Alejandra Duk-Rodkin

Late glacial drainage systems along the northwestern margin of the Laurentide Ice Sheet

Abstract The evolution of drainage systems along the retreating northwestern Laurentide Ice Sheet was complex. The interaction of ice-margin configuration, topography and glacioisostasy resulted in a network of meltwater rivers that variably overflowed to the Arctic and Pacific Oceans and to the Gulf of Mexico. Glacial lakes also changed dramatically in size and location during the period of deglaciation. At the last (and all time) glacial maximum, the ice sheet extended into the eastern Cordillera, blocking northward and eastward drainage to the Arctic Ocean. Some meltwater and most non-glacial runoff were diverted through the mountains to the Yukon River basin, into Alaska and the Pacific Ocean. Retreat from the glacial maximum prior to 21 ka BP allowed proglacial drainage from the western margin of the ice sheet to flow into the Beaufort Sea/Arctic Ocean. Deglaciation was rapid after about 13 ka BP, with the present route of the lower Mackenzie River established between 13 and 11.5 ka BP. Continued ice retreat led to significant southward expansion of the Mackenzie/Beaufort drainage basin at about 11.5 ka BP through drainage capture of glacial Lake Peace, which previously had drained southeastward into the Missouri River and to the Gulf of Mexico. Very rapid ice retreat between 10.5 and 10 ka BP allowed glacial lake McConnell to expand down-slope in contact with the ice margin. Numerous glacial lakes occurred along the northwestern margin of the ice sheet during the maximum and retreat phases. These include ice-dammed glacial Lake Old Crow, which occupied unglaciated terrain of the northern Yukon, and glacial Lake Peace, which utilized a number of outlets as it migrated eastward with the ice front along the Peace Valley. The largest glacial lakes in the region were the result of glacioisostatic depression reversing the regional drainage. The Mackenzie Phase of glacial Lake McConnell was the second largest Pleistocene lake in North America (> 215,000 km 2 ). Late glacial and post-glacial changes in drainage systems were largely in response to isostatic rebound.

Tertiary-quaternary drainage of the Pre-glacial Mackenzie basin

Abstract Pre-glacial drainage in the northwestern Mackenzie Region was established in early Tertiary time, with the uplift of the mountains during the Laramide Orogeny, and persisted until near the end of the Pleistocene. Rivers arose in the Mackenzie and Richardson mountains and followed a generally eastward direction across piedmont surfaces from the mountains towards the plains. The evidence to support this hypothesis includes geomorphology, provenance of sediments, reconstruction of paleo-surfaces and stratigraphy. Two major drainage systems developed — one to the Atlantic Ocean and another to the Arctic Ocean. The Atlantic drainage is composed of the northwestern tributaries of an integrated drainage system that flowed towards Hudson Bay. Traces of this drainage are found in the Canyon Ranges of the Mackenzie Mountains, Iroquois upland and in the Norman Range of the Franklin Mountains. The Arctic drainage included two main basins — the Porcupine Basin that drained from west to east across the Richardson Mountains and the Anderson basin that included the Peel and Snake rivers as its tributaries. Late Wisconsinan Laurentide glaciation deranged these drainage systems establishing the Porcupine River as a tributary of the Yukon River and integrating the Tertiary drainage systems into the Mackenzie River system flowing to the Arctic Ocean.

Chronology and extent of Late Cenozoic ice sheets in North America: A magnetostratigraphic assessment

Publisher Summary This chapter discusses the classic four-fold subdivision of the Pleistocene in both North America and Europe dominated terrestrial Quaternary palaeoclimate, which have been studied for many decades. With the exception of the last major continental glaciation, the understanding of the extent and timing of ice-sheet development in North America has remained uncertain. With the more widespread use of magnetostratigraphy and detailed mapping of superficial deposits, it has become possible to identify the approximate: spatial extent of ice sheets, as well as the timing of their appearance and disappearance. It reviews that with appropriate sampling and analytical techniques, geomagnetic polarity data can be obtained from tills, as well as from altered sediments such as palaeosols, to provide a direct assessment of the palaeomagnetism of glacial and interglacial environments. The chapter discusses the advances which have been made in dating and modeling of past terrestrial climatic events. It has been in particular, the contribution of magnetostratigraphy which has provided timelines for glacial and interglacial events of the last 3.0 million years, and assigned sediments to the Chrons and Subchrons of this period. In the absence of absolute dating tools, magnetostratigraphy affords a valuable means of assigning terrestrial ice age deposits to the geological timescale, and most importantly, allows a correlation to the more complete marine record. The distribution of past ice sheets and their chronologies will be better defined with future magneto-stratigraphical work.

