ABSTRACT: Cosmogenic nuclide burial dating of the Late Cenozoic Beaufort Formation and equivalent deposits in Western and High Canadian Arctic.

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Abstract

The Beaufort Formation and equivalent deposits of Pliocene age in the western and high Canadian arctic primarily comprises interbedded fluvial gravel and sand, with centimetre to decimetre-thick peat beds in increasing in abundance toward the top. The Beaufort Fm. conformably overlies the Miocene-Pliocene Ballast Brook Fm. but also unconformably covers older Cretaceous-Tertiary units. The upper surface is typically parallel to uppermost bedding which generally dip westward toward Beaufort Sea. Sedimentology, stratigraphy, faunal and floral paleontology, and paleoenvironmental reconstructions have been published for some previously accessed sections but geochronology has been limited to paleontological correlations and paleomagnetic stratigraphy. The formation has been interpreted as an extensive wedge of clastic sediments with provenance to the east. The sediment package, which is less than 100 m in most places, provides important Arctic paleoenvironmental change information regarding (i) paleoclimatic and paleoceaongraphic variations recorded by a period of extensive (pan-western archipelago) deposition followed by rapid incision; (ii) paleontological assemblages which document warmer and wetter than present-day conditions; which may have been (iii) punctuated by periods of cooling to a sufficient degree to cause ice wedging; and (iv) the possibility that the present-day interisland channels were filled with Beaufort Fm and equivalent and older sediments until late-Pliocene Arctic glaciation. Hypotheses regarding the genetic history of the Beaufort Fm. and equivalent packages—and its abandonment—include (i) sediment yield and discharge fluctuations due to arctic climate changes induced by the formation of the Isthmus of Panama or other oceanographic change, (ii) fluvial responses to sea level changes, (iii) changes in glaciofluvial flux and glacial erosion with variations in extent of ice sheets and ice caps in North America and Greenland, (iv) sedimentation related to tectonic or dynamic topographic forcing, or combination of these. We have begun a five year cosmogenic nuclide burial dating programme to improve the chronology of previous paleontological and sedimentary discoveries and to use chronological strategies to test some of these hypotheses. In 2008 we revisited two previously studied sections, the “Beaver Pond site” and the “100-metre section” exposed in Stathcona Fiord, southwestern Ellesmere Island. At both localities approximately 400 m of fluvial or glacial incision has exposed sediments that are similar to Beaufort Fm mapped in the western Archipelago. The fluvial sediments are capped with up to 2 m of till. We collected five coarse sand samples at >10 m depth below the surface at each site, for burial dating using 26Al/10Be in quartz. At the former site, the Beaver Pond peat is stratigraphically above the sampled sands so the burial age will provide a maximum age estimate for the fossil assemblage and for the first glaciation and valley incision. At the latter site, the samples were collected within a zone of thin leaf beds, below thick peat beds and till, and bracket ice wedge casts that appear between 27 and 91 m depth. Burial dating at other western and high Arctic localities is planned for future years.

Bibliography of Canadian Geomorphology