ORIGIN OF MIDDLE WALLACE BRECCIAS, MESOPROTEROZOIC BELT SUPERGROUP, MONTANA AND IDAHO

A series of enigmatic breccias in the Wallace Formation, Mesoproterozoic Belt Supergroup, have been interpreted as originating from either: (1) syndepositional downslope slumping; (2) syndepositional evaporite dissolution and collapse; or (3) post-depositional tectonic fracturing. 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. Rather than a single genesis mechanism, these breccia groups originate from distinctly different mechanisms. Group 1 breccias are concordant with surrounding strata, occur in conjunction with soft-sediment folding, have abundant detrital sand and silt in the matrix, and contain minor authigenic dolomite. These characteristics suggest a syndepositional downslope slump mechanism of brecciation, and associated hydrothermal overprinting. Group 2 breccias are extremely large breccia bodies that cross-cut surrounding strata, contain clasts >10 m, and are dominated by a matrix of authigenic quartz, feldspar, and dolomite. These breccias are clearly postdepositional and are interpreted to result from hydraulic fracturing of the host rock during Mesozoic-Early Tertiary emplacement of the Idaho/Boulder batholiths. No breccias show evidence of having originated via evaporite solution and collapse.

Group 1 breccias are likely associated with syndepositional extension of the Belt Basin and local remobilization of Middle Wallace strata. Although no evidence exists for a high depositional gradient, shelf-slope transition at this time, recent analysis of Helena/Wallace Formation cycles suggests episodic increases in basin subsidence consistent with syndepositional extension. Group 2 breccias are likely associated with those of the Coeur d'Alene mining district. mineralization. Proximity of Group 2 breccias to the Idaho batholith would have limited fluid contact with underlying shales of the Prichard Formation, which are believed to have sourced Pb, Zn, and Ag enrichment in the Coeur d'Alene.