Quaternary stratigraphy and glacial history of the Fort Nelson (southeast) and Fontas River (southwest) map areas (NTS 094J/SE and 0941/SW), northeastern British Columbia




Trommelen, Michelle Suzanne

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The study area in northeast British Columbia extends from the Rocky Mountains in the west to the Fort Nelson Lowland in the east, and includes the westernmost extend of the Laurentide Ice Sheet (LIS) and the easternmost extend of the Cordilleran Ice Sheet (CIS) in the Late Pleistocene. Surficial mapping conducted over portions of the Fontas and Fort Nelson map areas (NTS 094I/SW and 094J/SE, respectively) provides information on sediment distribution and characteristics as well as glacial history. This information has direct implications for geotechnical investigations, aggregate resources and diamond exploration in the region. Non-glacial pre-Late Wisconsinan sediments occur at multiple sites along the Prophet River, providing a pre-glacial or interglacial history for the area. Geochemical analysis and clast lithologies were used to differentiate between sediments derived from the LIS to the east, and Montane/CIS glaciers to the west. The Quaternary stratigraphy of the Prophet River valley indicates the presence of a paleo-Prophet River valley system. Nonglacial deposits in the paleovalley include overbank fines and floodplain sediments interbedded with fluvial gravels. Macrofossils within horizontally laminated organic-rich black clay and silt are interpreted to indicate deposition in the floodplain of the paleo-Prophet River within oxbow-lakes and possibly also sag ponds. The climate is interpreted to be similar to present within a dominantly spruce forest. Wood found at one site provided a radiocarbon date of 49 300±2000 BP, while wood obtained from five other sites provided non-finite radiocarbon ages. In the Late Wisconsinan, the LIS advanced west-southwest into the study area, blocking existing east-flowing regional drainage, and forming an ice-dammed proglacial lake in the Prophet River valley. Ice overrode these sediments and deposited clast-poor clayey-silt till over the entire region. Thicknesses range from less than one metre to greater than twenty metres in the Prophet River valley. In river-cut sections near the Rocky Mountains in the Fort Nelson and Tuchodi Lakes map areas, potassium-feldspar rich granitoid and gneissic clasts, derived from the Canadian Shield, are generally found only east of the foothills, except along the Tetsa River valley. Near the mountain front, in Tuchodi River valley, outwash from Montane/Cordilleran glacial meltwaters was deposited before the LIS advanced and ponded the valley in the Late Wisconsinan. The geochemistry of 303 till samples collected throughout the study area is used to evaluate the regional glacial history inferred from stratigraphic and geomorphic data. Three different geochemical populations are recognized and corroborated by clast lithology (relative percent) from 56 till and glaciofluvial samples. One population, covering the northeast portion of the study area, was likely deposited by the LIS where it extended west into the Rocky Mountain front during the Late Wisconsinan. The second population suggests that the eastern extent of Montane/Cordilleran ice during the Late Wisconsinan was at least to the Rocky Mountain Foothills; however its easternmost position remains unknown. The third population can be attributed to Late Wisconsinan LIS reworking sediment deposited on the Interior Plains by the CIS, either in the Late Wisconsinan or earlier. During early deglaciation, the ice retreated to the east-northeast, impounding local drainage at the ice margin, forming Glacial Lake Prophet in the Fort Nelson map area and Glacial Lake Hay in the Fontas map area. Glacial lakes followed the retreating ice margin and drained through a variety of meltwater channels. The exposed glacial lacustrine plain became a source for sand dunes oriented southeast indicating katabatic paleowinds from the northwest (NTS 094I/SE).



stratigraphic geology, quaternary, British Columbia