Controls on accommodation during the early-middle Campanian in southern Alberta, western Canada foreland system

Abstract

The Western Canada Sedimentary Basin is a retroarc foreland system produced by thrust sheet loading in the Canadian Cordillera and resultant flexural displacement of the lithosphere east of the orogenic front. Theoretical models of flexural tectonism and dynamic loading have long been used to predict the effects of these controls on the creation/destruction of accommodation in retroarc foreland settings, however, few field studies have attempted to evaluate their relative contributions for a given point in geologic time. As key regional drivers of subsidence/uplift and implicitly, relative sea level rise and fall, flexural tectonism and dynamic loading impart unique signatures (subsidence geometries) which can be discerned via the spatial and temporal analysis of event-significant surfaces preserved in the geologic record. We use magnetostratigraphy to constrain the timing of a major marine incursion (transgressive phase of the second-order Campanian Claggett Cycle, which spanned ∼8 Myr in its entirety) at three locations in the southern Alberta plains where the onset of this transgression is marked by a widespread ravinement surface. A chronostratigraphic framework is presented which includes new empirical ages for stratigraphic units above and below this key surface, informally termed the Milk River ‘shoulder’. The framework provides a basis for determining the earliest extents of the Claggett (Pakowki/Lea Park) sea in southern Alberta, and for characterization of its subsequent expansion during the early-middle Campanian. Although commonly depicted as a simple, westward-expanding seaway, we observe that the initial expansion of the Claggett sea in southern Alberta was more complicated and proceeded in multiple directions through time, prior to maximum transgression and the ensuing second-order regression. We interpret the Claggett sea as restricted to the Alberta foredeep at ∼82.0 Ma, before inundating a paleotopographic high associated with a prominent basement feature in southernmost Alberta (Sweetgrass Arch) at ∼81.0 Ma, likely establishing a connection with the Claggett sea in northern Montana. As relative sea level continued to rise, the sea reached the Rocky Mountain foothills in the west while also advancing more than 100 km eastward into the forebulge region during peak transgression (∼80.0 Ma). With this history established, we ascertain the overriding cause for the Claggett transgression was dynamic subsidence because (1) the magnitude, direction, and timing of relative sea level rise and the associated expansion of the Claggett sea fits published dynamic subsidence models for the Campanian of North America, and (2) several hundred metres of marine sediments are preserved on the early-middle Campanian forebulge, requiring accommodation which cannot be explained by widely accepted flexural models or eustatic estimates.