A pop-up structure exposed in the outer foothills, Crowsnest Pass area, Alberta

摘要

Recent mapping in the Blairmore East-Half map area indicates a pop-up structure transected by the Crowsnest River, near the leading edge of Eastern Cordilleran deformation. At scenic Lundbreck Falls, the sub-horizontal attitude of the Santonian Virgelle Formation (Milk River Group) is anomalous in comparison to fault-bounded slices to both the east and west, where strata display steep to moderate dips more typical of Foothills structures. The relatively broad (750 m across strike), flat-lying structure exposed at Lundbreck Falls is bounded downstream to the east by a foreland-directed thrust placing Virgelle Formation on Campanian Lundbreck Formation (Belly River Group). To the west, this sub-horizontal structure is overridden by a steeply west-dipping, foreland-directed thrust carrying Milk River and Belly River strata in its hanging wall. Stratigraphic offset across this thrust fault is apparently small to negligible at the Crowsnest River, but increases at higher elevations to the north and south. On either side of this thrust fault, north of the Crowsnest River, two anticlines involving the Virgelle Formation face each other. These atypical relationships suggest that, at the river and adjacent low elevations, the sub-horizontal structure may be bounded to the west by a cryptic, west-directed backthrust that is overridden up-slope by the foreland-directed fault just described. These features are inferred to comprise a pop-up structure, similar to those interpreted elsewhere within the triangle zone in the subsurface, but never before described in outcrop in the Foothills of Alberta. Although they appear to be very rare in the Foothills, the inferred backthrust is not alone in this immediate area. Two kilometres downstream, the hinge of an upright anticline, involving a well-exposed section of Milk River Group strata at the classic "Lundbreck transition outcrop", is removed to the north by another west-directed backthrust. This backthrust could also be associated with a pop-up structure, as suggested by its configuration in restored cross-sections. An association between the pop-up structures and the triangle zone is implied by their proximity, but relative timing relationships are unclear. The recognition of these structures, using a refined stratigraphy in an area mapped previously, suggests that pop-up structures associated with the triangle zone may be more common than published studies indicate, and therefore may be more important features of the Outer Foothills belt than is widely appreciated.%Une cartographie récente de la moitié-est de la carte de la région de Blairmore indique une structure d'un coin extrusif transecté par la rivière Crownest, près de la limite de la déformation de la cordilière-est. Au niveau des chutes pittoresques de Lundbreck, l'attitude sous-horizontale de la formation Virgelle du Santonien (groupe Milk River) est irrégulière comparée aux tranches limitées par des failles situées à l'est et à l'ouest, là où les couches présentent des pendages prononcés à modérés, plus typiques des structures des Foothills. La structure relativement large (750 m d'un bout à l'autre), et plate, exposée sur les chutes de Lundbreck, est limitée en aval à l'est par un chevauchement d'avant-chaîne qui superpose la formation Virgelle sur la formation Lundbreck du Campanien (groupe Belly River). A l'ouest, cette structure sous-horizontale est surmontée par un chevauchement d'avant-chaîne à pendage prononcé vers l'ouest, transportant des couches de Milk River et Belly River dans son compartiment supérieur.