Overdeepened Glacial Basins As Archives for the Quaternary Landscape Evolution of the Alps

Home , Glatt Valley, Oberglatt

Quaternary Science Journal GEOZON SCIENCE MEDIA Volume 66 / Number 1 / 2017 / th 3–5 / DOI 10.3285/eg.66.1.th2 ISSN 0424-7116 E&G www.quaternary-science.net

THESIS ABSTRACT Overdeepened glacial basins as archives for the Quaternary landscape evolution of the Alps

Marius W. Buechi

E-Mail: [email protected] University: Quaternary Geology & Paleoclimatology Research, Institute of Geological Sciences & Oeschger Centre for Climate Change Research, University of Bern, Switzerland Supervisors: Flavio S. Anselmetti

Dissertation online: doi.org/10.13140/RG.2.2.17415.80802

The Alps and the Alpine foreland have been shaped by re- ly eroded, ‘overdeepened’, basins may serve as important peated glaciations during Quaternary glacial-interglacial archives to complement the Quaternary stratigraphy over cycles. Extent, timing, and impact on landscape evolution several glacial-interglacial cycles. In this thesis, the infill of of these glaciations remain, however, poorly constrained deep subglacial basins in the Lower Glatt valley (N Swit- due to the fragmentary character of proximal terrestrial ar- zerland, Fig. 1) are explored to better constrain the Mid- chives. In this context, the sedimentary infill of subglacial- dle- to Late Pleistocene glaciation history and landscape

Bedrock surface & Study area Quaternary sediment thickness 500 Overdeepened areas Rhine R. 47°N 500 Drill core (this thesis)

500 400 400 Bedrock contours (m a.s.l.) Swiss Plateau Zweidlen 400 G la ? Bedrock surface uncertain tt 400 R The Alps . Glattfelden 500 4 Sediment thickness 0 500 0 400 Bedrock LGM 50 km 9°E 400 outcrops 500 Töss R. 500 600 400 0 m terrain surface 50 m a 100 m 150 m 200 m Quaternary valley fill ? B1/12 bedrock surface B2/12 5 0 Cross-section bedrock 0

400 a’ Bülach ? B3/12 W E WNW ESE

500 Stadler Berg

B4/12 B5/12 0

500 Dättenberg

6 0 OSM AZ-11-14 Strassberg

0 B2/12 B1/12 0 5 Stadel

400 400 OMM OMM 0 300 300 4 0

400 approx. lowest base level

Elevation (m a.s.l.) USM 0 a USM a’ Oberglatt 2 0 5x vertical exaggerated Strassberg trough Bülach trough

Dielsdorf 0 6 0

not exaggerated 2 km 200 N 0 1000 2000 3000 4000 5000 6000 7000 8000

Fig. 1: The bedrock topography in the Lower Glatt Valley (N-Switzerland) is characterized by narrow bedrock depressions forming subparallel to bifurcat- ing troughs attributed primarily to subglacial erosion. These troughs are inset into a broad valley geometry and cut up to ~100 m below lowest fluvial base level. The up to 200-m thick Quaternary sediments infilled into the eastern branch (Bülach Trough) have been recovered in five drill cores (B1-B5/12), which were analysed in detail for this thesis. Bedrock surface map is courtesy of Amt für Abfall, Wasser, Energie und Luft (AWEL), Zürich and Nationale Genos- senschaft für die Lagerung radioaktiver Abfälle (Nagra), Wettingen.

E&G / Vol. 66 / No. 1 / 2017 / th 3–5 / DOI 10.3285/eg.66.1.th2 / © Authors / Creative Commons Attribution License th 3 evolution. Five drill cores gave direct insight into to the up proglacial glacio-deltaic, glacio-lacustrine, and later lacus- to ~190 m thick valley fill at the study site and allowed for trine sediments that document glacier retreat from the ba- detailed analysis of sedimentary facies, age, and architec- sin. Interbedded subglacial tills indicate later glaciations ture of the basin fills. forming nested, or ‘inlaid’, overdeepened basins with par- In a first step, the characteristics and origin of the infilled tial preservation of the older valley fill. sediments were studied using core descriptions, micro-sed- In a second step, an absolute chronology of the dep- imentology in thin-sections, and compositional analysis ositional and erosional periods was developed using lu- of different grain-size fractions. The study by Buechi et al. minescence dating of glaciolacustrine silts. Quartz OSL (2017a) focusses on the sedimentology of coarse-grained was identified as the luminescence signal of choice as it diamictons with sorted interbeds, which overlie bedrock met all performance criteria and is least affected by in- in the deepest depressions and mark the onset of deposi- complete bleaching. The results suggest glaciolacustrine tion in many other glacial bedrock troughs. The macro- and deposition and infilling of the thick sedimentary sequence micro-sedimentology suggest that these sediments are em- during different stages of the MIS6 Beringen Glaciation placed subglacially and reflect deposition, re-working, and period. However, the resulting burial ages were relative- deformation in response to repeated coupling and decou- ly close to the upper dating limit of the method and the pling of the ice–bed-interface promoted by high basal wa- reliability had to be tested extensively in signal stability ter pressures (Fig. 2, see Buechi et al. 2017a). These results experiments (Buechi et al. 2017b). The burial ages of the therefore give insights into the subglacial conditions that lower glaciolacustrine units partly coincide with the ice- existed in these otherwise inaccessible bedrock troughs. free MIS7 interglacial period. This overlap is interpreted The overlying valley fill is dominated by submarginal to to result from an overestimation of the true burial age

