Paraglacial origin for terraced river sediments in Bow Valley, Alberta

Abstract

Fluvial terraces flank the course of the Bow River for 100 km from the eastern margin of the Rocky Mountain Front Ranges to Calgary and beyond. The terraces are cut predominantly in gravel fill, which ranges in thickness from approximately 10 m in the Calgary area to 30 m near the mountain front. Sedimentary structures in the gravels indicate a braided stream sedimentary environment in contrast to the present quasi-stable, sinuous, single-channel form of the Bow River. Radiocarbon dates on ungulate remains from the gravels indicate the main period of fill occurred ca. 11 500–10 000 RCYBP (radiocarbon years before present). Previous workers have postulated that the gravels originated directly as outwash from a glacial advance to or beyond the mountain front. This explanation has been refuted by recent stratigraphic and palynological investigations. A complex nonglaciofluvial origin is proposed for these terraces and the sediments that form them. The last glacial advance to reach the mountain front was well into retreat by as early as ca. 13 400 RCYBP. Spruce and pine forest was established in the Bow River drainage by ca. 10 400–10 000 RCYBP and glaciers were restricted to high cirques. It is probable that the early period of fill deposition (ca. 11 500–10 000 RCYBP) was initiated when mountain tributary trunk streams of the Bow River were choked with debris-flow-delivered sediment during the construction of paraglacial debris fans and related phenomena. The debris flows were distinctive features of early nonglacial times, when landforms left unstable by ice retreat mass-wasted into the valleys. Paraglacial processes explain the early postglacial history of the Bow drainage and this example provides a model readily applicable to other drainages in formerly glaciated terrain.