Mt Gee, Arkaroola, is 60m thick slab of crystalline quartz, resulting from hydrothermal activity. The prolific quartz cavities present a great variety of crystal form, colour and growth stages, including nail-hole quartz, only found at Arkaroola.
Iron hydroxides percolating through sandstone can form concentric patterns called Liesegang rings. These palm sized samples were seen in a localised area to the west of Leigh Creek in the Flinders Ranges.
The dark bands seen between the rock layers are goethite, an iron oxide mineral that forms through the oxidation of iron-rich fluids along bedding planes or fractures in the rock.
Located within the Merinjina Tillite Formation, the smaller embedded rocks were likely deposited in this massive matrix by glacial activity. This tillite has since undergone oxidation, and in some areas, has been partially replaced by botryoidal goethite—a rounded form of iron oxide.
This striking outcrop near Copley exhibits a range of sedimentary deposits. From the north to south, green shale and blocky siltstone meets weathered sandstone. This is followed by a distinct tillite layer between two dolomite layers, and a thin layer of quartzite. Further along, siltstone layers coloured by iron oxide, and more weathered sandstone.
Recent studies in the area suggest that the northern Flinders Ranges area was a shallow marine or lagoonal setting prior to the climate changing into one of the earth’s most severe glaciations ever recorded (the global Sturtian Glaciation), 720 million years ago . Over these older rocks, we find layers of pebbly, mixed rocks (called diamictites) that were likely deposited by a glacier at its edge, along with some sand layers (Bolla Bollana Tillite). These rock layers are thought to show the movement of the glacier as it advanced and retreated along the coastline. These layers then change into sandstones and mudstones with large rocks (called dropstones), which probably formed when the ice started to melt, in an area just in front of the retreating glacier (Wilyerpa Formation). After the glacier melted, the layers of rock that formed (Tapley Hill Formation) include thin, layered shales and carbonates that were deposited underwater.
Weathered sandstone:
Continue reading “Glacial geological formation: sedimentary deposits”
Example of barite sand nodules in a Rawnsley Quartzite boulder formed in shallow water below the surface of the sand, with quartz grains cemented by barium sulphate later replaced by fine grained quartz to form lumps.
Skillogalee Dolomite includes blue-grey dolomites, magnesites, bands of black cherts and stromatolites. Magnesites (magnesium carbonate) of this formation were formed in marginal lagoons of the Adelaide Geosyncline around 750 million years ago and occur as beds, commonly conglomerate in texture, as pictured below.
Iridescent coating is not a true mineral but a nanomineral, associated iron oxides (though not exclusively) such as this example of Turgite. The nano-crystal array of the fine iridescent coating layer diffracts and scatters beams of light, producing a rainbow effect.
Mt Neill Granite is exposed in a rocky creek near Paralana Hot Springs. The red feldspars of this 1580 million year old formation present in a variety of forms.
This lightweight Kaolinitic Granite at NE Arkaroola is an alteration of Mt Neil Granite. Hydrothermal fluids have destroyed the micas and replaced the feldspars of the original granite (1580 million years old) with Kaolinite.
For more information on how to locate this site, see Geological Field Excursion Guides.
