The Geology of the Clinton District, South Otago With Folding Geological Maps.

Transactions of the Royal Society of New Zealand : Geology, Volume 2, Issue 14, 11 February 1965, Page 205

The Geology of the Clinton District, South Otago With Folding Geological Maps.

D. G. Bishop,

By

tJniversity of Otago

[.Received by the Editor, October 21, 1963.]

Abstract A map and description of the geology of an area of 73 square miles around the township of Clinton are presented. The area is underlain by sedimentary rocks of Paleozoic and Mesozoic age, with a small area of Tertiary sediments occurring in the northern part of the district.

The stratigraphy and structure are essentially similar to those described by Wood (1956) from the contiguous Gore Subdivision. Additional structural complexities are apparent, but insufficient exposure precludes full elucidation of these. A magnetic isanomalic map provides some additional data.

The oldest rocks are the indurated greywackes of the Tuapeka Group, which grade northwards into the Otago schists. Younger Paleozoic rocks are divided into the Waipahi and Arthurton Groups, the new Popotunoa Formation being introduced for the youngest rocks of the Arthurton Group. The latter two groups comprise a marginal eugeosynclinal sequence of volcanic arenites, shales, conglomerates, and tuff's and in part belong to the zeolite facies of metamorphism.

The oldest Mesozoic rocks have been placed in the Wairuna Peak Group, equivalent to Wood’s (1956; 53) Wairuna Peak Beds, and subdivided into the Awakia (lower) and Waiwera (upper) Formations. The Waiwera Formation is conformably overlain by the fossiliferous Kaihikuan Stage, which is in turn succeeded by the Oretian Stage. The latter forms the southern boundary of the district. During the Rangitata Orogeny these rocks were thrown into a series of overturned folds trending north-west-south-east. The Waipahi Group is the oldest unit on the overturned north-eastern limb of the regional Southland Syncline. Preliminary evidence suggests that in the rocks of the Tuapeka Group, recumbent folds may have been present before the Rangitata deformation.

Freshwater or estuarine coal measures, the Gore Lignite Measures, possibly of Bortonian-Kaiatan age, overlie rocks of the Tuapeka Group near the Pomahaka River, and at one locality the lignite is on fire. Tertiary and later deformation in the northern part of the district has been mainly restricted to gentle warping about north-north-east and north-west axes which largely control the present topography and drainage pattern. Strike faulting along the Murihiku escarpment probably occurred during the early Tertiary, the result being uplift of the Kaihiku Ranges and possibly also of smaller blocks in the Paleozoic rocks.

Introduction

The Clinton District embraces 73 square miles of agricultural land in South Otago, and includes the township of Clinton, which lies on the main road and railway 63 miles south-west of Dunedin. A prominent group of hills, the Kaihiku Ranges, forms the southern part of the district, but north of these hills the countryside is gently rolling downland.

Previous Observers

Gar Vie (1859) recorded slate, conglomerate, and compact sandstones in the Kaihiku Ranges, and also noted that a seam of coal near Popotunoa was on fire, as it still is.

Hector (1864) mapped Popotunoa Hill as either “igneous rocks” or “igneous tuffas ”, but the legend on his map has faded with age and the similar colour schemes are now difficult to distinguish.

McKay (1877) visited the Kaihiku, Waiwera, and “Popatuna” Gorges in 1873. The litter locality corresponds to the Kuriwao Gorge and is probably a corruption of the name Popotunoa, a prominent hill nearby. Along the Kaihiku Ranges arid in the “ Popatuna ” Gorge he recorded a large conglomerate of crystalline rocks overlain by sandstones and then grits, both containing fossil ferns, while in the Waiwera Gorge he noticed strata dipping north-east for four miles along the strike.

Later Cox and McKay (1878) mapped 73 beds in the Hokonui Hills and showed the stratigraphic relationships and synclinal structure of the Mesozoic sediments.

Hector (1884) listed the “ Popotunoa or Waiwera beds” as a subdivision of his Permian formation in his “ Table of Sedimentary Beds ”, but the name has not been used since.

Marshall (1918) presented an account of the greywackes of the Tuapeka Series which he considered were of lower Mesozoic age.

In 1925, Marwick established the presence of Permian or older rocks when he described three fossils, including the pelecypod Atomodesma, from a locality found by Ongley at Clinton.

Ongley (1939) mapped much of the present area as part of the Green IslandKaitangata Subdivision. On the basis of the Permian fauna from Clinton and lithological evidence he separated the Clinton Series from the Tuapeka Series.

In 1956, Wood mapped the adjacent Gore Subdivision and further subdivided the Clinton Series into the Waipahi (lower) and Arthurton (upper) Groups, presenting for the first time detailed interpretations of the stratigraphy and structure of the rocks between the Triassic greywackes and the Otago schists.

PHYSIOGRAPHY

The district can be subdivided into two distinct topographic units separated by the Murihiku escarpment (Harrington, 1958). The Popotunoa Downlands lie to the north and the Kaihiku Ranges to the south of this feature.

The Popotunoa Downlands

The relief is low and landforms are subdued over this large area, and only Popotunoa Hill, near Clinton, and Anise Hill, in the north-western corner of the district, rise above an altitude of 600 ft.

As noted by Benson (1941: 212), Mackie (1935: 299), and Wood (1956: 19) Tertiary and later deformation about fold axes trending north-north-east or northwest control the topography over large areas in East and South Otago. In the Clinton District a rectilinear grid-like stream pattern has developed about these two directions, consequent streams developing in the synclinal warps and anticlines forming higher ground. Where the more prominent anticlines intersect the course of the Pomahaka River antecedent gorges have developed, while in the smaller creeks constricting gorges have formed, swampy bottleneck flats often being developed upstream. Anise Hill (721 ft), the highest point in the northern part of the area, is situated on the intersection of a north-north-east with a northwest trending anticline.

That such trends have been continued from earlier times is shown by the occurrence of the Gore Lignite Measures in a shallow structural basin whose long axis lies north-west-south-east (Wilson, 1950: 3), paralleling the even older structural trends of the Southland Syncline.

Popotunoa Hill

Popotunoa Hill (1010 ft), a steep ridge one mile north of Clinton, is a prominent landmark rising 600 ft above the surrounding countryside (Plate 1). The origin of this feature is puzzling. Outcrops where attitudes can be measured are lacking, but apparent dip slopes on the southern face suggest a strike of 150° and a dip of 30° south-west. Resistance to erosion may be a factor in the present configuration of the hill, as it consists of a hard volcanic greywacke, but it is unlikely that resistance alone is the full explanation.

The Kaihiku Ranges

The Kaihiku Ranges form a prominent series of Mesozoic strike ridges rising steeply to 2,000 ft across the southern part of the district. The frontal ranges, along the Murihiku escarpment, where the strata stand nearly vertically, are hogbacks, but farther south, where the dip becomes less steep, these grade into homoclines. The Kaihiku Ranges are separated from the low-lying country underlain by Permian rocks by a high-angle fault. The movement on this fault appears to have been distributed over a number of bedding planes in the older of the Triassic rocks. A benched appearance of the northern aspect of the range is believed to be due to the progressive overlap of successively younger strata involved in this type of movement.

The two larger streams in this block, the Kuriwao Stream and the Waiwera River, flow north-east across the strike and drain into the Clutha River. They may have been initiated as consequent streams in synclinal warps, and a strong near vertical joint system has also influenced their courses Steep-sided narrow valleys with frequent rapids and waterfalls and confined rocky gorges point to the immaturity of this cycle of erosion.

Tributary creeks almost invariably parallel the strike for most of their courses and have eroded steep-sided subsequent valleys along the lines of less resistant strata. With the development of short transverse reaches across strike ridges, this control by the underlying structure has given rise to the trellised drainage pattern first noted in this region by Marshall (1912).

PALEOZOIC STRATIGRAPHY

Wood (1956) has described in some detail the stratigraphy of contiguous Paleozoic rocks near Waipahi in the adjacent Gore Subdivision, and his stratigraphic units have been retained for most of the strata at Clinton. Outcrops are too widely separated, however, and lithologies are too variable, to allow accurate correlation of the rocks of the Clinton District with those at Waipahi; and the strata at Clinton are, with rare exceptions, insufficiently exposed to justify the establishment of new or smaller units.

The strata have been thrown into a series of folds about north-west—south-east axes, and overturned limbs and concomitant reversals in the direction of younging are common. It is possible, considering the outcrop density of the present area, that some such reversals remain undetected, and the relative age of some of the strata is thus open to question. In some places the beds flanking the southern belt of rocks mapped as belonging to the Waipahi Group young towards it, suggesting that these Waipahi rocks are younger than the Arthurton Group. This is the reverse of the situation described by Wood (1956), but the quality and quantity of the evidence is such that neither possibility (i.e., older or younger) can be ruled out. Strike faults involving considerable vertical displacement are one possible explanation of the stratigraphic relationships shown, but, while these do not appear unlikely, the evidence for their existence is scanty.

Repetition of individual beds due to folding is difficult to detect, and correlation on opposite limbs of folds can be made only on overall generalised sequences.

Order of Superposition

Two qualitative classes of evidence are recognised, Class 1, regarded as probable and indicated on the map by a solid triangle, was recognised at localities where a consistent direction was obtained in almost all cases and two or more observers were agreed. Less positive evidence was designated Glass 2 (open triangle) .

Of the beds in the Clinton District for which evidence is available, some 60 per cent are overturned, reflecting their structural location as subsidiary folds on the overturned north-eastern limb of the Southland Syncline.

Tuapeka Group

The Tuapeka Group contains the oldest rocks mapped in the district. They were first described in detail by Marshall (1918) as Tuapeka Series, the name being amended to Tuapeka Group by Wood (1956: 17) following the recommendations of the Stratigraphic Commission (Moore, 1947). Marshall considered that the rocks were of early Mesozoic age and that no great disconformity existed between them and the Triassic rocks of the Southland Syncline.

With the discovery of Permian fossils at Clinton by Ongley (in Marwick, 1925) an upper age limit was established. Considering the great thickness of probably Permian strata below the fossiliferous horizon and above the Tuapeka Group (15,300 ft in the Gore Subdivision) it appears possible that the age of at least some of the Tuapeka rocks is Carboniferous.

In the Clinton District the rocks are rather uniform fine-grained indurated greywackes, usually light blue-grey in colour. Finer-grained rocks are rare. Outcrops are generally massive and may show several sets of joints. Sedimentary structures are uncommon and the order of superposition was not established with certainty at any outcrop, the attitude of bedding itself being often obscure.

An insufficient thickness of rocks occurs within the district to detect the northward increase in metamorphic grade described by Marshall (1918) and Wood (1956). The regular attitudes observed by Wood in the Gore Subdivision do not persist across the Clinton District, sharp changes in attitude being recorded within two or three miles. The significance of these variations will be discussed later.

Waipahi Group

Rocks of the Waipahi Group outcrop in two strike belts crossing the area in a north-west and south-east direction. The northern belt generally dips northeastwards at angles between 40° and 60°, and the maximum possible thickness is in the order of B,oooft. The section has tentatively been mapped as simple but overturned, and is described below as such. In the absence of any supporting evidence no significance has been attached to the single upright “ Class 2 ” determination mentioned below.

The stratigraphically lowest rocks exposed are deeply weathered, rather shattered feldspathic arenites. The shattering is possibly an effect of the Macpherson Thrust Fault. Above these six feet of red shale are exposed in a road cutting at G.R. 171423, and are apparently overlain by crystal vitric tuffs and spheroidally weathered sandstones exposed in a quarry north of the road at G.R. Sl7O/162421* (west of the present area). Graded bedding in the tuff bands gives good evidence of overturning through 125°.

In the western part of the district and above the graded tuffs a coarse-grained lithic sandstone, the Mt Mistake Member, is well exposed along Mt Mistake Road. It appears to attain a thickness of 2,500 ft. On weathered surfaces small subangular pebbles stand out in high relief and give the rock a distinctive appearance. This member was not detected east of the abrupt change in strike near the comer of the Mt Mistake and Clinton-Glydevale Roads, in spite of its apparent thickness. At a quarry at G.R. 204377 it is overlain by black shales and finer-grained, more feldspathic arenites, but a quarter of a mile south the coarse lithology reappears in a small outcrop where thin red shales are also present.

Farther east outcrops are more limited. About 200 yards north of Old Lake Road three beds of roundstone conglomerate crop out, 130 ft of strata are intermittently exposed, and the conglomerate beds are intercalated with greywackes and shales. The whole sequence dips 49° north-east, and graded bedding shows that it is overturned. Four miles to the south-east a further outcrop of conglomerate was found and perhaps represents the same horizon, as conglomerates are otherwise uncommon within the Waipahi Group. At a quarry near Trig. U a greenish, sheared greywacke with minor finer beds is exposed. Small fragments of black shale, up to Icm across occur throughout this rock. Anastomosing veins of laumonite seam the greywacke, which also contains a good deal of secondary chlorite. The bedding tends to be somewhat undulose, and polished sections of finer beds suggest that the strata may be upright (Class 2). At several localities along Taumata Road, probably stratigraphically above the conglomerates and the Trig. U greywacke, more red shales occur, sometimes interbedded with weathered brown shales.

The southern belt, approximately one mile wide, is poorly exposed. The evidence for correlating it with the rest of the Waipahi Group, following Wood (1956), is not conclusive, and an alternative explanation, that the belt or part of it is an infolded outlier of Triassic rocks, seems equally feasible. Petrographic evidence given later tends to support this alternative, but the amount of information available is not sufficient to enable discussion of the additional complications arising from this concept. Possibly fold axes in the Permian rocks have an appreciable change in plunge in either direction along the regional strike. Popotunoa Hill and its subsidiary hills to the south-east are composed of a massive, hard, greyishgreen volcanic greywacke, which outcrops infrequently. This lithology does not outcrop farther to the south-east, due to at least three faults transverse to the strike which appear, on topographic evidence, to be successively downthrown on the south-eastern side. The rocks exposed here are red and grey-brown shales with sub-mesoscopic intraformational breccias and slump features. As the outcrop density is less than one per square mile over the area thought to be underlain by these rocks, no evidence as to the succession could be obtained.

Relationship to the Tuapeka Group

The contact between rocks of the Tuapeka and Waipahi Groups was considered by Wood (1956: 36) to be a strong thrust fault. No additional information was obtained during the present survey.

Correlation

Wood (1956: 52) has correlated the rocks of the Waipahi Group with the Te Anau Group by virtue of their similarity with igneous and metamorphic rocks at Bluff and in the Longwood Range which were tentatively placed in the Te Anau Group by Service (1937: 206). Wood also points out the occurrence of the rocks of the Waipahi Group and the type Te Anau Group on a common strike belt.

Mutch (1957: 501) has correlated the Takitimu Group with the Waipahi and Tuapeka Groups, considering that the rocks of the Tuapeka Group are the redeposited equivalents of those of the Takitimu Group. The rocks of the Waipahi Go up in the Clinton District are essentially a continuation of Wood’s units, but the evidence from the present area does not support Mutch’s conclusions. Rocks of the Takitimu Group collected by Mr C. T. Harper from the Aparima River and briefly examined by the writer have almost identical counterparts in the Waipahi Group of the Clinton District, the Clinton rocks differing only in a smaller grain size. It seems unlikely from their petrography that the Clinton

representatives of the Tuapeka Group could be derived from such rocks by redeposition—though Wood (1956: 70) notes that the Waipahi rocks are very similar in composition to the Tuapeka rocks.

Furthermore, the Takitimu Group contains fossils including Atomodesma, Terrakea, Strophalosia, and Euryphyllum (Waterhouse, 1958: 609), of which Atomodesma occurs in the Waipahi Group and also, with Euryphyllum, in the younger Arthurton Group. No fossils have been reported from the older Tuapeka Group. Although limited, the petrographic and paleontological affinities of the Takitimu Group are thus with the Waipahi or younger rocks and the TakitimuTuapeka correlation is not justified.

The Waipahi Group only, then, is correlated with the Takitimu Group, and these groups constitute a satisfactory lower limit to the Southland Syncline, as discussed later. The Takitimu Group is considered by Waterhouse (1958: 608) to be of Lower Permian age.

Arthurton Group

Lower, Undifferentiated Portion

Rocks of this group occur in four strike belts, due to repetition by folding. The northernmost belt, which is queried, does not outcrop, and its presence is inferred from structural and stratigraphic considerations. A second belt, north-east of Popotunoa Hill, is folded in an isoclinal anticline, and an anticline also occurs in the third belt, which passes through Clinton township.

The content of the Arthurton Group as mapped in the Clinton District is probably equivalent to the lower part of the Arthurton Group in the Gore Subdivision (Formations AGI.4). Individual formations have not been named, as exposures are insufficient to trace such units laterally for any distance.

Lithologies are more varied than in the Waipahi Group, and exposures are dominated by conglomeratic rock types, with tuffs also becoming more common.

