- Email: [email protected]
Reply to a comment by H.-J. Stephan
Correspondence / Quaternary Science Reviews 21 (2002) 1111–1116
towards east refolding the Saalian sequence and related to an ice lobe in the Hohwacht bay, as assumed by Van der Wateren (1999) (cf. Seifert, 1954). The dislocation in a drumlinized hill SE of this bay (in this relation cited by Van der Wateren as a personal communication by J.A. Piotrowski) in a new exposure visited during an excursion by the INQUA/PERIBALTIC group 1997 turned out to be a large S-fold overturned towards SW, the upper till wedging out on its north-eastern flank. This observation coincides perfectly with the model of landscape development published by Stephan (1994, Abb.2; cf. Lundershausen, 1997) for the youngest Weichselian glacier advance. The ice flow was E–W in the vicinity of Heiligenhafen and reaching the depression of the recent Hohwacht bay it turned then more towards WSW and SW, in response to the pre-existing relief. This overridden older relief has been reshaped and partly drumlinized by the same glacier that has deposited the UT at the Heiligenhafen cliff. The results of all investigations at the cliff section of Heiligenhafen contradict the main hypotheses of Van der Wateren (1999). References Kabel, C., 1982. Geschiebestratigraphische Untersuchungen im Pleistoz.an Schleswig-Holsteins and angrenzender Gebiete. Dissertation, University of Kiel, Kiel, 231 pp. and appendix.
1113
Lundershausen, S., 1997. Geologie, Sedimentologie and Genese der Drumlins von Wandelwitz, Ostholstein. M.Sc. Thesis, University of Kiel, Kiel, 90 pp. Seifert, G., 1954. Das mikroskopische Korngef.uge des Geschiebemergels als Abbild der Eisbewegung, zugleich Geschichte des Eisabbaues in Fehmarn, Ostholstein und dem D.anischen Wohld. Meyniana (Kiel) 2, 122–184. Stephan, H.-J., 1987. Heiligenhafen Kystklint. VARW 1987, No. (3), Geologisk Centralinstitut, K benhavn, pp. 88–95. Stephan, H.-J., 1992. Exkursion Ostseekuste/Ostholstein. . Erla. uterungen zur Route Kiel-Heiligenhafen. Das Hohe Ufer bei Heiligenhafen. Erl.auterungen zur Route Heiligenhafen-Brodten. DEUQUA 0 92, Exkursionsfuhrer . (Exk. B1), Geologisches Landesamt, Kiel, pp. 192–205. Stephan, H.-J., 1995. The Heiligenhafen cliff. In: Schirmer, W. (Ed.), Quaternary Field Trips in Central Europe (XIV. DEUQUA Congress), Vol. 1 (Regional Field Trips), Friedrich Pfeil, Munchen, . pp. 46–47 Stephan, H.-J., 1997. The ‘‘Hohe Ufer’’-cliff at Heiligenhafen. In: Piotrowski, J. A. (Ed.), The Peribaltic Group. Field Symposium on Glacial Geology at the Baltic Coast in Northern Germany. Excursion Guide, University of Kiel, Kiel pp. 20–23. Stephan, H.-J., 1998. Geschiebemergel als stratigraphische Leitho. rizonte in Schleswig-Holstein; ein Uberblick. Meyniana (Kiel) 50, 113–135. Stephan, H.-J., Kabel, C., Schluter, . G., 1983. Stratigraphical problems in the glacial deposits of Schleswig-Holstein. In: Ehlers, J. (Ed.), Glacial Deposits in North-West Europe. Balkema, Rotterdam, pp. 305–320 (Fig. 314–325). Van Der Wateren, F.M., 1999. Structural geology and sedimentology of the Heiligenhafen till section, North Germany. Quaternary Science Reviews 18 (14), 1625–1638.
