Physical and geochemical controls on sedimentation along an ancient continental margin: The deep-marine Old Fort Point Formation (Ediacaran), southern Canadian Cordillera

Abstract

Abstract The approximately 608 Ma Old Fort Point Formation (OFP) is a unique, mixed siliciclastic and carbonate stratigraphic marker horizon exposed locally over an area of about 35 000 km2 within the Neoproterozoic Windermere Supergroup (WSG) of the southern Canadian Cordillera. Paleogeographically, these strata were deposited by a variety of deep-marine processes in slope to basin-floor settings along the continental margin of Laurentia (ancestral North America). The OFP ranges from approximately 60 to 450 m thick and comprises three informal lithostratigraphic members, which stratigraphically upward are the Temple Lake (TLM), Geikie Siding (GSM) and Whitehorn Mountain (WMM) members. The TLM and GSM are generally uniform in thickness and lithofacies and are interpreted to reflect basin-wide, synchronous deposition. The TLM is characteristically composed of variably coloured fine-grained rocks that stratigraphically-upward grade from siltstone to rhythmically bedded limestone-siltstone couplets. These fine-grained strata represent deposition during a major, post-glacial eustatic rise that essentially terminated the supply of coarse-grained siliciclastic sediment into the deep-water part of the Windermere basin. These strata are then gradationally overlain, although over an interval only <0.5–2 m thick, by strata of the GSM that stratigraphically-upward become progressively more organic-rich. The GSM is interpreted to have accumulated during terminal transgressive and highstand conditions, likely in anoxic bottom-water below a postulated pycnocline. The youngest member of the OFP is the WMM. The base of the WMM is almost always sharp and locally marked by scours more than 100 m deep. This surface is interpreted to be the result of an abrupt fall of relative sea level related to regional tectonic uplift, which in turn caused widespread mass wasting on the slope, and locally the formation of deeply incised submarine canyons. Sediment that accumulated in the canyons and more distally on the basin floor during the ensuing lowstand and early transgression was composed of remobilized slope sediment mixed with texturally and mineralogically immature clastic sediment derived directly from the hinterland. As transgression proceeded, the shelf eventually became flooded and shut-off the hinterland sediment supply. At the same time (late transgression and/or highstand), however, new, more landward canyons formed, or existing canyon heads eroded headward and reactivated the slope-basin floor transport system. This time, however, sediment was made up of a mixture of texturally and mineralogically mature palimpsest siliciclastic and carbonate shelf sediments. Notwithstanding its unique lithological and geochemical characteristics, strata of the OFP are underlain and overlain by a thick monotonous succession of mostly siliciclastic deep-marine sandstones and mudstones, suggesting that deposition of the OFP represented only a short-term anomaly superimposed on a much longer-term history of physical and geochemical stability throughout the Neoproterozoic deep-water Windermere basin.