On the Relation between Subglacial Tills and the Substratum in Western Allgäu ( Germany )

[Zum Problem des Zusammenhanges zwi­ schen Grundmoränen-Ablagerungen und dem Substrat im westlichen Allgäu (Deutschland)] K u r z f a s s u n g : Korngrößen, Kies und Schwerminera l ­ ana lysen würmzei t l icher Grundmoränen -Ab lage rungen des Rheingle tschers im baden-wür t t emberg i schen Alpen­ vor land ze igen einen meis t relativ niedrigen Einfluß des aus M o l a s s e bes tehenden Substrats auf die Zusammense t ­ zung d ieser Sedimente. A u f G r a n d theore t i scher Überle­ g u n g e n und g roßmorpho log i s che r B e o b a c h t u n g e n wird ge sch lo s sen , daß auch d ie Aufnahme prä-exis t ierender quartärer Ablageningen un te rgeordne t war und d a ß die Pe t rograph ie der M o r ä n e n A b l a g e n i n g e n hauptsächl ich bes t immt wird vom w ä h r e n d de r letzten Vorland-Vergletsche rung vom Rheingle tscher ins Vorland transportierten Schutt .

berg (Austria) have been subject of many studies dur ing the last decades (a.o.CAMMERAAT & RAPPOL 1987;GERMAN 1976;GERMAN & MADER & KILGER 1979;DE JONG 1983;DE JONG & RAPPOL & RUPKE 1982;RAPPOL 1983;RAPPOL & VAN GIJSSEL 1988;SIMONS 1985;WEIN-HOI.D 1973).The study of sedimentary facies and pe trography has not only resulted in a better under standing of the various glacial s.l.processes active in the past, but also enabled, through a combination with morphological studies, a detailed reconstruc tion of the history of deglaciation after the last major Pleistocene ice advance (DE JONG 1983;DE JONG & DE GRAAFF & RLIPKF. in prep.).
The present paper focusses on the origin of the sub glacial tills in western Allgäti (Fig. 1), in particular on the relation between the tills and the local substra tum.The topic has been discussed by several au thors in the recent past.Based mainly on heavy min eral and gravel petrographic analyses, DE JONG (1983) argued that the influence of the Molasse sub stratum on the composition of most subglacial tills in western Allgäu was subordinate.He suggested that most till-forming debris had been transported by gla ciers to the foreland, assuming little local reworking from sub-till fluvial and/or glaciofluvial deposits.
Based mainly on granulometric analyses, CAMMERAAT & RAPPOL (1987;see also RAPPOL 1983) concluded that the till-forming debris in western AllgäLi and Vorarlberg is for a major part of local, bedrock, ori gin.Sedimentary petrographic work on glacial de posits in the Rotach valley, straddling the boundary between Germany and Austria, by RAPPOL & VAN GIJSSEL (1988) showed the influence both of local bed rock and pre-existing gravels and of far-travelled de bris on the composition of subglacial tills, the contri bution of the first being considered predominant in most cases by these authors.
The terms 'local' and 'far-travelled' or 'remote' in the discussions refer to the compositional relationship between the tills and the substratum, not to the ac tual transport distance.A till is termed 'local' when (the largest part of) its sediment-petrography reflects the composition of the substratum, irrespective of the actual transport distance.Conversely, the terms far-travelled' and 'remote' are used for tills with compositions which do not reflect the local substra tum.
The emphasis on sediment-petrography bears the risk of under-or overestimating the actual transport distances and may hamper the discussions.
The main glacier flow in Rhine and 111 valleys (Fig. 1) was at an angle with the SW-NE trending geological zones of the Alps and foreland.These zones show quite a variation in width (Fig. 1).It is conceivable that, for instance, Molasse debris in the Alpine foreland of Germany, which is underlain by Molasse bedrock, classifies as 'local' because of petrographic similari ties with the substratum although the actual subgla cial transport may have been tens of kilometers.Whereas glacial debris in the Alps after a short trans port across one or more geological zones would be coined 'far-travelled' based on compositional differ ences with the bedrock.To avoid confusion, the terms in the present paper are used with as much specification as possible.

Table 1: Heavy mineral compositions
It is not the scope of the paper to present a compre hensive overview of the relation between tills and their substratum.Emphasis will be on a critical re view of some of the aspects which have been in the centre of debate -grain-size distribution, gravel pe trography, heavy mineral weight percentages and heavy mineral composition.Inspite of this limita tion, a few general conclusions can be drawn.
Information on the methods of sampling and sample analysis has been presented by DE JONG (1983).The results of the analyses are tabulated in the same pu blication.An overview of the general glacier network in the area during the last glaciation is presented a.o.

