Geology of the Flatrocks site

by Lisa
Publish date
28 February 2012
Comments (5)

Lisa works in the Public Programs Department at Melbourne Museum but also volunteers in the Palaeontology Department and has been on several fossil digs.

By the tenth day of the annual Dinosaur Dreaming dig we had already catalogued more than 140 fossils. To know where to dig in the first place we need to understand the geology of the area because the types of rock and how they have been laid down can give us much information about the palaeoenvironment. Dr Alan Tait, Adjunct Research Fellow in the Department of Geosciences at Monash University is currently researching the sedimentology of the Flatrocks site and kindly explained its geology to me.

Today the site known as Flatrocks is a rocky beach dominated by light grey sandstone but 120 million years ago during the Cretaceous, the environment was very different. Australia was once part of a supercontinent called Gondwana which also comprised Antarctica, South America, Africa, New Zealand and India.

Much of Gondwana had broken up by the Cretaceous and a rift had started to form between Australia and Antarctic. The types of rocks and fossils we find along the coastline in Inverloch today tell us the story of the rift valley and the animals and plants that lived there.

The cliff face near the Flatrocks site The cliff face near the Flatrocks site. The grey mudstone is the remains of a flood plain which was on the floor of the rift valley. The layer where we find most of our fossils lies above this and at the top is massive sandstone. To the left of the mudstone you can see a fault where the rock layers have shifted dramatically from their original horizontal deposition.
Image: Lisa Nink
Source: Museum Victoria

The fossil layer itself consists of the sedimentary rocks, grey sandstone and conglomerate that were deposited during flooding of the rift valley. The conglomerate pebbles are made of clay eroded from the flood plain soils during flooding. The sandstone is grey because it contains grains of volcanic rock eroded from active volcanos some distance away and washed into the rift valley. The sediments also include the fossilised remains of dead animals, plants and trees. The time between the floods was long enough for large trees to grow, perhaps at least 100 years, and the floods were catastrophic.

Cliff at Inverloch “The main fossil bearing layer (under the red line) consists of grey sandstone with coal throughout it. The layer is bounded by a layer of mudstone below and massive sandstone above.
Image: Lisa Nink
Source: Museum Victoria

There are many fossilised tree stumps on the shore platform. Some of these trees lie horizontally with their fossilised roots still attached and are believed to have been knocked over by the force of the floods and washed down the river. We also find fossil leaves of ferns, gingkoes and monkey puzzle-like trees that once grew as part of a forest within the rift valley.

Fossil tree trunk A fossil tree trunk. If you look closely you can even see the growth rings.
Image: Lisa Nink
Source: Museum Victoria

The coal in the fossil layer is the remains of decomposing plants that once grew in the valley. Fossilised grains of pollen from these plants have also been found and by identifying their species, we can date the sediments surrounding them.

A nearby dyke (a long straight crack in the rocks through which magma from deep below the Earth's crust travels upwards and cools) is made up of basaltic rock, another igneous rock type. The dyke is 99.5 million years old and cuts through the grey sandstone, meaning it formed after the sedimentary rocks had been deposited. 

volcanic dyke at Inverloch Dale Nelson stands upon the basaltic dyke near the Flatrocks site.
Image: Lisa Nink
Source: Museum Victoria

We also find minerals at the site, like pyrite and calcite.

Crystals found at dinosaur dig Minerals found at the fossil dig site, shown with objects often found in geologists' pockets, for scale. Left: Pyrite crystals | Right: Calcite crystals
Image: Lisa Nink
Source: Museum Victoria


Dinosaur Dreaming blog

Infosheet: Dinosaur Dreaming - the Inverloch fossil site

Video: Dinosaur Dreaming

Comments (5)

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Glen warren 28 February, 2012 15:30
What is the dip of the rock formations and is there a date for the fault and was the fault effected by the dyke or a banolith body.
Calan Murphy 28 February, 2012 17:42
Great blog! Really interesting, and good pics too!
Wendy White 4 March, 2012 13:00
Love this piece, Lisa! A great overview.
Mike Cleeland 7 March, 2012 23:21
Nice work Lisa; good up to date info! @ Glen; the dip of the rock formations is about 10 degrees to the north. There is no known date for the fault, except that it is presumed to be quite old, as the suface has levellled off after the displacement. The fault is a couple of hunderd metres from the dyke and shows no indication of being affected by that or any batholith, the nearest of which is Wilsons Prom. *note Lisa, I think the dyke is more dolerite than basalt? xx Mike
Discovery Centre 20 March, 2012 16:03

I have spoken to Dr Alan Tait, a sedimentologist researching the geology of the Flatrocks site and Doris Seegets-Villiers, a PhD candidate studying the palynology and taphonomy of the Flatrocks site and they have very kindly informed me that the dip of the rocks is between 13 and 14 degrees at this part of the site. The sediments at the Flatrocks site were deposited sometime between 125 million years ago and 115 million years ago at the end of the Early Cretaceous when this part of the rift valley apparently stopped subsiding. The fault pictured in the above article is a normal fault which has been downthrown to the left of the photo and probably occurred between 115 million years ago and the end of the Early Cretaceous. We can’t see if or where this fault meets with the dyke although the dyke is probably younger than the fault pictured and as far as we can see has only affected the rocks immediately adjacent to it. There are no banoliths or other dykes nearby. There are many other larger, more impressive faults in the area but they lie under the sea so you’ll need to have a look at Google Earth or similar to view them. Mike, you are correct in saying that the dyke is not literally basalt which is why I have gone with the term ‘basaltic’ which covers a range of mafic rock types.

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