Sedimentary Petrography of Glacial Deposits in the Rotach Valley, Western Allgäu, Southern West Germany

52—68 Eiszeitalter u. Gegenwart 38 Hannover 1988 6 flg., 3 tab. Sedimentary Petrography of Glacial Deposits in the Rotach Valley, Western Allgäu, Southern West Germany MARTIN RAPPOL & KIER VAN GIJSSEL *) Glaciofluvial sedimentation, Würm, lithostratigraphy, section, gravel, sand, diamicton, calcareous composition, heavy mineral analysis, pebble analysis, gtanulometry, paleorelief Baden-Württemberg, Bavaria, Alpenvorland Abstract: Gravel, heavy mineral, carbonate, and gtain size 1. Introduction analyses of Würmian glacial sediments, mainly till, deposi ted by the Rhine-Valley glaciet in the Alpine Foreland, indi The study area is located in the northern alpine Fore cate that the compositional variation of till is controlled by land, east of Lake Constance (Bodensee), in southern several factors. Firstly, vertical and latetal vatiations are pre West Germany and the Austrian border area (Fig. 1). sent that relate to the incorporation of vatiable amounts of The international border between Allgäu (West Ger local bedrock or pre-existing glaciofluvial deposits. Secondly, a lateral variation in the composition of the far-travelled many) and Vorarlberg (Austria) lies partly along the material reflects the combined effects of variable rock types small streams of Kesselbach and Eyenbach, which in the source areas of the glaciers constituting the composite have cut deep gullies through Quaternary (mainly Rhine-Valley glacier, and the shifting position of theit flow glacial) deposits and bedrock. Many exposures in paths that is likely to have taken place during the course of these gullies offer good opportunities to study the the glaciation. complex lithostratigraphy of unconsolidated deposits in the Rotach valley. In this paper we present new results from sediment-petrographic analyses and dis [Sedimentpetrographie glazialer Ablagerungen cuss the vertical as well as cross-valley compositional im Rotachtal, westliches Allgäu, Südwest-Deutschland variation in till. Kurzfassung: Kies-, Schwermineral-, Karbonat- und Korn größenanalysen würmzeitlicher glazialen Ablagerungen, hauptsächlich Grundmoränen, des Rheintalgletschets im 2. Regional geology Alpenvorland zeigen, daß verschiedene Faktoren ihre Zu sammensetzung beeinflussen. Zum ersten gibt es laterale The pre-Quaternary basement of the area consists of und vertikale Variationen, welche mit der Aufnahme wech calcareous shales, sandstones, and conglomerates of selnder Mengen der lokalen Gesteinstypen oder prä-existie- the Tertiary Molasse series. For details the reader is render glazifluvialer Ablagerungen zusammenhängen. referred to recent summaries and discussions on the Zweitens zeigt sich eine latetale Variation im erratischen Bestandteil, welche die Auswirkung der verschiedenen Ge Molasse geology by e. g. VOLLMAYR & ZIEGLER (1976), steinstypen im Utsprungsgebiet der Teilgletscher des Rhein FUCHS (1980), and HERRMANN & SCHWERD (1983). talgletschers und die im Verlauf der Vergletscherung wahr Here we will only point out some features that are of scheinlich wechselnde Lage der Strombahnen der Teilglet- interest to the present study. schet widerspiegeln. In the Rotach Valley, the Molasse beds exhibit a tecto nic dip towards northwest (25—45°) and belong to *) Address of the authots: Dr. M. RAPPOL, Geological the Erected Outer Molasse in the concept developed Survey of Canada, 601 Booth Stteet, Ottawa, Ontario, Cana by FUCHS (1980). Several lithostratigraphic units can da K1A 0E8. — Drs. K. VAN GIJSSEL, Physical Geo graphy and Soil Science Laboratoty, University of Amstet- be distinguished, mainly on the basis of alternating dam, Dappetstraat 115, 1093 BS Amstetdam, The Nethet- marine and terrestrial conditions of sedimentation lands. (Fig. 2). Sedimentary Petrography of Glacial Deposits in the Rotach Valley, Western Allgäu, Southern West Getmany 53 Fig. 1: Location map with position of sample sites. Heavy line represent the approximate position of the Inner Younger Endmoraine complex. The southeastern slope of the Rotach valley (Sulzberg but evidence for multiple glaciation is scarce. ridge) is built mainly in calcareous shales and sand Multiple-till sections in the region have been descri stone interbeds of the Granitic Molasse, whereas the bed by e.g. WEINHOLD (1973), SCHREINER (1976), northeastern slope (Pfänder-Hirschberg ridge) consists HERRMANN & SCHWERE» (1983) and DE JONG (1983). mainly of conglomerates and sandstones of the Upper Sediment-petrographic analyses of such sections do Freshwater Molasse (VOLLMAYR & ZIEGLER 1976). In not provide us with stratigraphie tools, and at present between, the narrow zone of sandstones of the Upper C-14 dating of organic beds is the only stratigraphie Marine Molasse is present. Because the