People have natural affinities with the sea and coastlines, using them for work, recreation and aesthetic enjoyment. Interest in coastal areas has increased with growing awareness of environmental sustainability, issues such as natural coastal vulnerability and potential climatic changes, as well as the development of a greater appreciation of impacts on coasts due to urban and industrial development.

In order to make a contribution towards an understanding of the evolution of the South Australian coast and its current changes, and thereby contribute towards its better management, the authors are sharing their coastal knowledge and research. Collectively the contributors, who have been friends and academic collaborators over several decades, have accumulated a vast amount of experience related to the evolution of coastal features of South Australia. From this unique position they have synthesised this information in a manner to make it accessible to students, planners and the general public.

Geologically, the South Australian coast is very young, having evolved only over 1% of geological time, during the past 43 million years since the separation of Australia and Antarctica. It is also very dynamic, with the current shoreline position having been established from only 7000 years ago. There is a remarkable diversity of coastal landscapes in South Australia, ranging through rocky cliffed coasts, submarine canyons, high wave energy sandy beaches and estuarine environments to tidally dominated coasts with sandflats and mangrove woodlands. This diversity of coastal landforms has resulted from the interaction of tides, winds and wave-generated processes operating on a range of rock types impacted by relative movements of the land and sea. Highlighting past changes at the coastline such as erosion, siltation, land movements and fluctuations in sea level provides a sound basis for understanding future changes and instigating appropriate planning strategies. Some features of the South Australian coast have national and global research significance for understanding sea level changes, coastal evolution and management by providing present analogues of past landforms.

The main aim of this book is educational. By explaining the variable character of the coast and its long-term evolution, it is hoped that this book will provide people with background information and awaken curiosity about the coast, enabling them to understand and interpret coastal landscapes, or ‘to read the coastal landscape’.

Introduction

The coast of metropolitan Adelaide extends from Sellicks Beach in the south to Le Fevre Peninsula in the north (Figure 2.1). Situated in the most populated part of South Australia, the coast provides an excellent example of intense human use of coastal resources, illustrating the impact of urban development and artificial modification of the coast. A lack of understanding about coastal processes during European development has resulted in coastal degradation.

The dominant geological influence on this section of coast is a series of arcuate northeasterly trending faults (Figure 2.2), which extend from the hills and define the landward limit of the coastal plains. Differential faulting of Neoproterozoic to Cambrian strata and Paleogene and Neogene sedimentary rocks has formed the template for the metropolitan coastline. The uplifted zones are associated with prominent cliffs and headlands, while between these uplifted sections of coast, embayments occupy fault angle depressions producing sandy bays. Although important rivers such as the Onkaparinga and the Torrens have their outlets on this section of coast, they deliver minimal sediment to Gulf St Vincent. The exposure of differentially faulted rocks and sediments has provided a north-south sequence of beach compartments.

Geological setting

The geological influences on the coast of metropolitan Adelaide date back to the folded, metamorphosed and uplifted Neoproterozoic and Cambrian strata of the Adelaide Geosyncline, which broadly coincides with the modern Mount Lofty and Flinders Ranges. Through major crustal deformation events of the Delamerian Orogeny from 514 ± 3 to 490 ± 3 Ma, the region was transformed to an extensive fold mountain range of Himalayan proportions, named the Delamerides. From the Middle Ordovician (c. 470 Ma) through to earliest Permian time (299 Ma), the Delamerides were deeply eroded. The remnants of this major mountain chain influence the overall shape and character of the modern coast. Mainly during Early Permian times (299 to 290 Ma), the region experienced extensive glaciation. Evidence of this glaciation is spectacularly preserved at Hallett Cove, where the ice flow was from the south to the north with bedrock structures diverting the overall southeast-northwest movement (see Chapter Three — The Fleurieu Peninsula coast).

Erosion of the Delamerides continued for millions of years, exposing the core of the mountain range and reducing it to a planation surface of relatively low relief.

