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Volume 79, 1951
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Petrological Features of the Rocks of the Maruwenua District,
North Otago

[Read before the Otago Branch, September 12, 1950; received by the Editor, October 4, 1950]

Abstract

The basement rocks of the Maruwenua district are semi-schists of the Chl. 2 subzone, possibly of Triassic age. They are overlain by Cretaceous (?) and Tertiary sediments, and an account is given of the heavy minerals contained in these rocks. Many of the heavy minerals have been derived from the nearby schists, but the origin of sillimanite and kyanite is uncertain. Two dolerite sheets, probably of early Oligocene age, invaded the sediments while these were still in a wet condition, and pillow structure is locally developed. A brief petrographic description of the dolerites is given.

Table of Contents

I.

Introduction

II.

Summary of Previous Work

III.

Outline of Stratigraphy

IV.

Semi-schist Basement Rock

V.

The Dolerite Sills

VI.

Heavy Mineral Assemblages of the Sediments

VII.

List of References

I. Introduction

The general geology of the region adjacent to the Maruwenua valley, North Otago, was made the subject of the writer's M.Sc. thesis while he was a student at the University of Otago. As details of the Tertiary stratigraphy and palaeontology were being investigated by Drs. Finlay and Marwick, Mr. Amies prepared a paper for publication concerned chiefly with the petrological features encountered in his work. He left New Zealand to join the Geological Survey in Malaya before submitting his paper, and was killed by bandits on the Kemaman River, in July, 1949. Revision of his manuscript has since been carried out by former fellow students at the University of Otago, namely D. S. Coombs and W. A. Watters.

II. Summary of Previous Work

A detailed summary of early work is given by Uttley (1920). Local problems of Tertiary stratigraphy have since been dealt with by Park (1923), Thomson (1926), Allan (1933, 1938), and by Finlay and Marwick (1941, 1947), whose stage names are used in the following table of sediments, while descriptions of vertebrate remains have been published by Benham (1935) and Marples (1946).

In a survey of the basic igneous rocks of Eastern Otago, Benson (1943) refers to sills of olivine tholeiite at Maruwenua, while in a later paper (1944) he gives a detailed account of the petrology of other mid-Tertiary basic rocks in North-Eastern Otago. The structure and general geology of the area have been discussed by Uttley (1920) and Marwick (1935, 1946).

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III. Outline of Stratigraphy

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Table of Sediments
Alluvium Recent
Low Terrace gravels Pleistocene?
High Terrace gravels
Argillaceous sandstone Hutchinsonian
Waitaki limestone Waitakian Oligocene
Duntroon greensand Duntroonian
Calcareous argillaceous greensand Whaingaroan
Micaceous sandstones Kaiatan and Bortonian? Eocene
Quartz conglomerate Cretaceous?
Awamoko schists Lower Triassic?

The basal Awamoko schists, dipping at high angles, consist of reconstituted greywackes and shales. These are followed unconformably by a series of Cretaceous (?) and Tertiary sediments folded into a slightly pitching shallow syncline.

The lowest beds conformable with the Tertiary sequence are the Ngapara coal measures (Cretaceous?) consisting of limonitic quartz conglomerates. These grade upwards into Eocene (Kaiatan and Bortonian?) micaceous and glauconitic sandstones, about 300 feet thick, showing rapidly changing lithology. There are two dolerite sills injected into the sandstones. The Whaingaroan argillaceous greensand (10 feet) followed by markedly glauconitic Duntroonian greensands (5 feet) overlie the Eocene sandstones. The Waitakian limestone, about 300 feet thick, lies above the Duntroonian greensand. In its lowest portions it is glauconitic, but grades upwards into a pure white limestone, which in its uppermost levels passes into a calcareous mudstone. The highest sedimentary marine bed is the Hutchinsonian argillaceous sandstone, exposed only on the western side of the Maruwenua River, in the tableland south-west of Duntroon.

The area is extensively covered by river gravels, which may be conveniently divided into three groups, Recent, low terrace, and high terrace. Where Tertiary sediments are exposed, the physiography is dominated by a number of plateaux cut through by creeks and capped by gravels.

