Australia's Ancient Larapintine Sea

Imagine the heart of Australia not as red desert, but as a shallow blue sea. Wind ripples stir warm waters where today only dust devils dance. In this deep time vision, central Australia lies submerged beneath the Larapintine Sea, an inland ocean teeming with early life. Geologists and paleontologists have pieced together the story of this vanished sea from rocks and fossils scattered across the outback.

This is the story of a forgotten inland sea. Unlike the more recent Eromanga Sea—that existed around 100 million years ago and responsible for creating the vast Great Artesian Basin—the Larapintine Sea remains largely unknown. We’ve explored the Eromanga Sea and the Great Artesian Basin in previous videos, and you’ll find links to those in the description. But today, we’re turning our focus to the deeper past—to uncover the story of the Larapintine Sea, and the extraordinary legacy it left beneath Australia’s Red Centre.

Born of Tectonics and Rising Tides

The Larapintine Sea was born from a perfect confluence of tectonic fortune and high global sea levels. Over 500 million years ago, Australia was part of the Gondwanan supercontinent, and the crust beneath central Australia sagged and rifted, forming a broad basin. This depression – known as the Centralian Superbasin – opened the gates for ocean waters to transgress deep into the continent. As the Neoproterozoic ice ages waned and the climate warmed, global sea levels rose. Shallow marine incursions repeatedly flooded the interior lowlands. By the Early Ordovician period (~480–470 million years ago), geological evidence shows a shallow seaway had spread across what is now the Northern Territory, Queensland, and Western Australia

Tectonics also played a guiding role in shaping this sea. As Gondwana’s plates shifted, sections of the Australian craton buckled and bowed. A series of linked sedimentary basins – including the Amadeus, Georgina, Ngalia, and Canning basins – formed a corridor across the continent. Marine waters poured in along this corridor, establishing a shallow epeiric (inland) sea thousands of kilometers from any current coastline. The Larapintine Sea likely connected to open oceans at both ends: westward through the Canning Basin to waters off what is now Western Australia, and eastward through other basins toward the paleo-Pacific. In essence, it was a narrow inland ocean bisecting Australia, turning the center of the continent into a marine realm.

An Inland Ocean Across the Outback

At its height, the Larapintine Sea spanned a vast area of central Australia, covering parts of today’s Northern Territory, Queensland, South Australia, and Western Australia. The sea’s temporal extent stretched from the Cambrian into the Ordovician Period, with especially extensive coverage during Early to Middle Ordovician times. In the Amadeus Basin (central NT), up to 600 meters of shallow marine sandstone was laid down during this interval, attesting to prolonged inundation. To the northeast, the Georgina Basin and adjacent basins accumulated thick layers of limestone and shale from the same seaway. Even the relatively small Ngalia Basin (about 15,000 km² in area) records around six kilometers of sediment – marine and non-marine – deposited from the Neoproterozoic to the late Paleozoic.

Geologists envision the Larapintine Sea as a series of interconnected shallow bays and shelf seas. It likely resembled today’s Persian Gulf or the ancient Western Interior Seaway of North America, albeit on an Australian scale. The main axis of the seaway ran through what is now the Amadeus and Warburton basins, linking east and west. Broad shallow shelves flanked deeper troughs in places, creating environments from near-shore tidal flats to deeper subtidal zones. The sea’s waters were warm – central Australia sat near the paleoequator at the time. Indeed, fossils show that many Larapintine species were those restricted to low latitudes, implying tropical to subtropical conditions. Picture an outback where reef-like communities of algae and sponges grew on the seafloor, and soft-bodied organisms and early shellfish filtered nutrients in sunlit waters. This tranquil inland sea persisted for tens of millions of years, nurturing biodiversity and connecting marine life across Gondwana’s interior.

Life in the Larapintine Sea

Fossils recovered from Larapintine sediments paint a vivid picture of the life that once thrived in Central Australia’s inland ocean. In the Stairway Sandstone of the Amadeus Basin – deposited in the Middle Ordovician – paleontologists have found remains of diverse marine fauna. Bivalve molluscs dominate some fossil assemblages, alongside ancient gastropods, trilobites, brachiopods, and other invertebrates. The bivalves include species with strong Gondwanan affinities, and at least one (a nuculoid clam) found in both Amadeus and Georgina Basins, hinting that the two were connected by seawater at least intermittently. These creatures lived on a shallow marine shelf – part of the Larapintine Sea’s broad expanse – enjoying the nutrients and warmth of an epicontinental sea. The period corresponds to the Great Ordovician Biodiversification Event, a global surge in marine life diversity, and central Australia’s seaway contributed to this evolutionary blossoming by providing extensive new habitat.

