NEW INSIGHTS ON WISCONSINAN STRATIGRAPHY AND ICE DYNAMICS OF UPPER ST. LAWRENCE LOWLANDS NEAR MONTREAL

In the Upper St. Lawrence Lowlands, near Montreal, the classical interpretation had assigned the regional ice flow episode toward SE to the last glacial maximum (LGM) and the younger flow episodes toward S and SW to a gradual late-glacial shift. New data from the Lowlands west of Montreal indicate that this model must be revisited. The SE flow is now assigned to an early ice advance, most likely Early Wisconsinan in age, which shifted toward south subsequently. During and after the LGM, regional flow patterns were controlled by an ice stream and its catchment area which were feeding the Ontario Lobe. At that time, ice flow may have been briefly toward south but it was predominantly toward southwest. It is also inferred that this flow which had accelerated at the beginning of deglaciation as a result of glacial streaming slowed down with the development of a well organized system of subglacial channels. New subsurface data reveal two superposed tills overlying proximal glaciolacustrine sediments. Lithologic and geochemical results indicate that the lower till was deposited by ice flowing toward SE or S over its own proglacial suite of glaciolacustrine sediments while the upper till was clearly deposited by ice flowing toward SW. The contact between the two till units is sharp, indicating that the shift was probably rapid. Fan-shaped niobium and rare-earth element dispersal trains derived from known Nb-hosting rocks show that the different flow episodes were of equivalent magnitude. At Pointe-Fortune, Nb and REE concentrations in the upper till also increase near the top, which suggests a shift from SE or S toward SW. The newly-discovered till unit records an early SE glacial advance which only persisted until the development of the Ontario Lobe while the S and SW flow episodes affected the region for a much longer period than previously interpreted. This region thus lay inside the catchment area of the Ontario Lobe well before the start of deglaciation, which is in much better agreement with the dispersal of erratics from known sources and with till provenance studies from the Great Lakes region.