A comprehensive review and reappraisal of the Howell Creek Structure: indication of multiple extension events in the southern Canadian Rocky Mountains, southeastern British Columbia and southwestern Alberta

Abstract

The Howell Creek structure was first mapped 60 years ago as a structural window (fenster) through the Lewis Thrust, where Upper Cretaceous foreland basin strata are bounded on all sides by Mesoproterozoic through Triassic platformal strata. Later interpretations ranged from the involvement of cryptic thrust and normal faults to a coherent gravitational slide. None of these, however, has adequately accounted for all available observations and constraints within the context of structural styles and physiography normally encountered in thin-skinned thrust-and-fold belts. Recent GIS map compilation required a reappraisal of these interpretations. Rather than a structural window, the Howell Creek structure is interpreted to lie entirely within the Lewis Thrust sheet. The Howell Fault, which places Upper Cretaceous Alberta Group strata onto Mississippian to Triassic strata and bounds the Cretaceous exposures to the southeast, is considered to be a low-angle normal fault. It may have formed initially as a footwall cut-off, splaying from and merging structurally upward with a thrust fault (the Twentynine Mile Creek Fault) that bounds the Cretaceous exposures to the southwest. Much of the Twentynine Mile Creek Fault was reactivated as a normal fault prior to the well-known Oligocene normal fault motion on the large-offset Flathead Fault, which lies to the east and north of the Howell Creek structure. The Shepp Fault, which is antithetic to but closely associated with the Flathead Fault, truncates the Twentynine Mile Creek Fault north of the Howell Creek structure. The Twentynine Mile Creek Fault reappears north of the Flathead Fault as the Squaw Fault, which is also reinterpreted as a normal-sense-reactivated thrust fault, explaining unusual structures near Flathead Pass. The northwest boundary of the Cretaceous exposures in the Howell Creek structure is marked by the reinterpreted and newly named Fuel Creek Fault, which is a small-displacement down-to-the-southeast normal fault. It is probably contemporaneous with the larger Harvey Fault, which bounds the Cretaceous exposures to the northeast and is a down-to-the-southwest normal fault related to the development of the Flathead half-graben. Normal-sense motion on the Twentynine Mile Creek, Howell, and Squaw faults clearly predates the Flathead and Harvey faults, by virtue of high-angle cross-cutting relationships, and may also predate or even coincide with development of sub-Lewis Thrust contractional duplex structures. The age of the Flathead Fault is constrained by the Early to early Late Oligocene Kishenehn Formation that fills the half-graben above the fault. Isolated exposures of Kishenehn Formation lie within and immediately adjacent to the Howell Creek structure. Balanced structural reconstructions indicate erosion of a minimum of 1.5 km of Paleozoic and Mesozoic strata after initial normal-sense motion on the Twentynine Mile Creek and Howell faults, but prior to deposition of the Kishenehn Formation. Coupled with estimates of Oligocene and earlier erosion rates, this suggests that local normal-sense motion on the Twentynine Mile Creek, Howell, and Squaw faults could have occurred prior to the end of regional contractional deformation of the southern Canadian Rocky Mountains, perhaps developing above an active sub-Lewis Thrust duplex structure.