Perhaps the most astonishing discovery from the Larapintine Sea’s deposits is evidence of early vertebrate life. In 1959, geologists near Alice Springs stumbled upon curious fish-like fossils in the orange-red sandstone. These remains were later identified as Arandaspis prionotolepis, an primitive jawless fish about 480 million years old – making it one of the oldest known vertebrates on Earth. The fossil fragments of Arandaspis consist of bony armor plates that once protected this small filter-feeding fish. The Larapintine sea provided a cradle for early vertebrates at a time when life was just beginning to experiment with backbones and hard body parts. In addition to Arandaspis, Ordovician rocks in central Australia have yielded trilobite fossils with ornate exoskeletons and even traces of mysterious graptolites – colonial organisms that floated in the ancient sea. Each fossil recovered from the Amadeus or Georgina Basin – be it a shell, bone, or burrow – is a time capsule from an era when the outback was underwater.

Receding Waters and a Vanished Sea

But no sea lasts forever. The Larapintine Sea’s retreat was gradual, paced by shifting continents, climate change, and rising lands. By the Late Ordovician (around 450 million years ago), Earth’s climate took a cold turn – an Ice Age at the end of the Ordovician caused global sea levels to drop. As glaciers expanded near Gondwana’s south pole, shallow seaways like the Larapintine began to shrink. Marine shorelines retreated from the central basins, stranding ecosystems and turning broad swaths of seafloor into coastal plains. Sediments from this time show signs of exposure and erosion; for example, in parts of the Warburton Basin geologists find conglomerates that hint at sea-level fall and deltaic shallows at the end of the Early Ordovician. The once-continuous seaway fragmented into isolated embayments before finally disappearing as a connected ocean.

The final demise of the Larapintine Sea came with renewed tectonic upheaval in the mid-Paleozoic. Between about 400 and 300 million years ago, the region experienced the Alice Springs Orogeny – a mountain-building event that uplifted and folded the central Australian crust. This orogeny (named for its effects still visible near Alice Springs) effectively squeezed shut the basins that had hosted the inland sea. Former seafloor strata were buckled into gentle folds or pushed up into hills. Rivers began to replace waves, carrying sand and gravel from rising ranges into what remained of the basins. By around 340 million years ago (Late Devonian to Carboniferous), sedimentation in the Amadeus, Georgina and Ngalia Basins had turned predominantly continental; the marine chapter was over. The vast Centralian Superbasin that once unified these areas had been torn apart by tectonics, breaking into the separate basins we identify today.

As the waters receded, they left behind a rich geological legacy. Sturdy layers of sandstone, limestone, and siltstone that had formed under the sea now made up the bedrock of central Australia. Some formations, like the Horn Valley Siltstone in the Amadeus Basin and the Coolibah Formation in the Georgina Basin, are black shales laid down in quiet deep-water conditions during the height of the Larapintine Sea. They are laced with organic matter – the preserved remains of plankton and algae, recording episodes of anoxia (oxygen-poor bottom waters) in the ancient sea. Overlying these shales are extensive sandy strata such as the Stairway Sandstone, which represent the shallow, oxygen-rich shelf environment teeming with benthic life. In the Ngalia Basin, marine deposits of the Larapintine Sea underlie later Devonian red beds, illustrating the stark environmental shift from sea to desert over time.

The withdrawal of the Larapintine Sea also reshaped the landscape. Where waves once lapped, outback plains and ranges eventually emerged. Sediments that accumulated under water were now subject to wind and rain. Erosion sculpted the softer marine deposits into lowlands, while harder layers formed ridges. In some places, ancient marine limestone dissolved to create karst features and caverns. Elsewhere, trapped seawater evaporated, leaving behind minerals – it’s thought that salt deposits in parts of the Amadeus Basin (and the salt-crusted surface of present-day Lake Amadeus) owe their origin to these drying seas. The modern MacDonnell Ranges and other uplifts around Alice Springs owe some of their rugged form to the tectonic compression that ended the seaway, exposing and tilting the layered rocks of the former seabed. Central Australia’s iconic red sands themselves are largely derived from weathering of the Larapintine Sea’s sandstone deposits, their iron oxides painting the desert landscape a vivid ochre.

Aquifers Beneath the Red Centre – The Sea’s Hidden Legacy

One of the most important legacies of the long-vanished Larapintine Sea lies invisible beneath the ground: vast aquifers of water stored in the porous rocks of those ancient marine sediments. As the inland sea deposited sands and carbonates across central Australia, it unwittingly built natural reservoirs that today hold groundwater like giant sponges. Over geologic time, these deposits were buried, lithified into rock, and later uplifted and fractured – but crucially, they retained pore spaces and cracks filled with water. In the arid environment of central Australia, such groundwater is literally the water of life.

