Deep in the North Sea, researchers have mapped giant buried mounds and “sinkholes” that upend a fundamental rule of geology.
A new peer-reviewed study shows that heavy, younger sands sank while older, lighter layers rose, forming hills hidden beneath the seafloor.
The team assembled a massive three-dimensional seismic data set with evidence from numerous wells to show that the mounds occupy an area of approximately 19,000 square miles.
The work highlights a process that can scramble the usual order of layers and change the way scientists judge the safety of places that could store captured carbon.
Mysterious mounds of the North Sea
Geologists generally rely on the law that the oldest layers lie beneath the youngest, unless the rocks have been overturned.
In the case of the North Sea, the rule is broken locally because dense sand is pushed downward while lighter material rises.
The lightest layer is a stiff, low-density mud known as silt, derived from the remains of small marine life. The heavier layer is younger, loose sand that collapsed through cracks and then spread, supporting rafts of silt from below.
The result is a stratigraphic inversion, a reversal into the expected stacking order. The team names the sand bodies that sank the wells and the silt blocks that floated the flotites.
These features do not resemble simple landslides or classic sand intrusions. Their magnitude, shape and relationship to fractures in the mud indicate a different factor.
Basic physics at work
During strong shaking, wet sand can lose strength and move like a fluid, a process called liquefaction. If this liquefied sand sits on top of a stiffer but lighter layer, the dense mud tends to sink and the lighter layer tends to rise.
This pushing and pulling creates instability due to buoyancy. The silt, cut by natural polygons of small faults, breaks into rafts that heave as sand flows along the fractures.
Earthquakes that occurred in the region millions of years ago likely triggered several cycles of movement. Each pulse would have let more sand fall and more silt rise until the energy disappeared and the system locked into place.
How scientists spotted this pattern
Seismic reflections reveal sharp boundaries between silt and sand intrusions. Mounded areas are confined to a specific slice of the rock record, while the layers above and below remain essentially intact.
In places, thin sand-filled fractures connect the intrusions below to the sands above, indicating downward movement rather than sand injected from great depths.
The chemistry and grain composition of some buried sands also match nearby overlying sands, supporting the same conclusion.
The mounds form ridges and pods that reflect the size and orientation of the surrounding polygonal faults. This map view geometry fits the idea of sand flowing along fracture networks while rafts of silt rise between them.
North Sea Mounds and Carbon
Engineers are already injecting carbon dioxide into a large North Sea sandstone known as Utsira, part of the long-running Sleipner project.
Any new process that moves fluids or moves underground layers is important when selecting safe and sustainable storage targets.
Understanding whether sands can move and whether joints remain tight helps planners estimate their long-term behavior. It also shows where to avoid injecting and where to monitor more closely.
The reserves of pore space in the North Sea are enormous, but safety depends on the details of the rocks at each site. Results like these inform the checklist used to evaluate storage security.
More tests and answers needed
“This discovery reveals a geological process that we have never seen before at this scale. This research shows how fluids and sediments can move in the Earth’s crust in unexpected ways,” said Mads Huuse, a geophysicist at the University of Manchester who led the study.
The data favor a model in which sand sank as mud while silt rose as rigid rafts. Still, scientists want to test how often such reversals occur, the size of the units, and the levels of shaking needed to start the movement.
Another question is that of timing. Evidence suggests concentrated activity during the late Miocene and Pliocene, but the exact sequence of pulses and pauses varies across the basin.
North Sea Mounds and New Geology
Geologists mapping buried landscapes use shape, texture and context to decide whether a body is a channel, landslide or intrusion.
Sinkites add a new category with their own fingerprints, like jagged edges where sand fills polygonal fractures.
For industrial and storage projects, the work adds new clues to spot areas where density contrasts once the stack is rearranged. He also cautions against assuming that all thick sands formed where they are now.
Future studies can target other continental margins where light biogenic mud lies beneath younger sand. If similar structures appear elsewhere, the process is not an oddity of the North Sea but part of a larger pattern.
More laboratory tests and computer models are exploring how liquefied sand moves through fractured layers. This allows a striking field observation to be transformed into rules that predict when and where the reversal will occur.
The study is published in Earth and Environment Communications.
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