CLASSIFICATION AND ORIGIN OF MIDDLE WALLACE BRECCIAS, MESOPROTEROZOIC BELT SUPERGROUP, MT AND ID

A series of enigmatic breccias in the Wallace Formation, Mesoproterozoic Belt Supergroup, have been variously interpreted as originating from either: (1) syndepositional downslope slumping, (2) syndepositional evaporite dissolution and collapse, or (3) post-depositional tectonic faulting. Each hypothesis carries its own implications and broad significance regarding the depositional and diagenetic history of the Belt Basin. Examination of several Middle Wallace Breccia outcrops suggest that, rather than a single mode of origin, breccias can be grouped into three distinct categories based on size of breccia bodies, clast size, and relationships with surrounding strata. The first group, typified by the Trout Creek and surrounding breccias, consists of 2 cm to 2.5 m sandstone and argillite clasts in a matrix of quartz sand and carbonate. Numerous soft-sediment folds suggest these breccias originated as syndepositional slump features. The second group, typified by the Lolo #2 and Clear Creek breccias, consists of 2 cm to 3 m sandstone clasts in a matrix of quartz, feldspar and carbonate. Faulted contacts with surrounding strata suggest these are post-depositional tectonic breccias. Both group one and two breccias cut surrounding strata. The third group, typified by the Lolo #1 and Lostkey breccias, consist of bodies 200 to 400 m high and >200 m wide with 2 cm to >10 m sandstone, argillite and dolomite clasts in a carbonate and sand matrix. The origin of group three breccias is as yet indeterminate. All three groups contain hydrothermal dolomite and quartz in their matrix indicating secondary overprinting of breccia textures. Although additional research is needed to constrain the specific origins of each breccia grouping, preliminary results indicate that breccias originate from several distinct mechanisms, suggesting a diverse syn- and post-depositional history of the Wallace Formation.