Pebble deformation in the San Antonio formation, Rice Lake area, Manitoba

Abstract

The San Antonio formation is a sheared feldspathic quartzite which contains pebbles concentrated in conglomeratic beds and scattered throughout the quartzite. Field data supports the contention that the pebbles were all deformed under the same environmental conditions. Most of the pebbles were derived from granitic rocks, vein quartz, and volcanic rocks. Studies of the average shapes of pebbles of these compositions have indicated the following orthogonal dimension ratios (a:b:c): Granitic 1.95 : 1.45 : 1.0 Quartz 2.30 : 1.55 : 1.0 Volcanic 6.30 : 4.50 : 1.0 The long axes of the pebbles pitch an average of 64o to the east in the plane of the regional foliation of the quartzite, and are oriented approximately parallel to the interpreted direction of shearing movement on these planes. The mechanism of deformation of the pebbles is interpreted as brittle fracture followed by recrystallization, resulting in flattening and elongation in the shear planes during low temperature shear folding of the San Antonio formation. A comparison of compositional data versus the relative deformation indicates that the degree of deformation generally increases with increasing mica content in the pebbles, and decreases with increasing quartz and feldspar content.