THE MADISON, A NOMENCLATURAL REVIEW WITH A LOOK TO THE FUTURE

ABSTRACT The difficulty of extending nomenclature applied to Madison strata of the outcrops in Montana into the subsurface in the Williston Basin was recognized early and led to the addition of the Charles Formation to the Madison Group in the subsurface of eastern Montana. Recognition of facies changes across the Basin led to the use of marker horizons to delineate marker-defined units within Madison strata on the east flank of the Basin. Numerous terms were introduced for marker-defined units and different horizons were used in different areas for similar terms. Exploration has now provided a means for establishing the stratigraphic relationships of the marker-defined units, and a review of the stratigraphic code provides a basis for clarifying nomenclature. It is herein proposed that the lithostratigraphic terms Madison Group, Charles, Mission Canyon, and Lodgepole Formations be retained as formal stratigraphic units. The facies relationships of the evaporites and carbonates are recognized by placing the Charles-Mission Canyon contact at progressively lower stratigraphic horizons towards the east flank of the Basin, so that the lowest evaporites are in the Charles Formation. The marker-defined units or marker horizons are considered informal units and may be referred to as intervals, subintervals, cycles, formats, marker horizons, or facies.