A guide to the Geology of King IslandClive Calver

Contents - Part 1

Introduction....................................................................................................................................................2Geological Time Scale............................................................................................................................4Acknowledgements.................................................................................................................................4

Part 1: Geological history...........................................................................................................................6The Surprise Bay Formation: sediments in an ocean basin, 1300 m.y. ago...........................................6

How old, and how do we know?.........................................................................................................7Which way up are the beds?................................................................................................................8

Deep burial and metamorphism under a mountain belt, 1270 m.y. ago.................................................9The supercontinent Rodinia...............................................................................................................10

Dykes and sills in the Surprise Bay Formation.....................................................................................11The Fraser Formation – much the same, but less deformed.................................................................12

Amphibolite.......................................................................................................................................12The west coast granites: igneous intrusions, 750 m.y. ago..................................................................13The lower Grassy Group: ‘Snowball Earth’ (Cryogenian, 650- 635 m.y.)...........................................14

The Cottons Breccia..........................................................................................................................16The ‘Snowball Earth’ hypothesis......................................................................................................18Meltdown of Snowball Earth - when did it happen?.........................................................................19The cap carbonate: aftermath of Snowball Earth..............................................................................20The Yarra Creek Shale: prelude to volcanism...................................................................................20

The upper Grassy Group: vast volcanic outpourings (Ediacaran, 570 m.y.)........................................21Cambrian earth movements (510 m.y.).................................................................................................23The Sandblow Granite: a bonanza of tungsten from the Carboniferous (351 m.y.).............................23

Formation of the scheelite ore bodies at Grassy and Bold Head.......................................................23Mining History..................................................................................................................................24Other minerals associated with the Carboniferous granites..............................................................24

Late Carboniferous – Cretaceous: continental assembly and breakup (350 – 100 m.y.)......................25Volcanic activity, Cretaceous-Paleogene (90 – 60 m.y. ago)...............................................................26

Lamprophyre dykes...........................................................................................................................26Volcanic pipes of basalt.....................................................................................................................26

Uplift and erosion, Miocene limestone (20 m.y.).................................................................................27The Quaternary Period: movements of sea and sand (2.6 m.y. – present)............................................28

Peat, and giant wombats....................................................................................................................29The ‘calcified forest’ and tufa terraces..............................................................................................29Naracoopa heavy mineral sands........................................................................................................30Ironstone............................................................................................................................................31

Geophysics and remote sensing............................................................................................................32Further reading......................................................................................................................................34

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IntroductionThis guide is divided into two sections. The first is an account of the geology, written in the order in which King Island formed, starting with the oldest rocks and working towards the most recent events. Scientific jargon is kept to a minimum. Some technical terms (in bold where first used) are explained in a glossary at the end.

The second section is an excursion guide, outlining seven excursions that can be undertaken as half- to one-day trips by car and on foot. The interested visitor will be able to view for themselves some of the evidence for the island’s unique geological history.

Amongst the features of interest that you will see are:

The oldest rocks of the southeast Australian region: sediments deposited on an ancient sea floor some 1.3 billion years ago;

Metamorphic rocks, formed when some of these sediments were subjected to great heat and pressure deep within the crust;

Glacial deposits from Earth’s most severe ice age (‘Snowball Earth’) 636 million years ago;

Lava flows and breccias from ancient volcanic eruptions, exceptionally well preserved in coastal exposures;

Skarns – mineralogically interesting rocks that host Australia’s largest tungsten deposit, that formed when a large mass of granite intruded into older, calcareous rocks, 351 m. y. ago.

There have been notable advances in the last 10 years or so in our understanding of King Island’s geology, that make it an opportune time to write such a guide as this. Important radiometric dating work has been carried out by researchers at the University of Tasmania, and the federal and state government geoscience bodies (Geoscience Australia and Mineral Resources Tasmania (MRT), respectively). About half the island has recently been geologically mapped at 1:25,000 scale by MRT geologists C.R. Calver and J.L. Everard. A geological map of the whole island at a less detailed scale is shown at Fig. 1.

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Fig. 1: Geological map of King Island. Red numbers show areas covered by excursions (Part 2).

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Fig. 2: West-east geological cross section of King Island, from just offshore Ettrick Bay to City of Melbourne Bay. For key to colours, see Fig. 1. Major folds and faults shown. The cross section is drawn by extrapolating from surface geology. The two large subsurface granite bodies are conjectural, and inferred from weak variations in the gravity field.

Geological Time ScaleFig. 3 shows the geological timescale, and the major events in Australian and King Island geological history.

Note that the oldest rocks on King Island are only about a third of the age of the Earth, which itself is only about a third of the age of the Universe. Nevertheless these are the oldest rocks known in the southeast Australian region (east of approximately Adelaide).

Geological time is referred to in two ways. Firstly, by the geological period (Cambrian, Jurassic, etc.). These periods (and finer subdivisions not shown on Fig. 3) were worked out mainly by 19th century European and North American geologists, by looking at the succession of fossils in sedimentary sequences. The second way is by absolute (numerical) time, generally given as m.y. (millions of years) or Ma (mega-anna) before the present. This only became possible in the 20th century with the development of radiometric dating. Fossils are almost non-existent in Precambrian (i.e. Archaean and Proterozoic) rocks, so in these old rocks (older than 545 m.y.) we tend to use numerical time, even if only imprecisely known.

AcknowledgementsDonald Graham provided the impetus for this guide to be written, and advised on many aspects. Fig. 3 is adapted from a figure kindly provided by Keith Corbett. J. Everard helped with aspects of the west coast geology.

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Fig. 3: The geological timescale, with a simplified history of King Island, surrounding areas, and life on Earth. The left-most time scale divides the history of the Earth into a single 24-hour day, to give some idea of the immensity of geological (particularly Precambrian) time.

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Part 1: Geological history

The Surprise Bay Formation: sediments in an ocean basin, 1300 m.y. agoThe oldest rocks on King Island make up a belt of schist (a metamorphic rock type rich in mica, easily split into thin slabs with glossy surfaces) and quartzite, about 4 – 8 km wide, that extends all the way from Stokes Point in the south to Cape Wickham in the north, parallel to the west coast (grey on the geological map). Geologists tend to name rock formations after the place where they are best exposed, and this one is called the Surprise Bay Formation. It is also well exposed on the west coast around Fitzmaurice Bay - Millers Bay, near Currie, and at Cape Wickham. Inland, it is nearly everywhere covered up by sand (pale yellow on the map), or by thick, pale grey silty clay soil. The soils developed on Surprise Bay Formation tend to have a sparkly look caused by the abundant mica in the schist, which survives breakdown of the rock to form particles in the soil.

The distinct layering in these rocks (Fig. 4) is sedimentary bedding in rocks that were laid down on the sea floor 1300 m.y. ago. The steep dip (tilt) of the bedding is due to the fact that the originally flat-lying rocks have been tilted up on edge and folded (see below) some time after they were deposited.

Fig. 4: Bedding in the Surprise Bay Formation, consisting of alternating paler quartzite layers and darker schist layers. The originally horizontal bedding is tilted steeply to the west here (Surprise Bay).

The layering (alternating quartzite and schist) shows that this was originally a sedimentary rock, that accumulated as layers or beds (originally horizontal) on the floor of a sea or lake. A closer look shows other evidence of a sedimentary origin. The dark schist beds contain thin layers of silt, some with the distinctive cross-sections of sea-floor ripples produced by gentle currents (Fig. 5). The schist beds were originally mud that accumulated on the sea floor, later changed (metamorphosed) by heat and pressure to mica (muscovite and biotite). The more prominent, lighter-coloured quartzite beds in the Surprise Bay Formation (Fig. 4)

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started out as beds of fine-grained sand, and microscopic examination shows them to be made up of sand grains (~ ¼ mm in size) of mostly quartz. The sedimentary beds in the Surprise Bay Formation lack the telltale features found in lakes or shallow seas; rather it appears the sediments were deposited in deep seawater (at least 200 m), beyond the reach of tides or storms. The quartzite (sandstone) beds were deposited by a series of pulses of suspended sediment known as turbidity currents. Such beds are sometimes referred to as turbidites. Mapping out the whole formation at Surprise Bay shows it to be at least 4 kilometres thick: it was evidently deposited in a deep, continually and slowly subsiding, marine basin, probably much larger in extent than King Island.

