Burschil et al. 1#shareEGU20 ○ EGU2020-8386 ○ 04.-08.05.2020

Seismic response of three glacially overdeepened valleys – a comparison

T. Burschil1 · H. Buness1 · A.-C. Brandt2 · D.C. Tanner1 · G. Gabriel1,21 Leibniz Institute for Applied Geophysics, Hannover, Germany2 Institut für Geologie, Leibniz Universität Hannover, Germany

Basadingen Valley - BV

… a distal valley of the Rhine glacier.

… is incised in Tertiary Molasse.

Tannwald Basin - TB

… a distal branch basin of the Rhine glacier.

… is carved in Tertiary Molasse.

… contains sediments of several glacial periods.

Lienz Basin - LB

… an intra-mountainous basin at the intersection of two major faults and the paleo-confluence of three glaciers.

… has a substratum of paragneiss and dolomite.

… contains sediments of one glacial period.

BV

TB

LB

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The ICDP-project DOVE investigates the evolution of the past Alpine glaciations by drilling overdeepened valleys. In this context, we explore the erosional sur-faces and the sedimentary successions of three overdeepened valleys by the means of seismic reflection. A comparison reveals the similarities and differences of these three valleys.

Intr

od

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ion Source: 4t LIAG Minivib HVP-30

Sweep: 20-200 Hz, 10 sSource spacing: 5 mReceiver: 1-C geophoneReceiver spacing: 2.5 m# of ch.: up to 600

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uis

itio

n p

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eter 1. P-waves image the bedrock topography and the interior of

overdeepened valleys in high resolution.

2. Foreland and intra-mountainous valleys show the same overall structure for each accumulation cycle (from bottom to top: basal till, basin fines, fluvial deposits).

3. The sedimentary succession and thickness of the deposits vary among overdeepened valleys in detail. Even valleys in the same catchment area differ significantly (e.g. Tannwald Basin and Basadingen Valley in the Rhine Glacier arena).

4. A detailed study of each overdeepened valley is mandatory for a comprehensive understanding. We show that seismic reflection is a suitable method.

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A c k n o w l e d g e m e n tThe project was funded by DFG Deutsche Forschungsgemeinschaft (grants KR2073/3-1,GA749/5-1, BU2467/1-2). Supplemental equipment provided by G.I.P.P. (grant GIPP201623; GFZPotsdam). We thank D. Ellwanger, U. Wieland-Schuster (LGRB), and J.M. Reitner (GBA) forproductive cooperation; furthermore H. Haider, A. Gander (Baubezirksamt Lienz), P. Nagy, P.Herbst (GWU) for field support; C. Haberland, M. Stiller (GFZ Potsdam) for seismic advice; C.Schmelzbach (ETH Zürich) for support of Basadingen field survey and assistance with multi-component data; M. Fiebig (BOKU Wien), M. Linner (GBA) for geological discussion; as well asthe editors and reviewers of our publications.

R e l e v a n t p u b l i c a t i o n s (and references within)

Burschil et al. (2018). High-resolution reflection seismics reveal the structure and the evolutionof the Quaternary glacial Tannwald Basin. Near Surface Geophysics, 16, 593-610.

Burschil et al. (2019). Unravelling the shape and stratigraphy of a glacially-overdeepened valleywith reflection seismic: The Lienz Basin (Austria). Swiss Journal of Geosciences, 112, 341-355.

Burschil & Buness (2020). S-wave seismic imaging of near-surface sediments using tailoredprocessing strategies. Journal of Applied Geophysics, 173, 103927.

HVP-30

Burschil et al. 2#shareEGU20 ○ EGU2020-8386 ○ 04.-08.05.2020

Seismic response of three glacially overdeepened valleys – a comparison

T. Burschil1 · H. Buness1 · A.-C. Brandt2 · D.C. Tanner1 · G. Gabriel1,21 Leibniz Institute for Applied Geophysics, Hannover, Germany2 Institut für Geologie, Leibniz Universität Hannover, Germany

Upper Fresh-water MolassetOS

Dietmanns basin fines qDMb(Rissian)

Outwash plain deposits qRTN(Wurmian)

Scholterhaus Sfm. qDMSDürmentingen Sfm. qILD(Rissian)

Basal till(Hoßkirchian)

M/M’ ECBASe

ism

icfa

cies

Burschil et al. (2018)

vertical exaggeration: 2.5

Seismic imaging, supported by the stratigraphy of a nearby borehole, uncovers the bedrock morphology (D3, red line) and images the N-S elongatedvalley on the basis of five seismicprofiles. Within the sedimentarysuccession, we reveal allochthonousmolasse units (M´) at the bottom of thebasin that were presumablytransported during the erosion process. Arch-shaped stuctures indicate eskerdeposits (visible on profile TB-3). On top of basin fines (facies B), till and tillsequences of younger glaciations arefound (facies C).

