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. A preserved synsedimentary normal fault system forms the southern margin of an east-trending half-graben basin. The basin fill was derived from the south. The fill consists of fault-proximal talus breccia and as much as 1,100 m of debris-flow conglomerate deposited in coalescing alluvial fans. Intermediate and distal alluvial-fan facies include mid-to-lower fan conglomerate and sandstone deposited by sheetfloods, distal debris flows, and braided channel streamflows. A thick, coarsening-upward megasequence in the eastern part of the study area records the change from waterlaid (lacustrine?) red beds to lower alluvial-fan sandstones and conglomerates. Clastic deposition ended soon after volcanism was initiated, resulting in the overlying Mount Harper volcanic complex (ca. 750 Ma).
The half-graben basin was formed by the southern margin faulting. Comparison with strike-slip basin depositional patterns indicates significant differences, suggesting that faulting was predominantly dip-slip. Synsedimentary fault movements and minor back-stepping of the fault system controlled development of the megasequences. Volcanism postdates sedimentation in this half-graben, but north-derived coarse clastic rocks which intertongue with the volcanic rocks were deposited to the northwest and are not directly related to lower Mount Harper group deposition. A model is proposed in which diachronous, opposed-symmetry half-grabens formed, with volcanism concentrated in the accommodation zone between half-grabens. Basins in the western arm of the East Africa rift system provide geometric and evolutionary analogues.