Abstract

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.

First Page Preview

First page PDF preview
You do not currently have access to this article.