Palaeochannel
Encyclopedia
Palaeochannels or paleochannels are deposits of unconsolidated sediment
s 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.
, 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 meander
ing 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 erosion
al surface.
Palaeochannels can also be identified according to their age. For example, there are deposits of Tertiary
lignite
s situated in Tertiary river systems which are preserved on top of Archaean basement in the Yilgarn Craton
of Western Australia. These river systems have laid in place for ~15 to 50 million years and would be considered palaeochannels.
, 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 floodplain
s 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.
deposits of uranium
, lignite
, precious metal
s such as gold
and platinum
, heavy minerals such as tin
, tungsten
, and iron ore preserved as paleo-placer
deposits.
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s 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 RiverTodd River
The Todd River is an ephemeral river in the southern Northern Territory, central Australia. The origins of the Todd River begin in the MacDonnell Ranges, where it flows past the Telegraph Station, almost through the center of Alice Springs, through Heavitree Gap at the southern end of Alice Springs...
, Central Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
). 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 meander
Meander
A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the...
ing peneplain
Peneplain
A peneplain is a low-relief plain representing the final stage of fluvial erosion during times of extended tectonic stability. The existence of peneplains, and peneplanation as a geomorphological process, is not without controversy, due to a lack of contemporary examples and uncertainty in...
streams.
Thereafter, a palaeochannel may form part of the regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
of a region and, although it is unconsolidated or partly consolidated, is currently part of the erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
al surface.
Palaeochannels can also be identified according to their age. For example, there are deposits of Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
lignite
Lignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...
s situated in Tertiary river systems which are preserved on top of Archaean basement in the Yilgarn Craton
Yilgarn craton
The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...
of Western Australia. These river systems have laid in place for ~15 to 50 million years and would be considered palaeochannels.
Formation
Paleochannels form when river channels aggradeAggradation
Aggradation is the term used in geology for the increase in land elevation due to the deposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount of material that the system is able to transport...
, 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
Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
. Paleochannels may also be preserved in the short-term on non-net-depositional floodplain
Floodplain
A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...
s in which the river migrates or avulses
Avulsion (river)
In sedimentary geology and fluvial geomorphology, avulsion is the rapid abandonment of a river channel and the formation of a new river channel. Avulsions occur as a result of channel slopes that are much lower than the slope that the river could travel if it took a new course.-Deltaic and...
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
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
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, EoceneEoceneThe Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
and Recent sediments and fossilFossilFossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s within them, and are important locations for palaeontology, palaeobotany and archaeologyArchaeologyArchaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...
. - Preserving evidence of older erosional surfaces and levels, useful for estimating the net erosional budget of older regolithRegolithRegolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
- Preserving sedimentary records useful for understanding climatic conditions, including various isotopic indicators of past rainfall, temperatures and climates, which are used in understanding climate changeClimate changeClimate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...
and global warmingGlobal warmingGlobal warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...
Economic importance
Palaeochannels can host economic oreOre
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
deposits of uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
, lignite
Lignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...
, precious metal
Precious metal
A precious metal is a rare, naturally occurring metallic chemical element of high economic value.Chemically, the precious metals are less reactive than most elements, have high lustre, are softer or more ductile, and have higher melting points than other metals...
s such as gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
and platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
, heavy minerals such as tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
, tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
, and iron ore preserved as paleo-placer
Placer
Placer may refer to one of the following:*Placer deposit*Placer sheep*Placer mining*Placer, rugby league football role.Geographical names:* Placer, Masbate, Philippines* Placer, Surigao del Norte, Philippines...
deposits.
See also
- SedimentologySedimentologySedimentology encompasses the study of modern sediments such as sand, mud , and clay, and the processes that result in their deposition. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary...
- ErosionErosionErosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
- Ore genesisOre genesisThe various theories of ore genesis explain how the various types of mineral deposits form within the Earth's crust. Ore genesis theories are very dependent on the mineral or commodity....
- Placer miningPlacer miningPlacer mining is the mining of alluvial deposits for minerals. This may be done by open-pit or by various surface excavating equipment or tunneling equipment....
- Uranium ore deposits