Proceedings (Published version)

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Conference Proceedings
Limnogeological cruise on Lake Geneva : geological history of the
Lake Geneva basin and human settlements
GIRARDCLOS, Stéphanie, CORBOUD, Pierre Olivier, WILDI, Walter
Reference
GIRARDCLOS, Stéphanie, CORBOUD, Pierre Olivier, WILDI, Walter. Limnogeological cruise
on Lake Geneva : geological history of the Lake Geneva basin and human
settlements. Geneva (Switzerland) : University of Geneva, Section des Sciences de la Terre et
de l'Environnement, 2014, Guidebook for ISC 2014 Excursion (August 20th, 2014)
Available at:
http://archive-ouverte.unige.ch/unige:39357
Disclaimer: layout of this document may differ from the published version.
[ Downloaded 14/10/2016 at 06:31:52 ]
19th International Sedimentological Congress – Geneva, Switzerland, 18-22 August 2014
Mid Congress Excursion – 20th August
Limnogeological cruise on Lake Geneva :
geological history of the Lake Geneva basin and human settlements
By Stéphanie Girardclos*, Pierre Corboud** and Walter Widi***
*Section of Earth and Environmental Sciences and Institute for Environmental Sciences, **Institut F.-A. Forel,
***honorary professor, Institut F.-A. Forel
Table of content
Abstract, map of the tour
1
1.
Technical data of Lake Geneva
2
2.
Winds and currents of Lake Geneva
3
3.
Limnology of Lake Geneva
4
4.
Regional Chronology
5
5.
Tectonic, geological and sedimentological setting
6
6.
Formation of Lake Geneva basin
8
7.
Glacial stages and sediment infill of Western Lake Geneva
9
8.
The Rhone delta and canyons
14
9.
Extreme events and tsunamis in Lake Geneva
18
10.
Evolution of Lake Geneva level between glacial retreat and the present
21
11.
Holocene vegetation and landscape history
23
12.
Human occupation of the Geneva basin, relations with the environment
25
13.
Construction of Geneva city with local georesources
26
14.
Bibliography
28
Addresses of authors:
Stéphanie Girardclos • [email protected]
Department of Earth Sciences
University of Geneva
Rue des Maraichers 13
CH-1205 Geneva
SWITZERLAND
Walter Wildi • [email protected]
Pierre Corboud • [email protected]
Institut F.-A. Forel
University of Geneva
Route de Suisse 10 - CP 416
CH-1290 Versoix
SWITZERLAND
Thank you to our co-authors : Katrina Kremer, Guy Simpson, Juan Pablo Corella, Michael Hilbe, Jean-Luc
Loizeau, Angel Arantegui, Anne-Marie Rachoud Schneider, Flavio Anselmetti, François Marillier, Frédéric
Arlaud and Tonya DelSontro.
Thank you to our financing institutions : Swiss National Science Foundation, Fondation Ernest Boninchi,
Fondation Ernest and Lucie Schmidheiny.
This document is published thanks to the financial support of the
Société de physique et de science naturelle de Genève (SPHN)
Recommended citation : Girardclos (S.), Corboud (P.) and Wildi (W.). 2014. Limnogeological cruise on Lake Geneva :
geological history of the Lake Geneva basin and human settlements. Guidebook for ISC 2014 Excursion (August 20th
2014), Section des Sciences de la Terre et de l'Environnement. Geneva : University of Geneva.
Limnogeological cruise on Lake Geneva, ISC 20.08.2014
Abstract
Lake Geneva became famous in the scientific world since the publication of the pioneer limnology
book “Le Léman” by François-Alphonse Forel (Forel 1892-1904). We will take you onboard the
Neptun, a boat constructed in 1904, the same year Forel's book was published, for a short journey
through the geological and sedimentological history of the lake. We will show you how the Leman and
humans societies, including the palafittes (UNESCO world heritage), interacted through time and how
two tsunamis swept over the lake. We will also take the opportunity of the breathtaking view over the
surrounding landscape to give an overview of the regional geology (Prealps, Jura) and discuss how
the Rhone glacier shaped the Geneva basin during the last glaciation (Last Glacial Maximum ca.
20'000 BP).
Map of the tour
Departure: 10h
Arrival: 13h
1
1. Technical data of Lake Geneva
Average altitude of the lake :
Extreme levels (after the regularization of Lake Geneva, 1887) :
Surface water level :
Catchment area :
Average altitude of the watershed :
Average volume :
Average total flow of the tributaries of Lake Geneva :
Medium flow of the Rhone River upstream (at Porte du Scex Station)
Percentage of the contribution of the Rhône River :
Medium outflow of the Rhone River (Geneva)
Theoretical retention time of water
Maximum depth
Length of shoreline
372.05 m (1943-2008)
mini : 371.78 (1949)
maxi : 372.19 (1977)
mini : 370.85 (1921)
maxi : 372.91 (1937)
580.1 km2
7'419 km2 (less the lake)
1'670 m
89 billion m3 (89 km3)
220.5 m3/s (mean)
182 m3/s (1935-2008)
mini : 127 m3/s (1976)
maxi : 227 m3/s (1999)
83% of total of the tributaries
250 m3 /s (1935-2008)
mini : 166 m3/s (1976)
maxi : 327 m3/s (1995)
11.3 years (volume/medium flow)
308.9 m (updated)
200.2 km
Rapport de la Commission internationale pour la protection des Eaux du Léman contre pollution, survey 2012, 2013,
120-140. Office fédéral de l'environnement, données hydrologiques.
