Geomorphology and Late Quaternary evolution of Lake Manicouagan, a deep impounded impact crater-fjord-lake of the eastern Canadian Shield hinterland

Abstract

A geophysical survey of Manicouagan Reservoir (51.5°N; 68.3°W) has been undertaken using high-resolution swath bathymetry and acoustic subbottom profiling to explore the flooded Lake Manicouagan, Manikuakan in the Innu language. The drowned lake is a 60-km long arc-shaped basin lying within the Manicouagan impact crater, the so-called ‘Eye of Québec’. This dataset unravels the geomorphology and the late Quaternary environmental history of one of the deepest lakes in North America, which reached a natural depth > 320 m prior to the impoundment (> 100 m below modern sea level). The swath bathymetry imaged subaqueous landforms attributed to terrestrial inputs (e.g., sediment waves on drowned deltas, submerged ravines, deep lateral channels, subaqueous fans), slope features (e.g., gullies, slide scars, compression ridges), and water-level changes (terraces). The morphosedimentary record of Lake Manicouagan has no equivalent in regional fjord-lake surveys. The morphosedimentary record documents the successive paleoenvironments since the northward retreat of the Laurentide Ice Sheet margin 7.5 ka ago. The Late Quaternary history deduced from the acoustic dataset shows a three-stage evolution: deglacial, paraglacial/postglacial, and anthropogenic after the construction of the Manic-5 dam and the filling of the associated reservoir by ~140 m of water during the 1960s. The deposition of asymmetrical laminated infills during the paraglacial/postglacial stage probably resulted from the deflection of turbidity currents by the steep and sinuous sidewalls inherited from the impact structure, possibly under the joint influence of the Coriolis and centrifugal forces. Long-distance seismic source and/or large amplitude water-level fluctuations, either natural or man-made, are two potential triggers for lake-wide record of mass-movements initiated under and above the submerged shores. The 3D imagery of the drowned lakeshore is a means to promote a cultural landscape and preserve the memory of a heritage site for the Pessamiulnuat, the Innu of Pessamit, who occupied the lakeshore prior to the impoundment.