At the time these sediments were deposited (about 1300 million years ago, see below), the Earth was a quite different planet from today. It was the middle of the Proterozoic eon. Only microscopic (single-celled) organisms existed in the oceans, and there was no life on land. The oxygen content of the atmosphere was only a fraction of today’s. The continents were in a different configuration (Fig. 8), and it is uncertain even whether the area we now know as King Island was in the same position, relative to Tasmania and mainland Australia, as it is today.

How old, and how do we know?Many of King Island’s rocks have been dated radiometrically. Certain radioactive elements, such as uranium, decay at a known (very slow) rate into so-called daughter elements (such as lead), so by measuring the (usually very tiny) amounts of uranium and lead in a mineral, its age of formation can be calculated. (In the case of uranium, the story is more complicated than this, because there are two different forms, or isotopes, of uranium that decay at different rates). Only a few minerals have enough uranium that they can be dated this way. The one most often used is zircon (zirconium silicate, ZrSiO4). Zircon makes up a small proportion of many igneous rocks, where it formed by crystallising out of the molten magma. The crystal structure of zircon is hospitable to uranium (but not to lead), so the zircon may start off with a relatively high content (~0.1%) of uranium, but next to no lead (which means that any lead subsequently found in the zircon must have got there by decay of the uranium). Zircon is extremely hard, durable and resistant to high temperatures, so all in all it is the ideal ‘geological clock’ or geochronometer. Zircon grains are usually very small (~0.1 mm): rare large crystals, clear, yellow or brown (not known on King Island) are often used as gems.

Modern dating methods, generally using an expensive laboratory device known as a Sensitive High Resolution Ion Micro-Probe (or ‘SHRIMP’, first developed at the Australian National University, Canberra, in the 1980’s), can date individual very tiny zircon grains, or portions of grains, less than 1/20 mm across.

As well as being found in igneous rocks, zircon is also found as tiny grains in most sedimentary and metamorphic rocks. Dating the zircons in, say, a sandstone, however, won’t tell you the age of the rock (i.e. when the sand was deposited). It will tell you the ages of the (often much older) igneous rocks that were eroded to produce the sand (and zircon) grains making up the sandstone. And some of the grains might be derived from older sedimentary rocks, themselves containing grains from still older, igneous rocks. There will generally be a range of ages, but all of the sedimentary (detrital) zircon grains will be older than the sandstone in question.

Sixty-four zircons from a sample of Surprise Bay Formation from Fitzmaurice Bay were dated using a SHRIMP. There was a range of ages going back to 3400 million years (three-quarters of the age of the Earth). The youngest grains were about 1350 million years old, so the Surprise Bay Formation must have been deposited some time after that.

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The other piece of evidence that pins down the age of the Surprise Bay Formation comes from dating of monazite grains in several samples of the formation in the south of the island. Monazite (cerium phosphate), like zircon, contains a useful proportion of uranium, as well as thorium (another radioactive element), but unlike most zircon, monazite forms within sedimentary rocks during metamorphism: that is, once the rocks have become sufficiently deeply buried that high temperature and pressure start to change their mineralogical makeup.

The monazite grains that formed during metamorphism of the Surprise Bay Formation have been determined to be about 1270 m.y. old. So the rock was deposited some time between 1350 m.y. (the age of the youngest zircons) and 1270 m.y., and by 1270 m.y. the Surprise Bay Formation had become deeply buried, and was being subjected to high temperatures and pressures.

Which way up are the beds?The geologist looking at sedimentary rocks that have been steeply tilted by earth movements in the distant geological past needs some way of telling which way was originally up, because it is not uncommon for the beds, originally horizontal, to be tilted more than 90 degrees and become overturned. There are a couple of different types of sedimentary structure that can be seen here and there in the Surprise Bay Formation that reveal the “way-upness” of the beds. Cross-lamination has formed where a thin bed containing gently inclined laminations (typically formed by migrating ripples on the sea floor) has been slightly eroded and overlain by another bed, truncating the inclined laminations. The second bed must be younger (originally on top), as it truncates the earlier laminations (Fig. 5). The other type of sedimentary structure is graded bedding, which can be seen in many of the beds deposited by turbidity currents. When a turbidity current drops its load of sediment on the sea floor, the coarser sand grains are deposited first, followed by progressively finer sand, silt, then mud. The resulting ‘graded beds’ can be recognised here and there (Fig. 6), and give the original way-up of the bed.

Fig. 5: The pale siltstone layers in the middle of the photo are cross-laminated, and were created by gentle currents forming ripples on the sea floor. The very thin, inclined laminations are truncated by horizontal layers above, telling us that the beds (in this case) are the right way up.

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Fig. 6: Dark grey mudstone of the Surprise Bay Formation, with a graded sandstone bed in the lower half of the photo. Note the sharp base and gradational top of the pale sandstone layer.

In the Surprise Bay Formation, the bedding nearly everywhere dips steeply to the west. In some areas, such as the coast from Fitzmaurice Bay to Millers Bay, the beds are right-way-up, that is, they get younger to the west. In other areas, such as around Surprise Bay and up the Ettrick River, they are overturned and get younger to the east. Mapping out these areas enables us to discern huge, kilometre-scale folds in these rocks (Fig. 2, and below).

Deep burial and metamorphism under a mountain belt, 1270 m.y. agoThe layering (bedding) in the Surprise Bay Formation dips almost vertically into the ground, because the rocks were tightly folded by huge compressive forces in the crust about 1270 m.y. ago. The convergence of tectonic plates is the main process causing rocks to be compressed into folds, just like the wrinkles in a table cloth pushed across a table top. The process takes millions of years and is often accompanied by metamorphism. The folding happens in the solid state, by ductile flow; and the rocks end up being deformed as if they were putty or plastic (Fig. 7).

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Fig. 7: Small-scale folds in the Surprise Bay Formation.

Mountain belts form where tectonic plates are being pushed together, such as is happening today at the Alps or the Himalayas. Australia is currently tectonically stable, with no active mountain building or major fault systems. Tectonic activity (including earthquakes and volcanism) occurs mainly at the boundaries of the large plates that make up the outer part of the Earth.

Mapping of the Surprise Bay Formation has shown huge (kilometre-scale) folds that define the overall structure of the rocks in the south of the island (Fig. 2). The dramatic nature of these huge folds gives a sense of the magnitude of the tectonic forces involved in their creation.

The Surprise Bay Formation became deeply buried as the deforming crust thickened under lateral compression. Heat and pressure have brought about the growth of metamorphic minerals: biotite, muscovite, garnet and andalusite. Metamorphism does not involve melting, but rather recrystallization in the solid state, something like what happens to powdery new snow as it is buried and becomes crunchy. These particular minerals show that the metamorphic changes occurred at depths of between 5 and 10 km in the Earth’s crust. The fact that these rocks are now visible at the Earth’s surface shows they have been uplifted and deeply eroded over time.

The supercontinent RodiniaKing Island must have been only a small part of the mountain belt that formed by converging plates 1270 m.y. ago. Can rocks be found that were deformed at the same time, on nearby landmasses, that might represent a continuation of this mountain belt? On mainland Tasmania, the oldest known rocks are younger than this event. All the rocks in Victoria are much younger. We have to go as far afield as central Australia and Antarctica to find rocks that were deformed at about this time, that might have been part of the same mountain belt.

In the Precambrian, the continents were not arranged as they are today. In the period between about 1200 and 750 million years ago, Australia was part of a hypothetical supercontinent named Rodinia. Antarctica was joined to Australia, and ‘Laurentia’ – most of what is now North America – lay off to the east (Fig. 8). The position of Tasmania, and King Island, in this assemblage is uncertain, but they probably lay

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somewhere between Australia and Laurentia. The configuration of ancient continents is worked out from the geological similarity of rocks of the same age, in continents now widely separated, in jigsaw puzzle fashion. Some rocks also preserve the orientation of the Earth’s magnetic field at the time they were formed, and this paleomagnetic evidence can also be used to work out past changes in the positions of continents.

