Otago Land District
Four main geological divisions may be made in the Otago Land District. North-west Otago consists of steeply eroded alpine ranges of Fiordland-type rocks. Central Otago is underlain by peneplaned schists and is a range-and-basin terrain with flat-topped or rolling block-faulted mountain ranges, separated by broad alluviated basins. South-east Otago is low rolling country with elongated north-west – south east fold ridges of Upper Paleozoic and Mesozoic sedimentaries. Eastern and north-eastern Otago comprises rolling hills of Cretaceous and Tertiary and late Tertiary volcanics. Many of the rocks and structures seen in Otago are continuations of similar features in Southland.
The steep ranges of north-west Otago support permanent snowfields and glaciers and are dissected by deep glaciated valleys occupied by freshwater lakes, the largest of which are Lakes Wakatipu, Wanaka, and Hawea. This area consists of Fiordland metamorphic and igneous rocks flanked to the east by late Paleozoic volcanics and sedimentaries of the Southland Syncline. These rocks are well exposed in the Hollyford Valley and in the adjacent Humboldt and Ailsa Mountains. The seaward margin of this mountain region is formed by the still active Alpine Fault. This continues through from Westland to separate the ranges from the south-western extension of schistose greywackes of Paleozoic or Precambrian age (Greenland and Waiuta Groups), with their mid-Tertiary cover rocks, which compose the low-lying coastal strip of Martins Bay — Awarua Bay area. To the east the Southland Syncline rocks are separated from the Otago schists and schistose greywackes by the Livingstone Fault, which branches off the Alpine Fault and has probably infaulted a strip of Upper Jurassic rocks that is known from the Pyke River area.
By far the most extensive area in Otago is underlain by chlorite schists, forming the basement to the east and outcropping over much of Central Otago. They are high stress metamorphic rocks, originally greywackes and argillites of the New Zealand Geosyncline, probably Upper Paleozoic or Triassic in age and attaining a possible thickness of up to 9 miles. During the later stages of metamorphism they were intensely folded on a grand scale into a series of recumbent or semi-recumbent nappe-folds. This gave rise to subhorizontal schistosity and lamination over many hundreds of square miles, and steeply dipping schistosity near the edges of the schist. Metamorphism and folding of the schists is thought to have occurred during the Rangitata Orogeny (late Jurassic — early Cretaceous). Erosion rapidly stripped the overlying strata, exposing the schists towards the end of the Cretaceous; long-continued erosion extending into the Tertiary reduced the land to a peneplain; the covering forests are now preserved as lignites in central Otago. At this time beds of quartzose conglomerate, sand, and white clays were laid down by slow-flowing rivers and in shallow inland lakes. In many of these were concentrated deposits of valuable alluvial gold, originally from quartz veins in the schist.
The numerous flat-topped ranges of central Otago (for example, Dunstan Mountains, Raggedy Range) arose mainly by block faulting in the Kaikoura Orogeny (late Tertiary and Pleistocene). Stripping of the Tertiary covering beds from the Otago peneplain and exposure of the underlying schist led to the formation of the curious tor topography of the schist region. Warm climate conditions produced zones of deep weathering of the schist. These were subsequently more easily removed by soil creep during the glacial stages of the Pleistocene, leaving relatively unweathered residuals standing as tors.
To the north-west the Otago schist continues into Westland where it is cut by the Alpine Fault and, it is thought, displaced north-eastwards into Marl-borough. To the north-east and south-west the schist is flanked by tuffaceous greywackes thrown into large steeply dipping folds close to the schist. The north-eastern greywackes are mainly of Triassic age and continue into Canterbury to form the Southern Alps and foothills. The south-western greywackes are Permian and form the north-eastern limb of the Southland Syncline, continued in a sweeping arc through Southland to reappear in north-west Otago in the Hollyford and Pyke areas before being cut by the Alpine Fault. The Triassic and Jurassic rocks forming the core of the Southland Syncline, and seen in eastern Southland, continue to the south-eastern Otago coast. Particularly good sections are seen along the Catlins coast. The rocks of the Southland Syncline were strongly folded in the Rangitata Orogeny, probably at about the same time as the Otago schists were formed, the region being elevated above sea level and eroded to form hilly land. During this erosional phase talus fans and alluvium filled the valleys, and small areas of such deposits of Cretaceous age are now preserved at Kyeburn and Henley. On the east coast of Otago, quartz sands and conglomerates were deposited with coal beds in a swampy margin of the Cretaceous sea. These now comprise the Kaitangata, Green Island, and Shag Point coalfields.
During the middle and late Tertiary times, inland Otago remained above sea level seediagram 7) and was occupied by shallow lakes and clothed with forest. In eastern Otago mid-Tertiary sandstones, greenstones, and limestone were deposited; in the Oamaru district mudstones, greensands, and bryozoan limestones were interbedded with basaltic flows and volcanic ash layers; in the western Otago area shelly mudstones and limestones were deposited. During the late Tertiary the north-western and eastern coastal districts emerged slowly above sea level while, on the east coast, volcanic eruptions centering on the Dunedin district produced cones, extensive lava flows, and bouldery mud flows. These andesitic and basaltic volcanic rocks form the hills in the Dunedin district and are also scattered throughout eastern and northern Otago.
During the Pleistocene the ranges were heavily glaciated in the west by valley glaciers and in central Otago by high-altitude cirque-glaciers. Around the margins of the glaciated ranges of western Otago rigorous periglacial conditions caused mass-slumping of rock and development of characteristic solifluxion slopes. Throughout central Otago and eastern districts a covering of loess was deposited during cold-climate periods of the Pleistocene. Intermontane valleys of central Otago were filled with auriferous river alluvium and alluvial fans which were later dissected into terraces by the main rivers. Uplift of the ranges appears to have continued during deposition of the early Pleistocene alluvium, and many parts of the main rivers are now incised across rock barriers in deep gorges (for example, the lower Taieri gorge). In the last major glacial period of the Pleistocene valley glaciers occupying the present lake basins terminated at about the southern limits of the lakes and during the earlier stages had advanced only a few miles further down the valleys. At the same time the two major rivers (Clutha and Waitaki) built extensive deltas and plains of alluvium at their mouths on the east coast.
Excluding a region in the north-west, close to the Alpine Fault, Otago is seismically relatively quiet and, as may be seen in diagram 8, only a few recent fault traces are known — some from central Otago and one in the Milton-Taieri area.
Gold was formerly extensively worked in Otago – including both reef gold (for instance, in the Shotover district) and alluvial gold, which occurred almost everywhere, especially in the Arrowtown and Bannockburn areas. Today the most important economic minerals of Otago are mainly non-metallic and include limestone (Oamaru), coking coal (Shag Point), bright coal (Kaitangata, Green Island), sands for foundry and other industrial purposes, and clays for pottery manufacture (Benhar). Oilshale deposits in the Nevis Valley have proved of doubtful economic value.