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Sedimentation in isolated glaciomarine embayments during glacio-isostatically induced relative sea level fall (northern Champlain Sea basin)


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The nature of glaciomarine sediments deposited during ice margin retreat can vary according to physiographic setting and relative sea level fluctuations. To understand the effects of these two parameters on sedimentation, we analyzed the sediment records of four lakes located within former isolated glaciomarine embayments of the northern Champlain Sea basin. These lakes were initially inundated by marine water of the Champlain Sea, following deglaciation, and have subsequently experienced basin isolation owing to glacio-isostatic rebound. Three of these lakes reveal a common litho- and acoustic stratigraphic succession, characterized by an IRD-free glaciomarine to marine facies consisting of homogeneous to faintly laminated clayey silts grading into well-laminated silts with rapidly deposited layers. These two units recorded the transitional environment from glaciomarine sedimentation below multiyear shorefast ice to increased terrestrial runoff and rapid glacio-isostatic rebound once the ice margin retreated inland. During ice margin retreat, relative sea level fell concomitantly resulting in the deposition of coarser sediments in marine embayments. Upon the complete retreat of the ice margin, the supply of terrestrial sediments diminished and lake isolation, driven by relative sea level fall, led to higher biogenic content and increased bioturbation. This study provides a framework for sedimentation in isolated glaciomarine embayments which differs from deep-water sedimentation owing to the presence of shorefast sea-ice and their protected location from major ice-stream outlets.
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Sedimentation in isolated glaciomarine embayments during
glacio-isostatically induced relative sea level fall (northern
Champlain Sea basin)
Alexandre Normandeau, Patrick Lajeunesse, Annie-Pier Trottier, Antoine G. Poiré, and Reinhard Pienitz
Abstract: The nature of glaciomarine sediments deposited during ice margin retreat can vary according to physiographic setting
and relative sea level fluctuations. To understand the effects of these two parameters on sedimentation, we analyzed the
sediment records of four lakes located within former isolated glaciomarine embayments of the northern Champlain Sea basin.
These lakes were initially inundated by marine water of the Champlain Sea, following deglaciation, and have subsequently
experienced basin isolation owing to glacio-isostatic rebound. Three of these lakes reveal a common litho- and acoustic strati-
graphic succession, characterized by an IRD-free glaciomarine to marine facies consisting of homogeneous to faintly laminated
clayey silts grading into well-laminated silts with rapidly deposited layers. These two units recorded the transitional environ-
ment from glaciomarine sedimentation below multiyear shorefast ice to increased terrestrial runoff and rapid glacio-isostatic
rebound once the ice margin retreated inland. During ice margin retreat, relative sea level fell concomitantly resulting in the
deposition of coarser sediments in marine embayments. Upon the complete retreat of the ice margin, the supply of terrestrial
sediments diminished and lake isolation, driven by relative sea level fall, led to higher biogenic content and increased biotur-
bation. This study provides a framework for sedimentation in isolated glaciomarine embayments which differs from deep-water
sedimentation owing to the presence of shorefast sea-ice and their protected location from major ice-stream outlets.
Résumé : La nature des sédiments glaciomarins déposés durant le retrait des marges glaciaires peut varier selon le contexte
physiographique et les fluctuations du niveau marin relatif. Afin de comprendre les effets de ces deux paramètres sur la
sédimentation, nous avons analysé les sédiments de quatre lacs situés dans d’anciennes baies glaciomarines isolées de la partie
nord de la mer de Champlain. Ces lacs étaient initialement inondés par de l’eau marine de la mer de Champlain, après la
déglaciation, pour ensuite former des bassins isolés en raison du soulèvement glacio-isostatique. Trois de ces lacs révèlent une
séquence lithostratigraphique et acoustique commune caractérisée par un faciès glaciomarin a
`marin sans débris glaciels
consistant en des silts argileux homogènes a
`finement laminés passant progressivement a
`des silts bien laminés avec des couches
déposées rapidement. Ces deux unités témoignent d’un milieu de transition entre une sédimentation glaciomarine sous de la
glace de rive pluriannuelle a
`un ruissellement terrestre accru et un soulèvement glacio-isostatique rapide après que la marge
glaciaire se soit retirée vers l’intérieur des terres. Durant le retrait de la marge glaciaire, le niveau marin relatif a baissé,
entraînant le dépôt de sédiments plus grossiers dans des baies marines. Une fois le retrait de la marge glaciaire terminé, l’apport
de sédiments terrestres a diminué et l’isolement des lacs, causé par la baisse du niveau marin relatif, s’est traduit par une teneur
en matières biogéniques plus importante et plus de bioturbation. L'étude fournit un cadre pour la sédimentation dans des baies
glaciomarines isolées qui se distingue de la sédimentation en eau profonde en raison de la présence de glace de rive marine et
de leur emplacement protégé d’importants exutoires de coulées de glace. [Traduit par la Rédaction]
Lake basins in formerly glaciated terrains can provide a contin-
uous and high-resolution record of both abrupt events and grad-
ual changes in sedimentation (Hodder et al. 2006) that can be used
to reconstruct paleo-climates (e.g., Gajewski et al. 1997;Besonen
et al. 2008), paleoseismicity (e.g., Doughty et al. 2014;Brooks 2016;
Lajeunesse et al. 2017), and paleo-environments (Dix and Duck
2000;Chapron et al. 2007) from deglacial to modern times. Addi-
tionally, lake sediments are sometimes used to document envi-
ronmental changes related to past sea level transgressions and (or)
regressions (e.g., Snyder et al. 1997;Zwartz et al. 1998;Hutchinson
et al. 2004;Nutz et al. 2013;Narancic et al. 2016). During regres-
sions, topographic basins originally located below sea level can be
isolated from marine water and record a transition towards a
limnic environment.
The typical sedimentary succession observed in recently degla-
ciated marine basins consists of, from bottom to top, (i) ice-contact
diamicton, followed by (ii) laminated mud and sand with ice-
rafted debris (IRD), grading upwards into (iii) massive bioturbated
muds (Syvitski 1993;Syvitski and Praeg 1989;Andrews et al. 1991;
Josenhans and Lehman 1999;St-Onge et al. 2008;Duchesne et al.
2010). This succession, which is attributed to glaciomarine set-
tings, is viewed as a transition from ice-proximal to ice-distal dep-
ositional environments. It mainly reflects a decrease in transport
Received 4 January 2017. Accepted 4 June 2017.
Paper handled by Associate Editor Alan Trenhaile.
A. Normandeau. Geological Survey of Canada – Atlantic, 1 Challenger Drive, Dartmouth, NS B2Y 4A2, Canada; Centre d’études nordiques and
Département de géographie, Université Laval, 2405 rue de la Terrasse, Québec, QC G1V 0A6, Canada.
P. Lajeunesse, A.-P. Trottier, A.G. Poiré, and R. Pienitz. Centre d’études nordiques and Département de géographie, Université Laval, 2405 rue de la
Terrasse, Québec, QC G1V 0A6, Canada.
Corresponding author: Alexandre Normandeau (email:
Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink.
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capacity and sedimentation rates as well as an increase in biotur-
bation resulting from an increase of macrofauna (benthos) as the
ice margin retreats from the region (Ó Cofaigh and Dowdeswell
2001). However, this typical succession reflects sedimentation at
ice-stream outlets and in deep-water environments of glacial seas
or fjords and do not necessarily reflect glaciomarine sedimenta-
tion in isolated bays of the nearshore environment. The latter is
yet to be properly described.
Recent studies by Normandeau et al. (2013) and Nutz et al. (2013)
were conducted on the acoustic stratigraphy of lakes of southern
Québec located below marine limit, i.e, below the highest pre-
served shoreline from the Late Pleistocene transgression. These
studies reconstructed the impact of deglaciation on the nature
and pattern of sedimentation during a transition from a marine to
a lacustrine environment. During the retreat of the Laurentide Ice
Sheet (LIS), vast regions of northeastern America were inundated
by large proglacial lakes (e.g., Agassiz-Ojibway, Vermont) and seas
(e.g., Laflamme, Champlain, Goldthwait). These glacially influ-
enced lakes and seas acted as sediment traps and now contain
archives of past environmental change. The Laflamme and Gold-
thwait seas remained large bodies of water throughout the Holo-
cene (Lake Saint-Jean and the St. Lawrence Estuary, respectively)
that are easily studied using conventional hydroacoustic tech-
niques (Syvitski and Praeg 1989;Praeg et al. 1992;Josenhans and
Lehman 1999;St-Onge et al. 2008;Duchesne et al. 2010;Nutz et al.
2013). Conversely, the Champlain Sea retreated from the region
and was replaced by the St. Lawrence River hydrographic system,
which altered its sedimentary record. Nevertheless, small lakes on
its northern margin remained mostly unaltered by Holocene ero-
sion and geomorphic processes, allowing them to hold a complete
and continuous sedimentary record of the evolution from the
Champlain Sea during deglaciation to the establishment of mod-
ern lakes. They thus yield unaffected records of a marine to la-
custrine deglacial succession deposited during glacio-isostatic
rebound. In this respect, they hold a continuous record of glacio-
marine to lacustrine sedimentation in isolated bays.
In this study, we report on the geomorphology, stratigraphy,
and sedimentary infill of four lakes (Maskinongé, Mékinac, Aux-
Sables, and St-Joseph) located below marine limit in the northern
sector of the Champlain Sea basin to document the sedimentary
succession and architecture of isolated glaciomarine embayments
as well as the transition from a deglacial to a postglacial sequence
during a forced regression. The main objectives of this study are to
(i) examine the spatial distribution of sedimentation in lakes lo-
cated along the northern St. Lawrence River Valley between Qué-
bec City and Montréal, (ii) infer environmental conditions that led
to the deposition of contrasting sedimentary units, and (iii) pro-
vide a conceptual framework for the deposition of sediments in
isolated glaciomarine embayments that are progressively isolated
by relative sea level (RSL) fall to become lakes.
Regional setting
Location of lakes
The studied lakes are located along the southern limit of the
Laurentian Highlands (Fig. 1) near the northern limit of the former
Champlain Sea basin (marine limit ≤250 m above sea level (asl)).
