Palaeochannel
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A palaeochannel or paleochannel is a remnant of an inactive river or stream channel that has been either filled or buried by younger sediment. The sediments that the ancient channel is either cut into or buried by can be either unconsolidated, semi-consolidated, consolidated, or lithified. The word palaeochannel is formed from the words "palaeo" or 'old', and channel; i.e.; 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 peneplain streams.
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.[1] These river systems have laid in place for ~15 to 50 million years and would be considered palaeochannels.
Formation
Paleochannels form when river channels aggrade, depositing sediment on their bed. For these channel deposits to be preserved, the flow must not re-occupy and re-erode them. Examples of what may cause long-term preservation include the channels being in a net-depositional environment and/or being in a subsiding sedimentary basin. Paleochannels may also be preserved in the short-term on non-net-depositional floodplains in which the river migrates or avulses away from its previous course. This preservation is short-term because unless the channel deposits are buried, flow will eventually re-occupy its formerly-occupied course, re-working and eroding the channel deposits.
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 net 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,[2] lignite, precious metals such as gold and platinum, heavy minerals such as tin, tungsten, and iron ore preserved as paleo-placer deposits.
References
- ^ "Anand, R. and Paine M. Regolith Geology of the Yilgarn Craton, Western Australia (abstract)". Retrieved 2007-08-14.
- ^ "Douglas, Butt & Gray. 2003, CRC LEME" (PDF). Retrieved 2006-03-07.