A REINTERPRETATION OF FRACTURING AT TETON ANTICLINE, SAWTOOTH RANGE, MONTANA

Classic mapping of the Teton Anticline (Sawtooth Range, Montana) documented two fracture assemblages (i.e., Stearns, 1964; Friedman and Stearns, 1971). Each assemblage is characterized by one fracture in a plane defined by principal stresses and two fractures intersecting to make a modest acute angle bisected by the direction of the maximum principal stress. Using calcite for a dynamic analysis, F&S (1971) could find no evidence of a stress consistent with the strike-parallel fracture assemblage (#2). F&S did observe stress recorded by calcite that is consistent in orientation with an early layer-parallel shortening (LPS) in the flanks of Teton Anticline and the cross-fold fracture assemblage (#1). However, as an understanding mechanics matured, it became clear that any three-component fracture assemblage as envisioned by F&S is not co-genetic. Based on preliminary mapping during the summer of 2006 a number of observations will apply to a reinterpretation of fracturing at Teton Anticline. First, disjunctive cleavage appears on several scales and could have been responsible for the F&S interpretation that there was a strike-parallel assemblage (#2) and a bedding normal assemblage (#3). Second, jointing in the Jurassic Morrison is consistent with a prefolding regional joint set in the cross-fold orientation and the pre-folding LPS. Third, multiple joint sets are consistent with fold growth and may well have populated the cross-fold fracture assemblage of F&S. Fourth, the most common fracture type in the Mississippian Madison is a mini-fracture associated with bedding-parallel stylolitization during overburden compaction. These could have easily populated both F&S fracture assemblages.