10 km
t
th a
est dep
g
r
a
l
e
h
xis
Le Bouveret
Geneva
400.0
m asl
Geneva
Le Bouveret
372 m asl
350.0
76 m
300.0
250.0
310 m
200.0
150.0
100.0
vertical exaggeration : 50 x
50.0
0
10
20
30
40
2
50
60
70
km
Geological history of the Lake Geneva basin and human settlements
2. Winds and currents of Lake Geneva
SE
10 km
SE
BI
JO
RA
SE
BI
VENT
RE
AI
UD
VA
VE
NT
(S
UD
OI
S)
N
BISE
BI
Main winds of Lake Geneva, Atlas climatique de la Haute-Savoie (Bravard and Debray 1991).
Wind Velocity [m/s]
0.5 m/s
0
3
5
8
10
Field interpolated winds, 20 January 2005, typical situation of Bise
(interpolation data from COSMO 7, Le Thi et al. 2012).
Current Velocity [cm/s]
0.1 m/s
0
6
12
18
24
Average current velocities of surface water layer during October 2005
(Le Thi et al. 2012).
3
30
3. Limnology of Lake Geneva
Lake Geneva, also known in French as Le Léman, is a large deep-water basin in central Europe, on the
border between Switzerland and France. The lake is characterized by thermal stratification of the
water column from spring to autumn and homogenization from the end of winter to the beginning of
spring. Lake Geneva is a "monomictic" lake according to the classification of Hutchison.
The annual thermal cycle begins with isothermal conditions in winter, followed by thorough mixing in
late winter, and by stratification from May to October. The annual cycle of temperature distribution in
the Grand-Lac shows strong seasonal trends. Depending on the difference in volume and morphology
of the basin, cooling and warming during the transition periods (spring and fall) are slower in the GrandLac than in the Petit-Lac. Minor differences within the basins could be linked to local circulations
between coastal areas and deep areas, as well as the influence of fluvial input, including the Rhone.
Over much of the year, the waters of Grand-Lac circulate in a gyre turning counterclockwise (Le Thi et
al. 2012). The size and lifetime of the vortex varies depending on weather conditions. Appendices of
the main gyre are found in major bays of the northern shore, in Morges and Vidy (Lausanne). These
are less stable than the main gyre and their direction may switch depending on wind direction. In the
transitional area between the Grand-Lac and the Petit-Lac a gyre oriented clockwise is established.
The western end of the Petit-Lac is characterized by a counterclockwise circulation system. In the
Petit-Lac, flow towards the Rhone outlet mainly occurs in surface waters and along the borders of the
basin, while the return currents to the Grand-Lac are deeper-water currents and found in the central
part of the basin. In the Grand-Lac, the downstream flow from East to West follows the northern edge of
the basin, while the return current, from the western to the eastern part of the basin, predominantly
follows the southern border.
Temperature contours maps from monthly-observed data (left panel) and modeled data (right panel) at the
GE3 monitoring site in Petit-Lac (Le Thi et al. 2012).
4
Seismic units
Fiore et al. 2011; Girardclos
et al.
Kremer et al.
2003; 2005
2014
14
Seismic
Reflections
4. Regional Chronology
D
Swiss Plateau Biozones
(Lotter 1999;
1999, Ammann et al. 1996)
Younger Subatlantique
0
2 AD
1
1 AD
X
8
Older Subatlantique
Cal. Cal.
Archaeological
kyr kyr
cultures
BP BC
IX
2
0
Modern
period
Roman times
Iron Age
C3
VIII
4
2
5
3
6
4
7
5
8
6
9
7
Neolithic
10/9
Younger Atlantique
VII
11
C2
1
Bronze Age
Subboreal
13
3
12
Older Atlantique
VI
13
Boreal
V
10
8
Preboreal
IV
11
9
Younger Dryas
III
12
10
13
11
14
12
15
13
Mesolithic
14
C1
15
B3
B2
17
18
II
Bølling
Ib
Oldest Dryas
12.1
11
10
8
B1
A2
A1
glacial
deposits
14
Epipaleolithic
Upper
Paleolithic
?
19
20
21
Nyon stage
Coppet stage
22
23
7
2-6
1
Ia
Rhone glacier
deglaciation
12.2
16
Allerød
?
?
?
?
Reposoir stage
Geneva stage
End of last glacial maximum
ca. 22 ca. 20
Glacio-lacustrine and lacustrine units : age and correlation of units and reflections with the Swiss
Plateau chrono-biozones and archeological cultures. Fiore et al. 2011 modified.
5
5. Tectonic, geological and sedimentological setting
500000
510000
520000
530000
150000
U
EA SE
T
A AS
PL OL
M
540000
550000
MC MR
Lausanne
20
IN
E
NA
PP
ES
Pe
ti tLa
c
130000
27
0
LF
12
0
se
an
Dr
70
70
22
0
JU
Grand-Lac
E
IN E
P
L S
A S
B LA
SU O
M
e
on
Rh
140000
RA
170 PF
560000
LP
EA
PR
120000
+
Geneva
5 km
Legend:
Major thrust fault
Fault
Supposed fault
Anticline
Other thrust fault
Shaded digital elevation model of Lake Geneva region with main rivers, cities and tectonic units and faults (in
black, after Dupuy 2006). PF: Paudeze fault; LF: Lutrive fault; MC: Molasse à charbon; MR: Molasse rouge.