Sub-till substratum geology
The study area is underlain by calcareous sandstones and shales of the Tertiary Upper Freshwater Mo lasse (Obere Süßwasser Molasse) in the so-called Undisturbed Outer Molasse tectonic unit (FUCHS Interbedded conglomerates do occur in the Disturbed Outer Molasse and Younger Inner Molasse to the S and SE, in the Pfänder-Hirsch berg mountain ridge and in the Rotach and Weißach valleys (Fig. 1).Sediment-petrographically, the Molasse rocks in the study area are part of the so-call ed Bodensee fan ('Schüttung'), which includes the Pfänder fan, and the Hochgrat fan (FCCHT-BAUER 1954a, 1954b, 1967;HOF-MANN, 1957).The Bodensee fan is characterized by a garnet-zircon-rutile-ore-staurolite heavy mineral association (Table 1).The heavy mineral weight per centage is less than 0.5%.The sediments are quartz-rich, feld spar-poor, calcareous (30-50%) and poorly to non-dolomitic.In most respects similar to the Bodensee fan, the Hochgrat fan is characterized by a garnetstaurolite-apatite heavy mineral association.
The amount and distribution of these deposits is difficult to assess due to lack of exposures.Gravels, often sandy, appear to be predominant, with subordinate amounts of debris-flow deposits, fine-grained waterlain sediments, ablation tills and (older) subglacial tills.Based on a limited number of samples, DE JONG (1983) concluded that the gravel petrography of the 'older' deposits does not significantly differ from that of the subglacial tills (Table 2).

Grain-size distribution
The grain-size distribution of the subglacial tills (number of samples n=64) in the area of investigati on has been described by DE JONG (1983) and was la ter discussed by CAMMERAAT & RAPPOL (1987).In a nutshell, the distribution is characterized by a mode in the gravel fraction and a double-peak mode in the fine sand/silt fraction, with a deficiency in the coarse sand fraction.
CAMMERAAT & RAPPOL (1987: Fig. 2) showed the simi larity in the grain-size distribution for samples from the present study area and samples from the Molas se area to the S and SE, and concluded that the tillforming debris in both areas is for a major part of lo cal (bedrock) origin.As mentioned before, the Mo lasse substratum in the latter area is composed not only of sandstones and shales, but also of conglo merates.
The mode in the gravel fraction of the samples from the area of investigation cannot be attributed to the incorporation of erosion products from the Molasse bedrock.The Molasse sandstones and shales in the study area are only moderately lithified and quite friable, hence do not weather or erode to pebbleand boulder-size debris.Large Molasse sandstone and shale clasts are notably absent in the deposits of the study area, and the Molasse sandstone percen tage of the size grade 5-8mm is generally subordinate (Tab.2).No evidence exists for the present or for mer occurrence of Molasse conglomerates in the area.These factors have been somewhat underex posed in previous discussions on the relation be tween tills and substratum.They imply that the gra vel fraction has either been transported to the area from southerly sources or incorporated from locally present 'older' Quaternary deposits.

Gravel petrography
The gravel petrography of the fraction 5-8mm has been analysed for 79 samples from the so-called up per till (DE JONG 1983).The percentage of Molasse sandstone pebbles is considered the best gravel-petrographic measure for contribution by the local Mo lasse substratum.Molasse shale pebbles are nota bly absent.The sandstone percentage is generally low (Tab.2; Fig. 3), even if one takes a bias to low percentages into account due to the destruction of friable sandstone pebbles during sampling and sam ple preparation.The composition of most samples is dominated by non-Molasse debris.This is diffe rent from the observations of RAPPOL & VAN GIJSSEL (1988)  Reworking of glaciofluvial deposits into (the basal part of) an overlying till has been concluded by RAPPOL & VAN GIJSSEL (1988) for some deposits in the Rotach valley, based on corresponding, relatively high percentages of erratic material.The well-round ed nature of some pebbles in the tills of the Argen lo be area of the Rhine glacier suggests reworking of fluvial and/or glaciofluvial sediments.A 25 cm thick sheared layer at the base of a till mantle was report ed by DE JONG & RAPPOL & RUPKE (1982) for a drum-Lin in west Allgäu consisting largely of sorted drift.This evidence shows that debris from 'older' Quater nary deposits has been incorporated into the tills.
Below, it will be argued, however, that the degree of glacial erosion of pre-existing deposits was limited.

