Adelaide Rift Complex

The Adelaide Rift Complex (also known as 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 extending into Kangaroo Island, and composes the two major mountain ranges of the State: the Flinders Ranges and the Mount Lofty Ranges. The sediments in the rift complex were deposited between about 870 Ma (the middle Neoproterozoic) to ~500 Ma (the end of the Cambrian). They consist of a thick pile of sedimentary rocks and minor volcanic rocks that were deposited on the eastern margin of Australia during the time of breakup of the supercontinent Rodinia. A number of authors have noted the similarity in these sedimentary rocks with rocks found in western North America and have suggested that they were formerly adjacent to each other in Rodinia. This is one major correlation in the so-called SWEAT (SW USA against East Antarctica) reconstruction of Rodinia.^[1]^[2]

Formation

The Adelaide Rift Complex 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.

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. Foden et al. (2006) suggest that this orogeny lasted from ~514 Ma to 500 Ma. 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 Rift Complex 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.

Stratigraphic column

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
			Early	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
			Willouran	Callanna beds / Mount Painter complex	not exposed	Sandstones
	Unconformity underlain by metamorphosed Mesoproterozoic basement

Fossil life

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.

Recent geological history

The 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.

References

↑ Moores, E. M. (1991). "Southwest U.S.-East Antarctic (SWEAT) connection: A hypothesis". Geology 19 (5): 425. doi:10.1130/0091-7613(1991)019<0425:SUSEAS>2.3.CO;2.
↑ Dalziel, Ian W.D. (1991). "Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent". Geology 19 (6): 598. doi:10.1130/0091-7613(1991)019<0598:PMOLAE>2.3.CO;2.

External links

Atlas of South Australia

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