Grey contour lines for the bathymetry. Coordinates are in meters (Swiss Grid). Modified from Kremer et al.
2014.
SE
1000
Rhône
NW
PLATEAU DES BORNES
GENEVA BASIN
Salève
Salève-2
Humilly-2
Vuache
0m
CI
-1000
?
JS
-2000
JI
JM
-3000
5
?
JI
P-C
10 km
CRYSTALLINE BASEMENT
LEGEND for sedimentary rocks
JI = Lower Jurassic
CI = Lower Cretaceous, JS = Late Jurassic,
JM = Middle Jurassic, TR = Triassic
P-C = Permo-Carboniferous
Lausanne
Lake Geneva
N
Geneva
EV
SA
L
AC
HE
6
VU
Geological profile through the southern Geneva basin. The Tertiary
Molasse basin (yellow) was formed at the end of alpine formation.
Gorin and Moscariello 2013.
AL
P
S
20 Km
E
Faults
10
0
PR
E
Drillings
JU
RA
Tertiary Molasse
-1000
-2000
TR
CRYSTALLINE BASEMENT
0
?
CI
JS
?
TR
P-C
1000
0m
?
-3000
Lake Geneva lies between the Jura mountain and Prealps in a Tertiary Molasse basin which was
deposited in the alpine foreland on top of Lower Cretaceous formations. Its basin was carved during
Quaternary glaciations. In the Petit-Lac a deep and undulating valley typical of 'tunnel valleys' (Fiore et
al. 2011) built in its southwestern part. Lake Geneva Quaternary sediment infill is maximum 400 m
thick in the Grand-Lac and consists mainly of glacial and glacio-lacustrine deposits from the last
deglaciation deposited by the Rhone Glacier (Girardclos et al. 2005). The Holocene lacustrine
sediment infill is complex and varies greatly depending on currents (Girardclos et al. 2003 ; Lemmin
1998 ; Le Thi et al. 2012) and on clastic input from nearby rivers. Where hemipelagic processes are
dominant, Holocene sediment infill is only 5-10 m thick but it reaches over 100 m in the Rhone delta.
Geneva geologic basin is formed by three main geologic and tectonic units : sedimentary rocks of the
Jura mountain in the NW, Tertiary sandstone ('Molasse') in the centre, and thrusted Molasse in the SE.
The thrusted Molasses is topped by ultrahelvétique and Prealps units which rim the eastern end of the
lake.
Seismic and drilling data show that the formation of Molasse basin is younger than folding of Salève
and Jura mountains dated at the end of Miocene or at the beginning of Pliocene (between 10 to 2
millions years). But as Molasse basin formation is a consequence of alpine tectonics, it originated
much earlier and still presently continues to change at slower rate.
Top of the Molasse Geneva basin
500-525
250-275
475-500
225-250
450-475
200-225
425-450
175-200
400-425
150-175
375-400
125-150
350-375
m asl
0
275-300
Nyon
20
525-550
Nernier
30
0
300-325
0
> 550 m
400
135000
325-350
30
140000
Coppet secondary valley
0
40
0
50
Hauts-Monts area
Anières
Versoix
300
125000
500
0
20
75-100
200
400
Coppet
100-125
130000
Céligny
strike-slip fault
Anières secondary valley
0
30
0
40
400
400
400
120000
30
115000
30
0
0
0
40
0
50
Geneva
40
300
0
00
0
40
4
5 km
110000
490000
495000
500000
505000
510000
515000
Structural map of the top Molasse bedrock erosional surface (m asl, contour interval 25 m, Swiss coordinate
system). Modified from Fiore et al. 2011.
7
6. Formation of Lake Geneva basin
In the region of Ecoteaux (Prealps units, 10 km north of Vevey) glaciolacustrine sediments at 800 m asl
show that a periglacial lake was present in the area indicating a first origin of Lake Geneva basin ca.
730'000 years ago (Pugin et al. 1993). With time, this basin evolved from a wide and relatively shallow
depression into the present narrower and deeper Lake Geneva basin.
During the late Pleistocene glacial cycles, Lake Geneva basin deepened into the Molasse bedrock to
reach a maximal depth of 500-560 m offshore Lausanne and Meillerie (Grand-Lac : Dupuy 2006).
During the phase of Last Glacial Maximum, most of the former sediment infill was removed by Rhone
glacier and only rare and compacted sediment patches were preserved from former glaciations (Wildi
and Pugin 1998 ; Wildi et al. 1999).
Ecoteaux
Villeneuve
Lausanne
Lake Ecoteaux ca. 800 m, 730’000 years old
800
Morges
Bière
700
Armoy
600
500
Thonon
actuel
Present Lake level 372 m
400
rs
ea
0y
’00
20
300
ars
e
0y
200
0
0’0
12
0m
Glacio-lacustre and lacustrine infill
-100
-200
0 km
5 km
10 km
15 km
20 km
25 km
Profile of Lake Geneva basin 730'000, 120'000 years ago, 20'000 and at present (Corboud et al. 2006 ; from
Wildi and Pugin 1998).
