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To determine the attributable cost and length of stay of hospital-acquired Clostridioides difficile infection (HA-CDI) from the healthcare payer perspective using linked clinical, administrative, and microcosting data.
A retrospective, population-based, propensity-score–matched cohort study.
Acute-care facilities in Alberta, Canada.
Admitted adult (≥18 years) patients with incident HA-CDI and without CDI between April 1, 2012, and March 31, 2016.
Incident cases of HA-CDI were identified using a clinical surveillance definition. Cases were matched to noncases of CDI (those without a positive C. difficile test or without clinical CDI) on propensity score and exposure time. The outcomes were attributable costs and length of stay of the hospitalization where the CDI was identified. Costs were expressed in 2018 Canadian dollars.
Of the 2,916 HA-CDI cases at facilities with microcosting data available, 98.4% were matched to 13,024 noncases of CDI. The total adjusted cost among HA-CDI cases was 27% greater than noncases of CDI (ratio, 1.27; 95% confidence interval [CI], 1.21–1.33). The mean attributable cost was $18,386 (CAD 2018; USD $14,190; 95% CI, $14,312–$22,460; USD $11,046-$17,334). The adjusted length of stay among HA-CDI cases was 13% greater than for noncases of CDI (ratio, 1.13; 95% CI, 1.07–1.19), which corresponds to an extra 5.6 days (95% CI, 3.10–8.06) in length of hospital stay per HA-CDI case.
In this population-based, propensity score matched analysis using microcosting data, HA-CDI was associated with substantial attributable cost.
The Earth is dynamic, its continents wandering great distances over time, repeatedly coming together and splitting up as oceans close and open. Australia is an old and complex continent, subject to ever-changing global lithosphere dynamics, and still slowly evolving. Its history is written in its rocks. It was periodically subjected to compressional events as the supercontinents were assembled and tensional events as the supercontinents disintegrated. A convergent plate boundary lay immediately to the east of Australia for much of its more recent, post-Precambrian history. The dynamics of this boundary built much of the eastern Australian crust. Climate affecting Australia fluctuated considerably, with episodes of icy conditions separated by longer intervals of warmth. There were massive extinction events that shaped life on the Earth. The last of these occurred when Australia was progressing to be an island continent. This event, and the isolation that followed, resulted in the distinctive animals and plants that characterise Australia.
The story of Australia is about the journey of a landmass wandering across the globe through geological time. The more recent part of the journey, over the last 100 million years or so, back to the Cretaceous, is well established. This is because the seafloor spreading history of the oceans is known back to this time, recorded by the striped pattern of magnetic anomalies symmetrical about the mid-ocean ridges. By progressively removing the strips of ocean crust, from the youngest back to the oldest, we can project movements of the ocean-bordering continents back through time.
The earlier journey, preceding the Cretaceous, is less securely known and increasingly so the further back we go. Knowledge of it depends on paleomagnetic data, which tells us where rock bodies of various ages were located with respect to the Earth's magnetic field at their time of formation. By implication, they give a ‘fix’ for the location of a continent at that time. However, the ‘fix’ is only in terms of paleolatitude and is unconstrained for paleolongitude. This deficiency is reminiscent of the riddle that plagued ancient mariners. Doubtless we will have misplaced Australia in global reconstructions for the distant reaches of geological time, just as many ancient mariners misplaced the longitudinal position of their vessels, sometimes with disastrous consequences.
Continental geology provides important indications of how the jigsaw of continental configurations may have looked in the past.
Through the Paleozoic a convergent plate boundary was located adjacent to what is now eastern Australia. Dynamics of this boundary shaped and substantially added to the crust of eastern Australia. It was the site of mighty fold-and-thrust mountain ranges formed by crustal compression and seamed with a succession of continental magmatic arcs and associated explosive volcanism. The contrast between the Paleozoic and the present-day aspects of eastern Australia is profound.
ASSEMBLY OF THE GONDWANA SUPERCONTINENT
The Precambrian core of Australia provided the foundation of the continent and a frame of reference for later crustal addition, including the widespread development of sedimentary basins that formed on top of the Precambrian craton. At the close of the Precambrian, Australia was part of an extensive continental configuration, the supercontinent Gondwana, in which Africa, South America, India and Antarctica were also united (see Figure 5.1) Following the breakup of the earlier supercontinent Rodinia, amalgamation of continental fragments to form Gondwana progressed through the late Precambrian and continued into the early Paleozoic (650–500 Ma). Australia through this interval of time was already united with Antarctica and India and was little affected by collisional sutures which developed as a consequence of the amalgamation process and which characterise the geology of eastern Africa, in particular, and are also clearly evident in Madagascar and Antarctica.
