A sharp-based sandstone of the Viking Formation, Joffre Field, Alberta, Canada: Criteria for recognition of transgressively incised shoreface complexes

The Viking Formation of the Joffre field comprises parts of three discrete sequences. Sequence 2 overlies an erosional discontinuity, termed BD-1, which is incis ed into underlying marine parasequences of the informally named "Regional V iking''. The surface represents a sequence boundary that was transgressivel y modified during subsequent relative sea-level riser and is commonly demar cated by the Glossifungites ichnofacies.Up to three parasequences are trunc ated by BD-1, Sequence 2 comprises an incised sandstone body passing basinw ards into a granule to pebble-bearing sandy mudstone, A complete facies suc cession consists of a thin granule to pebble lag mantling BD-1, grading upw ards into thoroughly bioturbated gritty sandy mudstone, through intensely b urrowed muddy sandstone, and into interbedded hummocky cross stratified san dstone and burrowed sandstone. The facies contain diverse and uniformly dis tributed, open-marine trace-fossil suites displaying an upward progression from archetypal Cruziana through proximal Cruziana and into mixed Skolithos -Cruziana assemblages. The succession is interpreted to reflect a weakly st orm-influenced upper offshore to proximal lower-shoreface deposit. Incised shorefaces are allocyclically generated, and may be produced by for ced regressive (falling stage), lowstand, or transgressive scenarios. Seque nce 2 of the Joffre area is interpreted as a transgressively incised shoref ace. It is distinguished from the other two sharp-based shoreface types lar gely on the basis of the extent of the erosional component of its basal dis continuity. In distal positions, ED-1 remains erosional even where it is ov erlain by facies deposited below fair-weather wave base. This is inconsiste nt with forced regressive and lowstand conditions because in weakly storm-i nfluenced shorefaces, the regressive surface of erosion and the sequence bo undary, respectively, pass into correlative conformities seaward of fair-we ather wave base. Facies deposited below fair-weather wave base would theref ore overlie the non-erosional correlative conformity surface. In a transgre ssive scenario, however, ravinement during erosional shoreface retreat gene rates an erosional discontinuity that may lie seaward of fair-weather wave base during subsequent progradation, because the surface was cut prior to p rogradation and while sea level was considerably lower, As a result, facies deposited below fair-weather wave base can overlie the erosional discontin uity.