On Fig. 8, the darker bands show the distribution of rocks that were all deformed and metamorphosed around 1300 – 1100 m. y. ago, the same age as the deformation that affected the Surprise Bay Formation. At that time, these would have been mountain belts formed by the coming together of the various continental blocks (shown in pale grey) that make up Rodinia. Once formed, this supercontinent lasted until it started to break up about 750 m.y. ago. The positions of these mountain belts have been used to suggest that King Island (and perhaps the rest of Tasmania) were situated either near central Australia, or near Antarctica, when they were part of Rodinia.

Fig. 8: Rodinia around 750 million years ago. The black belts represent 1300 – 1100 m.y. old mountain belts formed during the assembly of Rodinia. Note Laurentia (= present day North America and Greenland) adjacent to Australia. Eastern Australia had not yet formed. The modern coastlines shown merely indicate the position of continents: actual coastlines 750 m. y. ago would have been quite different. Tasmania and King Island are not shown, mainly because their position at that time was very uncertain.

Dykes and sills in the Surprise Bay FormationIn many places, the Surprise Bay Formation is intruded by narrow igneous intrusions that either lie parallel to the bedding (they are sills) or they cut across it (dykes) (Fig. dyke). Generally a few metres wide, these intruded as magma along fractures, sourced from deep within the crust or upper mantle, at various times after the Surprise Bay Formation was deposited. Most of them are composed of dolerite, a fine-grained, dark or greenish-coloured rock. These dolerite intrusions cannot be dated radiometrically as they do not contain zircon or any other mineral that can be dated with current technology. Some of them must be younger than 750 million years, as they also cut across the granite of that age (see below).

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The Fraser Formation – much the same, but less deformedThe Precambrian mudstone and siltstone that make up most of eastern King Island are almost entirely covered by sand north of Naracoopa, but they can be seen on the coast at Naracoopa and Seal Point, and crop out in Grassy River, Yarra Creek and other creeks. They make up most of the Pegarah Plateau, where deep weathering has produced a thick, pale grey silty clay soil. These rocks have been called the Fraser Formation, after Fraser Beach at Naracoopa where they are well exposed. The Fraser Formation is only gently deformed - the folds are quite open (Fig. 2), and most of it is also less metamorphosed than the Surprise Bay Formation. Because of this, for many years it was thought that the Fraser Formation was younger than the Surprise Bay Formation. However, recent work has highlighted many similarities between the two formations. We now suspect the formations are one and the same, and that the Fraser Formation may have been at the margins of the mountain belt, or at a shallower level in the crust, and so escaped the worst effects of the deformation and metamorphism suffered by the Surprise Bay Formation. This hypothesis has yet to be confirmed by radiometric dating. The two formations adjoin along a major fault, the Pearshape Fault, which runs down the middle of the island (Fig. 1; Fig. 2).

Like the Surprise Bay Formation, the Fraser Formation was deposited as great thicknesses of mud, silt and fine sand on the sea floor, and much of it has the characteristics of turbidites, deposited in quite deep water (Fig. 9). Parts of the Fraser Formation in the west have been metamorphosed to the extent that they contain sparse, small garnets – these areas are shown on Fig. 1 as ‘metasediments’.

Fig. 9: Siltstone (paler) and mudstone (darker) of the Fraser Formation, at Seal Point. The letters indicate the parts of a single graded bed (turbidite), which is repeated on the right.

AmphiboliteSome areas of an unusual, metamorphosed igneous rock, known as amphibolite, are found within the Fraser Formation (green on Fig. 1). The two biggest areas occur north of Pegarah Road. The weathered surfaces are yellowish brown, but fresh (unweathered) rock is greenish black. It mainly consists of interlocking crystals of dark amphibole about 10 mm in size, with a pearly lustre. These rocks are large intrusions of coarse-grained dolerite that have been metamorphosed so that their original minerals have been turned into

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amphibole. Weathering and erosion of these intrusions is thought to have produced much of the ilmenite and rutile in the heavy mineral sands deposit at Naracoopa.

The west coast granites: igneous intrusions, 750 m.y. agoAfter the mountain belt phase of ~1270 m.y. ago, there is a long interval of time, over 500 million years, about which little is known, because there are no known (dated) rocks of this age range on King Island. The schists of the Surprise Bay Formation evidently stayed buried deep (several km) in the crust, because at about 750 million years ago, they were intruded by two large masses of granite – a phenomenon that must have taken place at depth. These two masses are known as the Loorana Granite and the Cape Wickham Granite (Fig. 1). The Loorana Granite forms a belt about 5 km wide extending down the west coast from Whistler Point to British Admiral Beach, with outliers at New Year Island/Christmas Island and Cataraqui Point. It is about 748 m.y. old, from dating of its zircons. The Cape Wickham Granite, on the north coast east of Cape Wickham, is mostly covered by sand. It is about 760 m.y. old, although there is an overlapping ‘margin of error’ on these age determinations, which means the two granites could actually be the same age.

Granites crystallize from large bodies of magma that slowly cool, deep in the crust. Slow cooling explains the large size of the crystals that make up the rock. The slow cooling of solidifying granite intrusions mean that they are sufficiently coarse-grained that their minerals can be identified by the unaided eye: in the case of the Loorana and Cape Wickham granites, they are typical in consisting of feldspar (whitish, often oblong crystals a few cm long), quartz (smaller, pale grey, slightly translucent grains) and a smaller amount of biotite (shiny black, platy or flake-like crystals) (Fig. 10). Also typical of most granites, these are rather uniform rocks, quite massive (not layered), looking pretty much the same wherever you encounter them.

Fig. 10: The Loorana Granite, near Porkys Beach. The pale oblong crystals are feldspar.

What caused the granites to be intruded at this time? Granites are a common component of continental crust. They generally form in the late stages of an orogeny (mountain-building event). Compressional deformation means that continental crust becomes much thicker than normal under a mountain belt, and rocks (mainly sedimentary) that become deeply buried in the crust become heated and metamorphosed (like

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the Surprise Bay Formation), and eventually approach their melting point. Surprisingly, there is still a small amount of water remaining in the rocks, locked in microscopic pores; this acts as a flux and helps to turn large volumes of the lower crust into a hot viscous mass of crystals and fluid. This magma is sufficiently buoyant to slowly rise through the crust like a large mass of rising dough, pushing aside the rocks above, a process that takes millions of years. Granites usually solidify before reaching the surface, as large blob-shaped intrusions several kilometres wide, at several kilometres depth. This type of magmatic activity is quite different to that of most volcanoes, in which the magmas generally have a lower water and silica content than granitic magmas, are hotter, and are consequently much less viscous (‘runnier’) and able to rapidly move up through the crust to erupt at the surface. Granites inherit their high silica and water contents from their sedimentary precursor rocks.

Although most granites are intruded during orogenies, the Cape Wickham and Loorana granites arrived 500 million years too late to be part of the orogeny that occurred 1270 m.y. ago. Some geologists have proposed that these granites do mark an orogeny on King Island 750 m.y. ago, that they have named the ‘Wickham Orogeny’, but there is little evidence of it other than the existence of the granites themselves. There is evidence for mild earth movements on mainland Tasmania at about this time, but nothing that resembles an orogeny in the usual sense. The reason for the existence of these granites remains something of a mystery. No granites of this age are known in neighbouring areas, with the possible exception of a small intrusion 777 m.y. old in north-west Tasmania.

Granites have a strong effect on the rocks they intrude. Heat from the large volume of magma (~900 degrees C) gives rise to a wide zone of thermal, or contact, metamorphism in the surrounding rocks. The contact metamorphic zone adjacent to the Cape Wickham Granite can be seen in the coastal outcrops between Victoria Cove and Cape Wickham (Excursion 5). Here the Surprise Bay Formation, already metamorphosed and turned to schist in the earlier (1270 Ma) phase of widespread ‘regional’ metamorphism, has been heated again to high temperatures at the time of granite intrusion.