Lake Maskinongé is the southernmost lake of the study area
(46.32°N, 73.39°W) and is located 60 km west of Trois-Rivières at
140 m asl (Table 1). Lake Mékinac is the northernmost lake
(47.05°N, 72.68°W) and is located at the limit of the Champlain Sea
basin, at an elevation of 165 m (Table 1). Lake Aux-Sables is located
between Québec City and Trois-Rivières (46.88°N, 72.37°W) at
150 m asl, while the easternmost Lake St-Joseph (46.92°N, 71.64°W)
is located 30 km northwest of Québec City at 160 m asl (Table 1).
The lakes are considered as gravity-driven water bodies because
their morphology is dominated by mass movement deposits in-
stead of aeolian or deltaic landforms (Nutz et al. 2017). Neverthe-
Table 1. Characteristics of the four studied lakes.
depth (m)
area (km
Maskinongé 36 10.2 140
Mékinac 145 23 165
Aux-Sables 42 5.2 150
St-Joseph 36 11.3 160
Fig. 1. (A) Location of the studied lakes in relation to the Champlain Sea limit and the Younger Dryas moraines. Red stars represent the study
lakes while dashed red lines represent major morainic systems related to the Younger Dryas (YD) episode. (B–E) Approximate marine limit of
the Champlain Sea around lakes (B) St-Joseph, (C) Aux-Sables, (D) Mékinac, and (E) Maskinongé. [Colour online.]
1050 Can. J. Earth Sci. Vol. 54, 2017
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less, their sedimentary infill provides valuable insights into the
deglacial dynamics of southern Québec.
Deglacial history
The epicontinental Champlain Sea is of glacio-isostatic origin
and lasted from 13 to 10.6 ka cal BP (Richard and Occhietti 2005). It
extended over the entire St. Lawrence River Valley, from Québec
City to the Lake Champlain Valley and west to the Ottawa Valley
(Cronin 1977;Gadd 1988)(Fig. 1), although it never covered the
entire basin simultaneously (Elson 1969). Indeed, while the ma-
rine limit along the southern shore of the Champlain Sea was
occupied almost instantly at the onset, the northern limit was
reached later and is widely diachronic (Parent and Occhietti 1988).
The diachronic aspect of the northern marine limit is due to a
standstill of the LIS margin after a rapid retreat across the St.
Lawrence River Lowlands and to variations in RSL owing to glacio-
isostatic rebound rates of ca. 12 cm/yr during land emergence
(Hillaire-Marcel and Occhietti 1980).
Prior to the invasion of the Champlain Sea, the St-Maurice lobe
likely blocked glacial Lake Candona that originated from the co-
alescence of glacial lakes Iroquois and Vermont (Occhietti and
Richard 2003)(Fig. 2). When this lobe retreated north-west, glacial
Lake Candona drained into the Goldthwait Sea and marine water
invaded the St. Lawrence valley at ca. 13 ka cal BP (Richard and
Occhietti 2005;Cronin et al. 2008). During the Younger Dryas (YD)
cold episode (12.9–11.4 ka cal BP; Parent and Occhietti 1988;
Occhietti 2007) the LIS margin was in contact with the Cham-
plain Sea along most of the northern basin and deposited the
St-Narcisse morainic complex, except near Québec City where it was
located ≥50 km inland (Fig. 1)(Lasalle and Elson 1975;Occhietti
2007). Surficial waters of the Champlain Sea were brackish at that
time while the deeper waters were more saline, except near the
ice margin, according to isotopic composition of sediments
(Hillaire-Marcel 1981). With time, the sea became fresher owing to
the input of glacial meltwater. The retreat of the LIS margin from
the St-Narcisse moraine then led to the invasion of the river val-
leys and to subsequent regression over the entire basin. In the
Québec City region, the Champlain Sea retreated from the Lake
St-Joseph sector at ca. 11–10.5 ka cal BP (Lamarche 2011).
According to Parent and Occhietti (1988), the Champlain Sea
inundation/regression deposited (i) glacial diamictons, (ii) glacio-
marine sediments, and (iii) marine sediments (Fig. 2). Glacial sed-
iments consist mainly of till, whereas glaciomarine sediments
consist primarily of proximal outwash fans and clayey diamic-
tons. Glaciomarine sediments also consist of massive to laminated
clays, silts, and sands and were deposited by suspension settling of
glacial meltwater plumes. Marine sediments consist of stratified
sands and gravels related to littoral sands, alluvial fans and bars
(Prichonet 1988). Some coastal and estuarine regions consist of
sediments deposited in brackish waters (Parent and Occhietti
Data and methods
High-resolution bathymetric maps of lakes Maskinongé, Mékinac,
Aux-Sables, and St-Joseph were produced from hydroacoustic sur-
veys using a GeoAcoustics Geoswath Plus compact (250 kHz)
swath sonar deployed on a 4.5 m inflated boat and a Reson Seabat
8101 on board R/V Louis-Edmond Hamelin. The Geoswath system
was coupled with a SMC motion sensor and a Hemisphere V101
DGPS (60 cm precision), while the Reson system was coupled
with a IXSea Octans III motion sensor and the same Hemisphere
V101 DGPS. GS+®and Hypack®softwares were used for naviga-
tion and data acquisition. Sub-bottom data were acquired using a
Knudsen 3212 echosounder operating at a frequency of 3.5 and
12 kHz, also deployed alternately on the inflated boat. This cover-
age allowed a detailed analysis of the geometry, extent, and dis-
tribution of the acoustic units present in the lakes (Fig. 3). The
sub-bottom data were interpreted using The Kingdom Suite®soft-
ware. The picked lines were exported to ArcGIS®to produce
isopach maps of the units.
Sediment cores were collected in the lakes using an Aquatic
Research®percussion corer. The coring sites were carefully se-
lected from the analysis of the sub-bottom profiles to sample
outcropping units. The cores were analyzed through a Siemens
SOMATOM Definition CT-Scan, allowing for a non-destructive
rapid visualization of sedimentary structures (St-Onge and Long
2009). CT-numbers (HU) were extracted using the ImageJ®soft-
ware and proved to have an excellent correlation with density
measurements (Fortin et al. 2013). The CT-numbers were thus con-
verted into bulk density values (
). The HU values were first con-
verted into positive CT-numbers (Amos et al. 1996)
(1) CT 1000 HU
The bulk density estimates were then obtained using the following
equation from Yamada et al. (2010):
t(g/cm³) 1.32CT 0.39 (R0.95)
Cores were then opened and visually described. Magnetic suscep-
tibility, using a Bartington MS2E surface scanning sensor, was
measured on the cores at 1 cm intervals. The cores were then
sampled for grain-size measurements at 10 cm intervals and ana-
lyzed using a Horiba laser sizer. Samples were diluted into a
Fig. 2. History of deglaciation and Champlain Sea inundation in
Québec and the lower St-Maurice (Trois-Rivières area) (modified
from Parent and Occhietti 1988). Ages are from Richard and
Occhietti (2005) and Occhietti et al. (2001). The St-Narcisse moraine
was not in contact with the Champlain Sea near Québec City while
it was in the lower St-Maurice region.
Normandeau et al. 1051
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calgon solution for 24 h, submitted to an ultrasound bath, and at
least three runs were averaged to obtain reliable grain-size distri-
bution data. Statistical parameters of the sediments were then
obtained using the Gradistat software (Blott and Pye 2001). End-
member modelling analysis (EMMA) was used to quantify the
grain-size characteristics of the samples following Dietze et al.
(2012). The EMMA algorithm unmixes grain-size distributions to
extract the processes and (or) the sources of sediment deposited in
the lakes. Three end-members were sufficient to describe 90%
of the grain-size variability.
The absence of organic material within the Champlain Sea
sediments precluded extensive dating in this study. A pine tree
twig was sent to the Radiochronology Laboratory of the Centre
d’études nordiques (Université Laval) for pre-treatment and prep-
aration of the sample. The dating itself was done by accelerator
mass spectrometry (AMS) at the Keck Carbon laboratory of the
University of California (UCIAMS). The only date obtained was
calibrated with Calib 7.0 (Stuiver and Reimer 1993) using the
IntCal13 database (Reimer et al. 2013) and is shown with 2in
Table 2.
Sediments sampled in different units of the Lake St-Joseph and
Aux-Sables cores were used to examine fossil diatom assemblage
composition for the reconstruction of past aquatic conditions
(fresh versus brackish or salt water) that prevailed when sedi-
ments were deposited. In the Aquatic Paleoecology Laboratory at
Centre d’études nordiques, fossil diatoms were extracted from
core sediment samples with hydrogen peroxide (30% H
) diges-
tion techniques, and microscope slides were mounted using Na-
phrax resin (Pienitz et al. 2003). Identification of diatom species
was made using a Leica DMRB microscope at 1000× magnification
under oil immersion objectives. The main reference floras used
were Campeau et al. (1999),Fallu et al. (2000), and Pienitz et al.
(2003). Diatoms were generally rare within the sediments and a
quantitative analysis of the diatom remains was not possible.
Therefore, this diatom analysis was a qualitative investigation
Fig. 3. Sub-bottom profile lines collected in the four studied lakes: (A) Maskinongé, (B) Mékinac, (C) Aux-Sables, and (D) St-Joseph.
(E–J) Stratigraphic context of cores presented in Figs. 6,7,9, and 10. [Colour online.]
Table 2. Accelerator mass spectrometry
C age of the dated material
collected in Lake Aux-Sables.
Core name
C age
age (cal BP) Material Laboratory No.
LAS2013-03P 120 8930±30 9920–10 195 Twig UCIAMS-137209
Note: Calibration was done using the IntCal13 curve and is presented with 2.
1052 Can. J. Earth Sci. Vol. 54, 2017
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where the presence or absence of species allowed the character-
ization of paleo-environments.
