Evidence for Changes in Paleoenvironments along the Lower Cap-Rouge River, Québec (Canada), in Relation to a High Water Stand during the Mid-Holocene Laurentian Transgression

摘要

At the outlet of the Cap-Rouge River, upstream of the Québec sound on the St. Lawrence north shore, a 5-m-high stratigraphic section was excavated within an urban area, specifically, in a schoolyard located on the 14 m terrace on the east side of the river. Our stratigraphic and high-resolution macrofossil data show that major paleoenvironmental changes occurred along the lower Cap-Rouge River during the mid-Holocene. Macrofossil assemblages indicate that during a low water stand between 6320 and 6230 cal YBP, eastern hemlock (Tsuga canadensis [L.] Carr.) and white pine (Pinus strobus L.) formed mixed stands on the sandy terraces flanking the river. They also indicate that these mesophytic forests shifted to hydrophytic plant communities (eastern larch and aquatic plants) ca. 6230 cal YBP. These rapid changes in plant communities occurred during a mid-Holocene high water stand. Whether waters flowing at the Cap-Rouge River’s outlet were brackish or soft waters during this highstand cannot be determined with certainty in absence of halophytes in the macrofossil records. Similar paleoenvironmental changes in plant cover have been well documented at several locations on the south shore of the St. Lawrence Estuary, where they were attributed to a rise in sea level coinciding with the Laurentian transgression. We also discuss the possibility that ice jams played a role in fluvial inundation and sedimentation over the last centuries at the mouth of several tributaries of the St. Lawrence River.