STRATIGRAPHIC RELATIONSHIPS BETWEEN THE WINDERMERE SUPERGROUP AND HYLAND GROUP IN THE RACKLA BELT OF EAST-CENTRAL YUKON: IMPLICATIONS FOR THE AGE OF SELWYN BASIN AND FORMATION OF THE LAURENTIAN PASSIVE MARGIN

The northern margin of Selwyn basin is marked by the north-vergent Dawson thrust – a Mesozoic contractional structure that coincides with the position of an antecedent fault that has been periodically active since the Proterozoic. The Dawson thrust separates Neoproterozoic-Cambrian rocks of the Hyland Group (Selwyn basin) from Proterozoic-Paleozoic shelf and slope rocks of the Ogilvie platform (Yukon stable block). Depositional relationships across the basin margin are generally obscured by faults and younger strata, but stratigraphic relationships are preserved at the eastern end of the Dawson thrust zone, in an area that is informally known as the Rackla belt.

The Neoproterozoic-early Cambrian Hyland Group, which is the oldest unit in Selwyn basin, includes three formations. The basal Yusezyu Formation comprises up to 3km of coarse siliciclastic rocks and brown shale. It is overlain by the carbonate Algae Formation, followed by the Narchilla Formation, which is dominated by maroon, green and brown shale. In the Rackla belt, the Algae and Narchilla formations can be traced across and around the eastern end of the Dawson thrust zone, where they overlie the Ediacaran Blueflower Formation of the Windermere Supergroup. This indicates equivalence of the Algae and Narchilla formations (Hyland Group) with the Risky and Ingta formations, which are the youngest units in the Windermere Supergroup. A carbonate unit near the top of the Yusezyu Formation is correlative with the Gametrail Formation of the Windermere Supergroup. δ13Ccarb analyses from the Gametrail Formation yield values as low as -13‰ (VPDB) and increase up-section to positive values near the top of the formation. This carbon isotope excursion has been correlated with the global Shuram/Wonoka anomaly, which is documented globally in rocks deposited around 560 Ma. These data provide a minimum age for deposition of most of the coarse clastic rocks of the Yusezyu Formation and the continental rifting that accompanied the formation of Selwyn basin.