Famous rocks of Maria Island, Tasmania; From Painted to Fossil Cliffs
Intro: An island off an island off an island; welcome to Maria! I recently jumped aboard the opportunity to go on a weekend field excursion run by Geological Association of Australia (GSA) and the university to this wonderful little island off the east coast of Tasmania, Australia. The whole island is a protected national park, and not only is it home to copious colonies of cute Tassie animals like wombats, kangaroos and Tasmanian devils, but it also contains some of the most beautiful and stunning geology you can find here in Tasmania! Ranging from ‘Painted’ to ‘Fossil’ Cliffs, I’ll give a brief background on why the rocks are just so spectacular on Maria Island.
Science Spiel: Tasmanian time travel (Geology)
Progressing up from the oldest and lowest rock units, underlying Maria island are folded and metamorphosed sedimentary and igneous rocks, referred to as the Mathina Supergroup. The age of these rocks range from Silurian to Devonian (~500 to 360 million years; Ma), and they are exposed as vertical cliffs on the north coast of island (best seen by boat). The Mathina basement rocks are abruptly cut off by what is known as the “great” unconformity of Tasmania (Corbett et al., 2014). Above the unconformity are flat-lying Permian (~280 Ma) mudstones, known as the Parmeener Supergroup. Thus, what this ‘great’ unconformity is is a missing (i.e. eroded away) ~100 Ma period in the geological record.
The Permian Parmeener group is host to the Fossil Cliffs; an extraordinary fossiliferous sequence of impure limestone, siltstone, and mudstone (Hughes, 1957; Reid, 2004). These beds are locally up to 110 metres thick and full of marine fossils such as bivalves (e.g. Eurydesma), brachiopods (e.g. Trigonotreta) and bryozoans (e.g. Fenstellids) just to name a few!
Disrupting the fossil beds are large rounded ice-rafted debris, interpreted as glacial derived dropstones (Johnston, 1888). What is so unique about Fossil Cliffs is not only the shear abundance of the fossils, but what they represent… This site is one of the rarest in the world (e.g. so rare that David Attenborough was here) as the fossil-rich limestone represents a productive sub-polar cold-water shallow marine environment, where melting glaciers dropped large rocks onto the sea floor (Hughes, 1957; Reid, 2004; Corbett et al., 2014).
Now progressing through time (and to the west of Maria Island) we move out of marine rocks into massive- and cross-bedded quartz sandstones of the Upper Permain Parmeener group. These rocks are exposed along the shore and known as the Painted Cliffs, and it’s obvious why once you see them. The sandstones have stunning patterns of red, orange and yellow bands and rings, which are a form of iron-oxide staining known as liesegang. The patterns are the result of repeated precipitation of colloidal iron from groundwater solutions (Clarke and Baillie, 1984). The groundwater flow is concentrated along an interplay between major fracture or joints and bedding layers within the sandstone, and thus is why they appear symmetrical in some localities.
Where did the iron in groundwater come from to produce these remarkable patterns? The answer may be from one of the youngest geological units. The top of Maria Island is comprised of two prominent hills known as Bishop and Clerk and Mt. Maria. These, along with being excellent day hikes, are composed by the profuse Jurassic aged (~ 180 Ma) dolerite that caps most of the peaks of Tasmania (Corbett et al., 2014). The weathering of these iron-rich dolerite rocks may have contributed to the iron-oxide staining on the Painted Cliffs.
Final Thoughts: For such a little island, Maria hosts an immense amount of geological diversity. In a couple days you can travel through Tasmanian geological time, with visiting rocks that are both scientifically interesting and visually beautiful… that is, if you can get away from taking picture of all the wild animals roaming in this island national park! Maria Island is stunning and definitely worth the trip over when you’re on the not-so-little-island of Tasmania.
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