Weichselian red tills in the Gardno Phase End Moraine ( De • bina Cliff ) – criteria or distinction , origin and f stratigraphic position , and implications for the origin and course of the Baltic Ice Stream

The paper presents the results of investigations of red tills from the South Baltic Middle Sea-Coast recorded in cliff exposures of Gardno Phase End Moraine. Distinctive features of these tills are: high percentage of Aland granitoids and porphyries identified in the 16-32 mm fraction, and large amounts of non-carbonate rocks, mostly red feldspars in relation to quartz, in the sandy fractions of 0,5-0,8 mm and 2-4 mm. Based on structural-textural features, the red till has been classified as flow till of local origin. The ice dynamism during Gardno Phase can be correlated with the Young Baltic Advance and most probably with the Belt Sea advance. On the basis of the petrographic properties of the Gardno red till and the results of radiocarbon dating, a new hypothesis regarding the origin, chronology and rate of flow of the Baltic Ice Stream is developed.

Red tills are a well-known phenomenon in northern Germany, and a significant criterion for establishing changes in the dynamics of ice sheets during individual glaciations.Their common feature is the dominance of rock derived from the eastern and mid Baltic regions.
In the majority of cases, the tills are grey, but red tills are also found in northern Germany.
According to KABEL (1982) and EHLERS (1992), four till types are recognisable, based on dominant pebble composition as follows: • tills containing abundant Devonian dolomites and intensively crushed Devonian siltstones, with a high percentage of Paleozoic limestone.Such tills are widespread in north-western Germany, directly on the older Saale glaciation tills.Similar tills of Elsterian age have been found in Lower Saxony (Niedersachsen); • tills with abundant Eocambrian sandstones; • tills with concentrations of red crystalline rocks, mainly large red feldspars; and • tills with a high percentage of Rotliegend siltstones and carbonate rocks.
The available information on Weichselian red tills in northern Germany has been summarised by STEPHAN (1985), with further information given subsequent publications (STEPHAN 1987(STEPHAN , 1998(STEPHAN , 2001)).KABEL (1982) has also published the results of fine gravel analyses of Weichselian red tills from northern Germany and Denmark.JASIEWICZ (1999JASIEWICZ ( , 2001aJASIEWICZ ( , b 2005) ) was the first to report red till in the middle part of the south Baltic coast, in a cliff exposure at Dębina (Fig. 1a).
The main aims of this paper are to describe the lithological properties of the red tills found at Dębina, indicate the criteria used in distinguishing them as a separate lithostratigraphic unit, and establish their lithostratigraphic position.Furthermore, the work seeks to explain the origin of the Weichselian red tills in the light of new insights into ice sheet movement at the edge of the Weichselian glaciation.
The stratigraphy of morainic tills exposed in the cliff cross-section of the Gardno Phase end moraine has been a subject of research for a long time.The first mention of tills in the Dębina cliff occurs in the work by BORN (1857).Later, JENTZSCH (1916) distinguished two levels of till, while HARTNACK (1926) and BÜLOW (1925BÜLOW ( , 1930) ) indicated three levels.GIEDROJĆ-JURAHA (1949) identified one more till level under the lacustrine deposits that he interpreted as a deposit from an older glaciation.
Results of the initial investigations into the stratigraphy of the Gardno end moraine were presented by PETELSKI (1976).He examined a 770-m long segment of the cliff and distin-guished three levels of till, which he referred to as g1, g2 and g3.The youngest, g3, is a light-brown till that may be associated with one of the phases of deglaciation of the Gardno Phase ice sheet.Level g2, a brown moraine till, was deposited during the maximum extent of the Gardno Phase, whereas the brownish till of level g1 is associated with an older glaciation according to PETELSKI (1976).The first petrographic study of the deposits of the cliff between Rowy and Orzechowo was carried out by RACINOWSKI et al. (1993).

