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“Lodgement till” and “deformation till”
Quaternary Science Reviews 20 (2001) 579}581
Viewpoint
`Lodgement tilla and `deformation tilla Hanna RuszczynH ska-Szenajch* Department of Geology, University of Warsaw, Al. ZQ wirki i Wigury 93, 02-089 Warszawa, Poland
Abstract The use of the terms &&lodgement till'' and &&deformation till'' are discussed. It is proposed that the term &&hard lodgement till'' and &&soft lodgement till'' are used for sediments, deposited subglacially by active ice, and existing tectonic names are used for glaciotectonically deformed substructures. 2001 Elsevier Science Ltd. All rights reserved.
Over the last decade, the problem of subglacial deformation has been an issue of discussion, including publications in Quaternary Science Reviews (Hart and Boulton 1991, Benn and Evans 1996, Boulton et al. 1996, Hart 1998). The subglacial processes and the resulting sediments and structures have also been the subject of the author's own studies (RuszczynH ska-Szenajch, 1983, 1987, 1988, 1998). The question which is addressed brie#y here is the problem with certain genetic names recently applied to subglacial sediments by those involved in the study of glacial geology. It is usual in developing branches of science to give new names to newly described phenomena. However, the processes responsible for the existence of such phenomena are sometimes already well-known and already named in other branches of the same science. In such cases, it is of course convenient to use the generally accepted terminology rather than to attribute a specialised meaning to the terms in general use. Otherwise, such usage may lead to a language of terms used exclusively within a narrow specialisation, which may be barely understandable for scientists in other branches of even the same science. In this case, I am referring to glacial geology and its relationship to sedimentology and tectonics. Recently, ambiguous terms have often been used for sediments accumulated underneath active glaciers moving upon soft substrata. These sediments have been thoroughly studied, described in detail, and interpreted in the works cited above (compare also a discussion by Dreimanis, 1989). It is already a truism that such sediments are the results of: (1) glaciosedimentary processes,
* Corresponding author. Tel.: #48-22-822-3051.
i.e. the release from the basal part of the ice (by melting) of debris transported within the ice, and (2) glaciotectonic processes, i.e. the deformation and re-deposition, underneath the ice, of debris supplied by melting together with the soft substratum (which is included in the deforming zone primarily by glaciotectonic processes) or the deformation and redeposition of the soft substratum alone. In previous decades, terminology for glaciogenic sediments was mainly related to glacio-sedimentary processes (Dreimanis, 1989), and the tills deposited in the subglacial environment by active ice were most commonly termed lodgement tills (same references). However, it then became clear that tills accumulated underneath active glaciers represented at least two di!erent kinds of sediments, depending on the sedimentary process and on glaciotectonic synsedimentary deformation. Thus, two di!erent names were given by RuszczynH ska-Szenajch (1983, 1987, 1998) to these kinds of sediments: hard lodgement till for deposits released from a glacier sole mainly due to friction, which represent the `classica lodgement till in the sense of Flint (1971) and Boulton (1975), and soft lodgement till for sediments released from the base of a glacier due to the melting of ice in a more watersaturated (and usually characterised by a higher temperature) subglacial environment. Both kinds of tills often contain inclusions of substratum (layers, lenses, lumps etc.). It was also stressed and documented (RuszczynH skaSzenajch, 1983, 1987, 1998) that both hard lodgement tills and especially soft lodgement tills su!ered synsedimentary glaciotectonic deformation. It is worth adding that hard lodgement tills mainly show the structures of brittle deformation, while soft lodgement tills show mainly those of ductile deformation. In the last decade the sediments called above soft lodgement tills have often been given names which only
0277-3791/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 9 7 - 4
580
H. Ruszczyn& ska-Szenajch / Quaternary Science Reviews 20 (2001) 579}581
