Big Snowy and Amsden groups and the Mississippian-Pennsylvanian boundary in Montana

Abstract

The Big Snowy Group is redefined to include the Kibbey, Otter, and Heath Formations, as the group was originally established. However, the Heath Formation, and consequently the top of the Big Snowy Group, is restricted to strata beneath a Late Mississippian to Early Pennsylvanian regional unconformity. This restriction limits the Big Snowy to three closely related formations that comprise one sedimentary cycle uncomplicated by intraformational structural movements. Also, the Big Snowy Group, as restricted, closely approximates a timestratigraphic unit of Late Misisissippian age.