By the time Gondwana came into existence the Paleo-Pacific Ocean had long been formed as a consequence of the Rodinian breakup. This ocean developed when eastern Australia separated from Laurentia. During the Cambrian, the Paleo-Pacific Ocean began to close, through subduction, along the entire ocean-facing edge of Gondwana. For Australia, the consequences of subduction along its eastern margin became clearly evident in the late Cambrian and persisted through the remainder of the Paleozoic, an interval of at least 250 Ma. As expected of subducting plate boundaries, large volumes of igneous rocks were generated throughout that interval.
OROGENIC PROCESSES AND PATTERNS
The details of the plate dynamics that affected eastern Australia through the Paleozoic are complex. There were times when the Paleo-Pacific plate boundary adjacent to eastern Australian continental crust retreated oceanwards (roll back) and the continental borderland stretched. Back arc basins, to be infilled with sediment and volcanics, developed as a consequence.
This chapter introduces the main ideas of geology and explains briefly the technical terms used in this book. It deals with the overall structure of the Earth, the theory of plate tectonics, the main mineral groups and rock types, the deformation of rocks into folds and faults, and the processes of erosion and formation of the landscape.
A MODEL OF THE EARTH
The Earth is a near-spherical planet slightly flattened at the poles (see Figure 2.1b). Our understanding of its inner nature comes largely from interpreting records of seismic waves generated by earthquakes that have passed through it. Three separate layers, with different compositions, occur within the Earth (see Figure 2.1a):
• a central, hot core, with a 3470 km radius, composed mainly of iron and nickel, with a density of 10–13 t/m3 (10000–13000 kg/m3), solid in the inner part but liquid in the outer part
• a mantle, 2900 km thick, composed of dense (3–6 t/m3), rocky material in a hot, semiplastic state, except for its uppermost layer, which is solid
• an outer, cool, solid crust, generally 8–50km thick, with a density of 2–3 t/m3
These three layers are separated by discontinuities identified from the passage of seismic waves. The dense and heavy core and mantle make up most of the Earth (99 per cent by volume), whereas the light crust in comparison forms a very thin skin.
The base of the crust is marked by a major seismic discontinuity where the speed at which earthquake waves travel is abruptly increased. Since the speed of such waves is directly dependent on the density of the material in which they are propagated, the crust–mantle boundary marks a major density contrast between material within the crust and that within the mantle. Crustal rocks are richer in silicon, a light element, and poorer in iron and magnesium, heavier elements, than those of the mantle. The boundary is referred to as the Mohorovičić discontinuity, or the Moho for short, after the Croatian seismologist who discovered it, in 1909. For the continents the crust averages 40 km thick but is much thinner for the ocean, at only 8 km. Depth to the Moho has been mapped for the Australian continent and averages 38 km.
Although restricted to northern Australia and of limited distribution, coral reefs are a striking feature of Australia's continental shelves. The Great Barrier Reef, on the northeastern shelf, is a reef complex of great international significance and one of Australia's prime assets. It is famous for the diversity and colour of its inhabitants, especially fish and corals. The history of the reef reflects its geological context, including Australia's journey into the tropics and the consequences of changing sea levels. The long-term future of the Great Barrier Reef will be influenced by processes that apply on the adjoining coastal fringe and on the continental shelf where it is developed.
The Australian coral reefs were great hazards to early seafarers. Even today, with highly accurate navigation equipment and powerful engines, boats and even ships still run aground within the Great Barrier Reef. For example, in 2010 the bulk coal carrier Shen Neng became grounded on Douglas Shoal of the Great Barrier Reef, east of Rockhampton. Navigating the waters must have been far more difficult for explorers like James Cook and Matthew Flinders, who had to command sailing vessels that were at the mercies of the winds and tidal currents. This was particularly the case when navigating the intricacies of the Great Barrier Reef. On 1 June 1770 Cook's vessel, HM Bark Endeavour, became firmly fixed on a reef, yet he was able to refloat it, make hasty repairs, sail to the mainland and there careen it on a river bank at present-day Cooktown. With the repairs complete he sailed the short distance to Lizard Island and climbed its granitic hill, 300 m high, to search for a way out of the reefs, taking a bearing on a channel to the safety of the open sea. Matthew Flinders, following Cook in early European exploration along the northeastern coastline, directed steerage of the Investigator from the masthead. Both men and their vessels survived their encounters with the reef.
