Palaeochannel

In the Earth sciences, a palaeochannel, also spelled paleochannel, is a significant length of a river or stream channel which no longer conveys fluvial discharge as part of an active fluvial system. The term palaeochannel is derived from the combination of two words, palaeo or old, and channel; i.e., a palaeochannel is an old channel. Palaeochannels may be preserved either as abandoned surface channels on the surface of river floodplains and terraces or infilled and partially or fully buried by younger sediments. The fill of a palaeochannel and its enclosing sedimentary deposits may consist of unconsolidated, semi-consolidated, or well-cemented sedimentary strata depending on the action of tectonics and diagenesis during their geologic history after deposition. The abandonment of an active fluvial channel and the resulting formation of a palaeochannel can be the result of tectonic processes, geomorphologic processes, anthropogenic activities, climatic changes, or a variable and interrelated combination of these factors.^[2][3]

Aerial view of exhumed fluvial palaeochannel, Emery County, Utah. The erosion of softer surrounding mudstone left this palaeochannel as a sandstone ridge.^[1]

Palaeochannel versus palaeovalley

In practice, the term palaeochannel has often been used both interchangeably and inconsistently with the terms palaeovalley and paleovalley. This has led to both potential and actual confusion in the published literature and studies of groundwater and mineral resources associated with palaeochannels.^[4][5] First, this distinction is important because not all valleys and palaeovalleys are fluvial in origin. Some of them may be either of glacial or tectonic origin.^[4] Other palaeovalleys are buried submarine canyons cut by turbidity currents and mass wasting.^[6] Second, even the deposits that fill a fluvial palaeovalley are not always fluvial sediments. Often, fluvial palaeovalleys are filled and buried by some combination of fluvial, volcanic, glacial, aeolian, lacustrine, estuarine, or marine deposits.^[4] Finally, even when filled largely by fluvial sediments, the channel deposits that fill a palaeochannel comprise only a small fraction of a valley fill, which mainly consists of the deposits of other fluvial environments.^[7] The nomenclature of palaeochannels must reflect their actual physical character, origin, and evolution if their relationship to mineral and groundwater resources is to be properly understood.^[4][5] Thus, it is recommended^[4][5] that palaeochannel be used for an inactive channel formed by a river; palaeochannel deposits for the sediments that infill a palaeochannel; and palaeovalley for a valley incised by an ancient river.

Formation

The avulsion of an active river or stream is the most common fluvial process resulting in the formation of palaeochannels. It is the process by which flow diverts out of an established river channel into a new permanent course on the adjacent floodplain. An avulsion can be either a full avulsion, in which in which all of the discharge is transferred out of the parent channel to a new one, or partial avulsion, in which only a portion of the discharge is transferred to a new one. Only the full avulsion results the formation of a palaeochannel. Partial avulsions result in the formation of anastomosing channels when the divided active channels rejoin downstream and distributary channels when the divided active channels do not rejoin downstream.^[8]

At least three broadly different types of avulsions, (a) avulsion by annexation; (b) avulsion by incision; and (c) avulsion by progradation, are recognized. First, an avulsion by annexation is an avulsion in which an existing active channel is appropriated or if an existing abandoned channel is reoccupied. Second, an avulsion by incision is an avulsion in which a new channel is created by the scouring into the floodplain surface as a direct result of the avulsion. Finally, an avulsion by progradation is an avulsion that results in the formation of an extensive deposition and multi-channeled distributive network. Of these types of avulsions only the avulsion by incision results in the complete abandonment and preservation of a fluvial channel as a palaeochannel.^[8]

The exact environmental conditions that favour incisional avulsions remain unsettled. However, it is generally agreed that they are promoted by a) rapid aggradation of the main channel and floodplain; b) wide unobstructed floodplain and down-valley drainage; and c) frequently recurring floods of high magnitude. In many floodplains, these conditions and frequent avulsions are correlated with superelevated alluvial ridges and river stages.^[8]

The event or factor that can trigger a specific avulsion may be either external or internal to a river system and quite varied. Factors external to a river system that might cause an avulsion include fault activity, sea-level rise, or an increase in flood peak discharge. Factors internal to a river system that might cause an avulsion include sediment influx, breakout along animal pathways, and blockage by ice jams, plant growth, log jams, and beaver dams.^[9]

Recognition

A variety of techniques have been used to recognize and map palaeochannels. At first, surficial data from aerial photography, soils maps, topographic maps, archaeological surveys and excavations, and field observations were integrated with subsurface data from geological and engineering borings and cores to recognize and map palaeochannels.^[10][11] As the importance of coarse-grained fluvial deposits associated with palaeochannels as sources of groundwater and favoured conveyance of subsurface water became appreciated, geophysical techniques sensing the physical properties of underlying ground and bedrock and groundwater and other fluids contained within them became more important and widely used.^[12][13] For example, palaeochannels can be identified using airborne electromagnetic surveys, as the coarse-grained sediments are more electrically resistive than surrounding materials.^[14] Also, lidar, more sophisticated remote sensing techniques, digital analysis, including computer modeling, of data were added to the various techniques use to detect and map palaeochannels.^[13]

Geological importance

Palaeochannels 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. A significant example is the Cadell Fault in Australia, which affected the course of the Murray River.^[15]
• Preserving Tertiary, Eocene and Holocene sediments 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 importance

Palaeochannels can host economic ore deposits of uranium,^[16] lignite, precious metals such as gold and platinum, heavy minerals such as tin, tungsten, and iron ore preserved as palaeo-placer deposits.

