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“Geometry” and Facies Characteristics of Stacked Shallow Marine Sandier-Upward Sequences in the Cardium Formation at Willesden Green, Alberta
Seven, stacked, marine, sandier-upward sequences comprise the Raven River Member (“A” sand) of the Upper Cretaceous (Turonian) Cardium Formation at Willesden Green, Alberta. Each sequence begins with bioturbated mudstone and passes upward into storm-dominated shallow marine sandstone. The seven sequences, each up to 12 m thick, are commonly capped by grit horizons. Each sequence also shows a lateral shaling out from hummocky cross-stratified sandstone into bioturbated mudstone. The sequences do not offlap systematically and each appears to have been supplied from a different source area. It is likely that each of the sandier-upward sequences of the Raven River Member formed in response to minor cycles of lowering and raising of relative sea level, which also controlled the grit supply. These relative sea-level changes were probably caused by small-scale tectonic movements in the basin.
The Raven River sandstone sequences are dissected by a basinwide erosion surface (E5) that formed following a major lowering of relative sea level. A seriecs of stepped shorefaces were cut into the underlying sandstones during ensuing transgression and may be either mud-draped or overlain by important conglomerate reservoirs. Reservoir characteristics of the Willesden Green oilfield are largely determined by the “geometry” and stacking of the sandstone sequences, with stratigraphic trapping being partly due to lateral shaling out, and partly due to incision by the mud-draped shorefaces. It should be emphasized that the long, narrow “geometry” of the field is not controlled by the primary deposition of sands as an offshore bar, but by subsequent erosional dissection of a sheet-like stack of sandstones.
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