SEQUENCE STRATIGRAPHY FROM CORE AND WIRELINE LOG DATA - THE VIKING FORMATION, ALBIAN, SOUTH CENTRAL ALBERTA, CANADA

An extensive core and wireline log database has allowed a regional (200000 km2) high resolution sequence stratigraphic analysis of the predominantly shallow marine, Viking Formation of south-western Alberta, Canada. The study has revealed new insights into: (i) the geometry and origin of sequence boundaries; (ii) temporal changes in shelf processes across key sequence stratigraphic surfaces; and (iii) the origin of the Viking Formation. Five regionally significant erosion surfaces (VE1-4 and VE3c) define six units, A-F, within the Viking Formation. Each unit is characterized by one or more of four facies associations: (i) tide-dominated shelf; (ii) storm-dominated shoreline-shelf; (iii) coastal plain; and (iv) estuarine. Sequence stratigraphic analysis of these units and their bounding surfaces defines five sequence boundaries and four sequences that correspond to units A, B and C, and D and E combined. Units A-C are lowstand deposits that are underlain by sequence boundaries and record stepped sea-level fall in a tidal shelf setting. Unit A is underlain by a correlative conformity, whereas units B and C are underlain by regionally extensive erosion surfaces that have a rolling topography and are considered to have been formed subaqueously by tidal scour. Unit D is an estuarine valley fill unit that records further sea-level fall. It is underlain by a laterally restricted erosion surface (VE3c) and capped by a regionally extensive transgressive surface (VE3). Unit E is a storm-dominated shoreline-shelf succession that forms a highstand systems tract to the VE3c sequence boundary. In turn unit E is capped by a stepped transgressive surface, coincident with a further sequence boundary. The potential for tidal currents to generate regional subaqueous erosion surfaces is great but not widely recognized. In tide-dominated environments, such surfaces are likely during relative sea-level fall and may be a common correlative surface to subaerial sequence boundaries. Similarly, changes in dominant shelf processes across key sequence stratigraphic surfaces may be more common than previously realized. Finally, the recognition of regionally extensive tidal deposits in units A-C is in marked contrast with several recent studies where the same successions have been interpreted as lowstand shoreline deposits.