Under pressure: clastic dykes in glacial settings Jaap J.M. van der Meer a, * , K.H. Kjær b , J. Kru ¨ ger c , J. Rabassa d , A.A. Kilfeather a, e a Dept of Geography, Queen Mary, University of London, Mile End Road, London E1 4NS, UK b Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark c Institute of Geography and Geology, University of Copenhagen, Øster Voldgade 10,1350 Copenhagen K, Denmark d Laboratorio de Geologia del Cuaternario, CADIC-CONICET, C.C. 92, 9410 Ushuaia, Argentina e Department of Geography, Durham University, South Road, Durham DH1 3LE, UK article info Article history: Received 14 February 2008 Accepted 10 July 2008 abstract 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. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Clastic dykes (dikes), also referred to as water escape structures (WES) or hydrofractures, have been described from many different environments (e.g. Johnson, 1986; Singh and Jain, 2007), sedi- mentary settings and ages (e.g. Mount, 1993; Parize and Frie `s, 2003). In general they are considered to be created by the filling of a space created by the separation of walls that were formerly in contact. Depending on the consolidation of the host rock, a closed fracture may have existed prior to the formation of the dyke, or it may have been an instantaneous cut-and-fill (Larsen and Man- gerud, 1992). The infill can come from above or from below, the latter often ascribed to liquefaction caused by earthquakes. More or less horizontal stretches of clastic dykes are also referred to as clastic sills. However, as water escape structures are often complex and consist of both (sub-) vertical and (sub-) horizontal elements of differential length, we use the term clastic dyke to describe all features. Field and thin section descriptions of the clastic dykes presented in this paper have previously been published by the authors in various wider studies (for instance van der Meer et al., 1999; Kjær et al., 2006). The aims of this paper are to draw this information together to (a) demonstrate the widespread occurrence of clastic dykes in (sub-) glacial settings, (b) to analyse their sedimentology at the macro- and microscale and (c) to discuss the role of clastic dykes in (sub-) glacial hydrology and the implication of their formation for glaciodynamics such as ice velocity and landform formation. 2. Methods All the dykes have been logged, described and photographed in the field at natural exposures and intact block samples of the dykes and their host sediments were taken for thin section production. Thin sections were prepared as well as their microscopic description is based on the methods outlined in van der Meer (1993), while the terminology follows that of van der Meer (1993) and Menzies (2000). The thin sections were photographically covered completely and the micro-images were projected onto paper so that accurate and detailed diagrams of sedimentary structures could be drawn for entire thin section samples by stitching the sketches. 3. Setting 3.1. Switzerland Clastic dykes have been described from several sites in western Switzerland, in a study covering several hundred exposures * Corresponding author. Tel.: þ44 207 882 8416. E-mail address: j.meer@qmul.ac.uk (J.J.M. van der Meer). Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev 0277-3791/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2008.07.017 Quaternary Science Reviews 28 (2009) 708–720