Paterson (1942). The general geographical relation between the quartz porphyry of Shag Valley and the coastal districts of Otago is shown in the writer's map (Benson, 1941, Plate 36), and in particular its geological relation to the immediately adjacent rock-formations is displayed on Figure 1 herewith, which is based on Paterson's (1942, Plate 7) map and section as modified by the present writer. McKay showed that two major faults run south-eastwards near the foot of the Kakanui Mountains or Horse Range, and considered that the quartz porphyry was injected into the more westerly of these two fissures. Paterson recognised that the quartz porphyry ran between the two faults mapped by McKay and himself, but did not notice that it occupied there a third fault-fissure, as is shown on Section YY in our Figure I, since no attempt was made by him to map separate units in the “Coal Measures” of his stratigraphical classification. Nor was this done by the present writer, the relations being readily described in general terms.

As will be seen from Figure I, the basement formations are schists to the south-west of the Cretaceous and Tertiary formations which occupy the lower portions of the Shag Valley, and less metamorphosed greywackes, semi-schists and phyllites, rising from beneath these covering formations to form the Horse Range. The sequence of Cretaceous and Tertiary formations in the lowland regions may be briefly described thus: A basal series of Upper Cretaceous (Upper Senonian) coal-measure conglomerates, etc., about 1200 ft. thick occurs on the north-eastern side of the valley, but is only about 100 ft. thick on the south-western side of the valley. Outliers of the basal portions of these coal-measures extend south-westward up the slopes of the schist, and north-eastward up those of the greywacke, etc., rocks forming the Horse Range. The stratigraphic subdivisions of these Upper Cretaceous rocks are distinctive and, being significant for our inquiry, are detailed below. Conformably on the Coal Measures there rest the “Katiki Beach” Sandstone and above the more or less glauconitic mudstones which together form the “Abbotsford” beds of Lower Eocene age and 600–900 feet thick. Upon them follow with apparent conformity the local equivalent of the “Green Island” Loose Sandstone about 120 feet thick, finer in grain-size and much more argillaceous than that of the type area near Dunedin, but containing the same characteristic assemblage of heavy minerals (Paterson, 1942, pp. 41, 44). Above this again is the Upper Eocene (Tahuian) “Burnside” Mudstone. All these formations occur within two miles of the porphyry intrusion. Elsewhere, chiefly south of the Shag River, the “Burnside” Mudstone is followed disconformably by the Lower Miocene “Caversham” Sandstone and overlying “Goodwood” Limestone, with which we are but little concerned. In compiling this generalised summary use was made of the observations of Brown (1938), Paterson (1942) and the recent age-determinations kindly communicated by Finlay, and by Finlay cited by Benson (1943).

The Upper Cretaceous (Senonian) Coal-Measure conglomerate in the neighbourhood of the quartz porphyry is about 1200 feet thick. As shown by McKay and Paterson its lowest portion (A) is a very massive irregular aggregate of more or less angular fragments several inches long of greywacke, semi-schist and schist, of phyllite and quartz pebbles derived from fault-scarps of the pre-Cretaceous formations which rose into relief during the strong early Cretaceous (?) crust-movements, and were deposited more or less as semi-talus or fan-conglomerate. A vivid and unconventionally humorous description of this portion of the Coal-Measures was given by McKay (1892). It is several hundred feet thick and passes up into (B) more or less friable quartz sands containing thin beds of quartz pebbles. Following on these and forming the upper portion of the Coal-Measure Conglomerates are: (C) a limonitic coarse-grained pebbly sandstone with abundant plant-fragments, (D) haematitic quartz conglomerate. (E) carbonaceous shales and coal-seams followed locally by (F) limonitic sandstone with rare Upper Senonian marine mollusca. It is not here asserted that the order C, D, E is the true sequence in the upper portion of the coal-measures near the porphyry, the essential point for our present purpose being that, whatever be the actual inter-relationship, the beds (C–F) all lie above (A) and (B).

