1214 Hydrobiologia 143: 227-232, (1986) 227 @ Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands Geology and sedimentary history of Lake Traunsee (Salzkammergut, Austria) 1 2 Jurgen Schneider , Jens Miiller & Michael Sturm3 1Institut fur Geologie und Dynamik der Lithosphiire, Goldschmidtstraj)e 3, D-3400 Gottingen, FRG 2Lehrstuhl fur Geologie, Technische Universitiit Munchen, Lichtenbergstraj)e 4, D-8046 Garching, FRG 3EAWAG-ETH Zurich, Oberlandstra}Je 133, CH-8600 Dubendorf, Switzerland Keywords: lake sediments, elastic deposits, sediment echography, turbidities, sedimentary history Abstract Traunsee was formed by glacial overdeepening of a pre-existing fault system. Present-day morphology is characterized by a deep (189 m) narrow trough with steep slopes (> 50°) in the southern part surrounded by the Northern Calcareous Alps. The northern part of the lake is bordered by flysch and glacial deposits with gentle slopes ( < 30°} and exhibits several ridges, basins and troughs. During the late and postglacial period, more than 45 m of sediment has accumulated in the central basin. Sedimentation in the southern part of Traunsee is mainly controlled by the river Traun forming a prograding delta in the south and within the past 50 years - by industrial tailings consisting mainly of calcite. Sedi- ments are distributed by undercurrents and by turbidites. Cores from the central basin thus show an intercala- tion of Traun-derived dolomite-rich sediments with anthropogenic muds from the tailings deposited up to more than 6 km from its source. Within the northern basin, land slides from the flysch region played an important role leading to drastic changes in the morphology of slopes and adjacent basins. These slumps have persisted until historic times. Sedimentation in the shallow sublittoral regions is dominated by benthic biogenic decalcification. The frequency of turbidite sedimentation within the profundal basin decreased during the last 200 years probably due to man's activities in the drainage area such as regulation of rivers and torrents. Sedimentation rates during the past decades range from 2-3 cm/a in the southern basin to 0.4 cm/a in the northern part as shown by 137Cs-dating. Geologic setting, morphology and hydrography even vertical walls with only local sediment cover. Accumulation of sediments from the major tribu- The Traunsee is situated at the northern rim of tary the river Traun along the eastern border the Alps. It is bordered to the south by the North- of the profundal zone causes a NW dipping of the ern Calcareous Alps and the northern part is sur- basinal plain in the southern part. The narrow rounded by flysch and glacial deposits (Fig. 1, trough widens in the central part forming a 189 m Baumgartner, 1984). The N-S extending basin fol- deep central basin plain which is bordered in the lows a major transverse fault system which has north by ridges. Slope angles decrease distinctly in been eroded and shaped during glaciation. Rock the northern part of the lake where moraines and types surrounding the lake reflect the morphology flysch border the lake. Within this part of the lake of the present-day basin (see Fig. 2 and Fig. 3 a/b ). several ridges reaching into shallow water depths The steep slopes of the Northern Calcareous are separated by smaller basins (see Fig. 2). Alps continue below the lake level down to the Traunsee has the second largest area (24.4 km2) profundal zone. Dives with the submarine GEO of all the Austrian Alpine lakes. Its volume totals showed the existence of steep slopes ( > 50°} and 2228x106 m3, the greatest depth is 189 m. Major 228 TRAUNSEE Fig. J. Geological sketch map of the region around Traunsee (from Baumgartner, 1984). inflow (800Jo) comes from the river Traun, draining an area of 1417 km 2• Mean discharge amounts to 65 m3/sec transporting approx. 43 000 t/a of sus- pended load into the lake. Bed load transport aver- ages 36000 t/a (Baumgartner, 1984; Millier, Sossau & Zeh, 1983). The flow of the river Traun into the lake causes a depression of the thermocline and a short resi- dence time (0.9 a). The resulting exchange rate keeps Traunsee at a mesotrophic level in spite of )llti,~,...6::9'ru~ substantial nutrient loading (Pechlaner & Sossau, N91"~·~1J~ *ilbittl~.r~ 1982). l"~.ci.w­ ~J:e~·Wf)1HJ-- Sediment sources and sedimentation processes Fig. 2. Bathymetric map of Traunsee showing the position of two sediment echography profiles (Figs. 5 and 7) (from Muller Sediments are supplied by different sources and & Schneider, 1984). 229 DlfFERENT SEOIMB:<'t' SUPPLIERS H;' THE i..AKE lr~AUN lElE Fig. 3a. 30 computer graph of the Traunsee, viewed from North to South (from Muller & Schneider, 1984). NA.Tl:RAJ, SEDIMENT sortiL!ERS CD Cl.astic input from river Traun @ elastic ir.put from rivulets cr.