Seismic‐stratigraphic record of a deglaciation sequence: from the marine Laflamme Gulf to Lake Saint‐Jean (late Quaternary, Québec, Canada)

Abstract

International audienceThe stratigraphy of the last deglaciation sequence is investigated in Lake Saint-Jean (Québec Province, Canada) based on 300 km of echo-sounder two dimensional seismic profiles. The sedimentary archive of this basin is documented from the Late Pleistocene Laurentidian ice-front recession to the present-day situation. Ten seismic units have been identified that reflect spatio-temporal variations in depositional processes characterizing different periods of the Saint-Jean basin evolution. During the postglacial marine flooding, a high deposition rate of mud settling, from proglacial glacimarine and then prodeltaic plumes in the Laflamme Gulf, produced an extensive, up to 50 m thick mud sheet draping the isostatically depressed marine basin floor. Subsequently, a closing of the water body due to glacio-isostatic rebound occurred at 8.5 cal. ka BP, drastically modifying the hydrodynamics. Hyperpycnal flows appeared because fresh lake water replaced dense marine water. River sediments were transferred towards the deeper part of the lake into river-related sediment drifts and confined lobes. The closing of the water body is also marked by the onset of a wind-driven internal circulation associating coastal hydrodynamics and bottom currents with sedimentary features including shoreface deposits, sediment drifts and a prograding shelf-type body. The fingerprints of a forced regression are well expressed by mouth-bar systems and by the shoreface-shelf system, the latter unexpected in such a lacustrine setting. In both cases, a regressive surface of lacustrine erosion (RSLE) has been identified, separating sandy mouth-bar from glaciomarine to prodeltaic muds, and sandy shoreface wedges from the heterolithic shelf-type body, respectively. The Lake Saint-Jean record is an example of a regressive succession driven by a glacio-isostatic rebound and showing the transition from late-glacial to post-glacial depositional systems

Forced regressive deposits of a deglaciation sequence: Example from the Late Quaternary succession in the Lake Saint‐Jean basin (Québec, Canada)

“…In general, Quaternary sediments of the Lake Saint‐Jean basin include from base to top: (i) sandy to conglomeratic paraglacial sediments (tills, eskers and sandy ice‐contact accumulations); (ii) an extensive mud sheet; (iii) sandy and gravelly beaches and deltas; overlain, in places, by (iv) aeolian sand dunes (Tremblay, ; Daigneault et al ., ; Nutz et al ., ). Extending over the entire basin (Daigneault et al ., ), the extensive mud sheet is referred to as ‘Ums’ and used as a relevant marker in order to identify the underlying sand and conglomerate as glacigenic material deposited prior to marine flooding.…”

“…Above, isolated sand/conglomerate mounds (transects 2 and 4) attributed to ice‐contact accumulations (F9a, F8, F6a, F4b, F4c, F3a and F1a) relate to ice‐front short‐term stillstands (Lajeunesse & Allard, ; Powell & Cooper, ) during the general retreating trend of the ice sheet and suggest a stepwise deglaciation. Ice‐contact accumulations are then capped by glacimarine muds throughout the Lake Saint‐Jean basin that correspond to the lower part of the extensive mud sheet referred to as the Laflamme Mud Formation (Laverdière & Mailloux, ; Daigneault et al ., ; Nutz et al ., ). Comparable successions marking ice‐front retreats have formed in marine as well as in lacustrine settings related to the LIS evolution (Syvitski & Praeg, ; Syvitski & Lee, ; St‐Onge et al ., ; Nutz et al ., ).…”

“…Ice‐contact accumulations are then capped by glacimarine muds throughout the Lake Saint‐Jean basin that correspond to the lower part of the extensive mud sheet referred to as the Laflamme Mud Formation (Laverdière & Mailloux, ; Daigneault et al ., ; Nutz et al ., ). Comparable successions marking ice‐front retreats have formed in marine as well as in lacustrine settings related to the LIS evolution (Syvitski & Praeg, ; Syvitski & Lee, ; St‐Onge et al ., ; Nutz et al ., ).…”

From outwash to coastal systems in the Portneuf–Forestville deltaic complex (Québec North Shore): Anatomy of a forced regressive deglacial sequence

“…Yet, a wider spectrum of depositional systems, mainly including turbiditic, deltaic, lacustrine, coastal, tidal or fluvial deposits, constitute the bulk of many other deglacial sequences related to continent‐scale ice sheet recessions, as evidenced in post‐Late Glacial Maximum (LGM) successions (e.g. Syvitski & Hein, ; Hansen, ; Fraser et al ., ; Corner, ; Eilertsen et al ., ; Nutz et al ., , ; Normandeau et al ., ) and in the deep time glacial record (e.g. Proust & Deynoux, ; Ghienne, ; Le Heron et al ., , , ; Isbell et al ., ; Loi et al ., ; Girard et al ., ).…”

Post‐Last Glacial Maximum evolution of a “fjord‐type” lake based on high‐resolution seismic data: the Lago Roca/Acigami (southern Tierra del Fuego, Argentina/Chile)

“…This model of basin stratigraphy usually includes a basal ice‐contact glacier‐derived depositional unit, overlain by proglacial glaciolacustrine sediments that transition into meltwater‐fed distal proglacial deposits. The disconnection of the discharge rivers from the glacier front marks the transition from a glacier‐fed lake to an alluvial‐fed lake with open conditions (Van Rensbergen et al ., , ; Nutz et al ., ).…”