Timing and extent of plio-pleistocene glaciations in north-western canada and east-central alaska

Abstract North-western Canada and eastern Alaska are recognised as having one of the oldest known continental glacial records (Late Pliocene) preserved in stratigraphical sections. These include the individual and complex records of Cordilleran, montane and continental glaciations. Regional scale glaciations (Cordilleran and continental) started in northwestern Canada and east-central Alaska between 2.9 and 2.6 million years ago. Overall, two Cordilleran glaciations and two plateau ice caps (Horton Ice Cap) developed in Late Pliocene (Gauss and Matuyama Chron). During the Early Pleistocene, three Cordilleran glaciations occurred, while one to five continental glaciations (Keewatin Ice Sheet and Horton Ice Cap) are inferred from the Banks Island stratigraphic record (late Matuyama Chron). Three Middle-Pleistocene glaciations are recorded for the Cordilleran (including the Reid Glaciation) as well as three continental (Keewatin Ice Sheet and Horton Ice Cap) events (early Brunhes Chron). During the Late Pleistocene (late Brunhes) a well defined, extensive continental ice sheet (Keewatin) covered western and northwestern Canada, while in the Yukon Cordillera and Yukon-Tanana Uplands, two glaciations (Early-Late Pleistocene Eagle Glaciation, and Late Pleistocene McConnell Glaciation) are recognised. Successive Cordilleran glaciations diminished in size, while continental glaciations increased. The moisture source for the Cordilleran ice was largely the Pacific Ocean, however, for the Horton Ice Cap, an open Arctic Ocean may have been a significant moisture source. The role of tectonics in the development of the two major physiographic barriers, Wrangell/St. Elias Mountains and the Continental Divide (Mackenzie/Selwyn Mountains) appears to have been an important controlling variable in moisture distribution in northwest Canada and east-central Alaska. The timing and interplay of tectonic uplift versus erosion of these barriers has very much controlled the growth, thickness and decay of ice masses in the interior of Yukon and valleys east of the continental divide.

Late pleistocene chronology of glacial lake old crow and the north-west margin of the laurentide ice sheet

Late Pleistocene advances of the Laurentide Ice Sheet in the northern Cordillera diverted westward the regional preglacial arctic drainage of the Porcupine River, northern Yukon, Canada. Glacial advance caused flooding of glacial Lake Old Crow which occupied Old Crow Basin, originally inundated by the northward diversion of Peel River drainage by a lobe of the Laurentide Ice Sheet that occupied Bonnet Plume Depression to the headwaters of Eagle River during the ice sheet maximum. Further glacial lake transgression expanded into Bell, Driftwood and Bluefish basins and Upper Rat River valley, covering an area of c. 13,000 km2. Radiocarbon dates from an exposure along the south-western edge of Bluefish Basin place ages on the deposition of near-shore deltaic sediments associated with the maximum glacial Lake Old Crow transgression, c. 18.8–16.4 ka BP. A synthesis of regional stratigraphic and geochronological data indicates two phases of glacial Lake Old Crow transgression; Stage 1, c. 35–22 ka BP, correlative to the all-time maximum advance of Laurentide ice; and Stage 2, c. 22–16 ka BP, correlative to the Katherine Creek phase readvance. Final drainage of glacial Lake Old Crow by 14.8 ka BP caused rearrangement of the regional drainage, establishing the present Porcupine River as a tributary of the Yukon River system into Alaska.

Late Quaternary vegetation history of the central Mackenzie Mountains, Northwest Territories, Canada

The sedimentary and pollen records from three lakes in the tundra, forest-tundra and open forest of the central Mackenzie Mountains of Canada, are described. Although two of the lakes lie beyond the mapped limits of late Quaternary glaciation, their records only extend to ∼ 12,000 yr B.P. The third site lies within the limits of the late Wisconsin Laurentide advance and a basal radiocarbon date provides a minimum date of ca. 11,500 yr B.P. for retreat of the northwestern margin of Laurentide ice. Artemisia and Salix-dominated herb-tundra occurred at all elevations from at least 12,000 yr B.P. until ca. 10,200 yr B.P., when shrub tundra dominated by Betula glandulosa expanded throughout the region. Populus balsamifera was also present by this time and probably grew up to and beyond the current treeline. Picea arrived in the central Mackenzie Mountains by 8500 yr B.P., and expanded to the elevation of the current treeline by 8000 yr B.P. Vegetation composition became similar to that of the present by 6000 yr B.P. with the expansion of Alnus at all elevations. Pollen accumulation rates and detrended correspondence analysis suggest Picea population densities in the forest-tundra were greater than present between ca. 8000 yr B.P. and 5000 yr B.P. and have since declined steadily. At the open forest site, Picea population density has not declined in the late Holocene, but the forest composition has shifted towards a greater affinity with higher elevation sites. In general, the changes apparent in the treeline vegetation of the Mackenzie Mountains during the postglacial are consistent with predicted changes in summer insolation based on the Milankovitch theory.

An extensive late Cenozoic terrestrial record of multiple glaciations preserved in the Tintina Trench of west-central Yukon: stratigraphy, paleomagnetism, paleosols, and pollenThis is a companion paper to Barendregt et al., also in this issue.Geological Survey of Canada Contribution 20100035.

The Tintina Trench in west-central Yukon is a late Miocene graben formed along the antecedent early Tertiary Tintina fault. Since its formation the trench has served as a sediment trap for alluvial and glacial deposits. An extensive record of preglacial, glacial, and interglacial sediments spanning the late Pliocene to late Pleistocene has been preserved and is exposed today in modern landslide scars. This sedimentary record comprises multiple sequences of tills, outwash, mudflows, loess, and paleosols. The glacial sediments are the product of both local (montane) and regional (Cordilleran) ice advances that channeled into the trench, while loess and well-developed paleosols (brunisols and luvisols) reflect nonglacial and interglacial conditions, respectively. The Tintina Trench exposures provide the most complete record of glaciations for the region. Paleomagnetism, paleosols, and palynology provide age constraints for the geological events. A formal stratigraphic nomenclature is proposed for this region...