ABSTRACT

Within basin-filling shales of the Frasnian Duvernay and Ireton Formations a number of inclined electric log markers indicate the presence of significant submarine topography during deposition. Log markers reflect submarine hardgrounds on the platform and in the basin, and may be traced laterally into thin carbonate-rich layers on the slope. Markers formed in response to a relaxation in terrigenous sedimentation against a background of continuous carbonate deposition, and may be considered essentially synchronous surfaces. The presence of synchronous stratigraphic markers allows precise quantitative depth estimates of lithofacies and biofacies ranges to be made.

Log markers may be used to divide the Duvernay and Ireton Formations into a total of seven informal chronostratigraphic units termed depo-units. Depo-units are broadly sigmoidal in cross section and imbricated basinward, their platform portions forming a stacked sequence of upward-shoaling units in the east of the basin. They indicate that deposition took place during a number of episodic rises in sea level, each of the order of 20 m. A reciprocal pattern of sedimentation involving rise and stillstand characterizes and explains depositional patterns in this stratigraphic interval.

Mapping of the upper surfaces of individual depo-units allows the paleogeography to be reconstructed at a number of stages during basin-filling. Formation of log markers and depositional episodes in basin-filling shales have been used to explain growth stages and eventual termination of hydrocarbon-producing Leduc reef buildups within and surrounding the Shale Basin.

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