Abstract

Deformed diamictites and associated sediments of the lower part of the Upper Proterozoic Windermere Group have been examined on all sides of the Purcell geanticline of southeastern British Columbia. In all areas examined the rocks are characterized by two early cleavages having a variable angle between them of 10° to 35°; the earlier of the two cleavages is always slaty, whereas the second may be slaty or fracture with offset, according to lithology.Clasts are aligned parallel to both cleavages; those parallel to the earlier cleavage have extension features perpendicular to both cleavages. Mineral overgrowths at the strain shadows of clasts are parallel to both cleavages and show similar time relations. Clast rotation, variation in flattening strain, and the earlier slaty cleavage with associated mineral growth are present throughout the whole region and were probably initiated by low angle buckling of water-saturated sediments. Continued buckling might have brought about further dewatering; associated flattening apparently gave rise to the second cleavage and mineral overgrowth, to rotation of the earlier cleavage, and to further rotation of some clasts parallel to the new cleavage.

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