Ecoteaux : laminated distal
glacio-lacustrine
sediments with organic
layers.
8
7. Glacial stages and sediment infill of Western Lake Geneva
plus de 1000 m
5 km
plus de 750 m
plus de 500 m
Situation before the Last Glacial Maximum, at ca. 40'000-37'000 BP (38'000-35'000 BC) thick units of
sand and gravels deposited in the Geneva basin in more than 10 km-sized sandur, forming the
'Alluvion ancienne' deposit (in grey).
1000 m
800
1000 m
1200
1800
2800
2600
1400
1200
2200
2000 m
2400
1600
1800
3000 m
1000 m
2000 m
Last Glacial Maximum in Switzerland around 22'000-20'000 years BP (20'000-18'000 BC). Modified
from Bini et al. 2009. Red lines indicate present lakes. Ice altitudes are in m asl. © 2014 swisstopo.
9
Jura ice cap
Rhone glacier
Arve glacier
Reconstruction of the Last Glacial maximum in the Geneva area with arrows representing main ice
fluxes.
plus de 1000 m
5 km
plus de 750 m
plus de 500 m
After the Last Glacial Maximum, the Rhone glacier retreated to form a moraine near Laconnex around
20'000 BP (18'000 BC) with a lake level at 475 m asl.
10
It was followed by a glacial stage in Geneva city which formed the Old Town hill. The final deglaciation
history is recorded in the seismic and sedimentary infill of western Lake Geneva.
plus de 1000 m
5 km
plus de 750 m
plus de 500 m
Geneva stage with lake-level at 410 m. Formation of a glacio-lacustrine delta at location of the Old City hill.
In the city harbor next to the Neptune, the so-called “Pierres du Niton” (Niton
rocks) represent additional traces of the Rhone and Arve glaciers pathway as
they were also deposited during the last deglaciation. They are foliated
porphyritic granites of Mont Blanc mountain dated at 303 ± 2 million years
(Sesiano et al. 2011). These stones were used since 1845 as altitude
reference level for topographic maps and still serve as base level mark for the
entire geodesy of Switzerland (373.6 m above sea level, Repère de la Pierre
du Niton : RPN).
Till-tongue model with debrisflow lenses coming from the
downstream flank of a
subaqueous moraine formed by
the pushing of proglacial
sediments (Stravers and Powell
1997).
11
The deglaciation happened in a fjord-like environment occupied by a glacier with a steep, calving ice
front. The lake-level decreased from an altitude of 430 to 380 m in a few steps but this chronology is still
not well known.
Seismic reflection data present the bedrock lake basin (unit U0) as well as sismostratigraphic units
with glacial- (U1-11), glaciolacustrine- (U8,10,12) and lacustrine facies (U13-14). The facies and
geometry of U9 and U11 indicate re-advances of the Rhone glacier. Units U9b and U11b are
interpreted as till-tongues (Fiore et al. 2011 ; Stravers and Powell 1997). Data points to at least three
stages/readvances during the deglaciation process (Reposoir stage, Coppet and Nyon re-advances).
Four drillings (S1-S4) done in 2010, 34 to 74-m-deep, confirm the seismic interpretation.
Palynological data from core (S3) show that ca. 60 m of sediment, out of the 75-m-thick infill, were
deposited during the Oldest Dryas. The asymmetry of unit U7a also shows that the glacier melted first
in the central and eastern part of the lake basin while it was still lying on the Molasse 'ramp' in the West.
Glacio-lacustrine deposits (U8-12) consist of variable sedimentary facies, alternating between
massive and laminated clayed silt with rare sand layers. Gravels of alpine origin and 'galets mous',
interpreted as ice-rafted debris, point to the recurrent presence of icebergs over the lake.
Late Glacial and Holocene infill of Geneva Bay was massively reworked as large mass transport
deposit in Bronze Age (3415 ± 35 14C BP ; see 'catastrophic events' chapter). Remaining Holocene
infill of the Petit-Lac is largely influenced by river input and strong underwater currents (Girardclos et
al. 2003 ; Girardclos et al. 2005).
SSW
S3
0.50
Distance (km)
15
Versoix
delta
5
10
0
Rhone glacier
Coppet readvance
ICE/ WATER FLOW
U7a
0.10
U12-14
0.15
U6b
TWT (s)
U11a U11c
U9a
U9c
U4
U1
U5
U3
0.30
U0: Molasse bedrock
U2
0.35
moraine
ice front
glacio-tectonic front
debris-flow extent
Nyon
readvance
Nyon
1000 m
Coppet
readvance
370 m
Nernier
Céligny
130000
Tougues
Coppet
Hermance
Synthesized compiled profile along the thalweg showing
the entire Quaternary record of western Lake Geneva.
Note the numerous till sequences (units U1-U5, U7, U9
and U11), separated by compaction levels corresponding
to glacial readvances. The main esker (sub-unit U6a) is
not located in the thalweg and is projected from the NW.
The latest two glacial readvances in the area ("Coppet"
and "Nyon") have built large push moraines (subunits U9a
and U11a) and associated proglacial debris-flow fans
(sub-units U9b and U11b). Left axis is in TWT (s) and
depth scale (right axis) is estimated using seismic
velocities of 1500 m/s for water (down to 100 ms) and
1800 m/s for sediments (Fiore et al. 2011).
