In the Ogilvie Mountains, west-central Yukon, the upper Proterozoic lower Mount Harper group (informal name) is stratigraphically equivalent to basal Windermere Supergroup rocks that occur in other areas of the North American Cordillera. Breccias at the base of the lower Mount Harper group are of two different, but in part coeval, origins. A normal fault that was active during late Proterozoic time forms the southern boundary of the lower Mount Harper group and is genetically related to a wedge of paleotalus breccia that thins rapidly northward from the fault. A different breccia layer is preserved on a disconformity surface that separates lower Harper group strata from a thick succession of underlying dolostones. This disconformity breccia contains silcretes and calcretes that provide evidence of several episodes of subaerial exposure. Rare interbedded debris flows display features that suggest coincidence of the last exposure event with initiation of deposition of the Mount Harper group sediments. Silcretes and calcretes in this succession suggest that, at the onset of Windermere deposition in this area, a temperate to equatorial, probably semi-arid to arid, climate regime existed. Basal breccias in Windermere-equivalent strata of other areas in the Cordillera generally have been interpreted as fault-related, but some of these breccias also contain features compatible with paleokarst origin.