Sedimentology and regional significance of the ‘argillite unit’, a probable Cryogenian map unit in southeast Yukon, Canada

Abstract

AbstractAn unnamed succession of volcaniclastic argillite, sandstone, and conglomerate (the ‘argillite unit’) is exposed in an inlier in southeastern Yukon (NTS 95C/5). These strata were hornfelsed during emplacement of the Pool Creek syenite (ca. 640–650 Ma) and are correlated with the late Cryogenian Hay Creek Group of the Mackenzie Mountains based on lithostratigraphic evidence and published constraints from detrital zircons. The unit preserves three argillite lithofacies, two sandstone lithofacies, and two conglomerate lithofacies. The argillite and sandstone can be grouped into three facies associations, reflecting differing volumes of sandstone and the presence or absence of chaotic bedding. Deposition was mainly from sediment‐gravity flows, particularly turbidity currents and debris flows. Post‐depositional slumping was common, and the succession is interpreted to have been deposited on a slope that trended NNW and dipped to the ENE. This orientation was subparallel to but facing towards the contemporaneous slope of the Hay Creek Group in the Mackenzie Mountains. Conglomeratic facies are dominated by angular to poorly rounded clasts of subalkali basalt that probably were entrained in debris flows during or soon after eruption and otherwise saw little transport or weathering. Geochemistry of the clasts is permissive of a rift‐related setting. Following deposition, but prior to deposition of overlying Cambro‐Ordovician strata, the argillite unit underwent compression that produced broad, open folds, consistent with recent proposals for late Neoproterozoic transtension–transpression on the present‐day northwest margin of Laurentia. The argillite unit provides a snapshot of the geological evolution of southeast Yukon during the late Cryogenian, providing a new data point for reconstructing the protracted and complex rifting history of Rodinia in western Canada. © 2016 Her Majesty the Queen in Right of Canada Geological Journal © 2016 John Wiley & Sons, Ltd.