The Eromanga Sea: An Inland Sea That Covered One Third of Australia

When early European settlers first ventured into Australia's harsh interior, they dreamed of discovering a vast inland sea—an immense body of water that would quench their thirst, nourish their livestock, and sustain their farms. Driven by hope and necessity, explorers set out on expeditions across deserts and barren plains, believing a glittering oasis might lie just over the next horizon. Yet each journey ended in disappointment: no shimmering inland sea awaited them. What these settlers didn't realize was that their search was millions of years too late. A vast inland sea had indeed existed, filling the continent's heart with shallow waters during the Early Cretaceous, long before humans walked the earth. This prehistoric ocean—the Eromanga Sea—had vanished around 95 million years ago, leaving behind only subtle geological clues hidden beneath Australia's sunburnt landscape.

Imagine the heart of Australia not as a sun-baked outback, but as a vast shallow sea stretching to the horizon. Around 120 million years ago, during the Early Cretaceous Period, central and eastern Australia lay beneath the waters of the Eromanga Sea, a great inland sea that flooded the continent’s interior. This was a time of high global sea levels and dynamic earth movements – conditions ripe for an ocean to pour into the middle of a continent. Over the next 25 million years, the Eromanga Sea expanded and contracted in pulses, transforming the landscape in phases. By about 95 million years ago, the sea finally withdrew for the last time, leaving behind a legacy recorded in rock, water, and even gemstone. What follows is the story of how tectonic forces, rising seas, and sediment shaped Australia’s last inland sea.

Tectonic Downwarp: A Continent Prepares for a Sea

Long before the first waves rolled in, the stage was set by movements of the Earth’s crust. In the Early Jurassic, Australia was part of the southern supercontinent Gondwana, and plate movements began to change the landscape. Distant tectonic forces to the east and north – where the Pacific Ocean floor was sinking under the edge of Gondwana – caused the land in the continental interior to slowly sag. Geologists call this process tectonic subsidence: a broad, gentle downwarping of the crust that created a huge low-lying basin. The Eromanga Basin, as it’s known, formed like a giant shallow bowl in the middle of the continent, covering parts of what are now Queensland, the Northern Territory, South Australia, and New South Wales. This basin was hundreds of kilometers across – a colossal depression in the land, imperceptibly sinking over millions of years.

During the Early Cretaceous, Australia's eastern margin experienced subduction, where oceanic plates slid beneath the continental edge and sank deep into Earth's mantle. The slow descent of these subducted oceanic slabs created large-scale downward mantle flow, generating a gentle yet immense pull on the crust above—a geological phenomenon known as dynamic subsidence. Although occurring deep beneath the Earth's surface, these invisible mantle currents profoundly influenced the continent, gradually pulling the land downward and forming extensive inland depressions, including the vast Eromanga Basin. As Australia continued drifting slowly northeastward, ongoing changes in mantle convection beneath the continent deepened these inland basins further. Geological modeling confirms that this mantle-driven subsidence created the broad, gently sloping landscape necessary for seawater to flood into Australia’s interior, ultimately giving rise to the expansive, shallow waters of the Eromanga Sea.

At the same time, the breakup of Gondwana was underway. To the south, Australia was slowly rifting away from Antarctica, and to the east, there may have been a volcanic cordillera – a mountain chain akin to the Andes – along the margin of the continent. These distant plate boundary forces caused the interior’s gentle downwarp (the Eromanga Basin) to continue deepening. By the late Early Cretaceous, a vast depression existed, waiting to be filled. And filled it would be – not just with water, but also with enormous volumes of sediment from the highlands at its edges.

Rising Seas Invade the Outback

Earth’s climate in the mid-Cretaceous was generally warm and seas were on the rise globally. As polar ice melted (if any remained at those times) and the shape of ocean basins changed, global sea level began to climb. Around 120 million years ago, the ocean finally found its way into Australia’s interior. The sea first crept in from the north: imagine shallow bays pushing inland from the ancient Carpentarian Basin (near today’s Gulf of Carpentaria) and from the east via the Surat Basin in what is now Queensland. What had been rivers and lakes on a low plain gradually turned brackish and then fully marine as saltwater flowed in. The Eromanga Basin became the Eromanga Sea, a true epicontinental sea (a sea on top of a continent) that spread out over one-third of the Australian landmass.

For millions of years, vast inland shallows covered this region. The Eromanga Sea was relatively shallow, often only tens of meters deep. Because Australia at that time sat much farther south (joined to Antarctica) – the water was cooler than in equatorial seas. In fact, scientists think the Eromanga Sea’s water was chill and muddy, not a clear blue tropical lagoon. It was also somewhat isolated, with narrow connections to the open ocean. This meant limited circulation, so its waters were often stagnant and low in oxygen in deeper spots. Picture a broad, calm inland sea under a cool sky, its silty bottom dark and still.

