The Z Marker is a distinctive and widespread wireline log marker within the thick Frasnian Ireton shale basin succession of west-central Alberta. 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 within a conformable succession of nodular Lime wackestones and corresponds to the base of a depositional cycle near the middle of the Lobstick Member of the Nisku Formation. Further shelfward, the Z Marker continues as a well-defined log marker until the Nisku shelf margin. Still further east, it corresponds to a level in the lower part of the Nisku Formation in its type locality (well 12-25-5026mT4) on the eastern shelf, along the underlying Rimbey-Meadowbrook Leduc reef chain. The Z Marker is a stratigraphically significant surface in the Ireton basinal succession because it more closely approximates the division between Woodbend and Winterburn Group deposits on the eastern shelf than any other widespread, correlatable marker. Our correlations clearly demonstrate that Ireton shales above the Z Marker in the West-Central Alberta Basin and coeval Fort Simpson deposits in northern Alberta are stratigraphically equivalent to the Nisku Formation and are not part of st basinally restricted wedge that predates most or all of the Nisku Formation. As well, the Z Marker is a genetically significant surface because it separates an underlying progradational uppermost Ireton-lower Lobstick succession from an overlying retreating to backstepping upper Lobstick to Bigoray-equivalent Nisku succession in the West Pembina-Nisku shelf-margin area. The Z Marker also marks the onset of isolated downslope Nisku reef growth in the West Pembina area. The correct recognition and correlation of this marker permits an understanding of basin evolution beyond that discernable from the existing Lithostratigraphic nomenclature alone.