Shear phenomena in ice-thrust gravels, central Alberta

Pressures exerted by the continental ice sheet produced sheared zones and a pervasive crushing of strong quartzite clasts in preglacial Saskatchewan Gravels of central Alberta. Where deformation is of moderate intensity clasts have fractured at points of impingement with other clasts, and clasts of 3–6 cm in the longest dimension are most likely to be shattered. In zones of intense shear nearly all clasts larger than coarse sand have been crushed and drawn out into lensoid masses.An index of fracturing was devised to measure the degree of ice pressure induced deformation where the gravels show no evidence of having been disturbed. A model that defines possible conditions at the time of deformation is presented.