Once the sea arrived, it tended to expand and retreat in pulses rather than simply staying put. The causes of these fluctuations were complex. Global sea levels continued to oscillate, with notable rises during the Aptian stage (~115 million years ago) and slight falls in between. If the basin subsided faster or the world’s oceans rose higher, the Eromanga Sea would spread further inland. When sea level fell or sediment filled in the basin shallows, the shoreline would retreat. At least four major marine incursions flooded central Australia during the Early Cretaceous, each separated by intervals when the sea drew back and coastal plains emerged. We can imagine a time when the inland sea might have shrunken into expansive lagoons and estuaries, ringed by swamps and river deltas – only for the ocean to surge landward again in the next high stand.

Sediments: Recording the Tides of Change

Every advance and retreat of the Eromanga Sea was recorded in the layers of sediment laid down in the basin. In the earliest phase of flooding, as the sea transgressed over the land, it would have pushed a wave of sand ahead of it – literally beach sand and shallow marine sand – into the basin. Indeed, the base of the Eromanga Sea sequence includes deposits of sandstone that formed the ancient seafloor. These sand layers are extensive and later became crucial aquifers as we’ll soon see. As the sea deepened in the middle of the basin, finer sediments settled out. Thick mudstones and siltstones accumulated from the clay and silt suspended in the calm waters. Geologists find bands of dark shale rich in organic matter – these were the gooey muds of a seafloor starved of oxygen. In some layers, fossils of marine creatures (like clams and ammonites) are preserved, marking the times when the sea was at its greatest extent.

However, unlike many shallow seas, the Eromanga Sea did not build extensive limestone reefs or chalky deposits. Typically, in warm clear waters, you’d expect coral reefs or lots of shell limestone. But the Eromanga Sea’s unique conditions – cool, muddy, and often stagnant – meant that carbonate sediments (like coral reefs) were scarce. Instead, almost all the deposits were clastic (particles of rock eroded from land): sand, silt, and clay washed in from the surrounding highlands. Many of these sediments were volcaniclastic, originating from volcanic activity. Ash and fine debris from distant volcanoes (along that eastern Gondwana volcanic arc) rained into the sea and became part of the sediment mix. This gave the marine mud a distinctive chemistry – rich in iron minerals and volcanic fragments. On the seafloor, it created an environment where certain bacteria thrived, converting the sulfate in seawater to sulfide and precipitating iron sulfide minerals like pyrite (“fool’s gold”) in the mud. All these ingredients – volcanic ash, pyrite, and organic ooze – would later play a role in the formation of opal, one of the sea’s most dazzling legacies.

When the sea periodically shrank back, the newly exposed seabed turned into broad coastal plains and swamps. During those times, rivers pouring in from the continent’s interior and eastern highlands deposited coarser sands and gravels, building deltas at the sea’s edge. In swampy areas, lush conifer forests and fern prairies grew, only to be submerged again when the sea returned. Those vegetated interludes left layers of coal and red beds (rusty-colored soils) intermingled with the marine strata. Thus, the stratigraphy of the Eromanga Basin is a grand layer-cake that alternates between marine and non-marine deposits, recording each advance and retreat of the inland sea.

Over time, the basin filled up with an enormous thickness of sediment – up to 3,000 meters of Jurassic and Cretaceous rocks in some areas. By the Late Albian age (~100 million years ago), the balance began to tip. So much sediment had been dumped into the basin that it started to effectively fill the available space. Huge river systems from the eastern highlands (some as large as the Amazon or Mississippi) fanned out into the basin, depositing massive volumes of sand and mud on what had become a vast riverine plain. Geologists call the final extensive sediment layer the Winton Formation – formed on a broad floodplain as the Eromanga Sea retreated for the last time. By about 95 million years ago, these rivers and floodplains had filled the basin so much that the sea could no longer sustain a footing; it was forced to withdraw completely to the northern coast. The inland sea was gone, leaving behind a landscape of wetlands and deltas that would eventually dry out and turn to the arid outback we know today.. By 95 Ma, the Eromanga Sea has vanished, never to return, as Australia’s interior is uplifted and begins to dry.

In a subsequent video, I’ll be discussing the Great Artesian Basin, the vast groundwater reservoir that lies beneath one-fifth of Australia's surface. We'll explore how this massive underground aquifer formed from sediments deposited by the ancient Eromanga Sea, how it sustains life in some of Australia’s most arid regions today, and the challenges it faces as we continue to draw from its ancient waters.

As we conclude our journey, consider how the Eromanga Sea has shaped Australia in ways subtle and profound. The flatness of much of the interior eastern Australia, the broad plains of the Channel Country and beyond, owe their origin to the infilling of a prehistoric sea. The soils in some of these areas carry salts and minerals from ancient marine incursions. The stratigraphy beneath the ground – alternating layers of hard sandstone and soft clay – controls where rivers flow, where water can be found, and even where communities could develop.

In an awe-inspiring sense, the Eromanga Sea is gone but not gone. Its waters have withdrawn, yet its imprint is everywhere in the continent’s geology. Next time you find yourself in the vast quiet of outback Queensland or South Australia, try to picture the scene 100 million years ago: waves lapping at a shoreline near where you stand, marine reptiles gliding through water above what is now endless scrubland, and distant volcanoes dusting the sky with ash that drifts into a Cretaceous sea. The rocks underfoot remember this story. And through scientific exploration, we too can envision this ancient inland sea that one covered half of Australia beneath sea water.

Here's the video that we made on the Eromanga Sea: