Megascopic and microscopic study of mid-Paleozoic carbonate formations in southwestern Montana shows that the Lodgepole Limestone (late Kinderhook to middle Osage), the lower formation of the Madison Group, is particularly suited to evaluation of paleogeographic history, and is useful in palinspastic reconstructions. The region of deposition was transitional from positive or shelf to geosyncline through several periods of geologic time. Isopach maps of the Madison and Lodgepole show thickening from the shelf into the miogeosyncline. This parameter, however, must be used with great caution in determining the paleogeography.

When surface sections distributed over a significantly large region are studied using the Energy Index (El) classification of limestones, subtle changes can be outlined in the energy of the depositional environments. Further, the environments can be defined in much greater detail than otherwise is possible. The research indicates that the shelf facies of northwest Wyoming and adjacent Montana were characterized by Type III limestone during deposition of the Lodgepole, indicating deposition in slightly agitated water. Some of the westernmost exposures in southwestern Montana are characterized by Type I limestone, indicating deposition in the quiet water of a low-energy environment characteristic of the miogeosyncline. The latter area must have been remote from any shelf, bank, or posit ve.

The megascopic appearance of the rocks in outcrop appears virtually identical throughout this region; thus, although the gross characteristics of the formation or any smaller lithologic unit may appear consistent, microscopic texture can vary widely. A study of this texture permits evaluation of the depositional environment.