Reply to a comment by H.-J. Stephan$ F.M. Van der Wateren Amstel AB to 256, 1011 PX Amsterdam, Netherlands
In my paper on the till section near Heiligenhafen (Van der Wateren, 1999) I argue that the probably Saalian Lower and Middle Tills (LT and MT) do not belong to two stratigraphically separate ice sheet advances as was suggested by Stephan et al. (1983). I suggested an alternative interpretation where the two lithologically and structurally distinct diamicts had formed as a single subglacial shear zone (deformation till). The structural and lithological variation which can be observed along the entire length of the 1 km long section results from upward increasing finite strain, (accumulated deformation) from the undeformed footwall sediments at the interface between the ice and the deforming bed (Van der Wateren, 1987, 1994, 1995). This subglacial shear zone model has recently been elaborated and now includes the full spectrum of $
PII of original articles S0277-3791(98)001206, S02773791(01)00063-4. E-mail address: [email protected] (F.M. Van der Wateren).
structures visible on the outcrop scale and on the microscale (Van der Wateren et al., 2000). Structural analysis of subglacially deformed sediments closely follows methods which have been developed for the study of shear zones in deformed low-to-medium grade metamorphic rocks (Passchier and Trouw, 1996). The rationale for this is the similarity of the kinematics (movement history) of lithified and unlithified sediments. Thus, simple shear will produce structures in sediments, e.g. subglacial till, which are very similar to those produced in rocks. The full range of (micro) structures which are known from mylonites and cataclastic shear zonesFincluding cleavages, Riedel shears, boudins and foldsFcan also be found in tills and be used to reconstruct the kinematics of the subglacial shear zone. Using the principles of structural analysis it is possible to distinguish between sediments which are undeformed and those which have undergone varying degrees of deformation, i.e. accumulated low to high finite strains.
1114
Correspondence / Quaternary Science Reviews 21 (2002) 1111–1116
In a shear zone, strain rates, and therefore finite strain, is at maximum in the centre and decreases towards the margins with the undeformed bedrock. With subglacial tills the highest strain rates are near the ice–sediment interface which effectively is the central part of the shear zone (Van der Wateren, 1994, 1995). We can thus expect to find a systematic upward increase in finite strain from the undeformed substratum to the top of the subglacial deforming bed. This has been confirmed by the . Breidamerkurj okull experiments (Boulton and Hindmarsh, 1987) where the largest displacements occurred just below the ice. The consequence of this subglacial shear zone (deformation till) model is that particles (sand grains, pebbles) higher up in the deforming bed tend to have travelled farther than those in the lower parts. Thus the top part of any single till may be expected to contain the more exotic elements (fartravelled erratics) while those near the bottom consists of more locally derived material (see also Rappol and Stoltenberg, 1985). Another consequence is that, although many tills show a lamination which is reminiscent of stratigraphic layering, stratigraphic principles do not apply for a single unit of subglacially deformed sediments. The layering is a transposed foliation resulting from a repetition of folding and stretching of sediment layers embedded in a more homogenized matrix, indicating relatively high finite strains. Intrafolial folds and boudins of internally undeformed or weakly deformed sediments are telltale signs of the origin of such a layering. In earlier papers (Van der Wateren, 1987, 1995) I have shown that laminated tills are intermediate between weakly deformed sediments and completely homogenized tills. The higher part of what deceptively mimics a stratigraphic sequence therefore is not younger than the underlying part, but coeval and only differs from it in the intensity of deformation and the distance it has travelled. Many natural outcrops clearly differ from the ideal outlined here. There may not always be a gradual increase in finite strain from the bottom to the top of a deformation till. In a recent paper (Van der Wateren et al., 2000) we discussed the causes of discontinuities within shear zones, which may be related to lithological or water pressure variation within the deforming layer which lead to variation in shear strength and thus strain rate. As a result, a homogenized diamict containing fartravelled erratics may overlie with a sharp contact a laminated till composed of mainly local sediments and still be the product of a single glacial event. Obviously, it has to be demonstrated that two or more overlying tills are in effect one single unit which has been deposited during one glacial event. I appear not to have succeeded in convincing Hans–Jurgen . Stephan that such is the case for the lower and middle tills in the Heiligenhafen section.