The lower Grassy Group: ‘Snowball Earth’ (Cryogenian, 650- 635 m.y.)After the two large granite bodies were intruded at about 750 m.y. ago, another blank page intervenes in the geologic history of the island, as no rocks are known that are aged between about 750 m.y. and 650 m.y. The rocks making up present-day King Island, which were quite deep within the crust at 750 Ma when the granites were intruded, were, however, during this long interregnum, being uplifted and gradually exhumed by erosion. Finally, at about 650 m.y., the erosion ceased, and an entirely new sequence of rocks began to be deposited – the Grassy Group (Fig. 11; coloured blue and green on Fig. 1). (A succession of formations that succeed one another in layer cake fashion is known geologically as a “Group”). The ancient, eroded land surface is preserved as the contact between the Fraser Formation and the Grassy Group in southeast King Island. This contact is an unconformity in geological parlance: where the tilted and eroded layers of the older rocks are truncated and overlain by a much younger series of rocks. One spot you can see this is in Yarra Creek (Fig. 12). The unconformity represents the surface of the land about 650 million years ago.

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Fig. 11: Diagram showing the succession of formations in the Grassy Group, and their approximate thicknesses. The ‘Grimes Intrusive Suite’ is an igneous intrusion that cuts across the lower three formations.

Beginning at about 650 m.y., the land surface (and underlying crust) began to slowly subside, making room for the accumulation of the sedimentary and volcanic rocks, three kilometres or more in total thickness, that we see along the southeast coast of the island. These rocks later became tilted to the east and south-east, and the story they tell can be deciphered from the extensive coastal outcrops between Barrier Creek and Bold Head. It is a story, 60- 80 m. y. long, punctuated by one of the most severe ice ages ever to befall the planet (~636 m.y.), and numerous episodes of volcanic activity. At the start of this story, life was still restricted to microbial forms living in the sea, that left virtually no visible trace. But soon after the ice retreated, fossils of strange marine life-forms appear in several places around the world, apparently unrelated to anything alive today. This assemblage of curious sea-floor dwelling creatures is known as the ‘Ediacara fauna’, after the locality in the Flinders Ranges where they were first found. None have yet been found on King Island.

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On King Island, the first layer to be deposited on the unconformity was conglomerate, just a few metres thick, with rounded pebbles and cobbles derived from the underlying Fraser Formation (Fig. 12). This must have been deposited from vigorously moving water, and could be the deposit of a river or stony beach. The next beds to be deposited were of mud, now thinly laminated brown or black, flaky shale. The conglomerate and shale are about 80 m thick altogether and are called the Robbins Creek Formation (Fig. 11). This formation contains a few thin (30 cm) basaltic lava flows and layers of volcanic breccia –modest forerunners of much more vigorous volcanic activity later on.

Fig. 12: The unconformity between mudstone of the Fraser Formation below, and the Robbins Creek Formation of the Grassy Group above, is at the level of the hammer handle. The basal bed of the Robins Creek Formation is conglomerate - note the large oval rounded cobble at the bottom right.

The Cottons Breccia The second formation in the Grassy Group is a distinctive layer, around 150 m thick, named the ‘Cottons Breccia’ by geologist J.B. Jago, who first named and described the formation in detail. Breccia is a loan word from Italian, meaning a rock composed of angular (broken) fragments of rock cemented together by a fine-grained matrix. Diamictite is the term now more usually used for these rocks, and means a sedimentary rock with clasts of all sizes, ranging from boulders to clay. The Cottons Breccia contains angular and rounded fragments suspended in a greyish or reddish, fine-grained mudstone matrix, like sultanas in a Christmas cake (Fig. 13).

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Fig. 13: Diamictite of the Cottons Breccia. Most of the clasts in this outcrop are carbonate rocktypes.

The unusual ‘plum-pudding’ texture of the Cottons Breccia shows that the boulders, sand and mud that made them were not subjected to the sorting action of moving water while they were being deposited. The largest boulders are 3 m wide – water alone is unlikely to transport boulders of this size. Rocks such as these are usually explained in one of two ways: a landslide (‘mass-flow’) deposit, or a glacial deposit. After some debate in the scientific literature, there is now a consensus that the Cottons Breccia was mostly deposited by ice. In places, glacial ‘dropstones’ can be seen – one of the hallmarks of glacial deposition. Dropstones are clasts that have been dropped from melting icebergs into laminated sediments that accumulated at the bottom of a body of water (Fig. 14). These are not features that could be produced by a mass-flow.

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Fig. 14: Dropstones in thinly laminated mudstone, above massive pale grey diamictite. Cottons Breccia, at Cottons Beach.

Another notable feature is the variety in the composition of the clasts (fragments) themselves. Many of them are carbonate rocktypes – limestone (calcium carbonate) or dolostone (calcium-magnesium carbonate). There are other rocktypes, including siltstone and mudstone identical to the Fraser Formation. The carbonate clasts, usually white or grey, can often be distinguished by the fact that they weather faster than the matrix, and are recessive, or leave holes, on weathered outcrop surfaces (Fig. 13). Where these carbonate fragments came from is a mystery, for there are no carbonates amongst the older rocks on King Island. They appear to have been transported from an unknown, more distant source area. Probably the ice flowed off a large landmass that lay to the west of King Island (Antarctica was still joined to Australia at that time).

Glacial deposits are unusual in the geological record, and tend to be restricted to certain time intervals (‘ice ages’) – (the most recent one, affecting the middle to high latitudes of the planet, ended only 12,000 years ago, and still holds sway in Greenland and Antarctica). Evidence for the nature and timing of the ice age represented by the Cottons Breccia comes, not so much from the Breccia itself, but from the formation that overlies it, the Cumberland Creek Dolostone.

The ‘Snowball Earth’ hypothesisFor over a century, geologists have known of widespread glacial deposits in late Precambrian strata – and their presence on every continent hinted at an unusually severe ice age. (Over 100 years ago, the young Douglas Mawson began work on the late Precambrian glacial deposits of South Australia, arousing a curiosity about glacial processes that led directly to his famous Antarctic explorations). It was also noted that most of these Precambrian glacial deposits are overlain by a relatively thin (a few metres) layer of carbonate, usually dolostone, with some unusual and distinctive features. These so-called cap carbonates were difficult to explain, because carbonates, in contrast to glacial deposits, are usually the product of warm climatic conditions. Until recently, it has also not been clear just how far the ice extended into low latitudes, given that the distribution of continents has changed through time. In the 1980’s, careful paleomagnetic studies of one of the South Australian glacial deposits – the Elatina Formation – showed that it was

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deposited within 15 degrees of the equator. Moreover, telltale sedimentary features showed deposition at the margins of the sea – this was no mere isolated highland glacier. It seemed that here was final proof of an ice age that covered all (or nearly all) of the Earth with ice.

However, climatologists had long recoiled from the idea of sea-level ice sheets in tropical latitudes. Such a scenario raised the spectre of a ‘Snowball Earth’ – a white, frozen planet, whose reflectivity would be so high that its frozen state would be permanent. All their calculations showed that a white, ice-covered planet would reflect nearly all the sun’s energy and stay frozen forever. Given that the planet is not a ‘snowball’ today, the climatologists surmised that ice sheets near the equator at any time in the past were impossible, or at least very difficult to explain. In 1992, a way out of this quandary was suggested. On an ice-covered planet, volcanoes would have been constantly emitting carbon dioxide, just as they do today. On an ice-covered planet this flux of CO2 into the atmosphere would not have been counteracted by weathering, photosynthesis and dissolution in the ocean - the processes that normally keep atmospheric CO2 in equilibrium. Somewhere between 4 and 12 million years (it was later calculated) would be enough time for the amount of volcanic CO2 in the atmosphere to accumulate to such a high level that the greenhouse effect would be sufficient to break the grip of the snowball glaciation maintained by the high reflectivity of the global ice cover. Melting of the ice, once started, would accelerate rapidly as increasing ice-free areas of land and ocean would absorb an ever-growing proportion of the sun’s energy. A large rise in sea level would have accompanied the melting of the ice.

For a few million years after the melting of the ice, a hot, greenhouse world would have existed, because the high levels of atmospheric CO2 would only slowly be absorbed by the oceans and continental weathering. Rain, unusually corrosive because of its high dissolved CO2 content, falling on the newly exposed continents, would have led to high rates of chemical weathering, and a lot of dissolved calcium and carbonate would have been washed into the oceans. This material was deposited to form the cap carbonate that nearly everywhere overlies the ‘Snowball’ glacial rocks.

This, in a nutshell, is how the Snowball Earth hypothesis explains the widespread distribution of late Precambrian glacial deposits, their occurrence near the (then) equator, and the presence of the cap carbonates. However, despite 15 years of intensive research, the hypothesis is still controversial and has not been universally accepted.

