ABSTRACT

The Howell Creek fenster lies six miles west of the Flathead fault and the westernmost exposures of the Lewis thrust, in south-eastern British Columbia. Between the fenster and the Flathead fault is the MacDonald dome, a broad anticline cut by many normal faults. Upper Cretaceous strata are exposed in the fenster, separated from the surrounding Precambrian to Triassic formations by both normal and thrust faults. The fenster may have been developed by displacement of the Lewis overthrust sheet, together with a slice of Cretaceous beds in the footwall, by a later low-angle thrust, followed by vertical offsetting of the superposed thrusts and the Cretaceous between them. The structure could also have been formed by fracturing and differential vertical and horizontal movements within a single thrust sheet, structurally higher than the Lewis thrust, underlying the MacDonald dome at shallow depth.