Crushed Pebble Conglomerate of Southwestern Montana

Abstract

Many quartzite pebbles in the Tertiary Beaverhead Formation in southwestern Montana are scarred or deformed in two distinctive ways: (1) at points of mutual pebble contact, dents (or depressions) have been formed as a result of squeezing of the strata; and (2) at localities where deformation has been the greatest, individual pebbles have been sheared (and commonly rehealed), in some instances showing ten or more shear planes in a single stone. The direction of motion inferred from sheared pebbles supports the hypothesis of pronounced lateral (strike-slip) displacement in the area. Drag folds yield the same result. Vertical offset, although more spectacular, must be relatively minor. Sandstone lenses and beds within the conglomerate show little evidence of deformation. If resistant pebbles in contact with each other had not been common in the Beaverhead, the large-scale horizontal displacement would have gone unnoticed, and only the vertical motion would have been recorded. The width of the zone of shattered pebbles indicates that much of the displacement was taken up in the strata, rather than by dislocation along an adjacent fault plane. These observations tend to confirm the hypothesis that, in deformed regions, measurements of offset produce results that are conservative by one or more orders of magnitude.