relate to subglacial glaciotectonic processes. Benn and Evans (1996) and Boulton et al. (1996) call such sediments deformation tills. However, most sedimentologists and tectonists would hardly regard deformation as a process which creates new sediment. Deformation may only cause a re-deposition and a change (in extreme cases, a metamorphosis) of the already existing deposits (compare RuszczynH ska-Szenajch, 1983, p. 116, and 1998, p. 40). In addition, it is also evident that all genetic kinds of tills } not only subglacial ones } may be subject to deformation (Pleistocene examples in: RuszczynH skaSzenajch, 1998): (1) melt-out tills may show pre-sedimentary glaciodynamic deformation inherited after transport within the ice, (2) both kinds of lodgement tills mentioned may be a!ected by synsedimentary glaciotectonic deformation, and (3) #ow tills often show synsedimentary gravity-induced deformation. The term deformable bed till, used by Hart (1998), is also unsatisfactory because it is possible that deformation may also a!ect subglacial sediment accumulated upon an undeformable bed. Boulton et al. (1996) also use the term deformation till for the pure (or almost pure) substratum squeezed from beneath the ice front, constituting glaciotectonic frontal moraines. The glaciotectonic end moraines squeezed from beneath frontal parts of the Pleistocene ice sheets are common in the European Lowlands. They are usually several tens of metres high. Once again, most sedimentologists or tectonists would hardly accept giving the name deformation till to the huge masses of up-thrusted Tertiary and/or Quaternary sediments of e.g. #uvial, lacustrine, or marine origin. They simply represent (glacio)tectonic structures composed of sediments of various, most often nonglacial origin. Why should they be called tills? Sediments composed of pure (or almost pure) substratum, which were deformed and transported by exclusively glaciotectonic processes (i.e. underneath and/or before a glacier), may be named using the tectonic terms which are already in use in glacial geology. These names are di!erentiated according to the degree of intensity of glacio-deformation, e.g. glaciotectonic structures (thrust sheets, folds, etc.) which refer to clearly marked tectonic units composed of substratum, and glaciotectonic melange concerning strongly deformed substratum (Banham, 1977). The sediments constituting such deformed units still retain their primary lithological features after which they are usually named, e.g. (deformed) marine clay, lacustrine silt, #uvial sand etc. The subglacially formed glaciotectonic structures and glaciotectonic melange built of substratum may also contain inclusions composed of syn-tectonic subglacial till (RuszczynH skaSzenajch, 1983). The occurrence however of such inclusions cannot be the reason of giving the name `tilla, to the whole deformed horizon. The substratum sediments subjected to the most advanced stage of glacio-deformation are supposed to form
an homogenised mixture within which neither the primary structures nor the primary lithology of the mixed sediments can be di!erentiated (Hart and Boulton, 1991). For such deposits the term glaciotectonic diamicton may be proposed (provided the mixture is composed of grains of various sizes, a particular feature attributed to diamictons). It is di$cult, however, to "nd documented examples in the glacial geological literature of homogenous diamictons composed of pure substratum material which underwent glaciotectonic transport only. Still, this remains a theoretical possibility. Hart and Boulton (1991) compare such sediments to metamorphic rocks, but the examples they discuss concern the `deformation tilla, which was being deformed simultaneously with a more or less continuous `inputa of material from the melting glacier sole. So, in such a case it must have been a synsedimentary metamorphism, something unusual in geology. In conclusion: it is an advantageous situation that glacio-sedimentological studies (examining the processes of debris release from ice and the accretion of till derived from a glacier) have recently been combined with those of tectonics and structural geology (explaining the mechanisms of subglacial deformation and re-deposition). However, the best terminology for the resulting sediments seems to be that related to glacio-sedimentary processes (often accompanied by deformation), while tectonic names ought to be used for sediments of substratum which have been deformed and re-deposited by glacial tectonics only (and which may contain only scanty admixtures of material released from the deforming glacier). The term `deformation tilla is misleading and may be misunderstood. The author recommends the usage of the above quoted terms hard lodgement till and soft lodgement till for sediments deposited subglacially by an active ice (which are often deformed during the process of deposition and often contain inclusions of substratum) and giving the already used tectonic names to the glaciotectonically deformed substratum sediments (which in some cases contain the inclusions of the syn-tectonic till). The term glaciotectonic diamicton is proposed for sediments representing pure glaciotectonic homogenization.
Acknowledgements The author thanks Jim Aber and Aleksis Dreimanis for the discussion concerning the topic of this article.
References Banham, P.H., 1977. Glacitectonites in till stratigraphy. Boreas 6, 101}105. Benn, D.I., Evans, D.J.A., 1996. The interpretation and classi"cation of subglacially-deformed materials. Quaternary Science Reviews 15, 23}52.