Study area
The Gardno Phase end moraine is a thrust moraine that form a series of hills enclosing the Gardno-Łeba Coastal Plain (ROTNICKI & BORÓWKA 1995).In its southern and eastern part, from the Łeba River to the Łupawa Valley, the moraine consists of a row of isolated hummocks that reach maximum heights of 30-50 m a.s.l. and are cut through by streams.Rowokół hill (115 m) is the culmination of the moraine.
In the western part, the moraine rampart turns into a characteristic lobate form oriented northeast to south-west.The western arm of the lobe, near the village of Dębina, stretches to the seashore where it is undercut by the sea.This undercut constitutes the study area chosen for detailed investigations (Fig. 1a).
Pleistocene deposits appear about 4.5 km southwest of Rowy, in the base of the dune cliff, and are displayed along a 1,5 km segment of a sand-till cliff that extends to the culmination of the moraine (43 m a.s.l.).The height of the cliff ranges from 8 m where the Pleistocene deposits begin and reach 43 m at the end.In addition, a forested, poorly exposed fragment of the cliff stretches to the mouth of the Orzechówka stream at Poddąbie.The stratigraphic study only focused on the segment that embraces the exposures of Pleistocene deposits along the cliff near the village of Dębina (Ustka commune, Pomeranian province), where the hinterland of the moraine and its proximal (close to the ice front) and axial parts are situated (Fig. 1b).

Methods
Field research was carried out at the exposures after identifying the geological structure of the entire cross-section.The material for this petrographic study was collected from selected exposures.The pebble material was extracted from the samples using sieves with mesh sizes of 16 and 5 mm.Sieving continued until 500 and 300 pebbles, respectively, were obtained in this way from a total of 34 samples and the pebbles in both classes were identified.The material for the study of fine fractions was obtained in the course of grain-size analyses.Altogether over 200 samples from 12 profiles were studied.The location of the sampling sites and the profiles is shown in Fig. 1b.The petrographic composition of the tills was investigated in order to determine the mutual stratigraphic position of the deformed glaciotectonic deposits present in the Gardno Phase end moraine.This involved mainly analysis of the 5-10 mm fraction (TREMBACZOWSKI 1967, RZECHOWSKI 1971, 1974, 1976), which is standard procedure in the Polish Geological Institute and commonly applied in geological cartography in Poland.However, due to the absence of carbonate rocks in the uppermost till, which lies discordantly on deformed Weichselian deposits, the following additional fractions were also used: 16-32 mm (RUTKOWSKI 1995), 2-4 mm (KRYGOWSKI 1967, HOUMARK-NIELSEN 1993) and 0,5-0,8 mm.The results from those fractions provided enough data to define the uppermost till as a separate lithostratigraphic unit.
Fraction 5-10 mm: The investigations carried out on this fraction followed the methodology used in the Polish Geological Institute (TREMBACZOWSKI 1967, RZECHOWSKI 1971, 1974, 1976).In the sample, selected types were distinguished and then placed into petrographic groups (Tab.1).Additionally, due to the characteristic pebble composition of the uppermost till, alkaline granites were distinguished as a separate category of crystalline rocks.Weichselian red tills in the Gardno Phase End Moraine Fraction 16-32 mm (Fig. 1): In this fraction a detailed study was conducted taking into consideration crystalline rocks and selected carbonate rocks.For this purpose, the sample was not divided into local and Baltic rocks, but considered as a whole.This fraction was analysed to study the lithological differences within the crystalline rocks and carbonates, with a view to making comparisons with the uppermost till, which lacks carbonates.Fine fraction: Within the 2-4 mm fraction the following petrographic groups were identified: quartz; carbonate rocks; alkaline granites; crystallines; and sedimentary rocks (Fig. 3).The basis for classification as alkaline granites was the presence of alkaline (red) feldspars.In this fraction the Ga/K indicator (alkaline granites/ other crystallines) was calculated.This factor does not include carbonates, which mostly depends on post-depositional processes.The 0,5-0,8 mm fraction consists of quartz, carbonates, and fragments of non-carbonate rocks.The determination was made using a binocular microscope.In this fraction, the Ok/Q indicator (clasts of non-carbonates/quartz) does not take carbonates into account, too.Analysis of the elevation of the Baltic Ice Stream was made using the GRASS GIS program, which is open source software.A digital elevation model Etopo2 was imported, the size of the isostatic depression (MÖRNER 1980) was subsequently digitalised and subtracted from the original elevation model.Finally hypsometric cross-sections were made.