Lake morphology
Lake Maskinongé is a relatively shallow lake with a maximum
depth of 36 m. Two main rivers, the Mastigouche and Matambin
rivers, flow into the lake along its northern shore. The shores are
generally steep (≥10°), favouring the presence of mass movement
deposits (MMD) (Fig. 4A). A particularly large MMD is observed in
the eastern part of the lake, near the Maskinongé River, the lake
outlet. Otherwise, most of the central part of the lake is undis-
turbed and relatively flat.
Lake Mékinac is a fjord-lake with steep sidewalls and a maxi-
mum depth of 145 m (Fig. 4B). Its morphology can be divided into
two parts: (1) the southern sector of the lake, consisting of a cha-
otic surface typical of hummocky moraines with two apparent
sills; and (2) the northern part of the lake, presenting a smoother
surface reflecting draping sedimentation. Two rivers also dis-
charge into the northern sector of the lake and contribute to the
shallower and smoother bottom morphology by delivering sedi-
ments to the basin plain. This draping sedimentation is disturbed
by MMDs to the north and by crescentic bedforms on the Du
Milieu River delta that are interpreted as cyclic steps (e.g.,
Normandeau et al. 2016).
Lake Aux-Sables is also a relatively shallow lake with a maxi-
mum depth of 42 m (Fig. 4C). Half of the lake is disturbed by
MMDs, especially in the southern half where slopes are steeper.
Residual mounds are observed in the sector affected by the largest
mass movement of the lake (Fig. 4C). Other small-scale mass move-
ment morphologies are present on the shores and on the different
plateaus bordering the lake. Lake Aux-Sables does not receive
considerable amounts of sediment from its tributaries, which are
mostly small rivers and streams.
Lake St-Joseph is composed of two very different basins. The
northern basin has a maximum depth of 36 m, whereas the south-
ern one has a maximum depth of 12 m (Normandeau et al. 2013)
(Fig. 4D). The southern basin has a smooth morphology, whereas
the northern one is disturbed by MMDs on half of its surface.
Residual mounds are also apparent within the MMDs.
Acoustic and litho-stratigraphy
Unit 1 is the lowermost unit and underlies the entire sedimen-
tary succession of the lakes. Acoustic penetration is limited to
absent and its surface morphology can be smooth or rugged
(Table 3;Fig. 5), representing the acoustic basement. This unit was
not cored during our surveys.
Unit 2 is a transparent acoustic facies and is observed only in
Lake St-Joseph (Table 3). It has a basin-fill geometry with a sharp
upper reflection. It is observed exclusively in the deeper parts of
the lake basin (Fig. 5D). Its thickness varies according to the depth
of the basin that it fills (0–10 m thick). This unit was not cored.
Unit 3 is a generally transparent acoustic facies containing a few
low amplitude reflections (Table 3) and is observed primarily in
lakes Aux-Sables and St-Joseph and appears to be present in Lake
Maskinongé (Fig. 5). The upper boundary of Unit 3 is sharp and
consists of a high amplitude reflection. It has a draping geometry
and has a mean thickness of 4 m, but it can reach greater thick-
nesses locally (7 m) at greater lake depths. Unit 3 was cored in
lakes St-Joseph and Aux-Sables and reveals similar lithofacies
(Fig. 6). In Lake St-Joseph, it consists of massive dark grey clayey
silt; in Lake Aux-Sables, it consists of fine and faint parallel lami-
nated dark grey clayey silt. The faintly laminated sediments are
gradational and individual laminae are generally ≤5 mm thick.
Density (derived from CT-numbers) and magnetic susceptibility
are relatively low and homogeneous in these two lakes, with val-
ues averaging 1.85–2 g/cm
(700–800 HU) and 400 SI, respectively
(Fig. 6). Grain-size distributions are mostly unimodal in the clayey
silt fraction (Fig. 7). EMMA reveals that EM1, centered on 5–10 m
is predominant in this unit. No diatoms were observed in core
LAS05P (Lake Aux-Sables) while clastic particles were abundant. In
core LSJ-01P (Lake St-Joseph), one sample revealed the presence of
elongate benthic freshwater diatoms (Fragilaria ssp.) (Fig. 6B).
Unit 4 consists of high amplitude reflections and drapes con-
formably the underlying Unit 3. These high-amplitude reflections
are present in the four lakes (Fig. 5). The thickness of Unit 4 is
generally between 3 and4m(Table 3) but reaches 7 m in Lake
St-Joseph and 30 m in the central part of Lake Aux-Sables (Fig. 8).
Unit 4 is not present throughout the entire lake floor, as it is
absent in some cases on steep nearshore slopes. Additionally, sev-
eral transparent to chaotic lens-shaped facies interpreted as
MMDs or rapidly deposited layers (RDL) are observed within this
unit (Figs. 5D,8). For example, nine MMDs are stacked within
Unit 4 in Lake Aux-Sables, increasing the thickness of the unit
to >20 m in the deeper areas (Fig. 8). Unit 4 was cored in lakes
Maskinongé, Aux-Sables, and St-Joseph and consists of grayish silt
and clay rhythmites responsible for a wide range of density and
magnetic susceptibility values (Fig. 6). Density values vary be-
tween 1.7 and 2.25 g/cm
(600 and 1000 HU) while magnetic sus-
ceptibility varies between 500 and 1700 SI. Grain-size distributions
are mostly bimodal, especially in Lake Aux-Sables, reflecting the
laminated nature of the sediments (Fig. 7). EM1, centered on
5–10 m, and EM2, centered on 20 m, are predominant within
this unit with the occasional presence of EM3 where coarser par-
ticles are present (centered on 40–70 m). The grain-size distribu-
tions generally have a broad mode centred on 5–10 m and a
narrower mode centred on 40–70 m, indicating fine clayey silt
laminae alternating with medium to coarse silts. The grain size
(D50) also shows an upward coarsening trend when compared to
Unit 3 (Fig. 6). CT-scan imagery reveals erosional surfaces, dis-
turbed bedding, and coarse layers of gravels within the unit
(Fig. 9). Diatoms are rare or absent within this unit, which is
mainly composed of clastic material. However, Diploneis smithii
ssp. and sponge spicules, typically observed in brackish waters,
were observed at the base of core LSJ-02P (Fig. 10B).
Unit 5 consists of medium amplitude acoustic reflections and
drapes the underlying unit (Table 3;Fig. 5). It is generally less than
2 m thick and in some cases cannot be differentiated from the
underlying Unit 4. Cores LAS-03P and LSJ-02P reveal faintly lami-
nated gray sediments with density and magnetic susceptibility
values decreasing upwards from 1.4 to 1 g/cm
(400–40 HU) and
400 to 10 SI (Fig. 10). Laminations are clearly visible at the base of
the unit and gradually become faint to absent upcore. The lami-
nated facies is replaced upcore by a more homogeneous one. Con-
sequently, the grain-size distribution becomes more unimodal in
the medium silt fraction (Fig. 10). End-members vary from one
core to the other, where in Lake Aux-Sables EM2 is predominant,
whereas in Lake St-Joseph EM1 and EM2 are present. Diatom as-
semblages identified within this unit are diverse and composed of
brackish water (e.g., Rhopalodia ssp., Tabularia ssp.) and freshwater
(e.g., Fragilaria ssp., Achnanthes ssp., Tabellaria ssp.) species. A pine
tree twig sampled at 120 cm depth in core LAS-03P (Lake Aux-
Sables) provided an age of 10 045 cal BP (Table 2), which indicates
that the deposition of Unit 5 occurred during the early Holocene.
The uppermost Unit 6 observed in the four lakes consists of
transparent to low amplitude reflections (Table 3;Fig. 5). It is
generally less than 4 m thick and conformably drapes the under-
lying units. The top of this unit (= lake bottom) is in some cases
difficult to identify on the subbottom profiles owing to the low
acoustic impedance of the surficial sediments (Fig. 5C). In Lake
Maskinongé, this unit is composed of low amplitude reflections
whereas it is completely transparent in Lake Aux-Sables and in the
southern basin of Lake St-Joseph. Sediment cores reveal homoge-
neous to faintly laminated silt (Fig. 10). Density and magnetic
susceptibility values are very low and homogeneous in the order
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Fig. 4. High-resolution swath bathymetric maps (2–5 m grid resolution) of the four study lakes: (A) Maskinongé, (B) Mékinac, (C) Aux-Sables,
and (D) St-Joseph (Normandeau et al. 2013). DS, draping sedimentation; MMD, mass movement deposit; HM, hummocky morphology; D, delta;
RM, residual mound; UB, undisturbed blocks. [Colour online.]
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Table 3. Description and interpretation of acoustic units with their lithofacies description, from oldest (Unit 1) to youngest (Unit 6).
Unit Acoustic properties
thickness (m) Lithofacies description
lithofacies Interpretation
6 Reflection free to medium
amplitude reflections
0–4 Dark brown silt, rich in
organic matter
Drapes lower unit
5 Transitional unit 0–2 Silty laminations transitioning
to dark brown silt, rich in
organic matter
Marine to lacustrine
sedimentation rates
Low to medium amplitude
Drapes lower unit
4 High amplitude reflections 3–4 Gray silt rythmites with
occasional coarse rapidly
deposited layers
sediment deposited
during high
Drapes lower unit
Interbedded with
transparent lenses
3 Reflection-free to low
amplitude reflections
2–4 Massive to finely laminated
clay and silt
sediment deposited
below multiyear ice
Drapes lower unit
2 Reflection-free 0–10 N/A N/A Pre-Champlain Sea
Basin fill geometry
Upper erosive contact?
1 Absence of penetration,
irregular morphology
N/A N/A N/A Glacial diamicton and
(or) bedrock
Fig. 5. Typical acoustic stratigraphy succession of the study lakes: (A) Profile from Lake Maskinongé. Unit 3 (U3) could only be observed near
the shores as acoustic penetration is not sufficient at the centre of the lake. (B) Profile from Lake Mékinac illustrating Unit 4 and Unit 5.
(C) Profile from Lake Aux-Sables illustrating the main units. (D) Profile from Lake St-Joseph illustrating the six main units. Note the presence
of Unit 2 as well as mass movement deposits (MMDs) and rapidly deposited layers (RDLs) within Unit 4. [Colour online.]
