Otago and Southland
Macpherson's views on the Otago and Southland districts have already been briefly outlined, and it is interesting to consider his views on late Cretaceous and Tertiary diastrophism in relation to earlier fold trends in that region.
It has long been known that the north-west trend of the Otago schists and the Hokonui syncline is of ancient origin. Cotton (1917, pp. 429–430) early recognised pre-Cretaceous faulting at the Shag Valley. Benson (1941, p. 216) summarises the position as regards the early history of the Shag Valley fault-zone, which strikes north-west and reaches the east coast some thirty miles north of Dunedin, as follows: “The fact that it has brought into apposition the Palaeozoic (?) Otago or Maniototo schist on the south-west with the Early Mesozoic (?) greywackes, etc., on the north-east, which were reduced
to a common peneplain level by the earlier Cretaceous erosion, indicates movement with a southerly upthrow during the closing Jurassic orogeny. Paterson (1941) pointed out that the semi-talus character of the oldest Upper Cretaceous rocks near Shag Point indicates the existence of a rejuvenated fault-scarp on the north-east side of this fault-zone.” Late Cretaceous and Tertiary marine sediments were later deposited across this ancient fault-zone. “Renewed movement, this time with a northerly upthrow, occurred in mid-Tertiary times,” and after the upthrown greywacke and the marine Tertiary sediments had been reduced to a common peneplain level, a “flow of basalt flooded over this peneplain crossing the (by then) obliterated fault trace …”
This north-west trend can be followed a considerable distance south and west from the Shag Valley Fault into the Otago schists and into the great Hokonui syncline of Triassic and Jurassic rocks. That the trend is in the first place of ancient origin is demonstrable in many places; the lineation of the Otago schists, both throughout Central and Eastern Otago as well as in the Shag Valley, as Turner and Paterson have shown by petrofabric analysis, is between north-west and north-north-west. The main metamorphism of the Otago schists has been reasonably placed by Turner (pp. 75, 189, 190, 1940) as late Palaeozoic or early Triassic, but he does consider it possible that part of the metamorphism may have been produced during the great post-Hokonui Orogeny. There is a distinct difference in the degree of metamorphism shown by the schists and by all rocks of known Triassic age, so that if we were to assume (an assumption not necessarily valid) the periods of deposition of the sediments now appearing as Otago schists to be approximately coeval, it may be suggested as most likely that metamorphism preceded the deposition of the Clinton conglomerate of Permian or even Carboniferous age (Ongley, 1939).
Mackie (1936), by a detailed study of bedding planes, schistosity planes, and joints within the schists and greywackes of Northern Otago, also demonstrates a north-west strike in that region.
Both the Otago schists and the Trias-Jura greywackes showing the north-west strike are in many places unconformably overlain by late Cretaceous beds showing a marked north-east strike. To the north of the Shag Valley the strike of the Cretaceous and Tertiary beds appears to persist as approximately north-west for some considerable distance through North Otago and South Canterbury; but south of Dunedin the strike in Cretaceous and Tertiary beds becomes very markedly north-east to north-north-east and this strike continues along the coastal belt to Kaitangata. Benson (1941) describes this region, summarising the work of others and giving as well much new structural detail. Turner (1940, p. 189) notes the tilting of the schists across a general north-east to north-north-east strike and observes that it has not affected the quartz fabrics of the Otago schists except, possibly, in the immediate vicinity of late Tertiary faults.
Cotton (1917) and Benson (1935) drew attention to widespread remnants of the great Cretaceous peneplain carved on the Otago schists, where is has been buried and exhumed. The plateau so formed is broken in many places so that Cretaceous and/or Tertiary strata are found in fault-angle depressions. In some places as at the Shag Valley already cited, it is evident that much faulting of the peneplain preceded as
well as followed the deposition of these later sediments; but at other places the faulting and folding may be entirely late Tertiary in date. Along the eastern fringe of Otago the peneplain surface carved on the schist shows a general south-easterly dip, in places interrupted by north-east-striking faults, and the schists dip under Cretaceous and Tertiary sediments which also strike north-east. The lowest of these beds, of upper Cretaceous age (at Kaitangata), consist largely of coarse conglomerates with interstratified sands, and thick seams of sub-bituminous coal; they are evidently terrestrial, but are only locally developed and are succeeded in some places (e.g. Kaitangata) conformably, in others unconformably, by a widespread thick series of quartz conglomerates containing rare marine fossils. These strata represent the denudation of an immediately adjacent land mass consisting almost entirely of the Otago schists, which are rich in quartz layers. Ongley's mapping for the Kaitangata bulletin, supported by detailed study of the coalfield (Lillie and Jenkins, unpublished report), shows that a major anticline follows the coast, with a core of schist well exposed in places. The thickest concentration of Cretaceous beds appears to lie between this anticline and the gently-south-east-dipping peneplain carved on the Otago schists to the east of a broad topographic depression which continues from Kaitangata almost to Dunedin. This depression, elongated along a north-east trend, is essentially a faulted syncline, and it formed the chief area of deposition in Cretaceous and Tertiary times. Benson remarks that “the present depression … is parallel to and at most a few miles west of the mid-Cretaceous faultbounded depression. Once again, the tectonic character of eastern Otago seems remarkably persistent”. Throughout his paper he points out evidence of repeated faulting and folding such as that already cited at the Shag Valley.