Geological structure of the glaciotectonite: lithostratigraphic units
In the section of the cliff near Dębina three series of deposits are exposed: (1) glaciotectonically deformed Weichselian deposits; (2) Weichselian red tills lying on deformed deposits; and (3) Late-glacial and Holocene aeolian cover sands, which cover Pleistocene deposits.These geological and glaciotectonic structures are shown in Fig. 1b.JASIEWICZ (2005) distinguishes the following lithostratigraphic units: Poddąbie formation, Dębina formation, and Wytowno formation.The Poddąbie formation includes till that relate to the advance of the main stadial of the Weichselian glaciation.The Dębina formation embraces tills that represent sub-and end-glacial facies of the late Pomeranian Phase.Finally, the Wytowno formation is a glacio-lacustrine series over 20 m thickness, exposed in the highest part of the cross-section.The origin and stratigraphic position of the last series is the subject of a separate research project.As it is of marginal significance to the matters under consideration in this paper, the results are not presented in detail.The red till covering the deformed Weichselian deposits is treated as a unit of a different formation, namely, the Gardno formation.

Podd• abie till (black or dark gey till) r
The Poddąbie till appears along the entire crosssection (Fig. 1).In the hinterland of the moraine and in its proximal part, the till forms a 1,5 m thick layer at the base of the cliff.Due to a glaciotectonic thrust, at the boundary between the proximal and the axial part, the ostensible thickness of this till increases to about 12 m.A similar till is exposed 600 m and 1200 m further in the south-westerly direction, at the base and on top of the glacio-lacustrine series.The thickness of this till fluctuates between 3 and 5 m.The gravel content of the 16-32 mm fraction is dominated by Palaeozoic limestones, mainly grey in colour, with visible Palaeozoic fauna such as Rugosa corals and Spirifer brachiopods.Light limestones, without a macroscopically visible fauna, are limited to the upper third of the deposit.The K/W indicator for the 5-10 mm fraction ranges between 0,6 to 0,9 (Table 1).
The Poddąbie formation also includes fine and medium sands overlying the till.Fragments of redeposited wood in these deposits have been dated to 27000±750 BP (Figs. 1b,2b).

De• bina till -the lwer unit o (brownish and green till)
This till -maximum thickness of 1,5 m -was observed only in the south-western part of the section, in the hinterland of the moraine.It is separated from the underlying Poddąbie tills (Fig. 2b) by a 20-cm thick sand-clay layer with an admixture of organic matter.In the proximal and axial parts of the moraine, deposits of this unit were not observed.Some thin lenses of this till were found within lacustrine deposits only in the cliff at Poddąbie, i.e. beyond the main study area (KRZYSZKOWSKI et al. 1998, JASIEWICZ 2005).The till discussed here is either layered (and brownish) or massive (and green).In the layered till, the thickness of the sandy layers ranges from several millimetres to a few centimetres.The green till is not laminated, but it does show traces of folded lenses of sands and organic matter (Fig. 2a).The fining-upward, grain-size distribution within the sandy laminae in the brownish till and the chaotic arrangement of the long axes of pebbles allows this till to be classified as a subaquatic till facies (HART & ROBERTS 1994), or waterlain till (DREMANIS 1980(DREMANIS , 1989)).In both the 5-10 mm and 16-32 mm fractions, the two variants of till are dominated by Mesozoic limestones, mostly chalk, which constitutes up to 40% of the whole fraction.The characteristic feature of the till is that both fractions are poor in Paleozoic limestones.

De• bina till -the upper unit (light-brown till)
This till appears predominantly in the proximal part of the moraine, in the middle and top parts of the glaciotectonite, and within deformation structures.In some areas, contact between the Dębina till and the overlying Gardno till was observed (Fig. 3g).This lithostratigraphic unit is dominated by light-brown sandy till (diamicton).In these deposits, lenses and layers of varigrained sands and gravels are frequently found.Not much can be said about the sedimentary structures noted in these deposits, because in most cases the original sedimentary structure has been overprinted by a new fabric resulting from deformation processes.
The characteristic quality of this till in the 5-10 mm fraction is a balance between crystalline rocks and Baltic carbonates (K/W 0,9-1,1).The till has been classified as a separate type on the basis of lithological differences and the results of study of the 16-32 mm fraction.Apart from crystallines, light limestones are another dominating component.Grey limestones constitute about a quarter of the carbonates, whereas dolomites (in the 5-10 mm and 16-32 mm fractions) constitute less than 0,5% or do not appear at all.

