Cardium Formation (U. Cretaceous) at Seebe, Alberta—storm-transported sandstones and conglomerates in shallow marine depositional environments below fair-weather wave base
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abstract
As a result of detailed mapping at Seebe, Alberta, we have defined five coarsening-upward sequences in the Turanian Cardium Formation. The sequences begin with bioturbated mudstones and coarsen into sandstones, commonly with conglomeratic veneers on top. In sequence 2, the conglomerate is up to 20 cm thick, and has been molded into sand waves a few centimetres high and with wavelengths of about 1 m.The sandstones are dominated by hummocky cross-stratification (HCS), a broad, low-angle undulating stratification now believed to be formed below fair-weather wave base by long-period storm waves. Sandstones with HCS occur in the upper parts of sequences 1, 2, 3, and 5, suggesting that all of these sequences terminate in water deeper than fair-weather wave base (10–15 m). The trace fossil assemblage of abundant Zoophycos and long, horizontal Rhizocorallium suggests similar depths. Foraminifera in sequences 3 and 4 show a high diveristy of agglutinated species, also suggesting deposition below fair-weather wave base.In view of the physical and biological evidence for deposition in a few tens of metres of water, and the total absence of medium-scale cross-bedding, we suggest that the entire Cardium Formation at Seebe was deposited below fair-weather wave base. The sandstones were emplaced by storm-generated density currents, the same storm both generating the flow and imprinting HCS on the deposit. Finally, we suggest that the conglomerates were also moved rapidly out to sea by density currents, there being no evidence for the traditional beach or transgressive lag interpretation of these rocks.