A fossiliferous tuffaceous greywacke stratigraphically overlying an overturned sequence of calcareous black shales outcrops as scattered blocks on a hilltop northwest of Clinton (Sl7O/665, G.R. 194301) and from this locality specimens of the following fossils were collected: Streblochondria sp., Atomodesma trabeculum Waterhouse, Etheripecten striatura Waterhouse, Lissochonetes n.sp., Plekonella sp., Martinia sp., Straparollus (Euomphalus ) n.sp. (Waterhouse, 1963: 78), Peruvispira sp., Euryphyllum sp., Crinoid (fragment).

Three hundred yards to the west, at locality SI 71/666, Atomodesma sp., Etheripecten striatura Waterhouse, Glabrocingulum (Ananias) campbelli Waterhouse, and Lissochonetes n.sp. were collected from a calcareous tuff which appears to be stratigraphically above the tuffaceous greywacke. The tuff outcrops again in Bell’s quarry (Steel’s quarry in Marwick, 1925), where a thickness of approximately 100 ft is exposed. From this locality (SI7O/664) specimens of Atomodesma sp., Lissochonetes n.sp,, and Glabro cingulum ( Ananias ) campbelli Waterhouse were obtained.

In a new quarry at Wairuna (Sl7O/667, G.R. Sl7O/150343), two miles west of the Clinton District, what appears to be the same sequence of calcareous black shales, tuffaceous greywacke containing Atomodesma and Lissochonetes, and scattered blocks of Atomodesma-hea,Tmg tuff is exposed. This sequence, which dips north-east at 30°, is shown by graded bedding to be upright (Plate 2, fig, 1).

This attitude cannot be reconciled with the structural position at the base of the Arthurton Group on the south-west dipping limb of a simple anticline, as shown on Wood’s (1956) map. Rather it appears that another anticline takes up in the Arthurton Group towards the eastern margin of the Gore Subdivision and the Wairuna quarry is situated on the north-east limb of this fold. North-west of Clinton the shale-fossiliferous greywacke-calcareous tuff-conglomerate sequence is overturned along the north-east limb of the same fold, upright beds appearing to the south. From this information it is difficult to determine whether the Wairuna quarry horizon and the Clinton fossil horizon are older or younger than the Arthurton limestone horizon (Wood, 1956: 46).

Dr J. B. Waterhouse, of the New Zealand Geological Survey, who is working on New Zealand Permian brachiopods, gastropods, and pelecypods has kindly supplied the following information, which tends to support the latter conclusion.

“Fauna from the Clinton ridge (GS fossil locality 1456)—Brachiopoda: Lissochonetes n.sp., Attenuatella n.sp., Martinia aff. mongolica Grabau, Martinia sp.; Bivalvia: Megadesmus sp., Conocardium sp., Atomodesma n.sp., Etheripecten striatura Waterhouse; Gastropoda: Straparollus n.sp., Glabro cingulum n.sp., Peruvispira sp., Cinclindonema sp.; Corals: Euryphyllum sp.; Bryozoa are also present.

“ Internal correlation. Several of these specimens are represented also in the faunas from AG 4 limestone at Arthurton (Wood, 1956), including Lissochonetes, Attenuatella, Etheripecten striatura, and Ananias; others including Martinia aff. mongolica and Straparollus are unique to the Clinton beds. The fauna differs considerably from the Productus Greek fauna described by Fletcher (1952) and from the upper Maitai fauna described by Trechmann (1917). Both of these latter faunas are dominated by cold-water forms akin to species of eastern Australia, whereas the Clinton fauna is rich in Tethyan affinities.

“ The Clinton species of Atomodesma suggests that the fauna is slightly younger than that from the AG 4 limestone at Arthurton. It compares well with Atomodesma n.sp. B from the upper Tramway Sandstone of Nelson and upper Annear Sandstone of the Eglinton Valley (Table I), and differs considerably from Atomodesma n.sp. A of Arthurton.

“External correlation. The Clinton fauna is Kazanian (Upper Permian). Lissochonetes compares with upper Permian species of Asia, Attenuatella is moderately close to A. attenuate (Cloud) from the Waagenoceras zone of Mexico, and M. mongolica occurs in the Jisu Honguer (Kazanian) limestone and Ghihsia Limestone (Artinskian) of China.”

Popotunoa Formation

This formation includes the youngest Paleozoic rocks of the Clinton District that are sufficiently well exposed to warrant naming. As the formation may be equivalent to Wood’s (1956: 48) AG 5 . 7 formations it is tentatively included in the Arthurton Group, though the thick sequence of purple shales that characterise the lower parts of the formation are apparently missing from the Gore Subdivision. Wood (1956: 48) notes 50ft of dark red and reddish-brown well-bedded sandstone exposed in the banks of the Waipahi River, but states that the beds have no correlatives elsewhere in the district. The purple shales of the Clinton District, on the other hand, have been observed by the writer at a number of localities between Clinton and Kaka Point.

The lower beds are well exposed in the artificial channel of Awakia Stream, north of Hillfoot Road. A thick sequence of closely laminated purple shales overlies a massive greywacke which is regarded as the highest bed of the remainder of the Arthurton Group. Thin bands of yellow feldspathic sandstone are occasionally interlaminated with the shales, and in such cases the order of superposition can usually be determined. Intercalated with the shales are, in order of probably decreasing age, brown shales, feldspathic sandstone, and a coarse lithic sandstone containing angular fragments of black shale up to lin in length. Several hundred feet above this sequence brown shales outcrop in the stream bed 400 yards north of Gibb’s farmhouse. The purple shales also outcrop in the Kuriwao Stream, where they have been thrust over younger beds, and at this latter locality thin, rather poorly defined greenish bands are sometimes present in the shales.

The rocks farther south are not as well exposed, and their relative stratigraphic position is uncertain. Several hundred feet above the brown shales in Awakia Stream a lithic greywacke is exposed in the creek bed and probably passes laterally into the sharpstone conglomerate at G.R. 229277. Another several hundred feet above this horizon in Awakia Stream a gritty blue-grey sandstone which is also exposed in the Kuriwao Stream outcrops. Blocks of coarse-grained crystal tuff occur near Hillfoot Road and isolated outcrops of grey and black shales, heulanditised vitric tuff, and volcanic greywackes occur near the top of the formation below the basal conglomerate of the Wairuna Peak Group.

A bed of Atomodesma-he?irmg tuff outcrops near the mouth of the Waiwera Gorge (G.R. 260257), probably as part of a wedge of Arthurton Group rocks exposed on the southern limb of a syncline which takes up in the eastern part of the district. Outside the present area, at G.R. 5179/285246, a distinctive sequence of red and green vitric tuffs occurs and may prove a useful marker horizon farther to the south-east.

Correlation

The colour banding of the purple shales of the lower part of the formation recalls the “ red and green argillites ” of the Waiau Formation in Nelson (Wellman, 1957), though the latter rocks have undergone more rigorous metamorphism, and also the “ laminated argillites ” of the Winton Formation in the Eglinton Valley (Grindley, 1958: 45).

MESOZOIC STRATIGRAPHY

The rather uniform overall lithology of the Mesozoic rocks encountered in the district renders it difficult to establish small rock-stratigraphic units, but this was achieved in the lower part of the sequence. It would be impractical if not impossible, however, to establish formations of a similar size for the younger rocks, and time-stratigraphic units have been employed instead for them.

Two new formations together constituting the Wairuna Peak Group are introduced for those rocks lacking characteristic fossils. The upper boundary of the lower formation is the base of a thick vitric tuff, one of the many in the Triassic sequence. These tuffs are water-deposited sediments (Coombs, 1954: 102; Wood, 1956: 54), but still contain delicate glass shards and bubbles which would be unlikely to survive even slight attrition (Plate 2, fig. 2). The source area of these tuffs is not known with certainty but probably lay many miles to the south-west (Coombs, 1954: 67), and consequently the pyroclastic material must have been aerially transported for long distances. In the Clinton District the tuffs appear to be rather persistent along the strike, and even the thinnest beds show no noticeable change of thickness in any one outcrop. This uniformity would be expected from large-scale ash showers. If this mode of origin is correct, individual beds of tuff would be for all practical purposes isochronous surfaces, and thus rockstratigraphic units bounded above and below by such beds would be true time stratigraphic units.

Wairuna Peak Group

This group is equivalent to the Wairuna Peak Beds and to part of the Kaihikuan Stage as described by Wood (1956: 53-8), and also to the greater part of the Kaihiku Series as described by Ongley (1939: 34), It consists of the lower, Awakia, and the upper, Waiwera Formations.

