Sedimentological and geochemical evolution of an Ediacaran mixed carbonate-siliciclastic continental slope system, Windermere Supergroup, southern Canadian Cordillera, British Columbia, Canada

Abstract

Abstract In this study a 165–265\u202fm-thick siliciclastic-carbonate (mixed) base-of-slope succession within the Ediacaran Isaac Formation (hereafter termed FIC) in the deep-marine Windermere Supergroup (WSG), southern Canadian Cordillera was analyzed. Excellent outcrop exposure at three study areas (Castle Creek, Hill Section and Milk River), spaced over a distance of 20\u202fkm parallel to depositional dip provided the opportunity to observe the stratigraphic and inorganic carbon (δ13Ccarb) isotopic evolution of an ancient mixed, passive margin deep-water system. Based on detailed sedimentological analysis six stratal elements make-up the FIC (channel complexes, levees, siliciclastic-to-carbonate tripartite deposits, scour dominated units, debrites and slides). Within these strata the most voluminous carbonate phase (C1) represents neomorphism of primary aragonite on the slope and during early burial, and thus preserves the primary shallow-marine isotopic signature even after downslope transport. At the base of the FIC δ13Ccarb is negative (−5.2‰) and then progressively increases to 2–3‰ in the middle of the succession, after which it plateaus for about 65\u202fm before declining to a nadir of −6.3‰ at its top. Coincident with these isotopic changes are systematic changes in the architecture and mineralogical composition of the component stratal elements that makeup the FIC, which collectively suggest long-term transgressive followed by regressive conditions that quite possibly are tied to rising and then falling sea-level, and accordingly the state of the continental shelf carbonate factory. Additionally, the upward change in δ13Ccarb closely resembles the trend reported in the upper June beds-Gametrail Formation of the WSG in the northern Canadian Cordillera, but the interpreted association of the latter with the Shuram-Wonoka anomaly (570–551\u202fMa) makes its correlation with the FIC unlikely. Instead, the FIC excursion potentially correlates with the EN2 excursion in the Doushantuo Formation of South China (and its potential global correlatives), which has been interpreted to be associated with the Gaskiers glaciation (580\u202fMa). This suggests that the FIC might be a stratigraphic and δ13Ccarb record of conditions that prevailed in the deep sea preceding the Gaskiers, which previously has been unreported from strata of the WSG.