In his comment Stephan disagrees with the main conclusions of my paper. He distinguishes three tills (MT, LT and a lowest till) below the uppermost (Weichselian) till which unconformably overlies the other ones. His arguments are: 1. (a) I do not know the section well enough since I started studying it in 1992, while Stephan has been studying the section annually since 1962. (b) I do not know all papers resulting from his nearly 40 years of fieldwork. (c) Since this Baltic Sea cliff is subjected to continuous wave erosion new structures and even formerly unknown beds regularly appear, requiring changes in the interpretation of the section over the years. (d) Over the years, new methods have been applied, notably petrographic analyses, which likewise led to new interpretations. Of course I may have missed some features which had washed away prior to the start of my fieldwork at Heiligenhafen in the early 1990’s. Yet, all of the evidence presented by Stephan to refute my hypothesis of the origin of the LT and MT has essentially been there up to today in the same or similar form, and has been incorporated in my paper. In fact, my present interpretation was inspired 10 years previous by a close study of the profile published by Stephan et al. (1983) and by visits on several occasions on field trips led by Stephan himself. 6 years is neither too short, nor are the strata too complex to come to meaningful interpretations of the field data. As to his publications, most of these are from sources not generally available, such as excursion guides and local journals. All but one were cited in my paper. I am not aware that Stephan has appreciatively changed his analytical methods since the start of his study of the Heiligenhafen section nearly 40 years ago. Then and now, his stratigraphy is mainly based on petrographic variation, as has been for many years a routine tool in Quaternary mapping in Germany. I hope to have demonstrated that relying primarily on petrographic analyses may lead to wrong interpretations in subglacial deforming sediments. Once it has been appreciated that any sediment, including diamicts interpreted as lodgement or meltout tills, which at some stage was overlain by glacier ice inevitably experienced some degree of simple shearing, the principal method will be structural analysis with additional study of the petrographic, isotopic etc. composition of the sediments. 2. I overlooked a till underlying my Lower Till. This lowermost till is rich in Eocene diatomite, the local bedrock and the oldest exposed in situ stratigraphic unit. It is underlain by ‘‘a subaquatic deposit with intercalations of flow till and layers with drop till’’ and separated from the LT by a ‘‘pebbly gravel or gravely sand ’’, also rich in Eocene rock components. I did not single out this lowermost sediment unit since it clearly belongs to the basal part of the shear zone
Correspondence / Quaternary Science Reviews 21 (2002) 1111–1116
where undeformed bedrock and in situ glaciofluvial sediment grades into weakly deformed sediment. The fact that the latter, Stephan’s ‘‘lowermost till’’, is rich in Eocene diatomite is evidence of its low degree of transport and finite strain. 3. The LT and MT units are separated by an ‘‘up to 2 m thick glaciofluvial sand’’ which is ‘‘evidence for the stratigraphical separation’’ of the two tills. Where large fragments of these relatively undeformed sediment bodies included in the tills can be studied in detail, the sedimentary structures indicate that they had once been part of one or several proglacial/ice-marginal deltas which had been overridden and torn apart by advancing ice. From east to west along the section these floating sediment bodies show a consistent decrease in grain size while structural associations grade from trough-shaped cross bedding to fine ripple lamination, including climbing ripples in the silty fine sands and fine parallel lamination in silt and clay. This is the typical progression from proximal to distal of a proglacial delta. It is not necessary to invoke a stratigraphic separation between the LT and the MT because of these delta sediment inclusions. Instead, I prefer the simplest explanation whereby the Saalian ice margin advanced over local unlithified and weakly lithified sediments, including its own ice-marginal delta and Eocene diatomite bedrock, respectively. In situ and weakly deformed remnants of this delta are found on top of Eocene bedrock, the same association which Stephan interprets as an older till below the LT. 4. Striations associated with the interface between MT and LT trend E–W, while those observed within the LT trend