Not so fortunate were those on the Pandora, which was carrying mutineers from the Bounty home to England for trial. In 1791 she struck Pandora Reef, at the entrance to Torres Strait, and was dragged across it through the night before being tipped into 34 m depth of water the next morning.
The begetting of Australia as we know it was a very protracted process, the pathway of which is written in the rocks. This is true of every continent, and for each the pathway was unique, although many of the treads were shared. The last steps in the Australian journey were of special importance, because they gave rise to the unique environmental attributes which characterise this island continent.
This volume tells the story of how Australia came to be the way it is. It gives a time and process context to the rich and involved preceding history – a forever-fascinating tale. Its main parts are long known and well established, but the story is embellished every year by new research. The pace of change, both in the unfolding of new parts to the story and in the achievement of a tighter focus on broad-scale perspectives, is considerable. In just the seven years or so since the second edition of The Geology of Australia was crafted, much new information has come to light through research undertaken in universities, by government agencies and in the private sector.
The considerable physical assets of Australia are indelibly vested in its origins. These have exerted a powerful influence on the development of Australia as a nation. Two complementary streams of discovery have been running. One is pure research, knowledge for its own sake, which has unlocked the begetting pathway, the types of rock systems involved and the nature of the Earth's processes that applied in their origins. The other is the discovery, documentation and utilisation of Earth resources – minerals, rocks, oil and gas, along with soil and subsurface water – with economic ends as the drivers. These two knowledge-based streams have intertwined from the days of early European settlement to the present. Their interaction has been highly productive, in discovering both how Australia came to be and how best to use its physical assets in nation building. The outcome has been to place Australian geoscience as top tier on a global scale, one of very few home-based science disciplines to achieve that international standing.
Time is the great canvas on which geology is painted. The Earth's surface, its rock systems and its resources result from a cavalcade through time of geological processes driven by a restless lithosphere. The immensity of geological time, like the scale of the universe, is difficult to imagine. Time relationships between rock bodies, and the structures they show, are the keys to understanding how the crust developed and the processes involved. Assigning ages to rocks, placing them in time sequence and unlocking their history are the very foundations of geology as a discipline.
At the time Australia was settled by Europeans the age of the Earth was agreed, at least in principle, and based on biblical genealogy reaching back to Adam and Eve. However, the fact that fossils reflect past life disturbingly different from that of the present became increasingly apparent during the 1800s. Darwin's On the origin of species, of 1859, combined with geological concepts already developed in Europe challenged the biblically based view. Geological mapping, particularly in England, established sediment thicknesses of many kilometres in what we now recognise as accumulations of sedimentary basin configuration. The time needed for such accumulations of this scale, based on observed sedimentation rates, changed our perspective of geological time. From multiplying sediment thicknesses established from mapping sedimentary rock layers by observed sedimentation rates, it became clear that biblical estimates of the age of the Earth departed from reality by several orders of magnitude. The sedimentary rock layers shown by the Grand Canyon, in the USA, well over 1 km thick, did not form in a mere 4000 years or so.
DEVELOPING A TIMESCALE
Individual sedimentary layers accumulate at the surface of the Earth. The accumulation of multiple layers, the younger on top of the older, registers the passage of time like the sequential pages of a novel as they are read. This simple fact is an important principle of geology, referred to as superposition: stacked sedimentary layers record the passage of time. Where the stacks are kilometres in thickness the time registered has been of very long duration (see Figure 3.1).
The contents of the very many sedimentary basins in Australia, Europe and elsewhere likewise represent very long spans of time.
Australia is currently the only continent in the world without active volcanoes, but this was not so in the past. There has been regular volcanism throughout Australia's geological history, especially of the silica-rich, felsic type along the eastern margin during the Paleozoic and Mesozoic, and the most recent mafic phase has barely finished. Basalt eruptions may have started in the late Cretaceous, as early as 90–85 Ma, were most active through much of the Cenozoic (from 55 Ma) and persisted until the most recent eruptions, in South Australia, only 4.6 ka ago. There is a close relationship between these basalts and the Great Divide of eastern Australia. Rich soil derived from the weathered basalt on elevated country combined with cooler and wetter climates along the eastern margin is the basis for many of Australia's finest agricultural and horticultural areas.