IV. Semi-schist Basement Rock

The basement rocks consist of an alternating series of phyllites and fissile greywackes of the semi-schist type. They outcrop in creek beds in the western and southern portions of the area mapped, and are extensively exposed in the more mountainous country of the Kakanui Range. The schistosity planes are parallel to the bedding planes and strike north-east, dipping at 70°–90° to the north-west. The rocks are cut by thin quartz veins developed along joint planes, and, in the phyllites especially, these veins are sometimes complexly folded. There is much evidence of post-metamorphic deformation, e.g. the schistosity planes of some phyllites are thrown into small sharp folds with a wave-length measured in inches, and striking oblique to the lineation developed in the deformed schistosity-surfaces. Quartz veins filling tension-joints are often broken by very small faults. Both the dominant schistosity-planes and lineation are steeply dipping, although it appears that there is a general tendency for the angle of dip to lessen north-westwards towards the Kyeburn area. (Williamson, 1939.) As

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concluded by Turner (1942) from consideration of schists further to the south, it is probable that rocks with originally moderately dipping schistosity-planes have been tilted by post-metamorphic movements into their present position.

Degree of Metamorphism

Semi-schists derived from relatively coarse-grained greywackes are found to consist of relict clastic fragments of quartz and twinned albite studded with alteration products, together with rarer biotite, augite, and sphene set in a matrix of finely re-constituted minerals—quartz, epidote, sericite, chlorite and pumpellyite—making up perhaps 60–70 per cent. of the rock. These features agree with those of semi-schists of subzone Chl. 2 as defined by Turner (1936), and correspond with similar rocks of the Kakanui Range described by him (in Williamson, 1939) and by Paterson (1941).

Petrography

A typical quartzo-feldspathic schist has the following composition: quartz 40%, albite 30%, epidote 10%, pumpellyite 5%, sericite 5%, chlorite 5%, with accessory stilpnomelane, sphene, ilmenite, apatite, zircon, tourmaline (8293*), and occasionally needles of actinolite (8289). In the phyllites the proportion of sericite and chlorite increases, and calcite is generally present as well, being concentrated into bands parallel to the schistosity planes.

Clastic fragments of quartz typically show undulose extinction and fracturing, the cracks sometimes being filled with minerals such as pumpellyite, epidote, chlorite and stilpnomelane. Much of the quartz is granulated and it is an important constituent of the matrix. Even in the least metamorphosed rocks alteration of clastic plagioclase produces albite studded with flaky sericite, chlorite, epidote and pumpellyite. These clouded relict fragments are almost invariably twinned, the twin planes being commonly bent or fractured, whereas the groundmass albite is clear and typically untwinned. Pleochroic ferruginous epidote is the commonest of the lime silicate minerals, but colourless clinozoisitic types are also found occasionally. Sometimes a central core of epidote is surrounded by small granular masses of a more clinozoisitic composition. The epidote minerals may be intimately associated with granular pumpellyite which is consistently present and sometimes forms dense aggregates. More rarely pumpellyite occurs as distinct prismatic crystals enclosed in recrystallised albite. The optical properties of the mineral differ little from those recorded by Hutton (1937, 1940) for the pumpellyite of schists from Western Otago.

Sericite and chlorite are universally present in the matrix, both minerals showing well-marked preferred orientation. They also occur as pseudomorphs after clastic biotite, relicts of which are sometimes preserved. Dense aggregates of white mica may represent original potash feldspar. Stilpnomelane is ubiquitous, although it is always in small amount (1–5%). It is characteristically developed as tiny wisps and sheaf-like aggregates (e.g. 8290) and is strongly pleochroic from deep reddish or sometimes deep olive brown (Y = Z) to golden

[Footnote] * Numbers refer to catalogued specimens, Geology Dept., University of Otago.

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yellow or straw colour (X). The optical properties suggest a composition near the stilpnomelane end of the series (cf. Hutton, 1938).

Separation of the Heavy Mineral Fraction

A number of typical schists (8282–8286) selected for this purpose were crushed and screened through a 50-mesh sieve (I.M.M. standard); the fragments were washed carefully to rid them of dust, and subsequently dried. By flotation in bromoform (S.G. 2·9 approx.) large crops of heavy minerals were obtained, due to the relative abundance of ilmenite and minerals of the epidote group in the original rocks.

(a)

Zircon is a constant member of the assemblage, the following types being present:

(i)

Short stumpy clear prisms terminated by pyramidal faces.

