In the Ogilvie Mountains, west-central Yukon, the upper Proterozoic to Lower Cambrian Mount Harper Group (informal name) contains strata equivalent to both basal Windermere Supergroup and Lower Cambrian rocks in other areas of the North American Cordillera. Strata of the Mount Harper group are directly time constrained: the basal Windermere equivalents by a ca. 750 Ma U-Pb age from a volcanic complex which both conformably overlies and intertongues with the sedimentary rocks; the disconformably overlying units by the presence of Lower Cambrian trace fossils. The lower Mount Harper Group (LMHG) unconformably overlies a thick succession of dolostones informally named the Fifteenmile Group. A breccia layer is discontinuously preserved on the unconformity surface. This breccia contains silcretes and calcretes that record several episodes of subaerial exposure. Rare interbedded debris flows suggest that the most recent of these episodes was coincident with initial deposition of the LMHG conglomerates. 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 prevailed. Elsewhere, basal breccias in Windermere-equivalent strata generally have been interpreted as fault-related, but some contain features compatible with a karstic origin. A synsedimentary normal fault forms the southern margin of an asymmetric, east-trending half- graben basin which contains the lower Mount Harper Group. Basin fill appears to have been derived exclusively from source areas to the south. Proximal facies consist of fault-talus breccia and up to 1100 m of debris-flow conglomerates that were deposited in coalescing alluvial fans. Intermediate and distal facies include mid-to-lower fan conglomerates and sandstones deposited by sheetfloods, distal debris flows and braided channel streamflows. A basin-fill coarsening-upward megasequence in the eastern part of thc study area records a change from lacustrine redbeds to lower alluvial fan sandstones and conglomerates. The LMHG half-graben basin was formed, and sedimentation was controlled, by normal faulting along the southern margin. Synsedimentary faulting and minor back-stepping of the fault controlled development of the internal sequences. Volcanism conformably postdates this sedimentation, but northerly derived coarse clastic rocks of the upper Mount Harper group (UMHG) .
