The search for underground water and mineral wealth was the principal stimulus to the growth of geological knowledge about South Australia.
For the first fifty years, exploration for minerals was conducted by private companies and individuals; it was not until 1882 that the Geological Survey was established to carry out systematic mapping and investigation of mineral deposits and underground water resources. H.Y.L. Brown was appointed the first Government Geologist. Travelling by camel, often under considerable hardship, he undertook extensive fieldwork which resulted in the first geological maps of South Australia. Early editions of the map in 1883 and 1886 were superseded in 1899 by a State geological map at a scale of sixteen miles to the inch. This map, still broadly accurate, was the foundation of future geological work.
Brown's pioneering work was supplemented later by contributions from R. Tate, W. Howchin, D. Mawson and R.L. Jack. Since then many other geologists in government organisations, mineral exploration companies and universities have significantly advanced our understanding of the geological evolution of South Australia.
The surface geology map shows the broad surface distribution of rocks in South Australia. The subdivisions used are based not only on rock or sediment type but also on time. Appreciating the importance of time in geological processes is fundamental to understanding the geological record. Natural radioactive isotopes provide a means for reducing geological time; they indicate that the earth formed at least 4500 million years ago (Ma). Major geological time subdivisions are Archaean, Proterozoic, Palaeozoic, Mesozoic and Cainozoic. Each of these is further subdivided, as shown on which depicts characteristic South Australian fossils for each time period.
The main theme for the surface geology map is the succession of sedimentary and metamorphosed sedimentary strata and igneous events. Sedimentary sequences are grouped so as to define periods which deposition in the basin or on a shelf was controlled by uniform or interrelated processes. Similarly, the igneous events defined distinct pulses of magnetism. On the surface geology map, each igneous event and sequence of sediments is assigned a colour or symbol.
Archaean (Pre-2500 Ma)
The oldest rocks in South Australia are restricted to southern and central Eyre Peninsula and the Tarcoola areas. The original sediments of quartzite, shale, limestone and ironstone were deposited about 2700 Ma and were highly folded, deformed and intruded by granites, granodiorites and amphibolites about 2500 Ma. Their antiquity was determined only in the 1970s.
Early-Middle Proterozoic (2500-1100 Ma)
After deformation and metamorphism of Archaean rocks, there was a 300 to 500 million year period of tectonic quiescence and erosion. On the eastern and possibly also the northern margins of this ancient continent were deposited sediments of the Hutchison Group and its equivalents. These classic and chemical sediments host iron ores in the Middleback Range, jade near Cowell and barite near Olary.
The interval 1800-1580 Ma saw a major period of folding, metamorphism, intrusion of granite and volcanic activity known as the Kimban Orogeny. This orogeny influenced all rocks older that 1580 Ma, particularly those of eastern Eyre Peninsula and the Olary-Broken Hill area (Willyama Inlier).
Immediately following the orogeny, there was a period of extensive volcanism (Gawler Range Volcanics), associated sediments and volcaniclastics (Corunna Conglomerate) and further granitic intrusions. Probably related to this phase is the Olympic Dam copper-uranium-gold depots.
Other significant events during this period include deposition of very thick fluvial sediments in the Mount Gunson area (Pandurra Formation), and high grade metamorphism and deformation accompanied by the intrusion of granite and the basic-ultrabasic rocks of the Giles Complex about 1200-1100 Ma.
تهیه توسط:علی اکتشافیادامه مطلب