Rocks in the Rocky River

Rocky River Monzogranite (Bungulla Suite).
The Monzogranite here contains large crystals of twinned pink K-feldspar.

The Rocky River Road is a very quiet, scenic and out of the way route to travel. It is slow and windy, but a pretty alternative to the Bruxner Highway route between Drake and Tenterfield. I had the pleasure of a trip along Long Gully Road and Rocky River Road just last week. I enjoyed it very much for the scenery and the clear water of the Rocky River (also known as the Timbarra River). The area is also very interesting in a geological sense. The rock that is found along Rocky River Road (the Rocky River Monzogranite) is actually remnants of outer part of a very large batholith that makes up Timbarra Tableland.

Previously, understanding of the inner rocks of the Timbarra Tableland were incorrectly thought to be Moonbi Supersuite, while the outer rocks were correctly part of the Stanthorpe Supersuite. Having two parts of an intrusion being apparently related to different Suites was all quite confused. Mustard (2004) suggested an informal renaming of the Bungulla Monzogranite in the area of Rocky River to the Rocky River Monzogranite. The Rocky River Monzogranite would in turn be part of the Bungulla Suite. The Bungulla Suite being rocks that are I-type (derived from melted igneous rocks) of the Stanthorpe Supersuite.  Although the nomenclature by Mustard (2004) was suggested as informal it is quite reasonable to adopt the name of Rocky Creek Monzogranite as formal. The previous identification of some rocks in the Timbarra Tableland as Moonbi Supersuite has since been shown to be incorrect - they are all Stanthorpe Supersuite.

The Rocky River Monzogranite is in the extensive eastern edge of the Timbarra Tablelands. It is comprised mainly of the rock monzogranite. This rock is comprised of abundant quartz and roughly equal proportions of plagioclase feldspar (sodium and calcium feldspar) and potassium feldspar. There are also smaller amounts of dark biotite mica and amphibole in the rock. The Rocky River Monzogranite is quite a course grained and the crystals are very, very large. The monzonite is notable as it has many 'inclusions' called xenoliths. These are blobs of rock are of a less granitic composition. They are very, very common in some areas as the rock comprises of about 10% or more xenoliths. The xenoliths indicate that mixing of different composition magmas was occurring when the intrusion formed.

A monzogranite tor in the sandy bed of the Rocky River.
Note different sized irregular shaped xenoliths.

Along the very margin of the intrusion (I didn't get to see this) the crystals are smaller in size and the feldspars are even more potassium rich forming the rock syenite. The central area of the Timbarra tablelands is comprised of granitic rocks that were high in fluids when the rock was crystallizing. These fluids (formed by residual enrichment of the original magma chamber), has resulted in the concentration of metals, most notably gold (Mustard 2004). The Timbarra gold mine targeted this inner zone of the tablelands as the outer granite (Rocky Creek Monzogranite) do not contain nearly as much gold. The erosion of the gold has led to alluvial gold deposits in the Rocky River and Clarence Rivers but the gold is very fine grained so fossickers panning can be tricky.

The many components of the Timbarra tablelands intrusion were emplaced in the Triassic period. They intruded the Drake Volcanics. The size of the granite plutons has caused significant contact metamorphism, creating a large metamorphic aureole around the intrusion.

There is much more to say about the zones in the Timbarra tablelands intrusion described by Mustard (2004). This includes the neatness of the tablelands cross section, the way that the slightly different granites tapped different parts of a deeper magma chamber and the way that differentiation of granite types occurred are all worthy of a discussion. Though, this needs more than just a few paragraphs and so I will have to cover these matters in future posts. In the mean time I hope this post gives a taste for some of the 'granite'.

References/bibliography:
*Mustard, R. 2004. Textural, mineralogical and geochemical variation in the zoned Timbarra Tablelands pluton, New South Wales. Australian Journal of Earth Sciences, 51.

