Australia: The Land Where Time Began

A biography of the Australian continent 

Submarine Canyons

Structurally, the the edges of the continents are the edges of the continental shelf and adjacent slopes, not the coastlines of the present that are ephemeral on geological time scales, and in places over much shorter periods of time. This is the edge of the continents, or sialic rafts, that are moved around by plate tectonics.

Submarine Canyons, that include some of the largest valleys on Earth, are deep gorges that have been cut into the continental slope, though in places they head back a considerable distance, almost to the present coastline where the continental shelf is narrow. They appear very similar to terrestrial canyons, being steep-sided and of a sinuous plan, and they are at their steepest in their headwater region, flattening towards the terminus. As with rivers on land, they have tributaries, mostly accordant, though some are discordant or hanging. In appearance they are V-shaped, though the authors suggest this may be a function of the survey method. The description of submarine canyons are derived from echo sounding surveys. The actual shape of the canyons may not all be V-shaped, visual surveys by divers in the headwater regions of submarine canyons off the coast of California has found that some of the valleys were flask-shaped in cross-section, having an upper sector that was relatively narrow and a wider lower sector. The authors suggest it is possible that the full length of the canyon is of this flask shape, as the survey method used would be unable to detect the difference between V-shaped canyons and flask-shaped canyons. Visual surveys of the deeper parts of the canyons away from the headwater regions have not been carried out.

Submarine canyons reach depths of 6 km, cutting into a variety of rock types, such as granite, and sediments dating to as young as the Pliocene. They are present around the margins of all continents, some being offshore from the mouths of major rivers on the land, such as the Tagus River, Indus River and the Hudson River. The Murray Canyons, that reach depths of 2 km, are among the deepest known, have been dated to times of lower sea level when the Murray River extended much further south of its present mouth, flowing over the edge of the continental slope. In Western Australia, the Perth Canyon is believed to be similarly related to the Swan River. On the southern margin of the Australian continent there are many canyons that cut into the continental slope and shelf that don't appear to be related to any river of the present, possibly being linked to rivers that no longer exist.

The authors suggest the Ceduna Canyon, or possibly a nearby one, could have been associated with a known system of rivers from the Eocene and Pliocene that drained the southern Gawler Ranges, as well as adjacent parts of northwestern Eyre Peninsula. Off the south coast of Western Australia there are many marine canyons, at such places as Esperance, Albany, Leeuwin, etc., that are suggested by the authors to possibly be related to a drainage system from the Early Tertiary, the only remaining traces of which are strings of salinas. Geophysical surveys have detected canyons at the northern margin of the Eucla Basin that have been filled by marine sedimentary deposits dating from the Eocene and Miocene. At the eastern end of Bass Strait there are a number of canyons with no apparent link to rivers on land, Flinders Canyon, Everard Canyon and Bass Canyon (almost 1.3 km deep).

Many origins have been proposed for the formation of marine canyons. According to the authors, several of the suggestions may be partial explanations or are possibly valid locally. One suggestion was that marine canyons were features that had been eroded by rivers during periods of low sea level during glacial periods, times when sea level was 100 m or more lower than at the present, then were drowned when the sea level rose after the glacial period. The authors say the depth reached by many marine canyons is too great to have been formed by this proposed mechanism, though some marine canyons off the coast of north Africa are in fact drowned river valleys that formed at a time when the region was much wetter than at the present and was drained by major rivers. The Mediterranean Basin of the present was dry at that time, being inundated by the opening of the Straits of Gibraltar, the Atlantic Ocean flooding the basin and inundating the river gorges. These canyons are exceptional.

Turbidity currents form most submarine canyons. A turbidity current is similar to a river, but where the river is flowing water carrying debris, turbidity currents are flowing debris carrying water (Source 1). The density of the mix of sediment and water is increased by the concentration of silt and sand, making it heavier than seawater leading to its flow along the sea floor, channels and gorges being eroded by its flow. The authors suggest it is not certain if turbidity currents are capable of eroding fresh rock such as granite, as in the canyons off the coast of California, some still doubting it can be eroded in this manner, though the authors say there is a lot of evidence supporting the erosion of granite by this process.

A large source of material for such currents would have been available on the exposed sea floors of the Pleistocene glacial period when the sea levels were lower, in a similar manner to the provision by fragmented sea shells as detritus for the formation of calcareous dunes accumulating on the coast. If this is the case, the link between land rivers and submarine canyons would be explicable, large volumes of detritus being brought to the ocean margin by the land rivers, that then become the basis of turbidity currents.

Many submarine canyons off the Californian coast head in embayments a bit to the north of projecting headlands. There are prevalent northwesterly winds and a southerly longshore drift, debris tending to accumulate to the north of barriers formed by the headlands, providing the material that is the basis of turbidity currents. Slumps and turbidity currents active in a canyon head can fool fishermen into thinking that have a big fish on their line.

According to the authors, the most dramatic deductions about turbidity currents came from the breakage of submarine telegraph cables that resulted from earthquakes. In the series of breaks resulting from an earthquake off southern Newfoundland on 18 November 1932, the east-west running cables across the continental shelf and slope were broken. It was first thought that faulting associated with the earthquake was responsible for the breaks, but the cables did not break simultaneously as would be expected if faulting was responsible, they broke progressively away from the earthquake epicentre. It has been suggested that the breaks were actually caused by a submarine slump, that was triggered by the earthquake, flowing at high speed down the continental slope, cutting the cables in succession and not simultaneously.

Sources & Further reading

  1. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd.

Links

  1. Geomorphology, sedimentology and Stratigraphy of Marine Canyons
  2. Deep sea canyon discovered off Western Australia
  3. Bass Strait submarine canyon
  4. Southern Australian submarine canyons: Their distribution and ages
  5. Submarine canyons of the continental margin, east Bass Strait (australia).
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Last updated 21/10/2016

 

 

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