AGE AND TECTONIC SIGNIFICANCE OF THE MID-NEOPROTEROZOIC CALLISON LAKE FORMATION OF YUKON, CANADA

The Callison Lake Formation (previously termed the Callison Lake dolostone) of the Coal Creek inlier, Yukon, Canada, is critical to our understanding of Neoproterozoic tectonism in northwestern Laurentia. Multiple measured stratigraphic sections through the Callison Lake Formation reveal a complex subsidence history in which episodic basinal restriction and facies change can be tied to local extension in the Mount Harper graben. The 4–75 m thick basal siliciclastic member hosts locally discontinuous stromatolitic bioherms and black shale with simple and poorly preserved vase-shaped microfossils. Here, we report a new Re-Os organic-rich rock isochron age from this horizon, which also broadly constrains the relative timing for the onset of Callison Lake subsidence. These strata are sharply overlain by a ~400 m thick medium- to thick-bedded platformal dolostone member, the basal 0­–80 m of which is locally associated with talc-rich shale and evaporite pseudomorphs suggestive of regional basin restriction. In some sections, these carbonate strata gradationally transition into finely laminated organic-rich black shale deposits that host an extraordinarily diverse vase-shaped microfossil horizon constrained by a Re-Os isochron age of 739.9±6.1 Ma. In other sections, this same depositional interval is marked by a significant paleokarst horizon, which is suggestive of differential subsidence and uplift associated with extensional fault rotation. The lateral facies variation in the Callison Lake Formation is closely matched with overlying alluvial fan and fan delta depositional patterns in the informal Seela Pass formation of the Mount Harper Group. These new Re-Os isochron ages, in combination with the overall tectonic setting of the Callison Lake Formation, suggest a clear stratigraphic correlation with the cupriferous Coates Lake Group of the Mackenzie Mountains, Canada. Given the stratigraphic, structural, and temporal relationships between these two successions, we suggest that the northwestern margin of Laurentia was characterized by dextral transtension and transpression between ca. 750–717 Ma.