Flexural bulges and foredeep basins: Dynamic tectonic control on shifting depocentres and condensed facies, Upper Cretaceous (Santonian), Western Canada Foreland Basin

Abstract

Upper Cretaceous (Santonian) shallow‐marine mudstone of the Puskwaskau Formation fills the foredeep of the Western Canada Foreland Basin and was deposited on a near‐planar, wave‐graded ramp. These strata span ~2.3 myr and form an 800‐km‐long prismatic wedge that, perpendicular to the Rocky Mountain Cordillera, thins from >360 to <40 m over 400 km. The Santonian succession was divided into 12 allomembers, A–L, bounded by marine flooding surfaces. Allomembers A–D, E–H and I–L group into three genetic ‘units’, bounded by subtle disconformities. Isopach maps of the three units, each inferred to span ~760 kyr, show pronounced along‐strike variation in thickness. The depocentre of Unit 1 lies between 49 and 53° N, suggesting that a region of active thrust loading lay immediately to the SW. The depocentre of unit 2 is offset 300 km to the NW yet traced 500 km along‐strike to the SE, 90 m of mudstone thins to 1 m of bioclast‐rich sandstone, which lies atop the forebulge, relatively close to the orogen. Unit 2 also thins 250 km NE (offshore) where the unit comprises 5 m of coccolith‐ and organic‐rich mudstone. Relative to unit 2, the depocentre of unit 3 is offset 300–400 km along‐strike to the SE. Isopach maps of individual allomembers, inferred to span ~190 kyr, show that depocentres can migrate laterally up to 300 km between individual allomembers. Mudstone depocentres are interpreted as regions of active flexural subsidence rather than point‐sourced accumulations, such as deltas or mud banks. Individual thrust sheets in the Rocky Mountain fold‐and‐thrust belt may have experienced spatially and temporally discrete episodes of internal thickening and advance; the resulting load produced a corresponding arcuate depocentre in the adjacent foredeep.