Bunya Phyllite
This Earthcache seeks to illustrate the geological processes which formed Bunya Phyllite, and the location you're visiting provides a great example of the exposed rock surface due to the hydraulic action of Albany Creek. Bunya Phyllite belongs to the Brisbane Metamorphics group and is considered one of the important basement rocks of Brisbane (see below), along with the Neranleigh-Fernvale Beds and Rocksberg Greenstone.
Bunya Phyllite was formed from sediments deposited in deep oceanic waters during the late Devonian and Carboniferous Periods, some 370 to 290 million years ago. During this period, the east coast of Australia lay to the west of Dalby and consisted of a lofty chain of volcanic mountains. The oceanic plate, to the east of Australia, was moving westward and being subducted beneath the continent. A shallow sea occurred over the continental shelf which was approximately 100 km wide and extended in a north-west south-east direction (see below). The ocean deepened rapidly from the edge of the continental shelf in an easterly direction. Sand and mud were washed down the rivers from the volcanic mountain chain and deposited on the continental shelf, some of which was carried by currents down submarine canyons onto the deep ocean floor. These sediments were deposited together with slowly accumulating mud from other sources and the remains of marine organisms.
During the Permian Period (290-240 million years ago), the Bunya Phyllite beds were uplifted and contorted under massive compressive forces and metamorphosed to form a mountain range which became the eastern margin of Australia (see below). Molten rock was intruded into these coastal mountains during the Triassic Period (240-220 million years ago); some of which cooled slowly deep below the land surface to form coarse crystalline granite bodies like Enoggera Granite, Samford Granodiorite, and Mount Samson granodiorite which intrude into Bunya Phyllite.
The Bunya Phyllite is an intensely crumpled and deformed metamorphic unit occurring up to 2000m deep, which is is light to medium grey consisting of slate, phyllite, arenite, and metabasalt banded with layers of quartz and mica. The quartz layers are commonly contorted by later small folds and then cut by narrow veinlets of quartz. The mica crystals lie in one direction giving the slaty cleavage and provide a distinct sheen to the rock. It can be delicate or crumbly in places and easily removed by erosion processes, but mostly the remaining exposed phyllite is highly compressed and somewhat resistant to erosion.
To log a find on this earthcache you must refer to the exposed Bunya Phyllite at the 2 waypoints and answer questions 1 to 4.
At the posted coordinates:
1. Would you say that the rock demonstrates conformity (do the layers appear to be laid in the same consistent direction)?
2. Does the rock appear to be stable, or is it fragile?
At Waypoint 2:
3. Describe the colours and textures of the rock in the lower section in contrast to those of the upper section.
4. What process has caused these differences?
Optional: Wander along the creek and take a reading using a compass/ GPS / smartphone. In which direction do the rock layers appear to be orientated?
It is expected that EACH TEAM wishing to log a find will SUBMIT THEIR OWN ANSWERS to the CO. Once your answers have been submitted, you are free to log your find. If there are problems with your answers you will be contacted. If you fail to submit answers, your log may be deleted.
For the Australian Stratigraphic Units Database reference sheet on Bunya Phyllite click here; for the Brisbane Metamorphics reference sheet click here; and for further research click here