Meltdown of Snowball Earth - when did it happen?The Snowball Earth hypothesis predicts that the meltdown happened quickly and at much the same time all over the Earth. So the stratigraphic contact between the glacial rocks (such as the Cottons Breccia) and cap carbonates should be the same age, everywhere. In 2004, the international geological community reached agreement that this contact would henceforth be the basis of sub-dividing geological time: it would be the boundary between rocks deposited in the Cryogenian Period (below the contact) and those of the Ediacaran Period, above (these were the first ‘new’ geological Periods to be formally defined since the 19th century). The Ediacaran Period lasted until the beginning of the Cambrian Period, 543 m.y. ago. To forestall any argument about the relative ages of glacial formations on different continents, the geological community decided that the actual definition point for the beginning of the Ediacaran Period would be the top of the glacial formation at a certain spot in the Flinders Ranges, South Australia. At first no-one was sure exactly how old this contact (and hence the end of the Cryogenian Period and the beginning of the Ediacaran Period) was. There are no datable igneous rocks, such as lava flows or volcanic ash deposits, in the sedimentary sequence in the Flinders Ranges. In 2004-2005, thin volcanic ash layers were dated in a glacial deposit in Namibia and a cap carbonate in China, that pointed to an age of 635 m.y. for the end of the glaciation in those places.

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King Island is the only other place in the world where the meltdown of Snowball Earth has been dated. Recent work indicates an age of 636 m.y. for the top of the Cottons Breccia, within the margin of error of the dates in Namibia and China. The dating is consistent with the Snowball Earth prediction of synchronous global meltdown. This date was obtained from zircons in a bed at the top of the Cottons Breccia at ‘the Gut’ (see Excursion 2).

The cap carbonate: aftermath of Snowball EarthGood exposures of the cap carbonate can be seen at a few places on the east coast (Fig. 15). It directly overlies the top of the Cottons Breccia. It is a pale yellowish or pinkish, laminated (thinly layered) fine-grained dolostone (similar to limestone but with added magnesium). Higher up in the formation, there are layers of pale grey limestone and shale. The whole formation is less than 10 m thick. It looks identical to cap carbonates, thought to be the same age, all over the world. Beside their appearance, there are other subtle, chemical similarities. It is this distinctive nature of the cap carbonates that appears to be the key to recognising the “Snowball Earth” deposits all over the world. They were deposited on the continental shelves under rising sea-levels in the aftermath of the melting of the ice.

Fig. 15: Dark reddish diamictite of the Cottons Breccia, overlain by pale yellowish dolostone of the cap carbonate, near the mouth of Grimes Creek on the east coast of King Island.

The Yarra Creek Shale: prelude to volcanismThe formation that overlies the cap carbonate tells us the next episode of the story. This is the Yarra Creek Shale, a pale yellow-brown or reddish, shale (or flaky mudstone). It was deposited by the slow accumulation of mud on the sea floor, in deep water, over a lengthy period of time (about 60 million years). Towards the end of this time, the crust came under tension, and began to fracture. Earthquakes occurred and faults broke through to the sea floor as different blocks of the crust moved relative to one another. Magma welled up towards the surface along some of these fractures. Some of it reached the newly deposited Yarra Creek Shale and spread out horizontally within the sediment as a sill, without actually erupting on the sea floor. A distinctive formation called the Grimes Intrusive Suite was formed in this way (Fig. 16). It contains zircons (unlike the other volcanic rocks of the Grassy Group), and so we know its age fairly

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accurately: 575 m.y. This gives us a good idea of the age of the tectonic activity and of the beginning of the major phase of volcanism that was to follow.

Fig. 16: A small sill of Grimes Intrusive Suite, between the dashed lines, intruding into the Yarra Creek Shale, just north of City of Melbourne Bay. The abrupt change in the thickness of the sill in the middle of the photo has fractured the overlying rocks (F); the same process has also occurred at a much larger (map) scale.

The upper Grassy Group: vast volcanic outpourings (Ediacaran, 570 m.y.)Rifting (pulling apart) of the crust is often accompanied by eruption of large amounts of basaltic lava (think of Iceland today), and King Island in the late Ediacaran Period was no exception. Basalts of similar age are found in northwest Tasmania, western Victoria and western NSW, along the line of what seems to have been the eastern margin of the Australian continent, as it was then. This may have been the final episode in the break-up of the supercontinent Rodinia. This line of rifting may signify that a continental fragment rifted away to the east, but if so, we are uncertain where that fragment is today, if indeed it still exists. In any case, King Island is the best place to see these late Ediacaran volcanic rocks, with superb coastal exposures, some looking as if they were only erupted yesterday.

The volcanic rocks overlie the Yarra Creek Shale (they lie to its east in these east-dipping exposures). In places such as City of Melbourne Bay, the first of the lavas violently erupted through, and onto, soft water-saturated sediment on the sea floor. Molten lava encountering seawater instantly froze and shattered into angular fragments to form volcanic breccias (Fig. 17). Higher up in the sequence (further east) there are great thicknesses of pillow lava (Fig. 18). These also form when hot magma enters the seawater, but in this case its surface instantly cools to form a skin; continued magma pressure expands the pillow much like a balloon, until the thickening skin prevents further expansion and the magma breaks out to form another pillow. Many of the lava flows lack pillows, and are entirely massive. Many of them are also vesicular: they contain gas bubbles, mostly now filled with white calcite (calcium carbonate) or quartz. Vesicular basalts reflect the fact that most volcanic eruptions contain significant amounts of gas (usually CO2 and steam) that exsolves like the bubbles in a fizzy drink when the pressure is released on eruption.

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Fig. 17: Volcanic breccia, consisting of angular chunks of pale brown basalt, set in greenish volcanic ash. This is the basal part of the formation known as the Shower Droplet Volcanics.

Fig. 18: Pillow lava, City of Melbourne Bay – the City of Melbourne Volcanics.

These volcanic lavas, interspersed with lesser amounts of volcanic breccia, ash and conglomerate, form a sequence on King Island that is at least 2 km thick. You can walk through most of this sequence along the coast between City of Melbourne Bay and Bold Point. Three volcanic formations have been recognised (Fig. 11). The middle one (Shower Droplet Volcanics) consists of an unusual variety of basalt, especially dense and rich in magnesium, known as picrite. In spite of all this activity, there were probably no high conical volcanoes like today’s Mt Fuji or Mt Vesuvius. The lavas are of a composition that meant that they had a low viscosity: that is, they were free-flowing and tended to flow sideways without building up large

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edifices. And most of them seem to have been erupted under the sea, so the great thickness we see today must have been accommodated by slow subsidence of the sea floor over time.

Cambrian earth movements (510 m.y.)Some time after the late Ediacaran rifting and volcanic activity, in the middle Cambrian, about 510 million years ago, King Island was affected by another phase of compressional tectonic activity – a mountain-building event that also affected mainland Tasmania, where it is called the Tyennan Orogeny, and western Victoria and South Australia, where it is known as the Delamerian Orogeny. King Island, together with the northwest corner of Tasmania, was not much affected by this event. On King Island there was some gentle folding and deformation of the Grassy Group and the older rocks. The main result that we see today is the easterly dip (tilt) of the layering in the Grassy Group, along the east coast.

The Sandblow Granite: a bonanza of tungsten from the Carboniferous (351 m.y.)Two roughly circular masses of granite, each about 5 km wide, intruded into eastern King Island in the early Carboniferous period, about 351 m.y. ago. One, the Mt Counsel Granite, just inland from Nine Mile Beach, is mostly covered by sand. The other, the Sandblow Granite, is well exposed along the coast at Grassy Harbour and south as far as Red Hut Point. There is a third, much smaller area of granite inland of Bold Head, which appears to be part of the Sandblow Granite that has been split off the main body and moved north by the Grassy River Fault (Fig. 1).

Granites of similar age, or a little older, are widespread in Tasmania and Victoria, where many of them are associated with important tin and gold deposits.