Lithological-petrographic characteristics of the red till
The red till is exposed along the whole section of the Gardno end moraine and lies discordantly on deformed older deposits.It does not have continuous cover, but appears as isolated beds with thickness of several centimetres up to two metres.The boundary between the Gardno

JAROSŁAW JASIEWICZ
till and the overlying series of Holocene cover sands is erosive.The original thickness of the till is impossible to establish.In the north-eastern part of the section, at the hinterland of the moraine (Fig. 1b), the till has a massive structure.It covers discordantly older deformed tills, that are separated in places by a series of silt deposits (Figs.2a, b, c).The thickness of that layer is up to 50 cm, and border between till and underlying deposits is distinct.In the proximal (middle) part of the moraine, Gardno deposits are formed as alternate layers of till, clay and sand that dip in various directions at gradients of 20°-22° and lying discordantly on older deposits, or laminated till lying directly on older Dębina till (Fig. 2g), with no discordance.In the axial part, the till appears in a few places as patches with massive structure, lying discordantly over the deformed lacustrine deposits (Fig. 2f).In some parts, the only traces of the red till are lag sediments of gravel.The course fraction in these gravels is very small.The structural-textural features of the red till enable it to be classified as a flow till of local origin.In all fractions, there was a complete lack of carbonate rocks and hence the use of the K/W, O/K and A/B indicators (Table 1) for the purpose of comparing the tills with other deposits was impossible.In order to test how it differs from the underlying deposits, indicators that disregard carbonates were employed.In the 16-32 mm fraction, the proportion of Aland rocks (Aland granitoids and porphyries) to other crystallines was estimated.This indicator varied between 0,91 and 0,45 with a mean of 0,62±0,18 (this contrasts with 0,34±0,18 in the upper unit of the Dębina till and 0,03 in the remaining tills).These data clearly demonstrate the uniqueness of the Gardno till, which was confirmed by analysis of variance.The uniqueness of the petrographic composition of the till is readily visible especially in the 0,5-0,8 mm fraction (Fig. 3).The value of the Ok/Q indicator (clasts of non-carbonates/quartz) is usually 0,59, which is three times higher than that found in other tills.Similar differences were observed in the 2-4 mm fraction, where the Ga/K indicator was also significantly bigger than in the other petrographic types (Fig. 3), and again they were confirmed by the analysis of variance.The lack of carbonates in the till is probably due to post-depositional weathering, which explains the high content of precipitated carbonate at the boundary between the Gardno and Dębina tills (Fig. 3).However, a low percentage of quartz in the 2-4 mm fraction and high of non-carbonate clasts in the finest fraction show that the untypical petrographic composition of this till is not only the effect of the post-depositional weathering of carbonates (WOŹNIAK 2004).
In the proximal part of the moraine a structure formed by Gardno till with an underlying series of sandy deposits was noted (Fig. 2e).The north-eastern part of the structure consists of a thin series of till with laminae of sand and gravel that dip in a south-westerly direction at a 40° angle.The south-western part of the structure is built of similar deposits, but they dip in the opposite, north-easterly, direction at an angle of 20°-30°.Between them lies a block of massive till, with maximum thickness of 2,2 m.

Criteria for distinguishing red tills
The Gardno formation was established as a separate lithostratigraphic unit on the basis of the deposits lying discordantly on deformed Weichselian deposits that build the glaciotectonite of the Gardno Phase end moraine.The evidence for this is as follows.The red till is sometimes found lying on the light brown till (Fig. 2g), but also on silt deposits (Figs.2a, d,  f).It is formed either as a flow till with frequent interbeddings of clay and sand, or as a massive till.In some places, a discordance between the red and the light brown till was noted.The upper unit of the Dębina till (light brown) is always found within the glaciotectonic structure, and its original fabric is often destroyed and overprinted by one of glaciotectonic origin (Fig. 2d), often in the form of a mixture of till and sand beds.
The lithological criteria of the red tills include a high concentration of Aland rocks in compari-Weichselian red tills in the Gardno Phase End Moraine son with other crystalline rocks components.This is confirmed by a high concentration of red feldspars in the fine gravel fraction.A large amount of non-carbonate clasts in comparison with quartz in the sandy fraction is the most distinctive feature of the red till that helps to differentiate it from other tills (Fig. 4).
This feature is helpful in two cases: when the Gardno till lies discordantly on deformed nontill deposits or on light brown till.Thus the lack of carbonates, caused by the post-depositional weathering, is not a serious impediment to certain identification of the red till.

