| Adelaide Geosyncline |
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FORMATION The Geosyncline is a great belt of sediments, deposited in a depression during a time of lithospheric stretching in an arc approximately a thousand kilometres long and several hundred kilometres wide. The thickest parts of the belt are approximately 24,000 m thick. The sediments deposited indicate a wide range of conditions existed in the depositional arc, with Limestones and Shales indicating shallow marine environments, and Sandstones alluvial floodplain conditions. DELAMERIAN OROGENY This sedimentation ended towards the Cambrian, when plate movements changed and the area experienced an Orogeny (mountain-building period) extending into the Ordovician . This event is called the Delamerian Orogeny, named after a small town on the Fleurieu Peninsula where evidence was found for the event. The orogeny caused substantial folding, buckling, and faulting of the strata, and resulted in the creation of a major mountain range, the eroded stumps of which can today be seen as the Mount Lofty and Flinders Ranges. Accompanying this folding and faulting were several were intruded at this time, as were those at Palmer in the eastern South Mount Lofty Ranges. Not all of the Geosyncline experienced tectonic activity; the deposits in the Stuart Shelf to the northwest remained undisturbed (and still do today), while limestones and shales were deposited in the Cooper and Pedirka Basins to the north and northeast. SEDIMENTARY GROUPS There are four major groups of sedimentary rocks which can be identified in both the Mount Lofty Ranges and Flinders Ranges. From oldest to youngest, they are: the Burra Group (after Burra ), the Umberatana Group (after a station near Arkaroola ), the Wilpena Group (after Wilpena Pound ), and the Hawker and Normanville Groups (after Hawker and Normanville ). Although these groups outcrop significantly at sites near Adelaide and in places throughout the Flinders, the names of specific rock strata within tends to reflect where outcrops are most common. Therefore, although Burra Group rocks make up outcropping strata, for example, southeast of Arkaroola , they also comprise the major scarp of the Mount Lofty Ranges overlooking the Adelaide Plains, and have names like Glen Osmond Slate, Beaumont Dolomite, Stonyfell Quartzite, and Aldgate Sandstone. It is the younger sediments that tend to outcrop in the Flinders Ranges, the most famous of which are the hardy quartzites: the Pound Quartzite, at the top of the Wilpena Group, and the ABC Range Quartzite just a little older than it: being more resistant to weathering than the older and more easily weathered shales which lie underneath them (like Brachina Shale, which, apart from the Flinders, also outcrops at . At the same time as the Hawker Group rocks were deposited in the north and west of the Geosyncline, conditions were different in the southeast, where faults had developed and a collapse occurred: sandstones were deposited. These were later Metamorphosed into the Schists and Gneisses of the Kanmantoo Group. FOSSIL LIFE Fossils are to be found in the Geosyncline; those discovered in the Ediacara Hills of the northern Flinders in 1946 - incidentally in Pound Quartzite! - are of worldwide significance for being some of the oldest examples of fossilised animal life ever found. They date from the very end of the Neoproterozoic , and in 2004 the location gave its name to the last geological period of the era, the Ediacaran . RECENT GEOLOGICAL HISTORY See Also: Mount_Lofty_Ranges#Geology_and_geography The ranges formed during the Delamerian orogeny continue to erode, and intra-plate subsidence is occuring. In the South Mount Lofty Ranges this has resulted in rifting and the formation of Graben structures, creating the long parallel faults which shape the Adelaide Plains . REFERENCES
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