Alpine Overdeepenings and Paleo-Ice Flow Changes
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Swiss J Geosci (2010) 103:385–405 DOI 10.1007/s00015-010-0046-9 Alpine overdeepenings and paleo-ice flow changes: an integrated geophysical-sedimentological case study from Tyrol (Austria) Ju¨rgen M. Reitner • Wilfried Gruber • Alexander Ro¨mer • Rainer Morawetz Received: 13 February 2008 / Accepted: 24 August 2010 / Published online: 1 December 2010 Ó Swiss Geological Society 2010 Abstract We present a study of the inneralpine basin of major glaciation of MIS 12. Limited glacial erosion during Hopfgarten focused on the analysis of basin fill in order to the LGM occurred only during the ice build-up phase. reveal its formation in relation to paleo-ice flow and tec- Further overdeepening was impeded due to topographic tonics. The study is based on geological mapping as well as barrier and mutual blockades of glaciers within this highly seismic (reflection and refraction) and geoelectrical sur- dissected landscape. The occurrence and relative timing of veys. The oldest sequence in the basin, identified by seismic the impediment was controlled by the onset of transfluences stratigraphy at 400 m below surface, consists of coarse and thus by the altitude of coles. The higher amount of grained sediments of supposedly Oligocene to Miocene age, overdeepening during older glacial periods is explained by which subsided along faults linked to the Inn fault. Three longer phases of free ice advance in the ice build up phase superimposed sequences, each displaying baselaps in due to higher transfluences routes at that time. Thus, the contact with a subglacially formed unconformity and sig- preservation of older Pleistocene sequences within the basin moid foresets, show pleniglacial conditions followed by a may be the result of the lowering of watersheds from one glaciolacustrine environment. The uppermost of these three glaciation to the next. Our model of an inverse relationship sequences lies on top of last glacial maximum till (LGM; between glacial shaping of the surface and the subsurface Wu¨rmian Pleniglacial; MIS 2) and represents Termination may apply to similar Alpine landscapes as well. I. The middle sequence is classified as Termination II following the Rissian Pleniglacial (MIS 6). The oldest Keywords Lithostratigraphy Á Seismic stratigraphy Á glacial sequence cannot be constrained chronostratigraphi- Glacial erosion Á Glacier reconstruction Á cally but might correlate with Termination V following the Landscape evolution Á Integrated interpretation Editorial handling: Markus Fiebig & Kurt Decker. Introduction J. M. Reitner (&) Á A. Ro¨mer Geologische Bundesanstalt, Neulinggasse 38, Estimates of the amount of subglacial erosion substantially 1030 Vienna, Austria rely on documents of the action of paleoglaciers especially e-mail: [email protected] in overdeepened valleys and basins. In the Alps, the first A. Ro¨mer direct evidence of glacial overdeepening was revealed by a e-mail: [email protected] tragic tunneling project beneath the Swiss Gasterntal in 1908 (Schweizerische Bauzeitung, 1908: cit. in Schlu¨chter, W. Gruber HOT Engineering, Parkstraße 8, 8700 Leoben, Austria 1983). But even drillings rarely document overdeepening e-mail: [email protected] since they are most often hydrogeologically motivated and do not reach down to the bedrock. Geophysical data R. Morawetz however, especially seismic surveys of alpine and perial- Joanneum Research Forschungsgesellschaft m.b.H, Roseggerstraße 17, 8700 Leoben, Austria pine valleys, basins and lakes, are well suited to reveal e-mail: [email protected] basement depths of up to 1,000 m and can increase our 386 J. M. Reitner et al. knowledge of overdeepened bedrock topography substan- 3. The combination between surface geology and geo- tially (e.g. Finkh et al. 1984; Wildi 1984; Weber et al. physical data, especially seismic stratigraphy, for 1990; Pfiffner et al. 1997). In combination with actualistic developing a climatostratigraphy. studies (e.g. Hooke 1991; Smith et al. 2007), this has led to 4. The formation and shaping of a small basin in context a better understanding of the contributing processes such as of Alpine morphogenetic development. erosion by basal ice and subglacial meltwater action. Geophysical data also elucidate the structure of sedi- ments in overdeepened inneralpine valleys, indicating a Study area rapid infill by deltaic sediments (e.g. Burgschwaiger and Schmid 2001) and partly by mass movements originating Morphological and geological setting from overdeepened slopes (e.g. Gruber and Weber 2004). In most cases, geophysical data accompanied by occasional The basin of Hopfgarten resembles the area around the drillings indicate a shaping of valleys during the last glacial village of Hopfgarten (622 m asl.) within the Kitzbu¨heler maximum (LGM) and subsequent sedimentary filling dur- Alpen and is located around 10 km south–east of the Inn ing the following