3-3 Heavy mineral weight percentages
Data of heavy mineral weight percentages is avail able for 39 so-called upper till and 3 so-called lower till samples (Fig. 1 and 2).The weight percentages of the heavy mineral fraction (size grade 0.06-0.4mm) of the Bodensee fan Molasse sandstones are less than 0.5% (HOFMANN 1957).
Samples 1143 and 1144, collected in pits near Stei negaden in the eastern part of the study area (Fig. 1), show heavy mineral weight percentages of 0.3% (in the decalcified size grade 0.105-0.15mm),which is in good agreement with the fore-mentioned percen- tage from HOFMANN (1957) for the Molasse bedrock.These samples also show garnet-dominated heavy mineral compositions and high Molasse sandstone percentages in the fraction 5-8mm (Fig. 2 and 3) and, hence, are considered typical representatives of lo cal, predominantly Molasse-sourced subglacial tills.
Non-Molasse till-forming debris could conceivably have been derived completely from the non-sedi mentary rocks of the Silvretta mountains (Fig. 1)with high heavy mineral weight percentages (HAHN 1969) -as well as completely from the calcareous rocks of Vorarlberg -with very low percentages (e.g.The heavy mineral concentrations of most subglaci al till samples in the study area are considerably larg er than those of the Molasse substratum and the Molasse-derived tills (Fig. 2) and are interpreted to result from a large if not predominant contribution from non-Molasse debris.A predominant influence of the Molasse substratum would have been reflect ed in substantially lower heavy mineral concentra tions.
Little to no information is available for the heavy min eral fraction of the 'older' Quaternary deposits (see also HERRMANN & SCHWERD 1983;RAPPOL & VAN GIJSSEL 1988).Based on the similarity in gravel petrography with the subglacial tills (Tab.2) and on an analogy with the fluvial sediments of the Rhine (see above), the weight fractions may be estimated at several per cents.This, then, means that reworking of older de posits into the tills is hard to distinguish from direct sourcing on the basis of weight percentages only.As will be argued below, however, the glacial erosion of the pre-existing deposits was probably not veiy large and, consequently, the contribution of such erosive products to the till petrography limited.

Heavy mineral composition
The heavy mineral composition of the Molasse bed rock is dominated by garnet (Tab.1).The subgla cial tills on average show a garnet-amphibole-epidote association.The cross-plot of the garnet per centage versus the heavy mineral weight percentage for 39 so-called upper and 3 so-called lower tills in the study area (Fig. 2) shows a decrease of garnet with an increase in heavy mineral weight percenta ge, reflecting the waning influence of the Molasse debris.For example, the samples 1143 and 1144 and, to a lesser degree, 19 and 32 show relatively high garnet percentages and low heavy mineral con centrations.The concentrations of Molasse sand stone pebbles are very high and high respectively (Fig. 3), as one would expect for samples derived from the local Molasse substratum.
As mentioned before, little to no information is avai lable for the heavy mineral fraction of the 'older' Quaternary deposits.Based on the similarity in gra vel petrography with the subglacial tills (Tab.2) and on an analogy with the fluvial sediments of the Rhine (see above), the heavy mineral compositions may be quite similar to those of the tills.This, then, means that local sourcing from the older deposits cannot be differentiated from sourcing by far-travelled debris on the basis of composition only.See also below.