Awakia Formation

The Awakia Formation is introduced for the oldest Triassic strata in the Clinton District. Rocks of this formation crop out along the northern slopes of the Kaihiku Ranges in a series of prominent strike ridges, following the general pattern of outcrop of the Triassic strata on the northern limb of the Southland Syncline. The lowest bed is a thick basal conglomerate which occurs sporadically along the strike. It is well developed at the mouth of the Waiwera Gorge, where its thickness is in excess of 40ft and possibly as much as 200 ft, but is absent in the type section at the Kuriwao Gorge, Several other bands of conglomerate occur in the lower part of the sequence and are interbedded with blue-grey tuffaceous and lithic greywackes and “ flinty ” vitric and vitric-crystal tuffs. The tuffs are often pale brown or yellow and vary in thickness from a fraction of an inch to 15ft. These rocks are all well indurated and dip steeply south, being overturned in some of the more northerly outcrops.

The upper limit of the formation is set at the base of a pale yellow-brown albitised vitric tuff 15ft thick which outcrops at a sharp bend in the Kuriwao Gorge Road 100 yards north of the first bridge south of Clinton, at G.R. 190279. A similar tuff was found at apparently the same stratigraphic horizon in both the Awakia Stream and the Waiwera Gorge. The thickness of the formation in the type section is 2,200 ft, and in Awakia Stream 2,400 ft. The sequence at Waiwera Gorge is not simple, but the apparent thickness is 3,100 ft.

No fossils were found, though borings and tracks of burrowing organisms are common particularly near the base of the sequence. McKay (1874) recorded fossil ferns from sandstones and grits overlying the basal conglomerate along the Kaihiku Ranges. Wood (1956: 54) collected Triassic fossils distinct from Kaihikuan forms from the conglomerate near Pukerau (15 miles north-west of the Clinton district), and also reported fossil plants from the overlying beds.

Waiwera Formation

This formation conformably succeeds the Awakia Formation. It consists of thin bedded, often vitric tuffs with thick blue-grey or brown fine-grained tuffaceous greywackes. Coarse-grained rocks are rare. The lithologies of rocks of this formation are rather similar to those of the Awakia Formation, the chief differences being in the absence of conglomerates and the larger percentage of tuffs and tuffaceous rocks. In the Kuriwao Gorge the type section is 2,200 ft thick and in the complex Waiwera Gorge section it is probably about 3,000 ft thick.

At locality Sl7B/587, G.R. 190278, in the Kuriwao Gorge, an indeterminable pagodiform gastropod was seen in but could not be extracted from the lowest bed of the formation. Impressions of logs several feet long were also present. In the upper levels of the formation several specimens of Daonella apteryx Marw. were obtained (SI7B/584, 585, 586) and also an indeterminable brachiopod from Sl7B/586. The upper limit is placed at the base of a fine blue-grey greywacke containing, at locality SI7B/581, G.R. 183274, a Kaihikuan assemblage.

In the Waiwera Gorge a nautiloid fragment, a poorly preserved gastropod, Mellarium cf. mutchi Waterhouse, and a small ? molluscan fossil as yet unclassified, were collected from a hill slope 100 yards east of the river at locality 5179/673, G.R. 248240, 100 ft above the base of the formation. No other fossils were found below locality 5179/672, G.R. 243224, where specimens of Daonella were obtained, 400 ft below an Oretian fauna.

Age. The gastropod Mellarium suggests an Etalian age for the lower part of the formation, while the presence of Daonella indicates that the upper levels may be Kaihikuan or Etalian.

Kaihikuan Stage

Only one diagnostic faunule of Kaihikuan age was collected, from a quarry beside the Kuriwao Gorge Road, two and a-half miles south of Clinton. Details are: Locality Sl7B/581, G.R. 183272: Mentzeliopsis parki (Wilck.), Halobia sp., Rhynchonella zealandica (Trech.), Daonella apteryx Marw.

The fossils were collected from loose blocks of a fine blue-grey greywacke on the floor of a quarry. No lithological change could be detected in the weathered outcrop, but the presence of Halobia suggests that strata of Oretian age are also present. Ovoid concretions up to a foot long occur in these rocks and are filled with a purple puggy material. Some 10-20 ft below this locality, at SI7B/584, Daonella occurs in large numbers, but no other fossils were found; while 170 ft above the quarry (581) an Oretian fauna occurs (Sl7B/583). In a poorly exposed outcrop in a road cut at the northern corner of the quarry the strata have an anomalous attitude and faulting is suspected. It appears that the Kaihikuan Stage is represented by only 50-100 ft of strata, although more than 750 ft of Daonella- containing rocks stratigraphically below locality 581 may be of Kaihikuan age.

Diagnostic Kaihikuan fossils were not found in the stream section in the Waiwera Gorge, and it seems likely that the stage has been faulted out, or at least the fossiliferous horizons in it. Many localised changes of strike and dip associated with slickensided and shattered rock occur in the area in which it would be expected to outcrop.

One mile east of the Waiwera Gorge, Ongley (1939, p. 36) has reported a Kaihikuan fauna from localities G.S. 1427 and 1437, The precise points are probably near G.R. 5179/271223, outside the present area. This is approximately half a mile north of the projected Kaihikuan horizon in the Waiwera Gorge, due to structural complexities to be discussed below.

Oretian Stage

The roundstone conglomerate tentatively mapped by Wood (1956: 58) as the base of the Oretian Stage is absent in the Kuriwao and Waiwera Gorges. Ongley (1939, p. 37) records an angular (sharpstone) conglomerate to the southeast of the Clinton district, which may be the equivalent horizon.

In the Kuriwao Gorge at locality (SI7B/583, G.R. 181270, 60ft above the Kaihikuan-Oretian locality 581, the following fossils were found in very weathered, well bedded, tuffaceous greywacke exposed in a road cutting: Athyris cf. wreyi (Suess), Halobia sp., ? Hokonuia sp., indet. cephalopod (fragment).

In the stream bed 200 yards south of the bridge and 170 ft above 583, at Sl7B/582, another collection was made. Details are: G.R. Sl7B/182267: Terebratula pachydentata Trech., Halobia sp., Anodontophora angulata Trech., Athyris sp., Triaphorus zealandicus (Trech,), indet. cephalopod (fragment).

From these two neighbouring localities Athyris and Halobia indicate an Oretian age, but Terebratula, Anodontophora, and Triaphorus all extend upwards into the Otamitan, The Otamitan form Hokonuia has previously been reported from the type Oretian by Campbell (1955).

In the Waiwera Gorge, an Oretian fauna was found two and a-half miles south of the end of Waiwera Gorge road in the river bed, at locality 5179/671, G.R. 242213. The fossils, from a fine blue-grey greywacke, have been identified as: Triaphorus zealandicus (Trech.), Anodontophora angulata Trech., Halobia sp. This locality is stratigraphically 400 ft above the lowest known occurrence of Daonella apteryx in the Waiwera Gorge.

The upper limit of the Oretian Stage lies outside the area mapped and was not determined. In the south-west part of the adjacent Gore Subdivision the stage is approximately 1,400 ft thick (Wood, 1956: 59).

PETROGRAPHY*

Many of the Permian and Triassic arenites of the Clinton District exhibit the lithological characters of greywackes. They are hard, dark-coloured rocks of sand grade, with coarse angular fragments of rock and mineral grains set in a finegrained matrix. The matrix is believed, however, to be largely due to the finer fraction of the vast quantity of pyroclastic material supplied to the developing geosyncline rather than the result of deposition from turbidity currents. Indeed the presence of plant fossils, worm trails and borings, and thin graded beds of tuff and the preservation of delicate glass shards suggest that deposition occurred under rather quiet conditions, probably by traction currents in moderately shallow water.

Such rocks, lacking the depositional characters of the greywacke suite, cannot, as realised by Crook (1960: 427), be included in his proposed classification of arenites. The following terms have therefore been employed:

Greywacke. Arenaceous rocks with more than 10 per cent of a fine-grained matrix but containing no evidence of fragments of pyroclastic origin and with volcanic-rock fragments present only in relatively subordinate amounts.

Volcanic sandstone. Those arenaceous rocks containing an abundance of volcanic-rock fragments but with less than 10 per cent fine-grained matrix.

Volcanic greywacke. Those arenaceous rocks with an abundance of volcanicrock fragments but with in addition a fine-grained matrix forming 10 per cent or more of the rock. No evidence of pyroclastic origin can be detected.

Tuffaceous greywacke. Those rocks similar to volcanic greywackes but with vitroclastic or relict vitroclastic structures in the groundmass. Idiomorphic crystals may add corroborative evidence of contemporaneous admixing of pyroclastic material.

Tuffs. When pyroclastic material becomes dominant the nomenclature suggested by Pettijohn (1957) is used—viz., crystal, crystal-vitric, vitric-crystal, and vitric tuff. The prefixes “ lithic ” and “ feldspathic ” may be used where appropriate.