deglaciation (Termination I; van Husen valley (village of Wo¨rgl in Figs. 1, 2, 3). It is drained by 1979). There are some locations along inneralpine valley the river Brixentaler Ache, a tributary of the river Inn. The flanks showing deposits of Termination II and older main feeder rivers of the Brixentaler Ache are the rivers (Gru¨ger 1979; Drescher-Schneider 2000; Draxler 2000; Windauer Ache and Kelchsauer Ache. Their drainage areas Reitner and Draxler 2002), which is in accordance with the are surrounded by up to *2,400 m high summits. Both evidence of multiple glaciations in the southern as well as valleys have cols in 1,686 and 1,983 m asl. (locations A in the northern Alpine foreland (van Husen 2004 cf). and B in Figs. 1, 2), respectively, that connect them to the However, in the Eastern Alps the initial valley formation E–W trending Salzach valley, which drains the mountain by fluvial incision was linked to the morphogenesis driven chain of the Hohen Tauern with still glaciated summits and by tectonical processes starting in the Paleocene and fol- cirques. lowed by major changes in drainage pattern during the The north-western edge of the Hopfgarten area consists of Miocene (Frisch et al. 1998). red sandstone (PSK), whereas carbonates of the Northern The impact of today’s tectonic movements, expressed Calcareous Alps (NCA) occur west of Wo¨rgl only on both by shortening trends (e.g. Grenerczy et al. 2005), on the Inn valley flanks (Ampferer and Ohnesorge 1918; Fig. 3). shaping of valleys during the quaternary is still a matter of The largest part of the Kitzbu¨heler Alpen around Hopfgarten debate. Indications of Oligocene to Miocene deposits at the is made up of schists and metasandstones (‘‘graywacke’’) of bottom of overdeepened valleys along major strike slip the Grauwackenzone (GWZ) and towards the south of faults, like the Inn valley, suggest tectonic subsidence quartzphyllite of the Innsbrucker Quarzphyllit Zone (Amp- linked to small-scale pull-apart basins (Poscher 1993). On ferer and Ohnesorge 1918; Heinisch and Panwitz 2007; the one hand, tectonically induced complex drainage sys- Fig. 3). The latter two are summarised under the term tems with valley trains running across or oblique to the ‘GWZ’ for reasons of simplicity. general ice flow, had a major impact on glacier flow and dynamics. On the other hand, glacial shaping and erosion Quaternary geology of the study area apparently altered the topography resulting in the broad- ening of valley floors, oversteepening of flanks as well as in The following description is based on Reitner (2005). a successive lowering of water divides and thus ice trans- Sedimentological logging of available sections was carried fluences routes for subsequent glaciations. out according to standard approaches (Jones et al. 1999; In this paper we present sedimentary and geophysical Evans and Benn 2004) using a modified lithofacies clas- data of a small Alpine basin in order to address the fol- sification of Keller (1996) based on Eyles et al. (1983) and lowing aspects: Miall (1996). 1. The effect of a highly dissected mountain topography In the basin of Hopfgarten terraces up to 150 m high on ice dynamics in terms of promoting ice-build up as (Figs. 3, 4) with different levels, which extend towards the well as restricting erosion. tributary valleys of the Brixental, the Windau valley and the 2. The limitations of paleo-glaciological models derived Kelchsau valley are found. They contain a tripartite sedi- from the youngest sequence, in this case the LGM mentary succession (unit A, B and C) which will be sequence, for explaining sedimentary remnants of characterised in terms of lithofacies, sedimentary processes, older glaciations. paleoclimatological conditions and chronostratigraphy. Alpine overdeepenings and paleo-ice flow changes 387 Fig. 1 Digital elevation model 12°10’ E (DEM) with the location and N morphology of the study area SamerbergSamerberg W E and its surroundings. Major River VIENNA faults related to Miocene S Inn tectonics are indicated (dashed Austria red lines) based on Pestal et al. INNSBRUCK (2004). Bridge-symbols indicate the cols and former ice transfluences routes from the Basin of Hopfgarten Salzach valley to A the Kelchsau valley in 1,983 m asl KufsteinKufstein B the Windau valley in 1,686 m asl. and C the Sperten valley (Spertental) in 1,713 m asl. The most WörglWörgl St.St. JohannJohann prominent similar feature is that ’N 47°30’ N BrixenB of Pass Thurn (1,274 m asl.) 47°30 ey r towards the Kitzbu¨hel valley ll ix va e HopfgartenHopfgarten InnInn valley n KitzbühelKitzbühel KramsachKramsach SpertenS WindauW p e i n r ZillerZ t u d e i a l s a n l h u e lc C r e B vallev KelchsauK A PassPass ThurnThurn a l l e y SalzachSalzach valleyvalley ’N 47°10’ N HOHEH O H E TAUERNTA U E R N 47°10 05km 10 15 12°10’ E 12°12’ E Special paleogeographical aspects will be considered in the organic debris (Flo, Sho), and occasionally coarse-grained following chapter.