The Sandblow Granite was associated with the formation of major scheelite (tungsten) orebodies at Grassy and Bold Head. Scheelite (calcium tungstate, CaWO4) is one of the two main ore minerals for tungsten, a strong, dense metal with a high melting point, used for electric filaments and armour-piercing ammunition, as well as hard tungsten carbide machine tools. It is an inconspicuous translucent yellowish mineral, with the unusual property that it fluoresces bright blue under ultra-violet light, making it easy to spot in the host rock on a moonless night (or if you are down a mine).

Formation of the scheelite ore bodies at Grassy and Bold HeadIn the final stages of the crystallisation of granite magma, superheated water is released, and percolates through microscopic pores or migrates along cracks, outward into the surrounding rocks. At such high temperatures and pressures, the water carries a high concentration of dissolved minerals, often including many of the rarer elements such as tin, tungsten, molybdenum and gold. The dissolved atoms of these elements do not readily fit into the crystal structure of the common granite minerals such as feldspar, so they become concentrated in the residual fluids in the late stages of granite solidification, through a kind of natural distillation process. Thus many of the world’s orebodies of these rare elements are associated with granite margins. But a number of factors have to be just right for an economic mineral deposit to form.

One essential factor was that the Sandblow Granite was of a particular chemical type of granite that favoured tungsten, and to a much lesser extent, molybdenum, mineralisation. Another factor was the presence of carbonate-rich beds (limestone, dolomite) in the rocks that the granite intruded near Grassy and Bold Head. These beds are none other than the carbonate-rich breccia and dolostone deposited during ‘Snowball Earth’ and its aftermath (Cottons Breccia and cap carbonate, see above). The hot fluids given off by the granite reacted with the carbonates, precipitating scheelite and other minerals. Only 0.3 to 1 % of the rock needs to be scheelite for it to be an economically minable ore. Another important factor in the

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formation of the Grassy orebody was the presence of a number of fractures, or faults, which became pathways for the migration of the hot mineral-rich fluids out of the granite and into the carbonate-rich layers. These faults included the Grassy River Fault, and a number of north-west trending faults branching off it.

The hot fluids, laden with dissolved minerals, reacted with the carbonates and replaced most of the original calcite and dolomite with a new set of distinctive and unusual minerals, creating a type of metamorphic rock known as skarn. The skarn at the Grassy mine is a colourful mix of grass-green epidote, reddish black garnet, and dark green diopside, while some of the calcite and dolomite transforms into sugary white marble. Less than 1% is the translucent yellowish scheelite.

All of these factors (the right type of granite, the presence of carbonate beds, the fractures) came together at Grassy to make one of Australia’s largest tungsten deposits.

Mining HistoryProspector Tom Farrell first discovered scheelite on the shore of Grassy Bay, in 1904, near the eastern end of what is now the abandoned open-cut mine.

Mining took place from 1917-1920 and 1938-1990, with development of the open-cut mine on the ‘No. 1 orebody’ between 1942 and 1974. The mine was operated by King Island Scheelite (1947) Ltd until purchased by Geopeko Ltd (a subsidiary of Peko-Wallsend Ltd) in 1969. As the open-cut deepened, the miners found that the grades of the ore improved at depth in an easterly direction (under the sea), following the trend of the dipping carbonate (skarn) layers, and the faults or fluid migration pathways. Waste rock from the open cut was used to reclaim land in Grassy Bay, from where drilling was undertaken to prove up the Dolphin orebody seaward of the original shoreline. In 1973, the underground Dolphin Mine was opened on a subsea portal within the open-cut. The Bold Head deposit, 3 km north, was discovered from drilling on a soil geochemical anomaly in 1968, and development commenced in 1972. The ore from both mines was milled and put through a gravity separation and flotation plant on the coast at Grassy. The scheelite concentrate was initially shipped from Currie, and after 1972, from the upgraded port of Grassy. The mines employed up to 300 people in the early 1970s. The population of Grassy peaked at 767 in 1971. Production from the open cut was phased out in 1974. Low tungsten prices led to closure of the Bold Head Mine in 1984 and the Dolphin Mine in November 1990, by which time the mines had produced a total of 60,000 tonnes of tungstate from 11.5 million tonnes of ore. A considerable amount of ore remains, comprising about 20 % of Australia’s total economic demonstrated resources of tungsten. Currently, the Dolphin and Bold Head deposits are being assessed for their potential to support a new mining operation, by King Island Scheelite Ltd.

Other minerals associated with the Carboniferous granitesThere have been tantalising hints of other kinds of mineralisation at various places around the island. In Barrier Creek, some narrow quartz veins with sulphide and carbonate minerals are reported to have been worked for lead, zinc, silver and gold in the early 20th century. Two adits (horizontal mine tunnels) can still be seen on the west bank of the creek. Recent work in this area has failed to turn up anything of interest.

Abandoned workings known as ‘McKie’s Gold Mine’ were noted by a visiting geologist in 1916, 2 miles from Pegarah Road. The location of this is now unknown, but it may be on the 2 m wide quartz reef that crops out along the crest of a low ridge just west of the Sea Elephant River. Quartz veins commonly host gold mineralisation, because they act as conduits for mineralising fluids to move through the crust. Gold is generally present at low concentrations in these fluids, picked up from the surrounding rocks, and it may

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precipitate out when a drop in pressure causes the dissolved minerals to crystallise. Alluvial gold has also been reported in this area.

There are some abandoned alluvial tin workings on Tin Creek in the centre of the island . The ‘tin’ is actually cassiterite (tin oxide, consisting of 70% tin metal by weight), a black, hard and heavy mineral that becomes naturally concentrated in creek beds. It was probably weathered from veins of quartz or pegmatite associated with the Mt Counsel Granite.

Late Carboniferous – Cretaceous: continental assembly and breakup (350 – 100 m.y.)After the early Carboniferous granites and orebodies, there is another long interval, this time about 250 million years, where an absence of evidence (in the form of rocks the right age on King Island) makes the local story rather hazy. During this time, the dispersed fragments of Rodinia reunited in a new configuration – the supercontinent Pangaea – which lasted from about 300 to 200 m.y. ago. In Tasmania, and elsewhere on the southern parts of Pangaea, a late Carboniferous to early Permian ice age left widespread glacial deposits, then rising sea levels left behind fossiliferous marine deposits, and in the Triassic, a time of low sea level, rivers and floodplains left behind a lot of sandstone and coal. None of these deposits are found on King Island. Perhaps they once existed and have been entirely eroded away. Likewise, there is no sign on King Island of the Jurassic dolerite that forms many of Tasmania’s mountains. About 200 m. y. ago, the smaller supercontinent of Gondwana split away from Pangaea. Gondwana included present-day Australia, Antarctica, India, Africa and South America. Around 100 m.y. ago, after a lengthy period of rifting and stretching of the crust, Gondwana broke up. Australia pulled away from Antarctica, and began its long journey – still in progress – northwards. The two continents are today moving apart at about 6 cm a year. Beginning at about 90 m.y. ago, King Island had a large expanse of ocean opening up to its west, where formerly Antarctica lay.

Fig. 19: The topography of the area around Tasmania, with the sea removed. The image has great vertical exaggeration.

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The rift along which Australia and Antarctica split apart just missed King Island, as shown by the alignment of the edge of the continental shelf, only 50 km to the west (Fig. 19). The north-south pulling-apart of the two continents led to spectacular undersea scarps, SW of Tasmania (Fig. 19).

The rifting and stretching led to subsidence of the crust in areas now under the sea, to the east and west of King Island. This subsidence continued for a long time after the splitting of Antarctica and Australia, and continues in mild form today. Sediments, eroded from the land areas of Tasmania, Victoria and King Island, were carried by rivers and progressively filled up these subsiding areas (basins). There are now thick (up to 10 km) accumulations of sediment – sandstone, mudstone and limestone – in these offshore basins. The Otway Basin lies west of King Island and extends northwards as far as southwest Victoria; the Bass Basin lies to the east under Bass Strait. Most of what we know about these basins comes from offshore seismic surveys and drilling conducted by petroleum exploration companies over the last 40 years or so. In 2001, Woodside Energy Ltd discovered a moderate sized gas field about 100 km NW of Cape Wickham. Gas from the Thylacine gas field is now piped to Victoria through an undersea pipeline. The gas-bearing reservoir there is Cretaceous in age (about 100 m.y.), and 2-3 km below the seabed. On King Island itself, the rocks are too old and too metamorphosed to contain gas or oil.

