Western Australia is well known for its very old rocks, but it also has many locations where younger rocks (Phanerozoic, less than 538.8 million years old) highlight important chapters of more recent Earth history.
Geologists working on trace fossils in the Tumblagooda Sandstone have identified a set of trackways and burrows that provide an intriguing insight into an arthropod-dominated ecosystem that existed well before dinosaurs ruled the Earth. The following describes the Tumblagooda Sandstone and its formation and landscape, as well as the traces of life it preserves.
The features described here are located within a National Park, where it is prohibited to collect any rocks, including fossils. Do your part to protect Western Australia’s fossils by staying up to date with the restrictions and responsibilities related to fossils and State geoheritage, following all laws and regulations, and spreading the word to others (including tourists).
Tumblagooda Sandstone
The Tumblagooda Sandstone is in Kalbarri National Park, 600km north of Perth and is a sedimentary rock, deposited as successive layers of sand. These layers form the colourful horizontal banding, or bedding, seen in gorges along the Murchison River and coastal cliff sections.
The iconic red and white striping and blotching is caused by iron oxides staining the rocks. The colouring probably formed shortly after the rock was deposited, in an oxygen-rich (rather than stagnant or swampy) environment. The colouring often highlights features formed in the original environment, such as burrows and ripples.
Interpreting the original Tumblagooda Sandstone landscape
The sediments that now make up the Tumblagooda Sandstone were deposited on a broad (more than 100km wide) coastal area covered by numerous constantly shifting river channels, with minimal land vegetation to bind the channel banks between.
When the Tumblagooda Sandstone was deposited, Australia was still adjacent to land masses now part of Asia, India and Antarctica, and the shore and its river channels were influenced by a large inland sea. The tidal range during this time may have been larger than today, and the boundary between the river plain and the tidal flats would have been irregular and constantly changing.
Abundant trackways indicate that a wide variety of arthropods were living in both the river channels and tidal flats in the Kalbarri region. This interpretation agrees with fossil evidence in other places around the world. Evidence suggests that this was a time of intense evolution and adaptation for arthropods, who rapidly became important parts of many different ecosystems, and were the dominant animals living on the land at the time.
Arthropods aren’t the only animals seen in the Tumblagooda Sandstone. Intensely burrowed layers of sedimentary rocks indicate worms mined the just beneath the flats in search of food as they do in coastal flats today.
Arthropods
Only one body fossil has been found so far in the Tumblagooda Sandstone — an impression of a strange arthropod called Kalbarria, which belongs to a now-extinct group called the euthycarcinoids. Body fossils are rare in the sandstone because the depositional environment did not favour the preservation of organic material. The active, high-energy river system would have quickly broken down any dead bodies and the water’s high oxygen level would have rapidly decayed flesh.
Despite this, the sandstones are rich in signs of life, preserving many tracks, traces, and burrows of organisms that lived while the sandstone was being deposited. These ichnofossils give us an insight into not only what organisms were present while the sandstone was forming, but also what the environment was like.
Ichnology
Ichnology is the study of trace fossils (ichnofossils), which indicate an organism’s behaviour, such as walking, feeding, burrowing, escaping and even defecating. Trace fossils can be used to understand the environment of deposition of sedimentary rocks, and sometimes their age.
An ichnofauna is a collective of traces left by organisms. Ichnofaunas, especially those formed in marine environments, can be restricted to very specific conditions or water depths. These types of studies can be used to understand past environments.
Ichnofossils of the Tumblagooda Sandstone are comparable to those of the Devonian Taylor Group in Antarctica and the Ordovician to Silurian Mereenie Sandstone at Kings Canyon in central Australia. Most of the traces in the Tumblagooda Sandstone were formed by animals living under water (aquatic or subaqueous). Some trackways seem to indicate that they were subaerial or formed above the water.
Subaerial traces
The subaerial traces seen in the Tumblagooda Sandstone consist of many types of Diplichnites, a walking trace. Diplichnites are trackways, made up of a pair of parallel rows of ‘pits or paddle-like impressions, which can extend over surfaces for many metres. Tracks made in sands that were either very dry or very wet often lack definition, but those made in just the right conditions have exceptional detail preserved.
Diplichnites traces are interpreted to be formed by arthropods walking over a sediment surface, so the range of walking trails indicates the area was populated by a diverse fauna of arthropods during deposition. When the trails are well preserved, we can count how many legs the trace-maker had, helping to identify the arthropods that made them.
Aquatic traces
There are many examples of aquatic or subaqueous burrows in the Tumblagooda Sandstone. A few of the more common types include:
- Skolithos: a simple vertical or near vertical burrow that can be tens of centimetres in length, with some examples up to 2m long
- Heimdallia: forms intensely burrowed beds with a ‘can of worms’ appearance, found in tidal sandflat deposits
- Psammichnites: a horizontal, gently meandering trail, possibly created by a crawling or grazing marine snail
- Tumblagoodichnus hockingi: a trace unique to the Tumblagooda Sandstone. A probable arthropod hunting burrow — named after GSWA geologist Roger Hocking, who discovered it during the first detailed study of the Tumblagooda Sandstone.
Age of the Tumblagooda Sandstone
The age of the Tumblagooda Sandstone is uncertain, with previously documented ages ranging from the Cambrian to the Cretaceous. The most likely age is middle Palaeozoic. We are working on new techniques to find out exactly when this formation was deposited.
