New evidence on the age of the top of the Madison Limestone (Mississippian), Bighorn Mountains, Wyoming and Montana

The youngest strata recognized in the Madison Limestone arc dated by foraminifers and corals at two localities on the west flank of the Bighorn Mountains in Wyoming and Montana. Fossils collected in situ from the Madison at Shell Canyon represent Zone 12 of Mamet and Skipp of late Salem age (middle Visean), previously thought to be unrepresented by strata in the northern Cordilleran region of the United States. A block of residual chert found at Devils Canyon contains fossils with a possible age range of microfaunal Zones 12 15 (late Salem to Ste. Genevieve, middle to late Visean), providing further evidence of an extended age range for Madison strata prior to post-Madison, pre-Amsden erosion.