Volcanic activity, Cretaceous-Paleogene (90 – 60 m.y. ago)

Lamprophyre dykesOn King Island there are rare dykes (narrow, vertical igneous intrusions) of an unusual, potassium-rich composition, called lamprophyre. A 50 cm wide dyke of lamprophyre on the east coast was dated at ~90 m.y. (middle Cretaceous). Another at Three Rivers Bay, a metre or so wide, is strongly magnetic and gives rise to an aeromagnetic anomaly several km long (D3 on Fig. 25). These dykes probably carried magma to volcanoes at the surface, that have long since been eroded away.

King Island was probably land at this time, at the end of a broad promontory extending from northwest Tasmania. We lay much closer to the south pole, at a latitude of about 70 degrees south. No doubt, judging from Cretaceous fossils in Victoria, there were hardy dinosaurs (varieties accustomed to the cold) running about, but no sedimentary beds that could preserve their fossils remain on King Island, if indeed they were ever deposited.

Volcanic pipes of basaltThe Paleogene period (65 - 23 m.y.) saw the eruption of numerous volcanoes across King Island. Scarcely a trace of this activity remains today. Only the volcanic feeder pipes remain: the lava flows and other volcanic deposits that spread out across the surface of the land have all eroded away. The evidence for these volcanic feeders comes mainly from the magnetic map (Fig. 25). Basalt is a relatively magnetic rocktype, so the vertical basalt pipes, or plugs, give rise to small, round, bullseye-like magnetic anomalies. There are about a dozen of these. In most cases, there is nothing to see at the surface – the pipes are completely covered up by sand and soil. Several of the anomalies were drilled, and found to be basalt, by hopeful mineral explorers, for example the one immediately north of Tathams Lagoon, and the one at Colliers Beach. Only one (to my knowledge) can be seen at the surface, just north of Adams Road, where basalt rubble is scattered across a low hill about 200 m across. A sample of this basalt has been dated at about 62 m.y., a mere 3 m.y. after the extinction of the dinosaurs. The magnetic maps also show that the lava flows (eroded away on King Island) still remain on parts of the sea floor to the southeast of the island, where remnants poke up above sea level to form Reid Rocks and Black Pyramid. These islands have basalt lava flows with spectacular columnar jointing, produced during cooling of the flows (Fig. 20).

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Fig. 20: Columnar-jointed basalt on Black Pyramid, 50 km SE of King Island.

Uplift and erosion, Miocene limestone (20 m.y.)In about the late Paleocene, the lava flows were eroded away. King Island was a part of a low-lying promontory jutting out from northwest Tasmania. No powerful rivers were available to carry out this erosion, so it seems more likely that the island was planed off flat by a period of marine erosion –as is happening again today by westerly wave action. Sea level continued to rise – (or perhaps it was the land that subsided – it doesn’t really matter which) until shallow sea covered the island in the early Miocene (about 20 m.y. ago). Abundant shells of marine organisms accumulated on the sea floor, leaving a layer of soft, porous marine limestone. Most of this limestone has been subsequently eroded away or covered by sand, but a large outcrop can still be seen at the Blowhole on the east coast. Other remnants are found inland, up to 70 m above present day sea-level. Fossil marine shells, snails, bryozoans, and foraminifera can be seen in these limestone outcrops (Fig. 21). ‘Foraminifera’ are single-celled marine animals that left tiny multichambered calcareous shells, and fossil species can be identified under the microscope by specialist palaeontologists, to tell the age of the rock. Similar limestone of about the same age is found at Fossil Bluff at Wynyard, and the Twelve Apostles, Victoria.

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Fig. 21: Surface of the limestone outcrop at the Blowhole, showing a rock that consists of mainly bryozoan fragments.

After the early Miocene the land was uplifted and the sea retreated again. This uplift, by as much as 160 m, happened all across the southern half of Australia. At the same time, the northern half of the continent subsided, and there was a relative rise in sea level in northern Australia. This large-scale tilting of the continent is the result of Australia colliding with the Asian tectonic plate, 10 -15 m.y. ago.

The uplift of King Island at this time seems to have been somewhat uneven. The plateau of bedrock was uplifted more in the south, and south of a line joining Currie and Naracoopa, the bedrock plateau is generally more elevated than to the north. This southern area (including the Pegarah Plateau) is gently tilted to the NW, so that a coastal escarpment up to 100 m high today extends along the southeast coast (particularly between Grassy and Naracoopa). Southeast-flowing streams such as Grassy River and Yarra Creek have cut down through this coastal escarpment, forming small steep-sided gorges.

The Quaternary Period: movements of sea and sand (2.6 m.y. – present)About 2.6 million years ago, the Earth’s climate entered into a cyclic pattern of ice ages and interglacials (warmer spells) that continues to the present day. Each glacial-interglacial cycle lasts about 120,000 years, and is thought to be driven by subtle variations in the Earth’s orbit. Mountain glaciers formed and retreated many times in Tasmania’s highlands, but the glacial periods have had little direct effect on King Island. At the height of the last ice age, 20,000 years ago, sea levels were 120 m lower than at present, because of the vast amounts of water held in ice caps, mainly in the northern hemisphere. Bass Strait was dry land, except for a shallow arm of the sea north of King Island. The west coast lay 50 km further west, near the edge of the continental shelf. Aboriginal people took this opportunity to walk across the low-lying sandy ‘Bassian Plain’ to reach Tasmania and King Island, although they did not remain on King Island once it became isolated again by rising sea levels 12,000 years ago.

The prevailing strong westerly winds and rising sea levels at the end of the last ice age combined to push large volumes of sand from the continental shelf west of King Island, eastward onto the island. A broad belt of sand dunes now extends along the west coast of the island, up to 4 km wide, in most places fronted by low rocky coast. These dunes are up to 80 m high (e.g. Huxley Hill, Haystack Hill). The milder and wetter

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climate of the Holocene led to stabilisation of these dunes by vegetation. The sand in these western dunes consists of up to 70% particles of calcium carbonate (fragments of marine shells). This sand is an important source of agricultural lime for the island.

On the eastern (leeward) side of the island, the more sheltered low sandy coast extending north from Naracoopa almost to Cape Wickham is of a quite different character. Sand accumulating on the long beaches and foredunes has caused the coast to build seawards by accretion, by up to 1 km since sea level stabilised in the early Holocene. The sand here is predominantly siliceous (mostly quartz), without much shell material. Near Naracoopa, deposits of ‘heavy minerals’ are of economic interest (see below).

Peat, and giant wombatsDeposits of peat and lignite are found within the sands in places. They result from the accumulation of plant matter in swamps and lagoons, in turn brought about by blocking of drainage by moving sand dunes. Egg Lagoon is a large area in the north of the island that has been artificially drained. Peat (the forerunner to brown coal) at least 2 m thick occurs in places under this area. Bones of diprotodonts (giant wombats: Zygomaturus trilobus) and other megafaunal remains, dating from the Pleistocene (2 million – 10 000 y), were recovered from beneath the peat when the drains were being excavated.

The ‘calcified forest’ and tufa terracesThe high calcium carbonate content of the western dunes has given rise to some interesting features, such as the ‘calcified forest’ in the south of the island, which is really not a calcified forest at all. The calcium carbonate shell fragments in the sand are weakly soluble in the rainwater that percolates through the sand, which means that part of them dissolves and can then precipitate out under the right conditions. Such conditions exist adjacent to certain thin, deep roots of vegetation that grows on the sand dunes. The resulting branching sandy encrustations, termed rhizoliths, are exposed when the surrounding sand is blown away (Fig. 51 (Part 2)).

Groundwater with dissolved calcium carbonate drains from beneath the sand dunes to emerge as springs at many places along the west coast, where the sand overlies bedrock just above high water mark. On emerging, the waters release CO2, resulting in an increase in pH which causes calcium carbonate to be precipitated as tufa in several places. Near Boggy Creek the tufa forms a rim around several shallow pools just above high tide mark (Fig. 22). At Dripping Wells, a much larger mass of tufa – several metres thick and a few hundred metres long – is exposed as a cliff just above high tide level. This tufa is highly porous, having grown around grass, leaves and roots that have rotted away to leave voids. Small caves have formed, in which stalactites are forming. Megafaunal remains have been found in similar deposits elsewhere in Tasmania.

