|One of the Nimbin Rocks, Cathederal Rock|
The Nimbin Rocks are comprised mainly of the quartz rich volcanic rock called rhyolite overlying a section of agglomerates (reworked volcanic rock) and volcanic glass known as perlite. Below the perlite lies basalts of the Kyogle Basalt. And here may lie the clue. The rocks appear to be layered because they are deposited on top of each other. First the Kyogle Basalt, then the perlite and agglomerates and then the rhyolite lavas (with some bands of perlite within it). The rhyolitic lavas are referred to as the Georgica Rhyolite Member according to Duggan and Mason (1974), or historically and more recently as Nimbin Rhyolite according to McElroy (1962) and Cotter (1998) and others.
If the Nimbin Rocks were related to a dyke they would have formed through pushing through the surrounding rocks such as those of the Kyogle Basalt or the Clarence Moreton Basin sediments, metamorphosing them and displaying different diagnostic textures than those I know about. However, it is still quite possible that the rocks may have been vents since the nearest identified vents seem to be about 8km away to the north east in the Nightcap Ranges and rhyolite lava flows tend to not move great distances, indeed rarely greater than 5km. However, the vents located further into the Nightcap Ranges are characterised by thick erosion resistant units of rhyolite which we don't see so much near Nimbin other than the Nimbin Rocks themselves. But conversely, the shape of the rock monoliths does imply a dyke.
So, what is the answer? Well the Nimbin Rocks are either one or more volcanic vents or they are the remnants of thick lava flows possibly from vents in the nightcap ranges located on the flanks of the Tweed Volcano. Which is almost not an answer at all. But one thing is obvious, it is interesting just how little we know about the landscape in which we live, work and see.
Blog Note: I like to provide photos for these sort of posts but recently where I store photos (skydrive and/or GoogleDocs) has changed its method for providing URLs to allow embedding of these files and Blogger doesn't like the new URLs. So, these next blogs might be a bit more bland looking until I figure out a better way to store and embed photos.
Since writing the above I have come across a report by Relph (1958) which says the following:
“Quartz-feldspar porphyry [granite, the intrusive equivalent of rhyolite] has intruded the sediments at Lillian Rock and the eastern portion of the Nimbin Rocks area. In the latter occurrence the porphyry forms two prominent pinnacles, with columnar jointing evident, and outcrops to the east, and in the bed of Goolmangar Creek. In neither case have the surrounding sediments been affected to any marked degree, but it is thought that it is intrusive and of dyke or plug form rather than of extrusive nature. Under the microscope this rock revealed no sign of flow structure.”
Although Relph considered two of the Nimbin Rocks intrusive he did not find any diagnostic evidence of them being either intrusive or extrusive.
*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*McElroy, C.T. 1969. The Clarence-Moreton Basin in New South Wales. In Packham, G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16
*Relph, R.E. 1958: Geology of the Nimbin area. Technical Report. Department of Mines NSW, 3.
*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.