A
palaeochannel, or
paleochannel, is a remnant of an inactive river or stream channel that has been filled or buried by younger sediment. The sediments that the ancient channel is cut into or buried by can be unconsolidated, semi-consolidated, consolidated or lithified. The word
palaeochannel is formed from the
palaeo, meaning "old," and
channel. It may be synonymous with
palaeovalley and
palaeoriver.
RecognitionEditA palaeochannel is distinct from the overbank deposits of currently-active river channels, including ephemeral water courses that do not regularly flow (such as the
Todd River, Central
Australia) because the river bed is filled with sedimentary deposits unrelated to the normal bed load of the current drainage pattern.
Many palaeochannels are arranged on old drainage patterns, distinct from the current drainage system of a catchment. For example, palaeochannels may relate to a system of rivers and creeks that drained east-west if the current drainage direction is north-south.
Palaeochannels can be most easily identified as broad erosional channels into a basement that underlies a system of depositional sequences, which may contain several episodes of deposition and represent
meanderingpeneplain streams.
Thereafter, a palaeochannel may form part of the
regolith of a region and although it is unconsolidated or partly consolidated, it is currently part of the
erosional surface.
Palaeochannels can also be identified according to their age. For example, there are deposits of
Tertiarylignites in the Tertiary river systems preserved on top of
Archaean basement in the
Yilgarn Craton of
Western Australia.
[1] The river systems have laid in place for 15 to 50 million years and would be considered palaeochannels.
FormationEditPaleochannels form when river channels
aggradeand so deposit sediment on their bed. For the channel deposits to be preserved, the flow must not occupy and erode them again. 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. The preservation is short-term because unless the channel deposits are buried, flow will eventually reoccupy its formerly-occupied course, reworking and
eroding the channel deposits.
Geological importanceEditPalaeochannels are important to geology for a number of reasons:
- Understanding movements of faults, which may redirect river systems and so form stranded channels that are, in essence, palaeochannels.
- Preserving Tertiary, Eocene and Holocenesediments and fossils within them, important locations for palaeontology, palaeobotany and archaeology.
- Preserving evidence of older erosional surfaces and levels, which is useful for estimating the net erosional budget of older regolith.
- Preserving sedimentary records, which is useful for understanding climatic conditions, including various isotopic indicators of past rainfall, temperatures and climates, used to understand climate change and global warming.
Economic importanceEditPalaeochannels can host economic
ore deposits of
uranium,
[2]lignite,
precious metals such as
goldand
platinum, heavy minerals such as
tin,
tungsten, and
iron ore preserved as paleo-
placer deposits.
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