The Amadeus Basin is a prime example. South of Alice Springs, the basin’s Paleozoic sandstones form a multi-layered aquifer system that is the sole water supply for the region. Within this basin, the main aquifers are the Mereenie Sandstone, Pacoota Sandstone, and Shannon Formation – all originally deposited when the area oscillated between shallow marine and coastal environments. These sandstone layers are relatively narrow and deep, lying about 150–200 meters below the desert surface. They are confined beneath less permeable siltstones and shales, which act as natural caps to keep water under pressure. Boreholes drilled through the confining layers strike water that often rises towards the surface.

Hydrogeologists describe the Amadeus aquifers as highly permeable and porous, especially where the formations are extensively fractured. Groundwater percolates through interconnected pores in the sandstone and along fractures or faults that formed during past tectonic events. In some areas near outcrop zones, the Mereenie Sandstone’s porosity can reach up to 20–30%, meaning a substantial portion of the rock’s volume is open space for water. More typically, the specific yield (effectively drainable porosity) of these aquifers is about 10–20%, which is still considerable for rocks of such age . Fractures provide the main highways for water movement, allowing the aquifer to transmit water readily. In practical terms, a single high-capacity well in the Amadeus Basin can yield on the order of 4–8 million liters per day to supply Alice Springs. This is impressive in an environment where surface water is scarce or seasonal at best.

The spatial extent of these aquifers is as remarkable as their capacity. The connected sandstones of the Amadeus Basin underlie tens of thousands of square kilometers of the central desert. Similar aquifer systems are found in the Georgina Basin, where thick Cambrian limestones act as water-bearing strata. In fact, the Cambrian Limestone Aquifer (CLA) of the Georgina and Wiso Basins is one of Australia’s largest groundwater systems, underlying roughly 474,000 km² of the Northern Territory and Queensland. This regional carbonate aquifer, also a product of the Larapintine Sea’s marine deposition, feeds numerous springs and waterholes. For example, water from the Georgina Basin portion of the CLA travels north and emerges at the Mataranka Thermal Springs – an oasis where clear warm water pours out of the ground, sustaining the Roper River. It is awe-inspiring to realize that these springs owe their flow to ancient rain and seawater soaked into Cambrian sea-floor sediments, now discharging many millennia later.

Water for 300 Years: Aquifers as an Outback Lifeline

In modern times, the groundwater stored in these fossil aquifers is a lifeline for communities and ecosystems across central Australia. Alice Springs, for instance, draws 100% of its drinking water from the Amadeus Basin aquifers. Infact Alice Springs owes its very existence to the legacy of the Larapintine Sea. The spring for which the town is named rises from groundwater stored deep within sandstone layers laid down over 450 million years ago, when this region lay beneath a vast inland sea. These ancient marine sediments, part of the Amadeus Basin, now form aquifers that continue to nourish the heart of the desert. In a very real sense, the vanished sea still flows beneath Alice Springs. Estimates suggest that at current rates of use, the Amadeus Basin Groundwater System holds enough water to supply Alice Springs for 300–400 years. To put it in numbers, the town uses roughly 14 gigaliters (billion liters) of water per year, and only a small fraction of that (perhaps 20%) is naturally recharged annually by scant rainfall. The remainder is “fossil water” – rain that fell thousands to tens of thousands of years ago when the climate was wetter, now stored in what was once the Larapintine seabed. By one approximation, well over 5,000 gigaliters of potable water might reside in the Amadeus aquifers alone (equivalent to about ten Sydney Harbors), although not all of it is economically recoverable.

Though over 400 million years have passed since waves last rolled over central Australia, the Larapintine Sea’s story is far from lost. Every fossil shell and every well that draws cool water on a hot desert day is an echo of that ancient sea. Geologists drilling core samples in the Amadeus or Georgina Basin often bring up dark marine shales or fossil-rich limestones, prompting awe that such rocks formed in what is now a desert. In some cores, they even detect the faint whiff of petroleum – another legacy, as the organic-rich muds of the Larapintine Sea have matured into oil and gas over time. (Indeed, central Australia’s modest oil fields, like the Mereenie field, owe their existence to algae and plankton that bloomed in the Larapintine Sea and were buried in its sediments.)

Finally, consider the surprising juxtaposition the Larapintine Sea offers: infinite water in the midst of a desert. Stand on the dry shores of a salt lake in Central Australia today, and you stand on what was once a shallow seabed. The parched clay beneath your feet holds salt from an evaporated ocean. Far below, in rock pores and crevices, the old sea’s groundwater still sloshes, slowly making its way through stone. The present desert ecosystem, with its resilient shrubs and secretive reptiles, lives on land that was once a marine paradise of wave-lapped beaches and strange Paleozoic creatures. This contrast imbues the landscape with a sense of wonder.

The Larapintine Sea may be gone, but it is not forgotten. Its footprints lie written in rock –and its spirit endures in water.

Here's a link to the video we made on the Larapintine Sea: