Neoproterozoic Carbonate Ramp Development in the Canadian Arctic (Shaler Supergroup): Sedimentology, Sequence Stratigraphy, and Chemostratigraphy

Abstract

This study describes Tonian-Cryogenian pre-rift strata of the upper Shaler Supergroup, deposited in the Amundsen Basin (Victoria Island, Canada). The Wynniatt Formation, part of the upper Shaler Supergroup, consists of three carbonate ramp successions separated by regional unconformities. Each cycle represents part of a distally steepened, storm-dominated carbonate ramp. Temporal changes in carbonate facies record depositional environments spanning from supratidal to outer ramp settings. Interpretation of an intertidal mudflat depositional environment for the Tawuia-Chuaria assemblage zone provides depositional context for interbasinal biostratigraphic correlation. Correlation between the Amundsen and Fifteenmile (Yukon) basins is complicated by differing rates and regimes of subsidence, with the exception of a regional basin-deepening event. Contrary to previous correlations, it is proposed that the upper Shaler Supergroup and Little Dal Group of the Mackenzie Mountains Supergroup (Mackenzie Basin) are equivalent to the entire Fifteenmile Group. Five transgressive-regressive (T-R) cycles were identified in the Jago Bay, Minto Inlet, and Wynniatt formations. Three tectonostratigraphic units for the upper Shaler Supergroup record an initial sag basin, followed by early extension and thermal doming, and finally rifting of the Amundsen Basin. Neoproterozoic successions of northwest Laurentia are an example where rift and intracontinental basins are spatially and temporally related. Subsidence possibly was related to multiple cycles of intra-plate extension that complemented coeval fault-controlled subsidence. An improved δ13Ccarb curve paired with δ13Corg values provides support for correlation of a significant negative δ13C excursion that has been linked to the Bitter Springs stage. In the Amundsen Basin, δ13Ccarb values drop to markedly negative values (> -14 ‰), which is attributed to local overprints whereby isotopically light carbon in pore waters was precipitated as authigenic carbonate cement fuelled by oxidation of methane or organic matter during sulphate and iron reduction. Redox proxies indicate local basin euxinia and anoxia. Pronounced Mo, V, and U enrichments in euxinic black shales suggest that the Bitter Springs stage was a transitional time, where the global oceans became more oxygenated relative to preceding portions of the mid-Proterozoic, but remained more reducing compared to the Phanerozoic.