Adelaide Geosyncline
The
Adelaide Geosyncline is a major geological province in central
South Australia. It stretches from the northernmost parts of the
Flinders Ranges, narrowing at the
Fleurieu Peninsula, and composes the two major mountain ranges of the State: the Flinders and the
Mount Lofty Ranges. The sediments in the
geosyncline were deposited in a period of geological time known as the Adelaidean, which stretches from sometime between 870 Ma and 700 Ma (the later
Neoproterozoic) to the end of the era, and 500 Ma (the end of the
Cambrian).
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.
Limestones,
shales, and
sandstones indicate a predominantly marine environment.
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
intrusions: the granites at
Victor Harbor 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.
| Eon | Era | Period | Local division | General name | Mt Lofty Ranges | Flinders Ranges | | | Phanerozoic | Palaeozoic | Unconformity overlaid by Permian glacial sediments |
| Cambrian | Mid-late | Lake Frome group | not exposed | Limestones, sandstones |
| Early | Kanmantoo group | Metamorphosed schists, greywackes, gneisses | Shales, siltstones |
| Normanville group / Hawker group | Limestones |
| Proterozoic | Neoproterozoic | Ediacaran | Marinoan | Wilpena group | | Pound subgroup: quartzites, massive sandstones |
| Limestones |
| ABC Range quartzite |
| Brachina formation: purple siltstones |
| Dolomites | Nuccaleena formation dolomite (marker bed) |
| Cryogenian | Umberatana group | Reynella formation: shales, siltstones | Glaciation event: Yerelina subgroup tillites |
| Angepena formation: shales, siltstones |
| Sturtian | Brighton limestone | Dolomites |
| First of the Sturtian-Varangian glaciations: Sturt tillite |
| Burra group | Belair subgroup: shales, sandstones | Shales, siltstones, etc. |
| Torrensian | Glen Osmond slate |
| Beaumont dolomite |
| Stonyfell quartzite |
| Dolomites |
| Aldgate sandstone | Conglomerates |
| Willouran | Callanna beds / Mount Painter complex | not exposed | Wooltana volcanics |
| Sandstones |
| Unconformity underlain by metamorphosed Mesoproterozoic basement |
Fossils are to be found in the Geosyncline; those discovered in the
Ediacara Hills of the northern Flinders in 1946 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.
See also: Geomorphology of the Mount Lofty RangesThe ranges formed during the Delamerian orogeny continue to erode, and intra-plate subsidence is occurring. 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.
*
Geology of Australia* N.H. Ludbrook.
A guide to the geology and mineral resources of South Australia. Government Printer, 1980.
*
Atlas of South Australia