It's a Demon of a Fault

Many people have requested that I do a post on the Demon Fault. I've struggled to put something together because structural geology is not one of my strong points and secondly because there was so little published information about it, except for some specific papers in the 1970’s. Thankfully, a few months ago Babaahmdi & Rosenbaum (2013) published a detailed paper summarising what was known in the 1970s, presenting how the fault appears, how it seems to have developed and how it may fit into the development of eastern Australia. It is worth noting that Gideon Rosenbaum from The University of Queensland has been the major researcher on New England structural geology for the last 5 years. If it was not for him and his student’s research we would be struggling to understand some of the basic features of the older rocks of the region including the Demon Fault.

Large faults are usually have quite distinctive landscape features. The Demon Fault is mainly a transverse type fault (movement on the fault horizontally rather than vertically) which displays very obvious topographical features.  Transverse faults often form valleys, where the rock of the fault has been broken down into what is called gouge or rock-flour. Gouge is very weak material. It is easily eroded and rivers often preferentially follow the route of the fault carving out the gouge into deep valleys.  The presence of deformational features in the surrounding rock can give an indication of how deep the fault was when it was active.

The Demon Fault is a prominent feature because it is evident from a series of valleys from near the Queensland Border to Dorrigo. At the Queensland end it is partly obscured by the Cenozoic Main Range Volcanics and in the Dorrigo area the end is obscured by the Cenozoic aged Ebor Volcanics. Geological maps of the area show a nice linear feature with obvious truncation of pre-existing geological units. Aerial photos also show the fault up nicely with streams preferentially flowing along the trace of the fault and contrasting with the rugged forested mountains surrounding it. I’ve never taken a photo of any part of the Demon Fault but a nice photo taken from an aeroplane can be found here: http://www.panoramio.com/photo/35890361

The Timbarra River has followed the Demon Fault creating linear valley
http://www.panoramio.com/photo/35890361 (used with permission)

Korsch et al (1978) observed that the Demon Fault had displaced several geological units including intrusions of the Bungulla Monzogranite (now known as the Rocky River Monzogranite), Dumbudgery Granodiorite and Newton Boyd Granodiorite as well as the Drake Volcanics. The fault was interpreted as a dextral strike-slip fault (a fault where the eastern side had moved south relative to the western side). Korsch et al (1978) calculated that the fault had displaced these units 17km which is substantial in Eastern Australia. Dating of the displaced granite intrusions provides a possible maximum date of within Triassic period (249-232 million years). The nature of deformation features adjacent to the faulting indicates that the fault was shallow and/or was created in a brittle environment. Badaahmadi & Rosenbaum (2013) speculate that the timing of the faulting may actually be similar to that of faulting and extension in the earth’s crust that formed the Ipswich and Clarence-Moreton Basins (more about this in future posts).

There are many factors in understanding the Demon Fault. It is interesting to note that other authors have come up with different lengths of displacement including 30km in the northern part of the fault and 23Km in the central part. Badaahmadi & Rosenbaum (2013) have calculated that the northern part of the fault displaced 35km, in the centre by 25km and south by 19km. Some components of reverse faulting (where one side of the fault slid down and away from the other side) were observed. Additionally, it was noted that the Demon Fault did not appear to follow one big long line but instead had numerous splays (deviations, splitting, etc) especially in the south. Badaahmadi & Rosenbaum (2013) suspect that there may be two causes to the different lengths:

1. Splays may have created movement of the fault which had a vertical component as well as horizontal.
2. There may have been some fault reactivation of the northern part of the fault as recently as the Cenozoic era.

Both of these possibilities really need a discussion in their own right, rather than cursory mention. So, I’ll get back to these in a future post.  I also want to cover the significance of the Demon Fault in formation of the Texas and Coffs Harbour Oroclines which are incredibly large features that I’ve briefly touched on in an earlier post about the South Solitary Island.

References/bibliography:
*Babaahmadi, A. & Rosenbaum, G. 2013. Kinematics of the Demon Fault: Implications for Mesozoic strike-slip faulting in eastern Australia. Australian Journal of Earth Sciences. V.60
*Korsch, R.J., Archer, R. & McConachy, G.W. 1978. The Demon Fault. Journal and Proceedings, Royal Society of New South Wales. V111.