NE–SW, indicating a change in ice flow direction from the LT to the MT from westward to southwestward. According to Stephan these flow directions agree with the different provenance of the two tills. According to my own measurements of the orientation of cleavages in the LT and MT there is no such dramatic shift in flow direction. All are in agreement with an ENE–WSW to E–W direction of shearing and thus ice flow. 5. Locally the upper part of the LT is laminated and contains lenses, layers and sand balls which to Stephan indicate that it is a supraglacial meltout sequence. This is an interpretation for which there is no foundation and which carefully avoids to explain why their internal structures (Riedel shears) as well as those around them all indicate dextral (E–W) shearing and why, along the section and parallel to the ice flow direction, they constitute a sequence from ice-proximal to ice-distal. An interpretation of these sediment lenses as boudins stretched in the direction of shearing is more in agreement with the observed structural associations. 6. The large fold structures relate to the youngest (Weichselian) E–W ice advance. Therefore the intense
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shearing within the layers also dates from this latest advance. The Heiligenhafen section clearly contains two deformational styles: (1) S–C cleavages, boudins and stretched layers indicating subhorizontal (layer-parallel) extension, and (2) large open folds indicating subhorizontal shortening. These two styles are the result of two entirely different kinematic regimes which cannot occur simultaneously. The former is the result of horizontal simple shear obviously associated with subglacial deformation, while the folding was produced by horizontal pure shear (horizontal compression), which since it overprints the simpler shear fabrics postdates deposition of the LT and MT. I cannot exclude that the folding dates from a Weichselian advance, rather than a late Saalian one in my preferred interpretation. Yet, this must predate deposition of the (probably Weichselian) UT which unconformably overlies the LT and MT, since horizontal compression and horizontal extension do not go together. 7. An outcrop SE from the area did not yield evidence of an eastward directed Weichselian ice advance as I postulated on the basis of a structure in the westernmost part of the cliff (Van der Wateren, 1999: Fig. 12). I have not been on the field trip where this was revealed (INQUA/Peribaltic Group 1997), but the structure shown in my Fig. 12 clearly indicates W to E simple shearing overprinting an older E to W shear which dominates the rest of the cliff. The most likely source for this eastward movement is an ice lobe situated in the Hohwachter Bucht, to the west of the Heiligenhafen section. It will be clear that, unlike Stephan, I hold that the entire sequence, between the undeformed Eocene bedrock and overlying delta sediments and the UT has been subjected to subglacial shear. I prefer the simplest possible explanation of the exposed till units which is in agreement with the evidence in the field and in thin sections: one Saalian till unit consisting of low-tomedium shear strain locally derived material in its bottom part (LT) and high shear strain far-travelled material in its upper part (MT). References Boulton, G.S., Hindmarsh, R.C.A., 1987. Sediment deformation beneath glaciers; rheology and geological consequences. Journal of Geophysical Research B, Solid Earth and Planets 92, 9059–9082. Passchier, C.W., Trouw, R.A.J., 1996. Microtectonics. Springer, Berlin, 289pp. Rappol, M., Stoltenberg, H.M., 1985. Compositional variability of Saalian till in The Netherlands and its origin. Boreas 14, 33–50. Stephan, H.-J., Kabel, Chr., Schluter, . G., 1983. Stratigraphic problems in the glacial deposits of Schleswig-Holstein. In: Ehlers, J. (Ed.), Glacial Deposits in North-West Europe. A.A. Balkema, Rotterdam, pp. 305–320.
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Correspondence / Quaternary Science Reviews 21 (2002) 1111–1116
Van der Wateren, F.M., 1987. Structural geology and sedimentation of the Dammer Berge push moraine, FRG. In: Van der Meer, J.J.M. (Ed.), Tills and Glaciotectonics. Balkema, Rotterdam, pp. 157–182. Van der Wateren, F.M., 1994. Processes of Glaciotectonism. In: Menzies, J. (Ed.), Glacial Environments. Processes, Sediments and Landforms. Pergamon Press, Oxford, pp. 309–335. Van der Wateren, F.M., 1995. Structural Geology and Sedimentology of Push Moraines. Processes of soft sediment deformation in a glacial environment and the distribution of glaciotectonic styles.