H. Ruszczyn& ska-Szenajch / Quaternary Science Reviews 20 (2001) 579}581 Boulton, G.S., 1975. Processes and patterns of subglacial sedimentation: a theoretical approach. In: Wright, A.E., Mosely, F. (Eds.), Ice Ages: Ancient and Modern. Seel House, London, pp. 7}42. Boulton, G.S., van der Meer, J.J.M., Hart, J.K., Beets, D., Ruegg, G.H.J., van der Wateren, F.M., Jarvis, J., 1996. Till and moraine emplacement in a deforming bed surge } an example from a marine environment. Quaternary Science Reviews 15, 961}987. Dreimanis, A., 1989. Tills: their genetic terminology and classi"cation. In: Goldthwait, R.P., Matsch, C.L. (Eds.), Genetic Classi"cation of Glacigenic Deposits. Balkema, Rotterdam, pp. 1117}1184. Flint, R.F., 1971. Glacial and Quaternary Geology. Wiley, New York. Hart, J.K., 1998. The deforming bed/debris-rich basal ice continuum and its implications for the formation of glacial landforms (#utes) and sediments (melt-out till). Quaternary Science Reviews 17, 737}754. Hart, J.K., Boulton, G.S., 1991. The interrelation of glaciotectonic and glaciodepositional processes within the glacial environment. Quaternary Science Reviews 10, 335}350.
581
RuszczynH ska-Szenajch, H., 1983. Lodgement tills and syndepositional glacitectonic processes related to subglacial thermal and hydrologic conditions. In: Evenson, E.B., SchluK chter, C., Rabassa, J. (Eds.), Tills and Related Deposits. Balkema, Rotterdam, pp. 113}117. RuszczynH ska-Szenajch, H., 1987. The origin of glacial rafts: detachment, transport, deposition. Boreas 16, 101}112. RuszczynH ska-Szenajch, H., 1988. Glacial tectonics and its relationship to other glaciogenic processes. In: Croot, D.G. (Ed.), Glaciotectonics: Forms and Processes. Balkema, Rotterdam, pp. 191}193. RuszczynH ska-Szenajch, H., 1998. Struktura glin lodowcowych jako istotny wskazH nik ich genezy (Structure of tills: a signi"cant indicator of their genesis } extended English summary). In: Mycielska-Dowgia""o, E. (Ed.), Struktury Sedymentacyjne i Postsedymentacyjne w Osadach Czwartorze1 dowych, Wydzia" Geogra"i i StudioH w Regionalnych Uniwersytetu. Warszawskiego, Warszawa, pp. 13}40.
Viewpoint
`Lodgement tilla and `deformation tilla Hanna RuszczynH ska-Szenajch* Department of Geology, University of Warsaw, Al. ZQ wirki i Wigury 93, 02-089 Warszawa, Poland
Abstract The use of the terms &&lodgement till'' and &&deformation till'' are discussed. It is proposed that the term &&hard lodgement till'' and &&soft lodgement till'' are used for sediments, deposited subglacially by active ice, and existing tectonic names are used for glaciotectonically deformed substructures. 2001 Elsevier Science Ltd. All rights reserved.
Over the last decade, the problem of subglacial deformation has been an issue of discussion, including publications in Quaternary Science Reviews (Hart and Boulton 1991, Benn and Evans 1996, Boulton et al. 1996, Hart 1998). The subglacial processes and the resulting sediments and structures have also been the subject of the author's own studies (RuszczynH ska-Szenajch, 1983, 1987, 1988, 1998). The question which is addressed brie#y here is the problem with certain genetic names recently applied to subglacial sediments by those involved in the study of glacial geology. It is usual in developing branches of science to give new names to newly described phenomena. However, the processes responsible for the existence of such phenomena are sometimes already well-known and already named in other branches of the same science. In such cases, it is of course convenient to use the generally accepted terminology rather than to attribute a specialised meaning to the terms in general use. Otherwise, such usage may lead to a language of terms used exclusively within a narrow specialisation, which may be barely understandable for scientists in other branches of even the same science. In this case, I am referring to glacial geology and its relationship to sedimentology and tectonics. Recently, ambiguous terms have often been used for sediments accumulated underneath active glaciers moving upon soft substrata. These sediments have been thoroughly studied, described in detail, and interpreted in the works cited above (compare also a discussion by Dreimanis, 1989). It is already a truism that such sediments are the results of: (1) glaciosedimentary processes,
* Corresponding author. Tel.: #48-22-822-3051.
i.e. the release from the basal part of the ice (by melting) of debris transported within the ice, and (2) glaciotectonic processes, i.e. the deformation and re-deposition, underneath the ice, of debris supplied by melting together with the soft substratum (which is included in the deforming zone primarily by glaciotectonic processes) or the deformation and redeposition of the soft substratum alone. In previous decades, terminology for glaciogenic sediments was mainly related to glacio-sedimentary processes (Dreimanis, 1989), and the tills deposited in the subglacial environment by active ice were most commonly termed lodgement tills (same references). However, it then became clear that tills accumulated underneath active glaciers represented at least two di!erent kinds of sediments, depending on the sedimentary process and on glaciotectonic synsedimentary deformation. Thus, two di!erent names were given by RuszczynH ska-Szenajch (1983, 1987, 1998) to these kinds of sediments: hard lodgement till for deposits released from a glacier sole mainly due to friction, which represent the `classica lodgement till in the sense of Flint (1971) and Boulton (1975), and soft lodgement till for sediments released from the base of a glacier due to the melting of ice in a more watersaturated (and usually characterised by a higher temperature) subglacial environment. Both kinds of tills often contain inclusions of substratum (layers, lenses, lumps etc.). It was also stressed and documented (RuszczynH skaSzenajch, 1983, 1987, 1998) that both hard lodgement tills and especially soft lodgement tills su!ered synsedimentary glaciotectonic deformation. It is worth adding that hard lodgement tills mainly show the structures of brittle deformation, while soft lodgement tills show mainly those of ductile deformation. In the last decade the sediments called above soft lodgement tills have often been given names which only
0277-3791/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 9 7 - 4
580
H. Ruszczyn& ska-Szenajch / Quaternary Science Reviews 20 (2001) 579}581
relate to subglacial glaciotectonic processes. Benn and Evans (1996) and Boulton et al. (1996) call such sediments deformation tills. However, most sedimentologists and tectonists would hardly regard deformation as a process which creates new sediment. Deformation may only cause a re-deposition and a change (in extreme cases, a metamorphosis) of the already existing deposits (compare RuszczynH ska-Szenajch, 1983, p. 116, and 1998, p. 40). In addition, it is also evident that all genetic kinds of tills } not only subglacial ones } may be subject to deformation (Pleistocene examples in: RuszczynH skaSzenajch, 1998): (1) melt-out tills may show pre-sedimentary glaciodynamic deformation inherited after transport within the ice, (2) both kinds of lodgement tills mentioned may be a!ected by synsedimentary glaciotectonic deformation, and (3) #ow tills often show synsedimentary gravity-induced deformation. The term deformable bed till, used by Hart (1998), is also unsatisfactory because it is possible that deformation may also a!ect subglacial sediment accumulated upon an undeformable bed. Boulton et al. (1996) also use the term deformation till for the pure (or almost pure) substratum squeezed from beneath the ice front, constituting glaciotectonic frontal moraines. The glaciotectonic end moraines squeezed from beneath frontal parts of the Pleistocene ice sheets are common in the European Lowlands. They are usually several tens of metres high. Once again, most sedimentologists or tectonists would hardly accept giving the name deformation till to the huge masses of up-thrusted Tertiary and/or Quaternary sediments of e.g. #uvial, lacustrine, or marine origin. They simply represent (glacio)tectonic structures composed of sediments of various, most often nonglacial origin. Why should they be called tills? Sediments composed of pure (or almost pure) substratum, which were deformed and transported by exclusively glaciotectonic processes (i.e. underneath and/or before a glacier), may be named using the tectonic terms which are already in use in glacial geology. These names are di!erentiated according to the degree of intensity of glacio-deformation, e.g. glaciotectonic structures (thrust sheets, folds, etc.) which refer to clearly marked tectonic units composed of substratum, and glaciotectonic melange concerning strongly deformed substratum (Banham, 1977). The sediments constituting such deformed units still retain their primary lithological features after which they are usually named, e.g. (deformed) marine clay, lacustrine silt, #uvial sand etc. The subglacially formed glaciotectonic structures and glaciotectonic melange built of substratum may also contain inclusions composed of syn-tectonic subglacial till (RuszczynH skaSzenajch, 1983). The occurrence however of such inclusions cannot be the reason of giving the name `tilla, to the whole deformed horizon. The substratum sediments subjected to the most advanced stage of glacio-deformation are supposed to form
