STRATIGRAPHIC TIES BETWEEN THE WINDERMERE SUPERGROUP AND HYLAND GROUP IN THE RACKLA BELT OF EAST-CENTRAL YUKON: IMPLICATIONS FOR THE AGE OF SELWYN BASIN

The Dawson thrust zone of central Yukon, Canada, separates Neoproterozoic-Cambrian slope and basinal rocks of the Hyland Group (Selwyn basin) from Proterozoic-Paleozoic shelf and slope rocks of the Ogilvie platform (Yukon block). The thrust zone hosts a variety of mineral occurrences, including recently discovered Carlin-type gold mineralization close to its eastern end. Stratigraphic relationships between Proterozoic-Cambrian successions on opposite sides of the Dawson fault zone have previously been uncertain. Here we provide the first documentation of stratigraphic relationships across the basin margin, in an area that is informally known as the Rackla belt. In this region, Proterozoic-Cambrian successions can be traced around the eastern tip of the Dawson thrust zone, demonstrating equivalence between the Hyland Group (Selwyn basin) and the upper part of the Windermere Supergroup (Yukon block).

The Neoproterozoic-early Cambrian Hyland Group, which comprises the oldest strata in Selwyn basin, includes three formations: the basal coarse-grained siliciclastic-dominated Yusezyu Formation, the overlying carbonate-dominated Algae Formation, and the uppermost fine-grained siliciclastic Narchilla Formation. The Yusezyu Formation is equivalent to at least three units in the Windermere succession – the Nadaleen, Gametrail and Blueflower formations. A late Ediacaran age (<~580 Ma) for these formations is indicated by the presence of Ediacaran body fossils in the Nadaleen formation, and by anomalous δ¹³C_carb values in the Gametrail Formation. This unit, and its equivalent carbonate member in the Yusezyu Formation are each characterised by δ¹³C_carb values up to -13‰ (VPDB). This δ¹³C_carbexcursion has been correlated with the Ediacaran Shuram/Wonoka anomaly, which is documented globally in ~580–560 Ma strata.

Although the base of the Hyland Group is not exposed elsewhere in Selwyn basin, these relationships indicate that most if not all of the Yusezyu Formation was deposited during the latter part of the Ediacaran. This in turn suggests a late Ediacaran age for the formation of Selwyn basin, possibly coincident with ~570 Ma rift-related volcanism in southern British Columbia.