Sharp-Based Shoreface Sequences and ‘Offshore Bars’ in the Cardium Formation of Alberta

Abstract: Under conditions of stable sea level, the progradation of a wave-dominated clastic shoreface will give rise to a coarsening-upward sequence, reflecting an increase in the frequency and volume of sand transport with time. Core and well-log data from the Cardium Formation (Turonian) of Alberta reveal two types of shelf-to-shoreface sequences: (1) gradational-based sequences that steadily coarsen upward from thin-bedded, wave-rippled sandstone and mudstone through hummocky cross-stratified (HCS) sandstone and mudstone into mud-free, swaley cross-stratified (SCS) sandstone capped by a root bed, and (2) sharp-based sequences that consist of SCS sandstone, which, near the base, may contain large mudstone intraclasts, sharply overlying thin-bedded sandstone and mudstone. The HCS interval is thin or absent. Log cross sections show that the change from a gradational to a sharp-based sequence is accompanied by a lowering of both the top and bottom of the SCS sandstone, relative to upper and lower markers. Simultaneously, the SCS sandstone may thin from 15 to 18 m to as little as 6 m. The sharp base, the presence of intraclasts, and the relative lowering of the SCS unit suggest deposition during a rapid fall of relative sea level during which the shoreface prograded over an erosion surface cut into the inner shelf by fair-weather wave scour. Tens of kilometers seaward of Cardium shoreface sandstones lies a series of shore-parallel, lenticular bodies (‘offshore bars’) of conglomeratic muddy sandstone. The lenticular bodies rest on regional erosion surfaces that can be traced landward into slightly older shoreface deposits. The conglomerates are here interpreted as lowstand shoreface deposits, which lie on erosion surfaces cut into the shelf by wave scour during a relative sea-level fall. The stratigraphic and lithologic relationships demonstrable from the Cardium Formation suggest that a number of sharp-based ‘offshore bar sandstones’ in other parts of the Western Interior Seaway may also be more satisfactorily explained as lowstand shoreface deposits.