MODELING REGIONAL VERSUS LOCAL SUBSIDENCE IN THE CANADIAN CORDILLERAN FORELAND BASIN

The development of accommodation in sedimentary basins falls into two broad categories; regional basin-scale subsidence and localized processes. Regional accommodation can result from lithospheric flexure, extensional faulting, the isostatic response to cooling, or mantle dynamics. Local factors include preexisting topography, differential weathering along unconformities, karst/salt collapse, and subsidence along inherited structures or crustal features. Understanding the contribution of these two end member mechanisms spatially may aid in predicting sediment facies patterns across a basin.

The objective of this study is to model the flexural response of the Canadian Cordilleran Foreland Basin, better known as the Western Canada Sedimentary Basin (WCSB), and compare the predicted subsidence model to actual sediment thickness maps. The WCSB is one of the most studied foreland basins in the world with a wealth of well data that can be used to constrain subsidence and develop geologic models. The foreland basin phase of the WCSB began in the Middle to Late Jurassic and continued into the Cenozoic as several terranes collided along the western margin of North America, forming the western North American Cordillera. In central and eastern Alberta and Saskatchewan, a major unconformity separates the foreland basin fill from underlying Paleozoic passive margin strata and Precambrian basement. In southern and western Alberta, this unconformity eroded into Mesozoic rocks. Differential weathering along this unconformity, as well as dissolution of Paleozoic salt layers by meteoric water contributed to the paleotopography over which the basal foreland basin sequence was deposited. Publically available well data was gridded and used to construct several cross-sections oriented perpendicular to the thrust front. These cross sections provide the observed sediment thickness distribution to which flexural subsidence models were compared. Model parameters include variable flexural rigidity across the basin, sediment density, maximum deflection, and eroded overburden. Small deviations from the ideal curve likely indicate areas were local processes contributed to total subsidence.