(ii)

Acicular prisms usually with elongated inclusions.

(iii)

Rounded prismatic grains with a slight yellowish tint.

(iv)

Rare spherical pink grains.

(v)

Occasional large euhedral grains (0·2 mm.) with spherical inclusions.

(b)

Apatite is found in most residues as prismatic euhedra with slightly rounded edges.

(c)

Tourmaline occurs as rare doubly terminated stout prisms with dichroism from light brown to very dark brown.

(d)

A few pink and colourless idiomorphic garnets are present.

(e)

Chlorite is found as irregular aggregates within the assemblage.

Age of Metamorphism

Trechmann (1918) assigns a Kaihiku (Upper middle Triassic) age to the Mount St. Mary beds, and since according to Park (1918) there is no visible unconformity between these beds and the underlying Awamoko schists, the latter may possibly be lower Triassic in age. The Awamoko schists are, it is believed, to be correlated with those of the Kakanui Range; Paterson (1941) considered the latter to be possibly Triassic in age. If so, these rocks must have undergone metamorphism during the late Jurassic or early Cretaceous orogeny. The present writer can offer no new evidence on this question.

V. Dolerite Sills

Two sills, about 40 feet apart and each 30–40 feet thick, invade the Eocene sandstones and form prominent plateaux in the southern portion of the area. Near Tokarahi the lower sill shows pillow structure, the pillows being generally about 3–4 feet in diameter. Further west the same sheet has well-developed vertical columnar jointing. The intrusive nature of the dolerite is indicated by the baked condition of the sandstone in contact with both the upper and lower surfaces of the lower sheet. The upper margin of the higher sill has not been seen, but fragments of baked mudstone were found on the slopes of Basalt Hill. The field evidence suggests that the magma was injected into wet marine sediments. The higher sheet occurs 20 feet below the base of the Whaingaroan greensapd, which is compatible with the suggestion of Benson (1943) that the intrusion of the sills may have occurred in the period immediately preceding the deposition of these greensands which are approximately co-eval with the Upper Ototaran limestone of the Oamaru district.

A typical specimen of the dolerite contains about 35% plagioclase (An50, sometimes with marginal zones of oligoclase), 30% augite,

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5% olivine (much altered), and accessory ilmenite and apatite, with intersertal patches of glassy and ferruginous material. Such a rock would be classed as an olivine tholeiite. (Harker, 1935.) Where the augite crystals are large (0·5 mm.) the structure is ophitic (e.g. 8392), whilst the smaller crystals are usually irregular and intergranular. The optic axial angle (2V) of the augite is about 46°. The phenocrysts show all stages of alteration. Ilmenite occurs in long plates or as small granules. Ferruginous materials ranging in colour from pale yellow-green to dark olive-brown fill vesicles and replace glass and olivine. The refractive index is variable, though commonly less than Canada balsam, values of 1·52 being frequent. Some of this material is isotropic and thus appears to be chlorophaeite as described by Peacock and Fuller (1928), who have explained the variability in colour and refractive index as being due to variations in the state of oxidation and water content. Certain deep green materials clearly replacing glass may be palagonite. However, most of the ferruginous material here described shows fibrolaminar structure and is markedly birefringent and pleochroic. Some of it may be bowlingite.

Sections of the glassy margin of the pillow lava (8296) display a clear, pale, buff-coloured glass (sideromelane) containing scattered idiomorphic crystals of olivine, augite, and plagioclase. Occasionally otherwise idiomorphic crystals of augite are penetrated ophitically by feldspar laths. (Fig. 1 A.) Ilmenite is not present in these rapidly chilled specimens, indicating that the ilmenite in the main mass of the dolerites commenced to crystallize at a relatively late stage, after olivine, augite and plagioclase had been crystallizing for some time. The same section (8296) contains an extraordinary row of subparallel, idiomorphic prisms of augite enclosed in the sideromelane, the prisms being arranged roughly at right angles to the length of the row. (Fig. 1, B and C.) No explanation of this occurrence is offered. Certainly the magma could not have flowed after their formation without breaking up the row.

