High-resolution seismo-stratigraphy and sedimentological properties of late- and postglacial sediments in Lac Guillaume-Delisle Estuary and Nastapoka Sound, eastern Hudson Bay

Abstract

Lac Guillaume-Delisle Estuary and Nastapoka Sound are two sedimentary basins that recorded the late Quaternary deglaciation on the eastern coast of Hudson Bay. Acoustic profiles reveal an average sediments thickness of 15 m in the estuary and 6 m in the sound. These sediments reach 70 m thick in deep glacial troughs. Within the studied basins, four seismo-stratigraphic units overlying the acoustic basement were recognized. Unit 1 (subaqueous ice-contact and draped glaciomarine deposits associated with the Quebec-Labrador Ice Sector (QLIS) of the Laurentide Ice Sheet (LIS) and the Tyrrell Sea) records the presence of a short ice-marginal stillstand during glacial retreat. Unit 2 (paraglacial and postglacial fluvial-deltaic deposits) and unit 3 (postglacial silty deposits) result from erosion of emerged sediments and redeposition in response to changes in relative sea level (RSL). Finally, unit 4 is composed of deformed deposits associated with a mass wasting event. The stratigraphic sequence and the spatial distribution patterns of deposits show that Lac Guillaume-Delisle is a good model to explain the dynamics of the QLIS margin during and after successive ice stillstands, continuous RSL fall, river discharge (ablation on land), and its final ablation inland. Additionally, seven cores were sampled in the southeast part of the estuary. Geochemical (organic carbon and total nitrogen) and carbon isotopic contents, used as alternative proxy, indicate that the sampled sediments correspond to the postglacial estuarine deposits of unit 3. Allochthonous sources of carbon dominate the supply to the sediments where the environment is regularly flushed by fresh and marine waters.Résumé : L'estuaire Lac Guillaume-Delisle et le chenal Nastapoka sont deux bassins sédimentaires où l'on retrouve des évi-dences de la déglaciation, au Quaternaire tarif, sur la côte Est de la baie d'Hudson. Des profils acoustiques indiquent une épaisseur moyenne de sédiments de 15 m dans l'estuaire et de 6 m dans le chenal. Ces sédiments atteignent une épaisseur de 70 m dans des fosses glaciaires profondes. Dans les bassins à l'étude, on reconnaît quatre unités sismostratigraphiques audessus du socle acoustique. L'unité 1 (des dépôts de contact glaciaire sous l'eau et des dépôts glaciomarins drapés associés au secteur glaciaire Québec-Labrador de la calotte glaciaire laurentidienne et à la mer de Tyrrell) enregistre la présence d'une brève pause de stabilization de la bordure glaciaire durant le retrait de la glace. L'unité 2 (des dépôts para-glaciaires et post-glaciaires fluvio-deltaïques) et l'unité 3 (des dépôts postglaciaires silteux) proviennent de l'érosion et de la redéposition de sédiments émergés en réponse aux changements du niveau relatif de la mer. Finalement, l'unité 4 est composée de dépôts déformés associés à un événement de mouvement de masse. La séquence lithologique et les patrons de répartition spatiale des dépôts montrent que le Lac Guillaume-Delisle est un bon modèle pour expliquer la dynamique de la bordure du secteur Québec-L...

Deglaciation landforms and C-14 chronology of the Lac Guillaume-Delisle area, eastern Hudson Bay: A report on field evidence

“…HillaireMarcel, 1976;Hillaire-Marcel and Fairbridge, 1978) were mainly developed over such deposits that provided the sediments for beach construction by the waves. In addition, the marine surveys show that these deposits observed on land have continuity below present-day sea level in the estuary and below the surface of Hudson Bay, in Nastapoka Sound (Lavoie et al, 2008).…”

“…This Delta R value is smaller than the ones used in other studies (310 14 C BP, Barber et al, 1999;365 14 C BP, Coulthard et al, 2010; 300 14 C BP, Roy et al, 2011) obtained by averaging reservoir age corrections from the southeastern Hudson Bay and James Bay region. The shell specimens used from James Bay are, in our opinion, potentially affected by the regional abundance of Paleozoic limestone in the bedrock (hard water effect), as reflected by the high contents in inorganic carbon (CaCO 3 ) found in emerged postglacial and glacio-marine sediments (Lavoie et al, 2008). Therefore, it is probable that the Delta R remained lower along eastern Hudson Bay than in the James Bay basin since deglaciation.…”

“…The region is characterized by the highest topographic relief along the entire eastern coast of Hudson Bay which makes it possible to observe the highest marine limit and precisely constrain the age of the beginning of the deglaciation on the coast. The Lac Guillaume-Delisle estuary was the last marine basin to be deglaciated when the western margin of the QLIS was retreating eastward (Lavoie et al, 2008). It was deglaciated along a broad retreating ice front rather than at the localized outlet of a concentrated ice stream.…”

“…The depression measures approximately 780 km 2 and it is between 12 and 150 m deep (in submerged glacial troughs). The estuary is a sedimentary basin that contains an average thickness of 15 m of sediments, but sediment thickness reaches 70 m in troughs where four seismo-stratigraphic units have been described Geomorphology 159-160 (2012) [142][143][144][145][146][147][148][149][150][151][152][153][154][155] by Lavoie et al (2008). The oldest unit consists of submarine ice-contact and draped glacio-marine deposits (Unit 1) that record the presence of a QLIS-marginal stillstand during ice regression.…”

Periphytic diatom community structure in thermokarst ecosystems of Nunavik (Québec, Canada)

“…(Allard and Séguin 1985). This deglaciation was followed by a marine transgression episode (postglacial Tyrrell Sea), which covered the landscape with marine (silt-clay) and coastal (sand) deposits that are widespread today in topographic depressions (Saulnier-Talbot et al 2003;Lavoie et al 2008a). Following ice retreat, strong isostatic rebound (up to 9 m per century) affected the landscape, resulting in marine regression (Allard and Séguin 1987;Lafortune et al 2006).…”