Mededelingen Rijks Geologische Dienst 54, 1–168. Van der Wateren, F.M., 1999. Structural geology and sedimentology of Saalian tills near Heiligenhafen, Germany. Quaternary Science Reviews 18, 1625–1639. Van der Wateren, F.M., Kluiving, S.J., Bartek, L.R., 2000. Kinematic indicators of subglacial shearing. In: Maltman, A.J., Hubbard, B.P., Hambrey, M.J. (Eds.), Deformation of Glacial Materials, Geological Society Special Publication, Vol. 176. The Geological Society of London, London, pp. 259–278.
towards east refolding the Saalian sequence and related to an ice lobe in the Hohwacht bay, as assumed by Van der Wateren (1999) (cf. Seifert, 1954). The dislocation in a drumlinized hill SE of this bay (in this relation cited by Van der Wateren as a personal communication by J.A. Piotrowski) in a new exposure visited during an excursion by the INQUA/PERIBALTIC group 1997 turned out to be a large S-fold overturned towards SW, the upper till wedging out on its north-eastern flank. This observation coincides perfectly with the model of landscape development published by Stephan (1994, Abb.2; cf. Lundershausen, 1997) for the youngest Weichselian glacier advance. The ice flow was E–W in the vicinity of Heiligenhafen and reaching the depression of the recent Hohwacht bay it turned then more towards WSW and SW, in response to the pre-existing relief. This overridden older relief has been reshaped and partly drumlinized by the same glacier that has deposited the UT at the Heiligenhafen cliff. The results of all investigations at the cliff section of Heiligenhafen contradict the main hypotheses of Van der Wateren (1999). References Kabel, C., 1982. Geschiebestratigraphische Untersuchungen im Pleistoz.an Schleswig-Holsteins and angrenzender Gebiete. Dissertation, University of Kiel, Kiel, 231 pp. and appendix.
1113
Lundershausen, S., 1997. Geologie, Sedimentologie and Genese der Drumlins von Wandelwitz, Ostholstein. M.Sc. Thesis, University of Kiel, Kiel, 90 pp. Seifert, G., 1954. Das mikroskopische Korngef.uge des Geschiebemergels als Abbild der Eisbewegung, zugleich Geschichte des Eisabbaues in Fehmarn, Ostholstein und dem D.anischen Wohld. Meyniana (Kiel) 2, 122–184. Stephan, H.-J., 1987. Heiligenhafen Kystklint. VARW 1987, No. (3), Geologisk Centralinstitut, K benhavn, pp. 88–95. Stephan, H.-J., 1992. Exkursion Ostseekuste/Ostholstein. . Erla. uterungen zur Route Kiel-Heiligenhafen. Das Hohe Ufer bei Heiligenhafen. Erl.auterungen zur Route Heiligenhafen-Brodten. DEUQUA 0 92, Exkursionsfuhrer . (Exk. B1), Geologisches Landesamt, Kiel, pp. 192–205. Stephan, H.-J., 1995. The Heiligenhafen cliff. In: Schirmer, W. (Ed.), Quaternary Field Trips in Central Europe (XIV. DEUQUA Congress), Vol. 1 (Regional Field Trips), Friedrich Pfeil, Munchen, . pp. 46–47 Stephan, H.-J., 1997. The ‘‘Hohe Ufer’’-cliff at Heiligenhafen. In: Piotrowski, J. A. (Ed.), The Peribaltic Group. Field Symposium on Glacial Geology at the Baltic Coast in Northern Germany. Excursion Guide, University of Kiel, Kiel pp. 20–23. Stephan, H.-J., 1998. Geschiebemergel als stratigraphische Leitho. rizonte in Schleswig-Holstein; ein Uberblick. Meyniana (Kiel) 50, 113–135. Stephan, H.-J., Kabel, C., Schluter, . G., 1983. Stratigraphical problems in the glacial deposits of Schleswig-Holstein. In: Ehlers, J. (Ed.), Glacial Deposits in North-West Europe. Balkema, Rotterdam, pp. 305–320 (Fig. 314–325). Van Der Wateren, F.M., 1999. Structural geology and sedimentology of the Heiligenhafen till section, North Germany. Quaternary Science Reviews 18 (14), 1625–1638.