Discussion and conclusions
The results of the petrographic analyses show that, with a few notable exceptions, the subglacial tills are composed largely of non-Molasse debris.The tills consist to a large degree of debris which was either transported directly to the sttidy area from southerly sources or reworked from locally present older QLUIternary deposits.Or, from a combination of these.
Theoretically, substantial erosion and deformation of (unconsolidated) porous gravels is likely to occur only at low effective normal pressures at the ice/bed interface (BOLILTON 1975;SMALLEY & UNWIN 1968).
Conditions of impeded internal drainage through poroLis 'older' Quaternary deposits resulting in low effective normal presstires could conceivably have existed in the Allgäu, dLie to the presence of imper meable layers or topographic obstructions.The pre sence, however, of drumlins partly or completely composed of pre-existing gravels (DE JONG & RAPPOL & REPKE 1982) and the preservation of sheet-like lay ers of gravels below the tills (DE JONG 1983) suggest that subglacial erosion of this type of SLibstratum was probably of limited extent only.
As mentioned before, the full extent of the "older' Quaternary deposits in the study area cannot be de termined due to lack of exposures.Full terrain cover ing geomorphological mapping of large parts of the west Allgäu at scale 1:25,000 (DEN BESTEN 1984;CASTEL 1982;GILLEBERT 1985;DE JONG 1983;WOLFERT 1984), however, has shown that in wide areas sub glacial till is not underlain by pre-existing deposits.
One could argue that this demonstrates that older deposits have been completely eroded to form the subglacial tills.This scenario, however, is consider ed less likely.The interpretation of a predominant direct sourcing from southerly areas is preferred by the present author.
The cirques, the wide glacial valleys -which are of ten overdeepened, e. g. the Upper Rhine, 111 and Großwalser valleys in Vorarlberg -and the overall glacial erosive character of the Alpine relief witness the importance of glacial erosion in creating the pre sent morphology.Glaciers have eroded the cirques and valleys and have swept freshly eroded and older glacial drift as well as the interglacial and interstadi al fluvial and mass movement debris to the foreland.
The result has been a net transport of debris from the Alps to the foreland (and beyond).A sizeable part of this material forms the subglacial tills in the view of the present author.
RAPPOL & VAN GIJSSEL (1988) argue that glacial erosion is also an important geomorphic process in the pied mont area.The topography of highs ('Riedel') and lows ('Senke') in the foreland, with differences in elevation up to 100m, probably reflects the relief of the Molasse surface (DE JONG 1983;see also GERMAN et al., 1967, MADER 1971, and WEINHOLD 1973).Alter nating fluvial and glacial erosion, as described by SCHREINER (1979) for the Bodensee basin, is held res ponsible for the formation of this morphological framework.
The degree of the (glacial) erosion, however, is clearly of a lower order than that in the Alps.
In summary, the petrography of the subglacial tills in the area, except for a few well-defined local tills, is dominated by far-travelled debris.The combination of petrographic evidence, theoretical considerations and broad-scale geomorphological observations leads to the conclusions that most till-forming debris was transported to the area during the last glaciation and that reworking of 'older' Quaternary deposits was subordinate.
These conclusions differ from those of RAPPOL (1983), CAMMERAAT & RAPPOL (1987) and RAPPOL & VAN GIJSSEL (1988), who conchidecl on average to a pre dominant infhience of the local substratum (see also GASSER & NABHOLZ 1969).It is stressed that the pre sent conclusions do not invalidate their work as far as the area outside the present area of investigation is concerned.All studies show that the petrography of subglacial tills can reflect any source from the wide range between remote and local.It is beyond the scope of this paper to analyse the causes for the va riations.It is noted, however, that the core of the stu dy areas of R\PPOL, CAMMERAAT & RAPPOL and RAPPOL & VAN GIJSSEL is situated in mountainous relief, where as the present study area lies completely in the hil ly Alpine foreland.In addition, the distribution of the samples throtighout the present area of investi gation shows some interesting features.The Molasse-derived samples (19, 32, 1143 and 1144) are from the eastern part of the area -the foot of the Pfänder mountain -where the glacier flow probably was confined by the bedrock relief and morphology.The samples from the western part of the area, where glacier flow was less constrained, do not show a strong Molasse influence.'Older' Quaternary depo sits occur in the latter area, but as argued before it re mains questionable if they should be considered an important source rock for the tills.

Acknowledgements
The paper is largely based on a Ph.D. thesis submit ted at the University of Amsterdam in 1983-The auth or is indebted to all those people who contributed to the thesis.Appreciation is especially extended to the members of the Alpine Geomorphology Re search Group at the University of Amsterdam for their stimulating cooperation.

1
Introduction The Quaternary deposits of the hilly and mountain ous area of western Allgäu (Germany) and of the adjacent mountains of central and northern Vorarl-*) Alpine Geomorphology Research Group, University of Amsterdam.Address of the author: Dr. M. G. G. DE JONG, Prinsengracht 38c, 1015 DW Amsterdam, The Netherlands.

Fig. 1 :
Fig. 1: Geological map of the region (after FUCHS 1980, D. RICHTER 1956, M. RICHTER 1969, and VOLLMAYR 1958).Also indicated are the sampling locations of the Quaternary deposits in the study area.
in the Rotach valley, who found in many sam ples from the Eyenbach and Kesselbach stream inci sions large percentages of local Molasse debrisconglomerates, sandstones and siltstones -and con cluded to an important contribution by the local Mo lasse substratum.The work by RAPPOL & VAN GIJSSEL SLiggests that, when important, a contribution by the local Molasse bedrock will be reflected in high per centages of Molasse components in the gravel pe trography.Conversely, the low percentages in most samples of the present study area may be seen as an indication for a relatively low influence of the local Molasse substratum.
SMIT SIBINGA-LOKKER 1965).Such 'extremes' then are likely to have undergone changes during transport due to admixing of new lithologies.It is, hence, difficult if not impossible to characterize the heavy mineral weight fraction of the far-travelled de bris with one figure.Using the analog of recent flu vial sediments of the river Rhine downstream from the confluence with the river 111(HAHN 1969), a fig ure or range of several percents might be a reason able approximation.