Benson (1935, 1941) has recognised also a late Tertiary peneplain which he considers, and Cotton admits (1938), may be widespread, though evidence may yet be forthcoming that very considerable areas of the interior plateau of Otago are formed by the exhumed fossil surface (of pre-Miocene, possibly Cretaceous, age) mentioned earlier instead of by the late-Tertiary peneplain (Raeside). In the Dunedin district Benson traces the faulted portions of this late Tertiary peneplain by means of covering lava flows. The surface, also with a gentle south-eastward dip, truncates the Cretaceous fossil peneplain generally at a slight angle, and may have been originally continuous with the peneplain described by Wellman and Willett as extending over the Southern Alps east of the Alpine Fault.
A further feature of Benson's paper (1941), whose main object is a detailed description of the igneous rocks, is the separation of eastern Otago into three tectonic districts according to the amount of deformation suffered. The least deformed district, occupying the eastern part of the field, is almost devoid of late Tertiary igneous rocks. The moderately deformed district which roughly follows the line of the great depression already described, contains many igneous rocks, chiefly of normal basaltic composition, and also many alkaline basic rocks, but the variety is not as great as in the strongly deformed central district around Dunedin. In this last, the alkaline rocks, earlier described by Marshall, occupy an important place and the rock types vary through numerous varieties from very basic olivine basalts to
phonolites, probably including also hybrid rocks. There are also many agglomerates and tuffs, giving evidence that this district constituted the main centre of volcanicity coincident with the maximum tectonic disturbance.
Benson's conception of the regional tectonics is “a series of broad but broken asymmetric folds of late or post-Tertiary age, in which many of the steeper limbs have been more or less replaced by faults,” as was shown by Cotton (1917, b, p. 252), in other parts of Otago. Benson notes that “usually the western limbs of the anticlines have the gentler slopes and provide the stripped and partially dissected back-slopes of broken anticlines or fault-blocks, the eastern and steeper limbs passing into fault scarps.” In the southern portion of the field, at Kaitangata, however, he remarks that the coastal anticline shows a steeper limb dipping to the west. Possibly this feature results from a depression of the basin to the west caused by the infilling load of Cretaceous conglomerates. The Tapanui Range also presents its steep face towards the west (Cotton, 1948, Otago's Physiography, pl. II) at the base of which a meridional strip of Tertiary beds is preserved in a syncline or fault-angle depression of the Otago plateau surface.
One of the most peculiar tectonic accidents in New Zealand has been re-examined by C. O. Hutton (1939). On the shores of Lake Wakatipu, at Bob's Cove, is an overturned sequence of Tertiary strata, 1,450 feet thick, forming, according to Hutton's interpretation, an upfold and a downfold, and flanked by the Otago schists. “Extending from the edge of Lake Wakatipu, for approximately 22 miles in a direction slightly east of north, is a narrow strip of the Tertiary sediments, never more than 150 feet thick, which have been caught in and thrust under the schists, by, it is believed, an easterly directed overthrust” (p. 73). The schist on each side of this Moonlight Thrust fault dips west at approximately 60°. Hutton's view that these sediments are compressed in a great thrust fault seems the only reasonable explanation. If the sequence at Bob's Cove is really inverted, as Hutton shows, then the upfold and downfold, which appear to have strong south-westerly pitch, represent respectively a syncline and an anticline, both upside down. The most logical conclusion to be drawn from Hutton's map is that the Tertiary beds at Bob's Cove represent only the inverted sedimentary cover of the western mass of schist. The Tertiary strata appear to be involved in schist, possibly to a depth of 3,000 feet or more (p. 85), and since the overpush appears to be directed eastwards it may be an important tectonic feature whose full significance in the regional tectonic pattern has been rather neglected.