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THE GEOLOGY OF OTAGO
Otago along with the rest of New Zealand has a great complies geological history and this is reflected in the great variety of interesting rock and mineral types.
Classification of Rocks:
Rocks are grouped according to their mode of origin.
Volcanic Rocks are the result of the outpouring onto land surface of molten rock. Sedimentary rocks result from the accumulation of the products of erosion under water or on land surface. Mud, silt, sand shingle and other materials settle on the sea floor and similar alluvium and loess on land. Intrusive and Metamorphic rocks are formed deep underground and are today visible on the surface where erosion has removed the overlying rock. They are grouped because of similarities.
The classical grouping of rocks was into three groups – Sedimentary, Metamorphic and Igneous. The latter group was divided into volcanic and plutonic, depending on whether they were formed on the surface or deep underground.
With the exception of the far western mountains, most of the basement rock of Otago is schist. It is a coarsely flakey material composed mainly of layers of white quartz and mica. To the north and south the rocks are less metamorphosed and grade out into greywacke, a kind of hard sandstone. This suggests that mica of the Otago or Haast Schist as it is now called, originated in greywacke which was subjected to great pressure and heat through being completely buried for a long period.
This caused the constituent minerals to separate into layers of different composition (foliation) and gave it the ability to split along the layers (schistosity).
The greywacke is believed to have been deposited during Triassic or Upper Permian times about 200 million years ago. The age of the metamorphism of the schist is late Jurassic or Early Cretaceous in the period of 100-140 million years ago. Subsequently crustal movements raised most of Otago and when the overlying rocks were removed the schist was exposed over most of the surface.
The Sedimentary Sequence of the Late Cretaceous – Tertiary Periods
Breccia is composed of sharp angled rock fragments cemented together. The Henley Breccia is probably composed of pieces of the rocks that once covered the Haast schist. When this covering material was removed, pieces of it were trapped in the hollow of the Titri fault angle on the east side of the Taieri Plain. The stone is greywacke, pebble and cobble conglomerate and mudstone with blocks of schist near the top.
The exposed Haast Schist was eroded by the sea which removed the soft mica material and rounded the hard quartz pebbles. The resulting coarse sand was transported by marine action along the coast just as beach sand is today. Where the sand was deposited at the mouths of bays and inlets, backwaters were formed which became swamps where plant material gathered and could eventually become coal. At Brighton and Green Island the Taratu formation includes seams of lignite coal. The sand is used in making mortar for bricklayer, plater and concrete building blocks. The age of the formation is about 90 million years and takes its name from the Taratu Coal mine near Milton where the formation was first recorded.
The Brighton Formation:
This overlies the Taratu coal measures and at Brighton , the type locality consists of about another 15 metres (50ft) of quartz conglomerate, followed by 4 metres (12ft) of coal , another 9 metres (30ft) of quartz conglomerate and 9 metres of shelly limestone. The fossils include ammonites and these and other fossil molluscs fix the formation in the Haumurian stage of the Mata period of the Upper Cretaceous Age or about 85 to 90 million years ago.
The formation consists of sandy mudstone such as is found in the deltas of large rivers. It is dark coloured and often rusty in colour. The thickness varies but reaches more than 295 metres (970ft) in the type locality a few miles West of Dunedin. It extends along the coast from-Brighton to Palmerston. Wherever it reaches the surface it is a trial and tribulation to road engineers because when wet it becomes very plastic and moves easily, sharing this bad reputation for slipping and slumping with Burnside Mudstone. Fossils indicate an age around 80 million years.
Green Island Loose Sand:
Towards the upper part of the Abbotsford Mudstone it becomes increasingly sandy until it merges into Green Island loose Sand. In the type locality and Shag Valley areas the sand may reach a depth of up to 53 metres (175ft) but in the middle of the area it may be absent. The sand is fine to medium grained and not unlike beach sand in appearance but may contain mica. Generally it is mainly silica and may have some economic importance in ceramic or glass industries.