Versoix
Anières
S3
120000
Reposoir
stage
Geneva
5 km
500000
300
0.25
U9b
U7c
U6a
0.20
U10
U8
Rhone glacier
Nyon readvance
U11b
depth (m) 0
20
100
0.00
NNE
200
Rhone glacier
Reposoir stage
510000
12
chrono-biozones
C dating
palynology
samples
14
lithology
core depth (m)
sediment deposit
seismic unit
(Fiore et al. 2011)
5
10
? ?
15
undefinded
Holocene
Late Glacial
and Holocene
mass transport
deposit
U12
U13-14
lacustrine
deposit
0
30
Lithology of drilled sediment core S3 from the ‘Petit Lac’
at 40 m water depth. The lower part (17-71 m depth)
belongs to the Oldest Dryas Ia chronobiozone and
consists of a glacial sequence with compacted till (U6b),
melt-out till linked to the Reposoir stage (U7a) and glaciolacustrine deposits (U8-10). Unit U12 (7-17 m) is
characterised by a mix of sand layers, pebbles, entire
and broken shells and vegetal remains interbedded with
silty – clayey lacustrine sediment which is interpreted as
a mass transport deposit and dated at 3415 ± 35 14C BP.
(Modified from Kremer et al. 2014.
45
50
Middle LPAZ
35
40
U7a
metl-out till linked to
Reposoir stage
Upper LPAZ
25
Oldest Dryas Ia
glacio-lacustrine deposit
U8-10
20
14
3415 ± 35 C BP
Legend for core S3 lithology
55
60
65
Basal LPAZ
U6b
compacted
melt-out till
sand
silt
shell
broken shell
bioturbation
clay
wood charcoal
empty
vegetation rest
70
13
8. The Rhone delta and canyons
Lake Geneva being a prominent landscape feature in Central Europe already figures in road map of
the Roman Empire and on early World map from the 10th century. The first scientific report of water
depth measurements in Lake Geneva was written by Horace-Bénédicte de Saussure in 1779. The first
comprehensive bathymetric map of Lake Geneva, a compilation from measurements made from
1873-1889 by various authors, was published in 1892 by F.-A. Forel. This map was later published in
various versions (including Delebecque 1898) and for all official topographic maps of the Swiss
Federal Office of Topography (at present too !).
First bathymetric map of Lake Geneva. Version published in the “Atlas des lacs français. Planche 1”
(Delebecque 1898). BGE, Centre d'iconographie genevoise.
Methological setting used to establish the bathymetric map of
Lake Geneva (Forel 1892 ; Delebecque 1898). 1880-1890 (date
undetermined).
14
Under the pressing need of updated and more detailed bathymetric data, a Digital Terrain Model
(DTM) and corresponding bathymetry map of eastern Lake Geneva were recorded with multibeam
echosounding methods in 2008 (Sastre et al. 2010). This dataset truly revealed the underwater Rhone
river delta morphology, showing numerous unknown canyons and structures such as slide scars and
dredging traces. Presently inactive canyons (Corella et al. 2014) are interpreted as traces of the former
Rhone river mouthing (Sastre et al. 2010), as shown by ancient map from 1781.
active Rhone canyon = aC
former Rhone canyons = fC
aC
fC
fC fC
aC
2 km
fC
fC fC
fC
dredging
fC
Bathymetric map recorded in 2008 with shaded relief of NW Lake Geneva and sublacustrine Rhone delta
morphology. Modified from Sastre et al. 2010.
Left : Map of the lower
Rhone valley and the
terrestrial part of the
Rhone Delta by Mallet
1781.
Right : Comparison with
modern coastline and
sublacustrine canyons
(C1-C8). Modified from
Sastre et al. 2010.
15
Rhone delta dynamics are mainly controlled by three sedimentation processes, which mainly depend
on the river discharge and water density. When discharge is low, suspended particles deposit as delta
foreset near the river mouth. If the incoming river flow density is comprised between the one of the
epilimnion and the one of the hypolimnion, it propagates along the thermocline, and brings fine silt
particles to the northern part of the delta. In contrast, when river inflow is denser than the hypolimnion,
underflow currents build up a distal delta morphology with canyons, levee and fans, thus transporting
coarse silt- and fine sand-sized sediment up to 15 km to the west. The turbidite record of distal Rhone
delta is due to both flood and slope failure events (Lambert and Giovanoli 1988 ; Corella et al. 2014).
The Rhone delta model with three types of
sedimentation :
1. the proximal sedimentation builds a delta
foreset ;
2. the erosion, transport and deposition by
underflow currents form channels and distal
fan ;
3. the interflow fine sediment transport is
deflected to the North.
Modified from Giovanoli (1990) and Loizeau
(1991).
The recent sedimentary infill of deep Lake Geneva (Grand-Lac) is characterized by hemipelagic
sediments intercalated with smaller turbidites originating from either the Rhone or Dranse delta area
(floods or delta collapses). The frequency of turbidites is varying spatially and temporally. From the 6th
century until 1675 ± 125 cal AD, the infill is dominated by turbidites from the Dranse delta most
probably because the Rhone was mouthing in the northern canyons too. Since 18th/19th century,
deep Lake Geneva sediment record shows an apparent 'progradation' of the Rhone turbidites (Kremer
2014). Additionally, a decrease in the frequency of turbidites upwards along the active canyon (C8)
wall suggests a progressive confinement of the flow through time (Corella et al. 2014). Both distal
turbidite progradation and decrease of proximal upwards turbidite frequency are interpreted as
consequences of the inland channelizing of the Rhone River between 1870-1880 and point to possible
ongoing lengthening and deepening of the underwater channel (Kremer 2014 ; Corella et al. 2014).