The narrow belt between the quartz porphyry and the greywackes, etc., of the Horse Range is a syncline bounded on each side by N.W.–S.E. faults and pitching south-eastwards. Its northern extremity consists wholly of the semi-talus (A). Where the belt is crossed by the Old North Road (Section YY, Figure 1), the centre of this belt is occupied by the quartz sands (B) which here lie nearly horizontally in the trough of the syncline, and further to the south-east are covered by the beds (C), (D), (E), and (F), and these by the Lower Eocene beds, according to the information available in the literature. The north-eastern boundary of the quartz porphyry intrusion abuts against formation A, which dips north-eastward at the contact, except at one point, namely, in the pine-covered ridge 250 yards north of the road, where is exposed, though badly, a narrow (10–20 yards?) strip of deeply-weathered high-grade schist [Chl 3 on Turner's (1938) classification]. This strip of schist is bounded on the east by a fault-breccia composed of schist-fragments, north-east again of which is the downthrown, steeply dipping semi-talus.

Though the schist in this small inlier adjacent to the porphyry is not of quite so high a grade of metamorphism as that (Chl 4) which characterises the Central Otago schist exposed south-west of the Shag Valley in the area illustrated by Figure 1, the distinction is not great. There is no evidence available to show whether there is a gradual transition from the Chl 4 to the Chl 3 grade beneath the Cretaceous-Tertiary sediments in the Shag Valley, or whether here, as in the Waipiata district thirty miles to the north-north-west, a block of Chl 3 schist was faulted down into the basement of Chl 4, before the Cretaceous peneplanation which truncated both (Turner, 1939, p. 40, and Figure 10). Since the quartz porphyry rising through the Chl 3 schist contains abundant xenoliths of rocks displaying lower grades of metamorphism (Chl 1–2) such as may occur in the deeper portions

of the Kakanui Series which forms the Horse Range, it is evident that the rising magma came into contact with such low-grade material, and hence probably followed the major pre-Cretaceous fault which brought the two very different grades [Otago (Chl 4–3) and Kakanui (Chl 2–1)] of material into apposition, though the intrusion of the magma must have occurred in post-Cretaceous times and was probably accompanied by a revival of differential crust-movement along this member of a frequently rejuvenated series of faults. That the porphyry should have diverged obliquely from the major fault, to traverse the higher grade schist only in the upper part of its course, may perhaps be the result of the opening of a branch fault-fissure during this rejuvenation of fault-movement. (See Figure 1.)

Immediately south-west of the porphyry is a second belt of Cretaceous rocks, only about a hundred yards wide where it crosses the roadway. It consists of nearly horizontal or gently dipping limonitic pebbly sandstone with plant fragments (C) by the roadway and haematitic quartz conglomerate (D) further north. The porphyry thus occupies a fault-fissure between Cretaceous formations. It is difficult to see any clear evidence of the inclination of this fault, but from various hints it appears probable that it is a very steep reversed fault, with a westerly downthrow of varying amount, generally several hundred feet.

A similar qualitative description might be given of the western member of the pair of faults mapped by McKay and Paterson. It runs parallel to the western boundary of the porphyry and about a hundred yards from it where it crosses the roadway. On the western side of this fault, fortunately exposed by a recent clearing of the roadside cutting, is a white, finely-granular, richly argillaceous sandstone spangled with muscovite, macroscopically identical with the Shag Valley facies of the Green Island Loose Sandstone which was recognised by Paterson, who did not, however, note the occurrence of this rock in the immediate vicinity of the porphyry. The boundary of an area of this formation which he recognised less than half a mile from the porphyry has therefore been extended in our Figure 1 to indicate its probable continuity with the newly discovered occurrence. The well-grassed slopes of the areas underlain by Tertiary sediments rarely afford the opportunity for the exact mapping of their boundaries. Though Paterson found this Shag Valley argillaceous equivalent of the Green Island sandstone contained, though but very sparsely, the heavy minerals, notably andalusite and kyanite, characteristic of the Green Island sandstone near Dunedin, Dr. Turner was unable to recognise their presence in the specimen collected by the present writer from near the porphyry. So far no fossils have been recorded from the Green Island Sandstone. It is therefore of interest to note that Dr. H. J. Finlay recognised in the writer's specimen numerous siliceous micro-organisms, especially radiolaria and several of an as yet undescribed species of the foraminifer Bolivinopsis which he has found in the Palaeocene-Lower Eocene Abbotsford beds from the Katiki formation upwards in the coastal region between the Shag Valley and Hampden, and in coeval formations and in the Te Uri district of the North Island, but which does not extend into the Mid

or Upper Eocene Bortonian and Tahuian formations. Unless the outcrop by the roadway is of a facies of sediment within the Abbotsford Mudstone elsewhere unknown, it seems appropriate for the present to regard it as the equivalent of the “Green Island” Loose Sandstone of the Dunedin district, into which the Abbotsford Mudstone passes upwards by gradual transition, and to be of latest Lower Eocene age.