ice-bed interface ice-bed interface COUPLED DE-COUPLED cavity with meltwater stream B ice flow ice

fluidized deformable glacier bed bedrock immobilized till A ice flow ice water flow with bed-load transport winnowing of fines soft bed CONTINUUM groundwater expulsion, piping

bedrock immobilized till cavity with ponded meltwater

fluidized deformable bed C ice flow bed erosion ice

LFA1.1: diamicts, partly stratified bedrock immobilized till LFA1.2: Gravel interbeds ponded subglacial water LFA1.3: Laminated interbeds debris flows turbidity flows and suspension settling Erosion (bedrock / immobilized till) melt-out re-coupling & re-establishment of deformable bed

Fig. 2. Schematic diagram showing different states of the ice–bed-interface in the deepest part of the subglacial basin (Buechi et al. 2017a). In this model, the interbedding of subglacial tills with gravels and laminated fines results of repeated switching between a coupled and decoupled ice–bed-interface in space and time.

th 4 E&G / Vol. 66 / No. 1 / 2017 / th 3–5 / DOI 10.3285/eg.66.1.th2 / © Authors / Creative Commons Attribution License most likely caused by admixture of unbleached older valley fill material. This litho- and chronostratigraphic framework was then used to reconstruct the local to regional glaciation history and landscape evolution (Buechi et al. in review). The valley fill is grouped into nine formations, which are related to the Birrfeld Glaciation (~MIS2), the Beringen Glaciation (~MIS6), and up to three earlier Middle Pleistocene glaciations, tentatively correlated to the Hagenholz, Habsburg, and Möhlin Glaciations (Preusser et al. 2011). The complex valley fill and bedrock architecture are therefore interpreted to be result of multiple erosion and infilling cycles and reflect the interplay of subglacial erosion, glacial to lacustrine infilling of overdeepened basins, and fluvial down- cutting and aggradation in the non-overdeepened valley fill. Evidence suggests that in the study area deep bedrock incision, and/or partial re-excavation, occurred mainly during the Beringen and Hagenholz Glaciation, while older structures may have existed. Together with the observa- tion of minor, ‘inlaid’ glacial basins, dynamic changes in the magnitude and focus of subglacial erosion over time are documented. The preservation of sediments in subglacial basins over several glacial-interglacial cycles demon- strates the importance of subglacial basins as archives for glaciation history and landscape evolution of the Alps and glaciated mountain ranges worldwide.

References

Buechi, M. W., Frank, S. M., Graf, H. R., Menzies, J. & Anselmetti, F. S. (2017a): Subglacial emplacement of tills and meltwater deposits at the base of overdeepened bedrock troughs. – Sedimentology 64, 658–685. DOI: 10.1111/sed.12319 Buechi, M. W., Lowick, S. E. & Anselmetti, F. S. (2017b): Luminescence dating of glaciolacustrine silt in overdeepened basin fills beyond the last interglacial. – Quaternary Geochronology 37, 55–67. DOI: 10.1016/j.quageo.2016.09.009 Buechi, M. W., Graf, H. R., Haldimann, P. A., Lowick, S. E., Anselmet- ti, F. S. (in review): Multiple Quaternary erosion and infill cycles in overdeepened basins of the northern Alpine foreland. – Swiss Journal of Geosciences. Preusser, F., Graf, H. R., Keller, O., Krayss, E. & Schlüchter, C. (2011): Quaternary glaciation history of northern Switzerland – E&G Qua- ternary Science Journal 60, 282–305. DOI: 10.3285/eg.60.2-3.06