ABSTRACT: Is glacial Lake Old Crow all ‘glacial’? Reconsidering constraints on the early NW Laurentide margin.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

During the late Pleistocene nearly 15,000 km2 of northern Yukon was occupied by a large proglacial lake system developed by blockage of easterly flowing tributaries to the Mackenzie that were re-routed into the unglaciated basins of northern Yukon. The sediments in these basins, their microfossils and associated radiocarbon ages have been used to put constraints on the timing of the NW Laurentide margin suggesting a maximum as early as 35 kyr. This age is seemingly at odds with the western margin elsewhere. Radiocarbon ages in the western plains suggest restricted Laurentide ice as late as 27 kyr in the Mackenzie valley and 22 kyr further south. Glacial lake Old Crow consists of two main sedimentary packages: a lower unit of laminated clays grading to stratified sands (Unit 2b of Morlan, 1996) and an upper glaciolacustrine unit of dark clays (Unit 3 of Morlan, 1996) found throughout the Old Crow and Bluefish basins. This latter unit represents the glacial lake highstand which ultimately drained into Alaska establishing modern westerly drainage. We investigated several sites in the Old Crow basin to address the issue, focusing on the lower stratified sediments since they host the ‘early’ ages for the glacial lake. In the southern Old Crow basin (CRH-11) a series of AMS ages from the stratified sands and silts (ie. Unit 3 of Morlan, 1996) range from ca. 40-44 kyr on discrete terrestrial plant macrofossils from the Keck and Oxford AMS labs. At CRH-106 an ice-wedge cast underlies the upper lake sediments with a composite fill including abundant terrestrial organics and beetle remains with ages ranging from ca. 32-22 kyr. These represent deformation of surface soil organic detritus into the wedge-fill and indicate non-lake conditions existed in the Old Crow basin, at least locally, prior to the upper lake, consistent with a series of large mammal ages in the basin from ca. 40-22 kyr. Palaeoecologically, these macrofossil assemblages include a range of boreal, tundra and steppe components – seemingly an interstadial assemblage. The appearance of the lower stratified sands and clays indicate a shallow braid-delta plain occupying the basin. It seems likely this unit represents a late Pleistocene tectonically-controlled lake associated with the Richardson fault existing prior to the proglacial lake. If this is correct, it would indicate the western Laurentide margin maximum extent is in fact quite late and may be after 16 kyr. Alternatively, could we have had a Laurentide-influenced margin around 40 kyr? This model would require at least two advances of the western Laurentide margin to the maximum position with retreat to near the edge of the shield, seemingly unlikely given the stratigraphic record of the Laurentide ice sheet in the Mackenzie valley and mountains to the south.

Bibliography of Canadian Geomorphology