It should be noted, however, that the distinction between pyroclastic ash and finely comminuted clastic material is often difficult to make, particularly as advancing mineralogical readjustment tends to obliterate the original textures.

Tuapeka Group

Greywackes of the Tuapeka Group consist predominantly of subangular grains of quartz and feldspar set in a dark-coloured, fine-grained matrix, the latter making up about 30 per cent of the rock. The feldspars are almost invariably albitised and clouded with minute sericitic inclusions, but rare, less altered grains appear to be about An i s in composition. Fine-grained sedimentary- and volcanicrock fragments are subordinate in most sections and plutonic rock fragments are absent. Prehnite and/or calcite often occur as interstitial material or in veins, where they may be associated with quartz or albite. Pale-green chlorite appears in some sections but never in quantity. The matrix is difficult to resolve but appears to consist of quartz and albite, chloritic, sericitic, and possibly graphitic material.

Waipahi Group

The majority of rocks examined were lithic volcanic sandstones of the Mt Mistake Member. Rock fragments constitute up to 80 per cent of these rocks, the predominant rock types being fine-grained porphyritic volcanics, sometimes with an opaque black or red groundmass. Granitoid rocks make their first appearance in rocks of this group, as do grains of strongly pleochroic brown hornblende and pale-green augite. The latter are quite fresh, but the hornblendes may be surrounded by a dusty rim containing minute granules of iron ore.

Pumpellyite is an authigenic mineral occurring as blebs in altered plagioclase, in veins with prehnite, or as granular masses in the groundmass, where it may be intimately associated with quartz. Chlorite occurs commonly as an alteration product of the volcanic fragments or as pools in quartz. Prehnite occurs abundantly in some totally reconstituted rock fragments. Calcite is generally restricted to individual rock fragments and appears to have been introduced into them before the incorporation of suck rock fragments into the present rocks.

In the southern belt a lithic volcanic greywacke from Trig. V (17231)* contains heulandite, as does 17256 from Popotunoa Hill. This latter rock contains a few distinctive intergrowths of quartz and feldspar.

Arthurton Group

In rocks of this group lithic fragments are generally less abundant except in the rudaceous rocks, Feldspathic volcanic greywackes and tuffs become more common. The feldspars again show severe alteration, but a somewhat larger percentage, generally of andesite composition, are more noticeable than in older rocks. Calcite is very common, often partly replacing plagioclase, and in 17247 and 17262 it completely replaces the cores of zoned crystals. Laumontite may occur in the groundmass or as a replacement of plagioclase, but was not observed in any quantity, though at Albert’s Gap, a few miles to the south-east, a vitric tuff now consists almost entirely of laumontite. Chlorite and prehnite are rare, and celadonite is a rather constant minor accessory.

Conglomerate pebbles from a number of localities near Clinton were collected, sectioned, and described by the late A. C. Amies, whose unpublished manuscript was available to the present writer. Amies made his fullest collections at or near G.R. 199294, 199311, and 250289. Of the 140 sections that he examined he found that 60 per cent were of igneous origin, 19 per cent pyroclastic, and 15 per cent sedimentary, and that the commonest rock types were granodiorites, keratophyres, quartz-keratophyric tuffs, and greywackes.

One hundred and twenty of Amies’s thin sections at present in the collection of the Geology Department, University of Otago, were briefly re-examined by the writer, 8458 being of special interest. Described by Amies as an augite andesite, it is considered by the present writer to be a crystal tuff. Confirmatory evidence is afforded by the presence of prismatic crystals of calcite that have almost certainly been derived from the shell of the pelecypod Atomodesma. Andesitic rock fragments are also present and are set in an opaque groundmass.

Many of the other porphyritic rocks, particularly the keratophyric types, may also be tuffs in which the groundmass has been replaced by quartz and albite and any vitroclastic textures have been destroyed.

Popotunoa Formation

Heulanditised vitric tuffs occur in this formation. In the purple shales quartz, albite, and celadonite can be identified with fair certainty. A diffuse peak at 14.9 A on a powder diffraction chart indicates the presence of a montmorillonoid.

Wairuna Peak Group

Of the rocks examined, the majority are albite- or analcime-rich tuffs. In analcime-rich varieties the vitroclastic texture is usually faithfully preserved. In 17214 (Plate 2, fig. 2) replacement of analcime by finely crystalline albite is occurring at the tips of the shards. Quartz is also present. Albitisation is generally more advanced in rocks lacking vitroclastic textures, but it is not clear whether albitisation is destroying the textures or is favoured by more finely comminuted rocks.

A volcanic greywacke (17235) consisting of rock fragments, quartz, albitized plagioclase, and small amounts of heulandite and celadonite also contains a few grains of graphic intergrowths of quartz and feldspar similar to those noted in 17256 from Popotunoa Hill.

Discussion

Two progressive changes are apparent in the petrography of the sediments, one being in the nature of the detrital grains and the other in the nature of the authigenic minerals. The first of these reflects the changing provenance of the sediments. In the rocks of the Waipahi Group the percentage of volcanic-rock fragments sharply increases from the amount present in the Tuapeka Group greywackes (cf. Wood, 1956: 31) and augite and hornblende make their first appearance. Unstable conditions prevailed during the deposition of rocks of the Arthurton Group and, perhaps related, vulcanism of explosive violence commenced in the source area resulting in the appearance of significant amounts of pyroclastic rocks*. Then followed a quieter period and apparently a temporary cessation in vulcanism while rocks of the Popotunoa Formation were deposited, but renewed crustal movements closely followed by intense vulcanism resulted in the conglomerates and tuffs of the Wairuna Peak Group.

The second trend, that of progressive mineralogical reconstitution in response to the imposed conditions of the zeolite and prehnite-pumpellyite metagreywacke facies is conveniently described in reverse stratigraphic order. The youngest rocks which contain assemblages of analcine or heulandite with quartz, correspond to the heulandite zone of the zeolite facies (Coombs, 1960). Some B,oooft of strata are involved in the present area but the younger rocks are overlain by possibly as much as another 15,000 ft of Triassic and Jurassic strata, and so the zone may be very much thicker.

The laumontite zone is relatively thin and not clearly defined. Many of the calcareous rocks of the Arthurton Group probably belong in this zone, but the formation of laumontite may have been inhibited by the chemical activity of C0 2 . Laumontite is widespread, however, as a vein mineral and in crush zones in many of the Paleozoic rocks.

The presence of an “ island ” of heulandite-bearing rocks forming the Popotunoa massif in the southern belt of strata mapped as belonging to the Waipahi Group perhaps suggests that these rocks have not been buried as deeply as those of the Arthurton Group, though alternative explanations are possible (Coombs et. al., 1959: 63). Petrographically the affinities of the Popotunoa Hill rocks are with those of the Awakia Formation, and although its proof is far from conclusive the possibility that the Popotunoa massif is of Triassic age cannot be denied.

The majority of the other rocks of the Waipahi Group, and of the Tuapeka Group belong to the prehnite-pumpellyite metagreywacke facies, as is indicated by the persistent occurrence of assemblages containing one or both of these minerals.

TERTIARY AND RECENT DEPOSITS

Tertiary rocks are restricted to a few square miles in the north-eastern part of the district, where they unconformably overlie rocks of the Tuapeka Group. They extend farther south-east as a continuous unit and also outcrop to the west in the Gore Subdivision. The outcrops included in the Clinton District form the western edge of a depositional basin whose long axis trends north-west-south-east (Wilson, 1950: 3). Wilson mapped them as the Pomahaka Goal Measures and

Ongley (1939: 77) as the Pomahaka Lignite Field, but Wood (1956) mapped those occurring in the Clinton District as the Gore Lignite Measures. The name Pomahaka has been used by Hector (1884), and later by Wood, for the nearby Pomahaka Estuarine Beds, and so Gore Lignite Measures will be used.

Gore Lignite Measures

Outcrops are rare and almost exclusively artificial, the majority being opencast coal pits. The topography tends to be somewhat more subdued than in other areas where the blanket of Tertiary rocks is absent.

The surface on which the Gore Lignite Measures were deposited has been seen at one locality, G.R. 223459, but the lower boundary of the Lignite Measures is not distinct. Relatively fresh greyish-brown siliceous greywacke of the Tuapeka Group is exposed in the river bed and is succeeded by 19ft of much weathered material. The lower three feet is still recognisable as light blue-grey greywacke which has split along joint planes to form small crudely sphenoidal splinters about an inch long, and this passes gradually into 4ft of lighter-coloured material with joint and bedding planes of the original greywacke still discernible. Above this there is 12ft of earthy brown clay with vague suggestions of relict bedding and jointing still visible, as is the sphenoidal structure in some instances.