THE VOLCANIC PROVINCES
The mainland has no active volcanoes, though there are two active volcanoes in the offshore Australian territory, which includes Heard Island and the McDonald Islands. These islands are part of the Kerguelen volcanic system, related to a large igneous province of mantle plume association, and lie about 4000 km southwest of Australia and 1500 km north of Antarctica. Big Ben volcano on Heard Island has a high point at Mawson Peak which is 2745 m in elevation. It has a thick mantle of snow and glacial ice and last erupted in 1992 and 2016. The McDonald Islands volcano is only 230 m high and erupted between 1997 and 2005, following a long hiatus in activity thought to have extended back tens of thousands of years.
Most of the volcanism in Australia produced rhyolite and andesite, rocks which formed mainly during a subduction-related active margin setting, and involved considerable melting of continental crust. In addition, three episodes of within-plate, mantle-derived basaltic igneous activity are evident: in the early to mid Cambrian (at about 515 Ma) in northern Australia, during the final assembly of Gondwana; in the Jurassic and early Cretaceous (185–132 Ma) in southern and western Australia, before and during the initial Gondwanan breakup; and in the late Cretaceous (about 85 Ma) to Holocene (5.6 ka) interval, along eastern Australia.
After the enormous time span of the Precambrian, lasting 4 billion years (4 Ga), complex life suddenly burst forth in an astonishing array within just a few million years. Eastern Australia came under the sway of a nearby convergent plate boundary which strongly influenced its character. It was episodically the site of widespread igneous activity and prominent mountains generating voluminous erosion products. The sedimentary rocks which resulted are now an important part of the continental fabric. The span of this chapter covers 182 Ma, comprising 56 million years in the Cambrian (541–485 Ma), 42 million years in the Ordovician (485–443 Ma), 24 million years in the Silurian (443–419 Ma) and 60 million years in the Devonian (419–359 Ma).
PART OF GONDWANA
During the Paleozoic, between 541 and 252 Ma, there was no Australia as it is known today. The present Australia was just a segment of eastern Gondwana embedded in continental crust of much larger extent within a single tectonic plate. Australia was continuous with Antarctica to the south and India to the west. Continental crust now embedded in Asia adjoined present Australia to the northwest. For most of the Cambrian the eastern Gondwanan margin that included Australia faced deep ocean built by seafloor spreading following the breakup of the supercontinent Rodinia, in the late Proterozoic (see Chapter 4). Contemporary research indicates that Tasmania was a separate microcontinent located in the ocean beyond Australia and independent in its history. In proto-Australia's northern part, stretching eastwards from the Kimberley region, within-plate, hotspot-related volcanism generated extensive basaltic fields.
However, in the late Cambrian, at about 500 Ma, the plate geometry changed. From that time onwards through the Paleozoic the ocean-facing part of eastern Gondwana, including the Australian segment, lay close to a convergent plate boundary, with the Paleo-Pacific Ocean floor being consumed by subduction (see Figure 5.1). As a consequence, eastern Australia was transformed into an active continental margin characterised by magmatism, the accumulation of very large-scale sedimentary systems and mountain building. Changing stress regimes along the margin reflected contrasting episodes of plate boundary advance characterised by crustal compression, widespread thickening and mountain building, and by plate boundary retreat characterised by crustal extension, widespread thinning and sedimentary basin formation (see Chapter 5).
The Carboniferous is so named because of the extensive carbon-bearing coal deposits formed at that time in Europe and also represented in North America. The climate in these Northern Hemisphere continental masses was tropical, with vast coastal peat swamps and reefal limestone forming offshore. The Permian continued to be hot, with seasonal river systems and large salt lakes in North America and Europe.
The climate was very different on the landmasses of Gondwana, which extended through high southern latitudes towards the South Pole. There, the climate was cold and in some places glacial. Permian Gondwanan coal in Australia, India and South Africa formed in these very different, cold-climate situations. A chain of volcanoes along eastern Australia erupted intermittently for over 60 Ma, and the last orogeny affecting Australia built a mountain chain at its eastern margin.