C-1

Seis

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ies

C-2

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mar

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Stratigraphy

M

M’

B-1

B-2

230 mP-wave S-wave 1-CS-wave 6-C3-D P/SBorehole

TB-1P

Tannwald BasinSW

Burschil et al. 3#shareEGU20 ○ EGU2020-8386 ○ 04.-08.05.2020

Seismic response of three glacially overdeepened valleys – a comparison

T. Burschil1 · H. Buness1 · A.-C. Brandt2 · D.C. Tanner1 · G. Gabriel1,21 Leibniz Institute for Applied Geophysics, Hannover, Germany2 Institut für Geologie, Leibniz Universität Hannover, Germany

Lienz Basin

Burschil et al. (2019)

Seismic stratigraphy/units based on seismic faciesE gravel/coarse sandD coarse clasticsC2 fluvial deposits

wo. clinoformsC1 fluvial deposits

w. clinoformsB lacustrine

depositsA basal till

Seismic imaging unravels the erosional surface and the basin base in 622 m depth. The trough of the LienzBasin along the Upper Drau Valley shows a local overdeepening of 146 m. The reverse slope of the outlet of the LB would cause freezing of subglacial meltwater and thus indicates erosion prior to the last glacial maximum. The sedimentary succession reveals the depositional history of one glacial cycle. From the sedimentary succession, we derive a paleo-water depth of 216 m.

Reverse slope of basin 2.6 %

2200

Gradient of ice surface during LGM 0.4-0.5 % (Reitner 2003)

Freezing of subglacial meltwater

P-wave S-wave 1-CS-wave 6-C

LB trough

Cross-section along the Upper Drau Valley

vertical exaggeration: 2.5

ele

vation

(m)

Burschil et al. 4#shareEGU20 ○ EGU2020-8386 ○ 04.-08.05.2020

Seismic response of three glacially overdeepened valleys – a comparison

T. Burschil1 · H. Buness1 · A.-C. Brandt2 · D.C. Tanner1 · G. Gabriel1,21 Leibniz Institute for Applied Geophysics, Hannover, Germany2 Institut für Geologie, Leibniz Universität Hannover, Germany

BV-1

BV-2

Basadingen Valley The overdeepened Basadingen Valley is carved (red line) into layers of Molasse. A 275 m deep inner-trough is located in the southern part of the profile. The bedrock shows indication of faulting (black dashed lines).

Seven seismic facies differentiate the sedimentary fill of the valley. We interpret facies A as basal till that contains slumping. The inner-trough is filled with deposits containing clinoforms (facies B), followed by mega-clinoforms of hypothetically different, most likely fluvial, deposits that migrated from S to N (facies C and D). Facies E and F show high reflectivity. The uppermost layer (facies G) shows several small-scale depressions (white stars) that could be refilled kettle holes.

BV-1P

NS

P-wave S-wave source test

vertical exaggeration: 2.5

Burschil et al. 5#shareEGU20 ○ EGU2020-8386 ○ 04.-08.05.2020

Seismic response of three glacially overdeepened valleys – a comparison

T. Burschil1 · H. Buness1 · A.-C. Brandt2 · D.C. Tanner1 · G. Gabriel1,21 Leibniz Institute for Applied Geophysics, Hannover, Germany2 Institut für Geologie, Leibniz Universität Hannover, Germany

Comparison

TB-1P

Tannwald Basin – TB

Basadingen Valley – BV

Lienz Basin – LB

Seismic imaging shows similarities and differences in different overdeepened valleys. The overall sedimentary succession consists of basal till, lacu-strine and fluvial deposits of the accumulation phase after the erosion. The thickness of each package varies significantly.

While the LB and BV show thick basal till with slumping structures at the bottom, the basal till in the TB is thin. The thick basin fines in the LB are laminated, while only little structure is present in the TB. Both foreland basins incise through a Molasse unit of high reflectivity. The reflection pattern of the upper part of the BV is like the

upper part of the LB. The seismic image of the BV shows evidence of faulting in the bedrock. We also presume that the intersection of the faults pro-moted the formation of the LB, but these cannot be seen.

Seismic sections have same scale and elevation.

BV-1P

NS

S

All three valleys are foreseen as drilling locations for a first phase of the ICDP-project DOVE.