Abstract

In the subsurface of the Northern Interior Plains, the contact between the dolomites of the Upper Cambrian to Lower Ordovician Franklin Mountain Formation and the overlying Upper Ordovician to Silurian Mount Kindle Formation is difficult to pick from drill cuttings or wireline logs, although the contact represents a stratigraphic hiatus of up to 40 Ma. No single criterion can be applied to pick this boundary from samples or logs. Locally, an empirical marker is present, formed by a small concentration of siliciclastic material in the carbonates, either as a thin sandstone bed, or as floating, rounded sand grains. This contact marker is persistent at several locations in the Northern Interior Plains.

Other characteristics used to determine this formation boundary are: a kick on the gamma-ray log, the presence of green waxy shales, and coloured dolomite. Part of the dolomite is light grey and laminated; elsewhere, it is green or “pastel” coloured, as a result of the carbonate crystals being coated with coloured shale material. The first occurrence of chert in abundance in the upper member of the Franklin Mountain Formation, and changes in the colour and crystal size of the dolomites can only be used as proximity indicators.