Above this there is a poorly exposed transition to a creamy-white clay with no original structures visible. Three hundred yards west of this outcrop a similar kaolinitic clay is quarried, and here there is little doubt of its sedimentary nature. Faint bedding is discernible in places and odd plant fragments can be found, often replaced or rimmed with marcasite. The lower boundary of the Gore Lignite Measures is drawn at the base of the sedimentary kaolinitic clay.

The lowest bed, of kaolinitic clay, seems to be restricted to lenses but in at least one locality, Jones’s quarry, G.R. 220461, it attains a thickness in excess of 30ft. It is massive and tough in artificial outcrop and exhibits a crude conchoidal fracture. It is creamy-white in colour, with slightly yellowish limonitic staining in some parts, and contains numerous fragments of carbonised twigs and other woody plant material. At Jones’s quarry the carbonised root-system of a tree that grew in the upper surface was found. The grain size is extremely fine, a suspended sample passing through a B.S. 350 sieve (44/x) with only organic matter retained. The clay has been analysed at the Dominion Laboratory {in Wood, 1956, p. 112).

This clay may be considered to represent the finer fraction of the reworked highly-weathered residual material of an early-Tertiary erosion surface. According to Neumann (1927) conditions prior to the deposition of such clays must have been of mild temperatures and heavy rainfall, with an abundant land flora so that the ground was continually soaked with organic and carbonic acids to give a white argillaceous residual soil such as is now exposed at G.R. 223459. Transport in a colloidal state would effectively winnow out the larger particles. Deposition occurred when water of sufficiently high salinity (or possibly possessing a high concentration of organic acids) was encountered, probably in an estuary.

At Jones’s quarry the clay is overlain by 6in of purple fire-clay with small weathered lenses of lignite above it. Associated with the fire-clay are thin bands of a bright red powdery material which X-ray powder diffraction patterns reveal to be practically pure hematite. Yellow limonitic bands are also present.

In the various coal pits in the district and in the seam exposed in the Pomahaka River the thickness of the lignite varies from 2 to 20ft. It is dull in colour and of low rank, and well preserved woody branches and trunks may be easily discerned. Resin appears to be concentrated near the top of the seam.

Microfloral samples examined from Soldier’s opencast pit, G.R. SI7O/187457, by Gouper (1960) suggest a Bortonian-Kaiatan age for this locality. Localities farther west are all of Duntroonian or Waitakian age, indicating that in the Clinton area the land subsided from east to west between Bortonian and Waitakian times.

Near the Burning Plains Road, at G.R. 215414, the lignite is on fire, as was observed in 1857 by Garvie (1859). The area overlying the burnt-out seam has slumped, rendering it easy to see the extent of the fire, which has been estimated by Wilson (1950) as approximately 1,000 acres. Scattered blocks of brick-red and black fused sediment occur over this area.

Mineralogy of the fused sediments

Petrological investigation of specimens of the fused sediments revealed the presence of the mineral phases cordierite, mullite, cristobalite, plagioclase, and relict quartz. The cordierite occurs as minute, virtually isotropic, hexagonal basal sections or rectangular length fast prisms. Most grains are apparently uniaxialnegative but one larger grain gave a biaxial-negative figure with a very small 2V. The crystals are non-pleochroic and the polarisation colours are first-order greys and whites. The refractive indices are less than balsam.

The structural state of the cordierite was determined by X-ray powder diffraction techniques. An average “A value ” of 0.08, similar to that found by Wylie (1959) for a cordierite from a Hebridean fused arkose, was obtained. A is a measure of the degree of distortion of the orthorhombic low cordierite structure from the hexagonal high cordierite (indialite) form. The low values of A—values as high as A = 0.28 have been recorded for cordierites from metamorphic rocks (Miyashiro, 1957) — support the optical evidence that the Burning Plains mineral is structurally close to the high-temperature hexagonal form.

Younger Beds

At G.R. 219460 the lignite is overlain by Gore Piedmont Gravels, but elsewhere the sequence above the lignite is not well exposed. Several feet of white kaolinitic clay, fine quartz sand, or rarely mudstone, appear to overlie the lignite in some areas. At two localities, G.R. 211479 and 215464, a thin layer of yellowish “ Chinamen ” boulders of recemented quartz directly overlie the lignite and are in turn overlain by loess. These have probably been derived from quartzite beds in the Lignite Measures such as have been described from Pukerau by Wood (1956, p. 82) in which case they are unconformable on the Lignite Measures and are perhaps equivalent to the Waimumu or Waikaka quartz gravels or, more likely, a phase of the Gore Piedmont Gravels.

Gore Piedmont Gravels

These are well developed in the Gore Subdivision (Wood, 1956) but form only a rare cover in the Clinton District. They consist of rusty-brown gravels with rounded pebbles up to 6in in diameter of greywackes of the Tuapeka Group, schist, metamorphic quartz and rare pebbles of clay and mudstone from the Gore Lignite Measures. Apart from the “ Chinamen ” boulders noted above, two outcrops only, at G.R. 219460 and 240461, are known in the district.

Loess

A superficial deposit of loess covers large areas of the district. At G.R.219460, 25ft of earthy yellow material is exposed which here exhibits a rough columnar jointing. Small fragmentary plant remains occur throughout and fibrous elements probably representing former root systems are also common. At this locality also a small pocket of well-rounded quartz pebbles, 5-15 mm, was found, the total number of pebbles being approximately 50. Dr R. R. Forster, of the Otago Museum, has confirmed that they are the gastroliths of a moa.

MAGNETIC SURVEY (Plate 4)

Part of the area was surveyed with a vertical-force variometer having a sensitivity of 29.93 gammas per scale division, 260 stations being occupied in an area of approximately 35 square miles. A base station at the Clinton football ground was checked three times a day for diurnal variation and instrumental drift. A reduced vertical field at Clinton of 58263 gammas with a deviation of + 463 gammas was deduced (Bishop, 1962) from observations there and at the Waipahi magnetic station (Cullington, 1954), Results have been reduced to the nearest hundred gammas, and plotted as positive and negative isanomalic contours about a zero of 3300 gammas (i.e., a first approximation of the instrumental value of the reduced field at Clinton).

Susceptibilities of a set of Clinton lithologies were kindly determined by Dr T. Hatherton, of the Geophysics Division. These range from 47 to 2170 (X 10~ 6 ) cgs units per cc, which would not account for all the observed magnetic relief (1600 gammas). However, as representative samples could not be obtained near or from the extreme “ highs ” it cannot be established whether the latter are due to surface lithology, deeper-seated rock bodies, or mineralised zones. The overall parallelism of the magnetic contours with the regional strike and the general correlation of low susceptibilities with negative fields and vice versa suggest that most of the variations are in fact due to the surface lithology, particularly as a variation of 1 per cent in the magnetite content could produce the total relief.

Uniformly low values recorded over the Triassic rocks can be correlated with the lesser amounts of ferromagnesians and ore minerals present in these rocks. Similar values were recorded on Popotunoa Hill and over some rocks of the Waipahi Group.

The most striking feature is the positive “ ridge ” immediately north of Popotunoa Hill. This feature was traced eastwards to G.R. SI7O/154364 (-J-400 gammas) but could not be detected at or near G.R. SI7O/120370. The possibility that it was due to an intercalated spilite body was investigated, but stations occupied on spilite outcrops in the Gore Subdivision showed that these rocks do not contribute towards a positive anomaly.

STRUCTURE

Regional Setting

The Southland Syncline

The first mention of a major synclinal structure was made by Hutton and Ulrich (1875) and Cox and McKay (1878). This structure has since become known as the Southland Syncline, a term used by Wellman (1956), Wood (1956), and others. Wellman notes that it is a major syncline involving rocks of probably Carboniferous to lower Jurassic age which extends across the southern part

of the South Island and plunges south-eastwards at a low angle. Wood (1956: 18) states “ the pre-Tertiary strata (of the Gore Subdivision) constitute part of the steeply-dipping north-east limb of the major Southland Syncline He also records the presence of subsidiary folds at Kaiwera, which plunge in the same direction as the main syncline. Examination of Wood’s map reveals that the nose of the Kaiwera Syncline plunges south-east at about 12°, considerably steeper than the overall plunge of the Southland Syncline, which is probably not more than s°.

