Chronostratigraphy of the Beaufort Formation, western Canadian Arctic Archipelago
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The Beaufort Formation (BF) braided river deposit contains exceptionally well-preserved logs, leaves, peat, insects, and vertebrate fossils that provide key evidence for Arctic environmental conditions during the Pliocene. Its wide geographic range along the western edge of the Canadian Arctic Archipelago suggests that its deposition and incision history were dictated by regional drivers of sediment transport (e.g., eustatic sea-level, permafrost thaw, ice sheet erosion, and dynamic topography). Hence, the BF provides clues about both environmental and depositional conditions, but available chronology in the last few decades has not been able to identify these as having occurred either 1) during the polar amplification of global warming (Pliocene Climate Optimum, 3.3-3.0 Ma) or 2) during the onset of northern hemisphere glaciations (Plio-Pleistocene transition, 2.6 Ma). We use cosmogenic nuclide burial dating at the southernmost BF locality (Ballast Brook on Banks Is.) to obtain: a) a minimum age of 2.72 (+0.34)⁄(-0.24) (1 σ) Ma, and b) a maximum catchment-wide paleo-erosion rate of 49-86 ± 2 cm/ka. The description of a previously unreported glaciofluvial gravel (which occurs at the same stratigraphic level as a potential ice-wedge pseudomorph and coincides with the base of a previously-mapped 3-km wide cut-and-fill channel) dates the earliest evidence of CAA glaciation, at 2.72 Ma. The presence of a large channel that runs parallel to the northern coast of Banks Is. also suggests that M’Clure Strait (and the Northwest Passage) was not open at that time. Furthermore, such a large (correlative with part of the 3-km Iperk Formation offshore) and quick (49-86 ± 2 cm/ka) deposition event must have required the stripping of unconsolidated material (e.g, part of the Eureka Sound Group or Hassel and Isachsen Formations). The deposition system was likely a transport-limited system, and like the White Channel Gravels of the Yukon (Hidy et al. 2013), may have been controlled by changing climate in the Pliocene (e.g., melting permafrost, increased precipitation).