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of 1 g/cm
(40 HU) and 0–20 SI, respectively (Fig. 10). Grain-size
distributions are mostly unimodal with D50 between 30 and
40 m. EM2 and EM3 generally predominate within this unit.
Diatom assemblages in this facies are species-rich and well pre-
served. Planktonic and benthic freshwater diatoms are observed,
including the centric plankters Cyclotella bodanica and Aulacoseira
ssp., as well as benthic and epiphytic Pinnularia ssp., Surirella ssp.,
Diploneis ssp., and Amphora ssp.
Depositional environments
Although the local chronology of deglaciation differs from one
lake to another according to LIS margin retreat models (Dyke
2004;Richard and Occhietti 2005;Occhietti et al. 2011), the overall
stratigraphy and architecture of their Late Quaternary sediment
infill is similar, indicating comparable sedimentation processes
and sequence of events along the northern margin of the Cham-
plain Sea basin. A conceptual model of sedimentation can thus be
developed for former isolated glaciomarine embayments that
gradually transitioned to limnic environments in the context of
glacio-isostatic rebound (Fig. 11). This conceptual model of sedi-
mentation consists of (i) a glacial phase, (ii) a glaciomarine with
sub-multiyear floating ice sedimentation to open marine phase,
(iii) a transitional postglacial marine to lacustrine phase, and (iv)a
full postglacial limnic phase. Some of these depositional environ-
ments have previously been described by Normandeau et al. (2013)
for the Lake St-Joseph basin. We nevertheless review them and
discuss their similarities and differences between lakes before
presenting a conceptual framework of sedimentation for isolated
glaciomarine embayments that are progressively isolated from
the sea.
Glacial phase
The pre-deglacial phase of the lakes is not well recorded nor
constrained. Its interpretation is solely based on the acoustic stra-
tigraphy analysis because sediment cores could not be collected.
Based on the absence of acoustic penetration at the base of the
sedimentary successions, the basal Unit 1 is interpreted as a de-
posit consisting of coarse-grained sediments that could be either
glacial diamicton and (or) glaciofluvial sands.
In Lake Mékinac, the morphological expression of the southern
part of the lake is typical of hummocky terrains consisting of till
(e.g., Eyles et al. 1999)(Fig. 4B). The fjord heritage of this lake and
the hummocky terrain suggest that during the retreat of the LIS,
an ice tongue remained in the Mékinac Valley. Sills are present on
the lake bottom where the valley narrows, suggesting that the ice
tongue stabilized in these sectors during its retreat. As the lake is
located at marine limit and residual ice likely remained in this
valley during deglaciation, it appears that the Champlain Sea may
have invaded only the southern part of the lake basin for a limited
time while it did not invade its northern part.
In Lake St-Joseph, the basin-fill geometry of acoustically trans-
parent Unit 2 was previously interpreted as homogeneous fine-
grained sediment (mud) deposited prior to final deglaciation
(Normandeau et al. 2013). It could thus consist of preserved sedi-
ments dating from an earlier phase of marine invasion, for exam-
ple, preceding the LIS margin stabilization and (or) readvance of
the YD (Occhietti 2007), or it could have been preserved through-
out the entire Wisconsinan glaciation and be of Upper Wisconsi-
nan or Sangamonien age. Unit 2 was not observed in the other
lakes, either owing to the absence of acoustic penetration (e.g.,
Maskinongé, Mékinac) or because they were not preserved owing
to the absence of deep and narrow basins (e.g., Aux-Sables).
Glaciomarine with multiyear shorefast ice to open marine phase
Unit 3 and Unit 4 are observed in lakes Maskinongé, Aux-Sables,
and St-Joseph and are interpreted as having been deposited in a
glaciomarine to marine environment (Fig. 11). In Lake Mékinac,
only Unit 4 is present, which is interpreted as glaciolacustrine in
Fig. 6. Physical sediment properties of Unit 3 and Unit 4 in lakes (A) Aux-Sables, (B) St-Joseph, and (C) Maskinongé. FW, freshwater diatoms.
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The faintly laminated to homogeneous appearance and blanket-
like draping of Unit 3 over the lake floors indicate that the sedi-
ments were deposited through suspension settling potentially
enhanced by flocculation (Normandeau et al. 2013). The presence
of faint laminations in the Aux-Sables cores precludes sediment
reworking by biological activity, hinting at hypoxic conditions
(Jenny et al. 2016). We suggest that these sediments were depos-
ited in a proximal setting relative to the ice margin because mac-
rofauna is unlikely within several hundred metres of the ice
margin (Syvitski et al. 1996), which prevents bioturbation (Jaeger
and Nittrouer 1999). The presence of freshwater diatoms also
Fig. 7. End-member modelling analysis (EMMA) of sediments from
lakes (A) Aux-Sables and (B) St-Joseph. End-member loadings
represent the three end-members used for each lake while the end-
member scores represent the percentage of each end-member in
each sample. The coefficient of determination (r
) represents the
error estimate that compares modeled results to the original
datasets. The EMMA shows an increase in coarse particle up-core in
both lake sequences as well as the prevalence of EM1 and EM2 in
Unit 3 (U3) and Unit 4 and EM2 and EM3 in Unit 5 and Unit 6.
Fig. 8. (A) Isopach map of mass movement deposits (MMDs) within
Unit 4 (U4) in Lake Aux-Sables superimposed on a shaded relief
image of the bathymetry. Note that the isopach map is surrounded
by thick black lines. The important thickness of this unit is due to
the presence of stacked mass movement deposits (B). [Colour
Fig. 9. (A) Erosional surface, (B) disturbed bedding, and (C) rapidly
deposited layer within Unit 4 in Lake Aux-Sables. Location of facies
in Figs. 6 and 10.
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Fig. 11. Depositional environments of sediments in a glaciomarine to lacustrine transition in former embayments of the northern Champlain
Sea basin. (A) Deposition of till below the Laurentide Ice Sheet (LIS); (B) Deposition of faintly laminated mud below multiyear shorefast sea-ice
(Unit 3); (C) Deposition of laminated mud and fine sand during ice margin retreat as well as mass movement deposits (MMDs) owing to glacio-
isostatic rebound (Unit 4); (D) Deposition of lacustrine sediments in modern lakes (Units 5-6). [Colour online.]
Fig. 10. Physical sediment properties of Unit 4 (U4), Unit 5, and Unit 6 in lakes (A) Aux-Sables, (B) St-Joseph, and (C) Maskinongé. BW, brackish
water diatoms; FW, freshwater diatoms.
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suggests dilution by abundant glacial meltwater inputs close to
the ice margin.
Sedimentation in glaciomarine settings can be dominated by
discharge plumes or melting of icebergs (Syvitski et al. 1996). Two
important criteria distinguish both environments: (i) the presence
versus absence of IRD, and (ii) the presence versus absence of
biogenic material (Domack et al. 1995,1999;Pudsey and Evans
2002). Sedimentation controlled by discharge is believed to have
dominated the northern Champlain Sea embayments as no IRD
were observed in any of the cores. The absence of IRD in glacio-
marine settings suggests the presence of a floating ice cover
(Reece 1950;Domack et al. 1995,1999;Dowdeswell et al. 2000;
Smith and Andrews 2000;Pudsey and Evans 2002;Pien´ kowski
et al. 2012), either as multiyear shorefast sea-ice or an ice-shelf
(Fig. 11B). During cooler climate periods, the presence of ice
prevents icebergs from reaching the shores, which in turn sup-
presses IRD in glaciomarine sediments (Dowdeswell et al. 2000;
Pien´kowski et al. 2012). For example, multiyear shorefast sea-ice
currently observed in many High Arctic regions (e.g., Syvitski
et al. 1996;Dowdeswell et al. 2000) is believed to represent condi-
tions analogous to those that prevailed during the Champlain Sea
episode. The absence of icebergs can also be inferred from the
absence of scouring on the acoustic profiles, even at paleo-depths
shallower than 50 m (i.e.,5mofthemodern lake depths). This
absence of scours and IRD contrasts sharply with sediments found
at greater paleo-depths near the St. Lawrence River, for example,
near Québec City (Occhietti et al. 2001). For all the above-
mentioned reasons and because the EMMA reveals the predomi-
nance of EM1 in Unit 3, we interpret EM1 as sediment deposited
below sea-ice.
Two competing hypotheses may thus explain the deposition of
Unit 3: sedimentation below an ice-shelf of below multiyear shore-
fast sea-ice. A floating ice-shelf extends across water from a land-
based glacier. It is bounded upstream by a grounding zone wedge
(GZW), some of which have been identified in the Estuary and Gulf
of St. Lawrence (Syvitski and Praeg 1989;Lajeunesse 2016), but
which have not been identified in the St. Lawrence River Valley to
date. In the immediate regions of the lakes, no GZW were identi-
fied in our study. Assuming the deposition of sediment below an
ice-shelf, the GWZ would have existed along the axis of major ice
streams flowing into the Champlain Sea. Our study lakes are not
located on this direct axis but in isolated bays next to the potential
pathways of ice streams (major glacially carved rivers). The loca-
tion of the lake basins in former embayments would also explain
the presence of muds and the absence of coarse sediment (diam-
icton) related to the basal debris zone of the GZW, corresponding
to a “null zone” as described by Domack et al., 1999. However,
studies on the Antarctic ice-shelves reveal that following cata-
strophic ice break-up, sedimentation returns to coarse diamicton
above the fine silts (Kilfeather et al. 2011;Reinardy and Lukas
2009), reflecting the release of supraglacial debris to the water
column and the release of IRD from the calving ice-front. The
absence of such a clast-rich layer favours the presence of landfast
sea-ice rather than ice-shelf conditions, which would have pre-
vented icebergs from entering the former bays. During the Cham-
plain sea invasion, shorefast sea-ice could have been present
(Harington et al. 2006;Paiement 2007), especially in low-energy,
isolated glaciomarine bays.
