General

A snippet of Kakadu NP geology

| Marianne Harvey

The areas of Ubirr and Nourlangie Rock I visited in May 2017 are dominated by the fluvial Kombolgie Formation sandstones and conglomerates. This sequence of sediments was deposited approximately 1.7 billion years ago by a braided river system flooding across a wide flat plain that had formed after a long period (~100 My) of erosion of even older rocks.

The conglomerates seen at Nourlangie are the basal units of the Formation and the size of the quartzite pebbles indicates deposition by a high-energy river system. As the energy of the system decreased over time and with the increasing thickness of material deposited, the sediment able to be carried by the river became finer resulting in the upper units grading to sands.  Flash-floods would have occurred from time to time depositing thick layers of coarser sand, and cross-bedding is evident in places where river banks may have been over-topped.  Ripple marks can be seen in the finest sands and apparently indicate material deposition from the North-West[1], although I did not get a photograph of these!  Will have to go back sometime…

The consolidation of this thick (up to 1km) pile of sediment took place over the next 1.5 billion years while the area remained stable. During the Mesozoic period (~140 Ma), a sea invaded the area causing rapid erosion of the sandstones by wave action.  This sea retreated around 100 million years ago leaving the major components of the landscape as they are seen today.  Shallow seas have inundated the area since, as climate patterns have changed, indicated by the presence of marine fossils – also not observed by me!

[1] http://learnline.cdu.edu.au/tourism/kakadu/values/pdf/geology.pdf

Isolated stacks of the Palaeo-Proterozoic Kombolgie sandstone at Ubirr. These stacks would have been islands off the sea cliffs of the (now) Arnhem Land Plateau during the Mesozoic, until the sea receded ~100 My ago.
Isolated stacks of the Palaeo-Proterozoic Kombolgie sandstone at Ubirr. These stacks would have been islands off the sea cliffs of the (now) Arnhem Land Plateau during the Mesozoic, until the sea receded ~100 My ago.

 

 

 

 

 

 

 

 

 

 

 

 

 

View across the Nardab (fresh water) wetlands from Ubirr lookout.
View across the Nardab (fresh water) wetlands from Ubirr lookout.

 

 

 

 

 

 

 

 

 

 

 

 

Cross bedding in the sandstone is evident in places at Ubirr.
Cross bedding in the sandstone is evident in places at Ubirr.

 

 

 

 

 

 

 

 

 

 

 

 

Conglomerate at Anbangbang Shelter, Nourlangie Rock area.
Conglomerate at Anbangbang Shelter, Nourlangie Rock area.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Close-up of conglomerate. The quartzite pebbles are not particularly well-rounded which indicates they either didn’t travel particularly far, or the energy of the river system must have been quite extreme.
Close-up of conglomerate. The quartzite pebbles are not particularly well-rounded which indicates they either didn’t travel particularly far, or the energy of the river system must have been quite extreme.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sandstone cliffs of Nourlangie Rock. Photo taken from Kunwardehwarde Lookout but still standing on conglomerate.
Sandstone cliffs of Nourlangie Rock. Photo taken from Kunwardehwarde Lookout but still standing on conglomerate.

 

 

 

 

 

 

 

 

 

 

 

 

 

Looking South East from Kunwardewarde Lookout to the Arnhem Land Escarpment, or +100 million year old ‘sea cliffs’.
Looking South East from Kunwardewarde Lookout to the Arnhem Land Escarpment, or +100 million year old ‘sea cliffs’.

 

 

 

 

 

 

 

 

 

 

 

 

 

References:

Kakadu National Park – Geology notes, from https://www.environment.gov.au/system/files/resources/ac9756b9-9f52-4a00-b964-25c33c41b8f7/files/geology.pdf

Geology of the Kakadu Region, from http://learnline.cdu.edu.au/tourism/kakadu/values/pdf/geology.pdf

Geology of the walking trails in Kakadu National Park, R.S.Needham, AGSO Record 1992/81, from https://d28rz98at9flks.cloudfront.net/14605/Rec1992_081.pdf

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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