Recently Speden (1961), in a note to accompany the geological map of the Papatowai Subdivision, has stated that the axial trace of the Southland Syncline separates two regions of different structure —a steeply-dipping north-east limb, and a shallow-dipping south-west limb which is modified by secondary folds. This division is somewhat artificial and tends to destroy the overall picture of a largescale asymmetric synclinorium. The term “ secondary folds ” is also unfortunate as such folds are almost certainly contemporaneous with, and form an integral part of, the “ primary ” synclinorial structure. In the present work the Southland Syncline is considered to involve only rocks younger than (and to include) those of the Waipahi Group. Rocks of this group are the oldest that have correlatives on the south-western side of the syncline established on admittedly thin paleontological evidence (in Wood, 1956: 52).

Folds

The Paleozoic rocks are folded about north-west-south-east axes sub-parallel to the axis of the Southland Syncline. These folds are horizontal or nearly so, except perhaps near the nose, as parallel conglomerate horizons crop out intermittently for thirty miles along the strike. Field data are too meagre to position the axes with any degree of certainty, but folds additional to those described by Wood (1956) from the Gore Subdivision are present. These folds are sub-isoclinal, often with overturned limbs, and strike faults may be present.

Although some strata have been rotated as much as 140° there has been little internal disturbance of the coarser beds apart from jointing. A weak cleavage is only occasionally developed in the finer beds, but extensive shattering and slickensiding are common.

In the younger Paleozoic rocks a number of sharp deviations from the regional strike occur, the majority of which are confined to a NNW-trending zone. Where this zone intersects the more pronounced topography of the Kaihiku Ranges transcurrent dislocation is indicated by the following evidence:

(1) The Murihiku escarpment has a sharp sinistral step immediately west of Conical Hill.

(2) A low saddle drained by only a minor stream is present between Conical Hill and Waiwera Cone, on the proposed dislocation zone.

(3) The stream entering the Waiwera River from the south-east, south of Conical Hill, is aligned for part of its length along the zone.

(4) A broad sloping terrace, capped with a thin veneer of alluvium, on the inside of the sharp bend in the Waiwera River to the south of Conical Hill, may be the result of offsetting of the river.

(5) The Kaihikuan Stage appears to be sinistrally offset about half a mile.

Projections of bedding attitudes obtained by the writer in the Waiwera Gorge and of others from the Warepa Survey District (Ongley, 1939) indicate an anticline plunging 25° south-east, which could account for some or all of this Kaihikuan offset. Directions of superposition on the northern limb are needed to confirm the existence of such a fold.

It seems possible, that in the Paleozoic rocks the transcurrent movement has produced a zone of “ kink ” folds with steep axes rather than a clean fracture.

Relationship of the Tuapeka Group to the Rocks of the Southland Syncline

In the rocks of the Tuapeka Group in the Gore Subdivision Wood (1956) has mapped a series of north-west-trending isoclinal folds with horizontal axes on the basis of upright and overturned beds. The axial planes of these folds dip between 40° and 50° north-east. In the adjacent Clinton District, however, such folds are not apparent. Rather, a pole diagram of the limited bedding attitudes available indicates a fold direction plunging at 45° to the north-north-east (Fig. 2a). As this trend is almost normal to the Gore folds, data for neighbouring areas obtained from the literature have been re-examined using equal-area projections. Although the quantity of data is not adequate for conclusive results from such statistical techniques, the structural complexity of this terrain is demonstrated and a preliminary interpretation is advanced. It should be emphasised, however, that further investigation of the areas concerned is required, and until more information is available discussion of possible alternatives to, or modifications of, this preliminary interpretation will not be attempted.

In the Hillend District (Ongley, 1939: map of the Hillend and part of the Waitahuna West and Waitahuna East Survey Districts) a poles-to-bedding diagram (Fig. 2b) reveals that a northerly-trending fold axis is again present, though in this case it is subhorizontal (170°/9° south).

To the north, from the Tuapeka District (Marshall: 1918), a further 60 observations are available. A complication arises with the northward increase in metamorphic grade (Marshall, 1918: 33-4) and the corresponding uncertainty in the nature of the S-surface measured. To reduce this uncertainty only attitudes from the less altered rocks south-west of the Milton-Roxburgh railway line have been plotted. Both Marshall and Ongley (1939: 29) considered that the S-surface is parallel to bedding, but Marshall (1918: 35) recorded one instance, at Stony Creek, where a “ slaty parting ” “ which certainly gradually develops into the foliation plane throughout the district ” could be seen intersecting bedding.

Nevertheless, the pole diagram (Fig. 2c) provides some useful information. Although there is considerable scatter, a single strong maximum defines a plane striking 035° and dipping south-east. Not only is this a similar orientation to the “ slaty parting ” at Stony Creek but it is also similar to the easterly-dipping planes in the Hillend District, those which dip west in the latter area paralleling the bedding at Stony Creek. These relationships are summarised in Fig. 2d. A style of folding which could produce this situation is illustrated in Fig. 3.

During the major Rangitata (i.e., post-Hokonui) Orogeny (Kingma, 1959: 5), of late Jurassic-early Crecaceous times, much of Otago and Southland was deformed about more or less horizontal north-west axes. This period of deformation, the only one to affect the Mesozoic rocks, gave rise to the Southland Syncline and the broad flat-crested Otago Anticlinorium, It should be emphasised, however, that the former is a structure in bedding and the latter in schistosity. Mutch (1957: 508) implied that the schists and the rocks of the Southland Syncline were essentially conformable and considered that the isoclinal folds at Gore and similar structures near Mossbum could be regarded as regional drag folds on the Otago Anticlinorium, but recent work (Means, 1963) presenting evidence of multiple deformation in the schists, has shown this concept to be an oversimplification. Indeed, Coombs (1954: 93) had previously emphasised the different types of metamorphism in the two suites. From a consideration of this difference, and of Turner’s work showing that deformation of the schists had proceeded in more than one stage, he suggested that the metamorphism of the schists may have occurred long before the Rangitata Orogeny. Subsequently (1959: 58) Coombs has pointed out that it is conceivable that two separate troughs of deposition existed in New Zealand in Lower Mesozoic time. Recent work is adding further support to this concept.

As the isoclinal folds at Gore are parallel and adjacent to the north-west folds of the Southland Syncline, it is probable that they were also produced during the Rangitata Orogeny. Subsequent earth movements in this region have been restricted to gentle warping and block faulting, and therefore it appears that the northerly trend in the Hillend District must predate the Rangitata phase. Means (1963, fig. 4) shows several north- to north-east-trending Li lineations occurring with a north-west trending L2 lineation in south-east Otago a few miles north of the present areas. He notes that in the coastal section south of Brighton and in the Waipori Gorge L 2 postdates the earliest lineation Li. Li, then, may well be related to the parallel northerly trend at Hillend and L 2 to the subsequent Rangitata folding. Means considers that the first phase of deformation of the schists probably gave rise to large recumbent folds, and it has already been indicated (Fig. 3) that recumbent folds are compatible with the evidence at present available for the Tuapeka rocks.

If recumbent folds trending north were refolded about north-west axes, bedding planes alone would not necessarily show any evidence of the earlier phase except near the hinges of the early-formed folds. It is suggested that the Clinton District may include an area of this type, the original subhorizontal northerly axis being rotated about an axis almost perpendicular to it; thus the trend is little altered, but a plunge equivalent to the dip of the axial plane of the later folds is acquired. If this is the case the Gore folds may have inherited their isoclinal character from the earlier folds, the profiles of which now appear as the outcrop pattern due to the flexuring of the earlier axes (Fig. 3).

The proximity of a multiply deformed to a simply deformed suite seems to add further weight to Macpherson’s original suggestion (1946: 11) that the schists have been driven south-west against the rocks of the Southland Syncline, presumably during the Rangitata Orogeny.

Summary of Geological History

Indurated greywackes of the Tuapeka Group are the oldest rocks in the district. The Clinton representatives belong to the prehnite-pumpellyite metagreywacke facies but north of the area the metamorphic grade increases and the rocks pass into the Otago schists. Regional structural considerations indicate that they are in tectonic contact with the younger Waipahi and Arthurton Groups. The latter are characterised by a higher content of volcanic and pyroclastic material, and conglomerates and tuffs become important rock types. Fossils of upper Permian age have been found in rocks of the Arthurton Group. By early Triassic times a vast quantity of pyroclastic debris was being supplied directly to the developing geosyncline and a thick sequence of often fossiliferous volcanic arenites and tuffs accumulated. Subsequent “burial metamorphism” (Coombs, 1961: 214) has led to the formation of large quantities of zeolites in these rocks.

All the pre-Tertiary strata have been folded during the late Jurassic-early Cretaceous Rangitata Orogeny into relatively minor overturned folds on the north-eastern limbs of the great Southland Syncline.

