Forced regressive deposits of a deglaciation sequence: Example from the Late Quaternary succession in the Lake Saint-Jean basin (Québec, Canada)
Résumé
Differentiating between forced regressive deposits from deglacial periods inhigh latitude domains and forced regressive deposits from the onset of glacialperiods in low latitude domains is fundamental for the accurate interpretationof glacial cycles within the geological record and then for thereconstruction of palaeogeography and palaeo-climate. A forced regressivedeglacial sequence is documented from the Lake Saint-Jean basin (Quebec,Canada). In this area, the Late Pleistocene to Holocene sediments haverecorded the Laurentide ice sheet retreat accompanied by the invasion ofmarine waters (Laflamme Gulf) from ca 129 cal kyr BP. Subsequently, fluviodeltaicand coastal prograding wedges were deposited; they followed thebase-level fall due to glacio-isostatic rebound. This succession, representinga transition from glacial to post-glacial periods within a previously glaciatedarea, was investigated through recent mapping, preserved landforms, faciesanalysis, and new optical stimulated luminescence and radiocarbon dates.Three basin-scale geological sections share a common lower part made ofisolated ice-contact fan deposits overlying bedrock. Throughout the entirebasin, ice-contact fans are capped by glacimarine muds. Above, fluvial andcoastal prograding systems were deposited and evolved through four steps:(i) deltaic systems progressively increased in width; (ii) coastal influence onsedimentation increased; (iii) hydrographic drainage systems became moreorganised; and (iv) deltas graded from steep (Gilbert delta) to low-angle foresets(mouth-bar delta). Deposited during the base-level fall from glacioisostaticrebound, the complete succession has been designated as a singlefalling stage system tract referred to as a deglacial falling stage system tract.It is representative of a deglaciation sequence in areas previously covered byice during glacial periods (i.e. medium to high latitude domains). Diagnosticcriteria are provided to identify such a deglacial falling stage system tract inthe geological record, which may aid identification of previously unknownglacial cycles.