Geology
Kakadu is an ancient landscape, it has some of the oldest exposed rocks in the world. Kakadu's people describe how landscapes were created by the journeys and activities of creation beings.
The spectacular and diverse geology of Kakadu is recognised in the World Heritage Listing of the park.
Timeline
- 2 500 mya oldest known rocks of Kakadu form as granite intrusions in the earth's crust.
- 2 000 mya erosion of the crust exposes the granite. Faulting forms a wide shallow depression, a geosyncline.
- 1 870 to 2 000 mya oxygenated atmosphere evolves. High areas erode and are deposited as sediments in the geosyncline. Faults sag and layers of sediment 10 km thick collect in the basin.
140 million years ago Jim Jim Falls was part of a sea cliff shoreline above a shallow sea.
- 1 860 mya the weight of sediments destabilises the earth's lower crust and mantle, leading to mountain building. Pressure and heat fold and metamorphose sediments to gneiss and schist rocks. New granites intrude. Faults open up rift valleys in southern Kakadu and volcanoes fill them with lava. There is very rapid erosion by high-energy rivers.
- 1 650 mya micro algae evolve. Flash flooding of large braided rivers spread a one km thick layer of sand from an unknown land mass to north west. This was followed by a long erosion period in an arid climate produces a flat, desert-like landscape with scattered low ridges and hills. Rocks are deeply leached.
- 1 000 mya sands consolidate to form the Kombolgie Sandstone Formation which now forms the Arnhem Land Plateau. The sandstone is often ripplemarked and cross-bedded and there is basalt lava near the top.
- 500 mya, the time of the first macro organisms. A long geologically stable period apart from crustal warping that forms faults and joints in sandstone (now weathered to gorges and waterfalls).
- 500 - 140 mya is the time of the first land plants and age of the dinosaurs. Mesozoic seas spread across the area, eroding older sandstone into sea cliffs (now the Arnhem Land escarpment) and islands (now outliers like Ubirr). Sandstone and siltstone containing fossils are deposited over lowlands.
- 100 mya Mesozoic seas receded and most of their sediments are eroded away. The major components of today's landscape are apparent.
- 50 mya ancient faults in southern Kakadu move once more to form local depressions. Swampy sediments with fossil tree palms are laid down and preserved. 10 to one mya continued erosion of highlands and deposition of sand on lowlands. Rocks are deeply leached and laterite forms.
- 10 000 years ago, in the Estuarine Period, the sea invades lowlands. Beach ridges show the sea advancing 20 to 30 cm a year.
- 7 000 years ago the sea level drops and freshwater swamps form as the coastline advances.
Kakadu's geological attractions
Escarpment
On top of Ubirr you can see ripple marks in the sandstone.
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.
Floodplains
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.
Unconformity
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 lightercoloured
sandstone
cap.
Igneous intrusions
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
Bardedjilidji near the East Alligator River
is a good place to see cross bedding.
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
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.
Conglomerates
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.
