The Mississippian sequence exposed at North Georgetown Canyon, Idaho is newly recognised as a facies belt, which adds to knowledge of Mississippian stratigraphy and petroleum geology in the Overthrust belt of Idaho, Wyoming, and Utah. In the newly recognized facies belt in the Aspen Range, the Madison Group is represented by the Lodgepole Limestone and Mission Canyon Limestone. The overlying beds of middle Osagean to middle Chesterian age are a unique combination of deep-basin shallowing upward to peritidal lithofacies (the Aspen Range Formation). An organic-rich phosphatic member at the base of this is a possible petroleum source rock. Westward thinning of the Mission Canyon Limestone suggests a pinch-out that may provide stratigraphic traps for petroleum.-from Authors