If this be correct, it will have an interesting bearing on the geology of the Dunedin district in that it will indicate the absence there of the Middle Eocene Bortonian formations, except as may be represented by the thick richly or purely glauconitic beds at the base of the “Burnside” mudstone, which may have accumulated there during a period of partial cessation of deposition of clastic sediments in the Dunedin district while the Bortonian sediments were accumulating north of the Shag Valley. Further, the throw of the westernmost fault in the Shag Valley will be therefore of the order of 800 feet, also with a westerly downthrow.

McKay followed by Paterson, mapped the quartz porphyry as a strip of almost uniform width (50–60 yards) extending from about 250 yards south of the roadway to about 2000 yards north thereof. The present writer found, however, that the intrusion was more nearly lenticular, reaching its greatest thickness of about 130–140 yards in the gully and spur-ridge 400–500 yards north of the roadway, but thinning out northwards, and could not be seen more than about 1000 yards from the road in this direction, though the continuation of the fault-fissure which it occupied was traceable by the contact of the Lower Coal Measure semi-talus (A) with the Upper Coal Measure haematitic quartz conglomerate (D) for at least a mile beyond the northernmost exposure of the porphyry.

It has not been possible to observe the contact of the porphyry with the formations which it invades. In several places, however, and especially in the spur-ridge 500 yards north of the roadway, the pale green ground-mass of the porphyry contains, though sparsely, small joint-bounded flaky xenoliths of dark grey micaceous phyllite comparable with the finer-grained or argillitic bands in the greywackes and more especially semi-schists of the Kakanui rocks of the Horse Range. These xenoliths may be as much as 8 mm. long, but range down to those of microscopic dimensions, the minute xenoliths being very abundant. These xenolithic flakes are more or less parallel to one another, their common direction being a plane of laminar flow in the enclosing porphyry, which is clearly marked by the orientation of the phenocrystic plates of biotite, and to a less extent by the more or less linear parallelism of the longer axes of the phenocrystic plagioclase-prisms, and by the streaks in the more or less glassy groundmass. This plane of laminar flow strikes about N.W.–S.E., i.e., parallel to the elongation of the intrusive mass of porphyry, but the northward-directed dips vary from about 20°–80°, are difficult if not impossible to measure by ordinary means, and the data obtained therefrom are insufficient to form an adequate basis for discussion of their structural significance. Nevertheless, they afford a suggestion that the intrusion as a whole forms a sheet which dips rather steeply

to the north-east, which accords with the view that it was injected into a steeply-dipping reversed fault-fissure under the influence of dominant pressure from the north-east. Indication of the effect of such pressure deforming the nearly rigid cooling mass is described below.

