Quaternary geology of Fort Liard map area, Northwest Territories

Abstract

Detailed mapping of the surfi cial geology of the Fort Liard area (NTS 95 B) provides insight into the dynamics of the Laurentide Ice Sheet as it impinged against the Cordilleran Ice Sheet during the last glacial maximum. Overall, ice advance over the plains was from the northeast, but as the ice sheet approached the mountain front, the fl ow was defl ected southward, up the lower Liard River valley. In the southern part of the map area, curving fl owlines record the collision and northward defl ection of the Laurentide Ice Sheet when it impinged on the Cordilleran Ice Sheet in the mountains west of the map area. High-elevation striations and Canadian Shield erratics show that the entire region was glaciated during this time. Once deglaciation began, an expanding corridor between the two ice sheets was instrumental in channelling meltwater from at least 500 km of the Laurentide ice margin, northward, into the Mackenzie Basin. During early deglaciation, drainage divides connecting northward-trending mountain valleys controlled proglacial lakes levels (>600 m a.s.l.). With eastward retreat of the Laurentide ice margin, glacial lake levels continued to fall as lower outlets became ice-free. About 11.5 ka BP, a large glacial lake formed in Liard River valley when the Laurentide ice margin disengaged from the mountain front. The entire map area was deglaciated shortly after 11 ka BP. Glacial Lake Liard fi nally drained into Mackenzie River valley when the ice margin retreated east of the confl uence of the Liard and Mackenzie rivers.