axis of the marker available. Dates from sub-till organic deposits, valley lies oblique to the structural zonation of the as summarized by DE JONG (1983), suggest a Middle- bedrock, the Upper Marine Molasse traverses the val Würm ice-free period prior to the Late-Würmian gla ley axis just south of Weiler (Fig. 2). cial maximum of about 22.000—16.000 yr. BP. For the larger part of the area north of the Alps, it appears Of importance to the present study is the occurrence that there is no consensus (see e. g. WEINHOLD 1973; of an epidote-rich zone at the base of the Uppet Mari WELTEN 1976; SCHLÜCHTER 1978; VAN DER MEER ne Molasse (,Luzerner Schichten'); the remaining part 1982; DE JONG 1983), and nor is there really any of the Molasse in the area contains a heavy-mineral strong evidence, concerning the age of till underlying assemblage in which epidote is virtually absent and is the dated horizons. It is variously classified as being of dominated instead by garnets (WOLETZ 1958, FÜCHT- Middle-Würmian, Early-Würmian (Turicum 2 and BAUER 1964). The total heavy mineral content of Turicum 1, respectively, in the concept of WELTEN, Molasse sandstones is low: generally the weight per 1976) or Rissian age. centage is less than 0.5% (HOFMANN 1957, FÜCHT- BAUER 1964, both using the 0.06—0.4 mm fraction). As far as we are aware, the only sub-till organic During the Pleistocene, the area was covered several material from the Rotach valley was discovered by the times by the eastern part of the Rhine-Valley glacier, second author in an exposure of the Weissebach, west 54 MARTIN RAPPOL & KIER VAN GIJSSEL Fig. 2: Simplified bedrock geology. 1. Upper Freshwater Molasse, 2. Upper Marine Molasse, 3. Granitic Molasse, 4. Inner Younger Molasse. Letters refer to topographic names shown on Fig. 1. of Weiler (site 64 in Fig. 1), and dated as minimal and HERRMANN & SCHWERD (1983), pre-Würmian 42.100 ± 24oo yr BP (GrN 12281). The organic mate moraines cover the top of these bedrock ridges, rial occurs in a section consisting of a subglacial till on whereas e.g. SCHMID (1955) and KRAYSS & KELLER top (about 3 m thick), overlying about 8 m of gravel (1983) assume that these ridges were completely with some diamictons of sediment-flow genesis, over covered by the Rhine-Valley glacier at its maximum. lying about 7 m of weathered gravel with a sandy layer An important morphologic feature is represented by near its base, containing the reworked organic debris. the so-called Inner Younger Endmoraine, which is Below this are shales of the Molasse (VAN GIJSSEL analogous to the sediment complex formed during the 1984). stadial complex Würm-Stein am Rhein as defined by Finally, in a section of the Bösenreutin Tobel (site 18 KELLER & KRAYSS (1980). In the study area, this ice- in Fig. 1) two tills overlie an organic-bearing deposit, marginal feature has been mapped and discussed by which is suggested to be pre-Rissian in age by HERR SCHMID (1955), VOLLMAYR & ZlEGLER (1976), KELLER MANN & SCHWERE» (1983). & KRAYSS (1980), DE JONG (1983) and VAN GIJSSEL The Quaternary cover seldom exceeds 20 m in thick (1985), among others. Its position is indicated in ness, and the contour pattern in Fig. 1 therefore Fig. 1. approximates the bedrock relief. A number of major Because deglaciation of the area proceeded down the relief features in the bedrock surface, such as the regional drainage slope, ice-marginal and proglacial Pfänder-Hirschberg ridge (1095 m) and Sulzberg lakes formed in front of the retreating glacier front ridge (1041 m), separated the Rhine-Valley glacier (e. g. VOLLMAYR & ZlEGLER 1976; VAN GlJSSEL 1985). entering the Foreland into several glacier tongues occupying the major valleys. From east to west the The last two major tributaries feeding the Rhine- Weissach, Rotach, Argen-Leiblach und Schüssen Valley glacier on its eastern flank were the 111 and glacier tongues can be distinguished. Landquart glaciers. According to KRASSER (1940), the Rotach valley was covered by part of the Landquart Whether the Sulzberg and Pfänder-Hirschberg ridges glacier, whereas the 111 branch occupied the Weissach. were completely covered with ice or remained as nuna- The source area of these two glaciers is underlain pre taker standing above the ice during the maximum dominantly by sedimentary rocks (mainly limestone, of the Late-Wurmian glaciation is still a matter of dolostone and calcareous shales), but in the upper dispute. According to VOLLMAYR & ZlEGLER (1976) reaches of their basins metamorphic rocks also occur. Sedimentary Petrography of Glacial Deposits in the Rotach Valley, Western Allgäu, Southern West Getmany 55 Especially important is the amphibolite of the Silvretta with the predominant local provenance, suggests massif as an erratic indicator in glacial deposits

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