Geometry of Facies Packages and E5 Erosian Surface in the Cardium Formation, Ferrier Field

Abstract

<p>The Upper Cretaceous (Turonian) Cardium Formation was deposited along the western margin of the Western Interior Seaway, within the Alberta Foreland Basin of the Canadian Cordillera. Like other Cretaceous formations within the Western Interior Seaway, it is characterized by a series of linear sandstone and conglomerate bodies encased in marine shales. Ferrier oil field is one of the western-most of the Cardium linear sand ridges. The Raven River Member in Ferrier field consists of two coarsening-upward sequences. The upper sequence contains hummocky cross-stratified sandstones, which suggest deposition below fairweather wave base. Cross sections show that the two sequences are scoured to variable depths by a major erosion surface (termed "E5"). The E5 erosion surface defines an undulating topography of gently and steeply dipping surfaces, termed "terraces" and "bevels",u respectively. Ferrier field and neighbouring Willesden Green field are terraces; an erosional bevel coincident with the northeastern margin of Ferrier separates the two terraces. These morphological elements cannot be explained by either totally subaqueous or totally subaerial erosion; erosion at the shoreface during stillstands of sea level is invoked. The terraces at Ferrier and Willesden Green have gentle southwestward dips relative to horizontal well log markers, but were probably cut horizontally at fairweather wave base during stillstand. This suggests that the Raven River sediments were dipping to the northeast during shoreface incision. Since the mean dip of the terrace at Willesden Green (0.07 degrees) exceeds that of the terrace at Ferrier (0.03 degrees), a downward flexing of the sediment surface is suggested in addition to an initial basinward tilt. The conglomerates which immediately overlie the E5 erosion surface at northeastern Ferrier are interpreted to be shoreface gravels. Based on the morphology of the E5 erosion surface basin-wide, and assuming a constant depth to fairweather wave base of meters, it is possible to separate the horizontal vertical erosion components of stillstand and steady sea level rise from one another. In total, meters of vertical sea level rise occurred over the 85 kilometers which separates Carrot Creek field from western Ferrier field. Assuming that the rate of erosion effective during incision of the E5 surface to be 1.2m/year, then it would have taken just over 70,000 years to cut the erosion surface from Carrot Creek to Ferrier. During this time, sea level rose at an average rate of 1.9 mm/year.</p>