A very short and basic story

Dr Victor Gostin, Earth & Environmental Sciences, University of Adelaide

Victor Gostin

Most of the Ranges consist of an extensive and very thick (15km) sequence of layered sedimentary rocks belonging to, and deposited in the Adelaide Geosyncline, also called the Adelaide Fold Belt or Adelaide Rift Complex, stretching (~600km) from Kangaroo Island north to Mt Painter, South Australia.

This structure was formed some 850 Ma (=million years ago) with the stretching, thinning and sinking of much older crust now seen at Mt Painter, in parts of the Adelaide Hills, and flanked on the west by the Gawler Craton, and on the east by the Curnamona & Broken Hill blocks.

After a very long 350 million years of almost continuous sedimentation, this layered sequence was squeezed and gently folded during the end-Cambrian Delamerian Orogeny (~500 Ma). Deep burial regionally metamorphosed the sediments, and granites were intruded, such as those at Victor Harbor, Reedy Creek, Olary, and Mt Painter. Today's Flinders Ranges are the deeply eroded and exhumed parts of this very thick sedimentary sequence, with soft shales in the valleys and harder sandstones or quartzites forming many of the high ridges.

The whole sedimentary sequence may be divided into five parts, the first two, Callana and Burra Groups, were formed in localised rift valleys involved with the break up of the supercontinent Rhodinia. Basaltic lavas, alluvial fans and saline lakes began the sedimentation. A widening rift sequence of the Burra Group involved extensive marine sandstones, shales and dolomites, with some magnesites as well. These outcrop extensively from Adelaide to Quorn, and northwest of Copley.

A breakup unconformity or erosion surface separates these from the succeeding Umberatana and Wilpena Groups, which together with the Early Cambrian sediments were deposited in shallow seas and along continental shelves. These include two massive ice ages, involving major sealevel fluctuations and a third cold period (580 Ma) during which a huge asteroid impact created the Acraman structure in the Gawler Ranges, ejecting rock fragments at least 400km distant. This may have contributed to the evolution of the famous soft-bodied Ediacaran fauna preserved in the latest Precambrian, the Ediacaran Period.

Cambrian sediments included shallow marine sands and silts with shelly fossils, and predominant fossiliferous limestones, some as reefs (with sponge-like Archaeocyatha). The youngest Cambrian sequences include basalts, and red deltaic sandstones and siltstones.

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