an homogenised mixture within which neither the primary structures nor the primary lithology of the mixed sediments can be di!erentiated (Hart and Boulton, 1991). For such deposits the term glaciotectonic diamicton may be proposed (provided the mixture is composed of grains of various sizes, a particular feature attributed to diamictons). It is di$cult, however, to "nd documented examples in the glacial geological literature of homogenous diamictons composed of pure substratum material which underwent glaciotectonic transport only. Still, this remains a theoretical possibility. Hart and Boulton (1991) compare such sediments to metamorphic rocks, but the examples they discuss concern the `deformation tilla, which was being deformed simultaneously with a more or less continuous `inputa of material from the melting glacier sole. So, in such a case it must have been a synsedimentary metamorphism, something unusual in geology. In conclusion: it is an advantageous situation that glacio-sedimentological studies (examining the processes of debris release from ice and the accretion of till derived from a glacier) have recently been combined with those of tectonics and structural geology (explaining the mechanisms of subglacial deformation and re-deposition). However, the best terminology for the resulting sediments seems to be that related to glacio-sedimentary processes (often accompanied by deformation), while tectonic names ought to be used for sediments of substratum which have been deformed and re-deposited by glacial tectonics only (and which may contain only scanty admixtures of material released from the deforming glacier). The term `deformation tilla is misleading and may be misunderstood. The author recommends the usage of the above quoted terms hard lodgement till and soft lodgement till for sediments deposited subglacially by an active ice (which are often deformed during the process of deposition and often contain inclusions of substratum) and giving the already used tectonic names to the glaciotectonically deformed substratum sediments (which in some cases contain the inclusions of the syn-tectonic till). The term glaciotectonic diamicton is proposed for sediments representing pure glaciotectonic homogenization.
Acknowledgements The author thanks Jim Aber and Aleksis Dreimanis for the discussion concerning the topic of this article.
References Banham, P.H., 1977. Glacitectonites in till stratigraphy. Boreas 6, 101}105. Benn, D.I., Evans, D.J.A., 1996. The interpretation and classi"cation of subglacially-deformed materials. Quaternary Science Reviews 15, 23}52.
H. Ruszczyn& ska-Szenajch / Quaternary Science Reviews 20 (2001) 579}581 Boulton, G.S., 1975. Processes and patterns of subglacial sedimentation: a theoretical approach. In: Wright, A.E., Mosely, F. (Eds.), Ice Ages: Ancient and Modern. Seel House, London, pp. 7}42. Boulton, G.S., van der Meer, J.J.M., Hart, J.K., Beets, D., Ruegg, G.H.J., van der Wateren, F.M., Jarvis, J., 1996. Till and moraine emplacement in a deforming bed surge } an example from a marine environment. Quaternary Science Reviews 15, 961}987. Dreimanis, A., 1989. Tills: their genetic terminology and classi"cation. In: Goldthwait, R.P., Matsch, C.L. (Eds.), Genetic Classi"cation of Glacigenic Deposits. Balkema, Rotterdam, pp. 1117}1184. Flint, R.F., 1971. Glacial and Quaternary Geology. Wiley, New York. Hart, J.K., 1998. The deforming bed/debris-rich basal ice continuum and its implications for the formation of glacial landforms (#utes) and sediments (melt-out till). Quaternary Science Reviews 17, 737}754. Hart, J.K., Boulton, G.S., 1991. The interrelation of glaciotectonic and glaciodepositional processes within the glacial environment. Quaternary Science Reviews 10, 335}350.
581
RuszczynH ska-Szenajch, H., 1983. Lodgement tills and syndepositional glacitectonic processes related to subglacial thermal and hydrologic conditions. In: Evenson, E.B., SchluK chter, C., Rabassa, J. (Eds.), Tills and Related Deposits. Balkema, Rotterdam, pp. 113}117. RuszczynH ska-Szenajch, H., 1987. The origin of glacial rafts: detachment, transport, deposition. Boreas 16, 101}112. RuszczynH ska-Szenajch, H., 1988. Glacial tectonics and its relationship to other glaciogenic processes. In: Croot, D.G. (Ed.), Glaciotectonics: Forms and Processes. Balkema, Rotterdam, pp. 191}193. RuszczynH ska-Szenajch, H., 1998. Struktura glin lodowcowych jako istotny wskazH nik ich genezy (Structure of tills: a signi"cant indicator of their genesis } extended English summary). In: Mycielska-Dowgia""o, E. (Ed.), Struktury Sedymentacyjne i Postsedymentacyjne w Osadach Czwartorze1 dowych, Wydzia" Geogra"i i StudioH w Regionalnych Uniwersytetu. Warszawskiego, Warszawa, pp. 13}40.