Origin and Classification of Middle Wallace Breccias, Mesoproterozoic Belt Supergroup, Montana and Idaho

A series of enigmatic breccias in the Wallace Formation, Mesoproterozoic Belt Supergroup, have been variously interpreted as originating from: (1) syndepositional downslope slumping; (2) syndepositional evaporite dissolution and collapse; or (3) post- depositional tectonic faulting. Each hypothesis carries its own implications regarding the depositional and diagenetic history of the Belt basin. Examination of several Middle Wallace breccia outcrops suggests that they can be grouped into two distinct categories based on size of breccia bodies, clast size, and relationships with surrounding strata, and matrix composition. Results indicate that rather than a single origin, these two breccia groups originate from distinctly different mechanisms, suggesting a varied syn- and post- depositional history of the Wallace Formation. Group 1 breccias, typified by the Trout Creek locality, are concordant with surrounding strata, occur in conjunction with soft- sediment folding, and contain detrital sand and silt in the matrix. These characteristics suggest a syndepositional downslope slump mechanism of brecciation. Group 2 breccias, typified by the Lostkey locality, are extremely large breccia bodies, contain clasts >10 m, and contain abundant authigenic quartz and feldspar in the matrix. These breccias are the result of a post-depositional tectonic mechanism of brecciation. Both groups show evidence of secondary overprinting by hydrothermal fluids. Group 1 breccias are associated with syndepositional faulting on the western edge of the Belt basin, and Group 2 breccias are related to pluton emplacement in the Late Cretaceous through Early Tertiary.