As there is some evidence that earlier recumbent folds existed in the rocks of the Tuapeka Group before the Rangitata deformation the Southland Syncline is considered to involve only rocks younger than those of the Tuapeka Group.

Tertiary coal measures, probably derived from the rising Murihiku escarpment, were deposited on the Paleozoic basement in a shallow tectonic depression. Continued warping gently folded these rocks and strongly influenced the presentday lowland topography, while a trellised drainage pattern developed on the uplifted Triassic block to the south. During the Pleistocene a thick but impersistent blanketing cover of loess was deposited over much of the area.

Acknowledgments

The writer is deeply indebted to Mr and Mrs R. W. Stuart, of Hillfoot Road, Clinton, for their ready hospitality during the course of fieldwork.

Valuable advice, discussion, and assistance in the field by staff and colleagues of the Geology Department, University of Otago, and also by officers of the New Zealand Geological Survey and Geophysics Division, in particular Dr T. Hatherton, Dr J. B. Waterhouse, and Mr B. L. Wood, was greatly appreciated.

The present work is based on a thesis presented for the degree of Master of Science at the University of Otago during 1962, and has been prepared for publication at the Department of Geology and Geophysics, University of Sydney. Professor D. S. Coombs, Dr W. D. Means, and Mr J. D. Campbell of the Geology Department, University of Otago, kindly read the manuscript and made many helpful suggestions.

Field expenses were defrayed by a grant from the Benson Memorial Fund.

PLATES IN POCKET

Plate 3

Geological map of the Clinton district.

Plate 4

Isanomalic overlay map of the vertical magnetic force near Clinton.

Plate 5

Geological cross-sections and equivalent magnetic profiles. Attitudes on or near line of section shown by bar, direction of younger beds indicated by triangle.

References

Amies, A. C., not dated. A description of ? Permian conglomerate pebbles from Clinton, Otago. Unpublished manuscript, Geology Department, University of Otago, Dunedin.

Benson, W. N., 1941. The basic igneous rocks of Eastern Otago and their tectonic environment: Part I, The general tectonic environment. Trans, roy. Soc. N.Z., 71: 208-22.

Bishop, D. G., 1962. The geology of the Clinton District. Unpublished M.Sc. thesis, University of Otago, Dunedin.

Campbell, J. D., 1955. The Oretian Stage of the New Zealand Triassic System. Trans, roy. Soc. N.Z., 82: 1033-47.

Coombs, D. S., 1954. The nature and alteration of some Triassic sediments from Southland, New Zealand. Trans, roy. Soc. N.Z., 82: 65-109. Congress, XIII : 339-51.

Coombs, D. S., Ellis, A. J., Fyfe, W. S., and Taylor, A. M., 1959. The zeolite facies, with comments on the interpretation of hydrothermal syntheses. Geochim. cosmochim. Acta., 17: 53-107.

Couper, R. A., 1960. New Zealand Mesozoic and Cainozoic plant microfossils. N.Z. geol. Surv., Pal. Bull. 32.

Cox, S. H., and McKay, A., 1878. Report on the geology of the Hokonui Ranges, Southland. N.Z. geol. Surv. Rep. geol. Explor., 1877-78 {II): 25-48.

Crook, K. A. W., 1960. Classification of arenites. Amer. J. Sci., 258 : 419-28.

Cullington, A. L., 1954. The geomagnetic field in New Zealand. N.Z. Dep. sci. industr. Res. Geophys. Mem. 2.

Garvie, A., 1859. Report on the reconnaissance survey of the South-eastern districts of the Province of Otago, 1857-58. Otago prov. Gaz. 3 {91): 276-82.

Grindley, G. W., 1958. The geology of the Eglinton Valley, Southland. N.Z. geol. Surv. Bull. n.s. 58.

Harrington, H. J., 1958. Geology of the Kaitangata Coalfield. N.Z. geol. Surv. Bull. n.s. 59.

Hector, J., 1864. Geological Map of the Province of Otago. In Geology Dept., University of Otago, Dunedin.

Hutton, F. W., and Ulrich, G. H. F., 1875. The Geology of Otago. Dunedin: Mills, Dick.

Kingma, J. T., 1959. The tectonic history of New Zealand. N.Z. J. Geol. Geophys., 2: 1-55.

McKay, A., 1877. Reports relative to collections of fossils in the S.E. district of the Province of Otago. N.Z. geol. Surv. Rep. geol. Explor., 1873-74 : 59-73.

Mackie, J. 8., 1935. The Geology of the Glenomaru Survey District, Otago, New Zealand. Trans, roy. Soc. N.Z., 64: 275-302.

Macpherson, E. 0., 1946. An outline of Late Cretaceous and Tertiary diastrophism in New Zealand. N.Z. geol. Surv. Mem. 6.

Marshall, P., 1912. Geology of New Zealand. Wellington: Government Printer. Surv. Bull. n.s. 19.

Marwick, J., 1925. Upper Paleozoic (Permian) fossils at Clinton (with a description of the locality by M. Ongley). N.Z. J. Sci. Tech., 7{6) : 362-4

Means, W. D. In press. Mesoscopic structures and multiple deformation in the Otago schist.

Miyashiro, A., 1957. Cordierite-indialite relations. Amer. J. Sci., 255: 43-62.

Moore, R. C., 1947. Nature and classes of stratigraphic units (Note 2of the American Commission of Stratigraphic Nomenclature). Amer. Ass. Petrol. Geol. Bull. 31 (3): 519-28.

Mutch, A. R., 1957. Facies and thickness of the Upper Paleozoic and Triassic sediments of Southland. Trans, roy. Soc. N.Z., 84: 499-511.

Neumann, F. R., 1927. Origin of the Cretaceous white clays of South Carolina. Econ. Geol., 22: 374-87.

Ongley, M., 1939. The Geology of the Kaitangata-Green Island Subdivision, Eastern and Central Otago Divisions. N.Z. geol. Surv., Bull. n.s. 38.

Packham, G. H., 1954. Sedimentary structures as an important factor in the classification of sandstones. Amer. J. Sci., 252: 466-76.

Pettijohn, F. J., 1957. Sedimentary Rocks. New York: Harper.

Service, H., 1937. An Intrusion of norite and its accompanying contact metamorphism at Bluff, New Zealand. Trans, roy. Soc. N.Z., 67: 185-217.

Speden, I. G., 1961. Sheet SIB 4 Papatowai (Ist Ed.) Geological Map of New Zealand I: 63360. N.Z. Dep. sci. industr. Res., Wellington, New Zealand.

Trechmann, C. T., 1917. The age of the Maitai Series of New Zealand. Geol. Mag., dec. 6, 4: 53-64.

Waterhouse, J. 8., 1958 a. The occurrence of Atomodesma Beyrich in New Zealand. N.Z. J. Geol. Geophys., I: 166-77. Geophys., I: 604-10. phalacea. N.Z. J. Geol. Geophys., 6: 88-109.

Wellman, H. W., 1952. The Permian-Jurassic stratified rocks. Symposium sur les sines de Gondwana (Nouvelle Zelande), XIX Congres geologique international, Algier, 1952. Vegetation and Agriculture of the Eastern Hills, Nelson. Nelson: Gawthron Inst.

Wilson, D. D., 1950. Geology of the Pomahaka Coalfield. N.Z. Dep. sci. industr. Res. Coal Res. Comm. Rep. 273: 1-8.

Wood, B. L., 1956. The geology of the Gore Subdivision. N.Z. geol. Surv., Bull. n.s. 53.

Wylie, P. J., 1959. Microscopic cordierite from fused Torridonian arkose. Am. Min., 44: 1039-46.

D. G. Bishop, N.Z. Geological Survey, P.O. Box 368, Lower Hutt.

* Sheet numbers (e.g., SI70) are used only for grid references lying outside the present area mapped.

* For more detailed petrography see Bishop (1962).

* Numbers of thin sections refer to the catalogue. Geology Department, University of Otago.

* Vitric tuffs and volcanics have been described from the older Waipahi Group by Wood (1956).

NELSON EQ LINTON ARTHUPTON CLINTON' 4 C i « ■D c 0 to > 0 1 , E 0 H AtomodL&Jsma n.&p. 3 tJ c 0 -U (0 V C 0 CO L 0 0 c c < Atom.odLe.smcc. n.sp. 3 Atomodesrnct n. sp. 3 A to modesma. tre.chman.ni A to modesma. trechmannL Wooded Peak Limestone Howden Limestone AG4, Limestone A to modes ma. n. sp. A

Table I.—Distribution of Atomodesma in the lower Kazanian (upper Permian) beds of the South Island, New Zealand.