Palaeochannel

Palaeochannels are deposits of unconsolidated or semi-consolidated sedimentary rocks deposited in ancient, currently inactive river and stream channel systems. The word palaeochannel is formed from the words "palaeo" or 'old', and channel; ie; a palaeochannel is an old channel. This may be synonymous with palaeovalley and palaeoriver.

Recognition

A palaeochannel is distinct from the overbank deposits of currently active river channels, including ephemeral water courses which do not regularly flow (such as the Todd River, Central Australia). A palaeochannel is distinct from such watercourses because the river bed is filled with sedimentary deposits which are unrelated to the normal bed load of the current drainage pattern.

Many palaeochannels are arranged on old drainage patterns which are distinct from the current drainage system of a catchment. For example, palaeochannels may relate to a system of rivers and creeks which drained east-west and the current drainage direction is north-south.

Palaeochannels can be most easily identified as broad erosional channels into a basement which underlies a system of depositional sequences which may contain several episodes of deposition and represent meandering peneplains.

Thereafter, a palaeochannel may form part of the regolith of a region and, although it is unconsolidated or partly consolidated, is currently part of the erosional surface.

Palaeochannels can also be identified according to their age. For example, there are deposits of Tertiary lignites situated in Tertiary river systems which are preserved on top of Archaean basement in the Yilgarn Craton of Western Australia [cite web|url=http://www.informaworld.com/smpp/content~content=a725292047~db=all|title=Anand, R. and Paine M. Regolith Geology of the Yilgarn Craton, Western Australia (abstract)|accessdate=2007-08-14] . These river systems have laid in place for ~15 to 50 million years and would be considered palaeochannels.

Formation

Palaeochannels form due to an equilibration of a river with its erosional level, whereupon the nett transport of sediment balances erosion. Palaochannels form by palaeovalleys and erosional channels in basement becoming filled with sediment.

For longer preservation, palaeochannels must have the source of river flows removed, either via a river changing course, climate change strangling inflows into the catchment, or perhaps faulting or tectonic movements altering the dynamics of a river system and/or its flow direction.

Palaeochannels are not necessarily permanent; it is possible for them to become eroded via reactivation of erosional activity or reactivation of the original river system.

Geological importance

Palaeochannels are important to geology for a number of reasons;
* Understanding movements of faults, which may redirect river systems and thus form stranded channels which are in essence palaeochannels.
* Preserving Tertiary, Eocene and Recent sediments and fossils within them, and are important locations for palaeontology, palaeobotany and archaeology.
* Preserving evidence of older erosional surfaces and levels, useful for estimating the nett erosional budget of older regolith
* Preserving sedimentary records useful for understanding climatic conditions, including various isotopic indicators of past rainfall, temperatures and climates, which are used in understanding climate change and global warming

Economic importance

Palaeochannels can host economic ore deposits of uranium [cite web|url=http://crcleme.org.au/RegExpOre/MulgaRock.pdf|title=Douglas, Butt & Gray. 2003, CRC LEME|accessdate=2006-03-07] , lignite, precious metals such as gold and platinum, heavy minerals such as tin, tungsten, and iron ore.

References

See also

* Sedimentology
* Erosion
* Ore genesis
* Placer mining
* Uranium ore deposits