Picture icon

A. Early stage in the development of ophitic structure, showing idiomorphism of augite embedded in sideromelane from pillow lava. Note narrow zone of reaction surrounding the feldspar crystals. (No. 8296, × 97, ordinary light.)
B. Glassy portion of pillow lava with string of idiomorphic augite crystals, idiomorphic crystals of plagioclase and pseudomorphs after olivine (showing dark). (No. 8296, × 22, ordinary light.)
C. Part of the row of idiomorphic augite crystals embedded in sideromelane from chilled outer portion of pillow lava. (No. 8296, × 100, ordinary light.)

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History of the Magma

It may be concluded that a hydrous undersaturated magma was injected near the surface and chilled moderately quickly. The first mineral to crystallise freely was olivine, followed by pyroxene and plagioclase. With continued slow cooling ilmenite, pyroxene and plagioclase continued to crystallise. During the final stages ferruginous alteration-products were produced by circulating magmatic waters.

VI. Heavy Mineral Assemblages of the Sediments

The sediments examined were disintegrated, sieved, washed by decantation and screened through bolting cloth; the resulting material was dried and immersed in bromoform (S.G. 2·9), and the heavy residue obtained was mounted and examined under a polarizing microscope. The mineral assemblages determined are listed in Table I.

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Table I. Heavy Mineral Assemblages in Maruwenua Sedimentary Rocks
Number of Slide Anatase Apatite Epidote Garnet Hornblende Kyanite Rutile Sillimanite Sphene Tourmaline Zircon Stage
8254 r m m fa r p
8255 m fa m m m a Ngapara
8256 m m a Conglomerate
8257 fa m r p
8258 r fa r m r r m fa
8259 fa m m a fa fa
8260 m r m r r fa a
8261 r m m fa Eocene
8262 fa m m r r fa fa
8263 m m r r r fa fa Sandstones
8264 r m fa m m m m fa
8265 m m r fa fa
8266 m r r r r m a
8267 fa m r r r fa a
8268 r fa r r r m fa p Whaingaroan
8269 fa m r m fa p
8270 r a r r r m m p Greensand
8271 r fa m m r m m fa fa
8272 m m r r r fa fa Duntroonian
8273 r fa m m fa a
8274 a m m m r fa Greensand
8275 fa m m m m fa
8276 r fa m r m fa p
8277 a r r r m m p Waitakian
8278 fa m r r m fa Limestone
8279 m p r r m Hutchinsonian
8280 r p r r m Sandstone

Note:—p = predominant (> 60%); a = abundant (20–60%); fa = fairly abundant (5–20%); m = minor (< 5%); r = rare,

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[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

8279–8281 Grid square S.43, 265945
8270, 3, 4, 6, 8 S.43, 329939. Koakam Hill Escarpment
8268, 8269, 8271μ S.43, 242932. Earthquake Rent
8272, 8275, 8277μ S.43, 242932. Earthquake Rent
8267 S.43, 217846. Greensand, Livingstone Sluicings
8265, 8266 Sandstone, 4 feet above 8267
8264 S.43, 238889. Greensand
8263 Sandstone, 25 feet below 8264
8262 Sandstone, 5 feet above 8264
8261 Brown sandstone, Livingstone Sluicings, 18 feet above 8267
8260 Greensand, 14 feet above 8267
8258, 8259 S.43, 238888. Greensand on lower slope of hill facing Sheep-wash Creek
8257 S.43, 209968. Conglomerate, Livingstone Sluicings
8256 S.43, 244968. Conglomerate
8254, 8255 Conglomerate, 10 feet above 8256

(Grid references refer to localities on Provisional One Mile Sheet, S.127.)

Characteristics of the minerals observed are given below.

Anatase

Anatase is found in rounded grains showing faint dichroism from light yellow to greenish yellow. One broken euhedral grain was also observed.

Apatite

Apatite was observed in the following two forms: (a) Short, thick, subidiomorphic prisms with rounded edges and terminated by basal planes. (b) Acicular prisms.

Epidote

Epidote is present in irregular grains. Both colourless and coloured types occur, the latter usually showing faint pleochroism from almost colourless to light apple green. The colourless epidote is optically negative or neutral, poorly birefringent, and is evidently rich in the clinozoisite molecule.

Garnet

Garnet is found mainly in subeuhedral grains with the edges, slightly rounded, occasionally in rounded broken fragments, or more rarely as euhedral octahedral grains. It is pink or colourless, and usually clear, although sometimes there are minute inclusions scattered through it. In one grain the inclusions assume a circular arrangement.

