GENETIC AND STRATIGRAPHIC SIGNIFICANCE OF THE UPPER DEVONIAN FRASNIANZ-MARKER, WEST-CENTRAL ALBERTA

The Z Marker is a distinctive and widespread wireline log marker withi n the thick Frasnian Ireton shale basin succession of west-central Alb erta. The marker represents an interval of condensed sedimentation and corresponds to an abrupt change from a calcareous signature below to an argillaceous character above. Toward the shelf, in the West Pembina area of west-central Alberta, the Z Marker correlates to a level with in a conformable succession of nodular Lime wackestones and correspond s to the base of a depositional cycle near the middle of the Lobstick Member of the Nisku Formation. Further shelfward, the Z Marker continu es as a well-defined log marker until the Nisku shelf margin. Still fu rther east, it corresponds to a level in the lower part of the Nisku F ormation in its type locality (well 12-25-5026mT4) on the eastern shel f, along the underlying Rimbey-Meadowbrook Leduc reef chain. The Z Mar ker is a stratigraphically significant surface in the Ireton basinal s uccession because it more closely approximates the division between Wo odbend and Winterburn Group deposits on the eastern shelf than any oth er widespread, correlatable marker. Our correlations clearly demonstra te that Ireton shales above the Z Marker in the West-Central Alberta B asin and coeval Fort Simpson deposits in northern Alberta are stratigr aphically equivalent to the Nisku Formation and are not part of st bas inally restricted wedge that predates most or all of the Nisku Formati on. As well, the Z Marker is a genetically significant surface because it separates an underlying progradational uppermost Ireton-lower Lobs tick succession from an overlying retreating to backstepping upper Lob stick to Bigoray-equivalent Nisku succession in the West Pembina-Nisku shelf-margin area. The Z Marker also marks the onset of isolated down slope Nisku reef growth in the West Pembina area. The correct recognit ion and correlation of this marker permits an understanding of basin e volution beyond that discernable from the existing Lithostratigraphic nomenclature alone.