Cardium Formation at Ricinus Field, Alberta: A Channel Cut and Filled by Turbidity Currents in Cretaceous Western Interior Seaway

ABSTRACT The Upper Cretaceous (Turonian) Cardium Formation forms a series of long, narrow reservoirs in the subsurface of southern Alberta. Most of the sandstones show evidence of deposition by storm currents many tens of kilometers offshore. However, Ricinus field is different from all other Cardium fields: the sandstone body (Ricinus Member: new name) is up to 20 m thick compared with a Cardium average of only a few meters, and well-log cross sections indicate that this 20-m sandstone rests in a channel up to 55 km long, 8 km wide (before palinspastic reconstruction) and 20-40 m deep. Study of 71 cores showed that the channel fill consists of massive sandstone beds up to several meters thick, and thinner graded beds that are either massive or contain the sequence of sedimentary structures from parallel lamination to ripple cross-lamination (ABC to BC beds in the Bouma terminology for turbidites). Most of the sandstones have sharp bases, many with rip-up mudstone clasts. A thin mudstone layer commonly occurs between individual sandstones. Wave-formed structures such as symmetrical ripple cross-lamination and hummocky cross-stratification are rare in the Ricinus sandstones, despite being characteristic of the Cardium in most other fields. Thus, the nature of the channel fill indicates deposition in depths mostly below storm wave base, probably by turbidity currents. The channel also may have been eroded by turbidity currents during a brief lowstand of sea level. This erosion during lowstand may have far-reaching implications for style and distance of sand transport, for sand-body geometry, and for the nature of reservoir margins in other linear sandstones in shallow epeiric seas.