Kakadu is an ancient landscape. It has some of the oldest exposed rocks in the world dating back some 2500 million years. The spectacular and diverse geology of Kakadu is another reason why the area was inscribed on the World Heritage list.
Geology of Kakadu by time
The region’s cave paintings, rock carvings and archaeological sites record their skills and way of life, from the hunter-gatherers of prehistoric times to the Aboriginal people - Bininj/Mungguy - who still live in the park today.
While the overall geological evolution of the park is the same, the geology of northern Kakadu is very different to the geology of southern Kakadu. For more information about the evolution of the Kakadu’s geology over time, download the Geology of Kakadu brochure.
Geology of Kakadu described by geological feature
Some 140 million years ago Kakadu was under a shallow sea. The sea cliffs forming the shoreline are now the dramatic escarpment wall that can be seen at Gunlom, Jim Jim, and Twin Falls and from the Gunwarde-warde Lookout at Nourlangie.
Vast floodplains stretch across Kakadu's lowlands. These dynamic environments are gradually built up by sand and silt eroding from rocks and being carried by wet season waters. You can see these relatively young landscapes at Yellow Water, Mamukala and from Ubirr.
An unconformity is where there has been a period of erosion between the formation of an older rock and the deposition of a younger rock. You can see an unconformity at the Budjmi walking track lookout near Twin Falls creek crossing. Here 1 800 million year old granite was exposed by erosion and then covered by the Kombolgie Sandstone formation. This unconformity represents a 100 million year break in time. Another unconformity can be seen near the Ikoymarrwa Lookout, north of the Goymarr Interpretive Centre at the Mary River Roadhouse. Here red weathered basalt sits under a lighter coloured sandstone cap.
Molten rock forced up between cracks in surrounding rocks can solidify to become an igneous intrusion. These intrusions are more resistant to erosion than the softer surrounding rocks.
Ripple marks were left by the currents of a huge river that deposited the Kombolgie sandstone formation 1 700 million years ago. Kombolgie sandstone was so named by geologists after the Kombolgie Creek in the south of the park.
Cross bedding occurs when there are changes in water flow while sediments are deposited. Cross beds are layers with different inclinations. For example, a horizontal layer develops during a quiet period with little water flow. Then, during faster water flow, the sediments deposit on a sloping surface.
Rivers and floods often carry rocks and deposit them where the water movement slows, resulting in rocks embedded in sandstone—conglomerate. The size of the embedded rocks indicates the rate of water flow, with larger rocks deposited in faster flowing water. The roundness of embedded rocks reflects how far they travelled, with rounder rocks having tumbled further.