SUBTILL ORGANIC NONGLACIAL DEPOSITS IN THE HUDSON BAY LOWLANDS, ONTARIO, CANADA: AN UPDATE

Subtill organic nonglacial deposits have been reported from riverbank exposures of all major river valleys in the Hudson Bay Lowlands. They developed in lakes, rivers, and inland seas. A common feature from these sites is that, despite the many reports in the literature, in any individual sections examined in the field so far in Ontario, only one such nonglacial deposit has been observed below the surface tills. Pollen from these deposits indicates exclusively a vegetation resembling the present-day boreal forest with intervening wetlands. Although local names are assigned, e.g., the Webequie Beds, the Fawn River Sediments, these nonglacial deposits are usually correlated with the Missinaibi Formation as defined in the Moose River drainage basin in the southern part of the lowlands. They are, in general, beyond the method limit of radiocarbon dating. Consequently, dating of them has been proven very challenging. The nonglacial deposits are often correlated to the Last Interglacial (MIS 5e or MIS 5) based on their stratigraphic position, i.e., the only organic deposits observed before the Holocene peat. However, radiometric dates with an affinity to this geological episode are rare. Previous luminescence dating (TL and OSL) has yielded various, inconclusive results. Recent OSL dating on the newly found subtill organic lacustrine deposits has returned consistent dates at 126 to 105 ka on the Winisk River and the Little Current a tributary of the Albany River, indicating, within the dating uncertainties, an MIS 5e affinity. The climate was likely significantly warmer than today as indicated by the change of lake geochemistry and development of numerous, sand-sized, sulphur-rich nodules on the floor of the former lakes. Current work is focused on 3 drill cores recently obtained in the Moose River drainage basin, which contain, between tills, organic lacustrine deposits. The drill cores are located near the type sites of the Missinaibi Formation and, therefore, provide a rare opportunity to further examine the subtill nonglacial deposits as to their age and climatic conditions.