The stratigraphic rank and position of the red till
The stratigraphic position of the red till, which lies discordantly on glaciotectonically disturbed Weichselian deposits, is easy to establish: it is as old as, or younger than, the disturbance zone (JAROSZEWSKI 1963, ROTNICKI 1983).Therefore, it can only be connected with the Gardno Phase, the youngest advance on the middle-south coast of the Baltic Sea.Additionally, the chronostratigraphic position of the Gardno advance is welldocumented by radiometric methods.According to ROTNICKI & BORÓWKA (1995), the oldest postglacial deposits from the Gardno-Łeba lowland has been 14 C dated to 14300±150 BP.On this basis, ROTNICKI (2001) estimated the time of the Gardno advance to be 15000 BP.
Correlation of till cover with the regional and trans-regional glaciology remains problematic.
In the hinterland of the Gardno Phase end moraine ROTNICKI (1995) and ROTNICKI & BORÓWKA (1995) suggested the existence of lags connected with Gardno Phase till.Recently, JASIEWICZ (2005) suggest the possible presence of Gardno till flows in the lower Łupawa ice-dammed area, in the foreland of the Gardno Phase.However, no Gardno till, was found on land yet, having originated as basal till, due to connected with the limited extent of this phase south of the contemporary coast of the Baltic Sea.The correlation of marginal zone deposits of the Gardno Phase end moraine is shown in Fig. 4. In turn, the correlation at a trans-regional scale based on the numerical values of the indicators is not possible because of the lack of carbonates makes the calculation of the O/K, K/W and A/B indicators impossible, and the rest of the indicators calculated for this till cannot be compared with other indices used in Polish geological cartography.Furthermore, a transregional correlation based solely on boulder indicators often fails because tills belonging to the same lithostratigraphic unit may have various petrographic compositions in different regions (LÜTTIG 1999).In this situation it is necessary to compare the lithological features of till, that are not directly dependent on the numerical values of the indicators.Crystalline rocks, dominated by material transported from a distant, relatively restricted area, and in the finest gravel fraction a high concentration of non-limestone rock clasts (mainly orthoclases) in proportion to quartz suggest that there was restricted interaction with the substratum during transport of the material.Both the chronostratigraphy (ca.15000 BP (ROTNICKI & BORÓWKA 1995) and the lithological features of the Gardno till suggest a correlation of the Gardno with the Young Baltic Advance (KJAER et al. 2003, cf. Fig. 1b) is appropriate.The Gardno advance is the last advance in the Middle-South-Baltic area that left the Baltic basin and it is probably with this advance that all glaciotectonic disturbances described from the islands of Rügen (PANZIG 1995) and Wolin (RUSZCZYŃSKA-SZENAJCH 1996) are connected.Pebble composition of the Young Baltic advances till also formed under conditions of limited interaction with the substratum (STEPHAN 2001, KJAER et al. 2003).The extent and direction of the advancing lobes were strictly controlled by substratum relief.A higher than usual concentration of Aland rocks in tills that relate to the Young Baltic Belt Sea Advance (Warleberg Advance in Germany) was also stressed by STEPHAN (2001) The Gardno Phase may well be connected with this advance.Similarly, the Gardno ice sheet probably took advantage of the natural depression of the Gardno-Łeba Coastal Plain.Thus, features that allow correlation of the Belt Sea and Gardno advances are the dynamics of the ice sheet, petrographic composition and the chronostratigraphic position.
All deposits in the region of Gardno-Łeba Lowland connected with this advance belong to the same formation, i.e. the Gardno formation, whatever their degree of preservation or weathering.The red till, which originated mostly in the flow facies, is regarded as one of the units in this formation.
rock fragments in the sandy fraction indicate that the red till retains features of material that derives from the Aland Islands, 600 km distant.The genesis of its petrographic composition may be explained in two ways, i.e. the original material did not scatter during glacial transport (DONNER 1995, STEPHAN 2001) or the red till has gained the above mentioned features due to a disintegration of a large granite raft.Since the second variant seems unlikely, it is useful to consider the conditions during the erosion-transport-accumulation cycle that might have resulted in till with such a composition.These include limited interaction between the basal ice and the substratum, and transport of the material high above the ice foot (EHLERS 1981, STEPHAN 2001) (1993,1995) suggest that the threshold of the Aland Islands was a great obstacle for the Baltic Ice Stream and was eroded during its functioning much more intensely than for example Gotland, which lies farther to the south (HOLMLUND & FASTOOK 1993, 1995).In the Younger Dryas, the Aland Islands could even have risen above the surface of the ice sheet as nunataks.However, a relief analysis that takes isostatic depression into consideration (Fig. 5) shows that at the height, and during the retreat, of the glaciation the Aland Islands had already stopped being an obstacle.This suggests that the incorporation of material into the ice sheet took place at the beginning of the glaciation.If we assume, after BOULTON et al. (2001), that the Aland Islands found themselves at the limit of the ice sheet in the Inter-Pleni-Weichselian, they would be an area of intense weathering during periglacial conditions.The presence of poorly scattered material proves that the Baltic Ice Stream began to function immediately after filling up the Baltic basin with ice, but due to time and distance the effects of its functioning are not clearly visible until the deposits of the final stage of the glaciation.