Palaeochannels have been proposed as a mechanism for managed aquifer recharge in California.^[14][17]

See also

• Sedimentology
• Erosion
• Ore genesis
• Paleoshoreline
• Placer mining
• Uranium ore deposits

References

1. Hayden, A.T., Lamb, M.P., Fischer, W.W., Ewing, R.C., McElroy, B.J. and Williams, R.M., 2019. Formation of sinuous ridges by inversion of river-channel belts in Utah, USA, with implications for Mars. Icarus, 332, pp.92-110.
2. Kumar, V., 2011. Palaeo-channel. In: Bishop, M.P., Björnsson, H., Haeberli, W., Oerlemans, J., Shroder, J.F. and Tranter, M., eds., p. 803, Encyclopedia of snow, ice and glaciers. Amsterdam, The Netherlands, Springer Science & Business Media. 1253 pp. ISBN 978-90-481-2641-5
3. Nash, D.J., 2000. Palaeochannel. In Thomas, D.S.G., and Goudie, A., eds., p. 354. The Dictionary of Physical Geology, 3rd ed. Oxford, United Kingdom, Blackwell Publishing. 610 pp. ISBN 978-0-631-20472-5
4. Clarke, J., 2009. Palaeovalley, palaeodrainage, and palaeochannel–what’s the difference and why does it matter?. Transactions of the Royal Society of South Australia, 133(1), pp.57-61.
5. Munday, T., Taylor, A., Raiber, M., Soerensen, C., Peeters, L., Krapf, C., Cui, T., Cahill, K., Flinchum, B., Smolanko, N. and Martinez, J., 2020. Integrated regional hydrogeophysical conceptualization of the Musgrave Province, South Australia. Goyder Institute for Water Research Technical Report Series No. 20/04. Adelaide, SA, Australia, Goyder Institute for Water Research. 108 pp.
6. Shepard, F.P., 1981. Submarine canyons: multiple causes and long-time persistence. American Association of Petroleum Geologist Bulletin, 65(6), pp.1062-1077.
7. Gibling, M.R., Fielding, C.R., and Sinha, R., 2011. Alluvial valleys and alluvial sequences: towards a geomorphic assessment. In: North, C., Davidson, S., and Leleu, S. eds., pp. 423–447, Rivers to Rocks. Special Publication. 97. Tulsa, Oklahoma, SEPM (Society for Sedimentary Geology) 447 pp. ISBN 978-1-56576-305-0
8. Slingerland, R., and Smith, N.D., 2004. River avulsions and their deposits. Annual Review of Earth and Planetary Sciences, 32, pp.257-285.
9. Gibling, M.R., Bashforth, A.R., Falcon-Lang, H.J., Allen, J.P. and Fielding, C.R., 2010. Log jams and flood sediment buildup caused channel abandonment and avulsion in the Pennsylvanian of Atlantic Canada. Journal of Sedimentary Research, 80(3), pp.268-287.
10. Fisk, H.N., 1944. Geological Investigation of the Alluvial Valley of the Lower Mississippi River. Vicksburg, Mississippi, Mississippi River Commission and Washington, DC, War Department, U. S. Army Corps of Engineers. 78 pp.
11. El Bastawesy, M., Gebremichael, E., Sultan, M., Attwa, M. and Sahour, H., 2020. Tracing Holocene channels and landforms of the Nile Delta through integration of early elevation, geophysical, and sediment core data. The Holocene , 30(8), pp.1129-1141.
12. Nimnate, P., Thitimakorn, T., Choowong, M. and Hisada, K., 2017. Imaging and locating paleo-channels using geophysical data from meandering system of the Mun River, Khorat Plateau, Northeastern Thailand. Open Geosciences, 9(1), pp.675-688.
13. Kirsch, R., 2011. Groundwater Geophysics: A Tool for Hydrogeology, 2nd. Berlin, New York, Springer. 493 pp. ISBN 978-3-540-29383-5
14. Knight, R., Steklova, K., Miltenberger, A., Kang, S., Goebel, M., and Fogg, G., 2022. Airborne geophysical method images fast paths for managed recharge of California’s groundwater. Environmental Research Letters, 17(12), no. 124021.
15. Heath A; Clarke D; Gibson G; McCue K; Van Dissen R (2005). "A palaeoseismological investigation of the Cadell Fault Zone, Victoria, Australia, New Zealand Society for Earthquake Engineering, 2005 conference" (PDF). Nzsee.org.nz. Retrieved 7 January 2024.
16. Douglas, G.B.; Butt, C.R.M.; Gray, D.J. (2003). "Mulga Rock Uranium and Multielement Deposits, Officer Basin, WA" (PDF). CRC LEME, CSIRO Exploration and Mining. Retrieved 7 March 2006.
17. • Gies, Erica (7 January 2023). "California Could Capture Its Destructive Floodwaters to Fight Drought". New York Times.
    • Lansdale, Amy LeVan (2005). Influence of a coarse-grained incised-valley fill on groundwater flow in fluvial fan deposits, Stanislaus County, Modesto, California, USA (Masters). Michigan State University. doi:10.25335/M5G44J01D.
    • He, Xiaogang; Bryant, Benjamin P.; Moran, Tara; Mach, Katharine J.; Wei, Zhongwang; Freyberg, David L. (2023). "Climate-informed hydrologic modeling and policy typology to guide managed aquifer recharge". Science Advances. 7 (17): eabe6025. doi:10.1126/sciadv.abe6025. PMC 8059926. PMID 33883132. S2CID 233349426.