The sharp but conformable transition between Unit 3 and Unit 4
indicates an increase in terrestrial runoff to the basins and a rapid
change in suspension settling dynamics reflecting a sudden
change in sea-ice dynamics. Unit 4 is believed to have been depos-
ited when the ice margin retreated inland, allowing an increase in
terrestrial runoff through enhanced glaciofluvial discharge to the
Champlain Sea. Therefore, EM2 and EM3 are interpreted as sedi-
ments originating from river floods that were deposited by sus-
pension settling and underflows (Fig. 11C). The presence of EM2
and EM3 indicates ice-free conditions during the summer months,
whereas the presence of EM1, which is associated with sedimen-
tation below a floating ice cover, reflects the presence of shorefast
sea-ice during winter.
According to the diatom remains, waters were brackish at the
time of deposition of Unit 4, which reinforces the interpretation
of a retreating LIS margin. As the ice margin retreat progressed,
the mixing of glacial meltwater with the Champlain Sea was re-
duced. As a result, coastal waters became more brackish.
Blanket-like draping indicates that Unit 4 was also deposited
primarily by suspension settling (e.g., Gilbert et al. 2002). How-
ever, Unit 4 is also ponded in the deeper lake areas, especially in
Lake Aux-Sables (Fig. 8B), owing to the presence of chaotic to
transparent lenses that suggest sediment deposition when the LIS
margin retreated towards the hinterland. These lenses are inter-
preted as mass movement deposits that were triggered by earth-
quakes related to rapid glacio-isostatic rebound (e.g., Hill et al.
1999;Lajeunesse and Allard 2002;Beck 2009;Brooks 2016)orby
important meltwater discharges during the retreat of the LIS.
Sediment cores collected within this unit also show typical ero-
sional surfaces on the slopes that are associated with mass move-
ments (Fig. 9). Lajeunesse and Allard (2002) and Hill et al. (1999)
showed that MMDs related to glacio-isostatic rebound were fre-
quent during the glaciomarine to marine transition in northern
Québec, which is also where we observe them in our study.
Because the Champlain Sea did not reach the northern Lake
Mékinac basin, the observed high amplitude reflections are rather
associated with glaciolacustrine sediments that are similar in ap-
pearance to glaciomarine sediments in acoustic stratigraphy and
widespread in southeastern Canadian lakes (e.g., Turgeon et al.
2003;Brooks 2016;Lajeunesse et al. 2017).
Transitional phase
As the LIS margin retreated inland, sedimentation rates were
reduced while glacio-isostatic rebound caused relative sea level to
fall over the lake areas. With increasing distance from a glacioflu-
vial sediment source and RSL fall, laminated sediments graded
into massive bioturbated mud (Unit 5), reflecting reduced influence
of meltwater fluxes, lower sedimentation rates, and an increase in
macrofaunal colonization (e.g., Ó Cofaigh and Dowdeswell 2001).
In Lake Aux-Sables, the deposition of Unit 5 occurred at
ca. 10 045 ka cal BP. At that time, brackish and freshwater diatoms
were deposited in the lake, representing the progressive isolation
of the lake which likely occurred earlier during the Holocene
based on known sea levels (Lamarche 2011), yet brackish waters
remained trapped in the lake as it was stratified with a residual
salty dense water at the bottom, until replacement with freshwa-
ter was complete.
Lacustrine phase
The lacustrine phase in each lake is characterized by the depo-
sition of organic-rich gyttja (Fig. 11D). However, the stratigraphy
revealed that these sediments have a transparent acoustic appear-
ance in Lake Aux-Sables and the southern basin of Lake St-Joseph,
whereas they have a laminated appearance in lakes Maskinongé,
Mékinac, and the northern basin of Lake St-Joseph. In these three
latter cases, rivers flowing on a relatively large delta plain deliver
sediments to the lakes during the spring snow melt, which trans-
port coarser sediments to the lake bottom. In contrast, the Lake
Aux-Sables watershed is much smaller and does not contain delta
plain sediments that can be flushed into the lake during floods.
Therefore, the difference in acoustic stratigraphy properties be-
tween lakes is mainly related to the surficial geology of their
watersheds and the rivers delivering clastic sediments. During the
lacustrine phase, large mass movements also occurred, although
infrequently, especially during the late Holocene, suggesting a
common trigger such as earthquakes.
Normandeau et al. 1059
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Sedimentary succession in isolated glaciomarine
embayments during relative sea level fall
The depositional environments described above allow us to
compare the sedimentary succession of isolated glaciomarine
bays to those of deep-water glaciomarine environments. In deep-
water glaciomarine settings, typical deglaciation successions con-
sist of ice-proximal to ice-distal laminated sediments, reflecting
energetic sedimentation dynamic near the ice margin, followed
by paraglacial homogeneous to massive sediment that reflect in-
creased bioturbation and the ablation of the terrestrial ice margin
(Syvitski 1993;Jaeger and Nittrouer 1999). Sediment grain-size and
sedimentation rates decrease as the ice margin retreats, favouring
bioturbation (Syvitski 1993;Aitken and Bell 1998). Additionally,
ice-proximal environments are often composed of IRD, reflecting
calving of the ice margin. For example, in the deeper water of the
nearby former Goldthwait Sea, the typical succession consists of
highly laminated sediments followed by finely laminated sedi-
ment representing ice-proximal to ice-distal environments (St-Onge
et al. 2008). This typical deglacial succession is not observed in the
isolated embayments of the northern Champlain Sea basin owing
to its particular physiographic setting and glacio-isostatic re-
bound dynamics that led to important changes in water depth
with time. In glaciomarine embayments, multiyear shorefast sea-
ice is more likely to occur than in deep-water environments or
near ice-stream outlets. The presence of shorefast sea-ice thus
prevents the deposition of relatively ice-proximal coarse sedi-
ments, leading to an initial reversed sedimentary succession.
Therefore, the sedimentary succession in former glaciomarine
embayments (Fig. 12) rather consists of the following:
(1) IRD-free, finely laminated sediments below multiyear ice in
relatively ice-proximal environments. This unit is believed to
have been deposited in embayments while outwash fans are
being constructed at the front of ice streams in adjacent val-
leys and while coarse glaciomarine diamicton are being de-
posited in deeper waters.
(2) Well-laminated sediment with rapidly deposited layers, ero-
sional surfaces, and mass movement deposits put in place
during the inland retreat of the LIS margin. The melting of
shorefast sea-ice during the summer allows small streams and
rivers fed by glaciogenic sediments to supply contrasting sed-
iment layers in the embayments. Additionally, the retreat of
the LIS margin leads to an initial rapid glacio-isostatic re-
bound, generating numerous mass movements along the
shores of the sea.
(3) Finely laminated to homogeneous sediment deposited during
RSL fall and LIS margin retreat from the watersheds. These
homogenous muds represent the progressive isolation of the
basins and the reworking of the sediment by macrofauna
(4) Finely laminated to homogenous gyttja deposited in postglacial
lakes, which represents the establishment of modern conditions.
In addition to this sedimentation framework, the progressive
retreat of the LIS margin during a RSL fall leads to a slight increase
in grain size with time. Therefore, the nearshore sediment of the
Champlain Sea reflects sea-ice breakup and the continued glacio-
isostatic rebound that favour the deposition of coarser sediments
in increasingly shallower areas. The effect of glacio-isostatic re-
bound counteracts the effect of a retreating ice-sheet margin in
the grain-size distribution of the sediments. The sediments then
gradually change from marine to lacustrine as the individual
basins/bays progressively emerge from the Champlain Sea in
response to glacio-isostatic rebound. Under this forced marine
regression, water depth continues to diminish and leads to a
slight increase in grain size, even in lacustrine settings. This in-
crease in grain size appears to be observed in other glacio-
isostatically uplifted lakes, such as in the High Arctic (e.g., Cuven
et al. 2011).
The morphological, stratigraphic, and sedimentary data col-
lected from four lakes located within the northern sector of the
former Champlain Sea basin (southern Québec) provide new
information on the depositional environments that marked
Fig. 12. Typical deglacial succession in the emerged study lakes owing to glacio-isostatic rebound. This succession reflects a retreating ice
margin combined with a forced marine regression. vf, very fine; f, fine; m, medium; c, coarse.
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deglaciation and land emergence in the region. These results dem-
onstrate the following:
(1) Lakes St-Joseph, Aux-Sables, and Maskinongé contain a sedi-
mentary infill of similar stratigraphy and architecture marked
by sedimentation below multiyear ice, LIS margin retreat, and
the deposition of terrestrially derived event beds (e.g., MMD
and turbidites) and lacustrine sediments. This similar succes-
sion in the lakes of the northern Champlain Sea suggests a
similar sequence of events, despite diachronic marine inun-
dation and regression.
(2) Glaciomarine to marine sediments consist of homogeneous
to faintly laminated muds without IRD followed by a lami-
nated facies. End-member modelling analysis combined with
a lack of IRD suggest that the lowermost glaciomarine unit
(Unit 3) was deposited below multiyear ice, more likely to be
shorefast sea-ice then an ice-shelf. Conversely, the sudden
input of coarser materials (Unit 4) is interpreted as being
associated with the break-up of multiyear ice and open sea-
water conditions during summer months, allowing the depo-
sition of coarser terrestrially derived sediments while the ice
margin was retreating inland.
(3) The transition from marine to lacustrine environments is
characterized by increasing bioturbation (i.e., increasing pri-
mary production), decreasing terrestrial runoff, and increas-
ing grain size. The latter is attributed to land emergence
owing to glacio-isostatic rebound. Although glacial influence
gradually diminishes, the falling RSL favoured the deposition
of coarser sediment with time.
The deglacial succession of the northern Champlain Sea sedi-
ments is slightly different than that observed in conventional
deglacial successions, which is attributed to the location of the
study lakes in former embayments and to postglacial land emer-
gence. Hence, sedimentary facies successions in embayments in
the context of glacio-isostatic rebound are notably different from
deep-water facies successions. This conceptual model based on
the northern Champlain Sea sediment records should provide
help in identifying sediment deposited in former shallow near-
shore embayments.
