Seven years ago, Zealandia was confirmed as Earth’s eighth continent—a slab of continental crust stretching from New Zealand to New Caledonia, 95% of it underwater. In 2023, it was fully mapped for the first time.
Now, scientists have pulled together decades of work by hundreds of researchers to consistently characterize and map of all New Zealand’s offshore sedimentary basins—offering clues about Zealandia’s evolution.
“A sedimentary basin is any hollow on the surface of the Earth,” explained Lorna Strachan, a geologist from the University of Auckland. “This can be anything from a puddle in your driveway to the Pacific Ocean.”
These basins accumulate sediments over time—layer upon layer of sand, mud, rocks, shells, pollen, and bones that tell myriad stories of past climates, ocean currents, plate tectonics, the birth of mountain ranges, ecology, and evolution.
“I’m biased, I’m a sedimentologist,” said Strachan. “But to me, they’re a really rich archive of past history—I’d argue the most important.”
In a new paper, researchers from GNS Science Te Pū Ao (the Institute of Geological and Nuclear Sciences Limited, a New Zealand Crown Research Institute) compiled decades of data to define the distribution, extent, and sediment thickness of 25 major offshore basins around New Zealand. The work is part of a much larger, likely decades-long project to unravel Zealandia’s geologic history.
Lead author Kyle Bland likened the endeavor to the Māori concept of whakapapa—tracing Zealandia’s ancestry or genealogy. “Why does it have the shape it does? When did it break away from Gondwana? Why do we have these central North Island volcanoes? Why do we have the Southern Alps? If you understand how everything came to be, you can explain—at least at a high level—why we have the structure of New Zealand that we do.”
Fragments and Forams
One key data source for the researchers was a collection of rocks drilled from the deep seafloor. The sediment cores were collected by prospecting petroleum companies as well as scientists participating in the International Ocean Discovery Program.
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Inside these 10-centimeter-diameter cylinders are millions of years of history, Bland said. Most of Zealandia’s basins formed during the mid- to Late Cretaceous (about 100 million to 66 million years ago), although younger basins date to as recently as 3 million years ago.
He held up a 3D printed seashell-shaped object, a model of a type of foraminifera (foram), a family of single-celled ocean-dwelling organisms. It’s not to scale: “This thing would be about the size of a human hair in terms of thickness,” Bland said. Small, yes, but mighty: Researchers are “reconstructing a continent, how we think it looked over 100 million years…mostly using these little things.”
Because forams evolved quite rapidly, diversifying into new species that are relatively easy to identify, their fossils can help geologists date the sedimentary rocks in which they’re found. “We have scientists here who sit at a microscope with a really fine little brush, picking and picking,” Bland said. “And by looking at the species we find, we can say, ‘Aha, this rock must be 50 million years old.’”
By applying the same dating methodology consistently to samples from all 25 basins, Bland’s team was able to identify when Zealandia began to break away from the rest of Gondwana—about 105 million years ago. “Across the basins…the number’s the same. You have to stand back and look at everything together to get that sense of a pattern.”
The sediments reveal what was happening above the waves, too. Researchers were able to identify when New Zealand’s Southern Alps started to rise from the South Island’s backbone 10–12 million years ago, for example, as a result of the collision of the Australian tectonic plate with the Pacific.
The cores reveal “incredible volumes of sediment being deposited in the areas surrounding the South Island and as far north as Taranaki,” Bland explained, referencing a western region on the North Island.
Similarly, around 100 million years ago, the sediments in basins across Zealandia were full of coarse riverbed conglomerates, he said. The pattern is suggestive of high fault activity, the growth of hills, and a landscape type called horst and graben.
In addition to the cores themselves, Bland’s team compiled seismic reflection imagery to compare sediment thicknesses across Zealandia’s basins. The deepest—reaching up to 11 kilometers thick—were the Taranaki, Raukumara, East Coast, and Pegasus Basins, those nearest the modern Australia-Pacific plate boundary. Knowing the depth to the harder basement rocks beneath will be crucial for future work to calculate seismic hazard risks in different places, Bland said.
A Little Climatic Dipstick
In a separate, but related, paper published in 2022, Bland and other colleagues, including James Crampton, a paleontologist from Te Herenga Waka Victoria University of Wellington, used analyses of the same sedimentary basins data to underpin and test a computer model. They then used the model to produce a series of paleogeographic maps showing our best current understanding of Zealandia’s evolution from the mid-Cretaceous to the present.
Inorganic sediments found in the cores helped to ground truth these interpretations. Fragments of volcanic rock and ash, for example, contain particles of iron, which align themselves with magnetic north. This alignment can help identify where on Earth’s surface the rock was formed, informing the maps. “They’re like little ancient compasses,” said Bland.
Together, the papers provide a great baseline and a springboard for future work, said Strachan, who was not involved in either study. Big questions remain about the tectonic evolution of Zealandia, she said, especially over the past 30 million years.
The deep-sea sediment cores we have are largely clustered around areas thought to have oil-drilling potential, leaving huge areas of the undersea continent essentially unexplored. “We’re trying to reconstruct an area the size or Wales or Switzerland based on one 10-centimeter hole,” acknowledged Bland.
“I’m not a jigsaw puzzle enthusiast,” said Strachan. “But I liken it to having a 10,000 piece jigsaw puzzle, and all you’ve got is five pieces—and the pictures have rubbed off. It’s difficult. There are lots of things missing. And so you put together the most logical interpretation based on what you’ve got.”
What scientists do have is a site that sheds light on an important corner of Earth’s surface. “We sit right across all these latitudinal, oceanographic, climate and atmospheric boundaries. New Zealand’s been this little climatic dipstick sitting in glorious isolation in the southwest Pacific, just recording it all for us—and that record is in the sedimentary basins,” Bland said.
This article originally appeared in Eos Magazine.
