Northern Rivers Geology

The landscapes of the mountains surrounding the Tweed Valley are very spectacular. I have discussed some of the facets of the Tweed Volcano and Mount Warning area in previous posts. However, I have not covered much on the main rock type that is mainly responsible for the rugged steep cliffs and valleys of the Nightcap National Park World Heritage Area. This rock is the Nimbin Rhyolite, a quartz rich lava that was dominant in the final phases of the Tweed Volcano. Because of its resistance to weathering it results in inspiring cliffs and rugged ranges.

Rhyolite is a volcanic rock that contains a high volume of silica (quartz) in it. Because of the silica content rhyolite lavas tend to be “sticky” and slow moving. This also causes gases to be trapped in the lava or magma chamber feeding the lava flows. The release of trapped gases can cause explosive eruptions. Therefore, accompanying the lava flows there are also deposits of volcanic ash and glass caused by the rapid cooling of lava during explosive eruptions. All of these features are present in the Nightcap Ranges and surrounding areas.

In a future post I will show a picture of a Nimbin Rhyolite lava which exhibits flow banding. There are many examples of flow banding in lava near Minyon Falls. It is a tricky lava to look at in hand specimen because it is very fine grained. You can only see occasional tiny specks that are crystals but most of the time it is just a grey mass. In outcrop you might see some flow structures like the one pictured, but generally it is a boring looking rock! The same rock is in the Mount Matheson area. Smith and Houston (1995) referred to this rhyolite as crystal-poor rhyolite. It compares very differently to the crystal-rich rhyolite identified elsewhere in the area.

As for the crystal rich rhyolite, I was lucky enough to go for a walk in a property that has just been purchased by the NSW National Parks and Wildlife Service. It is located in the valley between the Goonengerry and Nightcap National Parks. While inspecting the excellent work done to remove exotic weeds from this property and celebrate the inclusion of an important vegetative link between National Parks. I came across some good examples of the crystal-rich rhyolite. In these samples the rock contains large quartz crystals which are very evident (see the picture below). The more crystalline form of rhyolite occurs in about a third of the total area mapped as rhyolite. This includes the area from the Koonyum and Goonengerry ranges in the east to Whian Whian in the west.

Smith and Houston (1995) observe the crystal abundance is related to the vent (or group of vents) from which the lava was erupted. Only occasionally do crystal rich and crystal poor varieties occur on top or under each other indicating a high degree of lava mixing. The relationship between specific vents and crystal richness shows the vents must have been tapping different magma sources (different magma chambers). Alternatively the vents may have erupted magma from a single, somewhat heterogeneous magma chamber.

However, it is worth noting there is a third major form of rhyolite in the area and is known as the volcanic glass, obsidian. This volcanic glass occurs around the bases of the major lava flows and is often referred to as perlite. The glass is rarely a massive unit but tends to appear brecciated and as an agglomerate. I will discuss this obsidian further in a future post as many interesting features and textures are preserved showing the way that rhyolite lavas move across the lands surface. In the mean time, it is worth remembering that lavas ain’t just lavas. There can be many differences which provide a window into how the landscape was formed.

If you are familiar with the northern rivers you would be aware of grand waterfalls in Nightcap National Park. The grandest (in my opinion) are the Minyon Falls which drop Repentance Creek around 100metres into the gorge below. I remember when you used to be able to stand at the very top and jump over the streams to cross but the National Parks and Wildlife Service of N.S.W. have stopped access (for obvious safety reasons) except at a constructed viewing platform.

Minyon Falls are spectacular. Geologically they represent thick units of rhyolite known as the Nimbin Rhyolite erupted during the later phases of the tweed volcano during the period known as the Cenozoic which was centred on the nearby Mount Warning. Underlying the rhyolite is basalt and andesite of the Lismore Basalt which appears to be from the earlier main phase of eruption from the volcano. At Minyon Falls the Nimbin Rhyolite is greater in thickness than the height of the falls themselves. It mainly shows massive units of rhyolite lava inter-collated with units of volcanic glass (obsidian) darker, but still of similar composition to the rhyolite.

Rhyolite is the volcanic equivalent of granite (which forms underground). It is fine grained due to quick cooling due to its volcanic nature which stops crystals from becoming very large. Rhyolite is silica rich which means that minerals like quartz and feldspar are abundant and other minerals such as olivine that is commonly be present in some of the basalts nearby are absent. The high silica content makes the lava thick and viscus and therefore gas bubbles are commonly trapped in the lava and banding of the lava flows becomes more frequently observed. The composition of rhyolite often leads to violent eruptions which are represented by ash and volcanic glass which can form thick layers themselves (some of these glass layers are present at Minyon Falls too).

If you are fit enough for a big walk at the base of the Minyon Falls are unusual structures which show how viscus the lava can be. Brittle-ductile structures are evident to the trained eye in this area. Smith (1996) identified these as essentially these are structures which show that when the lava was flowing the lava had become almost solid with many small faults mixed in with folding and flow banding of the lava.

Fresh rhyolite lava is a hard, erosion resistant rock and for this reason is why we have rugged ranges surrounding the central core of the Tweed Volcano at Mount Warning. The highest portions of the volcano including the rhyolite have been eroded away from the area now occupied by the Tweed Valley. Most of the volcanic rock in the valley has been eroded right down to the much older Paleozoic aged rocks of the Neranleigh Fernvale Group. The creeks that start in the ranges such as Repentance Creek have slowly cut back the face of the rhyolite cliffs as the velocity and power of the waterfalls slowly breaks grains from the rocks and creates cracks that break off in large rock falls.

Are you in Northern Rivers? It might be worth climbing Mount Warning to see the shape of the Tweed Valley and the remnants of the shield volcano in the cliffs seen all around the edge of the valley. Or maybe a trip into the Nightcap Ranges to Minyon Falls. Have a look at rocky creek beds to see exposed rock and many structures.

Note: There are two large areas of rhyolite in the Northern Rivers. These are the Nimbin Rhyolite of Cenozoic age discussed in this post but there is also rhyolites within the Chillingham Volcanics which are much older and are probably the basal units of the Mesozoic Ipswich Basin.

References/Bibliography:

*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Smith, J.V. 1996.Ductile-brittle transition structures in the basal shear zone of a rhyolite lava flow, eastern Australia. Journal of Volcanology and Geothermal Research V72
*Smith, J.V. , Houston, E.C. 1995. Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales. Australian Journal of Earth Sciences V42(1) p69-74.