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Fig. 22: Calcium carbonate precipitates out of spring water here, forming small terraces of tufa.

Calcium carbonate, precipitated wholesale through large volumes of dune sand, has given rise to deposits of calcite-cemented sandstone in some places – for example at Seal Point (Fig. 23), on Stokes Point, and in Camp Creek Reserve, Currie. This sandstone can be an easily worked and attractive building stone.

Fig. 23: Dune sand, turned to stone by being cemented with calcium carbonate. Seal Point.

Naracoopa heavy mineral sands The black heavy mineral sands along the coast north of Naracoopa were initially prospected for tin, beginning (as far as records show) in 1905. Further investigations (1928 – 1969) focused on terraces (raised

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beach deposits) immediately inland of the present day beach, stretching for at least 2 km north of the Fraser River mouth. Test boring showed a substantial resource of the heavy minerals ilmenite, rutile and zircon, with some cassiterite (tin) and monazite. The deposits were mined between 1969 and 1977, first by Naracoopa Rutile Ltd and then by Kibuka Mines Pty Ltd. Total production was 20,000 tonnes of rutile and 23,000 tonnes of zircon from 3,070,000 tonnes of sand. Considerable resource remains. The less valuable component of the heavy minerals, comprising mostly ilmenite and garnet with lesser quantities of rutile and zircon, was stockpiled.

Dr Allan J. Bond & Associates Pty Ltd currently hold a mining lease over the deposits. In early 2013, intermittent production resumed from the ilmenite-rich old tailings stockpile.

Rutile (TiO2) and ilmenite (FeTiO3) are mainly used in the manufacture of white titanium dioxide pigment, in ceramics and for the production of titanium metal. Zircon (ZrSiO4) is mainly used in the ceramics industry, and monazite (a complex phosphate mineral) is a source of thorium and rare earth metals.

The heavy minerals appear to have been derived from weathering of the Proterozoic sedimentary rocks (Fraser Formation) and igneous intrusives which are the basement rocks for most of the area drained by the Fraser and Sea Elephant Rivers. The ilmenite and chromite are probably derived from the amphibolite bodies inland of Naracoopa, and the Ediacaran volcanic rocks along the coast to the south.

The heavy minerals get washed down the rivers to the coast. There, they tend to become concentrated in the upper, or swash, zone of the beach where surf washes up on the beach face and loses energy. In this zone, heavier grains accumulate because they are denser than the quartz grains they occur with and become stranded. Beach heavy mineral deposits are thus often referred to as ‘strand-line deposits’.

Exploration north of the main deposits has shown that the mineral content of the sand generally diminishes northwards from the mouth of the Fraser River. Two lower grade, as yet unexploited deposits, have been found southwest of Cowper Point.

IronstoneScattered outcrops and boulders of ironstone are common on King Island, especially on the Pegarah Plateau. In places the ironstone forms small plateaux raised 1-2 m above surrounding country. The ironstone has been extracted on a small scale in many places for surfacing roads and cow lanes.

The ironstone is a surface deposit, not part of the bedrock. It is formed by springs, and is still forming in many places. Where exposed, it is usually about 2 m thick, with an upper solid dark red-brown ironstone with small voids (Fig. 24), grading down into earthy rubbly yellow-brown ironstone, often with inclusions of quartz sand grains and iron-stained angular bedrock fragments. The ironstone formed by oxidation and precipitation of dissolved iron in groundwater that reaches the surface. Ironstone formed this way is known as ‘bog iron’. Bog iron was an important source of iron in early iron-age and medieval Europe. On King Island, groundwater picks up dissolved iron through subsurface weathering of fine-grained pyrite (iron sulphide) in Proterozoic bedrock of siltstone and mudstone (Fraser Formation). A series of ironstone bodies form a discontinuous low ridge crossing the Pegarah Road about 1 km W of Parenna. The ridge seems to coincide with the Grassy River Fault. These deposits probably came about from an enhanced flow of groundwater welling up out of the fractured fault rocks.

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Fig. 24: Typical King Island ironstone.

Geophysics and remote sensingIn the last few decades, aerial surveys using sensitive geophysical instruments have been flown over almost the whole of Australia. Much useful geological information can be gleaned from these ‘remote sensing’ surveys. The two images in Fig. 25 come from a survey by a light plane in 2001, which flew along east-west lines 200 m apart, over the whole of King Island and some of the surrounding sea. The aircraft carried a magnetometer, which measures slight variations in the Earth’s magnetic field, to produce the image on the right. The data has been processed by a computer to produce a false topographic image in which areas with a stronger magnetic field (positive anomalies) show up as red ‘peaks’, and those with a weaker magnetic field (negative anomalies) show up as blue ‘holes’. These reflect the distribution of magnetic rocktypes (such as basalt) and non-magnetic rocktypes (most sedimentary rocks), and this technique can ‘see’ through the cover of sand and soil, and even out under the sea. We cannot (yet) explain many of the features of the magnetic map, as many are completely covered and the rocks cannot be directly examined. Some of the strong N-S anomalies (e.g. that labelled S1) appear to be magnetic zones within the Surprise Bay and Fraser Formations. The NE-trending, straight, narrow anomalies (examples labelled D1, D2, D3) are dolerite or lamprophyre dykes intruding those rocks. Some of the small, round anomalies are Paleogene basalt pipes (examples indicated as P1, P2, P3). P3 shows up as a negative anomaly because it intruded at a time when the Earth’s magnetic field was reversed. The magnetic grains in this rock therefore point oppositely to normal and act to locally reduce (very slightly) the strength of the field. (Magnetic polarity reversals are common in the geologic record – why or how they happened, remains incompletely understood. The last reversal happened 780,000 years ago). Most of the Sandblow Granite and the volcanic rocks of the Grassy Group show up as positive anomalies (compare with Fig. 1). The magnetic map also shows up some features that are deeply buried (1 km or more). The ‘yellow’ anomaly just south of Naracoopa is thought to be a small granite intrusion at depth. Such features are of interest to mineral explorers.

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Fig. 25: Magnetic (left) and radiometric (right) images of King Island.

The image on the right was generated from the data gathered by an airbourne gamma-ray spectrometer. This instrument measures the very weak gamma radiation produced by three common, naturally radioactive elements, potassium, thorium and uranium. The computer-generated image shows areas relatively rich in potassium as red, thorium as green and uranium as blue. Areas rich in all three show up as white. This technique, unlike the magnetic method, cannot see to depth, in fact only the top 20 cm or so of the rock or soil is ‘seen’ by the gamma-ray spectrometer. Water bodies and peaty or swampy areas effectively block the radiation and show up dark. The rocky outcrops along the west coast show up as a narrow white band. Inland, the western belt of dunes shows up as dull pink or red, indicating presence of potassium, probably due to the presence in the sand of a small amount of feldspar eroded from the Proterozoic granites. Elsewhere the radiometric map shows an intricate pattern of whitish areas, reflecting mineral-rich soils (e.g.

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most of Pegarah Plateau) and darker areas blanketed by quartz-rich sand or swampland. The Naracoopa heavy mineral deposits, and probably similar deposits further north, show up as blue-green patches.

Further readingCalver, C.R. 2012. Explanatory Report for the Grassy and Naracoopa geological map sheets. 1:25 000 Digital Geological Map Series, Mineral Resources Tasmania. Explanatory Report 5. (A technical account of the geology of south-eastern King Island: available online at www.mrt.tas.gov.au).

Hoffman P. F., Calver C. R. & Halverson G. P. 2009. Cottons Breccia of King Island, Tasmania: glacial or non-glacial, Cryogenian or Ediacaran? Precambrian Research 172, 311-322. (Technical detail on the Cottons Breccia, and an account of the controversy surrounding its interpretation).

Johnson, David. 2004. The geology of Australia. Cambridge University Press. 276 pp. (Good general book on geology of Australia, and basic geological concepts, accessible to the lay person).

Walker, Gabrielle. 2004. Snowball Earth: The Story of a Maverick Scientist and His Theory of the Global Catastrophe That Spawned Life As We Know It. Broadway. 288pp. (Dramatic account of the development of the Snowball Earth hypothesis, written for the general reader).

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