Stratigraphic and geochemical evolution of the Old Fort Point Formation, southern Canadian Cordillera: The deep-marine perspective of Ediacaran post-glacial environmental change

The ∼608 Ma Old Fort Point Formation (OFP) is a unique, mixed siliciclastic and carbonate, stratigraphic marker that is exposed locally over an area of 35,000 km2 within the Neoproterozoic Windermere Supergroup (WSG) of the southern Canadian Cordillera. The OFP accumulated in bathyal to upper abyssal settings along the ancient western continental margin of Laurentia. It comprises three lithostratigraphic members that can easily be distinguished from the enveloping strata of the deep-marine WSG. The Temple Lake and Geikie Siding members are generally uniform in thickness and lithofacies and are interpreted to reflect basin-wide, synchronous deposition. These basal Ediacaran trangressive and highstand deposits, respectively, accumulated during a post-glacial eustatic rise that essentially shutdown the supply of coarse-graine siliciclastic sediment into the deep-marine part of the Windermere basin. Regionally widespread bottom-water anoxia developed during the post-glacial highstand and is indicated by prominent enrichments in organic carbon and seawater redox-proxies (e.g. Mo, V/Cr). Subsequent regional tectonic uplift and related relative sea-level fall is indicated by the diachronous deposition of the Whitehorn Mountain Member. Newly created sediment transport pathways such as submarine canyons, controlled the timing, location, and composition of Whitehorn Mountain Member strata which accumulated mostly during the ensuing relative sea-level rise. Limestone units of the OFP are characterized by negative delta 13C values that stratigraphically-upward exhibit a positive ∼12‰ shift, in addition to a 1 to 2‰ shift towards more negative values from shallow to deep water environments. These results are similar to other globally distributed, basal Ediacaran transgressive post-glacial carbonates related to melting of the ca. 635 Ma terminal Cryogenian glacial event, including correlative WSG strata exposed in the Mackenzie Mountains, northern Canadian Cordillera. The physical and isotopic characteristics of the OFP are interpreted to be most consistent with the hypothesis of post-glacial upwelling and oceanic stratification.