Reply to a comment by H.-J. Stephan$ F.M. Van der Wateren Amstel AB to 256, 1011 PX Amsterdam, Netherlands
In my paper on the till section near Heiligenhafen (Van der Wateren, 1999) I argue that the probably Saalian Lower and Middle Tills (LT and MT) do not belong to two stratigraphically separate ice sheet advances as was suggested by Stephan et al. (1983). I suggested an alternative interpretation where the two lithologically and structurally distinct diamicts had formed as a single subglacial shear zone (deformation till). The structural and lithological variation which can be observed along the entire length of the 1 km long section results from upward increasing finite strain, (accumulated deformation) from the undeformed footwall sediments at the interface between the ice and the deforming bed (Van der Wateren, 1987, 1994, 1995). This subglacial shear zone model has recently been elaborated and now includes the full spectrum of $
PII of original articles S0277-3791(98)001206, S02773791(01)00063-4. E-mail address: [email protected] (F.M. Van der Wateren).
structures visible on the outcrop scale and on the microscale (Van der Wateren et al., 2000). Structural analysis of subglacially deformed sediments closely follows methods which have been developed for the study of shear zones in deformed low-to-medium grade metamorphic rocks (Passchier and Trouw, 1996). The rationale for this is the similarity of the kinematics (movement history) of lithified and unlithified sediments. Thus, simple shear will produce structures in sediments, e.g. subglacial till, which are very similar to those produced in rocks. The full range of (micro) structures which are known from mylonites and cataclastic shear zonesFincluding cleavages, Riedel shears, boudins and foldsFcan also be found in tills and be used to reconstruct the kinematics of the subglacial shear zone. Using the principles of structural analysis it is possible to distinguish between sediments which are undeformed and those which have undergone varying degrees of deformation, i.e. accumulated low to high finite strains.
1114
Correspondence / Quaternary Science Reviews 21 (2002) 1111–1116
In a shear zone, strain rates, and therefore finite strain, is at maximum in the centre and decreases towards the margins with the undeformed bedrock. With subglacial tills the highest strain rates are near the ice–sediment interface which effectively is the central part of the shear zone (Van der Wateren, 1994, 1995). We can thus expect to find a systematic upward increase in finite strain from the undeformed substratum to the top of the subglacial deforming bed. This has been confirmed by the . Breidamerkurj okull experiments (Boulton and Hindmarsh, 1987) where the largest displacements occurred just below the ice. The consequence of this subglacial shear zone (deformation till) model is that particles (sand grains, pebbles) higher up in the deforming bed tend to have travelled farther than those in the lower parts. Thus the top part of any single till may be expected to contain the more exotic elements (fartravelled erratics) while those near the bottom consists of more locally derived material (see also Rappol and Stoltenberg, 1985). Another consequence is that, although many tills show a lamination which is reminiscent of stratigraphic layering, stratigraphic principles do not apply for a single unit of subglacially deformed sediments. The layering is a transposed foliation resulting from a repetition of folding and stretching of sediment layers embedded in a more homogenized matrix, indicating relatively high finite strains. Intrafolial folds and boudins of internally undeformed or weakly deformed sediments are telltale signs of the origin of such a layering. In earlier papers (Van der Wateren, 1987, 1995) I have shown that laminated tills are intermediate between weakly deformed sediments and completely homogenized tills. The higher part of what deceptively mimics a stratigraphic sequence therefore is not younger than the underlying part, but coeval and only differs from it in the intensity of deformation and the distance it has travelled. Many natural outcrops clearly differ from the ideal outlined here. There may not always be a gradual increase in finite strain from the bottom to the top of a deformation till. In a recent paper (Van der Wateren et al., 2000) we discussed the causes of discontinuities within shear zones, which may be related to lithological or water pressure variation within the deforming layer which lead to variation in shear strength and thus strain rate. As a result, a homogenized diamict containing fartravelled erratics may overlie with a sharp contact a laminated till composed of mainly local sediments and still be the product of a single glacial event. Obviously, it has to be demonstrated that two or more overlying tills are in effect one single unit which has been deposited during one glacial event. I appear not to have succeeded in convincing Hans–Jurgen . Stephan that such is the case for the lower and middle tills in the Heiligenhafen section.
