E. through the Devil's Bridge. (a) Tuffs and diatomaceous earth, intruded by dolerite (a1); (b) limestone; (c) greensand; (d) Awamoa beds; (e) gravels.
Park (1905, p. 518), in describing the beds here, placed the limestone above the Hutchinsonian and Awamoan; but the greensands lie hard upon the surface of the limestone, which is undoubtedly the Ototara stone. The rocks are conformable throughout, and dip towards the coast at an angle of from 10° to 16°.
2. Grant's Stream, Ardgowan. (Fig. 3.)
At Ardgowan, near the junction of Grant's Stream with the Oamaru Creek, a small section is exposed on the roadside. The lowest beds (a) consist of a brecciated pillow-lava, which can be traced in a continuous section along the banks of Oamaru Creek to the town of Oamaru. This bed is overlain by a limestone (b) 10 ft. thick, containing occasional water-worn masses of decomposed vesicular basaltic rock. This passes into an indurated limestone (c), which becomes nodular at its junction with the greensand (d). The nodular portion contains many fossils, but chiefly as casts. Stems of Mopsea also occur. The actual junction with the overlying greensand (d) is not seen, as the latter occur separated from the main exposure. From the nodular surface of the limestone I obtained the following forms: Turbo sp.; Polinices sp., Turritella sp., Lima lima (L.), Ostrea sp., Cardium sp., Liothyrella boehmi Thomson, Terebratulina suessi (Hutt.), Aetheia gualteri (Morris), Hemithyris sp. The section is interesting as it indicates clearly the horizon of the volcanic rocks, which are here about 20 ft. below the nodular bed.
3. Water-race Creek, Oamaru District. (Fig. 4.)
This creek is a tributary of Landon Creek. The rocks cover an area of about half a square mile, and the exposure is isolated, but it is quite possible to determine the horizon of the upper beds. The section is noteworthy, as it shows a bed of limestone of considerable thickness between two beds of fragmental volcanic rocks. The beds dip E. 30° S., at an angle varying from 10° to 16°.
The tuffs (a) are greenish-brown, much weathered, and cemented by crystalline calcite. They are finer than the breccia higher in the section, and do not contain the same variety of minerals. The limestone (b) is tufaceous in parts, sometimes containing distinct bands of calcareous tuff.
Bed (c) is a thick breccia similar in character to the “mineral breccia” of the Kakanui locality (cf. Uttley, 1916, p. 20). This is overlain by a limestone which becomes glauconitic and fossiliferous towards the top. The fossils obtained were: Epitonium lyratum (Zitt.), Pecten delicatulus Hutt., P. polymorphoides Zitt., Liothyrella boehmi Thomson, L. landonensis Thomson, Murravia catinuliformis (Tate), Terebratulina suessi (Hutt.), Pachymagas ellipticus Thomson, Rhizothyris rhizoida (Hutt.), Hemithyris sp., Aetheia gaulteri (Morris).
Fig. 4. — Section, Water-race Creek, Oamaru District. (a) Tuffs; (b) limestone; (c) calcareous mineral breccia; (d) limestone, glauconitic and fossiliferous in its upper portion; (e) limestone band; (f) greensand; (g) hard brachiopod band; (h) river gravels.
This bed is followed by a very hard white limestone (e) about 18 in. thick.
The greensand (f) overlying is crowded with brachiopods. The following fossils were collected here: Pecten hutchinsoni (Hutt.), P. huttoni (Park), Epitonium lyratum (Zitt), Pachymagas parki (Hutt.), Rhizothyris rhizoida (Hutt).
The bed (h), which is really the upper portion of (g), is a hardened glauconitic stone. It contained Pachymagas parki (Hutt.) and Pecten huttoni (Park).
This greensand (g) and (h) evidently represents the Hutchinson Quarry greensand, while the hard limestone band (e) represents the nodular band at Kakanui and All Day Bay (cf. Uttley, 1916, pp. 20, 21, 23).
4. Landon Creek, Papakaio Survey District. (Fig. 5.)
About a mile west of Trig. Station B, limestone and greensand occur on both banks of Landon Creek. The section across the creek is shown in fig. 5. The beds dip S. 30° E. at 8°, and this would take them beneath the Awamoa beds at Pukeuri cutting.
The lowest bed is a calcareous tuff, which, however, is not exposed in section. The material excavated from the tunnel for the Oamaru—Papakaio water-race is a tuff, and from the position of the tunnel it must lie beneath the limestone. This limestone is at least 50 ft. thick. It is pure and white in it lower portion, but in its upper 20 ft. it becomes glauconitic, and near the junction with the overlying greensand (c) there is an alternation of
hard limestone with looser glauconitic sand, giving the rock a flaggy appearance. At their base the greensand (c) is intermingled with limestone, and where the former has been removed by weathering an irregular nodular surface is exposed on the surface of the limestone.
From the upper glauconitic portion of the limestone (b) I collected the following forms: Liothyrella boehmi Thomson, L. landonensis Thomson, Terebratulina suessi (Hutt.), Pachymagas ellipticus Thomson, Rhizothyris rhizoida (Hutt.), Aetheia gualteri (Morris), and Hemithyris sp.