Evolution of the canyon activity in the Rhone
delta area and in the distal deposition zone of
the Rhone turbidites since 563 AD.
A. Between 563 AD and 1675 ± 125 cal AD
(Unit 1), the main active canyon is C5 with a
sedimentation zone located in the northern part
of the basin.
B. Between 1675 ± 125 cal AD to present (Unit
2) the main active canyon is C8 with a
sedimentation zone located in the southern part
of the basin. C5 was cut from the Rhone River
between 1870 and 1880 AD. From Kremer 2014.
16
Comparison of multibeam bathymetry datasets recorded in 2008 and 2012 show that the
sublacustrine Rhone canyon is currently very active : up to 4 m of sediment is either deposited or
eroded on the canyon floor in these 4 years. In the same time interval, floor erosions are associated to
new slide scars formed in the canyon walls and thus certainly triggered by strong (mud ? water ?) flows
in the canyon (Girardclos et al. 2012).
Detailed shaded multibeam bathymetry maps of the Rhone main channel, with panels of the
February 2008 (a, c and e ; from Sastre et al. 2010) and March 2012 (b, d and f) channel
morphology. Colored isolines indicate erosion (-) and deposition (+) in meters. New slide scars
appear close to areas of strong bottom erosion (d). Refer to upper panel for channel overview,
panel location and contour lines color scale. From Girardclos et al. 2012.
17
9. Extreme events and tsunamis in Lake Geneva
In the Grand-Lac, high resolution seismic reflection data (pinger 13 kHz) indicate that in the upper 30m-thick sequence consists of five large Mass Transport Deposits (MTD, lenses of transparent facies)
which alternate with the normal lacustrine sediment infill (Kremer et al. submitted).
MTD A (volume 130×106 m3), which is interpreted as a slump deposit starting offshore Lausanne at 100
m water depth, happened at the same event horizon than MTD C (volume 22×106 m3) which started on
the opposite (southern) shore. Both failures left scars on the topography which are still visible on
multibeam bathymetry data. These mass transport deposits are dated from Early Bronze Age (3395
± 35 14C BP) and might have happened coevally with the mass transport deposit in Geneva Bay (see
core S3, chapter 7) pointing to a possible earthquake trigger. Taking into account a minimum VII
intensity to trigger coeval slope failures in both the 'Grand' and 'Petit-Lac', the Early Bronze Age event
of Lake Geneva could be explained by a Mw ~ 6 earthquake potentially located south of the lake,
where magnitudes of historical events reached already Mw 4.6-5.5 during the last 200 years.
Independently of its trigger, MTD A displacement by itself induced a minimum 4-6 m high tsunami near
Lausanne and Evian and thus certainly impacted local palafitte settlements (Kremer et al. 2014).
MTD D to MTD F started in the area of the Rhone delta and are interpreted as delta failures (maybe
linked to sediment accumulation). Their trigger is unknown but their volume and comparison with
numerical modeling show that they were all large enough to produce m-sized tsunami waves over
Lake Geneva (MTD D volume 30×106 m3 ; MTD E volume 46×106 m3 ; MTD F volume 83×106 m3
(Kremer et al. submitted).
MTD G, with its volume of 250×106 m3, is the largest mass transport deposit in our record. It happened
in 563 AD, during the so-called 'Tauredunum' event (Montandon 1925) and when a large aerial
landslide of aseismic origin felt onto the former Rhone delta to destabilized its slopes and trigger a
huge sublacustrine failure. This failure induced a giant tsunami over Lake Geneva and deposited as a
large turbidite bed at the bottom of the Grand-Lac (Kremer et al. 2012).
Core data confirm the seismic interpretation and date MTD events thanks to radiocarbon dating and
the correlation with historical data for the 563 AD event. (Kremer et al. 2012 ; 2014 ; submitted).
High-resolution seismic reflection profile in deep Lake Geneva (Grand-Lac) imaging five large MTDs. Bold
lines represent time horizons that are considered as event horizons. Dotted lines delimit the base of the
MTDs. Vertical axis is TWT (s). From Kremer et al. submitted.
18
Photographs and lithology of Ku-IV sediment core with turbidite and mass transport deposits outlined in grey.
MTD A, interpreted as a slump, is composed of deformed and folded laminated sediment layers, topped by
mudclasts incorporated in a muddy matrix and a homogeneous layer. The dashed black line indicates the
age-depth curve in calibrated years BP and in AD/BC. Turbidite of 563 AD (Kremer et al. 2012) corresponds
to MTD G on seismic line (from Kremer et al. 2014).
19
Sketch of the geological and limnogeological
process describing the main phases of the 563 AD
event, from the aerial rockfall in the Rhone valley
(1), to the sublacustrine Rhone delta collapse,
followed by turbiditic and debris flow propagation
with wave generation (2), ending with water wave
passing over the old city walls of Burgundian
Geneva (3).
