Late Mesozoic reactivation of Precambrian basement structures and their resulting effects on the sequence stratigraphic architecture of the Viking Formation of east-central Alberta, Canada

Abstract

The Lower Cretaceous Viking Formation is a siliciclastic unit that occurs in the subsurface of Alberta in the Western Canadian sedimentary basin. This study focuses on a lowstand paleoshoreline trend extending along strike between two hydrocarbon-producing fields, Joarcam and Judy Creek (250 km NW). The Viking Formation in these fields records depositional thicknesses ranging from 20 to 30 m. Between these two fields, however, the formation is anomalously thick (45-60 m), complicating the recognition and correlation of key stratigraphic surfaces. Marine flooding surfaces above and below the Viking Formation are routinely employed as stratigraphic datums in order to remove postdepositional deformation and facilitate the development of a sequence stratigraphic framework. However, as each successive surface is employed as the datum, the other flooding surfaces within the formation become distorted, resulting in unrealistic depositional geometries. These geometries are best explained to be the result of structural readjustments during Viking deposition. The Precambrian lithosphere of the Canadian Shield forms the Western Canadian sedimentary basin basement, with major structures previously mapped using gravity and magnetic anomaly studies. Locally, the increased accommodation observed within the Viking Formation of central Alberta is attributed to differential reactivation of the Paleoproterozoic Snowbird tectonic zone basement structures, which flank the areas of anomalously thick deposits and trend approximately normal to the regional strike of the Western Canadian sedimentary basin. The Snowbird tectonic zone faults are interpreted to have been reactivated during renewed tectonic loading in the southern Canadian Cordillera during Aptian-Albian time, causing subtle readjustments along basement faults that caused variable syndepositional subsidence. By selecting successive datums, the gross Viking interval can be recognized to have accumulated prior to, during, and following structural reactivation.

Coeval deposition of transgressive and normal regressive stratal packages in a structurally controlled area of the Viking Formation, central Alberta, Canada

“…Ross et al ., ; Pilkington et al ., ; Lyatsky et al ., ) have the potential to have become reactivated, which may have led to the creation of variable accommodation along depositional strike in the basin. The reactivation of Precambrian basement structures during the deposition of marginal‐marine environments has been shown to have had a pronounced effect on the evolution and geometry of Mesozoic palaeoshorelines (Zaitlin et al ., ; Vakarelov & Bhattacharya, ; Schultz et al ., ).…”

“…Schematic of the potential basement reactivation recorded in central Alberta (Schultz et al ., ). Differential subsidence of the Snowbird Tectonic Zone (STZ) led to the creation of accommodation space in central Alberta.…”

“…During deposition of the Viking Formation in the Albian, loading on the Cordilleran deformation front led to differential subsidence along the STZ (Schultz et al ., ). Specifically, along depositional strike between the producing fields of Judy Creek and Joarcam (Fig.…”

“…), a marked increase in accommodation is recorded in the Viking Formation overlying the STZ, and accommodation creation generated pronounced facies variations along‐strike within over‐thickened intervals. The Viking deposits overlying the STZ range in thickness from 35 to 65 m, whereas the lower accommodation zones flanking the STZ successions generally show thicknesses of 15 to 30 m. The variations in accommodation space brought about by reactivation of the STZ are responsible for the marked complexities in stratal relations of the Viking Formation along depositional strike (Schultz et al ., ).…”

“…A sequence stratigraphic approach is employed herein to develop a high‐resolution stratigraphic framework for Viking deposits in central Alberta. In this area, the Viking has been regarded historically to record deposition in a low‐accommodation basin; however, recent studies have documented areas of the Viking that are markedly over‐thickened, leading to pronounced variability in the 3D architecture of the constituent systems tracts (Schultz et al ., ). The systems tracts in over‐thickened areas experienced pronounced changes in accommodation space generated by differential reactivation of the STZ, which led to significant along‐strike variability in their orientations, geometries, thicknesses and, ultimately, stratal stacking patterns (Schultz et al ., ).…”