Neoglacial (<3000 years) till and flutes at Saskatchewan Glacier, Canadian Rocky Mountains, formed by subglacial deformation of a soft bed

Abstract

Understanding the processes that deposit till below modern glaciers provides fundamental information for interpreting ancient subglacial deposits. A process-deposit-landform model is developed for the till bed of Saskatchewan Glacier in the Canadian Rocky Mountains. The glacier is predominantly hard bedded in its upper reaches and flows through a deep valley carved into resistant Palaeozoic carbonates but the ice margin rests on a thick (<6 m) soft bed of silt-rich deformation till that has been exposed as the glacier retreats from its Little Ice Age limit reached in 1854. In situ tree stumps rooted in a palaeosol under the till are dated between ca 2900 and 2700 yr BP and record initial glacier expansion during the Neoglacial. Sedimentological and stratigraphic observations underscore the importance of subglacial deformation of glaciofluvial outwash deposited in front of the advancing glacier and mixing with glaciolacustrine carbonate-rich silt to form a soft bed. The exposed till plain has a rolling drumlinoid topography inherited from overridden end moraines and is corrugated by more than 400 longitudinal flute ridges which record deformation of the soft bed and fall into three genetically related types: those developed in propagating incipient cavities in the lee of large subglacial boulders embedded in deformation till, and those lacking any originating boulder and formed by pressing of wet till up into radial crevasses under stagnant ice. A third type consists of U-shaped flutes akin to barchan dunes; these wrap around large boulders at the downglacier ends of longitudinal scours formed by the bulldozing of boulders by the ice front during brief winter readvances across soft till. Pervasive subglacial deformation during glacier expansion was probably facilitated by large boulders rotating within the soft bed ('glacioturbation').

The formation of a rectilinear ridge network, Múlajökull, Iceland

“…al. (2003) in support of a crevasse-related origin included coincidence of ridges with the location of former radial crevasses on the glacier and the emergence of some ridges from the foot of crevasses at the ice front (Eyles et al, 2014). Another similar hypothesis described a 'till esker' that formed by squeezing of a basal till into an elongated cavity or R-channel at the base of the glacier (Evans et al, 2010).…”

“…Longitudinal crevassing is a characteristic of many glaciers (Nye, 1952;Rea and Evans, 2011;Eyles et al, 2014). Piedmont glacier lobes are commonly characterized by a radial (i.e., longitudinal) crevasse pattern, which forms due to the lateral spreading of the ice (Benn and Evans, 2010).…”

“…AMS and clast fabrics indicating downglacier simple shear have long been documented in flutes (Eklund and Hart, 1996;Eyles et al, 2014), including those at Múlajökull (Ives, 2016 (Fig. 1.4), makes the crevasse-squeeze hypothesis especially viable.…”

Factors affecting the dynamics of the North Karelian/Oulu Ice Lobe, Central Finland, during the last deglaciation – a LiDAR and DEM interpretation of subglacial lineation patterns

“…Accordingly, the small flutings were most likely deposited under a relatively thin glacier (cf. Eyles et al, 2015).…”

High-resolution LiDAR mapping of glacial landforms and ice stream lobes in Finland

“…Such debris is partially reworked subglacially by canalized meltwater streams and left as esker-fan complexes or much larger sandur plains during ice retreat (Fig. 2) (see Boulton et al, 2001;Putkinen & Lunkka, 2008;Eyles et al, 2015;Ojala et al, 2016). Significant areas of dissected meltwater-deposited sediment terrain underlain by glaciotectonically-deformed and partially collapsed outwash interbedded with diamict facies deposited by mass flow at subaqueous-terminating ice margins at water depths of 50 m or more.…”