Possible inundated zone for an 8 m wave in the
reconstructed Burgundian city of Geneva (in white)
with 6th century buildings, walls and bridge, and
the past lake border (thick black line) with
background aerial photograph of Geneva.
Inundated region was estimated taking account of
the first arrival wave height (at the lake shore) and
the current topography without estimation of the
wave runup (from Kremer et al. 2012).
Geneva
The displacement of MTD G was simulated by numerical
modelling and confirms historical data on the
Tauredunum event. Historical reports mention waves
entering the Geneva Burgundian city (located on the hill
of the present Old City) and killing people (Montandon
1925 ; Kremer et al. 2012).
20
10. Evolution of Lake Geneva level between glacial retreat and the present
After the retreat of the Rhône glacier, and from the Mesolithic period (ca. 6500 BC), the level of Lake
Geneva stabilised at a mean water level close to the present lake-level. However, climatic variations
and geological events have caused secular and seasonal fluctuations within an interval of ca. 9 m.
373
4
372
3
371
8
370
10
?
369
5
12
9
11
15
13
5
1
2
2
368
7
19
14
21
17
18
16
20
1
6
367
Middle Neolithic II
366 m
-4000 BC -3800
-3600
Late Neolithic
-3400
-3200
-3000
-2800
-2600
-2400
-2200
Middle
Bronze
Early Bronze age
Campaniforme
-2000
-1800
-1600
-1400
Late Bronze age
-1200
-1000
-800
Chronological positions of pile dwelling settlements in the Lake Geneva with the probable water altitudes for
each station. Local archaeological periods. 1-21 : archaeological sites studied and dated by
dendrochronology. In grey : datation with reserve (Corboud 2012).
From the 19th century dams were built in Geneva on the Rhone River to 'feed' the hydraulic machines
of the local industry. These facilities caused important lake-level variations so that the Canton of Vaud
opened a court case against Geneva at the Swiss Federal Supreme Court. The «Lake Geneva trial»
was solved in 1884 by the signature of a Convention between both cantons that defined the limits of
possible lake-level variations. The construction of sluice gates of Pont-de-la-Machine aimed at
applying this convention. Since 1887, Lake Geneva was regulated by the Pont-de-la-Machine dam
and since 1995 by the downstream and present Seujet dam.
Regularization of the Lake Geneva : 1887
373
373 m
372
372
371
371
ZL
370.6
1810
1820
1830
1840
1850
1860
1870
1880
1890
1900
1910
1920
1930
1940
Extreme water level variations of Lake Geneva between 1806 and 1941 (annual maximum and minimum,
and smoothed curves, in red). Red circle : drought of summer 1921. According to data provided by FrançoisAlphonse Forel and by the “Service hydrologique et géologique national”.
21
Lake Geneva regime, which is largely influenced by glaciers from the upstream Rhone River
catchment, can change rapidly if the balance between precipitation, glacier melting and
evapotranspiration changes. Small variations in temperature and rainfall can cause rapid fluctuations
of mean level, that can happen a the scale of the half-century, with average levels betwen 367 and
372 m at least. On the other hand, highest lake-levels during prehistoric and Roman times were
probably caused by geological accidents, occurring downstream at the junction between the Rhone
and Arve River in Geneva.
Bassin du Petit-Lac bas niveau
du Léman au Bronze final
Geneva
Bassin du Petit-Lac haut niveau
du Léman à l'époque romaine
Geneva
5 km
Around 1000 yrs BC (3000 BP), at the time of pile
dwellings settlements, the lake level lies below
369 m asl.
5 km
During Roman times (58 BC to 476 AD) , the lake
level lies around 375 m asl (thus higher than today).
Low lake-level during summer 1921 (370.85 m), with view on the coastal bank with boat lying on it. In the
background : Chillon, Montreux and Clarens. La Patrie Suisse no 719, 13 avril 1921, p. 91.
22
11. Holocene vegetation and landscape history
Landscape of Geneva Bay at the time of the glacial retreat (approximately 15'000 years BC/ 17'000
BP). The hill of Geneva, a glacier-linked feature, emerges as a peninsula. Boulders are deposited on
the edges and at the bottom of the lake like the «Pierres du Niton», which were transported from the
Mont-Blanc area to Geneva by the Arve glacier. The lake level lowers progressively, approximately to
470, 430 m, then to 405 m. During the Late Glacial period, fauna, attracted by the grass, appears along
juniper (Juniperus), dwarf birch (Betula nana) and other rare pioneer vegetation. In this context, we
can imagine that the last Ice Age mammoths were coming to drink waters of the Geneva Bay. Around
14'000 BC the Rhone glacier left the lake basin with a water level 33 to 36 m higher than at present.
(Drawings Yves Reymond, from Corboud et al. 2006, 2008).
Forest of birches and pines in Geneva Bay during the Bølling, approximately 12'000 years BC (14'000
BP). The lake level stabilises at ca. 380 m asl, i.e. 8 m higher than at present. In this period, the first
phase of steppe reforestation happens, first with trees and shrubs, then birches settle. During Allerød,
(between 12'000 to 11'000 BC) dense pine and birch forests establish. During the climate cooling of
Younger Dryas (between 11'000 to 9500 BC) herbaceous and shrub vegetation increase with linked
forest opening.