Introduction

The River Murray Estuary is a complex series of waterways comprising Lake Alexandrina, Lake Albert, the Murray Mouth, Coorong Lagoon and the coastal barrier systems of Younghusband and Sir Richard Peninsulas. The region has long been a source of fascination because of its inherent natural beauty, its social and cultural history, and because of the records of explorations by Matthew Flinders, Nicholas Baudin, Charles Sturt, Collet Barker and others. The Coorong became immortalised as the setting for Colin Thiele's novel and film, Storm Boy. Aboriginal people had a finely developed understanding of their environment and occupied the area sustainably for many thousands of years before the arrival of Europeans.

In 1802, Matthew Flinders and Nicholas Baudin, during their charting of the southern Australian coastline, met offshore from the Murray Mouth in Encounter Bay, the outfall of Australia's largest exoreic river system, although neither navigator recognised it. This is not remarkable; they were many kilometres offshore, the coast is low-lying and there were no large freshwater flows containing sediment. Captain Charles Sturt reached the Murray Estuary in 1830 after an intrepid boat trip down the River Murray and was forced to return the same way after his efforts at accessing the sea were thwarted by sand bars in the Goolwa Channel. Hopes were high that the Lower Murray area would support a thriving port and that Goolwa would become the ‘New Orleans of Australia’. However, the mouth could not always be reliably navigated, there was no natural site for a deep-sea port, and the romance of the paddle steamers was finally quashed by the construction of more reliable railways. Today the area has become a focus for conservation, tourism, recreation and retirement.

Physical setting

The arcuate sweeping shoreline of Encounter Bay fringes the seaward margin of the Murray Estuary, stretching from the uplands of the Mount Lofty Ranges towards the southeast (Figure 4.1). It includes part of the longest beach (194 km) in Australia. Unconsolidated sand forms most of the shoreline as long, narrow coastal barriers (Sir Richard and Younghusband Peninsulas) separate the open ocean from the elongate back-barrier lagoons of the Goolwa Channel and Coorong Lagoon. The name 'Coorong’ has been anglicised from the Ngarrindjerii word ‘Kurangh’, which means a long neck of water.

Introduction

Northern Spencer Gulf encompasses the coastline that extends north from Whyalla to Port Augusta on the eastern Eyre Peninsula and from Port Augusta to Port Broughton on Yorke Peninsula (Figure 8.1). Northern Spencer Gulf is an inverse or negative estuary where evaporation exceeds freshwater input, with salinities (34 to 49 ppt, or parts per thousand) increasing in the northernmost portion of the gulf. Water temperatures are also elevated, ranging between 13 to 28°C. The gulf thus provides a refuge for plants and animals that colonised during warmer water conditions of the past, such as coralline algae near the bridge crossings at Port Augusta. Protected from the ocean swell, these northernmost gulf areas also experience diminished wave heights but amplified tidal ranges. The tidal ranges typical of the gulf are 2.5 to 3 m, but a maximum tidal range of 4.1 m has been recorded at Port Augusta, which is just into the macrotidal range. There are regular dodge tides every two weeks, when for 1 to 2 days there is no tidal movement due to the two main semi-diurnal tides, M2 and S2, cancelling each other out.

Tidal processes dominate the northern Spencer Gulf, and the coastline is characterised by thick seagrass meadows, wide intertidal sandflats, mangrove woodlands and supratidal saline marshland. Coastal development is intimately related to the massive production and accumulation of biogenic materials including algae, seagrass, molluscs and bryozoans; the site is a ‘major carbonate factory’, sequestering much CO2. Algal mats, seagrass meadows and mangrove woodlands are highly productive environments for the growth of rich and diverse marine organisms that promote the rapid accumulation of skeletal, calcareous, bioclastic debris when they die. Intertidal sandflats produce vast numbers of molluscs that also contribute to the vertical accretion and seaward progradation of the shoreline (Figure 8.2). These processes have been enhanced by a fall in relative sea level over the past 5000 years, stranding shell ridges and supratidal flats, which accumulate gypsum, dolomite and salt.

