Relationship of the Stratigraphy of the Western Canada Foreland Basin to Cordilleran Tectonics<subtitle>Insights from Geodynamic Models</subtitle>

Abstract

Geodynamic models of foreland basin development have enabled researchers to quantify the mechanical relationships between foreland basin subsidence and tectonic loads that cause downflexing of the lithosphere. These models also provide a qualitative framework for construction of foreland basin stratigraphic sequences that are anticipated as having resulted from loading events caused by emplacement of overthrust sheets, terrane accretions, and changes in plate-boundary dynamics. Initial terrane accretion and associated overthrust emplacement at a preexisting passive margin are anticipated to result in an unconformity-bounded foreland succession that will exhibit a shallowing-upward pattern similar to the classic Flysch to Molasse succession of the European Alpine basins. A basal unconformity develops as a result of cratonward migration of the peripheral bulge associated with lithospheric flexure caused by the tectonically advancing terrane. The shallowing of sedimentary facies occurs because initially low sediment supply, a consequence of little or no subaerial expression across the terrane at early stages, is succeeded by a progressively coarser and more voluminous supply as significant tectonic uplift occurs across the terrane. The upper unconformity may de elop primarily as a result of (1) relaxation of lithospheric bending stresses through time following overthrusting, resulting in migration of the peripheral bulge toward the orogene, and/or (2) reduction of the flexural load on the lithosphere through erosion or tectonic End_Page 107------------------------ denudation of the overthrust belt, causing regional uplift or basin rebound. Later clastic wedges, associated with subsequent overthrust events sufficient to significantly modify the size or position of the tectonic load depressing the foreland basin, may depart from this idealized sequence in that (1) basal unconformities may not develop if eustatic sea level is high, or if there is little or no time lag between events; and (2) a clear shallowing-upward trend may be precluded if sediment supply can keep pace with relative subsidence, impeding the normal nonmarine to marine transition between stacked clastic wedges. In the case of alternative (1), however, the transition from one clastic wedge to another may be marked by a transition from predominantly nonmarine to marine conditions. Some clastic units appear to correlate in time with known eustatic changes; o hers are considered indicative of changes in tectonic loading. The stratigraphy of the Alberta basin has been subdivided by comparing it with the idealized sequence resulting from an individual tectonic loading (overthrust and/or accretion) event, modified to account for conditions prevalent after initial accretion. The ages of the six clastic wedges recognized (Fernie and Kootenay groups; Mannville Group; upper Fort St. John Group and Dunvegan Formation; Smoky Group and Belly River Formation; Edmonton Group; Paskapoo Formation--and their lateral equivalents) are compared with the times of accretion of Cordilleran terranes (Intermontane superterrane, terranes of the North Cascades and the Coast belt, Insular superterrane, and outer terranes of the latest Cretaceous-early Tertiary thrust stack). Some mechanical implications of a cause-and-effect relationship between terrane collisions and clastic wedges, if one exists, are discussed; it is also shown that most accreted terranes are too distant from the foreland basin to have influenced subsidence directly through tectonic loading of the lithosphere. General discrepancies in timing between collisions and clastic wedges may result from many factors, but the relatively well-understood temporal relations in the foreland basin provide an opportunity to more thoroughly investigate the use of foreland stratigraphy to constrain models of deformation and crustal thickening in the Cordillera.