Under pressure: clastic dykes in glacial settings

Clastic dykes are widespread in glacial settings, and we provide examples from Switzerland, Patagonia, Iceland and the Antarctic, ranging in age from Tertiary to recent. On the basis of these examples we establish the general characteristics of clastic dykes and proceed to establish the direction of propagation on sedimentological grounds. Micromorphological analysis reveals that sedimentation in the dykes is ruled by pressure gradients and that ordinary sedimentological rules do not apply. Clastic dykes act like safety valves in the subglacial hydraulic system. Their development depends on specific subglacial conditions like water volume and pressure, the nature of the bed, sediments or bedrock, and the hydraulic properties of the bed. A number of scenarios for clastic dyke development are presented as there is not just one set of conditions under which they form. Development of clastic dykes affects glaciodynamics, like velocity and surging and landform formation, like till thickness and bedrock disruption.