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

Abstract

ABSTRACT The Viking Formation of the Joffre field comprises parts of three discrete sequences. Sequence 2 overlies an erosional discontinuity, termed BD-1, which is incised into underlying marine parasequences of the informally named Regional Viking. The surface represents a sequence boundary that was transgressively modified during subsequent relative sea-level rise, and is commonly demarcated by the Glossifungites ichnofacies. Up to three parasequences are truncated by BD-1. Sequence 2 comprises an incised sandstone body passing basinwards into a granule- to pebble-bearing sandy mudstone. A complete facies succession consists of a thin granule to pebble lag mantling BD-1, grading upwards into thoroughly bioturbated gritty sandy mudstone, through intensely burrowed muddy sandstone, and into interbedded hummocky cross-stratified sandstone and burrowed sandstone. The facies contain diverse and uniformly distributed, 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 storm-influenced upper offshore to proximal lower-shoreface deposit. Incised shorefaces are allocyclically generated, and may be produced by forced regressive (falling stage), lowstand, or transgressive scenarios. Sequence 2 of the Joffre area is interpreted as a transgressively incised shoreface. It is distinguished from the other two sharp-based shoreface types largely on the basis of the extent of the erosional component of its basal discontinuity. In distal positions, BD-1 remains erosional even where it is overlain by facies deposited below fair-weather wave base. This is inconsistent with forced regressive and lowstand conditions because in weakly storm-influenced shorefaces, the regressive surface of erosion and the sequence boundary, respectively, pass into correlative conformities seaward of fair-weather wave base. Facies deposited below fair-weather wave base would therefore overlie the non-erosional correlative conformity surface. In a transgressive scenario, however, ravinement during erosional shoreface retreat generates an erosional discontinuity that may lie seaward of fair-weather wave base during subsequent progradation, because the surface was cut prior to progradation and while sea level was considerably lower. As a result, facies deposited below fair-weather wave base can overlie the erosional discontinuity.