Abstract

Zones of breccia within the Madison group of SW. Montana consist of very poorly sorted angular fragments of calcilutite or silty limestone, which are set in a fine-grained matrix of crushed "rock-flour" and cemented by calcite. In places where the matrix was silty or sandy it weathers yellow-brown and contains abundant authigenic quartz crystals. The breccias are confined stratigraphically to zones which may be correlated with evaporites in subsurface sections. The upper contacts of the breccia zones are indistinct, but the lower contacts are well defined; in some localities, brecciation occurred in place with little displacement of the fragments. The brecciation resulted from the collapse and crushing of nonsoluble strata after the solution by ground water of beds of evaporite.

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