REACTIVATION OF PRECAMBRIAN BASEMENT STRUCTURES AND ITS EFFECTS ON THE REGIONAL STRATIGRAPHY OF THE EARLY CRETACEOUS (LATE ALBIAN) VIKING FORMATION, CENTRAL ALBERTA, CANADA

The late Albian Viking Formation is a siliciclastic unit in the subsurface of Alberta in the Western Canadian Sedimentary Basin (WCSB). The study focuses on one of the inferred lowstand paleoshoreline trends extending from the Joarcam Field (50 km SE of Edmonton) to the Judy Creek Field (350 km NW along strike). The Viking in these fields records deposition in a low-accommodation setting, with depositional thicknesses ranging from 15-25 m. Between these two fields, however, the formation is anomalously thick (40-60 m), complicating the recognition and correlation of key stratigraphic surfaces. Marine flooding surfaces above and below the Viking are employed as stratigraphic datums in order to remove post-depositional structure and deformation, facilitating the development of a sequence stratigraphic framework. However, as each successive surface is employed as a datum, the other flooding surfaces within the formation become distorted and the resulting depositional geometries are unrealistic. This indicates that there were structural controls active during deposition. The Precambrian lithosphere of the Canadian Shield forms the WCSB basement, with major structures previously identified and mapped through the use of gravity and magnetic anomaly studies coupled with limited drill core and U-Pb geochronology. The increased accommodation observed throughout central Alberta is attributed to differential reactivation of these basement structures that appear to flank the intervals of varying thickness in the Viking and trend across strike of the WCSB. These structures are interpreted to have become reactivated during renewed tectonic loading in the southern Canadian Cordillera during Aptian-Albian time, causing subtle movements along basement faults, leading to varying syndepositional subsidence. By selecting successive datums, parts of the Viking interval can be recognized to have accumulated prior to, during and following structural reactivation. Only through such stratigraphic “serial slicing” can the geometries of the different sequences and their systems tracts be resolved.