Hornblende

Actinolitic hornblende is present as irregular fibrous prisms showing pleochroism from yellowish green to emerald green. The extinction Z ∧ C is 15°, and elongation positive.

Kyanite

Kyanite was observed as colourless, irregular, prismatic grains, usually with rounded edges. There are two perfect cleavages almost at right angles, and oblique extinction with reference to them.

Rutile

Rutile in stout or slender prisms is a common accessory. The prisms are somewhat rounded and display vertical striations crossed by widely spaced lines oblique to the length of the prism. Occasionally stumpy prisms terminated by pyramidal faces or fragments derived from these show multiple or knee-shaped twinning.

There is usually no discernible dichroism.

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Sillimanite

Sillimanite was observed in Eocene sandstones in the form of clear colourless prisms with rectangular outline. They show striations parallel to the length of the prism, and a faint (010) cleavage.

Sphene

Sphene occurs as a minor constituent, in irregular or rounded highly refringent grains, which are usually cloudy in appearance. Birefringence is always high.

Tourmaline

Tourmaline is a common and important member of the heavy mineral assemblages of the sediments. It is possible to divide the crystals found into three general types:

(a) Slender prisms (0·2–0·4 mm.), singly terminated, showing vertical striations, and crowded with tiny opaque inclusions. The dichroism of this type is usually from light or dark brown to bluish or black; rarely it is from bluish-green to blue.

(b) Large, stout, clear prisms, sometimes doubly terminated. In one case an inclusion was found to be an obliquely oriented crystal of tourmaline. The dichroism is from light to dark brown, or occasionally from colourless to deep brown or deep reddish brown.

(c) Deep brown rather stout angular fragments, possibly derived from type (b) above.

These may be compared with very similar types recorded by Hutton and Turner (1936).

Zircon

Zircon is the most important mineral of the assemblages, both in abundance and in the number of habits assumed. These are listed in order of abundance below:

(a) Clear, colourless, acicular prisms doubly terminated by pyramidal faces, and generally euhedral. Inclusions are common, usually elongated parallel or sub-parallel to the length of the prism. Some inclusions are recognisable as tiny crystals of zircon.

(b) Clear euhedral stumpy prisms, doubly terminated by pyramids and commonly carrying rounded inclusions.

(c) Rounded grains derived from type (b) above.

(d) Large crystals (0·2 mm.) with recognisable inclusions of smaller crystals of zircon. The edges of the prisms are much rounded.

(e) Rare pink zircons are also present. These are well rounded and may be almost spherical.

(f) One grain of clear, euhedral, colourless zircon showed a knee-shaped twin.

(g) One grain showed pronounced zonary structure surrounding a corroded nucleus of zircon.

Provenance

(1) Apatite, epidote, garnet, hornblende, rutile, sphene, and zircon are all known to occur in the schists of Otago. Tourmaline of type (a) above is also found in the Otago schists (Hutton and Turner, 1936), whereas angular fragments are characteristic of the Kakanui schists (Paterson, 1941). Tourmaline of type (b) has been found in the heavy mineral assemblages of the schists described in this paper.

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(2) The clear acicular grains of zircon which Hutton and Turner (1936) found to be absent from the Central Otago schists are common in the schists of the area at present under discussion. This is further evidence for the correlation of the latter rocks with the Kakanui schists. The large zircon crystals are very similar to those found in the sandstones on the east side of Lake Manapouri by Hutton and Turner (1936, Plate 23, Fig. 4). It is possible that since this type has been found in the concentrates of the local schists, it has a long and complex history.

(3) The increase in the proportion of apatite in the more glauconitic types of sediment, e.g. the Whaingaroan and Duntroonian greensand, can be compared with a similar change noted by Paterson (1941).

(4) The present writer has nothing to add to the remarks of Hutton and Turner (1936) concerning the source of the kyanite, and their conclusions apply also to the rare occurrence of sillimanite in Eocene sandstones of the present area.

(5) A noteworthy feature is the predominance of epidote in concentrates from the Hutchinsonian sandstone.

(6) Minerals such as pyrite, limonite, glauconite and chloritic material as well as some of the idiomorphic anatase, apatite and rutile are mainly of authigenic origin.

