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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 and terraces or infilled and partially or fully buried by younger . 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 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.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. 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.
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.
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.
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. 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.
Palaeochannels often preserve the form, width, and sinuosity of prehistoric river channels when they were active. This is important in reconstructing prehistoric climate and hydrology because empirical equations developed using data collected from modern rivers and streams can be used to calculate the approximate past hydrologic regime of a palaeochannel and the palaeoclimate associated with it. Such empirical equations also allow the estimation of palaeochannel gradient, meander wavelength, sinuosity, and discharge from a palaeochannel exposed in cross-section in an outcrop.Schumm, S.A., 1972. Fluvial paleochannels. in Rigby, J.K., and Hamblin, W.K., eds., pp. 98-107, Recognition of Ancient Sedimentary Environments. SEPM Special Publication, 16. Tulsa, Oklahoma, Society for Sedimentary Geology (SEPM). 340 pp. Williams, G.P., 1988. Paleofluvial estimates from dimensions of former channels and meanders. in Baker, V.R., Kochel, R.C., and Patton, P.C., eds., pp.321-334, Flood Geomorphology. New York, New York, John Wiley. 503 pp. Sidorchuk, A.Y., and Borisova, O.K., 2000. Method of paleogeographical analogues in paleohydrological reconstructions. Quaternary International, 72(1), pp.95-106. The sediments or sedimentary rocks filling palaeochannels also often contain dateable material, micro- and megafossils, and palaeoenvironmental proxies. Fine-grained palaeochannel fills containing autochthonous vertebrate fossils may, in extremely favourable circumstances, contain unabraded, complete skeletons that are important for understanding habitat-specific palaeofaunas and associated palaeoenvironments.Behrensmeyer, A.K., 1988. Vertebrate preservation in fluvial channels. Palaeogeography, Palaeoclimatology, Palaeoecology, 63(1-3), pp.183-199.Behrensmeyer, A.K., and Hook, R.W. 1992. Paleoenvironmental contexts and taphonomic modes in the terrestrial fossil record. in Behrensmeyer, A. K., Damuth, J. D., DiMichele, W. A., Potts, R., Sues, H.-D., and Wing, S.L., eds., pp. 15-136, Terrestrial Ecosystems Through Time. Chicago, Illinois, University of Chicago Press. 588 pp. Fine-grained palaeochannel fills also frequently contain wood, leaves, and palynomorphs that can be used for geologic dating and understanding palaeoclimatic and other palaeoenvironmental conditions, including past rainfall, temperatures and climates, and prehistoric and historic climate change and global warming.Gastaldo, R.A., and Demko, T.M., 2011. The relationship between continental landscape evolution and the plant-fossil record: long term hydrology controls the plant fossil record. in Allison, P.A., and Bottjer, D.J., eds., pp. 249- 286, Taphonomy, Second Edition: Processes and Bias Through Time. The Netherlands, Springer. 612 pp. Simon, S., Gibling, M.R., DiMichele, W.A., Chaney, D.S., Looy, C.V. and Tabor, N.J., 2016. An abandoned-channel fill with exquisitely preserved plants in redbeds of the Clear Fork Formation, Texas, USA: an Early Permian water-dependent habitat on the arid plains of Pangea. Journal of Sedimentary Research, 86, 944–964. Finally, the theoretical equilibrium profiles of rivers and streams provide a datum by which to detect and quantify tectonic processes such as faulting, uplift, and subsidence. Examples of the displacement of palaeochannels by active faulting are shown by the lateral movement along the San Andreas fault where it crosses Wallace Creek in central California,Sieh, K.E. and Jahns, R.H., 1984. Holocene activity of the San Andreas fault at Wallace creek, California. Geological Society of America Bulletin, 95(8), pp.883-896.Dascher-Cousineau, K., Finnegan, N.J., and Brodsky, E.E., 2021. The life span of fault-crossing channels. Science, 373(6551), pp.204-207. and where a fault of the Baton Rouge fault zone vertically offsets a Pleistocene palaeochannel and palaeo-floodplain of the Amite River near Denham Springs, Louisiana.Shen, Z., Dawers, N.H., Törnqvist, T.E., Gasparini, N.M., Hijma, M.P. and Mauz, B., 2017. Mechanisms of late Quaternary fault throw rate variability along the north central Gulf of Mexico coast: Implications for coastal subsidence. Basin Research, 29(5), pp.557-570.
Although layers of lignite and other types of coal are sometimes part of the sedimentary fill of palaeochannels, they are typically too thin and narrow to be economically mined. Also, they actually occur in palaeovalleys, which have been mislabeled as palaeochannels. Typically, when palaeochannels formed, they often partially or totally removed any underlying peat, the precursor to coal. Thus, where present, they are directly associated with areas of thin or missing coal called either wash-outs or coal wants. Wash-outs are a major problem for coal mining because of the drastic decrease in the total tonnage of mineable coal, and disruption to mining techniques. Also, bedding and jointing within strata comprising palaeochannels typically result in hazardous conditions related to unstable highwalls in opencast mines and collapsable roof rock in coal .Jones, N.S., Guion, P.D., Fulton, I.M., 1995. Sedimentology and its applications within the UK opencast coal mining industry. in Whateley, M.K.G., and Spears, D.A., eds., pp. 115–135, European Coal Geology. Geological Society, London Special Publication, 82. London, England, Geological Society Publishing House. 331 pp. Sames, G.P. and Laird, R.B., 1987. Geologic Conditions Affecting Coal Mine Ground Control in the Western United States. US Department of the Interior, Bureau of Mines Report, IC-9172. 30 pp.Kane, W.F., Milici, R.C. and Gathright, T.M., 1993. Geologic factors affecting coal mine roof stability in the eastern United States. Bulletin of the Association of Engineering Geologists, 30(2), pp.165-179.
This distinction is important, first because not all and palaeovalleys are fluvial in origin; some of them may be either of glacial or tectonic origin. Other palaeovalleys are buried cut by turbidity currents and mass wasting.Shepard, F.P., 1981. Submarine canyons: multiple causes and long-time persistence. American Association of Petroleum Geologist Bulletin, 65(6), pp.1062-1077. 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. 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.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.
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