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.
Liesegang rings in sandstone
Iron-rich tillite
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.
Goethite boulders
This group of goethite–quartz boulders may have formed from a hydrothermal seam or vent, where silica and sulfur-rich fluids precipitated quartz and pyrite. As pyrite oxidizes, it typically forms sulfuric acid, which accelerates iron oxidation, in this case forming goethite. As the surrounding blocky shale erodes, the more resistant goethite-quartz boulders remain – a common sight in the Flinders Ranges.
Muddy limestone turbidite: Nantawarrina
Muddy limestone turbidite, formed from sediment deposits in offshore silty waters during the Cambrian period (around 550 million years ago). The dramatic localised buckling seen here may have occurred in situ during a turbulent underwater event. This rock is part of the Mernmerna Formation in the northern Flinders Ranges.
Glacial geological formation: sedimentary deposits
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”
Flute Casts: Wonoka Formation
These flute casts are part of the limey mud deposits of the Wonoka Formation. Flowing water in the marine trough (the Adelaide Geosyncline approximately 580 mya) hollowed grooves (flutes) into deep sea floor sediments. These flutes fulled with slurries of mud which are preserved as flute casts seen in Bunyeroo Creek in the Ikara- Flinders Ranges National Park.
Bonney Sandstone Formation
The Bonney Sandstone Formation is the result of deposition of iron-stained sand, silt and mica, eroded from older igneous rocks to the west and transported by rivers into a large delta. This formation has a distinctive reddish colour, and is often seen alongside the harder weathering prominent peaks of Rawnsley Quartzite which was formed from white sands, 600 million years ago. See the essay Legacy of Time by The Royal Geographical Society of South Australia for the riveting story of the formation of the Flinders Ranges.
Nodular limestone: Bunyeroo Creek
This outcrop of nodular limestone can be seen 4km into Bunyeroo Creek from the road. Part of the Wilkawillina Limestone Formation (dating 530 million years ago), the origins of this lumpy limestone remain a mystery.
Mt Neill Granite: Arkaroola
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.
Kaolinitic Granite: Arkaroola
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.