Financial support was provided by the Natural Sciences and
Engineering Research Council of Canada (NSERC) through a Dis-
covery grant to P.L. Hydroacoustic survey instruments were ac-
quired through research grants awarded to P.L. by the Canadian
Foundation for Innovation, the Ministère de l’Éducation du Québec,
and NSERC. We thank the journal editor Ali Poliat, the associate
editor Alan Trenhaile, as well as Pierre J. Richard, Mark Furze, and an
anonymous reviewer, for their comments that greatly improved this
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... However, the deglacial and postglacial dynamics that could be interpreted from the geomorphology and the stratigraphic sequence were not discussed since the stratigraphic sequence observed in Lake Mékinac stood out from the other studied lakes in southern Québec. Only 3 of the 6 acoustic units identified from Normandeau et al. (2017) framework were identified: U1 Absence of penetration and irregular morphology interpreted as icecontact sediments or bedrock; U4, High amplitude reflection that drapes lower unit and interbedded with transparent lenses, interpreted as glaciomarine sediments deposited during high glaciofluvial discharge; and the uppermost U6, reflection free to medium amplitude reflections, interpreted as gyttja and postglacial sediments. Lake Mékinac was also classified as a gravity-driven water body (Nutz et al., 2018) as MMD occur on its basin floor, but the triggering mechanisms were not discussed . ...
... Terrestrial glacial landforms around Lake Mékinac were interpreted by analyzing LiDAR imagery (distributed by the Government of Québec;, consulted in October 2019) and mapped in ESRI ArcGIS 10.4. High-resolution swath bathymetry data and acoustic sub-bottom profiles within the lake were acquired in 2014 ( Fig. 2; Normandeau et al., 2017). The shallower sectors of the lakes (5-80 m-depth) were mapped using a GeoAcoustics GeoSwath Plus Compact interferometric bathymetry side-scan sonar (250 kHz), coupled with a Valeport SMC motion sensor and a Hemisphere V101 DGPS (~60 cm precision) for positioning; these instruments were deployed using a pontoon boat. ...
... The deepest basins of the lake (>80 mdepth) were mapped using a Reson Seabat 8101 (240 kHz) echosounder combined with an IXSEA Octans III motion sensor for vessel movement correction and a Hemisphere VS131 DGPS (~60 cm precision) for positioning, all mounted on R/V Louis-Edmond-Hamelin. The swath bathymetry imagery and the 12 kHz acoustic sub-bottom profiles are the same as those presented by Normandeau et al. (2017), except for lower frequency (3.5 kHz) profiles ( Fig. 2) that were not included in their study. The 3.5 kHz acoustic sub-bottom profiler data were acquired using a dual-frequency Knudsen 3212 Chirp system mounted on a pontoon boat or on the R/V Louis-Edmond-Hamelin. Subbottom profiles were processed and analyzed in SegyJp2Viewer software from Natural Resources Canada using a sound velocity of 1500 m s-1 to compute depths. ...
Fjords can transition from marine embayments to lacustrine waterbodies under glacio-isostatic change following their deglaciation. However, little is known on how a transition from fjord to lake is influenced by glacial dynamics and how it influences sedimentation in and around the basin. Here, we analyse bathymetry imagery and subbottom imagery in Lake Mékinac in southern Quebec, and deglacial landforms from LiDAR imagery around the lake to document the evolution of a fjord basin into a lacustrine body during the transition from the Late Pleistocene to the Holocene. These analyses helped refine the glacial history of the area and show that the study area was probably under the up-ice reach of the St. Lawrence Ice Stream during full glacial conditions. The mapping of moraines in and around Lake Mékinac shows that deglaciation of the study area was marked by stabilization of the ice-margin during the Mars-Batiscan Event, which is traditionally correlated to the climate deterioration at the end of the Younger Dryas. The architecture of acoustic facies in the lake indicate a transition from glacial conditions (U1), to proglacial-paraglacial (U2) and to postglacial conditions (U3). The subbottom of Lake Mékinac also contains multiple mass movement deposits related to progradation of the Du-Milieu River delta and probably to late-Holocene earthquakes. The acoustic architecture of sediments within the lake indicates that the sedimentation regime was most dependant on the sediment input regime and that the transition from fjord to lake did not significantly influence sedimentation during deglaciation of the watershed. Further work on dating the mass movement deposits that characterise the sediment architecture should provide a better assessment of natural hazard associated with seismological events.
... In combination with basic sedimentary data (lithology, grain size, density of deposit, magnetic susceptibility), the acoustic signal indicates a deglaciation dynamic and postglacial evolution, defines erosive surfaces (Niessen et al., 2010;Brooks, 2016;Normandeau et al., 2017), explores changes in sediment supply (Van Rensbergen et al., 1999) or tectonic and soft sediment deformations, and subwater slides (Pirrotta et al., 2013). The sensitivity of the High Tatra Mountains lakes to regional climate change draws the attention of the limno-geologists and palaeolimnologists with an aim to reconstruct the past climatic and environmental changes (e.g. ...
... The cores were scanned on micro-CT GE phoenix v|tome|x L240 at the Earth Science Institute of the SAS in Banská Bystrica (Slovakia) (for scanning parameters see Data in Brief, in progress), after which they were split open for photo documentation, followed by sampling for radiometric dating and the analysis of diatoms, pollens, chironomids, biomarkers and mineralogical composition. The non-destructive CT scan allowed a visualization of sedimentary structures and extraction of uncalibrated CT numbers as the function of sediment density, grain-size, mineralogical composition, and were extracted using software ImageJ (Tanaka, et al., 2011;Normandeau et al., 2017;Reilly et al., 2017;Emmanoulidis et al., 2020). ...
An acoustic survey using sub-bottom sonar (SBS) and Autonomous Underwater Vehicle EcoMapper (AUVEM) combined with coring was used for the investigation of bottom topography, thickness of the sediments, and geodynamic factors influencing the postglacial deposition in Tatra Mts. lakes. This combined study of seven lakes shows four basic acoustic units and four subunits that are interpreted in terms of stratigraphy and lithology. However, the presence of particular acoustic units may not be encountered in all the lakes due to the effects of local and internal lake processes. Thick horizontal reflections with high amplitude reflectors characterize the bedrock or the infill with a thickness of more than 6 m. Chaotic internal reflection configuration and high amplitude reflectors are common and reflect the glacigene deposits of unknown thickness in the bottom of the lakes. Above the signals of glacigene sediments lie reflections with weak to moderate amplitude reflectors, and with a parallel and chaotic configuration. They correspond to the glaciolacustrine laminated clay and silt and postglacial gyttja that contain clastic outwash from various mass movement deposits. The transition zone between these two SBS reflections is formed by a discontinuous, parallel and wavy configuration with the acoustic bottom. This type of signal is interpreted as a mixture of cobbles, boulders, and fine-grained sediments. The exact lithological content and the presence of the acoustic units are determined by other factors that may not be visible around the lakes and can cause stratigraphic hiatuses. As a prominent and un-recognized factor in the Tatra Mts. until now is the presence of the bottom springs (pockmarks) of the non-karstic origin, which effectively disturb the fine clastic and organic matter deposition and hence can limit the use of the lake deposit for palaeoclimatic and palaeoecological studies. The mass movement processes – debris flow, rockfall – forming the mountain geo-relief are frequent and their products are visible in the marginal facies. The sonar inspection also provides evidence of mass movement deposits, which have no indication on the terrestrial environment and thereby represent intralacustrine landslides as a result of bedrock instability due to the late Pleistocene deglaciation or Holocene climatic variability.
... The modern SLFS evolves from a complex hydrosystem made of several river channels, branches, archipelagos and shallow fluvial lakes (RS) into a 1 to 5 km-wide estuary where fluvial processes formed bluffs and terraces that extend to Qu ebec City (FES). The modern physiography and surface geology of the SL is mainly inherited from the passage and retreat of the Laurentide Ice Sheet and subsequent marine invasion in the glacio-isostatically depressed basin located in the St. Lawrence Lowlands (Occhietti et al. 2011;Dalton et al. 2020), where a sequence of glacial, paraglacial and postglacial sediments was deposited and uplifted before the onset of modern river processes (Lamarche 2005;Occhietti 2007;Normandeau et al. 2017) (Figure 1(C,D)). ...
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Over the past decades, riparian land-use changes coupled to the multiplication of river infrastructures have enhanced vulnerability issues for societies and ecosystems located along large rivers. Exposure to geohazards is also changing due to the ongoing climate change, underlining the need for flexible management strategies for riparian environments. In this perspective, GIS-based mapping allows integrating a wide range of environmental data. However, such datasets are often incomplete and not homogeneous over large geographical scales, which can be problematic for the implementation of regional land-use planning strategies. Using the St. Lawrence fluvial system (SLFS) (Québec, Canada) as a case study, this article reports and describes a high-resolution approach to map position, characteristics and erosion susceptibility of natural and artificial riverbanks from a combination of field-based, remote sensing and local knowledge-derived data. This approach allowed identifying erosion-prone sites and highlighting dominant erosion processes and spatially constrain them along the SLFS. The proposed geospatial framework constitutes (1) an initial portrait of the riverscape that will allow an effective implementation of future monitoring and process-based studies; and (2) a first step in supporting land-use planning stakeholders in the selection of appropriate measures to ensure a greater resilience of riparian communities and ecosystems.
... It is considered that sediments deposited in silled and partly isolated basins are valuable records of past environmental and sea level changes (Lambeck and Purcell, 2005;Long et al., 2011). Prolific paleoenvironmental studies of silled basins have been conducted in Canada and Greenland (Long et al., 2011;Normandeau et al., 2017;Fedje et al., 2018), Scotland (Lloyd, 2000;Lloyd and Evans, 2002), Scandinavia (Balascio et al., 2011;Narančić et al., 2016), and Black and Marmara Sea (Çaǧatay et al., 2003;Bahr et al., 2005;Taviani et al., 2014;Filikci et al., 2017). There is, however, a lack of such studies in the Adriatic Sea. ...