In considering the age of the Shag Valley quartz porphyry it will be best to commence with an outline of the geological history of the Shag Valley. It is immaterial for our purpose to consider whether the Otago Schists (Chl 4–3) are the most strongly altered and deeper portions of the same huge complex of Palaeozoic to Early Mesozoic sediments as yielded the Kakanui semi-schists, etc. (Chl 2–1) in their less deeply buried members during one period of metamorphism, or alternatively whether two sedimentary series are involved, with an intense pre-Mesozoic orogeny causing the high grade of metamorphism of the Otago Schists derived from Palaeozoic sediments, and a much less powerful orogeny at the close of Jurassic times changing a largely Lower Mesozoic series of sediments into the Kakanui formations, and affecting to a minor degree the previously strongly metamorphosed Otago Schists. The point is discussed by Turner (1939, pp. 42–3), the balance of probability being thought by him to lie with the latter hypothesis. Be that as it may, it is clear that very strong block-faulting occurred along the Shag Valley fault-zone and its north-westward continuation into the northern portion of the Maniototo region, before the peneplanation of Early-Middle Cretaceous times which produced a surface of low relief, which was, as it were, a large-scale mosaic of planed fault-blocks wherein rocks of very varying grades of metamorphism were brought into apposition, and the dominant fault-fissures were ranged in a zone running N.W.–S.E. Rejuvenation of the movement of these fault-blocks in Middle Cretaceous times produced fault-scarps, during the rapid destruction of which the semi-talus of the Lower Coal Measures was laid down north of the present Shag River, and the more normal clastic sediments of the Middle and Upper portions of the Coal Measures transgressed beyond the fault-angle depression in which they formed to extend far over the Cretaceous peneplain on either side of it, chiefly as terrestrial deposits, but containing in their highest portions Upper Senonian marine sediments. Deposition of the Palaeocene-Lower Eocene “Abbotsford” marine formations, with the “Katiki” beds at their base, and the “Green Island” Loose Sandstone at their top and after a period of non-deposition, the formation of the Upper Eocene “Burnside” beds followed in the Shag Valley, and southwards thence to Dunedin, but there may be traced northwards from Moeraki the Middle Eocene “Bortonian” mudstones laid down during this period unrepresented by clastic sediments south of the Shag Valley. Again in Lower Oligocene times there was a marked difference between the geographic conditions on either side of the Shag Valley. To the south of it sediments of this age are practically absent, and the Upper Oligocene “Caversham” Sandstone is separated from the Upper Eocene “Burnside” Mudstone (on which it rests disconformably) by a thin bed of glauconite only. North of the valley, however, there is a very extensive record of Oligocene forma-

tions. Submarine eruptions breaking out in the later portion of Eocene times (Benson, 1943, p. 130) formed of the thick basic Waiarekan tuffs, with intercalated more normal marine sediments, and a diatomaceous band, were overlain (near Oamaru) by the Otataran limestone, through which the basalt magma rose to form the pillow lavas and breccias of the Deborah group, more or less simultaneously with an extensive development of sills and dykes of dolerite invading formations in and beneath the Ototaran limestone. On the cessation of eruptive activity the Lower Oligocee “Kakanui” Limestone was laid down near Oamaru, and, with minor disconformities and lacunae, late Oligocene sediments were laid thereon. None of these formations with an aggregate thickness of the order of 1000 feet, are represented south of the Shag River and the facies of the Upper Oligocene (“Hutchinsonian” greensand) and the overlying “Awamoan” beds near Oamaru are very different from those of the coeval “Caversham” Sandstone and “Goodwood” Limestones immediately south of the Shag River, affording palaeogeographic evidence of noteworthy crust-movements probably along the Shag Valley fault-zone between Eocene and Miocene times. It is in this fault-zone that the intrusion of quartz porphyry occurs. If this intrusion was genetically related to the Mid-Early Upper Oligoeene activity of basaltic magma beneath north-eastern Otago, crustal movement in the Shag Valley fault-zone must have been vigorous, possibly more so than would suffice to explain the palaeogeographic distinction between the regions north and south thereof in Late Oligocene-Early Miocene times deducible from stratigraphical considerations only.

Further crust-movements occurred in later Miocene-Early Pliocene times, both folding and faulting occurring throughout Eastern and North-Eastern Otago, and especially the Shag Valley district (see Benson, 1941, Figure 2). The resulting differential relief then produced was obliterated by peneplanation during the remainder of Pliocene times, and in particular the quartz porphyry, which is not now associated with any topographic features such as fault-scarps which may have formed at the time of its intrusion, was probably exposed by the removal of the enclosing Cretaceous formations during this second peneplanation. But if the quartz porphyry magma rose into its present position during the Late Miocene deformation it must have had but a slight relation if any with the basic magma which was active beneath North-Eastern Otago in Oligocene times, unless the activity of the latter were maintained or renewed (though without overt expression) until this later period, which is perhaps not beyond the bounds of possibility. The occurrence of rhyolites, etc., resting on the eroded surfaces of Mesozoic-Lower Miocene formations and overlain by the Pliocene-Pleistocene (?) basalts of Banks Peninsula may be recalled (Speight, 1935), though these rhyolites occur in an environment with a geological history very different from that of the Shag Valley. Nevertheless, on both sides of this valley the Pliocene peneplain was covered by effusions of basaltic lavas of Late Pliocene-Pleistocene (?) age, developed before the Pleistocene foldings and faulting which again rejuvenated the Shag Valley fault-zone