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Formation of Cardium Erosion Surface E5, and Associated Deposition of Conglomerate: Carrot Creek Field, Cretaceous Western Interior Seaway, Alberta
The Turonian (Upper Cretaceous) Cardium Formation can be divided into several stratigraphic units on the basis of regionally extensive erosion surfaces, designated E1 through E7. The erosion surfaces have up to 20 m of relative relief with respect to well log markers correlated as a horizontal datum. They are believed to be the result of stepwise shoreface erosion during episodes of transgression.
This paper concentrates on the fifth erosion surface, E5. Below this surface, the sandstones and mudstones are organized into two distinct sandier-upward sequences. Preserved lamination is interpreted as hummocky cross-stratification, indicating deposition in a storm-dominated shallow marine environment. The well log correlations show that these sandier-upward sequences are dissected by the E5 surface, and cores indicate that the thicker conglomerates are characteristic of the landward (southwestward) sides of the deeper areas of the erosion surface. In the seaward direction, the conglomerates are replaced by pebbly mudstones and, on the higher parts of the erosion surface, conglomerates are much thinner and represent a transgressive lag. The pebbles in the pebbly mudstones were probably derived from a shoreface and were transported by storms into areas of mud accumulation during initial stages of transgression. All of these coarser facies were finally buried by delicately laminated marine mudstones (the “laminated blanket”) that can be traced over a large part of the Cardium basin.
The morphology of the erosion surface, and the characteristics of the associated sediments, do not suggest erosion in an open marine environment, nor erosion subaerially. Instead, we suggest an initial marine regression, probably caused by uplift of the basin floor to the southwest, followed by transgression of this surface and the development of a series of incised shorefaces. The gravels were supplied to these shorefaces during a relative lowstand of sea level, when the rivers would have been incised and had steeper gradients. Thus the gravels form long, linear bodies that were deposited along paleoshorefaces; they are not “offshore bars”.
Understanding the origin of the erosion surfaces is vital to understanding the distribution of reservoir rocks in the Cardium. It also contributes to ideas of event stratigraphy, both in the Cardium, and in other areas of shallow marine “offshore bars”.
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