The results of a comprehensive study of submerged paleoenvironments developed along the karstified eastern Adriatic coast during the Late Quaternary are presented in this study. The Lošinj Channel is a drowned karst basin filled with sediments. A multi-proxy analysis of two sediment cores (LK-12 and LK-15) recovered from water depths of 62 and 64 m was conducted. We used magnetic susceptibility, grain size, mineralogy, XRF core scanning, organic and inorganic carbon, total nitrogen, and paleontological data, supplemented with AMS 14C dating results and high-resolution seismic data, to reconstruct the infill history of the Lošinj basin during the Late Pleistocene and Holocene. Our findings include the first detailed description of the presumed Marine Isotope Stage (MIS) 5a marine sediment succession along the eastern Adriatic coast. Deposition in the brackish-to-freshwater lacustrine body (Lošinj paleolake) occurred during MIS 3. Sea level lowstand that followed caused the formation of environmental conditions typical of a karst polje. The post-Last Glacial Maximum (LGM) sea level rise led to the establishment of a brackish marine lake with seawater seepage through the karstified sill at 13.7 cal kyr B.P. The transition to the present-day marine conditions commenced at 10.5 cal kyr B.P. Paleoenvironmental changes in the investigated area can be linked to the presence of a sill at −50 m depth that separates the Lošinj basin from the Kvarnerić Bay. The sill depth determines the isolation or inundation of the investigated basin in response to the changes in sea level. Paleoenvironments reacted sensitively to these changes, and therefore, the study area represents an ideal setting to track regional sea level and climate variabilty.
... These interpretations reflect previous acoustic stratigraphy studies in Quebec and Labrador lakes, as well as adjacent continental margins (e.g. Syvitski and Praeg, 1989;Syvitski and Lee, 1997;Normandeau et al., 2013Normandeau et al., , 2017Lajeunesse et al., 2017;Gagnon-Poiré et al., 2018;Trottier et al., 2018). ...
High‐resolution multibeam bathymetric data and acoustic sub‐bottom profiles were recently collected in Grand Lake (Labrador), one of the deepest lake basins in eastern North America, to reconstruct: (1) the retreat of the Laurentide Ice Sheet (LIS) west of Lake Melville, and (2) the history of sedimentation since deglaciation in this 54‐km long, 3‐km wide fjord‐lake. Our results provide a morphostratigraphical framework that brings new insights to the style and pattern of retreat of the LIS in the region as well as deglacial and postglacial sedimentary dynamics. Terrestrial glacial lineations observed on a digital elevation model (DEM) provide evidence of a previously undocumented ice stream in the Grand Lake area. This newly mapped ice stream suggests that the calving bay formed in Lake Melville triggered a reorganisation of the regional drainage pattern of the LIS. The sedimentary infill of Grand Lake consists of a sequence of deglacial to postglacial sediments that contain deposits related to a series of mass movements. The 8.2 cal ka BP cold event is recorded in Grand Lake by a series of closely spaced moraines deposited at the outlet of the fjord‐lake to form a morainic complex similar to the Cockburn morainic complex on Baffin Island. During deglaciation, a dense dendritic network of proglacial gullies incised into the steep sidewalls of the lake. Since deglaciation, paraglacial and postglacial sedimentation has led to the deposition of large prograding deltas at the fjord head, where density currents remain active today and have formed a series of sediment waves on the frontal slopes and a prodeltaic environment.
... Such multiple MTD event horizons are present in some eastern Canadian lake basins and within the Saguenay fjord, and are interpreted to be the stratigraphic signature of significant historical earthquakes (e.g., AD1663 M7 Charlevoix earthquake, Smith, 1987, 1988;Syvitski and Schafer, 1996;Locat et al., 2016; AD1935 M w 6.1 Timiskaming earthquake, Shilts, 1984;Shilts and Clague, 1992;Doughty et al., 2010a). Similarly, the presence of a multiple MTD event horizon(s) in the upper portion of the lacustrine sediments in some lakes has been interpreted as evidence of prehistoric earthquakes during the late Holocene (e.g., Shilts et al., 1989Ouellet, 1997;Lajeunesse et al., 2017;Normandeau et al., 2013Normandeau et al., , 2017Poir e et al., 2018). All of these examples are located within or close to the modern Charlevoix-Kamouraska, Western Quebec, or Lower St. Lawrence seismic zones ( Figs. 1 and 8). ...
Evidence of early postglacial, glacially-induced seismic activity should be abundant in eastern Canada, which was formerly covered by the Laurentide Ice Sheet during the late Wisconsinan Glaciation. This review identified only two 'probable' and at most nine 'possible' examples of glacially-induced faulting over an area of ~4 million km 2 , a far lower number than that expected from a long-term rate of background seismicity. The 'probable' sites are the Holy Grail scarp, Manitoba, and the series of faults buried in the sub-bottom of Round Lake, Ontario. The 'possible' nine glacially-induced faults include the Aspy fault, Nova Scotia, and Timiskaming East Shore fault, Ontario, but there is little published information on the other seven features. Despite this low number, the common occurrence of subaqueous mass transport deposits interbedded within or immediately overlying glaciolacustrine-glaciomarine deposits in many modern lake basins strongly hints at an elevated period of glacially-induced seismicity over widespread areas of eastern Canada. In southern Quebec, the age of these mass transport deposits suggests that glacially-induced earthquake activity began as early as about 13.5 ka cal BP, which corresponds to the time of local ice melt-back. This indicates that glacially-induced seismicity began at the time of local deglaciation in southern Quebec, rather then being delayed as existing fault instability models imply. It is anticipated that new candidate glacially-induced or younger faults will be identified in eastern Canada over the next decade or so because of the increasing availability of new high-resolution data imagery sources. Crown
In this study, depositional conditions in the glaciogenic Champlain Sea basin were inferred solely from evidence of computed tomography (CT). CT-scan images and Hounsfield unit (HU) profiles of two continuous cores in Ottawa, Canada, provided the data to identify five lithologically distinct, mud-dominated stratal units. Distinctively, all strata exhibit a repeating pattern of HU values. The upward increase and then decrease in HU values within each bed suggests upward increasing and then decreasing silt content, which collectively, is interpreted to reflect the consistent waxing followed by waning flow conditions during a single cycle of glaciogenic meltwater discharge. Mud rhythmites at the base of the succession (Unit 1) and beds of well-stratified mud at the top (Unit 4a) exhibit well-developed parallel-stratification in the lower part of each bed; they are characterized by a range of ~250–350 HU. This wide range in HU values reflects the effective partitioning of silt-rich and clay-rich parts in each bed, and therefore bed-surface and near-bed transport processes that sorted sediment mostly by grain size, followed by low-energy sediment fallout. The silt-rich part of each bed was deposited by glaciogenic meltwater-sourced hyperpycnal flows, which due to the freshwater composition of the basin, or at least in the upper part of the water column, were able to plunge to the basin floor and sort particles along the bed. In comparison, beds of bioturbated mud (Unit 2), banded mud (Unit 3), and diffusely stratified or structureless mud (Unit 4a) have a narrower range of ~50–120 HU, indicative of poorer particle sorting. These conditions, most clearly illustrated by the bioturbation, indicate a change to a seawater basin that caused meltwater inflows to instead form buoyant, sediment-laden hypopycnal flows, which aided by clay-silt particle flocculation and/or settling-driven convection, resulted in sediment deposition directly from suspension. In addition to characteristic sedimentary structures and textures, beds within each unit also exhibit similar thickness, which often doubles or triples in the abrupt transition from one unit to the next. These changes most probably reflect abrupt and systemic changes in deglacial dynamics and its control on sediment supply that often coincide with equally abrupt changes in salinity in the Champlain Sea basin.
This study investigates the potential of a new high-resolution, non-destructive, X-ray imaging technique for the Quaternary Sciences – computed laminography (CL). Greyscale properties are systematically extracted from digital X-radiographic CL images of cored glaciomarine sediments to analyse and characterise sediments at pixel-scale resolution (<0.1 mm). We show how this can be achieved manually, and also with an easy-to-use, automated statistical tool which we have devised specifically for use in glaciomarine sediments. This Sediment Characteristics tool, in the form of a plugin for the freely available FIJI/ImageJ programme, extracts mean or median X-ray grey values (GV) – a proxy for sediment density; and associated standard deviation (SD) – a proxy for sediment heterogeneity – at sub-mm resolution, across the width of sediment core CL images. We demonstrate how these properties (GV and SD) can be directly used to characterise sediment properties and in particular to quantifying the abundance of gravel clasts, or ice-rafted debris, in cored glaciomarine sediments. The tool’s effectiveness is compared with other, more traditional, X-radiographic methods for counting ice-rafted gravel clasts in glaciomarine sediment. We propose that the CL output and Sediment Characteristics tool also have the potential to quantitatively analyse other 3-dimensional structures, such as cyclic lamination (varve) geometry; deformation structures; bioturbation and void space (porosity). Finally, we present the raw code, allowing open-access, transparency and reproducibility in other formats.
The late Pleistocene to early Holocene Champlain Sea provides a unique opportunity to study the development of marine ecosystems in a context of global climatic change. This study presents radiocarbon ( ¹⁴ C) dates and stable isotope analyses on 15 vertebrate specimens from Champlain Sea sediments, including the Charlotte Whale, which is Vermont’s State marine fossil. Data are used in an attempt to investigate the timing of colonization and ecological dynamics in this newly formed sea. Using the average marine correction, ¹⁴ C dates on four specimens likely calibrate prior to or possibly synchronous with the accepted origination date for the Champlain Sea, implying larger marine reservoir effects than the average marine correction in the vertebrate tissues. Without knowing the specific marine reservoir offsets, it is not possible to calculate the timing of colonization or its relation to concurrent climatic change. Observed lower δ ¹³ C and δ ¹⁵ N values in walruses, a fin whale, and a right whale support consumption of prey from lower trophic levels such as bivalve mollusks, krill, and copepods. Higher isotopic values in beluga whales and a bird, the thick-billed murre, support consuming fish, such as cod and capelin. These isotopic data show comparable values and relationships as observed in modern arctic marine ecosystems.
