16 Apr 2012 2:07 PMSusan Guthrie
Whilst wandering out in the Tallebudgera Valley last September, one of the landowners mentioned the new Bally MT Conservation Reserve (see earlier blog for more info) that was being amalgamated by the GCCC at the time. After gaining permission to walk through his property, I followed a steep 4wd track up to a small plateau below the main peak and took a few photos of the interesting and unusual geology I saw.
I sent a couple of photos off to Warwick Willmott at the Queensland division of the Australian Geology Society whom by chance had recently been engaged by council to write a report on the mountain. As he previously had only been to the northern Bonogin side, he joined Geo-Nature Walks and Tours a few weeks later for a better view of the south and eastern end of the mountain.
This is a shortened version of his report.
The south-eastern ridge
This prominent feature is a series of separated, jagged towers, which at close range can be seen to be composed of a great mass of agglomerate (or breccia) of consolidated cobbles and boulders of rhyolitic rocks. Its topography has been created by the relative ease of disintegration and erosion of this agglomerate (Photo 1).
At the base of the ridge on both sides, solid rock in small, but well developed cooling columns, is exposed. It is of a black, very fine grained, glassy rhyolite, in places showing flow banding and containing small rock fragments up to 3 cm across (Photo 2).
In one place, steeply inclined flow banding was seen in the rock. Such black glassy rhyolite is unusual and signifies rapid cooling. The cooling columns are mainly horizontal, in contrast to the more usual vertical columns. As cooling contraction columns are thought to form at right angles to the cooling surface, this implies that the surface against which the lava cooled was vertical. Remarkably, the solid rock in the cooling columns appears to grade irregularly upwards into the agglomerate mass, with no obvious break.
On the western side of the ridge, the solid rock of the cooling columns extends further up the towers of the ridge than on the eastern side. Here small horizontal cooling columns are extremely well developed (Photo3), in places fanning out in spectacular patterns. Only at the very base are larger vertical cooling columns developed (Photo 4). These have an abrupt junction with the horizontal columns above, but they are clearly developed in the same mass. The significance of this abrupt transition is uncertain.
Interpretation of geological history of Bally Mountain
Bally Mountain appears to have developed in two stages of volcanic activity on the north-eastern flank of the Tweed Volcano.
Firstly, rhyolite magma appears to have risen along a narrow fracture oriented SE-NW within the surrounding meta-sediments of the Neranleigh-Fernvale Beds, beneath what is now the south-eastern ridge of the mountain. Evidence for this includes the horizontal cooling columns in the rhyolite at the base of both sides of this south-eastern ridge, indicating cooling against a vertical surface or wall; the glassy nature of the rhyolite, indicating rapid cooling within a small mass; and steeply inclined flow banding within the rhyolite in places.
As the magma approached the surface, it began to explode and fragment, probably in a small elongate volcanic vent. The fragmented material fell back into the top of the fracture or vent to form the thick deposit of agglomerate. As the less fragmented magma below the agglomerate cooled, it contracted into small columns, which inter-fingered with the agglomerate above. Possibly the glassy rhyolite of the north-eastern ridge was pushed up into another, narrower fracture at the same time. There is no evidence of agglomerate above this, but it could have been removed by erosion since. The two ridges have remained prominent as their rocks have better resisted erosion than the Neranleigh-Fernvale Beds which once surrounded their fissures.
It is not clear how extensive the eruptive vent and its agglomerate may have been beneath what is now the main mass of the mountain as it is obscured there by later basalt lavas. The vent appears to have been a smaller-scale version of what may have been over Pages Pinnacle in the Numinbah Valley. There, a thick elongate fracture has been filled with flow-banded rhyolite magma, although there is no evidence of agglomerate above.
Report kindly supplied by Warwick Willmott and the GCCC
Geo-Nature Walks and Tours offer small group tours to experience and view the spectacular geology, wildlife and interesting history of the wonderful Gold Coast.