The main deposits are easily recognised being light grey in colour, semi plastic and silty. It is mainly argillite but contains up to 20% lime at the type locality is used for the manufacture of cement. In places it reaches a thickness of 45 metres (150ft). Fossils indicate that the age is mid Eocene or about 50 million years.
Throughout the area the material between Burnside mudstone and the overlying Caversham Sandstone varies in thickness and texture. At the type locality the greensand is loose and granular containing calcite and dark green phosphatic concretions in two bands about 7cm (3 inches) thick.
At Waikouaiti the calcite and green phosphatic grains are lightly cemented together and reach depths of around 30 metres (100ft). Macro fossils include fish teeth and penguin bones including those of the giant penguin. Where the beds are thickest in the East Otago District around Palmerstone and Waikouaiti, investigations have been carried out to determine their value as a source of agricultural phosphate. The age is Oligocene or about 35 million years.
The stone is moderately hard and has been used for building, although not really suitable. It is white or light grey when first broken but weathers to a cream or light yellow colour seen in the high sea-cliffs of Otago. The base is glauconitic or argillaceous and the dominant quartz sand content varies from about 40% to 70% throughout the total depth of up to 228 metres (750ft). Many fossils are present including scallops, oysters, echinoderms, ostracods, rare corals, bryozoa and whales. The age is lower Miocene or upper Oligocene about 30 million years ago.
Along the coast north of Waikouaiti the Caversham sandstone grades into the overlying Goodwood Limestone which in places reaches a thickness of 30 metres (100ft). The formation consists of impure bands of limestone of 30 metres (100ft). The formation consists of impure bands of limestone up to a metre in thickness separated by mudstone layers of varying thickness but usually narrower. The fossils indicate that the bed formed early in the Miocene age or about 25 million years ago.
Dowling Bay Limestone:
The stone is found at Dowling Bay between Port Chalmers and the Otago heads and also at Hooper’s Inlet and at Sandymount as well as about 32 kilometres (20miles) further out to sea. The limestone is greyish and impure and contains bands of sandy mudstone at both Dowling Bay and Waipuna Bay. The fossils it contains show it to be similar in age to Goodwood Limestone.
This ends the Sedimentary rocks exposed along the Otago Coast in the Dunedin vicinity. It must be remembered that out at sea the process is still going on and the fossils being formed there are those of modern animals and plants.
Following the formation of the last of these sedimentary rocks a change in sea level took place as the land lifted, perhaps as a preliminary to the volcanic activity which soon followed.
The Dunedin Volcanic Complex:
Like most other volcanic areas the Dunedin complex had many vents and volcanic activity went on intermittently over a period of many millions of years.
The volcanic activity has been divided into three main stages. During the first stage outflows of trachytes from the Port Chalmers – Portobello vents covered an area about 6miles in diameter. This was followed by outflows of basalt and phonolite which spread for at least 24kilometres (15miles) from the vents. At the close of the first stage explosive eruptions blew out vents in the earlier flows where the harbour is now.
The second stage saw flows spreading widely from the Portobello eruptive centre. Many spread 29 kilometres (18miles) from the vents and new vents became active in the outlying hills. During this period Roslyn dolerite and Leith Valley andesite covered about 129 sq kilometres (50 sq miles) west of Dunedin while Waitati and Logans Point phonolites spread to the north for many miles. Outlying vents or flows were active in the Waikouaiti area.
The third main stage was centred on vents in the Mount Cargill area which spread basalts and phonolites over what is now Flagstaff ridge to the west of the City. Rocks from this flow have been dated by the potassium-argon method and show an age of about 1 million years.
Since then the movement in the earth’s crust have caused the lifting of the hills in the flagstaff ridge, the Peninsula hills and Silver Peaks Down folding has caused the valley to flood and become the harbour. At the same time the old volcano landscape has faulted and eroded until it is now very difficult for anyone other than a train geologist to trace its history.