Geological setting

Between Whyalla and Port Augusta, the basic shape of the shoreline reflects bedrock geology; resistant Proterozoic rocks form shore platforms and headlands that shelter sandy bays. From Point Lowly to Port Augusta, the coast is closely aligned with the Torrens Hinge Zone (see Figure 9.1), a major complex fault that separates the eastern side of the Gawler Craton from the rocks of the Adelaide Geosyncline.

Introduction

This chapter describes the coastline between Port Lincoln and Whyalla, the southern section of the Gulf Coast of Eyre Peninsula; the coast between Whyalla and Port Augusta is included in Chapter Eight on the northern Spencer Gulf shoreline. The eastern, sheltered Gulf Coast of Eyre Peninsula has a subdued morphological character in comparison with the exposed rugged, open ocean Bight Coast due to restrictions on oceanic swell, a much calmer wind and wave regime, and greater tidal influences. This has resulted in less development of modern sand dunes and aeolianite (lithified sand dunes), which on the Bight Coast are widespread and have been eroded into spectacular high cliffs. Ancient coarse crystalline rocks of the Gawler Craton, which underlie Eyre Peninsula and most of the Gulf Coast, have been essentially stable for millions of years. Tectonic deformation has been restricted to widespread uplift of the peninsula as a single unit, with more localised downfaulting near its eastern margin.

The shape and origin of Spencer Gulf is associated with many long-lived basement faults (Figure 9.1). The orientation of the coast southwest to northeast from Port Lincoln to Port Augusta is broadly related to tilted blocks and associated observed and inferred faults expressed in low escarpments. These are up to 70 km long with displacements of 100 to 150 m, separating uplands from coastal plains. Faulting has uplifted parts of the peninsula, such as the Lincoln and Cleve Uplands and the Blue Range, which merge with the eastern coastal plain that provides the backdrop to the coastline. In some locations, such as the shores of Boston Bay, the shoreline closely follows the Lincoln Fault. The faults are oriented to the northeast or to the east, and ancient structural features influence some of them. Faulting is continuing, as revealed by offset of the Late Pleistocene Pooraka Formation; there is also ongoing seismic activity. In places, faults are coincident with the coastline, whereas in others there is only an indirect influence, as the shoreline position is determined by the location of alluvial sediments derived from the fault-bounded uplands.

Most of the coastal plain comprises alluvial deposits overlying resistant Proterozoic rocks, which also produce rocky headlands and help to explain the more detailed character of the coastline (Figure 9.1). Weathering of the resistant rocks has added diversity to the coastline.

Introduction

South Australia is notable for a remarkable diversity of coastal landscapes, many of which are of national and global significance. Numerous landscape-forming processes have influenced the evolution of this coastline. The current shape of the coastline relates to geological processes operating on a wide range of geological timescales that extend as far back as Archaean time (>2.5 billion years). As well as possessing many scenic wonders, the South Australian coastline presents numerous opportunities for scientific investigators to unravel the evolution of the coastline, with national and international implications.

As the broader continental-scale features of Australia influence the coastal landscapes of South Australia, the coastline should not be viewed in isolation from its hinterland. Australia in many respects is an old, flat and highly denuded continent, with the lowest topographical relief of all continents. Its intra-plate setting, high degree of tectonic stability, regional aridity, inland drainage (up to half the continent) and absence of major mountain ranges have significantly reduced the supply of terrigenous sediment to much of the coastline of South Australia. Few rivers reach the sea along the entire coastline of South Australia. Much of the State's drainage trends inland, and therefore the production of temperate sedimentary carbonates on the surrounding continental shelves is enhanced. Even on the Adelaide Plains, when the principal rivers (Little Para, Gawler, Light, Wakefield) do flow vigorously, their waters tend to temporarily exceed bankfull discharge and flood the adjacent flood plains, rather than reach Gulf St Vincent.