In his comment Stephan disagrees with the main conclusions of my paper. He distinguishes three tills (MT, LT and a lowest till) below the uppermost (Weichselian) till which unconformably overlies the other ones. His arguments are: 1. (a) I do not know the section well enough since I started studying it in 1992, while Stephan has been studying the section annually since 1962. (b) I do not know all papers resulting from his nearly 40 years of fieldwork. (c) Since this Baltic Sea cliff is subjected to continuous wave erosion new structures and even formerly unknown beds regularly appear, requiring changes in the interpretation of the section over the years. (d) Over the years, new methods have been applied, notably petrographic analyses, which likewise led to new interpretations. Of course I may have missed some features which had washed away prior to the start of my fieldwork at Heiligenhafen in the early 1990’s. Yet, all of the evidence presented by Stephan to refute my hypothesis of the origin of the LT and MT has essentially been there up to today in the same or similar form, and has been incorporated in my paper. In fact, my present interpretation was inspired 10 years previous by a close study of the profile published by Stephan et al. (1983) and by visits on several occasions on field trips led by Stephan himself. 6 years is neither too short, nor are the strata too complex to come to meaningful interpretations of the field data. As to his publications, most of these are from sources not generally available, such as excursion guides and local journals. All but one were cited in my paper. I am not aware that Stephan has appreciatively changed his analytical methods since the start of his study of the Heiligenhafen section nearly 40 years ago. Then and now, his stratigraphy is mainly based on petrographic variation, as has been for many years a routine tool in Quaternary mapping in Germany. I hope to have demonstrated that relying primarily on petrographic analyses may lead to wrong interpretations in subglacial deforming sediments. Once it has been appreciated that any sediment, including diamicts interpreted as lodgement or meltout tills, which at some stage was overlain by glacier ice inevitably experienced some degree of simple shearing, the principal method will be structural analysis with additional study of the petrographic, isotopic etc. composition of the sediments. 2. I overlooked a till underlying my Lower Till. This lowermost till is rich in Eocene diatomite, the local bedrock and the oldest exposed in situ stratigraphic unit. It is underlain by ‘‘a subaquatic deposit with intercalations of flow till and layers with drop till’’ and separated from the LT by a ‘‘pebbly gravel or gravely sand ’’, also rich in Eocene rock components. I did not single out this lowermost sediment unit since it clearly belongs to the basal part of the shear zone
Correspondence / Quaternary Science Reviews 21 (2002) 1111–1116
where undeformed bedrock and in situ glaciofluvial sediment grades into weakly deformed sediment. The fact that the latter, Stephan’s ‘‘lowermost till’’, is rich in Eocene diatomite is evidence of its low degree of transport and finite strain. 3. The LT and MT units are separated by an ‘‘up to 2 m thick glaciofluvial sand’’ which is ‘‘evidence for the stratigraphical separation’’ of the two tills. Where large fragments of these relatively undeformed sediment bodies included in the tills can be studied in detail, the sedimentary structures indicate that they had once been part of one or several proglacial/ice-marginal deltas which had been overridden and torn apart by advancing ice. From east to west along the section these floating sediment bodies show a consistent decrease in grain size while structural associations grade from trough-shaped cross bedding to fine ripple lamination, including climbing ripples in the silty fine sands and fine parallel lamination in silt and clay. This is the typical progression from proximal to distal of a proglacial delta. It is not necessary to invoke a stratigraphic separation between the LT and the MT because of these delta sediment inclusions. Instead, I prefer the simplest explanation whereby the Saalian ice margin advanced over local unlithified and weakly lithified sediments, including its own ice-marginal delta and Eocene diatomite bedrock, respectively. In situ and weakly deformed remnants of this delta are found on top of Eocene bedrock, the same association which Stephan interprets as an older till below the LT. 4. Striations associated with the interface between MT and LT trend E–W, while those observed within the LT trend NE–SW, indicating a change in ice flow direction from the LT to the MT from westward to southwestward. According to Stephan these flow directions agree with the different provenance of the two tills. According to my