Adelaide Superbasin

Major geological province in central South Australia
The Adelaide Superbasin (previously known as the Adelaide Geosyncline and Adelaide Rift Complex) is a major Neoproterozoic to middle Cambrian geological province in central and south east South Australia, western New South Wales, and western Victoria. The Adelaide Superbasin consists 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 SWEAT (south west USA against East Antarctica) reconstruction of Rodinia. Particularly notable events that are preserved in the rock record of the Adelaide Superbasin are the two Neoproterozoic Snowball Earth events (the Sturtian and Marinoan Glaciations), the Neoproterozoic Oxygenation Event, the Ediacaran Acraman bollide ejecta layer and the rise of Ediacaran Fauna.

North–south it stretches over from the Peake and Denison Ranges in the central north of South Australia down through the Flinders Ranges and Mount Lofty Ranges, narrowing at the Fleurieu Peninsula and extending onto Kangaroo Island. West–east it stretches about from the eastern margin of Lake Gairdner across to the Barrier Ranges of New South Wales. The true northern and eastern extent of the Adelaide Superbasin is not well known Most of the outcropping rock today is within the two major mountain ranges of South Australia: the Flinders Ranges and the Mount Lofty Ranges. Much like the partly coeval Centralian Superbasin it contains several named basins and sub basins. The oldest and largest of these is the Adelaide Rift Complex, with the adjoining and relatively undeformed Stuart Shelf, Torrens Hinge Zone and Coombalarnie Platform making up the remaining Neoproterozoic provinces. The Stansbury Basin (including its sub province the Kanmantoo Province/Trough) and Arrowie Basin (including its sub province the Yalkalpo Sub basin) are the two known Cambrian provinces within the Adelaide Superbasin.

Deposition in the Adelaide Superbasin began just prior to 830 Ma as Laurentia (and possibly an intervening continent, see Rodinia for alternative reconstructions) began to rift away from Australia during the breakup of the supercontinent Rodinia. Deposition continued through to the Delamerian Orogeny c. 514–490 Ma at which time deposition in the Adelaide Superbasin stopped and the rocks were folded and deformed. Initially, deposition occurred from gradual subsidence of peneplained cratonic lithosphere before developing into a rift basin. After about 725 Ma, deposition continued in a mostly passive margin setting along the western edge of the Palaeo Pacific Ocean, with renewed rifting during the Ediacaran to the east (present day) of the main depocentres until being terminated by the Delamerian Orogeny. The most abundant rock types indicate a transition from evaporitic depositional environments through to proximal marine, glacial and marine depositional environments.

The Adelaide Rift Complex is the oldest and most central part of the Adelaide Superbasin. It is a series of rift troughs and passive margins basins with protracted development from c. 840 Ma to c. 550 Ma with the top of the sedimentary sequence marked by a major basin wide disconformity separating it from the Cambrian Arrowie and Stansbury Basins.

The Torrens Hinge Zone is a transitional area between the relatively undeformed platform deposits of the Stuart Shelf and the deformed rocks of the central Adelaide Rift Complex. It runs along the western margin of the primary rift basin.

The Stuart Shelf is a region of platform deposits on the western rift shoulder that overlies the Gawler Craton. It experienced deposition in the late Neoproterozoic after the Sturtian Glaciation during a period of marine transgression and remains relatively undeformed to this day.

The Coombalarnie Platform, like the Stuart Shelf, is a region of platform deposits; however, it overlies the Curnamona Province to the north east of the main depocentre. Deposition occurred only after a major marine transgression event during the late Neoproterozoic after the Sturtian Glaciation.

The Stansbury Basin is one of the two known Cambrian basins of the Adelaide Superbasin. It is exposed in the south of the superbasin, extending from Kangaroo Island and the Mount Lofty Ranges toward Victoria underneath the Murray Basin. The true eastward extent of this basin is not well understood and is a focus of current geological research in South Australia. It is likely that deposition was continuous with the Arrowie Basin to the north.

Sedimentation in the Adelaide Superbasin ended in the middle Cambrian, when tectonic inversion occurred along the eastern margin of Neoproterozoic and Cambrian cratonic Australia and the margin became a subduction zone of the Palaeo Pacific. although the first confirmed Animalia are the world renowned Ediacaran fauna, named for the Ediacara Hills where they were first discovered in South Australia in 1946. They occur at the end of the Neoproterozoic, and in 2004 the location gave its name to the last geological period of the era, the Ediacaran.

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.