Desert dune fields related to intensified aridity and the latitudinal expansion of the arid zone during successive glacial events4 occur throughout extensive regions of inland Australia and along parts of the coastal margin. At times of glacial low sea level, when South Australia's gulfs were dry land, longitudinal dune fields extended across this broad region. On the northern Adelaide Plains, and to the east of Lake Alexandrina and the northern Coorong Lagoon (Big Desert), as well as on Eyre and Yorke Peninsulas, the dunes are very notable features of the regional landscape, sometimes dramatically truncated at the coast. They have contributed to coastal sediments.

General overview

The modern coastline: A general overview

The modern coastline was broadly established some 7000 years ago with the culmination of the most recent phase of postglacial sea level rise.

Introduction

The northern coastline of Gulf St Vincent (GSV) is a low-energy environment dominated by wide tidal flats with a peritidal sequence of subtidal seagrass meadows, intertidal sandflats and mangroves, and supratidal saline marshland. The broad form of the coast reflects the dominance of intertidal processes and their interrelationships with peritidal fauna and flora. Seagrasses thrive in the subtidal and intertidal shallow warm waters, with extremely productive calcareous algae, foraminifers and molluscan organisms manufacturing vast amounts of calcareous sediment. The accumulation of these biogenically derived sediments has generated the resultant wide intertidal and supratidal flats visible today by causing the shoreline to aggrade (build up) and prograde (build seawards).

Most of the northern GSV coast is bordered by mangroves (Figure 6.1), which occur from the Port River Estuary on the eastern side of the gulf around to Price on the northwestern side of the gulf. The mangrove woodlands are quite extensive around Barker Inlet, Middle Beach and the Light River delta on the eastern side of the gulf, at the northernmost part of GSV north of Sandy Point, Port Wakefield and continuing through the head of GSV to its western side around Port Clinton, Mangrove Point and Price. In the Barker Inlet, Caton et al. estimated that over 900 hectares (ha) is remnant vegetation, most of which is mangrove.

An extensive area of the northern GSV mangroves occurs within the Clinton and Wills Creek Conservation Parks. The Clinton Conservation Park (1923 ha) is a boomerang-shaped reserve extending from Sandy Point through Port Wakefield and around the western side and head of the gulf, to just north of the township of Port Clinton. The Wills Creek Conservation Park (2130 ha) is situated at Mangrove Point on the northwestern shores of GSV and extends from Port Clinton south to the town of Price.

An equally dominant feature of the northern GSV coastline is the extensive salt marsh environment occupying the intertidal to supratidal areas resulting in the GSV coast being referred to locally as ‘the Samphire Coast’. This type of coast is particularly important for its conservation significance, given the loss of large areas of similar salt marsh environment on the metropolitan coast. Within the Gulf, it is estimated that there are 6000 ha of salt marsh on the eastern side, 2000 ha at the head of the gulf and around 4700 ha on the western GSV coast.

Introduction

Fleurieu Peninsula, the smallest of South Australia's three main peninsulas, was named by Nicolas Baudin in 1802 after Captain Charles Pierre Claret de Fleurieu, a Member of the Institute of France, which promoted Baudin's expedition. The Fleurieu coast, which extends about 140 km from Sellicks Beach in the north to Middleton in the southeast (Figure 3.1), is bedrock-dominated and exhibits cliffs, bluffs, rocky islands, shore platforms and reefs; sandy pocket beaches occupy small bays where valleys intersect the coast. More extensive beaches at Normanville, Tunkalilla and Victor Harbor front last interglacial embayments, formed when sea level was about 2 m higher than present. Vast arrays of geological features, past ice ages, climatic changes, prior shoreline positions and erosional and depositional stream activities have impacted on the coast.

Geological evolution

Part of the South Mount Lofty Ranges, Fleurieu Peninsula is underlain by resistant, folded rocks and granite intrusions, now exposed at the coast due to progressive tectonic uplift and erosion (Figure 3.2). During the Early Permian (299 to 290 Ma), an ice sheet some 1000 to 2000 m thick covered Fleurieu Peninsula, travelling towards the northwest. The ice sheet aided exposure of the Encounter Bay Granites, produced the present distribution of granite headland and islands of Encounter Bay, eroded deep valleys such as Backstairs Passage and deposited great thicknesses of glacial sediments, which today provide sand sources for many beaches.

Throughout the Mesozoic (253 to 66 Ma), Fleurieu Peninsula was exposed to continual weathering and erosion. After the separation of Australia and Antarctica, various marine incursions invaded the Paleogene and Neogene (65 to 2.6 Ma) basins fringing Fleurieu Peninsula and partially re-excavated Permian valleys within the ranges. Progressive tectonic uplift of Fleurieu Peninsula is revealed by the offset of these Cenozoic marine sediments. In the Myponga Basin, limestone 20 Ma old occurs at up to 240 m APSL, while at Sellicks Beach, only 12 km away, it extends below sea level. Active tectonism has uplifted the peninsula largely as a single tectonic block, tilted slightly to the south and exposing resistant bedrock at the coast to initiate and maintain the rugged, cliffed Fleurieu coast.

Introduction

Kangaroo Island, Australia's third-largest island after Tasmania and Melville Island, is approximately 140 km long (east-west) and 55 km wide (north-south), with a coastline of some 458 km, and a land area of approximately 3890 km2. Coastal cliffs that developed on pre-Cenozoic bedrock, Paleogene-Neogene limestones and Pleistocene aeolianite (dune limestone) comprise approximately 66% of the island's coastline, while the remaining 34% of the coastline consists of sandy beaches. The island has 218 beaches, which average only 700 m in length. Kangaroo Island is separated from the mainland by Backstairs Passage, a stretch of water about 14 km wide between Dudley Peninsula, on the easternmost portion of the island, and Fleurieu Peninsula, on the mainland. In places, Backstairs Passage is an oversteepened bedrock depression formed by glacial erosion during the Early Permian (299 to 290 Ma). In the central portion of the passage, water depth exceeds 40 m; and closer to the island's coast, approximately 2 km to the east of Cape St Albans, it exceeds 80 m (Figure 11.1). In this region, an elongate bedrock depression trends in a northwesterly direction. The north coast of the island is separated from southernmost Yorke Peninsula by Investigator Strait, a stretch of water 50 km long and less than 55 m deep on the easternmost Lincoln Shelf.

With its long axis trending east-west, the island attenuates the impact of high-energy swell waves on Gulf St Vincent. The northwestern coastline of the island is bounded by the Lincoln Shelf, and its southern and eastern coastlines are bordered by the Lacepede Shelf (Figure 11.1). The edge of the continental shelf is approximately 60 km south of the island, where a spectacular series of submarine canyons (Murray Canyons Group) occurs. Some of these are among the largest in the world and remain active conduits for the transportation of continental shelf sediment to the deep sea. In contrast, the west-facing coastline between Cape Borda and Cape du Couedic is bounded by a narrow shelf, less than 10 km wide.

Regional geological setting

The regional landscape of Kangaroo Island comprises a weathered, partially dissected plateau surface mantled by deeply weathered regolith, including ferricrete developed on pre-Cenozoic bedrock. The plateau surface has been differentially uplifted and tilted with higher relief (250 m APSL) in the northwest of the island compared with the southeast, where the plateau surface is 50 m APSL.

Introduction

The 400 km coastal sector from the mouth of the River Murray to the border of South Australia and Victoria is primarily a depositional coastline of low topographical relief (<50 m), dominated by coastal dunes of Late Quaternary age (<125 ka).

