Australia: The Land Where Time Began

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Adelaide Geosyncline (Adelaide Rift Complex) ARC                                                          

Aka the Adelaide Rift Complex, this is a major central South Australian geological province that stretches from the northern parts of the Flinders Ranges to Kangaroo Island. The Flinders Ranges and the Mt Loft Ranges, the 2 major ranges of South Australia, are included in this province. The sedimentary rocks were deposited in a depression that is believed to have formed as the crust was stretched during the breakup of the supercontinent of Rodinia. It was in the form of an arc about 1000 km long and several hundred km wide. At their deepest the deposits of limestones, shales and sandstones, as well as some volcanics, reach up to 24,000 m thick. The nature of the rocks suggest they were deposited in a mostly marine environment. At the time of their deposition, between the Middle Proterozoic and the end of the Cambrian (about 870 million years ago to about 500 million years ago) the area was on the east coast of the continent. Some have noticed that similar rocks are found on the west cost of North America, leading to the suggestion that before the fragmentation of Rodinia, North America was connected to eastern Gondwana.

In the Late Cambrian, 500 Ma, the Adelaide Rift Complex was inverted to become the Adelaide Fold Belt.

Twidale2

In South Australia the Mount Lofty Ranges and Kangaroo Island are part of a fold mountain belt (orogen), the Adelaide Geosyncline (Preiss, 1987). It had been recognised (Benson, 1909, 1911) that the dominant landform is an upfaulted block comprised on rocks of Proterozoic, Cambrian and Permian age. It is in the form of a relatively long, narrow block, a horst, that is aligned roughly north-south that has been uplifted along fractures or faults running in parallel that are straight or only gently arcuate in plan. Dislocations have been recorded of post-Eocene, post-Pliocene and post-Pleistocene  ages, as well as contemporary dislocations (e.g., Grant, 1976; Glaessner & Wade,1958; Bourman & Lindsay, 1989). A prominent high plain is preserved on it that is covered in places by a crust of ironstone or laterite (Fenner, 1930; Spring, 1945; Miles, 1952; Campana, 1958a).

Geological framework

According to the author1 the horst was raised by faulting over a considerable period between 500-550 Ma, and based on recent and contemporary earthquakes it is believed the same faults that were responsible for the Mt Lofty Ranges are still active (Greenhalgh et al., 1994; Love et al., 1995). During glaciation in the Late Proterozoic the entire area was affected by glaciation, with striated pavements and erratic blocks being well known from several sites such as Hallett Cove and Glacier Rock (Selwyn, 1860; Tate, 1879; Howchin, 1895; David & Howchin, 4897). Following this the area was reduced to low relief and finely weathered. Major faulting was renewed in the Late Cretaceous-earliest Tertiary (Miles, 1952; Campana, 1958a; Glaessner and Wade, 1958).

Summit surface age

Part of the northern Mt Lofty Ranges were investigated in the early 1920s and it was noted (Hossfeld, 1920) that the summit plain, that was lateritised, must be of considerable antiquity. The horst on which it stands is bounded on the east by the Murray Basin marine beds, of Miocene age, that are well exposed in the bluffs that border the gorge that was cut by the river during glacial periods of the Pleistocene in periods of low  sea level (Twidale et al., 1978), to be displaced later by recurrent faulting of the eastern scarp of the Ranges. To the west there are marine strata of Middle Miocene and younger ages (Reynolds, 1953; Glaessner & Wade, 1958) that were deposited  in embayments occupying fault-angle valleys or half-grabens, that border Gulf St Vincent. The author1 suggests the faulting that caused the the ferruginised surface to be uplifted, therefore probably predates the oldest of the basin sediments, and at least in part, occurring before the Early Tertiary (Middle Eocene). The surface of the summit, and the carapace of ironstone that developed on it, must be at least as old.

It might be expected that in the basins, as well as the uplifted block, remnants would be preserved if faulting had disrupted the uplifted block in the early Tertiary. Laterite debris has been reported from basal Tertiary sediments (Glaessner & Wade, 1958) though the laterised surface as such has not been found in bores penetrating through the rocks of Tertiary age and into the older strata. Examples are quarries to the northeast of Adelaide on the backslope of the Para Block, that have been excavated through the unconformity separating the sand of Eocene age from weathered shale and mudstones of Precambrian age encountered kaolinised rock but no ironstone horizon. It has been suggested that the ironstone may have been dissolved in groundwaters, the process possibly being facilitated by chemicals that are produced when eucalypt and other plant litter decay, some of which, the polyphenols, facilitate the dissolution of ferruginous compounds (Bloomfield, 1957; Higston, 1962). Eucalypt forests were dominant in the Australian vegetation by the Miocene.

Based on the evidence available at the time Hossfeld concluded that the surface of the summit of the Mt Lofty Ranges was probably at least of Cretaceous age. This turned out to be an underestimate. The 2 principles that guided Hossfeld were that he concluded that as the summit surface was higher in the landscape than the rocks of Tertiary age that had been deposited in the marginal basins that had been downfaulted, the surface was older. The topographic difference in this instance was the result of faulting or earth movements, though the same principles would have applied if the discrepancy had resulted from erosion.

According to his second principle, in adjudging the faulting that was responsible for Mt Lofty Ranges horst he considered the age of the basal beds laid down in the adjacent basins, as the older beds occur at the base of the sequence in an undisturbed sequence with the younger ones on top: the law of superimposition. As the marine beds of Middle Eocene age had been deposited in the downfaulted basins that flank the western scarp, the fracturing and subsidence, as well as the surface that was disrupted, must predate those basin deposits.

The rivers that drained and shaped it the summit surface of the Mt Lofty Ranges are indicated to be very old, based on the deduction of great age for the surface. Several major rivers drain the Mt Lofty Ranges, a prominent one being the Torrens. The River Torrens has deposited alluvium, as it debouches from its gorge, that is derived from the rocks exposed in the Ranges on a surface of Proterozoic age that has been cut into by the river. The altitude of the surface declines to the west, the sediments becoming finer in the same direction. The gravels in the northeastern suburbs of Adelaide grade laterally into fossiliferous sands of Eocene age (Twidale, 1968a, pp. 388-390). Therefore the River Torrens is at least of Early Tertiary age and, as it cut into the high plain of laterite it can be tentatively dated to the Late Mesozoic or Early Eocene, making it one of the oldest extant rivers in the world.

Adelaide Geosyncline, South Australia - Geochemistry & Petrology and Neoproterozoic Origin of Ironstones in the Eastern Part

Sources & Further reading

  1. Walter & Veevers in Veevers, J. J  (ed.), 2000, Billion-year earth history of Australia and neighbours in Gondwanaland, GEMOC Press Sydney.
  2. Twidale, C.R., 2007, Ancient Australian Landscapes, Rosenberg Publishing Pty. Ltd. , NSW
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 05/08/2013

Australian Neoproterozoic

Interglacial Carbonates, Umberatana Group, Flinders Ranges, South Australia

Delamerian Orogeny

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading