Lake Edgar (Tasmania)
Encyclopedia
Lake Edgar was a natural fault scarp
pond on the upper reaches of the Huon River
in South West
Tasmania
.
Lake Edgar was actually two small lakes that were created when the Edgar fault caused the plains below Mount Anne
, on the upper reaches of the Huon River, to move upwards by little more than 6 meters, causing the river to dam and divert northwards around Scotts Peak
. The lake was eventually inundated when the Lake Pedder
was flooded as part of a hydroelectric power scheme in 1972. Lake Edgar should not be confused with the Edgar pond which was created when the Edgar Dam
intersected one of the smaller tributaries of Lake Edgar.
The Lake Edgar Fault originally formed in the Cambrian
as a sinistral wrench fault with a displacement of 12 km. It was reactivated in the Quaternary
. The fault passes through the west part of the Edgar Dam. The fault is about 25–33 km long. the fault dips to the west 60–70 degrees. The fault ruptured twice in the Quaternary period, with the early rupture around 25 ka and the last rupture since glaciation (less than 10 ka). The two Quaternary ruptures each elevated the western side by 2.5 m. This is the only fault known to have ruptured twice in the Quaternary period.
The fault was discovered by Carey and Newstead in 1960. It cracks up Precambrian rocks in the Jubilee Element. The fault dammed up the water to form a lake. This was later split by partial draining. Lake Edgar is the southern of the two small lakes formed. Rainfall is 2500 mm per year. Sediment would have rapidly filled the lake if it was old. The earthquakes associated with the formation of the uplifts would be 6.6 to 7.
The fault separated graphitic phyllite
in the east from metasiltstone
, argillite, quartz
sandstone
conglomerate
on the west. The Hydro Electric Commission dug a trench through the fault and dug pits to determine if water would leak through the fault when the Edgar Dam was built. The fault possibly continues through weak talc containing ultrabasic rocks and into the Gell River Fault about 30 km north, which has an older Late Pleistocene displacement.
Fault scarp
A fault scarp is the topographic expression of faulting attributed to the displacement of the land surface by movement along faults. They are exhibited either by differential movement and subsequent erosion along an old inactive geologic fault , or by a movement on a recent active fault...
pond on the upper reaches of the Huon River
Huon River
The Huon River is the fourth largest river in Tasmania, Australia. It is 170 km in length, and runs through the fertile Huon Valley. From Scotts Peak Dam at Lake Pedder where it begins, it flows south-east to the Tahune Airwalk, where the Picton River joins, before heading through the rural...
in South West
South West Tasmania
South West Tasmania is a region in Tasmania that has evolved over the fifty years between its consideration as a potential resource for development to its consideration as World Heritage wilderness...
Tasmania
Tasmania
Tasmania is an Australian island and state. It is south of the continent, separated by Bass Strait. The state includes the island of Tasmania—the 26th largest island in the world—and the surrounding islands. The state has a population of 507,626 , of whom almost half reside in the greater Hobart...
.
Lake Edgar was actually two small lakes that were created when the Edgar fault caused the plains below Mount Anne
Mount Anne
Mount Anne is a mountain in the Southwest National Park in Tasmania, Australia. It is within the UNESCO World Heritage listed Tasmanian Wilderness....
, on the upper reaches of the Huon River, to move upwards by little more than 6 meters, causing the river to dam and divert northwards around Scotts Peak
Scotts Peak
thumb|right|Scotts PeakScotts Peak is a mountain in the South West Tasmania which is associated with the construction and flooding of the original Lake Pedder, as the lake now completely surrounds the peak...
. The lake was eventually inundated when the Lake Pedder
Lake Pedder
Lake Pedder was once a natural lake, located in the southwest of Tasmania, Australia but the name is now used in an official sense to refer to the much larger artificial impoundment and diversion lake formed when the original lake was expanded by damming in 1972 by the Hydro Electric Commission of...
was flooded as part of a hydroelectric power scheme in 1972. Lake Edgar should not be confused with the Edgar pond which was created when the Edgar Dam
Edgar Dam
The Edgar Dam was one of the component dams in the flooding of Lake Pedder in South West Tasmania, Australia. It was named after Lake Edgar. The dam was constructed by Hydro Tasmania as part of the Gordon River Power Development Scheme....
intersected one of the smaller tributaries of Lake Edgar.
The Lake Edgar Fault originally formed in the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
as a sinistral wrench fault with a displacement of 12 km. It was reactivated in the Quaternary
Quaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
. The fault passes through the west part of the Edgar Dam. The fault is about 25–33 km long. the fault dips to the west 60–70 degrees. The fault ruptured twice in the Quaternary period, with the early rupture around 25 ka and the last rupture since glaciation (less than 10 ka). The two Quaternary ruptures each elevated the western side by 2.5 m. This is the only fault known to have ruptured twice in the Quaternary period.
The fault was discovered by Carey and Newstead in 1960. It cracks up Precambrian rocks in the Jubilee Element. The fault dammed up the water to form a lake. This was later split by partial draining. Lake Edgar is the southern of the two small lakes formed. Rainfall is 2500 mm per year. Sediment would have rapidly filled the lake if it was old. The earthquakes associated with the formation of the uplifts would be 6.6 to 7.
The fault separated graphitic phyllite
Phyllite
Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the...
in the east from metasiltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
, argillite, quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
conglomerate
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
on the west. The Hydro Electric Commission dug a trench through the fault and dug pits to determine if water would leak through the fault when the Edgar Dam was built. The fault possibly continues through weak talc containing ultrabasic rocks and into the Gell River Fault about 30 km north, which has an older Late Pleistocene displacement.