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The objective of this 4-days excursion is to study different aspects of the Quaternary last deglaciation which relates to the recession and final disappearance of the Laurentide Ice Sheet during the latest Wisconsinan and Holocene along the Québec North Shore (North Shore of the Estuary of the St. Lawrence, Québec, Canada). A particular emphasis will be given to the observation and study of deglacial sedimentary successions and landforms along with associated relative sea level (RSL) fall inherent to this deglaciation episode. Different aspects of glacially-influenced sediment transport and depositional processes will also be discussed. Our excursion will lead us both in the hinterland for the observation of glacial landforms and along the shores where the modern erosion of coastal cliffs and river banks permits the direct observation of sedimentary facies and stratigraphic architectures.
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Lakes are common natural systems for which sedimentation is considered to be relatively simple, generally dominated by fluvial processes along the margin and prevailing low-energy settling in the central, deeper parts. However, for many lakes, higher-energy wind-driven processes dominate. As such, a new category of lakes is proposed, herein referred to as wind-driven waterbodies (WWB). WWB display a sedimentation largely dominated by wave related processes and wind-induced lake-scale water circulation evidenced by the construction of beach ridges, spits or cuspate spits along their shorelines, and by sediment drifts, sedimentary shelf progradation and erosional surfaces in their deeper, offshore domains. WWB are observed worldwide, they share a common physiography that favours wind-forced hydrodynamics and related sedimentation patterns. This physiography is expressed by the IWWB index, a ratio of the maximum representative fetch relative to mean basin depth. It is proposed that an index value greater than three favours the evolution of a lake as a WWB. The WWB concept represents a new end-member in an alternative, sedimentologically-based lake classification that is proposed in this paper.
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Nettilling Lake (Baffin Island, Nunavut) is currently the largest lake in the Canadian Arctic Archipelago. Despite its enormous size, this freshwater system remains little studied until the present-day. Existing records from southern Baffin Island indicate that in the early postglacial period, the region was submerged by the postglacial Tyrell Sea due to isostatic depression previously exerted by the Laurentide Ice Sheet. However, these records are temporally and spatially discontinuous, relying on qualitative extrapolation. This paper presents the first quantitative reconstruction of the postglacial environmental succession of the Nettilling Lake basin based on a 8300 yr-long high resolution sedimentary record. Our multi-proxy investigation of the glacio-isostatic uplift and subsequent changes in paleosalinity and sediment sources is based on analyses of sediment fabric, elemental geochemistry (μ-XRF), diatom assemblage composition, as well as on the first diatom-based oxygen isotope record from the eastern Canadian Arctic. Results indicate that the Nettilling Lake basin experienced a relatively rapid and uniform marine invasion in the early Holocene, followed by progressive freshening until about 6000 yr BP when limnological conditions similar to those of today were established. Our findings present evidence for deglacial processes in the Foxe Basin that were initiated at least 400yrs earlier than previously thought.
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A buried bedrock trough >350 m deep extends 100 km above Saguenay Fjord beneath the North Channel of the middle Estuary. Four of five regional seismographic units are recognized in and adjacent to the trough; unit 1 (glacial ice-contact) and older sediments might also be present beneath the largely unpenetrated trough axis. Units 2 and 3 represent thick glacial marine sediments deposited in the >550 m deep waters of the Goldthwait Sea after glacial withdrawal ca. 13 ka BP. Unit 4 corresponds to lobes over 30 m thick on both sides of the upper North Channel, recording marginal input from glacial fluvio-deltaic sources. Unit 5 (estuarine sands, gravels and muds ≤30 m thick) unconformably overlies glacial units. A smooth unconformity surface records erosion (at least 15 m, to axial depths >150 m) by strong currents; irregular relief above depths of 25-50 m might relate to relative sea levels below present ca. 7-6 ka BP. -from Authors
An integrated seismo- and chronostratigraphic investigation at Lac Dasserat, northwestern Quebec, identified 74 separate failures within eight event horizons. Horizons E and B, and H and G have strong or moderately-strong multi-landslide signatures, respectively, composed of 11–23 failures, while horizons F, D, C, and A have minor landslide signatures consisting of a single or pair of deposit(s). Cores collected at six sites recovered glacial Lake Ojibway varve deposits that are interbedded with the event horizons. The correlation of the varves to the regional Timiskaming varve series allowed varve ages or ranges of varve ages to be determined for the event horizons. Horizons H, G, E, and B are interpreted to be evidence of paleoearthquakes with differing levels of interpretative confidence, based on the relative strength of the multi-landslide signatures, the correlation to other disturbed deposits of similar age in the region, and the lack or possibility of alternative aseismic mechanisms. The four interpreted paleoearthquakes occurred between 9770 ± 200 and 8470 ± 200 cal yr BP, when glacial Lake Ojibway was impounded behind the Laurentide Ice Sheet during deglaciation. They probably represent an elevated period of seismicity at deglaciation that was driven by crustal unloading.
High-resolution swath bathymetry data collected in fjord-lakes Pentecôte, Walker and Pasteur (eastern Québec, Canada) allowed imaging in great detail the deltas of four rivers in order to understand the factors controlling the formation and downslope evolution of bedforms present on their slopes. The morphometry and morphology of 199 bedforms reflect the behaviour of sediment density flows. The shape of the bedforms, mostly crescentic, and the relationships between their morphological properties indicate that they were formed by supercritical density flows and that they are cyclic steps. The crescentic shape suggests an upslope migration while the aspect ratios and increasing wavelengths with distance from the shore (and decreasing slopes) are compatible with a cyclic step origin. At the rollover point, the acceleration of the density flows on steep slopes produces tightly spaced hydraulic jumps and favours short wavelength and symmetrical bedforms. Further downslope, decreasing slopes and increasing specific discharge increase the wavelength and asymmetry of the bedforms. The wavelength and asymmetry are increased because density flows require longer distances to become supercritical again on lower slopes after each successive hydraulic jump. Bedform morphometry and morphology are used to reconstruct density flow behaviour downslope. Froude numbers are high near the rollover point and gradually decrease downslope as the slope becomes gentler. Conversely, the specific discharge and flow depth are low near the rollover point and gradually increase downslope as the flow either erodes sediments or becomes more dilute due to sediment deposition and water entrainment. The supercritical density flows are believed to be triggered mainly by hyperpycnal flows but some evidence of delta-front slope failures is also observed. The differences in delta morphology and bedform development between the four deltas are linked to basin morphology and watershed hydrology, but also mainly to the fjord heritage of the lakes that allowed the focusing of sediment at the delta front. © 2016 The Authors. Sedimentology
Disturbed lacustrine deposits of eastern Canada have been associated with past seismic events in the intraplate Western Québec Seismic Zone (WQZ) and the Charlevoix–Kamouraska Seismic Zone (CKSZ). With five major earthquakes that occurred during historical times and hundreds of minor earthquakes, the CKSZ is the most active seismic zone in eastern Canada. Here we present new high-resolution multibeam bathymetric, subbottom profiler and sediment core data collected in Lake Témiscouata, southeastern Québec, the largest lake located within the influence of the CKSZ. The occurrence of mass-movements in the highly cohesive glaciolacustrine deposits of the lake produced a peculiar lacustrine landscape consisting of steep slide scars and residual mounds and very narrow residual crests. Our data indicate that postglacial times were marked by recurrent seismically-induced mass-movement events that cannot be individually identified in the sedimentary record due to the highly disturbed stratigraphy of the lake and to the absence of significant postglacial background sediments between events. The most recent recorded mass-movement is a distinct debris flow sediment layer on the lake floor that was deposited at ~ 1300 cal BP, very shortly after a distinct large-scale mass-movement event observed across the entire lake basin. The presence of silt layers in gyttja overlying these disturbed units suggests that the region was affected by other seismic events following this event, although no MMDs could be directly related to these layers.
A detailed high-resolution seismic stratigraphy, calibrated by core data and terrestrial geomorphological mapping, has been constructed for Loch Ainort, Isle of Skye. This study has provided a palaeoenvironmental history of the area as well as important corroborative evidence for the stepped deglaciation of the Loch Lomond Stadial ice-field on Skye. The Ainort Glacier reworked pre-Loch Lomond glacial deposits terminating in a grounded tidewater ice-front potentially 800 m beyond the previously extrapolated limit. The first stage of deglaciation was characterised by the formation of De Geer moraines indicative of a period of interrupted retreat. The second phase, by contrast, produced hummocky relief with sporadic linear moraines suggesting periods of uninterrupted retreat with occasional stillstands/readvances. Paraglacial reworking of terrestrial slopes resulted in the deposition of thick, subaqueous, debris flows which graded into fluvioglacial dominated sediments and ultimately modern fjordic deposits. The identification of an initial period of active retreat punctuated by numerous readvances correlates directly with the terrestrial record. However, the offshore stratigraphy suggests that although the second phase was dominated by uninterrupted retreat, occasional stillstands/ readvances did occur. Copyright (C) 2000 John Wiley & Sons, Ltd.
The spread of hypoxia is a threat to aquatic ecosystem functions and services as well as to biodiversity. However, sparse long-term monitoring of lake ecosystems has prevented reconstruction of global hypoxia dynamics while inhibiting investigations into its causes and assessing the resilience capacity of these systems. This study compiles the onset and duration of hypoxia recorded in sediments of 365 lakes worldwide since AD 1700, showing that lacustrine hypoxia started spreading before AD 1900, 70 years prior to hypoxia in coastal zones. This study also shows that the increase of human activities and nutrient release is leading to hypoxia onset. No correlations were found with changes in precipitation or temperature. There is no evidence for a post-1980s return to well-oxygenated lacustrine conditions in industrialized countries despite the implementation of restoration programs. The apparent establishment of stable hypoxic conditions prior to AD 1900 highlights the challenges of a growing nutrient demand, accompanied by increasing global nutrient emissions of our industrialized societies, and climate change.
In a tentative synthesis of statistical marine 14C chronology, isostatic and eustatic movements, continental glacial features and post-glacial marine limits, the paleogeographic evolution during the late and post-glacial times in eastern Canada, with special reference to Quebec, is here reassessed. Includes maps of isochrones of Laurentide ice retreat, maximum diachronous extension of post-glacial lakes and seas, emergence observed since 7 500 BP and of paleogeography at 10 000-10 500 BP. The post-glacial uplift of Quebec is reconsidered with new regional emergence curves. -from Authors