> Dislocation of sediments throuqt~ slidinqs @ rnogenic epilimn.ic decalclfi.c:ition @ a109enic benthic decalcification ARTH'lCIAt. SEDIMENT SUPPLIERS @ Input of residuals fro:n limestone quarry 0 Input of industrial wastes Fig. 4. Natural and artificial sediment suppliers of the Traunsee Fig. Jb. 30 computer graph of the Traunsee, viewed from Somh (modified after Schneider, Claes & Kersting, 1984). to North (from Muller & Schneider, 1984). rents often initiating additional slumps by erosion. mechanisms which are shown in Fig. 4 (see also Depending upon grain size, fine particles are trans- Claes & Kersting, 1981). Allochthonous compo- ported by currents further north before they settle nents are derived from natural sources such as the to the ground. Occasional turbidity underflows oc- Traun river or other smaller rivulets and by sliding cur when river water is cold and extremely loaded of unstable rock resp. sediment masses or by an- with fines. In these instances, sediments are trans- thropogenic wastes. The latter come from a lime- ported directly into the central basin. stone quarry and from the combined tailings of The extension of turbidites and turbidity under- soda works and salt works (Muller & Schneider, flows is controlled by the basin morphology. Turbi- 1984). dites originating from the south are thus restricted Autochthonous sediment sources are biogenic to the northern end of the central basin due to the epilimnetic decalcification and, in sublittoral ridge separating the northern basin from the deeper regions, biogenic benthic decalcification. southern basin (Sturm & Muller, 1984, Fig. 5). l. Due to the petrography of the drainage area, 2. Sediment input of rivulets is restricted both dolomite is the most important elastic component regionally and with respect to the amount deposit- of the river Traun. Deposition of Traun-derived ed. Due to differences in the geology of the respec- sediments can thus be traced by the high dolomite tive drainage area sediment distribution from the content of basin sediments since no other substan- rivulets can be traced by their specific mineralogi- tial dolomite source exists. Most of the coarse sus- cal association. Abundance in quartz characterizes pended load and the bed load of the river Traun ac- catchment areas with flysch outcrops while those cumulates in the delta area forming foresets with coming from moraine areas or from the Calcareous 30° dip slopes. Episodic slumps in the foreset area Alps are distinctly lower in quartz and dominated continue into the profundal basin as turbidity cur- by dolomite and calcite (Fig. 6). 230 'll ~I ~~ ~ f' l 1.if'i.il!•·*"LL4@!11 .&141,J ., H I I I INI I I .I l i I Ii I I I I I I I I I I I I I I I I I I I I I I Profundal vf l<tke I I I I I I I I I I I I I I I I I I I I I I I I I I I I·.. I I I I I , I~~~~ I I I I I ' l~ I I I I : I I 1 I I Fig. 5. Part of a longitudinal profile (see Fig. 2) from sediment echography (3.5 KHz ORE). The central ridge separates the profundal from the northern basin. Height of the' ridge: about 30 m (from Muller & Schneider, 1984). 3. Landslides reaching into the lake are a com- activity of macro- and microphytes leading to the mon feature in the flysch zone at the northeastern formation of lake marl and chalk in these areas side of the lake. These slumps have been active for (Schroder, 1982; Schneider, Schroder & Le centuries. The last catastrophic event occurred in Campion-Alsumard, 1983). 1910 when houses slid into the lake. Cores and 6. An additional source of calcite in the south- profiling with 3.5 Khz echosounder showed numer- ern part of the lake comes from a limestone quarry ous subaquatic slumps below a sequence of un- (Karbach) operated by the soda works. Approx. disturbed lake sediments in the adjacent slope and 12500 t/a of calcite < 1 mm is washed into the lake basin documenting the frequency of slumps in the forming a distinct fan in the bottom sediments. past (Fig. 7). 7. Calcite is also the major constituent of the in- 4. Biogenic epilimnetic decalcification is best de- dustrial tailings accumulating in the bay of Eben- veloped during the summer. Increased phytoplank- see in the southwestern corner of Traunsee. The ton production reduces C02 and HC03 leading to tailings come from soda works and salt works lo- an increase in pH and subsequent oversaturation cated in Ebensee which have been pumping their causing the precipitation of calcite (e.g. Schroder, solid and dissolved wastes into the lake for more Windolph & Schneider, 1983). Sediment input by than 50 years. These tailings are of special relevance this mechanism is however considered to be rather for the lake ecosystem with respect to their input low due to the dilution with allochthonous material rate (max.