own measurements of the orientation of cleavages in the LT and MT there is no such dramatic shift in flow direction. All are in agreement with an ENE–WSW to E–W direction of shearing and thus ice flow. 5. Locally the upper part of the LT is laminated and contains lenses, layers and sand balls which to Stephan indicate that it is a supraglacial meltout sequence. This is an interpretation for which there is no foundation and which carefully avoids to explain why their internal structures (Riedel shears) as well as those around them all indicate dextral (E–W) shearing and why, along the section and parallel to the ice flow direction, they constitute a sequence from ice-proximal to ice-distal. An interpretation of these sediment lenses as boudins stretched in the direction of shearing is more in agreement with the observed structural associations. 6. The large fold structures relate to the youngest (Weichselian) E–W ice advance. Therefore the intense
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shearing within the layers also dates from this latest advance. The Heiligenhafen section clearly contains two deformational styles: (1) S–C cleavages, boudins and stretched layers indicating subhorizontal (layer-parallel) extension, and (2) large open folds indicating subhorizontal shortening. These two styles are the result of two entirely different kinematic regimes which cannot occur simultaneously. The former is the result of horizontal simple shear obviously associated with subglacial deformation, while the folding was produced by horizontal pure shear (horizontal compression), which since it overprints the simpler shear fabrics postdates deposition of the LT and MT. I cannot exclude that the folding dates from a Weichselian advance, rather than a late Saalian one in my preferred interpretation. Yet, this must predate deposition of the (probably Weichselian) UT which unconformably overlies the LT and MT, since horizontal compression and horizontal extension do not go together. 7. An outcrop SE from the area did not yield evidence of an eastward directed Weichselian ice advance as I postulated on the basis of a structure in the westernmost part of the cliff (Van der Wateren, 1999: Fig. 12). I have not been on the field trip where this was revealed (INQUA/Peribaltic Group 1997), but the structure shown in my Fig. 12 clearly indicates W to E simple shearing overprinting an older E to W shear which dominates the rest of the cliff. The most likely source for this eastward movement is an ice lobe situated in the Hohwachter Bucht, to the west of the Heiligenhafen section. It will be clear that, unlike Stephan, I hold that the entire sequence, between the undeformed Eocene bedrock and overlying delta sediments and the UT has been subjected to subglacial shear. I prefer the simplest possible explanation of the exposed till units which is in agreement with the evidence in the field and in thin sections: one Saalian till unit consisting of low-tomedium shear strain locally derived material in its bottom part (LT) and high shear strain far-travelled material in its upper part (MT). References Boulton, G.S., Hindmarsh, R.C.A., 1987. Sediment deformation beneath glaciers; rheology and geological consequences. Journal of Geophysical Research B, Solid Earth and Planets 92, 9059–9082. Passchier, C.W., Trouw, R.A.J., 1996. Microtectonics. Springer, Berlin, 289pp. Rappol, M., Stoltenberg, H.M., 1985. Compositional variability of Saalian till in The Netherlands and its origin. Boreas 14, 33–50. Stephan, H.-J., Kabel, Chr., Schluter, . G., 1983. Stratigraphic problems in the glacial deposits of Schleswig-Holstein. In: Ehlers, J. (Ed.), Glacial Deposits in North-West Europe. A.A. Balkema, Rotterdam, pp. 305–320.
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Van der Wateren, F.M., 1987. Structural geology and sedimentation of the Dammer Berge push moraine, FRG. In: Van der Meer, J.J.M. (Ed.), Tills and Glaciotectonics. Balkema, Rotterdam, pp. 157–182. Van der Wateren, F.M., 1994. Processes of Glaciotectonism. In: Menzies, J. (Ed.), Glacial Environments. Processes, Sediments and Landforms. Pergamon Press, Oxford, pp. 309–335. Van der Wateren, F.M., 1995. Structural Geology and Sedimentology of Push Moraines. Processes of soft sediment deformation in a glacial environment and the distribution of glaciotectonic styles.
Mededelingen Rijks Geologische Dienst 54, 1–168. Van der Wateren, F.M., 1999. Structural geology and sedimentology of Saalian tills near Heiligenhafen, Germany. Quaternary Science Reviews 18, 1625–1639. Van der Wateren, F.M., Kluiving, S.J., Bartek, L.R., 2000. Kinematic indicators of subglacial shearing. In: Maltman, A.J., Hubbard, B.P., Hambrey, M.J. (Eds.), Deformation of Glacial Materials, Geological Society Special Publication, Vol. 176. The Geological Society of London, London, pp. 259–278.