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 (20 0000 km2) high resolution sequence stratigraphic analysis of the predo minantly shallow marine, Viking Formation of south-western Alberta, Ca nada. The study has revealed new insights into: (i) the geometry and o rigin of sequence boundaries; (ii) temporal changes in shelf processes across key sequence stratigraphic surfaces; and (iii) the origin of t he 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) coas tal plain; and (iv) estuarine. Sequence stratigraphic analysis of thes e units and their bounding surfaces defines five sequence boundaries a nd four sequences that correspond to units A, B and C, and D and E com bined. 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 r olling topography and are considered to have been formed subaqueously by tidal scour. Unit D is an estuarine valley fill unit that records f urther sea-level fall. It is underlain by a laterally restricted erosi on surface (VE3c) and capped by a regionally extensive transgressive s urface (VE3). Unit E is a storm-dominated shoreline-shelf succession t hat forms a highstand systems tract to the VE3c sequence boundary. In turn unit E is capped by a stepped transgressive surface, coincident w ith a further sequence boundary. The potential for tidal currents to g enerate regional subaqueous erosion surfaces is great but not widely r ecognized. In tide-dominated environments, such surfaces are likely du ring relative sea-level fall and may be a common correlative surface t o subaerial sequence boundaries. Similarly, changes in dominant shelf processes across key sequence stratigraphic surfaces may be more commo n than previously realized. Finally, the recognition of regionally ext ensive tidal deposits in units A-C is in marked contrast with several recent studies where the same successions have been interpreted as low stand shoreline deposits.