VII. List of References

Allan, R. S., 1933. On the System and Stage Names applied to Divisions of the Tertiary Strata in New Zealand. Trans, N.Z. Inst., vol. 63, pp. 81–108.

—— 1938. Appendix to Geology of the Mt. Somers District. N.Z. Geol. Surv. Memoir, No. 3, by R. Speight.

Benham, W. B., 1935. The Teeth of an Extinct Whale Microcetus hectori n.sp. Trans. Roy. Soc. N.Z., vol. 65, pp. 239–244.

Benson, W. N., 1943. The Basic Igneous Rocks of Eastern Otago and their Tectonic Environment. Part IV. Trans. Roy. Soc. N.Z., vol. 73, pp. 116–188.

—— 1944. Ibid. Part IVB, loc. cit., vol. 74, pp. 71–123.

Finlay, H. J., and Marwick, J., 1941. The Divisions of the Upper Cretaceous and Tertiary in New Zealand. Trans. Roy. Soc. N.Z., vol. 70, pp. 70–183.

—— 1947. New Divisions of the New Zealand Upper Cretaceous and Tertiary. N.Z. Journ. Sc. & Tech., vol. 28, no. 4 (sect. B), pp. 228–236.

Harker, A., 1935. Petrology for Students, 7th Edition. Cambridge Univ. Press.

Hutton, C. O., 1937. An Occurrence of the Mineral Pumpellyite in the Lake Wakatipu Region, Western Otago, N.Z. Miner. Mag., vol. 25, pp. 172–206.

—— 1938. The Stilpnomelane Group of Minerals. Miner. Mag., vol. 25, pp. 172–206.

—— 1940. Metamorphism in the Lake Wakatipu Region, Western Otago, New Zealand. D.S.I.R. Geol. Mem., No. 5.

—— and Turner, F. J., 1936. The Heavy Minerals of some Cretaceous and Tertiary Sediments from Otago and Southland. Trans. Roy. Soc. N.Z., vol. 66, pp. 254–274.

Marples, B. J., 1946. Notes on some Neognathous Bird Bones from the Early Tertiary of New Zealand. Trans. Roy. Soc. N.Z., vol. 76, pp. 132–134.

Marwick, J., 1935. Geology of Wharekuri Basin. N.Z. Journ. Sc. & Tech., pp. 321–338.

—— 1946. The Geology of North Otago and South Canterbury. School Publication Branch, Education Dept., New Zealand.

Park, J., 1918. Geology of the Oamaru District. N.Z. Geol. Survey Bull., No. 20.

—— 1923. On the Relation of Oamaru and Waitaki Stone. Trans. N.Z. Inst., vol. 54, pp. 82–87.

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Paterson, O. D., 1941. The Geology of the Lower Shag Valley, N.E. Otago. Trans. Roy. Soc. N.Z., vol. 71, pp. 32–58.

Peacock, M. A., and Fuller, R. K., 1928. Chlorophaeite, Sideromelane and Palagonite from Columbia River Plateau. Amer. Min., vol. 13, pp. 360–383.

Thomson, J. A., 1926. Marine Phosphatic Horizons in the Tertiary Limestones and Greensands of South Canterbury and North Otago, and Brachiopod Evidence of their Age. N.Z. Journ. Sc. & Tech., vol. 8, pp. 143–160.

Trechmann, C. T., 1918. The Trias of New Zealand. Quart. Jour. Geol. Soc., vol. 73, pp. 165–247.

Turner, F. J., 1936. Metamorphism of the Te Anau Series. Trans. Roy. Soc. N.Z., vol. 65, pp. 329–349.

—— 1938. Progressive Regional Metamorphism in Southern New Zealand. Geol. Mag., vol. 75, pp. 160–174.

—— 1942. Structural Petrology of Quartzose Veins in Schists of Eastern Otago. Trans. Roy. Soc. N.Z., vol. 71, pp. 307–324.

Uttley, G. H., 1920. Tertiary Geology of the Area between Otiake River and Duntroon, North Otago. Trans. N.Z. Inst., vol. 52, pp. 137–168.

Williamson, J. H., 1939. The Geology of the Naseby Subdivision, Central Otago. N.Z. Geol. Surv. Bull., No. 39.