Tectonically deformed pebbles in the Brazeau and Paskapoo formations, central Alberta Foothills, Canada

Abstract

Characteristic features of pebbles in the Entrance and High Divide Ridge conglomerates, of late Maastrichtian and mid-Paleocene age, respectively, are the common occurrence of pock marks, fractures and polished surfaces with striations. These features are due to tangentially oriented tectonic movements that affected the highly clast supported conglomerate composed of well-rounded quartzite pebbles. The stress was transmitted through the points of contact between the pebbles, causing solution of the silica and producing pock marks and tensile fractures extending through whole pebbles. The remobilized silica tended to migrate and recrystallize, forming cement in some parts of the conglomerate. Many pebbles have been reoriented to occupy the most stable position in relation to the maximum tectonic stress. Their componental movements within the matrix produced striations on their surfaces and lineations on their imprints. The resulting preferred orientation is expressed on most of the striated surfaces of the elongated pebbles. The direction of the maximum compressive stress, measured as a line parallel to the major tensile fractures joining two opposite pock marks, also shows a preferred orientation normal to the main NW-SE structural trend of the Rocky Mountains. Layer-parallel tectonic movements have also produced slickensides on the planes of sandstone interbeds in the Entrance and High Divide Ridge conglomerates. The occurrence of the deformed pebbles in the Brazeau-Paskapoo Formations of the Alberta Foothills may indicate repeated deformation accompanying the Laramide Orogeny in an area of the compressional foreland basin in the Cordillera. Deformed pebbles may be useful in dating Laramide deformation in the area of the Foothills and the Plains.