From the River Murray mouth area south to Cape Jaffa, the coastline is formed by the coastal barrier landform Younghusband Peninsula and the associated Coorong Lagoon. The sector from Cape Jaffa to the border is dominated by rocky coastal cliffs, developed on Pleistocene dune limestone (aeolianite) with sandy pocket beaches and shingle beaches, comprising flint cobbles to the east of Port MacDonnell. Extensive sand sheets with migrating dunes occur at many locations along this coast. In only a few localities does bedrock older than Paleogene age (>66 Ma) crop out along the coastline, such as ‘The Granites’ north of Kingston SE. No rivers reach the sea along this coastline. Relative sea level changes associated with glacial cycles of the Quaternary have accordingly promoted sediment exchange from the continental shelf to the adjacent coastline and given rise to the region's principal coastal landforms, coastal barrier systems, uplifted over tens of thousands of years. From Cape Jaffa, south to Discovery Bay in western Victoria, the coastline displays slightly higher topographical relief in places, due to uplifted outcrops of aeolianite of Late Pleistocene age. These sediments are more strongly cemented, and give rise to vertical coastal cliffs.

The sediments and landforms of the modern coastline form the latest stage of the geological development of a succession of similar but older coastal barrier landforms. The Younghusband Peninsula and the Coorong Lagoon thus provide a modern analogue of the older shoreline features, which extend 500 km inland from the modern coastline, and back in time to approximately 6 Ma as a later phase in the early sedimentary infill of the Murray and Gambier Basins (Murravian Gulf). These stranded coastal dune barriers developed at times of high sea levels during Pleistocene interglacials.

The generalised distinction between unconsolidated Holocene sediments dominating the modern coastline to the north of Cape Jaffa and consolidated Pleistocene aeolianite being more prevalent to the south reflects variable vertical crustal movements.

Introduction

Yorke Peninsula, with its distinctive leg-shaped form, was mapped by Matthew Flinders in 1802 and named after the First Lord of the Admiralty, Charles Philip Yorke. The peninsula covers an area of approximately 6800 km2: from north to south, it is 240 km long; its width varies from 50 km in its northern section to 32 km in the south between Hardwicke Bay and Wool Bay; and the east-west trending ‘foot’ is 80 km long (Figure 7.1).

Several distinctive geographical and geological features of the peninsula have had marked influences on the present coastline. The peninsula is of low and smooth relief, with the Arthurton Trig (229 m AHD, or Australian Height Datum, which approximates mean sea level) in the central north being the highest point. Generally, land surface elevations increase northwards from <100 m AHD in the south, rising gradually to 200 m in the north, partly reflecting tectonic tilting of the land. Elevations at the coast are well below 100 m, restricting the potential height of coastal cliffs. No permanent streams on the peninsula and few intermittent watercourses reflect the moderate rainfall of 355 to 500 mm, high evaporation rates associated with strong winds and hot summers, as well as the ubiquitous surface cover of rain-absorbing Cenozoic sediments with thin soil mantles, which cover more than 90% of the peninsula. Consequently, little continental sediment is delivered to the coast, and most beach sediment is derived from offshore sources and coastal erosion.

Why does Yorke Peninsula resemble ‘a very ill-shaped leg and foot’?

There is a strong structural control on the Yorke Peninsula coastline, due to tectonic activity (faulting), rock fabrics, structural factors, and varying degrees of susceptibility of rocks to erosion. These factors, together with wind, wave and tidal actions operating during repeated migrations of sea level, help to explain the character of the coastline. The most recent rise in sea level (18 to 7 ka), following the Last Glacial Maximum, essentially established the modern coastline of the peninsula.

Yorke Peninsula consists of a subdued north-south ridge of ancient coarsely crystalline basement rocks linked to the west and southwest Gawler Craton, with thin mantles of younger rocks and sediment. The oldest Gawler Craton rocks on Yorke Peninsula are those of the Corny Point Paragneiss (1920 to 1845 ± 6 Ma).