Person:

Smith, Emily

Ongoing mapping, chemostratigraphy, geochronology, and stratigraphic analysis of Neoproterozoic successions in the Ogilvie Mountains requires redefinition and correlation of the Fifteenmile Group across the Proterozoic inliers in Yukon. Here we present new stratigraphic logs through the lower Fifteenmile Group in the Coal Creek and Hart River inliers. Based on these data and new observations, we propose redefinition of the lower Fifteenmile Group. A succession dominated by sandstone, mapped as unit PPD1 in the Hart River inlier, is now recognized at the base of the Fifteenmile Group in the Coal Creek inlier. These strata unconformably overlie the Pinguicula Group and transition upward into a distinctive carbonate interval; together, these comprise the informally defined Gibben formation. The shallowing-upward carbonate sequence contains abundant oolitic grainstone and packstone and microbial laminated dolostone. It is capped by a distinct interval of mud-cracked maroon mudstone, siltstone, and fine-grained sandstone that forms the base of what we informally define as the Chandindu formation. The mud-cracked shale transitions upwards into interbedded shale, coarse-grained sandstone, and minor carbonate. The overlying informally defined Reefal assemblage consists of up to 1 km of complexly interbedded carbonate and shale, with variable truncation beneath the major angular unconformity at the base of the Callison Lake Dolostone. The lower Fifteenmile Group (now informally PPD1 through the Chandindu formation) likely correlates with the Hematite Creek Group in the Wernecke Mountains.

Geological mapping and stratigraphic anaylsis of the early Neoproterozoic Fifteenmile Group in the western Ogilvie Mountains of Yukon, Canada, has revealed large lateral facies changes in both carbonate and siliciclastic strata. Syn-sedimentary NNW-side-down normal faulting during deposition of the lower Fifteenmile Group generated local topographic relief and wedge-shaped stratal geometries. These strata were eventually capped by platformal carbonate after the establishment of a NNW-facing stromatolitic reef complex that formed adjacent to the coeval Little Dal Group of the Mackenzie Mountains, Northwest Territories. Correlations between specific formations within these groups are tested with carbon isotope chemostratigraphy. As there are no known 830-780 Ma stratigraphic successions south of 62°N, the basin-forming event that accommodated the Fifteenmile and Little Dal Groups of the Ogilvie and Mackenzie Mountains and equivalent strata in the Shaler Supergroup of Victoria Island was restricted to the northwest margin of Laurentia. Therefore, this event does not represent widespread rifting of the entire western margin of Laurentia and instead we propose that these strata were accommodated in a failed rift generated by localized subsidence associated with the emplacement of the coeval Guibei (China) and Gairdner (Australia) large igneous provinces. The northern margin of Laurentia was reactivated by renewed extension at ca. 720 Ma associated with the emplacement of the Franklin large igneous province. Significant crustal thinning and generation of a thermally subsiding passive margin on the western margin of Laurentia may not have occurred until the late Ediacaran.