THE SETTING AND RELATIONSHIPS
Through the Carboniferous and Permian interval (359–252 Ma), Australia formed the northeastern part and margin of Gondwana, the Southern Hemisphere sector of an even larger continental aggregation: Pangea. To the south lay Antarctica, to the southwest India; all contributed parts of an ancient continental interior. The crust of the Australian continent was more extensive than at present in its northwestern part, between Darwin and Exmouth Gulf. Fragmentation occurred there by rifting and seafloor spreading, which commenced as early as the Permian and continued into the Jurassic. Detached pieces of continental crust were transported northwards, eventually colliding with, and becoming part of, Southeast Asia. During the Carboniferous to Permian interval Australia moved progressively into a high southern latitude, lying between approximately 40 and 70° at the beginning of the Permian.
An ocean edge extended from Tasmania to east of Cape York, in far northern Queensland. The long-established convergent plate boundary to the east of Australia continued to strongly influence the setting of the continental margin. Plate boundary advance in the mid Carboniferous induced mountain building, and the sedimentary record for this period is poor. However, plate boundary retreat towards the close of the Carboniferous and into the early Permian induced widespread extension of the eastern Australian crust. A major episode of sedimentary basin formation was the consequence, most notable in the development of the Sydney-Bowen Basin, which is a striking feature of eastern Australian geology (see Figure 7.14). Infill of these features contains a rich sedimentary record of glacial association.
What do we know of the origin and age of the Earth and how it was organised in its formative stages? What do we know of climate in the very distant past? When did life first appear and what was it like? Australian rocks have much to contribute in answering these questions. The oldest rocks tell us that the continents, oceans and atmosphere are of great antiquity but have changed remarkably with time. Plate tectonics and its consequences are also of great antiquity and had a controlling influence on shaping the crust to form the original nucleus of Australia in very deep time.
THE ORIGINAL EARTH
The Earth's surface has been comprehensively remodelled from its initial state as it has evolved through time, with new rock systems replacing and covering those of greater age. As a consequence, the early stages of the Earth's development are poorly known, through progressive lack of evidence backwards to the beginning. The most accepted scientific theory is that following the big bang and the origin of the universe, and after the contraction of the matter that formed the Sun, a huge rotating disc of dust and gas, spread across billions of kilometres of space, was left behind. This disc of debris gradually collapsed due to gravity, coalescing into a series of variously sized lumps, or planetesimals. The planetesimals, which ranged in size from a few metres to Mars-like proportions, then accreted by gravity over a period estimated to have been 29–100 million years. They aggregated to form spinning balls of rock, gas and ice – the planets – and the Earth is one of them.
The most widely accepted hypothesis is that the Moon originated from a collision between the Earth and a similar-sized body, with the mass now forming the Moon ejected as a consequence. Moon rocks can be matched with igneous rocks of the Earth and are between about 4.5 and 3.1 Ga old, overlapping substantially with the age of the early Earth's rocks. The early Earth and Moon were bombarded with meteoroids, asteroids and comets, probably the debris left from formation of the Solar System.
Australia's journey continues. Its most recent history has seen it populated with humankind, whose members arrived in two separate rafts. The first arrivals sought to fit into the landscape and saw themselves as part of it. Their touch on the continent was light, and in the 50 000 or more years of their tenure they have experienced significant change, particularly to climate and coastal geography. And they have brought about some change themselves, through hunting and fire.
The second raft brought humans who developed a much more populous presence. Their tenure has been very short and their outlook quite different. They have viewed the continent as a fixed and stable entity, with changes to it squarely in their hands. Their touch has been much more heavy: the built environment, the transformation of landscapes through the application of agriculture and the mass movement of material in mining operations. Their experience has been too short for them to appreciate change wrought by the Earth's system both independent of, and within, the sphere of human influence. In some cases they have been surprised by change completely outside their control – weather events, floods, tidal surges and earthquakes. In many others they have interfered with, and changed, aspects of the Earth's system without a satisfactory understanding of the consequences – for example, mismanagement of groundwater and landscapes, causing their degradation.
Although we are slow to see it, change to Australia is an integral part of its being; this is the essential lesson to learn from the rocks. The surface of the continent is its most transient part and is subject to change on a whole range of timescales, both with direct human help and without it. Understanding the drivers of such change, and the rates at which they apply, and recognising those we can influence, are ongoing challenges.
An appreciation of the national estate, scaled expansively for time and space, is the gift of geology. The discipline also has relevance for change over shorter periods and at local scale, particularly in relation to landscape dynamics and the effective utilisation of the Earth's resources to service the needs of humankind.