Devonian salt dissolution-collapse breccias flooring the Cretaceous Athabasca oil sands deposit and development of lower McMurray Formation sinkholes, northern Alberta Basin, Western Canada

Abstract

The sub-Cretaceous paleotopography underlying giant Lower Cretaceous Athabasca oil sands, northern Alberta, has an orthogonal lattice pattern of troughs up to 50km long and 100m deep between pairs of cross-cutting lineaments. These structures are interpreted to have been inherited from a similar pattern of dissolution collapse-subsidence troughs in the underlying Middle Devonian salt beds. Removal of more than 100m of halite salt fragmented the overlying Upper Devonian strata into fault blocks and collapse breccias that subsided into the underlying dissolution troughs. The unusually low 1:2 to 1:3 thickness ratios of halite salts to the overlying strata resulted in the Upper Devonian strata collapse-subsidence into underlying salt dissolution troughs being more cataclysmic during the first phase of salt removal. The second phase of slower but complete salt removal between the earlier troughs resulted in a more gradual subsidence of the overlying strata. This obliterated the earlier pattern of giant cross-cutting dissolution troughs bounded by major lineaments. The collapse breccia fabrics underlying the earlier troughs differ from those from areas between the troughs. Collapse breccias underlying the large troughs often have crushed fabrics distributed in zones that rapidly pinched out between fault blocks. Breccias between troughs developed as giant mosaics of detached carbonate blocks that formed breccia pipe complexes.Multiple sinkholes up to 100m deep aligned along multi-km linear valley trends that dissected the sub-Cretaceous paleotopography. These sinkhole trends formed orthogonal patterns inherited from underlying lattice of NW–SE and NE–SW salt structured lineaments. These cross-cutting sinkhole trends have a smaller 5km scale reticulate pattern similar to the giant 50km scale pattern of collapse-subsidence troughs. Other sinkholes developed as lower McMurray strata sagged when underlying Devonian fault blocks and breccia pipes differentially subsided.