23
Landscape of the Geneva Bay at the beginning of the middle Neolithic (Younger Atlantic). A forest of
leafy trees, mixed oak grove, develops. The lake-level is approximately 372 m asl, like at present. At
that time, the mixed oak forest gradually loses ground and is replaced by beech and fir trees. From the
Neolithic (5000 to 4000 years BC), the presence of man becomes more evident, with remains of pile
dwellings prehistoric villages in the Geneva Bay and with a settlement found beneath the SaintGervais church. The discovery of cultivated cereal pollens, dated from the middle of the fifth
millennium BC, confirms agriculture practices. The presence of Neolithic farmers-breeders in the area
is attested at least around 4500 years BC.
rive ac
tuelle
During the Late Bronze Age, around 1000 BC, pile dwellings occupations are numerous. During the
lake low levels (about 3 m lower than at present) the Rhone river didn't flow out of the bay of Geneva.
Several villages were built next to others on the «Banc de Travers», a sandy and clayed shoal. At the
end of Bronze Age, the vegetation is enriched by new species such as white alder, maple and ash. The
forest extends, with a majority of oaks, hazel trees, as well as beeches. On surfaces released by water,
willows colonize the ground, while flooded banks are occupied by reeds which leave a little higher, for
bordering alders and ashes forest.
24
12. Human occupation of the Geneva basin, relations with the environment
After the glacial retreat, the pioneer herbaceous vegetation occupies the territory of the Geneva basin
which is favourable to the passage of reindeer and wild horses. This fauna attracts the first Upper
Paleolithic hunters in shelters under blocks collapsed from the Salève limestone cliffs. In addition to
reindeer and horse, Magdalenian of Veyrier were hunting many additional species such as hares,
marmots and ibex. Overall, with the climatic optimum of the Atlantic, at ca. 4500 BC, human influence
is growing in Geneva region.
As the forest occupies much of the plain, first farmers and stock breeders clear woodland with polished
stone axes to create proper land for agriculture. First pile dwellings settlements are known at 4000 BC
and certainly linked to a significant decrease in the lake-level.
The next climate phase (Subboreal, 3400 to 800 BC) corresponds to the final Neolithic and the Bronze
Age human occupations. The western part of the Geneva basin is still dominated by mixed oak forest.
Changes in the lake-level induce village displacements, from terraces overlooking the lake to coastal
banks, emerged during low waters (Corboud 1998 ; 2009).
Model of a pile dwelling settlement, during the Late Bronze age (ca. 1050-850 BC)
1. Between 1200 and 1100 BC, a significant drop of lake-level,
known throughout all Swiss Plateau lakes, also happens in Lake
Geneva. This important regression results in a lowering of at
least 3-m down, to 369 m. In Geneva Bay, two villages are built
and occupied as early as 1067 BC (Corboud 2003).
377 m
376
375
373
372
371
370
369
368
1
377 m
2. Most probably, Bise windstorms damage initial buildings which
require the building of an extra palisade against wind waves, a
few meters away from houses toward the lake at around 993 BC.
During summers, when waters are particularly high, these fences
act as protection for the slightly raised house floors from flooding
by strong wind waves.
376
3. Before 858 BC, the lake-level becomes higher during summer.
In the following years, the lake transgression is too great to allow
further human occupation, villages are abandoned and
constructions are quickly destroyed by severe Bise windstorms.
The level of Lake Geneva then reaches an altitude of 370 to 371
m asl.
375
373
372
371
370
369
368
2
377 m
376
375
4. The level of water still regularly rises. Each storm demolishes
ancient architectural structures and erodes remaining vertical
piles. Soon, all construction elements are dispersed by waves,
excepted piles which are firmly planted in compacted clayed
sediment.
373
372
371
370
369
368
3
377 m
At present in Geneva Bay, with the artificial lake-level
management at ca. 372 m asl, only wooden piles resist wave
erosion. But they are often broken at the sediment level and are
accompanied by only few archaeological bronze and stone
objects, which concentrate at the interface between sand and
glaciolacustrine clay layers.
In 2011, the prehistoric pile dwellings around the Alps were
nominated as UNESCO World Heritage. A representative selection
of 111 prehistoric pile dwellings sites from six countries (France,
Germany, Austria, Italia, Slovenia and Switzerland) is listed by
UNESCO.
376
375
373
372
371
For more information :
www.palafittes.org
370
369
368
4
25
Organisation
des Nations Unies
pour l’éducation,
la science et la culture
Sites palafittiques préhistoriques
autour des Alpes
Patrimoine mondial depuis 2011
13. Construction of Geneva city with local georesources
Underwater carry for Molasse mining used until 1700. Collex-Bossy, La Pierrière. Picture by Service
du cadastre Genève.
Molasse extraction in botanical garden of Geneva in 2010. Picture Mello & Fils SA, Carouge.
26
Geneva Harbor with limestone transported on boat from Meillerie (June 1899). BGE, Centre
d'iconographie genevoise.
The Palais Wilson, a former hotel on the right bank of the Geneva Bay, was built between 1873 and
1875 with Molasse sandstone. From 1920 to 1936, it became headquarters of the 'League of Nations',
until they moved to the new Palais des Nations. At present, the Palais Wilson serves as Office of the
High Commissioner for Human Rights. Photo taken between 1905 and 1919.
BGE, Centre d'iconographie genevoise.
27
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29
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