Conclusions
Weichselian red tills identified in the crosssection of the cliff of the Gardno Phase end moraine include flow tills that are connected with the advance of the Gardno ice sheet.Hence they constitute a component of the Gardno formation that is regarded as a series of deposits left during the advance and recession of the Gardno ice lobe.The stratigraphic status Weichselian red tills in the Gardno Phase End Moraine of the red tills was established on the basis of their discordant position on deformed Weichselian deposits, and confirmed by an analysis of their petrographic composition in the 16-32 mm and 0,5-0,8 mm fractions.Though the red tills lack carbonate rocks and dispersed carbonates (probably due to weathering), this is not an impediment to establishing lithostratigraphic identity or genesis of the red tills if the methods, referred to above, are applied.The unique composition of the red tills is probably the effect of specific erosion conditions, i.e. transport and accumulation in the area of the Baltic Ice Stream.These conditions are not present until the end of every glaciation (EHLERS 1992).Nevertheless, the hypothesis presented here concerning the dynamics of the Baltic Ice Stream still requires further research.

Fig. 2
Fig. 2: a) Red till lying discordantly over deformed Weichselian deposits.Under the till, a fine asymmetric fold in the Dębina till, lower unit.The scale used in this Figure is 2 m long.b) Red till over deformed Poddąbie till (black).Below the sands a piece of wood was dated to 27000 BP. c) Red till over mud deposits.Below Dębina till (brownish) and Poddąbie till (black) d) Dębina till -lower unit (light-brown) mixed with sands in a duplex structure.Red till over muds, over a deformation structure.

Fig. 2 Fig. 3 :
Fig.2: e) A red till lens, probably a remnant of dead ice f) Lenses of red till over deformed Wytowno formation (glacio-lacustrine deposits) in the culmination (highest point) of the moraine rampart.g) Red tills lying concordantly on the Dębina till, upper unit (light brown till).The difference between the tills was established on the basis of a petrographic analysis of the fine fractions (see Fig.3a).
Its functioning is strictly defined by the relief of its substratum, i.e. the Baltic basin south of the Aland Islands, by the considerable inclination of the basin slope in the west (the Scandinavian mountains), and by the slope in the east.The area of the Aland Islands today is a threshold that separates the thickest portions of the ice sheet and places of maximum isostatic depression (MÖRNER 1980) from the area of a distinct transport route in the Baltic basin.On the basis of the results of glacial process modelling, HOLMLUND & FASTOOK

Table 1 :
Petrographic types and groups used in the 5-10 mm fraction.