Geology of the Fassifern Peaks

Fassifern Peaks  - rocks and landscape

Extracted from Willmott, Warwick (2013), Rocks and Landscapes of the Boonah District, Geological Society of Australia Queensland Division 2013. With kind permission of the author.

After it broke away from Antarctica, the continent of Australia (including New Guinea) drifted north to its present position. As it did so, it moved over a 'hot-spot'. Between 30 - 23 million years ago, south-east Queensland moved over the 'hot-spot', and several volcanoes erupted sequentially along the line travelled by the crust. 
              

In the Fassifern district, the Main Range Volcano erupted first, probably from a line of craters or maybe even a fissure. The volcano was built up by numerous basalt lava flows, but there were also some flows of rhyolyte that are more resistant to erosion and cliff-forming. The Focal Peak Volcano erupted slightly later from a centre just to the west of Mount Barney, and also contributed lavas to the north and west.

           

The Tweed Valley Volcano then formed, followed by newer volcanoes further south as the northward drift continued.

Towards the end of many volcanoes, molten magma can be intruded within, and beneath the lava pile, in bodies of various shapes and sizes. These intrusions are composed of a great variety of rock types, which are thought to be ‘distilled off’ from the basalt in a deep magma chamber. The most obvious is of rhyolite, as it is resistant to erosion, and remains as peaks after the surrounding softer rocks are eroded away. Rhyolite is rich in silica (or quartz), with few dark minerals. They may form vertical ‘plugs’, filling former volcanic vents, or bulges pushing aside the surrounding rocks, or sheet-like’ sills’ and ‘dykes’ inserted between or cutting across the surrounding strata.

              

With few exceptions, the prominent peaks of the Fassifern Valley are composed of resistant rhyolite. Mount Greville and Mount Moon were probably domes of rhyolite magma intruded beneath the surface rather than vent fillings. Mount Edwards represents a body of trachyte similar to rhyolite. During its erosion it has been carved into two parts by Reynolds Creek gradually cutting down from a higher level. Mount Fraser is also composed of trachyte whereas Mt Toowoonan is formed from rhyolite. 

               

Some of the prominent peaks of the Fassifern are not composed of resistant rhyolite:

• The Sugarloaf southeast of Boonah, a plug of basalt; and
• Mt Walker, a flat-topped hill north-west of Warrill View, is formed by basalt lavas that are rare remnants of the central surface of the Main Range Volcano.

The group of peaks around Flinders Peak are intrusive bodies, dykes and sills formed within the Main Range Volcano, and composed of rhyolite of slightly different colour, grain-size and mineral content. Flinders Peak itself is a compound intrusion of various trachytes, cut by a thick vertical dyke on the northern side. Towards the top, fragmental debris of a volcanic breccia may be the products of a volcanic explosion when the magma reached the surface. The nearby Ivory’s Rock is a small plug of rhyolite, where some outcrops of glassy, black obsidian represent sections that have been chilled rapidly.

                           

Minto Crags at Croftby is a circular ‘ring’ dyke of rhyolite, while the flat topped Mount French is developed on several gently inclined sills of rhyolite.

                           

Mount Alford is a complex intrusion of micro-diorite and granophyre with a broken-up andesite capping, encircled by several ring-dykes of trachy-rhyolite, including Glennies Pulpit.

               

Towards the end of the Focal Peak Volcano, a large body of rhyolite magma was thrust upwards to cool slowly to the resistant coarser granophyre of the present Mount Barney. Large thick sills of rhyolite magma were also intruded in a circular pattern around the central mountain and now form Mount Maroon, Mount May, Mount Phillip and Minnages Mountain. Rhyolite also formed Mount Ernest. 
         
Erosion since then has exposed these plugs, sills and dykes. 

                                                                                                                                                                                                                                                                                                                                         

For further information refer to:

              

https://www.gsa.org.au/Public/Divisions/QLD_subpages/Rocks_and_Landscape_Notes.aspx?WebsiteKey=a8c3ae88-b6eb-48e1-bea8-adfed361add5&hkey=648b9b05-2706-4890-b3ab-3c306c82384d

Three eroded ‘hot-spot’ volcanoes of southeast Queensland  (by Ben Cohen)

Image: Wilmott, Warwick (2013), Rocks and Landscapes of the Boonah District, Geological Society of Australia Queensland Division 2013.