Debris-flow lobes on the distal flanks of terminal moraines in Spitsbergen fjords

Abstract

Terminal and recessional moraines formed at the margins of tidewater glaciers are often asymmetrical, with relatively steep ice-distal sides and lower-gradient ice-proximal slopes (e.g. Benn & Evans 2010; Dowdeswell & Vasquez 2013). Their ice-distal slopes, in particular, may be the locations of slope failure and the generation of debris flows that produce elongate lobes of sediment that flow down the distal moraine-ridge flanks. Glacier-derived diamictic sediment forms the major component of most moraine ridges, and debris flows derived from such material would also be expected to produce debrites of heterogeneous particle size and with abrupt lateral margins and terminations. In several Spitsbergen fjords (Fig. 1g), well-defined elongate lobes of sediment are found on the relatively steep (but usually <2°) ice-distal slopes of terminal moraine ridges (Fig. 1) (e.g. Boulton et al. 1996; Plassen et al. 2004; Ottesen & Dowdeswell 2006). The submarine crests of the terminal moraines are typically too shallow to be traversed by survey vessels, so the ridge crests themselves occur just beyond the swath coverage at the left side of each image in Figure 1 at water …