Fig. 5.—Section, Landon Creek, Papakaio district. (a) Tuffs; (b) limestone; (c) greensand; (d) hard glauconitic band; (e) river gravels.
The greensand (c) yielded Epitonium lyratum (Zitt.), Pecten polymorphoides Zitt., Terebratulina suessi (Hutt.), Pachymagas parki (Hutt.), Rhizothyris rhizoida (Hutt.), Aetheia gualteri (Morris), Hemithyris sp., and Mopsea hamiltoni (Thomson) (?). A hard glauconitic band (d) overlies, containing Pachymagas parki (Hutt.) and Rhizothyris rhizoida (Hutt.).
The sequence in this locality differs somewhat from the section in Water-race Creek, for the breccia is missing, and the hardened limestone in the latter locality is apparently represented by the flaggy limestone in the present section, and the latter is evidently at the horizon of the nodular band in the Kakanui locality.
5. Flume Creek, Papakaio District. (Fig. 6.)
This section is exposed near the township of Papakaio in a small gully, spanned by the flume of the water-race. The section is not continuous, and the dip of the rocks in the lower part of the creek varies somewhat. There are distinct signs of faulting in the neighbourhood. At the head of the gully a bed of diatomaceous earth crops out, and lower down a small exposure on the left shows the same bed lying beneath a fine calcareous tachylytic tuff, dipping N. 70° E. at 20°. Greenish-brown laminated tuffs (c) overlie, and then follows a flaggy limestone (d). There is a break in the section at this point, but tuffs containing minerals occur in situ at the bottom of the bank. Then follows a coarser and more glauconitic limestone, which has a steeper dip than the lower flaggy limestone. A hard band of limestone (g) about 15 ft. thick caps the more glauconitic stone. Greensand (h) lies hard upon (g), and the junction is marked by the great abundance of the stems of Mopsea.
The greensand yielded Epitonium lyratum (Zitt.), Terebratulina suessi (Hutt.), Pachymagas parki (Hutt.), Aetheia gualteri (Morris), and Hemithyris sp.
In this locality also we have the nodular band occurring at the base of the greensand, and notable for the abundance of alcyonarian stems. Dr. Thomson informs me that a nodule collected by him from this locality was analysed by Mr. B. C. Aston and showed 1.8 per cent. P2O5, equivalent to 2.9 per cent. Ca3P2O8.
(a.) There has been at least one period of vulcanicity subsequent to the Waiarekan tuffs. It is more than probable that there were two periods of eruption, the mineral breccia of Kakanui being the record of the first, while the volcanic rocks at Grant's Creek indicate the last phase. This is not so evident in the present area, as nowhere are the two types of volcanic rocks represented in the same section, although the breccia at Water-race Creek is certainly farther down in the limestone than the volcanic rocks at Ardgowan. In the town of Oamaru, however, the intrusive rocks cut across interbedded limestone and breccia beds.
(b.) The mineral breccia of Kakanui (see Thomson, 1906) extends into the Papakaio district, and forms a well-marked stratigraphical horizon throughout the whole Oamaru coastal district.
(c.) The diatomaecous-earth deposits occur at Papakaio associated with tachylite tuffs, as in the earlier known deposits in Cave Valley. This represents a considerable extension of its range.
(d.) The nodular band is persistent throughout the Oamaru and Papakaio districts from Kakanui to Papakaio, a distance of about twenty miles, and is phosphatic at both these localities. As a hardened band of limestone often underlies it, and has been proved phosphatic by Morgan (1915) at Hutchinson Quarry, it may reward investigation elsewhere.
(e.) In the Papakaio district there is no evidence of two distinct limestones separated by the Hutchinson Quarry and Awamoa beds, as contended by Park (1905).
(f.) Dr. Thomson informs me that the fauna beneath the Maerewhenua limestone, farther up the Waitaki Valley, bears a strong resemblance to that of the upper part of the Ototara stone in the Landon Creek area. Detailed stratigraphical work between Papakaio and the Maerewhenua districts should therefore go far towards settling the vexed question of the relationship between the Oamaru and Waitaki stones. There are many excellent natural sections in the Maerewhenua district that have not yet been described.
Hector, J., 1882. Index to Fossiliferous Localities in New Zealand, Rep. Geol. Explor. dur. 1881, pp. 118–28.
McKay, A., 1877. Oamaru and Waitaki Districts, Rep. Geol. Explor. dur. 1876–77, pp. 41–66.
Marshall, P., and Uttley, G. H., 1913. Some Localities for Fossils at Oamaru, Trans. N.Z. Inst., vol. 45, pp. 297–307.
Morgan, P. G., 1915. Phosphate Occurrences in the South Island, 9th Ann. Rep. (n.s.) N.Z. Geol. Surv., pp. 97–98.
Park, J., 1905. On the Marine Tertiaries of Otago and Canterbury, with Special Reference to the Relations existing between the Pareora and Oamaru Series, Trans. N.Z. Inst., vol. 37, pp. 489–551.
Thomson, J. A., 1906. The Gem Gravels of Kakanui, Trans. N.Z. Inst., vol. 38, pp. 582–95.
Uttley, G. H., 1916. The Geology of the Neighbourhood of Kakanui, Trans. N.Z. Inst., vol. 48, pp. 19–27.